Audio/Video Transport Working Group                         Tmima Koren
Internet Draft                                            Cisco Systems
January 28,
February 27, 2003                                        Stephen Casner
Expires August September 2003                                    Packet Design
draft-ietf-avt-crtp-enhance-07.txt                     John Geevarghese
                                                         Bruce Thompson
                                                          Patrick Ruddy
                                                          Cisco Systems

        Compressing IP/UDP/RTP headers on

        Enhanced Compressed RTP (CRTP) for links with high delay,
                 packet loss and reordering

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
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Copyright Notice

Copyright (C) The Internet Society (1999-2001). (1999-2003). All Rights Reserved.


This document describes a header compression scheme for point to
point links with packet loss and long delays. It is based on
Compressed Real-time Transport Protocol (CRTP), the IP/UDP/RTP
header compression described in RFC 2508. CRTP does not perform well
on such links: packet loss results in context corruption and due to
the long delay, many more packets are discarded before the context
is repaired. To correct the behavior of CRTP over such links, a few
extensions to the protocol are specified here. The extensions aim to
reduce context corruption by changing the way the compressor updates
the context at the decompressor: updates are repeated and include
updates to full and differential context parameters. With these
extensions, CRTP performs well over links with packet loss, packet
reordering and long delays.

1.0 Introduction

RTP header compression (CRTP) as described in RFC 2508 was designed
to reduce the header overhead of IP/UDP/RTP datagrams by compressing
the three headers. The IP/UDP/RTP headers are compressed to 2-4
bytes most of the time.

CRTP was designed for reliable point to point links with short
delays. It does not perform well over links with high rate of packet
loss, packet reordering and long delays.

An example of such a link is a PPP session that is tunneled using an
IP level tunneling protocol such as L2TP. Packets within the tunnel
are carried by an IP network and hence may get lost and reordered.
The longer the tunnel, the longer the round trip time.

Another example is an IP network that uses layer 2 technologies such
as ATM and Frame Relay for the access portion of the network. Layer
2 transport networks such as ATM and Frame Relay behave like point
to point serial links in that they do not reorder packets. In
addition, Frame Relay and ATM virtual circuits used as IP access
technologies often have a low bit rate associated with them. These
virtual circuits differ from low speed serial links in that they may
span a larger physical distance than a point to point serial link.
Speed of light delays within the layer 2 transport network will
result in higher round trip delays between the endpoints of the
circuit. In addition, congestion within the layer 2 transport
network may result in an effective drop rate for the virtual circuit
which is significantly higher than error rates typically experienced
on point to point serial links.

It may be desirable to extend existing CRTP implementations for use
also over IP tunnels and other virtual circuits, where packet
losses, reordering, and long delays are common characteristics.  To
address these scenarios, this document defines modifications and
extensions to CRTP to increase robustness to both packet loss and
misordering between the compressor and the decompressor.  This is
achieved by repeating updates and allowing the sending of absolute
(uncompressed) values in addition to delta values for selected
context parameters. Although these new mechanisms impose some
additional overhead, the overall compression is still substantial.
The enhanced CRTP, as defined in this document, is thus suitable for
many applications in the scenarios discussed above, e.g. tunneling
and other virtual circuits.

RFC 3095 defines another RTP header compression scheme called Robust
Header Compression [ROHC].  ROHC was developed with wireless links
as the main target, and introduced new compression mechanisms with
the primary objective to achieve the combination of robustness
against packet loss and maximal compression efficiency.  ROHC is
expected to be the preferred compression mechanism over links where
compression efficiency is important.  However, ROHC was designed
with the same link assumptions as CRTP, e.g. that the compression
scheme should not have to tolerate misordering of compressed packets
between the compressor and decompressor, which may occur when
packets are carried in an IP tunnel across multiple hops.

At some time in the future, enhancements may be defined for ROHC to
allow it to perform well in the presence of misordering of
compressed packets.  The result might be more efficient than the
compression protocol specified in this document.  However, there are
many environments for which the enhanced CRTP defined here may be
the preferred choice.  In particular, for those environments where
CRTP is already implemented, the additional effort required to
implement the extensions defined here is expected to be small.
There are also cases where the implementation simplicity of this
enhanced CRTP relative to ROHC is more important than the
performance advantages of ROHC.

1.1	CRTP Operation

During compression of an RTP stream, a session context is defined.
For each context, the session state is established and shared
between the compressor and the decompressor. Once the context state
is established, compressed packets may be sent.

The context state consists of the full IP/UDP/RTP headers, a few
first order differential values, a link sequence number, a
generation number and a delta encoding table.

The headers part of the context is set by the FULL_HEADER packet
that always starts a compression session. The first order
differential values (delta values) are set by sending COMPRESSED_RTP
packets that include updates to the delta values.

The context state must be synchronized between compressor and
decompressor for successful decompression to take place. If the
context gets out of sync, the decompressor is not able to restore
the compressed headers accurately. The decompressor invalidates the
context and sends a CONTEXT_STATE packet to the compressor
indicating that the context has been corrupted. To resume
compression, the compressor must reestablish the context.

During the time the context is corrupted, the decompressor discards
all the packets received for that context. Since the context repair
mechanism in CRTP involves feedback from the decompressor, context
repair takes at least as much time as the round trip time of the
link. If the round trip time of the link is long, and especially if
the link bandwidth is high, many packets will be discarded before
the context is repaired. On such links it is desirable to minimize
context invalidation.

1.2	How do contexts get corrupted?

As long as the fields in the combined IP/UDP/RTP headers change as
expected for the sequence of packets in a session, those headers can
be compressed, and the decompressor can fully restore the compressed
headers using the context state. When the headers don't change as
expected it's necessary to update some of the full or the delta
values of the context. For example, the RTP timestamp is expected to
increment by delta RTP timestamp (dT). If silence suppression is
used, packets are not sent during silence periods. Then when voice
activity resumes, packets are sent again, but the RTP timestamp is
incremented by a large value and not by dT. In this case an update
must be sent.

If a packet that includes an update to some context state values is
lost, the state at the decompressor is not updated. The shared state
is now different at the compressor and decompressor. When the next
packet arrives at the decompressor, the decompressor will fail to
restore the compressed headers accurately since the context state at
the decompressor is different than the state at the compressor.

1.3	Preventing context corruption

Note that the decompressor fails not when a packet is lost, but when
the next compressed packet arrives. If the next packet happens to
include the same context update as in the lost packet, the context
at the decompressor may be updated successfully and decompression
may continue uninterrupted. If the lost packet included an update to
a delta field such as the delta RTP timestamp (dT), the next packet
can't compensate for the loss since the update of a delta value is
relative to the previous packet which was lost. But if the update is
for an absolute value such as the full RTP timestamp or the RTP
payload type, this update can be repeated in the next packet
independently of the lost packet. Hence it is useful to be able to
update the absolute values of the context.

The next chapter describes several extensions to CRTP that add the
capability to selectively update absolute values of the context,
rather than sending a FULL_HEADER packet, in addition to the
existing updates of the delta values. This enhanced version of CRTP
is intended to minimize context invalidation and thus improve the
performance over lossy links with a long round trip time.

1.4 Specification of Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in [RFC2119].

2. Enhanced CRTP

This chapter specifies the changes in this enhanced version of CRTP.
They are:

-	Extensions to the COMPRESSED_UDP packet to allow updating the
differential RTP values in the decompressor context and to
selectively update the absolute IP IPv4 ID and the following RTP
values: sequence number, timestamp, payload type, and CSRC
count. This allows context sync to be maintained even with
some packet loss.

-	A "headers checksum" to be inserted by the compressor and
removed by the decompressor when the UDP checksum is not
present so that validation of the decompressed headers is
still possible. This allows the decompressor to verify that
context sync has not been lost after a packet loss.

An algorithm is then described to use these changes with repeated
updates to achieve robust operation over links with packet loss and
long delay.

2.1 Extended COMPRESSED_UDP packet

It is possible to accommodate some packet loss between the
compressor and decompressor using the "twice" algorithm in RFC 2508
so long as the context remains in sync. In that algorithm, the delta
values are added to the previous context twice (or more) to effect
the change that would have occurred if the missing packets had
arrived. The result is verified with the UDP checksum. Keeping the
context in sync requires reliably communicating both the absolute
value and the delta value whenever the delta value changes. For many
environments, sufficient reliability can be achieved by repeating
the update with each of several successive packets.

The COMPRESSED_UDP packet satisfies the need to communicate the
absolute values of the differential RTP fields, but it is specified
in RFC 2508 to reset the delta RTP timestamp. That limitation can be
removed with the following simple change: 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 enhanced version of CRTP changes
that specification to say that the T bit MAY be nonzero to indicate
that the delta RTP timestamp is included explicitly rather than
being reset to zero.

A second change adds another byte of flag bits to the COMPRESSED_UDP
packet to allow only selected individual uncompressed fields of the
RTP header to be included in the packet rather than carrying the
full RTP header as part of the UDP data. The additional flags do
increase computational complexity somewhat, but the corresponding
increase in bit efficiency is important when the differential field
updates are communicated multiple times in successive COMPRESSED_UDP
packets.  With this change, there are flag bits to indicate
inclusion of both delta values and absolute values, so 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. A new flag P indicates inclusion of the absolute RTP payload
type value and, as in the COMPRESSED_RTP packet, a four-bit CC field
copies the absolute value of the CC field in the RTP header.

The last of the three changes to the COMPRESSED_UDP packet deals
with updating the IP IPv4 ID field.  For this field, the COMPRESSED_UDP
packet as specified in RFC 2508 can already convey a new value for
the delta IP IPv4 ID, but not the absolute value which is only conveyed
by the FULL_HEADER packet. Therefore, a new flag I is added to the
COMPRESSED_UDP packet to indicate inclusion of the absolute IP IPv4 ID
value.  The I flag replaces the dS flag which is not needed in the
COMPRESSED_UDP packet since the delta RTP sequence number always
remains 1 in the decompressor context and hence does not need to be
updated. Note that IPv6 does not have an IP ID field, so when
compressing IPv6 packets both the I and the dI flags are always set
to 0.

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 for the COMPRESSED_UDP packet 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 : P :      CC       :  (if F = 1)

dI  = delta IP IPv4 ID
dT  = delta RTP timestamp
I   = absolute IP IPv4 ID
F   = additional flags byte
M   = marker bit
S   = absolute RTP sequence number
T   = absolute RTP timestamp
P   = RTP payload type
CC  = number of CSRC identifiers
CID = Context ID

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. As in RFC 2508, if dI=0, the decompressor does not change
deltaI. If dT=0, the decompressor sets deltaT to 0.

Some example packet formats will illustrate the use of the new
flags.  First, when F=0, the "traditional" COMPRESSED_UDP packet
which carries the full RTP header as part of the UDP data:

             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, P, 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. As in RFC 2508, if dI=0 the
decompressor does not change deltaI. However, in contrast to RFC
2508, if dT=0 the decompressor KEEPS THE CURRENT deltaT in the
context (DOES NOT set deltaT to 0).

An enhanced COMPRESSED_UDP packet is similar in contents and
behavior to a COMPRESSED_RTP 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 | P |      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 P = 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 enhanced COMPRESSED_UDP 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 and I=1, dI=1, dT=1, the COMPRESSED_UDP 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 always send absolute values
and never delta values, using COMPRESSED_UDP packets with the flags
F=1, and any of S, T, I as necessary.

2.2 CRTP Headers Checksum

RFC 2508, in Section 3.3.5, describes how the UDP checksum may be
used to validate header reconstruction periodically or when the
"twice" algorithm is used. When a UDP checksum is not present (has
value zero) in a stream, such validation would not be possible. To
cover that case, this enhanced CRTP provides an option whereby the
compressor MAY replace the null UDP checksum with a 16-bit headers
checksum (HDRCKSUM) which is subsequently removed by the
decompressor after validation. Note that this option is never used
with IPv6 since a null UDP checksum is not allowed.

A new flag C 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. The
compressor MAY set the C flag when UDP packet carried in the
FULL_HEADER packet originally contained a checksum value of zero.
If the C flag is set, the FULL_HEADER packet itself MUST also have
the 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 except
that it does not cover all of the UDP data. That is, the HDRCKSUM is
the 16-bit one's complement of the one's complement sum of the
pseudo-IP header (as defined for UDP), the UDP header, and the first
12 bytes of the UDP data which are assumed to hold the fixed part of
an RTP header. 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 packet where a UDP checksum would
have been. The decompressor MUST zero out the UDP checksum field in
the reconstructed packets.

For a non-RTP context, there may be fewer than 12 UDP data bytes
present. The IP and UDP headers can still be compressed into a
COMPRESSED_UDP packet. For this case, the HDRCKSUM is calculated
over the pseudo-IP header, the UDP header, and the UDP data bytes
that are present. If the number of data bytes is odd, then a zero
padding byte is appended for the purpose of calculating the
checksum, but not transmitted.

The HDRCKSUM does not validate the RTP data. If the link layer is
configured to deliver packets without checking for errors, then
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. The same holds for the
extended part of the RTP header.

Here is the format of the FULL_HEADER length fields with the new
flag C to indicate that a header checksum will be added in

For 8-bit context ID:

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

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

For 16-bit context ID:

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

|              CID              |  Second length field

2.3 Achieving robust operation

Enhanced CRTP achieves robust operation by sending changes multiple
times to keep the compressor and decompressor in sync. This method
is characterized by a number "N" that represents the quality of the
link between the hosts. What it means is that the probability of
more than N adjacent packets getting lost on this link is small. For
every change in a full value or a delta value, if the compressor
includes the change in N+1 consecutive packets, then the
decompressor can keep its context state in sync with the compressor
using the "twice" algorithm so long as no more than N adjacent
packets are lost.

Since updates are repeated in N+1 packets, if at least one of these
N+1 update packets is received by the decompressor, both the full
and delta values in the context at the decompressor will get updated
and its context will stay synchronized with the context at the
compressor. We can conclude that as long as less than N+1 adjacent
packets are lost, the context at the decompressor is guaranteed to
be synchronized with the context at the compressor, and use of the
"twice" algorithm to recover from packet loss will successfully
update the context and restore the compressed packets.

The link sequence number cycles in 16 packets, so it's not always
clear how many packets were lost. For example, if the previous link
sequence number was 5 and the current number is 4, one possibility
is that 15 packets were lost, but another possibility is that due to
misordering packet 5 arrived before packet 4 and they are really
adjacent. If there is an interpretation of the link sequence numbers
that could be a gap of less than N+1, the "twice" algorithm may be
applied that many times and verified with the UDP checksum (or the

When more than N packets are lost, all of the repetitions of an
update might have been lost. The context state may then be different
at the compressor and decompressor. The decompressor can still try
to recover by making one or more guesses for how many packets were
lost and then applying the "twice" algorithm that many times.
However, since the IPv4 ID field is not included in the checksum,
this does not validate the IPv4 ID.

The conclusion is that for IPv4 if more than N packets were lost,
the decompressor SHOULD NOT try to recover using the "twice"
algorithm and instead SHOULD invalidate the context and send a
CONTEXT_STATE packet. In IPv6 the decompressor MAY always try to
recover from packet loss by using the "twice" algorithm and
verifying the result with the UDP checksum.

It is up to the implementation to derive an appropriate N for a
link. The value is maintained independently for each context and is
not required to be the same for all contexts. When compressing a new
stream, the compressor sets a value of N for that context and sends
N+1 FULL_HEADER packets. The compressor MUST also repeat each
subsequent COMPRESSED_UDP update N+1 times. The value of N may be
changed for an existing context by sending a new sequence of
FULL_HEADER packets.

The decompressor learns the value of N by counting the number of
times the FULL_HEADER packet is repeated and storing the resulting
value in the corresponding context. If some of the FULL_HEADER
packets are lost, the decompressor may still be able to determine
the correct value of N by observing the change in the 4-bit sequence
number carried in the FULL_HEADER packets. Any inaccuracy in the
counting will lead the decompressor to assume a smaller value of N
than the compressor is sending. This is safe in that the only
negative consequence is that the decompressor might send a
CONTEXT_STATE packet when it was not really necessary to do so. In
response, the compressor will send FULL_HEADER packets again,
providing another opportunity for the decompressor to count the
correct N.

The sending of FULL_HEADER packets is also triggered by a change in
one of the fields held constant in the context, such as the IP TOS.
If such a change should occur while the compressor is in the middle
of sending the N+1 FULL_HEADER packets, then the compressor MUST
send N+1 FULL_HEADER packets after making the change. This could
cause the decompressor to receive more than N+1 FULL_HEADER packets
in a row with the result that it assumes a larger value for N than
is correct. That could lead to an undetected loss of context
synchronization. Therefore, the compressor MUST change the
"generation" number in the context and in the FULL_HEADER packet
when it begins sending the sequence of N+1 FULL_HEADER packets so
the decompressor can detect the new sequence. For IPv4, this is a
change in behavior relative to RFC 2508.

CONTEXT_STATE packets SHOULD also be repeated N+1 times (using the
same sequence number for each context) to provide a similar measure
of robustness against packet loss. Here N can be the largest N of
all contexts included in the CONTEXT_STATE packet, or any number the
decompressor finds necessary in order to ensure robustness.

2.3.1 Examples

Here are some examples to demonstrate the robust operation of
enhanced CRTP using N+1 repetitions of updates. In this stream the
audio codec sends a sample every 10 milliseconds. The first
talkspurt is 1 second long. Then there are 2 seconds of silence,
then another talkspurt. We also assume in this first example that
the IPv4 ID field does not increment at a constant rate because the
host is generating other uncorrelated traffic streams at the same
time and therefore the delta IP IPv4 ID changes for each packet.

In these examples, we will use some short notations:


When operating on a link with low loss, we can just use
COMPRESSED_RTP packets in the basic CRTP method specified in RFC
2508.  We might have the following packet sequence:

seq Time pkt    updates and comments
 #       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 unpredictable IP IPv4 ID) and the
context must be invalidated.

Here is the same example using the enhanced CRTP method specified in
this document, when N=2. Note that the compressor only sends the
absolute IP IPv4 ID (I) and not the delta IP IPv4 ID (dI).

seq Time pkt  CU flags            updates and comments
 #       type F I dT dI M S T P
1   10   FH
2   20   FH                             repeat constant fields
3   30   FH                             repeat constant fields
4   40   CU   1 1  1  0 M 0 1 0   I T=40 dT=10
5   50   CU   1 1  1  0 M 0 1 0   I T=50 dT=10 repeat update T & dT
6   60   CU   1 1  1  0 M 0 1 0   I T=60 dT=10 repeat update T & dT
7   70   CU   1 1  0  0 M 0 0 0   I
8   80   CU   1 1  0  0 M 0 0 0   I
100 1000 CU   1 1  0  0 M 0 0 0   I

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

This second example is the same sequence, but assuming the delta IP
ID is constant.  First the basic CRTP for a lossless link:

seq Time pkt    updates and comments
 #       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

For the equivalent sequence in enhanced CRTP, the more efficient
COMPRESSED_RTP packet can still be used once the deltas are all

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

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

Here is the second example when using IPv6.
First the basic CRTP for a lossless link:

seq Time pkt    updates and comments
 #       type
1   10   FH
2   20   CR     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

For the equivalent sequence in enhanced CRTP, the more efficient
COMPRESSED_RTP packet can still be used once the deltas are all

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

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

3. Negotiating usage of enhanced-CRTP

   The use of IP/UDP/RTP compression (CRTP) over a particular link is
   a function of the link-layer protocol.  It is expected that
   negotiation of the use of CRTP will be defined separately
   for each link layer.

   For link layers that already have defined a negotiation for the use
   of CRTP as specified in RFC 2508, an extension to that negotiation
   will be required to indicate use of the enhanced CRTP defined in
   this document since the syntax of the existing packet formats has
   been extended.

4. Security Considerations

Because encryption eliminates the redundancy that this compression
scheme tries to exploit, there is some inducement to forego
encryption in order to achieve operation over a low-bandwidth link.
However, for those cases where encryption of data and not headers is
satisfactory, RTP does specify an alternative encryption method in
which only the RTP payload is encrypted and the headers are left in
the clear. clear [SRTP]. That would allow compression to still be applied.

A malfunctioning or malicious compressor could cause the
decompressor to reconstitute packets that do not match the original
packets but still have valid IP, UDP and RTP headers and possibly
even valid UDP check-sums.  Such corruption may be detected with
end-to-end authentication and integrity mechanisms which will not be
affected by the compression. Constant portions of authentication
headers will be compressed as described in [IPHCOMP].

No authentication is performed on the CONTEXT_STATE control packet
sent by this protocol.  An attacker with access to the link between
the decompressor and compressor could inject false CONTEXT_STATE
packets and cause compression efficiency to be reduced, probably
resulting in congestion on the link.  However, an attacker with
access to the link could also disrupt the traffic in many other

A potential denial-of-service threat exists when using compression
techniques that have non-uniform receiver-end computational load.
The attacker can inject pathological datagrams into the stream which
are complex to decompress and cause the receiver to be overloaded
and degrading processing of other streams.  However, this
compression does not exhibit any significant non-uniformity.

5. Acknowledgements

The authors would like to thank Van Jacobson, co-author of RFC 2508,
and the authors of RFC 2507, 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, Alex Tweedly, Herb Wildfeuer, and
Dan Wing.

6. References

   Normative 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, T. Koren,
   "IP Header Compression over PPP",
   draft-koren-pppext-rfc2509bis-01.txt, February 2002.

   [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.

   Informative References

   [ROHC] Bormann, C., Burmeister, C., Degermark, M., Fukushima,
   H., Hannu, H., Jonsson, L., Hakenberg, R., Koren, T., Le,
   K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
   Wiebke, T., Yoshimura, T. and H. Zheng, "RObust Header
   Compression (ROHC): Framework and four profiles: RTP,
   UDP, ESP, and uncompressed", RFC 3095, July 2001.

   [SRTP] Baugher, McGrew, Oran, Blom, Carrara, Naslund, Norrman,
   "The Secure Real-time Transport Protocol",
   draft-ietf-avt-srtp-05.txt, June 2002

7. Authors' Addresses

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


   Stephen L. Casner
   Packet Design
   2465 Latham Street, Third Floor
   Mountain View, CA  94040
   United States of America


   John Geevarghese
   Telseon Inc.
   480 S. California
   Palo Alto, CA  94306
   United States of America


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


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


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