Discussion Overview

 

 

The following points were raised in the class discussion for this paper:

 

 

  1. Much of the discussion was focused on the concern that the flow semantics implemented by PCC make it very difficult to develop an application on top of, thus making it very unlikely that this TCP-friendly protocol will be used in place of UDP.  PCC requires that an application choose an appropriate value for the parameter T, which specifies the duration following a transition into the on or off states after which the random experiment determining the subsequent state of the flow will be repeated.  It is unlikely that small values of T will be suitable for many applications, as in this case the random experiment will have to be repeated many times over any reasonably long duration and so, the flow will be switched into the off-state several times over this period. Clearly, such frequent and abrupt interruptions are undesirable.  At the same time, however, large values of T are also unsuitable for most applications, as once a flow is switched into the off-state it remains blocked for a longer interval T.  In addition to the obvious inconvenience of having a flow blocked for long periods of time, longer off-times also result in poor intra-protocol fairness that severely punishes at least some PCC flows with very low average throughput (as illustrated in Figure 8 in the paper).

 

  1. It was argued that a potentially useful approach to TCP-friendly congestion control could be obtained by combining the binary on-or-off approach introduced by PCC with a more traditional rate-adaptive algorithm.  Under such a scheme, a rate-adaptive approach would be used until the congestion situation of the network became poor enough to require that the flow rate be pushed below the minimum rate acceptable to the application; at this point, the flow would be forced into the off state.  This advantage of this approach is that it avoids the frequent and abrupt switches to the off-state for T second intervals that makes PCC so difficult to use, but nevertheless retains all of the advantages of having an off-state for the flow – namely, an aggressive response to congestion that does not waste bandwidth by reducing the flow to a rate that is so low as to be useless to the application, but is still non-zero.

 

  1. An interesting point raised in the paper and clarified in the discussion, was that by varying the parameter T, PCC can be made to resemble different points along the spectrum from the one extreme of an admission control scheme based on the current congestion situation of the network, to the opposite extreme of a conventional rate-based congestion control algorithm.  In particular, for sufficiently small values of T, PCC begins to resemble TFRC, with the application rate being the capacity of the outgoing interface of the sender, the on-time being the time required to transmit a single packet and the off-time acting as the interval between packets.  For very large values of T, an admission control scheme is obtained, with an “admitted” flow being one accepted into the on-state given the prevailing network conditions.

 

  1. It was pointed out that the line of research presented in this paper seems to be a “terminal” one.  In particular, since its publication, the only material found to reference this work was an obscure technical report citing it as one of many examples of TCP-friendly transport protocols that application designers use UDP in favor of.

 

  1. Another concern raised in the discussion was the possibility of a dangerous “oscillatory” effect occurring when the starting times of several flows are aligned.  In particular, if several flows were to move into the on state on at the same time, they could overburden the network, create congestion and so, because they all started at the same time, all turn off at the same time.  After their T seconds in the off state, they would then simultaneously move into the on state and this pattern would repeat, with all of the flows effectively remaining blocked.  It was noted that this problem was mentioned in the paper and dealt with by ensuring that the start times of all flows are adjusted by a random offset in order to prevent their alignment with high probability.