%The following is a revised (and I hope final) version of the TCP/IP
%introduction I posted a week ago. It takes into account comments that
%I got from a number of helpful people, and adds a good deal of extra
%material. Originally, I intended it to give my own staff enough
%background to read the RFC's. It appears that there was a need for a
%bit more general introduction. I have tried to do that, to the extent
%that I could.
%
%Various people asked for permission to distribute it to their staff
%(or even customers). I have added a copyright that tries to make it
%clear that this is OK.
%
%If you don't have TCP/IP, and this convinces you that you should get
%it, the NIC (whose address is given in the last section) has a list of
%implementations. However these days, most equipment vendors can
%supply TCP/IP, or point you to a 3rd party who can.
%
%This posting is in two parts, because inews claimed that postings over
%64K bytes were a bad idea.
%
%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.
%
%Unix is a trademark of AT\&T Technologies, Inc.
%
\documentstyle{article}
\author{Charles Hedrick \\
Computer Science Facilities Group \\
Rutgers University\thanks{Copyright \copyright 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.}}
\title{Introduction to the Internet Protocols}
\date{July 3, 1987}
\begin{document}
\maketitle
\begin{abstract}
This document is a brief introduction to TCP/IP, followed by advice on
what to read for more information. This is not intended to be a
complete description. It can give you a reasonable idea of the
capabilities of the protocols. But if you need to know any details of
the technology, you will want to read the standards yourself.
Throughout the text, you will find references to the standards, in the
form of ``RFC'' or ``IEN'' numbers. These are document numbers. The final
section of this document tells you how to get copies of those
standards.
\end{abstract}
\section{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, 1987, 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, {\em 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:
\begin{itemize}
\item File Transfer. The file transfer protocol (FTP) allows a user on
any computer to get files from another computer, or to send files
to another computer. Security is handled by requiring the user
to specify a user name and password for the other computer.
Provisions are made for handling file transfer between machines
with different character set, end of line conventions, etc. This
is not quite the same thing as more recent ``network file system''
or ``netbios'' protocols, which will be described below. Rather,
FTP is a utility that you run any time you want to access a file
on another system. You use it to copy the file to your own
system. You then work with the local copy. (See RFC 959 for
specifications for FTP.)

\item Remote Login. The network terminal protocol (TELNET) allows a
user to log in on any other computer on the network. You start a
remote session by specifying a computer to connect to. From that
time until you finish the session, anything you type is sent to
the other computer. Note that you are really still talking to
your own computer. But the telnet program effectively makes your
computer invisible while it is running. Every character you type
is sent directly to the other system. Generally, the connection
to the remote computer behaves much like a dialup connection.
That is, the remote system will ask you to log in and give a
password, in whatever manner it would normally ask a user who had
just dialed it up. When you log off of the other computer, the
telnet program exits, and you will find yourself talking to your
own computer. Microcomputer implementations of telnet generally
include a terminal emulator for some common type of terminal.
(See RFC's 854 and 855 for specifications for telnet. By the
way, the telnet protocol should not be confused with Telenet, a
vendor of commercial network services.)

\item Computer Mail. This allows you to send messages to users on
other computers. Originally, people tended to use only one or
two specific computers. They would maintain ``mail files'' on
those machines. The computer mail system is simply a way for you
to add a message to another user's mail file. There are some
problems with this in an environment where microcomputers are
used. The most serious is that a micro is not well suited to
receive computer mail. When you send mail, the mail software
expects to be able to open a connection to the addressee's
computer, in order to send the mail. If this is a microcomputer,
it may be turned off, or it may be running an application other
than the mail system. For this reason, mail is normally handled
by a larger system, where it is practical to have a mail server
running all the time. Microcomputer mail software then becomes a
user interface that retrieves mail from the mail server. (See
RFC 821 and 822 for specifications for computer mail. See RFC
937 for a protocol designed for microcomputers to use in reading
mail from a mail server.) 
\end{itemize}

These services should be present in any implementation of TCP/IP,
except that micro-oriented implementations may not support computer
mail. These traditional applications still play a very important role
in TCP/IP-based networks. However more recently, the way in which
networks are used has been changing. The older model of a number of
large, self-sufficient computers is beginning to change. Now many
installations have several kinds of computers, including
microcomputers, workstations, minicomputers, and mainframes. These
computers are likely to be configured to perform specialized tasks.
Although people are still likely to work with one specific computer,
that computer will call on other systems on the net for specialized
services. This has led to the ``server/client'' model of network
services. A server is a system that provides a specific service for
the rest of the network. A client is another system that uses that
service. (Note that the server and client need not be on different
computers. They could be different programs running on the same
computer.) Here are the kinds of servers typically present in a
modern computer setup. Note that these computer services can all be
provided within the framework of TCP/IP.

\begin{itemize}
\item Network File Systems. This allows a system to access files on
another computer in a somewhat more closely integrated fashion
than FTP. A network file system provides the illusion that disks
or other devices from one system are directly connected to other
systems. There is no need to use a special network utility to
access a file on another system. Your computer simply thinks it
has some extra disk drives. These extra ``virtual'' drives refer
to the other system's disks. This capability is useful for
several different purposes. It lets you put large disks on a few
computers, but still give others access to the disk space. Aside
from the obvious economic benefits, this allows people working on
several computers to share common files. It makes system
maintenance and backup easier, because you don't have to worry
about updating and backing up copies on lots of different
machines. A number of vendors now offer high-performance
diskless computers. These computers have no disk drives at all.
They are entirely dependent upon disks attached to common ``file
servers''. (See RFC's 1001 and 1002 for a description of
PC-oriented NetBIOS over TCP. In the workstation and
minicomputer area, Sun's Network File System is more likely to be
used. Protocol specifications for it are available from Sun
Microsystems.)

\item Remote Printing. This allows you to access printers on other
computers as if they were directly attached to yours. (The most
commonly used protocol is the remote lineprinter protocol from
Berkeley Unix. Unfortunately, there is no protocol document for
this. However the C code is easily obtained from Berkeley, so
implementations are common.)

\item Remote Execution. This allows you to request that a particular
program be run on a different computer. This is useful when you
can do most of your work on a small computer, but a few tasks
require the resources of a larger system. There are a number of
different kinds of remote execution. Some operate on a command
by command basis. That is, you request that a specific command
or set of commands should run on some specific computer. (More
sophisticated versions will choose a system that happens to be
free.) However there are also ``remote procedure call'' systems
that allow a program to call a subroutine that will run on
another computer. (There are many protocols of this sort.
Berkeley Unix contains two servers to execute commands remotely:
rsh and rexec. The man pages describe the protocols that they
use. The user-contributed software with Berkeley 4.3 contains a
``distributed shell'' that will distribute tasks among a set of
systems, depending upon load. Remote procedure call mechanisms
have been a topic for research for a number of years, so many
organizations have implementations of such facilities. The most
widespread commercially-supported remote procedure call protocols
seem to be Xerox's Courier and Sun's RPC. Protocol documents are
available from Xerox and Sun. There is a public implementation
of Courier over TCP as part of the user-contributed software with
Berkeley 4.3. An implementation of RPC was posted to Usenet by
Sun, and also appears as part of the user-contributed software
with Berkeley 4.3.)

\item Name Servers. In large installations, there are a number of
different collections of names that have to be managed. This
includes users and their passwords, names and network addresses
for computers, and accounts. It becomes very tedious to keep
this data up to date on all of the computers. Thus the databases
are kept on a small number of systems. Other systems access the
data over the network. (RFC 822 and 823 describe the name server
protocol used to keep track of host names and Internet addresses
on the Internet. This is now a required part of any TCP/IP
implementation. IEN 116 describes an older name server protocol
that is used by a few terminal servers and other products to look
up host names. Sun's Yellow Pages system is designed as a
general mechanism to handle user names, file sharing groups, and
other databases commonly used by Unix systems. It is widely
available commercially. Its protocol definition is available
from Sun.)

\item Terminal servers. Many installations no longer connect terminals
directly to computers. Instead they connect them to terminal
servers. A terminal server is simply a small computer that only
knows how to run telnet (or some other protocol to do remote
login). If your terminal is connected to one of these, you
simply type the name of a computer, and you are connected to it.
Generally it is possible to have active connections to more than
one computer at the same time. The terminal server will have
provisions to switch between connections rapidly, and to notify
you when output is waiting for another connection. (Terminal
servers use the telnet protocol, already mentioned. However any
real terminal server will also have to support name service and a
number of other protocols.)

\item Network-oriented Window Systems. Until recently, high-performance
graphics programs had to execute on a computer that
had a bit-mapped graphics screen directly attached to it.
Network window systems allow a program to use a display on a
different computer. Full-scale network window systems provide an
interface that lets you distribute jobs to the systems that are
best suited to handle them, but still give you a single
graphically-based user interface. (The most widely-implemented
window system is X. A protocol description is available from
MIT's Project Athena. A reference implementation is publically
available from MIT. A number of vendors are also supporting
NeWS, a window system defined by Sun. Both of these systems are
designed to use TCP/IP.) 
\end{itemize}

Note that some of the protocols described above were designed by
Berkeley, Sun, or other organizations. Thus they are not officially
part of the Internet protocol suite. However they are implemented
using TCP/IP, just as normal TCP/IP application protocols are. Since
the protocol definitions are not considered proprietary, and since
commercially-support implementations are widely available, it is
reasonable to think of these protocols as being effectively part of
the Internet suite. Note that the list above is simply a sample of
the sort of services available through TCP/IP. However it does
contain the majority of the ``major'' applications. The other
commonly-used protocols tend to be specialized facilities for getting
information of various kinds, such as who is logged in, the time of
day, etc. However if you need a facility that is not listed here, we
encourage you to look through the current edition of Internet
Protocols (currently RFC 1011), which lists all of the available
protocols, and also to look at some of the major TCP/IP
implementations to see what various vendors have added.

\section{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, {\em 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
retransmits anything that did not get through. If any message is too
large for one datagram, {\em 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:

\begin{itemize}
\item an application protocol such as mail

\item a protocol such as TCP that provides services need by many applications

\item IP, which provides the basic service of getting datagrams to
their destination

\item the protocols needed to manage a specific physical medium, such
 as Ethernet or a point to point line. 

\end{itemize}

TCP/IP is based on the ``catenet model''. (This is described in more
detail in IEN 48.) This model assumes that there are a large number
of independent networks connected together by gateways. The user
should be able to access computers or other resources on any of these
networks. Datagrams will often pass through a dozen different
networks before getting to their final destination. The routing
needed to accomplish this should be completely invisible to the user.
As far as the user is concerned, all he needs to know in order to
access another system is an ``Internet address''. This is an address
that looks like 128.6.4.194. It is actually a 32-bit number. However
it is normally written as 4 decimal numbers, each representing 8 bits
of the address. (The term ``octet'' is used by Internet documentation
for such 8-bit chunks. The term ``byte'' is not used, because TCP/IP is
supported by some computers that have byte sizes other than 8 bits.)
Generally the structure of the address gives you some information
about how to get to the system. For example, 128.6 is a network
number assigned by a central authority to Rutgers University. Rutgers
uses the next octet to indicate which of the campus Ethernets is
involved. 128.6.4 happens to be an Ethernet used by the Computer
Science Department. The last octet allows for up to 254 systems on
each Ethernet. (It is 254 because 0 and 255 are not allowed, for
reasons that will be discussed later.) Note that 128.6.4.194 and
128.6.5.194 would be different systems. The structure of an Internet
address is described in a bit more detail later.

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 ({\em 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.

\subsection{The TCP level}

Two separate protocols are involved in handling TCP/IP datagrams. TCP
(the ``transmission control protocol'') is responsible for breaking up
the message into datagrams, reassembling them at the other end,
resending anything that gets lost, and putting things back in the
right order. IP (the ``internet protocol'') is responsible for routing
individual datagrams. It may seem like TCP is doing all the work.
And in small networks that is true. However in the Internet, simply
getting a datagram to its destination can be a complex job. A
connection may require the datagram to go through several networks at
Rutgers, a serial line to the John von Neuman Supercomputer Center, a
couple of Ethernets there, a series of 56Kbaud phone lines to another
NSFnet site, and more Ethernets on another campus. Keeping track of
the routes to all of the destinations and handling incompatibilities
among different transport media turns out to be a complex job. Note
that the interface between TCP and IP is fairly simple. TCP simply
hands IP a datagram with a destination. IP doesn't know how this
datagram relates to any datagram before it or after it.

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: 

\begin{verbatim}
 ......................................................
\end{verbatim}

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.) 

\begin{verbatim}
 .... .... .... .... .... .... .... ....
\end{verbatim}

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.

\begin{verbatim}
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 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 |
 | ......   |
\end{verbatim}

If we abbreviate the TCP header as ``T'', the whole file now looks like
this:

\begin{verbatim}
 T.... T.... T.... T.... T.... T.... T....
\end{verbatim}

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.

\subsection{The IP level}

TCP sends each of these datagrams to IP. Of course it has to tell IP
the Internet address of the computer at the other end. Note that this
is all IP is concerned about. It doesn't care about what is in the
datagram, or even in the TCP header. IP's job is simply to find a
route for the datagram and get it to the other end. In order to allow
gateways or other intermediate systems to forward the datagram, it
adds its own header. The main things in this header are the source
and destination Internet address (32-bit addresses, like 128.6.4.194),
the protocol number, and another checksum. The source Internet
address is simply the address of your machine. (This is necessary so
the other end knows where the datagram came from.) The destination
Internet address is the address of the other machine. (This is
necessary so any gateways in the middle know where you want the
datagram to go.) The protocol number tells IP at the other end to
send the datagram to TCP. Although most IP traffic uses TCP, there
are other protocols that can use IP, so you have to tell IP which
protocol to send the datagram to. Finally, the checksum allows IP at
the other end to verify that the header wasn't damaged in transit.
Note that TCP and IP have separate checksums. IP needs to be able to
verify that the header didn't get damaged in transit, or it could send
a message to the wrong place. For reasons not worth discussing here,
it is both more efficient and safer to have TCP compute a separate
checksum for the TCP header and data. Once IP has tacked on its
header, here's what the message looks like:

\begin{verbatim}
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |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 ...... |
 |   |
\end{verbatim}

If we represent the IP header by an ``I'', your file now looks like
this: 

\begin{verbatim}
 IT.... IT.... IT.... IT.... IT.... IT.... IT....
\end{verbatim}

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).

\subsection{The Ethernet level}

However most of our networks these days use Ethernet. So now we have
to describe Ethernet's headers. Unfortunately, Ethernet has its own
addresses. The people who designed Ethernet wanted to make sure that
no two machines would end up with the same Ethernet address.
Furthermore, they didn't want the user to have to worry about
assigning addresses. So each Ethernet controller comes with an
address builtin from the factory. In order to make sure that they
would never have to reuse addresses, the Ethernet designers allocated
48 bits for the Ethernet address. People who make Ethernet equipment
have to register with a central authority, to make sure that the
numbers they assign don't overlap any other manufacturer. Ethernet is
a ``broadcast medium''. That is, it is in effect like an old party line
telephone. When you send a packet out on the Ethernet, every machine
on the network sees the packet. So something is needed to make sure
that the right machine gets it. As you might guess, this involves the
Ethernet header. Every Ethernet packet has a 14-octet header that
includes the source and destination Ethernet address, and a type code.
Each machine is supposed to pay attention only to packets with its own
Ethernet address in the destination field. (It's perfectly possible
to cheat, which is one reason that Ethernet communications are not
terribly secure.) Note that there is no connection between the
Ethernet address and the Internet address. Each machine has to have a
table of what Ethernet address corresponds to what Internet address.
(We will describe how this table is constructed a bit later.) In
addition to the addresses, the header contains a type code. The type
code is to allow for several different protocol families to be used on
the same network. So you can use TCP/IP, DECnet, Xerox NS, etc. at
the same time. Each of them will put a different value in the type
field. Finally, there is a checksum. The Ethernet controller
computes a checksum of the entire packet. When the other end receives
the packet, it recomputes the checksum, and throws the packet away if
the answer disagrees with the original. The checksum is put on the
end of the packet, not in the header. The final result is that your
message looks like this:

\begin{verbatim}
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 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  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\end{verbatim}

If we represent the Ethernet header with ``E'', and the Ethernet
checksum with ``C'', your file now looks like this: 

\begin{verbatim}
 EIT....C EIT....C EIT....C EIT....C EIT....C
\end{verbatim}

When 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.)

\section{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, {\em 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 128.6.4.7. 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 128.6.4.7, 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:
\begin{center}
\begin{tabular}{lll}
 & Internet addresses & TCP ports \\
 connection 1 & 128.6.4.194, 128.6.4.7 & 1234, 21 \\
 connection 2 & 128.6.4.194, 128.6.4.7 & 1235, 21 \\
\end{tabular}
\end{center}

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 ({\em 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.

\subsection{An example application: SMTP}

In order to give a bit better idea what is involved in the application
protocols, I'm going to show an example of SMTP, which is the mail
protocol. (SMTP is ``simple mail transfer protocol.) We assume that a
computer called TOPAZ.RUTGERS.EDU wants to send the following message.

\begin{verbatim}
 Date: Sat, 27 Jun 87 13:26:31 EDT
 From: hedrick@topaz.rutgers.edu
 To: levy@red.rutgers.edu
 Subject: meeting

 Let's get together Monday at 1pm.

\end{verbatim}
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 ({\em 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 128.6.4.2. Then the mail program
opens a TCP connection to port 25 on 128.6.4.2. 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:

\begin{verbatim}
 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:<hedrick@topaz.rutgers.edu>
 RED 250 MAIL accepted
 TOPAZ RCPT To:<levy@red.rutgers.edu>
 RED 250 Recipient accepted
 TOPAZ DATA
 RED 354 Start mail input; end with <CRLF>.<CRLF>
 TOPAZ Date: Sat, 27 Jun 87 13:26:31 EDT
 TOPAZ From: hedrick@topaz.rutgers.edu
 TOPAZ To: levy@red.rutgers.edu
 TOPAZ Subject: meeting
 TOPAZ
 TOPAZ Let's get together Monday at 1pm.
 TOPAZ .
 RED 250 OK
 TOPAZ QUIT
 RED 221 RED.RUTGERS.EDU Service closing transmission channel
\end{verbatim}

First, 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.)

\section{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 to TCP.

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.

\section{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.)

\section{Routing}

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 128.6.3.12. Systems on net 128.6.3 will list 128.6.3.12 as
the gateway for many off-campus networks. However systems on net
128.6.4 will list 128.6.4.1 as the gateway to those same off-campus
networks. 128.6.4.1 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, 128.6.4.59 and 128.6.4.1. 128.6.4.59 leads to several other
internal Rutgers networks. 128.6.4.1 leads indirectly to the NSFnet.
Suppose we set 128.6.4.59 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, 128.6.4.59. As
it happens, this gateway is the wrong one. So it will forward the
datagram to 128.6.4.1. But it will also send back an error saying in
effect: ``to get to network 18, use 128.6.4.1''. Our software will then
add an entry to the routing table. Any future datagrams to MIT will
then go directly to 128.6.4.1. (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.

\section{Details about Internet addresses: subnets and broadcasting}

As indicated earlier, Internet addresses are 32-bit numbers, normally
written as 4 octets (in decimal), {\em e.g.} 128.6.4.7. 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 not defined).

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 128.6.4.255 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
128.6.4.255, 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 128.6.4.255. In addition, certain older
implementations may use 0 instead of 255 to form the broadcast
address. Such implementations would use 128.6.4.0 instead of
128.6.4.255 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 128.6.255.255 or 128.6.0.0. 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.

\section{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.

\section{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 128.6.4.194 and you want to connect to
system 128.6.4.7. Your system will first verify that 128.6.4.7 is on
the same network, so it can talk directly via Ethernet. Then it will
look up 128.6.4.7 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
128.6.4.7''. Every system listens to ARP requests. When a system sees
an ARP request for itself, it is required to respond. So 128.6.4.7
will see the request, and will respond with an ARP reply saying in
effect ``128.6.4.7 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, {\em 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 ({\em e.g.} 255.255.255.255 or 128.6.4.255)
are also sent with an Ethernet address that is all ones.

\section{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 matter.

Here is a list of the documents you are more likely to want:

\begin{tabular}{lp{4.0in}}
RFC-index & List of all RFC's \\

RFC1012 & Somewhat fuller list of all RFC's \\

RFC1011 & Official Protocols. It's useful to scan this to see
what tasks protocols have been built for. This defines
which RFC's are actual standards, as opposed to
requests for comments. \\

RFC1010 & Assigned Numbers. If you are working with TCP/IP, you
will probably want a hardcopy of this as a reference.
It's not very exciting to read. It lists all the
offically defined well-known ports and lots of other
things. \\

RFC1009 & NSFnet gateway specifications. A good overview of IP
routing and gateway technology. \\

RFC1001/2 & NetBIOS: networking for PC's \\

RFC973 & Update on domains \\

RFC959 & FTP (file transfer) \\

RFC950 & Subnets \\

RFC937 & POP2: protocol for reading mail on PC's \\

RFC894 & How IP is to be put on Ethernet, see also RFC825 \\

RFC882/3 & Domains (the database used to go from host names to
Internet address and back -- also used to handle UUCP
these days). See also RFC973 \\

RFC854/5 & Telnet - protocol for remote logins \\

RFC826 & ARP - protocol for finding out Ethernet addresses \\

RFC821/2 & Mail \\

RFC814 & Names and Ports - general concepts behind well-known ports \\

RFC793 & TCP \\

RFC792 & ICMP \\

RFC791 & IP \\

RFC768 & UDP \\

rip.doc & Details of the most commonly-used routing protocol \\

IEN-116 & Old name server (still needed by several kinds of system) \\

IEN-48 & The Catenet model, general description
of the philosophy behind TCP/IP  \\
\end{tabular}

The following documents are somewhat more specialized.

\begin{tabular}{lp{4.0in}}
RFC813 & Window and acknowledgement strategies in TCP \\

RFC815 & Datagram reassembly techniques \\

RFC816 & Fault isolation and resolution techniques \\

RFC817 & Modularity and efficiency in implementation \\

RFC879 & The maximum segment size option in TCP \\

RFC896 & Congestion control \\

RFC827,888,904,975,985 & EGP and related issues  \\
\end{tabular}

To those of you who may be reading this document remotely instead of
at Rutgers: The most important RFC's have been collected into a
three-volume set, the DDN Protocol Handbook. It is available from the
DDN Network Information Center, SRI International, 333 Ravenswood
Avenue, Menlo Park, California 94025 (telephone: 800-235-3155). You
should be able to get them via anonymous FTP from sri-nic.arpa. File
names are: \\
RFC's: \\
RFC:RFC-index.txt \\
RFC:RFCxxx.txt \\
IEN's: \\
ien:ien-index.txt \\
ien:ien-xxx.txt \\

rip.doc is available by anonymous FTP from topaz.rutgers.edu, as
/pub/tcp-ip-docs/rip.doc.

Note that SRI-NIC has the entire set of RFC's and IEN's, but rutgers
and topaz have only those specifically mentioned above.
\newpage
\tableofcontents
\end{document}
