[Ipsec] Saving of one exchange in case of DOS attack

PREAMBLE:

From draft-ietf-ipsec-ikev2-15.txt

“An expected attack against IKE is state and CPU exhaustion, where the

target is flooded with session initiation requests from forged IP

addresses. This attack can be made less effective if an

implementation of a responder uses minimal CPU and commits no state

to an SA until it knows the initiator can receive packets at the

address from which he claims to be sending them.”

Initiator Responder

----------- p; -----------

HDR(A,0), SAi1, KEi, Ni -->

<-- HDR(A,0), N(COOKIE)

HDR(A,0), N(COOKIE), SAi1, KEi, Ni -->

<-- HDR(A,B), SAr1, KEr, Nr, [CERTREQ]

HDR(A,B), SK {IDi, [CERT,] [CERTREQ,] [IDr,]

AUTH, SAi2, TSi, TSr} -->

<-- HDR(A,B), SK {IDr, [CERT,] AUTH,

SAr2, TSi, TSr}

Where:

A = Initiator’s SPI

B = Responder’s SPI

QUESTION:

My point of view is that why not we do in this way

In this case, Alice will not send her IKE_SPI value to Bob in message#1, instead Bob will send his IKE_SPI value (acts as Cookie) to Alice in message#2.

Bob will not commit any state until Alice sends her IKE_SPI value and Bob’s IKE_SPI value (acts as cookie) to Bob in message#3 (i.e., first message of second exchange).

ADVANTAGES:

1- We can save cost/time for one extra exchange.

2- IKE_SPI and Cookie both can be random values, we can get benefit of using Cookie as IKE_SPI or IKE_SPI as Cookie.

Initiator Responder

----------- p; -----------

HDR(0,0), SAi1, KEi, Ni -->

<-- HDR(0,B), SAr1, KEr, Nr, [CERTREQ]