LDP: Label Distribution
Protocol Overview
Label Distribution
Protocol (LDP) is a key protocol in the MPLS (Multi Protocol Label Switching)architecture. In the MPLS network, 2 label
switching routers (LSR) must agree on the meaning of the labels used to forward
traffic between and through them. LDP defines a set of procedures and messages
by which one LSR (Label Switched Router) informs another of the label bindings
it has made. The LSR uses this protocol to establish label switched paths
through a network by mapping network layer routing information directly to
data-link layer switched paths.
Two LSRs (Label Switched
Routers) which use LDP to exchange label mapping information are known as LDP peers
and they have an LDP session between them. In a single session, each peer is
able to learn about the others label mappings, in other words, the protocol is
bi-directional.
Protocol Structure - LDP Label Distribution Protocol
2 bytes
|
2 bytes
|
Version
|
PDU Length
|
LDP Identifier(6 bytes)
|
|
LDP Messages
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|
- Version -- LDP version number. The present number is 1.
- PDU Length -- The total length of the PDU excluding the
version and the PDU length field.
- LDP identifier -- This field uniquely identifies the
label space of the sending LSR for which this PDU applies. The first 4
octets encode the IP address assigned to the LSR. The lst 2 indicate a
label space within the LSR.
LDP messages -- All LDP
messages have the following format:
U
|
Message type
|
Message Ienght
|
Message ID
|
||
Parameters
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||
- U -- The U bit is an unknown message bit.
- Message type -- The type of message. The following
message types exist: Notification, Hello, Initialization, Keep Alive,
Address, Address Withdraw, Label Request, Label Withdraw, Label Release,
and Unknown Message name.
- Message length -- The length in octets of the message
ID, mandatory parameters and optional parameters
- Message ID -- 32-bit value used to identify the
message.
- Parameters -- The parameters contain the TLVs. There
are both mandatory and optional parameters. Some messages have no
mandatory parameters, and some have no optional parameters.
TLV format:
U
|
F
|
Type
|
Length
|
Value
|
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TLV format
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|||
- U -- The U bit is an unknown TLV bit.
- F -- Forward unknown TLV bit.
- 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.
