7.2. SSM and RFC 2401 IPsec Caveats

For existing implementations of RFC 2401 IPsec (now superseded by [RFC4301]), there are a few caveats related to SSM. They are listed here. In RFC 2401 IPsec, the source address is not used as part of the key in the SAD lookup. As a result, two senders that happen to use the same SSM destination address and the same Security Parameter Index (SPI) will "collide" in the SAD at any host that is receiving both channels. Because the channel addresses and SPIs are both allocated autonomously by the senders, there is no reasonable means to ensure that each sender uses a unique destination address or SPI.

A problem arises if a receiver subscribes simultaneously to two unrelated channels using IPsec whose sources happen to be using the same IP destination address (IPDA) and the same IPsec SPI. Because the channel destination addresses are allocated autonomously by the senders, any two hosts can simultaneously use the same destination address, and there is no reasonable means to ensure that this does not happen. The <IPDA,SPI> tuple, however, consists of 56 bits that are generally randomly chosen (24 bits of the IP destination and 32 bits of the SPI), and a conflict is unlikely to occur through random chance.

If such a collision occurs, a receiver will not be able to simultaneously receive IPsec-protected traffic from the two colliding sources. A receiver can detect this condition by noticing that it is receiving traffic from two different sources with the same SPI and the same SSM destination address.

Holbrook & Cain Standards Track [Page 12] RFC 4607 Source-Specific Multicast August 2006