RFC2598 - An Expedited Forwarding PHB

Network Working Group V. Jacobson

Request for Comments: 2598 K. Nichols

Category: Standards Track Cisco Systems

K. Poduri

Bay Networks

June 1999

An EXPedited Forwarding PHB

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.

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 memo 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. IANA Considerations

This document allocates one codepoint, 101110, in Pool 1 of the code

space defined by [RFC2474].

5. References

[Bradner97] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC2119, March 1997.

[RFC2474] 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.

[RFC2475] Black, D., Blake, S., Carlson, M., Davies, E., Wang, Z.

and W. Weiss, "An Architecture for Differentiated

Services", RFC2475, December 1998.

[2BIT] K. Nichols, V. Jacobson, and L. Zhang, "A Two-bit

Differentiated Services Architecture for the Internet",

Work in Progress, 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] Poduri, K. and K. Nichols, "Simulation Studies of

Increased Initial TCP Window Size", RFC2415, September

1998.

[LCN] K. Nichols, "Improving Network Simulation with Feedback",

Proceedings of LCN '98, October 1998.

6. Authors' Addresses

Van Jacobson

Cisco Systems, Inc

170 W. Tasman Drive

San Jose, CA 95134-1706

EMail: van@cisco.com

Kathleen Nichols

Cisco Systems, Inc

170 W. Tasman Drive

San Jose, CA 95134-1706

EMail: kmn@cisco.com

Kedarnath Poduri

Bay Networks, Inc.

4401 Great America Parkway

Santa Clara, CA 95052-8185

EMail: 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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