i did some more debugging and wireshark shows that the 3.0 sr does not
even try to send anything to the 3.1 sr over the tcp connection although
netstat now tells at both hosts that the connection is established.
instead sr 3.0 replies immediately after receiving invite from ua:
SIP/2.0 477 Unfortunately error on sending to next hop occurred (477/TM)
there is no related messages in syslog. perhaps tcp stack on 3.0 host
has not got acks for earlier packets and just waits there.
-- juha