When a "node X" (not SiteMaster) needs to send a message to a remote site, it forwards the message to the local SiteMaster ("node S"). If the route/bridge to the remote site is not available, "node S" sends a SITE_UNREACHABLE to "node X".

The problem is the Message.setDst() is set with the "node X" SiteUUID instead of using the [[ExtendedUUID}}. The Message is discarded in [1] because SiteUUID.equals(ExtendedUUID.) returns false.

RELAY2 needs to use the ExtendedUUID to send the SITE_UNREACHABLE.

Zulip Discussion: https://infinispan.zulipchat.com/#narrow/stream/118645-infinispan/topic/xsite/near/253721421

[1] https://github.com/belaban/JGroups/blob/78fc3aec2b15bab26a6c873e67d657e18994033c/src/org/jgroups/protocols/TP.java#L1500