Source Address Dependent Routing and Source Address Selection for IPv6 Hosts: Problem Space Overview

BCP 38 recommends ingress traffic routing to prohibit Denial-of-Service (DoS) attacks. As such, datagrams which have source addresses that do not match with the network where the host is attached are discarded . Avoiding packets to be dropped because of ingress filtering is difficult especially in multihomed networks where the host receives more than one prefix from the networks it is connected to, and consequently may have more than one source addresses. Based on BCP 38, BCP 84 introduced recommendations on the routing system for multihomed networks . Recommendations on the routing system for ingress filtering such as in BCP 84 inevitably involve source address checks. This leads to the source address dependent routing (SADR). Source address dependent routing is an issue especially when the host is connected to a multihomed network and is communicating with another host in another multihomed network. In such a case, the communication can be broken in both directions if Network Providers apply ingress filtering and the datagrams contain wrong source addresses (see for more details ). Hosts with simultaneously active interfaces receive multiple prefixes and have multiple source addresses. Datagrams originating from such hosts are likely to be dropped due to ingress filtering policies. Source address selection algorithm needs to be careful to try to avoid ingress filtering on the next-hop router . Many use cases have been reported for source/destination routing, for example . These use cases clearly indicate that the multihomed host or Customer Premises Equipment (CPE) router needs to be configured with correct source prefixes/addresses so that it can forward packets upstream correctly to avoid ingress filtering applied by an upstream Network Provider to drop the packets. In multihomed networks there is a need to do source address based routing if some providers are performing the ingress filtering defined in BCP38 . This requires the routers to consider the source addresses as well as the destination addresses in determining the next hop to send the packet to.

Based on the use cases defined in , the routers may be informed about the source addresses to use for forwarding using extensions to the routing protocols like IS-IS or OSPF . In this document, we describe the scenarios for source address dependent routing from the host perspective. Two flavors can be considered: A host may have a single interface with multiple addresses (from different prefixes or /64s). Each prefix is delegated from different exit routers, and this case can be called multi-prefix multihoming (MPMH). A host may have simultaneously connected multiple interfaces where each interface is connected to a different exit router and this case can be called multi-prefix multiple interface (MPMI). Several limitations arise in such NAT- and NPTv6-based () multihoming contexts (see for example ). NPTv6 is left out of scope of this document. This document was initially written to inform the community about the SADR problem space. It was updated to record the various set of alternate solutions to address that problem space. The 6man consensus is documented in .

SADR can be facilitated at the host with proper next-hop and source address selection. For this, each router connected to different interfaces of the host uses Router Advertisements (RAs, ) to distribute default route, next hop as well as source address/prefix information to the host. As a reminder, while Prefix Information Option (PIO) is defined in , Route Information Option (RIO) is defined in .

The scenario shown in is multi-prefix multi interface where "rtr1" and "rtr2" represent customer premises equipment/routers (CPE) and there are exit routers in both "network 1" and "network 2". If the packets from the host communicating with a remote destination are routed to the wrong exit router, i.e., carry wrong source address, they will get dropped due to ingress filtering.

There is a variant of that is often referred to as a corporate VPN, i.e., a secure tunnel from the host to a router attached to a corporate network. In this case "rtr2" gives access directly to the corporate network, and the link from the host to "rtr2" is a secure tunnel (for example an IPsec tunnel). The interface is therefore a virtual interface, with its own IP address/prefix assigned by the corporate network.

There are at least two sub-cases: Dedicated forwarding entries are created in the host such that only traffic directed to the corporate network is sent to "rtr2"; everything else is sent to "rtr1". All traffic is sent to "rtr2" and then routed to the Internet if necessary. This case doesn't need host routes but leads to unnecessary traffic and latency because of the path stretch via rtr2.

Another scenario is shown in . This one is a multi-prefix multihoming use case. "rtr" is CPE router which is connected to two Network Providers, each advertising their own prefixes. In this case, the host may have a single interface but it receives multiple prefixes from the connected Network Provider. Assuming that providers apply ingress filtering policy the packets for any external communication from the host should follow source address dependent routing in order to avoid getting dropped.

A variation of the scenario in is: specialized egress routing. Upstream networks offer different services with specific requirements, e.g., VoIP or IPTV. The hosts using this service need to use the service's source and destination addresses. No other service will accept this source address, i.e., those packets will be dropped .

The scenario shown in s a variation of multi-prefix multi interface scenario (). "rtr1", "rtr2" and "rtr3" are CPE routers. The networks apply ingress routing. Source address dependent routing should be used to avoid any external communications be dropped.

In the homenet scenario depicted in , representing a simple home network, there is a host connected to two CPEs which are connected to providers 1 and 2, respectively. Each network delegates a different prefix. Also each router provides a different prefix to the host. The issue in this scenario is also ingress filtering used by each provider.

The host has to select the source address from the prefixes of Providers 1 or 2 when communicating with other hosts in Provider 1 or 2. The next issue is to select the correct next hop router, rtr1 or rtr2 that can reach the correct provider, "Network Provider 1" or "Network Provider 2".

In this section we present an analysis of the scenarios of and then discuss the relevance of SADR to the provisioning domains.

As in we assume that the routers in use Router Advertisements to distribute default route and source address prefixes supported in each next hop to the hosts or the gateway/CPE router relays this information to the hosts. Referring to the scenario in , source address dependent routing can present a solution to the problem of the host wishes to reach a destination in network 2 and the host may choose rtr1 as the default router. The solution assumes the host is correctly configured. The host should be configured with the prefixes supported in these next hops. This way the host having received many prefixes will have the correct knowledge in selecting the right source address and next hop when sending packets to remote destinations. Note that similar considerations apply to the scenario in . In the configuration of the scenario () it is also useful to configure the host with the prefixes and source address prefixes they support. This will enable the host to select the right prefix when sending packets to the right next hop and avoid any ingress filtering. Source address dependent routing in the use case of specialized egress routing may work as follows. The specialized service router advertizes one or more specific prefixes with appropriate source prefixes, e.g., to the CPE Router, rtr in . The CPE router in turn advertizes the specific service's prefixes and source prefixes to the host. This will allow proper configuration at the host so that the host can use the service by sending the packets with the correct source and destination addresses. Let us analyze the scenario in . If a source address dependent routing protocol is used, the two routers (rtr1 and rtr2) are both able to route traffic correctly, no matter which next-hop router and source address the host selects. In case the host chooses the wrong next hop router, e.g., for provider 2 rtr1 is selected, rtr1 will forward the traffic to rtr2 to be sent to network provider 2 and no ingress filtering will happen. Note that home networks are expected to comply with requirements for source address dependent routing and the routers will be configured accordingly, no matter which routing protocol, e.g., OSPF is used . This would work but with issues. The host traffic to provider 2 will have to go over two links instead of one, i.e., the link bandwidth will be halved. Another possibility is rtr1 can send an ICMPv6 Redirect message to the host to direct the traffic to rtr2. Host would redirect provider 2 traffic to rtr2. The problem with redirects is that ICMPv6 Redirect message can only convey two addresses, i.e., in this case the router address, or rtr2 address and the destination address, or the destination host in provider 2. That means the source address will not be communicated. As a result, the host would send packets to the same destination using both source addresses which causes rtr2 to send a redirect message to rtr1, resulting in ping-pong redirects sent by rtr1 and rtr2. A solution to these issues is to configure the host with the source address prefixes that the next hop supports. In homenets, each interface of the host can be configured by its next hop router, so that all that is needed is to add the information on source address prefixes. This results in the hosts to select the right router no matter what.

Consistent set of network configuration information is called provisioning domain (PvD). In case of multi-prefix multihoming (MPMH), more than one provisioning domain is present on a single link. In case of multi-prefix multiple interface (MPMI) environments, elements of the same domain may be present on multiple links. PvD aware nodes support association of configuration information into PvDs and use these PvDs to serve requests for network connections, e.g., choosing the right source address for the packets. PvDs can be constructed from one of more DHCP or Router Advertisement (RA) options carrying such information as PvD identity and PvD container , . PvDs constructed based on such information are called explicit PvDs . Apart from PvD identity, PvD content may be encapsulated in separate RA or DHCP options called PvD Container Option. These options are placed in the container options of an explicit PvD. Explicit PvDs may be received from different interfaces. Single PvD may be accessible over one interface or simultaneously accessible over multiple interfaces. Explicit PvDs may be scoped to a configuration related to a particular interface, however in general this may not apply. What matters is PvD ID provided that PvD ID is authenticated by the node even in cases where the node has a single connected interface. The authentication of the PvD ID should meet the level required by the node policy. Single PvD information may be received over multiple interfaces as long as PvD ID is the same. This applies to the router advertisements (RAs) in which case a multi-homed host (that is, with multiple interfaces) should trust a message from a router on one interface to install a route to a different router on another interface.

We presented many topologies in which a host with multiple interfaces or a multihomed host is connected to various networks or Network Providers which in turn may apply ingress routing. The scenario analysis in shows that in order to avoid packets getting dropped due to ingress routing, source address dependent routing is needed. Also, source address dependent routing should be supported by routers throughout a site that has multiple exits. In this section, we provide some alternate solutions vis a vis the scenarios presented in . We start with source address selection rule 5.5 () and the scenarios it solves and continue with solutions that state exactly what information hosts need in terms of new router advertisement options for correct source address selection in those scenarios. No recommendation is made in this section.

There is a need to configure the host not only with the prefixes but also with the source prefixes the next hop routers support. Such a configuration may avoid the host getting ingress/egress policy error messages such as ICMP source address failure message. If host configuration is done using router advertisement messages then there is a need to define new router advertisement options for source address dependent routing. These options include Route Prefix with Source Address/Prefix Option. Other options such as Next Hop Address with Route Prefix option and Next Hop Address with Source Address and Route Prefix option will be considered in . As discussed in , the scenario in can be solved by defining a new router advertisement option. If host configuration is done using DHCP then there is a need to define new DHCP options for Route Prefix with Source Address/Prefix. As mentioned above, DHCP server configuration is interface specific. New DHCP options for source address dependent routing such as route prefix and source prefix need to be configured for each interface separately. The scenario in can be solved by defining a new DHCP option.

The source address selection rule 5.5 may possibly be a solution for selecting the right source addresses for each next hop but there are cases where the next hop routers on each interface of the host are not known by the host initially. Such use cases are out of scope. Guidelines for use cases that require router advertisement option set involving third party next hop addresses are also out of scope.

One possible solution is the default source address selection Rule 5.5 in which recommends to select source addresses advertized by the next hop. Considering the above scenarios, we can state that this rule can solve the problem in , and . Source address selection rules can be distributed by DHCP server using DHCP Option OPTION_ADDRSEL_TABLE defined in . In case of DHCP based host configuration, DHCP server can configure only the interface of the host to which it is directly connected. In order for Rule 5.5 to apply on other interfaces the option should be sent on those interfaces as well using . The default source address selection Rule 5.5 solves that problem when an application sends a packet with an unspecified source address. In the presence of two default routes, one route will be chosen, and Rule 5.5 will make sure the right source address is used. When the application selects a source address, i.e., the source address is chosen before next-hop selection, even though the source address is a way for the application to select the exit point, in this case that purpose will not be served. In the presence of multiple default routes, one will be picked, ignoring the source address which was selected by the application because it is known that IPv6 implementations are not required to remember which next-hops advertised which prefixes. Therefore, the next-hop router may not be the correct one, and the packets may be filtered. This implies that the hosts should register which next-hop router announced each prefix. It is required that RAs be sent by the routers and that they contain PIO on all links. It is also required that the hosts remember the source addresses of the routers that sent PIOs together with the prefixes advertised. This can be achieved by updating redirect rules specified in . further elaborates this to specify to which router a host should present its transmission. Source address dependent routing solution is not complete without support from the edge routers. All routers in edge networks need to be required to support routing based on not only the destination address but also the source address. All edge routers need to be required to satify BCP 38 filters.

This document describes some use cases and thus brings no additional security risks. Solution documents should further elaborate on specific security considerations.

None.

In writing this document, we benefited from the ideas expressed by the electronic mail discussion participants on 6man Working Group: Brian Carpenter, Ole Troan, Pierre Pfister, Alex Petrescu, Ray Hunter, Lorenzo Colitti and others. Pierre Pfister proposed the scenario in as well as some text for Rule 5.5. The text on corporate VPN in Section 3 was provided by Brian Carpenter.