Internet Engineering Task Force Juha Heinanen
INTERNET DRAFT Telia Finland
Expires April 1999 Fred Baker
Cisco Systems
Walter Weiss
Lucent Technologies
John Wroclawski
MIT LCS
October, 1998
Assured Forwarding PHB Group
<draft-ietf-diffserv-af-01.txt>
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Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This document proposes a general use Differentiated Services (DS)
[Blake] Per-Hop-Behavior (PHB) Group called Assured Forwarding (AF).
The AF PHB group provides delivery of IP packets in four
independently forwarded AF classes. Within each AF class, an IP
packet can be assigned one of three different levels of drop
precedence. A DS node does not reorder IP packets of the same
microflow if they belong to the same AF class.
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1. Purpose and Overview
There is a demand to offer assured delivery of IP packets over the
Internet. In a typical application, a company uses the Internet to
connect its geographically distributed sites and wants an assurance
that IP packets within this intranet are delivered with high
probability as long as the aggregate traffic from each site does not
exceed the subscribed information rate (profile). It is desirable
that a site may exceed the subscribed profile with the understanding
that the excess traffic is not delivered with as high a probability
as the traffic that is within the profile. It is also important that
the network does not reorder packets that belong to the same
microflow no matter if they are in or out of the profile.
Assured Forwarding (AF) PHB group is a means for a provider DS domain
to offer different levels of delivery assurances for IP packets
received from a customer DS domain. Four AF classes are defined,
where each AF class is in each DS node allocated a certain amount of
forwarding resources (buffer space, bandwidth). IP packets that wish
to use the services provided by the AF PHB group are assigned by the
customer or the provider DS domain into one or more of these AF
classes according to the subscribed services.
Within each AF class IP packets are marked (again by the customer or
the provider DS domain) with one of three possible drop precedence
values. In case of congestion, the drop precedence of a packet
determines the relative importance of the packet within the AF class.
A congested DS node tries to protect packets with a lower drop
precedence value from being lost by preferably discarding packets
with a higher drop precedence value.
In a DS node, the level of delivery assurance of an IP packet thus
depends on (1) how much forwarding resources has been allocated to
the AF class that the packet belongs to, (2) what is the current load
of the AF class, and, in case of congestion, (3) what is the drop
precedence of the packet.
For example, if traffic conditioning actions at the ingress of the
provider DS domain make sure that an AF class in the DS nodes is only
moderately loaded by packets with the lowest drop precedence value
and is not overloaded by packets with the two lowest drop precedence
values, then the AF class can offer a high level of delivery
assurance for packets that are within the subscribed profile and
offer up to two lower levels of delivery assurance for the excess
traffic.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in [Bradner].
2. The AF PHB Group
Assured Forwarding (AF) PHB group provides delivery of IP packets in
N independent AF classes. Within each AF class, an IP packet is
assigned one of M different levels of drop precedence. An IP packet
that belongs to an AF class i and has drop precedence j is marked
with the AF codepoint AFij, where 1 <= i <= N and 1 <= j <= M. At
this point, four classes (N=4) with three drop precedences in each
class (M=3) are defined for general use. More AF classes or levels
of drop precedence MAY be defined for local use.
A DS node MUST allocate forwarding resources (buffer space and
bandwidth) to AF classes so that, relative to the loads, an AF class
x has no more forwarding resources than an AF class y if x < y.
Similarly, within an AF class, an IP packet with drop precedence p
MUST NOT be delivered with smaller probability than an IP packet with
drop precedence q if p < q.
A DS node MUST NOT reorder AF packets of the same microflow when they
belong to the same AF class regardless of their drop precedence.
There are no timing requirements (delay or delay variation)
associated with the forwarding of AF packets.
The AF PHB group MAY be used to implement both end-to-end and domain
edge-to-domain edge services.
3. Traffic Conditioning Actions
A DS domain MAY at the edge of a domain control the amount of AF
traffic that enters or exists the domain at various levels of drop
precedence. The traffic conditioning actions MAY include discarding
of packets, increasing or decreasing the drop precedence of packets,
and reassigning of packets to other AF classes. The latter action
MUST NOT distribute packets of the same microflow to more than one AF
class.
4. Queueing and Discard Behavior
A DS node SHOULD implement all four general use AF classes. Packets
in one AF class MUST be forwarded independently from packets in
another AF class, i.e., a DS node MUST NOT aggregate two or more AF
classes together.
Within each AF class, the three drop precedence codepoints MUST yield
at least two different levels of loss probability. In some networks,
particularly in enterprise networks, where transient congestion is a
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rare and brief occurrence, it may be reasonable for a DS node to
support only two different levels of loss probability. While this
may suffice for some networks, three different levels of loss
probability SHOULD be supported in DS domains where congestion is a
common occurrence.
If a DS node only implements two different levels of loss probability
for an AF class x, the codepoint AFx1 MUST yield the lower loss
probability and the codepoints AFx2 and AFx3 MUST yield the higher
loss probability.
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 with three configurable levels
of drop precedence and a configurable averaging function (interval).
Future versions of this document may say more about specific aspects
of the desirable behavior.
5. Tunneling
When AF packets are tunneled, the PHB of the tunneling packet MUST
NOT reduce the delivery assurance of the tunneled AF packet nor cause
reordering of AF packets belonging to the same microflow.
6. Recommended Codepoints
It is RECOMMENDED that the AF codepoints AF11, AF21, AF31, and AF41,
i.e., the codepoints that denote the lowest drop precedence in each
AF class, are mapped to the Class Selector [Nichols] codepoints
'010000', '011000', '100000', '101000'. This is done in order to
save DS code space, because the forwarding rules associated with
these AF codepoints are consistent and compatible with the forwarding
rules of the corresponding Class Selector codepoints.
The RECOMMENDED values of the remaining AF codepoints are as follows:
AF12 = '010010', AF13 = '010100', AF22 = '011010', AF23 = '011100',
AF32 = '100010', AF33 = '100100', AF42 = '101010', and AF43 =
'101100'. The table below summarizes the recommended AF codepoint
values.
Class 1 Class 2 Class 3 Class 4
+----------+----------+----------+----------+
Low Drop Pref | 010000 | 011000 | 100000 | 101000 |
Medium Drop Pref | 010010 | 011010 | 100010 | 101010 |
High Drop Pref | 010100 | 011100 | 100100 | 101100 |
+----------+----------+----------+----------+
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7. Interactions with Other PHB Groups
The AF codepoint mappings recommended above do not interfere with the
local use spaces nor use the Class Selector codepoints '00x000' and
'11x000'. The PHBs selected by those Class Selector codepoints may
thus coexist with the AF PHB group, and retain the forwarding
behavior and relationships that was defined for them in [Nichols].
In particular, the Default PHB codepoint of '000000' may remain to be
used for conventional best effort traffic. Similarly, the codepoints
'11x000' may remain to be used for network control traffic.
In addition to the Class Selector PHBs, any other PHB groups may co-
exist with the AF group within the same DS domain provided that the
other PHB groups don't preempt the resources allocated to the AF
classes.
8. Security Implications
In order to protect itself against denial of service attacks, a
provider DS domain SHOULD limit the traffic entering the domain to
the subscribed profiles. 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
resources of the link are allocated to AF packets. If a service
offering requires that traffic marked with an AF codepoint be limited
by such attributes as source or destination address, it is the
responsibility of the ingress node in a network to verify validity of
such attributes.
Other security considerations are covered in [Blake] and [Nichols].
Appendix: Example Services
The AF PHB group may be used to implement, for example, the so-called
Olympic service, which consists of three service classes: bronze,
silver, and gold. Packets are assigned to these three classes so
that packets in the gold class experience lighter load (and thus have
greater probability for timely delivery) than packets assigned to the
silver class. Same kind of relationship exists between the silver
class and the bronze class. If desired, packets within each class
may be further separated by giving them either low, medium, or high
drop precedence.
The bronze, silver, and gold service classes may in the network be
mapped to the AF classes 1, 2, and 3. Similarly, low, medium, and
high drop precedence may be mapped to AF drop precedence indexes 1,
2, or 3.
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The drop precedence level of a packet may be assigned, for example,
by using a dual leaky bucket traffic policer, which has as its
parameters a rate and two burst sizes: a committed burst and an
excess burst. If a packet falls within the committed burst, it is
assigned low drop precedence. If a packet falls between the
committed burst and the excess burst, it is assigned medium drop
precedence. And finally, if the packet falls out of the excess burst,
it is assigned high drop precedence.
Another possibility would be to limit the user traffic of an Olympic
service class to a given peak rate and distribute it evenly across
each level of drop precedence. This would yield a proportional
bandwidth service, which equally apportions available capacity during
times of congestion under the assumption that customers with high
bandwidth microflows have subscribed to higher peak rates than
customers with low bandwidth microflows.
The AF PHB group could also be used to implement a low loss, low
delay, and low jitter service using an over provisioned AF class, if
the maximum arrival rate to that class is known a priori in each DS
node. Specification of the required admission control services,
however, is beyond the scope of this document.
References
[Blake] Blake, Steve, et al., An Architecture for Differentiated
Services. Internet draft draft-ietf-diffserv-arch-01.txt, August
1998.
[Bradner] Bradner, S., Key words for use in RFCs to Indicate
Requirement Levels. Internet RFC 2119, March 1997.
[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.
[Nichols] Nichols, Kathleen, et al., Definition of the Differentiated
Services Field (DS Field) in the IPv4 and IPv6 Headers. Internet
draft draft-ietf-diffserv-header-02.txt, August 1998.
Author Information
Juha Heinanen
Telia Finland
Myyrmaentie 2
01600 Vantaa, Finland
Email: jh@telia.fi
Heinanen Assured Forwarding PHB Group [Page 6]
INTERNET DRAFT October, 1998
Fred Baker
Cisco Systems
519 Lado Drive
Santa Barbara, California 93111
E-mail: fred@cisco.com
Walter Weiss
Lucent Technologies
300 Baker Avenue, Suite 100,
Concord, MA 01742-2168
E-mail: wweiss@lucent.com
John Wroclawski
MIT Laboratory for Computer Science
545 Technology Sq.
Cambridge, MA 02139
Email: jtw@lcs.mit.edu
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