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Network Working Group E. Krol
Request for Comments: 1118 University of Illinois Urbana
September 1989
The Hitchhikers Guide to the Internet
Status of this Memo
This RFC is being distributed to members of the Internet community in
order to make available some "hints" which will allow new network
participants to understand how the direction of the Internet is set,
how to acquire online information and how to be a good Internet
neighbor. While the information discussed may not be relevant to the
research problems of the Internet, it may be interesting to a number
of researchers and implementors. No standards are defined or
specified in this memo. Distribution of this memo is unlimited.
NOTICE:
The hitchhikers guide to the Internet is a very unevenly edited memo
and contains many passages which simply seemed to its editors like a
good idea at the time. It is an indispensable companion to all those
who are keen to make sense of life in an infinitely complex and
confusing Internet, for although it cannot hope to be useful or
informative on all matters, it does make the reassuring claim that
where it is inaccurate, it is at least definitively inaccurate. In
cases of major discrepancy it is always reality that's got it wrong.
And remember, DON'T PANIC. (Apologies to Douglas Adams.)
Purpose and Audience
This document assumes that one is familiar with the workings of a
non-connected simple IP network (e.g., a few 4.3 BSD systems on an
Ethernet not connected to anywhere else). Appendix A contains
remedial information to get one to this point. Its purpose is to get
that person, familiar with a simple net, versed in the "oral
tradition" of the Internet to the point that that net can be
connected to the Internet with little danger to either. It is not a
tutorial, it consists of pointers to other places, literature, and
hints which are not normally documented. Since the Internet is a
dynamic environment, changes to this document will be made regularly.
The author welcomes comments and suggestions. This is especially
true of terms for the glossary (definitions are not necessary).
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RFC 1118 The Hitchhikers Guide to the Internet September 1989
What is the Internet?
In the beginning there was the ARPANET, a wide area experimental
network connecting hosts and terminal servers together. Procedures
were set up to regulate the allocation of addresses and to create
voluntary standards for the network. As local area networks became
more pervasive, many hosts became gateways to local networks. A
network layer to allow the interoperation of these networks was
developed and called Internet Protocol (IP). Over time other groups
created long haul IP based networks (NASA, NSF, states...). These
nets, too, interoperate because of IP. The collection of all of
these interoperating networks is the Internet.
A few groups provide much of the information services on the
Internet. Information Sciences Institute (ISI) does much of the
standardization and allocation work of the Internet acting as the
Internet Assigned Numbers Authority (IANA). SRI International
provides the principal information services for the Internet by
operating the Network Information Center (NIC). In fact, after you
are connected to the Internet most of the information in this
document can be retrieved from the SRI-NIC. Bolt Beranek and Newman
(BBN) provides information services for CSNET (the CIC) and NSFNET
(the NNSC), and Merit provides information services for NSFNET (the
NIS).
Operating the Internet
Each network, be it the ARPANET, NSFNET or a regional network, has
its own operations center. The ARPANET is run by BBN, Inc. under
contract from DCA (on behalf of DARPA). Their facility is called the
Network Operations Center or NOC. Merit, Inc. operates NSFNET from
yet another and completely seperate NOC. It goes on to the regionals
having similar facilities to monitor and keep watch over the goings
on of their portion of the Internet. In addition, they all should
have some knowledge of what is happening to the Internet in total.
If a problem comes up, it is suggested that a campus network liaison
should contact the network operator to which he is directly
connected. That is, if you are connected to a regional network
(which is gatewayed to the NSFNET, which is connected to the
ARPANET...) and have a problem, you should contact your regional
network operations center.
RFCs
The internal workings of the Internet are defined by a set of
documents called RFCs (Request for Comments). The general process
for creating an RFC is for someone wanting something formalized to
write a document describing the issue and mailing it to Jon Postel
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(Postel@ISI.EDU). He acts as a referee for the proposal. It is then
commented upon by all those wishing to take part in the discussion
(electronically of course). It may go through multiple revisions.
Should it be generally accepted as a good idea, it will be assigned a
number and filed with the RFCs.
There are two independent categorizations of protocols. The first is
the state of standardization which is one of "standard", "draft
standard", "proposed", "experimental", or "historic". The second is
the status of this protocol which is one of "required",
"recommended", "elective", or "not recommended". One could expect a
particular protocol to move along the scale of status from elective
to required at the same time as it moves along the scale of
standardization from proposed to standard.
A Required Standard protocol (e.g., RFC-791, The Internet Protocol)
must be implemented on any host connected to the Internet.
Recommended Standard protocols are generally implemented by network
hosts. Lack of them does not preclude access to the Internet, but
may impact its usability. RFC-793 (Transmission Control Protocol) is
a Recommended Standard protocol. Elective Proposed protocols were
discussed and agreed to, but their application has never come into
wide use. This may be due to the lack of wide need for the specific
application (RFC-937, The Post Office Protocol) or that, although
technically superior, ran against other pervasive approaches. It is
suggested that should the facility be required by a particular site,
an implementation be done in accordance with the RFC. This insures
that, should the idea be one whose time has come, the implementation
will be in accordance with some standard and will be generally
usable.
Informational RFCs contain factual information about the Internet and
its operation (RFC-1010, Assigned Numbers). Finally, as the Internet
and technology have grown, some RFCs have become unnecessary. These
obsolete RFCs cannot be ignored, however. Frequently when a change
is made to some RFC that causes a new one to be issued obsoleting
others, the new RFC may only contains explanations and motivations
for the change. Understanding the model on which the whole facility
is based may involve reading the original and subsequent RFCs on the
topic. (Appendix B contains a list of what are considered to be the
major RFCs necessary for understanding the Internet).
Only a few RFCs actually specify standards, most RFCs are for
information or discussion purposes. To find out what the current
standards are see the RFC titled "IAB Official Protocol Standards"
(most recently published as RFC-1100).
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The Network Information Center (NIC)
The NIC is a facility available to all Internet users which provides
information to the community. There are three means of NIC contact:
network, telephone, and mail. The network accesses are the most
prevalent. Interactive access is frequently used to do queries of
NIC service overviews, look up user and host names, and scan lists of
NIC documents. It is available by using
%telnet nic.ddn.mil
on a BSD system, and following the directions provided by a user
friendly prompter. From poking around in the databases provided, one
might decide that a document named NETINFO:NUG.DOC (The Users Guide
to the ARPANET) would be worth having. It could be retrieved via an
anonymous FTP. An anonymous FTP would proceed something like the
following. (The dialogue may vary slightly depending on the
implementation of FTP you are using).
%ftp nic.ddn.mil
Connected to nic.ddn.mil
220 NIC.DDN.MIL FTP Server 5Z(47)-6 at Wed 17-Jun-87 12:00 PDT
Name (nic.ddn.mil:myname): anonymous
331 ANONYMOUS user ok, send real ident as password.
Password: myname
230 User ANONYMOUS logged in at Wed 17-Jun-87 12:01 PDT, job 15.
ftp> get netinfo:nug.doc
200 Port 18.144 at host 128.174.5.50 accepted.
150 ASCII retrieve of <NETINFO>NUG.DOC.11 started.
226 Transfer Completed 157675 (8) bytes transferred
local: netinfo:nug.doc remote:netinfo:nug.doc
157675 bytes in 4.5e+02 seconds (0.34 Kbytes/s)
ftp> quit
221 QUIT command received. Goodbye.
(Another good initial document to fetch is NETINFO:WHAT-THE-NIC-
DOES.TXT).
Questions of the NIC or problems with services can be asked of or
reported to using electronic mail. The following addresses can be
used:
NIC@NIC.DDN.MIL General user assistance, document requests
REGISTRAR@NIC.DDN.MIL User registration and WHOIS updates
HOSTMASTER@NIC.DDN.MIL Hostname and domain changes and updates
ACTION@NIC.DDN.MIL SRI-NIC computer operations
SUGGESTIONS@NIC.DDN.MIL Comments on NIC publications and services
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For people without network access, or if the number of documents is
large, many of the NIC documents are available in printed form for a
small charge. One frequently ordered document for starting sites is
a compendium of major RFCs. Telephone access is used primarily for
questions or problems with network access. (See appendix B for
mail/telephone contact numbers).
The NSFNET Network Service Center
The NSFNET Network Service Center (NNSC), located at BBN Systems and
Technologies Corp., is a project of the University Corporation for
Atmospheric Research under agreement with the National Science
Foundation. The NNSC provides support to end-users of NSFNET should
they have questions or encounter problems traversing the network.
The NNSC, which has information and documents online and in printed
form, distributes news through network mailing lists, bulletins, and
online reports. NNSC publications include a hardcopy newsletter, the
NSF Network News, which contains articles of interest to network
users and the Internet Resource Guide, which lists facilities (such
as supercomputer centers and on-line library catalogues) accessible
from the Internet. The Resource Guide can be obtained via anonymous
ftp to nnsc.nsf.net in the directory resource-guide, or by joining
the resource guide mailing list (send a subscription request to
Resource-Guide-Request@NNSC.NSF.NET.)
Mail Reflectors
The way most people keep up to date on network news is through
subscription to a number of mail reflectors (also known as mail
exploders). Mail reflectors are special electronic mailboxes which,
when they receive a message, resend it to a list of other mailboxes.
This in effect creates a discussion group on a particular topic.
Each subscriber sees all the mail forwarded by the reflector, and if
one wants to put his "two cents" in sends a message with the comments
to the reflector.
The general format to subscribe to a mail list is to find the address
reflector and append the string -REQUEST to the mailbox name (not the
host name). For example, if you wanted to take part in the mailing
list for NSFNET reflected by NSFNET-INFO@MERIT.EDU, one sends a
request to NSFNET-INFO-REQUEST@MERIT.EDU. This may be a wonderful
scheme, but the problem is that you must know the list exists in the
first place. It is suggested that, if you are interested, you read
the mail from one list (like NSFNET-INFO) and you will probably
become familiar with the existence of others. A registration service
for mail reflectors is provided by the NIC in the files
NETINFO:INTEREST-GROUPS-1.TXT, NETINFO:INTEREST-GROUPS-2.TXT,
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RFC 1118 The Hitchhikers Guide to the Internet September 1989
NETINFO:INTEREST-GROUPS-3.TXT, through NETINFO:INTEREST-GROUPS-9.TXT.
The NSFNET-INFO mail reflector is targeted at those people who have a
day to day interest in the news of the NSFNET (the backbone, regional
network, and Internet inter-connection site workers). The messages
are reflected by a central location and are sent as separate messages
to each subscriber. This creates hundreds of messages on the wide
area networks where bandwidth is the scarcest.
There are two ways in which a campus could spread the news and not
cause these messages to inundate the wide area networks. One is to
re-reflect the message on the campus. That is, set up a reflector on
a local machine which forwards the message to a campus distribution
list. The other is to create an alias on a campus machine which
places the messages into a notesfile on the topic. Campus users who
want the information could access the notesfile and see the messages
that have been sent since their last access. One might also elect to
have the campus wide area network liaison screen the messages in
either case and only forward those which are considered of merit.
Either of these schemes allows one message to be sent to the campus,
while allowing wide distribution within.
Address Allocation
Before a local network can be connected to the Internet it must be
allocated a unique IP address. These addresses are allocated by
SRI-NIC. The allocation process consists of getting an application
form. Send a message to Hostmaster@NIC.DDN.MIL and ask for the
template for a connected address. This template is filled out and
mailed back to the hostmaster. An address is allocated and e-mailed
back to you. This can also be done by postal mail (Appendix B).
IP addresses are 32 bits long. It is usually written as four decimal
numbers separated by periods (e.g., 192.17.5.100). Each number is
the value of an octet of the 32 bits. Some networks might choose to
organize themselves as very flat (one net with a lot of nodes) and
some might organize hierarchically (many interconnected nets with
fewer nodes each and a backbone). To provide for these cases,
addresses were differentiated into class A, B, and C networks. This
classification had to with the interpretation of the octets. Class A
networks have the first octet as a network address and the remaining
three as a host address on that network. Class C addresses have
three octets of network address and one of host. Class B is split
two and two. Therefore, there is an address space for a few large
nets, a reasonable number of medium nets and a large number of small
nets. The high order bits in the first octet are coded to tell the
address format. There are very few unallocated class A nets, so a
very good case must be made for them. So as a practical matter, one
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RFC 1118 The Hitchhikers Guide to the Internet September 1989
has to choose between Class B and Class C when placing an order.
(There are also class D (Multicast) and E (Experimental) formats.
Multicast addresses will likely come into greater use in the near
future, but are not frequently used yet).
In the past, sites requiring multiple network addresses requested
multiple discrete addresses (usually Class C). This was done because
much of the software available (notably 4.2BSD) could not deal with
subnetted addresses. Information on how to reach a particular
network (routing information) must be stored in Internet gateways and
packet switches. Some of these nodes have a limited capability to
store and exchange routing information (limited to about 700
networks). Therefore, it is suggested that any campus announce (make
known to the Internet) no more than two discrete network numbers.
If a campus expects to be constrained by this, it should consider
subnetting. Subnetting (RFC-950) allows one to announce one address
to the Internet and use a set of addresses on the campus. Basically,
one defines a mask which allows the network to differentiate between
the network portion and host portion of the address. By using a
different mask on the Internet and the campus, the address can be
interpreted in multiple ways. For example, if a campus requires two
networks internally and has the 32,000 addresses beginning
128.174.X.X (a Class B address) allocated to it, the campus could
allocate 128.174.5.X to one part of campus and 128.174.10.X to
another. By advertising 128.174 to the Internet with a subnet mask
of FF.FF.00.00, the Internet would treat these two addresses as one.
Within the campus a mask of FF.FF.FF.00 would be used, allowing the
campus to treat the addresses as separate entities. (In reality, you
don't pass the subnet mask of FF.FF.00.00 to the Internet, the octet
meaning is implicit in its being a class B address).
A word of warning is necessary. Not all systems know how to do
subnetting. Some 4.2BSD systems require additional software. 4.3BSD
systems subnet as released. Other devices and operating systems vary
in the problems they have dealing with subnets. Frequently, these
machines can be used as a leaf on a network but not as a gateway
within the subnetted portion of the network. As time passes and more
systems become 4.3BSD based, these problems should disappear.
There has been some confusion in the past over the format of an IP
broadcast address. Some machines used an address of all zeros to
mean broadcast and some all ones. This was confusing when machines
of both type were connected to the same network. The broadcast
address of all ones has been adopted to end the grief. Some systems
(e.g., 4.3 BSD) allow one to choose the format of the broadcast
address. If a system does allow this choice, care should be taken
that the all ones format is chosen. (This is explained in RFC-1009
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and RFC-1010).
Internet Problems
There are a number of problems with the Internet. Solutions to the
problems range from software changes to long term research projects.
Some of the major ones are detailed below:
Number of Networks
When the Internet was designed it was to have about 50 connected
networks. With the explosion of networking, the number is now
approaching 1000. The software in a group of critical gateways
(called the core gateways) are not able to pass or store much more
than that number. In the short term, core reallocation and
recoding has raised the number slightly.
Routing Issues
Along with sheer mass of the data necessary to route packets to a
large number of networks, there are many problems with the
updating, stability, and optimality of the routing algorithms.
Much research is being done in the area, but the optimal solution
to these routing problems is still years away. In most cases, the
the routing we have today works, but sub-optimally and sometimes
unpredictably. The current best hope for a good routing protocol
is something known as OSPFIGP which will be generally available
from many router manufacturers within a year.
Trust Issues
Gateways exchange network routing information. Currently, most
gateways accept on faith that the information provided about the
state of the network is correct. In the past this was not a big
problem since most of the gateways belonged to a single
administrative entity (DARPA). Now, with multiple wide area
networks under different administrations, a rogue gateway
somewhere in the net could cripple the Internet. There is design
work going on to solve both the problem of a gateway doing
unreasonable things and providing enough information to reasonably
route data between multiply connected networks (multi-homed
networks).
Capacity & Congestion
Some portions of the Internet are very congested during the busy
part of the day. Growth is dramatic with some networks
experiencing growth in traffic in excess of 20% per month.
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Additional bandwidth is planned, but delivery and budgets might
not allow supply to keep up.
Setting Direction and Priority
The Internet Activities Board (IAB), currently chaired by Vint Cerf
of NRI, is responsible for setting the technical direction,
establishing standards, and resolving problems in the Internet.
The current IAB members are:
Vinton Cerf - Chairman
David Clark - IRTF Chairman
Phillip Gross - IETF Chairman
Jon Postel - RFC Editor
Robert Braden - Executive Director
Hans-Werner Braun - NSFNET Liaison
Barry Leiner - CCIRN Liaison
Daniel Lynch - Vendor Liaison
Stephen Kent - Internet Security
This board is supported by a Research Task Force (chaired by Dave
Clark of MIT) and an Engineering Task Force (chaired by Phill Gross
of NRI).
The Internet Research Task Force has the following Research Groups:
Autonomous Networks Deborah Estrin
End-to-End Services Bob Braden
Privacy Steve Kent
User Interfaces Keith Lantz
The Internet Engineering Task Force has the following technical
areas:
Applications TBD
Host Protocols Craig Partridge
Internet Protocols Noel Chiappa
Routing Robert Hinden
Network Management David Crocker
OSI Interoperability Ross Callon, Robert Hagen
Operations TBD
Security TBD
The Internet Engineering Task Force has the following Working Groups:
ALERTMAN Louis Steinberg
Authentication Jeff Schiller
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CMIP over TCP Lee LaBarre
Domain Names Paul Mockapetris
Dynamic Host Config Ralph Droms
Host Requirements Bob Braden
Interconnectivity Guy Almes
Internet MIB Craig Partridge
Joint Management Susan Hares
LAN Mgr MIB Amatzia Ben-Artzi
NISI Karen Bowers
NM Serial Interface Jeff Case
NOC Tools Bob Enger
OSPF Mike Petry
Open Systems Routing Marianne Lepp
OSI Interoperability Ross Callon
PDN Routing Group CH Rokitansky
Performance and CC Allison Mankin
Point - Point IP Drew Perkins
ST and CO-IP Claudio Topolcic
Telnet Dave Borman
User Documents Karen Roubicek
User Services Karen Bowers
Routing
Routing is the algorithm by which a network directs a packet from its
source to its destination. To appreciate the problem, watch a small
child trying to find a table in a restaurant. From the adult point
of view, the structure of the dining room is seen and an optimal
route easily chosen. The child, however, is presented with a set of
paths between tables where a good path, let alone the optimal one to
the goal is not discernible.
A little more background might be appropriate. IP gateways (more
correctly routers) are boxes which have connections to multiple
networks and pass traffic between these nets. They decide how the
packet is to be sent based on the information in the IP header of the
packet and the state of the network. Each interface on a router has
an unique address appropriate to the network to which it is
connected. The information in the IP header which is used is
primarily the destination address. Other information (e.g., type of
service) is largely ignored at this time. The state of the network
is determined by the routers passing information among themselves.
The distribution of the database (what each node knows), the form of
the updates, and metrics used to measure the value of a connection,
are the parameters which determine the characteristics of a routing
protocol.
Under some algorithms, each node in the network has complete
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knowledge of the state of the network (the adult algorithm). This
implies the nodes must have larger amounts of local storage and
enough CPU to search the large tables in a short enough time
(remember, this must be done for each packet). Also, routing updates
usually contain only changes to the existing information (or you
spend a large amount of the network capacity passing around megabyte
routing updates). This type of algorithm has several problems.
Since the only way the routing information can be passed around is
across the network and the propagation time is non-trivial, the view
of the network at each node is a correct historical view of the
network at varying times in the past. (The adult algorithm, but
rather than looking directly at the dining area, looking at a
photograph of the dining room. One is likely to pick the optimal
route and find a bus-cart has moved in to block the path after the
photo was taken). These inconsistencies can cause circular routes
(called routing loops) where once a packet enters it is routed in a
closed path until its time to live (TTL) field expires and it is
discarded.
Other algorithms may know about only a subset of the network. To
prevent loops in these protocols, they are usually used in a
hierarchical network. They know completely about their own area, but
to leave that area they go to one particular place (the default
gateway). Typically these are used in smaller networks (campus or
regional).
Routing protocols in current use:
Static (no protocol-table/default routing)
Don't laugh. It is probably the most reliable, easiest to
implement, and least likely to get one into trouble for a small
network or a leaf on the Internet. This is, also, the only method
available on some CPU-operating system combinations. If a host is
connected to an Ethernet which has only one gateway off of it, one
should make that the default gateway for the host and do no other
routing. (Of course, that gateway may pass the reachability
information somehow on the other side of itself.)
One word of warning, it is only with extreme caution that one
should use static routes in the middle of a network which is also
using dynamic routing. The routers passing dynamic information
are sometimes confused by conflicting dynamic and static routes.
If your host is on an ethernet with multiple routers to other
networks on it and the routers are doing dynamic routing among
themselves, it is usually better to take part in the dynamic
routing than to use static routes.
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RIP
RIP is a routing protocol based on XNS (Xerox Network System)
adapted for IP networks. It is used by many routers (Proteon,
cisco, UB...) and many BSD Unix systems. BSD systems typically
run a program called "routed" to exchange information with other
systems running RIP. RIP works best for nets of small diameter
(few hops) where the links are of equal speed. The reason for
this is that the metric used to determine which path is best is
the hop-count. A hop is a traversal across a gateway. So, all
machines on the same Ethernet are zero hops away. If a router
connects connects two networks directly, a machine on the other
side of the router is one hop away. As the routing information is
passed through a gateway, the gateway adds one to the hop counts
to keep them consistent across the network. The diameter of a
network is defined as the largest hop-count possible within a
network. Unfortunately, a hop count of 16 is defined as infinity
in RIP meaning the link is down. Therefore, RIP will not allow
hosts separated by more than 15 gateways in the RIP space to
communicate.
The other problem with hop-count metrics is that if links have
different speeds, that difference is not reflected in the hop-
count. So a one hop satellite link (with a .5 sec delay) at 56kb
would be used instead of a two hop T1 connection. Congestion can
be viewed as a decrease in the efficacy of a link. So, as a link
gets more congested, RIP will still know it is the best hop-count
route and congest it even more by throwing more packets on the
queue for that link.
RIP was originally not well documented in the community and people
read BSD code to find out how RIP really worked. Finally, it was
documented in RFC-1058.
Routed
The routed program, which does RIP for 4.2BSD systems, has many
options. One of the most frequently used is: "routed -q" (quiet
mode) which means listen to RIP information, but never broadcast
it. This would be used by a machine on a network with multiple
RIP speaking gateways. It allows the host to determine which
gateway is best (hopwise) to use to reach a distant network. (Of
course, you might want to have a default gateway to prevent having
to pass all the addresses known to the Internet around with RIP.)
There are two ways to insert static routes into routed; the
/etc/gateways file, and the "route add" command. Static routes
are useful if you know how to reach a distant network, but you are
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not receiving that route using RIP. For the most part the "route
add" command is preferable to use. The reason for this is that
the command adds the route to that machine's routing table but
does not export it through RIP. The /etc/gateways file takes
precedence over any routing information received through a RIP
update. It is also broadcast as fact in RIP updates produced by
the host without question, so if a mistake is made in the
/etc/gateways file, that mistake will soon permeate the RIP space
and may bring the network to its knees.
One of the problems with routed is that you have very little
control over what gets broadcast and what doesn't. Many times in
larger networks where various parts of the network are under
different administrative controls, you would like to pass on
through RIP only nets which you receive from RIP and you know are
reasonable. This prevents people from adding IP addresses to the
network which may be illegal and you being responsible for passing
them on to the Internet. This type of reasonability checks are
not available with routed and leave it usable, but inadequate for
large networks.
Hello (RFC-891)
Hello is a routing protocol which was designed and implemented in
a experimental software router called a "Fuzzball" which runs on a
PDP-11. It does not have wide usage, but is the routing protocol
formerly used on the initial NSFNET backbone. The data
transferred between nodes is similar to RIP (a list of networks
and their metrics). The metric, however, is milliseconds of
delay. This allows Hello to be used over nets of various link
speeds and performs better in congestive situations.
One of the most interesting side effects of Hello based networks
is their great timekeeping ability. If you consider the problem
of measuring delay on a link for the metric, you find that it is
not an easy thing to do. You cannot measure round trip time since
the return link may be more congested, of a different speed, or
even not there. It is not really feasible for each node on the
network to have a builtin WWV (nationwide radio time standard)
receiver. So, you must design an algorithm to pass around time
between nodes over the network links where the delay in
transmission can only be approximated. Hello routers do this and
in a nationwide network maintain synchronized time within
milliseconds. (See also the Network Time Protocol, RFC-1059.)
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Gateway Gateway Protocol (GGP RFC-823)
The core gateways originally used GGP to exchange information
among themselves. This is a "distance-vector" algorithm. The new
core gateways use a "link-state" algorithm.
NSFNET SPF (RFC-1074)
The current NSFNET Backbone routers use a version of the ANSI IS-
IS and ISO ES-IS routing protocol. This is a "shortest path
first" (SPF) algorithm which is in the class of "link-state"
algorithms.
Exterior Gateway Protocol (EGP RFC-904)
EGP is not strictly a routing protocol, it is a reachability
protocol. It tells what nets can be reached through what gateway,
but not how good the connection is. It is the standard by which
gateways exchange network reachability information with the core
gateways. It is generally used between autonomous systems. There
is a metric passed around by EGP, but its usage is not
standardized formally. The metric's value ranges from 0 to 255
with smaller values considered "better". Some implementations
consider the value 255 to mean unreachable. Many routers talk EGP
so they can be used to interface to routers of different
manufacture or operated by different administrations. For
example, when a router of the NSFNET Backbone exchanges routing or
reachability information with a gateway of a regional network EGP
is used.
Gated
So we have regional and campus networks talking RIP among
themselves and the DDN and NSFNET speaking EGP. How do they
interoperate? In the beginning, there was static routing. The
problem with doing static routing in the middle of the network is
that it is broadcast to the Internet whether it is usable or not.
Therefore, if a net becomes unreachable and you try to get there,
dynamic routing will immediately issue a net unreachable to you.
Under static routing the routers would think the net could be
reached and would continue trying until the application gave up
(in 2 or more minutes). Mark Fedor, then of Cornell, attempted to
solve these problems with a replacement for routed called gated.
Gated talks RIP to RIP speaking hosts, EGP to EGP speakers, and
Hello to Hello'ers. These speakers frequently all live on one
Ethernet, but luckily (or unluckily) cannot understand each others
ruminations. In addition, under configuration file control it can
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filter the conversion. For example, one can produce a
configuration saying announce RIP nets via Hello only if they are
specified in a list and are reachable by way of a RIP broadcast as
well. This means that if a rogue network appears in your local
site's RIP space, it won't be passed through to the Hello side of
the world. There are also configuration options to do static
routing and name trusted gateways.
This may sound like the greatest thing since sliced bread, but
there is a catch called metric conversion. You have RIP measuring
in hops, Hello measuring in milliseconds, and EGP using arbitrary
small numbers. The big questions is how many hops to a
millisecond, how many milliseconds in the EGP number 3.... Also,
remember that infinity (unreachability) is 16 to RIP, 30000 or so
to Hello, and 8 to the DDN with EGP. Getting all these metrics to
work well together is no small feat. If done incorrectly and you
translate an RIP of 16 into an EGP of 6, everyone in the ARPANET
will still think your gateway can reach the unreachable and will
send every packet in the world your way. Gated is available via
anonymous FTP from devvax.tn.cornell.edu in directory pub/gated.
Names
All routing across the network is done by means of the IP address
associated with a packet. Since humans find it difficult to remember
addresses like 128.174.5.50, a symbolic name register was set up at
the NIC where people would say, "I would like my host to be named
uiucuxc". Machines connected to the Internet across the nation would
connect to the NIC in the middle of the night, check modification
dates on the hosts file, and if modified, move it to their local
machine. With the advent of workstations and micros, changes to the
host file would have to be made nightly. It would also be very labor
intensive and consume a lot of network bandwidth. RFC-1034 and a
number of others describe Domain Name Service (DNS), a distributed
data base system for mapping names into addresses.
We must look a little more closely into what's in a name. First,
note that an address specifies a particular connection on a specific
network. If the machine moves, the address changes. Second, a
machine can have one or more names and one or more network addresses
(connections) to different networks. Names point to a something
which does useful work (i.e., the machine) and IP addresses point to
an interface on that provider. A name is a purely symbolic
representation of a list of addresses on the network. If a machine
moves to a different network, the addresses will change but the name
could remain the same.
Domain names are tree structured names with the root of the tree at
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the right. For example:
uxc.cso.uiuc.edu
is a machine called "uxc" (purely arbitrary), within the subdomains
of the U of I, and "uiuc" (the University of Illinois at Urbana),
registered with "edu" (the set of educational institutions).
A simplified model of how a name is resolved is that on the user's
machine there is a resolver. The resolver knows how to contact
across the network a root name server. Root servers are the base of
the tree structured data retrieval system. They know who is
responsible for handling first level domains (e.g., 'edu'). What
root servers to use is an installation parameter. From the root
server the resolver finds out who provides 'edu' service. It
contacts the 'edu' name server which supplies it with a list of
addresses of servers for the subdomains (like 'uiuc'). This action
is repeated with the sub-domain servers until the final subdomain
returns a list of addresses of interfaces on the host in question.
The user's machine then has its choice of which of these addresses to
use for communication.
A group may apply for its own domain name (like 'uiuc' above). This
is done in a manner similar to the IP address allocation. The only
requirements are that the requestor have two machines reachable from
the Internet, which will act as name servers for that domain. Those
servers could also act as servers for subdomains or other servers
could be designated as such. Note that the servers need not be
located in any particular place, as long as they are reachable for
name resolution. (U of I could ask Michigan State to act on its
behalf and that would be fine.) The biggest problem is that someone
must do maintenance on the database. If the machine is not
convenient, that might not be done in a timely fashion. The other
thing to note is that once the domain is allocated to an
administrative entity, that entity can freely allocate subdomains
using what ever manner it sees fit.
The Berkeley Internet Name Domain (BIND) Server implements the
Internet name server for UNIX systems. The name server is a
distributed data base system that allows clients to name resources
and to share that information with other network hosts. BIND is
integrated with 4.3BSD and is used to lookup and store host names,
addresses, mail agents, host information, and more. It replaces the
/etc/hosts file or host name lookup. BIND is still an evolving
program. To keep up with reports on operational problems, future
design decisions, etc., join the BIND mailing list by sending a
request to Bind-Request@UCBARPA.BERKELEY.EDU. BIND can also be
obtained via anonymous FTP from ucbarpa.berkeley.edu.
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There are several advantages in using BIND. One of the most
important is that it frees a host from relying on /etc/hosts being up
to date and complete. Within the .uiuc.edu domain, only a few hosts
are included in the host table distributed by SRI. The remainder are
listed locally within the BIND tables on uxc.cso.uiuc.edu (the server
machine for most of the .uiuc.edu domain). All are equally reachable
from any other Internet host running BIND, or any DNS resolver.
BIND can also provide mail forwarding information for interior hosts
not directly reachable from the Internet. These hosts an either be
on non-advertised networks, or not connected to an IP network at all,
as in the case of UUCP-reachable hosts (see RFC-974). More
information on BIND is available in the "Name Server Operations Guide
for BIND" in UNIX System Manager's Manual, 4.3BSD release.
There are a few special domains on the network, like NIC.DDN.MIL.
The hosts database at the NIC. There are others of the form
NNSC.NSF.NET. These special domains are used sparingly, and require
ample justification. They refer to servers under the administrative
control of the network rather than any single organization. This
allows for the actual server to be moved around the net while the
user interface to that machine remains constant. That is, should BBN
relinquish control of the NNSC, the new provider would be pointed to
by that name.
In actuality, the domain system is a much more general and complex
system than has been described. Resolvers and some servers cache
information to allow steps in the resolution to be skipped.
Information provided by the servers can be arbitrary, not merely IP
addresses. This allows the system to be used both by non-IP networks
and for mail, where it may be necessary to give information on
intermediate mail bridges.
What's wrong with Berkeley Unix
University of California at Berkeley has been funded by DARPA to
modify the Unix system in a number of ways. Included in these
modifications is support for the Internet protocols. In earlier
versions (e.g., BSD 4.2) there was good support for the basic
Internet protocols (TCP, IP, SMTP, ARP) which allowed it to perform
nicely on IP Ethernets and smaller Internets. There were
deficiencies, however, when it was connected to complicated networks.
Most of these problems have been resolved under the newest release
(BSD 4.3). Since it is the springboard from which many vendors have
launched Unix implementations (either by porting the existing code or
by using it as a model), many implementations (e.g., Ultrix) are
still based on BSD 4.2. Therefore, many implementations still exist
with the BSD 4.2 problems. As time goes on, when BSD 4.3 trickles
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through vendors as new release, many of the problems will be
resolved. Following is a list of some problem scenarios and their
handling under each of these releases.
ICMP redirects
Under the Internet model, all a system needs to know to get
anywhere in the Internet is its own address, the address of where
it wants to go, and how to reach a gateway which knows about the
Internet. It doesn't have to be the best gateway. If the system
is on a network with multiple gateways, and a host sends a packet
for delivery to a gateway which feels another directly connected
gateway is more appropriate, the gateway sends the sender a
message. This message is an ICMP redirect, which politely says,
"I'll deliver this message for you, but you really ought to use
that gateway over there to reach this host". BSD 4.2 ignores
these messages. This creates more stress on the gateways and the
local network, since for every packet sent, the gateway sends a
packet to the originator. BSD 4.3 uses the redirect to update its
routing tables, will use the route until it times out, then revert
to the use of the route it thinks is should use. The whole
process then repeats, but it is far better than one per packet.
Trailers
An application (like FTP) sends a string of octets to TCP which
breaks it into chunks, and adds a TCP header. TCP then sends
blocks of data to IP which adds its own headers and ships the
packets over the network. All this prepending of the data with
headers causes memory moves in both the sending and the receiving
machines. Someone got the bright idea that if packets were long
and they stuck the headers on the end (they became trailers), the
receiving machine could put the packet on the beginning of a page
boundary and if the trailer was OK merely delete it and transfer
control of the page with no memory moves involved. The problem is
that trailers were never standardized and most gateways don't know
to look for the routing information at the end of the block. When
trailers are used, the machine typically works fine on the local
network (no gateways involved) and for short blocks through
gateways (on which trailers aren't used). So TELNET and FTP's of
very short files work just fine and FTP's of long files seem to
hang. On BSD 4.2 trailers are a boot option and one should make
sure they are off when using the Internet. BSD 4.3 negotiates
trailers, so it uses them on its local net and doesn't use them
when going across the network.
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Retransmissions
TCP fires off blocks to its partner at the far end of the
connection. If it doesn't receive an acknowledgement in a
reasonable amount of time it retransmits the blocks. The
determination of what is reasonable is done by TCP's
retransmission algorithm.
There is no correct algorithm but some are better than others,
where worse is measured by the number of retransmissions done
unnecessarily. BSD 4.2 had a retransmission algorithm which
retransmitted quickly and often. This is exactly what you would
want if you had a bunch of machines on an Ethernet (a low delay
network of large bandwidth). If you have a network of relatively
longer delay and scarce bandwidth (e.g., 56kb lines), it tends to
retransmit too aggressively. Therefore, it makes the networks and
gateways pass more traffic than is really necessary for a given
conversation. Retransmission algorithms do adapt to the delay of
the network after a few packets, but 4.2's adapts slowly in delay
situations. BSD 4.3 does a lot better and tries to do the best
for both worlds. It fires off a few retransmissions really
quickly assuming it is on a low delay network, and then backs off
very quickly. It also allows the delay to be about 4 minutes
before it gives up and declares the connection broken.
Even better than the original 4.3 code is a version of TCP with a
retransmission algorithm developed by Van Jacobson of LBL. He did
a lot of research into how the algorithm works on real networks
and modified it to get both better throughput and be friendlier to
the network. This code has been integrated into the later
releases of BSD 4.3 and can be fetched anonymously from
ucbarpa.berkeley.edu in directory 4.3.
Time to Live
The IP packet header contains a field called the time to live
(TTL) field. It is decremented each time the packet traverses a
gateway. TTL was designed to prevent packets caught in routing
loops from being passed forever with no hope of delivery. Since
the definition bears some likeness to the RIP hop count, some
misguided systems have set the TTL field to 15 because the
unreachable flag in RIP is 16. Obviously, no networks could have
more than 15 hops. The RIP space where hops are limited ends when
RIP is not used as a routing protocol any more (e.g., when NSFnet
starts transporting the packet). Therefore, it is quite easy for
a packet to require more than 15 hops. These machines will
exhibit the behavior of being able to reach some places but not
others even though the routing information appears correct.
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Solving the problem typically requires kernel patches so it may be
difficult if source is not available.
Appendix A - References to Remedial Information
-----------------------------------------------
[1] Quarterman and Hoskins, "Notable Computer Networks",
Communications of the ACM, Vol. 29, No. 10, pp. 932-971, October
1986.
[2] Tannenbaum, A., "Computer Networks", Prentice Hall, 1981.
[3] Hedrick, C., "Introduction to the Internet Protocols", Via
Anonymous FTP from topaz.rutgers.edu, directory pub/tcp-ip-docs,
file tcp-ip-intro.doc.
[4] Comer, D., "Internetworking with TCP/IP: Principles, Protocols,
and Architecture", Copyright 1988, by Prentice-Hall, Inc.,
Englewood Cliffs, NJ, 07632 ISBN 0-13-470154-2.
Appendix B - List of Major RFCs
-------------------------------
This list of key "Basic Beige" RFCs was compiled by J.K. Reynolds. This
is the 30 August 1989 edition of the list.
RFC-768 User Datagram Protocol (UDP)
RFC-791 Internet Protocol (IP)
RFC-792 Internet Control Message Protocol (ICMP)
RFC-793 Transmission Control Protocol (TCP)
RFC-821 Simple Mail Transfer Protocol (SMTP)
RFC-822 Standard for the Format of ARPA Internet Text Messages
RFC-826 Ethernet Address Resolution Protocol
RFC-854 Telnet Protocol
RFC-862 Echo Protocol
RFC-894 A Standard for the Transmission of IP
Datagrams over Ethernet Networks
RFC-904 Exterior Gateway Protocol
RFC-919 Broadcasting Internet Datagrams
RFC-922 Broadcasting Internet Datagrams in the Presence of Subnets
RFC-950 Internet Standard Subnetting Procedure
RFC-951 Bootstrap Protocol (BOOTP)
RFC-959 File Transfer Protocol (FTP)
RFC-966 Host Groups: A Multicast Extension to the Internet Protocol
RFC-974 Mail Routing and the Domain System
RFC-1000 The Request for Comments Reference Guide
RFC-1009 Requirements for Internet Gateways
RFC-1010 Assigned Numbers
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RFC-1011 Official Internet Protocols
RFC-1012 Bibliography of Request for Comments 1 through 999
RFC-1034 Domain Names - Concepts and Facilities
RFC-1035 Domain Names - Implementation
RFC-1042 A Standard for the Transmission of IP
Datagrams over IEEE 802 Networks
RFC-1048 BOOTP Vendor Information Extensions
RFC-1058 Routing Information Protocol
RFC-1059 Network Time Protocol (NTP)
RFC-1065 Structure and Identification of
Management Information for TCP/IP-based internets
RFC-1066 Management Information Base for Network
Management of TCP/IP-based internets
RFC-1084 BOOTP Vendor Information Extensions
RFC-1087 Ethics and the Internet
RFC-1095 The Common Management Information
Services and Protocol over TCP/IP (CMOT)
RFC-1098 A Simple Network Management Protocol (SNMP)
RFC-1100 IAB Official Protocol Standards
RFC-1101 DNS Encoding of Network Names and Other Types
RFC-1112 Host Extensions for IP Multicasting
RFC-1117 Internet Numbers
Note: This list is a portion of a list of RFC's by topic that may be
retrieved from the NIC under NETINFO:RFC-SETS.TXT (anonymous FTP, of
course).
The following list is not necessary for connection to the Internet,
but is useful in understanding the domain system, mail system, and
gateways:
RFC-974 Mail Routing and the Domain System
RFC-1009 Requirements for Internet Gateways
RFC-1034 Domain Names - Concepts and Facilities
RFC-1035 Domain Names - Implementation and Specification
RFC-1101 DNS Encoding of Network Names and Other Types
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Appendix C - Contact Points for Network Information
---------------------------------------------------
Network Information Center (NIC)
DDN Network Information Center
SRI International, Room EJ291
333 Ravenswood Avenue
Menlo Park, CA 94025
(800) 235-3155 or (415) 859-3695
NIC@NIC.DDN.MIL
NSF Network Service Center (NNSC)
NNSC
BBN Systems and Technology Corporation
10 Moulton St.
Cambridge, MA 02238
(617) 873-3400
NNSC@NNSC.NSF.NET
NSF Network Information Service (NIS)
NIS
Merit Inc.
University of Michigan
1075 Beal Avenue
Ann Arbor, MI 48109
(313) 763-4897
INFO@NIS.NSF.NET
CIC
CSNET Coordination and Information Center
Bolt Beranek and Newman Inc.
10 Moulton Street
Cambridge, MA 02238
(617) 873-2777
INFO@SH.CS.NET
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Glossary
--------
autonomous system
A set of gateways under a single administrative control and using
compatible and consistent routing procedures. Generally speaking,
the gateways run by a particular organization. Since a gateway is
connected to two (or more) networks it is not usually correct to
say that a gateway is in a network. For example, the gateways
that connect regional networks to the NSF Backbone network are run
by Merit and form an autonomous system. Another example, the
gateways that connect campuses to NYSERNET are run by NYSER and
form an autonomous system.
core gateway
The innermost gateways of the Internet. These gateways have a
total picture of the reachability to all networks known to the
Internet. They then redistribute reachability information to
their neighbor gateways speaking EGP. It is from them your EGP
agent (there is one acting for you somewhere if you can reach the
core of the Internet) finds out it can reach all the nets on the
Internet. Which is then passed to you via Hello, gated, RIP. The
core gateways mostly connect campuses to the ARPANET, or
interconnect the ARPANET and the MILNET, and are run by BBN.
count to infinity
The symptom of a routing problem where routing information is
passed in a circular manner through multiple gateways. Each
gateway increments the metric appropriately and passes it on. As
the metric is passed around the loop, it increments to ever
increasing values until it reaches the maximum for the routing
protocol being used, which typically denotes a link outage.
hold down
When a router discovers a path in the network has gone down
announcing that that path is down for a minimum amount of time
(usually at least two minutes). This allows for the propagation
of the routing information across the network and prevents the
formation of routing loops.
split horizon
When a router (or group of routers working in consort) accept
routing information from multiple external networks, but do not
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pass on information learned from one external network to any
others. This is an attempt to prevent bogus routes to a network
from being propagated because of gossip or counting to infinity.
DDN
Defense Data Network the collective name for the ARPANET and
MILNET. Used frequently because although they are seperate
networks the operational and informational foci are the same.
Security Considerations
Security and privacy protection is a serious matter and too often
nothing is done about it. There are some known security bugs
(especially in access control) in BSD Unix and in some
implementations of network services. The hitchhikers guide does not
discuss these issues (too bad).
Author's Address
Ed Krol
University of Illinois
195 DCL
1304 West Springfield Avenue
Urbana, IL 61801-4399
Phone: (217) 333-7886
EMail: Krol@UXC.CSO.UIUC.EDU
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