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

Status of this Memo

    This document is an Internet-Draft and is in full conformance
    with all provisions of Section 10 of RFC2026.

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    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

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

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,

    [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

    Kathleen Nichols
    Cisco Systems, Inc
    170 W. Tasman Drive
    San Jose, CA 95134-1706

    Kedarnath Poduri
    Bay Networks, Inc.
    4401 Great America Parkway
    Santa Clara, CA 95052-8185

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 and 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

    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