2.2.4 Message Types
This section defines the MMS message types. All messages MUST be transmitted over TCP.
MMS messages adhere to a common packet structure. MMS messages always start with the chunkLen field that specifies the size of the MMS message in multiples of 8 bytes. If the sum of the sizes of all fields in the MMS message is not a multiple of 8 bytes, padding MUST be appended after the final field. Whether padding is required or not, the amount of padding added MUST be chosen so that the total size of the MMS message, including the padding, becomes a multiple of 8 bytes. The value of the chunkLen field includes the size of the padding, if any.
The chunkLen field is followed by a Message ID (MID) field that specifies the type of message contained in the packet.