Re: [RFC PATCH] virtio-vsock: add description for datagram type

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Apr 14, 2021 at 05:43:52PM -0700, Cong Wang . wrote:
> On Wed, Mar 17, 2021 at 8:45 AM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > Please stick to UDP semantics as much as possible so that applications
> > can be ported easily and developers aren't surprised by unexpected
> > behavior. UDP packets sent to a destination that has no listen socket
> > result in a Connection Refused error. vsock dgrams should behave in the
> > same way.
> 
> There is no connection refused error for UDP, as it is clearly connectionless.
> What you suggested might be an ICMP unreachable error, however, vsock does
> not have anything equivalent. So, I think there is no error returned
> in this scenario
> for vsock datagram.

Yes, exactly. UDP uses ICMP unreachable:

  16:55:40.380292 IP 127.0.0.1.41519 > 127.0.0.1.1234: UDP, length 5
  16:55:40.380308 IP 127.0.0.1 > 127.0.0.1: ICMP 127.0.0.1 udp port 1234 unreachable, length 41

This is mentioned a bit here:
https://tools.ietf.org/html/rfc8085#section-5.2

Here is how Python's socket module produces an ConnectionRefused
exception:

  socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC, IPPROTO_IP) = 3
  sendto(3, "hello", 5, 0, {sa_family=AF_INET, sin_port=htons(1234), sin_addr=inet_addr("127.0.0.1")}, 16) = 5
  getsockname(3, {sa_family=AF_INET, sin_port=htons(41519), sin_addr=inet_addr("0.0.0.0")}, [16]) = 0
  getpeername(3, 0x7ffc43c99a20, [16])    = -1 ENOTCONN (Transport endpoint is not connected)

UDP itself may not carry this information but applications do rely on
the ICMP information as shown above.

The Linux network stack implementation is here:
net/ipv4/udp.c:__udp4_lib_err().

Please take a look and decide how vsock dgrams can have similar
semantics.

Stefan

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