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The Hitchhikers Guide to the Internet
25 August 1987
Ed Krol
krol@uxc.cso.uiuc.edu
This document was produced through funding of the National
Science Foundation.
Copyright (C) 1987, by the Board of Trustees of The University
of Illinois. Permission to duplicate this document, in whole
or part, is granted provided reference is made to the source
and this copyright is included in whole copies.
This document assumes that one is familiar with the workings
of a non-connected simple IP network (e.g. a few 4.2 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).
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 IP
(Internet Protocol). 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.
Two groups do much of the research and information work of
the Internet (ISI and SRI). ISI (the Informational Sciences
Institute) does much of the research, standardization, and
allocation work of the Internet. SRI International provides
information services for the Internet. In fact, after you
are connected to the Internet most of the information in
this document can be retrieved from the Network Information
Center (NIC) run by SRI.
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 DARPA. Their facility is
called the Network Operations Center or NOC. Cornell
University temporarily operates NSFnet (called the Network
Information Service Center, NISC). It goes on to the
-2-
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 (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.
The RFCs can be divided into five groups: required, suggested,
directional, informational and obsolete. Required RFC's (e.g.
RFC-791, The Internet Protocol) must be implemented on any host
connected to the Internet. Suggested RFCs 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 suggested RFC. Directional
RFCs 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 (RFC-891 Hello). It is suggested that should the
facility be required by a particular site, animplementation
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-990, 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 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.
-3-
(Appendix B contains a list of what are considered to be the
major RFCs necessary for understanding the Internet).
The Network Information Center
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 sri-nic.arpa
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 sri-nic.arpa
Connected to sri-nic.arpa.
220 SRI_NIC.ARPA FTP Server Process 5Z(47)-6 at Wed
17-Jun-87 12:00 PDT
Name (sri-nic.arpa: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@SRI-NIC.ARPA General user assistance, document requests
REGISTRAR@SRI-NIC.ARPA User registration and WHOIS updates
HOSTMASTER@SRI-NIC.ARPA Hostname and domain changes and updates
ACTION@SRI-NIC.ARPA SRI-NIC computer operations
SUGGESTIONS@SRI-NIC.ARPA Comments on NIC publications and services
-4-
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) is funded by NSF to
provide a first level of aid to users of NSFnet should they
have questions or encounter problems traversing the network.
It is run by BBN Inc. Karen Roubicek
(roubicek@nnsc.nsf.net) is the NNSC user liaison.
The NNSC, which currently has information and documents
online and in printed form, plans to distribute news through
network mailing lists, bulletins, newsletters, and online
reports. The NNSC also maintains a database of contact
points and sources of additional information about NSFnet
component networks and supercomputer centers.
Prospective or current users who do not know whom to call
concerning questions about NSFnet use, should contact the
NNSC. The NNSC will answer general questions, and, for
detailed information relating to specific components of the
Internet, will help users find the appropriate contact for
further assistance. (Appendix B)
Mail Reflectors
The way most people keep up to date on network news is
through subscription to a number of mail reflectors. 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@NNSC.NSF.NET, one sends a request to
-5-
NSFNET-REQUEST@NNSC.NSF.NET. 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) 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, and NETINFO:INTEREST-GROUPS-
3.TXT.
The NSFNET 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 ISI. The allocation process consists of getting
an application form received from ISI. (Send a message
to hostmaster@sri-nic.arpa and ask for the template for a
connected address). This template is filled out and mailed
back to 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. It was
seen from the beginning that some networks might choose to
organize themselves as very flat (one net with a lot of nodes)
and some might organize hierarchically
-6-
(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 top two bits in the first octet are coded to tell the address
format. All of the class A nets have been allocated. So one
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 now).
In the past sites requiring multiple network addresses
requested multiple discrete addresses (usually Class C).
This was done because much of the software available
(not ably 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 300
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-932) 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
-7-
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. 4.2 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
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 300. The software in a
group of critical gateways (called the core gateways of
the ARPAnet) 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. By the
summer of '88 the current PDP-11 core gateways will be
replaced with BBN Butterfly gateways which will solve
the problem.
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.
-8-
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
Many portions of the ARPAnet are very congested during
the busy part of the day. Additional links are planned
to alleviate this congestion, but the implementation
will take a few months.
These problems and the future direction of the Internet are
determined by the Internet Architect (Dave Clark of MIT)
being advised by the Internet Activities Board (IAB). This
board is composed of chairmen of a number of committees with
responsibility for various specialized areas of the Internet.
The committees composing the IAB and their chairmen are:
Committee Chair
Autonomous Networks Deborah Estrin
End-to-End Services Bob Braden
Internet Architecture Dave Mills
Internet Engineering Phil Gross
EGP2 Mike Petry
Name Domain Planning Doug Kingston
Gateway Monitoring Craig Partridge
Internic Jake Feinler
Performance & Congestion ControlRobert Stine
NSF Routing Chuck Hedrick
Misc. MilSup Issues Mike St. Johns
Privacy Steve Kent
IRINET Requirements Vint Cerf
Robustness & Survivability Jim Mathis
Scientific Requirements Barry Leiner
Note that under Internet Engineering, there are a set of
task forces and chairs to look at short term concerns. The
chairs of these task forces are not part of the IAB.
-9-
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
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, regional...).
-10-
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 reachablity
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.
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
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 net-
works 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 net-
work. The diameter of a network is defined as the
largest hop-count possible within a network. Unfor-
tunately, 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
-11-
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.
The protocol is not well documented. A group of people
are working on producing an RFC to both define the
current RIP and to do some extensions to it to allow it
to better cope with larger networks. Currently, the
best documentation for RIP appears to be the code to
BSD "routed".
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 infor-
mation but never broadcast it. This would be used by a
machine on a network with multiple RIP speaking gate-
ways. 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 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
-12-
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 currently used
on the 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.
Exterior Gateway Protocol (EGP RFC-904)
EGP is not strictly a routing protocol, it is a reacha-
bility protocol. It tells only if nets can be reached
through a particular gateway, not how good the connec-
tion is. It is the standard by which gateways to local
nets inform the ARPAnet of the nets they can reach.
There is a metric passed around by EGP but its usage is
not standardized formally. Its typical value is value
is 1 to 8 which are arbitrary goodness of link values
understood by the internal DDN gateways. The smaller
the value the better and a value of 8 being unreach-
able. A quirk of the protocol prevents distinguishing
between 1 and 2, 3 and 4..., so the usablity of this as
a metric is as three values and unreachable. Within
NSFnet the values used are 1, 3, and unreachable. Many
routers talk EGP so they can be used for ARPAnet gateways.
-13-
Gated
So we have regional and campus networks talking RIP
among themselves, the NSFnet backbone talking
Hello, and the DDN speaking EGP.
How do they interoperate? In the beginning there was
static routing, assembled into the Fuzzball software
configured for each site. 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 of Cornell
(fedor@devvax.tn.cornell.edu) 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
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. For
these reasons, Mark requests that you consult closely
with him when configuring and using "gated".
-14-
"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-882 and a number of others describe
domain name service, 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 connec-
tion 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 the right. For example:
uxc.cso.uiuc.edu
is a machine called 'uxc' (purely arbitrary), within the
subdomains method of allocation 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 subdomain servers until the final sub-
domain 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.
-15-
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 reach-
able 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 net-
work 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 for
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@ucbarp.Berkeley.EDU". BIND can also
be obtained via anonymous FTP from ucbarpa.berkley.edu.
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.
BIND can also provide mail forwarding information for inte-
rior hosts not directly reachable from the Internet. These
hosts can either be on non-advertised networks, or not con-
nected to a network at all, as in the case of UUCP-reachable
hosts. 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 SRI-
NIC.ARPA. The 'arpa' domain is historical, referring to
hosts registered in the old 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
-16-
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 ether-
nets and smaller Internets. There were deficiencies, how-
ever, 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 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 gate-
way over there to reach this host". BSD 4.2 ignores
these messages. This creates more stress on the gate-
ways and the local network, since for every packet
-17-
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.
Retransmissions
TCP fires off blocks to its partner at the far end of
the connection. If it doesn't receive an acknowledge-
ment 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 better is measured by the number of retransmis-
sions 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. Retransmis-
sion algorithms do adapt to the delay of the network
-18-
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.
-19-
Appendix A
References to Remedial Information
Quaterman and Hoskins, "Notable Computer Networks",
Communications of the ACM, Vol 29, #10, pp. 932-971
(October, 1986).
Tannenbaum, Andrew S., Computer Networks, Prentice
Hall, 1981.
Hedrick, Chuck, Introduction to the Internet Protocols,
Anonymous FTP from topaz.rutgers.edu, directory
pub/tcp-ip-docs, file tcp-ip-intro.doc.
-20-
Appendix B
List of Major RFCs
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-854 Telnet Protocol
RFC-917 * Internet Subnets
RFC-919 * Broadcasting Internet Datagrams
RFC-922 * Broadcasting Internet Datagrams in the Presence of Subnets
RFC-940 * Toward an Internet Standard Scheme for Subnetting
RFC-947 * Multi-network Broadcasting within the Internet
RFC-950 * Internet Standard Subnetting Procedure
RFC-959 File Transfer Protocol (FTP)
RFC-966 * Host Groups: A Multicast Extension to the Internet Protocol
RFC-988 * Host Extensions for IP Multicasting
RFC-997 * Internet Numbers
RFC-1010 * Assigned Numbers
RFC-1011 * Official ARPA-Internet Protocols
RFC's marked with the asterisk (*) are not included in
the 1985 DDN Protocol Handbook.
Note: This list is a portion of a list of RFC's by
topic 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-882 Domain Names - Concepts and Facilities
RFC-883 Domain Names - Implementation
RFC-973 Domain System Changes and Observations
RFC-974 Mail Routing and the Domain System
RFC-1009 Requirements for Internet Gateways
-21-
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@SRI-NIC.ARPA
NSF Network Service Center (NNSC)
NNSC
BBN Laboratories Inc.
10 Moulton St.
Cambridge, MA 02238
(617) 497-3400
NNSC@NNSC.NSF.NET
-22-
Glossary
core gateway
The innermost gateways of the ARPAnet. These
gateways have a total picture of the reacha-
bility to all networks known to the ARPAnet
with EGP. They then redistribute reachabil-
ity information to all those gateways speak-
ing EGP. It is from them your EGP agent
(there is one acting for you somewhere if you
can reach the ARPAnet) finds out it can reach
all the nets on the ARPAnet. Which is then
passed to you via Hello, gated, RIP....
count to infinity
The symptom of a routing problem where
routing information is passed in a circular
manner through multiple gateways. Each gate-
way increments the metric appropriately and
passes it on. As the metric is passed around
the loop, it increments to ever increasing
values til 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 pro-
pagation 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 mul-
tiple external networks, but do not 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.
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