Network Standards

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Network Standards

• Khaled M. Elleithy, Ph.D.
• elleithy_at_ccse.kfupm.edu.sa
• Department of Computer Engineering
• King Fahd University of Petroleum and Minerals
• Dhahran, Saudi Arabia

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Topics Covered in this Session

• Why Communication Standards?
• The OSI Reference Model

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Why Communications Standards?

• Incompatible products from different
  manufacturers
• The need to allow different manufacturers
  products to communicate
• The need to provide a framework for networks to
  be acceptable from all manufacturers

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Layered Architecture

• A layer is corresponding to a different
  abstraction level
• Each layer should perform a well defined
  function
• Layers should be chosen to minimize information
  exchange between different layers

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OSI Reference Model

• Application layer
• Presentation layer
• Session layer
• Transport layer
• Network layer
• Data link layer
• Physical layer

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OSI Reference Model
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Physical Layer

• The Physical Layer is simply responsible for
  sending bits from one computer to the another.
  The Physical Layer is not concerned with the
  meaning of the bits.
• This level defines physical and electrical
  details, such as what will represent 1 or 0, how
  many pins a network connector will have, how
  data will be synchronized, and when the network
  adapter may or may not transmit the data.

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Physical Layer

• Couplers, cables and cabling, connectors,
  multiplexers, transmitters, receivers and
  transceivers are devices associated with the
  physical layer.

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Physical Layer

• The following items are addressed at the physical
  layer
• Network connection types, including multipoint
  and point-to-point connections.
• Physical topologies, which are physical layouts
  of networks, such as bus, star or ring

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Physical Layer

• Analog and digital signaling, which include
  several methods for encoding data in analog and
  digital signals.
• Bit synchronization, which deals with
  synchronization between sender and receiver.
• Baseband and Broadband transmissions, which are
  different methods for using media bandwidth.

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Data Link Layer

• The data link layer provides for the flow of data
  over a single link from one device to another. It
  accepts packets from the network layer and
  packages the information into data units called
  frames to be presented to the physical layer for
  transmission.
• A Cyclic Redundancy Check (CRC) added to the data
  frame can detect damaged frames, and the data
  link layer in the receiving computer can request
  that the information be present. The data link
  layer can also detect when frames are lost and
  request that those frames be sent again.

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Data Link Layer

• A data frame thats all packaged and ready to go
  follows this format
• The start indicator
• The source address
• The destination address
• The control portion like special handling
  instructions
• The actual data
• The error control segment or the CRC

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Data Link Layer

• Bridges, intelligent hubs, and network interface
  cards are devices typically associated with the
  data link layer.
• Two sub-layers make up the data link layer
• Media Access Control (MAC)
• Logical Link Control (LLC)

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Network Layer

• The network layer makes routing decisions and
  forwards packets for devices that are farther
  away than a single link. (A link connects two
  network devices and is implemented by the data
  link layer. Two devices connected by a link
  communicate directly with each other and not
  through a third device.) In larger networks there
  may be intermediate systems between any two end
  systems, and the network layer makes it possible
  for the transport layer and layers above it to
  send packets without being concerned about
  whether the end system is immediately adjacent or
  several hops away.

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Network Layer

• The network layer translates logical network
  addresses into physical machine addresses (the
  numbers used as destination IDs in the physical
  network cards). This layer also determines the
  quality of service (such as the priority of the
  message) and the route a message will take if
  there are several ways a message can get to its
  destination.
• It also may break large packets into smaller
  chunks if the packet is larger than the largest
  data frame the data link layer will accept. The
  network reassembles the chunks into packets at
  the receiving end.
• Routers and gateways operate in the network layer.

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Network Layer

• The network layer is concerned with controlling
  the operation of the subnet. A key design issue
  is determining how packets are routed from source
  to destination.
• If too many packets are present in the subnet at
  the same time, they will get in each others way,
  forming bottlenecks. The control of such
  congestion also belongs to the network layer.

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Network Layer

• The network layer serves to support
  communications between logically separate
  networks. This layer is concerned with the
  following.
• Addressing, including logical network addresses
  and service addresses
• Circuit, message, and packet switching
• Route discovery and route selection
• Connection services, including network layer flow
  control, network layer error control, and packet
  sequence control.
• Gateway services.

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Transport Layer

• The transport layer ensures that packets are
  delivered error free, in sequence, and with no
  losses or duplications. The transport layer
  breaks large messages from the session layer into
  packets to be sent to the destination computer
  and reassembles packets into messages to be
  presented to the session layer in the destination
  layer.

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Transport Layer

• The transport layer typically sends an
  acknowledgment to the originator for messages
  received.
• The transport layer also determines what type of
  service to provide the session layer, and
  ultimately, the users of the network.
• The transport and network layers deal with the
  logical transmission of data.

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Session Layer

• The session layer allows application on separate
  computers to share a connection called a session.
  This layer provides services such as name lookup
  and security to allow two programs to find each
  other and establish the communications link.
• The session layer also provides for data
  synchronization and checkpointing so that in the
  event of a network failure, only the data sent
  after the point of failure need be resent.

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Session Layer

• This layer also controls the dialog between two
  processes, determining who can transmit and who
  can receive at what point during the
  communication. This session service is called
  token management. For some protocols, it is
  essential that both sides do not attempt the same
  operation at the same time. To manage these
  activities, the session layer provides a token
  that can be exchanged. Only the side holding the
  token may perform the critical operation.

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Presentation Layer

• Unlike all the lower layers, which are just
  interested in moving bits reliably from here to
  there, the presentation layer is concerned with
  the syntax and semantics of the information
  transmitted. A typical example of a presentation
  service is encoding data in a standard agreed
  upon way.

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Presentation Layer

• The presentation layer translates data between
  the formats the network requires and the formats
  the computer excepts. This layer does protocol
  conversion, data translation, compression and
  encryption, character set conversion, and the
  interpretation of graphics commands.

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Presentation Layer

• The network redirector operates at this level.
  The network redirector is what makes the files on
  a file server visible to the client computer. The
  network redirector also makes remote printers act
  as though they are attached to the local computer.

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Application Layer

• It provides services that directly support user
  application, such as database access, e-mail, and
  file transfers. It also allows applications to
  communicate with applications on other computers
  as though they were on the same computer.

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Application Layer

• Examples
• WWW
• FTP
• e-mail
• USENET
• Multimedia

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Network Layer

• Khaled M. Elleithy, Ph.D
• elleithy_at_ccse.kfupm.edu.sa
• Department of Computer Engineering
• King Fahd University of Petroleum and Minerals
• Dhahran, Saudi Arabia

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Topics Covered in this Session

• Services to the Transport layer
• Routing
• Congestion control

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Services

• The boundary between the network layer is the
  boundary between the subnet (the set of all
  routers) and the host. In other words, the
  services provided by the network layer are the
  services provided by the subnet.

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Services Specifications

• The services should be independent from the
  subnet technology
• The transport layer should be shield from the
  number, type, and topology of the subnets present
• A uniform numbering plan should be used across
  LANs and WANs

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Services types

• Connectionless
• The packets carry its full destination address
  and sent independently of other packets. All
  error detection, correction and flow control is
  done in the host.
• Argument the subnet is unreliable no matter who
  it is designed.

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Services types

• Connection oriented
• Establishment of connection
• Negotiating Qos
• Transmission session
• Connection oriented

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Services types

• The argument of connection oriented vs.
  connectionless is a question of where to put the
  complexity in the network layer or in the
  transport layer?
• Both classes of services are allowed in the OSI
  model.

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Internal Organization

• Virtual Circuits (VCs)
• Used with connection oriented service
• A route from the source machine to the
  destination machine is established in setup
• The route is used for all traffic
• When the connection is released, the VC is
  terminated

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Internal Organization

• Datagrams
• No routes are worked out in advance
• Each packet is routed independently
• Successive packets many follow different routes
• Adapt easily to failures and congestion

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Comparison
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Routing

• Design issues
• Correctness
• Simplicity
• Robustness
• stability
• Fairness
• Optimality

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• Adaptive
• Routing decisions are based on measurements or
  estimates of traffic and topology
• Non-adaptive
• Routing decisions are static. Dont depend on
  changing measurements

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Classes of Routing Algorithms

• Centralized Routing
• Collecting information from the entire subnet to
  make optimal decisions
• Isolated Routing
• Each router collects information from neighboring
  routers
• Distributed algorithms
• Using mixture of local and global information

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Shortest Path Routing

• Meaning of shortest path
• Number of hops
• Distance in kilometers
• Mean queue length and transmission delay
• In general, it is a weighted function using all
  the above

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Shortest Path Routing

• It is a centralized routing algorithm. The
  shortest path is computed between all pairs in
  the subnet. The paths are stored in routing
  tables in the routers.
• If a certain router fails, the algorithm has to
  compute all routes again.

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Multipath Routing

• When a packet arrives to a router, a certain path
  is chosen from the different alternatives in the
  routing table.
• In case of VCs, the route is chosen at the VC
  setup.

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Centralized Routing

• Examples
• Shortest path routing
• Multipath routing
• Centralized routing works fine if the traffic
  doesn't change frequently.
• A Routing Control Center (RCC) is used to compute
  the routing tables

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Centralized Routing

• Drawbacks
• Time consuming in delivering information,
  computing tables, delivering tables.
• A failure in the RCC will cause the network to
  crash.
• Routers closer to RCC get their tables faster
  than others.
• Heavy traffic near the RCC

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Isolated Routing

• Examples
• Hot potato
• Backward learning
• Delta routing
• Flooding

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Isolated Routing

• Hot potato
• When packets arrive to the router they are
  forwarded to the shortest queue without regard to
  where this queue leads to.
• A static weight of the route can be combined with
  the queue length as criterion for forwarding

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Isolated Routing

• Backward learning
• Include with each packet the identity of the
  source and a counter that is incremented at each
  hop. This counter is used by a router receiving
  the packet to know how many hops to the source.
  This information is kept in the routers table
  and is updated if better values are received
  later.

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Isolated Routing

• Delta routing
• A hybrid technique between centralized and
  isolated routing
• A centralized step is done first through the RCC
• Based on local measurements the router can chose
  among different alternatives provided by the RCC

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Isolated Routing

• Flooding
• An incoming packet is sent to all the lines
  except the incoming one
• A counter is initialized with the maximum number
  of hops in the subnet
• The counter is decremented at each hop
• The packet is discarded when the counter reaches
  zero to avoid circulation of packets.

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Congestion Control
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Congestion Control

• Congestion can be brought by several factors
• Routers are slow to keep the required
  bookkeeping.
• Output links are slower than input links

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Congestion Control

• Congestion tends to feed upon itself and become
  worse. If a router discards a packet because it
  doesnt have an empty buffer to queue the
  incoming packet, the discarded packet will be
  transmitted later. Extra traffic is pumped in the
  subnet which causes congestion to buildup.

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Algorithms

• Preallocation of buffers
• Packet discarding
• Choke packets

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Preallocation of Buffers

• The call request packet marks routing tables when
  a VC is setup.
• The call request packet reserve the required
  buffers for the VC.
• If the required buffers arent available in
  advance, the VC can't be established

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Packet discarding

• Nothing is reserved in advance.
• If a packet arrives to a router and there is no
  buffer to queue it, it is discarded.
• The discarded packet will be retransmitted later
  by the sender.
• Piggybacked acknowledgements may be lost due to
  packet discarding

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Choke packets

• A mechanism that is triggered only when the
  system starts to be congested.
• The router monitor the utilization levels of
  output lines.
• Whenever an output line exceeds a certain
  threshold, it becomes in a warning state.
• Choke packets are sent to hosts using lines in
  warning state to reduce the traffic.

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Choke packets

• The host reduces the traffic.
• After sometime the host can increase the traffic
  if no choke packets are coming from the same
  router.

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Flow Control

• Flow control is used to control the traffic
  between to hosts. Host (A) can't send traffic to
  host (B) that it can't handle although the subnet
  may be able to handle.
• Does flow control solves the congestion problem?
  Why?

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Other issues

• There are other issues related to the network
  layer that are discussed in other sessions, such
  as
• Internetworking (How networks differ,
  fragmentation, firewalls)
• The network layer in the Internet (IP protocol)

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