Networking and Communications

1
Networking and Communications

• October 22, 2002

2
Computer Networks

• Computer Networks
• "interconnected collection of autonomous
  computers" Tanenbaum 1996
• Types of Networks
• Local Area Networks (LANs)
• high-speed communication on proprietary grounds
  (on-campus)
• most typical solution Ethernet with 100 Mbps

3
Computer Networks

• Types of Networks (cont.)
• Metropolitan Area Networks
• high-speed communication for nodes distributed
  over medium-range distances, usually belonging to
  one organization
• providing "back-bone" to interconnect LAN's
• technology often based on ATM, FDDI or DSL

4
Computer Networks

• Types of Networks (cont.)
• Wide Area Networks
• communication over long distances
• covers computers of different organizations
• high degree of heterogeneity of underlying
  computing infrastructure involves routers
• speeds up to a few Mbps possible, but around
  50-100 Kbps more typical
• most prominent example the Internet

5
Computer Networks

• Types of Networks (cont.)
• Wireless Networks
• end user equipment accesses network through short
  or mid range radio or infrared signal
  transmission
• Wireless WANs
• GSM (up to about 20 Kbps)
• UMTS (up to Mbps)
• PCS
• Wireless LANs/MANs
• WaveLAN (2-11 Mbps, radio up to 150 meters)
• Wireless Personal Area Networks
• bluetooth (up to 2 Mbps on low power radio
  signal, lt 10 m distance)

6
Computer Networks

• Network Type Performance Characteristics

Range
Bandwidth (Mbps)
Latency (ms)
LAN
1-2 kms
10-1000
1-10
WAN
worldwide
0.010-600
100-500
MAN
2-50 kms
1-150
10
Wireless LAN
0.15-1.5 km
2-11
5-20
Wireless WAN
worldwide
0.010-2
100-500
Internet
worldwide
0.010-2
100-500
7
Protocols

• Agreement between two communicating parties how
  the communication is to proceed
• syntax
• message formats
• data representation
• semantics
• when to send which message
• appropriate responses
• how to detect and handle failures

8
Services

• provide functions to invoker of service
• use other services while providing abstraction
  from the particulars of the used services

9
Message Formats

• header sequence numbers, synchronization
  patterns, message types, etc.
• data user data
• trailer end sequence, error check sum

10
OSI-BRM

• Open Systems Interconnection Basic Reference
  Model (OSI-BRM)

11
OSI-BRM
12
OSI-BRM
13
OSI-BRM

• Application Layer
• Provide services that support the various types
  of distributed applications
• OSI protocols
• electronic mail (X.400, almost entirely extinct
  these days)
• name/directory services (X.500, some residual
  interest and some implementations)

14
OSI-BRM

• Application Layer
• Internet protocols
• SMTP (simple mail transfer protocol)
• FTP (file transfer)
• telnet (remote login)
• http (hypertext transfer protocol)

15
OSI-BRM

• Presentation Layer
• Problem different computers represent data in
  different formats
• In the Internet XDR (external data
  representation), fixed conventions for the
  representation of data
• all integers 4-byte big-endians
• floating point numbers in IEEE format
• texts in ASCII
• all fields aligned on 4-byte word boundaries

16
OSI-BRM

• Presentation Layer
• Problem may require two conversions
• XDR-to-C compiler exists

17
OSI-BRM

• Session Layer
• Support session oriented traffic (classical
  database applications, file transfer, etc.)
• Two main functions
• send token management
• synchronization/resynchronization after failures
• Non-existent in Internet RM
• functions are the responsibility of the
  application or the application layer protocols

18
OSI-BRM

• Transport Layer
• Provide services for application message
  exchanges between peer application entities
• Interface with the underlying network
• if application messages are too big for network
  layer, segment them and reassemble at the
  receiving end
• multiple network connections for one application
  connection (if higher bandwidth needed than what
  one network connection can deliver)
• multiplex multiple application connections via
  one network connection, if possible, to
  efficiently use network bandwidth

19
OSI-BRM

• Transport Layer
• Provide connection across network with
  well-defined qualities (QoS, quality of service)
• connection establishment delay
• connection establishment failure probability
• throughput
• transit delay
• residual error ratio
• protection
• priority

20
OSI-BRM

• Transport Layer
• Provide connection-oriented as well as
  connection-less services
• connection-oriented
• establish connection on a well-defined source
  service access point (or port) p and destination
  service access point p
• send messages to ports without providing target
  address

21
OSI-BRM

• Transport Layer
• Provide connection-oriented as well as
  connection-less services
• connection-less send messages providing target
  address for each message sent

22
OSI-BRM

• Transport Layer
• connection-less vs. connection-oriented
• connection-less
• no overhead for connection setup and release
• potential bandwidth-loss due to complete address
  information
• no possibility to perform error-correction
  (pushed into application)
• connection-oriented
• overhead, but no bandwidth loss
• need to reserve network resources
• facilitates ensuring connection properties
• order preservation
• retransmission

23
OSI-BRM

• Transport Layer
• Ports
• link an application process to a transport
  connection
• permit identifying a remote application process
  (or service)
• note use of process id in target node would be
  unsuitable since pids are generated and destroyed
  dynamically in most operating systems
• The internet protocol architecture defines
  reserved port numbers, e.g.
• FTP 21 (ftp connection establishment etc.)
• FTP-DATA 20 (ftp data transfer)
• TELNET 23 (terminal connection)
• SMTP 25 (mail delivery)
• HTTP 80 (http requests)

24
OSI-BRM

• Transport Layer
• Ports
• link an application process to a transport
  connection
• permit identifying a remote application process
  (or service)
• note use of process id in target node would be
  unsuitable since pids are generated and destroyed
  dynamically in most operating systems
• The internet protocol architecture defines
  reserved port numbers, e.g.
• FTP 21 (ftp connection establishment etc.)
• FTP-DATA 20 (ftp data transfer)
• TELNET 23 (terminal connection)
• SMTP 25 (mail delivery)
• HTTP 80 (http requests)

25
OSI-BRM

• Transport Layer
• UDP (User Datagram Protocol)
• provides unreliable, connectionless transport
  service
• no guarantee of order preservation, delivery
• message duplications are possible
• facilitates multicast
• application areas context-free protocols, simple
  client-server applications, i.e., one request -
  one reply
• Domain Name Server lookup
• SNMP requests
• NFS requests
• Multimedia protocols that do not require error
  correction

26
OSI-BRM

• Transport Layer
• UDP (User Datagram Protocol)
• UDP header format

27
OSI-BRM

• Transport Layer
• TCP (Transport Control Protocol)
• provides connection-oriented transport service
• error-correcting
• order preserving
• segmentation of application-layer data stream
• duplex communication
• transport connection uniquely identified through
• network (IP) addresses of sender end receiver
• port addresses of sender and receiver
• protocol identifier for TCP (6)

28
OSI-BRM

• TCP (header)

29
OSI-BRM

• TCP (pseudo-header)

30
OSI-BRM

• TCP
• despite complexity, allows for high data rates
  (experimentally up to 100 Mbit/s)
• useable in LAN/MAN/WAN environments
• typical applications
• e-mail (SMTP)
• file transfer (ftp)
• remote terminal (telnet)
• remote graphics terminal (X11 for X-Windows)
• http

31
OSI-BRM

• Network Layer
• Central questions
• addressing how to identify the target computer
• routing how to route the message most
  effectively through the network
• packet switching will there be a new path for
  every packet, or will there be predescribed paths
  from source to destination
• connection setup and release
• end-to-end error detection, ensuring packet
  ordering, flow-control
• General functionality network transparency for
  the transport layer
• provide for end-to-end transport connection
  independent of actual routing and switching
  decisions

32
OSI-BRM

• Network Layer
• Packet switching
• virtual circuit a fixed path for all packets of
  a connection will be determined at connection
  setup time
• facilitates order preservation
• route determination costs only once per
  connection
• inflexible to adapt to changing network loads and
  configurations
• datagram routing decision for every packet in
  every node
• full address information in every packet
• less overhead for connection establishment,
  easier to implement
• more flexible for short-lived connections

33
OSI-BRM

• Network Layer
• Routing algorithms
• objectives
• minimize average packet delay
• maximize total throughput
• efficient implementation
• conflicting, therefore often used minimize
  number of hops (visited nodes) per packet
• reduces delay
• reduces needed bandwidth
• increases throughput

34
OSI-BRM

• Network Layer
• Routing algorithms
• static (non-adaptive) algorithms
• determination of network routes for every pair of
  nodes at network setup time
• no consideration of current network status and
  load (average values used)
• no change of routes during network operation

35
OSI-BRM

• Network Layer
• Routing algorithms
• dynamic (adaptive) algorithms
• determination of network routes based on
  measurement/estimation of current network load
  and configuration
• centralized one central node makes routing
  decisions
• isolated decision on routing based solely on
  local traffic and load information (backward
  learning, routing, delta-routing)
• distributed nodes are exchanging routing
  information (distance vector routing)

36
OSI-BRM

• Data Link Layer
• error detection and correction
• physical media are prone to signal distortions
  due to external impulses and material properties
• typical value for error probability of a 32 bit
  block over telephone wire 0.0016

37
OSI-BRM

• Data Link Layer
• error detection and correction
• error detection using check sum (e.g., parity
  bits)
• to detect e bit errors one needs code with
  Hamming distance of e1
• error correction check sum plus exact
  information, which bit flipped
• to correct e bit errors, need 2e1 Hamming
  distance
• often used check sum generated through cyclic
  redundancy check
• detects all error burst with a length of up to
  16, 99.998 of all longer bursts

38
OSI-BRM

• Data Link Layer
• frames error-detecting and error-correcting
  codes need frame delimiters
• bit stuffing
• acknowledgement and retransmission of erroneous
  frames
• sequence numbers
• go-back-n

39
OSI-BRM

• Data Link Layer
• flow control nodes may receive more traffic than
  they can deliver to adjacent nodes, but have
  limited buffer capacity buffer overflow
• sliding window protocol (of size n)
• sending node may race ahead a number of n
  unacknowledged messages
• if last acknowledged packet is k, and new
  acknowledgement lgtk arrives, then sender may
  transmit up to sequence number kn
• includes acknowledgement/retransmission
  functionality

40
OSI-BRM

• Physical Layer
• defines the physical characteristics of the
  signal transmissions
• example bit encoding mechanisms

41
Internet Protocol Architecture

• Comparison OSI-BRM vs. Internet
• presentation and session layers not implemented
  in Internet architecture, will be implemented in
  application (e.g., XDR encoding)
• IP provides less functionality than network layer
  in OSI-BRM
• L2 and L1 not specified in Internet architecture

42
Addressing

• Addressing in the Internet Protocol
• addresses used in source and destination fields
  of the Internet Protocol
• requirements
• define a unique address for any node in the
  Internet
• no two nodes on the Internet may have the same
  address

43
Addressing

• Addressing in the Internet Protocol
• requirements
• define a sufficiently large address space
• IPv4 (1982) 32-bit addresses for 232 (appr. 4
  billion) addresses
• insufficient due to
• unforeseen growth of internet
• inefficient use of address space
• IPv6 (1994) 128-bit addresses for 2128 (appr.
  3x1038) addressable nodes
• max. 7x1023 IP addresses per m2 of entire earth
  surface
• if as inefficiently allocated as phone numbers
  103 per m2

44
Addressing

• Addressing in the Internet Protocol
• requirements
• support a flexible routing scheme, but addresses
  themselves should not contain routing information

45
Addressing

• IP address class structure

46
Decimal representation of Internet addresses
octet 1
octet 2
octet 3
Range of addresses
Network ID
Host ID
1.0.0.0 to
Class A
1 to 127
0 to 255
0 to 255
0 to 255
127.255.255.255
Network ID
Host ID
128.0.0.0 to
Class B
128 to 191
0 to 255
0 to 255
0 to 255
191.255.255.255
Network ID
Host ID
192.0.0.0 to
Class C
0 to 255
0 to 255
1 to 254
192 to 223
223.255.255.255
Multicast address
Multicast address
224.0.0.0 to
Class D (multicast)
0 to 255
0 to 255
1 to 254
224 to 239
239.255.255.255
240.0.0.0 to
Class E (reserved)
0 to 255
0 to 255
1 to 254
240 to 255
255.255.255.255
47
Addressing

• problems
• network administrators cannot predict growth of
  their subnets
• tend to apply for class B network addresses, even
  though subnets then tend to be smaller than 255
  nodes (i.e., class C would be sufficient)
• leads to inefficient use of address space

48
Routing
49
Routing

• find cost optimal path from one node to another,
  i.e., paths with minimum number of hops
• e.g., from C to D C-E-D has minimal cost of 2

50
Routing
Routings from A
Routings from B
Routings from C
To
Link
Cost
To
Link
Cost
To
Link
Cost
A
local
0
A
1
1
A
2
2
B
1
1
B
local
0
B
2
1
C
1
2
C
2
1
C
local
0
D
3
1
D
1
2
D
5
2
E
1
2
E
4
1
E
5
1
Routings from D
Routings from E
To
Link
Cost
To
Link
Cost
A
3
1
A
4
2
B
3
2
B
4
1
C
6
2
C
5
1
D
local
0
D
6
1
E
6
1
E
local
0
51
Routing

• cost column not used for routing decision, but
  for construction of routing table
• construction of routing tables Routing
  Information Protocol (RIP)
• each node specifies single hop routing
  information
• use of a distributed algorithm, based on
  Ford-Fulkerson shortest path algorithm

52
Routing

• construction of RIP tables
• each node shares its local routing table
  information with all its direct neighbors will
  send copy of local routing table to all adjacent
  nodes
• periodically, when a timer t expires (in internet
  typically t30 sec.)
• when local routing table changes

53
Routing

• construction of RIP tables
• when receiving a routing table from a node R
• update table with new route or with existing
  route with better cost (compare local value with
  Rs value 1)
• when received on link n, and received table shows
  different value for some local route starting
  with n, then copy the value from received table
  (R is closer to destination and therefore its
  table is more authoritative)

54
RIP Algorithm
Send Each t seconds or when Tl changes, send Tl
on each non-faulty outgoing link. Receive
Whenever a routing table Tr is received on link
n for all rows Rr in Tr if (Rr.link n)
Rr.cost Rr.cost 1 Rr.link n if
(Rr.destination is not in Tl) add Rr to Tl
// add new destination to Tl else for all rows Rl
in Tl if (Rr.destination Rl.destination and
(Rr.cost lt Rl.cost or Rl.link n)) Rl Rr //
Rr.cost lt Rl.cost remote node has better
route // Rl.link n remote node is more
authoritative
55
Routing

• extensions RIP-1
• cost represents actual bandwidth
• increased speed of convergence
• avoidance of loops

56
Internet Protocol (IP)

• network protocol of the Internet protocol stack
• provides network service with the following
  characteristics
• no guarantee of delivery
• duplication possible
• unbounded delay
• no order preservation

57
Internet Protocol (IP)

• address resolution
• IP addresses may need to be mapped to physical
  addresses (e.g., on Ethernet)
• use Address Resolution Protocol (ARP)
• either direct relation between IP and physical
  address, or mapping
• transport layer service provides data stream
  transmission service
• broken into network layer datagrams by transport
  layer
• IP max length of datagram 64 Kbytes, usually
  1500 bytes, if necessary by fragmenting and
  reassembling them further

58
IP version 6 (IPv6), 1994

• enlarged address space
• improved routing speed
• no checksum applied to body, only to header
• no datagram fragmentation occurs inside network
  (mechanism determining smallest datagram size
  along path before packet is transmitted)

59
IP version 6 (IPv6), 1994

• improved routing speed
• support for real-time traffic
• priority preferred handling of high-priority
  packets
• flow labels resource reservation for specific
  types of real-time traffic
• future evolution of protocol next header may
  point to special header inside packet body (e.g.,
  for router information)
• support for multicast, anycast, and security

60
MobileIP

• support for roaming of laptop computers, personal
  digital assistants (PDAs), wearable computing
  devices, etc.
• IP addresses are bound to subnet addresses, but
  roaming may leave subnet boundary

61
MobileIP
Sender
Subsequent IP packets
Mobile host MH
tunnelled to FA
Address of FA
returned to sender
First IP packet
addressed to MH
Internet
Foreign agent FA
Home
First IP packet
agent
tunnelled to FA
62
MobileIP

• every IP address is assigned to home domain
• home agent (HA) and foreign agent (FA)
• when devide is at home, will behave as local
  router
• when device leaves home area it informs HA
• HA behaves as proxy will answer ARP requests for
  mobile device with own local network address

63
MobileIP

• every IP address is assigned to home domain
• home agent (HA) and foreign agent (FA)
• when device arrives at a new side it informs FA
• FA assigns temporary, local address to mobile
  device
• FA then contacts HA and gives mobile devices
  temporary and permanent address
• arriving packet for mobile device
• routed to HA
• tunneled to FA and delivered to mobile device
• subsequent packets from same source tunneled
  directly from sender to FA

64
Break
65
Interprocess Communication

• Distributed Systems rely on exchanging data and
  achieving synchronization amongst autonomous
  distributed processes
• Inter process communication (IPC)
• shared variables
• message passing
• message passing in concurrent programming
  languages
• language extensions
• API calls

66
Interprocess Communication

• Synchronization
• concurrent processes on different computers
  execute at different speeds
• need for one process to influence computation in
  another process
• specify constraints on the ordering of events in
  concurrent processes
• message passing

67
Interprocess Communication

• Message Passing Primitives
• send expression_list to destination_designator
• evaluates expression_list
• adds a new message instance to channel
  destination_designator
• receive variable_list from source_designator
• assigns received values to variables in
  variable_list
• destroys received message
• Central questions
• how are destination designators specified?
• how is communication synchronized?

68
Interprocess Communication

• Destination Designation
• direct naming source and destination process
  names serve as designators (a pair of source and
  destination designators defines a channel)
• send cur_status to monitor or monitor!cur_status
• receive message from handler or handler?message
• easy to implement and use

69
Interprocess Communication

• Destination Designation
• allows a process easy control when to receive
  which message from which other process
• use to implement client/server applications
• well suited to implement client/server paradigm
  if there is one client and one server
• otherwise server should be capable of accepting
  invocations from any client at any time, and a
  client should be allowed to invoke many services
  at a time if more than one server available

70
Interprocess Communication

• Destination Designation
• global names or mailboxes process
  name-independent destination designator shared by
  many processes
• messages sent to a mailbox can be received by any
  process that executes a receive referring to that
  mailbox name
• to implement Client/Server applications
• clients send requests to mailbox, an available
  server picks them up

71
Interprocess Communication

• Destination Designation
• drawback costly implementation
• message sent to mailbox
• relayed to all other sites that could potentially
  receive from that mailbox
• if one site decides to receive, inform all other
  sites that message is no longer available for
  receipt
• mutual exclusion for concurrent access

72
Interprocess Communication

• Destination Designation
• ports mailbox, but only one process is permitted
  to receive from that mailbox
• easy to implement receives can occur in only one
  process, no distribution and coordination
  necessary
• suitable for multiple clients / single server
  applications
• Message destinations in Internet programming
• hybrid direct naming/port scheme
• (Internet_address, port_number)
• port corresponds to many sender/one receiver
  concept

73
Interprocess Communication

• Channel Naming
• static (at compile time)
• impossible for a program to communicate along a
  channel not known at compile time
• inflexibility if a process might ever need to
  communicate with a recipient, that channel must
  be available throughout the entire runtime of the
  sending program
• dynamic (at runtime)
• administrative overhead at runtime
• more flexible allocation of communication
  resources

74
Interprocess Communication

• Semantics of message passing primitives
• Blocking
• non-blocking the execution will never delay the
  invoking process
• blocking otherwise
• Synchronization
• asynchronous message passing message passing
  using buffers with unbounded capacity
• sender may race ahead an unbounded number of
  steps
• sender never blocks
• receiver blocks on empty queue

75
Interprocess Communication

• Semantics of message passing primitives
• Synchronization
• synchronous message passing no buffering between
  sender and receiver
• sender blocks until receiver ready to receive
• receiver blocks until sender ready to send
• buffered message passing buffers with bounded,
  finite capacity
• sender may race ahead a finite, bounded number of
  steps
• sender blocks on full buffer
• receiver blocks on empty buffer

76
Interprocess Communication

• Non-blocking primitives for asynchronous or
  buffered message passing
• receive
• background variant process continues, receives
  interrupt upon arrival
• overhead for implementation
• ignoring variant program polls for availability
• send
• sending process waits for empty buffer or drops
  message to be sent

77
Interprocess Communication

• Distributed Applications
• availability of a set of pre-implemented
  application services, like email, ftp, http, etc.
• what if you want to build your own, customized
  Internet application?
• access to Transport Layer services
• Services provided by Internet Transport Layer
• UDP message passing (datagram)
• TCP data stream

78
Interprocess Communication

• Sockets
• Internet IPC mechanism of Unix and other
  operating systems (BSD Unix, Solaris, Linux,
  Windows NT, Macintosh OS)
• processes in these OS can send and receive
  messages via a socket
• sockets are duplex
• sockets need to be bound to a port number and an
  internet address in order to be useable for
  sending and receiving messages
• each socket has a transport protocol attribute
  (TCP or UDP)
• messages sent to some internet address and port
  number can only be received by a process using a
  socket that is bound to this address and port
  number
• UDP socket can be connected to a remote IP
  address and port number
• processes cannot share ports (exception TCP
  multicast)

79
Interprocess Communication

• IPC based on UDP datagrams
• UDP datagram properties no guarantee of order
  preservation, message loss and duplications are
  possible
• necessary steps
• create socket
• bind socket to a port and local Internet address
• client arbitrary free port
• server server port

80
Interprocess Communication

• IPC based on UDP datagrams
• receive method returns Internet address and port
  of sender, plus message
• message size IP allows for messages of up to 216
  bytes
• most implementations restrict this to around 8
  Kbytes
• larger application messages applications
  responsibility to perform fragmentation/reassembli
  ng
• if arriving message is too big for array
  allocated to receive message content, truncation
  occurs

81
Interprocess Communication

• IPC based on UDP datagrams
• send non-blocking
• blocks only until message given to UDP/IP
• upon arrival placed in per-port queue
• receive blocking
• pre-emption by timeout possible
• if process wishes to continue while waiting for
  packet, use separate thread

82
Java API for UDP datagrams

• Classes
• DatagramPacket constructor generating message for
  sending from array of bytes
• message content (byte array)
• length of message
• Internet address and port number (destination)
• similar constructor for receiving a message

83
Java API for UDP datagrams

• Classes
• DatagramSocket class for sending and receiving of
  UDP datagrams
• one constructor with port number as argument,
  another without
• no-argument constructor to use free local port
• methods
• send and receive
• setSoTimeout
• connect for connecting a socket to a particular
  remote Internet address and port

84

• import java.net.
• import java.io.
• public class UDPClient
• public static void main(String args)
• // args give message contents and server
  hostname
• DatagramSocket aSocket null
• try
• aSocket new DatagramSocket()
• byte m args0.getBytes()
• InetAddress aHost InetAddress.getByName(args1
  )
• int serverPort 6789
  
• DatagramPacket request new DatagramPacket(m,
  args0.length(), aHost, serverPort)
• aSocket.send(request)
  
• byte buffer new byte1000
• DatagramPacket reply new DatagramPacket(buffer
  , buffer.length)
• aSocket.receive(reply)
• System.out.println("Reply " new
  String(reply.getData()))
• catch (SocketException e)System.out.println("
  Socket " e.getMessage())
• catch (IOException e)System.out.println("IO
  " e.getMessage())

85

• import java.net.
• import java.io.
• public class UDPServer
• public static void main(String args)
• DatagramSocket aSocket null
• try
• aSocket new DatagramSocket(6789)
• byte buffer new byte1000
• while(true)
• DatagramPacket request new
  DatagramPacket(buffer, buffer.length)
• aSocket.receive(request)
• DatagramPacket reply new
  DatagramPacket(request.getData(),
• request.getLength(), request.getAddress(),
  request.getPort())
• aSocket.send(reply)
• catch (SocketException e)System.out.println("
  Socket " e.getMessage())
• catch (IOException e) System.out.println("IO
  " e.getMessage())
• finally if(aSocket ! null)
  aSocket.close()

86
Interprocess Communication

• API for streams
• connection establishment using client/server
  approach, afterwards peer communication
• client issue connect requests
• server has listening port to receive connect
  request messages
• accept of a connection create new stream socket
  for new connection

87
Java API for TCP streams

• Classes
• ServerSocket class create socket at server side
  to listen for connect requests
• Socket class for processes with connections
• constructor to create a socket and connect it to
  remote host and port of a server
• methods for accessing input and output stream

88

• import java.net.
• import java.io.
• public class TCPServer
• public static void main (String args)
• try
• int serverPort 7896
• ServerSocket listenSocket new
  ServerSocket(serverPort)
• while(true)
• Socket clientSocket listenSocket.accept()
• Connection c new Connection(clientSocket)
• catch(IOException e) System.out.println("Liste
  n e.getMessage())

89
class Connection extends Thread
DataInputStream in DataOutputStream
out Socket clientSocket public Connection
(Socket aClientSocket) try
clientSocket aClientSocket in new
DataInputStream( clientSocket.getInputStream())
out new DataOutputStream( clientSocket.getOutput
Stream()) this.start()
catch(IOException e) System.out.println("
Connection"e.getMessage()) public void
run() try // an echo
server String data in.readUTF()
out.writeUTF(data)
catch(EOFException e) System.out.println("EOF"e
.getMessage()) catch(IOException e)
System.out.println("IO"e.getMessage())
finally try clientSocket.close() catch
(IOException e)/close failed/
90

• import java.net.
• import java.io.
• public class TCPClient
• public static void main (String args)
• // arguments supply message and hostname of
  destination
• Socket s null
• try
• int serverPort 7896
• s new Socket(args1, serverPort)
• DataInputStream in new DataInputStream(
  s.getInputStream())
• DataOutputStream out
• new DataOutputStream( s.getOutputStream())
• out.writeUTF(args0) // UTF is a string
  encoding see Sn 4.3
• String data in.readUTF()
• System.out.println("Received " data)
• catch (UnknownHostException e)
• System.out.println("Sock"e.getMessage())
• catch (EOFException e) System.out.println("
  EOF"e.getMessage())
• catch (IOException e)System.out.println("I
  O"e.getMessage())

91
Data Representation

• data representation problem
• use agreed external representation, two
  conversions necessary
• use senders or receivers format and convert at
  the other end
• transmission of structured data types
• data types may not change during transmission
• usage of a commonly understood flattened
  transfer format (structured types are reduced to
  their primitive components)

92
Data Representation

• marshalling/unmarshalling
• marshalling assembling a collection of data
  items in a form suitable for transmission
• unmarshalling disassembling and recovery of
  original data items
• usually performed automatically by middleware
  layer
• hand-programming error-prone
• use of compilers for programs working directly at
  transport API

93
Java Object Serialization

• flattening an object into a linear form such that
  it can be stored in a file or transmitted in a
  message
• linear format must be such that deserialization
  routine is capable of recovering the complete
  object structure and state
• inclusion of
• handles (references to other objects)
• name of class that an object belongs to
• version number of class
• note mark objects as non-serializable
  (transient) if they are not supposed to be
  serialized (e.g., socket references, files, etc.)

94
Java Object Serialization

• serialization create instance of class
  ObjectOutputStream and invoke writeObject method,
  passing object to be serialized as argument
• deserialization open ObjectInputStream on the
  serialized structure and use readObject method

95
Java Object Serialization

• reflection enables serialization/deserialization
  in a generic manner
• query on properties of a class,
• names and types of instance variables
• methods (including constructors)
• classes can be created based on their names
• a constructor with given argument types can be
  created
• serialization
• use reflection to find out the class name and
  name, type and value of its instance variables
• deserialization
• use class name to create a class
• used to create a new constructor with argument
  types corresponding to those in the serialized
  form
• use new constructor to create a new object with
  instance variables that are identical in value
  and type to those at serialization time

96
Remote Object References

• needed when a client invokes an object that is
  located on a remote server
• reference needed that is unique over space and
  time
• space where is the object located
• time correct version of an undeleted object

97
Remote Object References

• extension
• object references that are location transparent

98
Client-Server Communication

• often built over UDP datagrams
• client-server protocol consists of
  request/response pairs, hence no acknowledgements
  at transport layer are necessary
• avoidance of connection establishment overhead
• no need for flow control due to small amounts of
  data transferred
• generic protocol example (for RPC or RMI
  communication)

99
Client-Server Communication

• format of protocol messages
• message type (request/reply)
• request ID
• sending process identifier (e.g., IP address/port
  number)
• integer sequence number incremented by sender
  with every request
• object reference
• method ID and arguments

100
Client-Server Communication

• if implemented over UDP failure recovery
• omission failures
• use timeout and resend request when timeout
  expires and reply hasnt arrived
• server receives repeated request
• indempotent operations same result obtained on
  every invokation
• non-indempotent operations re-send result stored
  from previous request, requires maintenance of a
  history of replies
• loss of replies request - reply - ack reply
  protocol
• message duplication return request ID with reply

101
Client-Server Communication

• example Hypertext Transfer Protocol (HTTP)
• lightweight request - reply protocol for the
  exchange of network resources between web clients
  and web servers
• protocol steps
• connection establishment between client and
  server (likely TCP, but any reliable transport
  protocol is acceptable)
• client sends request
• server sends reply
• connection closure

102
Client-Server Communication

• example Hypertext Transfer Protocol (HTTP)
• inefficient scheme, therefore HTTP 1.1 allows
  persistent transport connections (remains open
  for successive request/reply pairs)
• Resources can have mime-type data types, e.g.
• text/plain
• text/html
• image/jpeg
• data is marshaled into ASCII transfer syntax

103
Client-Server Communication

• HTTP request

• GET request of resource, identified by URL, may
  refer to
• data server returns data
• program server runs program and returns output
  data
• HEAD request similar like GET, but only meta
  data on resource is returned (like date of last
  modification)
• POST specifies resource (for instance, a server
  program) that can deal with the client data
  provided with previous request
• PUT supplied data to be stored in given URL
• DELETE delete an identified resource on server

104
Client-Server Communication

• HTTP reply