1. Chapter 13 Introduction to the Transport Layer

2. 13.1 Transport-Layer Services
Process-to-Process Communication
Addressing : Port Numbers
Encapsulation and Decapsulation
Multiplexing and Demultiplexing
Flow Control
Error Control
Congestion Control
Connectionless and Connection-Oriented Services

3. Process-to-Process Communication
Process is an application-layer entity
Responsible for delivery of the message to the appropriate process

4. Addressing : Port Number
Client-server paradigm : the most common way to achieve process-to-process communication
Process on local host needs service from a process on the remote host
Both processes have the same name : Daytime process
To support several program at the same time, we must define
Local host
Local process
Remote host
Remote process

5. Addressing : Port Number
To define the hosts, we use IP addresses
To define the processes, we need second identifiers
Port number
In the TCP/IP protocol suite, port numbers are integers between 0 and 65,535
Ephemeral port number
Recommended to be greater than 1,023
Well-known port number
Universal port numbers for servers

6. Port Numbers

7. IP Addresses versus Port Numbers

8. ICANN Ranges Well-known ports : 0 ~ 1,023
Assigned and controlled by ICANN
Registered ports : 1,024 ~ 49,151
Not assigned and controlled by ICANN
Can be registered with ICANN to prevent duplication
Dynamic ports : 49,152 ~ 65,535
Can be used as temporary or private port number

9. Example 13.1
In UNIX, the well-known ports are stored in a file called /etc/services. Each line in this file gives the name of the server and the well-known port number. We can use the grep utility to extract the line corresponding to the desired application. The following shows the port for TFTP. Note that TFTP can use port 69 on either UDP or TCP. SNMP uses two port numbers (161 and 162), each for a different purpose.

10. Socket Address
The combination of an IP address and a port number

11. Encapsulation and Decapsulation
Happen at the sender site
Add the transport-layer header
Decapsulation
Happen at the receiver site
The header is dropped

12. Mutiplexing and Demultiplexing
Entity accepts items from more than one source
Demultiplexing
Entity deliver items to more than one source

13. Flow Control Balance between production and consumption rates
Delivery of items from a producer to a consumer
Pushing : sender delivers items whenever they produced
Pulling : producer delivers the items after the consumer has requested

14. Flow Control at the Transport Layer
One of solutions for flow control is normally to use buffers

15. Error Control Detect and discard corrupted packets.
Keep track of lost and discarded packets and resend them.
Recognize duplicate packets and discard them.
Buffer out-of-order packets until the missing packets arrive.

16. Error Control Sequence Number Acknowledgment
Which packet is to be resent
Which packet is a duplicate
Which packet has arrived out of order
This can be done if the packets are numbered
Acknowledgment
The receiver side can send an acknowledgement for each or a collection of packets that have arrived safe and sound

17. Combination of Flow and Error Control
Flow control requires the use of two buffers, one at sender site and other at the receiver site.
Error control requires the use of sequence and acknowledgment number by both sides.
Sliding window

18. Sliding window in circular format

19. Sliding window in linear format

20. Congestion Control
Occur if the load on the network is greater than the capacity of the network
Open-loop congestion control :prevent congestion before it happens
Retransmission policy : using retransmission timer
Window policy : selective repeat and go-back-N window
Acknowledgment policy : using timer, a ACK for N packets
Closed-loop congestion control
alleviating congestion after it happens
Flexible window size

21. Connectionless and Connection-Oriented Service
Connectionless service
Divide messages into chunks of data
No dependency between the packets
The packets may arrive out of order
Connection-oriented service
Establish a connection between a server and a client.
After data exchange, the connection needs to be teared down

22. Connectionless Service

23. Connection-Oriented Service⑤
Connection-close
request ⑤

24. 13.2 Transport-Layer Protocols
Simple connectionless protocol
No flow control and error control
Connection-oriented protocol
Stop-and-Wait protocol
Provide flow and error control
Go-Back-N protocol
Efficient version of Stop-and-Wait protocol
Selective-Repeat Protocol
Suited to handle packet loss
Piggybacking

25. Simple Protocol
No flow nor error control based connectionless protocol

26. Example 13.3
Figure shows an example of communication using the protocol. It is very simple. The sender sends packets one after another without even thinking about the receiver.

27. Stop-and-Wait Protocol
Use both flow and error control
Both the sender and the receiver use a sliding window of size 1.
The sender sends one packet and waits for an ACK before sending the next one.
Add a checksum to detect corrupted packets
When sender sends a packet, it starts a timer
If an ACK arrives, timer is stopped
Either the packet was lost or corrupted, the sender resends the previous packet

28. Stop-and-Wait Protocol

29. Stop-and-Wait Protocol
Sequence Number
To prevent duplicate packets
A field is added to the packet header to hold the sequence number
Use x as a sequence number, x+1 as a acknowledgment number
Acknowledgement number
Sequence number of the next packet expected by the receiver

30. Example 13.4
Following figure shows an example of Sop-and Wait protocol. Packet 0 is sent and acknowledged. Packet 1 is lost and resent after the time-out. The resent packet 1 is acknowledged and the timer stops. Packet 0 is sent and acknowledged, but the acknowledgment is lost. The sender has no idea if the packet or the acknowledgment is lost, so after the time-out, it resends packet 0, which is acknowledged.

31. Flow Diagram for Example 13.4

32. Go-Back-N Protocol
To improve the efficiency of transmission, we can send several packets before receiving acknowledgments.

33. Send Window for Go-Back-N
The send window is an abstract concept defining an imaginary box of maximum size = 2m-1 with three variable : sf, sn, and ssize.
The send window can slide one or more slot when an error-free ACK with ackNo between Sf and Sn arrives.

34. Sliding the send window

35. Receive Window for Go-Back-N
The receive window is an abstract concept defining an imaginary box of size 1 with one single variable Rn.
The window slides when a correct packet has arrived: sliding occurs one slot at a time.

36. Send Window Size for Go-Back-N
In the Go-Back-N protocol, the size of the send window must be less than 2m; the size of the receive window is always 1.

37. Example 13.7
Following figure shows an example of Go-Back-N. This is an example of a case where the forward channel is reliable, but the reverse is not. No data packets are lost, but some ACKs are delayed and one is lost. The example also shows how cumulative acknowledgements can help if acknowledgements are delayed of lost.

38. Flow Diagram for Example 13.7

39. Example 13.8
Following figure shows what happen when a packet is lost. Packets 0, 1, 2, and 3 are sent. However, packet 1 is lost. The receiver receives packets 2 and 3, but they are discarded because they are received out of order. When the receiver receives packets 2 and 3, it send ACK 1 to show that it expects to receive packet 1. However, these ACKs are nor useful for the sender because the ackNo is equal Sf, not greater that Sf. So the sender discards them. When the time-out occur, the sender resends packets 1, 2, and 3, which are acknowledged.

40. Flow Diagram for Example 13.8

41. Selective-Repeat Protocol
The Go-Back-N protocol simplifies the process of receiver
No need to buffer out-of-order packets
Inefficient when the underlying network protocol loses a lot of packets
SR(Selective-Repeat) protocol devised above.
Resend only selective packets

42. Outline of Selective-Repeat

43. Send Window for Selective-Repeat protocol

44. Receive Window for Selective-Repeat protocol

45. Example 13.9
Assume a sender sends 6 packets: packets 0, 1, 2, 3, 4, and 5. The sender receives an ACK with ackNo=3. What is the interpretation if the system is using GBN or SR ?
Solution
If the system is using GBN, it means that packet 0, 1, and 2 have been received uncorrupted and the receiver is expecting packet 3. in using SR, it means that packet 3 has been received uncorrupted; the ACK does not say anything about other pakcets.

46. Example 13.10
This example is similar to Example 3.8 in which packet 1 is lost. We show how Selective-Repeat behave in this case. Figure shows the situation.
At the sender, packet 0 is transmitted and acknowledged. Packet 1 is lost. Packet 2 and 3 arrive out of order and are acknowledged. When the timer times out, packet 1 is resent and is acknowledged. The send window then slides.

47. Flow Diagram of 13.10

48. Selective-Repeat Window Size

49. Bidirectional Protocols : Piggybacking
Previous protocols are unidirectional
Data packets flow in only one direction and acknowledgement travel in the other direction
In real life, data flow and acknowledgement need to flow in both direction.
Piggybacking is used to improved the efficiency of the bidirectional protocols
When a packet is carrying data from A to B, it can also carry acknowledgement feedback about arrived packets from B

50. Design of Piggybacking in Go-Back-N

51. Summary(1)
The main duty of a transport-layer protocol is to provide process-to-process communication. To define the processes, we need port number. The client program defines itself with an ephemeral port number. The server defines itself with a well-known port number.
To send a message from one process to another, the transport layer protocol encapsulates and decapsulates messages. Encapsulation happens at the sender site. Decapsulation happens at the receiver site.
The transport layer at the source perform multiplexing, collecting messages from server processes for transmission; the transport layer at the destination performs demultiplexing, delivering messages to different processes.
Flow control balances the exchange of data items between a producer and a consumer. At the transport layer, we use buffers to hold packets when the consumer is not ready to accept them. Reliability at the transport layer can be achieved by adding error control, which includes detection of corrupted packets, reordering packets that arrived out of order. To manage flow and error control, we use sequence numbers to number packets and use acknowledgement numbers to refer to the numbered packets.

52. Summary(2)
A transport layer can provide two types of congestion control: open-loop and closed-loop. In an open-loop congestion control, the protocol tries to avoid the congestion; in closed-loop congestion control, the protocol tries to detect and remove the congestion after it occurred.
A transport-layer protocol can provide two types of services: connectionless and connection-oriented. In a connectionless service, the sender sends packets to the receiver without any connection establishment. In a connection-oriented service, the client and the server first need to establish a connection between themselves. The data exchange can only happen after the connection establishment. After data exchange, the connection needs to be torn down.
We have discussed several common transport-layer protocols in this chapter. The simple connectionless protocol provides neither flow control nor error control. The connection-oriented Stop-and-Wait protocol provides both flow and error control, but is inefficient. The Go-Back-N protocol is more efficient version of the Stop-and-Wait protocol that takes advantage of pipelining. The Selective-Repeat protocol is a modification of the Go-Back-N protocol that is better suited to handle packet loss. All of these protocol can be implemented bidirectionally using piggybacking.