Multiple Interfaces Problem Statement
draft-ietf-mif-problem-statement-00.txt

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Abstract

A multihomed host receives node configuration information from each of its access networks. 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.

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.  Interactive Connectivity Establishment (ICE)
    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 Networks
5.  Problems
6.  Summary
7.  Security Considerations
8.  IANA Considerations
9.  Acknowledgements
10.  Discussion home for this draft
11.  Informative References
§  Authors' Addresses

1.  Introduction

A multihomed host has multiple network interfaces (physical and/or virtual). 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 that are simultaneously connected to networks.

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 access network, 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.mrw‑mif‑current‑practices] (Wasserman, M., “Current Practices for Multiple Interface Hosts,” March 2009.) discusses current practices.

2.  Terminology

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 active interfaces, 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 provisionning domains, or, at least, on a limited number of provisionning domains.
• While the MIF host may forward packets between its interfaces, forwarding packets is not taken into account in this definition.

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.

Proxy MIP allows 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. Furthermore, link aggregation done under IP where a single interace 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 [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 [RFC5112] (Garcia-Martin, M., “The Presence-Specific Static Dictionary for Signaling Compression (Sigcomp),” January 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.  Interactive Connectivity Establishment (ICE)

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.

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.

An API is also defined [RFC5014] (Nordmark, E., Chakrabarti, S., and J. Laganier, “IPv6 Socket API for Source Address Selection,” September 2007.) to influence the default address selection mechanism by specifying attributes of the source addresses it prefers.

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.

4.  Symptoms

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

These situations are also described in [I‑D.savolainen‑6man‑fqdn‑based‑if‑selection] (Savolainen, T., “Domain name based network interface selection,” October 2008.), [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 right one S1 would have given. Therefore, the application tries to connect to the wrong destination host, which may imply security issues.
4. TBD: example with different address families.

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 the IP1 address family, H1 has one default route (R1, R2) per network (N1, N2). IP1 is only reachable by N2. H1 stack uses R1 and tries to send through I1. 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‑6man‑fqdn‑based‑if‑selection] (Savolainen, T., “Domain name based network interface selection,” October 2008.), [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 networks and 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). The user or the application is trying to reach an IP address (IP1). Any of the following situations may occur:

1. 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.
2. TBD: add more

Merging address selection policies may have important impacts on routing.

4.4.  Single Interface on Multiple Networks

When a MIF host using a single interface is connected to multiple networks with different default routers, similar issues as described above happen.

5.  Problems

This section tries to list the underlying problems corresponding to the issues discussed in the previous section.

1. Routing tables are usually node-scoped.
2. DNS server addresses and other configuration objects (NTP servers, ...) are usually node-scoped.
3. Same configuration objects (eg DNS server addresses, NTP server addresses, ..) received from multiple interfaces or networks are usually overwritten.
4. Default Address Selection policies are usually node-scoped.
5. Default Address Selection policies may differ when received on different interfaces.
6. Host implementations usually do not implement the [RFC1122] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.) strong model where the source address is in the routing table.
7. Host implementations usually do not implement the [RFC1122] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.) model where the Type-of-Service are in the routing table.
8. Host implementations usually do not keep path characteristics, user or provider preferences in the routing table.
9. Applications usually do not use advanced APIs to specify the source IP address or to set preferences on the address selection policies.
10. DNS answers are usually not kept with the interface from which the answer comes from.
11. Host implementations usually do not keep separate network configuration (such as DNS server addresses) per interface.

6.  Summary

A MIF host receives node configuration information from each of its access networks. 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. 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.  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, Christian Vogt, Lars Eggert, Margaret Wasserman, Hui Deng, Ralph Droms, Ted Hardie, Christian Huitema, Rémi Denis-Courmont, Carl Williams, Pierrick Seite. Sorry if some contributors have not been named.

10.  Discussion home for this draft

This document is intended to define the problem space discussed in the mif@ietf.org mailing list.

11. Informative References

Authors' Addresses