Mpls-Multi Protocol label Switching
- 2. • A forwarding scheme designed to speed up IP packet forwarding • Idea: use a fixed length label in the packet header to decide packet forwarding • Label carried in an MPLS header between the link layer header and network layer header • Support any network layer protocol and link layer protocol 2
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- 4. • Label: 20-bit label value • Exp: experimental use • Can indicate class of service • S: bottom of stack indicator • 1 for the bottom label, 0 otherwise • TTL: time to live 4
- 5. • An MPLS capable router is called a labe l switching ro ute r (LSR) • Forwarding Equivalence Class (FEC): A subset of packets that are all treated the same way by an LSR • A packet is assigned to an FEC at the ingress of an MPLS domain 5
- 6. • A packet’s FEC can be determined by one or more of the following: • Source and/or destination IP address • Source and/or destination port number • Protocol ID • Differentiated services code point • Incoming interface • A particular PHB (scheduling and discard policy) can be defined for a given FEC 6
- 7. • At ingress LSR of an MPLS domain, an MPLS header is inserted to a packet before the packet is forwarded • Label in the MPLS header encodes the packet’s FEC • At subsequent LSRs • The label is used as an index into a forwarding table that specifies the next hop and a new label. • The old label is replaced with the new label, and the packet is forwarded to the next hop. • Egress LSR strips the label and forwards the packet to final destination based on the IP packet header 7
- 8. 8 Intf In Label In Intf Out 3 40 1 Intf In Label In Intf Out Label Out 3 50 1 40 1 2 3 1 2 1 2 3 3 FEC Intf Out Label Out a 1 50 50 40
- 9. • For each FEC, a specific path called Labe lSwitche d Path (LSP) is assigned • The LSP is unidirectional • To set up an LSP, each LSR must • Assign an incoming label to the LSP for the corresponding FEC • Labels have only local significance • Inform the upstream node of the assigned label • Learn the label that the downstream node has assigned to the LSP • Need a label distribution protocol so that an LSR can inform others of the label/FEC bindings it has made • A forwarding table is constructed as the result of label distribution. 9
- 10. 10 Intf In Label In Intf Out 3 40 1 Intf In Label In Intf Out Label Out 3 50 1 40 47.1 47.247.3 1 2 3 1 2 1 2 3 3 Dest Intf Out Label Out 47.1 1 50 Mapping: 40 Request: 47.1 Mapping: 50 Request: 47.1
- 11. • Hop-by-hop routing: use the route determined by the dynamic routing protocol • Explicit routing (ER): the sender LSR can specify an e xplicit ro ute for the LSP • Explicit route can be selected ahead of time or dynamically 11
- 12. • A packet may carry multiple labels, organized as a last- in-first-out stack • A label may be added to/removed from the stack at any LSR • Processing always done on the top label • Allow the aggregation of LSPs into a single LSP for a portion of the route, creating a tunnel • At the beginning of the tunnel, the LSR assigns the same label to packets from different LSPs by pushing the label onto each packet’s stack • At the end of the tunnel, the LSR pops the top label 12
- 13. When a failure occurs: • LSR at the beginning of the tunnel will • Switch packets received on the protected LSP x onto the bypass tunnel • Replace the old label with a new label that will be understood by the last node in the bypass tunnel to indicate LSP x • Push the bypass tunnel's label onto the label-stack of the redirected packets. • LSR at the end of the tunnel will • Pop the bypass tunnel's label • Examine the top label to determine the protected LSP that the packet is to follow. 13
- 14. • MPLS can be used together with Differentiated Services to provide QoS. • LSPs are configured between each ingress-egress pair. • For each ingress-egress pair, a separate LSP can be created for each traffic class, or • Can create a single LSP for each ingress-egress pair and use the Exp bits to differentiate packet classes. • Scalable: as the number of flows increases, the number of LSPs does not increase. 14
- 15. • IP restoration is very slow • Routing table convergence time on the order of seconds. • Looping and packet loss can occur during convergence • MPLS enables fast failure restoration 15
- 16. • End-to-End protection • A backup LSP is set up in advance from the source LSR to the destination LSR of the primary LSP. • The backup LSP is link and node disjoint with the primary LSP • Need reserve resources for the backup LSP • Source LSR responsible for restorationsender must be notified of the failure 16
- 17. • Local protection • When establishing a primary LSP, a backup LSP for each possible link or node failure is set up • Resources reserved for each backup LSP • Failure detecting LSR responsible for switching traffic to the backup LSR • Faster restoration than end-to-end protection 17
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Editor's Notes
- #2: Start with slide 37
- #5: P529 of Stalling for details of TTL RFC 3032
- #7: In IP, FEC is destination network
- #8: Once a packet is assigned to a FEC, no further header analysis is done by subsequent routers; all forwarding is driven by the labels.
- #10: Inform all potential upstream nodes of the assigned label