Network Working Group C. Alexander, Ed.
Internet-Draft J. Babiarz
Expires: August 15, 2005 Nortel
February 11, 2005
RTP Payload Format for ECN Probing
draft-alexander-rtp-payload-for-ecn-probing-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo defines a Real Time Transport Protocol (RTP) payload format
for use when probing for congestion using Explicit Congestion
Detection (ECN). This payload format is intended for use with the
probing mechanisms described in [3]. While defined in terms of the
specific application of admission control, it is desirable to overlay
this format with other probing mechanisms so as to reduce the number
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of probing packet formats.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. RTP Payload Format for Real-Time ECN Admission Control . . . . 6
4.1 Registration . . . . . . . . . . . . . . . . . . . . . . . 6
4.2 IP Header Fields . . . . . . . . . . . . . . . . . . . . . 6
4.3 RTP Header Fields . . . . . . . . . . . . . . . . . . . . 6
4.4 Payload Format . . . . . . . . . . . . . . . . . . . . . . 6
4.4.1 Version . . . . . . . . . . . . . . . . . . . . . . . 7
4.4.2 Sender Congestion Indication (SCI) . . . . . . . . . . 7
4.4.3 Responder Congestion Indication (RCI) . . . . . . . . 7
4.4.4 Sender Congestion Indication (SCI) Sequence Number . . 7
4.4.5 Reserved . . . . . . . . . . . . . . . . . . . . . . . 7
5. Considerations for New Payload Format . . . . . . . . . . . . 8
5.1 Extensibility Considerations . . . . . . . . . . . . . . . 8
5.2 Flexibility Considerations . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1 Normative References . . . . . . . . . . . . . . . . . . . 12
9.2 Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 14
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1. Introduction
This memo defines a new RTP payload format for use with applications
requiring congestion detection along the data path, verification of
data path connectivity, or measurement of Round Trip Time (RTT)
between two endpoints, for example, admission control of a real-time
session. The format described herein is intended for use with the
mechanisms described in "Congestion Notification Process for
Real-Time Traffic" [2], which defines the use of the Explicit
Congestion Detection (ECN) bits in the Internet Protocol (IP) header
as a means to detect congestion in the network for real-time
inelastic flows. The new format can be used to provide the
capabilities described in "Admission Control Use Case for Real-time
ECN" [3], although it may additionally be used in other contexts.
The new RTP payload format defined herein is called "admcntl".
Packets utilizing this payload are carried as RTP traffic through the
IP network. Packets carrying this payload are treated the same as
any other RTP packet with the exception of play-back by the receiving
device.
The advantages of using this new payload format are:
1. congestion detection can be performed using a simple probing
mechanism without having to extend other protocols;
2. the payload format accommodates both one-way and
two-way/loop-back mechanisms;
3. the payload format allows for limited detection of devices making
inappropriate changes to the ECN markings in the network;
4. the packet carrying the payload can vary in size from the minimum
necessary to carry the payload, to a size padded to mimic a
specific codec.
Applications will use this payload format to create and send RTP
probe packets through the IP network to determine the highest state
of congestion along the path taken by the packets. Depending on the
probing mechanism by an implementation, some of the fields defined
may not be used.
In all uses, applications receiving this payload MUST NOT attempt to
play it as actual media.
This memo only defines the new payload format. Examples of its usage
can be found in [3].
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2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [9] and
indicate requirement levels for compliant implementations.
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3. Definitions
The following terms are used in this document:
Cheater: A device in the network that makes inappropriate changes to
the ECN markings in the network. A cheating device might re-mark
the ECN bits in the IP header in order to hide congestion from an
endpoint (i.e., by lowering the ECN congestion marking), or might
force an endpoint to think congestion is present when it is not
(i.e., by raising the ECN congestion marking). Due to the nature
of ECN and how conformant network devices mark ECN for real-time
inelastic flows, it is possible to detect the presence of cheater
devices which lower the ECN marking, but not those that raise it.
Request Probe Packet: In the context of a one-way probing mechanism,
the Request Probe Packet is an RTP packet utilizing the "admcntl"
payload format defined herein. In the context of a two-way
probing mechanism, it is also an RTP packet using the "admcntl"
payload format, but it is limited by definition to flow only from
the Sender to the Responder.
Responder: In the context of a two-way/loop-back probing mechanism,
the Responder is an endpoint device which receives a Request Probe
Packet from the Sender, and generates a Response Probe Packet in
response. In the context of a one-way probing mechanism, the
Responder is simply an endpoint device which receives a Request
Probe Packet from the Sender. In this context, however, the
Responder does not actually respond with a Response Probe Packet.
Response Probe Packet: In the context of a two-way probing mechanism,
the Response Probe Packet is an RTP packet utilizing the "admcntl"
payload. It differs from the Request Probe Packet in that it is
limited by definition to flow only from the Responder to the
Sender. A Response Probe Packet has no definition in the context
of a one-way probing mechanism.
Sender: In the context of both a one-way and a two-way/loop-back
probing mechanism, the Sender is an endpoint device which
generates a Request Probe Packet.
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4. RTP Payload Format for Real-Time ECN Admission Control
The "admcntl" payload is transported in RTP packets. However, it is
not part of an RTP stream. It therefore has no requirements to use
similar properties of the media to be admitted.
4.1 Registration
The new RTP payload format is defined as "admcntl", with a MIME type
of "audio/admcntl" for audio and a MIME type of "video/admcntl" for
video. The RTP payload type for RTP packets carrying this payload is
determined dynamically through methods outside the scope of this
document.
4.2 IP Header Fields
DSCP: The DSCP set in the IP header is a critical component of the
ECN method as outlined in [2]. It should be set appropriately for
the session media for which admission control is being performed.
ECN: Unless attempting to detect for the presence of Cheaters along
the media path, an application MUST set the two-bit ECN field in
the IP header to 01, which indicates that it is an ECN-capable
transport, with no congestion experienced. If attempting to
detect for the presence of Cheaters, the ECN field SHOULD be set
as required by the detection method being used.
4.3 RTP Header Fields
Payload Type: The payload type field MUST be filled with a value
determined dynamically, and communicated to all application
devices involved.
4.4 Payload Format
The "admcntl" 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version|SCI|RCI| SCI Sequence Number | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: admcntl Payload Format
It consists of five fields: Version, SCI, RCI, SCI Sequence Number,
and Reserved.
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4.4.1 Version
The Version field designates the version of the payload format. At
the time of this writing, this field MUST be set to zero. This field
is provided for future extensibility of the payload to carry other
information.
4.4.2 Sender Congestion Indication (SCI)
This field contains a two-bit ECN value associated with the Sender.
It has meaning in both one-way and two-way/loop-back probing
mechanisms.
For both one-way and two-way/loop-back probing mechanisms, the Sender
MUST set this field in the Request Probe Packet to the two-bit ECN
value it uses to send the packet.
For a two-way/loop-back probing mechanism, the Responder MUST set
this field in the Response Probe Packet to the two-bit ECN value
retrieved from the ECN field in the IP header of the associated
Request Probe Packet.
4.4.3 Responder Congestion Indication (RCI)
This field contains a two-bit ECN value associated with the
Responder. It has meaning only with two-way/loop-back probing
mechanisms.
For two-way/loop-back probing mechanisms, the Responder MUST set this
field in the Response Probe Packet to the two-bit ECN value it uses
to send the packet.
For a one-way probing mechanism, this field is unused.
4.4.4 Sender Congestion Indication (SCI) Sequence Number
This field contains a 16-bit sequence number. It has meaning only
with two-way/loop-back probing mechanisms.
For two-way/loop-back probing mechanisms, the Responder MUST set this
field in the Response Probe Packet to the 16-bit sequence number in
the RTP header of the associated Request Probe Packet.
For a one-way probing mechanism, this field is unused.
4.4.5 Reserved
This field contains eight bits which are reserved for future use.
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5. Considerations for New Payload Format
There were two main considerations driving the new payload format
defined in this memo: extensibility and flexibility.
5.1 Extensibility Considerations
While the intended use for the new payload format is for admission
control using ECN, the payload format need not be limited to that
application. Even for admission control applications which will use
it, the payload format also need not be limited to the mechanisms
described in this memo. With that in mind, the four-bit Version
field is included to allow for extensibility for future applications
and/or implementations.
5.2 Flexibility Considerations
In addition to planning the payload format for extensibility, another
flexibility consideration is to allow the initial definition of the
payload to be used in as wide a range of implementations as possible.
In the simplest form of the two-way/loop-back mechanisms, probing for
ECN only requires the two-bit SCI field in which to return the
received ECN marking. However, the definition of this single field
does not limit the use of this payload format only to
two-way/loop-back mechanisms.
By adding the RCI field for a two-way/loop-back mechanism, and
extending the definition of the SCI field for one-way probing
mechanisms, both mechanisms can utilize these fields to perform
limited detection of Cheaters. The SCI Sequence Number field is also
intended to allow for such detection to be performed.
The minimum length of the payload is 4 octets, although it MAY be
padded to simulate a specific codec. In this case, the application
also needs to ensure that the packets carrying the padded payload are
sent at the appropriate rate corresponding to the codec being
mimicked.
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6. Security Considerations
Security considerations for the use of ECN for real-time inelastic
flows is covered in [2]. The main consideration to account for here
is that when the payload is carrying any relevant information for
admission control, the payload SHOULD be secured, e.g., using "The
Secure Real-time Transport Protocol (SRTP)" [5] or "Security
Architecture for the Internet Protocol" [6].
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7. IANA Considerations
The Version field in the admcntl payload format will need to be
administered. This field should be administered on a first come,
first served basis. Additional details will be provided in future
revisions of this memo.
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8. Acknowledgements
The authors acknowledge a great many inputs, including the following:
John Rutledge, Jeremy Matthews, Marvin Krym, Stephen Dudley, and Kwok
Ho Chan.
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9. References
9.1 Normative References
[1] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 3550, January 2005.
[2] Babiarz, J., Chan, K. and V. Firoiu, "Congestion Notification
Process for Real-Time Traffic, draft-babiarz-tsvwg-rtecn-03",
Internet-Draft Work in Progress, February 2005.
[3] Alexander, C., Ed., Babiarz, J. and J. Matthews, "Admission
Control Use Case for Real-time ECN,
draft-alexander-rtecn-admission-control-use-case-00",
Internet-Draft Work in Progress, February 2005.
9.2 Informative References
[4] Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001.
[5] Baugher, M., Carrara, E., McGrew, D., Naslund, M. and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
Internet-Draft Work in Progress, March 2004.
[6] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", Internet-Draft Work in Progress, November
1998.
Authors' Addresses
Corey W. Alexander (editor)
Nortel
MS 08704A30
2370 Performance Drive
Richardson, TX 75287
US
Phone: +1 972 684-8320
Fax: +1 972 684-1838
Email: coreya@nortel.com
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Jozef Babiarz
Nortel
MS 04331C04
3500 Carling Avenue
Ottawa, Ontario K2H 8E9
CA
Phone: +1 613 763-6098
Fax: +1 613 763-2231
Email: babiarz@nortel.com
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