1. Announcements Late homework policy New Add Updated on course website
Up to 1 HW can be late for up to 5 days without penalty
After that, late HW accepted and graded with discount of 10%/day for up to 5 days
Late HW will not be accepted after 5 days from deadline
New Add
STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!

2. Announcements Projects Details on the course website (will be updated)
Form project team by Fri Oct 12
If your team wants to work on EV projects, talk to Zach Lee or me ASAP
Let us know your background, esp software experience
Schedule weekly meetings with TA by Fri Oct 19
Meetings start week 4 (10% of course grade)
Project tutorials: Fri Oct 19 class
There will be 2 tutorials
Network simulator: ANB 105
ACN simulator: ANB 213
STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!

3. Recap Sharing Metrics Scalability Layering Application topology
Application & technology independence
Application topology
Little’s law: informal proof

4. Recap

5. cs/ee/ids 143 Communication Networks Chapter 3 Ethernet
Text: Walrand & Parakh, 2010
Steven Low
CMS, EE, Caltech
Economic Dispatch in Power Networks with Storage
March 10-12, 2010: Workshop on Distributed Decisions via Games and Price Mechanisms, Lund University, Sweden (Anders Rantzer)

6. Agenda Ethernet: MAC protocols Ethernet: routing protocol
History and performance
Ethernet: routing protocol
Spanning tree protocol

7. 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 ?

8. Ethernet
Medium Access Control (MAC) protocols

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

10. 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

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

12. 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)

13. Slotted Aloha utilization

14. Slotted Aloha utilization

15. Slotted Aloha utilization

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

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

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

19. Unslotted ALOHA utilization

20. Unslotted ALOHA utilization

21. Unslotted ALOHA utilization
reduces to slotted case
If K=1

22. Unslotted ALOHA utilization

23. Unslotted ALOHA utilization

24. Unslotted ALOHA utilization

25. Cable Ethernet ( )
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.

26. Cable Ethernet (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.

27. Cable Ethernet (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

28. Hub Ethernet (1980s)
CSMA/CD as in Ethernet cable

29. Hub Ethernet (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

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

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

32. Ethernet switch: forwarding table
Example: A  J
Send [J | A | data] to switch S1
S1 looks at table and sends it out on port 4
Etc.
What happen if a switch receives a packet for a destination not in its table?
e.g. if a new device has been added
The switch broadcasts it on all its output ports, except the port on which the packet arrived [simple, not security-conscious]
Whenever it gets a packet, it adds the corresponding source address to the table entry that corresponds to the port on which it came.
e.g. when switch S1 receives [J | A | data] from port 1, it adds an entry [A=1] to its table.
VLAN:
network admin can define a subset of links to be a VLAN.
Each link belongs to a set of VLANs.
Packets are restricted to its own VLAN
Benefit: security, service differentiation
x  y: [ y | x | data ]

33. Agenda Ethernet: MAC protocols Ethernet: routing protocol
History and performance
Ethernet: routing protocol
Spanning tree protocol

34. Ethernet bridges/switches
Cable Ethernet H1 H2 H3 H4 H1 H2 H4 B2 B3 B4 B5 B1 B6
Switched Ethernet H3
STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!

35. Ethernet routing: STP Goal Operation Example Performance STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!
Goal
Operation
Example
Performance

36. 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
STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!

37. 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 designated switch 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
STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!

38. 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: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!

39. STP: example I’m 3 I think root is 3 my distance to root is 0
STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!
I think root is 3
my distance to root is 0

40. STP: example I’m 3 I think root is 3 my distance to root is 0
STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!
I think root is 3
my distance to root is 0

41. STP: example a new initiation before previous converges STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!

42. STP: example a new initiation before previous converges STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!

43. STP: example a new initiation before previous converges
STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!
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)

44. STP: example a new initiation before previous converges
STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!
B5’s distance to root (B1) is 2 and no greater than “distance to root” on STP packets it received from both its neighbors
It stops forwarding (it will operate only as backup)

45. STP: designated switches
STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!
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 as B4 does

46. STP: result STP has converged
STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path! H1 H2
STP has converged
The switches form a tree rooted at switch with smallest ID
Every LAN segment is connected to exactly 1 designated switch

47. STP: result
STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path! H1 H2
Neither B4 nor B5 will be involved in forwarding data packets

48. Spanning tree protocol
Performance
Unique path between every source-destination pair
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
STP: goal
STP protocol allows the switches to find the shortest path tree rooted at the switch with the smallest ID (i.e. smallest MAC address on all its interfaces).
To break a tie, a switch selects, among interfaces with the same distance to the root, the next hop with the smallest ID.
STP: Operation
Switches send packets with [my ID | current root ID | distance from current root], *asynchronously*.
A switch relays packets whose “current root ID” is the smallest (& smaller than its current root ID) it has seen so far, and the switch adds one to the distance to current root ID.
STP: Example
Switch B3 starts by sending [3 | 3 | 1]
Followed by switch B5, etc.
Tie-breaking:
When eventually B5 receives from both B3 and B4 packets [3 | 1 | 2] and [4 | 1 | 2], it chooses B3 because it has a smaller ID.
STP: What is the performance?
Can be bad, e.g., in a ring of switches, the switch with the largest ID communicate with root via *longest* path!