Internet DRAFT - draft-heinanen-diffserv-af
draft-heinanen-diffserv-af
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Internet Engineering Task Force Juha Heinanen
INTERNET DRAFT Telia Finland, Inc.
Expires February 1999 August, 1998
Assured Forwarding PHB Group
<draft-heinanen-diffserv-af-00.txt>
Status of this Memo
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Abstract
This document proposes a general use Differentiated Services (DS)
[Black] Per-Hop-Behavior (PHB) Group called Assured Forwarding (AF).
AF PHB group consists of two code points AF1 and AF2. A DS node
delivers packets marked as AF1 with a very small propability of loss,
whereas packets marked as AF2 are delivered as best effort. The DS
node will not reorder AF packets of the same microflow no matter if
some of the packets are marked as AF1 and some as AF2. There is no
timing requirements associated with the delivery of AF packets.
1. Overview and Purpose
There is a demand for assured delivery of packets over IP networks,
for example, when a company uses the Internet to interconnect its
geographically distributes sites or wants to provide high quality
access to its Web pages. It is desirable that the delivery assurance
holds for a certain information rate, which can be exceeded on best
effort basis. It is also important that the IP network does not
reorder packets that belong to the same microflow no matter if the
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assurance holds only to some of the packets of the microflow.
Assured Forwarding (AF) PHB group is a means for a provider DS domain
to assure delivery of some portion of IP packets received from a
customer DS domain and offer best effort delivery for some other
portion of the packets without reordering packets that belong to the
same microflow. Those packets that belong to the assured portion of
the traffic are marked with AF code point AF1 and those packets that
belong to the best effort portion are marked as with AF code point
AF2. Together these two code points form the AF PHB group.
In order to avoid discarding AF1 packets in case of congestion (and
thus violating the SLA that the provider DS domain has given to the
customer DS domain), the assumption is that the volume of AF1 packets
that enter the provider DS domain is limited by traffic conditioning
actions at the ingress to the domain. Such an assumption is not
needed for AF2 packets, since the provider DS domain needs to deliver
them only as best effort on the basis of available resources.
The AF PHB group is proposed for general use. It can be used to
implement either end-to-end or domain edge-to-domain edge services.
2. Queueing and Discard Behavior
It is possible that some packets of the same microflow get marked
with code point AF1 and some with code point AF2. In order to avoid
reordering of packets within a microflow, a DS node must place all
packets of the same microflow to the same queue in the same order in
which they were received no matter if they are marked as AF1 or AF2.
In order to protect AF1 packets from being discarded due to
congestion, a DS node must start discarding AF2 packets well before
the queue space allocated for the particular AF1 packets starts to
fill up. The discard mechanism may be based, for example, on the RED
algorithm.
3. Packet Promotion/Demotion
AF packets may be promoted or demoted within the AF PHB group by
traffic conditioning actions at the edge of a DS domain. Promoting
means remarking an AF2 packet as an AF1 packet and demoting means
remarking an AF1 packet as an AF2 packet. No re-marking of AF
packets should occur inside the DS domain.
In addition to promoting and demoting AF packets, the traffic
conditioning actions at the edge of a DS domain may also include re-
marking packets marked with the default (best effort) PHB code point
with AF1 or AF2 code point. In order to avoid packet reordering,
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such re-marking must be applied to all or none of the packets of the
same microflow.
4. Tunneling
AF packets may be tunneled provided that the PHB of the tunneling
packet does not reduce the level of delivery assurance of the
tunneled AF packet and does not cause reordering of AF packets
belonging to the same microflow.
5. Interactions with Other PHB Groups
Other PHB groups may co-exist with the AF group within the same DS
domain provided that AF1 packets need not be discarded in a DS node
due to lack of resources caused by packets belonging to these other
PHB groups.
6. Security Implications
In order to protect itself against denial of service attacks, a DS
domain must limit at the edge the volume of incoming AF1 packets.
Also, in order to protect a link to a customer DS domain from denial
of service attacks, the provider DS domain should allow the customer
DS domain to specify how the link to the customer DS domain is
allocated to AF packets. Because source IP address may be one of the
allocation criterias, the provider DS domain should verify the source
addresses of all received AF packets.
References
[Black] Black, David, et al., An Architecture for Differentiated
Services. Internet draft draft-ietf-diffserv-arch-00.txt, May 1998.
Author Information
Juha Heinanen
Telia Finland, Inc.
Myyrmaentie 2
01600 VANTAA
Finland
Phone +358 303 944 808
Email jh@telia.fi
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