Internet-Draft EF RESOLVE DESIGN TEAM
(Grenville Armitage)
(Alessio Casati)
(Jon Crowcroft)
(Joel Halpern)
(Brijesh Kumar)
(John Schnizlein)
November 12th, 2000
A revised expression of the Expedited Forwarding PHB
<draft-ietf-diffserv-efresolve-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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 working documents as Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
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The list of current Internet-Drafts can be accessed at
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This document was submitted to the IETF Differentiated Services
(DiffServ) WG. Publication of this document does not imply
acceptance by the DiffServ WG of any ideas expressed within.
Comments should be submitted to the diffserv@ietf.org mailing list.
Distribution of this memo is unlimited.
Abstract
RFC 2598 is the DiffServ working group's current standards track
definition of the Expedited Forwarding (EF) Per Hop Behavior (PHB)
[1]. As part of the DiffServ working group's ongoing refinement of
the EF PHB, additional issues were raised with the text in RFC 2598
[2]. An 'EF design team' was formed after the Pittsburgh IETF meeting
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to synthesize a new expression of the EF PHB. This Internet Draft
captures our feedback to the DiffServ WG on a proposed revision to
the EF PHB definition. A formal revision to RFC 2598 will be derived
from this document.
Specification of Requirements
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 [3].
1 Introduction
RFC 2598 is the Differentiated Services (DiffServ) working group's
current standards track definition of the Expedited Forwarding (EF)
Per Hop Behavior (PHB) [1]. As part of the DiffServ working group's
ongoing refinement of the EF PHB, additional issues were raised with
the text in RFC 2598 [2]. An 'EF design team' was formed after the
Pittsburgh IETF meeting to synthesize a new expression of the EF PHB.
This Internet Draft captures our feedback to the DiffServ WG on a
proposed revision to the EF PHB definition.
A formal revision to RFC 2598 will be derived from this document.
Section 2 covers the minimum, necessary and sufficient description of
what qualifies as 'EF' 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 Expedited Forwarding
For a traffic stream not exceeding a configured rate the goal of the
EF PHB is a strict bound on the delay variation of packets through a
hop.
When a DS-compliant node claims to implement the EF PHB, the
implementation MUST conform to the specification given in this
document. However, the EF PHB is not a mandatory part of the
Differentiated Services architecture - a node is NOT REQUIRED to
implement the EF PHB in order to be considered DS-compliant.
This section will begin with the goals and necessary boundary
conditions for EF behavior, then provide a descriptive definition of
EF behavior itself, discuss what it means to conform to the EF
definition, and assign the default EF code point.
2.1 Goal and Scope of EF
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For a traffic stream not exceeding a configured rate the goal of the
EF PHB is a strict bound on the delay variation of packets through a
hop.
Traffic MUST be policed and/or shaped at the source edge (for
example, on ingress to the DS-domain as discussed in RFC 2475 [5]) in
order to get such a bound. However, specific policing and/or shaping
rules are outside the scope of the EF PHB definition. Such rules
MUST be defined in any per-domain behaviors (PDBs) composed from the
EF PHB.
A device (hop) delivers EF behavior to appropriately marked traffic
received on one or more interfaces (marking is specified in section
2.4). A device SHALL deliver the EF behavior on an interface to EF
marked traffic meeting (i.e. less than or equal) a certain arrival
rate limit R.
If more EF traffic arrives than is acceptable, the device is NOT
REQUIRED to deliver the EF behavior. However, although the original
source of EF 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 an EF implementation SHOULD have some
tolerance for burstiness - the ability to provide EF behavior even
when the arrival rate exceeds the rate limit R.
Different EF 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
EF behavior. However, since the goal of characterizing burstiness is
to allow useful comparison of EF implementations, vendors and users
of EF implementations MAY choose to utilize other burstiness
metrics.)
The EF PHB definition does NOT mandate or recommend any particular
method for achieving EF behavior. Rather, the EF PHB definition
identifies parameters that bound the operating range(s) over which an
implementation can deliver EF behavior. Implementors characterize
their implementations using these parameters, while network designers
and testers use these parameters to assess the utility of different
EF implementations.
2.2 Description of EF 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
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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 EF
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 EF traffic arriving at the device on
other ports, and all non-EF traffic arriving at the device on any
port.
EF 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 EF device is prepared to accept on
this interface.
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
the device.
2.3 Conformance to EF behavior
An implementation need not conform to the EF 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 EF 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 EF 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 EF behavior, and the
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conditions under which it might do so, is outside the scope of this
document.
The implementor MAY define additional constraints on the range of
configurations in which EF behavior is delivered. These constraints
MAY include limits on the total EF traffic across the device, or
total EF traffic targetted at a given interface from all inputs.
This document does not specify any requirements on an EF
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 EF behavior
One or more DiffServ codepoint (DSCP) values may be used to indicate
a requirement for EF behavior [4].
By default a DSCP of 101110 indicates that EF PHB is required.
3. Discussion
This section discusses some issues that might not be immediately
obvious from the definition in section 2.
3.1 Mutability
Packets marked for EF PHB MAY be remarked at a DS domain boundary
only to other codepoints that satisfy the EF PHB. Packets marked for
EF PHBs SHOULD NOT be demoted or promoted to another PHB by a DS
domain.
3.2 Tunneling
When EF packets are tunneled, the tunneling packets must be marked as
EF.
3.3 Interaction with other PHBs
Other PHBs and PHB groups may be deployed in the same DS node or
domain with the EF PHB as long as the requirement of section 2 is
met.
3.4 Output Rate not specified
The definition of EF behavior given in section 2 is quite explicitly
given in terms of input rate R and output delay variation D(i) -
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E(i). A scheduler's output rate does not need to be specified, since
it will be whatever is needed to achieve the target delay variation
bounds.
3.5 Jitter
Jitter is not the bounded parameter in EF 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, EF
behavior aims to bound a related but different parameter - the
variation in delay between the time packets would ideally depart,
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 EF
traffic is converging on a single output port from multiple input
ports. When evaluating the ability of an EF device to offer EF
behavior to traffic arriving on one port, EF traffic arriving on
other ports is factored in as competing traffic.
When considering EF 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 EF packets is being avoided
(either through some form of Path MTU discovery, or configuration),
and does not need to be specifically considered in the EF behavior
definition.
3.8 Interference with other traffic
If the EF 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 EF traffic
could inflict on other traffic. This will be reflected in the EF
device's burst tolerance described in section 2.1.
3.9 Micro flow awareness
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Some EF 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 EF PHB definition. However, such behavior is also NOT part of
the EF PHB definition. Vendors are free to characterize and publicize
the additional per micro flow capabilities of their EF
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 EF 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 allocates one codepoint, 101110, in Pool 1 of the code
space defined by [4].
5. Conclusion.
This document defines EF 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 EF 'conformance' for specific implementations of EF
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 EF traffic stream.
Security Considerations
To protect itself against denial of service attacks, the edge of a DS
domain MUST strictly police all EF 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 EF rate, the downstream domain MUST use 0 as the
rate (i.e., drop all EF marked packets).
Since PDBs constructed from the EF PHB will require that the upstream
domain police and shape EF marked traffic to meet the rate negotiated
with the downstream domain, the downstream domain's policer should
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never have to drop packets. Thus these drops SHOULD be noted (e.g.,
via SNMP traps) as possible security violations or serious
misconfiguration.
Overflow events on an EF queue MAY also be logged as indicating
possible denial of service attacks or serious network
misconfiguration.
Acknowledgments
This draft is the product of the EF Resolve design team, and builds
almost entirely on the works of V. Jacobson, K. Nichols, K. Poduri
[1] and A. Charny, F. Baker, J. Bennett, K. Benson, J.-Y. Le Boudec,
A. Chiu, W. Courtney, B. Davie, S. Davari, V. Firou, C. Kalmanek,
K.K. Ramakrishnan, and D. Stiliadis [2]. Non-contentious text (such
as the use of EF with tunnels, the security considerations, etc) were
drawn directly from RFC 2598.
EF Design Team Members
Grenville Armitage
Rm A234, 3180 Porter Drive
Palo Alto, CA 94061
email: gja@lucent.com
Brian E. Carpenter (team observer, WG co-chair)
IBM
iCAIR, Suite 150
1890 Maple Avenue
Evanston IL 60201, USA
email: brian@icair.org
Alessio Casati
Lucent Technologies
Swindon, WI SN5 7DJ United Kingdom
email: acasati@lucent.com
Jon Crowcroft
Department of Computer Science
University College London
Gower Street,
London WC1E 6BT, UK
email: J.Crowcroft@cs.ucl.ac.uk
Joel M. Halpern
Longitude Systems, Inc.
15000 Conference Center Drive
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Chantilly, VA 20151
email: joel@longsys.com
Brijesh Kumar
Ennovate Networks
email: bkumar@ennovatenetworks.com
John Schnizlein
Cisco Systems
9123 Loughran Road
Fort Washington, MD 20744
email: john.schnizlein@cisco.com
Intellectual Properties Considerations
<TBD>
References
[1] V. Jacobson, K. Nichols, K. Poduri, "An Expedited Forwarding
PHB", RFC 2598, June 1999
[2] A Charny, ed. "EF PHB Redefined", INTERNET DRAFT <draft-charny-
ef-definition-00.txt> (work in progress), July 2000
[3] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997
[4] K. Nichols, S. Blake, F. Baker, D. Black, "Definition of the
Differentiated Services Field (DS Field) in the IPv4 and IPv6
Headers", RFC 2474, December 1998.
[5] D. Black, S. Blake, M. Carlson, E. Davies, Z. Wang, W. Weiss, "An
Architecture for Differentiated Services", RFC 2475, December 1998.
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