rfc823.txt

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     routing  updates to its neighbors.  The gateway sends new updates

     to its neighbors if every one of the following  three  conditions

     occurs:   1)  one  of the gateway's interfaces changes state,  2)

     one of the gateway's neighbor gateways changes state, and  3) the

     gateway  receives  a  routing  update  from  a  neighbor  that is

     different from the update that it had  previously  received  from

     that  neighbor.   The  gateway  sends  routing  updates  only  to

     neighbors that are currently in the "up" state.


          The gateway requests a routing update  from  neighbors  that

     are  in  the  "up"  state,  but  from which it has yet received a



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     routing update.  Routing updates are  requested  by  setting  the

     appropriate  bit  in  the routing update being sent [Appendix A].

     Similarly, if a gateway receives from a neighbor a routing update

     in  which the bit requesting a routing update is set, the gateway

     sends the neighbor the most recent routing update.




     4.4.5  Non-Routing Gateways


          A Non-routing Gateway is a gateway  that  forwards  internet

     traffic,  but  does  not  implement  the  GGP  routing algorithm.

     Networks that are behind a Non-routing Gateway are known a-priori

     to  Routing Gateways.  There can be one or more of these networks

     which are considered to be directly connected to the  Non-routing

     Gateway.   A  Routing  Gateway  will forward a datagram to a Non-

     routing Gateway if it is addressed to a network behind  the  Non-

     routing   Gateway.    Routing  Gateways  currently  do  not  send

     Redirects for  Non-routing  Gateways.   A  Routing  Gateway  will

     always  use  another  Routing Gateway as a path instead of a Non-

     routing Gateways if both exist and are the same  number  of  hops

     away from the destination network.  The Non-routing Gateways path

     will be used only when the Routing Gateway path is down; when the

     Routing Gateway path comes back up, it will be used again.




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     4.4.6  Adding New Neighbors and Networks


          Gateways  dynamically  add  routing  information  about  new

     neighbors   and  new  networks  to  their  tables.   The  gateway

     maintains a list of neighbor gateway addresses.  When  a  routing

     update  is  received, the gateway searches this list of addresses

     for the Internet source address of the  routing  update  message.

     If  the  Internet  source  address  of  the routing update is not

     contained in the list of neighbor  addresses,  the  gateway  adds

     this  address  to  the  list  of  neighbor addresses and sets the

     neighbor's connectivity status to "down."   Routing  updates  are

     not  accepted  from neighbors until the GGP polling mechanism has

     determined that the neighbor is up.


          This strategy of adding  new  neighbors  requires  that  one

     gateway   in  each  pair  of  neighbor  gateways  must  have  the

     neighbor's address configured in its tables.  The newest  gateway

     can be given a complete list of neighbors, thus avoiding the need

     to re-configure older gateways when new gateways are installed.


          Gateways obtain routing information about  new  networks  in

     several  steps.   The  gateway has a list of all the networks for

     which it currently maintains routing information.  When a routing

     update  is  received,  if the routing update contains information



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     about a new network, the gateway adds this network to the list of

     networks  for  which it maintains routing information.  Next, the

     gateway adds  the  new  network  to  its  distance  matrix.   The

     distance  matrix comprises the is the matrix of distances (number

     of hops) to networks as reported  in  routing  updates  from  the

     neighbor  gateways.   The  gateway  sets  the distance to all new

     networks to "infinity," and then  computes  new  routes  and  new

     routing updates as outlined above.




     4.5  Exterior Gateway Protocol


          The Exterior Gateway Protocol (EGP) is used to permit  other

     gateways  and  gateway systems to pass routing information to the

     DARPA Internet gateways.  The use of the EGP permits the user  to

     perceive  all  of  the networks and gateways as part of one total

     Internet system, even though the "exterior" gateways are disjoint

     and  may  use  a  routing  algorithm  that  is  different and not

     compatible with  that  used  in  the  "interior"  gateways.   The

     important elements of the EGP are:


     o Neighbor Acquisition

          The procedure by which a gateway requests that it  become  a
          neighbor  of  another  gateway.  This is used when a gateway
          wants to become a neighbor  of  another  in  order  to  pass



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          routing information.  This includes the capability to accept
          or refuse the request.

     o Neighbor Up/Down

          The procedure by which a gateway decides if another  gateway
          is up or down.

     o Network Reachability Information

          The facility used to pass routing and  neighbor  information
          between gateways.

     o Gateway Going Down

          The ability of a gateway to inform other gateways that it is
          going  down  and  no  longer  has  any  routes  to any other
          networks.  This permits a gateway to go down in  an  orderly
          way without disrupting the rest of the Internet system.


     A complete description of the EGP can be found  in  IEN-209,  the

     "Exterior Gateway Protocol" [10].























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     5  GATEWAY SOFTWARE


          The DARPA Internet Gateway  runs  under  the  MOS  operating

     system [9] which provides facilities for:


          o Multiple processes
          o Interprocess communication
          o Buffer management
          o Asynchronous input/output
          o Shareable real-time clock


     There is a MOS process for  each  network  that  the  gateway  is

     directly  connected  to.   A  data  structure  called  a NETBLOCK

     contains variables of interest for each network and  pointers  to

     local  network  routines.   Network  processes run common gateway

     code while  network-specific  functions  are  dispatched  to  the

     routines  pointed  to  in the NETBLOCK.  There are also processes

     for gateway functions which require their own timing, such as GGP

     and HMP.




     5.1  Software Structure


          The gateway software can be divided conceptually into  three

     parts:   MOS Device Drivers, Network software, and Shared Gateway

     software.





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     5.1.1  Device Drivers


          The gateway has a set of  routines  to  handle  sending  and

     receiving  data  for  each type of hardware interface.  There are

     routines for initialization,  initiation,  and  interruption  for

     both  the  transmit  and  receive sides of a device.  The gateway

     supports the following types of devices:


          a)  ACC LSI-11 1822
          b)  DEC IMP11a 1822
          c)  ACC LHDH 1822
          d)  ACC VDH11E
          e)  ACC VDH11C
          f)  Proteon Ring Network
          g)  RSRE HDLC
          h)  Interlan Ethernet
          i)  BBN Fibernet
          j)  ACC XQ/CP X.25 **
          k)  ACC XQ/CP HDH  **




     5.1.2  Network Software


          For each connected network, the gateway has a set  of  eight

     routines  which  handle  local  network  functions.   The network

     routines and their functions are described briefly below.




     _______________
     ** Planned, not yet supported.




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          Up.net    Perform  local  network  initialization  such   as
                    flapping the 1822 ready line.

          Sg.net    Handle specific  local  network  timing  functions
                    such as timing out 1822 Destination Deads.

          Rc.net    A message  has  been  received  from  the  network
                    interface.  Check for any input errors.

          Wc.net    A message has  been  transmitted  to  the  network
                    interface.  Check for any output errors.

          Rs.net    Set up a buffer (or buffers) to  receive  messages
                    on the network interface.

          Ws.net    Transmit a message to the network interface.

          Hc.net    Check the local network  header  of  the  received
                    message.    Perform  any  local  network  protocol
                    tasks.

          Hb.net    Rebuild the local network header.


          There are  network  routines  for  the  following  types  of

     networks:


          o  Arpanet (a,b,c,k)
          o  Satnet (d,e,k)
          o  Proteon Ring Network (f)
          o  Packet Radio Network (a,b,c)
          o  Rsre HDLC Null Network (g)
          o  Ethernet (h)
          o  Fibernet (i)
          o  Telenet X.25 (j) **


     Note: The letters in parentheses refer to the device drivers used

     _______________
     ** Planned, not yet supported.




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     for each type of network as described in the previous section.




     5.1.3  Shared Gateway Software


          The internet processing of a datagram is performed by a body

     of  code  which  is  shared  by the network processes.  This code

     includes  routines  to  check   the   IP   header,   perform   IP

     fragmentation, calculate the IP checksum, forward a datagram, and

     implement the routing, monitoring, and error reporting protocols.




     5.2  Gateway Processes


     5.2.1  Network Processes


          When the gateway starts up, each network process  calls  its

     local network initialization routine and read start routine.  The

     read start routine sets up two maximum network size  buffers  for

     receiving datagrams.  The network process then waits for an input

     complete signal from the network device driver.


          When a message has been received, the MOS  Operating  System

     signals  the  appropriate  network process with an input complete

     signal.  The network process wakes up and executes the  net  read




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     complete  routine.   After  the  message  has been processed, the

     network process waits for more input.


          The  net  read  complete  routine  is  the   major   message

     processing  loop  in  the  gateway.   The  following  actions are

     performed when a message has been received:


          o  Call Local Network Read Complete Routine
          o  Start more reads
          o  Check local Network Header
          o  Check Internet header
          o  Check if datagram is for the gateway
          o  Forward the datagram if necessary
          o  Send ICMP error message if necessary.




     5.2.2  GGP Process


          The GGP process periodically sends GGP echos to each of  the

     gateway's neighbors to determine neighbor connectivity, and sends

     interface  status  messages  addressed  to  itself  to  determine

     network  connectivity.   The  GGP  process also sends out routing

     updates when necessary.  The details of the algorithms  currently

     implemented  by  the  GGP  process  are  given  in  Section  4.4,

     Gateway-to-Gateway Protocol, and in Appendix C.








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     5.2.3  HMP Process


          The  HMP  process  handles  timer-based  gateway  statistics

     collection and the periodic transmission of traps.









































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     APPENDIX A. GGP Message Formats


          Note that the GGP protocol is currently undergoing extensive

     changes to introduce the Exterior Gateway Protocol facility; this

     is the vehicle needed to permit  gateways  in  other  systems  to

     exchange  routing information with the gateways described in this

     document.


          Each GGP message consists of an Internet header followed  by

     one  of the messages explained below.  The values (in decimal) in

     the Internet header used in a GGP message are as follows.


     Version                  4.

     IHL                      Internet header length in 32-bit words.

     Type of Service          0.

     Total Length             Length of Internet header  and  data  in
                              octets.

     ID, Flags,
     Fragment Offset          0.

     Time to Live             Time to live in seconds.  This field  is
                              decremented   at   least  once  by  each
                              machine that processes the datagram.

     Protocol                 Gateway Protocol = 3.