From rgnet-owner Fri May 7 15:49:30 1993 Received: by fido.wps.com (5.67/1.34) id AA10958; Fri, 7 May 93 15:46:55 -0700 From: matthew@klinzhai.echo.com (matthew kaufman) To: rgnet@fido.wps.com Date: Fri, 7 May 93 15:47:23 PDT Subject: LONG description of routing. hope this is helpful. Message-Id: <9305072247.AA12965@klinzhai.echo.com> X-L2L: wps.com Status: OR disclaimer: this is my understanding of how routing works and i've simplified descriptions of how routing tables get made, because i don't want to write a whole paper about routing protocols. Routing An IP router is a device with two (or more) INTERFACES (physical data communications connections) Upon receiving an IP packet, the router examines the (network portion of the) DESTINATION address and determines which interface to retransmit (FORWARD) the packet on. The list that relates destination network addresses to which interface to use to forward that packet is the ROUTING TABLE. If there is not a corresponding entry, and there is a DEFAULT ROUTE (the default interface to send packets whose addresses which are not recognized to) it is forwarded to the default interface. If there is no default route, a Network Unreachable ICMP message is sent back to the source address contained in the packet. If there is more than one way to reach a destination, the PREFERRED ROUTE is used. It is important to remember that routing decisions are made by each router, all each router can do is pick which interface to forward the packet on, and that there are no "circuits"... your next packet might take a different route, and return packets probably donUt come back the same way. When everything is working properly, routes are relatively stable, so if your packet takes one path today, it is likely to take the same path tomorrow. Inter-Network routing tables are generated through a process that involves one router ADVERTISING (ANNOUNCING) networks that it knows that it can reach, and the cooperating routing LEARNING those routes. For instance, when TLG was connected to AlterNet, AlterNet began advertising that TLG's network number was reachable by sending packets to AlterNet. These advertisements were sent to the ANS network and to the CIX router. ANS and CIX learned these routes and then began advertising them appropriately. Advertising and learning of routes is a SELECTIVE process, however. For various reasons (policy, technical) a network may choose to not advertise certain destination networks to some of the routers it is connected to, and the routers it does advertise routes to have the option of learning or ignoring those routes. Selective advertising of routes is the primary way that long-term routing LOOPS are avoided. Routing preferences are typically determined on a policy basis, depending on whether the destination address is a "Commercial" or "Research & Education" network. Note that most commercial sites are actually classified for policy reasons as "Research", their actual use notwithstanding. In the IP Internet today, the following major networks interact in the following ways: the following physical paths exist: ANS is directly connected to BARRN, NEARN, CERF, PSI, Alternet, CIX and other "ANS-only" networks BARRN is directly connected to ANS, Alternet NEARN is directly connected to ANS, Alternet CIX is directly connected to ANS, CERF, PSI, Alternet, Sprint and other "CIX-only" networks Alternet is directly connected to ANS, BARRN, NEARN, CIX CERF is directly connected to ANS, CIX PSI is directly connected to ANS, CIX other ANS-only networks are connected to ANS other CIX-only networks are connected to CIX Here are the announce/learn/prefer policies of each: (this is of course subject to change, but reflects how things are configured in April of 1993) (from the CIX-ANS routing plan of 1992 and personal communication with Jordan Becker of ANS) CERF: Learn CIX routes from the CIX Learn ANS routes from ANS, including NEARN/BARRN DEFAULT route unknown destinations to the ANS network Announce all member networks to CIX Announce all research&education member networks to ANS Prefer ANS as the route to NEARN/BARRN research&education nets Prefer CIX as the route for CIX-attached networks and NEARN/BARRN "commercial" networks In all other cases, prefer ANS. PSI: Learn CIX routes from the CIX Learn ANS routes from ANS, including NEARN/BARRN DEFAULT route unknown destinations to the ANS network Announce all member networks to CIX Announce all research&education member networks to ANS SHOULD Prefer ANS as the route to NEARN/BARRN research&education nets, but doesn't do this! (thus causing ANS to change their policy, see later) so ends up using CIX as the route Prefer CIX as the route for CIX-attached networks and NEARN/BARRN "commercial" networks In all other cases, prefer ANS. NEARN & BARRN: Learn all Alternet routes from Alternet Learn all ANS routes from ANS Learn all CIX routes from ANS DEFAULT route to ANS Announce all member networks to Alternet Announce all member networks to ANS Prefer Alternet for Alternet networks In all other cases, prefer ANS. Alternet: Learn all CIX routes from CIX Learn all BARRN and NEARN routes directly from BARRN and NEARN Learn all ANS routes, including BARRN and NEARN, from ANS DEFAULT route to ANS Announce all member networks to CIX Announce all member networks to NEARN, BARRN Announce research&education member nets to ANS Prefer BARRN direct for BARRN Prefer NEARN direct for NEARN Prefer CIX for CIX-attached networks In all other cases, prefer ANS other CIX-only networks: Learn all CIX routes from CIX (including NEARN and BARRN) Announce all member networks to CIX Prefer CIX routes for all destinations other ANS-only networks: Learn all ANS routes from ANS, including NEARN and BARRN DEFAULT route to ANS Announce all research&education member networks to ANS Prefer ANS routes for all destinations ANS: Learn all research&education net routes from CERF Learn all research&education net routes from PSI Learn all research&education net routes from Alternet Learn all CIX routes from CIX Announce all ANS nets to NEARN and BARRN Announce all CIX nets to NEARN and BARRN Announce all BARRN member networks to CIX Announce all "Commercial" NEARN member networks to CIX (DOES NOT announce R&E NEARN networks to CIX because if they did, the PSI preference problem would cause unwanted routing asymmetry) Announce all ANS nets to CERF, PSI, Alternet, and other ANS-only Prefer direct routes to CERF, PSI, Alternet, BARRN, NEARN Prefer CIX routes for all other destinations CIX: Learn all CERF routes from CERF Learn all PSI routes from PSI Learn all Alternet routes from Alternet Learn all RCIX-onlyS routes from those networks Learn all BARRN routes from ANS Learn all NEARN routes from ANS (BUT CANNOT learn NEARN R&E nets because ANS has stopped announcing them to CIX because of the PSI problem) Announce all nets to CERF, PSI, Alternet, ANS Prefer direct routes to CERF, PSI, Alternet and other CIX-only networks Prefer ANS routes for NEARN and BARRN For all other destinations, discard packet. No default route available. Because CIX announces all member networks to CERF, PSI, Alternet AND ANS, and ANS properly learns that it can reach CIX member networks via the ANS-CIX interconnection, packets flowing TO any CIX member network will get through. But, packets sent from a CIX member network TO the CIX router with destinations other than [CERF,PSI,Alternet,BARRN,NEARN-commerical-only] will be discarded as undeliverable by the CIX router. Thus the need for an additional outbound path (or a change in CIX routing policies) for full connectivity to the Internet. -Matthew Kaufman (5/7/1993)