RFC2507 - IP Header Compression

Network Working Group M. Degermark
Request for Comments: 2507 Lulea University of Technology/SICS
Category: Standards Track B. Nordgren
Lulea University of Technology/Telia Research AB
S. Pink
Lulea University of Technology/SICS
February 1999
IP Header Compression
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
This document describes how to compress multiple IP headers and TCP
and UDP headers per hop over point to point links. The methods can be
applied to of IPv6 base and extension headers, IPv4 headers, TCP and
UDP headers, and encapsulated IPv6 and IPv4 headers.
Headers of typical UDP or TCP packets can be compressed down to 4-7
octets including the 2 octet UDP or TCP checksum. This largely
removes the negative impact of large IP headers and allows efficient
use of bandwidth on low and medium speed links.
The compression algorithms are specifically designed to work well
over links with nontrivial packet-loss rates. Several wireless and
modem technologies result in sUCh links.
TABLE OF CONTENTS
1. Introduction..............................................3
2. Terminology...............................................5
3. Compression method........................................7
3.1. Packet types.......................................8
3.2. Lost packets in TCP packet streams.................9
3.3. Lost packets in UDP and non-TCP packet streams....10
4. Grouping packets into packet streams.....................14
4.1. Guidelines for grouping packets...................15
5. Size Issues..............................................16
5.1. Context identifiers...............................16
5.2. Size of the context...............................17
5.3. Size of full headers..............................18
5.3.1. Length fields in full TCP headers............19
5.3.2. Length fields in full non-TCP headers........19
6. Compressed Header Formats................................20
7. Compression of subheaders................................22
7.1. IPv6 Header.......................................24
7.2. IPv6 Extension Headers............................25
7.3. Options...........................................25
7.4. Hop-by-hop Options Header.........................26
7.5. Routing Header....................................26
7.6. Fragment Header...................................27
7.7. Destination Options Header........................28
7.8. No Next Header....................................29
7.9. Authentication Header.............................29
7.10. Encapsulating Security Payload Header.............29
7.11. UDP Header........................................30
7.12. TCP Header........................................30
7.13. IPv4 Header.......................................33
7.14 Minimal Encapsulation header......................34
8. Changing context identifiers.............................35
9. Rules for dropping or temporarily storing packets........35
10. Low-loss header compression for TCP .....................36
10.1. The "twice" algorithm............................37
10.2. Header Requests..................................37
11. Links that reorder packets...............................38
11.1. Reordering in non-TCP packet streams.............39
11.2. Reordering in TCP packet streams.................39
12. Hooks for additional header compression..................40
13. Demultiplexing...........................................41
14. Configuration Parameters.................................42
15. Implementation Status....................................43
16. Acknowledgments..........................................44
17. Security Considerations..................................44
18. Authors" Addresses.......................................45
19. References...............................................46
20. Full Copyright Statement.................................47
1. Introduction
There are several reasons to do header compression on low- or
medium-speed links. Header compression can
* Improve interactive response time
For very low-speed links, echoing of characters may take longer
than 100-200 ms because of the time required to transmit large
headers. 100-200 ms is the maximum time people can tolerate
without feeling that the system is sluggish.
* Allow using small packets for bulk data with good line efficiency
This is important when interactive (for example Telnet) and bulk
traffic (for example FTP) is mixed because the bulk data should be
carried in small packets to decrease the waiting time when a
packet with interactive data is caught behind a bulk data packet.
Using small packet sizes for the FTP traffic in this case is a
global solution to a local problem. It will increase the load on
the network as it has to deal with many small packets. A better
solution might be to locally fragment the large packets over the
slow link.
* Allow using small packets for delay sensitive low data-rate traffic
For such applications, for example voice, the time to fill a
packet with data is significant if packets are large. To get low
end-to-end delay small packets are preferred. Without header
compression, the smallest possible IPv6/UDP headers (48 octets)
consume 19.2 kbit/s with a packet rate of 50 packets/s. 50
packets/s is equivalent to having 20 ms worth of voice samples in
each packet. IPv4/UDP headers consumes 11.2 kbit/s at 50
packets/s. Tunneling or routing headers, for example to support
mobility, will increase the bandwidth consumed by headers by 10-20
kbit/s. This should be compared with the bandwidth required for
the actual sound samples, for example 13 kbit/s with GSM encoding.
Header compression can reduce the bandwidth needed for headers
significantly, in the example to about 1.7 kbit/s. This enables
higher quality voice transmission over 14.4 and 28.8 kbit/s
modems.
* Decrease header overhead.
A common size of TCP segments for bulk transfers over medium-speed
links is 512 octets today. When TCP segments are tunneled, for
example because Mobile IP is used, the IPv6/IPv6/TCP header is 100
octets. Header compression will decrease the header overhead for
IPv6/TCP from 19.5 per cent to less than 1 per cent, and for
tunneled IPv4/TCP from 11.7 to less than 1 per cent. This is a
significant gain for line-speeds as high as a few Mbit/s.
The IPv6 specification prescribes path MTU discovery, so with IPv6
bulk TCP transfers should use segments larger than 512 octets when
possible. Still, with 1400 octet segments (RFC894 Ethernet
encapsulation allows 1500 octet payloads, of which 100 octets are
used for IP headers), header compression reduces IPv6 header
overhead from 7.1% to 0.4%.
* Reduce packet loss rate over lossy links.
Because fewer bits are sent per packet, the packet loss rate will
be lower for a given bit-error rate. This results in higher
throughput for TCP as the sending window can open up more between
losses, and in fewer lost packets for UDP.
The mechanisms described here are intended for a point-to-point link.
However, care has been taken to allow extensions for multi-Access
links and multicast.
Headers that can be compressed include TCP, UDP, IPv4, and IPv6 base
and extension headers. For TCP packets, the mechanisms of Van
Jacobson [RFC-1144] are used to recover from loss. Two additional
mechanisms that increase the efficiency of VJ header compression over
lossy links are also described. For non-TCP packets, compression
slow-start and periodic header refreshes allow minimal periods of
packet discard after loss of a header that changes the context. There
are hooks for adding header compression schemes on top of UDP, for
example compression of RTP headers.
Header compression relies on many fields being constant or changing
seldomly in consecutive packets belonging to the same packet stream.
Fields that do not change between packets need not be transmitted at
all. Fields that change often with small and/or predictable values,
e.g., TCP sequence numbers, can be encoded incrementally so that the
number of bits needed for these fields decrease significantly. Only
fields that change often and randomly, e.g., checksums or
authentication data, need to be transmitted in every header.
The general principle of header compression is to occasionally send a
packet with a full header; subsequent compressed headers refer to the
context established by the full header and may contain incremental
changes to the context.
This header compression scheme does not require that all packets in
the same stream passes over the compressed link. However, for TCP
streams the difference between subsequent headers can become more
irregular and the compression rate can decrease. Neither is it
required that corresponding TCP data and acknowledgment packets
traverse the link in opposite directions.
This header compression scheme is useful on first-hop or last-hop
links as well as links in the middle of the network. When many packet
streams (several hundred) traverse the link, a phenomenon that could
be called CID thrashing could occur, where headers seldom can be
matched with an existing context and have to be sent uncompressed or
as full headers. It is up to an implementation to use techniques such
as hysteresis to ensure that the packet streams that give the highest
compression rates keep their context. Such techniques are more
likely to be needed in the middle of the network.
2. Terminology
This section eXPlains some terms used in this document.
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 RFC2119.
Subheader
An IPv6 base header, an IPv6 extension header, an IPv4 header, a
UDP header, or a TCP header.
Header
A chain of subheaders.
Compress
The act of reducing the size of a header by removing header fields
or reducing the size of header fields. This is done in a way such
that a decompressor can reconstruct the header if its context
state is identical to the context state used when compressing the
header.
Decompress
The act of reconstructing a compressed header.
Context identifier (CID)
A small unique number identifying the context that should be used
to decompress a compressed header. Carried in full headers and
compressed headers.
Context
The state which the compressor uses to compress a header and the
decompressor uses to decompress a header. The context is the
uncompressed version of the last header sent (compressor) or
received (decompressor) over the link, except for fields in the
header that are included "as-is" in compressed headers or can be
inferred from, e.g., the size of the link-level frame.
The context for a packet stream is associated with a context
identifier. The context for non-TCP packet streams is also
associated with a generation.
Generation
For non-TCP packet streams, each new version of the context for a
given CID is associated with a generation: a small number that is
incremented whenever the context associated with that CID changes.
Carried by full and compressed non-TCP headers.
Packet stream
A sequence of packets whose headers are similar and share context.
For example, headers in a TCP packet stream have the same source
and final destination address, and the same port numbers in the
TCP header. Similarly, headers in a UDP packet stream have the
same source and destination address, and the same port numbers in
the UDP header.
Full header (header refresh)
An uncompressed header that updates or refreshes the context for a
packet stream. It carries a CID that will be used to identify the
context.
Full headers for non-TCP packet streams also carry the generation
of the context they update or refresh.
Regular header
A normal, uncompressed, header. Does not carry CID or generation
association.
Incorrect decompression
When a compressed and then decompressed header is different from
the uncompressed header. Usually due to mismatching context
between the compressor and decompressor or bit errors during
transmission of the compressed header.
Differential coding
A compression technique where the compressed value of a header
field is the difference between the current value of the field and
the value of the same field in the previous header belonging to
the same packet stream. A decompressor can thus oBTain the value
of the field by adding the value in the compressed header to its
context. This technique is used for TCP streams but not for non-
TCP streams.
3. Compression method
Much of the header information stays the same over the life-time of a
packet stream. For non-TCP packet streams almost all fields of the
headers are constant. For TCP many fields are constant and others
change with small and predictable values.
To initiate compression of the headers of a packet stream, a full
header carrying a context identifier, CID, is transmitted over the
link. The compressor and decompressor store most fields of this full
header as context. The context consists of the fields of the header
whose values are constant and thus need not be sent over the link at
all, or change little between consecutive headers so that it uses
fewer bits to send the difference from the previous value compared to
sending the absolute value.
Any change in fields that are expected to be constant in a packet
stream will cause the compressor to send a full header again to
update the context at the decompressor. As long as the context is the
same at compressor and decompressor, headers can be decompressed to
be exactly as they were before compression. However, if a full header
or compressed header is lost during transmission, the context of the
decompressor may become obsolete as it is not updated properly.
Compressed headers will then be decompressed incorrectly.
IPv6 is not meant to be used over links that can deliver a
significant fraction of damaged packets to the IPv6 module. This
means that links must have a very low bit-error rate or that link-
level frames must be protected by strong checksums, forward error
correction or something of that nature. Header compression SHOULD
not be used for IPv4 without strong link-level checksums. Damaged
frames will thus be discarded by the link layer. The link layer
implementation might indicate to the header compression module that a
frame was damaged, but it cannot say what packet stream it belonged
to as it might be the CID that is damaged. Moreover, frames may
disappear without the link layer implementation"s knowledge, for
example if the link is a multi-hop link where frames can be dropped
due to congestion at each hop. The kind of link errors that a header
compression module should deal with and protect against will thus be
packet loss.
So a header compression scheme needs mechanisms to update the context
at the decompressor and to detect or avoid incorrect decompression.
These mechanisms are very different for TCP and non-TCP streams, and
are described in sections 3.2 and 3.3.
The compression mechanisms in this document assume that packets are
not reordered between the compressor and decompressor. If the link
does reorder, section 11 describes mechanisms for ordering the
packets before decompression. It is also assumed that the link-layer
implementation can provide the length of packets, and that there is
no padding in UDP packets or tunneled packets.
3.1. Packet types
This compression method uses four packet types in addition to the
IPv4 and IPv6 packet types. The combination of link-level packet
type and the value of the first four bits of the packet uniquely
determines the packet type. Details on how these packet types are
represented are in section 13.
FULL_HEADER - indicates a packet with an uncompressed header,
including a CID and, if not a TCP packet, a generation. It
establishes or refreshes the context for the packet stream
identified by the CID.
COMPRESSED_NON_TCP - indicates a non-TCP packet with a compressed
header. The compressed header consists of a CID identifying what
context to use for decompression, a generation to detect an
inconsistent context and the randomly changing fields of the
header.
COMPRESSED_TCP - indicates a packet with a compressed TCP header,
containing a CID, a flag octet indentifying what fields have
changed, and the changed fields encoded as the difference from
the previous value.
COMPRESSED_TCP_NODELTA - indicates a packet with a compressed TCP
header where all fields that are normally sent as the difference
to the previous value are instead sent as-is. This packet type
is only sent as the response to a header request from the
decompressor. It must not be sent as the result of a
retransmission.
In addition to the packet types used for compression, regular IPv4
and IPv6 packets are used whenever a compressor decides to not
compress a packet. An additional packet type may be used to speed up
repair of TCP streams over links where the decompressor can send
packets to the compressor.
CONTEXT_STATE - indicates a special packet sent from the
decompressor to the compressor to communicate a list of (TCP)
CIDs for which synchronization has been lost. This packet is only
sent over a single link so it requires no IP header. The format
is shown in section 10.2.
3.2. Lost packets in TCP packet streams
Since TCP headers are compressed using the difference from the
previous TCP header, loss of a packet with a compressed or full
header will cause subsequent compressed headers to be decompressed
incorrectly because the context used for decompression was not
incremented properly.
Loss of a compressed TCP header will cause the TCP sequence numbers
of subsequently decompressed TCP headers to be off by k, where k is
the size of the lost segment. Such incorrectly decompressed TCP
headers will be discarded by the TCP receiver as the TCP checksum
reliably catches "off-by-k" errors in the sequence numbers for
plausible k.
TCP"s repair mechanisms will eventually retransmit the discarded
segment and the compressor peeks into the TCP headers to detect when
TCP retransmits. When this happens, the compressor sends a full
header on the assumption that the retransmission was due to
mismatching compression state at the decompressor. [RFC-1144] has a
good explanation of this mechanism.
The mechanisms of section 10 should be used to speed up the repair of
the context. This is important over medium speed links with high
packet loss rates, for example wireless. Losing a timeout"s worth of
packets due to inconsistent context after each packet lost over the
link is not acceptable, especially when the TCP connection is over
the wide area.
3.3. Lost packets in UDP and other non-TCP packet streams
Incorrectly decompressed headers of UDP packets and other non-TCP
packets are not so well-protected by checksums as TCP packets. There
are no sequence numbers that become "off-by-k" and virtually
guarantees a failed checksum as there are for TCP. The UDP checksum
only covers payload, UDP header, and pseudo header. The pseudo
header includes the source and destination addresses, the transport
protocol type and the length of the transport packet. Except for
those fields, large parts of the IPv6 header are not covered by the
UDP checksum. Moreover, other non-TCP headers lack checksums
altogether, for example fragments.
In order to safely avoid incorrect decompression of non-TCP headers,
each version of the context for non-TCP packet streams is identified
by a generation, a small number that is carried by the full headers
that establish and refresh the context. Compressed headers carry the
generation value of the context that were used to compress them.
When a decompressor sees that a compressed header carries a
generation value other than the generation of its context for that
packet stream, the context is not up to date and the packet must be
discarded or stored until a full header establishes correct context.
Differential coding is not used for non-TCP streams, so compressed
non-TCP headers do not change the context. Thus, loss of a
compressed header does not invalidate subsequent packets with
compressed headers. Moreover, the generation changes only when the
context of a full header is different from the context of the
previous full header. This means that losing a full header will make
the context of the decompressor obsolete only when the full header
would actually have changed the context.
The generation field is 6 bits long so the generation value repeats
itself after 64 changes to the context. To avoid incorrect
decompression after error bursts or other temporary disruptions, the
compressor must not reuse the same generation value after a shorter
time than MIN_WRAP seconds. A decompressor which has been
disconnected MIN_WRAP seconds or more must wait for the next full
header before decompressing. A compressor must wait at least MIN_WRAP
seconds after booting before compressing non-TCP headers. Instead of
reusing a generation value too soon, a compressor may switch to
another CID or send regular headers until MIN_WRAP seconds have
passed. The value of MIN_WRAP is found in section 14.
3.3.1. Compression Slow-Start
To allow the decompressor to recover quickly from loss of a full
header that would have changed the context, full headers are sent
periodically with an exponentially increasing period after a change
in the context. This technique avoids an exchange of messages between
compressor and decompressor used by other compression schemes, such
as in [RFC-1553]. Such exchanges can be costly for wireless mobiles
as more power is consumed by the transmitter and delay can be
introduced by switching between sending and receiving. Moreover,
techniques that require an exchange of messages cannot be used over
simplex links, such as direct-broadcast satellite channels or cable
TV systems, and are hard to adapt to multicast over multi-access
links.
.............................................................
^
Change Sent packets: with full header, . with compressed header
The picture shows how packets are sent after change. The compressor
keeps a variable for each non-TCP packet stream, F_PERIOD, that keeps
track of how many compressed headers may be sent between full
headers. When the headers of a non-TCP packet stream change so that
its context changes, a full header is sent and F_PERIOD is set to
one. After sending F_PERIOD compressed headers, a full header is
sent. F_PERIOD is doubled each time a full header is sent during
compression slow-start.
3.3.2. Periodic Header Refreshes
To avoid losing too many packets if a receiver has lost its context,
there is an upper limit, F_MAX_PERIOD, on the number of non-TCP
packets with compressed headers that may be sent between header
refreshes. If a packet is to be sent and F_MAX_PERIOD compressed
headers have been sent since the last full header for this packet
stream was sent, a full header must be sent.
To avoid long periods of disconnection for low data rate packet
streams, there is also an upper bound, F_MAX_TIME, on the time
between full headers in a non-TCP packet stream. If a packet is to be
sent and more than F_MAX_TIME seconds have passed since the last full
header was sent for this packet stream, a full header must be sent.
The values of F_MAX_PERIOD and F_MAX_TIME are found in section 14.
3.3.3. Rules for sending Full Headers
The following pseudo code can be used by the compressor to determine
when to send a full header for a non-TCP packet stream. The code
maintains two variables:
C_NUM -- a count of the number of compressed headers sent
since the last full header was sent.
F_LAST -- the time of sending the last full header.
and uses the functions
current_time() return the current time
min(a,b) return the smallest of a and b
the procedures send_full_header(), increment_generation_value(),
and send_compressed_header()
do the obvious thing.
if ( <this header changes the context> )
C_NUM := 0;
F_LAST := current_time();
F_PERIOD := 1;
increment_generation_value();
send_full_header();
elseif ( C_NUM >= F_PERIOD )
C_NUM := 0;
F_LAST := current_time();
F_PERIOD := min(2 * F_PERIOD, F_MAX_PERIOD);
send_full_header();
elseif ( current_time() > F_LAST + F_MAX_TIME )
C_NUM := 0;
F_LAST := current_time();
send_full_header();
else
C_NUM := C_NUM + 1
send_compressed_header();
endif
3.3.4. Cost of sending Header Refreshes
If every f"th packet carries a full header, H is the size of a full
header, and C is the size of a compressed header, the average header
size is
(H-C)/f + C
For f > 1, the average header size is (H-C)/f larger than a
compressed header.
In a diagram where the average header size is plotted for various f
values, there is a distinct knee in the curve, i.e., there is a limit
beyond which further increasing f gives diminishing returns.
F_MAX_PERIOD should be chosen to be a frequency well to the right of
the knee of the curve. For typical sizes of H and C, say 48 octets
for the full header (IPv6/UDP) and 4 octets for the compressed
header, setting F_MAX_PERIOD > 44 means that full headers will
contribute less than an octet to the average header size. With a
four-address routing header, F_MAX_PERIOD > 115 will have the same
effect.
The default F_MAX_PERIOD value of 256 (section 14) puts the full
header frequency well to the right of the knee and means that full
headers will typically contribute considerably less than an octet to
the average header size. For H = 48 and C = 4, full headers
contribute about 1.4 bits to the average header size after reaching
the steady-state header refresh frequency determined by the default
F_MAX_PERIOD. 1.4 bits is a very small overhead.
After a change in the context, the exponential bacKOFf scheme will
initially send full headers frequently. The default F_MAX_PERIOD
will be reached after nine full headers and 255 compressed headers
have been sent. This is equivalent to a little over 5 seconds for a
typical voice stream with 20 ms worth of voice samples per packet.
During the whole backoff period, full headers contribute 1.5 octets
to the average header size when H = 48 and C = 4. For 20 ms voice
samples, it takes less than 1.3 seconds until full headers contribute
less than one octet to the average header size, and during these
initial 1.3 seconds full headers add less than 4 octets to the
average header size. The cost of the exponential backoff is not
great and as the headers of non-TCP packet streams are expected to
change seldomly, it will be amortized over a long time.
The cost of header refreshes in terms of bandwidth are higher than
similar costs for hard state schemes like [RFC-1553] where full
headers must be acknowledged by the decompressor before compressed
headers may be sent. Such schemes typically send one full header plus
a few control messages when the context changes. Hard state schemes
require more types of protocol messages and an exchange of messages
is necessary. Hard state schemes also need to deal explicitly with
various error conditions that soft state handles automatically, for
instance the case of one party disappearing unexpectedly, a common
situation on wireless links where mobiles may go out of range of the
base station.
The major advantage of the soft state scheme is that no handshakes
are needed between compressor and decompressor, so the scheme can be
used over simplex links. The costs in terms of bandwidth are higher
than for hard state schemes, but the simplicity of the decompressor,
the simplicity of the protocol, and the lack of handshakes between
compressor and decompressor justifies this small cost. Moreover, soft
state schemes are more easily extended to multicast over multi-access
links, for example radio links.
4. Grouping packets into packet streams
This section explains how packets MAY be grouped together into packet
streams for compression. To achieve the best compression rates,
packets SHOULD be grouped together such that packets in the same
packet stream have similar headers. If this grouping fails, header
compression performance will be bad, since the compression algorithm
can rarely utilize the existing context for the packet stream and
full headers must be sent frequently.
Grouping is done by the compressor. A compressor may use whatever
criterion it finds appropriate to group packets into packet streams.
To determine what packet stream a packet belongs to, a compressor MAY
a) examine the compressible chain of subheaders (see section 7),
b) examine the contents of an upper layer protocol header that
follows the compressible chain of subheaders, for example ICMP
headers, DVMRP headers, or tunneled IPX headers,
c) use information obtained from a resource manager, for example if a
resource manager requests compression for a particular packet
stream and provides a way to identify packets belonging to that
packet stream,
d) use any other relevant information, for example if routes flap and
the hop limit (TTL) field in a packet stream changes frequently
between n and n+k, a compressor may choose to group the packets
into two different packet streams.
A compressor is also free not to group packets into packet streams
for compression, letting some packets keep their regular headers and
passing them through unmodified.
As long as the rules for when to send full headers for a non-TCP
packet stream are followed and subheaders are compressed as specified
in this document, the decompressor is able to reconstruct a
compressed header correctly regardless of how packets are grouped
into packet streams.
4.1 Guidelines for grouping packets
In this section we give OPTIONAL guidelines for how a compressor may
group packets into packet streams for compression.
Defining fields
The defining fields of a header should be present and identical in
all packets belonging to the same packet stream. These fields are
marked DEF in section 7. The defining fields include the flow
label, source and destination addresses of IP headers, final
destination address in routing headers, the next header fields
(for IPv6), the protocol field (IPv4), port numbers (UDP and TCP),
and the SPI in authentication and encryption headers.
Fragmented packets
Fragmented and unfragmented packets should never be grouped
together in the same packet stream. The Identification field of
the Fragment header or IPv4 header should not be used to identify
the packet stream. If it was, the first fragment of a new packet
would cause a compression slow-start.
No field after a Fragment Header, or an IPv4 header for a
fragment, should be used for grouping purposes.
Upper protocol identification
The first next header field identifying a header not described in
section 7 should be used for identifying packet streams, i.e., all
packets with the same DEF fields and the same upper protocol
should be grouped together.
TTL field (Hop Limit field)
A sophisticated implementation might monitor the TTL (Hop Limit)
field and if it changes frequently use it as a DEF field. This can
occur when there are frequent route flaps so that packets traverse
different paths through the internet.
Traffic Class field (IPv6), Type of Service field (IPv4)
It is possible that the Traffic Class field of the IPv6 header and
the Type of Service of the IPv4 header will change frequently
between packets with otherwise identical DEF fields. A
sophisticated implementation should watch out for this and be
prepared to use these fields as defining fields.
When IP packets are tunneled they are encapsulated with an additional
IP header at the tunnel entry point and then sent to the tunnel
endpoint. To group such packets into packet streams, the inner
headers should also be examined to determine the packet stream. If
this is not done, full headers will be sent each time the headers of
the inner IP packet changes. So when a packet is tunneled, the
identifying fields of the inner subheaders should be considered in
addition to the identifying fields of the initial IP header.
An implementation can use other fields for identification than the
ones described here. If too many fields are used for identification,
performance might suffer because more CIDs will be used and the wrong
CIDs might be reused when new flows need CIDs. If too few fields are
used for identification, performance might suffer because there are
too frequent changes to the context.
We stress that these guidelines are educated guesses. When IPv6 is
widely deployed and IPv6 traffic can be analyzed, we might find that
other grouping algorithms perform better. We also stress that if the
grouping fails, the result will be bad performance but not incorrect
decompression. The decompressor can do its task regardless of how the
grouping algorithm works.
5. Size Issues
5.1. Context Identifiers
Context identifiers can be 8 or 16 bits long. Their size is not
relevant for finding the context. An 8-bit CID with value two and a
16-bit CID with value two are equivalent.
The CID spaces for TCP and non-TCP are separate, so a TCP CID and a
non-TCP CID never identify the same context. Even if they have the
same value. This doubles the available CID space while using the same
number of bits for CIDs. It is always possible to tell whether a
full or compressed header is for a TCP or non-TCP packet, so no
mixups can occur.
Non-TCP compressed headers encode the size of the CID using one bit
in the second octet of the compressed header. The 8-bit CID allows a
minimum compressed header size of 2 octets for non-TCP packets, the
CID uses the first octet and the size bit and the 6-bit Generation
value fit in the second octet.
For TCP the only available CID size is 8 bits as in [RFC-1144]. 8
bits is probably sufficient as TCP connections are always point-to-
point.
The 16 bit CID size may not be needed for point-to-point links; it is
intended for use on multi-access links where a larger CID space may
be needed for efficient selection of CIDs.
The major difficulty with multi-access links is that several
compressors share the CID space of a decompressor. CIDs can no
longer be selected independently by the compressors as collisions may
occur. This problem may be resolved by letting the decompressors
have a separate CID space for each compressor. Having separate CID
spaces requires that decompressors can identify which compressor sent
the compressed packet, perhaps by utilizing link-layer information as
to who sent the link-layer frame. If such information is not
available, all compressors on the multi-access link may be
enumerated, automatically or otherwise, and supply their number as
part of the CID. This latter method requires a large CID space.
5.2. Size of the context
The size of the context SHOULD be limited to simplify implementation
of compressor and decompressor, and put a limit on their memory
requirements. However, there is no upper limit on the size of an
IPv6 header as the chain of extension headers can be arbitrarily
long. This is a problem as the context is essentially a stored
header.
The configurable parameter MAX_HEADER (see section 14) represents the
maximum size of the context, expressed as the maximum sized header
that can be stored as context. When a header is larger than
MAX_HEADER, only part of it is stored as context. An implementation
MUST NOT compress more than the initial MAX_HEADER octets of a
header. An implementation MUST NOT partially compress a subheader.
Thus, the part of the header that is stored as context and is
compressed is the longest initial sequence of entire subheaders that
is not larger than MAX_HEADER octets.
5.3. Size of full headers
It is desirable to avoid increasing the size of packets with full
headers beyond their original size, as their size may be optimized
for the MTU of the link. Since we assume that the link layer
implementation provides the length of packets, we can use the length
fields in full headers to pass the values of the CID and the
generation to the decompressor.
This requires that the link-layer must not add padding to the
payload, at least not padding that can be delivered to the
destination link user. It is also required that no extra padding is
added after UDP data or in tunneled packets. This allows values of
length fields to be calculated from the length of headers and the
length of the link-layer frame.
The generation requires one octet and the CID may require up to 2
octets. There are length fields of 2 octets in the IPv6 Base Header,
the IPv4 header, and the UDP header.
A full TCP header will thus have at least 2 octets available in the
IP header to pass the 8 bit CID, which is sufficient. There will be
more than two octets available if there is more than one IP header.
[RFC-1144] uses the 8 bit Protocol field of the IPv4 header to pass
the CID. We cannot use the corresponding method as the sequence of
IPv6 extension headers is not fixed and CID values are not disjoint
from the legal values of Next Header fields.
An IPv6/UDP or IPv4/UDP packet will have 4 octets available to pass
the generation and the CID, so all CID sizes may be used. Fragmented
or encrypted packet streams may have only 2 octets available to pass
the generation and CID. Thus, 8-bit CIDs may be the only CID sizes
that can be used for such packet streams. When IPv6/IPv4 or
IPv4/IPv6 tunneling is used, there will be at least 4 octets
available, and both CID sizes may be used.
The generation value is passed in the higher order octet of the first
length field in the full header. When only one length field is
available, the 8-bit CID is passed in the low order octet. When two
length fields are available, the lowest two octets of the CID are
passed in the second length field and the low order octet of the
first length field carries the highest octet of the CID.
5.3.1. Use of length fields in full TCP headers
Use of first length field:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Length field LSB of pkt nr CID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Use of second length field if available:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Second length field MSB of pkt nr 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Pkt nr is short for packet sequence number, described in section
11.2.
5.3.2. Use of length fields in full non-TCP headers
Full non-TCP headers with 8-bit CID:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
First length field 0D Generation CID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Second length field (if avail.) 0 Data (if D=1)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Full non-TCP headers with 16-bit CID:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
First length field 1D Generation Data (if D=1)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Second length field CID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first bit in the first length field indicates the length of the
CID. The Data field is zero if D is zero. The use of the D bit and
Data field is explained in section 12.
6. Compressed Header Formats
This section uses some terminology (DELTA, RANDOM) defined in section
7.
a) COMPRESSED_TCP format (similar to [RFC1144]):
+-+-+-+-+-+-+-+-+
CID
+-+-+-+-+-+-+-+-+
R O I P S A W U
+-+-+-+-+-+-+-+-+

+ TCP Checksum +

+-+-+-+-+-+-+-+-+
RANDOM fields, if any (see section 7) (implied)
- - - - - - - -
R-octet (if R=1)
- - - - - - - -
Urgent Pointer Value (if U=1)
- - - - - - - -
Window Delta (if W=1)
- - - - - - - -
Acknowledgment Number Delta (if A=1)
- - - - - - - -
Sequence Number Delta (if S=1)
- - - - - - - -
IPv4 Identification Delta (if I=1)
- - - - - - - -
Options (if O=1)
- - - - - - - -
The latter flags in the second octet (IPSAWU) have the same meaning
as in [RFC-1144], regardless of whether the TCP segments are carried
by IPv6 or IPv4. The C bit has been eliminated because the CID is
always present. The context associated with the CID keeps track of
the IP version and what RANDOM fields are present. The order between
delta fields specified here is exactly as in [RFC-1144]. An
implementation will typically scan the context from the beginning and
insert the RANDOM fields in order. The RANDOM fields are thus placed
before the DELTA fields of the TCP header in the same order as they
occur in the original uncompressed header.
The I flag is zero unless an IPv4 header immediately precedes the TCP
header. The combined IPv4/TCP header is then compressed as a unit as
described in [RFC-1144]. Identification fields in IPv4 headers that
are not immediately followed by a TCP header are RANDOM.
If the O flag is set, the Options of the TCP header were not the same
as in the previous header. The entire Option field are placed last in
the compressed TCP header.
If the R flag is set, there were differences between the context and
the Reserved field (6 bits) in the TCP header or bit 6 or 7 of the
TOS octet (Traffic Class octet) in a IPv4 header (IPv6 header) that
immediately precedes the TCP header. An octet with the actual values
of the Reserved field and bit 6 and 7 of the TOS or Traffic Class
field is then placed immediately after the RANDOM fields. Bits 0-5
of the passed octet is the actual value of the Reserved field, and
bits 6 and 7 are the actual values of bits 6 and 7 in the TOS or
Traffic Class field. If there is no preceding IP header, bits 6 and 7
are 0. The octet passed with the R flag MUST NOT update the context.
NOTE: The R-octet does not update the context because if it did, the
nTCP checksum would not guard the receiving TCP from erroneously
decompressed headers. Bits 6 and 7 of the TOS octet or Traffic Class
octet is expected to change frequently due to Explicit Congestion
Notification.
See section 7.12 and [RFC-1144] for further information on how to
compress TCP headers.
b) COMPRESSED_TCP_NODELTA header format
+-+-+-+-+-+-+-+-+
CID
+-+-+-+-+-+-+-+-+
RANDOM fields, if any (see section 7) (implied)
+-+-+-+-+-+-+-+-+
Whole TCP header except for Port Numbers
+-+-+-+-+-+-+-+-+
c) Compressed non-TCP header, 8 bit CID:
0 7
+-+-+-+-+-+-+-+-+
CID
+-+-+-+-+-+-+-+-+
0D Generation
+-+-+-+-+-+-+-+-+
data (if D=1)
- - - - - - - -
RANDOM fields, if any (section 7) (implied)
- - - - - - - -
d) Compressed non-TCP header, 16 bit CID:
0 7
+-+-+-+-+-+-+-+-+
msb of CID
+-+-+-+-+-+-+-+-+
1D Generation
+-+-+-+-+-+-+-+-+
lsb of CID
+-+-+-+-+-+-+-+-+
data (if D=1)
- - - - - - - -
RANDOM fields, if any (section 7) (implied)
- - - - - - - -
The generation, CID and optional one octet data are followed by
relevant RANDOM fields (see section 7) as implied by the compression
state, placed in the same order as they occur in the original
uncompressed header, followed by the payload.
7. Compression of subheaders
This section gives rules for how the compressible chain of subheaders
is compressed. These rules MUST be followed. Subheaders that may be
compressed include IPv6 base and extension headers, TCP headers, UDP
headers, and IPv4 headers. The compressible chain of subheaders
extends from the beginning of the header
a) up to but not including the first header that is not an IPv4
header, an IPv6 base or extension header, a TCP header, or a UDP
header, or
b) up to and including the first TCP header, UDP header, Fragment
Header, Encapsulating Security Payload Header, or IPv4 header for
a fragment,
whichever gives the shorter chain. For example, rules a) and b) both
fit a chain of subheaders that contain a Fragment Header and ends at
a tunneled IPX packet. Since rule b) gives a shorter chain, the
compressible chain of subheaders stops at the Fragment Header.
The following subsections are a systematic classification of how all
fields in subheaders are expected to change.
NOCHANGE The field is not expected to change. Any change means
that a full header MUST be sent to update the context.
DELTA The field may change often but usually the difference
from the field in the previous header is small, so that
it is cheaper to send the change from the previous value
rather than the current value. This type of compression
is only used for TCP packet streams.
RANDOM The field must be included "as-is" in compressed headers,
usually because it changes unpredictably.
INFERRED The field contains a value that can be inferred from
other values, for example the size of the frame carrying
the packet, and thus must not be included in the
compressed header.
The classification implies how a compressed header is constructed. No
field that is NOCHANGE or INFERRED is present in a compressed header.
A compressor obtains the values of NOCHANGE fields from the context
identified by the compression identifier, and obtains the values of
INFERRED fields from the link-layer implementation, e.g., from the
size of the link-layer frame, or from other fields, e.g., by
recalculating the IPv4 header checksum. DELTA fields are encoded as
the difference to the value in the previous packet in the same packet
stream. The decompressor must update the context by adding the value
in the compressed header to the value in its context. The result is
the proper value of the field. RANDOM fields must be sent "as-is" in
the compressed header. RANDOM fields must occur in the same order in
the compressed header as they occur in the full header.
Fields that may optionally be used to identify what packet stream a
packet belongs to according to section 4.1 are marked with the word
DEF. To a compressor using the optional guidelines from section 4.1,
any difference in corresponding DEF fields between two packets
implies that they belong to different packet streams. Moreover, if a
DEF field is present in one packet but not in another, the packets
belong to different packet streams.
7.1. IPv6 Header [IPv6, section 3]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version Traffic Class Flow Label
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Payload Length Next Header Hop Limit
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ +

+ Source Address +

+ +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ +

+ Destination Address +

+ +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version NOCHANGE (DEF)
Traffic Class NOCHANGE (might be DEF, see sect 4.1)
(see also sect 6 a)
Flow Label NOCHANGE (DEF)
Payload Length INFERRED
Next Header NOCHANGE
Hop Limit NOCHANGE (might be DEF, see sect 4.1)
Source Address NOCHANGE (DEF)
Destination Address NOCHANGE (DEF)
The Payload Length field of encapsulated headers must correspond to
the length value of the encapsulating header. If not, the header
chain MUST NOT be compressed.
NOTE: If this the IP header closest to a TCP header, bit 7 of the
Traffic Class field can be passed using the R-flag of the compressed
TCP header. See section 6 a).
This classification implies that the entire IPv6 base header will be
compressed away.
7.2. IPv6 Extension Headers [IPv6, section 4]
What extension headers are present and the relative order of them is
not expected to change in a packet stream. Whenever there is a
change, a full packet header must be sent. All Next Header fields in
IPv6 base header and IPv6 extension headers are NOCHANGE.
7.3. Options [IPv6, section 4.2]
The contents of Hop-by-hop Options and Destination Options extension
headers are encoded with TLV "options" (see [IPv6]):
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
Option Type Opt Data Len Option Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
Option Type and Opt Data Len fields are assumed to be fixed for a
given packet stream, so they are classified as NOCHANGE. The Option
data is RANDOM unless specified otherwise below.
Padding
Pad1 option
+-+-+-+-+-+-+-+-+
0
+-+-+-+-+-+-+-+-+
Entire option is NOCHANGE.
PadN option
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
1 Opt Data Len Option Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
All fields are NOCHANGE.
7.4. Hop-by-Hop Options Header [IPv6, section 4.3]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Hdr Ext Len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +

. .
. Options .
. .

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header NOCHANGE
Hdr Ext Len NOCHANGE
Options TLV coded values and padding.
Classified according to 7.3 above, unless
being a Jumbo Payload option (see below).
Jumbo Payload option
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
194 Opt Data Len=4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Jumbo Payload Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
First two fields are NOCHANGE and Jumbo Payload Length INFERRED.
(frame length must be supplied by link layer implementation).
NOTE: It is silly to compress the headers of a packet carrying a
Jumbo Payload Option since the relative header overhead is
negligible. Moreover, it is usually a bad idea to send such
large packets over low- and medium-speed links.
7.5. Routing Header [IPv6, section 4.4]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Hdr Ext Len Routing Type Segments Left
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

. .
. type-specific data .
. .

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
All fields of the Routing Header are NOCHANGE.
If the Routing Type is not recognized, it is impossible to determine
the final Destination Address unless the Segments Left field has the
value zero, in which case the Destination Address is the final
Destination Address in the basic IPv6 header.
In the Type 0 Routing Header, the last address is DEF if (Segments
Left > 0).
Routing Headers are compressed away completely. This is a big win as
the maximum size of the Routing Header is 392 octets. Moreover, Type
0 Routing Headers with one address, size 24 octets, are used by
Mobile IP.
7.6. Fragment Header [IPv6, section 4.5]
The first fragment of a packet has Fragment Offset = 0 and the chain
of subheaders extends beyond its Fragment Header. If a fragment is
not the first (Fragment Offset not 0), there are no subsequent
subheaders (unless the chain of subheaders in the first fragment
didn"t fit entirely in the first fragment).
Since packets may be reordered before reaching the compression point,
and some fragments may follow other routes through the network, a
compressor cannot rely on seeing the first fragment before other
fragments. This implies that information in subheaders following the
Fragment Header of the first fragment cannot be examined to determine
the proper packet stream for other fragments.
It is possible to design compression schemes that can compress
subheaders after the Fragment Header, at least in the first fragment,
but to avoid complicating the rules for sending full headers and the
rules for compression and decompression, the chain of subheaders that
follow a Fragment Header MUST NOT be compressed.
The fields of the Fragment Header are classified as follows.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Reserved Fragment Offset ResM
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Identification
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header NOCHANGE
Reserved NOCHANGE
Res RANDOM
M flag RANDOM
Fragment Offset RANDOM
Identification RANDOM
This classification implies that a Fragment Header is compressed down
to 6 octets. The minimum IPv6 MTU is 1280 octets so most fragments
will be at least 1280 octets. Since the 6 octet overhead of the
compressed fragment header is amortized over a fairly large packet,
the additional complexity of more sophisticated compression schemes
is not justifiable.
NOTE: The Identification field is RANDOM instead of NOCHANGE
to avoid one compression slow-start per original packet.
Grouping of fragments according to the optional guidelines in
section4.1:
Fragments and unfragmented packets should not be grouped
together.
Port numbers cannot be used to identify the packet stream because
port numbers are not present in every fragment. To adhere to the
uniqueness rules for the Identification value, a fragmented
packet stream is identified by the combination of Source Address
and (final) Destination Address.
NOTE: The Identification value is NOT used to identify the
packet stream. This avoids using a new CID for each packet and
saves the cost of the associated compression slow-start. We
expect that the unfragmentable part of the headers will not
change too frequently, if it does thrashing may occur.
7.7. Destination Options Header [IPv6, section 4.6]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Hdr Ext Len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +

. .
. Options .
. .

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header NOCHANGE
Hdr Ext Len NOCHANGE
Options TLV coded values and padding.
Compressed according to 7.3 above.
The only Destination Options defined in [IPv6] are the padding
options.
7.8. No Next Header [IPv6, section 4.7]
Covered by rules for IPv6 Header Extensions (7.2).
7.9. Authentication Header [RFC-2402, section 3.2]
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
+---------------+---------------+---------------+---------------+
Next Header Length RESERVED
+---------------+---------------+---------------+---------------+
Security Parameters Index (SPI)
+---------------+---------------+---------------+---------------+

+ Authentication Data (variable number of 32-bit words)

+---------------+---------------+---------------+---------------+
Next Header NOCHANGE
Length NOCHANGE
Reserved NOCHANGE
SPI NOCHANGE (DEF)
Authentication Data RANDOM
[RFC-1828] specifies how to do authentication with keyed MD5, the
authentication method all IPv6 implementations must support. For
this method, the Authentication Data is 16 octets.
7.10. Encapsulating Security Payload Header [RFC-2406, section 3.1]
This header implies that the subsequent parts of the packet are
encrypted. Thus, no further header compression is possible on
subsequent headers as encryption is typically already performed when
the compressor sees the packet.
However, when the ESP Header is used in tunnel mode an entire IP
packet is encrypted, and the headers of that packet MAY be compressed
before the packet is encrypted at the entry point of the tunnel.
This means that it must be possible to feed an IP packet and its
length to the decompressor, as if it came from the link-layer. The
mechanisms for dealing with reordering described in section 11 MUST
also be used, as packets can be reordered in a tunnel.
+---------------+---------------+---------------+---------------+
Security Association Identifier (SPI), 32 bits
+===============+===============+===============+===============+
Opaque Transform Data, variable length
+---------------+---------------+---------------+---------------+
SPI NOCHANGE (DEF)
Opaque Transform Data RANDOM
Everything after the SPI is encrypted and is not compressed.
7.11. UDP Header
The UDP header is described in [RFC-768].
The Next Header field (IPv6) or Protocol field (IPv4) in the
preceding subheader is DEF.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source Port Destination Port
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Length Checksum
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source Port NOCHANGE (DEF)
Destination Port NOCHANGE (DEF)
Length INFERRED
Checksum RANDOM, unless it is zero,
in which case it is NOCHANGE.
The Length field of the UDP header MUST match the Length field(s) of
preceding subheaders, i.e, there must not be any padding after the
UDP payload that is covered by the IP Length.
The UDP header is typically compressed down to 2 octets, the UDP
checksum. When the UDP checksum is zero (which it cannot be with
IPv6), it is likely to be so for all packets in the flow and is
defined to be NOCHANGE. This saves 2 octets in the compressed header.
7.12. TCP Header
The TCP header is described in [RFC-793].
The Next Header field (IPv6) or Protocol field (IPv4) in the
preceding subheader is DEF.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source Port Destination Port
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sequence Number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Acknowledgment Number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Offset Reserved UAPRSF Window
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum Urgent Pointer
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Options Padding
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
U, A, P, R, S, and F stands for Urg, Ack, Psh, Rst, Syn, and Fin.
There are two ways to compress the TCP header.
7.12.1. Compressed with differential encoding
Source Port NOCHANGE (DEF)
Destination Port NOCHANGE (DEF)
Sequence Number DELTA
Acknowledgment Number DELTA
Offset NOCHANGE
Reserved DELTA (if differs from context,
set R-flag in flag octet
and send absolute value
as described in 6 a.)
Urg,Psh RANDOM (placed in flag octet)
Ack INFERRED to be 1
Rst,Syn,Fin INFERRED to be 0
Window DELTA (if change in Window,
set W-flag in flag octet
and send difference)
Checksum RANDOM
Urgent Pointer DELTA (if Urg is set, send
absolute value)
Options, Padding DELTA (if change in Options,
set O-flag and send
whole Options, Padding)
A packet with a TCP header compressed according to the above must be
indicated to be of type COMPRESSED_TCP. The compressed header is
described in section 6.
This method is essentially the differential encoding techniques of
Jacobson, described in [RFC-1144], the differences being the placement
of the compressed TCP header fields (see section 6), the use of the
O-flag, the use of the R-flag, and elimination of the C-flag. The
O-flag allows compression of the TCP header when the Timestamp option
is used and the Options fields changes with each header.
DELTA values (except for Reserved field and Options, Padding) MUST be
coded as in [RFC-1144]. A Reserved field value passed with the R-flag
MUST NOT update the context at compressor or decompressor.
7.12.2. Without differential encoding
Source Port NOCHANGE (DEF)
Destination Port NOCHANGE (DEF)
(all the rest) RANDOM
The Identification field in a preceding IPv4 header is RANDOM.
A packet with a TCP header compresse