Network Working Group B. Davie
Request for Comments: 3246 A. Charny
Obsoletes: 2598 Cisco Systems, Inc.
Category: Standards Track J.C.R. Bennett
Motorola
K. Benson
Tellabs
J.Y. Le Boudec
EPFL
W. Courtney
TRW
S. Davari
PMC-Sierra
V. Firoiu
Nortel Networks
D. Stiliadis
LUCent Technologies
March 2002

An EXPedited Forwarding PHB (Per-Hop Behavior)

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 Notice

Copyright (C) The Internet Society (2001). All Rights Reserved.

Abstract

This document defines a PHB (per-hop behavior) called Expedited
Forwarding (EF). The PHB is a basic building block in the
Differentiated Services architecture. EF is intended to provide a
building block for low delay, low jitter and low loss services by
ensuring that the EF aggregate is served at a certain configured
rate. This document obsoletes RFC2598.

Table of Contents

1 Introduction ........................................... 2
1.1 Relationship to RFC2598 ............................... 3
2 Definition of EF PHB ................................... 3
2.1 Intuitive Description of EF ............................ 3
2.2 Formal Definition of the EF PHB ........................ 5
2.3 Figures of merit ....................................... 8
2.4 Delay and jitter ....................................... 8
2.5 Loss ................................................... 9
2.6 Microflow misordering .................................. 9
2.7 Recommended codepoint for this PHB ..................... 9
2.8 Mutability ............................................. 10
2.9 Tunneling .............................................. 10
2.10 Interaction with other PHBs ............................ 10
3 Security Considerations ................................ 10
4 IANA Considerations .................................... 11
5 Acknowledgments ........................................ 11
6 References ............................................. 11
Appendix: Implementation Examples .............................. 12
Authors' Addresses ............................................. 14
Full Copyright Statement ....................................... 16

1. Introduction

Network nodes that implement the differentiated services enhancements
to IP use a codepoint in the IP header to select a per-hop behavior
(PHB) as the specific forwarding treatment for that packet [3, 4].
This memo describes a particular PHB called expedited forwarding
(EF).

The intent of the EF PHB is to provide a building block for low loss,
low delay, and low jitter services. The details of exactly how to
build such services are outside the scope of this specification.

The dominant causes of delay in packet networks are fixed propagation
delays (e.g. those arising from speed-of-light delays) on wide area
links and queuing delays in switches and routers. Since propagation
delays are a fixed property of the topology, delay and jitter are
minimized when queuing delays are minimized. In this context, jitter
is defined as the variation between maximum and minimum delay. The
intent of the EF PHB is to provide a PHB in which suitably marked
packets usually encounter short or empty queues. Furthermore, if
queues remain short relative to the buffer space available, packet
loss is also kept to a minimum.

To ensure that queues encountered by EF packets are usually short, it
is necessary to ensure that the service rate of EF packets on a given
output interface exceeds their arrival rate at that interface over
long and short time intervals, independent of the load of other
(non-EF) traffic. This specification defines a PHB in which EF
packets are guaranteed to receive service at or above a configured
rate and provides a means to quantify the accuracy with which this
service rate is delivered over any time interval. It also provides a
means to quantify the maximum delay and jitter that a packet may
experience under bounded operating conditions.

Note that the EF PHB only defines the behavior of a single node. The
specification of behavior of a collection of nodes is outside the
scope of this document. A Per-Domain Behavior (PDB) specification
[7] may provide such information.

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.

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 RFC2119 [2].

1.1. Relationship to RFC2598

This document replaces RFC2598 [1]. The main difference is that it
adds mathematical formalism to give a more rigorous definition of the
behavior described. The full rationale for this is given in [6].

2. Definition of EF PHB

2.1. Intuitive Description of EF

Intuitively, the definition of EF is simple: the rate at which EF
traffic is served at a given output interface should be at least the
configured rate R, over a suitably defined interval, independent of
the offered load of non-EF traffic to that interface. Two
difficulties arise when we try to formalize this intuition:

- it is difficult to define the appropriate timescale at which to
measure R. By measuring it at short timescales we may introduce
sampling errors; at long timescales we may allow excessive
jitter.

- EF traffic clearly cannot be served at rate R if there are no
EF packets waiting to be served, but it may be impossible to
determine externally whether EF packets are actually waiting to
be served by the output scheduler. For example, if an EF
packet has entered the router and not exited, it may be
awaiting service, or it may simply have encountered some
processing or transmission delay within the router.

The formal definition below takes account of these issues. It
assumes that EF packets should ideally be served at rate R or faster,
and bounds the deviation of the actual departure time of each packet
from the "ideal" departure time of that packet. We define the
departure time of a packet as the time when the last bit of that
packet leaves the node. The "ideal" departure time of each EF packet
is computed iteratively.

In the case when an EF packet arrives at a device when all the
previous EF packets have already departed, the computation of the
ideal departure time is simple. Service of the packet should
(ideally) start as soon as it arrives, so the ideal departure time is
simply the arrival time plus the ideal time to transmit the packet at
rate R. For a packet of length L_j, that transmission time at the
configured rate R is L_j/R. (Of course, a real packet will typically
get transmitted at line rate once its transmission actually starts,
but we are calculating the ideal target behavior here; the ideal
service takes place at rate R.)

In the case when an EF packet arrives at a device that still contains
EF packets awaiting service, the computation of the ideal departure
time is more complicated. There are two cases to be considered. If
the previous (j-1-th) departure occurred after its own ideal
departure time, then the scheduler is running "late". In this case,
the ideal time to start service of the new packet is the ideal
departure time of the previous (j-1-th) packet, or the arrival time
of the new packet, whichever is later, because we cannot expect a
packet to begin service before it arrives. If the previous (j-1-th)
departure occurred before its own ideal departure time, then the
scheduler is running "early". In this case, service of the new
packet should begin at the actual departure time of the previous
packet.

Once we know the time at which service of the j-th packet should
(ideally) begin, then the ideal departure time of the j-th packet is
L_j/R seconds later. Thus we are able to express the ideal departure
time of the j-th packet in terms of the arrival time of the j-th
packet, the actual departure time of the j-1-th packet, and the ideal
departure time of the j-1-th packet. Equations eq_1 and eq_2 in
Section 2.2 capture this relationship.

Whereas the original EF definition did not provide any means to
guarantee the delay of an individual EF packet, this property may be
desired. For this reason, the equations in Section 2.2 consist of
two parts: an "aggregate behavior" set and a "packet-identity-aware"
set of equations. The aggregate behavior equations (eq_1 and eq_2)
simply describe the properties of the service delivered to the EF
aggregate by the device. The "packet-identity-aware" equations (eq_3
and eq_4) enable the bound on delay of an individual packet to be
calculated given a knowledge of the operating conditions of the
device. The significance of these two sets of equations is discussed
further in Section 2.2. Note that these two sets of equations provide
two ways of characterizing the behavior of a single device, not two
different modes of behavior.

2.2. Formal Definition of the EF PHB

A node that supports EF on an interface I at some configured rate R
MUST satisfy the following equations:

d_j <= f_j + E_a for all j > 0 (eq_1)

where f_j is defined iteratively by

f_0 = 0, d_0 = 0

f_j = max(a_j, min(d_j-1, f_j-1)) + l_j/R, for all j > 0 (eq_2)

In this definition:

- d_j is the time that the last bit of the j-th EF packet to
depart actually leaves the node from the interface I.

- f_j is the target departure time for the j-th EF packet to
depart from I, the "ideal" time at or before which the last bit
of that packet should leave the node.

- a_j is the time that the last bit of the j-th EF packet
destined to the output I actually arrives at the node.

- l_j is the size (bits) of the j-th EF packet to depart from I.
l_j is measured on the IP datagram (IP header plus payload) and
does not include any lower layer (e.g. MAC layer) overhead.

- R is the EF configured rate at output I (in bits/second).

- E_a is the error term for the treatment of the EF aggregate.
Note that E_a represents the worst case deviation between the
actual departure time of an EF packet and the ideal departure
time of the same packet, i.e. E_a provides an upper bound on
(d_j - f_j) for all j.

- d_0 and f_0 do not refer to a real packet departure but are
used purely for the purposes of the recursion. The time origin
should be chosen such that no EF packets are in the system at
time 0.

- for the definitions of a_j and d_j, the "last bit" of the
packet includes the layer 2 trailer if present, because a
packet cannot generally be considered available for forwarding
until such a trailer has been received.

An EF-compliant node MUST be able to be characterized by the range of
possible R values that it can support on each of its interfaces while
conforming to these equations, and the value of E_a that can be met
on each interface. R may be line rate or less. E_a MAY be specified
as a worst-case value for all possible R values or MAY be expressed
as a function of R.

Note also that, since a node may have multiple inputs and complex
internal scheduling, the j-th EF packet to arrive at the node
destined for a certain interface may not be the j-th EF packet to
depart from that interface. It is in this sense that eq_1 and eq_2
are unaware of packet identity.

In addition, a node that supports EF on an interface I at some
configured rate R MUST satisfy the following equations:

D_j <= F_j + E_p for all j > 0 (eq_3)

where F_j is defined iteratively by

F_0 = 0, D_0 = 0

F_j = max(A_j, min(D_j-1, F_j-1)) + L_j/R, for all j > 0 (eq_4)

In this definition:

- D_j is the actual departure time of the individual EF packet
that arrived at the node destined for interface I at time A_j,
i.e., given a packet which was the j-th EF packet destined for
I to arrive at the node via any input, D_j is the time at which
the last bit of that individual packet actually leaves the node
from the interface I.

- F_j is the target departure time for the individual EF packet
that arrived at the node destined for interface I at time A_j.

- A_j is the time that the last bit of the j-th EF packet
destined to the output I to arrive actually arrives at the
node.

- L_j is the size (bits) of the j-th EF packet to arrive at the
node that is destined to output I. L_j is measured on the IP
datagram (IP header plus payload) and does not include any
lower layer (e.g. MAC layer) overhead.

- R is the EF configured rate at output I (in bits/second).

- E_p is the error term for the treatment of individual EF
packets. Note that E_p represents the worst case deviation
between the actual departure time of an EF packet and the ideal
departure time of the same packet, i.e. E_p provides an upper
bound on (D_j - F_j) for all j.

- D_0 and F_0 do not refer to a real packet departure but are
used purely for the purposes of the recursion. The time origin
should be chosen such that no EF packets are in the system at
time 0.

- for the definitions of A_j and D_j, the "last bit" of the
packet includes the layer 2 trailer if present, because a
packet cannot generally be considered available for forwarding
until such a trailer has been received.

It is the fact that D_j and F_j refer to departure times for the j-th
packet to arrive that makes eq_3 and eq_4 aware of packet identity.
This is the critical distinction between the last two equations and
the first two.

An EF-compliant node SHOULD be able to be characterized by the range
of possible R values that it can support on each of its interfaces
while conforming to these equations, and the value of E_p that can be
met on each interface. E_p MAY be specified as a worst-case value
for all possible R values or MAY be expressed as a function of R. An
E_p value of "undefined" MAY be specified. For discussion of
situations in which E_p may be undefined see the Appendix and [6].

For the purposes of testing conformance to these equations, it may be
necessary to deal with packet arrivals on different interfaces that
are closely spaced in time. If two or more EF packets destined for
the same output interface arrive (on different inputs) at almost the

same time and the difference between their arrival times cannot be
measured, then it is acceptable to use a random tie-breaking method
to decide which packet arrived "first".

2.3. Figures of merit

E_a and E_p may be thought of as "figures of merit" for a device. A
smaller value of E_a means that the device serves the EF aggregate
more smoothly at rate R over relatively short timescales, whereas a
larger value of E_a implies a more bursty scheduler which serves the
EF aggregate at rate R only when measured over longer intervals. A
device with a larger E_a can "fall behind" the ideal service rate R
by a greater amount than a device with a smaller E_a.

A lower value of E_p implies a tighter bound on the delay experienced
by an individual packet. Factors that might lead to a higher E_p
might include a large number of input interfaces (since an EF packet
might arrive just behind a large number of EF packets that arrived on
other interfaces), or might be due to internal scheduler details
(e.g. per-flow scheduling within the EF aggregate).

We observe that factors that increase E_a such as those noted above
will also increase E_p, and that E_p is thus typically greater than
or equal to E_a. In summary, E_a is a measure of deviation from
ideal service of the EF aggregate at rate R, while E_p measures both
non-ideal service and non-FIFO treatment of packets within the
aggregate.

For more discussion of these issues see the Appendix and [6].

2.4. Delay and jitter

Given a known value of E_p and a knowledge of the bounds on the EF
traffic offered to a given output interface, summed over all input
interfaces, it is possible to bound the delay and jitter that will be
experienced by EF traffic leaving the node via that interface. The
delay bound is

D = B/R + E_p (eq_5)

where

- R is the configured EF service rate on the output interface

- the total offered load of EF traffic destined to the output
interface, summed over all input interfaces, is bounded by a
token bucket of rate r <= R and depth B

Since the minimum delay through the device is clearly at least zero,
D also provides a bound on jitter. To provide a tighter bound on
jitter, the value of E_p for a device MAY be specified as two
separate components such that

E_p = E_fixed + E_variable

where E_fixed represents the minimum delay that can be experienced by
an EF packet through the node.

2.5. Loss

The EF PHB is intended to be a building block for low loss services.
However, under sufficiently high load of EF traffic (including
unexpectedly large bursts from many inputs at once), any device with
finite buffers may need to discard packets. Thus, it must be
possible to establish whether a device conforms to the EF definition
even when some packets are lost. This is done by performing an
"off-line" test of conformance to equations 1 through 4. After
observing a sequence of packets entering and leaving the node, the
packets which did not leave are assumed lost and are notionally
removed from the input stream. The remaining packets now constitute
the arrival stream (the a_j's) and the packets which left the node
constitute the departure stream (the d_j's). Conformance to the
equations can thus be verified by considering only those packets that
successfully passed through the node.

In addition, to assist in meeting the low loss objective of EF, a
node MAY be characterized by the operating region in which loss of EF
due to congestion will not occur. This MAY be specified, using a
token bucket of rate r <= R and burstsize B, as the sum of traffic
across all inputs to a given output interface that can be tolerated
without loss.

In the event that loss does occur, the specification of which packets
are lost is beyond the scope of this document. However it is a
requirement that those packets not lost MUST conform to the equations
of Section 2.2.

2.6. Microflow misordering

Packets belonging to a single microflow within the EF aggregate
passing through a device SHOULD NOT experience re-ordering in normal
operation of the device.

2.7. Recommended codepoint for this PHB

Codepoint 101110 is RECOMMENDED for the EF PHB.

2.8. 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.

2.9. Tunneling

When EF packets are tunneled, the tunneling packets SHOULD be marked
as EF. A full discussion of tunneling issues is presented in [5].

2.10. Interaction with other PHBs

Other PHBs and PHB groups may be deployed in the same DS node or
domain with the EF PHB. The equations of Section 2.2 MUST hold for a
node independent of the amount of non-EF traffic offered to it.

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 (e.g., a token bucket rate limiter).
Traffic that exceeds this limit MUST be discarded. This maximum EF
rate, and burst size if appropriate, MUST be settable by a network
administrator (using whatever mechanism the node supports for non-
volatile configuration).

3. Security Considerations

To protect itself against denial of service attacks, the edge of a DS
domain SHOULD strictly police all EF marked packets to a rate
negotiated with the adjacent upstream domain. Packets in excess of
the negotiated rate SHOULD be dropped. If two adjacent domains have
not negotiated an EF rate, the downstream domain SHOULD use 0 as the
rate (i.e., drop all EF marked packets).

In addition, traffic conditioning at the ingress to a DS-domain MUST
ensure that only packets having DSCPs that correspond to an EF PHB
when they enter the DS-domain are marked with a DSCP that corresponds
to EF inside the DS-domain. Such behavior is as required by the
Differentiated Services architecture [4]. It protects against
denial-of-service and theft-of-service attacks which exploit DSCPs
that are not identified in any Traffic Conditioning Specification
provisioned at an ingress interface, but which map to EF inside the
DS-domain.

4. IANA Considerations

This document allocates one codepoint, 101110, in Pool 1 of the code
space defined by [3].

5. Acknowledgments

This document was the result of collaboration and discussion among a
large number of people. In particular, Fred Baker, Angela Chiu,
Chuck Kalmanek, and K. K. Ramakrishnan made significant contributions
to the new EF definition. John Wroclawski provided many helpful
comments to the authors. This document draws heavily on the original
EF PHB definition of Jacobson, Nichols, and Poduri. It was also
greatly influenced by the work of the EFRESOLVE team of Armitage,
Casati, Crowcroft, Halpern, Kumar, and Schnizlein.

6. References

[1] Jacobson, V., Nichols, K. and K. Poduri, "An Expedited
Forwarding PHB", RFC2598, June 1999.

[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.

[3] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of
the Differentiated Services Field (DS Field) in the IPv4 and
IPv6 Headers", RFC2474, December 1998.

[4] Black, D., Blake, S., Carlson, M., Davies, E., Wang, Z. and W.
Weiss, "An Architecture for Differentiated Services", RFC2475,
December 1998.

[5] Black, D., "Differentiated Services and Tunnels", RFC2983,
October 2000.

[6] Charny, A., Baker, F., Davie, B., Bennett, J.C.R., Benson, K.,
Le Boudec, J.Y., Chiu, A., Courtney, W., Davari, S., Firoiu,
V., Kalmanek, C., Ramakrishnan, K.K. and D. Stiliadis,
"Supplemental Information for the New Definition of the EF PHB
(Expedited Forwarding Per-Hop Behavior)", RFC3247, March 2002.

[7] Nichols K. and B. Carpenter, "Definition of Differentiated
Services Per Domain Behaviors and Rules for their
Specification", RFC3086, April 2001.

Appendix: Implementation Examples

This appendix is not part of the normative specification of EF.
However, it is included here as a possible source of useful
information for implementors.

A variety of factors in the implementation of a node supporting EF
will influence the values of E_a and E_p. These factors are
discussed in more detail in [6], and include both output schedulers
and the internal design of a device.

A priority queue is widely considered as the canonical example of an
implementation of EF. A "perfect" output buffered device (i.e. one
which delivers packets immediately to the appropriate output queue)
with a priority queue for EF traffic will provide both a low E_a and
a low E_p. We note that the main factor influencing E_a will be the
inability to pre-empt an MTU-sized non-EF packet that has just begun
transmission at the time when an EF packet arrives at the output
interface, plus any additional delay that might be caused by non-
pre-emptable queues between the priority queue and the physical
interface. E_p will be influenced primarily by the number of
interfaces.

Another example of an implementation of EF is a weighted round robin
scheduler. Such an implementation will typically not be able to
support values of R as high as the link speeds, because the maximum
rate at which EF traffic can be served in the presence of competing
traffic will be affected by the number of other queues and the
weights given to them. Furthermore, such an implementation is likely
to have a value of E_a that is higher than a priority queue
implementation, all else being equal, as a result of the time spent
serving non-EF queues by the round robin scheduler.

Finally, it is possible to implement hierarchical scheduling
algorithms, such that some non-FIFO scheduling algorithm is run on
sub-flows within the EF aggregate, while the EF aggregate as a whole
could be served at high priority or with a large weight by the top-
level scheduler. Such an algorithm might perform per-input
scheduling or per-microflow scheduling within the EF aggregate, for
example. Because such algorithms lead to non-FIFO service within the
EF aggregate, the value of E_p for such algorithms may be higher than
for other implementations. For some schedulers of this type it may
be difficult to provide a meaningful bound on E_p that would hold for
any pattern of traffic arrival, and thus a value of "undefined" may
be most appropriate.

It should be noted that it is quite acceptable for a Diffserv domain
to provide multiple instances of EF. Each instance should be
characterizable by the equations in Section 2.2 of this
specification. The effect of having multiple instances of EF on the
E_a and E_p values of each instance will depend considerably on how
the multiple instances are implemented. For example, in a multi-
level priority scheduler, an instance of EF that is not at the
highest priority may experience relatively long periods when it
receives no service while higher priority instances of EF are served.
This would result in relatively large values of E_a and E_p. By
contrast, in a WFQ-like scheduler, each instance of EF would be
represented by a queue served at some configured rate and the values
of E_a and E_p could be similar to those for a single EF instance.

Authors' Addresses

Bruce Davie
Cisco Systems, Inc.
300 Apollo Drive
Chelmsford, MA, 01824

EMail: bsd@cisco.com

Anna Charny
Cisco Systems
300 Apollo Drive
Chelmsford, MA 01824

EMail: acharny@cisco.com

Jon Bennett
Motorola
3 Highwood Drive East
Tewksbury, MA 01876

EMail: jcrb@motorola.com

Kent Benson
Tellabs Research Center
3740 Edison Lake Parkway #101
Mishawaka, IN 46545

EMail: Kent.Benson@tellabs.com

Jean-Yves Le Boudec
ICA-EPFL, INN
Ecublens, CH-1015
Lausanne-EPFL, Switzerland

EMail: jean-yves.leboudec@epfl.ch

Bill Courtney
TRW
Bldg. 201/3702
One Space Park
Redondo Beach, CA 90278

EMail: bill.courtney@trw.com

Shahram Davari
PMC-Sierra Inc
411 Legget Drive
Ottawa, ON K2K 3C9, Canada

EMail: shahram_davari@pmc-sierra.com

Victor Firoiu
Nortel Networks
600 Tech Park
Billerica, MA 01821

EMail: vfiroiu@nortelnetworks.com

Dimitrios Stiliadis
Lucent Technologies
101 Crawfords Corner Road
Holmdel, NJ 07733

EMail: stiliadi@bell-labs.com

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