[PCN] Comments on draft-ietf-pcn-architecture-02.txt
Michael Menth <menth@informatik.uni-wuerzburg.de> Fri, 23 November 2007 10:39 UTC
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Date: Fri, 23 Nov 2007 11:40:57 +0100
From: Michael Menth <menth@informatik.uni-wuerzburg.de>
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Subject: [PCN] Comments on draft-ietf-pcn-architecture-02.txt
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Hi Phil, here are my comments to the -02 version of the architecture draft. Regards, Michael ===================== Introduction "PCN-egress-nodes make measurements of the packet markings and send information as necessary to the nodes that make the decision about which PCN-flows to accept/reject or terminate, based on this information." [Michael] "PCN-egress nodes monitor the packet markings and ..." (measurement is not necessarily required, both for admission and termination.) ===================== "the admission control mechanism limits the PCN-traffic on each link to *roughly* its PCN-lower-rate and the flow termination mechanism limits the PCN-traffic on each link to *roughly* its PCN-upper-rate." [Michael] Assuming this sentence is based on agreement, I cannot refrain from pointing out that texts on PCN are better readable with terms "admissible rate" and "supportable rate" instead of PCN-lower-rate and PCN-upper-rate. ===================== "The PCN-lower-rates can be chosen small enough that admitted traffic can still be carried after a rerouting in most failure cases." [Michael] The general concept of resilient admission control has been proposed in [Menth04g] and applied to PCN in [PCN-Config] including algorithms how to set PCN rate thresholds appropriately. [Menth04g] M. Menth, S. Kopf, and J. Charzinski: "Network Admission Control for Fault-Tolerant QoS Provisioning" in IEEE High-Speed Networks for Multimedia Communication (HSNMC), June 2004 [PCN-Config] M. Menth, and M. Hartmann: "PCN-Based Resilient Network Admission Control: The Impact of a Single Bit", <http://www3.informatik.uni-wuerzburg.de/staff/menth/Publications/Menth07-PCN-Config.pdf>", 2007. ===================== Terminology "o Excess-rate-marking: a PCN-marking behaviour such that the amount of PCN-traffic that is PCN-marked is equal to the amount that exceeds a particular rate (either the PCN-lower-rate or PCN-upper-rate). NOTE: The definition reflects the overall intent rather than its instantaneous behaviour, since the rate measured at a particular moment depends on the behaviour, its implementation and the traffic's variance as well as its rate." [Michael] The term "Excess-rate-marking" implies that exactly the excess rate is marked. Modifications of this algorithm using e.g. marking frequency reduction don't deserve this name anymore. This is different with "Excess-traffic-marking": if all packets exceeding a particular rate are marked, it's exactly the excess traffic rate, but the algorithm may also mark fewer packets. "o Excess-traffic-marking: a PCN-marking behaviour marking packets exceeding a particular rate (either the PCN-lower-rate or PCN-upper-rate). ..." ===================== "o Pre-congestion: a condition of a link within a PCN-domain in which the PCN-node performs PCN-marking, in order to provide an "early warning" of potential congestion before there is any significant build-up of PCN-packets in the real queue." [Michael] Pre-congestion is always connected to a PCN rate threshold. "o X-Pre-congestion: a condition of a link within a PCN-domain in which the PCN-node performs PCN-marking since the PCN-traffic of the link has *roughly* exceeded the PCN rate threshold "X" (e.g. PCN-lower-rate or PCN-upper-rate); it is done to provide an "early warning" of potential congestion before there is any significant build-up of PCN-packets in the real queue. When the context is clear, "X" can be omitted." ===================== 4.1 "However, note that this description reflects the overall intent of the algorithm rather than its instantaneous behaviour, since the rate measured at a particular moment depends on the detailed algorithm, its implementation (eg virtual queue, token bucket...) and the traffic's variance as well as its rate (eg marking may well continue after a recent overload even after the instantaneous rate has dropped)." [Michael]: virtual queue and token bucket are absolutely equivalent paradigms on which metering and marking may be based. see also: http://www.ietf.org/internet-drafts/draft-charny-pcn-comparison-00.txt Just omit the example! ===================== (Hence, by analogy with ECN we call our mechanism Pre-Congestion Notification.) [Michael] This should come earlier in the document. E.g. in the terminology section. ===================== 5.5 o PCN-meter at PCN-egress-node - make "measurements of PCN-traffic" from a particular PCN-ingress-node. [Michael] Could you add "(if required)" to that point? 3sm http://www.ietf.org/internet-drafts/draft-babiarz-pcn-3sm-01.txt does not need metering or measurement of marked packets for flow termination. ===================== o Make decision about flow termination - use the "measurements of PCN-traffic" to decide which PCN-flow or PCN-flows to terminate. The decision takes account of policy and application layer requirements. [Michael] Similar issue: could you add "possibly" somewhere? ===================== 5.7 o NB the order of increasing severity is: unmarked; PCN-marking with first encoding (ie associated with the PCN-lower-rate); PCN- marking with second encoding (ie associated with the PCN-upper- rate) [Michael] What does "NB" stand for? ===================== 7.2 o (if required) Generate probe traffic - the PCN-ingress-node generates the probe traffic. The appropriate number (or rate) of probe packets will depend on the PCN-marking algorithm; for example an excess-rate-marking algorithm generates fewer PCN-marks than a threshold-marking algorithm, and so will need more probe packets. and 7.3 The open issues associated with this viewpoint include: o What rate and pattern of probe packets does the PCN-ingress-node need to generate, so that there's enough traffic to make the admission decision? [Michael] We've done some work on this. See Table 1 in 4.1 of http://www.ietf.org/internet-drafts/draft-menth-pcn-performance-01.txt ===================== 7.3 o Probing adds delay to the admission control process. o Sufficient probing traffic has to be generated to test the pre- congestion level of the ingress-egress-aggregate. But the probing traffic itself may cause pre-congestion, causing other PCN-flows to be blocked or even terminated - and in the flash crowd scenario there will be probing on many ingress-egress-aggregates. [Michael] Could you add "possibly" please? These statements are not true for the probing mechanisms in 2.3.2 of http://www.ietf.org/internet-drafts/draft-babiarz-pcn-3sm-01.txt ===================== The downsides of probing for this viewpoint are: o Probing adds delay to the admission control process. o Sufficient probing traffic has to be generated to test the pre- congestion level of the ECMP path. But there's the risk that the probing traffic itself may cause pre-congestion, causing other PCN-flows to be blocked or even terminated. [Michael] Could you please add "possibly" to these points? ===================== Viewpoint 3 or probably extra viewpoint: [Michael] Probing might be used because it is easier than measuring and signalling aggregate packet markings. This is the case when a signalling protocol already exists, e.g., to find out the correct ingress-egress-aggregate of a flow. Then, this signalling protocol might be reused for probing purposes saving complex operations in the egress node and signalling. An example is given in 2.4 of http://www.ietf.org/internet-drafts/draft-babiarz-pcn-3sm-01.txt Also 4.3 of http://tools.ietf.org/wg/pcn/draft-briscoe-pcn-boundary-behav-00.txt suggests to use a protocol to find out the right egress node such that probing would in fact lead to simplifications. ===================== 8.1.2 Parameters [Michael] A citation to our work on parameter settings would be appropriate. [PCN-Config] M. Menth, and M. Hartmann: "PCN-Based Resilient Network Admission Control: The Impact of a Single Bit", <http://www3.informatik.uni-wuerzburg.de/staff/menth/Publications/Menth07-PCN-Config.pdf>", 2007. -- Dr. Michael Menth, Assistant Professor University of Wuerzburg, Institute of Computer Science Am Hubland, D-97074 Wuerzburg, Germany, room B206 phone: (+49)-931/888-6644, fax: (+49)-931/888-6632 mailto:menth@informatik.uni-wuerzburg.de http://www3.informatik.uni-wuerzburg.de/research/ngn _______________________________________________ PCN mailing list PCN@ietf.org https://www1.ietf.org/mailman/listinfo/pcn
- [PCN] Comments on draft-ietf-pcn-architecture-02.… Michael Menth
- Re: [PCN] Comments on draft-ietf-pcn-architecture… philip.eardley