[e2e] What is the meaning of "end-to end"?

Bob Braden braden at ISI.EDU
Mon May 21 13:34:33 PDT 2001

>   *> Could you be so kind as to tell me what the meaning of "end-to-end"
>   *>
>   *> My native language is Spanish and I can´t grasp the meaning of the
>   *> expression
>   *>
>   *> Thank you
>   *>
>   *> Mirela Perusia
>   *> Argentina
>   *>
> Historically, it has meant "not unicast routing".
> Bob Braden

I want to apologize for my unintentionally cryptic comment (above),
which has generated a few private "Huh??" queries.  I was really
answering the question, "What is the range of topics covered by the
end2end Research Group and by its accompanying open mailing list,
end2end-interest at postel.org?"

As for the meaning of "end-to-end", I hope you will not think me
frivolous if I say that it means what it says.  In any computer
communication, there are n >= 2 end points, called "end systems" by the
OSI folks and "hosts" by the Internet crowd (or sometimes vice versa).
End-to-end protocol issues are those that operate between (or among)
two (or more) end systems.  In this (simplistic) view, the network has
two parts, the "ends" (hosts) and the middle (routers).  Because of the
statelessness of Internet routers, end-to-end issues are particularly
important for us.  End is where the state is ((mostly)).

Link layer protocols are architecturally not E2E.  The Internet
Protocol has both E2E and hop/hop significance, making it a bit
ambiguous.  The transport layer is clearly E2E, at least until various
recent clever hacks came along to add router code to improve TCP
performance.  The application layer is clearly end-to-end, at least
until middleboxes and active networking came along.  Routing is
something the routers do without asking the end systems, so it is
clearly not E2E.  (However, historically the E2E research group abetted
and encouraged the development of IP multicast, which tended to get ud
into multicast routing.  Hence my comment.)

I expect that you knew most of this, but I hope it helps.

Bob Braden

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