Problem: ZMTP 3.1 ping/pong context not implemented

[bluca]

ZMTP 3.1 adds PING/PONG commands as heartbeat options that libzmq implements. The PING commands can add a context following the TTL (an arbitrary payload) which the PONG is supposed to echo back, but libzmq does not implement this.

To invoke a single heartbeat, the peer MAY, at any point after the security handshake is complete, send a PING command:

ping = command-size %d4 "PING" ping-ttl ping-context
ping-ttl = 2OCTET
ping-context = *OCTET

The ping-ttl is a 16-bit unsigned integer in network order that MAY be zero or MAY contain a time-to-live measured in tenths of seconds. The ping-ttl provides a strong hint to the other peer to disconnect if no further traffic is received after that time. The maximum TTL is 6553 seconds.

When a peer receives a PING command it SHALL respond with a PONG command that echoes the ping-context, which may be empty:

pong = command-size %d4 "PONG" ping-context
ping-context = *OCTET

https://rfc.zeromq.org/spec:37/ZMTP

[eponsko]

What's the purpose of echoing the TTL back? One thing I thought could be neat is to use it to provide an estimate of the latency between the peers, to use as a weight in or alternative to the RR behavior.

[bluca]

It's not the TTL that's echoed back, it's an arbitrary content. I'm working on that, but I'll probably change the RFC to limit it - at the moment there's no size limit which might open it to abuse.