2. Multi-protocol Label Switching (MPLS)

3. RFC 3031 MPLS provides new capabilities: QoS support Traffic engineering VPN Multiprotocol support

4. MPLS-based Solutions Enable QoS in IP Networks –Support Diffserv using connection-oriented QoS –“Connections” can be flows or large aggregates IP Traffic Engineering –Use constraint-based routing to adapt to latest network loading and QoS performance Virtual Private Networks –Use controllable tunneling mechanisms L2/L3 Integration –Integrate with L1 and L2 technologies like Optical Cross Connects (OXC’s) and ATM Resilient Network Design –Automatic Failover and Backup

5. Overview (RFC 3031) MPLS is a forwarding scheme Motivation: speed up IP packet forwarding. Idea: use a fixed length label in the packet header to decide packet forwarding –Label is an index into an internal table Advantage: fast forwarding and resources can be reserved on the path Each MPLS packet has a header encapsulated between the link layer header and network layer header (MPLS is layer 2.5) –Can support any network layer protocol and link layer protocol A MPLS capable router is called label switched router (LSR), LSR examines only the label in forwarding the packet Header format?? Useful tool for traffic engineering

6. Label: locally significant label QoS: indicate class of service –Communicate DS information or PHB S: 1 for the bottom label, 0 for other labels TTL: time to live label QoS sTTL 20 318

7. TTL Processing P529: need detail?? copied from IP packet header at the ingress LSR of an MPLS domain At internal LSR: packet discarded when it hits 0

8. Label used as in index into a table to determine the outgoing line and the new label –Labels have only local significance Routers can group multiple flows that ends at a particular router and use a single label for them Flows that are grouped together under a single label belong to the same Forwarding Equivalent Class (FEC) –FEC determines the destination and the service class

9. Label Switched Routers (LSRs): switching packets based on their labels For each FEC, –a specific path though the MPLS network is defined –Each FEC associated with a traffic characterization that defines the QoS requirements for that flow LSRs do not examine IP header, simply forward packet based on its label value

10. Before delivering packets in a given FEC, a LSP must be defined and QoS parameters along the path must be established –How much resources to commit to the path –What queuing and discarding policy to establish at each LSR Label assignment: manually specify routes and assign label values, or, use a protocol to determine the route and establish label values

11. At ingress edge LSR of an MPLS domain, a packet is assigned to a FEC, append appropriate label to the packet and forward the packet Within an MPLS domain, when LSR receives a labeled packet, it removes the incoming label and attach the appropriate outgoing label to the packet, and forward the packet to the next LSR Egress edge LSR strips the label, forward the packet to final destination based on the IP packet header Use figure on p527 of stallings book

12. The FEC for a packet can be determined by one or more of the following –Source and/or destination IP addresses or IP network addresses –Source and/or destination port numbers –IP protocol ID –Differentiated services code point –IPv6 flow label A particular PHB can be defined at an LSR for a given FEC –PHB defined the queuing priority and the discard policy

13. LSP Need a protocol to distribute the labels to set up label switched paths (LSPs) –Set up table at each LSR, table format? LSP is unidirectional LSP set up can be –Control driven: triggered by control traffic such as routing updates –Data driven: triggered by the the request of a flow or a traffic trunk (an aggregation of flows with the same service class that can be put into a LSP)

14. An LSR maintains a forwarding table for each LSP passing through the LSR. –Table entries at ingress edge node: FEC, out iface, out label –Table entries at core node: in label, in iface, out iface, out label Ways to create the forwarding table entries –Data driven: when a packet arrives at a router, the router contact the router downstream and asks it to generate a label for the flow. This is applied recursively –Control driven: p417 of Tanenbaum

15. Label Stacking A packet may carry a number of labels, organized as a last-in-first-out stack, allow groups of flows to carry the same label for part of a route Processing based on the top label A label may be added to/removed from the stack at any LSR Allow the aggregation of LSPs into a single LSP, 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 Illustration?? P528 paragraph 2

16. Traffic grouped into FECs –Each traffic flow must be assigned to an FEC –A particular LSP is assigned to an FEC (support the QoS requirements of the FEC) –Individual LSRs must assign an incoming label to the LSP for a given FEC and communicate the label to other LSRs that may send it packets for this FEC Traffic in an FEC sent along an LSP Packets in an FEC identified by a locally significant label

17. Route Selection Hop-by-hop routing Explicit routing (strict and loose) –Benefits: able to do traffic engineering and policy routing –Explicit routes can be selected by configuration (ahead of time) or dynamically –Dynamic case: LSR need following info (p532 of Stallings) –Constraint based routing

18. Label Distribution To set up an LSP, each LSR must –Assign a label to the LSP to be used to recognize the incoming packets that belong to the corresponding FEC –Inform all potential upstream nodes of the label assigned by this LSR to this FEC –Learn the next hop for this LSP and learn the label that the downstream node has assigned to this FEC Item 2 and 3 can be done either by manual configuration or require a label distribution protocol (enable an LSR to inform others of the label/FEC bindings it has made)

19. The LSP between two routers can be the same as the L3 hop-by-hop route, or the sender LSR can specify an Explicit Route (ER) for the LSP. A forwarding table indexed by labels is constructed as the result of label distribution. Each forwarding table entry specifies ??

20. Packets are classified and MPLS headers are inserted at the ingress LSRs of a MPLS-capable domain. When a LSR receives a labeled packet, it will use the label as the index to look up the forwarding table. –This is faster than the process of parsing the routing table in search of the longest match done in IP routing –The incoming label is replaced by the outgoing label and the packet is switched to the next LSR. –Inside a MPLS domain, packet forwarding, classification and QoS service are determined by the labels and the COS fields. This makes core LSRs simple. –Before a packet leaves a MPLS domain, its MPLS label is removed.

21. After LSPs are set up, a packet’s path can be completely determined by the label assigned by the ingress LSR. MPLS is strategically significant because: 1. it provides faster packet classification and forwarding, 2. it provides an efficient tunneling mechanism. These features, particularly the second one, make MPLS useful for Traffic Engineering

22. A Service Architecture based on MPLS MPLS can be used together with Differentiated Services to provide QoS. LSPs are first configured between each ingress-egress pair. It is likely that for each ingress-egress pair, a separate LSP is created for each traffic class. In order to reduce the number of LSPs, the LSPs from all ingress routers to a single egress router can be merged into a Sink Tree. The total number of Sink Trees needed is C*N. It is also possible to use a single Sink Tree to transmit packets of different traffic classes, and use the COS bits to differentiate packet classes. Scalable: as the number of transiting flows increases, the number of flows in each LSP or Sink Tree also increases. But the number of LSPs or Sink Trees need not increase.

23. three differences in the processing of a packet. 1) At the ingress of the ISP network, in addition to all the processing described in the DS field-based architecture, a MPLS header is inserted into the packet. 2) Core routers process the packet based on its label and COS field rather than its DS field. 3) At the egress, unless inter-domain LSPs are configured, the MPLS header is removed.

24. Whether a ISP’s architecture is DS field-based or MPLS-based is transparent to other ISPs  the DS field based architecture and the MPLS based architecture can easily inter-operate. Each customer domain still needs a BB to allocate services, and to request for resources on behalf of the customer domain when the SLA is dynamic. But since LSPs are configured within the ISPs, resource requests can be easily hidden from the core routers by tunneling them from the ingress routers to the egress routers  BBs may not be needed in the MPLS-based ISP networks. Admission control is made the ingress routers