IPv6 and Quality of Service (QoS)

IPv6 addresses another challenge that has recently faced the aging IPv4 infrastructure: the need for uniform Quality of Service (QoS) levels.

In the old days, when the Internet primarily was used for email and FTP-style downloads, no one thought much about prioritizing data transmission. If an email message didn’t arrive in 2 seconds, it would arrive in 2 minutes—or possibly in an hour. No one really cared about specifying or limiting the time interval in which the message could arrive. In contrast, today’s Internet supports many different types of transmissions, some with rigid delivery requirements. Internet video and television and other real-time applications cannot operate properly with long delays as packets wind their way through router buffers. Even a small delay in an Internet phone connection can have the effect of distorting the speech of the participants.

In the Internet of the future, it will be possible to prioritize IP datagrams as they wait for delivery. A datagram from an interactive video application could move to the top of the queue as it waits in a router buffer, whereas an email datagram might pause for a momentary delay.

IPv6 is designed to support prioritizing through differentiated service levels. The Traffic Class and Flow Label fields of the IPv6 header provide a means for specifying the type and priority of data enclosed in the datagram (refer to Figure 13.1).