Add a SetDoNotFragment() method to DatagramSocket, and call this for

net/udp/udp_socket_posix.cc

Index: net/udp/udp_socket_posix.cc
diff --git a/net/udp/udp_socket_posix.cc b/net/udp/udp_socket_posix.cc
index 797e1a0bc1352b5a4a9a43c8c663c7acb05ab374..09774ee0033144077ac9a1646e30a637851fcadd 100644
--- a/net/udp/udp_socket_posix.cc
+++ b/net/udp/udp_socket_posix.cc
@@ -425,6 +425,24 @@ int UDPSocketPosix::SetSendBufferSize(int32_t size) {
   return rv == 0 ? OK : MapSystemError(errno);
 }
 
+int UDPSocketPosix::SetDoNotFragment(bool do_not_fragment) {
+  DCHECK_NE(socket_, kInvalidSocket);
+  DCHECK(CalledOnValidThread());
+
+  int val = do_not_fragment ? IP_PMTUDISC_DO : IP_PMTUDISC_DONT;

[Jeremy Dorfman, 2016/08/10 23:56:57]

Strictly speaking, the system default is configurable and can be changed with
/proc/sys/net/ipv4/ip_no_pmtu_disc. It isn't necessarily IP_PMTUDISC_DONT (it
could be _WANT). You may want to read the original value when the socket is
created and save it off somewhere.

[Ryan Hamilton, 2016/08/11 19:47:31]

Good point. I removed the argument so that this method only sets DF.

+  int rv = setsockopt(socket_, IPPROTO_IP, IP_MTU_DISCOVER, &val, sizeof(val));

[Jeremy Dorfman, 2016/08/10 23:56:57]

I'm not sure what will happen if the socket is v6-only (i.e. IPV6_V6ONLY was
set, or /proc/sys/net/ipv6/bindv6only is 1). I think it will probably fail. It
may be worth querying the socket to check the state of IPV6_V6ONLY first.

[Ryan Hamilton, 2016/08/11 19:47:31]

How 'bout we only do one of these codepaths depending on the address family?

[Ryan Hamilton, 2016/08/12 16:53:29]

As discussed offline, doing only one codepath is problematic. I've changed this
to call both if the socket is dual stack. The QUIC code which calls this method
ignores the error code, so we *could* skip the v6-only check but I suspect the
performance difference is not important relative to the correctness.

+  if (rv != OK)
+    return MapSystemError(errno);
+
+  if (addr_family_ == AF_INET6) {
+    val = do_not_fragment ? IPV6_PMTUDISC_DO : IPV6_PMTUDISC_DONT;
+    rv =
+        setsockopt(socket_, IPPROTO_IPV6, IPV6_MTU_DISCOVER, &val, sizeof(val));
+  }
+
+  return rv == 0 ? OK : MapSystemError(errno);
+}
+
 int UDPSocketPosix::AllowAddressReuse() {
   DCHECK_NE(socket_, kInvalidSocket);
   DCHECK(CalledOnValidThread());