rfc3246.txt

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   [6]   Charny, A., Baker, F., Davie, B., Bennett, J.C.R., Benson, K.,
         Le Boudec, J.Y., Chiu, A., Courtney, W., Davari, S., Firoiu,
         V., Kalmanek, C., Ramakrishnan, K.K. and D. Stiliadis,
         "Supplemental Information for the New Definition of the EF PHB
         (Expedited Forwarding Per-Hop Behavior)", RFC 3247, March 2002.

   [7]   Nichols K. and B. Carpenter, "Definition of Differentiated
         Services Per Domain Behaviors and Rules for their
         Specification", RFC 3086, April 2001.







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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
   be most appropriate.






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   It should be noted that it is quite acceptable for a Diffserv domain
   to provide multiple instances of EF.  Each instance should be
   characterizable by the equations in Section 2.2 of this
   specification.  The effect of having multiple instances of EF on the
   E_a and E_p values of each instance will depend considerably on how
   the multiple instances are implemented.  For example, in a multi-
   level priority scheduler, an instance of EF that is not at the
   highest priority may experience relatively long periods when it
   receives no service while higher priority instances of EF are served.
   This would result in relatively large values of E_a and E_p.  By
   contrast, in a WFQ-like scheduler, each instance of EF would be
   represented by a queue served at some configured rate and the values
   of E_a and E_p could be similar to those for a single EF instance.






































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Authors' Addresses

   Bruce Davie
   Cisco Systems, Inc.
   300 Apollo Drive
   Chelmsford, MA, 01824

   EMail: [e-mail]

   Anna Charny
   Cisco Systems
   300 Apollo Drive
   Chelmsford, MA 01824

   EMail: [e-mail]

   Jon Bennett
   Motorola
   3 Highwood Drive East
   Tewksbury, MA 01876

   EMail: [e-mail]

   Kent Benson
   Tellabs Research Center
   3740 Edison Lake Parkway #101
   Mishawaka, IN  46545

   EMail: [e-mail]

   Jean-Yves Le Boudec
   ICA-EPFL, INN
   Ecublens, CH-1015
   Lausanne-EPFL, Switzerland

   EMail: [e-mail]

   Bill Courtney
   TRW
   Bldg. 201/3702
   One Space Park
   Redondo Beach, CA 90278

   EMail: [e-mail]







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   Shahram Davari
   PMC-Sierra Inc
   411 Legget Drive
   Ottawa, ON K2K 3C9, Canada

   EMail: [e-mail]

   Victor Firoiu
   Nortel Networks
   600 Tech Park
   Billerica, MA 01821

   EMail: [e-mail]

   Dimitrios Stiliadis
   Lucent Technologies
   101 Crawfords Corner Road
   Holmdel, NJ 07733

   EMail: [e-mail]































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Full Copyright Statement

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Acknowledgement

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