12 myths about how the Internet works

A list of assumptions we make when we hit the send button or download a video

3. Multicast always works

Multicast allows you to send communications out to many systems simultaneously as long as the receivers indicate they can accept the communication. Many applications assume that multicast works within all types of links. But that isn't always true with 802.11 wireless LANs or across tunneling mechanisms such as Teredo or 6to4.

4. The time it takes to initiate communications between two systems is what you'll see throughout the communication

Thaler says many applications assume that the end-to-end delay of the first packet sent to a destination is typical of what will be experienced afterwards. For example, many applications ping servers and select the one that responds first. However, the first packet may have additional latency because of the look-ups it does. So applications may choose longer paths and have slower response times using this assumption. Increasingly, applications such as Mobile IPv6 and Protocol Independent Multicast send packets on one path and then switch to a shorter, faster path.

5. IP addresses rarely change.

Many applications assume that IP addresses are stable over long periods of time. These applications resolve names to addresses and then cache them without any notion of the lifetime of the name/address connection, Thaler says. This assumption isn't always true today because of the popularity of the Dynamic Host Configuration Protocol as well as roaming mechanisms and wireless communications.

6. A computer has only one IP address and one interface to the network.

This is an example of an assumption that was never true to begin with, Thaler says. From the onset of the Internet, hosts could have several physical interfaces to the network and each of those could have several logical Internet addresses. Today, computers are dealing with wired and wireless access, dual IPv4/IPv6 nodes and multiple IPv6 addresses on the same interface making this assumption truly a myth.

7. If you and I have addresses in a subnet, we must be near each other.

Some applications assume that the IP address used by an application is the same as the address used for routing. This means an application might assume two systems on the same subnet are nearby and would be better to talk to each other than a system far away. This assumption doesn't hold up because of tunneling and mobility. Increasingly, new applications are adopting a scheme known as an identifier/locator split that uses separate IP addresses to identify a system from the IP addresses used to locate a system.

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Carolyn Duffy Marsan

Network World
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