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TCP/IP Overview
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TCP/IP Overview
The generic term "TCP/IP" usually means anything and everything related to the specific protocols of TCP and IP. It can include other protocols, applications, and even the network medium. A sample of these protocols are: UDP, ARP, and ICMP. A sample of these applications are: TELNET, FTP, and rcp. A more accurate term is "internet technology". A network that uses internet technology is called an "internet".
To understand this technology you must first understand the following logical structure:
----------------------------
| network applications |
| |
|... \ | / .. \ | / ...|
| ----- ----- |
| |TCP| |UDP| |
| ----- ----- |
| \ / |
| -------- |
| | IP | |
| ----- -*------ |
| |ARP| | |
| ----- | |
| \ | |
| ------ |
| |ENET| |
| ---@-- |
----------|-----------------
|
----------o------------------
Ethernet Cable
Figure 1. Basic TCP/IP Network Node
This is the logical structure of the layered protocols inside a computer on an internet. Each computer that can communicate using internet technology has such a logical structure. It is this logical structure that determines the behavior of the computer on the internet. The boxes represent processing of the data as it passes through the computer, and the lines connecting boxes show the path of data. The horizontal line at the bottom represents the Ethernet cable; the "o" is the transceiver. The "*" is the IP address and the "@" is the Ethernet address. Understanding this logical structure is essential to understanding internet technology; it is referred to throughout this tutorial.
The name of a unit of data that flows through an internet is dependent upon where it exists in the protocol stack. In summary: if it is on an Ethernet it is called an Ethernet frame; if it is between the Ethernet driver and the IP module it is called a IP packet; if it is between the IP module and the UDP module it is called a UDP datagram; if it is between the IP module and the TCP module it is called a TCP segment (more generally, a transport message); and if it is in a network application it is called a application message.
These definitions are imperfect. Actual definitions vary from one publication to the next. More specific definitions can be found in RFC 1122, section 1.3.3.
A driver is software that communicates directly with the network interface hardware. A module is software that communicates with a driver, with network applications, or with another module.
The terms driver, module, Ethernet frame, IP packet, UDP datagram, TCP message, and application message are used where appropriate throughout this tutorial.
Let's follow the data as it flows down through the protocol stack shown in Figure 1. For an application that uses TCP (Transmission Control Protocol), data passes between the application and the TCP module. For applications that use UDP (User Datagram Protocol), data passes between the application and the UDP module. FTP (File Transfer Protocol) is a typical application that uses TCP. Its protocol stack in this example is FTP/TCP/IP/ENET. SNMP (Simple Network Management Protocol) is an application that uses UDP. Its protocol stack in this example is SNMP/UDP/IP/ENET.
The TCP module, UDP module, and the Ethernet driver are n-to-1 multiplexers. As multiplexers they switch many inputs to one output. They are also 1-to-n de-multiplexers. As de-multiplexers they switch one input to many outputs according to the type field in the protocol header.
1 2 3 ... n 1 2 3 ... n
\ | / | \ | | / ^
\ | | / | \ | | / |
------------- flow ---------------- flow
|multiplexer| of |de-multiplexer| of
------------- data ---------------- data
| | | |
| v | |
1 1
Figure 2. n-to-1 multiplexer and 1-to-n de-multiplexer
If an Ethernet frame comes up into the Ethernet driver off the network, the packet can be passed upwards to either the ARP (Address Resolution Protocol) module or to the IP (Internet Protocol) module. The value of the type field in the Ethernet frame determines whether the Ethernet frame is passed to the ARP or the IP module.
If an IP packet comes up into IP, the unit of data is passed upwards to either TCP or UDP, as determined by the value of the protocol field in the IP header.
If the UDP datagram comes up into UDP, the application message is passed upwards to the network application based on the value of the port field in the UDP header. If the TCP message comes up into TCP, the application message is passed upwards to the network application based on the value of the port field in the TCP header.
The downwards multiplexing is simple to perform because from each starting point there is only the one downward path; each protocol module adds its header information so the packet can be de- multiplexed at the destination computer.
Data passing out from the applications through either TCP or UDP converges on the IP module and is sent downwards through the lower network interface driver.
Although internet technology supports many different network media, Ethernet is used for all examples in this tutorial because it is the most common physical network used under IP. The computer in Figure 1 has a single Ethernet connection. The 6-byte Ethernet address is unique for each interface on an Ethernet and is located at the lower interface of the Ethernet driver.
The computer also has a 4-byte IP address. This address is located at the lower interface to the IP module. The IP address must be unique for an internet.
A running computer always knows its own IP address and Ethernet address.
If a computer is connected to 2 separate Ethernets it is as in Figure 3.
---------------------------- | network applications | | | |... \ | / .. \ | / ...| | ----- ----- | | |TCP| |UDP| | | ----- ----- | | \ / | | -------- | | | IP | | | ----- -*----*- ----- | | |ARP| | | |ARP| | | ----- | | ----- | | \ | | / | | ------ ------ | | |ENET| |ENET| | | ---@-- ---@-- | ----------|-------|--------- | | | ---o--------------------------- | Ethernet Cable 2 ---------------o---------- Ethernet Cable 1
Figure 3. TCP/IP Network Node on 2 Ethernets
Please note that this computer has 2 Ethernet addresses and 2 IP addresses.
It is seen from this structure that for computers with more than one physical network interface, the IP module is both a n-to-m multiplexer and an m-to-n de-multiplexer.
1 2 3 ... n 1 2 3 ... n
\ | | / | \ | | / ^
\ | | / | \ | | / |
------------- flow ---------------- flow
|multiplexer| of |de-multiplexer| of
------------- data ---------------- data
/ | | \ | / | | \ |
/ | | \ v / | | \ |
1 2 3 ... m 1 2 3 ... m
Figure 4. n-to-m multiplexer and m-to-n de-multiplexer
It performs this multiplexing in either direction to accommodate incoming and outgoing data. An IP module with more than 1 network interface is more complex than our original example in that it can forward data onto the next network. Data can arrive on any network interface and be sent out on any other.
TCP UDP
\ /
\ /
--------------
| IP |
| |
| --- |
| / \ |
| / v |
--------------
/ \
/ \
data data
comes in goes out
here here
Figure 5. Example of IP Forwarding a IP Packet
The process of sending an IP packet out onto another network is called "forwarding" an IP packet. A computer that has been dedicated to the task of forwarding IP packets is called an "IP-router".
As you can see from the figure, the forwarded IP packet never touches the TCP and UDP modules on the IP-router. Some IP-router implementations do not have a TCP or UDP module.
The IP module is central to the success of internet technology. Each module or driver adds its header to the message as the message passes down through the protocol stack. Each module or driver strips the corresponding header from the message as the message climbs the protocol stack up towards the application. The IP header contains the IP address, which builds a single logical network from multiple physical networks. This interconnection of physical networks is the source of the name: internet. A set of interconnected physical networks that limit the range of an IP packet is called an "internet".
IP hides the underlying network hardware from the network applications. If you invent a new physical network, you can put it into service by implementing a new driver that connects to the internet underneath IP. Thus, the network applications remain intact and are not vulnerable to changes in hardware technology.
If two computers on an internet can communicate, they are said to "interoperate"; if an implementation of internet technology is good, it is said to have "interoperability". Users of general-purpose computers benefit from the installation of an internet because of the interoperability in computers on the market. Generally, when you buy a computer, it will interoperate. If the computer does not have interoperability, and interoperability can not be added, it occupies a rare and special niche in the market.
With the background set, we will answer the following questions:
When sending out an IP packet, how is the destination Ethernet address determined?
How does IP know which of multiple lower network interfaces to use when sending out an IP packet?
How does a client on one computer reach the server on another?
Why do both TCP and UDP exist, instead of just one or the other?
What network applications are available?
These will be explained, in turn, after an Ethernet refresher.