Do you know what video codec is used for messengers?

I'd like to know the video codec used for MSN, yahoo, QQ or any other XMPP messengers.

Thanks for help.

For video codecs it's something like this:

• Skype: On2's VP7.
• Yahoo! Messenger: JPEG 2000, i.e. only I-frames.
• Google talk: H264 and others.
• MSN Messenger: Siren Codec.
• iChat: H.264/AVC.