Re: Need your help: different connection IDs in the same datagram

[Ian Swett <[e-mail]>]

I don't think this change would be difficult for our implementation, but I
also don't see it as necessary.  Given where we are in the process, that
alone argues for not changing it I believe.

On Wed, Jul 15, 2020 at 2:02 PM Dmitri Tikhonov <[e-mail]>
wrote:

> On Wed, Jul 15, 2020 at 05:23:57PM +1000, Martin Thomson wrote:
> > There has been some opposition to the proposed resolution in PR 3870.
> >
> > Apparently, for some, having multiple connection IDs in the same
> > datagram complicates processing.  I don't understand this objection.
> > It seems to me more difficult to retain state across packets than it
> > is to process each atomically.  I was hoping that Christian or Nick
> > can explain more about how this affects them.
>
> I can provide an example from lsquic.  The datagram is parsed into
> QUIC packets in one function, lsquic_engine_packet_in():
>
>
> https://github.com/litespeedtech/lsquic/blob/v2.18.1/src/liblsquic/lsquic_engine.c#L2781L2816
>
> Each QUIC packet is processed by process_packet_in(), where a
> connection is looked up:
>
>
> https://github.com/litespeedtech/lsquic/blob/v2.18.1/src/liblsquic/lsquic_engine.c#L1352L1360
>
> The DCID check is performed lsquic_engine_packet_in(), before
> process_packet_in() is called:
>
>
> https://github.com/litespeedtech/lsquic/blob/v2.18.1/src/liblsquic/lsquic_engine.c#L2793L2806
>
> The DCID information is readily available in the datagram parsing
> loop, while connection information is not.
>
> For lsquic to support the proposed change, it would have to remember
> the current connection and then query it whether it is indeed the
> owner of the next DCID (A) or look up DCID in the global hash (B):
>
>     conn = NULL;
>     while (quic_packet = parse_udp(pointers)) {
>         dcid = parse(quic_packet);
>         if (conn)
>         {
>   #if VARIANT_A
>             if (!conn_owns_scid(conn, dcid))
>   #else
>             if (conn != lookup_by_dcid(dcid))
>   #endif
>                 continue;
>         }
>         conn = process_packet(quic_packet);
>     }
>
> Not that it could not be done, of course, but it is both extra work
> to modify lsquic and a more inefficient mechanism: what was a simple
> CID comparison is now a hash lookup.
>
> That's why I argued [1] for having solid rationale behind the change
> rather than a personal preference.
>
>   - Dmitri.
>
> 1.
> https://github.com/quicwg/base-drafts/issues/3800#issuecomment-656851626
>
>