Network Working Group J. Heinanen
Request for Comments: 2597 Telia Finland
Category: Standards Track F. Baker
June 1999 Assured Forwarding PHB Group
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document defines 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.
1. Purpose and Overview
There is a demand to provide assured forwarding of IP packets over
the Internet. In a typical application, a company uses the Internet
to interconnect its geographically distributed sites and wants an
assurance that IP packets within this intranet are forwarded 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
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, as defined in [Nichols], 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 forwarding 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 and 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 services that the customer has subscribed
to. Further background about this capability and some ways to use it
may be found in [Clark].
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 forwarding 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 within the class, (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 forwarding
assurance for packets that are within the subscribed profile (i.e.,
marked with the lowest drop precedence value) and offer up to two
lower levels of forwarding assurance for the excess traffic.
This document describes the AF PHB group. An otherwise DS-compliant
node is not required to implement this PHB group in order to be
considered DS-compliant, but when a DS-compliant node is said to
implement an AF PHB group, it must conform to the specification in
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 [Bradner].
2. The AF PHB Group
Assured Forwarding (AF) PHB group provides forwarding 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.
Currently, four classes (N=4) with three levels of drop precedence 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 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
A DS node MUST allocate a configurable, minimum amount of forwarding
resources (buffer space and bandwidth) to each implemented AF class.
Each class SHOULD be serviced in a manner to achieve the configured
service rate (bandwidth) over both small and large time scales.
An AF class MAY also be configurable to receive more forwarding
resources than the minimum when excess resources are available either
from other AF classes or from other PHB groups. This memo does not
specify how the excess resources should be allocated, but
implementations MUST specify what algorithms are actually supported
and how they can be parameterized.
Within an AF class, a DS node MUST NOT forward an IP packet with
smaller probability if it contains a drop precedence value p than if
it contains a drop precedence value q when p < q. Note that this
requirement can be fulfilled without needing to dequeue and discard
Within each AF class, a DS node MUST accept all three drop precedence
codepoints and they MUST yield at least two different levels of loss
probability. In some networks, particularly in enterprise networks,
where transient congestion is a rare and brief occurrence, it may be
reasonable for a DS node to implement only two different levels of
loss probability per AF class. 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
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 quantifiable timing requirements (delay or delay
variation) associated with the forwarding of AF packets.
The relationship between AF classes and other PHBs is described in
Section 7 of this memo.
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 exits the domain at various levels of drop
precedence. Such traffic conditioning actions MAY include traffic
shaping, discarding of packets, increasing or decreasing the drop
precedence of packets, and reassigning of packets to other AF
classes. However, the traffic conditioning actions MUST NOT cause
reordering of packets of the same microflow.
4. Queueing and Discard Behavior
This section defines the queueing and discard behavior of the AF PHB
group. Other aspects of the PHB group's behavior are defined in
An AF implementation MUST attempt to minimize long-term congestion
within each class, while allowing short-term congestion resulting
from bursts. This requires an active queue management algorithm. An
example of such an algorithm is Random Early Drop (RED) [Floyd].
This memo does not specify the use of a particular algorithm, but
does require that several properties hold.
An AF implementation MUST detect and respond to long-term congestion
within each class by dropping packets, while handling short-term
congestion (packet bursts) by queueing packets. This implies the
presence of a smoothing or filtering function that monitors the
instantaneous congestion level and computes a smoothed congestion
level. The dropping algorithm uses this smoothed congestion level to
determine when packets should be discarded.
The dropping algorithm MUST be insensitive to the short-term traffic
characteristics of the microflows using an AF class. That is, flows
with different short-term burst shapes but identical longer-term
packet rates should have packets discarded with essentially equal
probability. One way to achieve this is to use randomness within the
The dropping algorithm MUST treat all packets within a single class
and precedence level identically. This implies that for any given
smoothed congestion level, the discard rate of a particular
microflow's packets within a single precedence level will be
proportional to that flow's percentage of the total amount of traffic
passing through that precedence level.
The congestion indication feedback to the end nodes, and thus the
level of packet discard at each drop precedence in relation to
congestion, MUST be gradual rather than abrupt, to allow the overall
system to reach a stable operating point. One way to do this (RED)
uses two (configurable) smoothed congestion level thresholds. When
the smoothed congestion level is below the first threshold, no
packets of the relevant precedence are discarded. When the smoothed
congestion level is between the first and the second threshold,
packets are discarded with linearly increasing probability, ranging
from zero to a configurable value reached just prior to the second
threshold. When the smoothed congestion level is above the second
threshold, packets of the relevant precedence are discarded with 100%
To allow the AF PHB to be used in many different operating
environments, the dropping algorithm control parameters MUST be
independently configurable for each packet drop precedence and for
each AF class.
Within the limits above, this specification allows for a range of
packet discard behaviors. Inconsistent discard behaviors lead to
inconsistent end-to-end service semantics and limit the range of
possible uses of the AF PHB in a multi-vendor environment. As
experience is gained, future versions of this document may more
tightly define specific aspects of the desirable behavior.
When AF packets are tunneled, the PHB of the tunneling packet MUST
NOT reduce the forwarding assurance of the tunneled AF packet nor
cause reordering of AF packets belonging to the same microflow.
6. Recommended Codepoints
Recommended codepoints for the four general use AF classes are given
below. These codepoints do not overlap with any other general use PHB
The RECOMMENDED values of the AF codepoints are as follows: AF11 = '
001010', AF12 = '001100', AF13 = '001110', AF21 = '010010', AF22 = '
010100', AF23 = '010110', AF31 = '011010', AF32 = '011100', AF33 = '
011110', AF41 = '100010', AF42 = '100100', and AF43 = '100110'. The
table below summarizes the recommended AF codepoint values.
Class 1 Class 2 Class 3 Class 4
Low Drop Prec | 001010 | 010010 | 011010 | 100010 |
Medium Drop Prec | 001100 | 010100 | 011100 | 100100 |
High Drop Prec | 001110 | 010110 | 011110 | 100110 |
7. Interactions with Other PHB Groups
The AF codepoint mappings recommended above do not interfere with the
local use spaces nor the Class Selector codepoints recommended in
[Nichols]. 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 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.
The AF PHB group, in conjunction with edge traffic conditioning
actions that limit the amount of traffic in each AF class to a
(generally different) percentage of the class's allocated resources,
can be used to obtain the overall behavior implied by the Class
Selector PHBs. In this case it may be appropriate within a DS domain
to use some or all of the Class Selector codepoints as aliases of AF
In addition to the Class Selector PHBs, any other PHB groups may co-
exist with the AF PHB group within the same DS domain. However, any
AF PHB group implementation should document the following:
(a) Which, if any, other PHB groups may preempt the forwarding
resources specifically allocated to each AF PHB class. This
preemption MUST NOT happen in normal network operation, but may be
appropriate in certain unusual situations - for example, the '11x000'
codepoint may preempt AF forwarding resources, to give precedence to
unexpectedly high levels of network control traffic when required.
(b) How "excess" resources are allocated between the AF PHB group and
other implemented PHB groups. For example, once the minimum
allocations are given to each AF class, any remaining resources could
be allocated evenly between the AF classes and the Default PHB. In
an alternative example, any remaining resources could be allocated to
forwarding excess AF traffic, with resources devoted to the Default
PHB only when all AF demand is met.
This memo does not specify that any particular relationship hold
between AF PHB groups and other implemented PHB groups; it requires
only that whatever relationship is chosen be documented.
Implementations MAY allow either or both of these relationships to be
configurable. It is expected that this level of configuration
flexibility will prove valuable to many network administrators.
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
Other security considerations are covered in [Blake] and [Nichols].
9. Intellectual Property Rights
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information, consult the online list of claimed
10. IANA Considerations
This document allocates twelve codepoints, listed in section 6, in
Pool 1 of the code space defined by [Nichols].
Appendix: Example Services
The AF PHB group could 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 forwarding) 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 could 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 levels 1, 2,
The drop precedence level of a packet could be assigned, for example,
by using a leaky bucket traffic policer, which has as its parameters
a rate and a size, which is the sum of two burst values: a committed
burst size and an excess burst size. A packet is assigned low drop
precedence if the number of tokens in the bucket is greater than the
excess burst size, medium drop precedence if the number of tokens in
the bucket is greater than zero, but at most the excess burst size,
and high drop precedence if the bucket is empty. It may also be
necessary to set an upper limit to the amount of high drop precedence
traffic from a customer DS domain in order to avoid the situation
where an avalanche of undeliverable high drop precedence packets from
one customer DS domain can deny service to possibly deliverable high
drop precedence packets from other domains.
Another way to assign the drop precedence level of a packet could 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
The AF PHB group could also be used to implement a loss and low
latency 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. If low loss is not an objective,
a low latency service could be implemented without over provisioning
by setting a low maximum limit to the buffer space available for an
[Blake] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z. and
W. Weiss, "An Architecture for Differentiated Services",
RFC 2475, December 1998.
[Bradner] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[Clark] Clark, D. and Fang, W., Explicit Allocation of Best Effort
Packet Delivery Service. IEEE/ACM Transactions on
Networking, Volume 6, Number 4, August 1998, pp. 362-373.
[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, K., Blake, S., Baker, F. and D. Black, "Definition
of the Differentiated Services Field (DS Field) in the IPv4
and IPv6 Headers", RFC 2474, December 1998.
01600 Vantaa, Finland
519 Lado Drive
Santa Barbara, California 93111
300 Baker Avenue, Suite 100,
Concord, MA 01742-2168
MIT Laboratory for Computer Science
545 Technology Sq.
Cambridge, MA 02139
Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Funding for the RFC Editor function is currently provided by the