Download 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
– Each correct TCP segment is acknowledged by
the receiver
TCP Segment
Source
Transport
Process
Destination
Transport
Process
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
3?
1
4
2
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
Reserved (6)
(4)
Flags (6)
TCP Checksum (16)
Window Size (16)
Urgent Pointer (16)
Options (if any)
Data Field
PAD
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)
TCP Sequence Numbers
• Data octets in data fields of all segments in a
connection are viewed as a long string
ISN
• TCP Segment 1 79
• TCP Segment 2 80
81
82
• TCP Segment 3 83
3 Octets in Data Field
2 Octets in Data Field
84
TCP Sequence Numbers
• Supervisory segments, which contain a header but
no data, are treated as carrying a single octet of
data
Carries data
• TCP seg 1898
899
Supervisory segment
• TCP seg 2900
• TCP seg 3901
Carries data
902
…
TCP Sequence Numbers
• Sequence number field gets the value of the
first octet in the data field
• TCP 1
79
79 is SeqNum Field Value
• TCP 2
80
80 is SeqNum Field Value
81
82
83 is SeqNum Field Value
• TCP 3
83
84
TCP Acknowledgements
• Acknowledgement must indicate which
TCP segment is being acknowledged
TCP Segment
Source
TCP
Process
Destination
TCP
Process
ACK
TCP Acknowledgements
• TCP header contains a 32-bit
Acknowledgement Number field to
designate the TCP segment being
Source Port # (16)
Destination Port # (16)
acknowledged
Sequence Number (32 bits)
Acknowledgement Number (32 bits)
Hdr Len
Reserved (6)
(4)
Flags (6)
TCP Checksum (16)
Window Size (16)
Urgent Pointer (16)
Options (if any)
Data Field
PAD
TCP Acknowledgment Numbers
• Acknowledgement Number field contains the next
byte expected--the last byte of the segment being
acknowledged, plus one
80 is AckNum Field Value
• TCP 1
79
• TCP 2
80
81
82
• TCP 3
83
84
83 is AckNum Field Value
85 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
Too Much
Data
TCP Process
TCP Process
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
Reserved (6)
(4)
Flags (6)
TCP Checksum (16)
Window Size (16)
Urgent Pointer (16)
Options (if any)
Data Field
PAD
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
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
May send through 480
400
1-44
45-79
80-419
May send through 920
420-630
420-630
500
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
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
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
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 Message
TCP Segment
TCP Segment
Application
Transport
Internet
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