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Efficient Flow Identification and Virtual Circuit Switching for Quality of Service Guarantee

This article delves into the mechanisms of flow identification essential for guaranteeing Quality of Service (QoS) in networking, particularly in managing minimum bandwidth and maximum delay for users. It explores methods to differentiate packet flows using source and destination addresses, the implementation of search trees, hash tables for rapid access to packet queues, and the utilization of virtual circuits (VCs) with unique identifiers. Through these techniques, better throughput and QoS can be achieved compared to datagram switching methods.

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Efficient Flow Identification and Virtual Circuit Switching for Quality of Service Guarantee

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  1. Flow Identification • Assume you want to guarantee some type of quality of service (minimum bandwidth, maximum end-to-end delay) to a user • Before you do that, you have to distinguish the packets of one “user” from another “user”, since they have different requirements (bandwidth, delay) • How to do this distinction? • We will call a flow to be the sequence of packets generated by one user in one application.

  2. Flow Identification Continued… • How to separate packets of one user and another? • Perhaps look at addresses: • Each packet has a source and destination address • Separate packets according to (source,destination), i.e., one packet queue per pair • How to quickly do this separation? • I.e., if a packet is received, to which queue I should add it?

  3. Flow Identification, search tree • How about a search tree, indexed by (S,D) • Searching takes multiple steps (O log N, N is the number of queues), too slow

  4. Flow Identification – (S,D) table • We need fast access to the queues • How about a table, (big array) indexed by (S,D)? • It would require 264 entries! 264

  5. Hash Table (key: (S,D) ) Flow Identification – hash table • We need a fast but small table • Perhaps a hash table (say, 1000 entries) • Hash(S,D) gives you the “bucket” or table entry where the queue of the packet is • How to handle collisions? • You have to revert to search trees for each bucket • Slow!

  6. Flow Identification – flow label • We still need a fast but small table • Say, 1000 entries • Each flow can have a label that is used as an index into that table. • Note that there are more than 1000 flows in the world • But a single router may be involved in only about 1000 flows at a time • Hence, flow labels could be local to the router • These are known as “virtual circuits” • BTW, they are not used in the Internet (in general)

  7. Virtual Circuit Switching • Also referred as connection-oriented model. • Explicit connection setup (and tear-down) phase • All packets follow same route as the setup message route. • Resources may be reserved along this path • The telephone network performs something similar for voice calls

  8. Virtual Circuit ID’s • Each virtual circuit (VC) (i.e., flow) has a unique virtual circuit identifier (VID) in each switch of its path • It is determined locally at the switch and is chosen independently of other values along the path. • No two VC’s on the same link (port, interface) can use the same VID • Note: VID's are reused periodically, since they are bounded, VID : 0 .. v-1 for some constant v Switch 1 Switch 2 vid = 10 vid = 2 vid = 29 Host B Two circuits: one from E to F another from A to B Host A vid = 11 vid = 100 vid = 9 Host E Host F

  9. Virtual Circuit Table • Each switch has a VC table • Table has an entry for each virtual circuit going through it • Contains (at least): • Input interface (port number, link number, or whatever you want to call it) • Input VID • Output interface • Output VID vid = 10 vid = 2 Output Port 2 vid = 11 Input Port 0 • Packets carry the VID in their header • The pair (port, VID) can be used as an index into the table • The table has v*n rows, n = # ports Input Port 1 vid = 9

  10. Virtual Circuit Switching (steps) • Virtual Circuit (VC) set-up or signaling phase: • Source sends setup() message towards destination. • Note that forwarding tables are necessary for the setup message, just like in datagram routing !!!! • Destination replies back with ack() . • All the switches along the path between source and destination create an entry in their virtual circuit table • Data Transfer Phase: • Switches use Virtual Circuit Tables to forward data.

  11. VC: Setup or signaling Phase Host E 0 Switch 1 Host F 3 1 Switch 2 2 Host C 2 3 1 0 Host A Switch 1 VC Table Switch 2 VC Table 0 Switch 3 Host B Host G 1 3 2 Switch 3 VC Table Host H Setup( ) message Ack() Message

  12. VC: Data Transfer Phase Host E 0 Switch 1 Host F 3 1 Switch 2 2 Host C 2 3 1 0 Host A Switch 1 VC Table Switch 2 VC Table 0 Switch 3 Host B Host G 1 3 2 Switch 3 VC Table Host H Data( ) message

  13. VC Vs Datagram switching

  14. Quality of Service Summary • Easier to guaranteed throughput to each connection in VC’s than using datagrams • E.g., serve each VC on an output line using round-robin (or weighted round robin) • In datagram, there is no way to determine which “connection” a datagram belongs to • In datagram, there is no mechanism to inform the switch of the QoS desired.

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