|Title||A Delay Bound alternative revision of RFC 2598
|Author||G. Armitage, B.
Carpenter, A. Casati, J. Crowcroft, J. Halpern, B. Kumar, J.
Network Working Group G. Armitage
Request for Comments: 3248 Swinburne University of Technology
Category: Informational B. Carpenter
University of Cambridge
Corona Networks Inc.
A Delay Bound alternative revision of RFC 2598
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (C) The Internet Society (2001). All Rights Reserved.
For historical interest, this document captures the EF Design Team's
proposed solution, preferred by the original authors of RFC 2598 but
not adopted by the working group in December 2000. The original
definition of EF was based on comparison of forwarding on an unloaded
network. This experimental Delay Bound (DB) PHB requires a bound on
the delay of packets due to other traffic in the network. At the
Pittsburgh IETF meeting in August 2000, the Differentiated Services
working group faced serious questions regarding RFC 2598 - the
group's standards track definition of the Expedited Forwarding (EF)
Per Hop Behavior (PHB). An 'EF Design Team' volunteered to develop a
re-expression of RFC 2598, bearing in mind the issues raised in the
DiffServ group. At the San Diego IETF meeting in December 2000 the
DiffServ working group decided to pursue an alternative re-expression
of the EF PHB.
Specification of Requirements
This document is for Informational purposes only. If implementors
choose to experiment with the DB PHB, key words "MUST", "MUST NOT",
"REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" are interpreted as described in
RFC 2119 .
RFC 2598 was the Differentiated Services (DiffServ) working group's
first standards track definition of the Expedited Forwarding (EF) Per
Hop Behavior (PHB) . As part of the DiffServ working group's
ongoing refinement of the EF PHB, various issues were raised with the
text in RFC 2598 .
After the Pittsburgh IETF meeting in August 2000, a volunteer 'EF
design team' was formed (the authors of this document) to propose a
new expression of the EF PHB. The remainder of this Informational
document captures our feedback to the DiffServ working group at the
San Diego IETF in December 2000. Our solution focussed on a Delay
Bound (DB) based re-expression of RFC 2598 which met the goals of RFC
2598's original authors. The DiffServ working group ultimately chose
an alternative re-expression of the EF PHB text, developed by the
authors of  and revised to additionally encompass our model
Our proposed Delay Bound solution is archived for historical
interest. Section 2 covers the minimum, necessary and sufficient
description of what we believed qualifies as 'DB' behavior from a
single node. Section 3 then discusses a number of issues and
assumptions made to support the definition in section 2.
2. Definition of Delay Bound forwarding
For a traffic stream not exceeding a particular configured rate, the
goal of the DB PHB is a strict bound on the delay variation of
packets through a hop.
This section will begin with the goals and necessary boundary
conditions for DB behavior, then provide a descriptive definition of
DB behavior itself, discuss what it means to conform to the DB
definition, and assign the experimental DB PHB code point.
2.1 Goal and Scope of DB
For a traffic stream not exceeding a configured rate the goal of the
DB PHB is a strict bound on the delay variation of packets through a
Traffic MUST be policed and/or shaped at the source edge (for
example, on ingress to the DS-domain as discussed in RFC 2475 ) in
order to get such a bound. However, specific policing and/or shaping
rules are outside the scope of the DB PHB definition. Such rules
MUST be defined in any per-domain behaviors (PDBs) composed from the
A device (hop) delivers DB behavior to appropriately marked traffic
received on one or more interfaces (marking is specified in section
2.4). A device SHALL deliver the DB behavior on an interface to DB
marked traffic meeting (i.e. less than or equal) a certain arrival
rate limit R.
If more DB traffic arrives than is acceptable, the device is NOT
REQUIRED to deliver the DB behavior. However, although the original
source of DB traffic will be shaped, aggregation and upstream jitter
ensure that the traffic arriving at any given hop cannot be assumed
to be so shaped. Thus a DB implementation SHOULD have some tolerance
for burstiness - the ability to provide EF behavior even when the
arrival rate exceeds the rate limit R.
Different DB implementations are free to exhibit different tolerance
to burstiness. (Burstiness MAY be characterized in terms of the
number of back-to-back wire-rate packets to which the hop can deliver
DB behavior. However, since the goal of characterizing burstiness is
to allow useful comparison of DB implementations, vendors and users
of DB implementations MAY choose to utilize other burstiness
The DB PHB definition does NOT mandate or recommend any particular
method for achieving DB behavior. Rather, the DB PHB definition
identifies parameters that bound the operating range(s) over which an
implementation can deliver DB behavior. Implementors characterize
their implementations using these parameters, while network designers
and testers use these parameters to assess the utility of different
2.2 Description of DB behavior
For simplicity the definition will be explained using an example
where traffic arrives on only one interface and is destined for
another (single) interface.
The crux of this definition is that the difference in time between
when a packet might have been delivered, and when it is delivered,
will never exceed a specifiable bound.
Given an acceptable (not exceeding arrival rate limit R) stream of DB
packets arriving on an interface:
There is a time sequence E(i) when these packets would be
delivered at the output interface in the absence of competing
traffic. That is, E(i) are the earliest times that the packets
could be delivered by the device.
In the presence of competing traffic, the packets will be delayed
to some later time D(i).
Competing traffic includes all DB traffic arriving at the device on
other ports, and all non-DB traffic arriving at the device on any
DB is defined as the behavior which ensures, for all i, that:
D(i) - E(i) <= S * MTU/R.
MTU is the maximum transmission unit (packet size) of the output. R
is the arrival rate that the DB device is prepared to accept on this
Note that D(i) and E(i) simply refer to the times of what can be
thought of as "the same packet" under the two treatments (with and
without competing traffic).
The score, S, is a characteristic of the device at the rate, R, in
order to meet this defined bound. This score, preferably a small
constant, depends on the scheduling mechanism and configuration of
2.3 Conformance to DB behavior
An implementation need not conform to the DB specification over an
arbitrary range of parameter values. Instead, implementations MUST
specify the rates, R, and scores S, for which they claim conformance
with the DB definition in section 2.2, and the implementation-
specific configuration parameters needed to deliver conformant
behavior. An implementation SHOULD document the traffic burstiness
it can tolerate while still providing DB behavior.
The score, S, and configuration parameters depend on the
implementation error from an ideal scheduler. Discussion of the
ability of any particular scheduler to provide DB behavior, and the
conditions under which it might do so, is outside the scope of this
The implementor MAY define additional constraints on the range of
configurations in which DB behavior is delivered. These constraints
MAY include limits on the total DB traffic across the device, or
total DB traffic targeted at a given interface from all inputs.
This document does not specify any requirements on DB
implementation's values for R, S, or tolerable burstiness. These
parameters will be bounded by real-world considerations such as the
actual network being designed and the desired PDB.
2.4 Marking for DB behavior
One or more DiffServ codepoint (DSCP) value may be used to indicate a
requirement for DB behavior .
By default we suggest an 'experimental' DSCP of 101111 be used to
indicate that DB PHB is required.
This section discusses some issues that might not be immediately
obvious from the definition in section 2.
Packets marked for DB PHB MAY be remarked at a DS domain boundary
only to other codepoints that satisfy the DB PHB. Packets marked for
DB PHBs SHOULD NOT be demoted or promoted to another PHB by a DS
When DB packets are tunneled, the tunneling packets must be marked as
3.3 Interaction with other PHBs
Other PHBs and PHB groups may be deployed in the same DS node or
domain with the DB PHB as long as the requirement of section 2 is
3.4 Output Rate not specified
The definition of DB behavior given in section 2 is quite explicitly
given in terms of input rate R and output delay variation D(i) -
E(i). A scheduler's output rate does not need to be specified, since
(by design) it will be whatever is needed to achieve the target delay
Jitter is not the bounded parameter in DB behavior. Jitter can be
understood in a number of ways, for example the variability in
inter-packet times from one inter-packet interval to the next.
However, DB behavior aims to bound a related but different parameter
- the variation in delay between the time packets would depart in the
absence of competing traffic, E(i), and when they would depart in the
presence of competing traffic, D(i).
3.6 Multiple Inputs and/or Multiple Outputs
The definition of 'competing traffic' in section 2.2 covers both the
single input/single output case and the more general case where DB
traffic is converging on a single output port from multiple input
ports. When evaluating the ability of an DB device to offer DB
behavior to traffic arriving on one port, DB traffic arriving on
other ports is factored in as competing traffic.
When considering DB traffic from a single input that is leaving via
multiple ports, it is clear that the behavior is no worse than if all
of the traffic could be leaving through each one of those ports
individually (subject to limits on how much is permitted).
3.7 Fragmentation and Rate
Where an ingress link has an MTU higher than that of an egress link,
it is conceivable packets may be fragmented as they pass through a
Diffserv hop. However, the unpredictability of fragmentation is
significantly counter to the goal of providing controllable QoS.
Therefore we assume that fragmentation of DB packets is being avoided
(either through some form of Path MTU discovery, or configuration),
and does not need to be specifically considered in the DB behavior
3.8 Interference with other traffic
If the DB PHB is implemented by a mechanism that allows unlimited
preemption of other traffic (e.g., a priority queue), the
implementation MUST include some means to limit the damage DB traffic
could inflict on other traffic. This will be reflected in the DB
device's burst tolerance described in section 2.1.
3.9 Micro flow awareness
Some DB implementations may choose to provide queuing and scheduling
at a finer granularity, (for example, per micro flow), than is
indicated solely by the packet's DSCP. Such behavior is NOT
precluded by the DB PHB definition. However, such behavior is also
NOT part of the DB PHB definition. Implementors are free to
characterize and publicize the additional per micro flow capabilities
of their DB implementations as they see fit.
3.10 Arrival rate 'R'
In the absence of additional information, R is assumed to be limited
by the slowest interface on the device.
In addition, an DB device may be characterized by different values of
R for different traffic flow scenarios (for example, for traffic
aimed at different ports, total incoming R, and possibly total per
output port incoming R across all incoming interfaces).
4. IANA Considerations
This document suggests one experimental codepoint, 101111. Because
the DSCP is taken from the experimental code space, it may be re-used
by other experimental or informational DiffServ proposals.
This document defines DB behavior in terms of a bound on delay
variation for traffic streams that are rate shaped on ingress to a DS
domain. Two parameters - capped arrival rate (R) and a 'score' (S),
are defined and related to the target delay variation bound. All
claims of DB 'conformance' for specific implementations of DB
behavior are made with respect to particular values for R, S, and the
implementation's ability to tolerate small amounts of burstiness in
the arriving DB traffic stream.
To protect itself against denial of service attacks, the edge of a DS
domain MUST strictly police all DB marked packets to a rate
negotiated with the adjacent upstream domain (for example, some value
less than or equal to the capped arrival rate R). Packets in excess
of the negotiated rate MUST be dropped. If two adjacent domains have
not negotiated an DB rate, the downstream domain MUST use 0 as the
rate (i.e., drop all DB marked packets).
Since PDBs constructed from the DB PHB will require that the upstream
domain police and shape DB marked traffic to meet the rate negotiated
with the downstream domain, the downstream domain's policer should
never have to drop packets. Thus these drops (or a summary of these
drops) SHOULD be noted (e.g., via rate-limited SNMP traps) as
possible security violations or serious misconfiguration.
Overflow events on an DB queue MAY also be logged as indicating
possible denial of service attacks or serious network
This document is the product of the volunteer 'EF Resolve' design
team, building on the work of V. Jacobson, K. Nichols, K. Poduri 
and clarified through discussions with members of the DiffServ
working group (particularly the authors of ). Non-contentious
text (such as the use of DB with tunnels, the security
considerations, etc.) were drawn directly from equivalent text in RFC
Intellectual Properties Considerations
To establish whether any considerations apply to the idea expressed
in this document, readers are encouraged to review notices filed with
the IETF and stored at:
 Jacobson, V., Nichols, K. and K. Poduri, "An Expedited Forwarding
PHB", RFC 2598, June 1999.
 Davie, B., Charny, A., Baker, F., Bennett, J.C.R., Benson, K., Le
Boudec, J.Y., Chiu, A., Courtney, W., Davari, S., Firoiu, V.,
Kalmanek, C., Ramakrishnan, K. and D. Stiliadis, "An Expedited
Forwarding PHB (Per-Hop Behavior)", RFC 3246, March 2002.
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 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.
 Black, D., Blake, S., Carlson, M., Davies, E., Wang, Z. and W.
Weiss, "An Architecture for Differentiated Services", RFC 2475,
Authors (volunteer EF Design Team members)
Center for Advanced Internet Architectures
Swinburne University of Technology,
Brian E. Carpenter (team observer, WG co-chair)
IBM Zurich Research Laboratory
Swindon, WI SN5 7DJ United Kingdom
Marconi Professor of Communications Systems
University of Cambridge
William Gates Building
J J Thomson Avenue
Phone: +44 (0)1223 763633
Joel M. Halpern
P. O. Box 6049
Leesburg, VA 20178
Corona Networks Inc.,
630 Alder Drive,
Milpitas, CA 95035
9123 Loughran Road
Fort Washington, MD 20744
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