5: DataLink Layer 5a-1 18: Ethernet, Hubs, Bridges, Switches Last Modified: 10/27/2015 1:29:46 PM

5: DataLink Layer 5a-2 Ethernet “dominant” LAN technology: r First widely used LAN technology r Kept up with speed race: 10, 100, 1000 Mbps Metcalfe’s Ethernet sketch

5: DataLink Layer 5a-3 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: r 7 bytes with pattern followed by one byte with pattern r used to synchronize receiver, sender clock rates

5: DataLink Layer 5a-4 Ethernet Frame Structure (more) r Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match r Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk) r CRC: checked at receiver, if error is detected, the frame is simply dropped

Data Link Layer 5-5 Ethernet: Unreliable, connectionless r connectionless: No handshaking between sending and receiving NICs r unreliable: receiving NIC doesn’t send acks or nacks to sending NIC m stream of datagrams passed to network layer can have gaps (missing datagrams) m gaps will be filled if app is using TCP m otherwise, app will see gaps r Ethernet’s MAC protocol: unslotted CSMA/CD

5: DataLink Layer 5a-6 Ethernet: uses CSMA/CD A: sense channel, if idle then { transmit and monitor the channel; If detect another transmission then { abort and send jam signal; update # collisions; delay as required by exponential backoff algorithm; goto A } else {done with the frame; set collisions to zero} } else {wait until ongoing transmission is over and goto A}

5: DataLink Layer 5a-7 Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Exponential Backoff: r Goal: adapt retransmission attempts to estimated current load m heavy load: random wait will be longer r first collision: choose K from {0,1}; delay is K x 512 bit transmission times r after second collision: choose K from {0,1,2,3}… r after ten or more collisions, choose K from {0,1,2,3,4,…,1023}

Data Link Layer 5-8 Manchester encoding r used in 10BaseT r each bit has a transition r allows clocks in sending and receiving nodes to synchronize to each other m no need for a centralized, global clock among nodes! r Hey, this is physical-layer stuff!

5: DataLink Layer 5a-9 Ethernet Technologies: 10Base2 r 10: 10Mbps; 2: under 200 meters max cable length r thin coaxial cable in a bus topology r repeaters used to connect up to multiple segments r repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!

5: DataLink Layer 5a-10 10BaseT and 100BaseT r 10/100 Mbps rate; latter called “fast ethernet” r T stands for Twisted Pair r Hub to which nodes are connected by twisted pair, thus “star topology” r CSMA/CD implemented at hub

5: DataLink Layer 5a-11 10BaseT and 100BaseT (more) r Max distance from node to Hub is 100 meters r Hub can disconnect “jabbering adapter” r Hub can gather monitoring information, statistics for display to LAN administrators

5: DataLink Layer 5a-12 Gbit Ethernet r use standard Ethernet frame format r allows for point-to-point links and shared broadcast channels r in shared mode, CSMA/CD is used; short distances between nodes to be efficient r uses hubs, called here “Buffered Distributors” r Full-Duplex at 1 Gbps for point-to-point links

5: DataLink Layer 5a-13 Repeaters r Physical Layer devices: operating at bit levels: repeat received bits on one interface to all other interfaces r Extend the range of a signal by amplifying r Useful when want to connect devices beyond the IEEE specifications for distance limitation of 328 feet or 100 meters r Examples – outdoor installations, mine shafts, remote locations, etc.

5: DataLink Layer 5a-14 Hubs r Also physical layer device, but may have some management r Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top r Hubs do not isolate collision domains: node may collide with any node residing at any segment in LAN r Hub Advantages: m Simple, inexpensive device m Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions m Extends maximum distance between node pairs (100m per Hub)

Data Link Layer 5-15 Hubs … physical-layer (“dumb”) repeaters: m bits coming in one link go out all other links at same rate m all nodes connected to hub can collide with one another m no frame buffering m no CSMA/CD at hub: host NICs detect collisions twisted pair hub

5: DataLink Layer 5a-16 Hub limitations r Single collision domain results in no increase in max throughput m multi-tier throughput same as single segment throughput m Also less secure – hear traffic from/to everyone on the hub r Individual LAN restrictions pose limits on number of nodes in same collision domain and on total allowed geographical coverage r Difficult to connect different Ethernet types, but can have dual speed hubs (e.g., 10BaseT and 100baseT)

Data Link Layer 5-17 Switch r link-layer device: smarter than hubs, take active role m store, forward Ethernet frames m examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment r transparent m hosts are unaware of presence of switches r plug-and-play, self-learning m switches do not need to be configured

Data Link Layer 5-18 Switch: allows multiple simultaneous transmissions r Switch isolates collision domains m Hosts have dedicated, direct connection to switch m A-to-A’ and B-to-B’ simultaneously, without collisions m not possible with dumb hub m Does not forward out all interfaces m Buffers frames r Ethernet protocol used on each incoming link, but no collisions; full duplex m each link is its own collision domain switch with six interfaces (1,2,3,4,5,6) A A’ B B’ C C’

r Collision domain m When I speak, who else can I prevent from speaking at the same time m Hub = one collision domain; Switch = collision domain per port r Broadcast domain m When I deliberately send a broadcast address, who all hears it m VLANs separate broadcast domains 5: DataLink Layer 5a-19

Managed vs Unmanaged r Switches more likely than hubs or repeaters to have sophisticated management features r Log in remotely and configure, get reports/statistics etc. r More control over what each port or group of ports can do (e.g. establish groups of ports into virtual LANs or VLANs that further divide the broadcast domain) 5: DataLink Layer 5a-20

5: DataLink Layer 5a-21 Switches (more) r Switch advantages: m Isolates collision domains resulting in higher total max throughput and more security m Can connect different type Ethernet since it is a store and forward device ( dual speed hub is compromise between full switch and hub that does this)

5: DataLink Layer 5a-22 Switch: frame filtering, forwarding r Switches filter packets m same-LAN -segment frames not forwarded onto other LAN segments r Forwarding: m how to know which LAN segment on which to forward frame? m looks like a routing problem?

Data Link Layer 5-23 Switch: self-learning r switch learns which hosts can be reached through which interfaces m when frame received, switch “learns” location of sender: incoming LAN segment m records sender/location pair in switch table A A’ B B’ C C’ A A’ Source: A Dest: A’ MAC addr interface TTL Switch table (initially empty) A 1 60

Data Link Layer 5-24 Switch: frame filtering/forwarding When frame received: 1. record link associated with sending host 2. index switch table using MAC dest address 3. if entry found for destination then { if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived

Data Link Layer 5-25 Self-learning, forwarding: example A A’ B B’ C C’ A A’ Source: A Dest: A’ MAC addr interface TTL Switch table (initially empty) A 1 60 A A’ r frame destination unknown: flood A’ A  destination A location known: A’ 4 60 selective send

r Generally on a switch only see traffic to/from your machine and broadcast traffic r Can attack switch by sending many MACs and overflowing its storage of which MACs on which port => will begin to act like hub ( flooding each packet out every port) 5: DataLink Layer 5a-26

Data Link Layer 5-27 Interconnecting switches r switches can be connected together A B  Q: sending from A to G - how does S 1 know to forward frame destined to F via S 4 and S 3 ?  A: self learning! (works exactly the same as in single-switch case!) S1S1 C D E F S2S2 S4S4 S3S3 H I G

Data Link Layer 5-28 Switches vs. Routers r both store-and- forward devices m routers: Layer 3 or network-layer devices (examine network-layer headers) m switches are Layer 2 or link-layer devices (examine link-layer headers) r routers maintain routing tables, implement routing algorithms r switches maintain switch tables, implement filtering, learning algorithms application transport network link physical network link physical link physical switch datagram application transport network link physical frame datagram

5: DataLink Layer 5a-29 Switch Pros and Cons + Switch operation is simpler requiring less processing bandwidth - Topologies are restricted with bridges: a spanning tree must be built to avoid cycles - Switch do not offer protection from broadcast storms (endless broadcasting by a host will be forwarded by a bridge)

5: DataLink Layer 5a-30 Routers Pros and Cons + arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols) + provide firewall protection against broadcast storms - require IP address configuration (not plug and play) - require higher processing bandwidth

5: DataLink Layer 5a-31 Network Diagrams Shared

Sample Icons r Icons for in network diagrams 5: DataLink Layer 5a-32

5: DataLink Layer 5a-33 Summary r Layer 3 Devices (Network Layer) m Router r Layer 2 Devices (Link Layer) m Bridge m Switch r Layer 1 Devices (Physical Layer) m Repeaters m Hubs

5: DataLink Layer 5a-34 Outtakes

Data Link Layer 5-35 Institutional network to external network router IP subnet mail server web server

5: DataLink Layer 5a-36 Switch Learning: example Suppose C sends frame to D and D replies back with frame to C r C sends frame, switch has no info about D, so floods to both LANs m switch notes that C is on port 1 m frame ignored on upper LAN m frame received by D

5: DataLink Layer 5a-37 Switch Learning: example r D generates reply to C, sends m switch sees frame from D m switch notes that D is on interface 2 m switch knows C on interface 1, so selectively forwards frame out via interface 1

5: DataLink Layer 5a-38 Spanning Tree r for increased reliability, desirable to have redundant, alternate paths from source to dest r with multiple simultaneous paths, cycles result - bridges may multiply and forward frame forever r solution: organize bridges in a spanning tree by disabling subset of interfaces Disabled

5: DataLink Layer 5a-39 Spanning Tree Algorithm

r VLAN tagging 5: DataLink Layer 5a-40

5: DataLink Layer 5a-41 Interconnection Without Backbone r Not recommended for two reasons: - single point of failure at Computer Science hub - all traffic between EE and SE must path over CS segment

5: DataLink Layer 5a-42 Backbone Switch

5: DataLink Layer 5a-43 Ethernet Switches r Sophisticated bridges m Switches usually switch in hardware, bridges in software m large number of interfaces r Like bridges, layer 2 (frame) forwarding, filtering using LAN addresses r Can support combinations of shared/dedicated, 10/100/1000 Mbps interfaces

5: DataLink Layer 5a-44 Switching r Switching: A-to-B and A’-to-B’ simultaneously, no collisions r cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame m slight reduction in latency r Store and forward switching: entire frame received before transmission out an output port r Fragment-free switching: compromise, before send out the output port receive enough of the packet to do some error checking (ex. detect and drop partial frames)

5: DataLink Layer 5a-45 Ethernet Limitations Q: Why not just one big Ethernet? r Limited amount of supportable traffic: on single LAN, all stations must share bandwidth r limited length: specifies maximum cable length r large “collision domain” (can collide with many stations)