[e2e] Why was hop by hop flow control eventually abandonded?

Loa Andersson loa at pi.nu
Wed Jul 17 03:24:28 PDT 2013

Michael and Jon,

While I agree that a combination of end to end and hop by hop is
a good idea for communication systems, I do don't buy the idea that
it must be so *just* because it has been done so in other system.

You may get good ideas from how it is done elsewhere, but it is not
always the right solution


On 2013-07-17 11:13, Michael Welzl wrote:
> i agree like i haven't agreed to anything in a long time! in fact i agree with every single word jon wrote here
> Sent from my iPhone
> On 17. juli 2013, at 10:36, Jon Crowcroft <Jon.Crowcroft at cl.cam.ac.uk> wrote:
>> so most systems in the world do hop by hop as well as end to end
>> e.g.
>> transportation systems  (traffic lights, stacks of planes, semaphores to
>> control trains entry/exit from track sections etc etc)
>> power systems (elec ant gas)
>> water systems (valves etc)
>> eco-systems (food chains/feast/famine etc)
>> political systems (you switch from feudal to democract, btut you still
>> have periodic elections - you switch from city states to countries,
>> but still have border and immigration/emigration controls...:)
>> so why do we think comms should be different?
>> in fact, I suggest we do some flow control inside nets - its called
>> traffic engineering and operates on aggregates - when we do
>> multipath routing, we also select a modest number of routes (obviously
>> more than 1 but a lot less than actually would give connectivity or
>> even some additional capacity)....
>> so i think the design decision to throw out all hop by hop flow
>> control was probably an error (not a disastrous one: as many people
>> have pointed out, it simplified early router design a lot to be
>> completely stateless - but you don't need to keep per-5-tuple based
>> e2e state to do hop by hop flow control if its on aggregates, right?)
>> In missive <1374014873.23736.140661256474709.49B0CF90 at webmail.messagingengine.c
>> om>, Mark Handley typed:
>>>> It's before my time, but I'd always assumed it was also influenced by
>>>> the NVP work, which would not have wanted hop-by-hop flow control.
>>>> Mark
>>>> On Tue, Jul 16, 2013, at 02:24 PM, Bob Braden wrote:
>>>>> I believe that hop-by-hop flow control only works with per-flow state in
>>>>> the routers (see X.25 for example). Once the decision was made that the
>>>>> routers should be stateless, end-to-end flow control was the only
>>>>> option. That is what the end-to-end principle is all about.
>>>>> Bob Braden
>> cheers
>>    jon


Loa Andersson                        email: loa at mail01.huawei.com
Senior MPLS Expert                          loa at pi.nu
Huawei Technologies (consultant)     phone: +46 739 81 21 64

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