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Re: Proxies and loops

From: Luigi Rizzo <luigi@labinfo.iet.unipi.it>
Date: Sun, 26 Nov 1995 01:26:34 +0100 (MET)
Message-Id: <199511260026.BAA00496@labinfo.iet.unipi.it>
To: "Roy T. Fielding" <fielding@avron.ICS.UCI.EDU>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
> > While the configuration of proxies does not really belong in the
> > HTTP protocol, it helps a lot having a way to detect the existence
> > of loops, or at least minimising its bad effects.
> The Forwarded header field was created for this purpose.
> Is it insufficient?

Finally I could get a copy of the 1.1 draft.

The Forwarded: header can actually be used to detect loops. However,

* it is not compulsory (although a node willing to avoid loops will
  certainly insert it and detect the loop);
* in principle, it does not prevent the occurrence of arbitrarily
  long paths (although a node can decide not to pass a request which
  has been "Forwarded:" too many times);

and these reasons make me like better the use of a *compulsory*
TTL field as a loop detector (which is also simpler to manage).

The "Forwarded:" header was inspired from email, but email loops
are different from http loops. The former are built one step at a
time, in a store-and-forward fashion:  it might take hours before
a loop is complete and detected, during this time the system does
not allocate resources other than the storage for a single copy of
the message.  But http allocates resources on all nodes, so loops
are more expensive for the network.

Luigi Rizzo                     Dip. di Ingegneria dell'Informazione
email: luigi@iet.unipi.it       Universita' di Pisa
tel: +39-50-568533              via Diotisalvi 2, 56126 PISA (Italy)
fax: +39-50-568522              http://www.iet.unipi.it/~luigi/
Received on Saturday, 25 November 1995 16:31:18 UTC

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