Re: IP fragmentation

пн, 20 июл. 2020 г. в 15:17, Edward Cree <[email protected]>:
>
> On 20/07/2020 09:15, Alexander Petrovsky wrote:
> > But, the main problem for us it's fragmented IP packets. Some times
> > ago I tried to use for such packets AF_XDP, fast pass them into the
> > user space, accumulate and after that pass back to the network, it was
> > a PoC.
> Not 100% sure this works because I haven't tried it, but as long as
>  packets aren't being re-ordered, you can do it without needing to
>  save the payload in a map.
> All the map needs to store is (for each IPID being tracked) what host
>  this connection goes to.
> If you receive a First Fragment (frag_off=0, MF=1), you look up the
>  tuple through the regular LB to pick a server, and record that host
>  in the map entry for the IPID.
> For any other fragment, you look up the IPID in the map to get the
>  destination host, and if MF=0 you delete the map entry.
>  (If the IPID wasn't found, either drop or punt to userspace.)
> Then TX/REDIRECT the packet to the appropriate host.
> You might want to add some kind of simple ageing to this so that map
>  entries from interrupted/spurious fragment chains don't stick around
>  and build up over time.
>
> The problem comes when 'middle' fragments can either come after the
>  last (MF=0) fragment (technically this can be handled by tracking
>  the byte range seen for the IPID, and not deleting from the map
>  until all bytes up to the frag_off+total_len of the last-frag have
>  been seen), or worse, before the first fragment.  If the frag_off=0
>  fragment isn't the first one received, then this doesn't work
>  because you don't know at the time of receiving fragments what L4
>  ports they belong to.  But I don't know how common that situation is
>  and whether having it take the slow-path is acceptable.
>
> HTH,
> -ed

Unfortunately, for UDP I can't pick some _random_ host in case the
first _seen_ fragment it's not a First Fragment (frag_off=0, MF=1). In
this case, I have to accumulate ALL fragments in map. And on each
received fragment check, is all fragments are collected. I did it in
my PoC with AF_XDP, but in PoC all seems unreliable.

-- 
Alexander Petrovsky