More on TCP Acknowledgements

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More on TCP Acknowledgements

• Sequence Number Field
• Initial Sequence Number
• Acknowledgement Number Field

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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
1
5
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TCP

• TCP is Reliable
• Each correct TCP segment is acknowledged by the
  receiver

TCP Segment
Source Transport Process
Destination Transport Process
ACK
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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

3?
1
4
2
5
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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)
Reserved (6)
TCP Checksum (16)
Urgent Pointer (16)
Options (if any)
PAD
Data Field
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TCP Sequence Numbers

• Initial Sequence Number is randomly selected by
  the sender Say, 79
• Sent in the sequence number field of the first
  TCP segment

TCP Header
79
TCP Data Field
Sequence Number Field with Initial Sequence
Number (79)
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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 81 82
• TCP Segment 3 83 84

ISN
3 Octets in Data Field
2 Octets in Data Field
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TCP Sequence Numbers

• Supervisory segments, which contain a header but
  no data, are treated as carrying a single octet
  of data
• TCP seg 1 898 899
• TCP seg 2 900
• TCP seg 3 901 902

Carries data
Supervisory segment
Carries data
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TCP Sequence Numbers

• Sequence number field gets the value of the first
  octet in the data field
• TCP 1 79
• TCP 2 80 81 82
• TCP 3 83 84

79 is SeqNum Field Value
80 is SeqNum Field Value
83 is SeqNum Field Value
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TCP Acknowledgements

• Acknowledgement must indicate which TCP segment
  is being acknowledged

TCP Segment
Source TCP Process
Destination TCP Process
ACK
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TCP Acknowledgements

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

Source Port (16)
Destination Port (16)
Sequence Number (32 bits)
Acknowledgement Number (32 bits)
Hdr Len (4)
Flags (6)
Window Size (16)
Reserved (6)
TCP Checksum (16)
Urgent Pointer (16)
Options (if any)
PAD
Data Field
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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 80 81 82
• TCP 3 83 84

80 is AckNum Field Value
83 is AckNum Field Value
85 is AckNum Field Value
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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?

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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

Too Much Data
TCP Process
TCP Process
Flow Control Message
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TCP Flow Control

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

Source Port (16)
Destination Port (16)
Sequence Number (32 bits)
Acknowledgement Number (32 bits)
Hdr Len (4)
Flags (6)
Window Size (16)
Reserved (6)
TCP Checksum (16)
Urgent Pointer (16)
Options (if any)
PAD
Data Field
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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

Data
TCP Process
TCP Process
Acknowledgement with Window Size Field
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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

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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
400
May send through 480
1-44
45-79
80-419
420-630
500
May send through 920
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TCP Fragmentation

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

Application Layer Message
TCP Header
TCP Data Field Max
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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 Header
TCP Data Field Max
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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 Header
TCP Data Field Max
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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
Application Message
Transport
TCP Segment
TCP Segment
Internet
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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
Application Message
Transport
TCP Segment
TCP Segment
Internet
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TCP Fragmentation

• What is the maximum TCP data field size?
• Complex
• Maximum Segment Size (MSS)
• Maximum size of a TCP segments data field
• NOT maximum size of the segment as its name would
  suggest!!!

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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

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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

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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 defaultthe value to use if no
  other value is specified by the other side