ELEN Lecture 13 LAN Bridges Routers Switches Gateways Network layer IP Reading 6 7 8 1 8 3 1 Switches A multi input multi output device that transfers packets from one input to an output The number of ports on a switch determine the number of hosts that can be connected Larger of hosts can be interconnected by interconnecting switches Point to point links can be used to extend the geographic reach Adding a new host does not limit or reduce the performance of other hosts 2 Scalable Networks Switch forwards packets from input port to output port port selected based on address in packet header T3 T3 Switch T3 T3 STS 1 STS 1 Input Output ports ports Advantages cover large geographic area tolerate latency support large numbers of hosts scalable bandwidth 3 Virtual Circuit Switching Explicit connection setup and tear down phase Subsequence packets follow same circuit Sometimes called connection oriented model 0 Switch 1 1 3 2 5 Analogy phone call 3 11 2 Switch 2 1 0 Host A Each switch maintains a VC table 7 1 0 Switch 3 3 2 4 Host B 4 Datagram Switching No connection setup phase Each packet forwarded independently Sometimes called connectionless model Host D Analogy postal system Each switch maintains a forwarding routing table 3 Host C Host E 0 Switch 1 1 2 Host F 3 2 Switch 2 1 0 Host A Host G 1 0 Switch 3 Host B 3 2 Host H 5 Virtual Circuit Model Typically wait full RTT for connection setup before sending first data packet While the connection request contains the full address for destination each data packet contains only a small identifier making the perpacket header overhead small If a switch or a link in a connection fails the connection is broken and a new one needs to be established Connection setup provides an opportunity to reserve resources 6 Datagram Model There is no round trip time delay waiting for connection setup a host can send data as soon as it is ready Source host has no way of knowing if the network is capable of delivering a packet or if the destination host is even up Since packets are treated independently it is possible to route around link and node failures Since every packet must carry the full address of the destination the overhead per packet is higher than for the connection oriented model 7 Bridges switches routers gateways Devices used to interconnect multiple networks Bridge device interconnecting two or more networks at MAC layer Router device interconnecting two or more networks at the network layer Gateway device interconnecting two or more networks at a higher layer 8 A Bridged LAN S1 S2 S3 LAN1 Bridge LAN2 S4 S5 S6 9 Interconnection by a Bridge Network Network Bridge LLC LLC MAC MAC MAC MAC Physical Physical Physical Physical 10 Example Configuration S1 S2 S3 LAN1 LAN2 port 1 B1 Address port 2 Port S5 S4 LAN3 port 1 B2 Address port 2 Port 11 S1 sends a frame to S5 S1 S1 S2 S3 S5 S4 S5 LAN1 LAN2 B1 port 1 Address S1 port 2 Port 1 LAN3 B2 port 1 Address S1 port 2 Port 1 12 S3 sends a frame to S2 S1 S2 S3 S3 LAN1 S2 LAN2 B1 port 1 port 2 S5 S4 LAN3 B2 port 1 port 2 Address Port Address Port S1 S3 1 2 S1 S3 1 1 13 S4 Sends a frame to S3 S1 S2 S3 S5 S4 S4 LAN1 LAN2 B1 port 1 Address S1 S3 S4 port 2 Port 1 2 2 LAN3 B2 port 1 port 2 Address Port S1 S3 S4 1 1 2 14 S3 S2 sends a frame to S1 S1 S2 S3 S2 S5 S4 S1 LAN1 LAN2 port 1 Bridge1 port 2 LAN3 port 1 Bridge 2 port 2 Address Port Address Port S1 S3 S4 S2 1 2 2 1 S1 S3 S4 1 1 2 15 Sample Topology LAN1 1 1 B1 B2 2 LAN2 1 2 B3 3 2 1 B4 2 LAN3 1 B5 2 LAN4 16 Spanning Tree LAN1 D R 1 1 B1 B2 2 D LAN2 R 1 2 B3 3 D D 2 R 1 B4 2 LAN3 R 1 B5 2 LAN4 17 Frame Format for Source Routing Routing Control 2 bytes Destination Address Route 1 Designator Route 2 Designator Route m Designator 2 bytes 2 bytes 2 bytes Source Address Routing Information Data FCS 18 LAN interconnection with source routing bridges S2 S1 B1 LAN 1 LAN 2 B4 LAN 4 B3 B5 B7 LAN 3 B6 LAN 5 B2 S3 19 Routes followed by single route broadcast frames LAN1 B3 LAN3 B4 LAN4 B6 LAN5 B1 20 Routes of all routes broadcast frames B6 LAN3 B2 LAN1 B3 LAN2 B5 LAN4 LAN5 B1 LAN2 B1 LAN1 B4 LAN4 B4 LAN2 B4 B7 LAN4 B5 B7 B2 B5 B7 B1 LAN1 B2 LAN1 B2 LAN3 B2 B5 B6 B1 LAN1 B3 B5 B6 B1 LAN2 LAN4 B5 B4 B3 B7 B1 B3 LAN3 B3 LAN3 B2 LAN1 B3 LAN2 B6 B1 B4 B3 B4 B2 LAN2 LAN1 21 IP Internet Concatenation of Networks Network 1 Ethernet H7 H2 H1 R3 H8 H3 Network 4 point to point Network 2 Ethernet R1 R2 Protocol Stack H4 Network 3 FDDI H5 H6 H1 H8 TCP R1 IP IP ETH R2 ETH R3 IP FDDI FDDI IP PPP PPP TCP IP ETH ETH 22 Service Model Connectionless datagram based Best effort delivery unreliable service packets are lost packets are delivered out of order duplicate copies of a packet are delivered packets can be delayed for a long time Datagram format 0 4 8 16 Version HLen TOS 31 Length Ident TTL 19 Flags Protocol Offset Checksum SourceAddr DestinationAddr Options variable Pad variable Data 23 Fragmentation and Reassembly Each network has some MTU Strategy fragment when necessary MTU Datagram try to avoid fragmentation at source host re fragmentation is possible fragments are self contained datagrams use CS PDU not cells for ATM delay reassembly until destination host do not recover from lost fragments 24 Example Start of header Ident x 0 Offset 0 Rest of header 1400 data bytes Start of header H1 R1 R2 R3 H8 Ident x 1 Offset 0 Rest of header 512 data bytes Start of header ETH IP 1400 FDDI IP 1400 PPP IP 512 ETH IP 512 PPP IP 512 ETH IP 512 Rest of header PPP IP 376 ETH IP 376 512 data bytes Ident x 1 Offset 512 Start of header Ident x 0 Offset 1024 Rest of header 376 data bytes 25 Global Addresses Properties globally unique hierarchical network host Dot Notation 10 3 2 4 128 96 33 81 192 12 69 77 A B C 0 7 24 Network Host 1 0 1 1 0 14 16 Network Host 21 8 Network Host 26 Datagram Forwarding Strategy every datagram contains destination s address if directly connected to destination network then forward to host if not directly connected to destination network then forward to some router forwarding table maps network number into next hop …