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Versions: (draft-linkova-v6ops-conditional-ras) 00 01 02 03 04 05 06 07 08 RFC 8475

IPv6 Operations                                               J. Linkova
Internet-Draft                                                    Google
Intended status: Informational                                M. Stucchi
Expires: February 22, 2019                                      RIPE NCC
                                                         August 21, 2018


   Using Conditional Router Advertisements for Enterprise Multihoming
                  draft-ietf-v6ops-conditional-ras-08

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 the "ietf-rtgwg-
   enterprise-pa-multihoming" draft 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 "ietf-rtgwg-enterprise-pa-
   multihoming" document proposes a solution to this problem by
   introducing a new routing functionality (Source Address Dependent
   Routing) to solve the uplink selection issue and using Router
   Advertisements to influence the host source address selection.  While
   the above-mentioned document focuses on solving the general problem
   and on covering various complex use cases, this document adopts the
   approach proposed in the "ietf-rtgwg-enterprise-pa-multihoming" draft
   to provide a solution for a limited number of common use cases.  In
   particular, the focus is on scenarios where 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 RFC8028.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."



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   This Internet-Draft will expire on February 22, 2019.

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 Router, Load Balancing Between Uplinks  . . . . .  12
       3.2.5.  Topologies with Dedicated Border Routers  . . . . . .  13
       3.2.6.  Intra-Site 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 . . . . . . . . . . . . . . . . . . .  17
     5.1.  Privacy Considerations  . . . . . . . . . . . . . . . . .  18
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  20



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   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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 [RFC1918] address space 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.  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 address 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 PA blocks in 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).

   The document [I-D.ietf-rtgwg-enterprise-pa-multihoming] discusses
   these and other related challenges in detail in relation to the
   general multihoming scenario for enterprise networks and proposes a
   solution which relies heavily on the rule 5.5 of the default address
   selection algorithm ([RFC6724]).  The 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 the Rule 5.5
   as optional, the recent [RFC8028] recommends that multihomed hosts
   SHOULD support it.  Unfortunately that rule has not been widely
   implemented when this document was written.  Therefore network
   administrators in enterprise networks can't yet assume that all
   devices in their network support the 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 provider-assigned (PA) address space for the most
   common use cases.  This document discusses how the general approach
   described in [I-D.ietf-rtgwg-enterprise-pa-multihoming] can be
   applied to solve multihoming scenarios when:




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   o  An enterprise network has two or more ISP uplinks;

   o  Those uplinks are used for Internet access in active/backup or
      load sharing mode w/o any sophisticated traffic engineering
      requirements;

   o  Each ISP assigns the network a subnet from its own PA address
      space

   o  Hosts in the enterprise network are not expected to support the
      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", "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 is using 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 primary one;

   o  Hosts in the enterprise network are not expected to support the
      Rule 5.5 of the default address selection algorithm ([RFC6724]).





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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 the
      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 [I-D.ietf-rtgwg-enterprise-pa-multihoming], 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 ISP_A uplink while
   packets sourced from subnet_B must be sent to 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 anti-spoofing ingress filtering
   ([RFC2827], [RFC3704]).

   While some work is being done in the Source Address Dependent Routing
   (SADR) (such as [I-D.ietf-rtgwg-dst-src-routing]), the simplest way
   to implement the desired functionality currently is to apply a policy
   which selects a next-hop or an egress interface based on the packet
   source address.  Most SMB/Enterprise grade routers have such
   functionality available currently.

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



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   responsible for choosing the correct uplink based on the source
   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 the
   particular prefixes and therefore such changes have to be signaled
   back to the hosts.  For example:

   o  An uplink to an ISP_A went down.  Hosts should not use addresses
      from ISP_A prefix;

   o  A primary uplink to ISP_A which was not operational has come back
      up.  Hosts should start using the source addresses from ISP_A
      prefix.

   [I-D.ietf-rtgwg-enterprise-pa-multihoming] provides a detailed
   explanation on why SLAAC (Stateless Address Autoconfiguration,
   [RFC4862]) and RAs (Router Advertisements, [RFC4861]) are the most
   suitable mechanism for signaling network topology changes to hosts
   and thereby influencing the source address selection.  Sending a
   router advertisement to change the preferred lifetime for a given
   prefix provides the following functionality:

   o  deprecating addresses (by sending an RA with the
      preferred_lifetime set to 0 in the corresponding PIO (Prefix
      Information option, [RFC4861])) to indicate to hosts that 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 non-zero preferred lifetime)
      to indicate to hosts that addresses from that prefix can be used
      again.

   It should be notes that only preferred lifetime for the affected
   prefix needs to be changed.  As the goal is to influence the source
   address selection algoorithm on hosts, not preventing them from
   forming addresses from a specific prefix, the valid lifetime should
   not be changed.  Actually it would not even be possible for
   unauthenticated RAs (which is the most common deployment scenario) as
   Section 5.5.3 of [RFC4862] prevents hosts from setting valid lifetime
   for addresses to zero unless RAs are authenticated.

   To provide the desired functionality, first-hop routers are required
   to

   o  send RA triggered by defined event policies in response to uplink
      status change event; and





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   o  while sending periodic or solicted 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.

   etc.

   In some scenarios, when two routers are providing first-hop
   redundancy via VRRP (Virtual Router Redundancy Protocol, [RFC5798]),
   the master-backup status can be considered as a condition for sending
   RAs and changing the preferred lifetime value.  See Section 3.2.2 for
   more details.

   If hosts are provided with ISP DNS servers IPv6 addresses via RDNSS
   (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 RA
   fields values (like PIO Preferred Lifetime or DNS Server Lifetime)
   changed by the particular trigger need to stay the same until another
   event happens causing the value to be updated.  E.g. if the 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 enterprise
   network scenario.







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3.2.  Example Scenarios

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

3.2.1.  Single Router, Primary/Backup Uplinks


                                                                 --------
                                               ,-------,       ,'        ',
                    +----+  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

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



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   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/if 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 router
   advertisement 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 on Fig.2


















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                                                                 --------
                                               ,-------,       ,'        ',
                      +----+ 2001:db8:1::/48 ,'         ',    :            :
  2001:db8:1:1::/64  _|    |----------------+    ISP_A    +--+:            :
                    | | R1 |                 ',         ,'    :            :
                    | +----+                   '-------'      :            :
H1------------------|                                         :  INTERNET  :
                    | +----+                   ,-------,      :            :
                    |_|    | 2001:db8:2::/48 ,'         ',    :            :
  2001:db8:2:1::/64   | R2 |----------------+    ISP_B    +--+:            :
                      +----+                 ',         ,'    :            :
                                               '-------'       ',        ,'
                                                                 --------


               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)
i 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 router advertisement.  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 ISP_A uplink is up).  If ISP_A
   uplink is down, then R1 becomes a backup (the VRRP interface status
   tracking is expected to be used to automatically modify the VRRP
   priorities and trigger the mastership switchover).  Router



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   advertisements 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.  Router advertsement on R2 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 like a route to the uplink
   interface subnet or a default route received from the uplink can be
   used.  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
     }

   R2:






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   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
   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 one described
   in Section 3.2.1.

3.2.4.  Two Router, 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.  It means
   that both R1 and R2 need to have the corresponding forwarding policy
   to forward packets based on their source addresses.

   Each router would send RAs with PIO for the corresponding prefix.
   setting preferred_lifetime to a non-zero value when the ISP uplink is
   up, and deprecating the prefix by setting the preferred lifetime to 0
   in case of uplink failure.  The uplink recovery would trigger another



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   RA with non-zero 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 can not be used as a trigger for conditional RAs and route
   presence as described above (Section 3.2.2) should be used instead.

   Let's look at the example topology shown on the Figure 3:

















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                                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 one
   then the following policy might be used to achieve the desired
   behaviour (H1 is using ISP_A address space, 2001:db8:1:1::/64 while
   ISP_A uplink is up and only using 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
    }

   For the load-balancing case the policy would look slightly different:
   each prefix has non-zero preferred_lifetime only if the correspoding
   ISP uplink route is present:






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   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.  Intra-Site Communication during Simultaneous Uplinks Outage

   Prefix deprecation as a result of an uplink status change might lead
   to a situation when 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 intra-site
   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 the
   intra-site communication.  However some OSes might just fall back to
   IPv4 if the network interface has no preferred IPv6 global addresses.
   Therefore if intra-site connectivity is vital during simultanious
   outages of multiple uplinks, administrators might consider using ULAs
   (Unique Local Addresses, [RFC4193]) or provisioning additional backup
   uplinks to protect the network from double-failure cases.

3.2.7.  Uplink Damping

   If an actively used uplink (primary one or one used in load balaning
   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 non-zero preferred PIO lifetime, and every time
   the uplink goes down all addresses in the affected prefix 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 non-zero



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   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 a multiple simultaneous uplink
   failure scenario, when all but one uplinks are down and the last
   remaining 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
   discourage but 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 behaviour 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 behaviour
   would be implementation-dependend 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]), and therefore the load between two uplinks
   most likely would not be evenly distributed.  (However, the proposed
   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 uplinks properties (such as latency etc), so
   egress traffic might not be sent to the nearest uplink if the



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   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.

   An uplink recovery, however, 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 memo asks the IANA for no new parameters.

5.  Security Considerations

   This memo introduces no new security considerations.  It relies on
   Router Advertisements ([RFC4861]) and 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 infuence
   source address selection processes on hosts.

   The potential attack vectors are including but not limited to:



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   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 undesirable (e.g. more expensive)
      uplink.

   Therefore the network administrators SHOULD secure Router
   Advertisements, e.g., by deploying RA guard [RFC6105].

5.1.  Privacy Considerations

   This memo introduces no new privacy considerations.

6.  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, Dave
   Thaler.

7.  References

7.1.  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>.

   [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>.





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   [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>.

   [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>.






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7.2.  Informative References

   [I-D.ietf-rtgwg-dst-src-routing]
              Lamparter, D. and A. Smirnov, "Destination/Source
              Routing", draft-ietf-rtgwg-dst-src-routing-06 (work in
              progress), October 2017.

   [I-D.ietf-rtgwg-enterprise-pa-multihoming]
              Baker, F., Bowers, C., and J. Linkova, "Enterprise
              Multihoming using Provider-Assigned Addresses without
              Network Prefix Translation: Requirements and Solution",
              draft-ietf-rtgwg-enterprise-pa-multihoming-07 (work in
              progress), 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>.

Appendix A.  Change Log

   Initial Version:  July 2017

Authors' Addresses

   Jen Linkova
   Google
   Mountain View, California  94043
   USA

   Email: furry@google.com






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   Massimiliano Stucchi
   RIPE NCC
   Stationsplein, 11
   Amsterdam  1012 AB
   The Netherlands

   Email: mstucchi@ripe.net












































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