14. - SIP SDP - MIME

Mime was defined to provide a way of attaching different document/file types to email messages, SIP takes advante of this existing standard by using it to inform a UA what content is present in a SIP message.

Content-Type: <type>/<subtype>

Content-Type: image/gif

As you have already seen Content-Type: application/SDP tell us that the MIME content of the SIP message is the application layer protocol SDP. THE UA then (if supporting SDP) can interpret the SDP information in the message to find out details such as Codecs supported in the 'Inviting Agent' and Port numbers for responses etc.

A sip message can actually contain multiple mime parts, on the previous image wee can see the following

Content-Type:multipart/mixed; boundary=boundary1 

the first Content-type:application/sdp belongs to an SDP MIME message, there is also a second Content-type: image/gif which could be an image of a business card, a photograph, etc.