Announcements. Late homework policy

Transcription

1 Announcements Late homework policy n Updated on course website n Up to 1 HW can be late for up to 5 days without penalty n After that, late HW accepted and graded with discount of 10%/day for up to 5 days n Late HW will not be accepted after 5 days from deadline

2 Announcements Projects n Details on the course website (will be updated) n Form project team by Fri Oct 12 n If your team wants to work on EV projects, talk to Zach Lee or me ASAP o Let us know your backgroun, esp software experience n Schedule weekly meetings with TA by Fri Oct 19 o Meetings start week 4 (10% of course grade) n Project tutorials: Fri Oct 19 class o There will be 2 tutorials n Network simulator: ANB 105 n ACN simulator: ANB 213

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

4 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

5 Agenda Broadcast Ethernet n History and performance Switched Ethernet n Spanning tree protocol

6 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. A key issue: how to share common medium, efficiently & fairly?

7 Ethernet Medium Access Control (MAC) protocols

8 Aloha network (1970) Randomized multiple access Send on frequency f1; receive ack on frequency f2.

9 Aloha network (1970) Randomized multiple access If an ack is not outstanding, transmit immediately If no ack after timeout, wait a random time and re-transmit

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

11 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) R(p) = np( 1 p) n 1

12 Slotted Aloha utilization max p R(p) = np( 1 p) n 1 0 = R'(p * ) = n( 1 p * ) n 1 n(n 1)p * ( 1 p * ) n 2 ( 1 p * ) = (n 1)p * p * = 1 n max utilization = R(p * ) " = $ 1 1 # n % ' & n 1 1 e as n

13 Slotted Aloha utilization max p R(p) = np( 1 p) n 1 0 = R'(p * ) = n( 1 p * ) n 1 n(n 1)p * ( 1 p * ) n 2 ( 1 p * ) = (n 1)p * p * = 1 n max utilization = R(p * ) " = $ 1 1 # n % ' & n 1 1 e as n

14 Slotted Aloha utilization max p R(p) = np( 1 p) n 1 0 = R'(p * ) = n( 1 p * ) n 1 n(n 1)p * ( 1 p * ) n 2 ( 1 p * ) = (n 1)p * p * = 1 n max utilization = R(p * ) " = $ 1 1 # n % ' & n 1 1 e as n

15 Unslotted Aloha utilization Model Fixed packet size, n stations Slotted time of duration τ << 1. pkt transmission time lasts 1/τ time slots In each τ slot, each station transmits independently with probability p Small τ => approximates unslotted operation Prob (slot τ has a successful transmission) R(p) = np( 1 p) n 1

16 Unslotted ALOHA utilization Prob (a pkt transmission started in an arbitrary τ-slot by station 1 is successful) ( ) n 1 ( 1 p) n 1 S(p) = p 1 p = p 1 p K:= 2 1 time slots τ ( ) ( n 1 )K

17 Unslotted ALOHA utilization Prob (a pkt transmission started in an arbitrary τ-slot by station 1 is successful) ( ) n 1 ( 1 p) n 1 S(p) = p 1 p = p 1 p K:= 2 1 time slots τ ( ) ( n 1 )K

18 Unslotted ALOHA utilization utilization := n τ S(p* ) = n τ 1 " (n 1)K % $ ' # (n 1)K +1& ( n 1)K = n n 1 1 τ K + τ n 1 " $ 1 # 1 % ' (n 1)K +1& ( n 1)K 1 2e as n and τ 0 τ k = 2 τ

19 Unslotted ALOHA utilization max p S(p) = p( 1 p) ( n 1 )K 0 = S'(p * ) = ( 1 p * ) ( n 1 )K ( 1 p * ) = (n 1)Kp * (n 1)Kp * ( 1 p * ) ( n 1 )K 1 p * = 1 (n 1)K +1

20 Unslotted ALOHA utilization max p S(p) = p( 1 p) ( n 1 )K 0 = S'(p * ) = ( 1 p * ) ( n 1 )K ( 1 p * ) = (n 1)Kp * (n 1)Kp * ( 1 p * ) ( n 1 )K 1 p * = 1 (n 1)K +1 reduces to slotted case If K=1

21 Unslotted ALOHA utilization utilization := n τ S(p* ) = n τ 1 " (n 1)K % $ ' # (n 1)K +1& ( n 1)K = n n 1 1 τ K + τ n 1 " $ 1 # 1 % ' (n 1)K +1& ( n 1)K 1 2e as n and τ 0 τ k = 2 τ

22 Unslotted ALOHA utilization utilization := n τ S(p* ) = n τ 1 " (n 1)K % $ ' # (n 1)K +1& ( n 1)K = n n 1 1 τ K + τ n 1 " $ 1 # 1 % ' (n 1)K +1& ( n 1)K 1 2e as n and τ 0 τ K = 2 τ

23 Unslotted ALOHA utilization utilization := n τ S(p* ) = n τ 1 " (n 1)K % $ ' # (n 1)K +1& ( n 1)K = n n 1 1 τ K + τ n 1 " $ 1 # 1 % ' (n 1)K +1& ( n 1)K 1 2e as n and τ 0 τ K = 2 τ

24 Ethernet cable ( ) CSMA/CD (carrier sensing multiple access/collision detection) 1. Wait till channel is idle; wait for a random time. 2. Transmit when the channel is idle following the random wait. 3. Abort if collision is detected, and goto 1.

25 Ethernet cable ( ) Truncated binary exponential backoff 1. Pick X uniformly at random from {0, 1,..., 2^n-1}, n = min (10, m), m = #collisions. Give up & declare error when m = Wait X x 512 bit times (4,096 bits for 1G) 3. If collide, increment m and repeat.

26 Ethernet cable ( ) 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

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

28 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 / ( A), A = PROP/TRAN

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

30 Agenda Broadcast Ethernet n History and performance Switched Ethernet n Spanning tree protocol

31 Ethernet switch: forwarding table (Ethernet) MAC address bit 2. Globally unique to MAC device, location independent (c.f. IP) 3. Broadcast address: 48 ones

32 Ethernet switch: forwarding table x à y: [ y x data ]

33 Ethernet switch routing: STP Goal Operation Example Performance x à y: [ y x data ]

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

35 Spanning tree protocol Three criteria 1. The root switch always forwards messages on all its ports 2. Each switch computes its shortest path (in #bridges) to root 3. All switches connected to a LAN elect a designated switch for the LAN to send packets towards root switch o A switch that is not elected for any of the LANs it is connected to will not receive nor forward any data packet

36 Spanning tree protocol n Switches send packets asynchronously with [ my ID current root ID distance to root ] n 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 n 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 n until protocol converges Completely decentralized, asynchronous, robust

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

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

39 STP: example a new initiation before previous converges

40 STP: example a new initiation before previous converges

41 STP: example a new initiation before previous converges During transient, B5 may connect to root B1 either via B3 or B4 which should B5 use? Ans: use switch with a smaller ID (B3)

42 STP: example a new initiation before previous converges B5 s distance to root (B1) is 2 and no greater than distance to root on STP packets it received from both its neighbors è It stops forwarding (it will operate only as backup)

43 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 B3 has smaller ID & connects the same LANs

44 Spanning tree for all switches STP has converged The switches form a tree rooted at switch with smallest ID Every LAN segment is connected to exactly 1 designated switch

45 Spanning tree for all switches Neither B4 nor B5 will be involved in forwarding data packets

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