Internet DRAFT - draft-boot-homenet-brdp
draft-boot-homenet-brdp
Network Working Group T. Boot
Internet-Draft Infinity Networks B.V.
Intended status: Informational October 15, 2012
Expires: April 18, 2013
BRDP for Homenet
draft-boot-homenet-brdp-00.txt
Abstract
This document describes the Border Router Discovery Protocol (BRDP)
and all of its related components. BRDP enables multi-homing for
small to medium sites, including Homenets, using Provider
Aggregatable IPv6 addresses. It describes a mechanism for automated
IP address configuration and renumbering, a mechanism for optimized
source address selection and a new paradigm for packet forwarding,
for support of multi-homed sites. BRDP prevents ingress filtering
problems with multi-homed sites and supports load-balancing for
multi-path transport protocols. This work also prevents routing
scalability problems in the provider network and Internet Default
Free Zone because small to medium multi-homed size sites would not
need to request Provider Independent address blocks.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 18, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Detection of Homenet Perimeter interfaces on Border
Routers . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Propagation of Border Router information . . . . . . . . . 5
1.3. Address configuration with Border Router information . . . 6
1.4. Address selection with Border Router information . . . . . 6
1.5. Routing based on Border Router information . . . . . . . . 7
1.6. Deployment of the Border Router Discovery Protocol . . . . 7
1.7. Requirements Language . . . . . . . . . . . . . . . . . . 8
2. Reference Scenarios . . . . . . . . . . . . . . . . . . . . . 8
2.1. Single-homed site . . . . . . . . . . . . . . . . . . . . 8
2.2. Small multi-homed site or DMZ . . . . . . . . . . . . . . 10
2.3. Medium multi-homed site . . . . . . . . . . . . . . . . . 12
2.4. Medium multi-homed site with ULAs and DHCP server . . . . 16
2.5. MANET site . . . . . . . . . . . . . . . . . . . . . . . . 18
3. Border Router Discovery Protocol (BRDP) . . . . . . . . . . . 20
3.1. Border Router Information Option (BRIO) . . . . . . . . . 21
3.2. BRDP processing . . . . . . . . . . . . . . . . . . . . . 22
3.2.1. BRDP message generation and transmission . . . . . . . 23
3.2.2. BRDP message reception . . . . . . . . . . . . . . . . 24
3.2.3. BRIO-Cache maintenance . . . . . . . . . . . . . . . . 25
3.2.4. BRDP loop prevention . . . . . . . . . . . . . . . . . 26
3.3. Unified Path Metric (UPM) . . . . . . . . . . . . . . . . 27
4. BRDP based Address Configuration and Prefix Delegation . . . . 27
4.1. Border Router selection . . . . . . . . . . . . . . . . . 28
4.1.1. Border Router Selection based on UPM . . . . . . . . . 28
4.2. Address autoconfiguration . . . . . . . . . . . . . . . . 29
4.2.1. Address and prefix configuration with SLAAC or DHCP . 29
4.2.2. Address generation and configuration for Routers . . . 29
4.2.3. Support for Unique Local Addresses (ULA) . . . . . . . 30
5. BRDP based Source Address Selection . . . . . . . . . . . . . 30
5.1. Address Selection for dynamic DNS . . . . . . . . . . . . 30
6. BRDP based Routing . . . . . . . . . . . . . . . . . . . . . . 30
6.1. Problems with default gateway routing . . . . . . . . . . 31
6.2. Default gateway routing replaced with BRDP Based
Routing . . . . . . . . . . . . . . . . . . . . . . . . . 31
7. BRDP and IRTF RRG goals . . . . . . . . . . . . . . . . . . . 32
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7.1. Scalability . . . . . . . . . . . . . . . . . . . . . . . 33
7.2. Traffic engineering . . . . . . . . . . . . . . . . . . . 33
7.3. Multi-homing . . . . . . . . . . . . . . . . . . . . . . . 33
7.4. Loc/id separation . . . . . . . . . . . . . . . . . . . . 33
7.5. Mobility . . . . . . . . . . . . . . . . . . . . . . . . . 34
7.6. Simplified renumbering . . . . . . . . . . . . . . . . . . 34
7.7. Modularity . . . . . . . . . . . . . . . . . . . . . . . . 34
7.8. Routing quality . . . . . . . . . . . . . . . . . . . . . 34
7.9. Routing security . . . . . . . . . . . . . . . . . . . . . 34
7.10. Deployability . . . . . . . . . . . . . . . . . . . . . . 35
8. Currently unaddressed issues . . . . . . . . . . . . . . . . . 35
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
11. Security Considerations . . . . . . . . . . . . . . . . . . . 35
12. Change log . . . . . . . . . . . . . . . . . . . . . . . . . . 36
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
13.1. Normative References . . . . . . . . . . . . . . . . . . . 36
13.2. Informative References . . . . . . . . . . . . . . . . . . 37
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 38
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1. Introduction
*** Note to the reader *** This Internet Draft is submitted as an
early version for a proposal for the Homenet working group. This
version is a merge from earlier documents. Now that is a singe
document, it is to be adjusted to comply to the Homenet scenario.
This is work in progress.
IPv6 provides basic functionality for multi-homing, since nodes can
have multiple addresses configured on their interfaces. However, it
is difficult to utilize the advantages of this, as there is a strong
tendency shielding the network topology from hosts and in general
routing does not support multi-homing very well. As a result, it is
difficult or impossible for a host to utilize available facilities of
the network, such as multi-path. Also scalability of the Internet
routing system is getting a problem due to a high demand of Provider
Independent (PI) addresses.
The Border Router Discovery Protocol (BRDP) enhances the IPv6 model
by enabling automated renumbering in dynamically changing multi-homed
environments, such that routers and hosts cooperate on address
configuration and path selection. BRDP utilizes Provider
Aggregatable (PA) addresses and uses them as locator. Mapping
identifiers to locators is out of scope of BRDP, also because other
solutions exists or are being worked on. All these solutions work
fine with BRDP, as long as they don't break IPv6.
BRDP applies to edge networks. These networks can be fixed, for
example enterprise networks, small offices / home offices (SOHO) or
home sites (Homenets). BRDP also can be used in wireless access
networks, for example wireless access networks such as 3G or 4G,
wireless LANs or mobile ad hoc networks (MANETs). A nice attribute
of BRDP is that it supports multi-homing in heterogeneous networks,
meaning that e.g. a Homenet network can have multiple wired broadband
and 3G/4G connections to the Internet simultaneously.
In a multi-homed network, nodes are connected to the Internet via
multiple exit points, possibly via multiple providers. [RFC5887]
argues that if a site is multi-homed, using multiple PA routing
prefixes, then the interior routers need a mechanism to learn which
upstream providers and corresponding PA prefixes are currently
reachable and valid. Next to that, these upstream providers or PA
prefixes may change over time. This requires a dynamic renumbering
mechanism that can handle planned or unplanned changes in the
prefixes used. BRDP proposes a mechanism for automated renumbering
in larger networks that goes beyond hosts in a single subnet.
BRDP uses the following key elements:
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o Propagation of available Border Routers and corresponding
prefixes, described in Section 3;
o Address autoconfiguration and prefix delegation, using BRDP
provided hints, described in Section 4;
o Source address selection, using BRDP provided hints, described in
Section 5;
o Packet forwarding to the Border Router that corresponds with the
source address prefix, in case the destination address is not
found in the routing domain, described in Section 6.
1.1. Detection of Homenet Perimeter interfaces on Border Routers
For fully automated deployment in Homenets, it is required that
routers can discover automatically their uplink interfaces, that
connect the Homenet to ISPs. Some mechanisms for automatic detection
are described in [I-D.kline-default-perimeter].
After detection of an uplink interface to an ISP and reception of a
prefix, the router starts acting as a border router. It starts
acting as a DHCP server, with support of prefix delegation. It also
configures at least an address out of the assigned prefix. This
address is used as Border Router address and DHCP server address.
The BRDP protocol can also be used to assist perimeter detection. A
router interface on which Border Router information is received
should not be identified as an uplink interface to an ISP.
1.2. Propagation of Border Router information
The propagation of available Border Routers and corresponding
prefixes is implemented as an extension on the Neighbor Discovery
Protocol [RFC4861]. Border Router Information Options (BRIOs) are
sent with Router Advertisements, and contain information about the
Border Router, such as:
o - the Border Router address;
o - the prefix that corresponds with that Border Router;
o - cost indication of the path via that Border Router to the core
network, i.e. the Internet Default Free Zone (DFZ).
BRIOs are disseminated downstream through the network. All nodes
store the information from BRIOs they receive in a BRIO cache.
Border routers with multiple prefixes send out a BRIO for each of
these prefixes. In a multi-homed network, nodes will receive
multiple prefix information, from multiple upstream Border Routers or
from a Border Router with multiple prefixes.
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1.3. Address configuration with Border Router information
Routers can generate IPv6 addresses, with regular SLAAC [RFC4862].
Generation is based on Prefix Information Option from upstream
routers and optionally on information in the BRIO cache, e.g. using
the prefix with the lowest cost to the Internet. In addition,
routers may generate /128 IPv6 address-prefixes for a management
interface, based on a Border Router prefix. Routers set up
reachability to these addresses automatically, by adding the
generated address or prefix in the routing protocol.
With BRDP, routers automatically learn Border Routers that act as
DHCP server or relay agent [RFC3633]. When routers detect an
alternate path to the DFZ, with no corresponding assigned address or
prefix already, new prefixes are requested for using this alternate
path.
Prefixes, of which the path to the DFZ is no longer available, are
put 'out of service' by routers, meaning they are not used for
address assignments anymore. Optionally, if the cost to the DFZ
through a Border Router is far higher than via other available paths,
a router can put the corresponding prefix out of service also.
Prefixes that are out of service are released.
Prefixes that are in service are configured on interfaces with a 64-
bit prefix length and advertised with a Prefix Information Option in
Router Advertisements. The Prefix Information lifetime is copied
from lifetime information in the BRIO cache.
Hosts can use the BRDP provided information together with the Prefix
Information to autoconfigure addresses, based on IPv6 Stateless
Address Autoconfiguration [RFC4862]. A host may also use DHCPv6 to
get addresses or "Other configuration", using multicast or with
unicast to the BRDP learned DHCP server address.
1.4. Address selection with Border Router information
Nodes with multiple configured addresses need to select a source
address for outgoing connections. Default Address Selection for IPv6
[RFC6724] defines a mechanism, used as default behavior. It is open
to more advanced mechanisms or site policies. BRDP provided
information can be used for a more advanced mechanism, where the
hosts select automatically a source address that corresponds with a
path with the lowest cost to the DFZ. When multiple Border Routers
are available, automatic load distribution and multi-path transport
becomes available.
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1.5. Routing based on Border Router information
Network Ingress Filtering [RFC2827] describes the need for ingress
filtering, to limit the impact of distributed denial of service
attacks, by denying traffic with spoofed source addresses access. It
also helps ensure that traffic is traceable to its correct source
network. Ingress Filtering for Multihomed Networks [RFC3704]
provides solutions for multi-homed sites. However, the proposal
applicable for PA addresses requires careful planning and
configuration. It suggests routing based on source address, and a
path on each Border Router to all ISPs in use, either with a direct
connection or with tunnels between all Border Routers. It is hard to
make such mechanisms work in an automated fashion, or mechanisms are
not supported on Border Routers used today. As an evolutionary
approach, BRDP provided information is to be used to forward packets
to their destination without ingress filtering problems. The BRIO
cache contains a mapping between Border Routers and the addresses
that do pass ingress filtering. So the packet forwarding heuristic
can be straightforward: send packets, where the destination is not in
the routing domain itself, to the Border Router that owns the prefix
of the source address.
Hosts use information in the Default Router List to select a default
router. For selecting the best paths, hosts may use next hop
selection based on source address and path costs to the corresponding
Border Router, if such information is available to the host. Such
next hop determination is useful for destinations outside the edge
network, i.e. the destination address does not belong to a prefix in
the BRIO cache.
1.6. Deployment of the Border Router Discovery Protocol
Enabling BRDP in an existing network is straightforward. First, all
routers have to be updated for BRDP support. At this step, Border
Router information is propagated in the network enabling BRDP
assisted address autoconfiguration and prefix delegation and BRDP
assisted source address selection. The second step is updating all
routers with the BRDP based routing mechanism. To enable this
mechanism the default gateway is removed from the routing table.
This second step is a flag day operation. Rolling back is easy, by
just re-inserting the default gateway.
After the update of the network, additional border routers can be
added to the network and will be used automatically. Also a
renumbering event will take place without any manual intervention.
BRDP does not provide session continuity when paths are broken.
Mobility solutions are in place, or are work in progress. Recently,
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interesting developments are work in progress, such as MPTCP
[I-D.ietf-mptcp-multiaddressed] and ILNP [I-D.rja-ilnp-intro]. BRDP
is very useful for both of these protocols.
1.7. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Reference Scenarios
This section describes the use of BRDP in five different scenarios: a
single homed Homenet, multi-homed site or DMZ, a medium multi-homed
site, a medium multi-homed site with ULA with DHCP server and a MANET
site.
2.1. Single-homed site
This scenario discusses BRDP operation for single-homed home
networks. The scenario is taken from [I-D.ietf-homenet-arch].
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/^^^^^^^^^^^^^^^^^^^^^^\
/ \
{ The Internet }
\ /
\______________________/
|
|(2001:08db:100::/40)
+-------+-------+
| Service |
| Provider |
| Router |
+-------+-------+
|
| (2001:8db:101::/48)
+------+--------+
| IPv6 CPE |
| BR_101 |
+----+-+---+----+
2001:8db:101:1::1 | | |
| | |
2001:8db:101:1::/64 | | | 2001:8db:101:3::/64
----+-------------+----+ | +---+-------------+------+
| | | | | | |
+----+-----+ +-----+----+ | +----+-----+ +-----+----+ |
|IPv6 Host | |IPv6 Host | | | IPv6 Host| |IPv6 Host | |
+----------+ +----------+ | +----------+ +----------+ |
| | | |
2001:8db:101:2::/64 | ---+------+------+-----+
| |
2001:8db:101:2::2 | |
+------+--------+ |
| IPv6 | |
| Interior +------+
| Router |
+---+-------+---+
2001:8db:101:4::/64 | | 2001:8db:101:5::/64
----+-------------+---+- --+---+-------------+---
| | | |
+----+-----+ +-----+----+ +----+-----+ +-----+----+
|IPv6 Host | |IPv6 Host | | IPv6 Host| |IPv6 Host |
+----------+ +----------+ +----------+ +----------+
Figure 1: Scenario 1: Single-homed site
The CPE device has to discover the link to the ISP and has to get
assigned an IPv6 prefix, in this scenario 2001:8db:101::/48. The CPE
configures itself a global unique address prefix 2001:8db:101:1::
1/64, assumed on the first interface in the homenet. It may
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configure additional global unique address on other interfaces, but
this is not required. This is existing functionality which is not
updated by BRDP.
The CPE starts sending router advertisements. It also checks
received router advertisements on already existing prefixes for the
/48 prefix it has assigned by the ISP. In this scenario there is no
other CPE, so no on-link prefixes exist. The CPE allocates and bind
additional prefixes for all its interfaces, and send Router
Advertisements with the Prefix Information Option. By then it has
configured 2001:8db:101:1::/64, 2001:8db:101:2::/64 and 2001:8db:101:
3::/64. The CPE router also acts as DHCP server, for the ISP
provided prefix.
Now, the IPv6 hosts in the middle row learn these prefixes from
Prefix Information Options sent by the CPE. They can configure IPv6
addresses, either with SLAAC or DHCP. Also, the IPv6 Interior Router
can configure an IPv6 address, in this scenario on the link with
prefix 2001:8db:101:2::/64.
The IPv6 Interior Router also receives the router advertisement with
the onlink prefix 2001:8db:101:3::/64 on its interface on the right.
It could configure an address in thes prefix, but because it has
already a globally unique address configured, there is no need for
this. Question is if the router should echo the prefix as on-link.
In this BRDP proposal, it doesn't. It is not the "delegated prefix
holder".
Before the IPv6 hosts on the lower row can get their addresses, the
IPv6 Interior Router has to be assigned two more prefixes. Here,
BRDP starts playing its role. The CPE router advertises itself with
Border Router Information Option, in its Router Advertisement. The
IPv6 Interior Router learns this information, and gets the two needed
prefixes from the CPE Router, using unicasted DHCP messages to the
(CPE) Border Router. Two prefixes are assigned and configured, 2001:
8db:101:4::/64 and 2001:8db:101:5::/64.
For full connectivity, the homenet uses an interior routing protocol.
BRDP is agnostic on the routing protocol used.
2.2. Small multi-homed site or DMZ
This scenario discusses BRDP operation for multi-homed Small Office -
Home Office (SOHO) networks and De-Militarized Zones (DMZ). The
scenario is shown in Figure 2. Each provider assigns a PA /48 prefix
to its customers. All addresses and prefixes are configured
completely automatically. The feature of BRDP that adds value in
this scenario is BRDP based Border Router selection for multi-homed
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hosts. This is enabled by using BRDP based forwarding.
/^^^^^^^^^^^^^^^^^^^^^^\
/ \
{ The Internet }
\ /
\______________________/
/ \
/(2001:08db:100::/40) \(2001:8db:200::/40)
+=======+ +=======+
[ ISP_1 ] [ ISP_2 ]
+=======+ +=======+
| |
|(2001:8db:101::/48) |(2001:8db:201::/48)
+--------+ +--------+
| BR_101 | | BR_201 |
+--------+ +--------+
| fe80::101/64 | fe80::201/64
| 2001:8db:101:1::101/64 | 2001:8db:201:1::201/64
| |
-+---------------------+---+-
|
2001:8db:101:1::1234/64 | 2001:8db:201:1::1234/64
fe80::1234/64 |
+-----------+
| Host_1234 |
+-----------+
Figure 2: Scenario 2: multi-homed Small Office - Home Office (SOHO)
network or DMZ
In this scenario, Host_1234 has configured two addresses using SLAAC
[RFC4862], one with prefix 2001:8db:101:1::/64 from Border Router
BR_101 and one with prefix 2001:8db:201:1::/64 from Border Router
BR_201. Host_1234 has learned these prefixes from Prefix Information
Options sent by both Border Routers according to [RFC4861]. The host
has learned via BRIOs that these prefixes belong to Border Routers.
The host can use optimal paths by selecting BR_101 as default router
for all packets with a source address with prefix 2001:8db:101:1::/64
and default gateway BR_201 for all packets with a source address with
prefix 2001:8db:201:1::/64. Non-optimal default router selection on
hosts is handled by the routers, "misdirected" packets are forwarded
to the correct Border Router.
BRDP enables routers to deliver non-optimal directed packets from
attached hosts towards a Border Router that owns the prefix of the
source address, if such a Border Router exists. In the above
scenario, a packet sent from Host_1234 with source address 2001:8db:
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201:1::1234 to default router BR_101 would be dropped due to on an
ingress filter, when no mechanism is in place to redirect the packet.
BRDP based forwarding provides such a mechanism automatically.
Instead of dropping the packet, BR_101 forwards it to BR_201.
2.3. Medium multi-homed site
This scenario discusses BRDP operation for medium sized multi-homed
networks. The difference with the previous scenario is that the
network paths between hosts and the Border Routers have intermediate
routers. The scenario is shown in Figure 3. The added value of BRDP
in this scenario is the discovery of Border Routers for hosts and
routers beyond the first hop as well as Border Router Selection for
hosts and routers.
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/^^^^^^^^^^^^^^^^^^^^^^\
/ \
{ The Internet }
\ /
\______________________/
/ \
/(2001:8db:100::/40) \(2001:8db:200::/40)
+=======+ +=======+
[ ISP_1 ] [ ISP_2 ]
+=======+ +=======+
| |
|(2001:8db:101::/48) |(2001:8db:201::/48)
+--------+ +--------+
| BR_101 | | BR_201 |
+--------+ +--------+
| fe80::101/64 | fe80::201/64
| 2001:8db:101:1::101/64 | 2001:8db:201:1::201/64
| |
-+-+-----------------------+-+-
| |
| 2001:8db:201:1::1/64 |
| 2001:8db:101:1::1/64 | 2001:8db:201:1::2/64
| fe80::1/64 | fe80::2/64
+----------+ +--------+
| R_1 | | R_2 |
+----------+ +--------+
|fe80:: | fe80::2:1/64 | fe80::1:2/64
|1:1/64 | |
| -+--------------------+-+-
| |
| 2001:8db:101:2::1234/64 |
| 2001:8db:201:3::1234/64 |
| fe80::1234/64 |
+-----------+ | 2001:8db:101:3::ABCD/64
| Host_1234 | | fe80::ABCD/64
+-----------+ +-----------+
| Host_ABCD |
+-----------+
Figure 3: Scenario 3: medium sized multi-homed network
Routers can learn advertised on-link prefixes automatically via the
Prefix Information Option in IPv6 ND RAs. In this scenario, routers
R_1 and R_2 learn prefix 2001:8db:101:1::/64 from BR_101 and prefix
2001:8db:201:1::/64 from BR_201. Routers may autoconfigure addresses
on their interfaces. In this example, R_1 has configured addresses
from both providers on its upstream interface, R_2 only configured an
address based on the prefix of BR_201. If the routers run a routing
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protocol, the learned prefixes are made reachable in the network. In
the next steps of the autoconfiguration proces, the prefixes and
addresses on the other links are automatically configured, but first
we discuss the BRDP messages that are disseminated through the
network.
Routers automatically learn Border Routers and mapping between
prefixes and Border Routers using BRDP. The diagram in Figure 4
depicts BRIO message dissemination in scenario 2. The two Border
Routers advertise their own address and corresponding prefix with an
address prefix. Nothing prevents them from forwarding each other's
BRIO message, although resending BRIO information on non-MANET
interfaces is not useful. Both routers R_1 and R_2 forward both
Border Router address prefixes, using separate BRIOs in RAs, on
downstream interfaces. In this way all routers and hosts in the
network are aware of all reachable Border Routers and corresponding
prefixes.
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/^^^^^^^^^^^^^^^^^^^^^^\
/ \
{ The Internet }
\ /
\______________________/
/ \
/(2001:8db:100::/40) \(2001:8db:200::/40)
+=======+ +=======+
[ ISP_1 ] [ ISP_2 ]
+=======+ +=======+
| |
|(2001:8db:101::/48) |(2001:8db:201::/48)
+--------+ +--------+
| BR_101 | | BR_201 |
+--------+ +--------+
| : | :
| V 2001:8db:101:1::101/48 | V 2001:8db:201:1::201/48
| |
-+-+-------------------------+-+-
| |
| |
+--------+ +--------+
| R_1 | | R_2 |
+--------+ +--------+
| : | : | :
| : | : 2001:8db:101:1::101/48 | : 2001:8db:101:1::101/48
| : | V 2001:8db:201:1::201/48 | V 2001:8db:201:1::201/48
| : | |
| : -+--------------------------+-
| :
| : 2001:8db:101:1::101/48
| V 2001:8db:201:1::201/48
|
-+----
Figure 4: BRIO dissemination in Scenario 3
Routers are not required to configure global addresses on each
interface. In the example, only the interface pointing to the
Internet has configured global addresses. Routers may also use a
(logical) management interface for global reachability.
So, the one-hop neighbours of BR_101 and BR_201, being R_1 and R_2,
have learned the prefixes and configured addresses on their upstream
interfaces. And all nodes in the network have learned the Border
Router prefixes. The next step is to get configured addresses on the
hosts in Figure 2. This is done by using DHCP Prefix Delegation.
R_1 and R_2 request a prefix from either or both BR_101 and BR_201
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for binding as on-link prefix on the links, and advertise those using
Prefix Information Options to the hosts. This will result in a
maximum of four prefixes that are advertised on the downlink of R_1
and R_2. Having multiple prefixes from the same ISP bound on a link
is not useful. So a router requests a prefix from a Border Router
only if no other prefix of that Border Router is advertised already
by another router on this network segment.
In this example, R_1 has been delegated two prefixes by DHCP PD for
the link with host Host_1234; 2001:8db:101:2::/64 and 2001:8db:201:
3::/64. No other router is on this link. R_1 or R_2 has also been
delegated a prefix on the link to host Host_ABCD; 2001:8db:101:
3::/64. It cannot be seen in Figure 2 which router has been
delegated the prefix, nor if another prefix for this link has been
delegated. No redundant prefix is delegated, as the routers learned
with RA PIO already delegated prefixes for known Border Routers.
Now, Host_1234 and Host_ABCD can autoconfigure addresses for their
interfaces. Host_1234 configures two addresses, one for each Border
Router. Host_ABCD chooses not to use ISP_2.
Nodes R_1 and Host_1234 can use both providers, by using two
configured global addresses. Any multi-path facility can be used,
either on an application layer or with a multi-path transport
protocol.
Host_ABCD may forward packets to the Internet via router R_1 or R_2.
If R_2 is selected as default router, R_2 forwards the packets to
BR_101 as this Border Router corresponds to the prefix of the source
address 2001:8db:101:3::ABCD. This works well, even in this case
where R_2 hasn't configured an address with a BR_101 prefix for
itself, and selected a global address from the BR_201 prefix only.
2.4. Medium multi-homed site with ULAs and DHCP server
In this example, the scenario 2 is extended by adding Unique Local
Addresses (ULA) for communication within the site itself. For
simplicity there is only one ISP present. The ULA IP configuration,
with prefix fd00:8db::/48, is managed by DHCPv6 server DHCP_201. The
scenario is shown in Figure 5.
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/^^^^^^^^^^^^^^^^^^^^^^\
/ \
{ The Internet }
\ /
\______________________/
/
/(2001:8db:100::/40)
+=======+
[ ISP_1 ]
+=======+
|
|(2001:8db:101::/48)
+--------+ +---------+
| BR_101 | | DHCP_201|
+--------+ +---------+
| fe80::101/64 | fe80::201/64
| 2001:8db:101:1::101/64 | fd00:8db:201:1::201/64
| fd00:8db:201:1::101/64 | (acme.com,fd00:8db:201:1:201:/48)
| |
-+-+-----------------------+-+-
| |
| fd00:8db:201:1::1/64 | fd00:8db:201:1::2/64
| 2001:8db:101:1::1/64 | 2001:8db:101:1::2/64
| fe80::1/64 | fe80::2/64
+--------+ +--------+
| R_1 | | R_2 |
+--------+ +--------+
|FE80 | fe80::1/64 | fe80::2/64
|::1 | |
|/64 -+--------------------+-+-
| |
| 2001:8db:101:2::1234/64 |
| fd00:8db:201:3::1234/64 |
| fe80::1234/64 |
+-----------+ | 2001:8db:101:3::ABCD/64
| Host_1234 | | fd00:8db:201:3::ABCD/64
+-----------+ | fe80::ABCD/64
+-----------+
| Host_ABCD |
+-----------+
Figure 5: Scenario 4: a medium sized multi-homed site with ULAs and
DHCP server.
In this scenario, all nodes have configured a ULA and a Global
Unicast Address using prefix delegation in the way that was described
in Section 2.2. ULA prefix delegation is automated just like PA
addresses. The DHCP server is therefore implemented on a router, in
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this case DHCP_201. This router advertises the ULA prefix with BRDP,
here fd00:8db:201::/48.
Although BRDP provides automatic prefix and address configuration for
ULA, a network administrator is free to configure it manually, along
using BRDP for global addresses.
BRDP based ULA configuration with BRDP based routing would result in
routing packets with ULA destinations outside the site to the
originator of the ULA prefix, in this case router DHCP_201. DHCP_201
is not connected to the Internet or another site owning the ULA, so
packets to non-existing destinations are dropped. DHCP_201 indicates
such with the BRIO F-bit set, meaning the Border Router is floating.
This scenario, it is demonstrated that BRDP and DHCPv6 cooperate in
address configuration. BRDP provides announcements of Border Routers
and DHCP servers. Routers request prefixes with DHCP, and can
request other parameters also. Such parameters are disseminated to
other nodes, either with router advertisements or acting as DHCP
server itself. Routers may also act as DHCP relay, redirecting
address requests to the Border Router(s). The Router Advertisement
M-bit and O-bit indicates availability of DHCPv6 services to attached
nodes.
Difficulties may arise when both ULA and global addresses are used
for Internet connectivity, e.g. when address translation is used. To
distinguish, the Border Router not providing Internet connectivity
informs nodes in the network using Service Selection suboption,
similar to "Service Selection for Mobile IPv6" [RFC5149]. This
procedure helps also for extranet connectivity. In this scenario,
the ULA is used within the ACME Corporation, nodes are made aware by
adding "acme.com" in the BRIO Service Selection Option.
It is for the reader to work out extensions for this scenario, where
the ULA prefix originator is a Border Router to another site, e.g. a
link from a branch office to a head quarter, or a ULA-only side
connected to the Internet with NAT66.
2.5. MANET site
BRDP was developed for address autoconfiguration in MANETs. This
scenario, see Figure 6 demonstrates the powerful multi-homing
facilities provided to the MANET nodes.
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/^^^^^^^^^^^^^^^^^^^^^^\
/ \
{ The Internet }
\ /
\______________________/
/ \
/(2001:8db:100::/40) \(2001:8db:200::/40)
+=======+ +=======+
[ ISP_1 ] [ ISP_2 ]
+=======+ +=======+
| |
|c=1 |c=1
|(2001:8db:101::/48) |(2001:8db:201::/48)
+--------+ +--------+
| BR_101 | | BR_201 |
+-+------+ +---+----+
| fe80::101/64 | fe80::201/64
| 2001:8db:101:1::101/128 | 2001:8db:201:1::201/128
/|\ /|\
: . c=5 . :
: . . . . . . . . . :c=1
:c=2 . :
: . . . . . . . . . . \|/
: . c=5 | 2001:8db:201:1::2/128
\|/ | 2001:8db:201:1::2/128
| 2001:8db:201:1::1/128 | fe80::2/64
| 2001:8db:101:1::1/128+--+--+
| fe80::102/64 . .| R_2 |
+--+--+ . . . . +-----+.
| R_1 |. . . . c=5 : . c=4
+-----+. . . . . . . :c=1 . . . . .
c=4 . : .
\|/ \|/
2001:8db:201:1::3/128 | 2001:8db:201:1::4/128 |
2001:8db:101:1::3/128 | 2001:8db:201:1::4/128 |
fe80::3/64 | fe80::4/64 |
+--+--+ +--+--+
| R_3 | . . . . . . . . .| R_4 |
+-----+ c=4 +-----+
Figure 6: Scenario 5: a MANET site
On the MANET interfaces, addresses are configured using a 64-bit
prefix provided by BRDP, appending it with a 64-bit Interface
Identifier according to BRDP based address autoconfiguration. This
creates a 128-bit prefix length as recommended in IP Addressing Model
in Ad Hoc Networks [RFC5889]. Each MANET node has configured two
global addresses, one for each ISP. With BRDP, the nodes are aware
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of the cost of the path to the DFZ, defined as dimensionless metric
for both directions of the patch. This enables optimized source
address selection, and as an implicit result a Border Router and ISP
selection. In the scenario, R_1 is near to BR_101 and the cost via
this Border Router is lower than via BR_201. The table below shows
costs to the DFZ for all nodes, via both ISPs. Paths with lowest
costs are marked with *.
+---------+-------+-------+
| Costs | Via | Via |
| to DFZ | ISP_1 | ISP_2 |
+---------+-------+-------+
| BR_101 | 1* | 7 |
| BR_201 | 7 | 1* |
| R_1 | 3* | 6 |
| R_2 | 6 | 2* |
| R_3 | 7 | 3* |
| R_4 | 10 | 6* |
+---------+-------+-------+
The optimized source address selection facility is also of utility in
the other scenarios. For example, the cost of the link to the ISP
could be set depending of bandwidth and optionally on utilization.
Nodes would use a near uplink to an ISP, and as a result some form of
load distribution is enabled. Note that nodes still can use the
alternative addresses, in fact it is recommended to use multi-path
transport protocols for better load balancing and improved
robustness.
For isolated MANETs, a DHCP server election mechanism can be used.
Nodes may initiate to advertise a self-generated ULA. In such cases,
it is recommended that a prefix is used with a 56-bit random ULA
identifier (including random 16-bit Subnet ID) and 64-bit prefix
length. Other nodes join this prefix, although some may wish to
start or continue using their own prefix. The latter would occur in
cases of a merge of previous isolated MANETs.
3. Border Router Discovery Protocol (BRDP)
BRDP is an extension to the IPv6 ND mechanism [RFC4861] that provides
information about the reachability, availability, prefix information,
quality and cost of upstream providers, and enables automated
(re)numbering of possibly multi-homed routers and hosts.
BRDP adds the Border Router Information Option (BRIO) to the Router
Advertisement (RA) of IPv6 ND. A BRIO contains all relevant
information of an upstream Border Router and the corresponding
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provider.
Border Routers initiate sending BRIO messages, other routers in the
network disseminate the messages downstream through the network.
Nodes store the information from received BRIOs in a BRIO cache, to
be used for address generation, DHCP server discovery, address
selection or packet forwarding.
A BRIO cache entry records reception of a BRIO for a single
advertised prefix, received via a neighbor router. Border Routers
that need to advertise multiple prefixes simply use multiple BRIOs,
each with its own address prefix. For further processing of BRIO
entries, only the entry with the lowest cost to a Border Router is
used, for each Border Router.
When a node is multi-homed, it will receive BRIOs from multiple
upstream Border Routers.
3.1. Border Router Information Option (BRIO)
The Border Router Information Option carries information that allows
a nodes in the edge network to select and utilize a Border Router.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Prefix Length |D| reserverd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number | Hopcount | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Uniform Path Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ ( Border Router prefix ) +
| |
+ Border Router Address +
| |
+ ( rest of Border Router Address ) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: BRIO base option
Fields:
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Type:
8-bit identifier of the Border Router Information Option type.
The value of this option identifier is to be determined.
Length:
8-bit unsigned integer. The length of the option (including the
type and length fields) in units of 8 octets. A BRIO has a length
value of 4.
Prefix Length:
8-bit unsigned integer. The number of leading bits in the Border
Router Address, that indicates the assigned prefix for that Border
Router. The Prefix Length is used for BRDP Based Routing, as
described in Section 6.
DHCP (D):
When the D-flag is set, the Border Router is acting as a DHCP
server or DHCP relay agent [RFC3315].
reserved:
Reserved bits. Currently unused, set to 0.
Sequence Number:
16-bit unsigned integer. It is set by the Border Router and
incremented with each new BRIO it sends on a link. When
forwarding downstream, the sequence number is not changed.
Hopcount:
8-bit field registering the number of hops from the advertizing
Router to the Border Router. Border Routers send the initial BRIO
with its Hopcount set to zero. Routers increment the Hopcount by
one when forwarding a BRIO.
Uniform Path Metric (UPM):
A measure for the cost of the bi-directional path between the
upstream Router and the Default Free Zone of the Internet.
Uniform Path Metric is set to some initial value by the Border
Router and is incremented by each Router forwarding the BRIO.
Border Router Address:
128-bit address of the Border Router. For reachability, the
Border Router is expected to add this own address (prefix) in the
routing system.
3.2. BRDP processing
The main BRDP processing functions of a Router are BRDP message
generation, transmission and reception and the maintenance of a BRIO-
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Cache. Routers forward BRDP messages using ICMP ND Router
Advertisements.
3.2.1. BRDP message generation and transmission
A BRDP message is part of a Router Advertisement and includes a set
of BRIOs. It provides the current state of (paths to) the Border
Routers listed in the set of BRIOs. BRIOs originate from a Border
Router, and contain initially metric information on connectivity to
the Internet. BRIOs are forwarded downstream in the edge network.
When a Router sends a ICMP ND Router Advertisement, it SHOULD include
a set of BRIOs by appending them to the message. The maximum number
of BRIOs in a single BRDP message is a Router configuration
parameter. BRIO selection for advertisement is done based on the
information stored in the BRIO-Cache. BRIOs that do not pass the
loop prevention check described in Section 3.2.4 SHOULD NOT be
selected.
The UPM and Hopcount fields of the advertised BRIOs are updated. An
UPM-increment, based on uniformed bi-directional link metrics, is
added to the UPM and the Hopcount is incremented by 1. UPM-increment
MAY be governed by a hysteresis and dampening mechanism. Also
forecasted information MAY be used.
Each BRIO originating from a Border Router has an increased Sequence
Number. This BRIO is forwarded in the edge network and refreshes
entries in BRIO-Caches of downstream Routers.
Router Advertisements are sent in response to Router Solicitation
messages or unsolicited with a uniformly-distributed random interval
between MinRtrAdvInterval and MaxRtrAdvInterval [RFC4861]. The
MaxRtrAdvInterval falls between a minimum of 30 milliseconds,
specified in [RFC6275] and a maximum of 1800 seconds, specified in
[RFC4861]. In addition, the Router MAY send a Router Advertisement
when an important change in a to be sent BRIO would occur.
When a Router sends Router Advertisements more frequently than an
upstream Router, this Router MAY repeatedly send BRIOs with a
constant Sequence Number but with an updated UPM or Hopcount.
The ICMP ND Router Advertisement MAY include the Advertisement
Interval Option [RFC6275]. This option contains the interval at
which the sending router sends unsolicited multicast Router
Advertisements.
A Router SHOULD inform downstream Routers in case the path to a
previous advertised Border Router is lost, by at least 3 times
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retransmitting the previously sent BRIO with a UPM value of
4294967295.
In case a Border Router loses its connection to the infrastructure it
will lose its Border Router functionality and become a normal Router.
In that case it performs the same procedure as a Router that has lost
the path to a previous advertised Border Router.
For each Border Router listed in the BRIO-Cache, the UPM-loop-
prevention-threshold and the Hopcount-loop-prevention-threshold
variables are maintained. These variables are used by the loop
prevention mechanism described in Section 3.2.4. The thresholds are
set or updated when sending BRDP messages. When sending a BRIO with
a higher Sequence Number than the previously sent BRIO for that
Border Router, the threshold variables are set to the UPM and
Hopcount values in BRIO to be sent. When sending a BRIO with the
same Sequence Number as the previously sent BRIO, the loop-
prevention-thresholds are independently updated if either the UPM or
Hopcount of the outgoing BRIO is lower than their thresholds.
A Router that detects an attractive candidate BRIO but is prohibited
from using it because of the loop prevention check, MAY send a
(unicast) Router Solicitation message to the Border Router. The
Border Router responds to such a Router Solicitation message with a
new BRIO. Sending Router Solicitations MUST be rate limited. A next
version of this document would include a specification for sending
the unicast Router Solicitation message.
3.2.2. BRDP message reception
When a BRDP message is received, the Sequence Number fields of the
contained BRIOs are checked; the Sequence Number of a received BRIO
MUST be equal to or higher than the Sequence Number in the cache for
an existing entry in the cache, with wrap-around checking.
Otherwise, the BRIO will be discarded.
BRIO messages do not need to be forwarded at fixed time intervals,
because the RA intervals on different Routers are not synchronized.
Therefore, large gaps in Sequence Numbers may occur. Increment
values between 0 and 65000 are accepted. Increment values between
65001 and 65535 are rejected.
Information in received BRIOs is stored in a BRIO-Cache table. Other
information is stored as well, such as the BRIO upstream node, a
timestamp indicating when the most recent message was received and
the measured or signaled RA interval.
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3.2.3. BRIO-Cache maintenance
Each Router maintains a BRIO-Cache that stores all information on
Border Routers. Unique cache entries are maintained on (Border
Router Address, address of the upstream router that forwarded the
BRIO) tuples. This information is obtained by receiving BRIOs, or,
in case of a Border Router, by getting information from the interface
that connects to the Internet. The BRIO-Cache also maintains context
information for the BRIO such as the BRIO sender, link metrics and
UPM-increment for this sender, history, statistics and status
information. History information includes a timestamp indicating
when the most recent message was received and a measured or signaled
RA interval. Status information includes the BRIO selection outcome
for BRIO forwarding as explained in Section 3.2.1 and the Border
Router selected for address generation as explained in Section 4.
BRIO entries in the BRIO-Cache stay valid for a certain period of
time. During this period, they can be used for Border Router
selection by the Router, for forwarding BRIOs and for address
generation. BRIO-Cache information could also be useful for source
address selection [RFC6724]. The lifetime of a BRIO is determined by
using the timing information sent along with the RA ([RFC6275],
section 7.3) or statistics of received BRIOs.
Some values in the BRIO-Cache can be updated independent of incoming
BRDP messages. A Router MAY update the UPM-increment based on link
quality measurements performed in an environment with changing link
metrics. A Router SHOULD indicate in its BRIO-Cache which BRIO
entries are currently selected for forwarding and for address
generation. Border Router Selection MAY take place after the UPM of
a BRIO entry has been updated.
In case the link to the Router from which a BRIO has been received is
broken, the UPM and the Hopcount of the BRIO entry in the cache are
set to the maximum value, i.e. 4294967295 and 255.
A cache cleanup routine SHOULD run at regular intervals to get rid of
stale entries. Stale entries are removed when the entry is not
updated for 5400 seconds or all of the following conditions are met:
o The stale entry is not used by the Router itself for address
generation.
o The stale entry was not selected for forwarding in the last three
Router Advertisement.
o The stale entry was not recently updated by a received BRIO. In
this context, recently is defined as the maximum of a) three times
its own unsolicited multicast Router Advertisements interval and
b) three times the senders unsolicited multicast Router
Advertisements interval.
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Cache entries MAY also be removed, under the condition that the BRIO-
Cache has reached a configured maximum number of entries and a new,
to be stored BRIO is received. A removal candidate is selected based
on:
o The candidate entry is not used by the Router itself.
o The candidate entry was not selected for forwarding in the last
Router Advertisement.
o The candidate entry is redundant; other information for the same
Border Router is stored in the cache with a better UPM and / or
was received more recently.
o The candidate entry is redundant; other information for the same
Service Selection Identifier is stored in the cache with a better
UPM and / or was received more recently.
o The candidate entry is less attractive; other Border Routers are
stored in the cache with better UPM and / or were received more
recently.
3.2.4. BRDP loop prevention
A BRDP loop check mechanism prevents that a Router forwards an
earlier advertized BRIO.
BRDP loop-free operation is guaranteed as long as at least one of the
following conditions is true:
o The to be sent BRIO has a higher Sequence Number than a BRIO for
this Border Router that was sent before. The loop check mechanism
uses wrap-around logic. Increments up to 32768 are acceptable
(wrap-around logic needs checking).
o The to be sent BRIO is generated from the same BRIO-Cache entry as
the BRIO that was sent most recently.
o The to be sent BRIO has the same Sequence Number as the BRIO for
this Border Router that was sent before but the BRIO-Cache entry
UPM is equal to or lower than the UPM-loop-prevention-threshold
for this Border Router.
o The to be sent BRIO has the same Sequence Number as the BRIO for
this Border Router that was sent before but the BRIO-Cache entry
Hopcount is equal to or lower than the Hopcount-loop-prevention-
threshold for this Border Router.
In some circumstances, a Router would select a BRIO for forwarding
that fails the loop prevention check. For example, the link to the
upstream neighbor is lost and an alternative path is available, with
a higher UPM and a higher Hopcount or with a lower Sequence Number.
The Router cannot assure this candidate BRIO is not reflecting its
own advertized message, therefore it should not send this BRIO.
Instead, it sends a unicast Router Solicitation message to that
Border Router.
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3.3. Unified Path Metric (UPM)
Unified Path Metric (UPM) is a measure for the cost of the path
between the Router and the Internet Default Free Zone. It is a
united metric for both inbound and outbound paths. On each hop, the
UPM is incremented with an UPM-increment, which is derived from the
routing protocol and / or is obtained from lower layers.
It is on forehand not known what is more important; Border Router
selection based on path metric to the Border Router or the path
metric for the reverse path. In BRDP, UPM is used for optimizing
Border Router selection for both the inbound and the outbound
traffic. Note that actual traffic will use the path provided by the
routing protocols, not by BRDP.
Since the UPM uses 32 bits, its maximum value is 4294967295. On each
hop, an UPM-increment is calculated for each Router from which a BRIO
has been received. UPM-increments have a value between 1 and
16777215, to support a 255 hop path, with maximum UPM increments.
Further discussion on metrics and how the UPM-increment value is
determined is outside the scope of this document.
4. BRDP based Address Configuration and Prefix Delegation
BRDP supports stateless address autoconfiguration [RFC4862], DHCP
managed IP configuration [RFC3315] and DHCP Prefix Delegation
[RFC3633]. Routers can also use a variant of stateless address
autoconfiguration, where BRDP provided information is used to
configure Router management interfaces or used to configure off-link
addresses, used in ad hoc networks [RFC5889].
BRDP adds topology awareness in address configuration. Nodes can
configure multiple addresses, each to support a different facility.
ULAs can be used for site internal traffic. Global addresses are
mandatory for access to the Internet, assuming address translation is
not used.
A node that is offered multiple prefixes for stateless address
autoconfiguration or multiple addresses by DHCP chooses to configure
one or more addresses. BRDP provides information for the candidate
addresses. An important criterium is the costs of the path to the
Internet DFZ. A node would prefer addresses with lower costs.
BRDP does not modify stateless address autoconfiguration and DHCP
protocols, except that in a edge network, Routers may perform
stateless address autoconfiguration from the Border Router
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Information Option (BRIO), for their management or MANET interfaces.
This enables edge network-wide address configuration, because BRIOs
are disseminated over multiple hops in the edge network, while PIOs
are link local messages only.
When a BRIO is stored in the BRIO cache table, the node checks if a
corresponding address already exists for the Border Router from which
this BRIO originates. If not, and a corresponding address for that
Border Router is beneficial, address generation for that Border
Router is triggered.
4.1. Border Router selection
When a node needs to communicate to nodes on the Internet, it MUST
select a (set of) Border Router(s) for address generation. A node
MAY generate multiple addresses for smooth handover implementing
make-before-break or distributing traffic over multiple Border
Routers. A description how Border Routers can be used concurrently
is out-of-scope for this document.
Information concerning available Border Routers is kept in the BRIO-
Cache.
The Border Router selection mechanism MAY be triggered by received
BRDP messages, changes in metrics on links to neighbors advertising
BRDP messages, changes in costs to Border Routers used or on a time-
driven basis.
The Border Router selection algorithm SHOULD be based on UPM. UPM is
used for selecting the Border Router with the best connectivity to
the Internet. The Border Router selection algorithm MAY be extended
with any other information. Future defined BRIO suboptions could
provide additional information, such authorization and service
selection. Border Router selection MAY be based on the type of the
Border Router Address, e.g. a globally unique address or a unique
local address.
Border Router selection does not provide nor select a routing path to
the Border Router.
4.1.1. Border Router Selection based on UPM
The node uses the UPM for Border Router selection preferring the best
bi-directional paths between the node and the Internet. Note that
the BRIO UPM includes the initial metric set by the Border Router and
is not solely a metric between the node and the Border Router. The
initial metric set by Border Routers can be used for Border Router
preference and for load balancing.
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In order to use an up-to-date UPM in the selection procedure the UPM-
increment is calculated by the node before selecting a Border Router.
UPM is discussed in Section 3.3.
4.2. Address autoconfiguration
Nodes should use a topologically correct address when communicating
with corresponding nodes on the Internet. Topologically correct
addresses should be configured for each Border Router used.
4.2.1. Address and prefix configuration with SLAAC or DHCP
Nodes can use existing IPv6 address configuration protocols, such as
SLAAC [RFC4862] and DHCP [RFC3315]. Nodes can use SLAAC based on
prefix information, provided by the upstream router. Nodes may use
DHCP multicast and neighbor routers will relay those packets to
selected Border Routers with D-flag set or reply with DHCP parameters
it has received from a Border Router before for itself.
Nodes using SLAAC may also query a DHCP server on a Border Router
themselves for additional parameters, using the BRDP learned address
of the DHCP server.
A Router should request a prefix for attached subnetworks, with
DHCP-PD [RFC3633], where there is at that moment no on-link prefix
for a selected Border Router.
4.2.2. Address generation and configuration for Routers
A generated address for a Router management interface or a MANET has
a /128 prefix. It is constructed from a 64-bit Interface Identifier
and a 64-bit prefix from the Border Router Address. The generated
128-bit address SHOULD be advertised in the routing system. The
generated address may be used for user traffic, either inside the
edge network or traffic towards the Internet.
For the Interface Identifier used, the BRDP-based Address Generation
MUST implement a mechanism for generating a highly unique Interface
Identifier. Known mechanisms are:
o Modified EUI-64 format-based Interface Identifier, [RFC4291],
based on IEEE 802 48-bit MAC address or IEEE EUI-64 identifier.
However, this method does not guarantee identifiers are unique as
duplicate MAC addresses can occur.
o Generation of randomized Interface Identifiers, [RFC4941].
o Well-distributed hash function, [RFC3972].
After Address Generation, RFC4429 Optimistic Duplicate Address
Detection [RFC4429] should be used. A passive Duplicate Address
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Detection, based on information in the routing protocol information
bases could be used as an alternative. Still, uniqueness is not
fully guaranteed. Main reasons for non-uniqueness are merging of
edge network segments, node movement, node misbehavior or address
spoofing attacks. Details on handling a duplicate address condition
are out-of-scope for this document.
A generated addresses clean-up routine SHOULD run at regular
intervals to get rid of stale addresses.
4.2.3. Support for Unique Local Addresses (ULA)
Address generation for globally unique addresses and unique local
addresses (ULA) [RFC4193] is equivalent. If no BRIO for a unique
local addresses is available, a router may start as a Border Router
and DHCP server for a self generated 48-bit ULA prefix.
5. BRDP based Source Address Selection
As a next step, multi-homed nodes perform source address selection
for new, self-initiated connections. The algorithm described in
Default Address Selection for IPv6 [RFC6724] uses the concept of a
"candidate set" of potential source addresses. Rule 8 of source
address selection is "Uses longest match prefix". The goal of this
rule is to select the address with good communications performance.
If other means of choosing among source addresses for better
performance is available, that should be used.
BRDP provides attributes for prefix, such as a cost metric to the
Internet. This information van be used to select the "best" source
address. For multi-path transport protocols, it is also important to
have a mechanism to select alternative addresses. For example, rule
4 gives preference to a Home Address. Alternate addresses can be
used for route optimization and to avoid overhead of the Mobile IP
tunnel.
5.1. Address Selection for dynamic DNS
BRDP provided information can also be utilized by address lookup
protocols such as DNS. A node can register its addresses
dynamically, with support of preference and load balancing if the
mechanism used support such.
6. BRDP based Routing
BRDP introduces a new paradigm for packet forwarding for multi-homed
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sites, where forwarding to a default gateway is replaced by source
address based forwarding towards a corresponding Border Router. This
enforces that packets will be sent via the selected upstream
provider, without the need of tunneling. As such, it prevents
problems with ingress filters in multi-homed edge networks [RFC3704].
The BRDP Based Routing mechanism provides basic support for load
distribution over multiple Border Routers. BRDP Based Routing
forwards the packets to the Border Router that corresponds with the
source address. As a result, nodes can utilize multiple paths, if
available. Standardization of this load balancing functionality is
work in progress in the IETF MPTCP working group.
When a router forwards a packet to a next-hop node, via the interface
where this packet was received, and the next-hop address was selected
using BRDP based routing, then the router should not send an ICMP
redirect message to that host. This is because the upstream node
would cache the redirect for the destination address, while the
forwarding decision was based on the source address.
6.1. Problems with default gateway routing
Usually, the nexthop selection is based on the destination address.
In case of default gateway routing and multiple exit routers to
multiple providers, the source has no influence on what exit router
is used. In case of ingress filtering and lack of a mechanism to
redirect packets to exit routers that correspond to the source
address, packets may be dropped.
This default gateway routing behavior blocks incremental enhancement
of the Internet, e.g. through the addition of support for more
dynamic networks and / or host based load distribution mechanisms.
In a MANET, it also also prevents the use of make-before-break
[RFC3753] mechanisms.
6.2. Default gateway routing replaced with BRDP Based Routing
Default gateway based routing for IPv4 is defined in [RFC1812],
section 5.2.4.3:
(5) Default Route: This is a route to all networks for which there
are no explicit routes. It is by definition the route whose
prefix length is zero.
With BRDP Based Routing, another type of route is introduced:
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(6) BRDP Route: This is a route to all networks for which there
are no explicit routes, and a default route is not used.
The nexthop IP address is found by means of a Border Router
Information Cache (BRIO-Cache) lookup based on the source
address and, if a matching BRIO-Cache entry is found, a
subsequent FIB lookup based on the selected Border Router
address.
Note that route types (3) and (4) are not defined in RFC1812.
BRDP Based Routing can be turned on and off with the existence of a
default route in the IGP. This switch function might be useful in
migration scenarios towards BRDP Based Routing.
The Border Router should run the IGP on the interface with the BRDP
advertized Border Router address, to make sure this address is
reachable in the edge network.
In the edge network, all interior routers should run BRDP and BRDP
Based Routing. All interior routers will have a BRIO-Cache with
information for selecting Border Routers as exit points to the
Internet. A BRIO-Cache entry contains a Border Router address and a
summary prefix assigned to that Border Router. BRIO-Cache lookup
follows the longest-match rule.
Forwarding is solely based on FIB lookups, the nexthop IP address is
found either by a FIB lookup with the destination address or by a FIB
lookup with the address of the Border Router that corresponds with
the source address. If the nexthop IP address lookup fails, the
packet is discarded.
7. BRDP and IRTF RRG goals
The IRTF Routing Research Group (RRG) was chartered to explore
solutions for problems on routing and addressing, when the Internet
continues to evolve. It has explored a number of proposed solutions,
but did not reach consensus on a single, best approach
[I-D.irtf-rrg-recommendation]. In fulfillment of the routing
research group's charter, the co-chairs recommend that the IETF
pursue work in three areas, "Evolution" [I-D.zhang-evolution],
"Identifier/Locator Network Protocol (ILNP) [I-D.rja-ilnp-intro] and
"Renumbering" [RFC5887] . BRDP fits in all three approaches.
BRDP is an evolution in IPv6 address configuration and address
selection, as well as forwarding to destinations outside the routing
domain. As a result, it removes a demand for Provider Independent
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addresses for (small) multi-homed edge networks. BRDP enables sites
to use multiple Provider Aggregatable address blocks, while being
able to utilize multi-homing for improved redundancy of
communications and enlarged capacity. Each site that continues to
use Provider Aggregatable addresses when getting multi-homed, instead
of using its own Provider Independent address space, reduces the
growth of the routing tables in the Default Free Zone.
BRDP can cooperate or live next many other solutions. ILNP is a good
example for cooperation, BRDP provides multi-path transport
capabilities to ILNP nodes. This multi-path transport capability
applies to many other approaches also, such as map&encap and nat66.
Because BRDP provides automatic address and prefix configuration,
Renumbering is far less problematic. That said, legacy (IPv4) hosts,
applications and network equipment is not BRDP enabled and manual
address configuration will be used for many years to come.
In Design Goals for Scalable Internet Routing
[I-D.irtf-rrg-design-goals], a number of design goals are defined.
The role BRDP can play for these goals are briefly described in the
next sections.
7.1. Scalability
Because BRDP is implemented in edge networks, and not in the core,
scalability of BRDP is less an issue. BRDP solves the Internet
routing problem at the source, by reducing the demand for PI
addresses.
7.2. Traffic engineering
BRDP provides traffic engineering options to end-nodes. End-nodes
can configure multiple addresses and use these for utilizing multi-
path capabilities of the network. Using multi-path is being worked
on by the IETF MPTCP working group.
7.3. Multi-homing
The core function of BRDP is providing support for IPv6 multi-homing,
without any problems caused by ingress filtering [RFC3704].
7.4. Loc/id separation
BRDP does not mandate any approach for location / identification.
For packet forwarding, addresses are used as locator. If addresses
are used as identifiers also, for example in Mobile IP, BRDP supports
route optimization where traffic uses the Home Address as identifier
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and care-of addresses as locator. MPTCP provides the route
optimization capability.
7.5. Mobility
BRDP was defined as a solution for address autoconfiguration for ad
hoc networks. With BRDP, nodes can easily configure topology correct
addresses in a multi-homes ad hoc network. BRDP does not provide
session continuity functions. Mobility solutions are already in
place or new approaches are proposed. All approaches should work
well with BRDP, as BRDP does not modify the IPv6 protocol.
7.6. Simplified renumbering
BRDP makes site renumbering fully automatic. This applies to node
address configuration on the IPv6 stack and prefix delegation and
configuration on routers. IP addresses could be configured on many
other places, either manually or using specific protocols for such
purpose. Complete automatic numbering is possible if all mechanisms
in use support dynamic addresses. There is definitely more work to
do [RFC5887].
7.7. Modularity
BRDP is a small, but important piece of the puzzle. It applies to
edge networks only. It helps other mechanisms to work well in a
multi-homed network using PA addresses, but also provides multi-path
capabilities in multi-homed networks with PI addresses or multi-
homing with connections to Extranets.
7.8. Routing quality
BRDP is not a routing protocol, so it has no influence on routing
quality. But the functionality of routing to a default gateway is
changed. BRDP based routing supports paths to multiple Border
Routers, where hosts can select which Border Router to use. In such
scheme, nodes can select the route to use, based on quality of
available routes. MPTCP provides this route selection functionality.
7.9. Routing security
BRDP doesn't update any routing protocols. BRDBP based routing
modifies the default gateway heuristic, the route to prefix ::/0 is
replaced by a route to a Border Router, which corresponds with the
source address of a packet. As a result, ingress filtering is
distributed over all routers in the edge network and invalid packets
are dropped as near to the source as possible.
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The BRDP protocol runs on IPv6 NDP and inherits all security aspects.
BRDP messages are disseminated in the edge network, which may enlarge
the needs for protection. Implementing SeND [RFC3971] is
recommended.
7.10. Deployability
BRDP deployment takes place edge network by edge network. Each
network that migrates to BRDP, instead of getting a PI address bock,
reduces the load on the Internet routing infrastructure.
For implementing BRDP on an edge network, all routers in the network
must support BRDP. BRDP support for hosts is optional. Enterprise
networks can migrate site by site.
8. Currently unaddressed issues
BRDP based routing may have impact on multicast routing, e.g.
selecting the route to a RP.
It is not fully understood how BRDP may influence host behavior on RA
M and O bits, and may bypass a 1-hop router DHCP relay server for
getting information for a BRDP-learned DHCP server.
Currently unaddressed issues are to be addressed in a next version of
this document.
9. Acknowledgements
BRDP is inspired by MANEMO technology; thanks to all who contributed
to it. Thanks to Arjen Holtzer (TNO), co-author of earlier Internet
drafts on BRDP. Thanks to Ran Atkinson, who guided me towards a BRDP
Based Routing mechanism that does not rely on routing headers or
encapsulation.
10. IANA Considerations
TBD
11. Security Considerations
TBD
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12. Change log
This -00 version is gathering the material of BRDP, produced for
Autoconf and RRG. It is a bit cleaned up, with removal of some
details for MANET and with removal of options for emergency services,
service selection and authorization.
13. References
13.1. Normative References
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3753] Manner, J. and M. Kojo, "Mobility Related Terminology",
RFC 3753, June 2004.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD)
for IPv6", RFC 4429, April 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[RFC6724] Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, September 2012.
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13.2. Informative References
[I-D.ietf-homenet-arch]
Chown, T., Arkko, J., Brandt, A., Troan, O., and J. Weil,
"Home Networking Architecture for IPv6",
draft-ietf-homenet-arch-04 (work in progress), July 2012.
[I-D.ietf-mptcp-multiaddressed]
Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", draft-ietf-mptcp-multiaddressed-10 (work in
progress), October 2012.
[I-D.irtf-rrg-design-goals]
Li, T., "Design Goals for Scalable Internet Routing",
draft-irtf-rrg-design-goals-06 (work in progress),
January 2011.
[I-D.irtf-rrg-recommendation]
Li, T., "Recommendation for a Routing Architecture",
draft-irtf-rrg-recommendation-16 (work in progress),
November 2010.
[I-D.kline-default-perimeter]
Kline, E., "Default Perimeter Identification",
draft-kline-default-perimeter-00 (work in progress),
July 2012.
[I-D.rja-ilnp-intro]
Atkinson, R., "ILNP Concept of Operations",
draft-rja-ilnp-intro-11 (work in progress), July 2011.
[I-D.zhang-evolution]
Zhang, B. and L. Zhang, "Evolution Towards Global Routing
Scalability", draft-zhang-evolution-02 (work in progress),
October 2009.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
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[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC5149] Korhonen, J., Nilsson, U., and V. Devarapalli, "Service
Selection for Mobile IPv6", RFC 5149, February 2008.
[RFC5887] Carpenter, B., Atkinson, R., and H. Flinck, "Renumbering
Still Needs Work", RFC 5887, May 2010.
[RFC5889] Baccelli, E. and M. Townsley, "IP Addressing Model in Ad
Hoc Networks", RFC 5889, September 2010.
[RFC6275] Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
in IPv6", RFC 6275, July 2011.
Author's Address
Teco Boot
Infinity Networks B.V.
Email: teco@inf-net.nl
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