This commit introduces additional platform-dependent checking when the
kernel returns an error. Previously, the session is terminated when
PingFrame sends a discovery packet larger than the limit. With this
commit, the error is checked, and if it is "datagram too large", the
error is ignored.
Additionally,
1. This commit re-enables MTU discovery on Windows unless it
is disabled explicitly by user (Undo #3276),
2. Set IP_DONTFRAGMENT and IPV6_DONTFRAG with error checking on Windows,
and
3. Set IP_MTU_DISCOVERY to PMTUDISC_DO for both IPv4 and IPv6 on Linux
so that the kernel will return "message too long".
Fixes#3273#3327
When the server is listening on multiple interfaces or interfaces with
multiple IPs, the outgoing datagrams are sometime delivered with the
wrong source IP address.
In order to fix that, each quic connection needs to extract the
destination IP (and optionally interface id) of the received datagrams,
and set it as source IP (and interface) on the sent datagrams.
On most platforms, this can be done using ancillary data with recvmsg()
and sendmsg(). Some of the machinery for this is already there for ECN,
this change extends it to read the destination IP info and write it to
the outgoing packets.
Fix#1736
