CS 330 Architecture Class 9

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CS 330 Architecture Class 9

• Plan for today TCP/ UDP, Loshin Ch 9, 10

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(Wikepedia)
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Review Where Are We?

• Layer 1 Physical
• transmits changes in voltage, light waves, etc
• Layer 2 Data Link
• organizes interprets layer 1 signals
• transmits them from one local interface to
  another on the same physical network
• Layer 3 Network
• transmits data across networks from one host to
  another host
• no attempt at end-to-end reliability
• Layer 4 Transport
• transmits data from a service on a host to a
  service on another to provide a reliable
  connection between the two (end-to-end)

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Transport Layer Protocols

• UDP and TCP
• end-to-end connectivity between hosts
• the ability to address processes on the
  communicating hosts (not just between whole
  computers as in internet layer).
• TCP
• a mechanism for creating reliability in a
  connection
• a virtual circuit that applications on the
• layer above it can rely on as if the
• hosts were hard-wired

(source)
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Function 1 Addressing Processes

• As in all layers, addressing is a key function
• Network layer provides a way to address a host
• (i.e. a computer as a whole)
• Transport layer provides a way to addressing
  specific processes running on the host
• (telnet, FTP, HTTP sessions)
• As usual, addresses include both recipient and
  sender

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Ports

• Each process on a computer is given an address
  (port)
• (a process is an instance of an application)
• Some ports are well-known
• 80 (HTTP)
• 23 (telnet)
• 21 (FTP)
• 53 (DNS).
• A Web connection to www.wells.edu is really to
  http//www.wells.edu80.
• Try http//calendar.wells.edu591

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Sockets

• The combination of host IP address and port
  number a socket
• The sending computer creates a port number on the
  fly
• ephemeral port

Connections /Virtual Circuits

• The combination of the destination and sending
  host sockets forms a connection (virtual circuit)
  for tracking traffic between services on the two
  machines
• This connection will always be unique
• even if the same client opens two telnet sessions
  with the same host
• even if two clients open sessions with the same
  port on a server
• why?

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Function Two Reliability

• What does reliability mean in the context of
  networking?
• assurance that all packets are received
• assurance that they can be put back together in
  the right order
• IP is unreliable by design
• a best effort protocol
• hosts send out packets in the right direction,
  but dont know whether they actually arrive
• Reliability pushed up the stack to a higher layer
  

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Costs of Reliability

• Always involves a process for acknowledge and
  confirm
• when one host sends a packet, the other must send
  a packet back to acknowledge receipt
• A mechanism for retransmitting lost packets
• A mechanism for ordering packets
• In principle can double the amount of traffic
• Can also slow the rate of transmission
• And traffic must always flow both ways
• cf. the problems of TCP/IP for cable TV or
  satellite

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http//www.skullbox.net/tcpudp.php
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UDP

• UDP (User Datagram Protocol) raises the question
  again when is unreliable better than reliable?
• Economizes on resources by pushing
  responsibility for reliability up the stack to
  the application layer
• Used for simple applications and applications
  where getting most packets through quickly is
  more important than getting all packets through
  slowly
• DNS send a single packet request, receive a
  single packet reply. No response try somewhere
  else. No cost to set up a circuit
• TFTP Trivial FTP for devices that are not full
  computers (e.g. switches, firewalls, traffic
  shapers)
• Real-time applications e.g. audio, video speed
  more important than perfection

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UDP

• Simpler than TCP
• Does not track each packet sent
• Faster
• Used for data that does not necessarily have to
  be reliable
• Supports broadcast and multicast
• Format (pink is optional)

(source)
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UDP used for DNS Protocol

• DNS protocol query and reply messages, both
  with same message format

• DNS message header
• Identification 16 bit for query, reply to
  query uses same
• Flags
• Query or reply
• Recursion desired
• Recursion available
• Reply is authoritative
• (plus UDP header on prev slide)
• Example name server queries sent as a UDP message

recursivenameserver can process requests for
which it is not authoritative
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TCP

• Transmission Control Protocol
• The protocol that provides reliability in TCP/IP
• Creates a virtual circuit from the point of view
  of the application that sits on top of it
• The application sees a hard-wired connection to
  the application on the other host
• Bytes are organized into segments which are sent
  down to IP
• Responsibilities guarantee that
• all segments are delivered
• all segments can be restored to their proper
  order, no matter when they arrive
• both hosts are notified of which segments have
  been sent and which have been received

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How does TCP work? (1)

• Negotiation the processes on the two hosts must
  agree to communicate using particular ports and
  particular options
• Start with a three-way handshake
• Host A requests a connection, including a
  randomly-generated sequence number
• Host B sends an ACK confirming it has received
  the request, and also sends its own arbitrary
  sequence number
• Host A sends an ACK confirming it has received
  the sequence number of host B
• Once started each host acknowledges every
  segment received and each acknowledges the
  acknowledgements
• If the sending host doesnt get an ACK for a
  particular segment within a stated time, it
  resends it

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How does TCP work? (2)

• The TCP header includes sending and receiving
  port number, its own
• sequence number (incremented by one) the ACK
  number acknowledging
• receipt of the other hosts sequence number
  other fields, including options

(source)
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How does TCP work? (3)

• A host doesnt wait for an ACK before sending the
  next segment
• Instead, a sliding window
• a host can send a number of segments in the
  expectation that they will be acknowledged within
  a certain time
• if they are not, it resends them and reduces the
  window size
• Flow control
• if segments are not being acknowledged properly,
  the link slows down
• Slow start
• imagine a fast host on a fast network sending to
  a slow host
• slowly increase window size to the point of
  failure, then back off

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How does TCP work? (4)

• Sounds inefficient
• The requirement for bi-directional
  acknowledgements led many to believe TCP/IP
  unusable for real-time audio/video
• Not quite so bad
• the TCP header contains both the new segment
  number and the acknowledgement of the last
  segment (as well as data, except at very
  beginning and end)
• elaborate timing algorithms to attempt to send as
  many packets as possible, but not so many that
  packets have to be resent
• Why does physical proximity of a server means
  faster communication?
• Nice summary

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UDP Header
TCP Header