3. Protocol Specification

Previous sections that describe LDP operation have discussed scenarios that involve the exchange of messages among LDP peers. This section specifies the message encodings and procedures for processing the messages.

LDP message exchanges are accomplished by sending LDP protocol data units (PDUs) over LDP session TCP connections.

Each LDP PDU can carry one or more LDP messages. Note that the messages in an LDP PDU need not be related to one another. For example, a single PDU could carry a message advertising FEC-label bindings for several FECs, another message requesting label bindings for several other FECs, and a third Notification message signaling some event.

3.1. LDP PDUs

Each LDP PDU is an LDP header followed by one or more LDP messages. The LDP header is:

 0                   1                   2                   3
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Version                      |         PDU Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         LDP Identifier                        |
+                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version Two octet unsigned integer containing the version number of the protocol. This version of the specification specifies LDP protocol version 1.

PDU Length Two octet integer specifying the total length of this PDU in octets, excluding the Version and PDU Length fields.

The maximum allowable PDU Length is negotiable when an LDP session is initialized. Prior to completion of the negotiation, the maximum allowable length is 4096 bytes.

LDP Identifier Six octet field that uniquely identifies the label space of the sending LSR for which this PDU applies. The first four octets identify the LSR and MUST be a globally unique value. It SHOULD be a 32-bit router Id assigned to the LSR and also used to identify it in Loop Detection Path Vectors. The last two octets identify a label space within the LSR. For a platform-wide label space, these SHOULD both be zero.

Note that there is no alignment requirement for the first octet of an LDP PDU.

3.2. LDP Procedures

LDP defines messages, TLVs, and procedures in the following areas:

• Peer discovery
• Session management
• Label distribution
• Notification of errors and advisory information

The sections that follow describe the message and TLV encodings for these areas and the procedures that apply to them.

The label distribution procedures are complex and are difficult to describe fully, coherently, and unambiguously as a collection of separate message and TLV specifications.

Appendix A, "LDP Label Distribution Procedures", describes the label distribution procedures in terms of label distribution events that may occur at an LSR and how the LSR must respond. Appendix A is the specification of LDP label distribution procedures. If a procedure described elsewhere in this document conflicts with Appendix A, Appendix A specifies LDP behavior.

3.3. Type-Length-Value Encoding

LDP uses a Type-Length-Value (TLV) encoding scheme to encode much of the information carried in LDP messages.

An LDP TLV is encoded as a 2 octet field that uses 14 bits to specify a Type and 2 bits to specify behavior when an LSR doesn't recognize the Type, followed by a 2 octet Length field, followed by a variable length Value field.

 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|F|        Type               |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
|                             Value                             |
~                                                               ~
|                                                               |
|                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

U-bit Unknown TLV bit. Upon receipt of an unknown TLV, if U is clear (=0), a notification MUST be returned to the message originator and the entire message MUST be ignored; if U is set (=1), the unknown TLV MUST be silently ignored and the rest of the message processed as if the unknown TLV did not exist. The sections following that define TLVs specify a value for the U-bit.

F-bit Forward unknown TLV bit. This bit applies only when the U-bit is set and the LDP message containing the unknown TLV is to be forwarded. If F is clear (=0), the unknown TLV is not forwarded with the containing message; if F is set (=1), the unknown TLV is forwarded with the containing message. The sections following that define TLVs specify a value for the F-bit. By setting both the U- and F-bits, a TLV can be propagated as opaque data through nodes that do not recognize the TLV.

Type Encodes how the Value field is to be interpreted.

Length Specifies the length of the Value field in octets.

Value Octet string of Length octets that encodes information to be interpreted as specified by the Type field.

Note that there is no alignment requirement for the first octet of a TLV.

Note that the Value field itself may contain TLV encodings. That is, TLVs may be nested.

The TLV encoding scheme is very general. In principle, everything appearing in an LDP PDU could be encoded as a TLV. This specification does not use the TLV scheme to its full generality. It is not used where its generality is unnecessary and its use would waste space unnecessarily. These are usually places where the type of a value to be encoded is known, for example by its position in a message or an enclosing TLV, and the length of the value is fixed or readily derivable from the value encoding itself.

Some of the TLVs defined for LDP are similar to one another. For example, there is a Generic Label TLV, an ATM Label TLV, and a Frame Relay TLV; see Sections "Generic Label TLV", "ATM Label TLV", and "Frame Relay TLV".

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

The specification assigns type values for related TLVs, such as the label TLVs, from a contiguous block in the 16-bit TLV type number space.

Section "TLV Summary" lists the TLVs defined in this version of the protocol and the section in this document that describes each.