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INFORMATIONAL
Network Working Group                                            E. KrolRequest for Comments: 1118                 University of Illinois Urbana                                                          September 1989The Hitchhikers Guide to the InternetStatus 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 1989What 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 PostelKrol                                                            [Page 2]

RFC 1118         The Hitchhikers Guide to the Internet    September 1989   (Postel@ISI.EDU).  He acts as a referee for the proposal.  It is then   commented upon by all those wishing to take part in the discussion   (electronically of course).  It may go through multiple revisions.   Should it be generally accepted as a good idea, it will be assigned a   number and filed with the RFCs.   There are two independent categorizations of protocols.  The first is   the state of standardization which is one of "standard", "draft   standard", "proposed", "experimental", or "historic".  The second is   the status of this protocol which is one of "required",   "recommended", "elective", or "not recommended".  One could expect a   particular protocol to move along the scale of status from elective   to required at the same time as it moves along the scale of   standardization from proposed to standard.   A Required Standard protocol (e.g.,RFC-791, The Internet Protocol)   must be implemented on any host connected to the Internet.   Recommended Standard protocols are generally implemented by network   hosts.  Lack of them does not preclude access to the Internet, but   may impact its usability.RFC-793 (Transmission Control Protocol) is   a Recommended Standard protocol.  Elective Proposed protocols were   discussed and agreed to, but their application has never come into   wide use.  This may be due to the lack of wide need for the specific   application (RFC-937, The Post Office Protocol) or that, although   technically superior, ran against other pervasive approaches.  It is   suggested that should the facility be required by a particular site,   an implementation be done in accordance with the RFC.  This insures   that, should the idea be one whose time has come, the implementation   will be in accordance with some standard and will be generally   usable.   Informational RFCs contain factual information about the Internet and   its operation (RFC-1010, Assigned Numbers).  Finally, as the Internet   and technology have grown, some RFCs have become unnecessary.  These   obsolete RFCs cannot be ignored, however.  Frequently when a change   is made to some RFC that causes a new one to be issued obsoleting   others, the new RFC may only contains explanations and motivations   for the change.  Understanding the model on which the whole facility   is based may involve reading the original and subsequent RFCs on the   topic.  (Appendix B contains a list of what are considered to be the   major RFCs necessary for understanding the Internet).   Only a few RFCs actually specify standards, most RFCs are for   information or discussion purposes.  To find out what the current   standards are see the RFC titled "IAB Official Protocol Standards"   (most recently published asRFC-1100).Krol                                                            [Page 3]

RFC 1118         The Hitchhikers Guide to the Internet    September 1989The Network Information Center (NIC)   The NIC is a facility available to all Internet users which provides   information to the community.  There are three means of NIC contact:   network, telephone, and mail.  The network accesses are the most   prevalent.  Interactive access is frequently used to do queries of   NIC service overviews, look up user and host names, and scan lists of   NIC documents.  It is available by using      %telnet nic.ddn.mil   on a BSD system, and following the directions provided by a user   friendly prompter.  From poking around in the databases provided, one   might decide that a document named NETINFO:NUG.DOC (The Users Guide   to the ARPANET) would be worth having.  It could be retrieved via an   anonymous FTP.  An anonymous FTP would proceed something like the   following.  (The dialogue may vary slightly depending on the   implementation of FTP you are using).     %ftp nic.ddn.mil     Connected to nic.ddn.mil     220 NIC.DDN.MIL FTP Server 5Z(47)-6 at Wed 17-Jun-87 12:00 PDT     Name (nic.ddn.mil:myname): anonymous     331 ANONYMOUS user ok, send real ident as password.     Password: myname     230 User ANONYMOUS logged in at Wed 17-Jun-87 12:01 PDT, job 15.     ftp> get netinfo:nug.doc     200 Port 18.144 at host 128.174.5.50 accepted.     150 ASCII retrieve of <NETINFO>NUG.DOC.11 started.     226 Transfer Completed 157675 (8) bytes transferred     local: netinfo:nug.doc  remote:netinfo:nug.doc     157675 bytes in 4.5e+02 seconds (0.34 Kbytes/s)     ftp> quit     221 QUIT command received. Goodbye.   (Another good initial document to fetch is NETINFO:WHAT-THE-NIC-   DOES.TXT).   Questions of the NIC or problems with services can be asked of or   reported to using electronic mail.  The following addresses can be   used:     NIC@NIC.DDN.MIL         General user assistance, document requests     REGISTRAR@NIC.DDN.MIL   User registration and WHOIS updates     HOSTMASTER@NIC.DDN.MIL  Hostname and domain changes and updates     ACTION@NIC.DDN.MIL      SRI-NIC computer operations     SUGGESTIONS@NIC.DDN.MIL Comments on NIC publications and servicesKrol                                                            [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.  (Seeappendix B for   mail/telephone contact numbers).The NSFNET Network Service Center   The NSFNET Network Service Center (NNSC), located at BBN Systems and   Technologies Corp., is a project of the University Corporation for   Atmospheric Research under agreement with the National Science   Foundation.  The NNSC provides support to end-users of NSFNET should   they have questions or encounter problems traversing the network.   The NNSC, which has information and documents online and in printed   form, distributes news through network mailing lists, bulletins, and   online reports.  NNSC publications include a hardcopy newsletter, the   NSF Network News, which contains articles of interest to network   users and the Internet Resource Guide, which lists facilities (such   as supercomputer centers and on-line library catalogues) accessible   from the Internet.  The Resource Guide can be obtained via anonymous   ftp to nnsc.nsf.net in the directory resource-guide, or by joining   the resource guide mailing list (send a subscription request to   Resource-Guide-Request@NNSC.NSF.NET.)Mail Reflectors   The way most people keep up to date on network news is through   subscription to a number of mail reflectors (also known as mail   exploders).  Mail reflectors are special electronic mailboxes which,   when they receive a message, resend it to a list of other mailboxes.   This in effect creates a discussion group on a particular topic.   Each subscriber sees all the mail forwarded by the reflector, and if   one wants to put his "two cents" in sends a message with the comments   to the reflector.   The general format to subscribe to a mail list is to find the address   reflector and append the string -REQUEST to the mailbox name (not the   host name).  For example, if you wanted to take part in the mailing   list for NSFNET reflected by NSFNET-INFO@MERIT.EDU, one sends a   request to NSFNET-INFO-REQUEST@MERIT.EDU.  This may be a wonderful   scheme, but the problem is that you must know the list exists in the   first place.  It is suggested that, if you are interested, you read   the mail from one list (like NSFNET-INFO) and you will probably   become familiar with the existence of others.  A registration service   for mail reflectors is provided by the NIC in the files   NETINFO:INTEREST-GROUPS-1.TXT, NETINFO:INTEREST-GROUPS-2.TXT,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 Hostmaster@NIC.DDN.MIL and ask for the   template for a connected address.  This template is filled out and   mailed back to the hostmaster.  An address is allocated and e-mailed   back to you.  This can also be done by postal mail (Appendix B).   IP addresses are 32 bits long.  It is usually written as four decimal   numbers separated by periods (e.g., 192.17.5.100).  Each number is   the value of an octet of the 32 bits.  Some networks might choose to   organize themselves as very flat (one net with a lot of nodes) and   some might organize hierarchically (many interconnected nets with   fewer nodes each and a backbone).  To provide for these cases,   addresses were differentiated into class A, B, and C networks.  This   classification had to with the interpretation of the octets.  Class A   networks have the first octet as a network address and the remaining   three as a host address on that network.  Class C addresses have   three octets of network address and one of host.  Class B is split   two and two.  Therefore, there is an address space for a few large   nets, a reasonable number of medium nets and a large number of small   nets.  The high order bits in the first octet are coded to tell the   address format.  There are very few unallocated class A nets, so a   very good case must be made for them.  So as a practical matter, oneKrol                                                            [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 inRFC-1009Krol                                                            [Page 7]

RFC 1118         The Hitchhikers Guide to the Internet    September 1989   andRFC-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 SchillerKrol                                                            [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 BowersRouting   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 completeKrol                                                           [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 inRFC-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 areKrol                                                           [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 (GGPRFC-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 (EGPRFC-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 canKrol                                                           [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 atKrol                                                           [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 Bind-Request@UCBARPA.BERKELEY.EDU.  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 (seeRFC-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 tricklesKrol                                                           [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.  Thisis 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 MessagesRFC-826       Ethernet Address Resolution ProtocolRFC-854       Telnet ProtocolRFC-862       Echo ProtocolRFC-894       A Standard for the Transmission of IP              Datagrams over Ethernet NetworksRFC-904       Exterior Gateway ProtocolRFC-919       Broadcasting Internet DatagramsRFC-922       Broadcasting Internet Datagrams in the Presence of SubnetsRFC-950       Internet Standard Subnetting ProcedureRFC-951       Bootstrap Protocol (BOOTP)RFC-959       File Transfer Protocol (FTP)RFC-966       Host Groups: A Multicast Extension to the Internet ProtocolRFC-974       Mail Routing and the Domain SystemRFC-1000      The Request for Comments Reference GuideRFC-1009      Requirements for Internet GatewaysRFC-1010      Assigned NumbersKrol                                                           [Page 20]

RFC 1118         The Hitchhikers Guide to the Internet    September 1989RFC-1011      Official Internet ProtocolsRFC-1012      Bibliography of Request for Comments 1 through 999RFC-1034      Domain Names - Concepts and FacilitiesRFC-1035      Domain Names - ImplementationRFC-1042      A Standard for the Transmission of IP              Datagrams over IEEE 802 NetworksRFC-1048      BOOTP Vendor Information ExtensionsRFC-1058      Routing Information ProtocolRFC-1059      Network Time Protocol (NTP)RFC-1065      Structure and Identification of              Management Information for TCP/IP-based internetsRFC-1066      Management Information Base for Network              Management of TCP/IP-based internetsRFC-1084      BOOTP Vendor Information ExtensionsRFC-1087      Ethics and the InternetRFC-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 StandardsRFC-1101      DNS Encoding of Network Names and Other TypesRFC-1112      Host Extensions for IP MulticastingRFC-1117      Internet NumbersNote:  This list is a portion of a list of RFC's by topic that may beretrieved from the NIC under NETINFO:RFC-SETS.TXT (anonymous FTP, ofcourse).The following list is not necessary for connection to the Internet,but is useful in understanding the domain system, mail system, andgateways:RFC-974        Mail Routing and the Domain SystemRFC-1009       Requirements for Internet GatewaysRFC-1034       Domain Names - Concepts and FacilitiesRFC-1035       Domain Names - Implementation and SpecificationRFC-1101       DNS Encoding of Network Names and Other TypesKrol                                                           [Page 21]

RFC 1118         The Hitchhikers Guide to the Internet    September 1989Appendix C - Contact Points for Network Information---------------------------------------------------Network Information Center (NIC)      DDN Network Information Center      SRI International, Room EJ291      333 Ravenswood Avenue      Menlo Park, CA 94025      (800) 235-3155 or (415) 859-3695      NIC@NIC.DDN.MILNSF Network Service Center (NNSC)      NNSC      BBN Systems and Technology Corporation      10 Moulton St.      Cambridge, MA 02238      (617) 873-3400      NNSC@NNSC.NSF.NETNSF Network Information Service (NIS)      NIS      Merit Inc.      University of Michigan      1075 Beal Avenue      Ann Arbor, MI 48109      (313) 763-4897      INFO@NIS.NSF.NETCIC      CSNET Coordination and Information Center      Bolt Beranek and Newman Inc.      10 Moulton Street      Cambridge, MA 02238      (617) 873-2777      INFO@SH.CS.NETKrol                                                           [Page 22]

RFC 1118         The Hitchhikers Guide to the Internet    September 1989Glossary--------   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 notKrol                                                           [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: Krol@UXC.CSO.UIUC.EDUKrol                                                           [Page 24]

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