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3.5. LDP Messages

All LDP messages have the following format:

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|   Message Type              |      Message Length           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Message ID                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                     Mandatory Parameters                      |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                     Optional Parameters                       |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

U-bit Unknown message bit. Upon receipt of an unknown message, if U is clear (=0), a notification is returned to the message originator; if U is set (=1), the unknown message is silently ignored. The sections following that define messages specify a value for the U-bit.

Message Type Identifies the type of message.

Message Length Specifies the cumulative length in octets of the Message ID, Mandatory Parameters, and Optional Parameters.

Message ID 32-bit value used to identify this message. Used by the sending LSR to facilitate identifying Notification messages that may apply to this message. An LSR sending a Notification message in response to this message SHOULD include this Message ID in the Status TLV carried by the Notification message; see Section "Notification Message".

Mandatory Parameters Variable length set of required message parameters. Some messages have no required parameters.

For messages that have required parameters, the required parameters MUST appear in the order specified by the individual message specifications in the sections that follow.

Optional Parameters Variable length set of optional message parameters. Many messages have no optional parameters.

For messages that have optional parameters, the optional parameters may appear in any order.

Note that there is no alignment requirement for the first octet of an LDP message and that there is no padding at the end of a message; that is, parameters can end at odd-byte boundaries.

The following message types are defined in this version of LDP:

Message Name Section Title

   Notification            "Notification Message"
   Hello                   "Hello Message"
   Initialization          "Initialization Message"
   KeepAlive               "KeepAlive Message"
   Address                 "Address Message"
   Address Withdraw        "Address Withdraw Message"
   Label Mapping           "Label Mapping Message"
   Label Request           "Label Request Message"
   Label Abort Request     "Label Abort Request Message"
   Label Withdraw          "Label Withdraw Message"
   Label Release           "Label Release Message"

The sections that follow specify the encodings and procedures for these messages.

Some of the above messages are related to one another, for example the Label Mapping, Label Request, Label Withdraw, and Label Release messages.

While it is possible to think about messages related in this way in terms of a message type that specifies a message class and a message subtype that specifies a particular kind of message within that class, this specification does not formalize the notion of a message subtype.

The specification assigns type values for related messages, such as the Label messages, from of a contiguous block in the 16-bit message type number space.