From: Sowmini Varadhan <sowmini.varad...@oracle.com> Date: Wed, 18 Apr 2018 08:31:03 -0400
> However, I share Sridhar's concerns about the very fundamental change > to UDP message boundary semantics here. There is actually no such thing > as a "segment" in udp, so in general this feature makes me a little > uneasy. Well behaved udp applications should already be sending mtu > sized datagrams. And the not-so-well-behaved ones are probably relying > on IP fragmentation/reassembly to take care of datagram boundary semantics > for them? > > As Sridhar points out, the feature is not really "negotiated" - one side > unilaterally sets the option. If the receiver is a classic/POSIX UDP > implementation, it will have no way of knowing that message boundaries > have been re-adjusted at the sender. There are no "semantics". What ends up on the wire is the same before the kernel/app changes as afterwards. The only difference is that instead of the application doing N - 1 sendmsg() calls with mtu sized writes, it's giving everything all at once and asking the kernel to segment. It even gives the application control over the size of the packets, which I think is completely prudent in this situation.
