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rfc:rfc4815

Network Working Group L-E. Jonsson Request for Comments: 4815 K. Sandlund Updates: 3095, 3241, 3843, 4019, 4362 G. Pelletier Category: Standards Track P. Kremer

                                                         February 2007
                 RObust Header Compression (ROHC):
             Corrections and Clarifications to RFC 3095

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

 RFC 3095 defines the RObust Header Compression (ROHC) framework and
 profiles for IP (Internet Protocol), UDP (User Datagram Protocol),
 RTP (Real-Time Transport Protocol), and ESP (Encapsulating Security
 Payload).  Some parts of the specification are unclear or contain
 errors that may lead to misinterpretations that may impair
 interoperability between different implementations.  This document
 provides corrections, additions, and clarifications to RFC 3095; this
 document thus updates RFC 3095.  In addition, other clarifications
 related to RFC 3241 (ROHC over PPP), RFC 3843 (ROHC IP profile) and
 RFC 4109 (ROHC UDP-Lite profiles) are also provided.

Jonsson, et al. Standards Track [Page 1] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

Table of Contents

 1. Introduction and Terminology ....................................3
 2. CRC Calculation and Coverage ....................................4
    2.1. CRC Calculation ............................................4
    2.2. Padding Octet and CRC Calculations .........................4
    2.3. CRC Coverage in CRC Feedback Options .......................5
    2.4. CRC Coverage of the ESP NULL Header ........................5
 3. Mode Transition .................................................5
    3.1. Feedback During Mode Transition to U- and O-Mode ...........5
         3.1.1. Mode Transition Procedures Allowing Sparse Feedback .6
         3.1.2. Transition from Reliable to Optimistic Mode .........7
         3.1.3. Transition to Unidirectional Mode ...................8
    3.2. Feedback During Mode Transition ............................8
    3.3. Packet Decoding During Mode Transition .....................9
 4. Timestamp Encoding ..............................................9
    4.1. Encoding Used for Compressed TS Bits .......................9
    4.2. (De)compression of TS without Transmitted TS Bits .........10
    4.3. Interpretation Intervals for TS Encoding ..................11
    4.4. Scaled RTP Timestamp Encoding .............................11
         4.4.1. TS_STRIDE for Scaled Timestamp Encoding ............11
         4.4.2. TS Wraparound with Scaled Timestamp Encoding .......12
         4.4.3. Algorithm for Scaled Timestamp Encoding ............12
    4.5. Recalculating TS_OFFSET ...................................14
    4.6. TS_STRIDE and the Tsc Flag in Extension 3 .................14
    4.7. Using Timer-Based Compression .............................15
 5. List Compression ...............................................15
    5.1. CSRC List Items in RTP Dynamic Chain ......................15
    5.2. Multiple Occurrences of the CC Field ......................15
    5.3. Bit Masks in List Compression .............................16
    5.4. Headers Compressed with List Compression ..................16
    5.5. ESP NULL Header List Compression ..........................17
    5.6. Translation Tables and Indexes for IP Extension Headers ...17
    5.7. Reference List ............................................17
    5.8. Compression of AH and GRE Sequence Numbers ................18
 6. Updating Properties ............................................19
    6.1. Implicit Updates ..........................................19
    6.2. Updating Properties of UO-1* ..............................20
    6.3. Context Updating Properties for IR Packets ................20
    6.4. RTP Padding Field (R-P) in Extension 3 ....................20
    6.5. RTP eXtension bit (X) in dynamic part .....................21
 7. Context management and CID/context Reuse .......................21
    7.1. Persistence of Decompressor Contexts ......................21
    7.2. CID/Context Reuse .........................................21
         7.2.1. Reusing a CID/Context with the Same Profile ........22
         7.2.2. Reusing a CID/Context with a Different Profile .....23
 8. Other Protocol Clarifications ..................................23
    8.1. Meaning of NBO ............................................23

Jonsson, et al. Standards Track [Page 2] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

    8.2. IP-ID .....................................................23
    8.3. Extension-3 in UOR-2* Packets .............................24
    8.4. Multiple Occurrences of the M Bit .........................24
    8.5. Multiple SN options in one feedback packet ................24
    8.6. Multiple CRC Options in One Feedback Packet ...............25
    8.7. Responding to Lost Feedback Links .........................25
    8.8. UOR-2 in Profile 0x0002 (UDP) and Profile 0x0003 (ESP) ....25
    8.9. Sequence Number LSB's in IP Extension Headers .............25
    8.10. Expecting UOR-2 ACKs in O-Mode ...........................26
    8.11. Context Repairs, TS_STRIDE and TIME_STRIDE ...............26
 9. ROHC Negotiation ...............................................27
 10. PROFILES Sub-option in ROHC-over-PPP ..........................27
 11. Constant IP-ID Encoding in IP-only and UPD-Lite Profiles ......27
 12. Security Considerations .......................................28
 13. Acknowledgments ...............................................28
 14. References ....................................................28
    14.1. Normative References .....................................28
    14.2. Informative References ...................................29
 Appendix A. Sample CRC Algorithm ..................................30

1. Introduction and Terminology

 RFC 3095 [1] defines the RObust Header Compression (ROHC) framework
 and profiles for IP (Internet Protocol) [8][9], UDP (User Datagram
 Protocol) [10], RTP (Real-Time Transport Protocol) [11], and ESP
 (Encapsulating Security Payload) [12].  During implementation and
 interoperability testing of RFC 3095, some ambiguities and common
 misinterpretations have been identified, as well as a few errors.
 This document summarizes identified issues and provides corrections
 needed for implementations of RFC 3095 to interoperate, i.e., it
 constitutes an update to RFC 3095.  This document also provides other
 clarifications related to common misinterpretations of the
 specification.  References to RFC 3095 should, therefore, also
 include this document.
 In addition, some clarifications and corrections are also provided
 for RFC 3241 (ROHC over PPP) [2], RFC 3843 (ROHC IP-only profile)
 [4], and RFC 4019 (ROHC UDP-Lite profiles) [5], which are thus also
 updated by this document.  Furthermore, RFC 4362 (ROHC Link-Layer
 Assisted Profile) [7] is implicitly updated by this document, since
 RFC 4362 is also based on RFC 3095.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [6].

Jonsson, et al. Standards Track [Page 3] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

 When a section of this document makes formal corrections, additions
 or invalidations to text in RFC 3095, this is clearly summarized.
 The text from RFC 3095 that is being addressed is given and labeled
 "INCOMPLETE", "INCORRECT", or "INCORRECT AND INVALIDATED", followed
 by the correct text labeled "CORRECTED", where applicable.  When text
 is added that does not simply correct text in previous
 specifications, it is given with the label "FORMAL ADDITION".
 In this document, a reference to a section in RFC 3095 [1] is written
 as RFC 3095-Section <number>.

2. CRC Calculation and Coverage

2.1. CRC Calculation

 RFC 3095-Section 5.9 defines polynomials for 3-, 7-, and 8-bit Cyclic
 Redundancy Checks (CRCs), but it does not specify what algorithm is
 used.  The 3-, 7- and 8-bit CRCs are calculated using the CRC
 algorithm defined in [3].
 A Perl implementation of the algorithm can be found in Appendix A of
 this document.

2.2. Padding Octet and CRC Calculations

 RFC 3095-Section 5.9.1 is incomplete, as it does not mention how to
 handle the padding octet in CRC calculations for IR and IR-DYN
 packets.  Padding isn't meant to be a meaningful part of a packet and
 is not included in the CRC calculation.  As a result, the CRC does
 not cover the Add-CID octet for CID 0, either.
 INCOMPLETE RFC 3095 TEXT (RFC 3095-Section 5.9.1):
    "The CRC in the IR and IR-DYN packet is calculated over the entire
     IR or IR-DYN packet, excluding Payload and including CID or any
     Add-CID octet."
 CORRECTED TEXT:
    "The CRC in the IR and IR-DYN packet is calculated over the entire
     IR or IR-DYN packet, excluding Payload, Padding and including CID
     or any Add-CID octet, except for the add-CID octet for CID 0."

Jonsson, et al. Standards Track [Page 4] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

2.3. CRC Coverage in CRC Feedback Options

 RFC 3095-Section 5.7.6.3 is incomplete, as it does not mention how
 the "size" field is handled when calculating the 8-bit CRC used in
 the CRC feedback option.  Since the "size" field is an extension of
 the "code" field, it must be treated in the same way.
 INCOMPLETE RFC 3095 TEXT (RFC 3095-Section 5.7.6.3):
    "The CRC option contains an 8-bit CRC computed over the entire
     feedback payload, without the packet type and code octet, but
     including any CID fields, using the polynomial of section 5.9.1."
 CORRECTED TEXT:
    "The CRC option contains an 8-bit CRC computed over the entire
     feedback payload including any CID fields but excluding the
     packet type, the 'Size' field and the 'Code' octet, using the
     polynomial of Section 5.9.1."

2.4. CRC Coverage of the ESP NULL Header

 RFC 3095-Section 5.8.7 gives the CRC coverage of the ESP NULL [13]
 header as "Entire ESP header".  This must be interpreted as including
 only the initial part of the header (i.e., Security Parameter Index
 (SPI) and sequence number), and not the trailer part at the end of
 the payload.  Therefore, the ESP NULL header has the same CRC
 coverage as the ESP header used in the ESP profile (RFC 3095-Section
 5.7.7.7).

3. Mode Transition

3.1. Feedback During Mode Transition to U- and O-Mode

 RFC 3095-Section 5.6.1 states that during mode transitions, while the
 D_TRANS parameter is I, the decompressor sends feedback for each
 received packet.  This restrictive behavior prevents the decompressor
 from using a sparse feedback algorithm during mode transitions.
 To reduce transmission overhead and computational complexity
 (including CRC calculation) associated with feedback packets sent for
 each decompressed packet during mode transition, a decompressor MAY
 be implemented with slightly modified mode transition procedures
 compared to those defined in [1], as described in this section.
 These enhanced procedures should be considered only as a possible
 improvement to a decompressor implementation, since interoperability
 is not affected in any way.  A decompressor implemented according to

Jonsson, et al. Standards Track [Page 5] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

 the optimized procedures will interoperate with an RFC 3095
 compressor, as well as a decompressor implemented according to the
 procedures described in RFC 3095.

3.1.1. Mode Transition Procedures Allowing Sparse Feedback

 The purpose of these enhanced transition procedures is to allow the
 decompressor to sparsely send feedback for packets decompressed
 during the second half of the transition procedure, i.e., after an
 appropriate IR/IR-DYN/UOR-2 packet has been received from the
 compressor.  This is achieved by allowing the decompressor transition
 parameter (D_TRANS) to be set to P (Pending) at that stage, as shown
 in the transition diagrams of Sections 3.1.2 and 3.1.3 below.
 This enhanced transition, where feedback need not be sent for every
 decompressed packet, does however introduce some considerations in
 case feedback messages would be lost.  Specifically, there is a risk
 for a deadlock situation when a transition from R-mode is performed;
 if no feedback message successfully reaches the compressor, the
 transition is never completed.  For transition between U-mode and
 O-mode, there is also a small risk for reduced compression
 efficiency.
 To avoid this, the decompressor MUST continue to send feedback at
 least periodically, as well as when in a Pending transition state.
 This is equivalent to enhancing the definition of the D_TRANS
 parameter in RFC 3095-Section 5.6.1, to include the definition of a
 Pending state:
  1. D_TRANS:

Possible values for the D_TRANS parameter are (I)NITIATED,

    (P)ENDING, and (D)ONE.  D_TRANS MUST be initialized to D, and a
    mode transition can be initiated only when D_TRANS is D.  While
    D_TRANS is I, the decompressor sends a NACK or ACK carrying a CRC
    option for each packet received.  When D_TRANS is set to P, the
    decompressor does not have to send a NACK or ACK for each packet
    received, but it MUST continue to send feedback with some
    periodicity, and all feedback packets sent MUST include the CRC
    option.  This ensures that all mode transitions will be completed
    also in case of feedback losses.
 The modifications affect transitions to Optimistic and Unidirectional
 modes of operation (i.e., the transitions described in RFC 3095-
 Section 5.6.5 and RFC 3095-Section 5.6.6) and make those transition
 diagrams more consistent with the diagram describing the transition
 to R-mode.

Jonsson, et al. Standards Track [Page 6] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

3.1.2. Transition from Reliable to Optimistic Mode

 The enhanced procedure for transition from Reliable to Optimistic
 mode is shown below:
           Compressor                     Decompressor
          ----------------------------------------------
                |                               |
                |        ACK(O)/NACK(O) +-<-<-<-| D_TRANS = I
                |       +-<-<-<-<-<-<-<-+       |
    C_TRANS = P |-<-<-<-+                       |
    C_MODE = O  |                               |
                |->->->-+ IR/IR-DYN/UOR-2(SN,O) |
                |       +->->->->->->->-+       |
                |->-..                  +->->->-| D_TRANS = P
                |->-..                          | D_MODE = O
                |           ACK(SN,O)   +-<-<-<-|
                |       +-<-<-<-<-<-<-<-+       |
    C_TRANS = D |-<-<-<-+                       |
                |                               |
                |->->->-+  UO-0, UO-1*          |
                |       +->->->->->->->-+       |
                |                       +->->->-| D_TRANS = D
                |                               |

Jonsson, et al. Standards Track [Page 7] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

3.1.3. Transition to Unidirectional Mode

 The enhanced procedure for transition to Unidirectional mode is shown
 on the following figure:
               Compressor                     Decompressor
              ----------------------------------------------
                |                               |
                |        ACK(U)/NACK(U) +-<-<-<-| D_TRANS = I
                |       +-<-<-<-<-<-<-<-+       |
    C_TRANS = P |-<-<-<-+                       |
    C_MODE = U  |                               |
                |->->->-+ IR/IR-DYN/UOR-2(SN,U) |
                |       +->->->->->->->-+       |
                |->-..                  +->->->-| D_TRANS = P
                |->-..                          |
                |           ACK(SN,U)   +-<-<-<-|
                |       +-<-<-<-<-<-<-<-+       |
    C_TRANS = D |-<-<-<-+                       |
                |                               |
                |->->->-+  UO-0, UO-1*          |
                |       +->->->->->->->-+       |
                |                       +->->->-| D_TRANS = D
                |                               | D_MODE= U

3.2. Feedback During Mode Transition

 RFC 3095-Section 5.6.1 states that feedback is always used during
 mode transitions.  However, the text then continues by making
 concrete applications of the rule in an inconsistent way, making it
 unclear when CRCs are used.  Further, the text does not define how
 the compressor should act during mode transitions based on feedback
 not protected by CRCs, i.e., whether or not to carry out mode
 transition actions.  The proper behavior from the compressor is to
 perform any action related to mode transitions only when the feedback
 is protected by the CRC option.
 INCOMPLETE RFC 3095 TEXT (RFC 3095-Section 5.6.1):
    "As a safeguard against residual errors, all feedback sent during
     a mode transition MUST be protected by a CRC, i.e., the CRC
     option MUST be used."
 CORRECTED TEXT:
     "As a safeguard against residual errors, all feedback sent by the
     decompressor during a mode transition MUST be protected by a CRC,
     i.e., the CRC option MUST be used.  The compressor MUST ignore

Jonsson, et al. Standards Track [Page 8] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

     feedback information related to mode transition if the feedback
     is not protected by the CRC option."
 One more related issue that requires clarifications comes from the
 following text at the end of RFC 3095-Section 5.6.1:
    "While D_TRANS is I, the decompressor sends a NACK or ACK carrying
     a CRC option for each received packet."
 However, RFC 3095-Section 5.5.2.2 already stated that for R-mode,
 feedback is never sent for packets that do not update the context,
 i.e., for packets that do not carry a CRC, such as R-0 and R-1*.
 This means that when D_TRANS=I during mode transition, a decompressor
 operating in R-mode sends an acknowledgement for each packet it
 receives and MUST use the sequence number that corresponds to the
 packet that last updated the context, i.e., the decompressor MUST NOT
 use the sequence number of the R-0 or the R-1* packet.

3.3. Packet Decoding During Mode Transition

 The purpose of a mode transition is to ensure that the compressor and
 the decompressor coherently move from one mode of operation to
 another using a three-way handshake.  At one point during the mode
 transition, the decompressor acknowledges the reception of one (or
 more) IR, IR-DYN or UOR-2 packet(s) that have mode bits set to the
 new mode.  Packets of type 0 or type 1 that are received up to this
 point are decompressed using the old mode, while afterwards they are
 decompressed using the new mode.  If the enhanced transition
 procedures described in Section 3.1 are used, the setting of the
 D_TRANS parameter to P represents this breakpoint.  The successful
 decompression of a packet of type 0 or type 1 completes the mode
 transition.

4. Timestamp Encoding

4.1. Encoding Used for Compressed TS Bits

 RTP Timestamp (TS) values are always encoded using W-LSB encoding,
 both when sent scaled and unscaled.  When no TS bits are transmitted
 in a compressed packet, TS is always scaled.  If a compressed packet
 carries an Extension 3 and field(Tsc)=0, the compressed packet must
 thus always carry unscaled TS bits.  For TS values sent in Extension
 3, W-LSB encoded values are sent using the self-describing variable-
 length format (RFC 3095-Section 4.5.6), and this applies to both
 scaled and unscaled values.

Jonsson, et al. Standards Track [Page 9] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

4.2. (De)compression of TS without Transmitted TS Bits

 When ROHC RTP operates using its most efficient packet types, apart
 from packet type identification and the error detection CRC, only RTP
 sequence number (SN) bits are transmitted in RTP compressed headers.
 All other fields are then omitted either because they are unchanged
 or because they can be reconstructed through a function from the SN
 (i.e., by combining the transmitted SN bits with state information
 from the context).  Fields that can be inferred from the SN are the
 IP Identification (IP-ID) and the RTP Timestamp (TS).
 IP-ID compression and decompression, both with and without
 transmitted IP-ID bits in the compressed header, are well defined in
 RFC 3095-Section 4.5.5 (see Section 8.2).  For the TS field, however,
 RFC 3095 only defines how to decompress based on actual TS bits in
 the compressed header, either scaled or unscaled, but not how to
 infer the TS from the SN when there are no TS bits present in the
 compressed header.
 When no TS bits are received in the compressed header, the scaled TS
 value is reconstructed assuming a linear extrapolation from the SN,
 i.e., delta_TS = delta_SN * default-slope, where delta_SN and
 delta_TS are both signed integers.  RFC 3095-Section 5.7 defines the
 potential values for default-slope.
 INCOMPLETE RFC 3095 TEXT (RFC 3095-Section 5.7):
    "If value(Tsc) = 1, Scaled RTP Timestamp encoding is used before
     compression (see section 4.5.3), and default-slope(TS) = 1.
     If value(Tsc) = 0, the Timestamp value is compressed as-is, and
     default-slope(TS) = value(TS_STRIDE)."
 CORRECTED TEXT:
    "When a compressed header with no TS bits is received, the scaled
     TS value is reconstructed assuming a linear extrapolation from
     the SN, i.e., delta_TS = delta_SN * default-slope(TS).
     If value(Tsc) = 1, Scaled RTP Timestamp encoding is used before
     compression (see Section 4.5.3), and default-slope(TS) = 1.
     If value(Tsc) = 0, the Timestamp value is compressed as-is, and
     default-slope(TS) = value(TS_STRIDE).  If a packet with no TS
     bits is received with Tsc = 0, the decompressor MUST discard the
     packet."

Jonsson, et al. Standards Track [Page 10] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

 INCORRECT AND INVALIDATED RFC 3095 TEXT (Section RFC 3095-5.5.1.2):
     "For example, in a typical case where the string pattern has the
     form of non-SN-field = SN * slope + offset, one ACK is enough if
     the slope has been previously established by the decompressor
     (i.e., only the new offset needs to be synchronized).  Otherwise,
     two ACKs are required since the decompressor needs two headers to
     learn both the new slope and the new offset."
 Consequently, there is no other slope value than the default-slope,
 as defined in RFC 3095-Section 5.7.

4.3. Interpretation Intervals for TS Encoding

 RFC 3095-Section 4.5.4 defines the interpretation interval, p, for
 timer-based compression of the RTP timestamp.  However, RFC 3095-
 Section 5.7 defines a different interpretation interval, which is
 defined as the interpretation interval to use for all TS values.  It
 is thus unclear which p-value to use, at least for timer-based
 compression.
 The way this should be interpreted is that the p-value differs
 depending on whether or not timer-based compression is enabled.
 For timer-based compression (TIME_STRIDE set to a non-zero value),
 the interpretation interval is:
    p = 2^(k-1) - 1 (as per RFC 3095-Section 4.5.4)
 Otherwise, the interpretation interval is:
    p = 2^(k-2) - 1 (as per RFC 3095-Section 5.7)

4.4. Scaled RTP Timestamp Encoding

 This section redefines the algorithm for scaled RTP timestamp
 encoding, defined as a 5-step procedure in RFC 3095-Section 4.5.3.
 Two formal errors have been corrected, as described in sub-sections
 4.4.1 and 4.4.2 below, and the whole algorithm has been reworked to
 be more concise and to use well-defined terminology.  The resulting
 text can be found in 4.4.3 below.

4.4.1. TS_STRIDE for Scaled Timestamp Encoding

 RFC 3095 defines the timestamp stride (TS_STRIDE) as the expected
 increase in the timestamp value between two RTP packets with
 consecutive sequence numbers.  TS_STRIDE is set by the compressor and

Jonsson, et al. Standards Track [Page 11] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

 explicitly communicated to the decompressor, and it is used as the
 scaling factor for scaled TS encoding.
 The relation between TS and TS_SCALED, given by the following
 equality in RFC 3095-Section 4.5.3, defines the mathematical meaning
 of TS_STRIDE:
    TS = TS_SCALED * TS_STRIDE + TS_OFFSET
 TS_SCALED is incompletely written as TS / TS_STRIDE in the
 compression step following the above core equality.  This formula is
 incorrect both because it excludes TS_OFFSET and because it would
 prevent a TS_STRIDE value of 0, which is an alternative not excluded
 by the definition or by the core equality above.  If "/" were a
 generally unambiguously defined operation meaning "the integral part
 of the result from dividing X by Y", the absence of TS_OFFSET could
 be explained, but the formula would still lack a proper output for
 TS_STRIDE equal to 0.  The formula of "2. Compression" is thus valid
 only with the following requirements:
   a) "/" means "the integral part of the result from dividing X by Y"
   b) TS_STRIDE>0 (TS is never sent scaled when TS_STRIDE=0)

4.4.2. TS Wraparound with Scaled Timestamp Encoding

 RFC 3095-Section 4.5.3 states in points 4 and 5 that the compressor
 is not required to initialize TS_OFFSET at wraparound, but that it is
 required to increase the number of bits sent for the scaled TS value
 when there is a TS wraparound.  The decompressor is also required to
 detect and cope with TS wraparound, including updating TS_OFFSET.
 This method is not interoperable and not robust.  The gain is also
 insignificant, as TS wraparound happens very seldomly.  Therefore,
 the compressor should reinitialize TS_OFFSET upon TS wraparound, by
 sending an unscaled TS.

4.4.3. Algorithm for Scaled Timestamp Encoding

 INCORRECT RFC 3095 TEXT (RFC 3095-Section 4.5.3):
   "1. Initialization: The compressor sends to the decompressor the
       value of TS_STRIDE and the absolute value of one or several TS
       fields.  The latter are used by the decompressor to initialize
       TS_OFFSET to (absolute value) modulo TS_STRIDE.  Note that
       TS_OFFSET is the same regardless of which absolute value is
       used, as long as the unscaled TS value does not wrap around;
       see 4) below.

Jonsson, et al. Standards Track [Page 12] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

    2. Compression: After initialization, the compressor no longer
       compresses the original TS values.  Instead, it compresses the
       downscaled values: TS_SCALED = TS / TS_STRIDE.  The compression
       method could be either W-LSB encoding or the timer-based
       encoding described in the next section.
    3. Decompression: When receiving the compressed value of
       TS_SCALED, the decompressor first derives the value of the
       original TS_SCALED.  The original RTP TS is then calculated as
       TS = TS_SCALED * TS_STRIDE + TS_OFFSET.
    4. Offset at wraparound: Wraparound of the unscaled 32-bit TS will
       invalidate the current value of TS_OFFSET used in the equation
       above.  For example, let us assume TS_STRIDE = 160 = 0xA0 and
       the current TS = 0xFFFFFFF0.  TS_OFFSET is then 0x50 = 80.
       Then if the next RTP TS = 0x00000130 (i.e., the increment is
       160 * 2 = 320), the new TS_OFFSET should be 0x00000130 modulo
       0xA0 = 0x90 = 144.  The compressor is not required to re-
       initialize TS_OFFSET at wraparound.  Instead, the decompressor
       MUST detect wraparound of the unscaled TS (which is trivial)
       and update TS_OFFSET to TS_OFFSET = (Wrapped around unscaled
       TS) modulo TS_STRIDE"
 CORRECTED TEXT:
   "1. Initialization and updating of RTP TS scaling function:  The
       compressor sends to the decompressor the value of TS_STRIDE
       along with an unscaled TS.  These are both needed by the
       decompressor to initialize TS_OFFSET as hdr(TS) modulo
       field(TS_STRIDE).  Note that TS_OFFSET is the same for any TS
       as long as TS_STRIDE does not change and as long as the
       unscaled TS value does not wrap around; see 4) below.
    2. Compression: After initialization, the compressor no longer
       compresses the unscaled TS values.  Instead, it compresses the
       scaled values.  The compression method can be either W-LSB
       encoding or timer-based encoding.
    3. Decompression: When receiving a (compressed) TS_SCALED, the
       field is first decompressed, and the unscaled RTP TS is then
       calculated as TS = TS_SCALED * TS_STRIDE + TS_OFFSET.
    4. Offset at wraparound: If the value of TS_STRIDE is not equal to
       a power of two, wraparound of the unscaled 32-bit TS will
       change the value of TS_OFFSET.  When this happens, the
       compressor SHOULD reinitialize TS_OFFSET by sending unscaled
       TS, as in 1 above."

Jonsson, et al. Standards Track [Page 13] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

 INCORRECT AND INVALIDATED RFC 3095 TEXT (RFC 3095-Section 4.5.3):
    The entire point 5, i.e. the entire text starting from "5.
    Interpretation interval at wraparound ...", down to and including
    the block of text that starts with "Let a be the number of LSBs"
    and that ends with "...interpretation interval is b." is incorrect
    and is thus invalid.

4.5. Recalculating TS_OFFSET

 TS can be sent unscaled if the TS value change does not match the
 established TS_STRIDE, but the TS_STRIDE might still stay unchanged.
 To ensure correct decompression of subsequent packets, the
 decompressor MUST therefore always recalculate TS_OFFSET (RTP TS
 modulo TS_STRIDE) when a packet with an unscaled TS value is
 received.

4.6. TS_STRIDE and the Tsc Flag in Extension 3

 The Tsc flag in Extension 3 indicates whether or not TS is scaled.
 The value of the Tsc flag thus applies to all TS bits, as well as if
 there are no TS bits in the extension itself.  When TS is scaled, it
 is always scaled using context(TS_STRIDE).  The legend for Extension
 3 in RFC 3095-Section 5.7.5 incorrectly states that value(TS_STRIDE)
 is used for scaled TS.
 If TS_STRIDE is present in Extension 3, as indicated by the Tss flag
 being set, the compressed header SHOULD carry unscaled TS bits; i.e.,
 the Tsc flag SHOULD NOT be set when Tss is set since an unscaled TS
 is needed together with TS_STRIDE to recalculate the TS_OFFSET.  If
 TS_STRIDE is included in a compressed header with scaled TS, the
 decompressor must ignore and discard field(TS_STRIDE).
 INCORRECT RFC 3095 TEXT (RFC 3095-Section 5.7.5):
    "Tsc: Tsc = 0 indicates that TS is not scaled;
          Tsc = 1 indicates that TS is scaled according to section
           4.5.3, using value(TS_STRIDE).
           Context(Tsc) is always 1.  If scaling is not desired, the
           compressor will establish TS_STRIDE = 1."
 CORRECTED TEXT:
    "Tsc: Tsc = 0 indicates that TS is not scaled;
          Tsc = 1 indicates that TS is scaled according to Section
          4.5.3, using context(TS_STRIDE).

Jonsson, et al. Standards Track [Page 14] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

          Context(Tsc) is always 1.  If scaling is not desired, the
          compressor will establish TS_STRIDE = 1.
          If field(Tsc) = 1, and if TSS = 1 (meaning that TS_STRIDE is
          present in the extension), field(TS_STRIDE) MUST be ignored
          and discarded."
 When the compressor re-establishes a new value for TS_STRIDE using
 Extension 3, it should send unscaled TS bits together with TS_STRIDE.

4.7. Using Timer-Based Compression

 Timer-based compression of the RTP timestamp, as described in RFC
 3095-Section 4.5.4, may be used to reduce the number of transmitted
 timestamp bits (bytes) needed when the timestamp cannot be inferred
 from the SN.  Timer-based compression is only used for decompression
 of compressed headers that contains a TS field; otherwise, when no
 timestamp bits are present, the timestamp is linearly inferred from
 the SN (see Section 4.2 of this document).
 Whether or not to use timer-based compression is controlled by the
 TIME_STRIDE control field, which can be set by either an IR, an IR-
 DYN, or a compressed packet with Extension 3.  Before timer-based
 compression can be used, the decompressor has to inform the
 compressor (on a per-channel basis) about its clock resolution by
 sending a CLOCK feedback option for any CID on the channel.  The
 compressor can then initiate timer-based compression by sending (on a
 per-context basis) a non-zero TIME_STRIDE to the decompressor.  When
 the compressor is confident that the decompressor has received the
 TIME_STRIDE value, it can switch to timer-based compression.

5. List Compression

5.1. CSRC List Items in RTP Dynamic Chain

 RFC 3095-Section 5.7.7.6 defines the static and dynamic parts of the
 RTP header.  This section indicates a 'Generic CSRC list' field in
 the dynamic chain, which has a variable length (see RFC 3095-Section
 5.8.6).  This field is always at least one octet in size, even if the
 list is empty (as opposed to the CSRC list in the uncompressed RTP
 header, which is not present when the RTP CC field is set to 0).

5.2. Multiple Occurrences of the CC Field

 The static and the dynamic parts of the RTP header are defined in RFC
 3095-Section 5.7.7.6.  In the dynamic part, a CC field indicates the
 number of CSRC items present in the 'Generic CSRC list'.  Another CC
 field also appears within the 'Generic CSRC list' (RFC 3095-Section

Jonsson, et al. Standards Track [Page 15] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

 5.8.6.1), because Encoding Type 0 is always used in the dynamic
 chain.  Both CC fields have the same meaning: the value of the CC
 field determines the number of XI items in the CSRC list for Encoding
 Type 0, and it is not used otherwise.  Therefore, the following
 applies:
 FORMAL ADDITION TO RFC 3095:
    "The first octet in the dynamic part of the RTP header contains a
     CC field, as defined in Section 5.7.7.6.  A second occurrence
     appears in the 'Generic CSRC list', which is also in the dynamic
     part of the RTP header, where Encoding Type 0 is used according
     to the format defined in RFC 3095-5.8.6.1.
     The compressor MUST set both occurrences of the CC field to the
     same value.
     The decompressor MUST use the value of the CC field from the
     Encoding Type 0 within the Generic CRSC list, and it MUST thus
     ignore the first occurrence of the CC field."

5.3. Bit Masks in List Compression

 The insertion and/or removal schemes, described in RFC 3095-Sections
 5.8.6.2 - 5.8.6.4, use bit masks to indicates insertion or removal
 positions within the reference list.  The size of the bit mask can be
 7 bits or 15 bits.
 The compressor MAY use a 7-bit mask, even if the reference list has
 more than seven items, provided that changes to the list are only
 applied to items within the first seven items of the reference list,
 leaving items with an index not covered by the 7-bit mask unchanged.
 The decompressor MUST NOT modify items with an index not covered by
 the 7-bit mask, when a 7-bit mask is received for a reference list
 that contains more than seven items.

5.4. Headers Compressed with List Compression

 In RFC 3095-Section 5.8, it states that headers that can be part of
 extension header chains "include" AH [14], ESP NULL [13], minimal
 encapsulation (MINE) [15], GRE [16][17], and IPv6 [9] extensions.
 This list of headers that can be compressed is correct, but the word
 "include" should not be there, since only the header types listed can
 actually be handled.  It should further be noted that for the Minimal
 Encapsulation (MINE) header, there is no explicit discussion of how
 to compress it, as the header is sent either uncompressed or fully
 compressed away.

Jonsson, et al. Standards Track [Page 16] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

5.5. ESP NULL Header List Compression

 Due to the offset of the fields in the trailer part of the ESP
 header, a compressor MUST NOT compress packets containing more than
 one NULL ESP [13] header, unless the second-outermost header is
 treated as a regular ESP [12] header and the packets are compressed
 using profile 0x0003.

5.6. Translation Tables and Indexes for IP Extension Headers

 RFC 3095-Section 5.8.4 describes how list indexes are associated to
 list items and how table lists are built for IP extension headers.
 The text incorrectly states that one index per type is used, since
 the same type can appear several times with different content in one
 single chain.
 In IP extension header list compression, an index is associated with
 each individual extension header of an extension header chain.  When
 there are multiple non-identical occurrences of the same extension
 type (Protocol Number) within a header chain, each MUST be given its
 own index.
 In the case where there are multiple identical occurrences of the
 same extension type, the compressor can associate them to the same
 index.  When the value of an item whose index occurs more than once
 in the list is updated, the compressor MUST send the value for each
 occurrence of that index in the list.
 When content of extension headers changes, an implementation can
 choose to either use a different index or update the existing one.
 Some extensions can be compressed away even when some fields change,
 as those changes can be conveyed to the decompressor implicitly (e.g.
 sequence numbers in extension headers that can be inferred from the
 RTP SN) or explicitly (e.g., as part of the 'IP extension header(s)'
 field in Extension 3).
 When there is more than one IP header, there is more than one list of
 extension headers, and a translation table is maintained for each
 list independently of one another.

5.7. Reference List

 A list compressed using encoding type 1 (insertion), type 2
 (removal), or type 3 (removal/insertion) uses a coding scheme that is
 based on the use of a reference list in the context (identified as
 ref_id).

Jonsson, et al. Standards Track [Page 17] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

 While it could seem to be a fair choice to send a type 1 list when
 ref_id is an empty list, there is nothing gained in doing so with
 respect to using a type 0 list.  Sending a type 2 list when ref_id is
 an empty list would lead to a failure, while sending a type 3 list
 has very little meaning.  All these alternatives could be seen as
 possible, based on how list compression is specified in RFC 3095.
 If these alternatives were allowed, a decompressor would become
 required to maintain a sliding window of ref_id lists in R-mode, even
 for the case where no items are sent in the compressed list, and this
 is not a desirable requirement.  Using list encoding type 1, type 2,
 and type 3 is therefore only allowed for non-empty reference lists.
 FORMAL ADDITION TO RFC 3095:
    "Regardless of the operating mode, for list encoding of type 1,
     type 2, and type 3 lists, ref_id MUST refer to a non-empty list."

5.8. Compression of AH and GRE Sequence Numbers

 RFC 3095-Section 5.8.4.2 and RFC 3095-Section 5.8.4.4 describe how to
 compress the Authentication Header (AH) [14] and the Generic Routing
 Encapsulation (GRE) [16][17] header.  Both these sections present a
 possibility to omit the AH/GRE sequence number in the compressed
 header, under certain circumstances.  However, the specific
 conditions for omitting the AH/GRE sequence number, as well as the
 concrete compression and decompression procedures to apply, are not
 clearly defined to guarantee robustness and facilitate interoperable
 implementation.
 Proper rules are provided for the ESP case, i.e.,:
    "Sequence Number: Not sent when the offset from the sequence
     number of the compressed header is constant, when the compressor
     has confidence that the decompressor has established the correct
     offset.  When the offset is not constant, the sequence number may
     be compressed by sending LSBs"
 The same logic applies to the AH/GRE sequence numbers.
 INCORRECT RFC 3095 TEXT (RFC 3095-Section 5.8.4.2):
    "If the sequence number in the AH linearly increases as the RTP
     Sequence Number increases, and the compressor is confident that
     the decompressor has obtained the pattern, the sequence number in
     AH need not be sent.  The decompressor applies linear
     extrapolation to reconstruct the sequence number in the AH."

Jonsson, et al. Standards Track [Page 18] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

 CORRECTED TEXT:
    "The AH sequence number can be omitted from the compressed header
     when the offset from the sequence number (SN) of the compressed
     header is constant, when the compressor has confidence that the
     decompressor has established the correct offset."
 INCORRECT RFC 3095 TEXT (RFC 3095-Section 5.8.4.4):
    "If the sequence number in the GRE header linearly increases as
     the RTP Sequence Number increases and the compressor is confident
     that the decompressor has received the pattern, the sequence
     number in GRE need not be sent.  The decompressor applies linear
     extrapolation to reconstruct the sequence number in the GRE
     header."
 CORRECTED TEXT:
    "The GRE sequence number can be omitted from the compressed header
     when the offset from the sequence number (SN) of the compressed
     header is constant, when the compressor has confidence that the
     decompressor has established the correct offset."

6. Updating Properties

6.1. Implicit Updates

 A context updating packet that contains compressed sequence number
 information may also carry information about other fields; in such
 cases, these fields are updated according to the content of the
 packet.  The updating packet also implicitly updates inferred fields
 (e.g., RTP Timestamp) according to the current mode and the
 appropriate mapping function of the updated and inferred fields.
 An updating packet thus updates the reference values of all header
 fields, either explicitly or implicitly, except for the UO-1-ID
 packet (see Section 6.2 of this document).  In UO-mode, all packets
 are updating packets, while in R-mode, all packets with a CRC are
 updating packets.
 For example, a UO-0 packet contains the compressed RTP sequence
 number (SN).  Such a packet also implicitly updates RTP timestamp,
 IPv4 ID, and sequence numbers of IP extension headers.

Jonsson, et al. Standards Track [Page 19] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

6.2. Updating Properties of UO-1*

 RFC 3095-Section 5.7.3 states that the values provided in extensions
 carried by a UO-1-ID packet do not update the context, except for SN,
 TS, or IP-ID fields.  However, RFC 3095-Section 5.8.1 correctly
 states that the translation table in the context is updated whenever
 an (Index, item) pair is received, something that is contradicted by
 the statement in RFC 3095-5.7.3 because the UO-1-ID packet can carry
 Extension 3 with (Index, item) pair items within the 'Compressed CSRC
 list' field.  In addition to this contradiction, the text does not
 mention what to do with the other sequence numbers inferred from the
 SN, which are also to be implicitly updated.  The updating properties
 of UO-1* as stated by RFC 3095-Section 5.7.3 are thus incomplete.
 INCOMPLETE RFC 3095 TEXT (RFC 3095-Section 5.7.3):
    "Values provided in extensions, except those in other SN, TS, or
     IP-ID fields, do not update the context."
 CORRECTED TEXT:
    "UO-1-ID packets only updates TS, SN, IP-ID, and sequence numbers
     of IP extension headers.  Other values provided in extensions do
     not update the context.
     The decompressor MUST update its translation table whenever an
     (Index, item) pair is received, as per RFC 3095-Section 5.8.1,
     and this rule applies also to UO-1-ID packets."

6.3. Context Updating Properties for IR Packets

 IR packets do not clear the whole context, but update all fields
 carried in the IR header.  Similarly, an IR without a dynamic chain
 simply updates the static part of the context, while the rest of the
 context is left unchanged.
 A consequence of this is that fields that are not updated by the IR
 packet, e.g., the translation tables for list compression, MUST NOT
 be invalidated by the decompressor when it assumes context damage.

6.4. RTP Padding Field (R-P) in Extension 3

 RFC 3095-Section 5.7.5 defines the properties of RTP header flags and
 fields in Extension 3.  These get updated when the rtp flag of the
 Extension 3 is set, i.e., when rtp = 1; otherwise, they are not
 updated.  However, it is unclear how Extension 3 updates the R-P bit
 in the context.

Jonsson, et al. Standards Track [Page 20] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

 INCOMPLETE RFC 3095 TEXT (RFC 3095-Section 5.7.5):
    "R-P: RTP Padding bit, absolute value (presumed zero if absent)."
 CORRECTED TEXT:
    "R-P: RTP Padding bit.  If R-PT = 1, R-P is the absolute value of
          the RTP padding bit and this value updates context(R-P).  If
          R-PT = 0, context(R-P) is updated to zero."

6.5. RTP eXtension bit (X) in dynamic part

 RFC 3095-Section 5.7.7.6 defines the properties of the RTP header
 flags and fields in the RTP part of the dynamic chain of IR and IR-
 DYN packets.  However, it is unclear how the X bit is updated in the
 context.
 INCOMPLETE RFC 3095 TEXT (RFC 3095-Section 5.7.7.6):
    "X: Copy of X bit from RTP header (presumed 0 if RX = 0)"
 CORRECTED TEXT:
    "X: X bit from RTP header.  If RX = 1, X is the X bit from the RTP
        header and this value updates context(X).  If RX = 0,
        context(X) is updated to zero."

7. Context management and CID/context Reuse

7.1. Persistence of Decompressor Contexts

 As part of the negotiated channel parameters, compressor and
 decompressor have, through the MAX_CID parameter, agreed on the
 highest context identification (CID) number to be used.  By agreeing
 on 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 MUST be kept by the decompressor
 until either the CID gets reused, or the channel is taken down or
 renegotiated.

7.2. CID/Context Reuse

 As part of the channel negotiation, the maximal number of active
 contexts supported is negotiated between the compressor and the
 decompressor through the MAX_CID parameter.  The value of MAX_CID can
 differ significantly from one link application to another, as well as
 the load in terms of the number of packet streams to compress.  The
 lifetime of a ROHC channel can also vary, from almost permanent to

Jonsson, et al. Standards Track [Page 21] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

 rather short-lived.  However, in general, it is not expected that
 resources will be allocated for more contexts than what can
 reasonably be expected to be active concurrently over the link.  As a
 consequence hereof, context identifiers (CIDs) and context memory are
 resources that will have to be reused by the compressor as part of
 what can be considered normal operation.
 How context resources are reused is left unspecified in RFC 3095 [1]
 and subsequent 3095-based ROHC specifications.  This document does
 not intend to change that, i.e., ROHC resource management is still
 considered an implementation detail.  However, reusing a CID and its
 allocated memory is not always as simple as initiating a context with
 a previously unused CID.  Because some profiles can be operating in
 various modes where packet formats vary depending on current mode,
 care has to be taken to ensure that the old context data will be
 completely and safely overwritten, eliminating the risk of undesired
 side effects from interactions between old and new context data.
 This document therefore points out some important core aspects to
 consider when implementing resource management in ROHC compressors
 and decompressors.
 On a high level, CID/context reuse can be of two kinds, either reuse
 for a new context based on the same profile as the old context, or
 for a new context based on a different profile.  These cases are
 discussed separately in the following two sub-sections.

7.2.1. Reusing a CID/Context with the Same Profile

 For multi-mode profiles, such as those defined in RFC 3095 [1], mode
 transitions are performed using a decompressor-initiated handshake
 procedure, as defined in RFC 3095-Section 5.6.  When a CID/context is
 reused for a new context based on the same profile as the old
 context, the current mode of operation SHOULD be inherited from the
 old to the new context.  Specifically, the compressor SHOULD continue
 to operate using the mode of operation of the old context also with
 the new context.  The reason for this is that there is no reliable
 way for the compressor to inform the decompressor that a CID/context
 reuse is happening.  The decompressor can thus not be expected to
 clear the context memory for the CID (see Section 6.3), and there is
 no way to trigger a safe mode switching (which requires the
 decompressor-initiated handshake procedure).
 The rule of mode inheritance applies also when the
 CONTEXT_REINITIALIZATION signal (RFC 3095-Section 6.3.1) is used to
 reinitiate an entire context.

Jonsson, et al. Standards Track [Page 22] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

7.2.2. Reusing a CID/Context with a Different Profile

 When a CID is reused for a new context based on a different profile
 than the old context, both the compressor and the decompressor MUST
 start operation with that context in the initial mode of the profile
 (if it is a multi-mode profile).  This applies both to IR-initiated
 new contexts and profile downgrades with IR-DYN (e.g., the profile
 0x0001 -> profile 0x0002 downgrade in RFC 3095-Section 5.11.1).
 Type 0 and type 1 packets have different formats in U/O- and R-mode,
 and these R-mode packets have no CRC.  When initiating a new context
 on a reused R-mode CID, there is a risk that the decompressor will
 misinterpret compressed packets if the initiating IR packets are
 lost.
 A CID for a context currently operating in R-mode SHOULD therefore
 not be reused for a new context based on a different profile than the
 old context.  A compressor doing otherwise should minimize the risk
 for misinterpretation of R-0/R-1 by, e.g., not using packets of types
 beginning with 00 or 10 before it is highly confident that the new
 context has successfully been initiated at the decompressor.

8. Other Protocol Clarifications

8.1. Meaning of NBO

 In IPv4 dynamic part (RFC 3095-Section 5.7.7.4), if the 'NBO' bit is
 set, it means that network byte order is used.

8.2. IP-ID

 According to RFC 3095-Section 5.7, IP-ID means the compressed value
 of the IPv4 header's 'Identification' field.  Compressed packets
 contain this compressed value (IP-ID), while IR packets with dynamic
 chain and IR-DYN packets transmit the original, uncompressed
 Identification field value.  The IP-ID field always represents the
 Identification value of the innermost IPv4 header whose corresponding
 RND flag is not 1.
 If RND or RND2 is set to 1, the corresponding IP-ID(s) is (are) sent
 as 16-bit uncompressed Identification value(s) at the end of the
 compressed base header, according to the IP-ID description (see the
 beginning of RFC 3095-Section 5.7).  When there is no compressed IP-
 ID, i.e., for IPv6 or when all IP Identification information is sent
 as is (as indicated by RND/RND2 being set to 1), the decompressor
 ignores IP-ID bits sent within compressed base headers.

Jonsson, et al. Standards Track [Page 23] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

 When RND=RND2=0, IP-ID is compressed, i.e., expressed as an SN offset
 and byte-swapped if NBO=0.  This is the case also when 16 bits of
 IP-ID is sent in Extension 3.
 When RND=0 but no IP-ID bits are sent in the compressed header, the
 SN offset for IP-ID stays unchanged, meaning that Offset_m equals
 Offset_ref, as described in Section 4.5.5.  This is further expressed
 in a slightly different way (with the same meaning) in Section 5.7,
 where it is said that "default-slope(IP-ID offset) = 0", meaning, if
 no bits are sent for IP-ID, its SN offset slope defaults to 0.

8.3. Extension-3 in UOR-2* Packets

 Some flags of the IP header in the extension (e.g., NBO or RND) may
 change the interpretation of fields in UOR-2* packets.  In such
 cases, when a flag changes in Extension 3, a decompressor MUST re-
 parse the UOR-2* packet.

8.4. Multiple Occurrences of the M Bit

 The RTP header part of Extension 3, as defined by RFC 3095-Section
 5.7.5, includes a one-bit field for the RTP Marker bit.  This field
 is also present in all compressed base header formats except for UO-
 1-ID; meaning, there may be two occurrences of the field within one
 single compressed header.  In such cases, the two M fields must have
 the same value.
 FORMAL ADDITION TO RFC 3095:
    "When there are two occurrences of the M field in a compressed
     header (both in the compressed base header and in the RTP part of
     Extension 3), the compressor MUST set both these occurrences of
     the M field to the same value.
     At the decompressor, if the two M field values of such a packet
     are not identical, the packet MUST be discarded."

8.5. Multiple SN options in one feedback packet

 The length of the sequence number field in the original ESP [12]
 header is 32 bits.  The format of the SN feedback option (RFC 3095-
 Section 5.7.6.6) allows for 8 additional SN bits to the 12 SN bits of
 the FEEDBACK-2 format (RFC 3095-Section 5.7.6.1).  One single SN
 feedback option is thus not enough for the decompressor to send back
 all the 32 bits of the ESP sequence number in a feedback packet,
 unless it uses multiple SN options in one feedback packet.

Jonsson, et al. Standards Track [Page 24] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

 RFC 3095-Section 5.7.6.1 declares that a FEEDBACK-2 packet can
 contain a variable number of feedback options, and the options can
 appear in any order.
 When processing multiple SN options in one feedback packet, the SN
 would be given by concatenating the fields.

8.6. Multiple CRC Options in One Feedback Packet

 Although it is not useful to have more than one single CRC option in
 a feedback packet, having multiple CRC options is still allowed.  If
 multiple CRC options are included, all such CRC options MUST be
 identical, as they will be calculated over the same header; the
 compressor MUST otherwise discard the feedback packet.

8.7. Responding to Lost Feedback Links

 Although this is neither desirable or expected, it may happen that a
 link used to carry feedback between two associated instances becomes
 unavailable.  If the compressor can be notified of such an event, the
 compressor SHOULD restart compression for each flow that is operating
 in R-mode.  When restarting compression, the compressor SHOULD use a
 different CID for each flow being restarted; this is useful to avoid
 the possibility of misinterpreting the type of the compressed header
 for the packet type identifiers that are common to both U/O-mode and
 R-mode, when the flow is restarted in U-mode (see also Section 7.2).
 Generally, feedback links are not expected to disappear once present,
 but it should be noted that this might be the case for certain link
 technologies.

8.8. UOR-2 in Profile 0x0002 (UDP) and Profile 0x0003 (ESP)

 One single new format is defined for UOR-2 in profile 0x0002 and
 profile 0x0003, which replaces all three (UOR-2, UOR-2-ID, UOR-2-TS)
 formats from profile 0x0001.  The same UOR-2 format is thus used
 independent of whether or not there are IP headers with a
 corresponding RND=1.  This also applies to the IP profile [4] and the
 IP/UDP-Lite profile [5].

8.9. Sequence Number LSB's in IP Extension Headers

 In RFC 3095-Section 5.8.5, formats are defined for compression of IP
 extension header fields.  These include compressed sequence number
 fields, and these fields contain the "LSB of sequence number".  These
 sequence numbers are not "LSB-encoded" as, e.g., the RTP sequence
 number, but are the LSB's of the uncompressed fields.

Jonsson, et al. Standards Track [Page 25] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

8.10. Expecting UOR-2 ACKs in O-Mode

 Usage of UOR-2 ACKs in O-mode, as discussed in RFC 3095-Section
 5.4.1.1.2, is optional.  A decompressor can also send ACKs for
 purposes other than to acknowledge the UOR-2, without having to
 continue sending ACKs for all UOR-2.  Similarly, a compressor
 implementation can ignore UOR-2s ACKs for the purpose of adapting the
 optimistic approach strategies.
 It is thus NOT RECOMMENDED to use the optional ACK mechanism in O-
 mode, either in compressor or in decompressor implementations.
 Using an incorrect expectation on UOR-2 ACKs as a basis for
 compressor behavior will significantly degrade the compression
 performance.  This is because UOR-2 ACKs can be sent from a
 decompressor for other purposes than to acknowledge the UOR-2 packet,
 e.g., to send feedback such as clock resolution, or to initiate a
 mode transition.  If an implementation does use the optional
 acknowledgment algorithm described in Section 5.4.1.1.2, it must make
 sure to set the k_3 and n_3 parameters to much larger values than 1
 to ensure that the compressor performance is not degraded due to the
 problem described above.

8.11. Context Repairs, TS_STRIDE and TIME_STRIDE

 The 7-bit CRC used to verify the outcome of the decompression attempt
 covers the original uncompressed header.  The CRC verification thus
 excludes TS_STRIDE and TIME_STRIDE, as these fields are not part of
 the original uncompressed header.
 The UOR-2 packet type can be used to update the value of the
 TS_STRIDE and/or the TIME_STRIDE, with the Extension 3.  However,
 these fields are not used for decompression of the RTP TS field for
 this packet type and their respective value is thus not verified,
 either implicitly or explicitly.
 When the compressor receives a negative acknowledgement, it thus
 cannot determine whether the failure may be caused by an unsuccessful
 update to the TS_STRIDE and/or the TIME_STRIDE field(s), for which a
 previous header that last attempted to update their value had
 previously been acknowledged.
 FORMAL ADDITION TO RFC 3095:
    "When the compressor receives a NACK and uses the UOR-2 header
     type to repair the decompressor context, it SHOULD include fields
     that update the value of both the TS_STRIDE and the TIME_STRIDE
     whose value it has updated at least once since the establishment

Jonsson, et al. Standards Track [Page 26] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

     of that context, i.e., since the CID was first associated with
     its current profile.
     When the compressor receives a static-NACK, it MUST include in
     the IR header fields for both the TS_STRIDE and the TIME_STRIDE
     whose value it has updated at least once since the establishment
     of that context, i.e., since the CID was first associated with
     its current profile."

9. ROHC Negotiation

 RFC 3095-Section 4.1 states that the link layer must provide means to
 negotiate, e.g., the channel parameters listed in RFC 3095-Section
 5.1.1.  One of these parameters is the PROFILES parameter, which is a
 set of non-negative integers where each integer indicates a profile
 supported by the decompressor.
 Each 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 (see RFC 3095-Section 8).  In the ROHC headers sent
 over the link, the profile used is identified only with the 8 LSB
 bits, which means that the compressor and decompressor must have
 agreed on which variant to use for each profile.
 The negotiation protocol must thus be able to communicate to the
 compressor the set of profiles supported by the decompressor.  When
 multiple variants of the same profile are available, the negotiation
 protocol must provide the means for the decompressor to know which
 variant will be used by the compressor.  This basically means that
 the PROFILES set after negotiation MUST NOT include more than one
 variant of a profile.

10. PROFILES Sub-option in ROHC-over-PPP

 The logical union of sub-options for IPCP and IPV6CP negotiations, as
 specified by ROHC over PPP [2], cannot be used for the PROFILES
 suboption, as the whole union would then have to be considered within
 each of the two IPCP negotiations to avoid getting an ambiguous
 profile set.  An implementation of RFC 3241 MUST therefore ensure
 that the same profile set is negotiated for both IPv4 and IPv6
 (IPCP/IPV6CP).

11. Constant IP-ID Encoding in IP-only and UPD-Lite Profiles

 In the ROHC IP-only profile, Section 3.3 of RFC 3843 [4], a mechanism
 for encoding of a constant Identification value in IPv4 (constant
 IP-ID) is defined.  This mechanism is also used by the ROHC UDP-Lite
 profiles, RFC 4019 [5].

Jonsson, et al. Standards Track [Page 27] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

 The "Constant IP-ID" mechanism applies to both the inner and outer IP
 header, when present, meaning that there will be both a SID and a
 SID2 context value.

12. Security Considerations

 This document provides a number of corrections and clarifications to
 [1], but it does not make any changes with regard to the security
 aspects of the protocol.  As a consequence, the security
 considerations of [1] apply without additions.

13. Acknowledgments

 The authors would like to thank Vicknesan Ayadurai, Carsten Bormann,
 Mikael Degermark, Zhigang Liu, Abigail Surtees, Mark West, Tommy
 Lundemo, Alan Kennington, Remi Pelland, Lajos Zaccomer, Endre Szalai,
 Mark Kalmanczhelyi, and Arpad Szakacs for their contributions and
 comments.  Thanks also to the committed document reviewers, Carl
 Knutsson and Biplab Sarkar, who reviewed the document during working
 group last-call.

14. References

14.1. Normative References

 [1]  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.
 [2]  Bormann, C., "Robust Header Compression (ROHC) over PPP", RFC
      3241, April 2002.
 [3]  Simpson, W., "PPP in HDLC-like Framing", STD 51, RFC 1662, July
      1994.
 [4]  Jonsson, L-E. and G. Pelletier, "RObust Header Compression
      (ROHC): A Compression Profile for IP", RFC 3843, June 2004.
 [5]  Pelletier, G., "RObust Header Compression (ROHC): Profiles for
      User Datagram Protocol (UDP) Lite", RFC 4019, April 2005.
 [6]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.

Jonsson, et al. Standards Track [Page 28] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

14.2. Informative References

 [7]  Jonsson, L-E., Pelletier, G., and K. Sandlund, "RObust Header
      Compression (ROHC): A Link-Layer Assisted Profile for
      IP/UDP/RTP", RFC 4362, January 2006.
 [8]  Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
 [9]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
      Specification", RFC 2460, December 1998.
 [10] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
      1980.
 [11] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
      "RTP: A Transport Protocol for Real-Time Applications", STD 64,
      RFC 3550, July 2003.
 [12] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
      December 2005.
 [13] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and Its
      Use With IPsec", RFC 2410, November 1998.
 [14] Kent, S., "IP Authentication Header", RFC 4302, December 2005.
 [15] Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
      October 1996.
 [16] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,
      "Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.
 [17] Dommety, G., "Key and Sequence Number Extensions to GRE", RFC
      2890, September 2000.

Jonsson, et al. Standards Track [Page 29] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

Appendix A. Sample 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 CRCs 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 30] RFC 4815 Corrections and Clarifications to RFC 3095 February 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 that this is complemented like for PPP
 # (this differs from 8-, 7-, and 3-bit CRCs)
 #
 #      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 31] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

Authors' Addresses

 Lars-Erik Jonsson
 Optand 737
 SE-831 92 Ostersund, Sweden
 Phone: +46 70 365 20 58
 EMail: lars-erik@lejonsson.com
 Kristofer Sandlund
 Ericsson AB
 Box 920
 SE-971 28 Lulea, Sweden
 Phone: +46 8 404 41 58
 EMail: kristofer.sandlund@ericsson.com
 Ghyslain Pelletier
 Ericsson AB
 Box 920
 SE-971 28 Lulea, Sweden
 Phone: +46 8 404 29 43
 EMail: ghyslain.pelletier@ericsson.com
 Peter Kremer
 Conformance and Software Test Laboratory
 Ericsson Hungary
 H-1300 Bp. 3., P.O. Box 107, HUNGARY
 Phone: +36 1 437 7033
 EMail: peter.kremer@ericsson.com

Jonsson, et al. Standards Track [Page 32] RFC 4815 Corrections and Clarifications to RFC 3095 February 2007

Full Copyright Statement

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 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.
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Jonsson, et al. Standards Track [Page 33]

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