[e2e] Congestion control decision frequency + scaling of related signaling

[Michael Welzl]

Hi all,

One major issue of Explicit Rate (and any other congestion control
related) signaling is its scalability: whether it is designed for
edge2edge or end2end usage, it must remain scalable.

Some key factors are obvious:
- treatment of ER signaling packets in routers must be very efficient
- per flow state (or even flow counting) in routers should definitely
  be avoided

Another important factor is the amount of packets. It is well known
that adapting the sending rate faster than once per RTT can lead to
oscillations whereas adapting slower can lead to "unresponsive behaviour".
So the RTT (SRTT) could be seen as an optimal choice for any congestion
control related signaling / decision frequency.

There are two problems I have with that:

1.) Shouldn't the optimal decision frequency for congestion control
    generally be 2 RTT instead of 1 RTT?

    Here's a quote from "Congestion Avoidance in Computer Networks With
    a Connectionless Network Layer", Raj Jain / K. K. Ramakrishnan /
    Dah-Ming Chiu:
    "System control theory tells us that the optimal control frequency
     depends upon the feedback delay - the time between applying a control
     (change window) and getting feedback (bits) from the network
     corresponding to this control. In computer networks, it takes one
     round-trip delay to affect the control, that is, for the new window
     to take effect and another round-trip delay to get the resulting
     change fed back from the network to the users. This leads us to
     the recommendation that windows should be adjusted once every two
     round-trip delays (two window turns) (..)."

2.) A common method to make signaling scale is to make its traffic a
    certain percentage of a network node's generated traffic (or the
    related traffic). For a network wide given value (say 30% of the
    traffic), the overall signaling traffic in the network then does
    not exceed this percentage. This method is used in some experimental
    ABR schemes and in RTP / RTCP. But what is a reasonable value?

Cheers,
Michael