< draft-ietf-rtgwg-enterprise-pa-multihoming-08.txt   draft-ietf-rtgwg-enterprise-pa-multihoming-09.txt >
Routing Working Group F. Baker Routing Working Group F. Baker
Internet-Draft Internet-Draft
Intended status: Informational C. Bowers Intended status: Informational C. Bowers
Expires: November 18, 2019 Juniper Networks Expires: January 2, 2020 Juniper Networks
J. Linkova J. Linkova
Google Google
May 17, 2019 July 1, 2019
Enterprise Multihoming using Provider-Assigned IPv6 Addresses without Enterprise Multihoming using Provider-Assigned IPv6 Addresses without
Network Prefix Translation: Requirements and Solution Network Prefix Translation: Requirements and Solutions
draft-ietf-rtgwg-enterprise-pa-multihoming-08 draft-ietf-rtgwg-enterprise-pa-multihoming-09
Abstract Abstract
Connecting an enterprise site to multiple ISPs using provider- Connecting an enterprise site to multiple ISPs over IPv6 using
assigned addresses is difficult without the use of some form of provider-assigned addresses is difficult without the use of some form
Network Address Translation (NAT). Much has been written on this of Network Address Translation (NAT). Much has been written on this
topic over the last 10 to 15 years, but it still remains a problem topic over the last 10 to 15 years, but it still remains a problem
without a clearly defined or widely implemented solution. Any without a clearly defined or widely implemented solution. Any
multihoming solution without NAT requires hosts at the site to have multihoming solution without NAT requires hosts at the site to have
addresses from each ISP and to select the egress ISP by selecting a addresses from each ISP and to select the egress ISP by selecting a
source address for outgoing packets. It also requires routers at the source address for outgoing packets. It also requires routers at the
site to take into account those source addresses when forwarding site to take into account those source addresses when forwarding
packets out towards the ISPs. packets out towards the ISPs.
This document attempts to define a complete solution to this problem. This document examines currently available mechanisms for providing a
solution to this problem for a broad range of enterprise topologies.
It covers the behavior of routers to forward traffic taking into It covers the behavior of routers to forward traffic taking into
account source address, and it covers the behavior of host to select account source address, and it covers the behavior of hosts to select
appropriate source addresses. It also covers any possible role that appropriate source addresses. It also covers any possible role that
routers might play in providing information to hosts to help them routers might play in providing information to hosts to help them
select appropriate source addresses. In the process of exploring select appropriate source addresses. In the process of exploring
potential solutions, this document also makes explicit requirements potential solutions, this document also makes explicit requirements
for how the solution would be expected to behave from the perspective for how the solution would be expected to behave from the perspective
of an enterprise site network administrator. of an enterprise site network administrator.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
skipping to change at page 2, line 7 skipping to change at page 2, line 10
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 18, 2019. This Internet-Draft will expire on January 2, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 6 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 6
3. Enterprise Multihoming Requirements . . . . . . . . . . . . . 6 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Simple ISP Connectivity with Connected SERs . . . . . . . 6 4. Enterprise Multihoming Use Cases . . . . . . . . . . . . . . 8
3.2. Simple ISP Connectivity Where SERs Are Not Directly 4.1. Simple ISP Connectivity with Connected SERs . . . . . . . 8
Connected . . . . . . . . . . . . . . . . . . . . . . . . 8 4.2. Simple ISP Connectivity Where SERs Are Not Directly
3.3. Enterprise Network Operator Expectations . . . . . . . . 9 Connected . . . . . . . . . . . . . . . . . . . . . . . . 9
3.4. More complex ISP connectivity . . . . . . . . . . . . . . 12 4.3. Enterprise Network Operator Expectations . . . . . . . . 11
3.5. ISPs and Provider-Assigned Prefixes . . . . . . . . . . . 14 4.4. More complex ISP connectivity . . . . . . . . . . . . . . 13
3.6. Simplified Topologies . . . . . . . . . . . . . . . . . . 15 4.5. ISPs and Provider-Assigned Prefixes . . . . . . . . . . . 15
4. Generating Source-Prefix-Scoped Forwarding Tables . . . . . 15 4.6. Simplified Topologies . . . . . . . . . . . . . . . . . . 16
5. Mechanisms For Hosts To Choose Good Source Addresses In A 5. Generating Source-Prefix-Scoped Forwarding Tables . . . . . 16
Multihomed Site . . . . . . . . . . . . . . . . . . . . . . . 22 6. Mechanisms For Hosts To Choose Good Source Addresses In A
5.1. Source Address Selection Algorithm on Hosts . . . . . . . 24 Multihomed Site . . . . . . . . . . . . . . . . . . . . . . . 23
5.2. Selecting Source Address When Both Uplinks Are Working . 27 6.1. Source Address Selection Algorithm on Hosts . . . . . . . 25
5.2.1. Distributing Address Selection Policy Table with 6.2. Selecting Source Address When Both Uplinks Are Working . 28
DHCPv6 . . . . . . . . . . . . . . . . . . . . . . . 27 6.2.1. Distributing Address Selection Policy Table with
5.2.2. Controlling Source Address Selection With Router DHCPv6 . . . . . . . . . . . . . . . . . . . . . . . 28
Advertisements . . . . . . . . . . . . . . . . . . . 27 6.2.2. Controlling Source Address Selection With Router
5.2.3. Controlling Source Address Selection With ICMPv6 . . 29 Advertisements . . . . . . . . . . . . . . . . . . . 29
5.2.4. Summary of Methods For Controlling Source Address 6.2.3. Controlling Source Address Selection With ICMPv6 . . 31
Selection To Implement Routing Policy . . . . . . . . 31 6.2.4. Summary of Methods For Controlling Source Address
5.3. Selecting Source Address When One Uplink Has Failed . . . 32 Selection To Implement Routing Policy . . . . . . . . 33
5.3.1. Controlling Source Address Selection With DHCPv6 . . 32 6.3. Selecting Source Address When One Uplink Has Failed . . . 33
5.3.2. Controlling Source Address Selection With Router 6.3.1. Controlling Source Address Selection With DHCPv6 . . 34
Advertisements . . . . . . . . . . . . . . . . . . . 34 6.3.2. Controlling Source Address Selection With Router
Advertisements . . . . . . . . . . . . . . . . . . . 35
5.3.3. Controlling Source Address Selection With ICMPv6 . . 35 6.3.3. Controlling Source Address Selection With ICMPv6 . . 36
5.3.4. Summary Of Methods For Controlling Source Address 6.3.4. Summary Of Methods For Controlling Source Address
Selection On The Failure Of An Uplink . . . . . . . . 35 Selection On The Failure Of An Uplink . . . . . . . . 37
5.4. Selecting Source Address Upon Failed Uplink Recovery . . 36 6.4. Selecting Source Address Upon Failed Uplink Recovery . . 37
5.4.1. Controlling Source Address Selection With DHCPv6 . . 36 6.4.1. Controlling Source Address Selection With DHCPv6 . . 37
5.4.2. Controlling Source Address Selection With Router 6.4.2. Controlling Source Address Selection With Router
Advertisements . . . . . . . . . . . . . . . . . . . 36
5.4.3. Controlling Source Address Selection With ICMP . . . 37
5.4.4. Summary Of Methods For Controlling Source Address
Selection Upon Failed Uplink Recovery . . . . . . . . 37
5.5. Selecting Source Address When All Uplinks Failed . . . . 37
5.5.1. Controlling Source Address Selection With DHCPv6 . . 38
5.5.2. Controlling Source Address Selection With Router
Advertisements . . . . . . . . . . . . . . . . . . . 38 Advertisements . . . . . . . . . . . . . . . . . . . 38
5.5.3. Controlling Source Address Selection With ICMPv6 . . 39 6.4.3. Controlling Source Address Selection With ICMP . . . 38
5.5.4. Summary Of Methods For Controlling Source Address 6.4.4. Summary Of Methods For Controlling Source Address
Selection When All Uplinks Failed . . . . . . . . . . 39 Selection Upon Failed Uplink Recovery . . . . . . . . 39
5.6. Summary Of Methods For Controlling Source Address 6.5. Selecting Source Address When All Uplinks Failed . . . . 39
Selection . . . . . . . . . . . . . . . . . . . . . . . . 39 6.5.1. Controlling Source Address Selection With DHCPv6 . . 39
5.7. Other Configuration Parameters . . . . . . . . . . . . . 40 6.5.2. Controlling Source Address Selection With Router
5.7.1. DNS Configuration . . . . . . . . . . . . . . . . . . 40 Advertisements . . . . . . . . . . . . . . . . . . . 39
6. Deployment Considerations . . . . . . . . . . . . . . . . . . 42 6.5.3. Controlling Source Address Selection With ICMPv6 . . 40
7. Other Solutions . . . . . . . . . . . . . . . . . . . . . . . 43 6.5.4. Summary Of Methods For Controlling Source Address
7.1. Shim6 . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Selection When All Uplinks Failed . . . . . . . . . . 40
7.2. IPv6-to-IPv6 Network Prefix Translation . . . . . . . . . 43 6.6. Summary Of Methods For Controlling Source Address
7.3. Multipath Transport . . . . . . . . . . . . . . . . . . . 43 Selection . . . . . . . . . . . . . . . . . . . . . . . . 40
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44 6.7. Solution Limitations . . . . . . . . . . . . . . . . . . 42
9. Security Considerations . . . . . . . . . . . . . . . . . . . 44 6.7.1. Connections Preservation . . . . . . . . . . . . . . 42
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 44 6.8. Other Configuration Parameters . . . . . . . . . . . . . 43
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 44 6.8.1. DNS Configuration . . . . . . . . . . . . . . . . . . 43
11.1. Normative References . . . . . . . . . . . . . . . . . . 44 7. Deployment Considerations . . . . . . . . . . . . . . . . . . 44
11.2. Informative References . . . . . . . . . . . . . . . . . 46 7.1. Deploying SADR Domain . . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 47 7.2. Hosts-Related Considerations . . . . . . . . . . . . . . 45
8. Other Solutions . . . . . . . . . . . . . . . . . . . . . . . 45
8.1. Shim6 . . . . . . . . . . . . . . . . . . . . . . . . . . 45
8.2. IPv6-to-IPv6 Network Prefix Translation . . . . . . . . . 46
8.3. Multipath Transport . . . . . . . . . . . . . . . . . . . 46
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
10. Security Considerations . . . . . . . . . . . . . . . . . . . 47
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 47
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 47
12.1. Normative References . . . . . . . . . . . . . . . . . . 47
12.2. Informative References . . . . . . . . . . . . . . . . . 49
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 50
1. Introduction 1. Introduction
Site multihoming, the connection of a subscriber network to multiple Site multihoming, the connection of a subscriber network to multiple
upstream networks using redundant uplinks, is a common enterprise upstream networks using redundant uplinks, is a common enterprise
architecture for improving the reliability of its Internet architecture for improving the reliability of its Internet
connectivity. If the site uses provider-independent (PI) addresses, connectivity. If the site uses provider-independent (PI) addresses,
all traffic originating from the enterprise can use source addresses all traffic originating from the enterprise can use source addresses
from the PI address space. Site multihoming with PI addresses is from the PI address space. Site multihoming with PI addresses is
commonly used with both IPv4 and IPv6, and does not present any new commonly used with both IPv4 and IPv6, and does not present any new
technical challenges. technical challenges.
It may be desirable for an enterprise site to connect to multiple It may be desirable for an enterprise site to connect to multiple
ISPs using provider-assigned (PA) addresses, instead of PI addresses. ISPs using provider-assigned (PA) addresses, instead of PI addresses.
Multihoming with provider-assigned addresses is typically less Multihoming with provider-assigned addresses is typically less
expensive for the enterprise relative to using provider-independent expensive for the enterprise relative to using provider-independent
addresses. PA multihoming is also a practice that should be addresses as it does not require obtaining and maintaining PI address
facilitated and encouraged because it does not add to the size of the space as well as running BGP between the enterprise and the ISPs (for
Internet routing table, whereas PI multihoming does. Note that PA is small/meduim networks running BGP might be not just undesirable but
also used to mean "provider-aggregatable". In this document we impossible, especially if residential-type ISP connections are used).
assume that provider-assigned addresses are always provider- PA multihoming is also a practice that should be facilitated and
aggregatable. encouraged because it does not add to the size of the Internet
routing table, whereas PI multihoming does. Note that PA is also
used to mean "provider-aggregatable". In this document we assume
that provider-assigned addresses are always provider-aggregatable.
With PA multihoming, for each ISP connection, the site is assigned a With PA multihoming, for each ISP connection, the site is assigned a
prefix from within an address block allocated to that ISP by its prefix from within an address block allocated to that ISP by its
National or Regional Internet Registry. In the simple case of two National or Regional Internet Registry. In the simple case of two
ISPs (ISP-A and ISP-B), the site will have two different prefixes ISPs (ISP-A and ISP-B), the site will have two different prefixes
assigned to it (prefix-A and prefix-B). This arrangement is assigned to it (prefix-A and prefix-B). This arrangement is
problematic. First, packets with the "wrong" source address may be problematic. First, packets with the "wrong" source address may be
dropped by one of the ISPs. In order to limit denial of service dropped by one of the ISPs. In order to limit denial of service
attacks using spoofed source addresses, BCP38 [RFC2827] recommends attacks using spoofed source addresses, BCP38 [RFC2827] recommends
that ISPs filter traffic from customer sites to only allow traffic that ISPs filter traffic from customer sites to only allow traffic
skipping to change at page 4, line 33 skipping to change at page 4, line 43
source address in prefix-A may be dropped by ISP-B. source address in prefix-A may be dropped by ISP-B.
However, even if ISP-B does not implement BCP38 or ISP-B adds However, even if ISP-B does not implement BCP38 or ISP-B adds
prefix-A to its list of allowed source addresses on the uplink from prefix-A to its list of allowed source addresses on the uplink from
the multihomed site, two-way communication may still fail. If the the multihomed site, two-way communication may still fail. If the
packet with source address in prefix-A was sent to ISP-B because the packet with source address in prefix-A was sent to ISP-B because the
uplink to ISP-A failed, then if ISP-B does not drop the packet and uplink to ISP-A failed, then if ISP-B does not drop the packet and
the packet reaches its destination somewhere on the Internet, the the packet reaches its destination somewhere on the Internet, the
return packet will be sent back with a destination address in prefix- return packet will be sent back with a destination address in prefix-
A. The return packet will be routed over the Internet to ISP-A, but A. The return packet will be routed over the Internet to ISP-A, but
it will not be delivered to the multihomed site because its link with it will not be delivered to the multihomed site because the site
ISP-A has failed. Two-way communication would require some uplink with ISP-A has failed. Two-way communication would require
arrangement for ISP-B to advertise prefix-A when the uplink to ISP-A some arrangement for ISP-B to advertise prefix-A when the uplink to
fails. ISP-A fails.
Note that the same may be true with a provider that does not Note that the same may be true with a provider that does not
implement BCP 38, if his upstream provider does, or has no implement BCP 38, if his upstream provider does, or has no
corresponding route. The issue is not that the immediate provider corresponding route to deliver the ingress traffic to the multihomed
implements ingress filtering; it is that someone upstream does, or site. The issue is not that the immediate provider implements
lacks a route. ingress filtering; it is that someone upstream does (so egress
traffic is blocked), or lacks a route (causing blackholing of the
ingress traffic).
Another issue with asymmetric traffic flow (when the egress traffic
leaves the site via one ISP but the return traffic enters the site
via another uplink) is related to stateful firewalls/middleboxes.
Keeping state in that case might be problematic, even impossible.
With IPv4, this problem is commonly solved by using [RFC1918] private With IPv4, this problem is commonly solved by using [RFC1918] private
address space within the multi-homed site and Network Address address space within the multi-homed site and Network Address
Translation (NAT) or Network Address/Port Translation (NAPT) on the Translation (NAT) or Network Address/Port Translation (NAPT) on the
uplinks to the ISPs. However, one of the goals of IPv6 is to uplinks to the ISPs. However, one of the goals of IPv6 is to
eliminate the need for and the use of NAT or NAPT. Therefore, eliminate the need for and the use of NAT or NAPT. Therefore,
requiring the use of NAT or NAPT for an enterprise site to multihome requiring the use of NAT or NAPT for an enterprise site to multihome
with provider-assigned addresses is not an attractive solution. with provider-assigned addresses is not an attractive solution.
[RFC6296] describes a translation solution specifically tailored to [RFC6296] describes a translation solution specifically tailored to
meet the requirements of multi-homing with provider-assigned IPv6 meet the requirements of multi-homing with provider-assigned IPv6
addresses. With the IPv6-to-IPv6 Network Prefix Translation (NPTv6) addresses. With the IPv6-to-IPv6 Network Prefix Translation (NPTv6)
solution, within the site an enterprise can use Unique Local solution, within the site an enterprise can use Unique Local
Addresses [RFC4193] or the prefix assigned by one of the ISPs. As Addresses [RFC4193] or the prefix assigned by one of the ISPs. As
traffic leaves the site on an uplink to an ISP, the source address traffic leaves the site on an uplink to an ISP, the source address
gets translated to an address within the prefix assigned by the ISP gets translated to an address within the prefix assigned by the ISP
on that uplink in a predictable and reversible manner. [RFC6296] is on that uplink in a predictable and reversible manner. [RFC6296] is
currently classified as Experimental, and it has been implemented by currently classified as Experimental, and it has been implemented by
several vendors. See Section 7.2, for more discussion of NPTv6. several vendors. See Section 8.2, for more discussion of NPTv6.
This document defines routing requirements for enterprise multihoming This document defines routing requirements for enterprise multihoming
using provider-assigned IPv6 addresses. We have made no attempt to This document focuses on the following general class of solutions.
write these requirements in a manner that is agnostic to potential
solutions. Instead, this document focuses on the following general
class of solutions.
Each host at the enterprise has multiple addresses, at least one from Each host at the enterprise has multiple addresses, at least one from
each ISP-assigned prefix. Each host, as discussed in Section 5.1 and each ISP-assigned prefix. Each host, as discussed in Section 6.1 and
[RFC6724], is responsible for choosing the source address applied to [RFC6724], is responsible for choosing the source address applied to
each packet it sends. A host SHOULD be able respond dynamically to each packet it sends. A host is expected to be able respond
the failure of an uplink to a given ISP by no longer sending packets dynamically to the failure of an uplink to a given ISP by no longer
with the source address corresponding to that ISP. Potential sending packets with the source address corresponding to that ISP.
mechanisms for the communication of changes in the network to the Potential mechanisms for the communication of changes in the network
host are Neighbor Discovery Router Advertisements, DHCPv6, and to the host are Neighbor Discovery Router Advertisements ([RFC4861]),
ICMPv6. DHCPv6 ([RFC8415]), and ICMPv6 ([RFC4443]).
The routers in the enterprise network are responsible for ensuring The routers in the enterprise network are responsible for ensuring
that packets are delivered to the "correct" ISP uplink based on that packets are delivered to the "correct" ISP uplink based on
source address. This requires that at least some routers in the site source address. This requires that at least some routers in the site
network are able to take into account the source address of a packet network are able to take into account the source address of a packet
when deciding how to route it. That is, some routers must be capable when deciding how to route it. That is, some routers must be capable
of some form of Source Address Dependent Routing (SADR), if only as of some form of Source Address Dependent Routing (SADR), if only as
described in [RFC3704]. At a minimum, the routers connected to the described in the section 4.3 of [RFC3704]. At a minimum, the routers
ISP uplinks (the site exit routers or SERs) must be capable of Source connected to the ISP uplinks (the site exit routers or SERs) must be
Address Dependent Routing. Expanding the connected domain of routers capable of Source Address Dependent Routing. Expanding the connected
capable of SADR from the site exit routers deeper into the site domain of routers capable of SADR from the site exit routers deeper
network will generally result in more efficient routing of traffic into the site network will generally result in more efficient routing
with external destinations. of traffic with external destinations.
This document is organized as follows. Section 3 looks in more This document is organized as follows. Section 4 looks in more
detail at the enterprise networking environments in which this detail at the enterprise networking environments in which this
solution is expected to operate. The discussion of Section 3 uses solution is expected to operate. The discussion of Section 4 uses
the concepts of source-prefix-scoped routing advertisements and the concepts of source-prefix-scoped routing advertisements and
forwarding tables without stopping to provide a precise description forwarding tables and provides a description of how source-prefix-
of how source-prefix-scoped routing advertisements are used to scoped routing advertisements are used to generate source-prefix-
generate source-prefix-scoped forwarding tables. Instead, this scoped forwarding tables. Instead, this detailed description is
detailed description is provided in Section 4. Section 5 discusses provided in Section 5. Section 6 discusses existing and proposed
existing and proposed mechanisms for hosts to select the source mechanisms for hosts to select the source address applied to packets.
address applied to packets. It also discusses the requirements for It also discusses the requirements for routing that are needed to
routing that are needed to support these enterprise network scenarios support these enterprise network scenarios and the mechanisms by
and the mechanisms by which hosts are expected to select source which hosts are expected to select source addresses dynamically based
addresses dynamically based on network state. Section 6 discusses on network state. Section 7 discusses deployment considerations,
deployment considerations, while Section 7 discussed other solutions. while Section 8 discusses other solutions.
2. Requirements Language 2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
3. Enterprise Multihoming Requirements 3. Terminology
3.1. Simple ISP Connectivity with Connected SERs PA (provider-assigned or provider-aggregatable) address space: a
block of IP addresses assigned by an Regional Internet Registry (RIR)
to a Local Internet Registry (LIR), used to create allocations to end
sites. Can be aggregated and present in the routing table as one
route.
PI (provider-independent) address space: a block of IP addresses
assigned by an Regional Internet Registry (RIR) directly to end site/
end customer.
ISP: Internet Service Provider.
LIR (Local Internet Registry): an organisation (usually an ISP or an
enterprise/academic) which receives IP addresses allocation from its
Regional Internet Regsitry, then assign parts of that allocation to
its customers.
RIR (Regional Internet Registry): an organization which manages the
Internet number resources (such as IP addresses and AS numbers)
within a geographical region of the world.
SADR (Source Address Dependent Routing): Routing which takes into
account the source address of a packet in addition to the packet
destination address.
SADR domain: a routing domain where some (or all) routers exchange
source-dependent routing information.
Source-Prefix-Scoped Routing/Forwarding Table: a routing (or
forwarding) table which contains routing (or forwarding) information
which is applicable to packets with source addresses from the
specific prefix only.
Unscoped Routing/Forwarding Table: a routing (or forwarding) table
which can be used to route/forward packets with any source addresses.
SER (Site Edge Router): a router which connects the site to an ISP
(terminates an ISP uplink)..
LLA (Link-Local Address): IPv6 Unicast Address from fe80::/10 prefix
([RFC4291]).
ULA (Unique Local IPv6 Unicast Address): IPv6 unicast addresses from
FC00::/7 prefix. They are globally unique and intended for local
communications ([RFC4193]).
GUA (Global Unicast Address): globally routable IPv6 addresses of the
global scope ([RFC4291]).
SLAAC (IPv6 Stateless Address Autoconfiguration): a stateless process
of configuring network stack on IPv6 hosts ([RFC4862]).
RA (Router Advertisement): a message sent by an IPv6 router to
advertise its presence to hosts together with various network-related
parameters required for hosts to perform SLAAC ([RFC4861]).
PIO (Prefix Information Option): a part of RA message containing
information about IPv6 prefixes which could be used by hosts to
generate global IPv6 addresses ([RFC4862]).
RIO (Route Information Option): a part of RA message containing
information about more specific IPv6 prefixes reachable via the
advertising router ([RFC4191]).
4. Enterprise Multihoming Use Cases
4.1. Simple ISP Connectivity with Connected SERs
We start by looking at a scenario in which a site has connections to We start by looking at a scenario in which a site has connections to
two ISPs, as shown in Figure 1. The site is assigned the prefix two ISPs, as shown in Figure 1. The site is assigned the prefix
2001:db8:0:a000::/52 by ISP-A and prefix 2001:db8:0:b000::/52 by ISP- 2001:db8:0:a000::/52 by ISP-A and prefix 2001:db8:0:b000::/52 by ISP-
B. We consider three hosts in the site. H31 and H32 are on a LAN B. We consider three hosts in the site. H31 and H32 are on a LAN
that has been assigned subnets 2001:db8:0:a010::/64 and that has been assigned subnets 2001:db8:0:a010::/64 and
2001:db8:0:b010::/64. H31 has been assigned the addresses 2001:db8:0:b010::/64. H31 has been assigned the addresses
2001:db8:0:a010::31 and 2001:db8:0:b010::31. H32 has been assigned 2001:db8:0:a010::31 and 2001:db8:0:b010::31. H32 has been assigned
2001:db8:0:a010::32 and 2001:db8:0:b010::32. H41 is on a different 2001:db8:0:a010::32 and 2001:db8:0:b010::32. H41 is on a different
subnet that has been assigned 2001:db8:0:a020::/64 and subnet that has been assigned 2001:db8:0:a020::/64 and
skipping to change at page 7, line 33 skipping to change at page 8, line 48
+--+ +--+ +--+ \ / +--+ +--+ +--+ \ /
H41------|R3|--|R5|--|R6| -------- H41------|R3|--|R5|--|R6| --------
+--+ +--+ +--+ +--+ +--+ +--+
2001:db8:0:a020::41 2001:db8:0:a020::41
2001:db8:0:b020::41 2001:db8:0:b020::41
Figure 1: Simple ISP Connectivity With Connected SERs Figure 1: Simple ISP Connectivity With Connected SERs
We refer to a router that connects the site to an ISP as a site edge We refer to a router that connects the site to an ISP as a site edge
router(SER). Several other routers provide connectivity among the router (SER). Several other routers provide connectivity among the
internal hosts (H31, H32, and H41), as well as connecting the internal hosts (H31, H32, and H41), as well as connecting the
internal hosts to the Internet through SERa and SERb. In this internal hosts to the Internet through SERa and SERb. In this
example SERa and SERb share a direct connection to each other. In example SERa and SERb share a direct connection to each other. In
Section 3.2, we consider a scenario where this is not the case. Section 4.2, we consider a scenario where this is not the case.
For the moment, we assume that the hosts are able to make good For the moment, we assume that the hosts are able to make good
choices about which source addresses through some mechanism that choices about which source addresses through some mechanism that
doesn't involve the routers in the site network. Here, we focus on doesn't involve the routers in the site network. Here, we focus on
primary task of the routed site network, which is to get packets primary task of the routed site network, which is to get packets
efficiently to their destinations, while sending a packet to the ISP efficiently to their destinations, while sending a packet to the ISP
that assigned the prefix that matches the source address of the that assigned the prefix that matches the source address of the
packet. In Section 5, we examine what role the routed network may packet. In Section 6, we examine what role the routed network may
play in helping hosts make good choices about source addresses for play in helping hosts make good choices about source addresses for
packets. packets.
With this solution, routers will need some form of Source Address With this solution, routers will need some form of Source Address
Dependent Routing, which will be new functionality. It would be Dependent Routing, which will be new functionality. It would be
useful if an enterprise site does not need to upgrade all routers to useful if an enterprise site does not need to upgrade all routers to
support the new SADR functionality in order to support PA multi- support the new SADR functionality in order to support PA multi-
homing. We consider if this is possible and what are the tradeoffs homing. We consider if this is possible and what are the tradeoffs
of not having all routers in the site support SADR functionality. of not having all routers in the site support SADR functionality.
skipping to change at page 8, line 29 skipping to change at page 9, line 45
In Figure 1, when only SERa and SERb are capable of source address In Figure 1, when only SERa and SERb are capable of source address
dependent routing, PA multi-homing will work. However, the paths dependent routing, PA multi-homing will work. However, the paths
over which the packets are sent will generally not be the shortest over which the packets are sent will generally not be the shortest
paths. The forwarding paths will generally be more efficient as more paths. The forwarding paths will generally be more efficient as more
routers are capable of SADR. For example, if R4, R2, and R6 are routers are capable of SADR. For example, if R4, R2, and R6 are
upgraded to support SADR, then can exchange source-scoped routes with upgraded to support SADR, then can exchange source-scoped routes with
SERa and SERb. They will then know to send traffic with a source SERa and SERb. They will then know to send traffic with a source
address matching prefix 2001:db8:0:b000::/52 directly to SERb, address matching prefix 2001:db8:0:b000::/52 directly to SERb,
without sending it to SERa first. without sending it to SERa first.
3.2. Simple ISP Connectivity Where SERs Are Not Directly Connected 4.2. Simple ISP Connectivity Where SERs Are Not Directly Connected
In Figure 2, we modify the topology slightly by inserting R7, so that In Figure 2, we modify the topology slightly by inserting R7, so that
SERa and SERb are no longer directly connected. With this topology, SERa and SERb are no longer directly connected. With this topology,
it is not enough to just enable SADR routing on SERa and SERb to it is not enough to just enable SADR routing on SERa and SERb to
support PA multi-homing. There are two solutions to ways to enable support PA multi-homing. There are two solutions to enable PA
PA multihoming in this topology. multihoming in this topology.
2001:db8:0:1234::101 H101 2001:db8:0:1234::101 H101
| |
| |
2001:db8:0:a010::31 -------- 2001:db8:0:a010::31 --------
2001:db8:0:b010::31 ,-----. / \ 2001:db8:0:b010::31 ,-----. / \
+--+ +--+ +----+ ,' `. : : +--+ +--+ +----+ ,' `. : :
+---|R1|---|R4|---+---|SERa|-+ ISP-A +--+-- : +---|R1|---|R4|---+---|SERa|-+ ISP-A +--+-- :
H31--+ +--+ +--+ | +----+ `. ,' : : H31--+ +--+ +--+ | +----+ `. ,' : :
| | `-----' : Internet : | | `-----' : Internet :
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H41------|R3|--|R5|--|R6| -------- H41------|R3|--|R5|--|R6| --------
+--+ +--+ +--+ | +--+ +--+ +--+ |
| |
2001:db8:0:a020::41 2001:db8:0:5678::501 H501 2001:db8:0:a020::41 2001:db8:0:5678::501 H501
2001:db8:0:b020::41 2001:db8:0:b020::41
Figure 2: Simple ISP Connectivity Where SERs Are Not Directly Figure 2: Simple ISP Connectivity Where SERs Are Not Directly
Connected Connected
One option is to effectively modify the topology by creating a One option is to effectively modify the topology by creating a
logical tunnel between SERa and SERb, using GRE for example. logical tunnel between SERa and SERb, using GRE ([RFC7676]) for
Although SERa and SERb are not directly connected physically in this example. Although SERa and SERb are not directly connected
topology, they can be directly connected logically by a tunnel. physically in this topology, they can be directly connected logically
by a tunnel.
The other option is to enable SADR functionality on R7. In this way, The other option is to enable SADR functionality on R7. In this way,
R7 will exchange source-scoped routes with SERa and SERb, making the R7 will exchange source-scoped routes with SERa and SERb, making the
three routers act as a single SADR domain. This illustrates the three routers act as a single SADR domain. This illustrates the
basic principle that the minimum requirement for the routed site basic principle that the minimum requirement for the routed site
network to support PA multi-homing is having all of the site exit network to support PA multi-homing is having all of the site exit
routers be part of a connected SADR domain. Extending the connected routers be part of a connected SADR domain. Extending the connected
SADR domain beyond that point can produce more efficient forwarding SADR domain beyond that point can produce more efficient forwarding
paths. paths.
3.3. Enterprise Network Operator Expectations 4.3. Enterprise Network Operator Expectations
Before considering a more complex scenario, let's look in more detail Before considering a more complex scenario, let's look in more detail
at the reasonably simple multihoming scenario in Figure 2 to at the reasonably simple multihoming scenario in Figure 2 to
understand what can reasonably be expected from this solution. As a understand what can reasonably be expected from this solution. As a
general guiding principle, we assume an enterprise network operator general guiding principle, we assume an enterprise network operator
will expect a multihomed network to behave as close as to a single- will expect a multihomed network to behave as close as to a single-
homed network as possible. So a solution that meets those homed network as possible. So a solution that meets those
expectations where possible is a good thing. expectations where possible is a good thing.
For traffic between internal hosts and traffic from outside the site For traffic between internal hosts and traffic from outside the site
to internal hosts, an enterprise network operator would expect there to internal hosts, an enterprise network operator would expect there
be no visible change in the path taken by this traffic, since this be no visible change in the path taken by this traffic, since this
traffic does not need to be routed in a way that depends on source traffic does not need to be routed in a way that depends on source
address. It is also reasonable to expect that internal hosts should address. It is also reasonable to expect that internal hosts should
be able to communicate with each other using either of their source be able to communicate with each other using either of their source
addresses without restriction. For example, H31 should be able to addresses without restriction. For example, H31 should be able to
communicate with H41 using a packet with S=2001:db8:0:a010::31, communicate with H41 using a packet with S=2001:db8:0:a010::31,
D=2001:db8:0:b010::41, regardless of the state of uplink to ISP-B. D=2001:db8:0:b020::41, regardless of the state of uplink to ISP-B.
These goals can be accomplished by having all of the routers in the These goals can be accomplished by having all of the routers in the
network continue to originate normal unscoped destination routes for network continue to originate normal unscoped destination routes for
their connected networks. If we can arrange so that these unscoped their connected networks. If we can arrange so that these unscoped
destination routes get used for forwarding this traffic, then we will destination routes get used for forwarding this traffic, then we will
have accomplished the goal of keeping forwarding of traffic destined have accomplished the goal of keeping forwarding of traffic destined
for internal hosts, unaffected by the multihoming solution. for internal hosts, unaffected by the multihoming solution.
For traffic destined for external hosts, it is reasonable to expect For traffic destined for external hosts, it is reasonable to expect
that traffic with a source address from the prefix assigned by ISP-A that traffic with a source address from the prefix assigned by ISP-A
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Implementing the same routing policy is more difficult with the PA Implementing the same routing policy is more difficult with the PA
multihoming solution described in this document since it doesn't use multihoming solution described in this document since it doesn't use
NAT. By design, the only way to control where a packet exits this NAT. By design, the only way to control where a packet exits this
network is by setting the source address of the packet. Since the network is by setting the source address of the packet. Since the
network cannot modify the source address without NAT, the host must network cannot modify the source address without NAT, the host must
set it. To implement this routing policy, each host needs to use the set it. To implement this routing policy, each host needs to use the
source address from the prefix assigned by ISP-A to send traffic source address from the prefix assigned by ISP-A to send traffic
destined for H101. Mechanisms have been proposed to allow hosts to destined for H101. Mechanisms have been proposed to allow hosts to
choose the source address for packets in a fine grained manner. We choose the source address for packets in a fine grained manner. We
will discuss these proposals in Section 5. However, interacting with will discuss these proposals in Section 6. However, interacting with
host operating systems in some manner to ensure a particular source host operating systems in some manner to ensure a particular source
address is chosen for a particular destination prefix is not what an address is chosen for a particular destination prefix is not what an
enterprise network administrator would expect to have to do to enterprise network administrator would expect to have to do to
implement this routing policy. implement this routing policy.
3.4. More complex ISP connectivity 4.4. More complex ISP connectivity
The previous sections considered two variations of a simple The previous sections considered two variations of a simple
multihoming scenario where the site is connected to two ISPs offering multihoming scenario where the site is connected to two ISPs offering
only Internet connectivity. It is likely that many actual enterprise only Internet connectivity. It is likely that many actual enterprise
multihoming scenarios will be similar to this simple example. multihoming scenarios will be similar to this simple example.
However, there are more complex multihoming scenarios that we would However, there are more complex multihoming scenarios that we would
like this solution to address as well. like this solution to address as well.
It is fairly common for an ISP to offer a service in addition to It is fairly common for an ISP to offer a service in addition to
Internet access over the same uplink. Two variations of this are Internet access over the same uplink. Two variations of this are
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administrator trying to configure, maintain, and trouble-shoot this administrator trying to configure, maintain, and trouble-shoot this
multihoming solution, it seems much clearer to have SERb2 originate multihoming solution, it seems much clearer to have SERb2 originate
the source-prefix-scoped destination route correspond to the service the source-prefix-scoped destination route correspond to the service
offered by ISP-B. In this way, all of the traffic leaving the site offered by ISP-B. In this way, all of the traffic leaving the site
is determined by the source-prefix-scoped routes, and all of the is determined by the source-prefix-scoped routes, and all of the
traffic within the site or arriving from external hosts is determined traffic within the site or arriving from external hosts is determined
by the unscoped destination routes. Therefore, for this multihoming by the unscoped destination routes. Therefore, for this multihoming
solution we choose to originate source-prefix-scoped routes for all solution we choose to originate source-prefix-scoped routes for all
traffic leaving the site. traffic leaving the site.
3.5. ISPs and Provider-Assigned Prefixes 4.5. ISPs and Provider-Assigned Prefixes
While we expect that most site multihoming involves connecting to While we expect that most site multihoming involves connecting to
only two ISPs, this solution allows for connections to an arbitrary only two ISPs, this solution allows for connections to an arbitrary
number of ISPs to be supported. However, when evaluating scalable number of ISPs to be supported. However, when evaluating scalable
implementations of the solution, it would be reasonable to assume implementations of the solution, it would be reasonable to assume
that the maximum number of ISPs that a site would connect to is five. that the maximum number of ISPs that a site would connect to is five
(topologies with two redundant routers each having two uplinks to
different ISPs plus a tunnel to a headoffice acting as fifth one are
not unheard of).
It is also useful to note that the prefixes assigned to the site by It is also useful to note that the prefixes assigned to the site by
different ISPs will not overlap. This must be the case, since the different ISPs will not overlap. This must be the case, since the
provider-assigned addresses have to be globally unique. provider-assigned addresses have to be globally unique.
3.6. Simplified Topologies 4.6. Simplified Topologies
The topologies of many enterprise sites using this multihoming The topologies of many enterprise sites using this multihoming
solution may in practice be simpler than the examples that we have solution may in practice be simpler than the examples that we have
used. The topology in Figure 1 could be further simplified by having used. The topology in Figure 1 could be further simplified by having
all hosts directly connected to the LAN connecting the two site exit all hosts directly connected to the LAN connecting the two site exit
routers, SERa and SERb. The topology could also be simplified by routers, SERa and SERb. The topology could also be simplified by
having the uplinks to ISP-A and ISP-B both connected to the same site having the uplinks to ISP-A and ISP-B both connected to the same site
exit router. However, it is the aim of this draft to provide a exit router. However, it is the aim of this document to provide a
solution that applies to a broad a range of enterprise site network solution that applies to a broad a range of enterprise site network
topologies, so this draft focuses on providing a solution to the more topologies, so this document focuses on providing a solution to the
general case. The simplified cases will also be supported by this more general case. The simplified cases will also be supported by
solution, and there may even be optimizations that can be made for this solution, and there may even be optimizations that can be made
simplified cases. This solution however needs to support more for simplified cases. This solution however needs to support more
complex topologies. complex topologies.
We are starting with the basic assumption that enterprise site We are starting with the basic assumption that enterprise site
networks can be quite complex from a routing perspective. However, networks can be quite complex from a routing perspective. However,
even a complex site network can be multihomed to different ISPs with even a complex site network can be multihomed to different ISPs with
PA addresses using IPv4 and NAT. It is not reasonable to expect an PA addresses using IPv4 and NAT. It is not reasonable to expect an
enterprise network operator to change the routing topology of the enterprise network operator to change the routing topology of the
site in order to deploy IPv6. site in order to deploy IPv6.
4. Generating Source-Prefix-Scoped Forwarding Tables 5. Generating Source-Prefix-Scoped Forwarding Tables
So far we have described in general terms how the routers in this So far we have described in general terms how the routers in this
solution that are capable of Source Address Dependent Routing will solution that are capable of Source Address Dependent Routing will
forward traffic using both normal unscoped destination routes and forward traffic using both normal unscoped destination routes and
source-prefix-scoped destination routes. Here we give a precise source-prefix-scoped destination routes. Here we give a precise
method for generating a source-prefix-scoped forwarding table on a method for generating a source-prefix-scoped forwarding table on a
router that supports SADR. router that supports SADR.
1. Compute the next-hops for the source-prefix-scoped destination 1. Compute the next-hops for the source-prefix-scoped destination
prefixes using only routers in the connected SADR domain. These prefixes using only routers in the connected SADR domain. These
skipping to change at page 16, line 8 skipping to change at page 17, line 8
3. Augment each less specific source-prefix-scoped forwarding table 3. Augment each less specific source-prefix-scoped forwarding table
with all more specific source-prefix-scoped forwarding tables with all more specific source-prefix-scoped forwarding tables
entries based on the following rule. If the destination prefix entries based on the following rule. If the destination prefix
of the less specific source-prefix-scoped forwarding entry of the less specific source-prefix-scoped forwarding entry
exactly matches the destination prefix of an existing more exactly matches the destination prefix of an existing more
specific source-prefix-scoped forwarding entry (including specific source-prefix-scoped forwarding entry (including
destination prefix length), then do not add the less specific destination prefix length), then do not add the less specific
source-prefix-scoped forwarding entry. If the destination prefix source-prefix-scoped forwarding entry. If the destination prefix
does NOT match an existing entry, then add the entry to the more does NOT match an existing entry, then add the entry to the more
source-prefix-scoped forwarding table. As the unscoped specific source-prefix-scoped forwarding table. As the unscoped
forwarding table is considered to be scoped to ::/0 this process forwarding table is considered to be scoped to ::/0 this process
starts with propagating routes from the unscoped forwarding table starts with propagating routes from the unscoped forwarding table
to source-prefix-scoped forwarding tables and then continues with to source-prefix-scoped forwarding tables and then continues with
propagating routes to more-specific-source-prefix-scoped propagating routes to more-specific-source-prefix-scoped
forwarding tables should they exist. forwarding tables should they exist.
The forward tables produced by this process are used in the following The forwarding tables produced by this process are used in the
way to forward packets. following way to forward packets.
1. Select the most specific (longest prefix match) source-prefix- 1. Select the most specific (longest prefix match) source-prefix-
scoped forwarding table entries that matches the source address scoped forwarding table that matches the source address of the
of the packet (again, the unscoped forwarding table is considered packet (again, the unscoped forwarding table is considered to be
to be scoped to ::/0). scoped to ::/0).
2. Look up the destination address of the packet in the selected 2. Look up the destination address of the packet in the selected
forwarding table to determine the next-hop for the packet. forwarding table to determine the next-hop for the packet.
The following example illustrates how this process is used to create The following example illustrates how this process is used to create
a forwarding table for each provider-assigned source prefix. We a forwarding table for each provider-assigned source prefix. We
consider the multihomed site network in Figure 3. Initially we consider the multihomed site network in Figure 3. Initially we
assume that all of the routers in the site network support SADR. assume that all of the routers in the site network support SADR.
Figure 4 shows the routes that are originated by the routers in the Figure 4 shows the routes that are originated by the routers in the
site network. site network.
skipping to change at page 18, line 41 skipping to change at page 19, line 41
forwarding entries. As unscoped forwarding table is considered being forwarding entries. As unscoped forwarding table is considered being
scoped to ::/0 and both 2001:db8:0:a000::/52 and 2001:db8:0:b000::/52 scoped to ::/0 and both 2001:db8:0:a000::/52 and 2001:db8:0:b000::/52
are more specific prefixes of ::/0, the unscoped (scoped to ::/0) are more specific prefixes of ::/0, the unscoped (scoped to ::/0)
forwarding table needs to be augmented with both more specific forwarding table needs to be augmented with both more specific
source-prefix-scoped tables. If a less specific scoped forwarding source-prefix-scoped tables. If a less specific scoped forwarding
entry has the exact same destination prefix as a more specific entry has the exact same destination prefix as a more specific
source-prefix-scoped forwarding entry (including destination prefix source-prefix-scoped forwarding entry (including destination prefix
length), then the more specific source-prefix-scoped forwarding entry length), then the more specific source-prefix-scoped forwarding entry
wins. wins.
As as an example of how the source scoped forwarding entries are As an example of how the source scoped forwarding entries are
augmented, we consider how the two entries in the first table in 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 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 augmented with entries from the third table in Figure 5 (the table of
unscoped or scoped to ::/0 forwarding entries). The first four unscoped or scoped to ::/0 forwarding entries). The first four
unscoped forwarding entries (D=2001:db8:0:a010::/64, 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:b010::/64, D=2001:db8:0:a020::/64, and
D=2001:db8:0:b020::/64) are not an exact match for any of the D=2001:db8:0:b020::/64) are not an exact match for any of the
existing entries in the forwarding table for source prefix existing entries in the forwarding table for source prefix
2001:db8:0:a000::/52. Therefore, these four entries are added to the 2001:db8:0:a000::/52. Therefore, these four entries are added to the
final forwarding table for source prefix 2001:db8:0:a000::/52. The final forwarding table for source prefix 2001:db8:0:a000::/52. The
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D=2001:db8:0:b010::/64 NH=R8 D=2001:db8:0:b010::/64 NH=R8
D=2001:db8:0:a020::/64 NH=R3 D=2001:db8:0:a020::/64 NH=R3
D=2001:db8:0:b020::/64 NH=R3 D=2001:db8:0:b020::/64 NH=R3
D=2001:db8:0:5555::/64 NH=R8 D=2001:db8:0:5555::/64 NH=R8
D=2001:db8:0:6666::/64 NH=SERb2 D=2001:db8:0:6666::/64 NH=SERb2
D=::/0 NH=R8 D=::/0 NH=R8
Figure 8: Forwarding Table For R5, Which Doesn't Understand Source- Figure 8: Forwarding Table For R5, Which Doesn't Understand Source-
Prefix-Scoped Routes Prefix-Scoped Routes
Any traffic that needs to exit the site will eventually hit a SADR- As all SERs belong to the SADR domain any traffic that needs to exit
capable router. Once that traffic enters the SADR-capable domain, the site will eventually hit a SADR-capable router. To prevent
then it will not leave that domain until it exits the site. This routing loops involving SADR-capable and non-SADR-capable routers,
property is required in order to guarantee that there will not be traffic that enters the SADR-capable domain does not leave the domain
routing loops involving SADR-capable and non-SADR-capable routers. until it exits the site. Therefore all SADR-capable routers with the
domain MUST be logically connected.
Note that the mechanism described here for converting source-prefix- Note that the mechanism described here for converting source-prefix-
scoped destination prefix routing advertisements into forwarding scoped destination prefix routing advertisements into forwarding
state is somewhat different from that proposed in state is somewhat different from that proposed in
[I-D.ietf-rtgwg-dst-src-routing]. The method described in the [I-D.ietf-rtgwg-dst-src-routing]. The method described in the
current document is functionally equivalent, but it is intended to be current document is functionally equivalent, but it is based on
easier to understand for enterprise network operators. application of existing mechanisms for the described scenarios.
An interesting side-effect of deploying SADR is if all routers in a
given network support SADR and have a scoped forwarding table, then
the unscoped forwarding table can be eliminated which ensures that
packets with legitimate source addresses only can leave the network
(as there are no scoped forwarding tables for spoofed/bogon source
addresses). It would prevent accidental leaks of ULA/reserved/link-
local sources to the Internet as well as ensures that no spoofing is
possible from the SADR-enabled network.
5. Mechanisms For Hosts To Choose Good Source Addresses In A Multihomed 6. Mechanisms For Hosts To Choose Good Source Addresses In A Multihomed
Site Site
Until this point, we have made the assumption that hosts are able to Until this point, we have made the assumption that hosts are able to
choose the correct source address using some unspecified mechanism. choose the correct source address using some unspecified mechanism.
This has allowed us to just focus on what the routers in a multihomed 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 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 ISP based on source address. Now we look at possible mechanisms for
hosts to choose the correct source address. We also look at what hosts to choose the correct source address. We also look at what
role, if any, the routers may play in providing information that role, if any, the routers may play in providing information that
helps hosts to choose source addresses. helps hosts to choose source addresses.
It should be noted that this section discussed how hosts could select
the 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 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 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 prefix assigned by that ISP. The host will control which ISP is used
for its traffic by selecting one of the addresses configured on the for its traffic by selecting one of the addresses configured on the
host as the source address for outgoing traffic. It is the host as the source address for outgoing traffic. It is the
responsibility of the site network to ensure that a packet with 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. 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 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 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 uplinks, or it may be driven by the desire to take advantage of
certain properties of a particular uplink or ISP. If any of the ISP certain properties of a particular uplink or ISP (if some information
uplinks is not working, then the choice of source address by the host about various path properties has been made availabe to the host
can determine if packets get dropped. 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 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 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 solve this problem in this document. Instead we discuss the current
state of affairs with respect to standardized solutions and state of affairs with respect to standardized solutions and
implementation of those solutions. We also look at proposed implementation of those solutions. We also look at proposed
solutions for this problem. solutions for this problem.
An external host initiating communication with a host internal to a An external host initiating communication with a host internal to a
PA multihomed site will need to know multiple addresses for that host PA multihomed site will need to know multiple addresses for that host
in order to communicate with it using different ISPs to the in order to communicate with it using different ISPs to the
multihomed site. These addresses are typically learned through DNS. multihomed site (knowing just one address would undermine all
(For simplicity, we assume that the external host is single-homed.) benefits of redundant connectivity provided by multihoming). These
The external host chooses the ISP that will be used at the remote addresses are typically learned through DNS. (For simplicity, we
multihomed site by setting the destination address on the packets it assume that the external host is single-homed.) The external host
transmits. For a session originated from an external host to an chooses the ISP that will be used at the remote multihomed site by
internal host, the choice of source address used by the internal host setting the destination address on the packets it transmits. For a
is simple. The internal host has no choice but to use the session originated from an external host to an internal host, the
destination address in the received packet as the source address of choice of source address used by the internal host is simple. The
the transmitted packet. 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 internal to the multi-homed site, For a session originated by a host inside the multi-homed site, the
the decision of what source address to select is more complicated. decision of what source address to select is more complicated. We
We consider three main methods for hosts to get information about the consider three main methods for hosts to get information about the
network. The two proactive methods are Neighbor Discovery Router network. The two proactive methods are Neighbor Discovery Router
Advertisements and DHCPv6. The one reactive method we consider is Advertisements and DHCPv6. The one reactive method we consider is
ICMPv6. Note that we are explicitly excluding the possibility of ICMPv6. Note that we are explicitly excluding the possibility of
having hosts participate in or even listen directly to routing having hosts participate in or even listen directly to routing
protocol advertisements. protocol advertisements.
First we look at how a host is currently expected to select the First we look at how a host is currently expected to select the
source and destination address with which it sends a packet. source and destination address with which it sends a packet for a new
connection.
5.1. Source Address Selection Algorithm on Hosts 6.1. Source Address Selection Algorithm on Hosts
[RFC6724] defines the algorithms that hosts are expected to use to [RFC6724] defines the algorithms that hosts are expected to use to
select source and destination addresses for packets. It defines an select source and destination addresses for packets. It defines an
algorithm for selecting a source address and a separate algorithm for algorithm for selecting a source address and a separate algorithm for
selecting a destination address. Both of these algorithms depend on selecting a destination address. Both of these algorithms depend on
a policy table. [RFC6724] defines a default policy which produces a policy table. [RFC6724] defines a default policy which produces
certain behavior. certain behavior.
The rules in the two algorithms in [RFC6724] depend on many different The rules in the two algorithms in [RFC6724] depend on many different
properties of addresses. While these are needed for understanding properties of addresses. While these are needed for understanding
skipping to change at page 24, line 29 skipping to change at page 25, line 29
sending a packet to a remote host. Returning to the example in sending a packet to a remote host. Returning to the example in
Figure 3, we look at what the default algorithms in [RFC6724] say Figure 3, we look at what the default algorithms in [RFC6724] say
about the source address that internal host H31 should use to send about the source address that internal host H31 should use to send
traffic to external host H101, somewhere on the Internet. traffic to external host H101, somewhere on the Internet.
There is no choice to be made with respect to destination address. 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 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 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 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 for this packet. We go through the rules for source address
selection in Section 5 of [RFC6724]. Rule 1 (Prefer same address) is selection in Section 5 of [RFC6724].
not useful to break the tie between source addresses, because neither
the candidate source addresses equals the destination address. Rule Rule 1 (Prefer same address) is not useful to break the tie between
2 (Prefer appropriate scope) is also not used in this scenario, 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 because both source addresses and the destination address have global
scope. scope.
Rule 3 (Avoid deprecated addresses) applies to an address that has Rule 3 (Avoid deprecated addresses) applies to an address that has
been autoconfigured by a host using stateless address been autoconfigured by a host using stateless address
autoconfiguration as defined in [RFC4862]. An address autoconfigured autoconfiguration as defined in [RFC4862]. An address autoconfigured
by a host has a preferred lifetime and a valid lifetime. The address by a host has a preferred lifetime and a valid lifetime. The address
is preferred until the preferred lifetime expires, after which it is preferred until the preferred lifetime expires, after which it
becomes deprecated. A deprecated address is not used if there is a becomes deprecated. A deprecated address is not used if there is a
preferred address of the appropriate scope available. When the valid preferred address of the appropriate scope available. When the valid
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We'll discuss later how Rule 5.5 can be used to influence a source 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 address selection in single-router topologies (e.g. when H41 is
sending traffic using R3 as a default gateway). sending traffic using R3 as a default gateway).
Rule 6 (Prefer matching label) refers to the Label value determined Rule 6 (Prefer matching label) refers to the Label value determined
for each source and destination prefix as a result of applying the for each source and destination prefix as a result of applying the
policy table to the prefix. With the default policy table defined in policy table to the prefix. With the default policy table defined in
Section 2.1 of [RFC6724], Label(2001:db8:0:a010::31) = 5, 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. 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, So with the default policy, Rule 6 does not break the tie. However,
the algorithms in [RFC6724] are defined in such as way that non- the algorithms in [RFC6724] are defined in such a way that non-
default address selection policy tables can be used. [RFC7078] default address selection policy tables can be used. [RFC7078]
defines a way to distribute a non-default address selection policy defines a way to distribute a non-default address selection policy
table to hosts using DHCPv6. So even though the application of rule 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 to this scenario using the default policy table is not useful, rule
6 may still be a useful tool. 6 may still be a useful tool.
Rule 7 (Prefer temporary addresses) has to do with the technique Rule 7 (Prefer temporary addresses) has to do with the technique
described in [RFC4941] to periodically randomize the interface described in [RFC4941] to periodically randomize the interface
portion of an IPv6 address that has been generated using stateless portion of an IPv6 address that has been generated using stateless
address autoconfiguration. In general, if H31 were using this address autoconfiguration. In general, if H31 were using this
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Rule 8 (Use longest matching prefix) dictates that between two Rule 8 (Use longest matching prefix) dictates that between two
candidate source addresses the one which has longest common prefix candidate source addresses the one which has longest common prefix
length with the destination address. For example, if H31 were length with the destination address. For example, if H31 were
selecting the source address for sending packets to H101, this rule selecting the source address for sending packets to H101, this rule
would not be a tie breaker as for both candidate source addresses 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 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 with the destination is 48. However if H31 were selecting the source
address for sending packets H41 address 2001:db8:0:a020::41, then 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 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: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:a000::/58, while for 2001:db8:0:b101::31 and
2001:db8:0:a020::41 the common prefix is 2001:db8:0:a000::/51). 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 Therefore rule 8 might be useful for selecting the correct source
address in some but not all scenarios (for example if ISP-B services 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 belong to 2001:db8:0:b000::/59 then H31 would always use
2001:db8:0:b010::31 to access those destinations). 2001:db8:0:b010::31 to access those destinations).
So we can see that of the 8 source selection address rules from So we can see that of the 8 source selection address rules from
[RFC6724], four actually apply to our basic site multihoming [RFC6724], four actually apply to our basic site multihoming
scenario. The rules that are relevant to this scenario are scenario. The rules that are relevant to this scenario are
summarized below. summarized below.
skipping to change at page 27, line 5 skipping to change at page 28, line 5
another strategy is to choose a source address, send the packet, get another strategy is to choose a source address, send the packet, get
feedback from the network about whether or not the source address is feedback from the network about whether or not the source address is
correct, and try another source address if it is not. correct, and try another source address if it is not.
We consider four scenarios where a host needs to select the correct We consider four scenarios where a host needs to select the correct
source address. The first is when both uplinks are working. The source address. The first is when both uplinks are working. The
second is when one uplink has failed. The third one is a situation 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 when one failed uplink has recovered. The last one is failure of
both (all) uplinks. both (all) uplinks.
5.2. Selecting Source Address When Both Uplinks Are Working It should be noted that [RFC6724] defines the default behaviour for
IPv6 hosts. The applications and uppler-layer protocols can make
their own choices on selecting source addresses. However 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.
6.2. Selecting Source Address When Both Uplinks Are Working
Again we return to the topology in Figure 3. Suppose that the site 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 administrator wants to implement a policy by which all hosts need to
use ISP-A to reach H01 at D=2001:db8:0:1234::101. So for example, 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. H31 needs to select S=2001:db8:0:a010::31.
5.2.1. Distributing Address Selection Policy Table with DHCPv6 6.2.1. Distributing Address Selection Policy Table with DHCPv6
This policy can be implemented by using DHCPv6 to distribute an This policy can be implemented by using DHCPv6 to distribute an
address selection policy table that assigns the same label to address selection policy table that assigns the same label to
destination address that match 2001:db8:0:1234::/64 as it does 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 source addresses that match 2001:db8:0:a000::/52. The following two
entries accomplish this. entries accomplish this.
Prefix Precedence Label Prefix Precedence Label
2001:db8:0:1234::/64 50 33 2001:db8:0:1234::/64 50 33
2001:db8:0:a000::/52 50 33 2001:db8:0:a000::/52 50 33
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This requires that the hosts implement [RFC6724], the basic source This requires that the hosts implement [RFC6724], the basic source
and destination address framework, along with [RFC7078], the DHCPv6 and destination address framework, along with [RFC7078], the DHCPv6
extension for distributing a non-default policy table. Note that it extension for distributing a non-default policy table. Note that it
does NOT require that the hosts use DHCPv6 for address assignment. does NOT require that the hosts use DHCPv6 for address assignment.
The hosts could still use stateless address autoconfiguration for The hosts could still use stateless address autoconfiguration for
address configuration, while using DHCPv6 only for policy table address configuration, while using DHCPv6 only for policy table
distribution (see [RFC8415]). However this method has a number of distribution (see [RFC8415]). However this method has a number of
disadvantages: disadvantages:
o DHCPv6 support is not a mandatory requirement for IPv6 hosts, so o DHCPv6 support is not a mandatory requirement for IPv6 hosts
this method might not work for all devices. ([RFC6434]), so this method might not work for all devices.
o Network administrators are required to explicitly configure the o Network administrators are required to explicitly configure the
desired network access policies on DHCPv6 servers. While it might desired network access policies on DHCPv6 servers. While it might
be feasible in the scenario of a single multihomed network, such be feasible in the scenario of a single multihomed network, such
approach might have some scalability issues, especially if the approach might have some scalability issues, especially if the
centralized DHCPv6 solution is deployed to serve a large number of centralized DHCPv6 solution is deployed to serve a large number of
multiomed sites. multiomed sites.
5.2.2. Controlling Source Address Selection With Router Advertisements 6.2.2. Controlling Source Address Selection With Router Advertisements
Neighbor Discovery currently has two mechanisms to communicate prefix Neighbor Discovery currently has two mechanisms to communicate prefix
information to hosts. The base specification for Neighbor Discovery information to hosts. The base specification for Neighbor Discovery
(see [RFC4861]) defines the Prefix Information Option (PIO) in the (see [RFC4861]) defines the Prefix Information Option (PIO) in the
Router Advertisement (RA) message. When a host receives a PIO with 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 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 from which it assigns itself an IP address using stateless address
autoconfiguration (SLAAC) procedures described in [RFC4862]. In the autoconfiguration (SLAAC) procedures described in [RFC4862]. In the
example of Figure 3, if the site network is using SLAAC, we would 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 expect both R1 and R2 to send RA messages with PIOs for both source
skipping to change at page 28, line 37 skipping to change at page 29, line 46
Information option for Neighbor Discovery Router Advertisements which Information option for Neighbor Discovery Router Advertisements which
would associate a source prefix and with a destination prefix. The would associate a source prefix and with a destination prefix. The
details of [I-D.pfister-6man-sadr-ra] might need tweaking to address 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 this use case. However, in order to be able to use Neighbor
Discovery Router Advertisements to implement this routing policy, an Discovery Router Advertisements to implement this routing policy, an
extension that allows R1 and R2 to explicitly communicate to H31 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 association between S=2001:db8:0:a000::/52 D=2001:db8:0:1234::/64
would be needed. would be needed.
However, Rule 5.5 of the source address selection algorithm However, Rule 5.5 of the source address selection algorithm
(discussed in Section 5.1 above), together with default router (discussed in Section 6.1 above), together with default router
preference (specified in [RFC4191]) and RIO can be used to influence preference (specified in [RFC4191]) and RIO can be used to influence
a source address selection on a host as described below. Let's look 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 at source address selection on the host H41. It receives RAs from R3
with PIOs for 2001:db8:0:a020::/64 and 2001:db8:0:b020::/64. At that 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) 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 so Rule 5.5 does not apply. Now let's assume that R3 supports SADR
and has two scoped forwarding tables, one scoped to 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. 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 If R3 generates two different link-local addresses for its interface
facing H41 (one for each scoped forwarding table, LLA_A and LLA_B) facing H41 (one for each scoped forwarding table, LLA_A and LLA_B)
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The description of the mechanism above assumes SADR support by the The description of the mechanism above assumes SADR support by the
first-hop routers as well as SERs. However, a first-hop router can first-hop routers as well as SERs. However, a first-hop router can
still provide a less flexible version of this mechanism even without still provide a less flexible version of this mechanism even without
implementing SADR. This could be done by providing configuration implementing SADR. This could be done by providing configuration
knobs on the first-hop router that allow it to generate different knobs on the first-hop router that allow it to generate different
link-local addresses and to send individual RAs for each prefix. link-local addresses and to send individual RAs for each prefix.
The mechanism described above relies on Rule 5.5 of the default The mechanism described above relies on Rule 5.5 of the default
source address selection algorithm defined in [RFC6724]. [RFC8028] source address selection algorithm defined in [RFC6724]. [RFC8028]
recommends that a host SHOULD select default routers for each prefix states that "A host SHOULD select default routers for each prefix it
in which it is assigned an address. It also recommends that hosts is assigned an address in". It also recommends that hosts should
SHOULD implement Rule 5.5. of [RFC6724]. Hosts following the implement Rule 5.5. of [RFC6724]. Hosts following the
recommendations specified in [RFC8028] therefore should be able to recommendations specified in [RFC8028] therefore should be able to
benefit from the solution described in this document. No standards benefit from the solution described in this document. No standards
need to be updated in regards to host behavior. need to be updated in regards to host behavior.
5.2.3. Controlling Source Address Selection With ICMPv6 6.2.3. Controlling Source Address Selection With ICMPv6
We now discuss how one might use ICMPv6 to implement the routing 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, 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 even when uplinks to both ISPs are working. If H31 started sending
traffic to H101 with S=2001:db8:0:b010::31 and 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 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 is traffic is not uplink to ISP-B. SERb1 could recognize that this traffic is not
following the desired routing policy and react by sending an ICMPv6 following the desired routing policy and react by sending an ICMPv6
message back to H31. message back to H31.
In this example, we could arrange things so that SERb1 drops the 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 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 then sends to H31 an ICMPv6 Destination Unreachable message with Code
5 (Source address failed ingress/egress policy). When H31 receives 5 (Source address failed ingress/egress policy). When H31 receives
this packet, it would then be expected to try another source address this packet, it would then be expected to try another source address
to reach the destination. In this example, H31 would then send a 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 packet with S=2001:db8:0:a010::31 and D=2001:db8:0:1234::101, which
skipping to change at page 30, line 46 skipping to change at page 32, line 8
that meet the policy. In its current form, when SERb1 sends an that meet the policy. In its current form, when SERb1 sends an
ICMPv6 Destination Unreachable Code 5 message, it is basically ICMPv6 Destination Unreachable Code 5 message, it is basically
saying, "This source address is wrong. Try another source address." saying, "This source address is wrong. Try another source address."
In the absence of a clear indication which address to try next, the In the absence of a clear indication which address to try next, the
host will iterate over all addresses assigned to the interface (e.g. host will iterate over all addresses assigned to the interface (e.g.
various privacy addresses) which would lead to significant delays and various privacy addresses) which would lead to significant delays and
degraded user experience. It would be better is if the ICMPv6 degraded user experience. It would be better is if the ICMPv6
message could say, "This source address is wrong. Instead use a message could say, "This source address is wrong. Instead use a
source address in S=2001:db8:0:a000::/52.". source address in S=2001:db8:0:a000::/52.".
However using ICMPv6 for signalling source address information back However using ICMPv6 for signaling source address information back to
to hosts introduces new challenges. Most routers currently have hosts introduces new challenges. Most routers currently have
software or hardware limits on generating ICMP messages. An site software or hardware limits on generating ICMP messages. A site
administrator deploying a solution that relies on the SERs generating administrator deploying a solution that relies on the SERs generating
ICMP messages could try to improve the performance of SERs for ICMP messages could try to improve the performance of SERs for
generating ICMP messages. However, in a large network, it is still generating ICMP messages. However, in a large network, it is still
likely that ICMP message generation limits will be reached. As a likely that ICMP message generation limits will be reached. As a
result hosts would not receive ICMPv6 back which in turn leads to result hosts would not receive ICMPv6 back which in turn leads to
traffic blackholing and poor user experience. To improve the traffic blackholing and poor user experience. To improve the
scalability of ICMPv6-based signalling hosts SHOULD cache the scalability of ICMPv6-based signaling hosts SHOULD cache the
preferred source address (or prefix) for the given destination (which preferred source address (or prefix) for the given destination (which
in turn might cause issues in case of the corresponding ISP uplinks in turn might cause issues in case of the corresponding ISP uplinks
failure - see Section 5.3). In addition, the same source prefix failure - see Section 6.3). In addition, the same source prefix
SHOULD be used for other destinations in the same /64 as the original SHOULD be used for other destinations in the same /64 as the original
destination address. The source prefix SHOULD have a specific destination address. The source prefix to the destination mapping
lifetime. Expiration of the lifetime SHOULD trigger the source SHOULD have a specific lifetime. Expiration of the lifetime SHOULD
address selection algorithm again. trigger the source address selection algorithm again.
Using ICMPv6 Destination Unreachable Messages with Code 5 to Using ICMPv6 Destination Unreachable Messages with Code 5 to
influence source address selection allows an attacker to exhaust the influence source address selection allows an attacker to exhaust the
list of candidate source addresses on the host by sending spoofed list of candidate source addresses on the host by sending spoofed
ICMPv6 Code 5 for all prefixes known on the network (therefore ICMPv6 Code 5 for all prefixes known on the network (therefore
preventing a victim from establishing a communication with the preventing a victim from establishing a communication with the
destination host). To protect from an attack of this kind, hosts destination host). To protect from an attack of this kind, hosts
SHOULD verify that the original packet header included into ICMPv6 SHOULD verify that the original packet header included into ICMPv6
error message was actually sent by the host. error message was actually sent by the host.
As currently standardized in [RFC4443], the ICMPv6 Destination As currently standardized in [RFC4443], the ICMPv6 Destination
Unreachable Message with Code 5 would allow for the iterative Unreachable Message with Code 5 would allow for the iterative
approach to retransmitting packets using different source addresses. approach to retransmitting packets using different source addresses.
As currently defined, the ICMPv6 message does not provide a mechanism As currently defined, the ICMPv6 message does not provide a mechanism
to communication information about which source prefix should be used to communication information about which source prefix should be used
for a retransmitted packet. The current document does not define for a retransmitted packet. The current document does not define
such a mechanism. However, we note that this might be a useful such a mechanism but it might be a useful extension to define in a
extension to define in a different document. 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.
5.2.4. Summary of Methods For Controlling Source Address Selection To 6.2.4. Summary of Methods For Controlling Source Address Selection To
Implement Routing Policy Implement Routing Policy
So to summarize this section, we have looked at three methods for So to summarize this section, we have looked at three methods for
implementing a simple routing policy where all traffic for a given implementing a simple routing policy where all traffic for a given
destination on the Internet needs to use a particular ISP, even when destination on the Internet needs to use a particular ISP, even when
the uplinks to both ISPs are working. the uplinks to both ISPs are working.
The default source address selection policy cannot distinguish The default source address selection policy cannot distinguish
between the source addresses needed to enforce this policy, so a non- between the source addresses needed to enforce this policy, so a non-
default policy table using associating source and destination default policy table using associating source and destination
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A mechanism exists for DHCPv6 to distribute a non-default policy A mechanism exists for DHCPv6 to distribute a non-default policy
table but such solution would heavily rely on DHCPv6 support by host table but such solution would heavily rely on DHCPv6 support by host
operating system. Moreover there is no mechanism to translate operating system. Moreover there is no mechanism to translate
desired routing/traffic engineering policies into policy tables on desired routing/traffic engineering policies into policy tables on
DHCPv6 servers. Therefore using DHCPv6 for controlling address DHCPv6 servers. Therefore using DHCPv6 for controlling address
selection policy table is not recommended and SHOULD NOT be used. selection policy table is not recommended and SHOULD NOT be used.
At the same time Router Advertisements provide a reliable mechanism At the same time Router Advertisements provide a reliable mechanism
to influence source address selection process via PIO, RIO and to influence source address selection process via PIO, RIO and
default router preferences. As all those options have been default router preferences. As all those options have been
standardized by IETF and are supported by various operating systems, standardized by IETF and are supported by various operating systems
no changes are required on hosts. First-hop routers in the no changes are required on hosts. First-hop routers in the
enterprise network need to be able of sending different RAs for enterprise network need to be able of sending different RAs for
different SLAAC prefixes (either based on scoped forwarding tables or different SLAAC prefixes (either based on scoped forwarding tables or
based on pre-configured policies). based on pre-configured policies).
SERs can enforce the routing policy by sending ICMPv6 Destination SERs can enforce the routing policy by sending ICMPv6 Destination
Unreachable messages with Code 5 (Source address failed ingress/ Unreachable messages with Code 5 (Source address failed ingress/
egress policy) for traffic that is being sent with the wrong source egress policy) for traffic that is being sent with the wrong source
address. The policy distribution can be automated by defining an address. The policy distribution can be automated by defining an
EXCLUSIVE flag for the source-prefix-scoped route which can be set on EXCLUSIVE flag for the source-prefix-scoped route which can be set on
the SER that originates the route. As ICMPv6 message generation can the SER that originates the route. As ICMPv6 message generation can
be rate-limited on routers, it SHOULD NOT be used as the only be rate-limited on routers, it SHOULD NOT be used as the only
mechanism to influence source address selection on hosts. While mechanism to influence source address selection on hosts. While
hosts SHOULD select the correct source address for a given hosts SHOULD select the correct source address for a given
destination the network SHOULD signal any source address issues back destination the network SHOULD signal any source address issues back
to hosts using ICMPv6 error messages. to hosts using ICMPv6 error messages.
5.3. Selecting Source Address When One Uplink Has Failed 6.3. Selecting Source Address When One Uplink Has Failed
Now we discuss if DHCPv6, Neighbor Discovery Router Advertisements, Now we discuss if DHCPv6, Neighbor Discovery Router Advertisements,
and ICMPv6 can help a host choose the right source address when an 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 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 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 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 that the uplink from SERa to ISP-A is working and that the uplink
from SERb1 to ISP-B is working. from SERb1 to ISP-B is working.
We assume there is no particular routing policy desired, so H31 is We assume there is no particular routing policy desired, so H31 is
free to send packets with S=2001:db8:0:a010::31 or 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 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 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 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 link from SERb1 to ISP-B fails. How should H31 learn that it needs
to start sending the packet to H501 with S=2001:db8:0:a010::31 in 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 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 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. S=2001:db8:0:b010::31 in order to reach H61 at D=2001:db8:0:6666::61.
5.3.1. Controlling Source Address Selection With DHCPv6 6.3.1. Controlling Source Address Selection With DHCPv6
For this example we assume that the site network in Figure 3 has a 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 centralized DHCP server and all routers act as DHCP relay agents. We
assume that both of the addresses assigned to H31 were assigned via assume that both of the addresses assigned to H31 were assigned via
DHCP. DHCP.
We could try to have the DHCP server monitor the state of the uplink 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 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 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 zero. The DHCP server could initiate this process by sending a
Reconfigure Message to H31 as described in Section 18.3 of [RFC8415]. Reconfigure Message to H31 as described in Section 18.3 of [RFC8415].
Or the DHCP server can assign addresses with short lifetimes in order Or the DHCP server can assign addresses with short lifetimes in order
to force clients to renew them often. to force clients to renew them often.
This approach would prevent H31 from using S=2001:db8:0:b010::31 to This approach would prevent H31 from using S=2001:db8:0:b010::31 to
reach the a host on the Internet. However, it would also prevent H31 reach a host on the Internet. However, it would also prevent H31
from using S=2001:db8:0:b010::31 to reach H61 at from using S=2001:db8:0:b010::31 to reach H61 at
D=2001:db8:0:6666::61, which is not desirable. D=2001:db8:0:6666::61, which is not desirable.
Another potential approach is to have the DHCP server monitor the Another potential approach is to have the DHCP server monitor the
uplink from SERb1 to ISP-B and control the choice of source address uplink from SERb1 to ISP-B and control the choice of source address
on H31 by updating its address selection policy table via the on H31 by updating its address selection policy table via the
mechanism in [RFC7078]. The DHCP server could initiate this process mechanism in [RFC7078]. The DHCP server could initiate this process
by sending a Reconfigure Message to H31. Note that [RFC8415] by sending a Reconfigure Message to H31. Note that [RFC8415]
requires that Reconfigure Message use DHCP authentication. DHCP requires that Reconfigure Message use DHCP authentication. DHCP
authentication could be avoided by using short address lifetimes to authentication could be avoided by using short address lifetimes to
skipping to change at page 33, line 44 skipping to change at page 35, line 16
Prefix Precedence Label Prefix Precedence Label
::/0 50 44 ::/0 50 44
2001:db8:0:a000::/52 50 44 2001:db8:0:a000::/52 50 44
2001:db8:0:6666::/64 50 55 2001:db8:0:6666::/64 50 55
2001:db8:0:b000::/52 50 55 2001:db8:0:b000::/52 50 55
Figure 10: Policy Table Needed On Failure Of Uplink From SERb1 Figure 10: Policy Table Needed On Failure Of Uplink From SERb1
The described solution has a number of significant drawbacks, some of The described solution has a number of significant drawbacks, some of
them already discussed in Section 5.2.1. them already discussed in Section 6.2.1.
o DHCPv6 support is not required for an IPv6 host and there are o DHCPv6 support is not required for an IPv6 host and there are
operating systems which do not support DHCPv6. Besides that, it operating systems which do not support DHCPv6. Besides that, it
does not appear that [RFC7078] has been widely implemented on host does not appear that [RFC7078] has been widely implemented on host
operating systems. operating systems.
o [RFC7078] does not clearly specify this kind of a dynamic use case o [RFC7078] does not clearly specify this kind of a dynamic use case
where address selection policy needs to be updated quickly in where address selection policy needs to be updated quickly in
response to the failure of a link. In a large network it would response to the failure of a link. In a large network it would
present scalability issues as many hosts need to be reconfigured present scalability issues as many hosts need to be reconfigured
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for mid/large distributed scale enterprise networks. In addition for mid/large distributed scale enterprise networks. In addition
to that, the policy table needs to be manually configured by to that, the policy table needs to be manually configured by
administrators which makes that solution prone to human error. administrators which makes that solution prone to human error.
o No mechanism exists for making DHCPv6 servers aware of network o No mechanism exists for making DHCPv6 servers aware of network
topology/routing changes in the network. In general DHCPv6 topology/routing changes in the network. In general DHCPv6
servers monitoring network-related events sounds like a bad idea servers monitoring network-related events sounds like a bad idea
as completely new functionality beyond the scope of DHCPv6 role is as completely new functionality beyond the scope of DHCPv6 role is
required. required.
5.3.2. Controlling Source Address Selection With Router Advertisements 6.3.2. Controlling Source Address Selection With Router Advertisements
The same mechanism as discussed in Section 5.2.2 can be used to 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 control the source address selection in the case of an uplink
failure. If a particular prefix should not be used as a source for failure. If a particular prefix should not be used as a source for
any destinations, then the router needs to send RA with Preferred any destinations, then the router needs to send RA with Preferred
Lifetime field for that prefix set to 0. Lifetime field for that prefix set to 0.
Let's consider a scenario when all uplinks are operational and H41 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 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 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 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. router preference set to 01 (high) and RIO for 2001:db8:0:6666::/64.
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hop and 2001:db8:0:b020::41 as a source address. For all traffic 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 following the default route, LLA_A will be used as a next-hop and
2001:db8:0:a020::41 as a source address. 2001:db8:0:a020::41 as a source address.
If all uplinks to ISP-B have failed and therefore source addresses 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 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 table scoped S=2001:db8:0:b000::/52 contains no entries. Hosts can
be instructed to stop using source addresses from that block by be instructed to stop using source addresses from that block by
sending RAs containing PIO with Preferred Lifetime set to 0. sending RAs containing PIO with Preferred Lifetime set to 0.
5.3.3. Controlling Source Address Selection With ICMPv6 6.3.3. Controlling Source Address Selection With ICMPv6
Now we look at how ICMPv6 messages can provide information back to 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 H31. We assume again that at the time of the failure H31 is sending
packets to H501 using (S=2001:db8:0:b010::31, 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, 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 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 default destination (S=2001:db8:0:b000::/52, D=::/0) as well as its
unscoped default destination route. With these routes no longer in 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) 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 would end up at SERa based on the unscoped default destination route
skipping to change at page 35, line 30 skipping to change at page 36, line 50
address to be forwarded to ISP-A, SERa would drop it and send a address to be forwarded to ISP-A, SERa would drop it and send a
Destination Unreachable message with Code 5 (Source address failed Destination Unreachable message with Code 5 (Source address failed
ingress/egress policy) back to H31. H31 would then know to use ingress/egress policy) back to H31. H31 would then know to use
another source address for that destination and would try with 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 (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 forwarded to SERa based on the source-prefix-scoped default
destination route still being originated by SERa, and SERa would destination route still being originated by SERa, and SERa would
forward it to ISP-A. As discussed above, if we are willing to extend 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 ICMPv6, SERa can even tell H31 what source address it should use to
reach that destination. The expected host behaviour has been reach that destination. The expected host behaviour has been
discussed in Section 5.2.3. Potential issue with using ICMPv6 for discussed in Section 6.2.3. Using ICMPv6 would have the same
signalling source address issues back to hosts is that uplink to an scalability/rate limiting issues discussed in Section 6.2.3. ISP-B
ISP-B failure immediately invalidates source addresses from uplink failure immidiately makes source addresses from
2001:db8:0:b000::/52 for all hosts which triggers a large number of 2001:db8:0:b000::/52 unsuitable for external communication and might
ICMPv6 being sent back to hosts - the same scalability/rate limiting trigger a large number of ICMPv6 packets being sent to hosts in that
issues discussed in Section 5.2.3 would apply. subnet.
5.3.4. Summary Of Methods For Controlling Source Address Selection On 6.3.4. Summary Of Methods For Controlling Source Address Selection On
The Failure Of An Uplink The Failure Of An Uplink
It appears that DHCPv6 is not particularly well suited to quickly It appears that DHCPv6 is not particularly well suited to quickly
changing the source address used by a host in the event of the changing the source address used by a host in the event of the
failure of an uplink, which eliminates DHCPv6 from the list of failure of an uplink, which eliminates DHCPv6 from the list of
potential solutions. On the other hand Router Advertisements potential solutions. On the other hand Router Advertisements
provides a reliable mechanism to dynamically provide hosts with a provides a reliable mechanism to dynamically provide hosts with a
list of valid prefixes to use as source addresses as well as prevent list of valid prefixes to use as source addresses as well as prevent
particular prefixes to be used. While no additional new features are particular prefixes to be used. While no additional new features are
required to be implemented on hosts, routers need to be able to send required to be implemented on hosts, routers need to be able to send
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Therefore it is highly desirable that hosts are able to select the Therefore it is highly desirable that hosts are able to select the
correct source address in case of uplinks failure with ICMPv6 being correct source address in case of uplinks failure with ICMPv6 being
an additional mechanism to signal unexpected failures back to hosts. an additional mechanism to signal unexpected failures back to hosts.
The current behavior of different host operating system when The current behavior of different host operating system when
receiving ICMPv6 Destination Unreachable message with code 5 (Source receiving ICMPv6 Destination Unreachable message with code 5 (Source
address failed ingress/egress policy) is not clear to the authors. address failed ingress/egress policy) is not clear to the authors.
Information from implementers, users, and testing would be quite Information from implementers, users, and testing would be quite
helpful in evaluating this approach. helpful in evaluating this approach.
5.4. Selecting Source Address Upon Failed Uplink Recovery 6.4. Selecting Source Address Upon Failed Uplink Recovery
The next logical step is to look at the scenario when a failed uplink 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 on SERb1 to ISP-B is coming back up, so hosts can start using source
addresses belonging to 2001:db8:0:b000::/52 again. addresses belonging to 2001:db8:0:b000::/52 again.
5.4.1. Controlling Source Address Selection With DHCPv6 6.4.1. Controlling Source Address Selection With DHCPv6
The mechanism to use DHCPv6 to instruct the hosts (H31 in our The mechanism to use DHCPv6 to instruct the hosts (H31 in our
example) to start using prefixes from ISP-B space (e.g. 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 S=2001:db8:0:b010::31 for H31) to reach hosts on the Internet is
quite similar to one discussed in Section 5.3.1 and shares the same quite similar to one discussed in Section 6.3.1 and shares the same
drawbacks. drawbacks.
5.4.2. Controlling Source Address Selection With Router Advertisements 6.4.2. Controlling Source Address Selection With Router Advertisements
Let's look at the scenario discussed in Section 5.3.2. If the Let's look at the scenario discussed in Section 6.3.2. If the
uplink(s) failure caused the complete withdrawal of prefixes from uplink(s) failure caused the complete withdrawal of prefixes from
2001:db8:0:b000::/52 address space by setting Preferred Lifetime 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 value to 0, then the recovery of the link should just trigger new RA
being sent with non-zero Preferred Lifetime. In another scenario being sent with non-zero Preferred Lifetime. In another scenario
discussed in Section 5.3.2, the SERb1 uplink to ISP-B failure leads 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 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, 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 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 recovery of the SERb1 uplink to ISP-B leads to
(S=2001:db8:0:b000::/52, D=::/0) scoped forwarding entry re- (S=2001:db8:0:b000::/52, D=::/0) scoped forwarding entry re-
appearance and instructs R3 that it should advertise itself as a appearance and instructs R3 that it should advertise itself as a
default router for ISP-B address space domain (send RAs from LLA_B default router for ISP-B address space domain (send RAs from LLA_B
with non-zero Router Lifetime). with non-zero Router Lifetime).
5.4.3. Controlling Source Address Selection With ICMP 6.4.3. Controlling Source Address Selection With ICMP
It looks like ICMPv6 provides a rather limited functionality to It looks like ICMPv6 provides a rather limited functionality to
signal back to hosts that particular source addresses have become signal back to hosts that particular source addresses have become
valid again. Unless the changes in the uplink state a particular valid again. Unless the changes in the uplink state a particular
(S,D) pair, hosts can keep using the same source address even after (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 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 SERb1 to ISP-B had failed, H31 received ICMPv6 Code 5 message (as
described in Section 5.3.3) and allegedly started using 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 (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 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 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 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 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 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 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 of the network topology change and keep using S=2001:db8:0:a010::31
for Internet destinations, including H51. for Internet destinations, including H51.
One of the possible option may be using a scoped route with EXCLUSIVE One of the possible option may be using a scoped route with EXCLUSIVE
flag as described in Section 5.2.3. SERa1 uplink recovery would 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 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 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 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- source addresses from 2001:db8:0:b000::/52. When the route re-
appears SERb1 would reject those packets and sends ICMPv6 back as appears SERb1 would reject those packets and sends ICMPv6 back as
discussed in Section 5.2.3. Practically it might lead to scalability discussed in Section 6.2.3. Practically it might lead to scalability
issues which have been already discussed in Section 5.2.3 and issues which have been already discussed in Section 6.2.3 and
Section 5.4.3. Section 6.4.3.
5.4.4. Summary Of Methods For Controlling Source Address Selection Upon 6.4.4. Summary Of Methods For Controlling Source Address Selection Upon
Failed Uplink Recovery Failed Uplink Recovery
Once again DHCPv6 does not look like reasonable choice to manipulate Once again DHCPv6 does not look like reasonable choice to manipulate
source address selection process on a host in the case of network source address selection process on a host in the case of network
topology changes. Using Router Advertisement provides the flexible topology changes. Using Router Advertisement provides the flexible
mechanism to dynamically react to network topology changes (if mechanism to dynamically react to network topology changes (if
routers are able to use routing changes as a trigger for sending out routers are able to use routing changes as a trigger for sending out
RAs with specific parameters). ICMPv6 could be considered as a RAs with specific parameters). ICMPv6 could be considered as a
supporting mechanism to signal incorrect source address back to hosts supporting mechanism to signal incorrect source address back to hosts
but should not be considered as the only mechanism to control the but should not be considered as the only mechanism to control the
address selection in multihomed environments. address selection in multihomed environments.
5.5. Selecting Source Address When All Uplinks Failed 6.5. Selecting Source Address When All Uplinks Failed
One particular tricky case is a scenario when all uplinks have 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 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- any external destinations while it might be desirable to have intra-
site connectivity. site connectivity.
5.5.1. Controlling Source Address Selection With DHCPv6 6.5.1. Controlling Source Address Selection With DHCPv6
From DHCPv6 perspective uplinks failure should be treated as two From DHCPv6 perspective uplinks failure should be treated as two
independent failures and processed as described in Section 5.3.1. At independent failures and processed as described in Section 6.3.1. At
this stage it is quite obvious that it would result in quite this stage it is quite obvious that it would result in quite
complicated policy table which needs to be explicitly configured by complicated policy table which needs to be explicitly configured by
administrators and therefore seems to be impractical. administrators and therefore seems to be impractical.
5.5.2. Controlling Source Address Selection With Router Advertisements 6.5.2. Controlling Source Address Selection With Router Advertisements
As discussed in Section 5.3.2 an uplink failure causes the scoped As discussed in Section 6.3.2 an uplink failure causes the scoped
default entry to disappear from the scoped forwarding table and default entry to disappear from the scoped forwarding table and
triggers RAs with zero Router Lifetime. Complete disappearance of triggers RAs with zero Router Lifetime. Complete disappearance of
all scoped entries for a given source prefix would cause the prefix all scoped entries for a given source prefix would cause the prefix
being withdrawn from hosts by setting Preferred Lifetime value to being withdrawn from hosts by setting Preferred Lifetime value to
zero in PIO. If all uplinks (SERa, SERb1 and SERb2) failed, hosts zero in PIO. If all uplinks (SERa, SERb1 and SERb2) failed, hosts
either lost their default routers and/or have no global IPv6 either lost their default routers and/or have no global IPv6
addresses to use as a source. (Note that 'uplink failure' might mean addresses to use as a source. (Note that 'uplink failure' might mean
'IPv6 connectivity failure with IPv4 still being reachable', in which 'IPv6 connectivity failure with IPv4 still being reachable', in which
case hosts might fall back to IPv4 if there is IPv4 connectivity to case hosts might fall back to IPv4 if there is IPv4 connectivity to
destinations). As a result, intra-site connectivity is broken. One destinations). As a result, intra-site connectivity is broken. One
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table. In the absence of (S=ULA_Prefix; D=::/0) first-hop routers 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 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 PIO from ULA address space, RIO for the ULA prefix and Router
Lifetime set to zero. The behaviour is consistent with the situation Lifetime set to zero. The behaviour is consistent with the situation
when SERb1 lost the uplink to ISP-B (so there is no Internet 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 connectivity from 2001:db8:0:b000::/52 sources) but those sources can
be used to reach some specific destinations. In the case of ULA be used to reach some specific destinations. In the case of ULA
there is no Internet connectivity from ULA sources but they can be there is no Internet connectivity from ULA sources but they can be
used to reach another ULA destinations. Note that ULA usage could 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 particularly useful if all ISPs assign prefixes via DHCP-PD. In the
absence of ULAs uplinks failure hosts would lost all their GUAs upon absence of ULAs upon the all uplinks failure hosts would lost all
prefix lifetime expiration which again makes intra-site communication their GUAs upon prefix lifetime expiration which again makes intra-
impossible. site communication impossible.
It should be noted that the Rule 5.5 (prefer a prefix advertised by 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 the selected next-hop) takes precedence over the Rule 6 (prefer
matching label, which ensures that GUA source addresses are preferred matching label, which ensures that GUA source addresses are preferred
over ULAs for GUA destinations). Therefore if ULAs are used, the over ULAs for GUA destinations). Therefore if ULAs are used, the
network administrator needs to ensure that while the site has an network administrator needs to ensure that while the site has an
Internet connectivity, hosts do not select a router which advertises Internet connectivity, hosts do not select a router which advertises
ULA prefixes as their default router. ULA prefixes as their default router.
5.5.3. Controlling Source Address Selection With ICMPv6 6.5.3. Controlling Source Address Selection With ICMPv6
In case of all uplinks failure all SERs will drop outgoing IPv6 In case of all uplinks failure all SERs will drop outgoing IPv6
traffic and respond with ICMPv6 error message. In the large network traffic and respond with ICMPv6 error message. In the large network
when many hosts are trying to reach Internet destinations it means when many hosts are trying to reach Internet destinations it means
that SERs need to generate an ICMPv6 error to every packet they that SERs need to generate an ICMPv6 error to every packet they
receive from hosts which presents the same scalability issues receive from hosts which presents the same scalability issues
discussed in Section 5.3.3 discussed in Section 6.3.3
5.5.4. Summary Of Methods For Controlling Source Address Selection When 6.5.4. Summary Of Methods For Controlling Source Address Selection When
All Uplinks Failed All Uplinks Failed
Again, combining SADR with Router Advertisements seems to be the most Again, combining SADR with Router Advertisements seems to be the most
flexible and scalable way to control the source address selection on flexible and scalable way to control the source address selection on
hosts. hosts.
5.6. Summary Of Methods For Controlling Source Address Selection 6.6. Summary Of Methods For Controlling Source Address Selection
To summarize the scenarios and options discussed above: To summarize the scenarios and options discussed above:
While DHCPv6 allows administrators to manipulate source address While DHCPv6 allows administrators to manipulate source address
selection policy tables, this method has a number of significant selection policy tables, this method has a number of significant
disadvantages which eliminates DHCPv6 from a list of potential disadvantages which eliminates DHCPv6 from a list of potential
solutions: solutions:
1. It required hosts to support DHCPv6 and its extension (RFC7078); 1. It required hosts to support DHCPv6 and its extension (RFC7078);
2. DHCPv6 server needs to monitor network state and detect routing 2. DHCPv6 server needs to monitor network state and detect routing
changes. changes.
3. The use of policy tables requires manual configuration and might 3. The use of policy tables requires manual configuration and might
be extremely complicated, especially in the case of distributed be extremely complicated, especially in the case of distributed
network when large number of remote sites are being served by network when large number of remote sites are being served by
centralized DHCPv6 servers. centralized DHCPv6 servers.
4. Network topology/routing policy changes could trigger 4. Network topology/routing policy changes could trigger
simultaneous re-configuration of large number of hosts which simultaneous re-configuration of large number of hosts which
skipping to change at page 40, line 30 skipping to change at page 41, line 45
to implement SADR and be able to send dedicated RAs per scoped to implement SADR and be able to send dedicated RAs per scoped
forwarding table as discussed above, reacting to network changes with forwarding table as discussed above, reacting to network changes with
sending new RAs. It should be noted that the proposed solution would 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 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 send individual RAs for each ISP prefix and react to topology changes
as discussed above (e.g. via configuration knobs). as discussed above (e.g. via configuration knobs).
The RA-based solution relies heavily on hosts correctly implementing The RA-based solution relies heavily on hosts correctly implementing
default address selection algorithm as defined in [RFC6724]. While default address selection algorithm as defined in [RFC6724]. While
the basic (and most common) multihoming scenario (two or more the basic (and most common) multihoming scenario (two or more
Internet uplinks, no 'wall gardens') would work for any host Internet uplinks, no 'walled gardens') would work for any host
supporting the minimal implementation of [RFC6724], more complex use supporting the minimal implementation of [RFC6724], more complex use
cases (such as "wall garden" and other scenarios when some ISP cases (such as "walled garden" and other scenarios when some ISP
resources can only be reached from that ISP address space) require resources can only be reached from that ISP address space) require
that hosts support Rule 5.5 of the default address selection that hosts support Rule 5.5 of the default address selection
algorithm. There is some evidence that not all host OSes have that algorithm. There is some evidence that not all host OSes have that
rule implemented currently. However it should be noted that rule implemented currently. However it should be noted that
[RFC8028] states that Rule 5.5 SHOULD be implemented. [RFC8028] states that Rule 5.5 should be implemented.
ICMPv6 Code 5 error message SHOULD be used to complement RA-based ICMPv6 Code 5 error message SHOULD be used to complement RA-based
solution to signal incorrect source address selection back to hosts, solution to signal incorrect source address selection back to hosts,
but it SHOULD NOT be considered as the stand-alone solution. To but it SHOULD NOT be considered as the stand-alone solution. To
prevent scenarios when hosts in multihomed envinronments incorrectly prevent scenarios when hosts in multihomed envinronments incorrectly
identify onlink/offlink destinations, hosts should treat ICMPv6 identify onlink/offlink destinations, hosts SHOULD treat ICMPv6
Redirects as discussed in [RFC8028]. Redirects as discussed in [RFC8028].
5.7. Other Configuration Parameters 6.7. Solution Limitations
5.7.1. DNS Configuration 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
(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 In mutihomed envinronment each ISP might provide their own list of
DNS servers. For example, in the topology shown in Figure 3, ISP-A 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 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 ISP-B might provide H61 2001:db8:0:6666::61 as a recursive DNS
server. [RFC8106] defines IPv6 Router Advertisement options to allow server. [RFC8106] defines IPv6 Router Advertisement options to allow
IPv6 routers to advertise a list of DNS recursive server addresses IPv6 routers to advertise a list of DNS recursive server addresses
and a DNS Search List to IPv6 hosts. Using RDNSS together with 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 '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 the Figure 3) to send DNS server addresses and search lists provided
by each ISP (or the corporate DNS servers addresses if the enterprise by each ISP (or the corporate DNS servers addresses if the enterprise
is running its own DNS servers). 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 5.5.2, failure of all ISP uplinks would cause 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 deprecation of all addresses assigned to a host from the address
space of all ISPs. If any intra-site IPv6 connectivity is still space of all ISPs. If any intra-site IPv6 connectivity is still
desirable (most likely to be the case for any mid/large scare desirable (most likely to be the case for any mid/large scare
network), then ULAs should be used as discussed in Section 5.5.2. In 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 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 DNS server(s) and provide its ULA addresses to hosts via RDNSS in RAs
send for ULA-scoped forwarding table as described in Section 5.5.2. send for ULA-scoped forwarding table as described in Section 6.5.2.
There are some scenarios when the final outcome of the name There are some scenarios when the final outcome of the name
resolution might be different depending on: resolution might be different depending on:
o which DNS server is used; o which DNS server is used;
o which source address the client uses to send a DNS query to the o which source address the client uses to send a DNS query to the
server (DNS split horizon). server (DNS split horizon).
There is no way currently to instruct a host to use a particular DNS There is no way currently to instruct a host to use a particular DNS
skipping to change at page 42, line 7 skipping to change at page 44, line 19
For example, if it is desirable that host H31 reaches the ISP-A DNS 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 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 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 containing the route to 2001:db8:0:5555::51 (or covering route) in
their 'scoped' RAs, containing LLA_A as the default router address 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 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- 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 hop and then chose 2001:db8:0:a010::31 as the source address for
packets to the DNS server. packets to the DNS server.
It should be noted that [RFC8106] explicitly prohibits using DNS It should be noted that [RFC6106] explicitly prohibits using DNS
information if the RA router Lifetime expired: "An RDNSS address or a 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 DNSSL domain name MUST be used only as long as both the RA router
Lifetime (advertised by a Router Advertisement message) and the Lifetime (advertised by a Router Advertisement message) and the
corresponding option Lifetime have not expired.". Therefore hosts corresponding option Lifetime have not expired.". Therefore hosts
might ignore RDNSS information provided in ULA-scoped RAs as those might ignore RDNSS information provided in ULA-scoped RAs as those
RAs would have router lifetime set to 0. However the updated version RAs would have router lifetime set to 0. However the updated version
of RFC6106 ([RFC8106]) has that requirement removed. of RFC6106 ([RFC8106]) has that requirement removed.
6. Deployment Considerations 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 The solution described in this document requires certain mechanisms
to be supported by the network infrastructure and hosts. It requires to be supported by the network infrastructure and hosts. It requires
some routers in the enterprise site to support some form of Source some routers in the enterprise site to support some form of Source
Address Dependent Routing (SADR). It also requires hosts to be able Address Dependent Routing (SADR). It also requires hosts to be able
to learn when the uplink to an ISP changes its state so the to learn when the uplink to an ISP changes its state so the
corresponding source addresses should (or should not) be used. corresponding source addresses should (or should not) be used.
Ongoing work to create mechanisms to accomplish this are discussed in Ongoing work to create mechanisms to accomplish this are discussed in
this document, but they are still a work in progress. 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:
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.
7.2. Hosts-Related Considerations
The solution discussed in this document relies on the default address The solution discussed in this document relies on the default address
selection algorithm ([RFC6724]) Rule 5.5. While [RFC6724] considers selection algorithm ([RFC6724]) Rule 5.5. While [RFC6724] considers
this rule as optional, the recent [RFC8028] recommends that a host this rule as optional, the recent [RFC8028] states that "A host
SHOULD select default routers for each prefix in which it is assigned SHOULD select default routers for each prefix it is assigned an
an address. It also recommends that hosts SHOULD implement Rule 5.5. address in". It also recommends that hosts should implement Rule
of [RFC6724]. Therefore while RFC8028-compliant hosts already have 5.5. of [RFC6724]. Therefore while RFC8028-compliant hosts already
mechanism to learn about ISP uplinks state changes and selecting the have mechanism to learn about ISP uplinks state changes and selecting
source addresses accordingly, many hosts do not have such mechanism the source addresses accordingly, many hosts do not have such
supported yet. mechanism supported yet.
It should be noted that multihomed enterprise network utilizing It should be noted that multihomed enterprise network utilizing
multiple ISP prefixes can be considered as a typical mutiple multiple ISP prefixes can be considered as a typical multiple
provisioning domain (mPVD) scenario, as described in [RFC7556]. This provisioning domain (mPVD) scenario, as described in [RFC7556]. This
document defines a way for network to provide the PVD information to document defines a way for the network to provide the PVD information
hosts indirectly, using the existing mechanisms. At the same time to hosts indirectly, using the existing mechanisms. At the same time
[I-D.ietf-intarea-provisioning-domains]takes one step further and [I-D.ietf-intarea-provisioning-domains] takes one step further and
describes a comprehensive mechanism for hosts to discover the whole describes a comprehensive mechanism for hosts to discover the whole
set of configuration information associated with different PVD/ISPs. set of configuration information associated with different PVD/ISPs.
[I-D.ietf-intarea-provisioning-domains] complements this document in [I-D.ietf-intarea-provisioning-domains] complements this document in
terms of making hosts being able to learn about ISP uplink states and terms of making hosts being able to learn about ISP uplink states and
selecting the corresponding source addresses. selecting the corresponding source addresses.
7. Other Solutions 8. Other Solutions
7.1. Shim6 8.1. Shim6
The Shim6 working group specified the Shim6 protocol [RFC5533] which The Shim6 working group specified the Shim6 protocol [RFC5533] which
allows a host at a multihomed site to communicate with an external allows a host at a multihomed site to communicate with an external
host and exchange information about possible source and destination host and exchange information about possible source and destination
address pairs that they can use to communicate. It also specified address pairs that they can use to communicate. It also specified
the REAP protocol [RFC5534] to detect failures in the path between the REAP protocol [RFC5534] to detect failures in the path between
working address pairs and find new working address pairs. A working address pairs and find new working address pairs. A
fundamental requirement for Shim6 is that both internal and external fundamental requirement for Shim6 is that both internal and external
hosts need to support Shim6. That is, both the host internal to the 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 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 support Shim6 in order for there to be any benefit for the internal
host to run Shim6. The Shim6 protocol specification was published in host to run Shim6. The Shim6 protocol specification was published in
2009, but it has not been widely implemented. Therefore Shim6 is not 2009, but it has not been widely implemented. Therefore Shim6 is not
considered as a viable solution for enterprise multihoming. considered as a viable solution for enterprise multihoming.
7.2. IPv6-to-IPv6 Network Prefix Translation 8.2. IPv6-to-IPv6 Network Prefix Translation
IPv6-to-IPv6 Network Prefix Translation (NPTv6) [RFC6296] is not the IPv6-to-IPv6 Network Prefix Translation (NPTv6) [RFC6296] is not the
focus of this document. NPTv6 suffers from the same fundamental focus of this document. NPTv6 suffers from the same fundamental
issue as any other address translation approaches: it breaks end-to- issue as any other address translation approaches: it breaks end-to-
end connectivity. Therefore NPTv6 is not considered as desirable end connectivity. Therefore NPTv6 is not considered as desirable
solution and this document intentionally focuses on solving solution and this document intentionally focuses on solving
enterprise multihoming problem without any form of address enterprise multihoming problem without any form of address
translations. translations.
With increasing interest and ongoing work in bringing path awareness With increasing interest and ongoing work in bringing path awareness
to transport and application layer protocols hosts might be able to to transport and application layer protocols hosts might be able to
determine the properties of the various network paths and choose determine the properties of the various network paths and choose
among paths available to them. As selecting the correct source among paths available to them. As selecting the correct source
address is one of the possible mechanisms path-aware hosts may address is one of the possible mechanisms path-aware hosts may
utilize, address translation negatively affects hosts path-awareness utilize, address translation negatively affects hosts path-awareness
which makes NTPv6 even more undesirable solution. which makes NTPv6 even more undesirable solution.
7.3. Multipath Transport 8.3. Multipath Transport
Using multipath transport might solve the problems discussed in Using multipath transport (such as MPTCP, [RFC6824] or multipath
Section 5 since it would allow hosts to use multiple source addresses capabilities in QUIC) might solve the problems discussed in Section 6
for a single connection and switch between source addresses when a 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 particular address becomes unavailable or a new address gets assigned
to the host interface. Therefore if all hosts in the enterprise to the host interface. Therefore if all hosts in the enterprise
network are only using multipath transport for all connections, the network are only using multipath transport for all connections, the
signalling solution described in Section 5 might not be needed (it signaling solution described in Section 6 might not be needed (it
should be noted that the Source Address Dependent Routing would still should be noted that the Source Address Dependent Routing would still
be required to deliver packets to the correct uplinks). At the time be required to deliver packets to the correct uplinks). At the time
this document was written, multipath transport alone could not be this document was written, multipath transport alone could not be
considered a solution for the problem of selecting the source address considered a solution for the problem of selecting the source address
in a multihomed environment. There are significant number of hosts in a multihomed environment. There are significant number of hosts
which do not use multipath transport currently and it seems unlikely which do not use multipath transport currently and it seems unlikely
that the situation is going to change in any foreseeable future. The that the situation is going to change in any foreseeable future (even
solution for enterprise multihoming needs to work for the least if new releases of operatin systems get multipath protocols support
common denominator: hosts without multipath transport support. In there will be a long tail of legacy hosts). The solution for
addition, not all protocols are using multipath transport. While enterprise multihoming needs to work for the least common
multipath transport would complement the solution described in denominator: hosts without multipath transport support. In addition,
Section 5, it could not be considered as a sole solution to the not all protocols are using multipath transport. While multipath
problem of source address selection in multihomed environments. 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.
8. IANA Considerations 9. IANA Considerations
This memo asks the IANA for no new parameters. This memo asks the IANA for no new parameters.
9. Security Considerations 10. Security Considerations
Section 5.2.3 discusses a mechanism for controlling source address Section 6.2.3 discusses a mechanism for controlling source address
selection on hosts using ICMPv6 messages. It describes how an selection on hosts using ICMPv6 messages. It describes how an
attacker could exploit this mechansim by sending spoofed ICMPv6 attacker could exploit this mechansim by sending spoofed ICMPv6
messages. It recommends that a given host verify the original packet messages. It recommends that a given host verify the original packet
header included into ICMPv6 error message was actually sent by the header included into ICMPv6 error message was actually sent by the
host itself. host itself.
The security considerations of using stateless address The security considerations of using stateless address
autoconfiguration are discussed in [RFC4862]. autoconfiguration are discussed in [RFC4862].
10. Acknowledgements 11. Acknowledgements
The original outline was suggested by Ole Troan. The original outline was suggested by Ole Troan.
The authors would like to thank the following people (in alphabetical The authors would like to thank the following people (in alphabetical
order) for their review and feedback: Olivier Bonaventure, Brian E order) for their review and feedback: Olivier Bonaventure, Deborah
Carpenter, Lorenzo Colitti, David Lamparter, Nicolai Leymann, Acee Brungard, Brian E Carpenter, Lorenzo Colitti, Roman Danyliw, Benjamin
Lindem, Philip Matthewsu, Robert Raszuk, Dave Thaler, Martin Kaduk, Suresh Krishnan, Mirja Kuhlewind, David Lamparter, Nicolai
Vigoureux. Leymann, Acee Lindem, Philip Matthewsu, Robert Raszuk, Alvaro Retana,
Dave Thaler, Michael Tuxen, Martin Vigoureux, Eric Vyncke, Magnus
Westerlund.
11. References 12. References
11.1. Normative References 12.1. Normative References
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G., [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets", and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996, BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
<https://www.rfc-editor.org/info/rfc1918>. <https://www.rfc-editor.org/info/rfc1918>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
skipping to change at page 45, line 23 skipping to change at page 48, line 13
May 2000, <https://www.rfc-editor.org/info/rfc2827>. May 2000, <https://www.rfc-editor.org/info/rfc2827>.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191, More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
November 2005, <https://www.rfc-editor.org/info/rfc4191>. November 2005, <https://www.rfc-editor.org/info/rfc4191>.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast [RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005, Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
<https://www.rfc-editor.org/info/rfc4193>. <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>.
[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 [RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011, Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,
<https://www.rfc-editor.org/info/rfc6296>. <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 [RFC7556] Anipko, D., Ed., "Multiple Provisioning Domain
Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015, Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
<https://www.rfc-editor.org/info/rfc7556>. <https://www.rfc-editor.org/info/rfc7556>.
[RFC8028] Baker, F. and B. Carpenter, "First-Hop Router Selection by [RFC8028] Baker, F. and B. Carpenter, "First-Hop Router Selection by
Hosts in a Multi-Prefix Network", RFC 8028, Hosts in a Multi-Prefix Network", RFC 8028,
DOI 10.17487/RFC8028, November 2016, DOI 10.17487/RFC8028, November 2016,
<https://www.rfc-editor.org/info/rfc8028>. <https://www.rfc-editor.org/info/rfc8028>.
[RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, [RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
skipping to change at page 46, line 5 skipping to change at page 49, line 29
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., [RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters, Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018, RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>. <https://www.rfc-editor.org/info/rfc8415>.
11.2. Informative References 12.2. Informative References
[I-D.ietf-intarea-provisioning-domains] [I-D.ietf-intarea-provisioning-domains]
Pfister, P., Vyncke, E., Pauly, T., Schinazi, D., and W. Pfister, P., Vyncke, E., Pauly, T., Schinazi, D., and W.
Shao, "Discovering Provisioning Domain Names and Data", Shao, "Discovering Provisioning Domain Names and Data",
draft-ietf-intarea-provisioning-domains-04 (work in draft-ietf-intarea-provisioning-domains-05 (work in
progress), March 2019. progress), June 2019.
[I-D.ietf-rtgwg-dst-src-routing] [I-D.ietf-rtgwg-dst-src-routing]
Lamparter, D. and A. Smirnov, "Destination/Source Lamparter, D. and A. Smirnov, "Destination/Source
Routing", draft-ietf-rtgwg-dst-src-routing-07 (work in Routing", draft-ietf-rtgwg-dst-src-routing-07 (work in
progress), March 2019. progress), March 2019.
[I-D.pfister-6man-sadr-ra] [I-D.pfister-6man-sadr-ra]
Pfister, P., "Source Address Dependent Route Information Pfister, P., "Source Address Dependent Route Information
Option for Router Advertisements", draft-pfister-6man- Option for Router Advertisements", draft-pfister-6man-
sadr-ra-01 (work in progress), June 2015. sadr-ra-01 (work in progress), June 2015.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed [RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
2004, <https://www.rfc-editor.org/info/rfc3704>. 2004, <https://www.rfc-editor.org/info/rfc3704>.
[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>.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<https://www.rfc-editor.org/info/rfc4941>. <https://www.rfc-editor.org/info/rfc4941>.
[RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming [RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", RFC 5533, DOI 10.17487/RFC5533, Shim Protocol for IPv6", RFC 5533, DOI 10.17487/RFC5533,
June 2009, <https://www.rfc-editor.org/info/rfc5533>. June 2009, <https://www.rfc-editor.org/info/rfc5533>.
[RFC5534] Arkko, J. and I. van Beijnum, "Failure Detection and [RFC5534] Arkko, J. and I. van Beijnum, "Failure Detection and
Locator Pair Exploration Protocol for IPv6 Multihoming", Locator Pair Exploration Protocol for IPv6 Multihoming",
RFC 5534, DOI 10.17487/RFC5534, June 2009, RFC 5534, DOI 10.17487/RFC5534, June 2009,
<https://www.rfc-editor.org/info/rfc5534>. <https://www.rfc-editor.org/info/rfc5534>.
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, [RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
"Default Address Selection for Internet Protocol Version 6 Requirements", RFC 6434, DOI 10.17487/RFC6434, December
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012, 2011, <https://www.rfc-editor.org/info/rfc6434>.
<https://www.rfc-editor.org/info/rfc6724>.
[RFC7078] Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
Address Selection Policy Using DHCPv6", RFC 7078, "TCP Extensions for Multipath Operation with Multiple
DOI 10.17487/RFC7078, January 2014, Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<https://www.rfc-editor.org/info/rfc7078>. <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 Authors' Addresses
Fred Baker Fred Baker
Santa Barbara, California 93117 Santa Barbara, California 93117
USA USA
Email: FredBaker.IETF@gmail.com Email: FredBaker.IETF@gmail.com
Chris Bowers Chris Bowers
skipping to change at page 47, line 34 skipping to change at page 51, line 4
USA USA
Email: FredBaker.IETF@gmail.com Email: FredBaker.IETF@gmail.com
Chris Bowers Chris Bowers
Juniper Networks Juniper Networks
Sunnyvale, California 94089 Sunnyvale, California 94089
USA USA
Email: cbowers@juniper.net Email: cbowers@juniper.net
Jen Linkova Jen Linkova
Google Google
Mountain View, California 94043 1 Darling Island Rd
USA Pyrmont, NSW 2009
AU
Email: furry@google.com Email: furry@google.com
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