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

[Gorry Fairhurst <[e-mail]>]

On 14/06/2021 17:17, Tom Herbert wrote:
> On Sun, Jun 13, 2021 at 9:31 PM Joseph Touch <[e-mail]> wrote:
>>
>>
>> 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
> Only if the device supports header/data split where the headers are in
> one buffer and UDP data is in aligned buffer.
>
>> - 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
>>
> An obvious feature we'd want is NIC hardware to do UDP options
> fragementation and reassembly, analogous to existing UDP Fragmentation
> Offload (UFO) which performs IP fragmentation of UDP packets. The
> impediment with supporting this is that hardware devices would need to
> perform protocol processing on trailers as opposed to headers. Nearly
> all hardware devices, including switches and NICs, are optimized to
> process protocol headers and in modern devices they are quite
> programmable in that regard. However, they typically rely on a parsing
> buffer that holds the first N bytes of the packet and assume that all
> the protocol headers lie within that. They wouldn't process data after
> that header in the fast path at least, and almost certainly would have
> capability to process protocol headers at that end of a large packet.
> I am doubtful we'll ever see hardware support for trailer protocols,
> and hence it's unlikely we'd see accelerations for UDP options like we
> have for TCP.
>
> Tom

OK.... Is there any way that we could design to enable this?

I'm "fishing" for ideas because I know you've talked about the various 
offload methods.

So for options in the trailer, this is clearly an impediment.

For UDP-Opt fragmentation, I understand there is no standard UDP payload,

.... only an option containing a fragment, so the Fragment information 
would actually be in the" first N bytes of the packet".

So, what do you think  could be most likely helpful to enable fastpath 
accelleration for the fragments?

Gorry

>> 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
>>
>>
>>
>>