MPLS _Lecture 11_ by zhouwenjuan


• Some notations:
  –   LSP: Label Switched Path
  –   LSR: Label Switched Router
  –   Ingress: first LSR
  –   Egress: last LSR
  –   Upstream/downstream
  –   FEC: forwarding equivalency classes
• An example for a packet to pass the
  following MPLS domain:
                    LSP 1

    LSR A           LSR B           LSR C

            LSP 2           LSP 3

                    LSR D
• Key concepts in MPLS:
  – Label:
     • a short, fixed-length, locally significant identifier.
     • Labels may be mapped to some fields in the layer 2 header.
     • Otherwise use standard MPLS encapsulation (a thin layer
       between layer 3 and layer 2).
     • Each label identifies an LSP and is associated with an FEC.
  – Hierarchical label stack
     • Each packet may have one or more labels.
     • Labels in a packet are organized as a stack (LSRs may push or
       pop a label.
     • This is needed for nested tunnels
• Key concepts in MPLS (continue):
  – Label-switching table
     Incoming label     outgoing label next-hop per-label state

     • Also called imcoming label map (ILM)
     • Each entry is called next-hop label-forwarding entry (NHLFE)
     • May have multiple NHLFEs for an incoming label for load sharing
       (exploiting multi-paths).
  – Label Distribution Protocols
     • Before LSPs can be used, the label switching table must be set. This
       process is called LSP setup or label distribution.
     • LSRs learn each other’s capabilities and exchange label-mapping
       information through label distribution protocols.
     • Signaling in MPLS networks.
     • LDP for hop-by-hop label distribution, CR-LDP and RSVP-TE for
       explicitly routed LSPs.
• Key concepts in MPLS (continue):
  – Label Assignment and Distribution
     • Label assignment is always done by the downstream LSR.
       Why not the upstream LSR?
     • Two modes: donwstream on demand and unsolicited
  – Label merging
     •   Two or more LSPs may be merged into one.
     •   Label merging may lose some information about a packet.
     •   Label merging reduce the requirement of label space
     •   Not all LSRs support label merging. (e.g. ATM cannot support
         merged circuit).
• Key concepts in MPLS (continue):
  – Route selection and explicit routing
     • Hop-by-hop routing (follow whatever IP does)
     • Explicit routing (need to use constraint based
  – FEC
     • A set of packets that are treated identifically in the
       forwarding process
     • Packet classification: MPLS move this functionality
       from the data plane to the control plane.
     • Some common FECs in MPLS networks:
        – IP prefix, egress router, application flow
        – A matter of scalability and controllability.
• Key concepts in MPLS (continue):
  – Label Stack Encoding

  PPP/HDLC header       MPLS header IP packet HDLC trailer

  MPLS header IP packet AAL5 pad and trailer

  – The MPLS label stack contains one or more label stack entries,
    each is 32 bits long.

  Label (20 bits)   exp (3 bits) s (1 bit) TTL (8 bits)
• Key concepts in MPLS (continue):
  – Determining the network layer protocol
     • Network layer protocol information is not in the
       MPLS header.
  – Loop detection.
     • IP routing form loops easily
     • TTL in the MPLS header
     • Some link layer may not be able to handle TTL (e.g
        – Path vector in the label distribution messages
        – Hop count in the label distribution messages.

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