The first number is the session id, an unique identifier for the session . The number in second position - 2 - is the session version: if a new offer/answer negotiation is needed during this media session this number will be increased by one. This will happen when any parameter need to be changed in the media session such as on-hold, codec-change, add-remove media track. The three following fields are the network type (Internet), IP address type (version 4) and unicast address of the machine which created the SDP. These three values are not relevant for the negotiation.