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 3 LSP 2 LSR D

Key concepts in MPLS: Label: Hierarchical label stack 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.