CS472 Computer Networks

1
CS472Computer Networks

• Professor Mike Kain
• Class 3 Slides
• Spring 2008-2009

2
Tonights Topics

• Review last weeks class
• Tonights topics
• Finish application layer
• HTTP, DNS, SMTP, FTP, BitTorrent, P2P
• Start Transport Layer
• Reliable Data Transfer
• TCP, UDP.

3
How Processes Actually Communicate

• Interface between Processes and Computer Networks
• Sockets Application Programming interfaces

internet
host or server
host or server
4
Port Usage

• Port is represented by a positive (16-bit)
  integer value
• Some ports have been reserved to support
  common/well known services
• ftp 21/tcp
• telnet 23/tcp
• smtp 25/tcp
• User level process/services generally use port
  number value gt 1024

5
Internet Endpoints Connections

• How do we uniquely identify a process on a host?
• INTERNET ENDPOINT
• (IP, port , TCP/UDP)
• How do we uniquely identify a connection?
• INTERNET CONNECTION (socketpair)
• (src IP, src port, dst IP, dst port, TCP/UDP)
• Netstat -a

6
HTTP

• RFC 1945 and 2616
• Client Server Architecture
• Communication using HTTP messages
• HTTP defines how Web Clients request Web Pages
  and how Servers transfer Web Pages back
• Web Page composed of objects ,
• HTML is the data
• Objects addressable by URLs

7
HTTP

• Stateless (Important)
• Versions
• 1.0 - Non- Persistent
• 1.1 Persistent

From http//commons.wikimedia.org/wiki/ImageHTTP_
persistent_connection.svg
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HTTP Request Message
9
HTTP Response Message
10
Cookies
11
The Conditional GET

• GET /sample.html HTTP/1.1Host www.example.com
• HTTP/1.1 200 OKDate Tue, 27 Dec 2005 052511
  GMTContent-Type text/html charsetiso-8859-1Se
  rver Apache/1.3.33 (Unix) PHP/4.3.10Last-Modifie
  d Thur, 10 Apr 2008 132452 GMT
• GET /sample.html HTTP/1.1Host
  www.example.comIf-Modified-Since Thur, 10 Apr
  2008 132452 GMT
• HTTP/1.1 304 Not ModifiedDate Tue, 27 Dec 2005
  052519 GMTServer Apache/1.3.33 (Unix)
  PHP/4.3.10

12
URL Resolution

• http//www.cs.drexel.edu/mkain/cs472/index.html
• The browsers operation
• Takes first part (hostname) and resolves through
  DNS (if it doesnt already know).
• Takes method (http) and looks up port (unless
  overwritten by after hostname.
• Connects to that host and port number.
• Sends GET with the rest (/mkain/cs472/index.htm)
  to server and waits for answer.
• Then it looks through what it gets back and sends
  requests for any objects contained.

13
Socket Programming - TCP

• Sockets are a protocol independent method of
  creating a connection between processes. Sockets
  can be either
• connection based or connectionless
• packet based or streams based
• reliable or unreliable
• The TCP and UDP protocols use ports to map
  incoming data to a particular process running on
  a computer.

14
Client/Server Implementation TCP
Server (running on hostid)
Client
15
TCP Client (JAVA)

• Socket clientSocket new Socket( hostname, port
  )
• DataOutputStream outToServer new
  DataOutputStream( clientSocket.getOutputStream()
• BufferedReader inFromServer new BufferedReader(
  clientSocket.getInputStream()
• While (protocol)
• ltconstruct PDUgt
• outToServer.writeBytes( PDU, buflen )
• inFromServer.readLine( PDU )
• ltparse response PDUgt
• clientSocket.close()

16
TCP Client (C)

• int mysocket
• mysocket socket( AF_INET, SOCK_STREAM, 0)
• if ( mysocket lt 0 )
• perror(socket() called failed.\n)
• exit(-1)
• struct sockaddr_in you
• sockaddr.s_family AF_INET
• sockaddr.s_port ntohs(port)
• sockaddr.s_addr ltfill in with IP addressgt
• if ( connect( mysocket, you, sizeof(sockaddr_in)
  ) lt 0 )
• perror(connect failed.\n)
• close(mysocket)
• exit(-1)

17
TCP Client (C), continued

• while (protocol)
• ltconstruct PDUgt
• send( mysocket, buffer, buflen )
• rcvlen recv( mysocket, buffer, maxbuflen )
• ltparse response PDUgt
• close( mysocket )
• exit(0)

18
Example Java Server
import java.io. import java.net. class
TCPServer public static void main(String
argv) throws Exception String
clientSentence String capitalizedSentence
ServerSocket listeningSocket new
ServerSocket(6789) while(true)
Socket connectionSocket
listeningSocket.accept()
BufferedReader inFromClient new
BufferedReader(new
InputStreamReader(connectionSocket.getInputStream(
)))
Create Listening socket at port 6789
Wait, on listening socket for contact by client
Create input stream, attached to socket
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DataOutputStream outToClient
new DataOutputStream(connectionSocket.get
OutputStream()) clientSentence
inFromClient.readLine()
capitalizedSentence clientSentence.toUpperCase()
'\n' outToClient.writeBytes(capit
alizedSentence)
Create output stream, attached to socket
Read in line from socket
Write out line to socket
End of while loop, loop back and wait for another
client connection
20
TCP Server (JAVA)

• ServerSocket groupSocket new ServerSocket(6789)
  
• While (true)
• Socket connectedSocket groupSocket.accept()
• ltconstruct reader and writer from
  connectedSocket just like clientgt
• ltrun protocolgt

21
Example in C
22
TCP Server (C)

• Groupsocket Socket( ltsame parameters as
  clientgt)
• Bind() using specific port , and addr
  IPADDR_ANY
• Listen( groupsocket, 5 )
• While (TRUE)
• Newsocket accept( groupsocket )
• Do protocol with newsocket
• Close newsocket

23

• while (protocol)
• send( mysocket, buffer, buflen )
• rcvlen recv( mysocket, buffer, maxbuflen )
• close( mysocket )
• exit(0)

24
Server types

• Iterative handles one client at a time
• Concurrent handles multiple clients
  simultaneously
• Two methods
• Process/thread server executes one
  process/thread for each client for 1-1
  communication/protocol
• select model server maintains list of active
  sockets and polls for activity through select()
  call.
• Affects models after accept and before
  execution of protocol.

25
UDP Clients

• Similar to TCP clients, but no connection to
  worry about.
• C gt no connect, just sendto() and recvfrom()
• JAVA gt use DatagramSocket and DatagramPacket to
  send and recv.

26
UDP Servers

• UDP servers have the same algorithm as the UDP
  client, but the send recv are in the other
  order (usually servers wait for the next message
  to be read and then send a response)

27
UDP
Server (running on hostid)
28
JAVA CLIENT -UDP
import java.io. import java.net. class
UDPClient public static void main(String
args) throws Exception
BufferedReader inFromUser new
BufferedReader(new InputStreamReader(System.in))
DatagramSocket clientSocket new
DatagramSocket() InetAddress IPAddress
InetAddress.getByName("hostname")
byte sendData new byte1024 byte
receiveData new byte1024 String
sentence inFromUser.readLine() sendData
sentence.getBytes()
Create input stream
Create client socket
Translate hostname to IP address using DNS
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Create datagram with data-to-send, length, IP
addr, port
DatagramPacket sendPacket new
DatagramPacket(sendData, sendData.length,
IPAddress, 9876) clientSocket.send(send
Packet) DatagramPacket receivePacket
new DatagramPacket(receiveData,
receiveData.length) clientSocket.receiv
e(receivePacket) String
modifiedSentence new
String(receivePacket.getData())
System.out.println("FROM SERVER"
modifiedSentence) clientSocket.close()

Send datagram to server
Read datagram from server
30
JAVA SERVER -UDP
import java.io. import java.net. class
UDPServer public static void main(String
args) throws Exception
DatagramSocket serverSocket new
DatagramSocket(9876) byte
receiveData new byte1024 byte
sendData new byte1024 while(true)
DatagramPacket
receivePacket new
DatagramPacket(receiveData, receiveData.length)
serverSocket.receive(receivePacket)
Create datagram socket at port 9876
Create space for received datagram
Receive datagram
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String sentence new
String(receivePacket.getData())
InetAddress IPAddress receivePacket.getAddress()
int port receivePacket.getPort()
String
capitalizedSentence sentence.toUpperCase()
sendData capitalizedSentence.getBytes()
DatagramPacket sendPacket
new DatagramPacket(sendData,
sendData.length, IPAddress,
port) serverSocket.send(s
endPacket)
Get IP addr port , of sender
Create datagram to send to client
Write out datagram to socket
End of while loop, loop back and wait for another
client connection
32
Example C
33
FTP
file transfer
user or host
Remote file system

• transfer file to/from remote host
• client/server architecture
• ftp RFC 959
• ftp server port 21

34
FTP

• Data and control connection
• ftp/21
• ftp-data/20 (either direction)
• 4 character commands
• GET, PUT, MDIR, AUTH, ADAT
• Implicit vs Explicit SSL model
• 990 vs 21

35
DNS (Domain Name System)

• Takes names and translates to IP addresses
• www.cs.drexel.edu gt 129.25.6.158 or IPv6 address
  (FEC042F4800BFFFE3A211C)
• Can do other things
• Host aliasing mailserver.drexel.edu and
  webserver.drexel.edu might be the same machine
• bob_at_drexel.edu bob_at_mailrelay.irt.drexel.edu
• www.drexel.edu may be more than one IP address
• Problems with 3?

36
DNS Record Types

• A IPv4 address record
• AAAA IPv6 address record
• CNAME Canonical Name (alias, processing
  continues)
• MX Mail server record
• PTR Name record (processing stops)
• LOC geographic location
• RFC 1035, 3596 (Ipv6)

37
SMTP

• Push protocol used to deliver mail
• RFC 2821 (used to be RFC 821)
• MIME extensions (822, 2405, 2406)
• Content-type image/jpeg, text/ascii, etc.
• Content-Transfer-Encoding base64
• 4 character ASCII commands
• HELO, EHLO, etc.

38
SMTP errors

• 1yz Positive Preliminary reply
• 2yz Positive Completion reply
• 3yz Positive Intermediate reply
• 4yz Transient Negative Completion
• 5yz Permanent Negative Completion
• X0z Syntax
• X1z Information
• X2z Connection
• X5z mail system

39
Peer-to-Peer protocols

• Perform both client and server functionality at
  the same time
• BitTorrent
• Web Proxy services (ICP, etc.)
• Content distribution

40
Transport Layer

• What the network provides to the user
• Hides any network characteristics
• Make believe that the applications are directly
  connected (logically), NOT HOSTS
• Uniquely identify each process on each host
• TCP/IP gives us at least two
• TCP and UDP

41
UDP

• RFC 768 (1980)
• Connectionless, message-oriented transport
• Adds
• Checksum for segment
• Port numbers for source and destination
• Up to 64KB data
• Used by protocols which cant or dont want
  reliability or delay
• DNS, SNMP, RTP, TFTP, real-time

42
Reliable Data Transfer

• Concept that is used by many layers (data link
  above) to guarantee quality of service
• Uses a SLIDING WINDOW PROTOCOL (SWP) or AUTOMATIC
  REPEAT REQUEST PROTOCOL (ARQ) different names
  for the same thing

43
SWP/ARQ basics

• Assume one direction (for now)
• Each PDU (or some addressable part of the
  message) has a sequence number that increases
• Have ACKs and NAKs
• Have timeouts
• Have errors (each PDU has checksum)

44
Stop-And-Wait

• Send PDU. Wait for ACK. If ACK received, send
  next PDU.
• How does this protocol deal with lost duplicate
  frames?

45
Add sequence numbers

• Send PDU 0. Wait for ACK. If ACK then send next
  sequence numbered PDU.
• What does ACK have in it?
• What happens if frame corrupted?

46
Add NAK retransmission

• Send PDU 0. Wait for ACK. If ACK 0 received,
  then send PDU 1.
• If NAK received, then resend PDU 0.
• What happens if frame is lost?

47
Add timeout

• Send PDU 0. Wait for ACK or timeout
  (preconfigured).
• If ACK received before timeout, then send PDU 1.
• If timeout or NAK, then retransmit PDU 0.
• How does receiver deal if he sees two or more PDU
  0?

48
Restrict sequence numbers

• Solution is to make sure that a sequence number
  can only be valid for a period of time.
• Sequence numbers are really a field of the PDU
  which is a certain size (3 bits, 32 bits, etc.)
• Restrict reuse of sequence numbers within timeout
  interval.

49
So, what do we have?

• One-directional reliable protocol
• Send PDU 0. Wait for ACK or timeout.
• If ACK received, send PDU 1.
• If NAK received, or timeout, then resend PDU 0
  and wait again.
• Receiver throws away any subsequent copies of PDU
  0 once successfully received.
• sender and receiver have agreed on what numbers
  to start

50
More sequence numbers

• More than one PDU outstanding
• ACK/NAK has for PDU.
• Window of what PDUs are outstanding at any
  time.
• Pipelining having more than one PDU outstanding
  at any time.

51
Pipelined ACKs

• Can ACK last PDU and that implies ACKs of all
  outstanding PDU cumulative ACK.
• Really next to receive how is this better?
• SENDING WINDOW
• RECEIVING WINDOW

52
Two way transfer

• But, we really have bidirectional transfer. Each
  end has a SENDING and RECEIVING WINDOW.
• Each PDU has reserved spot for both PDU sequence
  number and ACK number (piggybacking)

53
So, what does it look like?
54
Retransmission

• With multiple PDUs outstanding, what happens when
  one (or more) get lost?
• Go Back N
• Selective Repeat
• Some protocols do a combination of both
• Fast retransmission

55
Other types of SWP

• Does the protocol have to be reliable?
• Do we really care about timeout?

56
The Big Picture

• Technology doesnt really drive network design,
  new protocols, etc.
• Social / Society / networking
• Business
• Legal / Criminal
• Others?

57
Next two weeks

• NO SCHOOL NEXT WEEK (10/13), but homework 2a
  DUE! (if not before)
• Homework 2b due 10/20 (posted)
• Finish reliable data transfer
• TCP
• Connection Management
• Congestion Control (Reno/Tahoe/Vegas)
• RTT/RTO problems (sockstress)
• Flow Control
• Other types SCTP, DCCP
• 1st midterm examination in 3 weeks (October 27th)