Re: [Cerowrt-devel] [Bloat] DOCSIS 3+ recommendation?

[David Lang <david@lang.hm>]

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On Fri, 20 Mar 2015, Michael Welzl wrote:

>> On 20. mar. 2015, at 17.31, Jonathan Morton <chromatix99@gmail.com> wrote:
>> 
>> 
>>> On 20 Mar, 2015, at 16:54, Michael Welzl <michawe@ifi.uio.no> wrote:
>>> 
>>> I'd like people to understand that packet loss often also comes with delay - for having to retransmit.
>> 
>> Or, turning it upside down, it’s always a win to drop packets (in the service of signalling congestion) if the induced delay exceeds the inherent RTT.
>
> Actually, no: as I said, the delay caused by a dropped packet can be more than 1 RTT - even much more under some circumstances. Consider this quote from the intro of https://tools.ietf.org/html/draft-dukkipati-tcpm-tcp-loss-probe-01  :

You are viewing this as a question to drop a packet or not drop a packet.

The problem is that isn't the actual question.

The question is to drop a packet early and have the sender slow down, or wait 
until the sender has filled the buffer to the point that all traffic (including 
acks) is experiencing multi-second latency and then drop a bunch of packets.

In theory ECN would allow for feedback to the sender to have it slow down 
without any packet being dropped, but in the real world it doesn't work that 
well.

1. If you mark packets as congested if they have ECN and drop them if they 
don't, programmers will mark everything ECN (and not slow transmission) because 
doing so gives them an advantage over applications that don't mark their packets 
with ECN

   marking packets with ECN gives an advantage to them in mixed environments

2. If you mark packets as congested at a lower level than where you drop them, 
no programmer is going to enable ECN because flows with ECN will be prioritized 
below flows without ECN

If everyone use ECN you don't have a problem, but if only some 
users/applications do, there's no way to make it equal, so one or the other is 
going to have an advantage, programmers will game the system to do whatever 
gives them the advantage

David Lang