Advanced Computer Networks: Lecture 14. This lecture will cover the following: moving on from direct to indirect networks; introducing switches which provide indirect connectivity; store-and-forward switches; bridges and extended LANs; cell switching; segmentation and reassembly;...
CS716 Advanced Computer Networks By Dr. Amir Qayyum 1 Lecture No. 14 What we know … • Elements of networks: nodes and links • Building a packet abstraction on a link • Transmission, and units of communication data • Detecting transmission errors • Simulating an errorfree, reliable channel – Sliding window mechanism • Arbitrating access to a shared medium • Design issues of direct link networks – Functionality of network adaptors What Next …? • Moving on from direct to indirect networks • Introducing switches which provide indirect connectivity Switching and Forwarding Outline StoreandForward Switches Bridges and Extended LANs Cell Switching Segmentation and Reassembly Why Switching ? • Motivation: – Why not just one direct link network ? • Basic approach: – How can we extend the direct link abstraction (provide illusion of one physical network) ? • Challenges: – What problems must we address ? Why Switching ? • Examples: – Where are these issues addressed in real networks ? • Details of the switch: – What are the goals in design / how are they addressed ? • Heterogeneity – switching allows multiple physical netwrk After Switching – Are We Done ? • Scale – direct link networks: O(100) hosts – packetswitched networks: O(100,000) hosts – Internet: O(2 year1974) hosts • Beyond the basics – quality of service – congestion and performance analysis – network trends and their importance Connecting Large Networks • Assert: want to use one direct link network • Limitations of directly connected networks: – Limited Scale number of hosts that can be attached • 1024 in Ethernet; only 2 in pointtopoint link – Limited geographical area that can be covered • 2500 m in Ethernet; Pointtopoint links also limited Connecting Large Networks • Alternative: provide illusion of one physical network • Solution: Indirect connectivity by using switches – Packet switches in computer networks control frame flow – Multiple direct link networks, transparent to application 10 Datagram Switching • Managing tables in large, complex networks with dynamically changing topologies is a real challenge for the routing protocol Host E At switch 1: Dest Port#/Interface A 2 B 1 C 3 D 0 E 1 … … Host D Host F Switch Switch 1 Host C Host A Host G Switch Host B 26 Host H Datagram Switching • What happens if the destination is unknown ? • Network discards packet – Possibly notifying the sender (“no route to host”) E C B A Host E C A data Host D data C F F data A K ? ? B K Switch 3 Host C D B Host F Switch 2 data data Host A Host G Switch Host B 27 Host H Datagram Model • No round trip time delay waiting for connection setup – Host can send data anywhere, anytime as soon as it is ready – Source has no way of knowing if the network is capable of delivering a packet or if the destination host is even up • Packets are treated independently – Possible to route around link and node failures dynamically 28 Datagram Model • Every packet carry full address of the destination – Overhead per packet is higher than for the connectionoriented model – Global address to path translation requires storage – Might not be possible to deliver packet (dest unknown) 29 Virtual Circuit Switching • Explicit connection setup (& teardown) phase from source to destination: connectionoriented model – Subsequence packets follow established circuit • Supporting “connections” in network layer may be useful for service notions 30 Virtual Circuit Switching • Each switch maintains a VC table (connection state) perlink or per switch • Analogy – Phone call: each packet follows an established path – Following a known route to reach the 31 destination VC Tables in VC Switching • VC table contains information for each connection – incoming / outgoing interface (port) – incoming / outgoing VCI (virtual circuit identifier) • Permanent (PVC) or switched (signaled) virtual circuit (SVC) 32 VC Tables in VC Switching • Setup message in signaling process (to create VC table) is forwarded like a datagram • Acknowledgment of connection setup to downstream neighbors to complete signaling – Data transfer phase can start after 33 ACK is received Signaling in VC Switching I/F VCI I/F VCI in in out out • Setup message is forwarded from Host A to Host B I/F VCI I/F VCI in in out out 3 9 0 2 5 1 Switch • On connection request, each switch creates an entry in VC table with a VCI for the connection setup 1 Switch setup setup B B B Host A Switch Host B I/F VCI I/F VCI in in out out 2 7 3 34 setup B Signaling in VC Switching • Host B accepts connection from Host A, and sends back an ACK I/F VCI I/F VCI in in out out I/F VCI I/F VCI in in out out 2 5 1 • In ACK, everyone communicates its choice of VCI to its upstream neighbor 9 ACK Switch Switch 1 7 3 9 0 ACK ACK Host A Switch Host B I/F VCI I/F VCI in in out out 2 7 3 4 35 ACK Data Transfer in VC Switching I/F VCI I/F VCI in in out out • Host A knows that everything is in place all the way to Host B I/F VCI I/F VCI in in out out 2 5 1 9 Switch data Switch 1 • In data packets, each node then puts the VCI of its downstream neighbor 3 9 0 7 data data Host A Switch Host B I/F VCI I/F VCI in in out out 2 7 3 4 36 data Virtual Circuit Model • Typically wait full RTT for connection setup before sending first data packet – Can not avoid failures dynamically; must reestablish connection (old one is torn down to free storage space) 37 Virtual Circuit Model • Each data packet contains only a small identifier, making the perpacket header overhead small – Global address to path information still necessary • Connection setup provides an opportunity to know network conditions and/or reserve resources – Avoiding congestion but underutilizing 38 switch Review Lecture 14 • • • • • • • Direct to indirect nets: switches provide ic Motivation, approach, challenges Heterogeneity, scale, QoS, congestion Limits: scale, area, Sol: illusion of 1 net Packet switches: MIMO, star Challenges: forwarding, routing, contention Forwardg: decision process at intersections, selecting appropriate output port 39 Review Lecture 14 • 3 approaches: DG, VC, source • pkt cont enough info, switch translates addr to out port, pkt forwarded independen, no con setup ph, no RTT delay, dest unknown, full addr overhead, route around failure • Con setup, pkt follow establi circuit, switch maintain VC table, signaling (in datagram) + ack, wait 1 RTT for data, not avoid 40 failures, small overhead, reserve resources .. .Lecture? ?No.? ?14 What we know … • Elements of? ?networks:? ?nodes and links • Building a packet abstraction on a link • Transmission, and units of communication ... switching allows multiple physical netwrk After Switching – Are We Done ? • Scale – direct link? ?networks:? ?O(100) hosts – packetswitched? ?networks:? ?O(100,000) hosts – Internet: O(2 year1974) hosts • Beyond the basics... forwards packets from input port to output port – port selected based on address in packet header T3 T3 STS-1 Input ports Switch T3 T3 STS-1 Output ports • Advantages – cover large geographic area (tolerate latency) –