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			      Introduction
				   to
			 the Internet Protocols


		       C                       R
			       C       S
		   Computer Science Facilities Group
			       C       I
		       L                       S

				RUTGERS
		   The State University of New Jersey

			      3 July 1987

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.


			   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
  6. Routing
  7. Details about Internet addresses: subnets and broadcasting
  8. Datagram fragmentation and reassembly
  9. Ethernet encapsulation: ARP
 10. Getting more information


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.


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:

   -    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.)

   -    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.)

   -    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.)

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.

   -    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.)

   -    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.)

   -    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.)

   -    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.)

   -    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.)

   -    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.)

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.


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:

  -  an application protocol such as mail

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

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

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

 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 (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.



2.1 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:


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.

	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	|          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                             |
	|   ......                                                    |

If  we abbreviate the TCP header as "T", the whole file now looks like
this:

   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.


2.2 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:

	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	|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 ......                          |
	|                                                             |

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

   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).



2.3 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:

	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	|     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                       |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   EIT....C   EIT....C   EIT....C   EIT....C   EIT....C

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.)



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  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:

			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

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.


3.1 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.

  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.

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 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:

	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

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.)


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 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.


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.)


6. 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.


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. 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.


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  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,  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 128.6.4.255)
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 matter.

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

	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

The following documents are somewhat more specialized.

	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

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.

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

   RFC's:
     /topaz/pub/pub/tcp-ip-docs/rfc-index.txt
     /topaz/pub/pub/tcp-ip-docs/rfcxxx.txt
   IEN's:
     /topaz/pub/pub/tcp-ip-docs/ien-index.txt
     /topaz/pub/pub/tcp-ip-docs/ien-xxx.txt
   /topaz/pub/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.

