rfc3242.txt

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   The figure above shows the various levels, as defined in [ROHC] and
   this document, constituting a complete implementation of the LLA
   profile.  The figure also underlines the need for additional
   documents to specify how to implement these interfaces for a link
   technology for which this profile is relevant.

   This section defines the information to be exchanged between the LLA
   compressor and the assisting layer for this profile to operate
   properly.  While it does define semantics, it does not specify how
   these interfaces are to be implemented.

4.2.1.  Interface, Compressor to Assisting Layer

   This section defines the interface semantics between the compressor
   and the assisting layer, providing rules for packet delivery from the
   compressor.

   The interface defines the following parameters: RRP, RRP segmentation
   flag, CSP, CSP segmentation flag, NHP, and RTP Sequence Number.  All
   parameters, except the NHP, MUST always be delivered to the assisting
   layer.  This leads to two possible delivery scenarios:

   a. RRP, CSP, CCP, NHP and RTP Sequence Number are delivered, along
      with the corresponding segmentation flags set accordingly.



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      This corresponds to the case when the compressor allows sending of
      an NHP packet, with or without segmentation being applied to the
      corresponding RRP/CSP packets.

      Recall that delivery of an NHP packet occurs when the ROHC RTP
      compressor would have used a ROHC UO-0.

   b. RRP, CSP, CCP and RTP Sequence Number are delivered, along with
      the corresponding segmentation flags set accordingly.

      This corresponds to the case when the compressor does not allow
      sending of an NHP packet.  Segmentation might be applied to the
      corresponding RRP and CSP packets.

   Segmentation may be applied independently to an RRP or a CSP packet
   if its size exceeds the largest value provided in the PREFERRED
   PACKET_SIZES list and if the LARGE_PACKET_ALLOWED parameter is set to
   false.  The segmentation flags are explicitly stated in the interface
   definition to emphasize that the RRP and the CSP may be delivered by
   the compressor as segmented packets.

   The RTP SN MUST be delivered for each packet by the compressor to
   allow the assisting layer to maintain the necessary sequencing
   information.

4.2.2.  Interface, Assisting Layer to Decompressor

   Here the interface semantics between the assisting layer and the
   decompressor are defined, providing simple rules for the delivery of
   received packets to the decompressor.  The decompressor needs a way
   to distinguish NHP packets from RHP packets.  Also, when receiving
   packets without a header, the decompressor needs a way to infer the
   sequencing information to keep synchronization between the received
   payload and the sequence information of the decompressed headers.  To
   achieve this, the decompressor MUST receive the following from the
   assisting layer:

   -  an indication for each packet loss over the link between the
      compressing and decompressing sides for CID=0
   -  the received packet together with an indication whether the packet
      received is an NHP or not

   Note that the context is updated from a packet loss indication.








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4.3.  Optimistic Approach Agreement

   ROHC defines an optimistic approach for updates to reduce the header
   overhead.  This approach is fully exploited in the Optimistic and
   Unidirectional modes of operation.  Due to the presence of a CRC in
   all compressed headers, the optimistic approach is defined as a
   compressor issue only because the decompressor will always be able to
   detect an invalid context through the CRC verification.

   However, no CRC is present in the NHP packet defined by the LLA
   profile.  Therefore the loss of an RHP packet updating the context
   may not always be detected.  To avoid this problem, the compressing
   and decompressing sides must agree on the principles for the
   optimistic approach, and the agreed principles MUST be enforced not
   only by the compressor but also by the transmitting assisting layer.
   If, for example, three consecutive updates are sent to convey a
   header field change, the decompressor must know this and invalidate
   the context in case of three or more consecutive physical packet
   losses.  Note that the mechanism used to enforce the optimistic
   approach must be reinitialized if a new field change needs to be
   conveyed while the compressing side is already sending packets to
   convey non-linear context updates.

   An LLA decompressor MUST use the optimistic approach knowledge to
   detect possible context loss events.  If context loss is suspected it
   MUST invalidate the context and not forward any packets before the
   context has been synchronized.

   It is REQUIRED that all documents, describing how the LLA profile is
   implemented over a certain link technology, define how the optimistic
   approach is agreed between the compressing side and the decompressing
   side.  It could be handled with a fixed principle, negotiation at
   startup, or by other means, but the method must be unambiguously
   defined.

4.4.  Fast Context Initialization, IR Redefinition

   As initial IR packets might overrun the channel bandwidth and
   significantly delay decompressor context establishment, it might be
   beneficial to initially discard the payload.  This allows state
   transitions and higher compression efficiency to be achieved with
   minimal delay.

   To serve this purpose, the D-bit from the basic structure of the ROHC
   RTP IR packet [ROHC section 5.7.7.1] is redefined for the LLA
   profile.  For D=0 (no dynamic chain), the meaning of the D-bit is





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   extended to indicate that the payload has been discarded when
   assembling the IR packet.  All other fields keep their meanings as
   defined for ROHC RTP.

   The resulting structure, using small CIDs and CID=0, becomes:

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1 | 1 | 1 | 1 | 1 | 1 | 0 | D |
   +---+---+---+---+---+---+---+---+
   |            Profile            | 1 octet
   +---+---+---+---+---+---+---+---+
   |              CRC              | 1 octet
   +---+---+---+---+---+---+---+---+
   |            Static             | variable length
   |             chain             |
    - - - - - - - - - - - - - - - -
   |            Dynamic            | not present if D = 0
   |             chain             | present if D = 1, variable length
    - - - - - - - - - - - - - - - -
   |            Payload            | not present if D = 0
   |                               | present if D = 1, variable length
    - - - - - - - - - - - - - - - -

        D:   D = 0 indicates that the dynamic chain is not present
             and the payload has been discarded.

   After an IR packet with D=0 has been processed by the decompressor,
   the packet MUST be discarded.

4.5.  Feedback Option, CV-REQUEST

   The CV-REQUEST option MAY be used by the decompressor to request an
   RRP or CSP for context verification.  This option should be used if
   only NHP have been received for a long time and the context therefore
   has not been verified recently.

   +---+---+---+---+---+---+---+---+
   |  Opt Type = 8 |  Opt Len = 0  |
   +---+---+---+---+---+---+---+---+

   If the compressor receives a feedback packet with this option, the
   next packet compressed SHOULD NOT be delivered to the assisting layer
   as an NHP.







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4.6.  Periodic Context Verification

   As described in section 3.3, transparency is expected to be
   guaranteed by the functionality provided by the lower layers.  This
   ROHC profile would therefore be at least as reliable as the older
   header compression schemes [VJHC, IPHC, CRTP], which do not make use
   of a header compression CRC.  However, since ROHC RTP normally is
   extremely safe to use from a transparency point of view, it would be
   desirable to be able to achieve this with LLA also.

   To provide an additional guarantee for transparency and also catch
   unexpected errors, such as errors due to faulty implementations, it
   is RECOMMENDED to periodically send context updating packets, even
   when the compressor logic allows NHP packets to be used.

4.7.  Use of Context Identifier

   Since an NHP cannot carry a context identifier (CID), there is a
   restriction on how this profile may be used, related to context
   identification.  Independent of which CID size has been negotiated,
   NHP packets can only be used for CID=0.  If the decompressor receives
   an NHP packet, it can only belong to CID=0.

   Note that if multiple packet streams are handled by a compressor
   operating using LLA, the assisting layer must in case of physical
   packet loss be able to tell for which CID the loss occurred, or at
   least it MUST be able to tell if packets with CID=0 (packet stream
   with NHPs) have been lost.

5.  Implementation Issues

   This document specifies mechanisms for the protocol and leaves
   details on the use of these mechanisms to the implementers.  The
   present chapter aims to provide guidelines, ideas and suggestions for
   implementation of LLA.

5.1.  Implementation Parameters and Signals

   As described in [ROHC, section 6.3], implementations use parameters
   to set up configuration information and to stipulate how a ROHC
   implementation is to operate.  The following parameters are
   additions, useful to LLA, to the parameter set defined for ROHC RTP
   implementations.  Note that if the PREFERRED_PACKET_SIZES parameters
   defined here are used, they obsolete all PACKET_SIZE and PAYLOAD_SIZE
   parameters of ROHC RTP.






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5.1.1.  Implementation Parameters at the Compressor

   ALWAYS_PAD -- value: boolean

      This parameter may be set by an external entity to specify to the
      compressor that every RHP packet MUST be padded with a minimum of
      one octet ROHC padding.

      The assisting layer MUST provide a packet type identification.  If
      no field is available for this purpose from the protocol at the
      link layer, then a leading sequence may be used to distinguish RHP
      packets from NHP packets.  Although the use of a leading sequence
      is obviously not efficient, since it sacrifices efficiency for RHP
      packets, the efficiency loss should be insignificant because the
      leading sequence applies only to packets with headers in order to
      favor the use of packets without headers.  If a leading sequence
      is desired for RHP identification, the lower layer MAY use ROHC
      padding for the leading sequence by setting the ALWAYS_PAD
      parameter. Note that in such cases, possible collisions of the
      padding with the NHP payload must be avoided.

      By default, this parameter is set to FALSE.

   PREFERRED_PACKET_SIZES -- list of:
         SIZE -- value: integer (octets)
         RESTRICTED_TYPE -- values: [NHP_ONLY, RHP_ONLY, NO_RESTRICTION]

      This parameter set governs which packet sizes are preferred by the
      assisting layer.  If this parameter set is used, all RHP packets
      MUST be padded to fit the smallest possible preferred size.  If
      the size of the unpadded packet (or, in the case of ALWAYS_PAD
      being set, the packet with minimal one octet padding) is larger
      than the maximal preferred packet size, the compressor has two
      options.  Either, it may deliver this larger packet with an
      arbitrary size, or it may split the packet into several segments
      using ROHC segmentation and pad each segment to one of the
      preferred sizes.  Which method to use depends on the value of the
      LARGE_PACKETS_ALLOWED parameter below.