Skype and SIP
In a comment on a previous post, muppetmaster pointed to an ongoing discussion in the Skype Forum. In it, a Skype staffer says directly that "skype client does not implement/use SIP". Apparently a field in the shared.xml Skype config file is named
Of course, it's possible that the Skype developers simply borrowed SIP terminology in developing their own protocol. (Developers like to borrow things.) It's also possible that the Skype proprietary protocol is 99.999% SIP. (As I said, developers like to borrow things.) And except to the protocol zealots, it doesn't really matter either way. The Skype protocol is unpublished and its transmission is encrypted, so unless Skype decides they want to allow arbitrary SIP clients to interwork with Skype, it's not going to happen.
And as long as Skype has a larger user base than the SIP world, they're not going to want it to happen. (For a detailed examination of why this is the case, see this paper, quoted below:)
...if network value scales like n log(n), as we argue ... then relative gains from interconnection depend on the sizes of the networks. In this case the smaller network gains considerably more than the larger one. This produces an incentive for larger networks to refuse to interconnect without payment...