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 34215

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

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 ?

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

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 79 TCP Data Field TCP Header Sequence Number Field with 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 179 TCP Segment TCP Segment Octets in Data Field 2 Octets in Data Field ISN

TCP Sequence Numbers Supervisory segments, which contain a header but no data, are treated as carrying a single octet of data TCP seg TCP seg 2900 TCP seg … Supervisory segment Carries data

TCP Sequence Numbers Sequence number field gets the value of the first octet in the data field TCP 179 TCP TCP is SeqNum Field Value 83 is SeqNum Field Value 79 is SeqNum Field Value

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

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) Options (if any)PAD Reserved (6) TCP Checksum (16)Urgent Pointer (16) Data Field

TCP Acknowledgment Numbers Acknowledgement Number field contains the next byte expected--the last byte of the segment being acknowledged, plus one TCP 179 TCP TCP is AckNum Field Value 85 is AckNum Field Value 80 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 Too Much Data Flow Control Message

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) Options (if any)PAD Reserved (6) TCP Checksum (16)Urgent Pointer (16) 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 Data Acknowledgement with Window Size Field

TCP Flow Control Example –TCP segment contained octets –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 May send through May 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 TCP HeaderTCP Data Field Max App Frag 1App Frag 2App Frag 3

TCP Fragmentation Note that, in TCP fragmentation, the TCP segment is NOT fragmented –The application layer message is fragmented TCP HeaderTCP Data Field Max App Frag 1App Frag 2App 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 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 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