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Jacobson, et al.            Standards Track                     [Page 6]

RFC 2598              An Expedited Forwarding PHB              June 1999


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.




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RFC 2598              An Expedited Forwarding PHB              June 1999


   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)





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RFC 2598              An Expedited Forwarding PHB              June 1999


                           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.











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   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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RFC 2598              An Expedited Forwarding PHB              June 1999


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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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