Network Working Group                                   R. Braden
Request for Comments: 831               University College London
                                                    December 1982

Backup Access to the European Side of SATNET

Robert Braden


The purpose of this RFC is to focus discussion on a particular Internet problem: a backup path for software maintenance of the European sector of the Internet, for

       use   when   SATNET   is  partitioned.   We  propose  a
       mechanism, based upon the Source Routing option of  IP,
       to  reach  European  Internet sites via the VAN Gateway
       and UCL.

This proposal is not intended as a standard at this time.

Network Working Group R. Braden
Request for Comments: 831 University College London
December 1982

1. Introduction

  During  several  previous  SATNET  meetings,  it  has   been
  observed  that  it  would  be  useful  for BBN to be able to
  access the European side of SATNET indirectly  via  the  VAN
  Gateway,  when  direct  SATNET  connectivity  has been lost.
  This short  paper  proposes  a  possible  approach  to  such
  "backup" access, using the source routing option of IP.

Figure 1 illustrates the problem we wish to solve. The US host H is used for diagnosis and control of the SATNET SIMP's S1 and S2 as well as the gateways B and G and the UCL TAC (not shown, but connected to G).


    ARPANET/SATNET          __     __               UCL
    Gateway           Simp (   \  \  )  Simp        Gateway
               ____    ___(    /  /   )____          ____
              | B  |__| S1 |   \  \   | S2 |________| G  |_____ rsre
              |____|  |____|   /  /   |____|        |____|
                |         (    \  \   )                |
                |          (__ /  /__)          _______|____
        ________|____                          (             )
       (             )                        (               )
      (   ARPANET     )                      (     UCL NET     )
      (               )                       (                 )
       (_____________)                         (               )
        |        |                              (_____________)
      __|_       |            VAN/                     .
     | H  |      |         Public Data Nets            .
     |____|      |          _____________              .
    Diagnostic   |         (             )             .
    Host       __|__      (    VANNET     )           _.___
              | VAN |* * (* * * * * * * * *)*  * * * |     |
              | gw------(--- IP Tunnel -----)--------|  U  |
              |_____|* * (* * * * * * * * *)*  * *   |_____|
          VAN             (               )
          Gateway          (_____________)

Figure 1. US/UK Connectivity with Partitioned SATNET

RFC 831 - 1 - [Braden]

Request for Comments: 831 University College London
December 1982

VANgw is the VAN Gateway which encapsulates IP datagrams in X25 packets for transmission over VAN/PTT virtual circuits. The collection of these paths, called "IP tunnels" by UCL, is addressed from the Internet as a distinct network, VANNET.

U is a UCL host, the Terminal Protocol Converter, which provides a path to UK X25 networks. However, to the Internet world U looks like a host on VANNET, so the path from U to UCLNET (shown dotted) does not appear to exist.

Now suppose SATNET is partitioned between S1 and S2. Then we wish host H to be able to exchange IP datagrams with S2 via the "back door" path:

H - Internet - VANgw - VANNET - U - UCLNET - G - S2

There are some important rules in this game, however.

   (1)    U may only be a host, not a gateway.

This is because we do not want the Internet to route ALL its traffic (e.g. rsre traffic and UCL traffic that is required to use SATNET) via the IP Tunnel. So the VAN Gateway (VANgw) must not discover it can get to UCLNET through U.

   (2)    To implement the  back  door  path  to  S2,  we  are
          willing  to have some special code in H and/or in U,
          but not in G, S2, or VANgw.

Note: Jack Haverty is allowed to violate this assumption, though we doubt that he will want to. But we must stick to it.

Given these constraints, we claim that the only possible solution is to "mung" the headers of IP datagrams at UCL. Thus, when SATNET is partitioned:

   (1)    The IP addresses of S2,  G,  and  the  UCL  TAC  are
          unreachable  from  all US gateways.  Therefore, if H
          sends  a  packet   addressed   to   one   of   these
          destinations,  it  will  be  discarded  and  an ICMP
          unreachable message returned.

RFC 831 - 2 - [Braden]

Request for Comments: 831 University College London
December 1982

   (2)    Similarly, the IP address of H is  unreachable  from
          the  UK  side.   Hence, if the XNET debugger in a UK
          host emits a return  packet  addressed  to  H,  that
          packet will be dropped.

Therefore, the destination address of each packet from H must be changed in order to reach the UCL side of SATNET (S2 or G), and the source address of each of these packets must be changed so that return packets can reach H. For this purpose, we introduce the Munger host M (see Figure 2).


            BBN             __     __               UCL
            Gateway   Simp (   \  \  )  Simp        Gateway
               ____    ___(    /  /   )____          ____
              | B  |__| S1 |   \  \   | S2 |________| G  |_____ rsre
              |____|  |____|   /  /   |____|        |____|
                |         (    \  \   )                |
                |          (__ /  /__)          _______|____
        ________|____                          (             )
       (             )                        (               )
      (   ARPANET     )                     (     UCL NET     )
      (               )                      (                 )
       (_____________)                        (               )
        |        |                             (_____________)
      __|_       |                                         |
     | H  |      |        Public Data Nets                 |
     |____|      |          _______________               _|___
    Diagnostic   |         (               )             | M1  |
    Host       __|__      (                 )            |:::::|
              | VAN |* * (* * * * * * * * * *) * *       |:::::|
              | gw------(--- IP Tunnel -----)------------| M2  |
              |_____|* * (* * * * * * * * * *) * *       |_____|
          VAN             (   VANNET        )              M
          Gateway          (_______________)             "Header

Figure 2. Introduction of Header Munger at UCL

Host "M" (M1/M2) is mulit-homed, appearing as host M2 on VANNET and as host M1 on UCLNET. Like host U (shown in Figure 1), host M2 is the end of an IP Tunnel which communicates with VANgw over an X25 virtual call.

RFC 831 - 3 - [Braden]

Request for Comments: 831 University College London
December 1982

Suppose for example that host H desiollege London
December 1982

Suppose for example that host H desires to reach the XNET debugger in the SIMP S2. H must send its packets with destination address M1; these will be routed to M1 via VANgw and the IP Tunnel. Host M will change the headers of these datagrams to contain source address M1 and destination S2. S2 will return packets to M1, and M1 will change them back to M2->H packets and launch them back through the VANNET to H.

How does M know how to change the headers?

   (1)    M could respond to a range of M1 and  M2  addresses,
          and have a fixed table of correspondence.

   (2)    We propose instead to use the SOURCE ROUTING  option
          in  the  datagrams.   This assumes that H is able to
          build source-routed datagrams, and is not upset that
          the intermediate host in the route is not a gateway.

If we further assume that the IP layers in G and S2 can handle source and return routes, then the task

          is  simple.  M  must  contain  the  source   routing
          algorithm  of  a  gateway,  but otherwise act as two
          hosts (no routing updates, etc).

   (3)    Although G supports source routing, S2 and  the  TAC
          may  not.   In that case, S2 and the TAC will not be
          able to recognise the return  route  in  a  received
          packet  and use it as a source route in packets sent
          in reply.

This possibility calls for additional complexity in M, a combination of (1) and (2):

           *      In  the  US  ->  UK  direction,  the  Source
                  Routing option would be used.

           *      In  the   reverse  direction  (UK  ->   US),
                  mapping   of  datagram  addresses  would  be
                  controlled by a table in M.

RFC 831 - 4 - [Braden]

Request for Comments: 831 University College London
December 1982

We suggest that M use source routing to get packets from H to S2, and meanwhile build a "soft state" table showing this mapping. When a packet comes from S2 without source routing, M would consult this soft state table to discover how to alter the addresses to reach H again. This would allow only one US host at a time to access a given SATNET host, but surely this is no restriction.

In practice, M2 and U should have different IP tunnels and hence different DTE addresses. Since the caller pays the X25 charges, the IP Tunnel for U will normally be opened only by UCL. On the other hand, the IP Tunnel to M2 will be opened from the US end. Since UCL has only one PSS line, this requires the use of separate X25 subaddresses. The VAN gateway must handle 14 digit X121 addresses, as well as 12 digit addresses.

2. Acknowledgment

Robert Cole of UCL has made major contributions to the contents of this paper. In particular, he suggested the use of the Source Routing option.

RFC 831 - 5 - [Braden]