Label Distribution Protocol

Kashif Latif

Label Distribution Protocol (LDP) is a protocol in which routers capable of Multiprotocol Label Switching (MPLS) exchange label mapping information.

MPLS is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table.

What is LDP…?

MPLS

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 Overview

LSR Routers

LDP Protocol

Purpose of LDPThe Label Distribution Protocol (LDP) is a

protocol defined by the IETF (RFC 5036) for the purpose of distributing labels in an MPLS environment.

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.

LSR Working

LSR Routers

LDP Peers

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

1. Version -- LDP version number. The present number is 1.

2. PDU Length -- The total length of the PDU excluding the version and the PDU length field.

3. 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 last 2 indicate a label space within the LSR.

Protocol Detail

All LDP messages have the following format:

LDP Messages

1. U -- The U bit is an unknown message bit.

2. 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.

3. Message Length -- The length in octets of the message ID, mandatory parameters and optional parameters.

4. Message ID -- 32-bit value used to identify the message.

5. 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.

LDP Message Detail

TLV Format

1. U -- The U bit is an unknown TLV bit.

2. F -- Forward unknown TLV bit.

3. Type -- Encodes how the Value field is to be interpreted.

4. Length -- Specifies the length of the Value field in octets.

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

TLV Format Detail

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

Message Name Section TitleNotification Notification MessageHello Hello MessageInitialization Initialization MessageKeepAlive KeepAlive MessageAddress Address MessageAddress Withdraw Address Withdraw MessageLabel Mapping Label Mapping MessageLabel Request Label Request MessageLabel Abort Request Label Abort Request MessageLabel Withdraw Label Withdraw MessageLabel Release Label Release Message

LDP Message Types

LDP provides a standard methodology for hop-by-hop, or dynamic label, distribution in an MPLS network by assigning labels to routes that have been chosen by the underlying Interior Gateway Protocol (IGP) routing protocols.

The resulting labeled paths, called Label Switch Paths (LSPs), forward label traffic across an MPLS backbone to particular destinations.

MPLS LDP Functional Overview

When you enable MPLS LDP, the LSRs send out messages to try to find other LSRs with which they can create LDP sessions.

The following sections explain the differences between sessions.1. Directly Connected MPLS LDP Sessions2. Nondirectly Connected MPLS LDP Sessions

Introduction to LDP Sessions

If an LSR is one hop from its neighbor, it is directly connected to its neighbor. The LSR sends out LDP link Hello messages as User Datagram Protocol (UDP) packets to all the routers on the subnet (multicast).

A neighboring LSR may respond to the link Hello message, allowing the two routers to establish an LDP session. This is called Basic Discovery.

Directly Connected Sessions

Session Establishment

To initiate an LDP session between routers, the routers determine which router will take the active role and which router will take the passive role. The router that takes the active role establishes the LDP TCP connection session and initiates the negotiation of the LDP session parameters.

To determine the roles, the two routers compare their Transport Addresses. The router with the higher IP address takes the active role and establishes the session.

Active/Passive Role

Methods of Label DistributionAfter the LDP TCP connection session is established, the LSRs negotiate the session parameters, including the method of label distribution to be used. Two methods are available:

1. Downstream Unsolicited: An LSR advertises label mappings to peers without being asked to.

2. Downstream on Demand: An LSR advertises label mappings to a peer only when the peer asks for them.

If the LSR is more than one hop from its neighbor, it is nondirectly connected to its neighbor. For these nondirectly connected neighbors, the LSR sends out a targeted Hello message as a UDP packet, but as a unicast message specifically addressed to that LSR.

The nondirectly connected LSR responds to the Hello message and the two routers begin to establish an LDP session. This is called Extended Discovery.

Nondirectly Connected Sessions

The exchange of targeted Hello messages between two nondirectly connected neighbors can occur in several ways, including the following:

1. Router 1 sends targeted Hello messages carrying a response request to Router 2. Router 2 sends targeted Hello messages in response if its configuration permits. In this situation, Router 1 is considered to be active and Router 2 is considered to be passive.

2. Router 1 and Router 2 both send targeted Hello messages to each other. Both routers are considered to be active. Both, one, or neither router can also be passive, if they have been configured to respond to requests for targeted Hello messages from each other.

Exchange of Message

An LDP label binding is an association between a destination prefix and a label.

LDP Label Binding

There are four categories of LDP messages:

1. Discovery messages, used to announce and maintain the presence of an LSR in a network.

2. Session messages, used to establish, maintain, and terminate sessions between LDP peers.

3. Advertisement messages, used to create, change, and delete label mappings for FECs.

4. Notification messages, used to provide advisory information and to signal error information.

LDP Message Exchange

LDP errors and other events of interest are signaled to an LDP peer by Notification messages.There are two kinds of LDP Notification messages:

1. Error Notifications2. Advisory Notifications

LDP Error Handling

Error Notifications, used to signal fatal errors. If an LSR receives an Error Notification from a peer for an LDP session, it terminates the LDP session by closing the TCP transport connection for the session and discarding all label mappings learned via the session.

Error Notifications

Advisory Notifications, used to pass on LSR information about the LDP session or the status of some previous message received from the peer.

Advisory Notifications

LDP uses TCP as a reliable transport for sessions.

When multiple LDP sessions are required between two LSRs, there is one TCP session for each LDP session.

LDP Transport

Thank

You…!

Kashif Latif