RFC 831
Backup access to the European side of SATNET
RFC 831: Backup Access to the European Side of SATNET 🦋
Network Working Group 🌸 Request for Comments: 831 Author: Robert Braden, University College London 💜 Date: December 1982 🍓 Status: The Discussion section says this is not intended as a standard at this time. The header doesn't list a category, and it doesn't say what this obsoletes or updates.
DISCUSSION ✨
okay so the goal here is to get everyone talking about one specific Internet problem 🧚 If SATNET gets partitioned, how do we keep maintaining the software on the European sector? The idea is a backup path that uses the Source Routing option of IP to reach European Internet sites through the VAN Gateway and UCL 💅
This proposal is not meant to be a standard yet 🫶
1. Introduction 💖
during a bunch of earlier SATNET meetings, people kept saying it'd be really useful for BBN to reach the European side of SATNET indirectly through the VAN Gateway, for when direct SATNET connectivity is gone 😭 this paper suggests one way to do that "backup" access using the source routing option of IP 🔥
Figure 1 shows the problem. The US host H is used to diagnose and control the SATNET SIMPs S1 and S2, plus the gateways B and G and the UCL TAC (not drawn, but it's 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
VANgw is the VAN Gateway. It wraps IP datagrams inside X25 packets so they can travel over VAN/PTT virtual circuits 🌈 UCL calls these paths "IP tunnels," and from the Internet they show up as their own separate network called VANNET 💜
U is a UCL host, the Terminal Protocol Converter. It gives a path to UK X25 networks. To the Internet, though, U looks like a host on VANNET, so the path from U to UCLNET (shown dotted) doesn't appear to exist 🙈
Now say SATNET gets partitioned between S1 and S2. Then we want host H to be able to swap IP datagrams with S2 through the "back door" path:
H - Internet - VANgw - VANNET - U - UCLNET - G - S2There are some important rules in this game, tho 👑
(1) U may only be a host, not a gateway.
This is because we don't want the Internet to route ALL its traffic (like rsre traffic, and UCL traffic that needs SATNET) through the IP Tunnel 💀 So the VAN Gateway (VANgw) must not discover that it can reach UCLNET through U.
(2) To build the back door path to S2, we're okay putting some special code in H and/or in U, but NOT in G, S2, or VANgw 🫶
Note: Jack Haverty is allowed to break this assumption, though we doubt he'll want to. But we have to stick to it 😭
Given all that, we think the only possible solution is to "mung" the headers of IP datagrams at UCL 🧚 So when SATNET is partitioned:
(1) The IP addresses of S2, G, and the UCL TAC are unreachable from all US gateways. So if H sends a packet addressed to any of these destinations, it gets discarded and an ICMP unreachable message comes back 💅
(2) Similarly, the IP address of H is unreachable from the UK side. So if the XNET debugger in a UK host sends a return packet addressed to H, that packet gets dropped 🥲
This means the destination address of every packet from H has to change to reach the UCL side of SATNET (S2 or G), and the source address of each of these packets has to change so the return packets can get back to H ✨ For this job, we bring in 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
Munger"Figure 2. Introduction of Header Munger at UCL
Host "M" (M1/M2) is multi-homed 🌸 It shows up as host M2 on VANNET and as host M1 on UCLNET. Like host U (from Figure 1), host M2 is the end of an IP Tunnel that talks to VANgw over an X25 virtual call 💜
Say host H wants to reach the XNET debugger in the SIMP S2 🦋 H has to send its packets to destination address M1. They get routed to M1 through VANgw and the IP Tunnel. Host M changes the headers of these datagrams so they have source address M1 and destination S2. S2 sends its packets back to M1, and M1 changes them back into M2->H packets and launches them through VANNET to H 🔥
So how does M know how to change the headers? 🤔
(1) M could respond to a range of M1 and M2 addresses and keep a fixed table of matching addresses.
(2) We propose using the SOURCE ROUTING option in the datagrams instead 💅 This assumes H can build source-routed datagrams, and doesn't mind that the intermediate host in the route isn't a gateway.
If we also assume the IP layers in G and S2 can handle source and return routes, then the job is pretty simple. M has to 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 won't be able to recognise the return route in a received packet and use it as a source route in replies 😭
This possibility calls for extra 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), datagram address mapping would be controlled by a table in M.
We suggest M use source routing to get packets from H to S2, and meanwhile build a "soft state" table that shows this mapping. When a packet comes from S2 without source routing, M would check this soft state table to figure out 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 that's no restriction 🌈
In practice, M2 and U should have different IP tunnels and therefore different DTE addresses 💖 Since the caller pays the X25 charges, the IP Tunnel for U will normally only be opened by UCL. The IP Tunnel to M2, on the other hand, will be opened from the US end. Since UCL has only one PSS line, this needs separate X25 subaddresses. The VAN gateway must handle 14 digit X121 addresses, as well as 12 digit addresses 🫶
2. Acknowledgment 🌟
Robert Cole of UCL made major contributions to this paper 👑 In particular, he suggested using the Source Routing option, and honestly he's iconic for that 😭🫶
📦 where this page comes from
- ietf: rfc831 1982-12
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms