5.5. Cache Response

The cache responds to queries with zero or more payload PDUs. When replying to a Serial Query (Section 5.3), the cache sends the set of announcements and withdrawals that have occurred since the Serial Number sent by the client router. When replying to a Reset Query (Section 5.4), the cache sends the set of all data records it has; in this case, the withdraw/announce field in the payload PDUs MUST have the value 1 (announce).

In response to a Reset Query, the new value of the Session ID tells the router the instance of the cache session for future confirmation. In response to a Serial Query, the Session ID being the same reassures the router that the Serial Numbers are commensurate, i.e., the cache session has not been changed.

0          8          16         24        31
.-------------------------------------------.
| Protocol |   PDU    |                     |
| Version  |   Type   |     Session ID      |
|    1     |    3     |                     |
+-------------------------------------------+
|                                           |
|                 Length=8                  |
|                                           |
`-------------------------------------------'