Internet DRAFT - draft-firoiu-diffserv-af-ammend
draft-firoiu-diffserv-af-ammend
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Internet Engineering Task Force Victor Firoiu,
Bay Networks/Nortel Networks
Alessio Casati,
Lucent Technologies
INTERNET DRAFT
Document: <draft-firoiu-diffserv-af-ammend-00.txt> November, 1998
Expires: May 1999
Amendments to the Assured Forwarding PHB Group
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
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1. Abstract
This note was motivated by the ongoing discussion in the
Differentiated Services Working Group regarding the definition of
the Assured Forwarding (AF) Per-Hop Behavior (PHB) Group. We
consider two issues with the current proposal for AF PHB Group in
[Heinanen]: the recommendation for AF discard mechanism, and the
definition of AF classes. We discuss the implications of the
definitions and recommendations that have raised comments and
propose alternative texts.
2. Conventions used in this document
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 [RFC-2119].
Firoiu, Casati 1
Amendments to AF PHB Group November 1998
3. Introduction
We are concerned with two aspects in the current AF PHB proposal in
[Heinanen]. The first issue regards the recommendation for AF
discard mechanism. The second issue relates to the definition of AF
classes. In Section 4, we discuss the first issue, and in Section
5, the second.
4. Recommendation for AF discard mechanism
[Heinanen] Section 4 has the following text:
Text 1. "Inconsistent discard behaviors lead to inconsistent end-
to-end service semantics. It is RECOMMENDED that the discard
mechanism is based on a RED-like [Floyd] algorithm."
Consistency of local packet treatment is clearly needed for
providing a defined end-to-end service. However, a uniform
implementation is neither necessary nor sufficient for providing
such consistent per-hop behavior. Specifically, it is possible to
provide consistent discard behaviors with implementations different
than RED-based. Moreover, recommending RED-based implementations
does not guarantee consistent discard behavior, since it is possible
to construct pathological configurations of RED.
We consider that, in order to ensure consistency, we need to
recommend the discard behavior, and not an implementation. We
propose to define the AF drop preference behavior as follows,
replacing Text 1:
"Inconsistent discard behaviors lead to inconsistent end-to-end
service semantics. It is RECOMMENDED that the discard mechanism be
based on active queue management algorithms. An example of such a
discard mechanism is a RED-like [Floyd] algorithm with three
configurable levels of drop precedence."
5. Definition of AF classes
The second issue relates to the definition of AF classes. The
following text in [Heinanen] is the most relevant to this issue.
Text 2. "A DS node MUST allocate forwarding resources (buffer space
and bandwidth) to AF classes so that, under reasonable operating
conditions and traffic loads, packets of an AF class x do not have
higher probability of timely forwarding than packets of an AF class
y if x < y."
Firoiu, Casati 2
Amendments to AF PHB Group November 1998
Timely forwarding is thus the criterion differentiating the four AF
classes. Unfortunately, this is not a well-defined criterion, and
can give way to different and inconsistent interpretations. For
example, what is a conformance test for "packets of AF class 1 do
not have higher probability of timely forwarding than packets of AF
class 2"? Is timely delivery the average (or minimum, or maximum)
packet forwarding time at a DS node? An end-to-end service
constructed from DS nodes using different interpretations can be
inconsistent or unpredictable.
We consider that this issue is essential for the definition of AF
PHB Group, and that more discussion is needed in the Working Group
to clarify it.
As a basis for discussion, we propose to define the timeliness of
packet forwarding in an AF Class at a DS node to be the average
forwarding time of non-dropped packets. The forwarding time is the
time between the input and output of a packet at the DS node. The
averaging time interval is to be defined or may be a differentiation
parameter among different providers.
Observe that a precise definition of timely forwarding does not make
the AF a quantitative PHB. For example "average forwarding time of
Class 1 smaller than average forwarding time of Class 2" is a
qualitative characterization.
Another issue is that AF class differentiation through forwarding
time can be realized in many different ways. For example, one way
to provide small average forwarding time for an AF class is a
combination of low average load and large amount of resources.
Another way is to have an aggressive drop policy on low priority
packets, thus a small average queue length and ultimately small
average queuing delay. Therefore, we believe that it is better to
differentiate AF classes by a behavioral attribute, the average
forwarding time, leaving configuration details open to innovation.
In conclusion, we propose to define the forwarding behavior provided
to packets belonging to different AF classes in the following way,
replacing Text 2:
"In a DS node, packets of AF class x do not have higher average
forwarding time (delay) than packets of AF class y if x < y, the
averaging being over the same time interval."
6. References
[Heinanen], Heinanen, J, Baker, F, Weiss, W, Wroclawski, J,
"Assured Forwarding PHB Group", draft-ietf-diffserv-af-03.txt,
November, 1998.
Firoiu, Casati 3
Amendments to AF PHB Group November 1998
[Floyd], Floyd, S., and Jacobson, V., Random Early Detection
gateways for Congestion Avoidance. IEEE/ACM Transactions on
Networking, Volume 1, Number 4, August 1993, pp. 397-413.
[RFC-2119], Bradner, S, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, Harvard University, March 1997
7. Author's Addresses
Victor Firoiu
Bay Networks / Nortel Networks
3 Federal St
Billerica MA, 01821
Phone: +1 (978) 916-4354
Email: vfiroiu@baynetworks.com
Alessio Casati
Lucent Technologies
Sigma
Windmill Hill Business Park
Swindon
Wiltshire SN5 6PP
United Kingdom
Phone: +44 (1793) 88-3861
Email: acasati@lucent.com
Firoiu, Casati 4
