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.


   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

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.


   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

   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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RFC 1118         The Hitchhikers Guide to the Internet    September 1989

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


   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).

     Connected to
     220 NIC.DDN.MIL FTP Server 5Z(47)-6 at Wed 17-Jun-87 12:00 PDT
     Name ( 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 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-

   Questions of the NIC or problems with services can be asked of or
   reported to using electronic mail.  The following addresses can be

     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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RFC 1118         The Hitchhikers Guide to the Internet    September 1989

   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 in the directory resource-guide, or by joining
   the resource guide mailing list (send a subscription request to

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

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   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.,  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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   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

   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

           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 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

   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

   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

   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 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

      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.


      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"

   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.


      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 in directory pub/gated.


   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, 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:


   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

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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 domain, only a few hosts
   are included in the host table distributed by SRI.  The remainder are
   listed locally within the BIND tables on (the server
   machine for most of the 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.


      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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      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 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

  [2]  Tannenbaum, A., "Computer Networks", Prentice Hall, 1981.

  [3]  Hedrick, C., "Introduction to the Internet Protocols", Via
       Anonymous FTP from, 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

The following list is not necessary for connection to the Internet,
but is useful in understanding the domain system, mail system, and

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


NSF Network Service Center (NNSC)

      BBN Systems and Technology Corporation
      10 Moulton St.
      Cambridge, MA 02238
      (617) 873-3400


NSF Network Information Service (NIS)

      Merit Inc.
      University of Michigan
      1075 Beal Avenue
      Ann Arbor, MI 48109
      (313) 763-4897



      CSNET Coordination and Information Center
      Bolt Beranek and Newman Inc.
      10 Moulton Street
      Cambridge, MA 02238
      (617) 873-2777


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   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.


      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


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