Emails

• SMTP, similar to HTTP
• IMAP (retrieval, deletion, folders, etc)
• HTTP interface onto of SMTP to send, etc.
• Email application use agent

DNS

• Find IP address, given hostname (translation)
• Hostname for humans,
• IP address for routers (addressing datagrams)
• Services
  • Translation
  • Host aliasing
  • Mail server aliasing
  • Load distribution
• Why not centralize: single point of failure, volume, database, maintenance
• Hierarchy of many name servers
• Application layer DNS protocol
• Core internet implemented as application
• Scale up rule
• Names are hierarchical
• Nameservers are also organized by hierarchy
• Name resolution follows hierarchy to resolve names
• Hierarchy:
  • Root
  • Top level
  • 2nd level
  • Authoritative
• Contact of last resort (13 root servers replicated ~200 times)