Chapter 7
The Application Layer
Where are we now?
DNS: Domain Name System
availability
performance
a) An example partitioning of the DNS name space, including Internet-accessible files,
into three layers.
b) Caching: for performance and 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)
e.g. Europe
e.g. America
a) The comparison between recursive and iterative name resolution with respect
to communication costs.
SMTP: Simple Mail Transfer Protocol
User Agent (UA): interacts with the user
Message Transfer Agent (MTA): actual communication
SMTP
MTAs within the Internet act as gateways that transfer (and convert) messages
from/to other networks.
SMTP
Entire Email System:
SMTP
Multipurpose Internet Mail Extension (MIME): Helper application that
supports SMTP with treatment of non-ASCII messages.
SMTP
Post 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.
SNMP
To 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