services.txt
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Internet services

 Client/server: client (a user) requests service, server provides service 
  Example: web browser is client, server at www.google.com provides search
  Client/server is asymmetric: 
   Client opens, closes temporary session
   Server runs all the time, waits for clients to connect,
    handles multiple client sessions simultaneously















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Internet service - Hosts

 Services are identified by host and port

 Hosts are identified by names or IP addresses
  Uniquely identifies each host on the planet (er, solar system, see DINET)
  Must be issued by central authority, starting at top with ICANN
   Names and numbers are hierarchical, so authority can be delegated locally
  Local machine can always be reached at localhost 127.0.0.1
   good for developement and debugging

 Domain Name Service - DNS - associates names with IP addresses
  nslookup command: nslookup staff.washington.edu
    (then point your browser at the numeric IP address)








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Internet services - Ports

 Services are identified by host and port
  port numbers distinguish different services at the same host
 
 Ports numbers range from 0 to 65535

 Common services are at well-known port numbers:
   7 echo, 13 daytime, 22 ssh, 25 email, 53 DNS, 80 web server,  ...

 Use high port numbers for your own custom services












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Internet services - Protocols

 Each service implements a protocol: in effect, a command language

 Telnet client can demo any service that runs on TCP/IP 
  telnet command opens TCP/IP connection to host and port,
   then you can just type the protocol commands by hand
  example: telnet staff.washington.edu 80
    (type garbage, see what happens.  Then type using HTTP protocol).

 Services to try - 
  Any web site, port 80, HTTP protocol
  Very simple: date/time service
   List of servers at http://tf.nist.gov/tf-cgi/servers.cgi
    telnet nist1-sj.ustiming.org 13
      ...