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´╗┐Title: Hitchhiker's Guide to the Internet
Author: Krol, Ed
Language: English
As this book started as an ASCII text book there are no pictures available.
Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

*** Start of this Doctrine Publishing Corporation Digital Book "Hitchhiker's Guide to the Internet" ***

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            The Hitchhikers Guide to the Internet
                        25 August 1987

                           Ed Krol

 This document was produced through funding of the National
 Science Foundation.

 Copyright (C) 1987, by the Board of Trustees of The University
 of Illinois.  Permission to duplicate this document, in whole
 or part, is granted provided reference is made to the source
 and this copyright is included in whole copies.

 This document assumes that one is familiar with the workings
 of a non-connected simple IP network (e.g. a few 4.2 BSD
 systems on an Ethernet not connected to anywhere else).
 Appendix A contains remedial information to get one to this
 point.  Its purpose is to get that person, familiar with a
 simple net, versed in the "oral tradition" of the Internet
 to the point that that net can be connected to the Internet
 with little danger to either.  It is not a tutorial, it
 consists of pointers to other places, literature, and hints
 which are not normally documented.  Since the Internet is a
 dynamic environment, changes to this document will be made
 regularly.  The author welcomes comments and suggestions.
 This is especially true of terms for the glossary (definitions
 are not necessary).

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

 Two groups do much of the research and information work of
 the Internet (ISI and SRI).  ISI (the Informational Sciences
 Institute) does much of the research, standardization, and
 allocation work of the Internet.  SRI International provides
 information services for the Internet.  In fact, after you
 are connected to the Internet most of the information in
 this document can be retrieved from the Network Information
 Center (NIC) run by SRI.

 Operating the Internet

 Each network, be it the ARPAnet, NSFnet or a regional network,
 has its own operations center.  The ARPAnet is run by
 BBN, Inc. under contract from DARPA.  Their facility is
 called the Network Operations Center or NOC.  Cornell
 University temporarily operates NSFnet (called the Network
 Information Service Center, NISC).  It goes on to the


 regionals having similar facilities to monitor and keep
 watch over the goings on of their portion of the Internet.
 In addition, they all should have some knowledge of what is
 happening to the Internet in total. If a problem comes up,
 it is suggested that a campus network liaison should contact
 the network operator to which he is directly connected. That
 is, if you are connected to a regional network (which is
 gatewayed to the NSFnet, which is connected to the
 ARPAnet...)  and have a problem, you should contact your
 regional network operations center.


 The internal workings of the Internet are defined by a set
 of documents called RFCs (Request for Comments).  The general
 process for creating an RFC is for someone wanting something
 formalized to write a document describing the issue and mailing
 it to Jon Postel (postel@isi.edu).  He acts as a referee for
 the proposal.  It is then commented upon by all those wishing
 to take part in the discussion (electronically of course).
 It may go through multiple revisions.  Should it be generally
 accepted as a good idea, it will be assigned a number and
 filed with the RFCs.

 The RFCs can be divided into five groups: required, suggested,
 directional, informational and obsolete.  Required RFC's (e.g.
 RFC-791, The Internet Protocol) must be implemented on any host
 connected to the Internet.  Suggested RFCs are generally
 implemented by network hosts.  Lack of them does not preclude
 access to the Internet, but may impact its usability.  RFC-793
 (Transmission Control Protocol) is a suggested RFC.  Directional
 RFCs were discussed and agreed to, but their application has never
 come into wide use.  This may be due to the lack of wide need for
 the specific application (RFC-937 The Post Office Protocol) or
 that, although technically superior, ran against other pervasive
 approaches (RFC-891 Hello).  It is suggested that should the
 facility be required by a particular site, animplementation
 be done in accordance with the RFC.  This insures that, should
 the idea be one whose time has come, the implementation will be
 in accordance with some standard and will be generally usable.
 Informational RFCs contain factual information about the
 Internet and its operation (RFC-990, Assigned Numbers).
 Finally, as the Internet and technology have grown, some
 RFCs have become unnecessary.  These obsolete RFCs cannot
 be ignored, however.  Frequently when a change is made to
 some RFC that causes a new one to be issued obsoleting others,
 the new RFC only contains explanations and motivations for the
 change.  Understanding the model on which the whole facility
 is based may involve reading the original and subsequent RFCs
 on the topic.


 (Appendix B contains a list of what are considered to be the
 major RFCs necessary for understanding the Internet).

 The Network Information Center

 The NIC is a facility available to all Internet users which
 provides information to the community.  There are three
 means of NIC contact: network, telephone, and mail.  The
 network accesses are the most prevalent.  Interactive access
 is frequently used to do queries of NIC service overviews,
 look up user and host names, and scan lists of NIC documents.
 It is available by using

      %telnet sri-nic.arpa

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

      %ftp sri-nic.arpa
      Connected to sri-nic.arpa.
      220 SRI_NIC.ARPA FTP Server Process 5Z(47)-6 at Wed
17-Jun-87 12:00 PDT
      Name (sri-nic.arpa:myname): anonymous
      331 ANONYMOUS user ok, send real ident as password.
      Password: myname
      230 User ANONYMOUS logged in at Wed 17-Jun-87 12:01 PDT,
job 15.
      ftp> get netinfo:nug.doc
      200 Port 18.144 at host accepted.
      150 ASCII retrieve of 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

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

      NIC@SRI-NIC.ARPA         General user assistance, document requests
      REGISTRAR@SRI-NIC.ARPA   User registration and WHOIS updates
      HOSTMASTER@SRI-NIC.ARPA  Hostname and domain changes and updates
      ACTION@SRI-NIC.ARPA      SRI-NIC computer operations
      SUGGESTIONS@SRI-NIC.ARPA Comments on NIC publications and services


 For people without network access, or if the number of documents
 is large, many of the NIC documents are available in printed
 form for a small charge.  One frequently ordered document for
 starting sites is a compendium of major RFCs.  Telephone access is
 used primarily for questions or problems with network access.
 (See appendix B for mail/telephone contact numbers).

 The NSFnet Network Service Center

 The NSFnet Network Service Center (NNSC) is funded by NSF to
 provide a first level of aid to users of NSFnet should they
 have questions or encounter problems traversing the network.
 It is run by BBN Inc.  Karen Roubicek
 (roubicek@nnsc.nsf.net) is the NNSC user liaison.

 The NNSC, which currently has information and documents
 online and in printed form, plans to distribute news through
 network mailing lists, bulletins, newsletters, and online
 reports.  The NNSC also maintains a database of contact
 points and sources of additional information about NSFnet
 component networks and supercomputer centers.

 Prospective or current users who do not know whom to call
 concerning questions about NSFnet use, should contact the
 NNSC.  The NNSC will answer general questions, and, for
 detailed information relating to specific components of the
 Internet, will help users find the appropriate contact for
 further assistance.  (Appendix B)

 Mail Reflectors

 The way most people keep up to date on network news is
 through subscription to a number of mail reflectors.  Mail
 reflectors are special electronic mailboxes which, when they
 receive a message, resend it to a list of other mailboxes.
 This in effect creates a discussion group on a particular
 topic.  Each subscriber sees all the mail forwarded by the
 reflector, and if one wants to put his "two cents" in sends
 a message with the comments to the reflector....

 The general format to subscribe to a mail list is to find
 the address reflector and append the string -REQUEST to the
 mailbox name (not the host name).  For example, if you
 wanted to take part in the mailing list for NSFnet reflected
 by NSFNET@NNSC.NSF.NET, one sends a request to


 NSFNET-REQUEST@NNSC.NSF.NET.  This may be a wonderful scheme,
 but the problem is that you must know the list exists in the
 first place.  It is suggested that, if you are interested,
 you read the mail from one list (like NSFNET) and you will
 probably become familiar with the existence of others.
 A registration service for mail reflectors is provided by

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

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

 Address Allocation

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

 IP addresses are 32 bits long.  It is usually written as
 four decimal numbers separated by periods (e.g.,
 Each number is the value of an octet of the 32 bits.  It was
 seen from the beginning that some networks might choose to
 organize themselves as very flat (one net with a lot of nodes)
 and some might organize hierarchically


 (many interconnected nets with fewer nodes each and a backbone).
 To provide for these cases, addresses were differentiated into
 class A, B, and C networks.  This classification had to with the
 interpretation of the octets.  Class A networks have the first
 octet as a network address and the remaining three as a host
 address on that network.  Class C addresses have three octets of
 network address and one of host.  Class B is split two and two.
 Therefore, there is an address space for a few large nets, a
 reasonable number of medium nets and a large number of small nets.
 The top two bits in the first octet are coded to tell the address
 format.  All of the class A nets have been allocated.  So one
 has to choose between Class B and Class C when placing an order.
 (There are also class D (Multicast) and E (Experimental) formats.
 Multicast addresses will likely come into greater use in the near
 future, but are not frequently used now).

 In the past sites requiring multiple network addresses
 requested multiple discrete addresses (usually Class C).
 This was done because much of the software available
 (not ably 4.2BSD) could not deal with subnetted addresses.
 Information on how to reach a particular network (routing
 information) must be stored in Internet gateways and packet
 switches.  Some of these nodes have a limited capability to
 store and exchange routing information (limited to about 300
 networks).  Therefore, it is suggested that any campus
 announce (make known to the Internet) no more than two
 discrete network numbers.

 If a campus expects to be constrained by this, it should
 consider subnetting.  Subnetting (RFC-932) allows one to
 announce one address to the Internet and use a  set of
 addresses on the campus.  Basically, one defines a mask
 which allows the network to differentiate between the
 network portion and host portion of the address.  By using a
 different mask on the Internet and the campus, the address
 can be interpreted in multiple ways.  For example, if a
 campus requires two networks internally and has the 32,000
 addresses beginning 128.174.X.X (a Class B address) allocated
 to it,  the campus could allocate 128.174.5.X to one part
 of campus and 128.174.10.X to another.  By advertising
 128.174 to the Internet with a subnet mask of FF.FF.00.00,
 the Internet would treat these two addresses as one. Within
 the campus a mask of FF.FF.FF.00 would be used, allowing the
 campus to treat the addresses as separate entities. (In reality
 you don't pass the subnet mask of FF.FF.00.00 to the Internet,
 the octet meaning is implicit in its being a class B address).
 A word of warning is necessary.  Not all systems know how to
 do subnetting.  Some 4.2BSD systems require additional
 software.  4.3BSD systems subnet as released.  Other devices


 and operating systems vary in the problems they have dealing
 with subnets.  Frequently these machines can be used as a
 leaf on a network but not as a gateway within the subnetted
 portion of the network.  As time passes and more systems
 become 4.3BSD based, these problems should disappear.

 There has been some confusion in the past over the format of
 an IP broadcast address.  Some machines used an address of
 all zeros to mean broadcast and some all ones.  This was
 confusing when machines of both type were connected to the
 same network. The broadcast address of all ones has been
 adopted to end the grief.  Some systems (e.g. 4.2 BSD) allow
 one to choose the format of the broadcast address.  If a
 system does allow this choice, care should be taken that the
 all ones format is chosen.  (This is explained in RFC-1009
 and RFC-1010).

 Internet Problems

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

 Number of Networks

      When the Internet was designed it was to have about 50
      connected networks.  With the explosion of networking,
      the number is now approaching 300.  The software in a
      group of critical gateways (called the core gateways of
      the ARPAnet) are not able to pass or store much more
      than that number.  In the short term, core reallocation
      and recoding has raised the number slightly.  By the
      summer of '88 the current PDP-11 core gateways will be
      replaced with BBN Butterfly gateways which will solve
      the problem.

 Routing Issues

      Along with sheer mass of the data necessary to route
      packets to a large number of networks, there are many
      problems with the updating, stability, and optimality
      of the routing algorithms.  Much research is being done
      in the area, but the optimal solution to these routing
      problems is still years away.  In most cases the the
      routing we have today works, but sub-optimally and
      sometimes unpredictably.


 Trust Issues

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

 Capacity & Congestion

      Many portions of the ARPAnet are very congested during
      the busy part of the day.  Additional links are planned
      to alleviate this congestion, but the implementation
      will take a few months.

 These problems and the future direction of the Internet are
 determined by the Internet Architect (Dave Clark of MIT)
 being advised by the Internet Activities Board (IAB).  This
 board is composed of chairmen of a number of committees with
 responsibility for various specialized areas of the Internet.
 The committees composing the IAB and their chairmen are:

         Committee                            Chair
      Autonomous Networks                  Deborah Estrin
      End-to-End Services                  Bob Braden
      Internet Architecture                Dave Mills
      Internet Engineering                 Phil Gross
           EGP2                            Mike Petry
           Name Domain Planning            Doug Kingston
           Gateway Monitoring              Craig Partridge
           Internic                        Jake Feinler
           Performance & Congestion ControlRobert Stine
           NSF Routing                     Chuck Hedrick
           Misc. MilSup Issues             Mike St. Johns
      Privacy                              Steve Kent
      IRINET Requirements                  Vint Cerf
      Robustness & Survivability           Jim Mathis
      Scientific Requirements              Barry Leiner

 Note that under Internet Engineering, there are a set of
 task forces and chairs to look at short term concerns.  The
 chairs of these task forces are not part of the IAB.


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

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

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

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


 Routing protocols in current use:

 Static (no protocol-table/default routing)

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

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


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

      The other problem with hop-count metrics is that if
      links have different speeds, that difference is not


      reflected in the hop-count. So a one hop satellite link
      (with a .5 sec delay) at 56kb would be used instead of
      a two hop T1 connection. Congestion can be viewed as a
      decrease in the efficacy of a link. So, as a link gets
      more congested, RIP will still know it is the best
      hop-count route and congest it even more by throwing
      more packets on the queue for that link.

      The protocol is not well documented.  A group of people
      are working on producing an RFC to both define the
      current RIP and to do some extensions to it to allow it
      to better cope with larger networks.  Currently, the
      best documentation for RIP appears to be the code to
      BSD "routed".


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

      There are two ways to insert static routes into "routed",
      the "/etc/gateways" file and the "route add" command.
      Static routes are useful if you know how to reach a
      distant network, but you are not receiving that route
      using RIP.  For the most part the "route add" command is
      preferable to use.  The reason for this is that the
      command adds the route to that machine's routing table
      but does not export it through RIP.  The "/etc/gateways"
      file takes precedence over any routing information
      received through a RIP update.  It is also broadcast as
      fact in RIP updates produced by the host without question,
      so if a mistake is made in the "/etc/gateways" file,
      that mistake will soon permeate the RIP space and
      may bring the network to its knees.

      One of the problems with "routed" is that you have very
      little control over what gets broadcast and what
      doesn't.  Many times in larger networks where various
      parts of the network are under different administrative
      controls, you would like to pass on through RIP only nets
      which you receive from RIP and you know are reasonable.
      This prevents people from adding IP addresses to
      the network which may be illegal and you being
      responsible for passing them on to the Internet.  This


      type of reasonability checks are not available with "routed"
      and leave it usable, but inadequate for large networks.

 Hello (RFC-891)

      Hello is a routing protocol which was designed and
      implemented in a experimental software router called a
      "Fuzzball" which runs on a PDP-11. It does not have
      wide usage, but is the routing protocol currently used
      on the NSFnet backbone.  The data transferred between
      nodes is similar to RIP (a list of networks and their
      metrics).  The metric, however, is milliseconds of delay.
      This allows Hello to be used over nets of various link
      speeds and performs better in congestive situations.

      One of the most interesting side effects of Hello based
      networks is their great timekeeping ability.  If you
      consider the problem of measuring delay on a link for
      the metric, you find that it is not an easy thing to
      do.  You cannot measure round trip time since the
      return link may be more congested, of a different
      speed, or even not there.  It is not really feasible
      for each node on the network to have a builtin WWV
      (nationwide radio time standard) receiver.  So, you
      must design an algorithm to pass around time between
      nodes over the network links where the delay in
      transmission can only be approximated.  Hello routers
      do this and in a nationwide network maintain synchronized
      time within milliseconds.

 Exterior Gateway Protocol (EGP RFC-904)

      EGP is not strictly a routing protocol, it is a reacha-
      bility protocol. It tells only if nets can be reached
      through a particular gateway, not how good the connec-
      tion is.  It is the standard by which gateways to local
      nets inform the ARPAnet of the nets they can reach.
      There is a metric passed around by EGP but its usage is
      not standardized formally.  Its typical value is value
      is 1 to 8 which are arbitrary goodness of link values
      understood by the internal DDN gateways. The smaller
      the value the better and a value of 8 being unreach-
      able.  A quirk of the protocol prevents distinguishing
      between 1 and 2, 3 and 4..., so the usablity of this as
      a metric is as three values and unreachable.  Within
      NSFnet the values used are 1, 3, and unreachable.  Many
      routers talk EGP so they can be used for ARPAnet gateways.



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

      "Gated" talks RIP to RIP speaking hosts, EGP to EGP
      speakers, and Hello to Hello'ers.  These speakers
      frequently all live on one Ethernet, but luckily (or
      unluckily) cannot understand each others ruminations.
      In addition, under configuration file control it can
      filter the conversion.  For example, one can produce a
      configuration saying announce RIP nets via Hello only
      if they are specified in a list and are reachable by
      way of a RIP broadcast as well.  This means that if a
      rogue network appears in your local site's RIP space,
      it won't be passed through to the Hello side of the
      world.  There are also configuration options to do
      static routing and name trusted gateways.

      This may sound like the greatest thing since sliced
      bread, but there is a catch called metric conversion.
      You have RIP measuring in hops, Hello measuring in
      milliseconds, and EGP using arbitrary small numbers.
      The big questions is how many hops to a millisecond,
      how many milliseconds in the EGP number 3....  Also,
      remember that infinity (unreachability) is 16 to RIP,
      30000 or so to Hello, and 8 to the DDN with EGP.
      Getting all these metrics to work well together is no
      small feat.  If done incorrectly and you translate an
      RIP of 16 into an EGP of 6, everyone in the ARPAnet
      will still think your gateway can reach the unreachable
      and will send every packet in the world your way.  For
      these reasons, Mark requests that you consult closely
      with him when configuring and using "gated".



 All routing across the network is done by means of the IP
 address associated with a packet. Since humans find it
 difficult to remember addresses like, a symbolic
 name register was set up at the NIC where people would say
 "I would like my host to be named 'uiucuxc'".  Machines
 connected to the Internet across the nation would connect to
 the NIC in the middle of the night, check modification dates
 on the hosts file, and if modified move it to their local
 machine.  With the advent of workstations and micros,
 changes to the host file would have to be made nightly.  It
 would also be very labor intensive and consume a lot of
 network bandwidth. RFC-882 and a number of others describe
 domain name service, a distributed data base system for
 mapping names into addresses.

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

 Domain names are tree structured names with the root of the
 tree at the right.  For example:


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

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


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

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

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

 BIND can also provide mail forwarding information for inte-
 rior hosts not directly reachable from the Internet.  These
 hosts can either be on non-advertised networks, or not con-
 nected to a network at all, as in the case of UUCP-reachable
 hosts.  More information on BIND is available in the "Name
 Server Operations Guide for BIND" in "UNIX System Manager's
 Manual", 4.3BSD release.

 There are a few special domains on the network, like SRI-
 NIC.ARPA.  The 'arpa' domain is historical, referring to
 hosts registered in the old hosts database at the NIC.
 There are others of the form NNSC.NSF.NET.  These special
 domains are used sparingly and require ample justification.
 They refer to servers under the administrative control of


 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

 What's wrong with Berkeley Unix

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

 ICMP redirects

      Under the Internet model, all a system needs to know to
      get anywhere in the Internet is its own address, the
      address of where it wants to go, and how to reach a
      gateway which knows about the Internet.  It doesn't
      have to be the best gateway.  If the system is on a
      network with multiple gateways, and a host sends a
      packet for delivery to a gateway which feels another
      directly connected gateway is more appropriate, the
      gateway sends the sender a message.  This message is an
      ICMP redirect, which politely says "I'll deliver this
      message for you, but you really ought to use that gate-
      way over there to reach this host".  BSD 4.2 ignores
      these messages.  This creates more stress on the gate-
      ways and the local network, since for every packet


      sent, the gateway sends a packet to the originator.
      BSD 4.3 uses the redirect to update its routing tables,
      will use the route until it times out, then revert to
      the use of the route it thinks is should use.  The
      whole process then repeats, but it is far better than
      one per packet.


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


      TCP fires off blocks to its partner at the far end of
      the connection.  If it doesn't receive an acknowledge-
      ment in a reasonable amount of time it retransmits the
      blocks.  The determination of what is reasonable is
      done by TCP's retransmission algorithm.  There is no
      correct algorithm but some are better than others,
      where better is measured by the number of retransmis-
      sions done unnecessarily.  BSD 4.2 had a retransmission
      algorithm which retransmitted quickly and often.  This
      is exactly what you would want if you had a bunch of
      machines on an ethernet (a low delay network of large
      bandwidth).  If you have a network of relatively longer
      delay and scarce bandwidth (e.g. 56kb lines), it tends
      to retransmit too aggressively.  Therefore, it makes
      the networks and gateways pass more traffic than is
      really necessary for a given conversation.  Retransmis-
      sion algorithms do adapt to the delay of the network


      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.

                            Appendix A
                References to Remedial Information

      Quaterman and Hoskins, "Notable Computer Networks",
      Communications of the ACM, Vol 29, #10, pp. 932-971
      (October, 1986).

      Tannenbaum, Andrew S., Computer Networks, Prentice
      Hall, 1981.

      Hedrick, Chuck, Introduction to the Internet Protocols,
      Anonymous FTP from topaz.rutgers.edu, directory
      pub/tcp-ip-docs, file tcp-ip-intro.doc.


                            Appendix B
                        List of Major RFCs

RFC-768        User Datagram Protocol (UDP)
RFC-791        Internet Protocol (IP)
RFC-792        Internet Control Message Protocol (ICMP)
RFC-793        Transmission Control Protocol (TCP)
RFC-821        Simple Mail Transfer Protocol (SMTP)
RFC-822        Standard for the Format of ARPA Internet Text Messages
RFC-854        Telnet Protocol
RFC-917 *      Internet Subnets
RFC-919 *      Broadcasting Internet Datagrams
RFC-922 *      Broadcasting Internet Datagrams in the Presence of Subnets
RFC-940 *      Toward an Internet Standard Scheme for Subnetting
RFC-947 *      Multi-network Broadcasting within the Internet
RFC-950 *      Internet Standard Subnetting Procedure
RFC-959        File Transfer Protocol (FTP)
RFC-966 *      Host Groups: A Multicast Extension to the Internet Protocol
RFC-988 *      Host Extensions for IP Multicasting
RFC-997 *      Internet Numbers
RFC-1010 *     Assigned Numbers
RFC-1011 *     Official ARPA-Internet Protocols

      RFC's marked with the asterisk (*) are not included in
      the 1985 DDN Protocol Handbook.

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

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

RFC-882        Domain Names - Concepts and Facilities
RFC-883        Domain Names - Implementation
RFC-973        Domain System Changes and Observations
RFC-974        Mail Routing and the Domain System
RFC-1009       Requirements for Internet Gateways


                            Appendix C
              Contact Points for Network Information

 Network Information Center (NIC)

      DDN Network Information Center
      SRI International, Room EJ291
      333 Ravenswood Avenue
      Menlo Park, CA 94025
      (800) 235-3155 or (415) 859-3695

 NSF Network Service Center (NNSC)

      BBN Laboratories Inc.
      10 Moulton St.
      Cambridge, MA 02238
      (617) 497-3400



 core gateway

The innermost gateways of the ARPAnet.  These
gateways have a total picture of the reacha-
bility to all networks known to the ARPAnet
with EGP.  They then redistribute reachabil-
ity information to all those gateways speak-
ing EGP.  It is from them your EGP agent
(there is one acting for you somewhere if you
can reach the ARPAnet) finds out it can reach
all the nets on the ARPAnet. Which is then
passed to you via Hello, gated, RIP....

 count to infinity

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

 hold down

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

 split horizon

When a router (or group of routers working in
consort) accept routing information from mul-
tiple external networks, but do not pass on
information learned from one external network
to any others.  This is an attempt to prevent
bogus routes to a network from being propagated
because of gossip or counting to infinity.


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