Client-Server Applications

Derek Bridge

Department of Computer Science,
University College Cork

Client-Server Applications

Aims:

• to know what client-server computing is
• to know about servers and ports
• to understand the Domain Name System

Client-Server Networking

Peer-to-Peer Networking

Servers

• Servers have to run, unattended, continuously under possibly heavy loads
• We need high performance and redundancy

Discussion: Is there a difference between the hardware needed for servers and the hardware needed for 'normal' computers?

Ports

• One server might be offering ('hosting') more than one service
• E.g. a server computer may be running web server software and email server software
• How does a client indicate which server program is to handle its requests?
• Each server program is assigned an identification number, called a port number

Class exercise: When transmitting a packet, where does the client put the port number? The IP address?

Listening on ports

Well-known port numbers

• The IANA oversees the use of port numbers
• E.g. well-known port numbers:

  20,21:	ftp	22:	ssh	23:	telnet
  25:	smtp	53:	dns	80:	http
  110:	pop3	143:	imap

  http://www.iana.org/assignments/port-numbers

Telnet and ssh

• The telnet and ssh application protocols allow a user to access and use a remote computer
• Commands you type are
  • captured by the telnet/ssh client software
  • sent over the network, and
  • executed by the remote computer's operating system
• Telnet/ssh are client-server applications; at the transport layer, they use TCP; and they use well-known ports 23 and 22, resp.

The Domain Name System (DNS)

• Numeric IP addresses are cumbersome for humans. Why exactly?
• The Domain Name System (DNS) defines:
  • a name space
  • a name registration scheme, and
  • a name resolution scheme
• In DNS, these are organised hierarchically

The DNS name space

• The set of names is split into subsets, called domains.
• The top-level domains (TLDs) are:
  • .com, .edu, .gov, .mil, .net, .org (.arpa)
  • .int
  • .aero, .biz, .coop, .info, .museum, .name, .pro
  • .mobi
  • two-letter country codes (.ie, .uk, .de, etc). http://www.iana.org/cctld/cctld-whois.htm

The DNS name space

• E.g. www.cs.ucc.ie, cosmos.ucc.ie
• The top-level domains (TLDs) are split into second-level domains
• Each second-level domain may be split further
• At the lowest-level are hostnames
• A name must be unique within its parent domain

DNS name registration

• The IANA decides what TLDs there will be
• For each TLD, there is an authority for managing names within the TLD
• E.g. in Ireland the IE Domain Registry: http://www.domainregistry.ie/
• Once an organization has a registered lower-level domain, it becomes the authority for that domain
• It can sell further lower-level domains or use it for its own internal structure
• Each authority is responsible for ensuring that names are unique within their parent domain

DNS name resolution

• DNS uses a client-server name resolution scheme
• DNS name servers receive names and, if they can, they respond with the corresponding IP address from their database
• But, no DNS name server knows every domain name
• E.g. your local DNS name server(s) might know just the names and IP addresses of hosts connected to your LAN

DNS name resolution

• If you are running an application that needs to translate a name to a numeric IP address:
  • Your application launches a DNS client program
  • The DNS client contacts (one of) your local DNS name server(s)
  • Your local DNS name server responds with the address if it knows it
  • If not, it asks another DNS name server, which will send back either the IP address we want, or the address of another DNS name server
  • And so on, until the local server gets the address and sends it to your DNS client, which gives it to your application

Class exercise:Which do you think DNS uses for name resolution: TCP or UDP?

DNS name resolution

Efficiency: caching

• DNS name servers store the results of recent name resolution requests
• If the request occurs again it can be satisfied from the cache
• Your own machine might even cache results for your DNS client software

Class exercise: What problem do you think this brings? How do you think it is solved?