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

[Joe Touch <[e-mail]>]

FYI that’s what fragments look like. We can’t do this for non fragments. 

> On Jun 14, 2021, at 10:03 AM, Tom Herbert <[e-mail]> wrote:
> 
> On Mon, Jun 14, 2021 at 9:31 AM Gorry Fairhurst <[e-mail]> wrote:
>> 
>>> 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.
>> 
> 
> Gorry,
> 
> My suggestion was to place UDP options after the UDP header. Instead
> of just placing fragment header after the UDP header, place all the
> UDP options there and then follow that by the Payload. So packet looks
> like:
> 
> +-------------------+
> |   UDP header  |
> +-------------------+
> |  UDP options  |
> +-------------------+
> |     Payload      |
> +-------------------+
> 
> Now this looks a lot like a TCP packet and other variable length
> headers which we know how to handle. For zero copy we can do
> header/split by programming emerging smart devices to split through
> UDP options in one buffer and payload in another thereby also
> eliminating any need to move headers or data around.
> 
> Tom
> 
>> 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
>>>> 
>>>> 
>>>> 
>>>> 
>>