Transmission Control Protocol: TCP
Transmission Control Protocol: TCP
Wolfgang Schreiner
Engineering for Computer-based Learning
Polytechnic University Hagenberg
Wolfgang.Schreiner@fhs-hagenberg.ac.at
http://cbl.fhs.hagenberg.ac.at/˜schreine
Wolfgang Schreiner
FH Hagenberg — CBL
Transmission Control Protocol: TCP
Transmission Control Protocol: TCP
• IP provides a connectionless packet delivery service.
– Unreliable packet delivery to specific host.
– UDP adds delivery to multiple ports within a host.
• TCP bases a reliable stream delivery service on IP.
– Delivery to multiple ports within a host (like UDP).
– Streams: connections between application programs through which sequences of bytes flow.
– Reliable: the sequence of bytes sent is the same as the sequence of bytes received.
• Application programming interface (API) to use TCP.
– Not part of standard: specific to OS/programming lanugage.
Protocol used in most application programs.
Wolfgang Schreiner
1
Transmission Control Protocol: TCP
Stream Service
• Stream Orientation.
– Application programs see stream of bytes flowing from sender to receiver.
• Virtual circuit connection.
– Sender and receiver interact with their OS to set up a connection.
– Protocol software on both sides exchanges messages to make sure that both sides are ready
and to allocate resources for the communication.
– During communication, messages are exchanged to make sure that data are received correctly.
• Buffered transfer.
– Application software may transmit individual bytes across the stream.
– To make transfer more efficient and to minimize network traffic, protocol software collects
data from stream to fill large datagrams before transmission.
– Push mechanisms may be used to force transmission of not yet transmitted data.
Wolfgang Schreiner
2
Transmission Control Protocol: TCP
Stream Service
• Unstructured streams.
– Protocol software considers a stream as a sequence of bytes without any structure.
– Application programs must agree on the interpretation of stream format (record structures)
for communication of application data.
• Full duplex connection.
– A connection provides two streams, one stream in each direction.
– From the application point of view, the two streams do not interact with each other.
– Internally, the control software can piggyback control information for one stream into the
datagrams of the other stream.
– Larger efficiency than having a separate connection in each direction.
Main characteristics of the TCP delivery service.
Wolfgang Schreiner
3
Transmission Control Protocol: TCP
TCP
Sophisticated transport protocol for reliable stream delivery.
• Various facets:
– Format of data and acknowledgements.
– Initialization and termination of stream transfer.
– Procedure that sender and receiver follow to guarantuee reliability.
– Distinction among multiple destinations of a given machine.
– Recovery from errors like lost or duplicated packets.
• Very few assumptions on underlying network:
– Implementation on top of the IP datagram delivery service.
– Implementation on physical networks: Ethernet, phone lines, . . .
Well investigated protocol implemented on various delivery systems.
Wolfgang Schreiner
4
Transmission Control Protocol: TCP
TCP Connections
Use of protocol port numbers.
• A TCP connection is identified by a pair of endpoints.
– An endpoint is a pair (host IP address, port number).
• Examples:
– Connection 1: (18.26.0.36, 1069) and (128.10.2.3, 25).
– Connection 2: (128.9.0.32, 1184) and (128.10.2.3, 53).
– Connection 3: (127.2.254.139, 1184) and (128.10.2.3, 53).
• Different connections may share the endpoints!
– Multiple clients may be connected to the same service.
The basic TCP abstraction is the connection, not the endpoint.
Wolfgang Schreiner
5
Transmission Control Protocol: TCP
TCP Port Numbers
Linux/Unix: /etc/services
ftp-data
ftp
ssh
telnet
smtp
http
pop3
nntp
ntp
imap
snmp
20/tcp
21/tcp
22/tcp
23/tcp
25/tcp
80/tcp
110/tcp
119/tcp
123/tcp
143/tcp
161/tcp
mail
imap2
#
#
#
#
#
#
#
#
#
#
#
File Transfer [Default Data]
File Transfer [Control]
SSH Remote Login Protocol
Telnet
Simple Mail Transfer
World Wide Web HTTP
Post Office Protocol - Version 3
Network News Transfer Protocol
Network Time Protocol
Internet Message Access Protocol
Simple Network Management Protocol
Reserved TCP ports as well as freely available ports.
Wolfgang Schreiner
6
Transmission Control Protocol: TCP
TCP Aspects
• Reliability:
– Packages are received as they have been sent.
– Technique: sliding window protocol.
• Flow control:
– Packages are not sent faster than they can be received.
– Technique: TCP variant of sliding window protocol.
• Timeout and retransmission:
– Lost packages are retransmitted as soon as possible but not more than necessary.
– Technique: Adaptive retransmission.
• Congestion control:
– Packages are not sent faster than they can be forwarded by routers.
– Technique: Slow startup and multiplicative decrease.
Wolfgang Schreiner
7
Transmission Control Protocol: TCP
Reliability
How to build a reliable service on top of unreliable packet delivery?
• Recipient sends acknowledgement (ACK) messages to sender.
– On receipt of each packet, an ACK message is sent.
– Sender waits for ACK message before sending next packet.
• Sender may have to retransmit a packet.
– Sender starts a timer when sending a packet.
– If timer expires before ACK message is received, packet is retransmitted.
• Sequence numbers are used to detect packet duplication.
– Each packet receives a sequence number.
– Each ACK message takes sequence number from received packet.
Positive acknowledgement with retransmission.
Wolfgang Schreiner
8
Transmission Control Protocol: TCP
Normal Situation
Events At Sender Site
Network Messages
Events At Receiver Site
Send Packet 1
Receive Packet 1
Send ACK 1
Receive ACK 1
Send Packet 2
Receive Packet 2
Send ACK 2
Receive ACK 2
Figure 13.1 A protocol using positive acknowledgement with retransmission
in which the sender awaits an acknowledgement for each packet
sent. Vertical distance down the figure represents increasing
time and diagonal lines across the middle represent network
packet transmission.
Wolfgang Schreiner
9
Transmission Control Protocol: TCP
Error Situation
Events At Sender Site
Send Packet 1
Start Timer
Network Messages
Events At Receiver Site
Packet lost
Packet should arrive
ACK should be sent
ACK would normally
arrive at this time
Timer Expires
Retransmit Packet 1
Start Timer
Receive Packet 1
Send ACK 1
Receive ACK 1
Cancel Timer
Figure 13.2 Timeout and retransmission that occurs when a packet is lost.
The dotted lines show the time that would be taken by the
transmission of a packet and its acknowledgement, if the packet
was not lost.
Wolfgang Schreiner
10
Transmission Control Protocol: TCP
Simple Positive Acknowledgement Protocol
Simple protocol does not utilize the network very well.
• There is only one message at a time in the network.
– Either a packet or an ACK message.
– But network is capable of simultaneous communication in both directions.
• The network is idle at some times.
– Software needs time to construct outgoing datagrams and to process incoming datagrams.
Network bandwidth is wasted by waiting for ACK messages.
Wolfgang Schreiner
11
Transmission Control Protocol: TCP
The Sliding Window Protocol
• Better form of positive acknowledgement and retransmission.
– Sender may transmit multiple packets before waiting for an acknowledgement.
• Windows of packets of small fixed size N .
– All packets within window may be transmitted.
– If first packet inside the window is acknowledged, window slides one element further.
– At most N packets have not yet been acknowledged and may have to be retransmitted.
initial window
1
2
3
4
5
6
7
8
9
10 . . .
7
8
9
10 . . .
(a)
window slides
1
2
3
4
5
6
(b)
Figure 13.3 (a) A sliding window protocol with eight packets in the window,
and (b) The window sliding so that packet 9 can be sent when
an acknowledgement has been received for packet 1. Only
unacknowledged packets are retransmitted.
Wolfgang Schreiner
12
Transmission Control Protocol: TCP
Performance of Sliding Window Protocol
Events At Sender Site
Network Messages
Events At Receiver Site
Send Packet 1
Send Packet 2
Receive Packet 1
Send ACK 1
Send Packet 3
Receive Packet 2
Send ACK 2
Receive ACK 1
Receive Packet 3
Send ACK 3
Receive ACK 2
Receive ACK 3
Figure 13.4 An example of three packets transmitted using a sliding window
protocol. The key concept is that the sender can transmit all
packets in the window without waiting for an acknowledgement.
• Depends on window size N and on speed of packet delivery.
– N = 1: simple positive acknowledgement protocol.
– By increasing N , network idle time may be eliminiated.
– Steady state: sender transmits packets as fast as network can transfer them.
Well tuned sliding window protocol keeps network saturated.
Wolfgang Schreiner
13
Transmission Control Protocol: TCP
The TCP Sliding Window Mechanism
current window
1
2
• Operates on byte level:
3
4
5
6
.
.
.
.
.
.
.
.
.
.
.
7
8
9
10 11 . . .
Figure 13.6 An example of the TCP sliding window. Octets through 2 have
been sent and acknowledged, octets 3 through 6 have been sent
but not acknowledged, octets 7 though 9 have not been sent but
will be sent without delay, and octets 10 and higher cannot be
sent until the window moves.
– Bytes in window are sent from first to last.
– First bytes have to be acknowledged before window may slide.
• Bytes are not transmitted individually but in segments.
– Segment = sequence of bytes transmitted as an IP datagram.
– Each ACK message for a segment describes how many bytes have been received so far.
TCP translates byte stream into sequence of segments.
Wolfgang Schreiner
14
Transmission Control Protocol: TCP
The TCP Sliding Window Mechanism
The size of a window is variable.
• Receiver delivers window advertisements to sender:
– On setup of a connection, the receiver informs the sender about the size of its window (i.e.,
how many bytes the receiver is willing to accept = size of buffer on receiver).
– The sender sends at most as many bytes as determined by the window advertisement.
– Every ACK message from the receiver contains a new window advertisement; the sender
adapts its window size correspondingly.
• Solves the problem of flow control:
– As the window slides, the receiver may adjust the speed of the sender to its own speed by
modifying the window size.
Flow control in addition to reliable transfer.
Wolfgang Schreiner
15
Transmission Control Protocol: TCP
Variable Window Size
Sender
Receiver
Application
does a 2K
write
Receiver's
buffer
0
4K
Empty
2K S
EQ =
0
2K
ACK = 2048 WIN = 2048
Application
does a 3K
write
2K
SEQ = 2048
Full
Sender is
blocked
Application
reads 2K
IN = 0
096 W
4
ACK =
40
ACK =
96 WIN
= 2048
2K
Sender may
send up to 2K
1K
Wolfgang Schreiner
SEQ =
4096
1K
2K
16
Transmission Control Protocol: TCP
Variable Window Size
Sender Events
Receiver Events
advertise window=2500
send data octets 1-1000
send data octets 1001-2000
send data octets 2001-2500
ack up to 1000, window=1500
ack up to 2000, window=500
ack up to 2500, window=0
receive ack for 1000
receive ack for 2000
receive ack for 2500
application reads 2000 octets
ack up to 2500, window=2000
send data octets 2501-3500
send data octets 3501-4500
ack up to 3500, window=1000
ack up to 4500, window=0
receive ack for 3500
receive ack for 4500
application reads 1000 octets
ack up to 4500, window=1000
receive ack for 4500
..
.
Wolfgang Schreiner
17
Transmission Control Protocol: TCP
TCP Segment Format
• Segment = unit of TCP transfer.
– Part of a stream, acknowledgements, opening and closing connections, . . .
• Piggybacking
– Acknowledgment may travel in same segment as stream data.
0
4
10
16
SOURCE PORT
24
31
DESTINATION PORT
SEQUENCE NUMBER
ACKNOWLEDGEMENT NUMBER
HLEN
RESERVED CODE BITS
CHECKSUM
WINDOW
URGENT POINTER
OPTIONS (IF ANY)
PADDING
DATA
...
Figure 13.7 The format of a TCP segment with a TCP header followed by
data. Segments are used to establish connections as well as to
carry data and acknowledgements.
Wolfgang Schreiner
18
Transmission Control Protocol: TCP
TCP Header Fields
SOURCE/DESTINATION PORT: 16 bit.
• Identification of connection.
SEQUENCE NUMBER: 32 bit.
• Position of segment in sender’s byte stream.
• Refers to stream from sender to receiver.
ACKNOWLEDGEMENT NUMBER: 32 bit.
• Number of byte that sender expects to receive next.
• Refers to stream from receiver to sender!
WINDOW: 16 bit.
• Window announcement: how many bytes is sender willing to accept?
• Refers to stream from receiver to sender!
Wolfgang Schreiner
19
Transmission Control Protocol: TCP
TCP Header Fields
HLEN: 4 bit.
• Number of words in header.
CODE BITS: 6 bit.
• Determine purpose and contents of segment.
• ACK: acknowledgment field is valid; URG: urgent pointer field is valid; SYN: synchronize
sequence numbers; PSH: push data to application; . . .
URGENT POINTER 16 bit.
• Support for sending out of band data; e.g. a keyboard sequence that interrupts the program
at the other end.
• URGENT Pointer specifies position in segment where urgent data ends.
• On receipt of segment with urgent data, TCP software tells application program to go into
“urgent mode” and immediately delivers the urgent data.
Wolfgang Schreiner
20
Transmission Control Protocol: TCP
TCP Checksum Computation
Similar to UDP checksum computation.
• A UDP pseudo header is added before the checksum is computed:
– Contains IP destination address.
– Checksum is computed over UDP pseudo header and UDP datagram.
– Receiver can decide whether segment has reached the correct destination.
0
8
16
31
SOURCE IP ADDRESS
DESTINATION IP ADDRESS
ZERO
PROTOCOL
TCP LENGTH
Figure 13.9 The format of the pseudo header used in TCP checksum computations. At the receiving site, this information is extracted from
the IP datagram that carried the segment.
Wolfgang Schreiner
21
Transmission Control Protocol: TCP
Acknowledgements and Retransmission
What exactly does an ACK message acknowledge?
• ACK message refers to position in stream.
– Number of bytes from the beginning of stream that have arrived so far (without gap).
– There may be additional bytes later in the stream (after a gap).
• Cumulative scheme.
– ACK message reports how much of the stream has accumulated.
– Position in subsequent ACK messages may only increase.
• Example:
– DATA 0 → DATA 1500 → DATA 3000 → DATA 4500 → . . .
– ACK 1500 → ACK 3000 → ACK 4500 → ACK 6000 → . . .
Cumulative scheme has advantages and disadvantages.
Wolfgang Schreiner
22
Transmission Control Protocol: TCP
Acknowledgements and Retransmission
• Advantage:
– Lost ACK messages do not necessarily force re-transmission of data segment.
– DATA 0 → DATA 1500 → DATA 3000 → DATA 4500 → . . .
– ACK 1500 → × → ACK 4500 → ACK 6000 → . . .
– ACK 4500 supersedes the lost ACK 3000.
• Disadvantage:
– If data segment is lost, ACKs of later segments cannot increase value.
– DATA 0 → DATA 1500 → × → DATA 4500 → DATA 6000
– ACK 1500 → ACK 3000 → ACK 3000 → ACK 3000
– Sender does not know whether datagrams after the lost DATA 3000 have been received.
– Sender transmits DATA 3000 again and waits for ACK message before it transmits the next
datagram (ACK 7500 → DATA 7500).
If segment is lost, we get simple positive acknowledgement protocol.
Wolfgang Schreiner
23
Transmission Control Protocol: TCP
Timeout
What happens if a data segment is lost?
• TCP starts a timer for each segment sent.
– If segment is acknowledged by some ACK message, everything is fine.
– If timer expires without acknowledgement (timeout), the segment is retransmitted.
• What is an appropriate timeout value?
– We operate in an internet with possibly long and varying delays!
10 s
x
.
..
. .
6s
Time
4s
x.. x. xx..
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
x
2s
xx.
.
.
.
.
.
x. ..
.
x.. x. ..
8s
x.
.
x.. ..
x. .
.
.
xx. x. x. .
.
x..
x. . ..
. x
.
.
.
.
x
.
.
.
.
.
.
.
.
.
.
.
x
.
.
.
.
.
.
.
.
.
.
.
x.
x..
.
.
xx
. .
.
.
.
. .
.
.
.
.
.
. .
. .
. .
xx
xx
10
. .
. .
. .
. .
. .
.
.
.
.
x
x
x. . x
.
x.
x..
.
.
.
.
.
.
.
x.
x. .
.
x..
.
.
.
.
. ..
.
. .
.
.
.
.
.
. . .
.
.
. . .
. . .
.
.
.
.
.
.
.
. .
.
.
.
. .
. .
. .
.
x
x
x
xx xxxxxx
x..
.
.
.
.
.
.
.
.
.
.
. .
. .
.
. .
. . .
.
.
..
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. .
.
.
. . .
. .
.
.
.
.
.
.
.
.
.
.
..
.
. . .
. .
. . .
.
. .
.
. .
.
..
.
.
xx
x
30
40
x
x
x x
x
x
xxx
x
x x
x
xx
50
xx
x x
x
x
20
.
.
.
.
.
.
.
.
.
.
.
.
.
.
..
.
..
.
..
.
..
.
..
.
. .
.
. .
. .
.
.
. .
.
.
. . .
.
.
. . .
.
.
.
.
. .
. .
.
.
. .
.
.
.
.
.
. . . .
.
.
. .
. .
.
. .
..
.
..
.
.
..
..
.
.
.
.
. .
. ..
. .
x
.
.
x
x
x
x.. x. x. x.
x. .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. .
. . .
. . .
.
.
.
.
..
.
..
.
.
.
.
.
.
.
.
.
.
60
70
x
xxx
xx
x
x
x
x
x
xx x
x
x
80
90
100
Datagram Number
Wolfgang Schreiner
Figure 13.10 A plot of Internet round trip times as measured for 100 successive IP datagrams. Although the Internet now operates with
much lower delay, the delays still vary over time.
24
Transmission Control Protocol: TCP
Adaptive Retransmission
TCP monitors performance of connection and deduces timeout values.
• TCP collects data needed for adaptation:
– Time at which data segment is sent and time at which ACK message arrives.
– Difference gives the round trip time RTT .
– From all sample round trip times, TCP computes an average round trip time.
– 0 < α < 1 (e.g. α = 0.78):
new RTT = (α ∗ old RTT ) + ((1 − α) ∗ sample RTT )
– New samples let RTT change slowly.
• Timeout value is a multiple of the roundtrip time.
– β > 1 (e.g. β = 2):
Timeout = β ∗ RTT .
– To cope with large variations, TCP uses RTT variance instead of constant β.
Wolfgang Schreiner
25
Transmission Control Protocol: TCP
Adaptive Retransmission
100
80
60
40
............. ............. . .
.
... ...... ......
.. . ... .
.. ... .
. .. .
..... ... .......
. .
.
... .
.. .
. .
.
..
.. . ...... .....
.. ..
. . .. .
.
.
..... ...
..... .. ..... ...
.
. . .
.. . ...
.
.
.
.
.
..
.
.....
. .. .. ........ ..
. ....... . . ...
... .......
20
20
40
60
80
100
120
140
160
180
200
Datagram Number
Figure 13.11 A set of 200 (randomly generated) roundtrip times shown as
dots, and the TCP retransmission timer shown as a solid line.
The timeout increases when delay varies.
Wolfgang Schreiner
26
Transmission Control Protocol: TCP
Cngestion Control
• TCP must react to congestion in network:
– Severe delay caused by overload of datagrams at a a router.
– Delays increase because router begins to enqueue datagrams until it can route them.
– Since router has only finite memory, it ultimately starts to drop datagrams.
– Since datagrams are dropped, sender retransmit datagrams which yields a congestion collapse.
• Solution: reduce transmission rate when congestion occurs.
– In addition to window size announced by receiver, sender maintains a congestion window:
window size = min(announced window , congestion window ).
– Slow-start: start the congestion window at the size of a single segment; increase it by one
segment each time an ACK message arrives until a treshold of 64 KB is reached.
– Congestion avoidance: if treshold is reached, increase congestion window by one segment only
if all segments in window have been acknowledged.
– Multiplicative decrease: if segment is lost, set treshold to half the size of congestion window,
set congestion window to 1, double timeout value, and go into slow-start mode again.
Wolfgang Schreiner
27
Transmission Control Protocol: TCP
Congestion Control
44
Timeout
40
Congestion window (kilobytes)
36
Threshold
32
28
Threshold
24
20
16
12
8
4
0
0
Wolfgang Schreiner
2
4
6
8
10
12
14
Transmission number
16
18
20
22
24
28
Transmission Control Protocol: TCP
Router Response to Congestion
What should a router do when too many packets arrive?
• Early strategy: tail drop.
– If a new packet arrives and the buffer is full, drop the packet.
– Disadvantage: since datagrams are multiplexed from different connections, one segment is
dropped from each connection; all connections go into slow-start at the same time.
• Nowadays strategy: random early discard.
– If the buffer is not full but the size exceeds a minimum threshold T , discard an arriving
datagram with propability p.
– Consequence: router slowly starts dropping datagrams as congestion increases (without waiting that buffer becomes full).
– Key is careful choice of threshold value T and probability p.
Router operation is optimized to TCP congestion control strategy.
Wolfgang Schreiner
29
Transmission Control Protocol: TCP
Three-Way Handshake
How is a TCP connection initially established?
• Both sides must signal that they are ready to transfer data.
– Both sides must also agree on initial sequence numbers.
– Initial sequence numbers are randomly chosen; if they were always 0, problems might arise
due to duplicated packets from previous connection requests.
Host 1
Host 2
SYN (SEQ
Host 1
Host 2
SYN (SE
Q = x)
= x)
)
EQ = y
Time
SYN (S
EQ
SYN (S
K=x+
= y, AC
1)
(SEQ
SYN
SYN
(SE
Q=
CK =
= y, A
x,A
CK
(SEQ = x
+ 1, ACK
= y + 1)
=y
x + 1)
+1
)
Similar protocol for closing TCP connections.
Wolfgang Schreiner
30