RFC 3095
RObust Header Compression (ROHC): Framework and four profiles: RTP, UDP, ESP, and uncompressed
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..........................................1681. 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.
(next page on part 2)
