Re: [tsvwg] A review of draft-ietf-tsvwg-udp-options-12

[Joseph Touch <[e-mail]>]

> On Jun 13, 2021, at 7:20 PM, C. M. Heard <[e-mail]> wrote:
> 
> If we DO support zero-copy and thus want to allow non-terminal fragments to have post-fragoption options that operate on each fragment, then we would add THISFRAGLEN to the nonterminal format and issue different KIND numbers to nonterminal/terminal fragment.
> 
> 
> I for one would appreciate further discussion of these last points. I admit that I have failed to grasp Joe's message on the RDMA thread, and I would appreciate some time to think about it.

Sure - here’s how it all works. Note that this is relevant mostly for long transfers with persistent UDP fragmentation; if that is assumed to be ‘adjusted’ at the app layer (as QUIC does), then we don’t need zero-copy support...

- right now, UDP data can be zero-copied when received into user space, starting with the user data
- if we add options, UDP data can still be zero-copied because it hasn’t moved (it still begins the payload
- however, fragments are different because (esp given the merging of frag and lite) they don’t start at the beginning of data
	- they always start after OCS (which I think we should make fit the uniform KIND/LEN/OCS format of 4 bytes)
	- if the FRAG comes next, then we can move the frag content around a little and still support zero-copy

		notably, we move the first 10 bytes of the fragment to the end
			4 for OCS
			6 for FRAG (assuming FRAG includes KIND/OPTLEN/FRAGOFFSET/ID/FRAGLEN)
		that way we can zero-copy the frag packet into place, then just copy those last 8 bytes over OCS and the FRAG header

This method assumes that we try to keep FRAG early in the packet - preferably right after OCS. The later it comes, the more additional bytes we need to move to “fix” the copy (beyond the 8 bytes noted above).

—

This method is the only reason we would want to allow options after non-terminal fragments - basically to keep the fragment toward the front of the packet, using the rule that post-noninitial frag options still operate on the fragment, rather than waiting for reassembly. The exception is the terminal fragment, where post-terminal fragment options operate on the reassembled packet. 

Joe