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MPLS • 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 downstream – 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 routing). – 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 ATM) – Path vector in the label distribution messages – Hop count in the label distribution messages.
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