Introduction to TCP
A first look at the sockets API for
‘connection-oriented’ client/server
application programs
Benefits of TCP
• Communication is ‘transparently reliable’
• Data is delivered in the proper sequence
• An application programmer does not need
to worry about issues such as:
– Lost or delayed packets
– Timeouts and retransmissions
– Duplicated packets
– Packets arriving out-of-sequence
– Flow and congestion control
Overview
process A
… port P …
application layer
transport layer
process B
… port Q …
reliable TCP
TCP
IP
byte-stream connection
network layer
unreliable IP datagrams
TCP
IP
Transport-Layer’s duties
• Create the illusion of a reliable two-way
point-to-point connection linking a client
application with a server application
– Manage the ‘error-control’ mechanism
– Manage the ‘flow-control’ mechanism
– Manage the connection’s ‘persistence’
– Manage the connection’s ‘shutdown’
Interaction overview
The ‘server’ application
socket()
bind()
The ‘client’ application
socket()
listen()
bind()
accept()
read()
write()
close()
3-way handshake
data flow to server
data flow to client
4-way handshake
connect()
write()
read()
close()
What is a ‘connection’?
• An application’s socket is ‘connected’ if it
has a defined pair of socket-addresses:
– An IP-address and port-number for the ‘host’
– An IP-address and port-numbet for the ‘peer’
‘hrn23501.usfca.edu’
‘hopper.usfca.edu’
138.202.171.14
port 53124
138.202.192.14
port 80
2-way data stream
138.202.192.14
port 80
138.202.171.14
port 53124
classroom workstation
USF’s web-server
Layout of TCP header
32-bits
source port address
destination port address
sequence number
acknowledgment number
Header
Length
reserved
U A P R S F
R C S S Y I
G K H T N N
checksum
window size
urgent pointer
options and padding
Sequence number
segmented stream of 1000 bytes
ISN
data
data
data
data
(256 bytes)
(256 bytes)
(256 bytes)
(232 bytes)
ISN + 256
ISN + 512
ISN + 768
The sequence number field defines the number being assigned to the
first byte of data contained in this segment. During connection setup,
each party to the connection uses a random number generator to get
the value it will assign to the first byte of data it will transmit, called its
initial sequence number (ISN). Thereafter, the sequence number in
each succeeding segment will equal the sequence number used in the
prior segment plus the number of data bytes in that prior segment. By
this scheme the receiver can arrange all the incoming data bytes in the
proper order, even if some segments happen to arrive out-of-order.
Acknowledgment number
• This field holds the number of the byte that
the source of this segment is expecting to
receive next from its connection partner
9, 8, 7
6, 5 
4, 3, 2, 1
sender
receiver
ACK 5
• This field’s value is meaningful only when
this segment’s ACK control flag bit is set
TCP Header Length
• Like IP headers, the TCP Header’s length is
expressed in multiples of 32-bits: it’s at least 5
(i.e., 20-bytes) if there aren’t any ‘TCP Options’
included in the TCP header
IP header
TCP header
DATA
Total Length (in bytes)
• The amount of DATA in a TCP packet can be
calculated from the IP Header’s ‘Total Length’
field, minus the number of bytes that comprise
these two headers (IP header + TCP header)
Control flags
5
4
3
2
1
0
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Legend:
FIN = Terminate the connection
SYN = Synchronize sequence numbers
RST = Reset the connection
PSH = Push the data
ACK = The value in the acknowledgement field is valid
URG = The value in the urgent pointer field is valid
Establishing the connection
timeline
SYN J
SYN K, ACK J+1
server
application
(passive)
ACK K+1
The 3-way Handshake
client
application
(active)
Exchanging data
timeline
PSH+ACK
ACK
server
application
(passive)
PSH+ACK
ACK
A typical Request and Reply transaction
client
application
(active)
Connection shutdown
timeline
ACK+FIN
ACK
server
application
(passive)
ACK+FIN
ACK
The 4-way Handshake
client
application
(active)
3-way handshake
SYN
ACK + SYN
ACK
‘request-and-reply’
ACK+PSH
ACK
ACK+PSH
ACK+FIN
4-way handshake
ACK+FIN
ACK
ACK+FIN
ACK
TCP Timers
• To achieve transparent reliability, the TCP
subsystem maintains some internal timers
• One of these is the ‘Retransmission Timer’
• If a packet is sent, but its ACK does not
arrive before this timer expires, then the
packet will be ‘retransmitted’
• Of course, this could result in the receiver
getting duplicate packets (if it’s a bit slow)
‘lost’ versus ‘late’
timeline
PSH+ACK
retransmit
timeout
PSH+ACK
ACK
ACK arrives late
same PSH arrives twice
client
application
Busy server might be ‘slow’ to acknowledge
server
application
‘piggyback’
• To reduce traffic-flow when possible, TCP
delays sending an immediate ACK for an
arriving data-packet, in case the receiver
might soon have some data of its own to
send back – in which case the ACK can
‘piggyback’ on the outgoing data PSH
• This mechanism, of course, requires TCP
to maintain a ‘Delayed ACK’ timer
Delayed ACK senario
process A
application layer
write
read
buffer for incoming data
buffer for outgoing data
Delayed
ACK
timer
Retransmit
timer
Window
Probe
timer
Keep
Alive
timer
… port P …
TCP
to/from the IP layer
transport layer
Window Size
• During the ‘connection setup’ handshake,
each host communicates to its partner a
‘window size’ parameter, to let be known
some information about its capacity for
buffering packets
• It also conveys its ‘MSS’ parameter (as a
‘TCP header option’) to inform its partner
of its buffers’ Maximum Segment Size
MSS versus MTU
• A diagram shows the distinction between
the protocol’s MSS (Maximum Segment
Size) and the interface’s MTU (Maximum
Transmission Unit); no TCP packets will
be sent with a segment-size that’s larger
MTU
MSS
datalink
header
IP
header
TCP
header
packet
DATA
FCS
Main TCP option types
• The TCP Header contains an options list,
occupying from 0 to 11 longword values
• Each option is identified by an 8-bit ‘type’:
•
•
•
•
•
•
•
Type 0: End of the options list: data follows this
Type 1: No option: used for alignment padding
Type 2: Maximum Segment Size (MSS)
Type 3: Window scaling option (WSOPT)
Type 4: Selective Acknowledgments supported
Type 5: Selective Acknowledgment (SACK)
Type 8: Timestamp value and echo reply (TSOPT)
Option formats
• Option types 0 and 1 are single-bytes
• All other option types are at least 2-bytes,
with the second byte containing the length
type
2
length
4
type
3
MSS
value
4 bytes
type
8
length
10
length
3
3 bytes
timestamp
value
10 bytes
WSOPT
value
type
4
length
2
2 bytes
timestamp echo reply
value
Looking at TCP options
HLEN
HLEN
HLEN
‘Wrapped’ sequences
• The TCP header’s ‘Sequence Number’ is
a 32-bit value, initially chosen at random
• It could happen that a large number gets
selected as an Initial Sequence Number
and that a large amount of data gets sent,
thus causing the 32-bit field to ‘overflow’
• So how does the receiver tell a ‘wrapped
sequence’ from a ‘late-arriving’ segment?
Type 8: TSOPT
16 bits
TYPE (=8)
LENGTH (=10)
Timestamp
Value
Timestamp
Echo Reply
• The TCP timestamps have two purposes:
– RTTM: Round-Trip Time Measurement
– PAWS: Protect Against Wrapped Sequences
The SACK option
• It conveys extended ‘acknowledgment’
information from a receiver to a sender
about ‘gaps’ in the received data-stream
Type (=5)
Length
Left Edge of first Block
no gaps here
Right Edge of first Block
...
no gaps here
Left Edge of n-th Block
Right Edge of n-th Block
32-bits
2+n*8 bytes
Demo programs
• We put ‘tcpserver.cpp’ and ‘tcpclient.cpp’
on our class website, so you can watch
actual TCP packets being exchanged by
using our ‘nicwatch’ application (or some
other packet-sniffer, e.g., ‘wireshark’)
• We deliberately used loops which write to,
or read from, sockets one-byte-at-a-time
so that you can observe ‘TCP buffering’!