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Versions: 00 01                                                         
Audio/Video Transport Working Group                         Tmima Koren
Internet Draft                                           Stephen Casner
March 9, 2000                                             Patrick Ruddy
Expires October 2000                                     Bruce Thompson
draft-koren-avt-crtp-enhance-01.txt                        Alex Tweedly
                                                               Dan Wing
                                                          Cisco Systems
                                                       John Geevarghese
                                                         Motorola India

              Enhancements to IP/UDP/RTP Header Compression

Status of this memo

This document is an Internet Draft and is in full conformance with all
provisions of Section 10 of RFC 2026. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas, and
its Working Groups. Note that other groups may also distribute working
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Copyright Notice

Copyright (C) The Internet Society (1999-2000). All Rights Reserved.



Abstract

This document describes enhancements to CRTP, the header compression
algorithm for RTP streams described in [RFC2508]. Each enhancement
addresses issues with RFC2508 in different deployment scenarios. Each
section below provides a description of the proposed enhancement, the
scenario where it is useful and the justification for its use.

Each of these enhancements could be evaluated separately.
The enhancements are applicable both for IPv4 and IPv6.

The IPCP option æIP header compressionÆ (described in RFC2509) is also
extended to negotiate using the CRTP enhancements.


1.0 Introduction

As IP/UDP/RTP header compression becomes more widely deployed, it is
being used in scenarios where a compressed link could extend over a
long physical distance and involve multiple layer-2 switching points.
An example of such a link is RTP transport over ATM AAL-5, where the
"link" would actually traverse through multiple layer-2 switching
points on the path from the CRTP transmitter (compressor) to the CRTP
receiver (decompressor). Another example is a wireless link. Such links
may experience significant packet loss and/or long round trip delays.
Contexts get invalidated due to packet loss, but the CRTP error
recovery mechanism using CONTEXT_STATE messages is not efficient due to
the long round trip delay.

In scenarios such as this, it is desirable to minimize context
invalidation. This document suggests several methods of error
prevention and recovery. The suggested enhancements make CRTP more
robust and resilient to packet loss, which in turn will reduce context
invalidation.

1.1 Specification of Requirements

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].


2. CRTP Enhancements

2.1 The negative cache stream flag

Certain streams, known or suspected to not be RTP, can be placed in a
"negative cache" at the compressor, so only the IP and UDP headers are
compressed. It is beneficial to notify the decompressor that the
compressed stream is in the negative cache: for such streams the
context is shorter - there is no need to include the RTP header, and
all RTP-related calculations can be avoided.

In this enhancement, a new flag bit "N" is added to the FULL_HEADER
packet that initializes a context at the decompressor.  The bit
occupied by the new flag was previously always set to zero.  If the N
flag is set to 1, this indicates that no COMPRESSED_RTP packets will be
transmitted in this context.  This flag is only an optimization and the
decompressor may choose to ignore it.

Format of the FULL_HEADER length fields with the negative cache flag:

For 8-bit context ID:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1| Generation|      CID      |  First length field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            0        |N|  seq  |  Second length field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  N=1: negative cache stream


For 16-bit context ID:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1| Generation|   0 |N|  seq  |  First length field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  N=1: negative cache stream

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



2.2 Reject a new compressed stream

In a point to point link the two nodes can agree on the number of
compressed sessions they are prepared to support for this link. In an
end-to-end scheme a host may have compressed sessions with many hosts
and eventually may run out of resources.  When the end-to-end tunnel is
negotiated, the number of contexts needed may not be predictable.  This
enhancement allows the negotiated number of contexts to be larger than
could be accommodated if many tunnels are established.  Then, as
context resources are consumed, an attempt to set up a new context may
be rejected.
The compressor initiates a compression of a stream by sending a
FULL_HEADER packet. Currently if the decompressor has insufficient
resources to decompress the new stream, it can send a CONTEXT_STATE
packet to invalidate the newly compressed stream. The compressor does
not know the reason for the invalidation: usually this happens when the
decompressor gets out of synchronization due to packet loss. The
compressor will most likely reattempt to compress this stream by
sending another FULL_HEADER.
This enhancement specifies that the decompressor may reject the
compression of a stream by sending a REJECT message to the compressor.
A REJECT message tells the compressor to stop compressing this stream.
The REJECT message is a CONTEXT_STATE message with an additional flag:

   Type code = 1 :CONTEXT_STATE for 8-bit CID streams
   Type code = 2 :  CONTEXT_STATE for16-bit CID streams

Here is the format of CONTEXT_STATE packets with REJECT flags:

             0   1   2   3   4   5   6   7
           +---+---+---+---+---+---+---+---+
           |Type code=1: CS, 8-bit CID |
           +---+---+---+---+---+---+---+---+
           |         context count         |
           +---+---+---+---+---+---+---+---+
           |       session context ID      |
           +---+---+---+---+---+---+---+---+
           | 1 |R=1| 0 | 0 |    sequence   |   R is the REJECT flag
           +---+---+---+---+---+---+---+---+
           | 0 | 0 |       generation      |
           +---+---+---+---+---+---+---+---+
                         . . .
           +---+---+---+---+---+---+---+---+
           |       session context ID      |
           +---+---+---+---+---+---+---+---+
           | 1 |R=1| 0 | 0 |    sequence   |   R is the REJECT flag
           +---+---+---+---+---+---+---+---+
           | 0 | 0 |       generation      |
           +---+---+---+---+---+---+---+---+




             0   1   2   3   4   5   6   7
           +---+---+---+---+---+---+---+---+
           |Type code=2: CS, 16-bit CID|
           +---+---+---+---+---+---+---+---+
           |         context count         |
           +---+---+---+---+---+---+---+---+
           |                               |
           +       session context ID      +
           |                               |
           +---+---+---+---+---+---+---+---+
           | 1 |R=1| 0 | 0 |    sequence   |   R is the REJECT flag
           +---+---+---+---+---+---+---+---+
           | 0 | 0 |       generation      |
           +---+---+---+---+---+---+---+---+
                         . . .
           +---+---+---+---+---+---+---+---+
           |                               |
           +       session context ID      +
           |                               |
           +---+---+---+---+---+---+---+---+
           | 1 |R=1| 0 | 0 |    sequence   |   R is the REJECT flag
           +---+---+---+---+---+---+---+---+
           | 0 | 0 |       generation      |
           +---+---+---+---+---+---+---+---+

The session CID, sequence and generation are taken from the
FULL_HEADER.

The compressor may decide to wait for a while before attempting to
compress additional streams destined to the rejecting host.



2.3 Including IP ID in the UDP checksum

A UDP checksum can be used by the decompressor to verify validity of
the packet it reconstructed, especially when the 'twice' algorithm is
used. When the ætwiceÆ algorithm was defined in RFC 2507 and
subsequently incorporated into RFC 2508, the fact that the IP ID field
is not included in the checksum was overlooked. Since the IP ID field
is conveyed with a delta value, accurate reconstruction of the IP ID
field cannot be verified using the current specifications.

This enhancement modifies the function of the UDP checksum to include
the IP ID value, but only between the compressor and decompressor. That
is, when a UDP checksum is present (nonzero), the compressor will 1Æs
complement subtract the IP ID value from the UDP checksum before
compression and the decompressor will 1Æs complement add the IP ID
value to the UDP checksum after any validation operations and before
delivering the packet further downstream.




2.4 CRTP Headers Checksum

When a UDP checksum is not present (has value zero) in a stream, the
compressor MAY replace it with a 16-bit headers checksum (HDRCKSUM).
The HDRCKSUM can be used to validate the IP ID and all the headers in
the reconstructed packet. Hence it can be used by the decompressor to
validate reconstructed packets when ætwiceÆ is used, and to validate
every 16Æth packet as recommended in RFC2508, Section 3.3.5.

A new flag in the FULL_HEADER packet, as specified below, indicates
when set that all COMPRESSED_UDP and COMPRESSED_RTP packets sent in
that context will have HDRCKSUM inserted. If a packet in the same
stream subsequently arrives at the compressor with a UDP checksum
present, then a new FULL_HEADER packet must be sent with the flag
cleared to re-establish the context.

The HDRCKSUM is calculated in the same way as a UDP checksum, but
includes only the pseudo-IP header (as defined for UDP), the IP ID (as
in Section 2.3), the UDP header and for COMPRESSED_RTP packets, the
fixed part of the RTP header (first 12 bytes). The extended part of the
RTP header and the RTP data will not be included in the HDRCKSUM. The
HDRCKSUM is placed in the COMPRESSED_UDP or COMPRESSED_RTP packets
where a UDP checksum would have been.
The decompressor MUST zero out the UDP checksum field in the
reconstructed packets.

The HDRCKSUM does not validate the RTP data. If the link layer is
configured to deliver packets without checking for errors, errors in
the RTP data will not be detected. Over such links, the compressor
SHOULD add the HDRCKSUM if a UDP checksum is not present, and the
decompressor SHOULD validate each reconstructed packet to make sure
that at least the headers are correct. This ensures that the packet
will be delivered to the right destination. If only HDRCKSUM is
available, the RTP data will be delivered even if it includes errors.
This might be a desirable feature for applications that can tolerate
errors in the RTP data. Same holds for the extended part of the RTP
header.


Here is the format of the FULL_HEADER length fields with the new flag
that indicates that a header checksum will be added in COMPRESSED_UDP
and COMPRESSED_RTP packets:

For 8-bit context ID:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1| Generation|      CID      |  First length field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            0      |C|N|  seq  |  Second length field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  C=1: HDRCKSUM will be added


For 16-bit context ID:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1| Generation| 0 |C|N|  seq  |  First length field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  C=1: HDRCKSUM will be added

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




2.5 Enhancement to COMPRESSED_UDP packet format (CU*)

The COMPRESSED_UDP packet includes the whole RTP header, so it can
restore all RTP-related parameters at the decompressor. It is also
specified to reset the delta RTP timestamp to zero and the delta RTP
sequence number to zero.It can also convey a new value for the delta IP
ID.

It is possible to accommodate some packet loss between the compressor
and decompressor using the "twice" algorithm in RFC 2508, but this
requires reliably communicating the absolute values and the deltas for
the differential fields.  The reliability of communication of the
absolute values in the RTP header can be increased by sending a
COMPRESSED_UDP packet repeatedly, but this resets the delta timestamp.
RFC 2508 describes the format of COMPRESSED_UDP as being the same as
COMPRESSED_RTP except that the M, S and T bits are always 0 and the
corresponding delta fields are never included.  This enhancement
changes that specification to say that the T bit may be nonzero to
indicate that the RTP timestamp delta is included explicitly rather
than being reset to zero.

Sometimes it is necessary to change just a few fields of the RTP
header. A second part of this enhancement adds more flag bits to the
COMPRESSED_UDP packet to select individual uncompressed fields of the
RTP header to be included in the packet.  Since there are flag bits to
indicate inclusion of both delta values and absolute values, the flag
nomenclature is changed.  The original S,T,I bits which indicate the
inclusion of deltas are renamed dS, dT, dI, and the inclusion of
absolute values is indicated by S,T,I.  The M bit is absolute as
before.


The format of the flags/sequence byte for the original COMPRESSED_UDP
packet is shown here for reference:

            +---+---+---+---+---+---+---+---+
            | 0 | 0 | 0 |dI | link sequence |
            +---+---+---+---+---+---+---+---+


The new definition of the flags/sequence byte plus an extension flags
byte is as follows, where the new F flag indicates the inclusion of the
extension flags byte:

            +---+---+---+---+---+---+---+---+
            | F | I |dT |dI | link sequence |
            +---+---+---+---+---+---+---+---+
            : M : S : T :pt :      CC       :  (if F = 1)
            +...+...+...+...+...............+


dI = delta IP ID
dT = delta RTP timestamp
I  = absolute IP ID
F  = additional flags byte
M  = marker bit
S  = absolute RTP sequence number
T  = absolute RTP timestamp
pt = RTP payload type
CC = number of CSRC identifiers


Some short notations:

FH    FULL_HEADER
CR    COMPRESSED_RTP
CR+   COMPRESSED_RTP with delta fields
CU    COMPRESSED_UDP
CU*   enhanced COMPRESSED_UDP


When F=0, there is only one flags byte, and the only available flags
are: dI, dT and I.
In this case the packet includes the full RTP header
If dT=0, the decompressor sets deltaT to 0
If dI=0, the decompressor sets deltaI to 1


Some example packet formats will illustrate the use of the new flags.
First, a 'traditional' COMPRESSED_UDP with full RTP header, when F=0:

             0   1   2   3   4   5   6   7
           +...............................+
           :   msb of session context ID   :  (if 16-bit CID)
           +-------------------------------+
           |   lsb of session context ID   |
           +---+---+---+---+---+---+---+---+
           |F=0| I |dT |dI | link sequence |
           +---+---+---+---+---+---+---+---+
           :                               :
           +         UDP checksum          +  (if nonzero in context)
           :                               :
           +...............................+
           :                               :
           +        "RANDOM" fields        +  (if encapsulated)
           :                               :
           +...............................+
           :         delta IPv4 ID         :  (if dI = 1)
           +...............................+
           :      delta RTP timestamp      :  (if dT = 1)
           +...............................+
           :                               :
           +           IPv4 ID             +  (if I = 1)
           :                               :
           +...............................+
           |           UDP data            |
           :   (uncompressed RTP header)   :


When F=1, there is an additional flags byte and the available flags
are: dI, dT, I, M, S, T, pt, CC. In this case the packet does not
include the full RTP header, but includes selected fields from the RTP
header as specified by the flags. The delta values of the context are
not reset even if they are not specified in the packet:
If dT=0, the decompressor KEEPS THE CURRENT deltaT
   (and DOES NOT set the deltaT to 0)
If dI=0, the decompressor KEEPS THE CURRENT deltaI
   (and DOES NOT set the the deltaI to 1)


A CU* packet is similar in contents and behavior to a CR packet, but it
has more flag bits, some of which correspond to absolute values for RTP
header fields.


COMPRESSED_UDP with individual RTP fields, when F=1 :

             0   1   2   3   4   5   6   7
           +...............................+
           :   msb of session context ID   :  (if 16-bit CID)
           +-------------------------------+
           |   lsb of session context ID   |
           +---+---+---+---+---+---+---+---+
           |F=1| I |dT |dI | link sequence |
           +---+---+---+---+---+---+---+---+
           : M : S : T :pt :      CC       :
           +...+...+...+...+...............+
           :                               :
           +         UDP checksum          +  (if nonzero in context)
           :                               :
           +...............................+
           :                               :
           :        "RANDOM" fields        :  (if encapsulated)
           :                               :
           +...............................+
           :         delta IPv4 ID         :  (if dI = 1)
           +...............................+
           :      delta RTP timestamp      :  (if dT = 1)
           +...............................+
           :                               :
           +           IPv4 ID             +  (if I = 1)
           :                               :
           +...............................+
           :                               :
           +     RTP sequence number       +  (if S = 1)
           :                               :
           +...............................+
           :                               :
           +                               +
           :                               :
           +         RTP timestamp         +  (if T = 1)
           :                               :
           +                               +
           :                               :
           +...............................+
           :       RTP payload type        :  (if pt = 1)
           +...............................+
           :                               :
           :           CSRC list           :  (if CC > 0)
           :                               :
           +...............................+
           :                               :
           :      RTP header extension     :  (if X set in context)
           :                               :
           +-------------------------------+
           |                               |
           /           RTP data            /
           /                               /
           |                               |
           +-------------------------------+
           :            padding            :  (if P set in context)
           +...............................+


Usage for the CU* packet:

It is useful for the compressor to periodically refresh the state of
the decompressor to avoid having the decompressor send CONTEXT_STATE
messages in the case of unrecoverable packet loss.
Using the flags F=0 I dI dT, this CU* packet refreshes all the context
parameters.

When compression is done over a lossy link with a long round trip
delay, we want to minimize context invalidation. If the delta values
are changing frequently, the context might get invalidated often. In
such cases the compressor may choose to include absolute values in the
CRTP packets instead of delta values, using CU* packets with the flags:
F=1, and any of S, T, I as necessary.



2.6 Acknowledgement packet (ACK packet)

The ACK packet will be sent from decompressor to compressor to indicate
receipt of a compressed packet with the ACK'd RTP sequence number.
ItÆs a CONTEXT_STATE packet with type codes 4 and 5.  The ACK packet is
to be used in a separately negotiated mode of operation as described in
the next section.

Type code = 4 :  ACK a packet of a context with 8-bit CID
Type code = 5 :  ACK a packet of a context with 16-bit CID

The format for the ACK packet is:

             0   1   2   3   4   5   6   7
           +---+---+---+---+---+---+---+---+
           |  Type code=4: ACK, 8-bit CID  |
           +---+---+---+---+---+---+---+---+
           |         context count         |
           +---+---+---+---+---+---+---+---+
           |       session context ID      |
           +---+---+---+---+---+---+---+---+

           |                               |
           +      RTP sequence number      +
           |                               |
           +---+---+---+---+---+---+---+---+
                         . . .
           +---+---+---+---+---+---+---+---+
           |       session context ID      |
           +---+---+---+---+---+---+---+---+

           |                               |
           +      RTP sequence number      +
           |                               |
           +---+---+---+---+---+---+---+---+


             0   1   2   3   4   5   6   7
           +---+---+---+---+---+---+---+---+
           |  Type code=5: ACK, 16-bit CID |
           +---+---+---+---+---+---+---+---+
           |         context count         |
           +---+---+---+---+---+---+---+---+
           |                               |
           +       session context ID      +
           |                               |
           +---+---+---+---+---+---+---+---+

           |                               |
           +      RTP sequence number      +
           |                               |
           +---+---+---+---+---+---+---+---+
                         . . .
           +---+---+---+---+---+---+---+---+
           |                               |
           +       session context ID      +
           |                               |
           +---+---+---+---+---+---+---+---+

           |                               |
           +      RTP sequence number      +
           |                               |
           +---+---+---+---+---+---+---+---+



2.6.1 CRTP operation in ACK mode

This mode of operation is optional and must be negotiated per link.


2.6.1.1   Description of the ACK mode

The ACK mode is a mode of operation in which the compressor and
decompressor continuously verify that their context states are
synchronized. The compressor repeatedly notifies the decompressor about
changes in the context state, until the decompressor acknowledges
reception of the changes by sending ACK packets to the decompressor.
This effort of synchronizing the context states helps minimize context
invalidation.

The context state shared between the compressor and decompressor
includes all the fields of the uncompressed headers and the first order
differences (delta fields) of the fields that change by a constant
value from packet to packet. Each field follows its known change
pattern: either stays constant or is incremented by its corresponding
delta field. Fields that follow their change pattern are compressed.
They are reconstructed by the decompresor from the context state at the
decompressor. Correct decompression of a packet depends on whether the
context state at the compressor when the packet is compressed and sent
is identical to the context state at the decompressor when that packet
is received and decompressed.

When a field changes in a way that is different from its change
pattern, the compressor assigns a new value to the field, and stores it
in the context state at the compressor side. The decompressor must be
informed about the change so that it can update the state on its side
to match the state at the compressor. The compressor notifies the
decompressor about such changes by including information about the
changed field in the compressed packet. (for example if dT was assigned
a new value, the compressor can send a CR+ packet that includes dT).
The context is not synchronized until the decompressor receives the
packet that includes the changed field and updates its state
accordingly.

The decompressor indicates reception of the change by sending an ACK
packet to the compressor. The ACK packet includes the RTP sequence
number of the packet that it is ACKÆing, so the compressor can identify
which packet is ACKÆd. The compressor canÆt assume that the
decompressor received the change until the ACK packet is received.

Depending on the round trip delay of the link, the compressor might
have to send a few more packets before the ACK from the decompressor
arrives. In this case the compressor must repeat the change in all
subsequent packets. Reception of the ACK is an indication that the
decompressor updated its context with the changed value. Now that their
contexts are synchronized again, the compressor can stop including the
changed field in the compressed packets.


The decompressor must be able to recognize the repeat packets (the
packets that repeat the same change and were sent while the compressor
was waiting for the ACK packet). Those repeat packets donÆt require an
ACK.

If in the process of changing some fields additional changes are
required, the compressor will switch to send packets that include all
changes. The decompressor must ACK one of the packets that include all
the changes.

The compressor and decompressor must be in full agreement about which
packets must be ACKÆd: packets that include changes are larger in size,
and if they are not ACKÆd, the changes are repeated in all subsequent
packets, and bandwidth is wasted.

LetÆs summarize which packets require an ACK:

1. A Packet that assigns a value to any context state field  must be
ACKÆd. This includes FH and CU packets because they initialize
fields in the context state.
2. Repeat packets donÆt require an ACK

How are repeat packets identified?

A packet is considered to be a repeat packet if:
1. It updates the same fields as the previous packet
2. Each field is updated by a value that is equal to the one assigned
to this field in the previous packet plus the corresponding delta
for this field, when applicable.



2.6.1.2   The Random IP ID

The IP ID change pattern is to be incremented by dI. In some
implementations, the IP ID counter is shared across multiple streams,
so as a result of the varying mix of packets the increment for any
particular stream is not constant. When compressing such a stream, the
compressor must include in each packet either dI or I. It is
recommended to include I rather then dI because a loss of a packet that
includes a new delta value dI will invalidate the context.
According to the rules set above, each packet will have to be ACKÆd.

To correct this weÆll define a new change pattern for the IP ID: random
value. The IP ID assumes this change pattern when dI is set to be 0.

We add a rule to the ACK rules:
3. When the value of dI is 0, packets that update only the IP ID field
donÆt require an ACK.

And add to rule 2 of the repeat packet rules:
2. Each field is updated by a value that is equal to the one assigned
to this field in the previous packet plus the corresponding delta for
this field, when applicable. An exception to this rule is the IP ID
field: if the value of dI is 0, the IP ID may be assigned any value.


2.6.1.3   Implementation hints when using the ACK scheme

1. When a delta field is updated, add the matching absolute field too
(dT and T, dI and I). Loss of a packet that updates only the delta
value can easily cause context invalidation.
2. Set dI=0 when the IP ID is changing randomly, and include I in all
packets.
3. If you ACKÆd a packet, but the number of repeat packets exceed your
estimate, ACK again (your previous ACK was probably lost)


Here is an example to demonstrate the usage of the ACK scheme.
In this stream the audio codec sends a sample every 10 msec
The first talk spurt is 1 second long. Then there are 2 seconds
silence, then another talk spurt.

When there is no loss on the link, we can use the following sequence:
(The deltaID is not constant so we send deltaID in each packet)

seq#  Time  pkt type
1     10    FH
2     20    CR+     dI dT=10
3     30    CR+     dI
4     40    CR+     dI
...
100   1000  CR+     dI

101   3010  CR+     dI dT=2010
102   3020  CR+     dI dT=10
103   3030  CR+     dI
104   3040  CR+     dI
...

In the above sequence if a packet is lost, we cannot recover ('twice'
will not work due to the random IP ID) and the context must be
invalidated.


Here is the same sequence using the ACK scheme(CU* is the enhanced CU):


seq#  Time  pkt type  flags
1     10    FH                                       FH must be ACK'd
2     20    FH                                       repeat
ACK 1
3     30    CU*   1 I dT dI M 0 T 0   I T=30 dT=10 dI=0  dI,dT changed
 (packet 3 was lost)                               (I and T sent too)
4     40    CU*   1 I dT dI M 0 T 0   I T=40 dT=10 dI=0  repeat
5     50    CU*   1 I dT dI M 0 T 0   I T=50 dT=10 dI=0  repeat
6     60    CU*   1 I dT dI M 0 T 0   I T=60 dT=10 dI=0  repeat
ACK 4 == got new dI=0 and dT=10 at T=40.
          dI was set to 0, so I does not require an ACK.
          No need to ACK 5 and 6: repeat packets
7     70    CU*   1 I  0 0  M 0 0 0   I
8     80    CU*   1 I  0 0  M 0 0 0   I
...
100   1000  CU*   1 I  0 0  M 0 0 0   I

101   3010  CU*   1 I  0 0  M 0 T 0   I T=3010   T changed, keep
deltas!
102   3020  CU*   1 I  0 0  M 0 T 0   I T=3020   repeat
ACK 101 == got new T at sequence 101
           No need to ACK packet 102 because 3010 + dT = 3020
           If 101 is lost, 102 will be ACK'd
103   3030  CU*   1 I  0 0  M 0 0 0   I
104   3040  CU*   1 I  0 0  M 0 0 0   I
...


The same sequences, when delta IP ID is constant:

seq#  Time  pkt type
1     10    FH
2     20    CR+     dI dT=10
3     30    CR
4     40    CR
...
100   1000  CR

101   3010  CR+    dT=2010
102   3020  CR+    dT=10
103   3030  CR
104   3040  CR
...




seq#  Time  pkt type  flags
1     10    FH                                       FH must be ACK'd
2     20    FH                                       repeat
ACK 1
3     30    CU*   1 I dT dI M 0 T 0   I dI T=30 dT=10  dI,dT changed
  (packet 3 was lost)                                 (I and T sent
too)
4     40    CU*   1 I dT dI M 0 T 0   I dI T=40 dT=10  repeat
5     50    CU*   1 I dT dI M 0 T 0   I dI T=50 dT=10  repeat
6     60    CU*   1 I dT dI M 0 T 0   I dI T=60 dT=10  repeat
ACK 4 == got new dI and dT=10 at T=40.
          No need to ACK 5 and 6: no changes
7     70    CR
8     80    CR
...
100   1000  CR

101   3010  CU*   1 0  0 0  M 0 T 0   T=3010    T changed, keep deltas!
102   3020  CU*   1 0  0 0  M 0 T 0   T=3020    repeat
ACK 101 == got new T at sequence 101
          No need to ACK packet 102 because 3010 + dT = 3020
          If 101 is lost, 102 will be ACK'd
103   3030  CR
104   3040  CR
...




2.8 CRTP operation in 'N' mode

This scheme is similar to the ACK scheme in that the compressor tries
to keep the decompressor in sync by sending changes multiple times. The
'N' is a number that represents the quality of the link between the
hosts, and it means that the probability of more than 'N' adjacent
packets getting lost on this link is small. For every change in a base
value or a delta value, if the compressor includes the change in N+1
consecutive packets, there is a very good chance that the compressor
and decompressor can  stay in sync using the 'twice' algorithm.
CONTEXT_STATE packets should also be repeated N+1 times (using the same
sequence number).
It is up to the implementation to find a scheme to derive an
appropriate N for a link.

This scheme may be used at any time and does not require negotiation.

Here is the same example in 'N' mode, when N=2 and deltaID is constant:

seq#  Time  pkt type  flags
1     10    FH
2     20    FH                                  repeat constant fields
3     30    FH                                  repeat constant fields
4     40    CU*   1 I dT dI M 0 T 0   I dI T=40 dT=10
5     50    CU*   1 I dT dI M 0 T 0   I dI T=50 dT=10  repeat delta
6     60    CU*   1 I dT dI M 0 T 0   I dI T=60 dT=10  repeat delta
7     70    CR
8     80    CR
...
100   1000  CR

101   3010  CU*   1 0  0 0  M 0 T 0   T=3010    T changed, keep deltas!
102   3020  CU*   1 0  0 0  M 0 T 0   T=3020    repeat updated T
103   3030  CU*   1 0  0 0  M 0 T 0   T=3030    repeat updated T
104   3040  CR
105   3050  CR
...




2.9 Negotiating usage of enhanced-CRTP and ACK scheme

RFC 2509 [IPCPHP] specifies how the use of CRTP is negotiated on PPP
links using the IP Compression Protocol option of IPCP:

    IPCP option 2: IP compression protocol
    protocol 0x61 indicates RFC 2507 header compression
    sub-option 1 enables use of COMPRESSED_RTP, COMPRESSED_UDP and
                  CONTEXT_STATE as specified in RFC 2508

For the enhancements defined in this document, two new sub-options are
added:

    sub-option 2 (length=2) :  enables use of CRTP with
                               enhancements 2.1 - 2.5
    sub-option 3 (length=2) :  enables use of CRTP with
                               enhancements 2.1 - 2.6 (ACK scheme)


3. Acknowledgements
   The authors would like to thank Van Jacobson, co-author of RFC2508,
and the authors of RFC2507, Mikael Degermark, Bjorn Nordgren, and
Stephen Pink. The authors would also like to thank Dana Blair, Francois
Le Faucheur, Tim Gleeson, Matt Madison, Hussein Salama, Mallik
Tatipamula, Mike Thomas, and Herb Wildfeuer.



4. References

   [CRTP] S. Casner, V. Jacobson, "Compressing IP/UDP/RTP Headers for
   Low-Speed Serial Links", RFC2508, February 1999.

   [IPHCOMP] M. Degermark, B. Nordgren, S. Pink,
   "IP Header Compression", RFC2507, February 1999.

   [IPCPHC] M. Engan, S. Casner, C. Bormann,
   "IP Header Compression over PPP", RFC2509, February 1999.

   [KEYW] S. Bradner, "Key words for use in RFCs to Indicate
   Requirement Levels", RFC2119, BCP 14, March 1997.

   [RTP] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson,
   "RTP: A Transport Protocol for Real-Time Applications", RFC1889,
   January 1996.



5. Authors' Addresses

   Stephen L. Casner
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134-1706
   United States of America

   Email: casner@cisco.com


   John Geevarghese
   Motorola India Electronics Ltd,
   Ulsoor Road
   Bangalore - 42
   India

   Email: gvjohn@miel.mot.com


   Tmima Koren
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134-1706
   United States of America
   Email: tmima@cisco.com


   Patrick Ruddy
   Cisco Systems, Inc.
   3rd Floor, 96 Commercial Street
   Edinburgh
   EH6 6LX
   Scotland

   Email: pruddy@cisco.com


   Bruce Thompson
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134-1706
   United States of America

   Email: brucet@cisco.com


   Alex Tweedly
   Cisco Systems, Inc.
   3 The Square, Stockley Park
   Uxbridge, Middlesex
   UB11 1BN
   United Kingdom

   Email: agt@cisco.com


   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134-1706
   United States of America

   Email: dwing@cisco.com




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Casner, Geevarghese, Koren, Ruddy, Thompson, Tweedly, Wing
Expires Oct 2000