Re: [tsvwg] UDP options and header-data split (zero copy)

[Joseph Touch <[e-mail]>]

Hi, Tom,

> On Jul 15, 2021, at 2:50 PM, Tom Herbert <[e-mail]> wrote:
> 
> On Thu, Jul 15, 2021 at 2:32 PM Joseph Touch <[e-mail]> wrote:
>> 
>> ...
>> And there is no way to avoid trailers for non-fragmented packets.
>> 
> Right trailers are only needed in that case. In the case of fragment
> using both headers and trailers simultaneously is unnecessary
> complexity that provides no benefit.

If I put per-fragment as headers and post-fragment as trailers, I can do the following processing:

Outbound:
	A- add/process all (safe) per segment options
	B- if there are any unsafe options or fragmentation is needed for space:
		B.1- split as needed
		B.2- process per-fragment / unsafe options on the resulting fragments
	C- transmit

Inbound does the converse.

In both cases, I can simply skip step B if fragmentation isn’t used/needed.

If all the headers come first for fragments - even the per-segment ones, I either need two versions of step A (one for fragments, one for not) or I need to shuffle the trailer to the header between steps A and B.

—

Your claim that “trailers are ONLY needed…” (emphasis mine) does not consider that “trailer processing IS needed”.

And whenever there are trailers, there are both headers (the current UDP header) and trailers (UDP options), by necessity.

So having two versions of the option location complicates things in the middle of option processing.

The only objection you have raised is that this will hinder processing that’s already hindered. If there’s a slow path here, IMO it ought to be for fragments, not non-fragments.

Joe