module a panko and panko business data networks and security, 9 th edition © 2013 pearson

More on TCPModule A

Panko and PankoBusiness Data Networks and Security, 9th Edition© 2013 Pearson

TCP is reliable.◦ IP packets carrying TCP segments may arrive out

of order.

◦ TCP must put the TCP segments in order.

2

TCP

3 4 2 15

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TCP is reliable.◦ Each correct TCP segment is acknowledged by the

receiver.

3

TCP

SourceTransportProcess

SourceTransportProcess

DestinationTransportProcess

DestinationTransportProcess

TCP SegmentTCP Segment

ACKACK

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Each TCP segment sent by a side must have a sequence number.◦ Simplest approach to sequence numbers:

1,2,3,4,5,6,7, and so on

◦ To detect lost or out-of-sequence messages

◦ (TCP uses a more complex approach than this, as we will see)

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TCP Segment

11 44 22 55

3?

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TCP header has a 32-bit sequence number field.

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TCP Sequence Numbers

Source Port # (16) Destination Port # (16)

Sequence Number (32 bits)

Acknowledgement Number (32 bits)

Hdr Len(4) Flags (6) Window Size (16)

Options (if any) PAD

Reserved (6)

TCP Checksum (16) Urgent Pointer (16)

Data Field

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Initial Sequence Number is randomly selected by the sender; say, 79.

Sent in the sequence number field of the first TCP segment.

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79TCP Data Field

TCP Header

Sequence Number Fieldwith Initial Sequence Number (79)

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Data octets in data fields of all segments in a connection are viewed as a long string.

TCP Segment 1 79 TCP Segment 2 80

8182

TCP Segment 3 8384

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3 Octets in Data Field

2 Octets in Data Field

ISN

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Supervisory segments, which contain a header but no data, are treated as carrying a single octet of data.

TCP seg 1 898899

TCP seg 2 900 TCP seg 3 901

902…

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Supervisory Segment

Carries Data

Carries Data

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Sequence number field gets the value of the first octet in the data field.

TCP 1 79 TCP 2 80

8182

TCP 3 8384

9


80 is SeqNum Field Value



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Acknowledgement must indicate which TCP segment is being acknowledged.

10

TCP Acknowledgements

SourceTCP

Process

SourceTCP

Process

DestinationTCP

Process

DestinationTCP

Process

TCP SegmentTCP Segment

ACKACK

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TCP header contains a 32-bit Acknowledgement Number field to designate the TCP segment being acknowledged.

11

TCP Acknowledgements



Acknowledgement Number (32 bits)Hdr Len

(4) Flags (6) Window Size (16)


Reserved (6)


Data Field

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Acknowledgement Number field contains the next byte expected—the last byte of the segment being acknowledged, plus one.

TCP 1 79

TCP 2 808182

TCP 3 8384

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TCP Acknowledgment Numbers

83 is AckNum Field Value



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Quiz: A TCP segment contains the following data octets:◦ 567, 568, 569, 570, 571, 572, 573, 574

What will be in the sequence number field of the TCP segment delivering the data?

What will be in the acknowledgement number field of the TCP segment acknowledging the TCP segment that delivers these octets?

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TCP Acknowledgement Number

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Flow Control

◦ One TCP process transmits too fast.

◦ Other TCP process is overwhelmed.

◦ Receiver must control transmission rate.

◦ This is flow control.

14

TCP Flow Control

TCP Process TCP Process

Too MuchData

Flow Control Message

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A TCP segment has a Window Size field.◦ Used in acknowledgements

15

TCP Flow Control



Acknowledgement Number (32 bits)

Hdr Len(4) Flags (6) Window Size (16)


Reserved (6)


Data Field

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A TCP segment has a Window Size field.◦ Tell how many more octets the sender can send

beyond the segment being acknowledged

16

TCP Flow Control

TCP Process TCP Process

Data

Acknowledgement with Window Size Field

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Example

◦ TCP segment contained octets 45–89

◦ Acknowledgement number for TCP segment acknowledging the segment is 90

◦ If Window Size field value is 50, then

◦ Sender may send through octet 140

◦ Must then stop unless the window has been extended in another acknowledgement

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TCP Flow Control

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Each Acknowledgement extends the window of octets that may be sent.◦ Called a sliding window protocol

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TCP Flow Control

1–44 45–79 80–419 420–630

400May send through 480

1–44 45–79 80–419 420–630

500May send through 920

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TCP Segments have maximum data field sizes.◦ (Size limit details are discussed later.)◦ What if an application layer message is too large?

19

TCP Fragmentation

TCP HeaderTCP Data Field Max

Application Layer Message

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Application layer message must be fragmented.◦ Broken into several pieces◦ Delivered in separate TCP segments

20

TCP Fragmentation


App Frag 1 App Frag 2 App Frag 3

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Note that, in TCP fragmentation, the TCP segment is not fragmented.◦ The application layer message is fragmented.

21

TCP Fragmentation


App Frag 1 App Frag 2 App Frag 3

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Transport layer process on the source host does the fragmentation.◦ Application layer on the source host is not

involved◦ Transparent to the application layer

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TCP Fragmentation

Application

Transport

Internet

Application Message

TCP Segment TCP Segment

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Transport layer process on the destination host does the reassembly.◦ Application layer on the destination host is

not involved; gets original application layer message.

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TCP Fragmentation

Application

Transport

Internet

Application Message

TCP Segment TCP Segment

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What is the maximum TCP data field size?◦ Complex

Maximum Segment Size (MSS)◦ Maximum size of a TCP segment’s data field

◦ NOT maximum size of the segment as its name would suggest!!!

24

TCP Fragmentation

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MSS Default is 536 octets.◦ Maximum IP packet size any network must

support is 576 octets. Larger IP packets MAY be fragmented

◦ IP and TCP headers are 20 octets each if there are no options.

◦ This gives the default MSS of 536.

◦ Smaller if there are options in the IP or TCP header.

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TCP Fragmentation

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MSS Default is 536 octets.

◦ Suppose the application layer process is 1,000 octets long.

◦ Two TCP segments will be needed to send the data.

◦ The first can send the first 536 octets.

◦ The second can carry the remaining 464 octets of the application layer message.

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TCP Fragmentation

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Each side may announce a larger MSS.

◦ An option usually used in the initial SYN message it sends to the other.

◦ If announces MSS of 2,048, this many octets of data may be sent in each TCP segment.

◦ 536 is only the default—the value to use if no other value is specified by the other side.

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TCP Fragmentation

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module a panko and panko business data networks and security, 9 th edition © 2013 pearson

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