More on TCP Acknowledgements Sequence Number Field Initial Sequence Number Acknowledgement Number Field

More on TCP Acknowledgements

Sequence Number Field

Initial Sequence Number

Acknowledgement Number Field

TCP

• TCP is Reliable

– IP packets carrying TCP segments may arrive out of order

– TCP must put the TCP segments in order

3 4 2 15

TCP

• TCP is Reliable

– Each correct TCP segment is acknowledged by the receiver

SourceTransportProcess

SourceTransportProcess

DestinationTransportProcess

DestinationTransportProcess

TCP SegmentTCP Segment

ACKACK

TCP Segment• Each TCP segment sent by a side must

have a sequence number

– Simplest: 1,2,3,4,5,6,7

– To detect lost or out-of-sequence messages

– TCP uses a more complex approach

11 44 22 55

3?

TCP Sequence Numbers

• TCP header has a 32-bit sequence number field

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


• Initial Sequence Number is randomly selected by the sender; Say, 79

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

79

TCP Data Field

TCP Header

Sequence Number Fieldwith Initial Sequence Number (79)

TCP Sequence Numbers• 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

3 Octets in Data Field

2 Octets in Data Field

ISN


• 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…

Supervisory segment

Carries data

Carries data

TCP Sequence Numbers• Sequence number field gets the value of the

first octet in the data field• TCP 1 79• TCP 2 80

8182

• TCP 3 8384

80 is SeqNum Field Value



TCP Acknowledgements

• Acknowledgement must indicate which TCP segment is being acknowledged

SourceTCP

Process

SourceTCP

Process

DestinationTCP

Process

DestinationTCP

Process

TCP SegmentTCP Segment

ACKACK

TCP Acknowledgements

• TCP header contains a 32-bit Acknowledgement Number field to designate the TCP segment being acknowledgedSource Port # (16) Destination Port # (16)





Reserved (6)


Data Field

TCP Acknowledgment Numbers

• 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

83 is AckNum Field Value



TCP Acknowledgement Number

• 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?

TCP Flow Control

• Flow Control– One TCP process transmits too fast– Other TCP process is overwhelmed– Receiver must control transmission rate– This is flow control

TCP Process TCP Process

Too MuchData

Flow Control Message

TCP Flow Control

• A TCP segment has a Window Size field– Used in acknowledgements

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





Reserved (6)


Data Field

TCP Flow Control

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

beyond the segment being acknowledged

TCP Process TCP Process

Data

Acknowledgement with Window Size Field

TCP Flow Control

• 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

TCP Flow Control

• Each Acknowledgement extends the window of octets that may be sent– Called a sliding window protocol

1-44 45-79 80-419 420-630

400May send through 480

1-44 45-79 80-419 420-630

500May send through 920

TCP Fragmentation• TCP Segments have maximum data field

sizes– (Size limit details are discussed later)– What if an application layer message is too

large?

TCP HeaderTCP Data Field Max

Application Layer Message

TCP Fragmentation

• Application layer message must be fragmented– Broken into several pieces– Delivered in separate TCP segments


App Frag 1 App Frag 2 App Frag 3

TCP Fragmentation

• Note that, in TCP fragmentation, the TCP segment is NOT fragmented– The application layer message is fragmented


App Frag 1 App Frag 2 App Frag 3

TCP Fragmentation• Transport layer process on the source

host does the fragmentation– Application layer on the source host is not

involved– Transparent to the application layer

Application

Transport

Internet

Application Message

TCP Segment TCP Segment

TCP Fragmentation• Transport layer process on the

destination host does the reassembly– Application layer on the destination host is

not involved; Gets original application layer message

Application

Transport

Internet

Application Message

TCP Segment TCP Segment

TCP Fragmentation

• 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!!!

TCP Fragmentation• MSS Default is 536 octets

– Minimum 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

TCP Fragmentation• 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

TCP Fragmentation

• 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 segments

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

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More on TCP Acknowledgements Sequence Number Field Initial Sequence Number Acknowledgement Number Field