复旦大学：《计算机网络 Computer Networking》课程电子教案（PPT课件讲稿）15 Virtual Circuits, ATM, MPLS

Lecture 8 Virtual circuits ATM mPls Dejian Ye Software school Fudan University

1 Lecture 8 Virtual Circuits, ATM, MPLS Dejian Ye Software School Fudan University

Outline Exam discussion Layering review(bridges, routers, etc. 》 Exam section c Circuit switching refresher Virtual circuits- general Why virtual circuits? > How virtual circuits?- tag switching! Two modern implementations >)ATM-teleco-style virtual circuits >)MPLS-IP-style virtual circuits 2

2 Outline Exam discussion Layering review (bridges, routers, etc.) » Exam section C. Circuit switching refresher Virtual Circuits - general » Why virtual circuits? » How virtual circuits? -- tag switching! Two modern implementations » ATM - teleco-style virtual circuits » MPLS - IP-style virtual circuits

Exam stats Maxavg/min 90/63/20 B 19.6 17.9 12.8 11.0 57.6% 74.8% 58.3% 68.6% 876 321 3

3 Exam stats Max/avg/min: 90 / 63 / 20 A B C D 19.6 17.9 12.8 11.0 57.6% 74.8% 58.3% 68.6%

Common Exam Problems Routing: No one big problem; many small misunderstandings. Please check your scores Short answer: Many incorrect round-trip times vs one-way times DNS >)Always sends the full query!(e. g ra1 streaming. npr. org", not just"npr. org") Clients dont recurse: the local recursive dNs server does. Could run on clients, but usually doesnt. Routing and bridging and addressing

4 Common Exam Problems Routing: No one big problem; many small misunderstandings. Please check your scores. Short answer: Many incorrect round-trip times vs. one-way times. DNS » Always sends the full query! (e.g. “ra1.streaming.npr.org”, not just “npr.org”) » Clients don’t recurse; the local recursive DNS server does. Could run on clients, but usually doesn’t. Routing and bridging and addressing…

Packet Switching Source sends information as self-contained packets that have an address )Source may have to break up single message in multiple Each packet travels independently to the destination host >) Routers and switches use the address in the packet to determine how to forward the packets Destination recreates the message Analogy: a letter in surface mail 5

5 Packet Switching Source sends information as self-contained packets that have an address. » Source may have to break up single message in multiple Each packet travels independently to the destination host. » Routers and switches use the address in the packet to determine how to forward the packets Destination recreates the message. Analogy: a letter in surface mail

Circuit Switching Source first establishes a connection (circuit) to the destination )) Each router or switch along the way may reserve some bandwidth for the data flow Source sends the data over the circuit ) No need to include the destination address with the data since the routers know the path The connection is torn down Example: telephone network. 6

6 Circuit Switching Source first establishes a connection (circuit) to the destination. » Each router or switch along the way may reserve some bandwidth for the data flow Source sends the data over the circuit. » No need to include the destination address with the data since the routers know the path The connection is torn down. Example: telephone network

Circuit Switching Discussion Traditional circuits: on each hop, the circuit has a dedicated wire or slice of bandwidth Physical connection clearly no need to include addresses with the data Advantages, relative to packet switching: >)Implies guaranteed bandwidth, predictable performance >)Simple switch design: only remembers connection formation, no longest-prefix destination address look up Disadvantages >)Inefficient for bursty traffic(wastes bandwidth) >)Delay associated with establishing a circuit Can we get the advantages without (all)the disadvantages?

7 Circuit Switching Discussion Traditional circuits: on each hop, the circuit has a dedicated wire or slice of bandwidth. » Physical connection - clearly no need to include addresses with the data Advantages, relative to packet switching: » Implies guaranteed bandwidth, predictable performance » Simple switch design: only remembers connection information, no longest-prefix destination address look up Disadvantages: » Inefficient for bursty traffic (wastes bandwidth) » Delay associated with establishing a circuit Can we get the advantages without (all) the disadvantages?

Virtual circuits Each wire carries many" virtual'circuits >)Forwarding based on virtual circuit (vc)identifier iP header: src, dst, etc Virtual circuit header: just Vc >)A path through the network is determined for each vc when the VC is established >Use statistical multiplexing for efficiency Can support wide range of quality of service ))No guarantees: best effort serv ice 》 Weak guarantees: delay<300msec,… Strong guarantees: e.g. equivalent of physical circuit 8

8 Virtual Circuits Each wire carries many “virtual” circuits. » Forwarding based on virtual circuit (VC) identifier – IP header: src, dst, etc. – Virtual circuit header: just “VC” » A path through the network is determined for each VC when the VC is established » Use statistical multiplexing for efficiency Can support wide range of quality of service. » No guarantees: best effort service » Weak guarantees: delay < 300 msec, … » Strong guarantees: e.g. equivalent of physical circuit

Packet Switching and Virtual circuits: similarities ) Address is either the destination address or a vc identifier Must have buffer space to tem porarily store packets >)E.g. multiple packets for some destination arrive simultaneously Multiplexing on a link is similar to time sharing > No reservations: multiplexing is statistical, i.e. packets are interleaved without a fixed pattern Reservations: some flows are guaranteed to get a certain number of“ slots” DBCIBAA 9

9 Packet Switching and Virtual Circuits: Similarities “Store and forward” communication based on an address. » Address is either the destination address or a VC identifier Must have buffer space to temporarily store packets. » E.g. multiple packets for some destination arrive simultaneously Multiplexing on a link is similar to time sharing. » No reservations: multiplexing is statistical, i.e. packets are interleaved without a fixed pattern » Reservations: some flows are guaranteed to get a certain number of “slots” D B C B A A

Virtual circuits versus Packet Switching Circuit switching: > Uses short connection identifiers to forward packets > Switches know about the connections so they can more easily implement features such as quality of service >) Virtual circuits form basis for traffic engineering: Vc identifies long-lived stream of data that can be scheduled Packet switching >)Use full destination addresses for forwarding packets >) Can send data right away no need to establish a connection first Switches are stateless: easier to recover from failures 》 Adding Qos is hard > Traffic engineering is hard: too many packets 10

10 Virtual Circuits Versus Packet Switching Circuit switching: » Uses short connection identifiers to forward packets » Switches know about the connections so they can more easily implement features such as quality of service » Virtual circuits form basis for traffic engineering: VC identifies long-lived stream of data that can be scheduled Packet switching: » Use full destination addresses for forwarding packets » Can send data right away: no need to establish a connection first » Switches are stateless: easier to recover from failures » Adding QoS is hard » Traffic engineering is hard: too many packets!