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Network Working Group                       Ghyslain Pelletier, Ericsson
INTERNET-DRAFT                               Lars-Erik Jonsson, Ericsson
Expires: December 2004                        Kristofer Sandlund, Effnet
                                                           June 14, 2004


                      RObust Header Compression (ROHC):
                 ROHC over Channels that can Reorder Packets
                <draft-pelletier-rohc-over-reordering-00.txt>


Status of this memo

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Abstract

   RObust Header Compression (ROHC), RFC 3095, defines a framework for
   header compression, along with a number of compression protocols
   (profiles). One operating assumption for the profiles defined in RFC
   3095 is that the channel between compressor and decompressor is
   required to maintain packet ordering. This document discusses aspects
   of using ROHC over channels that can reorder packets. It provides
   guidelines on how to implement existing profiles over such channels,
   as well as suggestions for the design of new profiles.



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Table of Contents

   1. Introduction.....................................................3
   2. Terminology......................................................3
   3. Applicability of this Document to ROHC Profiles..................5
      3.1. Profiles within Scope.......................................5
      3.2. Profiles with Special Considerations........................5
      3.3. Profiles Incompatible with Reordering.......................5
   4. Background.......................................................6
      4.1. Reordering Channels.........................................6
      4.2. Robustness Principles of ROHC...............................6
         4.2.1. Optimistic Approach (U/O-mode).........................6
         4.2.2. Secure Reference Principle (R-mode)....................7
   5. Problem Description..............................................7
      5.1. ROHC and Reordering Channels................................7
         5.1.1. LSB Interpretation Interval and Reordering.............7
         5.1.2. Reordering of Packets in R-mode........................9
            5.1.2.1. Updating Packets..................................9
            5.1.2.2. Non-Updating Packets..............................9
         5.1.3. Reordering of Packets in U/O-mode.....................10
         5.1.4. Reordering on the Feedback Channel....................10
         5.1.5. List Compression......................................10
         5.1.6. Reordering and Mode Transitions.......................11
      5.2. Consequences of Reordering.................................11
         5.2.1. Functionality Incompatible with Reordering............11
         5.2.2. Context Damage (Loss of Synchronization)..............12
         5.2.3. Detected Decompression Failures (U/O/R-mode)..........12
         5.2.4. Undetected Decompression Failures (R-mode only).......12
   6. Making ROHC Tolerant against Reordering.........................13
      6.1. Properties of ROHC Implementations.........................13
         6.1.1. Compressing Headers with Robustness against Reordering13
            6.1.1.1. Reordering and the Optimistic Approach...........13
            6.1.1.2. Reordering and the Secure Reference Principle....14
            6.1.1.3. Robust Selection of Compressed Header............14
         6.1.2. Implementing a Reordering Tolerant Decompressor.......15
            6.1.2.1. Bi-directional Reliable Mode (R-mode)............15
            6.1.2.2. Decompressor Feedback Considerations.............16
            6.1.2.3. Considerations for Local Repair Mechanisms.......16
      6.2. Specifying ROHC Profiles with Robustness against Reordering16
         6.2.1. Profiles with Interpretation Interval Offset p = -1...16
         6.2.2. Modifying the Interpretation Interval Offset..........17
            6.2.2.1. Example profile for handling reordering..........17
            6.2.2.2. Defining the values of p for new profiles........17
         6.2.3. TCP Profile Considerations............................18
   7. Security Consideration..........................................18
   8. IANA Considerations.............................................18
   9. Acknowledgments.................................................18
   10. Authors' Addresses.............................................18
   11. Informative References.........................................19





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1. Introduction

   RObust Header Compression (ROHC), RFC 3095 [2], defines a framework
   for header compression, along with a number of compression protocols
   (profiles). One operating assumption for the profiles defined in RFC
   3095 is that the channel between compressor and decompressor is
   required to maintain packet ordering for each compressed flow. The
   motivation behind this assumption was that the primary candidate
   channels considered did guarantee in-order delivery of header-
   compressed packets; making this assumption made it possible to
   improve the compression efficiency and the tolerance to packet loss,
   objectives that were on top of the requirements list at the time.

   Since the publication of RFC 3095 in 2001, the question about ROHC
   operation over channels that do not guarantee in-order delivery has
   surfaced several times; arguments that ROHC cannot perform adequately
   over such channels have even been heard. Specifically, this has been
   raised as a weakness when compared to other header compression
   alternatives, as RFC 3095 explicitly states its inability to operate
   if in-order delivery is not guaranteed. For those familiar with the
   details of ROHC and of other header compression schemes, it is clear
   that this is a misconception; but it can also be easily understood
   that the wording used in RFC 3095 can lead to such interpretation.

   This document discusses the various aspects of implementing ROHC over
   channels that can reorder header-compressed packets. It explains
   different ways of implementing the profiles found in RFC 3095, as
   well as other profiles based on those profiles, over reordering
   channels. This can be achieved either by ensuring that compressor
   implementations uses compressed headers that are sufficiently robust
   to the expected possible reordering, and/or by modifying decompressor
   implementations to tolerate reordered packets. Ideas regarding how
   existing profiles could be updated and how new profiles can be
   defined to cope efficiently with reordering are also discussed.


2. Terminology

   This document uses terminology consistent with RFC 3759 [3], and is
   in itself only informative. Although it does discuss technical
   aspects of implementing the ROHC specifications in particular
   environments, it does not specify any new technology.

   However, the document discusses possible ways of modifying existing
   ROHC implementations and/or specifications to address its objectives.
   In those parts of the document, the key words "MUST", "MUST NOT",
   "REQUIRED", "SHALL", "SHALL NOT", "SHOULD, "SHOULD NOT",
   "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in RFC 2119 [1].





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   ROHC

      The term "ROHC" herein refers to the following profiles:

         - 0x0001, 0x0002 and 0x0003 defined in RFC 3095 [2];
         - 0x0004 for compression of IP-only headers [5];
         - 0x0007 and 0x0008 for compression of UDP-Lite headers [6].

      The term "ROHC" excludes the following profiles, that are either
      not affected by reordering or that have the assumption of in-order
      delivery as a fundamental requirement for their proper operation:

         - 0x0000 (uncompressed) [2];
         - 0x0005 (LLA) [7] and 0x0105 (R-mode extension to LLA) [8];

   Reordering

      A type of transmission taking place between compressor and
      decompressor where in-order delivery of header-compressed
      packets is not guaranteed.

   Reordering Channel

      A connection over which reordering, as defined above, can occur.

   Sequentially early packet

      A packet that reaches the decompressor before one or several
      packets of the same CID that were delayed on the link. At the time
      of the arrival of a sequentially early packet, the packet(s)
      delayed on the link cannot be differentiated from lost packet(s).

   Sequentially late packet

      A packet is late within its sequence if it reaches the
      decompressor after one or several other packets belonging to the
      same CID have been received, although the sequentially late packet
      was sent from the compressor before the other packet(s).

   Updating packet

      A packet that updates the context of the decompressor, i.e. all
      packets carrying a CRC calculated over the uncompressed header.

   Non-updating packet

      A packet that carries a CRC calculated over the uncompressed
      header updates the context of the decompressor when it is
      successfully decompressed. A packet without such a CRC is thus
      referred to as a non-updating packet.




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   Change packet

      A packet that updates one or more fields of the context other than
      the fields pertaining to the functions established with respect to
      the sequence number (SN). Specifically, it is a packet that
      updates fields other than the SN, IP-ID or RTP timestamp (TS).


3. Applicability of this Document to ROHC Profiles

   This document addresses general reordering issues for ROHC profiles.
   The foremost objectives are to ensure that ROHC implementations will
   not forward packets with incorrectly decompressed headers to upper
   layers, as well as to limit the possible increase in the rate of
   decompression failures or in events leading to context damage, when
   compression is applied over reordering channels.


3.1. Profiles within Scope

   The solutions outlined in following sections are generally applicable
   to profiles 0x0001 (RTP), 0x0002 (UDP) and 0x0003 (ESP) defined in
   RFC 3095 [2]. Profile 0x0000 (uncompressed) is not affected by
   reordering, as the headers are sent uncompressed. The solutions also
   apply to profiles for IP-only (0x0004) [5] and for UDP-Lite (0x0007
   and 0x0008) [6]. These profiles are based on the profiles of RFC 3095
   [2] and inherently make the same in-order delivery assumption.


3.2. Profiles with Special Considerations

   Special considerations are needed to make some of the implementation
   solutions of sections 6.1 and 6.2 applicable to profiles 0x0002 (UDP)
   [2], 0x0004 (IP-only) [5], and 0x0008 (UDP-Lite) [6]. For these
   profiles, the SN is generated at the compressor, as it is not present
   in headers being compressed. For the least significant bit (LSB)
   encoding method, the interpretation interval offset (p) is always
   p = -1 (see section 5.1.1) when interpreting the SN. The SN is thus
   required to increase for each packet received at the decompressor,
   which means that reordered packets cannot be decompressed.

3.3. Profiles Incompatible with Reordering

   The ROHC LLA profiles defined in RFC 3242 [7] and RFC 3408 [8] have
   been explicitly designed with in-order delivery as a fundamental
   requirement to their proper operation. Profiles 0x0005 and 0x0105 can
   therefore not be implemented over channels where reordering can
   occur; this document therefore does not apply to these profiles.






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4. Background

   ROHC was designed with the assumption that packets are delivered in-
   order from compressor to decompressor. This was considered as a
   reasonable working assumption for links where it was expected that
   ROHC would be used. However, many have expressed that it would be
   desirable to use ROHC also over connections where in-order delivery
   is not guaranteed [9].


4.1. Reordering Channels

   The reordering channels that are potential candidates to use ROHC are
   single-hop channels and multi-hop virtual channels.

   A single-hop channel is a point-to-point link that constitutes a
   single IP hop. Note that one IP hop could be one or multiple physical
   links. For example, a single-hop reordering channel could be a
   wireless link that applies error detection and performs
   retransmissions to guarantee error-free delivery of all data. Another
   example could be a wireless connection that performs bicasting of
   data during a handoff procedure.

   A multi-hop virtual channel is a virtual point-to-point link that
   traverses multiple IP hops. A multi-hop virtual channel would
   typically be an IP tunnel, where compression is applied over the
   tunnel by the endpoints of the tunnel (not to be confused with single
   link compression of tunneled packets).


4.2. Robustness Principles of ROHC

   Robustness is based on the optimistic approach in the unidirectional
   and optimistic modes of operation (U/O-mode), and on the secure
   reference principle in the bi-directional reliable mode (R-mode).

   Both approaches have different characteristics in the presence of
   reordering between compressor and decompressor. However, in any mode,
   decompression of sequentially early packets will generally be handled
   quite well since they will be perceived and treated by the
   decompressor as if there had been one or more packet losses.


4.2.1. Optimistic Approach (U/O-mode)

   A ROHC compressor uses the optimistic approach to reduce header
   overhead when performing context updates in U/O-mode. The compressor
   normally repeats the same update until it is fairly confident that
   the decompressor has successfully received the information. The
   number of consecutive packets needed to obtain this confidence is
   open to implementations, and this number is normally related to the



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   packet loss characteristics of the link where header compression is
   used (see also [2], section 5.3.1.1.1).

   All packet types used in U/O-mode are context updating.


4.2.2. Secure Reference Principle (R-mode)

   A ROHC compressor uses the secure reference principle in R-mode, to
   ensure that context synchronization between ROHC peers cannot be lost
   due to packet losses. The compressor obtains its confidence that the
   decompressor has successfully updated the context from a packet
   carrying a 7- or 8-bit CRC based on acknowledgements received from
   the decompressor (see also [2], section 5.5.1.2).

   The secure reference principle makes it possible for a compressor to
   use packets that do not update the context (i.e. R-0 and R-1* [2]).


5. Problem Description

5.1. ROHC and Reordering Channels

   This section reviews different aspects of ROHC susceptible of being
   impacted by reordering of compressed packets between ROHC peers.


5.1.1. LSB Interpretation Interval and Reordering

   The LSB encoding method defined in RFC 3095 ([2], section 5.7)
   specifies the interpretation interval offset, called p, as follow:


   For profiles 0x0001, 0x0003 and 0x0007:

      p = 1, when bits(SN) <= 4;
      p = 2^(bits(SN)-5) - 1 otherwise.

      The resulting table describing the interpretation interval is:

         +-----------+--------------+--------------+
         | bits (SN) |   Offset p   | (2^k-1) - p  |
         |     k     | (reordering) |   (losses)   |
         +-----------+--------------+--------------+
         |     4     |      1       |      7       |
         |     5     |      0       |      16      |
         |     6     |      1       |      31      |
         |     7     |      3       |      61      |
         |     8     |      7       |      121     |
         |     9     |      15      |      241     |
         +-----------+--------------+--------------+



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      As shown in the table above, the ability for ROHC to handle
      sequentially late packets depends on the number of bits sent in
      each packet. For example, a sequentially late packet of type 0
      (with either 4 or 6 bits of SN) sets the limit to one packet out
      of sequence for successful decompression to be possible.


   For profiles 0x0002, 0x0004 and 0x0008:

      p = - 1, independently of bits(SN).

      A value of p = -1 means that the interpretation interval offset
      can only take positive values, and that no sequentially late
      packet can be decompressed if reordering occurs over the link.


   The trade-off between reordering and robustness

      The ability of ROHC to handle sequentially late packets is limited
      by the interval interpretation offset of the sliding window used
      for LSB encoding. This offset has a very small value for packets
      with a small number of sequence number (SN) bits, but grows with
      the number of SN bits transmitted.

      For channels where both packet losses and reordering can occur,
      modifications to the interpretation interval faces a trade-off
      between the amount of reordering and the number of consecutive
      packets losses that can be handled by the decompressor. If the
      negative offset (i.e. p) is increased to handle a larger amount of
      reordering, the value of the positive offset of the interpretation
      interval must be decreased. This may impact the compression
      efficiency when the channel has a high loss rate.

      This is shown in the figure:


         <--- interpretation interval (size is 2^k) ---->
         |------------------+---------------------------|
       Lower              v_ref                       Upper
       Bound                                          Bound
         <--- reordering --> <--------- losses --------->
          max delta(SN) = p   max delta(SN) = (2^k-1) - p

         where v_ref is the reference value as per [2].


      In practice, the maximum variation in SN value (max delta(SN))
      due to reordering that can be handled will normally correspond to
      the maximum number of packets that can be reordered. The same
      applies to the maximum number of consecutive packet losses covered
      by the robustness interval.



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5.1.2. Reordering of Packets in R-mode

5.1.2.1. Updating Packets

   The compressor always adds references in the sliding window for all
   updating packets sent. The compressor removes values smaller than
   values for which it has received an acknowledgement, to shrink the
   window and thereby increase the compression efficiency.

   The decompressor always updates the context when receiving an
   updating packet, and uses the new reference for decompression.
   Acknowledgements are sent to allow the compressor to shrink its
   sliding window.

   Reordering between updating packets

      The decompressor can update its context from the reception of a
      sequentially late updating packet. The decompressor reference is
      then updated with a value that is no longer in the sliding window
      of the compressor. This "missing reference" can be caused by
      reordering when operating in R-mode.

      The result is that the compressor and the decompressor lose
      synchronization with each other. When the decompressor
      acknowledges the sequentially late packet, the compressor might
      already have discarded the reference to this sequence number, and
      continue to compress packets based on more recent references (in
      packet arrival time). Decompression will then be attempted using
      the wrong reference.


5.1.2.2. Non-Updating Packets

   Reordering between non-updating packets only

      A non-updating packet that reaches the decompressor out-of-
      sequence with respect to other non-updating packets only can
      always be decompressed properly.

   Reordering between non-updating packets and updating packets

      When a non-updating packet is reordered and becomes sequentially
      late with respect to an updating packet, the decompressor may have
      already updated the context with a new reference when the late
      packet is received. It is thus possible for a non-updating packet
      to be decompressed based on the wrong reference because of
      reordering when operating in R-mode.

      Since decompression of non-updating packets cannot be verified,
      this can lead to a packet erroneously decompressed being forwarded
      to upper layers.



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5.1.3. Reordering of Packets in U/O-mode

   Sequentially late packets

      The ability to decompress sequentially late packets is limited by
      the offset p of the interpretation interval (see section 5.1.1).
      Decompression of a sequentially late packet with SN = x is
      possible if the value of the SN of the packet that last updated
      the context was less than or equal to x + p.

      Problems occur if context(SN) has increased by more than p with
      respect to field(SN) carried within the packet to decompress.

      This means that for a well-behaved stream with a constant unit
      increase in the RTP SN, a packet can arrive up to p packets out of
      sequence and still be correctly decompressed. Otherwise, it cannot
      be properly decompressed. It also means that if the compressor
      sends two consecutive packets with SN(packet1)=100 and
      SN(packet2)=108 when p=7, packet1 cannot be decompressed if it
      arrives even one packet late due to reordering.

   Decompression can always be verified since all U/O-mode packet types
   are context updating. Consequently, reordering of packets is not
   deemed problematic when operating in U/O-mode. For channels known to
   reorder packets, the U/O-mode should therefore be the preferred mode
   of operation. The additional risk of losing context synchronization
   or for erroneous packet to be delivered to upper layers is limited.


5.1.4. Reordering on the Feedback Channel

   For R-mode, upon reception of an acknowledgement, the compressor
   searches the sliding window to locate an updating packet with the
   corresponding SN; if it is not found, the acknowledgement is invalid
   and is discarded ([2], section 5.5.1.2). In other words, feedback
   received out-of-order either is still useful or is discarded.

   In U/O-mode, if the compressor updates its context based on feedback,
   the same logic as for R-mode applies in practice.

   Reordering on the feedback channel has thus no impact in either mode.


5.1.5. List Compression

   <# Editor's Note:    This is for further study.                    #>
   <#                                                                 #>
   <#                                                                 #>
   <#                                                                 #>





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5.1.6. Reordering and Mode Transitions

   Transition from U/O-mode to R-mode

      This transition can be affected by reordering if a packet type 0
      (UO-0) is reordered and delayed by at least one round-trip time
      (RTT). If the decompressor initiates a mode change request to R-
      mode in the meantime, the reordered UO-0 packet may be handled as
      an R-0 packet; it can be erroneously decompressed and forwarded to
      upper layers. This is because the decompressor can switch to
      R-Mode as soon as it sends the acknowledgement Ack(SN, R) to the
      compressor (see also [2], section 5.6).

   Transition from R-mode to U/O-mode

      A similar situation as above can occur during this transition.
      However, because the outcome of the decompression is always
      verified using a CRC verification in U/O-mode, the reordered
      packet will most likely fail decompression and will be discarded.

   The above situation, while it is not deemed to occur frequently, is
   still possible; thus mode transitions from U/O-mode to R-mode should
   be avoided when reordering can occur.


5.2. Consequences of Reordering

   The context updating properties of the packets exchanged between ROHC
   peers are the most important factors to consider when deriving the
   impacts of reordering. For this reason, the robustness properties of
   the U/O-mode and of the R-mode are affected differently.

   The effects of reordering on ROHC can be summarized as follow:

   - Functionality incompatible with reordering;
   - Increased probability of context damage (loss of synchronization);
   - Increased number of decompression failures - Detected (U/O/R-mode);
   - Increased number of decompression failures - Undetected (R-mode).


5.2.1. Functionality Incompatible with Reordering

   There are some optional ROHC functions that cannot work in the
   presence of reordering between ROHC peers.

   The ROHC segmentation scheme (see [2], section 5.2.5) relies entirely
   on the in-order delivery of each segment, as there is no sequencing
   information in the segments. Therefore segmentation should not be
   used if there can be reordering between the ROHC peers.





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   Timer-based compression of RTP TS (see [2], section 4.5.4) is built
   on an assumption of timely (minimal jitter) delivery. Therefore it
   should be used with care over links where reordering can occur, with
   respect to the amount of jitter that can be introduced by reordering.

   The use of these optional features is open to implementations and is
   local to the compressor only; it does not impact the decompressor.


5.2.2. Context Damage (Loss of Synchronization)

   Reordering of packets between ROHC peers can impact the robustness
   properties of the optimistic approach (U/O-mode) as well as the
   reliability of the secure reference principle (R-mode).

   The successful decompression of a sequentially late change packet
   (U/O-mode) and/or updating packet (R-mode) can update the context of
   the decompressor in a manner unexpected by the compressor. This can
   lead to a loss of context synchronization between the ROHC peers.


5.2.3. Detected Decompression Failures (U/O/R-mode)

   Reordering of packets between ROHC peers can lead to an increase in
   the number of decompression failures for context updating packets
   (see sections 5.1.2.1 and 5.1.3). Fortunately, as the outcome of the
   decompression of updating packets can be verified, the decompressor
   can reliably detect decompression failures caused by reordering and
   discard the packet. Note that local repairs, subject to the
   limitations stated in [2] section 5.3.2.3, can still be performed.


5.2.4. Undetected Decompression Failures (R-mode only)

   Reordering of packets between ROHC peers can lead to an increase in
   the number of decompression errors for non-updating packets. For R-
   mode, decompression of R-0 and R-1* packets cannot be verified. If
   reordering occurs and decompression is performed using the wrong
   secure reference (see section 5.1.2.1 and 5.1.2.2), the decompressor
   cannot reliably detect such errors. As a result, erroneous packets
   may be forwarded to upper layers.













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6. Making ROHC Tolerant against Reordering

   This chapter describes different approaches that can improve the
   performance of ROHC when used over reordering channels and minimize
   the effects of reordering. Examples are provided to guide
   implementers and designers of new profiles. The solutions target
   either the properties of ROHC implementations or the specifications
   of profiles. This is covered by sections 6.1 and 6.2 respectively.


6.1. Properties of ROHC Implementations

   Existing ROHC profiles can be implemented with the capability to
   properly handle packet reordering. The methods described in this
   section conform with, and thus do not require any modifications to,
   the ROHC specifications within scope of this document (see section
   3). Specifically, the methods presented in this section can be
   implemented without any impairment to interoperability with other
   ROHC implementations that do not use these methods.

   The methods suggested here may however lower compression efficiency,
   and these modifications should not be used when reordering is known
   not to occur. Some of these methods aim to increase the decompression
   success rate at the decompressor, while others aim to avoid context
   damages causing loss of context synchronization between compressor
   and decompressor.

   The methods proposed are each addressing specific issues listed in
   section 5, and can be combined to achieve better robustness against
   reordering.


6.1.1. Compressing Headers with Robustness against Reordering

   The methods described in this section are methods local only to the
   compressor implementation. They can be used without modifications or
   impact to the decompressor.


6.1.1.1. Reordering and the Optimistic Approach

   The optimistic approach is affected by the reordering characteristics
   of the channel when operating over a reordering channel. Compressor
   implementations should therefore adjust their optimistic approach
   strategy to match both packet loss and reordering characteristics.

   For example, the number of repetitions for each context update can be
   increased. The compressor should ensure that each update is repeated
   until it is reasonably confident that a least one change packet in
   the sequence of repetitions has reached the decompressor before the
   first packet sent after this sequence.



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6.1.1.2. Reordering and the Secure Reference Principle

   Fundamental to the secure reference principle is that only values
   acknowledged by the decompressor can be used as reference for
   compression. In addition, some of the packet types used in R-mode do
   not include a CRC over the original uncompressed header, and the
   decompressor has no means to verify the outcome of the decompression.

   Decompression of non-updating packet types thus entirely relies on
   the cumulative effect of previous updates to the secure reference,
   and the compressed data is based on the current value of the
   reference. This reference must be synchronized between ROHC peers.
   For R-0 and R-1* packets, the reception of the encoded bits applied
   to the secure reference is sufficient for correct decompression, but
   only when in-order delivery between ROHC peers is guaranteed.

   Avoiding the "missing reference" problem (section 5.1.2.1)

      A compressor implementation can delay the advance in the sliding
      window to a reference acknowledged by the decompressor, until it
      has confidence that no acknowledgement for any of the values that
      could be discarded can be received. This confidence can be based
      on the maximum delay that reordering can introduce over the
      channel. It can also be based on the knowledge that the
      decompressor implements the context updating logic of section
      6.1.2.1 (e.g. by means of standardization).


6.1.1.3. Robust Selection of Compressed Header

   The interpretation interval for the LSB encoded sequence number can
   be adjusted to allow for larger negative offsets (see section 5.1.1).
   This would provide the capability to decompress sequentially late
   packets with a greater amount of reordering.

   To achieve this, the compressor should be implemented conservatively
   in terms of the choice of packet types to send, by transmitting
   packets with more sequence number bits. As shown in the table of
   section 5.1.1, using eight bits of SN allows a packet to be
   decompressed when the reordering leads to up to seven units in
   sequence number variation (i.e. delta(SN)). Increasing the number of
   SN bits (i.e. using a larger SN_k [2]) transmitted will make ROHC
   even more tolerant to reordering.











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   For example, a conservative compressor implementation could use the
   packet types as shown in the table below:

      +----------------------+-------------------------+
      | Optimal Packet Type  | Alternative Packet Type |
      | (without reordering) |  (reordering possible)  |
      +----------------------+-------------------------+
      | UO-0                 | UOR-2*-ext0             |
      | R-0                  | R-1*-ext0               |
      | R-0-CRC              | UOR-2*-ext0             |
      | R-1*                 | R-1*-ext0               |
      | UO-1                 | UOR-2-ext0              |
      | UO-1-TS              | UOR-2-TS-ext0           |
      | UO-1-ID              | UO-1-ID-ext3 (with S=1) |
      |                      | UOR-2-ID-ext0           |
      | UOR-2*               | UOR-2*-ext0             |
      +----------------------+-------------------------+


   Such a compressor implementation would thus always be sending at
   least 3 octets (R-mode) or 4 octets (U/O-mode). This is a trade-off
   when compared to the 1 octet that can be sent by a more aggressive
   implementation operating on a channel with no reordering.

   Note that since the interpretation interval for profiles 0x0002,
   0x0004 and 0x0008 is always p = -1 independently of bits(SN), the
   methods suggested in this section will not work for these profiles
   unless this value is modified (section 6.2.1).


6.1.2. Implementing a Reordering Tolerant Decompressor

   The methods described in this section are methods local only to the
   decompressor implementation. They can be used without modifications
   or impact to the compressor.


6.1.2.1. Bi-directional Reliable Mode (R-mode)

   The "missing reference" problem described in section 5.1.2.1 can be
   avoided. If the decompressor can detect when two updating packets
   (packets including CRCs) are reordered with respect to each other, it
   should not update the context with the values of the sequentially
   late update packet.










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6.1.2.2. Decompressor Feedback Considerations

   Reducing the feedback rate when the flow behaves linearly

      The decompressor should reduce its feedback rate when a large
      number of UOR-2 packets with extensions a received, when the flow
      behaves linearly (i.e. when only fields pertaining to the
      functions established with respect to the sequence number are
      changing.

      In particular, if the compressor implementation makes a more
      conservative selection of packet types (section 6.1.1.3) in order
      to handle reordering, the decompressor should try to avoid sending
      more feedback than it would have if the more optimal packet types
      had been used.

      Note that if the decompressor does not make this adjustment,
      packet losses or context damages will not increase. It might
      however reduce link efficiency.

   Acknowledgements and sequentially late packets

      Reordered feedback (or feedback for packets received out-of-order)
      will not cause problems (see section 5.1.4). However, the
      decompressor should not send feedback for sequentially late
      packets, as the current state of the context will better reflect
      the compressor context than the content of the reordered packet.


6.1.2.3. Considerations for Local Repair Mechanisms

   When decompression fails, and if reordering can be the cause of this
   failure, a local repair may be attempted for the sequentially late
   packet by going backward in the interpretation interval (as opposed
   to moving forward as for packet losses).


6.2. Specifying ROHC Profiles with Robustness against Reordering


6.2.1. Profiles with Interpretation Interval Offset p = -1

   New revisions of profiles 0x0002 (UDP) [2], 0x0004 (IP-only) [5], and
   0x0008 (UDP-Lite) [6] should redefine how the value of the offset p
   is determined, and use the same algorithm as in profile 0x0001 [2]
   instead of p = -1 independently of bits(SN) (section 5.1.1).

   While such a change would make these updated profiles slightly less
   robust to packet losses, they would still be no less robust than
   profile 0x0001.




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6.2.2. Modifying the Interpretation Interval Offset

   The interpretation interval offset p could be modified for existing
   profile in order to handle reordering while improving the compression
   efficiency when compared to the solution of section 6.1.1.3.


6.2.2.1. Example profile for handling reordering

   The value of the interpretation interval offset p can be adjusted to
   achieve a robustness against reordering similar to the effect of
   selecting packet types as suggested in section 6.1.1.3.

   For example, assuming that having a value p=7 is enough while still
   considering robustness against packet losses a priority, a ratio
   where the positive offset is about twice as large as the negative
   offset can be used. This leaves a value of p = 2^k/ 3.

   The resulting values are shown in the following table:

         +-----------+--------------+----------------+
         | bits (SN) |   Offset p   | Positive range |
         |     k     | (reordering) |    (losses)    |
         +-----------+--------------+----------------+
         |     4     |        5     |        10      |
         |     5     |       10     |        21      |
         |     6     |       21     |        42      |
         |     7     |       42     |        85      |
         |     8     |       85     |       170      |
         |     9     |      170     |       341      |
         +-----------+--------------+----------------+

   Using this value for p, a fair amount of reordering can be handled
   without having to send UOR-2 packets most of the time. The trade-off
   is that this is at the expense of robustness against packet losses.


6.2.2.2. Defining the values of p for new profiles

   As described in RFC3095, the interpretation interval when sending k
   bits of SN is defined as:

      f(v_ref, k) = [v_ref - p, v_ref + (2^k - 1) - p]

   The negative bound (v_ref - p) limits the ability to handle
   reordering, while the positive bound (v_ref + (2^k - 1) - p) limits
   the ability to handle packet losses.

   Adjusting p will increase one of these ranges, while the other range
   will decrease. When designing ROHC profiles, considerations on how
   these correlate with each other should be taken.



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   For example, if it is desirable for a profile to be as robust against
   reordering (negative range) and against packet losses (positive
   range), these range can be made equal by setting p near (2^k / 2).


6.2.3. TCP Profile Considerations

   The current draft for the ROHC TCP profile [4] contains packet
   formats that allow sending as little as 1 bit of MSN (master sequence
   number). Since the MSN is used in the same fashion as the sequence
   number in profile 0x0002, it will not be possible to decompress
   reordered packets if used over a reordering channel.

   The work on the ROHC-TCP profile should consider using more bits of
   MSN to enable simple implementation modifications when operating over
   a reordering channel.


7. Security Consideration

   This document does not include additional security risks to [2]. In
   addition, it may lower risks related to context damage in R-mode with
   injected packets when sequentially late packets do not update the
   context (section 6.1.2.1).


8. IANA Considerations

   This document does not require any IANA action.


9. Acknowledgments

   The authors would appreciate feedback on this document, as input from
   others would certainly help us improve it significantly.


10. Authors' Addresses

   Ghyslain Pelletier           Tel:    +46 920 20 24 32
   Ericsson AB                  EMail:  ghyslain.pelletier@ericsson.com
   Box 920
   S-971 28 Lulea
   Sweden

   Lars-Erik Jonsson            Tel:    +46 920 20 21 07
   Ericsson AB                  EMail:  lars-erik.jonsson@ericsson.com
   Box 920
   S-971 28 Lulea
   Sweden




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   Kristofer Sandlund           Tel:    +46 920 609 17
   Effnet AB                    EMail:  kristofer.sandlund@effnet.com
   Stationsgatan 69
   S-972 34 Lulea
   Sweden


11. Informative References

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

   [2]  C. Bormann, et. al, "RObust Header Compression (ROHC): Framework
        and four profiles: RTP, UDP, ESP, and uncompressed", RFC 3095,
        July 2001.

   [3]  Jonsson, L-E., " RObust Header Compression (ROHC): Terminology
        and Channel Mapping Examples", RFC 3759, April 2004.

   [4]  G. Pelletier, et. al, "RObust Header Compression (ROHC): TCP/IP
        Profile (ROHC-TCP)", Internet-Draft (work in progress),
        <draft-ietf-rohc-tcp-06.txt>, April 2004.

   [5]  Jonsson, L-E. and G. Pelletier, "RObust Header Compression
        (ROHC): A compression profile for IP", Internet draft (work in
        progress), <draft-ietf-rohc-ip-only-05.txt>, October 2003.

   [6]  G. Pelletier, "RObust Header Compression (ROHC): Profiles for
        UDP-Lite", Internet draft (work in progress), <draft-ietf-rohc-
        udp-lite-04.txt>, June 2004.

   [7]  Jonsson, L-E. and G. Pelletier, "RObust Header Compression
        (ROHC): A Link-Layer Assisted Profile for IP/UDP/RTP", RFC 3242,
        April 2002.

   [8]  Liu, Z. and K. Le, "Zero-byte Support for Bidirectional Reliable
        More (R-mode) in Extended Link-Layer Assisted Profile for RObust
        Header Compression (ROHC) Profile", RFC 3408, December 2002.

   [9]  Ash, J., Goode B. and J. Hand, "Requirements for Header
        Compression over MPLS", Internet draft (work in progress),
        <draft-ietf-avt-hc-mpls-reqs-02.txt>, June 2004.












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