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

Backup Access to the European Side of SATNET

Robert Braden

DISCUSSION

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).

SATNET
(partitioned)
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]

Network Working Group R. 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]

Network Working Group R. 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).

SATNET
(partitioned)
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
Munger"

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]

Network Working Group R. 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]

Network Working Group R. 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]