Chapter 1 Review

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Chapter 1 Review

• Csc4220/6220
• Computer Networks
• Instructor Akshaye Dhawan

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Network Core

• 2 ways of transferring data thru the core
• a. Circuit Switching
• b. Packet Switching
• Circuit Switching Dedicated resources per
  call. How do we divide resources?
• a. TDM
• b. FDM

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Circuit Switching FDM and TDM
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Packet Switching

• Resources shared as needed
• aka. Statistical multiplexing
• Advantages
• 1. More users
• 2. Great for bursty traffic patterns
• Disadvantage No guarantees. Congestion issues.

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Packet-switching store-and-forward
L
R
R
R

• Takes L/R seconds to transmit (push out) packet
  of L bits on to link or R bps
• Entire packet must arrive at router before it
  can be transmitted on next link store and
  forward
• delay 3L/R

• Example
• L 7.5 Mbits
• R 1.5 Mbps
• delay 15 sec

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Network Taxonomy
Telecommunication networks

• Datagram network is not either
  connection-oriented
• or connectionless.
• Internet provides both connection-oriented (TCP)
  and
• connectionless services (UDP) to apps.

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Access Networks and Physical Media

• Dialup
• ADSL Asymmetric. Why?
• Still use phone line. Distance between ISP and
  user reduced.
• Coaxial cable
• LANs
• Wireless LANs

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Physical Media

• Twisted Pair
• Coaxial Cable
• Fiber
• Radio
• ISP structuring model

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Delays

• 1. nodal processing
• check bit errors
• determine output link

• 2. queueing
• time waiting at output link for transmission
• depends on congestion level of router

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• 4. Propagation delay
• d length of physical link
• s propagation speed in medium (2x108 m/sec)
• propagation delay d/s

• 3. Transmission delay
• Rlink bandwidth (bps)
• Lpacket length (bits)
• time to send bits into link L/R

Note s and R are very different quantities!
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Total delay

• dproc processing delay
• typically a few microsecs or less
• dqueue queuing delay
• depends on congestion
• dtrans transmission delay
• L/R, significant for low-speed links
• dprop propagation delay
• a few microsecs to hundreds of msecs

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Queuing delay

• Rlink bandwidth (bps)
• Lpacket length (bits)
• aaverage packet arrival rate

traffic intensity La/R

• La/R 0 average queueing delay small
• La/R -gt 1 delays become large
• La/R gt 1 more work arriving than can be
  serviced, average delay infinite!

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Internet protocol stack

• application supporting network applications
• FTP, SMTP, STTP
• transport host-host data transfer
• TCP, UDP
• network routing of datagrams from source to
  destination
• IP, routing protocols
• link data transfer between neighboring network
  elements
• PPP, Ethernet
• physical bits on the wire

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Chapter 2 Review
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Architectures for Applications

• Client-server
• Peer-to-peer (P2P)
• Hybrid of client-server and P2P

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Socket Model

• process sends/receives messages to/from its
  socket
• socket analogous to door
• sending process shoves message out door
• sending process relies on transport
  infrastructure on other side of door which brings
  message to socket at receiving process
• Addressing needs IP address (host) Port
  (process)

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Main services needed from Transport Layer

• Data Loss
• Timing
• Band Width

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Internet transport protocols services

• UDP service
• unreliable data transfer between sending and
  receiving process
• does not provide connection setup, reliability,
  flow control, congestion control, timing, or
  bandwidth guarantee
• Q why bother? Why is there a UDP?

• TCP service
• connection-oriented setup required between
  client and server processes
• reliable transport between sending and receiving
  process
• flow control sender wont overwhelm receiver
• congestion control throttle sender when network
  overloaded
• does not provide timing, minimum bandwidth
  guarantees

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HTTP

• Non Persistent -At most one object is sent over a
  TCP connection.
• Persistent - Multiple objects can be sent over
  single TCP connection between client and server.
• Persistent without pipelining
• client issues new request only when previous
  response has been received
• one RTT for each referenced object
• Persistent with pipelining
• default in HTTP/1.1
• client sends requests as soon as it encounters a
  referenced object
• as little as one RTT for all the referenced
  objects

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HTTP contd

• Packet Types (Request, Response)
• Methods (GET, POST etc.)
• Response codes

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Add ons

• Cookies
• Way of adding some state
• Advantages?
• Disadvantages/Concerns?

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Caching

• How to cache?
• Why should we cache?
• Caching issues. Keeping cache coherent?
• (Conditional GET)

origin server
Proxy server
HTTP request
HTTP request
client
HTTP response
HTTP response
HTTP request
HTTP response
client
origin server
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Email

• User agents
• Mail servers
• SMTP steps, contrast with HTTP
• SMTP Headers
• MIME

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Mail access protocols

• POP download and keep
• POP download and delete
• POP Is stateless across sessions
• IMAP everything organized and stored at server.
• Keeps state across sessions

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DNS

• Name -gt IP address mapping
• What other services?
• Model Distributed Hierarchical

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Distributed, Hierarchical Database
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Structure

• Root servers
• TLD servers
• Authoritative servers.
• Also, Local Name Server

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Querying
root DNS server
2

• Iterative
• Recursive
• DNS caching, updating cache (TTL)

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TLD DNS server
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5
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7
1
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gaia.cs.umass.edu
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P2P

• Centralized Directory Napster Model
• Full y Distributed Gnutella model
• Gnutella Overlay network, limited flooding?
• How does a peer join/leave a distributed Network?

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P2P

• Kazaa Group leader model
• Distinguish between powerful/less powerful peers
• Graph model Connected Dominating Set

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Socket Programming

• Socket Programming with TCP
• Socket Programming with UDP