QMCS 370 - Class Today

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QMCS 370 - Class Today

• Essentials of packets
• Protocol stack structures
• ISO
• TCP/IP

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Essentials of packets

• (Review)
• What do packets accomplish?
• How does this work?
• What errors must packets tolerate?

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Packet notes

• Elements
• Error detection
• ACKs (what about NAKs?)
• Flow control
• Packet Duplication
• Practical Objectives
• Error detection not always error recovery
• Flow control
• Efficiency
• Addressability

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Protocol Layering

• Why break things into layers (modules)?
• What kinds of layers make sense in a network?

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Layering Example

• Pizza Delivery
• Figure 5.18
• Give protocol interactions at each layer

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Layering and Net Architecture

• Layering is reflected in software architecture
  and in the packet formatting
• (what do we mean by layering?)
• Details may be different between packets and
  software, but they follow a parallel structure

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Protocol Stacks

• Specific cases of layering
• TCP/IP Protocol Stack
• ISO Reference Model
• ISO Protocol Stack

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TCP/IP Stack

• Applications at the top
• TCP/UDP
• IP
• Link layers
• Device Drivers

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Software and Packets

• Software and flexibility
• Extensible applications library
• Interchangeable link layers
• IP provides the glue for the network
• TCP/UDP provide application flexibility
• Packet formatting
• Data for higher layers embedded in lower-layer
  packets
• Physical packets sent on local network
• Internet packets travel largely unchanged
• Physical packet changes from one net to next

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How the Internet developed

• Similar to open source model
• Early days before commercialization
• Protocols became Internet Protocols if they
  were demonstrated to work by multiple members of
  the community
• Rough Consensus and Running Code
• Reference Implementations required of new
  protocols
• Others used these implementations to try to
  deploy the new protocol
• Protocol specs were revised based on practical
  experience

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ISO/OSI Model

• Major driver for the layering concept
• Used layering concept from software design
• Developed before the Internet
• Early in network development
• Driven by phone companies
• Based on needs and expectations, not experience
• Seven (7!) layers
• It is fashionable to map other protocol
  architectures onto this model, however
  imperfectly they might match
• Mapping TCP/IP to the 7 layers

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ISO Protocol Stack

• A real stack implementation based on the 7 layer
  ISO model
• It was supposed to replace TCP/IP
• Governments, DOD, phone companies promoted it
• It failed
• Standardize first, implement second
• Interoperability problems too many options, no
  accepted profiles of common feature
  implementations
• Software was expensive compared to
  quasi-open-source Internet protocols
• Desktops embraced TCP/IP (free source code that
  worked)
• Then came the Web....

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TCP/IP in Practice

• Stacking for Routers
• Role of IP vs TCP vs UDP
• IP talks between hosts
• TCP/UDP connects IP to processes

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Router Stacking

• Which layers are in endpoint hosts?
• Which stack layers are in routers?

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IP Internet Protocol

• Applications dont use IP directly
• IP talks between hosts, not processes
• IP only has host-related addressing
• Key point what do addresses mean at a given
  level of the protocol stack?
• Missing information must be filled in by other
  layers

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UDP User Datagram Protocol

• Easy protocol just carries the port number
• NO guarantee of delivery
• NO guarantee of delivery in order
• Packet duplication is possible

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TCP

• The reliable protocol
• Flow control Error detection and correction
• Assembles data into a stream of bits and bytes
• Packets are placed in order of transmission
• Setting up a connection
• SYN
• SYN-ACK
• ACK
• Reliability and flow control
• Sequence Numbers
• Sliding Window
• Ending a connection
• FIN in each direction

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