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Oct 11, 2022 •330 likes •437 views

Lecture 6: Bridging & Switching CSE 123: Computer Networks Chris Kanich Project 1 countdown: 5 days Last time How do multiple hosts share a single channel? Medium Access Control (MAC) protocols Channel partitioning

  1. Lecture 6: Bridging & Switching CSE 123: Computer Networks Chris Kanich Project 1 countdown: 5 days Last time  How do multiple hosts share a single channel?  Medium Access Control (MAC) protocols  Channel partitioning (FDMA,TDMA,CDMA)  Contention-based protocols (CSMA/CD) CSE 123 -- Lecture 6 – Hubs, Bridges, Switches Today  What if one wire isn’t enough?  Interconnecting different LANs  Hubs/Repeaters: bit-for-bit rebroadcast  Bridges: selective rebroadcast  Switches: multi-port selective rebroadcast CSE 123 -- Lecture 6 – Hubs, Bridges, Switches 1

  2. Limits of a LAN  One shared LAN can limit us in terms of:  Distance » Max Ethernet segment is 2500m  Number of nodes » Max nodes for Ethernet is 1024  Performance nodes (wire)  What to do? CSE 123 -- Lecture 6 – Hubs, Bridges, Switches Hubs/Repeaters  Hubs are multiway repeaters  Physical layer device (layer 1)  One “port” for each LAN (local area network)  Repeat received bits on one port out all other ports  “Amplifies” signal Hub LAN1 LAN2 LAN3 CSE 123 -- Lecture 6 – Hubs, Bridges, Switches Benefits of hubs  Hubs can be arranged into hierarchies to create larger networks  Ethernet rules » Up to four hubs between pair of nodes  Most of LAN continues to operate if “leaf” hub dies  Simple, cheap Leaf hub CSE 123 -- Lecture 6 – Hubs, Bridges, Switches 2

  3. Limitations of the “One Big LAN” approach  Single collision domain  All hosts compete for access to same physical link  No improvement in max throughput  Average throughput decreases as # of nodes increases  Why?  Still limited in distance and # of hosts  Collision detection requirements  Synchronization requirements  Requires performance homogeneity  Can’t connect 10BaseT and 100BaseT networks CSE 123 -- Lecture 6 – Hubs, Bridges, Switches Bridges to the rescue  Data-link layer device (layer 2)  Key difference between bridges and hubs  Bridges buffer entire packet/frame and then rebroadcast it on other ports (“store and forward” device) » Uses CSMA/CD for access to each LAN » Can accommodate different speed interfaces  Creates separate collision domains » Improves throughput  Total bandwidth increased » Single Ethernet segment can carry 10 Mbps » Bridges can support 10n Mbps for n ports CSE 123 -- Lecture 6 – Hubs, Bridges, Switches Bridges 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 CSE 123 -- Lecture 6 – Hubs, Bridges, Switches 3

  4. Selective forwarding optimization  Only rebroadcast a packet to the LAN where its destination resides  If A sends packet to X, then bridge should forward packet  If A sends packet to B, then bridge shouldn’t  Benefits? LAN 2 LAN 1 A W B X bridge C Y D Z CSE 123 -- Lecture 6 – Hubs, Bridges, Switches How to make this work? Need to know “destination” of packet   Destination address in packet header (48bit in Ethernet) Need know which destinations are on which LANs   Could be statically configured by hand  Forwarding table mapping address to output port (i.e. LAN) 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*/ CSE 123 -- Lecture 6 – Hubs, Bridges, Switches “Learning” bridges Eliminate manual configuration and table creation by  “learning” which addresses are on which LANs Host Port Basic approach A 1   Start with empty table B 1  If a packet arrives on a port, then associate its source address with that port C 1  As each host transmits, the table becomes accurate D 1 Tricky problem: moving offices  W 2  Solution: table aging » Associate a timestamp with each table entry X 2 » Refresh timestamp for each new packet with same source Y 2 » If entry is older than x (stale), then delete entry For packets destined to hosts not in table, forward Z 2  CSE 123 -- Lecture 6 – Hubs, Bridges, Switches 4

  5. Bridge learning: example Suppose C sends frame to D and D replies back with frame to C Host Port C 1 C sends frame, bridge has no info about D, so floods to both LANs   bridge notes that C is on port 1  Bridge sends packet out port 2 and port 3  frame ignored on upper LAN  frame received by D CSE 123 -- Lecture 6 – Hubs, Bridges, Switches Bridge learning: example Host Port C 1 D 2 D generates reply to C, sends   bridge sees frame from D  bridge notes that D is on port 2  bridge knows C on port 1, so selectively forwards frame out via port 1 CSE 123 -- Lecture 6 – Hubs, Bridges, Switches Issues w/network architecture  Linear organization  Inter-bridge hubs (e.g. CS) are single points of failure  Unnecessary transit (e.g. EE<->SE must traverse CS)  Backbone/tree  Can survive LAN failure  Manages all inter-LAN communication  Requires more ports (3 vs 2) CSE 123 -- Lecture 6 – Hubs, Bridges, Switches 5

  6. Why aren’t we done? A  Learning works well in B tree topologies B3 C B5  Trees are fragile D B7 B2  Net admins like E K F redundant/backup paths B1  Cycles? G H  Where should B1 forward B6 packets destined for LAN A? B4 I J CSE 123 -- Lecture 6 – Hubs, Bridges, Switches Potential solutions  Don’t allow redundant links (no loops allowed)  Distributed routing protocol (SPF) [future lecture]  Create a temporary “virtual tree” on the physical topology  Spanning Tree algorithm CSE 123 -- Lecture 6 – Hubs, Bridges, Switches Spanning Tree A  Spanning tree uses B subset of bridges so B3 there are no cycles C B5  Prune some ports D B7  Only one tree B2 K E F  Q: How do we find a B1 spanning tree? G H  Automatically B6 B4 I J CSE 123 -- Lecture 6 – Hubs, Bridges, Switches 6

  7. Spanning Tree Algorithm Elect a root node of the tree (lowest address)  Grow tree as shortest distances from the root  (use lowest address to break distance ties)  All bridges send periodic configuration messages over ports for which they are the “best” path  Then turn off ports that aren’t on “best” paths CSE 123 -- Lecture 6 – Hubs, Bridges, Switches Spanning tree details  Each bridge sends periodic configuration messages  (RootID, Distance to Root, BridgeID)  Special multicast address (all bridges on this LAN)  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  Add 1 to distance, send configs where still “best” (designated bridge)  Turn off forwarding on ports except those that send/receive “best” CSE 123 -- Lecture 6 – Hubs, Bridges, Switches Spanning Tree Example Message format:  (RootID, Distance to Root, BridgeID) A B root B3 Sample messages sequences to  and from B3: C B5 B3 sends (B3, 0, B3) to B2 and B5 1. D B7 K B2 root B3 receives (B2, 0, B2) and (B5, 0, 2. B5) and accepts B2 as root (2<3) E F B3 sends (B2, 1, B3) to B5 3. B3 receives (B1, 1, B2) and (B1, 1, 4. root B1 B5) and accepts B1 as root G B3 wants to send (B1, 2, B3 ) but H 5. doesn’t as its nowhere “best” B3 receives (B1, 1, B2) and (B1, 1, B6 6. B4 B5) again … stable I – Data forwarding is turned off to the J LAN A CSE 123 -- Lecture 6 – Hubs, Bridges, Switches 7

  8. Some other tricky details  What if root bridge fails?  Age configuration info » If not refreshed for MaxAge seconds, then delete root and recalculate spanning tree » If config message is received with more recent age, then recalculate spanning tree  Applies to all bridges (not just root)  Temporary loops  When topology changes, takes a bit for new configuration messages to spread through the system  Don’t start forwarding packets immediately -> wait some time for convergence  We send broadcast packets everywhere  Out each “active” port CSE 123 -- Lecture 6 – Hubs, Bridges, Switches So, what’s a switch then? A multi-port bridge   learning + spanning tree protocol Parallel switching between different  ports:  A-to- B and A’ -to- B’ simultaneously Typically   Supports Full-Duplex communication A->B and B->A simultaneously  Connect individual hosts No collisions… doesn’t look anything  like CSMA/CD CSE 123 -- Lecture 6 – Hubs, Bridges, Switches Some switching details  Cut through switching optimization  Only buffer packet header (for output port lookup)  Then forward remaining bits directly  Reduced latency, but may forward bad packets  Backpressure flow control  Input port=1Gbps, output port = 100Mbps  Buffer can only absorb temporary bursts  Send JAM signal on input port when buffer gets too full  Aggregate bandwidth is function of topology & workload  Bridges are a specific kind of switch – called a LAN switch CSE 123 -- Lecture 6 – Hubs, Bridges, Switches 8

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