Transcript TCP: Software for Reliable Communication

TCP: Software for Reliable
Communication
Internet: a Collection of
Disparate Networks
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Different goals:
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Speed, cost, distance;
Different standards for:
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Expected carrier;
Coding bits;
Detecting and recovering from errors;
Protocols for transmitting messages: bus, token
ring,…
Packets sizes, and encoding for the start/end of
packets,…
Types of computer addresses
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Solution: Routers
Router
High speed
connection
Router
Router
Routers: computers design to interconnect different networks
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Solution: Internet Protocol (IP)
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Divide a message in small blocks, called
packets;
IP protocol for transmitting packets;
IP hides the details of physical networks;
Every computer connected to the Internet
must run IP software
IP specifies:
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Packet format;
How routers should forward packets
Define address format
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IP can be overrun
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Consider the following scenario:
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Computers X and Y send simultaneously
messages to a computer Z across the same
network, d;
Both X and Y send 5000 packets/s
Network d may transmit 5000 packets/s;
There is an excess of 5000 packets each
second which are discarded.
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IP can be overrun (cont.)
X
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Y
Z
TCP (Transmission Control Protocol) designed
to handle this problem.
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TCP
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Checks for lost datagrams;
Routers may fail TCP chooses new paths;
Because datagrams may travel different
paths, they can arrive at the destination in a
different order  TCP assembles them in the
correct order
Network failure may result in multiple copies
of the same datagram TCP checks for
duplicate datagrams, and accepts only the
first copy
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TCP- a connection oriented
protocol
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IP: a connectionless protocol, i.e. there is no
connection between sender and destination;
In order to achieve its goals TCP establishes a
connection between two computers;
A data transmission proceeds much like a
telephone call:
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The sender program contact the destination
The destination accepts the incoming call;
The sender and destination may exchange arbitrary amounts of
data;
Any of the party may terminate the communication
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Recovering lost datagrams
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Problem: datagrams may be lost by a router
far away from the sender or the destination;
Solution:
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TCP includes a unique ID in each datagram;
Whenever data arrives at the destination, it sends
an acknowledgment (ack) back to the source,
containing the ID of the datagram;
If an ack is not received in a certain time the
sender retransmits the message.
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Example of retransmission
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Retransmission is adaptive
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Different values for waiting times:
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Short, if source and destination are “close”
to each other;
Longer, if they are far apart.
Adjusted if delays occur
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TCP measures the delay in sending a
message and adjusts the timer
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Retransmission examples
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Detecting duplicate datagram
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The receiver maintains a table with all
datagrams received so far;
When the destination, receives a
datagram, checks the ID of incoming
with the IDs of the datagrams received
so far;
If it is a duplicate, the datagram is
discarded.
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TCP and IP work together
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IP specifies how a packet should be
transmitted from the sender to the
destination;
TCP specifies ways for making packet
transmission reliable;
TCP/IP often used and sold as a single
software packet.
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Designing Protocols
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Two possible solutions:
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A single, giant protocol that specifies all
details;
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Divide the problem into subpieces, and
design a special protocol for each piece;
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Cons: difficult to design, and to update
Each piece is called a layer;
The result: a suite of protocols;
Requires that different parts share information.
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TCP/IP---a layered protocol
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Has seven layers;
Lowest layer refers
to hardware;
Top layers refer to
the software;
The level of
abstraction
increases bottom to
top.
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TCP/IP layers:
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Layer 1 (Physical layer):
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Corresponds to network hardware;
defines the carrier, the way to encode bits,
etc..
Example of a carrier
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TCP/IP layers (cont.)
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the carrier is reduced to 2/3 full strength to encode a 1 bit, and
1/3 strength to encode a 0 bit;
Two successive pulses needed to encode one bit; an unchanged
pulse separates two bits.
Digital signal
The resulting
wave encoding
The signal above.
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TCP/IP layers (cont.)
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Layer 2: data link
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Specifies how to organize data into
packets, and how to transmit packets over
a network
For ex:
maximum packet size,
 format packet header,
 checksum computation
are defined at this layer.
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TCP/IP layers (cont.)
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Layer 3: Network
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Specifies how addresses are formed
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How packets are forwarded:
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Ex: IP addresses
Ex: store and forward technique
Layer 4: Transport
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Handles details of reliable transfer;
EX: format of acks, retransmission times, rules for
changing it
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TCP/IP layers (cont.)
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Layer5: Session
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Specifies how to establish a communication with a
remote system; ex: telnet
Authentication details; ex: passwords
Layer 6: Presentation
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Specifies how to represent data;
Different computers use different internal
representation (Ex: ASCII, EBDIC) for integers and
characters;
How to translate from one representation to
another
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TCP/IP layers (cont.)
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Layer 7: Application
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Specifies how one particular application
uses a network;
Ex: FTP
Specifies request format (how to name a
file) and how the application on another
machine responds.
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How layered software works
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Each layer solves one part of the problem;
To do so, each layer on the sending computer adds
information to the outgoing data;
The same layer in the receiving computer uses the
additional information to process data;
Ex: checksums in data layer;
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How layered software works
(cont.)
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Layering Principle:
Layer N software on the
destination computer, must
receive the exact message sent
by layer N software on the
sending computer.
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Ex: if one layer adds a header,
the corresponding layer has to
remove it.
If one layer encrypts data, the
receiving computer layer has to
decrypt it.
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Why layering?
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Each layer can be:
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Designed,
Implemented
Tested
independently of other layers.
creating and evolving protocols, much
easier task.
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