Internet DRAFT - draft-carlberg-avtext-classifier
draft-carlberg-avtext-classifier
AVTEXT Working Group Ken Carlberg
INTERNET-DRAFT G11
Intended Status: Standards Track Toerless Eckert
Expires: April 21, 2014 Cisco
Oct 21, 2013
A Real-time Transport Protocol (RTP) Classifier Header Extension
draft-carlberg-avtext-classifier-01
Abstract
This document defines a new RTP header extension. The purpose of the
extension is to provide additional information that further
distinguishes the RTP datagram (and its payload) from other datagrams
containing the same type of payload. The information may be used to
assist functions performed by application layer gateways (such as
Session Border Controller or MCUs) and/or by routers/switches through
deep packet inspection. Examples of these functions include intelligent
dropping of packets or (re)setting the IP header diff-serv code points
at ingress/egress boundaries of a diff-serv domain.
Status of this Memo
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of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 14, 2014.
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1. Introduction
This document defines a new RTP Header extension. The purpose of the
extension it to provide additional information that distinguishes the
RTP datagram (and its payload) from other datagrams containing the same
type of payload. This distinction can be viewed as a generalized
abbreviation of the significance of the payload.
It is important to note that this document uses the term classification,
NOT priority, in distinguishing payloads. This is because the word
priority tends to convey a definitive importance of the packet, as well
as an expected Quality of Service (QoS). In addition, the concept of
priority may be different on per-application or per-user community
basis. Hence, local policy is required to determine the relationship of
various classifications. This policy may be associated with the
administrative policy defined for a domain. The form, support of, and
dissemination of local policy is outside the scope of this document.
Another advantage in appending a classifier extension to the RTP header
is that it provides a means by which a forwarding node acquires
information from the source without the need to breach confidentiality
(through the use of Secure RTP) or support of the codec used to produce
the RTP payload.
1.2 Terminology and Abbreviations
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [rfc2119].
2. Related Work
There have been several efforts that have added classification, in the
more narrow scope of priority to other applications. These efforts
include: (1) a Resource Priority Header in the Session Initiation
Protocol (SIP) [rfc4412] and (2) a priority extension for the Simple
Mail Transfer Protocol (SMTP) [rfc6710], and (3) a priority policy
element for the Resource Reservation Protocol (RSVP) [rfc6401].
In each of these examples, the priority classification was accomplished
by dividing the solution space into two parts. The first identified a
namespace associated with the set of priorities. The second part
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identified the specific priority. Initial example values would be
defined in the respective RFC, while future values would be placed in a
registry maintained by IANA. The advantage in using this two-part
solution was that various "communities of interest" had the freedom to
define the form of the classification (in their case, priority) and the
number of classifications. In addition, the registry provided a common
place where various vendors and user groups could access and agree on a
single set of values that assisted in interoperability.
3. Classifier Header Extension
The classifier header extension for RTP is divided into two parts: a
Namespace entry and a Value entry. This information is carried in an
RTP header extension element as by "A General Mechanism for RTP Header
Extensions" [rfc5285].
The payload of the classifier header extension element can be encoded
using either the one-byte or two-byte header defined in [rfc5285] and
shown below in Figure 1 and 2 below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | len=1 | Namespace | Value | 0 (pad) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Classifier Using the One-Byte Header
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | len=2 | Namespace | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Classifier Using the Two-Byte Header
Author's notes: There have been recent discussions in the Transport
Services working group (TSVWG) on the relative importance of different
types of payloads. There has also been recent work concerning the
mapping of diff-serv code points related to RTP payloads. In both of
these cases, the focus has been with the characteristics of specific
types of applications.
On the other hand, and as discussed above in section 2, previous related
work has gravitated to supporting classifications (specifically,
priorities) based on a user community. One can easily observe that
these are two different and possibly divergent motivations in adding
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classification information to an RTP payload. A question to the
community is should both interests be supported by a new RTP classifier
header? (the author's position is yes)
Examples exist in the case of Namespaces correlating to a user
community. This section should, at a minimum, present an example
Namespace that correlates to either a specific application or a set of
applications. Another question to the community is whether the latter
can be achieved since it would reduce the number of Namespaces that
would need to be supported by implementors. The author's position is
that this could be achieved by having a Namespace and set of values that
correspond to the existing set of defined differentiated services code
points. As such, we recommend a Namespace assigned to a per-hop
behaviour (e.g., the AFxx set of code points)
Finally, we anticipate the possibility that two sets of users groups may
choose to inject their own classifier information: one that corresponds
to hop-by-hop forwarding, and the other at the destination end-point.
However, we should encourage a minimalistic approach and discourage more
than two (namespace, value) entries.
3.1 Example: AF Namespace
TBD
4. SDP Signaling
TBD
5. IANA Considerations
At present, this section is listed as To Be Done. Eventually, a
description and statement requirement of a registry will be needed.
6. Security Considerations
To Be Done
7. Acknowledgements
An earlier work-in-progress related effort concerning the specification
of a classifier extension header for RTP was presented to the IETF
community in 2002. The author thanks James Polk and Dave Oran for
earlier discussions on this topic. The authors also thanks Cheng-Jai
Lai for recent discussion on the topic.
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8. References
8.1 Normative
[rfc2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[rfc5285] Singer, D., et. al., "A General Mechanism for RTP
Header Extensions", RFC 5285, IETF, July 2008
8.2 Informative
[rfc4412] Schulzrinne, H., J. Polk, "Communications Resource
Priority for the Session Inititiation Protocol (SIP)"
RFC 4412, IETF, Feb 2006
[rfc6401] Le Faucher, F, et al, "RSVP Extensions for Admission
Priority", RFC 6401, IETF, Oct 2011
[rfc6710] Melnikov, A., K. Carlberg, "Simple Mail Transfer
Protocol Extension for Message Transfer Priorities",
RFC 6710, IETF, Aug 2012
Author?s Addresses
Ken Carlberg
G11
Arlington VA
USA
Email: carlberg@g11.org.uk
Toerless Eckert
Cisco Systems, Inc.
San Jose
USA
Email: eckert@cisco.com
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