Label%20Distribution%20Protocols

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

• An Example

PATH LSP_TUNNEL LABEL_REQEUST EXPLICIT_ROUTE(B,C
) RECORD_ROUTE(B)
PATH LSP_TUNNEL LABEL_REQEUST EXPLICIT_ROUTE(C)
RECORD_ROUTE(B,C)
LSR C
LSR A
LSR B
RESV LSP_TUNNEL LABEL(15) RECORD_ROUTE(B,C)
RESV LSP_TUNNEL LABEL(12) RECORD_ROUTE(B,C)