Editing Short Message Peer-to-Peer (section)

==PDU format (after version 3.4)==

The SMPP PDUs are [[Binary protocol|binary encoded]] for efficiency. They start with a header which may be followed by a body:

{| class="wikitable" style="text-align:center"
|- 
| style="background: black; color: white; font-weight: bold" colspan="5" | SMPP PDU
|-
| colspan="4" | PDU Header (mandatory)
| PDU Body (Optional)
|-
| Command <br/> length || Command <br/> Id || Command <br/> Status || Sequence <br/> Number || PDU Body
|- 
| 4 octets
|  colspan="4" | Length = (Command Length value - 4) octets
|}

===PDU header===

Each PDU starts with a header. The header consists of 4 fields, each of length of 4 octets:

;<code>command_length</code>: Is the overall length of the PDU in octets (including command_length field itself); must be ≥ 16 as each PDU must contain the 16 octet header
;<code>command_id</code>: Identifies the SMPP operation (or command). If the most significant bit is cleared, this is a request operation. Otherwise it is a response.
;<code>command_status</code>: Always has a value of 0 in requests; in responses it carries information about the result of the operation
;<code>sequence_number</code>: Is used to correlate requests and responses within an SMPP session; allows asynchronous communication (using a [[Sliding window protocol|sliding window]] method)

All numeric fields in SMPP use the [[big endian]] order, which means that the first octet is the Most Significant Byte (MSB).