Request for Comments: 3582
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Goals for IPv6 Site-Multihoming Architectures
Status of this Memo
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
Copyright © The Internet Society (2003). All Rights Reserved.
This document outlines a set of goals for proposed new IPv6 site- multihoming architectures. It is recognised that this set of goals is ambitious and that some goals may conflict with others. The solution or solutions adopted may only be able to satisfy some of the goals presented here.
Site-multihoming, i.e., connecting to more than one IP service provider, is an essential component of service for many sites which are part of the Internet.
Current IPv4 site-multihoming practices have been added on to the CIDR architecture , which assumes that routing table entries can be aggregated based upon a hierarchy of customers and service providers.
However, it appears that this hierarchy is being supplanted by a dense mesh of interconnections . Additionally, there has been an enormous growth in the number of multihomed sites. For purposes of redundancy and load-sharing, the multihomed address blocks are introduced into the global table even if they are covered by a provider aggregate. This contributes to the rapidly-increasing size of both the global routing table and the turbulence exhibited within it, and places stress on the inter-provider routing system.
Continued growth of both the Internet and the practice of site- multihoming will seriously exacerbate this stress. The site- multihoming architecture for IPv6 should allow the routing system to scale more pleasantly.
A "site" is an entity autonomously operating a network using IP, and in particular, determining the addressing plan and routing policy for that network. This definition is intended to be equivalent to "enterprise" as defined in .
A "transit provider" operates a site that directly provides connectivity to the Internet to one or more external sites. The connectivity provided extends beyond the transit provider's own site. A transit provider's site is directly connected to the sites for which it provides transit.
A "multihomed" site is one with more than one transit provider. "Site-multihoming" is the practice of arranging a site to be multihomed.
The term "re-homing" denotes a transition of a site between two states of connectedness due to a change in the connectivity between the site and its transit providers' sites.
3. Multihoming Goals
3.1. Capabilities of IPv4 Multihoming
The following capabilities of current IPv4 multihoming practices should be supported by an IPv6 multihoming architecture.
By multihoming, a site should be able to insulate itself from certain failure modes within one or more transit providers, as well as failures in the network providing interconnection among one or more transit providers.
Infrastructural commonalities below the IP layer may result in connectivity which is apparently diverse, sharing single points of failure. For example, two separate DS3 circuits ordered from different suppliers and connecting a site to independent transit providers may share a single conduit from the street into a building; in this case, physical disruption (sometimes referred to as "backhoe-fade") of both circuits may be experienced due to a single incident in the street. The two circuits are said to "share fate".
The multihoming architecture should accommodate (in the general case, issues of shared fate notwithstanding) continuity of connectivity during the following failures:
- Physical failure, such as a fiber cut, or router failure,
- Logical link failure, such as a misbehaving router interface,
- Routing protocol failure, such as a BGP peer reset,
- Transit provider failure, such as a backbone-wide IGP failure, and
- Exchange failure, such as a BGP reset on an inter-provider peering.
3.1.2. Load Sharing
By multihoming, a site should be able to distribute both inbound and outbound traffic between multiple transit providers. This goal is for concurrent use of the multiple transit providers, not just the usage of one provider over one interval of time and another provider over a different interval.
By multihoming, a site should be able to protect itself from performance difficulties directly between the site's transit providers.
For example, suppose site E obtains transit from transit providers T1 and T2, and there is long-term congestion between T1 and T2. The multihoming architecture should allow E to ensure that in normal operation, none of its traffic is carried over the congested interconnection T1-T2. The process by which this is achieved should be a manual one.
A multihomed site should be able to distribute inbound traffic from particular multiple transit providers according to the particular address range within their site which is sourcing or sinking the traffic.
A customer may choose to multihome for a variety of policy reasons beyond technical scope (e.g., cost, acceptable use conditions, etc.) For example, customer C homed to ISP A may wish to shift traffic of a certain class or application, NNTP, for example, to ISP B as matter of policy. A new IPv6 multihoming proposal should provide support for site-multihoming for external policy reasons.
As any proposed multihoming solution must be deployed in real networks with real customers, simplicity is paramount. The current multihoming solution is quite straightforward to deploy and maintain.
A new IPv6 multihoming solution should not be substantially more complex to deploy and operate (for multihomed sites or for the rest of the Internet) than current IPv4 multihoming practices.
3.1.6. Transport-Layer Survivability
Multihoming solutions should provide re-homing transparency for transport-layer sessions; i.e., exchange of data between devices on the multihomed site and devices elsewhere on the Internet may proceed with no greater interruption than that associated with the transient packet loss during the re-homing event.
New transport-layer sessions should be able to be created following a re-homing event.
Transport-layer sessions include those involving transport-layer protocols such as TCP, UDP and SCTP over IP. Applications which communicate over raw IP and other network-layer protocols may also enjoy re-homing transparency.
3.1.7. Impact on DNS
Multi-homing solutions either should be compatible with the observed dynamics of the current DNS system, or the solutions should demonstrate that the modified name resolution system required to support them is readily deployable.
3.1.8. Packet Filtering
Multihoming solutions should not preclude filtering packets with forged or otherwise inappropriate source IP addresses at the administrative boundary of the multihomed site, or at the administrative boundaries of any site in the Internet.
3.2. Additional Requirements
Current IPV4 multihoming practices contribute to the significant growth currently observed in the state held in the global inter- provider routing system; this is a concern, both because of the hardware requirements it imposes, and also because of the impact on the stability of the routing system. This issue is discussed in great detail in .
A new IPv6 multihoming architecture should scale to accommodate orders of magnitude more multihomed sites without imposing unreasonable requirements on the routing system.
3.2.2. Impact on Routers
The solutions may require changes to IPv6 router implementations, but these changes should be either minor, or in the form of logically separate functions added to existing functions.
Such changes should not prevent normal single-homed operation, and routers implementing these changes should be able to interoperate fully with hosts and routers not implementing them.
3.2.3. Impact on Hosts
The solution should not destroy IPv6 connectivity for a legacy host implementing RFC 3513 , RFC 2460 , RFC 3493 , and other basic IPv6 specifications current in April 2003. That is to say, if a host can work in a single-homed site, it should still be able to work in a multihomed site, even if it cannot benefit from site- multihoming.
It would be compatible with this goal for such a host to lose connectivity if a site lost connectivity to one transit provider, despite the fact that other transit provider connections were still operational.
If the solution requires changes to the host stack, these changes should be either minor, or in the form of logically separate functions added to existing functions.
If the solution requires changes to the socket API and/or the transport layer, it should be possible to retain the original socket API and transport protocols in parallel, even if they cannot benefit from multihoming.
The multihoming solution may allow host or application changes if that would enhance transport-layer survivability.
3.2.4. Interaction between Hosts and the Routing System
The solution may involve interaction between a site's hosts and its routing system; such an interaction should be simple, scalable and securable.
3.2.5. Operations and Management
It should be possible for staff responsible for the operation of a site to monitor and configure the site's multihoming system.
3.2.6. Cooperation between Transit Providers
A multihoming strategy may require cooperation between a site and its transit providers, but should not require cooperation (relating specifically to the multihomed site) directly between the transit providers.
The impact of any inter-site cooperation that might be required to facilitate the multihoming solution should be examined and assessed from the point of view of operational practicality.
3.2.7. Multiple Solutions
There may be more than one approach to multihoming, provided all approaches are orthogonal (i.e., each approach addresses a distinct segment or category within the site multihoming problem). Multiple solutions will incur a greater management overhead, however, and the adopted solutions should attempt to cover as many multihoming scenarios and goals as possible.
4. Security Considerations
A multihomed site should not be more vulnerable to security breaches than a traditionally IPv4-multihomed site.
Any changes to routing practices made to accommodate multihomed sites should not cause non-multihomed sites to become more vulnerable to security breaches.
5. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director.
6. Normative References
 Fuller, V., Li, T., Yu, J. and K. Varadhan, "Classless Inter- Domain Routing (CIDR): an Address Assignment and Aggregation Strategy", RFC 1519, September 1993.
 Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and E.
Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996.
 Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6) Addressing Architecture", RFC 3513, April 2003.  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998.  Gilligan, R., Thomson, S., Bound, J., McCann, J. and W. Stevens, "Basic Socket Interface Extensions for IPv6", RFC 3493, February 2003.  Huston, G., "Commentary on Inter-Domain Routing in the Internet", RFC 3221, December 2001.
7. Authors' Addresses
Joe Abley Internet Software Consortium 950 Charter Street Redwood City, CA 94063 USA Phone: +1 650 423 1317 EMail: firstname.lastname@example.org
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