HSS generate multiple authentication vector as it has been asked from MME .
MME can ask authentication vector in between 1 to 5 in one short from hss. After that MME will keep those authentication vector in its context w.r.t to time. As the time elapsed MME will refresh its authentication vector context.
KSI ( Key set identifier ) table will be maintained at MME to store all KASME OR K'ASME along with dl/ul count , GUTI,
AVP structure used by MME to ask for EPS vectors
Requested- EUTRAN-Authentication-Info ::=
**[ Number-Of-Requested-Vectors]**( This parameter tells how many authentication vector hss has to send back to MME , if nothing is mention in this then hss will only send one authentication vector to MME.
[ Immediate-Response-Preferred ]
[ Re-synchronization-Info ]
The reason of asking for multiple authentication vector is to reduce the communication between MME and HSS.
for more details http://diameter-protocol.blogspot.in/2013/06/airaia-authentication-information.html . one can refer it .