Advanced Computer Networks: Lecture 28. This lecture will cover the following: multiprotocol label switching; destination based forwarding; label distribution protocol (LDP); label edge router (LER); methods of attaching labels; conventional network;...
CS716 Advanced Computer Networks By Dr. Amir Qayyum 1 Lecture No. 28 Internetworking • Basics of internetworking (heterogeneity) – IP protocol, address resolution, control messages … • Routing • Global internets (scale) – Virtual geography and addresses – Hierarchical routing • Future internetworking: IPv6 • Multicast traffic • MPLS Multiprotocol Label Switching • Combines properties of virtual circuits with flexibility and robustness of datagrams – Relies on IP addresses and IP routing protocols – Forwards packets using short, fixed length labels with local scope • Marriage of two seemingly opposed technologies MPLS Capabilities • Enable IP capabilities on devices that do not have the capability to forward IP datagrams • Forward IP packets along explicit routes: routes precalculated separate to IP r protocol • Support certain types of VPN services • Where is performance improvement ? – Depends on factors other than header processing Destination Based Forwarding • MPLS – attaching labels with IP datagrams • Router allocates a label for each prefix in its routing table Destination Based Forwarding • Advertisement of label and prefixes to neighbors via Label Distribution Protocol (LDP) – Attach corresponding label to all packets sent to that router for that prefix Destination Based Forwarding • Other routers store these labels as remote labels in their routing table along with prefixes Destination Based Forwarding • Other routers store these labels as remote labels in their routing table along with prefixes Destination Based Forwarding Example • R1 acts as Label Edge Router (LER) – Applies labels to arriving IP packets after complete IP lookup • Packet destined to 10.1.1.5 arrives at R1 – R1 matches the prefix 10.1.1, attaches label ‘15’ to packet, send to R2 10 Destination Based Forwarding Example • R2 checks the label of incoming packet, consults its table for outgoing interface (1), updates the label value to ‘24’ and forwards the packet to R3 • R2 doesn’t consult IP address for forwarding packet to R3 !!! 11 MPLS – Achievements • Fixedlength label lookup instead of variable length IP prefix lookup – Simpler to implement exact match instead of longest match algorithm • Only forwarding algorithm is changed – Any standard routing algorithm may still be used – Packets will follow the path selected by IProute 12 How to Attach Labels to Packets ? • Depends upon type of link carrying packets • When IP packets carried as complete frames – Label is inserted as a “shim” between layer 2 header and layer 3 header – Like on most link types: Ethernet, Token Ring, PPP • When the switch function as an MPLS LSR – Label need to be in a place where switch can use it – In ATM, labels are inserted in ATM cell header (combination of VPI and VCI) 13 Methods of Attaching Labels 14 An MPLS Network • Mixture of conventional IP routers, label edge routers (LER) and ATM switches (as LSRs) – All are using the same routing protocols • Reduction in number of adjacencies that each router must maintain – Greatly reduce amount of work of each router • Edge routers have a full view of complete topology of the network – Edge routers pick a new path in case of node failure 15 MPLS – Example of MPLS Network • Conventional Network – Overlay of virtual circuits – Each router connect to other by a virtual circuit 16 MPLS – Example of MPLS Network • MPLS Network – Routers peer directly with LSRs – No virtual circuits interconnecting routers – Each router has only one adjacency 17 MPLS – Explicit Routing • MPLS provides a convenient way to add capabilities similar to source routing to IP networks – Explicit routing • Not really source of packet that picks the route – different from source routing • One of the routers inside the service provider’s network does this • Application – Traffic Engineering 18 Example – Explicit Routing • • • • • “Fish” network Route for traffic from R1 to R7 is R1R3R6R7 Route for traffic from R2 to R7 is R2R3R4R5R7 Good use of capacity available along two distinct paths Different from normal IP routing 19 MPLS – Explicit Routing • With MPLS enabled routers it is very easy to achieve desired routing • How can we make sure to choose different paths? – If labels of R1 and R2 are different, R3 can send the packets along different paths • How to agree on labels in a network? – Normal label switch forwarding doesn’t work – RSVP (Resource Reservation Protocol) 20 MPLS – Explicit Routing • Fast reroute – a capability to make networks more resilient in the face of failure • Possible to precalculate a path (backup path) between router ‘A’ to router ‘B’ that explicitly avoids a certain link ‘L’ • Can significantly reduce the time taken to reroute packets around a point of failure • CSPF (Constrained Shortest Path First): Most common algorithm to calculate 21 explicit routes MPLS VPN An ATM Circuit • Pseudowire Emulation •Routers interconnected by a tunnel • Tunnel header 22 Forwarding ATM Cells in MPLS VPN 23 Example of Layer 3 VPN 24 .. .Lecture? ?No.? ?28 Internetworking • Basics of internetworking (heterogeneity) – IP protocol, address resolution, control messages …