RMAN backup hangs during implicit resync (Doc ID 1070012.1)

RMAN backup hangs.
Debug trace shows we hang on a call to pcicmp during an implicit partial resync:

DBGRPC: CMD type=partial resync id=1 status=STARTED
DBGRPC: 1 STEP id=1 status=STARTED chid=default
DBGRPC: krmqgns: no work found for channel d1 (krmqgns)
DBGRPC: (krmqgns)
DBGRPC: EXITED krmqgns with status 1
DBGRPC: krmxcis: xc=436847736 chid=default calling pcicmp

10046 trace on the rman target default channel shows the target session is idle.
10046 on the rman catalog session shows the catalog is also idle.
The catalog database is accessed remotely via SQL through a firewall.

Cause

This is an SQLNet issue caused when "inspect sqlnet" or "deep packet inspection" is enabled on the firewall.
This setting can sometimes result in failed or mis-formed packets over the network.
It is the same problem as described in Note 805088.1: ORA-3135 with Recovery Catalog Creation Across the Network (Firewall included).

Solution

Disable "inspect sqlnet" or "deep packet inspection" on the firewall.
Once done, the backup that had been consistently hanging at the same point , completed successfully.