1. Lecture 21 and 22
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2. Overview TCP = Transmission Control Protocol
Connection-oriented protocol
Provides a reliable unicast end-to-end byte stream over an unreliable internetwork.
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3. Connection-Oriented
Before any data transfer, TCP establishes a connection:
One TCP entity is waiting for a connection (“server”)
The other TCP entity (“client”) contacts the server
The actual procedure for setting up connections is more complex.
Each connection is full duplex
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4. Reliable
Byte stream is broken up into chunks which are called seg-ments
Receiver sends acknowledgements (ACKs) for segments
TCP maintains a timer. If an ACK is not received in time, the segment is retransmitted
Detecting errors:
TCP has checksums for header and data. Segments with invalid checksums are discarded
Each byte that is transmitted has a sequence number
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5. TCP header fields Port Number:
A port number identifies the endpoint of a connection.
A pair <IP address, port number> identifies one endpoint of a connection.
Two pairs <client IP address, server port number> and <server IP address, server port number> identify a TCP connection.
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6. TCP overview TCP is the most widely used Internet protocol
Web, Peer-to-peer, FTP, telnet, …
A two way, reliable, byte stream oriented end-to-end protocol
Includes flow and congestion control
Closely tied to the Internet Protocol (IP)
A focus of intense study for many years.
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7. TCP Features Connection-oriented Byte-stream Reliable data transfer
app writes bytes
TCP sends segments
app reads bytes
Reliable data transfer
Full duplex
Flow control: keep sender from overrunning receiver
Congestion control: keep sender from overrunning network
Application process W
rite
bytes
TCP
Send buffer
Segment T
ransmit segments
Read
Receive buffer …
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8. TCP Format
TCP segments have a 20 byte header with >= 0 bytes of data.
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9. Segment Format
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10. Segment Format Each connection identified with 4-tuple:
(SrcPort, SrcIPAddr, DsrPort, DstIPAddr)
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11. TCP header fields Sequence Number (SeqNo):
Sequence number is 32 bits long.
So the range of SeqNo is
0 <= SeqNo <=  4.3 Gbyte
Each sequence number identifies a byte in the byte stream
Initial Sequence Number (ISN) of a connection is set during connection establishment
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12. TCP header fields Acknowledgement Number (AckNo):
Acknowledgements are piggybacked, I.e
a segment from A -> B can contain an acknowledgement for a data sent in the B -> A direction
A hosts uses the AckNo field to send acknowledgements. (If a host sends an AckNo in a segment it sets the “ACK flag”)
The AckNo contains the next SeqNo that a hosts wants to receive Example: The acknowledgement for a segment with sequence numbers is AckNo=1501
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13. TCP header fields Acknowledge Number (cont’d) Example:
TCP uses the sliding window flow protocol (see CS 457) to regulate the flow of traffic from sender to receiver
TCP uses the following variation of sliding window:
no NACKs (Negative ACKnowledgement)
only cumulative ACKs
Example:
Assume: Sender sends two segments with “ ” and “ ”, but receiver only gets the second segment.
In this case, the receiver cannot acknowledge the second packet. It can only send AckNo=1
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14. TCP header fields Header Length ( 4bits):
Length of header in 32-bit words
Note that TCP header has variable length (with minimum 20 bytes)
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15. TCP header fields Flag bits: URG: Urgent pointer is valid
If the bit is set, the following bytes contain an urgent message in the range:
SeqNo <= urgent message <= SeqNo+urgent pointer
ACK: Acknowledgement Number is valid
PSH: PUSH Flag
Notification from sender to the receiver that the receiver should pass all data that it has to the application.
Normally set by sender when the sender’s buffer is empty
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16. TCP header fields Flag bits: RST: Reset the connection
The flag causes the receiver to reset the connection
Receiver of a RST terminates the connection and indicates higher layer application about the reset
SYN: Synchronize sequence numbers
Sent in the first packet when initiating a connection
FIN: Sender is finished with sending
Used for closing a connection
Both sides of a connection must send a FIN
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17. TCP header fields Window Size: TCP Checksum: Urgent Pointer:
Each side of the connection advertises the window size
Window size is the maximum number of bytes that a receiver can accept.
Maximum window size is 216-1= bytes
TCP Checksum:
TCP checksum covers over both TCP header and TCP data (also covers some parts of the IP header)
Urgent Pointer:
Only valid if URG flag is set
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18. TCP Connection-oriented
Sets up the connection prior to data transmission
SYN and 3-way handshake
Guarantees delivery of data
Sender holds a copy of the data for retransmission if necessary
Receiver ACKS specific byte positions in the stream so sender can resend from any byte position
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19. TCP Connection Establishment
TCP uses a three-way handshake to open a connection:
(1) ACTIVE OPEN: Client sends a segment with
SYN bit set *
port number of client
initial sequence number (ISN) of client
(2) PASSIVE OPEN: Server responds with a segment with
initial sequence number of server
ACK for ISN of client
(3) Client acknowledges by sending a segment with:
ACK ISN of server (* counts as one byte)
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20. Connection Establishment
Active participant
Passive participant
(client)
(server)
SYN, SequenceNum = x y , + 1
SYN + ACK, SequenceNum = x
Acknowledgment =
ACK, Acknowledgment = y + 1
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21. Connection Termination
Active participant
Passive participant
(server)
(client)
FIN, SequenceNum = x + 1 x
Acknowledgment = y
FIN, SequenceNum=
Acknowledgment = y + 1
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22. UDP
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23. User datagram format
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24. Example 14.1
The following is a dump of a UDP header in hexadecimal format.
a. What is the source port number?
b. What is the destination port number?
c. What is the total length of the user datagram?
d. What is the length of the data?
e. Is the packet directed from a client to a server or vice versa?
f. What is the client process?
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25. Example 14.1 Continued Solution
a. The source port number is the first four hexadecimal digits (CB84)16 or
b. The destination port number is the second four hexadecimal digits (000D)16 or 13.
c. The third four hexadecimal digits (001C)16 define the length of the whole UDP packet as 28 bytes.
d. The length of the data is the length of the whole packet minus the length of the header, or 28 – 8 = 20 bytes.
e. Since the destination port number is 13 (well-known port), the packet is from the client to the server.
f. The client process is the Daytime (see Table 14.1).
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26. 5/29/2019

27. Figure 14.5 Encapsulation and decapsulation
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28. Figure 14.7 Multiplexing and demultiplexing
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29. UDP APPLICATION
A client-server application such as DNS uses the services of UDP because a client needs to send a short request to a server and to receive a quick response from it. The request and response can each fit in one user datagram. Since only one message is exchanged in each direction, the connectionless feature is not an issue; the client or server does not worry that messages are delivered out of order.
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30. Example
A client-server application such as SMTP, which is used in electronic mail, cannot use the services of UDP because a user can send a long message, which may include multimedia (images, audio, or video). If the application uses UDP and the message does not fit in one single user datagram, the message must be split by the application into different user datagrams. Here the connectionless service may create problems. The user datagrams may arrive and be delivered to the receiver application out of order. The receiver application may not be able to reorder the pieces. This means the connectionless service has a disadvantage for an application program that sends long messages.
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31. Example
Assume we are downloading a very large text file from the Internet. We definitely need to use a transport layer that provides reliable service. We don’t want part of the file to be missing or corrupted when we open the file. The delay created between the delivery of the parts are not an overriding concern for us; we wait until the whole file is composed before looking at it. In this case, UDP is not a suitable transport layer.
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32. Example
Assume we are watching a real-time stream video on our computer. Such a program is considered a long file; it is divided into many small parts and broadcast in real time. The parts of the message are sent one after another. If the transport layer is supposed to resend a corrupted or lost frame, the synchronizing of the whole transmission may be lost. The viewer suddenly sees a blank screen and needs to wait until the second transmission arrives. This is not tolerable. However, if each small part of the screen is sent using one single user datagram, the receiving UDP can easily ignore the corrupted or lost packet and deliver the rest to the application program. That part of the screen is blank for a very short period of the time, which most viewers do not even notice. However, video cannot be viewed out of order, so streaming audio, video, and voice applications that run over UDP must reorder or drop frames that are out of sequence.
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