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Versions: 00                                                            
Internet Engineering Task Force                            Peter Parnes
INTERNET-DRAFT                                                 LuTH/CDT
draft-parnes-rtp-ext-srm-00.txt                       November 16, 1996
                                                     Expires: May, 1997


             RTP extension for Scalable Reliable Multicast



                          Status of this Memo

       This document is an Internet-Draft. Internet-Drafts are working
     documents of the Internet Engineering Task Force (IETF), its areas,
     and its working groups. Note that other groups may also distribute
     working documents as Internet-Drafts.

     Internet-Drafts are draft documents valid for a maximum of six
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     as reference material or to cite them other than as "work in
     progress."

     To learn the current status of any Internet-Draft, please check the
     "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
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     munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
     ftp.isi.edu (US West Coast).

     Distribution of this document is unlimited.

Abstract

     This document describes how the Real-time Transport Protocol, RTP
     (RFC1189), could be extended to include support for parts of the
     framework called Scalable Reliable Multicast. The scheme proposed
     could be used for transporting a data flow reliably over the
     transport protocols supported by RTP in a light-weight way. This
     could be used for numerous applications, for instance white-boards,
     semi-reliable audio/video and messaging/data-transfers within
     group-ware applications.










Peter Parnes                                                    [Page 1]


INTERNET-DRAFT                RTP-EXT-SRM                  November 1996


1. Introduction

   The Real-time Transport Protocol, (RTP) [1] is based on UDP which is
   a best-effort transport protocol meaning that packets can be lost.
   For some applications this is acceptable but for other, for instance
   white-board-applications, it is necessary to do retransmission so an
   end-user can rely on that he has received everything that everybody
   else has sent.

   This document describe how RTP could be extended to include support
   for the framework of the so called Scalable Reliable Multicast [2].
   The scheme proposed could be used for transporting a data flow
   reliably over the transport protocols supported by RTP.

   This new RTP/SRM-platform could be used for numerous applications,
   including for instance semi-reliable audio/video and distributed
   group-ware applications.


2. The Scalable Reliable Multicast Framework

   Scalable Reliable Multicast,(SRM) [2] is a reliable multicast
   framework for application level framing and light-weight sessions.
   The algorithms of this framework are efficient, robust and scale well
   to both very large networks and very large sessions. The framework
   has been prototyped in wb, a distributed white-board application, and
   has been extensively tested on a global scale with sessions ranging
   from a few to more than 1000 participants.

2.1 The SRM ideas

   The ideas of SRM can briefly be described as follows:

     1) Every packet transmitted is uniquely identified by a sender-
        identification and a sequence-number that is incremented by one
        for each transmitted packet.

     2) Every member of the session buffers a certain amount of packets,
        even if she is only a receiver and not a sender, so if necessary
        she can participate in 'repairs' of lost packets.

     3) When a receiver notices that she has lost packets (by checking
        if the difference of the sequence-numbers of the incoming packet
        and the last heard packet before that is greater than 1) she
        sends a negative acknowledgment, NACK, using multicasting so all
        members of the session sees the NACK. But before sending the
        NACK she waits for a random time determined by the distance of
        the original source and if she hears a NACK for the same packet



Peter Parnes                                                    [Page 2]


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        from another member she suppress her own NACK-transmission. See
        section 2.2 for a discussion of how the timers and the distance
        is calculated.

     4) Any member that gets a NACK and has the packet requested can
        send a repair but just as in the NACK-case she waits a random
        time before sending the repair. Again see section 2.2 for the
        calculation of the repair-timers.

     5) Loss is detected by finding a gap in the sequence space but
        since it is possible the last data-packet is dropped, each
        sender sends a low-rate periodic message, a heartbeat,
        announcing the highest sequence number used.

   Please note that this only gives the basic ideas of the algorithms
   used and implementers should read the SRM-paper first.


2.2  Calculating timers in SRM

   Every timer is based on the distance, D, between the sender and the
   receiver. A member that only is active in repairs is also called a
   sender.

2.2.1 NACK timers (point 6)

   When loss is detected a NACK-timer is started. The expire-time is
   chosen from the uniform distribution of

                         2^i*[C1*Dsa, (C1+C2)*Dsa]

   seconds, where Dsa is host A's estimate of the one-way delay to the
   original source S of the missing data, C1 and C2 are parameters of
   the request algorithm and i is the count of how many times back-off
   has been issued. See section 2.2.3 for initial values of C1 and C2.
   The initial value of i is 0. When the request timer expires, host A
   multicasts the NACK for the missing data, and doubles the request-
   timer by increasing i by one to wait for the repair.

   If host A receives a NACK for the missing data before its own
   request-timer for that missing data expires, host A does a
   exponential back-off, and resets its request timer. This means that
   the new timers expire-time is randomly chosen from the uniform
   distribution of

                      2^(i+1)*[C1*Dsa, (C1+C2)*Dsa].

   To improve the repair time we don't back-off the request timer for



Peter Parnes                                                    [Page 3]


INTERNET-DRAFT                RTP-EXT-SRM                  November 1996


   every duplicate request that is received. For example, if a member
   receives several duplicate requests immediately after receiving the
   initial request, that member only backs off its request timer once.
   After the initial timer back-off, we only back-off the timer again if
   a request is received close to the time when the timer is set to
   expire. More precisely, when we back-off the request timer, we set an
   ignore-back-off variable to a time halfway between the current time
   and the expiration time, and we ignore additional duplicate requests
   until the ignore-back-off time.

2.2.2 Repair-timers (point 7)

   When host B receives a request from A that host B is capable of
   answering, host B sets a repair timer to a random value from the
   uniform distribution of

                           [D1*Dab, (D1+D2)*Dab]

   seconds where Dab is host B's estimate of the one-way delay to host
   A, and D1 and D2 are parameters of the repair algorithm. See section
   2.2.3 for initial values of D1 and D2. If host B receives a repair
   for the missing data before its repair timer expires, host B cancels
   the timer. Otherwise, when host B's repair timer for that data
   expires host B multicasts the repair.

   A host could receive duplicate NACKs immediately after sending a
   repair. In order to prevent these duplicate NACKs from triggering a
   responding set of duplicate repairs, host B ignores NACKs for data d
   for 3*Dsb seconds after sending or receiving a repair for that data.
   Host s is either the original source of data d or the source of the
   first NACK.

2.2.3 Initial values of the timer parameters

   The initial values of the timer parameters should be set to C1=C2=2,
   D1=D2=log10(G), where G is is the current number of members in the
   session.

   An adaptive algorithm for changing these parameters is presented in
   [2].


2.3 Calculating the host-to-host distances

   In order to be able to calculate the NACK and repair timers we need
   to have some knowledge of the host-to-host distance. This distance is
   calculated in seconds based on how long it takes for a packet to
   travel from host A to host B.



Peter Parnes                                                    [Page 4]


INTERNET-DRAFT                RTP-EXT-SRM                  November 1996


   To be able to do this each member of the session sends a time-stamp
   which is used in the following way to calculate the host-to-host
   distance:

     Assume that host A sends a session packet P1 at time t1 and host B
     receives P1 at time t2. At some later time, t3, host B generates a
     session packet P2, containing (t1, d), where d = t3 - t2 (time t1
     is included in P2 to make the algorithm robust to lost session
     packets). Upon receiving P2 at time t4, host A can estimate the
     latency from host B to host A as (t4 - t1 - d)/2. Note that while
     this estimate does assume that the paths are symmetric, it does not
     assume synchronized clocks.


3. How does SRM fit into RTP?

   Here follows a description of how the SRM ideas fit into RTP
   according to the points in the earlier sections. Points marked with a
   needed changes.

     1)  RTP has a unique sender-ID, the Synchronization Source (SSRC)
         and each data-packet has a sequence-number.

     2)  The buffering can be accomplished without interfering with the
         protocol itself.

     3*) The transmission of NACKs has to be incorporated into RTP using
         for instance the Real-time Transport Control Protocol, RTCP.

     4*) The transmission of repairs have to be incorporated into RTP.

     5*) The heartbeats have to be incorporated into RTP/RTCP.

     6)  The timer calculations don't imply any changes to RTP/RTCP.

     7*) The time-stamps should be incorporated into RTCP. The NTP-
         time-stamp in the RTCP/SR packet is to be used but this implies
         that all clients has to implement the usage of this field (the
         RTP specification has left it optional for clients to use this
         field). In order to calculate the host-to-host delay all
         clients need to have some notion of wall-clock time or elapsed
         time.

   Out-of-order packets could initially be seen as lost packets and lead
   to a started NACK-timer but when the "lost" packet(s) arrives the
   NACK-timer should be cancelled and an ignore flag should be raised
   for a time of 3*Dsa, where Dsa is the receivers notion of the
   distance to the sender. All NACKs received within this time should be



Peter Parnes                                                    [Page 5]


INTERNET-DRAFT                RTP-EXT-SRM                  November 1996


   ignored.


4. What extensions are needed to RTP/RTCP

   This section explains the needed additions to RTP/RTCP. The new
   needed packet-formats are discussed section 5.

   The additional SRM-generated-traffic can be incorporated into RTP
   basically in two ways; either incorporate all additional packets and
   data into the same channel as the original RTP or send all traffic on
   a separate multicast-group as a new channel.

     The first method would allow for clients that don't understand the
     SRM additions to benefit from the retransmitted repairs but would
     destroy their jitter-calculations and traffic-monitoring
     applications.

     The second method means that all additional SRM-generated-packets
     are sent on a separate RTP-data/RTCP channel. Only "standard" RTP-
     data/RTCP packets are sent on the base-channel. This would allow
     clients that doesn't understand the SRM extension to join and
     listen to the session. This is for instance preferable when doing
     semi-reliable audio/video transfers where clients understanding the
     extension could get extra value.

   The second method is the one chosen and discussed in the rest of this
   draft. The added channel is called the "SRM-channel".

4.1 NACKs (point 3)

   The NACK-transmissions should be implemented using RTCP by adding a
   new RTCP packet-type, 205 is proposed for now.

4.2 Retransmission of data-packets (point 4)

   The retransmissions can be handled in three ways:

     * Either we just let applications that don't understand the SRM-
       extension to see the retransmitted packets as original data and
       interpret them as duplicates or late packets. This would be nice
       because applications that don't understand SRM would benefit, but
       it would unfortunately 'break' traffic estimation and analysis
       programs. It would also break applications, like for instance the
       MBone-tool VAT, because they adjust the play-out delay according
       to the jitter of the incoming packets.

     * Or all the retransmitted packets could be encapsulated using a



Peter Parnes                                                    [Page 6]


INTERNET-DRAFT                RTP-EXT-SRM                  November 1996


       new payload-type for redundant data. This would not break
       existing applications as RTP states that unknown payload-types
       should be ignored.

     * Or the extra data-packets could be sent on a separate RTP-channel
       using layered encoding. This means that the extension-
       understanding client would listen to at least two channels, the
       base-channel (an ordinary RTP-channel) and a secondary channel,
       the SRM-channel, where all SRM-originated traffic is sent. The
       SRM-channel should be run on a separate multicast group to
       benefit from group pruning.

       The third method is to be used.

4.3 Heartbeats (point 5)

   The heartbeats could be incorporated into the SR-packets using the
   "profile-specific-extensions" but two things argue against this:

     * This isn't inter-operable with other payload-specific-extensions
       as there can only be one header-extension.

     * The SR-packets are only sent during and once right after a sender
       transmits data but the heartbeats should be sent all the time, so
       a receiver that have lost several packets directly after the end
       of a data-flow would notice that she has lost packets.

   Instead the heartbeats should be sent using the new RTCP-SRM packet-
   type on the SRM-channel.

   These heartbeats should be sent more often right after a sender has
   stopped sending so receivers can notice that they have lost packets
   more quickly. This isn't a problem for small groups as the dynamic
   delay between RTCP-packets is small but in larger groups the delay
   could become a problem.

4.4 Time-stamps (point 7)

   Time-stamps for active senders on the RTP-channel are calculated
   using the Sender- and Receiver reports (SR and RR) in RTP as
   described in section 6.3.1 of [RFC1889]. No extra information is
   needed for this.

   This section describe how time-stamps for passive members should be
   calculated.

   The time-stamps are added into the new RTCP-packet-type and divided
   into two types.



Peter Parnes                                                    [Page 7]


INTERNET-DRAFT                RTP-EXT-SRM                  November 1996


   The first type is "time-stamp queries" where a member sends out his
   wall-clock time-stamp and every other member of the session is
   expected to answer this query using the second type "time-stamp
   replies".

   A time-stamp query should be included in the first RTCP-packet that a
   new member sends out after joining the session.

   Replies to pending queries (if any) should be sent each time we send
   a RTCP-packet. Several replies can be contained in the same RTCP-
   packet as it would lower the overhead. The time-stamp replies for a
   new member should have higher priority than replies to an old member.

   When old receivers see a new member they should set a query-timer
   chosen randomly from the interval [0.5 , 5]*current_RTCP_interval
   with a minimum of 5 seconds and a maximum of 2 minutes. If they
   before the expire don't see another query they send out a query of
   their own. If they on the other hand do see a query they reset their
   query-timer and choose a new random expire-time.

   Each 5*current_RTCP_interval since last query, (minimum 2 minutes,
   maximum 5 minutes) the member sends out a new query.


5. Packet format

   This section explains the format of the new RTCP-SRM packets.

   A new RTCP packet type is defined, PT = 205.

5.1 Header format

   The header is the same for all RTCP-SRM packets:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V=2|CM | Count |       PT      |             length            | header
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

     Version (V): 2 bits
          Identifies the version of RTP, two (2).

     Command (CM): 2 bits
          The SRM-command as defined below.

     Count: 4 bits
          The number of command-data-fields minus one included in this



Peter Parnes                                                    [Page 8]


INTERNET-DRAFT                RTP-EXT-SRM                  November 1996


          packet. How this field is used depends on the command, see
          below.

     Packet type (PT): 8 bits
          Contains the constant 205 to identify this RTCP packet as a
          SRM packet.

     Packet length (length): 16 bits
          The length of this RTCP packet in 32-bit words minus one,
          including the header. (The offset of one makes zero a valid
          length and avoids a possible infinite loop in scanning a
          compound RTCP packet, while counting 32-bit words avoids a
          validity check for a multiple of 4.)

   Depending on the Command (CM) field the header is followed by any of
   the following:

5.2 Heartbeat (CM=0)

   For heartbeats CM contains the constant 0 and the header is followed
   by 16-bits containing the latest sequence number used by the sender
   when this report was issued. The next 16 bits contain the cycle
   number which indicate how many times the sender has wrapped his
   sequence number. This allows for clients detect that they have lost
   more that 2^16 packets (when the sequence number counter wraps
   around) which can happen during net-failure and/or high
   transmission-speeds.

   The sender is identified by the SSRC field in the SR or RR-report
   that must come before this SRM-packet.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |        Sequence number        |         Cycle number          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Sequence number: 16 bits
          The highest sequence number used by the sender when sending
          this report.

     Cycle number: 16 bits
          How many times the sender has wrapped his sequence number.

5.3 Time-stamp queries (CM=1)

   To be able to calculate the host-to-host delay a member has to send
   out a time-stamp. The time-stamp is composed of the 32 middle bits of



Peter Parnes                                                    [Page 9]


INTERNET-DRAFT                RTP-EXT-SRM                  November 1996


   a 64 bits NTP time-stamp, meaning the 16 first bits signify the
   seconds and the later 16 bits signify the fraction.

   The CM-field in the header contains the constant 1 and the count
   field is not used.

   The header is followed by the following:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                           time-stamp                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     time-stamp: 32 bits
          The senders current wall-clock time as defined above.

5.4 Time-stamp replies (CM=2)

   Several time-stamp replies can be contained in the same SRM-packet
   and the number of replies are count+1.

   A reply-packet for a certain receiver should only be issued if we
   earlier have received a time-stamp query.

   The header is followed by the following:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                             SSRC_1                            |block1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Last time-stamp(LTQ)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              DLTQ                             |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                             SSRC_2                            |block2
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ....                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     SSRC_n: 32 bits
          The SSRC of the sender to whom we are answering.

     Last time-stamp query (LTQ): 32 bits
          The time-stamp that SSRC_n sent out earlier

     Delay since last time-stamp query (DLTQ): 32 bits



Peter Parnes                                                   [Page 10]


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          The delay, expressed in units of 1/65536 seconds, between
          receiving the last time-stamp query packet from source SSRC_n
          and sending this reply.

   If SSRC_n receives this reply at time A the host-to-host delay, D,
   can be calculated as

                         D = (A - LTQ - DLTQ) / 2

5.5 NACKs (CM=3)

   Three different types of NACK-packets are currently defined:

     Single NACKs for specific packets from one sender.

     Sequential NACKs requesting a series of lost packets.

     Application specific NACKs.

   Each RTCP-NACK can include NACKs for one sender, but several NACKs
   for different senders may be included into a compound RTCP-packet.

   Different NACK-types are distinguished by the NACK-Type-field, NT.

   Each NACK-request also include an urgent-flag (U) indicating (if
   raised) that this request should be prioritized over requests that
   don't have the flag set. How this flag is used is application
   specific (see section 6).

5.5.1 Single NACKs, (NT=0)

   This type of NACK is used for requesting lost packets by specifying
   each lost packet.

   The CM-field contains the constant 0 and the count field contains the
   number of NACKs minus one included in this SRM-packet. This makes
   zero a useful number as it doesn't make much sense to send an empty
   NACK packet just containing the SSRC.

   The header is followed by the following:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |NT |U|        not used         |          Cycle count          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              SSRC                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



Peter Parnes                                                   [Page 11]


INTERNET-DRAFT                RTP-EXT-SRM                  November 1996


   |                     First Sequence number                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                              ....                             :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Last Sequence number                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     NACK-Type (NT): 2 bits
          Indicates the type of the NACK as a single NACK, 0.

     Urgent (U): 1 bit
          Indicating that this is an urgent request.

     Cycle count: 16 bits
          The cycle count for the first sequence number as reported in
          an earlier heartbeat.

     SSRC: 32 bits
          The SSRC of the sender from whom we have lost the packets.

     First/Last Sequence number: 32 bits
          The sequence numbers of the packets lost from this SSRC. The
          number of sequence numbers is determined by the count-field in
          the header.

5.5.2 Sequential NACKs, (NT=1)

   This type of NACK is used for requesting lost packets by specifying a
   sequence number and the number of following lost packets.

   The CM-field contains the constant 1 and the count field contains the
   number of NACKs minus one included in this SRM-packet. This makes
   zero a useful number as it doesn't make much sense to send an empty
   NACK packet just containing the SSRC.

   The header is followed by the following:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |NT |U| Number of lost packets  |          Cycle count          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              SSRC                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     First Sequence number                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     NACK-Type (NT): 2 bits



Peter Parnes                                                   [Page 12]


INTERNET-DRAFT                RTP-EXT-SRM                  November 1996


          Indicates the type of the NACK as a sequential NACK, 1.

     Urgent (U): 1 bit
          Indicating that this is an urgent request.

     Number of lost packets: 13 bits
          The number of lost packets following the specified sequence
          number.

     Cycle count: 16 bits
          The cycle count for the first sequence number as reported in
          an earlier heartbeat.

     SSRC: 32 bits
          The SSRC of the sender from whom we have lost the packets.

     First Sequence number: 32 bits
          The sequence number of the first packets lost from this SSRC
          in this sequence of lost packets.

5.5.2 Application specific NACKs, (NT=2)

   This type of NACK is used for requesting lost packets in a way that
   is specific for the application using this RTP/SRM scheme. It can be
   used by applications to optimize the buffering by allowing repairers
   to reconstruct repair-packets from some other representation of the
   data. This could be used for file-transmissions where the incoming
   data is transformed into a file.

   The CM-field contains the constant 2 and the length of this packet is
   determined by the length field in the header.

   The header is at-least followed by 4 bytes where the first byte is
   defined as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |NT |                      Un-specified                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                              ...                              :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                              ...                              :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     NACK-Type (NT): 2 bits
          Indicates the type of the NACK as a application specific NACK,
          2.



Peter Parnes                                                   [Page 13]


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          6. Semi-reliable sessions

          Not all applications can wait a long time-period before
          receiving a repair for a lost packet. For instance, real-time
          data as audio and video that is played out almost immediately
          as received would have to get their repairs almost
          immediately. This leads to that the NACKs for such streams has
          to be prioritized over other NACKs if contained within the
          same RTP-session. The U-flag in the NACK-RTCP-packet can be
          used for this.

          For some applications it might not make any sense in receiving
          the repair after a certain time-period (as the real-time data
          might already have been played out). These applications might
          decide not to retransmit a certain repair if the time since
          they received the NACK plus the network distance is larger
          that some number. This number is application specific.

          This type of sessions are called semi-reliable sessions.


7. Further issues

   [2] presents algorithms for dynamically adjusting the timer
   parameters C1,C2,D1 and D2. These algorithms should be included but
   do not imply any changes to the actual packet-format.

   [2] also presents methods for "local recovery" meaning that we don't
   multicast NACKs to the whole session but instead minimize the scope
   of the NACKs by adjusting the TTL. This TTL is adaptively changing as
   clients get to know their "loss neighbourhood".


8. Acknowledgments

   I'd like to thank Jon Crowcroft, Anders Klemets and Todd Montgomery
   for creative comments and encouragements.

   This work is done within the Multimedia Assisted distributed Tele-
   Engineering Services, MATES, project (ESPRIT 20598). I want to thank
   the Department of Computer Science and the Centre for Distance-
   spanning Technology at the Lulea Technical University for giving me
   the opportunity to do this work.


9. Author's Address

        Peter Parnes



Peter Parnes                                                   [Page 14]


INTERNET-DRAFT                RTP-EXT-SRM                  November 1996


        Dept. of Computer Science/Centre for Distance-spanning Technology
        Lulea Technical University
        S-971 87 Lulea, Sweden
        Tel: +46 920 72421
        Fax: +46 920 72191
        E-Mail: peppar@cdt.luth.se
        WWW: <URL:http://www.cdt.luth.se/~peppar/>


10. References

      [1] Schulzrine/Casner/Frederic/Jacobson, "RTP: A Transport
          Protocol for Real-Time Applications", RFC 1889, January 1996

      [2] Floyd/Jacobson/McCanne/Liu/Zhang, "A Reliable Multicast
          Framework for Light-weight Sessions and Application Level
          Framing", Proceedings of ACM SIGCOMM 95,
          <URL:ftp://ftp.ee.lbl.gov/papers/srm_sigcomm.ps.Z>. An
          extended and corrected version is submitted to IEEE/ACM
          Transactions on Networking,
          <URL:ftp://ftp.ee.lbl.gov/papers/srm1.tech.ps.Z>






























Peter Parnes                                                   [Page 15]