[e2e] TCP Framing
jonathan at DSG.Stanford.EDU
Fri Mar 23 16:11:03 PST 2001
In message <200103232320.f2NNK0j17500 at calcite.rhyolite.com>,
Vernon Schryver writes:
>In other words, contrary to various claims, no black magic was required
>to "page flip" on both input and output more than 10 years ago.
Yes, provided that the MSS is a multiple of the pagesize, (or the
sender rounded down to that), and that you already DMAed the packet
into memory, aligned such that the TCP (or whatever) payload
ended up page-aligned.
That is, it works provided the receiver's guess about alignment and
preceding header sizes pays off. Reading between the lines, one of the
aims of this proposal is to address the cases where such guesses would
fail. The "RDMA" makes me wonder if this isn't just about preserving
record boundaries, but about preserving in-memory alignment of
each record, too.
Alignment constraints might be why byte-stuffing (or Stuart Cheshire's
COBS) was not proposed. More explanation of WARP (the target
remote-DMA ULP) might give some insight. Maybe the authors of the
draft could comment?
That said -- it seems an awful lot of effort, in an Internet where
both Ethernet-sized MTUs, and signficantly larger alignment
constraints -- pagesizes of 4K, 8k, 16k or larger -- are common.
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