Internet DRAFT - draft-bormann-6lowpan-ghc
draft-bormann-6lowpan-ghc
6LoWPAN Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Standards Track March 29, 2013
Expires: September 30, 2013
6LoWPAN Generic Compression of Headers and Header-like Payloads
draft-bormann-6lowpan-ghc-06
Abstract
This short I-D provides a simple addition to 6LoWPAN Header
Compression that enables the compression of generic headers and
header-like payloads, without a need to define a new header
compression scheme for each new such header or header-like payload.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 30, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Bormann Expires September 30, 2013 [Page 1]
Internet-Draft 6lowpan-ghc March 2013
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. The Header Compression Coupling Problem . . . . . . . . . 2
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
1.3. Notation . . . . . . . . . . . . . . . . . . . . . . . . 3
2. 6LoWPAN-GHC . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Integrating 6LoWPAN-GHC into 6LoWPAN-HC . . . . . . . . . . . 5
3.1. Compressing payloads (UDP and ICMPv6) . . . . . . . . . . 5
3.2. Compressing extension headers . . . . . . . . . . . . . . 5
3.3. Indicating GHC capability . . . . . . . . . . . . . . . . 6
3.4. Using the 6CIO Option . . . . . . . . . . . . . . . . . . 7
4. IANA considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security considerations . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
1.1. The Header Compression Coupling Problem
6LoWPAN-HC [RFC6282] defines a scheme for header compression in
6LoWPAN [RFC4944] packets. As with most header compression schemes,
a new specification is needed for every new kind of header that needs
to be compressed. In addition, [RFC6282] does not define an
extensibility scheme like the ROHC profiles defined in ROHC [RFC3095]
[RFC5795]. This leads to the difficult situation that 6LoWPAN-HC
tended to be reopened and reexamined each time a new header receives
consideration (or an old header is changed and reconsidered) in the
6LoWPAN/roll/CoRE cluster of IETF working groups. While [RFC6282]
finally got completed, the underlying problem remains unsolved.
The purpose of the present contribution is to plug into [RFC6282] as
is, using its NHC (next header compression) concept. We add a
slightly less efficient, but vastly more general form of compression
for headers of any kind and even for header-like payloads such as
those exhibited by routing protocols, DHCP, etc. The objective is an
extremely simple specification that can be defined on a single page
and implemented in a small number of lines of code, as opposed to a
general data compression scheme such as that defined in [RFC1951].
1.2. Terminology
Bormann Expires September 30, 2013 [Page 2]
Internet-Draft 6lowpan-ghc March 2013
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].
The term "byte" is used in its now customary sense as a synonym for
"octet".
1.3. Notation
This specification uses a trivial notation for code bytes and the
bitfields in them the meaning of which should be mostly obvious.
More formally speaking, the meaning of the notation is:
Potential values for the code bytes themselves are expressed by
templates that represent 8-bit most-significant-bit-first binary
numbers (without any special prefix), where 0 stands for 0, 1 for 1,
and variable segments in these code byte templates are indicated by
sequences of the same letter such as kkkkkkk or ssss, the length of
which indicates the length of the variable segment in bits.
In the notation of values derived from the code bytes, 0b is used as
a prefix for expressing binary numbers in most-significant-bit first
notation (akin to the use of 0x for most-significant-digit-first
hexadecimal numbers in the C programming language). Where the
abovementioned sequences of letters are then referenced in such a
binary number in the text, the intention is that the value from these
bitfields in the actual code byte be inserted.
Example: The code byte template
101nssss
stands for a byte that starts (most-significant-bit-first) with the
bits 1, 0, and 1, and continues with five variable bits, the first of
which is referenced as "n" and the next four are referenced as
"ssss". Based on this code byte template, a reference to
0b0ssss000
means a binary number composed from a zero bit, the four bits that
are in the "ssss" field (for 101nssss, the four least significant
bits) in the actual byte encountered, kept in the same order, and
three more zero bits.
Bormann Expires September 30, 2013 [Page 3]
Internet-Draft 6lowpan-ghc March 2013
2. 6LoWPAN-GHC
The format of a GHC-compressed header or payload is a simple
bytecode. A compressed header consists of a sequence of pieces, each
of which begins with a code byte, which may be followed by zero or
more bytes as its argument. Some code bytes cause bytes to be laid
out in the destination buffer, some simply modify some decompression
variables.
At the start of decompressing a header or payload within a L2 packet
(= fragment), variables "sa" and "na" are initialized as zero.
The code bytes are defined as follows:
+------------+------------------------------------------+-----------+
| code byte | Action | Argument |
+------------+------------------------------------------+-----------+
| 0kkkkkkk | Append k = 0b0kkkkkkk bytes of data in | k bytes |
| | the bytecode argument (k < 96) | of data |
| | | |
| 1000nnnn | Append 0b0000nnnn+2 bytes of zeroes | |
| | | |
| 10010000 | STOP code (end of compressed data, see | |
| | Section 3.2) | |
| | | |
| 101nssss | Set up extended arguments for a | |
| | backreference: sa += 0b0ssss000, na += | |
| | 0b0000n000 | |
| | | |
| 11nnnkkk | Backreference: n = na+0b00000nnn+2; s = | |
| | 0b00000kkk+sa+n; append n bytes from | |
| | previously output bytes, starting s | |
| | bytes to the left of the current output | |
| | pointer; set sa = 0, na = 0 | |
+------------+------------------------------------------+-----------+
Note that the following bit combinations are reserved at this time:
011xxxxx, and 1001nnnn (where 0b0000nnnn > 0).
For the purposes of the backreferences, the expansion buffer is
initialized with a predefined dictionary, at the end of which the
target buffer begins. This dictionary is composed of the pseudo-
header for the current packet as defined in [RFC2460], followed by a
16-byte static dictionary (Figure 1). These dictionary bytes are
therefore available for backreferencing, but not copied into the
final result.
Bormann Expires September 30, 2013 [Page 4]
Internet-Draft 6lowpan-ghc March 2013
16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00
Figure 1: The 16 bytes of static dictionary (in hex)
3. Integrating 6LoWPAN-GHC into 6LoWPAN-HC
6LoWPAN-GHC is intended to plug in as an NHC format for 6LoWPAN-HC
[RFC6282]. This section shows how this can be done (without
supplying the detailed normative text yet, although it could be
implemented from this page).
3.1. Compressing payloads (UDP and ICMPv6)
GHC is by definition generic and can be applied to different kinds of
packets. All the examples given in Appendix A are for ICMPv6
packets; a single NHC value suffices to define an NHC format for
ICMPv6 based on GHC (see below).
In addition it may be useful to include an NHC format for UDP, as
many headerlike payloads (e.g., DHCPv6) are carried in UDP.
[RFC6282] already defines an NHC format for UDP (11110CPP). What
remains to be done is to define an analogous NHC byte formatted, e.g.
as shown in Figure 2, and simply reference the existing
specification, indicating that for 0b11010cpp NHC bytes, the UDP
payload is not supplied literally but compressed by 6LoWPAN-GHC.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | 1 | 0 | 1 | 0 | C | P |
+---+---+---+---+---+---+---+---+
Figure 2: Proposed NHC byte for UDP GHC (actual value to be allocated
by IANA)
To stay in the same general numbering space, we propose 0b11011111 as
the NHC byte for ICMPv6 GHC (Figure 3).
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 |
+---+---+---+---+---+---+---+---+
Figure 3: Proposed NHC byte for ICMPv6 GHC (actual value to be
allocated by IANA)
3.2. Compressing extension headers
Bormann Expires September 30, 2013 [Page 5]
Internet-Draft 6lowpan-ghc March 2013
If the compression of specific extension headers is considered
desirable, this can be added in a similar way, e.g. as in Figure 4
(however, probably only EID 0 to 3 need to be assigned). As there is
no easy way to extract the length field from the GHC-encoded header
before decoding, this would make detecting the end of the extension
header somewhat complex. The easiest (and most efficient) approach
is to completely elide the length field (in the same way NHC already
elides the next header field in certain cases) and reconstruct it
only on decompression. To serve as a terminator for the extension
header, the reserved bytecode 0b10010000 has been assigned as a stop
marker. Note that the stop marker is only needed for extension
headers, not for final payloads, the decompression of which is
automatically stopped by the end of the packet.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | 0 | 1 | 1 | EID |NH |
+---+---+---+---+---+---+---+---+
Figure 4: Proposed NHC byte for extension header GHC
3.3. Indicating GHC capability
The 6LoWPAN baseline includes just [RFC4944], [RFC6282], [RFC6775]
(see [I-D.bormann-6lowpan-roadmap]). To enable the use of GHC,
6LoWPAN nodes need to know that their neighbors implement it. While
this can also simply be administratively required, a transition
strategy as well as a way to support mixed networks is required.
One way to know a neighbor does implement GHC is receiving a packet
from that neighbor with GHC in it ("implicit capability detection").
However, there needs to be a way to bootstrap this, as nobody ever
would start sending packets with GHC otherwise.
To minimize the impact on [RFC6775], we propose adding an ND option
6LoWPAN Capability Indication (6CIO), as illustrated in Figure 5.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length = 1 |_____________________________|G|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|_______________________________________________________________|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: 6LoWPAN Capability Indication Option (6CIO)
Bormann Expires September 30, 2013 [Page 6]
Internet-Draft 6lowpan-ghc March 2013
The G bit indicates whether the node sending the option is GHC
capable.
Once a node receives either an explicit or an implicit indication of
GHC capability from another node, it may send GHC-compressed packets
to that node. Where that capability has not been recently confirmed,
similar to the way PLPMTUD [RFC4821] finds out about changes in the
network, a node SHOULD make use of NUD (neighbor unreachability
detection) failures to switch back to basic 6LoWPAN header
compression [RFC6282].
3.4. Using the 6CIO Option
The 6CIO option will typically only be ever sent in 6LoWPAN-ND RS
packets (it then cannot itself be GHC compressed unless the host
desires to limit itself to talking to GHC capable routers); the
resulting 6LoWPAN-ND RA can already make use of GHC and thus indicate
GHC capability implicitly, which in turn allows the nodes to use GHC
in the 6LoWPAN-ND NS/NA exchange.
6CIO can also be used for future options that need to be negotiated
between 6LoWPAN peers; an IANA registry will administrate the flags.
(Bits marked by underscores in Figure 5 are reserved for future
allocation, i.e., they MUST be sent as zero and MUST be ignored on
reception until allocated. Length values larger than 1 MUST be
accepted by implementations in order to enable future extensions; the
additional bits in the option are then reserved in the same way. For
the purposes of the IANA registry, the bits are numbered in msb-first
order from the 16th bit of the option onward, i.e., the G bit is flag
number 15.) (Additional bits may also be used by a follow-on version
of this document if some bit combinations that have been left
reserved here are then used in an upward compatible manner.)
Where the use of this option by other specifications is envisioned,
the following items have to be kept in mind:
o The option can be used in any ND packet.
o Specific bits are set in the option to indicate that a capability
is present in the sender. (There may be other ways to infer this
information, as is the case in this specification.) Bit
combinations may be used as desired. The absence of the
capability _indication_ is signaled by setting these bits to zero;
this does not necessarily mean that the capability is absent.
o The intention is not to modify the semantics of the specific ND
packet carrying the option, but to provide the general capability
indication described above.
Bormann Expires September 30, 2013 [Page 7]
Internet-Draft 6lowpan-ghc March 2013
o Specifications have to be designed such that receivers that do not
receive or do not process such a capability indication can still
interoperate (presumably without exploiting the indicated
capability).
o The option is meant to be used sparsely, i.e. once a sender has
reason to believe the capability indication has been received,
there no longer is a need to continue sending it.
4. IANA considerations
[This section to be removed/replaced by the RFC Editor.]
In the IANA registry for the "LOWPAN_NHC Header Type" (in the "IPv6
Low Power Personal Area Network Parameters"), IANA needs to add the
assignments in Figure 6.
10110IIN: Extension header GHC [RFCthis]
11010CPP: UDP GHC [RFCthis]
11011111: ICMPv6 GHC [RFCthis]
Figure 6: IANA assignments for the NHC byte
IANA needs to allocate an ND option number for the 6CIO ND option
format in the Registry "IPv6 Neighbor Discovery Option Formats"
[RFC4861].
IANA needs to create a registry for "6LoWPAN capability bits" within
the "Internet Control Message Protocol version 6 (ICMPv6)
Parameters". The bits are allocated by giving their numbers as small
non-negative integers as defined in section Section 3.4, preferably
in the range 0..47. The policy is "RFC Required" [RFC5226]. The
initial content of the registry is as in Figure 7:
0..14: unassigned
15: GHC capable bit (G bit) [RFCthis]
16..47: unassigned
Figure 7
Bormann Expires September 30, 2013 [Page 8]
Internet-Draft 6lowpan-ghc March 2013
5. Security considerations
The security considerations of [RFC4944] and [RFC6282] apply. As
usual in protocols with packet parsing/construction, care must be
taken in implementations to avoid buffer overflows and in particular
(with respect to the back-referencing) out-of-area references during
decompression.
One additional consideration is that an attacker may send a forged
packet that makes a second node believe a third victim node is GHC-
capable. If it is not, this may prevent packets sent by the second
node from reaching the third node (at least until robustness features
such as those discussed in Section 3.3 kick in).
No mitigation is proposed (or known) for this attack, except that a
victim node that does implement GHC is not vulnerable. However, with
unsecured ND, a number of attacks with similar outcomes are already
possible, so there is little incentive to make use of this additional
attack. With secured ND, 6CIO is also secured; nodes relying on
secured ND therefore should use 6CIO bidirectionally (and limit the
implicit capability detection to secured ND packets carrying GHC)
instead of basing their neighbor capability assumptions on receiving
any kind of unprotected packet.
6. Acknowledgements
Colin O'Flynn has repeatedly insisted that some form of compression
for ICMPv6 and ND packets might be beneficial. He actually wrote his
own draft, [I-D.oflynn-6lowpan-icmphc], which compresses better, but
addresses basic ICMPv6/ND only and needs a much longer spec (around
17 pages of detailed spec, as compared to the single page of core
spec here). This motivated the author to try something simple, yet
general. Special thanks go to Colin for indicating that he indeed
considers his draft superseded by the present one.
The examples given are based on pcap files that Colin O'Flynn, Owen
Kirby, Olaf Bergmann and others provided.
The static dictionary was developed, and the bit allocations
validated, based on research by Sebastian Dominik.
Erik Nordmark provided input that helped shaping the 6CIO option.
Yoshihiro Ohba insisted on clarifying the notation used for the
definition of the bytecodes and their bitfields.
Bormann Expires September 30, 2013 [Page 9]
Internet-Draft 6lowpan-ghc March 2013
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S.E. and R.M. Hinden, "Internet Protocol, Version
6 (IPv6) Specification", RFC 2460, December 1998.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
September 2011.
[RFC6775] Shelby, Z., Chakrabarti, S., Nordmark, E., and C. Bormann,
"Neighbor Discovery Optimization for IPv6 over Low-Power
Wireless Personal Area Networks (6LoWPANs)", RFC 6775,
November 2012.
7.2. Informative References
[I-D.bormann-6lowpan-roadmap]
Bormann, C., "6LoWPAN Roadmap and Implementation Guide",
draft-bormann-6lowpan-roadmap-03 (work in progress),
October 2012.
[I-D.oflynn-6lowpan-icmphc]
O'Flynn, C., "ICMPv6/ND Compression for 6LoWPAN Networks",
draft-oflynn-6lowpan-icmphc-00 (work in progress), July
2010.
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
Bormann Expires September 30, 2013 [Page 10]
Internet-Draft 6lowpan-ghc March 2013
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.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795, March
2010.
Appendix A. Examples
This section demonstrates some relatively realistic examples derived
from actual PCAP dumps taken at previous interops. (TBD: Add a
couple DHCP examples.)
Figure 8 shows an RPL DODAG Information Solicitation, a quite short
RPL message that obviously cannot be improved much.
IP header:
60 00 00 00 00 08 3a ff fe 80 00 00 00 00 00 00
02 1c da ff fe 00 20 24 ff 02 00 00 00 00 00 00
00 00 00 00 00 00 00 1a
Payload:
9b 00 6b de 00 00 00 00
Dictionary:
fe 80 00 00 00 00 00 00 02 1c da ff fe 00 20 24
ff 02 00 00 00 00 00 00 00 00 00 00 00 00 00 1a
00 00 00 08 00 00 00 3a 16 fe fd 17 fe fd 00 01
00 00 00 00 00 01 00 00
copy: 04 9b 00 6b de
4 nulls: 82
Compressed:
04 9b 00 6b de 82
Was 8 bytes; compressed to 6 bytes, compression factor 1.33
Figure 8: A simple RPL example
Figure 9 shows an RPL DODAG Information Object, a longer RPL control
message that is improved a bit more. Note that the compressed output
exposes an inefficiency in the simple-minded compressor used to
generate it; this does not devalue the example since constrained
nodes are quite likely to make use of simple-minded compressors.
IP header:
60 00 00 00 00 5c 3a ff fe 80 00 00 00 00 00 00
Bormann Expires September 30, 2013 [Page 11]
Internet-Draft 6lowpan-ghc March 2013
02 1c da ff fe 00 30 23 ff 02 00 00 00 00 00 00
00 00 00 00 00 00 00 1a
Payload:
9b 01 7a 5f 00 f0 01 00 88 00 00 00 20 02 0d b8
00 00 00 00 00 00 00 ff fe 00 fa ce 04 0e 00 14
09 ff 00 00 01 00 00 00 00 00 00 00 08 1e 80 20
ff ff ff ff ff ff ff ff 00 00 00 00 20 02 0d b8
00 00 00 00 00 00 00 ff fe 00 fa ce 03 0e 40 00
ff ff ff ff 20 02 0d b8 00 00 00 00
Dictionary:
fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
ff 02 00 00 00 00 00 00 00 00 00 00 00 00 00 1a
00 00 00 5c 00 00 00 3a 16 fe fd 17 fe fd 00 01
00 00 00 00 00 01 00 00
copy: 06 9b 01 7a 5f 00 f0
ref(9): 01 00 -> ref 11nnnkkk 0 7: c7
copy: 01 88
3 nulls: 81
copy: 04 20 02 0d b8
7 nulls: 85
ref(68): ff fe 00 -> ref 101nssss 0 8/11nnnkkk 1 1: a8 c9
copy: 08 fa ce 04 0e 00 14 09 ff
ref(39): 00 00 01 00 00 -> ref 101nssss 0 4/11nnnkkk 3 2: a4 da
5 nulls: 83
copy: 06 08 1e 80 20 ff ff
ref(2): ff ff -> ref 11nnnkkk 0 0: c0
ref(4): ff ff ff ff -> ref 11nnnkkk 2 0: d0
4 nulls: 82
ref(48): 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 fa ce
-> ref 101nssss 1 4/11nnnkkk 6 0: b4 f0
copy: 03 03 0e 40
ref(9): 00 ff -> ref 11nnnkkk 0 7: c7
ref(28): ff ff ff -> ref 101nssss 0 3/11nnnkkk 1 1: a3 c9
ref(24): 20 02 0d b8 00 00 00 00
-> ref 101nssss 0 2/11nnnkkk 6 0: a2 f0
Compressed:
06 9b 01 7a 5f 00 f0 c7 01 88 81 04 20 02 0d b8
85 a8 c9 08 fa ce 04 0e 00 14 09 ff a4 da 83 06
08 1e 80 20 ff ff c0 d0 82 b4 f0 03 03 0e 40 c7
a3 c9 a2 f0
Was 92 bytes; compressed to 52 bytes, compression factor 1.77
Figure 9: A longer RPL example
Similarly, Figure 10 shows an RPL DAO message. One of the embedded
addresses is copied right out of the pseudo-header, the other one is
effectively converted from global to local by providing the prefix
FE80 literally, inserting a number of nulls, and copying (some of)
Bormann Expires September 30, 2013 [Page 12]
Internet-Draft 6lowpan-ghc March 2013
the IID part again out of the pseudo-header. Note that a simple
implementation would probably emit fewer nulls and copy the entire
IID; there are multiple ways to encode this 50-byte payload into 27
bytes.
IP header:
60 00 00 00 00 32 3a ff 20 02 0d b8 00 00 00 00
00 00 00 ff fe 00 33 44 20 02 0d b8 00 00 00 00
00 00 00 ff fe 00 11 22
Payload:
9b 02 58 7d 01 80 00 f1 05 12 00 80 20 02 0d b8
00 00 00 00 00 00 00 ff fe 00 33 44 06 14 00 80
f1 00 fe 80 00 00 00 00 00 00 00 00 00 ff fe 00
11 22
Dictionary:
20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 33 44
20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 11 22
00 00 00 32 00 00 00 3a 16 fe fd 17 fe fd 00 01
00 00 00 00 00 01 00 00
copy: 0c 9b 02 58 7d 01 80 00 f1 05 12 00 80
ref(68): 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 33 44
-> ref 101nssss 1 6/11nnnkkk 6 4: b6 f4
copy: 08 06 14 00 80 f1 00 fe 80
9 nulls: 87
ref(74): ff fe 00 11 22 -> ref 101nssss 0 8/11nnnkkk 3 5: a8 dd
Compressed:
0c 9b 02 58 7d 01 80 00 f1 05 12 00 80 b6 f4 08
06 14 00 80 f1 00 fe 80 87 a8 dd
Was 50 bytes; compressed to 27 bytes, compression factor 1.85
Figure 10: An RPL DAO message
Figure 11 shows the effect of compressing a simple ND neighbor
solicitation.
IP header:
60 00 00 00 00 30 3a ff 20 02 0d b8 00 00 00 00
00 00 00 ff fe 00 3b d3 fe 80 00 00 00 00 00 00
02 1c da ff fe 00 30 23
Payload:
87 00 a7 68 00 00 00 00 fe 80 00 00 00 00 00 00
02 1c da ff fe 00 30 23 01 01 3b d3 00 00 00 00
1f 02 00 00 00 00 00 06 00 1c da ff fe 00 20 24
Dictionary:
20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 3b d3
fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
00 00 00 30 00 00 00 3a 16 fe fd 17 fe fd 00 01
00 00 00 00 00 01 00 00
Bormann Expires September 30, 2013 [Page 13]
Internet-Draft 6lowpan-ghc March 2013
copy: 04 87 00 a7 68
4 nulls: 82
ref(48): fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
-> ref 101nssss 1 4/11nnnkkk 6 0: b4 f0
copy: 04 01 01 3b d3
4 nulls: 82
copy: 02 1f 02
5 nulls: 83
copy: 02 06 00
ref(24): 1c da ff fe 00 -> ref 101nssss 0 2/11nnnkkk 3 3: a2 db
copy: 02 20 24
Compressed:
04 87 00 a7 68 82 b4 f0 04 01 01 3b d3 82 02 1f
02 83 02 06 00 a2 db 02 20 24
Was 48 bytes; compressed to 26 bytes, compression factor 1.85
Figure 11: An ND neighbor solicitation
Figure 12 shows the compression of an ND neighbor advertisement.
IP header:
60 00 00 00 00 30 3a fe fe 80 00 00 00 00 00 00
02 1c da ff fe 00 30 23 20 02 0d b8 00 00 00 00
00 00 00 ff fe 00 3b d3
Payload:
88 00 26 6c c0 00 00 00 fe 80 00 00 00 00 00 00
02 1c da ff fe 00 30 23 02 01 fa ce 00 00 00 00
1f 02 00 00 00 00 00 06 00 1c da ff fe 00 20 24
Dictionary:
fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 3b d3
00 00 00 30 00 00 00 3a 16 fe fd 17 fe fd 00 01
00 00 00 00 00 01 00 00
copy: 05 88 00 26 6c c0
3 nulls: 81
ref(64): fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
-> ref 101nssss 1 6/11nnnkkk 6 0: b6 f0
copy: 04 02 01 fa ce
4 nulls: 82
copy: 02 1f 02
5 nulls: 83
copy: 02 06 00
ref(24): 1c da ff fe 00 -> ref 101nssss 0 2/11nnnkkk 3 3: a2 db
copy: 02 20 24
Compressed:
05 88 00 26 6c c0 81 b6 f0 04 02 01 fa ce 82 02
1f 02 83 02 06 00 a2 db 02 20 24
Was 48 bytes; compressed to 27 bytes, compression factor 1.78
Bormann Expires September 30, 2013 [Page 14]
Internet-Draft 6lowpan-ghc March 2013
Figure 12: An ND neighbor advertisement
Figure 13 shows the compression of an ND router solicitation. Note
that the relatively good compression is not caused by the many zero
bytes in the link-layer address of this particular capture (which are
unlikely to occur in practice): 7 of these 8 bytes are copied from
the pseudo-header (the 8th byte cannot be copied as the universal/
local bit needs to be inverted).
IP header:
60 00 00 00 00 18 3a ff fe 80 00 00 00 00 00 00
ae de 48 00 00 00 00 01 ff 02 00 00 00 00 00 00
00 00 00 00 00 00 00 02
Payload:
85 00 90 65 00 00 00 00 01 02 ac de 48 00 00 00
00 01 00 00 00 00 00 00
Dictionary:
fe 80 00 00 00 00 00 00 ae de 48 00 00 00 00 01
ff 02 00 00 00 00 00 00 00 00 00 00 00 00 00 02
00 00 00 18 00 00 00 3a 16 fe fd 17 fe fd 00 01
00 00 00 00 00 01 00 00
copy: 04 85 00 90 65
ref(11): 00 00 00 00 01 -> ref 11nnnkkk 3 6: de
copy: 02 02 ac
ref(58): de 48 00 00 00 00 01
-> ref 101nssss 0 6/11nnnkkk 5 3: a6 eb
6 nulls: 84
Compressed:
04 85 00 90 65 de 02 02 ac a6 eb 84
Was 24 bytes; compressed to 12 bytes, compression factor 2.00
Figure 13: An ND router solicitation
Figure 14 shows the compression of an ND router advertisement. The
indefinite lifetime is compressed to four bytes by backreferencing;
this could be improved (at the cost of minor additional decompressor
complexity) by including some simple runlength mechanism.
IP header:
60 00 00 00 00 60 3a ff fe 80 00 00 00 00 00 00
10 34 00 ff fe 00 11 22 fe 80 00 00 00 00 00 00
ae de 48 00 00 00 00 01
Payload:
86 00 55 c9 40 00 0f a0 1c 5a 38 17 00 00 07 d0
01 01 11 22 00 00 00 00 03 04 40 40 ff ff ff ff
ff ff ff ff 00 00 00 00 20 02 0d b8 00 00 00 00
00 00 00 00 00 00 00 00 20 02 40 10 00 00 03 e8
20 02 0d b8 00 00 00 00 21 03 00 01 00 00 00 00
Bormann Expires September 30, 2013 [Page 15]
Internet-Draft 6lowpan-ghc March 2013
20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 11 22
Dictionary:
fe 80 00 00 00 00 00 00 10 34 00 ff fe 00 11 22
fe 80 00 00 00 00 00 00 ae de 48 00 00 00 00 01
00 00 00 60 00 00 00 3a 16 fe fd 17 fe fd 00 01
00 00 00 00 00 01 00 00
copy: 0c 86 00 55 c9 40 00 0f a0 1c 5a 38 17
2 nulls: 80
copy: 06 07 d0 01 01 11 22
4 nulls: 82
copy: 06 03 04 40 40 ff ff
ref(2): ff ff -> ref 11nnnkkk 0 0: c0
ref(4): ff ff ff ff -> ref 11nnnkkk 2 0: d0
4 nulls: 82
copy: 04 20 02 0d b8
12 nulls: 8a
copy: 04 20 02 40 10
ref(38): 00 00 03 -> ref 101nssss 0 4/11nnnkkk 1 3: a4 cb
copy: 01 e8
ref(24): 20 02 0d b8 00 00 00 00
-> ref 101nssss 0 2/11nnnkkk 6 0: a2 f0
copy: 02 21 03
ref(84): 00 01 00 00 00 00
-> ref 101nssss 0 9/11nnnkkk 4 6: a9 e6
ref(40): 20 02 0d b8 00 00 00 00 00 00 00
-> ref 101nssss 1 3/11nnnkkk 1 5: b3 cd
ref(136): ff fe 00 11 22
-> ref 101nssss 0 15/101nssss 0 1/11nnnkkk 3 3: af a1 db
Compressed:
0c 86 00 55 c9 40 00 0f a0 1c 5a 38 17 80 06 07
d0 01 01 11 22 82 06 03 04 40 40 ff ff c0 d0 82
04 20 02 0d b8 8a 04 20 02 40 10 a4 cb 01 e8 a2
f0 02 21 03 a9 e6 b3 cd af a1 db
Was 96 bytes; compressed to 59 bytes, compression factor 1.63
Figure 14: An ND router advertisement
Figure 15 shows the compression of a DTLS application data packet
with a net payload of 13 bytes of cleartext, and 8 bytes of
authenticator (note that the IP header is not relevant for this
example and has been set to 0). This makes good use of the static
dictionary, and is quite effective crunching out the redundancy in
the TLS_PSK_WITH_AES_128_CCM_8 header, leading to a net reduction by
15 bytes.
IP header:
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Bormann Expires September 30, 2013 [Page 16]
Internet-Draft 6lowpan-ghc March 2013
00 00 00 00 00 00 00 00
Payload:
17 fe fd 00 01 00 00 00 00 00 01 00 1d 00 01 00
00 00 00 00 01 09 b2 0e 82 c1 6e b6 96 c5 1f 36
8d 17 61 e2 b5 d4 22 d4 ed 2b
Dictionary:
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 2a 00 00 00 00 16 fe fd 17 fe fd 00 01
00 00 00 00 00 01 00 00
ref(13): 17 fe fd 00 01 00 00 00 00 00 01 00
-> ref 101nssss 1 0/11nnnkkk 2 1: b0 d1
copy: 01 1d
ref(10): 00 01 00 00 00 00 00 01 -> ref 11nnnkkk 6 2: f2
copy: 15 09 b2 0e 82 c1 6e b6 96 c5 1f 36 8d 17 61 e2
copy: b5 d4 22 d4 ed 2b
Compressed:
b0 d1 01 1d f2 15 09 b2 0e 82 c1 6e b6 96 c5 1f
36 8d 17 61 e2 b5 d4 22 d4 ed 2b
Was 42 bytes; compressed to 27 bytes, compression factor 1.56
Figure 15: A DTLS application data packet
Figure 16 shows that the compression is slightly worse in a
subsequent packet (containing 6 bytes of cleartext and 8 bytes of
authenticator, yielding a net compression of 13 bytes). The total
overhead does stay at a quite acceptable 8 bytes.
IP header:
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
Payload:
17 fe fd 00 01 00 00 00 00 00 05 00 16 00 01 00
00 00 00 00 05 ae a0 15 56 67 92 4d ff 8a 24 e4
cb 35 b9
Dictionary:
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 23 00 00 00 00 16 fe fd 17 fe fd 00 01
00 00 00 00 00 01 00 00
ref(13): 17 fe fd 00 01 00 00 00 00 00
-> ref 101nssss 1 0/11nnnkkk 0 3: b0 c3
copy: 03 05 00 16
ref(10): 00 01 00 00 00 00 00 05 -> ref 11nnnkkk 6 2: f2
copy: 0e ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9
Compressed:
b0 c3 03 05 00 16 f2 0e ae a0 15 56 67 92 4d ff
Bormann Expires September 30, 2013 [Page 17]
Internet-Draft 6lowpan-ghc March 2013
8a 24 e4 cb 35 b9
Was 35 bytes; compressed to 22 bytes, compression factor 1.59
Figure 16: Another DTLS application data packet
Figure 17 shows the compression of a DTLS handshake message, here a
client hello. There is little that can be compressed about the 32
bytes of randomness. Still, the net reduction is by 14 bytes.
IP header:
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
Payload:
16 fe fd 00 00 00 00 00 00 00 00 00 36 01 00 00
2a 00 00 00 00 00 00 00 2a fe fd 51 52 ed 79 a4
20 c9 62 56 11 47 c9 39 ee 6c c0 a4 fe c6 89 2f
32 26 9a 16 4e 31 7e 9f 20 92 92 00 00 00 02 c0
a8 01 00
Dictionary:
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 43 00 00 00 00 16 fe fd 17 fe fd 00 01
00 00 00 00 00 01 00 00
ref(16): 16 fe fd -> ref 101nssss 0 1/11nnnkkk 1 5: a1 cd
9 nulls: 87
copy: 01 36
ref(16): 01 00 00 -> ref 101nssss 0 1/11nnnkkk 1 5: a1 cd
copy: 01 2a
7 nulls: 85
copy: 23 2a fe fd 51 52 ed 79 a4 20 c9 62 56 11 47 c9
copy: 39 ee 6c c0 a4 fe c6 89 2f 32 26 9a 16 4e 31 7e
copy: 9f 20 92 92
3 nulls: 81
copy: 05 02 c0 a8 01 00
Compressed:
a1 cd 87 01 36 a1 cd 01 2a 85 23 2a fe fd 51 52
ed 79 a4 20 c9 62 56 11 47 c9 39 ee 6c c0 a4 fe
c6 89 2f 32 26 9a 16 4e 31 7e 9f 20 92 92 81 05
02 c0 a8 01 00
Was 67 bytes; compressed to 53 bytes, compression factor 1.26
Figure 17: A DTLS handshake packet (client hello)
Bormann Expires September 30, 2013 [Page 18]
Internet-Draft 6lowpan-ghc March 2013
Author's Address
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Bormann Expires September 30, 2013 [Page 19]
