Hello, hello ?

More information is in the Wireshark dissector for Apple MIDI at http://anonsvn.wireshark.org/wireshark/trunk/epan/dissectors/packet-applemidi.c

Intial packet capture shows that the Audio Midi software makes the initial invitation:

Apple Network-MIDI Session Protocol
 Signature: 0xffff
 Command: Invitation (0x494e)
 Protocol Version: 2
 Initiator Token: 0x6b8b4567
 Sender SSRC: 0x1c037394
 Name: AC7

To which the application replies:

Apple Network-MIDI Session Protocol
 Signature: 0xffff
 Command: Invitation Accepted (0x4f4b)
 Protocol Version: 2
 Initiator Token: 0x6b8b4567
 Sender SSRC: 0xa3a202f2
 Name: Ravelox iPad

The commands are IN and OK.

The Initiator Token is a value that is randomly generated by the session initiator and is used throughout the conversation, the Sender SSRC is generated by each side and remains the same.