





Network Working Group                                   T. Friedman, Ed.
Request for Comments: 3611                                       Paris 6
Category: Standards Track                                R. Caceres, Ed.
                                                            IBM Research
                                                           A. Clark, Ed.
                                                                Telchemy
                                                           November 2003


            RTP Control Protocol Extended Reports (RTCP XR)

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 Internet Society (2003).  All Rights Reserved.

Abstract

   This document defines the Extended Report (XR) packet type for the
   RTP Control Protocol (RTCP), and defines how the use of XR packets
   can be signaled by an application if it employs the Session
   Description Protocol (SDP).  XR packets are composed of report
   blocks, and seven block types are defined here.  The purpose of the
   extended reporting format is to convey information that supplements
   the six statistics that are contained in the report blocks used by
   RTCP's Sender Report (SR) and Receiver Report (RR) packets.  Some
   applications, such as multicast inference of network characteristics
   (MINC) or voice over IP (VoIP) monitoring, require other and more
   detailed statistics.  In addition to the block types defined here,
   additional block types may be defined in the future by adhering to
   the framework that this document provides.













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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
       1.1.  Applicability. . . . . . . . . . . . . . . . . . . . . .  4
       1.2.  Terminology. . . . . . . . . . . . . . . . . . . . . . .  7
   2.  XR Packet Format . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Extended Report Block Framework. . . . . . . . . . . . . . . .  8
   4.  Extended Report Blocks . . . . . . . . . . . . . . . . . . . .  9
       4.1.  Loss RLE Report Block. . . . . . . . . . . . . . . . . .  9
             4.1.1.  Run Length Chunk . . . . . . . . . . . . . . . . 15
             4.1.2.  Bit Vector Chunk . . . . . . . . . . . . . . . . 15
             4.1.3.  Terminating Null Chunk . . . . . . . . . . . . . 16
       4.2.  Duplicate RLE Report Block . . . . . . . . . . . . . . . 16
       4.3.  Packet Receipt Times Report Block. . . . . . . . . . . . 18
       4.4.  Receiver Reference Time Report Block . . . . . . . . . . 20
       4.5.  DLRR Report Block. . . . . . . . . . . . . . . . . . . . 21
       4.6.  Statistics Summary Report Block. . . . . . . . . . . . . 22
       4.7.  VoIP Metrics Report Block. . . . . . . . . . . . . . . . 25
             4.7.1.  Packet Loss and Discard Metrics. . . . . . . . . 27
             4.7.2.  Burst Metrics. . . . . . . . . . . . . . . . . . 27
             4.7.3.  Delay Metrics. . . . . . . . . . . . . . . . . . 30
             4.7.4.  Signal Related Metrics . . . . . . . . . . . . . 31
             4.7.5.  Call Quality or Transmission Quality Metrics . . 33
             4.7.6.  Configuration Parameters . . . . . . . . . . . . 34
             4.7.7.  Jitter Buffer Parameters . . . . . . . . . . . . 36
   5.  SDP Signaling. . . . . . . . . . . . . . . . . . . . . . . . . 36
       5.1.  The SDP Attribute. . . . . . . . . . . . . . . . . . . . 37
       5.2.  Usage in Offer/Answer. . . . . . . . . . . . . . . . . . 40
       5.3.  Usage Outside of Offer/Answer. . . . . . . . . . . . . . 42
   6.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 42
       6.1.  XR Packet Type . . . . . . . . . . . . . . . . . . . . . 42
       6.2.  RTCP XR Block Type Registry. . . . . . . . . . . . . . . 42
       6.3.  The "rtcp-xr" SDP Attribute. . . . . . . . . . . . . . . 43
   7.  Security Considerations. . . . . . . . . . . . . . . . . . . . 44
   A.  Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . 46
       A.1.  Sequence Number Interpretation . . . . . . . . . . . . . 46
       A.2.  Example Burst Packet Loss Calculation. . . . . . . . . . 47
   Intellectual Property Notice . . . . . . . . . . . . . . . . . . . 49
   Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . 50
   Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
   References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
   Normative References . . . . . . . . . . . . . . . . . . . . . . . 51
   Informative References . . . . . . . . . . . . . . . . . . . . . . 51
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 53
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 55






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1.  Introduction

   This document defines the Extended Report (XR) packet type for the
   RTP Control Protocol (RTCP) [9], and defines how the use of XR
   packets can be signaled by an application if it employs the Session
   Description Protocol (SDP) [4].  XR packets convey information beyond
   that already contained in the reception report blocks of RTCP's
   sender report (SR) or Receiver Report (RR) packets.  The information
   is of use across RTP profiles, and so is not appropriately carried in
   SR or RR profile-specific extensions.  Information used for network
   management falls into this category, for instance.

   The definition is broken out over the three sections that follow the
   Introduction.  Section 2 defines the XR packet as consisting of an
   eight octet header followed by a series of components called report
   blocks.  Section 3 defines the common format, or framework,
   consisting of a type and a length field, required for all report
   blocks.  Section 4 defines several specific report block types.
   Other block types can be defined in future documents as the need
   arises.

   The report block types defined in this document fall into three
   categories.  The first category consists of packet-by-packet reports
   on received or lost RTP packets.  Reports in the second category
   convey reference time information between RTP participants.  In the
   third category, reports convey metrics relating to packet receipts,
   that are summary in nature but that are more detailed, or of a
   different type, than that conveyed in existing RTCP packets.

   All told, seven report block formats are defined by this document.
   Of these, three are packet-by-packet block types:

   -  Loss RLE Report Block (Section 4.1): Run length encoding of
      reports concerning the losses and receipts of RTP packets.

   -  Duplicate RLE Report Block (Section 4.2): Run length encoding of
      reports concerning duplicates of received RTP packets.

   -  Packet Receipt Times Report Block (Section 4.3): A list of
      reception timestamps of RTP packets.

   There are two reference time related block types:

   -  Receiver Reference Time Report Block (Section 4.4): Receiver-end
      wallclock timestamps.  Together with the DLRR Report Block
      mentioned next, these allow non-senders to calculate round-trip
      times.




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   -  DLRR Report Block (Section 4.5): The delay since the last Receiver
      Reference Time Report Block was received.  An RTP data sender that
      receives a Receiver Reference Time Report Block can respond with a
      DLRR Report Block, in much the same way as, in the mechanism
      already defined for RTCP [9, Section 6.3.1], an RTP data receiver
      that receives a sender's NTP timestamp can respond by filling in
      the DLSR field of an RTCP reception report block.

   Finally, this document defines two summary metric block types:

   -  Statistics Summary Report Block (Section 4.6): Statistics on RTP
      packet sequence numbers, losses, duplicates, jitter, and TTL or
      Hop Limit values.

   -  VoIP Metrics Report Block (Section 4.7): Metrics for monitoring
      Voice over IP (VoIP) calls.

   Before proceeding to the XR packet and report block definitions, this
   document provides an applicability statement (Section 1.1) that
   describes the contexts in which these report blocks can be used.  It
   also defines (Section 1.2) the normative use of key words, such as
   MUST and SHOULD, as they are employed in this document.

   Following the definitions of the various report blocks, this document
   describes how applications that employ SDP can signal their use
   (Section 5).  The document concludes with a discussion (Section 6) of
   numbering considerations for the Internet Assigned Numbers Authority
   (IANA), of security considerations (Section 7), and with appendices
   that provide examples of how to implement algorithms discussed in the
   text.

1.1.  Applicability

   The XR packets are useful across multiple applications, and for that
   reason are not defined as profile-specific extensions to RTCP sender
   or Receiver Reports [9, Section 6.4.3].  Nonetheless, they are not of
   use in all contexts.  In particular, the VoIP metrics report block
   (Section 4.7) is specific to voice applications, though it can be
   employed over a wide variety of such applications.

   The VoIP metrics report block can be applied to any one-to-one or
   one-to-many voice application for which the use of RTP and RTCP is
   specified.  The use of conversational metrics (Section 4.7.5),
   including the R factor (as described by the E Model defined in [3])
   and the mean opinion score for conversational quality (MOS-CQ), in
   applications other than simple two party calls is not defined; hence,
   these metrics should be identified as unavailable in multicast
   conferencing applications.



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   The packet-by-packet report block types, Loss RLE (Section 4.1),
   Duplicate RLE (Section 4.2), and Packet Receipt Times (Section 4.3),
   have been defined with network tomography applications, such as
   multicast inference of network characteristics (MINC) [11], in mind.
   MINC requires detailed packet receipt traces from multicast session
   receivers in order to infer the gross structure of the multicast
   distribution tree and the parameters, such as loss rates and delays,
   that apply to paths between the branching points of that tree.

   Any real time multicast multimedia application can use the packet-
   by-packet report block types.  Such an application could employ a
   MINC inference subsystem that would provide it with multicast tree
   topology information.  One potential use of such a subsystem would be
   for the identification of high loss regions in the multicast tree and
   the identification of multicast session participants well situated to
   provide retransmissions of lost packets.

   Detailed packet-by-packet reports do not necessarily have to consume
   disproportionate bandwidth with respect to other RTCP packets.  An
   application can cap the size of these blocks.  A mechanism called
   "thinning" is provided for these report blocks, and can be used to
   ensure that they adhere to a size limit by restricting the number of
   packets reported upon within any sequence number interval.  The
   rationale for, and use of this mechanism is described in [13].
   Furthermore, applications might not require report blocks from all
   receivers in order to answer such important questions as where in the
   multicast tree there are paths that exceed a defined loss rate
   threshold.  Intelligent decisions regarding which receivers send
   these report blocks can further restrict the portion of RTCP
   bandwidth that they consume.

   The packet-by-packet report blocks can also be used by dedicated
   network monitoring applications.  For such an application, it might
   be appropriate to allow more than 5% of RTP data bandwidth to be used
   for RTCP packets, thus allowing proportionately larger and more
   detailed report blocks.

   Nothing in the packet-by-packet block types restricts their use to
   multicast applications.  In particular, they could be used for
   network tomography similar to MINC, but using striped unicast packets
   instead.  In addition, if it were found useful, they could be used
   for applications limited to two participants.

   One use to which the packet-by-packet reports are not immediately
   suited is for data packet acknowledgments as part of a packet
   retransmission mechanism.  The reason is that the packet accounting
   technique suggested for these blocks differs from the packet
   accounting normally employed by RTP.  In order to favor measurement



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   applications, an effort is made to interpret as little as possible at
   the data receiver, and leave the interpretation as much as possible
   to participants that receive the report blocks.  Thus, for example, a
   packet with an anomalous SSRC ID or an anomalous sequence number
   might be excluded by normal RTP accounting, but would be reported
   upon for network monitoring purposes.

   The Statistics Summary Report Block (Section 4.6) has also been
   defined with network monitoring in mind.  This block type can be used
   equally well for reporting on unicast and multicast packet reception.

   The reference time related block types were conceived for receiver-
   based TCP-friendly multicast congestion control [18].  By allowing
   data receivers to calculate their round trip times to senders, they
   help the receivers estimate the downstream bandwidth they should
   request.  Note that if every receiver is to send Receiver Reference
   Time Report Blocks (Section 4.4), a sender might potentially send a
   number of DLRR Report Blocks (Section 4.5) equal to the number of
   receivers whose RTCP packets have arrived at the sender within its
   reporting interval.  As the number of participants in a multicast
   session increases, an application should use discretion regarding
   which participants send these blocks, and how frequently.

   XR packets supplement the existing RTCP packets, and may be stacked
   with other RTCP packets to form compound RTCP packets [9, Section 6].
   The introduction of XR packets into a session in no way changes the
   rules governing the calculation of the RTCP reporting interval [9,
   Section 6.2].  As XR packets are RTCP packets, they count as such for
   bandwidth calculations.  As a result, the addition of extended
   reporting information may tend to increase the average RTCP packet
   size, and thus the average reporting interval.  This increase may be
   limited by limiting the size of XR packets.

   The SDP signaling defined for XR packets in this document (Section 5)
   was done so with three use scenarios in mind: a Real Time Streaming
   Protocol (RTSP) controlled streaming application, a one-to-many
   multicast multimedia application such as a course lecture with
   enhanced feedback, and a Session Initiation Protocol (SIP) controlled
   conversational session involving two parties.  Applications that
   employ SDP are free to use additional SDP signaling for cases not
   covered here.  In addition, applications are free to use signaling
   mechanisms other than SDP.









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1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119 [1]
   and indicate requirement levels for compliance with this
   specification.

2.  XR Packet Format

   An XR packet consists of a header of two 32-bit words, followed by a
   number, possibly zero, of extended report blocks.  This type of
   packet is laid out in a manner consistent with other RTCP packets, as
   concerns the essential version, packet type, and length information.
   XR packets are thus backwards compatible with RTCP receiver
   implementations that do not recognize them, but that ought to be able
   to parse past them using the length information.  A padding field and
   an SSRC field are also provided in the same locations that they
   appear in other RTCP packets, for simplicity.  The format is 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V=2|P|reserved |   PT=XR=207   |             length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              SSRC                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                         report blocks                         :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   version (V): 2 bits
         Identifies the version of RTP.  This specification applies to
         RTP version two.

   padding (P): 1 bit
         If the padding bit is set, this XR packet contains some
         additional padding octets at the end.  The semantics of this
         field are identical to the semantics of the padding field in
         the SR packet, as defined by the RTP specification.

   reserved: 5 bits
         This field is reserved for future definition.  In the absence
         of such definition, the bits in this field MUST be set to zero
         and MUST be ignored by the receiver.






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   packet type (PT): 8 bits
         Contains the constant 207 to identify this as an RTCP XR
         packet.  This value is registered with the Internet Assigned
         Numbers Authority (IANA), as described in Section 6.1.

   length: 16 bits
         As described for the RTCP Sender Report (SR) packet (see
         Section 6.4.1 of the RTP specification [9]).  Briefly, the
         length of this XR packet in 32-bit words minus one, including
         the header and any padding.

   SSRC: 32 bits
         The synchronization source identifier for the originator of
         this XR packet.

   report blocks: variable length.
         Zero or more extended report blocks.  In keeping with the
         extended report block framework defined below, each block MUST
         consist of one or more 32-bit words.

3.  Extended Report Block Framework

   Extended report blocks are stacked, one after the other, at the end
   of an XR packet.  An individual block's length is a multiple of 4
   octets.  The XR header's length field describes the total length of
   the packet, including these extended report blocks.

   Each block has block type and length fields that facilitate parsing.
   A receiving application can demultiplex the blocks based upon their
   type, and can use the length information to locate each successive
   block, even in the presence of block types it does not recognize.

   An extended report block has the following format:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      BT       | type-specific |         block length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :             type-specific block contents                      :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   block type (BT): 8 bits
         Identifies the block format.  Seven block types are defined in
         Section 4.  Additional block types may be defined in future
         specifications.  This field's name space is managed by the
         Internet Assigned Numbers Authority (IANA), as described in
         Section 6.2.



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   type-specific: 8 bits
         The use of these bits is determined by the block type
         definition.

   block length: 16 bits
         The length of this report block, including the header, in 32-
         bit words minus one.  If the block type definition permits,
         zero is an acceptable value, signifying a block that consists
         of only the BT, type-specific, and block length fields, with a
         null type-specific block contents field.

   type-specific block contents: variable length
         The use of this field is defined by the particular block type,
         subject to the constraint that it MUST be a multiple of 32 bits
         long.  If the block type definition permits, It MAY be zero
         bits long.

4.  Extended Report Blocks

   This section defines seven extended report blocks: block types for
   reporting upon received packet losses and duplicates, packet
   reception times, receiver reference time information, receiver
   inter-report delays, detailed reception statistics, and voice over IP
   (VoIP) metrics.  An implementation SHOULD ignore incoming blocks with
   types not relevant or unknown to it.  Additional block types MUST be
   registered with the Internet Assigned Numbers Authority (IANA) [16],
   as described in Section 6.2.

4.1.  Loss RLE Report Block

   This block type permits detailed reporting upon individual packet
   receipt and loss events.  Such reports can be used, for example, for
   multicast inference of network characteristics (MINC) [11].  With
   MINC, one can discover the topology of the multicast tree used for
   distributing a source's RTP packets, and of the loss rates along
   links within that tree, or they could be used to provide raw data to
   a network management application.

   Since a Boolean trace of lost and received RTP packets is potentially
   lengthy, this block type permits the trace to be compressed through
   run length encoding.  To further reduce block size, loss event
   reports can be systematically dropped from the trace in a mechanism
   called thinning that is described below and that is studied in [13].

   A participant that generates a Loss RLE Report Block should favor
   accuracy in reporting on observed events over interpretation of those
   events whenever possible.  Interpretation should be left to those who
   observe the report blocks.  Following this approach implies that



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   accounting for Loss RLE Report Blocks will differ from the accounting
   for the generation of the SR and RR packets described in the RTP
   specification [9] in the following two areas: per-sender accounting
   and per-packet accounting.

   In its per-sender accounting, an RTP session participant SHOULD NOT
   make the receipt of a threshold minimum number of RTP packets a
   condition for reporting upon the sender of those packets.  This
   accounting technique differs from the technique described in Section
   6.2.1 and Appendix A.1 of the RTP specification that allows a
   threshold to determine whether a sender is considered valid.

   In its per-packet accounting, an RTP session participant SHOULD treat
   all sequence numbers as valid.  This accounting technique differs
   from the technique described in Appendix A.1 of the RTP specification
   that suggests ruling a sequence number valid or invalid on the basis
   of its contiguity with the sequence numbers of previously received
   packets.

   Sender validity and sequence number validity are interpretations of
   the raw data.  Such interpretations are justified in the interest,
   for example, of excluding the stray old packet from an unrelated
   session from having an effect upon the calculation of the RTCP
   transmission interval.  The presence of stray packets might, on the
   other hand, be of interest to a network monitoring application.

   One accounting interpretation that is still necessary is for a
   participant to decide whether the 16 bit sequence number has rolled
   over.  Under ordinary circumstances this is not a difficult task.
   For example, if packet number 65,535 (the highest possible sequence
   number) is followed shortly by packet number 0, it is reasonable to
   assume that there has been a rollover.  However, it is possible that
   the packet is an earlier one (from 65,535 packets earlier).  It is
   also possible that the sequence numbers have rolled over multiple
   times, either forward or backward.  The interpretation becomes more
   difficult when there are large gaps between the sequence numbers,
   even accounting for rollover, and when there are long intervals
   between received packets.

   The per-packet accounting technique mandated here is for a
   participant to keep track of the sequence number of the packet most
   recently received from a sender.  For the next packet that arrives
   from that sender, the sequence number MUST be judged to fall no more
   than 32,768 packets ahead or behind the most recent one, whichever
   choice places it closer.  In the event that both choices are equally
   distant (only possible when the distance is 32,768), the choice MUST
   be the one that does not require a rollover.  Appendix A.1 presents
   an algorithm that implements this technique.



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   Each block reports on a single RTP data packet source, identified by
   its SSRC.  The receiver that is supplying the report is identified in
   the header of the RTCP packet.

   Choice of beginning and ending RTP packet sequence numbers for the
   trace is left to the application.  These values are reported in the
   block.  The last sequence number in the trace MAY differ from the
   sequence number reported on in any accompanying SR or RR report.

   Note that because of sequence number wraparound, the ending sequence
   number MAY be less than the beginning sequence number.  A Loss RLE
   Report Block MUST NOT be used to report upon a range of 65,534 or
   greater in the sequence number space, as there is no means of
   identifying multiple wraparounds.

   The trace described by a Loss RLE report consists of a sequence of
   Boolean values, one for each sequence number of the trace.  A value
   of one represents a packet receipt, meaning that one or more packets
   having that sequence number have been received since the most recent
   wraparound of sequence numbers (or since the beginning of the RTP
   session if no wraparound has been judged to have occurred).  A value
   of zero represents a packet loss, meaning that there has been no
   packet receipt for that sequence number as of the time of the report.
   If a packet with a given sequence number is received after a report
   of a loss for that sequence number, a later Loss RLE report MAY
   report a packet receipt for that sequence number.

   The encoding itself consists of a series of 16 bit units called
   chunks that describe sequences of packet receipts or losses in the
   trace.  Each chunk either specifies a run length or a bit vector, or
   is a null chunk.  A run length describes between 1 and 16,383 events
   that are all the same (either all receipts or all losses).  A bit
   vector describes 15 events that may be mixed receipts and losses.  A
   null chunk describes no events, and is used to round out the block to
   a 32 bit word boundary.

   The mapping from a sequence of lost and received packets into a
   sequence of chunks is not necessarily unique.  For example, the
   following trace covers 45 packets, of which the 22nd and 24th have
   been lost and the others received:

      1111 1111 1111 1111 1111 1010 1111 1111 1111 1111 1111 1









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   One way to encode this would be:

      bit vector 1111 1111 1111 111
      bit vector 1111 1101 0111 111
      bit vector 1111 1111 1111 111
      null chunk

   Another way to encode this would be:

      run of 21 receipts
      bit vector 0101 1111 1111 111
      run of 9 receipts
      null chunk

   The choice of encoding is left to the application.  As part of this
   freedom of choice, applications MAY terminate a series of run length
   and bit vector chunks with a bit vector chunk that runs beyond the
   sequence number space described by the report block.  For example, if
   the 44th packet in the same sequence was lost:

      1111 1111 1111 1111 1111 1010 1111 1111 1111 1111 1110 1

   This could be encoded as:

      run of 21 receipts
      bit vector 0101 1111 1111 111
      bit vector 1111 1110 1000 000
      null chunk


   In this example, the last five bits of the second bit vector describe
   a part of the sequence number space that extends beyond the last
   sequence number in the trace.  These bits have been set to zero.

   All bits in a bit vector chunk that describe a part of the sequence
   number space that extends beyond the last sequence number in the
   trace MUST be set to zero, and MUST be ignored by the receiver.

   A null packet MUST appear at the end of a Loss RLE Report Block if
   the number of run length plus bit vector chunks is odd.  The null
   chunk MUST NOT appear in any other context.

   Caution should be used in sending Loss RLE Report Blocks because,
   even with the compression provided by run length encoding, they can
   easily consume bandwidth out of proportion with normal RTCP packets.
   The block type includes a mechanism, called thinning, that allows an
   application to limit report sizes.




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   A thinning value, T, selects a subset of packets within the sequence
   number space: those with sequence numbers that are multiples of 2^T.
   Packet reception and loss reports apply only to those packets.  T can
   vary between 0 and 15.  If T is zero, then every packet in the
   sequence number space is reported upon.  If T is fifteen, then one in
   every 32,768 packets is reported upon.

   Suppose that the trace just described begins at sequence number
   13,821.  The last sequence number in the trace is 13,865.  If the
   trace were to be thinned with a thinning value of T=2, then the
   following sequence numbers would be reported upon: 13,824, 13,828,
   13,832, 13,836, 13,840, 13,844, 13,848, 13,852, 13,856, 13,860,
   13,864.  The thinned trace would be as follows:

      1    1    1    1    1    0    1    1    1    1    0

   This could be encoded as follows:

      bit vector 1111 1011 1100 000
      null chunk

   The last four bits in the bit vector, representing sequence numbers
   13,868, 13,872, 13,876, and 13,880, extend beyond the trace and are
   thus set to zero and are ignored by the receiver.  With thinning, the
   loss of the 22nd packet goes unreported because its sequence number,
   13,842, is not a multiple of four.  Packet receipts for all sequence
   numbers that are not multiples of four also go unreported.  However,
   in this example thinning has permitted the Loss RLE Report Block to
   be shortened by one 32 bit word.

   Choice of the thinning value is left to the application.




















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   The Loss RLE Report Block has the following format:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     BT=1      | rsvd. |   T   |         block length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        SSRC of source                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          begin_seq            |             end_seq           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          chunk 1              |             chunk 2           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                              ...                              :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          chunk n-1            |             chunk n           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   block type (BT): 8 bits
         A Loss RLE Report Block is identified by the constant 1.

   rsvd.: 4 bits
         This field is reserved for future definition.  In the absence
         of such definition, the bits in this field MUST be set to zero
         and MUST be ignored by the receiver.

   thinning (T): 4 bits
         The amount of thinning performed on the sequence number space.
         Only those packets with sequence numbers 0 mod 2^T are reported
         on by this block.  A value of 0 indicates that there is no
         thinning, and all packets are reported on.  The maximum
         thinning is one packet in every 32,768 (amounting to two
         packets within each 16-bit sequence space).

   block length: 16 bits
         Defined in Section 3.

   SSRC of source: 32 bits
         The SSRC of the RTP data packet source being reported upon by
         this report block.

   begin_seq: 16 bits
         The first sequence number that this block reports on.

   end_seq: 16 bits
         The last sequence number that this block reports on plus one.





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   chunk i: 16 bits
         There are three chunk types: run length, bit vector, and
         terminating null, defined in the following sections.  If the
         chunk is all zeroes, then it is a terminating null chunk.
         Otherwise, the left most bit of the chunk determines its type:
         0 for run length and 1 for bit vector.

4.1.1.  Run Length Chunk

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |C|R|        run length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   chunk type (C): 1 bit
         A zero identifies this as a run length chunk.

   run type (R): 1 bit
         Zero indicates a run of 0s.  One indicates a run of 1s.

   run length: 14 bits
         A value between 1 and 16,383.  The value MUST not be zero for a
         run length chunk (zeroes in both the run type and run length
         fields would make the chunk a terminating null chunk).  Run
         lengths of 15 or less MAY be described with a run length chunk
         despite the fact that they could also be described as part of a
         bit vector chunk.

4.1.2.  Bit Vector Chunk

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |C|        bit vector           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   chunk type (C): 1 bit
         A one identifies this as a bit vector chunk.

   bit vector: 15 bits
         The vector is read from left to right, in order of increasing
         sequence number (with the appropriate allowance for
         wraparound).







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4.1.3.  Terminating Null Chunk

   This chunk is all zeroes.

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

4.2.  Duplicate RLE Report Block

   This block type permits per-sequence-number reports on duplicates in
   a source's RTP packet stream.  Such information can be used for
   network diagnosis, and provide an alternative to packet losses as a
   basis for multicast tree topology inference.

   The Duplicate RLE Report Block format is identical to the Loss RLE
   Report Block format.  Only the interpretation is different, in that
   the information concerns packet duplicates rather than packet losses.
   The trace to be encoded in this case also consists of zeros and ones,
   but a zero here indicates the presence of duplicate packets for a
   given sequence number, whereas a one indicates that no duplicates
   were received.

   The existence of a duplicate for a given sequence number is
   determined over the entire reporting period.  For example, if packet
   number 12,593 arrives, followed by other packets with other sequence
   numbers, the arrival later in the reporting period of another packet
   numbered 12,593 counts as a duplicate for that sequence number.  The
   duplicate does not need to follow immediately upon the first packet
   of that number.  Care must be taken that a report does not cover a
   range of 65,534 or greater in the sequence number space.

   No distinction is made between the existence of a single duplicate
   packet and multiple duplicate packets for a given sequence number.
   Note also that since there is no duplicate for a lost packet, a loss
   is encoded as a one in a Duplicate RLE Report Block.













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   The Duplicate RLE Report Block has the following format:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     BT=2      | rsvd. |   T   |         block length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        SSRC of source                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          begin_seq            |             end_seq           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          chunk 1              |             chunk 2           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                              ...                              :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          chunk n-1            |             chunk n           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   block type (BT): 8 bits
         A Duplicate RLE Report Block is identified by the constant 2.

   rsvd.: 4 bits
         This field is reserved for future definition.  In the absence
         of such a definition, the bits in this field MUST be set to
         zero and MUST be ignored by the receiver.

   thinning (T): 4 bits
         As defined in Section 4.1.

   block length: 16 bits
         Defined in Section 3.

   SSRC of source: 32 bits
         As defined in Section 4.1.

   begin_seq: 16 bits
         As defined in Section 4.1.

   end_seq: 16 bits
         As defined in Section 4.1.

   chunk i: 16 bits
         As defined in Section 4.1.








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4.3.  Packet Receipt Times Report Block

   This block type permits per-sequence-number reports on packet receipt
   times for a given source's RTP packet stream.  Such information can
   be used for MINC inference of the topology of the multicast tree used
   to distribute the source's RTP packets, and of the delays along the
   links within that tree.  It can also be used to measure partial path
   characteristics and to model distributions for packet jitter.

   Packet receipt times are expressed in the same units as in the RTP
   timestamps of data packets.  This is so that, for each packet, one
   can establish both the send time and the receipt time in comparable
   terms.  Note, however, that as an RTP sender ordinarily initializes
   its time to a value chosen at random, there can be no expectation
   that reported send and receipt times will differ by an amount equal
   to the one-way delay between sender and receiver.  The reported times
   can nonetheless be useful for the purposes mentioned above.

   At least one packet MUST have been received for each sequence number
   reported upon in this block.  If this block type is used to report
   receipt times for a series of sequence numbers that includes lost
   packets, several blocks are required.  If duplicate packets have been
   received for a given sequence number, and those packets differ in
   their receipt times, any time other than the earliest MUST NOT be
   reported.  This is to ensure consistency among reports.

   Times reported in RTP timestamp format consume more bits than loss or
   duplicate information, and do not lend themselves to run length
   encoding.  The use of thinning is encouraged to limit the size of
   Packet Receipt Times Report Blocks.





















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   The Packet Receipt Times Report Block has the following format:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     BT=3      | rsvd. |   T   |         block length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        SSRC of source                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          begin_seq            |             end_seq           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Receipt time of packet begin_seq                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Receipt time of packet (begin_seq + 1) mod 65536        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                              ...                              :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Receipt time of packet (end_seq - 1) mod 65536          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   block type (BT): 8 bits
         A Packet Receipt Times Report Block is identified by the
         constant 3.

   rsvd.: 4 bits
         This field is reserved for future definition.  In the absence
         of such a definition, the bits in this field MUST be set to
         zero and MUST be ignored by the receiver.

   thinning (T): 4 bits
         As defined in Section 4.1.

   block length: 16 bits
         Defined in Section 3.

   SSRC of source: 32 bits
         As defined in Section 4.1.

   begin_seq: 16 bits
         As defined in Section 4.1.

   end_seq: 16 bits
         As defined in Section 4.1.








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   Packet i receipt time: 32 bits
         The receipt time of the packet with sequence number i at the
         receiver.  The modular arithmetic shown in the packet format
         diagram is to allow for sequence number rollover.  It is
         preferable for the time value to be established at the link
         layer interface, or in any case as close as possible to the
         wire arrival time.  Units and format are the same as for the
         timestamp in RTP data packets.  As opposed to RTP data packet
         timestamps, in which nominal values may be used instead of
         system clock values in order to convey information useful for
         periodic playout, the receipt times should reflect the actual
         time as closely as possible.  For a session, if the RTP
         timestamp is chosen at random, the first receipt time value
         SHOULD also be chosen at random, and subsequent timestamps
         offset from this value.  On the other hand, if the RTP
         timestamp is meant to reflect the reference time at the sender,
         then the receipt time SHOULD be as close as possible to the
         reference time at the receiver.

4.4.  Receiver Reference Time Report Block

   This block extends RTCP's timestamp reporting so that non-senders may
   also send timestamps.  It recapitulates the NTP timestamp fields from
   the RTCP Sender Report [9, Sec. 6.3.1].  A non-sender may estimate
   its round trip time (RTT) to other participants, as proposed in [18],
   by sending this report block and receiving DLRR Report Blocks (see
   next section) in reply.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     BT=4      |   reserved    |       block length = 2        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              NTP timestamp, most significant word             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             NTP timestamp, least significant word             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   block type (BT): 8 bits
         A Receiver Reference Time Report Block is identified by the
         constant 4.

   reserved: 8 bits
         This field is reserved for future definition.  In the absence
         of such definition, the bits in this field MUST be set to zero
         and MUST be ignored by the receiver.





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   block length: 16 bits
         The constant 2, in accordance with the definition of this field
         in Section 3.

   NTP timestamp: 64 bits
         Indicates the wallclock time when this block was sent so that
         it may be used in combination with timestamps returned in DLRR
         Report Blocks (see next section) from other receivers to
         measure round-trip propagation to those receivers.  Receivers
         should expect that the measurement accuracy of the timestamp
         may be limited to far less than the resolution of the NTP
         timestamp.  The measurement uncertainty of the timestamp is not
         indicated as it may not be known.  A report block sender that
         can keep track of elapsed time but has no notion of wallclock
         time may use the elapsed time since joining the session
         instead.  This is assumed to be less than 68 years, so the high
         bit will be zero.  It is permissible to use the sampling clock
         to estimate elapsed wallclock time.  A report sender that has
         no notion of wallclock or elapsed time may set the NTP
         timestamp to zero.

4.5.  DLRR Report Block

   This block extends RTCP's delay since the last Sender Report (DLSR)
   mechanism [9, Sec. 6.3.1] so that non-senders may also calculate
   round trip times, as proposed in [18].  It is termed DLRR for delay
   since the last Receiver Report, and may be sent in response to a
   Receiver Timestamp Report Block (see previous section) from a
   receiver to allow that receiver to calculate its round trip time to
   the respondent.  The report consists of one or more 3 word sub-
   blocks: one sub-block per Receiver Report.

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     BT=5      |   reserved    |         block length          |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                 SSRC_1 (SSRC of first receiver)               | sub-
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
 |                         last RR (LRR)                         |   1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   delay since last RR (DLRR)                  |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                 SSRC_2 (SSRC of second receiver)              | sub-
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
 :                               ...                             :   2
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+




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   block type (BT): 8 bits
         A DLRR Report Block is identified by the constant 5.

   reserved: 8 bits
         This field is reserved for future definition.  In the absence
         of such definition, the bits in this field MUST be set to zero
         and MUST be ignored by the receiver.

   block length: 16 bits
         Defined in Section 3.

   last RR timestamp (LRR): 32 bits
         The middle 32 bits out of 64 in the NTP timestamp (as explained
         in the previous section), received as part of a Receiver
         Reference Time Report Block from participant SSRC_n.  If no
         such block has been received, the field is set to zero.

   delay since last RR (DLRR): 32 bits
         The delay, expressed in units of 1/65536 seconds, between
         receiving the last Receiver Reference Time Report Block from
         participant SSRC_n and sending this DLRR Report Block.  If a
         Receiver Reference Time Report Block has yet to be received
         from SSRC_n, the DLRR field is set to zero (or the DLRR is
         omitted entirely).  Let SSRC_r denote the receiver issuing this
         DLRR Report Block.  Participant SSRC_n can compute the round-
         trip propagation delay to SSRC_r by recording the time A when
         this Receiver Timestamp Report Block is received.  It
         calculates the total round-trip time A-LRR using the last RR
         timestamp (LRR) field, and then subtracting this field to leave
         the round-trip propagation delay as A-LRR-DLRR.  This is
         illustrated in [9, Fig. 2].

4.6.  Statistics Summary Report Block

   This block reports statistics beyond the information carried in the
   standard RTCP packet format, but is not as finely grained as that
   carried in the report blocks previously described.  Information is
   recorded about lost packets, duplicate packets, jitter measurements,
   and TTL or Hop Limit values.  Such information can be useful for
   network management.

   The report block contents are dependent upon a series of flag bits
   carried in the first part of the header.  Not all parameters need to
   be reported in each block.  Flags indicate which are and which are
   not reported.  The fields corresponding to unreported parameters MUST
   be present, but are set to zero.  The receiver MUST ignore any
   Statistics Summary Report Block with a non-zero value in any field
   flagged as unreported.



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   The Statistics Summary Report Block has the following format:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     BT=6      |L|D|J|ToH|rsvd.|       block length = 9        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        SSRC of source                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          begin_seq            |             end_seq           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        lost_packets                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        dup_packets                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         min_jitter                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         max_jitter                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         mean_jitter                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         dev_jitter                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | min_ttl_or_hl | max_ttl_or_hl |mean_ttl_or_hl | dev_ttl_or_hl |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   block type (BT): 8 bits
         A Statistics Summary Report Block is identified by the constant
         6.

   loss report flag (L): 1 bit
         Bit set to 1 if the lost_packets field contains a report, 0
         otherwise.

   duplicate report flag (D): 1 bit
         Bit set to 1 if the dup_packets field contains a report, 0
         otherwise.

   jitter flag (J): 1 bit
         Bit set to 1 if the min_jitter, max_jitter, mean_jitter, and
         dev_jitter fields all contain reports, 0 if none of them do.

   TTL or Hop Limit flag (ToH): 2 bits
         This field is set to 0 if none of the fields min_ttl_or_hl,
         max_ttl_or_hl, mean_ttl_or_hl, or dev_ttl_or_hl contain
         reports.  If the field is non-zero, then all of these fields
         contain reports.  The value 1 signifies that they report on
         IPv4 TTL values.  The value 2 signifies that they report on



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         IPv6 Hop Limit values.  The value 3 is undefined and MUST NOT
         be used.

   rsvd.: 3 bits
         This field is reserved for future definition.  In the absence
         of such a definition, the bits in this field MUST be set to
         zero and MUST be ignored by the receiver.

   block length: 16 bits
         The constant 9, in accordance with the definition of this field
         in Section 3.

   SSRC of source: 32 bits
         As defined in Section 4.1.

   begin_seq: 16 bits
         As defined in Section 4.1.

   end_seq: 16 bits
         As defined in Section 4.1.

   lost_packets: 32 bits
         Number of lost packets in the above sequence number interval.

   dup_packets: 32 bits
         Number of duplicate packets in the above sequence number
         interval.

   min_jitter: 32 bits
         The minimum relative transit time between two packets in the
         above sequence number interval.  All jitter values are measured
         as the difference between a packet's RTP timestamp and the
         reporter's clock at the time of arrival, measured in the same
         units.

   max_jitter: 32 bits
         The maximum relative transit time between two packets in the
         above sequence number interval.

   mean_jitter: 32 bits
         The mean relative transit time between each two packet series
         in the above sequence number interval, rounded to the nearest
         value expressible as an RTP timestamp.

   dev_jitter: 32 bits
         The standard deviation of the relative transit time between
         each two packet series in the above sequence number interval.




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   min_ttl_or_hl: 8 bits
         The minimum TTL or Hop Limit value of data packets in the
         sequence number range.

   max_ttl_or_hl: 8 bits
         The maximum TTL or Hop Limit value of data packets in the
         sequence number range.

   mean_ttl_or_hl: 8 bits
         The mean TTL or Hop Limit value of data packets in the sequence
         number range, rounded to the nearest integer.

   dev_ttl_or_hl: 8 bits
         The standard deviation of TTL or Hop Limit values of data
         packets in the sequence number range.

4.7.  VoIP Metrics Report Block

   The VoIP Metrics Report Block provides metrics for monitoring voice
   over IP (VoIP) calls.  These metrics include packet loss and discard
   metrics, delay metrics, analog metrics, and voice quality metrics.
   The block reports separately on packets lost on the IP channel, and
   those that have been received but then discarded by the receiving
   jitter buffer.  It also reports on the combined effect of losses and
   discards, as both have equal effect on call quality.

   In order to properly assess the quality of a Voice over IP call, it
   is desirable to consider the degree of burstiness of packet loss
   [14].  Following a Gilbert-Elliott model [3], a period of time,
   bounded by lost and/or discarded packets with a high rate of losses
   and/or discards, is a "burst", and a period of time between two
   bursts is a "gap".  Bursts correspond to periods of time during which
   the packet loss rate is high enough to produce noticeable degradation
   in audio quality.  Gaps correspond to periods of time during which
   only isolated lost packets may occur, and in general these can be
   masked by packet loss concealment.  Delay reports include the transit
   delay between RTP end points and the VoIP end system processing
   delays, both of which contribute to the user perceived delay.
   Additional metrics include signal, echo, noise, and distortion
   levels.  Call quality metrics include R factors (as described by the
   E Model defined in [6,3]) and mean opinion scores (MOS scores).

   Implementations MUST provide values for all the fields defined here.
   For certain metrics, if the value is undefined or unknown, then the
   specified default or unknown field value MUST be provided.






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   The block is encoded as seven 32-bit words:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     BT=7      |   reserved    |       block length = 8        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        SSRC of source                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   loss rate   | discard rate  | burst density |  gap density  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       burst duration          |         gap duration          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     round trip delay          |       end system delay        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | signal level  |  noise level  |     RERL      |     Gmin      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   R factor    | ext. R factor |    MOS-LQ     |    MOS-CQ     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   RX config   |   reserved    |          JB nominal           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          JB maximum           |          JB abs max           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   block type (BT): 8 bits
         A VoIP Metrics Report Block is identified by the constant 7.

   reserved: 8 bits
         This field is reserved for future definition.  In the absence
         of such a definition, the bits in this field MUST be set to
         zero and MUST be ignored by the receiver.

   block length: 16 bits
         The constant 8, in accordance with the definition of this field
         in Section 3.

   SSRC of source: 32 bits
         As defined in Section 4.1.

   The remaining fields are described in the following six sections:
   Packet Loss and Discard Metrics, Delay Metrics, Signal Related
   Metrics, Call Quality or Transmission Quality Metrics, Configuration
   Metrics, and Jitter Buffer Parameters.








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4.7.1.  Packet Loss and Discard Metrics

   It is very useful to distinguish between packets lost by the network
   and those discarded due to jitter.  Both have equal effect on the
   quality of the voice stream, however, having separate counts helps
   identify the source of quality degradation.  These fields MUST be
   populated, and MUST be set to zero if no packets have been received.

   loss rate: 8 bits
         The fraction of RTP data packets from the source lost since the
         beginning of reception, expressed as a fixed point number with
         the binary point at the left edge of the field.  This value is
         calculated by dividing the total number of packets lost (after
         the effects of applying any error protection such as FEC) by
         the total number of packets expected, multiplying the result of
         the division by 256, limiting the maximum value to 255 (to
         avoid overflow), and taking the integer part.  The numbers of
         duplicated packets and discarded packets do not enter into this
         calculation.  Since receivers cannot be required to maintain
         unlimited buffers, a receiver MAY categorize late-arriving
         packets as lost.  The degree of lateness that triggers a loss
         SHOULD be significantly greater than that which triggers a
         discard.

   discard rate: 8 bits
         The fraction of RTP data packets from the source that have been
         discarded since the beginning of reception, due to late or
         early arrival, under-run or overflow at the receiving jitter
         buffer.  This value is expressed as a fixed point number with
         the binary point at the left edge of the field.  It is
         calculated by dividing the total number of packets discarded
         (excluding duplicate packet discards) by the total number of
         packets expected, multiplying the result of the division by
         256, limiting the maximum value to 255 (to avoid overflow), and
         taking the integer part.

4.7.2.  Burst Metrics

   A burst is a period during which a high proportion of packets are
   either lost or discarded due to late arrival.  A burst is defined, in
   terms of a value Gmin, as the longest sequence that (a) starts with a
   lost or discarded packet, (b) does not contain any occurrences of
   Gmin or more consecutively received (and not discarded) packets, and
   (c) ends with a lost or discarded packet.

   A gap, informally, is a period of low packet losses and/or discards.
   Formally, a gap is defined as any of the following: (a) the period
   from the start of an RTP session to the receipt time of the last



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   received packet before the first burst, (b) the period from the end
   of the last burst to either the time of the report or the end of the
   RTP session, whichever comes first, or (c) the period of time between
   two bursts.

   For the purpose of determining if a lost or discarded packet near the
   start or end of an RTP session is within a gap or a burst, it is
   assumed that the RTP session is preceded and followed by at least
   Gmin received packets, and that the time of the report is followed by
   at least Gmin received packets.

   A gap has the property that any lost or discarded packets within the
   gap must be preceded and followed by at least Gmin packets that were
   received and not discarded.  This gives a maximum loss/discard rate
   within a gap of: 1 / (Gmin + 1).

   A Gmin value of 16 is RECOMMENDED, as it results in gap
   characteristics that correspond to good quality (i.e., low packet
   loss rate, a minimum distance of 16 received packets between lost
   packets), and hence differentiates nicely between good and poor
   quality periods.

   For example, a 1 denotes a received packet, 0 a lost packet, and X a
   discarded packet in the following pattern covering 64 packets:

      11110111111111111111111X111X1011110111111111111111111X111111111
      |---------gap----------|--burst---|------------gap------------|

   The burst consists of the twelve packets indicated above, starting at
   a discarded packet and ending at a lost packet.  The first gap starts
   at the beginning of the session and the second gap ends at the time
   of the report.

   If the packet spacing is 10 ms and the Gmin value is the recommended
   value of 16, the burst duration is 120 ms, the burst density 0.33,
   the gap duration 230 ms + 290 ms = 520 ms, and the gap density 0.04.

   This would result in reported values as follows (see field
   descriptions for semantics and details on how these are calculated):

      loss rate             12, which corresponds to 5%
      discard rate          12, which corresponds to 5%
      burst density         84, which corresponds to 33%
      gap density           10, which corresponds to 4%
      burst duration       120, value in milliseconds
      gap duration         520, value in milliseconds





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   burst density: 8 bits
         The fraction of RTP data packets within burst periods since the
         beginning of reception that were either lost or discarded.
         This value is expressed as a fixed point number with the binary
         point at the left edge of the field.  It is calculated by
         dividing the total number of packets lost or discarded
         (excluding duplicate packet discards) within burst periods by
         the total number of packets expected within the burst periods,
         multiplying the result of the division by 256, limiting the
         maximum value to 255 (to avoid overflow), and taking the
         integer part.  This field MUST be populated and MUST be set to
         zero if no packets have been received.

   gap density: 8 bits
         The fraction of RTP data packets within inter-burst gaps since
         the beginning of reception that were either lost or discarded.
         The value is expressed as a fixed point number with the binary
         point at the left edge of the field.  It is calculated by
         dividing the total number of packets lost or discarded
         (excluding duplicate packet discards) within gap periods by the
         total number of packets expected within the gap periods,
         multiplying the result of the division by 256, limiting the
         maximum value to 255 (to avoid overflow), and taking the
         integer part.  This field MUST be populated and MUST be set to
         zero if no packets have been received.

   burst duration: 16 bits
         The mean duration, expressed in milliseconds, of the burst
         periods that have occurred since the beginning of reception.
         The duration of each period is calculated based upon the
         packets that mark the beginning and end of that period.  It is
         equal to the timestamp of the end packet, plus the duration of
         the end packet, minus the timestamp of the beginning packet.
         If the actual values are not available, estimated values MUST
         be used.  If there have been no burst periods, the burst
         duration value MUST be zero.

   gap duration: 16 bits
         The mean duration, expressed in milliseconds, of the gap
         periods that have occurred since the beginning of reception.
         The duration of each period is calculated based upon the packet
         that marks the end of the prior burst and the packet that marks
         the beginning of the subsequent burst.  It is equal to the
         timestamp of the subsequent burst packet, minus the timestamp
         of the prior burst packet, plus the duration of the prior burst
         packet.  If the actual values are not available, estimated
         values MUST be used.  In the case of a gap that occurs at the
         beginning of reception, the sum of the timestamp of the prior



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         burst packet and the duration of the prior burst packet are
         replaced by the reception start time.  In the case of a gap
         that occurs at the end of reception, the timestamp of the
         subsequent burst packet is replaced by the reception end time.
         If there have been no gap periods, the gap duration value MUST
         be zero.

4.7.3.  Delay Metrics

   For the purpose of the following definitions, the RTP interface is
   the interface between the RTP instance and the voice application
   (i.e., FEC, de-interleaving, de-multiplexing, jitter buffer).  For
   example, the time delay due to RTP payload multiplexing would be
   considered part of the voice application or end-system delay, whereas
   delay due to multiplexing RTP frames within a UDP frame would be
   considered part of the RTP reported delay.  This distinction is
   consistent with the use of RTCP for delay measurements.

   round trip delay: 16 bits
         The most recently calculated round trip time between RTP
         interfaces, expressed in milliseconds.  This value MAY be
         measured using RTCP, the DLRR method defined in Section 4.5 of
         this document, where it is necessary to convert the units of
         measurement from NTP timestamp values to milliseconds, or other
         approaches.  If RTCP is used, then the reported delay value is
         the time of receipt of the most recent RTCP packet from source
         SSRC, minus the LSR (last SR) time reported in its SR (Sender
         Report), minus the DLSR (delay since last SR) reported in its
         SR.  A non-zero LSR value is required in order to calculate
         round trip delay.  A value of 0 is permissible; however, this
         field MUST be populated as soon as a delay estimate is
         available.

   end system delay: 16 bits
         The most recently estimated end system delay, expressed in
         milliseconds.  End system delay is defined as the sum of the
         total sample accumulation and encoding delay associated with
         the sending direction and the jitter buffer, decoding, and
         playout buffer delay associated with the receiving direction.
         This delay MAY be estimated or measured.  This value SHOULD be
         provided in all VoIP metrics reports.  If an implementation is
         unable to provide the data, the value 0 MUST be used.









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   Note that the one way symmetric VoIP segment delay may be calculated
   from the round trip and end system delays is as follows; if the round
   trip delay is denoted, RTD and the end system delays associated with
   the two endpoints are ESD(A) and ESD(B) then:

    one way symmetric voice path delay  =  ( RTD + ESD(A) + ESD(B) ) / 2

4.7.4.  Signal Related Metrics

   The following metrics are intended to provide real time information
   related to the non-packet elements of the voice over IP system to
   assist with the identification of problems affecting call quality.
   The values identified below must be determined for the received audio
   signal.  The information required to populate these fields may not be
   available in all systems, although it is strongly recommended that
   this data SHOULD be provided to support problem diagnosis.

   signal level: 8 bits
         The voice signal relative level is defined as the ratio of the
         signal level to a 0 dBm0 reference [10], expressed in decibels
         as a signed integer in two's complement form.  This is measured
         only for packets containing speech energy.  The intent of this
         metric is not to provide a precise measurement of the signal
         level but to provide a real time indication that the signal
         level may be excessively high or low.

         signal level = 10 Log10 ( rms talkspurt power (mW) )

         A value of 127 indicates that this parameter is unavailable.
         Typical values should generally be in the -15 to -20 dBm range.

   noise level: 8 bits
         The noise level is defined as the ratio of the silent period
         background noise level to a 0 dBm0 reference, expressed in
         decibels as a signed integer in two's complement form.

         noise level = 10 Log10 ( rms silence power (mW) )

         A value of 127 indicates that this parameter is unavailable.

   residual echo return loss (RERL): 8 bits
         The residual echo return loss value may be measured directly by
         the VoIP end system's echo canceller or may be estimated by
         adding the echo return loss (ERL) and echo return loss
         enhancement (ERLE) values reported by the echo canceller.

         RERL(dB) = ERL (dB) + ERLE (dB)




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         In the case of a VoIP gateway, the source of echo is typically
         line echo that occurs at 2-4 wire conversion points in the
         network.  This can be in the 8-12 dB range.  A line echo
         canceler can provide an ERLE of 30 dB or more and hence reduce
         this to 40-50 dB.  In the case of an IP phone, this could be
         acoustic coupling between handset speaker and microphone or
         residual acoustic echo from speakerphone operation, and may
         more correctly be termed terminal coupling loss (TCL).  A
         typical handset would result in 40-50 dB of echo loss due to
         acoustic feedback.

         Examples:

         -  IP gateway connected to circuit switched network with 2 wire
            loop.  Without echo cancellation, typical 2-4 wire converter
            ERL of 12 dB.  RERL = ERL + ERLE = 12 + 0 = 12 dB.

         -  IP gateway connected to circuit switched network with 2 wire
            loop.  With echo canceler that improves echo by 30 dB.
            RERL = ERL + ERLE = 12 + 30 = 42 dB.

         -  IP phone with conventional handset.  Acoustic coupling from
            handset speaker to microphone (terminal coupling loss) is
            typically 40 dB.  RERL = TCL = 40 dB.

         If we denote the local end of the VoIP path as A and the remote
         end as B, and if the sender loudness rating (SLR) and receiver
         loudness rating (RLR) are known for A (default values 8 dB and
         2 dB respectively), then the echo loudness level at end A
         (talker echo loudness rating or TELR) is given by:

         TELR(A) = SRL(A) + ERL(B) + ERLE(B) + RLR(A)

         TELR(B) = SRL(B) + ERL(A) + ERLE(A) + RLR(B)

         Hence, in order to incorporate echo into a voice quality
         estimate at the A end of a VoIP connection, it is desirable to
         send the ERL + ERLE value from B to A using a format such as
         RTCP XR.

         Echo related information may not be available in all VoIP end
         systems.  As echo does have a significant effect on
         conversational quality, it is recommended that estimated values
         for echo return loss and terminal coupling loss be provided (if
         sensible estimates can be reasonably determined).






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         Typical values for end systems are given below to provide
         guidance:

         -  IP Phone with handset: typically 45 dB.

         -  PC softphone or speakerphone: extremely variable, consider
            reporting "undefined" (127).

         -  IP gateway with line echo canceller: typically has ERL and
            ERLE available.

         -  IP gateway without line echo canceller: frequently a source
            of echo related problems, consider reporting either a low
            value (12 dB) or "undefined" (127).

   Gmin
         See Configuration Parameters (Section 4.7.6, below).

4.7.5.  Call Quality or Transmission Quality Metrics

   The following metrics are direct measures of the call quality or
   transmission quality, and incorporate the effects of codec type,
   packet loss, discard, burstiness, delay etc.  These metrics may not
   be available in all systems, however, they SHOULD be provided in
   order to support problem diagnosis.

   R factor: 8 bits
         The R factor is a voice quality metric describing the segment
         of the call that is carried over this RTP session.  It is
         expressed as an integer in the range 0 to 100, with a value of
         94 corresponding to "toll quality" and values of 50 or less
         regarded as unusable.  This metric is defined as including the
         effects of delay, consistent with ITU-T G.107 [6] and ETSI TS
         101 329-5 [3].

         A value of 127 indicates that this parameter is unavailable.
         Values other than 127 and the valid range defined above MUST
         not be sent and MUST be ignored by the receiving system.

   ext. R factor: 8 bits
         The external R factor is a voice quality metric describing the
         segment of the call that is carried over a network segment
         external to the RTP segment, for example a cellular network.
         Its values are interpreted in the same manner as for the RTP R
         factor.  This metric is defined as including the effects of
         delay, consistent with ITU-T G.107 [6] and ETSI TS 101 329-5
         [3], and relates to the outward voice path from the Voice over
         IP termination for which this metrics block applies.



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         A value of 127 indicates that this parameter is unavailable.
         Values other than 127 and the valid range defined above MUST
         not be sent and MUST be ignored by the receiving system.

   Note that an overall R factor may be estimated from the RTP segment R
   factor and the external R factor, as follows:

   R total = RTP R factor + ext. R factor - 94

   MOS-LQ: 8 bits
         The estimated mean opinion score for listening quality (MOS-LQ)
         is a voice quality metric on a scale from 1 to 5, in which 5
         represents excellent and 1 represents unacceptable.  This
         metric is defined as not including the effects of delay and can
         be compared to MOS scores obtained from listening quality (ACR)
         tests.  It is expressed as an integer in the range 10 to 50,
         corresponding to MOS x 10.  For example, a value of 35 would
         correspond to an estimated MOS score of 3.5.

         A value of 127 indicates that this parameter is unavailable.
         Values other than 127 and the valid range defined above MUST
         not be sent and MUST be ignored by the receiving system.

   MOS-CQ: 8 bits
         The estimated mean opinion score for conversational quality
         (MOS-CQ) is defined as including the effects of delay and other
         effects that would affect conversational quality.  The metric
         may be calculated by converting an R factor determined
         according to ITU-T G.107 [6] or ETSI TS 101 329-5 [3] into an
         estimated MOS using the equation specified in G.107.  It is
         expressed as an integer in the range 10 to 50, corresponding to
         MOS x 10, as for MOS-LQ.

         A value of 127 indicates that this parameter is unavailable.
         Values other than 127 and the valid range defined above MUST
         not be sent and MUST be ignored by the receiving system.

4.7.6.  Configuration Parameters

   Gmin: 8 bits
         The gap threshold.  This field contains the value used for this
         report block to determine if a gap exists.  The recommended
         value of 16 corresponds to a burst period having a minimum
         density of 6.25% of lost or discarded packets, which may cause
         noticeable degradation in call quality; during gap periods, if
         packet loss or discard occurs, each lost or discarded packet
         would be preceded by and followed by a sequence of at least 16
         received non-discarded packets.  Note that lost or discarded



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         packets that occur within Gmin packets of a report being
         generated may be reclassified as part of a burst or gap in
         later reports.  ETSI TS 101 329-5 [3] defines a computationally
         efficient algorithm for measuring burst and gap density using a
         packet loss/discard event driven approach.  This algorithm is
         reproduced in Appendix A.2 of the present document.  Gmin MUST
         not be zero, MUST be provided, and MUST remain constant across
         VoIP Metrics report blocks for the duration of the RTP session.

   receiver configuration byte (RX config): 8 bits
         This byte consists of the following fields:

             0 1 2 3 4 5 6 7
            +-+-+-+-+-+-+-+-+
            |PLC|JBA|JB rate|
            +-+-+-+-+-+-+-+-+

   packet loss concealment (PLC): 2 bits
         Standard (11) / enhanced (10) / disabled (01) / unspecified
         (00).  When PLC = 11, then a simple replay or interpolation
         algorithm is being used to fill-in the missing packet; this
         approach is typically able to conceal isolated lost packets at
         low packet loss rates.  When PLC = 10, then an enhanced
         interpolation algorithm is being used; algorithms of this type
         are able to conceal high packet loss rates effectively.  When
         PLC = 01, then silence is being inserted in place of lost
         packets.  When PLC = 00, then no information is available
         concerning the use of PLC; however, for some codecs this may be
         inferred.

   jitter buffer adaptive (JBA): 2 bits
         Adaptive (11) / non-adaptive (10) / reserved (01)/ unknown
         (00).  When the jitter buffer is adaptive, then its size is
         being dynamically adjusted to deal with varying levels of
         jitter.  When non-adaptive, the jitter buffer size is
         maintained at a fixed level.  When either adaptive or non-
         adaptive modes are specified, then the jitter buffer size
         parameters below MUST be specified.

   jitter buffer rate (JB rate): 4 bits
         J = adjustment rate (0-15).  This represents the implementation
         specific adjustment rate of a jitter buffer in adaptive mode.
         This parameter is defined in terms of the approximate time
         taken to fully adjust to a step change in peak to peak jitter
         from 30 ms to 100 ms such that:

         adjustment time = 2 * J * frame size (ms)




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         This parameter is intended only to provide a guide to the
         degree of "aggressiveness" of an adaptive jitter buffer and may
         be estimated.  A value of 0 indicates that the adjustment time
         is unknown for this implementation.

   reserved: 8 bits
         This field is reserved for future definition.  In the absence
         of such a definition, the bits in this field MUST be set to
         zero and MUST be ignored by the receiver.

4.7.7.  Jitter Buffer Parameters

   The values reported in these fields SHOULD be the most recently
   obtained values at the time of reporting.

   jitter buffer nominal delay (JB nominal): 16 bits
         This is the current nominal jitter buffer delay in
         milliseconds, which corresponds to the nominal jitter buffer
         delay for packets that arrive exactly on time.  This parameter
         MUST be provided for both fixed and adaptive jitter buffer
         implementations.

   jitter buffer maximum delay (JB maximum): 16 bits
         This is the current maximum jitter buffer delay in milliseconds
         which corresponds to the earliest arriving packet that would
         not be discarded.  In simple queue implementations this may
         correspond to the nominal size.  In adaptive jitter buffer
         implementations, this value may dynamically vary up to JB abs
         max (see below).  This parameter MUST be provided for both
         fixed and adaptive jitter buffer implementations.

   jitter buffer absolute maximum delay (JB abs max): 16 bits
         This is the absolute maximum delay in milliseconds that the
         adaptive jitter buffer can reach under worst case conditions.
         If this value exceeds 65535 milliseconds, then this field SHALL
         convey the value 65535.  This parameter MUST be provided for
         adaptive jitter buffer implementations and its value MUST be
         set to JB maximum for fixed jitter buffer implementations.

5.  SDP Signaling

   This section defines Session Description Protocol (SDP) [4] signaling
   for XR blocks that can be employed by applications that utilize SDP.
   This signaling is defined to be used either by applications that
   implement the SDP Offer/Answer model [8] or by applications that use
   SDP to describe media and transport configurations in connection





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   with such protocols as the Session Announcement Protocol (SAP) [15]
   or the Real Time Streaming Protocol (RTSP) [17].  There exist other
   potential signaling methods that are not defined here.

   The XR blocks MAY be used without prior signaling.  This is
   consistent with the rules governing other RTCP packet types, as
   described in [9].  An example in which signaling would not be used is
   an application that always requires the use of one or more XR blocks.
   However, for applications that are configured at session initiation,
   the use of some type of signaling is recommended.

   Note that, although the use of SDP signaling for XR blocks may be
   optional, if used, it MUST be used as defined here.  If SDP signaling
   is used in an environment where XR blocks are only implemented by
   some fraction of the participants, the ones not implementing the XR
   blocks will ignore the SDP attribute.

5.1.  The SDP Attribute

   This section defines one new SDP attribute "rtcp-xr" that can be used
   to signal participants in a media session that they should use the
   specified XR blocks.  This attribute can be easily extended in the
   future with new parameters to cover any new report blocks.

   The RTCP XR blocks SDP attribute is defined below in Augmented
   Backus-Naur Form (ABNF) [2].  It is both a session and a media level
   attribute.  When specified at session level, it applies to all media
   level blocks in the session.  Any media level specification MUST
   replace a session level specification, if one is present, for that
   media block.

    rtcp-xr-attrib = "a=" "rtcp-xr" ":" [xr-format *(SP xr-format)] CRLF

     xr-format = pkt-loss-rle
               / pkt-dup-rle
               / pkt-rcpt-times
               / rcvr-rtt
               / stat-summary
               / voip-metrics
               / format-ext

     pkt-loss-rle   = "pkt-loss-rle" ["=" max-size]
     pkt-dup-rle    = "pkt-dup-rle" ["=" max-size]
     pkt-rcpt-times = "pkt-rcpt-times" ["=" max-size]
     rcvr-rtt       = "rcvr-rtt" "=" rcvr-rtt-mode [":" max-size]
     rcvr-rtt-mode  = "all"
                    / "sender"
     stat-summary   = "stat-summary" ["=" stat-flag *("," stat-flag)]



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     stat-flag      = "loss"
                    / "dup"
                    / "jitt"
                    / "TTL"
                    / "HL"
     voip-metrics   = "voip-metrics"
     max-size       = 1*DIGIT ; maximum block size in octets
     DIGIT          = %x30-39
     format-ext     = non-ws-string

     non-ws-string  = 1*(%x21-FF)
     CRLF           = %d13.10

   The "rtcp-xr" attribute contains zero, one, or more XR block related
   parameters.  Each parameter signals functionality for an XR block, or
   a group of XR blocks.  The attribute is extensible so that parameters
   can be defined for any future XR block (and a parameter should be
   defined for every future block).

   Each "rtcp-xr" parameter belongs to one of two categories.  The first
   category, the unilateral parameters, are for report blocks that
   simply report on the RTP stream and related metrics.  The second
   category, collaborative parameters, are for XR blocks that involve
   actions by more than one party in order to carry out their functions.

   Round trip time (RTT) measurement is an example of collaborative
   functionality.  An RTP data packet receiver sends a Receiver
   Reference Time Report Block (Section 4.4).  A participant that
   receives this block sends a DLRR Report Block (Section 4.5) in
   response, allowing the receiver to calculate its RTT to that
   participant.  As this example illustrates, collaborative
   functionality may be implemented by two or more different XR blocks.
   The collaborative functionality of several XR blocks may be governed
   by a single "rtcp-xr" parameter.

   For the unilateral category, this document defines the following
   parameters.  The parameter names and their corresponding XR formats
   are as follows:

      Parameter name    XR block (block type and name)
      --------------    ------------------------------------
      pkt-loss-rle      1  Loss RLE Report Block
      pkt-dup-rle       2  Duplicate RLE Report Block
      pkt-rcpt-times    3  Packet Receipt Times Report Block
      stat-summary      6  Statistics Summary Report Block
      voip-metrics      7  VoIP Metrics Report Block





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   The "pkt-loss-rle", "pkt-dup-rle", and "pkt-rcpt-times" parameters
   MAY specify an integer value.  This value indicates the largest size
   the whole report block SHOULD have in octets.  This shall be seen as
   an indication that thinning shall be applied if necessary to meet the
   target size.

   The "stat-summary" parameter contains a list indicating which fields
   SHOULD be included in the Statistics Summary report blocks that are
   sent.  The list is a comma separated list, containing one or more
   field indicators.  The space character (0x20) SHALL NOT be present
   within the list.  Field indicators represent the flags defined in
   Section 4.6.  The field indicators and their respective flags are as
   follows:

      Indicator    Flag
      ---------    ---------------------------
      loss         loss report flag (L)
      dup          duplicate report flag (D)
      jitt         jitter flag (J)
      TTL          TTL or Hop Limit flag (ToH)
      HL           TTL or Hop Limit flag (ToH)

   For "loss", "dup", and "jitt", the presence of the indicator
   indicates that the corresponding flag should be set to 1 in the
   Statistics Summary report blocks that are sent.  The presence of
   "TTL" indicates that the corresponding flag should be set to 1.  The
   presence of "HL" indicates that the corresponding flag should be set
   to 2.  The indicators "TTL" and "HL" MUST NOT be signaled together.

   Blocks in the collaborative category are classified as initiator
   blocks or response blocks.  Signaling SHOULD indicate which
   participants are required to respond to the initiator block.  A party
   that wishes to receive response blocks from those participants can
   trigger this by sending an initiator block.

   The collaborative category currently consists only of one
   functionality, namely the RTT measurement mechanism for RTP data
   receivers.  The collective functionality of the Receiver Reference
   Time Report Block and DLRR Report Block is represented by the "rcvr-
   rtt" parameter.  This parameter takes as its arguments a mode value
   and, optionally, a maximum size for the DLRR report block.  The mode
   value "all" indicates that both RTP data senders and data receivers
   MAY send DLRR blocks, while the mode value "sender" indicates that
   only active RTP senders MAY send DLRR blocks, i.e., non RTP senders
   SHALL NOT send DLRR blocks.  If a maximum size in octets is included,
   any DLRR Report Blocks that are sent SHALL NOT exceed the specified
   size.  If size limitations mean that a DLRR Report Block sender
   cannot report in one block upon all participants from which it has



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   received a Receiver Reference Time Report Block then it SHOULD report
   on participants in a round robin fashion across several report
   intervals.

   The "rtcp-xr" attributes parameter list MAY be empty.  This is useful
   in cases in which an application needs to signal that it understands
   the SDP signaling but does not wish to avail itself of XR
   functionality.  For example, an application in a SIP controlled
   session could signal that it wishes to stop using all XR blocks by
   removing all applicable SDP parameters in a re-INVITE message that it
   sends.  If XR blocks are not to be used at all from the beginning of
   a session, it is RECOMMENDED that the "rtcp-xr" attribute not be
   supplied at all.

   When the "rtcp-xr" attribute is present, participants SHOULD NOT send
   XR blocks other than the ones indicated by the parameters.  This
   means that inclusion of a "rtcp-xr" attribute without any parameters
   tells a participant that it SHOULD NOT send any XR blocks at all.
   The purpose is to conserve bandwidth.  This is especially important
   when collaborative parameters are applied to a large multicast group:
   the sending of an initiator block could potentially trigger responses
   from all participants.  There are, however, contexts in which it
   makes sense to send an XR block in the absence of a parameter
   signaling its use.  For instance, an application might be designed so
   as to send certain report blocks without negotiation, while using SDP
   signaling to negotiate the use of other blocks.

5.2.  Usage in Offer/Answer

   In the Offer/Answer context [8], the interpretation of SDP signaling
   for XR packets depends upon the direction attribute that is signaled:
   "recvonly", "sendrecv", or "sendonly" [4].  If no direction attribute
   is supplied, then "sendrecv" is assumed.  This section applies only
   to unicast media streams, except where noted.  Discussion of
   unilateral parameters is followed by discussion of collaborative
   parameters in this section.

   For "sendonly" and "sendrecv" media stream offers that specify
   unilateral "rtcp-xr" attribute parameters, the answerer SHOULD send
   the corresponding XR blocks.  For "sendrecv" offers, the answerer MAY
   include the "rtcp-xr" attribute in its response, and specify any
   unilateral parameters in order to request that the offerer send the
   corresponding XR blocks.  The offerer SHOULD send these blocks.

   For "recvonly" media stream offers, the offerer's use of the "rtcp-
   xr" attribute in connection with unilateral parameters indicates that
   the offerer is capable of sending the corresponding XR blocks.  If




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   the answerer responds with an "rtcp-xr" attribute, the offerer SHOULD
   send XR blocks for each specified unilateral parameter that was in
   its offer.

   For multicast media streams, the inclusion of an "rtcp-xr" attribute
   with unilateral parameters means that every media recipient SHOULD
   send the corresponding XR blocks.

   An SDP offer with a collaborative parameter declares the offerer
   capable of receiving the corresponding initiator and replying with
   the appropriate responses.  For example, an offer that specifies the
   "rcvr-rtt" parameter means that the offerer is prepared to receive
   Receiver Reference Time Report Blocks and to send DLRR Report Blocks.
   An offer of a collaborative parameter means that the answerer MAY
   send the initiator, and, having received the initiator, the offerer
   SHOULD send the responses.

   There are exceptions to the rule that an offerer of a collaborative
   parameter should send responses.  For instance, the collaborative
   parameter might specify a mode that excludes the offerer; or
   congestion control or maximum transmission unit considerations might
   militate against the offerer's response.

   By including a collaborative parameter in its answer, the answerer
   declares its ability to receive initiators and to send responses.
   The offerer MAY then send initiators, to which the answerer SHOULD
   reply with responses.  As for the offer of a collaborative parameter,
   there are exceptions to the rule that the answerer should reply.

   When making an SDP offer of a collaborative parameter for a multicast
   media stream, the offerer SHOULD specify which participants are to
   respond to a received initiator.  A participant that is not specified
   SHOULD NOT send responses.  Otherwise, undue bandwidth might be
   consumed.  The offer indicates that each participant that is
   specified SHOULD respond if it receives an initiator.  It also
   indicates that a specified participant MAY send an initiator block.

   An SDP answer for a multicast media stream SHOULD include all
   collaborative parameters that are present in the offer and that are
   supported by the answerer.  It SHOULD NOT include any collaborative
   parameter that is absent from the offer.

   If a participant receives an SDP offer and understands the "rtcp-xr"
   attribute but does not wish to implement XR functionality offered,
   its answer SHOULD include an "rtcp-xr" attribute without parameters.
   By doing so, the party declares that, at a minimum, is capable of
   understanding the signaling.




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5.3.  Usage Outside of Offer/Answer

   SDP can be employed outside of the Offer/Answer context, for instance
   for multimedia sessions that are announced through the Session
   Announcement Protocol (SAP) [15], or streamed through the Real Time
   Streaming Protocol (RTSP) [17].  The signaling model is simpler, as
   the sender does not negotiate parameters, but the functionality
   expected from specifying the "rtcp-xr" attribute is the same as in
   Offer/Answer.

   When a unilateral parameter is specified for the "rtcp-xr" attribute
   associated with a media stream, the receiver of that stream SHOULD
   send the corresponding XR block.  When a collaborative parameter is
   specified, only the participants indicated by the mode value in the
   collaborative parameter are concerned.  Each such participant that
   receives an initiator block SHOULD send the corresponding response
   block.  Each such participant MAY also send initiator blocks.

6.  IANA Considerations

   This document defines a new RTCP packet type, the Extended Report
   (XR) type, within the existing Internet Assigned Numbers Authority
   (IANA) registry of RTP RTCP Control Packet Types.  This document also
   defines a new IANA registry: the registry of RTCP XR Block Types.
   Within this new registry, this document defines an initial set of
   seven block types and describes how the remaining types are to be
   allocated.

   Further, this document defines a new SDP attribute, "rtcp-xr", within
   the existing IANA registry of SDP Parameters.  It defines a new IANA
   registry, the registry of RTCP XR SDP Parameters, and an initial set
   of six parameters, and describes how additional parameters are to be
   allocated.

6.1.  XR Packet Type

   The XR packet type defined by this document is registered with the
   IANA as packet type 207 in the registry of RTP RTCP Control Packet
   types (PT).

6.2.  RTCP XR Block Type Registry

   This document creates an IANA registry called the RTCP XR Block Type
   Registry to cover the name space of the Extended Report block type
   (BT) field specified in Section 3.  The BT field contains eight bits,
   allowing 256 values.  The RTCP XR Block Type Registry is to be
   managed by the IANA according to the Specification Required policy of




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   RFC 2434 [7].  Future specifications SHOULD attribute block type
   values in strict numeric order following the values attributed in
   this document:

      BT  name
      --  ----
       1  Loss RLE Report Block
       2  Duplicate RLE Report Block
       3  Packet Receipt Times Report Block
       4  Receiver Reference Time Report Block
       5  DLRR Report Block
       6  Statistics Summary Report Block
       7  VoIP Metrics Report Block

      The BT value 255 is reserved for future extensions.

   Furthermore, future specifications SHOULD avoid the value 0.  Doing
   so facilitates packet validity checking, since an all-zeros field
   might commonly be found in an ill-formed packet.

   Any registration MUST contain the following information:

   -  Contact information of the one doing the registration, including
      at least name, address, and email.

   -  The format of the block type being registered, consistent with the
      extended report block format described in Section 3.

   -  A description of what the block type represents and how it shall
      be interpreted, detailing this information for each of its fields.

6.3.  The "rtcp-xr" SDP Attribute

   The SDP attribute "rtcp-xr" defined by this document is registered
   with the IANA registry of SDP Parameters as follows:

   SDP Attribute ("att-field"):

     Attribute name:     rtcp-xr
     Long form:          RTP Control Protocol Extended Report Parameters
     Type of name:       att-field
     Type of attribute:  session and media level
     Subject to charset: no
     Purpose:            see Section 5 of this document
     Reference:          this document
     Values:             see this document and registrations below





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   The attribute has an extensible parameter field and therefore a
   registry for these parameters is required.  This document creates an
   IANA registry called the RTCP XR SDP Parameters Registry.  It
   contains the six parameters defined in Section 5.1: "pkt-loss-rle",
   "pkt-dup-rle", "pkt-rcpt-times", "stat-summary", "voip-metrics", and
   "recv-rtt".

   Additional parameters are to be added to this registry in accordance
   with the Specification Required policy of RFC 2434 [7].  Any
   registration MUST contain the following information:

   -  Contact information of the one doing the registration, including
      at least name, address, and email.

   -  An Augmented Backus-Naur Form (ABNF) [2] definition of the
      parameter, in accordance with the "format-ext" definition of
      Section 5.1.

   -  A description of what the parameter represents and how it shall be
      interpreted, both normally and in Offer/Answer.

7.  Security Considerations

   This document extends the RTCP reporting mechanism.  The security
   considerations that apply to RTCP reports [9, Appendix B] also apply
   to XR reports.  This section details the additional security
   considerations that apply to the extensions.

   The extensions introduce heightened confidentiality concerns.
   Standard RTCP reports contain a limited number of summary statistics.
   The information contained in XR reports is both more detailed and
   more extensive (covering a larger number of parameters).  The per-
   packet report blocks and the VoIP Metrics Report Block provide
   examples.

   The per-packet information contained in Loss RLE, Duplicate RLE, and
   Packet Receipt Times Report Blocks facilitates multicast inference of
   network characteristics (MINC) [11].  Such inference can reveal the
   gross topology of a multicast distribution tree, as well as
   parameters, such as the loss rates and delays, along paths between
   branching points in that tree.  Such information might be considered
   sensitive to autonomous system administrators.

   The VoIP Metrics Report Block provides information on the quality of
   ongoing voice calls.  Though such information might be carried in an
   application specific format in standard RTP sessions, making it
   available in a standard format here makes it more available to
   potential eavesdroppers.



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   No new mechanisms are introduced in this document to ensure
   confidentiality.  Encryption procedures, such as those being
   suggested for a Secure RTCP (SRTCP) [12] at the time that this
   document was written, can be used when confidentiality is a concern
   to end hosts.  Given that RTCP traffic can be encrypted by the end
   hosts, autonomous systems must be prepared for the fact that certain
   aspects of their network topology can be revealed.

   Any encryption or filtering of XR report blocks entails a loss of
   monitoring information to third parties.  For example, a network that
   establishes a tunnel to encrypt VoIP Report Blocks denies that
   information to the service providers traversed by the tunnel.  The
   service providers cannot then monitor or respond to the quality of
   the VoIP calls that they carry, potentially creating problems for the
   network's users.  As a default, XR packets should not be encrypted or
   filtered.

   The extensions also make certain denial of service attacks easier.
   This is because of the potential to create RTCP packets much larger
   than average with the per packet reporting capabilities of the Loss
   RLE, Duplicate RLE, and Timestamp Report Blocks.  Because of the
   automatic bandwidth adjustment mechanisms in RTCP, if some session
   participants are sending large RTCP packets, all participants will
   see their RTCP reporting intervals lengthened, meaning they will be
   able to report less frequently.  To limit the effects of large
   packets, even in the absence of denial of service attacks,
   applications SHOULD place an upper limit on the size of the XR report
   blocks they employ.  The "thinning" techniques described in Section
   4.1 permit the packet-by-packet report blocks to adhere to a
   predefined size limit.





















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A.  Algorithms

A.1.  Sequence Number Interpretation

   This is the algorithm suggested by Section 4.1 for keeping track of
   the sequence numbers from a given sender.  It implements the
   accounting practice required for the generation of Loss RLE Report
   Blocks.

   This algorithm keeps track of 16 bit sequence numbers by translating
   them into a 32 bit sequence number space.  The first packet received
   from a source is considered to have arrived roughly in the middle of
   that space.  Each packet that follows is placed either ahead of or
   behind the prior one in this 32 bit space, depending upon which
   choice would place it closer (or, in the event of a tie, which choice
   would not require a rollover in the 16 bit sequence number).

   // The reference sequence number is an extended sequence number
   // that serves as the basis for determining whether a new 16 bit
   // sequence number comes earlier or later in the 32 bit sequence
   // space.
   u_int32 _src_ref_seq;
   bool    _uninitialized_src_ref_seq;

   // Place seq into a 32-bit sequence number space based upon a
   // heuristic for its most likely location.
   u_int32 extend_seq(const u_int16 seq) {

           u_int32 extended_seq, seq_a, seq_b, diff_a, diff_b;
           if(_uninitialized_src_ref_seq) {

                   // This is the first sequence number received.  Place
                   // it in the middle of the extended sequence number
                   // space.
                   _src_ref_seq                = seq | 0x80000000u;
                   _uninitialized_src_ref_seq  = false;
                   extended_seq                = _src_ref_seq;
           }
           else {

                   // Prior sequence numbers have been received.
                   // Propose two candidates for the extended sequence
                   // number: seq_a is without wraparound, seq_b with
                   // wraparound.
                   seq_a = seq | (_src_ref_seq & 0xFFFF0000u);
                   if(_src_ref_seq < seq_a) {
                           seq_b  = seq_a - 0x00010000u;
                           diff_a = seq_a - _src_ref_seq;



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                           diff_b = _src_ref_seq - seq_b;
                   }
                   else {
                           seq_b  = seq_a + 0x00010000u;
                           diff_a = _src_ref_seq - seq_a;
                           diff_b = seq_b - _src_ref_seq;
                   }

                   // Choose the closer candidate.  If they are equally
                   // close, the choice is somewhat arbitrary: we choose
                   // the candidate for which no rollover is necessary.
                   if(diff_a < diff_b) {
                           extended_seq = seq_a;
                   }
                   else {
                           extended_seq = seq_b;
                   }

                   // Set the reference sequence number to be this most
                   // recently-received sequence number.
                   _src_ref_seq = extended_seq;
           }

           // Return our best guess for a 32-bit sequence number that
           // corresponds to the 16-bit number we were given.
           return extended_seq;
   }

A.2.  Example Burst Packet Loss Calculation.

   This is an algorithm for measuring the burst characteristics for the
   VoIP Metrics Report Block (Section 4.7).  The algorithm, which has
   been verified against a working implementation for correctness, is
   reproduced from ETSI TS 101 329-5 [3].  The algorithm, as described
   here, takes precedence over any change that might eventually be made
   to the algorithm in future ETSI documents.

   This algorithm is event driven and hence extremely computationally
   efficient.

   Given the following definition of states:

      state 1 = received a packet during a gap
      state 2 = received a packet during a burst
      state 3 = lost a packet during a burst
      state 4 = lost an isolated packet during a gap





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   The "c" variables below correspond to state transition counts, i.e.,
   c14 is the transition from state 1 to state 4.  It is possible to
   infer one of a pair of state transition counts to an accuracy of 1
   which is generally sufficient for this application.

   "pkt" is the count of packets received since the last packet was
   declared lost or discarded, and "lost" is the number of packets lost
   within the current burst.  "packet_lost" and "packet_discarded" are
   Boolean variables that indicate if the event that resulted in this
   function being invoked was a lost or discarded packet.

   if(packet_lost) {
           loss_count++;
   }
   if(packet_discarded) {
           discard_count++;
   }
   if(!packet_lost && !packet_discarded) {
           pkt++;
   }
   else {
           if(pkt >= gmin) {
                   if(lost == 1) {
                           c14++;
                   }
                   else {
                           c13++;
                   }
                   lost = 1;
                   c11 += pkt;
           }
           else {
                   lost++;
                   if(pkt == 0) {
                           c33++;
                   }
                   else {
                           c23++;
                           c22 += (pkt - 1);
                   }
           }
           pkt = 0;
   }

   At each reporting interval the burst and gap metrics can be
   calculated as follows.





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   // Calculate additional transition counts.
   c31 = c13;
   c32 = c23;
   ctotal = c11 + c14 + c13 + c22 + c23 + c31 + c32 + c33;

   // Calculate burst and densities.
   p32 = c32 / (c31 + c32 + c33);
   if((c22 + c23) < 1) {
           p23 = 1;
   }
   else {
           p23 = 1 - c22/(c22 + c23);
   }
   burst_density = 256 * p23 / (p23 + p32);
   gap_density = 256 * c14 / (c11 + c14);

   // Calculate burst and gap durations in ms
   m = frameDuration_in_ms * framesPerRTPPkt;
   gap_length = (c11 + c14 + c13) * m / c13;
   burst_length = ctotal * m / c13 - lgap;

   /* calculate loss and discard rates */
   loss_rate = 256 * loss_count / ctotal;
   discard_rate = 256 * discard_count / ctotal;

Intellectual Property Notice

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP 11 [5].  Copies
   of claims of rights made available for publication and any assurances
   of licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.





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Acknowledgments

   We thank the following people: Colin Perkins, Steve Casner, and
   Henning Schulzrinne for their considered guidance; Sue Moon for
   helping foster collaboration between the authors; Mounir Benzaid for
   drawing our attention to the reporting needs of MLDA; Dorgham Sisalem
   and Adam Wolisz for encouraging us to incorporate MLDA block types;
   and Jose Rey for valuable review of the SDP Signaling section.

Contributors

   The following people are the authors of this document:

     Kevin Almeroth, UCSB
     Ramon Caceres, IBM Research
     Alan Clark, Telchemy
     Robert G. Cole, JHU Applied Physics Laboratory
     Nick Duffield, AT&T Labs-Research
     Timur Friedman, Paris 6
     Kaynam Hedayat, Brix Networks
     Kamil Sarac, UT Dallas
     Magnus Westerlund, Ericsson

   The principal people to contact regarding the individual report
   blocks described in this document are as follows:

   sec. report block                         principal contributors
   ---- ------------                         ----------------------
   4.1  Loss RLE Report Block                Friedman, Caceres, Duffield
   4.2  Duplicate RLE Report Block           Friedman, Caceres, Duffield
   4.3  Packet Receipt Times Report Block    Friedman, Caceres, Duffield
   4.4  Receiver Reference Time Report Block Friedman
   4.5  DLRR Report Block                    Friedman
   4.6  Statistics Summary Report Block      Almeroth, Sarac
   4.7  VoIP Metrics Report Block            Clark, Cole, Hedayat

   The principal person to contact regarding the SDP signaling described
   in this document is Magnus Westerlund.













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References

Normative References

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

   [2]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [3]  ETSI, "Quality of Service (QoS) measurement methodologies", ETSI
        TS 101 329-5 V1.1.1 (2000-11), November 2000.

   [4]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [5]  Hovey, R. and S. Bradner, "The Organizations Involved in the
        IETF Standards Process", BCP 11, RFC 2028, October 1996.

   [6]  ITU-T, "The E-Model, a computational model for use in
        transmission planning", Recommendation G.107, January 2003.

   [7]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

   [8]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
        the Session Description Protocol (SDP)", RFC 3264, June 2002.

   [9]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
        "RTP: A Transport Protocol for Real-Time Applications", RFC
        3550, July 2003.

   [10] TIA/EIA-810-A Transmission Requirements for Narrowband Voice
        over IP and Voice over PCM Digital Wireline Telephones, December
        2000.

Informative References

   [11] Adams, A., Bu, T., Caceres, R., Duffield, N.G., Friedman, T.,
        Horowitz, J., Lo Presti, F., Moon, S.B., Paxson, V. and D.
        Towsley, "The Use of End-to-End Multicast Measurements for
        Characterizing Internal Network Behavior", IEEE Communications
        Magazine, May 2000.

   [12] Baugher, McGrew, Oran, Blom, Carrara, Naslund and Norrman, "The
        Secure Real-time Transport Protocol", Work in Progress.





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   [13] Caceres, R., Duffield, N.G. and T. Friedman, "Impromptu
        measurement infrastructures using RTP", Proc. IEEE Infocom 2002.

   [14] Clark, A.D., "Modeling the Effects of Burst Packet Loss and
        Recency on Subjective Voice Quality", Proc. IP Telephony
        Workshop 2001.

   [15] Handley, M., Perkins, C. and E. Whelan, "Session Announcement
        Protocol", RFC 2974, October 2000.

   [16] Reynolds, J., Ed., "Assigned Numbers: RFC 1700 is Replaced by an
        On-line Database", RFC 3232, January 2002.

   [17] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
        Protocol (RTSP)", RFC 2326, April 1998.

   [18] Sisalem D. and A. Wolisz, "MLDA: A TCP-friendly Congestion
        Control Framework for Heterogeneous Multicast Environments",
        Proc. IWQoS 2000.
































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Authors' Addresses

   Kevin Almeroth
   Department of Computer Science
   University of California
   Santa Barbara, CA 93106 USA

   EMail: almeroth@cs.ucsb.edu


   Ramon Caceres
   IBM Research
   19 Skyline Drive
   Hawthorne, NY 10532 USA

   EMail: caceres@watson.ibm.com


   Alan Clark
   Telchemy Incorporated
   3360 Martins Farm Road, Suite 200
   Suwanee, GA 30024 USA

   Phone: +1 770 614 6944
   Fax:   +1 770 614 3951
   EMail: alan@telchemy.com


   Robert G. Cole
   Johns Hopkins University Applied Physics Laboratory
   MP2-S170
   11100 Johns Hopkins Road
   Laurel, MD 20723-6099 USA

   Phone: +1 443 778 6951
   EMail: robert.cole@jhuapl.edu


   Nick Duffield
   AT&T Labs-Research
   180 Park Avenue, P.O. Box 971
   Florham Park, NJ 07932-0971 USA

   Phone: +1 973 360 8726
   Fax:   +1 973 360 8050
   EMail: duffield@research.att.com





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   Timur Friedman
   Universite Pierre et Marie Curie (Paris 6)
   Laboratoire LiP6-CNRS
   8, rue du Capitaine Scott
   75015 PARIS France

   Phone: +33 1 44 27 71 06
   Fax:   +33 1 44 27 74 95
   EMail: timur.friedman@lip6.fr


   Kaynam Hedayat
   Brix Networks
   285 Mill Road
   Chelmsford, MA 01824 USA

   Phone: +1 978 367 5600
   Fax:   +1 978 367 5700
   EMail: khedayat@brixnet.com


   Kamil Sarac
   Department of Computer Science (ES 4.207)
   Eric Jonsson School of Engineering & Computer Science
   University of Texas at Dallas
   Richardson, TX 75083-0688 USA

   Phone: +1 972 883 2337
   Fax:   +1 972 883 2349
   EMail: ksarac@utdallas.edu


   Magnus Westerlund
   Ericsson Research
   Ericsson AB
   SE-164 80 Stockholm Sweden

   Phone: +46 8 404 82 87
   Fax:   +46 8 757 55 50
   EMail: magnus.westerlund@ericsson.com











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Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
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Acknowledgement

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