Johan J' Lukkien, j'j'lukkientue'nl

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Computer Networks2002/2003

• Layering protocol stacks
• Johan Lukkien

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Architectural pattern layers

• The Layers architectural pattern helps to
  structure applications
• that can be decomposed into groups of subtasks
• in which each group of subtask is at a particular
  level of abstraction
• from Pattern-oriented software architecture,
  Buschmann et.al
• Layering supports
• stable interfaces
• re-use
• mastering complexity separation of concerns

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Layering

• Packets impose additional structure onto
  signals...
• ...Several packets can form again a higher level
  structure...
• Network software is organized in layers of
  protocols, services and interfaces
• separation of concerns
• separation of what is provided (the service) from
  how it is accessed (the interface) and internally
  realized (the protocol)
• different layers have different concerns
• ease of design

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Layering in communication
provided service
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Protocol stacks
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Protocol stacks

• Level k message is payload of level k-1
• add headers and trailers of level k-1
• Binding exact description of
• A mapping one protocol onto another
• messages, communication behavior
• B mapping abstract functions onto a service
  interface
• connecting layers at the interface
• Tunneling level k-1 tunnels level k
• transport without interpretation of payload
• term tunneling usually reserved for a stack in
  which a service occurs twice

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OSI reference model

• Open Systems Interconnection
• Abstract architecture of communicating systems
• Names and identifies relevant concepts
• services, protocols and interfaces
• service data units (xSDU) and protocol data units
  (xPDU), x is the layer
• Introduces layer semantics
• issues solved by a layer layer services
• Description of how it should be done

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The OSI model
PSDU
SSDU
TSDU
NSDU
DSDU
PhSDU
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TCP/IP model
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Remarks

• TCP/IP model
• named after the dominating protocols
• model constructed after the fact
• host-to-network
• not really a layer just get IP packets across
• OSI model
• well-discussed, before designing protocols
• OSI protocol stack
• not widely used

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Some stacks and the OSI model
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Model in the book...
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Communication modes

• Sender and receiver
• One to one
• One to many
• un-addressed broadcast
• collection of destinations multicast
• Many to one
• messages appear at receiver in undetermined order
• Fragmentation synchronization
• send and receive of message combined
• fixed units (packets, frames, cells)
• streaming
• synchronization on availability of data and
  buffers

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Communication modes

• Connections
• Connection oriented
• Setup connection, communicate, destroy connection
• control reliability, flow, congestion
• Connection less
• Single shot just send a message/packet
• Message existence
• Persistent subsystem will deliver messages
• Transient message life depends on life of
  sender, receiver, subsystem
• Timing
• Time-independent
• Time-dependent (real-time)
• soft real-time, hard real-time

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Issues, to be resolved by the layers

• Error correction
• Flow control
• Addressing
• Multiplexing
• Naming
• Congestion control
• Mobility
• Routing
• Fragmentation
• Security
• ....

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Layer semantics trade-offs

• Important design decision the choice in which
  level to solve a particular issue
• too high inefficient
• error correction at the application layer
• broadcasting at the transport layer
• too low issue may re-occur
• multiplexing at the link layer
• connection orientation below the network layer
• Transparency hiding lower layer details
• good for portability, generality etc.
• but must be considered carefully
• e.g. mobility, location, multi-casting

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Layer generality re-usability

• Layer k cooperates with many different types
  layers k-1
• moderate required service
• pitfall layer becomes
• complex and large
• Layer k provides useful services to many
  different types k1
• powerful provided service
• must be possible to indicate several next layers
• pitfall layer becomes complex and large
• Hence, work towards standardization as quickly as
  possible

Service required by layer k
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Definition service

• Service an overall functionality (semantics) of
  an object.
• Service quality non-functional properties of a
  service (e.g. speed, reliability, ...).
• Service interface actions (primitives) and
  responses that make the service available these
  responses can be autonomous (events). In
  addition, a specification that
• describes their effect on state variables and
  parameters, as well as their results
• describes rules as how and in what sequence to
  call them
• describes the functional and non-functional
  properties of sequences of calls.

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Service examples (cntd)

• Typically, the communication modes
• Quality dimensions
• reliable, acknowledged, guaranteed bandwidth, low
  latency

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Example primitives for connection oriented
service
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Service examples (cntd)

• Frame relay
• connection oriented
• no error or flow control
• typically, for connecting LANs
• X.25
• connection oriented, typically across phone lines
• still supported in e.g. GPRS

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Definition protocol

• Protocol A formal set of rules that dictates how
  information exchange as well as interaction
  between objects (can be devices, execution
  threads, etc.) should take place.
• The rules specify
• the format of the messages exchanged
• a number of different protocol states and what
  messages are allowed to be sent in each state
  these states determine, among others, the order
  of the messages.
• timing constraints and other non-functional
  properties, if any.

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Exercises

• Exercises 11, 12, 13, 19, 20 on p82
• Discuss the transmission of a letter through the
  postal system. Describe service, interface and
  protocol.
• Discuss the trade-offs of providing reliable
  versus datagram service at the data link layer
  when the transport layer must provide reliable
  connection oriented service. Take into account
• the probability of packet loss per link
• the number of hops
• the delay needed to judge when a retransmission
  is needed
• the resulting throughput at the transport layer