Network Working Group Bruce Davie, Editor
Internet Draft Anna Charny
Expiration Date: August 2001 Fred Baker
Cisco Systems, Inc.
Jon Bennet
Riverdelta Networks
Kent Benson Jean-Yves Le Boudec
Tellabs EPFL
Angela Chiu William Courtney
AT&T Labs TRW
Shahram Davari Victor Firoiu
PMC-Sierra Nortel Networks
Charles Kalmanek K.K. Ramakrishnam
AT&T Research TeraOptic Networks
Dimitrios Stiliadis
Lucent Technologies
February 2001
An Expedited Forwarding PHB
draft-ietf-diffserv-rfc2598bis-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
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Internet-Drafts are draft documents valid for a maximum of six months
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
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The list of Internet-Draft Shadow Directories can be accessed at
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This document is a product of the Diffserv working group of the
Internet Engineering Task Force. Please address comments to the
group's mailing list at diffserv@ietf.org, with a copy to the
authors.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
The PHB (per-hop behavior) is a basic building block in the
Differentiated Services architecture. This document defines a PHB
called Expedited Forwarding (EF). EF is intended to provide a
building block for low delay and low loss services by ensuring that
the EF aggregate is served at a certain configured rate.
Contents
1 Introduction ........................................... 3
2 Definition of EF PHB ................................... 4
2.1 Intuitive Description of EF ............................ 4
2.2 Formal Definition of the EF PHB ........................ 5
2.3 Figures of merit ....................................... 8
2.4 Delay and jitter ....................................... 9
2.5 Loss ................................................... 9
2.6 Microflow misordering .................................. 10
2.7 Recommended codepoint for this PHB ..................... 10
2.8 Mutability ............................................. 10
2.9 Tunneling .............................................. 10
2.10 Interaction with other PHBs ............................ 10
3 Security Considerations ................................ 11
4 IANA Considerations .................................... 11
5 Acknowledgments ........................................ 11
6 References ............................................. 11
7 Full Copyright ......................................... 15
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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
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 [RFC2474,
RFC2475]. 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 speed-of-light
propagation 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 queueing 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.
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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.
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 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 to 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,
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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 to a device which 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 can't 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 jth packet should
(ideally) begin, then the ideal departure time of the jth packet is
L_j/R seconds later. Thus we are able to express the ideal departure
time of the jth packet in terms of the arrival time of the jth
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: a "colorblind" set and a "packet-identity-aware" set of
equations. The colorblind 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:
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d_j <= f_j + E_a (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 to arrive 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 actual
departure time of an EF packet and 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.
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 jth packet to arrive may not be the jth
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packet to depart. It is in this sense that eq_1 and eq_2 are
colorblind with regard to 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 (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 actual the departure time of the individual EF packet
that arrived 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
which arrived 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
actual departure time of an EF packet and 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.
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It is the fact that D_j and F_j refer to departure times for the jth
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].
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].
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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 provided a tighter bound on
jitter, a device MAY advertise E_p 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
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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 SHOULD include some means to limit the damage EF
traffic could inflict on other traffic. This will be reflected in the
range of supported R values as described in section 2.2.
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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).
4. IANA Considerations
This document allocates one codepoint, 101110, in Pool 1 of the code
space defined by [RFC2474].
5. Acknowledgments
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] V. Jacobson, K. Nichols, K. Poduri, "An Expedited Forwarding
PHB", RFC 2598, June 1999
[2] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997
[3] 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.
[4] D. Black, S. Blake, M. Carlson, E. Davies, Z. Wang, W. Weiss, "An
Architecture for Differentiated Services", RFC 2475, December 1998.
[5] D. Black, "Differentiated Services and Tunnels", RFC 2983,
October 2000.
[6] A. Charny et al., "Supplemental Information for the New
Definition of the EF PHB", Work in Progress, February 2001.
[7] K. Nichols and B. Carpenter, "Definition of Differentiated
Services Per Domain Behaviors and Rules for their Specification",
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Work in Progress, January 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
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be most appropriate.
Authors' Addresses
Bruce Davie
Cisco Systems, Inc.
300 Apollo Drive
Chelmsford, MA, 01824
E-mail: bsd@cisco.com
Anna Charny
Cisco Systems
300 Apollo Drive
Chelmsford, MA 01824
E-mail: acharny@cisco.com
Fred Baker
Cisco Systems
170 West Tasman Dr.
San Jose, CA 95134
E-mail: fred@cisco.com
Jon Bennett
RiverDelta Networks
3 Highwood Drive East
Tewksbury, MA 01876
E-mail: jcrb@riverdelta.com
Kent Benson
Tellabs Research Center
3740 Edison Lake Parkway #101
Mishawaka, IN 46545
E-mail: Kent.Benson@tellabs.com
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Jean-Yves Le Boudec
ICA-EPFL, INN
Ecublens, CH-1015
Lausanne-EPFL, Switzerland
E-mail: leboudec@epfl.ch
Angela Chiu
AT&T Labs
100 Schulz Dr. Rm 4-204
Red Bank, NJ 07701
E-mail: alchiu@att.com
Bill Courtney
TRW
Bldg. 201/3702
One Space Park
Redondo Beach, CA 90278
E-mail: bill.courtney@trw.com
Shahram Davari
PMC-Sierra Inc
411 Legget Drive
Ottawa, ON K2K 3C9, Canada
E-mail: shahram_davari@pmc-sierra.com
Victor Firoiu
Nortel Networks
600 Tech Park
Billerica, MA 01821
E-mail: vfirou@nortelnetworks.com
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Charles Kalmanek
AT&T Labs-Research
180 Park Avenue, Room A113,
Florham Park NJ
E-mail: crk@research.att.com.
K.K. Ramakrishnan
TeraOptic Networks, Inc.
686 W. Maude Ave
Sunnyvale, CA 94086
E-mail: kk@teraoptic.com
Dimitrios Stiliadis
Lucent Technologies
1380 Rodick Road
Markham, Ontario, L3R-4G5, Canada
E-mail: stiliadi@bell-labs.com
7. Full Copyright
Copyright (C) The Internet Society 2001. 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
English.
The limited permissions granted above are perpetual and will not be
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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.
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