Internet DRAFT - draft-alexander-rtp-payload-for-ecn-probing
draft-alexander-rtp-payload-for-ecn-probing
Network Working Group C. Alexander, Ed.
Internet-Draft J. Babiarz
Expires: April 26, 2006 Nortel
October 23, 2005
RTP Payload Format for ECN Probing
draft-alexander-rtp-payload-for-ecn-probing-02.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 draft "Real-time ECN Use Cases".
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 of probing packet formats.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. History . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Version 00 . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Version 01 . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Alternatives Under Consideration . . . . . . . . . . . . . . . 7
5.1 Define New RTP Format . . . . . . . . . . . . . . . . . . 7
5.2 UDP . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.3 ICE/STUN . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.4 Re-use Existing RTP Format . . . . . . . . . . . . . . . . 8
6. RTP Payload Format for Real-Time ECN Admission Control . . . . 9
6.1 Registration . . . . . . . . . . . . . . . . . . . . . . . 9
6.2 IP Header Fields . . . . . . . . . . . . . . . . . . . . . 9
6.3 RTP Header Fields . . . . . . . . . . . . . . . . . . . . 9
6.4 Payload Format . . . . . . . . . . . . . . . . . . . . . . 9
6.4.1 Version . . . . . . . . . . . . . . . . . . . . . . . 10
6.4.2 Explicit Congestion Notification (ECN) . . . . . . . . 10
6.4.3 Initial RTP Sequence Number (IRSN) . . . . . . . . . . 10
6.4.4 Reserved . . . . . . . . . . . . . . . . . . . . . . . 10
7. Considerations for Payload Format . . . . . . . . . . . . . . 11
7.1 Extensibility Considerations . . . . . . . . . . . . . . . 11
7.2 Flexibility Considerations . . . . . . . . . . . . . . . . 11
8. Considerations for Direct Feedback . . . . . . . . . . . . . . 12
8.1 Feedback via RTP . . . . . . . . . . . . . . . . . . . . . 12
8.2 Feedback via RTCP . . . . . . . . . . . . . . . . . . . . 12
9. MIME Registration . . . . . . . . . . . . . . . . . . . . . . 13
9.1 audio/ecnprobe . . . . . . . . . . . . . . . . . . . . . . 13
9.2 video/ecnprobe . . . . . . . . . . . . . . . . . . . . . . 13
10. Security Considerations . . . . . . . . . . . . . . . . . . 15
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . 16
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
13.1 Normative References . . . . . . . . . . . . . . . . . . . 18
13.2 Informative References . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . 20
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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 and verification
of data path connectivity, 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 Notification (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 "Real-time ECN Use Cases" [3], although it
may also be used in other contexts.
The new RTP payload format defined herein is called "ecnprobe".
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 payload format are:
1. congestion detection can be performed using a simple probing
mechanism without having to extend other protocols;
2. the payload format allows for limited detection of devices making
inappropriate changes to the ECN markings in the network;
3. 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.
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. History
2.1 Version 00
Initial submission.
2.2 Version 01
Adding sections describing alternatives under investigation, as well
as the reasons why we believe RTP is necessary. No other major
content changes over -00 version.
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3. 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 RFC2119 [7] and
indicate requirement levels for compliant implementations.
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4. 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.
Probe Packet: The Probe Packet is an RTP packet utilizing the
"ecnprobe" payload format defined herein.
Receiver: The Receiver is simply an endpoint device which receives
one or more Probe Packets from the Sender. As defined here, the
Responder does not respond directly to the Probe Packet.
Sender: The Sender is an endpoint device which generates one or more
Probe Packets.
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5. Alternatives Under Consideration
While we believe it is necessary to use RTP as the packet format for
probing, there are other options. This section outlines the various
options currently being investigated, including RTP.
5.1 Define New RTP Format
RTP is the mechanism we originally specified, and the overall content
of this document still presumes RTP as the packet format for probing.
What we have not adequately answered is why RTP? There is certainly
no media content being delivered via the packets, resulting in the
use of RTP for this coming into question.
RTP is preferred primarily because real media utilizes it and the
underlying Real-time ECN mechanism for admission control of new
sessions can benefit from it. The RTP probe flow has several other
functions besides congestion monitoring. First, probing verifies the
media path end-to-end, ensuring that both endpoints can reliably send
RTP packets before alerting. An unreliable path can be detected via
RTP mechanisms, allowing such flows to not be admitted.
Second, probing verifies that the measured traffic level is within
the pre-engineered limits to provide good service. For this purpose,
ECN metering and marking must be performed on the RTP probe flow
packets along the same path that the RTP media packets will use.
Within the network, routers must treat probe packets as close to real
RTP media packets as possible. Using RTP for the probe packets
should ensure this. It has been pointed out that routers can filter
traffic based on traffic type, for example, routing RTP traffic one
way and other traffic another, and if this were to occur when the
probe packet were not using RTP, then the Real-time ECN mechanism
would not function as intended. While acknowledging again that the
probes would carry no media, RTP is necessary to ensure appropriate
treatment of the probe packets in the network.
Third, since endpoints already utilize RTP, there is a minimal
development effort to add ECN capabilities to RTP.
By using RTP, we are also able to leverage use of the RTP sequence
number field during conformance verification, as described in
"Congestion Notification Process for Real-Time Traffic" [2]. We do
acknowledge that there are other alternatives to accomplishing this
particular aspect of the mechanism, but it is a piece of RTP that can
be re-used.
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5.2 UDP
UDP has long been an alternative to using RTP. While everything
necessary can certainly be built with a new protocol or packet format
on top of UDP, what is lost with this approach is the assurance that
the probe packets will be treated as expected in the network.
5.3 ICE/STUN
ICE/STUN is another suggested alternative that is still being
investigated.
5.4 Re-use Existing RTP Format
This alternative is preferred at this time if the need for ECN-
specific payload is dissolved. In such a case, another RTP packet
format could be leveraged for use as the probe packet format.
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6. RTP Payload Format for Real-Time ECN Admission Control
The "ecnprobe" 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 it represents.
6.1 Registration
The new RTP payload format is defined as "ecnprobe", with a MIME type
of "audio/ecnprobe" for audio and a MIME type of "video/ecnprobe" for
video. The payload type for RTP packets carrying this payload is
determined dynamically through methods outside the scope of this
document.
6.2 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 '10', 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.
6.3 RTP Header Fields
Payload Type: The payload type field MUST be filled with a value
determined dynamically.
6.4 Payload Format
The "ecnprobe" 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|ECN| Initial RTP Sequence Number | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: ecnprobe Payload Format
It consists of five fields: Version, ECN, Initial RTP Sequence Number
(ISRN), and Reserved.
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6.4.1 Version
The Version field designates the version of the payload format. This
field is provided for future extensibility of the payload to carry
additional information. The following value is defined by this
document for the Version field:
0: Initial version defined by this document.
6.4.2 Explicit Congestion Notification (ECN)
This field contains a two-bit ECN value. If an application is trying
to detect Cheaters, the Sender SHOULD set this field to the two-bit
ECN value used in the IP header when sending the Probe Packet.
6.4.3 Initial RTP Sequence Number (IRSN)
In order to perform cheater detection and/or compliance testing in a
unidirectional fashion, the receiving endsystem needs to know which
packets to use for cheater detection and/or compliance testing.
During probing, this is less important as the probe payload will
contain the actual ECN value set in the IP header. But during media
exchange, the receiver must know exactly which packets are intended
for cheater detection and/or compliance testing. The IRSN value
represents the initial sequence number used on the outgoing probe
and/or media packets. This value is used as described in [2] to
allow both ends to know which packets in the media stream are marked
for cheater detection. In the event of lost or out-of-order packets,
the receiver needs only to check the sequence number of the incoming
packet against the calculated sequence number that it expects to find
in packets being used for cheater detection and/or compliance
testing.
6.4.4 Reserved
This field contains 10 bits reserved for future use.
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7. Considerations for Payload Format
There were two main considerations driving the new payload format
defined in this memo: extensibility and flexibility.
7.1 Extensibility Considerations
While the intended use for this 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/
implementations.
7.2 Flexibility Considerations
In addition to extensibility, another consideration is the
flexibility to allow the initial definition of the payload to be used
in as wide a range of implementations as possible.
While the default and minimum size of the payload is 4 octets, an
application MAY pad to the payload in order to simulate a specific
codec. In this case, the application needs to ensure that the
packets carrying the padded payload are sent at the appropriate rate
corresponding to the codec being simulated.
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8. Considerations for Direct Feedback
The payload format is currently defined with no explicit mechanism to
provide feedback on the Probe Packet(s) from the Receiver to the
Sender. This section discusses the options that have been
considered, and describes why they are not included along with the
payload format definition.
8.1 Feedback via RTP
The payload format was originally envisioned to be used with either a
unidirectional probe or a bidirectional probe.
This document specifies a unidirectional probe.
A bidirectional probe flows from the Sender to the Receiver, with the
Receiver then generating its own probe in response, with the payload
additionally carrying the ECN value from the IP header of the
received probe packet. While this is feasible, the unidirectional
probing model results in a simpler implementation.
8.2 Feedback via RTCP
Also considered was an approach whereby feedback is provided via RTCP
from the Receiver to the Sender. While possible, implementing
feedback via RTCP in real time as described in "Admission Control Use
Case for Real-time ECN" [3] would necessitate violation of the rules
governing the RTCP transmission interval described in RFC3550 [1].
The RTCP transmission interval deliberately paces RTCP transmissions
to be no more frequent than every 5 seconds, but for an admission
control application, the transmission interval would potentially need
to be much shorter.
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9. MIME Registration
This section registeres MIME types for audio/ecnprobe and video/
ecnprobe.
9.1 audio/ecnprobe
MIME media type name: audio
MIME subtype name: ecnprobe
Required Parameters: none
Optional Parameters: none
Encoding considerations: This type is only defined for transfer via
RTP [1] or Secure RTP [5].
Security considerations: See "Security Considerations" (Section 10).
Interoperability considerations: none
Published specification: This document.
Applications which use this media: The "ecnprobe" application subtype
is used to perform ECN probing and data path connectivity for
admission control, although it is not limited solely to this
application.
Additional information:
1. Magic number(s): N/A
2. File extensions(s): N/A
3. Macintosh file type code(s): N/A
9.2 video/ecnprobe
MIME media type name: video
MIME subtype name: ecnprobe
Required Parameters: none
Optional Parameters: none
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Encoding considerations: This type is only defined for transfer via
RTP [1] or Secure RTP [5].
Security considerations: See "Security Considerations" (Section 10).
Interoperability considerations: none
Published specification: This document.
Applications which use this media: The "ecnprobe" application subtype
is used to perform ECN probing and data path connectivity for
admission control, although it is not limited solely to this
application.
Additional information:
1. Magic number(s): N/A
2. File extensions(s): N/A
3. Macintosh file type code(s): N/A
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10. 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]. If an application is
attempting to detect Cheaters and the payload is not secured, a
cheating device will be able to inspect and modify the ECN field in
the payload, thereby circumventing the detection method.
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11. IANA Considerations
The Version field described in "Version" (Section 6.4.1) will need to
be administered. This field should be administered on a first come,
first served basis.
IANA is requested to make MIME type registrations as specified above
in "MIME Registration" (Section 9).
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12. Acknowledgements
The authors acknowledge a great many inputs, including the following:
Francois Audet, Amy Pendleton, Tom Taylor, John Rutledge, Jeremy
Matthews, Marvin Krym, Stephen Dudley, and Kwok Ho Chan.
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13. References
13.1 Normative References
[1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 3550, July 2003.
[2] Babiarz, J., Chan, K., and V. Firoiu, "Congestion Notification
Process for Real-Time Traffic", Internet-Draft
draft-babiarz-tsvwg-rtecn-04.txt (Work in Progress), July 2005.
[3] Alexander, C., Ed., Babiarz, J., and J. Matthews, "Real-time ECN
Use Cases", Internet-Draft
draft-alexander-rtecn-use-cases-00.txt (Work in Progress),
July 2005.
13.2 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)",
RFC 3711, March 2004.
[6] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
Authors' Addresses
Corey W. Alexander (editor)
Nortel
MS 08704A30
2370 Performance Drive
Richardson, TX 75082
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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