Set `TCP_USER_TIMEOUT` by default.

Read https://github.com/grpc/proposal/blob/master/A18-tcp-user-timeout.md for the background. We could do the same for all TCP/IP sockets we use. We could:

• Add the default value of TCP_USER_TIMEOUT option to Flags
• Set it in the constructors of ClientFactoryBuilder and ServerBuilder.

[trustin]

Thanks for the detailed idea. That is very useful!

Won’t think about server-side for this iteration since TCP_USER_TIMEOUT (and other timeout related options) only apply to client-side.

I thought TCP_USER_TIMEOUT is applicable to server-side as well? Can’t you set it by using ServerBootstrap.childOption()?

if idleTimeoutMillis > 0 or maxConnectionAgeMillis > 0:
    TCP_USER_TIMEOUT = max(idleTimeoutMillis, maxConnectionAgeMillis)
  if pingIntervalMillis > 0:
    SO_KEEPALIVE = 1
    TCP_KEEPIDLE = pingIntervalMillis
    TCP_KEEPINTVL = pingIntervalMillis # setting TCP_KEEPCNT may not be too important - TCP_USER_TIMEOUT will dominate if enabled anyways.

• maxConnectionAgeMillis is often a very large value. Shouldn’t it just be TCP_USER_TIMEOUT = idleTimeoutMillis?
• KEEPALIVE settings sounds reasonable.

[jrhee17]

Some useful links for personal reference

• tcp user timeout rfc: https://datatracker.ietf.org/doc/html/rfc5482
• linux tcp manual: https://man7.org/linux/man-pages/man7/tcp.7.html
• grpc-side PR: https://github.com/grpc/grpc/pull/16419
• original netty PR: https://github.com/netty/netty/pull/4175/files
• internet hosts rfc - section on tcp retransmission/timeouts: https://datatracker.ietf.org/doc/html/rfc1122#section-4.2.3.5