Internet Engineering Task Force          Audio-Video Transport Working Group
INTERNET-DRAFT                                                H. Schulzrinne
draft-ietf-avt-issues-01.txt                          AT&T Bell Laboratories
                                                            October 20, 1993
                                                          Expires:  03/01/94

Issues in Designing a Transport Protocol for Audio and Video Conferences and
               other Multiparticipant Real-Time Applications

Status of this Memo

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

Internet Drafts are draft documents valid for a maximum of six months.
Internet Drafts may be updated, replaced, or obsoleted by other documents
at any time.   It is not appropriate to use Internet Drafts as reference
material or to cite them other than as a ``working draft'' or ``work in

Please check the I-D abstract listing contained in each Internet Draft
directory to learn the current status of this or any other Internet Draft.

Distribution of this document is unlimited.


     This memorandum is a companion document to the current version
    of the RTP protocol specification draft-ietf-avt-rtp-*.{txt,ps}.
    It discusses aspects of transporting real-time services (such as
    voice or video) over the Internet.   It compares and evaluates
    design alternatives for a real-time transport protocol, providing
    rationales for the design decisions made for RTP. Also covered are
    issues of port assignment and multicast address allocation.   A
    comprehensive glossary of terms related to multimedia conferencing
    is provided.

This document 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 and/or the
INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993



1 Introduction                                                            4

  1.1 T. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Goals                                                                   7

3 Services                                                                9

  3.1 Duplex or Simplex? . . . . . . . . . . . . . . . . . . . . . . . . 12

  3.2 Framing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

  3.3 Version Identification . . . . . . . . . . . . . . . . . . . . . . 14

  3.4 Conference Identification. . . . . . . . . . . . . . . . . . . . . 14

    3.4.1Demultiplexing. . . . . . . . . . . . . . . . . . . . . . . . . 15

    3.4.2Aggregation . . . . . . . . . . . . . . . . . . . . . . . . . . 15

  3.5 Media Encoding Identification. . . . . . . . . . . . . . . . . . . 16

    3.5.1Audio Encodings . . . . . . . . . . . . . . . . . . . . . . . . 17

    3.5.2Video Encodings . . . . . . . . . . . . . . . . . . . . . . . . 19

  3.6 Playout Synchronization. . . . . . . . . . . . . . . . . . . . . . 19

    3.6.1Synchronization Methods . . . . . . . . . . . . . . . . . . . . 21

    3.6.2Detection of Synchronization Units. . . . . . . . . . . . . . . 22

    3.6.3Interpretation of Synchronization Bit . . . . . . . . . . . . . 24

    3.6.4Interpretation of Timestamp . . . . . . . . . . . . . . . . . . 25

    3.6.5End-of-talkspurt indication . . . . . . . . . . . . . . . . . . 29

    3.6.6Recommendation. . . . . . . . . . . . . . . . . . . . . . . . . 30

  3.7 Segmentation and Reassembly. . . . . . . . . . . . . . . . . . . . 30

  3.8 Source Identification. . . . . . . . . . . . . . . . . . . . . . . 31

H. Schulzrinne                   Expires 03/01/94                   [Page 2]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

    3.8.1Bridges, Translators and End Systems. . . . . . . . . . . . . . 31

    3.8.2Address Format Issues . . . . . . . . . . . . . . . . . . . . . 33

    3.8.3Globally unique identifiers . . . . . . . . . . . . . . . . . . 34

    3.8.4Locally unique addresses. . . . . . . . . . . . . . . . . . . . 35

  3.9 Energy Indication. . . . . . . . . . . . . . . . . . . . . . . . . 37

  3.10Error Control. . . . . . . . . . . . . . . . . . . . . . . . . . . 37

  3.11Security and Privacy . . . . . . . . . . . . . . . . . . . . . . . 39

    3.11.1Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . 39

    3.11.2Confidentiality . . . . . . . . . . . . . . . . . . . . . . . . 40

    3.11.3Message Integrity and Authentication. . . . . . . . . . . . . . 41

  3.12Security for RTP vs.  PEM. . . . . . . . . . . . . . . . . . . . . 42

  3.13Quality of Service Control . . . . . . . . . . . . . . . . . . . . 44

    3.13.1QOS Measures. . . . . . . . . . . . . . . . . . . . . . . . . . 44

    3.13.2Remote measurements . . . . . . . . . . . . . . . . . . . . . . 45

    3.13.3Monitoring by Third Party . . . . . . . . . . . . . . . . . . . 46

4 Conference Control Protocol                                            46

5 The Use of Profiles                                                    46

6 Port Assignment                                                        47

7 Multicast Address Allocation                                           48

  7.1 Channel Sensing. . . . . . . . . . . . . . . . . . . . . . . . . . 49

  7.2 Global Reservation Channel with Scoping. . . . . . . . . . . . . . 50

  7.3 Local Reservation Channel. . . . . . . . . . . . . . . . . . . . . 50

    7.3.1Hierarchical Allocation with Servers. . . . . . . . . . . . . . 51

    7.3.2Distributed Hierarchical Allocation . . . . . . . . . . . . . . 51

  7.4 Restricting Scope by Limiting Time-to-Live . . . . . . . . . . . . 52

H. Schulzrinne                   Expires 03/01/94                   [Page 3]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

8 Security Considerations                                                52

A Glossary                                                               52

B Address of Author                                                      62

1 Introduction

This memorandum

1.1 T

he transport protocol for real-time applications (RTP) discussed in the pr
this memorandum aims to provide services commonly required by interactive
multimedia conferences, such as playout synchronization, demultiplexing,
media identification and active-party identification.  However, RTP is not
restricted to multimedia conferences; it is anticipated that other real-time
services such as remote data acquisition and control may find its services
of use.

In this context, a conference describes associations that are characterized
by the participation of two or more agents, interacting in real time
with one or more media of potentially different types.   The agents are
anticipated to be human, but may also be measurement devices, remote media
servers, simulators and the like.    Both two-party and multiple-party
associations are to be supported, where one or more agents can take active
roles, i.e., generate data.  Thus, applications not commonly considered a
conference fall under this wider definition, for example, one-way media such
as the network equivalent of closed-circuit television or radio, traditional
two-party telephone conversations or real-time distributed simulations.
Even though intended for real-time interactive applications, the use of
RTP for the storage and transmission of recorded real-time data should be
possible, with the understanding that the interpretation of some fields such
as timestamps may be affected by this off-line mode of operation.

RTP uses the services of an end-to-end transport protocol such as UDP,
TCP, OSI TP1 or TP4, ST-II or the like(1) .   The services used are:
end-to-end delivery, framing, demultiplexing and multicast.  The underlying
network is not assumed to be reliable and can be expected to lose, corrupt,
arbitrarily delay and reorder packets.   However, the use of RTP within
 1. ST-II is not properly a transport protocol, as it is visible to
intermediate nodes, but it provides services such as process demultiplexing
commonly associated with transport protocols.

H. Schulzrinne                   Expires 03/01/94                   [Page 4]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

quality-of-service (e.g., rate) controlled networks is anticipated to be of
particular interest.  Network layer support for multicasting is desirable,
but not required.  RTP is supported by a real-time control protocol (RTCP)
in a relationship similar to that between IP and ICMP. However, RTP can be
used, with reduced functionality, without a control protocol.  The control
protocol RTCP provides minimum functionality for maintaining conference
state for one or more flows within a single transport association.  RTCP
is not guaranteed to be reliable; each participant simply sends the local
information periodically to all other conference participants.

As an alternative, RTP could be used as a transport protocol layered
directly on top of IP, potentially increasing performance and reducing
header overhead.  This may be attractive as the services provided by UDP,
checksumming and demultiplexing, may not be needed for multicast real-time
conferencing applications.   This aspect remains for further study.   The
relationships between RTP and RTCP to other protocols of the Internet
protocol suite are depicted in Fig. 1.

|                          |     conference controller   |
|    media application     |-------------------+         |
|                          |  conf. ctl. prot. |         |
|                |       RTCP        |                   |
|                +-------------------+                   |
|                         RTP                            |
+--------+-----------------+                             |
|        |       UDP       |                             |
| ST-II  +-----------------+-------------+               |
|                |         IP            |               |
|                         AAL5                           |
      Figure 1:  Embedding of RTP and RTCP in Internet protocol stack

Conferences  encompassing  several  media  are  managed  by  a  (reliable)
conference control protocol, whose definition is outside the scope of this
note.    Some aspects of its functionality, however, are described in
Section 4.

Within this working group, some common encoding rules and algorithms for
media have been specified, keeping in mind that this aspect is largely
independent of the remainder of the protocol.  Without this specification,
interoperability cannot be achieved.   It is intended, however, to keep
the two aspects as separate RFCs as changes in media encoding should
be independent of the transport aspects.    The encoding specification
includes issues such as byte order for multi-byte samples, sample order
for multi-channel audio, the format of state information for differential
encodings, the segmentation of encoded video frames into packets, and the

H. Schulzrinne                   Expires 03/01/94                   [Page 5]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993


When used for multimedia services, RTP sources will have to be able to
convey the type of media encoding used to the receivers.   The number
of encodings potentially used is rather large, but a single application
will likely restrict itself to a small subset of that.   To allow the
participants in conferences to unambiguously communicate to each other the
current encoding, the working group is defining a set of encoding names to
be registered with the Internet Assigned Numbers Authority (IANA). Also,
short integers for a default mapping of common encodings are specified.

The issue of port assignment will be discussed in more detail in Section 6.
It should be emphasized, however, that UDP port assignment does not imply
that all underlying transport mechanisms share this or a similar port

This memorandum aims to summarize some of the discussions held within the
audio-video transport (AVT) working group chaired by Stephen Casner, but
the opinions are the author's own.  Where possible, references to previous
work are included, but the author realizes that the attribution of ideas is
far from complete.   The memorandum builds on operational experience with
Van Jacobson's and Steve McCanne's vat audio conferencing tool as well as
implementation experience with the author's Nevot network voice terminal.
This note will frequently refer to NVP [1], the network voice protocol,
a protocol used in two versions for early Internet wide-area packet voice
experiments.   CCITT has standardized as recommendations G.764 and G.765
a packet voice protocol stack for use in digital circuit multiplication

The  name  RTP  was  chosen  to  reflect  the  fact  that  audio  and  video
conferences may not be the only applications employing its services, while
the real-time nature of the protocol is important, setting it apart from
other multimedia-transport mechanisms, such as the MIME multimedia mail
effort [2].

The remainder of this memorandum is organized as follows.    Section 2
summarizes the design goals of this real-time transport protocol.   Then,
Section 3 describes the services to be provided in more detail.  Section 4
briefly outlines some of the services added by a higher-layer conference
control protocol;  a more detailed description is outside the scope of
this document.   Two appendices discuss the issues of port assignment and
multicast address allocation, respectively.  A glossary defines terms and
acronyms, providing references for further detail.   The actual protocol
specification embodying the recommendation and conclusions of this report is
contained in a separate document.

H. Schulzrinne                   Expires 03/01/94                   [Page 6]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

2 Goals

Design decisions should be measured against the following goals,  not
necessarily listed in order of importance:

content flexibility: While  the  primary  applications  that  motivate  the
    protocol  design  are  conference  voice  and  video,  it  should  be
    anticipated  that  other  applications  may  also  find  the  services
    provided by the protocol useful.   Some examples include distribution
    audio/video (for example, the ``Radio Free Ethernet''application by
    Sun), distributed simulation and some forms of (loss-tolerant) remote
    data acquisition (for example, active badge systems [3,4]).  Note that
    it is possible that the same packet header field may be interpreted in
    different ways depending on the content (e.g., a synchronization bit
    may be used to indicate the beginning of a talkspurt for audio and the
    beginning of a frame for video).   Also, new formats of established
    media, for example, high-quality multi-channel audio or combined audio
    and video sources, should be anticipated where possible.

extensible: Researchers and implementors within the Internet community are
    currently only beginning to explore real-time multimedia services such
    as video conferences.   Thus, the RTP should be able to incorporate
    additional  services  as  operational  experience  with  the  protocol
    accumulates and as applications not originally anticipated find its
    services useful.   The same mechanisms should also allow experimental
    applications  to  exchange  application-specific  information  without
    jeopardizing interoperability with other applications.   Extensibility
    is also desirable as it will hopefully speed along the standardization
    effort, making the consequences of leaving out some group's favorite
    fixed header field less drastic.

    It should be understood that extensibility and flexibility may conflict
    with the goals of bandwidth and processing efficiency.

independent of lower-layer protocols: RTP should make as few assumptions
    about the underlying transport protocol as possible.  It should, for
    example, work reasonably well with UDP, TCP, ST-II, OSI TP, VMTP and
    experimental protocols, for example, protocols that support resource
    reservation and quality-of-service guarantees.    Naturally, not all
    transport protocols are equally suited for real-time services;  in
    particular, TCP may introduce unacceptable delays over anything but
    low-error-rate LANs.  Also, protocols that deliver streams rather than
    packets needs additional framing services as discussed in Section 3.2.

    It remains to be discussed whether RTP may use services provided by the
    lower-layer protocols for its own purposes (time stamps and sequence
    numbers, for example).

H. Schulzrinne                   Expires 03/01/94                   [Page 7]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

    The goal of independence from lower-layer considerations also affects
    the issue of address representation.   In particular, anything too
    closely tied to the current IP 4-byte addresses may face early
    obsolescence.  It is to be anticipated, however, that experience gained
    will suggest a new protocol revision in any event by that time.

bridge-compatible: Operational experience has shown that RTP-level bridges
    are necessary and desirable for a number of reasons.    First, it
    may be desirable to aggregate several media streams into a single
    stream and then retransmit it with possibly different encoding, packet
    size or transport protocol.   A packet ``translator'' that achieves
    multicasting by user-level copying may be needed where multicast
    tunnels or IP connectivity are unavailable or the end-systems are not

bandwidth efficient: It is anticipated that the protocol will be used in
    networks with a wide range of bandwidths and with a variety of media
    encodings.  Despite increasing bandwidths within the national backbone
    networks,  bandwidth efficiency will continue to be important for
    transporting conferences across 56 kb links, office-to-home high-speed
    modem connections and international links.   To minimize end-to-end
    delay and the effect of lost packets, packetization intervals have to
    be limited, which, in combination with efficient media encodings, leads
    to short packet sizes.  Generally, packets containing 16 to 32 ms of
    speech are considered optimal [5--7].  For example, even with a 65 ms
    packetization interval, a 4800 b/s encoding produces 39 byte packets.
    Current Internet voice experiments use packets containing around 20 ms
    of audio, which translates into 160 bytes of audio information coded
    at 64 kb/s.  Video packets are typically much longer, so that header
    overhead is less of a concern.

    For UDP multicast (without counting the overhead of source routing as
    currently used in tunnels or a separate IP encapsulation as planned),
    IPv4 incurs 20 bytes and UDP an additional 8 bytes of header overhead,
    to which datalink layer headers of at least 4 bytes must be added.
    With RTP header lengths between 4 and 8 bytes, the total overhead
    amounts to between 36 and 40 (or more) bytes per audio or video packet.
    For 160-byte audio packets, the overhead of 8-byte RTP headers together
    with UDP, IP and PPP (as an example of a datalink protocol) headers is
    25%.   For low bitrate coding, packet headers can easily double the
    necessary bit rate.

    Thus, it appears that any fixed headers beyond eight bytes would have
    to make a significant contribution to the protocol's capabilities as
    such long headers could stand in the way of running RTP applications
    over low-speed links.   The current fixed header lengths for NVP and
    vat are 4 and 8 bytes, respectively.  It is interesting to note that
    G.764 has a total header overhead, including the LAPD data link layer,
    of only 8 bytes, as the voice transport is considered a network-layer
    protocol.  The overhead is split evenly between layers 2 and 3.

H. Schulzrinne                   Expires 03/01/94                   [Page 8]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

    Bandwidth efficiency can be achieved by transporting non-essential or
    slowly changing protocol state in optional fields or in a separate
    low-bandwidth control protocol.   Also, header compression [8] may be

international: Even now, audio and video conferencing tools are used far
    beyond the North American continent.  It would seem appropriate to give
    considerations to internationalization concerns, for example to allow
    for the European A-law audio companding and non-US-ASCII character sets
    in textual data such as site identification.

processing efficient: With arrival rates of on the order of 40 to 50
    packets per second for a single voice or video source, per-packet
    processing  overhead  may  become  a  concern,  particularly  if  the
    protocol is to be implemented on other than high-end workstations.
    Multiplication and division operations should be avoided where possible
    and fields should be aligned to their natural size, i.e., an n-byte
    integer is aligned on an n-byte multiple, where possible.

implementable now: Given the anticipated lifetime and experimental nature
    of the protocol, it must be implementable with current hardware and
    operating systems.  That does not preclude that hardware and operating
    systems geared towards real-time services may improve the performance
    or  capabilities  of  the  protocol,  e.g.,  allow  better  intermedia

3 Services

The services that may be provided by RTP are summarized below.  Note that
not all services have to be offered.  Services anticipated to be optional
are marked with an asterisk.

  o framing (*)

  o demultiplexing by conference/association (*)

  o demultiplexing by media source

  o demultiplexing by conference

  o determination of media encoding

  o playout synchronization between a source and a set of destinations

  o error detection (*)

  o encryption (*)

H. Schulzrinne                   Expires 03/01/94                   [Page 9]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

  o quality-of-service monitoring (*)

In the following sections, we will discuss how these services are reflected
in the proposed packet header.   Information to be conveyed within the
conference can be roughly divided into information that changes with every
data packet and other information that stays constant for longer time
periods.  State information that does not change with every packet can be
carried in several different ways:

as a fixed part of the RTP header: This method is easiest to decode and
    ensures state synchronization between sender and receiver(s), but can
    be bandwidth inefficient or restrict the amount of state information to
    be conveyed.

as a header option: The information is only carried when needed.    It
    requires more processing by the sending and receiving application.  If
    contained in every packet, it is also less bandwidth-efficient than the
    first method.

within RTCP packets: This approach is roughly equivalent to header options
    in terms of processing and bandwidth efficiency.    Some means of
    identifying when a particular option takes effect within the data
    stream may have to be provided.

within a multicast conference announcement: Instead of residing at a well-
    known  conference  server,  information  about  on-going  or  upcoming
    conferences may be multicast to a well-known multicast address.

within conference control: The  state  information  is  conveyed  when  the
    conference is established or when the information changes.  As for RTCP
    packets, a synchronization mechanism between data and control may be
    required for certain information.

through a conference directory: This is a variant of the conference control
    mechanism, with a (distributed) directory at a well-known (multicast)
    address maintaining state information about on-going or scheduled
    conferences.    Changing state information during a conference is
    probably more difficult than with conference control as participants
    need to be told to look at the directory for changed information.
    Thus, a directory is probably best suited to hold information that will
    persist through the life of the conference, for example, its multicast
    group, list of media encodings, title and organizer.

The first two methods are examples of in-band signaling, the others of
out-of-band signaling.

Options can be encoded in a number of ways, resulting in different tradeoffs
between flexibility,  processing overhead and space requirements.    In

H. Schulzrinne                  Expires 03/01/94                  [Page 10]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

general, options consists of a type field, possibly a length field, and
the actual option value.   The length field can be omitted if the length
is implied by the option type.   Implied-length options save space, but
require special treatment while processing.   While options with explicit
length that are added in later protocol versions are backwards-compatible
(the receiver can just skip them), implied-length options cannot be added
without modifying all receivers, unless they are marked as such and all have
a known length.  As an example, IP defines two implied-length options, no-op
and end-of-option, both with a length of one octet.  Both CLNP and IP follow
the type-length-data model, with different substructure of the type field.

For indicating the extent of options, a number of alternatives have been

option length: The fixed header contains a field containing the length of
    the options, as used for IP. This makes skipping over options easy, but
    consumes precious header space.

end-of-options bit: Each option contains a special bit that is set only for
    the last option in the list.  In addition, the fixed header contains
    a flag indicating that options are present.   This conserves space
    in the fixed header, at the expense of reducing usable space within
    options, e.g., reducing the number of possible option types or the
    maximum option length.  It also makes skipping options somewhat more
    processing-intensive, particulary if some options have implied lengths
    and others have explicit lengths.  Skipping through the options list
    can be accelerated slightly by starting options with a length field.

end-of-options option: A special option type indicates the end of the
    option list, with a bit in the fixed header indicating the presence of
    options.  The properties of this approach are similar to the previous
    one, except that it can be expected to take up more header space.

options directory: An options-present bit in the fixed header indicates
    the presence of an options directory.    The options directory in
    turn contains a length field for the options list and possibly bits
    indicating the presence of certain options or option classes.   The
    option length makes skipping options fast, while the presence bits
    allow a quick decision whether the options list should be scanned for
    relevant options.  If all options have a known, fixed length, the bit
    mask can be used to directly access certain options, without having
    to traverse parts of the options list.   The drawback is increased
    header space and the necessity to create the directory.  If options are
    explicitly coded in the bit mask, the type, number and numbering of
    options is restricted.  This approach is used by PIP [9].

H. Schulzrinne                  Expires 03/01/94                  [Page 11]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

3.1 Duplex or Simplex?

In terms of information flow, protocols can be roughly divided into three

1. For one instance of a protocol, packets travel only in one direction;
    i.e., the receiver has no way to directly influence the sender.  UDP is
    an example of such a protocol.

2. While data only travels in one direction, the receiver can send back
    control packets, for example, to accept or reject a connection, or
    request retransmission.   ST-II in its standard simplex mode is an
    example; TCP is symmetric (see next item), but during a file transfer,
    it typically operates in this mode, where one side sends data and the
    receiver of the data returns acknowledgements.

3. The protocol is fully symmetric during the data transfer phase, with
    user data and control information travelling in both directions.  TCP
    is a symmetric protocol.

Note that bidirectional data flow can usually be simulated by two or more
one-directional data flows in opposite directions, however, if the data
sinks need to transmit control information to the source, a decoupled stream
in the reverse direction will not do without additional machinery to bridge
the gap between the two protocol state machines.

For  most  of  the  anticipated  applications  for  a  real-time  transport
protocol, one-directional data flow appears sufficient.  Also, in general,
bidirectional flows may be difficult to maintain in one-to-many settings
commonly found in conferences.    Real-time requirements combined with
network latency make achieving reliability through retransmission difficult,
eliminating another reason for a bidirectional communication channel.  Thus,
we will focus only on control flow from the receiver of a data flow to its
sender.   For brevity, we will refer to packets of this control flow as
reverse control packets.

There are at least two areas within multimedia conferences where a receiver
needs to communicate control information back to the source.  First, the
sender may want or need to know how well the transmission is proceding,
as traditional feedback through acknowledgements is missing (and usually
infeasible due to acknowledgment implosion).  Secondly, the receiver should
be able to request a selective update of its state, for example, to obtain
missing image blocks after joining an on-going conference.  Note that for
both uses, unicast rather than multicast is appropriate.

Three approaches allowing the sender to distinguish reverse control packets
from data packets are compared here:

H. Schulzrinne                  Expires 03/01/94                  [Page 12]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

sender port equals reverse port, marked packet: The  same  port  number  is
    used both for data and return control messages.  Packets then have to
    be marked to allow distinguishing the two.   Either the presence of
    certain options would indicate a reverse control packet, or the options
    themselves would be interpreted as reverse control information, with
    the rest of the packet treated as regular data.  The latter approach
    appears to be the most flexible and symmetric, and is similar in
    spirit to transport protocols with piggy-backed acknowledgements as in
    TCP. Also, since several conferences with different multicast addresses
    may be using the same port number, the receiver has to include the
    multicast address in its reverse control messages.    As a final
    identification, the control packets have to bear the flow identifier
    they belong to.   The scheme has the grave disadvantage that every
    application on a host has to receive the reverse control messages and
    decide whether it involves a flow it is responsible for.

single reverse port: Reverse control packets for all flows use a single
    port that differs from the data port.  Since the type of the packet
    (control vs.    data) is identified by the port number, only the
    multicast address and flow number still needs to be included, without a
    need for a distinguishing packet format.  Adding a port means that port
    negotiation is somewhat more complicated; also, as in the first scheme,
    the application still has to demultiplex incoming control messages.

different reverse port for each flow: This method requires that each source
    makes it known to all receivers on which port it wishes to receive
    reverse control messages.  Demultiplexing based on flow and multicast
    address is no longer necessary.   However, each participant sending
    data and expecting return control messages has to communicate the port
    number to all other participants.   Since the reverse control port
    number should remain constant throughout the conference (except after
    application restarts), a periodic dissemination of that information is
    sufficient.  Distributing the port information has the advantage that
    it gives applications the flexibility to designate only certain flows
    as potential recipients of reverse control information.

    Unfortunately, the delay in acquiring the reverse control port number
    when  joining  an  on-going  conference  may  make  one  of  the  more
    interesting uses of a reverse control channel difficult to implement,
    namely the request by a new arrival to the sender to transmit the
    complete current state (e.g., image) rather than changes only.

3.2 Framing

To satisfy the goal of transport independence, we cannot assume that the
lower layer provides framing.   (Consider TCP as an example; it would
probably not be used for real-time applications except possibly on a local
network, but it may be useful in distributing recorded audio or video
segments.)  It may also be desirable to pack several RTPDUs into a single

H. Schulzrinne                  Expires 03/01/94                  [Page 13]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993


The obvious solution is to provide for an optional message length prefixed
to the actual packet.    If the underlying protocol does not message
delineation, both sender and receiver would know to use the message length.
If used to carry multiple RTPDUs, all participants would have to arrive
at a mutual agreement as to its use.   A 16-bit field should cover most
needs, but appears to break the 4-byte alignment for the rest of the header.
However, an application would read the message length first and then copy
the appropriate number of bytes into a buffer, suitably aligned.

3.3 Version Identification

Humility suggests that we anticipate that we may not get the first iteration
of the protocol right.   In order to avoid ``flag days'' where everybody
shifts to a new protocol, a version identifier could ensure continued
interoperability.  Alternatively, a new port could be used, as long as only
one port (or at most a few ports) is used for all media.  The difficulty in
interworking between the current vat and NVP protocols further affirms the
desirability of a version identifier.  However, the version identifier can
be anticipated to be the most static of all proposed header fields.  Since
the length of the header and the location and meaning of the option length
field may be affected by a version change, encoding the version within an
optional field is not feasible.

Putting the version number into the control protocol packets would make RTCP
mandatory and would make rapid scanning of conferences significantly more

vat currently offers a 2-bit version field, while this capability is missing
from NVP. Given the low bit usage and their utility in other contexts (IP,
ST-II), it may be prudent to include a version identifier.  To be useful,
any version field must be placed at the very beginning of the header.
Assigning an initial version value of one to RTP allows interoperability
with the current vat protocol.

3.4 Conference Identification

A conference identifier (conference ID) could serve two mutually exclusive
functions:   providing another level of demultiplexing or a means of
logically aggregating flows with different network addresses and port
numbers.  vat specifies a 16-bit conference identifier.

H. Schulzrinne                  Expires 03/01/94                  [Page 14]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

3.4.1 Demultiplexing

Demultiplexing by RTP allows one association characterized by destination
address and port number to carry several distinct conferences.   However,
this appears to be necessary only if the number of conferences exceeds the
demultiplexing capability available through (multicast) addresses and port

Efficiency arguments suggest that combining several conferences or media
within a single multicast group is not desirable.    Combining several
conferences or media within a single multicast address reduces the bandwidth
efficiency  afforded  by  multicasting  if  the  sets  of  destinations  are
different.   Also, applications that are not interested in a particular
conference or capable of dealing with particular medium are still forced to
handle the packets delivered for that conference or medium.  Consider as an
example two separate applications, one for audio, one for video.  If both
share the same multicast address and port, being differentiated only by the
conference identifier, the operating system has to copy each incoming audio
and video packet into two application buffers and perform a context switch
to both applications, only to have one immediately discard the incoming

Given that application-layer demultiplexing has strong negative efficiency
implications and given that multicast addresses are not an extremely
scarce commodity, there seems to be no reason to burden every application
with maintaining and checking conference identifiers for the purpose of
demultiplexing.   However, if this protocol is to be used as a transport
protocol, demultiplexing capability is required.

It is also not recommended to use a conference identifier to distinguish
between different encodings, as it would be difficult for the application
to decide whether a new conference identifier means that a new conference
has arrived or simply all participants should be moved to the new conference
with a different encoding.   Since the encoding may change for some but
not all participants, we could find ourselves breaking a single logical
conference into several pieces, with a fairly elaborate control mechanism to
decide which conferences logically belong together.

3.4.2 Aggregation

Particularly within a network with a wide range of capacities, differing
multicast groups for each media component of a conference allows to
tailor the media distribution to the network bandwidths and end-system
capabilities.  It appears useful, however, to have a means of identifying
groups that logically belong together, for example for purposes of time

A conference identifier used in this manner would have to be globally

H. Schulzrinne                  Expires 03/01/94                  [Page 15]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

unique.  It appears that such logical connections would better be identified
as part of the higher-layer control protocol by identifying all multicast
addresses belonging to the same logical conference, thereby avoiding the
assignment of globally unique identifiers.

3.5 Media Encoding Identification

This field plays a similar role to the protocol field in data link
or network protocols, indicating the next higher layer (here, the media
decoder) that the data is meant for.  For RTP, this field would indicate the
audio or video or other media encoding.  In general, the number of distinct
encodings should be kept as small as possible to increase the chance that
applications can interoperate.   A new encoding should only be recognized
if it significantly enhances the range of media quality or the types of
networks conferences can be conducted over.  The unnecessary proliferation
of encodings can be reduced by making reference implementations of standard
encoders and decoders widely available.

It should be noted that encodings may not be enumerable as easily as, say,
transport protocols.  A particular family of related encoding methods may
be described by a set of parameters, as discussed below in the sections on
audio and video encoding.

Encodings may change during the duration of a conference.   This may be
due to changed network conditions, changed user preference or because the
conference is joined by a new participant that cannot decode the current
encoding.    If the information necessary for the decoder is conveyed
out-of-band, some means of indicating when the change is effective needs to
be incorporated.  Also, the indication that the encoding is about to change
must reach all receivers reliably before the first packet employing the new
encoding.   Each receiver needs to track pending changes of encodings and
check for every incoming packet whether an encoding change is to take effect
with this packet.

Conveying media encodings rapidly is also important to allow scanning of
conferences or broadcast media.  Note that it is not necessary to convey
the whole encoder description, with all parameters; an index into a table of
well-known encodings is probably preferable.  An index would also make it
easier to detect whether the encoding has changed.

Alternatively, a directory or announcement service could provide encoding
information for on-going conferences, without carrying the information in
every packet.  This may not be sufficient, however, unless all participants
within a conference use the same encoding.    As soon as the encoding
information is separated from the media data, a synchronization mechanism
has to be devised that ensures that sender and receiver interpret the data
in the same manner after the out-of-band information has been updated.

There are at least two approaches to indicating media encoding, either

H. Schulzrinne                  Expires 03/01/94                  [Page 16]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

in-band or out-of-band:

conference-specific: Here, the media identifier is an index into a table
    designating the approved or anticipated encodings (together with any
    particular version numbers or other parameters) for a particular
    conference  or  user  community.     The  table  can  be  distributed
    through RTCP, a higher-layer conference control protocol, a conference
    announcement service or some other out-of-band means.  Since the number
    of encodings used during a single conference is likely to be small, the
    field width in the header can likewise be small.  Also, there is no
    need to agree on an Internet-wide list of encodings.   It should be
    noted that conveying the table of encodings through RTCP forces the
    application to maintain a separate mapping table for each sender as
    there can be no guarantee that all senders will use the same table.
    Since the control protocol proposed here is unreliable, changing the
    meaning of encoding indices dynamically is fraught with possibilities
    for misinterpretation and lost data unless this mapping is carried in
    every packet.

global: Here,  the media identifier is an index into a global table
    of encodings.    A global list reduces the need for out-of-band
    information.  Transmitting the parameters associated with an encoding
    may be difficult, however, if it has to be done within the header space
    constraints of per-packet signaling.

To make detecting coder mismatches easier, encodings for all media should
be drawn from the same numbering space.  To facilitate experimentation with
new encodings, a part of any global encoding numbering space should be
set aside for experimental encodings, with numbers agreed upon within the
community experimenting with the encoding, with no Internet-wide guarantee
of uniqueness.

3.5.1 Audio Encodings

Audio data is commonly characterized by three independent descriptors:
encoding (the translation of one or more audio samples into a channel
symbol), the number of channels (mono, stereo, :::) and the sampling rate.

Theoretically, sampling rate and encoding are (largely) independent.   We
could, for example, apply mu-law encoding to any sampling rate even though
it is traditionally used with a rate of 8,000 Hz.  In practical terms, it
may be desirable to limit the combinations of encoding and sampling rate to
the values the encoding was designed for.(2)    Channel counts between 1 and
 2. Given the wide availability of mu-law encoding and its low overhead,
using it with a sampling rate of 16,000 or 32,000 Hz might be quite

H. Schulzrinne                  Expires 03/01/94                  [Page 17]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

6 should be sufficient even for surround sound.

The audio encodings listed in Table 1 appear particularly interesting,
even though the list is by no means exhaustive and does not include some
experimental encodings currently in use, for example a non-standard form of
LPC. The bit rate is shown per channel.   k samples/s, b/sample and kb/s
denote kilosamples per second, bits per sample and kilobits per second,
respectively.  If sampling rates are to be specified separately, the values
of 8, 16, 32, 44.1, and 48 kHz suggest themselves, even though other
values (11.025 and 22.05 kHz) are supported on some workstations (the
Silicon Graphics audio hardware and the Apple Macintosh, for example).
Clearly, little is to be gained by allowing arbitrary sampling rates, as
conversion particularly between rates not related by simple fractions is
quite cumbersome and processing-intensive [10].

Org.     Name    k samples/s b/sample  kb/s description
CCITT    G.711           8.0        8    64 mu-law PCM
CCITT    G.711           8.0        8    64 A-law PCM
CCITT    G.721           8.0        4    32 ADPCM
Intel    DVI             8.0        4    32 APDCM
CCITT    G.723           8.0        3    24 ADPCM
CCITT    G.726                                 ADPCM
CCITT    G.727                                 ADPCM
NIST/GSA FS 1015         8.0             2.4 LPC-10E
NIST/GSA FS 1016         8.0             4.8 CELP
NADC     IS-54           8.0            7.95 N. American Digital Cellular, VSELP
CCITT    G.728           8.0              16 LD-CELP
GSM                       8.0              13 RPE-LTP
CCITT    G.722           8.0              64 7 kHz, SB-ADPCM
ISO      3-11172                          256 MPEG audio
                          32.0       16   512 DAT
                          44.1       16 705.6 CD, DAT playback
                          48.0       16   786 DAT record

             Table 1:  Standardized and common audio encodings
appropriate for high-quality audio conferences, even though there are other
encodings, such as G.722, specifically designed for such applications.  Note
that the signal-to-noise ratio of mu-law encoding is about 38 dB, equivalent
to an AM receiver.  The ``telephone quality'' associated with G.711 is due
primarily to the limitation in frequency response to the 200 to 3500 Hz

H. Schulzrinne                  Expires 03/01/94                  [Page 18]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

3.5.2 Video Encodings

Common video encodings are listed in Table 2.  Encodings with tunable rate
can be configured for different rates, but produce a fixed-rate stream.
The average bit rate produced by variable-rate codecs depends on the source

        Org.       name     rate                remarks
        CCITT      JPEG     tunable
        CCITT      MPEG     variable, tunable
        CCITT      H.261    tunable, px64 kb/s
        Bolter               variable, tunable
        PictureTel           ??
        Cornell U. CU-SeeMe variable
        Xerox Parc nv       variable, tunable
        BBN        DVC      variable, tunable   block differences

                      Table 2:  Common video encodings

3.6 Playout Synchronization

A major purpose of RTP is to provide the support for various forms of
synchronization, without necessarily performing the synchronization itself.
We can distinguish three kinds of synchronization:

playout synchronization: The receiver plays out the medium a fixed time
    after it was generated at the source (end-to-end delay).    This
    end-to-end delay may vary from synchronization unit to synchronization
    unit.  In other words, playout synchronization assures that a constant
    rate source at the sender again becomes a constant rate source at the
    receiver, despite delay jitter in the network.

intra-media synchronization: All receivers play the same segment of a
    medium at the same time.   Intra-media synchronization may be needed
    during simulations and wargaming.

inter-media synchronization: The timing relationship between several media
    sources is reconstructed at the receiver.   The primary example is
    the synchronization between audio and video (lip-sync).   Note that
    different receivers may experience different delays between the media
    generation time and their playout time.

Playout synchronization is required for most media, while intra-media and
inter-media synchronization may or may not be implemented.  In connection

H. Schulzrinne                  Expires 03/01/94                  [Page 19]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

with playout synchronization, we can group packets into playout units, a
number of which in turn form a synchronization unit.  More specifically, we

synchronization unit: A  synchronization  unit  consists  of  one  or  more
    playout units (see below) that, as a group, share a common fixed delay
    between generation and playout of each part of the group.  The delay
    may change at the beginning of such a synchronization unit.  The most
    common synchronization units are talkspurts for voice and frames for
    video transmission.

playout unit: A playout unit is a group of packets sharing a common
    timestamp.   (Naturally, packets whose timestamps are identical due
    to timestamp wrap-around are not considered part of the same playout
    unit.)  For voice, the playout unit would typically be a single voice
    segment, while for video a video frame could be broken down into
    subframes, each consisting of packets sharing the same timestamp and
    ordered by some form of sequence number.

Two concepts related to synchronization and playout units are absolute and
relative timing.   Absolute timing maintains a fixed timing relationship
between sender and receiver, while relative timing ensures that the spacing
between packets at the sender is the same as that at the receiver, measured
in terms of the sampling clock.  Playout units within the synchronization
unit maintain relative timing with respect to each other; absolute timing is
undesirable if the receiver clock runs at a (slightly) different rate than
the sender clock.

Most proposed synchronization methods require a timestamp.  The timestamp
has to have a sufficient range that wrap-arounds are infrequent.    It
is desirable that the range exceeds the maximum expected inactive (e.g.,
silence) period.  Otherwise, if the silence period lasts a full timestamp
range, the first packet of the next talkspurt would have a timestamp one
larger than the last packet of the current talkspurt.  In that case, the
new talkspurt could not be readily discerned if the difference in increment
between timestamps and sequence numbers is used to detect a new talkspurt.

The 10-bit timestamp used by NVP is generally agreed to be too small as it
wraps around after only 20.5 s (for 20 ms audio packets), while a 32-bit
timestamp should serve all anticipated needs, even if the timestamp is
expressed in units of samples or other sub-packet entities.

A timestamp may be useful not only at the transport, but also at the network
layer, for example, for scheduling packets based on urgency.  The playout
timestamp would be appropriate for such a scheduling timestamp, as it would
better reflect urgency than a network-level departure timestamp.  Thus, it
may make sense to use a network-level timestamp such as the one provided by
ST-II at the transport layer.

H. Schulzrinne                  Expires 03/01/94                  [Page 20]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

3.6.1 Synchronization Methods

The necessary header components are determined to some extent by the method
of synchronizing sender and receivers.    In this section, we formally
describe some of the popular approaches, building on the exposition and
terminology of Montgomery [11].

We define a number of variables describing the synchronization process.  In
general, the subscript n represents the nth packet in a synchronization
unit, n=1;2;:::.  Let a , d , p  and t  be the arrival time, variable
                          n   n   n      n
delay, playout time and generation time of the nth packet, respectively.
Let o denote the fixed delay from sender to receiver.   Finally, d
describes the estimated maximum variable delay within the network.   The
estimate is typically chosen in such a way that only a very small fraction
(on the order of 1%) of packets take more than o+d    time units.  For best
performance under changing network load conditions, the estimate should be
refined based on the actual delays experienced.  The variable delay in a
network consists of queueing and media access delays, while propagation and
processing delays make up the fixed delay.   Additional end-to-end fixed
delay is unavoidably introduced by packetization; the non-real-time nature
of most operating systems adds a variable delay both at the transmitting and
receiving end.   All variables are expressed in sample unit of time, be
that seconds or samples, for example.  For simplicity, we ignore that the
sender and receiver clocks may not run at exactly the same speed.   The
relationship between the variables is depicted in Fig. 2.  The arrows in the
figure indicate the transmission of the packet across the network, occurring
after the packetization delay.  The packet with sequence number 5 misses the
playout deadline and, depending on the algorithm used by the receiver, is
either dropped or treated as the beginning of a new talkspurt.

Figure only available in PostScript version of document.
                Figure 2:  Playout Synchronization Variables

Given the above definitions, the relationship

                               a =t +d +o                            (1)
                                n  n  n
holds for every packet.  For brevity, we also define l  as the ``laxity''
of packet n, i.e., the time p -a  between arrival and playout.  Note that
                             n  n
it may be difficult to measure a  with resolution below a packetization
interval, particularly if the measurement is to be in units related to the
playback process (e.g., samples).  All synchronization methods differ only
in how much they delay the first packet of a synchronization unit.   All
packets within a synchronization unit are played out based on the position
of the first packet:
                       p =p   +(t -t   ) for n>1
                        n          n
                            n-1       n-1
Three synchronization methods are of interest.  We describe below how they
compute the playout time for the first packet in a synchronization unit and

H. Schulzrinne                  Expires 03/01/94                  [Page 21]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

what measurement is used to update the delay estimate d   .

blind delay: This method assumes that the first packet in a talkspurt
    experiences only the fixed delay, so that the full d    has to be
    added to allow for other packets within the talkspurt experiencing more
                                 p =a +d   :                          (2)
                                  1  1
    The estimate for the variable delay is derived from measurements
    of the laxity l ,  so that the new estimate after n packets is
    computed d     =f(l ;:::;l ), where the function f(.) is a suitably
              max;n             n
    chosen smoothing function.   Note that blind delay does not require
    timestamps to determine p , only an indication of the beginning of
    a synchronization unit.   Timestamps may be required to compute p ,
    however, unless t -t    is a known constant.
absolute timing: If the packet carries a timestamp measured in time units
    known to the receiver, we can improve our determination of the playout
                                p =t +o+d   :
                                 1  1
    This is, clearly, the best that can be accomplished.  Here, instead of
    estimating d   , we estimate o+d    as some function of p -t .  For
                max                  max                       n  n
    this computation, it does not matter whether p and t are measured with
    clocks sharing a common starting point.

added variable delay: Each node adds the variable delay experienced within
    it to a delay accumulator within the packet, yielding d .
                                p =a -d +d
                                 1  1  1
    From Eq. 1, it is readily apparent that absolute delay and added
    variable delay yield the same playout time.  The estimate for d    is
    based on the measurements for d.   Given a clock with suitably high
    resolution, these estimates can be better than those based on the
    difference between a and p; however, it requires that all routers can
    recognize RTP packets.  Also, determining the residence time within a
    router may not be feasible.

In summary, absolute timing is to be preferred due to its lower delays
compared to blind delay, while synchronization using added variable delays
is currently not feasible within the Internet (it is, however, used for

3.6.2 Detection of Synchronization Units

The receiver must have a way of readily detecting the beginning of a
synchronization unit, as the playout scheduling of the first packet in a
synchronization unit differs from that in the remainder of the unit.  This

H. Schulzrinne                  Expires 03/01/94                  [Page 22]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

detection has to work reliably even with packet reordering; for example,
reordering at the beginning of a talkspurt is particularly likely since
common silence detection algorithms send a group of stored packets at the
beginning of the talkspurt to prevent front clipping.

Two basic methods have been proposed:

timestamp and sequence number: The sequence number increases by one with
    each packet transmitted, while the timestamp reflects the total time
    covered, measured in some appropriate unit.  A packet is declared to
    start a new synchronization unit if (a) it has the highest timestamp
    and sequence number seen so far (within this wraparound cycle) and
    (b) the difference in timestamp values (converted into a packet count)
    between this and the previous packet is greater than the difference in
    sequence number between those two packets.

    This approach has the disadvantage that it may lead to erroneous packet
    scheduling with blind delay if packets are reordered.  An example is
    shown in Table 3.  In the example, the playout delay is set at 50 time
    units for blind timing and 550 time units for absolute timing.   The
    packet intergeneration time is 20 time units.

                               blind timing             absolute timing
                      no reordering   with reordering
   seq. timestamp arrival playout arrival playout arrival playout
    200      1020    1520    1570    1520    1570    1520    1570
    201      1040    1530    1590    1530    1590    1530    1590
    202      1220    1720    1770    1725    1750    1725    1770
    203      1240    1725    1790    1720    1770    1720    1790
    204      1260    1792    1810    1791    1790    1791    1810

Table 3:  Example where out-of-order arrival leads to packet loss for blind

    More significantly, detecting synchronization units requires that the
    playout mechanism can translate timestamp differences into packet
    counts,  so  that  it  can  compare  timestamp  and  sequence  number
    differences.   If the timespan ``covered'' by a packet changes with
    the encoding or even varies for each packet, this may be cumbersome.
    NVP provides the timestamp/sequence number combination for detecting
    talkspurts.  The following method avoids these drawbacks, at the cost
    of one additional header bit.

synchronization bit: The beginning of a synchronization unit is indicated
    by setting a synchronization bit within the header.   The receiver,
    however, can only use this information if no later packet has already
    been processed.    Thus,  packet reordering at the beginning of a
    talkspurt leads to missing opportunities for delay adjustment.   With

H. Schulzrinne                  Expires 03/01/94                  [Page 23]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

    the synchronization bit, a sequence number is not necessary to detect
    the beginning of a synchronization unit, but a sequence number remains
    useful for detecting packet loss and ordering packets bearing the same
    timestamp.  With just a timestamp, it is impossible for the receiver
    to get an accurate count of the number of packets that it should have
    received.  While gaps within a talkspurt give some indication of packet
    loss, the receiver cannot tell what part of the tail of a talkspurt
    has been transmitted.   (Example:  consider the talkspurts with time
    stamps 100, 101, 102, 110, 111.  Packets with timestamp 100 and 110
    have the synchronization bit set.  The receiver has no way of knowing
    whether it was supposed to have received two talkspurts with a total of
    five packets, or two or more talkspurts with up to 12 packets.)  The
    synchronization bit is used by vat, without a sequence number.  It is
    also contained in the original version of NVP [12].  A special sequence
    number, as used by G.764, is equivalent.

3.6.3 Interpretation of Synchronization Bit

Two possibilities for implementing a synchronization bit are discussed here.

start of synchronization unit: The first packet in a synchronization unit
    is  marked  with  a  set  synchronization  bit.     With  this  use  of
    the synchronization bit,  the receiver detects the beginning of a
    synchronization unit with the following simple algorithm:

      if synchronization bit = 1
         and current sequence number > maximum sequence number seen so far
        this packet starts a new synchronization unit

      if current sequence number > maximum sequence number
        maximum sequence number := current sequence number

    Comparisons and arithmetic operations are modulo the sequence number

end of synchronization unit: The last packet in a synchronization unit is
    marked.   As pointed out elsewhere, this information may be useful
    for initiating appropriate fill-in during silence periods and to start
    processing a completed video frame.  If a voice silence detector uses
    no hangover, it may have difficulty deciding which is the last packet
    in a talkspurt until it judges the first packet to contain no speech.
    The detection of a new synchronization unit by the receiver is only
    slightly more complicated than with the previous method:

H. Schulzrinne                  Expires 03/01/94                  [Page 24]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

      if sync_flag then
        if sequence number >= sync_seq then
          sync_flag := FALSE
        if sequence number = sync_seq then
          signal beginning of synchronization unit

      if synchronization bit = 1 then
        sync_seq  := sequence number + 1
        sync_flag := TRUE

    By changing the equal sign in the second comparison to 'if sequence
    number > syncseq', a new synchronization unit is detected even if
    packets at the beginning of the synchronization unit are reordered.  As
    reordering at the beginning of a synchronization unit is particularly
    likely,  for  example  when  transmitting  the  packets  preceding  the
    beginning of a talkspurt, this should significantly reduce the number
    of missed talkspurt beginnings.

3.6.4 Interpretation of Timestamp

Several proposals as to the interpretation of the timestamp have been

packet or frame interval: Each packetization or (video/audio) frame inter-
    val increments the timestamp.  This approach very efficient in terms
    of processing and bit-use, but cannot be used without out-of-band
    information if the time interval of media ``covered'' by a packet
    varies  from  packet  to  packet.     This  occurs  for  example  with
    variable-rate encoders or if the packetization interval is changed
    during a conference.  This interpretation of a timestamp is assumed by
    NVP, which defines a frame as a block of PCM samples or a single LPC
    frame.  Note that there is no inherent necessity that all participants
    within a conference use the same packetization interval.    Local
    implementation considerations such as available clocks may suggest
    different intervals.   As another example, consider a conference with
    feedback.   For the lecture audio, a long packetization interval may
    be desirable to better amortize packet headers.    For side chats,
    delays are more important, thus suggesting a shorter packetization
 3. Nevot  for example,  allows each participant to have a different
packetization interval, independent of the packetization interval used by

H. Schulzrinne                  Expires 03/01/94                  [Page 25]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

sample: This method simply counts samples, allowing a direct translation
    between time stamp and playout buffer insertion point.   It is just
    as easily computable as the per-packet timestamp.  However, for some
    media and encodings(4) , it may not be quite clear what a sample is.
    Also, some care must be taken at the receiver and sender if streams use
    different sampling rates.  This method is currently used by vat.

Milliseconds: A timestamp incremented every millisecond would wrap around
    once  every  49  days.     The  resolution  is  sufficient  for  most
    applications,  except  that  the  natural  packetization  interval  for
    LPC-coded speech is 22.5 ms.  Also, with a video frame rate of 30 Hz,
    an internal timestamp of higher resolution would need to be truncated
    to millisecond resolution to approximate 33.3 ms intervals.  This time
    increment has the advantage of being used by some Unix delay functions,
    which might be useful for playing back video frames with proper timing.
    It might be useful to take the second value from the current system
    clock to allow delay estimates for synchronized clocks.

subset of NTP timestamp: 16 bits encode seconds relative to midnight (0
    hours), January 1, 1900 (modulo 65536) and 16 bits encode fractions of
    a second, with a resolution of approximately 15.2 microseconds, which
    is smaller than any anticipated audio sampling or video frame interval.
    This timestamp is the same as the middle 32 bits of the 64-bit NTP
    timestamp [13].  It wraps around every 18.2 hours.  If it should be
    desirable to reconstruct absolute transmission time at the receiver for
    logging or recording purposes, it should be easy to determine the most
    significant 16 bits of the timestamp.  Otherwise, wrap-arounds are not
    a significant problem as long as they occur 'naturally', i.e., at a 16
    or 32 bit boundary, so that explicit checking on arithmetic operations
    is not required.  Also, since the translation mechanism would probably
    treat the timestamp as a single integer without accounting for its
    division into whole and fractional part, the exact bit allocation
    between seconds and fractions thereof is less important.   However,
    the 16/16 approach simplifies extraction from a full NTP timestamp.
    Sixteen bits of fractional seconds also allows a timestamp without
    wrap-around, i.e, with 32 bits of full seconds encoding time since
    January 1, 1990, to fit into the 52 bits of a IEEE floating point

    The NTP-like timestamp has the disadvantage that its resolution does
    not map into any of the common sample or packetization intervals.
    Thus, there is a potential uncertainty of one sample at the receiver
Nevot for its outgoing audio.  Only the packetization interval for outgoing
audio for all conferences this Nevot participates in must be the same.
 4. Examples include frame-based encodings such as LPC and CELP. Here, given
that these encodings are based on 8,000 Hz input samples, the preferred
interpretation would probably be in terms of audio samples, not frames, as
samples would be used for reconstruction and mixing.

H. Schulzrinne                  Expires 03/01/94                  [Page 26]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

    as to where to place the beginning of the received packet, resulting
    in the equivalent of a one-sample slip.   CCITT recommendation G.821
    postulates a mean slip rate of less than 1 slip in 5 hours, with
    degraded but acceptable service for less than 1 slip in 2 minutes.
    Tests with appropriate rounding conducted by the author showed that
    this uncertainty is not likely to cause problems.   In any event, a
    double-precision floating point multiplication is needed to translate
    between this timestamp and the integer sample count available on
    transmission and required for playout.(5)

MPEG timestamps: MPEG uses a 33 bit clock with a resolution of 90 kHz [14]
    as the system clock reference and for presentation time stamps.  The
    frequency was chosen based on the divisibility by the nominal video
    picture rates of 24 Hz, 25 Hz, 29.97 Hz and 30 Hz [14, p.42].   The
    frequency would also fit nicely with the 20 ms audio packetization
    interval.  The length of 33 bit is clearly inappropriate, however, for
    software implementations.  32 bit timestamps still cover more than half
    a day and thus can be readily extended to full unique timestamps or 33
    bits if needed.

Microseconds: A 32-bit timestamp incremented every microsecond wraps around
    once every 71.5 minutes.  The resolution is high enough that round-off
    errors for video frame intervals and such should be tolerable without
    maintaining a higher-precision internal counter.   This resolution is
    also provided, at least nominally, by the Unix gettimeofday() system

QuickTime: The Apple QuickTime file format is a generalization of the
    previous formats as it combines a 32-bit counter with a 32-bit media
    time scale expressed in time units per second.   The four previously
    mentioned timestamps can be represented by time scales of 1000, 65536,
    90,000 and 1,000,000.  For the sample and packet-based case, the value
    would depend on the media content, e.g., 8,000 for standard PCM-coded

Timestamps based on wallclock time rather than samples or frames have the
advantage that a receiver does not necessarily need to know about the
meaning of the encoding contained in the packet in order to process the
timestamp.   For example, a quality-of-service monitor within the network
could measure delay variance easily, without caring what kind of audio
information, say, is contained in the packet.   Other tools, such as a
recording and playback tool, can also be written without concern about the
mapping between timestamp and wallclock units.
 5. The multiplication with an appropriate factor can be approximated
to the desired precision by an integer multiplication and division, but
multiplication by a floating point value is actually much faster on some
modern processors.

H. Schulzrinne                  Expires 03/01/94                  [Page 27]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

A time stamp could reflect either real time or sample time.  A real time
timestamp is defined to track wallclock time plus or minus a constant
offset.   Sample time increases by the nominal sampling interval for each
sample.  The two clocks in general do not agree since the clock source used
for sampling will in all likelihood be slightly off the nominal rate.  For
example, typical crystals without temperature control are only accurate to
 50 -- 100 ppm (parts per million), yielding a potential drift of 0.36
seconds per hour between the sampling clock and wallclock time.

It has been suggested to use timestamps relative to the beginning of
first transmission from a source.   This makes correlation between media
from different participants difficult and seems to have no technical or
implementation advantages,  except for avoiding wrap-around during most
conferences.   As pointed out above, that seems to be of little benefit.
Clearly, the reliability of a wallclock-synchronized timestamps depends on
how closely the system clocks are synchronized, but that does not argue for
giving up potential real-time synchronization in all cases.

Using real time rather than sample time allows for easier synchronization
between different media and users (e.g., during playback of a recorded
conference) and to compensate for slow or fast sample clocks.  Note that it
is neither desirable nor necessary to obtain the wall clock time when each
packet was sampled.  Rather, the sender determines the wallclock time at the
beginning of each synchronization unit (e.g., a talkspurt for voice and a
frame for video) and adds the nominal sample clock duration for all packets
within the talkspurt to arrive at the timestamp value carried in packets.
The real time at the beginning of a talkspurt is determined by estimating
the true sample rate for the duration of the conference.

The sample rate estimate has to be accurate enough to allow placing the
beginning of a talkspurt, say, to within at most 50 to 100 ms, otherwise the
lack of synchronization may be noticeable, delay computations are confused
and successive talkspurts may be concatenated.

Estimating the true sampling instant to within a few milliseconds is
surprisingly difficult for current operating systems.  The sample rate r can
to be estimated as
                                 r=     :
Here, t is the current time, t  the time elapsed since the first sample
was acquired, s is the number of samples read, q is the number of samples
ready to be read (queued) at time t.  Let p denote the number of samples
in a packet.   The timestamp in the synchronization packet reflects the
sampling instant of the first sample of that packet and is computed as
t-(p+q)=r.  Unfortunately, only s and p are known precisely.  The accuracy
of the estimate for t  and t depend on how accurately the beginning of
sampling and the last reading from the audio device can be measured.  There
is a non-zero probability that the process will get preempted between the
time the audio data is read and the instant the system clock is sampled.
It remains unclear whether indications of current buffer occupancy, if
available, can be trusted.  Even with increasing sample count, the absolute

H. Schulzrinne                  Expires 03/01/94                  [Page 28]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

accuracy of the timestamp is roughly the same as the measurement accuracy of
t, as differentiating with respect to t shows.  Experiments with the SunOS
audio driver showed significant variations of the estimated sample rate,
with discontinuities of the computed timestamps of up to 25 ms.   Kernel
support is probably required for meaningful real time measurements.

Sample time increments with the sampling interval for every sample or
(sub)frame received from the audio or video hardware.   It is easy to
determine, as long as care is taken to avoid cumulative round-off errors
incurred by simply repeatedly adding the approximate packetization interval.
However, synchronization between media and end-to-end delay measurements are
then no longer feasible.  (Example:  Consider an audio and a video stream.
If the audio sample clock is slightly faster than the real clock and the
video sampling clock, a video and audio frame belonging together would be
marked by different timestamps, thus played out at different instants.)

If we choose to use sample time, the advantage of using an NTP-format
timestamp  disappears,  as  the  receiver  can  easily  reconstruct  a  NTP
sample-based timestamp from the sample count if needed, but would not have
to if no cross-media synchronization is required.   RTCP could relate the
time increment per sample in full precision.  The definition of a ``sample''
will depend on the particular medium, and could be a audio sample, a video
or a voice frame (as produced by a non-waveform coder).  The mapping fails
if there is no time-invariant mapping between sample units and time.

It should be noted that it may not be possible to associate an meaningful
notion of time with every packet.   For example, if a video frame is
broken into several fragments, there is no natural timestamp associated
with anything but the first fragment, particularly if there is not even
a sequential mapping from screen scan location into packets.   Thus, any
timestamp used would be purely artificial.  A synchronization bit could be
used in this particular case to mark beginning of synchronization units.
For packets within synchronization units, there are two possible approaches:
first, we can introduce an auxiliary sequence number that is only used to
order packets within a frame.  Secondly, we could abuse the timestamp field
by incrementing it by a single unit for each packet within the frame, thus
allowing a variable number of frames per packet.  The latter approach is
barely workable and rather kludgy.

3.6.5 End-of-talkspurt indication

An end-of-talkspurt indication is useful to distinguish silence from lost
packets.   The receiver would want to replace silence by an appropriate
background noise level to avoid the ``noise-pumping'' associated with
silence  detection.     On  the  other  hand,  missing  packets  should  be
reconstructed from previous packets.   If the silence detector makes use
of hangover, the transmitter can easily set the end-of-talkspurt indicator
on the last bit of the last hangover packet.   If the talkspurts follow
end-to-end, the end-of-talkspurt indicator has no effect except in the

H. Schulzrinne                  Expires 03/01/94                  [Page 29]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

case where the first packet of a talkspurt is lost.   In that case, the
indicator would erroneously trigger noise fill instead of loss recovery.
The end-of-talkspurt indicator is implemented in G.764 as a ``more'' bit
which is set to one for all but the last packet within a talkspurt.

3.6.6 Recommendation

Given the ease of cross-media synchronization and the media independence,
the use of 32-bit 16/16 timestamps representing the middle part of the NTP
timestamp is suggested.   Generally, a wallclock-based timestamp appears
to be preferable to a sample-based one, but it may only be approximately
realizable for some current operating systems.  Inter-media synchronization
to below 10 to 20 ms has to await mechanisms that can accurately determine
when a particular sample was actually received by the A/D converter.
Particularly with sample- or wallclock-based timestamp, a synchronization
bit simplifies the detection of the beginning of a synchronization unit.
Indicating either the end or beginning of a synchronization unit is roughly
equivalent, with tradeoffs between the two.

3.7 Segmentation and Reassembly

For high-bandwidth video, a single frame may not fit into the maximum
transport unit (MTU). Thus, some form of frame sequence number is needed.
If possible, the same sequence number should be used for synchronization and
fragmentation.  Six possibilities suggest themselves:

overload the timestamp: No sequence number is used.   Within a frame, the
    timestamp has no meaning.  Since it is used for synchronization only
    when the synchronization bit is set, the other timestamps can just
    increase by one for each packet.   However, as soon as the first
    frame gets lost or reordered, determining positions and timing becomes
    difficult or impossible.

packet count: The sequence number is incremented for every packet, without
    regard to frame boundaries.  If a frame consists of a variable number
    of packets, it may not be clear what position the packet occupies
    within the frame if packets are lost or reordered.  Continuous sequence
    numbers make it possible to determine if all packets for a particular
    frame have arrived, but only after the first packet of the next frame,
    distinguished by a new timestamp, has arrived.

packet count within a frame: The sequence number is reset to zero at the
    beginning of each frame.  This approach has properties complementary to
    continuous sequence numbers.

H. Schulzrinne                  Expires 03/01/94                  [Page 30]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

packet count and first-packet sequence number: Packets use a continuously
    incrementing sequence number plus an option field in every packet
    indicating the initial sequence number within the playout unit(6) .
    Carrying both a continuous and packet-within-frame count achieves the
    same effect.

packet count with last-packet sequence number: Packets carry a continuous
    sequence number plus an option in every packet indicating the last
    sequence number within the playout unit.  This has the advantage that
    the receiver can readily detect when the last packet for a playout unit
    has been received.   The transmitter may not know, however, at the
    beginning of a playout unit how many packets it will comprise.  Also,
    the position within the playout unit is more difficult to determine if
    the initial packet and the previous frame is lost.

packet count and frame count: The sequence number counts packets, without
    regard to frame boundaries.  A separate counter increments with each
    frame.  Detecting the end of a frame is delayed until the first packet
    belonging to the next frame.   Also, the frame count cannot help to
    determe the position of the packet within a frame.

It could be argued that encoding-specific location information should be
contained within the media part, as it will likely vary in format and use
from one media to the next.  Thus, frame count, the sequence number of the
last or first packet in a frame etc.  belong into the media-specific header.

The size of the sequence number field should be large enough to allow
unambiguous counting of expected vs.  received packets.  A 16-bit sequence
number would wrap around every 20 minutes for a 20 ms packetization
interval.  Using 16 bits may also simplify modulo arithmetic.

3.8 Source Identification

3.8.1 Bridges, Translators and End Systems

It is necessary to be able to identify the origin of the real-time data in
terms meaningful to the application.  First, this is required to demultiplex
sites (or sources) within the same conference.   Secondly, it allows an
indication of the currently active source.

Currently, NVP makes no explicit provisions for this, assuming that the
network source address can be used.  This may fail if intermediate agents
intervene between the content source and final destination.  Consider the
example in Fig. 3.   An RTP-level bridge is defined as an entity that
 6. suggested by Steve Casner

H. Schulzrinne                  Expires 03/01/94                  [Page 31]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

transforms either the RTP header or the RTP media data or both.   Such
a bridge could for example merge two successive packets for increased
transport efficiency or, probably the most common case, translate media
encodings for each stream, say from PCM to LPC (called transcoding).
A synchronizing bridge is defined here as a bridge that recreates a
synchronous media stream, possibly after mixing several sources.    An
application that mixes all incoming streams for a particular conference,
recreates a synchronous audio stream and then forwards it to a set of
receivers is an example of a synchronizing bridge.  A synchronizing bridge
could be built from two end system applications, with the first application
feeding the media output to the media input of the second application and
vice versa.

In figure 3, the bridges are used to translate audio encodings, from PCM
and ADPCM to LPC. The bridge could be either synchronizing or not.  Note
that a resynchronizing bridge is only necessary if audio packets depend on
their predecessors and thus cannot be transcoded independently.  It may be
advantageous if the packetization interval can be increased.  Also, for low
speed links that are barely able to handle one active source at a time,
mixing at the bridge avoids excessive queueing delays when several sources
are active at the same time.  A synchronizing bridge has the disadvantage
that it always increases the end-to-end delay.

We define translators as transport-level entities that translate between
transport protocols, but leave the RTP protocol unit untouched.   In the
figure, the translator connects a multicast group to a group of hosts that
are not multicast capable by performing transport-level replication.

We define an end system as an entity that receives and generates media
content, but does not forward it.

We define three types of sources:  the content source is the actual origins
of the media, e.g., the talker in an audiocast; a synchronization source
is the combination of several content sources with its own timing; network
source is the network-level origin as seen by the end system receiving the

The end system has to synchronize its playout with the synchronization
source, indicate the active party according to the content source and return
media to the network source.   If an end system receives media through a
resynchronizing bridge, the end system will see the bridge as the network
and synchronization source, but the content sources should not be affected.
The translator does not affect the media or synchronization sources, but the
translator becomes the network source.   (Note that having the translator
change the IP source address is not possible since the end systems need
to be able to return their media to the translator.)   In the (common)
case where no bridge or translator intercepts packets between sender and
receiver, content, synchronization and network source are identical.   If
there are several bridges or translators between sender and receiver, only
the last one is visible to the receiver.

H. Schulzrinne                  Expires 03/01/94                  [Page 32]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

/-------"        +------+
|       |  ADPCM |      |
| group |<------>|  GW  |--" LPC
|       |        |      |   "    /------ end system
"-------/        +------+    "|"/
                    reflector | >------- end system
/-------"        +------+    /|/"
|       |  PCM   |      |   /    "------ end system
| group |<------>|  GW  |--/ LPC
|       |        |      |
"-------/        +------+

<---> multicast
                         Figure 3:  Bridge topology

vat audio packets include a variable-length list of at most 64 4-byte
identifiers containing all content sources of the packet.  However, there is
no convenient way to distinguish the synchronization source from the network
source.   The end system needs to be able to distinguish synchronization
sources because jitter computation and playout delay differ for each
synchronization source.

3.8.2 Address Format Issues

The limitation to four bytes of addressing information may not be desirable
for a number of reasons.  Currently, it is used to hold an IP address.  This
works as long as four bytes are sufficient to hold an identifier that is
unique throughout the conference and as long as there is only one media
source per IP address.   The latter assumption tends to be true for many
current workstations, but it is easy to imagine scenarios where it might not
be, e.g., a system could hold a number of audio cards, could have several
audio channels (Silicon Graphics systems, for example) or could serve as a
multi-line telephone interface.(7)

The combination of IP address and source port can identify multiple sources
per site if each content source uses a different source port.  For a small
number of sources, it appears feasible, if inelegant, to allocate ports just
to distinguish sources.   In the PBX example a single output port would
appear to be the appropriate method for sending all incoming calls across
the network.  The mechanisms for allocating unique file names could also be
used.  The difficult part will be to convince all applications to draw from
 7. If we are willing to forego the identification with a site, we could
have a multiple-audio channel site pick unused IP addresses from the local
network and associate it with the second and following audio ports.

H. Schulzrinne                  Expires 03/01/94                  [Page 33]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

the same numbering space.

For efficiency in the common case of one source per workstation, the
convention (used in vat) of using the network source address, possibly
combined with the user id or source port, as media and synchronization
source should be maintained.

There are several possible approaches to naming sources.  We compare here
two examples representing naming through globally unique network addresses
and through a concatenation of locally unique identifiers.

The receiver needs to be able to uniquely identify the content source so
that speaker indication and labeling work.  For playout synchronization, the
synchronization source needs to be determined.  The identification mechanism
has to continue to work even if the path between sender and receiver
contains multiple bridges and translators.

Also, in the common case of no bridges or translators, the only information
available at the receiver is the network address and source port.   This
can cause difficulties if there is more than one participant per host in a
certain conference.  If this can occur, it is necessary that the application
opens two sockets, one for listening bound to the conference port number and
one for sending, bound to some locally unique port.  That randomly chosen
port should also be used for reverse application data, i.e., requests from
the receiver back to the content source.  Only the listening socket needs
to be a member of the IP multicast group.  If an application multiplexes
several locally generated sources, e.g., an interface to an audio bridge,
it should follow the rules for bridges, that is, insert content source

3.8.3 Globally unique identifiers

Sources are identified by their network address and the source port number.
The source port number rather than some other integer has to be chosen for
the common case that RTP packets contain no SSRC or CSRC options.  Since
the SDES option contains an address, it has to be the network address
plus source port, no other information being available to the receiver
for matching.   (The SDES address is not strictly needed unless a bridge
with mixing is involved, but carrying it keeps the receiver from having
to distinguish those cases.)   Since tying a protocol too closely to one
particular network protocol is considered a bad idea (witness the difficulty
of adopting parts of FTP for non-IP protocols), the address should probably
have the form of a type-lenght-value field.  To avoid having to manage yet
another name space, it appears possible to re-use the Ethertype values, as
all commonly used protocols with their own address space appear to have been
assigned such a value.   Other alternatives, such as using the BSD Unix
AF constants suffer from the drawback that there does not appear to be a
universally agreed-upon numbering.  NSAPs can contain other addresses, but
not every address format (such as IP) has an NSAP representation.   The

H. Schulzrinne                  Expires 03/01/94                  [Page 34]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

receiver application does not need to interpret the addresses themselves; it
treats address format identifier (e.g., the Ethertype field) and address as
a globally unique byte string.  We have to assure a single host does not use
two network addresses, one for transmission and a different one in the SDES

The rules for adding CSRC and SSRC options are simple:

end system: End systems do not insert CSRC or SSRC options.  The receiver
    remembers the CSRC address for each site;  if none is explicitly
    specified, the SSRC address is used.   If that is also missing, the
    network address is used.   SDES options are matched to this content
    source address.

bridge: A  bridge  adds  the  network  source  address  of  all  sources
    contributing to a particular outgoing packet as CSRC options.  A bridge
    that receives a packet containing CSRC options may decide to copy those
    CSRC options into an outgoing packet that contains data from that

translator: The translator checks whether the packet already contains a
    SSRC (inserted by an earlier translator).    If so, no action is
    required.   Otherwise, the translator inserts an SSRC containing the
    network address of the host from which the packet was received.

The SSRC option is set only by the translator, unless the packet already
bears such an option.

Globally unique identifiers based on network addresses have the advantage
that they simplify debugging, for example, allowing to determine which
bridge processed a message, even after the packet has passed through a

3.8.4 Locally unique addresses

In this scheme, the SSRC, CSRC and SDES options contain locally unique
identifiers of some length.    For lengths of at least four bytes, it
is sufficient to have the application pick one at random, without local
coordination, with sufficiently low probability of collision within a single
host.  The receiver creates a globally unique identifier by concatenating
the network address and one or more random identifiers.  The synchronization
source is identified by the concatenation of the SSRC identifier and the
network address.  Only translators are allowed to set the SSRC option.  If a
translator receives an RTP packet which already contains an SSRC option, as
can occur if a packet traverses several translators, the translator has to
choose a new set of values, mapping packets with the same network source,
but different incoming SSRC value into different outgoing SSRC values.  Note

H. Schulzrinne                  Expires 03/01/94                  [Page 35]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

that the SSRC constitute a label-swapping scheme similar to that used for
ATM networks, except that the assocation setup is implicit.  If a translator
loses state (say, after rebooting), the mapping is simply reestablished as
packets arrive from end systems or other translators.  Until the receivers
timeout, a single source may appear twice and there may be a temporary
confusion of sources and their descriptors.

The rules are:

end system: An end system never inserts CSRC options and typically does not
    insert an SSRC option.  An end system application may insert an SSRC
    option if it originates more than one stream for a single conference
    through a single network and transport address, e.g., a single UDP
    port.  The SDES option contains a zero for the identifier, indicating
    that the receiver is to much on network address only.   The receiver
    determines the synchronization source as the concatenation of network
    source and synchronization source.

bridge: A bridge assigns each source its own CSRC identifier (non-zero),
    which is then used also in the SDES option.

translator: The translator maintains a list of all incoming sources, with
    their network and SSRC, if present.  Sources without SSRC are assigned
    an SSRC equal to zero.  Each of these sources is assigned a new local
    identifier, which is then inserted into the SSRC option.

Local identifiers have advantages:  the length of the identifiers within
the packet are significantly shorter (four to six vs.    at least ten
bytes with padding);  comparison of content and synchronization source
are quicker (integer comparison vs.   variable-length string comparison).
The identifiers are meaningless for debugging.    In particular, it is
not easy for the receiver sitting behind a translator and a bridge to
determine where a bridge is located, unless the bridge identifies itself
periodically, possibly with another SDES-like option containing the actual
network address.

The major drawbacks appear to be the additional translator complexity:
translators needs to maintain a mapping from incoming network/SSRC to
outgoing SSRC.

Note that using IP addresses as ``random'' local identifiers is not workable
if there is any possibility that two sources participating in the same
conference can coexist on the same host.

A somewhat contrived scenaria is shown in Fig. 4.

H. Schulzrinne                  Expires 03/01/94                  [Page 36]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

Figure only available in PostScript version.
    Figure 4:  Complicated topology with translators (R) and bridges (G)

3.9 Energy Indication

G.764 contains a 4-bit noise energy field, which encodes the white noise
energy to be played by the receiver in the silences between talkspurts.
Playing silence periods as white noise reduces the noise-pumping where the
background noise audible during the talkspurt is audibly absent at the
receiver during silence periods.   Substituting white noise for silence
periods at the receiver is not recommended for multi-party conferences, as
the summed background noise from all silent parties would be distractive.
Determining the proper noise level appears to be difficult.  It is suggested
that the receiver simply takes the energy of the last packet received before
the beginning of a silence period as an indication of the background noise.
With this mechanism, an explicit indication in the packet header is not

3.10 Error Control

In principle, the receiver has four choices in handling packets with bit
errors [15]:

no checking: the receiver provides no indication whether a data packet
    contains bit errors, either because a checksum is not present or is not

discard: the receiver discards errored packets, with no indication to the

receive: the  receiver  delivers  and  flags  errored  packets  to  the

correct: the receiver drops errored packets and requests retransmission.

It remains to be decided whether the header, the whole packet or neither
should be protected by checksums.  NVP protects its header only, while G.764
has a single 16-bit check sequence covering both datalink and packet voice
header.  However, if UDP is used as the transport protocol, a checksum over
the whole packet is already computed by the receiver.  (Checksumming for UDP
can typically be disabled by the sending or receiving host, but usually not
on a per-port basis.)  ST-II does not compute checksums for its payload.
Many data link protocols already discard packets with bit errors, so that

H. Schulzrinne                  Expires 03/01/94                  [Page 37]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

packets are rarely rejected due to higher-layer checksums.

Bit errors within the data part may be easier to tolerate than a lost
packet, particularly since some media encoding formats may provide built-in
error correction.  The impact of bit errors within the header can vary; for
example, errors within the timestamp may cause the audio packet to be played
out at the wrong time, probably much more noticeable than discarding the
packet.  Other noticeable effects are caused by a wrong flow or encoding
identifier.   If a separate checksum is desired for the cases where the
underlying protocols do not already provide one, it should be optional.
Once optional, it would be easy to define several checksum options, covering
just the header, the header plus a certain part of the body or the whole

A checksum can also be used to detect whether the receiver has the correct
decryption key, avoiding noise or (worse) denial-of-service attacks.  For
that application, the checksum should be computed across the whole packet,
before encrypting the content.  Alternatively, a well-known signature could
be added to the packet and included in the encryption, as long as known
plaintext does not weaken the encryption security.

Embedding a checksum as an option may lead to undiscovered errors if
the the presence of the checksum is masked by errors.   This can occur
in a number of ways, for example by an altered option type field, a
final-option bit erroneously set in options prior to the checksum option or
an erroneous field length field.   Thus, it may be preferable to prefix
the RTP packet with a checksum as part of the specification of running
RTP over some network or transport protocol.   To avoid the overhead of
including a checksum even in the common case where it is not needed, it
might be appropriate to distinguish two RTP protocol variations through the
next-protocol value in the lower-layer protocol header; the first would
include a checksum, the second would not.   The checksum itself offers a
number of encoding possibilities(8) :

  o have two 16-bit checksums, one covering the header, the other the data

  o combine a 16-bit checksum with a byte count indicating its coverage,
    thus allowing either a header-only or a header-plus-data checksum

The latter has the advantage that the checksum can be computed without
determining the header length.

The error detection performance and computational cost of some common 16-bit
checksumming algorithms are summarized in Table 4.  The implementations were
drawn from [16] and compiled on a SPARC IPX using the Sun ANSI C compiler
 8. suggested by S. Casner

H. Schulzrinne                  Expires 03/01/94                  [Page 38]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

with optimization.    The checksum computation was repeated 100 times;
thus, due to data cache effects, the execution times shown are probably
better than would be measured in an actual application.   The relative
performance, however, should be similar.  Among the algorithms, the CRC has
the strongest error detection properties, particularly for burst errors,
while the remaining algorithms are roughly equivalent [16].  The Fletcher
algorithm with modulo 255 (shown here) has the peculiar property that a
transformation of a byte from 0 to 255 remains undetected.   CRC, the IP
checksum and Fletcher's algorithm cannot detect spurious zeroes at the end
of a variable-length message [17].  The non-CRC checksums have the advantage
that they can be updated incrementally if only a few bytes have changed.
The latter property is important for translators that insert synchronization
source indicators.

              algorithm                                   ms
              IP checksum                              0.093
              Fletcher's algorthm, optimized [17]      0.192
              CRC CCITT                                0.310
              Fletcher's algorithm, non-optimized [18] 2.044

Table 4:  Execution time of common 16-bit checksumming algorithms, for a
1024-byte packet, in milliseconds

3.11 Security and Privacy

3.11.1 Introduction

The discussions in this sections are based on the work of the privacy
enhanced mail (PEM) working group within the Internet Engineering Task
Force, as documented in [19,20] and related documents.   The reader is
referred to RFC 1113 [19] or its successors for terminology.  Also relevant
is the work on security for SNMP Version 2.   We discuss here how the
following security-related services may be implemented for packet voice and

Confidentiality: Measures that ensure that only the intended receiver(s)
    can decode the received audio/video data; for others, the data contains
    no useful information.

Authentication: Measures  that  allow  the  receiver(s)  to  ascertain  the
    identity of the sender of data or to verify that the claimed originator
    is indeed the originator of the data.

Message integrity: Measures that allow the receiver(s) to detect whether
    the received data has been altered.

H. Schulzrinne                  Expires 03/01/94                  [Page 39]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

As for PEM [19], the following privacy-related concerns are not addressed at
this time:

  o access control

  o traffic flow confidentiality

  o routing control

  o assurance of data receipt and non-deniability of receipt

  o duplicate  detection,  replay  prevention,  or  other  stream-oriented

These services either require connection-oriented services or support from
the lower layers that is currently unavailable.  A reasonable goal is to
provide privacy at least equivalent to that provided by the public telephone
system (except for traffic flow confidentiality).

As  for  privacy-enhanced  mail,  the  sender  determines  which  privacy
enhancements  are  to  be  performed  for  a  particular  part  of  a  data
transmission.   Therefore, mechanisms should be provided that allow the
sender to determine whether the desired recipients are equipped to process
any privacy-enhancements.  This is functionally similar to the negotiation
of,  say,  media encodings and should probably be handled by similar
mechanisms.   It is anticipated that privacy-enhanced mail will be used
in the absence of or in addition to session establishment protocols and
agents to distributed keys or negotiate the enhancements to be used during a

3.11.2 Confidentiality

Only data encryption can provide confidentiality as long as intruders can
monitor the channel.  It is desirable to specify an encryption algorithm and
provide implementations without export restrictions.  Although DES is widely
available outside the United States, its use within software in both source
and binary form remains difficult.

We have the choice of either encrypting and/or authenticating the whole
packet or only the options and payload.  Encrypting the fixed header denies
the intruder knowledge about some conference details (such as timing and
format) and protects against replay attacks.  Encrypting the fixed header
also allows some heuristic detection of key mismatches, as the version
identifier, timestamp and other header information are somewhat predictable.
However, header encryption makes packet traces and debugging by external
programs difficult.  Also, since translators may need to inspect and modify
the header, but do not have access to the sender's key, at least part of

H. Schulzrinne                  Expires 03/01/94                  [Page 40]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

the header needs to remain unencrypted, with the ability for the receiver
to discern which part has been encrypted.   Given these complications and
the uncertain benefits of header encryption, it appears appropriate to limit
encryption to the options and payload part only.

In public key cryptography, the sender uses the receiver's public key for
encryption.   Public key cryptography does not work for true multicast
systems since the public encoding key for every recipient differs, but it
may be appropriate when used in two-party conversations or application-level
multicast.  In that case, mechanisms similar to privacy enhanced mail will
probably be appropriate.  Key distribution for symmetric-key encryption such
as DES is beyond the scope of this recommendation, but the services of
privacy enhanced mail [19,21] may be appropriate.

For one-way applications, it may desirable to prohibit listeners from
interrupting the broadcast.   (After all, since live lectures on campus
get disrupted fairly often, there is reason to fear that a sufficiently
controversial lecture carried on the Internet could suffer a similar fate.)
Again, asymmetric encryption can be used.   Here, the decryption key is
made available to all receivers, while the encryption key is known only
to the legitimate sender.  Current public-key algorithms are probably too
computationally intensive for all but low-bit-rate voice.  In most cases,
filtering based on sources will be sufficient.

3.11.3 Message Integrity and Authentication

The usual message digest methods are applicable if only the integrity of the
message is to be protected against tampering.  Again, services similar to
that of privacy-enhanced mail [22] may be appropriate.   The MD5 message
digest [23] appears suitable.  It translates any size message into a 128-bit
(16-byte) signature.   On a SPARCstation IPX (Sun 4/50), the computation
of a signature for a 180-byte audio packet takes approximately 0.378 ms(9)
Defining the signature to apply to all data beginning at the signature
option allows operation when translators change headers.  The receiver has
to be able to locate the public key of the claimed sender.  This poses two
problems:  first, a way of identifying the sender unambiguously needs to be
found.  The current methods of identification, such as the SMTP (e-mail)
address, are not unambiguous.  Use of a distinguished name as described in
RFC 1255 [24] is suggested.

The authentication process is described in RFC 1422 [21]:
 9. The processing rates for Sun 4/50 (40 MHz clock) and SPARCstation 10's
(36 MHz clock) are 0.95 and 2.2 MB/s, respectively, measured for a single
1000-byte block.  Note that timing the repeated application of the algorithm
for the same block of data gives optimistic results since the data then
resides in the cache.

H. Schulzrinne                  Expires 03/01/94                  [Page 41]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

     In  order  to  provide  message  integrity  and  data  origin
    authentication, the originator generates a message integrity code
    (MIC), signs (encrypts) the MIC using the private component of his
    public-key pair, and includes the resulting value in the message
    header in the MIC-Info field.  The certificate of the originator
    is (optionally) included in the header in the Certificate field
    as described in RFC 1421.   This is done in order to facilitate
    validation in the absence of ubiquitous directory services.  Upon
    receipt of a privacy enhanced message, a recipient validates the
    originator's certificate (using the IPRA public component as the
    root of a certification path), checks to ensure that it has not
    been revoked, extracts the public component from the certificate,
    and uses that value to recover (decrypt) the MIC. The recovered
    MIC is compared against the locally calculated MIC to verify the
    integrity and data origin authenticity of the message.

For audio/video applications with loose control, the certificate could be
carried periodically to allow new listeners to obtain it and to achieve a
measure of reliability.

Symmetric key methods such as DES can also be used.   Here, the key is
simply prefixed to the message when computing the message digest (MIC), but
not transmitted.   The receiver has to obtain the sender's key through a
secure channel, e.g., a PEM message.  The method has the advantage that no
cryptography is involved, thus alleviating export-control concerns.  It is
used for SNMP Version 2 authentication.

3.12 Security for RTP vs.  PEM

It is the author's opinion that RTP should aim to reuse as much of the
PEM technology and syntax as possible, unless there are strong reasons in
the nature of real-time traffic to deviate.  This has the advantage that
terminology, implementation experience, certificate mechanisms and possibly
code can be reused.  Also, since it is hoped that RTP finds use in a range
of applications, a broad spectrum of security mechanisms should be provided,
not necessarily limited by what is appropriate for large-distribution audio
and video conferences.

It should be noted that connection-oriented security architectures are
probably unsuitable for RTP applications as they rely on reliable stream
transmission and an explicit setup phase with typically only a single sender
and receiver.

There are a number of differences between the security requirements of PEM
and RTP that should be kept in mind:

Transparency: Unlike electronic mail, it is safe to assume that the channel

H. Schulzrinne                  Expires 03/01/94                  [Page 42]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

    will carry 8 bit data unaltered.   Thus, a conversion to a canonical
    form or encoding binary data into a 64-element subset as done for PEM
    is not required.

Time: As outlined at the beginning of this document, processing speed and
    packet overhead have to be major considerations, much more so than with
    store-and-forward electronic mail.  Message digest algorithms and DES
    can be implemented sufficiently fast even in software to be used for
    voice and possibly for low-bit rate video.  Even for short signatures,
    RSA encryption is fairly slow.

    Note that the ASN.1/BER encoding of asymmetrically-encrypted MICs and
    certificates adds no significant processing load.  For the MICs, the
    ASN.1 algorithm yields only additional constant bytes which a paranoid
    program can check, but does not need to decode.   Certificates are
    carried much more infrequently and are relatively simple structures.
    It would seem unnecessary to supply a complete ASN.1/BER parser for any
    of the datastructures.

Space: Encryption algorithm require a minimum data input equal to their
    keylength.   Thus, for the suggested key length for RSA encryption
    of 508 to 1024 bits, the 16-byte message digest expands to a 53
    to 128 byte MIC. This is clearly rather burdensome for short audio
    packets.   Applying a single message digest to several packets seems
    possible if the packet loss rates are sufficiently low, even though it
    does introduce minor security risks in the case where the receiver is
    forced to decide between accepting as authentic an incomplete sequence
    of packets or rejecting the whole sequence.    Note that it would
    not be necessary to wait with playback until a complete authenticated
    block has been received; in general, a warning that authentication has
    failed would be sufficient for human users.   The application should
    also issue a warning if no complete block could be authenticated for
    several blocks, as that might indicate that an impostor was feigning
    the presence of MIC-protected data by strategically dropping packets.

    The initialization vector for DES in cipher block mode adds another
    eight bytes.

Scale: The symmetric key authentication algorithm used by PEM does not
    scale well for a large number of receivers as the message has to
    contain a separate MIC for each receiver, encrypted with the key for
    that particular sender-receiver pair.   If we forgo the ability to
    authenticate an individual user, a single session key shared by all
    participants can thwart impostors from outside the group holding the
    shared secret.

H. Schulzrinne                  Expires 03/01/94                  [Page 43]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

3.13 Quality of Service Control

Because real-time services cannot afford retransmissions, they are directly
affected by packet loss and delays.   Delay jitter and packet loss, for
example, provide a good indication of network congestion and may suggest
switching to a lower-bandwidth coding.   To aid in fault isolation and
performance monitoring,  quality-of-service (QOS) measurement support is
useful.  QOS of service monitoring is useful for the receiver of real-time
data, the sender of that data and possibly a third-party monitor, e.g.,
the network provider,  that is itself not part of the real-time data

3.13.1 QOS Measures

For real-time services, a number of QOS measures are of interest, roughly in
order of importance:

  o packet loss

  o packet delay variation (variance, minimum/maximum)

  o relative clock drift (delay between sender and receiver timestamp)

In the following, the terms receiver and sender pertain to the real-time
data, not any returned QOS data.  If the receiver is to measure packet loss,
an indication of the number of packets actually transmitted is required.
If the receiver itself does not need to compute packet loss percentages,
it is sufficient for the receiver to indicate to the sender the number of
packets received and the range timestamps covered, thus avoiding the need
for sequence numbers.   Translation into loss at the sender is somewhat
complicated, however, unless restrictions on permissible timestamps (e.g.,
those starting a synchronization unit) are enforced.  If sequence numbers
are available, the receiver has to track the number of times that the
sequence number has wrapped around, even in the face of packet reordering.
If c denotes the cycle count, M the sequence number modulus and s  the
sequence number of the n received packet, where s  is not necessarily
larger than s   , we can write:

                   c =c   +1 for -M<s -s   <-M=2
                    n                    n
                        n-1                  n-1
                   c =c   -1 for M=2<s -s   <M
                    n                    n
                        n-1                  n-1
                   c =c       otherwise

H. Schulzrinne                  Expires 03/01/94                  [Page 44]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

For example, the sequence number sequence 65534;2;65535;1;3;5;4 would
yield the cycle number sequence 0;1;0;1;1;1;1 for M=65536, i.e., 16-bit
sequence numbers.   The total number of expected packets is then computed
simply as s +M*c -s +1, where the first received packet has index 0.
           n     n
The user of the measurements should also have some indication as to the time
period they cover so that the degree of confidence in these statistical
meassurements can be established.

3.13.2 Remote measurements

It may be desirable for the sender, interested multicast group members
or  a  non-group  member  (third  party)  to  have  automatic  access  to
quality-of-service measurements.   In particular, it is necessary for the
sender to gather a number of reception reports from different parts of the
Internet to ``triangulate'' where packets get lost or delayed.

Two  modes  of  operation  can  be  distinguished:    monitor-driven  or
receiver-driven.  In the monitor-driven case, a site interested in QOS data
for a particular sender contacts the receiver through a back channel and
requests a reception report.  Alternatively, each site can send reception
reports to a monitoring multicast group or as session data, along with
the ``regular station identification'' to the same multicast group used
for data.   The first approach requires the most implementation effort,
but produces the least amount of data.   The other two approaches have
complementary properties.

In most cases, sender-specific quality of service information is more useful
for tracking network problems than aggregrate data for all senders.  Since
a site cannot transmit reception reports for all senders it has ever heard
from, some selection mechanism is needed, such as most-recently-heard or
cycling through sites.

Source identification poses some difficulties since the network address seen
by the receiver may not be meaningful to other members of the multicast
group, e.g., after IP-SIP address translation.  On the other hand, network
addresses are easier to correlate with other network-level tools such as
those used for Mbone mapping.

minimum and maximum difference between departure and arrival timestamp.
This has the advantage that the fixed delay can also be estimated if
sender and receiver clocks are known to be synchronized.   Unfortunately,
delay extrema are noisy measurement that give only limited indication of
the delay variability.   The receiver could also return the playout delay
value it uses, although for absolute timing, that again depends on the
clock differential, as well as on the particular delay estimation algorithm
employed by the receiver.  In summary, a minimal set of useful measurements
appears to be the expected and received packet count, combined with the
minimum and maximum timestamp difference.

H. Schulzrinne                  Expires 03/01/94                  [Page 45]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

3.13.3 Monitoring by Third Party

Except for delay estimates based on sequence number ranges, the above
section applies for this case as well.

4 Conference Control Protocol

Currently, only conference control functions used for loosely controlled
conferences (open admission,  no explicit conference set-up) have been
considered in depth.  Support for the following functionality needs to be

  o authentication

  o floor control, token passing

  o invitations, calls

  o call forwarding, call transfer

  o discovery of conferences and resources (directory service)

  o media, encoding and quality-of-service negotiation

  o voting

  o conference scheduling

  o user locator

The functional specification of a conference control protocol is beyond the
scope of this memorandum.

5 The Use of Profiles

RTP is intended to be a rather 'thin' protocol, partially because it aims
to serve a wide variety of real-time services.   The RTP specification
intentionally leaves a number of issues open for other documents (profiles),
which in turn have the goal of making it easy to build interoperable
applications for a particular application domain, for example, audio and
video conferences.

Some of the issues that a profile should address include:

H. Schulzrinne                  Expires 03/01/94                  [Page 46]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

  o the interpretation of the 'content' field with the CDESC option

  o the structure of the content-specific part at the end of the CDESC

  o the mechanism by which applications learn about and define the mapping
    between the 'content' field in the RTP fixed header and its meaning

  o the use of the optional framing field prefixed to RTP packets (not
    used, used only if underlying transport protocol does not provide
    framing, used by some negotiation mechanism, always used)

  o any RTP-over-x issues, that is, definitions needed to allow RTP to use
    a particular underlying protocol

  o content-specific RTP, RTCP or reverse control options

  o port assignments for data and reverse control

6 Port Assignment

Since it is anticipated that UDP and similar port-oriented protocols will
play a major role in carrying RTP traffic, the issue of port assignment
needs to be addressed.   The way ports are assigned mainly affects how
applications can extract the packets destined for them.  For each medium,
there also needs to be a mechanism for distinguishing data from control

For unicast UDP, only the port number is available for demultiplexing.
Thus, each media will need a separate port number pair unless a separate
demultiplexing agent is used.    However,  for one-to-one connections,
dynamically negotiating a port number is easy.  If several UDP streams are
used to provide multicast by transport-level replication, the port number
issue becomes somewhat more difficult.  For ST-II, a common port number has
to be agreed upon by all participants, which may be difficult particularly
if a new site wants to join an on-going connection, but is already using the
port number in a different connection.

For UDP multicast, an application can select to receive only packets with a
particular port number and multicast address by binding to the appropriate
multicast address(10) .   Thus, for UDP multicast, there is no need to
distinguish media by port numbers, as each medium could have its designated
and unique multicast group.   Any dynamic port allocation mechanism would
fail for large, dynamic multicast groups, but might be appropriate for small
10. This extension to the original multicast socket semantics is currently
in the process of being deployed.

H. Schulzrinne                  Expires 03/01/94                  [Page 47]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

conferences and two-party conversations.

Data and control packets for a single medium can either share a single
port or use two different port numbers.   (Currently, two adjacent port
numbers, 3456 and 3457, are used.)   A single port for data and control
simplifies the receiver code and translators and, less important, conserves
port numbers.  With the proliferation of firewalls, limiting the number of
ports has assumed additional importance.   Sharing a single port requires
some other means of identifying control packets, for example as a special
encoding code.  Alternatively, all control data could be carried as options
within data packets, akin to the NVP protocol options.   Since control
messages are also transmitted if no actual medium data is available, header
content of packets without media data needs to be determined.  With the use
of a synchronization bit, the issue of how sequence numbers and timestamps
are to be treated for these packets is less critical.  It is suggested to
use a zero timestamp and to increment the sequence number normally.  Due to
the low bandwidth requirements of typical control information, the issue of
accomodating control information in any bandwidth reservation scheme should
be manageable.   The penalty paid is the eight-byte overhead of the RTP
header for control packets that do not require time stamps, encoding and
sequence number information.

Using a single RTCP stream for several media may be advantageous to
avoid duplicating, for example, the same identification information for
voice, video and whiteboard streams.   This works only if there is one
multicast group that all members of a conference subscribe to.   Given
the relatively low frequency of control messages, the coordination effort
between applications and the necessity to designate control messages for a
particular medium are probably reasons enough to have each application send
control messages to the same multicast group as the data.

In conclusion, for multicast UDP, one assigned port number, for both data
and control, seems to offer the most advantages, although the data/control
split may offer some bandwidth savings.

7 Multicast Address Allocation

A fixed, permanent allocation of network multicast addresses to invidual
conferences by some naming authority such as the Internet Assigned Numbers
Authority is clearly not feasible, since the lifetime of conferences is
unknown, the potential number of conferences is rather large and the
                                           28             16
available number space limited to about 2  , of which 2   have been set
aside for dynamic allocation by conferences.

The alternative to permanent allocation is a dynamic allocation, where an
initiator of a multicast application obtains an unused multicast address in
some manner (discussed below).  The address is then made available again,
either implicitly or explicitly, as the application terminates.

H. Schulzrinne                  Expires 03/01/94                  [Page 48]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

The address allocation may or may not be handled by the same mechanism that
provides conference naming and discovery services.  Separating the two has
the advantage that dynamic (multicast) address allocation may be useful
to applications other than conferencing.  Also, different mechanisms (for
example, periodic announcements vs.  servers) may be appropriate for each.

We can distinguish two methods of multicast address assignment:

function-based: all applications of a certain type share a common, global
    address space.  Currently, a reservation of a 16-bit address space for
    conferences is one example.   The advantage of this scheme is that
    directory functions and allocation can be readily combined, as is done
    in the sd tool by Van Jacobson.   A single namespace spanning the
    globe makes it necessary to restrict the scope of addresses so that
    allocation does not require knowing about and distributing information
    about the existence of all global conferences.

hierarchical: Based on the location of the initiator, only a subset of
    addresses are available.    This limits the number of hosts that
    could be involved in resolving collisions, but, like most hierarchical
    assignment, leads to sparse allocation.  Allocation is independent of
    the function the address is used for.

Clearly, combinations are possible, for example, each local namespace could
be functionally divided if sufficiently large.  With the current allocation
of 2   addresses to conferences, hierarchical division except on a very
coarse scale is not feasible.

To a limited extent, multicast address allocation can be compared to the
well-known channel multiple access problem.   The multicast address space
plays the role of the common channel, with each address representing a time

All the following schemes require cooperation from all potential users of
the address space.  There is no protection against an ignorant or malicious
user joining a multicast group.

7.1 Channel Sensing

In this approach, the initiator randomly selects a multicast address from a
given range, joins the multicast group with that address and listens whether
some other host is already transmitting on that address.  This approach does
not require a separate address allocation protocol or an address server,
but it is probably infeasible for a number of reasons.   First, a user
process can only bind to a single port at one time, making 'channel sensing'
difficult.  Secondly, unlike listening to a typical broadcast channel, the

H. Schulzrinne                  Expires 03/01/94                  [Page 49]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

act of joining the multicast group can be quite expensive both for the
listening host and the network.   Consider what would happen if a host
attached through a low-bandwidth connection joins a multicast group carrying
video traffic, say.

Channel sensing may also fail if two sections of the network that were
separated at the time of address allocation rejoin later.   Changes in
time-to-live values can make multicast groups 'visible' to hosts that
previously were outside their scope.

7.2 Global Reservation Channel with Scoping

Each range of multicast addresses has an associated well-known multicast
address and port where all initiators (and possibly users) advertise the use
of multicast addresses.   An initiator first picks a multicast address at
random, avoiding those already known to be in use.   Some mechanism for
collision resolution has to be provided in the unlikely event that two
initiators simultaneously choose the same address.   Also, since address
advertisement will have to be sent at fairly long intervals to keep traffic
down, an application wanting to start a conference, for example, has to
wait for an extended period of time unless it continuously monitors the
allocation multicast group.

To limit traffic, it may seem advisable to only have the initiator multicast
the address usage advertisement.  This, however, means that there needs to
be a mechanism for another site to take over advertising the group if the
initiator leaves, but the multicast group continues to exist.  Time-to-live
restrictions pose another problem.  If only a single source advertises the
group, the advertisement may not reach all those sites that could be reached
by the multicast transmissions themselves.

The possibility of collisions can be reduced by address reuse with scoping,
discussed further below, and by adding port numbers and other identifiers
as further discriminators.   The latter approach appears to defeat the
purpose of using multicast to avoid transmitting information to hosts that
have no interest in receiving it.  Routers can only filter based on group
membership, not ports or other higher-layer demultiplexing identifiers.
Thus, even though two conferences with the same multicast address and
different ports, say, could coexist at the application layer, this would
force hosts and networks that are interested in only one of the conferences
to deal with the combined traffic of the two conferences.

7.3 Local Reservation Channel

Instead of sharing a global namespace for each application, this scheme
divides the multicast address space hierarchically, allowing an initiator
within a given network to choose from a smaller set of multicast addresses,

H. Schulzrinne                  Expires 03/01/94                  [Page 50]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

but independent of the application.  As with many allocation problems, we
can devise both server-based and fully distributed versions.

7.3.1 Hierarchical Allocation with Servers

By some external means, address servers, distributed throughout the network,
are provided with non-overlapping regions of the multicast address space.
An initiator asks its favorite address server for an address when needed.
When it no longer needs the address, it returns it to the server.   To
prevent addresses from disappearing when the requestor crashes and looses
its memory about allocated addresses, requests should have an associated
time-out period.  This would also (to some extent) cover the case that the
initiator leaves the conference, without the conference itself disbanding.
To decrease the chances that an initiator cannot be provided with an
address, either the local server could 'borrow' an address from another
server or could point the initiator to another server, somewhat akin to the
methods used by the Domain Name Service (DNS). Provisions have to be made
for servers that crash and may loose knowledge about the status of its block
of addresses, in particular their expiration times.   The impact of such
failures could be mitigated by limiting the maximum expiration time to a few
hours.  Also, the server could try to request status by multicast from its

7.3.2 Distributed Hierarchical Allocation

Instead of a server,  each network is allocated a set of multicast
addresses.   Within the current IP address space, both class A, B and C
networks would get roughly 120 addresses, taking into account those that
have been permanently assigned.   Contention for addresses works like the
global reservation channel discussed earlier, but the reservation group is
strictly limited to the local network.   (Since the address ranges are
disjoint, address information that inadvertently leaks outside the network,
is harmless.)

This method avoids the use of servers and the attendant failure modes, but
introduces other problems.  The division of the address space leads to a
barely adequate supply of addresses (although larger address formats will
probably make that less of an issue in the future).  As for any distributed
algorithm, splitting of networks into temporarily unconnected parts can
easily destroy the uniqueness of addresses.  Handling initiators that leave
on-going conferences is probably the most difficult issue.

H. Schulzrinne                  Expires 03/01/94                  [Page 51]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

7.4 Restricting Scope by Limiting Time-to-Live

Regardless of the address allocation method,  it may be desirable to
distinguish multicast addresses with different reach.  A local address would
be given out with the restriction of a maximum time-to-live value and could
thus be reused at a network sufficiently removed, akin to the combination
of cell reuse and power limitation in cellular telephony.  Given that many
conferences will be local or regional (e.g., broadcasting classes to nearby
campuses of the same university or a regional group of universities, or an
electronic town meeting), this should allow significant reuse of addresses.
Reuse of addresses requires careful engineering of thresholds and would
probably only be useful for very small time-to-live values that restrict
reach to a single local area network.  Using time-to-live fields to restrict
scope rather than just prevent looping introduces difficult-to-diagnose
failure modes into multicast sessions.  In particular, reachability is no
longer transitive, as B may have A and C in its scope, but A and B may be
outside each other's scope (or A may be in the scope of B, but not vice
versa, due to asymmetric routes, etc.).  This problem is aggravated by the
fact that routers (for obvious reasons) are not supposed to return ICMP time
exceeded messages, so that the sender can only guess why multicast packets
do not reach certain receivers.

8 Security Considerations

Security issues are discussed in Section 3.11.


This draft is based on discussion within the AVT working group chaired by
Stephen Casner.  Eve Schooler and Stephen Casner provided valuable comments.

This work was supported in part by the Office of Naval Research under
contract N00014-90-J-1293, the Defense Advanced Research Projects Agency
under contract NAG2-578 and a National Science Foundation equipment grant,
CERDCR 8500332.

A Glossary

The glossary below briefly defines the acronyms used within the text.
Further definitions can be found in RFC 1392, ``Internet User's Glossary''.
Some of the general Internet definitions below are copied from that
glossary.    The quoted passages followed by a reference of the form

H. Schulzrinne                  Expires 03/01/94                  [Page 52]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

``(G.701)'' are drawn from the CCITT Blue Book, Fascicle I.3, Definitions.
The glossary of the document ``Recommended Practices for Enhancing Digital
Audio Compatibility in Multimedia Systems'', published by the Interactive
Multimedia Association was used for some terms marked with [IMA]. The
section on MPEG is based on text written by Mark Adler (Caltech).

4:1:1 Refers to degree of subsampling of the two chrominance signals with
    respect to the luminance signal.  Here, each color difference component
    has one quarter the resolution of the luminance component.

4:2:2 Refers to degree of subsampling of the two chrominance signals with
    respect to the luminance signal.  Here, each color difference component
    has half the resolution of the luminance component.

16/16 timestamp: a 32-bit integer timestamp consisting of a 16-bit field
    containing the number of seconds followed by a 16-bit field containing
    the binary fraction of a second.  This timestamp can measure about 18.2
    hours with a resolution of approximately 15 microseconds.

n=m timestamp: a n+m bit timestamp consisting of an n-bit second count and
    an m-bit fraction.

ADPCM: Adaptive  differential  pulse  code  modulation.      Rather  than
    transmitting ! PCM samples directly,  the difference between the
    estimate of the next sample and the actual sample is transmitted.  This
    difference is usually small and can thus be encoded in fewer bits than
    the sample itself.   The ! CCITT recommendations G.721, G.723, G.726
    and G.727 describe ADPCM encodings.   ``A form of differential pulse
    code modulation that uses adaptive quantizing.  The predictor may be
    either fixed (time invariant) or variable.   When the predictor is
    adaptive, the adaptation of its coefficients is made from the quantized
    difference signal.''  (G.701)

adaptive quantizing: ``Quantizing  in  which  some  parameters  are  made
    variable according to the short term statistical characteristics of the
    quantized signal.''  (G.701)

A-law: a type of audio !companding popular in Europe.

CCIR: Comite Consultativ International de Radio.   This organization is
    part of the United Nations International Telecommunications Union (ITU)
    and is responsible for making technical recommendations about radio,
    television and frequency assignments.   The CCIR has recently changed
    its name to ITU-TR; we maintain the more familiar name.  !CCITT

CCIR-601: The CCIR-601 digital television standard is the base for all the
    subsampled interchange formats such as SIF, CIF, QCIF, etc.  For NTSC
    (PAL/SECAM), it is 720 (720) pixels by 243 (288) lines by 60 (50)
    fields per second, where the fields are interlaced when displayed.
    The chrominance channels horizontally subsampled by a factor of two,

H. Schulzrinne                  Expires 03/01/94                  [Page 53]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

    yielding 360 (360) pixels by 243 (288) lines by 60 (50) fields a

CCITT: Comite Consultatif International de Telegraphique et Telephonique
    (CCITT). This organization is part of the United Nations International
    Telecommunications Union (ITU) and is responsible for making technical
    recommendations about telephone and data communications systems.  X.25
    is an example of a CCITT recommendation.  Every four years CCITT holds
    plenary sessions where they adopt new recommendations.  Recommendations
    are known by the color of the cover of the book they are contained in.
    (The 1988 edition is known as the Blue Book.)  The CCITT has recently
    changed its name to ITU-TS; we maintain the familiar name.  !CCIR

CELP: code-excited linear prediction; audio encoding method for low-bit
    rate codecs; !LPC.

CD: compact disc.

chrominance: color information in a video image.   For !H.261, color is
    encoded as two color differences:   CR (``red'') and CB (``blue'').

CIF: common interchange format; interchange format for video images with
    288 lines with 352 pixels per line of luminance and 144 lines with 176
    pixel per line of chrominance information.  !QCIF, SCIF

CLNP: ISO connectionless network-layer protocol (ISO 8473), similar in
    functionality to !IP.

codec: short for coder/decoder; device or software that ! encodes and
    decodes audio or video information.

companding: contraction of compressing and expanding; reducing the dynamic
    range of audio or video by a non-linear transformation of the sample
    values.   The best known methods for audio are mu-law, used in North
    America, and A-law, used in Europe and Asia.   !G.711 For a given
    number of bits, companded data uses a greater number of binary codes to
    represent small signal levels than linear data, resulting in a greater
    dynamic range at the expense of a poorer signal-to-nose ratio. [25]

DAT: digital audio tape.

decimation: reduction of sample rate by removal of samples [IMA].

delay jitter: Delay jitter is the variation in end-to-end network delay,
    caused  principally  by  varying  media  access  delays,  e.g.,  in  an
    Ethernet, and queueing delays.  Delay jitter needs to be compensated
    by adding a variable delay (refered to as ! playout delay) at the

H. Schulzrinne                  Expires 03/01/94                  [Page 54]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

DVI: (trademark)  digital  video  interactive.     Audio/video  compression
    technology developed by Intel's DVI group.  [IMA]

dynamic range: a ratio of the largest encodable audio signal to the
    smallest encodable signal, expressed in decibels.   For linear audio
    data types, the dynamic range is approximately six times the number of
    bits, measured in dB.

encoding: transformation of the media content for transmission, usually to
    save bandwidth, but also to decrease the effect of transmission errors.
    Well-known encodings are G.711 (mu-law PCM), and ADPCM for audio, JPEG
    and MPEG for video.  ! encryption

encryption: transformation of the media content to ensure that only the
    intended recipients can make use of the information.  ! encoding

end system: host where conference participants are located.   RTP packets
    received by an end system are played out, but not forwarded to other
    hosts (in a manner visible to RTP).

FIR: finite (duration) impulse response.  A signal processing filter that
    does not use any feedback components [IMA].

frame: unit of information.  Commonly used for video to refer to a single
    picture.  For audio, it refers to a data that forms a encoding unit.
    For example, an LPC frame consists of the coefficients necessary to
    generate a specific number of audio samples.

frequency response: a system's ability to encode the spectral content of
    audio data.  The sample rate has to be at least twice as large as the
    maximum possible signal frequency.

G.711: ! CCITT recommendation for ! PCM audio encoding at 64 kb/s using
    mu-law or A-law companding.

G.721: ! CCITT recommendation for 32 kbit/s adaptive differential pulse
    code modulation (! ADPCM, PCM).

G.722: ! CCITT recommendation for audio coding at 64 kbit/s; the audio
    bandwidth is 7 kHz instead of 3.5 kHz for G.711, G.721, G.723 and

G.723: ! CCITT recommendation for extensions of Recommendation G.721
    adapted  to  24  and  40  kbit/s  for  digital  circuit  multiplication

G.728: ! CCITT recommendation for voice coding using code-excited linear
    prediction (CELP) at 16 kbit/s.

G.764: ! CCITT recommendation for packet voice; specifies both ! HDLC-like
    data link and network layer.   In the draft stage, this standard was

H. Schulzrinne                  Expires 03/01/94                  [Page 55]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

    referred to as G.PVNP. The standard is primarily geared towards digital
    circuit multiplication equipment used by telephone companies to carry
    more voice calls on transoceanic links.

G.821: !  CCITT  recommendation  for  the  error  performance  of  an
    international digital connection forming part of an integrated services
    digital network.

G.822: ! CCITT recommendation for the controlled !slip rate objective on
    an international digital connection.

G.PVNP: designation of CCITT recommendation ! G.764 while in draft status.

GOB: (H.261) groups of blocks; a !CIF picture is divided into 12 GOBs, a
    QCIF into 3 GOBs.   A GOB is composed of 3 macro blocks (!MB) and
    contains luminance and chrominance information for 8448 pixels.

GSM: Group Speciale Mobile.   In general, designation for European mobile
    telephony standard.   In particular, often used to denote the audio
    coding used.   Formally known as the European GSM 06.10 provisional
    standard for full-rate speech transcoding, prI-ETS 300 036.  It uses
    RPE/LTP (residual pulse excitation/long term prediction) at 13 kb/s
    using frames of 160 samples covering 20 ms.

H.261: ! CCITT recommendation for the compression of motion video at rates
    of Px64 kb/s (where p=1:::30.   Originally intended for narrowband

hangover:  [26] Audio data transmitted after the silence detector indicates
    that no audio data is present.   Hangover ensures that the ends of
    words, important for comprehension, are transmitted even though they
    are often of low energy.

HDLC: high-level data link control;  standard data link layer protocol
    (closely related to LAPD and SDLC).

IMA: Interactive  Multimedia  Assocation;  trade  association  located  in
    Annapolis, MD.

ICMP: Internet Control Message Protocol;  ICMP is an extension to the
    Internet Protocol.   It allows for the generation of error messages,
    test packets and informational messages related to ! IP.

in-band: signaling information is carried together (in the same channel or
    packet) with the actual data.  ! out-of-band.

interpolation: increase  in  sample  rate  by  introduction  of  processed

IP: internet protocol; the Internet Protocol, defined in RFC 791, is the
    network layer for the TCP/IP Protocol Suite.  It is a connectionless,

H. Schulzrinne                  Expires 03/01/94                  [Page 56]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

    best-effort packet switching protocol [27].

IP address: four-byte binary host interface identifier used by !IP for
    addressing.    An IP address consists of a network portion and a
    host portion.   RTP treats IP addresses as globally unique, opaque

IPv4: current version (4) of ! IP.

ISDN: integrated services digital network; refers to an end-to-end circuit
    switched digital network intended to replace the current telephone
    network.   ISDN offers circuit-switched bandwidth in multiples of 64
    kb/s (B or bearer channel), plus a 16 kb/s packet-switched data (D)

ISO: International  Standards  Organization.     A  voluntary,  nontreaty
    organization  founded  in  1946.     Its  members  are  the  national
    standardards organizations of the 89 member countries, including ANSI
    for the U.S. (Tanenbaum)

ISO 10646: !ISO standard for the encoding of characters from all languages
    into a single 32-bit code space (Universal Character Set).    For
    transmission and storage, a one-to-five octet code (UTF) has been
    defined which is upwardly compatible with US-ASCII.

JPEG: ISO/CCITT joint photographic experts group.    Designation of a
    variable-rate compression algorithm using discrete cosine transforms
    for still-frame color images.

jitter: ! delay jitter.

linear encoding: a mapping from signal values to binary codes where each
    binary level represents the same signal increment !companding.

loosely controlled conference: Participants  can  join  and  leave  the
    conference without connection establishment or notifying a conference
    moderator.  The identity of conference participants may or may not be
    known to other participants.  See also:  tightly controlled conference.

low-pass filter: a signal processing function that removes spectral content
    above a cutoff frequency.  [IMA]

LPC: linear predictive coder.  Audio encoding method that models speech as
    a parameters of a linear filter; used for very low bit rate codecs.

luminance: brightness information in a video image.    For black-and-
    white (grayscale) images,  only luminance information is required.

MB: (H.261) macroblock,  consisting of six blocks,  four eight-by-eight
    luminance blocks and two chrominance blocks.

H. Schulzrinne                  Expires 03/01/94                  [Page 57]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

MPEG: ISO/CCITT motion picture experts group JTC1/SC29/WG11.  Designates a
    variable-rate compression algorithm for full motion video at low bit
    rates; uses both intraframe and interframe coding.  It defines a bit
    stream for compressed video and audio optimized to fit into a bandwidth
    (data rate) of 1.5 Mbits/s.  This rate is special because it is the
    data rate of (uncompressed) audio CD's and DAT's.   The draft is in
    three parts, video, audio, and systems, where the last part gives the
    integration of the audio and video streams with the proper timestamping
    to allow synchronization of the two.   MPEG phase II is to define a
    bitstream for video and audio coded at around 3 to 10 Mbits/s.

    MPEG compresses YUV SIF images.   Motion is predicted from frame to
    frame, while DCTs of the difference signal with quantization make use
    of spatial redundancy.  DCTs are performed on 8 by 8 blocks, the motion
    prediction on 16 by 16 blocks of the luminance signal.  Quantization
    changes for every 16 by 16 macroblock.

    There are three types of coded frames.  Intra (``I'') frames are coded
    without motion prediction, Predicted (``P'') frames are difference
    frames to the last P or I frame.   Each macroblock in a P frame can
    either come with a vector and difference DCT coefficients for a close
    match in the last I or P frame, or it can just be intra coded (like
    in the I frames) if there was no good match.  Lastly, there are "B"
    or bidirectional frames.  They are predicted from the closest two I or
    P frames, one in the past and one in the future.  These are searched
    for matching blocks in those frames, and three different things tried
    to see which works best:  the forward vector, the backward vector, and
    the average of the two blocks from the future and past frames, and
    subtracting that from the block being coded.   If none of those work
    well, the block is intra-coded.

    There are 12 frames from I to I, based on random access requirements.

MPEG-1: Informal name of proposed !MPEG (ISO standard DIS 1172).

media source: entity (user and host) that produced the media content.
    It is the entity that is shown as the active participant by the

MTU: maximum transmission unit; the largest frame length which may be sent
    on a physical medium.

Nevot: network voice terminal; application written by the author.

network source: entity denoted by address and port number from which the !
    end system receives the RTP packet and to which the end system send any
    RTP packets for that conference in return.

NTP timestamp: ``NTP  timestamps  are  represented  as  a  64-bit  unsigned
    fixed-point number, in seconds relative to 0 hours on 1 January 1900.
    The integer part is in the first 32 bits and the fraction part in the

H. Schulzrinne                  Expires 03/01/94                  [Page 58]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

    last 32 bits.'' [13] NTP timestamps do not include leap seconds, i.e.,
    each and every day contains exactly 86,400 NTP seconds.

NVP: network voice protocol; original packet format used in early packet
    voice experiments; defined in [1].

octet: An octet is an 8-bit datum, which may contain values 0 through 255
    decimal.   Commonly used in ISO and CCITT documents, also known as a

OSI: Open System Interconnection;  a suite of protocols,  designed by
    ISO committees,  to be the international standard computer network

out of band: signaling and control information is carried in a separate
    channel or separate packets from the actual data.  For example, ICMP
    carries control information out-of-band, that is, as separate packets,
    for IP, but both ICMP and IP usually use the same communication channel
    (in band).

parametric coder: coder that encodes parameters of a model representing the
    input signal.  For example, LPC models a voice source as segments of
    voice and unvoiced speech, represented by a set of

parametric coder: coder that encodes parameters of a model representing the
    input signal.  For example, LPC models a voice source as segments of
    voice and unvoiced speech, represented by filter parameters.  Examples
    include LPC, CELP and GSM. !waveform coder.

PCM: pulse-code modulation; speech coding where speech is represented by a
    given number of fixed-width samples per second.   Often used for the
    coding employed in the telephone network:  64,000 eight-bit samples per

pel, pixel: picture element.    ``Smallest graphic element that can be
    independently addressed within a picture; (an alternative term for
    raster graphics element).''  (T.411)

playout: Delivery of the medium content to the final consumer within the
    receiving host.  For audio, this implies digital-to-analog conversion,
    for video display on a screen.

playout unit: A playout unit is a group of packets sharing a common
    timestamp.   (Naturally, packets whose timestamps are identical due
    to timestamp wrap-around are not considered part of the same playout
    unit.)  For voice, the playout unit would typically be a single voice
    segment, while for video a video frame could be broken down into
    subframes, each consisting of packets sharing the same timestamp and
    ordered by some form of sequence number.  !synchronization unit

H. Schulzrinne                  Expires 03/01/94                  [Page 59]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

plesiochronous: ``The essential characteristic of time-scales or signals
    such that their corresponding significant instants occur at nominally
    the same rate, any variation in rate being constrained within specified
    limits.   Two signals having the same nominal digit rate, but not
    stemming from the same clock or homochronous clocks,  are usually
    plesiochronous.   There is no limit to the time relationship between
    corresponding significant instants.''   (G.701, Q.9) In other words,
    plesiochronous  clocks  have  (almost)  the  same  rate,  but  possibly
    different phase.

pulse code modulation (PCM): ``A process in which a signal is sampled, and
    each sample is quantized independently of other samples and converted
    by encoding to a digital signal.''  (G.701)

PVP: packet video protocol; extension of ! NVP to video data [28]

QCIF: quarter common interchange format; format for exchanging video images
    with half as many lines and half as many pixels per line as CIF, i.e.,
    luminance information is coded at 144 lines and 176 pixels per line.
    !CIF, SIF

RTCP: real-time control protocol; adjunct to ! RTP.

RTP: real-time transport protocol; discussed in this memorandum.

sampling rate: ``The number of samples taken of a signal per unit time.''

SB: subband; as in subband codec.  Audio or video encoding that splits the
    frequency content of a signal into several bands and encodes each band
    separately, with the encoding fidelity matched to human perception for
    that particular frequency band.

SCIF: standard video interchange format;  consists of four !CIF images
    arranged in a square.  !CIF, QCIF

SIF: standard interchange format; format for exchanging video images of 240
    lines with 352 pixels each for NTSC, and 288 lines by 352 pixels for
    PAL and SECAM. At the nominal field rates of 60 and 50 fields/s, the
    two formats have the same data rate.  !CIF, QCIF

slip: In digital communications, slip refers to bit errors caused by the
    different clock rates of nominally synchronous sender and receiver.  If
    the sender clock is faster than the receiver clock, occasionally a bit
    will have to be dropped.  Conversely, a faster receiver will need to
    insert extra bits.   The problem also occurs if the clock rates of
    encoder and decoder are not matched precisely.  Information loss can be
    avoided if the duration of pauses (silence periods between talkspurts
    or the inter-frame duration) can be adjusted by the receiver.  ``The
    repetition or deletion of a block of bits in a synchronous or
    plesiochronous bit stream due to a discrepancy in the read and write

H. Schulzrinne                  Expires 03/01/94                  [Page 60]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

    rates at a buffer.''  (G.810) !G.821, G.822

ST-II: stream  protocol;   connection-oriented  unreliable,  non-sequenced
    packet-oriented network and transport protocol with process demulti-
    plexing and provisions for establishing flow parameters for resource
    control; defined in RFC 1190 [29,30].

Super CIF: video format defined in Annex IV of !H.261 (1992), comprising
    704 by 576 pixels.

synchronization unit: A  synchronization  unit  consists  of  one  or  more
    !playout units that, as a group, share a common fixed delay between
    generation and playout of each part of the group.  The delay may change
    at the beginning of such a synchronization unit.   The most common
    synchronization units are talkspurts for voice and frames for video

TCP: transmission control protocol; an Internet Standard transport layer
    protocol  defined  in  RFC  793.     It  is  connection-oriented  and
    stream-oriented, as opposed to UDP [31].

TPDU: transport protocol data unit.

tightly controlled conference: Participants can join the conference only
    after an invitation from a conference moderator.  The identify of all
    conference participants is known to the moderator.  !loosely controlled

transcoder: device  or  application  that  translates  between  several
    encodings, for example between ! LPC and ! PCM.

UDP: user  datagram  protocol;  unreliable,  non-sequenced  connectionless
    transport protocol defined in RFC 768 [32].

vat: visual audio tool written by Steve McCanne and Van Jacobson, Lawrence
    Berkeley Laboratory.

vt: voice terminal software written at the Information Sciences Institute.

VMTP: Versatile message transaction protocol; defined in RFC 1045 [33].

waveform coder: a  coder  that  tries  to  reproduce  the  waveform  after
    decompression; examples include PCM and ADPCM for audio and video and
    discrete-cosine-transform based coders for video; !parametric coder.

Y: Common abbreviation for the luminance or luma signal.

YCbCr: YCbCr coding is employed by D-1 component video equipment.

H. Schulzrinne                  Expires 03/01/94                  [Page 61]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

B Address of Author

Henning Schulzrinne
AT&T Bell Laboratories
MH 2A244
600 Mountain Avenue
Murray Hill, NJ 07974-0636
telephone:  +1 908 582 2262
facsimile:  +1 908 582 5809
electronic mail:


 [1] D. Cohen, ``A network voice protocol:  NVP-II,'' technical report,
     University of Southern California/ISI, Marina del Ray, California,
     Apr. 1981.

 [2] N.  Borenstein  and  N.  Freed,  ``MIME  (multipurpose  internet  mail
     extensions) mechanisms for specifying and describing the format of
     internet message bodies,'' Network Working Group Request for Comments
     RFC 1341, Bellcore, June 1992.

 [3] R. Want, A. Hopper, V. Falcao, and J. Gibbons, ``The active badge
     location system,'' ACM Transactions on Information Systems, vol. 10,
     pp. 91--102, Jan. 1992.

 [4] R. Want and A. Hopper,  ``Active badges and personal interactive
     computing objects,'' Technical Report ORL 92-2, Olivetti Research,
     Cambridge, England, Feb. 1992. also in IEEE Transactions on Consumer
     Electronics, Feb. 1992.

 [5] J. G. Gruber and L. Strawczynski, ``Subjective effects of variable
     delay and speech clipping in dynamically managed voice systems,'' IEEE
     Transactions on Communications, vol. COM-33, pp. 801--808, Aug. 1985.

 [6] N. S. Jayant, ``Effects of packet losses in waveform coded speech and
     improvements due to an odd-even sample-interpolation procedure,'' IEEE
     Transactions on Communications, vol. COM-29, pp. 101--109, Feb. 1981.

 [7] D. Minoli, ``Optimal packet length for packet voice communication,''
     IEEE Transactions on Communications, vol. COM-27, pp. 607--611, Mar.

 [8] V.  Jacobson,  ``Compressing  TCP/IP  headers  for  low-speed  serial
     links,'' Network Working Group Request for Comments RFC 1144, Lawrence
     Berkeley Laboratory, Feb. 1990.

H. Schulzrinne                  Expires 03/01/94                  [Page 62]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

 [9] P. Francis, ``A near-term architecture for deploying Pip,'' IEEE
     Network, vol. 7, pp. 30--37, May 1993.

[10] IMA Digital Audio Focus and Technical Working Groups, ``Recommended
     practices for enhancing digital audio compatibility in multimedia
     systems,'' tech. rep., Interactive Multimedia Association, Annapolis,
     Maryland, Oct. 1992.

[11] W. A. Montgomery, ``Techniques for packet voice synchronization,''
     IEEE  Journal  on  Selected  Areas  in  Communications,  vol.  SAC-1,
     pp. 1022--1028, Dec. 1983.

[12] D. Cohen, ``A protocol for packet-switching voice communication,''
     Computer Networks, vol. 2, pp. 320--331, September/October 1978.

[13] D. L. Mills, ``Network time protocol (version 3) -- specification,
     implementation and analysis,''  Network Working Group Request for
     Comments RFC 1305, University of Delaware, Mar. 1992.

[14] ISO/IEC JTC 1, ISO/IEC DIS 11172:  Information technology --- coding
     of moving pictures and associated audio for digital storage media up
     to about 1.5 Mbit/s. International Organization for Standardization
     and International Electrotechnical Commission, 1992.

[15] L. Delgrossi, C. Halstrick, R. G. Herrtwich, and H. St"uttgen, ``HeiTP:
     a transport protocol for ST-II,'' in Proceedings of the Conference on
     Global Communications (GLOBECOM), (Orlando, Florida), pp. 1369--1373
     (40.02), IEEE, Dec. 1992.

[16] G. J. Holzmann, Design and Validation of Computer Protocols. Englewood
     Cliffs, New Jersey:  Prentice Hall, 1991.

[17] A. Nakassis, ``Fletcher's error detection algorithm:  how to implement
     it efficiently and how to avoid the most common pitfalls,'' ACM
     Computer Communication Review, vol. 18, pp. 63--88, Oct. 1988.

[18] J. G. Fletcher, ``An arithmetic checksum for serial transmission,''
     IEEE Transactions on Communications, vol. COM-30, pp. 247--252, Jan.

[19] J. Linn, ``Privacy enhancement for Internet electronic mail:  Part III
     --- algorithms, modes and identifiers,'' Network Working Group Request
     for Comments RFC 1115, IETF, Aug. 1989.

[20] D. Balenson, ``Privacy enhancement for internet electronic mail:  Part
     III: Algorithms,  modes,  and identifiers,''  Network Working Group
     Request for Comments RFC 1423, IETF, Feb. 1993.

[21] S. Kent, ``Privacy enhancement for internet electronic mail:  Part II:
     Certificate-based key management,'' Network Working Group Request for
     Comments RFC 1422, IETF, Feb. 1993.

H. Schulzrinne                  Expires 03/01/94                  [Page 63]

INTERNET-DRAFT         draft-ietf-avt-issues-01.txt        October 20, 1993

[22] J. Linn, ``Privacy enhancement for Internet electronic mail:  Part
     I --- message encipherment and authentication procedures,'' Network
     Working Group Request for Comments RFC 1113, IETF, Aug. 1989.

[23] R. Rivest, ``The MD5 message-digest algorithm,'' Network Working Group
     Request for Comments RFC 1321, IETF, Apr. 1992.

[24] North American Directory Forum, ``A naming scheme for c=US,'' Network
     Working Group Request for Comments RFC 1255, North American Directory
     Forum, Sept. 1991.

[25] N. S. Jayant and P. Noll, Digital Coding of Waveforms. Englewood
     Cliffs, New Jersey:  Prentice Hall, 1984.

[26] P. T. Brady, ``A model for generating on-off speech patterns in
     two-way conversation,''  Bell System Technical Journal,  vol. 48,
     pp. 2445--2472, Sept. 1969.

[27] J. Postel, ``Internet protocol,'' Network Working Group Request for
     Comments RFC 791, Information Sciences Institute, Sept. 1981.

[28] R. Cole, ``PVP - a packet video protocol,'' W-Note 28, Information
     Sciences Institute, University of Southern California, Los Angeles,
     California, Aug. 1981.

[29] C. Topolcic, S. Casner, C. Lynn, Jr., P. Park, and K. Schroder,
     ``Experimental internet stream protocol, version 2 (ST-II),'' Network
     Working  Group  Request  for  Comments  RFC  1190,  BBN  Systems  and
     Technologies, Oct. 1990.

[30] C. Topolcic, ``ST II,'' in First International Workshop on Network and
     Operating System Support for Digital Audio and Video, no. TR-90-062 in
     ICSI Technical Reports, (Berkeley, California), 1990.

[31] J. B. Postel, ``DoD standard transmission control protocol,'' Network
     Working Group Request for Comments RFC 761, Information Sciences
     Institute, Jan. 1980.

[32] J. B. Postel,  ``User datagram protocol,''  Network Working Group
     Request for Comments RFC 768, ISI, Aug. 1980.

[33] D.  R.  Cheriton,  ``VMTP:  Versatile  Message  Transaction  Protocol
     specification,'' in Network Information Center RFC 1045, (Menlo Park,
     California), pp. 1--123, SRI International, Feb. 1988.

H. Schulzrinne                  Expires 03/01/94                  [Page 64]