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Limiting header block size

From: Martin Thomson <martin.thomson@gmail.com>
Date: Thu, 22 May 2014 05:53:54 -0700
Message-ID: <CABkgnnWXJrYd_Lxkz9a0NZ6iiGa01pRmfSJc6tSWAYGR2Fjq0w@mail.gmail.com>
To: HTTP Working Group <ietf-http-wg@w3.org>
The discussion on header blocks and flow control raised the question
of whether it is appropriate to allow an endpoint to limit the size of
header block it accepts.

Due to the design of HPACK, there are many cases where a server or
intermediary is forced to buffer an entire header block.  This is
because critical routing information like :path, :authority and
:scheme can be placed anywhere in a header block.  More importantly,
some of these are highly likely to be in the reference set, causing
them to be emitted only at the end of the block.

This means that processing a header block can require an essentially
unbounded amount of state for many implementations.  Being able to
limit this commitment would be good.

However, it's not clear what an announced limit would enable at a
sender.  Conforming to a limit ultimately requires a sender to drop
header fields.  It seems unlikely that a sender would be able to
arbitrarily drop header fields in order to comply with an arbitrary
limit.  Header fields are there to express semantics, and it's
difficult to know what fields are safe to drop without knowing
application requirements in some detail.

(Someone suggested that a block could be split, but that doesn't work
due to the structure of the protocol.)

Thus, I conclude that changing the protocol is not advisable.
Implementations that receive more header information than they are
willing to tolerate can process any updates to the header table and
then reset the corresponding stream (which for a PUSH_PROMISE would be
the promised stream, of course).

I will raise an editorial issue to note the DoS implications of
excessively large header blocks and the above method for handling
Received on Thursday, 22 May 2014 12:54:22 UTC

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