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SETTINGS frame - not documented as peer-to-peer only

From: Robert Collins <robertc@robertcollins.net>
Date: Tue, 30 Sep 2014 10:17:14 +1300
Message-ID: <CAJ3HoZ364rqvEuqqzGkFR-T1H=8uM4jmQMCd4npMoo1nLauK5w@mail.gmail.com>
To: HTTP Working Group <ietf-http-wg@w3.org>
When reading the websockets draft, it occured to me that perhaps I
misunderstand SETTINGS. AIUI it is peer to peer, not end to end.

That is:

Client<->intermediary<->server.
A               B                        C

settings are exchanged between A and B, and B and C, but never A and
C, and settings from C are not propogated to A.

Further, because A doesn't know for any particular stream whether it
will traverse BC or perhaps BD (were D might be a load balanced
secondary IP for the same origin as C, or be a new ALT-SVC being
warmed up), there is no way in the HTTP/2 model to talk about the
BC/BD capabilities to A.

Case in point, if C is capable of e.g. websockets, and so is B, we
don't know if D is. So B can't hide its websocket readiness until it
sees C is ready too - thats conflating stream capabilities with peer
capabilities?

Perhaps in section 6.5, after
"SETTINGS parameters are not negotiated; they describe characteristics
of the sending peer, which are used by the receiving peer. Different
values for the same parameter can be advertised by each peer. For
example, a client might set a high initial flow control window,
whereas a server might set a lower value to conserve resources."

"SETTINGS frames MUST NOT be forwarded - they are solely used to
negotiate the characteristics of a single connection between two
endpoints.

It also raises questions in my mind about SETTINGS_ENABLE_PUSH

Given

Client1
Client2
Proxy
Server

If client1 has SETTINGS_ENABLE_PUSH=1, and client2 has
SETTINGS_ENABLE_PUSH=0, the Proxy has SETTINGS_ENABLE_PUSH=1, then the
server may push resources for both clients, and the proxy will have to
cancel streams when they are for client2 ?

That is surely doable but seems a little wasteful. I wonder if we can do better?

-Rob


-- 
Robert Collins <rbtcollins@hp.com>
Distinguished Technologist
HP Converged Cloud
Received on Monday, 29 September 2014 21:17:43 UTC

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