Multi Protocol Label Switching (MPLS) Lecture 8: Multi Protocol Label Switching (MPLS)

Routing vs Switching routing: based on address lookup, max prefix match search operation complexity O(logn) - 0(n) switching: based on circuit numbers indexing operation complexity O(1) scalable to large networks  MPLS

MPLS connection MPLS introduces a connection-oriented structure into the connectionless IP network. An MPLS-ready IP router does not forward IP packets based on the destination address in the header, but on a label. Consider an MPLS-enabled IP network that runs over Ethernet: A special MPLS header is sandwiched between the IP header and the LLC header. The MPLS header contains a label that is a short, fixed-length connection identifier.

MPLS connection The MPLS-ready IP router, known as a label switched router (LSR), maintains a table of labels. When an IP packet arrives at the LSR, the label carried in the MPLS header is cross-referenced to the table of labels to find the next hop. The IP packet is then switched to the destination output port of the LSR that connects to the next hop LSR. The table contains labels for only the existing connections, and therefore it is not as large as the forwarding routing table in an IP router.

MPLS connection In order for a user to transmit over an MPLS-enabled IP network, it has to first request the establishment of a connection. This is done using a signaling protocol, such LDP (later CR-LDP) or RSVP (later RSVP-TE). The connection is a label switched path (LSP). LSR is aware of all of the connections that pass through its switch fabric; therefore, it can decide whether to accept a new connection or not based on the amount of traffic that will be transmitted and the requested QoS. The LSR allocates a portion of its bandwidth to a new connection, and it stops accepting new connections when it either runs out of bandwidth or reaches a certain percentage of utilization.

Q in Q Tunneling PBB

Tag Switching Key difference: tags setup in background using IP routing protocols (I.e. control-driven VC setup)

MPLS Concept: Route at Edge, Switch in Core IP IP #L1 IP #L2 IP #L3 IP IP Forwarding LABEL SWITCHING IP Forwarding

MPLS Terminology LDP: Label Distribution Protocol LSP: Label Switched Path FEC: Forwarding Equivalence Class LSR: Label Switching Router LER: Label Edge Router (Useful term not in standards) MPLS “multi-protocol” both in terms of protocols it supports ABOVE and BELOW in protocol stack!

MPLS Header IP packet encapsulated in MPLS header and sent down LSP IP packet restored at end of LSP by egress router TTL adjusted by default … IP Packet 32-bit MPLS Header MPLS is responsible for directing a flow of IP packets along a predetermined path across a network. This path is called a label-switched path. Label-switched paths are similar to ATM PVCs in that they are simplex in nature; that is, the traffic flows in one direction from the ingress router to a egress router. Duplex traffic requires two label-switched paths; that is, one path to carry traffic in each direction. A label-switched path is created by the concatenation of one or more label-switched hops, allowing a packet to be forwarded from one label-switching router to another label-switching router across the MPLS domain. A label-switching router is a router that supports MPLS-based forwarding. When an IP packet enters a label-switched path, the ingress router examines the packet and assigns it a label based on its destination, placing the label in the packet’s header. The label transforms the packet from one that is forwarded based on its IP routing information to one that is forwarded based on information associated with the label. The packet is then forwarded to the next router in the label-switched path. The key point in this scheme is that the physical path of the LSP is not limited to what the IGP would choose as the shortest path to reach the destination IP address.

MPLS Header label experimental bits TTL label used to match packet to LSP experimental bits carries packet queuing priority (CoS) stacking bit: can build “stacks” of labels qoal: nested tunnels! time to live copied from IP TTL An MPLS header consists of: 20-bit label—Used to identify the packet to a particular LSP Class of service value—Indicates queuing priority through the network. At each hop along the way, the class of service value determines which packets receive preferential treatment within the tunnel. Stacking bit—Indicates that this MPLS packet has more than one label associated with it. The MPLS implementation in JUNOS Software supports a stacking depth of one. Time to live value—Contains a limit on the number of router “hops” this MPLS packet may travel through the network. It is decremented at each hop, and if the TTL value drops below one, the packet is discarded.

MPLS Forwarding: Example IP packet destined to 134.112.1.5/32 arrives to SF San Francisco has route for 134.112/16 next hop is LSP to New York 134.112/16 IP New York 134.112.1.5 San Francisco 1965 1026 Santa Fe

MPLS Forwarding Example San Francisco pre-pends MPLS header onto IP packet, sends packet to first transit router on path 134.112/16 New York San Francisco IP 1965 Santa Fe

MPLS Forwarding Example because packet arrived to Santa Fe with MPLS header, Santa Fe forwards it using MPLS forwarding table 134.112/16 New York San Francisco IP 1026 Santa Fe

MPLS Forwarding Example packet arrives from penultimate router with label 0 egress router sees label 0, strips MPLS header egress router performs standard IP forwarding IP 134.112/16 Labels 0 through 15 are reserved labels, as specified in draft-ietf-mpls-label-encaps-07.txt. A value of 0 represents the "IPv4 Explicit NULL Label". This label value is only legal when it is the sole label stack entry. It indicates that the label stack must be popped, and the forwarding of the packet must then be based on the IPv4 header. A value of 1 represents the "Router Alert Label". This label value is legal anywhere in the label stack except at the bottom. When a received packet contains this label value at the top of the label stack, it is delivered to a local software module for processing. The actual forwarding of the packet is determined by the label beneath it in the stack. However, if the packet is forwarded further, the Router Alert Label should be pushed back onto the label stack before forwarding. The use of this label is analogous to the use of the "Router Alert Option" in IP packets. Since this label cannot occur at the bottom of the stack, it is not associated with a particular network layer protocol. A value of 2 represents the "IPv6 Explicit NULL Label".This label value is only legal when it is the sole label stack entry. It indicates that the label stack must be popped, and the forwarding of the packet must then be based on the IPv6 header. A value of 3 represents the "Implicit NULL Label". This is a label that an LSR may assign and distribute, but which never actually appears in the encapsulation. When an LSR would otherwise replace the label at the top of the stack with a new label, but the new label is "Implicit NULL", the LSR will pop the stack instead of doing the replacement. Although this value may never appear in the encapsulation, it needs to be specified in the Label Distribution Protocol, so a value is reserved. Values 4-15 are reserved for future use. New York IP San Francisco Santa Fe

Regular IP Forwarding 1 47.1 IP 47.1.1.1 1 2 IP 47.1.1.1 3 2 IP 47.1.1.1 1 3 47.2 47.3 2 IP 47.1.1.1 IP destination address unchanged in packet header!

MPLS Label Distribution 1 47.1 3 Request: 47.1 3 Request: 47.1 2 1 Mapping: 0.40 1 2 Mapping: 0.50 47.3 3 47.2 2

Label Switched Path (LSP) IP 47.1.1.1 1 47.1 3 3 2 1 1 2 47.3 3 47.2 2 IP 47.1.1.1

A General Vanilla LSP #963 #14 #99 #311 #216 #14 #963 #612 #311 #462 #99 #5 - Vanilla LSP actually part of tree from every source to destination (unidirectional) - Vanilla LDP builds tree using existing IP forwarding tables to route control messages

Forwarding Equivalence Classes FEC - group of IP packets forwarded over same path, with same forwarding treatment FEC may correspond to destination IP subnet source, destination IP subnet QoS class

Example