Diameter Dispatches: S6a Authentication Information Request / Answer

This is part of a series of posts focusing on common Diameter request pairs, looking at what’s inside and what they do.

The Authentication Information Request (AIR) and Authentication Information Answer (AIA) are one of the first steps in authenticating a subscriber, and a very common Diameter transaction.

The Process

The Authentication Information Request (AIR) is sent by the MME to the HSS to request when a Subscriber begins to attach containing the IMSI of the subscriber trying to connect.

If the subscriber’s IMSI is known to the HSS, the AuC will generate Authentication Vectors for the Subscriber, and repond back to the MME in an Authentication Information Answer (AIA).

For more information on how the Authentication process works and what the authentication vectors do, I’ve written about that quite extensively here.- HSS & USIM Authentication in LTE.

The Authentication Information Request (AIR)

The AIR is a comparatively simple request, without many AVPs;

The Session-Id, Auth-Session-State, Origin-Host, Origin-Realm & Destination-Realm are all common AVPs that have to be included.

The Username AVP (AVP 1) contains the username of the subscriber, which in this case is the IMSI.

The Requested-EUTRAN-Authentication-Info AVP ( AVP 1408 ) contains information in regards to what authentication info the MME is requesting from the subscriber, typically this indicates the MME is requesting 1 vector (Number-Of-Requested-Vectors (AVP 1410)), an immediate response is preferred (Immediate-Response-Preferred (AVP 1412)), and if the subscriber is re-resyncing the SQN will include a Re-Synchronization-Info AVP (AVP 1411).

The Visited-PLMN-Id AVP (AVP 1407) contains information regarding the PLMN of the RAN the Subscriber is connecting to.

The Authentication Information Answer (AIA)

The Authentication Information Answer contains several mandatory AVPs that would be expected, The Session-Id, Auth-Session-State, Origin-Host and Origin-Realm.

The Result Code (AVP 268) indicates if the request was successful or not, 2001 indicates DIAMETER SUCCESS.

The Authentication-Info (AVP 1413) contains the returned vectors, in LTE typically only one vector is returned, a sub AVP called E-UTRAN-Vector (AVP 1414), which contains AVPs with the RAND, XRES, AUTN and KASME keys.

Further Reading & References

3GPP TS 29.272 version 15.10.0 Release 15

Example Packet Capture (PCAP) of Message Flow

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