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

[Joseph Touch <[e-mail]>]

Hi, Tom,


> On Jul 15, 2021, at 8:13 AM, Tom Herbert <[e-mail]> wrote:
> 
> Hello,
> 
> Header-data split is a common technique in modern NICs where header
> data and payload are DMA'ed into separate sets of receive buffers. The
> header data goes to buffers that will be read and processed by CPU,
> the data buffers are targeted to application processing (e.g. 4K
> buffers for page cache, GPU memory). With sufficient capabilities,
> NICs can split header-data at the ULP header to precisely provide the
> application just its data. Header-data split is a foundation of zero
> copy receive (https://legacy.netdevconf.info/0x14/pub/slides/62/Implementing%20TCP%20RX%20zero%20copy.pdf)
> 
> There is no such thing as header-data-trailer split.

And there is no way to avoid trailers for non-fragmented packets.

We are designing options for UDP, not options only for fragmentation. 

> If a device
> receives a packet with protocol trailers and is performing header-data
> split, the trailers would be in the data buffers. The CPU may not be
> able to read this data, e.g. the data was DMA into GPU memory which is
> unreadable by the host CPU. So in the case of UDP options, this means
> that when options are in trailers they might nor be visible to be
> processed.

Then that endpoint cannot implement UDP options, because we are not *requiring* fragmentation just to use UDP options.

At that point, either we have systems that can handle both headers (the UDP header never goes away anyway) and trailers, or they don’t support UDP options.

Joe