RFC1118

Network Working Group E. Krol Request for Comments: 1118 University of Illinois Urbana

                                                         September 1989

                The Hitchhikers Guide to the Internet

Status of this Memo

  This RFC is being distributed to members of the Internet community in
  order to make available some "hints" which will allow new network
  participants to understand how the direction of the Internet is set,
  how to acquire online information and how to be a good Internet
  neighbor.  While the information discussed may not be relevant to the
  research problems of the Internet, it may be interesting to a number
  of researchers and implementors.  No standards are defined or
  specified in this memo.  Distribution of this memo is unlimited.

NOTICE:

  The hitchhikers guide to the Internet is a very unevenly edited memo
  and contains many passages which simply seemed to its editors like a
  good idea at the time.  It is an indispensable companion to all those
  who are keen to make sense of life in an infinitely complex and
  confusing Internet, for although it cannot hope to be useful or
  informative on all matters, it does make the reassuring claim that
  where it is inaccurate, it is at least definitively inaccurate.  In
  cases of major discrepancy it is always reality that's got it wrong.
  And remember, DON'T PANIC.  (Apologies to Douglas Adams.)

Purpose and Audience

  This document assumes that one is familiar with the workings of a
  non-connected simple IP network (e.g., a few 4.3 BSD systems on an
  Ethernet not connected to anywhere else).  Appendix A contains
  remedial information to get one to this point.  Its purpose is to get
  that person, familiar with a simple net, versed in the "oral
  tradition" of the Internet to the point that that net can be
  connected to the Internet with little danger to either.  It is not a
  tutorial, it consists of pointers to other places, literature, and
  hints which are not normally documented.  Since the Internet is a
  dynamic environment, changes to this document will be made regularly.
  The author welcomes comments and suggestions.  This is especially
  true of terms for the glossary (definitions are not necessary).

Krol [Page 1]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

What is the Internet?

  In the beginning there was the ARPANET, a wide area experimental
  network connecting hosts and terminal servers together.  Procedures
  were set up to regulate the allocation of addresses and to create
  voluntary standards for the network.  As local area networks became
  more pervasive, many hosts became gateways to local networks.  A
  network layer to allow the interoperation of these networks was
  developed and called Internet Protocol (IP).  Over time other groups
  created long haul IP based networks (NASA, NSF, states...).  These
  nets, too, interoperate because of IP.  The collection of all of
  these interoperating networks is the Internet.

  A few groups provide much of the information services on the
  Internet.  Information Sciences Institute (ISI) does much of the
  standardization and allocation work of the Internet acting as the
  Internet Assigned Numbers Authority (IANA).  SRI International
  provides the principal information services for the Internet by
  operating the Network Information Center (NIC).  In fact, after you
  are connected to the Internet most of the information in this
  document can be retrieved from the SRI-NIC.  Bolt Beranek and Newman
  (BBN) provides information services for CSNET (the CIC) and NSFNET
  (the NNSC), and Merit provides information services for NSFNET (the
  NIS).

Operating the Internet

  Each network, be it the ARPANET, NSFNET or a regional network, has
  its own operations center.  The ARPANET is run by BBN, Inc. under
  contract from DCA (on behalf of DARPA).  Their facility is called the
  Network Operations Center or NOC.  Merit, Inc. operates NSFNET from
  yet another and completely seperate NOC.  It goes on to the regionals
  having similar facilities to monitor and keep watch over the goings
  on of their portion of the Internet.  In addition, they all should
  have some knowledge of what is happening to the Internet in total.
  If a problem comes up, it is suggested that a campus network liaison
  should contact the network operator to which he is directly
  connected.  That is, if you are connected to a regional network
  (which is gatewayed to the NSFNET, which is connected to the
  ARPANET...) and have a problem, you should contact your regional
  network operations center.

RFCs

  The internal workings of the Internet are defined by a set of
  documents called RFCs (Request for Comments).  The general process
  for creating an RFC is for someone wanting something formalized to
  write a document describing the issue and mailing it to Jon Postel

Krol [Page 2]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

  ([email protected]).  He acts as a referee for the proposal.  It is then
  commented upon by all those wishing to take part in the discussion
  (electronically of course).  It may go through multiple revisions.
  Should it be generally accepted as a good idea, it will be assigned a
  number and filed with the RFCs.

  There are two independent categorizations of protocols.  The first is
  the state of standardization which is one of "standard", "draft
  standard", "proposed", "experimental", or "historic".  The second is
  the status of this protocol which is one of "required",
  "recommended", "elective", or "not recommended".  One could expect a
  particular protocol to move along the scale of status from elective
  to required at the same time as it moves along the scale of
  standardization from proposed to standard.

  A Required Standard protocol (e.g., RFC-791, The Internet Protocol)
  must be implemented on any host connected to the Internet.
  Recommended Standard protocols are generally implemented by network
  hosts.  Lack of them does not preclude access to the Internet, but
  may impact its usability.  RFC-793 (Transmission Control Protocol) is
  a Recommended Standard protocol.  Elective Proposed protocols were
  discussed and agreed to, but their application has never come into
  wide use.  This may be due to the lack of wide need for the specific
  application (RFC-937, The Post Office Protocol) or that, although
  technically superior, ran against other pervasive approaches.  It is
  suggested that should the facility be required by a particular site,
  an implementation be done in accordance with the RFC.  This insures
  that, should the idea be one whose time has come, the implementation
  will be in accordance with some standard and will be generally
  usable.

  Informational RFCs contain factual information about the Internet and
  its operation (RFC-1010, Assigned Numbers).  Finally, as the Internet
  and technology have grown, some RFCs have become unnecessary.  These
  obsolete RFCs cannot be ignored, however.  Frequently when a change
  is made to some RFC that causes a new one to be issued obsoleting
  others, the new RFC may only contains explanations and motivations
  for the change.  Understanding the model on which the whole facility
  is based may involve reading the original and subsequent RFCs on the
  topic.  (Appendix B contains a list of what are considered to be the
  major RFCs necessary for understanding the Internet).

  Only a few RFCs actually specify standards, most RFCs are for
  information or discussion purposes.  To find out what the current
  standards are see the RFC titled "IAB Official Protocol Standards"
  (most recently published as RFC-1100).

Krol [Page 3]

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

     %telnet nic.ddn.mil

  on a BSD system, and following the directions provided by a user
  friendly prompter.  From poking around in the databases provided, one
  might decide that a document named NETINFO:NUG.DOC (The Users Guide
  to the ARPANET) would be worth having.  It could be retrieved via an
  anonymous FTP.  An anonymous FTP would proceed something like the
  following.  (The dialogue may vary slightly depending on the
  implementation of FTP you are using).

    %ftp nic.ddn.mil
    Connected to nic.ddn.mil
    220 NIC.DDN.MIL FTP Server 5Z(47)-6 at Wed 17-Jun-87 12:00 PDT
    Name (nic.ddn.mil:myname): anonymous
    331 ANONYMOUS user ok, send real ident as password.
    Password: myname
    230 User ANONYMOUS logged in at Wed 17-Jun-87 12:01 PDT, job 15.
    ftp> get netinfo:nug.doc
    200 Port 18.144 at host 128.174.5.50 accepted.
    150 ASCII retrieve of <NETINFO>NUG.DOC.11 started.
    226 Transfer Completed 157675 (8) bytes transferred
    local: netinfo:nug.doc  remote:netinfo:nug.doc
    157675 bytes in 4.5e+02 seconds (0.34 Kbytes/s)
    ftp> quit
    221 QUIT command received. Goodbye.

  (Another good initial document to fetch is NETINFO:WHAT-THE-NIC-
  DOES.TXT).

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

    [email protected]         General user assistance, document requests
    [email protected]   User registration and WHOIS updates
    [email protected]  Hostname and domain changes and updates
    [email protected]      SRI-NIC computer operations
    [email protected] Comments on NIC publications and services

Krol [Page 4]

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 nnsc.nsf.net in the directory resource-guide, or by joining
  the resource guide mailing list (send a subscription request to
  [email protected].)

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 [email protected], one sends a
  request to [email protected].  This may be a wonderful
  scheme, but the problem is that you must know the list exists in the
  first place.  It is suggested that, if you are interested, you read
  the mail from one list (like NSFNET-INFO) and you will probably
  become familiar with the existence of others.  A registration service
  for mail reflectors is provided by the NIC in the files
  NETINFO:INTEREST-GROUPS-1.TXT, NETINFO:INTEREST-GROUPS-2.TXT,

Krol [Page 5]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

  NETINFO:INTEREST-GROUPS-3.TXT, through NETINFO:INTEREST-GROUPS-9.TXT.

  The NSFNET-INFO mail reflector is targeted at those people who have a
  day to day interest in the news of the NSFNET (the backbone, regional
  network, and Internet inter-connection site workers).  The messages
  are reflected by a central location and are sent as separate messages
  to each subscriber.  This creates hundreds of messages on the wide
  area networks where bandwidth is the scarcest.

  There are two ways in which a campus could spread the news and not
  cause these messages to inundate the wide area networks.  One is to
  re-reflect the message on the campus.  That is, set up a reflector on
  a local machine which forwards the message to a campus distribution
  list.  The other is to create an alias on a campus machine which
  places the messages into a notesfile on the topic.  Campus users who
  want the information could access the notesfile and see the messages
  that have been sent since their last access.  One might also elect to
  have the campus wide area network liaison screen the messages in
  either case and only forward those which are considered of merit.
  Either of these schemes allows one message to be sent to the campus,
  while allowing wide distribution within.

Address Allocation

  Before a local network can be connected to the Internet it must be
  allocated a unique IP address.  These addresses are allocated by
  SRI-NIC.  The allocation process consists of getting an application
  form.  Send a message to [email protected] and ask for the
  template for a connected address.  This template is filled out and
  mailed back to the hostmaster.  An address is allocated and e-mailed
  back to you.  This can also be done by postal mail (Appendix B).

  IP addresses are 32 bits long.  It is usually written as four decimal
  numbers separated by periods (e.g., 192.17.5.100).  Each number is
  the value of an octet of the 32 bits.  Some networks might choose to
  organize themselves as very flat (one net with a lot of nodes) and
  some might organize hierarchically (many interconnected nets with
  fewer nodes each and a backbone).  To provide for these cases,
  addresses were differentiated into class A, B, and C networks.  This
  classification had to with the interpretation of the octets.  Class A
  networks have the first octet as a network address and the remaining
  three as a host address on that network.  Class C addresses have
  three octets of network address and one of host.  Class B is split
  two and two.  Therefore, there is an address space for a few large
  nets, a reasonable number of medium nets and a large number of small
  nets.  The high order bits in the first octet are coded to tell the
  address format.  There are very few unallocated class A nets, so a
  very good case must be made for them.  So as a practical matter, one

Krol [Page 6]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

  has to choose between Class B and Class C when placing an order.
  (There are also class D (Multicast) and E (Experimental) formats.
  Multicast addresses will likely come into greater use in the near
  future, but are not frequently used yet).

  In the past, sites requiring multiple network addresses requested
  multiple discrete addresses (usually Class C).  This was done because
  much of the software available (notably 4.2BSD) could not deal with
  subnetted addresses.  Information on how to reach a particular
  network (routing information) must be stored in Internet gateways and
  packet switches.  Some of these nodes have a limited capability to
  store and exchange routing information (limited to about 700
  networks).  Therefore, it is suggested that any campus announce (make
  known to the Internet) no more than two discrete network numbers.

  If a campus expects to be constrained by this, it should consider
  subnetting.  Subnetting (RFC-950) allows one to announce one address
  to the Internet and use a set of addresses on the campus.  Basically,
  one defines a mask which allows the network to differentiate between
  the network portion and host portion of the address.  By using a
  different mask on the Internet and the campus, the address can be
  interpreted in multiple ways.  For example, if a campus requires two
  networks internally and has the 32,000 addresses beginning
  128.174.X.X (a Class B address) allocated to it, the campus could
  allocate 128.174.5.X to one part of campus and 128.174.10.X to
  another.  By advertising 128.174 to the Internet with a subnet mask
  of FF.FF.00.00, the Internet would treat these two addresses as one.
  Within the campus a mask of FF.FF.FF.00 would be used, allowing the
  campus to treat the addresses as separate entities. (In reality, you
  don't pass the subnet mask of FF.FF.00.00 to the Internet, the octet
  meaning is implicit in its being a class B address).

  A word of warning is necessary.  Not all systems know how to do
  subnetting.  Some 4.2BSD systems require additional software.  4.3BSD
  systems subnet as released.  Other devices and operating systems vary
  in the problems they have dealing with subnets.  Frequently, these
  machines can be used as a leaf on a network but not as a gateway
  within the subnetted portion of the network.  As time passes and more
  systems become 4.3BSD based, these problems should disappear.

  There has been some confusion in the past over the format of an IP
  broadcast address.  Some machines used an address of all zeros to
  mean broadcast and some all ones.  This was confusing when machines
  of both type were connected to the same network.  The broadcast
  address of all ones has been adopted to end the grief.  Some systems
  (e.g., 4.3 BSD) allow one to choose the format of the broadcast
  address.  If a system does allow this choice, care should be taken
  that the all ones format is chosen.  (This is explained in RFC-1009

Krol [Page 7]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

  and RFC-1010).

Internet Problems

  There are a number of problems with the Internet.  Solutions to the
  problems range from software changes to long term research projects.
  Some of the major ones are detailed below:

  Number of Networks

     When the Internet was designed it was to have about 50 connected
     networks.  With the explosion of networking, the number is now
     approaching 1000.  The software in a group of critical gateways
     (called the core gateways) are not able to pass or store much more
     than that number.  In the short term, core reallocation and
     recoding has raised the number slightly.

  Routing Issues

     Along with sheer mass of the data necessary to route packets to a
     large number of networks, there are many problems with the
     updating, stability, and optimality of the routing algorithms.
     Much research is being done in the area, but the optimal solution
     to these routing problems is still years away.  In most cases, the
     the routing we have today works, but sub-optimally and sometimes
     unpredictably.  The current best hope for a good routing protocol
     is something known as OSPFIGP which will be generally available
     from many router manufacturers within a year.

  Trust Issues

     Gateways exchange network routing information.  Currently, most
     gateways accept on faith that the information provided about the
     state of the network is correct.  In the past this was not a big
     problem since most of the gateways belonged to a single
     administrative entity (DARPA).  Now, with multiple wide area
     networks under different administrations, a rogue gateway
     somewhere in the net could cripple the Internet.  There is design
     work going on to solve both the problem of a gateway doing
     unreasonable things and providing enough information to reasonably
     route data between multiply connected networks (multi-homed
     networks).

  Capacity & Congestion

     Some portions of the Internet are very congested during the busy
     part of the day.  Growth is dramatic with some networks
     experiencing growth in traffic in excess of 20% per month.

Krol [Page 8]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

     Additional bandwidth is planned, but delivery and budgets might
     not allow supply to keep up.

Setting Direction and Priority

  The Internet Activities Board (IAB), currently chaired by Vint Cerf
  of NRI, is responsible for setting the technical direction,
  establishing standards, and resolving problems in the Internet.

  The current IAB members are:

          Vinton Cerf          - Chairman
          David Clark          - IRTF Chairman
          Phillip Gross        - IETF Chairman
          Jon Postel           - RFC Editor
          Robert Braden        - Executive Director
          Hans-Werner Braun    - NSFNET Liaison
          Barry Leiner         - CCIRN Liaison
          Daniel Lynch         - Vendor Liaison
          Stephen Kent         - Internet Security

  This board is supported by a Research Task Force (chaired by Dave
  Clark of MIT) and an Engineering Task Force (chaired by Phill Gross
  of NRI).

  The Internet Research Task Force has the following Research Groups:

           Autonomous Networks            Deborah Estrin
           End-to-End Services            Bob Braden
           Privacy                        Steve Kent
           User Interfaces                Keith Lantz

  The Internet Engineering Task Force has the following technical
  areas:

          Applications                    TBD
          Host Protocols                  Craig Partridge
          Internet Protocols              Noel Chiappa
          Routing                         Robert Hinden
          Network Management              David Crocker
          OSI Interoperability            Ross Callon, Robert Hagen
          Operations                      TBD
          Security                        TBD

  The Internet Engineering Task Force has the following Working Groups:

           ALERTMAN                       Louis Steinberg
           Authentication                 Jeff Schiller

Krol [Page 9]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

           CMIP over TCP                  Lee LaBarre
           Domain Names                   Paul Mockapetris
           Dynamic Host Config            Ralph Droms
           Host Requirements              Bob Braden
           Interconnectivity              Guy Almes
           Internet MIB                   Craig Partridge
           Joint Management               Susan Hares
           LAN Mgr MIB                    Amatzia Ben-Artzi
           NISI                           Karen Bowers
           NM Serial Interface            Jeff Case
           NOC Tools                      Bob Enger
           OSPF                           Mike Petry
           Open Systems Routing           Marianne Lepp
           OSI Interoperability           Ross Callon
           PDN Routing Group              CH Rokitansky
           Performance and CC             Allison Mankin
           Point - Point IP               Drew Perkins
           ST and CO-IP                   Claudio Topolcic
           Telnet                         Dave Borman
           User Documents                 Karen Roubicek
           User Services                  Karen Bowers

Routing

  Routing is the algorithm by which a network directs a packet from its
  source to its destination.  To appreciate the problem, watch a small
  child trying to find a table in a restaurant.  From the adult point
  of view, the structure of the dining room is seen and an optimal
  route easily chosen.  The child, however, is presented with a set of
  paths between tables where a good path, let alone the optimal one to
  the goal is not discernible.

  A little more background might be appropriate.  IP gateways (more
  correctly routers) are boxes which have connections to multiple
  networks and pass traffic between these nets.  They decide how the
  packet is to be sent based on the information in the IP header of the
  packet and the state of the network.  Each interface on a router has
  an unique address appropriate to the network to which it is
  connected.  The information in the IP header which is used is
  primarily the destination address.  Other information (e.g., type of
  service) is largely ignored at this time.  The state of the network
  is determined by the routers passing information among themselves.
  The distribution of the database (what each node knows), the form of
  the updates, and metrics used to measure the value of a connection,
  are the parameters which determine the characteristics of a routing
  protocol.

  Under some algorithms, each node in the network has complete

Krol [Page 10]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

  knowledge of the state of the network (the adult algorithm).  This
  implies the nodes must have larger amounts of local storage and
  enough CPU to search the large tables in a short enough time
  (remember, this must be done for each packet).  Also, routing updates
  usually contain only changes to the existing information (or you
  spend a large amount of the network capacity passing around megabyte
  routing updates).  This type of algorithm has several problems.
  Since the only way the routing information can be passed around is
  across the network and the propagation time is non-trivial, the view
  of the network at each node is a correct historical view of the
  network at varying times in the past.  (The adult algorithm, but
  rather than looking directly at the dining area, looking at a
  photograph of the dining room.  One is likely to pick the optimal
  route and find a bus-cart has moved in to block the path after the
  photo was taken).  These inconsistencies can cause circular routes
  (called routing loops) where once a packet enters it is routed in a
  closed path until its time to live (TTL) field expires and it is
  discarded.

  Other algorithms may know about only a subset of the network.  To
  prevent loops in these protocols, they are usually used in a
  hierarchical network.  They know completely about their own area, but
  to leave that area they go to one particular place (the default
  gateway).  Typically these are used in smaller networks (campus or
  regional).

  Routing protocols in current use:

  Static (no protocol-table/default routing)

     Don't laugh.  It is probably the most reliable, easiest to
     implement, and least likely to get one into trouble for a small
     network or a leaf on the Internet.  This is, also, the only method
     available on some CPU-operating system combinations.  If a host is
     connected to an Ethernet which has only one gateway off of it, one
     should make that the default gateway for the host and do no other
     routing.  (Of course, that gateway may pass the reachability
     information somehow on the other side of itself.)

     One word of warning, it is only with extreme caution that one
     should use static routes in the middle of a network which is also
     using dynamic routing.  The routers passing dynamic information
     are sometimes confused by conflicting dynamic and static routes.
     If your host is on an ethernet with multiple routers to other
     networks on it and the routers are doing dynamic routing among
     themselves, it is usually better to take part in the dynamic
     routing than to use static routes.

Krol [Page 11]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

  RIP

     RIP is a routing protocol based on XNS (Xerox Network System)
     adapted for IP networks.  It is used by many routers (Proteon,
     cisco, UB...) and many BSD Unix systems.  BSD systems typically
     run a program called "routed" to exchange information with other
     systems running RIP.  RIP works best for nets of small diameter
     (few hops) where the links are of equal speed.  The reason for
     this is that the metric used to determine which path is best is
     the hop-count.  A hop is a traversal across a gateway.  So, all
     machines on the same Ethernet are zero hops away.  If a router
     connects connects two networks directly, a machine on the other
     side of the router is one hop away.  As the routing information is
     passed through a gateway, the gateway adds one to the hop counts
     to keep them consistent across the network.  The diameter of a
     network is defined as the largest hop-count possible within a
     network.  Unfortunately, a hop count of 16 is defined as infinity
     in RIP meaning the link is down.  Therefore, RIP will not allow
     hosts separated by more than 15 gateways in the RIP space to
     communicate.

     The other problem with hop-count metrics is that if links have
     different speeds, that difference is not reflected in the hop-
     count.  So a one hop satellite link (with a .5 sec delay) at 56kb
     would be used instead of a two hop T1 connection.  Congestion can
     be viewed as a decrease in the efficacy of a link.  So, as a link
     gets more congested, RIP will still know it is the best hop-count
     route and congest it even more by throwing more packets on the
     queue for that link.

     RIP was originally not well documented in the community and people
     read BSD code to find out how RIP really worked.  Finally, it was
     documented in RFC-1058.

  Routed

     The routed program, which does RIP for 4.2BSD systems, has many
     options.  One of the most frequently used is: "routed -q" (quiet
     mode) which means listen to RIP information, but never broadcast
     it.  This would be used by a machine on a network with multiple
     RIP speaking gateways.  It allows the host to determine which
     gateway is best (hopwise) to use to reach a distant network.  (Of
     course, you might want to have a default gateway to prevent having
     to pass all the addresses known to the Internet around with RIP.)

     There are two ways to insert static routes into routed; the
     /etc/gateways file, and the "route add" command.  Static routes
     are useful if you know how to reach a distant network, but you are

Krol [Page 12]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

     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.)

Krol [Page 13]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

  Gateway Gateway Protocol (GGP RFC-823)

     The core gateways originally used GGP to exchange information
     among themselves.  This is a "distance-vector" algorithm.  The new
     core gateways use a "link-state" algorithm.

  NSFNET SPF (RFC-1074)

     The current NSFNET Backbone routers use a version of the ANSI IS-
     IS and ISO ES-IS routing protocol.  This is a "shortest path
     first" (SPF) algorithm which is in the class of "link-state"
     algorithms.

  Exterior Gateway Protocol (EGP RFC-904)

     EGP is not strictly a routing protocol, it is a reachability
     protocol.  It tells what nets can be reached through what gateway,
     but not how good the connection is.  It is the standard by which
     gateways exchange network reachability information with the core
     gateways.  It is generally used between autonomous systems.  There
     is a metric passed around by EGP, but its usage is not
     standardized formally.  The metric's value ranges from 0 to 255
     with smaller values considered "better".  Some implementations
     consider the value 255 to mean unreachable.  Many routers talk EGP
     so they can be used to interface to routers of different
     manufacture or operated by different administrations.  For
     example, when a router of the NSFNET Backbone exchanges routing or
     reachability information with a gateway of a regional network EGP
     is used.

  Gated

     So we have regional and campus networks talking RIP among
     themselves and the DDN and NSFNET speaking EGP.  How do they
     interoperate?  In the beginning, there was static routing.  The
     problem with doing static routing in the middle of the network is
     that it is broadcast to the Internet whether it is usable or not.
     Therefore, if a net becomes unreachable and you try to get there,
     dynamic routing will immediately issue a net unreachable to you.
     Under static routing the routers would think the net could be
     reached and would continue trying until the application gave up
     (in 2 or more minutes).  Mark Fedor, then of Cornell, attempted to
     solve these problems with a replacement for routed called gated.

     Gated talks RIP to RIP speaking hosts, EGP to EGP speakers, and
     Hello to Hello'ers.  These speakers frequently all live on one
     Ethernet, but luckily (or unluckily) cannot understand each others
     ruminations.  In addition, under configuration file control it can

Krol [Page 14]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

     filter the conversion.  For example, one can produce a
     configuration saying announce RIP nets via Hello only if they are
     specified in a list and are reachable by way of a RIP broadcast as
     well.  This means that if a rogue network appears in your local
     site's RIP space, it won't be passed through to the Hello side of
     the world.  There are also configuration options to do static
     routing and name trusted gateways.

     This may sound like the greatest thing since sliced bread, but
     there is a catch called metric conversion.  You have RIP measuring
     in hops, Hello measuring in milliseconds, and EGP using arbitrary
     small numbers.  The big questions is how many hops to a
     millisecond, how many milliseconds in the EGP number 3....  Also,
     remember that infinity (unreachability) is 16 to RIP, 30000 or so
     to Hello, and 8 to the DDN with EGP.  Getting all these metrics to
     work well together is no small feat.  If done incorrectly and you
     translate an RIP of 16 into an EGP of 6, everyone in the ARPANET
     will still think your gateway can reach the unreachable and will
     send every packet in the world your way.  Gated is available via
     anonymous FTP from devvax.tn.cornell.edu in directory pub/gated.

Names

  All routing across the network is done by means of the IP address
  associated with a packet.  Since humans find it difficult to remember
  addresses like 128.174.5.50, a symbolic name register was set up at
  the NIC where people would say, "I would like my host to be named
  uiucuxc".  Machines connected to the Internet across the nation would
  connect to the NIC in the middle of the night, check modification
  dates on the hosts file, and if modified, move it to their local
  machine.  With the advent of workstations and micros, changes to the
  host file would have to be made nightly.  It would also be very labor
  intensive and consume a lot of network bandwidth.  RFC-1034 and a
  number of others describe Domain Name Service (DNS), a distributed
  data base system for mapping names into addresses.

  We must look a little more closely into what's in a name.  First,
  note that an address specifies a particular connection on a specific
  network.  If the machine moves, the address changes.  Second, a
  machine can have one or more names and one or more network addresses
  (connections) to different networks.  Names point to a something
  which does useful work (i.e., the machine) and IP addresses point to
  an interface on that provider.  A name is a purely symbolic
  representation of a list of addresses on the network.  If a machine
  moves to a different network, the addresses will change but the name
  could remain the same.

  Domain names are tree structured names with the root of the tree at

Krol [Page 15]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

  the right.  For example:

                            uxc.cso.uiuc.edu

  is a machine called "uxc" (purely arbitrary), within the subdomains
  of the U of I, and "uiuc" (the University of Illinois at Urbana),
  registered with "edu" (the set of educational institutions).

  A simplified model of how a name is resolved is that on the user's
  machine there is a resolver.  The resolver knows how to contact
  across the network a root name server.  Root servers are the base of
  the tree structured data retrieval system.  They know who is
  responsible for handling first level domains (e.g., 'edu').  What
  root servers to use is an installation parameter. From the root
  server the resolver finds out who provides 'edu' service.  It
  contacts the 'edu' name server which supplies it with a list of
  addresses of servers for the subdomains (like 'uiuc').  This action
  is repeated with the sub-domain servers until the final subdomain
  returns a list of addresses of interfaces on the host in question.
  The user's machine then has its choice of which of these addresses to
  use for communication.

  A group may apply for its own domain name (like 'uiuc' above).  This
  is done in a manner similar to the IP address allocation.  The only
  requirements are that the requestor have two machines reachable from
  the Internet, which will act as name servers for that domain.  Those
  servers could also act as servers for subdomains or other servers
  could be designated as such.  Note that the servers need not be
  located in any particular place, as long as they are reachable for
  name resolution.  (U of I could ask Michigan State to act on its
  behalf and that would be fine.)  The biggest problem is that someone
  must do maintenance on the database.  If the machine is not
  convenient, that might not be done in a timely fashion.  The other
  thing to note is that once the domain is allocated to an
  administrative entity, that entity can freely allocate subdomains
  using what ever manner it sees fit.

  The Berkeley Internet Name Domain (BIND) Server implements the
  Internet name server for UNIX systems.  The name server is a
  distributed data base system that allows clients to name resources
  and to share that information with other network hosts.  BIND is
  integrated with 4.3BSD and is used to lookup and store host names,
  addresses, mail agents, host information, and more.  It replaces the
  /etc/hosts file or host name lookup.  BIND is still an evolving
  program.  To keep up with reports on operational problems, future
  design decisions, etc., join the BIND mailing list by sending a
  request to [email protected].  BIND can also be
  obtained via anonymous FTP from ucbarpa.berkeley.edu.

Krol [Page 16]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

  There are several advantages in using BIND.  One of the most
  important is that it frees a host from relying on /etc/hosts being up
  to date and complete.  Within the .uiuc.edu domain, only a few hosts
  are included in the host table distributed by SRI.  The remainder are
  listed locally within the BIND tables on uxc.cso.uiuc.edu (the server
  machine for most of the .uiuc.edu domain).  All are equally reachable
  from any other Internet host running BIND, or any DNS resolver.

  BIND can also provide mail forwarding information for interior hosts
  not directly reachable from the Internet.  These hosts an either be
  on non-advertised networks, or not connected to an IP network at all,
  as in the case of UUCP-reachable hosts (see RFC-974).  More
  information on BIND is available in the "Name Server Operations Guide
  for BIND" in UNIX System Manager's Manual, 4.3BSD release.

  There are a few special domains on the network, like NIC.DDN.MIL.
  The hosts database at the NIC.  There are others of the form
  NNSC.NSF.NET.  These special domains are used sparingly, and require
  ample justification.  They refer to servers under the administrative
  control of the network rather than any single organization.  This
  allows for the actual server to be moved around the net while the
  user interface to that machine remains constant.  That is, should BBN
  relinquish control of the NNSC, the new provider would be pointed to
  by that name.

  In actuality, the domain system is a much more general and complex
  system than has been described.  Resolvers and some servers cache
  information to allow steps in the resolution to be skipped.
  Information provided by the servers can be arbitrary, not merely IP
  addresses.  This allows the system to be used both by non-IP networks
  and for mail, where it may be necessary to give information on
  intermediate mail bridges.

What's wrong with Berkeley Unix

  University of California at Berkeley has been funded by DARPA to
  modify the Unix system in a number of ways.  Included in these
  modifications is support for the Internet protocols.  In earlier
  versions (e.g., BSD 4.2) there was good support for the basic
  Internet protocols (TCP, IP, SMTP, ARP) which allowed it to perform
  nicely on IP Ethernets and smaller Internets.  There were
  deficiencies, however, when it was connected to complicated networks.
  Most of these problems have been resolved under the newest release
  (BSD 4.3).  Since it is the springboard from which many vendors have
  launched Unix implementations (either by porting the existing code or
  by using it as a model), many implementations (e.g., Ultrix) are
  still based on BSD 4.2.  Therefore, many implementations still exist
  with the BSD 4.2 problems.  As time goes on, when BSD 4.3 trickles

Krol [Page 17]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

  through vendors as new release, many of the problems will be
  resolved.  Following is a list of some problem scenarios and their
  handling under each of these releases.

  ICMP redirects

     Under the Internet model, all a system needs to know to get
     anywhere in the Internet is its own address, the address of where
     it wants to go, and how to reach a gateway which knows about the
     Internet.  It doesn't have to be the best gateway.  If the system
     is on a network with multiple gateways, and a host sends a packet
     for delivery to a gateway which feels another directly connected
     gateway is more appropriate, the gateway sends the sender a
     message.  This message is an ICMP redirect, which politely says,
     "I'll deliver this message for you, but you really ought to use
     that gateway over there to reach this host".  BSD 4.2 ignores
     these messages.  This creates more stress on the gateways and the
     local network, since for every packet sent, the gateway sends a
     packet to the originator.  BSD 4.3 uses the redirect to update its
     routing tables, will use the route until it times out, then revert
     to the use of the route it thinks is should use.  The whole
     process then repeats, but it is far better than one per packet.

  Trailers

     An application (like FTP) sends a string of octets to TCP which
     breaks it into chunks, and adds a TCP header.  TCP then sends
     blocks of data to IP which adds its own headers and ships the
     packets over the network.  All this prepending of the data with
     headers causes memory moves in both the sending and the receiving
     machines.  Someone got the bright idea that if packets were long
     and they stuck the headers on the end (they became trailers), the
     receiving machine could put the packet on the beginning of a page
     boundary and if the trailer was OK merely delete it and transfer
     control of the page with no memory moves involved.  The problem is
     that trailers were never standardized and most gateways don't know
     to look for the routing information at the end of the block.  When
     trailers are used, the machine typically works fine on the local
     network (no gateways involved) and for short blocks through
     gateways (on which trailers aren't used).  So TELNET and FTP's of
     very short files work just fine and FTP's of long files seem to
     hang.  On BSD 4.2 trailers are a boot option and one should make
     sure they are off when using the Internet.  BSD 4.3 negotiates
     trailers, so it uses them on its local net and doesn't use them
     when going across the network.

Krol [Page 18]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

  Retransmissions

     TCP fires off blocks to its partner at the far end of the
     connection.  If it doesn't receive an acknowledgement in a
     reasonable amount of time it retransmits the blocks.  The
     determination of what is reasonable is done by TCP's
     retransmission algorithm.

     There is no correct algorithm but some are better than others,
     where worse is measured by the number of retransmissions done
     unnecessarily.  BSD 4.2 had a retransmission algorithm which
     retransmitted quickly and often.  This is exactly what you would
     want if you had a bunch of machines on an Ethernet (a low delay
     network of large bandwidth).  If you have a network of relatively
     longer delay and scarce bandwidth (e.g., 56kb lines), it tends to
     retransmit too aggressively.  Therefore, it makes the networks and
     gateways pass more traffic than is really necessary for a given
     conversation.  Retransmission algorithms do adapt to the delay of
     the network after a few packets, but 4.2's adapts slowly in delay
     situations.  BSD 4.3 does a lot better and tries to do the best
     for both worlds.  It fires off a few retransmissions really
     quickly assuming it is on a low delay network, and then backs off
     very quickly.  It also allows the delay to be about 4 minutes
     before it gives up and declares the connection broken.

     Even better than the original 4.3 code is a version of TCP with a
     retransmission algorithm developed by Van Jacobson of LBL.  He did
     a lot of research into how the algorithm works on real networks
     and modified it to get both better throughput and be friendlier to
     the network.  This code has been integrated into the later
     releases of BSD 4.3 and can be fetched anonymously from
     ucbarpa.berkeley.edu in directory 4.3.

  Time to Live

     The IP packet header contains a field called the time to live
     (TTL) field.  It is decremented each time the packet traverses a
     gateway.  TTL was designed to prevent packets caught in routing
     loops from being passed forever with no hope of delivery.  Since
     the definition bears some likeness to the RIP hop count, some
     misguided systems have set the TTL field to 15 because the
     unreachable flag in RIP is 16.  Obviously, no networks could have
     more than 15 hops.  The RIP space where hops are limited ends when
     RIP is not used as a routing protocol any more (e.g., when NSFnet
     starts transporting the packet).  Therefore, it is quite easy for
     a packet to require more than 15 hops.  These machines will
     exhibit the behavior of being able to reach some places but not
     others even though the routing information appears correct.

Krol [Page 19]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

     Solving the problem typically requires kernel patches so it may be
     difficult if source is not available.

Appendix A - References to Remedial Information

---

 [1]  Quarterman and Hoskins, "Notable Computer Networks",
      Communications of the ACM, Vol. 29, No. 10, pp. 932-971, October
      1986.

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

 [3]  Hedrick, C., "Introduction to the Internet Protocols", Via
      Anonymous FTP from topaz.rutgers.edu, directory pub/tcp-ip-docs,
      file tcp-ip-intro.doc.

 [4]  Comer, D., "Internetworking with TCP/IP: Principles, Protocols,
      and Architecture", Copyright 1988,  by Prentice-Hall, Inc.,
      Englewood Cliffs, NJ,  07632 ISBN 0-13-470154-2.

Appendix B - List of Major RFCs

---

This list of key "Basic Beige" RFCs was compiled by J.K. Reynolds. This is the 30 August 1989 edition of the list.

RFC-768 User Datagram Protocol (UDP) RFC-791 Internet Protocol (IP) RFC-792 Internet Control Message Protocol (ICMP) RFC-793 Transmission Control Protocol (TCP) RFC-821 Simple Mail Transfer Protocol (SMTP) RFC-822 Standard for the Format of ARPA Internet Text Messages RFC-826 Ethernet Address Resolution Protocol RFC-854 Telnet Protocol RFC-862 Echo Protocol RFC-894 A Standard for the Transmission of IP

             Datagrams over Ethernet Networks

RFC-904 Exterior Gateway Protocol RFC-919 Broadcasting Internet Datagrams RFC-922 Broadcasting Internet Datagrams in the Presence of Subnets RFC-950 Internet Standard Subnetting Procedure RFC-951 Bootstrap Protocol (BOOTP) RFC-959 File Transfer Protocol (FTP) RFC-966 Host Groups: A Multicast Extension to the Internet Protocol RFC-974 Mail Routing and the Domain System RFC-1000 The Request for Comments Reference Guide RFC-1009 Requirements for Internet Gateways RFC-1010 Assigned Numbers

Krol [Page 20]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

RFC-1011 Official Internet Protocols RFC-1012 Bibliography of Request for Comments 1 through 999 RFC-1034 Domain Names - Concepts and Facilities RFC-1035 Domain Names - Implementation RFC-1042 A Standard for the Transmission of IP

             Datagrams over IEEE 802 Networks

RFC-1048 BOOTP Vendor Information Extensions RFC-1058 Routing Information Protocol RFC-1059 Network Time Protocol (NTP) RFC-1065 Structure and Identification of

             Management Information for TCP/IP-based internets

RFC-1066 Management Information Base for Network

             Management of TCP/IP-based internets

RFC-1084 BOOTP Vendor Information Extensions RFC-1087 Ethics and the Internet RFC-1095 The Common Management Information

             Services and Protocol over TCP/IP (CMOT)

RFC-1098 A Simple Network Management Protocol (SNMP) RFC-1100 IAB Official Protocol Standards RFC-1101 DNS Encoding of Network Names and Other Types RFC-1112 Host Extensions for IP Multicasting RFC-1117 Internet Numbers

Note: This list is a portion of a list of RFC's by topic that may be retrieved from the NIC under NETINFO:RFC-SETS.TXT (anonymous FTP, of course).

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

RFC-974 Mail Routing and the Domain System RFC-1009 Requirements for Internet Gateways RFC-1034 Domain Names - Concepts and Facilities RFC-1035 Domain Names - Implementation and Specification RFC-1101 DNS Encoding of Network Names and Other Types

Krol [Page 21]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

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

     [email protected]

NSF Network Service Center (NNSC)

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

     [email protected]

NSF Network Information Service (NIS)

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

     [email protected]

CIC

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

     [email protected]

Krol [Page 22]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

Glossary

---

  autonomous system

     A set of gateways under a single administrative control and using
     compatible and consistent routing procedures.  Generally speaking,
     the gateways run by a particular organization.  Since a gateway is
     connected to two (or more) networks it is not usually correct to
     say that a gateway is in a network.  For example, the gateways
     that connect regional networks to the NSF Backbone network are run
     by Merit and form an autonomous system.  Another example, the
     gateways that connect campuses to NYSERNET are run by NYSER and
     form an autonomous system.

  core gateway

     The innermost gateways of the Internet.  These gateways have a
     total picture of the reachability to all networks known to the
     Internet.  They then redistribute reachability information to
     their neighbor gateways speaking EGP.  It is from them your EGP
     agent (there is one acting for you somewhere if you can reach the
     core of the Internet) finds out it can reach all the nets on the
     Internet.  Which is then passed to you via Hello, gated, RIP.  The
     core gateways mostly connect campuses to the ARPANET, or
     interconnect the ARPANET and the MILNET, and are run by BBN.

  count to infinity

     The symptom of a routing problem where routing information is
     passed in a circular manner through multiple gateways.  Each
     gateway increments the metric appropriately and passes it on.  As
     the metric is passed around the loop, it increments to ever
     increasing values until it reaches the maximum for the routing
     protocol being used, which typically denotes a link outage.

  hold down

     When a router discovers a path in the network has gone down
     announcing that that path is down for a minimum amount of time
     (usually at least two minutes).  This allows for the propagation
     of the routing information across the network and prevents the
     formation of routing loops.

  split horizon

     When a router (or group of routers working in consort) accept
     routing information from multiple external networks, but do not

Krol [Page 23]

RFC 1118 The Hitchhikers Guide to the Internet September 1989

     pass on information learned from one external network to any
     others.  This is an attempt to prevent bogus routes to a network
     from being propagated because of gossip or counting to infinity.

  DDN

     Defense Data Network the collective name for the ARPANET and
     MILNET.  Used frequently because although they are seperate
     networks the operational and informational foci are the same.

Security Considerations

  Security and privacy protection is a serious matter and too often
  nothing is done about it.  There are some known security bugs
  (especially in access control) in BSD Unix and in some
  implementations of network services.  The hitchhikers guide does not
  discuss these issues (too bad).

Author's Address

  Ed Krol
  University of Illinois
  195 DCL
  1304 West Springfield Avenue
  Urbana, IL  61801-4399

  Phone: (217) 333-7886

  EMail: [email protected]

Krol [Page 24]