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Cole, Shur & Villamizar Informational [Page 16] RFC 1932 IP over ATM: A Framework Document April 1996 .----------. ---------< Non-ATM : .-------. / /-< Subnet >-\ :Sub-ES >--/ : ---------- : ------- : : : : .--^---. .--^---. :Router: :Router: -v-v-- -v-v-- : : : : .--------. : : .--------. : : .--------. .-------. : >-/ \-< >-/ \-< : .-------. :Sub-ES :---: Subnet :-----: Subnet :-----: Subnet :---:Sub-ES : ------- : : : : : : ------- -------- ---v---- -------- : .--^----. :Sub-ES : ------- Figure 3: A configuration with both ATM-based and non-ATM based subnets. For example, figure 3 shows an end-to-end configuration consisting of four components, three of which are ATM technology based, while the fourth is a standard IP subnet based on non-ATM technology. End- systems (either hosts or routers) attached to the ATM-based networks may communicate either using the Classical IP model or directly via ATM (subject to policy constraints). Such nodes may communicate directly at the IP level without necessarily needing an intermediate router, even if end-systems do not share a common IP-level network prefix. Communication with end-systems on the non-ATM-based Classical IP subnet takes place via a router, following the Classical IP model (see Section 8.1 below). Many of the problems and issues associated with creating such direct connections across subnet boundaries were originally being addressed in the IETF's IPLPDN working group and the IP over ATM working group. This area is now being addressed in the Routing over Large Clouds working group. Examples of work performed in the IPLPDN working group include short-cut routing (proposed by P. Tsuchiya) and directed ARP RFC-1433 [5] over SMDS networks. The ROLC working group has produced the distributed ARP server architectures and the NBMA Address Resolution Protocol (NARP) [7]. The Next Hop Resolution Protocol (NHRP) is still work in progress, though the ROLC WG is considering advancing the current document. Questions/issues specifically related to defining a capability to cross IP subnet boundaries include:Cole, Shur & Villamizar Informational [Page 17] RFC 1932 IP over ATM: A Framework Document April 1996 o How can routing be optimized across multiple logical IP subnets over both a common ATM based and a non-ATM based infrastructure. For example, in Figure 3, there are two gateways/routers between the non-ATM subnet and the ATM subnets. The optimal path from end-systems on any ATM-based subnet to the non ATM-based subnet is a function of the routing state information of the two routers. o How to incorporate policy routing constraints. o What is the proper coupling between routing and address resolution particularly with respect to off-subnet communication. o What are the local procedures to be followed by hosts and routers. o Routing between hosts not sharing a common IP-level (or L3) network prefix, but able to be directly connected at the NBMA media level. o Defining the details for an efficient address resolution architecture including defining the procedures to be followed by clients and servers (see RFC-1433 [5], RFC-1735 [7] and NHRP). o How to identify the need for and accommodate special purpose SVCs for control or routing and high bandwidth data transfers. For ATM (unlike other NBMA media), an additional complexity in supporting IP routing over these ATM internets lies in the multiplicity of address formats in UNI 3.0 [4]. NSAP modeled address formats only are supported on "private ATM" networks, while either 1) E.164 only, 2) NSAP modeled formats only, or 3) both are supported on "public ATM" networks. Further, while both the E.164 and NSAP modeled address formats are to be considered as network points of attachment, it seems that E.164 only networks are to be considered as subordinate to "private networks", in some sense. This leads to some confusion in defining an ARP mechanism in supporting all combinations of end-to-end scenarios (refer to the discussion in Appendix A on the possible scenarios to be supported by ARP).7. Extensions to IP Routing RFC-1620 [3] describes the problems and issues associated with direct connections across IP subnet boundaries in greater detail, as well as possible solution approaches. The ROLC WG has identified persistent routing loop problems that can occur if protocols which lose information critical to path vector routing protocol loop suppression are used to accomplish direct connections across IP subnetCole, Shur & Villamizar Informational [Page 18] RFC 1932 IP over ATM: A Framework Document April 1996 boundaries. The problems may arise when a destination network which is not on the NBMA network is reachable via different routers attached to the NBMA network. This problem occurs with proposals that attempt to carry reachability information, but do not carry full path attributes (for path vector routing) needed for inter-AS path suppression, or full metrics (for distance vector or link state routing even if path vector routing is not used) for intra-AS routing. For example, the NHRP protocol may be used to support the establishment of direct connections across subnetwork boundaries. NHRP assumes that routers do run routing protocols (intra and/or inter domain) and/or static routing. NHRP further assumes that forwarding tables constructed by these protocols result in a steady state loop-free forwarding. Note that these two assumptions do not impose any additional requirements on routers, beyond what is required in the absence of NHRP. NHRP runs in addition to routing protocols, and provides the information that allows the elimination of multiple IP hops (the multiple IP hops result from the forwarding tables constructed by the routing protocols) when traversing an NBMA network. The IPATM and ROLC WGs have both expended considerable effort in discussing and coming to understand these limitations. It is well-known that truncating path information in Path Vector protocols (e.g., BGP) or losing metric information in Distance Vector protocols (e.g., RIP) could result in persistent forwarding loops. These loops could occur without ATM and without NHRP. The combination of NHRP and static routing alone cannot be used in some topologies where some of the destinations are served by multiple routers on the NBMA. The combination of NHRP and an intra-AS routing protocol that does not carry inter-AS routing path attributes alone cannot be used in some topologies in which the NBMA will provide inter-AS transit connectivity to destinations from other AS served by multiple routers on the NBMA. Figure 4 provides an example of the routing loops that may be formed in these circumstances. The example illustrates how the use of NHRP in the environment where forwarding loops could exist even without NHRP (due to either truncated path information or loss of metric information) would still produce forwarding loops. There are many potential scenarios for routing loops. An example is given in Figure 4. It is possible to produce a simpler example where a loop can form. The example in Figure 4 illustrates a loop whichCole, Shur & Villamizar Informational [Page 19] RFC 1932 IP over ATM: A Framework Document April 1996 will persist even if the protocol on the NBMA supports redirects or can invalidate any route which changes in any way, but does not support the communication of full metrics or path attributes. .----. .----. : H1 >----< S1 : Notes: ---- vvvv H#n == host #n / : \ R#n == router #n / : \ S#n == subnet #n /------/ : \ : : \ S2 to R3 breaks .--^---. .----. .-^--. : : : R4 : : R6 : : NBMA : --v- --v- See the text for : : : : details of the -v--v- = = looping conditions : \ = SLOW = and mechanisms : .-^--. = LINK = : : R2 : = = : --v- : : : : .--^-. .--^-. .-^--. : : R5 : : R7 : : R8 : : --v- --v- --v- \ : : : \ / : \ .-^^-. .--^-. \ : S2 : : S4 : \ --v- --v- \ \ / \ \ / \ .^--^. \ : R3 : path before the break is \ -v-- H1->S1->R1->NBMA->R2->S2->R3->H2 \ / .----. .-^^-. path after the break is : H2 >---< S3 : H1->S1->R1->NBMA->R2->S2->R5->R4->S1 ---- ---- \------<--the-loop--<-------/ Figure 4: A Routing Loop Due to Lost PV Routing Attributes. In the example in Figure 4, Host 1 is sending traffic toward Host 2. In practice, host routes would not be used, so the destination for the purpose of routing would be Subnet 3. The traffic travels by way of Router 1 which establishes a "cut-through" SVC to the NBMA next- hop, shown here as Router 2. Router 2 forwards traffic destined for Subnet 3 through Subnet 2 to Router 3. Traffic from Host 1 would then reach Host 2.Cole, Shur & Villamizar Informational [Page 20] RFC 1932 IP over ATM: A Framework Document April 1996 Router 1's cut-through routing implementation caches an association between Host 2's IP address (or more likely all of Subnet 3) and Router 2's NBMA address. While the cut-through SVC is still up, Link 1 fails. Router 5 loses it's preferred route through Router 3 and must direct traffic in the other direction. Router 2 loses a route through Router 3, but picks up an alternate route through Router 5. Router 1 is still directing traffic toward Router 2 and advertising a means of reaching Subnet 3 to Subnet 1. Router 5 and Router 2 will see a route, creating a loop. This loop would not form if path information normally carried by interdomain routing protocols such as BGP and IDRP were retained across the NBMA. Router 2 would reject the initial route from Router 5 due to the path information. When Router 2 declares the route to Subnet 3 unreachable, Router 1 withdraws the route from routing at Subnet 1, leaving the route through Router 4, which would then reach Router 5, and would reach Router 2 through both Router 1 and Router 5. Similarly, a link state protocol would not form such a loop. Two proposals for breaking this form of routing loop have been discussed. Redirect in this example would have no effect, since Router 2 still has a route, just has different path attributes. A second proposal is that is that when a route changes in any way, the advertising NBMA cut-through router invalidates the advertisement for some time period. This is similar to the notion of Poison Reverse in distance vector routing protocols. In this example, Router 2 would eventually readvertise a route since a route through Router 6 exists. When Router 1 discovers this route, it will advertise it to Subnet 1 and form the loop. Without path information, Router 1 cannot distinguish between a loop and restoration of normal service through the link L1. The loop in Figure 4 can be prevented by configuring Router 4 or Router 5 to refuse to use the reverse path. This would break backup connectivity through Router 8 if L1 and L3 failed. The loop can also be broken by configuring Router 2 to refuse to use the path through Router 5 unless it could not reach the NBMA. Special configuration of Router 2 would work as long as Router 2 was not distanced from Router 3 and Router 5 by additional subnets such that it could not determine which path was in use. If Subnet 1 is in a different AS or RD than Subnet 2 or Subnet 4, then the decision at Router 2 could be based on path information.Cole, Shur & Villamizar Informational [Page 21] RFC 1932 IP over ATM: A Framework Document April 1996 .--------. .--------. : Router : : Router : --v-v--- ---v-v-- : : : : .--------. .--------. : : .--------. : : .--------. .--------. : Sub-ES :---: Subnet :-/ \-: Subnet :-/ \-: Subnet :---: Sub-ES : -------- -------- -------- -------- -------- Figure 5: The Classical IP model as a concatenation of three separate ATM IP subnets. In order for loops to be prevented by special configuration at the NBMA border router, that router would need to know all paths that could lead back to the NBMA. The same argument that special⌨️ 快捷键说明
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