Network Working Group T. Koren

Network Working Group T. Koren
Request for Comments: 3545 Cisco Systems
Category: Standards Track S. Casner
Packet Design
J. Geevarghese
Motorola India Electronics Ltd.
B. Thompson
P. Ruddy
Cisco Systems
July 2003

Enhanced Compressed RTP (CRTP) for Links with High Delay,
Packet Loss and Reordering

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

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

Abstract

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.

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Table of Contents

1. Introduction ................................................. 2
1.1. CRTP Operation ......................................... 4
1.2. How do contexts get corrupted? ......................... 4
1.3. Preventing context corruption .......................... 5
1.4. Specification of Requirements .......................... 5
2. Enhanced CRTP ................................................ 5
2.1. Extended COMPRESSED_UDP packet ......................... 6
2.2. CRTP Headers Checksum .................................. 11
2.3. Achieving robust operation ............................. 13
2.3.1. Examples ....................................... 15
3. Negotiating usage of enhanced-CRTP ........................... 18
4. Security Considerations ...................................... 18
5. Acknowledgements ............................................. 19
6. References ................................................... 19
6.1. Normative References ................................... 19
6.2. Informative References ................................. 20
7. Intellectual Property Rights Notice .......................... 20
8. Authors' Addresses ........................................... 21
9. Full Copyright Statement ..................................... 22

1. 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

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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.

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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 re-establish 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.

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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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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 IPv4 ID and the following RTP
values: sequence number, timestamp, payload type, CSRC count and
CSRC list. This allows context sync to be maintained even with
some packet loss.

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- 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

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flag P indicates inclusion of the absolute RTP payload type value and
another flag C indicates the inclusion of the CSRC count. When C=1,
an additional byte is added following the two flag bytes to include
the absolute value of the four-bit CC field in the RTP header.

The last of the three changes to the COMPRESSED_UDP packet deals with
updating the IPv4 ID field. For this field, the COMPRESSED_UDP
packet as specified in RFC 2508 can already convey a new value for
the delta 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 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 : C : 0 : 0 : 0 : (if F = 1)
+...+...+...+...+...+...+...+...+

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

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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.

When C=1, an additional byte is added following the two flag bytes.
This byte includes the CC, the count of CSRC identifiers, in its
lower 4 bits:

+---+---+---+---+---+---+---+---+
| F | I |dT |dI | link sequence |
+---+---+---+---+---+---+---+---+
: M : S : T : P : C : 0 : 0 : 0 : (if F = 1)
+...+...+...+...+...+...+...+...+
: 0 : 0 : 0 : 0 : CC : (if C = 1)
+...+...+...+...+...............+

The bits marked "0" in the second flag byte and the CC byte SHOULD be
set to zero by the sender and SHOULD be ignored by the receiver.

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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, C. If C=1, there is an additional byte
that includes the number of CSRC identifiers. When F=1, 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.

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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 | C | 0 | 0 | 0 |
+---+---+---+---+---+---+---+---+
: 0 : 0 : 0 : 0 : CC : (if C = 1)
+...+...+...+...+...............+
: :
+ 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)
: :
+...............................+

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: :
: 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.

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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, the first 12
bytes of the UDP data which are assumed to hold the fixed part of an
RTP header, and the CSRC list. The extended part of the RTP header
beyond the CSRC list 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 beyond the CSRC list.

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 COMPRESSED_UDP
and COMPRESSED_RTP packets:

For 8-bit context ID:

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

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

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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
HDRCKSUM).

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.

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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.

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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 IPv4 ID changes for each packet.

In these examples, we will use some short notations:

FH FULL_HEADER
CR COMPRESSED_RTP
CU COMPRESSED_UDP

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 IPv4 ID) and the context must
be invalidated.

Koren, et al. Standards Track [Page 15]

RFC 3545 Enhanced Compressed RTP (CRTP) July 2003

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 IPv4 ID (I) and not the delta 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
...

Koren, et al. Standards Track [Page 16]

RFC 3545 Enhanced Compressed RTP (CRTP) July 2003

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

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
...

Koren, et al. Standards Track [Page 17]

RFC 3545 Enhanced Compressed RTP (CRTP) July 2003

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

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 [SRTP]. That would allow compression to still be applied.

Koren, et al. Standards Track [Page 18]

RFC 3545 Enhanced Compressed RTP (CRTP) July 2003

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 ways.

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,
Andrew Johnson, and Dan Wing.

6. References

6.1. Normative References

[CRTP] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers
for Low-Speed Serial Links", RFC 2508, February 1999.

[IPHCOMP] Degermark, M., Nordgren, B. and S. Pink, "IP Header
Compression", RFC 2507, February 1999.

[IPCPHC] Koren, T., Casner, S. and C. Bormann, "IP Header
Compression over PPP", RFC 3544, July 2003.

[KEYW] Bradner, S. "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

Koren, et al. Standards Track [Page 19]

RFC 3545 Enhanced Compressed RTP (CRTP) July 2003

[RTP] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
3550, July 2003.

6.2. 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, M., McGrew, D., Carrara, E., Naslund, M. and K.
Norrman, "The Secure Real-time Transport Protocol", Work in
Progress.

7. Intellectual Property Rights Notice

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.

Koren, et al. Standards Track [Page 20]

RFC 3545 Enhanced Compressed RTP (CRTP) July 2003

8. Authors' Addresses

Tmima Koren
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
USA

EMail: [email protected]

Stephen L. Casner
Packet Design
3400 Hillview Avenue, Building 3
Palo Alto, CA 94304
USA

EMail: [email protected]

John Geevarghese
Motorola India Electronics Ltd.
No. 33 A Ulsoor Road
Bangalore, India

EMail: [email protected]

Bruce Thompson
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
USA

EMail: [email protected]

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

EMail: [email protected]

Koren, et al. Standards Track [Page 21]

RFC 3545 Enhanced Compressed RTP (CRTP) July 2003

9. Full Copyright Statement

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

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

Koren, et al. Standards Track [Page 22]