Re: why the SAs are unidirectional

[Bill Sommerfeld <sommerfeld@east.sun.com>]

> I have a query regarding SAs (Security Associations ), why SAs are defined
> in one direction. Separate for inbound and outbond traffic. Why are they not
> defined in both ways.

1) reusing the same key in both directions makes reflection attacks
easier; using a different key makes them much harder.

2) reusing the same SPI in both directions is impossible in general
since the owner of each destination address controls/allocates its own
inbound SPI space.

Most key management protocols create pairs of SA's, one in each
direction.