TCP/IP basics, Part 2
 
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Continuing our discussion of TCP/IP, let's take a look at the 32-byte header of IP version 4.
We mentioned last time that IP places address numbers so that routers can forward user information to the correct receiving device. But the IP header also contains other important information, including:
* Version field, indicating which IP version is being used;
* Header length, currently limited to 60 bytes in IP version 4 (IPv4);
* Type of service, used to show the traffic requirements of the packet;
* Length, indicating the length of the entire datagram;
* Identification, which uniquely shows which IP packet belongs to which datagram; and
* Source address and destination address, each limited to 32 bits.
Please note, there are five important items when considering IP as a convergence protocol. First, we've detailed information for IPv4, which is the most widely deployed version, but IPv4 will soon be displaced by IPv6 so that additional information can be included in the future IP version header.
Second, IP contains a type of service (TOS) indicator that allows for a user's data to be assigned a relative priority. Exactly how this TOS byte will be used is still a topic of great discussion by the Internet Engineering Task Force. Assignment of TOS byte values forms the basis for DiffServ and Multi-protocol Label Switching, which we will begin to cover in the coming weeks.
Third, IP frames are both variable in length and can arrive " out of sequence, " which allows for variable delay and presents special challenges when trying to use IP for delay-sensitive services like voice.
Fourth, IPv4 has up to 60 bytes of overhead in the IP header, so IP data frames must be properly filled to prevent the excessive " overhead tax " we previewed last week in our perspective on ATM.
Fifth, with only 32 bits available for unique addresses, IPv4 presents special challenges when trying to use the limited address space to create universally available unique number plans.
Now that we have a better IP foundation, next time, we'll begin to look at some more details on how IP advocates are addressing some of the problems inherent in IP as a convergence protocol.
RELATED LINKS
Steve Taylor is President of Distributed Networking Associates and Publisher/Editor-in-Chief of Webtorials.Com. For more detailed information on most of the topics discussed in this newsletter, connect to Webtorials.Com, the first Web site dedicated exclusively to market studies and technology tutorials in the Broadband Packet areas of Frame Relay, ATM, and IP.
Larry Hettick is an independent consultant, with 19 years of experience in telecommunications and data communications marketing and product management for service providers and equipment vendors. He can be reached at larry@larryhettick.com
You can reach the authors at taylor@webtorials.com or larry@larryhettick.com.
Convergence archive
Past newsletters.
Gary Kessler's TCP/IP tutorial
IP-centric conferencing offers flexibility
Network World, 10/22/01
Cisco, IBM team to provide voice-over-IP package
Network World, 10/22/01
