CMPE 150 Introduction to Computer Networks Dr Chane L Fullmer chane cse ucsc edu Spring 2003 UCSC cmpe150 1 Mid Term Next Tuesday April 29th Bring Scantron large pink one 30 50 questions Multiple choice Questions come from both text and lectures Some may be similar to homework Closed book closed notes Spring 2003 UCSC cmpe150 2 Homework Assignments Homework assignment 2 Due by May 1 Spring 2003 UCSC cmpe150 3 CMPE 150 Introduction to Computer Networks LECTURE 8 Logical Link Control Part II Internetworking with Bridges Spring 2003 UCSC cmpe150 4 Initializing a Link Operation of initialization protocol leads to up and down periods of the link S and R can have different views of the state of the link S and R should not accept packets from prior incarnations of the link We can have a master slave or peer to peer protocol which consists of two M S protocols Master sends two types of packets INIT and DISC and must stop for ACKs This is the same as the SWP protocol Spring 2003 UCSC cmpe150 5 Initializing a Link initializing S link up INIT disconnecting DISC ACKI R time ACKD link up Spring 2003 link down UCSC link down cmpe150 6 Initialization of ARQ The basic problem is that an initialization protocol between two parties requires feedback The simplest scheme is a link up packet and its ACK used to set up a link followed by a link down packet and its ACK to tear down the link This however is just the SWP which needs to be initialized Ad hoc approach Node starts assuming that link is down and must wait a long time period after coming up before it accepts or sends an INIT packet to the link In e t e protocols e g TCP the connection is assigned an ID randomly Spring 2003 UCSC cmpe150 7 LAN Interconnection below Network Layer Hubs Bridges Switches Section 5 6 of textbook Spring 2003 UCSC cmpe150 8 Interconnecting with hubs Backbone hub interconnects LAN segments Extends maximum distance between nodes Individual segment collision domains become one large collision domain If a node in CS and a node EE transmit at same time collision Cannot interconnect 10BaseT 100BaseT Spring 2003 UCSC cmpe150 9 Internetworking with Bridges Bridges are used to interconnect LANs at the link layer Frame forwarding from one LAN to another is based on the destination s link level address MAC address without making any changes to the frame A MAC address is a name and for a bridge the address of the destination is the adjacent LAN over which the frames to the destination should be forwarded Plug and play self learning bridges do not need to be configured Spring 2003 UCSC cmpe150 10 Traffic Isolation with Bridges Bridge installation breaks LAN into LAN segments Bridges filter packets Same LAN segment frames not usually forwarded onto other LAN segments LAN segments become separate collision domains collision domain collision domain bridge LAN segment hub host LAN segment LAN IP network Spring 2003 UCSC cmpe150 11 Internetworking with Bridges To which LAN segment should the bridge forward a frame A routing problem There are two types of bridges that have been used Transparent Source routing Spring 2003 UCSC cmpe150 12 Transparent Bridges Summary The purpose of transparent bridges is to keep the packet forwarding functionality transparent to the hosts Transparent bridges establish and manage a spanning tree of the network to eliminate packet looping The address of a station is always the LAN over which packets from that station came last this is a dynamic process If no address is known a bridge broadcasts packets for a station over all its ports or those in the spanning tree Spring 2003 UCSC cmpe150 13 Addressing in Transparent Bridges Assume for now that the topology of the internet is a tree Bridge listens to every packet it receives over any LAN It builds a station cache consisting of the source addresses of packets it hears and the IDs of the ports over which the packets were heard For the bridge the address of a station is the port over which packets from the station were received B assigns port x as the address of station S after hearing the packet from S S port x B port y D S D Spring 2003 UCSC cmpe150 14 Routing in Transparent Bridges Consider a tree topology When a bridge receives a packet it looks up its station cache for the destination MAC address in the packet If match is found then If port in the cache is the same port over which packet came the packet is filtered dropped Otherwise the bridge forwards the packet to the port specified in the cache If no matching is found the bridge forwards the packet over all ports other than the port from which the packet came B has station cache entry D port y or B does not know about D and forwards to port y and other ports other than x Spring 2003 UCSC S port x S D B port y D S D cmpe150 15 Looping Problems S D S D S D B S D S D S D S D S D Looping with bridges is REALLY BAD and must be avoided A S C S D S D S D S D S D Packet is replicated three times at each LAN S D S D S D S D S D S D S D The topology of the internet need not be a tree The address learning process is such that packets will traverse loops and worse replicas of such packets will be produced and sent over the same loops S sends a packet to D D is silent and bridges do not know about D S D S D S D S D S D D Spring 2003 UCSC cmpe150 16 Bridges in Mesh Topologies Alternative paths from source to destination LANs are desirable for increased reliability Disabled Spring 2003 UCSC cmpe150 17 Spanning Tree Algorithm STA The objective is to define a single spanning tree in the internet over which packets flow without looping Basis of operation Perlman 1992 part of IEEE standard Elect distributedly a single bridge as the root of the tree Calculate distance in hops on a shortest path to root Elect a designated bridge for each LAN e g closest to the root in the LAN Allow only designated bridge to forward packets to and from its LAN A distributed election process is used to build the spanning tree root Spring 2003 UCSC cmpe150 18 STA Operation Each bridge has multiple MAC addresses one per port A bridge has a bridge wide ID one of the MAC addresses HELLOs messages used to build tree sent to all bridges of a LAN HELLO specifies Root ID The MAC address of the bridge assumed to be the root Transmitting bridge ID MAC address of bridge sending HELLO Cost Length in hops of path from bridge to root A bridge starts by considering itself the proposed root Bridge starts election process by sending HELLO own ID 0 own ID
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