|Academic Computing and Communications Center|
Introduction to the Internet Protocols
Computer Science Facilities Group
This is an introduction to the Internet networking protocols (TCP/IP). It includes a summary of the facilities available and brief descriptions of the major protocols in the family.
Copyright (C) 1987, Charles L. Hedrick. Anyone may reproduce this document, in whole or in part, provided that: (1) any copy or republication of the entire document must show Rutgers University as the source, and must include this notice; and (2) any other use of this material must reference this manual and Rutgers University, and the fact that the material is copyright by Charles Hedrick and is used by permission.
Unix is a trademark of AT&T Technologies, Inc
|Table of Contents|
1. What is TCP/IP?
2. General description of the TCP/IP protocols
-- 2.1 The TCP level
-- 2.2 The IP level
-- 2.3 The Ethernet level
3. Well-known sockets and the applications layer
-- 3.1 An example application: SMTP
4. Protocols other than TCP: UDP and ICMP
5. Keeping track of names and information: the domain system
7. Details about Internet addresses: subnets and broadcasting
8. Datagram fragmentation and reassembly
9. Ethernet encapsulation: ARP
10. Getting more information
|1. What is TCP/IP?|
TCP/IP is a set of protocols developed to allow cooperating computers to
share resources across a network. It was developed by a community of researchers
centered around the ARPAnet. Certainly the ARPAnet is the best-known TCP/IP
network. However as of June, 87, at least 130 different vendors had products
that support TCP/IP, and thousands of networks of all kinds use it.
First some basic definitions. The most accurate name for the set of protocols we are describing is the "Internet protocol suite". TCP and IP are two of the protocols in this suite. (They will be described below.) Because TCP and IP are the best known of the protocols, it has become common to use the term TCP/IP or IP/TCP to refer to the whole family. It is probably not worth fighting this habit. However this can lead to some oddities. For example, I find myself talking about NFS as being based on TCP/IP, even though it doesn't use TCP at all. (It does use IP. But it uses an alternative protocol, UDP, instead of TCP. All of this alphabet soup will be unscrambled in the following pages.)
The Internet is a collection of networks, including the Arpanet, NSFnet, regional networks such as NYsernet, local networks at a number of University and research institutions, and a number of military networks. The term "Internet" applies to this entire set of networks. The subset of them that is managed by the Department of Defense is referred to as the "DDN" (Defense Data Network). This includes some research-oriented networks, such as the Arpanet, as well as more strictly military ones. (Because much of the funding for Internet protocol developments is done via the DDN organization, the terms Internet and DDN can sometimes seem equivalent.) All of these networks are connected to each other. Users can send messages from any of them to any other, except where there are security or other policy restrictions on access. Officially speaking, the Internet protocol documents are simply standards adopted by the Internet community for its own use. More recently, the Department of Defense issued a MILSPEC definition of TCP/IP. This was intended to be a more formal definition, appropriate for use in purchasing specifications. However most of the TCP/IP community continues to use the Internet standards. The MILSPEC version is intended to be consistent with it.
Whatever it is called, TCP/IP is a family of protocols. A few provide "low-level" functions needed for many applications. These include IP, TCP, and UDP. (These will be described in a bit more detail later.) Others are protocols for doing specific tasks, e.g. transferring files between computers, sending mail, or finding out who is logged in on another computer. Initially TCP/IP was used mostly between minicomputers or mainframes. These machines had their own disks, and generally were self-contained. Thus the most important "traditional" TCP/IP services are:
|2. General description of the TCP/IP protocols|
TCP/IP is a layered set of protocols. In order to understand what this
means, it is useful to look at an example. A typical situation is sending
mail. First, there is a protocol for mail. This defines a set of commands
which one machine sends to another, e.g. commands to specify who the sender
of the message is, who it is being sent to, and then the text of the message.
However this protocol assumes that there is a way to communicate reliably
between the two computers. Mail, like other application protocols, simply
defines a set of commands and messages to be sent. It is designed to be
used together with TCP and IP. TCP is responsible for making sure that
the commands get through to the other end. It keeps track of what is sent,
and retransmitts anything that did not get through. If any message is too
large for one datagram, e.g. the text of the mail, TCP will split it up
into several datagrams, and make sure that they all arrive correctly. Since
these functions are needed for many applications, they are put together
into a separate protocol, rather than being part of the specifications
for sending mail. You can think of TCP as forming a library of routines
that applications can use when they need reliable network communications
with another computer. Similarly, TCP calls on the services of IP. Although
the services that TCP supplies are needed by many applications, there are
still some kinds of applications that don't need them. However there are
some services that every application needs. So these services are put together
into IP. As with TCP, you can think of IP as a library of routines that
TCP calls on, but which is also available to applications that don't use
TCP. This strategy of building several levels of protocol is called "layering".
We think of the applications programs such as mail, TCP, and IP, as being
separate "layers", each of which calls on the services of the layer below
it. Generally, TCP/IP applications use 4 layers:
Of course we normally refer to systems by name, rather than by Internet address. When we specify a name, the network software looks it up in a database, and comes up with the corresponding Internet address. Most of the network software deals strictly in terms of the address. (RFC 882 describes the name server technology used to handle this lookup.)
TCP/IP is built on "connectionless" technology. Information is transfered as a sequence of "datagrams". A datagram is a collection of data that is sent as a single message. Each of these datagrams is sent through the network individually. There are provisions to open connections (i.e. to start a conversation that will continue for some time). However at some level, information from those connections is broken up into datagrams, and those datagrams are treated by the network as completely separate. For example, suppose you want to transfer a 15000 octet file. Most networks can't handle a 15000 octet datagram. So the protocols will break this up into something like 30 500-octet datagrams. Each of these datagrams will be sent to the other end. At that point, they will be put back together into the 15000-octet file. However while those datagrams are in transit, the network doesn't know that there is any connection between them. It is perfectly possible that datagram 14 will actually arrive before datagram 13. It is also possible that somewhere in the network, an error will occur, and some datagram won't get through at all. In that case, that datagram has to be sent again.
Note by the way that the terms "datagram" and "packet" often seem to be nearly interchangable. Technically, datagram is the right word to use when describing TCP/IP. A datagram is a unit of data, which is what the protocols deal with. A packet is a physical thing, appearing on an Ethernet or some wire. In most cases a packet simply contains a datagram, so there is very little difference. However they can differ. When TCP/IP is used on top of X.25, the X.25 interface breaks the datagrams up into 128-byte packets. This is invisible to IP, because the packets are put back together into a single datagram at the other end before being processed by TCP/IP. So in this case, one IP datagram would be carried by several packets. However with most media, there are efficiency advantages to sending one datagram per packet, and so the distinction tends to vanish.
It may have occurred to you that something is missing here. We have talked about Internet addresses, but not about how you keep track of multiple connections to a given system. Clearly it isn't enough to get a datagram to the right destination. TCP has to know which connection this datagram is part of. This task is referred to as "demultiplexing." In fact, there are several levels of demultiplexing going on in TCP/IP. The information needed to do this demultiplexing is contained in a series of "headers". A header is simply a few extra octets tacked onto the beginning of a datagram by some protocol in order to keep track of it. It's a lot like putting a letter into an envelope and putting an address on the outside of the envelope. Except with modern networks it happens several times. It's like you put the letter into a little envelope, your secretary puts that into a somewhat bigger envelope, the campus mail center puts that envelope into a still bigger one, etc. Here is an overview of the headers that get stuck on a message that passes through a typical TCP/IP network:
We start with a single data stream, say a file you are trying to send to some other computer:
......................................................TCP breaks it up into manageable chunks. (In order to do this, TCP has to know how large a datagram your network can handle. Actually, the TCP's at each end say how big a datagram they can handle, and then they pick the smallest size.)
.... .... .... .... .... .... .... ....TCP puts a header at the front of each datagram. This header actually contains at least 20 octets, but the most important ones are a source and destination "port number" and a "sequence number". The port numbers are used to keep track of different conversations. Suppose 3 different people are transferring files. Your TCP might allocate port numbers 1000, 1001, and 1002 to these transfers. When you are sending a datagram, this becomes the "source" port number, since you are the source of the datagram. Of course the TCP at the other end has assigned a port number of its own for the conversation. Your TCP has to know the port number used by the other end as well. (It finds out when the connection starts, as we will explain below.) It puts this in the "destination" port field. Of course if the other end sends a datagram back to you, the source and destination port numbers will be reversed, since then it will be the source and you will be the destination. Each datagram has a sequence number. This is used so that the other end can make sure that it gets the datagrams in the right order, and that it hasn't missed any. (See the TCP specification for details.) TCP doesn't number the datagrams, but the octets. So if there are 500 octets of data in each datagram, the first datagram might be numbered 0, the second 500, the next 1000, the next 1500, etc. Finally, I will mention the Checksum. This is a number that is computed by adding up all the octets in the datagram (more or less - see the TCP spec). The result is put in the header. TCP at the other end computes the checksum again. If they disagree, then something bad happened to the datagram in transmission, and it is thrown away. So here's what the datagram looks like now.
If we abbreviate the TCP header as "T", the whole file now looks like this:+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Port | Destination Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Acknowledgment Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data | |U|A|P|R|S|F| | | Offset| Reserved |R|C|S|S|Y|I| Window | | | |G|K|H|T|N|N| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum | Urgent Pointer | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | your data ... next 500 octets | | ...... |
T.... T.... T.... T.... T.... T.... T....You will note that there are items in the header that I have not described above. They are generally involved with managing the connection. In order to make sure the datagram has arrived at its destination, the recipient has to send back an "acknowledgement". This is a datagram whose "Acknowledgement number" field is filled in. For example, sending a packet with an acknowledgement of 1500 indicates that you have received all the data up to octet number 1500. If the sender doesn't get an acknowledgement within a reasonable amount of time, it sends the data again. The window is used to control how much data can be in transit at any one time. It is not practical to wait for each datagram to be acknowledged before sending the next one. That would slow things down too much. On the other hand, you can't just keep sending, or a fast computer might overrun the capacity of a slow one to absorb data. Thus each end indicates how much new data it is currently prepared to absorb by putting the number of octets in its "Window" field. As the computer receives data, the amount of space left in its window decreases. When it goes to zero, the sender has to stop. As the receiver processes the data, it increases its window, indicating that it is ready to accept more data. Often the same datagram can be used to acknowledge receipt of a set of data and to give permission for additional new data (by an updated window). The "Urgent" field allows one end to tell the other to skip ahead in its processing to a particular octet. This is often useful for handling asynchronous events, for example when you type a control character or other command that interrupts output. The other fields are beyond the scope of this document.
If we represent the IP header by an "I", your file now looks like this:+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version| IHL |Type of Service| Total Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification |Flags| Fragment Offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time to Live | Protocol | Header Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TCP header, then your data ...... |
IT.... IT.... IT.... IT.... IT.... IT.... IT....Again, the header contains some additional fields that have not been discussed. Most of them are beyond the scope of this document. The flags and fragment offset are used to keep track of the pieces when a datagram has to be split up. This can happen when datagrams are forwarded through a network for which they are too big. (This will be discussed a bit more below.) The time to live is a number that is decremented whenever the datagram passes through a system. When it goes to zero, the datagram is discarded. This is done in case a loop develops in the system somehow. Of course this should be impossible, but well-designed networks are built to cope with "impossible" conditions.
At this point, it's possible that no more headers are needed. If your computer happens to have a direct phone line connecting it to the destination computer, or to a gateway, it may simply send the datagrams out on the line (though likely a synchronous protocol such as HDLC would be used, and it would add at least a few octets at the beginning and end).
If we represent the Ethernet header with "E", and the Ethernet checksum with "C", your file now looks like this:+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ethernet destination address (first 32 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ethernet dest (last 16 bits) |Ethernet source (first 16 bits)| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ethernet source address (last 32 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IP header, then TCP header, then your data | | | ... | | | end of your data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ethernet Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
EIT....C EIT....C EIT....C EIT....C EIT....CWhen these packets are received by the other end, of course all the headers are removed. The Ethernet interface removes the Ethernet header and the checksum. It looks at the type code. Since the type code is the one assigned to IP, the Ethernet device driver passes the datagram up to IP. IP removes the IP header. It looks at the IP protocol field. Since the protocol type is TCP, it passes the datagram up to TCP. TCP now looks at the sequence number. It uses the sequence numbers and other information to combine all the datagrams into the original file.
The ends our initial summary of TCP/IP. There are still some crucial concepts we haven't gotten to, so we'll now go back and add details in several areas. (For detailed descriptions of the items discussed here see, RFC 793 for TCP, RFC 791 for IP, and RFC's 894 and 826 for sending IP over Ethernet.)
|3. Well-known sockets and the applications layer|
So far, we have described how a stream of data is broken up into datagrams,
sent to another computer, and put back together. However something more
is needed in order to accomplish anything useful. There has to be a way
for you to open a connection to a specified computer, log into it, tell
it what file you want, and control the transmission of the file. (If you
have a different application in mind, e.g. computer mail, some analogous
protocol is needed.) This is done by "application protocols". The application
protocols run "on top" of TCP/IP. That is, when they want to send a message,
they give the message to TCP. TCP makes sure it gets delivered to the other
end. Because TCP and IP take care of all the networking details, the applications
protocols can treat a network connection as if it were a simple byte stream,
like a terminal or phone line.
Before going into more details about applications programs, we have to describe how you find an application. Suppose you want to send a file to a computer whose Internet address is 22.214.171.124. To start the process, you need more than just the Internet address. You have to connect to the FTP server at the other end. In general, network programs are specialized for a specific set of tasks. Most systems have separate programs to handle file transfers, remote terminal logins, mail, etc. When you connect to 126.96.36.199, you have to specify that you want to talk to the FTP server. This is done by having "well-known sockets" for each server. Recall that TCP uses port numbers to keep track of individual conversations. User programs normally use more or less random port numbers. However specific port numbers are assigned to the programs that sit waiting for requests. For example, if you want to send a file, you will start a program called "ftp". It will open a connection using some random number, say 1234, for the port number on its end. However it will specify port number 21 for the other end. This is the official port number for the FTP server. Note that there are two different programs involved. You run ftp on your side. This is a program designed to accept commands from your terminal and pass them on to the other end. The program that you talk to on the other machine is the FTP server. It is designed to accept commands from the network connection, rather than an interactive terminal. There is no need for your program to use a well-known socket number for itself. Nobody is trying to find it. However the servers have to have well-known numbers, so that people can open connections to them and start sending them commands. The official port numbers for each program are given in "Assigned Numbers".
Note that a connection is actually described by a set of 4 numbers: the Internet address at each end, and the TCP port number at each end. Every datagram has all four of those numbers in it. (The Internet addresses are in the IP header, and the TCP port numbers are in the TCP header.) In order to keep things straight, no two connections can have the same set of numbers. However it is enough for any one number to be different. For example, it is perfectly possible for two different users on a machine to be sending files to the same other machine. This could result in connections with the following parameters:
Since the same machines are involved, the Internet addresses are the same. Since they are both doing file transfers, one end of the connection involves the well-known port number for FTP. The only thing that differs is the port number for the program that the users are running. That's enough of a difference. Generally, at least one end of the connection asks the network software to assign it a port number that is guaranteed to be unique. Normally, it's the user's end, since the server has to use a well-known number.
Now that we know how to open connections, let's get back to the applications programs. As mentioned earlier, once TCP has opened a connection, we have something that might as well be a simple wire. All the hard parts are handled by TCP and IP. However we still need some agreement as to what we send over this connection. In effect this is simply an agreement on what set of commands the application will understand, and the format in which they are to be sent. Generally, what is sent is a combination of commands and data. They use context to differentiate. For example, the mail protocol works like this: Your mail program opens a connection to the mail server at the other end. Your program gives it your machine's name, the sender of the message, and the recipients you want it sent to. It then sends a command saying that it is starting the message. At that point, the other end stops treating what it sees as commands, and starts accepting the message. Your end then starts sending the text of the message. At the end of the message, a special mark is sent (a dot in the first column). After that, both ends understand that your program is again sending commands. This is the simplest way to do things, and the one that most applications use.
File transfer is somewhat more complex. The file transfer protocol involves two different connections. It starts out just like mail. The user's program sends commands like "log me in as this user", "here is my password", "send me the file with this name". However once the command to send data is sent, a second connection is opened for the data itself. It would certainly be possible to send the data on the same connection, as mail does. However file transfers often take a long time. The designers of the file transfer protocol wanted to allow the user to continue issuing commands while the transfer is going on. For example, the user might make an inquiry, or he might abort the transfer. Thus the designers felt it was best to use a separate connection for the data and leave the original command connection for commands. (It is also possible to open command connections to two different computers, and tell them to send a file from one to the other. In that case, the data couldn't go over the command connection.)
Remote terminal connections use another mechanism still. For remote logins, there is just one connection. It normally sends data. When it is necessary to send a command (e.g. to set the terminal type or to change some mode), a special character is used to indicate that the next character is a command. If the user happens to type that special character as data, two of them are sent.
We are not going to describe the application protocols in detail in this document. It's better to read the RFC's yourself. However there are a couple of common conventions used by applications that will be described here. First, the common network representation: TCP/IP is intended to be usable on any computer. Unfortunately, not all computers agree on how data is represented. There are differences in character codes (ASCII vs. EBCDIC), in end of line conventions (carriage return, line feed, or a representation using counts), and in whether terminals expect characters to be sent individually or a line at a time. In order to allow computers of different kinds to communicate, each applications protocol defines a standard representation. Note that TCP and IP do not care about the representation. TCP simply sends octets. However the programs at both ends have to agree on how the octets are to be interpreted. The RFC for each application specifies the standard representation for that application. Normally it is "net ASCII". This uses ASCII characters, with end of line denoted by a carriage return followed by a line feed. For remote login, there is also a definition of a "standard terminal", which turns out to be a half-duplex terminal with echoing happening on the local machine. Most applications also make provisions for the two computers to agree on other representations that they may find more convenient. For example, PDP-10's have 36-bit words. There is a way that two PDP-10's can agree to send a 36-bit binary file. Similarly, two systems that prefer full-duplex terminal conversations can agree on that. However each application has a standard representation, which every machine must support.
Date: Sat, 27 Jun 87 13:26:31 EDT From: email@example.com To: firstname.lastname@example.org Subject: meeting
Let's get together Monday at 1pm.First, note that the format of the message itself is described by an Internet standard (RFC 822). The standard specifies the fact that the message must be transmitted as net ASCII (i.e. it must be ASCII, with carriage return/linefeed to delimit lines). It also describes the general structure, as a group of header lines, then a blank line, and then the body of the message. Finally, it describes the syntax of the header lines in detail. Generally they consist of a keyword and then a value.
Note that the addressee is indicated as LEVY@RED.RUTGERS.EDU. Initially, addresses were simply "person at machine". However recent standards have made things more flexible. There are now provisions for systems to handle other systems' mail. This can allow automatic forwarding on behalf of computers not connected to the Internet. It can be used to direct mail for a number of systems to one central mail server. Indeed there is no requirement that an actual computer by the name of RED.RUTGERS.EDU even exist. The name servers could be set up so that you mail to department names, and each department's mail is routed automatically to an appropriate computer. It is also possible that the part before the @ is something other than a user name. It is possible for programs to be set up to process mail. There are also provisions to handle mailing lists, and generic names such as "postmaster" or "operator".
The way the message is to be sent to another system is described by RFC's 821 and 974. The program that is going to be doing the sending asks the name server several queries to determine where to route the message. The first query is to find out which machines handle mail for the name RED.RUTGERS.EDU. In this case, the server replies that RED.RUTGERS.EDU handles its own mail. The program then asks for the address of RED.RUTGERS.EDU, which is 188.8.131.52. Then the mail program opens a TCP connection to port 25 on 184.108.40.206. Port 25 is the well-known socket used for receiving mail. Once this connection is established, the mail program starts sending commands. Here is a typical conversation. Each line is labelled as to whether it is from TOPAZ or RED. Note that TOPAZ initiated the connection:
RED 220 RED.RUTGERS.EDU SMTP Service at 29 Jun 87 05:17:18 EDT TOPAZ HELO topaz.rutgers.edu RED 250 RED.RUTGERS.EDU - Hello, TOPAZ.RUTGERS.EDU TOPAZ MAIL From: RED 250 MAIL accepted TOPAZ RCPT To: RED 250 Recipient accepted TOPAZ DATA RED 354 Start mail input; end withFirst, note that commands all use normal text. This is typical of the Internet standards. Many of the protocols use standard ASCII commands. This makes it easy to watch what is going on and to diagnose problems. For example, the mail program keeps a log of each conversation. If something goes wrong, the log file can simply be mailed to the postmaster. Since it is normal text, he can see what was going on. It also allows a human to interact directly with the mail server, for testing. (Some newer protocols are complex enough that this is not practical. The commands would have to have a syntax that would require a significant parser. Thus there is a tendency for newer protocols to use binary formats. Generally they are structured like C or Pascal record structures.) Second, note that the responses all begin with numbers. This is also typical of Internet protocols. The allowable responses are defined in the protocol. The numbers allow the user program to respond unambiguously. The rest of the response is text, which is normally for use by any human who may be watching or looking at a log. It has no effect on the operation of the programs. (However there is one point at which the protocol uses part of the text of the response.) The commands themselves simply allow the mail program on one end to tell the mail server the information it needs to know in order to deliver the message. In this case, the mail server could get the information by looking at the message itself. But for more complex cases, that would not be safe. Every session must begin with a HELO, which gives the name of the system that initiated the connection. Then the sender and recipients are specified. (There can be more than one RCPT command, if there are several recipients.) Finally the data itself is sent. Note that the text of the message is terminated by a line containing just a period. (If such a line appears in the message, the period is doubled.) After the message is accepted, the sender can send another message, or terminate the session as in the example above.
Generally, there is a pattern to the response numbers. The protocol defines the specific set of responses that can be sent as answers to any given command. However programs that don't want to analyze them in detail can just look at the first digit. In general, responses that begin with a 2 indicate success. Those that begin with 3 indicate that some further action is needed, as shown above. 4 and 5 indicate errors. 4 is a "temporary" error, such as a disk filling. The message should be saved, and tried again later. 5 is a permanent error, such as a non-existent recipient. The message should be returned to the sender with an error message.
(For more details about the protocols mentioned in this section, see RFC's 821/822 for mail, RFC 959 for file transfer, and RFC's 854/855 for remote logins. For the well-known port numbers, see the current edition of Assigned Numbers, and possibly RFC 814.)
|4. Protocols other than TCP: UDP and ICMP|
So far, we have described only connections that use TCP. Recall that TCP
is responsible for breaking up messages into datagrams, and reassembling
them properly. However in many applications, we have messages that will
always fit in a single datagram. An example is name lookup. When a user
attempts to make a connection to another system, he will generally specify
the system by name, rather than Internet address. His system has to translate
that name to an address before it can do anything. Generally, only a few
systems have the database used to translate names to addresses. So the
user's system will want to send a query to one of the systems that has
the database. This query is going to be very short. It will certainly fit
in one datagram. So will the answer. Thus it seems silly to use TCP. Of
course TCP does more than just break things up into datagrams. It also
makes sure that the data arrives, resending datagrams where necessary.
But for a question that fits in a single datagram, we don't need all the
complexity of TCP to do this. If we don't get an answer after a few seconds,
we can just ask again. For applications like this, there are alternatives
The most common alternative is UDP ("user datagram protocol"). UDP is designed for applications where you don't need to put sequences of datagrams together. It fits into the system much like TCP. There is a UDP header. The network software puts the UDP header on the front of your data, just as it would put a TCP header on the front of your data. Then UDP sends the data to IP, which adds the IP header, putting UDP's protocol number in the protocol field instead of TCP's protocol number. However UDP doesn't do as much as TCP does. It doesn't split data into multiple datagrams. It doesn't keep track of what it has sent so it can resend if necessary. About all that UDP provides is port numbers, so that several programs can use UDP at once. UDP port numbers are used just like TCP port numbers. There are well-known port numbers for servers that use UDP. Note that the UDP header is shorter than a TCP header. It still has source and destination port numbers, and a checksum, but that's about it. No sequence number, since it is not needed. UDP is used by the protocols that handle name lookups (see IEN 116, RFC 882, and RFC 883), and a number of similar protocols.
Another alternative protocol is ICMP ("Internet control message protocol"). ICMP is used for error messages, and other messages intended for the TCP/IP software itself, rather than any particular user program. For example, if you attempt to connect to a host, your system may get back an ICMP message saying "host unreachable". ICMP can also be used to find out some information about the network. See RFC 792 for details of ICMP. ICMP is similar to UDP, in that it handles messages that fit in one datagram. However it is even simpler than UDP. It doesn't even have port numbers in its header. Since all ICMP messages are interpreted by the network software itself, no port numbers are needed to say where a ICMP message is supposed to go.
|5. Keeping track of names and information: the domain system|
As we indicated earlier, the network software generally needs a 32-bit
Internet address in order to open a connection or send a datagram. However
users prefer to deal with computer names rather than numbers. Thus there
is a database that allows the software to look up a name and find the corresponding
number. When the Internet was small, this was easy. Each system would have
a file that listed all of the other systems, giving both their name and
number. There are now too many computers for this approach to be practical.
Thus these files have been replaced by a set of name servers that keep
track of host names and the corresponding Internet addresses. (In fact
these servers are somewhat more general than that. This is just one kind
of information stored in the domain system.) Note that a set of interlocking
servers are used, rather than a single central one. There are now so many
different institutions connected to the Internet that it would be impractical
for them to notify a central authority whenever they installed or moved
a computer. Thus naming authority is delegated to individual institutions.
The name servers form a tree, corresponding to institutional structure.
The names themselves follow a similar structure. A typical example is the
name BORAX.LCS.MIT.EDU. This is a computer at the Laboratory for Computer
Science (LCS) at MIT. In order to find its Internet address, you might
potentially have to consult 4 different servers. First, you would ask a
central server (called the root) where the EDU server is. EDU is a server
that keeps track of educational institutions. The root server would give
you the names and Internet addresses of several servers for EDU. (There
are several servers at each level, to allow for the possibly that one might
be down.) You would then ask EDU where the server for MIT is. Again, it
would give you names and Internet addresses of several servers for MIT.
Generally, not all of those servers would be at MIT, to allow for the possibility
of a general power failure at MIT. Then you would ask MIT where the server
for LCS is, and finally you would ask one of the LCS servers about BORAX.
The final result would be the Internet address for BORAX.LCS.MIT.EDU. Each
of these levels is referred to as a "domain". The entire name, BORAX.LCS.MIT.EDU,
is called a "domain name". (So are the names of the higher-level domains,
such as LCS.MIT.EDU, MIT.EDU, and EDU.)
Fortunately, you don't really have to go through all of this most of the time. First of all, the root name servers also happen to be the name servers for the top-level domains such as EDU. Thus a single query to a root server will get you to MIT. Second, software generally remembers answers that it got before. So once we look up a name at LCS.MIT.EDU, our software remembers where to find servers for LCS.MIT.EDU, MIT.EDU, and EDU. It also remembers the translation of BORAX.LCS.MIT.EDU. Each of these pieces of information has a "time to live" associated with it. Typically this is a few days. After that, the information expires and has to be looked up again. This allows institutions to change things.
The domain system is not limited to finding out Internet addresses. Each domain name is a node in a database. The node can have records that define a number of different properties. Examples are Internet address, computer type, and a list of services provided by a computer. A program can ask for a specific piece of information, or all information about a given name. It is possible for a node in the database to be marked as an "alias" (or nickname) for another node. It is also possible to use the domain system to store information about users, mailing lists, or other objects.
There is an Internet standard defining the operation of these databases, as well as the protocols used to make queries of them. Every network utility has to be able to make such queries, since this is now the official way to evaluate host names. Generally utilities will talk to a server on their own system. This server will take care of contacting the other servers for them. This keeps down the amount of code that has to be in each application program.
The domain system is particularly important for handling computer mail. There are entry types to define what computer handles mail for a given name, to specify where an individual is to receive mail, and to define mailing lists.
(See RFC's 882, 883, and 973 for specifications of the domain system. RFC 974 defines the use of the domain system in sending mail.)
The description above indicated that the IP implementation is responsible
for getting datagrams to the destination indicated by the destination address,
but little was said about how this would be done. The task of finding how
to get a datagram to its destination is referred to as "routing". In fact
many of the details depend upon the particular implementation. However
some general things can be said.
First, it is necessary to understand the model on which IP is based. IP assumes that a system is attached to some local network. We assume that the system can send datagrams to any other system on its own network. (In the case of Ethernet, it simply finds the Ethernet address of the destination system, and puts the datagram out on the Ethernet.) The problem comes when a system is asked to send a datagram to a system on a different network. This problem is handled by gateways. A gateway is a system that connects a network with one or more other networks. Gateways are often normal computers that happen to have more than one network interface. For example, we have a Unix machine that has two different Ethernet interfaces. Thus it is connected to networks 128.6.4 and 128.6.3. This machine can act as a gateway between those two networks. The software on that machine must be set up so that it will forward datagrams from one network to the other. That is, if a machine on network 128.6.4 sends a datagram to the gateway, and the datagram is addressed to a machine on network 128.6.3, the gateway will forward the datagram to the destination. Major communications centers often have gateways that connect a number of different networks. (In many cases, special-purpose gateway systems provide better performance or reliability than general-purpose systems acting as gateways. A number of vendors sell such systems.)
Routing in IP is based entirely upon the network number of the destination address. Each computer has a table of network numbers. For each network number, a gateway is listed. This is the gateway to be used to get to that network. Note that the gateway doesn't have to connect directly to the network. It just has to be the best place to go to get there. For example at Rutgers, our interface to NSFnet is at the John von Neuman Supercomputer Center (JvNC). Our connection to JvNC is via a high-speed serial line connected to a gateway whose address is 220.127.116.11. Systems on net 128.6.3 will list 18.104.22.168 as the gateway for many off-campus networks. However systems on net 128.6.4 will list 22.214.171.124 as the gateway to those same off-campus networks. 126.96.36.199 is the gateway between networks 128.6.4 and 128.6.3, so it is the first step in getting to JvNC.
When a computer wants to send a datagram, it first checks to see if the destination address is on the system's own local network. If so, the datagram can be sent directly. Otherwise, the system expects to find an entry for the network that the destination address is on. The datagram is sent to the gateway listed in that entry. This table can get quite big. For example, the Internet now includes several hundred individual networks. Thus various strategies have been developed to reduce the size of the routing table. One strategy is to depend upon "default routes". Often, there is only one gateway out of a network.
This gateway might connect a local Ethernet to a campus-wide backbone network. In that case, we don't need to have a separate entry for every network in the world. We simply define that gateway as a "default". When no specific route is found for a datagram, the datagram is sent to the default gateway. A default gateway can even be used when there are several gateways on a network. There are provisions for gateways to send a message saying "I'm not the best gateway -- use this one instead." (The message is sent via ICMP. See RFC 792.) Most network software is designed to use these messages to add entries to their routing tables. Suppose network 128.6.4 has two gateways, 188.8.131.52 and 184.108.40.206. 220.127.116.11 leads to several other internal Rutgers networks. 18.104.22.168 leads indirectly to the NSFnet. Suppose we set 22.214.171.124 as a default gateway, and have no other routing table entries. Now what happens when we need to send a datagram to MIT? MIT is network 18. Since we have no entry for network 18, the datagram will be sent to the default, 126.96.36.199. As it happens, this gateway is the wrong one. So it will forward the datagram to 188.8.131.52. But it will also send back an error saying in effect: "to get to network 18, use 184.108.40.206". Our software will then add an entry to the routing table. Any future datagrams to MIT will then go directly to 220.127.116.11. (The error message is sent using the ICMP protocol. The message type is called "ICMP redirect.")
Most IP experts recommend that individual computers should not try to keep track of the entire network. Instead, they should start with default gateways, and let the gateways tell them the routes, as just described. However this doesn't say how the gateways should find out about the routes. The gateways can't depend upon this strategy. They have to have fairly complete routing tables. For this, some sort of routing protocol is needed. A routing protocol is simply a technique for the gateways to find each other, and keep up to date about the best way to get to every network. RFC 1009 contains a review of gateway design and routing. However rip.doc is probably a better introduction to the subject. It contains some tutorial material, and a detailed description of the most commonly-used routing protocol.
|7. Details about Internet addresses: subnets and broadcasting|
As indicated earlier, Internet addresses are 32-bit numbers, normally written
as 4 octets (in decimal), e.g. 18.104.22.168. There are actually 3 different
types of address. The problem is that the address has to indicate both
the network and the host within the network. It was felt that eventually
there would be lots of networks. Many of them would be small, but probably
24 bits would be needed to represent all the IP networks. It was also felt
that some very big networks might need 24 bits to represent all of their
hosts. This would seem to lead to 48 bit addresses. But the designers really
wanted to use 32 bit addresses. So they adopted a kludge. The assumption
is that most of the networks will be small. So they set up three different
ranges of address. Addresses beginning with 1 to 126 use only the first
octet for the network number. The other three octets are available for
the host number. Thus 24 bits are available for hosts. These numbers are
used for large networks. But there can only be 126 of these very big networks.
The Arpanet is one, and there are a few large commercial networks. But
few normal organizations get one of these "class A" addresses. For normal
large organizations, "class B" addresses are used. Class B addresses use
the first two octets for the network number. Thus network numbers are 128.1
through 191.254. (We avoid 0 and 255, for reasons that we see below. We
also avoid addresses beginning with 127, because that is used by some systems
for special purposes.) The last two octets are available for host addesses,
giving 16 bits of host address. This allows for 64516 computers, which
should be enough for most organizations. (It is possible to get more than
one class B address, if you run out.) Finally, class C addresses use three
octets, in the range 192.1.1 to 223.254.254. These allow only 254 hosts
on each network, but there can be lots of these networks. Addresses above
223 are reserved for future use, as class D and E (which are currently
Many large organizations find it convenient to divide their network number into "subnets". For example, Rutgers has been assigned a class B address, 128.6. We find it convenient to use the third octet of the address to indicate which Ethernet a host is on. This division has no significance outside of Rutgers. A computer at another institution would treat all datagrams addressed to 128.6 the same way. They would not look at the third octet of the address. Thus computers outside Rutgers would not have different routes for 128.6.4 or 128.6.5. But inside Rutgers, we treat 128.6.4 and 128.6.5 as separate networks. In effect, gateways inside Rutgers have separate entries for each Rutgers subnet, whereas gateways outside Rutgers just have one entry for 128.6. Note that we could do exactly the same thing by using a separate class C address for each Ethernet. As far as Rutgers is concerned, it would be just as convenient for us to have a number of class C addresses. However using class C addresses would make things inconvenient for the rest of the world. Every institution that wanted to talk to us would have to have a separate entry for each one of our networks. If every institution did this, there would be far too many networks for any reasonable gateway to keep track of. By subdividing a class B network, we hide our internal structure from everyone else, and save them trouble. This subnet strategy requires special provisions in the network software. It is described in RFC 950.
0 and 255 have special meanings. 0 is reserved for machines that don't know their address. In certain circumstances it is possible for a machine not to know the number of the network it is on, or even its own host address. For example, 0.0.0.23 would be a machine that knew it was host number 23, but didn't know on what network.
255 is used for "broadcast". A broadcast is a message that you want every system on the network to see. Broadcasts are used in some situations where you don't know who to talk to. For example, suppose you need to look up a host name and get its Internet address. Sometimes you don't know the address of the nearest name server. In that case, you might send the request as a broadcast. There are also cases where a number of systems are interested in information. It is then less expensive to send a single broadcast than to send datagrams individually to each host that is interested in the information. In order to send a broadcast, you use an address that is made by using your network address, with all ones in the part of the address where the host number goes. For example, if you are on network 128.6.4, you would use 22.214.171.124 for broadcasts. How this is actually implemented depends upon the medium. It is not possible to send broadcasts on the Arpanet, or on point to point lines. However it is possible on an Ethernet. If you use an Ethernet address with all its bits on (all ones), every machine on the Ethernet is supposed to look at that datagram.
Although the official broadcast address for network 128.6.4 is now 126.96.36.199, there are some other addresses that may be treated as broadcasts by certain implementations. For convenience, the standard also allows 255.255.255.255 to be used. This refers to all hosts on the local network. It is often simpler to use 255.255.255.255 instead of finding out the network number for the local network and forming a broadcast address such as 188.8.131.52. In addition, certain older implementations may use 0 instead of 255 to form the broadcast address. Such implementations would use 184.108.40.206 instead of 220.127.116.11 as the broadcast address on network 128.6.4. Finally, certain older implementations may not understand about subnets. Thus they consider the network number to be 128.6. In that case, they will assume a broadcast address of 18.104.22.168 or 22.214.171.124. Until support for broadcasts is implemented properly, it can be a somewhat dangerous feature to use.
Because 0 and 255 are used for unknown and broadcast addresses, normal hosts should never be given addresses containing 0 or 255. Addresses should never begin with 0, 127, or any number above 223. Addresses violating these rules are sometimes referred to as "Martians", because of rumors that the Central University of Mars is using network 225.
|8. Datagram fragmentation and reassembly|
TCP/IP is designed for use with many different kinds of network. Unfortunately,
network designers do not agree about how big packets can be. Ethernet packets
can be 1500 octets long. Arpanet packets have a maximum of around 1000
octets. Some very fast networks have much larger packet sizes. At first,
you might think that IP should simply settle on the smallest possible size.
Unfortunately, this would cause serious performance problems. When transferring
large files, big packets are far more efficient than small ones. So we
want to be able to use the largest packet size possible. But we also want
to be able to handle networks with small limits. There are two provisions
for this. First, TCP has the ability to "negotiate" about datagram size.
When a TCP connection first opens, both ends can send the maximum datagram
size they can handle. The smaller of these numbers is used for the rest
of the connection. This allows two implementations that can handle big
datagrams to use them, but also lets them talk to implementations that
can't handle them. However this doesn't completely solve the problem. The
most serious problem is that the two ends don't necessarily know about
all of the steps in between. For example, when sending data between Rutgers
and Berkeley, it is likely that both computers will be on Ethernets. Thus
they will both be prepared to handle 1500-octet datagrams. However the
connection will at some point end up going over the Arpanet. It can't handle
packets of that size. For this reason, there are provisions to split datagrams
up into pieces. (This is referred to as "fragmentation".) The IP header
contains fields indicating the a datagram has been split, and enough information
to let the pieces be put back together. If a gateway connects an Ethernet
to the Arpanet, it must be prepared to take 1500-octet Ethernet packets
and split them into pieces that will fit on the Arpanet. Furthermore, every
host implementation of TCP/IP must be prepared to accept pieces and put
them back together. This is referred to as "reassembly".
TCP/IP implementations differ in the approach they take to deciding on datagram size. It is fairly common for implementations to use 576-byte datagrams whenever they can't verify that the entire path is able to handle larger packets. This rather conservative strategy is used because of the number of implementations with bugs in the code to reassemble fragments. Implementors often try to avoid ever having fragmentation occur. Different implementors take different approaches to deciding when it is safe to use large datagrams. Some use them only for the local network. Others will use them for any network on the same campus. 576 bytes is a "safe" size, which every implementation must support.
|9. Ethernet encapsulation: ARP|
There was a brief discussion earlier about what IP datagrams look like
on an Ethernet. The discussion showed the Ethernet header and checksum.
However it left one hole: It didn't say how to figure out what Ethernet
address to use when you want to talk to a given Internet address. In fact,
there is a separate protocol for this, called ARP ("address resolution
protocol"). (Note by the way that ARP is not an IP protocol. That is, the
ARP datagrams do not have IP headers.) Suppose you are on system 126.96.36.199
and you want to connect to system 188.8.131.52. Your system will first verify
that 184.108.40.206 is on the same network, so it can talk directly via Ethernet.
Then it will look up 220.127.116.11 in its ARP table, to see if it already knows
the Ethernet address. If so, it will stick on an Ethernet header, and send
the packet. But suppose this system is not in the ARP table. There is no
way to send the packet, because you need the Ethernet address. So it uses
the ARP protocol to send an ARP request. Essentially an ARP request says
"I need the Ethernet address for 18.104.22.168". Every system listens to ARP
requests. When a system sees an ARP request for itself, it is required
to respond. So 22.214.171.124 will see the request, and will respond with an
ARP reply saying in effect "126.96.36.199 is 8:0:20:1:56:34". (Recall that
Ethernet addresses are 48 bits. This is 6 octets. Ethernet addresses are
conventionally shown in hex, using the punctuation shown.) Your system
will save this information in its ARP table, so future packets will go
directly. Most systems treat the ARP table as a cache, and clear entries
in it if they have not been used in a certain period of time.
Note by the way that ARP requests must be sent as "broadcasts". There is no way that an ARP request can be sent directly to the right system. After all, the whole reason for sending an ARP request is that you don't know the Ethernet address. So an Ethernet address of all ones is used, i.e. ff:ff:ff:ff:ff:ff. By convention, every machine on the Ethernet is required to pay attention to packets with this as an address. So every system sees every ARP requests. They all look to see whether the request is for their own address. If so, they respond. If not, they could just ignore it. (Some hosts will use ARP requests to update their knowledge about other hosts on the network, even if the request isn't for them.) Note that packets whose IP address indicates broadcast (e.g. 255.255.255.255 or 188.8.131.52) are also sent with an Ethernet address that is all ones.
|10. Getting more information|
This directory contains documents describing the major protocols. There
are literally hundreds of documents, so we have chosen the ones that seem
most important. Internet standards are called RFC's. RFC stands for Request
for Comment. A proposed standard is initially issued as a proposal, and
given an RFC number. When it is finally accepted, it is added to Official
Internet Protocols, but it is still referred to by the RFC number. We have
also included two IEN's. (IEN's used to be a separate classification for
more informal documents. This classification no longer exists -- RFC's
are now used for all official Internet documents, and a mailing list is
used for more informal reports.) The convention is that whenever an RFC
is revised, the revised version gets a new number. This is fine for most
purposes, but it causes problems with two documents: Assigned Numbers and
Official Internet Protocols. These documents are being revised all the
time, so the RFC number keeps changing. You will have to look in rfc-index.txt
to find the number of the latest edition. Anyone who is seriously interested
in TCP/IP should read the RFC describing IP (791). RFC 1009 is also useful.
It is a specification for gateways to be used by NSFnet. As such, it contains
an overview of a lot of the TCP/IP technology. You should probably also
read the description of at least one of the application protocols, just
to get a feel for the way things work. Mail is probably a good one (821/822).
TCP (793) is of course a very basic specification. However the spec is
fairly complex, so you should only read this when you have the time and
patience to think about it carefully. Fortunately, the author of the major
RFC's (Jon Postel) is a very good writer. The TCP RFC is far easier to
read than you would expect, given the complexity of what it is describing.
You can look at the other RFC's as you become curious about their subject
Here is a list of the documents you are more likely to want:
Sites with access to UUCP but not FTP may be able to retreive them via UUCP from UUCP host rutgers. The file names would be
|2010-5-24 ACCC documentation||