Part 7 - TCP Versus IP

In my last post I mentioned that there is a difference between TCP/IP, TCP, and IP.  The difference is this: TCP and IP are Internet protocols and TCP/IP is a suite of Internet protocols.  The TCP/IP suite includes not just the TCP and IP protocols, but also, dozens of other protocols such as, HTTP, POP, SMTP, and ISDN to name a few.  I'm not going to elaborate on the meaning of those acronyms nor am I going to explain all of the protocols in the Internet Protocol Suite in this series.  I will only be going over six of these starting with TCP and IP.  So what are they?

Transmissions Control Protocol (TCP) and Internet Protocol (IP) were created by the Department of Defense(DoD) during the infancy of the Internet [1].  In 1983 the United States military invaded the island of Grenada in an operation called "Operation Urgent Fury," which was an attempt to protect and rescue US medical students who lived there.  However, the military had a hard time finding the students due to lack of intelligence [2].  This lack of intelligence was caused in part by the inability of the different branches (Army, Navy, and Air Force) computer systems to communicate over the communications system (ARPANET) that the Defense Advanced Research Projects Agency (DARPA) had created and been using since the late 1960s [3].

The problem was that by law, defense contracts have to go to the lowest bidder.  When each branch launched bids to build their systems, they did so separately, so three different companies won three different bids to build systems; Digital Equipment Corporation won the Army's bid, International Business Machines won the Air Force's bid, and Unisys won the Navy's bid.  After the systems were built, each branch had an effective local area network (LAN), however, the LAN had trouble communicating to say the least, because there were no standards protocols set to allow them to.  Each network operated independently, which led to the trouble with the joint military operation in Grenada [1].

This led to the DoD setting out to create protocols that would allow these independent networks to communicate as if they were a part of one big LAN.  They wanted to create a language that would allow multiple LAN to communicate with one another creating a wide area network (WAN).  Not only did this language make things like file transfers and email possible between branches, it provided a fail safe so that if any one LAN or phone line went down or was destroyed in battle, the WAN could still communicate without having to be rebuilt.  Essentially, the WAN would self-recover [1].

Now with the history lesson out of the way, here's what each protocol does.  On LAN, there are various protocols used to assign addresses to individual computers depending on what vendor is used to build the network.  These protocols are generally not compatible with one another.  In other words, LAN using Apple computers use Appletalk and LAN using Novell machines use IPX to communicate, but these two LAN could not communicate without a common protocol.  Internet Protocol (IP) is that protocol.  Internet Protocol is the main protocol used in Internet communications. Unique 4-byte (32-bit) addresses (with each byte having a value of 0-254) are provided for all computers on the network using IP and when I say unique, I mean unique.  The intent of IP is to assign every computer in the world that is connected to the Internet a different IP address [1].  That is how the cops can find you if you download that illegal copy of Photoshop (I kid of course. I know none of you would do anything illegal).  This is done to ensure that the data is being sent to the right computer.

"Wait!  Every computer in the world? Surely there are more computers in the world than can be provided for with a 4-byte address."  Well, each byte has a value of 0-254 or 255 possible combinations for just the one byte.  Since we are combining four bytes that makes 255⁴ or 4,228,250,625 unique addresses.  Although some of these addresses are reserved, 4.2 billion is a lot of computers.  Despite that, IP addresses are in fact running out [4].  The IP we have been using is referred to as "IPv4" or Internet Protocol version 4, and most of us do have a IPv4 address, but since around 2005 or so IPv6 has been rolling out all over the world.  IPv6 provides 128-bit addresses [5].  Since as we know, a bit can only be a one or a zero, that would make 2¹²⁸ or 3.4x10³⁸ different combinations.

But I digress. We were discussing how IP works.  In addition to assigning each computer a unique identifying address, transmitted data is organized into packets with a header that contains source and destination IP addresses as well as other information about the data. This helps TCP do its job which we will get to in a minute.  Internet Protocol operates on Layer 3 of the Open Systems Interconnection (OSI) model, which allows it to function on different mediums such as Ethernet or Wi-Fi [6].  I do not want to delve into a discussion of the OSI model as it is beyond the scope of this series, but just so you know, Layer 3 is the the level on which LAN communication and routing occurs.  In other words, we just have LAN only transmissions and preparation for WAN transmission occurring on this Layer, no interpretation or application of data goes on [7].

Transmissions Control Protocol (TCP) provides guaranteed client/server transmission by supporting timeouts and re-transmissions when data is lost or errors occur. It is used when applications require guaranteed delivery of data. This protocol uses full-duplex transmission and each client is identified with an IP address and a TCP port number. Transmissions Control Protocol also provides basic data transfer, reliability, flow control, and multiplexing [6].  Essentially, TCP is the protocol that ensures that the illegal copy of Photoshop that you are downloading gets to your computer intact with no bits of the file missing. That way the cops have evidence that you did in fact download an illegal copy of Photoshop.  Transmissions Control Protocol operates on Layer 4 of the OSI model also called the Transport layer.  This is the Layer that is responsible for transmission and error checking [7].

This concludes Part 7 of the the Data Transmission and Protocol series.  In Part 8 I will explain Serial Line Internet Protocol and Point-To-Point Protocol.  After reading Part 8 you will understand why PPP is so widely used while SLIP is barely used at all.

[1] Gilbert, H. (1995). Introduction to TCP/IP. Retrieved from http://bit.ly/pSChlt

[2] Military Advantage. (2011). Invasion of Granada: Operation Urgent Fury. Retrieved from http://bit.ly/nNfhaA

[3] Howe, W. (2010). A brief history of the Internet. Retrieved from http://bit.ly/qUfkcb

[4] Huston, G. (2011). IPv4 Address Report. Retrieved from August 4, 2011 http://bit.ly/n4OZ78

[5] Das, K. (2008). IPv6 deployment around the world. Retrieved from http://bit.ly/oIH9lK

[6] Mitchell, B. (2010). IP - Internet Protocol. Retrieved from http://bit.ly/qTodlE

[7] Tomsho, G., Tittel, E., & Johnson, G. (2004). Guide to networking essentials. Thomson Course Technology: Boston, MA. ISBN-13: 978-1-4188-3718-1