18: Ethernet, Hubs, Bridges, Switches Last Modified: 4/6/2019 8:57:37 PM 5: DataLink Layer

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

Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 used to synchronize receiver, sender clock rates 5: DataLink Layer

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

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

Ethernet: uses CSMA/CD A: sense channel, if idle then { transmit and monitor the channel; If detect another transmission 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

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

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

Repeaters Physical Layer devices: operating at bit levels: repeat received bits on one interface to all other interfaces Extend the range of a signal by amplifying Useful when want to connect devices beyond the IEEE 802.3 specifications for distance limitation of 328 feet or 100 meters 5: DataLink Layer

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

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

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

Switch link-layer device: smarter than hubs, take active role store, forward Ethernet frames 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 transparent hosts are unaware of presence of switches plug-and-play, self-learning switches do not need to be configured Data Link Layer

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

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

Managed vs Unmanaged Switches more likely than hubs or repeaters to have sophisticated management features Log in remotely and configure, get reports/statistics etc. 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

Switches (more) Switch advantages: Isolates collision domains resulting in higher total max throughput and more security 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

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

Switch: self-learning Source: A Dest: A’ A A A’ switch learns which hosts can be reached through which interfaces when frame received, switch “learns” location of sender: incoming LAN segment records sender/location pair in switch table C’ B 1 2 6 3 4 5 C B’ A’ MAC addr interface TTL A 1 60 Switch table (initially empty) Data Link Layer

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

Self-learning, forwarding: example Source: A Dest: A’ Self-learning, forwarding: example A A A’ C’ B frame destination unknown: 1 2 flood 6 3 A A’ A A’ A A’ A A’ A A’ 4 5 destination A location known: C A’ A selective send B’ A’ MAC addr interface TTL A 1 60 Switch table (initially empty) A’ 4 60 Data Link Layer

Generally on a switch only see traffic to/from your machine and broadcast traffic 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

Interconnecting switches switches can be connected together D E F S2 S4 S3 H I G S1 A B C Q: sending from A to G - how does S1 know to forward frame destined to F via S4 and S3? A: self learning! (works exactly the same as in single-switch case!) Data Link Layer

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

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

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

Summary Layer 3 Devices (Network Layer) Layer 2 Devices (Link Layer) Router Layer 2 Devices (Link Layer) Bridge Switch Layer 1 Devices (Physical Layer) Repeaters Hubs 5: DataLink Layer

Outtakes 5: DataLink Layer

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

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

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

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

Spanning Tree Algorithm 5: DataLink Layer

VLAN tagging 5: DataLink Layer

Interconnection Without Backbone 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

Backbone Switch 5: DataLink Layer

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

Switching Switching: A-to-B and A’-to-B’ simultaneously, no collisions cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame slight reduction in latency Store and forward switching: entire frame received before transmission out an output port 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

Ethernet Limitations Q: Why not just one big Ethernet? Limited amount of supportable traffic: on single LAN, all stations must share bandwidth limited length: 802.3 specifies maximum cable length large “collision domain” (can collide with many stations) 5: DataLink Layer