Konzepte und Methoden der Systemsoftware Kapitel 9: Rechnernetze

1
Konzepte und Methoden der Systemsoftware Kapitel
9 Rechnernetze

• Holger Karl

2
Goals of this chapter

• Previous chapter has introduced the notion of
  channel objects to abstract away how data is
  exchanged between two processes
• This abstraction is OK for two processes on the
  same machine but what about communication
  across machine boundaries?
• This chapter treats this problem it starts from
  the simple case of two directly connected devices
  and generalizes to larger networks
• Goal is to provide a rough understanding on
• Which typical problems in computer networks
  exist,
• Why they occur,
• How a solution could look like.
• Details on solutions will be treated in a
  separate lecture

3
Overview

• Examples
• Direct connection between two devices
• Multiple devices
• Errors

4
How to bridge across devices with a channel
object?

• Channel objects used as a bridge
• Need some functionality on both sides of such a
  bridge stubs
• How to organize these stubs?

5
Basic example 1 World Wide Web

• What happens when you enter http//www.uni-paderb
  orn.de into a Web browser?

??
6
Basic example 2 Telephony

• What happens when picking up a telephone and
  making a phone call?
• How to find the peers phone? How to transmit
  speech?

?

• What are the crucial differences between
  transferring a Web page and a phone call?
• Web Bunch of data that has to be transmitted
  correctly
• Phone Continuous flow of information, must
  arrive in time, limited amount of errors
  acceptable

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What to communicate Information, data

• Data
• A formalized representation of facts, concepts,
  ideas, descriptions,
• Example text, speech
• Information
• A human interpretation of data, conferring
  meaning to data
• In this sense, a human-oriented term
• Only data can be communicated, information is
  restored by the human receiver
• Human to human, machine to machine, human to
  machine

InformationFacts, concepts,ideas,
Conventionsfor representation
Abstractworld
DataFormalized representation of information
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Overview

• Examples
• Direct connection between two devices
• Signals
• Low-level communication properties
• Duplex
• Multiple devices
• Errors

9
Simplest communication Direct physical connection

• Web example Browser (i.e. client) and server
• Simplest case directly connect them by a (pair
  of) cable
• Server provides data, client consumes it
• Telephony Connect two telephones via a (pair of)
  cable

Client
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What good is a physical connection? Signals

• Data has to be represented by signals
• Signal Representation of data by characteristic
  changes (in time and/or space) of physical
  variables
• Material example Letters on paper
• Immaterial example
• Acoustic waves when speaking
• Current or voltage in a wire
• Immaterial signals in physical media enable data
  communication between remote senders and receivers

Information
Conventionsfor representation
Abstractworld
Data
Conventionsfor representation
Physicalworld
Signals
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Bits and signals

• What should be communicated Data, represented as
  bits
• What can be communicated between remote entities
  Signals
• Needed a means to transform bits into signals
• And from signals back into bits at the receiver
• A simple convention for a copper wire
• A 1 is represented by current
• A 0 is represented by no current
• (Not practical, more sophisticated conversions
  necessary)
• Questions How to detect bits, decide on their
  length, handle errors?

12
Low-level properties of communication

• Transmitting signals on a physical carrier
  results in some low-level properties
• Propagation delay d How long does it take for a
  signal to reach receiver?
• Propagation speed v How fast does a signal
  travel in the medium?
• Electromagnetic waves in vacuum speed of light
  vc, in copper v¼ 2/3 c
• d distance / v
• Data rate r With which data rate (in
  bits/second) can a sender transmit?
• (EOT SOT) Data size / data rate
• Error rate What is the rate of incorrect bits
  arriving at the receiver?
• Also what error patterns?

Message Sequence Charts (MSC)
Start oftransmission
End oftransmission
Time
Delay d
Distance
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How can a wire store data bandwidth-delay
product

• What happens in the first d seconds after
  transmission starts?
• Bits are transmitted and propagate towards the
  receiver
• Sender keeps sending bits
• First bit arrives after d seconds
• In this time, sender has transmitted dr bits
• Where are they? Stored in the wire!
• dr is the product of delay and data rate
• Commonly called bandwidth/delay product
• Crucial property of high-performance networks

Start oftransmission
Delay d
Time

• Example (transcontinental)
• Data rate 100 Mbit/s
• Delay 6000km/(2/3c) ¼ 0.03 s
• dr ¼ 3 Mbit (in the wire)

14
Two physical connections for two-way
communication?

• Two-way communication
• Telephony Both parties want to say something
• WWW Server needs to know which webpage shall be
  delivered

• Different cases possible
• Simplex Only one party transmits
• Example Radio broadcast many recipients at the
  same time
• Half duplex Parties alternatively send data
• Example Conversation
• Full duplex Both parties send all the time

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How to realize duplexing

• Simplex operation trivial
• Half duplex
• Two pairs of cables, one for each direction
  wasteful
• Use one cable intelligently participants
  alternatively transmit, wait their time until it
  is their turn
• Both sending at the same time would not work,
  signals interfere
• Problem How can one node decide that the other
  is done sending?

A ! B
B ! A
A ! B
A ! B
B ! A
B ! A
B ! A
Time

• Time division duplex TDD

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How to realize duplexing

• Full duplex
• Two pairs of cables would work, but still
  overhead (installation, maintenance, ) does it
  work with one cable also?
• Exploit some properties of the physical medium
• Here transmissions in different frequencies do
  not interfere
• Idea use different frequencies for transmission
  in different directions

B ! A
Frequency
A ! B
Time

• Frequency Division Duplex FDD

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How to realize duplexing

• Full duplex by time division duplexing?
• Sounds like a contradiction both A and B always
  have data to send, but have to take turns?
• Having data to send corresponds to a
  certaindata rate bits per second
• How about intermediately storing data when the
  other station is currently sending? Then quickly
  send all stored new data

Data to be transmitted
A
B
A
B
A ! B
B ! A

• TDD can realize full duplex if transmission over
  medium is at least twice as fast as data is to be
  transmitted

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Lessons learned from duplexing

• It is useful to distinguish between
• Requirements on what should be possible
• Rules and methods how to implement such
  requirements
• Example Implement a full duplex requirement
  using TDD
• This distinction will become very important
• Formalized later as service versus protocol
• Buffering is an important means to decouple
  different dynamics in time
• Questions of buffer overflow have to be considered

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Overview

• Examples
• Direct connection between two devices
• Multiple devices
• Multi-hop connections
• Switching
• Forwarding
• Multiplexing
• Multiple access
• Routing
• Errors

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But there are more than two computers/telephones

• Connect each telephone/computer with each other
  one?

With eleven computers
With four computers

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Beirut connections

• Connecting many phones in real life

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Beirut connections
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Put some structure into a network

• Pair wise connecting all entities does not work
• Need some structure
• Distinguish between end systems/terminals/user
  devices on one hand, switching
  elements/routers on the other hand

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Typical network structures for local installations

• Busses
• All nodes connected to a single wireline
• Cheap, but relatively inefficient, error-prone
• Rings
• Nodes connected to a ring-shape network
• Can compensate for a single break of pairwise
  connection
• Star
• All nodes directly connected to a central cabling
  hub
• Again error-prone, but easy to administer, manage

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How to communicate over a switching element

• Using switching elements, there is no longer a
  direct physical connection between two terminals
  How to send signals nonetheless?
• Option 1 Have the switching element dynamically,
  on demand configure an electrical circuit between
  terminals
• Act as a real switch compare Fräulein vom Amt
• Resulting circuit lasts for duration of
  communication
• Circuit switching

http//www.wdrcobg.com/switchboard.html
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Circuit switching evaluation

• Advantages of circuit switching
• Simple
• Once circuit is established, resources are
  guaranteed to participating terminals
• Once circuit is established, data only has to
  follow the circuit
• Disadvantages
• Resources are dedicated what if there is a
  pause in the communication?
• Circuit has to be set up before communication can
  commence
• Alternatives?

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Packet switching

• Avoid setting up a circuit for a complete
  communication
• Instead chop up data into packets
• Packets contain some actual data that is to be
  delivered to the recipient (can have different
  size)
• Also need administrative information, e.g., who
  the recipient is
• Sender then occasionally sends out a packet,
  instead of a continuous flow of data
• Problems How to detect start and end of a
  packet, which information to put into a packet,

Packet 1
Packet 2
Packet 3
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Packet switching II
To Z

• Switches take on additional tasks
• Receive a complete packet
• Store the packet in a buffer
• Find out the packets destination
• Decide where the packet should be sent next to
  reach its destination
• Information about the network graph necessary
• Forward the packet to this next hop of its
  journey
• Also called store-and-forward network

To Y
To X
To Y
To Z
To Z
Buffer
Pick next hop
To Z
To Y
To X
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Multiplexing

• Previous example had two packets at the head of
  the queue destined for terminal Z
• Similar situation a switching element has only a
  single outgoing connection

• Such a special case is called a multiplexer
• Organizing the forwarding of packets over such a
  single, shared connection is called multiplexing
• Multiplexers in general need buffer space as well

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Multiplexing II

• Obvious option Time Division Multiplexing (TDM)
• Serve one packet after the other divide the use
  of the connection in time

To Z
To Z

• Alternative Frequency Division Multiplexing
  (FDM)
• Use different frequencies to transmit several
  packets at the same time

(in frequency 2)
To Z
(in frequency 1)
To Z
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Multiplexing III

• Some other alternatives exist
• Code Division Multiplexing (CDM), Space Division
  Multiplexing (SDM)
• Mostly relevant in wireless communication only,
  not covered here
• Obvious parallels/relations to duplex operation!
• Multiplexing describes how to operate several
  pairs of communicating entities
• Duplexing describes how a given pair of
  communicating entities can exchange data

Question How to combine different duplex and
multiplexing schemes?
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Mutliplexing virtualization

• In essence, mutiplexing serves one particular
  purpose
• It abstracts away that a physical connection has
  to be shared with other contending entities, each
  sending a logical flow
• It allows the entities connected to the switch to
  imagine that they alone are using the physical
  connection
• At somewhat lesser properties
• Multiplexing virtualizes the actual, physical
  connection
• It needs a peer entity, a demultiplexer, to
  restore the original flows
• This concept of virtualizing and enriching the
  properties of a simpler subsystem is a pivotal
  technique for communication networks
• As it is in software engineering, operating
  systems, etc.
• It allows us to think in terms of increasingly
  more complex communication systems, build one
  atop the other

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Multiplexing shared resources
Virtually shared, but exclusively controlled!

• Multiplexing can be viewed as a means to regulate
  the access to a resource that is shared by
  multiple users
• The switching element/its outgoing line
• With the switching element as the controller
• Are there other examples of shared resources?
• Classroom, with air as physical medium
• A shared copper wire, as opposed to direct
  connection
• Characteristic a broadcast medium!

Shared!
Shared!
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Broadcast medium and multiple access

• Common characteristic of a broadcast mediumOnly
  a single sender at a time!
• Exclusive access is necessary
• There are some exceptions using CDMA, but thats
  advanced material
• Exclusive access is simple to achieve with a
  multiplexer
• What if no multiplexer is available?
• Exclusive access has to be ensured by all
  participants working together
• The problem of multiple access to a shared medium
• Medium access for short
• Rules have to be agreed upon
• Classroom approach only speak when asked to,
  central instance

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An intermediate summary

• So far, we have a rough idea about
• Converting bits to signals and back again
• Duplexing, switching, multiplexing
• Packets as unit of data transport
• Multiple access of several entities to a shared
  medium
• In essence, we know how to connect several
  entities into a flat or hierarchical network
• Missing piece for hierarchical networks How to
  know where to send a packet
• For circuit switching how to setup the circuit

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Forwarding and next hop selection

• Recall A switching element/a router forwards a
  packet onto the next hop towards its destination
• How does a router know which of its neighbors is
  the best possible one towards a given
  destination?
• What is a good neighbor, anyway?

P
Z
V
A
X
U
W
M
Y
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Options for next hop selection

• Some simple options
• Flooding send to all neighbors
• Hot potato routing send to a randomly chosen
  neighbor
• Simple options not convincing
• Try to find good, i.e., short routes few hops
• Try to learn about the structure of the network,
  interpreted as a graph

Routing table of W
Destination

• Construct routing tables
• For each switching element separately
• Separate entry for each destination
• Contains information about the (conjectured)
  shortest distance to a given destination via each
  neighbor

M P Z
U 2 3 4
V 3 2 3
X 4 3 2
Y 4 4 3
Neighbor
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Routing tables

• Criteria
• Good/perfect estimate of real distances, absence
  of loops,
• Constructing routing tables
• Initially, typically empty how should a new
  node know anything?
• Passive observe ongoing traffic (e.g., from hot
  potato routing) and try to extract information,
  successively improve table correctness
• Actively exchange information between routers to
  try to learn network structure routing
  protocols
• Problem Size!
• In large networks, maintaining routing entries
  for all possible destinations quickly becomes
  infeasible
• Solution hierarchy treat similar nodes
  identically (divide et impera) ! internetworking

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Overview

• Examples
• Direct connection between two devices
• Multiple devices
• Errors

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Handling errors, overload,

• So we are done building a network unless
  something goes wrong!
• Source of errors/abnormal situations
• Conversion from signals to bits can fail
• Access to a shared medium might not work
• Packets can be lost, e.g., because buffers
  overflow
• Packets can be misrouted (because of incorrect
  routing tables), delayed, reordered
• Receiver might not be able to keep up with
  incoming stream of packets
• Routers can fail, resulting in incorrect routing
  tables
• and many more

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Handling errors, overload,

• Error control at various abstraction levels
  needed
• Between two direct neighbors, over a given
  connection
• Between end systems, to compensate for errors not
  detected locally e.g., incorrect order of
  packets
• Overload control
• Protect the network against buffer overflows,
  regulate the number of packets injected into the
  network Congestion control
• Protect end system against too many packets
  coming in Flow control
• Where and how to implement error and overload
  control is a principal architectural decision
• Main options in the end system or in the network
• Big difference between telephony system and
  Internet
• Telephony carriers are (traditionally) interested
  in network-based solutions to be able to charge
  for it

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Intermediate summary Basic required functions

• Bit-to-signal and signal-to-bit conversion
• Grouping bits into packets
• Accessing a shared medium
• Switching, duplexing, multiplexing
• Controlling errors on a connection between two
  systems
• Forwarding incoming packets, consulting routing
  tables
• Constructing routing tables, maintaining them
• Controlling errors not detectable between two
  neighboring systems
• Protecting the network against overload
• Protecting end systems against overload
• Ensuring correct order and possibly timeliness of
  packets
• Making these functions accessible from
  application programs
• Controlling the actual hardware that connects a
  wire to a computer
• and more!

43
System structure

• Any chance to build a single, monolithic
  system/piece of software that realizes all these
  functions, from wire to application? No way!
• To give an idea Size of telecommunication and
  astronautic software (Siemens telephony switches)

44
System structure layers

• Only feasible option Build virtual subsystems
  of increasing capabilities and abstraction levels
• Example Multiplexing abstracted away the
  physical connections capability to support a
  single transmission into a logical link that can
  be used by several entities
• Subsystems are called layers
• Each layer uses capabilities of a lower
  subsystem, adds own rules and procedures, to
  provide a more useful, advanced service
• Services expose a well defined interface to
  higher layers
• Services are accessible at their Service Access
  Point (SAP)
• A service is a promise what is going to happen to
  data when delivered to the SAP

45
Service

• Service Any act or performance that one party
  can offer to another that is essentially
  intangible and does not result in the ownership
  of anything. Its production may or may not be
  tied to a physical product.
• D. Jobber, Principles and Practice of Marketing
• Focus is on the output, the result of the service
• NOT the means to achieve it

46
Overview

• Structuring communication systems into layers
• Services
• Service primitives
• Properties of service primitives
• Example sockets
• Protocols
• Reference models
• Standardization

47
Services and layers

• Layers provide services to their users, at their
  interface
• Convention consecutive numbering, higher numbers
  for more abstract layers/services
• Service can be accessed at different places by
  different users

Service Access Point (SAP) of Layer 1
Transmission of bit sequences (possibly
erroneous)
Layer 1Bit ! Current/Current ! Bit conversion
Uses
Service Access Point (SAP) of Layer 0 Imperfect
transmission of electromagnetic current
Layer 0Physical connection
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Service primitives

• Service primitives set of operations available
  at the interface of a service (channel object
  API)
• Formal definition of the service
• Example for Bit Transmission Layer
• SEND_BIT sender can ask for the delivery of a
  bit to the receiver
• RECEIVE_BIT receive can wait for the arrival of
  a bit (blocking)
• INDICATE_BIT indicate to the receiver that bit
  is now available (asynchronous)
• Many different ways to structure service
  primitives conceivable
• Already covered Request, Indication, Response,
  Confirmation
• Unit of data individual bytes, messages

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Service primitives correctness requirements

• For both message and byte stream oriented (set
  of) service primitives, correctness requirements
  are important
• Completeness All data that is sent is eventually
  delivered
• Correctness If data is delivered, its is
  correct, i.e., the data that has been actually
  sent
• Messages are not modified, original version is
  delivered
• Byte sequence is free of errors
• In order Byte sequence/sequence of messages is
  delivered in the order it has been sent
• Dependable secure, available,
• Confirmed Reception of data is acknowledged to
  the sender
• Not all requirements are always necessary

50
Connection-oriented vs. connection-less service

• Recall telephony vs. postal service
• Service can require a preliminary setup phase,
  e.g., to determine receiver ! connection-oriented
  service
• Three phases connect, data exchange, release
  connection
• Alternative Invocation of a service primitive
  can happen at any time, with all necessary
  information provided in the invocation !
  connection-less service
• Note This distinction does NOT depend on circuit
  or packet switching connection-oriented
  services can be implemented on top of packet
  switching (and vice versa, even though a bit
  awkward)
• Connection-oriented services must provide
  primitives to handle connection
• CONNECT setup a connection to the communication
  partner
• LISTEN wait for incoming connection requests
• INCOMING_CONN indicate an incoming connection
  request
• ACCEPT accept a connection
• DISCONNECT terminate a connection

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Typical examples of services

• Datagram service
• Unit of data are messages
• Correct, but not necessarily complete or in order
• Connection-less
• Usually insecure/not dependable, not confirmed
• Reliable byte stream
• Byte stream
• Correct, complete, in order, confirmed
• Sometimes, but not always secure/dependable
• Connection-oriented
• Almost all possible combinations are conceivable!

52
BSD sockets as service interface for computer
networks

• Weve already covered BSD sockets as one service
  interface for intra-device channel objects
• They should also apply to inter-device channel
  objects and they do!
• Model the network service like the access to a
  file
• Sending data Writing to a file
• Receiving data Reading from a file
• Opening a file Connecting to a communication
  peer
• Files in UNIX are treated via so-called file
  handles
• By analogy, file handles will represent
  communication peers
• ! Same thing as treated already!

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Datagram communication over sockets

• Simplest possible service unreliable datagrams

• Sender
• int s socket ()
• sendto (s, buffer, datasize, 0, to_addr, ad
  dr_length)
• to_addr and addr_length specify the destination

• Receiver
• int s socket ()
• bind (s, local_addr, )
• recv (s, buffer, max_buff_length, 0)
• Will wait until data is available on socket s and
  put the data into buffer

54
Byte streams over a connection-oriented socket

• For reliable byte streams, sockets have to be
  connected first
• Receiver has to accept connection

• Client
• int s socket ()
• connect (s, destination_addr, addr_length)
• send (s,buffer, datasize, 0)
• Arbitrary recv()/send()
• close (s)
• Connected sockets use a send without address
  information

• Server
• int s socket ()
• bind (s, local_addr, )
• listen (s, )
• int newsock accept (s, remote_addr, )
• recv (newsock, buffer, max_buff_length, 0)
• Arbitrary recv()/send()
• close (newsock)

55
Sockets UNIX domain or cross-device?

• The previous two slides looked almost identical
  to the sockets example in the previous chapter
• How does the service (behind the interface) know
  what type of function is required?
• Solution socket creation function makes the
  difference!
• UNIX domain stream socket (AF_LOCAL,
  SOCK_STREAM, 0)
• UNIX domain datagram socket (AF_LOCAL,
  SOCK_DGRAM, 0)
• Cross-device socket, stream socket (AF_INET,
  SOCK_STREAM, 0)
• Cross-device socket, datagram socket (AF_INET,
  SOCK_DGRAM, 0)
• Anything else Identical (more or less)!

56
Overview

• Structuring communication systems into layers
• Services
• Protocols
• Reference models
• Standardization

57
Layers are distributed

• Previous example Bit sequence layer has to be
  present at both transmitter (bit ! electrical
  current) and at receiver (electrical current !
  bit)

Receiver
Sender
Bits
Bits
SAP of Layer 1
SAP of Layer 1
Layer 1 Bit ! Current conversion
Layer 1 Current ! Bit conversion
Layer 0 Physical connection
Pair of copper wires
58
Distributed layers need to follow rules
protocols

• Distributed parts of the layer 1 implementation
  have to follow the same set of rules protocols
• Suppose sender represented a 1 by current
  there receiver by no current

Receiver
Sender
Identical rules of operation
Matching rules of operation
Bits
Bits
SAP of Layer 1
SAP of Layer 1
Layer 1 Bit ! Current conversion
Layer 1 Current ! Bit conversion
Layer 0 Physical connection
Pair of copper wires
59
Protocols

• Protocols are a set of rules
• Describe how two (or more) remote parts of a
  layer cooperate to implement the service of the
  given layer
• Behavior, packet formats,
• These remote parts are called peer protocol
  entities or simply peers
• Use the service of the underlying layer to
  exchange data with peer

Use
Use

• Protocol is implementation of the service
• Not visible to service user

Protocol
Use
Use
60
Protocol specification

• The formal behavior, the rules which constitute
  the protocol have to be precisely specified
• One popular method (Extended) Finite State
  Machine (FSM)
• A protocol instance/protocol engine at each
  entity
• Protocol instance has several states
• E.g., for a protocol implementing a connection
  oriented service IDLE, CONNECTED,
  RELEASING_CONNECTION
• Events/transitions between states
• Message arrivals
• Real time / timer events
• Transition can have conditions
• Actions during transition are
• Send new message
• Set timer, delete timer,

STATE1
Condition Event Action
STATE2
61
Protocols and FSMs

• Finite state machines implement actual behavioral
  rules of a protocol
• Have to communicate with their remote peer
• Cannot do so directly, have to use service of the
  underlying communication layer
• Via service primitives, which can also provide
  arriving data to the protocol
• E.g., indications from lower layer are events to
  higher layer protocol engine

Layer n1
Use service primitives
Layer n
62
Protocols and messages

• When using lower-layer services to communicate
  with the remote peer, administrative data is
  usually included in those messages

• Typical example
• Protocol receives data from higher layer
• Adds own administrative data
• Passes the extended message down to the lower
  layer
• Receiver will receive original message plus
  administrative data
• Encapsulating
• Header ortrailer

Layer n1
Layer n1
Layer n
63
Protocol stacks

• Typically, there are several layers and thus
  several protocols in a real system
• Layers/protocols are arranged as a (protocol)
  stack
• One atop the other, only using services from
  directly beneath
• This is called strict layering

L4 protocol
Layer 4
Layer 4
L3 protocol
Layer 3
Layer 3
Cross-layer communicationObject of current
researchNot considered here
L2 protocol
Layer 2
Layer 2
L1 protocol
Layer 1
Layer 1
Physical medium (layer 0)
64
Layers do not care about distributed lower layers

• A given layer n1 does not care about that its
  lower layer is actually distributed, has remote
  FSMs,
• Layer n1 imagines layer n as something that
  just works, has service access points where
  they are necessary (a channel object!)
• In reality, layer n of course is distributed in
  turn, relying on yet lower layers
• At the end, the physical medium (layer 0) is
  transporting data/signals

65
Analogy Nested layers as nested translations

• Layers rely on services of lower layers
• Intermediate representation of data changes
• Vertical vs. horizontal communication
• Vertical always real
• Horizontal may be real or virtual

Verticalcomm.
Horizontal (real!) communication
66
Overview

• Structuring communication systems into layers
• Services
• Protocols
• Reference models
• ISO/OSI
• TCP/IP
• Standardization

67
How to structure functions/layers in real
systems?

• Many functions have to be realized
• How to actually group them so as to obtain a
  real, working communication system?
• Layering structure and according protocols define
  the communication architecture
• Two main reference models exist
• ISO/OSI reference model (International Standards
  Organization Open Systems Interconnection)
• TCP/IP reference model (by IETF Internet
  Engineering Taskforce)

68
ISO/OSI reference model

• Basic design principles
• One layer per abstraction
• Each layer has a well-defined function
• Choose layer boundaries such that information
  flow across the boundary is minimized (minimize
  inter-layer interaction)
• Enough layers to keep separate things separate,
  few enough to keep architecture manageable
• Result 7-layer model
• Not strictly speaking an architecture, because
  protocol details are not specified
• Only general duties of each layer are defined

69
ISO/OSI 7-layer reference model (two entities)
PDUProtocol Data Unit
Network protocol
Data link protocol
Physical layer protocol
70
ISO/OSI 7-layer reference model (complete network)
71
7 layers in brief

• Physical layer Transmit raw bits over a physical
  medium
• Data Link layer Provide a (more or less)
  error-free transmission service for data frames
  over a shared medium
• Network layer Solve the forwarding and routing
  problem for a network
• Transport layer Provide (possibly reliable, in
  order) end-to-end communication, overload
  protection, fragmentation
• Session layer Group communication into sessions
  which can be synchronized, checkpointed,
• Presentation layer Ensure that syntax and
  semantic of data is uniform between all types of
  terminals
• Application layer Actual application, e.g.,
  protocols to transport Web pages

72
ISO/OSI reference model Critique

• The reference model as such, in its structuring
  of functions into layers, is very influential
  until today
• Actual protocols developed for it are irrelevant
  in practice
• ISO failed in gaining actual market acceptance
  for its model
• Bad timing competing approaches already in
  market, lack of industry support
• Bad technology too big, too complex some
  design flaws
• Bad implementations initial implementations low
  quality
• Bad politics ISO/OSI conceived of as a
  bureaucratic thing

73
TCP/IP reference model

• Historically based on the ARPANET, later to
  become the Internet
• Started out as little university networks, which
  had to be interconnected
• In effect, only really defines two layers
• Internet layer packet switching, addressing,
  routing forwarding. Particularly for
  hierarchically organized networks (networks of
  networks) Internet Protocol (IP) defined
• Transport layer two services protocols defined
• Reliable byte stream Transport Control Protocol
  (TCP)
• Unreliable datagram User Datagram Protocol (UDP)
• In addition, (de)multiplexing
• Lower and higher layers not really defined
• Host to host communication assumed as a given
• Applications assumed

74
TCP/IP protocol stack
Nothing statedby TCP/IP model
75
TCP/IP Suite of protocols

• Over time, a suite of protocols has evolved
  around the core TCP/IP protocols

So-called hourglass model Thin waist of the
protocol stack at IP, above the technological
layers
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Naming addressing in the TPC/IP reference model

• Names Data to identify an entity
• Exist on different levels
• Alphanumerical names for machines
  info-stud.uni-paderborn.de
• Numeric names for a network device
• Depends on standard, e.g. Ethernet 48 bits,
  hexadecimal notation, example 080020aefd7e
• Usually called an address (partially justifiable,
  but actually a bit sloppy)
• Names for service access points in an end system
  Name of end system plus a numerical identifier
  for the SAP (a port number) www.google.de80
• Some of these port numbers are assigned by
  convention for standard SAPs, 80 web server
• Used to distinguish multiple processes/their
  sockets in a single end system

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Naming addressing in the TPC/IP reference model

• Address Data how/where to find an entity
• Address of a network device in an IP network An
  IP address
• IPv4 32 bits, structured into 4x8 bits
• Example 131.234.20.99 (dotted decimal notation)
• Address of a network Some of the initial bits of
  an IP address
• Note Other networks (notably, POTS/GSM) have
  very different naming/addressing architectures!
• E.g., a much clearer separation between identity
  and location of entities, cryptographic
  protection,

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Mapping
Web server process service access point

• Needed Mapping from name to address
• Realized by separate protocols
• From alphanumerical name to IP address Domain
  Name System (DNS)
• From MAC name/address to IP address Reverse
  Address Resolution Protocol
• Often also useful Mapping from IP address to MAC
  name/address Address Resolution Protocol (ARP)

wwwcs.uni-paderborn.de80
DNS
131.234.25.2780
ARP/RARP
080020aefd7e
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TCP/IP reference model Critique

• No clear distinction between service, protocol,
  interface
• Reliable byte stream is equated with TCP,
  although there is a clear difference
• Particularly below IP
• Very specialized stack, does not allow a
  generalization to other technologies/situations
• Great void below IP
• Many ad hoc, wildly hacked solutions in may
  places, without careful design
• Mobility support is a typical area where problems
  result later on

80
ISO/OSI versus TCP/IP

• ISO/OSI Very useful model, non-existing
  protocols
• TCP/IP Non-existing model, very useful protocols
• Hence Use a simplified ISO/OSI model, but treat
  the TCP/IP protocol stack in the context of this
  model
• With suitable add-ons especially for the lower
  layers

81
Some example protocols

• A communication architectures needs standard
  protocols in addition to a layering structure
• And some generic rules principles which are
  not really a protocol but needed nonetheless
• Example principle end-to-end
• Example rule Naming addressing scheme
• Popular protocols of the 5-layer reference model
• Data link layer Ethernet CSMA/CD
• Network layer Internet Protocol (IP)
• Transport layer Transmission Control Protocol
  (TCP)

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Medium Access à la Ethernet

• Access to a shared medium a resource management
  problem (Betriebsmittelverwaltung)
• First idea Just start transmission whenever
  anything to say
• Problem Collision, message is lost
• Better idea Listen first when nobody says
  anything, start own transmission, else wait
• Carrier Sense Multiple Access (CSMA)
• Problem What happens if two people start
  listening at about the same time? ! Collision!
• Even better Check whether a collision happened
  during transmission if so, abort transmission
• Carrier Sense Multiple Access with Collision
  Detection (CSMA/CD)
• ! Ethernet (IEEE 802.3) is CSMA/CD plus other
  rules

83
Repairing errors Repeat packet

• What happens if a packet is lost on the way?
• Idea 1 Have the receiver tell the sender that
  the packet was lost
• But how would the receiver know that a packet was
  on the way in the first place?
• ! Doesnt work!
• Idea 2 Turn idea 1 upside down receiver tells
  sender when a packet has arrived
• An acknowledgement
• When packet lost, acknowledgement will not arrive
• ! Sender can wait for acknowledgment, when it
  not arrives at expected time, resend the packet
• A timeout is used, forming an Automated Repeat
  Request (ARQ) protocol

84
Problem How can different error situations be
distinguished?

• How to tell apart
• Looks the same for the sender, but receiver will
  get different sequences
• Receiver needs to know whether a packet is a new
  one or one that is repeated
• Introduce sequence numbers to identify packets

85
Internet protocol (IP)

• Network is structured hierarchically to assist in
  routing/forwarding
• autonomous networks, sub-networks,
• Separate sub-protocols for routing table
  computation
• E.g., Border Gateway Protocol (BGP) between
  different high-level network regions
• Relatively complicated in detail
• Basic idea based on e.g. Dijkstras all-pairs
  shortest-path algorithm
• Forwarding
• Consult routing tables
• Relatively straightforward

86
IP packet format

• IP packet header
• Payload follows

32 Bit
87
Transmission control protocol

• Offered service Connection-oriented, reliable
  byte-stream transmission, provides flow
  congestion control
• Essential protocol mechanisms
• Connection establishment via handshaking
• Reliability via acknowledgements, timer,
  retransmissions
• Flow control Receiver can announce how much data
  it can consume sender must not send more than
  this amount
• Congestion control If packets are lost in the
  network, network is assumed to overloaded !
  reduce sending rate
• Basic idea Network is dumb, only offers
  best-effort forwarding of packets
• Added properties (e.g., reliability) have to be
  implemented in the end system

88
Overview

• Structuring communication systems into layers
• Services
• Protocols
• Reference models
• Standardization

89
Standardization

• To build large networks, standardization is
  necessary
• Traditional organization from a
  telecommunication/ telephony background
• Well established, world-wide, relatively slow
  time to market
• Internet
• Mostly centered around the Internet Engineering
  Task Force (IETF) with associated bodies
  (Internet Architectural Board IAB, Internet
  Research Task Force IRTF, Internet Engineering
  Steering Group IESG)
• Consensus oriented, heavy focus on working
  implementations
• Hope is quick time to market, but has slowed down
  considerably in recent years
• Manufacturer bodies

90
Standardization Traditional organizations

• ITU International Telecommunication Union
  (formerly CCITT und CCIR)
• CCITT Consultative Committee on International
  Telegraphy and Telephony (Comité Consultatif
  International Télégraphique et Téléphonique)
• CCIR Consultative Committee on International
  Radio
• CEPT Conférence Européenne des Administrations
  des Postes et des Télécommunications
• ISO International Organization for
  Standardization
• DIN Deutsches Institut für Normung
• German partner organization of ISO

91
ISO standardization work

• WG-Meetings
• Every 6-9 months
• National organizations have time to accept
  proposed concepts
• Then actual standarization process
• DP Draft Proposal
• DIS Draft International Standard
• IS International Standard
• Move a proposal to a higher level by
  incorporating all dissenting voices and
  international consensus
• Very slow process

ISO
Technical Committee (TC)
. . . . . . . .
SubCommittee (SC)
. . . . . . . .
Working Group (WG)
. . . . . . . .
92
IETF http//www.ietf.org

• IETF is organized in areas and Working Groups
• Volunteers from industry, academia, government
  organizations participate
• Drafts/proposal can be made by anybody
• An on-demand process
• To move beyond draft status, two independent,
  interoperating implementations are required
• Informal voting in working groups
• Humming
• Three meetings a year
• Result
• RFC request for comment, the actual standard
• FYI informal or informational
• In June 2005, there are 4114 RFCs
• Thats what the Internet is build on

Proposal, draft
Experimental
Informal
Proposed Standard
Draft Standard
Full Standard
93
Conclusions

• To keep complexity of communication systems
  tractable, division in subsystems with clearly
  assigned responsibilities is necessary layering
  
• Each layer (and the communication system as a
  whole) offers a particular service
• Services become more abstract and more powerful
  the higher up in the layering hierarchy
• To provide a service, a layer has to be
  distributed over remote devices
• Remote parts of a layer use a protocol to
  cooperate
• Make use of service of the underlying layer to
  exchange data
• Protocol is a horizontal relationship, service a
  vertical relationship
• Two important reference models how to group
  functionalities into layers, which services to
  offer at each layer, how to structure protocols

94
Conclusions

• Communication networks have to solve many
  problems and need a lot of functionality
• The most basic of these problems, and an idea
  about their solution, should have become clear
• How to group these functions, how to solve these
  problems, will be the topic of the remainder of
  this course

95
An experiment Constructing a spanning tree

• Nodes communicate wirelessly with each other
• Goal Construct a spanning tree, root node
  attached to laptop

LEDs encode level in the tree Red Root
node Yellow 1 hop to root Green 1 hops to
root Cyan 3 hops to root Purple 4 hops or more
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Lehrveranstaltungen FG Rechnernetze
Ad hoc/Sensor-netze
Seminar
VIII
Projektgruppe
Festnetz
Drahtlos/mobil
Methodik
Legende
97
Combinations of duplexing multiplexing
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