Internet DRAFT - draft-ietf-tsvwg-fecframe-ext
draft-ietf-tsvwg-fecframe-ext
TSVWG V. Roca
Internet-Draft INRIA
Updates: 6363 (if approved) A. Begen
Intended status: Standards Track Networked Media
Expires: July 15, 2019 January 11, 2019
Forward Error Correction (FEC) Framework Extension to Sliding Window
Codes
draft-ietf-tsvwg-fecframe-ext-08
Abstract
RFC 6363 describes a framework for using Forward Error Correction
(FEC) codes to provide protection against packet loss. The framework
supports applying FEC to arbitrary packet flows over unreliable
transport and is primarily intended for real-time, or streaming,
media. However, FECFRAME as per RFC 6363 is restricted to block FEC
codes. This document updates RFC 6363 to support FEC Codes based on
a sliding encoding window, in addition to Block FEC Codes, in a
backward-compatible way. During multicast/broadcast real-time
content delivery, the use of sliding window codes significantly
improves robustness in harsh environments, with less repair traffic
and lower FEC-related added latency.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 15, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 4
3. Summary of Architecture Overview . . . . . . . . . . . . . . 7
4. Procedural Overview . . . . . . . . . . . . . . . . . . . . . 10
4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Sender Operation with Sliding Window FEC Codes . . . . . 10
4.3. Receiver Operation with Sliding Window FEC Codes . . . . 13
5. Protocol Specification . . . . . . . . . . . . . . . . . . . 15
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.2. FEC Framework Configuration Information . . . . . . . . . 16
5.3. FEC Scheme Requirements . . . . . . . . . . . . . . . . . 16
6. Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7. Transport Protocols . . . . . . . . . . . . . . . . . . . . . 17
8. Congestion Control . . . . . . . . . . . . . . . . . . . . . 17
9. Implementation Status . . . . . . . . . . . . . . . . . . . . 17
10. Security Considerations . . . . . . . . . . . . . . . . . . . 17
11. Operations and Management Considerations . . . . . . . . . . 18
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
14.1. Normative References . . . . . . . . . . . . . . . . . . 18
14.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. About Sliding Encoding Window Management
(informational) . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
Many applications need to transport a continuous stream of packetized
data from a source (sender) to one or more destinations (receivers)
over networks that do not provide guaranteed packet delivery. In
particular packets may be lost, which is strictly the focus of this
document: we assume that transmitted packets are either lost (e.g.,
because of a congested router, of a poor signal-to-noise ratio in a
wireless network, or because the number of bit errors exceeds the
correction capabilities of the physical-layer error correcting code)
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or received by the transport protocol without any corruption (i.e.,
the bit-errors, if any, have been fixed by the physical-layer error
correcting code and therefore are hidden to the upper layers).
For these use-cases, Forward Error Correction (FEC) applied within
the transport or application layer is an efficient technique to
improve packet transmission robustness in presence of packet losses
(or "erasures"), without going through packet retransmissions that
create a delay often incompatible with real-time constraints. The
FEC Building Block defined in [RFC5052] provides a framework for the
definition of Content Delivery Protocols (CDPs) that make use of
separately-defined FEC schemes. Any CDP defined according to the
requirements of the FEC Building Block can then easily be used with
any FEC Scheme that is also defined according to the requirements of
the FEC Building Block.
Then FECFRAME [RFC6363] provides a framework to define Content
Delivery Protocols (CDPs) that provide FEC protection for arbitrary
packet flows over an unreliable datagram service transport such as
UDP. It is primarily intended for real-time or streaming media
applications, using broadcast, multicast, or on-demand delivery.
However, [RFC6363] only considers block FEC schemes defined in
accordance with the FEC Building Block [RFC5052] (e.g., [RFC6681],
[RFC6816] or [RFC6865]). These codes require the input flow(s) to be
segmented into a sequence of blocks. Then FEC encoding (at a sender
or an encoding middlebox) and decoding (at a receiver or a decoding
middlebox) are both performed on a per-block basis. For instance, if
the current block encompasses the 100's to 119's source symbols
(i.e., a block of size 20 symbols) of an input flow, encoding (and
decoding) will be performed on this block independently of other
blocks. This approach has major impacts on FEC encoding and decoding
delays. The data packets of continuous media flow(s) may be passed
to the transport layer immediately, without delay. But the block
creation time, that depends on the number of source symbols in this
block, impacts both the FEC encoding delay (since encoding requires
that all source symbols be known), and mechanically the packet loss
recovery delay at a receiver (since no repair symbol for the current
block can be generated and therefore received before that time).
Therefore a good value for the block size is necessarily a balance
between the maximum FEC decoding latency at the receivers (which must
be in line with the most stringent real-time requirement of the
protected flow(s), hence an incentive to reduce the block size), and
the desired robustness against long loss bursts (which increases with
the block size, hence an incentive to increase this size).
This document updates [RFC6363] in order to also support FEC codes
based on a sliding encoding window (A.K.A. convolutional codes)
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[RFC8406]. This encoding window, either of fixed or variable size,
slides over the set of source symbols. FEC encoding is launched
whenever needed, from the set of source symbols present in the
sliding encoding window at that time. This approach significantly
reduces FEC-related latency, since repair symbols can be generated
and passed to the transport layer on-the-fly, at any time, and can be
regularly received by receivers to quickly recover packet losses.
Using sliding window FEC codes is therefore highly beneficial to
real-time flows, one of the primary targets of FECFRAME. [RLC-ID]
provides an example of such FEC Scheme for FECFRAME, built upon the
simple sliding window Random Linear Codes (RLC).
This document is fully backward compatible with [RFC6363]. Indeed:
o this FECFRAME update does not prevent nor compromise in any way
the support of block FEC codes. Both types of codes can nicely
co-exist, just like different block FEC schemes can co-exist;
o each sliding window FEC Scheme is associated to a specific FEC
Encoding ID subject to IANA registration, just like block FEC
Schemes;
o any receiver, for instance a legacy receiver that only supports
block FEC schemes, can easily identify the FEC Scheme used in a
FECFRAME session. Indeed, the FEC Encoding ID that identifies the
FEC Scheme is carried in the FEC Framework Configuration
Information (see section 5.5 of [RFC6363]). For instance, when
the Session Description Protocol (SDP) is used to carry the FEC
Framework Configuration Information, the FEC Encoding ID can be
communicated in the "encoding-id=" parameter of a "fec-repair-
flow" attribute [RFC6364]. This mechanism is the basic approach
for a FECFRAME receiver to determine whether or not it supports
the FEC Scheme used in a given FECFRAME session;
This document leverages on [RFC6363] and re-uses its structure. It
proposes new sections specific to sliding window FEC codes whenever
required. The only exception is Section 3 that provides a quick
summary of FECFRAME in order to facilitate the understanding of this
document to readers not familiar with the concepts and terminology.
2. Definitions and Abbreviations
The following list of definitions and abbreviations is copied from
[RFC6363], adding only the Block/sliding window FEC Code and
Encoding/Decoding Window definitions (tagged with "ADDED"):
Application Data Unit (ADU): The unit of source data provided as
payload to the transport layer. For instance, it can be a
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payload containing the result of the RTP packetization of a
compressed video frame.
ADU Flow: A sequence of ADUs associated with a transport-layer flow
identifier (such as the standard 5-tuple {source IP address,
source port, destination IP address, destination port, transport
protocol}).
AL-FEC: Application-layer Forward Error Correction.
Application Protocol: Control protocol used to establish and control
the source flow being protected, e.g., the Real-Time Streaming
Protocol (RTSP).
Content Delivery Protocol (CDP): A complete application protocol
specification that, through the use of the framework defined in
this document, is able to make use of FEC schemes to provide FEC
capabilities.
FEC Code: An algorithm for encoding data such that the encoded data
flow is resilient to data loss. Note that, in general, FEC codes
may also be used to make a data flow resilient to corruption, but
that is not considered in this document.
Block FEC Code: (ADDED) An FEC Code that operates on blocks, i.e.,
for which the input flow MUST be segmented into a sequence of
blocks, FEC encoding and decoding being performed independently
on a per-block basis.
Sliding Window FEC Code: (ADDED) An FEC Code that can generate
repair symbols on-the-fly, at any time, from the set of source
symbols present in the sliding encoding window at that time.
These codes are also known as convolutional codes.
FEC Framework: A protocol framework for the definition of Content
Delivery Protocols using FEC, such as the framework defined in
this document.
FEC Framework Configuration Information: Information that controls
the operation of the FEC Framework.
FEC Payload ID: Information that identifies the contents and
provides positional information of a packet with respect to the
FEC Scheme.
FEC Repair Packet: At a sender (respectively, at a receiver), a
payload submitted to (respectively, received from) the transport
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protocol containing one or more repair symbols along with a
Repair FEC Payload ID and possibly an RTP header.
FEC Scheme: A specification that defines the additional protocol
aspects required to use a particular FEC code with the FEC
Framework.
FEC Source Packet: At a sender (respectively, at a receiver), a
payload submitted to (respectively, received from) the transport
protocol containing an ADU along with an optional Explicit Source
FEC Payload ID.
Repair Flow: The packet flow carrying FEC data.
Repair FEC Payload ID: A FEC Payload ID specifically for use with
repair packets.
Source Flow: The packet flow to which FEC protection is to be
applied. A source flow consists of ADUs.
Source FEC Payload ID: A FEC Payload ID specifically for use with
source packets.
Source Protocol: A protocol used for the source flow being
protected, e.g., RTP.
Transport Protocol: The protocol used for the transport of the
source and repair flows, using an unreliable datagram service
such as UDP.
Encoding Window: (ADDED) Set of Source Symbols available at the
sender/coding node that are used to generate a repair symbol,
with a Sliding Window FEC Code.
Decoding Window: (ADDED) Set of received or decoded source and
repair symbols available at a receiver that are used to decode
erased source symbols, with a Sliding Window FEC Code.
Code Rate: The ratio between the number of source symbols and the
number of encoding symbols. By definition, the code rate is such
that 0 < code rate <= 1. A code rate close to 1 indicates that a
small number of repair symbols have been produced during the
encoding process.
Encoding Symbol: Unit of data generated by the encoding process.
With systematic codes, source symbols are part of the encoding
symbols.
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Packet Erasure Channel: A communication path where packets are
either lost (e.g., in our case, by a congested router, or because
the number of transmission errors exceeds the correction
capabilities of the physical-layer code) or received. When a
packet is received, it is assumed that this packet is not
corrupted (i.e., in our case, the bit-errors, if any, are fixed
by the physical-layer code and therefore hidden to the upper
layers).
Repair Symbol: Encoding symbol that is not a source symbol.
Source Block: Group of ADUs that are to be FEC protected as a single
block. This notion is restricted to Block FEC Codes.
Source Symbol: Unit of data used during the encoding process.
Systematic Code: FEC code in which the source symbols are part of
the encoding symbols.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Summary of Architecture Overview
The architecture of [RFC6363], Section 3, equally applies to this
FECFRAME extension and is not repeated here. However, we provide
hereafter a quick summary to facilitate the understanding of this
document to readers not familiar with the concepts and terminology.
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+----------------------+
| Application |
+----------------------+
|
| (1) Application Data Units (ADUs)
|
v
+----------------------+ +----------------+
| FEC Framework | | |
| |-------------------------->| FEC Scheme |
|(2) Construct source |(3) Source Block | |
| blocks | |(4) FEC Encoding|
|(6) Construct FEC |<--------------------------| |
| Source and Repair | | |
| Packets |(5) Explicit Source FEC | |
+----------------------+ Payload IDs +----------------+
| Repair FEC Payload IDs
| Repair symbols
|
|(7) FEC Source and Repair Packets
v
+----------------------+
| Transport Protocol |
+----------------------+
Figure 1: FECFRAME architecture at a sender.
The FECFRAME architecture is illustrated in Figure 1 from the
sender's point of view, in case of a block FEC Scheme. It shows an
application generating an ADU flow (other flows, from other
applications, may co-exist). These ADUs, of variable size, must be
somehow mapped to source symbols of fixed size (this fixed size is a
requirement of all FEC Schemes that comes from the way mathematical
operations are applied to symbols content). This is the goal of an
ADU-to-symbols mapping process that is FEC-Scheme specific (see
below). Once the source block is built, taking into account both the
FEC Scheme constraints (e.g., in terms of maximum source block size)
and the application's flow constraints (e.g., in terms of real-time
constraints), the associated source symbols are handed to the FEC
Scheme in order to produce an appropriate number of repair symbols.
FEC Source Packets (containing ADUs) and FEC Repair Packets
(containing one or more repair symbols each) are then generated and
sent using an appropriate transport protocol (more precisely
[RFC6363], Section 7, requires a transport protocol providing an
unreliable datagram service, such as UDP). In practice FEC Source
Packets may be passed to the transport layer as soon as available,
without having to wait for FEC encoding to take place. In that case
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a copy of the associated source symbols needs to be kept within
FECFRAME for future FEC encoding purposes.
At a receiver (not shown), FECFRAME processing operates in a similar
way, taking as input the incoming FEC Source and Repair Packets
received. In case of FEC Source Packet losses, the FEC decoding of
the associated block may recover all (in case of successful decoding)
or a subset potentially empty (otherwise) of the missing source
symbols. After source-symbol-to-ADU mapping, when lost ADUs are
recovered, they are then assigned to their respective flow (see
below). ADUs are returned to the application(s), either in their
initial transmission order (in that case ADUs received after an
erased one will be delayed until FEC decoding has taken place) or not
(in that case each ADU is returned as soon as it is received or
recovered), depending on the application requirements.
FECFRAME features two subtle mechanisms:
o ADUs-to-source-symbols mapping: in order to manage variable size
ADUs, FECFRAME and FEC Schemes can use small, fixed size symbols
and create a mapping between ADUs and symbols. To each ADU this
mechanism prepends a length field (plus a flow identifier, see
below) and pads the result to a multiple of the symbol size. A
small ADU may be mapped to a single source symbol while a large
one may be mapped to multiple symbols. The mapping details are
FEC-Scheme-dependent and must be defined in the associated
document;
o Assignment of decoded ADUs to flows in multi-flow configurations:
when multiple flows are multiplexed over the same FECFRAME
instance, a problem is to assign a decoded ADU to the right flow
(UDP port numbers and IP addresses traditionally used to map
incoming ADUs to flows are not recovered during FEC decoding). To
make it possible, at the FECFRAME sending instance, each ADU is
prepended with a flow identifier (1 byte) during the ADU-to-
source-symbols mapping (see above). The flow identifiers are also
shared between all FECFRAME instances as part of the FEC Framework
Configuration Information. This (flow identifier + length +
application payload + padding), called ADUI, is then FEC
protected. Therefore a decoded ADUI contains enough information
to assign the ADU to the right flow.
A few aspects are not covered by FECFRAME, namely:
o [RFC6363] section 8 does not detail any congestion control
mechanism, but only provides high level normative requirements;
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o the possibility of having feedbacks from receiver(s) is considered
out of scope, although such a mechanism may exist within the
application (e.g., through RTCP control messages);
o flow adaptation at a FECFRAME sender (e.g., how to set the FEC
code rate based on transmission conditions) is not detailed, but
it needs to comply with the congestion control normative
requirements (see above).
4. Procedural Overview
4.1. General
The general considerations of [RFC6363], Section 4.1, that are
specific to block FEC codes are not repeated here.
With a Sliding Window FEC Code, the FEC Source Packet MUST contain
information to identify the position occupied by the ADU within the
source flow, in terms specific to the FEC Scheme. This information
is known as the Source FEC Payload ID, and the FEC Scheme is
responsible for defining and interpreting it.
With a Sliding Window FEC Code, the FEC Repair Packets MUST contain
information that identifies the relationship between the contained
repair payloads and the original source symbols used during encoding.
This information is known as the Repair FEC Payload ID, and the FEC
Scheme is responsible for defining and interpreting it.
The Sender Operation ([RFC6363], Section 4.2.) and Receiver Operation
([RFC6363], Section 4.3) are both specific to block FEC codes and
therefore omitted below. The following two sections detail similar
operations for Sliding Window FEC codes.
4.2. Sender Operation with Sliding Window FEC Codes
With a Sliding Window FEC Scheme, the following operations,
illustrated in Figure 2 for the generic case (non-RTP repair flows),
and in Figure 3 for the case of RTP repair flows, describe a possible
way to generate compliant source and repair flows:
1. A new ADU is provided by the application.
2. The FEC Framework communicates this ADU to the FEC Scheme.
3. The sliding encoding window is updated by the FEC Scheme. The
ADU-to-source-symbols mapping as well as the encoding window
management details are both the responsibility of the FEC Scheme
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and MUST be detailed there. Appendix A provides non-normative
hints about what FEC Scheme designers need to consider;
4. The Source FEC Payload ID information of the source packet is
determined by the FEC Scheme. If required by the FEC Scheme,
the Source FEC Payload ID is encoded into the Explicit Source
FEC Payload ID field and returned to the FEC Framework.
5. The FEC Framework constructs the FEC Source Packet according to
[RFC6363] Figure 6, using the Explicit Source FEC Payload ID
provided by the FEC Scheme if applicable.
6. The FEC Source Packet is sent using normal transport-layer
procedures. This packet is sent using the same ADU flow
identification information as would have been used for the
original source packet if the FEC Framework were not present
(e.g., the source and destination addresses and UDP port numbers
on the IP datagram carrying the source packet will be the same
whether or not the FEC Framework is applied).
7. When the FEC Framework needs to send one or several FEC Repair
Packets (e.g., according to the target Code Rate), it asks the
FEC Scheme to create one or several repair packet payloads from
the current sliding encoding window along with their Repair FEC
Payload ID.
8. The Repair FEC Payload IDs and repair packet payloads are
provided back by the FEC Scheme to the FEC Framework.
9. The FEC Framework constructs FEC Repair Packets according to
[RFC6363] Figure 7, using the FEC Payload IDs and repair packet
payloads provided by the FEC Scheme.
10. The FEC Repair Packets are sent using normal transport-layer
procedures. The port(s) and multicast group(s) to be used for
FEC Repair Packets are defined in the FEC Framework
Configuration Information.
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+----------------------+
| Application |
+----------------------+
|
| (1) New Application Data Unit (ADU)
v
+---------------------+ +----------------+
| FEC Framework | | FEC Scheme |
| |-------------------------->| |
| | (2) New ADU |(3) Update of |
| | | encoding |
| |<--------------------------| window |
|(5) Construct FEC | (4) Explicit Source | |
| Source Packet | FEC Payload ID(s) |(7) FEC |
| |<--------------------------| encoding |
|(9) Construct FEC | (8) Repair FEC Payload ID | |
| Repair Packet(s) | + Repair symbol(s) +----------------+
+---------------------+
|
| (6) FEC Source Packet
| (10) FEC Repair Packets
v
+----------------------+
| Transport Protocol |
+----------------------+
Figure 2: Sender Operation with Sliding Window FEC Codes
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+----------------------+
| Application |
+----------------------+
|
| (1) New Application Data Unit (ADU)
v
+---------------------+ +----------------+
| FEC Framework | | FEC Scheme |
| |-------------------------->| |
| | (2) New ADU |(3) Update of |
| | | encoding |
| |<--------------------------| window |
|(5) Construct FEC | (4) Explicit Source | |
| Source Packet | FEC Payload ID(s) |(7) FEC |
| |<--------------------------| encoding |
|(9) Construct FEC | (8) Repair FEC Payload ID | |
| Repair Packet(s) | + Repair symbol(s) +----------------+
+---------------------+
| |
|(6) Source |(10) Repair payloads
| packets |
| + -- -- -- -- -+
| | RTP |
| +-- -- -- -- --+
v v
+----------------------+
| Transport Protocol |
+----------------------+
Figure 3: Sender Operation with Sliding Window FEC Codes and RTP
Repair Flows
4.3. Receiver Operation with Sliding Window FEC Codes
With a Sliding Window FEC Scheme, the following operations,
illustrated in Figure 4 for the generic case (non-RTP repair flows),
and in Figure 5 for the case of RTP repair flows. The only
differences with respect to block FEC codes lie in steps (4) and (5).
Therefore this section does not repeat the other steps of [RFC6363],
Section 4.3, "Receiver Operation". The new steps (4) and (5) are:
4. The FEC Scheme uses the received FEC Payload IDs (and derived FEC
Source Payload IDs when the Explicit Source FEC Payload ID field
is not used) to insert source and repair packets into the
decoding window in the right way. If at least one source packet
is missing and at least one repair packet has been received, then
FEC decoding is attempted to recover missing source payloads.
The FEC Scheme determines whether source packets have been lost
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and whether enough repair packets have been received to decode
any or all of the missing source payloads.
5. The FEC Scheme returns the received and decoded ADUs to the FEC
Framework, along with indications of any ADUs that were missing
and could not be decoded.
+----------------------+
| Application |
+----------------------+
^
|(6) ADUs
|
+----------------------+ +----------------+
| FEC Framework | | FEC Scheme |
| |<--------------------------| |
|(2)Extract FEC Payload|(5) ADUs |(4) FEC Decoding
| IDs and pass IDs & |-------------------------->| |
| payloads to FEC |(3) Explicit Source FEC +----------------+
| scheme | Payload IDs
+----------------------+ Repair FEC Payload IDs
^ Source payloads
| Repair payloads
|(1) FEC Source
| and Repair Packets
+----------------------+
| Transport Protocol |
+----------------------+
Figure 4: Receiver Operation with Sliding Window FEC Codes
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+----------------------+
| Application |
+----------------------+
^
|(6) ADUs
|
+----------------------+ +----------------+
| FEC Framework | | FEC Scheme |
| |<--------------------------| |
|(2)Extract FEC Payload|(5) ADUs |(4) FEC Decoding|
| IDs and pass IDs & |-------------------------->| |
| payloads to FEC |(3) Explicit Source FEC +----------------+
| scheme | Payload IDs
+----------------------+ Repair FEC Payload IDs
^ ^ Source payloads
| | Repair payloads
|Source pkts |Repair payloads
| |
+-- |- -- -- -- -- -- -+
|RTP| | RTP Processing |
| | +-- -- -- --|-- -+
| +-- -- -- -- -- |--+ |
| | RTP Demux | |
+-- -- -- -- -- -- -- -+
^
|(1) FEC Source and Repair Packets
|
+----------------------+
| Transport Protocol |
+----------------------+
Figure 5: Receiver Operation with Sliding Window FEC Codes and RTP
Repair Flows
5. Protocol Specification
5.1. General
This section discusses the protocol elements for the FEC Framework
specific to Sliding Window FEC schemes. The global formats of source
data packets (i.e., [RFC6363], Figure 6) and repair data packets
(i.e., [RFC6363], Figures 7 and 8) remain the same with Sliding
Window FEC codes. They are not repeated here.
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5.2. FEC Framework Configuration Information
The FEC Framework Configuration Information considerations of
[RFC6363], Section 5.5, equally applies to this FECFRAME extension
and is not repeated here.
5.3. FEC Scheme Requirements
The FEC Scheme requirements of [RFC6363], Section 5.6, mostly apply
to this FECFRAME extension and are not repeated here. An exception
though is the "full specification of the FEC code", item (4), that is
specific to block FEC codes. The following item (4-bis) applies in
case of Sliding Window FEC schemes:
4-bis. A full specification of the Sliding Window FEC code
This specification MUST precisely define the valid FEC-Scheme-
Specific Information values, the valid FEC Payload ID values, and
the valid packet payload sizes (where packet payload refers to
the space within a packet dedicated to carrying encoding
symbols).
Furthermore, given valid values of the FEC-Scheme-Specific
Information, a valid Repair FEC Payload ID value, a valid packet
payload size, and a valid encoding window (i.e., a set of source
symbols), the specification MUST uniquely define the values of
the encoding symbol (or symbols) to be included in the repair
packet payload with the given Repair FEC Payload ID value.
Additionally, the FEC Scheme associated to a Sliding Window FEC Code:
o MUST define the relationships between ADUs and the associated
source symbols (mapping);
o MUST define the management of the encoding window that slides over
the set of ADUs. Appendix A provides non normative hints about
what FEC Scheme designers need to consider;
o MUST define the management of the decoding window. This usually
consists in managing a system of linear equations (in case of a
linear FEC code);
6. Feedback
The discussion of [RFC6363], Section 6, equally applies to this
FECFRAME extension and is not repeated here.
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7. Transport Protocols
The discussion of [RFC6363], Section 7, equally applies to this
FECFRAME extension and is not repeated here.
8. Congestion Control
The discussion of [RFC6363], Section 8, equally applies to this
FECFRAME extension and is not repeated here.
9. Implementation Status
Editor's notes: RFC Editor, please remove this section motivated by
RFC 7942 before publishing the RFC. Thanks!
An implementation of FECFRAME extended to Sliding Window codes
exists:
o Organisation: Inria
o Description: This is an implementation of FECFRAME extended to
Sliding Window codes and supporting the RLC FEC Scheme [RLC-ID].
It is based on: (1) a proprietary implementation of FECFRAME, made
by Inria and Expway for which interoperability tests have been
conducted; and (2) a proprietary implementation of RLC Sliding
Window FEC Codes.
o Maturity: the basic FECFRAME maturity is "production", the
FECFRAME extension maturity is "under progress".
o Coverage: the software implements a subset of [RFC6363], as
specialized by the 3GPP eMBMS standard [MBMSTS]. This software
also covers the additional features of FECFRAME extended to
Sliding Window codes, in particular the RLC FEC Scheme.
o Licensing: proprietary.
o Implementation experience: maximum.
o Information update date: March 2018.
o Contact: vincent.roca@inria.fr
10. Security Considerations
This FECFRAME extension does not add any new security consideration.
All the considerations of [RFC6363], Section 9, apply to this
document as well. However, for the sake of completeness, the
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following goal can be added to the list provided in Section 9.1
"Problem Statement" of [RFC6363]:
o Attacks can try to corrupt source flows in order to modify the
receiver application's behavior (as opposed to just denying
service).
11. Operations and Management Considerations
This FECFRAME extension does not add any new Operations and
Management Consideration. All the considerations of [RFC6363],
Section 10, apply to this document as well.
12. IANA Considerations
No IANA actions are required for this document.
A FEC Scheme for use with this FEC Framework is identified via its
FEC Encoding ID. It is subject to IANA registration in the "FEC
Framework (FECFRAME) FEC Encoding IDs" registry. All the rules of
[RFC6363], Section 11, apply and are not repeated here.
13. Acknowledgments
The authors would like to thank Christer Holmberg, David Black, Gorry
Fairhurst, and Emmanuel Lochin, Spencer Dawkins, Ben Campbell,
Benjamin Kaduk, Eric Rescorla, Adam Roach, and Greg Skinner for their
valuable feedback on this document. This document being an extension
to [RFC6363], the authors would also like to thank Mark Watson as the
main author of that RFC.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363,
DOI 10.17487/RFC6363, October 2011,
<https://www.rfc-editor.org/info/rfc6363>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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14.2. Informative References
[MBMSTS] 3GPP, "Multimedia Broadcast/Multicast Service (MBMS);
Protocols and codecs", 3GPP TS 26.346, March 2009,
<http://ftp.3gpp.org/specs/html-info/26346.htm>.
[RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error
Correction (FEC) Building Block", RFC 5052,
DOI 10.17487/RFC5052, August 2007,
<https://www.rfc-editor.org/info/rfc5052>.
[RFC6364] Begen, A., "Session Description Protocol Elements for the
Forward Error Correction (FEC) Framework", RFC 6364,
DOI 10.17487/RFC6364, October 2011,
<https://www.rfc-editor.org/info/rfc6364>.
[RFC6681] Watson, M., Stockhammer, T., and M. Luby, "Raptor Forward
Error Correction (FEC) Schemes for FECFRAME", RFC 6681,
DOI 10.17487/RFC6681, August 2012,
<https://www.rfc-editor.org/info/rfc6681>.
[RFC6816] Roca, V., Cunche, M., and J. Lacan, "Simple Low-Density
Parity Check (LDPC) Staircase Forward Error Correction
(FEC) Scheme for FECFRAME", RFC 6816,
DOI 10.17487/RFC6816, December 2012,
<https://www.rfc-editor.org/info/rfc6816>.
[RFC6865] Roca, V., Cunche, M., Lacan, J., Bouabdallah, A., and K.
Matsuzono, "Simple Reed-Solomon Forward Error Correction
(FEC) Scheme for FECFRAME", RFC 6865,
DOI 10.17487/RFC6865, February 2013,
<https://www.rfc-editor.org/info/rfc6865>.
[RFC8406] Adamson, B., Adjih, C., Bilbao, J., Firoiu, V., Fitzek,
F., Ghanem, S., Lochin, E., Masucci, A., Montpetit, M-J.,
Pedersen, M., Peralta, G., Roca, V., Ed., Saxena, P., and
S. Sivakumar, "Taxonomy of Coding Techniques for Efficient
Network Communications", RFC 8406, DOI 10.17487/RFC8406,
June 2018, <https://www.rfc-editor.org/info/rfc8406>.
[RLC-ID] Roca, V. and B. Teibi, "Sliding Window Random Linear Code
(RLC) Forward Erasure Correction (FEC) Scheme for
FECFRAME", Work in Progress, Transport Area Working Group
(TSVWG) draft-ietf-tsvwg-rlc-fec-scheme (Work in
Progress), September 2018, <https://tools.ietf.org/html/
draft-ietf-tsvwg-rlc-fec-scheme>.
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Appendix A. About Sliding Encoding Window Management (informational)
The FEC Framework does not specify the management of the sliding
encoding window which is the responsibility of the FEC Scheme. This
annex only provides a few informational hints.
Source symbols are added to the sliding encoding window each time a
new ADU is available at the sender, after the ADU-to-source-symbol
mapping specific to the FEC Scheme.
Source symbols are removed from the sliding encoding window, for
instance:
o after a certain delay, when an "old" ADU of a real-time flow times
out. The source symbol retention delay in the sliding encoding
window should therefore be initialized according to the real-time
features of incoming flow(s) when applicable;
o once the sliding encoding window has reached its maximum size
(there is usually an upper limit to the sliding encoding window
size). In that case the oldest symbol is removed each time a new
source symbol is added.
Several considerations can impact the management of this sliding
encoding window:
o at the source flows level: real-time constraints can limit the
total time source symbols can remain in the encoding window;
o at the FEC code level: theoretical or practical limitations (e.g.,
because of computational complexity) can limit the number of
source symbols in the encoding window;
o at the FEC Scheme level: signaling and window management are
intrinsically related. For instance, an encoding window composed
of a non-sequential set of source symbols requires an appropriate
signaling to inform a receiver of the composition of the encoding
window, and the associated transmission overhead can limit the
maximum encoding window size. On the opposite, an encoding window
always composed of a sequential set of source symbols simplifies
signaling: providing the identity of the first source symbol plus
their number is sufficient, which creates a fixed and relatively
small transmission overhead.
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Authors' Addresses
Vincent Roca
INRIA
Univ. Grenoble Alpes
France
EMail: vincent.roca@inria.fr
Ali Begen
Networked Media
Konya
Turkey
EMail: ali.begen@networked.media
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