Lecture 6: Bridging & Switching. Last time. Today. CSE 123: Computer Networks Chris Kanich. How do multiple hosts share a single channel?

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1 Lecture 6: ridging & Switching SE 3: omputer Networks hris Kanich Project countdown: 5 days Last time How do multiple hosts share a single channel? Medium ccess ontrol (M) protocols hannel partitioning (FM,TM,M) ontention-based protocols (SM/) Today What if one wire isn t enough? Interconnecting different LNs Hubs/Repeaters: bit-for-bit rebroadcast ridges: selective rebroadcast Switches: multi-port selective rebroadcast

2 Limits of a LN One shared LN can limit us in terms of: istance» Max Ethernet segment is 500m Number of nodes» Max nodes for Ethernet is 04 Performance nodes (wire) What to do? Hubs/Repeaters Hubs are multiway repeaters Physical layer device (layer ) One port for each LN (local area network) Repeat received bits on one port out all other ports mplifies signal LN Hub LN LN3 enefits of hubs Hubs can be arranged into hierarchies to create larger networks Ethernet rules» Up to four hubs between pair of nodes Most of LN continues to operate if leaf hub dies Simple, cheap Leaf hub

3 Limitations of the One ig LN approach Single collision domain ll hosts compete for access to same physical link No improvement in max throughput verage throughput decreases as # of nodes increases Why? Still limited in distance and # of hosts ollision detection requirements Synchronization requirements Requires performance homogeneity an t connect 0aseT and 00aseT networks ridges to the rescue ata-link layer device (layer ) Key difference between bridges and hubs ridges buffer entire packet/frame and then rebroadcast it on other ports ( store and forward device)» Uses SM/ for access to each LN» an accommodate different speed interfaces reates separate collision domains» Improves throughput Total bandwidth increased» Single Ethernet segment can carry 0 Mbps» ridges can support 0n Mbps for n ports ridges to the rescue New opportunity: selective forwarding Why not with a hub?» Hubs send packets to all hosts connected to it» Hubs have no choice they are at physical link layer and don t know anything about destination addresses 3

4 Selective forwarding optimization Only rebroadcast a packet to the LN where its destination resides If sends packet to X, then bridge should forward packet If sends packet to, then bridge shouldn t enefits? LN LN W bridge X Y Z How to make this work? Need to know destination of packet estination address in packet header (48bit in Ethernet) Need know which destinations are on which LNs ould be statically configured by hand Forwarding table mapping address to output port (i.e. LN) Simple algorithm receive packet p on port q lookup p.dest for output port if p.dest found then if output port is q then drop packet /* already delivered */ else forward the packet on output port; else flood; /* forward on all ports but the one on which the frame arrived*/ Learning bridges Eliminate manual configuration and table creation by learning which addresses are on which LNs asic approach Start with empty table If a packet arrives on a port, then associate its source address with that port s each host transmits, the table becomes accurate Tricky problem: moving offices Solution: table aging» ssociate a timestamp with each table entry» Refresh timestamp for each new packet with same source» If entry is older than x (stale), then delete entry For packets destined to hosts not in table, forward Host W X Y Z Port 4

5 ridge learning: example Suppose sends frame to and replies back with frame to Host Port sends frame, bridge has no info about, so floods to both LNs bridge notes that is on port ridge sends packet out port and port 3 frame ignored on upper LN frame received by ridge learning: example Host Port generates reply to, sends bridge sees frame from bridge notes that is on port bridge knows on port, so selectively forwards frame out via port Issues w/network architecture Linear organization Inter-bridge hubs (e.g. S) are single points of failure Unnecessary transit (e.g. EE<->SE must traverse S) ackbone/tree an survive LN failure Manages all inter-ln communication Requires more ports (3 vs ) 5

6 Why aren t we done? Learning works well in tree topologies 3 5 Trees are fragile Net admins like redundant/backup paths E 7 F K ycles? G H Where should forward packets destined for LN? I 6 4 J Potential solutions on t allow redundant links (no loops allowed) istributed routing protocol (SPF) [future lecture] reate a temporary virtual tree on the physical topology Spanning Tree algorithm Spanning Tree Spanning tree uses subset of bridges so there are no cycles Prune some ports Only one tree K E F Q: How do we find a spanning tree? utomatically G H 6 4 I J 6

7 Spanning Tree lgorithm Elect a root node of the tree (lowest address) Grow tree as shortest distances from the root (use lowest address to break distance ties) ll bridges send periodic configuration messages over ports for which they are the best path Then turn off ports that aren t on best paths Spanning tree details Each bridge sends periodic configuration messages (RootI, istance to Root, ridgei) Special multicast address (all bridges on this LN) Each bridge receives messages, updates best config. Smaller root address is better, then shorter distance To break ties, bridge with smaller address is better Initially, each bridge thinks it is the root Sends configuration messages on all ports Later, bridges send only best configs dd to distance, send configs where still best (designated bridge) Turn off forwarding on ports except those that send/receive best Spanning Tree Example Message format: (RootI, istance to Root, ridgei) Sample messages sequences to and from 3:. 3 sends (3, 0, 3) to and 5. 3 receives (, 0, ) and (5, 0, 5) and accepts as root (<3) 3. 3 sends (,, 3) to receives (,, ) and (,, 5) and accepts as root 5. 3 wants to send (,, 3 ) but doesn t as its nowhere best 6. 3 receives (,, ) and (,, 5) again stable ata forwarding is turned off to the LN E G root I 3 root 6 root K F H J 7

8 Some other tricky details What if root bridge fails? ge configuration info» If not refreshed for Maxge seconds, then delete root and recalculate spanning tree» If config message is received with more recent age, then recalculate spanning tree pplies to all bridges (not just root) Temporary loops When topology changes, takes a bit for new configuration messages to spread through the system on t start forwarding packets immediately -> wait some time for convergence We send broadcast packets everywhere Out each active port So, what s a switch then? multi-port bridge learning + spanning tree protocol Parallel switching between different ports: -to- and -to- simultaneously Typically Supports Full-uplex communication -> and -> simultaneously onnect individual hosts No collisions doesn t look anything like SM/ Some switching details ut through switching optimization Only buffer packet header (for output port lookup) Then forward remaining bits directly Reduced latency, but may forward bad packets ackpressure flow control Input port=gbps, output port = 00Mbps uffer can only absorb temporary bursts Send JM signal on input port when buffer gets too full ggregate bandwidth is function of topology & workload ridges are a specific kind of switch called a LN switch 8

9 VLNs Scaling problem with switches ll LNs in same broadcast domain s # hosts grows, broadcast traffic becomes an issue Virtual LNs (VLNs) created to address this issue Each port optionally configured with a VLN I Inbound packets tagged with this I ll switches will only forward on ports that are part of the same VLN reates independent broadcast domains within a single tree ridge/switch summary reate spanning tree across LNs Learn which ports to use to reach which addresses enefits Higher aggregate throughput (parallel communication) Improved fault tolerance (redundant paths) Limitations Requires homogeneous link layer (e.g. all Ethernet) an t control forwarding topology» ll traffic must traverse root; what if its poorly connected? ottom line: we can scale LNs a lot, but there are real limitations; motivates internetworking & routing For next time Read 4. on Internetworking Make sure you can access ieng6.ucsd.edu for testing and turnin purposes for project 9