INTERNET DRAFT M. Ohta draft-ohta-e2e-multihoming-04.txt Tokyo Institute of Technology June 2003 The Architecture of End to End Multihoming Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Abstract This memo describes the architecture of end to end multihoming. End to end multihoming does not burden routing system for multihoming. That is, even extensive use of end to end multihoming does not increase the number of entries in a global routing table. Traditionally with IPv4, multihoming capability is offered by an intelligent routing system, which, as is always the case with violating the end to end principle, lacks scalability on a global routing table size and robustness against link failures. On the other hand, with end to end multihoming, multihoming is supported by transport (TCP) or application layer (UDP etc.) of end systems and does not introduce any problem in the network and works as long as there is some connectivity between the end systems. Because end to end multihoming is performed in end systems, the architecture needs no routing protocol changes Instead, APIs and applications must be modified to some extent. 1. Introduction M. Ohta Expires on December 20, 2003 [Page 1]
INTERNET DRAFT End to End Multihoming June 2003 Multihoming is a way for hosts have robust connectivity to the Internet. Traditionally with IPv4, multihoming has been offered through intelligence of routing system, that is, the end to end principle has been ignored. However, as discussed in section 2, with the explosive deployment of the Internet, as is always the case with intelligent networking, IPv4 style multihoming was revealed its lack of scalability and robustness. Instead, multihoming can be supported based on the end to end principle by assigning multiple addresses to an interface of a host and let end systems choose an appropriate address at the transport or the application layer. To support the end to end multihoming, no change is necessary on routing protocols. Instead, APIs and applications must be modified to detect and react against the loss of connection. In case of TCP where there is a network wide agreement on the semantics of the loss of connectivity, most of the work can be done by the kernel code at the transport layer, though some timing may be adjusted for some application. However, in general, the condition of "loss of connectivity" varies application by application that the multihoming must directly be controlled by application programs. With IPV6, there is a partial effort of end to end multihoming that it is of course that an interface has multiple addresses. However, because the principle of the end to end multihoming is recognized merely subconsciously and because the current routing architecture violates the end to end principle in various ways as a result of partial attempt to avoid the lack of scalability of routing table size, there are a lot of attempts: to keep the APIs and application programs as is to modify routing system intelligent to let it automatically offer end to end multihoming which makes IPv6 multihoming as bad as that of IPv4. This memo describes why multihoming by intelligent routing system is harmful, how the current routing architecture is damaged and how the APIs and the applications should be modified to implement the end to end multihoming. 2. Multihoming by Intelligent Routing System is Harmful M. Ohta Expires on December 20, 2003 [Page 2]
INTERNET DRAFT End to End Multihoming June 2003 2.1 Routing Table Size See IAB Network Layer Workshop. 2.2 Lack of Robustness With route aggregation, routing information can be carried only for an aggregated area that a loss of connectivity for a part of the area can not be detected. For example, if a multihomed site with multiple subnets has a single global routing table entry, and if a site is partitioned, only a part of the partitioned can be reached with the global routing table entry. A solution is to let all the subnets of the site (or, ultimately, hosts) individually have global routing table entry, scalability of which is so absurd that no one bothers to try. 3. The Problems of Current Routing Architecture End systems (hosts) are end systems. To make the end to end principle effectively work, the end systems must have all the available knowledge to make decisions by the end systems themselves. With regard to multihoming, when an end system want to communicate with a multihomed end system, the end system must be able to select most appropriate (based on the local information) destination address of the multihomed end system. However, some think it of course to separate routers and nodes and let hosts not have routing information (which means the current IPv6 architecture is broken) and, worse, let most routers use default routes. With detailed route information, end systems can use the information as a hint to select the best destination address. However, with default route, end systems have no idea on what is the best address and must blindly try all the possibilities at random. It was partly because full routing table was large and was not be able to be held in a chip on end systems and partly because hosts should not be affected by routing protocol changes. In the past, IPv4 address was not assigned with hierarchy and scale of integration was small. But, with smaller full routing table and larger scale of integration, M. Ohta Expires on December 20, 2003 [Page 3]
INTERNET DRAFT End to End Multihoming June 2003 there is no reason not to have full routing table on every end system. As there are a lot of routers used in LAN or even in home, it is not so meaningful that we don't have to upgrade software on hosts, if we have to upgrade software on routers. The situation is worse with multicasting. For example, IGMP, which separates routers and nodes, is a total nonsense as IGMP has been revised several times upon multicast routing protocol changes. Moreover, all the legacy multicast routing protocols use intelligent routing system to deliver group specific information and does not scale. Multicast architecture must be redone with the end to end principle in mind. SM (Static Multicast) proposed by the Authors is such a proposal but latter proposals (such as "simple multicast" or "AS based static allocation") modified it without understanding the underlying end to end principle and are useless. Once a full routing table is available on all the end systems, it is easy for the end systems try all the destination addresses, from the most and to the least favorable ones, based on the routing metric. Note that end to end multihoming works with the separation between inter domain BGP and intra domain routing protocols, if BGP routers, based on domain policy, assign external routes preference values (metric) of intra domain routing protocols. One may still be allowed, though discouraged, to have local configuration with dumb end systems and an intelligent proxy. But, such configuration should be implemented with a protocol for purely local use without damaging the global protocol. 4. Modifications on APIs and Applications With TCP, applications must be able to pass multiple addresses to transport layer (e.g. BSD socket). All the other processing can be performed by transport layer (typically in kernel) using default or application specific timing of TCP. TCP itself must be modified that all the possible addresses of a host is transmitted to its peer through a TCP option. TCP connections are identified with all the addresses constitute an identical connection. Without TCP, applications must be able to detect loss of connectivity in application dependent way and try other addresses by themselves or tell transport layer to do so. Applications must still be able to pass multiple addresses of the destination to transport layer (e.g. BSD socket) to receive a packet to alternative addresses sent from M. Ohta Expires on December 20, 2003 [Page 4]
INTERNET DRAFT End to End Multihoming June 2003 the other end of the communication. The easiest way for applications know all the addresses of the destination is to use DNS. With DNS reverse, followed by forward, lookup, applications can get a list of all the addresses of the destination from an address of the destination. Note that policy based selection of a source address is incompatible to the end to end principle, because it must be selected as the destination address by the policy of the other end of the communication using information local to the other end of the communication. With so much asymmetric routing of the Internet today, proper destination address to reply can not be guessed by the querier. Thus, DNS query should be modified to carry all the addresses of clients and servers should try from the most favorable to the least ones, based on locally available information such as metric in intra domain full routing table. Any source address may be selected. However, to enable source address filtering to discard packets with source addresses not belonging to an ISP, it is useful to enable a host, not some intelligent intermediate router, select a source address compatible with an outgoing ISP. For that purpose, intra domain routing protocols should maintain routing table entries with not only preference values of an external routes, but also proper prefixes to be selected for source addresses, if the entries are chosen by a host. With DNS, it is also required that DNS reverse lookup works properly. But, as the reverse lookup is the mechanism to delegate IP addresses, the requirement is no more demanding than assigning valid IP addresses. A problem is that locally scoped address (IPv6 link and site local addresses) can not be used for reverse look up. Use of 8+8 addressing proposed by one of an Author with globally unique IID (Internet ID) and ILOC (Internet Locator) is strongly encouraged. With 8+8 addressing, DNS reverse lookup can be performed with IID part only. Note that 8+8 proposal must not be confused by latter proposal of routing goof by Mike O'dell, which is a proposal to use intelligent routers to rewrite source addresses to prevent source address spoofing and to tunnel between intelligent routers for pseudo multihoming, both of which are against the end to end principle and, thus, lacks robustness and/or scalability. M. Ohta Expires on December 20, 2003 [Page 5]
INTERNET DRAFT End to End Multihoming June 2003 5. Conclusions For robust and scalable multihoming, IPv6 separation of nodes and routers must be removed and the transition to the end to end multihoming should occur with the transition to IPv6. The modification is not so difficult, because most of the applications, which must be modified for IPV6 anyway, use TCP only and most of the UDP applications are DNS, which already tries all the addresses, or multicast capable ones, which are hopeless. One may argue that we can't further delay the transition to IPv6 merely because of a random proposal on multihoming. Then, it is a good idea to start another transition, separated from ones with legacy IPv6, by allocating a new address prefix of IPv6 address space for the end to end multihoming with 8+8 addressing. It is important to make the end to end principle work by keeping the number of top level routing prefix under the new address prefix small Politics of address space allocation may be avoided if those who are allocated IPv6 address space with the current prefix are automatically allocated corresponding address space with the new prefix. It should be noted that, because of the end to end nature, the architecture can be implemented purely on end systems without modifying routing functionality of existing IPv4 or IPv6 routers. 6. Security Considerations The author believes there is no special security concern. 7. Author's Address Masataka Ohta Graduate School of Information Science and Engineering Tokyo Institute of Technology 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, JAPAN Phone: +81-3-5734-3299 Fax: +81-3-5734-3299 EMail: mohta@necom830.hpcl.titech.ac.jp M. Ohta Expires on December 20, 2003 [Page 6]