1. Dan Harkins Trapeze Networks
July 2003
Naming Cached PMKs
Dan Harkins
Trapeze Networks
Dan Harkins, Trapeze Networks

2. July 2003
Current PMK Caching
Supplicant sets a “cached PMK” bit in the RSN Capabilities bitfield in the RSN IE in the associate request.
Authenticator sends associate response and
Begins 4-way handshake if it has a cached PMK for the supplicant
Begins 802.1X authentication if it does not have a cached PMK for the supplicant
Dan Harkins, Trapeze Networks

3. July 2003
Current PMK Caching
Relies on no ambiguity on which PMK to use in the 4-way handshake
Problematic to use for fast handoff
Proactive (push) techniques can provide an AP with a PMK for the supplicant prior to the associate request being received
Reactive (pull) techniques can allow the AP to retrieve a PMK for the supplicant
This can introduce ambiguity!
Dan Harkins, Trapeze Networks

4. July 2003
Name Cached PMKs!
pmkname = HMAC-SHA1-128(PMK, “Key Identifier” |
AP-mac | STA-mac)
If supplicant sets “cached PMK” bit in associate request, a list of pmknames, and the number of pmknames, is appended to the request.
If authenticator has one of the named PMKs in the list it appends the pmkname to the first message of the 4-way handshake.
Dan Harkins, Trapeze Networks

5. Name Cached PMKs! Semantics: use PMK named by “dbnier7owfurn7w”
July 2003
Name Cached PMKs!
I have cached PMKs:
fjkdkleifjcjd8w2
984oeruwonwru
dbnier7owfurn7w
8qo8awq8t348h4
dbnier7owfurn7w
Semantics: use PMK named by “dbnier7owfurn7w”
in the 4-way handshake
Dan Harkins, Trapeze Networks

6. Advantages of Naming Cached PMKs for fast handoff
July 2003
Advantages of Naming Cached PMKs for fast handoff
No new key hierarchies
No new service primitives
No new PRFs
No new key exchanges
No new management frames
Minimal, simple, change to existing mechanisms– add a list, append a blob
Dan Harkins, Trapeze Networks

7. Advantages of Naming Cached PMKs for Fast Handoff
July 2003
Advantages of Naming Cached PMKs for Fast Handoff
Can work with any scheme for distributing PMKs
IAPP
Neighbor graphs
It doesn’t matter how the PMK got there, just that it got there.
Protocol does not assume existence of PMKs. Either side can delete a PMK from its cache for any reason and at any time.
Dan Harkins, Trapeze Networks

8. Advantages of Naming Cached PMKs for Fast Handoff
July 2003
Advantages of Naming Cached PMKs for Fast Handoff
STA authenticates to A, hibernates and wakes up at D where it authenticates again. PMKs were delivered by AS to B and E for first authentication and different PMKs were delivered to B and E for the second. The STA will assert both when it moves to B. A B C E D
Dan Harkins, Trapeze Networks

9. Advantages of Naming Cached PMKs for Fast Handoff
July 2003
Advantages of Naming Cached PMKs for Fast Handoff
B will select one and initiate the 4-way handshake. If the STA moves to C it will again assert two named PMKs. Depending on the neighbor graph C may have one– in which case C will chose it– or none– in which case C will begin 802.1X authentication of the STA. A B C E D
Dan Harkins, Trapeze Networks

10. Advantages of Naming Cached PMKs for Fast Handoff
July 2003
Advantages of Naming Cached PMKs for Fast Handoff
Can work with any scheme for deriving AP-specific PMKs.
Is independent of whatever key hierarchy may be defined.
It doesn’t matter how the key was derived as long as the STA and AS are using the same technique. AP is out-of-the-loop and therefore the protocol does not care.
Dan Harkins, Trapeze Networks

11. Advantages of Naming Cached PMKs for Fast Handoff
July 2003
Advantages of Naming Cached PMKs for Fast Handoff
Can be used with PSKs too!
Dan Harkins, Trapeze Networks

12. July 2003
Discussion
Dan Harkins, Trapeze Networks

13. Motion! Insert changes described in 03/484-r1 to draft. July 2003
Dan Harkins, Trapeze Networks