Enterprise Multihoming using Provider-Assigned IPv6 Addresses without Network Prefix Translation: Requirements and Solutions
draft-ietf-rtgwg-enterprise-pa-multihoming-11
The information below is for an old version of the document.
Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8678.
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Authors | Fred Baker , Chris Bowers , Jen Linkova | ||
Last updated | 2019-07-03 | ||
Replaces | draft-bowbakova-rtgwg-enterprise-pa-multihoming | ||
RFC stream | Internet Engineering Task Force (IETF) | ||
Formats | |||
Reviews |
TSVART Last Call review
(of
-08)
by Michael Tüxen
Ready w/nits
GENART Last Call review
(of
-08)
by Pete Resnick
Almost ready
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Additional resources | Mailing list discussion | ||
Stream | WG state | Submitted to IESG for Publication | |
Document shepherd | Ron Bonica | ||
Shepherd write-up | Show Last changed 2018-05-11 | ||
IESG | IESG state | Became RFC 8678 (Informational) | |
Consensus boilerplate | Yes | ||
Telechat date |
(None)
Needs a YES. |
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Responsible AD | Martin Vigoureux | ||
Send notices to | Ron Bonica <rbonica@juniper.net> | ||
IANA | IANA review state | Version Changed - Review Needed |
draft-ietf-rtgwg-enterprise-pa-multihoming-11
forwarding entries scoped to source prefix = 2001:db8:0:a000::/52 ============================================ D=2001:db8:0:5555/64 NH=R7 D=::/0 NH=R7 forwarding entries scoped to source prefix = 2001:db8:0:b000::/52 ============================================ D=2001:db8:0:6666/64 NH=SERb2 D=::/0 NH=SERb1 unscoped forwarding entries ============================================ D=2001:db8:0:a010::/64 NH=R2 D=2001:db8:0:b010::/64 NH=R2 D=2001:db8:0:a020::/64 NH=R5 D=2001:db8:0:b020::/64 NH=R5 D=2001:db8:0:5555::/64 NH=R7 D=2001:db8:0:6666::/64 NH=SERb2 D=::/0 NH=SERb1 Figure 5: Forwarding Entries Computed at R8 The final step is for R8 to augment the more specific source-prefix- scoped forwarding tables with entries from less specific source- prefix-scoped forwarding tables. The unscoped forwarding table is considered as being scoped to ::/0, so both 2001:db8:0:a000::/52 and 2001:db8:0:b000::/52 are more specific prefixes of ::/0. Therefore, entries in the unscoped forwarding table will be evaluated to be added to these two more specific source-prefix-scoped forwarding tables. If a forwarding entry from the less specific source-prefix- scoped forwarding table has the exact same destination prefix (including destination prefix length) as the forwarding entry from the more specific source-prefix-scoped forwarding table, then the existing forwarding entry in the more specific source-prefix-scoped forwarding table wins. As an example of how the source scoped forwarding entries are augmented, we consider how the two entries in the first table in Figure 5 (the table for source prefix = 2001:db8:0:a000::/52) are augmented with entries from the third table in Figure 5 (the table of unscoped or scoped to ::/0 forwarding entries). The first four unscoped forwarding entries (D=2001:db8:0:a010::/64, D=2001:db8:0:b010::/64, D=2001:db8:0:a020::/64, and D=2001:db8:0:b020::/64) are not an exact match for any of the existing entries in the forwarding table for source prefix 2001:db8:0:a000::/52. Therefore, these four entries are added to the Baker, et al. Expires January 4, 2020 [Page 19] Internet-Draft Enterprise PA Multihoming July 2019 final forwarding table for source prefix 2001:db8:0:a000::/52. The result of adding these entries is reflected in the first four entries the first table in Figure 6. The next less specific scoped (scope is ::/0) forwarding table entry is for D=2001:db8:0:5555::/64. This entry is an exact match for the existing entry in the forwarding table for the more specific source prefix 2001:db8:0:a000::/52. Therefore, we do not replace the existing entry with the entry from the unscoped forwarding table. This is reflected in the fifth entry in the first table in Figure 6. (Note that since both scoped and unscoped entries have R7 as the next hop, the result of applying this rule is not visible.) The next less specific prefix scoped (scope is ::/0) forwarding table entry is for D=2001:db8:0:6666::/64. This entry is not an exact match for any existing entries in the forwarding table for source prefix 2001:db8:0:a000::/52. Therefore, we add this entry. This is reflected in the sixth entry in the first table in Figure 6. The next less specific prefix scoped (scope is ::/0) forwarding table entry is for D=::/0. This entry is an exact match for the existing entry in the forwarding table for more specific source prefix 2001:db8:0:a000::/52. Therefore, we do not overwrite the existing source-prefix-scoped entry, as can be seen in the last entry in the first table in Figure 6. Baker, et al. Expires January 4, 2020 [Page 20] Internet-Draft Enterprise PA Multihoming July 2019 if source address matches 2001:db8:0:a000::/52 then use this forwarding table ============================================ D=2001:db8:0:a010::/64 NH=R2 D=2001:db8:0:b010::/64 NH=R2 D=2001:db8:0:a020::/64 NH=R5 D=2001:db8:0:b020::/64 NH=R5 D=2001:db8:0:5555::/64 NH=R7 D=2001:db8:0:6666::/64 NH=SERb2 D=::/0 NH=R7 else if source address matches 2001:db8:0:b000::/52 then use this forwarding table ============================================ D=2001:db8:0:a010::/64 NH=R2 D=2001:db8:0:b010::/64 NH=R2 D=2001:db8:0:a020::/64 NH=R5 D=2001:db8:0:b020::/64 NH=R5 D=2001:db8:0:5555::/64 NH=R7 D=2001:db8:0:6666::/64 NH=SERb2 D=::/0 NH=SERb1 else if source address matches ::/0 use this forwarding table ============================================ D=2001:db8:0:a010::/64 NH=R2 D=2001:db8:0:b010::/64 NH=R2 D=2001:db8:0:a020::/64 NH=R5 D=2001:db8:0:b020::/64 NH=R5 D=2001:db8:0:5555::/64 NH=R7 D=2001:db8:0:6666::/64 NH=SERb2 D=::/0 NH=SERb1 Figure 6: Complete Forwarding Tables Computed at R8 The forwarding tables produced by this process at R8 have the desired properties. A packet with a source address in 2001:db8:0:a000::/52 will be forwarded based on the first table in Figure 6. If the packet is destined for the Internet at large or the service at D=2001:db8:0:5555/64, it will be sent to R7 in the direction of SERa. If the packet is destined for an internal host, then the first four entries will send it to R2 or R5 as expected. Note that if this packet has a destination address corresponding to the service offered by ISP-B (D=2001:db8:0:5555::/64), then it will get forwarded to SERb2. It will be dropped by SERb2 or by ISP-B, since the packet has a source address that was not assigned by ISP-B. However, this is expected behavior. In order to use the service offered by ISP-B, the host needs to originate the packet with a source address assigned by ISP-B. Baker, et al. Expires January 4, 2020 [Page 21] Internet-Draft Enterprise PA Multihoming July 2019 In this example, a packet with a source address that doesn't match 2001:db8:0:a000::/52 or 2001:db8:0:b000::/52 must have originated from an external host. Such a packet will use the unscoped forwarding table (the last table in Figure 6). These packets will flow exactly as they would in absence of multihoming. We can also modify this example to illustrate how it supports deployments where not all routers in the site support SADR. Continuing with the topology shown in Figure 3, suppose that R3 and R5 do not support SADR. Instead they are only capable of understanding unscoped route advertisements. The SADR routers in the network will still originate the routes shown in Figure 4. However, R3 and R5 will only understand the unscoped routes as shown in Figure 7. Routes originated by SERa: (D=2001:db8:0:5555::/64) (D=::/0) Routes originated by SERb1: (D=::/0) Routes originated by SERb2: (D=2001:db8:0:6666::/64) Routes originated by R1: (D=2001:db8:0:a010::/64) (D=2001:db8:0:b010::/64) Routes originated by R2: (D=2001:db8:0:a010::/64) (D=2001:db8:0:b010::/64) Routes originated by R3: (D=2001:db8:0:a020::/64) (D=2001:db8:0:b020::/64) Figure 7: Routes Advertisements Understood by Routers that do no Support SADR With these unscoped route advertisements, R5 will produce the forwarding table shown in Figure 8. Baker, et al. Expires January 4, 2020 [Page 22] Internet-Draft Enterprise PA Multihoming July 2019 forwarding table ============================================ D=2001:db8:0:a010::/64 NH=R8 D=2001:db8:0:b010::/64 NH=R8 D=2001:db8:0:a020::/64 NH=R3 D=2001:db8:0:b020::/64 NH=R3 D=2001:db8:0:5555::/64 NH=R8 D=2001:db8:0:6666::/64 NH=SERb2 D=::/0 NH=R8 Figure 8: Forwarding Table For R5, Which Doesn't Understand Source- Prefix-Scoped Routes As all SERs belong to the SADR domain any traffic that needs to exit the site will eventually hit a SADR-capable router. To prevent routing loops involving SADR-capable and non-SADR-capable routers, traffic that enters the SADR-capable domain does not leave the domain until it exits the site. Therefore all SADR-capable routers within the domain MUST be logically connected. Note that the mechanism described here for converting source-prefix- scoped destination prefix routing advertisements into forwarding state is somewhat different from that proposed in [I-D.ietf-rtgwg-dst-src-routing]. The method described in the current document is functionally equivalent, but it is based on application of existing mechanisms for the described scenarios. 6. Mechanisms For Hosts To Choose Good Source Addresses In A Multihomed Site Until this point, we have made the assumption that hosts are able to choose the correct source address using some unspecified mechanism. This has allowed us to just focus on what the routers in a multihomed site network need to do in order to forward packets to the correct ISP based on source address. Now we look at possible mechanisms for hosts to choose the correct source address. We also look at what role, if any, the routers may play in providing information that helps hosts to choose source addresses. It should be noted that this section discussed how hosts could select the default source address for new connections. Any connection which already exists on a host is bound to the specific source address which can not be changed. Section 6.7 discusses the connections preservation issue in more details. Any host that needs to be able to send traffic using the uplinks to a given ISP is expected to be configured with an address from the prefix assigned by that ISP. The host will control which ISP is used Baker, et al. Expires January 4, 2020 [Page 23] Internet-Draft Enterprise PA Multihoming July 2019 for its traffic by selecting one of the addresses configured on the host as the source address for outgoing traffic. It is the responsibility of the site network to ensure that a packet with the source address from an ISP is now sent on an uplink to that ISP. If all of the ISP uplinks are working, the choice of source address by the host may be driven by the desire to load share across ISP uplinks, or it may be driven by the desire to take advantage of certain properties of a particular uplink or ISP (if some information about various path properties has been made availabe to the host somehow - see [I-D.ietf-intarea-provisioning-domains] as an example). If any of the ISP uplinks is not working, then the choice of source address by the host can cause packets to get dropped. How a host should make good decisions about source address selection in a multihomed site is not a solved problem. We do not attempt to solve this problem in this document. Instead we discuss the current state of affairs with respect to standardized solutions and implementation of those solutions. We also look at proposed solutions for this problem. An external host initiating communication with a host internal to a PA multihomed site will need to know multiple addresses for that host in order to communicate with it using different ISPs to the multihomed site (knowing just one address would undermine all benefits of redundant connectivity provided by multihoming). These addresses are typically learned through DNS. (For simplicity, we assume that the external host is single-homed.) The external host chooses the ISP that will be used at the remote multihomed site by setting the destination address on the packets it transmits. For a session originated from an external host to an internal host, the choice of source address used by the internal host is simple. The internal host has no choice but to use the destination address in the received packet as the source address of the transmitted packet. For a session originated by a host inside the multi-homed site, the decision of what source address to select is more complicated. We consider three main methods for hosts to get information about the network. The two proactive methods are Neighbor Discovery Router Advertisements and DHCPv6. The one reactive method we consider is ICMPv6. Note that we are explicitly excluding the possibility of having hosts participate in or even listen directly to routing protocol advertisements. First we look at how a host is currently expected to select the default source and destination addresses to be used for a new connection. Baker, et al. Expires January 4, 2020 [Page 24] Internet-Draft Enterprise PA Multihoming July 2019 6.1. Source Address Selection Algorithm on Hosts [RFC6724] defines the algorithms that hosts are expected to use to select source and destination addresses for packets. It defines an algorithm for selecting a source address and a separate algorithm for selecting a destination address. Both of these algorithms depend on a policy table. [RFC6724] defines a default policy which produces certain behavior. The rules in the two algorithms in [RFC6724] depend on many different properties of addresses. While these are needed for understanding how a host should choose addresses in an arbitrary environment, most of the rules are not relevant for understanding how a host should choose among multiple source addresses in multihomed environment when sending a packet to a remote host. Returning to the example in Figure 3, we look at what the default algorithms in [RFC6724] say about the source address that internal host H31 should use to send traffic to external host H101, somewhere on the Internet. There is no choice to be made with respect to destination address. H31 needs to send a packet with D=2001:db8:0:1234::101 in order to reach H101. So H31 have to choose between using S=2001:db8:0:a010::31 or S=2001:db8:0:b010::31 as the source address for this packet. We go through the rules for source address selection in Section 5 of [RFC6724]. Rule 1 (Prefer same address) is not useful to break the tie between source addresses, because neither the candidate source addresses equals the destination address. Rule 2 (Prefer appropriate scope) is also not used in this scenario, because both source addresses and the destination address have global scope. Rule 3 (Avoid deprecated addresses) applies to an address that has been autoconfigured by a host using stateless address autoconfiguration as defined in [RFC4862]. An address autoconfigured by a host has a preferred lifetime and a valid lifetime. The address is preferred until the preferred lifetime expires, after which it becomes deprecated. A deprecated address is not used if there is a preferred address of the appropriate scope available. When the valid lifetime expires, the address cannot be used at all. The preferred and valid lifetimes for an autoconfigured address are set based on the corresponding lifetimes in the Prefix Information Option in Neighbor Discovery Router Advertisements. So a possible tool to control source address selection in this scenario would be for a host to make an address deprecated by having routers on that link, R1 and R2 in Figure 3, send a Router Advertisement message containing a Baker, et al. Expires January 4, 2020 [Page 25] Internet-Draft Enterprise PA Multihoming July 2019 Prefix Information Option for the source prefix to be discouraged (or prohibited) with the preferred lifetime set to zero. This is a rather blunt tool, because it discourages or prohibits the use of that source prefix for all destinations. However, it may be useful in some scenarios. For example, if all uplinks to a particular ISP fail, it is desirable to prevent hosts from using source addresses from that ISP address space. Rule 4 (Avoid home addresses) does not apply here because we are not considering Mobile IP. Rule 5 (Prefer outgoing interface) is not useful in this scenario, because both source addresses are assigned to the same interface. Rule 5.5 (Prefer addresses in a prefix advertised by the next-hop) is not useful in the scenario when both R1 and R2 will advertise both source prefixes. However potentially this rule may allow a host to select the correct source prefix by selecting a next-hop. The most obvious way would be to make R1 to advertise itself as a default router and send PIO for 2001:db8:0:a010::/64, while R2 is advertising itself as a default router and sending PIO for 2001:db8:0:b010::/64. We'll discuss later how Rule 5.5 can be used to influence a source address selection in single-router topologies (e.g. when H41 is sending traffic using R3 as a default gateway). Rule 6 (Prefer matching label) refers to the Label value determined for each source and destination prefix as a result of applying the policy table to the prefix. With the default policy table defined in Section 2.1 of [RFC6724], Label(2001:db8:0:a010::31) = 5, Label(2001:db8:0:b010::31) = 5, and Label(2001:db8:0:1234::101) = 5. So with the default policy, Rule 6 does not break the tie. However, the algorithms in [RFC6724] are defined in such a way that non- default address selection policy tables can be used. [RFC7078] defines a way to distribute a non-default address selection policy table to hosts using DHCPv6. So even though the application of rule 6 to this scenario using the default policy table is not useful, rule 6 may still be a useful tool. Rule 7 (Prefer temporary addresses) has to do with the technique described in [RFC4941] to periodically randomize the interface portion of an IPv6 address that has been generated using stateless address autoconfiguration. In general, if H31 were using this technique, it would use it for both source addresses, for example creating temporary addresses 2001:db8:0:a010:2839:9938:ab58:830f and 2001:db8:0:b010:4838:f483:8384:3208, in addition to 2001:db8:0:a010::31 and 2001:db8:0:b010::31. So this rule would prefer the two temporary addresses, but it would not break the tie between the two source prefixes from ISP-A and ISP-B. Baker, et al. Expires January 4, 2020 [Page 26] Internet-Draft Enterprise PA Multihoming July 2019 Rule 8 (Use longest matching prefix) dictates that between two candidate source addresses the one which has longest common prefix length with the destination address. For example, if H31 were selecting the source address for sending packets to H101, this rule would not be a tie breaker as for both candidate source addresses 2001:db8:0:a101::31 and 2001:db8:0:b101::31 the common prefix length with the destination is 48. However if H31 were selecting the source address for sending packets H41 address 2001:db8:0:a020::41, then this rule would result in using 2001:db8:0:a101::31 as a source (2001:db8:0:a101::31 and 2001:db8:0:a020::41 share the common prefix 2001:db8:0:a000::/58, while for 2001:db8:0:b101::31 and 2001:db8:0:a020::41 the common prefix is 2001:db8:0:a000::/51). Therefore rule 8 might be useful for selecting the correct source address in some but not all scenarios (for example if ISP-B services belong to 2001:db8:0:b000::/59 then H31 would always use 2001:db8:0:b010::31 to access those destinations). So we can see that of the 8 source selection address rules from [RFC6724], four actually apply to our basic site multihoming scenario. The rules that are relevant to this scenario are summarized below. o Rule 3: Avoid deprecated addresses. o Rule 5.5: Prefer addresses in a prefix advertised by the next-hop. o Rule 6: Prefer matching label. o Rule 8: Prefer longest matching prefix. The two methods that we discuss for controlling the source address selection through the four relevant rules above are SLAAC Router Advertisement messages and DHCPv6. We also consider a possible role for ICMPv6 for getting traffic- driven feedback from the network. With the source address selection algorithm discussed above, the goal is to choose the correct source address on the first try, before any traffic is sent. However, another strategy is to choose a source address, send the packet, get feedback from the network about whether or not the source address is correct, and try another source address if it is not. We consider four scenarios where a host needs to select the correct source address. The first is when both uplinks are working. The second is when one uplink has failed. The third one is a situation when one failed uplink has recovered. The last one is failure of both (all) uplinks. Baker, et al. Expires January 4, 2020 [Page 27] Internet-Draft Enterprise PA Multihoming July 2019 It should be noted that [RFC6724] only defines the behavior of IPv6 hosts to select default addresses that applications and upper-layer protocols can use. Applications and upper-layer protocols can make their own choices on selecting source addresses. The mechanism proposed in this document attempts to ensure that the subset of source addresses available for applications and upper-layer protocols is selected with the up-to-date network state in mind. The rest of the document discusses various aspects of the default source address selection defined in [RFC6724], calling it for the sake of brevity "the source address selection". 6.2. Selecting Source Address When Both Uplinks Are Working Again we return to the topology in Figure 3. Suppose that the site administrator wants to implement a policy by which all hosts need to use ISP-A to reach H101 at D=2001:db8:0:1234::101. So for example, H31 needs to select S=2001:db8:0:a010::31. 6.2.1. Distributing Address Selection Policy Table with DHCPv6 This policy can be implemented by using DHCPv6 to distribute an address selection policy table that assigns the same label to destination address that match 2001:db8:0:1234::/64 as it does to source addresses that match 2001:db8:0:a000::/52. The following two entries accomplish this. Prefix Precedence Label 2001:db8:0:1234::/64 50 33 2001:db8:0:a000::/52 50 33 Figure 9: Policy table entries to implement a routing policy This requires that the hosts implement [RFC6724], the basic source and destination address framework, along with [RFC7078], the DHCPv6 extension for distributing a non-default policy table. Note that it does NOT require that the hosts use DHCPv6 for address assignment. The hosts could still use stateless address autoconfiguration for address configuration, while using DHCPv6 only for policy table distribution (see [RFC8415]). However this method has a number of disadvantages: o DHCPv6 support is not a mandatory requirement for IPv6 hosts ([RFC6434]), so this method might not work for all devices. o Network administrators are required to explicitly configure the desired network access policies on DHCPv6 servers. While it might be feasible in the scenario of a single multihomed network, such approach might have some scalability issues, especially if the Baker, et al. Expires January 4, 2020 [Page 28] Internet-Draft Enterprise PA Multihoming July 2019 centralized DHCPv6 solution is deployed to serve a large number of multiomed sites. 6.2.2. Controlling Source Address Selection With Router Advertisements Neighbor Discovery currently has two mechanisms to communicate prefix information to hosts. The base specification for Neighbor Discovery (see [RFC4861]) defines the Prefix Information Option (PIO) in the Router Advertisement (RA) message. When a host receives a PIO with the A-flag set, it can use the prefix in the PIO as source prefix from which it assigns itself an IP address using stateless address autoconfiguration (SLAAC) procedures described in [RFC4862]. In the example of Figure 3, if the site network is using SLAAC, we would expect both R1 and R2 to send RA messages with PIOs for both source prefixes 2001:db8:0:a010::/64 and 2001:db8:0:b010::/64 with the A-flag set. H31 would then use the SLAAC procedure to configure itself with the 2001:db8:0:a010::31 and 2001:db8:0:b010::31. Whereas a host learns about source prefixes from PIO messages, hosts can learn about a destination prefix from a Router Advertisement containing Route Information Option (RIO), as specified in [RFC4191]. The destination prefixes in RIOs are intended to allow a host to choose the router that it uses as its first hop to reach a particular destination prefix. As currently standardized, neither PIO nor RIO options contained in Neighbor Discovery Router Advertisements can communicate the information needed to implement the desired routing policy. PIO's communicate source prefixes, and RIO communicate destination prefixes. However, there is currently no standardized way to directly associate a particular destination prefix with a particular source prefix. [I-D.pfister-6man-sadr-ra] proposes a Source Address Dependent Route Information option for Neighbor Discovery Router Advertisements which would associate a source prefix and with a destination prefix. The details of [I-D.pfister-6man-sadr-ra] might need tweaking to address this use case. However, in order to be able to use Neighbor Discovery Router Advertisements to implement this routing policy, an extension that allows R1 and R2 to explicitly communicate to H31 an association between S=2001:db8:0:a000::/52 D=2001:db8:0:1234::/64 would be needed. However, Rule 5.5 of the default source address selection algorithm (discussed in Section 6.1 above), together with default router preference (specified in [RFC4191]) and RIO can be used to influence a source address selection on a host as described below. Let's look at source address selection on the host H41. It receives RAs from R3 Baker, et al. Expires January 4, 2020 [Page 29] Internet-Draft Enterprise PA Multihoming July 2019 with PIOs for 2001:db8:0:a020::/64 and 2001:db8:0:b020::/64. At that point all traffic would use the same next-hop (R3 link-local address) so Rule 5.5 does not apply. Now let's assume that R3 supports SADR and has two scoped forwarding tables, one scoped to S=2001:db8:0:a000::/52 and another scoped to S=2001:db8:0:b000::/52. If R3 generates two different link-local addresses for its interface facing H41 (one for each scoped forwarding table, LLA_A and LLA_B) and starts sending two different RAs: one is sent from LLA_A and includes PIO for 2001:db8:0:a020::/64, another is sent from LLA_B and includes PIO for 2001:db8:0:b020::/64. Now it is possible to influence H41 source address selection for destinations which follow the default route by setting default router preference in RAs. If it is desired that H41 reaches H101 (or any destinations in the Internet) via ISP-A, then RAs sent from LLA_A should have default router preference set to 01 (high priority), while RAs sent from LLA_B should have preference set to 11 (low). Then LLA_A would be chosen as a next-hop for H101 and therefore (as per rule 5.5) 2001:db8:0:a020::41 would be selected as the source address. If, at the same time, it is desired that H61 is accessible via ISP-B then R3 should include a RIO for 2001:db8:0:6666::/64 to its RA sent from LLA_B. H41 would chose LLA_B as a next-hop for all traffic to H61 and then as per Rule 5.5, 2001:db8:0:b020::41 would be selected as a source address. If in the above mentioned scenario it is desirable that all Internet traffic leaves the network via ISP-A and the link to ISP-B is used for accessing ISP-B services only (not as ISP-A link backup), then RAs sent by R3 from LLA_B should have Router Lifetime set to 0 and should include RIOs for ISP-B address space. It would instruct H41 to use LLA_A for all Internet traffic but use LLA_B as a next-hop while sending traffic to ISP-B addresses. The description of the mechanism above assumes SADR support by the first-hop routers as well as SERs. However, a first-hop router can still provide a less flexible version of this mechanism even without implementing SADR. This could be done by providing configuration knobs on the first-hop router that allow it to generate different link-local addresses and to send individual RAs for each prefix. The mechanism described above relies on Rule 5.5 of the default source address selection algorithm defined in [RFC6724]. [RFC8028] states that "A host SHOULD select default routers for each prefix it is assigned an address in". It also recommends that hosts should implement Rule 5.5. of [RFC6724]. Hosts following the recommendations specified in [RFC8028] therefore should be able to benefit from the solution described in this document. No standards need to be updated in regards to host behavior. Baker, et al. Expires January 4, 2020 [Page 30] Internet-Draft Enterprise PA Multihoming July 2019 6.2.3. Controlling Source Address Selection With ICMPv6 We now discuss how one might use ICMPv6 to implement the routing policy to send traffic destined for H101 out the uplink to ISP-A, even when uplinks to both ISPs are working. If H31 started sending traffic to H101 with S=2001:db8:0:b010::31 and D=2001:db8:0:1234::101, it would be routed through SER-b1 and out the uplink to ISP-B. SERb1 could recognize that this traffic is not following the desired routing policy and react by sending an ICMPv6 message back to H31. In this example, we could arrange things so that SERb1 drops the packet with S=2001:db8:0:b010::31 and D=2001:db8:0:1234::101, and then sends to H31 an ICMPv6 Destination Unreachable message with Code 5 (Source address failed ingress/egress policy). When H31 receives this packet, it would then be expected to try another source address to reach the destination. In this example, H31 would then send a packet with S=2001:db8:0:a010::31 and D=2001:db8:0:1234::101, which will reach SERa and be forwarded out the uplink to ISP-A. However, we would also want it to be the case that SERb1 does not enforce this routing policy when the uplink from SERa to ISP-A has failed. This could be accomplished by having SERa originate a source-prefix-scoped route for (S=2001:db8:0:a000::/52, D=2001:db8:0:1234::/64) and have SERb1 monitor the presence of that route. If that route is not present (because SERa has stopped originating it), then SERb1 will not enforce the routing policy, and it will forward packets with S=2001:db8:0:b010::31 and D=2001:db8:0:1234::101 out its uplink to ISP-B. We can also use this source-prefix-scoped route originated by SERa to communicate the desired routing policy to SERb1. We can define an EXCLUSIVE flag to be advertised together with the IGP route for (S=2001:db8:0:a000::/52, D=2001:db8:0:1234::/64). This would allow SERa to communicate to SERb that SERb should reject traffic for D=2001:db8:0:1234::/64 and respond with an ICMPv6 Destination Unreachable Code 5 message, as long as the route for (S=2001:db8:0:a000::/52, D=2001:db8:0:1234::/64) is present. The definition of an EXCLUSIVE flag for SADR advertisements in IGPs would require future standardization work. Finally, if we are willing to extend ICMPv6 to support this solution, then we could create a mechanism for SERb1 to tell the host what source address it should be using to successfully forward packets that meet the policy. In its current form, when SERb1 sends an ICMPv6 Destination Unreachable Code 5 message, it is basically saying, "This source address is wrong. Try another source address." In the absence of a clear indication which address to try next, the Baker, et al. Expires January 4, 2020 [Page 31] Internet-Draft Enterprise PA Multihoming July 2019 host will iterate over all addresses assigned to the interface (e.g. various privacy addresses) which would lead to significant delays and degraded user experience. It would be better is if the ICMPv6 message could say, "This source address is wrong. Instead use a source address in S=2001:db8:0:a000::/52.". However using ICMPv6 for signaling source address information back to hosts introduces new challenges. Most routers currently have software or hardware limits on generating ICMP messages. A site administrator deploying a solution that relies on the SERs generating ICMP messages could try to improve the performance of SERs for generating ICMP messages. However, in a large network, it is still likely that ICMP message generation limits will be reached. As a result hosts would not receive ICMPv6 back which in turn leads to traffic blackholing and poor user experience. To improve the scalability of ICMPv6-based signaling hosts SHOULD cache the preferred source address (or prefix) for the given destination (which in turn might cause issues in case of the corresponding ISP uplinks failure - see Section 6.3). In addition, the same source prefix SHOULD be used for other destinations in the same /64 as the original destination address. The source prefix to the destination mapping SHOULD have a specific lifetime. Expiration of the lifetime SHOULD trigger the source address selection algorithm again. Using ICMPv6 Destination Unreachable Messages with Code 5 to influence source address selection introduces some security challenges which are discussed in Section 10. As currently standardized in [RFC4443], the ICMPv6 Destination Unreachable Message with Code 5 would allow for the iterative approach to retransmitting packets using different source addresses. As currently defined, the ICMPv6 message does not provide a mechanism to communication information about which source prefix should be used for a retransmitted packet. The current document does not define such a mechanism but it might be a useful extension to define in a different document. However this approach has some security implications such as an ability for an attacker to send spoofed ICMPv6 messages to signal invalid/unreachable source prefix causing DoS-type attack. 6.2.4. Summary of Methods For Controlling Source Address Selection To Implement Routing Policy So to summarize this section, we have looked at three methods for implementing a simple routing policy where all traffic for a given destination on the Internet needs to use a particular ISP, even when the uplinks to both ISPs are working. Baker, et al. Expires January 4, 2020 [Page 32] Internet-Draft Enterprise PA Multihoming July 2019 The default source address selection policy cannot distinguish between the source addresses needed to enforce this policy, so a non- default policy table using associating source and destination prefixes using Label values would need to be installed on each host. A mechanism exists for DHCPv6 to distribute a non-default policy table but such solution would heavily rely on DHCPv6 support by host operating system. Moreover there is no mechanism to translate desired routing/traffic engineering policies into policy tables on DHCPv6 servers. Therefore using DHCPv6 for controlling address selection policy table is not recommended and SHOULD NOT be used. At the same time Router Advertisements provide a reliable mechanism to influence source address selection process via PIO, RIO and default router preferences. As all those options have been standardized by IETF and are supported by various operating systems no changes are required on hosts. First-hop routers in the enterprise network need to be able of sending different RAs for different SLAAC prefixes (either based on scoped forwarding tables or based on pre-configured policies). SERs can enforce the routing policy by sending ICMPv6 Destination Unreachable messages with Code 5 (Source address failed ingress/ egress policy) for traffic that is being sent with the wrong source address. The policy distribution could be automated by defining an EXCLUSIVE flag for the source-prefix-scoped route which can be set on the SER that originates the route. As ICMPv6 message generation can be rate-limited on routers, it SHOULD NOT be used as the only mechanism to influence source address selection on hosts. While hosts SHOULD select the correct source address for a given destination the network SHOULD signal any source address issues back to hosts using ICMPv6 error messages. 6.3. Selecting Source Address When One Uplink Has Failed Now we discuss if DHCPv6, Neighbor Discovery Router Advertisements, and ICMPv6 can help a host choose the right source address when an uplink to one of the ISPs has failed. Again we look at the scenario in Figure 3. This time we look at traffic from H31 destined for external host H501 at D=2001:db8:0:5678::501. We initially assume that the uplink from SERa to ISP-A is working and that the uplink from SERb1 to ISP-B is working. We assume there is no particular routing policy desired, so H31 is free to send packets with S=2001:db8:0:a010::31 or S=2001:db8:0:b010::31 and have them delivered to H501. For this example, we assume that H31 has chosen S=2001:db8:0:b010::31 so that the packets exit via SERb to ISP-B. Now we see what happens when the link from SERb1 to ISP-B fails. How should H31 learn that it needs Baker, et al. Expires January 4, 2020 [Page 33] Internet-Draft Enterprise PA Multihoming July 2019 to start sending the packet to H501 with S=2001:db8:0:a010::31 in order to start using the uplink to ISP-A? We need to do this in a way that doesn't prevent H31 from still sending packets with S=2001:db8:0:b010::31 in order to reach H61 at D=2001:db8:0:6666::61. 6.3.1. Controlling Source Address Selection With DHCPv6 For this example we assume that the site network in Figure 3 has a centralized DHCP server and all routers act as DHCP relay agents. We assume that both of the addresses assigned to H31 were assigned via DHCP. We could try to have the DHCP server monitor the state of the uplink from SERb1 to ISP-B in some manner and then tell H31 that it can no longer use S=2001:db8:0:b010::31 by settings its valid lifetime to zero. The DHCP server could initiate this process by sending a Reconfigure Message to H31 as described in Section 18.3 of [RFC8415]. Or the DHCP server can assign addresses with short lifetimes in order to force clients to renew them often. This approach would prevent H31 from using S=2001:db8:0:b010::31 to reach a host on the Internet. However, it would also prevent H31 from using S=2001:db8:0:b010::31 to reach H61 at D=2001:db8:0:6666::61, which is not desirable. Another potential approach is to have the DHCP server monitor the uplink from SERb1 to ISP-B and control the choice of source address on H31 by updating its address selection policy table via the mechanism in [RFC7078]. The DHCP server could initiate this process by sending a Reconfigure Message to H31. Note that [RFC8415] requires that Reconfigure Message use DHCP authentication. DHCP authentication could be avoided by using short address lifetimes to force clients to send Renew messages to the server often. If the host is not obtaining its IP addresses from the DHCP server, then it would need to use the Information Refresh Time option defined in [RFC8415]. If the following policy table can be installed on H31 after the failure of the uplink from SERb1, then the desired routing behavior should be achieved based on source and destination prefix being matched with label values. Baker, et al. Expires January 4, 2020 [Page 34] Internet-Draft Enterprise PA Multihoming July 2019 Prefix Precedence Label ::/0 50 44 2001:db8:0:a000::/52 50 44 2001:db8:0:6666::/64 50 55 2001:db8:0:b000::/52 50 55 Figure 10: Policy Table Needed On Failure Of Uplink From SERb1 The described solution has a number of significant drawbacks, some of them already discussed in Section 6.2.1. o DHCPv6 support is not required for an IPv6 host and there are operating systems which do not support DHCPv6. Besides that, it does not appear that [RFC7078] has been widely implemented on host operating systems. o [RFC7078] does not clearly specify this kind of a dynamic use case where address selection policy needs to be updated quickly in response to the failure of a link. In a large network it would present scalability issues as many hosts need to be reconfigured in very short period of time. o Updating DHCPv6 server configuration each time an ISP uplink changes its state introduces some scalability issues, especially for mid/large distributed scale enterprise networks. In addition to that, the policy table needs to be manually configured by administrators which makes that solution prone to human error. o No mechanism exists for making DHCPv6 servers aware of network topology/routing changes in the network. In general DHCPv6 servers monitoring network-related events sounds like a bad idea as completely new functionality beyond the scope of DHCPv6 role is required. 6.3.2. Controlling Source Address Selection With Router Advertisements The same mechanism as discussed in Section 6.2.2 can be used to control the source address selection in the case of an uplink failure. If a particular prefix should not be used as a source for any destinations, then the router needs to send RA with Preferred Lifetime field for that prefix set to 0. Let's consider a scenario when all uplinks are operational and H41 receives two different RAs from R3: one from LLA_A with PIO for 2001:db8:0:a020::/64, default router preference set to 11 (low) and another one from LLA_B with PIO for 2001:db8:0:a020::/64, default router preference set to 01 (high) and RIO for 2001:db8:0:6666::/64. Baker, et al. Expires January 4, 2020 [Page 35] Internet-Draft Enterprise PA Multihoming July 2019 As a result H41 is using 2001:db8:0:b020::41 as a source address for all Internet traffic and those packets are sent by SERs to ISP-B. If SERb1 uplink to ISP-B failed, the desired behavior is that H41 stops using 2001:db8:0:b020::41 as a source address for all destinations but H61. To achieve that R3 should react to SERb1 uplink failure (which could be detected as the scoped route (S=2001:db8:0:b000::/52, D=::/0) disappearance) by withdrawing itself as a default router. R3 sends a new RA from LLA_B with Router Lifetime value set to 0 (which means that it should not be used as default router). That RA still contains PIO for 2001:db8:0:b020::/64 (for SLAAC purposes) and RIO for 2001:db8:0:6666::/64 so H41 can reach H61 using LLA_B as a next- hop and 2001:db8:0:b020::41 as a source address. For all traffic following the default route, LLA_A will be used as a next-hop and 2001:db8:0:a020::41 as a source address. If all uplinks to ISP-B have failed and therefore source addresses from ISP-B address space should not be used at all, the forwarding table scoped S=2001:db8:0:b000::/52 contains no entries. Hosts can be instructed to stop using source addresses from that block by sending RAs containing PIO with Preferred Lifetime set to 0. 6.3.3. Controlling Source Address Selection With ICMPv6 Now we look at how ICMPv6 messages can provide information back to H31. We assume again that at the time of the failure H31 is sending packets to H501 using (S=2001:db8:0:b010::31, D=2001:db8:0:5678::501). When the uplink from SERb1 to ISP-B fails, SERb1 would stop originating its source-prefix-scoped route for the default destination (S=2001:db8:0:b000::/52, D=::/0) as well as its unscoped default destination route. With these routes no longer in the IGP, traffic with (S=2001:db8:0:b010::31, D=2001:db8:0:5678::501) would end up at SERa based on the unscoped default destination route being originated by SERa. Since that traffic has the wrong source address to be forwarded to ISP-A, SERa would drop it and send a Destination Unreachable message with Code 5 (Source address failed ingress/egress policy) back to H31. H31 would then know to use another source address for that destination and would try with (S=2001:db8:0:a010::31, D=2001:db8:0:5678::501). This would be forwarded to SERa based on the source-prefix-scoped default destination route still being originated by SERa, and SERa would forward it to ISP-A. As discussed above, if we are willing to extend ICMPv6, SERa can even tell H31 what source address it should use to reach that destination. The expected host behaviour has been discussed in Section 6.2.3. Using ICMPv6 would have the same scalability/rate limiting issues discussed in Section 6.2.3. ISP-B uplink failure immediately makes source addresses from 2001:db8:0:b000::/52 unsuitable for external communication and might Baker, et al. Expires January 4, 2020 [Page 36] Internet-Draft Enterprise PA Multihoming July 2019 trigger a large number of ICMPv6 packets being sent to hosts in that subnet. 6.3.4. Summary Of Methods For Controlling Source Address Selection On The Failure Of An Uplink It appears that DHCPv6 is not particularly well suited to quickly changing the source address used by a host in the event of the failure of an uplink, which eliminates DHCPv6 from the list of potential solutions. On the other hand Router Advertisements provides a reliable mechanism to dynamically provide hosts with a list of valid prefixes to use as source addresses as well as prevent particular prefixes to be used. While no additional new features are required to be implemented on hosts, routers need to be able to send RAs based on the state of scoped forwarding tables entries and to react to network topology changes by sending RAs with particular parameters set. The use of ICMPv6 Destination Unreachable messages generated by the SER (or any SADR-capable) routers seem like they have the potential to provide a support mechanism together with RAs to signal source address selection errors back to hosts, however scalability issues may arise in large networks in case of sudden topology change. Therefore it is highly desirable that hosts are able to select the correct source address in case of uplinks failure with ICMPv6 being an additional mechanism to signal unexpected failures back to hosts. The current behavior of different host operating system when receiving ICMPv6 Destination Unreachable message with code 5 (Source address failed ingress/egress policy) is not clear to the authors. Information from implementers, users, and testing would be quite helpful in evaluating this approach. 6.4. Selecting Source Address Upon Failed Uplink Recovery The next logical step is to look at the scenario when a failed uplink on SERb1 to ISP-B is coming back up, so hosts can start using source addresses belonging to 2001:db8:0:b000::/52 again. 6.4.1. Controlling Source Address Selection With DHCPv6 The mechanism to use DHCPv6 to instruct the hosts (H31 in our example) to start using prefixes from ISP-B space (e.g. S=2001:db8:0:b010::31 for H31) to reach hosts on the Internet is quite similar to one discussed in Section 6.3.1 and shares the same drawbacks. Baker, et al. Expires January 4, 2020 [Page 37] Internet-Draft Enterprise PA Multihoming July 2019 6.4.2. Controlling Source Address Selection With Router Advertisements Let's look at the scenario discussed in Section 6.3.2. If the uplink(s) failure caused the complete withdrawal of prefixes from 2001:db8:0:b000::/52 address space by setting Preferred Lifetime value to 0, then the recovery of the link should just trigger new RA being sent with non-zero Preferred Lifetime. In another scenario discussed in Section 6.3.2, the SERb1 uplink to ISP-B failure leads to disappearance of the (S=2001:db8:0:b000::/52, D=::/0) entry from the forwarding table scoped to S=2001:db8:0:b000::/52 and, in turn, caused R3 to send RAs from LLA_B with Router Lifetime set to 0. The recovery of the SERb1 uplink to ISP-B leads to (S=2001:db8:0:b000::/52, D=::/0) scoped forwarding entry re- appearance and instructs R3 that it should advertise itself as a default router for ISP-B address space domain (send RAs from LLA_B with non-zero Router Lifetime). 6.4.3. Controlling Source Address Selection With ICMP It looks like ICMPv6 provides a rather limited functionality to signal back to hosts that particular source addresses have become valid again. Unless the changes in the uplink state a particular (S,D) pair, hosts can keep using the same source address even after an ISP uplink has come back up. For example, after the uplink from SERb1 to ISP-B had failed, H31 received ICMPv6 Code 5 message (as described in Section 6.3.3) and allegedly started using (S=2001:db8:0:a010::31, D=2001:db8:0:5678::501) to reach H501. Now when the SERb1 uplink comes back up, the packets with that (S,D) pair are still routed to SERa1 and sent to the Internet. Therefore H31 is not informed that it should stop using 2001:db8:0:a010::31 and start using 2001:db8:0:b010::31 again. Unless SERa has a policy configured to drop packets (S=2001:db8:0:a010::31, D=2001:db8:0:5678::501) and send ICMPv6 back if SERb1 uplink to ISP-B is up, H31 will be unaware of the network topology change and keep using S=2001:db8:0:a010::31 for Internet destinations, including H51. One of the possible option may be using a scoped route with EXCLUSIVE flag as described in Section 6.2.3. SERa1 uplink recovery would cause (S=2001:db8:0:a000::/52, D=2001:db8:0:1234::/64) route to reappear in the routing table. In the absence of that route packets to H101 which were sent to ISP-B (as ISP-A uplink was down) with source addresses from 2001:db8:0:b000::/52. When the route re- appears SERb1 would reject those packets and sends ICMPv6 back as discussed in Section 6.2.3. Practically it might lead to scalability issues which have been already discussed in Section 6.2.3 and Section 6.4.3. Baker, et al. Expires January 4, 2020 [Page 38] Internet-Draft Enterprise PA Multihoming July 2019 6.4.4. Summary Of Methods For Controlling Source Address Selection Upon Failed Uplink Recovery Once again DHCPv6 does not look like reasonable choice to manipulate source address selection process on a host in the case of network topology changes. Using Router Advertisement provides the flexible mechanism to dynamically react to network topology changes (if routers are able to use routing changes as a trigger for sending out RAs with specific parameters). ICMPv6 could be considered as a supporting mechanism to signal incorrect source address back to hosts but should not be considered as the only mechanism to control the address selection in multihomed environments. 6.5. Selecting Source Address When All Uplinks Failed One particular tricky case is a scenario when all uplinks have failed. In that case there is no valid source address to be used for any external destinations while it might be desirable to have intra- site connectivity. 6.5.1. Controlling Source Address Selection With DHCPv6 From DHCPv6 perspective uplinks failure should be treated as two independent failures and processed as described in Section 6.3.1. At this stage it is quite obvious that it would result in quite complicated policy table which needs to be explicitly configured by administrators and therefore seems to be impractical. 6.5.2. Controlling Source Address Selection With Router Advertisements As discussed in Section 6.3.2 an uplink failure causes the scoped default entry to disappear from the scoped forwarding table and triggers RAs with zero Router Lifetime. Complete disappearance of all scoped entries for a given source prefix would cause the prefix being withdrawn from hosts by setting Preferred Lifetime value to zero in PIO. If all uplinks (SERa, SERb1 and SERb2) failed, hosts either lost their default routers and/or have no global IPv6 addresses to use as a source. (Note that 'uplink failure' might mean 'IPv6 connectivity failure with IPv4 still being reachable', in which case hosts might fall back to IPv4 if there is IPv4 connectivity to destinations). As a result, intra-site connectivity is broken. One of the possible way to solve it is to use ULAs. All hosts have ULA addresses assigned in addition to GUAs and used for intra-site communication even if there is no GUA assigned to a host. To avoid accidental leaking of packets with ULA sources SADR- capable routers SHOULD have a scoped forwarding table for ULA source for internal routes but MUST NOT have an entry for D=::/0 in that Baker, et al. Expires January 4, 2020 [Page 39] Internet-Draft Enterprise PA Multihoming July 2019 table. In the absence of (S=ULA_Prefix; D=::/0) first-hop routers will send dedicated RAs from a unique link-local source LLA_ULA with PIO from ULA address space, RIO for the ULA prefix and Router Lifetime set to zero. The behaviour is consistent with the situation when SERb1 lost the uplink to ISP-B (so there is no Internet connectivity from 2001:db8:0:b000::/52 sources) but those sources can be used to reach some specific destinations. In the case of ULA there is no Internet connectivity from ULA sources but they can be used to reach another ULA destinations. Note that ULA usage could be particularly useful if all ISPs assign prefixes via DHCP-PD. In the absence of ULAs, upon the all uplinks failure hosts would lost all their GUAs upon prefix lifetime expiration which again makes intra- site communication impossible. It should be noted that the Rule 5.5 (prefer a prefix advertised by the selected next-hop) takes precedence over the Rule 6 (prefer matching label, which ensures that GUA source addresses are preferred over ULAs for GUA destinations). Therefore if ULAs are used, the network administrator needs to ensure that while the site has an Internet connectivity, hosts do not select a router which advertises ULA prefixes as their default router. 6.5.3. Controlling Source Address Selection With ICMPv6 In case of all uplinks failure all SERs will drop outgoing IPv6 traffic and respond with ICMPv6 error message. In the large network when many hosts are trying to reach Internet destinations it means that SERs need to generate an ICMPv6 error to every packet they receive from hosts which presents the same scalability issues discussed in Section 6.3.3 6.5.4. Summary Of Methods For Controlling Source Address Selection When All Uplinks Failed Again, combining SADR with Router Advertisements seems to be the most flexible and scalable way to control the source address selection on hosts. 6.6. Summary Of Methods For Controlling Source Address Selection To summarize the scenarios and options discussed above: While DHCPv6 allows administrators to manipulate source address selection policy tables, this method has a number of significant disadvantages which eliminates DHCPv6 from a list of potential solutions: 1. It required hosts to support DHCPv6 and its extension (RFC7078); Baker, et al. Expires January 4, 2020 [Page 40] Internet-Draft Enterprise PA Multihoming July 2019 2. DHCPv6 server needs to monitor network state and detect routing changes. 3. The use of policy tables requires manual configuration and might be extremely complicated, especially in the case of distributed network when large number of remote sites are being served by centralized DHCPv6 servers. 4. Network topology/routing policy changes could trigger simultaneous re-configuration of large number of hosts which present serious scalability issues. The use of Router Advertisements to influence the source address selection on hosts seem to be the most reliable, flexible and scalable solution. It has the following benefits: 1. no new (non-standard) functionality needs to be implemented on hosts (except for [RFC4191] RIO support, which remains at the time of this writing not widely implemented); 2. no changes in RA format; 3. routers can react to routing table changes by sending RAs which would minimize the failover time in the case of network topology changes; 4. information required for source address selection is broadcast to all affected hosts in case of topology change event which improves the scalability of the solution (comparing to DHCPv6 reconfiguration or ICMPv6 error messages). To fully benefit from the RA-based solution, first-hop routers need to implement SADR, belong to the SADR domain and be able to send dedicated RAs per scoped forwarding table as discussed above, reacting to network changes with sending new RAs. It should be noted that the proposed solution would work even if first-hop routers are not SADR-capable but still able to send individual RAs for each ISP prefix and react to topology changes as discussed above (e.g. via configuration knobs). The RA-based solution relies heavily on hosts correctly implementing default address selection algorithm as defined in [RFC6724]. While the basic (and most common) multihoming scenario (two or more Internet uplinks, no 'walled gardens') would work for any host supporting the minimal implementation of [RFC6724], more complex use cases (such as "walled garden" and other scenarios when some ISP resources can only be reached from that ISP address space) require that hosts support Rule 5.5 of the default address selection Baker, et al. Expires January 4, 2020 [Page 41] Internet-Draft Enterprise PA Multihoming July 2019 algorithm. There is some evidence that not all host OSes have that rule implemented currently. However it should be noted that [RFC8028] states that Rule 5.5 should be implemented. ICMPv6 Code 5 error message SHOULD be used to complement RA-based solution to signal incorrect source address selection back to hosts, but it SHOULD NOT be considered as the stand-alone solution. To prevent scenarios when hosts in multihomed envinronments incorrectly identify onlink/offlink destinations, hosts SHOULD treat ICMPv6 Redirects as discussed in [RFC8028]. 6.7. Solution Limitations 6.7.1. Connections Preservation The proposed solution is not designed to preserve connection state in case of an uplink failure. When all uplinks to an ISP go down all transport connections established to/from that ISP address space will be interrupted (unless the transport protocol has specific multihoming support). That behaviour is similar to the scenario of IPv4 multihoming with NAT when an uplink failure causes all connections to be NATed to completely different public IPv4 addresses. While it does sound suboptimal, it is determined by the nature of PA address space: if all uplinks to the particular ISP have failed, there is no path for the ingress traffic to reach the site and the egress traffic is supposed to be dropped by the BCP38 [RFC2827] ingress filters. The only potential way to overcome this limitation would be running BGP with all ISPs and advertise all site prefixes to all uplinks - a solution which shares all drawbacks of using PI address space without having its benefits. Networks willing and capable of running BGP and using PI are out of scope of this document. It should be noted that in case of IPv4 NAT-based multihoming uplink recovery could cause connection interruptions as well (unless packet forwarding is integrated with existing NAT sessions tracking so the egress interface for the existing sessions is not changed). However the proposed solution has a benefit of preserving the existing sessions during/after the failed uplink restoration. Unlike the uplink failure event which causes all addresses from the affected prefix to be deprecated the recovery would just add new preferred addresses to a host without making any addresses unavailable. Therefore connections estavlished to/from those addresses do not have to be interrupted. While it's desirable for active connections to survive ISP failover events, for sites using PA address space such events affect the reachability of IP addresses assigned to hosts. Unless the transport Baker, et al. Expires January 4, 2020 [Page 42] Internet-Draft Enterprise PA Multihoming July 2019 (or even higher level protocols) are capable of suviving the host renumbering, the active connections will be broken. The proposed solution focuses on minimizing the impact of failover for new connections and for multipath-aware protocols. 6.8. Other Configuration Parameters 6.8.1. DNS Configuration In mutihomed envinronment each ISP might provide their own list of DNS servers. For example, in the topology shown in Figure 3, ISP-A might provide recursive DNS server H51 2001:db8:0:5555::51, while ISP-B might provide H61 2001:db8:0:6666::61 as a recursive DNS server. [RFC8106] defines IPv6 Router Advertisement options to allow IPv6 routers to advertise a list of DNS recursive server addresses and a DNS Search List to IPv6 hosts. Using RDNSS together with 'scoped' RAs as described above would allow a first-hop router (R3 in the Figure 3) to send DNS server addresses and search lists provided by each ISP (or the corporate DNS servers addresses if the enterprise is running its own DNS servers - as discussed below DNS split-horizon problem is to hard to solve without running a local DNS server). As discussed in Section 6.5.2, failure of all ISP uplinks would cause deprecation of all addresses assigned to a host from the address space of all ISPs. If any intra-site IPv6 connectivity is still desirable (most likely to be the case for any mid/large scare network), then ULAs should be used as discussed in Section 6.5.2. In such a scenario, the enterprise network should run its own recursive DNS server(s) and provide its ULA addresses to hosts via RDNSS in RAs send for ULA-scoped forwarding table as described in Section 6.5.2. There are some scenarios when the final outcome of the name resolution might be different depending on: o which DNS server is used; o which source address the client uses to send a DNS query to the server (DNS split horizon). There is no way currently to instruct a host to use a particular DNS server out of the configured servers list for resolving a particular name. Therefore it does not seem feasible to solve the problem of DNS server selection on the host (it should be noted that this particular issue is protocol-agnostic and happens for IPv4 as well). In such a scenario it is recommended that the enterprise runs its own local recursive DNS server. Baker, et al. Expires January 4, 2020 [Page 43] Internet-Draft Enterprise PA Multihoming July 2019 To influence host source address selection for packets sent to a particular DNS server the following requirements must be met: o the host supports RIO as defined in [RFC4191]; o the routers send RIO for routes to DNS server addresses. For example, if it is desirable that host H31 reaches the ISP-A DNS server H51 2001:db8:0:5555::51 using its source address 2001:db8:0:a010::31, then both R1 and R2 should send the RIO containing the route to 2001:db8:0:5555::51 (or covering route) in their 'scoped' RAs, containing LLA_A as the default router address and the PO for SLAAC prefix 2001:db8:0:a010::/64. In that case the host H31 (if it supports the Rule 5.5) would select LLA_A as a next- hop and then chose 2001:db8:0:a010::31 as the source address for packets to the DNS server. It should be noted that [RFC6106] explicitly prohibits using DNS information if the RA router Lifetime expired: "An RDNSS address or a DNSSL domain name MUST be used only as long as both the RA router Lifetime (advertised by a Router Advertisement message) and the corresponding option Lifetime have not expired.". Therefore hosts might ignore RDNSS information provided in ULA-scoped RAs as those RAs would have router lifetime set to 0. However the updated version of RFC6106 ([RFC8106]) has that requirement removed. As discussed above the DNS split-horizon problem and selecting the correct DNS server in a multihomed envinroment is not an easy one to solve. The proper solution would require hosts to support the concept of multiple Provisioning Domains (PvD, a set of configuration information associated with a network, [RFC7556]). 7. Deployment Considerations The solution described in this document requires certain mechanisms to be supported by the network infrastructure and hosts. It requires some routers in the enterprise site to support some form of Source Address Dependent Routing (SADR). It also requires hosts to be able to learn when the uplink to an ISP changes its state so the corresponding source addresses should (or should not) be used. Ongoing work to create mechanisms to accomplish this are discussed in this document, but they are still a work in progress. 7.1. Deploying SADR Domain The proposed solution provides does not prescribe particular details regarding deploying an SADR domain within a multihomed enterprise network. However the following guidelines could be applied: Baker, et al. Expires January 4, 2020 [Page 44] Internet-Draft Enterprise PA Multihoming July 2019 o The SADR domain is usually limited by the multihomed site border. o The minimal deployable scenario requires enabling SADR on all SERs and including them into a single SADR domain. o As discussed in Section 4.2, extending the connected SADR domain beyond that point down to the first-hop routers can produce more efficient forwarding paths and allow the network to fully benefit from SADR. it would also simplify the operation of the SADR domain. o During the incremental SADR domain expansion from the SERs down towards first-hop routers it's important to ensure that at any moment of time all SADR-capable routers within the domain are logically connected (see Section 5). 7.2. Hosts-Related Considerations The solution discussed in this document relies on the default address selection algorithm ([RFC6724]) Rule 5.5. While [RFC6724] considers this rule as optional, the recent [RFC8028] states that "A host SHOULD select default routers for each prefix it is assigned an address in". It also recommends that hosts should implement Rule 5.5. of [RFC6724]. Therefore while RFC8028-compliant hosts already have mechanism to learn about ISP uplinks state changes and selecting the source addresses accordingly, many hosts do not have such mechanism supported yet. It should be noted that multihomed enterprise network utilizing multiple ISP prefixes can be considered as a typical multiple provisioning domain (mPVD) scenario, as described in [RFC7556]. This document defines a way for the network to provide the PVD information to hosts indirectly, using the existing mechanisms. At the same time [I-D.ietf-intarea-provisioning-domains] takes one step further and describes a comprehensive mechanism for hosts to discover the whole set of configuration information associated with different PVD/ISPs. [I-D.ietf-intarea-provisioning-domains] complements this document in terms of making hosts being able to learn about ISP uplink states and selecting the corresponding source addresses. 8. Other Solutions 8.1. Shim6 The Shim6 working group specified the Shim6 protocol [RFC5533] which allows a host at a multihomed site to communicate with an external host and exchange information about possible source and destination address pairs that they can use to communicate. It also specified Baker, et al. Expires January 4, 2020 [Page 45] Internet-Draft Enterprise PA Multihoming July 2019 the REAP protocol [RFC5534] to detect failures in the path between working address pairs and find new working address pairs. A fundamental requirement for Shim6 is that both internal and external hosts need to support Shim6. That is, both the host internal to the multihomed site and the host external to the multihomed site need to support Shim6 in order for there to be any benefit for the internal host to run Shim6. The Shim6 protocol specification was published in 2009, but it has not been widely implemented. Therefore Shim6 is not considered as a viable solution for enterprise multihoming. 8.2. IPv6-to-IPv6 Network Prefix Translation IPv6-to-IPv6 Network Prefix Translation (NPTv6) [RFC6296] is not the focus of this document. NPTv6 suffers from the same fundamental issue as any other address translation approaches: it breaks end-to- end connectivity. Therefore NPTv6 is not considered as desirable solution and this document intentionally focuses on solving enterprise multihoming problem without any form of address translations. With increasing interest and ongoing work in bringing path awareness to transport and application layer protocols hosts might be able to determine the properties of the various network paths and choose among paths available to them. As selecting the correct source address is one of the possible mechanisms path-aware hosts may utilize, address translation negatively affects hosts path-awareness which makes NTPv6 even more undesirable solution. 8.3. Multipath Transport Using multipath transport (such as MPTCP, [RFC6824] or multipath capabilities in QUIC) might solve the problems discussed in Section 6 since it would allow hosts to use multiple source addresses for a single connection and switch between source addresses when a particular address becomes unavailable or a new address gets assigned to the host interface. Therefore if all hosts in the enterprise network are only using multipath transport for all connections, the signaling solution described in Section 6 might not be needed (it should be noted that the Source Address Dependent Routing would still be required to deliver packets to the correct uplinks). At the time this document was written, multipath transport alone could not be considered a solution for the problem of selecting the source address in a multihomed environment. There are significant number of hosts which do not use multipath transport currently and it seems unlikely that the situation is going to change in any foreseeable future (even if new releases of operatin systems get multipath protocols support there will be a long tail of legacy hosts). The solution for enterprise multihoming needs to work for the least common Baker, et al. Expires January 4, 2020 [Page 46] Internet-Draft Enterprise PA Multihoming July 2019 denominator: hosts without multipath transport support. In addition, not all protocols are using multipath transport. While multipath transport would complement the solution described in Section 6, it could not be considered as a sole solution to the problem of source address selection in multihomed environments. On the other hand PA-based multihoming could provide additional benefits for multipath protocol, should those protocols be deployed in the network. Multipath protocols could leverage source address selection to achieve maximum path diversity (and potentially improved performance). Therefore deploying multipath protocols could not be considered as an alternative to the approach proposed in this document. Instead both solutions complement each other so deploying multipath protocols in PA-based multihomed network proves mutually beneficial. 9. IANA Considerations This memo asks the IANA for no new parameters. 10. Security Considerations Section 6.2.3 discusses a mechanism for controlling source address selection on hosts using ICMPv6 messages. Using ICMPv6 to influence source address selection allows an attacker to exhaust the list of candidate source addresses on the host by sending spoofed ICMPv6 Code 5 for all prefixes known on the network (therefore preventing a victim from establishing a communication with the destination host). Another possible attack vector is using ICMPv6 Destination Unreachable Messages with Code 5 to steer the egress tra ffic towards the particular ISP (for example if the attacker has the ability of doing traffic sniffing or man-in-the-middle attack in that ISP network). To prevent those attacks hosts SHOULD verify that the original packet header included into ICMPv6 error message was actually sent by the host (to ensure that the ICMPv6 message was triggered by a packet sent by the host). As ICMPv6 Destination Unreachable Messages with Code 5 could be originated by any SADR-capable router within the domain (or even come from the Internet), GTSM ([RFC5082]) can not be applied. Filtering such ICMOv6 messages at the site border can not be recommended as it would break the legitimate end2end error signalling mechanism ICMPv6 is designed for. Baker, et al. Expires January 4, 2020 [Page 47] Internet-Draft Enterprise PA Multihoming July 2019 The security considerations of using stateless address autoconfiguration are discussed in [RFC4862]. 11. Acknowledgements The original outline was suggested by Ole Troan. The authors would like to thank the following people (in alphabetical order) for their review and feedback: Olivier Bonaventure, Deborah Brungard, Brian E Carpenter, Lorenzo Colitti, Roman Danyliw, Benjamin Kaduk, Suresh Krishnan, Mirja Kuhlewind, David Lamparter, Nicolai Leymann, Acee Lindem, Philip Matthewsu, Robert Raszuk, Alvaro Retana, Dave Thaler, Michael Tuxen, Martin Vigoureux, Eric Vyncke, Magnus Westerlund. 12. References 12.1. Normative References [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G., and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996, <https://www.rfc-editor.org/info/rfc1918>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, May 2000, <https://www.rfc-editor.org/info/rfc2827>. [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191, November 2005, <https://www.rfc-editor.org/info/rfc4191>. [RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005, <https://www.rfc-editor.org/info/rfc4193>. [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2006, <https://www.rfc-editor.org/info/rfc4291>. Baker, et al. Expires January 4, 2020 [Page 48] Internet-Draft Enterprise PA Multihoming July 2019 [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", STD 89, RFC 4443, DOI 10.17487/RFC4443, March 2006, <https://www.rfc-editor.org/info/rfc4443>. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, <https://www.rfc-editor.org/info/rfc4861>. [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, DOI 10.17487/RFC4862, September 2007, <https://www.rfc-editor.org/info/rfc4862>. [RFC6106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, "IPv6 Router Advertisement Options for DNS Configuration", RFC 6106, DOI 10.17487/RFC6106, November 2010, <https://www.rfc-editor.org/info/rfc6106>. [RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011, <https://www.rfc-editor.org/info/rfc6296>. [RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, "Default Address Selection for Internet Protocol Version 6 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012, <https://www.rfc-editor.org/info/rfc6724>. [RFC7078] Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing Address Selection Policy Using DHCPv6", RFC 7078, DOI 10.17487/RFC7078, January 2014, <https://www.rfc-editor.org/info/rfc7078>. [RFC7556] Anipko, D., Ed., "Multiple Provisioning Domain Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015, <https://www.rfc-editor.org/info/rfc7556>. [RFC8028] Baker, F. and B. Carpenter, "First-Hop Router Selection by Hosts in a Multi-Prefix Network", RFC 8028, DOI 10.17487/RFC8028, November 2016, <https://www.rfc-editor.org/info/rfc8028>. [RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, "IPv6 Router Advertisement Options for DNS Configuration", RFC 8106, DOI 10.17487/RFC8106, March 2017, <https://www.rfc-editor.org/info/rfc8106>. Baker, et al. Expires January 4, 2020 [Page 49] Internet-Draft Enterprise PA Multihoming July 2019 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. [RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., Richardson, M., Jiang, S., Lemon, T., and T. Winters, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 8415, DOI 10.17487/RFC8415, November 2018, <https://www.rfc-editor.org/info/rfc8415>. 12.2. Informative References [I-D.ietf-intarea-provisioning-domains] Pfister, P., Vyncke, E., Pauly, T., Schinazi, D., and W. Shao, "Discovering Provisioning Domain Names and Data", draft-ietf-intarea-provisioning-domains-05 (work in progress), June 2019. [I-D.ietf-rtgwg-dst-src-routing] Lamparter, D. and A. Smirnov, "Destination/Source Routing", draft-ietf-rtgwg-dst-src-routing-07 (work in progress), March 2019. [I-D.pfister-6man-sadr-ra] Pfister, P., "Source Address Dependent Route Information Option for Router Advertisements", draft-pfister-6man- sadr-ra-01 (work in progress), June 2015. [RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March 2004, <https://www.rfc-editor.org/info/rfc3704>. [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, <https://www.rfc-editor.org/info/rfc4941>. [RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C. Pignataro, "The Generalized TTL Security Mechanism (GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007, <https://www.rfc-editor.org/info/rfc5082>. [RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming Shim Protocol for IPv6", RFC 5533, DOI 10.17487/RFC5533, June 2009, <https://www.rfc-editor.org/info/rfc5533>. Baker, et al. Expires January 4, 2020 [Page 50] Internet-Draft Enterprise PA Multihoming July 2019 [RFC5534] Arkko, J. and I. van Beijnum, "Failure Detection and Locator Pair Exploration Protocol for IPv6 Multihoming", RFC 5534, DOI 10.17487/RFC5534, June 2009, <https://www.rfc-editor.org/info/rfc5534>. [RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node Requirements", RFC 6434, DOI 10.17487/RFC6434, December 2011, <https://www.rfc-editor.org/info/rfc6434>. [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013, <https://www.rfc-editor.org/info/rfc6824>. [RFC7676] Pignataro, C., Bonica, R., and S. Krishnan, "IPv6 Support for Generic Routing Encapsulation (GRE)", RFC 7676, DOI 10.17487/RFC7676, October 2015, <https://www.rfc-editor.org/info/rfc7676>. Authors' Addresses Fred Baker Santa Barbara, California 93117 USA Email: FredBaker.IETF@gmail.com Chris Bowers Juniper Networks Sunnyvale, California 94089 USA Email: cbowers@juniper.net Jen Linkova Google 1 Darling Island Rd Pyrmont, NSW 2009 AU Email: furry@google.com Baker, et al. Expires January 4, 2020 [Page 51]