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.
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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
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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
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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
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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).
----------------------------------------------------------------[Page 15]
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].
----------------------------------------------------------------[Page 16]
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.
----------------------------------------------------------------[Page 17]
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.)
----------------------------------------------------------------[Page 18]
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.
----------------------------------------------------------------[Page 19]
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
----------------------------------------------------------------[Page 20]
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)
----------------------------------------------------------------[Page 21]
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.
----------------------------------------------------------------[Page 22]
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.
----------------------------------------------------------------[Page 23]
+----------------------+
| 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).
----------------------------------------------------------------[Page 24]
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.
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