Internet DRAFT - draft-pelletier-rohc-rohcv2-profiles
draft-pelletier-rohc-rohcv2-profiles
Robust Header Compression G. Pelletier
Internet-Draft K. Sandlund
Expires: December 20, 2006 Ericsson
June 18, 2006
RObust Header Compression Version 2 (RoHCv2): Profiles for RTP, UDP, IP,
ESP and UDP Lite
draft-pelletier-rohc-rohcv2-profiles-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document specifies ROHC (Robust Header Compression) profiles
that efficiently compress RTP/UDP/IP (Real-Time Transport Protocol,
User Datagram Protocol, Internet Protocol), RTP/UDP-Lite/IP (User
Datagram Protocol Lite), UDP/IP, UDP-Lite/IP, IP and ESP/IP
(Encapsulating Security Payload) headers.
This specification update the profiles defined in RFC 3095, RFC 3843
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and RFC 4019 to their second version (RoHCv2 profiles). The profiles
herein thus supersede their earlier definition, but they do not
obsolete them.
The RoHCv2 specification introduce a number of simplifications to the
rules and algorithms that govern the behavior of the compression
endpoints. It also defines robustness mechanisms that may be used by
a compressor implementation to increase the probability of
decompression success when packets can be lost and/or reordered on
the ROHC channel. Finally, the RoHCv2 profiles define its own
specific set of packet formats, using the ROHC formal notation.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Background . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Classification of header fields . . . . . . . . . . . . . 8
4.2. Operational Characteristics of RoHCv2 Profiles . . . . . 9
5. Overview of the RoHCv2 Profiles . . . . . . . . . . . . . . . 9
5.1. General Concepts . . . . . . . . . . . . . . . . . . . . 9
5.1.1. Control Fields and Context Updates . . . . . . . . . 10
5.2. Compressor Concepts . . . . . . . . . . . . . . . . . . . 10
5.2.1. Optimistic Approach . . . . . . . . . . . . . . . . . 10
5.2.2. Tradeoff between robustness to losses and to
reordering . . . . . . . . . . . . . . . . . . . . . 11
5.2.3. Interactions with the Decompressor Context . . . . . 12
5.3. Decompressor Concepts . . . . . . . . . . . . . . . . . . 13
5.3.1. Decompressor State Machine . . . . . . . . . . . . . 13
5.3.2. Decompressor Context Management . . . . . . . . . . . 16
5.3.3. Feedback logic . . . . . . . . . . . . . . . . . . . 17
6. RoHCv2 Profiles (Normative) . . . . . . . . . . . . . . . . . 18
6.1. Profile Operation, per-context . . . . . . . . . . . . . 18
6.2. Control Fields . . . . . . . . . . . . . . . . . . . . . 19
6.2.1. Master Sequence Number (MSN) . . . . . . . . . . . . 19
6.2.2. IP-ID behavior . . . . . . . . . . . . . . . . . . . 20
6.3. Reconstruction and Verification . . . . . . . . . . . . . 20
6.4. Compressed Header Chains . . . . . . . . . . . . . . . . 20
6.5. Packet Formats and Encoding Methods . . . . . . . . . . . 22
6.5.1. baseheader_extension_headers . . . . . . . . . . . . 22
6.5.2. baseheader_outer_headers . . . . . . . . . . . . . . 22
6.5.3. inferred_udp_length . . . . . . . . . . . . . . . . . 22
6.5.4. inferred_ip_v4_header_checksum . . . . . . . . . . . 22
6.5.5. inferred_mine_header_checksum . . . . . . . . . . . . 23
6.5.6. inferred_ip_v4_length . . . . . . . . . . . . . . . . 24
6.5.7. inferred_ip_v6_length . . . . . . . . . . . . . . . . 24
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6.5.8. Scaled RTP Timestamp Encoding . . . . . . . . . . . . 25
6.5.9. inferred_scaled_field . . . . . . . . . . . . . . . . 25
6.5.10. control_crc3 . . . . . . . . . . . . . . . . . . . . 26
6.5.11. inferred_sequential_ip_id . . . . . . . . . . . . . . 26
6.5.12. list_csrc(cc_value) . . . . . . . . . . . . . . . . . 27
6.6. Packet Formats . . . . . . . . . . . . . . . . . . . . . 30
6.6.1. Initialization and Refresh Packet (IR) . . . . . . . 31
6.6.2. IR Packet Payload Discard (IR-PD) . . . . . . . . . 32
6.6.3. IR Dynamic Packet (IR-DYN) . . . . . . . . . . . . . 33
6.6.4. Compressed Packet Formats (CO) . . . . . . . . . . . 34
6.7. Feedback Formats and Options . . . . . . . . . . . . . . 85
6.7.1. Feedback Formats . . . . . . . . . . . . . . . . . . 85
6.7.2. Feedback Options . . . . . . . . . . . . . . . . . . 87
7. Security Considerations . . . . . . . . . . . . . . . . . . . 89
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 89
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 90
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 90
10.1. Normative References . . . . . . . . . . . . . . . . . . 90
10.2. Informative References . . . . . . . . . . . . . . . . . 92
Appendix A. Detailed classification of header fields . . . . 92
Appendix A.1. General classification . . . . . . . . . . . . . 93
Appendix A.1.1. IPv4 header fields . . . . . . . . . . . . . . . 93
Appendix A.1.2. IPv6 header fields . . . . . . . . . . . . . . . 95
Appendix A.1.3. UDP header fields . . . . . . . . . . . . . . . 96
Appendix A.1.4. UDP-Lite header fields . . . . . . . . . . . . . 96
Appendix A.1.5. RTP header fields . . . . . . . . . . . . . . . 97
Appendix A.2. Analysis of change patterns of header fields . . 98
Appendix A.2.1. IPv4 Identification . . . . . . . . . . . . . . 100
Appendix A.2.2. IP Traffic Class / Type-Of-Service . . . . . . . 101
Appendix A.2.3. IP Hop-limit / Time-To-Live . . . . . . . . . . 101
Appendix A.2.4. IPv4 Don't Fragment . . . . . . . . . . . . . . 101
Appendix A.2.5. UDP Checksum . . . . . . . . . . . . . . . . . . 102
Appendix A.2.6. UDP-Lite Checksum Coverage . . . . . . . . . . . 102
Appendix A.2.7. UDP-Lite Checksum . . . . . . . . . . . . . . . 102
Appendix A.2.8. RTP CSRC Counter . . . . . . . . . . . . . . . . 102
Appendix A.2.9. RTP Marker . . . . . . . . . . . . . . . . . . . 102
Appendix A.2.10. RTP Padding . . . . . . . . . . . . . . . . . . 102
Appendix A.2.11. RTP Extension . . . . . . . . . . . . . . . . . 103
Appendix A.2.12. RTP Payload Type . . . . . . . . . . . . . . . . 103
Appendix A.2.13. RTP Sequence Number . . . . . . . . . . . . . . 103
Appendix A.2.14. RTP Timestamp . . . . . . . . . . . . . . . . . 103
Appendix A.2.15. RTP Contributing Sources (CSRC) . . . . . . . . 103
Appendix A.3. Header compression strategies . . . . . . . . . 104
Appendix A.3.1. Do not send at all . . . . . . . . . . . . . . . 104
Appendix A.3.2. Transmit only initially . . . . . . . . . . . . 104
Appendix A.3.3. Transmit initially, be prepared to update . . . 104
Appendix A.3.4. Be prepared to update, or send as-is
frequently . . . . . . . . . . . . . . . . . . . 105
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Appendix A.3.5. Guarantee continuous robustness . . . . . . . . 105
Appendix A.3.6. Transmit as-is in all packets . . . . . . . . . 105
Appendix A.3.7. Establish and be prepared to update delta . . . 105
Appendix B. Differences between RoHCv2 and RFC3095
profiles . . . . . . . . . . . . . . . . . . . . 106
Appendix C. Sample CRC algorithm . . . . . . . . . . . . . . 106
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 109
Intellectual Property and Copyright Statements . . . . . . . . . 110
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1. Introduction
The ROHC WG has developed a header compression framework on top of
which various profiles can be defined for different protocol sets or
compression requirements. The ROHC framework was first documented in
[RFC3095], together with profiles for compression of RTP/UDP/IP
(Real-Time Transport Protocol, User Datagram Protocol, Internet
Protocol), UDP/IP, IP and ESP/IP (Encapsulating Security Payload)
headers. Additional profiles for compression of IP headers [RFC3843]
and UDP-Lite (User Datagram Protocol Lite) headers [RFC4019] were
later specified to complete the initial set of ROHC profiles.
This document defines an updated version for each of the above
mentionned profiles, and its definition is based on the specification
of the RoHC framework as found in [I-D.ietf-rohc-rfc3095bis-
framework].
Specifically, this document defines header compression schemes for:
o RTP/UDP/IP : profile 0x0101 (updates profile 0x0001 [RFC3095])
o UDP/IP : profile 0x0102 (updates profile 0x0002 [RFC3095])
o ESP/IP : profile 0x0103 (updates profile 0x0003 [RFC3095])
o IP : profile 0x0104 (updates profile 0x0004 [RFC3843])
o RTP/UDP-Lite/IP : profile 0x0107 (updates profile 0x0007 [RFC4019])
o UDP-Lite/IP : profile 0x0108 (updates profile 0x0008 [RFC4019])
ROHCv2 compresses the following type of extension headers:
o AH [RFC4302]
o GRE [RFC2784][RFC2890]
o MINE [RFC2004]
o NULL-encrupted ESP [RFC4303]
o IPv6 Destination Options header[RFC2460]
o IPv6 Hop-by-hop Options header[RFC2460]
o IPv6 Routing header [RFC2460].
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
This document is consistent with the terminology found in the ROHC
framework [I-D.ietf-rohc-rfc3095bis-framework] and in the formal
notation for ROHC [I-D.ietf-rohc-formal-notation]. In addition, this
document uses or defines the following terms:
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Chaining of Items
A chain groups fields based on similar characteristics. ROHCv2
defines chain items for static, dynamic and irregular fields.
Chaining is done by appending an item for e.g. each header to the
chain in their order of appearance in the uncompressed packet.
Chaining is useful to construct compressed headers from an
arbitrary number of any of the protocol headers for which a ROHCv2
profile defines a compressed format.
CRC-8 validation
The CRC-8 validation refers to the validation of the integrity
against bit error(s) of the received IR and in the IR-DYN header,
using the 8-bit CRC that is included in the header.
CRC verification
The CRC verification refers to the verification of the result of a
decompression attempt, using the 3-bit CRC or 7-bit CRC included
in the header of a compressed packet format (CO).
Delta
The delta refers to the difference in terms of the absolute value
of a field between two consecutive packets and processed by the
same compression endpoint.
Reordering Depth
The number of packets by which a packet made late in its sequence.
See definition of sequentially late packet below.
ROHCv2 packet types
ROHCv2 profiles use two different packet types: the Initialization
and Refresh (IR) packet type, and the Compressed packet type (CO).
Sequentially early packet
A packet that reaches the decompressor before one or several
packets that were delayed over the channel, whereas all of the
said packets belong to the same header-compressed flow and are
associated to the same compression context (CID). At the time of
the arrival of a sequentially early packet, the packet(s) delayed
on the link cannot be differentiated from lost packet(s).
Sequentially late packet
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A packet is late within its sequence if it reaches the
decompressor after one or several other packets belonging to the
same CID have been received, although the sequentially late packet
was sent from the compressor before the other packet(s).
Timestamp stride (ts_stride)
The timestamp stride (TS_STRIDE) is the expected increase in the
timestamp value between two RTP packets with consecutive sequence
numbers.
3. Acronyms
This section lists most acronyms used for reference, in addition to
those defined in [I-D.ietf-rohc-rfc3095bis-framework].
AH Authentication Header.
ESP Encapsulating Security Payload.
GRE Generic Routing Encapsulation. RFC 2784, RFC 2890.
IC Initial Context state (decompressor)
FC Full Context state (decompressor)
IP Internet Protocol.
LSB Least Significant Bits.
MINE Minimal Encapsulation in IP
MSB Most Significant Bits.
MSN Master Sequence Number.
NC No Context state (decompressor).
OA Optimistic Approach.
ROHCv2 Set of header compression profiles defined in this document
RTP Real-time Transport Protocol.
SSRC Synchronization source. Field in RTP header.
CSRC Contributing source. Optional list of CSRCs in RTP header.
TC Traffic Class. Octet in IPv6 header. See also TOS.
TOS Type Of Service. Octet in IPv4 header. See also TC.
TS RTP Timestamp.
UDP User Datagram Protocol.
UDP-Lite User Datagram Protocol Lite.
4. Background
This section provides background information on the compression
profiles defined in this document. The fundamentals of general
header compression and of the ROHC framework may be found in section
3 and 4 of [I-D.ietf-rohc-rfc3095bis-framework], respectively. The
fundamentals of the formal notation for ROHC are defined in
[I-D.ietf-rohc-formal-notation]. [RFC4224] describes the impacts of
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out-of-order delivery on profiles based on [RFC3095].
4.1. Classification of header fields
Section 3.1 of [I-D.ietf-rohc-rfc3095bis-framework] explains that
header compression is possible due to the fact that there is much
redundancy between field values within the headers of a packet, but
especially between the headers of consecutive packets.
Appendix A lists and classifies in detail all the header fields
relevant to this document. The appendix concludes with
recommendations on how the various fields should be handled by header
compression algorithms.
The main conclusion is that most of the header fields can easily be
compressed away since they never or seldom change. A small number of
fields however need more sophisticated mechanisms.
These fields are:
- IPv4 Identification (16 bits) - IP-ID
- ESP Sequence Number (32 bits) - ESP SN
- UDP Checksum (16 bits) - Checksum
- UDP-Lite Checksum (16 bits) - Checksum
- UDP-Lite Checksum Coverage (16 bits) - CC
- RTP Marker ( 1 bit ) - M-bit
- RTP Sequence Number (16 bits) - RTP SN
- RTP Timestamp (32 bits) - TS
In particular, for RTP, the analysis in Appendix A reveals that the
values of the RTP Timestamp (TS) field usually have a strong
correlation to the RTP Sequence Number (SN), which increments by one
for each packet emitted by an RTP source. The RTP M-bit is expected
to have the same value most of the time, but it needs to be
communicated explicitly on occasion.
For UDP, the Checksum field cannot be inferred or recalculated at the
receiving end without violating its end-to-end properties, and is
thus sent as-is when enabled (mandatory with IPv6). The same applies
to the UDP-Lite Checksum (mandatory with both IPv4 and IPv6), while
the UDP-Lite Checksum Coverage may in some cases be compressible.
For IPv4, a similar correlation as the one of the RTP TS to the RTP
SN is often observed between the Identifier field (IP-ID) and the
master sequence number used for compression (e.g. the RTP SN when
compressing RTP headers).
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4.2. Operational Characteristics of RoHCv2 Profiles
Robust header compression can be used over many type of link
technologies. Section 4.4 of [I-D.ietf-rohc-rfc3095bis-framework]
lists the operational characteristics of the ROHC channel. The
RoHCv2 profiles address a wide range of applications, and this
section summarizes some of the operational characteristics that are
specific to these profiles.
Packet length
ROHCv2 profiles assume that the lower layer indicates the length
of a compressed packet. ROHCv2 compressed headers do not contain
length information for the payload.
Out-of-order delivery between compression endpoints
The definition of the RoHCv2 profiles places no strict requirement
on the delivery sequence between the compression endpoints, i.e.
packets may be received in a different order than the compressor
sent them with a fair chance of successfully be decompressed.
However, frequent out-of-order delivery and/or significant
reordering depth will negatively impact the compression
efficiency. More specifically, if the channel state includes
parameters that provide a proper estimate of such significant out-
of-order delivery, larger headers can be sent more often to
increase the robustness against decompression failures due to
reordering. Otherwise if the compressor cannot operate with
sufficient knowledge for such reordering, the efficiency will be
impaired from an increase in the frequency of decompression
failures and recovery attempts.
5. Overview of the RoHCv2 Profiles
This section provides an overview of important and useful concepts of
ROHCv2 profiles. These concepts may be used as guidelines for
implementations but they are not part of the normative definition of
the profiles, as these concepts relates to the compression efficiency
of the protocol without impacting the interoperability
characteristics of an implementation.
5.1. General Concepts
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5.1.1. Control Fields and Context Updates
Control fields have the same attributes and properties as
uncompressed fields [I-D.ietf-rohc-formal-notation]. These fields
are used for compression and decompression of some of the
uncompressed header fields. Updating the value of one or more
control field(s) is thus no less important than updating context
values for header fields. Control fields are defined in [I-D.ietf-
rohc-formal-notation].
Packet types that initialize or update the value of one or more
control field(s) thus include an additional 3-bit CRC, as defined by
the packet formats in Section 6.6. The CRC is calculated using the
UVALUE of the control field(s) that it covers.
This CRC validates all the control fields that are updated. Failure
to verify this CRC should be interpreted by the decompressor as a
decompression failure, in the algorithm it implements to assess the
validity of its context.
5.2. Compressor Concepts
Header compression can be conceptually characterized as the
interaction of a compressor with a decompressor state machine, one
per context. The responsability of the compressor is to minimally
send the information needed to successfully decompress a packet,
based on a certain confidence regarding the state of the decompressor
context.
This confidence is obtained from the frequency and the type of
information the compressor sends when updating the decompressor
context, from the optimistic approach and optionally from feedback
messages received from the decompressor.
5.2.1. Optimistic Approach
A compressor always uses the optimistic approach when it performs
context updates. The compressor normally repeats the same type of
update until it is fairly confident that the decompressor has
successfully received the information. If the decompressor
successfully receives any of the headers containing this update,
state will be available for the decompressor to process smaller
compressed headers.
If field X in the uncompressed header changes value, the compressor
uses a packet type that contains an encoding of field X until it has
gained confidence that the decompressor has received at least one
packet containing the new value for X. The compressor normally
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selects a compressed format with the smallest header that can convey
the changes needed to achieve confidence.
The number N of repetitions for the optimistic approach that is
needed to obtain this confidence is normally related to the packet
loss and to the out-of-order delivery characteristics of the link
where header compression is used; it is thus not defined in this
document and is left open to implementations.
5.2.2. Tradeoff between robustness to losses and to reordering
The ability of a header compression algorithm to handle sequentially
late packets is mainly limited by two factors: the interpretation
interval offset of the sliding window used for LSB encoded fields
[I-D.ietf-rohc-formal-notation], and the optimistic approach
Section 5.2.1 for seldom changing fields.
The interpretation interval offset specifies an upper limit for the
maximum reordering depth, by which is it possible for decompressor to
recover the original value of a dynamically changing field that is
encoded using W-LSB. Its value is bound to the number of LSB
compressed bits in the compressed header format, and grows with the
number of bits transmitted. However, the offset and the LSB encoding
only provide robustness for the field that it compresses, and
(implicitly) for other sequentially changing fields that are derived
from that field.
This is shown in the figure below:
<--- interpretation interval (size is 2^k) ---->
|------------------+---------------------------|
v_ref-p v_ref v_ref + (2^k-1) - p
Lower Upper
Bound Bound
<--- reordering --> <--------- losses --------->
where delta(SN) = p is the maximum negative delta, corresponding
to the maximum reordering depth for which the lsb encoding can
recover the original value of the field;
where delta(SN) = (2^k-1) - p is the maximum positive delta,
corresponding to the maximum number of consecutive losses for
which the lsb encoding can recover the original value of the
field;
where v_ref is the reference value, as defined in the lsb encoding
method in [I-D.ietf-rohc-formal-notation].
The optimistic approach Section 5.2.1 provides the upper limit for
the maximum reordering depth for seldom changing fields.
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There is thus a tradeoff between the robustness against reordering
and the robustness against packet losses, with respect to the number
of MSN bits needed and the distribution of the interpretation
interval between negative and positive deltas in the MSN.
There is also a tradeoff between compression efficiency and
robustness. When only information on the MSN needs to be conveyed to
the decompressor, the tradeoff relates to the number of compressed
MSN bits in the compressed header format. Otherwise, the tradeoff
relates to the implementation of the optimistic approach.
5.2.3. Interactions with the Decompressor Context
The compressor normally starts compression with the initial
assumption that the decompressor has no useful information to process
the new flow, and sends Initialization and Refresh (IR) packets.
The compressor can then adjust the compression level based on its
confidence that the decompressor has the necessary information to
successfully process the compressed headers that it selects. In
other words, the responsability of the compressor is to ensure that
the decompressor operates with state information that is sufficient
to allow decompression of the most efficient compressed header(s),
and to allow the decompressor to successfully recover that state
information as soon as possible otherwise.
The compressor thus has the entire responsability to ensure that the
decompressor has the proper information to decompress the type of
compressed header that it sends. In other words, the choice of
compressed header depends on the following factors:
o the outcome of the encoding method applied to each field;
o the optimistic approach, with respect to the characteristics of
the channel;
o the presence or not of an established feedback channel, and if
present, feedback received from the decompressor (ACKs, NACKs,
Static-NACK and options).
Encoding methods normally use previous value(s) from a history of
packets whose headers it has previously compressed. The optimistic
approach is meant to ensure that at least one compressed header
containing the information to update the state for a field is
received. Finally, feedback indicates what actions the decompressor
has taken with respect its assumptions regarding the validity of its
context Section 5.3.2; it indicates what type of compressed header
the decompressor can or cannot decompress.
The decompressor has the means to detect decompression failures for
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any type of compressed (CO) header, using the CRC verification.
Depending on the frequency and/or on the type of the failure, it
might send a negative acknowledgement (NACK) or an explicit request
for a complete context update (Static-NACK). However, the
decompressor does not have the means to identify the cause of the
failure, and in particular decompression of what field(s) is
responsible for the failure. The compressor is thus always
reponsible to figure out what is the most suitable response to a
negative acknowledgement, using the confidence it has in the state of
the decompressor context, when selecting the type of compressed
header it will use when compressing a header.
5.3. Decompressor Concepts
Initially, when sending the first IR packet for a compressed flow,
the compressor does not expect to receive feedback for that flow,
until such feedback is first received. At this point, the compressor
may then assume that the decompressor will continue to send feedback
in order to repair its context when necessary. The former is
referred to as unidirectional operation, while the latter is called
bidirectional operation.
The decompressor normally always uses the last received and
successfully validated (IR or IR-DYN packets) or verified (CO
packets) header as the reference for future decompression. If the
received packet is older than the current reference packet based on
the MSN in the compressed header, the decompressor may refrain from
using this packet as the new reference packet, even if the
correctness of its header was successfully verified.
The decompressor's responsability is thus to minimally consistently
verify the outcome of the decompression attempt, update its context
when successful and finally to request context repairs by making
coherent usage of feedback, once it starts using it.
Specifically, the outcome of every decompression attempt is verified
using the CRC present in the compressed header; the decompressor
updates the context information when this outcome is successfully
verified; finally if the decompressor uses feedback once for a
compressed flow then it will continue to do so for as long as the
corresponding context is associated with the same profile.
5.3.1. Decompressor State Machine
The decompressor operation may be represented as a state machine
defining three states: No Context (NC), Initial Context (IC) and Full
Context (FC).
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The decompressor starts with no valid context, the NC state.
Successful CRC-8 validation of an IR packet moves the decompressor to
the IC state, where it stays until it successfully verifies a
decompression attempt for compressed header with a 7-bit CRC. The
decompressor state machine normally does not leave the FC state once
it has entered this state; only repeated decompression failures will
force the decompressor to transit downwards to a lower state.
Below is the state machine for the decompressor. Details of the
transitions between states and decompression logic are given in the
sub-sections following the figure.
CRC-8(IR) or
CRC-8(IR-DYN)
Validation
CRC-8(IR) or CRC-7(CO) or
CRC-8(IR) CRC-8(IR) CRC-8(IR-DYN) CRC-7(CO) CRC-3(CO)
Failure Validation Validation Verification Verification
+--->---+ +-->---->--+ +-->----->--+ +-->---->--+ +-->---->--+
| | | | | | | | | |
| v | v | v | v | v
+-----------------+ +----------------------+ +-------------------+
| No Context (NC) | | Initial Context (IC) | | Full Context (FC) |
+-----------------+ +----------------------+ +-------------------+
^ | ^ CRC-7(CO) | ^ |
| Static Context | | Failure or | | Context Damage |
| Damage Detected | | PT not allowed | | Detected |
+--<-----<-----<--+ +----<------<----+ +--<-----<-----<--+
where:
CRC-8(IR) and/or CRC-8(IR-DYN) validation: successful CRC-8
validation for the IR header and the IR-DYN header, respectively.
CRC-7(CO) and/or CRC-3(CO) verification: successful CRC
verification for the CO header, based on the number of CRC bits
carried in the CO header.
CRC-7(CO) failure: failure to CRC verify the decompression of a CO
header carrying a 7-bit CRC.
PT not allowed: the decompressor has received a packet type (PT)
for which the decopressor's current context does not provide
enough valid state information for that packet to be decompressed.
Static Context Damaged Detected: see definition in Section 5.3.2.
Context Damage Detected: see definition in Section 5.3.2.
5.3.1.1. No Context (NC) State
Initially, while working in the No Context (NC) state, the
decompressor has not yet successfully validated an IR packet.
Attempting decompression:
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In the NC state, only packets carrying sufficient information on
the static fields (i.e. IR packets) can be decompressed.
Upward transition:
Upon receiving an IR packet, the decompressor validates the
integrity of its header using the CRC-8 validation. If the IR
packet is successfully validated, the decompressor updates the
context and use this packet as the reference packet. Once an IR
packet has initialized the context, the decompressor can transit
to the IC state.
Feedback logic:
In the No Context state, the decompressor should send a STATIC-
NACK if a packet of a type other than IR is received, or if an IR
packet has failed the CRC-8 validation, subject to the feedback
rate limitation as described in Section 5.3.3.
5.3.1.2. Initial Context (IC) State
In the IC state, the decompressor has successfully validated a IR
packet.
Attempting decompression:
In the Initial Context state, only packets carrying sufficient
information on the dynamic fields covered by an 8-bit CRC (e.g.
IR and IR-DYN) or CO packets carrying a 7-bit CRC can be
decompressed.
Upward transition:
The decompressor can move to the Full Context (FC) state when the
CRC verification succeeds for a CO header carrying a 7-bit CRC.
Downward transition:
The decompressor moves back to the NC state if it assumes static
context damage.
Feedback logic:
In the IC state, the decompressor should send a STATIC-NACK when
CRC-8 validation of an IR/IR-DYN fails, or when a CO header
carrying a 7-bit CRC fails and if static context damage is
assumed, subject to the feedback rate limitation as described
Section 5.3.3. If any other packet type is received, the
decompressor should treat it as a CRC verification failure when
deciding if a NACK is to be sent.
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5.3.1.3. Full Context (FC) State
In the FC state, the decompressor has successfully verified a CO
header with a 7-bit CRC.
Attempting decompression:
In the Full Context state, decompression can be attempted
regardless of the type of packet received.
Downward transition:
I.
Feedback logic:
In the Full Context state, the decompressor should send a NACK
when CRC-8 validation or CRC verification of any packet type fails
and if context damage is assumed, subject to the feedback rate
limitation as described in Section 5.3.3.
5.3.2. Decompressor Context Management
All header formats carry a CRC and are context updating. A packet
for which the CRC succeeds updates the reference values of all header
fields, either explicitly (from the information about a field carried
within the compressed header) or implicitly (fields that are inferred
from other fields).
The decompressor may assume that some or the entire context is
invalid, following one or more failures to validate or verify a
header using the CRC. Because the decompressor cannot know the exact
reason(s) of a CRC failure or what field caused it, the validity of
the context hence does not refer to what exact context entry is
deemed valid or not.
Validity of the context rather relates to the detection of a problem
with the context. The decompressor first assume that the type of
information that most likely caused the failure(s) is the state that
normally changes for each packet, i.e. context damage of the dynamic
part of the context. Upon repeated failures and unsuccessful
repairs, the decompressor then assume that the entire context,
including the static part, needs to be repaired, i.e. static context
damage.
Context Damage Detection
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The assumption of context damage means that the decompressor will
not attemp decompression of a CO headers that carries a 3-bit CRC,
and only attempt decompression of IR or IR-DYN headers, or CO
headers protected by a CRC-7.
Static Context Damage Detection
The assumption of static context damage means that the
decompressor refrains from attempting decompression of any type of
header other than the IR header, as it cannot know what part of
its context can be relied upon after first assuming context damage
and failed to repair its context, and as a result of too many
failures.
How these assumptions are made, i.e. how context damage is detected,
is open to implementations. It can be based on the residual error
rate, where a low error rate makes the decompressor assume damage
more often than on a high rate link.
The decompressor implements these assumptions by selecting the type
of compressed header for it may attempt decompression. In other
words, validity of the context refers to the ability of a
decompressor to attempt or not decompression of specific packet
types.
5.3.3. Feedback logic
RoHCv2 profiles may be used in environments with or without feedback
capabilities from decompressor to compressor. RoHCv2 however assumes
that if a ROHC feedback channel is available and if this channel is
used at least once by the decompressor for a specific context, this
channel will be used during the entire compression operation for that
context. If the connection is broken and the feedback channel
disappears, compression should be restarted.
The RoHC framework defines 3 types of feedback messages: ACKs, NACKs
and STATIC-NACKs. The semantics of each message if defined insection
5.2.3.1. [I-D.ietf-rohc-rfc3095bis-framework] What feedback to send
is coupled to the context management of the decompressor, i.e. to the
implementation of the context damage detection algorithms as
described in Section 5.3.2.
The decompressor should send a NACK when it assumes context damage,
and it should send a STATIC-NACK when it assumes static context
damage. The decompressor is not strictly expected to send ACK
feedback upon successful decompression, other than for the purpose of
improving compression efficiency.
The decompressor should limit the rate at which it sends feedback ,
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for both ACKs and STATIC-NACK/NACKs, and should avoid sending
unnecessary duplicates of the same type of feedback message that may
be associated to the same event.
6. RoHCv2 Profiles (Normative)
6.1. Profile Operation, per-context
RoHCv2 profiles operates differently, per context, depending on how
the decompressor uses of a feedback channel. Once the decompressor
uses the feedback channel for a context, it establishes the feedback
channel for that CID.
The compressor always start assuming that the decompressor will not
send feedback when it initializes a new context (see also , section
5.1.1.) [I-D.ietf-rohc-rfc3095bis-framework], i.e. there is no
established feedback channel for the new context. There will always
be a possibility of decompression failure with the optimistic
approach, because the decompressor may not have received sufficient
information for correct decompression. Therefore, until the
decompressor has established a feedback channel, the compressor
SHOULD periodically send IR packets. The periodicity can be based on
timeouts, on the number of compressed packets sent for the flow, or
any other strategy the implementer chooses.
The reception of either positive feedback (ACKs) or negative feedback
(NACKs) establishes the feedback channel from the decompressor for
the context (CID) for which the feedback was received. Once there is
an established feedback channel for a specific context, the
compressor can make use of this feedback to estimate the current
state of the decompressor. This helps increasing the compression
efficiency by providing the information needed for the compressor to
achieve the necessary confidence level. When the feedback channel is
established, it becomes superfluous for the compressor to send
periodic refreshes, and instead it can rely entirely on the
optimistic approach and feedback from the decompressor.
The decompressor MAY send positive feedback (ACKs) to initially
establish the feedback channel for a particular flow. Either
positive feedback (ACKs) or negative feedback (NACKs) establishes
this channel. The decompressor is REQUIRED to continue sending
feedback once it has established a feedback channel for a CID, for
the lifetime of the context, i.e. until the CID is associated with a
different profile from the reception of an IR packet, to send error
recovery requests and (optionally) acknowledgments of significant
context updates.
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Due to the periodic refreshes and the lack of feedback for initiation
of error recovery, compression without an established feedback
channel will be less efficient and have a slightly higher probability
of loss propagation compared to the decompressor making use of
feedback.
6.2. Control Fields
RoHCv2 defines a number of control fields that are used by the
decompressor in its interpretation of the packet formats received
from the compressor.
A control field is a field that is transmitted from the compressor to
the decompressor, but is not part of the uncompressed header. Values
for control fields can be set up in the context of both the
compressor and the decompressor. Once established at the
decompressor, the values of these fields MUST be kept until updated
by another packet.
6.2.1. Master Sequence Number (MSN)
The Master Sequence Number (MSN) field is either taken from a field
that already exists in each of the headers of the protocol that the
profile compresses (e.g. RTP SN), or alternatively it is created at
the compressor.
The MSN field has the following two functions:
o Differentiating between packets when sending feedback data.
o Inferring the value of incrementing fields (e.g. IPv4
Identifier).
The MSN field is present in every packet sent by the compressor. The
MSN is sent in full in IR and IR-DYN packets, while it is sent LSB
encoded within CO header formats. The decompressor always sends the
MSN as part of the feedback information. The compressor can later
use the MSN to infer which packet the decompressor is acknowledging.
When the MSN is initialized, it is initialized to a random value.
The compressor should only initialize a new MSN for the initial IR
packet sent for a new context, i.e. for a CID that corresponds to a
context that is not already associated with the profile used in the
IR header. In other words, if the compressor reuses the same CID
with the same profile to compress many flows one after the other, the
MSN is not reinitialized but rather continues to increment
monotonically.
For profiles for which the MSN is created by the compressor, the
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following rules applies:
o The compressor should only initialize a new MSN for the initial IR
sent for a CID that corresponds to a context that is not already
associated with this profile;
o When the MSN is initialized, it is initialized to a random value;
o The value of the MSN is incremented by one for each packet that
the compressor sends.
6.2.2. IP-ID behavior
The IP-ID field of the IPv4 header can have different change
patterns: sequential in network byte order, sequential byte-swapped,
random and constant (a constant value of zero, although not
conformant with [RFC0791], as been observed in practice).The control
field for the IP-ID behavior determines which set of packet formats
will be used. Note that these control fields are also used to
determine the contents of the irregular chain item for each IP
header.
If more than one level of IP headers is present, RoHCv2 profiles can
assign a sequential behavior (network byte order or byte-swapped)
only to the IP-ID of innermost IP header. This is because only this
IP-ID can possibly have a sufficiently close correlation with the MSN
to compress it as a sequentially changing field. Therefore, a
compressor MUST assign either the constant zero IP-ID or the random
IP-ID behavior to tunneling headers.
6.3. Reconstruction and Verification
The CRC carried within compressed headers MUST be used to verify
decompression. When the decompression is verified and successful,
the decompressor updates the context with the information received in
the current header; otherwise if the reconstructed header fails the
CRC verification, these updates MUST NOT be performed.
A packet for which the decompression attempt cannot be verified using
the CRC MUST NOT be delivered to upper layers.
Note: Decompressor implementations may attempt corrective or repair
measures prior to performing the above actions, and the result of any
decompression attempt MUST be verified using the CRC.
6.4. Compressed Header Chains
Some packet types use one or more chains containing sub-header
information. The function of a chain is to group fields based on
similar characteristics, such as static, dynamic or irregular fields.
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Chaining is done by appending an item for each header to the chain in
their order of appearance in the uncompressed packet, starting from
the fields in the outermost header.
Static chain:
The static chain consists of one item for each header of the chain
of protocol headers to be compressed, starting from the outermost
IP header. In the formal description of the packet formats, this
static chain item for each header type is labelled
<protocol_name>_static. The static chain is only used in IR
packets.
Dynamic chain:
The dynamic chain consists of one item for each header of the
chain of protocol headers to be compressed, starting from the
outermost IP header. In the formal description of the packet
formats, the dynamic chain item for each header type is labelled
<protocol_name>_dynamic. The dynamic chain is used both in IR and
IR-DYN packet
Irregular chain:
The structure of the irregular chain is analogous to the structure
of the static chain. For each compressed packet, the irregular
chain is appended at the specified location in the general format
of the compressed packets as defined in Section 6.6. The
irregular chain is used for all CO packets.
The format of the irregular chain for the innermost IP header
differs from the format of the one for the outer IP headers, since
this header is part of the compressed base header. What irregular
chain items to use is determined by the argument "is_innermost",
which is passed as an argument to the corresponding encoding
method (ipv4 or ipv6). The format of the irregular chain item for
the outer IP headers is also determined using one flag for TTL/Hop
Limit and one for TOS/TC. These flags are defined in the format
of some of the compressed base headers.
RoHCv2 profiles compresses extension headers as other headers, and
thus extension headers have a static chain, a dynamic chain and an
irregular chain.
Chains are defined for all headers compressed by RoHCv2 profiles,
i.e. RTP [RFC3550], UDP [RFC0768], UDP Lite [RFC3828], IPv4
[RFC0791], IPv6 [RFC2460], AH [RFC4302], GRE [RFC2784][RFC2890], MINE
[RFC2004], NULL-encrupted ESP [RFC4303], IPv6 Destination Options
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header[RFC2460], IPv6 Hop-by-hop Options header[RFC2460] and IPv6
Routing header [RFC2460].
6.5. Packet Formats and Encoding Methods
The packet formats used for are defined using the ROHC formal
notation. Some of the encoding methods used in the packet formats
are defined in [I-D.ietf-rohc-formal-notation], while other methods
are defined in this section.
6.5.1. baseheader_extension_headers
In CO packets (see Section 6.6.4), the innermost IP header can be
combined with other header(s) (i.e. UDP, UDP Lite, RTP) to create
the compressed base header. In such case, the IP header may have a
number of extension headers between itself and the other headers.
The base header defines some representation of these extension
headers, to comply with the syntax of the formal notation; this
encoding method provides this representation. The
baseheader_extension_headers encoding method skips over all fields of
the extension headers of the innermost IP header, without encoding
any of the them. Fields in these extension headers are instead
encoded in the irregular chain.
6.5.2. baseheader_outer_headers
This encoding method, similarly to the baseheader_extension_headers
encoding method above, is needed to keep the definition of the packet
formats syntactically correct. It describe tunneling IP headers and
their respective extension headers (i.e. all headers located before
the innermost IP header) for CO headers (see Section 6.6.4). The
baseheader_outer_headers encoding method skips over all the fields of
the extension header(s) that do not belong to the innermost IP
header, without encoding any of them. Changed fields in outer
headers are instead handled by the irregular chain.
6.5.3. inferred_udp_length
The UDP length field is inferred by the decompressor to be the size
of the UDP payload. This also means that the compressor MUST make
sure that the UDP length field is consistent with the length field(s)
of preceeding subheaders, i.e., there must not be any padding after
the UDP payload that is covered by the IP Length.
6.5.4. inferred_ip_v4_header_checksum
This encoding method compresses the header checksum field of the IPv4
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header. This checksum is defined in RFC 791 [RFC0791] as follows:
Header Checksum: 16 bits
A checksum on the header only. Since some header fields change
(e.g., time to live), this is recomputed and verified at each
point that the internet header is processed.
The checksum algorithm is:
The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header. For purposes
of computing the checksum, the value of the checksum field is
zero.
As described above, the header checksum protects individual hops from
processing a corrupted header. When almost all IP header information
is compressed away, and when decompression is verified by a CRC
computed over the original header for every compressed packet, there
is no point in having this additional checksum; instead it can be
recomputed at the decompressor side.
The "inferred_ip_v4_header_checksum" encoding method thus compresses
the IPv4 header checksum down to a size of zero bit, i.e. no bits are
transmitted in compressed headers for this field. Using this
encoding method, the decompressor infers the value of this field
using the computation above.
The compressor MAY use the header checksum to validate the
correctness of the header before compressing it, to avoid compressing
a corrupted header.
6.5.5. inferred_mine_header_checksum
This encoding method compresses the minimal encapsulation header
checksum. This checksum is defined in RFC 2004 [RFC2004] as follows:
Header Checksum
The 16-bit one's complement of the one's complement sum of all
16-bit words in the minimal forwarding header. For purposes of
computing the checksum, the value of the checksum field is 0.
The IP header and IP payload (after the minimal forwarding
header) are not included in this checksum computation.
The "inferred_mine_header_checksum" encoding method compresses the
minimal encapsulation header checksum down to a size of zero bit,
i.e. no bits are transmitted in compressed headers for this field.
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Using this encoding method, the decompressor infers the value of this
field using the above computation.
The motivations for inferring this checksum are similar to the ones
explained above in Section 6.5.4.
The compressor MAY use the minimal encapsulation header checksum to
validate the correctness of the header before compressing it, to
avoid compressing a corrupted header.
6.5.6. inferred_ip_v4_length
This encoding method compresses the total length field of the IPv4
header. The total length field of the IPv4 header is defined in RFC
791 [RFC0791] as follows:
Total Length: 16 bits
Total Length is the length of the datagram, measured in octets,
including internet header and data. This field allows the
length of a datagram to be up to 65,535 octets.
The "inferred_ip_v4_length" encoding method compresses the IPv4
header checksum down to a size of zero bit, i.e. no bits are
transmitted in compressed headers for this field. Using this
encoding method, the decompressor infers the value of this field by
counting in octets the length of the entire packet after
decompression.
6.5.7. inferred_ip_v6_length
This encoding method compresses the payload length field in the IPv6
header. This length field is defined in RFC 2460 [RFC2460] as
follows:
Payload Length: 16-bit unsigned integer
Length of the IPv6 payload, i.e., the rest of the packet
following this IPv6 header, in octets. (Note that any
extension headers present are considered part of the payload,
i.e., included in the length count.)
The "inferred_ip_v6_length" encoding method compresses the payload
length field of the IPv6 header down to a size of zero bit, i.e. no
bits are transmitted in compressed headers for this field. Using
this encoding method, the decompressor infers the value of this field
by counting in octets the length of the entire packet after
decompression.
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6.5.8. Scaled RTP Timestamp Encoding
The RTP timestamp usually increases by a multiple of the RTP Sequence
Number's increase and is therefore a suitable candidate for scaled
encoding. The scaling factor is decided by the compressor by
observing the increase in Timestamp compared to the RTP Sequence
Number. This scaling factor is labeled ts_stride in the definition
of the profile in ROHC-FNSection 6.6.
For the compressor to use the scaled timestamps, it MUST first
explicitly transmit the value of ts_stride to the decompressor, using
one of the packet types that can carry this information. Once the
value of the scaling factor is established, before using this scaled
encoding the compressor must have enough confidence that the
decompressor has successfully calculated the residue (ts_offset) of
the scaling function for the timestamp. This is done by sending
unscaled timestamp values to allow the compressor to establish the
residue based on the ts_stride established.
Once the compressor has gained enough confidence that both the value
of the scaling factor and the value of the residue have been
established in the decompressor, the compressor can start compressing
packets using the scaled representation of the timestamp. The
compressor MUST NOT use the scaled timestamp encoding with the value
of the ts_stride is set to zero.
If the compressor notices that the residue (ts_offset) value changes,
the compressor cannot use scaled timestamp packet formats until it
has re-established the residue as described above.
When the value of the timestamp field wraps around, the value of the
residue of the scaling function is likely to change. When this
occurs, the compressor re-establishes the new residue value, e.g.
using the unscaled representation of the field as described above.
The compressor MAY use the scaled timestamp encoding; what value it
will use as the scaling factor is up to the compressor
implementation, but to achive any gains from the scaling, the
ts_stride should be set to the value of the expected incease in
timestamp between consecutive sequence numbers.
When scaled timestamp encoding is used for packet formats that do not
transmit any LSB-encoded timestamp bits at all, the Section 6.5.9 is
used for encoding the timestamp.
6.5.9. inferred_scaled_field
The "inferred_scaled_field" encoding method is used to encode a field
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that is defined as changing in relation to the MSN but for each
increase is scaled by an established scaling factor. This encoding
method is to be used in the case when a packet format contains MSN
bits, but does not contain any bits for the scaled field. In this
case, the new value for the field being scaled is calculated
according to the following formula:
unscaled_value = delta_msn * stride + previous_unscaled_value
Where "delta_msn" is the difference is MSN between the previous value
of MSN in the context and the value of the MSN decompressed from this
packet, "previous_unscaled_value" is the value of the field being
scaled in the context, and "stride" is the scaling value for this
field.
For example, when this encoding method is applied to the RTP
timestamp in the RTP profile, the calculation above becomes:
timestamp = delta_msn * ts_stride + previous_timestamp
6.5.10. control_crc3
Some control fields that can be transmit by the co_common packet type
of each profile might not be used when decompressing this packet, and
therefore will not be covered by the included 7-bit CRC. If such a
control field has been corrupted on the link between compressor and
decompressor, the decompressor might send an ACK for this packet
which would be interpreted by the compressor as if the control fields
included in this packet were successfully decompressed. To avoid
such a situation, an additional 3-bit CRC is included in the
co_common packets.
This 3-bit CRC uses the same polynomial as the crc3 encoding method
defined in the formal notation, but has a different coverage. This
CRC should be calculated over the following field, in the order that
they are listed below:
o reorder_ratio, padded by 6 MSB of zeroes
o ts_stride, 16 bits (if applicable for this profile)
6.5.11. inferred_sequential_ip_id
This encoding method is used when a sequential IP-ID behavior is used
(sequential or sequential byte-swapped) and no coded IP-ID bits are
present in the compressed header. When these packet types are used,
the IP-ID offset from the MSN will be constant, and therefore, the
IP-ID will increase by the same amount as the MSN increases by
(similar to the inferred_scaled_field encoding method).
Therefore, the new value for the IP-ID is calculated according to the
following formula:
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IP-ID = delta_msn + previous_IP_ID_value
Where "delta_msn" is the difference is MSN between the previous value
of MSN in the context and the value of the MSN decompressed from this
packet, "previous_IP_ID_value" is the value of the IP-ID in the
context.
If the IP-ID behavior is random or zero, this encoding method does
not update any fields.
6.5.12. list_csrc(cc_value)
This encoding method describes how the list of CSRC identifiers can
be compressed using list compression. This list compression operates
by establishing content for the different CSRC identifiers (items)
and list describing the order that they appear.
The argument to this encoding method (cc_value) is the CC field from
the RTP header which the compressor passes to this encoding method.
The decompressor is reuired to bind the value of this argument to the
number of items in the list, which will allow the decompressor to
corectly reconstruct the CC field.
6.5.12.1. List Compression
The CSRC identifiers in the uncompressed packet can be represented as
an ordered list, whose order and presence are usually constant
between packets. The generic structure of such a list is as follows:
+--------+--------+--...--+--------+
list: | item 1 | item 2 | | item n |
+--------+--------+--...--+--------+
When performing list compression on a CSRC list, each item is the
uncompressed value of one CSRC identifier.
The basic principles of list-based compression are the following:
1) When a context is being initialized, a complete representation
of the compressed list of options is transmitted. All items that
have any content are present in the compressed list of items sent
by the compressor.
Then, once the context has been initialized:
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2) When the structure of the list is unchanged no information
about the list is sent in compressed headers.
3) When the structure of the list changes, a compressed list is
sent in the compressed header, including a representation of its
structure and order. Previously unknown items are sent
uncompressed in the list, while previously known items are only
represented by an index pointing to the context.
6.5.12.2. Table-based Item Compression
The Table-based item compression compresses individual items sent in
compressed lists. The compressor assigns a unique identifier,
"Index", to each item "Item" of a list.
Compressor Logic
The compressor conceptually maintains an Item Table containing all
items, indexed using "Index". The (Index, Item) pair is sent
together in compressed lists until the compressor gains enough
confidence that the decompressor has observed the mapping between
items and their respective index. Confidence is obtained from the
reception of an acknowledgment from the decompressor, or by
sending (Index, Item) pairs using the optimistic approach. Once
confidence is obtained, the index alone is sent in compressed
lists to indicate the presence of the item corresponding to this
index.
The compressor may reassign an existing index to a new item, by
re-establishing the mapping using the procedure described above.
Decompressor Logic
The decompressor conceptually maintains an Item Table that
contains all (Index, Item) pairs received. The Item Table is
updated whenever an (Index, Item) pair is received and
decompression is successfully verified using the CRC. The
decompressor retrieves the item from the table whenever an Index
without an accompanying Item is received.
If an index without an accompanying item is received and the
decompressor does not have any context for this index, the packet
MUST NOT be delivered to upper layers.
6.5.12.3. Encoding of Compressed Lists
Each item present in a compressed list is represented by:
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o an index into the table of items, and
o a presence bit indicating if a compressed representation of the
item is present in the list.
o an item (if the presence bit is set)
If the presence bit is not set, the item must already be known by the
decompressor.
A compressed list of items uses the following encoding:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Reserved |PS | m |
+---+---+---+---+---+---+---+---+
| XI_1, ..., XI_m | m octets, or m * 4 bits
/ --- --- --- ---/
| : Padding : if PS = 0 and m is odd
+---+---+---+---+---+---+---+---+
| |
/ item_1, ..., item_n / variable
| |
+---+---+---+---+---+---+---+---+
Reserved: Must be set to zero.
PS: Indicates size of XI fields:
PS = 0 indicates 4-bit XI fields;
PS = 1 indicates 8-bit XI fields.
m: Number of XI item(s) in the compressed list. Also the value of
the cc_value argument.
XI_1, ..., XI_m: m XI items. Each XI represents one item in the
list of item of the uncompressed header, in the same order as they
appear in the uncompressed header.
The format of an XI item is as follows:
+---+---+---+---+
PS = 0: | X | Index |
+---+---+---+---+
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
PS = 1: | X | Reserved | Index |
+---+---+---+---+---+---+---+---+
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X: Indicates whether the item present in the list:
X = 1 indicates that the item corresponding to the Index is
sent in the item_1, ..., item_n list;
X = 0 indicates that the item corresponding to the Index is
not sent.
Reserved: Set to zero when sending, ignored when received.
Index: An index into the item table. See Section 6.5.12.4
When 4-bit XI items are used and, the XI items are placed in
octets in the following manner:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| XI_k | XI_k + 1 |
+---+---+---+---+---+---+---+---+
Padding: A 4-bit padding field is present when PS = 0 and the
number of XIs is odd. The Padding field is set to zero when
sending and ignored when receiving.
Item 1, ..., item n: Each item corresponds to an XI with X = 1 in
XI 1, ..., XI m. Each entry in the item list is the uncompressed
representation of one CSRC identifier.
6.5.12.4. Item Table Mappings
The item table for list compression is limited to 16 different items,
since the RTP header can only carry at most 15 simultaneous CSRC
identifiers. The effect of having more than 16 items will only cause
a slight overhead to the compressor when items are swappen in/out of
the item table.
6.5.12.5. Compressed Lists in Dynamic Chain
A compressed list that is part of the dynamic chain (e.g. in IR or
IR-DYN packets) must have all its list items present, i.e. all X-bits
in the XI list MUST be set.
6.6. Packet Formats
ROHCv2 profiles use two different packet types: the Initialization
and Refresh (IR) packet type, and the Compressed packet type (CO).
Each packet type defines a number of packet formats: two packet
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formats are defined for the IR type, and two sets base header formats
are defined for the CO type with one additional format that is common
to both sets.
When the number of bits available in compressed header fields exceeds
the number of bits in the value, the most significant field is padded
with zeroes in its most significant bits.
Updating Properties: all packet types carry a CRC and are context
updating. Packets update the entire context besides the fields for
which they explicitly convey information for, since the context can
be expressed as the collection of the reference value of each field
together with the function established with respect to the MSN.
6.6.1. Initialization and Refresh Packet (IR)
The IR packet format uses the structure of the ROHC IR packet as
defined in [I-D.ietf-rohc-rfc3095bis-framework], section 5.2.2.1.
Packet type: IR
This packet type communicates the static part and the dynamic part
of the context.
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The ROHCv2 IR packet has the following format:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 1 0 1 | IR type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ Static chain / variable length
| |
- - - - - - - - - - - - - - - -
| |
/ Dynamic chain / variable length
| |
- - - - - - - - - - - - - - - -
| |
/ Payload / variable length
| |
- - - - - - - - - - - - - - - -
Static chain: See Section 6.4.
Dynamic chain: See Section 6.4.
Payload: The payload of the corresponding original packet, if any.
The presence of a payload is inferred from the packet length.
6.6.2. IR Packet Payload Discard (IR-PD)
The IR-PD packet format uses the structure of the ROHC IR packet as
defined in [I-D.ietf-rohc-rfc3095bis-framework], section 5.2.2.1.
Packet type: IR-PD
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This packet type communicates the static part and the dynamic part
of the context, but without the payload of the original packet for
which it carries the header information.
The ROHCv2 IR packet has the following format:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 1 0 0 | IR type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ Static chain / variable length
| |
- - - - - - - - - - - - - - - -
| |
/ Dynamic chain / variable length
| |
- - - - - - - - - - - - - - - -
Static chain: See Section 6.4.
Dynamic chain: See Section 6.4.
6.6.3. IR Dynamic Packet (IR-DYN)
The IR-DYN packet format uses the structure of the ROHC IR-DYN packet
as defined in [I-D.ietf-rohc-rfc3095bis-framework], section 5.2.2.2.
Packet type: IR-DYN
This packet type communicates the dynamic chains of the header(s)
that it compresses.
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The RoHCv2 IR-DYN packet has the following format:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 0 0 0 | IR-DYN type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ Dynamic chain / variable length
| |
- - - - - - - - - - - - - - - -
| |
/ Payload / variable length
| |
- - - - - - - - - - - - - - - -
Dynamic chain: See Section 6.4.
Payload: The payload of the corresponding original packet, if any.
The presence of a payload is inferred from the packet length.
6.6.4. Compressed Packet Formats (CO)
6.6.4.1. Design rationale for compressed base headers
The compressed packet formats are defined as two separate sets for
each profile: one set for the packets where the innermost IP header
contains a sequential IP-ID (either network byte order or byte
swapped), and one set for the packets without sequential IP-ID
(either random, zero, or no IP-ID).
The design of the packet formats is derived from the field behavior
analysis found in Appendix A.
All of the compressed base headers transmit LSB-encoded MSN bits and
a CRC. In addition, each base header in the sequential packet format
set contains LSB encoded IP-ID bits.
The following packet formats exist in both the sequential and random
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packet format sets:
o Format 1: This packet format transmits changes [Author's note:
TBW]
o Format 2: This packet format transmits changes [Author's note:
TBW]
o Common packet format: The common packet format can be used
indenpendently of the type of IP-ID behavior. It should also be
useful when some of the more rarely changing fields in the IP
header changes. Since this packet format modify the value of the
control fields that determine how the decompressor interprets
different compressed header format, it carries a 7-bit CRC to
reduce the probability of context corruption. This packet can
change most of the dynamic fields in the IP header, and it uses a
large set of flags to provide information about which fields are
present in the packet format.
6.6.4.2. General CO Header Format
The CO packets communicate irregularities in the packet header. All
CO packets carry a CRC and can update the context.
The general format for a compressed header is as follows:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and CID 1-15
+---+---+---+---+---+---+---+---+
| first octet of base header | (with type indication)
+---+---+---+---+---+---+---+---+
: :
/ 0, 1, or 2 octets of CID / 1-2 octets if large CIDs
: :
+---+---+---+---+---+---+---+---+
/ remainder of base header / variable number of octets
+---+---+---+---+---+---+---+---+
: :
/ Irregular Chain / variable
: :
--- --- --- --- --- --- --- ---
The base header in the figure above is the compressed representation
of the innermost IP header and other header(s), if any, in the
uncompressed packet.
Upon receiving other types of packet, the decompressor will
decompress it. The decompressor MUST verify the correctness of the
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decompressed packet by CRC check. If this verification succeeds, the
decompressor passes the decompressed packet to the system's network
layer. The decompressor will then use this packet as the reference
packet.
The entire set of base headers are described in the remainder of this
section.
////////////////////////////////////////////
// Constants
////////////////////////////////////////////
// IP-ID behavior constants
IP_ID_BEHAVIOR_SEQUENTIAL = 0;
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED = 1;
IP_ID_BEHAVIOR_RANDOM = 2;
IP_ID_BEHAVIOR_ZERO = 3;
// UDP-lite checksum coverage behavior constants
UDP_LITE_COVERAGE_INFERRED = 0;
UDP_LITE_COVERAGE_STATIC = 1;
UDP_LITE_COVERAGE_IRREGULAR = 2;
UDP_LITE_COVERAGE_RESERVED = 3;
// Variable reordering offset
REORDERING_NONE = 0;
REORDERING_QUARTER = 1;
REORDERING_HALF = 2;
REORDERING_THREEQUARTERS = 3;
// Profile names and versions
PROFILE_RTP_0101 = 1;
PROFILE_UDP_0102 = 2;
PROFILE_ESP_0103 = 3;
PROFILE_IP_0104 = 4;
PROFILE_RTP_0107 = 7; // With UDP-LITE
PROFILE_UDPLITE_0108 = 8; // Without RTP
////////////////////////////////////////////
// Global control fields
////////////////////////////////////////////
CONTROL {
msn [ 16 ];
reorder_ratio [ 2 ];
}
///////////////////////////////////////////////
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// Encoding methods not specified in FN syntax:
///////////////////////////////////////////////
baseheader_extension_headers "defined in Section X.Y.Z";
baseheader_outer_headers "defined in Section X.Y.Z";
inferred_udp_length "defined in Section X.Y.Z";
inferred_ip_v4_header_checksum "defined in Section X.Y.Z";
inferred_mine_header_checksum "defined in Section X.Y.Z";
inferred_ip_v4_length "defined in Section X.Y.Z";
inferred_ip_v6_length "defined in Section X.Y.Z";
list_csrc(cc_value) "defined in Section X.Y.Z";
inferred_scaled_field "defined in Section X.Y.Z";
inferred_sequential_ip_id "defined in Section X.Y.Z";
control_crc3 "defined in Section X.Y.Z";
////////////////////////////////////////////
// General encoding methods
////////////////////////////////////////////
reorder_ratio_choice
{
UNCOMPRESSED {
ratio [ 2 ];
}
DEFAULT {
ratio =:= irregular(2);
}
COMPRESSED none {
ratio [ 2 ];
ENFORCE(ratio.UVALUE == REORDERING_NONE);
}
COMPRESSED quarter {
ratio [ 2 ];
ENFORCE(ratio.UVALUE == REORDERING_QUARTER);
}
COMPRESSED half {
ratio [ 2 ];
ENFORCE(ratio.UVALUE == REORDERING_HALF);
}
COMPRESSED three_quarters {
ratio [ 2 ];
ENFORCE(ratio.UVALUE == REORDERING_THREEQUARTERS);
}
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}
static_or_irreg(flag, width)
{
UNCOMPRESSED {
field [ width ];
}
COMPRESSED irreg_enc {
field =:= irregular(width) [ width ];
ENFORCE(flag == 1);
}
COMPRESSED static_enc {
field =:= static [ 0 ];
ENFORCE(flag == 0);
}
}
optional32(flag)
{
UNCOMPRESSED {
item [ 0, 32 ];
}
COMPRESSED present {
item =:= irregular(32) [ 32 ];
ENFORCE(flag == 1);
}
COMPRESSED not_present {
item =:= compressed_value(0, 0) [ 0 ];
ENFORCE(flag == 0);
}
}
// Self-describing variable length encoding
sdvl(field_width)
{
UNCOMPRESSED {
field [ field_width ];
}
COMPRESSED lsb7 {
discriminator =:= '0' [ 1 ];
field =:= lsb(7, 63) [ 7 ];
}
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COMPRESSED lsb14 {
discriminator =:= '10' [ 2 ];
field =:= lsb(14, 16383) [ 14 ];
}
COMPRESSED lsb21 {
discriminator =:= '110' [ 3 ];
field =:= lsb(21, 65535) [ 21 ];
}
COMPRESSED lsb29 {
discriminator =:= '110' [ 3 ];
field =:= lsb(29, 65535) [ 29 ];
}
}
optional_stride(flag, value)
{
UNCOMPRESSED {
field [ 32 ];
}
COMPRESSED present {
field =:= sdvl(32);
ENFORCE(flag == 1);
}
COMPRESSED not_present {
field =:= static;
ENFORCE(flag == 0);
}
}
optional_scaled_timestamp(tss_flag, tsc_flag)
{
UNCOMPRESSED {
timestamp [ 32 ];
}
COMPRESSED present {
timestamp =:= sdvl(32);
ENFORCE((tss_flag == 0) && (tsc_flag == 1));
}
COMPRESSED not_present {
ENFORCE(((tss_flag == 1) && (tsc_flag == 0)) ||
((tss_flag == 0) && (tsc_flag == 0)));
}
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}
optional_unscaled_timestamp(tss_flag, tsc_flag)
{
UNCOMPRESSED {
timestamp [ 32 ];
}
COMPRESSED present {
timestamp =:= sdvl(32);
ENFORCE(((tss_flag == 1) && (tsc_flag == 0)) ||
((tss_flag == 0) && (tsc_flag == 0)));
}
COMPRESSED not_present {
ENFORCE((tss_flag == 0) && (tsc_flag == 1));
}
}
lsb_7_or_31
{
UNCOMPRESSED {
item [ 32 ];
}
COMPRESSED lsb_7 {
discriminator =:= '0' [ 1 ];
item =:= lsb(7, 8) [ 7 ];
}
COMPRESSED lsb_31 {
discriminator =:= '1' [ 1 ];
item =:= lsb(31, 256) [ 31 ];
}
}
opt_lsb_7_or_31(flag)
{
UNCOMPRESSED {
item [ 0, 32 ];
}
COMPRESSED present {
item =:= lsb_7_or_31 [ 8, 32 ];
ENFORCE(flag == 1);
}
COMPRESSED not_present {
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item =:= compressed_value(0, 0) [ 0 ];
ENFORCE(flag == 0);
}
}
crc3(data_value, data_length)
{
UNCOMPRESSED {
}
COMPRESSED {
crc_value =:=
crc(3, 0x06, 0x07, data_value, data_length) [ 3 ];
}
}
crc7(data_value, data_length)
{
UNCOMPRESSED {
}
COMPRESSED {
crc_value =:=
crc(7, 0x79, 0x7f, data_value, data_length) [ 7 ];
}
}
optional_pt(flag)
{
UNCOMPRESSED {
payload_type [ 7 ];
}
COMPRESSED not_present {
payload_type =:= static [ 0 ];
ENFORCE(flag == 0);
}
COMPRESSED present {
reserved =:= compressed_value(1, 0) [ 1 ];
payload_type =:= irregular(7) [ 7 ];
ENFORCE(flag == 1);
}
}
csrc_list_presence(presence, cc_value)
{
UNCOMPRESSED {
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csrc_list;
}
COMPRESSED no_list {
csrc_list =:= static [ 0 ];
ENFORCE(presence == 0);
}
COMPRESSED list_present {
csrc_list =:= list_csrc(cc_value) [ VARIABLE ];
ENFORCE(presence == 1);
}
}
// Variable reordering offset used for MSN
msn_lsb(k)
{
UNCOMPRESSED {
master [ 16 ];
}
COMPRESSED none {
master =:= lsb(k, -1);
ENFORCE(reorder_ratio.UVALUE == REORDERING_NONE);
}
COMPRESSED quarter {
master =:= lsb(k, ((2^k) / 4) - 1);
ENFORCE(reorder_ratio.UVALUE == REORDERING_QUARTER);
}
COMPRESSED half {
master =:= lsb(k, ((2^k) / 2) - 1);
ENFORCE(reorder_ratio.UVALUE == REORDERING_HALF);
}
COMPRESSED threequarters {
master =:= lsb(k, (((2^k) * 3) / 4) - 1);
ENFORCE(reorder_ratio.UVALUE == REORDERING_THREEQUARTERS);
}
}
// Encoding method for updating a scaled field and its associated
// control fields. Should be used both when the value is scaled
// or unscaled in a compressed format.
field_scaling(stride_value, scaled_value, unscaled_value)
{
UNCOMPRESSED {
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residue_field [ 32 ];
}
COMPRESSED no_scaling {
ENFORCE(stride_value == 0);
ENFORCE(residue_field.UVALUE == unscaled_value);
ENFORCE(scaled_value == 0);
}
COMPRESSED scaling_used {
ENFORCE(stride_value != 0);
ENFORCE(residue_field.UVALUE == (unscaled_value % stride_value));
ENFORCE(unscaled_value ==
scaled_value * stride_value + residue_field.UVALUE);
}
}
////////////////////////////////////////////
// IPv6 Destination options header
////////////////////////////////////////////
ip_dest_opt
{
UNCOMPRESSED {
next_header [ 8 ];
length [ 8 ];
value [ VARIABLE ];
}
DEFAULT {
length =:= static;
next_header =:= static;
value =:= static;
}
COMPRESSED dest_opt_static {
next_header =:= irregular(8) [ 8 ];
length =:= irregular(8) [ 8 ];
}
COMPRESSED dest_opt_dynamic {
value =:= irregular(length.UVALUE * 64 + 48) [ VARIABLE ];
}
}
////////////////////////////////////////////
// IPv6 Hop-by-Hop options header
////////////////////////////////////////////
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ip_hop_opt
{
UNCOMPRESSED {
next_header [ 8 ];
length [ 8 ];
value [ VARIABLE ];
}
DEFAULT {
length =:= static;
next_header =:= static;
value =:= static;
}
COMPRESSED hop_opt_static {
next_header =:= irregular(8) [ 8 ];
length =:= irregular(8) [ 8 ];
}
COMPRESSED hop_opt_dynamic {
value =:= irregular(length.UVALUE*64+48) [ VARIABLE ];
}
}
////////////////////////////////////////////
// IPv6 Routing header
////////////////////////////////////////////
ip_rout_opt
{
UNCOMPRESSED {
next_header [ 8 ];
length [ 8 ];
value [ VARIABLE ];
}
DEFAULT {
length =:= static;
next_header =:= static;
value =:= static;
}
COMPRESSED rout_opt_static {
next_header =:= irregular(8) [ 8 ];
length =:= irregular(8) [ 8 ];
value =:= irregular(length.UVALUE*64+48) [ VARIABLE ];
}
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COMPRESSED rout_opt_dynamic {
}
}
////////////////////////////////////////////
// GRE Header
////////////////////////////////////////////
optional_checksum(flag_value)
{
UNCOMPRESSED {
value [ 0, 16 ];
reserved1 [ 0, 16 ];
}
COMPRESSED cs_present {
value =:= irregular(16) [ 16 ];
reserved1 =:= uncompressed_value(16, 0) [ 0 ];
ENFORCE(flag_value == 1);
}
COMPRESSED not_present {
value =:= compressed_value(0, 0) [ 0 ];
reserved1 =:= compressed_value(0, 0) [ 0 ];
ENFORCE(flag_value == 0);
}
}
gre_proto
{
UNCOMPRESSED {
protocol [ 16 ];
}
COMPRESSED ether_v4 {
discriminator =:= compressed_value(1, 0) [ 1 ];
protocol =:= uncompressed_value(16, 0x0800);
}
COMPRESSED ether_v6 {
discriminator =:= compressed_value(1, 1) [ 1 ];
protocol =:= uncompressed_value(16, 0x86DD);
}
}
gre
{
UNCOMPRESSED {
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c_flag [ 1 ];
r_flag =:= uncompressed_value(1, 0) [ 1 ];
k_flag [ 1 ];
s_flag [ 1 ];
reserved0 =:= uncompressed_value(9, 0) [ 9 ];
version =:= uncompressed_value(3, 0) [ 3 ];
protocol [ 16 ];
checksum_and_res [ 0, 32 ];
key [ 0, 32 ];
sequence_number [ 0, 32 ];
}
DEFAULT {
c_flag =:= static;
k_flag =:= static;
s_flag =:= static;
protocol =:= static;
key =:= static;
sequence_number =:= static;
}
COMPRESSED gre_static {
protocol =:= gre_proto [ 1 ];
c_flag =:= irregular(1) [ 1 ];
k_flag =:= irregular(1) [ 1 ];
s_flag =:= irregular(1) [ 1 ];
padding =:= compressed_value(4, 0) [ 4 ];
key =:= optional32(k_flag.UVALUE) [ 0, 32 ];
}
COMPRESSED gre_dynamic {
checksum_and_res =:=
optional_checksum(c_flag.UVALUE) [ 0, 16 ];
sequence_number =:= optional32(s_flag.UVALUE) [ 0, 32 ];
}
COMPRESSED gre_irregular {
checksum_and_res =:=
optional_checksum(c_flag.UVALUE) [ 0, 16 ];
sequence_number =:=
opt_lsb_7_or_31(s_flag.UVALUE) [ 0, 8, 32 ];
}
}
/////////////////////////////////////////////
// MINE header
/////////////////////////////////////////////
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mine
{
UNCOMPRESSED {
next_header [ 8 ];
s_bit [ 1 ];
res_bits [ 7 ];
checksum [ 16 ];
orig_dest [ 32 ];
orig_src [ 0, 32 ];
}
DEFAULT {
next_header =:= static;
s_bit =:= static;
res_bits =:= static;
checksum =:= inferred_mine_header_checksum;
orig_dest =:= static;
orig_src =:= static;
}
COMPRESSED mine_static {
next_header =:= irregular(8) [ 8 ];
s_bit =:= irregular(1) [ 1 ];
// Reserved are included - no benefit in removing them
res_bits =:= irregular(7) [ 7 ];
orig_dest =:= irregular(32) [ 32 ];
orig_src =:= optional32(s_bit.UVALUE) [ 0, 32 ];
}
COMPRESSED mine_dynamic {
}
}
/////////////////////////////////////////////
// Authentication Header (AH)
/////////////////////////////////////////////
ah
{
UNCOMPRESSED {
next_header [ 8 ];
length [ 8 ];
res_bits [ 16 ];
spi [ 32 ];
sequence_number [ 32 ];
auth_data [ VARIABLE ];
}
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DEFAULT {
next_header =:= static;
length =:= static;
res_bits =:= static;
spi =:= static;
sequence_number =:= static;
}
COMPRESSED ah_static {
next_header =:= irregular(8) [ 8 ];
length =:= irregular(8) [ 8 ];
spi =:= irregular(32) [ 32 ];
}
COMPRESSED ah_dynamic {
res_bits =:= irregular(16) [ 16 ];
sequence_number =:= irregular(32) [ 32 ];
auth_data =:=
irregular(length.UVALUE*32-32) [ VARIABLE ];
}
COMPRESSED ah_irregular {
sequence_number =:= lsb_7_or_31 [ 8, 32 ];
auth_data =:=
irregular(length.UVALUE*32-32) [ VARIABLE ];
}
}
/////////////////////////////////////////////
// ESP header (NULL encrypted)
/////////////////////////////////////////////
// Since the "next header" field is located in the packet trailer
// and ROHC-FN requires all UNCOMPRESSED fields to be contiguous,
// the values of the next header field is passed as a parameter.
// To avoid forcing the decompression to access the trailer part of
// the packet, the next header is istead handled with a control field
esp_null(next_header_value)
{
UNCOMPRESSED {
spi [ 32 ];
sequence_number [ 32 ];
}
CONTROL {
nh_field [ 8 ];
}
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DEFAULT {
spi =:= static;
sequence_number =:= static;
nh_field =:= static;
}
COMPRESSED esp_static {
nh_field =:= compressed_value(8, next_header_value) [ 8 ];
spi =:= irregular(32) [ 32 ];
}
COMPRESSED esp_dynamic {
sequence_number =:= irregular(32) [ 32 ];
}
COMPRESSED esp_irregular {
sequence_number =:= lsb_7_or_31 [ 8, 32 ];
}
}
/////////////////////////////////////////////
// IPv6 Header
/////////////////////////////////////////////
fl_enc
{
UNCOMPRESSED {
flow_label [ 20 ];
}
COMPRESSED fl_zero {
discriminator =:= '0' [ 1 ];
flow_label =:= uncompressed_value(20, 0) [ 0 ];
reserved =:= '0000' [ 4 ];
}
COMPRESSED fl_non_zero {
discriminator =:= '1' [ 1 ];
flow_label =:= irregular(20) [ 20 ];
}
}
// The is_innermost flag should be true if this is the innermost
// IP header to be compressed.
// If extracting the irregular chain for an compressed packet,
// the TTL&TOS arguments must have the same value as it had when
// processing co_baseheader. If extracting any other chain
// items, this argument is not used.
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ipv6(profile, is_innermost,
ttl_irregular_chain_flag, tos_irregular_chain_flag)
{
UNCOMPRESSED {
version =:= uncompressed_value(4, 6) [ 4 ];
tos_tc [ 8 ];
flow_label [ 20 ];
payload_length [ 16 ];
next_header [ 8 ];
ttl_hopl [ 8 ];
src_addr [ 128 ];
dst_addr [ 128 ];
}
DEFAULT {
tos_tc =:= static;
flow_label =:= static;
payload_length =:= inferred_ip_v6_length;
next_header =:= static;
ttl_hopl =:= static;
src_addr =:= static;
dst_addr =:= static;
}
COMPRESSED ipv6_static {
version_flag =:= '1' [ 1 ];
reserved =:= '00' [ 2 ];
flow_label =:= fl_enc [ 5, 21 ];
next_header =:= irregular(8) [ 8 ];
src_addr =:= irregular(128) [ 128 ];
dst_addr =:= irregular(128) [ 128 ];
}
COMPRESSED ipv6_endpoint_dynamic {
tos_tc =:= irregular(8) [ 8 ];
ttl_hopl =:= irregular(8) [ 8 ];
reserved =:= compressed_value(6, 0) [ 6 ];
reorder_ratio =:= reorder_choice [ 2 ];
ENFORCE((is_innermost == true) &&
(profile == PROFILE_IP_0104));
}
COMPRESSED ipv6_regular_dynamic {
tos_tc =:= irregular(8) [ 8 ];
ttl_hopl =:= irregular(8) [ 8 ];
ENFORCE((is_innermost == false) ||
(profile != PROFILE_IP_0104));
}
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COMPRESSED ipv6_outer_irregular {
tos_tc =:=
static_or_irreg(tos_irregular_chain_flag) [ 0, 8 ];
ttl_hopl =:=
static_or_irreg(ttl_irregular_chain_flag) [ 0, 8 ];
ENFORCE(is_innermost == false);
}
COMPRESSED ipv6_innermost_irregular {
ENFORCE(is_innermost == true);
}
}
/////////////////////////////////////////////
// IPv4 Header
/////////////////////////////////////////////
ip_id_enc_dyn(behavior)
{
UNCOMPRESSED {
ip_id [ 16 ];
}
COMPRESSED ip_id_seq {
ip_id =:= irregular(16) [ 16 ];
ENFORCE((behavior == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED) ||
(behavior == IP_ID_BEHAVIOR_RANDOM));
}
COMPRESSED ip_id_zero {
ip_id =:= uncompressed_value(16, 0) [ 0 ];
ENFORCE(behavior == IP_ID_BEHAVIOR_ZERO);
}
}
ip_id_enc_irreg(behavior)
{
UNCOMPRESSED {
ip_id [ 16 ];
}
COMPRESSED ip_id_seq {
ENFORCE(behavior == IP_ID_BEHAVIOR_SEQUENTIAL);
}
COMPRESSED ip_id_seq_swapped {
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ENFORCE(behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED);
}
COMPRESSED ip_id_rand {
ip_id =:= irregular(16) [ 16 ];
ENFORCE(behavior == IP_ID_BEHAVIOR_RANDOM);
}
COMPRESSED ip_id_zero {
ip_id =:= uncompressed_value(16, 0) [ 0 ];
ENFORCE(behavior == IP_ID_BEHAVIOR_ZERO);
}
}
ip_id_behavior_choice
{
UNCOMPRESSED {
behavior [ 2 ];
}
DEFAULT {
behavior =:= irregular(2);
}
COMPRESSED sequential {
behavior [ 2 ];
ENFORCE(behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL);
}
COMPRESSED sequential_swapped {
behavior [ 2 ];
ENFORCE(behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED);
}
COMPRESSED random {
behavior [ 2 ];
ENFORCE(behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM);
}
COMPRESSED zero {
behavior [ 2 ];
ENFORCE(behavior.UVALUE == IP_ID_BEHAVIOR_ZERO);
}
}
// The is_innermost flag should be true if this is the innermost
// IP header to be compressed.
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// If extracting the irregular chain for an compressed packet,
// the TTL&TOS arguments must have the same value as it had when
// processing co_baseheader. If extracting any other chain
// items, this argument is not used.
ipv4(profile, is_innermost,
ttl_irregular_chain_flag, tos_irregular_chain_flag)
{
UNCOMPRESSED {
version =:= uncompressed_value(4, 4) [ 4 ];
hdr_length =:= uncompressed_value(4, 5) [ 4 ];
tos_tc [ 8 ];
length [ 16 ];
ip_id [ 16 ];
rf =:= uncompressed_value(1, 0) [ 1 ];
df [ 1 ];
mf =:= uncompressed_value(1, 0) [ 1 ];
frag_offset =:= uncompressed_value(13, 0) [ 13 ];
ttl_hopl [ 8 ];
protocol [ 8 ];
checksum [ 16 ];
src_addr [ 32 ];
dst_addr [ 32 ];
}
CONTROL {
ip_id_behavior [ 2 ];
}
DEFAULT {
tos_tc =:= static;
length =:= inferred_ip_v4_length;
df =:= static;
ttl_hopl =:= static;
protocol =:= static;
checksum =:= inferred_ip_v4_header_checksum;
src_addr =:= static;
dst_addr =:= static;
ip_id_behavior =:= static;
}
COMPRESSED ipv4_static {
version_flag =:= '0' [ 1 ];
reserved =:= '0000000' [ 7 ];
protocol =:= irregular(8) [ 8 ];
src_addr =:= irregular(32) [ 32 ];
dst_addr =:= irregular(32) [ 32 ];
}
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COMPRESSED ipv4_endpoint_dynamic {
reserved =:= '000' [ 5 ];
reorder_ratio =:= reorder_choice [ 2 ];
df =:= irregular(1) [ 1 ];
ip_id_behavior =:= ip_id_behavior_choice [ 2 ];
tos_tc =:= irregular(8) [ 8 ];
ttl_hopl =:= irregular(8) [ 8 ];
ip_id =:=
ip_id_enc_dyn(ip_id_behavior.UVALUE) [ 0, 16 ];
ENFORCE((is_innermost == true) &&
(profile == PROFILE_IP_0104));
}
COMPRESSED ipv4_regular_dynamic {
reserved =:= '00000' [ 5 ];
df =:= irregular(1) [ 1 ];
ip_id_behavior =:= ip_id_behavior_choice [ 2 ];
tos_tc =:= irregular(8) [ 8 ];
ttl_hopl =:= irregular(8) [ 8 ];
ip_id =:=
ip_id_enc_dyn(ip_id_behavior.UVALUE) [ 0, 16 ];
ENFORCE((is_innermost == false) ||
(profile != PROFILE_IP_0104));
}
COMPRESSED ipv4_outer_irregular {
ip_id =:=
ip_id_enc_irreg(ip_id_behavior.UVALUE) [ 0, 16 ];
tos_tc =:=
static_or_irreg(tos_irregular_chain_flag) [ 0, 8 ];
ttl_hopl =:=
static_or_irreg(ttl_irregular_chain_flag) [ 0, 8 ];
ENFORCE(is_innermost == false);
}
COMPRESSED ipv4_innermost_irregular {
ip_id =:=
ip_id_enc_irreg(ip_id_behavior.UVALUE) [ 0, 16 ];
ENFORCE(is_innermost == true);
}
}
/////////////////////////////////////////////
// UDP Header
/////////////////////////////////////////////
udp(profile)
{
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UNCOMPRESSED {
src_port [ 16 ];
dst_port [ 16 ];
udp_length [ 16 ];
checksum [ 16 ];
ENFORCE((profile == PROFILE_RTP_0101) ||
(profile == PROFILE_UDP_0102));
}
DEFAULT {
src_port =:= static;
dst_port =:= static;
udp_length =:= inferred_udp_length;
checksum =:= irregular(16);
}
COMPRESSED udp_static {
src_port =:= irregular(16) [ 16 ];
dst_port =:= irregular(16) [ 16 ];
}
COMPRESSED udp_endpoint_dynamic {
checksum [ 16 ];
msn =:= irregular(16) [ 16 ];
reserved =:= uncompressed_value(6, 0);
reorder_ratio =:= reorder_choice [ 2 ];
ENFORCE(profile == PROFILE_UDP_0102);
}
COMPRESSED udp_regular_dynamic {
checksum [ 16 ];
}
COMPRESSED udp_zero_checksum_irregular {
ENFORCE(checksum.UVALUE == 0);
checksum =:= uncompressed_value(16, 0);
}
COMPRESSED udp_with_checksum_irregular {
ENFORCE(checksum.UVALUE == 1);
checksum [ 16 ];
}
}
/////////////////////////////////////////////
// RTP Header
/////////////////////////////////////////////
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csrc_list_dynchain(presence, cc_value)
{
UNCOMPRESSED {
csrc_list;
}
COMPRESSED no_list {
csrc_list =:= uncompressed_value(0, 0) [ 0 ];
ENFORCE(cc_value == 0);
ENFORCE(presence == 0);
}
COMPRESSED list_present {
csrc_list =:= list_csrc(cc_value) [ VARIABLE ];
ENFORCE(presence == 1);
}
}
rtp(profile, ts_stride_value)
{
UNCOMPRESSED {
rtp_version =:= uncompressed_value(2, 0) [ 2 ];
pad_bit [ 1 ];
extension [ 1 ];
cc [ 4 ];
marker [ 1 ];
payload_type [ 7 ];
sequence_number [ 16 ];
timestamp [ 32 ];
ssrc [ 32 ];
csrc_list [ VARIABLE ];
ENFORCE((profile == PROFILE_RTP_0101) ||
(profile == PROFILE_RTP_0107));
}
CONTROL {
// The ts_stride has an initial UVALUE=1, which means that it
// can be encoded with 'static' even if it has not been
// previously established in the context.
ts_stride [ 32 ];
ts_scaled [ 32 ];
ts_offset =:=
field_scaling(ts_stride.UVALUE, ts_scaled.UVALUE,
timestamp.UVALUE) [ 32 ];
}
DEFAULT {
pad_bit =:= static;
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extension =:= static;
cc =:= static;
marker =:= static;
payload_type =:= static;
sequence_number =:= static;
timestamp =:= static;
ssrc =:= static;
csrc_list =:= static;
}
COMPRESSED rtp_static {
ssrc =:= irregular(32) [ 32 ];
}
COMPRESSED rtp_dynamic {
reserved =:= compressed_value(1, 0) [ 1 ];
reorder_ratio =:= reorder_choice [ 2 ];
list_present =:= irregular(1) [ 1 ];
tss_indicator =:= irregular(1) [ 1 ];
pad_bit =:= irregular(1) [ 1 ];
extension =:= irregular(1) [ 1 ];
marker =:= irregular(1) [ 1 ];
payload_type =:= irregular(7) [ 7 ];
sequence_number =:= irregular(16) [ 16 ];
timestamp =:= irregular(32) [ 32 ];
ts_stride =:=
optional_stride(tss_indicator,
ts_stride_value) [ VARIABLE ];
csrc_list =:=
csrc_list_dynchain(list_present, cc.UVALUE) [ VARIABLE ];
}
COMPRESSED rtp_irregular {
}
}
/////////////////////////////////////////////
// UDP-Lite Header
/////////////////////////////////////////////
checksum_coverage_dynchain(behavior)
{
UNCOMPRESSED {
checksum_coverage [ 16 ];
}
COMPRESSED inferred_coverage {
checksum_coverage =:= inferred_udp_length [ 0 ];
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ENFORCE(behavior == UDP_LITE_COVERAGE_INFERRED);
}
COMPRESSED static_coverage {
checksum_coverage =:= irregular(16) [ 16 ];
ENFORCE(behavior == UDP_LITE_COVERAGE_STATIC);
}
COMPRESSED irregular_coverage {
checksum_coverage =:= irregular(16) [ 16 ];
ENFORCE(behavior == UDP_LITE_COVERAGE_IRREGULAR);
}
}
checksum_coverage_irregular(behavior)
{
UNCOMPRESSED {
checksum_coverage [ 16 ];
}
COMPRESSED inferred_coverage {
checksum_coverage =:= inferred_udp_length [ 0 ];
ENFORCE(behavior == UDP_LITE_COVERAGE_INFERRED);
}
COMPRESSED static_coverage {
checksum_coverage =:= static [ 0 ];
ENFORCE(behavior == UDP_LITE_COVERAGE_STATIC);
}
COMPRESSED irregular_coverage {
checksum_coverage =:= irregular(16) [ 16 ];
ENFORCE(behavior == UDP_LITE_COVERAGE_IRREGULAR);
}
}
udp_lite(profile)
{
UNCOMPRESSED {
src_port [ 16 ];
dst_port [ 16 ];
checksum_coverage [ 16 ];
checksum [ 16 ];
ENFORCE((profile == PROFILE_RTP_0107) ||
(profile == PROFILE_UDPLITE_0108));
}
CONTROL {
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coverage_behavior [ 2 ];
}
DEFAULT {
src_port =:= static;
dst_port =:= static;
checksum_coverage =:= irregular(16);
checksum =:= irregular(16);
}
COMPRESSED udp_lite_static {
src_port =:= irregular(16) [ 16 ];
dst_port =:= irregular(16) [ 16 ];
}
COMPRESSED udp_lite_endpoint_dynamic {
reserved =:= compressed_value(4, 0) [ 4 ];
coverage_behavior =:= irregular(2) [ 2 ];
reorder_ratio =:= reorder_choice [ 2 ];
checksum_coverage =:=
checksum_coverage_dynchain(coverage_behavior.UVALUE) [ 16 ];
checksum [ 16 ];
msn =:= irregular(16) [ 16 ];
ENFORCE(profile == PROFILE_UDPLITE_0108);
}
COMPRESSED udp_lite_regular_dynamic {
coverage_behavior =:= irregular(2) [ 2 ];
reserved =:= compressed_value(6, 0) [ 6 ];
checksum_coverage =:=
checksum_coverage_dynchain(coverage_behavior.UVALUE) [ 16 ];
checksum [ 16 ];
}
COMPRESSED udp_lite_irregular {
checksum_coverage =:=
checksum_coverage_dynchain(coverage_behavior.UVALUE) [ 0, 16 ];
checksum [ 16 ];
}
}
/////////////////////////////////////////////
// ESP Header (Non-NULL encrypted
// i.e. only used for the ESP profile
/////////////////////////////////////////////
esp(profile)
{
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UNCOMPRESSED {
spi [ 32 ];
sequence_number [ 32 ];
ENFORCE(profile == PROFILE_ESP_0103);
}
DEFAULT {
spi =:= static;
sequence_number =:= static;
}
COMPRESSED esp_static {
spi =:= irregular(32) [ 32 ];
}
COMPRESSED esp_dynamic {
sequence_number =:= irregular(32) [ 32 ];
msn =:= irregular(16) [ 16 ];
reserved =:= uncompressed_value(6, 0) [ 6 ];
reorder_ratio =:= reorder_choice [ 2 ];
}
COMPRESSED esp_irregular {
}
}
///////////////////////////////////////////////////
// Encoding methods used in the profiles' CO packets
///////////////////////////////////////////////////
ip_id_lsb(behavior, k, p)
{
UNCOMPRESSED {
ip_id [ 16 ];
}
CONTROL {
ip_id_offset [ 16 ];
ip_id_nbo [ 16 ];
}
COMPRESSED nbo {
ip_id_offset =:= lsb(k, p) [ VARIABLE ];
ENFORCE(behavior == IP_ID_BEHAVIOR_SEQUENTIAL);
ENFORCE(ip_id_offset.UVALUE == ip_id.UVALUE - msn.UVALUE);
}
COMPRESSED non_nbo {
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ip_id_offset =:= lsb(k, p) [ VARIABLE ];
ENFORCE(behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED);
ENFORCE(ip_id_nbo.UVALUE ==
(ip_id.UVALUE / 256) + (ip_id.UVALUE % 256) * 256);
ENFORCE(ip_id_nbo.ULENGTH == 16);
ENFORCE(ip_id_offset.UVALUE == ip_id_nbo.UVALUE - msn.UVALUE);
}
}
optional_ip_id_lsb(behavior, indicator)
{
UNCOMPRESSED {
ip_id [ 16 ];
}
COMPRESSED short {
ip_id =:= ip_id_lsb(behavior, 8, 3) [ 8 ];
ENFORCE((behavior == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
ENFORCE(indicator == 0);
}
COMPRESSED long {
ip_id =:= irregular(16) [ 16 ];
ENFORCE((behavior == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
ENFORCE(indicator == 1);
}
COMPRESSED not_present {
ENFORCE((behavior == IP_ID_BEHAVIOR_RANDOM) ||
(behavior == IP_ID_BEHAVIOR_ZERO));
}
}
dont_fragment(version)
{
UNCOMPRESSED {
df [ 1 ];
}
COMPRESSED v4 {
df =:= irregular(1) [ 1 ];
ENFORCE(version == 4);
}
COMPRESSED v6 {
df =:= compressed_value(1, 0) [ 1 ];
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ENFORCE(version == 6);
}
}
////////////////////////////////////////////
// RTP profile
////////////////////////////////////////////
// ttl_irregular_chain_flag is set by the user if the TTL/Hop Limit
// of an outer header. The same value must be passed as an argument
// to the ipv4/ipv6 encoding methods when extracting the irregular
// chain items. The same applies to the tos_irregular_chain_flag
rtp_baseheader(profile, ts_stride_value,
ttl_irregular_chain_flag, tos_irregular_chain_flag)
{
UNCOMPRESSED v4 {
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
version =:= uncompressed_value(4, 4) [ 4 ];
header_length =:= uncompressed_value(4, 5) [ 4 ];
tos_tc [ 8 ];
length [ 16 ];
ip_id [ 16 ];
rf =:= uncompressed_value(1, 0) [ 1 ];
df [ 1 ];
mf =:= uncompressed_value(1, 0) [ 1 ];
frag_offset =:= uncompressed_value(13, 0) [ 13 ];
ttl_hopl [ 8 ];
next_header [ 8 ];
checksum [ 16 ];
src_addr [ 32 ];
dest_addr [ 32 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
dst_port [ 16 ];
udp_length [ 16 ];
checksum [ 16 ];
rtp_version =:= uncompressed_value(2, 0) [ 2 ];
pad_bit [ 1 ];
extension [ 1 ];
cc [ 4 ];
marker [ 1 ];
payload_type [ 7 ];
sequence_number [ 16 ];
timestamp [ 32 ];
ssrc [ 32 ];
csrc_list [ VARIABLE ];
ENFORCE(msn.UVALUE == sequence_number.UVALUE);
}
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UNCOMPRESSED v6 {
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
version =:= uncompressed_value(4, 6) [ 4 ];
tos_tc [ 8 ];
flow_label [ 20 ];
payload_length [ 16 ];
next_header [ 8 ];
ttl_hopl [ 8 ];
src_addr [ 128 ];
dest_addr [ 128 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
dst_port [ 16 ];
udp_length [ 16 ];
checksum [ 16 ];
rtp_version =:= uncompressed_value(2, 0) [ 2 ];
pad_bit [ 1 ];
extension [ 1 ];
cc [ 4 ];
marker [ 1 ];
payload_type [ 7 ];
sequence_number [ 16 ];
timestamp [ 32 ];
ssrc [ 32 ];
csrc_list [ VARIABLE ];
ENFORCE(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM);
ENFORCE(msn.UVALUE == sequence_number.UVALUE);
}
CONTROL {
// The ts_stride has an initial UVALUE=1, which means that it
// can be encoded with 'static' even if it has not been
// previously established in the context.
ts_stride [ 32 ];
ts_scaled [ 32 ];
ts_offset =:=
field_scaling(ts_stride.UVALUE, ts_scaled.UVALUE,
timestamp.UVALUE) [ 32 ];
ip_id_behavior [ 2 ];
ENFORCE(ts_stride.UVALUE == ts_stride_value);
ENFORCE(profile == PROFILE_RTP_0101);
}
DEFAULT {
tos_tc =:= static;
dest_addr =:= static;
version =:= static;
ttl_hopl =:= static;
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src_addr =:= static;
df =:= static;
ip_id_behavior =:= static;
payload_length =:= inferred_ip_v6_length;
checksum =:= inferred_ip_v4_header_checksum;
length =:= inferred_ip_v4_length;
flow_label =:= static;
next_header =:= static;
src_port =:= static;
dst_port =:= static;
udp_length =:= inferred_udp_length;
checksum =:= irregular(16);
pad_bit =:= static;
extension =:= static;
cc =:= static;
// When marker not present in packets, it is assumed 0
marker =:= uncompressed_value(1, 0);
payload_type =:= static;
sequence_number =:= static;
timestamp =:= static;
ssrc =:= static;
csrc_list =:= static;
ENFORCE(ttl_irregular_chain_flag == 0);
ENFORCE(tos_irregular_chain_flag == 0);
}
// Replacement for UOR-2-ext3
COMPRESSED co_common {
discriminator =:= '1111101' [ 7 ];
marker =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ip_id_indicator =:= irregular(1) [ 1 ];
ip_id_behavior =:= ip_id_behavior_choice [ 2 ];
reorder_ratio =:= reorder_choice [ 2 ];
df =:= dont_fragment(version.UVALUE) [ 1 ];
control_crc3 =:= control_crc3 [ 3 ];
ttl_hopl_outer_flag =:= irregular(1) [ 1 ];
ttl_hopl_present =:= irregular(1) [ 1 ];
tos_tc_outer_flag =:= irregular(1) [ 1 ];
tos_tc_present =:= irregular(1) [ 1 ];
ts_indicator =:= irregular(1) [ 1 ];
tss_indicator =:= irregular(1) [ 1 ];
pt_present =:= irregular(1) [ 1 ];
list_present =:= irregular(1) [ 1 ];
pad_bit =:= irregular(1) [ 1 ];
extension =:= irregular(1) [ 1 ];
reserved =:= compressed_value(6, 0) [ 6 ];
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ip_id =:=
optional_ip_id_lsb(ip_id_behavior.UVALUE,
ip_id_indicator.CVALUE) [ 0, 8, 16 ];
tos_tc =:=
tos_tc_enc(tos_tc_present.CVALUE) [ 0, 8 ];
ttl_hopl =:=
static_or_irreg(ttl_hopl_present.CVALUE,
ttl_hopl.ULENGTH) [ 0, 8 ];
sequence_number =:= sdvl(sequence_number.ULENGTH) [ 8, 16 ];
// Either scaled or unscaled timestamp
ts_scaled =:=
optional_scaled_timestamp(tss_indicator,
tsc_indicator) [ VARIABLE ];
ts_scaled =:=
optional_scaled_timestamp(tss_indicator,
tsc_indicator) [ VARIABLE ];
payload_type =:= optional_pt(pt_present) [ 0, 8 ];
ts_stride =:=
optional_stride(tss_indicator,
ts_stride_value) [ VARIABLE ];
csrc_list =:= list_csrc(cc.UVALUE) [ VARIABLE ];
ENFORCE(ttl_irregular_chain_flag == ttl_hopl_outer_flag.UVALUE);
ENFORCE(tos_irregular_chain_flag == tos_tc_outer_flag.UVALUE);
}
// UO-0
COMPRESSED pt_0_crc3 {
discriminator =:= '0' [ 1 ];
msn =:= msn_lsb(4, 4) [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
timestamp =:= inferred_scaled_field [ 0 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// New format, Type 0 with strong CRC and more SN bits
COMPRESSED pt_0_crc7 {
discriminator =:= '100' [ 3 ];
msn =:= msn_lsb(6, 16) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
timestamp =:= inferred_scaled_field [ 0 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// UO-1 replacement
COMPRESSED pt_1_rnd {
discriminator =:= '101' [ 3 ];
msn =:= msn_lsb(5, 8) [ 5 ];
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marker =:= irregular(1) [ 1 ];
ts_scaled =:= lsb(4, 3) [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
}
// UO-1-ID replacement
COMPRESSED pt_1_seq_id {
discriminator =:= '1010' [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
msn =:= msn_lsb(5, 8) [ 5 ];
ip_id =:= ip_id_lsb(ip_id_behavior.UVALUE, 4, 3) [ 4 ];
timestamp =:= inferred_scaled_field [ 0 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
// UO-1-TS replacement
COMPRESSED pt_1_seq_ts {
discriminator =:= '1011' [ 4 ];
marker =:= irregular(1) [ 1 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
msn =:= msn_lsb(4, 4) [ 4 ];
ts_scaled =:= lsb(4, 3) [ 4 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
// UOR-2 replacement
COMPRESSED pt_2_rnd {
discriminator =:= '110' [ 3 ];
msn =:= msn_lsb(7, 32) [ 7 ];
ts_scaled =:= lsb(6, 15) [ 6 ];
marker =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
}
// UOR-2-ID replacement
COMPRESSED pt_2_seq_id {
discriminator =:= '1100' [ 4 ];
msn =:= msn_lsb(7, 32) [ 7 ];
ip_id =:= ip_id_lsb(ip_id_behavior.UVALUE, 6, 3) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
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timestamp =:= inferred_scaled_field [ 0 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
// UOR-2-TS replacement
COMPRESSED pt_2_seq_ts {
discriminator =:= '1101' [ 4 ];
msn =:= msn_lsb(7, 32) [ 7 ];
ts_scaled =:= lsb(5, 7) [ 5 ];
marker =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
}
////////////////////////////////////////////
// UDP profile
////////////////////////////////////////////
// ttl_irregular_chain_flag is set by the user if the TTL/Hop Limit
// of an outer header. The same value must be passed as an argument
// to the ipv4/ipv6 encoding methods when extracting the irregular
// chain items. The same applies to the tos_irregular_chain_flag
udp_baseheader(profile,
ttl_irregular_chain_flag, tos_irregular_chain_flag)
{
UNCOMPRESSED v4 {
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
version =:= uncompressed_value(4, 4) [ 4 ];
header_length =:= uncompressed_value(4, 5) [ 4 ];
tos_tc [ 8 ];
length [ 16 ];
ip_id [ 16 ];
rf =:= uncompressed_value(1, 0) [ 1 ];
df [ 1 ];
mf =:= uncompressed_value(1, 0) [ 1 ];
frag_offset =:= uncompressed_value(13, 0) [ 13 ];
ttl_hopl [ 8 ];
next_header [ 8 ];
checksum [ 16 ];
src_addr [ 32 ];
dest_addr [ 32 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
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dst_port [ 16 ];
udp_length [ 16 ];
checksum [ 16 ];
}
UNCOMPRESSED v6 {
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
version =:= uncompressed_value(4, 6) [ 4 ];
version [ 4 ];
tos_tc [ 8 ];
flow_label [ 20 ];
payload_length [ 16 ];
next_header [ 8 ];
ttl_hopl [ 8 ];
src_addr [ 128 ];
dest_addr [ 128 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
dst_port [ 16 ];
udp_length [ 16 ];
checksum [ 16 ];
ENFORCE(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM);
}
CONTROL {
ip_id_behavior [ 2 ];
ENFORCE(profile == PROFILE_UDP_0102);
}
DEFAULT {
tos_tc =:= static;
dest_addr =:= static;
version =:= static;
ttl_hopl =:= static;
src_addr =:= static;
df =:= static;
ip_id_behavior =:= static;
payload_length =:= inferred_ip_v6_length;
checksum =:= inferred_ip_v4_header_checksum;
length =:= inferred_ip_v4_length;
flow_label =:= static;
next_header =:= static;
src_port =:= static;
dst_port =:= static;
udp_length =:= inferred_udp_length;
checksum =:= irregular(16);
ENFORCE(ttl_irregular_chain_flag == 0);
ENFORCE(tos_irregular_chain_flag == 0);
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}
// Replacement for UOR-2-ext3
COMPRESSED co_common {
discriminator =:= '1111101' [ 7 ];
ip_id_indicator =:= irregular(1) [ 1 ];
reorder_ratio =:= reorder_choice [ 2 ];
msn =:= msn_lsb(6, 16) [ 6 ];
df =:= dont_fragment(version.UVALUE) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ttl_hopl_outer_flag =:= irregular(1) [ 1 ];
ttl_hopl_present =:= irregular(1) [ 1 ];
tos_tc_outer_flag =:= irregular(1) [ 1 ];
ip_id_behavior =:= ip_id_behavior_choice [ 2 ];
control_crc3 =:= control_crc3 [ 3 ];
tos_tc_present =:= irregular(1) [ 1 ];
reserved =:= compressed_value(7, 0) [ 7 ];
ip_id =:=
optional_ip_id_lsb(ip_id_behavior.UVALUE,
ip_id_indicator.CVALUE) [ 0, 8, 16 ];
tos_tc =:=
tos_tc_enc(tos_tc_present.CVALUE) [ 0, 8 ];
ttl_hopl =:=
static_or_irreg(ttl_hopl_present.CVALUE,
ttl_hopl.ULENGTH) [ 0, 8 ];
ENFORCE(ttl_irregular_chain_flag == ttl_hopl_outer_flag.UVALUE);
ENFORCE(tos_irregular_chain_flag == tos_tc_outer_flag.UVALUE);
}
// UO-0
COMPRESSED pt_0_crc3 {
discriminator =:= '0' [ 1 ];
msn =:= msn_lsb(4, 4) [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// New format, Type 0 with strong CRC and more SN bits
COMPRESSED pt_0_crc7 {
discriminator =:= '100' [ 3 ];
msn =:= msn_lsb(6, 16) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// UO-1-ID replacement (PT-1 only used for sequential)
COMPRESSED pt_1_seq_id {
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discriminator =:= '101' [ 3 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
msn =:= msn_lsb(6, 16) [ 6 ];
ip_id =:= ip_id_lsb(ip_id_behavior.UVALUE, 4, 3) [ 4 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
// UOR-2 replacement
COMPRESSED pt_2_rnd {
discriminator =:= '110' [ 3 ];
msn =:= msn_lsb(6, 16) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
}
// UOR-2-ID replacement
COMPRESSED pt_2_seq_id {
discriminator =:= '1100' [ 4 ];
ip_id =:= ip_id_lsb(ip_id_behavior.UVALUE, 5, 3) [ 5 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
msn =:= msn_lsb(8, 64) [ 8 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
}
////////////////////////////////////////////
// ESP profile
////////////////////////////////////////////
// ttl_irregular_chain_flag is set by the user if the TTL/Hop Limit
// of an outer header. The same value must be passed as an argument
// to the ipv4/ipv6 encoding methods when extracting the irregular
// chain items. The same applies to the tos_irregular_chain_flag
esp_baseheader(profile,
ttl_irregular_chain_flag, tos_irregular_chain_flag)
{
UNCOMPRESSED v4 {
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
version =:= uncompressed_value(4, 4) [ 4 ];
header_length =:= uncompressed_value(4, 5) [ 4 ];
tos_tc [ 8 ];
length [ 16 ];
ip_id [ 16 ];
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rf =:= uncompressed_value(1, 0) [ 1 ];
df [ 1 ];
mf =:= uncompressed_value(1, 0) [ 1 ];
frag_offset =:= uncompressed_value(13, 0) [ 13 ];
ttl_hopl [ 8 ];
next_header [ 8 ];
checksum [ 16 ];
src_addr [ 32 ];
dest_addr [ 32 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
spi [ 32 ];
sequence_number [ 32 ];
ENFORCE(msn.UVALUE == sequence_number.UVALUE);
}
UNCOMPRESSED v6 {
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
version =:= uncompressed_value(4, 6) [ 4 ];
version [ 4 ];
tos_tc [ 8 ];
flow_label [ 20 ];
payload_length [ 16 ];
next_header [ 8 ];
ttl_hopl [ 8 ];
src_addr [ 128 ];
dest_addr [ 128 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
spi [ 32 ];
sequence_number [ 32 ];
ENFORCE(msn.UVALUE == (sequence_number.UVALUE % 65536));
ENFORCE(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM);
}
CONTROL {
ip_id_behavior [ 2 ];
ENFORCE(profile == PROFILE_ESP_0103);
}
DEFAULT {
tos_tc =:= static;
dest_addr =:= static;
version =:= static;
ttl_hopl =:= static;
src_addr =:= static;
df =:= static;
ip_id_behavior =:= static;
payload_length =:= inferred_ip_v6_length;
checksum =:= inferred_ip_v4_header_checksum;
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length =:= inferred_ip_v4_length;
flow_label =:= static;
next_header =:= static;
spi =:= static;
sequence_number =:= static;
ENFORCE(ttl_irregular_chain_flag == 0);
ENFORCE(tos_irregular_chain_flag == 0);
}
// Replacement for UOR-2-ext3
COMPRESSED co_common {
discriminator =:= '1111101' [ 7 ];
ip_id_indicator =:= irregular(1) [ 1 ];
df =:= dont_fragment(version.UVALUE) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ttl_hopl_outer_flag =:= irregular(1) [ 1 ];
ttl_hopl_present =:= irregular(1) [ 1 ];
tos_tc_outer_flag =:= irregular(1) [ 1 ];
tos_tc_present =:= irregular(1) [ 1 ];
reorder_ratio =:= reorder_choice [ 2 ];
ip_id_behavior =:= ip_id_behavior_choice [ 2 ];
control_crc3 =:= control_crc3 [ 3 ];
reserved =:= compressed_value(5, 0) [ 5 ];
sequence_number =:=
sdvl(sequence_number.ULENGTH) [ 8, 16, 24, 32 ];
ip_id =:=
optional_ip_id_lsb(ip_id_behavior.UVALUE,
ip_id_indicator.CVALUE) [ 0, 8, 16 ];
tos_tc =:=
tos_tc_enc(tos_tc_present.CVALUE) [ 0, 8 ];
ttl_hopl =:=
static_or_irreg(ttl_hopl_present.CVALUE,
ttl_hopl.ULENGTH) [ 0, 8 ];
ENFORCE(ttl_irregular_chain_flag == ttl_hopl_outer_flag.UVALUE);
ENFORCE(tos_irregular_chain_flag == tos_tc_outer_flag.UVALUE);
}
// UO-0
COMPRESSED pt_0_crc3 {
discriminator =:= '0' [ 1 ];
msn =:= msn_lsb(4, 4) [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// New format, Type 0 with strong CRC and more SN bits
COMPRESSED pt_0_crc7 {
discriminator =:= '100' [ 3 ];
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msn =:= msn_lsb(6, 16) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// UO-1-ID replacement (PT-1 only used for sequential)
COMPRESSED pt_1_seq_id {
discriminator =:= '101' [ 3 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
msn =:= msn_lsb(6, 16) [ 6 ];
ip_id =:= ip_id_lsb(ip_id_behavior.UVALUE, 4, 3) [ 4 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
// UOR-2 replacement
COMPRESSED pt_2_rnd {
discriminator =:= '110' [ 3 ];
msn =:= msn_lsb(6, 16) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
}
// UOR-2-ID replacement
COMPRESSED pt_2_seq_id {
discriminator =:= '1100' [ 4 ];
ip_id =:= ip_id_lsb(ip_id_behavior.UVALUE, 5, 3) [ 5 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
msn =:= msn_lsb(8, 64) [ 8 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
}
////////////////////////////////////////////
// IP-only profile
////////////////////////////////////////////
// ttl_irregular_chain_flag is set by the user if the TTL/Hop Limit
// of an outer header. The same value must be passed as an argument
// to the ipv4/ipv6 encoding methods when extracting the irregular
// chain items. The same applies to the tos_irregular_chain_flag
iponly_baseheader(profile,
ttl_irregular_chain_flag, tos_irregular_chain_flag)
{
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UNCOMPRESSED v4 {
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
version =:= uncompressed_value(4, 4) [ 4 ];
header_length =:= uncompressed_value(4, 5) [ 4 ];
tos_tc [ 8 ];
length [ 16 ];
ip_id [ 16 ];
rf =:= uncompressed_value(1, 0) [ 1 ];
df [ 1 ];
mf =:= uncompressed_value(1, 0) [ 1 ];
frag_offset =:= uncompressed_value(13, 0) [ 13 ];
ttl_hopl [ 8 ];
next_header [ 8 ];
checksum [ 16 ];
src_addr [ 32 ];
dest_addr [ 32 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
}
UNCOMPRESSED v6 {
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
version =:= uncompressed_value(4, 6) [ 4 ];
version [ 4 ];
tos_tc [ 8 ];
flow_label [ 20 ];
payload_length [ 16 ];
next_header [ 8 ];
ttl_hopl [ 8 ];
src_addr [ 128 ];
dest_addr [ 128 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
ENFORCE(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM);
}
CONTROL {
ip_id_behavior [ 2 ];
ENFORCE(profile == PROFILE_IP_0104);
}
DEFAULT {
tos_tc =:= static;
dest_addr =:= static;
version =:= static;
ttl_hopl =:= static;
src_addr =:= static;
df =:= static;
ip_id_behavior =:= static;
payload_length =:= inferred_ip_v6_length;
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checksum =:= inferred_ip_v4_header_checksum;
length =:= inferred_ip_v4_length;
flow_label =:= static;
next_header =:= static;
ENFORCE(ttl_irregular_chain_flag == 0);
ENFORCE(tos_irregular_chain_flag == 0);
}
// Replacement for UOR-2-ext3
COMPRESSED co_common {
discriminator =:= '1111101' [ 7 ];
ip_id_indicator =:= irregular(1) [ 1 ];
reorder_ratio =:= reorder_choice [ 2 ];
msn =:= msn_lsb(6, 16) [ 6 ];
df =:= dont_fragment(version.UVALUE) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ttl_hopl_outer_flag =:= irregular(1) [ 1 ];
ttl_hopl_present =:= irregular(1) [ 1 ];
tos_tc_outer_flag =:= irregular(1) [ 1 ];
ip_id_behavior =:= ip_id_behavior_choice [ 2 ];
control_crc3 =:= control_crc3 [ 3 ];
tos_tc_present =:= irregular(1) [ 1 ];
reserved =:= compressed_value(7, 0) [ 7 ];
ip_id =:=
optional_ip_id_lsb(ip_id_behavior.UVALUE,
ip_id_indicator.CVALUE) [ 0, 8, 16 ];
tos_tc =:=
tos_tc_enc(tos_tc_present.CVALUE) [ 0, 8 ];
ttl_hopl =:=
static_or_irreg(ttl_hopl_present.CVALUE,
ttl_hopl.ULENGTH) [ 0, 8 ];
ENFORCE(ttl_irregular_chain_flag == ttl_hopl_outer_flag.UVALUE);
ENFORCE(tos_irregular_chain_flag == tos_tc_outer_flag.UVALUE);
}
// UO-0
COMPRESSED pt_0_crc3 {
discriminator =:= '0' [ 1 ];
msn =:= msn_lsb(4, 4) [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// New format, Type 0 with strong CRC and more SN bits
COMPRESSED pt_0_crc7 {
discriminator =:= '100' [ 3 ];
msn =:= msn_lsb(6, 16) [ 6 ];
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header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// UO-1-ID replacement (PT-1 only used for sequential)
COMPRESSED pt_1_seq_id {
discriminator =:= '101' [ 3 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
msn =:= msn_lsb(6, 16) [ 6 ];
ip_id =:= ip_id_lsb(ip_id_behavior.UVALUE, 4, 3) [ 4 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
// UOR-2 replacement
COMPRESSED pt_2_rnd {
discriminator =:= '110' [ 3 ];
msn =:= msn_lsb(6, 16) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
}
// UOR-2-ID replacement
COMPRESSED pt_2_seq_id {
discriminator =:= '1100' [ 4 ];
ip_id =:= ip_id_lsb(ip_id_behavior.UVALUE, 5, 3) [ 5 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
msn =:= msn_lsb(8, 64) [ 8 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
}
////////////////////////////////////////////
// UDP-lite/RTP profile
////////////////////////////////////////////
// ttl_irregular_chain_flag is set by the user if the TTL/Hop Limit
// of an outer header. The same value must be passed as an argument
// to the ipv4/ipv6 encoding methods when extracting the irregular
// chain items. The same applies to the tos_irregular_chain_flag
udplite_rtp_baseheader(profile, ts_stride_value,
ttl_irregular_chain_flag, tos_irregular_chain_flag)
{
UNCOMPRESSED v4 {
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outer_headers =:= baseheader_outer_headers [ VARIABLE ];
version =:= uncompressed_value(4, 4) [ 4 ];
header_length =:= uncompressed_value(4, 5) [ 4 ];
tos_tc [ 8 ];
length [ 16 ];
ip_id [ 16 ];
rf =:= uncompressed_value(1, 0) [ 1 ];
df [ 1 ];
mf =:= uncompressed_value(1, 0) [ 1 ];
frag_offset =:= uncompressed_value(13, 0) [ 13 ];
ttl_hopl [ 8 ];
next_header [ 8 ];
checksum [ 16 ];
src_addr [ 32 ];
dest_addr [ 32 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
dst_port [ 16 ];
checksum_coverage [ 16 ];
checksum [ 16 ];
version =:= uncompressed_value(2, 0) [ 2 ];
pad_bit [ 1 ];
extension [ 1 ];
cc [ 4 ];
marker [ 1 ];
payload_type [ 7 ];
sequence_number [ 16 ];
timestamp [ 32 ];
ssrc [ 32 ];
csrc_list [ VARIABLE ];
ENFORCE(msn.UVALUE == sequence_number.UVALUE);
}
UNCOMPRESSED v6 {
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
version =:= uncompressed_value(4, 6) [ 4 ];
version [ 4 ];
tos_tc [ 8 ];
flow_label [ 20 ];
payload_length [ 16 ];
next_header [ 8 ];
ttl_hopl [ 8 ];
src_addr [ 128 ];
dest_addr [ 128 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
dst_port [ 16 ];
checksum_coverage [ 16 ];
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checksum [ 16 ];
version =:= uncompressed_value(2, 0) [ 2 ];
pad_bit [ 1 ];
extension [ 1 ];
cc [ 4 ];
marker [ 1 ];
payload_type [ 7 ];
sequence_number [ 16 ];
timestamp [ 32 ];
ssrc [ 32 ];
csrc_list [ VARIABLE ];
ENFORCE(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM);
}
CONTROL {
ip_id_behavior [ 2 ];
coverage_behavior [ 2 ];
ts_stride [ 32 ];
ts_scaled [ 32 ];
ts_offset =:=
field_scaling(ts_stride.UVALUE, ts_scaled.UVALUE,
timestamp.UVALUE) [ 32 ];
ENFORCE(ts_stride.UVALUE == ts_stride_value);
ENFORCE(profile == PROFILE_RTP_0107);
}
DEFAULT {
tos_tc =:= static;
dest_addr =:= static;
version =:= static;
ttl_hopl =:= static;
src_addr =:= static;
df =:= static;
ip_id_behavior =:= static;
payload_length =:= inferred_ip_v6_length;
checksum =:= inferred_ip_v4_header_checksum;
length =:= inferred_ip_v4_length;
flow_label =:= static;
next_header =:= static;
src_port =:= static;
dst_port =:= static;
checksum_coverage =:= irregular(16);
checksum =:= irregular(16);
pad_bit =:= static;
extension =:= static;
cc =:= static;
// When marker not present in packets, it is assumed 0
marker =:= uncompressed_value(1, 0);
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payload_type =:= static;
sequence_number =:= static;
timestamp =:= static;
ssrc =:= static;
csrc_list =:= static;
ENFORCE(ttl_irregular_chain_flag == 0);
ENFORCE(tos_irregular_chain_flag == 0);
}
// Replacement for UOR-2-ext3
COMPRESSED co_common {
discriminator =:= '1111101' [ 7 ];
marker =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ip_id_indicator =:= irregular(1) [ 1 ];
ip_id_behavior =:= ip_id_behavior_choice [ 2 ];
reorder_ratio =:= reorder_choice [ 2 ];
df =:= dont_fragment(version.UVALUE) [ 1 ];
control_crc3 =:= control_crc3 [ 3 ];
ttl_hopl_outer_flag =:= irregular(1) [ 1 ];
ttl_hopl_present =:= irregular(1) [ 1 ];
tos_tc_outer_flag =:= irregular(1) [ 1 ];
tos_tc_present =:= irregular(1) [ 1 ];
ts_indicator =:= irregular(2) [ 2 ];
tss_indicator =:= irregular(2) [ 2 ];
pt_present =:= irregular(1) [ 1 ];
list_present =:= irregular(1) [ 1 ];
pad_bit =:= irregular(1) [ 1 ];
extension =:= irregular(1) [ 1 ];
coverage_behavior =:= irregular(2) [ 2 ];
reserved =:= compressed_value(4, 0) [ 4 ];
sequence_number =:= sdvl(sequence_number.ULENGTH) [ 8, 16 ];
ip_id =:=
optional_ip_id_lsb(ip_id_behavior.UVALUE,
ip_id_indicator.CVALUE) [ 0, 8, 16 ];
tos_tc =:=
tos_tc_enc(tos_tc_present.CVALUE) [ 0, 8 ];
ttl_hopl =:=
static_or_irreg(ttl_hopl_present.CVALUE,
ttl_hopl.ULENGTH) [ 0, 8 ];
// Either scaled or unscaled timestamp
ts_scaled =:=
optional_scaled_timestamp(tss_indicator,
tsc_indicator) [ VARIABLE ];
ts_scaled =:=
optional_scaled_timestamp(tss_indicator,
tsc_indicator) [ VARIABLE ];
payload_type =:= optional_pt(pt_present) [ 0, 8 ];
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ts_stride =:=
optional_stride(tss_indicator,
ts_stride_value) [ VARIABLE ];
csrc_list =:= list_csrc(cc.UVALUE) [ VARIABLE ];
ENFORCE(ttl_irregular_chain_flag == ttl_hopl_outer_flag.UVALUE);
ENFORCE(tos_irregular_chain_flag == tos_tc_outer_flag.UVALUE);
}
// UO-0
COMPRESSED pt_0_crc3 {
discriminator =:= '0' [ 1 ];
msn =:= msn_lsb(4, 4) [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
timestamp =:= inferred_scaled_field [ 0 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// New format, Type 0 with strong CRC and more SN bits
COMPRESSED pt_0_crc7 {
discriminator =:= '100' [ 3 ];
msn =:= msn_lsb(6, 16) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
timestamp =:= inferred_scaled_field [ 0 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// UO-1 replacement
COMPRESSED pt_1_rnd {
discriminator =:= '101' [ 3 ];
msn =:= msn_lsb(5, 8) [ 5 ];
marker =:= irregular(1) [ 1 ];
ts_scaled =:= lsb(4, 3) [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
}
// UO-1-ID replacement
COMPRESSED pt_1_seq_id {
discriminator =:= '1010' [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
msn =:= msn_lsb(5, 8) [ 5 ];
ip_id =:= ip_id_lsb(ip_id_behavior.UVALUE, 4, 3) [ 4 ];
timestamp =:= inferred_scaled_field [ 0 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
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}
// UO-1-TS replacement
COMPRESSED pt_1_seq_ts {
discriminator =:= '1011' [ 4 ];
marker =:= irregular(1) [ 1 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
msn =:= msn_lsb(4, 4) [ 4 ];
ts_scaled =:= lsb(4, 3) [ 4 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
// UOR-2 replacement
COMPRESSED pt_2_rnd {
discriminator =:= '110' [ 3 ];
msn =:= msn_lsb(7, 32) [ 7 ];
ts_scaled =:= lsb(6, 15) [ 6 ];
marker =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
}
// UOR-2-ID replacement
COMPRESSED pt_2_seq_id {
discriminator =:= '1100' [ 4 ];
msn =:= msn_lsb(7, 32) [ 7 ];
ip_id =:= ip_id_lsb(ip_id_behavior.UVALUE, 6, 3) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
timestamp =:= inferred_scaled_field [ 0 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
// UOR-2-TS replacement
COMPRESSED pt_2_seq_ts {
discriminator =:= '1101' [ 4 ];
msn =:= msn_lsb(7, 32) [ 7 ];
ts_scaled =:= lsb(5, 7) [ 5 ];
marker =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
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}
////////////////////////////////////////////
// UDP-lite profile
////////////////////////////////////////////
// ttl_irregular_chain_flag is set by the user if the TTL/Hop Limit
// of an outer header. The same value must be passed as an argument
// to the ipv4/ipv6 encoding methods when extracting the irregular
// chain items. The same applies to the tos_irregular_chain_flag
udplite_baseheader(profile,
ttl_irregular_chain_flag, tos_irregular_chain_flag)
{
UNCOMPRESSED v4 {
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
version =:= uncompressed_value(4, 4) [ 4 ];
header_length =:= uncompressed_value(4, 5) [ 4 ];
tos_tc [ 8 ];
length [ 16 ];
ip_id [ 16 ];
rf =:= uncompressed_value(1, 0) [ 1 ];
df [ 1 ];
mf =:= uncompressed_value(1, 0) [ 1 ];
frag_offset =:= uncompressed_value(13, 0) [ 13 ];
ttl_hopl [ 8 ];
next_header [ 8 ];
checksum [ 16 ];
src_addr [ 32 ];
dest_addr [ 32 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
dst_port [ 16 ];
checksum_coverage [ 16 ];
checksum [ 16 ];
}
UNCOMPRESSED v6 {
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
version =:= uncompressed_value(4, 6) [ 4 ];
version [ 4 ];
tos_tc [ 8 ];
flow_label [ 20 ];
payload_length [ 16 ];
next_header [ 8 ];
ttl_hopl [ 8 ];
src_addr [ 128 ];
dest_addr [ 128 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
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src_port [ 16 ];
dst_port [ 16 ];
checksum_coverage [ 16 ];
checksum [ 16 ];
ENFORCE(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM);
}
CONTROL {
ip_id_behavior [ 2 ];
coverage_behavior [ 2 ];
ENFORCE(profile == PROFILE_UDPLITE_0108);
}
DEFAULT {
tos_tc =:= static;
dest_addr =:= static;
version =:= static;
ttl_hopl =:= static;
src_addr =:= static;
df =:= static;
ip_id_behavior =:= static;
payload_length =:= inferred_ip_v6_length;
checksum =:= inferred_ip_v4_header_checksum;
length =:= inferred_ip_v4_length;
flow_label =:= static;
next_header =:= static;
src_port =:= static;
dst_port =:= static;
checksum_coverage =:= irregular(16);
checksum =:= irregular(16);
ENFORCE(ttl_irregular_chain_flag == 0);
ENFORCE(tos_irregular_chain_flag == 0);
}
// Replacement for UOR-2-ext3
COMPRESSED co_common {
discriminator =:= '1111101' [ 7 ];
ip_id_indicator =:= irregular(1) [ 1 ];
reorder_ratio =:= reorder_choice [ 2 ];
msn =:= msn_lsb(6, 16) [ 6 ];
df =:= dont_fragment(version.UVALUE) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ttl_hopl_outer_flag =:= irregular(1) [ 1 ];
ttl_hopl_present =:= irregular(1) [ 1 ];
tos_tc_outer_flag =:= irregular(1) [ 1 ];
ip_id_behavior =:= ip_id_behavior_choice [ 2 ];
control_crc3 =:= control_crc3 [ 3 ];
tos_tc_present =:= irregular(1) [ 1 ];
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coverage_behavior =:= irregular(2) [ 2 ];
reserved =:= compressed_value(5, 0) [ 5 ];
ip_id =:=
optional_ip_id_lsb(ip_id_behavior.UVALUE,
ip_id_indicator.CVALUE) [ 0, 8, 16 ];
tos_tc =:=
tos_tc_enc(tos_tc_present.CVALUE) [ 0, 8 ];
ttl_hopl =:=
static_or_irreg(ttl_hopl_present.CVALUE,
ttl_hopl.ULENGTH) [ 0, 8 ];
ENFORCE(ttl_irregular_chain_flag == ttl_hopl_outer_flag.UVALUE);
ENFORCE(tos_irregular_chain_flag == tos_tc_outer_flag.UVALUE);
}
// UO-0
COMPRESSED pt_0_crc3 {
discriminator =:= '0' [ 1 ];
msn =:= msn_lsb(4, 4) [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// New format, Type 0 with strong CRC and more SN bits
COMPRESSED pt_0_crc7 {
discriminator =:= '100' [ 3 ];
msn =:= msn_lsb(6, 16) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// UO-1-ID replacement (PT-1 only used for sequential)
COMPRESSED pt_1_seq_id {
discriminator =:= '101' [ 3 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
msn =:= msn_lsb(6, 16) [ 6 ];
ip_id =:= ip_id_lsb(ip_id_behavior.UVALUE, 4, 3) [ 4 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
// UOR-2 replacement
COMPRESSED pt_2_rnd {
discriminator =:= '110' [ 3 ];
msn =:= msn_lsb(6, 16) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
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(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
}
// UOR-2-ID replacement
COMPRESSED pt_2_seq_id {
discriminator =:= '1100' [ 4 ];
ip_id =:= ip_id_lsb(ip_id_behavior.UVALUE, 5, 3) [ 5 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
msn =:= msn_lsb(8, 64) [ 8 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
}
}
6.7. Feedback Formats and Options
6.7.1. Feedback Formats
This section describes the feedback format for ROHCv2 profiles, using
the formats described in section 5.2.3 of [I-D.ietf-rohc-rfc3095bis-
framework].
All feedback formats carry a field labelled MSN, which contain LSBs
of the MSN described in Section 6.2.1. The sequence number to use is
the MSN corresponding to the last header that was successfully CRC-8
validated or CRC verified.
FEEDBACK-1
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| MSN |
+---+---+---+---+---+---+---+---+
MSN: The lsb-encoded master sequence number.
A FEEDBACK-1 is an ACK. In order to send a NACK or a STATIC-NACK,
FEEDBACK-2 must be used.
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FEEDBACK-2
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
|Acktype| MSN |
+---+---+---+---+---+---+---+---+
| MSN |
+---+---+---+---+---+---+---+---+
| CRC |
+---+---+---+---+---+---+---+---+
/ Feedback options /
+---+---+---+---+---+---+---+---+
Acktype:
0 = ACK
1 = NACK
2 = STATIC-NACK
3 is reserved (MUST NOT be used for parsability)
MSN: The lsb-encoded master sequence number.
CRC: 8-bit CRC computed over the entire feedback payload including
any CID fields but excluding the packet type, the 'Size' field and
the 'Code' octet, using the polynomial defined in [I-D.ietf-rohc-
rfc3095bis-framework]. If the CID is given with an Add-CID octet,
the Add-CID octet immediately precedes the FEEDBACK-1 or
FEEDBACK-2 format. For purposes of computing the CRC, the CRC
field is zero.
Feedback options: A variable number of feedback options, see
Section 6.7.2. Options may appear in any order.
A FEEDBACK-2 of type NACK or STATIC-NACK is always implicitely an
acknowlegement for a successfully decompressed packet, which packet
corresponds to the MSN of the feedback element, unless the MSN-NOT-
VALID option Section 6.7.2.2 appears in the feedback element.
The FEEDBACK-2 format always carry a CRC and is thus more robust than
the FEEDBACK-1 format. When receiving FEEDBACK-2, the compressor
MUST verify the information by computing the CRC and comparing the
result with the CRC carried in the feedback format. If the two are
not identical, the feedback element MUST be discarded.
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6.7.2. Feedback Options
A feedback option has variable length and the following general
format:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Opt Type | Opt Len |
+---+---+---+---+---+---+---+---+
/ option data / Opt Length (octets)
+---+---+---+---+---+---+---+---+
The CRC option contains an 8-bit CRC computed over the entire
feedback payload including any CID fields but excluding the packet
type, the 'Size' field and the 'Code' octet, using the polynomial of
[I-D.ietf-rohc-rfc3095bis-framework], section 5.3.1.1.
6.7.2.1. The REJECT option
The REJECT option informs the compressor that the decompressor does
not have sufficient resources to handle the flow.
+---+---+---+---+---+---+---+---+
| Opt Type = 2 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+
When receiving a REJECT option, the compressor MUST stop compressing
the packet flow, and SHOULD refrain from attempting to increase the
number of compressed packet flows for some time. Any FEEDBACK packet
carrying a REJECT option MUST also carry a CRC option. The REJECT
option MUST NOT appear more than once in the FEEDBACK-2 format,
otherwise the decompressor MUST discard the entire feedback element.
6.7.2.2. The MSN-NOT-VALID option
The MSN-NOT-VALID option indicates that the MSN of the feedback is
not valid.
+---+---+---+---+---+---+---+---+
| Opt Type = 3 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+
A compressor MUST NOT use the MSN of the feedback to find the
corresponding sent header when this option is present. Consequently,
a NACK or a STATIC-NACK feedback type sent with the MSN-NOT-VALID
option is equivalent to a STATIC-NACK with respect to the type of
context repair requested by the decompressor.
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The MSN-NOT-VALID option MUST NOT appear more than once in the
FEEDBACK-2 format and MUST NOT appear in the same feedback element as
the MSN option, otherwise the decompressor MUST discard the entire
feedback element.
6.7.2.3. The MSN option
The MSN option provides 8 additional bits of MSN.
+---+---+---+---+---+---+---+---+
| Opt Type = 4 | Opt Len = 1 |
+---+---+---+---+---+---+---+---+
| MSN |
+---+---+---+---+---+---+---+---+
the bits in the MSN option are concatenated with the MSN bits in the
FEEDBACK-2 format, with the bits in the FEEDBACK-2 format being the
most significant bits. The MSN option MAY appear more than once in
the FEEDBACK-2 format, in which case the MSN is given by
concatenating the MSN fields of each occurance of the MSN option.
The MSN option MUST NOT appear in the same feedback element as the
MSN-NOT-VALID option, otherwise the decompressor MUST discard the
entire feedback element.
6.7.2.4. The CONTEXT_MEMORY Feedback Option
The CONTEXT_MEMORY option informs the compressor that the
decompressor does not have sufficient memory resources to handle the
context of the packet flow, as the flow is currently compressed.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Opt Type = 9 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+
When receiving a CONTEXT_MEMORY option, the compressor SHOULD take
actions to compress the packet flow in a way that requires less
decompressor memory resources, or stop compressing the packet flow.
The CONTEXT_MEMORY option MUST NOT appear more than once in the
FEEDBACK-2 format, otherwise the decompressor MUST discard the entire
feedback element.
6.7.2.5. Unknown option types
If an option type unknown to the compressor is encountered, it must
continue parsing the rest of the FEEDBACK packet, which is possible
since the length of the option is explicit, but MUST otherwise ignore
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the unknown option.
7. Security Considerations
Because encryption eliminates the redundancy that header compression
schemes try to exploit, there is some inducement to forego encryption
of headers in order to enable operation over low-bandwidth links.
However, for those cases where encryption of data (and not headers)
is sufficient, RTP does specify an alternative encryption method in
which only the RTP payload is encrypted and the headers are left in
the clear. That would still allow header compression to be applied.
ROHC compression is transparent with regard to the RTP Sequence
Number and RTP Timestamp fields, so the values of those fields can be
used as the basis of payload encryption schemes (e.g., for
computation of an initialization vector).
A malfunctioning or malicious header compressor could cause the
header decompressor to reconstitute packets that do not match the
original packets but still have valid IP, UDP and RTP headers and
possibly also valid UDP checksums. Such corruption may be detected
with end-to-end authentication and integrity mechanisms which will
not be affected by the compression. Moreover, this header
compression scheme uses an internal checksum for verification of
reconstructed headers. This reduces the probability of producing
decompressed headers not matching the original ones without this
being noticed.
Denial-of-service attacks are possible if an intruder can introduce
(for example) bogus IR, IR-DYN, IR-PD or FEEDBACK packets onto the
link and thereby cause compression efficiency to be reduced.
However, an intruder having the ability to inject arbitrary packets
at the link layer in this manner raises additional security issues
that dwarf those related to the use of header compression.
8. IANA Considerations
The ROHC profile identifiers 0x00XX <# Editor's Note: To be replaced
before publication #> has been reserved by the IANA for the profile
defined in this document.
<# Editor's Note: To be removed before publication #>
A ROHC profile identifier must be reserved by the IANA for the
updated profiles defined in this document. Profiles 0x0000-0x0004
have previously been reserved, and since there is no changes to
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profile 0x0000, this document should thus update profiles 0x0001-
0x0004. As for previous ROHC profiles, profile numbers 0xnnXX must
also be reserved for future updates of this profile. A suggested
registration in the "RObust Header Compression (ROHC) Profile
Identifiers" name space would then be:
Profile Usage Reference
0x0000 ROHC uncompressed RFC 3095
0x0001 ROHC RTP RFC 3095
0x0101 ROHCv2 RTP [RFCXXXX (this)]
0xn101 - 0xn2nn Reserved
0x0002 ROHC UDP RFC 3095
0x0102 ROHCv2 UDP [RFCXXXX (this)]
0xn102 - 0xn2nn Reserved
0x0003 ROHC ESP RFC 3095
0x0103 ROHCv2 ESP [RFCXXXX (this)]
0xn103 - 0xn2nn Reserved
0x0004 ROHC IP RFC 3843
0x0104 ROHCv2 IP [RFCXXXX (this)]
0xn104 - 0xn7nn Reserved
0x0005 ROHC LLA RFC 3242
0x0105 ROHC LLA with R-mode RFC 3408
0xn105 - 0xn7nn Reserved
0x0007 ROHC RTP/UDP-Lite RFC 4019
0x0107 ROHCv2 RTP/UDP-Lite [RFCXXXX (this)]
0xn107 - 0xn2nn Reserved
0x0008 ROHC UDP-Lite RFC 4019
0x0108 ROHCv2 UDP-Lite [RFCXXXX (this)]
0xn108 - 0xn2nn Reserved
Author's note: The list above is incorrect and incomplete.
It must be updated before sending to IANA.
9. Acknowledgements
The authors would like to thank the many people who have contributed
to the ROHC specifications. The sample Perl implementation of
Appendix A was written by Carsten Bormann.
10. References
10.1. Normative References
[I-D.ietf-rohc-formal-notation]
Pelletier, G. and R. Finking, "Formal Notation for Robust
Header Compression (ROHC-FN)",
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draft-ietf-rohc-formal-notation-09 (work in progress),
June 2005.
[I-D.ietf-rohc-rfc3095bis-framework]
Jonsson, L., Pelletier, G., and K. Sandlund, "The RObust
Header Compression (ROHC) Framework",
draft-ietf-rohc-rfc3095bis-framework-01 (work in
progress), December 2005.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC2004] Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
March 2000.
[RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE",
RFC 2890, September 2000.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
G. Fairhurst, "The Lightweight User Datagram Protocol
(UDP-Lite)", RFC 3828, July 2004.
[RFC4019] Pelletier, G., "RObust Header Compression (ROHC): Profiles
for User Datagram Protocol (UDP) Lite", RFC 4019,
April 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
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10.2. Informative References
[I-D.ietf-rohc-rfc3095bis-improvements]
Jonsson, L., "Improvements for the ROHC Profile Set
Update", draft-ietf-rohc-rfc3095bis-improvements-02 (work
in progress), March 2006.
[I-D.ietf-rohc-rtp-impl-guide]
Jonsson, L., Pelletier, G., and K. Sandlund, "RObust
Header Compression (ROHC): Corrections and Clarifications
to RFC 3095", May 2006.
[I-D.ietf-rohc-tcp]
Pelletier, G., Sandlund, K., and M. West, "RObust Header
Compression (ROHC): A Profile for TCP/IP (ROHC-TCP)",
draft-ietf-rohc-tcp-11 (work in progress), January 2006.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
Compression (ROHC): Framework and four profiles: RTP, UDP,
ESP, and uncompressed", RFC 3095, July 2001.
[RFC3843] Jonsson, L-E. and G. Pelletier, "RObust Header Compression
(ROHC): A Compression Profile for IP", RFC 3843,
June 2004.
[RFC4224] Pelletier, G., Jonsson, L-E., and K. Sandlund, "RObust
Header Compression (ROHC): ROHC over Channels That Can
Reorder Packets", RFC 4224, January 2006.
Appendix A. Detailed classification of header fields
Header compression is possible thanks to the fact that most header
fields do not vary randomly from packet to packet. Many of the
fields exhibit static behavior or change in a more or less
predictable way. When designing a header compression scheme, it is
of fundamental importance to understand the behavior of the fields in
detail.
In this appendix, all IP, UDP, UDP-Lite and RTP header fields are
classified and analyzed in two steps. First, we have a general
classification in [REF] where the fields are classified on the basis
of stable knowledge and assumptions. The general classification does
not take into account the change characteristics of changing fields
because those will vary more or less depending on the implementation
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and on the application used. A less stable but more detailed
analysis of the change characteristics is then done in [REF].
Finally, [REF] summarizes this appendix with conclusions about how
the various header fields should be handled by the header compression
scheme to optimize compression and functionality.
Appendix A.1. General classification
INFERRED These fields contain values that can be inferred from other
values, for example the size of the frame carrying the packet, and
thus do not have to be handled at all by the compression scheme.
STATIC These fields are expected to be constant throughout the
lifetime of the packet stream. Static information must in some way
be communicated once.
STATIC-DEF STATIC fields whose values define a packet stream. They
are in general handled as STATIC.
STATIC-KNOWN These STATIC fields are expected to have well-known
values and therefore do not need to be communicated at all.
CHANGING These fields are expected to vary in some way: randomly,
within a limited value set or range, or in some other manner.
In this section, each of the IP, UDP and RTP header fields is
assigned to one of these classes. For all fields except those
classified as CHANGING, the motives for the classification are also
stated. In section A.2, CHANGING fields are further examined and
classified on the basis of their expected change behavior.
Appendix A.1.1. IPv4 header fields
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+---------------------+-------------+----------------+
| Field | Size (bits) | Class |
+---------------------+-------------+----------------+
| Version | 4 | STATIC-KNOWN |
| Header Length | 4 | STATIC-KNOWN |
| Type Of Service | 8 | CHANGING |
| Packet Length | 16 | INFERRED |
| Identification | 16 | CHANGING |
| Reserved flag | 1 | STATIC-KNOWN |
| Don't Fragment flag | 1 | CHANGING |
| More Fragments flag | 1 | STATIC-KNOWN |
| Fragment Offset | 13 | STATIC-KNOWN |
| Time To Live | 8 | CHANGING |
| Protocol | 8 | STATIC |
| Header Checksum | 16 | INFERRED |
| Source Address | 32 | STATIC-DEF |
| Destination Address | 32 | STATIC-DEF |
+---------------------+-------------+----------------+
Version
The version field states which IP version is used. Packets with
different values in this field must be handled by different IP
stacks. All packets of a packet stream must therefore be of the
same IP version. Accordingly, the field is classified as STATIC.
Header Length
As long no options are present in the IP header, the header length
is constant and well known. If there are options, the fields
would be STATIC, but it is assumed here that there are no options.
The field is therefore classified as STATIC-KNOWN.
Packet Length
Information about packet length is expected to be provided by the
link layer. The field is therefore classified as INFERRED.
Flags
The Reserved flag must be set to zero and is therefore classified
as STATIC-KNOWN. The Don't Fragment (DF) flag will changes rarely
and is therefore classified as CHANGING. Finally, the More
Fragments (MF) flag is expected to be zero because fragmentation
is NOT expected, due to the small packet size expected. The More
Fragments flag is therefore classified as STATIC-KNOWN.
Fragment Offset
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Under the assumption that no fragmentation occurs, the fragment
offset is always zero. The field is therefore classified as
STATIC-KNOWN.
Protocol
This field will have the same value in all packets of a packet
stream. It encodes the type of the subsequent header.
Header Checksum
The header checksum protects individual hops from processing a
corrupted header. When almost all IP header information is
compressed away, there is no point in having this additional
checksum; instead it can be regenerated at the decompressor side.
The field is therefore classified as INFERRED.
Source and Destination addresses
These fields are part of the definition of a stream and must thus
be constant for all packets in the stream. The fields are
therefore classified as STATIC-DEF.
Appendix A.1.2. IPv6 header fields
+---------------------+-------------+----------------+
| Field | Size (bits) | Class |
+---------------------+-------------+----------------+
| Version | 4 | STATIC-KNOWN |
| Traffic Class | 8 | CHANGING |
| Flow Label | 20 | STATIC-DEF |
| Payload Length | 16 | INFERRED |
| Next Header | 8 | STATIC |
| Hop Limit | 8 | CHANGING |
| Source Address | 128 | STATIC-DEF |
| Destination Address | 128 | STATIC-DEF |
+---------------------+-------------+----------------+
Version
The version field states which IP version is used. Packets with
different values in this field must be handled by different IP
stacks. All packets of a packet stream must therefore be of the
same IP version. Accordingly, the field is classified as STATIC.
Flow Label
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This field may be used to identify packets belonging to a specific
packet stream. If not used, the value should be set to zero.
Otherwise, all packets belonging to the same stream must have the
same value in this field, it being one of the fields that define
the stream. The field is therefore classified as STATIC-DEF.
Payload Length
Information about packet length (and, consequently, payload
length) is expected to be provided by the link layer. The field
is therefore classified as INFERRED.
Next Header
This field will have the same value in all packets of a packet
stream. It encodes the type of the subsequent header.
Source and Destination addresses
These fields are part of the definition of a stream and must thus
be constant for all packets in the stream. The fields are
therefore classified as STATIC-DEF.
Appendix A.1.3. UDP header fields
+------------------+-------------+-------------+
| Field | Size (bits) | Class |
+------------------+-------------+-------------+
| Source Port | 16 | STATIC-DEF |
| Destination Port | 16 | STATIC-DEF |
| Length | 16 | INFERRED |
| Checksum | 16 | CHANGING |
+------------------+-------------+-------------+
Source and Destination ports
These fields are part of the definition of a stream and must thus
be constant for all packets in the stream. The fields are
therefore classified as STATIC-DEF.
Length
This field is redundant and is therefore classified as INFERRED.
Appendix A.1.4. UDP-Lite header fields
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+-------------------+-------------+-------------+
| Field | Size (bits) | Class |
+-------------------+-------------+-------------+
| Source Port | 16 | STATIC-DEF |
| Destination Port | 16 | STATIC-DEF |
| Checksum Coverage | 16 | INFERRED |
| | | STATIC |
| | | CHANGING |
| Checksum | 16 | CHANGING |
+-------------------+-------------+-------------+
Source and Destination Port
Same as for UDP Appendix A.1.3.
Source and Destination ports
Same as for UDP Appendix A.1.3.
Checksum Coverage
This field specifies which part of the UDP-Lite datagram is
covered by the checksum. It may have a value of zero or be equal
to the datagram length if the checksum covers the entire datagram,
or it may have any value between eight octets and the length of
the datagram to specify the number of octets protected by the
checksum, calculated from the first octet of the UDP-Lite header.
The value of this field may vary for each packet, and this makes
the value unpredictable from a header-compression perspective.
Checksum
The information used for the calculation of the UDP-Lite checksum
is governed by the value of the checksum coverage and minimally
includes the UDP-Lite header. The checksum is a changing field
that must always be sent as-is.
Appendix A.1.5. RTP header fields
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+-----------------+-------------+----------------+
| Field | Size (bits) | Class |
+-----------------+-------------+----------------+
| Version | 2 | STATIC-KNOWN |
| Padding | 1 | CHANGING |
| Extension | 1 | CHANGING |
| CSRC Counter | 4 | CHANGING |
| Marker | 1 | CHANGING |
| Payload Type | 7 | CHANGING |
| Sequence Number | 16 | CHANGING |
| Timestamp | 32 | CHANGING |
| SSRC | 32 | STATIC-DEF |
| CSRC | 0(-480) | CHANGING |
+-----------------+-------------+----------------+
Version
Only one working RTP version exists, namely version 2. The field
is therefore classified as STATIC-KNOWN.
Padding
The use of this field is application-dependent, but when payload
padding is used it is likely to be present in most or all packets.
The field is classified as CHANGING to allow for the rare case
where this field is updated.
Extension
If RTP extensions are used by the application, these extensions
are likely to be present in all packets (but the use of extensions
is very uncommon). However, for safety's sake this field is
classified as CHANGING to allow for the rare case where this field
is changed during the flow.
SSRC
This field is part of the definition of a stream and must thus be
constant for all packets in the stream. The field is therefore
classified as STATIC-DEF.
Appendix A.2. Analysis of change patterns of header fields
To design suitable mechanisms for efficient compression of all header
fields, their change patterns must be analyzed. For this reason, an
extended classification is done based on the general classification
in A.1, considering the fields which were labeled CHANGING in that
classification. Different applications will use the fields in
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different ways, which may affect their behavior. For the fields
whose behavior is variable, typical behavior for conversational audio
and video will be discussed.
The CHANGING fields are separated into five different subclasses:
STATIC These are fields that were classified as CHANGING on a
general basis, but are classified as STATIC here due to certain
additional assumptions.
SEMISTATIC These fields are STATIC most of the time. However,
occasionally the value changes but will revert to its original
value.
RARELY-CHANGING (RC) These are fields that change their values
occasionally and then keep their new values.
ALTERNATING These fields alternate between a small number of
different values.
IRREGULAR These, finally, are the fields for which no useful
change pattern can be identified.
When the classification is done, other details are also stated
regarding possible additional knowledge about the field values and/or
field deltas, according to the classification. For fields classified
as STATIC or SEMISTATIC, the case could be that the value of the
field is not only STATIC but also well KNOWN a priori (two states for
SEMISTATIC fields). For fields with non-irregular change behavior,
it could be known that changes usually are within a LIMITED range
compared to the maximal change for the field. For other fields, the
values are completely UNKNOWN.
Table A.1 classifies all the CHANGING fields on the basis of their
expected change patterns, especially for conversational audio and
video.
+------------------------+-------------+-------------+-------------+
| Field | Value/Delta | Class | Knowledge |
+========================+=============+=============+=============+
| Sequential | Delta | RC | LIMITED |
| -----------+-------------+-------------+-------------+
| IPv4 Id: Seq. swap | Delta | RC | LIMITED |
| -----------+-------------+-------------+-------------+
| Random | Value | IRREGULAR | UNKNOWN |
| -----------+-------------+-------------+-------------+
| Zero | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| IP TOS / Tr. Class | Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| IP TTL / Hop Limit | Value | ALTERNATING | LIMITED |
+------------------------+-------------+-------------+-------------+
| IP Don't Fragment | Value | RC | KNOWN |
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+------------------------+-------------+-------------+-------------+
| Disabled | Value | STATIC | KNOWN |
| UDP Checksum: ---------+-------------+-------------+-------------+
| Enabled | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| UDP-Lite Checksum | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| Case #1 | Value | CHANGING | INFERRED |
| UDP-Lite ----------+-------------+-------------+-------------+
| Checksum: Case #2 | Value | RC | UNKNOWN |
| Coverage ----------+-------------+-------------+-------------+
| Case #3 | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| No mix | Value | STATIC | KNOWN |
| RTP CSRC Count: -------+-------------+-------------+-------------+
| Mixed | Value | RC | LIMITED |
+------------------------+-------------+-------------+-------------+
| RTP Marker | Value | SEMISTATIC | KNOWN/KNOWN |
+------------------------+-------------+-------------+-------------+
| RTP Payload Type | Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| RTP Extension | Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| RTP Padding | Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| RTP Sequence Number | Delta | STATIC | KNOWN |
+------------------------+-------------+-------------+-------------+
| RTP Timestamp | Delta | RC | LIMITED |
+------------------------+-------------+-------------+-------------+
| No mix | - | - | - |
| RTP CSRC List: -------+-------------+-------------+-------------+
| Mixed | Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+
Table A.1 : Classification of CHANGING header fields
The following subsections discuss the various header fields in
detail. Note that table A.1 and the discussions below do not
consider changes caused by loss or reordering before the compression
point.
Appendix A.2.1. IPv4 Identification
The Identification field (IP ID) of the IPv4 header is there to
identify which fragments constitute a datagram when reassembling
fragmented datagrams. The IPv4 specification does not specify
exactly how this field is to be assigned values, only that each
packet should get an IP ID that is unique for the source-destination
pair and protocol for the time the datagram (or any of its fragments)
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could be alive in the network. This means that assignment of IP ID
values can be done in various ways, which we have separated into
three classes.
Sequential
In this behavior, the IP-ID is expected to increment by one for
most packets, but may increment by a value larger than one,
depending on the behavior of the transmitting IPv4 stack.
Sequential Swapped
When using this behavior, the IP-ID behaves as in the Sequential
bahvior, but the two bytes of IP-ID are byte swapped. Therefore,
the IP-ID can be swapped before compression to make it behave
exactly as the Sequential behavior.
Random
Some IP stacks assign IP ID values using a pseudo-random number
generator. There is thus no correlation between the ID values of
subsequent datagrams, and therefore there is no way to predict the
IP ID value for the next datagram. For header compression
purposes, this means that the IP ID field needs to be sent
uncompressed with each datagram, resulting in two extra octets of
header.
Zero
This behavior, although not a legal implementation of IPv4 is
sometimes seen in existing IPv4 stacks. When this behavior is
used, all IP packets have the IP-ID value set to zero.
Appendix A.2.2. IP Traffic Class / Type-Of-Service
The Traffic-Class (IPv6) or Type-Of-Service (IPv4) field is expected
to be constant during the lifetime of a packet stream or to change
relatively seldom.
Appendix A.2.3. IP Hop-limit / Time-To-Live
The Hop-Limit (IPv6) or Time-To-Live (IPv4) field is expected to be
constant during the lifetime of a packet stream or to alternate
between a limited number of values due to route changes.
Appendix A.2.4. IPv4 Don't Fragment
The Don't Fragment flag in IPv4 will seldom change, and is therefore
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classified as RC.
Appendix A.2.5. UDP Checksum
The UDP checksum is optional. If disabled, its value is constantly
zero and could be compressed away. If enabled, its value depends on
the payload, which for compression purposes is equivalent to it
changing randomly with every packet.
Appendix A.2.6. UDP-Lite Checksum Coverage
The Checksum Coverage field may behave in different ways: it may have
a value of zero, it may be equal to the datagram length, or it may
have any value between eight octets and the length of the datagram.
From a compression perspective, this field is expected to either be
entirely predictable (for the cases where it follows the same
behavior as the UDP Length field or where it takes on a constant
value) or either to change randomly for each packet (making the value
unpredictable from a header-compression perspective). For all cases,
the behavior itself is not expected to change for this field during
the lifetime of a packet flow, or to change relatively seldom.
Appendix A.2.7. UDP-Lite Checksum
As opposed to the UDP checksum, the UDP-Lite checksum is not optional
and it cannot be disabled. Its value depends on the payload and on
the checksum coverage field, which for compression purposes is
equivalent to it changing randomly with every packet.
Appendix A.2.8. RTP CSRC Counter
This is a counter indicating the number of CSRC items present in the
CSRC list. This number is expected to be almost constant on a
packet-to-packet basis and change by small amounts. As long as no
RTP mixer is used, the value of this field is zero.
Appendix A.2.9. RTP Marker
For audio the marker bit should be set only in the first packet of a
talkspurt, while for video it should be set in the last packet of
every picture. This means that in both cases the RTP marker is
classified as SEMISTATIC with well-known values for both states.
Appendix A.2.10. RTP Padding
If padding is used, it is expected to be present in most packets, but
is classified as RC to allow efficient compression even when this
field changes.
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Appendix A.2.11. RTP Extension
If extensions are used, it is expected to be used in most packets,
but is classified as RC to allow efficient compression even when this
field changes.
Appendix A.2.12. RTP Payload Type
Changes of the RTP payload type within a packet stream are expected
to be rare. Applications could adapt to congestion by changing
payload type and/or frame sizes, but that is not expected to happen
frequently.
Appendix A.2.13. RTP Sequence Number
The RTP Sequence Number will be incremented by one for each packet
sent.
Appendix A.2.14. RTP Timestamp
In the audio case:
As long as there are no pauses in the audio stream, the RTP
Timestamp will be incremented by a constant delta, corresponding
to the number of samples in the speech frame. It will thus mostly
follow the RTP Sequence Number. When there has been a silent
period and a new talkspurt begins, the timestamp will jump in
proportion to the length of the silent period. However, the
increment will probably be within a relatively limited range.
In the video case:
Between two consecutive packets, the timestamp will either be
unchanged or increase by a multiple of a fixed value corresponding
to the picture clock frequency. The timestamp can also decrease
by a multiple of the fixed value for certain coding schemes. The
delta interval, expressed as a multiple of the picture clock
frequency, is in most cases very limited.
Appendix A.2.15. RTP Contributing Sources (CSRC)
The participants in a session, which are identified by the CSRC
fields, are expected to be almost the same on a packet-to-packet
basis with relatively few additions and removals. As long as RTP
mixers are not used, no CSRC fields are present at all.
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Appendix A.3. Header compression strategies
This section elaborates on what has been done in previous sections.
On the basis of the classifications, recommendations are given on how
to handle the various fields in the header compression process.
Seven different actions are possible; these are listed together with
the fields to which each action applies.
Appendix A.3.1. Do not send at all
The fields that have well known values a priori do not have to be
sent at all. These are:
o IPv6 Payload Length
o IPv4 Header Length
o IPv4 Reserved Flag
o IPv4 Last Fragment Flag
o IPv4 Fragment Offset
o UDP Checksum (if disabled)
o RTP Version
Appendix A.3.2. Transmit only initially
The fields that are constant throughout the lifetime of the packet
stream have to be transmitted and correctly delivered to the
decompressor only once. These are:
o IP Version
o IPv6 Next Header
o IPv4 Protocol
o IP Source Address
o IP Destination Address
o IPv6 Flow Label
o UDP Source Port
o UDP Destination Port
o UDP-Lite Source Port
o UDP-Lite Destination Port
o RTP SSRC
Appendix A.3.3. Transmit initially, be prepared to update
The fields that are changing only occasionally must be transmitted
initially but there must also be a way to update these fields with
new values if they change. These fields are:
o IPv6 Traffic Class
o IPv4 Don't Fragment Flag
o IPv6 Hop Limit
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o IPv4 Type Of Service (TOS)
o IPv4 Time To Live (TTL)
o UDP-Lite Checksum Coverage (if constant or assigned to datagram
length)
o RTP CSRC Counter
o RTP Padding Flag
o RTP Extension Flag
o RTP Payload Type
o RTP CSRC List
Appendix A.3.4. Be prepared to update, or send as-is frequently
For fields that normally either are constant or have values deducible
from some other field, but that frequently diverge from that
behavior, there must be an efficient way to update the field value or
send it as-is in some packets. These fields are:
o IPv4 Identification (if not sequentially assigned)
o RTP Marker
o RTP Timestamp
Appendix A.3.5. Guarantee continuous robustness
For fields that behave like a counter with a fixed delta for ALL
packets, the only requirement on the transmission encoding is that
packet losses between compressor and decompressor must be tolerable.
If several such fields exist, all these can be communicated together.
Such fields can also be used to interpret the values for fields
listed in the previous section. Fields that have this counter
behavior are:
o IPv4 Identification (if sequentially assigned)
o RTP Sequence Number
Appendix A.3.6. Transmit as-is in all packets
Fields that have completely random values for each packet must be
included as-is in all compressed headers. Those fields are:
o IPv4 Identification (if randomly assigned)
o UDP Checksum (if enabled)
o UDP-Lite Checksum
o UDP-Lite Checksum Coverage (if randomly assigned)
Appendix A.3.7. Establish and be prepared to update delta
Finally, there is a field that is usually increasing by a fixed delta
and is correlated to another field. For this field it would make
sense to make that delta part of the context state. The delta must
then be initiated and updated in the same way as the fields listed in
A.3.3. The field to which this applies is:
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o RTP Timestamp
Appendix B. Differences between RoHCv2 and RFC3095 profiles
To be Written
Profiles defined in RFC3095 were designed with the assumption that
the channel between compressor and decompressor maintains packet
ordering, i.e., that the decompressor always receive packets in the
same order as the compressor sent them. RoHCv2 profiles does not
make this assumption, i.e. reordering before and after the
compression point is handled as part of the compression algorithm
itself.
Appendix C. Sample CRC algorithm
#!/usr/bin/perl -w
use strict;
#=================================
#
# ROHC CRC demo - Carsten Bormann cabo@tzi.org 2001-08-02
#
# This little demo shows the three types of CRC in use in
# RFC3095, the RoHC Framework and ROHC profiles that
# specificy robust header compression.
# Type your data in hexadecimal form and then
# press Control+D.
#
#---------------------------------
#
# utility
#
sub dump_bytes($) {
my $x = shift;
my $i;
for ($i = 0; $i < length($x); ) {
printf("%02x ", ord(substr($x, $i, 1)));
printf("\n") if (++$i % 16 == 0);
}
printf("\n") if ($i % 16 != 0);
}
#---------------------------------
#
# The CRC calculation algorithm.
#
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sub do_crc($$$) {
my $nbits = shift;
my $poly = shift;
my $string = shift;
my $crc = ($nbits == 32 ? 0xffffffff : (1 << $nbits) - 1);
for (my $i = 0; $i < length($string); ++$i) {
my $byte = ord(substr($string, $i, 1));
for( my $b = 0; $b < 8; $b++ ) {
if (($crc & 1) ^ ($byte & 1)) {
$crc >>= 1;
$crc ^= $poly;
} else {
$crc >>= 1;
}
$byte >>= 1;
}
}
printf "%2d bits, ", $nbits;
printf "CRC: %02x\n", $crc;
}
#---------------------------------
#
# Test harness
#
$/ = undef;
$_ = <>; # read until EOF
my $string = ""; # extract all that looks hex:
s/([0-9a-fA-F][0-9a-fA-F])/$string .= chr(hex($1)), ""/eg;
dump_bytes($string);
#---------------------------------
#
# 32-bit segmentation CRC
# Note that the text implies this is complemented like for PPP
# (this differs from 8, 7, and 3-bit CRC)
#
# C(x) = x^0 + x^1 + x^2 + x^4 + x^5 + x^7 + x^8 + x^10 +
# x^11 + x^12 + x^16 + x^22 + x^23 + x^26 + x^32
#
do_crc(32, 0xedb88320, $string);
#---------------------------------
#
# 8-bit IR/IR-DYN CRC
#
# C(x) = x^0 + x^1 + x^2 + x^8
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#
do_crc(8, 0xe0, $string);
#---------------------------------
#
# 7-bit FO/SO CRC
#
# C(x) = x^0 + x^1 + x^2 + x^3 + x^6 + x^7
#
do_crc(7, 0x79, $string);
#---------------------------------
#
# 3-bit FO/SO CRC
#
# C(x) = x^0 + x^1 + x^3
#
do_crc(3, 0x6, $string);
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Authors' Addresses
Ghyslain Pelletier
Ericsson
Box 920
Lulea SE-971 28
Sweden
Phone: +46 (0) 8 404 29 43
Email: ghyslain.pelletier@ericsson.com
Kristofer Sandlund
Ericsson
Box 920
Lulea SE-971 28
Sweden
Phone: +46 (0) 8 404 41 58
Email: kristofer.sandlund@ericsson.com
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