6LoWPAN Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Standards Track September 06, 2012
Expires: March 10, 2013
6LoWPAN Generic Compression of Headers and Header-like Payloads
draft-bormann-6lowpan-ghc-05
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. The Header Compression Coupling Problem . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. 6LoWPAN-GHC . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Integrating 6LoWPAN-GHC into 6LoWPAN-HC . . . . . . . . . . . 6
3.1. Compressing payloads (UDP and ICMPv6) . . . . . . . . . . 6
3.2. Compressing extension headers . . . . . . . . . . . . . . 6
3.3. Indicating GHC capability . . . . . . . . . . . . . . . . 7
3.4. Using the 6CIO Option . . . . . . . . . . . . . . . . . . 8
4. IANA considerations . . . . . . . . . . . . . . . . . . . . . 10
5. Security considerations . . . . . . . . . . . . . . . . . . . 11
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. Normative References . . . . . . . . . . . . . . . . . . . 13
7.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 15
Appendix B. Things we probably won't do . . . . . . . . . . . . . 22
B.1. Context References . . . . . . . . . . . . . . . . . . . . 22
B.2. Nibblecode . . . . . . . . . . . . . . . . . . . . . . . . 22
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25
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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
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
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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.
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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 | Action | Argument |
| byte | | |
+----------+---------------------------------------------+----------+
| 0kkkkkkk | Append k = 0b0kkkkkkk bytes of data in the | k bytes |
| | 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 (possibly for Appendix B.1), and 1001nnnn (where 0b0000nnnn
> 0, possibly for Appendix B.2).
For the purposes of the backreferences, the expansion buffer is
initialized with the pseudo-header as defined in [RFC2460], at the
end of which the target buffer begins. These pseudo-header bytes are
therefore available for backreferencing, but not copied into the
final result.
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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 1, 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 1: 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 2).
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 |
+---+---+---+---+---+---+---+---+
Figure 2: Proposed NHC byte for ICMPv6 GHC (actual value to be
allocated by IANA)
3.2. Compressing extension headers
If the compression of specific extension headers is considered
desirable, this can be added in a similar way, e.g. as in Figure 3
(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
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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 -- this is only needed for extension headers, not for final
payloads.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | 0 | 1 | 1 | EID |NH |
+---+---+---+---+---+---+---+---+
Figure 3: Proposed NHC byte for extension header GHC
3.3. Indicating GHC capability
The 6LoWPAN baseline includes just [RFC4944], [RFC6282],
[I-D.ietf-6lowpan-nd] (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 [I-D.ietf-6lowpan-nd], we propose adding an
ND option 6LoWPAN Capability Indication (6CIO), as illustrated in
Figure 4.
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 4: 6LoWPAN Capability Indication Option (6CIO)
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
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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]. (Where context information is used in a
potential future version of this specification, as in Appendix B.1,
robustness may also be increased by making use of checksum error
indications that might point out errors in decompression.)
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 4 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.
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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.
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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 5.
10110IIN: Extension header GHC [RFCthis]
11010CPP: UDP GHC [RFCthis]
11011111: ICMPv6 GHC [RFCthis]
Figure 5: 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 6:
0..14: unassigned
15: GHC capable bit (G bit) [RFCthis]
16..47: unassigned
Figure 6
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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
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.
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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 and
Owen Kirby provided.
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.
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7. References
7.1. Normative References
[I-D.ietf-6lowpan-nd]
Shelby, Z., Chakrabarti, S., and E. Nordmark, "Neighbor
Discovery Optimization for Low Power and Lossy Networks
(6LoWPAN)", draft-ietf-6lowpan-nd-21 (work in progress),
August 2012.
[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.
[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.
7.2. Informative References
[I-D.bormann-6lowpan-roadmap]
Bormann, C., "6LoWPAN Roadmap and Implementation Guide",
draft-bormann-6lowpan-roadmap-01 (work in progress),
March 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.,
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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.
[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.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, August 2012.
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Appendix A. Examples
This section demonstrates some relatively realistic examples derived
from actual PCAP dumps taken at previous interops. Unfortunately,
for these dumps, no context information was available, so the
relatively powerful effect of context-based compression is not shown.
(TBD: Add a couple DHCP examples.)
Figure 7 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
Pseudoheader:
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
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 7: A simple RPL example
Figure 8 shows an RPL DODAG Information Object, a longer RPL control
message that is improved a bit more (but would likely benefit
additionally from a context reference). 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.
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IP header:
60 00 00 00 00 5c 3a ff 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
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
Pseudoheader:
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
copy: 09 9b 01 7a 5f 00 f0 01 00 88
3 nulls: 81
copy: 04 20 02 0d b8
7 nulls: 85
ref(52): ff fe 00 -> ref 101nssss 0 6/11nnnkkk 1 1: a6 c9
copy: 08 fa ce 04 0e 00 14 09 ff
2 nulls: 80
copy: 01 01
7 nulls: 85
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:
09 9b 01 7a 5f 00 f0 01 00 88 81 04 20 02 0d b8
85 a6 c9 08 fa ce 04 0e 00 14 09 ff 80 01 01 85
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 53 bytes, compression factor 1.74
Figure 8: A longer RPL example
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Similarly, Figure 9 shows an RPL DAO message. One of the embedded
addresses is copied right out of the pseudoheader, 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)
the IID part again out of the pseudoheader. 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
Pseudoheader:
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
copy: 0c 9b 02 58 7d 01 80 00 f1 05 12 00 80
ref(52): 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 33 44
-> ref 101nssss 1 4/11nnnkkk 6 4: b4 f4
copy: 08 06 14 00 80 f1 00 fe 80
9 nulls: 87
ref(58): ff fe 00 11 22 -> ref 101nssss 0 6/11nnnkkk 3 5: a6 dd
Compressed:
0c 9b 02 58 7d 01 80 00 f1 05 12 00 80 b4 f4 08
06 14 00 80 f1 00 fe 80 87 a6 dd
Was 50 bytes; compressed to 27 bytes, compression factor 1.85
Figure 9: An RPL DAO message
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Figure 10 shows the effect of compressing a simple ND neighbor
solicitation (again, no context-based compression).
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
Pseudoheader:
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
copy: 04 87 00 a7 68
4 nulls: 82
ref(32): fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
-> ref 101nssss 1 2/11nnnkkk 6 0: b2 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 b2 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 10: An ND neighbor solicitation
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Figure 11 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
Pseudoheader:
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
copy: 05 88 00 26 6c c0
3 nulls: 81
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 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 b4 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
Figure 11: An ND neighbor advertisement
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Figure 12 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
Pseudoheader:
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
copy: 04 85 00 90 65
ref(33): 00 00 00 00 01 -> ref 101nssss 0 3/11nnnkkk 3 4: a3 dc
copy: 02 02 ac
ref(42): de 48 00 00 00 00 01
-> ref 101nssss 0 4/11nnnkkk 5 3: a4 eb
6 nulls: 84
Compressed:
04 85 00 90 65 a3 dc 02 02 ac a4 eb 84
Was 24 bytes; compressed to 13 bytes, compression factor 1.85
Figure 12
Figure 13 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.
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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
20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 11 22
Pseudoheader:
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
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 -> ref 101nssss 0 9/11nnnkkk 3 7: a9 df
ref(40): 00 20 02 0d b8 00 00 00 00 00 00 00
-> ref 101nssss 1 3/11nnnkkk 2 4: b3 d4
ref(120): ff fe 00 11 22
-> ref 101nssss 0 14/11nnnkkk 3 3: ae 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 df b3 d4 ae db
Was 96 bytes; compressed to 58 bytes, compression factor 1.66
Figure 13: An ND router advertisement
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Appendix B. Things we probably won't do
This appendix documents parts of the proposal that so far have not
proven themselves sufficiently using real-life packets. They may
come back if they turn out to be useful; otherwise, they are to be
removed on the way to RFC.
B.1. Context References
A previous version of GHC also allowed the use of context references.
However, it appears that context references are more useful at the
IPv6/NHC level than here - contexts that are useful often already
have been unpacked into the pseudoheader, so they can be used by
backreferences. So none of the examples in Appendix A strongly need
this capability. Context references might be more useful if we find
good ways to populate the 6LoWPAN context with certain strings that
are likely to turn up in a certain LoWPAN.
+----------+---------------------------------------------+----------+
| code | Action | Argument |
| byte | | |
+----------+---------------------------------------------+----------+
| 0110iiii | Append all bytes (possibly filling an | |
| | incomplete byte with zero bits) from | |
| | Context i | |
| | | |
| 0111iiii | Append 8 bytes from Context i; i.e., the | |
| | context value truncated/zero-extended to 8 | |
| | bytes, and then append 0000 00FF FE00 | |
| | (i.e., 14 bytes total) | |
+----------+---------------------------------------------+----------+
B.2. Nibblecode
(It is to be decided whether the mechanism described in this section
is worth its additional complexity. To make this decision, it would
be useful to obtain more packet captures, in particular those that do
include ASCII data - the packet-capture-based examples in Appendix A
currently do not include nibblecode.)
Some headers/header-like structures, such as those used in CoAP or
DNS, may use ASCII data. There is very little redundancy by
repetition in these (DNS actually has its own compression mechanism
for repetition), so the backreferencing mechanism provided in the
bytecode is not very effective.
Efficient stateless compression for small amounts of ASCII data of
this kind is pretty much confined to Huffman (or, for even more
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complexity, arithmetic) coding. The complexity can be reduced
significantly by moving to n-ary Huffman coding, i.e., optimizing not
to the bit level, but to a larger level of granularity. Informal
experiments by the author show that a 16ary Huffman coding is close
to optimal at least for a small corpus of URI data. In other words,
basing the encoding on nibbles (4-bit half-bytes) is both nearly
optimal and relatively inexpensive to implement.
The actual letter frequencies that will occur in more general 6LoWPAN
ASCII data are hard to predict. As a first indication, the author
has analyzed an HTTP-based URI corpus and found the following lower
case letters to be the ASCII characters that occur with highest
frequency: aeinorst - it is therefore most useful to compress these.
In the encoding proposed, each byte representing one of these eight
highly-compressed characters is represented by a single 4-bit nibble
from the range 0x8 to 0xF. Bytes representing printable ASCII
characters, more specifically bytes from 0x20 to 0x7F, are
represented by both of their nibbles. Bytes from 0x00 to 0x1F and
from 0x80 to 0xFF are represented by a 0x1 nibble followed by both
nibbles of the byte. An 0x0 nibble terminates the nibblecode
sequence and returns to bytecode on the next byte boundary.
The first nibble of the nibblecode is transmitted right in the "enter
nibblecode" bytecode (0x9x - note that since it is never useful to
immediately return to bytecode, the bytecode 0x90 is allocated for a
different purpose). All other nibbles of the nibblecode are
transmitted as a sequence of bytes in most-significant-nibble-first
order; any unused nibble in the last byte of a nibblecode sequence is
set to 0x0.
The encoding is summarized in Figure 14.
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0 1
0 1 2 3 4 5 6 7 8 9 0 1
+---+---+---+---+
| 8-F | aeinorst
+---+---+---+---+ 89ABCDEF
+---+---+---+---+---+---+---+---+
| 2-7 | 0-F | other ASCII
+---+---+---+---+---+---+---+---+
+---+---+---+---+---+---+---+---+---+---+---+---+
| 1 | 0-F | 0-F | 0xHH
+---+---+---+---+---+---+---+---+---+---+---+---+
+---+---+---+---+
| 0 | return to bytecode
+---+---+---+---+
Figure 14: A nibble-based encoding
As an example for what level of compression can be expected, the 121
bytes of ASCII text shown in Figure 15 (taken from [RFC6690]) are
compressed into 183 nibbles of nibblecode, which (including delimiter
and padding overhead) need 93 bytes, resulting in a net compression
factor of 1.30. (Note that RFC 4944/6LoWPAN-HC supports compression
only in the first of a sequence of adaptation layer fragments; 93
bytes may not all fit into the first fragment, so any remaining
payload would be sent without the benefit of compression.)
<http://www.example.com/sensors/temp123>;anchor="/sensors/temp"
;rel=describedby,
</t>;anchor="/sensors/temp";rel=alternate
Figure 15: Example input text (line-wrapped)
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Author's Address
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Fax: +49-421-218-7000
Email: cabo@tzi.org
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