[Rpm] [Bloat] Relentless congestion control for testing purposes

[Jonathan Morton]

> On 29 Sep, 2021, at 2:17 am, Dave Taht <dave.taht at gmail.com> wrote:
> 
> In today's rpm meeting I didn't quite manage to make a complicated point. This long-ago proposal of matt mathis's has often intrigued (inspired? frightened?) me:
> 
> https://datatracker.ietf.org/doc/html/draft-mathis-iccrg-relentless-tcp-00
> 
> where he proposed that a tcp variant have no response at all to loss or markings, merely replacing lost segments as they are requested, continually ramping up until the network basically explodes.

I think "no response at all" is overstating it.  Right in the abstract, it is described as removing the lost segments from the cwnd; ie. only acked segments result in new segments being transmitted (modulo the 2-segment minimum).  In this sense, Relentless TCP is an AIAD algorithm much like DCTCP, to be classified distinctly from Reno (AIMD) and Scalable TCP (MIMD).

   Relentless congestion control is a simple modification that can be
   applied to almost any AIMD style congestion control: instead of
   applying a multiplicative reduction to cwnd after a loss, cwnd is
   reduced by the number of lost segments.  It can be modeled as a
   strict implementation of van Jacobson's Packet Conservation
   Principle.  During recovery, new segments are injected into the
   network in exact accordance with the segments that are reported to
   have been delivered to the receiver by the returning ACKs.

Obviously, an AIAD congestion control would not coexist nicely with AIMD based traffic.  We know this directly from experience with DCTCP.  It cannot therefore be recommended for general use on the Internet.  This is acknowledged extensively in Mathis' draft.

> In the context of *testing* bidirectional network behaviors in particular, seeing tcp tested more than unicast udp has been, in more labs, has long been on my mind.

Yes, as a tool specifically for testing with, and distributed with copious warnings against attempting to use it more generally, this might be interesting.

 - Jonathan Morton