Network Working Group Peter Kremer, Ericsson
INTERNET-DRAFT Lars-Erik Jonsson, Ericsson
Expires: December 2002 June, 2002
ROHC Implementer's Guide
<draft-ietf-rohc-rtp-impl-guide-01.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This document is an individual submission to the IETF. Comments
should be directed to the authors.
Abstract
This document describes common misinterpretations and some ambiguous
points of ROHC [RFC 3095], which defines the framework and four
profiles of a robust and efficient header compression scheme. These
points have been identified by the members of the ROHC working group
during different interoperability test events.
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Table of Contents
1. Introduction....................................................2
2. CRC Calculation.................................................2
3. Enhanced mode transition procedures.............................3
3.1. Modified transition logic for enhanced transitions............3
3.2. Transition from Reliable to Optimistic mode...................4
3.3. Transition to Unidirectional mode.............................5
4. Other clarifications............................................5
4.1. Padding octet in CRC..........................................5
4.2. Transition to timer-based compression.........................5
4.3. Generic extension header list.................................5
4.4. Generic CSRC list.............................................6
4.5. RTP dynamic chain.............................................6
4.6. Meaning of NBO................................................6
4.7. Implicit updates..............................................6
4.8. IP-ID.........................................................7
4.9. Extension-3 in UO-1-ID packets................................7
4.10. Extension-3 in UOR-2* packets................................7
4.11. Multiple SN options in one feedback packet...................7
4.12. Packet decoding during mode transition.......................7
4.13. How to respond to lost feedback links?.......................8
4.14. What does "presumed zero if absent" mean on page 88?.........8
4.15. CSRC list in UO-1-ID packets.................................8
5. Test Configuration..............................................9
6. Acknowledgment..................................................9
7. References.....................................................10
8. Authors' Addresses.............................................10
Appendix A. Sample CRC algorithm..................................11
1. Introduction
ROHC [RFC 3095] addresses a robust and efficient header compression
algorithm and as a such its description is long and complex. During
the implementation and the interoperability tests of the algorithm
some unclear areas have been identified. This document tries to
collect and clarify these points.
2. CRC Calculation
ROHC uses CRC checksum in order to provide some protection against
bit errors. CRC is used in the segmentation protocol and in the
compressed packets, as well.
Section 5.2.5.2 describes the segmentation protocol and refers to
[RFC 1662], which describes a well-defined CRC algorithm for 32 bit
checksums. Although, Section 5.9 only defines the polynomials for 3,
7 and 8-bit long checksum, the same algorithm can be used in these
cases as well.
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A PERL implementation of the algorithm (written by Carsten Bormann)
can be found in Appendix A.
3. Enhanced mode transition procedures
To reduce transmission overhead and computational complexity
(including CRC calculation) associated with feedback packets sent for
each decompressed packet during mode transition, a decompressor can
be implemented with slightly modified mode transition procedures,
compared to those defined in [RFC3095].
These modifications affect transitions to Optimistic and
Unidirectional modes of operation, i.e. the transitions described in
sections 5.6.5 and 5.6.6 of [RFC3095], and make those transition
diagrams more consistent with the diagram describing the transition
to R-mode. However, the differences between the original diagrams of
[RFC3095] were motivated by robustness concerns for mode transitions
to Optimistic and Unidirectional modes. To avoid deadlock situations
in mode transitions, these aspects must be addressed also when a
decompressor implements the enhanced transition procedures, and that
is done by following a slightly modified definition of the
decompressor transition states. All aspects related to implementation
of the enhanced transition procedures are described in subsequent
chapters.
Note that these modified operations should be seen only as an
improved decompressor implementation, since interoperability is not
at all affected. A decompressor implemented according to the
optimized procedures would be able to interoperate with an RFC3095
compressor, as well as a decompressor implemented according to the
procedures described in RFC3095 would do.
3.1. Modified transition logic for enhanced transitions
The intent with these enhanced transition procedures is to allow the
decompressor to stop sending feedback packets for all packets
decompressed during the second half of the transition procedure, i.e.
after an appropriate IR/IR-DYN/UOR-2 packet has been received from
the compressor. In the transition diagrams, sections 3.2 and 3.3
below, this is realized by allowing the decompressor transition
parameter (D_TRANS) to be set to P (Pending) at that stage. However,
as mentioned above, there are robustness concerns related to this
optimization, and to avoid deadlock situations with never completed
transitions as a result of feedback losses, the decompressor must
continue to send feedback at least periodically, also when in Pending
transition state. That would be the equivalence of enhancing the
D_TRANS parameter definition in section 5.6.1 of [RFC3095], to
include a definition of Pending state operation.
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- D_TRANS:
Possible values for the D_TRANS parameter are (I)NITIATED,
(P)ENDING and (D)ONE. D_TRANS MUST be initialized to D, and a mode
transition can be initiated only when D_TRANS is D. While D_TRANS
is I, the decompressor sends a NACK or ACK carrying a CRC option
for each packet received. When D_TRANS is set to P, the
decompressor do not have to send a NACK or ACK for each packet
received, but it MUST continue to send feedback on a regular
basis, and all feedback packets sent MUST include the CRC option.
This ensures that all mode transitions will be completed also in
case of feedback losses.
3.2. Transition from Reliable to Optimistic mode
The enhanced procedure for transition from Reliable to Optimistic
mode is shown below:
Compressor Decompressor
----------------------------------------------
| |
| ACK(O)/NACK(O) +-<-<-<-| D_TRANS = I
| +-<-<-<-<-<-<-<-+ |
C_TRANS = P |-<-<-<-+ |
C_MODE = O | |
|->->->-+ IR/IR-DYN/UOR-2(SN,O) |
| +->->->->->->->-+ |
|->-.. +->->->-| D_TRANS = P
|->-.. | D_MODE = O
| ACK(SN,O) +-<-<-<-|
| +-<-<-<-<-<-<-<-+ |
C_TRANS = D |-<-<-<-+ |
| |
|->->->-+ UO-0, UO-1* |
| +->->->->->->->-+ |
| +->->->-| D_TRANS = D
| |
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3.3. Transition to Unidirectional mode
The enhanced procedure for transition to Unidirectional mode is shown
on the following figure:
Compressor Decompressor
----------------------------------------------
| |
| ACK(U)/NACK(U) +-<-<-<-| D_TRANS = I
| +-<-<-<-<-<-<-<-+ |
C_TRANS = P |-<-<-<-+ |
C_MODE = U | |
|->->->-+ IR/IR-DYN/UOR-2(SN,U) |
| +->->->->->->->-+ |
|->-.. +->->->-| D_TRANS = P
|->-.. |
| ACK(SN,U) +-<-<-<-|
| +-<-<-<-<-<-<-<-+ |
C_TRANS = D |-<-<-<-+ |
| |
|->->->-+ UO-0, UO-1* |
| +->->->->->->->-+ |
| +->->->-| D_TRANS = D
| | D_MODE= U
4. Other clarifications
4.1. Padding octet in CRC
According to Section 5.9.1, in case of IR and IR-DYN packets the CRC
"is calculated over the entire IR or IR-DYN packet, excluding Payload
and including CID or Add-CID octet". Padding isn't meant to be
meaningful part of a packet and not included in CRC calculation. As
a result, CRC doesn't cover the Add-CID octet for CID 0, either.
4.2. Transition to timer-based compression
During transition from window-based compression to timer-based
compression it is necessary to keep k large enough to cover both
interpretation intervals.
4.3. Generic extension header list
Section 5.7.7.4 defines the static and dynamic parts of the IPv4
header. This section indicates a 'Generic extension header list'
field in the dynamic chain, which has a variable length. The
detailed description of this field can be found in Section 5.8.6.1.
The generic extension header list starts with an octet that is always
present, so its length is one octet, at least. If the 'GP' bit in
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the first octet is set to 1 then the length of the list is two
octets, even if the list is empty.
4.4. Generic CSRC list
Section 5.7.7.6 defines the static and dynamic parts of the RTP
header. This section indicates a 'Generic CSRC list' field in the
dynamic chain, which has a variable length. This field uses the same
encoding rules as the 'Generic extension header list' in the IPv4
header, so the same rules apply to its length.
4.5. RTP dynamic chain
Section 5.7.7.6 defines the static and dynamic parts of the RTP
header. This section indicates a 'CC' field in the dynamic chain.
The same field can be found in the 'Generic CSRC list' with the same
meaning. In order to decode a compressed packet correctly it's
necessary to know the 'CC' value because it has serious impact on the
packet's length. In normal case, the values of the fields are equal.
Proposed behavior if the values are different:
Both fields are within the RTP dynamic part but only the second
'CC' field resides inside the 'Generic CSRC list' together with
the XI items. Since the 'CC' value determines the number of XI
items in the CSRC list and isn't used otherwise, the first CC
field should be ignored and only the second field (inside the CSRC
list) should be used for incoming packets. For outgoing packets
both fields should be set to the same value.
4.6. Meaning of NBO
In general, an unset flag indicates the normal operation and a set
flag indicates unusual behavior. In IPv4 dynamic part (Section
5.7.7.4), if the 'NBO' bit is set, it means that network byte order
is used. Although, network byte order is the more common byte
alignment.
4.7. Implicit updates
If an updating packet (e.g. R-0-CRC or UO-0) contains information
about a specific field X (compressed RTP sequence number, typically),
then X is updated according to the content of that packet. But this
packet implicitly updates all other inferred fields (i.e. RTP
timestamp) according to the current mode and the appropriate mapping
function of the updated and the inferred fields.
For example, a UO-0 packet contains the compressed RTP sequence
number (SN). Such a packet also updates RTP timestamp and IPv4 ID,
implicitly.
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4.8. IP-ID
According to Section 5.7 IP-ID means the compressed value of the IPv4
header's 'Identification' field. Type 0, 1 and 2 packets contain the
compressed value (IP-ID), however IR packets with dynamic chain and
IR-DYN packets transmit the original, uncompressed value. This is
because the dynamic part of the IPv4 header (Section 5.7.7.4)
contains the 'Identification' field instead of the IP-ID.
4.9. Extension-3 in UO-1-ID packets
Extension-3 is applied to give values and indicate changes to fields
other than SN, TS and IP-ID in IP header(s) and RTP header. In case
of UO-1-ID packets, values provided in extensions do not update the
context, except those in other SN, TS and IP-ID fields (Section
5.7.3.). Note, that SN, TS and IP-ID fields can also be sent in
Extension-0, -1 and -2 and the other fields of Extension-3 don't
update the context.
Besides, usage of Extension-3 in UO-1-ID can be useful to compress a
transient change in a packet stream. For example, if a field's value
changes in the current packet but in the next packet it returns to
its regular behavior. E.g. changes in TTL.
4.10. Extension-3 in UOR-2* packets
If Extension-3 is used in a UOR-2* packet then the information of the
extension updates the context (Section). Some flags of the IP header
in the extension (e.g. NBO or RND) can change the interpretation of
fields in UOR-2* packets.
In these cases, when a flag changes in Extension-3, a decompressor
should re-parse the UOR-2* packet.
4.11. Multiple SN options in one feedback packet
The length of the sequence number field in the original RTP header is
32 bits. A decompressor can't send back all the 32 bits in a
feedback packet unless it uses multiple SN options in one feedback
packet. Section 5.7.6.1 declares that a FEEDABCK-2 packet can
contain variable number of feedback options and the options can
appear in any order.
A compressor - that wants to be conform to the specification - should
be able to process multiple SN options in one feedback packet.
4.12. Packet decoding during mode transition
Each ROHC profile defines its own set of packet formats, and also its
own feedback packets. The use of various operational modes is also
defined by each specific profile. A decompressor can therefore not
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initiate a mode transfer request before at least one packet of a new
context has been correctly decompressed, establishing the context
based on one specific profile (as specified in IR packets). First
then the context has been established, the decompressor knows the
profile used, which modes are defined by that profile, and the
feedback packet formats available.
If the transition procedures in sections 5.6.5 and 5.6.6 of [RFC3095]
are followed (and not the enhanced procedures described in section 3
of this document), it is important to note that type 0 or type 1
packets may be received by the decompressor during the first half of
the transition procedure, and these packets must not mistakenly be
interpreted as the packets sent by the compressor to indicate
completed transition. The decompressor side must therefore keep track
of the transition status, e.g. with an additional parameter. If the
enhanced transition procedures described in section 3 of this
document are used, the D_TRANS parameter can serve this purpose since
its definition and usage is slightly modified.
4.13. How to respond to lost feedback links?
One potential issue that might have to be considered, depending on
link technology, is whether feedback links might get lost, and in
such cases how this is handled. When the compressor is notified that
the feedback channel is down, the compressor must be able to handle
it by restarting compression with creating a new context. Creating a
new context also implies to use a new CID value.
Generally, feedback links are not expected to disappear when once
present, but it should be noted that this might be the case for
certain link technologies.
4.14. What does "presumed zero if absent" mean on page 88?
On page 88, RFC 3095 says that R-P contains the absolute value of RTP
Padding bit and it's presumed zero if absent. It could be absent
from RTP header flags and fields, from the extension type 3 or from
the ROHC packet. It's been agreed that the RTP padding bit is
presumed zero if absent from the RTP header flags.
4.15. CSRC list in UO-1-ID packets
This section describes the situation when a UO-1-ID packet carries a
CSRC list. Compressed CSRC lists are encoded using Encoding Type 0
(section 5.7.5 and 5.8.6.1) and every list may have a unique
identifier (gen_id). The identifier is present in U/O-mode when the
compressor decides that it may use this list as a future reference.
On one hand, the decompressor shouldn't save the list because UO-1-ID
packets doesn't update the context. On the other hand, the
decompressor updates its translation table whenever an (Index, item)
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pair is received (section 5.8.1). The decompressor must be able to
handle such a packet, thus it has to behave as described in the
latter case. According to section 5.8.1.2, the compressor must
increment Counter by 1.
5. Test Configuration
ROHC is used to compress IP/UDP/RTP, IP/UDP and IP/ESP headers, thus
every ROHC implementation has an interface that can send and receive
IP packets (i.e. Ethernet). On the other hand, there must be an
interface (a serial link for example) or other means of transport (an
IP/UDP flow), which can transmit ROHC packets. Having these two
interfaces several configurations can be set up. The figure below
shows sample configurations that can be used for testing a ROHC
implementation:
IP/UDP/RTP +------------+ ROHC +--------------+ IP/UDP/RTP
packets | ROHC | packets | ROHC | packets
---------->| Compressor |-----> ----->| Decompressor |---------->
+------------+ +--------------+
Unfortunately, comparing the IP/UDP/RTP packets at the endpoints can
only show whether the reconstructed stream differs from the original
or not. In order to identify the place of the error more detailed
tests are needed. The next figure shows another possible scenario:
IP/UDP/RTP +------------+ ROHC IP/UDP/RTP +--------------+ ROHC
packets | ROHC | packets packets | ROHC | packets
+----->| Compressor |----->+ +<-----| Decompressor |<-----+
| +------------+ | | +--------------+ |
| | | |
| +------------+ | | +------------+ |
| | Test | | | | Test | |
+<-----| Equipment |<-----+ +------>| Equipment |------>+
+------------+ +------------+
In the first case, the test equipment generates the RTP stream and
also acts as a ROHC decompressor. The tester must recognize if the
original RTP stream was compressed in a bad way and gives an alarm.
In the second case, it is the test equipment that sends the
compressed ROHC packets and the Decompressor reconstructs the RTP
stream. Since the tester knows the ROHC packets and the
reconstructed RTP stream it can detect if the Decompressor makes a
mistake.
6. Acknowledgment
The authors would like thank Vicknesan Ayadurai, Carsten Bormann,
Zhigang Liu, Abigail Surtees and Mark West for their contributions
and comments.
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7. References
[RFC-3095] C. Bormann, Editor, RObust Header Compression (ROHC),
RFC 3095, July 2001.
[RFC-1662] W. Simpson, Editor, PPP in HDLC-like Framing, RFC 1662,
July 1994.
8. Authors' Addresses
Peter Kremer
Conformance and Software Test Laboratory
Ericsson Hungary
H-1300 Bp. 3., P.O. Box 107, HUNGARY
Phone: +36-1-437-7033
Fax: +36-1-437-7767
Email: Peter.Kremer@ericsson.com
Lars-Erik Jonsson
Box 920
Ericsson AB
SE-971 28 Lulea, Sweden
Phone: +46 920 20 21 07
Fax: +46 920 20 20 99
EMail: lars-erik.jonsson@ericsson.com
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Appendix A. Sample CRC algorithm
#!/usr/bin/perl -w
use strict;
#=================================
#
# ROHC CRC demo - Carsten Bormann cabo@tzi.org 2001-08-02
#
# This little demo shows the three types of CRC in use in RFC 3095,
# the robust header compression standard. Type your data in
# hexadecimal form and the press Control+D.
#
#---------------------------------
#
# utility
#
sub dump_bytes($) {
my $x = shift;
my $i;
for ($i = 0; $i < length($x); ) {
printf("%02x ", ord(substr($x, $i, 1)));
printf("\n") if (++$i % 16 == 0);
}
printf("\n") if ($i % 16 != 0);
}
#---------------------------------
#
# The CRC calculation algorithm.
#
sub do_crc($$$) {
my $nbits = shift;
my $poly = shift;
my $string = shift;
my $crc = ($nbits == 32 ? 0xffffffff : (1 << $nbits) - 1);
for (my $i = 0; $i < length($string); ++$i) {
my $byte = ord(substr($string, $i, 1));
for( my $b = 0; $b < 8; $b++ ) {
if (($crc & 1) ^ ($byte & 1)) {
$crc >>= 1;
$crc ^= $poly;
} else {
$crc >>= 1;
}
$byte >>= 1;
}
}
printf "%2d bits, ", $nbits;
printf "CRC: %02x\n", $crc;
}
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#---------------------------------
#
# Test harness
#
$/ = undef;
$_ = <>; # read until EOF
my $string = ""; # extract all that looks hex:
s/([0-9a-fA-F][0-9a-fA-F])/$string .= chr(hex($1)), ""/eg;
dump_bytes($string);
#---------------------------------
#
# 32-bit segmentation CRC
# Note that the text implies this is complemented like for PPP
# (this differs from 8, 7, and 3-bit CRC)
#
# C(x) = x^0 + x^1 + x^2 + x^4 + x^5 + x^7 + x^8 + x^10 +
# x^11 + x^12 + x^16 + x^22 + x^23 + x^26 + x^32
#
do_crc(32, 0xedb88320, $string);
#---------------------------------
#
# 8-bit IR/IR-DYN CRC
#
# C(x) = x^0 + x^1 + x^2 + x^8
#
do_crc(8, 0xe0, $string);
#---------------------------------
#
# 7-bit FO/SO CRC
#
# C(x) = x^0 + x^1 + x^2 + x^3 + x^6 + x^7
#
do_crc(7, 0x79, $string);
#---------------------------------
#
# 3-bit FO/SO CRC
#
# C(x) = x^0 + x^1 + x^3
#
do_crc(3, 0x6, $string);
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