WDIFF

draft-degermark-ipv6-hc-03.txt --> draft-degermark-ipv6-hc-04.txt

Network Working Group Mikael Degermark /Lulea University
INTERNET-DRAFT Bjorn Nordgren /Telia Research AB
Expires: Jan May 1998 Stephen Pink /Swedish Institute of Computer Science
Sweden
July 30,
Nov 18, 1997

IP Header Compression for IPv6
<draft-degermark-ipv6-hc-03.txt>
<draft-degermark-ipv6-hc-04.txt>

Status of this Memo

Publication of this document does not imply acceptance by the IPng
Area of any ideas expressed within. Comments should be submitted to
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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 byte 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.

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TABLE OF CONTENTS
1. Introduction..............................................3
2. Terminology...............................................5
3. Compression method........................................7
3.1. Packet types.......................................7 types.......................................8
3.2. Lost packets in TCP packet streams.................8 streams.................9
3.3. Lost packets in UDP and non-TCP packet streams.....9
4. Grouping packets into packet streams.....................13
4.1. Guidelines for grouping packets...................14
5. Size Issues..............................................16
5.1. Compression identifiers...........................16 Context identifiers...............................16
5.2. Size of compression state.........................16 the context...............................17
5.3. Size of full headers..............................17
5.3.1. Length fields in full TCP headers............18 headers............19
5.3.2. Length fields in full non-TCP headers........18 headers........19
6. Compressed Header Formats................................19 Formats................................20
7. Compression of subheaders................................21 subheaders................................22
7.1. IPv6 Header.......................................23 Header.......................................24
7.2. IPv6 Extension Headers............................23 Headers............................24
7.3. Options...........................................24 Options...........................................25
7.4. Hop-by-hop Options Header.........................25 Header.........................26
7.5. Routing Header....................................26 Header....................................27
7.6. Fragment Header...................................27 Header...................................28
7.7. Destination Options Header........................28 Header........................29
7.8. No Next Header....................................28 Header....................................29
7.9. Authentication Header.............................29 Header.............................30
7.10. Encapsulating Security Payload Header.............29 Header.............30
7.11. UDP Header........................................30 Header........................................31
7.12. TCP Header........................................31 Header........................................32
7.13. IPv4 Header.......................................33 Header.......................................34
7.14 Minimal Encapsulation within IP...................36
8. Changing compression identifiers.........................34 context identifiers.............................36
9. Rules for dropping or temporarily storing packets........35 packets........36
10. Low-loss header compression for TCP .....................36 .....................37
10.1. The twice algorithm..............................36 algorithm..............................38
10.2. Header Requests..................................37 Requests..................................38
11. Links that reorder packets...............................38 packets...............................39
11.1. Reordering in non-TCP packet streams.............38 streams.............40
11.2. Reordering in TCP packet streams.................38 streams.................40
12. Hooks for additional header compression..................40 compression..................41
13. Demultiplexing...........................................41
14. Configuration Parameters.................................42
15. Implementation Status....................................44
16. Acknowledgments..........................................44
17. Security Considerations..................................44
18. Author's Addresses.......................................44
19. References...............................................45

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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 at least 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.

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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 header is
100 octets. Header compression will decrease the header
overhead for IPv6/TCP from 19.5 per cent to about 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
bytes
octets when possible. Still, with 1400 octet segments (RFC 894
Ethernet encapsulation allows 1500 octet payloads, of which 100
octets are used for IP headers), header compression reduces IP
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
slow-start and periodic header refreshes allow minimal periods of
packet discard after loss of a header that changes the compression state. context. There
are hooks for adding header compression schemes on top of UDP, for
example compressed compression of RTP now being developed by Casner and
Jacobson. 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
full header and may contain incremental changes to the full header.

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context established by the full header and may contain incremental
changes to the full header.

2. Terminology

This section explains some terms used in this document.

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
compression
context state is identical to the compression context state used when
compressing the header.

Decompress

The act of reconstructing a compressed header.

Compression

Context identifier (CID)

A small unique number identifying the compression state context that should be
used to decompress a compressed header. Carried in full
headers and compressed headers.

Compression state

Context

The state which the compressor uses to compress a header and
the decompressor uses to decompress a header. The compression
state 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 compression state context for a packet stream is associated with a
compression context
identifier. The compression state context for non-TCP packet streams is also
associated with a generation.

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Generation

For non-TCP packet streams, each new version of the compression
state context for
a given CID is associated with a generation: a small number
that is incremented whenever the compression state 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
compression state.
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
compression state context
for a packet stream. It carries a CID that will be used to
identify the compression state. context.

Full headers for non-TCP packet streams also carry the
generation of the compression state 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
compression state
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 compression state. context. This technique is used for
TCP streams but not for non-TCP streams.

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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 compression context identifier, CID, is transmitted over the
link. The compressor and decompressor store most fields of this full
header as compression state. context. The compression state 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 compression state context at the decompressor. As long as the
compression state 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 compression state 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
compression state 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

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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 compression state 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
compression state
context to use for decompression, a generation to detect an
inconsistent compression state context and the randomly changing fields of the
header.

COMPRESSED_TCP - indicates a packet with a compressed TCP header,
containing a CID, a flag byte 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 will be are used whenever a compressor decides to not
compress a packet.

3.2. Lost packets in TCP An additional packet streams

Since type may be used to speed up
repair of TCP headers are compressed using streams over links where the difference from decompressor can send
packets to the
previous TCP header, loss of compressor.

CONTEXT_STATE - indicates a special packet with a compressed or full
header will cause subsequent compressed headers sent from the
decompressor to be decompressed
incorrectly because the compression state used for decompression was 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.

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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

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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 compression state. 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 compression state 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 because
differential coding is not used and there packets. There
are no sequence numbers. 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 compression state 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 compression state. context. Compressed headers carry the
generation value of the compression
state context that were used to compress them.
When a decompressor sees that a compressed header carries a

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generation value other than the generation of its compression state context for that
packet stream, the
compression state context is not up to date and the packet must be
discarded or stored until a full header establishes correct compression state. context.

Differential coding is not used for non-TCP streams, so compressed
non-TCP headers do not change the compression state. context. Thus, loss of a
compressed header does not invalidate subsequent packets with
compressed headers. Moreover, the generation field changes only when the compression state
context of a full header is different from the
compression state context of the
previous full header. This means that losing

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the compression state context of the decompressor obsolete only when the full header
would actually have changed the
compression state. context.

The generation field is 6 bits long so the generation value repeats
itself after 64 changes to the compression state. 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 compression state, context, full headers are sent
periodically with an exponentially increasing period after a change
in the compression state. 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

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its compression state 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
compression state, context,
there is an upper limit, F_MAX_PERIOD, on the

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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() and send_compressed_header()
do the obvious thing.

if ( <this header changes the compression state> context> )

C_NUM := 0;
F_LAST := current_time();
F_PERIOD := 1;
send_full_header(); -- generation value incremented

elseif ( C_NUM >= F_PERIOD )

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C_NUM := 0;
F_LAST := current_time();
F_PERIOD := min(2 * F_PERIOD, F_MAX_PERIOD);
send_full_header(); -- generation value unchanged

elseif ( current_time() > F_LAST + F_MAX_TIME )

C_NUM := 0;
F_LAST := current_time();

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send_full_header(); -- generation value unchanged

else

C_NUM := C_NUM + 1
send_compressed_header(); -- with current generation value

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 a byte an octet to the average header size. With a four-
address
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.

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After a change in compression state, 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

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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 compression state 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 our 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 we feel that 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, the header
compression performance will be bad, since the compression algorithm
can rarely utilize the existing compression state 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.

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To determine what packet stream a packet belongs to, a compressor
might

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,

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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 the absence of specific instructions as to which packet streams to
compress, we offer the following quidelines for how a compressor may
group packets into packet streams for compression.

Defining fields

All

The defining fields in subheaders that are marked with DEF in section 7 of a header should be present and identical
in all packets belonging to the same packet stream. The DEF 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
preceding a UDP or TCP header, (IPv4), port numbers, numbers (UDP
and TCP), and the SPI in authentication and encryption headers.

Fragmented packets

Fragmented and unfragmented packets are never grouped together

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in the same packet stream. The Identification field of the
Fragment header or IPv4 header is not 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 Header, or an IPv4 header for a
fragment
fragment, should be used for grouping purposes.

Upper protocol identification

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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

A sophisticated implementation can monitor the TTL field and if
it changes frequently use it as a DEF field. This might occur
when there are frequent route flaps so that packets traverse
different paths through the internet.

Priority

Class field

It is concievable that the Priority Class field of the IPv6 header can
change between packets with identical DEF fields when the Flow
Label is zero. A sophisticated implementation can watch out for
this and be prepared to use the Priority Class field as a DEF defining
field.

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 in to the compression state. context.

We stress that these guidlines guidelines are only educated guesses, when IPv6 is
widely deployed and IPv6 traffic can be analyzed, we might find that

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other grouping algorithms perform better. We also stress that if the
grouping fails, the result will be bad performance and but not incorrect
decompression. The decompressor can do its task regardless of how the
grouping algorithm works.

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5. Size Issues

5.1. Compression Context Identifiers

Compression

Context identifiers can be 8 or 16 bits long. Their size is not
relevant for finding the compression state. context. An 8-bit CID with value two and an 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 compression state, 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 first byte 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 first octet and
the CID uses the second octet.

For TCP the only available CID size is 8 bits. bits as in [RFC-1144]. 8
bits is probably sufficient as TCP connections are always point-to-point. point-to-
point.

The 16 bit CID size is probably 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.

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5.2. Size of Compression State the context

The size of the compression state 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 compression state context is essentially a stored
header.

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The configurable parameter MAX_HEADER (see section 14) represents the
maximum size of the compression state, context, expressed as the maximum sized header
that can be stored as compression state. context. When an IPv6 a header is larger than
MAX_HEADER, only part of it is stored as compression
state. 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
compression state 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 so that the packets. This allows values of
length fields can 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. Length fields of 2 octets occur 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
IPv6
IP base header to pass the 8 bit CID, which is sufficient. [RFC-
1144] uses the 8 bit Protocol field of the IPv4 header to pass the
CID. We cannot use the corresponding method for IPv6 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

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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

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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.

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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

See section 11 for a description of how the pkt

Pkt nr field is used. 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 |0|D| Generation| CID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Second length field (if avail.) | 0 | Data (if D=1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Full non-TCP headers with 16-bit CID:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
First length field |1|D| Generation| Data (if D=1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Second length field | CID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The first bit inthe 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.

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6. Compressed Header Formats

This section uses some terminology (DELTA, RANDOM) defined in section
7.

a) COMPRESSED_TCP format (similar to [RFC 1144]):

The latter flags in the second byte 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 compression state 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 compression state 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

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as in the previous header. The entire Option field are placed last in
the compressed TCP header. The first bit in the flag byte octet is
reserved. It is always zero.

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 |
+-+-+-+-+-+-+-+-+
|0|D| 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 |
+-+-+-+-+-+-+-+-+
|1|D| Generation|
+-+-+-+-+-+-+-+-+
| lsb of CID |
+-+-+-+-+-+-+-+-+
| data | (if D=1)
- - - - - - - -
| RANDOM fields, if any (section 7) (implied)
- - - - - - - -

The generation, CID and optional one byte 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.

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7. Compression of subheaders

This section gives rules for how the compressible chain of subheaders
is compressed. 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 compression
state. 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 not only used for non-TCP TCP packet streams.

RANDOM The field should 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 need not be included in the
compressed header.

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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
compression state 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 adds the value in the compressed header to
the value in its compression state context to obtain the proper value. RANDOM fields
are sent "as-is" in the compressed header. DELTA and RANDOM fields occur in
the same order in the compressed header as they occur in the full
header.

There is currently little experience with actual IPv6 traffic, so
this classification may change as IPv6 traffic can be observed.

Fields that may 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 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.

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7.1. IPv6 Header [IPv6, section 3]

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Prio. Class | Flow Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Length | Next Header | Hop Limit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Source Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version NOCHANGE (DEF)
Prio
Class NOCHANGE (might be DEF)
Flow Label NOCHANGE (DEF)
Payload Length INFERRED
Next Header NOCHANGE
Hop Limit NOCHANGE (might be DEF)
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 cannot be compressed.

This classification implies that the whole entire IPv6 base header can 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.

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7.3. Options [IPv6, section 4.2]

The contents of Hop-by-hop Options and Destination Options extension
headers are encoded with TLV "options":

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| 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.

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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.

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7.5. Routing Header [IPv6, section 4.4]

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.

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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 |Res|M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 576 octets so most fragments
will be at least 576 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.

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Grouping of fragments according to the guidelines in section 4.1:

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 hope 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 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. When further Destination Options are defined, more clever
compression techniques may be defined.

7.8. No Next Header [IPv6, section 4.7]

Covered by rules for IPv6 Header Extensions (7.2).

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7.9. Authentication Header [RFC-1826, 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-1827, 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 are likely to 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.

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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 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.

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7.12. TCP Header

The TCP header is described in [RFC-793].

The Next Header field (IbPv6) or Protocol field (IPv4) in the
preceding subheader is DEF.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset| Reserved |U|A|P|R|S|F| 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 NOCHANGE
Urg,Psh RANDOM (placed in flag byte) octet)
Ack INFERRED to be 1
Rst,Syn,Fin INFERRED to be 0
Window DELTA (if change in Window,
set W-flag in flag byte 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)

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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
Jacobsson, described in [RFC-1144], the differences being the
placement of the compressed TCP header fields (see section 6), the
use of the O-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.

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 compressed according to the above must be
indicated to be of type COMPRESSED_TCP_NODELTA. It uses the same CID
space as COMPRESSED_TCP packets, and the header is saved as
compression state. The compressed header is described in section 6.

This packet type can be sent as the response to a header request
instead of sending a full header, can be used over links that reorder
packets, and can be sent instead of a full header when there are
changes that cannot be represented by a compressed header. A
sophisticated compressor can switch to sending only
COMPRESSED_TCP_NODELTA headers when the packet loss frequency is
high.

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INTERNET-DRAFT Header Compression for IPv6 July 30, Nov 18, 1997

7.13. IPv4 header [RFC-791, section 3.1]

As we expect many IPv6 packets to be encapsulated in IPv4 packets,
and many IPv4 packets to be encapsulated in IPv6 packets, it is
important to be able to compress IPv4 headers.

There are two ways to compress the IPv4 header

a) If the IPv4 header is not for a fragment (MF flag is not set and
Fragment Offset is zero) and there are no options (IHL is 5), it
is classified as follows

Version NOCHANGE (DEF)
IHL NOCHANGE (DEF, must be 5)
Type of Service NOCHANGE
Total Length INFERRED (from link-layer implementation
or encapsulating IP header)

Identification DELTA/ (If the Protocol field has the
value corresponding to TCP)
DELTA/ (for UDP when UDP Checksum = 0)
RANDOM (otherwise)

Flags NOCHANGE (MF flag must not be set)
Fragment Offset NOCHANGE (must be zero)
Time to Live NOCHANGE
Protocol NOCHANGE
Header Checksum INFERRED (calculated from other fields)
Source Address NOCHANGE (DEF)
Destination Address NOCHANGE (DEF)
Options, Padding (not present)

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Note: When a TCP header immediately follows, the IPv4 and TCP
header are compressed as a unit as described in section 7.12.

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Note: When the UDP Checksum is zero, the Identification field need
not be maintained between compressor and decompressor, so the
value of the Identification field is simply by default increased by 1 for
each decompressed packet.

b) If the IPv4 header is for a fragment (MF bit set or Fragment
Offset nonzero), or there are options (IHL > 5), all fields are
RANDOM (i.e., they are sent as-is and not compressed). If the
IPv4 header is for a fragment it ends the compressible chain of
subheaders, i.e., it is the last subheader to be compressed. If
the IPv4 header has options but is not for a fragment it does not
end the compressible chain of subheaders, so subsequent subheaders
will be compressed.

A compressor that follows the guidelines of section 4.1 will in case
a) use the Version, Source Address and Destination Address to define
the packet stream, together with the fact that there are no IPv4
options and that this is not a fragment.

Case b) can define two kinds of packet streams depending on whether
the IPv4 header is for a fragment or not.

If the IPv4 header in case b) is for a fragment, the compressor uses
that fact together with the Version, Source Address, and Destination
Address to determine the packet stream.

If the IPv4 header in case b) is not for a fragment, it must have
options. The compressor uses that fact, but not the size of the
options, together with the Version, Source Address, and Destination
Address to determine the packet stream.

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7.14. Minimal Encapsulation header [RFC-2004, section 3.1]

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol |S| reserved | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Original Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: (if present) Original Source Address :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Protocol NOCHANGE
Original Source Address Present (S) NOCHANGE
reserved NOCHANGE
Header Checksum INFERRED (calculated from
other values)
Original Destination Address NOCHANGE
Original Source Address NOCHANGE (present only
if S=1)

This header is likely to be used by Mobile IP.

8. Changing compression context identifiers

On a point-to-point link, the compressor has total knowledge of what
CIDs are in use at the decompressor and can change what CID a packet
stream uses or reuse CIDs at will.

Each non-TCP CID is associated with a compression state context with a generation
value. To avoid too rapid generation wrap-around and potential
incorrect decompression, an implementation must avoid wrap-around of
the generation value in less than MIN_WRAP seconds (see section 14).

To aid in avoiding wrap-around, the generation value associated with
a CID must not be reset when changing to a new packet stream.

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Instead, a compressor must increment the generation value by one when
using the CID for a new non-TCP packet stream.

9. Rules for dropping or temporarily storing packets

When a decompressor receives a packet with a compressed TCP header
with CID C, it must be discarded when the compression state context for C has not been
initialized by a full header.

When a decompressor receives a packet with a compressed non-TCP
header with CID C and generation G, the header must not be
decompressed using the current compression state context when

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a) the decompressor has been disconnected from the compressor for
more than MIN_WRAP seconds, because the compression state context might be
obsolete even if it has generation G.

b) the compression state context for C has a generation other than G.

In case a) and b) the packet can either be

i) discarded immediately, or else

ii) stored temporarily until the compression state context is updated by a packet
with a full non-TCP header with CID C and generation G, after
which the header can be decompressed.

Packets stored in this manner must be discarded when

*) receiving full or compressed non-TCP headers with CID C
and a generation other than G,

*) the decompressor has not received packets with CID C in
the last MIN_WRAP seconds.

When full headers are lost, a decompressor may receive compressed
non-TCP headers with a generation value other than the generation of
its compression state. context. Rule ii) allows the decompressor to store such headers
until they can be decompressed using the correct compression
state.

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10. Low-loss header compression for TCP

Since fewer bits are transmitted per packet with header compression,
the packet loss rate is lower with header compression than without,
for a fixed bit-error rate. This is beneficial for links with high
bit-error rates such as wireless links.

However, since TCP headers are compressed using differential
encoding, a single lost TCP segment can ruin an entire TCP sending
window because the compression state context is not incremented properly at the
decompressor. Subsequent headers will therefore be decompressed to
be different than before compression and discarded by the TCP
receiver because the TCP checksum fails.

A TCP connection in the wide area where the last hop is over a
medium-speed lossy link, for example a wireless LAN, will then have
poor performance with traditional header compression because the
delay-bandwidth product is relatively large and the bit-error rate
relatively high. For a 2 Mbit/s wireless LAN and a RTT of 200 ms, the
delay-bandwidth product is 50 kbyte. That is equivalent to about 97
512-byte
512-octet segments with compressed headers. Each loss can thus be

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multiplied by a factor of 100.

This section describes two simple mechanisms for quick repair of the
compression state.
context. With these mechanisms header compression will improve TCP
throughput over lossy links as well as links with low bit-error
rates.

10.1. The "twice" algorithm

The decompressor can compute the TCP checksum to determine if its
compression state
contex is not updated properly. If the checksum fails, the error is
assumed to be caused by a lost segment that did not update the
compression state
context properly. The delta of the current segment is then added to
the compression state context again on the assumption that the lost segment contained
the same delta as the current. By decompressing and computing the TCP
checksum again, the decompressor checks if the repair succeeded or if
the delta should be applied once more.

Analysis of traces of various TCP bulk transfers show that applying
the delta of the current segment one or two times will repair the
compression state
context for between 83 and 99 per cent of all single-
segment single-segment losses
in the data stream. For the acknowledgment stream, the success rate
is smaller due to the delayed ack mechanism of TCP. The "twice"
mechanism repairs the compression state context for 99 - 53 per cent of the losses in
the acknowledgment stream. A sophisticated implementation of this
idea would determine whether the TCP stream is an acknowledgment or
data stream and determine the segment size by

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INTERNET-DRAFT Header Compression for IPv6 July 30, 1997 observing the stream of
full and compressed headers. Trying deltas that are small multiples
of the segment size will result in even higher repair success rates of successful
repairs for acknowledgment streams.

10.2. Header Requests

The relativley low success rate for the "twice" algorithm for TCP
acknowledgment streams calls for an additional mechanism for
repairing the compression state context at the decompressor. When the decompressor
fails to repair the compression state context after a loss, the decompressor requests may
optionally request a full header from the compressor.

Node This is
possible on links where the decompressor can identify the compressor
and send packets to it.

On such links, a decompressor may send a CONTEXT_STATE packet back to
the compressor to indicate that one or more contexts are invalid. A lossy link Node B
------------------ ------------------
| | Acks | |
>---> Compressor >--------------------> Decompressor >---->
| |
decompressor should not transmit a CONTEXT_STATE packet every time a
compressed packet refers to an invalid context, but instead should
limit the rate of transmission of CONTEXT_STATE packets to avoid
flooding the reverse channel. A CONTEXT_STATE packet can indicate
that several contexts are out of date, this technique should be used

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instead of sending several separate packets. The following diagram
shows the format of a CONTEXT_STATE packet.

0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| TCP header request = 2 |
+---+---+---+---+---+---+---+---+
| CID count | Data
+---+---+---+---+---+---+---+---+
| CID |
<---< Decompressor <--------------------< Compressor <----<
+---+---+---+---+---+---+---+---+
| CID |
+---+---+---+---+---+---+---+---+
...
+---+---+---+---+---+---+---+---+
| CID |
------------------ ------------------
+---+---+---+---+---+---+---+---+

The most common configuration first octet is likely a type code to allow the CONTEXT_STATE packet type
to be shared for other compression protocols that the are (see [CRTP]) or
may be defined in parallel with this one. When used for TCP
acknowledgment and data streams pass through the same nodes on each
side header
requests the type code has the value 2, and the remainder of the
packet is a lossy link such as a wireless link. There will then be sequence of CIDs preceded by a
compressor/decompressor pair on each side one-octet count of the link.

Assume that an acknowledgement is damaged on the lossy link from node
A to node B. The link-level checksum detects
number of CIDs.

On receipt of a CONTEXT_STATE packet, the damaged frame and
discards it. Also, assume that compressor should mark the decompressor in node B fails
CIDs invalid to
repair its compression state when ensure that the next compressed acknowledgment
arrives. The decompressor in node B will then ask the compressor in
node B to set a bit packet emitted in those packet
streams are FULL_HEADER or COMPRESSED_TCP_NODELTA packets.

Header requests are an optimization, so loss of a full header in CONTEXT_STATE
packet does not affect the corresponding correct operation of TCP stream
going in the opposite direction, in this case the data stream. header
compression. When
the decompressor in node A sees the bit, it asks its companion
compressor to send a full header in the CONTEXT_STATE packet stream with the
corresponding IP addresses is lost, eventually a new
one will be transmitted or TCP will timeout and port numbers. retransmit. The full big
advantage of using header updates
the compression state for the decompressor in node B and
acknowledgments start to flow again.

In this manner the requests is that TCP acknowledgment stream is streams
can be repaired after a roundtrip-time over the lossy link, and most of the window link. This
will get
through undamaged. typically avoid a TCP timeout and unnecessary retransmissions.
The lower packet loss rate due to smaller packets will then result in
higher throughput because the TCP window can grow larger between
losses.

The header request mechanism will not work when routes are not
symmetric and the TCP streams do not visit the same nodes. Such

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situations are perfectly normal. Sending a header request is a hint
that it is a good idea to send a full header in the corresponding TCP
packet stream. However, header requests is only a performance
improving mechanism and it is safe to ignore such hints if no
corresponding TCP packet stream can be found. Compressors must
silently ignore requests for full headers in TCP streams they are not
compressing. Decompressors must ignore requests for full headers when
they cannot contact a suitable compressor.

What bit to use for requesting a full header is specified in section
13. The corresponding TCP stream going in the opposite direction is
identified by examining the source and final destination addresses of
the innermost IP header plus the TCP source and destination port
numbers. If one TCP stream has the values (SA1, DA1, SP1, DP1) in
these fields and another has the values (SA2, DA2, SP2, DP2), they
correspond to each other if and only if SA1 = DA2, DA1 = SA2, SP1 =
DP2, and DP1 = SP2.

11. Links that reorder packets

Some links reorder packets, for example multi-hop radio links that
use deflection routing to route around congested nodes. Packets
routed different ways can then arrive at the destination in a
different order than they were sent.

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11.1. Reordering in non-TCP packet streams

Compressed non-TCP headers do not change the compression state, context, and neither do
full headers that refresh it. It is There can be problems only when a full
header that changes the compression state context arrives out of order that
there can be problems. order. There are two
cases:

- A packet with a full header with generation G arrives *after* a
packet with a compressed header with generation G. This case
is covered by rule b) ii) in section 9.

- A packet with a full header with generation G arrives *before* a
packet with a compressed header with generation G-1 (modulo
128).
64). The decompressor can then keep both versions of the
compression state
context around for a while to be able to decompress subsequent
compressed headers with generation G-1 (modulo 128). 64). The old compression state
context must be discarded after MIN_WRAP seconds.

11.2. Reordering in TCP packet streams

A compressor can avoid sending COMPRESSED_TCP headers and only send
COMPRESSED_TCP_NODELTA headers when there is reordering over the

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link. Compressed headers will typically be 17 bytes octets with that
method, significantly larger than the usual 4-7 bytes. octets.

To achieve better compression rates the following method, adding only
two bytes octets to the compressed header for a total of 6-9 bytes, octets, can be
used. A packet sequence number, incremented by one for every packet
in the TCP stream, is associated with each compressed and full
header. This allows the decompressor to place the packets in the
correct sequence and apply their deltas to the compression state the context in the correct
order. A simple sliding window scheme can be used to place the
packets in the correct order.

Two bytes octets are needed for the packet sequence numbers. One byte octets
gives only 256 sequence numbers. In a sliding window scheme the
window should be no larger than half of the sequence number space, so
packets can not arrive more than 127 positions out-of-sequence. This
is equivalent to a delay of 260 ms on 2 Mbit/s links with 512 byte octet
segments. Delays of that order are not uncommon over wide-are
Internet connections. However, two bytes octets giving 2^16 = 65536 values
should be sufficient.

Full TCP headers will only have space for one byte octet of sequence
number when there is no tunneling. It is not feasible to increase the
size of full headers since the packet size might be optimized for the
MTU of the link. Therefore only the least significant byte octet of the
packet sequence number can be placed in such full headers. We believe

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that such full headers can be positioned correctly frequently enough
with only the least significant byte octet of the packet sequence number
available.

The packet sequence number zero is skipped over. Avoiding zero takes
care of a problem that can occur when the TCP window scale option is
used to enlarge the TCP window. When exactly 2^16 bytes octets of TCP data
is lost, a compressed header will be decompressed incorrectly without
being detected by the TCP checksum. TCP segments are often a power of
two. So by using a packet sequence number space that is not a power
of two either the sequence number or the packet sequence number will
differ when 2^16 bytes octets are lost. Whenever a compressor sees the
window scale option on a SYN segment, it must use packet sequence
numbers when subsequently compressing that packet stream.

In compressed TCP headers the two byte octet packet sequence number is
placed immediately after the TCP Checksum. See section 5.3 for
placement of packet sequence numbers in full headers.

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12. Hooks for additional header compression

The following hook is supplied to allow additional header compression
schemes for headers on top of UDP. The initial chain of subheaders is
then compressed as described here, and the other header compression
scheme is applied to the header above the UDP header. An example of
such additional header compression would be Compressed RTP by
Jacobson and Casner [CRTP]. To allow some error detection, such
schemes typically need a sequence number that may need to be passed
in full headers as well as compressed UDP headers.

The D-bit and Data byte octet (see section 6) provides the necessary
mechanism. When a sequence number, say, needs to be passed in a full
header
FULL_HEADER or COMPRESSED_NON_TCP header, the D-bit is set and the
sequence number is placed in the Data field. The decompressor must
then extract and make the Data field available to the additional
header compression scheme.

Use of additional header compression schemes like CRTP must be
negotiated. The D-bit and Data byte octet mechanism is automatically
enabled whenever use of additional header compression schemes has
been negotiated.

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13. Demultiplexing

It is necessary to distinguish packets with regular IPv4 headers,
regular IPv6 headers, full IPv6 packets, full IPv4 packets,
compressed TCP packets, and compressed non-TCP packets, and CONTEXT_STATE
packets. It is also
desirable to find one bit

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INTERNET-DRAFT Header Compression for requesting a header refresh as
described in section 10.2. IPv6 Nov 18, 1997

The decision to use a distinct ethertype (or equivalent) for IPv6 has
already been taken, which means that link-layers must be able to
indicate that a packet is an IPv6 packet.

IPv6 header compression requires that the link-layer implementation
can indicate three new four kinds of packets: COMPRESSED_TCP for format a) in
section 6, COMPRESSED_TCP_NODELTA for format b), and COMPRESSED_NON_TCP
for formats c) and d). d), and CONTEXT_STATE as described in section
11.2. It is also desirable to indicate FULL_HEADERS at the link
layer.

Full headers are can be indicated by special encodings of setting the first four
bits (Version field) bit of the Version
field in a packet indicated to be an IPv6 packet by packet. In addition, one
bit of the link layer. Version field is used to indicate if the first subheader
is an IPv6 or an IPv4 header, and one bit is used to indicate if this
full header carries a TCP CID or a non-TCP CID. The first four bits
are encoded as follows:

Version Meaning
------- -------

0110 regular IPv6 header

1T**

1T*0 T=1 indicates a TCP header, T=0 indicates a non-TCP header
1*V*
1*V0 V=1 indicates a IPv6 header, V=0 indicates a IPv4 header
1**R R=1 requests a full header

If the link-layer cannot indicate these the packet types, all packets
with types for the compressed
headers must or CONTEXT_STATE, packet types that cannot be indicated could
start with a byte an octet indicating the packet type, followed by the compressed
header.

First byte octet Type of compressed header
----------
----------- -------------------------

00000000

0 COMPRESSED_TCP
00000001
1 COMPRESSED_TCP_NODELTA
00000002
2 COMPRESSED_NON_TCP

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3 CONTEXT_STATE

14. Configuration Parameters

The following parameters are parameter is fixed for all implementations of this
header compression scheme.

MIN_WRAP - minimum time of generation value wrap around

3 seconds.

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The following parameters can be negotiated between the compressor and
decompressor. If not negotiated their values must be as specified by
DEFAULT.

F_MAX_PERIOD - Largest number of compressed non-TCP headers that
may be sent without sending a full header.

DEFAULT is 256

F_MAX_PERIOD must be at least 1 and at most 65535.

F_MAX_TIME - Compressed headers may not be sent more than
F_MAX_TIME seconds after sending last full header.

DEFAULT is 5

F_MAX_TIME must be at least 1 and at most 255.

NOTE: F_MAX_PERIOD and F_MAX_TIME should be lower when it is
likely that a decompressor loses its state.

MAX_HEADER - The largest header size in octets that may
be compressed.

DEFAULT is 168 octets, which covers

- Two IPv6 base headers
- A Keyed MD5 Authentication Header
- A maximum-sized TCP header

MAX_HEADER must be at least 60 octets and
at most 65535 octets.

TCP_SPACE - Maximum CID value for TCP.

DEFAULT is 15 (which gives 16 CID values)

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TCP_SPACE must be at least 3 and at most 255.

NON_TCP_SPACE - Maximum CID value for non-TCP.

DEFAULT is 15 (which gives 16 CID values)

NON_TCP_SPACE must be at least 3 and at most 65535.

EXPECT_REORDERING - The mechanisms in section 11 are used.

DEFAULT no.

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EXPECT_REORDERING - The mechanisms in section 11 are used.

DEFAULT no.

15. Implementation Status

A prototype using UDP as the link layer has been operational since
March 1996. A NetBSD implementation for PPP has been operational
since October 1996.

16. Acknowledgments

This protocol uses many ideas originated by Van Jacobson in the
design of header compression for TCP/IP [RFC-1144].

We thank Craig Partridge for pointing out a problem that can occur
when the TCP window scale option is used. A solution to this problem
relying on the packet sequence numbers used for reordering is
described in section 11.2.

17. Security Considerations

We advise against identifying packet streams with the aid of
information that is encrypted even if such information happens to be
available to the compressor. Doing so would expose traffic patterns.

18. Author's Addresses

Mikael Degermark Tel: +46 920 91188
CDT/Dept of Computer Communications Fax: +46 920 72801
Lulea University Mobile: +46 70 648 8121
S-971 87 Lulea, Sweden EMail: micke@sm.luth.se

Bjorn Nordgren Tel: +46 920 75400
CDT/Telia Research AB Fax: +46 920 75490
Aurorum 6 EMail: bcn@lulea.trab.se
S-977 75 Lulea, Sweden

Stephen Pink Tel: +46 8 752 15 59
CDT/Swedish Institute of Computer Science Fax: +46 8 751 72 30
PO Box 1263 Mobile: +46 70 532 0007
S-164 28 Kista, Sweden EMail: steve@sics.se

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19. References

[RFC-768] J. Postel, User Datagram Protocol, RFC 761, August 1980.

[RFC-791] J. Postel, Internet Protocol, RFC 791, September 1981.

[RFC-793] J. Postel, Transmission Control Protocol, RFC 793,
September 1981.

[RFC-1144] V. Jacobson, Compressing TCP/IP Headers for Low-Speed
Serial Links, RFC 1144, February 1990.

[RFC-1553] A. Mathur, M. Lewis, Compressing IPX Headers Over WAN
Media (CIPX), RFC 1553, December 1993.

[RFC-1700] J. Reynolds and J. Postel, Assigned Numbers, RFC-1700,
October 1994.

[RFC-1826] R. Atkinson, IP Authentication Header, RFC 1826, August
1995.

[RFC-1827] R. Atkinson, IP Encapsulating Security Protocol (ESP),
RFC 1827, August 1995.

[RFC-1828] Metzger, W. Simpson, IP Authentication using Keyed MD5,
RFC 1828, August 1995.

[IPv6] S. Deering, R. Hinden, Internet Protocol, Version 6
(IPv6) Specification, RFC 1883, December 1995.

[ICMPv6] A. Conta, S. Deering, Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6), RFC
1885, December 1995.

[RFC-2004] C. Perkins, Minimal Encapsulation within IP, RFC 2004,
October 1996.

[CRTP] S. Casner, V. Jacobson, Compressing IP/UDP/RTP Headers for
Low-Speed Serial Links. Internet-Draft (Work in
progress), July 11, 1997.

This draft expires in January May 1998.

Degermark, Nordgren, Pink [Page 45]

----