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RFC 3095 - RObust Header Compression (ROHC): Framework and four profiles: RTP, UDP, ESP, and uncompressed
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RFC3095 - RObust Header Compression (ROHC): Framework and four
Network Working Group C. Bormann, Editor, TZI/Uni Bremen
Request for Comments: 3095 C. Burmeister, Matsushita
Category: Standards Track M. Degermark, Univ. of Arizona
H. Fukushima, Matsushita
H. Hannu, Ericsson
L-E. Jonsson, Ericsson
R. Hakenberg, Matsushita
T. Koren, Cisco
K. Le, Nokia
Z. Liu, Nokia
A. Martensson, Ericsson
A. Miyazaki, Matsushita
K. Svanbro, Ericsson
T. Wiebke, Matsushita
T. Yoshimura, NTT DoCoMo
H. Zheng, Nokia
July 2001
RObust Header Compression (ROHC):
Framework and four profiles: RTP, UDP, ESP, and uncompressed
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 (2001). All Rights Reserved.
Abstract
This document specifies a highly robust and efficient header
compression scheme for RTP/UDP/IP (Real-Time Transport Protocol, User
Datagram Protocol, Internet Protocol), UDP/IP, and ESP/IP
(Encapsulating Security Payload) headers.
Existing header compression schemes do not work well when used over
links with significant error rates and long round-trip times. For
many bandwidth limited links where header compression is essential,
such characteristics are common.
This is done in a framework designed to be extensible. For example,
a scheme for compressing TCP/IP headers will be simple to add, and is
in development. Headers specific to Mobile IPv4 are not subject to
special treatment, but are expected to be compressed sufficiently
well by the provided methods for compression of sequences of
extension headers and tunneling headers. For the most part, the same
will apply to work in progress on Mobile IPv6, but future work might
be required to handle some extension headers, when a standards track
Mobile IPv6 has been completed.
Table of Contents
1. Introduction....................................................6
2. Terminology.....................................................8
2.1. Acronyms.....................................................13
3. Background.....................................................14
3.1. Header compression fundamentals..............................14
3.2. Existing header compression schemes..........................14
3.3. Requirements on a new header compression scheme..............16
3.4. Classification of header fields..............................17
4. Header compression framework...................................18
4.1. Operating assumptions........................................18
4.2. Dynamicity...................................................19
4.3. Compression and decompression states.........................21
4.3.1. Compressor states..........................................21
4.3.1.1. Initialization and Refresh (IR) State....................22
4.3.1.2. First Order (FO) State...................................22
4.3.1.3. Second Order (SO) State..................................22
4.3.2. Decompressor states........................................23
4.4. Modes of operation...........................................23
4.4.1. Unidirectional mode -- U-mode..............................24
4.4.2. Bidirectional Optimistic mode -- O-mode....................25
4.4.3. Bidirectional Reliable mode -- R-mode......................25
4.5. Encoding methods.............................................25
4.5.1. Least Significant Bits (LSB) encoding .....................25
4.5.2. Window-based LSB encoding (W-LSB encoding).................28
4.5.3. Scaled RTP Timestamp encoding .............................28
4.5.4. Timer-based compression of RTP Timestamp...................31
4.5.5. Offset IP-ID encoding......................................34
4.5.6. Self-describing variable-length values ....................35
4.5.7. Encoded values across several fields in compressed headers 36
4.6. Errors caused by residual errors.............................36
4.7. Impairment considerations....................................37
5. The protocol...................................................39
5.1. Data structures..............................................39
5.1.1. Per-channel parameters.....................................39
5.1.2. Per-context parameters, profiles...........................40
5.1.3. Contexts and context identifiers ..........................41
5.2. ROHC packets and packet types................................41
5.2.1. ROHC feedback .............................................43
5.2.2. ROHC feedback format ......................................45
5.2.3. ROHC IR packet type .......................................47
5.2.4. ROHC IR-DYN packet type ...................................48
5.2.5. ROHC segmentation..........................................49
5.2.5.1. Segmentation usage considerations........................49
5.2.5.2. Segmentation protocol....................................50
5.2.6. ROHC initial decompressor processing.......................51
5.2.7. ROHC RTP packet formats from compressor to decompressor....53
5.2.8. Parameters needed for mode transition in ROHC RTP..........54
5.3. Operation in Unidirectional mode.............................55
5.3.1. Compressor states and logic (U-mode).......................55
5.3.1.1. State transition logic (U-mode)..........................55
5.3.1.1.1. Optimistic approach, upwards transition................55
5.3.1.1.2. Timeouts, downward transition..........................56
5.3.1.1.3. Need for updates, downward transition..................56
5.3.1.2. Compression logic and packets used (U-mode)..............56
5.3.1.3. Feedback in Unidirectional mode..........................56
5.3.2. Decompressor states and logic (U-mode).....................56
5.3.2.1. State transition logic (U-mode)..........................57
5.3.2.2. Decompression logic (U-mode).............................57
5.3.2.2.1. Decide whether decompression is allowed................57
5.3.2.2.2. Reconstruct and verify the header......................57
5.3.2.2.3. Actions upon CRC failure...............................58
5.3.2.2.4. Correction of SN LSB wraparound........................60
5.3.2.2.5. Repair of incorrect SN updates.........................61
5.3.2.3. Feedback in Unidirectional mode..........................62
5.4. Operation in Bidirectional Optimistic mode...................62
5.4.1. Compressor states and logic (O-mode).......................62
5.4.1.1. State transition logic...................................63
5.4.1.1.1. Negative acknowledgments (NACKs), downward transition..63
5.4.1.1.2. Optional acknowledgments, upwards transition...........63
5.4.1.2. Compression logic and packets used.......................63
5.4.2. Decompressor states and logic (O-mode).....................64
5.4.2.1. Decompression logic, timer-based timestamp decompression.64
5.4.2.2. Feedback logic (O-mode)..................................64
5.5. Operation in Bidirectional Reliable mode.....................65
5.5.1. Compressor states and logic (R-mode).......................65
5.5.1.1. State transition logic (R-mode)..........................65
5.5.1.1.1. Upwards transition.....................................65
5.5.1.1.2. Downward transition....................................66
5.5.1.2. Compression logic and packets used (R-mode)..............66
5.5.2. Decompressor states and logic (R-mode).....................68
5.5.2.1. Decompression logic (R-mode).............................68
5.5.2.2. Feedback logic (R-mode)..................................68
5.6. Mode transitions.............................................69
5.6.1. Compression and decompression during mode transitions......70
5.6.2. Transition from Unidirectional to Optimistic mode..........71
5.6.3. From Optimistic to Reliable mode...........................72
5.6.4. From Unidirectional to Reliable mode.......................72
5.6.5. From Reliable to Optimistic mode...........................72
5.6.6. Transition to Unidirectional mode..........................73
5.7. Packet formats...............................................74
5.7.1. Packet type 0: UO-0, R-0, R-0-CRC .........................78
5.7.2. Packet type 1 (R-mode): R-1, R-1-TS, R-1-ID ...............79
5.7.3. Packet type 1 (U/O-mode): UO-1, UO-1-ID, UO-1-TS ..........80
5.7.4. Packet type 2: UOR-2 ......................................82
5.7.5. Extension formats..........................................83
5.7.5.1. RND flags and packet types...............................88
5.7.5.2. Flags/Fields in context..................................89
5.7.6. Feedback packets and formats...............................90
5.7.6.1. Feedback formats for ROHC RTP............................90
5.7.6.2. ROHC RTP Feedback options................................91
5.7.6.3. The CRC option...........................................92
5.7.6.4. The REJECT option........................................92
5.7.6.5. The SN-NOT-VALID option..................................92
5.7.6.6. The SN option............................................93
5.7.6.7. The CLOCK option.........................................93
5.7.6.8. The JITTER option........................................93
5.7.6.9. The LOSS option..........................................94
5.7.6.10. Unknown option types....................................94
5.7.6.11. RTP feedback example....................................94
5.7.7. RTP IR and IR-DYN packets..................................96
5.7.7.1. Basic structure of the IR packet.........................96
5.7.7.2. Basic structure of the IR-DYN packet.....................98
5.7.7.3. Initialization of IPv6 Header [IPv6].....................99
5.7.7.4. Initialization of IPv4 Header [IPv4, section 3.1].......100
5.7.7.5. Initialization of UDP Header [RFC-768]..................101
5.7.7.6. Initialization of RTP Header [RTP]......................102
5.7.7.7. Initialization of ESP Header [ESP, section 2]...........103
5.7.7.8. Initialization of Other Headers.........................104
5.8. List compression............................................104
5.8.1. Table-based item compression..............................105
5.8.1.1. Translation table in R-mode.............................105
5.8.1.2. Translation table in U/O-modes..........................106
5.8.2. Reference list determination..............................106
5.8.2.1. Reference list in R-mode and U/O-mode...................107
5.8.3. Encoding schemes for the compressed list..................109
5.8.4. Special handling of IP extension headers..................112
5.8.4.1. Next Header field.......................................112
5.8.4.2. Authentication Header (AH)..............................114
5.8.4.3. Encapsulating Security Payload Header (ESP).............115
5.8.4.4. GRE Header [RFC 2784, RFC 2890].........................117
5.8.5. Format of compressed lists in Extension 3.................119
5.8.5.1. Format of IP Extension Header(s) field..................119
5.8.5.2. Format of Compressed CSRC List..........................120
5.8.6. Compressed list formats...................................120
5.8.6.1. Encoding Type 0 (generic scheme)........................120
5.8.6.2. Encoding Type 1 (insertion only scheme).................122
5.8.6.3. Encoding Type 2 (removal only scheme)...................123
5.8.6.4. Encoding Type 3 (remove then insert scheme).............124
5.8.7. CRC coverage for extension headers........................124
5.9. Header compression CRCs, coverage and polynomials...........125
5.9.1. IR and IR-DYN packet CRCs.................................125
5.9.2. CRCs in compressed headers................................125
5.10. ROHC UNCOMPRESSED -- no compression (Profile 0x0000).......126
5.10.1. IR packet................................................126
5.10.2. Normal packet............................................127
5.10.3. States and modes.........................................128
5.10.4. Feedback.................................................129
5.11. ROHC UDP -- non-RTP UDP/IP compression (Profile 0x0002)....129
5.11.1. Initialization...........................................130
5.11.2. States and modes.........................................130
5.11.3. Packet types.............................................131
5.11.4. Extensions...............................................132
5.11.5. IP-ID....................................................133
5.11.6. Feedback.................................................133
5.12. ROHC ESP -- ESP/IP compression (Profile 0x0003)............133
5.12.1. Initialization...........................................133
5.12.2. Packet types.............................................134
6. Implementation issues.........................................134
6.1. Reverse decompression.......................................134
6.2. RTCP........................................................135
6.3. Implementation parameters and signals.......................136
6.3.1. ROHC implementation parameters at compressor..............137
6.3.2. ROHC implementation parameters at decompressor............138
6.4. Handling of resource limitations at the decompressor........139
6.5. Implementation structures...................................139
6.5.1. Compressor context........................................139
6.5.2. Decompressor context......................................141
6.5.3. List compression: Sliding windows in R-mode and U/O-mode..142
7. Security Considerations.......................................143
8. IANA Considerations...........................................144
9. Acknowledgments...............................................145
10. Intellectual Property Right Claim Considerations.............145
11. References...................................................146
11.1. Normative References.......................................146
11.2. Informative References.....................................147
12. Authors' Addresses...........................................148
Appendix A. Detailed classification of header fields.............152
A.1. General classification......................................153
A.1.1. IPv6 header fields........................................153
A.1.2. IPv4 header fields........................................155
A.1.3. UDP header fields.........................................157
A.1.4. RTP header fields.........................................157
A.1.5. Summary for IP/UDP/RTP....................................159
A.2. Analysis of change patterns of header fields................159
A.2.1. IPv4 Identification.......................................162
A.2.2. IP Traffic-Class / Type-Of-Service........................163
A.2.3. IP Hop-Limit / Time-To-Live...............................163
A.2.4. UDP Checksum..............................................163
A.2.5. RTP CSRC Counter..........................................164
A.2.6. RTP Marker................................................164
A.2.7. RTP Payload Type..........................................164
A.2.8. RTP Sequence Number.......................................164
A.2.9. RTP Timestamp.............................................164
A.2.10. RTP Contributing Sources (CSRC)..........................165
A.3. Header compression strategies...............................165
A.3.1. Do not send at all........................................165
A.3.2. Transmit only initially...................................165
A.3.3. Transmit initially, but be prepared to update.............166
A.3.4. Be prepared to update or send as-is frequently............166
A.3.5. Guarantee continuous robustness...........................166
A.3.6. Transmit as-is in all packets.............................167
A.3.7. Establish and be prepared to update delta.................167
Full Copyright Statement..........................................168
1. Introduction
During the last five years, two communication technologies in
particular have become commonly used by the general public: cellular
telephony and the Internet. Cellular telephony has provided its
users with the revolutionary possibility of always being reachable
with reasonable service quality no matter where they are. The main
service provided by the dedicated terminals has been speech. The
Internet, on the other hand, has from the beginning been designed for
multiple services and its flexibility for all kinds of usage has been
one of its strengths. Internet terminals have usually been general-
purpose and have been attached over fixed connections. The
experienced quality of some services (such as Internet telephony) has
sometimes been low.
Today, IP telephony is gaining momentum thanks to improved technical
solutions. It seems reasonable to believe that in the years to come,
IP will become a commonly used way to carry telephony. Some future
cellular telephony links might also be based on IP and IP telephony.
Cellular phones may have become more general-purpose, and may have IP
stacks supporting not only audio and video, but also web browsing,
email, gaming, etc.
One of the scenarios we are envisioning might then be the one in
Figure 1.1, where two mobile terminals are communicating with each
other. Both are connected to base stations over cellular links, and
the base stations are connected to each other through a wired (or
possibly wireless) network. Instead of two mobile terminals, there
could of course be one mobile and one wired terminal, but the case
with two cellular links is technically more demanding.
Mobile Base Base Mobile
Terminal Station Station Terminal
| ~ ~ ~ \ / \ / ~ ~ ~ ~ |
| | | |
+--+ | | +--+
| | | | | |
| | | | | |
+--+ | | +--+
| |
|=========================|
Cellular Wired Cellular
Link Network Link
Figure 1.1 : Scenario for IP telephony over cellular links
It is obvious that the wired network can be IP-based. With the
cellular links, the situation is less clear. IP could be terminated
in the fixed network, and special solutions implemented for each
supported service over the cellular link. However, this would limit
the flexibility of the services supported. If technically and
economically feasible, a solution with pure IP all the way from
terminal to terminal would have certain advantages. However, to make
this a viable alternative, a number of problems have to be addressed,
in particular problems regarding bandwidth efficiency.
For cellular phone systems, it is of vital importance to use the
scarce radio resources in an efficient way. A sufficient number of
users per cell is crucial, otherwise deployment costs will be
prohibitive. The quality of the voice service should also be as good
as in today's cellular systems. It is likely that even with support
for new services, lower quality of the voice service is acceptable
only if costs are significantly reduced.
A problem with IP over cellular links when used for interactive voice
conversations is the large header overhead. Speech data for IP
telephony will most likely be carried by RTP [RTP]. A packet will
then, in addition to link layer framing, have an IP [IPv4] header (20
octets), a UDP [UDP] header (8 octets), and an RTP header (12 octets)
for a total of 40 octets. With IPv6 [IPv6], the IP header is 40
octets for a total of 60 octets. The size of the payload depends on
the speech coding and frame sizes being used and may be as low as
15-20 octets.
From these numbers, the need for reducing header sizes for efficiency
reasons is obvious. However, cellular links have characteristics
that make header compression as defined in [IPHC,CRTP] perform less
than well. The most important characteristic is the lossy behavior
of cellular links, where a bit error rate (BER) as high as 1e-3 must
be accepted to keep the radio resources efficiently utilized. In
severe operating situations, the BER can be as high as 1e-2. The
other problematic characteristic is the long round-trip time (RTT) of
the cellular link, which can be as high as 100-200 milliseconds. An
additional problem is that the residual BER is nontrivial, i.e.,
lower layers can sometimes deliver frames containing undetected
errors. A viable header compression scheme for cellular links must
be able to handle loss on the link between the compression and
decompression point as well as loss before the compression point.
Bandwidth is the most costly resource in cellular links. Processing
power is very cheap in comparison. Implementation or computational
simplicity of a header compression scheme is therefore of less
importance than its compression ratio and robustness.
2. Terminology
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 RFC 2119.
BER
Bit Error Rate. Cellular radio links can have a fairly high BER.
In this document BER is usually given as a probability, but one
also needs to consider the error distribution as bit errors are
not independent.
Cellular links
Wireless links between mobile terminals and base stations.
Compression efficiency
The performance of a header compression scheme can be described
with three parameters: compression efficiency, robustness and
compression transparency. The compression efficiency is
determined by how much the header sizes are reduced by the
compression scheme.
Compression transparency
The performance of a header compression scheme can be described
with three parameters: compression efficiency, robustness, and
compression transparency. The compression transparency is a
measure of the extent to which the scheme ensures that the
decompressed headers are semantically identical to the original
headers. If all decompressed headers are semantically identical
to the corresponding original headers, the transparency is 100
percent. Compression transparency is high when damage propagation
is low.
Context
The context of the compressor is the state it uses to compress a
header. The context of the decompressor is the state it uses to
decompress a header. Either of these or the two in combination
are usually referred to as "context", when it is clear which is
intended. The context contains relevant information from previous
headers in the packet stream, such as static fields and possible
reference values for compression and decompression. Moreover,
additional information describing the packet stream is also part
of the context, for example information about how the IP
Identifier field changes and the typical inter-packet increase in
sequence numbers or timestamps.
Context damage
When the context of the decompressor is not consistent with the
context of the compressor, decompression may fail to reproduce the
original header. This situation can occur when the context of the
decompressor has not been initialized properly or when packets
have been lost or damaged between compressor and decompressor.
Packets which cannot be decompressed due to inconsistent contexts
are said to be lost due to context damage. Packets that are
decompressed but contain errors due to inconsistent contexts are
said to be damaged due to context damage.
Context repair mechanism
Context repair mechanisms are mechanisms that bring the contexts
in sync when they were not. This is needed to avoid excessive
loss due to context damage. Examples are the context request
mechanism of CRTP, the NACK mechanisms of O- and R-mode, and the
periodic refreshes of U-mode.
Note that there are also mechanisms that prevent (some) context
inconsistencies from occurring, for example the ACK-based updates
of the context in R-mode, the repetitions after change in U- and
O-mode, and the CRCs which protect context updating information.
CRC-DYNAMIC
Opposite of CRC-STATIC.
CRC-STATIC
A CRC over the original header is the primary mechanism used by
ROHC to detect incorrect decompression. In order to decrease
computational complexity, the fields of the header are
conceptually rearranged when the CRC is computed, so that it is
first computed over octets which are static (called CRC-STATIC in
this document) and then over octets whose values are expected to
change between packets (CRC-DYNAMIC). In this manner, the
intermediate result of the CRC computation, after it has covered
the CRC-STATIC fields, can be reused for several packets. The
restarted CRC computation only covers the CRC-DYNAMIC octets. See
section 5.9.
Damage propagation
Delivery of incorrect decompressed headers, due to errors in
(i.e., loss of or damage to) previous header(s) or feedback.
Loss propagation
Loss of headers, due to errors in (i.e., loss of or damage to)
previous header(s)or feedback.
Error detection
Detection of errors. If error detection is not perfect, there
will be residual errors.
Error propagation
Damage propagation or loss propagation.
Header compression profile
A header compression profile is a specification of how to compress
the headers of a certain kind of packet stream over a certain kind
of link. Compression profiles provide the details of the header
compression framework introduced in this document. The profile
concept makes use of profile identifiers to separate different
profiles which are used when setting up the compression scheme.
All variations and parameters of the header compression scheme
that are not part of the context state are handled by different
profile identifiers.
Packet
Generally, a unit of transmission and reception (protocol data
unit). Specifically, when contrasted with "frame", the packet
compressed and then decompressed by ROHC. Also called
"uncompressed packet".
Packet Stream
A sequence of packets where the field values and change patterns
of field values are such that the headers can be compressed using
the same context.
Pre-HC links
The Pre-HC links are all links that a packet has traversed before
the header compression point. If we consider a path with cellular
links as first and last hops, the Pre-HC links for the compressor
at the last link are the first cellular link plus the wired links
in between.
Residual error
Error introduced during transmission and not detected by lower-
layer error detection schemes.
Robustness
The performance of a header compression scheme can be described
with three parameters: compression efficiency, robustness, and
compression transparency. A robust scheme tolerates loss and
residual errors on the link over which header compression takes
place without losing additional packets or introducing additional
errors in decompressed headers.
RTT
The RTT (round-trip time) is the time elapsing from the moment the
compressor sends a packet until it receives feedback related to
that packet (when such feedback is sent).
Spectrum efficiency
Radio resources are limited and expensive. Therefore they must be
used efficiently to make the system economically feasible. In
cellular systems this is achieved by maximizing the number of
users served within each cell, while the quality of the provided
services is kept at an acceptable level. A consequence of
efficient spectrum use is a high rate of errors (frame loss and
residual bit errors), even after channel coding with error
correction.
String
A sequence of headers in which the values of all fields being
compressed change according to a pattern which is fixed with
respect to a sequence number. Each header in a string can be
compressed by representing it with a ROHC header which essentially
only carries an encoded sequence number. Fields not being
compressed (e.g., random IP-ID, UDP Checksum) are irrelevant to
this definition.
Timestamp stride
The timestamp stride (TS_STRIDE) is the expected increase in the
timestamp value between two RTP packets with consecutive sequence
numbers.
2.1. Acronyms
This section lists most acronyms used for reference.
AH Authentication Header.
CID Context Identifier.
CRC Cyclic Redundancy Check. Error detection mechanism.
CRTP Compressed RTP. RFC 2508.
CTCP Compressed TCP. Also called VJ header compression. RFC 1144.
ESP Encapsulating Security Payload.
FC Full Context state (decompressor).
FO First Order state (compressor).
GRE Generic Routing Encapsulation. RFC 2784, RFC 2890.
HC Header Compression.
IPHC IP Header Compression. RFC 2507.
IPX Flag in Extension 2.
IR Initiation and Refresh state (compressor). Also IR packet.
IR-DYN IR-DYN packet.
LSB Least Significant Bits.
MRRU Maximum Reconstructed Reception Unit.
MTU Maximum Transmission Unit.
MSB Most Significant Bits.
NBO Flag indicating whether the IP-ID is in Network Byte Order.
NC No Context state (decompressor).
O-mode Bidirectional Optimistic mode.
PPP Point-to-Point Protocol.
R-mode Bidirectional Reliable mode.
RND Flag indicating whether the IP-ID behaves randomly.
ROHC RObust Header Compression.
RTCP Real-Time Control Protocol. See RTP.
RTP Real-Time Protocol. RFC 1889.
RTT Round Trip Time (see section 2).
SC Static Context state (decompressor).
SN (compressed) Sequence Number. Usually RTP Sequence Number.
SO Second Order state (compressor).
SPI Security Parameters Index.
SSRC Sending source. Field in RTP header.
CSRC Contributing source. Optional list of CSRCs in RTP header.
TC Traffic Class. Octet in IPv6 header. See also TOS.
TOS Type Of Service. Octet in IPv4 header. See also TC.
TS (compressed) RTP Timestamp.
U-mode Unidirectional mode.
W-LSB Window based LSB encoding. See section 4.5.2.
3. Background
This chapter provides a background to the subject of header
compression. The fundamental ideas are described together with
existing header compression schemes. Their drawbacks and
requirements are then discussed, providing motivation for new header
compression solutions.
3.1. Header compression fundamentals
The main reason why header compression can be done at all is the fact
that there is significant redundancy between header fields, both
within the same packet header but in particular between consecutive
packets belonging to the same packet stream. By sending static field
information only initially and utilizing dependencies and
predictability for other fields, the header size can be significantly
reduced for most packets.
Relevant information from past packets is maintained in a context.
The context information is used to compress (decompress) subsequent
packets. The compressor and decompressor update their contexts upon
certain events. Impairment events may lead to inconsistencies
between the contexts of the compressor and decompressor, which in
turn may cause incorrect decompression. A robust header compression
scheme needs mechanisms for avoiding context inconsistencies and also
needs mechanisms for making the contexts consistent when they were
not.
3.2. Existing header compression schemes
The original header compression scheme, CTCP [VJHC], was invented by
Van Jacobson. CTCP compresses the 40 octet IP+TCP header to 4
octets. The CTCP compressor detects transport-level retransmissions
and sends a header that updates the context completely when they
occur. This repair mechanism does not require any explicit signaling
between compressor and decompressor.
A general IP header compression scheme, IP header compression [IPHC],
improves somewhat on CTCP and can compress arbitrary IP, TCP, and UDP
headers. When compressing non-TCP headers, IPHC does not use delta
encoding and is robust. When compressing TCP, the repair mechanism
of CTCP is augmented with a link-level nacking scheme which speeds up
the repair. IPHC does not compress RTP headers.
CRTP [CRTP, IPHC] by Casner and Jacobson is a header compression
scheme that compresses 40 octets of IPv4/UDP/RTP headers to a minimum
of 2 octets when the UDP Checksum is not enabled. If the UDP
Checksum is enabled, the minimum CRTP header is 4 octets. CRTP
cannot use the same repair mechanism as CTCP since UDP/RTP does not
retransmit. Instead, CRTP uses explicit signaling messages from
decompressor to compressor, called CONTEXT_STATE messages, to
indicate that the context is out of sync. The link round-trip time
will thus limit the speed of this context repair mechanism.
On lossy links with long round-trip times, such as most cellular
links, CRTP does not perform well. Each lost packet over the link
causes several subsequent packets to be lost since the context is out
of sync during at least one link round-trip time. This behavior is
documented in [CRTPC]. For voice conversations such long loss events
will degrade the voice quality. Moreover, bandwidth is wasted by the
large headers sent by CRTP when updating the context. [CRTPC] found
that CRTP did not perform well enough for a lossy cellular link. It
is clear that CRTP alone is not a viable header compression scheme
for IP telephony over cellular links.
To avoid losing packets due to the context being out of sync, CRTP
decompressors can attempt to repair the context locally by using a
mechanism known as TWICE. Each CRTP packet contains a counter which
is incremented by one for each packet sent out by the CRTP
compressor. If the counter increases by more than one, at least one
packet was lost over the link. The decompressor then attempts to
repair the context by guessing how the lost packet(s) would have
updated it. The guess is then verified by decompressing the packet
and checking the UDP Checksum -- if it succeeds, the repair is deemed
successful and the packet can be forwarded or delivered. TWICE
derives its name from the observation that when the compressed packet
stream is regular, the correct guess is to apply the update in the
current packet twice. [CRTPC] found that even with TWICE, CRTP
doubled the number of lost packets. TWICE improves CRTP performance
significantly. However, there are several problems with using TWICE:
1) It becomes mandatory to use the UDP Checksum:
- the minimal compressed header size increases by 100% to 4
octets.
- most speech codecs developed for cellular links tolerate errors
in the encoded data. Such codecs will not want to enable the
UDP Checksum, since they do want damaged packets to be
delivered.
- errors in the payload will make the UDP Checksum fail when the
guess is correct (and might make it succeed when the guess is
wrong).
2) Loss in an RTP stream that occurs before the compression point
will make updates in CRTP headers less regular. Simple-minded
versions of TWICE will then perform badly. More sophisticated
versions would need more repair attempts to succeed.
3.3. Requirements on a new header compression scheme
The major problem with CRTP is that it is not sufficiently robust
against packets being damaged between compressor and decompressor. A
viable header compression scheme must be less fragile. This
increased robustness must be obtained without increasing the
compressed header size; a larger header would make IP telephony over
cellular links economically unattractive.
A major cause of the bad performance of CRTP over cellular links is
the long link round-trip time, during which many packets are lost
when the context is out of sync. This problem can be attacked
directly by finding ways to reduce the link round-trip time. Future
generations of cellular technologies may indeed achieve lower link
round-trip times. However, these will probably always be fairly
high. The benefits in terms of lower loss and smaller bandwidth
demands if the context can be repaired locally will be present even
if the link round-trip time is decreased. A reliable way to detect a
successful context repair is then needed.
One might argue that a better way to solve the problem is to improve
the cellular link so that packet loss is less likely to occur. Such
modifications do not appear to come for free, however. If links were
made (almost) error free, the system might not be able to support a
sufficiently large number of users per cell and might thus be
economically infeasible.
One might also argue that the speech codecs should be able to deal
with the kind of packet loss induced by CRTP, in particular since the
speech codecs probably must be able to deal with packet loss anyway
if the RTP stream crosses the Internet. While the latter is true,
the kind of loss induced by CRTP is difficult to deal with. It is
usually not possible to completely hide a loss event where well over
100 ms worth of sound is completely lost. If such loss occurs
frequently at both ends of the end-to-end path, the speech quality
will suffer.
A detailed description of the requirements specified for ROHC may be
found in [REQ].
3.4. Classification of header fields
As mentioned earlier, header compression is possible due to the fact
that there is much redundancy between header field values within
packets, but especially between consecutive packets. To utilize
these properties for header compression, it is important to
understand the change patterns of the various header fields.
All header fields have been classified in detail in appendix A. The
fields are first classified at a high level and then some of them are
studied more in detail. Finally, the appendix concludes with
recommendations on how the various fields should be handled by header
compression algorithms. The main conclusion that can be drawn is
that most of the header fields can easily be compressed away since
they never or seldom change. Only 5 fields, with a combined size of
about 10 octets, need more sophisticated mechanisms. These fields
are:
- IPv4 Identification (16 bits) - IP-ID
- UDP Checksum (16 bits)
- RTP Marker (1 bit) - M-bit
- RTP Sequence Number (16 bits) - SN
- RTP Timestamp (32 bits) - TS
The analysis in Appendix A reveals that the values of the TS and IP-
ID fields can usually be predicted from the RTP Sequence Number,
which increments by one for each packet emitted by an RTP source.
The M-bit is also usually the same, but needs to be communicated
explicitly occasionally. The UDP Checksum should not be predicted
and is sent as-is when enabled.
The way ROHC RTP compression operates, then, is to first establish
functions from SN to the other fields, and then reliably communicate
the SN. Whenever a function from SN to another field changes, i.e.,
the existing function gives a result which is different from the
field in the header to be compressed, additional information is sent
to update the parameters of that function.
Headers specific to Mobile IP (for IPv4 or IPv6) do not receive any
special treatment in this document. They are compressible, however,
and it is expected that the compression efficiency for Mobile IP
headers will be good enough due to the handling of extension header
lists and tunneling headers. It would be relatively painless to
introduce a new ROHC profile with special treatment for Mobile IPv6
specific headers should the completed work on the Mobile IPv6
protocols (work in progress in the IETF) make that necessary.
4. Header compression framework
4.1. Operating assumptions
Cellular links, which are a primary target for ROHC, have a number of
characteristics that are described briefly here. ROHC requires
functionality from lower layers that is outlined here and more
thoroughly described in the lower layer guidelines document [LLG].
Channels
ROHC header-compressed packets flow on channels. Unlike many
fixed links, some cellular radio links can have several channels
connecting the same pair of nodes. Each channel can have
different characteristics in terms of error rate, bandwidth, etc.
Context identifiers
On some channels, the ability to transport multiple packet streams
is required. It can also be feasible to have channels dedicated
to individual packet streams. Therefore, ROHC uses a distinct
context identifier space per channel and can eliminate context
identifiers completely for one of the streams when few streams
share a channel.
Packet type indication
Packet type indication is done in the header compression scheme
itself. Unless the link already has a way of indicating packet
types which can be used, such as PPP, this provides smaller
compressed headers overall. It may also be less difficult to
allocate a single packet type, rather than many, in order to run
ROHC over links such as PPP.
Reordering
The channel between compressor and decompressor is required to
maintain packet ordering, i.e., the decompressor must receive
packets in the same order as the compressor sent them.
(Reordering before the compression point, however, is dealt with,
i.e., there is no assumption that the compressor will only receive
packets in sequence.)
Duplication
The channel between compressor and decompressor is required to not
duplicate packets. (Duplication before the compression point,
however, is dealt with, i.e., there is no assumption that the
compressor will receive only one copy of each packet.)
Packet length
ROHC is designed under the assumption that lower layers indicate
the length of a compressed packet. ROHC packets do not contain
length information for the payload.
Framing
The link layer must provide framing that makes it possible to
distinguish frame boundaries and individual frames.
Error detection/protection
The ROHC scheme has been designed to cope with residual errors in
the headers delivered to the decompressor. CRCs and sanity checks
are used to prevent or reduce damage propagation. However, it is
RECOMMENDED that lower layers deploy error detection for ROHC
headers and do not deliver ROHC headers with high residual error
rates.
Without giving a hard limit on the residual error rate acceptable
to ROHC, it is noted that for a residual bit error rate of at most
1E-5, the ROHC scheme has been designed not to increase the number
of damaged headers, i.e., the number of damaged headers due to
damage propagation is designed to be less than the number of
damaged headers caught by the ROHC error detection scheme.
Negotiation
In addition to the packet handling mechanisms above, the link
layer MUST provide a way to negotiate header compression
parameters, see also section 5.1.1. (For unidirectional links,
this negotiation may be performed out-of-band or even a priori.)
4.2. Dynamicity
The ROHC protocol achieves its compression gain by establishing state
information at both ends of the link, i.e., at the compressor and at
the decompressor. Different parts of the state are established at
different times and with different frequency; hence, it can be said
that some of the state information is more dynamic than the rest.
Some state information is established at the time a channel is
established; ROHC assumes the existence of an out-of-band negotiation
protocol (such as PPP), or predefined channel state (most useful for
unidirectional links). In both cases, we speak of "negotiated
channel state". ROHC does not assume that this state can change
dynamically during the channel lifetime (and does not explicitly
support such changes, although some changes may be innocuous from a
protocol point of view). An example of negotiated channel state is
the highest context ID number to be used by the compressor (MAX_CID).
Other state information is associated with the individual packet
streams in the channel; this state is said to be part of the context.
Using context identifiers (CIDs), multiple packet streams with
different contexts can share a channel. The negotiated channel state
indicates the highest context identifier to be used, as well as the
selection of one of two ways to indicate the CID in the compressed
header.
It is up to the compressor to decide which packets to associate with
a context (or, equivalently, which packets constitute a single
stream); however, ROHC is efficient only when all packets of a stream
share certain properties, such as having the same values for fields
that are described as "static" in this document (e.g., the IP
addresses, port numbers, and RTP parameters such as the payload
type). The efficiency of ROHC RTP also depends on the compressor
seeing most RTP Sequence Numbers.
Streams need not share all characteristics important for compression.
ROHC has a notion of compression profiles: a compression profile
denotes a predefined set of such characteristics. To provide
extensibility, the negotiated channel state includes the set of
profiles acceptable to the decompressor. The context state includes
the profile currently in use for the context.
Other elements of the context state may include the current values of
all header fields (from these one can deduce whether an IPv4 header
is present in the header chain, and whether UDP Checksums are
enabled), as well as additional compression context that is not part
of an uncompressed header, e.g., TS_STRIDE, IP-ID characteristics
(incrementing as a 16-bit value in network byte order? random?), a
number of old reference headers, and the compressor/decompressor
state machines (see next section).
This document actually defines four ROHC profiles: One uncompressed
profile, the main ROHC RTP compression profile, and two variants of
this profile for compression of packets with header chains that end
in UDP and ESP, respectively, but where RTP compression is not
applicable. The descriptive text in the rest of this section is
referring to the main ROHC RTP compression profile.
4.3. Compression and decompression states
Header compression with ROHC can be characterized as an interaction
between two state machines, one compressor machine and one
decompressor machine, each instantiated once per context. The
compressor and the decompressor have three states each, which in many
ways are related to each other even if the meaning of the states are
slightly different for the two parties. Both machines start in the
lowest compression state and transit gradually to higher states.
Transitions need not be synchronized between the two machines. In
normal operation it is only the compressor that temporarily transits
back to lower states. The decompressor will transit back only when
context damage is detected.
Subsequent sections present an overview of the state machines and
their corresponding states, respectively, starting with the
compressor.
4.3.1. Compressor states
For ROHC compression, the three compressor states are the
Initialization and Refresh (IR), First Order (FO), and Second Order
(SO) states. The compressor starts in the lowest compression state
(IR) and transits gradually to higher compression states. The
compressor will always operate in the highest possible compression
state, under the constraint that the compressor is sufficiently
confident that the decompressor has the information necessary to
decompress a header compressed according to that state.
+----------+ +----------+ +----------+
| IR State | <--------> | FO State | <--------> | SO State |
+----------+ +----------+ +----------+
Decisions about transitions between the various compression states
are taken by the compressor on the basis of:
- variations in packet headers
- positive feedback from decompressor (Acknowledgments -- ACKs)
- negative feedback from decompressor (Negative ACKs -- NACKs)
- periodic timeouts (when operating in unidirectional mode, i.e.,
over simplex channels or when feedback is not enabled)
How transitions are performed is explained in detail in chapter 5 for
each mode of operation.
4.3.1.1. Initialization and Refresh (IR) State
The purpose of the IR state is to initialize the static parts of the
context at the decompressor or to recover after failure. In this
state, the compressor sends complete header information. This
includes all static and nonstatic fields in uncompressed form plus
some additional information.
The compressor stays in the IR state until it is fairly confident
that the decompressor has received the static information correctly.
4.3.1.2. First Order (FO) State
The purpose of the FO state is to efficiently communicate
irregularities in the packet stream. When operating in this state,
the compressor rarely sends information about all dynamic fields, and
the information sent is usually compressed at least partially. Only
a few static fields can be updated. The difference between IR and FO
should therefore be clear.
The compressor enters this state from the IR state, and from the SO
state whenever the headers of the packet stream do not conform to
their previous pattern. It stays in the FO state until it is
confident that the decompressor has acquired all the parameters of
the new pattern. Changes in fields that are always irregular are
communicated in all packets and are therefore part of what is a
uniform pattern.
Some or all packets sent in the FO state carry context updating
information. It is very important to detect corruption of such
packets to avoid erroneous updates and context inconsistencies.
4.3.1.3. Second Order (SO) State
This is the state where compression is optimal. The compressor
enters the SO state when the header to be compressed is completely
predictable given the SN (RTP Sequence Number) and the compressor is
sufficiently confident that the decompressor has acquired all
parameters of the functions from SN to other fields. Correct
decompression of packets sent in the SO state only hinges on correct
decompression of the SN. However, successful decompression also
requires that the information sent in the preceding FO state packets
has been successfully received by the decompressor.
The compressor leaves this state and goes back to the FO state when
the header no longer conforms to the uniform pattern and cannot be
independently compressed on the basis of previous context
information.
4.3.2. Decompressor states
The decompressor starts in its lowest compression state, "No Context"
and gradually transits to higher states. The decompressor state
machine normally never leaves the "Full Context" state once it has
entered this state.
+--------------+ +----------------+ +--------------+
| No Context | <---> | Static Context | <---> | Full Context |
+--------------+ +----------------+ +--------------+
Initially, while working in the "No Context" state, the decompressor
has not yet successfully decompressed a packet. Once a packet has
been decompressed correctly (for example, upon reception of an
initialization packet with static and dynamic information), the
decompressor can transit all the way to the "Full Context" state, and
only upon repeated failures will it transit back to lower states.
However, when that happens it first transits back to the "Static
Context" state. There, reception of any packet sent in the FO state
is normally sufficient to enable transition to the "Full Context"
state again. Only when decompression of several packets sent in the
FO state fails in the "Static Context" state will the decompressor go
all the way back to the "No Context" state.
When state transitions are performed is explained in detail in
chapter 5.
4.4. Modes of operation
The ROHC scheme has three modes of operation, called Unidirectional,
Bidirectional Optimistic, and Bidirectional Reliable mode.
It is important to understand the difference between states, as
described in the previous chapter, and modes. These abstractions are
orthogonal to each other. The state abstraction is the same for all
modes of operation, while the mode controls the logic of state
transitions and what actions to perform in each state.
+----------------------+
| Unidirectional Mode |
| +--+ +--+ +--+ |
| |IR| |FO| |SO| |
| +--+ +--+ +--+ |
+----------------------+
^ ^
/ \
/ \
v v
+----------------------+ +----------------------+
| Optimistic Mode | | Reliable Mode |
| +--+ +--+ +--+ | | +--+ +--+ +--+ |
| |IR| |FO| |SO| | <--------------> | |IR| |FO| |SO| |
| +--+ +--+ +--+ | | +--+ +--+ +--+ |
+----------------------+ +----------------------+
The optimal mode to operate in depends on the characteristics of the
environment of the compression protocol, such as feedback abilities,
error probabilities and distributions, effects of header size
variation, etc. All ROHC implementations MUST implement and support
all three modes of operation. The three modes are briefly described
in the following subsections.
Detailed descriptions of the three modes of operation regarding
compression and decompression logic are given in chapter 5. The mode
transition mechanisms, too, are described in chapter 5.
4.4.1. Unidirectional mode -- U-mode
When in the Unidirectional mode of operation, packets are sent in one
direction only: from compressor to decompressor. This mode therefore
makes ROHC usable over links where a return path from decompressor to
compressor is unavailable or undesirable.
In U-mode, transitions between compressor states are performed only
on account of periodic timeouts and irregularities in the header
field change patterns in the compressed packet stream. Due to the
periodic refreshes and the lack of feedback for initiation of error
recovery, compression in the Unidirectional mode will be less
efficient and have a slightly higher probability of loss propagation
compared to any of the Bidirectional modes.
Compression with ROHC MUST start in the Unidirectional mode.
Transition to any of the Bidirectional modes can be performed as soon
as a packet has reached the decompressor and it has replied with a
feedback packet indicating that a mode transition is desired (see
chapter 5).
4.4.2. Bidirectional Optimistic mode -- O-mode
The Bidirectional Optimistic mode is similar to the Unidirectional
mode. The difference is that a feedback channel is used to send
error recovery requests and (optionally) acknowledgments of
significant context updates from decompressor to compressor (not,
however, for pure sequence number updates). Periodic refreshes are
not used in the Bidirectional Optimistic mode.
O-mode aims to maximize compression efficiency and sparse usage of
the feedback channel. It reduces the number of damaged headers
delivered to the upper layers due to residual errors or context
invalidation. The frequency of context invalidation may be higher
than for R-mode, in particular when long loss/error bursts occur.
Refer to section 4.7 for more details.
4.4.3. Bidirectional Reliable mode -- R-mode
The Bidirectional Reliable mode differs in many ways from the
previous two. The most important differences are a more intensive
usage of the feedback channel and a stricter logic at both the
compressor and the decompressor that prevents loss of context
synchronization between compressor and decompressor except for very
high residual bit error rates. Feedback is sent to acknowledge all
context updates, including updates of the sequence number field.
However, not every packet updates the context in Reliable mode.
R-mode aims to maximize robustness against loss propagation and
damage propagation, i.e., minimize the probability of context
invalidation, even under header loss/error burst conditions. It may
have a lower probability of context invalidation than O-mode, but a
larger number of damaged headers may be delivered when the context
actually is invalidated. Refer to section 4.7 for more details.
4.5. Encoding methods
This chapter describes the encoding methods used for header fields.
How the methods are applied to each field (e.g., values of associated
parameters) is specified in section 5.7.
4.5.1. Least Significant Bits (LSB) encoding
Least Significant Bits (LSB) encoding is used for header fields whose
values are usually subject to small changes. With LSB encoding, the
k least significant bits of the field value are transmitted instead
of the original field value, where k is a positive integer. After
receiving k bits, the decompressor derives the original value using a
previously received value as reference (v_ref).
The scheme is guaranteed to be correct if the compressor and the
decompressor each use interpretation intervals
1) in which the original value resides, and
2) in which the original value is the only value that has the
exact same k least significant bits as those transmitted.
The interpretation interval can be described as a function f(v_ref,
k). Let
f(v_ref, k) = [v_ref - p, v_ref + (2^k - 1) - p]
where p is an integer.
<------- interpretation interval (size is 2^k) ------->
|-------------+---------------------------------------|
v_ref - p v_ref v_ref + (2^k-1) - p
The function f has the following property: for any value k, the k
least significant bits will uniquely identify a value in f(v_ref, k).
The parameter p is introduced so that the interpretation interval can
be shifted with respect to v_ref. Choosing a good value for p will
yield a more efficient encoding for fields with certain
characteristics. Below are some examples:
a) For field values that are expected always to increase, p can be
set to -1. The interpretation interval becomes
[v_ref + 1, v_ref + 2^k].
b) For field values that stay the same or increase, p can be set to
0. The interpretation interval becomes [v_ref, v_ref + 2^k - 1].
c) For field values that are expected to deviate only slightly from a
constant value, p can be set to 2^(k-1) - 1. The interpretation
interval becomes [v_ref - 2^(k-1) + 1, v_ref + 2^(k-1)].
d) For field values that are expected to undergo small negative
changes and larger positive changes, such as the RTP TS for video,
or RTP SN when there is misordering, p can be set to 2^(k-2) - 1.
The interval becomes [v_ref - 2^(k-2) + 1, v_ref + 3 * 2^(k-2)],
i.e., 3/4 of the interval is used for positive changes.
The following is a simplified procedure for LSB compression and
decompression; it is modified for robustness and damage propagation
protection in the next subsection:
1) The compressor (decompressor) always uses v_ref_c (v_ref_d), the
last value that has been compressed (decompressed), as v_ref;
2) When compressing a value v, the compressor finds the minimum value
of k such that v falls into the interval f(v_ref_c, k). Call this
function k = g(v_ref_c, v). When only a few distinct values of k
are possible, for example due to limitations imposed by packet
formats (see section 5.7), the compressor will instead pick the
smallest k that puts v in the interval f(v_ref_c, k).
3) When receiving m LSBs, the decompressor uses the interpretation
interval f(v_ref_d, m), called interval_d. It picks as the
decompressed value the one in interval_d whose LSBs match the
received m bits.
Note that the values to be encoded have a finite range; for example,
the RTP SN ranges from 0 to 0xFFFF. When the SN value is close to 0
or 0xFFFF, the interpretation interval can straddle the wraparound
boundary between 0 and 0xFFFF.
The scheme is complicated by two factors: packet loss between the
compressor and decompressor, and transmission errors undetected by
the lower layer. In the former case, the compressor and decompressor
will lose the synchronization of v_ref, and thus also of the
interpretation interval. If v is still covered by the
intersection(interval_c, interval_d), the decompression will be
correct. Otherwise, incorrect decompression will result. The next
section will address this issue further.
In the case of undetected transmission errors, the corrupted LSBs
will give an incorrectly decompressed value that will later be used
as v_ref_d, which in turn is likely to lead to damage propagation.
This problem is addressed by using a secure reference, i.e., a
reference value whose correctness is verified by a protecting CRC.
Consequently, the procedure 1) above is modified as follows:
1) a) the compressor always uses as v_ref_c the last value that has
been compressed and sent with a protecting CRC.
b) the decompressor always uses as v_ref_d the last correct
value, as verified by a successful CRC.
Note that in U/O-mode, 1) b) is modified so that if decompression of
the SN fails using the last verified SN reference, another
decompression attempt is made using the last but one verified SN
reference. This procedure mitigates damage propagation when a small
CRC fails to detect a damaged value. See section 5.3.2.2.3 for
further details.
4.5.2. Window-based LSB encoding (W-LSB encoding)
This section describes how to modify the simplified algorithm in
4.5.1 to achieve robustness.
The compressor may not be able to determine the exact value of
v_ref_d that will be used by the decompressor for a particular value
v, since some candidates for v_ref_d may have been lost or damaged.
However, by using feedback or by making reasonable assumptions, the
compressor can limit the candidate set. The compressor then
calculates k such that no matter which v_ref_d in the candidate set
the decompressor uses, v is covered by the resulting interval_d.
Since the decompressor always uses as the reference the last received
value where the CRC succeeded, the compressor maintains a sliding
window containing the candidates for v_ref_d. The sliding window is
initially empty. The following operations are performed on the
sliding window by the compressor:
1) After sending a value v (compressed or uncompressed) protected by
a CRC, the compressor adds v to the sliding window.
2) For each value v being compressed, the compressor chooses k =
max(g(v_min, v), g(v_max, v)), where v_min and v_max are the
minimum and maximum values in the sliding window, and g is the
function defined in the previous section.
3) When the compressor is sufficiently confident that a certain value
v and all values older than v will not be used as reference by the
decompressor, the window is advanced by removing those values
(including v). The confidence may be obtained by various means.
In R-mode, an ACK from the decompressor implies that values older
than the ACKed one can be removed from the sliding window. In
U/O-mode there is always a CRC to verify correct decompression,
and a sliding window with a limited maximum width is used. The
window width is an implementation dependent optimization
parameter.
Note that the decompressor follows the procedure described in the
previous section, except that in R-mode it MUST ACK each header
received with a succeeding CRC (see also section 5.5).
4.5.3. Scaled RTP Timestamp encoding
The RTP Timestamp (TS) will usually not increase by an arbitrary
number from packet to packet. Instead, the increase is normally an
integral multiple of some unit (TS_STRIDE). For example, in the case
of audio, the sample rate is normally 8 kHz and one voice frame may
cover 20 ms. Furthermore, each voice frame is often carried in one
RTP packet. In this case, the RTP increment is always n * 160 (=
8000 * 0.02), for some integer n. Note that silence periods have no
impact on this, as the sample clock at the source normally keeps
running without changing either frame rate or frame boundaries.
In the case of video, there is usually a TS_STRIDE as well when the
video frame level is considered. The sample rate for most video
codecs is 90 kHz. If the video frame rate is fixed, say, to 30
frames/second, the TS will increase by n * 3000 (= n * 90000 / 30)
between video frames. Note that a video frame is often divided into
several RTP packets to increase robustness against packet loss. In
this case several RTP packets will carry the same TS.
When using scaled RTP Timestamp encoding, the TS is downscaled by a
factor of TS_STRIDE before compression. This saves
floor(log2(TS_STRIDE))
bits for each compressed TS. TS and TS_SCALED satisfy the following
equality:
TS = TS_SCALED * TS_STRIDE + TS_OFFSET
TS_STRIDE is explicitly, and TS_OFFSET implicitly, communicated to
the decompressor. The following algorithm is used:
1. Initialization: The compressor sends to the decompressor the value
of TS_STRIDE and the absolute value of one or several TS fields.
The latter are used by the decompressor to initialize TS_OFFSET to
(absolute value) modulo TS_STRIDE. Note that TS_OFFSET is the
same regardless of which absolute value is used, as long as the
unscaled TS value does not wrap around; see 4) below.
2. Compression: After initialization, the compressor no longer
compresses the original TS values. Instead, it compresses the
downscaled values: TS_SCALED = TS / TS_STRIDE. The compression
method could be either W-LSB encoding or the timer-based encoding
described in the next section.
3. Decompression: When receiving the compressed value of TS_SCALED,
the decompressor first derives the value of the original
TS_SCALED. The original RTP TS is then calculated as TS =
TS_SCALED * TS_STRIDE + TS_OFFSET.
4. Offset at wraparound: Wraparound of the unscaled 32-bit TS will
invalidate the current value of TS_OFFSET used in the equation
above. For example, let us assume TS_STRIDE = 160 = 0xA0 and the
current TS = 0xFFFFFFF0. TS_OFFSET is then 0x50 = 80. Then if
the next RTP TS = 0x00000130 (i.e., the increment is 160 * 2 =
320), the new TS_OFFSET should be 0x00000130 modulo 0xA0 = 0x90 =
144. The compressor is not required to re-initialize TS_OFFSET at
wraparound. Instead, the decompressor MUST detect wraparound of
the unscaled TS (which is trivial) and update TS_OFFSET to
TS_OFFSET = (Wrapped around unscaled TS) modulo TS_STRIDE
5. Interpretation interval at wraparound: Special rules are needed
for the interpretation interval of the scaled TS at wraparound,
since the maximum scaled TS, TSS_MAX, (0xFFFFFFFF / TS_STRIDE) may
not have the form 2^m - 1. For example, when TS_STRIDE is 160,
the scaled TS is at most 26843545 which has LSBs 10011001. The
wraparound boundary between the TSS_MAX may thus not correspond to
a natural boundary between LSBs.
interpretation interval
|<------------------------------>|
unused scaled TS
------------|--------------|---------------------->
TSS_MAX zero
When TSS_MAX is part of the interpretation interval, a number of
unused values are inserted into it after TSS_MAX such that their
LSBs follow naturally upon each other. For example, for TS_STRIDE
= 160 and k = 4, values corresponding to the LSBs 1010 through
1111 are inserted. The number of inserted values depends on k and
the LSBs of the maximum scaled TS. The number of valid values in
the interpretation interval should be high enough to maintain
robustness. This can be ensured by the following rule:
Let a be the number of LSBs needed if there was no
wraparound, and let b be the number of LSBs needed to
disambiguate between TSS_MAX and zero where the a LSBs of
TSS_MAX are set to zero. The number of LSB bits to send
while TSS_MAX or zero is part of the interpretation interval
is b.
This scaling method can be applied to many frame-based codecs.
However, the value of TS_STRIDE might change during a session, for
example as a result of adaptation strategies. If that happens, the
unscaled TS is compressed until re-initialization of the new
TS_STRIDE and TS_OFFSET is completed.
4.5.4. Timer-based compression of RTP Timestamp
The RTP Timestamp [RFC 1889] is defined to identify the number of the
first sample used to generate the payload. When 1) RTP packets carry
payloads corresponding to a fixed sampling interval, 2) the sampling
is done at a constant rate, and 3) packets are generated in lock-step
with sampling, then the timestamp value will closely approximate a
linear function of the time of day. This is the case for
conversational media, such as interactive speech. The linear ratio
is determined by the source sample rate. The linear pattern can be
complicated by packetization (e.g., in the case of video where a
video frame usually corresponds to several RTP packets) or frame
rearrangement (e.g., B-frames are sent out-of-order by some video
codecs).
With a fixed sample rate of 8 kHz, 20 ms in the time domain is
equivalent to an increment of 160 in the unscaled TS domain, and to
an increment of 1 in the scaled TS domain with TS_STRIDE = 160.
As a consequence, the (scaled) TS of headers arriving at the
decompressor will be a linear function of time of day, with some
deviation due to the delay jitter (and the clock inaccuracies)
between the source and the decompressor. In normal operation, i.e.,
no crashes or failures, the delay jitter will be bounded to meet the
requirements of conversational real-time traffic. Hence, by using a
local clock the decompressor can obtain an approximation of the
(scaled) TS in the header to be decompressed by considering its
arrival time. The approximation can then be refined with the k LSBs
of the (scaled) TS carried in the header. The value of k required to
ensure correct decompression is a function of the jitter between the
source and the decompressor.
If the compressor knows the potential jitter introduced between
compressor and decompressor, it can determine k by using a local
clock to estimate jitter in packet arrival times, or alternatively it
can use a fixed k and discard packets arriving too much out of time.
The advantages of this scheme include:
a) The size of the compressed TS is constant and small. In
particular, it does NOT depend on the length of silence intervals.
This is in contrast to other TS compression techniques, which at
the beginning of a talkspurt require sending a number of bits
dependent on the duration of the preceding silence interval.
b) No synchronization is required between the clock local to the
compressor and the clock local to the decompressor.
Note that although this scheme can be made to work using both scaled
and unscaled TS, in practice it is always combined with scaled TS
encoding because of the less demanding requirement on the clock
resolution, e.g., 20 ms instead of 1/8 ms. Therefore, the algorithm
described below assumes that the clock-based encoding scheme operates
on the scaled TS. The case of unscaled TS would be similar, with
changes to scale factors.
The major task of the compressor is to determine the value of k. Its
sliding window now contains not only potential reference values for
the TS but also their times of arrival at the compressor.
1) The compressor maintains a sliding window
{(T_j, a_j), for each header j that can be used as a reference},
where T_j is the scaled TS for header j, and a_j is the arrival
time of header j. The sliding window serves the same purpose as
the W-LSB sliding window of section 4.5.2.
2) When a new header n arrives with T_n as the scaled TS, the
compressor notes the arrival time a_n. It then calculates
Max_Jitter_BC =
max {|(T_n - T_j) - ((a_n - a_j) / TIME_STRIDE)|,
for all headers j in the sliding window},
where TIME_STRIDE is the time interval equivalent to one
TS_STRIDE, e.g., 20 ms. Max_Jitter_BC is the maximum observed
jitter before the compressor, in units of TS_STRIDE, for the
headers in the sliding window.
3) k is calculated as
k = ceiling(log2(2 * J + 1),
where J = Max_Jitter_BC + Max_Jitter_CD + 2.
Max_Jitter_CD is the upper bound of jitter expected on the
communication channel between compressor and decompressor (CD-CC).
It depends only on the characteristics of CD-CC.
The constant 2 accounts for the quantization error introduced by
the clocks at the compressor and decompressor, which can be +/-1.
Note that the calculation of k follows the compression algorithm
described in section 4.5.1, with p = 2^(k-1) - 1.
4) The sliding window is subject to the same window operations as in
section 4.5.2, 1) and 3), except that the values added and removed
are paired with their arrival times.
Decompressor:
1) The decompressor uses as its reference header the last correctly
(as verified by CRC) decompressed header. It maintains the pair
(T_ref, a_ref), where T_ref is the scaled TS of the reference
header, and a_ref is the arrival time of the reference header.
2) When receiving a compressed header n at time a_n, the
approximation of the original scaled TS is calculated as:
T_approx = T_ref + (a_n - a_ref) / TIME_STRIDE.
3) The approximation is then refined by the k least significant bits
carried in header n, following the decompression algorithm of
section 4.5.1, with p = 2^(k-1) - 1.
Note: The algorithm does not assume any particular pattern in the
packets arriving at the compressor, i.e., it tolerates reordering
before the compressor and nonincreasing RTP Timestamp behavior.
Note: Integer arithmetic is used in all equations above. If
TIME_STRIDE is not equal to an integral number of clock ticks,
time must be normalized such that TIME_STRIDE is an integral
number of clock ticks. For example, if a clock tick is 20 ms and
TIME_STRIDE is 30 ms, (a_n - a_ref) in 2) can be multiplied by 3
and TIME_STRIDE can have the value 2.
Note: The clock resolution of the compressor or decompressor can
be worse than TIME_STRIDE, in which case the difference, i.e.,
actual resolution - TIME_STRIDE, is treated as additional jitter
in the calculation of k.
Note: The clock resolution of the decompressor may be communicated
to the compressor using the CLOCK feedback option.
Note: The decompressor may observe the jitter and report this to
the compressor using the JITTER feedback option. The compressor
may use this information to refine its estimate of Max_Jitter_CD.
4.5.5. Offset IP-ID encoding
As all IPv4 packets have an IP Identifier to allow for fragmentation,
ROHC provides for transparent compression of this ID. There is no
explicit support in ROHC for the IPv6 fragmentation header, so there
is never a need to discuss IP IDs outside the context of IPv4.
This section assumes (initially) that the IPv4 stack at the source
host assigns IP-ID according to the value of a 2-byte counter which
is increased by one after each assignment to an outgoing packet.
Therefore, the IP-ID field of a particular IPv4 packet flow will
increment by 1 from packet to packet except when the source has
emitted intermediate packets not belonging to that flow.
For such IPv4 stacks, the RTP SN will increase by 1 for each packet
emitted and the IP-ID will increase by at least the same amount.
Thus, it is more efficient to compress the offset, i.e., (IP-ID - RTP
SN), instead of IP-ID itself.
The remainder of section 4.5.5 describes how to compress/decompress
the sequence of offsets using W-LSB encoding/decoding, with p = 0
(see section 4.5.1). All IP-ID arithmetic is done using unsigned
16-bit quantities, i.e., modulo 2^16.
Compressor:
The compressor uses W-LSB encoding (section 4.5.2) to compress a
sequence of offsets
Offset_i = ID_i - SN_i,
where ID_i and SN_i are the values of the IP-ID and RTP SN of
header i. The sliding window contains such offsets and not the
values of header fields, but the rules for adding and deleting
offsets from the window otherwise follow section 4.5.2.
Decompressor:
The reference header is the last correctly (as verified by CRC)
decompressed header.
When receiving a compressed packet m, the decompressor calculates
Offset_ref = ID_ref - SN_ref, where ID_ref and SN_ref are the
values of IP-ID and RTP SN in the reference header, respectively.
Then W-LSB decoding is used to decompress Offset_m, using the
received LSBs in packet m and Offset_ref. Note that m may contain
zero LSBs for Offset_m, in which case Offset_m = Offset_ref.
Finally, the IP-ID for packet m is regenerated as
IP-ID for m = decompressed SN of packet m + Offset_m
Network byte order:
Some IPv4 stacks do use a counter to generate IP ID values as
described, but do not transmit the contents of this counter in
network byte order, but instead send the two octets reversed. In
this case, the compressor can compress the IP-ID field after
swapping the bytes. Consequently, the decompressor also swaps the
bytes of the IP-ID after decompression to regenerate the original
IP-ID. This requires that the compressor and the decompressor
synchronize on the byte order of the IP-ID field using the NBO or
NBO2 flag (see section 5.7).
Random IP Identifier:
Some IPv4 stacks generate the IP Identifier values using a
pseudo-random number generator. While this may provide some
security benefits, it makes it pointless to attempt compressing
the field. Therefore, the compressor should detect such random
behavior of the field. After detection and synchronization with
the decompressor using the RND or RND2 flag, the field is sent
as-is in its entirety as additional octets after the compressed
header.
4.5.6. Self-describing variable-length values
The values of TS_STRIDE and a few other compression parameters can
vary widely. TS_STRIDE can be 160 for voice and 90 000 for 1 f/s
video. To optimize the transfer of such values, a variable number of
octets is used to encode them. The number of octets used is
determined by the first few bits of the first octet:
First bit is 0: 1 octet.
7 bits transferred.
Up to 127 decimal.
Encoded octets in hexadecimal: 00 to 7F
First bits are 10: 2 octets.
14 bits transferred.
Up to 16 383 decimal.
Encoded octets in hexadecimal: 80 00 to BF FF
First bits are 110: 3 octets.
21 bits transferred.
Up to 2 097 151 decimal.
Encoded octets in hexadecimal: C0 00 00 to DF FF FF
First bits are 111: 4 octets.
29 bits transferred.
Up to 536 870 911 decimal.
Encoded octets in hexadecimal: E0 00 00 00 to FF FF FF FF
4.5.7. Encoded values across several fields in compressed headers
When a compressed header has an extension, pieces of an encoded value
can be present in more than one field. When an encoded value is
split over several fields in this manner, the more significant bits
of the value are closer to the beginning of the header. If the
number of bits available in compressed header fields exceeds the
number of bits in the value, the most significant field is padded
with zeroes in its most significant bits.
For example, an unscaled TS value can be transferred using an UOR-2
header (see section 5.7) with an extension of type 3. The Tsc bit of
the extension is then unset (zero) and the variable length TS field
of the extension is 4 octets, with 29 bits available for the TS (see
section 4.5.6). The UOR-2 TS field will contain the three most
significant bits of the unscaled TS, and the 4-octet TS field in the
extension will contain the remaining 29 bits.
4.6. Errors caused by residual errors
ROHC is designed under the assumption that packets can be damaged
between the compressor and decompressor, and that such damaged
packets can be delivered to the decompressor ("residual errors").
Residual errors may damage the SN in compressed headers. Such damage
will cause generation of a header which upper layers may not be able
to distinguish from a correct header. When the compressed header
contains a CRC, the CRC will catch the bad header with a probability
dependent on the size of the CRC. When ROHC does not detect the bad
header, it will be delivered to upper layers.
Damage is not confined to the SN:
a) Damage to packet type indication bits can cause a header to be
interpreted as having a different packet type.
b) Damage to CID information may cause a packet to be interpreted
according to another context and possibly also according to
another profile. Damage to CIDs will be more harmful when a large
part of the CID space is being used, so that it is likely that the
damaged CID corresponds to an active context.
c) Feedback information can also be subject to residual errors, both
when feedback is piggybacked and when it is sent in separate ROHC
packets. ROHC uses sanity checks and adds CRCs to vital feedback
information to allow detection of some damaged feedback.
Note that context damage can also result in generation of
incorrect headers; section 4.7 elaborates further on this.
4.7. Impairment considerations
Impairments to headers can be classified into the following types:
(1) the lower layer was not able to decode the packet and did not
deliver it to ROHC,
(2) the lower layer was able to decode the packet, but discarded
it because of a detected error,
(3) ROHC detected an error in the generated header and discarded
the packet, or
(4) ROHC did not detect that the regenerated header was damaged
and delivered it to upper layers.
Impairments cause loss or damage of individual headers. Some
impairment scenarios also cause context invalidation, which in turn
results in loss propagation and damage propagation. Damage
propagation and undetected residual errors both contribute to the
number of damaged headers delivered to upper layers. Loss
propagation and impairments resulting in loss or discarding of single
packets both contribute to the packet loss seen by upper layers.
Examples of context invalidating scenarios are:
(a) Impairment of type (4) on the forward channel, causing the
decompressor to update its context with incorrect information;
(b) Loss/error burst of pattern update headers: Impairments of
types (1),(2) and (3) on consecutive pattern update headers; a
pattern update header is a header carrying a new pattern
information, e.g., at the beginning of a new talk spurt; this
causes the decompressor to lose the pattern update
information;
(c) Loss/error burst of headers: Impairments of types (1),(2) and
(3) on a number of consecutive headers that is large enough to
cause the decompressor to lose the SN synchronization;
(d) Impairment of type (4) on the feedback channel which mimics a
valid ACK and makes the compressor update its context;
(e) a burst of damaged headers (3) erroneously triggers the "k-
out-of-n" rule for detecting context invalidation, which
results in a NACK/update sequence during which headers are
discarded.
Scenario (a) is mitigated by the CRC carried in all context updating
headers. The larger the CRC, the lower the chance of context
invalidation caused by (a). In R-mode, the CRC of context updating
headers is always 7 bits or more. In U/O-mode, it is usually 3 bits
and sometimes 7 or 8 bits.
Scenario (b) is almost completely eliminated when the compressor
ensures through ACKs that no context updating headers are lost, as in
R-mode.
Scenario (c) is almost completely eliminated when the compressor
ensures through ACKs that the decompressor will always detect the SN
wraparound, as in R-mode. It is also mitigated by the SN repair
mechanisms in U/O-mode.
Scenario (d) happens only when the compressor receives a damaged
header that mimics an ACK of some header present in the W-LSB window,
say ACK of header 2, while in reality header 2 was never received or
accepted by the decompressor, i.e., header 2 was subject to
impairment (1), (2) or (3). The damaged header must mimic the
feedback packet type, the ACK feedback type, and the SN LSBs of some
header in the W-LSB window.
Scenario (e) happens when a burst of residual errors causes the CRC
check to fail in k out of the last n headers carrying CRCs. Large k
and n reduces the probability of scenario (e), but also increases the
number of headers lost or damaged as a consequence of any context
invalidation.
ROHC detects damaged headers using CRCs over the original headers.
The smallest headers in this document either include a 3-bit CRC
(U/O-mode) or do not include a CRC (R-mode). For the smallest
headers, damage is thus detected with a probability of roughly 7/8
for U/O-mode. For R-mode, damage to the smallest headers is not
detected.
All other things (coding scheme at lower layers, etc.) being equal,
the rate of headers damaged by residual errors will be lower when
headers are compressed compared when they are not, since fewer bits
are transmitted. Consequently, for a given ROHC CRC setup the rate
of incorrect headers delivered to applications will also be reduced.
The above analysis suggests that U/O-mode may be more prone than R-
mode to context invalidation. On the other hand, the CRC present in
all U/O-mode headers continuously screens out residual errors coming
from lower layers, reduces the number of damaged headers delivered to
upper layers when context is invalidated, and permits quick detection
of context invalidation.
R-mode always uses a stronger CRC on context updating headers, but no
CRC in other headers. A residual error on a header which carries no
CRC will result in a damaged header being delivered to upper layers
(4). The number of damaged headers delivered to the upper layers
depends on the ratio of headers with CRC vs. headers without CRC,
which is a compressor parameter.
5. The protocol
5.1. Data structures
The ROHC protocol is based on a number of parameters that form part
of the negotiated channel state and the per-context state. This
section describes some of this state information in an abstract way.
Implementations can use a different structure for and representation
of this state. In particular, negotiation protocols that set up the
per-channel state need to establish the information that constitutes
the negotiated channel state, but it is not necessary to exchange it
in the form described here.
5.1.1. Per-channel parameters
MAX_CID: Nonnegative integer; highest context ID number to be used by
the compressor (note that this parameter is not coupled to, but in
effect further constrained by, LARGE_CIDS).
LARGE_CIDS: Boolean; if false, the short CID representation (0 bytes
or 1 prefix byte, covering CID 0 to 15) is used; if true, the
embedded CID representation (1 or 2 embedded CID bytes covering CID 0
to 16383) is used.
PROFILES: Set of nonnegative integers, each integer indicating a
profile supported by the decompressor. The compressor MUST NOT
compress using a profile not in PROFILES.
FEEDBACK_FOR: Optional reference to a channel in the reverse
direction. If provided, this parameter indicates which channel any
feedback sent on this channel refers to (see 5.7.6.1).
MRRU: Maximum reconstructed reception unit. This is the size of the
largest reconstructed unit in octets that the decompressor is
expected to reassemble from segments (see 5.2.5). Note that this
size includes the CRC. If MRRU is negotiated to be 0, no segment
headers are allowed on the channel.
5.1.2. Per-context parameters, profiles
Per-context parameters are established with IR headers (see section
5.2.3). An IR header contains a profile identifier, which determines
how the rest of the header is to be interpreted. Note that the
profile parameter determines the syntax and semantics of the packet
type identifiers and packet types used in conjunction with a specific
context. This document describes profiles 0x0000, 0x0001, 0x0002,
and 0x0003; further profiles may be defined when ROHC is extended in
the future.
Profile 0x0000 is for sending uncompressed IP packets. See section
5.10.
Profile 0x0001 is for RTP/UDP/IP compression, see sections 5.3
through 5.9.
Profile 0x0002 is for UDP/IP compression, i.e., compression of the
first 12 octets of the UDP payload is not attempted. See section
5.11.
Profile 0x0003 is for ESP/IP compression, i.e., compression of the
header chain up to and including the first ESP header, but not
subsequent subheaders. See section 5.12.
Initially, all contexts are in no context state, i.e., all packets
referencing this context except IR packets are discarded. If defined
by a "ROHC over X" document, per-channel negotiation can be used to
pre-establish state information for a context (e.g., negotiating
profile 0x0000 for CID 15). Such state information can also be
marked read-only in the negotiation, which would cause the
decompressor to discard any IR packet attempting to modify it.
5.1.3. Contexts and context identifiers
Associated with each compressed flow is a context, which is the state
compressor and decompressor maintain in order to correctly compress
or decompress the headers of the packet stream. Contexts are
identified by a context identifier, CID, which is sent along with
compressed headers and feedback information.
The CID space is distinct for each channel, i.e., CID 3 over channel
A and CID 3 over channel B do not refer to the same context, even if
the endpoints of A and B are the same nodes. In particular, CIDs for
any pairs of forward and reverse channels are not related (forward
and reverse channels need not even have CID spaces of the same size).
Context information is conceptually kept in a table. The context
table is indexed using the CID which is sent along with compressed
headers and feedback information. The CID space can be negotiated to
be either small, which means that CIDs can take the values 0 through
15, or large, which means that CIDs take values between 0 and 2^14 -
1 = 16383. Whether the CID space is large or small is negotiated no
later than when a channel is established.
A small CID with the value 0 is represented using zero bits. A small
CID with a value from 1 to 15 is represented by a four-bit field in
place of a packet type field (Add-CID) plus four more bits. A large
CID is represented using the encoding scheme of section 4.5.6,
limited to two octets.
5.2. ROHC packets and packet types
The packet type indication scheme for ROHC has been designed under
the following constraints:
a) it must be possible to use only a limited number of packet sizes;
b) it must be possible to send feedback information in separate ROHC
packets as well as piggybacked on forward packets;
c) it is desirable to allow elimination of the CID for one packet
stream when few packet streams share a channel;
d) it is anticipated that some packets with large headers may be
larger than the MTU of very constrained lower layers.
These constraints have led to a design which includes
- optional padding,
- a feedback packet type,
- an optional Add-CID octet which provides 4 bits of CID, and
- a simple segmentation and reassembly mechanism.
A ROHC packet has the following general format (in the diagram,
colons ":" indicate that the part is optional):
--- --- --- --- --- --- --- ---
: Padding : variable length
--- --- --- --- --- --- --- ---
: Feedback : 0 or more feedback elements
--- --- --- --- --- --- --- ---
: Header : variable, with CID information
--- --- --- --- --- --- --- ---
: Payload :
--- --- --- --- --- --- --- ---
Padding is any number (zero or more) of padding octets. Either of
Feedback or Header must be present.
Feedback elements always start with a packet type indication.
Feedback elements carry internal CID information. Feedback is
described in section 5.2.2.
Header is either a profile-specific header or an IR or IR-DYN header
(see sections 5.2.3 and 5.2.4). Header either
1) does not carry any CID information (indicating CID zero), or
2) includes one Add-CID Octet (see below), or
3) contains embedded CID information of length one or two octets.
Alternatives 1) and 2) apply only to compressed headers in channels
where the CID space is small. Alternative 3) applies only to
compressed headers in channels where the CID space is large.
Padding Octet
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 0 0 0 0 0 |
+---+---+---+---+---+---+---+---+
Add-CID Octet
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 0 | CID |
+---+---+---+---+---+---+---+---+
CID: 0x1 through 0xF indicates CIDs 1 through 15.
Note: The Padding Octet looks like an Add-CID octet for CID 0.
Header either starts with a packet type indication or has a packet
type indication immediately following an Add-CID Octet. All Header
packet types have the following general format (in the diagram,
slashes "/" indicate variable length):
0 x-1 x 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if (CID 1-15) and (small CIDs)
+---+--- --- --- ---+--- --- ---+
| type indication | body | 1 octet (8-x bits of body)
+---+--- ---+---+---+--- --- ---+
: :
/ 0, 1, or 2 octets of CID / 1 or 2 octets if (large CIDs)
: :
+---+---+---+---+---+---+---+---+
/ body / variable length
+---+---+---+---+---+---+---+---+
The large CID, if present, is encoded according to section 4.5.6.
5.2.1. ROHC feedback
Feedback carries information from decompressor to compressor. The
following principal kinds of feedback are supported. In addition to
the kind of feedback, other information may be included in profile-
specific feedback information.
ACK : Acknowledges successful decompression of a packet,
which means that the context is up-to-date with a high
probability.
NACK : Indicates that the dynamic context of the
decompressor is out of sync. Generated when several
successive packets have failed to be decompressed
correctly.
STATIC-NACK : Indicates that the static context of the decompressor
is not valid or has not been established.
It is anticipated that feedback to the compressor can be realized in
many ways, depending on the properties of the particular lower layer.
The exact details of how feedback is realized is to be specified in a
"ROHC over X" document, for each lower layer X in question. For
example, feedback might be realized using
1) lower-layer specific mechanisms
2) a dedicated feedback-only channel, realized for example by the
lower layer providing a way to indicate that a packet is a
feedback packet
3) a dedicated feedback-only channel, where the timing of the
feedback provides information about which compressed packet caused
the feedback
4) interspersing of feedback packets among normal compressed packets
going in the same direction as the feedback (lower layers do not
indicate feedback)
5) piggybacking of feedback information in compressed packets going
in the same direction as the feedback (this technique may reduce
the per-feedback overhead)
6) interspersing and piggybacking on the same channel, i.e., both 4)
and 5).
Alternatives 1-3 do not place any particular requirements on the ROHC
packet type scheme. Alternatives 4-6 do, however. The ROHC packet
type scheme has been designed to allow alternatives 4-6 (these may be
used for example over PPP):
a) The ROHC scheme provides a feedback packet type. The packet type
is able to carry variable-length feedback information.
b) The feedback information sent on a particular channel is passed
to, and interpreted by, the compressor associated with feedback on
that channel. Thus, the feedback information must contain CID
information if the associated compressor can use more than one
context. The ROHC feedback scheme requires that a channel carries
feedback to at most one compressor. How a compressor is
associated with feedback on a particular channel needs to be
defined in a "ROHC over X" document.
c) The ROHC feedback information format is octet-aligned, i.e.,
starts at an octet boundary, to allow using the format over a
dedicated feedback channel, 2).
d) To allow piggybacking, 5), it is possible to deduce the length of
feedback information by examining the first few octets of the
feedback. This allows the decompressor to pass piggybacked
feedback information to the associated same-side compressor
without understanding its format. The length information
decouples the decompressor from the compressor in the sense that
the decompressor can process the compressed header immediately
without waiting for the compressor to hand it back after parsing
the feedback information.
5.2.2. ROHC feedback format
Feedback sent on a ROHC channel consists of one or more concatenated
feedback elements, where each feedback element has the following
format:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 0 | Code | feedback type octet
+---+---+---+---+---+---+---+---+
: Size : if Code = 0
+---+---+---+---+---+---+---+---+
/ feedback data / variable length
+---+---+---+---+---+---+---+---+
Code: 0 indicates that a Size octet is present.
1-7 indicates the size of the feedback data field in
octets.
Size: Optional octet indicating the size of the feedback data
field in octets.
feedback data: Profile-specific feedback information. Includes
CID information.
The total size of the feedback data field is determinable upon
reception by the decompressor, by inspection of the Code field and
possibly the Size field. This explicit length information allows
piggybacking and also sending more than one feedback element in a
packet.
When the decompressor has determined the size of the feedback data
field, it removes the feedback type octet and the Size field (if
present) and hands the rest to the same-side associated compressor
together with an indication of the size. The feedback data received
by the compressor has the following structure (feedback sent on a
dedicated feedback channel MAY also use this format):
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
: :
/ large CID (4.5.6 encoding) / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
/ feedback /
+---+---+---+---+---+---+---+---+
The large CID, if present, is encoded according to section 4.5.6.
CID information in feedback data indicates the CID of the packet
stream for which feedback is sent. Note that the LARGE_CIDS
parameter that controls whether a large CID is present is taken from
the channel state of the receiving compressor's channel, NOT from
that of the channel carrying the feedback.
It is REQUIRED that the feedback field have either of the following
two formats:
FEEDBACK-1
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| profile specific information | 1 octet
+---+---+---+---+---+---+---+---+
FEEDBACK-2
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
|Acktype| |
+---+---+ profile specific / at least 2 octets
/ information |
+---+---+---+---+---+---+---+---+
Acktype: 0 = ACK
1 = NACK
2 = STATIC-NACK
3 is reserved (MUST NOT be used. Otherwise unparseable.)
The compressor can use the following logic to parse the feedback
field.
1) If for large CIDs, the feedback will always start with a CID
encoded according to section 4.5.6. If the first bit is 0, the
CID uses one octet. If the first bit is 1, the CID uses two
octets.
2) If for small CIDs, and the size is one octet, the feedback is a
FEEDBACK-1.
3) If for small CIDs, and the size is larger than one octet, and the
feedback starts with the two bits 11, the feedback starts with an
Add-CID octet. If the size is 2, it is followed by FEEDBACK-1.
If the size is larger than 2, the Add-CID is followed by
FEEDBACK-2.
4) Otherwise, there is no Add-CID octet, and the feedback starts with
a FEEDBACK-2.
5.2.3. ROHC IR packet type
The IR header associates a CID with a profile, and typically also
initializes the context. It can typically also refresh (parts of)
the context. It has the following general format.
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 1 0 | x | IR type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ profile specific information / variable length
| |
+---+---+---+---+---+---+---+---+
x: Profile specific information. Interpreted according to the
profile indicated in the Profile field.
Profile: The profile to be associated with the CID. In the IR
packet, the profile identifier is abbreviated to the 8 least
significant bits. It selects the highest-number profile in the
channel state parameter PROFILES that matches the 8 LSBs given.
CRC: 8-bit CRC computed using the polynomial of section 5.9.1. Its
coverage is profile-dependent, but it MUST cover at least the
initial part of the packet ending with the Profile field. Any
information which initializes the context of the decompressor
should be protected by the CRC.
Profile specific information: The contents of this part of the IR
packet are defined by the individual profiles. Interpreted
according to the profile indicated in the Profile field.
5.2.4. ROHC IR-DYN packet type
In contrast to the IR header, the IR-DYN header can never initialize
an uninitialized context. However, it can redefine what profile is
associated with a context, see for example 5.11 (ROHC UDP) and 5.12
(ROHC ESP). Thus the type needs to be reserved at the framework
level. The IR-DYN header typically also initializes or refreshes
parts of a context, typically the dynamic part. It has the following
general format:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 0 0 0 | IR-DYN type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ profile specific information / variable length
| |
+---+---+---+---+---+---+---+---+
Profile: The profile to be associated with the CID. This is
abbreviated in the same way as with IR packets.
CRC: 8-bit CRC computed using the polynomial of section 5.9.1.
Its coverage is profile-dependent, but it MUST cover at least
the initial part of the packet ending with the Profile field.
Any information which initializes the context of the
decompressor should be protected by the CRC.
Profile specific information: This part of the IR packet is
defined by individual profiles. It is interpreted according
to the profile indicated in the Profile field.
5.2.5. ROHC segmentation
Some link layers may provide a much more efficient service if the set
of different packet sizes to be transported is kept small. For such
link layers, these sizes will normally be chosen to transport
frequently occurring packets efficiently, with less frequently
occurring packets possibly adapted to the next larger size by the
addition of padding. The link layer may, however, be limited in the
size of packets it can offer in this efficient mode, or it may be
desirable to request only a limited largest size. To accommodate the
occasional packet that is larger than that largest size negotiated,
ROHC defines a simple segmentation protocol.
5.2.5.1. Segmentation usage considerations
The segmentation protocol defined in ROHC is not particularly
efficient. It is not intended to replace link layer segmentation
functions; these SHOULD be used whenever available and efficient for
the task at hand.
ROHC segmentation should only be used for occasional packets with
sizes larger than what is efficient to accommodate, e.g., due to
exceptionally large ROHC headers. The segmentation scheme was
designed to reduce packet size variations that may occur due to
outliers in the header size distribution. In other cases,
segmentation should be done at lower layers. The segmentation scheme
should only be used for packet sizes that are larger than the maximum
size in the allowed set of sizes from the lower layers.
In summary, ROHC segmentation should be used with a relatively low
frequency in the packet flow. If this cannot be ensured,
segmentation should be performed at lower layers.
5.2.5.2. Segmentation protocol
Segment Packet
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 1 1 | F |
+---+---+---+---+---+---+---+---+
/ Segment / variable length
+---+---+---+---+---+---+---+---+
F: Final bit. If set, it indicates that this is the last segment of
a reconstructed unit.
The segment header may be preceded by padding octets and/or feedback.
It never carries a CID.
All segment header packets for one reconstructed unit have to be sent
consecutively on a channel, i.e., any non-segment-header packet
following a nonfinal segment header aborts the reassembly of the
current reconstructed unit and causes the decompressor to discard the
nonfinal segments received on this channel so far. When a final
segment header is received, the decompressor reassembles the segment
carried in this packet and any nonfinal segments that immediately
preceded it into a single reconstructed unit, in the order they were
received. The reconstructed unit has the format:
Reconstructed Unit
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| |
/ Reconstructed ROHC packet / variable length
| |
+---+---+---+---+---+---+---+---+
/ CRC / 4 octets
+---+---+---+---+---+---+---+---+
The CRC is used by the decompressor to validate the reconstructed
unit. It uses the FCS-32 algorithm with the following generator
polynomial: 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 [HDLC]. If the reconstructed
unit is 4 octets or less, or if the CRC fails, or if it is larger
than the channel parameter MRRU (see 5.1.1), the reconstructed unit
MUST be discarded by the decompressor.
If the CRC succeeds, the reconstructed ROHC packet is interpreted as
a ROHC Header, optionally followed by a payload. Note that this
means that there can be no padding and no feedback in the
reconstructed unit, and that the CID is derived from the initial
octets of the reconstructed unit.
(It should be noted that the ROHC segmentation protocol was inspired
by SEAL by Steve Deering et al., which later became ATM AAL5. The
same arguments for not having sequence numbers in the segments but
instead providing a strong CRC in the reconstructed unit apply here
as well. Note that, as a result of this protocol, there is no way in
ROHC to make any use of a segment that has residual bit errors.)
5.2.6. ROHC initial decompressor processing
The following packet types are reserved at the framework level in the
ROHC scheme:
1110: Padding or Add-CID octet
11110: Feedback
11111000: IR-DYN packet
1111110: IR packet
1111111: Segment
Other packet types can be used at will by individual profiles.
The following steps is an outline of initial decompressor processing
which upon reception of a ROHC packet can determine its contents.
1) If the first octet is a Padding Octet (11100000),
strip away all initial Padding Octets and goto next step.
2) If the first remaining octet starts with 1110, it is an Add-CID
octet:
remember the Add-CID octet; remove the octet.
3) If the first remaining octet starts with 11110, and an Add-CID
octet was found in step 2),
an error has occurred; the header MUST be discarded without
further action.
4) If the first remaining octet starts with 11110, and an Add-CID
octet was not found in step 2), this is feedback:
find the size of the feedback data, call it s;
remove the feedback type octet;
remove the Size octet if Code is 0;
send feedback data of length s to the same-side associated
compressor;
if packet exhausted, stop; otherwise goto 2).
5) If the first remaining octet starts with 1111111, this is a
segment:
attempt reconstruction using the segmentation protocol
(5.2.5). If a reconstructed packet is not produced, this
finishes the processing of the original packet. If a
reconstructed packet is produced, it is fed into step 1)
above. Padding, segments, and feedback are not allowed in
reconstructed packets, so when processing them, steps 1),
4), and 5) are modified so that the packet is discarded
without further action when their conditions match.
6) Here, it is known that the rest is forward information (unless the
header is damaged).
7) If the forward traffic uses small CIDs, there is no large CID in
the packet. If an Add-CID immediately preceded the packet type
(step 2), it has the CID of the Add-CID; otherwise it has CID 0.
8) If the forward traffic uses large CIDs, the CID starts with the
second remaining octet. If the first bit(s) of that octet are not
0 or 10, the packet MUST be discarded without further action. If
an Add-CID octet immediately preceded the packet type (step 2),
the packet MUST be discarded without further action.
9) Use the CID to find the context.
10) If the packet type is IR, the profile indicated in the IR packet
determines how it is to be processed. If the CRC fails to verify
the packet, it MUST be discarded. If a profile is indicated in
the context, the logic of that profile determines what, if any,
feedback is to be sent. If no profile is noted in the context,
no further action is taken.
11) If the packet type is IR-DYN, the profile indicated in the IR-DYN
packet determines how it is to be processed.
a) If the CRC fails to verify the packet, it MUST be discarded.
If a profile is indicated in the context, the logic of that
profile determines what, if any, feedback is to be sent. If no
profile is noted in the context, no further action is taken.
b) If the context has not been initialized by an IR packet, the
packet MUST be discarded. The logic of the profile indicated
in the IR-DYN header (if verified by the CRC), determines what,
if any, feedback is to be sent.
12) Otherwise, the profile noted in the context determines how the
rest of the packet is to be processed. If the context has not
been initialized by an IR packet, the packet MUST be discarded
without further action.
The procedure for finding the size of the feedback data is as
follows:
Examine the three bits which immediately follow the feedback packet
type. When these bits are
1-7, the size of the feedback data is given by the bits;
0, a Size octet, which explicitly gives the size of the
feedback data, is present after the feedback type octet.
5.2.7. ROHC RTP packet formats from compressor to decompressor
ROHC RTP uses three packet types to identify compressed headers, and
two for initialization/refresh. The format of a compressed packet
can depend on the mode. Therefore a naming scheme of the form
<modes format is used in>-<packet type number>-<some property>
is used to uniquely identify the format when necessary, e.g., UOR-2,
R-1. For exact formats of the packet types, see section 5.7.
Packet type zero: R-0, R-0-CRC, UO-0.
This, the minimal, packet type is used when parameters of all SN-
functions are known by the decompressor, and the header to be
compressed adheres to these functions. Thus, only the W-LSB
encoded RTP SN needs to be communicated.
R-mode: Only if a CRC is present (packet type R-0-CRC) may the
header be used as a reference for subsequent decompression.
U-mode and O-mode: A small CRC is present in the UO-0 packet.
Packet type 1: R-1, R-1-ID, R-1-TS, UO-1, UO-1-ID, UO-1-TS.
This packet type is used when the number of bits needed for the SN
exceeds those available in packet type zero, or when the
parameters of the SN-functions for RTP TS or IP-ID change.
R-mode: R-1-* packets are not used as references for subsequent
decompression. Values for other fields than the RTP TS or IP-ID
can be communicated using an extension, but they do not update the
context.
U-mode and O-mode: Only the values of RTP SN, RTP TS and IP-ID can
be used as references for future compression. Nonupdating values
can be provided for other fields using an extension (UO-1-ID).
Packet type 2: UOR-2, UOR-2-ID, UOR-2-TS
This packet type can be used to change the parameters of any SN-
function, except those for most static fields. Headers of packets
transferred using packet type 2 can be used as references for
subsequent decompression.
Packet type: IR
This packet type communicates the static part of the context,
i.e., the value of the constant SN-functions. It can optionally
also communicate the dynamic part of the context, i.e., the
parameters of the nonconstant SN-functions.
Packet type: IR-DYN
This packet type communicates the dynamic part of the context,
i.e., the parameters of nonconstant SN-functions.
5.2.8. Parameters needed for mode transition in ROHC RTP
The packet types IR (with dynamic information), IR-DYN, and UOR-2 are
common for all modes. They can carry a mode parameter which can take
the values U = Unidirectional, O = Bidirectional Optimistic, and R =
Bidirectional Reliable.
Feedback of types ACK, NACK, and STATIC-NACK carry sequence numbers,
and feedback packets can also carry a mode parameter indicating the
desired compression mode: U, O, or R.
As a shorthand, the notation PACKET(mode) is used to indicate which
mode value a packet carries. For example, an ACK with mode parameter
R is written ACK(R), and an UOR-2 with mode parameter O is written
UOR-2(O).
5.3. Operation in Unidirectional mode
5.3.1. Compressor states and logic (U-mode)
Below is the state machine for the compressor in Unidirectional mode.
Details of the transitions between states and compression logic are
given subsequent to the figure.
Optimistic approach
+------>------>------>------>------>------>------>------>------+
| |
| Optimistic approach Optimistic approach |
| +------>------>------+ +------>------>------+ |
| | | | | |
| | v | v v
+----------+ +----------+ +----------+
| IR State | | FO State | | SO State |
+----------+ +----------+ +----------+
^ ^ | ^ | |
| | Timeout | | Timeout / Update | |
| +------<------<------+ +------<------<------+ |
| |
| Timeout |
+------<------<------<------<------<------<------<------<------+
5.3.1.1. State transition logic (U-mode)
The transition logic for compression states in Unidirectional mode is
based on three principles: the optimistic approach principle,
timeouts, and the need for updates.
5.3.1.1.1. Optimistic approach, upwards transition
Transition to a higher compression state in Unidirectional mode is
carried out according to the optimistic approach principle. This
means that the compressor transits to a higher compression state when
it is fairly confident that the decompressor has received enough
information to correctly decompress packets sent according to the
higher compression state.
When the compressor is in the IR state, it will stay there until it
assumes that the decompressor has correctly received the static
context information. For transition from the FO to the SO state, the
compressor should be confident that the decompressor has all
parameters needed to decompress according to a fixed pattern.
The compressor normally obtains its confidence about decompressor
status by sending several packets with the same information according
to the lower compression state. If the decompressor receives any of
these packets, it will be in sync with the compressor. The number of
consecutive packets to send