Download Lect03

Address Resolution Protocol
• Internet uses IP address to recognize a computer.
• But IP address needs to be translated to physical
address (NIC).
• ARP does this job.
• It takes IP address and gives corresponding
physical address.
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How ARP Works?
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There are three methods
1) Table look up
2) Closed form computation
3) Message Exchange
In table look up the mapping information is stored
in a table in the computer’s memory.
• In closed form method an algorithm is used to
map IP address to a physical address.
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How ARP Works?
• In message exchange, a message is broadcast to all
the computer on the network with the IP address.
• The computer with that IP address sends its
physical address which is maintained in a cache.
• ARP is local, that is, it is done on local network.
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Transmission Control Protocol
• Transmission Control Protocol or TCP
provides:
– Reliability
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Error Control
Loss Control
Sequence Control
Duplication Control
– Point to Point or Port to Port Communication
– Full duplex
– Connection Oriented
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???
• TCP uses IP for data delivery (like UDP)
• Endpoints are identified by ports (like UDP)
– Allows multiple connections on each host
– Ports may be associated with an application or a
process
• IP treats TCP messages like data and does
not interpret any contents of the TCP
message.
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???
• TCP message travels in IP datagrams.
• Internet routers only look at IP header to
forward datagrams.
• TCP at destination interprets TCP messages.
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TCP Segment Format
• Same header format used in both directions
• Segment can carry both data and
acknowledgment.
• Flag field has 6 1-bit flags. SYN = 1 means
request to establish connection. FIN = 1
means end of connection.
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Ports and Socket
• A single computer can be running several
applications (email, FTP, HTTP and others).
• TCP is application to application (port to port)
protocol, IP is computer to computer protocol.
• TCP uses port numbers to identify different
application running on a single computer.
• A port is a 16-bit number allocated to a particular
application.
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Ports and Socket
• A port identifies a single application on a
computer.
• A socket or socket address is IP address and port
number concatenated together. Example:
192.20.10.10:80
• This enables multiple application on one machine
to communicate simultaneously.
• Normally server port numbers are fixed and
known as well known ports so that client can send
a request to a known application.
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Ports and Socket
A Typical Scenario
• When a client application sends a TCP request to a
server application, it provides server application
port number.
• The TCP software on client allocates an unused
port number to that client application.
• The server receives the request and TCP software
passes it to the intended server application.
• The server replies with its own port number and
the client port number.
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Ports and Socket
A Typical Scenario
Client
Source port number = 23
Destination port number = 80
Source
Source port number = 80
Destination port number = 23
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Delivery in TCP
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Three-way Handshake
• TCP uses three-way handshake for reliable
connection establishment and termination
– Host 1 sends segment with SYN bit set and random
sequence number.
– Host 2 responds with segment with SYN bit set,
acknowledgment to Host 1 and random sequence
number.
– Host 1 responds with acknowledgment.
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Closing a Connection
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TCP Connection Types
• TCP has two types of connections:
– Non Persistent connection
– Persistent connection
• In non persistent connection TCP
connection is closed by the server once the
client request is served.
• In persistent connection the connection is
not closed and client can use it as many
time as needed.
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TCP Connection Types
• There are two types of persistent
connections.
– Without pipelining
– With pipelining
• Without pipelining the new request could be
sent to the sever only once the previous
request is served
• With pipelining several requests can be
made simeltaneously.
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Reliable Delivery
• TCP uses many techniques described earlier to
provide reliable delivery
• Recovers from
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Lost packets
Duplicate packets
Delayed packets
Corrupted data
Transmission speed mismatches
Congestion
System reboots
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Lost Packets
• TCP uses positive acknowledgment with
retransmission to achieve reliable data delivery
• Recipient sends acknowledgment control messages
(ACK) to sender to verify successful receipt of
data
• Sender sets timer when data transmitted; if timer
expires before acknowledgment arrives, sender
retransmits (with new timer)
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Lost Packets
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TCP Segments
• Application delivers arbitrarily large chunks
of data (blocks) to TCP as a “stream”.
• TCP breaks this data into segments, each of
which fits into an IP datagram.
• Original stream is numbered by bytes.
• Segment contains sequence number of data
bytes.
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Acknowledgements
• Receiver sends segment with sequence
number of acknowledged data (not
segments).
• One ACK can acknowledge many
segments.
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Timeout
• Inappropriate timeout can cause poor
performance:
– Too long - sender waits longer than necessary before retransmitting
– Too short - sender generates unnecessary traffic
• Timeout must be different for each connection and
set dynamically
– Host on same LAN should have shorter timeout than
host 20 hops away
– Delivery time across internet may change over time;
timeout must accommodate changes
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RTOs for Different Network
Delays
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Choosing RTO
• Timeout should be based on round trip time or
delay (RTT)
• Sender estimates RTT for each active connection
by measuring the time needed to receive a
response. Updated dynamically.
• Sender picks retransmission timeout (RTO) based
on previous RTTs
• Specific method is call adaptive retransmission
algorithm
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TCP Sliding Window
• TCP uses sliding window for flow control.
• Receiver specifies window:
– Called window advertisement
– Specifies which bytes in the data stream can be sent
– Carried in segment along with ACK
• Sender can transmit any bytes, in any size
segment, between last acknowledged byte and
within window size.
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TCP Sliding Window
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TCP Sliding Window with Acknowledgements
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Congestion Control
• Excessive traffic can cause packet loss.
– Transport protocols respond with retransmission.
– Excessive retransmission can cause congestion
collapse.
• TCP interprets packet (segment) loss as an
indicator of congestion.
• Sender uses TCP congestion control and slows
transmission of packets when a packet is lost.
– Sends single packet.
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Congestion Control
– If ACK returns without loss, sends two packets, and if
two ACKs return, sends four packets, and so on,
doubling each time until….one half window size.
– When TCP sends one-half window size, rate of increase
slows.
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Summary
• UDP provides end-to-end best-effort message
delivery
– IP used for delivery to destination host
– Protocol ports demultiplex to destination application
• TCP provides end-to-end reliable byte stream
delivery
– IP used for delivery to destination host
– Protocol ports demultiplex to destination application
– Additional techniques develop reliable delivery from IP
messages.
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Summary
• Positive acknowledgment with
retransmission
• Sequence numbers detect missing, duplicate
and out-of-order data
• Sliding window flow control
• Three-way handshake
• Congestion control
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