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1. Tag Switching

1. Tag Switching. RFC 2105 - Cisco systems Tag Switching architecture overview. Switching In IP Networks - B.Davie, P.Doolan, Y.Rekhter. Presnted By - Shmuel Vagner. 2. Overview. A label swapping protocol that allows: Destination based routing. Hierarchies of routing knowledge.

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1. Tag Switching

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  1. 1. Tag Switching RFC 2105 - Cisco systems Tag Switching architecture overview. Switching In IP Networks - B.Davie, P.Doolan, Y.Rekhter. Presnted By - Shmuel Vagner.

  2. 2. Overview • A label swapping protocol that allows: • Destination based routing. • Hierarchies of routing knowledge. • Multicast routing. • Explicit routes.

  3. 3. Overview • Advantages: • Better performance. • Hardware and data-link independent but at the same time can benefit from fast data-link protocols like ATM. • Scalable. • Flexible.

  4. 4. Overview • TSR - Tag Switching router. • TSER - Tag Switching edge router.

  5. 5. Components • Forwarding Component • Responsible for forwarding incoming packets: • The routing entry is found using a tag, carried by the incoming packet. • A new data-link address and a new tag are swapped into the outgoing packet. • The altered packet is sent over the outgoing interface specified in the routing entry.

  6. 6. Components • Control Component • Responsible for creation of tags. • Responsible for binding between tags and routing entries. • Responsible for distribution of tags to other TSRs.

  7. 7. Destination Based Routing • A typical routing table consist of routing entries that correspond to forwarding equivalence classes (FEC). • The routing table is populated using standard routing protocols like: RIP, OSPF, BGP etc… • Tag switching uses the information obtained by these protocols to construct it’s tag forwarding information base (TFIB)

  8. 8. Destination Based Routing (Tag Distribution) • Three techniques for tag distribution: • Downstream distribution. • Downstream on demand distribution. • Upstream distribution.

  9. 9. Destination Based Routing (Tag Distribution) • Downstream distribution: • An FEC entry is obtained by a routing protocol. • A Tag is allocated for the entry from a pool of free tags. • The FEC entry is placed in the TFIB under the allocated tag. • A <Tag, Address prefix> tupple is sent to each adjacent TSR. • Upon receiving a tupple from the next hop TSR, the TSR updates the TFIB entry.

  10. 10. Destination Based Routing (Tag Distribution) • On demand downstream distribution: • Same as downstream, with the exception that TSR explicitly request for outgoing tag information. • Upstream distribution: • The TSR allocates outgoing tags and receives incoming tag information from other TSRs.

  11. 11. Destination Based Routing (Tag Distribution) • Weak points: • Upstream requires a TFIB per interface. • Upstream will work only with point to point links. • Upstream requires the TSRs to know the tag ranges of each other. • Downstream send more messages then necessary. • Downstream on demand makes the protocol more complex and cannot be piggy-backed.

  12. 12. Destination Based Routing (Tag Distribution) • Tags can be distributed by standard routing messages. • Will work well with BGP (as a BGP attribute). • Not suitable for OSPF (Only adjacent nodes should be updated). • Not suitable for RIP (The protocol software should be upgraded). • Tags can be distributed using a dedicated Tag Distribution Protocol (TDP).

  13. 13. Example

  14. 14. Routing Changes Example

  15. 15. Routing Hierarchies • Interior Routers in transit domains currently have to store enormous routing tables that contain FEC entries to each possible destination. • Storing FEC entries to edge routers should be enough! • Advantages: • Fault Isolation. • Better Performance

  16. 16. Routing Hierarchies • The TFIB of internal TSRs contains only FEC entries that associated with destinations within the domain. • TSERs maintain full routing tables and thus know to what TSERs to route packets that enter the domain. • TSERs can also use tag switching between themselves by stacking tags.

  17. Intra-domain entries Inter-domain entries 17. Example V:5 T:2,10 X:2,12 Y:2,17 W:6

  18. 18. Multicast • Key features: • Allows a source to send a single message to multiple sources. • Structured as a tree. • Hello messages are sent to advertise a multicast tree. • Join messages are sent to join a multicast tree. • Allows efficient use of link - layer protocols with multicast capabilities (Ethernet).

  19. 19. Multicast • Tag Switching related issues: • No two TSRs on a common sub-network may bind the same tag to different multicast trees. • All TSRs in a sub-network belonging to a common multicast tree, should agree on the same tag for the tree.

  20. 20. Multicast • Solution: • The solution is PIM specific. • TFIBs are stored on a per interface basis. • TSRs advertise the range of tags they use for local bindings via PIM Hello messages. • When TSR wants to join a tree it advertise it’s tag for the tree via PIM Join message. • TSRs listen to all join messages over a common sub-network, and thus learn which tags belong to which trees.

  21. 21. Example A B C D E

  22. 22. RSVP • Tags are associated to locally to flows. • The TFIB entry contains resource reservation information. • A new RSVP object, a tag object is defined. • Tags are distributed via RSVP Resv messages in Tag objects.

  23. 23. Explicit Routing • Allows a source router to predefine the route that a class of incoming packets will take. • The route is described as a sequence of entries. • The advertised route is carried in RSVP Path messages. • The tag binding is achieved via RSVP Resv messages as before.

  24. 24. Tag Switching over ATM • Tag switching can be performed using ATM hardware. • The ATM Control plane is replaced with a Tag switching component. • Tag information is carried in the VCI field (And the VPI field if routing hierarchies are supported) of the ATM header. • The same TSR can act both as tag switch and ATM switch by dividing tag ranges.

  25. 25. Tag Switching over ATM • Cell Interleaving Problem: • Packets with same tags are broken into cells. • Cells may interleave at the next hop router. • Packets cannot be reconstructed at destination.

  26. 26. Tag Switching Over ATM • Call Interleave solutions: • VC Merge • Cells are not forwarded until end of packet detected. • Disadvantage - May require Hardware Modifications. • Multiple Tags per Route: • Use Upstream tag distribution. • Disadvantage - Consumes many tags, bad scalability.

  27. 27. Tag Distribution Protocol • Uses Incremental approach. • Protocol Messages: • TDP_PIE_OPEN - TSR say hello to neighbor. • TDP_PIE_BIND - A binding distribution message. • TDP_PIE_WITHDROW_BIND - Deprecation of previously advertised binding. • TDP_PIE_REQUEST_BIND - An explicit request for binding information.

  28. 28. Tag Distribution Protocol • Protocol Messages (Cont.): • TDP_PIE_RELEASE_REQUEST - Release of previously requested binding. TSR may choose to delete a binding after receiving this message. • TDP_PIE_NOTIFICATION - Conveys errors. • TDP_PIE_KEEP_ALIVE - Obvious.

  29. 29. Summary • Based on label swapping. • Supports a big variety of routing functions. • Can be implemented by ATM and Non-ATM hardware. • Enables Routing Hierarchies.

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