Internet Routing (COS 598A) Today: Multi-Protocol Label Switching

1. Internet Routing (COS 598A)Today: Multi-Protocol Label Switching Jennifer Rexford http://www.cs.princeton.edu/~jrex/teaching/spring2005 Tuesdays/Thursdays 11:00am-12:20pm

2. Outline • Circuit switching • Packet switching vs. circuit switching • Virtual circuits • MPLS • Labels and label-switching • Forwarding Equivalence Classes • Label distribution • MPLS applications • Feedback forms • Fill out during last 20 minutes

3. Packet Switching vs. Circuit Switching • Packet switching • Data traffic divided into packets • Each packet contains its own header (with address) • Packets sent separately through the network • Destination reconstructs the message • Example: sending a letter through postal system • Circuit switching • Source first establishes a connection to the destination • Each router on the path may reserve bandwidth • Source ends data over the connection • No destination address, since routers know the path • Source tears down the connection when done • Example: voice conversation on telephone network

4. Advantages of Circuit Switching • Guaranteed bandwidth • Predictable communication performance • Not “best-effort” delivery with no real guarantees • Simple abstraction • Reliable communication channel between hosts • No worries about lost or out-of-order packets • Simple forwarding • Forwarding based on time slot or frequency • No “longest prefix match” on each packet • Low per-packet overhead • Forwarding based on time slot or frequency • No IP (and TCP/UDP) header on each packet

5. Disadvantages of Circuit Switching • Wasted bandwidth • Bursty traffic leads to idle connection during silent period • Unable to achieve gains from statistical multiplexing • Blocked connections • Connection refused when resources are not sufficient • Unable to offer “okay” service to everybody • Connection set-up delay • No communication until the connection is set up • Unable to avoid extra latency for small data transfers • Network state • Routers must store per-connection information • Unable to avoid per-connection storage and state failover

6. Virtual Circuits • Hybrid of packet and circuit switching • Logical circuit between a source and destination • Packets from different VCs multiplex on a link • Virtual Circuit Identifier (VC ID) • Source set-up: establish path for the VC • Switch: mapping VC ID to an outgoing link • Packet: fixed length label in the header 1: 7 2: 7 1: 14 2: 8 link 7 1 link 14 2 link 8

7. Swapping the Label at Each Hop • Problem: using VC ID along the whole path • Each virtual circuit consumes a unique ID • Starts to use up all of the ID space in the network • Label swapping • Map the VC ID to a new value at each hop • Table has old ID, next link, and new ID • Allows reuse of the IDs at different links 1: 7: 20 2: 7: 53 20: 14: 78 53: 8: 42 link 7 1 link 14 2 link 8

8. Virtual Circuits Similar to IP Datagrams • Data divided in to packets • Sender divides the data into packets • Packet has an address (e.g., IP address or VC ID) • Store-and-forward transmission • Multiple packets may arrive at once • Need buffer space for temporary storage • Multiplexing on a link • No reservations: statistical multiplexing • Packets are interleaved without a fixed pattern • Reservations: resources for group of packets • Guarantees to get a certain number of “slots”

9. Virtual Circuits Differ from IP Datagrams • Forwarding look-up • Virtual circuits: fixed-length connection id • IP datagrams: destination IP address • Initiating data transmission • Virtual circuits: must signal along the path • IP datagrams: just start sending packets • Router state • Virtual circuits: routers know about connections • IP datagrams: no state, easier failure recovery • Quality of service • Virtual circuits: resources and scheduling per VC • IP datagrams: difficult to provide QoS

10. Wide Range of Quality-of-Service Models • Policies for allocating resources • Admission control: whether or not to accept the VC • Link scheduling: what order to send packets • Buffer management: which packets to drop • One extreme: best-effort service • Accept all connections (unless table is full) • Put all packets in a first-in-first-out queue • Drop any packet arriving when queue is full • Another extreme: strict bandwidth guarantees • Virtual circuit reserves bandwidth along the path • Network edge must shape/police to enforce this rate • Each link has a queue for packets from each VC • Link schedules the packets using weighted fair queuing

11. Multi-Protocol Label Switching

12. Multi-Protocol Label Switching • Multi-Protocol • Encapsulate a data packet • Could be IP, or some other protocol (e.g., IPX) • Put an MPLS header in front of the packet • Actually, can even build a stack of labels… • Label Switching • MPLS header includes a label • Label switching between MPLS-capable routers MPLS header IP packet

13. Pushing Popping Swapping IP IP IP IP C A R2 R1 IP edge R4 B R3 D MPLS core Pushing, Swapping, and Popping • Pushing: add the initial “in” label • Swapping: map “in” label to “out” label • Popping: remove the “out” label

14. Forwarding Equivalence Class (FEC) • Rule for grouping packets • Packets that should be treated the same way • Identified just once, at the edge of the network • Example FECs • Destination prefix • Longest-prefix match in forwarding table at entry point • Useful for conventional destination-based forwarding • Src/dest address, src/dest port, and protocol • Five-tuple match at entry point • Useful for fine-grain control over the traffic • Sent by a particular customer site • Incoming interface at entry point • Useful for virtual private networks A label is just a locally-significant identifier for a FEC

15. Label Distribution Protocol • Distributing labels • Learning the mapping from FEC to label • Told by the downstream router • Example: destination-based forwarding I’m using label 43 for 12.1.1.0/24 I’m using label 10 for 12.1.1.0/24 Pick in-label 10 for 12.1.1.0/24 In: Link: Out 43: to R4: 10 R2 Map destinations in 12.1.1.0/24 to out-label 43 and link to R2 12.1.1.0/24 R1 R4 R3

16. Supporting Explicitly-Routed Paths • Explicitly routing from ingress to egress • Set an explicit path (e.g., based on load) • Perhaps reserve resources along the path • Extend a protocol for resource reservation • Start with ReSource Reservation Protocol (RSVP) • Used for reserving resources along an IP path • Extensions for label distribution & explicit routing • Extend a protocol for distributing labels • Start with Label Distribution Protocol (LDP) • Extensions for explicit routing & reservation • Two competing proposed standards

17. Applications of MPLS

18. TE With Constraint-Based Routing • Path calculation • Constrained shortest-path first • Compute shortest path based on weights • But, exclude paths that do not satisfy constraints • E.g., do not consider links with insufficient bandwidth • Information dissemination • Extend OSPF/IS-IS to carry the extra information • E.g., link-state attributes for available bandwidth • Path signaling • Establish label-switched path on explicit route • Forwarding: MPLS labels

19. Surviving Failures: Path Protection • Path protection • Reserve bandwidth on an alternate route • Protect a label-switched path by having a stand-by • Much better than conventional IP routing • Precise control over where the traffic will go • Stand-by path can be chosen to be disjoint

20. Surviving Failures: Fast Reroute • Ensure fast recovery from a link failure • Protect a link by having a protection sub-path • Much faster recovery than switching paths • Affected router can detect the link failure • … and start redirecting to the protection sub-path

21. BGP-Free Core iBGP eBGP 12.1.1.0/24 C A R2 R1 R4 B R3 D FEC based on the destination prefix Routers R2 and R3 don’t need to speak BGP

22. VPNs With Private Addresses 10.1.0.0/24 10.1.0.0/24 C A R2 Two FECs R1 R4 B R3 D Direct traffic to orange 10.1.0.0/24 10.1.0.0/24 MPLS tags can differentiate green VPN from orange VPN.

23. Status of MPLS • Deployed in practice • BGP-free core • Virtual Private Networks • Traffic engineering • Challenges • Protocol complexity • Configuration complexity • Difficulty of collecting measurement data • Continuing evolution • Standards • Operational practices and tools

24. Conclusion • MPLS is an overlay • Tunneling on top of the network • Built on top of an underlying routing algorithm • Flexibility in mapping traffic to paths • Associating packets with FECs, and then labels • New protocols for creating label-switching tables • Binding FECs to labels across a path • Establishing explicit routes • Many open questions • Makes operations easier vs. harder? • Trade-offs in exploiting the flexibility? • Interdomain routing with MPLS?

25. Rest of the Semester • Rest of class • Feedback forms • Thanks (in advance) for your feedback • Written reports for course projects • Due Dean’s Date (May 10) by end of day • Submitting via e-mail would be fine • Oral presentations for course projects • Monday May 16 at 1:30pm in room 302 • 15 minutes for single-person, 20 for groups