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Label Distribution Protocols

Label Distribution Protocols. LDP: hop-by-hop routing RSVP-TE: explicit routing CR-LDP: another explicit routing protocol, no longer under development. LDP LDP peers: two LSRs that use LDP to exchange label/FRC mapping information. Four types of LDP messages:

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Label Distribution Protocols

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  1. Label Distribution Protocols • LDP: hop-by-hop routing • RSVP-TE: explicit routing • CR-LDP: another explicit routing protocol, no longer under development.

  2. LDP • LDP peers: two LSRs that use LDP to exchange label/FRC mapping information. • Four types of LDP messages: • Discovery messages for announcing and maintaining the presence of an LSR in a network. • Session message for establishing, maintaining, or terminating session between LDP peers. • Advertisement messages for creating, changing, or deleting label mapping for FEC. • Notification messages for distributing advisory information and error information. • Discovery messages are sent as UDP packets to LDP port at the all-router-on-this-subnet group multicast address. After a session is established, all messages are exchanged using TCP.

  3. LDP discovery: two mechanisms • Discover LSR neighbors that are directed connected • Send LDP Link Hello messages on each interface. • UDP packets addressed to LDP discovery port with the all-routers-on-this-subnet group multicast address. • Discover LSR neighbors that are remotely connected. • Send Targeted Hello messages to a specific IP address at the LDP discovery port. • When a LSR want to establish a peer relationship, it will reply. • The exchanges of Hello messages establish the adacency.

  4. LDP session management • After two LSRs establishes the adjacency, they can set up TCP connections and start the initialization process. • Negotiation of protocol version. Label distribution method, timer values, VPI/VCI ranges for label-controlled ATM. • Hello and Keepalive messages are sent periodically. • Label distribution and management • downstream on demand and downstream unsolicited. • Configurable options: Independent LSPs setup or in order LSP setup. • Independent LSP setup: LSRs may advertise label mappings to its neighbors at any time • In order LSP setup: LSP setup from egress to ingress. An LSR may send a label mapping for a FEC only when (1) the LSR has a label mapping for the FEC for the nexthop, or (2) the LSR is the egress for the FEC.

  5. FEC to LSP mapping • A LSR requests a label mapping from a neighboring LSR when it needs one and advertise the label mapping when I wants the neighbor to use a label. • Two types of FECs in LDP: address prefix and host address. • Special things must be done to deal with loops. • LDP identifiers: • An LSR may have multiple label spaces (one for each interface). It must identify the label spaces with LDP identifiers.

  6. RSVP-TE: • How RSVP works? • the difference between the paths setup by RSVP and LSP • Forward based on IP header, path associated with a distination and a transport-layer protocol. • LSP: packets are opaque to the intermediate nodes. LSP tunnel. • New features added to the original RSVP: • Label distribution, Explicit routing, Bandwidth reservation for LSPs, Rerouting LSPs after failures, Tracking of the actual route of an LSP, The concept of Nodal abstraction, Preemption options. • New objects in RSVP-TE: LABEL_REQUEST PATH LABEL RESV EXPLICIT_ROUTE PATH RECORD_ROUTE PATH, RESV SESSION_ATTRIBUTE PATH

  7. PATH: LSP_TUNNEL: • LABEL_REQEUST • EXPLICIT_ROUTE(B,C) • RECORD_ROUTE(B) • PATH: LSP_TUNNEL: • LABEL_REQEUST • EXPLICIT_ROUTE(C) • RECORD_ROUTE(B,C) • An Example LSR C LSR A LSR B • RESV: LSP_TUNNEL: • LABEL(12) • RECORD_ROUTE(B,C) • RESV: LSP_TUNNEL: • LABEL(15) • RECORD_ROUTE(B,C)

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