Time To Live

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Time To Live

The Time to Live (TTL) specification is set in Windows NT to a default of 128. This represents either 128 hops or 128 seconds, or a combination of the two. Each time a router handles a datagram, it decrements the TTL by one. If a datagram is held up at a router for longer than one second before it is transmitted, the router can decrement the TTL by more than one.

One way to visualize how the TTL works is to think of a deadly poison. Each time a datagram is sent out on to the network, it is injected with this deadly poison. The datagram has only the length of time specified in the TTL to get to its destination and receive the antidote for the poison. If the datagram gets routed through congested routers, traffic jams, narrow bandwidth communication avenues, and so on it just might not make it. If the TTL expires before the datagram reaches its destination, it is discarded from the network.

Although this concept may seem strange at first, in reality it prevents datagrams from running around a network indefinitely wreaking havoc with bandwidth and the synchronization of data. Imagine a scenario in which 100 datagrams are sent to a machine. Twenty-five of them have to be resent because the Retransmit Timer on the sending machine expired. After the communication is complete and the session broken down, suddenly 25 packets appear out of nowhere hitting the destination machine. It may be that these 25 packets got rerouted through some extremely slow network path and were never discarded. At least in this case the destination machine can just ignore the datagrams. However, in routed environments it would be pretty easy to set up infinite loops where packets would bounce in between two routers indefinitely.

So here we have TCP, UDP, and IP working together to provide both connection-oriented and non-connection-oriented communication. These three protocols work together to provide communication between two machines.

Consider an example that helps to illustrate exactly how each of these protocols works and the functionality required by them. In this illustration, Bob would like to send a message to Kim. The message is an invitation to a New Year’s Eve party.

Think of TCP as being similar to a telephone call. Bob picks up the phone and dials Kim’s number. If Kim is home and wants to receive calls, she picks up the phone, indicating that she is home and available to communicate. Kim answers with a greeting of some sort, such as “Hi,” indicating to Bob that he should speak. Bob then chooses the appropriate response to Kim’s “Hi,” such as, “Hey, this is Bob, I’m glad you’re home.” After this pleasant exchange, the session has been created and Bob can send his message of friendship to Kim. If, however, Bob replies with a response such as “Goodbye,” rather than “Hey,” the communication breaks down because Kim would think, “How strange,” and hang up the phone before any communication can occur. And certainly, if Kim is not at home, no data can be transferred.

Think of UDP now. Here communication can be achieved simply by Bob placing a written invitation in an envelope, addressing it properly, and placing the envelope in his mailbox with the flag up. Bob does not have to verify that Kim is currently at home to send the message. Delivery of the message is not guaranteed, however, because the only way for Bob to know whether the message got there would be for him to receive some indication from Kim, either by mail or by phone. Until Kim responds, Bob has no idea whether the mailman was attacked by a dog, or if perhaps the invitation is currently sitting under a stack of bills on Kim’s desk. The point is, Bob has no way to know.

Now how did IP play into the picture? IP serves both kinds of communications methods, but in and of itself does not guarantee delivery of data. Consider how this applies to the example. For the telephone conversation, IP acts similarly to the old-style operator who connects the call. The operator can make the connection and deliver the available resources, but it’s still up to Bob to say the correct things, and it’s still up to Kim to answer the phone. For the mailed invitation, IP acts as the mailman. The mailman checks to see whether the address is properly formatted and routes it to the appropriate delivery method, until it eventually lands in Kim’s mailbox. No guarantees are made here in terms of delivery of the mail.

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