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rfc:rfc4995

Network Working Group L-E. Jonsson Request for Comments: 4995 G. Pelletier Category: Standards Track K. Sandlund

                                                              Ericsson
                                                             July 2007
           The RObust Header Compression (ROHC) Framework

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 IETF Trust (2007).

Abstract

 The Robust Header Compression (ROHC) protocol provides an efficient,
 flexible, and future-proof header compression concept.  It is
 designed to operate efficiently and robustly over various link
 technologies with different characteristics.
 The ROHC framework, along with a set of compression profiles, was
 initially defined in RFC 3095.  To improve and simplify the ROHC
 specifications, this document explicitly defines the ROHC framework
 and the profile for uncompressed separately.  More specifically, the
 definition of the framework does not modify or update the definition
 of the framework specified by RFC 3095.

Table of Contents

 1. Introduction ....................................................3
 2. Terminology .....................................................4
    2.1. Acronyms ...................................................4
    2.2. ROHC Terminology ...........................................4
 3. Background (Informative) ........................................7
    3.1. Header Compression Fundamentals ............................7
    3.2. A Short History of Header Compression ......................7
 4. Overview of Robust Header Compression (ROHC) (Informative) ......8
    4.1. General Principles .........................................8
    4.2. Compression Efficiency, Robustness, and Transparency ......10
    4.3. Developing the ROHC Protocol ..............................10

Jonsson, et al. Standards Track [Page 1] RFC 4995 The ROHC Framework July 2007

    4.4. Operational Characteristics of the ROHC Channel ...........11
    4.5. Compression and Master Sequence Number (MSN) ..............13
    4.6. Static and Dynamic Parts of a Context .....................13
 5. The ROHC Framework (Normative) .................................14
    5.1. The ROHC Channel ..........................................14
         5.1.1. Contexts and Context Identifiers ...................14
         5.1.2. Per-Channel Parameters .............................15
         5.1.3. Persistence of Decompressor Contexts ...............16
    5.2. ROHC Packets and Packet Types .............................16
         5.2.1. General Format of ROHC Packets .....................17
                5.2.1.1. Format of the Padding Octet ...............17
                5.2.1.2. Format of the Add-CID Octet ...............18
                5.2.1.3. General Format of Header ..................18
         5.2.2. Initialization and Refresh (IR) Packet Types .......19
                5.2.2.1. ROHC IR Packet Type .......................20
                5.2.2.2. ROHC IR-DYN Packet Type ...................20
         5.2.3. ROHC Initial Decompressor Processing ...............21
         5.2.4. ROHC Feedback ......................................22
                5.2.4.1. ROHC Feedback Format ......................23
         5.2.5. ROHC Segmentation ..................................25
                5.2.5.1. Segmentation Usage Considerations .........25
                5.2.5.2. Segmentation Protocol .....................26
    5.3. General Encoding Methods ..................................27
         5.3.1. Header Compression CRCs, Coverage and Polynomials ..27
                5.3.1.1. 8-bit CRCs in IR and IR-DYN Headers .......27
                5.3.1.2. 3-bit CRC in Compressed Headers ...........27
                5.3.1.3. 7-bit CRC in Compressed Headers ...........28
                5.3.1.4. 32-bit Segmentation CRC ...................28
         5.3.2. Self-Describing Variable-Length Values .............29
    5.4. ROHC UNCOMPRESSED -- No Compression  (Profile 0x0000) .....29
         5.4.1. IR Packet ..........................................30
         5.4.2. Normal Packet ......................................31
         5.4.3. Decompressor Operation .............................31
         5.4.4. Feedback ...........................................32
 6. Overview of a ROHC Profile (Informative) .......................32
 7. Security Considerations ........................................33
 8. IANA Considerations ............................................34
 9. Acknowledgments ................................................35
 10. References ....................................................35
    10.1. Normative References .....................................35
    10.2. Informative References ...................................35
 Appendix A.  CRC Algorithm ........................................37

Jonsson, et al. Standards Track [Page 2] RFC 4995 The ROHC Framework July 2007

1. Introduction

 For many types of networks, reducing the deployment and operational
 costs by improving the usage of the bandwidth resources is of vital
 importance.  Header compression over a link is possible because some
 of the information carried within the header of a packet becomes
 compressible between packets belonging to the same flow.
 For links where the overhead of the IP header(s) is problematic, the
 total size of the header may be significant.  Applications carrying
 data carried within RTP [13] will then, in addition to link-layer
 framing, have an IPv4 [10] header (20 octets), a UDP [12] header (8
 octets), and an RTP header (12 octets), for a total of 40 octets.
 With IPv6 [11], the IPv6 header is 40 octets for a total of 60
 octets.  Applications transferring data using TCP [14] will have 20
 octets for the transport header, for a total size of 40 octets for
 IPv4 and 60 octets for IPv6.
 The relative gain for specific flows (or applications) depends on the
 size of the payload used in each packet.  For applications such as
 Voice-over-IP, where the size of the payload containing coded speech
 can be as small as 15-20 octets, this gain will be quite significant.
 Similarly, relative gains for TCP flows carrying large payloads (such
 as file transfers) will be less than for flows carrying smaller
 payloads (such as application signaling, e.g., session initiation).
 As more and more wireless link technologies are being deployed to
 carry IP traffic, care must be taken to address the specific
 characteristics of these technologies within the header compression
 algorithms.  Legacy header compression schemes, such as those defined
 in [16] and [17], have been shown to perform inadequately over links
 where both the lossy behavior and the round-trip times are non-
 negligible, such as those observed for example in wireless links and
 IP tunnels.
 In addition, a header compression scheme should handle the often
 non-trivial residual errors, i.e., where the lower layer may pass a
 packet that contains undetected bit errors to the decompressor.  It
 should also handle loss and reordering before the compression point,
 as well as on the link between the compression and decompression
 points [7].
 The Robust Header Compression (ROHC) protocol provides an efficient,
 flexible, and future-proof header compression concept.  It is
 designed to operate efficiently and robustly over various link
 technologies with different characteristics.

Jonsson, et al. Standards Track [Page 3] RFC 4995 The ROHC Framework July 2007

 RFC 3095 [3] defines the ROHC framework along with an initial set of
 compression profiles.  To improve and simplify the specification, the
 framework and the profiles' parts have been split into separate
 documents.  This document explicitly defines the ROHC framework, but
 it does not modify or update the definition of the framework
 specified by RFC 3095; both documents can be used independently of
 each other.  This also implies that implementations based on either
 definition will be compatible and interoperable with each other.
 However, it is the intent to let this specification replace RFC 3095
 as the base specification for all profiles defined in the future.

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 [1].

2.1. Acronyms

 This section lists most acronyms used for reference.
    ACK    Acknowledgment.
    CID    Context Identifier.
    CO     Compressed Packet Format.
    CRC    Cyclic Redundancy Check.
    IR     Initialization and Refresh.
    IR-DYN Initialization and Refresh, Dynamic part.
    LSB    Least Significant Bit(s).
    MRRU   Maximum Reconstructed Reception Unit.
    MSB    Most Significant Bit(s).
    MSN    Master Sequence Number.
    NACK   Negative Acknowledgment.
    ROHC   RObust Header Compression.

2.2. ROHC Terminology

 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 flow, such as static fields and possible
    reference values for compression and decompression.  Moreover,
    additional information describing the packet flow is also part of

Jonsson, et al. Standards Track [Page 4] RFC 4995 The ROHC Framework July 2007

    the context, for example, information about the change behavior of
    fields (e.g., the IP Identifier behavior, or the typical inter-
    packet increase in sequence numbers and 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 the 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 used to resynchronize the contexts,
    an important task since context damage causes loss propagation.
    Examples of such mechanisms are NACK-based mechanisms, and the
    periodic refreshes of important context information, usually done
    in unidirectional operation.  There are also mechanisms that can
    reduce the context inconsistency probability, for example,
    repetition of the same type of information in multiple packets and
    CRCs that protect context-updating information.
 CRC-8 validation
    The CRC-8 validation refers to the validation of the integrity
    against bit error(s) in a received IR and IR-DYN header using the
    8-bit CRC included in the IR/IR-DYN header.
 CRC verification
    The CRC verification refers to the verification of the result of a
    decompression attempt using the 3-bit CRC or 7-bit CRC included in
    the header of a compressed packet format.
 Damage propagation
    Delivery of incorrect decompressed headers due to context damage,
    that is, due to errors in (i.e., loss of or damage to) previous
    header(s) or feedback.

Jonsson, et al. Standards Track [Page 5] RFC 4995 The ROHC Framework July 2007

 Error detection
    Detection of errors by lower layers.  If error detection is not
    perfect, there will be residual errors.
 Error propagation
    Damage propagation or loss propagation.
 ROHC profile
    A ROHC profile is a compression protocol, which specifies how to
    compress specific header combinations.  A ROHC profile may be
    tailored to handle a specific set of link characteristics, e.g.,
    loss characteristics, reordering between compression points, etc.
    ROHC profiles provide the details of the header compression
    framework defined in this document, and each compression profile
    is associated with a unique ROHC profile identifier [21].  When
    setting up a ROHC channel, the set of profiles supported by both
    endpoints of the channel is negotiated, and when initializing new
    contexts, a profile identifier from this negotiated set is used to
    associate each compression context with one specific profile.
 Link
    A physical transmission path that constitutes a single IP hop.
 Loss propagation
    Loss of headers, due to errors in (i.e., loss of or damage to)
    previous header(s) or feedback.
 Packet flow
    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.
 Residual error
    Errors introduced during transmission and not detected by lower-
    layer error detection schemes.
 ROHC channel
    A logical unidirectional point-to-point channel carrying ROHC
    packets from one compressor to one decompressor, optionally
    carrying ROHC feedback information on the behalf of another

Jonsson, et al. Standards Track [Page 6] RFC 4995 The ROHC Framework July 2007

    compressor-decompressor pair operating on a separate ROHC channel
    in the opposite direction.  See also [5].
 This document also makes use of the conceptual terminology defined by
 "ROHC Terminology and Channel Mapping Examples", RFC 3759 [5].

3. Background (Informative)

 This section provides a background to the subject of header
 compression.  The fundamental ideas are described together with a
 discussion about the history of header compression schemes.  The
 motivations driving the development of the various schemes are
 discussed and their drawbacks identified, thereby providing the
 foundations for the design of the ROHC framework and profiles [3].

3.1. Header Compression Fundamentals

 Header compression is possible because there is significant
 redundancy between header fields; within the headers of a single
 packet, but in particular between consecutive packets belonging to
 the same flow.  On the path end-to-end, the entire header information
 is necessary for all packets in the flow, but over a single link,
 some of this information becomes redundant and can be reduced, as
 long as it is transparently recovered at the receiving end of the
 link.  The header size can be reduced by first sending field
 information that is expected to remain static for (at least most of)
 the lifetime of the packet flow.  Further compression is achieved for
 the fields carrying information that changes more dynamically by
 using compression methods tailored to their respective assumed change
 behavior.
 To achieve compression and decompression, some necessary information
 from past packets is maintained in a context.  The compressor and the
 decompressor update their respective contexts upon certain, not
 necessarily synchronized, events.  Impairment events may lead to
 inconsistencies in the decompressor context (i.e., context damage),
 which in turn may cause incorrect decompression.  A Robust Header
 Compression scheme needs mechanisms to minimize the possibility of
 context damage, in combination with mechanisms for context repair.

3.2. A Short History of Header Compression

 The first header compression scheme, compressed TCP (CTCP) [15], was
 introduced by Van Jacobson.  CTCP, also often referred to as VJ
 compression, compresses the 40 octets of the TCP/IP header down to 4
 octets.  CTCP uses delta encoding for sequentially changing fields.
 The CTCP compressor detects transport-level retransmissions and sends
 a header that updates the entire context when they occur.  This

Jonsson, et al. Standards Track [Page 7] RFC 4995 The ROHC Framework July 2007

 repair mechanism does not require any explicit signaling between the
 compressor and decompressor.
 A general IP header compression scheme, IP header compression [16],
 improves somewhat on CTCP.  IP Header Compression (IPHC) can compress
 arbitrary IP, TCP, and UDP headers.  When compressing non-TCP
 headers, IPHC does not use delta encoding and is robust.  The repair
 mechanism of CTCP is augmented with negative acknowledgments, called
 CONTEXT_STATE messages, which speeds up the repair.  This context
 repair mechanism is thus limited by the round-trip time of the link.
 IPHC does not compress RTP headers.
 CRTP [17] is an RTP extension to IPHC.  CRTP compresses the 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.
 On lossy links with long round-trip times, CRTP does not perform well
 [20].  Each packet lost over the link causes decompression of several
 subsequent packets to fail, because the context becomes invalidated
 during at least one link round-trip time from the lost packet.
 Unfortunately, the large headers that CRTP sends when updating the
 context waste additional bandwidth.
 CRTP uses a local repair mechanism known as TWICE, which was
 introduced by IPHC.  TWICE derives its name from the observation that
 when the flow of compressed packets is regular, the correct guess
 when one packet is lost between the compression points is to apply
 the update in the current packet twice.  While TWICE improves CRTP
 performance significantly, [20] also found that even with TWICE, CRTP
 doubled the number of lost packets.
 An enhanced variant of CRTP, called eCRTP [19], means to improve the
 robustness of CRTP in the presence of reordering and packet losses,
 while keeping the protocol almost unchanged from CRTP.  As a result,
 eCRTP does provide better means to implement some degree of
 robustness, albeit at the expense of additional overhead, leading to
 a reduction in compression efficiency in comparison to CRTP.

4. Overview of Robust Header Compression (ROHC) (Informative)

4.1. General Principles

 As mentioned earlier, header compression is possible per-link due to
 the fact that there is much redundancy between header field values
 within packets, and especially between consecutive packets belonging
 to the same flow.  To utilize these properties for header
 compression, there are a few essential steps to consider.

Jonsson, et al. Standards Track [Page 8] RFC 4995 The ROHC Framework July 2007

 The first step consists of identifying and grouping packets together
 into different "flows", so that packet-to-packet redundancy is
 maximized in order to improve the compression ratio.  Grouping
 packets into flows is usually based on source and destination host
 (IP) addresses, transport protocol type (e.g., UDP or TCP), process
 (port) numbers, and potentially additional unique application
 identifiers, such as the synchronization source (SSRC) in RTP [13].
 The compressor and decompressor each establish a context for the
 packet flow and identify the context with a Context Identifier (CID)
 included in each compressed header.
 The second step is to understand the change patterns of the various
 header fields.  On a high level, header fields fall into one of the
 following classes:
 INFERRED      These fields contain values that can be inferred from
               other fields or external sources, for example, the size
               of the frame carrying the packet can often be derived
               from the link layer protocol, and thus does not have to
               be transmitted by the compression scheme.
 STATIC        Fields classified as STATIC are assumed to be constant
               throughout the lifetime of the packet flow.  The value
               of each field is thus only communicated initially.
 STATIC-DEF    Fields classified as STATIC-DEF are used to define a
               packet flow as discussed above.  Packets for which
               respective values of these fields differ are treated as
               belonging to different flows.  These fields are in
               general compressed as STATIC fields.
 STATIC-KNOWN  Fields classified as STATIC-KNOWN are expected to have
               well-known values, and therefore their values do not
               need to be communicated.
 CHANGING      These fields are expected to vary randomly, either
               within a limited value set or range, or in some other
               manner.  CHANGING fields are usually handled in more
               sophisticated ways based on a more detailed
               classification of their expected change patterns.
 Finally, the last step is to choose the encoding method(s) that will
 be applied onto different fields based on classification.  The
 encoding methods, in combination with the identified field behavior,
 provide the input to the design of the compressed header formats.
 The analysis of the probability distribution of the identified change
 patterns then provides the means to optimize the packet formats,

Jonsson, et al. Standards Track [Page 9] RFC 4995 The ROHC Framework July 2007

 where the most frequently occurring change patterns for a field
 should be encoded within the most efficient format(s).
 However, compression efficiency has to be traded against two other
 properties: the robustness of the encoding to losses and errors
 between the compressor and the decompressor, and the ability to
 detect and cope with errors in the decompression process.

4.2. Compression Efficiency, Robustness, and Transparency

 The performance of a header compression protocol can be described
 with three parameters: its compression efficiency, its robustness,
 and its compression transparency.
 Compression efficiency
    The compression efficiency is determined by how much the average
    header size is reduced by applying the compression protocol.
 Robustness
    A robust protocol tolerates packet losses, residual bit errors,
    and out-of-order delivery on the link over which header
    compression takes place, without losing additional packets or
    introducing additional errors in decompressed headers.
 Compression transparency
    The compression transparency is a measure of the extent to which
    the scheme maintains the semantics of the original headers.  If
    all decompressed headers are bitwise identical to the
    corresponding original headers, the scheme is transparent.

4.3. Developing the ROHC Protocol

 The challenge in developing a header compression protocol is to
 conciliate compression efficiency and robustness while maintaining
 transparency, as increasing robustness will always come at the
 expense of a lower compression efficiency, and vice-versa.  The
 scheme should also be flexible enough in its design to minimize the
 impacts from the varying round-trip times and loss patterns of links
 where header compression will be used.
 To achieve this, the header compression scheme must provide
 facilities for the decompressor to verify decompression and detect
 potential context damage, as well as context recovery mechanisms such
 as feedback.  Header compression schemes prior to the ones developed

Jonsson, et al. Standards Track [Page 10] RFC 4995 The ROHC Framework July 2007

 by the Robust Header Compression (ROHC) WG were not designed with the
 above high-level objectives in mind.
 The ROHC WG has developed header compression solutions to meet the
 needs of present and future link technologies.  While special
 attention has been put towards meeting the more stringent
 requirements stemming from the characteristics of wireless links, the
 results are equally applicable to many other link technologies.
 RFC 3095 [3], "RObust Header Compression (ROHC): Framework and four
 profiles: RTP, UDP, ESP, and uncompressed", was published in 2001, as
 the first output of the ROHC WG.  ROHC is a general and extendable
 framework for header compression, on top of which profiles can be
 defined for compression of different protocols headers.  RFC 3095
 introduced a number of new compression techniques, and was successful
 at living up to the requirements placed on it, as described in [18].
 Interoperability testing of RFC 3095 confirms the capabilities of
 ROHC to meet its purposes, but feedback from implementers has also
 indicated that the protocol specification is complex and sometimes
 obscure.  Most importantly, a clear distinction between framework and
 profiles is not obvious in [3], which also makes development of
 additional profiles troublesome.  This document therefore aims at
 explicitly specifying the ROHC framework, while a companion document
 [8] specifies revised versions of the compression profiles of RFC
 3095.

4.4. Operational Characteristics of the ROHC Channel

 Robust header compression can be used over many type of link
 technologies.  The ROHC framework provides flexibility for profiles
 to address a wide range of applications, and this section lists some
 of the operational characteristics of the ROHC channel (see also
 [5]).
 Multiplexing over a single logical channel
    The ROHC channel provides a mechanism to identify a context within
    the general ROHC packet format.  The CID makes it possible for a
    logical channel that supports ROHC to transport multiple header-
    compressed flows, while still making it possible for a channel to
    be dedicated to one single packet flow without any CID overhead.
    More specifically, ROHC uses a distinct context identifier space
    per logical channel, and the context identifier can be omitted for
    one of the flows over the ROHC channel when configured to use a
    small CID space.

Jonsson, et al. Standards Track [Page 11] RFC 4995 The ROHC Framework July 2007

 Establishment of channel parameters
    A link layer defining support for the ROHC channel must provide
    the means to establish header compression channel parameters (see
    Section 5.1).  This can be achieved through a negotiation
    mechanism, static provisioning, or some out-of-band signaling.
 Packet type identification
    The ROHC channel defines a packet type identifier space, and puts
    restrictions with respect to the use of a number of identifiers
    that are common for all ROHC profiles.  Identifiers that have no
    restrictions, i.e., identifiers that are not defined by this
    document, are available to each profile.  The identifier is part
    of each compressed header, and this makes it possible for the link
    that supports the ROHC channel to allocate one single link layer
    payload type for ROHC.
 Out-of-order delivery between compression endpoints
    Each profile defines its own level of robustness, including
    tolerance to reordering of packets before but especially between
    compression endpoints, if any.
    For profiles specified in [3], the channel between the compressor
    and decompressor is required to maintain in-order delivery of the
    packets, i.e., the definition of these profiles assumes that the
    decompressor always receives packets in the same order as the
    compressor sent them.  The impacts of reordering on the
    performance of these profiles is described in [7].  However,
    reordering before the compression point is handled, i.e., these
    profiles make no assumption that the compressor will receive
    packets in-order.
    For the ROHCv2 profiles specified in [8], their definitions assume
    that the decompressor can receive packets out-of-order, i.e., not
    in the same order that the compressor sent them.  Reordering
    before the compression point is also dealt with.
 Duplication of packets
    The link supporting the ROHC channel is required to not duplicate
    packets (however, duplication of packets can occur before they
    reach the compressor, i.e., there is no assumption that the
    compressor will receive only one copy of each packet).

Jonsson, et al. Standards Track [Page 12] RFC 4995 The ROHC Framework July 2007

 Framing
    The link layer must provide framing that makes it possible to
    distinguish frame boundaries and individual frames.
 Error detection/protection
    ROHC profiles should be designed to cope with residual errors in
    the headers delivered to the decompressor.  CRCs are used to
    detect decompression failures and to prevent or reduce damage
    propagation.  However, it is recommended that lower layers deploy
    error detection for ROHC headers and that ROHC headers with high
    residual error rates not be delivered.

4.5. Compression and Master Sequence Number (MSN)

 Compression of header fields is based on the establishment of a
 function to a sequence number, called the master sequence number
 (MSN).  This function describes the change pattern of the field with
 respect to a change in the MSN.
 Change patterns include, for example, fields that increase
 monotonically or by a small value, fields that seldom change,and
 fields that remain unchanging for the entire lifetime of the packet
 flow, in which case the function to the MSN is equivalent to a
 constant value.
 The compressor first establishes functions for each of the header
 fields, and then reliably communicates the MSN.  When the change
 pattern of the field does not match the established function, i.e.,
 the existing function gives a result that is different from the field
 in the header being compressed, additional information can be sent to
 update the parameters of that function.
 The MSN is defined per profile.  It can be either derived directly
 from one of the fields of the protocol being compressed (e.g., the
 RTP SN [8]), or it can be created and maintained by the compressor
 (e.g., the MSN for compression of UDP in profile 0x0102 [8] or the
 MSN in ROHC-TCP [9]).

4.6. Static and Dynamic Parts of a Context

 A compression context can be conceptually divided into two different
 parts, the static context and the dynamic context, each based on the
 properties of the fields that are being compressed.

Jonsson, et al. Standards Track [Page 13] RFC 4995 The ROHC Framework July 2007

 The static part includes the information necessary to compress and
 decompress the fields whose change behavior is classified as STATIC,
 STATIC-KNOWN, or STATIC-DEF (as described in Section 4.1 above).
 The dynamic part includes the state maintained for all the other
 fields, i.e., those that are classified as CHANGING.

5. The ROHC Framework (Normative)

 This section normatively defines the parts common to all ROHC
 profiles, i.e., the framework.  The framework specifies the
 requirements and functionality of the ROHC channel, including how to
 handle multiple compressed packet flows over the same channel.
 Finally, this section specifies encoding methods used in the packet
 formats that are common to all profiles.  These encoding methods may
 be reused within profile specifications for encoding fields in
 profile-specific parts of a packet format, without requiring their
 redefinition.

5.1. The ROHC Channel

5.1.1. Contexts and Context Identifiers

 Associated with each compressed flow is a context.  The context is
 the state that the compressor and the decompressor maintain in order
 to correctly compress or decompress the headers of the packet in the
 flow.  Each context is identified using a CID.
 A context is considered to be a new context when the CID is
 associated with a profile for the first time since the creation of
 the ROHC channel, or when the CID gets associated from the reception
 of an IR (this does not apply to the IR-DYN) with a different profile
 than the profile in the context.
 Context information is conceptually kept in a table.  The context
 table is indexed using the CID, which is sent along with compressed
 headers and feedback information.
 The CID space can be either small, which means that CIDs can take the
 values 0 through 15, or large, which means that CIDs take values
 between 0 and 2^14 - 1 = 16383.  Whether the CID space is large or
 small MUST be established, possibly by negotiation, before any
 compressed packet may be sent over the ROHC channel.
 The CID space is distinct for each channel, i.e., CID 3 over channel
 A and CID 3 over channel B do not refer to the same context, even if
 the endpoints of A and B are the same nodes.  In particular, CIDs for

Jonsson, et al. Standards Track [Page 14] RFC 4995 The ROHC Framework July 2007

 any pair of ROHC channels are not related (two associated ROHC
 channels serving as feedback channels for one another do not even
 need to have CID spaces of the same size).

5.1.2. Per-Channel Parameters

 The ROHC channel is based on a number of parameters that form part of
 the established channel state and the per-context state.  The state
 of the ROHC channel MUST be established before the first ROHC packet
 may be sent, which may be achieved using negotiation protocols
 provided by the link layer (see also [4], which describes an option
 for negotiation of ROHC parameters for PPP).  This section describes
 some of this channel state information in an abstract way:
 LARGE_CIDS: Boolean; if false, the small CID representation (0 octets
    or 1 prefix octet, covering CID 0 to 15) is used; if true, the
    large CID representation (1 or 2 embedded CID octets covering CID
    0 to 16383) is used.  See also 5.1.1 and 5.2.1.3.
 MAX_CID: Non-negative integer; highest CID number to be used by the
    compressor (note that this parameter is not coupled to, but in
    effect further constrained by, LARGE_CIDS).  This value represents
    an agreement by the decompressor that it can provide sufficient
    memory resources to host at least MAX_CID+1 contexts; the
    decompressor MUST maintain established contexts within this space
    until either the CID gets re-used by the establishment of a new
    context, or until the channel is taken down.
 PROFILES: Set of non-negative integers, where each integer indicates
    a profile supported by both the compressor and the decompressor.
    A profile is identified by a 16-bit value, where the 8 LSB bits
    indicate the actual profile, and the 8 MSB bits indicate the
    variant of that profile.  The ROHC compressed header format
    identifies the profile used with only the 8 LSB bits; this means
    that if multiple variants of the same profile are available for a
    ROHC channel, the PROFILES set after negotiation MUST NOT include
    more than one variant of the same profile.  The compressor MUST
    NOT compress using a profile that is not in PROFILES.
 FEEDBACK_FOR: Optional reference to a ROHC channel in the opposite
    direction between the same compression endpoints.  If provided,
    this parameter indicates to which other ROHC channel any feedback
    sent on this ROHC channel refers (see [5]).
 MRRU: Non-negative integer.  Maximum Reconstructed Reception Unit.
    This is the size of the largest reconstructed unit in octets that
    the decompressor is expected to reassemble from segments (see
    Section 5.2.5).  This size includes the segmentation CRC.  If MRRU

Jonsson, et al. Standards Track [Page 15] RFC 4995 The ROHC Framework July 2007

    is negotiated to be 0, segmentation MUST NOT be used on the
    channel, and received segments MUST be discarded by the
    decompressor.

5.1.3. Persistence of Decompressor Contexts

 As part of the negotiated channel parameters, the compressor and
 decompressor have through the MAX_CID parameter agreed on the highest
 context identification (CID) number to be used.  By agreeing on the
 MAX_CID, the decompressor also agrees to provide memory resources to
 host at least MAX_CID+1 contexts, and an established context with a
 CID within this negotiated space SHOULD be kept by the decompressor
 until either the CID gets re-used, or the channel is taken down or
 re-negotiated.

5.2. ROHC Packets and Packet Types

 This section uses the following convention in the diagrams when
 representing various ROHC packet types, formats, and fields:
  1. colons ":" indicate that the part is optional
  2. slashes "/" indicate variable length
 The ROHC packet type indication scheme has been designed to provide
 optional padding, a feedback packet type, an optional Add-CID octet
 (which includes 4 bits of CID), and a simple segmentation and
 reassembly mechanism.
 The following packet types are reserved at the ROHC framework level:
    11100000 : Padding
    1110nnnn : Add-CID octet (nnnn=CID with values 0x1 through 0xF)
    11110    : Feedback
    11111000 : IR-DYN packet
    1111110  : IR packet
    1111111  : Segment
 Other packet types can be defined and used by individual profiles:
    0        : available (not reserved by ROHC framework)
    10       : available (not reserved by ROHC framework)
    110      : available (not reserved by ROHC framework)
    1111101  : available (not reserved by ROHC framework)
    11111001 : available (not reserved by ROHC framework)

Jonsson, et al. Standards Track [Page 16] RFC 4995 The ROHC Framework July 2007

5.2.1. General Format of ROHC Packets

 A ROHC packet has the following general format:
  1. – — — — — — — —

: Padding :

  1. – — — — — — — —

: Feedback :

  1. – — — — — — — —

: Header :

  1. – — — — — — — —

: Payload :

  1. – — — — — — — —
 Padding: Any number (zero or more) of padding octets, where the
    format of a padding octet is as defined in Section 5.2.1.1.
 Feedback: Any number (zero or more) of feedback elements, where the
    format of a feedback element is as defined in Section 5.2.4.1.
 Header: Either a profile-specific CO header (see Section 5.2.1.3), an
    IR or IR-DYN header (see Section 5.2.2), or a ROHC Segment (see
    Section 5.2.5).  There can be at most one Header in a ROHC packet,
    but it may also be omitted (if the packet contains Feedback only).
 Payload: Corresponds to zero or more octets of payload from the
    uncompressed packet, starting with the first octet in the
    uncompressed packet after the last header compressible by the
    current profile.
 At least one of Feedback or Header MUST be present.

5.2.1.1. Format of the Padding Octet

 Padding octet:
   0   1   2   3   4   5   6   7
 +---+---+---+---+---+---+---+---+
 | 1   1   1   0   0   0   0   0 |
 +---+---+---+---+---+---+---+---+
 Note: The Padding octet MUST NOT be interpreted as an Add-CID octet
 for CID 0.

Jonsson, et al. Standards Track [Page 17] RFC 4995 The ROHC Framework July 2007

5.2.1.2. Format of the Add-CID Octet

 Add-CID octet:
   0   1   2   3   4   5   6   7
 +---+---+---+---+---+---+---+---+
 | 1   1   1   0 |      CID      |
 +---+---+---+---+---+---+---+---+
 CID: 0x1 through 0xF indicates CIDs 1 through 15.
 Note: The Padding octet looks like an Add-CID octet for CID 0.

5.2.1.3. General Format of Header

 All ROHC packet types have the following general Header format:
   0              x-1  x       7
  --- --- --- --- --- --- --- ---
 :         Add-CID octet         :  if CID 1-15 and small CIDs
 +--- --- --- --- ---+--- --- ---+
 | type indication   |   body    |  1 octet (8-x bits of body)
 +--- --- --- --- ---+--- --- ---+
 :                               :
 /    0, 1, or 2 octets of CID   /  1 or 2 octets if large CIDs
 :                               :
 +---+---+---+---+---+---+---+---+
 /             body              /  variable length
 +---+---+---+---+---+---+---+---+
 type indication: ROHC packet type.
 body: Interpreted according to the packet type indication and CID
    information, as defined by individual profiles.
 Thus, the header either starts with a packet type indication or has a
 packet type indication immediately following an Add-CID octet.
 When the ROHC channel is configured with a small CID space:
    o  If an Add-CID immediately precedes the packet type indication,
       the packet has the CID of the Add-CID; otherwise, it has CID 0.
    o  A small CID with the value 0 is represented using zero bits;
       therefore, a flow associated with CID 0 has no CID overhead in
       the compressed header.  In such case, Header starts with a
       packet type indication.

Jonsson, et al. Standards Track [Page 18] RFC 4995 The ROHC Framework July 2007

    o  A small CID with a value from 1 to 15 is represented using the
       Add-CID octet as described above.  The Header starts with the
       Add-CID octet, followed by a packet type indication.
    o  There is no large CID in the Header.
 When the ROHC channel is configured with a large CID space:
    o  The large CID is always present and is represented using the
       encoding scheme of Section 5.3.2, limited to two octets.  In
       this case, the Header starts with a packet type indication.

5.2.2. Initialization and Refresh (IR) Packet Types

 IR packet types contain a profile identifier, which determines how
 the rest of the header is to be interpreted.  They also associate a
 profile with a context.  The stored profile parameter further
 determines the syntax and semantics of the packet type identifiers
 and packet types used with a specific context.
 The IR and IR-DYN packets always update the context for all context-
 updating fields carried in the header.  They never clear the context,
 except when initializing a new context (see Section 5.1.1), or unless
 the profile indicated in the Profile field specifies otherwise.

Jonsson, et al. Standards Track [Page 19] RFC 4995 The ROHC Framework July 2007

5.2.2.1. ROHC IR Packet Type

 The IR header associates a CID with a profile, and typically also
 initializes the context.  It can typically also refresh all (or parts
 of) the context.  For IR, Header has the following general format:
   0   1   2   3   4   5   6   7
  --- --- --- --- --- --- --- ---
 :         Add-CID octet         :  if CID 1-15 and small CID
 +---+---+---+---+---+---+---+---+
 | 1   1   1   1   1   1   0 | x |  IR type octet
 +---+---+---+---+---+---+---+---+
 :                               :
 /      0-2 octets of CID        /  1 or 2 octets if large CIDs
 :                               :
 +---+---+---+---+---+---+---+---+
 |            Profile            |  1 octet
 +---+---+---+---+---+---+---+---+
 |              CRC              |  1 octet
 +---+---+---+---+---+---+---+---+
 |                               |
 / profile specific information  /  variable length
 |                               |
 +---+---+---+---+---+---+---+---+
 x: Profile specific information.  Interpreted according to the
    profile indicated in the Profile field of the IR header.
 Profile: The profile associated with the CID.  In the IR header, the
    profile identifier is abbreviated to the 8 least significant bits
    (see Section 5.1.2).
 CRC: 8-bit CRC (see Section 5.3.1.1).
 Profile specific information: The content of this part of the IR
    header is defined by the individual profiles.  It is interpreted
    according to the profile indicated in the Profile field.

5.2.2.2. ROHC IR-DYN Packet Type

 In contrast to the IR header, the IR-DYN header can never initialize
 a non-initialized context.  However, it can redefine what profile is
 associated with a context, if the profile indicated in the IR-DYN
 header allows this.  Thus, this packet type is also reserved at the
 framework level.  The IR-DYN header typically also initializes or
 refreshes parts of a context.  For IR-DYN, Header has the following
 general format:

Jonsson, et al. Standards Track [Page 20] RFC 4995 The ROHC Framework July 2007

   0   1   2   3   4   5   6   7
  --- --- --- --- --- --- --- ---
 :         Add-CID octet         :  if CID 1-15 and small CID
 +---+---+---+---+---+---+---+---+
 | 1   1   1   1   1   0   0   0 |  IR-DYN type octet
 +---+---+---+---+---+---+---+---+
 :                               :
 /      0-2 octets of CID        /  1 or 2 octets if large CIDs
 :                               :
 +---+---+---+---+---+---+---+---+
 |            Profile            |  1 octet
 +---+---+---+---+---+---+---+---+
 |              CRC              |  1 octet
 +---+---+---+---+---+---+---+---+
 |                               |
 / profile specific information  /  variable length
 |                               |
 +---+---+---+---+---+---+---+---+
 Profile: The profile associated with the CID.  This is abbreviated in
    the same way as in IR packets.
 CRC: 8-bit CRC (see Section 5.3.1.1).
 Profile specific information: The content of this part of the IR-DYN
    header is defined by the individual profiles.  It is interpreted
    according to the profile indicated in the Profile field.

5.2.3. ROHC Initial Decompressor Processing

 Initially, all contexts are in no context state.  Thus, all packets
 referencing a non-initialized context, except packets that have
 enough information on the static fields, cannot be decompressed by
 the decompressor.
 When the decompressor receives a packet of type IR, the profile
 indicated in the IR packet determines how it is to be processed.
    o  If the 8-bit CRC fails to verify the integrity of the Header,
       the packet MUST NOT be decompressed and delivered to upper
       layers.  If a profile is indicated in the context, the logic of
       that profile determines what, if any, feedback is to be sent.
       If no profile is noted in the context, the logic used to
       determine what, if any, feedback to send is up to the
       implementation.  However, it may be suitable to take no further
       actions, as any part of the IR header covered by the CRC may
       have caused the failure.

Jonsson, et al. Standards Track [Page 21] RFC 4995 The ROHC Framework July 2007

 When the decompressor receives a packet of type IR-DYN, the profile
 indicated in the IR-DYN packet determines how it is to be processed.
    o  If the 8-bit CRC fails to verify the integrity of the header,
       the packet MUST NOT be decompressed and delivered to upper
       layers.  If a profile is indicated in the context, the logic of
       that profile determines what, if any, feedback is to be sent.
       If no profile is noted in the context, the logic used to
       determine what, if any, feedback to send is up to the
       implementation.  However, it may be suitable to take no further
       actions, as any part of the IR-DYN header covered by the CRC
       may have caused the failure.
    o  If the context has not already been initialized, the packet
       MUST NOT be decompressed and delivered to upper layers.  The
       logic of the profile indicated in the IR-DYN header (if
       verified by the 8-bit CRC), determines what, if any, feedback
       is to be sent.
 If a parsing error occurs for any packet type, the decompressor MUST
 discard the packet without further processing.  For example, a CID
 field is present in the compressed header when the large CID space is
 used for the ROHC channel, and the field is coded using the self-
 describing variable-length encoding of Section 5.3.2; if the field
 starts with 110 or 111, this would generate a parsing error for the
 decompressor because this field must not be encoded with a size
 larger than 2 octets.
 It is RECOMMENDED that profiles disallow the decompressor to make a
 decompression attempt for packets carrying only a 3-bit CRC after it
 has invalidated some or all of the entire dynamic context, until a
 packet that contains sufficient information on the dynamic fields is
 received, decompressed, and successfully verified by a 7- or 8-bit
 CRC.

5.2.4. ROHC Feedback

 Feedback carries information from the decompressor to compressor.
 Feedback can be sent over a ROHC channel that operates in the same
 direction as the feedback.
 The general ROHC packet format allows transport of feedback using
 interspersion or piggybacking (see [5]), or a combination of both,
 over a ROHC channel.  This is facilitated by the following
 properties:

Jonsson, et al. Standards Track [Page 22] RFC 4995 The ROHC Framework July 2007

 Reserved packet type:
    A feedback packet type is reserved at the framework level.  The
    packet type can carry variable-length feedback information.
 CID information:
    The feedback information sent on a particular channel is passed
    to, and interpreted by, the compressor associated with feedback on
    that channel.  Thus, each feedback element contains CID
    information from the channel for which the feedback is sent.  The
    ROHC feedback scheme thus requires that a channel carries feedback
    to at most one compressor.  How a compressor is associated with
    the feedback for a particular channel is outside the scope of this
    specification.  See also [5].
 Length information:
    The length of a feedback element can be determined by examining
    the first few octets of the feedback.  This enables piggybacking
    of feedback, and also the concatenation of more than one feedback
    element in a packet.  The length information thus decouples the
    decompressor from the associated same-side compressor, as the
    decompressor can extract the feedback information from the
    compressed header without parsing its content and hand over the
    extracted information.
 The association between compressor-decompressor pairs operating in
 opposite directions, for the purpose of exchanging piggyback and/or
 interspersed feedback, SHOULD be maintained for the lifetime of the
 ROHC channel.  Otherwise, it is RECOMMENDED that the compressor be
 notified if the feedback channel is no longer available: the
 compressor SHOULD then restart compression by creating a new context
 for each packet flow, and SHOULD use a CID value that was not
 previously associated with the profile used to compress the flow.

5.2.4.1. ROHC Feedback Format

 ROHC defines three different categories of feedback messages:
 acknowledgment (ACK), negative ACK (NACK), and NACK for the entire
 context (STATIC-NACK).  Other types of information may be defined in
 profile-specific feedback information.
 ACK         : Acknowledges successful decompression of a packet.
               Indicates that the decompressor considers its context
               to be valid.

Jonsson, et al. Standards Track [Page 23] RFC 4995 The ROHC Framework July 2007

 NACK        : Indicates that the decompressor considers some or all
               of the dynamic part of its context invalid.
 STATIC-NACK : Indicates that the decompressor considers its entire
               static context invalid, or that it has not been
               established.
 Feedback sent on a ROHC channel consists of one or more concatenated
 feedback elements, where each feedback element has the following
 format:
   0   1   2   3   4   5   6   7
 +---+---+---+---+---+---+---+---+
 | 1   1   1   1   0 |   Code    |  feedback type
 +---+---+---+---+---+---+---+---+
 :             Size              :  if Code = 0
 +---+---+---+---+---+---+---+---+
 :         Add-CID octet         :  if for small CIDs and (CID != 0)
 +---+---+---+---+---+---+---+---+
 :                               :
 /  large CID (5.3.2 encoding)   /  1-2 octets if for large CIDs
 :                               :
 +---+---+---+---+---+---+---+---+
 /         FEEDBACK data         /  variable length
 +---+---+---+---+---+---+---+---+
 Code: 0 indicates that a Size octet is present.
       1-7 indicates the size of the feedback data field, in octets.
 Size: Indicates the size of the feedback data field, in octets.
 FEEDBACK data: FEEDBACK-1 or FEEDBACK-2 (see below).
 CID information in a feedback element indicates the context for which
 feedback is sent.  The LARGE_CIDS parameter that controls whether a
 large CID is present is taken from the channel state of the receiving
 compressor's channel, not from the state of the channel carrying the
 feedback.
 The large CID field, if present, is encoded according to Section
 5.3.2, and it MUST NOT be encoded using more than 2 octets.

Jonsson, et al. Standards Track [Page 24] RFC 4995 The ROHC Framework July 2007

 The FEEDBACK data field can have either of the following two formats:
 FEEDBACK-1:
   0   1   2   3   4   5   6   7
 +---+---+---+---+---+---+---+---+
 | profile specific information  |  1 octet
 +---+---+---+---+---+---+---+---+
 FEEDBACK-2:
   0   1   2   3   4   5   6   7
 +---+---+---+---+---+---+---+---+
 |Acktype|                       |
 +---+---+   profile specific    /  at least 2 octets
 /             information       |
 +---+---+---+---+---+---+---+---+
 Acktype:  0 = ACK
           1 = NACK
           2 = STATIC-NACK
           3 is reserved (MUST NOT be used.  Otherwise unparseable.)

5.2.5. ROHC Segmentation

 ROHC defines a simple segmentation protocol.  The compressor may
 perform segmentation, e.g., to accommodate packets that are larger
 than a specific size configured for the channel.

5.2.5.1. Segmentation Usage Considerations

 The ROHC segmentation protocol is not particularly efficient.  It is
 not intended to replace link layer segmentation functions; these
 SHOULD be used whenever available and efficient for the task at hand.
 The ROHC segmentation protocol has been designed with an assumption
 of in-order delivery of packets between the compressor and the
 decompressor, using only a CRC for error detection, and no sequence
 numbers.  If in-order delivery cannot be guaranteed, ROHC
 segmentation MUST NOT be used.
 The segmentation protocol also assumes that all segments of a ROHC
 packet corresponding to one context are received without interference
 from other ROHC packets over the channel, including any ROHC packet
 corresponding to a different context.  Based on this assumption,
 segments do not carry CID information, and therefore cannot be
 associated with a specific context until all segments have been
 received and the whole unit has been reconstructed.

Jonsson, et al. Standards Track [Page 25] RFC 4995 The ROHC Framework July 2007

5.2.5.2. Segmentation Protocol

 ROHC segmentation is applied to the combination of the Header and the
 Payload fields of the ROHC packet, as defined in Section 5.2.1.
 Segment format:
   0   1   2   3   4   5   6   7
 +---+---+---+---+---+---+---+---+
 | 1   1   1   1   1   1   1 | F |  segment type
 +---+---+---+---+---+---+---+---+
 /           Segment             /  variable length
 +---+---+---+---+---+---+---+---+
 F: Final bit.  If set, it indicates that this is the last segment of
    a reconstructed unit.
 Padding and/or Feedback may precede the segment type octet.  There is
 no per-segment CID, but CID information is of course part of the
 reconstructed unit.  The reconstructed unit MUST NOT contain padding,
 segments, or feedback.
 When a final segment is received, the decompressor reassembles the
 segment carried in this packet and any non-final segments that
 immediately preceded it into a single reconstructed unit, in the
 order they were received.  All segments for one reconstructed unit
 have to be received consecutively and in the correct order by the
 decompressor.  If a non-segment ROHC packet directly follows a non-
 final segment, the reassembly of the current reconstructed unit is
 aborted and the decompressor MUST discard the non-final segments so
 far received on this channel.
 Reconstructed unit:
   0   1   2   3   4   5   6   7
 +---+---+---+---+---+---+---+---+
 /            Header             /  (see Section 5.2.1)
 +---+---+---+---+---+---+---+---+
 :            Payload            :  (see Section 5.2.1)
 +---+---+---+---+---+---+---+---+
 /              CRC              /  4 octets
 +---+---+---+---+---+---+---+---+
 CRC: 32-bit CRC computed using the polynomial of Section 5.3.1.4.
 If the reconstructed unit is 4 octets or less, or if the CRC fails,
 or if it is larger than the channel parameter MRRU (see Section

Jonsson, et al. Standards Track [Page 26] RFC 4995 The ROHC Framework July 2007

 5.1.2), the reconstructed unit MUST be discarded by the decompressor.
 If the CRC succeeds, the reconstructed unit can be further processed.

5.3. General Encoding Methods

5.3.1. Header Compression CRCs, Coverage and Polynomials

 This section describes how to calculate the CRCs used by ROHC.  For
 all CRCs, the algorithm used to calculate the CRC is the same as the
 one used in [2], defined in Appendix A of this document, with the
 polynomials specified in subsequent sections.

5.3.1.1. 8-bit CRCs in IR and IR-DYN Headers

 The coverage for the 8-bit CRC in the IR and IR-DYN headers is
 profile-dependent, but it MUST cover at least the initial part of the
 header ending with the Profile field, including the CID or an Add-CID
 octet.  Feedback and padding are not part of Header (Section 5.2.1)
 and are thus not included in the CRC calculation.  As a rule of thumb
 for profile specifications, any other information that initializes
 the decompressor context SHOULD also be covered by a CRC.
 More specifically, the 8-bit CRC does not cover only and entirely the
 original uncompressed header; therefore, it does not provide the
 means for the decompressor to verify a decompression attempt, or the
 means to verify the correctness of the entire decompressor context.
 However, when successful, it does provide enough robustness for the
 decompressor to update its context with the information carried
 within the IR or the IR-DYN header.
 The CRC polynomial for the 8-bit CRC is:
    C(x) = 1 + x + x^2 + x^8
 When computing the CRC, the CRC field in the header is set to zero,
 and the initial content of the CRC register is set to all 1's.

5.3.1.2. 3-bit CRC in Compressed Headers

 The 3-bit CRC in compressed headers is calculated over all octets of
 the entire original header, before compression, in the following
 manner.
 The initial content of the CRC register is set to all 1's.
 The polynomial for the 3-bit CRC is:
    C(x) = 1 + x + x^3

Jonsson, et al. Standards Track [Page 27] RFC 4995 The ROHC Framework July 2007

 The purpose of the 3-bit CRC is to provide the means for the
 decompressor to verify the outcome of a decompression attempt for
 small compressed headers, and to detect context damage based on
 aggregated probability over a number of decompression attempts.  It
 is however too weak to provide enough success guarantees from the
 decompression of one single header.  Therefore, compressed headers
 carrying a 3-bit CRC are normally not suitable to perform context
 repairs at the decompressor; hence, profiles should refrain from
 allowing decompression of such a header when some or the entire
 decompressor context is assumed invalid.

5.3.1.3. 7-bit CRC in Compressed Headers

 The 7-bit CRC in compressed headers is calculated over all octets of
 the entire original header, before compression, in the following
 manner.
 The initial content of the CRC register is set to all 1's.
 The polynomial for the 7-bit CRC is:
    C(x) = 1 + x + x^2 + x^3 + x^6 + x^7
 The purpose of the 7-bit CRC is to provide the means for the
 decompressor to verify the outcome of a decompression attempt for a
 larger compressed header, and to provide enough protection to
 validate a context repair at the decompressor.  The 7-bit CRC is
 strong enough to assume a repair to be successful from the
 decompression of one single header; hence, profiles may allow
 decompression of a header carrying a 7-bit CRC when some of the
 decompressor context is assumed invalid.

5.3.1.4. 32-bit Segmentation CRC

 The 32-bit CRC is used by the segmentation scheme to verify the
 reconstructed unit, and it is thus calculated over the segmented
 unit, i.e., over the Header and the Payload fields of the ROHC
 packet.
 The initial content of the CRC register is set to all 1's.
 The polynomial for the 32-bit CRC is:
    C(x) = x^0 + x^1 + x^2 + x^4 + x^5 + x^7 + x^8 + x^10 +
           x^11 + x^12 + x^16 + x^22 + x^23 + x^26 + x^32.
 The purpose of the 32-bit CRC is to verify the reconstructed unit.

Jonsson, et al. Standards Track [Page 28] RFC 4995 The ROHC Framework July 2007

5.3.2. Self-Describing Variable-Length Values

 The values of many fields and compression parameters can vary widely.
 To optimize the transfer of such values, a variable number of octets
 are used to encode them.  The first few bits of the first octet
 determine the number of octets used:
 First bit is 0: 1 octet.
          7 bits transferred.
          Up to 127 decimal.
          Encoded octets in hexadecimal: 00 to 7F
 First bits are 10: 2 octets.
          14 bits transferred.
          Up to 16 383 decimal.
          Encoded octets in hexadecimal: 80 00 to BF FF
 First bits are 110: 3 octets.
          21 bits transferred.
          Up to 2 097 151 decimal.
          Encoded octets in hexadecimal: C0 00 00 to DF FF FF
 First bits are 111: 4 octets.
          29 bits transferred.
          Up to 536 870 911 decimal.
          Encoded octets in hexadecimal: E0 00 00 00 to FF FF FF FF

5.4. ROHC UNCOMPRESSED – No Compression (Profile 0x0000)

 This section describes the uncompressed ROHC profile.  The profile
 identifier for this profile is 0x0000.
 Profile 0x0000 provides a way to send IP packets without compressing
 them.  This can be used for any packet for which a compression
 profile is not available in the set of profiles supported by the ROHC
 channel, or for which compression is not desirable for some reason.
 After initialization, the only overhead for sending packets using
 Profile 0x0000 is the size of the CID.  When uncompressed packets are
 frequent, Profile 0x0000 should be associated with a CID the size of
 zero or one octet.  Profile 0x0000 SHOULD be associated with at most
 one CID.

Jonsson, et al. Standards Track [Page 29] RFC 4995 The ROHC Framework July 2007

5.4.1. IR Packet

 The initialization and refresh packet (IR packet) for Profile 0x0000
 has the following Header format:
   0   1   2   3   4   5   6   7
  --- --- --- --- --- --- --- ---
 :         Add-CID octet         : if for small CIDs and (CID != 0)
 +---+---+---+---+---+---+---+---+
 | 1   1   1   1   1   1   0 |res|
 +---+---+---+---+---+---+---+---+
 :                               :
 /    0-2 octets of CID info     / 1-2 octets if for large CIDs
 :                               :
 +---+---+---+---+---+---+---+---+
 |         Profile = 0x00        | 1 octet
 +---+---+---+---+---+---+---+---+
 |              CRC              | 1 octet
 +---+---+---+---+---+---+---+---+
 res: MUST be set to zero; otherwise, the decompressor MUST discard
      the packet.
 Profile: 0x00
 CRC: 8-bit CRC, computed using the polynomial of Section 5.3.1.1.
      The CRC covers the first octet of the IR Header through the
      Profile octet of the IR Header, i.e., it does not cover the CRC
      itself.  Neither does it cover any preceding Padding or
      Feedback, nor the Payload.
 For the IR packet, Payload has the following format:
  1. – — — — — — — —

: : (optional)

 /           IP packet           / variable length
 :                               :
  --- --- --- --- --- --- --- ---
 IP packet: An uncompressed IP packet may be included in the IR
    packet.  The decompressor determines if the IP packet is present
    by considering the length of the IR packet.

Jonsson, et al. Standards Track [Page 30] RFC 4995 The ROHC Framework July 2007

5.4.2. Normal Packet

 A Normal packet is a normal IP packet plus CID information.  For the
 Normal Packet, the following format corresponds to the Header and
 Payload (as defined in Section 5.2.1):
   0   1   2   3   4   5   6   7
  --- --- --- --- --- --- --- ---
 :         Add-CID octet         : if for small CIDs and (CID != 0)
 +---+---+---+---+---+---+---+---+
 |   first octet of IP packet    |
 +---+---+---+---+---+---+---+---+
 :                               :
 /    0-2 octets of CID info     / 1-2 octets if for large CIDs
 :                               :
 +---+---+---+---+---+---+---+---+
 |                               |
 /       rest of IP packet       / variable length
 |                               |
 +---+---+---+---+---+---+---+---+
 Note that the first octet of the IP packet starts with the bit
 pattern 0100 (IPv4) or 0110 (IPv6).  This does not conflict with any
 reserved packet types.
 When the channel uses small CIDs, and profile 0x0000 is associated
 with a CID > 0, an Add-CID octet precedes the IP packet.  When the
 channel uses large CIDs, the CID is placed so that it starts at the
 second octet of the combined Header/Payload format above.
 A Normal Packet may carry Padding and/or Feedback as any other ROHC
 packet, preceding the combined Header/Payload.

5.4.3. Decompressor Operation

 When an IR packet is received, the decompressor first validates its
 header using the 8-bit CRC.
 o  If the header fails validation, the decompressor MUST NOT deliver
    the IP packet to upper layers.
 o  If the header is successfully validated, the decompressor
       1) initializes the context if it has no valid context for the
          given CID already associated to the specified profile,
       2) delivers the IP packet to upper layers if present,

Jonsson, et al. Standards Track [Page 31] RFC 4995 The ROHC Framework July 2007

       3) MAY send an ACK.
 When any other packet is received while the decompressor has no
 context, it is discarded without further action.
 When a Normal packet is received and the decompressor has a valid
 context, the IP packet is extracted and delivered to upper layers.

5.4.4. Feedback

 The only kind of feedback defined by Profile 0x0000 is ACK, using the
 FEEDBACK-1 format of Section 5.2.4.1, where the value of the profile-
 specific octet in the FEEDBACK-1 is 0 (zero).  The FEEDBACK-2 format
 is thus not defined for Profile 0x0000.

6. Overview of a ROHC Profile (Informative)

 The ROHC protocol consists of a framework part and a profile part.
 The framework defines the mechanisms common to all profiles, while
 the profile defines the compression algorithm and profile specific
 packet formats.
 Section 5 specifies the details of the ROHC framework.  This section
 provides an informative overview of the elements that make a profile
 specification.  The normative specification of individual profiles is
 outside the scope of this document.
 A ROHC profile defines the elements that build up the compression
 protocol.  A ROHC profile consists of:
 Packet formats:
 o  Bits-on-the-wire
    The profile defines the layout of the bits for profile-specific
    packet types that it defines, and for the profile-specific parts
    of packet types common to all profiles (e.g., IR and IR-DYN).
 o  Field encodings
    Bits and groups of bits from the packet format layout, referred to
    as Compressed fields, represents the result of an encoding method
    specific for that compressed field within a specific packet
    format.  The profile defines these encoding methods.

Jonsson, et al. Standards Track [Page 32] RFC 4995 The ROHC Framework July 2007

 o  Updating properties
    The profile-specific packet formats may update the state of the
    decompressor, and may do so in different ways.  The profile
    defines how individual profile-specific fields, or entire
    profile-specific packet types, update the decompressor context.
 o  Verification
    Packets that update the state of the decompressor are verified to
    prevent incorrect updates to the decompressor context.  The
    profile defines the mechanisms used to verify the decompression of
    a packet.
 Context management:
 o  Robustness logic
    Packets may be lost or reordered between the compressor and the
    decompressor.  The profile defines mechanism to minimize the
    impacts of such events and prevent damage propagation.
 o  Repair mechanism
    Despite the robustness logic, impairment events may still lead to
    decompression failure(s), and even to context damage at the
    decompressor.  The profile defines context repair mechanisms,
    including feedback logic if used.

7. 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.
 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 headers and possibly also valid
 transport checksums.  Such corruption may be detected with end-to-end
 authentication and integrity mechanisms, which will not be affected
 by the compression.  Moreover, the ROHC header compression scheme
 uses an internal checksum for verification of reconstructed headers,
 which 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, IR-DYN, or FEEDBACK packets onto the link and
 thereby cause compression efficiency to be reduced.  However, an

Jonsson, et al. Standards Track [Page 33] RFC 4995 The ROHC Framework July 2007

 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.

8. IANA Considerations

 An IANA registry for "RObust Header Compression (ROHC) Profile
 Identifiers" [21] was created by RFC 3095 [3].  The assignment
 policy, as outlined by RFC 3095, is the following:
 The ROHC profile identifier is a non-negative integer.  In many
 negotiation protocols, it will be represented as a 16-bit value.  Due
 to the way the profile identifier is abbreviated in ROHC packets, the
 8 least significant bits of the profile identifier have a special
 significance: Two profile identifiers with identical 8 LSBs should be
 assigned only if the higher-numbered one is intended to supersede the
 lower-numbered one.  To highlight this relationship, profile
 identifiers should be given in hexadecimal (as in 0x1234, which would
 for example supersede 0x0A34).
 Following the policies outlined in [22], the IANA policy for
 assigning new values for the profile identifier shall be
 Specification Required: values and their meanings must be documented
 in an RFC or in some other permanent and readily available reference,
 in sufficient detail that interoperability between independent
 implementations is possible.  In the 8 LSBs, the range 0 to 127 is
 reserved for IETF standard-track specifications; the range 128 to 254
 is available for other specifications that meet this requirement
 (such as Informational RFCs).  The LSB value 255 is reserved for
 future extensibility of the present specification.
 The following profile identifiers have so far been allocated:
 Profile Identifier    Usage                      Reference
 ------------------    ----------------------     ---------
 0x0000                ROHC uncompressed          RFC 4995
 0x0001                ROHC RTP                   RFC 3095
 0x0002                ROHC UDP                   RFC 3095
 0x0003                ROHC ESP                   RFC 3095
 0x0004                ROHC IP                    RFC 3843
 0x0005                ROHC LLA                   RFC 3242
 0x0105                ROHC LLA with R-mode       RFC 3408
 0x0006                ROHC TCP                   RFC 4996
 0x0007                ROHC RTP/UDP-Lite          RFC 4019
 0x0008                ROHC UDP-Lite              RFC 4019
 New profiles will need new identifiers to be assigned by the IANA,
 but this document does not require any additional IANA action.

Jonsson, et al. Standards Track [Page 34] RFC 4995 The ROHC Framework July 2007

9. Acknowledgments

 The authors would like to acknowledge all who have contributed to
 previous ROHC work, and especially to the authors of RFC 3095 [3],
 which is the technical basis for this document.  Thanks also to the
 various individuals who contributed to the RFC 3095 corrections and
 clarifications document [6], from which technical contents, when
 applicable, have been incorporated into this document.  Committed WG
 document reviewers were Carl Knutsson and Biplab Sarkar, who reviewed
 the document during working group last-call.

10. References

10.1. Normative References

 [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.

10.2. Informative References

 [2]  Simpson, W., "PPP in HDLC-like Framing", STD 51, RFC 1662, July
      1994.
 [3]  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.
 [4]  Bormann, C., "Robust Header Compression (ROHC) over PPP", RFC
      3241, April 2002.
 [5]  Jonsson, L-E., "RObust Header Compression (ROHC): Terminology
      and Channel Mapping Examples", RFC 3759, April 2004.
 [6]  Jonsson, L-E., Sandlund, K., Pelletier, G., and P. Kremer,
      "RObust Header Compression (ROHC): Corrections and
      Clarifications to RFC 3095", RFC 4815, February 2007.
 [7]  Pelletier, G., Jonsson, L-E., and K. Sandlund, "RObust Header
      Compression (ROHC): ROHC over Channels That Can Reorder
      Packets", RFC 4224, January 2006.
 [8]  Pelletier, G. and K. Sandlund, "RObust Header Compression
      Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP, and UDP
      Lite", Work in Progress, September 2006.

Jonsson, et al. Standards Track [Page 35] RFC 4995 The ROHC Framework July 2007

 [9]  Pelletier, G., Sandlund, K., Jonsson, L-E., and M. West, "RObust
      Header Compression (ROHC): A Profile for TCP/IP (ROHC-TCP)", RFC
      4996, July 2007.
 [10] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
 [11] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
      Specification", RFC 2460, December 1998.
 [12] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
      1980.
 [13] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
      "RTP: A Transport Protocol for Real-Time Applications", STD 64,
      RFC 3550, July 2003.
 [14] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
      September 1981.
 [15] Jacobson, V., "Compressing TCP/IP headers for low-speed serial
      links", RFC 1144, February 1990.
 [16] Degermark, M., Nordgren, B., and S. Pink, "IP Header
      Compression", RFC 2507, February 1999.
 [17] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for
      Low-Speed Serial Links", RFC 2508, February 1999.
 [18] Degermark, M., "Requirements for robust IP/UDP/RTP header
      compression", RFC 3096, July 2001.
 [19] Koren, T., Casner, S., Geevarghese, J., Thompson, B., and P.
      Ruddy, "Enhanced Compressed RTP (CRTP) for Links with High
      Delay, Packet Loss and Reordering", RFC 3545, July 2003.
 [20] Degermark, M., Hannu, H., Jonsson, L.E., and K. Svanbro,
      "Evaluation of CRTP Performance over Cellular Radio Networks",
      IEEE Personal Communication Magazine, Volume 7, number 4, pp.
      20-25, August 2000.
 [21] IANA registry, "RObust Header Compression (ROHC) Profile
      Identifiers", http://www.iana.org/assignments/rohc-pro-ids
 [22] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
      Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

Jonsson, et al. Standards Track [Page 36] RFC 4995 The ROHC Framework July 2007

Appendix A. CRC Algorithm

 #!/usr/bin/perl -w
 use strict;
 #=================================
 #
 # ROHC CRC demo - Carsten Bormann cabo@tzi.org 2001-08-02
 #
 # This little demo shows the four types of CRC in use in RFC 3095,
 # the specification for robust header compression.  Type your data in
 # hexadecimal form and then press Control+D.
 #
 #---------------------------------
 #
 # utility
 #
 sub dump_bytes($) {
     my $x = shift;
     my $i;
     for ($i = 0; $i < length($x); ) {
   printf("%02x ", ord(substr($x, $i, 1)));
   printf("\n") if (++$i % 16 == 0);
     }
     printf("\n") if ($i % 16 != 0);
 }
 #---------------------------------
 #
 # The CRC calculation algorithm.
 #
 sub do_crc($$$) {
     my $nbits = shift;
     my $poly = shift;
     my $string = shift;
     my $crc = ($nbits == 32 ? 0xffffffff : (1 << $nbits) - 1);
     for (my $i = 0; $i < length($string); ++$i) {
       my $byte = ord(substr($string, $i, 1));
       for( my $b = 0; $b < 8; $b++ ) {
         if (($crc & 1) ^ ($byte & 1)) {
           $crc >>= 1;
           $crc ^= $poly;
         } else {
         $crc >>= 1;
         }
         $byte >>= 1;
       }
     }

Jonsson, et al. Standards Track [Page 37] RFC 4995 The ROHC Framework July 2007

     printf "%2d bits, ", $nbits;
     printf "CRC: %02x\n", $crc;
 }
 #---------------------------------
 #
 # Test harness
 #
 $/ = undef;
 $_ = <>;         # read until EOF
 my $string = ""; # extract all that looks hex:
 s/([0-9a-fA-F][0-9a-fA-F])/$string .= chr(hex($1)), ""/eg;
 dump_bytes($string);
 #---------------------------------
 #
 # 32-bit segmentation CRC
 # Note that the text implies this is complemented like for PPP
 # (this differs from 8, 7, and 3-bit CRC)
 #
 #      C(x) = x^0 + x^1 + x^2 + x^4 + x^5 + x^7 + x^8 + x^10 +
 #             x^11 + x^12 + x^16 + x^22 + x^23 + x^26 + x^32
 #
 do_crc(32, 0xedb88320, $string);
 #---------------------------------
 #
 # 8-bit IR/IR-DYN CRC
 #
 #      C(x) = x^0 + x^1 + x^2 + x^8
 #
 do_crc(8, 0xe0, $string);
 #---------------------------------
 #
 # 7-bit FO/SO CRC
 #
 #      C(x) = x^0 + x^1 + x^2 + x^3 + x^6 + x^7
 #
 do_crc(7, 0x79, $string);
 #---------------------------------
 #
 # 3-bit FO/SO CRC
 #
 #      C(x) = x^0 + x^1 + x^3
 #
 do_crc(3, 0x6, $string);

Jonsson, et al. Standards Track [Page 38] RFC 4995 The ROHC Framework July 2007

Authors' Addresses

 Lars-Erik Jonsson
 Optand 737
 SE-831 92 Ostersund, Sweden
 Phone: +46 70 365 20 58
 EMail: lars-erik@lejonsson.com
 Ghyslain Pelletier
 Ericsson AB
 Box 920
 SE-971 28 Lulea, Sweden
 Phone: +46 8 404 29 43
 Fax:   +46 920 996 21
 EMail: ghyslain.pelletier@ericsson.com
 Kristofer Sandlund
 Ericsson AB
 Box 920
 SE-971 28 Lulea, Sweden
 Phone: +46 8 404 41 58
 Fax:   +46 920 996 21
 EMail: kristofer.sandlund@ericsson.com

Jonsson, et al. Standards Track [Page 39] RFC 4995 The ROHC Framework July 2007

Full Copyright Statement

 Copyright (C) The IETF Trust (2007).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

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Jonsson, et al. Standards Track [Page 40]

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