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

[Dmitri Tikhonov <[e-mail]>]

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