Network Working Group Ghyslain Pelletier, Editor, Ericsson INTERNET-DRAFT Qian Zhang, Microsoft Research Asia Expires: November 2003 Lars-Erik Jonsson, Ericsson HongBin Liao, Microsoft Research Asia Mark A West, Siemens/Roke Manor May 23, 2003 RObust Header Compression (ROHC): TCP/IP Profile (ROHC-TCP) Status of this memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Abstract This document specifies a ROHC (Robust Header Compression) profile for compression of TCP/IP packets. The profile, called ROHC-TCP, is a robust header compression scheme for TCP/IP that provides improved compression efficiency and enhanced capabilities for compression of various header fields including TCP options. Existing TCP/IP 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. In addition, existing schemes [RFC-1144, RFC-2507] have not addressed how to compress TCP options such as SACK (Selective Acknowledgements) [RFC-2018, RFC- 2883] and Timestamps [RFC-1323]. Pelletier, et al. [Page 1] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 Table of contents 1. Introduction....................................................3 2. Terminology.....................................................4 3. Background......................................................5 3.1. Existing TCP/IP header compression schemes................5 3.2. Classification of TCP/IP header fields....................6 3.3. Characteristics of short-lived TCP transfers..............7 4. Overview of the TCP/IP profile..................................8 4.1. General concepts..........................................8 4.1.1. Feedback channel considerations.........................8 4.1.2. Master sequence number (MSN)............................8 4.1.3. Context replication.....................................9 4.2. ROHC-TCP operation........................................9 4.3. Encoding methods..........................................9 5. ROHC-TCP - TCP/IP compression (Profile 0x0006).................10 5.1. Initialization...........................................10 5.2. Packet types.............................................11 5.2.1. Initialization and Refresh packets (IR)................11 5.2.2. Compressed packets (CO)................................11 5.3. Compression logic........................................11 5.3.1. Compressor states and logic............................11 5.3.2. Initialization and Refresh (IR) state..................12 5.3.3. Compression (CO) state.................................12 5.3.4. Feedback logic.........................................12 5.3.5. State transition logic.................................13 5.3.5.1. Optimistic approach, upward transition...............13 5.3.5.2. Optional acknowledgements (ACKs), upward transition..13 5.3.5.3. Timeouts, downward transition........................13 5.3.5.4. Negative ACKs (NACKs), downward transition...........14 5.3.5.5. Need for updates, downward transition................14 5.4. Decompression logic......................................14 5.4.1. Decompressor states and logic..........................14 5.4.2. No Context (NC) state..................................15 5.4.3. Full Context (FC) state................................15 5.4.4. Static Context (SC) state..............................16 5.4.5. Allowing decompression.................................16 5.4.6. Reconstruction and verification........................16 5.4.7. Actions upon CRC failure...............................16 5.4.8. Feedback logic.........................................17 5.5. Packet formats...........................................17 5.5.1. Basic structure of the ROHC-TCP IR and IR-DYN packets..17 5.5.1.1. Initialization of the IPv6 Header [RFC-2460].........17 5.5.1.2. Initialization of the IPv4 Header [RFC-791]..........17 Pelletier, et al. [Page 2] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 5.5.1.3. Initialization of the TCP Header [RFC-793]...........18 5.5.2. IP-ID..................................................18 5.5.3. Basic structure of the ROHC-TCP IR-REPLICATE packet....18 5.5.4. Feedback formats and options for ROHC-TCP..............20 5.5.4.1. Feedback formats.....................................20 5.5.4.2. Feedback options.....................................21 6. Security considerations........................................21 7. IANA considerations............................................21 8. Acknowledgements...............................................22 9. References.....................................................22 9.1. Normative references......................................22 9.2. Informative references....................................23 10. Authors' addresses............................................24 1. Introduction There are several reasons to perform header compression on low- or medium-speed links for TCP/IP traffic, and these have already been discussed in [RFC-2507]. [TCP-REQ] introduces additional considerations making robustness an important objective for a TCP compression scheme. Finally, existing TCP/IP header compression schemes [RFC-1144, RFC-2507] are limited in their handling of the TCP options field and cannot compress the headers of handshaking packets (SYNs and FINs). It is thus desirable for a header compression scheme to be able to handle loss on the link between the compression and decompression point as well as loss before the compression point. The header compression scheme also needs to consider how to efficiently compress short-lived TCP transfers and TCP options, such as SACK [RFC-2018, RFC-2883] and Timestamps [RFC-1323]. The ROHC WG has developed a header compression framework on top of which various profiles can be defined for different protocol sets, or for different compression strategies. This document defines a TCP/IP compression profile for the ROHC framework [RFC-3095], compliant with the requirements on ROHC TCP/IP header compression [TCP-REQ]. Specifically, it describes a header compression scheme for TCP/IP header compression (ROHC-TCP) that is robust against packet loss and that offers enhanced capabilities, in particular for the compression of header fields including TCP options. The profile identifier for TCP/IP compression is 0x0006. Pelletier, et al. [Page 3] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 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 RFC2119. This document reuses some of the terminology found in [RFC-3095]. In addition, this document defines the following terms: Base context The base context is a context that has been validated by both the compressor and the decompressor. A base context can be used as the reference when building a new context using replication. Base CID The Base Context Identifier is the CID used to identify the Base Context, where information needed for context replication can be extracted from. Context replication Context replication is the mechanism that establishes and initializes a new context based on another existing valid context (a base context). This mechanism is introduced to reduce the overhead of the context establishment procedure, and is especially useful for compression of multiple short-lived TCP connections that may be occurring simultaneously or near-simultaneously. Short-lived TCP Transfer Short-lived TCP transfers refer to the TCP connections transmitting only small amounts of data for each single connection. Short TCP flows seldom need to operate beyond the slow-start phase of TCP to complete their transfer, which also means that the transmission ends before any significant increase of the TCP congestion window may occur. Pelletier, et al. [Page 4] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 3. Background This chapter provides some background information on TCP/IP header compression. The fundamentals of general header compression may be found in [RFC-3095]. In the following sections, two existing TCP/IP header compression schemes are first described along with a discussion of their limitations, followed by the classification of TCP/IP header fields. Finally, some of the characteristics of short- lived TCP transfers are summarized. The behavior analysis of TCP/IP header fields among multiple short- lived connections may be found in [TCP-BEH]. 3.1. Existing TCP/IP header compression schemes Compressed TCP (CTCP) and IP Header Compression (IPHC) are two different schemes that may be used to compress TCP/IP headers. Both schemes transmit only the differences from the previous header in order to reduce the large overhead of the TCP/IP header. The CTCP [RFC-1144] compressor detects transport-level retransmissions and sends a header that updates the context completely when they occur. While CTCP works well over reliable links, it is vulnerable when used over less reliable links as even a single packet loss results in loss of synchronization between the compressor and the decompressor. This in turn leads to the TCP receiver discarding all remaining packets in the current window because of a checksum error. This effectively prevents the TCP Fast Retransmit algorithm [RFC-2001] from being triggered. In such case, the compressor must wait until the TCP timeout to resynchronize. To reduce the errors due to the inconsistent contexts between compressor and decompressor when compressing TCP, IPHC [RFC-2507] improves somewhat on CTCP by augmenting the repair mechanism of CTCP with a local repair mechanism called TWICE and with a link-level nacking mechanism to request a header that updates the context. The TWICE algorithm assumes that only the Sequence Number field of TCP segments are changing with the deltas between consecutive packets being constant in most cases. This assumption is however not always true, especially when TCP Timestamps and SACK options are used. The full header request mechanism requires a feedback channel that may be unavailable in some circumstances. This channel is used to explicitly request that the next packet be sent with an uncompressed header to allow resynchronization without waiting for a TCP timeout. In addition, this mechanism does not perform well on links with long round-trip time. Both CTCP and IPHC are also limited in their handling of the TCP options field. For IPHC, any change in the options field (caused by Pelletier, et al. [Page 5] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 timestamps or SACK, for example) renders the entire field uncompressible, while for CTCP such a change in the options field effectively disables TCP/IP header compression altogether. Finally, existing TCP/IP compression schemes do not compress the headers of handshaking packets (SYNs and FINs). Compressing these packets may greatly improve the overall header compression ratio for the cases where many short-lived TCP connections share the same link. 3.2. Classification of TCP/IP header fields Header compression is possible due to the fact that there is much redundancy between header field values within packets, especially between consecutive packets. To utilize these properties for TCP/IP header compression, it is important to understand the change patterns of the various header fields. All fields of the TCP/IP packet header have been classified in detail in [TCP-BEH]. The main conclusion is that most of the header fields can easily be compressed away since they never or seldom change. The following fields do however require more sophisticated mechanisms: - IPv4 Identification (16 bits) - IP-ID - TCP Sequence Number (32 bits) - SN - TCP Acknowledgement Number (32 bits) - ACKN - TCP Reserved (4 bits) - TCP ECN flags (2 bits) - ECN - TCP Window (16 bits) - WINDOW - TCP Options - Maximum Segment Size (4 octets) - MSS - Window Scale (3 octets) - WSopt - SACK Permitted (2 octets) - TCP SACK - SACK - TCP Timestamp (32 bits) - TS The assignment of IP-ID values can be done in various ways, which are Sequential jump, Random, and Sequential, respectively. However, designers of IPv4 stacks for cellular terminals should use an assignment policy close to Sequential. In [RFC-3095], the IP-ID is generally inferred from the RTP Sequence Number. However, with regard to TCP compression, the analysis in [TCP-BEH] reveals that there is no obvious candidate to this purpose among the TCP fields. The change pattern of several TCP fields (Sequence Number, Acknowledgement Number, Window, etc.) are very hard to predict and differs entirely from the behavior of RTP fields discussed in [RFC- 3095]. Of particular importance to a TCP/IP header compression scheme is the understanding of the sequence and acknowledgement number [TCP- BEH]. Specifically, at any point on the path (i.e. wherever a compressor might be deployed), the sequence number can be anywhere within a range defined by the TCP window. Missing packets or Pelletier, et al. [Page 6] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 retransmissions can cause the TCP sequence number to fluctuate within the limits of this window. The jumps in acknowledgement number are also bounded by this TCP window. Another important behavior of the TCP/IP header is the dependency between the sequence number and the acknowledgment number. It is well-known that most TCP connections only have one-way traffic (web browsing and FTP downloading, for example). This means that on the forward path (from server to client), only the sequence number is changing while the acknowledgement number remains constant for most packets; on the backward path (from client to server), only the sequence number is changing and the acknowledgement number remains constant for most packets. With respect to TCP options, it is noted that most options (such as MSS, WSopt, SACK-permitted, etc.) may appear only on a SYN segment. Every implementation should (and we expect most will) ignore unknown options on SYN segments. Headers specific to Mobile IP (for IPv4 or IPv6) do not receive any special treatment in this document, for similar reasons as those described in [RFC-3095]. 3.3. Characteristics of short-lived TCP transfers Recent studies shows that the majority of TCP flows are short-lived transfers with an average and a median size no larger than 10KB. Short-lived TCP transfers will degrade the performance of header compression schemes that establish a new context by initially sending full headers. It is hard to improve the performance for a single, unpredictable, short-lived connection. However, there are common cases where there will be multiple TCP connections between the same pair of hosts. A mobile user browsing several web pages from the same web server (this is more the case with HTTP/1.0 than HTTP/1.1) is one example. In such case, multiple short-lived TCP/IP flows occur simultaneously or near simultaneously within a relatively short time interval. It may be expected that most (if not all) of the IP header of the these connections will be almost identical to each other, with only small relative jumps for the IP-ID field. Furthermore, a subset of the TCP fields may also be very similar from one connection to another. For example, one of the port numbers may be reused (the service port) while the other (the ephemeral port) may be changed only by a small amount relative to the just-closed connection. With regard to header compression, this means that parts of a compression context used for a TCP connection may be reusable for Pelletier, et al. [Page 7] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 another TCP connection. A mechanism supporting context replication, where a new context is initialized from an existing one, provide useful optimizations for a sequence of short-lived TCP connections. Context replication is possible due to the fact that there is much similarity in header field values and context values among multiple simultaneous or near simultaneous connections. All header fields and related context values have been classified in detail in [TCP-BEH]. The main conclusion is that most part of the IP sub-context, some TCP fields, and some context values can easily be replicated since they seldom change or change with only a small jump. 4. Overview of the TCP/IP profile 4.1. General concepts Many of the concepts behind the ROHC-TCP profile are similar to those described in [RFC-3095]. Like for other ROHC profiles, ROHC-TCP makes use of the ROHC protocol as described in [RFC-3095, sections 5.1 to 5.2.6 inclusively]. This include data structures, general packet formats, reserved packet types, segmentation and initial decompressor processing. ROHC-TCP also integrally reuse some of the encoding methods defined in [RFC-3095, section 4.5]. 4.1.1. Feedback channel considerations The ROHC-TCP profile may be used in environments with or without feedback capabilities from decompressor to compressor. ROHC-TCP however assumes that if a ROHC feedback channel is available and is used at least once by the decompressor, this channel will be present during the entire compression operation. The occurrence of this channel will be further referred as the "established" feedback channel. Otherwise, if the connection is broken and the channel disappears, header compression should be restarted. To parallel [RFC-3095], this is similar to allowing only one transition per compressor state machine: from the initial unidirectional mode to the bi-directional mode of operation, with the transition being triggered by the reception of the first packet containing feedback from the decompressor. This effectively means that ROHC-TCP does not explicitly define any operational modes. 4.1.2. Master sequence number (MSN) Feedback packets of types ACK and NACK carry information about sequence number or acknowledgement number from decompressor to compressor. Unfortunately, there is no guarantee that sequence number and acknowledgement number fields will be used by every IP protocol stack. In addition, the combined size of the sequence number field Pelletier, et al. [Page 8] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 and the acknowledgement number field is rather large, and they can therefore not be carried efficiently within the feedback packet. To overcome this problem, ROHC-TCP introduces a control field called the Master Sequence Number (MSN) field. The MSN field is created at the compressor, rather than using one of the fields already present in the uncompressed header. If a feedback channel is established, the MSN field is present in every packets sent by the compressor when in the Initialization and Refresh state (IR) as well as in every m compressed header. The decompressor always sends the MSN as part of the feedback information. The MSN can later be used by the compressor to infer which packet is being acknowledged by the decompressor. The value of m is chosen as trade-off between compression efficiency and acknowledgement efficiency. 4.1.3. Context replication For ROHC-TCP, context replication may be particularly useful for short-lived TCP flows [TCP-REQ]. ROHC-TCP therefore supports context replication as defined in [ROHC-CR], and the basic structure of the IR-REPLICATE packet is defined in section 5.5.3. 4.2. ROHC-TCP operation 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. For ROHC-TCP compression, the compressor has two states and the decompressor has three states. The two compressor states are the Initialization and Refresh (IR) state, and the Compression (CO) state. The three states of the decompressor are No Context (NC), Static Context (SC) and Full Context (FC). Transitions need not be synchronized between the two state machines. 4.3. Encoding methods <# Editor's Note: This section needs to be completed and formatted #> As mentioned earlier, ROHC-TCP integrally reuse some of the encoding methods defined in [RFC-3095, section 4.5]. Considering the changing pattern of several TCP fields, such as sequence number, acknowledgement number, etc., Window-based LSB encoding [RFC-3095], which does not assume the linear changing pattern of the target header fields, is more suitable to encode those TCP fields both efficiently and robustly. Pelletier, et al. [Page 9] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 Fixed-payload encoding If the compressor finds that the payload size of consecutive packets is a constant value and one of such packets has been removed from the context window, which means the decompressor has known the exact value of the constant size, it may use fixed-payload encoding scheme to improve the compression efficiency. For some applications, such as bulk data transfer, the payload size of each packet is usually a constant value, e.g. 1460 bytes. In such case, the sequence number and acknowledgment number can be represented using the following equation: SEQ (or ACK) = m * PAYLOAD + n. If all the packets in context window have the same 'n', only 'm' needs to be transmitted to the decompressor. The decompressor can assign the value of æPAYLOADÆ using the packet size of the reference packet. The decompressor can then obtain the sequence number or acknowledgment number after correctly decoding 'm', and use those as reference values. This encoding method is called fixed-payload encoding. 5. ROHC-TCP - TCP/IP compression (Profile 0x0006) This section describes a ROHC profile for TCP/IP compression. The profile identifier for ROHC-TCP is 0x0006. <# Editor's Note: This chapter needs to be completed #> 5.1. Initialization The static context of ROHC TCP streams can be initialized in either two ways: 1) By using an IR packet as in section 5.4.1, where the profile is six (6) and the static chain ends with the static part of a TCP packet. At the compressor, the MSN is initialized to a random value when the IR packet is sent. 2) By reusing an existing context where using context replication [ROHC-CR]. This is done with an IR-REPLICATE packet as in section 5.4.3, where the profile number is six (6) and the static replication chain ends with the static part of a TCP packet. At the compressor, the MSN is either reinitialized to <# Editor's Note: Initial value of the MSN is TBD #> value if it is sent within the IR-REPLICATE packet, or the MSN value already present in the base context is used if the it is not sent. Pelletier, et al. [Page 10] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 5.2. Packet types ROHC-TCP defines two different packet types: the Initialization and Refresh (IR) packet type, and the Compressed packet type (CO). Each type correspond to one of the possible states of the compressor. Each packet type also define a number of packet formats: [#TBD] packet formats are defined for compressed headers (CO), and three for initialization/refresh/replication (IR). 5.2.1. Initialization and Refresh packets (IR) The ROHC-TCP IR packet follows the general format of the ROHC IR packet, as defined in [RFC-3095, section 5.2.3]. Packet type: IR This packet type communicates the static part of the context. It can optionally also communicate the dynamic part of the context. Packet type: IR-DYN This packet type communicates the dynamic part of the context. Packet type: IR-REPLICATE This packet type communicates the static and dynamic parts of the replicated context. 5.2.2. Compressed packets (CO) <# Editor's Note: #> <# To be written once the ROHC-TCP packet formats are defined #> <# A decision must be taken by the ROHC WG regarding the #> <# method to use when defining the packet formats, in #> <# particular in relation to the proposed formal notation. #> 5.3. Compression logic 5.3.1. Compressor states and logic For ROHC-TCP, the two compressor states are the Initialization and Refresh (IR) state, and the Compression (CO) state. The compressor always start in the lower compression state (IR). The compressor will normally operate in the higher compression state (CO), under the constraint that the compressor is sufficiently confident that the decompressor has the information necessary to reconstruct a header compressed according to this state. Pelletier, et al. [Page 11] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 The figure below shows the state machine for the compressor. The details of each state, state transitions, and compression logic are given in sub-sections following the figure. Optimistic approach / ACK ACK +------>------>------>------+ +->-+ | | | | | v | v +----------+ +----------+ | IR State | | CO State | +----------+ +----------+ ^ | | Timeout / NACK / STATIC-NACK | +-------<-------<-------<--------+ The transition from IR state to CO state is based on the following principles: the need for update and the optimistic approach principle or, if a feedback channel is established, feedback received from the decompressor. In ROHC-TCP, the compressor will start in the IR state. The following sub-sections will describe further the logic for the compressor. 5.3.2. Initialization and Refresh (IR) state <# Editor's Note: To be defined #> 5.3.3. Compression (CO) state <# Editor's Note: To be defined #> 5.3.4. Feedback logic ROHC-TCP makes use of feedback from decompressor to compressor for transitions in the backward direction, and optionally to improve the forward transition. The reception of either positive feedback (ACKs) or negative feedback (NACKs) establishes the feedback channel from the decompressor. Once there is an established feedback channel, the compressor makes use of this feedback for optionally improving the transitions among different states. This helps increasing the compression efficiency by providing the information necessary for the compressor to achieve the necessary confidence level. When the feedback channel is established, it becomes superfluous for the compressor to send periodic refreshes. In the IR state, the compressor can transit to the CO state once it receives a valid ACK for an IR/IR-REPLICATE packet sent (an ACK can only be valid if it refers to a packet sent earlier). If the packet referred by the feedback is in the context window, the compressor will remove packets older than the referred packet from the context Pelletier, et al. [Page 12] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 window. Because ACK means that the packet referred by feedback has been the reference of the decompressor, the compressor doesn't need to keep older packets. If the compressor is in the CO state, it will remove the packets older than the referred packet by the feedback from the context window. Upon receiving an NACK, the compressor transits back to IR state. 5.3.5. State transition logic Decisions about transitions between the IR and the CO states are taken by the compressor on the basis of: - variations in the packet headers - positive feedback from decompressor (Acknowledgements -- ACKs) - negative feedback from decompressor (Negative ACKS -- NACKs) - confidence level regarding error-free decompression of a packet 5.3.5.1. Optimistic approach, upward transition Transition to the CO state is carried out according to the optimistic approach principle. This means that the compressor transits to the CO state when it is fairly confident that the decompressor has received enough information to correctly decompress packets sent according to the higher compression state. In general, there are many approaches where the compressor can obtain such information. A simple and general approach can be achieved by sending uncompressed or partial full headers periodically. 5.3.5.2. Optional acknowledgements (ACKs), upward transition The compressor can also transit to the CO state based on feedback received by the decompressor. If a feedback channel is available, positive feedback (ACKs) MAY be used for acknowledging successful decompression of packets. Upon reception of an ACK for a context updating packet, the compressor knows that the decompressor has received the acknowledged packet and the transition to the CO state can be carried out immediately. This functionality is optional, so a compressor MUST NOT expect to get such ACKs initially or during normal operation, even if a feedback channel is available or established. 5.3.5.3. Timeouts, downward transition When the optimistic approach is used, e.g. until a feedback channel is established, there will always be a possibility of failure since the decompressor may not have received sufficient information for correct decompression. Therefore, unless a feedback channel has been Pelletier, et al. [Page 13] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 established, the compressor MUST periodically transit to the IR state. 5.3.5.4. Negative ACKs (NACKs), downward transition Negative acknowledgments (NACKs) are also called context requests. Upon reception of a NACK the compressor transits back to the IR state and sends updates (IR-DYN, or possibly IR) to the decompressor. NACKs carry the MSN of the latest packet successfully decompressed. 5.3.5.5. Need for updates, downward transition When the header to be compressed does not conform to the established pattern or the compressor is not confident whether the decompressor has the synchronized context, the compressor will transit to the IR state. 5.4. Decompression logic 5.4.1. Decompressor states and logic The three states of the decompressor are No Context (NC), Static Context (SC) and Full Context (FC). The decompressor starts in its lowest compression state, the NC state. Successful decompression will always move the decompressor to the FC state. The decompressor state machine normally never leaves the FC state once it has entered this state; only repeated decompression failures will force the decompressor to transit downwards to a lower state. The decompressor does not attempt to decompress headers at all in the NC state and SC states unless sufficient information is included in the received packet itself. Below is the state machine for the decompressor. Details of the transitions between states and decompression logic are given in the sub-sections following the figure. Success +-->------>------>------>------>------>--+ | | No Static | No Dynamic Success | Success +-->--+ | +-->--+ +--->----->---+ +-->--+ | | | | | | | | | | v | | v | v | v +-----------------+ +---------------------+ +-------------------+ | No Context (NC) | | Static Context (SC) | | Full Context (FC) | +-----------------+ +---------------------+ +-------------------+ ^ | ^ | | k_2 out of n_2 failures | | k_1 out of n_1 failures | +-----<------<------<-----+ +-----<------<------<-----+ Pelletier, et al. [Page 14] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 5.4.2. No Context (NC) state Initially, while working in the NC state, the decompressor has not yet successfully decompressed a packet. Upon receiving an IR-STATIC, IR-DYN or IR-REPLICATE packet, the decompressor will verify the correctness of this packet by validating its header using the CRC check. For an IR-REPLICATE packet, the decompressor builds a new context from the existing base context and make the necessary update. For an IR-STATIC or an IR-DYN packet, the decompressor simply updates the context. Finally, the decompressor uses the successfully decompressed packet as the reference packet. When an IR-REPLICATE packet passes the verification, the decompressor should send an ACK. When an IR, an IR-DYN or any other packet is correctly decompressed, the compressor may optionally send an ACK. In either cases, the feedback packet will carry the master sequence number (MSN) information corresponding to the latest correctly decompressed packet. In the NC state, when any packet fails the verification, the decompressor should send a NACK. The decompressor discards all packets until a static update (IR-STATIC) or replication (IR- REPLICATE) that passes the verification check is received. Once a packet has been decompressed correctly, the decompressor can transit to the FC state, and only upon repeated failures will it transit back to a lower state. Only IR, IR-DYN or IR-REPLICATE packets may be decompressed in the NC state. 5.4.3. Full Context (FC) state Upon receiving an IR, IR-DYN or IR-REPLICATE packet, the decompressor should verify the correctness of its header by CRC check. If the verification succeeds, the decompressor will update the context and use this packet as the reference packet. Consequently, the decompressor will convert the packet into the original packet and pass it to the network layer of the system. Upon receiving other types of packet, the decompressor will decompress it. The decompressor MUST verify the correctness of the decompressed packet. If this verification succeeds, the decompressor passes the decompressed packet to the system's network layer. The decompressor will then use this packet as the reference value, if it is not older than the current reference packet (by checking the MSN of the compressed packet, or the sequence number and/or the acknowledgement number field of the TCP header). When the verification check of k_1 out of the last n_1 decompressed packets have failed, context damage SHOULD be assumed and a NACK Pelletier, et al. [Page 15] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 SHOULD be sent. The decompressor moves to the SC state and discards all packets until an update that successfully passes the verification check is received. 5.4.4. Static Context (SC) state In the SC state, when the verification check of k_2 out of the last n_2 decompressed packets have failed, context damage is assumed and a STATIC-NACK SHOULD be sent. The decompressor moves to the NC state and discards all packets until an IR, IR-DYN or IR-REPLICATE that successfully passes the verification check is received. Note that appropriate values for k and n, are related to the residual error rate of the link. When the residual error rate is close to zero, k = n = 1 may be appropriate. <# Editor's Note: Parts if this logic may have to be refined #> ># based on the packet formats and types to be defined, and #> <# when the context replication mechanism will be defined. #> 5.4.5. Allowing decompression In the No Context state, only IR packets (including IR-REPLICATE), which carry information on the static fields, may be decompressed. <# Editor's Note: the Static Context is dependant on the packet #> <# format and particularly if and what kind of CRC #> <# will be used. This is still to be defined. #> In the Full Context state, decompression may be attempted regardless of the type of packet received. If decompression may not be performed, the packet is discarded. 5.4.6. Reconstruction and verification <# Editor's Note: To be written - depends on the packet formats #> 5.4.7. Actions upon CRC failure The actions to be taken when a CRC verification fails following the decompression of an IR-REPLICATE packet are specified in [ROHC-CR]. For other IR packets, the actions to be taken are specified in [RFC- 3095, section 5.3.2.2.3]. For other ROHC-TCP packet types carrying a CRC, the following actions MUST be taken when a CRC verification fails: <# Editor's Note: To be defined - depends on packet formats, #> <# and particularly if and what kind of CRC will #> <# be used. This is still to be defined. #> Pelletier, et al. [Page 16] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 5.4.8. Feedback logic The decompressor may send positive feedback (ACKs) to initially establish the feedback channel. Either positive feedback (ACKs) or negative feedback (NACKs) will establish the feedback channel between decompressor and compressor. Once a feedback channel is established, it will be used by the decompressor to send error recovery requests and (optionally) acknowledgements of significant context updates. When the feedback channel is established, it becomes superfluous for the compressor to send periodic refreshes. 5.5. Packet formats <# Editor's Note: To be defined #> 5.5.1. Basic structure of the TCP IR and IR-DYN packets ROHC-TCP uses the same basic structure for the IR and IR-DYN packets as defined in [RFC-3095, section 5.7.7.1. and section 5.7.7.2]. 5.5.1.2. Initialization of IPv6 Header [RFC-2460] Same as [RFC-3095, section 5.7.7.3]. 5.5.1.3. Initialization of IPv4 Header [RFC-791] Same as [RFC-3095, section 5.7.7.4]. Pelletier, et al. [Page 17] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 5.5.1.4. Initialization of TCP Header [RFC-793] Static part: +---+---+---+---+---+---+---+---+ / Source Port / 2 octets +---+---+---+---+---+---+---+---+ / Destination Port / 2 octets +---+---+---+---+---+---+---+---+ Dynamic part: +---+---+---+---+---+---+---+---+ / Master Sequence Number / 2 octets +---+---+---+---+---+---+---+---+ / Sequence Number / 4 octets +---+---+---+---+---+---+---+---+ / Acknowledgement Number / 4 octets +---+---+---+---+---+---+---+---+ | Data Offset | Reserved | 1 octet +---+---+---+---+---+---+---+---+ |CWR|ECE|URG|ACK|PSH|RST|SYN|FIN| 1 octet +---+---+---+---+---+---+---+---+ / Window / 2 octets +---+---+---+---+---+---+---+---+ / Checksum / 2 octets +---+---+---+---+---+---+---+---+ / Urgent Pointer / 2 octets +---+---+---+---+---+---+---+---+ / Options / variable length +---+---+---+---+---+---+---+---+ Eliminated: Nothing. Extra: Master Sequence Number See section 4.1.2. 5.5.2. IP-ID The IP-ID is handled as in [RFC-3095, section 4.5.5], but the offset is from the MSN (see also section 4.1.2). 5.5.3. Basic structure of the TCP IR-REPLICATE packet ROHC-TCP supports the context replication mechanism defined in [ROHC- CR]. Context replication requires a dedicated IR packet format that uniquely identifies the IR-REPLICATE packet for this profile. Pelletier, et al. [Page 18] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 With consideration to the extensibility of the IR packet type defined in [RFC-3095], the ROHC-TCP profile supports context replication through the profile specific part of the IR packet. This is achieved using the bit (x) left in the IR packet header for "Profile specific information". For ROHC-TCP, this bit is defined as a flag indicating whether this packet is an IR packet or an IR-REPLICATE packet. 0 1 2 3 4 5 6 7 --- --- --- --- --- --- --- --- | Add-CID octet | if for small CIDs and CID != 0 +---+---+---+---+---+---+---+---+ | 1 1 1 1 1 1 0 1 | +---+---+---+---+---+---+---+---+ | | / 0-2 octets of CID info / 1-2 octets if for large CIDs | | +---+---+---+---+---+---+---+---+ | Profile | 1 octet +---+---+---+---+---+---+---+---+ | CRC | 1 octet +---+---+---+---+---+---+---+---+ | B | CRC* | 1 octet +---+---+---+---+---+---+---+---+ | | present if B = 1, / Base CID / 1 octet if for small CIDs, or | | 1-2 octets if for large CIDs +---+---+---+---+---+---+---+---+ | | | Static replication chain / variable length | | +---+---+---+---+---+---+---+---+ | | / Dynamic replication chain / variable length | | - - - - - - - - - - - - - - - - | | / Payload / variable length | | - - - - - - - - - - - - - - - - B: B = 1 indicates that the Base CID field is present. Profile: Profile identifier. CRC: 8-bit CRC computed using the polynomial of [ROHC-CR, section 3.4.1.2]. The 8-bit CRC in the IR-REPLICATE packet is calculated over the initial part of the IR-REPLICATE packet ending with the Base CID (if present). The calculation of this CRC therefore excludes both replication chains as well as the payload but includes the B bit, CRC*, CID or any Add-CID octet. Pelletier, et al. [Page 19] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 CRC*: The CRC over the original, uncompressed, header. This 7-bit CRC is computed according to [ROHC-CR, section 3.4.1.1]. Static replication chain: A chain of static subheader information used for replication. Dynamic replication chain: A chain of dynamic subheader information used for replication. What dynamic information is present is inferred from the static replication chain. Payload: The payload of the corresponding original packet, if any. The presence of a payload is inferred from the packet length. 5.5.4. Feedback packets and formats 5.5.4.1. Feedback formats for ROHC-TCP This section describes the feedback format for ROHC-TCP. ROHC-TCP uses the ROHC feedback format described in [ROHC, section 5.2.2]. All feedback formats carry a field labeled SN. The SN field contains LSBs of the Master Sequence Number (MSN) described in section 4.1.3. The sequence number to use is the MSN corresponding to the header that caused the feedback information to be sent. If that MSN cannot be determined, for example when decompression fails, the MSN to use is that corresponding to the latest successfully decompressed header. FEEDBACK-1 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | SN | +---+---+---+---+---+---+---+---+ A FEEDBACK-1 is an ACK. In order to send a NACK or a STATIC-NACK, FEEDBACK-2 must be used. FEEDBACK-2 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ |Acktype| <# To Be Defined #> | +---+---+---+---+---+---+---+---+ | SN | +---+---+---+---+---+---+---+---+ / Feedback options / +---+---+---+---+---+---+---+---+ Acktype: 0 = ACK 1 = NACK 2 = STATIC-NACK Pelletier, et al. [Page 20] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 3 is reserved (MUST NOT be used for parseability) <# Editor's Note: The exact format is to be defined #> Feedback options: A variable number of feedback options, see section 5.5.4.2. Options may appear in any order. 5.5.4.2. ROHC-TCP Feedback options ROHC-TCP uses the same feedback options as the options defined in [RFC-3095, section 5.7.6], with the following exceptions: 1) The MSN replaces RTP SN in the feedback information. 2) The CLOCK option [RFC-3095, section 5.7.6.6] is not used. 3) The JITTER option [RFC-3095, section 5.7.6.7] is not used. 6. Security considerations Because encryption eliminates the redundancy that header compression schemes try to exploit, there is some inducement to forego encryption of headers in order to enable operation over low-bandwidth links. However, for those cases where encryption of data (and not headers) is sufficient, TCP does specify an alternative encryption method in which only the TCP payload is encrypted and the headers are left in the clear. That would still allow header compression to be applied. A malfunctioning or malicious header compressor could cause the header decompressor to reconstitute packets that do not match the original packets but still have valid IP, and TCP headers and possibly also valid TCP checksums. Such corruption may be detected with end-to-end authentication and integrity mechanisms which will not be affected by the compression. Moreover, this header compression scheme uses an internal checksum for verification of reconstructed headers. This reduces the probability of producing decompressed headers not matching the original ones without this being noticed. Denial-of-service attacks are possible if an intruder can introduce (for example) bogus IR, CO or FEEDBACK packets onto the link and thereby cause compression efficiency to be reduced. However, an intruder having the ability to inject arbitrary packets at the link layer in this manner raises additional security issues that dwarf those related to the use of header compression. 7. IANA Considerations ROHC profile identifier 0x00XX <# Editor's Note: To be replaced before publication #> has been reserved by the IANA for the profile defined in this document. Pelletier, et al. [Page 21] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 <# Editor's Note: To be removed before publication #> A ROHC profile identifier must be reserved by the IANA for the profile defined in this document. Profiles 0x0000-0x0005 have previously been reserved, which means this profile could be 0x0006. As for previous ROHC profiles, profile numbers 0xnnXX must also be reserved for future updates of this profile. A suggested registration in the "RObust Header Compression (ROHC) Profile Identifiers" name space would then be: Profile Usage Document identifier 0x0006 ROHC TCP [RFCXXXX (this)] 0xnn06 Reserved 8. Acknowledgements Header compression schemes from [RFC-1144, RFC-2507, RFC-3095] have been important sources of ideas and knowledge. The authors would like to thank [TBW] for valuable input. 9. References 9.1. Normative References [RFC-3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T., Yoshimura, T. and H. Zheng, "RObust Header Compression (ROHC): Framework and four profiles: RTP, UDP, ESP, and uncompressed", RFC 3095, July 2001. [ROHC-CR] Pelletier, G., "Robust Header Compression (ROHC): Context replication for ROHC profiles", Internet Draft (work in progress), , May 2003. [RFC-791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC-793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC-2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. Pelletier, et al. [Page 22] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 9.2. Informative References [TCP-REQ] Jonsson, L-E., "Requirements on ROHC IP/TCP header compression", Internet Draft (work in progress),, October 2002. [TCP-BEH] West, M. and S. McCann, "TCP/IP Field Behavior", Internet Draft (work in progress), , March 2003. [IP-ONLY] Jonsson, L., "RObust Header Compression (ROHC): A compression profile for IP", Internet draft (work in progress), January 2003, [RFC-768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. [RFC-1072] Jacobson, V., and R. Braden, "TCP Extensions for Long- Delay Paths", LBL, ISI, October 1988. [RFC-1144] Jacobson, V.,"Compressing TCP/IP Headers for Low-Speed Serial Links", RFC 1144, February 1990. [RFC-1323] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992. [RFC-1644] Braden, R. "T/TCP -- TCP Extensions for Transactions Functional Specification", ISI, July 1994. [RFC-1693] Connolly, T., et al, "An Extension to TCP : Partial Order Service", University of Delaware, November 1994. [RFC-1889] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996. [RFC-2001] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms", NOAO, January 1997. [RFC-2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP Selective Acknowledgment Options", RFC 2018, October 1996. [RFC-2026] Bradner, S., "The Internet Standards Process û Revision 3", BCP 9, RFC 2026, October 1996. [RFC-2119] Bradner, S.,"Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC-2507] Degermark, M., Nordgren, B. and S. Pink, "IP Header Pelletier, et al. [Page 23] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 Compression", RFC 2507, February 1999. [RFC-2883] Floyd, S., Mahdavi, J., Mathis, M. and M. Podolsky, "An Extension to the Selective Acknowledgement (SACK) Option for TCP", RFC 2883, July 2000. [E2E] Jacobson, V., "Fast Retransmit", Message to the end2end- interest mailing list, April 1990. [Mobi96] Degermark, M., Engan, M., Nordgren, B. and S. Pink, "Low- loss TCP/IP header compression for wireless networks", In the Proceedings of MobiCom, 1996. 10. Authors' addresses Ghyslain Pelletier Ericsson AB Box 920 SE-971 28 Lulea, Sweden Phone: +46 920 20 24 32 Fax: +46 920 20 20 99 Email: ghyslain.pelletier@epl.ericsson.se Qian Zhang Microsoft Research Asia Beijing Sigma Center No.49, Zhichun Road, Haidian District Beijing 100080, P.R.C. Phone: +86 10 62617711-3135Email: qianz@microsoft.com Lars-Erik Jonsson Ericsson AB Box 920 SE-971 28 Lulea, Sweden Phone: +46 920 20 21 07 Fax: +46 920 20 20 99 Email: lars-erik.jonsson@ericsson.com HongBin Liao Microsoft Research Asia Beijing Sigma Center No.49, Zhichun Road, Haidian District Beijing 100080, P.R.C. Pelletier, et al. [Page 24] INTERNET-DRAFT ROHC Profile for TCP May 23, 2003 Phone: +86 10 62617711-3156 Email: i-hbliao@microsoft.com Mark A West Roke Manor Research Ltd Romsey, Hants, SO51 0ZN United Kingdom Phone: +44 1794 833311 Email: mark.a.west@roke.co.uk Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. This Internet-Draft expires November 23, 2003. Pelletier, et al. [Page 25]