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More on TCP Acknowledgements

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. 5. 3. 4. 2. 1. TCP. TCP is Reliable

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More on TCP Acknowledgements

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

  2. TCP • TCP is Reliable • IP packets carrying TCP segments may arrive out of order • TCP must put the TCP segments in order 5 3 4 2 1

  3. TCP • TCP is Reliable • Each correct TCP segment is acknowledged by the receiver TCP Segment Source Transport Process Destination Transport Process ACK

  4. 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

  5. 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) Reserved (6) Flags (6) Window Size (16) TCP Checksum (16) Urgent Pointer (16) Options (if any) PAD Data Field

  6. 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)

  7. 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

  8. 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

  9. 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

  10. TCP Acknowledgements • Acknowledgement must indicate which TCP segment is being acknowledged TCP Segment Source TCP Process Destination TCP Process ACK

  11. 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) Reserved (6) Flags (6) Window Size (16) TCP Checksum (16) Urgent Pointer (16) Options (if any) PAD Data Field

  12. 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

  13. 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?

  14. 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

  15. 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) Reserved (6) Flags (6) Window Size (16) TCP Checksum (16) Urgent Pointer (16) Options (if any) PAD Data Field

  16. 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

  17. 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

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

  19. 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 Data Field Max TCP Header

  20. 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 Data Field Max TCP Header

  21. 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 Data Field Max TCP Header

  22. 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

  23. 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

  24. 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!!!

  25. 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

  26. 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

  27. 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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