11. DLEP Signal and Message Structure
DLEP defines two protocol units used in two different ways: Signals and Messages. Signals are only used in the Discovery mechanism and are carried in UDP datagrams. Messages are used bidirectionally over a TCP connection between the participants, in the Session Initialization, In-Session, and Session Termination states.
Both Signals and Messages consist of a Header followed by an unordered list of Data Items. Headers consist of Type and Length information, while Data Items are encoded as TLV (Type-Length-Value) structures. In this document, the Data Items following a Signal or Message Header are described as being "contained in" the Signal or Message.
There is no restriction on the order of Data Items following a Header, and the acceptability of duplicate Data Items is defined by the definition of the Signal or Message declared by the type in the Header.
All integers in Header fields and values MUST be in network byte order.
11.1. DLEP Signal Header
The DLEP Signal Header contains the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 'D' | 'L' | 'E' | 'P' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signal Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: DLEP Signal Header
"DLEP": Every Signal MUST start with the following characters: U+0044, U+004C, U+0045, U+0050.
Signal Type: A 16-bit unsigned integer containing one of the DLEP Signal Type values defined in this document.
Length: The length in octets, expressed as a 16-bit unsigned integer, of all of the DLEP Data Items contained in this Signal. This length MUST NOT include the length of the Signal Header itself.
The DLEP Signal Header is immediately followed by zero or more DLEP Data Items, encoded in TLVs, as defined in this document.
11.2. DLEP Message Header
The DLEP Message Header contains the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: DLEP Message Header
Message Type: A 16-bit unsigned integer containing one of the DLEP Message Type values defined in this document.
Length: The length in octets, expressed as a 16-bit unsigned integer, of all of the DLEP Data Items contained in this Message. This length MUST NOT include the length of the Message Header itself.
The DLEP Message Header is immediately followed by zero or more DLEP Data Items, encoded in TLVs, as defined in this document.
11.3. DLEP Generic Data Item
All DLEP Data Items contain the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Item Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: DLEP Generic Data Item
Data Item Type: A 16-bit unsigned integer field specifying the type of Data Item being sent.
Length: The length in octets, expressed as a 16-bit unsigned integer, of the Value field of the Data Item. This length MUST NOT include the length of the Data Item Type and Length fields.
Value: A field of