Fix bizarre StayRTR behavior #84
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…d report an error
Previously StayRTR would copy the client's Session ID back into the Cache
Response send to the router, even though the cache's internal Session ID
was something different.
The purpose of the Session ID is to help both router and cache understand
whether they are synchronized or not. There are two opportunities to fix
desyncs: if the cache recognises the router is desynced, the cache informs
the router (through an Error Report) to reconnect and send a Reset Query.
If the router recognises it is out of sync with the cache, the router can
send a Reset Query.
According to RFC 8210 section 5.1 the cache should send "Corrupt Data" when
a router sends a Serial Query with an unknown Session ID:
```
Session ID: A 16-bit unsigned integer. When a cache server is
started, it generates a Session ID to identify the instance of the
cache and to bind it to the sequence of Serial Numbers that cache
instance will generate. This allows the router to restart a
failed session knowing that the Serial Number it is using is
commensurate with that of the cache. If, at any time after the
protocol version has been negotiated (Section 7), either the
router or the cache finds that the value of the Session ID is not
the same as the other's, the party which detects the mismatch MUST
immediately terminate the session with an Error Report PDU with
code 0 ("Corrupt Data"), and the router MUST flush all data
learned from that cache.
```
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Previously StayRTR would copy the client's Session ID back into the Cache Response to the client, even though the server's internal Session ID was something different.
The purpose of the Session ID is to help both client and server understand whether they are synchronized or not. There are two opportunities to fix desyncs: if the server recognises the client is desynced, the server informs the client (through an Error Report) to reconnect and and send a Reset Query. If the client recognises it is out of sync with the server, the client can send a Reset Query.
Related #82