Label Distribution Protocol

1. Label Distribution Protocol Kashif Latif

2. 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…?

3. MPLS

4. Label Distribution Protocol (LDP) is a key protocol in the  MPLS (Multi Protocol Label Switching ) architecture . In the MPLS network, 2 Label S witching R outers (LSR) must agree on the meaning of the labels used to forward traffic between and through them . LDP Overview

5. LSR Routers LDP Protocol

6. Purpose of LDP The Label Distribution Protocol (LDP) is a protocol defined by the IETF (RFC 5036) for the purpose of distributing labels in an MPLS environment.

7. 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

8. LSR Routers

9. 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.

10. Protocol Structure

11. 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 last 2 indicate a label space within the LSR . Protocol Detail

12. All LDP messages have the following format: LDP Messages

13. U -- The U bit is an unknown message bit. Message T ype -- 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 L ength -- 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 . LDP Message Detail

14. TLV Format

15. 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 . TLV Format Detail

16. 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 LDP Message Types

17. 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 L abel S witch P aths (LSPs) , forward label traffic across an MPLS backbone to particular destinations . MPLS LDP Functional Overview

18. 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. Directly Connected MPLS LDP Sessions Nondirectly Connected MPLS LDP Sessions Introduction to LDP Sessions

19. 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 B asic D iscovery . Directly Connected Sessions

20. Session Establishment

21. 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 T ransport A ddresses . The router with the higher IP address takes the active role and establishes the session. Active/Passive Role

22. Methods of Label Distribution After 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: Downstream Unsolicited : An LSR advertises label mappings to peers without being asked to. Downstream on Demand : An LSR advertises label mappings to a peer only when the peer asks for them .

23. 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 E xtended D iscovery . Nondirectly Connected Sessions

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

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

26. There are four categories of LDP messages: Discovery messages , used to announce and maintain the presence of an LSR in a network. Session messages , used to establish, maintain, and terminate sessions between LDP peers. Advertisement messages , used to create, change, and delete label mappings for FECs. Notification messages , used to provide advisory information and to signal error information. LDP Message Exchange

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

28. 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

29. 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

30. 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

31. Thank You…! Kashif Latif