Computer Networks with Internet Technology William Stallings

1
Computer Networks with Internet
TechnologyWilliam Stallings

• Chapter 04
• Modern Applications

2
Hypertext Transfer ProtocolHTTP

• Underlying protocol of the World Wide Web
• Not a protocol for transferring hypertext
• For transmitting information with efficiency
  necessary for hypertext jumps
• Can transfer plain text, hypertext, audio,
  images, and Internet accessible information

3
HTTP Overview

• Transaction oriented client/server protocol
• Usually between Web browser (clinet) and Web
  server
• Uses TCP connections
• Stateless
• Each transaction treated independently
• Each new TCP connection for each transaction
• Terminate connection when transaction complete

4
Key Terms

• Cache
• Client
• Connection
• Entity
• Gateway
• Message
• Origin server
• Proxy
• Resource
• Server
• Tunnel
• User agent

5
Figure 4.1Examples of HTTP Operation
6
Figure 4.2Intermediate HTTP Systems
7
HTTP Messages

• Requests
• Client to server
• Responses
• Server to client
• Request line
• Response line
• General header
• Request header
• Response header
• Entity header
• Entity body

8
Figure 4.3HTTP Message Structure
9
General Header Fields

• Cache control
• Connection
• Data
• Forwarded
• Keep alive
• MIME version
• Pragma
• Upgrade

10
Request Methods

• Request-Line Method ltSPgt Request_URL ltSPgt
  HTTP-Version ltCRLFgt
• Methods
• Options
• Get
• Head
• Post
• Put
• Patch
• Copy
• Move
• Delete
• Link
• Unlink
• Trace
• Wrapped
• Extension-method

11
Request Header Field

• Accept
• Accept charset
• Accept encoding
• Accept language
• Authorization
• From
• Host
• If modified since
• Proxy authentication
• Range
• Referrer
• Unless
• User agent

12
Response Messages

• Status line followed by one or more general,
  response and entity headers, followed by optional
  entity body
• Status-Line HTTP-Version ltSPgt Status-Code ltSPgt
  Reason-Phrase ltCRLFgt

13
Status Codes

• Informational
• Successful
• Redirection
• Client error
• Server error

14
Response Header Fields

• Location
• Proxy authentication
• Public
• Retry after
• Server
• WWW-Authenticate

15
Entity Header Fields

• Allow
• Content encoding
• Content language
• Content length
• Content MD5
• Content range
• Content type
• Content version
• Derived from

• Expires
• Last modified
• Link
• Title
• Transfer encoding
• URL header
• Extension header

16
Entity Body

• Arbitrary sequence of octets
• HTTP transfers any type of data including
• text
• binary data
• audio
• images
• video
• Interpretation of data determined by header
  fields
• Content encoding, content type, transfer encoding

17
Internet Directory Services DNS

• Directory lookup service
• Provides mapping between host name and numerical
  address
• Essential to functioning of Internet
• RFCs 1034 and 1035.
• Four elements
• Domain name space
• Tree-structured
• DNS database
• Each node and leaf in name space tree structure
  names set of information (e.g., IP address, type
  of resource) in resource record
• Name servers
• Servers that hold information about portion of
  tree
• Resolvers
• Programs that extract information from name
  servers

18
Domain Names

• 32-bit IP address uniquely identifies devices
• Two components
• Network number
• Host address
• Problems
• Routers devise routes based on network number
• Cant hold table of every network and path
• Networks group to simplify routing
• 32-bit address usually written as four decimal
  numbers
• Effective for computer processing
• Not convenient for users
• Problems are addressed by concept of domain
• Group of networks are under control of single
  entity
• Organized hierarchically
• Names assigned reflect organization

19
Domain Name Example

• edu is college-level U.S. educational
  institutions
• mit.edu is M.I.T.
• lcs.mit.edu is Laboratory for Computer Science at
  M.I.T.
• Eventually get to leaf nodes
• Identify specific hosts
• Hosts assigned Internet (IP) addresses
• Internet-wide organization assigns domain names
• Delegated down the hierarchy
• mit.edu, has four IP addresses 18.7.21.77,
  18.7.21.69, 18.7.21.70, and 18.7.21.110
• Subordinate domain lcs.mit.edu has IP address
  18.26.0.36

20
Figure 4.4Portion of Internet Domain Tree
21
DNS Database

• Variable-depth unlimited levels hierarchy for
  names
• Delimited by period (.)
• Distributed database
• Distribution controlled by database
• Thousands of separately managed zones
• DNS servers provide name-to-address directory
  service for network applications

22
Resource Record

• Domain name
• Human readable form
• Series of labels of alphanumeric characters or
  hyphens
• Each pair separated by period
• Type
• E.g. A Host address, MX Mail exchange
• Class
• Usually IN, for Internet
• Time to live
• How long to hold result in local cache
• Zero means dont cache
• Rdata field length
• Rdata
• Description of resource
• For A type, Rdata is 32-bit IP address

23
Figure 4.5DNS Resource Record Format
24
DNS Operation

• User program requests IP address for domain name
• Resolver module in local host or local ISP
  formulates query for local name server
• In same domain as resolver
• Local name server checks for name in local
  database or cache
• If so, returns IP address to requestor
• Otherwise, query other available name servers
• Starting down from root of DNS tree or as high up
  as possible
• Local name server caches reply
• Depending on Time to live field
• User program given IP address or error message
• DNS name servers automatically send out updates
  to other relevant name servers as conditions
  warrant

25
Figure 4.6DNS Name Resolution
26
Server Hierarchy

• Name servers operated by any organization that
  has domain
• Each name server holds subset of name space (a
  zone)
• One or more (or all) subdomains within domain
• Authoritative
• This name server maintains accurate data for this
  portion hierarchy
• Can extend to any depth
• 13 root name servers share responsibility for top
  level zones
• Replication prevents root server bottleneck
• Individual root servers are busy
• Internet Software Consortium server (F) answers
  almost 300 million DNS requests daily
  (www.isc.org/services/public/F-root-server.html)
• Typically, single queries carried over UDP
• Queries for group of names carried over TCP

27
Name Resolution

• Resolver knows name and address of local DNS
  server
• If resolver does not have name in cache, it sends
  DNS query to local server
• Either returns address or after querying one or
  more other servers
• Server (A) forwards request to server (B)
• If B has name in cache or database, it can return
  result
• If not, B can
• Query another name server and send result back to
  A
• Recursive
• Tell A address of next server (C) to ask
• A then asks to C
• Iterative
•  Server exchanges use can either
• Name resolvers use recursive

28
Figure 4.7DNS Message Format
29
DNS Message Fields - Header

• Header always present 
• Identifier to match queries and responses.
• Query Response is message query or response
• Opcode Standard, inverse query (address to
  name), or server status request
• Authoritative Answer
• Truncated was response truncated
• Requestor will use TCP to resend query
• Recursion Desired
• Recursion Available
• Response Code e.g. no error, format error,
  refused
• QDcount entries in question section (zero or
  more)
• ANcount RRs in answer section (zero or more)
• NScount RRs in authority section (zero or more)
• ARcount RRs in additional records section (zero
  or more)

30
DNS Message Fields Question and Answer

• If present, question typically contains only one
  entry
• Domain Name
• Sequence of labels
• Length octet followed by that number of octets
• Terminates with the zero length octet for null
  label of root
• Query Type
• Values include all values valid for Type field in
  RR format plus general codes that match more than
  one type of RR
• Query Class typically Internet.
• Answer section contains RRs that answer question
• Authority section contains RRs that point toward
  an authoritative name server
• Additional records section contains RRs that
  relate to query but not strictly answers

31
Session Initiation ProtocolOverview

• RFC 3261
• Application-level control protocol
• Setting up, modifying, and terminating real-time
  sessions
• Enable Internet telephony,(voice over IP, VoIP
• Supports single or multimedia session, including
  teleconferencing
• Facets of SIP
• User location Users can access application
  features from remote locations
• User availability Willingness of called party to
  communicate
• User capabilities Media and parameters to be
  used
• Session setup Point-to-point and multiparty
  calls
• Session management Transfer and termination,
  modifying session parameters, and invoking
  services

32
Session Initiation ProtocolFeatures

• Based on HTTP-like request/response transaction
  model
• Client request invokes function on server
• At least one response
• Uses most HTTP header fields, encoding rules, and
  status codes
• Readable format for displaying information
• Uses concepts similar to recursive and iterative
  searches of DNS
• Incorporates Session Description Protocol (SDP)
• Defines session content using types similar to
  MIME

33
Components and Protocols (1)

• Client
• Sends requests and receives responses
• User agent clients and proxies are clients
• Server
• Receives requests and sends back responses
• Proxies, user agent servers, redirect servers,
  and registrars
• User Agent
• In every SIP end station
• User agent client (UAC) Issues requests
• User agent server (UAS) Receives requests and
  reponds
• Redirect Server
• Determines address of called device
• Like iterative searches in DNS

34
Components and Protocols (2)

• Proxy Server
• Server and client
• Makes requests for other clients
• Routing
• Enforcing policy
• Like recursive searches in DNS
• Registrar
• Server that accepts REGISTER requests
• Places information it receives in requests
  inlocation service for domain
• SIP address, associated IP address of device
• Location Service
• Used by redirect or proxy server to obtain
  information about a callee's possible location(s)
• Maintains database of SIP-address/IP-address
  mappings

35
Components and Protocols (3)

• Servers defined (RFC 3261) as logical devices
• Implemented as separate servers or combined into
  single application
• Proxy servers may act as redirect servers
• Need to consult location service database
• May be on proxy server or not
• Communication between proxy server and location
  service beyond scope of SIP standard
• Proxy consults DNS server to find target domain
  proxy
• SIP typically runs on UDP for performance
• Own reliability mechanisms
• May also use TCP
• May use Transport Layer Security (TLS) protocol
  for secure connection

36
Session Description ProtocolSDP

• RFC 2327
• SIP invites participants to session
• SDP-encoded body of SIP message contains
  information about what media encodings (e.g.,
  voice, video) parties can and will use
• Then data transmission begins, using appropriate
  transport protocol
• Real-Time Transport Protocol (RTP)
• Participants can make changes to session
  parameters using SIP

37
Figure 4.8 SIP Components and Protocols
38
Uniform Resource Identifier URI

• Identifies SIP resource
• User of online service
• Appearance on multiline phone
• Mailbox on messaging system
• Telephone number at gateway service
• Group (such as "sales" or "helpdesk") in an
  organization
• Format based on email address formats
• user_at_domain
• sipbob_at_biloxi.com
•  May also include password, port number, and
  related parameters
• If secure transmission required, use "sips
• SIP messages are transported over TLS
• URI is generic identifier for resource on
  Internet
• URL, for Web addresses is type of URI

39
Figure 4.9 SIP Call Setup Attempt Scenario
40
Figure 4.10 SIP Presence Example
41
Figure 4.11 SIP Registration and Notification
Example
42
Figure 4.12 SIP Successful Call Setup
43
SIP Messages

• Requests and responses
• Difference between types in first line
• Request
• Method nature of request
• Request-URI where request should be sent
• Response has response code
• All messages include header
• Number of lines
• Beginning with header label
• Message can also contain body e.g. SDP media
  description

44
SIP Messages - Requests

• Methods
• REGISTER notify SIP network of IP address and
  URLs for which it would like to receive calls
• INVITE establish session between user agents
• ACK Confirms reliable message exchanges
• CANCEL Terminates pending request, but does not
  undo completed call
• BYE Terminates session between two users in
  conference
• OPTIONS Solicits information about callee
  capabilities

45
SIP Message Request Example

• INVITE sipbob_at_biloxi.com SIP/2.0
• Via SIP/2.0/UDP 12.26.17.915060
• Max-Forwards 70
• To Bob ltsipbob_at_biloxi.comgt
• From Alice ltsipalice_at_atlanta.comgttag192830177
  4
• Call-ID a84b4c76e66710_at_12.26.17.91
• CSeq 314159 INVITE
• Contact ltsipalice_at_atlanta.comgt
• Content-Type application/sdp
• Content-Length 142

46
SIP Messages - Response

• Provisional (1xx) Request received and being
  processed
• Success (2xx) Action successfully received,
  understood, and accepted
• Redirection (3xx) Further action needed
• Client Error (4xx) Request contains bad syntax
  or cannot be fulfilled at this server
• Server Error (5xx) Server failed to fulfill
  apparently valid request
• Global Failure (6xx) Request cannot be fulfilled
  at any server

47
SIP Response Example

• SIP/2.0 200 OK
• Via SIP/2.0/UDP server10.biloxi.com
• Via SIP/2.0/UDP bigbox3.site3.atlanta.com
• Via SIP/2.0/UDP 12.26.17.915060
• To Bob ltsipbob_at_biloxi.comgttaga6c85cf
• From Alice ltsipalice_at_atlanta.comgttag192830177
  4
• Call-ID a84b4c76e66710_at_12.26.17.91
• CSeq 314159 INVITE
• Contact ltsipbob_at_biloxi.comgt
• Content-Type application/sdp
• Content-Length 131

48
SDP Information

• Media streams
• Session can include multiple streams of differing
  content
• Currently defines audio, video, data, control,
  and application
• Addresses
• Destination addresses,
• May be multicast
• Ports
• For each stream
• Payload types
• For each media stream type
• Start and stop times
• For broadcast sessions e.g. television or radio
  program
• Originator
• For broadcast sessions

49
Sockets

• 1980s UNIX Berkeley Sockets Interface
• Socket enables communications between client and
  server process
• Connection-oriented or connectionless
• Endpoint in communication
• Client socket in one computer uses address to
  call server socket on another computer
• Once appropriate sockets engaged, can exchange
  data
• Server sockets keep TCP or UDP port open
• Once connected server switches dialogue to
  different port

50
Sockets API (1)

• Sockets can be constructed from within program in
  most languages
• Berkeley Sockets Interface is de facto standard
  API
• Windows Sockets (WinSock) based on Berkeley
• TCP and UDP header includes source port and
  destination port fields
• Identify respective users (applications)
• IPv4 and IPv6 header includes source address and
  destination address fields
• Identify host systems
• Port value with IP address forms socket
• Unique throughout Internet
• When used as API, socket is identified by triple
• Protocol, local-address (IP), local-process (port)

51
Sockets API

• Sockets API recognizes three types of sockets
• Stream sockets for TCP, connection-oriented
  reliable
• Datagram sockets for UDP, connectionless
• Raw sockets
• Direct access to lower layer protocols, e.g. IP
  and ICMP

52
Socket Interface Calls

• Gethostname
• Gethostbyname
• Setup
• Connect
• Client
• Listen/accept
• Server
• Send
• Receive
• Close

53
Figure 4.13 Socket System Calls for
Connection-Oriented Protocol
54
Required Reading

• Stallings chapter 4
• RFCs
• WWW Consortium
• Loads of books and web sites on sockets
• E.g. Comer, D.E. and Stevens, D.L.
  Internetworking with TCP/IP Volume III, Prentice
  Hall
• Comes in three versions
• Windows Sockets
• BSD Sockets
• ATT TLI (not sockets)