[e2e] What should e2e protocols know about lower layers?

[RJ Atkinson]

At 10:34 10/10/01, Tim Moors wrote:
>One thread of the discussion questioned how an instance of an e2e 
>protocol like TCP can determine whether its communicating peer is
>"local".

I do not think "local" can be determined above layer-2.  I can't 
even convince myself that it is always something that can be
determined at layer-2.  Depending on what "local" means, even
a single Ethernet might not be "local" (e.g. 1 GigE products
often have 70-100 Km reach now a days; most shipping GigE 
products also support MTUs larger than IEEE-standard size).

Even in the old days there were often mixed L2 technologies
(e.g. FDDI & Ethernet, with different link speeds and different 
MTUs) that were bridged together on a single subnet.  In later
days, one might have had the wacky ATM LAN Emulation stuff bridged
with real Ethernet.

As I also read the TSVWG list, I'll note that I think that congestion
avoidance and control mechanisms should always be turned on.
I disagree with folk disabling congestion avoidance and control
just because a given link might be perceived to be "local"
(particularly where "local" seems to have the de facto meaning
of "on-subnet").

I'm not sure that followups belong on all the original lists,
so I've trimmed this reply to just the E2E list.

Ran
rja at inet.org