Internet Engineering Task Force Van Jacobson
Differentiated Services Working Group Kathleen Nichols
Internet Draft Cisco Systems
Expires August, 1999 Kedarnath Poduri
Bay Networks
February, 1999
An Expedited Forwarding PHB
<draft-ietf-diffserv-phb-ef-02.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
This document is a product of the IETF Differentiated Services
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Abstract
The definition of PHBs (per-hop forwarding behaviors) is a
critical part of the work of the Diffserv Working Group. This
document describes a PHB called Expedited Forwarding. We show the
generality of this PHB by noting that it can be produced by more
than one mechanism and give an example of its use to produce at
least one service, a Virtual Leased Line. A recommended
codepoint for this PHB is given.
A pdf version of this document is available at
ftp://ftp.ee.lbl.gov/papers/ef_phb.pdf
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 draft describes a
particular PHB called expedited forwarding (EF). The EF PHB can
be used to build a low loss, low latency, low jitter, assured
bandwidth, end-to-end service through DS domains. Such a service
appears to the endpoints like a point-to-point connection or a
?virtual leased line?. This service has also been described as
Premium service [2BIT].
Loss, latency and jitter are all due to the queues traffic
experiences while transiting the network. Therefore providing
low loss, latency and jitter for some traffic aggregate means
ensuring that the aggregate sees no (or very small) queues.
Queues arise when (short-term) traffic arrival rate exceeds
departure rate at some node. Thus a service that ensures no
queues for some aggregate is equivalent to bounding rates such
that, at every transit node, the aggregate's maximum arrival rate
is less than that aggregate's minimum departure rate.
Creating such a service has two parts:
1) Configuring nodes so that the aggregate has a well-defined
minimum departure rate. (?Well-defined? means independent of the
dynamic state of the node. In particular, independent of the
intensity of other traffic at the node.)
2) Conditioning the aggregate (via policing and shaping) so that its
arrival rate at any node is always less than that node's
configured minimum departure rate.
The EF PHB provides the first part of the service. The network
boundary traffic conditioners described in [RFC2475] provide the
second part.
The EF PHB is not a mandatory part of the Differentiated Services
architecture, i.e., a node is not required to implement the EF
PHB in order to be considered DS-compliant. However, when a DS-
compliant node claims to implement the EF PHB, the implementation
must conform to the specification given in this document.
The next sections describe the EF PHB in detail and give examples
of how it might be implemented. The keywords ?MUST?, ?MUST NOT?,
?REQUIRED?, ?SHOULD?, ?SHOULD NOT?, and ?MAY? that appear in this
document are to be interpreted as described in [Bradner97].
2. Description of EF per-hop behavior
The EF PHB is defined as a forwarding treatment for a particular
diffserv aggregate where the departure rate of the aggregate's
packets from any diffserv node must equal or exceed a
configurable rate. The EF traffic SHOULD receive this rate
independent of the intensity of any other traffic attempting to
transit the node. It SHOULD average at least the configured rate
when measured over any time interval equal to or longer than the
time it takes to send an output link MTU sized packet at the
configured rate. (Behavior at time scales shorter than a packet
time at the configured rate is deliberately not specified.) The
configured minimum rate MUST be settable by a network
administrator (using whatever mechanism the node supports for
non-volatile configuration).
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). The minimum and maximum
rates may be the same and configured by a single parameter.
The Appendix describes how this PHB can be used to construct end-
to-end services.
2.2 Example Mechanisms to Implement the EF PHB
Several types of queue scheduling mechanisms may be employed to
deliver the forwarding behavior described in section 2.1 and thus
implement the EF PHB. A simple priority queue will give the
appropriate behavior as long as there is no higher priority queue
that could preempt the EF for more than a packet time at the
configured rate. (This could be accomplished by having a rate
policer such as a token bucket associated with each priority
queue to bound how much the queue can starve other traffic.)
It's also possible to use a single queue in a group of queues
serviced by a weighted round robin scheduler where the share of
the output bandwidth assigned to the EF queue is equal to the
configured rate. This could be implemented, for example, using
one PHB of a Class Selector Compliant set of PHBs [RFC2474].
Another possible implementation is a CBQ [CBQ] scheduler that
gives the EF queue priority up to the configured rate.
All of these mechanisms have the basic properties required for
the EF PHB though different choices result in different ancillary
behavior such as jitter seen by individual microflows. See
Appendix A.3 for simulations that quantify some of these differences.
2.3 Recommended codepoint for this PHB
Codepoint 101110 is recommended for the EF PHB.
2.4 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.
2.5 Tunneling
When EF packets are tunneled, the tunneling packets must be
marked as EF.
2.6 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 2.1
is met.
3. 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. (This rate must be
<= the EF PHB configured rate.) 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 the end-to-end premium service constructed from the EF PHB
requires 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 never have to drop
packets. Thus these drops SHOULD be noted (e.g., via SNMP traps)
as possible security violations or serious misconfiguration.
Similarly, since the aggregate EF traffic rate is constrained at
every interior node, the EF queue should never overflow so if it
does the drops SHOULD be noted as possible attacks or serious
misconfiguration.
4. References
[Bradner97] S. Bradner, ?Key words for use in RFCs to Indicate
Requirement Levels?, Internet RFC 2119, March 1997.
[RFC2474] K. Nichols, S. Blake, F. Baker, and D. Black,
?Definition of the Differentiated Services Field (DS Field) in
the IPv4 and IPv6 Headers?, Internet RFC 2474, December 1998.
[RFC2475] D. Black, S. Blake, M. Carlson, E. Davies, Z. Wang, and
W. Weiss, ?An Architecture for Differentiated Services?,
Internet RFC 2475, December 1998.
[2BIT] K. Nichols, V. Jacobson, and L. Zhang, ?A Two-bit
Differentiated Services Architecture for the Internet?, Internet
Draft <draft-nichols-diff-svc-arch-00.txt>, November 1997,
ftp://ftp.ee.lbl.gov/papers/dsarch.pdf
[CBQ] S. Floyd and V. Jacobson, ?Link-sharing and Resource
Management Models for Packet Networks?, IEEE/ACM Transactions on
Networking, Vol. 3 no. 4, pp. 365-386, August 1995.
[RFC2415] K. Poduri and K. Nichols, ?Simulation Studies of
Increased Initial TCP Window Size?, Internet RFC 2415,
September 1998.
[LCN] K. Nichols, ?Improving Network Simulation with
Feedback?, Proceedings of LCN '98, October, 1998
5. Authors' Addresses
Van Jacobson
Cisco Systems, Inc
170 W. Tasman Drive
San Jose, CA 95134-1706
van@cisco.com
Kathleen Nichols
Cisco Systems, Inc
170 W. Tasman Drive
San Jose, CA 95134-1706
kmn@cisco.com
Kedarnath Poduri
Bay Networks, Inc.
4401 Great America Parkway
Santa Clara, CA 95052-8185
kpoduri@baynetworks.com
Appendix A: Example use of and experiences with the EF PHB
A.1 Virtual Leased Line Service
A VLL Service, also known as Premium service [2BIT], is
quantified by a peak bandwidth.
A.2 Experiences with its use in ESNET
A prototype of the VLL service has been deployed on DOE's ESNet
backbone. This uses weighted-round-robin queuing features of
Cisco 75xx series routers to implement the EF PHB. The early
tests have been very successful and work is in progress to make
the service available on a routine production basis (see
ftp://ftp.ee.lbl.gov/talks/vj-doeqos.pdf and
ftp://ftp.ee.lbl.gov/talks/vj-i2qos-may98.pdf for details).
A.3 Simulation Results
A.3.1 Jitter variation
In section 2.2, we pointed out that a number of mechanisms might
be used to implement the EF PHB. The simplest of these is a
priority queue (PQ) where the arrival rate of the queue is
strictly less than its service rate. As jitter comes from the
queuing delay along the path, a feature of this implementation is
that EF-marked microflows will see very little jitter at their
subscribed rate since packets spend little time in queues. The EF
PHB does not have an explicit jitter requirement but it is clear
from the definition that the expected jitter in a packet stream
that uses a service based on the EF PHB will be less with PQ than
with best-effort delivery. We used simulation to explore how
weighted round-robin (WRR) compares to PQ in jitter. We chose
these two since they?re the best and worst cases, respectively,
for jitter and we wanted to supply rough guidelines for EF
implementers choosing to use WRR or similar mechanisms.
Our simulation model is implemented in a modified ns-2 described
in [RFC2415] and [LCN]. We used the CBQ modules included with ns-
2 as a basis to implement priority queuing and WRR. Our topology
has six hops with decreasing bandwidth in the direction of a
single 1.5 Mbps bottleneck link (see figure 6). Sources produce
EF-marked packets at an average bit rate equal to their
subscribed packet rate. Packets are produced with a variation of
+-10% from the interpacket spacing at the subscribed packet rate.
The individual source rates were picked aggregate to 30% of the
bottleneck link or 450 Kbps. A mixture of FTPs and HTTPs is then
used to fill the link. Individual EF packet sources produce
either all 160 byte packets or all 1500 byte packets. Though we
present the statistics of flows with one size of packet, all of
the experiments used a mixture of short and long packet EF
sources so the EF queues had a mix of both packet lengths.
We defined jitter as the absolute value of the difference between
the arrival times of two adjacent packets minus their departure
times, |(aj-dj) - (ai-di)|. For the target flow of each
experiment, we record the median and 90th percentile values of
jitter (expressed as % of the subscribed EF rate) in a table. The
pdf version of this document contains graphs of the jitter
percentiles.
Our experiments compared the jitter of WRR and PQ implementations
of the EF PHB. We assessed the effect of different choices of WRR
queue weight and number of queues on jitter. For WRR, we define
the service-to-arrival rate ratio as the service rate of the EF
queue (or the queue?s minimum share of the output link) times the
output link bandwidth divided by the peak arrival rate of EF-
marked packets at the queue. Results will not be stable if the
WRR weight is chosen to exactly balance arrival and departure
rates thus we used a minimum service-to-arrival ratio of 1.03. In
our simulations this means that the EF queue gets at least 31% of
the output links. In WRR simulations we kept the link full with
other traffic as described above, splitting the non-EF-marked
traffic among the non-EF queues. (It should be clear from the
experiment description that we are attempting to induce worst-
case jitter and do not expect these settings or traffic to
represent a ?normal? operating point.)
Our first set of experiments uses the minimal service-to-arrival
ratio of 1.06 and we vary the number of individual microflows
composing the EF aggregate from 2 to 36. We compare these to a PQ
implementation with 24 flows. First, we examine a microflow at a
subscribed rate of 56 Kbps sending 1500 byte packets, then one at
the same rate but sending 160 byte packets. Table 1 shows the
50th and 90th percentile jitter in percent of a packet time at the
subscribed rate. Figure 1 plots the 1500 byte flows and figure 2
the 160 byte flows. Note that a packet-time for a 1500 byte
packet at 56 Kbps is 214 ms, for a 160 byte packet 23 ms. The
jitter for the large packets rarely exceeds half a subscribed
rate packet-time, though most jitters for the small packets are
at least one subscribed rate packet-time. Keep in mind that the
EF aggregate is a mixture of small and large packets in all cases
so short packets can wait for long packets in the EF queue. PQ
gives a very low jitter.
Table 1: Variation in jitter with number of EF flows:
Service/arrival ratio of 1.06 and subscription rate of 56 Kbps
(all values given as % of subscribed rate)
1500 byte pack. 160 byte packet
# EF flows 50th % 90th % 50th % 90th %
PQ (24) 1 5 17 43
2 11 47 96 513
4 12 35 100 278
8 10 25 96 126
24 18 47 96 143
Next we look at the effects of increasing the service-to-arrival
ratio. This means that EF packets should remain enqueued for less
time though the bandwidth available to the other queues remains
the same. In this set of experiments the number of flows in the
EF aggregate was fixed at eight and the total number of queues at
five (four non-EF queues). Table 2 shows the results for 1500 and
160 byte flows. Figures 3 plots the 1500 byte results and figure
4 the 160 byte results. Performance gains leveled off at service-
to-arrival ratios of 1.5. Note that the higher service-to-arrival
ratios do not give the same performance as PQ, but now 90% of
packets experience less than a subscribed packet-time of jitter
even for the small packets.
Table 2: Variation in Jitter of EF flows: service/arrival ratio varies,
8 flow aggregate, 56 Kbps subscribed rate
WRR 1500 byte pack. 160 byte packet
Ser/Arr 50th % 90th % 50th % 90th %
PQ 1 3 17 43
1.03 14 27 100 178
1.30 7 21 65 113
1.50 5 13 57 104
1.70 5 13 57 100
2.00 5 13 57 104
3.00 5 13 57 100
Increasing the number of queues at the output interfaces can lead
to more variability in the service time for EF packets so we
carried out an experiment varying the number of queues at each
output port. We fixed the number of flows in the aggregate to
eight and used the minimal 1.03 service-to-arrival ratio. Results
are shown in figure 5 and table 3. Figure 5 includes PQ with 8
flows as a baseline.
Table 3: Variation in Jitter with Number of Queues at Output Interface:
Service-to-arrival ratio is 1.03, 8 flow aggregate
# EF 1500 byte packet
flows 50th % 90th %
PQ (8) 1 3
2 7 21
4 7 21
6 8 22
8 10 23
It appears that most jitter for WRR is low and can be reduced by
a proper choice of the EF queue's WRR share of the output link
with respect to its subscribed rate. As noted, WRR is a worst
case while PQ is the best case. Other possibilities include WFQ
or CBQ with a fixed rate limit for the EF queue but giving it
priority over other queues. We expect the latter to have
performance nearly identical with PQ though future simulations
are needed to verify this. We have not yet systematically
explored effects of hop count, EF allocations other than 30% of
the link bandwidth, or more complex topologies. The information
in this section is not part of the EF PHB definition but provided
simply as background to guide implementers.
A.3.2 VLL service
We used simulation to see how well a VLL service built from the
EF PHB behaved, that is, does it look like a `leased line' at the
subscribed rate. In the simulations of the last section, none of
the EF packets were dropped in the network and the target rate
was always achieved for those CBR sources. However, we wanted to
see if VLL really looks like a `wire' to a TCP using it. So we
simulated long-lived FTPs using a VLL service. Table 4 gives the
percentage of each link allocated to EF traffic (bandwidths are
lower on the links with fewer EF microflows), the subscribed VLL
rate, the average rate for the same type of sender-receiver pair
connected by a full duplex dedicated link at the subscribed rate
and the average of the VLL flows for each simulation (all sender-
receiver pairs had the same value). Losses only occur when the
input shaping buffer overflows but not in the network. The target
rate is not achieved due to the well-known TCP behavior.
Table 4: Performance of FTPs using a VLL service
% link Average delivered rate (Kbps)
to EF Subscribed Dedicated VLL
20 100 90 90
40 150 143 143
60 225 213 215
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draft-ietf-diffserv-phb-ef-02
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