Internet Engineering Task Force      Audio/Video Transport Working Group
Internet Draft                     Schulzrinne/Casner/Frederick/Jacobson
ietf-avt-rtp-new-03.txt                    Columbia U./Cisco/Xerox/Cisco
February 26, 1999
Expires: August 26, 1999


          RTP: A Transport Protocol for Real-Time Applications

STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


                                 ABSTRACT


         This memorandum is a revision of RFC 1889 in preparation
         for advancement from Proposed Standard to Draft Standard
         status. Readers are encouraged to use the PostScript form
         of this draft to see where changes from RFC 1889 are
         marked by change bars.

         This memorandum describes RTP, the real-time transport
         protocol. RTP provides end-to-end network transport
         functions suitable for applications transmitting real-
         time data, such as audio, video or simulation data, over
         multicast or unicast network services. RTP does not
         address resource reservation and does not guarantee
         quality-of-service for real-time services. The data
         transport is augmented by a control protocol (RTCP) to
         allow monitoring of the data delivery in a manner
         scalable to large multicast networks, and to provide
         minimal control and identification functionality. RTP and
         RTCP are designed to be independent of the underlying
         transport and network layers. The protocol supports the
         use of RTP-level translators and mixers.


   This specification is a product of the Audio/Video Transport working
   group within the Internet Engineering Task Force. Comments are
   solicited and should be addressed to the working group's mailing list
   at rem-conf@es.net and/or the authors.


   Resolution of Open Issues

   [Note to the RFC Editor: This section is to be deleted when this
   draft is published as an RFC but is shown here for reference during
   the Last Call.]

   Readers are directed to Appendix B, Changes from RFC 1889, for a
   listing of the changes that have been made in this draft. The changes
   are marked with change bars in the PostScript form of this draft.

   The revisions in this draft are intended to be complete for Working
   Group last call; the open issues from previous drafts have been
   addressed:

       o A fudge factor has been added to the RTCP unconditional
         reconsideration algorithm to compensate for the fact that it
         settles to a steady state bandwidth that is below the desired
         level.

       o As agreed at the Chicago IETF, the conditional and hybrid
         reconsideration schemes have been removed in favor of
         unconditional reconsideration.

       o The SSRC sampling algorithm has been extracted to a separate
         draft as agreed at the Chicago IETF. That draft describes the
         "bin" mechanism that avoids a temporary underestimate in group
         size when the group size is decreasing.

       o The "reverse reconsideration" algorithm does not prevent the
         group size estimate from incorrectly dropping to zero for a
         short time when most participants of a large session leave at
         once but some remain. This has just been noted as only a
         secondary concern.




Schulzrinne/Casner/Frederick/Jacobson                         [Page 2]


Internet Draft                    RTP                  February 26, 1999


       o Scaling of the minimum RTCP interval inversely proportional to
         the session bandwidth parameter has been added, but only in the
         direction of smaller intervals for higher bandwidth. Scaling to
         longer intervals for low bandwidths would cause a problem
         because this is an optional step. Some participants might be
         timed out prematurely if they scaled to a longer interval while
         others kept the nominal 5 seconds. The benefit of scaling
         longer was not considered great in any case.

       o No change was specified for the jitter computation for media
         with several packets with the same timestamp. There is not a
         clear answer as to what should be done, or that any change
         would make a significant improvement.

       o As proposed without objection at the Los Angeles IETF,
         definition of additional SDES items such as PHOTO URL and
         NICKNAME will be deferred to subsequent registration through
         IANA since that method has been established. This is in the
         spirit of minimizing changes to the protocol in the transition
         from Proposed to Draft.

       o Nothing was added about allowing a translator to add its own
         random offsets to the sequence number and timestamp fields
         because it would likely cause more trouble than good.

1 Introduction

   This memorandum specifies the real-time transport protocol (RTP),
   which provides end-to-end delivery services for data with real-time
   characteristics, such as interactive audio and video. Those services
   include payload type identification, sequence numbering, timestamping
   and delivery monitoring. Applications typically run RTP on top of UDP
   to make use of its multiplexing and checksum services; both protocols
   contribute parts of the transport protocol functionality. However,
   RTP may be used with other suitable underlying network or transport
   protocols (see Section 10). RTP supports data transfer to multiple
   destinations using multicast distribution if provided by the
   underlying network.

   Note that RTP itself does not provide any mechanism to ensure timely
   delivery or provide other quality-of-service guarantees, but relies
   on lower-layer services to do so. It does not guarantee delivery or
   prevent out-of-order delivery, nor does it assume that the underlying
   network is reliable and delivers packets in sequence. The sequence
   numbers included in RTP allow the receiver to reconstruct the
   sender's packet sequence, but sequence numbers might also be used to
   determine the proper location of a packet, for example in video
   decoding, without necessarily decoding packets in sequence.



Schulzrinne/Casner/Frederick/Jacobson                         [Page 3]


Internet Draft                    RTP                  February 26, 1999


   While RTP is primarily designed to satisfy the needs of multi-
   participant multimedia conferences, it is not limited to that
   particular application. Storage of continuous data, interactive
   distributed simulation, active badge, and control and measurement
   applications may also find RTP applicable.

   This document defines RTP, consisting of two closely-linked parts:

       o the real-time transport protocol (RTP), to carry data that has
         real-time properties.

       o the RTP control protocol (RTCP), to monitor the quality of
         service and to convey information about the participants in an
         on-going session. The latter aspect of RTCP may be sufficient
         for "loosely controlled" sessions, i.e., where there is no
         explicit membership control and set-up, but it is not
         necessarily intended to support all of an application's control
         communication requirements.  This functionality may be fully or
         partially subsumed by a separate session control protocol,
         which is beyond the scope of this document.

   RTP represents a new style of protocol following the principles of
   application level framing and integrated layer processing proposed by
   Clark and Tennenhouse [1]. That is, RTP is intended to be malleable
   to provide the information required by a particular application and
   will often be integrated into the application processing rather than
   being implemented as a separate layer. RTP is a protocol framework
   that is deliberately not complete.  This document specifies those
   functions expected to be common across all the applications for which
   RTP would be appropriate. Unlike conventional protocols in which
   additional functions might be accommodated by making the protocol
   more general or by adding an option mechanism that would require
   parsing, RTP is intended to be tailored through modifications and/or
   additions to the headers as needed. Examples are given in Sections
   5.3 and 6.4.3.

   Therefore, in addition to this document, a complete specification of
   RTP for a particular application will require one or more companion
   documents (see Section 12):

       o a profile specification document, which defines a set of
         payload type codes and their mapping to payload formats (e.g.,
         media encodings). A profile may also define extensions or
         modifications to RTP that are specific to a particular class of
         applications.  Typically an application will operate under only
         one profile. A profile for audio and video data may be found in
         the companion RFC 1890 (updated by Internet-Draft draft-ietf-
         avt-profile-new [2]).



Schulzrinne/Casner/Frederick/Jacobson                         [Page 4]


Internet Draft                    RTP                  February 26, 1999


       o payload format specification documents, which define how a
         particular payload, such as an audio or video encoding, is to
         be carried in RTP.

   A discussion of real-time services and algorithms for their
   implementation as well as background discussion on some of the RTP
   design decisions can be found in [3].

1.1 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 RFC 2119 [4] and
   indicate requirement levels for compliant RTP implementations.

2 RTP Use Scenarios

   The following sections describe some aspects of the use of RTP. The
   examples were chosen to illustrate the basic operation of
   applications using RTP, not to limit what RTP may be used for. In
   these examples, RTP is carried on top of IP and UDP, and follows the
   conventions established by the profile for audio and video specified
   in the companion RFC 1890 (updated by Internet-Draft draft-ietf-avt-
   profile-new ).

2.1 Simple Multicast Audio Conference

   A working group of the IETF meets to discuss the latest protocol
   draft, using the IP multicast services of the Internet for voice
   communications. Through some allocation mechanism the working group
   chair obtains a multicast group address and pair of ports. One port
   is used for audio data, and the other is used for control (RTCP)
   packets.  This address and port information is distributed to the
   intended participants. If privacy is desired, the data and control
   packets may be encrypted as specified in Section 9.1, in which case
   an encryption key must also be generated and distributed.  The exact
   details of these allocation and distribution mechanisms are beyond
   the scope of RTP.

   The audio conferencing application used by each conference
   participant sends audio data in small chunks of, say, 20 ms duration.
   Each chunk of audio data is preceded by an RTP header; RTP header and
   data are in turn contained in a UDP packet. The RTP header indicates
   what type of audio encoding (such as PCM, ADPCM or LPC) is contained
   in each packet so that senders can change the encoding during a
   conference, for example, to accommodate a new participant that is
   connected through a low-bandwidth link or react to indications of
   network congestion.



Schulzrinne/Casner/Frederick/Jacobson                         [Page 5]


Internet Draft                    RTP                  February 26, 1999


   The Internet, like other packet networks, occasionally loses and
   reorders packets and delays them by variable amounts of time. To cope
   with these impairments, the RTP header contains timing information
   and a sequence number that allow the receivers to reconstruct the
   timing produced by the source, so that in this example, chunks of
   audio are contiguously played out the speaker every 20 ms. This
   timing reconstruction is performed separately for each source of RTP
   packets in the conference. The sequence number can also be used by
   the receiver to estimate how many packets are being lost.

   Since members of the working group join and leave during the
   conference, it is useful to know who is participating at any moment
   and how well they are receiving the audio data. For that purpose,
   each instance of the audio application in the conference periodically
   multicasts a reception report plus the name of its user on the RTCP
   (control) port. The reception report indicates how well the current
   speaker is being received and may be used to control adaptive
   encodings. In addition to the user name, other identifying
   information may also be included subject to control bandwidth limits.
   A site sends the RTCP BYE packet (Section 6.6) when it leaves the
   conference.

2.2 Audio and Video Conference

   If both audio and video media are used in a conference, they are
   transmitted as separate RTP sessions RTCP packets are transmitted for
   each medium using two different UDP port pairs and/or multicast
   addresses. There is no direct coupling at the RTP level between the
   audio and video sessions, except that a user participating in both
   sessions should use the same distinguished (canonical) name in the
   RTCP packets for both so that the sessions can be associated.

   One motivation for this separation is to allow some participants in
   the conference to receive only one medium if they choose. Further
   explanation is given in Section 5.2. Despite the separation,
   synchronized playback of a source's audio and video can be achieved
   using timing information carried in the RTCP packets for both
   sessions.

2.3 Mixers and Translators

   So far, we have assumed that all sites want to receive media data in
   the same format. However, this may not always be appropriate.
   Consider the case where participants in one area are connected
   through a low-speed link to the majority of the conference
   participants who enjoy high-speed network access. Instead of forcing
   everyone to use a lower-bandwidth, reduced-quality audio encoding, an
   RTP-level relay called a mixer may be placed near the low-bandwidth



Schulzrinne/Casner/Frederick/Jacobson                         [Page 6]


Internet Draft                    RTP                  February 26, 1999


   area. This mixer resynchronizes incoming audio packets to reconstruct
   the constant 20 ms spacing generated by the sender, mixes these
   reconstructed audio streams into a single stream, translates the
   audio encoding to a lower-bandwidth one and forwards the lower-
   bandwidth packet stream across the low-speed link. These packets
   might be unicast to a single recipient or multicast on a different
   address to multiple recipients. The RTP header includes a means for
   mixers to identify the sources that contributed to a mixed packet so
   that correct talker indication can be provided at the receivers.

   Some of the intended participants in the audio conference may be
   connected with high bandwidth links but might not be directly
   reachable via IP multicast. For example, they might be behind an
   application-level firewall that will not let any IP packets pass. For
   these sites, mixing may not be necessary, in which case another type
   of RTP-level relay called a translator may be used. Two translators
   are installed, one on either side of the firewall, with the outside
   one funneling all multicast packets received through a secure
   connection to the translator inside the firewall. The translator
   inside the firewall sends them again as multicast packets to a
   multicast group restricted to the site's internal network.

   Mixers and translators may be designed for a variety of purposes. An
   example is a video mixer that scales the images of individual people
   in separate video streams and composites them into one video stream
   to simulate a group scene. Other examples of translation include the
   connection of a group of hosts speaking only IP/UDP to a group of
   hosts that understand only ST-II, or the packet-by-packet encoding
   translation of video streams from individual sources without
   resynchronization or mixing. Details of the operation of mixers and
   translators are given in Section 7.

2.4 Layered Encodings

   Multimedia applications should be able to adjust the transmission
   rate to match the capacity of the receiver or to adapt to network
   congestion. Many implementations place the responsibility of rate-
   adaptivity at the source. This does not work well with multicast
   transmission because of the conflicting bandwidth requirements of
   heterogeneous receivers. The result is often a least-common
   denominator scenario, where the smallest pipe in the network mesh
   dictates the quality and fidelity of the overall live multimedia
   "broadcast".

   Instead, responsibility for rate-adaptation can be placed at the
   receivers by combining a layered encoding with a layered transmission
   system. In the context of RTP over IP multicast, the source can
   stripe the progressive layers of a hierarchically represented signal



Schulzrinne/Casner/Frederick/Jacobson                         [Page 7]


Internet Draft                    RTP                  February 26, 1999


   across multiple RTP sessions each carried on its own multicast group.
   Receivers can then adapt to network heterogeneity and control their
   reception bandwidth by joining only the appropriate subset of the
   multicast groups.

   Details of the use of RTP with layered encodings are given in
   Sections 6.3.9, 8.3 and 10.

3 Definitions

   RTP payload: The data transported by RTP in a packet, for example
        audio samples or compressed video data. The payload format and
        interpretation are beyond the scope of this document.

   RTP packet: A data packet consisting of the fixed RTP header, a
        possibly empty list of contributing sources (see below), and the
        payload data. Some underlying protocols may require an
        encapsulation of the RTP packet to be defined. Typically one
        packet of the underlying protocol contains a single RTP packet,
        but several RTP packets MAY be contained if permitted by the
        encapsulation method (see Section 10).

   RTCP packet: A control packet consisting of a fixed header part
        similar to that of RTP data packets, followed by structured
        elements that vary depending upon the RTCP packet type. The
        formats are defined in Section 6. Typically, multiple RTCP
        packets are sent together as a compound RTCP packet in a single
        packet of the underlying protocol; this is enabled by the length
        field in the fixed header of each RTCP packet.

   Port: The "abstraction that transport protocols use to distinguish
        among multiple destinations within a given host computer. TCP/IP
        protocols identify ports using small positive integers." [5] The
        transport selectors (TSEL) used by the OSI transport layer are
        equivalent to ports.  RTP depends upon the lower-layer protocol
        to provide some mechanism such as ports to multiplex the RTP and
        RTCP packets of a session.

   Transport address: The combination of a network address and port that
        identifies a transport-level endpoint, for example an IP address
        and a UDP port. Packets are transmitted from a source transport
        address to a destination transport address.

   RTP media type: An RTP media type is the collection of payload types
        which can be carried within a single RTP session. The RTP
        Profile assigns RTP media types to RTP payload types.

   RTP session: The association among a set of participants



Schulzrinne/Casner/Frederick/Jacobson                         [Page 8]


Internet Draft                    RTP                  February 26, 1999


        communicating with RTP. For each participant, the session is
        defined by a particular pair of destination transport addresses
        (one network address plus a port pair for RTP and RTCP). The
        destination transport address pair may be common for all
        participants, as in the case of IP multicast, or may be
        different for each, as in the case of individual unicast network
        addresses and port pairs.  In a multimedia session, each medium
        is carried in a separate RTP session with its own RTCP packets.
        The multiple RTP sessions are distinguished by different port
        number pairs and/or different multicast addresses.

   Synchronization source (SSRC): The source of a stream of RTP packets,
        identified by a 32-bit numeric SSRC identifier carried in the
        RTP header so as not to be dependent upon the network address.
        All packets from a synchronization source form part of the same
        timing and sequence number space, so a receiver groups packets
        by synchronization source for playback. Examples of
        synchronization sources include the sender of a stream of
        packets derived from a signal source such as a microphone or a
        camera, or an RTP mixer (see below). A synchronization source
        may change its data format, e.g., audio encoding, over time. The
        SSRC identifier is a randomly chosen value meant to be globally
        unique within a particular RTP session (see Section 8). A
        participant need not use the same SSRC identifier for all the
        RTP sessions in a multimedia session; the binding of the SSRC
        identifiers is provided through RTCP (see Section 6.5.1). If a
        participant generates multiple streams in one RTP session, for
        example from separate video cameras, each MUST be identified as
        a different SSRC.

   Contributing source (CSRC): A source of a stream of RTP packets that
        has contributed to the combined stream produced by an RTP mixer
        (see below). The mixer inserts a list of the SSRC identifiers of
        the sources that contributed to the generation of a particular
        packet into the RTP header of that packet. This list is called
        the CSRC list. An example application is audio conferencing
        where a mixer indicates all the talkers whose speech was
        combined to produce the outgoing packet, allowing the receiver
        to indicate the current talker, even though all the audio
        packets contain the same SSRC identifier (that of the mixer).

   End system: An application that generates the content to be sent in
        RTP packets and/or consumes the content of received RTP packets.
        An end system can act as one or more synchronization sources in
        a particular RTP session, but typically only one.

   Mixer: An intermediate system that receives RTP packets from one or
        more sources, possibly changes the data format, combines the



Schulzrinne/Casner/Frederick/Jacobson                         [Page 9]


Internet Draft                    RTP                  February 26, 1999


        packets in some manner and then forwards a new RTP packet. Since
        the timing among multiple input sources will not generally be
        synchronized, the mixer will make timing adjustments among the
        streams and generate its own timing for the combined stream.
        Thus, all data packets originating from a mixer will be
        identified as having the mixer as their synchronization source.

   Translator: An intermediate system that forwards RTP packets with
        their synchronization source identifier intact. Examples of
        translators include devices that convert encodings without
        mixing, replicators from multicast to unicast, and application-
        level filters in firewalls.

   Monitor: An application that receives RTCP packets sent by
        participants in an RTP session, in particular the reception
        reports, and estimates the current quality of service for
        distribution monitoring, fault diagnosis and long-term
        statistics. The monitor function is likely to be built into the
        application(s) participating in the session, but may also be a
        separate application that does not otherwise participate and
        does not send or receive the RTP data packets. These are called
        third party monitors.

   Non-RTP means: Protocols and mechanisms that may be needed in
        addition to RTP to provide a usable service. In particular, for
        multimedia conferences, a conference control application may
        distribute multicast addresses and keys for encryption,
        negotiate the encryption algorithm to be used, and define
        dynamic mappings between RTP payload type values and the payload
        formats they represent for formats that do not have a predefined
        payload type value. For simple applications, electronic mail or
        a conference database may also be used. The specification of
        such protocols and mechanisms is outside the scope of this
        document.

4 Byte Order, Alignment, and Time Format

   All integer fields are carried in network byte order, that is, most
   significant byte (octet) first. This byte order is commonly known as
   big-endian. The transmission order is described in detail in [6].
   Unless otherwise noted, numeric constants are in decimal (base 10).

   All header data is aligned to its natural length, i.e., 16-bit fields
   are aligned on even offsets, 32-bit fields are aligned at offsets
   divisible by four, etc. Octets designated as padding have the value
   zero.

   Wallclock time (absolute date and time) is represented using the



Schulzrinne/Casner/Frederick/Jacobson                        [Page 10]


Internet Draft                    RTP                  February 26, 1999


   timestamp format of the Network Time Protocol (NTP), which is in
   seconds relative to 0h UTC on 1 January 1900 [7]. The full resolution
   NTP timestamp is a 64-bit unsigned fixed-point number with the
   integer part in the first 32 bits and the fractional part in the last
   32 bits. In some fields where a more compact representation is
   appropriate, only the middle 32 bits are used; that is, the low 16
   bits of the integer part and the high 16 bits of the fractional part.
   The high 16 bits of the integer part must be determined
   independently.

   The NTP timestamp will wrap around to zero some time in the year
   2036, but for RTP purposes, only differences between pairs of NTP
   timestamps are used. So long as the pairs of timestamps can be
   assumed to be within 68 years of each other, using modulo arithmetic
   for subtractions and comparisons makes the wraparound irrelevant.

5 RTP Data Transfer Protocol

5.1 RTP Fixed Header Fields

   The RTP header 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V=2|P|X|  CC   |M|     PT      |       sequence number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           synchronization source (SSRC) identifier            |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |            contributing source (CSRC) identifiers             |
   |                             ....                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The first twelve octets are present in every RTP packet, while the
   list of CSRC identifiers is present only when inserted by a mixer.
   The fields have the following meaning:

   version (V): 2 bits
        This field identifies the version of RTP. The version defined by
        this specification is two (2). (The value 1 is used by the first
        draft version of RTP and the value 0 is used by the protocol
        initially implemented in the "vat" audio tool.)

   padding (P): 1 bit
        If the padding bit is set, the packet contains one or more
        additional padding octets at the end which are not part of the



Schulzrinne/Casner/Frederick/Jacobson                        [Page 11]


Internet Draft                    RTP                  February 26, 1999


        payload. The last octet of the padding contains a count of how
        many padding octets should be ignored, including itself.
        Padding may be needed by some encryption algorithms with fixed
        block sizes or for carrying several RTP packets in a lower-layer
        protocol data unit.

   extension (X): 1 bit
        If the extension bit is set, the fixed header MUST be followed
        by exactly one header extension, with a format defined in
        Section 5.3.1.

   CSRC count (CC): 4 bits
        The CSRC count contains the number of CSRC identifiers that
        follow the fixed header.

   marker (M): 1 bit
        The interpretation of the marker is defined by a profile. It is
        intended to allow significant events such as frame boundaries to
        be marked in the packet stream. A profile MAY define additional
        marker bits or specify that there is no marker bit by changing
        the number of bits in the payload type field (see Section 5.3).

   payload type (PT): 7 bits
        This field identifies the format of the RTP payload and
        determines its interpretation by the application. A profile MAY
        specify a default static mapping of payload type codes to
        payload formats. Additional payload type codes MAY be defined
        dynamically through non-RTP means (see Section 3).  A set of
        default mappings for audio and video is specified in the
        companion RFC 1890 (updated by Internet-Draft draft-ietf-avt-
        profile-new [2]).  An RTP source MAY change the payload type
        during a session, but this field SHOULD NOT be used for
        multiplexing separate media streams (see Section 5.2).

   A receiver MUST ignore packets with payload types that it does not
   understand.

   sequence number: 16 bits
        The sequence number increments by one for each RTP data packet
        sent, and may be used by the receiver to detect packet loss and
        to restore packet sequence. The initial value of the sequence
        number SHOULD be random (unpredictable) to make known-plaintext
        attacks on encryption more difficult, even if the source itself
        does not encrypt according to the method in Section 9.1, because
        the packets may flow through a translator that does. Techniques
        for choosing unpredictable numbers are discussed in [9].

   timestamp: 32 bits



Schulzrinne/Casner/Frederick/Jacobson                        [Page 12]


Internet Draft                    RTP                  February 26, 1999


        The timestamp reflects the sampling instant of the first octet
        in the RTP data packet. The sampling instant MUST be derived
        from a clock that increments monotonically and linearly in time
        to allow synchronization and jitter calculations (see Section
        6.4.1).  The resolution of the clock MUST be sufficient for the
        desired synchronization accuracy and for measuring packet
        arrival jitter (one tick per video frame is typically not
        sufficient). The clock frequency is dependent on the format of
        data carried as payload and is specified statically in the
        profile or payload format specification that defines the format,
        or MAY be specified dynamically for payload formats defined
        through non-RTP means. If RTP packets are generated
        periodically, the nominal sampling instant as determined from
        the sampling clock is to be used, not a reading of the system
        clock. As an example, for fixed-rate audio the timestamp clock
        would likely increment by one for each sampling period. If an
        audio application reads blocks covering 160 sampling periods
        from the input device, the timestamp would be increased by 160
        for each such block, regardless of whether the block is
        transmitted in a packet or dropped as silent.

   The initial value of the timestamp SHOULD be random, as for the
   sequence number. Several consecutive RTP packets will have equal
   timestamps if they are (logically) generated at once, e.g., belong to
   the same video frame. Consecutive RTP packets MAY contain timestamps
   that are not monotonic if the data is not transmitted in the order it
   was sampled, as in the case of MPEG interpolated video frames. (The
   sequence numbers of the packets as transmitted will still be
   monotonic.)

   SSRC: 32 bits
        The SSRC field identifies the synchronization source. This
        identifier SHOULD be chosen randomly, with the intent that no
        two synchronization sources within the same RTP session will
        have the same SSRC identifier. An example algorithm for
        generating a random identifier is presented in Appendix A.6.
        Although the probability of multiple sources choosing the same
        identifier is low, all RTP implementations must be prepared to
        detect and resolve collisions.  Section 8 describes the
        probability of collision along with a mechanism for resolving
        collisions and detecting RTP-level forwarding loops based on the
        uniqueness of the SSRC identifier. If a source changes its
        source transport address, it must also choose a new SSRC
        identifier to avoid being interpreted as a looped source (see
        Section 8.2).

   CSRC list: 0 to 15 items, 32 bits each
        The CSRC list identifies the contributing sources for the



Schulzrinne/Casner/Frederick/Jacobson                        [Page 13]


Internet Draft                    RTP                  February 26, 1999


        payload contained in this packet. The number of identifiers is
        given by the CC field. If there are more than 15 contributing
        sources, only 15 can be identified. CSRC identifiers are
        inserted by mixers (see Section 7.1), using the SSRC identifiers
        of contributing sources. For example, for audio packets the SSRC
        identifiers of all sources that were mixed together to create a
        packet are listed, allowing correct talker indication at the
        receiver.

5.2 Multiplexing RTP Sessions

   For efficient protocol processing, the number of multiplexing points
   should be minimized, as described in the integrated layer processing
   design principle [1]. In RTP, multiplexing is provided by the
   destination transport address (network address and port number) which
   define an RTP session. For example, in a teleconference composed of
   audio and video media encoded separately, each medium SHOULD be
   carried in a separate RTP session with its own destination transport
   address.  Separate audio and video streams SHOULD NOT be carried in a
   single RTP session and demultiplexed based on the payload type or
   SSRC fields. Interleaving packets with different RTP media types but
   using the same SSRC would introduce several problems:

        1.   If, say, two audio streams shared the same RTP session and
             the same SSRC value, and one were to change encodings and
             thus acquire a different RTP payload type, there would be
             no general way of identifying which stream had changed
             encodings.

        2.   An SSRC is defined to identify a single timing and sequence
             number space. Interleaving multiple payload types would
             require different timing spaces if the media clock rates
             differ and would require different sequence number spaces
             to tell which payload type suffered packet loss.

        3.   The RTCP sender and receiver reports (see Section 6.4) can
             only describe one timing and sequence number space per SSRC
             and do not carry a payload type field.

        4.   An RTP mixer would not be able to combine interleaved
             streams of incompatible media into one stream.

        5.   Carrying multiple media in one RTP session precludes: the
             use of different network paths or network resource
             allocations if appropriate; reception of a subset of the
             media if desired, for example just audio if video would
             exceed the available bandwidth; and receiver
             implementations that use separate processes for the



Schulzrinne/Casner/Frederick/Jacobson                        [Page 14]


Internet Draft                    RTP                  February 26, 1999


             different media, whereas using separate RTP sessions
             permits either single- or multiple-process implementations.

   Using a different SSRC for each medium but sending them in the same
   RTP session would avoid the first three problems but not the last
   two.

5.3 Profile-Specific Modifications to the RTP Header

   The existing RTP data packet header is believed to be complete for
   the set of functions required in common across all the application
   classes that RTP might support. However, in keeping with the ALF
   design principle, the header MAY be tailored through modifications or
   additions defined in a profile specification while still allowing
   profile-independent monitoring and recording tools to function.

       o The marker bit and payload type field carry profile-specific
         information, but they are allocated in the fixed header since
         many applications are expected to need them and might otherwise
         have to add another 32-bit word just to hold them. The octet
         containing these fields MAY be redefined by a profile to suit
         different requirements, for example with a more or fewer marker
         bits. If there are any marker bits, one SHOULD be located in
         the most significant bit of the octet since profile-independent
         monitors may be able to observe a correlation between packet
         loss patterns and the marker bit.

       o Additional information that is required for a particular
         payload format, such as a video encoding, SHOULD be carried in
         the payload section of the packet. This might be in a header
         that is always present at the start of the payload section, or
         might be indicated by a reserved value in the data pattern.

       o If a particular class of applications needs additional
         functionality independent of payload format, the profile under
         which those applications operate SHOULD define additional fixed
         fields to follow immediately after the SSRC field of the
         existing fixed header.  Those applications will be able to
         quickly and directly access the additional fields while
         profile-independent monitors or recorders can still process the
         RTP packets by interpreting only the first twelve octets.

   If it turns out that additional functionality is needed in common
   across all profiles, then a new version of RTP should be defined to
   make a permanent change to the fixed header.

5.3.1 RTP Header Extension




Schulzrinne/Casner/Frederick/Jacobson                        [Page 15]


Internet Draft                    RTP                  February 26, 1999


   An extension mechanism is provided to allow individual
   implementations to experiment with new payload-format-independent
   functions that require additional information to be carried in the
   RTP data packet header. This mechanism is designed so that the header
   extension may be ignored by other interoperating implementations that
   have not been extended.

   Note that this header extension is intended only for limited use.
   Most potential uses of this mechanism would be better done another
   way, using the methods described in the previous section. For
   example, a profile-specific extension to the fixed header is less
   expensive to process because it is not conditional nor in a variable
   location. Additional information required for a particular payload
   format SHOULD NOT use this header extension, but SHOULD be carried in
   the payload section of the packet.


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      defined by profile       |           length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        header extension                       |
   |                             ....                              |


   If the X bit in the RTP header is one, a variable-length header
   extension MUST be appended to the RTP header, following the CSRC list
   if present. The header extension contains a 16-bit length field that
   counts the number of 32-bit words in the extension, excluding the
   four-octet extension header (therefore zero is a valid length). Only
   a single extension can be appended to the RTP data header. To allow
   multiple interoperating implementations to each experiment
   independently with different header extensions, or to allow a
   particular implementation to experiment with more than one type of
   header extension, the first 16 bits of the header extension are left
   open for distinguishing identifiers or parameters. The format of
   these 16 bits is to be defined by the profile specification under
   which the implementations are operating. This RTP specification does
   not define any header extensions itself.

6 RTP Control Protocol -- RTCP

   The RTP control protocol (RTCP) is based on the periodic transmission
   of control packets to all participants in the session, using the same
   distribution mechanism as the data packets. The underlying protocol
   MUST provide multiplexing of the data and control packets, for
   example using separate port numbers with UDP. RTCP performs four



Schulzrinne/Casner/Frederick/Jacobson                        [Page 16]


Internet Draft                    RTP                  February 26, 1999


   functions:

        1.   The primary function is to provide feedback on the quality
             of the data distribution. This is an integral part of the
             RTP's role as a transport protocol and is related to the
             flow and congestion control functions of other transport
             protocols. The feedback may be directly useful for control
             of adaptive encodings [10,11], but experiments with IP
             multicasting have shown that it is also critical to get
             feedback from the receivers to diagnose faults in the
             distribution. Sending reception feedback reports to all
             participants allows one who is observing problems to
             evaluate whether those problems are local or global. With a
             distribution mechanism like IP multicast, it is also
             possible for an entity such as a network service provider
             who is not otherwise involved in the session to receive the
             feedback information and act as a third-party monitor to
             diagnose network problems. This feedback function is
             performed by the RTCP sender and receiver reports,
             described below in Section 6.4.

        2.   RTCP carries a persistent transport-level identifier for an
             RTP source called the canonical name or CNAME, Section
             6.5.1. Since the SSRC identifier may change if a conflict
             is discovered or a program is restarted, receivers require
             the CNAME to keep track of each participant. Receivers may
             also require the CNAME to associate multiple data streams
             from a given participant in a set of related RTP sessions,
             for example to synchronize audio and video.  Inter-media
             synchronization also requires the NTP and RTP timestamps
             included in RTCP packets by data senders.

        3.   The first two functions require that all participants send
             RTCP packets, therefore the rate must be controlled in
             order for RTP to scale up to a large number of
             participants. By having each participant send its control
             packets to all the others, each can independently observe
             the number of participants. This number is used to
             calculate the rate at which the packets are sent, as
             explained in Section 6.2.

        4.   A fourth, OPTIONAL function is to convey minimal session
             control information, for example participant identification
             to be displayed in the user interface. This is most likely
             to be useful in "loosely controlled" sessions where
             participants enter and leave without membership control or
             parameter negotiation. RTCP serves as a convenient channel
             to reach all the participants, but it is not necessarily



Schulzrinne/Casner/Frederick/Jacobson                        [Page 17]


Internet Draft                    RTP                  February 26, 1999


             expected to support all the control communication
             requirements of an application. A higher-level session
             control protocol, which is beyond the scope of this
             document, may be needed.

   Functions 1-3 SHOULD be used in all environments, but particularly in
   the IP multicast environment. RTP application designers SHOULD avoid
   mechanisms that can only work in unicast mode and will not scale to
   larger numbers. Transmission of RTCP MAY be controlled separately for
   senders and receivers, as described in Section 6.2, for cases such as
   unidirectional links where feedback from receivers is not possible.

6.1 RTCP Packet Format

   This specification defines several RTCP packet types to carry a
   variety of control information:

   SR: Sender report, for transmission and reception statistics from
        participants that are active senders

   RR: Receiver report, for reception statistics from participants that
        are not active senders

   SDES: Source description items, including CNAME

   BYE: Indicates end of participation

   APP: Application specific functions

   Each RTCP packet begins with a fixed part similar to that of RTP data
   packets, followed by structured elements that MAY be of variable
   length according to the packet type but MUST end on a 32-bit
   boundary. The alignment requirement and a length field in the fixed
   part of each packet are included to make RTCP packets "stackable".
   Multiple RTCP packets can be concatenated without any intervening
   separators to form a compound RTCP packet that is sent in a single
   packet of the lower layer protocol, for example UDP. There is no
   explicit count of individual RTCP packets in the compound packet
   since the lower layer protocols are expected to provide an overall
   length to determine the end of the compound packet.

   Each individual RTCP packet in the compound packet may be processed
   independently with no requirements upon the order or combination of
   packets. However, in order to perform the functions of the protocol,
   the following constraints are imposed:

       o Reception statistics (in SR or RR) should be sent as often as
         bandwidth constraints will allow to maximize the resolution of



Schulzrinne/Casner/Frederick/Jacobson                        [Page 18]


Internet Draft                    RTP                  February 26, 1999


         the statistics, therefore each periodically transmitted
         compound RTCP packet MUST include a report packet.

       o New receivers need to receive the CNAME for a source as soon
         as possible to identify the source and to begin associating
         media for purposes such as lip-sync, so each compound RTCP
         packet MUST also include the SDES CNAME.

       o The number of packet types that may appear first in the
         compound packet needs to be limited to increase the number of
         constant bits in the first word and the probability of
         successfully validating RTCP packets against misaddressed RTP
         data packets or other unrelated packets.

   Thus, all RTCP packets MUST be sent in a compound packet of at least
   two individual packets, with the following format RECOMMENDED:

   Encryption prefix:  If and only if the compound packet is to be
        encrypted according to the method in Section 9.1, it MUST be
        prefixed by a random 32-bit quantity redrawn for every compound
        packet transmitted.  If padding is required for the encryption,
        it MUST be added to the last packet of the compound packet.

   SR or RR:  The first RTCP packet in the compound packet MUST always
        be a report packet to facilitate header validation as described
        in Appendix A.2. This is true even if no data has been sent nor
        received, in which case an empty RR MUST be sent, and even if
        the only other RTCP packet in the compound packet is a BYE.

   Additional RRs:  If the number of sources for which reception
        statistics are being reported exceeds 31, the number that will
        fit into one SR or RR packet, then additional RR packets SHOULD
        follow the initial report packet.

   SDES:  An SDES packet containing a CNAME item MUST be included in
        each compound RTCP packet. Other source description items MAY
        optionally be included if required by a particular application,
        subject to bandwidth constraints (see Section 6.3.9).

   BYE or APP:  Other RTCP packet types, including those yet to be
        defined, MAY follow in any order, except that BYE SHOULD be the
        last packet sent with a given SSRC/CSRC. Packet types MAY appear
        more than once.

   It is RECOMMENDED that translators and mixers combine individual RTCP
   packets from the multiple sources they are forwarding into one
   compound packet whenever feasible in order to amortize the packet
   overhead (see Section 7). An example RTCP compound packet as might be



Schulzrinne/Casner/Frederick/Jacobson                        [Page 19]


Internet Draft                    RTP                  February 26, 1999


   produced by a mixer is shown in Fig. 1. If the overall length of a
   compound packet would exceed the maximum transmission unit (MTU) of
   the network path, it SHOULD be segmented into multiple shorter
   compound packets to be transmitted in separate packets of the
   underlying protocol. Note that each of the compound packets MUST
   begin with an SR or RR packet.

   An implementation SHOULD ignore incoming RTCP packets with types
   unknown to it. Additional RTCP packet types may be registered with
   the Internet Assigned Numbers Authority (IANA) as described in
   Section 11.3.



   if encrypted: random 32-bit integer
    |
    |[------- packet -------][----------- packet -----------][-packet-]
    |
    |             receiver              chunk        chunk
    V             reports            item  item    item  item
   --------------------------------------------------------------------
   |R[SR|# sender #site#site][SDES|# CNAME PHONE |#CNAME LOC][BYE##why]
   |R[  |# report #  1 #  2 ][    |#             |#         ][   ##   ]
   |R[  |#        #    #    ][    |#             |#         ][   ##   ]
   |R[  |#        #    #    ][    |#             |#         ][   ##   ]
   --------------------------------------------------------------------
   |<------------------  UDP packet (compound packet) --------------->|

   #: SSRC/CSRC

   Figure 1: Example of an RTCP compound packet



6.2 RTCP Transmission Interval

   RTP is designed to allow an application to scale automatically over
   session sizes ranging from a few participants to thousands. For
   example, in an audio conference the data traffic is inherently self-
   limiting because only one or two people will speak at a time, so with
   multicast distribution the data rate on any given link remains
   relatively constant independent of the number of participants.
   However, the control traffic is not self-limiting. If the reception
   reports from each participant were sent at a constant rate, the
   control traffic would grow linearly with the number of participants.
   Therefore, the rate must be scaled down by dynamically calculating
   the interval between RTCP packet transmissions.




Schulzrinne/Casner/Frederick/Jacobson                        [Page 20]


Internet Draft                    RTP                  February 26, 1999


   For each session, it is assumed that the data traffic is subject to
   an aggregate limit called the "session bandwidth" to be divided among
   the participants. This bandwidth might be reserved and the limit
   enforced by the network.  If there is no reservation, there may be
   other constraints, depending on the environment, that establish the
   "reasonable" maximum for the session to use, and that would be the
   session bandwidth.  The session bandwidth may be chosen based or some
   cost or a priori knowledge of the available network bandwidth for the
   session.  It is somewhat independent of the media encoding, but the
   encoding choice may be limited by the session bandwidth.  Often, the
   session bandwidth is the sum of the nominal bandwidths of the senders
   expected to be concurrently active. For teleconference audio, this
   number would typically be one sender's bandwidth. For layered
   encodings, each layer is a separate RTP session with its own session
   bandwidth parameter.

   The session bandwidth parameter is expected to be supplied by a
   session management application when it invokes a media application,
   but media applications MAY set a default based on the single-sender
   data bandwidth for the encoding selected for the session. The
   application MAY also enforce bandwidth limits based on multicast
   scope rules or other criteria. All participants MUST use the same
   value for the session bandwidth so that the same RTCP interval will
   be calculated.

   Bandwidth calculations for control and data traffic include lower-
   layer transport and network protocols (e.g., UDP and IP) since that
   is what the resource reservation system would need to know. The
   application can also be expected to know which of these protocols are
   in use. Link level headers are not included in the calculation since
   the packet will be encapsulated with different link level headers as
   it travels.

   The control traffic should be limited to a small and known fraction
   of the session bandwidth: small so that the primary function of the
   transport protocol to carry data is not impaired; known so that the
   control traffic can be included in the bandwidth specification given
   to a resource reservation protocol, and so that each participant can
   independently calculate its share. It is RECOMMENDED that the
   fraction of the session bandwidth allocated to RTCP be fixed at 5%.
   It is also RECOMMENDED that 1/4 of the RTCP bandwidth be dedicated to
   participants that are sending data so that in sessions with a large
   number of receivers but a small number of senders, newly joining
   participants will more quickly receive the CNAME for the sending
   sites. When the proportion of senders is greater than 1/4 of the
   participants, the senders get their proportion of the full RTCP
   bandwidth.  While the values of these and other constants in the
   interval calculation are not critical, all participants in the



Schulzrinne/Casner/Frederick/Jacobson                        [Page 21]


Internet Draft                    RTP                  February 26, 1999


   session MUST use the same values so the same interval will be
   calculated. Therefore, these constants SHOULD be fixed for a
   particular profile.

   A profile MAY specify that the control traffic bandwidth may be a
   separate parameter of the session rather than a strict percentage of
   the session bandwidth. Using a separate parameter allows rate-
   adaptive applications to set an RTCP bandwidth consistent with a
   "typical" data bandwidth that is lower than the maximum bandwidth
   specified by the session bandwidth parameter.

   The profile MAY further specify that the control traffic bandwidth
   may be divided into two separate session parameters for those
   participants which are active data senders and those which are not.
   Following the recommendation that 1/4 of the RTCP bandwidth be
   dedicated to data senders, the RECOMMENDED default values for these
   two parameters would be 1.25% and 3.75%, respectively. When the
   proportion of senders is greater than 1/4 of the participants, the
   senders get their proportion of the sum of these parameters. Using
   two parameters allows RTCP reception reports to be turned off
   entirely for a particular session by setting the RTCP bandwidth for
   non-data-senders to zero while keeping the RTCP bandwidth for data
   senders non-zero so that sender reports can still be sent for inter-
   media synchronization. This may be appropriate for systems operating
   on unidirectional links or for sessions that don't require feedback
   on the quality of reception.

   The calculated interval between transmissions of compound RTCP
   packets SHOULD also have a lower bound to avoid having bursts of
   packets exceed the allowed bandwidth when the number of participants
   is small and the traffic isn't smoothed according to the law of large
   numbers.  It also keeps the report interval from becoming too small
   during transient outages like a network partition such that
   adaptation is delayed when the partition heals. At application
   startup, a delay SHOULD be imposed before the first compound RTCP
   packet is sent to allow time for RTCP packets to be received from
   other participants so the report interval will converge to the
   correct value more quickly.  This delay MAY be set to half the
   minimum interval to allow quicker notification that the new
   participant is present. The RECOMMENDED value for a fixed minimum
   interval is 5 seconds.

   An implementation MAY scale the minimum RTCP interval to a smaller
   value inversely proportional to the session bandwidth parameter with
   the following limitations:

       o For multicast sessions, only active data senders MAY use the
         reduced minimum value to calculate the interval for



Schulzrinne/Casner/Frederick/Jacobson                        [Page 22]


Internet Draft                    RTP                  February 26, 1999


         transmission of compound RTCP packets.

       o For unicast sessions, the reduced value MAY be used by
         participants that are not active data senders as well, and the
         delay before sending the initial compound RTCP packet MAY be
         zero.

       o For all sessions, the fixed minimum SHOULD be used when
         calculating the participant timeout interval (see Section 6.3.5
         so that implementations which do not to use the reduced value
         for transmitting RTCP packets are not timed out by other
         participants prematurely.

       o The RECOMMENDED value for the reduced minimum in seconds is
         360 divided by the session bandwidth in kilobits/second. This
         minimum is smaller than 5 seconds for bandwidths greater than
         72 kb/s.

   The algorithm described in Section 6.3 and Appendix A.7 was designed
   to meet the goals outlined above. It calculates the interval between
   sending compound RTCP packets to divide the allowed control traffic
   bandwidth among the participants. This allows an application to
   provide fast response for small sessions where, for example,
   identification of all participants is important, yet automatically
   adapt to large sessions. The algorithm incorporates the following
   characteristics:

       o The calculated interval between RTCP packets scales linearly
         with the number of members in the group. It is this linear
         factor which allows for a constant amount of control traffic
         when summed across all members.

       o The interval between RTCP packets is varied randomly over the
         range [0.5,1.5] times the calculated interval to avoid
         unintended synchronization of all participants [12].  The first
         RTCP packet sent after joining a session is also delayed by a
         random variation of half the minimum RTCP interval.

       o A dynamic estimate of the average compound RTCP packet size is
         calculated, including all those received and sent, to
         automatically adapt to changes in the amount of control
         information carried.

       o Since the calculated interval is dependent on the number of
         observed group members, there may be undesirable startup
         effects when a new user joins an existing session, or many
         users simultaneously join a new session. These new users will
         initially have incorrect estimates of the group membership, and



Schulzrinne/Casner/Frederick/Jacobson                        [Page 23]


Internet Draft                    RTP                  February 26, 1999


         thus their RTCP transmission interval will be too short. This
         problem can be significant if many users join the session
         simultaneously. To deal with this, an algorithm called "timer
         reconsideration" is employed. This algorithm implements a
         simple back-off mechanism which causes users to hold back RTCP
         packet transmission if the group sizes are increasing.

       o When users leave a session, either with a BYE or by timeout,
         the group membership decreases, and thus the calculated
         interval should decrease. A "reverse reconsideration" algorithm
         is used to allow members to more quickly reduce their intervals
         in response to group membership decreases.

       o BYE packets are given different treatment than other RTCP
         packets. When a user leaves a group, and wishes to send a BYE
         packet, it may do so before its next scheduled RTCP packet.
         However, transmission of BYE's follows a back-off algorithm
         which avoids floods of BYE packets should a large number of
         members simultaneously leave the session.

   This algorithm may be used for sessions in which all participants are
   allowed to send. In that case, the session bandwidth parameter is the
   product of the individual sender's bandwidth times the number of
   participants, and the RTCP bandwidth is 5% of that.

   Details of the algorithm's operation are given in the sections that
   follow. Appendix A.7 gives an example implementation.

6.2.1 Maintaining the number of session members

   Calculation of the RTCP packet interval depends upon an estimate of
   the number of sites participating in the session. New sites are added
   to the count when they are heard, and an entry for each SHOULD be
   created in a table indexed by the SSRC or CSRC identifier (see
   Section 8.2) to keep track of them. New entries MAY be considered not
   valid until multiple packets carrying the new SSRC have been received
   (see Appendix A.1), or until an SDES RTCP packet containing a CNAME
   for that SSRC has been received.  Entries MAY be deleted from the
   table when an RTCP BYE packet with the corresponding SSRC identifier
   is received, except that some straggler data packets might arrive
   after the BYE and cause the entry to be recreated. Instead, the entry
   SHOULD be marked as having received a BYE and then deleted after an
   appropriate delay.

   A participant MAY mark another site inactive, or delete it if not yet
   valid, if no RTP or RTCP packet has been received for a small number
   of RTCP report intervals (5 is RECOMMENDED).  This provides some
   robustness against packet loss. All sites MUST have the same value



Schulzrinne/Casner/Frederick/Jacobson                        [Page 24]


Internet Draft                    RTP                  February 26, 1999


   for this multiplier and must calculate roughly the same value for the
   RTCP report interval in order for this timeout to work properly.
   Therefore, this multiplier SHOULD be fixed for a particular profile.

   For sessions with a very large number of participants, it may be
   impractical to maintain a table to store the SSRC identifier and
   state information for all of them. An implementation MAY use SSRC
   sampling, as described in [13], to reduce the storage requirements.
   An implementation MAY use any other algorithm with similar
   performance. A key requirement is that any algorithm considered
   SHOULD NOT substantially underestimate the group size, although it
   MAY overestimate.

6.3 RTCP Packet Send and Receive Rules

   The rules for how to send, and what to do when receiving an RTCP
   packet are outlined here.  An implementation that allows operation in
   a multicast environment or a multipoint unicast environment MUST meet
   the scalability goals described in Section 6.2. Such an
   implementation MAY use an algorithm other than the one defined here
   so long as it provides equivalent or better performance. An
   implementation which is constrained to two-party unicast operation
   MAY omit this algorithm.

   To execute these rules, a session participant must maintain several
   pieces of state:

   tp: the last time an RTCP packet was transmitted;

   tc: the current time;

   tn: the next scheduled transmission time of an RTCP packet;

   pmembers: the estimated number of session members at the time tn was
        last recomputed;

   members: the most current estimate for the number of session members;

   senders: the most current estimate for the number of senders in the
        session;

   rtcp_bw: The target RTCP bandwidth, i.e., the total bandwidth that
        will be used for RTCP packets by all members of this session, in
        octets per second. This will be a specified fraction of the
        "session bandwidth" parameter supplied to the application at
        startup.

   we_sent: Flag that is true if the application has sent data since the



Schulzrinne/Casner/Frederick/Jacobson                        [Page 25]


Internet Draft                    RTP                  February 26, 1999


        2nd previous RTCP report was transmitted.

   avg_rtcp_size: The average compound RTCP packet size, in octets, over
        all RTCP packets sent and received by this participant.

   initial: Flag that is true if the application has not yet sent an
        RTCP packet.

   Many of these rules make use of the "calculated interval" between
   packet transmissions. This interval is described in the following
   section.

6.3.1 Computing the RTCP transmission interval

   To maintain scalability, the average interval between packets from a
   session participant should scale with the group size. This interval
   is called the calculated interval. It is obtained by combining a
   number of the pieces of state described above. The calculated
   interval T is then determined as follows:

        1.   If there are any senders (senders > 0) in the session, but
             the number of senders is less than 25% of the membership
             (members), the interval depends on whether the participant
             is a sender or not (based on the value of we_sent). If the
             participant is a sender (we_sent true), the constant C is
             set to the average RTCP packet size (avg_rtcp_size) divided
             by 25% of the RTCP bandwidth (rtcp_bw), and the constant n
             is set to the number of senders. If we_sent is not true,
             the constant C is set to the average RTCP packet size
             divided by 75% of the RTCP bandwidth. The constant n is set
             to the number of receivers (members - senders). If the
             number of senders is greater than 25%, senders and
             receivers are treated together. The constant C is set to
             the total RTCP bandwidth and n is set to the total number
             of members.

        2.   If the participant has not yet sent an RTCP packet (the
             variable initial is true), the constant Tmin is set to 2.5
             seconds, else it is set to 5 seconds.

        3.   The deterministic calculated interval Td is set to
             max(Tmin, n*C).

        4.   The calculated interval T is set to a number uniformly
             distributed between 0.5 and 1.5 times the deterministic
             calculated interval.

        5.   The resulting value of T is divided by e-3/2=1.21828 to



Schulzrinne/Casner/Frederick/Jacobson                        [Page 26]


Internet Draft                    RTP                  February 26, 1999


             compensate for the fact that the unconditional
             reconsideration algorithm converges to a value below the
             intended average.

   This procedure results in an interval which is random, but which, on
   average, gives 25% of the RTCP bandwidth to senders, and 75% to
   receivers.

6.3.2 Initialization

   Upon joining the session, the participant initializes tp to 0, tc to
   0, senders to 0, pmembers to 1, members to 1, we_sent to false,
   rtcp_bw to the specified fraction of the session bandwidth, initial
   to true, and avg_rtcp_size to the size of the very first packet
   constructed by the application. The calculated interval T is then
   computed, and the first packet is scheduled for time tn = T. This
   means that a transmission timer is set which expires at time T. Note
   that an application MAY use any desired approach for implementing
   this timer.

   The participant adds its own SSRC to the member table.

6.3.3 Receiving an RTP or non-BYE RTCP packet

   When an RTP or RTCP packet is received from a participant whose SSRC
   is not in the member table, the SSRC is added to the table, and the
   value for members is updated once the participant has been validated
   as described in Section 6.2.1. The same processing occurs for each
   CSRC in a validated RTP packet.

   When an RTP packet is received from a participant whose SSRC is not
   in the sender table, the SSRC is added to the table, and the value
   for senders is updated.

   For each compound RTCP packet received, the value of avg_rtcp_size is
   updated: avg_rtcp_size = (1/16)*packet_size + (15/16)* avg_rtcp_size,
   where packet_size is the size of the RTCP packet just received.

6.3.4 Receiving an RTCP BYE packet

   Except as described in Section 6.3.7 for the case when an RTCP BYE is
   to be transmitted, if the received packet is an RTCP BYE packet, the
   SSRC is checked against the member table. If present, the entry is
   removed from the table, and the value for members is updated. The
   SSRC is then checked against the sender table. If present, the entry
   is removed from the table, and the value for senders is updated.

   Furthermore, to make the transmission rate of RTCP packets more



Schulzrinne/Casner/Frederick/Jacobson                        [Page 27]


Internet Draft                    RTP                  February 26, 1999


   adaptive to changes in group membership, the following "reverse
   reconsideration" algorithm SHOULD be executed when a BYE packet is
   received that reduces members to a value less than pmembers:

       o The value for tn is updated according to the following
         formula:  tn = tc + (members/pmembers)(tn - tc).

       o The value for tp is updated according the following formula:
         tp = tc - (members/pmembers)(tc - tp).

       o The next RTCP packet is rescheduled for transmission at time
         tn, which is now earlier.

       o The value of pmembers is set equal to members.

   This algorithm does not prevent the group size estimate from
   incorrectly dropping to zero for a short time due to premature
   timeouts when most participants of a large session leave at once but
   some remain. The algorithm does make the estimate return to the
   correct value more rapidly. This situation is unusual enough and the
   consequences are sufficiently harmless that this problem is deemed
   only a secondary concern.

6.3.5 Timing Out an SSRC

   At occassional intervals, the participant MUST check to see if any of
   the other participants time out. To do this, the participant computes
   the deterministic calculated interval (without the randomization
   factor) Td. Any other session member who has not sent a packet since
   time tc - MTd (M is the timeout multiplier, and defaults to 5) is
   timed out. This means that its SSRC is removed from the member list,
   and members is updated. A similar check is performed on the sender
   list. Any member on the sender list who has not sent an RTP packet
   since time tc - 2T (within the last two RTCP report intervals) is
   removed from the sender list, and senders is updated.

   If any members time out, the reverse reconsideration algorithm
   described in Section 6.3.4 SHOULD be performed.

   The participant MUST perform this check at least once per RTCP
   transmission interval.

6.3.6 Expiration of transmission timer

   When the packet transmission timer expires, the participant performs
   the following operations:

       o The transmission interval T is computed as described in



Schulzrinne/Casner/Frederick/Jacobson                        [Page 28]


Internet Draft                    RTP                  February 26, 1999


         Section 6.3.1, including the randomization factor.

       o If tp + T is less than or equal to tc, an RTCP packet is
         transmitted. tp is set to tc, then another value for T is
         calculated as in the previous step and tn is set to tc + T. The
         transmission timer is set to expire again at time tn. If tp + T
         is greater than tc, tn is set to tp + T. No RTCP packet is
         transmitted. The transmission timer is set to expire at time
         tn.

       o pmembers is set to members.

   If an RTCP packet is transmitted, the value of initial is set to
   FALSE. Furthermore, the value of avg_rtcp_size is updated:
   avg_rtcp_size = (1/16)*packet_size + (15/16)* avg_rtcp_size, where
   packet_size is the size of the RTCP packet just transmitted.

6.3.7 Transmitting a BYE packet

   When a participant wishes to leave a session, a BYE packet is
   transmitted to inform the other participants of the event. In order
   to avoid a flood of BYE packets when many participants leave the
   system, a participant MUST execute the following algorithm if the
   number of members is more than 50 when the participant chooses to
   leave. This algorithm usurps the normal role of the members variable
   to count BYE packets instead:

       o When the participant decides to leave the system, tp is reset
         to tc, the current time, members and pmembers are initialized
         to 1, initial is set to 1, we_sent is set to 0, senders is set
         to 0, and avg_rtcp_size is set to the size of the BYE packet.
         The calculated interval T is computed. The BYE packet is then
         scheduled for time tn = tc + T.

       o Every time a BYE packet from another participant is received,
         members is incremented by 1 regardless of whether that
         participant exists in the member table or not, and when SSRC
         sampling is in use, regardless of whether or not the BYE SSRC
         would be included in the sample.  members is NOT incremented
         when other RTCP packets or RTP packets are received, but only
         for BYE packets.

       o Transmission of the BYE packet then follows the rules for
         transmitting a regular RTCP packet, as above.

   This allows BYE packets to be sent right away, yet controls their
   total bandwidth usage. In the worst case, this could cause RTCP
   control packets to use twice the bandwidth as normal (10%) -- 5% for



Schulzrinne/Casner/Frederick/Jacobson                        [Page 29]


Internet Draft                    RTP                  February 26, 1999


   non BYE RTCP packets and 5% for BYE.

   A participant that does not want to wait for the above mechanism to
   allow transmission of a BYE packet MAY leave the group without
   sending a BYE at all. That participant will eventually be timed out
   by the other group members.

   If the group size estimate members is less than 50 when the
   participant decides to leave, the participant MAY send a BYE packet
   immediately. Alternatively, the participant MAY choose to execute the
   above BYE backoff algorithm.

   In either case, a participant which never sent an RTP or RTCP packet
   MUST NOT send a BYE packet when they leave the group.

6.3.8 Updating we_sent

   The variable we_sent contains true if the participant has sent an RTP
   packet recently, false otherwise. This determination is made by using
   the same mechanisms for managing the senders table and sending SR
   packets. If the participant sends an RTP packet when we_sent is
   false, it adds itself to the sender table and sets we_sent to true.
   Every time another RTP packet is sent, the time of transmission of
   that packet is maintained in the table. The normal sender timeout
   algorithm is then applied to the participant -- if an RTP packet has
   not been transmitted since time tc - 2T, the participant removes
   itself from the sender table, decrements the sender count, and sets
   we_sent to false.

6.3.9 Allocation of source description bandwidth

   This specification defines several source description (SDES) items in
   addition to the mandatory CNAME item, such as NAME (personal name)
   and EMAIL (email address). It also provides a means to define new
   application-specific RTCP packet types. Applications should exercise
   caution in allocating control bandwidth to this additional
   information because it will slow down the rate at which reception
   reports and CNAME are sent, thus impairing the performance of the
   protocol. It is RECOMMENDED that no more than 20% of the RTCP
   bandwidth allocated to a single participant be used to carry the
   additional information.  Furthermore, it is not intended that all
   SDES items will be included in every application. Those that are
   included SHOULD be assigned a fraction of the bandwidth according to
   their utility. Rather than estimate these fractions dynamically, it
   is recommended that the percentages be translated statically into
   report interval counts based on the typical length of an item.

   For example, an application may be designed to send only CNAME, NAME



Schulzrinne/Casner/Frederick/Jacobson                        [Page 30]


Internet Draft                    RTP                  February 26, 1999


   and EMAIL and not any others. NAME might be given much higher
   priority than EMAIL because the NAME would be displayed continuously
   in the application's user interface, whereas EMAIL would be displayed
   only when requested. At every RTCP interval, an RR packet and an SDES
   packet with the CNAME item would be sent. For a small session
   operating at the minimum interval, that would be every 5 seconds on
   the average. Every third interval (15 seconds), one extra item would
   be included in the SDES packet. Seven out of eight times this would
   be the NAME item, and every eighth time (2 minutes) it would be the
   EMAIL item.

   When multiple applications operate in concert using cross-application
   binding through a common CNAME for each participant, for example in a
   multimedia conference composed of an RTP session for each medium, the
   additional SDES information MAY be sent in only one RTP session.  The
   other sessions would carry only the CNAME item.  In particular, this
   approach should be applied to the multiple sessions of a layered
   encoding scheme (see Section 2.4).

6.4 Sender and Receiver Reports

   RTP receivers provide reception quality feedback using RTCP report
   packets which may take one of two forms depending upon whether or not
   the receiver is also a sender. The only difference between the sender
   report (SR) and receiver report (RR) forms, besides the packet type
   code, is that the sender report includes a 20-byte sender information
   section for use by active senders. The SR is issued if a site has
   sent any data packets during the interval since issuing the last
   report or the previous one, otherwise the RR is issued.

   Both the SR and RR forms include zero or more reception report
   blocks, one for each of the synchronization sources from which this
   receiver has received RTP data packets since the last report. Reports
   are not issued for contributing sources listed in the CSRC list. Each
   reception report block provides statistics about the data received
   from the particular source indicated in that block. Since a maximum
   of 31 reception report blocks will fit in an SR or RR packet,
   additional RR packets MAY be stacked after the initial SR or RR
   packet as needed to contain the reception reports for all sources
   heard during the interval since the last report.

   The next sections define the formats of the two reports, how they may
   be extended in a profile-specific manner if an application requires
   additional feedback information, and how the reports may be used.
   Details of reception reporting by translators and mixers is given in
   Section 7.

6.4.1 SR: Sender report RTCP packet



Schulzrinne/Casner/Frederick/Jacobson                        [Page 31]


Internet Draft                    RTP                  February 26, 1999



    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|    RC   |   PT=SR=200   |             length            | header
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         SSRC of sender                        |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |              NTP timestamp, most significant word             | sender
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ info
   |             NTP timestamp, least significant word             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         RTP timestamp                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     sender's packet count                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      sender's octet count                     |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                 SSRC_1 (SSRC of first source)                 | report
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
   | fraction lost |       cumulative number of packets lost       |   1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           extended highest sequence number received           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      interarrival jitter                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         last SR (LSR)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   delay since last SR (DLSR)                  |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                 SSRC_2 (SSRC of second source)                | report
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
   :                               ...                             :   2
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                  profile-specific extensions                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The sender report packet consists of three sections, possibly
   followed by a fourth profile-specific extension section if defined.
   The first section, the header, is 8 octets long. The fields have the
   following meaning:

   version (V): 2 bits
        Identifies the version of RTP, which is the same in RTCP packets
        as in RTP data packets. The version defined by this
        specification is two (2).

   padding (P): 1 bit



Schulzrinne/Casner/Frederick/Jacobson                        [Page 32]


Internet Draft                    RTP                  February 26, 1999


        If the padding bit is set, this individual RTCP packet contains
        some additional padding octets at the end which are not part of
        the control information but are included in the length field.
        The last octet of the padding is a count of how many padding
        octets should be ignored, including itself (it will be a
        multiple of four). Padding may be needed by some encryption
        algorithms with fixed block sizes. In a compound RTCP packet,
        padding is only required on one individual packet because the
        compound packet is encrypted as a whole for the method in
        Section 9.1.  Thus, padding MUST only be added to the last
        individual packet, and if padding is added to that packet, the
        padding bit MUST be set only on that packet. This convention
        aids the header validity checks described in Appendix A.2 and
        allows detection of packets from some early implementations that
        incorrectly set the padding bit on the first individual packet
        and add padding to the last individual packet.

   reception report count (RC): 5 bits
        The number of reception report blocks contained in this packet.
        A value of zero is valid.

   packet type (PT): 8 bits
        Contains the constant 200 to identify this as an RTCP SR packet.

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

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

   The second section, the sender information, is 20 octets long and is
   present in every sender report packet. It summarizes the data
   transmissions from this sender. The fields have the following
   meaning:

   NTP timestamp: 64 bits
        Indicates the wallclock time (see Section 4) when this report
        was sent so that it may be used in combination with timestamps
        returned in reception reports 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



Schulzrinne/Casner/Frederick/Jacobson                        [Page 33]


Internet Draft                    RTP                  February 26, 1999


        it may not be known.  On a system that has no notion of
        wallclock time but does have some system-specific clock such as
        "system uptime", a sender MAY use that clock as a reference to
        calculate relative NTP timestamps. It is important to choose a
        commonly used clock so that if separate implementations are used
        to produce the individual streams of a multimedia session, all
        implementations will use the same clock.  Until the year 2036,
        relative and absolute timestamps will differ in the high bit so
        (invalid) comparisons will show a large difference; by then one
        hopes relative timestamps will no longer be needed.  A sender
        that has no notion of wallclock or elapsed time MAY set the NTP
        timestamp to zero.

   RTP timestamp: 32 bits
        Corresponds to the same time as the NTP timestamp (above), but
        in the same units and with the same random offset as the RTP
        timestamps in data packets. This correspondence may be used for
        intra- and inter-media synchronization for sources whose NTP
        timestamps are synchronized, and may be used by media-
        independent receivers to estimate the nominal RTP clock
        frequency. Note that in most cases this timestamp will not be
        equal to the RTP timestamp in any adjacent data packet. Rather,
        it MUST be calculated from the corresponding NTP timestamp using
        the relationship between the RTP timestamp counter and real time
        as maintained by periodically checking the wallclock time at a
        sampling instant.

   sender's packet count: 32 bits
        The total number of RTP data packets transmitted by the sender
        since starting transmission up until the time this SR packet was
        generated.  The count SHOULD be reset if the sender changes its
        SSRC identifier.

   sender's octet count: 32 bits
        The total number of payload octets (i.e., not including header
        or padding) transmitted in RTP data packets by the sender since
        starting transmission up until the time this SR packet was
        generated. The count SHOULD be reset if the sender changes its
        SSRC identifier. This field can be used to estimate the average
        payload data rate.

   The third section contains zero or more reception report blocks
   depending on the number of other sources heard by this sender since
   the last report. Each reception report block conveys statistics on
   the reception of RTP packets from a single synchronization source.
   Receivers SHOULD NOT carry over statistics when a source changes its
   SSRC identifier due to a collision. These statistics are:




Schulzrinne/Casner/Frederick/Jacobson                        [Page 34]


Internet Draft                    RTP                  February 26, 1999


   SSRC_n (source identifier): 32 bits
        The SSRC identifier of the source to which the information in
        this reception report block pertains.

   fraction lost: 8 bits
        The fraction of RTP data packets from source SSRC_n lost since
        the previous SR or RR packet was sent, expressed as a fixed
        point number with the binary point at the left edge of the
        field. (That is equivalent to taking the integer part after
        multiplying the loss fraction by 256.) This fraction is defined
        to be the number of packets lost divided by the number of
        packets expected, as defined in the next paragraph. An
        implementation is shown in Appendix A.3.  If the loss is
        negative due to duplicates, the fraction lost is set to zero.
        Note that a receiver cannot tell whether any packets were lost
        after the last one received, and that there will be no reception
        report block issued for a source if all packets from that source
        sent during the last reporting interval have been lost.

   cumulative number of packets lost: 24 bits
        The total number of RTP data packets from source SSRC_n that
        have been lost since the beginning of reception. This number is
        defined to be the number of packets expected less the number of
        packets actually received, where the number of packets received
        includes any which are late or duplicates. Thus packets that
        arrive late are not counted as lost, and the loss may be
        negative if there are duplicates.  The number of packets
        expected is defined to be the extended last sequence number
        received, as defined next, less the initial sequence number
        received. This may be calculated as shown in Appendix A.3.

   extended highest sequence number received: 32 bits
        The low 16 bits contain the highest sequence number received in
        an RTP data packet from source SSRC_n, and the most significant
        16 bits extend that sequence number with the corresponding count
        of sequence number cycles, which may be maintained according to
        the algorithm in Appendix A.1. Note that different receivers
        within the same session will generate different extensions to
        the sequence number if their start times differ significantly.

   interarrival jitter: 32 bits
        An estimate of the statistical variance of the RTP data packet
        interarrival time, measured in timestamp units and expressed as
        an unsigned integer. The interarrival jitter J is defined to be
        the mean deviation (smoothed absolute value) of the difference D
        in packet spacing at the receiver compared to the sender for a
        pair of packets. As shown in the equation below, this is
        equivalent to the difference in the "relative transit time" for



Schulzrinne/Casner/Frederick/Jacobson                        [Page 35]


Internet Draft                    RTP                  February 26, 1999


        the two packets; the relative transit time is the difference
        between a packet's RTP timestamp and the receiver's clock at the
        time of arrival, measured in the same units.

   If Si is the RTP timestamp from packet i, and Ri is the time of
   arrival in RTP timestamp units for packet i, then for two packets i
   and j, D may be expressed as D(i,j) = (R_j - R_i) - (S_j - S_i) =
   (R_j - S_j) - (R_i - S_i)

   The interarrival jitter SHOULD be calculated continuously as each
   data packet i is received from source SSRC_n, using this difference D
   for that packet and the previous packet i-1 in order of arrival (not
   necessarily in sequence), according to the formula J_i = J_i-1 +
   (|D(i-1,i)| - J_i-1)/16
   Whenever a reception report is issued, the current value of J is
   sampled.

   The jitter calculation MUST conform to the formula specified here in
   order to allow profile-independent monitors to make valid
   interpretations of reports coming from different implementations.
   This algorithm is the optimal first-order estimator and the gain
   parameter 1/16 gives a good noise reduction ratio while maintaining a
   reasonable rate of convergence [14].  A sample implementation is
   shown in Appendix A.8.

   last SR timestamp (LSR): 32 bits
        The middle 32 bits out of 64 in the NTP timestamp (as explained
        in Section 4) received as part of the most recent RTCP sender
        report (SR) packet from source SSRC_n. If no SR has been
        received yet, the field is set to zero.

   delay since last SR (DLSR): 32 bits
        The delay, expressed in units of 1/65536 seconds, between
        receiving the last SR packet from source SSRC_n and sending this
        reception report block. If no SR packet has been received yet
        from SSRC_n, the DLSR field is set to zero.

   Let SSRC_r denote the receiver issuing this receiver report. Source
   SSRC_n can compute the round propagation delay to SSRC_r by recording
   the time A when this reception report block is received.  It
   calculates the total round-trip time A-LSR using the last SR
   timestamp (LSR) field, and then subtracting this field to leave the
   round-trip propagation delay as (A- LSR - DLSR). This is illustrated
   in Fig. 2.


   This may be used as an approximate measure of distance to cluster
   receivers, although some links have very asymmetric delays.



Schulzrinne/Casner/Frederick/Jacobson                        [Page 36]


Internet Draft                    RTP                  February 26, 1999




   [10 Nov 1995 11:33:25.125]           [10 Nov 1995 11:33:36.5]
   n                 SR(n)              A=b710:8000 (46864.500 s)
   ---------------------------------------------------------------->
                      v                 ^
   ntp_sec =0xb44db705 v               ^ dlsr=0x0005.4000 (    5.250s)
   ntp_frac=0x20000000  v             ^  lsr =0xb705:2000 (46853.125s)
     (3024992016.125 s)  v           ^
   r                      v         ^ RR(n)
   ---------------------------------------------------------------->
                          |<-DLSR->|
                           (5.250 s)

   A     0xb710:8000 (46864.500 s)
   DLSR -0x0005:4000 (    5.250 s)
   LSR  -0xb705:2000 (46853.125 s)
   -------------------------------
   delay 0x   6:2000 (    6.125 s)

   Figure 2: Example for round-trip time computation


6.4.2 RR: Receiver report RTCP packet



























Schulzrinne/Casner/Frederick/Jacobson                        [Page 37]


Internet Draft                    RTP                  February 26, 1999



    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|    RC   |   PT=RR=201   |             length            | header
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     SSRC of packet sender                     |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                 SSRC_1 (SSRC of first source)                 | report
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
   | fraction lost |       cumulative number of packets lost       |   1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           extended highest sequence number received           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      interarrival jitter                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         last SR (LSR)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   delay since last SR (DLSR)                  |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                 SSRC_2 (SSRC of second source)                | report
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
   :                               ...                             :   2
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                  profile-specific extensions                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format of the receiver report (RR) packet is the same as that of
   the SR packet except that the packet type field contains the constant
   201 and the five words of sender information are omitted (these are
   the NTP and RTP timestamps and sender's packet and octet counts). The
   remaining fields have the same meaning as for the SR packet.

   An empty RR packet (RC = 0) MUST be put at the head of a compound
   RTCP packet when there is no data transmission or reception to
   report.

6.4.3 Extending the sender and receiver reports

   A profile SHOULD define profile-specific extensions to the sender
   report and receiver report if there is additional information that
   needs to be reported regularly about the sender or receivers. This
   method SHOULD be used in preference to defining another RTCP packet
   type because it requires less overhead:

       o fewer octets in the packet (no RTCP header or SSRC field);

       o simpler and faster parsing because applications running under



Schulzrinne/Casner/Frederick/Jacobson                        [Page 38]


Internet Draft                    RTP                  February 26, 1999


         that profile would be programmed to always expect the extension
         fields in the directly accessible location after the reception
         reports.

   The extension is a fourth section in the sender- or receiver-report
   packet which comes at the end after the reception report blocks, if
   any. If additional sender information is required, then for sender
   reports it would be included first in the extension section, but for
   receiver reports it would not be present.  If information about
   receivers is to be included, that data SHOULD be structured as an
   array of blocks parallel to the existing array of reception report
   blocks; that is, the number of blocks would be indicated by the RC
   field.

6.4.4 Analyzing sender and receiver reports

   It is expected that reception quality feedback will be useful not
   only for the sender but also for other receivers and third-party
   monitors.  The sender may modify its transmissions based on the
   feedback; receivers can determine whether problems are local,
   regional or global; network managers may use profile-independent
   monitors that receive only the RTCP packets and not the corresponding
   RTP data packets to evaluate the performance of their networks for
   multicast distribution.

   Cumulative counts are used in both the sender information and
   receiver report blocks so that differences may be calculated between
   any two reports to make measurements over both short and long time
   periods, and to provide resilience against the loss of a report. The
   difference between the last two reports received can be used to
   estimate the recent quality of the distribution. The NTP timestamp is
   included so that rates may be calculated from these differences over
   the interval between two reports. Since that timestamp is independent
   of the clock rate for the data encoding, it is possible to implement
   encoding- and profile-independent quality monitors.

   An example calculation is the packet loss rate over the interval
   between two reception reports. The difference in the cumulative
   number of packets lost gives the number lost during that interval.
   The difference in the extended last sequence numbers received gives
   the number of packets expected during the interval. The ratio of
   these two is the packet loss fraction over the interval. This ratio
   should equal the fraction lost field if the two reports are
   consecutive, but otherwise it may not. The loss rate per second can
   be obtained by dividing the loss fraction by the difference in NTP
   timestamps, expressed in seconds. The number of packets received is
   the number of packets expected minus the number lost. The number of
   packets expected may also be used to judge the statistical validity



Schulzrinne/Casner/Frederick/Jacobson                        [Page 39]


Internet Draft                    RTP                  February 26, 1999


   of any loss estimates.  For example, 1 out of 5 packets lost has a
   lower significance than 200 out of 1000.

   From the sender information, a third-party monitor can calculate the
   average payload data rate and the average packet rate over an
   interval without receiving the data. Taking the ratio of the two
   gives the average payload size. If it can be assumed that packet loss
   is independent of packet size, then the number of packets received by
   a particular receiver times the average payload size (or the
   corresponding packet size) gives the apparent throughput available to
   that receiver.

   In addition to the cumulative counts which allow long-term packet
   loss measurements using differences between reports, the fraction
   lost field provides a short-term measurement from a single report.
   This becomes more important as the size of a session scales up enough
   that reception state information might not be kept for all receivers
   or the interval between reports becomes long enough that only one
   report might have been received from a particular receiver.

   The interarrival jitter field provides a second short-term measure of
   network congestion. Packet loss tracks persistent congestion while
   the jitter measure tracks transient congestion. The jitter measure
   may indicate congestion before it leads to packet loss. Since the
   interarrival jitter field is only a snapshot of the jitter at the
   time of a report, it may be necessary to analyze a number of reports
   from one receiver over time or from multiple receivers, e.g., within
   a single network.

6.5 SDES: Source description RTCP packet


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V=2|P|    SC   |  PT=SDES=202  |             length            | header
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                          SSRC/CSRC_1                          | chunk
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   1
   |                           SDES items                          |
   |                              ...                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                          SSRC/CSRC_2                          | chunk
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   2
   |                           SDES items                          |
   |                              ...                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+




Schulzrinne/Casner/Frederick/Jacobson                        [Page 40]


Internet Draft                    RTP                  February 26, 1999


   The SDES packet is a three-level structure composed of a header and
   zero or more chunks, each of of which is composed of items describing
   the source identified in that chunk. The items are described
   individually in subsequent sections.

   version (V), padding (P), length:
        As described for the SR packet (see Section 6.4.1).

   packet type (PT): 8 bits
        Contains the constant 202 to identify this as an RTCP SDES
        packet.

   source count (SC): 5 bits
        The number of SSRC/CSRC chunks contained in this SDES packet. A
        value of zero is valid but useless.

   Each chunk consists of an SSRC/CSRC identifier followed by a list of
   zero or more items, which carry information about the SSRC/CSRC. Each
   chunk starts on a 32-bit boundary. Each item consists of an 8-bit
   type field, an 8-bit octet count describing the length of the text
   (thus, not including this two-octet header), and the text itself.
   Note that the text can be no longer than 255 octets, but this is
   consistent with the need to limit RTCP bandwidth consumption.

   The text is encoded according to the UTF-8 encoding specified in RFC
   2279 [15].  US-ASCII is a subset of this encoding and requires no
   additional encoding. The presence of multi-octet encodings is
   indicated by setting the most significant bit of a character to a
   value of one.

   Items are contiguous, i.e., items are not individually padded to a
   32-bit boundary. Text is not null terminated because some multi-octet
   encodings include null octets. The list of items in each chunk MUST
   be terminated by one or more null octets, the first of which is
   interpreted as an item type of zero to denote the end of the list.
   No length octet follows the null item type octet, but additional null
   octets MUST be included if needed to pad until the next 32-bit
   boundary.  Note that this padding is separate from that indicated by
   the P bit in the RTCP header.  A chunk with zero items (four null
   octets) is valid but useless.

   End systems send one SDES packet containing their own source
   identifier (the same as the SSRC in the fixed RTP header). A mixer
   sends one SDES packet containing a chunk for each contributing source
   from which it is receiving SDES information, or multiple complete
   SDES packets in the format above if there are more than 31 such
   sources (see Section 7).




Schulzrinne/Casner/Frederick/Jacobson                        [Page 41]


Internet Draft                    RTP                  February 26, 1999


   The SDES items currently defined are described in the next sections.
   Only the CNAME item is mandatory. Some items shown here may be useful
   only for particular profiles, but the item types are all assigned
   from one common space to promote shared use and to simplify profile-
   independent applications. Additional items may be defined in a
   profile by registering the type numbers with IANA as described in
   Section 11.3.

6.5.1 CNAME: Canonical end-point identifier SDES item


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    CNAME=1    |     length    | user and domain name         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The CNAME identifier has the following properties:

       o Because the randomly allocated SSRC identifier may change if a
         conflict is discovered or if a program is restarted, the CNAME
         item MUST be included to provide the binding from the SSRC
         identifier to an identifier for the source that remains
         constant.

       o Like the SSRC identifier, the CNAME identifier SHOULD also be
         unique among all participants within one RTP session.

       o To provide a binding across multiple media tools used by one
         participant in a set of related RTP sessions, the CNAME SHOULD
         be fixed for that participant.

       o To facilitate third-party monitoring, the CNAME SHOULD be
         suitable for either a program or a person to locate the source.

   Therefore, the CNAME SHOULD be derived algorithmically and not
   entered manually, when possible. To meet these requirements, the
   following format SHOULD be used unless a profile specifies an
   alternate syntax or semantics. The CNAME item SHOULD have the format
   "user@host", or "host" if a user name is not available as on single-
   user systems.  For both formats, "host" is either the fully qualified
   domain name of the host from which the real-time data originates,
   formatted according to the rules specified in RFC 1034 [16], RFC 1035
   [17] and Section 2.1 of RFC 1123 [18]; or the standard ASCII
   representation of the host's numeric address on the interface used
   for the RTP communication. For example, the standard ASCII
   representation of an IP Version 4 address is "dotted decimal", also
   known as dotted quad. Other address types are expected to have ASCII



Schulzrinne/Casner/Frederick/Jacobson                        [Page 42]


Internet Draft                    RTP                  February 26, 1999


   representations that are mutually unique. The fully qualified domain
   name is more convenient for a human observer and may avoid the need
   to send a NAME item in addition, but it may be difficult or
   impossible to obtain reliably in some operating environments.
   Applications that may be run in such environments SHOULD use the
   ASCII representation of the address instead.

   Examples are "doe@sleepy.megacorp.com" or "doe@192.0.2.89" for a
   multi-user system. On a system with no user name, examples would be
   "sleepy.megacorp.com" or "192.0.2.89".

   The user name SHOULD be in a form that a program such as "finger" or
   "talk" could use, i.e., it typically is the login name rather than
   the personal name. The host name is not necessarily identical to the
   one in the participant's electronic mail address.

   This syntax will not provide unique identifiers for each source if an
   application permits a user to generate multiple sources from one
   host.  Such an application would have to rely on the SSRC to further
   identify the source, or the profile for that application would have
   to specify additional syntax for the CNAME identifier.

   If each application creates its CNAME independently, the resulting
   CNAMEs may not be identical as would be required to provide a binding
   across multiple media tools belonging to one participant in a set of
   related RTP sessions. If cross-media binding is required, it may be
   necessary for the CNAME of each tool to be externally configured with
   the same value by a coordination tool.

   Application writers should be aware that private network address
   assignments such as the Net-10 assignment proposed in RFC 1597 [19]
   may create network addresses that are not globally unique. This would
   lead to non-unique CNAMEs if hosts with private addresses and no
   direct IP connectivity to the public Internet have their RTP packets
   forwarded to the public Internet through an RTP-level translator.
   (See also RFC 1627 [20].) To handle this case, applications MAY
   provide a means to configure a unique CNAME, but the burden is on the
   translator to translate CNAMEs from private addresses to public
   addresses if necessary to keep private addresses from being exposed.

6.5.2 NAME: User name SDES item


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     NAME=2    |     length    | common name of source        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



Schulzrinne/Casner/Frederick/Jacobson                        [Page 43]


Internet Draft                    RTP                  February 26, 1999


   This is the real name used to describe the source, e.g., "John Doe,
   Bit Recycler, Megacorp". It may be in any form desired by the user.
   For applications such as conferencing, this form of name may be the
   most desirable for display in participant lists, and therefore might
   be sent most frequently of those items other than CNAME. Profiles MAY
   establish such priorities.  The NAME value is expected to remain
   constant at least for the duration of a session. It SHOULD NOT be
   relied upon to be unique among all participants in the session.

6.5.3 EMAIL: Electronic mail address SDES item


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    EMAIL=3    |     length    | email address of source      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The email address is formatted according to RFC 822 [21], for
   example, "John.Doe@megacorp.com". The EMAIL value is expected to
   remain constant for the duration of a session.

6.5.4 PHONE: Phone number SDES item


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    PHONE=4    |     length    | phone number of source       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The phone number SHOULD be formatted with the plus sign replacing the
   international access code.  For example, "+1 908 555 1212" for a
   number in the United States.

6.5.5 LOC: Geographic user location SDES item


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     LOC=5     |     length    | geographic location of site  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Depending on the application, different degrees of detail are
   appropriate for this item. For conference applications, a string like
   "Murray Hill, New Jersey" may be sufficient, while, for an active
   badge system, strings like "Room 2A244, AT&T BL MH" might be



Schulzrinne/Casner/Frederick/Jacobson                        [Page 44]


Internet Draft                    RTP                  February 26, 1999


   appropriate. The degree of detail is left to the implementation
   and/or user, but format and content MAY be prescribed by a profile.
   The LOC value is expected to remain constant for the duration of a
   session, except for mobile hosts.

6.5.6 TOOL: Application or tool name SDES item


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     TOOL=6    |     length    | name/version of source appl. ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A string giving the name and possibly version of the application
   generating the stream, e.g., "videotool 1.2". This information may be
   useful for debugging purposes and is similar to the Mailer or Mail-
   System-Version SMTP headers. The TOOL value is expected to remain
   constant for the duration of the session.

6.5.7 NOTE: Notice/status SDES item


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     NOTE=7    |     length    | note about the source        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The following semantics are suggested for this item, but these or
   other semantics MAY be explicitly defined by a profile. The NOTE item
   is intended for transient messages describing the current state of
   the source, e.g., "on the phone, can't talk". Or, during a seminar,
   this item might be used to convey the title of the talk. It should be
   used only to carry exceptional information and SHOULD NOT be included
   routinely by all participants because this would slow down the rate
   at which reception reports and CNAME are sent, thus impairing the
   performance of the protocol. In particular, it SHOULD NOT be included
   as an item in a user's configuration file nor automatically generated
   as in a quote-of-the-day.

   Since the NOTE item may be important to display while it is active,
   the rate at which other non-CNAME items such as NAME are transmitted
   might be reduced so that the NOTE item can take that part of the RTCP
   bandwidth. When the transient message becomes inactive, the NOTE item
   SHOULD continue to be transmitted a few times at the same repetition
   rate but with a string of length zero to signal the receivers.
   However, receivers SHOULD also consider the NOTE item inactive if it



Schulzrinne/Casner/Frederick/Jacobson                        [Page 45]


Internet Draft                    RTP                  February 26, 1999


   is not received for a small multiple of the repetition rate, or
   perhaps 20-30 RTCP intervals.

6.5.8 PRIV: Private extensions SDES item


      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     PRIV=8    |     length    | prefix length | prefix string...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...              |                  value string                ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This item is used to define experimental or application-specific SDES
   extensions. The item contains a prefix consisting of a length-string
   pair, followed by the value string filling the remainder of the item
   and carrying the desired information. The prefix length field is 8
   bits long. The prefix string is a name chosen by the person defining
   the PRIV item to be unique with respect to other PRIV items this
   application might receive. The application creator might choose to
   use the application name plus an additional subtype identification if
   needed.  Alternatively, it is RECOMMENDED that others choose a name
   based on the entity they represent, then coordinate the use of the
   name within that entity.

   Note that the prefix consumes some space within the item's total
   length of 255 octets, so the prefix should be kept as short as
   possible. This facility and the constrained RTCP bandwidth SHOULD NOT
   be overloaded; it is not intended to satisfy all the control
   communication requirements of all applications.

   SDES PRIV prefixes will not be registered by IANA. If some form of
   the PRIV item proves to be of general utility, it SHOULD instead be
   assigned a regular SDES item type registered with IANA so that no
   prefix is required. This simplifies use and increases transmission
   efficiency.

6.6 BYE: Goodbye RTCP packet












Schulzrinne/Casner/Frederick/Jacobson                        [Page 46]


Internet Draft                    RTP                  February 26, 1999



    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|    SC   |   PT=BYE=203  |             length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           SSRC/CSRC                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                              ...                              :
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |     length    |               reason for leaving             ... (opt)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The BYE packet indicates that one or more sources are no longer
   active.

   version (V), padding (P), length:
        As described for the SR packet (see Section 6.4.1).

   packet type (PT): 8 bits
        Contains the constant 203 to identify this as an RTCP BYE
        packet.

   source count (SC): 5 bits
        The number of SSRC/CSRC identifiers included in this BYE packet.
        A count value of zero is valid, but useless.

   The rules for when a BYE packet should be sent are specified in
   Sections 6.3.7 and 8.2.

   If a BYE packet is received by a mixer, the mixer SHOULD forward the
   BYE packet with the SSRC/CSRC identifier(s) unchanged. If a mixer
   shuts down, it SHOULD send a BYE packet listing all contributing
   sources it handles, as well as its own SSRC identifier. Optionally,
   the BYE packet MAY include an 8-bit octet count followed by that many
   octets of text indicating the reason for leaving, e.g., "camera
   malfunction" or "RTP loop detected". The string has the same encoding
   as that described for SDES. If the string fills the packet to the
   next 32-bit boundary, the string is not null terminated. If not, the
   BYE packet MUST be padded with null octets to the next 32-bit
   boundary. This padding is separate from that indicated by the P bit
   in the RTCP header.

6.7 APP: Application-defined RTCP packet






Schulzrinne/Casner/Frederick/Jacobson                        [Page 47]


Internet Draft                    RTP                  February 26, 1999



    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| subtype |   PT=APP=204  |             length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           SSRC/CSRC                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          name (ASCII)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   application-dependent data                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The APP packet is intended for experimental use as new applications
   and new features are developed, without requiring packet type value
   registration. APP packets with unrecognized names SHOULD be ignored.
   After testing and if wider use is justified, it is RECOMMENDED that
   each APP packet be redefined without the subtype and name fields and
   registered with IANA using an RTCP packet type.

   version (V), padding (P), length:
        As described for the SR packet (see Section 6.4.1).

   subtype: 5 bits
        May be used as a subtype to allow a set of APP packets to be
        defined under one unique name, or for any application-dependent
        data.

   packet type (PT): 8 bits
        Contains the constant 204 to identify this as an RTCP APP
        packet.

   name: 4 octets
        A name chosen by the person defining the set of APP packets to
        be unique with respect to other APP packets this application
        might receive. The application creator might choose to use the
        application name, and then coordinate the allocation of subtype
        values to others who want to define new packet types for the
        application.  Alternatively, it is RECOMMENDED that others
        choose a name based on the entity they represent, then
        coordinate the use of the name within that entity. The name is
        interpreted as a sequence of four ASCII characters, with
        uppercase and lowercase characters treated as distinct.

   application-dependent data: variable length
        Application-dependent data may or may not appear in an APP
        packet. It is interpreted by the application and not RTP itself.
        It MUST be a multiple of 32 bits long.



Schulzrinne/Casner/Frederick/Jacobson                        [Page 48]


Internet Draft                    RTP                  February 26, 1999


7 RTP Translators and Mixers

   In addition to end systems, RTP supports the notion of "translators"
   and "mixers", which could be considered as "intermediate systems" at
   the RTP level. Although this support adds some complexity to the
   protocol, the need for these functions has been clearly established
   by experiments with multicast audio and video applications in the
   Internet. Example uses of translators and mixers given in Section 2.3
   stem from the presence of firewalls and low bandwidth connections,
   both of which are likely to remain.

7.1 General Description

   An RTP translator/mixer connects two or more transport-level
   "clouds". Typically, each cloud is defined by a common network and
   transport protocol (e.g., IP/UDP) plus a multicast address and
   transport level destination port or a pair of unicast addresses and
   ports.  (Network-level protocol translators, such as IP version 4 to
   IP version 6, may be present within a cloud invisibly to RTP.) One
   system may serve as a translator or mixer for a number of RTP
   sessions, but each is considered a logically separate entity.

   In order to avoid creating a loop when a translator or mixer is
   installed, the following rules MUST be observed:

       o Each of the clouds connected by translators and mixers
         participating in one RTP session either MUST be distinct from
         all the others in at least one of these parameters (protocol,
         address, port), or MUST be isolated at the network level from
         the others.

       o A derivative of the first rule is that there MUST NOT be
         multiple translators or mixers connected in parallel unless by
         some arrangement they partition the set of sources to be
         forwarded.

   Similarly, all RTP end systems that can communicate through one or
   more RTP translators or mixers share the same SSRC space, that is,
   the SSRC identifiers MUST be unique among all these end systems.
   Section 8.2 describes the collision resolution algorithm by which
   SSRC identifiers are kept unique and loops are detected.

   There may be many varieties of translators and mixers designed for
   different purposes and applications. Some examples are to add or
   remove encryption, change the encoding of the data or the underlying
   protocols, or replicate between a multicast address and one or more
   unicast addresses. The distinction between translators and mixers is
   that a translator passes through the data streams from different



Schulzrinne/Casner/Frederick/Jacobson                        [Page 49]


Internet Draft                    RTP                  February 26, 1999


   sources separately, whereas a mixer combines them to form one new
   stream:

   Translator: Forwards RTP packets with their SSRC identifier intact;
        this makes it possible for receivers to identify individual
        sources even though packets from all the sources pass through
        the same translator and carry the translator's network source
        address. Some kinds of translators will pass through the data
        untouched, but others MAY change the encoding of the data and
        thus the RTP data payload type and timestamp. If multiple data
        packets are re-encoded into one, or vice versa, a translator
        MUST assign new sequence numbers to the outgoing packets. Losses
        in the incoming packet stream may induce corresponding gaps in
        the outgoing sequence numbers. Receivers cannot detect the
        presence of a translator unless they know by some other means
        what payload type or transport address was used by the original
        source.

   Mixer: Receives streams of RTP data packets from one or more sources,
        possibly changes the data format, combines the streams in some
        manner and then forwards the combined stream. Since the timing
        among multiple input sources will not generally be synchronized,
        the mixer will make timing adjustments among the streams and
        generate its own timing for the combined stream, so it is the
        synchronization source. Thus, all data packets forwarded by a
        mixer MUST be marked with the mixer's own SSRC identifier. In
        order to preserve the identity of the original sources
        contributing to the mixed packet, the mixer SHOULD insert their
        SSRC identifiers into the CSRC identifier list following the
        fixed RTP header of the packet. A mixer that is also itself a
        contributing source for some packet SHOULD explicitly include
        its own SSRC identifier in the CSRC list for that packet.

   For some applications, it MAY be acceptable for a mixer not to
   identify sources in the CSRC list. However, this introduces the
   danger that loops involving those sources could not be detected.

   The advantage of a mixer over a translator for applications like
   audio is that the output bandwidth is limited to that of one source
   even when multiple sources are active on the input side. This may be
   important for low-bandwidth links. The disadvantage is that receivers
   on the output side don't have any control over which sources are
   passed through or muted, unless some mechanism is implemented for
   remote control of the mixer. The regeneration of synchronization
   information by mixers also means that receivers can't do inter-media
   synchronization of the original streams. A multi-media mixer could do
   it.




Schulzrinne/Casner/Frederick/Jacobson                        [Page 50]


Internet Draft                    RTP                  February 26, 1999




         [E1]                                    [E6]
          |                                       |
    E1:17 |                                 E6:15 |
          |                                       |   E6:15
          V  M1:48 (1,17)         M1:48 (1,17)    V   M1:48 (1,17)
         (M1)-------------><T1>-----------------><T2>-------------->[E7]
          ^                 ^     E4:47           ^   E4:47
     E2:1 |           E4:47 |                     |   M3:89 (64,45)
          |                 |                     |
         [E2]              [E4]     M3:89 (64,45) |
                                                  |        legend:
   [E3] --------->(M2)----------->(M3)------------|        [End system]
          E3:64        M2:12 (64)  ^                       (Mixer)
                                   | E5:45                 <Translator>
                                   |
                                  [E5]          source: SSRC (CSRCs)
                                                ------------------->


   Figure 3: Sample RTP network with end systems, mixers and translators



   A collection of mixers and translators is shown in Figure 3 to
   illustrate their effect on SSRC and CSRC identifiers. In the figure,
   end systems are shown as rectangles (named E), translators as
   triangles (named T) and mixers as ovals (named M). The notation "M1:
   48(1,17)" designates a packet originating a mixer M1, identified with
   M1's (random) SSRC value of 48 and two CSRC identifiers, 1 and 17,
   copied from the SSRC identifiers of packets from E1 and E2.

7.2 RTCP Processing in Translators

   In addition to forwarding data packets, perhaps modified, translators
   and mixers MUST also process RTCP packets. In many cases, they will
   take apart the compound RTCP packets received from end systems to
   aggregate SDES information and to modify the SR or RR packets.
   Retransmission of this information may be triggered by the packet
   arrival or by the RTCP interval timer of the translator or mixer
   itself.

   A translator that does not modify the data packets, for example one
   that just replicates between a multicast address and a unicast
   address, MAY simply forward RTCP packets unmodified as well. A
   translator that transforms the payload in some way MUST make
   corresponding transformations in the SR and RR information so that it



Schulzrinne/Casner/Frederick/Jacobson                        [Page 51]


Internet Draft                    RTP                  February 26, 1999


   still reflects the characteristics of the data and the reception
   quality. These translators MUST NOT simply forward RTCP packets. In
   general, a translator SHOULD NOT aggregate SR and RR packets from
   different sources into one packet since that would reduce the
   accuracy of the propagation delay measurements based on the LSR and
   DLSR fields.

   SR sender information:  A translator does not generate its own sender
        information, but forwards the SR packets received from one cloud
        to the others. The SSRC is left intact but the sender
        information MUST be modified if required by the translation. If
        a translator changes the data encoding, it MUST change the
        "sender's byte count" field. If it also combines several data
        packets into one output packet, it MUST change the "sender's
        packet count" field. If it changes the timestamp frequency, it
        MUST change the "RTP timestamp" field in the SR packet.

   SR/RR reception report blocks:  A translator forwards reception
        reports received from one cloud to the others. Note that these
        flow in the direction opposite to the data.  The SSRC is left
        intact. If a translator combines several data packets into one
        output packet, and therefore changes the sequence numbers, it
        MUST make the inverse manipulation for the packet loss fields
        and the "extended last sequence number" field. This may be
        complex. In the extreme case, there may be no meaningful way to
        translate the reception reports, so the translator MAY pass on
        no reception report at all or a synthetic report based on its
        own reception. The general rule is to do what makes sense for a
        particular translation.

   A translator does not require an SSRC identifier of its own, but MAY
   choose to allocate one for the purpose of sending reports about what
   it has received. These would be sent to all the connected clouds,
   each corresponding to the translation of the data stream as sent to
   that cloud, since reception reports are normally multicast to all
   participants.

   SDES:  Translators typically forward without change the SDES
        information they receive from one cloud to the others, but MAY,
        for example, decide to filter non-CNAME SDES information if
        bandwidth is limited. The CNAMEs MUST be forwarded to allow SSRC
        identifier collision detection to work. A translator that
        generates its own RR packets MUST send SDES CNAME information
        about itself to the same clouds that it sends those RR packets.

   BYE:  Translators forward BYE packets unchanged. A translator that is
        about to cease forwarding packets SHOULD send a BYE packet to
        each connected cloud containing all the SSRC identifiers that



Schulzrinne/Casner/Frederick/Jacobson                        [Page 52]


Internet Draft                    RTP                  February 26, 1999


        were previously being forwarded to that cloud, including the
        translator's own SSRC identifier if it sent reports of its own.

   APP:  Translators forward APP packets unchanged.

7.3 RTCP Processing in Mixers

   Since a mixer generates a new data stream of its own, it does not
   pass through SR or RR packets at all and instead generates new
   information for both sides.

   SR sender information:  A mixer does not pass through sender
        information from the sources it mixes because the
        characteristics of the source streams are lost in the mix. As a
        synchronization source, the mixer SHOULD generate its own SR
        packets with sender information about the mixed data stream and
        send them in the same direction as the mixed stream.

   SR/RR reception report blocks:  A mixer generates its own reception
        reports for sources in each cloud and sends them out only to the
        same cloud. It MUST NOT send these reception reports to the
        other clouds and MUST NOT forward reception reports from one
        cloud to the others because the sources would not be SSRCs there
        (only CSRCs).

   SDES:  Mixers typically forward without change the SDES information
        they receive from one cloud to the others, but MAY, for example,
        decide to filter non-CNAME SDES information if bandwidth is
        limited. The CNAMEs MUST be forwarded to allow SSRC identifier
        collision detection to work. (An identifier in a CSRC list
        generated by a mixer might collide with an SSRC identifier
        generated by an end system.) A mixer MUST send SDES CNAME
        information about itself to the same clouds that it sends SR or
        RR packets.

   Since mixers do not forward SR or RR packets, they will typically be
   extracting SDES packets from a compound RTCP packet. To minimize
   overhead, chunks from the SDES packets MAY be aggregated into a
   single SDES packet which is then stacked on an SR or RR packet
   originating from the mixer. The RTCP packet rate MAY be different on
   each side of the mixer.

   A mixer that does not insert CSRC identifiers MAY also refrain from
   forwarding SDES CNAMEs. In this case, the SSRC identifier spaces in
   the two clouds are independent. As mentioned earlier, this mode of
   operation creates a danger that loops can't be detected.

   BYE:  Mixers MUST forward BYE packets. A mixer that is about to cease



Schulzrinne/Casner/Frederick/Jacobson                        [Page 53]


Internet Draft                    RTP                  February 26, 1999


        forwarding packets SHOULD send a BYE packet to each connected
        cloud containing all the SSRC identifiers that were previously
        being forwarded to that cloud, including the mixer's own SSRC
        identifier if it sent reports of its own.

   APP:  The treatment of APP packets by mixers is application-specific.

7.4 Cascaded Mixers

   An RTP session may involve a collection of mixers and translators as
   shown in Figure 3. If two mixers are cascaded, such as M2 and M3 in
   the figure, packets received by a mixer may already have been mixed
   and may include a CSRC list with multiple identifiers. The second
   mixer SHOULD build the CSRC list for the outgoing packet using the
   CSRC identifiers from already-mixed input packets and the SSRC
   identifiers from unmixed input packets. This is shown in the output
   arc from mixer M3 labeled M3:89(64,45) in the figure. As in the case
   of mixers that are not cascaded, if the resulting CSRC list has more
   than 15 identifiers, the remainder cannot be included.

8 SSRC Identifier Allocation and Use

   The SSRC identifier carried in the RTP header and in various fields
   of RTCP packets is a random 32-bit number that is required to be
   globally unique within an RTP session. It is crucial that the number
   be chosen with care in order that participants on the same network or
   starting at the same time are not likely to choose the same number.

   It is not sufficient to use the local network address (such as an
   IPv4 address) for the identifier because the address may not be
   unique. Since RTP translators and mixers enable interoperation among
   multiple networks with different address spaces, the allocation
   patterns for addresses within two spaces might result in a much
   higher rate of collision than would occur with random allocation.

   Multiple sources running on one host would also conflict.

   It is also not sufficient to obtain an SSRC identifier simply by
   calling random() without carefully initializing the state. An example
   of how to generate a random identifier is presented in Appendix A.6.

8.1 Probability of Collision

   Since the identifiers are chosen randomly, it is possible that two or
   more sources will choose the same number. Collision occurs with the
   highest probability when all sources are started simultaneously, for
   example when triggered automatically by some session management
   event. If N is the number of sources and L the length of the



Schulzrinne/Casner/Frederick/Jacobson                        [Page 54]


Internet Draft                    RTP                  February 26, 1999


   identifier (here, 32 bits), the probability that two sources
   independently pick the same value can be approximated for large N
   [22] as 1 - exp(-N**2 / 2**(L+1)). For N=1000, the probability is
   roughly 10**-4.

   The typical collision probability is much lower than the worst-case
   above. When one new source joins an RTP session in which all the
   other sources already have unique identifiers, the probability of
   collision is just the fraction of numbers used out of the space.
   Again, if N is the number of sources and L the length of the
   identifier, the probability of collision is N / 2**L. For N=1000, the
   probability is roughly 2*10**-7.

   The probability of collision is further reduced by the opportunity
   for a new source to receive packets from other participants before
   sending its first packet (either data or control). If the new source
   keeps track of the other participants (by SSRC identifier), then
   before transmitting its first packet the new source can verify that
   its identifier does not conflict with any that have been received, or
   else choose again.

8.2 Collision Resolution and Loop Detection

   Although the probability of SSRC identifier collision is low, all RTP
   implementations MUST be prepared to detect collisions and take the
   appropriate actions to resolve them. If a source discovers at any
   time that another source is using the same SSRC identifier as its
   own, it MUST send an RTCP BYE packet for the old identifier and
   choose another random one.  (As explained below, this step is taken
   only once in case of a loop.)  If a receiver discovers that two other
   sources are colliding, it MAY keep the packets from one and discard
   the packets from the other when this can be detected by different
   source transport addresses or CNAMEs. The two sources are expected to
   resolve the collision so that the situation doesn't last.

   Because the random SSRC identifiers are kept globally unique for each
   RTP session, they can also be used to detect loops that may be
   introduced by mixers or translators. A loop causes duplication of
   data and control information, either unmodified or possibly mixed, as
   in the following examples:

       o A translator may incorrectly forward a packet to the same
         multicast group from which it has received the packet, either
         directly or through a chain of translators. In that case, the
         same packet appears several times, originating from different
         network sources.

       o Two translators incorrectly set up in parallel, i.e., with the



Schulzrinne/Casner/Frederick/Jacobson                        [Page 55]


Internet Draft                    RTP                  February 26, 1999


         same multicast groups on both sides, would both forward packets
         from one multicast group to the other. Unidirectional
         translators would produce two copies; bidirectional translators
         would form a loop.

       o A mixer can close a loop by sending to the same transport
         destination upon which it receives packets, either directly or
         through another mixer or translator. In this case a source
         might show up both as an SSRC on a data packet and a CSRC in a
         mixed data packet.

   A source may discover that its own packets are being looped, or that
   packets from another source are being looped (a third-party loop).

   Both loops and collisions in the random selection of a source
   identifier result in packets arriving with the same SSRC identifier
   but a different source transport address, which may be that of the
   end system originating the packet or an intermediate system.
   Therefore, if a source changes its source transport address, it MUST
   also choose a new SSRC identifier to avoid being interpreted as a
   looped source.  Note that if a translator restarts and consequently
   changes the source transport address (e.g., changes the UDP source
   port number) on which it forwards packets, then all those packets
   will appear to receivers to be looped because the SSRC identifiers
   are applied by the original source and will not change. This problem
   MAY be avoided by keeping the source transport addressed fixed across
   restarts, but in any case will be resolved after a timeout at the
   receivers.

   Loops or collisions occurring on the far side of a translator or
   mixer cannot be detected using the source transport address if all
   copies of the packets go through the translator or mixer, however
   collisions may still be detected when chunks from two RTCP SDES
   packets contain the same SSRC identifier but different CNAMEs.

   To detect and resolve these conflicts, an RTP implementation MUST
   include an algorithm similar to the one described below. It ignores
   packets from a new source or loop that collide with an established
   source. It resolves collisions with the participant's own SSRC
   identifier by sending an RTCP BYE for the old identifier and choosing
   a new one. However, when the collision was induced by a loop of the
   participant's own packets, the algorithm will choose a new identifier
   only once and thereafter ignore packets from the looping source
   transport address. This is required to avoid a flood of BYE packets.

   This algorithm requires keeping a table indexed by the source
   identifier and containing the source transport addresses from the
   first RTP packet and first RTCP packet received with that identifier,



Schulzrinne/Casner/Frederick/Jacobson                        [Page 56]


Internet Draft                    RTP                  February 26, 1999


   along with other state for that source. Two source transport
   addresses are required since, for example, the UDP source port
   numbers may be different on RTP and RTCP packets. However, it may be
   assumed that the network address is the same in both source transport
   addresses.

   Each SSRC or CSRC identifier received in an RTP or RTCP packet is
   looked up in the source identifier table in order to process that
   data or control information. The source transport address from the
   packet is compared to the corresponding source transport address in
   the table to detect a loop or collision if they don't match. For
   control packets, each element with its own SSRC id, for example an
   SDES chunk, requires a separate lookup. (The SSRC id in a reception
   report block is an exception because it identifies a source heard by
   the reporter, and that SSRC id is unrelated to the source transport
   adddress of the RTCP packet sent by the reporter.) If the SSRC or
   CSRC is not found, a new entry is created. These table entries are
   removed when an RTCP BYE packet is received with the corresponding
   SSRC id and validated by a matching source transport address, or
   after no packets have arrived for a relatively long time (see Section
   6.2.1).

   Note that if two sources on the same host are transmitting with the
   same source identifier at the time a receiver begins operation, it
   would be possible that the first RTP packet received came from one of
   the sources while the first RTCP packet received came from the other.
   This would cause the wrong RTCP information to be associated with the
   RTP data, but this situation should be sufficiently rare and harmless
   that it may be disregarded.

   In order to track loops of the participant's own data packets, the
   implementation MUST also keep a separate list of source transport
   addresses (not identifiers) that have been found to be conflicting.
   As in the source identifier table, two source transport addresses
   MUST be kept to separately track conflicting RTP and RTCP packets.
   Note that the conflicting address list should be a short, usually
   empty. Each element in this list stores the source addresses plus the
   time when the most recent conflicting packet was received. An element
   MAY be removed from the list when no conflicting packet has arrived
   from that source for a time on the order of 10 RTCP report intervals
   (see Section 6.2).

   For the algorithm as shown, it is assumed that the participant's own
   source identifier and state are included in the source identifier
   table. The algorithm could be restructured to first make a separate
   comparison against the participant's own source identifier.





Schulzrinne/Casner/Frederick/Jacobson                        [Page 57]


Internet Draft                    RTP                  February 26, 1999


       IF the SSRC or CSRC identifier is not found in the source
          identifier table:
       THEN create a new entry storing the data or control source
            transport address, the SSRC or CSRC id and other state.
            CONTINUE with normal processing.

       (identifier is found in the table)

       IF the table entry was created on receipt of a control packet
          and this is the first data packet or vice versa:
       THEN store the source transport address from this packet.
            CONTINUE with normal processing.
       IF the source transport address from the packet matches
          the one saved in the table entry for this identifier:
       THEN CONTINUE with normal processing.

       (an identifier collision or a loop is indicated)

       IF the source identifier is not the participant's own:
       THEN IF the source identifier is from an RTCP SDES chunk
               containing a CNAME item that differs from the CNAME
               in the table entry:
            THEN (optionally) count a third-party collision.
            ELSE (optionally) count a third-party loop.
            ABORT processing of data packet or control element.

       (a collision or loop of the participant's own packets)

       IF the source transport address is found in the list of
          conflicting data or control source transport addresses:
       THEN IF the source identifier is not from an RTCP SDES chunk
               containing a CNAME item OR if that CNAME is the
               participant's own:
            THEN (optionally) count occurrence of own traffic looped.
            mark current time in conflicting address list entry.
            ABORT processing of data packet or control element.
       log occurrence of a collision.
       create a new entry in the conflicting data or control source
          transport address list and mark current time.
       send an RTCP BYE packet with the old SSRC identifier.
       choose a new identifier.
       create a new entry in the source identifier table with the
          old SSRC plus the source transport address from the data
          or control packet being processed.
       CONTINUE with normal processing.






Schulzrinne/Casner/Frederick/Jacobson                        [Page 58]


Internet Draft                    RTP                  February 26, 1999


   In this algorithm, packets from a newly conflicting source address
   will be ignored and packets from the original source will be kept.
   (If the original source was through a mixer and later the same source
   is received directly, the receiver may be well advised to switch
   unless other sources in the mix would be lost.) If no packets arrive
   from the original source for an extended period, the table entry will
   be timed out and the new source will be able to take over. This might
   occur if the original source detects the collision and moves to a new
   source identifier, but in the usual case an RTCP BYE packet will be
   received from the original source to delete the state without having
   to wait for a timeout.

   When a new SSRC identifier is chosen due to a collision, the
   candidate identifier SHOULD first be looked up in the source
   identifier table to see if it was already in use by some other
   source. If so, another candidate MUST be generated and the process
   repeated.

   A loop of data packets to a multicast destination can cause severe
   network flooding. All mixers and translators MUST implement a loop
   detection algorithm like the one here so that they can break loops.
   This should limit the excess traffic to no more than one duplicate
   copy of the original traffic, which may allow the session to continue
   so that the cause of the loop can be found and fixed. However, in
   extreme cases where a mixer or translator does not properly break the
   loop and high traffic levels result, it may be necessary for end
   systems to cease transmitting data or control packets entirely. This
   decision may depend upon the application. An error condition SHOULD
   be indicated as appropriate. Transmission MAY be attempted again
   periodically after a long, random time (on the order of minutes).

8.3 Use with Layered Encodings

   For layered encodings transmitted on separate RTP sessions (see
   Section 2.4), a single SSRC identifier space SHOULD be used across
   the sessions of all layers and the core (base) layer SHOULD be used
   for SSRC identifier allocation and collision resolution. When a
   source discovers that it has collided, it transmits an RTCP BYE
   message on only the base layer but changes the SSRC identifier to the
   new value in all layers.

9 Security

   Lower layer protocols may eventually provide all the security
   services that may be desired for applications of RTP, including
   authentication, integrity, and confidentiality. These services have
   been specified for IP in [23]. Since the initial audio and video
   applications using RTP needed a confidentiality service before such



Schulzrinne/Casner/Frederick/Jacobson                        [Page 59]


Internet Draft                    RTP                  February 26, 1999


   services were available for the IP layer, the confidentiality service
   described in the next section was defined for use with RTP and RTCP.
   That description is included here to codify existing practice. New
   applications of RTP MAY implement this RTP-specific confidentiality
   service for backward compatibility, and/or they MAY implement IP
   layer security services. The overhead on the RTP protocol for this
   confidentiality service is low, so the penalty will be minimal if
   this service is obsoleted by lower layer services in the future.

   Alternatively, other services, other implementations of services and
   other algorithms may be defined for RTP in the future if warranted.
   The selection presented here is meant to simplify implementation of
   interoperable, secure applications and provide guidance to
   implementors. No claim is made that the methods presented here are
   appropriate for a particular security need. A profile may specify
   which services and algorithms should be offered by applications, and
   may provide guidance as to their appropriate use.

   Key distribution and certificates are outside the scope of this
   document.

9.1 Confidentiality

   Confidentiality means that only the intended receiver(s) can decode
   the received packets; for others, the packet contains no useful
   information. Confidentiality of the content is achieved by
   encryption.

   When encryption of RTP or RTCP is desired, all the octets that will
   be encapsulated for transmission in a single lower-layer packet are
   encrypted as a unit. For RTCP, a 32-bit random number MUST be
   prepended to the unit before encryption to deter known plaintext
   attacks. For RTP, no prefix is required because the sequence number
   and timestamp fields are initialized with random offsets.

   For RTCP, an implementation MAY split a compound RTCP packet into two
   lower-layer packets, one to be encrypted and one to be sent in the
   clear. For example, SDES information might be encrypted while
   reception reports were sent in the clear to accommodate third-party
   monitors that are not privy to the encryption key. In this example,
   depicted in Fig. 4, the SDES information MUST be appended to an RR
   packet with no reports (and the encrypted) to satisfy the requirement
   that all compound RTCP packets begin with an SR or RR packet.


   The presence of encryption and the use of the correct key are
   confirmed by the receiver through header or payload validity checks.
   Examples of such validity checks for RTP and RTCP headers are given



Schulzrinne/Casner/Frederick/Jacobson                        [Page 60]


Internet Draft                    RTP                  February 26, 1999





                 UDP packet                        UDP packet
   -------------------------------------  -------------------------
   [32-bit ][       ][     #           ]  [    # sender # receiver]
   [random ][  RR   ][SDES # CNAME, ...]  [ SR # report # report  ]
   [integer][(empty)][     #           ]  [    #        #         ]
   -------------------------------------  -------------------------
                 encrypted                       not encrypted

   #: SSRC

   Figure 4: Encrypted and non-encrypted RTCP packets


   in Appendices A.1 and A.2.

   The default encryption algorithm is the Data Encryption Standard
   (DES) algorithm in cipher block chaining (CBC) mode, as described in
   Section 1.1 of RFC 1423 [24], except that padding to a multiple of 8
   octets is indicated as described for the P bit in Section 5.1. The
   initialization vector is zero because random values are supplied in
   the RTP header or by the random prefix for compound RTCP packets. For
   details on the use of CBC initialization vectors, see [25].
   Implementations that support encryption SHOULD always support the DES
   algorithm in CBC mode as the default to maximize interoperability.
   This method is chosen because it has been demonstrated to be easy and
   practical to use in experimental audio and video tools in operation
   on the Internet. Other encryption algorithms MAY be specified
   dynamically for a session by non-RTP means.

   As an alternative to encryption at the IP level or at the RTP level
   as described above, profiles MAY define additional payload types for
   encrypted encodings.  Those encodings MUST specify how padding and
   other aspects of the encryption should be handled. This method allows
   encrypting only the data while leaving the headers in the clear for
   applications where that is desired. It may be particularly useful for
   hardware devices that will handle both decryption and decoding.

9.2 Authentication and Message Integrity

   Authentication and message integrity services are not defined at the
   RTP level since these services would not be directly feasible without
   a key management infrastructure. It is expected that authentication
   and integrity services will be provided by lower layer protocols.

10 RTP over Network and Transport Protocols



Schulzrinne/Casner/Frederick/Jacobson                        [Page 61]


Internet Draft                    RTP                  February 26, 1999


   This section describes issues specific to carrying RTP packets within
   particular network and transport protocols. The following rules apply
   unless superseded by protocol-specific definitions outside this
   specification.

   RTP relies on the underlying protocol(s) to provide demultiplexing of
   RTP data and RTCP control streams. For UDP and similar protocols, RTP
   SHOULD use an even port number and the corresponding RTCP stream
   SHOULD use the next higher (odd) port number. If an application is
   supplied with an odd number for use as the RTP port, it SHOULD
   replace this number with the next lower (even) number.

   In a unicast session, applications SHOULD be prepared to receive RTP
   data and control on one port pair and send to another.

   It is RECOMMENDED that layered encoding applications (see Section
   2.4) use a set of contiguous port numbers.  The port numbers MUST be
   distinct because of a widespread deficiency in existing operating
   systems that prevents use of the same port with multiple multicast
   addresses, and for unicast, there is only one permissible address.
   Thus for layer n, the data port is P + 2n, and the control port is P
   + 2n + 1. When IP multicast is used, the addresses MUST also be
   distinct because multicast routing and group membership are managed
   on an address granularity. However, allocation of contiguous IP
   multicast addresses cannot be assumed because some groups may require
   different scopes and may therefore be allocated from different
   address ranges.

   RTP data packets contain no length field or other delineation,
   therefore RTP relies on the underlying protocol(s) to provide a
   length indication. The maximum length of RTP packets is limited only
   by the underlying protocols.

   If RTP packets are to be carried in an underlying protocol that
   provides the abstraction of a continuous octet stream rather than
   messages (packets), an encapsulation of the RTP packets MUST be
   defined to provide a framing mechanism. Framing is also needed if the
   underlying protocol may contain padding so that the extent of the RTP
   payload cannot be determined. The framing mechanism is not defined
   here.

   A profile MAY specify a framing method to be used even when RTP is
   carried in protocols that do provide framing in order to allow
   carrying several RTP packets in one lower-layer protocol data unit,
   such as a UDP packet. Carrying several RTP packets in one network or
   transport packet reduces header overhead and may simplify
   synchronization between different streams.




Schulzrinne/Casner/Frederick/Jacobson                        [Page 62]


Internet Draft                    RTP                  February 26, 1999


11 Summary of Protocol Constants

   This section contains a summary listing of the constants defined in
   this specification.

   The RTP payload type (PT) constants are defined in profiles rather
   than this document. However, the octet of the RTP header which
   contains the marker bit(s) and payload type MUST avoid the reserved
   values 200 and 201 (decimal) to distinguish RTP packets from the RTCP
   SR and RR packet types for the header validation procedure described
   in Appendix A.1. For the standard definition of one marker bit and a
   7-bit payload type field as shown in this specification, this
   restriction means that payload types 72 and 73 are reserved.

11.1 RTCP packet types


   abbrev.    name                   value
   SR         sender report            200
   RR         receiver report          201
   SDES       source description       202
   BYE        goodbye                  203
   APP        application-defined      204


   These type values were chosen in the range 200-204 for improved
   header validity checking of RTCP packets compared to RTP packets or
   other unrelated packets. When the RTCP packet type field is compared
   to the corresponding octet of the RTP header, this range corresponds
   to the marker bit being 1 (which it usually is not in data packets)
   and to the high bit of the standard payload type field being 1 (since
   the static payload types are typically defined in the low half). This
   range was also chosen to be some distance numerically from 0 and 255
   since all-zeros and all-ones are common data patterns.

   Since all compound RTCP packets MUST begin with SR or RR, these codes
   were chosen as an even/odd pair to allow the RTCP validity check to
   test the maximum number of bits with mask and value.

   Additional RTCP packet types may be registered through IANA (see
   Section 11.3).

11.2 SDES types


   abbrev.    name                              value
   END        end of SDES list                      0
   CNAME      canonical name                        1



Schulzrinne/Casner/Frederick/Jacobson                        [Page 63]


Internet Draft                    RTP                  February 26, 1999


   NAME       user name                             2
   EMAIL      user's electronic mail address        3
   PHONE      user's phone number                   4
   LOC        geographic user location              5
   TOOL       name of application or tool           6
   NOTE       notice about the source               7
   PRIV       private extensions                    8


   Additional SDES types may be registered through IANA (see Section
   11.3).

11.3 IANA Considerations

   Additional RTCP packet types and SDES types may be registered through
   the Internet Assigned Numbers Authority (IANA). Since these number
   spaces are small, allowing unconstrained registration of new values
   would not be prudent. To facilitate review of requests and to promote
   shared use of new types among multiple applications, requests for
   registration of new values must be documented in an RFC or other
   permanent and readily available reference such as the product of
   another cooperative standards body (e.g., ITU-T). Other requests may
   also be accepted, under the advice of a "designated expert." (Contact
   the IANA for the contact information of the current expert.)

A Algorithms

   We provide examples of C code for aspects of RTP sender and receiver
   algorithms. There may be other implementation methods that are faster
   in particular operating environments or have other advantages. These
   implementation notes are for informational purposes only and are
   meant to clarify the RTP specification.

   The following definitions are used for all examples; for clarity and
   brevity, the structure definitions are only valid for 32-bit big-
   endian (most significant octet first) architectures. Bit fields are
   assumed to be packed tightly in big-endian bit order, with no
   additional padding. Modifications would be required to construct a
   portable implementation.












Schulzrinne/Casner/Frederick/Jacobson                        [Page 64]


Internet Draft                    RTP                  February 26, 1999



   /*
    * rtp.h  --  RTP header file (RFC XXXX)
    */
   #include <sys/types.h>

   /*
    * The type definitions below are valid for 32-bit architectures and
    * may have to be adjusted for 16- or 64-bit architectures.
    */
   typedef unsigned char  u_int8;
   typedef unsigned short u_int16;
   typedef unsigned int   u_int32;
   typedef          short int16;

   /*
    * Current protocol version.
    */
   #define RTP_VERSION    2

   #define RTP_SEQ_MOD (1<<16)
   #define RTP_MAX_SDES 255      /* maximum text length for SDES */

   typedef enum {
       RTCP_SR   = 200,
       RTCP_RR   = 201,
       RTCP_SDES = 202,
       RTCP_BYE  = 203,
       RTCP_APP  = 204
   } rtcp_type_t;

   typedef enum {
       RTCP_SDES_END   = 0,
       RTCP_SDES_CNAME = 1,
       RTCP_SDES_NAME  = 2,
       RTCP_SDES_EMAIL = 3,
       RTCP_SDES_PHONE = 4,
       RTCP_SDES_LOC   = 5,
       RTCP_SDES_TOOL  = 6,
       RTCP_SDES_NOTE  = 7,
       RTCP_SDES_PRIV  = 8
   } rtcp_sdes_type_t;

   /*
    * RTP data header
    */
   typedef struct {
       unsigned int version:2;   /* protocol version */



Schulzrinne/Casner/Frederick/Jacobson                        [Page 65]


Internet Draft                    RTP                  February 26, 1999


       unsigned int p:1;         /* padding flag */
       unsigned int x:1;         /* header extension flag */
       unsigned int cc:4;        /* CSRC count */
       unsigned int m:1;         /* marker bit */
       unsigned int pt:7;        /* payload type */
       unsigned int seq:16;      /* sequence number */
       u_int32 ts;               /* timestamp */
       u_int32 ssrc;             /* synchronization source */
       u_int32 csrc[1];          /* optional CSRC list */
   } rtp_hdr_t;

   /*
    * RTCP common header word
    */
   typedef struct {
       unsigned int version:2;   /* protocol version */
       unsigned int p:1;         /* padding flag */
       unsigned int count:5;     /* varies by packet type */
       unsigned int pt:8;        /* RTCP packet type */
       u_int16 length;           /* pkt len in words, w/o this word */
   } rtcp_common_t;

   /*
    * Big-endian mask for version, padding bit and packet type pair
    */
   #define RTCP_VALID_MASK (0xc000 | 0x2000 | 0xfe)
   #define RTCP_VALID_VALUE ((RTP_VERSION << 14) | RTCP_SR)

   /*
    * Reception report block
    */
   typedef struct {
       u_int32 ssrc;             /* data source being reported */
       unsigned int fraction:8;  /* fraction lost since last SR/RR */
       int lost:24;              /* cumul. no. pkts lost (signed!) */
       u_int32 last_seq;         /* extended last seq. no. received */
       u_int32 jitter;           /* interarrival jitter */
       u_int32 lsr;              /* last SR packet from this source */
       u_int32 dlsr;             /* delay since last SR packet */
   } rtcp_rr_t;

   /*
    * SDES item
    */
   typedef struct {
       u_int8 type;              /* type of item (rtcp_sdes_type_t) */
       u_int8 length;            /* length of item (in octets) */
       char data[1];             /* text, not null-terminated */



Schulzrinne/Casner/Frederick/Jacobson                        [Page 66]


Internet Draft                    RTP                  February 26, 1999


   } rtcp_sdes_item_t;

   /*
    * One RTCP packet
    */
   typedef struct {
       rtcp_common_t common;     /* common header */
       union {
           /* sender report (SR) */
           struct {
               u_int32 ssrc;     /* sender generating this report */
               u_int32 ntp_sec;  /* NTP timestamp */
               u_int32 ntp_frac;
               u_int32 rtp_ts;   /* RTP timestamp */
               u_int32 psent;    /* packets sent */
               u_int32 osent;    /* octets sent */
               rtcp_rr_t rr[1];  /* variable-length list */
           } sr;

           /* reception report (RR) */
           struct {
               u_int32 ssrc;     /* receiver generating this report */
               rtcp_rr_t rr[1];  /* variable-length list */
           } rr;

           /* source description (SDES) */
           struct rtcp_sdes {
               u_int32 src;      /* first SSRC/CSRC */
               rtcp_sdes_item_t item[1]; /* list of SDES items */
           } sdes;

           /* BYE */
           struct {
               u_int32 src[1];   /* list of sources */
               /* can't express trailing text for reason */
           } bye;
       } r;
   } rtcp_t;

   typedef struct rtcp_sdes rtcp_sdes_t;











Schulzrinne/Casner/Frederick/Jacobson                        [Page 67]


Internet Draft                    RTP                  February 26, 1999



   /*
    * Per-source state information
    */
   typedef struct {
       u_int16 max_seq;        /* highest seq. number seen */
       u_int32 cycles;         /* shifted count of seq. number cycles */
       u_int32 base_seq;       /* base seq number */
       u_int32 bad_seq;        /* last 'bad' seq number + 1 */
       u_int32 probation;      /* sequ. packets till source is valid */
       u_int32 received;       /* packets received */
       u_int32 expected_prior; /* packet expected at last interval */
       u_int32 received_prior; /* packet received at last interval */
       u_int32 transit;        /* relative trans time for prev pkt */
       u_int32 jitter;         /* estimated jitter */
       /* ... */
   } source;


A.1 RTP Data Header Validity Checks

   An RTP receiver SHOULD check the validity of the RTP header on
   incoming packets since they might be encrypted or might be from a
   different application that happens to be misaddressed. Similarly, if
   encryption according to the method described in Section 9 is enabled,
   the header validity check is needed to verify that incoming packets
   have been correctly decrypted, although a failure of the header
   validity check (e.g., unknown payload type) may not necessarily
   indicate decryption failure.

   Only weak validity checks are possible on an RTP data packet from a
   source that has not been heard before:

       o RTP version field must equal 2.

       o The payload type must be known, in particular it must not be
         equal to SR or RR.

       o If the P bit is set, then the last octet of the packet must
         contain a valid octet count, in particular, less than the total
         packet length minus the header size.

       o The X bit must be zero if the profile does not specify that
         the header extension mechanism may be used. Otherwise, the
         extension length field must be less than the total packet size
         minus the fixed header length and padding.

       o The length of the packet must be consistent with CC and



Schulzrinne/Casner/Frederick/Jacobson                        [Page 68]


Internet Draft                    RTP                  February 26, 1999


         payload type (if payloads have a known length).

   The last three checks are somewhat complex and not always possible,
   leaving only the first two which total just a few bits. If the SSRC
   identifier in the packet is one that has been received before, then
   the packet is probably valid and checking if the sequence number is
   in the expected range provides further validation. If the SSRC
   identifier has not been seen before, then data packets carrying that
   identifier may be considered invalid until a small number of them
   arrive with consecutive sequence numbers.

   The routine update_seq shown below ensures that a source is declared
   valid only after MIN_SEQUENTIAL packets have been received in
   sequence. It also validates the sequence number seq of a newly
   received packet and updates the sequence state for the packet's
   source in the structure to which s points.

   When a new source is heard for the first time, that is, its SSRC
   identifier is not in the table (see Section 8.2), and the per-source
   state is allocated for it, s->probation should be set to the number
   of sequential packets required before declaring a source valid
   (parameter MIN_SEQUENTIAL ) and s->max_seq initialized to seq-1 s-
   >probation marks the source as not yet valid so the state may be
   discarded after a short timeout rather than a long one, as discussed
   in Section 6.2.1.

   After a source is considered valid, the sequence number is considered
   valid if it is no more than MAX_DROPOUT ahead of s->max_seq nor more
   than MAX_MISORDER behind. If the new sequence number is ahead of
   max_seq modulo the RTP sequence number range (16 bits), but is
   smaller than max_seq , it has wrapped around and the (shifted) count
   of sequence number cycles is incremented. A value of one is returned
   to indicate a valid sequence number.

   Otherwise, the value zero is returned to indicate that the validation
   failed, and the bad sequence number is stored. If the next packet
   received carries the next higher sequence number, it is considered
   the valid start of a new packet sequence presumably caused by an
   extended dropout or a source restart. Since multiple complete
   sequence number cycles may have been missed, the packet loss
   statistics are reset.

   Typical values for the parameters are shown, based on a maximum
   misordering time of 2 seconds at 50 packets/second and a maximum
   dropout of 1 minute. The dropout parameter MAX_DROPOUT SHOULD be a
   small fraction of the 16-bit sequence number space to give a
   reasonable probability that new sequence numbers after a restart will
   not fall in the acceptable range for sequence numbers from before the



Schulzrinne/Casner/Frederick/Jacobson                        [Page 69]


Internet Draft                    RTP                  February 26, 1999


   restart.


















































Schulzrinne/Casner/Frederick/Jacobson                        [Page 70]


Internet Draft                    RTP                  February 26, 1999



   void init_seq(source *s, u_int16 seq)
   {
       s->base_seq = seq - 1;
       s->max_seq = seq;
       s->bad_seq = RTP_SEQ_MOD + 1;
       s->cycles = 0;
       s->received = 0;
       s->received_prior = 0;
       s->expected_prior = 0;
       /* other initialization */
   }

   int update_seq(source *s, u_int16 seq)
   {
       u_int16 udelta = seq - s->max_seq;
       const int MAX_DROPOUT = 3000;
       const int MAX_MISORDER = 100;
       const int MIN_SEQUENTIAL = 2;

       /*
        * Source is not valid until MIN_SEQUENTIAL packets with
        * sequential sequence numbers have been received.
        */
       if (s->probation) {
           /* packet is in sequence */
           if (seq == s->max_seq + 1) {
               s->probation--;
               s->max_seq = seq;
               if (s->probation == 0) {
                   init_seq(s, seq);
                   s->received++;
                   return 1;
               }
           } else {
               s->probation = MIN_SEQUENTIAL - 1;
               s->max_seq = seq;
           }
           return 0;
       } else if (udelta < MAX_DROPOUT) {
           /* in order, with permissible gap */
           if (seq < s->max_seq) {
               /*
                * Sequence number wrapped - count another 64K cycle.
                */
               s->cycles += RTP_SEQ_MOD;
           }
           s->max_seq = seq;



Schulzrinne/Casner/Frederick/Jacobson                        [Page 71]


Internet Draft                    RTP                  February 26, 1999


       } else if (udelta <= RTP_SEQ_MOD - MAX_MISORDER) {
           /* the sequence number made a very large jump */
           if (seq == s->bad_seq) {
               /*
                * Two sequential packets -- assume that the other side
                * restarted without telling us so just re-sync
                * (i.e., pretend this was the first packet).
                */
               init_seq(s, seq);
           }
           else {
               s->bad_seq = (seq + 1) & (RTP_SEQ_MOD-1);
               return 0;
           }
       } else {
           /* duplicate or reordered packet */
       }
       s->received++;
       return 1;
   }


   The validity check can be made stronger requiring more than two
   packets in sequence. The disadvantages are that a larger number of
   initial packets will be discarded and that high packet loss rates
   could prevent validation. However, because the RTCP header validation
   is relatively strong, if an RTCP packet is received from a source
   before the data packets, the count could be adjusted so that only two
   packets are required in sequence. If initial data loss for a few
   seconds can be tolerated, an application MAY choose to discard all
   data packets from a source until a valid RTCP packet has been
   received from that source.

   Depending on the application and encoding, algorithms may exploit
   additional knowledge about the payload format for further validation.
   For payload types where the timestamp increment is the same for all
   packets, the timestamp values can be predicted from the previous
   packet received from the same source using the sequence number
   difference (assuming no change in payload type).

   A strong "fast-path" check is possible since with high probability
   the first four octets in the header of a newly received RTP data
   packet will be just the same as that of the previous packet from the
   same SSRC except that the sequence number will have increased by one.
   Similarly, a single-entry cache may be used for faster SSRC lookups
   in applications where data is typically received from one source at a
   time.




Schulzrinne/Casner/Frederick/Jacobson                        [Page 72]


Internet Draft                    RTP                  February 26, 1999


A.2 RTCP Header Validity Checks

   The following checks SHOULD be applied to RTCP packets.

       o RTP version field must equal 2.

       o The payload type field of the first RTCP packet in a compound
         packet must be equal to SR or RR.

       o The padding bit (P) should be zero for the first packet of a
         compound RTCP packet because padding should only be applied, if
         it is needed, to the last packet.

       o The length fields of the individual RTCP packets must total to
         the overall length of the compound RTCP packet as received.
         This is a fairly strong check.

   The code fragment below performs all of these checks. The packet type
   is not checked for subsequent packets since unknown packet types may
   be present and should be ignored.


       u_int32 len;        /* length of compound RTCP packet in words */
       rtcp_t *r;          /* RTCP header */
       rtcp_t *end;        /* end of compound RTCP packet */

       if ((*(u_int16 *)r & RTCP_VALID_MASK) != RTCP_VALID_VALUE) {
           /* something wrong with packet format */
       }
       end = (rtcp_t *)((u_int32 *)r + len);

       do r = (rtcp_t *)((u_int32 *)r + r->common.length + 1);
       while (r < end && r->common.version == 2);

       if (r != end) {
           /* something wrong with packet format */
       }


A.3 Determining the Number of RTP Packets Expected and Lost

   In order to compute packet loss rates, the number of packets expected
   and actually received from each source needs to be known, using per-
   source state information defined in struct source referenced via
   pointer s in the code below. The number of packets received is simply
   the count of packets as they arrive, including any late or duplicate
   packets. The number of packets expected can be computed by the
   receiver as the difference between the highest sequence number



Schulzrinne/Casner/Frederick/Jacobson                        [Page 73]


Internet Draft                    RTP                  February 26, 1999


   received ( s->max_seq ) and the first sequence number received ( s-
   >base_seq ). Since the sequence number is only 16 bits and will wrap
   around, it is necessary to extend the highest sequence number with
   the (shifted) count of sequence number wraparounds ( s->cycles ).
   Both the received packet count and the count of cycles are maintained
   the RTP header validity check routine in Appendix A.1.


       extended_max = s->cycles + s->max_seq;
       expected = extended_max - s->base_seq + 1;



   The number of packets lost is defined to be the number of packets
   expected less the number of packets actually received:


       lost = expected - s->received;



   Since this signed number is carried in 24 bits, it SHOULD be clamped
   at 0x7fffff for positive loss or 0xffffff for negative loss rather
   than wrapping around.

   The fraction of packets lost during the last reporting interval
   (since the previous SR or RR packet was sent) is calculated from
   differences in the expected and received packet counts across the
   interval, where expected_prior and received_prior are the values
   saved when the previous reception report was generated:


       expected_interval = expected - s->expected_prior;
       s->expected_prior = expected;
       received_interval = s->received - s->received_prior;
       s->received_prior = s->received;
       lost_interval = expected_interval - received_interval;
       if (expected_interval == 0 || lost_interval <= 0) fraction = 0;
       else fraction = (lost_interval << 8) / expected_interval;



   The resulting fraction is an 8-bit fixed point number with the binary
   point at the left edge.

A.4 Generating SDES RTCP Packets

   This function builds one SDES chunk into buffer b composed of argc



Schulzrinne/Casner/Frederick/Jacobson                        [Page 74]


Internet Draft                    RTP                  February 26, 1999


   items supplied in arrays type , value and length b


   char *rtp_write_sdes(char *b, u_int32 src, int argc,
                        rtcp_sdes_type_t type[], char *value[],
                        int length[])
   {
       rtcp_sdes_t *s = (rtcp_sdes_t *)b;
       rtcp_sdes_item_t *rsp;
       int i;
       int len;
       int pad;

       /* SSRC header */
       s->src = src;
       rsp = &s->item[0];

       /* SDES items */
       for (i = 0; i < argc; i++) {
           rsp->type = type[i];
           len = length[i];
           if (len > RTP_MAX_SDES) {
               /* invalid length, may want to take other action */
               len = RTP_MAX_SDES;
           }
           rsp->length = len;
           memcpy(rsp->data, value[i], len);
           rsp = (rtcp_sdes_item_t *)&rsp->data[len];
       }

       /* terminate with end marker and pad to next 4-octet boundary */
       len = ((char *) rsp) - b;
       pad = 4 - (len & 0x3);
       b = (char *) rsp;
       while (pad--) *b++ = RTCP_SDES_END;

       return b;
   }


A.5 Parsing RTCP SDES Packets

   This function parses an SDES packet, calling functions find_member()
   to find a pointer to the information for a session member given the
   SSRC identifier and member_sdes() to store the new SDES information
   for that member. This function expects a pointer to the header of the
   RTCP packet.




Schulzrinne/Casner/Frederick/Jacobson                        [Page 75]


Internet Draft                    RTP                  February 26, 1999



   void rtp_read_sdes(rtcp_t *r)
   {
       int count = r->common.count;
       rtcp_sdes_t *sd = &r->r.sdes;
       rtcp_sdes_item_t *rsp, *rspn;
       rtcp_sdes_item_t *end = (rtcp_sdes_item_t *)
                               ((u_int32 *)r + r->common.length + 1);
       source *s;

       while (--count >= 0) {
           rsp = &sd->item[0];
           if (rsp >= end) break;
           s = find_member(sd->src);

           for (; rsp->type; rsp = rspn ) {
               rspn = (rtcp_sdes_item_t *)((char*)rsp+rsp->length+2);
               if (rspn >= end) {
                   rsp = rspn;
                   break;
               }
               member_sdes(s, rsp->type, rsp->data, rsp->length);
           }
           sd = (rtcp_sdes_t *)
                ((u_int32 *)sd + (((char *)rsp - (char *)sd) >> 2)+1);
       }
       if (count >= 0) {
           /* invalid packet format */
       }
   }


A.6 Generating a Random 32-bit Identifier

   The following subroutine generates a random 32-bit identifier using
   the MD5 routines published in RFC 1321 [26]. The system routines may
   not be present on all operating systems, but they should serve as
   hints as to what kinds of information may be used. Other system calls
   that may be appropriate include

       o getdomainname() ,

       o getwd() , or

       o getrusage()

   "Live" video or audio samples are also a good source of random
   numbers, but care must be taken to avoid using a turned-off



Schulzrinne/Casner/Frederick/Jacobson                        [Page 76]


Internet Draft                    RTP                  February 26, 1999


   microphone or blinded camera as a source [9].

   Use of this or similar routine is RECOMMENDED to generate the initial
   seed for the random number generator producing the RTCP period (as
   shown in Appendix A.7), to generate the initial values for the
   sequence number and timestamp, and to generate SSRC values.  Since
   this routine is likely to be CPU-intensive, its direct use to
   generate RTCP periods is inappropriate because predictability is not
   an issue. Note that this routine produces the same result on repeated
   calls until the value of the system clock changes unless different
   values are supplied for the type argument.








































Schulzrinne/Casner/Frederick/Jacobson                        [Page 77]


Internet Draft                    RTP                  February 26, 1999



   /*
    * Generate a random 32-bit quantity.
    */
   #include <sys/types.h>   /* u_long */
   #include <sys/time.h>    /* gettimeofday() */
   #include <unistd.h>      /* get..() */
   #include <stdio.h>       /* printf() */
   #include <time.h>        /* clock() */
   #include <sys/utsname.h> /* uname() */
   #include "global.h"      /* from RFC 1321 */
   #include "md5.h"         /* from RFC 1321 */

   #define MD_CTX MD5_CTX
   #define MDInit MD5Init
   #define MDUpdate MD5Update
   #define MDFinal MD5Final

   static u_long md_32(char *string, int length)
   {
       MD_CTX context;
       union {
           char   c[16];
           u_long x[4];
       } digest;
       u_long r;
       int i;

       MDInit (&context);
       MDUpdate (&context, string, length);
       MDFinal ((unsigned char *)&digest, &context);
       r = 0;
       for (i = 0; i < 3; i++) {
           r ^= digest.x[i];
       }
       return r;
   }                               /* md_32 */


   /*
    * Return random unsigned 32-bit quantity. Use 'type' argument if you
    * need to generate several different values in close succession.
    */
   u_int32 random32(int type)
   {
       struct {
           int     type;
           struct  timeval tv;



Schulzrinne/Casner/Frederick/Jacobson                        [Page 78]


Internet Draft                    RTP                  February 26, 1999


           clock_t cpu;
           pid_t   pid;
           u_long  hid;
           uid_t   uid;
           gid_t   gid;
           struct  utsname name;
       } s;

       gettimeofday(&s.tv, 0);
       uname(&s.name);
       s.type = type;
       s.cpu  = clock();
       s.pid  = getpid();
       s.hid  = gethostid();
       s.uid  = getuid();
       s.gid  = getgid();
       /* also: system uptime */

       return md_32((char *)&s, sizeof(s));
   }                               /* random32 */


A.7 Computing the RTCP Transmission Interval

   The following functions implement the RTCP transmission and reception
   rules described in Section 6.2. These rules are coded in several
   functions:

       o rtcp_interval() computes the deterministic calculated
         interval, measured in seconds.  The parameters are defined in
         Section 6.3.

       o OnExpire() is called when the RTCP transmission timer expires.

       o OnReceive() is called whenever an RTCP packet is received.

   Both OnExpire() and OnReceive() have event e as an argument. This is
   the next scheduled event for that participant, either an RTCP report
   or a BYE packet.  It is assumed that the following functions are
   available:

       o Schedule(time t, event e) schedules an event e to occur at
         time t. When time t arrives, the funcion OnExpire is called
         with e as an argument.

       o ReSchedule(time t, event e) reschedules a previously scheduled
         event e for time t.




Schulzrinne/Casner/Frederick/Jacobson                        [Page 79]


Internet Draft                    RTP                  February 26, 1999


       o SendRTCPReport() sends an RTCP report.

       o SendBYEPacket() sends a BYE packet.

       o TypeOfEvent(event e) returns EVENT_BYE if the event being
         processed is for a BYE packet to be sent, else it returns
         EVENT_REPORT.

       o PacketType(p) returns PACKET_RTCP_REPORT if packet p is an
         RTCP report (not BYE), PACKET_BYE if its a BYE RTCP packet, and
         PACKET_RTP if its a regular RTP data packet.

       o ReceivedPacketSize() and SentPacketSize() return the size of
         the referenced packet in octets.

       o NewMember(p) returns a 1 if the participant who sent packet p
         is not currently in the member list, 0 otherwise. Note this
         function is not sufficient for a complete implementation
         because each CSRC identifier in an RTP packet and each SSRC in
         a BYE packet should be processed.

       o NewSender(p) returns a 1 if the participant who sent packet p
         is not currently in the sender sublist of the member list, 0
         otherwise.

       o AddMember() and RemoveMember() to add and remove participants
         from the member list.

       o AddSender() and RemoveSender() to add and remove participants
         from the sender sublist of the member list.





















Schulzrinne/Casner/Frederick/Jacobson                        [Page 80]


Internet Draft                    RTP                  February 26, 1999



   double rtcp_interval(int members,
                        int senders,
                        double rtcp_bw,
                        int we_sent,
                        double avg_rtcp_size,
                        int initial)
   {
       /*
        * Minimum average time between RTCP packets from this site (in
        * seconds).  This time prevents the reports from `clumping' when
        * sessions are small and the law of large numbers isn't helping
        * to smooth out the traffic.  It also keeps the report interval
        * from becoming ridiculously small during transient outages like
        * a network partition.
        */
       double const RTCP_MIN_TIME = 5.;
       /*
        * Fraction of the RTCP bandwidth to be shared among active
        * senders.  (This fraction was chosen so that in a typical
        * session with one or two active senders, the computed report
        * time would be roughly equal to the minimum report time so that
        * we don't unnecessarily slow down receiver reports.) The
        * receiver fraction must be 1 - the sender fraction.
        */
       double const RTCP_SENDER_BW_FRACTION = 0.25;
       double const RTCP_RCVR_BW_FRACTION = (1-RTCP_SENDER_BW_FRACTION);
       /*
       /* To compensate for "unconditional reconsideration" converging to a
        * value below the intended average.
        */
       double const COMPENSATION = 2.71828 - 1.5;

       double t;                   /* interval */
       double rtcp_min_time = RTCP_MIN_TIME;
       int n;                      /* no. of members for computation */

       /*
        * Very first call at application start-up uses half the min
        * delay for quicker notification while still allowing some time
        * before reporting for randomization and to learn about other
        * sources so the report interval will converge to the correct
        * interval more quickly.
        */
       if (initial) {
           rtcp_min_time /= 2;
       }




Schulzrinne/Casner/Frederick/Jacobson                        [Page 81]


Internet Draft                    RTP                  February 26, 1999


       /*
        * If there were active senders, give them at least a minimum
        * share of the RTCP bandwidth.  Otherwise all participants share
        * the RTCP bandwidth equally.
        */
       n = members;
       if (senders > 0 && senders < members * RTCP_SENDER_BW_FRACTION) {
           if (we_sent) {
               rtcp_bw *= RTCP_SENDER_BW_FRACTION;
               n = senders;
           } else {
               rtcp_bw *= RTCP_RCVR_BW_FRACTION;
               n -= senders;
           }
       }

       /*
        * The effective number of sites times the average packet size is
        * the total number of octets sent when each site sends a report.
        * Dividing this by the effective bandwidth gives the time
        * interval over which those packets must be sent in order to
        * meet the bandwidth target, with a minimum enforced.  In that
        * time interval we send one report so this time is also our
        * average time between reports.
        */
       t = avg_rtcp_size * n / rtcp_bw;
       if (t < rtcp_min_time) t = rtcp_min_time;

       /*
        * To avoid traffic bursts from unintended synchronization with
        * other sites, we then pick our actual next report interval as a
        * random number uniformly distributed between 0.5*t and 1.5*t.
        */
       t = t * (drand48() + 0.5);
       t = t / COMPENSATION;
       return t;
   }














Schulzrinne/Casner/Frederick/Jacobson                        [Page 82]


Internet Draft                    RTP                  February 26, 1999




   void OnExpire(event e,
                 int    members,
                 int    senders,
                 double rtcp_bw,
                 int    we_sent,
                 double *avg_rtcp_size,
                 int    *initial,
                 time_tp   tc,
                 time_tp   *tp,
                 int    *pmembers)
   {
       /* This function is responsible for deciding whether to send
        * an RTCP report or BYE packet now, or to reschedule transmission.
        * It is also responsible for updating the pmembers, initial, tp,
        * and avg_rtcp_size state variables. This function should be called
        * upon expiration of the event timer used by Schedule(). */

       double t;     /* Interval */
       double tn;    /* Next transmit time */

       /* In the case of a BYE, we use "unconditional reconsideration" to
        * reschedule the transmission of the BYE if necessary */

       if (TypeOfEvent(e) == EVENT_BYE) {
           t = rtcp_interval(members,
                             senders,
                             rtcp_bw,
                             we_sent,
                             *avg_rtcp_size,
                             *initial);
           tn = *tp + t;
           if (tn <= tc) {
               SendBYEPacket();
               exit(1);
           } else {
               Schedule(tn, e);
           }

       } else if (TypeOfEvent(e) == EVENT_REPORT) {
           t = rtcp_interval(members,
                             senders,
                             rtcp_bw,
                             we_sent,
                             *avg_rtcp_size,
                             *initial);
           tn = *tp + t;



Schulzrinne/Casner/Frederick/Jacobson                        [Page 83]


Internet Draft                    RTP                  February 26, 1999


           if (tn <= tc) {
               SendRTCPReport();
               *avg_rtcp_size = (1./16.)*SentPacketSize(e) +
                   (15./16.)*(*avg_rtcp_size);
               *tp = tc;

               /* We must redraw the interval. Don't reuse the
                  one computed above, since its not actually
                  distributed the same, as we are conditioned
                  on it being small enough to cause a packet to
                  be sent */

               t = rtcp_interval(members,
                                 senders,
                                 rtcp_bw,
                                 we_sent,
                                 *avg_rtcp_size,
                                 *initial);

               Schedule(t+tc,e);
               *initial = 0;
           } else {
               Schedule(tn, e);
           }
           *pmembers = members;
       }
   }
























Schulzrinne/Casner/Frederick/Jacobson                        [Page 84]


Internet Draft                    RTP                  February 26, 1999



   void OnReceive(packet p,
                  event e,
                  int *members,
                  int *pmembers,
                  int *senders,
                  double *avg_rtcp_size,
                  double *tp,
                  double tc,
                  double tn)
   {
       /* What we do depends on whether we have left the group, and
        * are waiting to send a BYE (TypeOfEvent(e) == EVENT_BYE) or
        * an RTCP report. p represents the packet that was just received. */

       if (PacketType(p) == PACKET_RTCP_REPORT) {
           if (NewMember(p) && (TypeOfEvent(e) == EVENT_REPORT)) {
               AddMember(p);
               *members += 1;
           }
           *avg_rtcp_size = (1./16.)*ReceivedPacketSize(p) +
               (15./16.)*(*avg_rtcp_size);
       } else if (PacketType(p) == PACKET_RTP) {
           if (NewMember(p) && (TypeOfEvent(e) == EVENT_REPORT)) {
               AddMember(p);
               *members += 1;
           }
           if (NewSender(p) && (TypeOfEvent(e) == EVENT_REPORT)) {
               AddSender(p);
               *senders += 1;
           }
       } else if (PacketType(p) == PACKET_BYE) {
           *avg_rtcp_size = (1./16.)*ReceivedPacketSize(p) +
               (15./16.)*(*avg_rtcp_size);

           if (TypeOfEvent(e) == EVENT_REPORT) {
               if (NewSender(p) == FALSE) {
                   RemoveSender(p);
                   *senders -= 1;
               }

               if (NewMember(p) == FALSE) {
                   RemoveMember(p);
                   *members -= 1;
               }

               if(*members < *pmembers) {
                   tn = tc + (((double) *members)/(*pmembers))*(tn - tc);



Schulzrinne/Casner/Frederick/Jacobson                        [Page 85]


Internet Draft                    RTP                  February 26, 1999


                   *tp = tc - (((double) *members)/(*pmembers))*(tc - *tp);

                   /* Reschedule the next report for time tn */

                   Reschedule(e, tn);
                   *pmembers = *members;
               }

           } else if (TypeOfEvent(e) == EVENT_BYE) {
               *members += 1;
           }
       }
   }



A.8 Estimating the Interarrival Jitter

   The code fragments below implement the algorithm given in Section
   6.4.1 for calculating an estimate of the statistical variance of the
   RTP data interarrival time to be inserted in the interarrival jitter
   field of reception reports. The inputs are r->ts , the timestamp from
   the incoming packet, and arrival , the current time in the same
   units. Here s points to state for the source; s->transit holds the
   relative transit time for the previous packet, and s->jitter holds
   the estimated jitter. The jitter field of the reception report is
   measured in timestamp units and expressed as an unsigned integer, but
   the jitter estimate is kept in a floating point. As each data packet
   arrives, the jitter estimate is updated:


       int transit = arrival - r->ts;
       int d = transit - s->transit;
       s->transit = transit;
       if (d < 0) d = -d;
       s->jitter += (1./16.) * ((double)d - s->jitter);



   When a reception report block (to which rr points) is generated for
   this member, the current jitter estimate is returned:


       rr->jitter = (u_int32) s->jitter;



   Alternatively, the jitter estimate can be kept as an integer, but



Schulzrinne/Casner/Frederick/Jacobson                        [Page 86]


Internet Draft                    RTP                  February 26, 1999


   scaled to reduce round-off error. The calculation is the same except
   for the last line:


       s->jitter += d - ((s->jitter + 8) >> 4);



   In this case, the estimate is sampled for the reception report as:


       rr->jitter = s->jitter >> 4;



B Changes from RFC 1889

   Most of this RFC is identical to RFC 1889. The changes are listed
   below.

       o The algorithm for calculating the RTCP transmission interval
         specified in Sections 6.2 and 6.3 and illustrated in Appendix
         A.7 is augmented to include "reconsideration" to minimize
         transmission over the intended rate when many participants join
         a session simultaneously, and "reverse reconsideration" to
         reduce the incidence and duration of false participant timeouts
         when the number of participants drops rapidly.

       o Section 6.3.7 specifies new rules controlling when an RTCP BYE
         packet should be sent in order to avoid a flood of packets when
         many participants leave a session simultaneously.  Sections 7.2
         and 7.3 specify that translators and mixers should send BYE
         packets for the sources they are no longer forwarding.

       o Section 6.2.1 specifies that implementations may store only a
         sampling of the participants' SSRC identifiers to allow scaling
         to very large sessions. Algorithms are specified in a separate
         RFC.

       o In Section 6.2 it is specified that RTCP sender and receiver
         bandwidths to be set as separate parameters of the session
         rather than a strict percentage of the session bandwidth, and
         may be set to zero. The requirement that RTCP was mandatory for
         RTP sessions using IP multicast was relaxed.

       o Also in Section 6.2 it is specified that the minimum RTCP
         interval may be scaled to smaller values for high bandwidth
         sessions, and may be set to zero for unicast sessions.



Schulzrinne/Casner/Frederick/Jacobson                        [Page 87]


Internet Draft                    RTP                  February 26, 1999


       o The requirement to retain state for inactive participants for
         a period long enough to span typical network partitions was
         removed from Section 6.2.1. In a session where many
         participants join for a brief time and fail to send BYE, this
         requirement would cause a significant overestimate of the
         number of participants. The reconsideration algorithm added in
         this revision compensates for the large number of new
         participants joining simultaneously when a partition heals.

       o Rule changes for layered encodings are defined in Sections
         2.4, 6.3.9, 8.3 and 10.

       o An indentation bug in the RFC 1889 printing of the pseudo-code
         for the collision detection and resolution algorithm in Section
         8.2 is corrected, and the algorithm has been modified to remove
         the restriction that both RTP and RTCP must be sent from the
         same source port number.

       o For unicast RTP sessions, distinct port pairs may be used for
         the two ends (Sections 3 and 7.1).

       o The description of the padding mechanism for RTCP packets was
         clarified and it is specified that padding MUST be applied to
         the last packet of a compound RTCP packet.

       o It is specified that a receiver MUST ignore packets with
         payload types it does not understand.

       o The specification of "relative" NTP timestamp in the RTCP SR
         section now defines these timestamps to be based on the most
         common system-specific clock, such as system uptime, rather
         than on session elapsed time which would not be the same for
         multiple applications started on the same machine at different
         times.

       o The inconsequence of NTP timestamps wrapping around in the
         year 2036 is explained.

       o The policy for registration of RTCP packet types and SDES
         types was clarified in a new Section 11.3, IANA Considerations.
         The suggestion that experimenters register the numbers they
         need and then unregister those which prove to be unneeded has
         been removed in in favor of using APP and PRIV.

       o The reference for the UTF-8 character set was changed to be
         RFC 2279.

       o The last paragraph of the introduction in RFC 1889, which



Schulzrinne/Casner/Frederick/Jacobson                        [Page 88]


Internet Draft                    RTP                  February 26, 1999


         cautioned implementers to limit deployment in the Internet, was
         removed because it was deemed no longer relevant.

       o Small clarifications of the text have been made in several
         places, some in response to questions from readers. In
         particular:

         -A definition for "RTP media type" is given in Section 3 to
          allow the explanation of multiplexing RTP sessions in Section
          5.2 to be more clear regarding the multiplexing of multiple
          media.

         -The description of the session bandwidth parameter is expanded
          in Section 6.2.

         -The method for terminating and padding a sequence of SDES
          items is clarified in Section 6.5.

         -The Security section adds a formal reference to IPSEC now that
          it is available, and says that the confidentiality method
          defined in this specification is primarily to codify existing
          practice.

         -The terms MUST, SHOULD, MAY, etc. are used as defined in RFC
          2119.

C Security Considerations

   RTP suffers from the same security liabilities as the underlying
   protocols. For example, an impostor can fake source or destination
   network addresses, or change the header or payload. Within RTCP, the
   CNAME and NAME information may be used to impersonate another
   participant. In addition, RTP may be sent via IP multicast, which
   provides no direct means for a sender to know all the receivers of
   the data sent and therefore no measure of privacy. Rightly or not,
   users may be more sensitive to privacy concerns with audio and video
   communication than they have been with more traditional forms of
   network communication [27]. Therefore, the use of security mechanisms
   with RTP is important. These mechanisms are discussed in Section 9.

   RTP-level translators or mixers may be used to allow RTP traffic to
   reach hosts behind firewalls. Appropriate firewall security
   principles and practices, which are beyond the scope of this
   document, should be followed in the design and installation of these
   devices and in the admission of RTP applications for use behind the
   firewall.

D Full Copyright Statement



Schulzrinne/Casner/Frederick/Jacobson                        [Page 89]


Internet Draft                    RTP                  February 26, 1999


   Copyright (C) The Internet Society (1999). 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 implmentation may be prepared, copied, published and
   distributed, in whole or in part, without restriction of any kind,
   provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

E Addresses of Authors

   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue
   New York, NY 10027
   USA
   electronic mail: schulzrinne@cs.columbia.edu

   Stephen L. Casner
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134
   United States
   electronic mail: casner@cisco.com

   Ron Frederick
   Xerox Palo Alto Research Center
   3333 Coyote Hill Road
   Palo Alto, CA 94304
   United States



Schulzrinne/Casner/Frederick/Jacobson                        [Page 90]


Internet Draft                    RTP                  February 26, 1999


   electronic mail: frederic@parc.xerox.com

   Van Jacobson
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134
   United States
   electronic mail: van@cisco.com


   Acknowledgments

   This memorandum is based on discussions within the IETF Audio/Video
   Transport working group chaired by Stephen Casner. The current
   protocol has its origins in the Network Voice Protocol and the Packet
   Video Protocol (Danny Cohen and Randy Cole) and the protocol
   implemented by the vat application (Van Jacobson and Steve McCanne).
   Christian Huitema provided ideas for the random identifier generator.
   Extensive analysis and simulation of the timer reconsideration
   algorithm was done by Jonathan Rosenberg.

F Bibliography

   [1] D. D. Clark and D. L. Tennenhouse, "Architectural considerations
   for a new generation of protocols," in SIGCOMM Symposium on
   Communications Architectures and Protocols , (Philadelphia,
   Pennsylvania), pp. 200--208, IEEE, Sept. 1990.  Computer
   Communications Review, Vol. 20(4), Sept. 1990.

   [2] H. Schulzrinne, "RTP Profile for Audio and Video Conferences with
   Minimal Control," Internet Draft, Internet Engineering Task Force,
   Feb. 1999 Work in progress, revision to RFC 1890.

   [3] H. Schulzrinne, "Issues in designing a transport protocol for
   audio and video conferences and other multiparticipant real-time
   applications." expired Internet draft, Oct. 1993.

   [4] S. Bradner, "Key words for use in RFCs to Indicate Requirement
   Levels,"  RFC 2119, Internet Engineering Task Force, Mar. 1997.

   [5] D. E. Comer, Internetworking with TCP/IP , vol. 1.  Englewood
   Cliffs, New Jersey: Prentice Hall, 1991.

   [6] J. Postel, "Internet protocol,"  RFC 791, Internet Engineering
   Task Force, Sept. 1981.

   [7] D. Mills, "Network time protocol (v3),"  RFC 1305, Internet
   Engineering Task Force, Apr. 1992.



Schulzrinne/Casner/Frederick/Jacobson                        [Page 91]


Internet Draft                    RTP                  February 26, 1999


   [8] J. Reynolds and J. Postel, "Assigned numbers,"  STD 2, RFC 1700,
   Internet Engineering Task Force, Oct. 1994.

   [9] D. Eastlake, S. Crocker, and J. Schiller, "Randomness
   recommendations for security," RFC 1750, Internet Engineering Task
   Force, Dec. 1994.

   [10] J.-C. Bolot, T. Turletti, and I. Wakeman, "Scalable feedback
   control for multicast video distribution in the internet," in SIGCOMM
   Symposium on Communications Architectures and Protocols , (London,
   England), pp. 58--67, ACM, Aug. 1994.

   [11] I. Busse, B. Deffner, and H. Schulzrinne, "Dynamic QoS control
   of multimedia applications based on RTP," Computer Communications ,
   Jan. 1996.

   [12] S. Floyd and V. Jacobson, "The synchronization of periodic
   routing messages," in SIGCOMM Symposium on Communications
   Architectures and Protocols (D. P. Sidhu, ed.), (San Francisco,
   California), pp. 33--44, ACM, Sept. 1993.  also in [28].

   [13] J. Rosenberg and H. Schulzrinne, "Sampling of the Group
   Membership in RTP," Internet Draft, Internet Engineering Task Force,
   November 1998.  Work in progress.

   [14] J. A. Cadzow, Foundations of digital signal processing and data
   analysis New York, New York: Macmillan, 1987.

   [15] F. Yergeau, "UTF-8, a transformation format of ISO 10646,"  RFC
   2279, Internet Engineering Task Force, Jan. 1998.

   [16] P. Mockapetris, "Domain names - concepts and facilities,"  STD
   13, RFC 1034, Internet Engineering Task Force, Nov. 1987.

   [17] P. Mockapetris, "Domain names - implementation and
   specification,"  STD 13, RFC 1035, Internet Engineering Task Force,
   Nov. 1987.

   [18] R. Braden, "Requirements for internet hosts - application and
   support,"  STD 3, RFC 1123, Internet Engineering Task Force, Oct.
   1989.

   [19] Y. Rekhter, R. Moskowitz, D. Karrenberg, and G. de Groot,
   "Address allocation for private internets,"  RFC 1597, Internet
   Engineering Task Force, Mar. 1994.

   [20] E. Lear, E. Fair, D. Crocker, and T. Kessler, "Network 10
   considered harmful (some practices shouldn't be codified),"  RFC



Schulzrinne/Casner/Frederick/Jacobson                        [Page 92]


Internet Draft                    RTP                  February 26, 1999


   1627, Internet Engineering Task Force, July 1994.

   [21] D. Crocker, "Standard for the format of ARPA internet text
   messages,"  STD 11, RFC 822, Internet Engineering Task Force, Aug.
   1982.

   [22] W. Feller, An Introduction to Probability Theory and its
   Applications, Volume 1 , vol. 1.  New York, New York: John Wiley and
   Sons, third ed., 1968.

   [23] S. Kent and R. Atkinson, "Security Architecture for the Internet
   Protocol," Internet Draft, Internet Engineering Task Force, July
   1998.  Work in progress.

   [24] D. Balenson, "Privacy enhancement for internet electronic mail:
   Part III: algorithms, modes, and identifiers,"  RFC 1423, Internet
   Engineering Task Force, Feb. 1993.

   [25] V. L. Voydock and S. T. Kent, "Security mechanisms in high-level
   network protocols," ACM Computing Surveys , vol. 15, pp. 135--171,
   June 1983.

   [26] R. Rivest, "The MD5 message-digest algorithm,"  RFC 1321,
   Internet Engineering Task Force, Apr. 1992.

   [27] S. Stubblebine, "Security services for multimedia conferencing,"
   in 16th National Computer Security Conference , (Baltimore,
   Maryland), pp. 391--395, Sept. 1993.

   [28] S. Floyd and V. Jacobson, "The synchronization of periodic
   routing messages," IEEE/ACM Transactions on Networking , vol. 2, pp.
   122--136, Apr. 1994.




                           Table of Contents



   1          Introduction ........................................    3
   1.1        Terminology .........................................    5
   2          RTP Use Scenarios ...................................    5
   2.1        Simple Multicast Audio Conference ...................    5
   2.2        Audio and Video Conference ..........................    6
   2.3        Mixers and Translators ..............................    6
   2.4        Layered Encodings ...................................    7
   3          Definitions .........................................    8



Schulzrinne/Casner/Frederick/Jacobson                        [Page 93]


Internet Draft                    RTP                  February 26, 1999


   4          Byte Order, Alignment, and Time Format ..............   10
   5          RTP Data Transfer Protocol ..........................   11
   5.1        RTP Fixed Header Fields .............................   11
   5.2        Multiplexing RTP Sessions ...........................   14
   5.3        Profile-Specific Modifications to the RTP Header
   ................................................................   15
   5.3.1      RTP Header Extension ................................   15
   6          RTP Control Protocol -- RTCP ........................   16
   6.1        RTCP Packet Format ..................................   18
   6.2        RTCP Transmission Interval ..........................   20
   6.2.1      Maintaining the number of session members ...........   24
   6.3        RTCP Packet Send and Receive Rules ..................   25
   6.3.1      Computing the RTCP transmission interval ............   26
   6.3.2      Initialization ......................................   27
   6.3.3      Receiving an RTP or non-BYE RTCP packet .............   27
   6.3.4      Receiving an RTCP BYE packet ........................   27
   6.3.5      Timing Out an SSRC ..................................   28
   6.3.6      Expiration of transmission timer ....................   28
   6.3.7      Transmitting a BYE packet ...........................   29
   6.3.8      Updating we_sent ....................................   30
   6.3.9      Allocation of source description bandwidth ..........   30
   6.4        Sender and Receiver Reports .........................   31
   6.4.1      SR: Sender report RTCP packet .......................   31
   6.4.2      RR: Receiver report RTCP packet .....................   37
   6.4.3      Extending the sender and receiver reports ...........   38
   6.4.4      Analyzing sender and receiver reports ...............   39
   6.5        SDES: Source description RTCP packet ................   40
   6.5.1      CNAME: Canonical end-point identifier SDES item .....   42
   6.5.2      NAME: User name SDES item ...........................   43
   6.5.3      EMAIL: Electronic mail address SDES item ............   44
   6.5.4      PHONE: Phone number SDES item .......................   44
   6.5.5      LOC: Geographic user location SDES item .............   44
   6.5.6      TOOL: Application or tool name SDES item ............   45
   6.5.7      NOTE: Notice/status SDES item .......................   45
   6.5.8      PRIV: Private extensions SDES item ..................   46
   6.6        BYE: Goodbye RTCP packet ............................   46
   6.7        APP: Application-defined RTCP packet ................   47
   7          RTP Translators and Mixers ..........................   49
   7.1        General Description .................................   49
   7.2        RTCP Processing in Translators ......................   51
   7.3        RTCP Processing in Mixers ...........................   53
   7.4        Cascaded Mixers .....................................   54
   8          SSRC Identifier Allocation and Use ..................   54
   8.1        Probability of Collision ............................   54
   8.2        Collision Resolution and Loop Detection .............   55
   8.3        Use with Layered Encodings ..........................   59
   9          Security ............................................   59
   9.1        Confidentiality .....................................   60



Schulzrinne/Casner/Frederick/Jacobson                        [Page 94]


Internet Draft                    RTP                  February 26, 1999


   9.2        Authentication and Message Integrity ................   61
   10         RTP over Network and Transport Protocols ............   61
   11         Summary of Protocol Constants .......................   63
   11.1       RTCP packet types ...................................   63
   11.2       SDES types ..........................................   63
   11.3       IANA Considerations .................................   64
   A          Algorithms ..........................................   64
   A.1        RTP Data Header Validity Checks .....................   68
   A.2        RTCP Header Validity Checks .........................   73
   A.3        Determining the Number of RTP Packets Expected and
   Lost ...........................................................   73
   A.4        Generating SDES RTCP Packets ........................   74
   A.5        Parsing RTCP SDES Packets ...........................   75
   A.6        Generating a Random 32-bit Identifier ...............   76
   A.7        Computing the RTCP Transmission Interval ............   79
   A.8        Estimating the Interarrival Jitter ..................   86
   B          Changes from RFC 1889 ...............................   87
   C          Security Considerations .............................   89
   D          Full Copyright Statement ............................   89
   E          Addresses of Authors ................................   90
   F          Bibliography ........................................   91






























Schulzrinne/Casner/Frederick/Jacobson                        [Page 95]