Workshop

This is reasonably easy with TCP (Transmission Control Protocol) sockets. A TCP connection is a
connection-oriented call—like a true phone call, where someone dials someone else and establishes a
two-way link.
A UDP (User Datagram Protocol) conversation, on the other hand, is basically like when I throw you a
crumpled-up piece of paper containing a message, and you throw one back at me. This is called a
connectionless session. You can think of the difference between TCP and UDP as the difference between
you and me using two tin cans and a string (TCP) to communicate versus you and me passing notes in
class (UDP). Keep in mind that a note can be easily misdirected.
It’s fairly trivial for TCP connections to be limited, because the reply to the connection is basically
within the same connection. It’s a lot harder to do this with UDP sockets. A UDP connection throws out
a packet and then waits for the reply. Because no connection exists, the firewall must be configured to
accept random UDP packets, any of which might be a reply. Typically, a range of UDP ports has to be
allowed in through the firewall, which implies a lot of trust.
Some administrators (depending on their site security policies) disallow UDP through the
firewall—period. Others rely on SMLI (Stateful Multi-Level Inspection) firewalls, which “remember”
the packets that have passed through them and accept responses appropriately.
Previous Table of Contents Next