cs / ee 143 Communication Networks Chapter 3 Ethernet

cs/ee 143 Communication NetworksChapter 3 Ethernet Text: Walrand & Parakh, 2010 Steven Low CMS, EE, Caltech

Warning These notes are not self-contained, probably not understandable, unless you also were in the lecture They are supplement to not replacement for class attendance

Agenda Ethernet history/devices Switch Ethernet forwarding table Spanning tree protocol Little’s theorem (informal proof)

Ethernet • Each (layer 2) network • full connectivity: every node can reach every other node • broadcast capable: every node (inc. router) can broadcast to all other nodes • e.g. Ethernet, WiFi, cable network, etc.

Aloha network (1970) • Randomized multiple access • Send on frequency f1; receive ack on frquency f2. • If no ack after timeout, wait a random time and re-transmit

Aloha network (1970) • Randomized multiple access • If an ack is not outstanding, transmit immediately • If no ack, re-transmit after a random delay

Aloha network (1970) Randomized multiple access Max utilization (prob of success) ~ 1/e ~ 37%

Slotted Aloha utilization • Model • Slotted time, fixed packet size, n stations • 1 slot = 1 pkt transmission time • In each slot, each station transmits • independently with probability p • Prob (slot t has a successful transmission)

Slotted Aloha utilization

Unslotted Aloha utilization • Model • Fixed packet size, n stations • Slotted time of duration t << 1. • pkt transmission time = 1/t • In each t-slot, each station transmits • independently with probability p • Prob (slot t has a successful transmission)

Unslotted ALOHA utilization Prob (a pkt transmission started in an arbitrary t-slot by station 1 is successful)

Unslotted ALOHA utilization

Ethernet cable (1973-76) CSMA/CD (carrier sensing multiple access/collision detection) Wait till channel is idle; wait for a random time. Transmit when the channel is idle following the random wait. Abort if collision is detected, and goto 1.

Ethernet cable (1973-76) Truncated binary exponential backoff Pick X uniformly at random from {0, 1, ..., 2^n-1}, n = min (10, m), m = #collisions. Give up & declare error when m = 16. Wait X x 512 bit times (4,096 bits for 1G) If collide, increment m and repeat.

Ethernet cable (1973-76) • Capture or winner-takes-all effect • A station that collides is more likely to pick a larger random backoff time. • A station that successfully transmits is more likely to pick a smaller backoff time and hence more likely to successfully transmit again

Ethernet hub (1980s) CSMA/CD as in Ethernet cable

Ethernet hub (1980s) • A station waits a random multiples of T = 2 PROP before transmitting • When n stations transmit independently with prob p, then prob of success is <= 1/e when n is large • Hence avg time till first success = e T • Utilization = TRANS / (TRANS + (e-1)T) = 1 / (1 + 3.4A), A = PROP/TRAN

Ethernet switch Ethernet switch eliminates collision, provided switch buffer is big enough.

Ethernet switch: forwarding table (Ethernet) MAC address 48 bit Globally unique to MAC device, location independent (c.f. IP) Broadcast address: 48 ones

Ethernet switch: forwarding table x  y: [ y | x | data ]

Ethernet switch routing: STP • Goal • Operation • Example • Performance x  y: [ y | x | data ]

Spanning tree protocol Goal: for all switches in a LAN to compute a shortest-path tree • used to route layer-2 packets • one tree for entire LAN • rooted at the switch with the smallest ID (MAC address) • decentralized, asynchronous, robust computation

Spanning tree protocol Three criteria • The root switch always forwards messages on all its ports • Each switch computes its shortest path (in #bridges) to root • All switches connected to a LAN elect a designatedswitch for the LAN to send packets towards root switch • A switch that is not elected for any of the LANs it is connected to will not receive nor forward any data packet

Spanning tree protocol • Switches send packets asynchronously with [ my ID | current root ID | distance to root ] • A switch relays packets whose “current root ID” is the smallest it has seen so far (& smaller than its own “current root ID”), and adds 1 to “distance to root” • If the “distances to root” on STP packets received by a switch on all its ports are the same or smaller than what it believes its distance is, then the switch stops forwarding • … until protocol converges Completely decentralized, asynchronous, robust

STP: example I’m 3 I think root is 3 my distance to root is 0

a new initiation before previous converges STP: example

STP: example During transient, B5 may connect to root B1 either via B3or B4 – which should B5 use? Ans: use switch with a smaller ID (B3)

Spanning tree for all switches x  y: [ y | x | data ]

STP: designated switches • B4 believes its distance to root B1 is 2 • The STP packets from both its ports have distances equal or less. So it does not forward and is not a designated switch for neither LAN • Neither B4 nor B5 will be involved in forwarding data packets

Spanning tree protocol Performance • Unique path between every source-destination path • Can potentially be bad since 2 switches may communicate only via root • e.g. in a ring of switches, the switch with the largest ID communicates with root via the longest path • Penalty is usually not too bad since it is in a LAN

Little’s law

Queueing system random arrival process with rate random service time with average • Little’s law • Verifies directly for M/M/1, but holds much moregenerally • Extremely useful because of its generality

cs / ee 143 Communication Networks Chapter 3 Ethernet