Multiple Interfaces Problem Statement
draft-ietf-mif-problem-statement-04.txt

Abstract

A multihomed host receives node configuration information from each of its provisioning domain. Some configuration objects are global to the node, some are local to the interface. Various issues arise when multiple conflicting node-scoped configuration objects are received on multiple interfaces. Similar situations also happen with single interface host connected to multiple networks. This document describes these issues.

Status of this Memo

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Table of Contents

1.  Introduction
2.  Terminology
3.  Scope and Existing Work
    3.1.  Below IP Interaction
    3.2.  Hosts Requirements
    3.3.  Mobility and other IP protocols
    3.4.  Address Selection
    3.5.  Finding and Sharing IP Addresses with Peers
    3.6.  Socket API
    3.7.  Above IP Layers
4.  Symptoms
    4.1.  DNS resolution issues
    4.2.  Routing
    4.3.  Address Selection Policy
    4.4.  Single Interface on Multiple Provisioning Domains
5.  Problems
6.  Summary
7.  Security Considerations
8.  IANA Considerations
9.  Authors
10.  Acknowledgements
11.  Informative References
§  Authors' Addresses

1.  Introduction

A multihomed host have multiple provisioning domains (via physical and/or virtual interfaces). For example, a node may be simultaneously connected to a wired Ethernet LAN, a 802.11 LAN, a 3G cell network, one or multiple VPN connections or one or multiple automatic or manual tunnels. Current laptops and smartphones typically have multiple access network interfaces and, thus, may be simultaneously connected to different provisioning domains.

A multihomed host receives node configuration information from each of its access networks, through various mechanims such as DHCPv4 [RFC2131] (Droms, R., “Dynamic Host Configuration Protocol,” March 1997.), DHCPv6 [RFC3315] (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.), PPP [RFC1661] (Simpson, W., “The Point-to-Point Protocol (PPP),” July 1994.) and IPv6 Router Advertisements [RFC4861] (Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” September 2007.). Some received configuration objects are specific to an interface such as the IP address and the link prefix. Others are typically considered by implementations as being global to the node, such as the routing information (e.g. default gateway), DNS servers IP addresses and address selection policies.

When the received node-scoped configuration objects have different values from each provisioning domains, such as different DNS servers IP addresses, different default gateways or different address selection policies, the node has to decide which it will use or how it will merge them.

Several issues regarding how the node-scoped configuration objects are used in a multihomed node environment have been raised. The following sections define the MIF host and the scope of this document, describe related work, list the symptoms and then the underlying problems.

A companion document [I‑D.ietf‑mif‑current‑practices] (Wasserman, M., “Current Practices for Multiple Interface Hosts,” October 2009.) discusses current practices.

2.  Terminology

Administrative domain

A group of hosts, routers, and networks operated and managed by a single organization [RFC1136] (Hares, S. and D. Katz, “Administrative Domains and Routing Domains: A model for routing in the Internet,” December 1989.).

Provisioning domain

A set of consistent configuration information (e.g. Default router, Network prefixes, DNS,...). 0ne administrative domain can contain multiple provisioning domains.

A MIF host is defined as:

• A [RFC1122] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.) IPv4 and/or [RFC4294] (Loughney, J., “IPv6 Node Requirements,” April 2006.) IPv6 compliant host
• Configured with more than one IP addresses (excluding loopback, link-local)
• On one or more provisioning domains, as presented to the IP stack.
• The interfaces may be logical, virtual or physical.
• The IP addresses come from more than one administrative domains.
• The IP addresses may be from the same or from different address families, such as IPv4 and IPv6.
• Communications using these IP addresses may happen simultaneously and independently.
• Communications using these IP addresses may be tied on all the possible provisioning domains, or, at least, on a limited number of provisioning domains.
• While the MIF host may forward packets between its interfaces, forwarding packets is not taken into account in this definition.

Reference to IP version

When a protocol keyword such as IP, PPP, DHCP is used without any reference to a specific IP version, then it implies both IPv4 and IPv6. A specific IP version keyword such as DHCPv4 or DHCPv6 is specific to that IP version.

3.  Scope and Existing Work

This section describes existing related work and defines the scope of the problem.

3.1.  Below IP Interaction

Network discovery and selection on lower layers as defined by [RFC5113] (Arkko, J., Aboba, B., Korhonen, J., and F. Bari, “Network Discovery and Selection Problem,” January 2008.) is out of scope of this document. Moreover, lower layer interaction such as IEEE 802.21 is also out of scope.

Some mechanisms (e.g., based on a logical IP interface) allow sharing a single IP address across multiple interfaces  (e.g., WiMAX and CDMA, LTE and HSPA, etc.) to disparate networks. From the IP stack view on the node, there is only a single interface and single IP address. Therefore, this situation is out of scope of this current problem statement. Furthermore, link aggregation done under IP where a single interface is shown to the IP stack is also out of scope.

3.2.  Hosts Requirements

The requirements for Internet Hosts [RFC1122] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.) describe the multihomed host as if it has multiple IP addresses, which may be associated with one or more physical interfaces connected to the same or different networks.

The host maintains a route cache table where each entry contains the local IP address, the destination IP address, Type-of-Service and Next-hop gateway IP address. The route cache entry would have data about the properties of the path, such as the average round-trip delay measured by a transport protocol.

As per [RFC1122] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.), two models are defined:

• The "Strong" host model defines a multihomed host as a set of logical hosts within the same physical host. In this model a packet must be sent on an interface that corresponds to the source address of that packet.
• The "Weak" host model describes a host that has some embedded gateway functionality. In the weak host model, the host can send and receive packets on any interface.

The multihomed host computes routes for outgoing datagrams differently depending on the model. Under the strong model, the route is computed based on the source IP address, the destination IP address and the Type-of-Service. Under the weak model, the source IP address is not used, but only the destination IP address and the Type-of-Service.

3.3.  Mobility and other IP protocols

This document assumes hosts only implementing [RFC1122] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.) for IPv4 and [RFC4294] (Loughney, J., “IPv6 Node Requirements,” April 2006.) for IPv6, and not using any kind of new transport protocols. It is not required for the host to support additional IP mobility or multihoming protocols, such as SHIM6, SCTP, Mobile IP, HIP, RRG, LISP or else. Moreover, the peer of the connection is also not required to use these mechanisms.

3.4.  Address Selection

The Default Address Selection specification [RFC3484] (Draves, R., “Default Address Selection for Internet Protocol version 6 (IPv6),” February 2003.) defines algorithms for source and destination IP address selections. It is mandatory to be implemented in IPv6 nodes, which also means dual-stack nodes. A node-scoped policy table managed by the IP stack is defined. Provisions are made to change or update the policy table, however, no mechanism is defined.

Issues on using the Default Address Selection were found [RFC5220] (Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama, “Problem Statement for Default Address Selection in Multi-Prefix Environments: Operational Issues of RFC 3484 Default Rules,” July 2008.) in the context of multiple prefixes on the same link. New work [I‑D.chown‑addr‑select‑considerations] (Chown, T., “Considerations for IPv6 Address Selection Policy Changes,” July 2009.) discusses the multiple attached networks scenarios and how to update the policy table.

3.5.  Finding and Sharing IP Addresses with Peers

Interactive Connectivity Establishment (ICE (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” October 2007.) [I‑D.ietf‑mmusic‑ice]) is a technique for NAT traversal for UDP-based (and TCP) media streams established by the offer/answer model. The multiplicity of IP addresses and ports in SDP offers are tested for connectivity by peer-to-peer connectivity checks. The result is candidate IP addresses and ports for establishing a connection with the other peer. ICE does not solve the MIF issues, such as the incompatible configuration objects received on different interfaces. However, ICE may be of use for address selection if the application is ICE-enabled.

Some application protocols do referrals (i.e. provides reachability information to itself or to a third-part) of IP addresses and port numbers for further exchanges. Grobj (Carpenter, B., Boucadair, M., Halpern, J., Jiang, S., and K. Moore, “A Generic Referral Object for Internet Entities,” October 2009.) [I‑D.carpenter‑behave‑referral‑object] defines the problem with referrals in today's IP networks. While referrals feature does not solve the MIF issues, it is related since, in a multiple provisioning domain context, referrals must provide consistent information depending on which provisioning domain is used.

3.6.  Socket API

Application Programming Interface (API) may expose objects that user applications may use for the MIF purpose. For example, [RFC3542] (Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, “Advanced Sockets Application Program Interface (API) for IPv6,” May 2003.) shows how an application using the Advanced sockets API can specify the interface or the source IP address, through simple bind() operation or IPV6_PKTINFO socket option.

There are other examples of API dealing with similar issues to MIF. For instance, [RFC5014] (Nordmark, E., Chakrabarti, S., and J. Laganier, “IPv6 Socket API for Source Address Selection,” September 2007.) defines API to influence the default address selection mechanism by specifying attributes of the source addresses it prefers. [I‑D.ietf‑shim6‑multihome‑shim‑api] (Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto, “Socket Application Program Interface (API) for Multihoming Shim,” February 2010.) gives another example in a multihoming context, by defining a socket API enabling interactions between applications and the multihoming shim layer for advanced locator management, and access to information about failure detection and path exploration.

In the MIF context, some implementations, specially in the mobile world, rely on higher-level connection managers to deal with issues brought by multiple provisioning domains. Typically, the connection manager can select the provisioning domain when application is domain scoped. Connection managers usually leverage on API to gather information and/or for control purpose. If examples exist, as reminded above, there is no set of high level API to provide all required services for a connection manager expected to address IP configuration issues in a context of multiple provisioning domains. Moreover, various operation system implementations deliver different sets of high level API. In addition, these mechanisms do not necessarily behave the same way across different platform and OS in the presence of the MIF problems [I‑D.ietf‑mif‑current‑practices] (Wasserman, M., “Current Practices for Multiple Interface Hosts,” October 2009.). This lack of harmonization is an issue since it may lead to multiple instantiation of a cross platform/OS connection manager or application.

3.7.  Above IP Layers

The MIF issues discussed in this document assume no changes in transport protocols or applications. However, fixing the issues might involve these layers. For instance, an application may implement the connection management function (as decribed in preceding section).

4.  Symptoms

This section describes the various symptoms found using a MIF host that has already received configuration objects from its various provisioning domains.

These situations are also described in [I‑D.savolainen‑mif‑dns‑server‑selection] (Savolainen, T., “DNS Server Selection on Multi-Homed Hosts,” February 2010.), [I‑D.yang‑mif‑req] (Yang, P., Seite, P., Williams, C., and J. Qin, “Requirements on multiple Interface (MIF) of simple IP,” March 2009.) and [RFC4477] (Chown, T., Venaas, S., and C. Strauf, “Dynamic Host Configuration Protocol (DHCP): IPv4 and IPv6 Dual-Stack Issues,” May 2006.). They occur, for example, when:

1. one interface is on the Internet and one is on a corporate private network. The latter may be through VPN.
2. one interface is on one access network (i.e. wifi) and the other one is on another access network (3G) with specific services.

4.1.  DNS resolution issues

A MIF host (H1) has an active interface(I1) connected to a network (N1) which has its DNS server (S1) and another active interface (I2) connected to a network (N2) which has its DNS server (S2). S1 serves with some private namespace "private.example.com". The user or the application uses a name "a.private.example.com" which is within the private namespace of S1 and only resolvable by S1. Any of the following situations may occur:

1. H1 stack, based on its routing table, uses I2 to reach S1 to resolve "a.private.example.com". H1 never reaches S1. The name is not resolved.
2. H1 keeps only one set of DNS server addresses from the received configuration objects and kept S2 address. H1 sends the DNS A query for a.private.example.com to S2. S2 responds with an error for an non-existant domain (NXDOMAIN). The name is not resolved.
3. H1 keeps only one set of DNS server addresses from the received configuration objects and kept S2 address. H1 sends the DNS A query for a.private.example.com to S2. S2 asks its upstream DNS and gets an IP address for a.private.example.com. However, the IP address is not the same one S1 would have given. Therefore, the application tries to connect to the wrong destination host, or to the wrong interface of the latter, which may imply security issues or result in lack of service.
4. S1 or S2 has been used to resolve "a.private.example.com" to an [RFC1918] (Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, “Address Allocation for Private Internets,” February 1996.) address. Both N1 and N2 are [RFC1918] (Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, “Address Allocation for Private Internets,” February 1996.) addressed networks. IPv4 source address selection may face challenges, as due address overlapping the source/destination IP addresses do not necessarily provide enough information for making proper address selection decisions.
5. H1 has resolved an FQDN to locally valid IP address when connected to N1. After movement from N1 to N2, the host tries to connect to the same IP address as earlier, but as the address was only locally valid, connection setup fails.
6. H1 requests AAAA record from a DNS server on a network that uses protocol translators and DNS64 [I‑D.ietf‑behave‑dns64] (Bagnulo, M., Sullivan, A., Matthews, P., and I. Beijnum, “DNS64: DNS extensions for Network Address Translation from IPv6 Clients to IPv4 Servers,” March 2010.). If the H1 receives synthesized AAAA record, it is guaranteed to be valid only on the network it was learned from. If the H1 uses synthesized AAAA on an network interface it is not valid on, the packets will be dropped by the network.

4.2.  Routing

A MIF host (H1) has an active interface(I1) connected to a network (N1) and another active interface (I2) connected to a network (N2). The user or the application is trying to reach an IP address (IP1). Any of the following situations may occur:

1. For IP1 , H1 has one default route (R1) via network (N1). So, trying to reach IP1, H1 stack uses R1 and sends through I1. If IP1 is only reachable by N2, IP1 is never reached or is not the right target.
2. For the IP1 address family, H1 has one default route (R1, R2) per network (N1, N2). IP1 is reachable by both networks, but N2 path has better characterictics, such as better round-trip time, least cost, better bandwidth, etc.... These preferences could be defined by user, by the provider, by discovery or else. H1 stack uses R1 and tries to send through I1. IP1 is reached but the service would be better by I2.
3. For the IP1 address family, H1 has a default route (R1), a specific X.0.0.0/8 route R1B (eg. RFC1918 prefix) to N1 and a default route (R2) to N2. IP1 is reachable by N2 only, but the prefix (X.0.0.0/8) is used in both networks. Because of the most specific route R1B, H1 stack sends through I2 and never reach the target.

A MIF host may have multiple routes to a destination. However, by default, it does not have any hint concerning which interface would be the best to use for that destination. For example, as discussed in [I‑D.savolainen‑mif‑dns‑server‑selection] (Savolainen, T., “DNS Server Selection on Multi-Homed Hosts,” February 2010.), [I‑D.hui‑ip‑multiple‑connections‑ps] (Hui, M. and H. Deng, “Problem Statement and Requirement of Simple IP Multi-homing of the Host,” March 2009.) and [I‑D.yang‑mif‑req] (Yang, P., Seite, P., Williams, C., and J. Qin, “Requirements on multiple Interface (MIF) of simple IP,” March 2009.), a service provider might want to influence the routing table of the host connecting to its network.

A host usually has a node-scoped routing table. Therefore, when a MIF host is connected to multiple provisioning domains where each service provider wants to influence the routing table of the host, then conflicts might arise from the multiple routing information being pushed to the host.

A user on such multihomed host might want a local policy to influence which interface will be used based on various conditions.

On a MIF host, some source addresses are not valid if used on some interfaces. For example, an RFC1918 source address might be appropriate on the VPN interface but not on the public interface of the MIF host. If the source address is not chosen appropriately, then sent packets might be filtered in the path if source address filtering is in place ([RFC2827] (Ferguson, P. and D. Senie, “Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing,” May 2000.),[RFC3704] (Baker, F. and P. Savola, “Ingress Filtering for Multihomed Networks,” March 2004.)) and reply packets might never come back to the source.

4.3.  Address Selection Policy

A MIF host (H1) has an active interface(I1) connected to a network (N1) and another active interface (I2) connected to a network (N2). When the user or the application is trying to reach an IP address (IP1), the following situations may occur:

H1 receives from both networks (N1 and N2) an update of its default address selection policy. However, the policies are specific to each network. The policies are merged by H1 stack. Based on the merged policy, the chosen source address is from N1 but packets are sent to N2. The source address is not reachable from N2, therefore the return packet is lost.

Merging address selection policies may have important impacts on routing.

4.4.  Single Interface on Multiple Provisioning Domains

When a MIF host using a single interface is connected to multiple networks with different default routers, similar issues as described above happen. Even with a single interface, a node may wish to connect to more than one configuration domain: that node may use more than one IP source address and may have more than one default router. The node may want to access services that can only be reached using one of the provisioning domain, so it needs to use the right outgoing source address and default gateway to reach that service. In this situation, that node may also need to use different DNS servers to get domain names in those different provisioning domains.

5.  Problems

This section tries to list the underlying problems corresponding to the issues discussed in the previous section. The problems can be divided into five categories: 1) Configuration 2) DNS resolution 3) Routing 4) Address selection and 5) connection management. They are shown as below:

1. Configuration
   1. Configuration objects (e.g. DNS servers, NTP servers, ...) are usually node-scoped.
   2. Same configuration objects (e.g. DNS server addresses, NTP server addresses, ..) received from multiple provisioning domains are usually overwritten.
   3. Host implementations usually do not keep separate network configuration (such as DNS server addresses) per provisioning domain.
   4. Referrals must provide consistent information depending on which provisioning domain is concerned.
2. DNS resolution
   1. DNS server addresses are usually node-scoped.
   2. DNS answers are usually not kept with the interface from which the answer comes from.
3. Routing
   1. Routing tables are usually node-scoped.
   2. Host implementations usually do not implement the [RFC1122] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.) models where the Type-of-Service are in the routing table.
   3. Host implementations usually do not keep path characteristics, user or provider preferences in the routing table.
4. Address selection
   1. Default Address Selection policies are usually node-scoped.
   2. Default Address Selection policies may differ when received on different provisioning domains.
   3. Host implementations usually do not implement the strong host model [RFC1122] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.) where the source address is in the routing table.
   4. Applications usually do not use advanced APIs to specify the source IP address or to set preferences on the address selection policies.
5. Connection management
   1. Some implementations, specially in the mobile world, have higher-level API and/or connection manager to address MIF issues. These mechanisms are not standardized and do not necessarily behave the same way across different OS, and/or platorms, in the presence of the MIF problems.
   2. Provisioning domain selection is a feature of connection management. Domain selection can be tricky due to lot of different situations and selection criteria: some applications are domain-scoped, or may have a preferred provisioning domain (e.g. according to avalaible QoS). Each actor (end-user, operator, service provider, etc.) may also have their preferred provisioning domains (e.g. single out lower cost domain), possibly per application.
   3. The different actors may provide different, and sometimes contradictory, domain selection policies to the connection management function. The connection manager can typically address the issue, but not all connection managers are able to.
   4. A MIF host can support different connection managers, which may have contradictory ways to solve the MIF issues. For instance, because of different selection algorithms, two different connection managers could select different domains in a same context. Or, when dealing with different domain selection policies, a connection manager may give precedence to user policy while another could favour mobile operator policy.

6.  Summary

A MIF host receives node configuration information from each of its provisioning domains. Some configuration objects are global to the node, some are local to the interface. Various issues arise when multiple conflicting node-scoped configuration objects are received via multiple provisioning domains. Similar situations also happen with single interface host connected to multiple networks. Therefore, there is a need to define the appropriate behavior of an IP stack and possibly define protocols to manage these cases.

7.  Security Considerations

The problems discussed in this document have security implications, such as when the packets sent on the wrong interface might be leaking some confidential information. Moreover, the undetermined behavior of IP stacks in the multihomed context bring additional threats where an interface on a multihomed host might be used to conduct attacks targeted to the networks connected by the other interfaces.

8.  IANA Considerations

This document has no actions for IANA.

9.  Authors

This document is a joint effort with authors of the MIF requirements draft [I‑D.yang‑mif‑req] (Yang, P., Seite, P., Williams, C., and J. Qin, “Requirements on multiple Interface (MIF) of simple IP,” March 2009.). The authors of this document, in alphabetical order, include: Marc Blanchet, Jacqni Qin, Pierrick Seite, Carl Williams and Peny Yang.

10.  Acknowledgements

The initial Internet-Drafts prior to the MIF working group and the discussions during the MIF BOF meeting and on the mailing list around the MIF charter scope on the mailing list brought very good input to the problem statement. This draft steals a lot of text from these discussions and the initial drafts. Therefore, the editor would like to acknowledge the following people (in no specific order), from which some text has been taken from: Jari Arkko, Keith Moore, Sam Hartman, George Tsirtsis, Scott Brim, Ted Lemon, Bernie Volz, Giyeong Son, Gabriel Montenegro, Julien Laganier, Teemu Savolainen, Christian Vogt, Lars Eggert, Margaret Wasserman, Hui Deng, Ralph Droms, Ted Hardie, Christian Huitema, Rémi Denis-Courmont, Alexandru Petrescu, Zhen Cao. Sorry if some contributors have not been named.

11. Informative References

Authors' Addresses