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rfc:rfc8475

Internet Engineering Task Force (IETF) J. Linkova Request for Comments: 8475 Google Category: Informational M. Stucchi ISSN: 2070-1721 RIPE NCC

                                                          October 2018
 Using Conditional Router Advertisements for Enterprise Multihoming

Abstract

 This document discusses the most common scenarios of connecting an
 enterprise network to multiple ISPs using an address space assigned
 by an ISP and how the approach proposed in "Enterprise Multihoming
 using Provider-Assigned Addresses without Network Prefix Translation:
 Requirements and Solution" could be applied in those scenarios.  The
 problem of enterprise multihoming without address translation of any
 form has not been solved yet as it requires both the network to
 select the correct egress ISP based on the packet source address and
 hosts to select the correct source address based on the desired
 egress ISP for that traffic.  The aforementioned document proposes a
 solution to this problem by introducing a new routing functionality
 (Source Address Dependent Routing) to solve the uplink selection
 issue.  It also proposes using Router Advertisements to influence the
 host source address selection.  It focuses on solving the general
 problem and covering various complex use cases, and this document
 adopts its proposed approach to provide a solution for a limited
 number of common use cases.  In particular, the focus of this
 document is on scenarios in which an enterprise network has two
 Internet uplinks used either in primary/backup mode or simultaneously
 and hosts in that network might not yet properly support multihoming
 as described in RFC 8028.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are candidates for any level of Internet
 Standard; see Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8475.

Linkova & Stucchi Informational [Page 1] RFC 8475 Conditional RAs October 2018

Copyright Notice

 Copyright (c) 2018 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
 2.  Common Enterprise Multihoming Scenarios . . . . . . . . . . .   4
   2.1.  Two ISP Uplinks, Primary and Backup . . . . . . . . . . .   4
   2.2.  Two ISP Uplinks, Used for Load-Balancing  . . . . . . . .   5
 3.  Conditional Router Advertisements . . . . . . . . . . . . . .   5
   3.1.  Solution Overview . . . . . . . . . . . . . . . . . . . .   5
     3.1.1.  Uplink Selection  . . . . . . . . . . . . . . . . . .   5
     3.1.2.  Source Address Selection and Conditional RAs  . . . .   5
   3.2.  Example Scenarios . . . . . . . . . . . . . . . . . . . .   8
     3.2.1.  Single Router, Primary/Backup Uplinks . . . . . . . .   8
     3.2.2.  Two Routers, Primary/Backup Uplinks . . . . . . . . .   9
     3.2.3.  Single Router, Load-Balancing between Uplinks . . . .  12
     3.2.4.  Two Routers, Load-Balancing between Uplinks . . . . .  12
     3.2.5.  Topologies with Dedicated Border Routers  . . . . . .  13
     3.2.6.  Intrasite Communication during Simultaneous Uplinks
             Outage  . . . . . . . . . . . . . . . . . . . . . . .  15
     3.2.7.  Uplink Damping  . . . . . . . . . . . . . . . . . . .  15
     3.2.8.  Routing Packets When the Corresponding Uplink Is
             Unavailable . . . . . . . . . . . . . . . . . . . . .  16
   3.3.  Solution Limitations  . . . . . . . . . . . . . . . . . .  16
     3.3.1.  Connections Preservation  . . . . . . . . . . . . . .  17
 4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
 5.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   5.1.  Privacy Considerations  . . . . . . . . . . . . . . . . .  18
 6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
   6.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
   6.2.  Informative References  . . . . . . . . . . . . . . . . .  20
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  20
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

Linkova & Stucchi Informational [Page 2] RFC 8475 Conditional RAs October 2018

1. Introduction

 Multihoming is an obvious requirement for many enterprise networks to
 ensure the desired level of network reliability.  However, using more
 than one ISP (and address space assigned by those ISPs) introduces
 the problem of assigning IP addresses to hosts.  In IPv4, there is no
 choice but using address space [RFC1918] and NAT [RFC3022] at the
 network edge [RFC4116].  Using Provider Independent (PI) address
 space is not always an option, since it requires running BGP between
 the enterprise network and the ISPs.  The administrative overhead of
 obtaining and managing PI address space can also be a concern.  As
 IPv6 hosts can, by design, have multiple addresses of the global
 scope [RFC4291], multihoming using provider addresses looks even
 easier for IPv6: each ISP assigns an IPv6 block (usually /48), and
 hosts in the enterprise network have addresses assigned from each ISP
 block.  However, using IPv6 provider-assigned (PA) blocks in a
 multihoming scenario introduces some challenges, including, but not
 limited to:
 o  Selecting the correct uplink based on the packet source address;
 o  Signaling to hosts that some source addresses should or should not
    be used (e.g., an uplink to the ISP went down or became available
    again).
 [PROVIDER-ASSIGNED] discusses these and other related challenges in
 detail in relation to the general multihoming scenario for enterprise
 networks.  It proposes a solution that relies heavily on Rule 5.5 of
 the default address selection algorithm [RFC6724].  Rule 5.5 makes
 hosts prefer source addresses in a prefix advertised by the next hop
 and, therefore, is very useful in multihomed scenarios when different
 routers may advertise different prefixes.  While [RFC6724] defines
 Rule 5.5 as optional, the recent [RFC8028] recommends that multihomed
 hosts SHOULD support it.  Unfortunately, that rule has not been
 widely implemented at the time of writing.  Therefore, network
 administrators in enterprise networks can't yet assume that all
 devices in their network support Rule 5.5, especially in the quite
 common BYOD ("Bring Your Own Device") scenario.  However, while it
 does not seem feasible to solve all the possible multihoming
 scenarios without relying on Rule 5.5, it is possible to provide IPv6
 multihoming using PA address space for the most common use cases.
 This document discusses how the general approach described in
 [PROVIDER-ASSIGNED] can be applied to solve multihoming scenarios
 when:
 o  An enterprise network has two or more ISP uplinks;

Linkova & Stucchi Informational [Page 3] RFC 8475 Conditional RAs October 2018

 o  Those uplinks are used for Internet access in active/backup or
    load-sharing mode without any sophisticated traffic engineering
    requirements;
 o  Each ISP assigns the network a subnet from its own PA address
    space; and
 o  Hosts in the enterprise network are not expected to support Rule
    5.5 of the default address selection algorithm [RFC6724].

1.1. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

2. Common Enterprise Multihoming Scenarios

2.1. Two ISP Uplinks, Primary and Backup

 This scenario has the following key characteristics:
 o  The enterprise network uses uplinks to two (or more) ISPs for
    Internet access;
 o  Each ISP assigns IPv6 PA address space for the network;
 o  Uplink(s) to one ISP is a primary (preferred) one.  All other
    uplinks are backup and are not expected to be used while the
    primary one is operational;
 o  If the primary uplink is operational, all Internet traffic should
    flow via that uplink;
 o  When the primary uplink fails, the Internet traffic needs to flow
    via the backup uplinks;
 o  Recovery of the primary uplink needs to trigger the traffic
    switchover from the backup uplinks back to the primary one;
 o  Hosts in the enterprise network are not expected to support Rule
    5.5 of the default address selection algorithm [RFC6724].

Linkova & Stucchi Informational [Page 4] RFC 8475 Conditional RAs October 2018

2.2. Two ISP Uplinks, Used for Load-Balancing

 This scenario has the following key characteristics:
 o  The enterprise network is using uplinks to two (or more) ISPs for
    Internet access;
 o  Each ISP assigns an IPv6 PA address space;
 o  All the uplinks may be used simultaneously, with the traffic flows
    being randomly (not necessarily equally) distributed between them;
 o  Hosts in the enterprise network are not expected to support Rule
    5.5 of the default address selection algorithm [RFC6724].

3. Conditional Router Advertisements

3.1. Solution Overview

3.1.1. Uplink Selection

 As discussed in [PROVIDER-ASSIGNED], one of the two main problems to
 be solved in the enterprise multihoming scenario is the problem of
 the next-hop (uplink) selection based on the packet source address.
 For example, if the enterprise network has two uplinks, to ISP_A and
 ISP_B, and hosts have addresses from subnet_A and subnet_B (belonging
 to ISP_A and ISP_B, respectively), then packets sourced from subnet_A
 must be sent to the ISP_A uplink while packets sourced from subnet_B
 must be sent to the ISP_B uplink.  Sending packets with source
 addresses belonging to one ISP address space to another ISP might
 cause those packets to be filtered out if those ISPs or their uplinks
 implement antispoofing ingress filtering [RFC2827][RFC3704].
 While some work is being done in the Source Address Dependent Routing
 (SADR) (such as [DESTINATION]), the simplest way to implement the
 desired functionality currently is to apply a policy that selects a
 next hop or an egress interface based on the packet source address.
 Currently, most SMB/Enterprise-grade routers have such functionality
 available.

3.1.2. Source Address Selection and Conditional RAs

 Another problem to be solved in the multihoming scenario is the
 source address selection on hosts.  In the normal situation (all
 uplinks are up/operational), hosts have multiple global unique
 addresses and can rely on the default address selection algorithm
 [RFC6724] to pick up a source address, while the network is
 responsible for choosing the correct uplink based on the source

Linkova & Stucchi Informational [Page 5] RFC 8475 Conditional RAs October 2018

 address selected by a host, as described in Section 3.1.1.  However,
 some network topology changes (i.e., changing uplink status) might
 affect the global reachability for packets sourced from particular
 prefixes; therefore, such changes have to be signaled back to the
 hosts.  For example:
 o  An uplink to ISP_A went down.  Hosts should not use addresses from
    an ISP_A prefix;
 o  A primary uplink to ISP_A that was not operational has come back
    up.  Hosts should start using the source addresses from an ISP_A
    prefix.
 [PROVIDER-ASSIGNED] provides a detailed explanation of why Stateless
 Address Autoconfiguration (SLAAC) [RFC4862] and Router Advertisements
 (RAs) [RFC4861] are the most suitable mechanisms for signaling
 network topology changes to hosts, thereby influencing the source
 address selection.  Sending an RA to change the preferred lifetime
 for a given prefix provides the following functionality:
 o  Deprecating addresses by sending an RA with preferred_lifetime set
    to 0 in the corresponding Prefix Information option (PIO)
    [RFC4861].  This indicates to hosts that addresses from that
    prefix should not be used;
 o  Making a previously unused (deprecated) prefix usable again by
    sending an RA containing a PIO with nonzero preferred lifetime.
    This indicates to hosts that addresses from that prefix can be
    used again.
 It should be noted that only the preferred lifetime for the affected
 prefix needs to be changed.  As the goal is to influence the source
 address selection algorithm on hosts rather than prevent them from
 forming addresses from a specific prefix, the valid lifetime should
 not be changed.  Actually, changing the valid lifetime would not even
 be possible for unauthenticated RAs (which is the most common
 deployment scenario), because Section 5.5.3 of [RFC4862] prevents
 hosts from setting the valid lifetime for addresses to zero unless
 RAs are authenticated.
 To provide the desired functionality, first-hop routers are required
 to:
 o  Send RAs triggered by defined event policies in response to an
    uplink status change event; and

Linkova & Stucchi Informational [Page 6] RFC 8475 Conditional RAs October 2018

 o  While sending periodic or solicited RAs, set the value in the
    given RA field (e.g., PIO preferred lifetime) based on the uplink
    status.
 The exact definition of the "uplink status" depends on the network
 topology and may include conditions like:
 o  Uplink interface status change;
 o  Presence of a particular route in the routing table;
 o  Presence of a particular route with a particular attribute (next
    hop, tag, etc.) in the routing table;
 o  Protocol adjacency change.
 In some scenarios, when two routers are providing first-hop
 redundancy via Virtual Router Redundancy Protocol (VRRP) [RFC5798],
 the master-backup status can be considered to be a condition for
 sending RAs and changing the preferred lifetime value.  See
 Section 3.2.2 for more details.
 If hosts are provided with the IPv6 addresses of ISP DNS servers via
 a Recursive DNS Server (RDNSS) (see "IPv6 Router Advertisement
 Options for DNS Configuration" [RFC8106]), it might be desirable for
 the conditional RAs to update the Lifetime field of the RDNSS option
 as well.
 The trigger is not only forcing the router to send an unsolicited RA
 to propagate the topology changes to all hosts.  Obviously, the
 values of the RA fields (like PIO Preferred Lifetime or DNS Server
 Lifetime) changed by the particular trigger need to stay the same
 until another event causes the value to be updated.  For example, if
 an ISP_A uplink failure causes the prefix to be deprecated, all
 solicited and unsolicited RAs sent by the router need to have the
 preferred lifetime for that PIO set to 0 until the uplink comes back
 up.
 It should be noted that the proposed solution is quite similar to the
 existing requirement L-13 for IPv6 Customer Edge Routers [RFC7084]
 and the documented behavior of homenet devices [RFC7788].  It is
 using the same mechanism of deprecating a prefix when the
 corresponding uplink is not operational, applying it to an
 enterprise-network scenario.

Linkova & Stucchi Informational [Page 7] RFC 8475 Conditional RAs October 2018

3.2. Example Scenarios

 This section illustrates how the conditional RAs solution can be
 applied to the most common enterprise multihoming scenarios,
 described in Section 2.

3.2.1. Single Router, Primary/Backup Uplinks

  1. ——-

,——-, / \

                 +----+ 2001:db8:1::/48  ,'         ',    :          :
                 |    |-----------------+    ISP_A    +--+:          :

2001:db8:1:1::/64 | | ', ,' : :

                 |    |                    '-------'      :          :

H1—————–| R1 | : INTERNET :

                 |    |                    ,-------,      :          :

2001:db8:2:1::/64 | | 2001:db8:2::/48 ,' ', : :

                 |    |-----------------+    ISP_B    +--+:          :
                 +----+                  ',         ,'    :          :
                                           '-------'       \        /
                                                            --------
            Figure 1: Single Router, Primary/Backup Uplinks
 Let's look at a simple network topology where a single router acts as
 a border router to terminate two ISP uplinks and as a first-hop
 router for hosts.  Each ISP assigns a /48 to the network, and the
 ISP_A uplink is a primary one, to be used for all Internet traffic,
 while the ISP_B uplink is a backup, to be used only when the primary
 uplink is not operational.
 To ensure that packets with source addresses from ISP_A and ISP_B are
 only routed to ISP_A and ISP_B uplinks, respectively, the network
 administrator needs to configure a policy on R1:
 IF (packet_source_address is in 2001:db8:1::/48)
     and
     (packet_destination_address is not in
     (2001:db8:1::/48 or 2001:db8:2::/48))
     THEN
         default next hop is ISP_A_uplink

Linkova & Stucchi Informational [Page 8] RFC 8475 Conditional RAs October 2018

 IF (packet_source_address is in 2001:db8:2::/48)
     and
     (packet_destination_address is not in
     (2001:db8:1::/48 or 2001:db8:2::/48))
     THEN
         default next hop is ISP_B_uplink
 Under normal circumstances, it is desirable that all traffic be sent
 via the ISP_A uplink; therefore, hosts (the host H1 in the example
 topology figure) should be using source addresses from
 2001:db8:1:1::/64.  When or if the ISP_A uplink fails, hosts should
 stop using the 2001:db8:1:1::/64 prefix and start using
 2001:db8:2:1::/64 until the ISP_A uplink comes back up.  To achieve
 this, the RA configuration on the R1 device for the interface facing
 H1 needs to have the following policy:
 prefix 2001:db8:1:1::/64 {
     IF (ISP_A_uplink is up)
         THEN
             preferred_lifetime = 604800
         ELSE
             preferred_lifetime = 0
 }
 prefix 2001:db8:2:1::/64 {
     IF (ISP_A_Uplink is up)
         THEN
             preferred_lifetime = 0
         ELSE
             preferred_lifetime = 604800
 }
 A similar policy needs to be applied to the RDNSS lifetime if ISP_A
 and ISP_B DNS servers are used.

3.2.2. Two Routers, Primary/Backup Uplinks

 Let's look at a more complex scenario where two border routers are
 terminating two ISP uplinks (one each), acting as redundant first-hop
 routers for hosts.  The topology is shown in Figure 2.

Linkova & Stucchi Informational [Page 9] RFC 8475 Conditional RAs October 2018

  1. ——-

,——-, / \

2001:db8:1:1::/64 +----+ 2001:db8:1::/48 ,'         ',    :          :
                 _|    |----------------+    ISP_A    +--+:          :
                | | R1 |                 ',         ,'    :          :
                | +----+                   '-------'      :          :

H1—————-| : INTERNET :

                | +----+                   ,-------,      :          :
                |_|    | 2001:db8:2::/48 ,'         ',    :          :
                  | R2 |----------------+    ISP_B    +--+:          :

2001:db8:2:1::/64 +—-+ ', ,' : :

                                           '-------'       \        /
                                                            --------
             Figure 2: Two Routers, Primary/Backup Uplinks
 In this scenario, R1 sends RAs with PIO for 2001:db8:1:1::/64 (ISP_A
 address space), and R2 sends RAs with PIO for 2001:db8:2:1::/64
 (ISP_B address space).  Each router needs to have a forwarding policy
 configured for packets received on its hosts-facing interface:
 IF (packet_source_address is in 2001:db8:1::/48)
     and
     (packet_destination_address is not in
     (2001:db8:1::/48 or 2001:db8:2::/48))
     THEN
         default next hop is ISP_A_uplink
 IF (packet_source_address is in 2001:db8:2::/48)
     and
     (packet_destination_address is not in
     (2001:db8:1::/48 or 2001:db8:2::/48))
     THEN
         default next hop is ISP_B_uplink
 In this case, there is more than one way to ensure that hosts are
 selecting the correct source address based on the uplink status.  If
 VRRP is used to provide first-hop redundancy, and the master router
 is the one with the active uplink, then the simplest way is to use
 the VRRP mastership as a condition for RA.  So, if ISP_A is the
 primary uplink, the routers R1 and R2 need to be configured in the
 following way:
 R1 is the VRRP master by default (when the ISP_A uplink is up).  If
 the ISP_A uplink is down, then R1 becomes a backup (the VRRP
 interface-status tracking is expected to be used to automatically

Linkova & Stucchi Informational [Page 10] RFC 8475 Conditional RAs October 2018

 modify the VRRP priorities and trigger the mastership switchover).
 RAs on R1's interface facing H1 needs to have the following policy
 applied:
 prefix 2001:db8:1:1::/64 {
     IF (vrrp_master)
         THEN
             preferred_lifetime = 604800
         ELSE
             preferred_lifetime = 0
 }
 R2 is VRRP backup by default.  RA on R2's interface facing H1 needs
 to have the following policy applied:
 prefix 2001:db8:2:1::/64 {
     IF(vrrp_master)
         THEN
             preferred_lifetime = 604800
         ELSE
             preferred_lifetime = 0
 }
 If VRRP is not used or interface status tracking is not used for
 mastership switchover, then each router needs to be able to detect
 the uplink failure/recovery on the neighboring router, so that RAs
 with updated preferred lifetime values are triggered.  Depending on
 the network setup, various triggers can be used, such as a route to
 the uplink interface subnet or a default route received from the
 uplink.  The obvious drawback of using the routing table to trigger
 the conditional RAs is that some additional configuration is
 required.  For example, if a route to the prefix assigned to the ISP
 uplink is used as a trigger, then the conditional RA policy would
 have the following logic:
 R1:
 prefix 2001:db8:1:1::/64 {
     IF (ISP_A_uplink is up)
         THEN
             preferred_lifetime = 604800
         ELSE
            preferred_lifetime = 0
 }

Linkova & Stucchi Informational [Page 11] RFC 8475 Conditional RAs October 2018

 R2:
 prefix 2001:db8:2:1::/64 {
     IF (ISP_A_uplink_route is present)
         THEN
             preferred_lifetime = 0
         ELSE
             preferred_lifetime = 604800
 }

3.2.3. Single Router, Load-Balancing between Uplinks

 Let's look at the example topology shown in Figure 1, but with both
 uplinks used simultaneously.  In this case, R1 would send RAs
 containing PIOs for both prefixes, 2001:db8:1:1::/64 and
 2001:db8:2:1::/64, changing the preferred lifetime based on
 particular uplink availability.  If the interface status is used as
 an uplink availability indicator, then the policy logic would look
 like the following:
 prefix 2001:db8:1:1::/64 {
     IF (ISP_A_uplink is up)
         THEN
             preferred_lifetime  = 604800
         ELSE
             preferred_lifetime = 0
 }
 prefix 2001:db8:2:1::/64 {
     IF (ISP_B_uplink is up)
         THEN
             preferred_lifetime  = 604800
         ELSE
             preferred_lifetime = 0
 }
 R1 needs a forwarding policy to be applied to forward packets to the
 correct uplink based on the source address, similar to the policy
 described in Section 3.2.1.

3.2.4. Two Routers, Load-Balancing between Uplinks

 In this scenario, the example topology is similar to the one shown in
 Figure 2, but both uplinks can be used at the same time.  This means
 that both R1 and R2 need to have the corresponding forwarding policy
 to forward packets based on their source addresses.

Linkova & Stucchi Informational [Page 12] RFC 8475 Conditional RAs October 2018

 Each router would send RAs with PIO for the corresponding prefix,
 setting preferred_lifetime to a nonzero value when the ISP uplink is
 up and deprecating the prefix by setting preferred_lifetime to 0 in
 the case of uplink failure.  The uplink recovery would trigger
 another RA with a nonzero preferred lifetime to make the addresses
 from the prefix preferred again.  The example RA policy on R1 and R2
 would look like:
 R1:
 prefix 2001:db8:1:1::/64 {
     IF (ISP_A_uplink is up)
         THEN
             preferred_lifetime  = 604800
         ELSE
             preferred_lifetime = 0
 }
 R2:
 prefix 2001:db8:2:1::/64 {
     IF (ISP_B_uplink is up)
         THEN
             preferred_lifetime  = 604800
         ELSE
             preferred_lifetime = 0
 }

3.2.5. Topologies with Dedicated Border Routers

 For simplicity, all topologies above show the ISP uplinks terminated
 on the first-hop routers.  Obviously, the proposed approach can be
 used in more complex topologies when dedicated devices are used for
 terminating ISP uplinks.  In that case, VRRP mastership or interface
 status cannot be used as a trigger for conditional RAs.  Route
 presence as described in Section 3.2.2 should be used instead.

Linkova & Stucchi Informational [Page 13] RFC 8475 Conditional RAs October 2018

 Let's look at the example topology shown in Figure 3:
                              2001:db8:1::/48              --------
  2001:db8:1:1::/64                     ,-------,        ,'        ',
            +----+  +---+  +----+     ,'         ',     :            :
           _|    |--|   |--| R3 |----+    ISP_A    +---+:            :
          | | R1 |  |   |  +----+     ',         ,'     :            :
          | +----+  |   |               '-------'       :            :
H1--------|         |LAN|                               :  INTERNET  :
          | +----+  |   |               ,-------,       :            :
          |_|    |  |   |  +----+     ,'         ',     :            :
            | R2 |--|   |--| R4 |----+    ISP_B    +---+:            :
            +----+  +---+  +----+     ',         ,'     :            :
2001:db8:2:1::/64                       '-------'        ',        ,'
                              2001:db8:2::/48              --------
                  Figure 3: Dedicated Border Routers
 For example, if ISP_A is a primary uplink and ISP_B is a backup, then
 the following policy might be used to achieve the desired behavior
 (H1 is using ISP_A address space, 2001:db8:1:1::/64, while the ISP_A
 uplink is up and only using the ISP_B 2001:db8:2:1::/64 prefix if the
 uplink is non-operational):
 R1 and R2 policy:
 prefix 2001:db8:1:1::/64 {
     IF (ISP_A_uplink_route is present)
         THEN
             preferred_lifetime = 604800
         ELSE
             preferred_lifetime = 0
 }
 prefix 2001:db8:2:1::/64 {
     IF (ISP_A_uplink_route is present)
         THEN
             preferred_lifetime = 0
         ELSE
             preferred_lifetime = 604800
 }

Linkova & Stucchi Informational [Page 14] RFC 8475 Conditional RAs October 2018

 For the load-balancing case, the policy would look slightly
 different: each prefix has a nonzero preferred_lifetime only if the
 corresponding ISP uplink route is present:
 prefix 2001:db8:1:1::/64 {
     IF (ISP_A_uplink_route is present)
         THEN
             preferred_lifetime = 604800
         ELSE
             preferred_lifetime = 0
 }
 prefix 2001:db8:2:1::/64 {
     IF (ISP_B_uplink_route is present)
         THEN
             preferred_lifetime = 604800
         ELSE
             preferred_lifetime = 0
 }

3.2.6. Intrasite Communication during Simultaneous Uplinks Outage

 Prefix deprecation as a result of an uplink status change might lead
 to a situation in which all global prefixes are deprecated (all ISP
 uplinks are not operational for some reason).  Even when there is no
 Internet connectivity, it might be still desirable to have intrasite
 IPv6 connectivity (especially when the network in question is an
 IPv6-only one).  However, while an address is in a deprecated state,
 its use is discouraged, but not strictly forbidden [RFC4862].  In
 such a scenario, all IPv6 source addresses in the candidate set
 [RFC6724] are deprecated, which means that they still can be used (as
 there are no preferred addresses available), and the source address
 selection algorithm can pick up one of them, allowing intrasite
 communication.  However, some operating systems might just fall back
 to IPv4 if the network interface has no preferred IPv6 global
 addresses.  Therefore, if intrasite connectivity is vital during
 simultaneous outages of multiple uplinks, administrators might
 consider using Unique Local Addresses (ULAs) [RFC4193] or
 provisioning additional backup uplinks to protect the network from
 double-failure cases.

3.2.7. Uplink Damping

 If an actively used uplink (a primary one or one used in a load-
 balancing scenario) starts flapping, it might lead to the undesirable
 situation of flapping addresses on hosts: every time the uplink goes
 up, hosts receive an RA with a nonzero preferred PIO lifetime, and
 every time the uplink goes down, all addresses in the affected prefix

Linkova & Stucchi Informational [Page 15] RFC 8475 Conditional RAs October 2018

 become deprecated.  This would, undoubtedly, negatively impact the
 user experience, not to mention the impact of spikes of duplicate
 address detection traffic every time an uplink comes back up.
 Therefore, it's recommended that router vendors implement some form
 of damping policy for conditional RAs and either postpone sending an
 RA with a nonzero lifetime for a PIO when the uplink comes up for a
 number of seconds or (even) introduce accumulated penalties/
 exponential backoff algorithm for such delays.  (In the case of
 multiple simultaneous uplink failure, when all but one of the uplinks
 are down and the last remaining one is flapping, it might result in
 all addresses being deprecated for a while after the flapping uplink
 recovers.)

3.2.8. Routing Packets When the Corresponding Uplink Is Unavailable

 Deprecating IPv6 addresses by setting the preferred lifetime to 0
 discourages but does not strictly forbid its usage in new
 communications.  A deprecated address may still be used for existing
 connections [RFC4862].  Therefore, when an ISP uplink goes down, the
 corresponding border router might still receive packets with source
 addresses belonging to that ISP address space while there is no
 available uplink to send those packets to.
 The expected router behavior would depend on the uplink selection
 mechanism.  For example, if some form of SADR is used, then such
 packets will be dropped as there is no route to the destination.  If
 policy-based routing is used to set a next hop, then the behavior
 would be implementation dependent and may vary from dropping the
 packets to forwarding them based on the routing table entries.  It
 should be noted that there is no return path to the packet source (as
 the ISP uplink is not operational).  Therefore, even if the outgoing
 packets are sent to another ISP, the return traffic might not be
 delivered.

3.3. Solution Limitations

 It should be noted that the proposed approach is not a "silver
 bullet" for all possible multihoming scenarios.  It would work very
 well for networks with relatively simple topologies and
 straightforward routing policies.  The more complex the network
 topology and the corresponding routing policies, the more
 configuration would be required to implement the solution.
 Another limitation is related to the load-balancing between the
 uplinks.  In the scenario in which both uplinks are active, hosts
 would select the source prefix using the Default Address Selection
 algorithm [RFC6724]; therefore, the load between two uplinks most
 likely would not be evenly distributed.  (However, the proposed

Linkova & Stucchi Informational [Page 16] RFC 8475 Conditional RAs October 2018

 mechanism does allow a creative way of controlling uplinks load in
 software-defined networks where controllers might selectively
 deprecate prefixes on some hosts but not others to move egress
 traffic between uplinks).  Also, the prefix selection does not take
 into account any other properties of uplinks (such as latency), so
 egress traffic might not be sent to the nearest uplink if the
 corresponding prefix is selected as a source.  In general, if not all
 uplinks are equal, and some uplinks are expected to be preferred over
 others, then the network administrator should ensure that prefixes
 from non-preferred ISP(s) are kept deprecated (so primary/backup
 setup is used).

3.3.1. Connections Preservation

 The proposed solution is not designed to preserve connection state
 after an uplink failure.  If all uplinks to an ISP go down, all
 sessions to/from addresses from that ISP address space are
 interrupted as there is no egress path for those packets and there is
 no return path from the Internet to the corresponding prefix.  In
 this regard, it is similar to IPv4 multihoming using NAT, where an
 uplink failure and failover to another uplink means that a public
 IPv4 address changes and all existing connections are interrupted.
 However, an uplink recovery does not necessarily lead to connections
 interruption.  In the load-sharing/balancing scenario, an uplink
 recovery does not affect any existing connections at all.  In the
 active/backup topology, when the primary uplink recovers from the
 failure and the backup prefix is deprecated, the existing sessions
 (established to/from the backup ISP addresses) can be preserved if
 the routers are configured as described in Section 3.2.1 and send
 packets with the backup ISP source addresses to the backup uplink,
 even when the primary one is operational.  As a result, the primary
 uplink recovery makes the usage of the backup ISP addresses
 discouraged but still possible.
 It should be noted that in IPv4 multihoming with NAT, when the egress
 interface is chosen without taking packet source address into account
 (as internal hosts usually have addresses from [RFC1918] space),
 sessions might not be preserved after an uplink recovery unless
 packet forwarding is integrated with existing NAT sessions tracking.

4. IANA Considerations

 This document has no IANA actions.

Linkova & Stucchi Informational [Page 17] RFC 8475 Conditional RAs October 2018

5. Security Considerations

 This memo introduces no new security considerations.  It relies on
 RAs [RFC4861] and the SLAAC [RFC4862] mechanism and inherits their
 security properties.  If an attacker is able to send a rogue RA, they
 could deprecate IPv6 addresses on hosts or influence source-address-
 selection processes on hosts.
 The potential attack vectors include, but are not limited to:
 o  An attacker sends a rogue RA deprecating IPv6 addresses on hosts;
 o  An attacker sends a rogue RA making addresses preferred while the
    corresponding ISP uplink is not operational;
 o  An attacker sends a rogue RA making addresses preferred for a
    backup ISP, steering traffic to an undesirable (e.g., more
    expensive) uplink.
 Therefore, the network administrators SHOULD secure RAs, e.g., by
 deploying an RA guard [RFC6105].

5.1. Privacy Considerations

 This memo introduces no new privacy considerations.

6. References

6.1. 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>.

Linkova & Stucchi Informational [Page 18] RFC 8475 Conditional RAs October 2018

 [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
            Address Translator (Traditional NAT)", RFC 3022,
            DOI 10.17487/RFC3022, January 2001,
            <https://www.rfc-editor.org/info/rfc3022>.
 [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>.
 [RFC4116]  Abley, J., Lindqvist, K., Davies, E., Black, B., and V.
            Gill, "IPv4 Multihoming Practices and Limitations",
            RFC 4116, DOI 10.17487/RFC4116, July 2005,
            <https://www.rfc-editor.org/info/rfc4116>.
 [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>.
 [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>.
 [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
            Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
            DOI 10.17487/RFC6105, February 2011,
            <https://www.rfc-editor.org/info/rfc6105>.
 [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>.
 [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>.

Linkova & Stucchi Informational [Page 19] RFC 8475 Conditional RAs October 2018

 [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>.

6.2. Informative References

 [DESTINATION]
            Lamparter, D. and A. Smirnov, "Destination/Source
            Routing", Work in Progress,
            draft-ietf-rtgwg-dst-src-routing-06, October 2017.
 [PROVIDER-ASSIGNED]
            Baker, F., Bowers, C., and J. Linkova, "Enterprise
            Multihoming using Provider-Assigned Addresses without
            Network Prefix Translation: Requirements and Solution",
            Work in Progress,
            draft-ietf-rtgwg-enterprise-pa-multihoming-07, June 2018.
 [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>.
 [RFC5798]  Nadas, S., Ed., "Virtual Router Redundancy Protocol (VRRP)
            Version 3 for IPv4 and IPv6", RFC 5798,
            DOI 10.17487/RFC5798, March 2010,
            <https://www.rfc-editor.org/info/rfc5798>.
 [RFC7084]  Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
            Requirements for IPv6 Customer Edge Routers", RFC 7084,
            DOI 10.17487/RFC7084, November 2013,
            <https://www.rfc-editor.org/info/rfc7084>.
 [RFC7788]  Stenberg, M., Barth, S., and P. Pfister, "Home Networking
            Control Protocol", RFC 7788, DOI 10.17487/RFC7788, April
            2016, <https://www.rfc-editor.org/info/rfc7788>.

Acknowledgements

 Thanks to the following people (in alphabetical order) for their
 review and feedback: Mikael Abrahamsson, Lorenzo Colitti, Marcus
 Keane, Erik Kline, David Lamparter, Dusan Mudric, Erik Nordmark, and
 Dave Thaler.

Linkova & Stucchi Informational [Page 20] RFC 8475 Conditional RAs October 2018

Authors' Addresses

 Jen Linkova
 Google
 Mountain View, California  94043
 United States of America
 Email: furry@google.com
 Massimiliano Stucchi
 RIPE NCC
 Stationsplein, 11
 Amsterdam  1012 AB
 The Netherlands
 Email: mstucchi@ripe.net

Linkova & Stucchi Informational [Page 21]

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