1. Introduction to Communication Networks – 67594
Dr. Michael Schapira
Rothberg A413

2. Chapter 3 Transport Layer
A note on the use of these ppt slides:
We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:
If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)
If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.
Thanks and enjoy! JFK/KWR
All material copyright
J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009.

3. Pipelined protocols
Pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledged pkts
range of sequence numbers must be increased
buffering at sender and/or receiver
Two generic forms of pipelined protocols: go-Back-N, selective repeat

4. Pipelining: increased utilization
sender
receiver
first packet bit transmitted, t = 0
last bit transmitted, t = L / R
first packet bit arrives
RTT
last packet bit arrives, send ACK
last bit of 2nd packet arrives, send ACK
last bit of 3rd packet arrives, send ACK
ACK arrives, send next
packet, t = RTT + L / R
Increase throughput
by a factor of 3!

5. Pipelining Protocols Go-back-N: overview
sender: up to N unACKed pkts in pipeline
receiver: only sends cumulative ACKs
doesn’t ACK pkt if there’s a gap
sender: has timer for oldest unACKed pkt
if timer expires: retransmit all unACKed packets
Selective Repeat: overview
sender: up to N unACKed packets in pipeline
receiver: ACKs individual pkts
sender: maintains timer for each unACKed pkt
if timer expires: retransmit only unACKed packet
GBN: many timers, one value in the ack
SR: N timers and window of acks in the senders, reassembly buffer in the receivers.

6. Selective repeat receiver sender data from above :
if next available seq # in window, send pkt
timeout(n):
resend pkt n, restart timer
ACK(n) in [sendbase,sendbase+N]:
mark pkt n as received
if n smallest unACKed pkt, advance window base to next unACKed seq #
pkt n in [rcvbase, rcvbase+N-1]
send ACK(n)
out-of-order: buffer
in-order: deliver (also deliver buffered, in-order pkts), advance window to next not-yet-received pkt
pkt n in [rcvbase-N,rcvbase-1]
ACK(n)
otherwise:
ignore

7. Selective repeat in action
Transport Layer

8. Selective repeat: dilemma
Example:
seq #’s: 0, 1, 2, 3
window size=3
receiver sees no difference in two scenarios!
incorrectly passes duplicate data as new in (a)
Q: what relationship between seq # size and window size?

9. Receive Window for Selective Repeat
A: The window size (=the number of pkts being handled ) should be at most half the seq # size

10. Summary of rdt mechanisms
Detection code (CRC, Checksum, etc.)
Can also be error correction codes
Timer
Need to deal with premature timeouts
Sequence number
To detect duplicates
Acknowledgment (ACK)
With/without seq. number; individual/cumulative
Negative Acknowledgment (NAK)
(Sliding) window, pipelining

11. Chapter 5 Link Layer and LANs
A note on the use of these ppt slides:
We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:
If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)
If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.
Thanks and enjoy! JFK/KWR
All material copyright
J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009.
5: DataLink Layer

12. Link Layer 5.1 Introduction and services
5.2 Error detection and correction
5.3 Multiple access protocols
-- Reliable Protocols
5.4 Link-Layer Addressing
5.5 Ethernet
5.6 Link-layer switches
5.7 PPP

13. Adaptors Communicating
datagram
datagram
controller
controller
sending host
receiving host
datagram
frame
sending side:
encapsulates datagram in frame
adds error checking bits, rdt, flow control, etc.
receiving side
looks for errors, flow control, etc.
extracts datagram, passes to upper layer at receiving side
5: DataLink Layer

14. Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today: star topology prevails
active switch in center
each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other)
We will talk about switches later…
switch
bus: coaxial cable
star

15. MAC Addresses MAC (or LAN or physical or Ethernet) address:
function: get frame from one interface to another physically-connected interface (same network)
48 bit MAC address (for most LANs)
burned in NIC ROM, also sometimes software settable
In Ethernet frame (reminder):

16. Each adapter on LAN has unique LAN address
LAN Addresses
Each adapter on LAN has unique LAN address
1A-2F-BB AD
Broadcast address =
FF-FF-FF-FF-FF-FF
LAN
(wired or
wireless)
= adapter
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
5: DataLink Layer

17. LAN Address (more) MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
Analogy: MAC address: like מספר תעודת זהות
MAC flat address ➜ portability
can move LAN card from one LAN to another
We will see later other addressing structures that are hierarchical (IP address) and therefore NOT portable
5: DataLink Layer

18. Other Types of Addresses
MAC address usually is not enough. Hosts usually have only another type of address - the IP address  translation is needed
We will learn about IP address when we discuss global networks
We will learn how source learn about IP addresses when we discuss application protocols (DNS)
5: DataLink Layer

19. ARP: Address Resolution Protocol
Question: how to determine
MAC address of B
knowing B’s IP address?
Each node (host, router) on LAN has ARP table
ARP table: IP/MAC address mappings for some LAN nodes
< IP address; MAC address; TTL>
TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)
1A-2F-BB AD
LAN
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
5: DataLink Layer

20. 5: DataLink Layer

21. ARP protocol: Same LAN (network)
A wants to send datagram to B, and B’s MAC address not in A’s ARP table.
A broadcasts ARP query packet, containing B's IP address
dest MAC address = FF-FF-FF-FF-FF-FF
all machines on LAN receive ARP query
B receives ARP packet, replies to A with its (B's) MAC address
frame sent to A’s MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state: information that times out (goes away) unless refreshed
ARP is “plug-and-play”:
nodes create their ARP tables without intervention from net administrator
5: DataLink Layer

22. Addressing: routing to another LAN
walkthrough: send datagram from A to B via R
assume A knows B’s IP address
two ARP tables in router R, one for each IP network (LAN) R
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D A
C-E8-FF-55
88-B2-2F-54-1A-0F B
49-BD-D2-C7-56-2A
5: DataLink Layer

23. This is a really important
A creates IP datagram with source A, destination B
A uses ARP to get R’s MAC address for
A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram
A’s NIC sends frame
R’s NIC receives frame
R removes IP datagram from Ethernet frame, sees its destined to B
R uses ARP to get B’s MAC address
R creates frame containing A-to-B IP datagram sends to B
This is a really important
example – make sure you
understand! R
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D A
C-E8-FF-55
88-B2-2F-54-1A-0F B
49-BD-D2-C7-56-2A
5: DataLink Layer

24. Link Layer 5.1 Introduction and services
5.2 Error detection and correction
5.3 Multiple access protocols
-- Reliable Protocols
5.4 Link-Layer Addressing
5.5 Ethernet
5.6 Link-layer switches
5.7 PPP

25. 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
5: DataLink Layer

26. 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
5: DataLink Layer

27. Switch: allows multiple simultaneous transmissions
hosts have dedicated, direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link, but no collisions; full duplex
each link is its own collision domain
switching: A-to-A’ and B-to-B’ simultaneously, without collisions
not possible with dumb hub C’ B 1 2 6 3 4 5 C B’ A’
switch with six interfaces
(1,2,3,4,5,6)
5: DataLink Layer

28. switch with six interfaces
Switch Table A
Q: how does switch know that A’ reachable via interface 4, B’ reachable via interface 5?
A: each switch has a switch table, each entry:
(MAC address of host, interface to reach host, time stamp)
Q: how are entries created, maintained in switch table? C’ B 1 2 6 3 4 5 C B’ A’
switch with six interfaces
(1,2,3,4,5,6)
5: DataLink Layer

29. 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)
5: DataLink Layer

30. 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
5: DataLink Layer

31. 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
5: DataLink Layer

32. Interconnecting switches
switches can be connected 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!)
5: DataLink Layer

33. Self-learning multi-switch example
Suppose C sends frame to I, I responds to C S4 1 S1 2 S3 S2 A F D I B C G H E
S4 contains ABCDEFGHI Mac
Q: show switch tables and packet forwarding in S1, S2, S3, S4
5: DataLink Layer

34. Institutional network
mail server
to external
network
web server
router
IP subnet
Heterogeneous links
5: DataLink Layer

35. Interconnection Without Backbone
Hub
Hub A F D I B C G H E
Not recommended for two reasons:
- single point of failure at middle hub
- all traffic between left segment and right segment must path over the middle segment

36. What will happen with loops? Incorrect learningB 1 2
A , 1 2 2

37. What will happen with loops? Frame loopingC 2 2
C,??
C,?? 1 1 A

38. What will happen with loops? Frame loopingB 2 2
B,2
B,1 1 1 A

39. A message from A will mark A’s location
Loop-free: tree C B
A message from A will mark A’s location A

40. A message from A will mark A’s location
Loop-free: tree C B
A: 
A message from A will mark A’s location A

41. A message from A will mark A’s location
Loop-free: tree
A:  C B
A: 
A message from A will mark A’s location A

42. A message from A will mark A’s location
Loop-free: tree
A: 
A: 
A:  C B
A: 
A: 
A message from A will mark A’s location A

43. A message from A will mark A’s location
Loop-free: tree
A: 
A: 
A:  C B
A: 
A: 
A message from A will mark A’s location A

44. Loop-free: tree A:  A:  A:  C B A:  A: 
So a message to A will go by marks…
A message from A will mark A’s location A

45. Spanning Tree
for increased reliability, desirable to have redundant, alternative paths from source to dest
with multiple paths, cycles result – switches may multiply and forward frame forever
solution: organize switches in a spanning tree by disabling subset of interfaces
Hub
Hub A F D I B C G H E