MAC Addresses and ARP. Mac Addressing, Ethernet, and Interconnections. LAN Addresses and ARP. LAN Address (more) ARP protocol: Same LAN (network)

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1 M ddresses and RP Mac ddressing, thernet, and Interconnections 32-bit IP address: network-layer address used to get datagram to destination IP subnet M (or LN or physical or thernet) address: used to get datagram from one interface to another physically-connected interface (same network) 48 bit M address (for most LNs) burned in the adapter ROM 5: DataLink Layer 5-5: DataLink Layer 5-2 LN ddresses and RP ach adapter on LN has unique LN address 7-65-F LN (wired or wireless) -2F D D7-F F-3-98 roadcast address = FF-FF-FF-FF-FF-FF = adapter 5: DataLink Layer 5-3 LN ddress (more) M address allocation administered by I manufacturer buys portion of M address space (to assure uniqueness) nalogy: (a) M address: like Social Security Number (b) IP address: like postal address M flat address portability can move LN card from one LN to another IP hierarchical address NOT portable depends on IP subnet to which node is attached 5: DataLink Layer 5-4 RP: ddress Resolution Protocol RP protocol: Same LN (network) Question: how to determine M address of knowing s IP address? F LN F D D7-F F-3-98 ach IP node (Host, Router) on LN has RP table RP Table: IP/M address mappings for some LN nodes < IP address; M address; TTL> TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) 5: DataLink Layer 5-5 wants to send datagram to, and s M address not in s RP table. broadcasts RP query packet, containing 's IP address Dest M address = FF- FF-FF-FF-FF-FF all machines on LN receive RP query receives RP packet, replies to with its ('s) M address frame sent to s M address (unicast) caches (saves) IP-to-M address pair in its RP table until information becomes old (times out) soft state: information that times out (goes away) unless refreshed RP is plug-and-play : nodes create their RP tables without intervention from net administrator 5: DataLink Layer 5-6

2 Routing to another LN walkthrough: send datagram from to via R assume know s IP address creates datagram with source, destination uses RP to get R s M address for...0 creates link-layer frame with R's M address as dest, frame contains -to- IP datagram s adapter sends frame R s adapter receives frame R removes IP datagram from thernet frame, sees its destined to R uses RP to get s M address R creates frame containing -to- IP datagram sends to R Two RP tables in router R, one for each IP network (LN) In routing table at source Host, find router...0 In RP table at source, find M address , etc 5: DataLink Layer 5-7 R 5: DataLink Layer 5-8 thernet Star topology dominant wired LN technology: cheap $20 for 00Mbs! first widely used LN technology Simpler, cheaper than token LNs and TM Kept up with speed race: 0 Mbps 0 bps us topology popular through mid 90s Now star topology prevails onnection choices: or (more later) Metcalfe s thernet sketch or 5: DataLink Layer 5-9 5: DataLink Layer 5-0 thernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in thernet frame Preamble: 7 bytes with pattern 0000 followed by one byte with pattern 000 used to synchronize receiver, sender clock rates thernet Frame Structure (more) ddresses: 6 bytes if adapter receives frame with matching destination address, or with broadcast address (eg RP packet), it passes data in frame to net-layer protocol otherwise, adapter discards frame Type: indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and ppletalk) R: checked at receiver, if error is detected, the frame is simply dropped 5: DataLink Layer 5-5: DataLink Layer 5-2 2

3 Manchester encoding Unreliable, connectionless service Used in 0aseT ach bit has a transition llows clocks in sending and receiving nodes to synchronize to each other no need for a centralized, global clock among nodes! onnectionless: No handshaking between sending and receiving adapter. Unreliable: receiving adapter doesn t send acks or nacks to sending adapter stream of datagrams passed to network layer can have gaps gaps will be filled if app is using TP otherwise, app will see the gaps Hey, this is physical-layer stuff! 5: DataLink Layer 5-4 5: DataLink Layer 5-3 thernet uses SM/D thernet SM/D algorithm No slots adapter doesn t transmit if it senses that some other adapter is transmitting, that is, carrier sense transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection efore attempting a retransmission, adapter waits a random time, that is, random access 5: DataLink Layer 5-5. daptor receives datagram from net layer & creates frame 2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits 3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame! 4. If adapter detects another transmission while transmitting, aborts and sends jam signal 5. fter aborting, adapter enters exponential backoff: after the mth collision, adapter chooses a K at random from {0,,2,,2 m -}. dapter waits K 52 bit times and returns to Step 2 5: DataLink Layer 5-6 thernet s SM/D (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits it time:. microsec for 0 Mbps thernet ; for K=023, wait time is about 50 msec See/interact with Java applet on WL Web site: highly recommended! xponential ackoff: oal: adapt retransmission attempts to estimated current load heavy load: random wait will be longer first collision: choose K from {0,}; delay is K 52 bit transmission times after second collision: choose K from {0,,2,3} after ten collisions, choose K from {0,,2,3,4,,023} 5: DataLink Layer 5-7 0aseT and 00aseT 0/00 Mbps rate; latter called fast ethernet T stands for Twisted Pair Nodes connect to a : star topology ; 00 m max distance between nodes and twisted pair 5: DataLink Layer 5-8 3

4 Hubs Hubs are essentially physical-layer repeaters: bits coming from one link go out all other links at the same rate no frame buffering no SM/D at : adapters detect collisions provides net management functionality twisted pair bit thernet uses standard thernet frame format allows for point-to-point links and shared broadcast channels in shared mode, SM/D is used; short distances between nodes required for efficiency uses s, called here uffered Distributors Full-Duplex at bps for point-to-point links 0 bps now! 5: DataLink Layer 5-9 5: DataLink Layer 5-20 Interconnecting with s ackbone interconnects LN segments xtends max distance between nodes ut individual segment collision domains become one large collision domain an t interconnect 0aseT & 00aseT Switch Link layer device stores and forwards thernet frames examines frame header and selectively forwards frame based on M dest address when frame is to be forwarded on segment, uses SM/D to access segment transparent hosts are unaware of presence of es plug-and-play, self-learning es do not need to be configured 5: DataLink Layer 5-2 5: DataLink Layer 5-22 Forwarding hu b 2 3 How do determine onto which LN segment to forward frame? Looks like a routing problem... 5: DataLink Layer 5-23 Self learning has a table entry in table: (M ddress, Interface, Time Stamp) stale entries in table dropped (TTL can be 60 min) learns which hosts can be reached through which interfaces when frame received, learns location of sender: incoming LN segment records sender/location pair in table 5: DataLink Layer

5 Filtering/Forwarding When receives a frame: Switch example Suppose sends frame to D index table using M dest address 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 5: DataLink Layer 5-25 Switch receives frame from from notes in bridge table that is on interface because D is not in table, forwards frame into interfaces 2 and 3 frame received by D D 2 3 F H address I interface 2 3 5: DataLink Layer 5-26 Switch example Suppose D replies back with frame to. Switch receives frame from from D notes in bridge table that D is on interface 2 because is in table, forwards frame only to interface frame received by D F H address I interface 2 3 5: DataLink Layer 5-27 Switch: traffic isolation installation breaks subnet into LN segments filters packets: same-ln-segment frames not usually forwarded onto other LN segments segments become separate collision domains collision domain collision domain collision domain 5: DataLink Layer 5-28 Switches: dedicated access Switch with many interfaces Hosts have direct connection to No collisions; full duplex Switching: -to- and -to- simultaneously, no collisions More on Switches cut-through ing: frame forwarded from input to output port without first collecting entire frame slight reduction in latency combinations of shared/dedicated, 0/00/000 Mbps interfaces 5: DataLink Layer : DataLink Layer

6 Institutional network Switches vs. Routers both store-and-forward devices to external network router mail server web server routers: network layer devices (examine network layer headers) es are link layer devices routers maintain routing tables, implement routing algorithms es maintain tables, implement filtering, learning algorithms IP subnet hu b 5: DataLink Layer 5-3 Switch 5: DataLink Layer 5-32 Summary comparison traffic isolation s routers es no yes yes plug & play yes no yes optimal routing cut through no yes no yes no yes 5: DataLink Layer