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   Internet Engineering Task Force                  Flemming Andreasen
   MMUSIC Working Group                                     David Oran
   INTERNET-DRAFT                                             Dan Wing
   Expires: August 2004                                  Cisco Systems
                                                        February, 2004

                       RTP No-Op Payload Format
                   <draft-wing-avt-rtp-noop-00.txt>


Status of this memo

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

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

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document defines an no-op payload format for the Real-time
   Transport Protocol (RTP), and a mechanism to request an immediate
   RTCP report.  This can be used to verify RTP connectivity and to
   keep Network Address Translator (NAT) bindings and Firewall pinholes
   open.



INTERNET-DRAFT             RTP No-Op Payload            February 2004


   TABLE OF CONTENTS

1.   Introduction.....................................................2
1.1 Notational Conventions............................................2
2. RTP Payload Format for No-Op.......................................3
2.2 Registration......................................................3
2.3 Use of RTP Header Fields..........................................3
2.4 Payload Format....................................................3
2.5 Sender Operation..................................................4
2.6 Mixer, Translator Operation.......................................4
2.7 Receiver Operation................................................4
2.8 Indication of No-OP Capability using SDP..........................5
3. MIME Registration..................................................5
3.1. audio/no-op......................................................5
4. Security Considerations............................................6
5. Acknowledgements...................................................6
6. Authors' Addresses.................................................6
7. Normative References...............................................6
8. Informative References.............................................6
Intellectual Property Statement.......................................7
Full Copyright Statement..............................................7
Acknowledgement.......................................................8

1. Introduction

   This memo defines a new RTP payload format called "no-op".  This
   payload behaves like a normal RTP payload, except that it isn't
   played by the receiver.

   This new payload format is useful for:

     *  bearer continuity testing, such as at the beginning of a call;

     *  keepalives to keep NAT bindings open when RTP media traffic is
        not otherwise being transmitted;

   For testing the RTP path, an RTP sender may transmit several No-Op
   payload packets with the Request Immediate RTCP bit set to 0,
   followed by one No-Op payload packet with the Request Immediate RTCP
   bit set to 1.  This would cause the RTP receiver to send an RTCP
   report indicating the quality of the RTP path.  The RTP sender could
   then decide to continue with call setup, abort the session, or
   perform some other action.

1.1 Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].




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2. RTP Payload Format for No-Op

   The no-op payload format is carried as part of the RTP stream, and
   MUST use the same sequence number space, SSRC, and timestamp base as
   the regular media.

2.2 Registration

   The RTP payload format is designated as "no-op" and the MIME type as
   "audio/no-op".  The default clock rate is 8000 Hz, but other rates
   MAY be used.  In accordance with current practice, this payload
   format does not have a static payload type number, but uses a RTP
   payload type number established dynamically and out-of-band.

2.3 Use of RTP Header Fields

        Timestamp:  The RTP timestamp reflects the measurement point
                    for the current packet. The receiver calculates
                    jitter for RTCP receiver reports based on all
                    packets with a given timestamp. Note: The jitter
                    value should primarily be used as a means for
                    comparing the reception quality between two users
                    or two time-periods, not as an absolute measure.

       Marker bit: The RTP marker bit has no special significance for
                    this payload type.

2.4 Payload Format

   The payload format is shown in Figure 1.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |R|                         reserved                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      padding (OPTIONAL)                       |
      |                             ....                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The payload contains at least 4 bytes.  The first 32 bits are
   defined as follows:

     bit 0:      "R", "Request immediate RTCP", is used to request
                 transmission of an immediate RTCP report (see section
                 2.7).

     bits 1-31:  Reserved, and all bits MUST be 0.

   Additional padding bytes MAY be appended up to the negotiated ptime
   value in SDP (see section 2.6).  These bytes MUST contain all 0



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   bits.  Padding may be useful to generate RTP packets that are the
   same size as another payload (such as a normal voice payload).

2.5 Sender Operation

   A source MAY send normal RTP audio and the no-op payload format for
   the same time instants (but with different sequence numbers of
   course).  This might be done in conjunction with this payload
   format's "Request Immediate RTCP" opcode.

2.6 Mixer, Translator Operation

   An RTP mixer or unicast-to-unicast RTP translator SHOULD forward RTP
   No-Op payload packets normally.  A unicast-to-multicast RTP
   translator SHOULD replicate RTP No-Op payload packets normally.

   A multicast-to-unicast RTP translator SHOULD NOT replicate an RTP
   No-Op packet with the Request Immediate RTCP bit set, because the
   receivers won't be able to prevent flooding of the multicast RTP
   sender.

2.7 Receiver Operation

   Upon receipt of an RTP packet with the No-Op payload format and the
   Send Immediate RTCP Report bit set to 0, the receiver performs
   normal RTP receive operations on it -- incrementing the RTP receive
   counter, calculating jitter, and so on.  The receiver then discards
   the packet -- it is not used to play out data.

   Upon receipt of an RTP packet with the No-Op payload format and the
   Send Immediate RTCP Report bit set to 1, the receiver performs the
   steps above and:

     *  if listening on a multicast IP address, the receiver MUST not
        send an immediate RTCP report, and the receiver MUST follow the
        normal RTCP transmission rules [RFC3550, sections 6.2 and 6.3].

     * if listening on a unicast IP address and sending RTP traffic,
       the receiver prepares to send an RTCP sender report, and

     * if listening on a unicast IP address and receiving RTP traffic,
       the receiver prepares to send an RTCP receiver report.

   In all cases, before actually sending its RTCP report, the RTCP
   bandwidth limits and randomization interval MUST be observed
   [RFC3550, sections 6.2 and 6.3], most especially when multiple SSRCs
   are in the same session.




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2.8 Indication of No-OP Capability using SDP

   Senders and receivers may indicate support for the No-Op payload
   format, for example, by using the Session Description Protocol
   ([SDP]).

   If successful completion of RTP No-Op is required before completing
   call establishment -- such as to verify the existence or quality of
   the bearer path -- No-Op preconditions can be used [Andreasen].

   The default packetization interval for this payload type is 20ms
   (ptime:20) but alternate values can be advertised in SDP using the
   ptime attribute value [SDP].

3. MIME Registration

3.1. audio/no-op

         MIME media type name: audio

         MIME subtype name: no-op

         Required parameters: none

         Optional parameters: none

         Encoding considerations: This type is only defined for
              transfer via RTP [1].

         Security considerations: See the "Security Considerations"
              section in this document.

         Interoperability considerations: none

         Published specification: This document.

         Applications which use this media: The "no-op" audio subtype
               is used to maintain network state or verify network
               connectivity, when a more traditional RTP payload type
               cannot be used.

         Additional information:

              1. Magic number(s): N/A

              2. File extension(s): N/A

              3. Macintosh file type code: N/A




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4. Security Considerations

   Without security of the RTP stream (via SRTP [SRTP], IPsec, or other
   means), it is possible for an attacker to spoof RTP packets,
   including this new packet type.  As this new RTP payload type
   includes a method to request immediate transmission of RTCP, this
   could be used to cause endpoints to flood the network with RTCP
   reports.  Thus, the RTCP transmissions MUST NOT exceed the bandwidth
   recommendations described in section 6.3 of [RFC3550].

5. Acknowledgements

   Thanks to Henning Schulzrinne for suggesting using RTCP as a
   feedback mechanism.

6. Authors' Addresses

   Flemming Andreasen
   Cisco Systems, Inc.
   499 Thornall Street, 8th Floor
   Edison, NJ 08837  USA

   EMail: fandreas@cisco.com


   David Oran
   Cisco Systems, Inc.
   7 Ladyslipper Lane
   Acton, MA 01720  USA

   EMail: oran@cisco.com


   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134  USA

   EMail: dwing@cisco.com


7. Normative References

   [RFC3550] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson,
   "RTP: A Transport Protocol for Real-Time Applications",
   http://www.ietf.org/rfc/rfc3550.txt.

8. Informative References

   [Andreasen] F. Andreasen, "No-Op Preconditions", Work In Progress.




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   [RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple
   Capability Declaration", October 2002,
   http://www.ietf.org/rfc/rfc3407.txt

   [SDP] M. Handley and V. Jacobson, "SDP: Session Description
   Protocol", April 1998, http://www.ietf.org/rfc/rfc2327.txt.

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   This document and the information contained herein is provided on an
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




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