1. The Transport Layer

2. The Transport Service
Services provided to the application layer by using the network layer service
Transport header of a segment exchanged between two users is not read by routers and packet switches.

3. The nesting of TPDUs, packets, and frames.
Transport Service
The nesting of TPDUs, packets, and frames.

4. The socket primitives for TCP.
Berkeley Sockets
The socket primitives for TCP.

5. Elements of Transport Protocols
Addressing
Connection Establishment
Connection Release
Flow and Congestion Control
Buffering
Multiplexing, Demultiplexing

6. UDP and TCP
User Datagram Protocol (UDP) provides only of multiplexing/ demultiplexing of data exchange between different applications. It is connectionless service.
Transmission Control Protocol (TCP) provides a reliable service to the applications. It is connection oriented service.

7. UDP and TCP Addressing
Whenever a UDP socket is created, an available or a specified port number is assigned to it.
The UDP socket is determined only by the destination port number. The source port number serves as a return address.
The TCP server application has a “welcoming socket” for accepting connection requests. When a client socket is created, a port number is assigned to it and a connection is requested.
TCP server socket is determined by the source IP address, source port number, destination IP address, destination port number.

8. The TCP Service Model Some assigned ports. Port Protocol Use 21 FTP
File transfer 23
Telnet
Remote login 25
SMTP 69
TFTP
Trivial File Transfer Protocol 79
Finger
Lookup info about a user 80
HTTP
World Wide Web
110
POP-3
Remote access
119
NNTP
USENET news
Some assigned ports.

9. UDP Segment Header
The UDP header.

10. Real-Time Transport Protocol (RTP)
(a) The position of RTP in the protocol stack. (b) Packet nesting.

11. RTP Header
Add Hdr
4B pad
Sources #
Coding Type
The RTP header.

12. Transport Control Protocol (TCP)
TCP Connection Establishment
TCP Connection Release
TCP Connection Management Modeling
TCP Transmission Policy
TCP Flow and Congestion Control
TCP Timer Management

13. The TCP Segment Header
TCP Header.

14. The pseudoheader included in the TCP checksum.
The TCP Segment Header
The pseudoheader included in the TCP checksum.

15. TCP Connection Establishment
6-31
(a) TCP connection establishment in the normal case.
(b) Call collision.

16. TCP Connection Management Modeling
Client starts TCP
Wait 30s
CLOSED
Send SYN
TIME_WAIT
SYN_SENT
Receive FIN,
Send ACK
Receive SYN & ACK
Send ACK
FIN_WAIT_2
ESTABLISHED
Send FIN
Receive ACK
FIN_WAIT_1

17. TCP Connection Management Modeling
Server application creates LISTEN socket
Receive ACK
CLOSED
LAST_ACK
LISTEN
Send FIN
Receive SYN
Send SYN & ACK
CLOSE_WAIT
SYN_RCVD
Receive ACK
Receive FIN
Send ACK
ESTABLISHED

18. TCP Mechanisms
TCP is a combination of Go Back N and Selective Repeat protocols.
Receiver sends ack with the sequence number of the last received byte. It is cumulative, meaning that it acks all previously received segments.
Sender keeps only the info about the first unack-ed segment, and retransmits the previous segment when it the timer expires or when it receives 3 identical ACKs.
Receiver has buffer and may receive segments out of order.

19. TCP ACK Generation
When a segment arrives in order and all previous data are acked, receiver waits for 500ms and sends ACK.
When a segment arrives in order and there is one segment waiting for ACK, the ACK is sent immediately.
When a segment arrives out of order, immediately send ACK with the next expected sequence number.
Arrival of segment that fills the gap. Immediately send ACK.

20. Flow Control
Processor of the receiver often handles multiple applications and tasks, so the speed of handling of a specific application is variable and limited.
Receiver sends the receive window with each ACK, that specifies the empty space in the receiver buffer.
Sender makes sure that the difference between the last byte sent and the last byte acked is smaller than the receive window, so that the receive buffer is not overflowed.

21. TCP Transmission Policy
Window management in TCP.

22. Congestion Control
When a timeout period expires, sender retransmits a packet, and timer is triggered again but it is set to twice the previous value. When a new segment is generated, or when ACK is received, the timer is set to the estimated timeout period.
Window size (w) is initially set to 0 and threshold size (t) is set to some large value.
When w<t, w grows exponentially fast (incremented by one whenever it receives ACK)-slow-start phase.
When w>t, w grows linearly (roughly by 1 once per RTT).
When triple ACK occcurs, the t=w/2, w=t.
If timeout happens, t=w/2, w=1.

23. Congestion Control Sender must ensure that:
Last byte sent – Last byte ACKed ≤
min{Congestion Window, Received Window}

24. TCP Congestion Control
An example of the Internet congestion algorithm.

25. TCP Timer Management
(a) Probability density of ACK arrival times in the data link layer.
(b) Probability density of ACK arrival times for TCP.

26. Timeout Calculation Timeout is Timeout=2RTT
where RTT is a round-trip time:
RTT=αRTT+(1-α)M, α=7/8
Alternatively timeout is:
Timeout = RTT+4xD
where D is estimated variance:
D=αD+(1-α)|RTT-M|

27. TCP Versions TCP Reno TCP Vegas is modification of Reno
TCP Tahoe is older version

28. Splitting a TCP connection into two connections.
Wireless TCP and UDP
Splitting a TCP connection into two connections.