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RECURSOS SOBRE LA SUITE DE PROTOCOLOS TCP/IP

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RECURSOS SOBRE LA SUITE DE PROTOCOLOS TCP/IP

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© 2002 Rus Shuler @ Pomeroy IT Solutions, all rights reserved

Contents

1. Introduction

2. Where to Begin? Internet Addresses

3. Protocol Stacks and Packets

4. Networking Infrastructure

5. Internet Infrastructure

6. The Internet Routing Hierarchy

7. Domain Names and Address Resolution

8. Internet Protocols Revisited

9. Application Protocols: HTTP and the World Wide Web

10. Application Protocols: SMTP and Electronic Mail

11. Transmission Control Protocol

12. Internet Protocol

13. Bibliography

Introduction
How does the Internet work? Good question! The Internet's growth has become explosive and it seems impossible to escape the bombardment of www.com's seen constantly on television, heard on radio, and seen in magazines. Because the Internet has become such a large part of our lives, a good understanding is needed to use this new tool most effectively.
This whitepaper explains the underlying infrastructure and technologies that make the Internet work. It does not go into great depth, but covers enough of each area to give a basic understanding of the concepts involved. For any unanswered questions, a list of resources is provided at the end of the paper. Any comments, suggestions, questions, etc. are encouraged and may be directed to the author at rshuler@gobcg.com. Because the Internet is a global network of computers each computer connected to the Internet must have a unique address. Internet addresses are in the form nnn.nnn.nnn.nnn where nnn must be a number from 0 - 255. This address is known as an IP address. (IP stands for Internet Protocol; more on this later.)

The picture below illustrates two computers connected to the Internet; your computer with IP address 1.2.3.4 and another computer with IP address 5.6.7.8. The Internet is represented as an abstract object in-between. (As this paper progresses, the Internet portion of Diagram 1 will be explained and redrawn several times as the details of the Internet are exposed.)
If you connect to the Internet through an Internet Service Provider (ISP), you are usually assigned a temporary IP address for the duration of your dial-in session. If you connect to the Internet from a local area network (LAN) your computer might have a permanent IP address or it might obtain a temporary one from a DHCP (Dynamic Host Configuration Protocol) server. In any case, if you are connected to the Internet, your computer has a unique IP address.

If you're using Microsoft Windows or a flavor of Unix and have a connection to the Internet, there is a handy program to see if a computer on the Internet is alive. It's called ping, probably after the sound made by older submarine sonar systems. ¹ If you are using Windows, start a command prompt window. If you're using a flavor of Unix, get to a command prompt. Type ping www.yahoo.com. The ping program will send a 'ping' (actually an ICMP (Internet Control Message Protocol) echo request message) to the named computer. The pinged computer will respond with a reply. The ping program will count the time expired until the reply comes back (if it does). Also, if you enter a domain name (i.e. www.yahoo.com) instead of an IP address, ping will resolve the domain name and display the computer's IP address. More on domain names and address resolution later.

Protocol Stacks and Packets
So your computer is connected to the Internet and has a unique address. How does it 'talk' to other computers connected to the Internet? An example should serve here: Let's say your IP address is 1.2.3.4 and you want to send a message to the computer 5.6.7.8. The message you want to send is "Hello computer 5.6.7.8!". Obviously, the message must be transmitted over whatever kind of wire connects your computer to the Internet. Let's say you've dialed into your ISP from home and the message must be transmitted over the phone line. Therefore the message must be translated from alphabetic text into electronic signals, transmitted over the Internet, then translated back into alphabetic text. How is this accomplished? Through the use of a protocol stack. Every computer needs one to communicate on the Internet and it is usually built into the computer's operating system (i.e. Windows, Unix, etc.). The protocol stack used on the Internet is refered to as the TCP/IP protocol stack because of the two major communication protocols used. The TCP/IP stack looks like this:

Protocol Layer

Comments

Application Protocols Layer

Protocols specific to applications such as WWW, e-mail, FTP, etc.

Transmission Control Protocol Layer

TCP directs packets to a specific application on a computer using a port number.

Internet Protocol Layer

IP directs packets to a specific computer using an IP address.

Hardware Layer

Converts binary packet data to network signals and back.
(E.g. ethernet network card, modem for phone lines, etc.)

If we were to follow the path that the message "Hello computer 5.6.7.8!" took from our computer to the computer with IP address 5.6.7.8, it would happen something like this:

1. The message would start at the top of the protocol stack on your computer and work it's way downward.

2. If the message to be sent is long, each stack layer that the message passes through may break the message up into smaller chunks of data. This is because data sent over the Internet (and most computer networks) are sent in manageable chunks. On the Internet, these chunks of data are known as packets.

3. The packets would go through the Application Layer and continue to the TCP layer. Each packet is assigned a port number. Ports will be explained later, but suffice to say that many programs may be using the TCP/IP stack and sending messages. We need to know which program on the destination computer needs to receive the message because it will be listening on a specific port.

4. After going through the TCP layer, the packets proceed to the IP layer. This is where each packet receives it's destination address, 5.6.7.8.

5. Now that our message packets have a port number and an IP address, they are ready to be sent over the Internet. The hardware layer takes care of turning our packets containing the alphabetic text of our message into electronic signals and transmitting them over the phone line.

6. On the other end of the phone line your ISP has a direct connection to the Internet. The ISPs router examines the destination address in each packet and determines where to send it. Often, the packet's next stop is another router. More on routers and Internet infrastructure later.

7. Eventually, the packets reach computer 5.6.7.8. Here, the packets start at the bottom of the destination computer's TCP/IP stack and work upwards.

8. As the packets go upwards through the stack, all routing data that the sending computer's stack added (such as IP address and port number) is stripped from the packets.

9. When the data reaches the top of the stack, the packets have been re-assembled into their original form, "Hello computer 5.6.7.8!"

Networking Infrastructure
So now you know how packets travel from one computer to another over the Internet. But what's in-between? What actually makes up the Internet? Let's look at another diagram:

Here we see Diagram 1 redrawn with more detail. The physical connection through the phone network to the Internet Service Provider might have been easy to guess, but beyond that might bear some explanation.
The ISP maintains a pool of modems for their dial-in customers. This is managed by some form of computer (usually a dedicated one) which controls data flow from the modem pool to a backbone or dedicated line router. This setup may be refered to as a port server, as it 'serves' access to the network. Billing and usage information is usually collected here as well.

After your packets traverse the phone network and your ISP's local equipment, they are routed onto the ISP's backbone or a backbone the ISP buys bandwidth from. From here the packets will usually journey through several routers and over several backbones, dedicated lines, and other networks until they find their destination, the computer with address 5.6.7.8. But wouldn't it would be nice if we knew the exact route our packets were taking over the Internet? As it turns out, there is a way...

Check It Out - The Traceroute Program

If you're using Microsoft Windows or a flavor of Unix and have a connection to the Internet, here is another handy Internet program. This one is called traceroute and it shows the path your packets are taking to a given Internet destination. Like ping, you must use traceroute from a command prompt. In Windows, use tracert www.yahoo.com. From a Unix prompt, type traceroute www.yahoo.com. Like ping, you may also enter IP addresses instead of domain names. Traceroute will print out a list of all the routers, computers, and any other Internet entities that your packets must travel through to get to their destination.

If you use traceroute, you'll notice that your packets must travel through many things to get to their destination. Most have long names such as sjc2-core1-h2-0-0.atlas.digex.net and fddi0-0.br4.SJC.globalcenter.net. These are Internet routers that decide where to send your packets. Several routers are shown in Diagram 3, but only a few. Diagram 3 is meant to show a simple network structure. The Internet is much more complex.
The Internet backbone is made up of many large networks which interconnect with each other. These large networks are known as Network Service Providers or NSPs. Some of the large NSPs are UUNet, CerfNet, IBM, BBN Planet, SprintNet, PSINet, as well as others. These networks peer with each other to exchange packet traffic. Each NSP is required to connect to three Network Access Points or NAPs. At the NAPs, packet traffic may jump from one NSP's backbone to another NSP's backbone. NSPs also interconnect at Metropolitan Area Exchanges or MAEs. MAEs serve the same purpose as the NAPs but are privately owned. NAPs were the original Internet interconnect points. Both NAPs and MAEs are referred to as Internet Exchange Points or IXs. NSPs also sell bandwidth to smaller networks, such as ISPs and smaller bandwidth providers. Below is a picture showing this hierarchical infrastructure.

This is not a true representation of an actual piece of the Internet. Diagram 4 is only meant to demonstrate how the NSPs could interconnect with each other and smaller ISPs. None of the physical network components are shown in Diagram 4 as they are in Diagram 3. This is because a single NSP's backbone infrastructure is a complex drawing by itself. Most NSPs publish maps of their network infrastructure on their web sites and can be found easily. To draw an actual map of the Internet would be nearly impossible due to it's size, complexity, and ever changing structure.
So how do packets find their way across the Internet? Does every computer connected to the Internet know where the other computers are? Do packets simply get 'broadcast' to every computer on the Internet? The answer to both the preceeding questions is 'no'. No computer knows where any of the other computers are, and packets do not get sent to every computer. The information used to get packets to their destinations are contained in routing tables kept by each router connected to the Internet.

Routers are packet switches. A router is usually connected between networks to route packets between them. Each router knows about it's sub-networks and which IP addresses they use. The router usually doesn't know what IP addresses are 'above' it. Examine Diagram 5 below. The black boxes connecting the backbones are routers. The larger NSP backbones at the top are connected at a NAP. Under them are several sub-networks, and under them, more sub-networks. At the bottom are two local area networks with computers attached.

When a packet arrives at a router, the router examines the IP address put there by the IP protocol layer on the originating computer. The router checks it's routing table. If the network containing the IP address is found, the packet is sent to that network. If the network containing the IP address is not found, then the router sends the packet on a default route, usually up the backbone hierarchy to the next router. Hopefully the next router will know where to send the packet. If it does not, again the packet is routed upwards until it reaches a NSP backbone. The routers connected to the NSP backbones hold the largest routing tables and here the packet will be routed to the correct backbone, where it will begin its journey 'downward' through smaller and smaller networks until it finds it's destination.
But what if you don't know the IP address of the computer you want to connect to? What if the you need to access a web server referred to as www.anothercomputer.com? How does your web browser know where on the Internet this computer lives? The answer to all these questions is the Domain Name Service or DNS. The DNS is a distributed database which keeps track of computer's names and their corresponding IP addresses on the Internet.

Many computers connected to the Internet host part of the DNS database and the software that allows others to access it. These computers are known as DNS servers. No DNS server contains the entire database; they only contain a subset of it. If a DNS server does not contain the domain name requested by another computer, the DNS server re-directs the requesting computer to another DNS server.

The Domain Name Service is structured as a hierarchy similar to the IP routing hierarchy. The computer requesting a name resolution will be re-directed 'up' the hierarchy until a DNS server is found that can resolve the domain name in the request. Figure 6 illustrates a portion of the hierarchy. At the top of the tree are the domain roots. Some of the older, more common domains are seen near the top. What is not shown are the multitude of DNS servers around the world which form the rest of the hierarchy.
When an Internet connection is setup (e.g. for a LAN or Dial-Up Networking in Windows), one primary and one or more secondary DNS servers are usually specified as part of the installation. This way, any Internet applications that need domain name resolution will be able to function correctly. For example, when you enter a web address into your web browser, the browser first connects to your primary DNS server. After obtaining the IP address for the domain name you entered, the browser then connects to the target computer and requests the web page you wanted.

If you're using Windows 95/NT and access the Internet, you may view your DNS server(s) and even disable them.

If you use Dial-Up Networking:
Open your Dial-Up Networking window (which can be found in Windows Explorer under your CD-ROM drive and above Network Neighborhood). Right click on your Internet connection and click Properties. Near the bottom of the connection properties window press the TCP/IP Settings... button.

If you have a permanent connection to the Internet:
Right click on Network Neighborhood and click Properties. Click TCP/IP Properties. Select the DNS Configuration tab at the top.

You should now be looking at your DNS servers' IP addresses. Here you may disable DNS or set your DNS servers to 0.0.0.0. (Write down your DNS servers' IP addresses first. You will probably have to restart Windows as well.) Now enter an address into your web browser. The browser won't be able to resolve the domain name and you will probably get a nasty dialog box explaining that a DNS server couldn't be found. However, if you enter the corresponding IP address instead of the domain name, the browser will be able to retrieve the desired web page. (Use ping to get the IP address prior to disabling DNS.) Other Microsoft operating systems are similar.

Internet Protocols Revisited
As hinted to earlier in the section about protocol stacks, one may surmise that there are many protocols that are used on the Internet. This is true; there are many communication protocols required for the Internet to function. These include the TCP and IP protocols, routing protocols, medium access control protocols, application level protocols, etc. The following sections describe some of the more important and commonly used protocols on the Internet. Higher level protocols are discussed first, followed by lower level protocols.
Application Protocols: HTTP and the World Wide Web
One of the most commonly used services on the Internet is the World Wide Web (WWW). The application protocol that makes the web work is Hypertext Transfer Protocol or HTTP. Do not confuse this with the Hypertext Markup Language (HTML). HTML is the language used to write web pages. HTTP is the protocol that web browsers and web servers use to communicate with each other over the Internet. It is an application level protocol because it sits on top of the TCP layer in the protocol stack and is used by specific applications to talk to one another. In this case the applications are web browsers and web servers.
HTTP is a connectionless text based protocol. Clients (web browsers) send requests to web servers for web elements such as web pages and images. After the request is serviced by a server, the connection between client and server across the Internet is disconnected. A new connection must be made for each request. Most protocols are connection oriented. This means that the two computers communicating with each other keep the connection open over the Internet. HTTP does not however. Before an HTTP request can be made by a client, a new connection must be made to the server.

When you type a URL into a web browser, this is what happens:

1. If the URL contains a domain name, the browser first connects to a domain name server and retrieves the corresponding IP address for the web server.

2. The web browser connects to the web server and sends an HTTP request (via the protocol stack) for the desired web page.

3. The web server receives the request and checks for the desired page. If the page exists, the web server sends it. If the server cannot find the requested page, it will send an HTTP 404 error message. (404 means 'Page Not Found' as anyone who has surfed the web probably knows.)

4. The web browser receives the page back and the connection is closed.

5. The browser then parses through the page and looks for other page elements it needs to complete the web page. These usually include images, applets, etc.

6. For each element needed, the browser makes additional connections and HTTP requests to the server for each element.

7. When the browser has finished loading all images, applets, etc. the page will be completely loaded in the browser window.

Check It Out - Use Your Telnet Client to Retrieve a Web Page Using HTTP

Telnet is a remote terminal service used on the Internet. It's use has declined lately, but it is a very useful tool to study the Internet. In Windows find the default telnet program. It may be located in the Windows directory named telnet.exe. When opened, pull down the Terminal menu and select Preferences. In the preferences window, check Local Echo. (This is so you can see your HTTP request when you type it.) Now pull down the Connection menu and select Remote System. Enter www.google.com for the Host Name and 80 for the Port. (Web servers usually listen on port 80 by default.) Press Connect. Now type

and press Enter twice. This is a simple HTTP request to a web server for it's root page. You should see a web page flash by and then a dialog box should pop up to tell you the connection was lost. If you'd like to save the retrieved page, turn on logging in the Telnet program. You may then browse through the web page and see the HTML that was used to write it.

Most Internet protocols are specified by Internet documents known as a Request For Comments or RFCs. RFCs may be found at several locations on the Internet. See the Resources section below for appropriate URL's. HTTP version 1.0 is specified by RFC 1945.
Application Protocols: SMTP and Electronic Mail
Another commonly used Internet service is electronic mail. E-mail uses an application level protocol called Simple Mail Transfer Protocol or SMTP. SMTP is also a text based protocol, but unlike HTTP, SMTP is connection oriented. SMTP is also more complicated than HTTP. There are many more commands and considerations in SMTP than there are in HTTP.
When you open your mail client to read your e-mail, this is what typically happens:

1. The mail client (Netscape Mail, Lotus Notes, Microsoft Outlook, etc.) opens a connection to it's default mail server. The mail server's IP address or domain name is typically setup when the mail client is installed.

2. The mail server will always transmit the first message to identify itself.

3. The client will send an SMTP HELO command to which the server will respond with a 250 OK message.

4. Depending on whether the client is checking mail, sending mail, etc. the appropriate SMTP commands will be sent to the server, which will respond accordingly.

5. This request/response transaction will continue until the client sends an SMTP QUIT command. The server will then say goodbye and the connection will be closed.

A simple 'conversation' between an SMTP client and SMTP server is shown below. R: denotes messages sent by the server (receiver) and S: denotes messages sent by the client (sender).
      This SMTP example shows mail sent by Smith at host USC-ISIF, to
      Jones, Green, and Brown at host BBN-UNIX.  Here we assume that
      host USC-ISIF contacts host BBN-UNIX directly.  The mail is
      accepted for Jones and Brown.  Green does not have a mailbox at
      host BBN-UNIX.

      -------------------------------------------------------------

         R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
         S: HELO USC-ISIF.ARPA
         R: 250 BBN-UNIX.ARPA

         S: MAIL FROM:<Smith@USC-ISIF.ARPA>
         R: 250 OK

         S: RCPT TO:<Jones@BBN-UNIX.ARPA>
         R: 250 OK

         S: RCPT TO:<Green@BBN-UNIX.ARPA>
         R: 550 No such user here

         S: RCPT TO:<Brown@BBN-UNIX.ARPA>
         R: 250 OK

         S: DATA
         R: 354 Start mail input; end with <CRLF>.<CRLF>
         S: Blah blah blah...
         S: ...etc. etc. etc.
         S: .
         R: 250 OK

         S: QUIT
         R: 221 BBN-UNIX.ARPA Service closing transmission channel

This SMTP transaction is taken from RFC 821, which specifies SMTP.
Transmission Control Protocol
Under the application layer in the protocol stack is the TCP layer. When applications open a connection to another computer on the Internet, the messages they send (using a specific application layer protocol) get passed down the stack to the TCP layer. TCP is responsible for routing application protocols to the correct application on the destination computer. To accomplish this, port numbers are used. Ports can be thought of as seperate channels on each computer. For example, you can surf the web while reading e-mail. This is because these two applications (the web browser and the mail client) used different port numbers. When a packet arrives at a computer and makes its way up the protocol stack, the TCP layer decides which application receives the packet based on a port number.
TCP works like this:

When the TCP layer receives the application layer protocol data from above, it segments it into manageable 'chunks' and then adds a TCP header with specific TCP information to each 'chunk'. The information contained in the TCP header includes the port number of the application the data needs to be sent to.

When the TCP layer receives a packet from the IP layer below it, the TCP layer strips the TCP header data from the packet, does some data reconstruction if necessary, and then sends the data to the correct application using the port number taken from the TCP header.

This is how TCP routes the data moving through the protocol stack to the correct application.
TCP is not a textual protocol. TCP is a connection-oriented, reliable, byte stream service. Connection-oriented means that two applications using TCP must first establish a connection before exchanging data. TCP is reliable because for each packet received, an acknowledgement is sent to the sender to confirm the delivery. TCP also includes a checksum in it's header for error-checking the received data. The TCP header looks like this:

Notice that there is no place for an IP address in the TCP header. This is because TCP doesn't know anything about IP addresses. TCP's job is to get application level data from application to application reliably. The task of getting data from computer to computer is the job of IP.

Internet Protocol
Unlike TCP, IP is an unreliable, connectionless protocol. IP doesn't care whether a packet gets to it's destination or not. Nor does IP know about connections and port numbers. IP's job is too send and route packets to other computers. IP packets are independent entities and may arrive out of order or not at all. It is TCP's job to make sure packets arrive and are in the correct order. About the only thing IP has in common with TCP is the way it receives data and adds it's own IP header information to the TCP data. The IP header looks like this:

Above we see the IP addresses of the sending and receiving computers in the IP header. Below is what a packet looks like after passing through the application layer, TCP layer, and IP layer. The application layer data is segmented in the TCP layer, the TCP header is added, the packet continues to the IP layer, the IP header is added, and then the packet is transmitted across the Internet.

Wrap Up
Now you know how the Internet works. But how long will it stay this way? The version of IP currently used on the Internet (version 4) only allows 2 ³² addresses. Eventually there won't be any free IP addresses left. Surprised? Don't worry. IP version 6 is being tested right now on a research backbone by a consortium of research institutions and corporations. And after that? Who knows. The Internet has come a long way since it's inception as a Defense Department research project. No one really knows what the Internet will become. One thing is sure, however. The Internet will unite the world like no other mechanism ever has. The Information Age is in full stride and I am glad to be a part of it.
Rus Shuler, 1998
Updates made 2002 Below are some interesting links associated with some of the topics discussed. (I hope they all still work. All open in new window.)

http://www.ietf.org/ is the home page of the Internet Engineering Task Force. This body is greatly responsible for the development of Internet protocols and the like.

http://www.internic.org/ is the organization responsible for administering domain names.

http://www.nexor.com/public/rfc/index/rfc.html is an excellent RFC search engine useful for finding any RFC.

http://www.internetweather.com/ shows animated maps of Internet latency.

http://routes.clubnet.net/iw/ is Internet Weather from ClubNET. This page shows packet loss for various carriers.

http://navigators.com/isp.html is Russ Haynal's ISP Page. This is a great site with links to most NSPs and their backbone infrastructure maps.

The following books are excellent resources and helped greatly in the writing of this paper. I believe Stevens' book is the best TCP/IP reference ever and can be considered the bible of the Internet. Sheldon's book covers a much wider scope and contains a vast amount of networking information.

TCP/IP Illustrated, Volume 1, The Protocols.
W. Richard Stevens.
Addison-Wesley, Reading, Massachusetts. 1994.

Encyclopedia of Networking.
Tom Sheldon.
Osbourne McGraw-Hill, New York. 1998.

Although not used for writing this paper, here are some other good books on the topics of the Internet and networking:

Firewalls and Internet Security; Repelling the Wiley Hacker.
William R. Cheswick, Steven M. Bellovin.
Addison-Wesley, Reading, Massachusetts. 1994.

Data Communications, Computer Networks and Open Systems. Fourth Edition.
Fred Halsall.
Addison-Wesley, Harlow, England. 1996.

Telecommunications: Protocols and Design.
John D. Spragins with Joseph L. Hammond and Krzysztof Pawlikowski.
Addison-Wesley, Reading, Massachusetts. 1992.

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internet how does whitepaper topology addressing protocol stack infrastructure backbone dwight marion rus shuler

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A slightly technical whitepaper explaining what makes the Internet tick.

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TeleGeography’s comprehensive and regularly updated interactive map of the world’s major submarine cable systems and landing stations.

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Una red de nivel 1 es una red de Protocolo de Internet (IP) que participa en Internet exclusivamente a través de acuerdos de interconexión libres, también conocido como acuerdo de peering libre.​ Estas redes pueden intercambiar tráfico sin pagar ninguna tarifa.​ En cambio, algunas redes de nivel 2 y todas las redes de nivel 3 deben pagar para transmitir tráfico en otras redes.​

Definición

Relación entre los diferentes niveles de los proveedores de Internet

En el Internet público, hay diferentes redes de acceso que se conectan al resto de Internet formando una jerarquía de capas o niveles de proveedores de servicios de Internet ( ISP). En el extremo más alto de la jerarquía hay un número relativamente pequeño de los llamados ISP de nivel 1 (Tier 1). Un ISP de nivel 1 es lo mismo que cualquier red: tiene enlaces y routers, y está conectado a otras redes. Sus routers deben ser capaces de transmitir una cantidad muy elevada de datos al mismo tiempo. Los ISP de nivel 1 (Tier 1) están caracterizados por:

Estar conectados directamente a cada uno de los demás ISP de nivel 1.

Estar conectados a un gran número de ISP de nivel 2 (Tier 2) y otras redes de usuario.

Tener cobertura internacional.

Un ISP de nivel 2 (Tier 2) normalmente tiene una cobertura regional o nacional, y se conecta a solamente unos pocos ISP de nivel 1. Por tanto, con el fin de alcanzar una porción grande de Internet global, un ISP de nivel 2 necesita encaminar su tráfico a través de los ISP de nivel 1 a los que está conectado. Un ISP de nivel 2 se dice que es un usuario de los ISP de nivel 1 los que está conectado y el ISP de nivel 1 se dice que es un proveedor del usuario. Una red de nivel 2 puede elegir también conectarse a otras redes de nivel 2, en cuyo caso el tráfico puede fluir entre las dos redes sin tener que pasar por una red de nivel 1. Por debajo de los ISP de nivel 2 están los ISP de nivel más bajo, que se conectan al Internet más grande a través de uno o más ISPs de nivel 2. En el extremo inferior de la jerarquía están los ISP de acceso. Cuando dos ISP están conectados directamente entre sí hay un acuerdo de peering.

Existen muchas razones por las que los profesionales del establecimiento de una red utilizan la "jerarquía de capas o niveles" para describir redes, pero quizás la más importante sea una comprensión mejor de las motivaciones políticas y económicas de una red. Una red de nivel 1 se caracteriza por no comprar tránsito IP de ninguna otra red para alcanzar cualquier otra porción de Internet. Una nueva red no puede convertirse en una Tier 1 sin la aprobación implícita de cada otra red de nivel 1, puesto que la denegación de cualquier red del mismo nivel evitará que la nueva red sea considerada una red de nivel 1. Las redes de nivel 1 intentan típicamente proteger su estado relativamente exclusivo previniendo que nuevas redes se conviertan en redes de nivel 1. Los Tier 1 logran a menudo su objetivo estableciendo unos "requisitos" excesivamente altos, lo cual impide que las nuevas redes puedan alcanzar su mismo estatus.[cita requerida]

Estas redes son reconocidas como redes de nivel 1, porque pueden llegar a todo Internet (IPv4 e IPv6) a través de acuerdos de peering libres. CAIDA es una clasificación que estima la importancia de una red en Internet.​

Existen algunas redes que a menudo se consideran y se acercan al estatus de Tier 1, porque pueden llegar a la mayoría (50%+) de Internet a través de peering libre. Sin embargo, las rutas a uno o más Tier 1 no existen o son de pago.

Véase también

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Colaboradores de los proyectos Wikimedia

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Red tier 1 - Wikipedia, la enciclopedia libre

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En las dos décadas que han pasado desde su invención, la heterogeneidad de las redes ha aumentado aún más con la implementación de Ethernet, Token Ring, Fiber Distributed Data Interface (FDDI), X.25, Frame Relay, Switched Multimegabit Data Service (SMDS), el Integrated Services Digital Network (ISDN), y más recientemente, con Asynchronous Transfer Mode (ATM). Los protocolos de Internet constituyen la mejor aproximación que haya sido probada para interconectar esta variada gama de tecnologías LAN y WAN.

El conjunto de Protocolos de Internet incluye no solo las especificaciones de nivel inferior, como el Protocolo de control de transmisión (TCP) y el Protocolo de Internet (IP), sino también las especificaciones para aplicaciones comunes, como correo electrónico, emulación de terminales y transferencia de archivos. La Figura 1 muestra el conjunto de protocolos TCP/IP en relación con el modelo de Interconexión de Sistemas Abiertos (OSI) de referencia. La Figura 2 muestra algunos de los Protocolos de Internet importantes y su relación con el modelo de OSI de referencia. Para obtener información sobre el modelo de OSI de referencia y la función de cada capa, consulte el documento Internetworking Basics (Principios de conexión entre redes).

Los protocolos de Internet representan el conjunto de protocolos de diversos proveedores más ampliamente implementado que se utiliza en la actualidad. Casi todos los proveedores de PC pueden proporcionar soporte para al menos parte del conjunto de Protocolos de Internet.

Esta sección describe los aspectos técnicos de TCP, IP, los protocolos relacionados y los entornos en los que operan estos protocolos. Dado que el enfoque principal de este documento es el routing (función de capa 3), la explicación de TCP (protocolo de capa 4) será relativamente breve.

TCP es un protocolo transporte orientado por conexión que envía datos como un flujo de bytes sin estructura. Mediante los números de secuencia y los mensajes de acuse de recibo, TCP puede proporcionar un nodo de envío con información de entrega sobre los paquetes transmitidos a un nodo de destino. Si se han perdido datos en el tránsito desde el origen hacia el destino, TCP puede retransmitir los datos hasta alcanzar una condición de tiempo de espera o hasta lograr la entrega satisfactoria. TCP también puede reconocer mensajes duplicados y los descartará adecuadamente. Si la PC de origen transmite demasiado rápido para la computadora de destino, TCP puede emplear mecanismos de control de flujo para ralentizar la transferencia de datos. TCP también puede comunicar información de entrega a los protocolos y las aplicaciones de capa superior que admite. Todas estas características convierten a TCP en un confiable protocolo de transporte de extremo a extremo. Las especificaciones de TCP se pueden encontrar en RFC 793 .

Figura 1 â€" Conjunto de protocolos TCP/IP en relación con el modelo de OSI de referencia.

Figura 2 â€" Protocolos de Internet importantes en relación con el modelo de OSI de referencia

Refiérase a la sección TCP de Protocolos de Internet para obtener más información.

IP es el protocolo de capa 3 principal del conjunto de Internet. Además del routing de redes interconectadas, el IP proporciona informes de errores, fragmentación y rearmado de unidades de información denominadas datagramas para su transmisión a través de redes con diferentes tamaños de unidades de datos máximos. El IP representa el corazón del conjunto de Protocolos de Internet.

Nota: El término IP de la sección se refiere a IPv4 a menos que se indique lo contrario explícitamente.

Las direcciones IP son números globalmente únicos de 32 bits asignados por el Centro de Información de la Red. Las direcciones globalmente únicas permiten que las redes IP se comuniquen entre sí en cualquier lugar del mundo.

Una dirección IP se divide en dos partes. La primera parte designa la dirección de red, mientras que la segunda parte designa la dirección de host.

El espacio de direcciones IP se divide en diferentes clases de red. Las redes de clase A están pensadas principalmente para su uso con algunas redes muy grandes, ya que solo proporcionan 8 bits para el campo de la dirección de red. Las redes de clase B asignan 16 bits, y las redes de clase C asignan 24 bits para el campo de la dirección de red. Sin embargo, las redes de clase C solo proporcionan 8 bits para el campo de host, por lo que la cantidad de hosts por red puede ser un factor de limitación. En los tres casos, la clase de red se reconoce por la cantidad máxima de bits a la izquierda. Las direcciones IP se escriben en formato decimal (con puntos); por ejemplo: 34.0.0.1. La Figura 3 muestra los formatos de dirección para las redes IP de clase A, B y C.

Figura 3 â€" Formatos de dirección para las redes IP de clase A, B y C.

Las redes IP también se pueden dividir en unidades más pequeñas denominadas subredes. Las subredes le brindan más flexibilidad al administrador de la red. Por ejemplo, supongamos que se asignó una dirección de clase A a una red y que todos los nodos de la red utilizan una dirección de clase A. Supongamos que la representación con puntos decimales de esta dirección de red es 34.0.0.0. (Todos los ceros en el campo de host de una dirección especifican toda la red). El administrador puede subdividir la red en subredes. Esto se hace al "pedir prestados" bits a la porción del host de la dirección y utilizarlos como campo de subred, como se muestra en la Figura 4.

Figura 4 â€" "Préstamo" de bits

Si el administrador de red eligió utilizar 8 bits en las subredes, el segundo octeto de una dirección IP de clase A proporciona el número de subred. En nuestro ejemplo, la dirección 34.1.0.0 se refiere a la red 34, subred 1; la dirección 34.2.0.0 se refiere a la red 34, subred 2, y así sucesivamente.

La cantidad de bits que pueden pedirse prestados para la dirección de subred varía. Para especificar la cantidad de bits que se utilizan para representar la red y la porción de subred de la dirección, el IP proporciona máscaras de subred. Las máscaras de subred utilizan el mismo formato y técnica de representación que las direcciones IP. Las máscaras de subred tiene unos en todos los bits excepto en aquéllos que especifican el campo host. Por ejemplo, la máscara de subred que especifica 8 bits de subredes para la dirección de clase A 34.0.0.0 es 255.255.0.0. La máscara de subred que especifica 16 bits de división en subredes para la dirección 34.0.0.0 de Clase A es 255.255.255.0. Ambas máscaras de subred se muestran en la Figura 5. Las máscaras de subred se pueden pasar a través de una red a demanda para que los nuevos nodos puedan aprender cuántos bits de subredes se utilizan en su red.

Figura 5 â€" Máscaras de subred

Tradicionalmente, todas las subredes del mismo número de red utilizaban la misma máscara de subred. En otras palabras, un administrador de redes elegiría una máscara de 8 bits para todas las subredes de la red. Esta estrategia es fácil de gestionar tanto para los administradores de redes como para los protocolos de routing. Sin embargo, esta práctica desperdicia espacio de dirección en algunas redes. Algunas subredes tienen varios hosts y otras tienen sólo unos pocos pero cada una consume un número entero de subred. Las líneas en serie son el ejemplo más extremo, ya que cada una tiene solo dos hosts que se pueden conectar a través de una subred de línea en serie.

A medida que las subredes IP han crecido, los administradores han buscado formas de utilizar el espacio de dirección de manera más eficaz. Una de las técnicas que ha dado resultado es la denominada Máscaras de subred de longitud variable (VLSM). Gracias a VLSM, un administrador de redes puede utilizar una máscara larga en redes con pocos hosts y una máscara corta en subredes con muchos hosts. Sin embargo, esta técnica es más compleja que la de usar un solo tamaño para todo, y las direcciones se deben asignar con cuidado.

Por supuesto, para usar VLSM, un administrador de redes debe utilizar un protocolo de routing que la admita. Los routers Cisco admiten VLSM con el protocolo abrir primero la ruta más corta (OSPF), el sistema intermedio a sistema intermedio integrado (IS-IS integrado), el protocolo de routing de puerta de enlace interior mejorado (IGRP mejorado) y el routing estático. Consulte IP Addressing and Subnetting for New Users (Direcciones IP y subredes para usuarios nuevos) para obtener más información sobre direcciones IP y subredes.

En algunos medios, como LAN IEEE 802, las direcciones IP se detectan de forma dinámica mediante el uso de otros dos miembros del conjunto de Protocolos de Internet: el Protocolo de resolución de direcciones (ARP) y el Protocolo de resolución de direcciones inversas (RARP). ARP utiliza mensajes de difusión para determinar la dirección del hardware (capa MAC) asociada con una dirección de capa de red en particular. ARP es lo suficientemente genérico como para permitir el uso de IP con prácticamente cualquier tipo de mecanismo de acceso a medios subyacentes. RARP utiliza mensajes de difusión para determinar la dirección de capa de red asociada con una dirección de hardware en particular. RARP es especialmente importante para los nodos sin disco, para los que las direcciones de capa de red normalmente no se conocen al momento del inicio.

Una "interconexión de redes" es un grupo de redes interconectadas. La Internet, por otro lado, es el conjunto de redes que permite la comunicación entre la mayoría de las instituciones de investigación, universidades y muchas otras organizaciones de todo el mundo. Los routers dentro de Internet están organizados jerárquicamente. Algunos routers se utilizan para mover información a través de un grupo particular de redes con igual control y autoridad de índole administrativa. (Esta entidad se denomina sistema autónomo). Los routers que se utilizan para el intercambio de información dentro de los sistemas autónomos se denominan routers interiores y utilizan una variedad de protocolos de puerta de enlace interior (IGP) para lograr este fin. Los routers que mueven información entre sistemas autónomos se denominan routers exteriores; utilizan el protocolo de puerta de enlace exterior (EGP) o el protocolo BGP (Border Gateway Protocol). La Figura 6 muestra la arquitectura de Internet.

Figura 6 â€" Representación de la arquitectura de Internet

Los protocolos de ruteo que se usan con la IP son de naturaleza dinámica. El routing dinámico requiere el software en los dispositivos de routing para calcular las rutas. Los algoritmos de routing dinámico se adaptan a los cambios en la red y seleccionan automáticamente las mejores rutas. En comparación con el routing dinámico, el routing estático requiere que el administrador de redes establezca las rutas. Las rutas estáticas no cambian hasta que el administrador de red no las cambia.

Las tablas de routing de IP constan de pares de dirección de destino y salto siguiente. Este ejemplo de tabla de routing de un router Cisco muestra que la primera entrada se interpreta como "para acceder a la red 34.1.0.0 (subred 1 en la red 34), la siguiente parada es el nodo en la dirección 54.34.23.12":

R6-2500#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is not set 34.0.0.0/16 is subnetted, 1 subnetsO  34.1.0.0 [110/65] via 54.34.23.12, 00:00:51, Serial0 54.0.0.0/24 is subnetted, 1 subnets C  54.34.23.0 is directly connected, Serial0 R6-2500#

Como hemos visto, el routing de IP especifica que los datagramas del IP viajan a través de redes interconectadas con un salto de router por vez. Al principio del recorrido no se conoce toda la ruta. En cambio, en cada parada, el siguiente salto del router se determina al hacer coincidir la dirección de destino dentro del datagrama con una entrada en la tabla de routing del nodo actual. La participación de cada nodo en el proceso de routing consiste solo en reenviar paquetes según la información interna. El IP no devuelve un informe de errores al origen cuando se producen las anomalías de routing. Esta tarea se deja a otro Protocolo de Internet, el Protocolo de mensajes de control de Internet (ICMP).

ICMP realiza una serie de tareas dentro de una conexión de redes de IP. Además del motivo principal para el que se creó (informar las fallas de routing al origen), ICMP proporciona un método para probar la disponibilidad de los nodos en la interconexión de redes (mensajes de eco y respuesta de ICMP), un método para aumentar la eficiencia del routing (el mensaje de redirección de ICMP), un método para informar a las fuentes que un datagrama ha excedido el tiempo asignado de duración en una interconexión de redes (el mensaje de tiempo superado de ICMP) y otros mensajes útiles. En resumen, ICMP es una parte integral de cualquier implementación de IP, en especial las que se ejecutan en los routers. Consulte Información relacionada.

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En las dos décadas que han pasado desde su invención, la heterogeneidad de las redes ha aumentado aún más con la implementación de Ethernet, Token Ring, Fiber Distributed Data Interface (FDDI), X.25, Frame Relay, Switched Multimegabit Data Service (SMDS), el Integrated Services Digital Network (ISDN), y más recientemente, con Asynchronous Transfer Mode (ATM).

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Join a game of kahoot here. Kahoot! is a free game-based learning platform that makes it fun to learn - any subject, in any language, on any device, for all ages!

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The Internet of Things (IoT) is the network of physical objects or "things" embedded with electronics, software, sensors, actuators, and connectivity to enable objects to exchange data with the manufacturer, operator, and/or other connected devices.

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The Internet of Things (IoT) is the network of physical objects or "things" embedded with electronics, software, sensors, actuators, and connectivity to enable objects to exchange data with the manufacturer, operator, and/or other connected devices.

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ICANN - Corporación para la Asignación de Nombres y Números en Internet

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La ICANN es una corporación de beneficio público, sin fines de lucro, con participantes de todo el mundo dedicados a mantener una Internet segura, estable e interoperable.

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Management of the DNS Root Zone (assignments of ccTLDs and gTLDs) along with other functions such as the .int and .arpa zones. Coordination of the global IP and AS number spaces, such as allocations made to Regional Internet Registries. The central repository for protocol name and number registries used in many Internet protocols.