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

[Joe Touch <[e-mail]>]

> On Aug 2, 2021, at 11:13 AM, C. M. Heard <[e-mail]> wrote:
> 
> As I understand it, the procedure (in concept) is to create a datagram
> in legacy format with its per-datagram options, and then split that up
> into fragments, each of which can have its own per-fragment options.
> The per-fragment options precede the corresponding fragment data; in
> the current draft, the  per-datagram options follow the data in the
> final fragment.
> 
> The upcoming revision will change this a bit ... FRAG END in the
> terminal FRAG option will be replaced with USER DATA LENGTH. In this
> case the per-datagram options will be carried within the fragment
> data. It won't be possible, in general, to tell where the user data
> ends and the options begin until the reassembly process is complete

Yup.  

There are special cases, eg where the trailing options fit in the terminal frag and the data gram is split to make that happen. 

The design has been intended so the sender ‘makes the legacy-style datagram first, then splits into fragments and add per frag options. The receiver would do that in reverse. That’s generally required logically because the trailing options can depend on the whole datagram and the option values in some cases (auth). 

Joe