Download Transport Control Protocol

Transport Control
Protocol
TCP
Connection-Oriented Service
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Connection-oriented service has a
handshake period
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During this time, a logical connection is made with
the destination node
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The connection is ‘logical’, since all packets are
forwarded individually, just like with UDP
Typically, connection-oriented service provides
reliability, meaning:
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Acknowledgements are used to ensure packets arrive
Checksums/CRCs are used to ensure data integrity
Transport Control Protocol
(TCP)
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Like X.25, TCP provides connection-oriented
delivery at a high level layer
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X.25 provides it at the Transport OSI layer
TCP provides it at the Transport IP layer
Providing connection-oriented delivery at a high level
allows TCP to be applied to any network
 Thus the ability to use TCP/IP over Ethernet, Token Ring,
etc.
However, providing connection-orientation at a high level
means that the network is not necessarily optimized for
connection-oriented delivery
 For example, Ethernet is optimized for connectionless
delivery
TCP
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The essence of TCP is to provide an apparently
continuous stream of data
Thus, above the transport layer:
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Data is not fragmented (into packets)
Data is in order
Lost packets do not occur
Thus, the transport layer (and layers below it) must
handle:
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Segmentation and reassembly (SAR)
Acknowledgements
Segmentation vs. Fragmentation
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Segmentation is basically the same as
fragmentation, with a few differences:
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Fragmentation (IP layer):
 …only occurs when transmitting a packet whose size is
larger than the MTU of the destination network
 Any router (connecting two different network types) could
theoretically fragment packets
 Fragmentation can almost be considered an emergency
practice (what to do when something goes wrong)
Segmentation (TCP layer):
 Occurs for all data streams, to divide the data into packets
(above TCP layer data is continuous)
 Only the source host will segment packets
 Segmentation is a normal part of TCP’s job
TCP
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TCP is a reliable protocol
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All data sent through TCP is automatically divided into
packets
Each of these packets is ensured to be sent by requiring
the destination acknowledge the packets when they are
received
The destination, knowing it will eventually receive all
messages, only has to reorder those messages into an
apparently continuous stream of data flow
TCP: Stream Delivery Protocol
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TCP abstracts data communication to appear as an
apparent stream of flowing data:
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The source sends data as a stream into the network
The destination node receives data from the network in an
identical form
 The data arrives in the same order as it was sent
 All data sent, arrives (in its proper position)
This is known as ‘stream orientation’, a format where the
data is oriented in such a way as to appear as a direct
stream from source to destination
 In reality, however, the data is sent as packets (using IP
datagrams, for example)
TCP: Stream Delivery Protocol
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TCP is normally achieved by using buffering
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Data is collected from the stream (and stored into
memory), until a certain amount has been obtained
This data is packaged into one or more network packets
(e.g. IP datagrams) and sent to the destination using
connectionless delivery
The destination should send an acknowledgement back to
the source
If this acknowledgement fails to arrive after a specified
length of time, the source will retransmit the packet
The destination node buffers the incoming packets into
memory, where they can be read (byte by byte)
TCP: Stream Delivery Protocol
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The TCP/IP service layers do not contain a
Presentation layer
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For this reason, both UDP and TCP require that the
applications recognize their own data formats
For example, using TCP to connect and send an E-Mail
message can be achieved using the existing data format
(or language) known as SMTP (Simple Mail Transfer
Protocol)
Using TCP to connect and request WWW pages can be
achieved using HTTP (HyperText Transfer Protocol)
These protocols are implemented (generated and
recognized) inside the applications themselves
TCP: Stream Delivery Protocol
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Since TCP is actually implemented using
packets (e.g. IP datagrams), it was possible
for TCP to ensure bi-directional
communication across its connections
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Transfer across TCP streams is full duplex
Connection Establishment
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TCP uses a three-way handshake to establish a
connection
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This means 3 messages are exchanged before a
connection exists
The first message (SYN), sent by the machine issuing the
‘active open’ request (A), is a request for connection to the
destination (B)
The second message (SYN/ACK), both an
acknowledgement of the first message as well as a request
for connection to A, is sent by B
The third message (ACK) is an acknowledgement to B
(from A) for the second message
TCP Handshake
SYN
SYN/ACK
Connection
Establishment
(Handshake)
ACK
Transmission
of data
TCP Reliability
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TCP provides reliability by requiring recipient nodes
to send acknowledgments
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Acknowledgements are sometimes called ACKs
When a packet is received by the destination, an
ACK is sent back to the source
When the source receives the ACK, it sends the
next packet
And so on, and so on, …
TCP Reliability
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TCP Reliability
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If a packet is sent, and no ACK is received within a
certain time, the message will be retransmitted
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This time is called the ‘timeout’
It is possible that the original packet was received,
but the ACK was somehow lost
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TCP networks treat both situations identically
The destination will receive the packet again, ignore it (it
already has the data), and acknowledge it again
 Hopefully this time, the acknowledgement will be received
TCP Reliability
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TCP Reliability
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If each node waited for acknowledgements
without transmitting data, it would involve
wasteful delays between packets in a series
TCP uses a scheme called the ‘sliding
window technique’ to solve this problem
Sliding Window Technique
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The sliding window technique allows a transmitting node to
transmit more than one packet without waiting for an ACK
Nodes cannot transmit more than S packets beyond the first
unacknowledged packet
 S is known as the window size
Thus, transmitting nodes have a ‘window’ of up to S packets, all
of which have already been sent
 Some of these packets may be acknowledged
 At least the first packet is unacknowledged (but sent)
When an ACK is received for the first packet in the window,
another packet can be sent
 The window index can be increased by one
Sliding Window Technique
Let’s see an example without a sliding window:
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Packet size: 4, Data: abcd efgh ijkl mnop
Passage of Time
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Connection established
…etc…
Sliding Window Technique
Let’s see the same example with a sliding window:
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Packet size: 4, Window size: 3, Data: abcd efgh ijkl mnop
Passage of Time
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Connection established
Piggybacked ACKs
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Often two node communicate back and forth
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When an acknowledgement is to be sent from A to B, as
well as a data packet, the ACK can be added to the packet
and sent to B as one packet
Essentially, only the sequencing number is required to
indicate that a message has been received
 Sequencing numbers are discussed later
Rather than send a small packet (ACK) followed by
a larger packet (data), the node sends a single
larger packet (data with piggybacked
acknowledgement)
TCP Layers
Application
Transport Control Protocol (TCP)
Internet Protocol (IP)
Network Interface
Hardware
TCP/UDP Layers
Application
TCP
UDP
Internet Protocol (IP)
Network Interface
Hardware
TCP Header Information
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As stated previously, TCP is built on top of IP
datagrams
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These datagrams must arrive correctly
Therefore, TCP streams are often created using
the same information as would be used in the IP
datagram header:
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Address (network and machine portion)
Header checksum
etc,
Ports
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Multiple TCP streams can be active on any
machine
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Therefore, ports should be used to represent
which stream is which
These ports are the same ones used for UDP
This makes sense, considering both use IP
datagrams for their implementation
Sockets
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Sockets, to programmers, represent
connections to the network
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In some sense, a socket are associated with a
network port on the machine
A machine (and even a single program) may
have several open sockets at any time
Sockets
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In UDP, sockets can be shared:
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Datagrams from different destinations can be received on
the same socket
Datagrams can be sent to multiple destinations through the
same socket
In TCP, sockets can not be shared:
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TCP sockets (or stream sockets) represent an active
connection with the other side
Both source and destination must have an active socket
open for communication to occur
Stream Sockets
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With stream sockets, one side must initiate the connection
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The side that will accept a connection requests a ‘passive
open’ with its operating system
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This indicates that the OS should accept incoming connection
requests
A port is associated with the passive open, and can be used by
the initiating node when requesting the connection
The side that initiates a connection requests an ‘active open’
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The initiating node requests a connection with a given machine
(specified by its address) at a particular port
If the machine has a passive open registered at that port, the
connection will be accepted, otherwise it will not
Stream Sockets
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The socket that represents a ‘passive
open’:
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Is called a server socket
Represents the willingness to accept
connections
The socket that represents an ‘active open’:
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Is called a client socket
Represents the act of actually connecting to a
server socket
Data Corruption
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TCP uses a 16 octet checksum to ensure that data
has not been corrupted
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If data is changed in any way, the checksum computed
using the data at the destination will be different than the
checksum computed on the source side (and transmitted
along with the data)
If checksums do not match:
 Data is corrupt
 The checksum is corrupt
 Both situations are treated identically in TCP, data is
retransmitted
Retransmission
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Similar to re-collision avoidance backoff,
unacknowledged packets are sent after increasing
timeouts
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This prevents packets from being indefinitely lost because
the timeout value is too short for extremely high network
usage situations
Unless a message is undeliverable, in any amount of time,
the message will eventually reach its destination and be
acknowledged
Connection Use
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Once a connection has been made,
sequence numbers are used to represent
packets that make up the data stream
Sequence numbers indicate the position of
the data in the packet in the data stream