The Application Layer

Chapter 7

Where are we now?

DNS: Domain Name System

a) An example partitioning of the DNS name space, including Internet-accessible files, into three layers.

b) Caching: for performance and availability

performance

availability

Implementation of Name Resolution (1)

The principle of iterative name resolution.

+: less performance requirements for name servers

-: effective caching only at client

-: may induce high communication overhead

Implementation of Name Resolution (2)

The principle of recursive name resolution.-: high performance demands on name servers+: more effective caching possible (in different places)+: high availability due to cached information (e.g. #<vu> down use #<vu,cs>)+: may reduce communication costs

Implementation of Name Resolution (3)

a) The comparison between recursive and iterative name resolution with respect to communication costs.

e.g. America

e.g. Europe

SMTP: Simple Mail Transfer Protocol

User Agent (UA): interacts with the userMessage Transfer Agent (MTA): actual communication

SMTPMTAs within the Internet act as gateways that transfer (and convert) messages from/to other networks.

SMTPEntire Email System:

SMTPMultipurpose Internet Mail Extension (MIME): Helper application that supports SMTP with treatment of non-ASCII messages.

SMTPPost Office Protocol (POP3): Supports desktop computers that do not continuously run an MTA. A mail server (of an organization) is always “on”, and desktop computers connect to the mail sever via POP3 to get their (received) messages. Desktop computers can send messages directly using MTA (via SMTP).

SNMP: Simple Network Management Protocol

Manager: client that manages (part of) the internet (e.g. host)Agent: Machine or device to be managed (e.g. router, host). It acts as a server.Basic Management Functions: 1) Manager gets management information from an agent e.g. x = # of forwarded packets, y = # of received packets Manager checks: y-x high? router congested 2) Manager can set some critical variables with agent to enforce it to perform some tasks. e.g. manager sets the variable reboot = 1 3) Agent can notify manager about critical situations in its environment.

SNMPTo accomplish management tasks at least two other protocols are needed (besides SNMP):

SMI (Structure of Management Information): It defines the structure of the information provided by the agents and defines encoding rules for the data to be transferred (BER: Basic Encoding Rules)

MIB (Management Information Base): Database with an agent that includes the management information needed by managers. Information about 8 different groups can be stored. These are: system, interface, address translation, ip, icmp, tcp, udp, egp.

SNMP