wdiff draft-ietf-v6ops-design-choices-01.txt draft-ietf-v6ops-design-choices-02.txt

V6OPS Working Group P. Matthews
Internet-Draft Alcatel-Lucent
Intended status: Informational V. Kuarsingh
Expires: September 7, 2014 Dyn March 6, 8, 2015 Dyn
September 4, 2014

Design Choices for IPv6 Networks
draft-ietf-v6ops-design-choices-01
draft-ietf-v6ops-design-choices-02

Abstract

This document presents advice on the design choices that arise when
designing IPv6 networks (both dual-stack and IPv6-only). The
intended audience is someone designing an IPv6 network who is
knowledgeable about best current practices around IPv4 network
design, and wishes to learn the corresponding practices for IPv6.

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 September 7, 2014. March 8, 2015.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Design Choices . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Links . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.1. Mix IPv4 and IPv6 on the Same Link? . . . . . . . . . . . 3
2.2.
2.1.2. Links with Only Link-Local Addresses? . . . . . . . . 4
2.2. Static Routes . . . 4
2.3. . . . . . . . . . . . . . . . . . . . 6
2.2.1. Link-Local Next-Hop in a Static Route? . . . . . . . 6
2.3. IGPs . . . . 5
2.4. Separate or Combined eBGP Sessions? . . . . . . . . . . . 6
2.5. eBGP Endpoints: Global or Link-Local Addresses? . . . . . 7
2.6. . . . . . . 6
2.3.1. IGP Choice . . . . . . . . . . . . . . . . . . . . . 6
2.4. BGP . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4.1. BGP Sessions for Unlabeled Routes . . . . . . . . . . 10
2.4.2. BGP sessions for Labeled or VPN Routes . . . . . . . 11
2.4.3. eBGP Endpoints: Global or Link-Local Addresses? . . . 11
3. General Observations . . . . . . . . . . . . . . . . . . . . 9 12
3.1. Use of Link-Local Addresses . . . . . . . . . . . . . . . 9 12
3.2. Separation of IPv4 and IPv6 . . . . . . . . . . . . . . . 10 13
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 14
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 14
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 14
7. Informative References . . . . . . . . . . . . . . . . . . . 11 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 16

1. Introduction

The focus of the document is on design choices where there are
differences between IPv4 and IPv6, either in the range of possible
alternatives (e.g. the extra possibilities introduced by link-local
addresses in IPv6) or the recommended alternative. The document
presents the alternatives and discusses the pros and cons in detail.
Where consensus currently exists around the best practice, this is
documented; otherwise the document simply summarizes the current
state of the discussion. Thus this document serves to both to
document the reasoning behind best current practices for IPv6, and to
allow a designer to make an intelligent choice where no such
consensus exists.

This document does not present advice on strategies for adding IPv6
to a network, nor does it discuss transition mechanisms. For advice
in these areas, see [RFC6180] for general advice, [RFC6782] for
wireline service providers, [RFC6342] for mobile network providers,
[RFC5963] for exchange point operators, [RFC6883] for content
providers, and both [RFC4852] and
[I-D.ietf-v6ops-enterprise-incremental-ipv6] for enterprises. Nor
does the document cover the ins and outs of creating an IPv6
addressing plan; for advice in this area, see [RFC5375].

This document focuses on unicast network design only. It does not
cover multicast, nor supporting infrastructure such as DNS.

The current version is still work in progress, and it is expected
that the presentation and discussion of additional design choices
will be added as the document matures.

2. Design Choices

This section consists of a list of specific design choices a network
designer faces when designing an IPv6-only or dual-stack network,
along with guidance and advice to the designer when making a choice.

2.1. Links

2.1.1. Mix IPv4 and IPv6 on the Same Link?

Should IPv4 and IPv6 traffic be logically separated on a link? That
is:

a. Mix IPv4 and IPv6 traffic on the same layer 2 connection, OR

b. Separate IPv4 and IPv6 by using separate physical or logical
links (e.g., two physical links or two VLANs on the same link)?

Option (a) implies a single layer 3 interface at each end with both
IPv4 and IPv6 addresses; while option (b) implies two layer 3
interfaces, one for IPv4 addresses and one with IPv6 addresses.

The advantages of option (a) include:

o Requires only half as many layer 3 interfaces as option (b), thus
providing better scaling;

o May require fewer physical ports, thus saving money;
o Can make the QoS implementation much easier (for example, rate-
limiting the combined IPv4 and IPv6 traffic to or from a
customer);

o Provides better support for the expected future of increasing Works well in practice, as any increase in IPv6 traffic and decreasing is usually
counter-balanced by a corresponding decrease in IPv4 traffic; traffic to or
from the same host (ignoring the common pattern of an overall
increase in Internet usage);

o And is generally conceptually simpler.

For these reasons, there is a pretty strong consensus in the operator
community that option (a) is the preferred way to go. Most networks
today use option (a) wherever possible.

However, there can be times when option (b) is the pragmatic choice.
Most commonly, option (b) is used to work around limitations in
network equipment. One big example is the generally poor level of
support today for individual statistics on IPv4 traffic vs IPv6
traffic when option (a) is used. Other, device-specific, limitations
exist as well. It is expected that these limitations will go away as
support for IPv6 matures, making option (b) less and less attractive
until the day that IPv4 is finally turned off.

Most networks today use option (a) wherever possible.

2.2.

2.1.2. Links with Only Link-Local Addresses?

Should the link:

a. Use only link-local addresses ("unnumbered"), OR

b. Have global or unique-local addresses assigned in addition to
link-locals?

There are two advantages of unnumbered links. The first advantage is
ease of configuration. In a network with a large number of
unnumbered links, the operator can just enable an IGP on each router,
without going through the tedious process of assigning and tracking
the addresses for each link. The second advantage is security.
Since link-local addresses are unroutable, the associated interfaces
cannot be attacked from an off-link device. This implies less effort
around maintaining security ACLs.

Countering this advantage are various disadvantages to unnumbered
links in IPv6:

o It is not possible to ping an interface that has only a link-local
address from a device that is not directly attached to the link.
Thus, to troubleshoot, one must typically log into a device that
is directly attached to the device in question, and execute the
ping from there.

o A traceroute passing over the unnumbered link will return the
loopback or system address of the router, rather than the address
of the interface itself.

o On some devices, by default the link-layer address In cases of the
interface is derived from the MAC address assigned parallel point to interface.
When this point links it is done, swapping out the interface hardware (e.g.

interface card) will cause difficult to
determine which of the link-layer address parallel links was taken when attempting to change. In
some cases
troubleshoot unless one sends packets directly between the two
attached link-locals on the specific interfaces. Since many
network problems behave differently for traffic to/from a router
than for traffic through the router(s) in question, this can pose
a significant hurdle to some troubleshooting scenarios.

o On some devices, by default the link-layer address of the
interface is derived from the MAC address assigned to interface.
When this is done, swapping out the interface hardware (e.g.
interface card) will cause the link-layer address to change. In
some cases (peering config, ACLs, etc) this may require additional
changes. However, many devices allow the link-layer address of an
interface to be explicitly configured, which avoids this issue.

o The practice of naming router interfaces using DNS names is
difficult-to-impossible
difficult and not recommended when using LLAs link-locals only. More
generally, it is not recommended to put link-local addresses into
DNS; see [RFC4472].

o It is not often not possible to identify the interface or link (in
a database, email, etc) by just giving just its address. address without also
specifying the link in some manner.

It should be noted that it is quite possible for the same link-local
address to be assigned to multiple interfaces. This can happen
because the MAC address is duplicated (due to manufacturing process
defaults or the use of virtualization), because a device deliberately
re-uses automatically-assigned link-local addresses on different
links, or because an operator manually assigns the same easy-to-type
link-local address to multiple interfaces. All these are allowed in
IPv6 as long as the addresses are used on different links.

For more discussion on the pros and cons, see
[I-D.ietf-opsec-lla-only].

Today, most operators use numbered links (option b).

2.3.

2.2. Static Routes

2.2.1. Link-Local Next-Hop in a Static Route?

What form of next-hop address should one use in a static route?

a. Use the far-end's link-local address as the next-hop address, OR

b. Use the far-end's GUA/ULA address as the next-hop address?

Recall that the IPv6 specs for OSPF [RFC5340] and ISIS [RFC5308]
dictate that they always use link-locals for next-hop addresses. For
static routes, [RFC4861] section 8 says:

A router MUST be able to determine the link-local address for each
of its neighboring routers in order to ensure that the target
address in a Redirect message identifies the neighbor router by
its link-local address. For static routing, this requirement
implies that the next-hop router's address should be specified
using the link-local address of the router.

This implies that using a GUA or ULA as the next hop will prevent a
router from sending Redirect messages for packets that "hit" this
static route. All this argues for using a link-local as the next-hop
address in a static route.

However, there are two cases where using a link-local address as the
next-hop clearly does not work. One is when the static route is an
indirect (or multi-hop) static route. The second is when the static
route is redistributed into another routing protocol. In these
cases, the above text from RFC 4861 notwithstanding, either a GUA or
ULA must be used.

Furthermore, many network operators are concerned about the
dependency of the default link-local address on an underlying MAC
address, as described in the previous section.

Today most operators use GUAs as next-hop addresses.

2.4. Separate or Combined eBGP Sessions?

For a dual-stack peering connection where eBGP is used as

2.3. IGPs

2.3.1. IGP Choice

One of the routing
protocol, then one can either:

a. Use one BGP session to carry both IPv4 and IPv6 routes, OR

b. Use two BGP sessions, a session over IPv4 carrying IPv4 routes
and a session over IPv6 carrying IPv6 routes.

The main advantage of (a) decisions for an IPv6 implementor is a reduction in the number choice of BGP
sessions compared with (b).

However, there
IGP (Interior Gateway Protocol) within the network. The primary
choices are a number the IETF protocols of concerns with option (a):

o On most existing implementations, adding or removing an address
family RIP [RFC2080], OSPF [RFC2328]
[RFC5340] and IS-IS [RFC5120] [RFC5308], though some operators may
consider non-IETF protocols. Here we limit our discussion to an established BGP session will cause the router to tear
down
pros and re-establish cons of OSPF vs. IS-IS.

Considering just OSPF vs. IS-IS, the session. Thus adding discussion in this section
revolves around the options in the table below:

+--------+--------+---------+---------+------------+----------------+
| Option | IGP | IGP for | Known | Hard | Similar |
| | for | IPv6 family | to
an existing session carrying just work | separation | configuration |
| | IPv4 routes will disrupt the
session, and the eventual removal | | well | | possible |
+--------+--------+---------+---------+------------+----------------+
| | | | | | |
+--------+--------+---------+---------+------------+----------------+
| a | IS-IS | IS-IS | YES | - | YES |
+--------+--------+---------+---------+------------+----------------+
| b | IS-IS | OSPFv3 | - | YES | - |
+--------+--------+---------+---------+------------+----------------+
| | | | | | |
+--------+--------+---------+---------+------------+----------------+
| c | OSPFv2 | IS-IS | YES | YES | - |
+--------+--------+---------+---------+------------+----------------+
| d | OSPFv2 | OSPFv3 | YES | YES | YES |
+--------+--------+---------+---------+------------+----------------+
| | | | | | |
+--------+--------+---------+---------+------------+----------------+
| e | OSPFv3 | IS-IS | - | YES | - |
+--------+--------+---------+---------+------------+----------------+
| f | OSPFv3 | OSPFv3 | - | - | YES |
+--------+--------+---------+---------+------------+----------------+

Three of IPv4 from the dual IPv4/IPv6
session will also disrupt the session. This disruption problem
will persist until something similar options above are marked as "Known to [I-D.ietf-idr-dynamic-cap]
or [I-D.ietf-idr-bgp-multisession] work well". These
options have seen significant deployments and are generally
considered to be good choices. The other options represent valid
choices, but have not seen widespread use, so it is widely deployed.

o Whatever selection you make for the underlying transport protocol
(v4 or v6) will likely look funny at some date. Using two
sessions hard to offer
comments on how well they work. In particular, options (e) and (f)
use OSPFv3 to route IPv4 [RFC5838], which is appropriate both now still rather new and in the future.

o Carrying (for example)
untested.

A number of options are marked "Gives hard separation". These
options use a different IGP for IPv4 vs IPv6. With these options, a
problem with routing IPv6 routes over is unlikely to affect IPv4 or visa-versa.

Three options are marked "Similar configuration possible". This
means that route
information it is transported over a different transport plane than possible (but not required) to use very similar IGP
configuration for IPv4 and IPv6: for example, the data packets themselves. same area
boundaries, area numbering, link costing, etc. If v6 connectivity goes down locally
without v4 also going down, you are happy with
your IPv4 IGP design, then v6 routes this will still likely be
exchanged, thus leading to a blackhole.

o In some implementations, carrying v4 routes in a BGP session over
v6 transport (or vica-versa) results in the BGP next-hops in the
wrong address family, which must be fixed using routing policy
before the routes can be used.

Given these disadvantages, option (b) is the better choice in most
situations, and this is consideration. By
contrast, the choice selected in most networks today.

2.5. eBGP Endpoints: Global or Link-Local Addresses?

When running eBGP over IPv6, there are two options for the addresses
to use at each end of the eBGP session (or more properly, the
underlying TCP session):

a. Use link-local addresses for the eBGP session, OR

b. Use global addresses for the eBGP session.

Note that the choice here is the addresses to use uses IS-IS for the eBGP
sessions, one IP version and not whether OSPF for
the link itself has global (or unique-
local) addresses. In particular, it is other version will require quite possible for different configuration, and
will also require the eBGP
session operations staff to use link-local addresses even when become familiar with the link has global
addresses.

The big attraction for
difference between the two protocols.

With option (a) (a), there is security: an eBGP session using
link-local addresses is impossible additional choice of whether to attack from run IS-
IS in single-topology mode (where IPv4 and IPv6 share a device that is
off-link. This provides very strong protection against TCP RST single
topology and
similar attacks. Though there are other ways to get an equivalent
level a single set of security (e.g. GTSM [RFC5082], MD5 [RFC5925], or ACLs),
these other ways require additional configuration which can be
forgotten link costs[RFC5308]) or multi-topology
mode (where IPv4 and IPv6 have separate topologies and potentially mis-configured.

However, there are a number of small disadvantages to using link-
local addresses:

o Using link-local addresses only works for single-hop eBGP
sessions;
different link costs[RFC5120]). A big problem with single-topology
mode is that it does not work cannot easily accommodate devices that support
IPv4-only or IPv6-only. Thus, today there is general agreement that
multi-topology is the right choice as this gives the greatest
flexibility in network design.

It should be noted that a number of ISPs have run OSPF as their IPv4
IGP for multi-hop sessions.

o One must use "next-hop self" quite a few years, but have selected IS-IS as their IPv6 IGP.
However, there are very few (none?) that have made the reverse
choice. This is, in part, because routers generally support more
nodes in an IS-IS area than in the corresponding OSPF area, and
because IS-IS is seen as more secure because it runs at both endpoints, otherwise
redistributing routes learned via eBGP into iBGP will not work.
(Some products enable "next-hop self" layer 2.

2.4. BGP

The discussion in this situation
automatically). section revolves around the following table.

+----------------+-----------+-------------------+
| Route Family | Transport | Comments |
+----------------+-----------+-------------------+
| | | |
+----------------+-----------+-------------------+
| Unlabeled IPv4 | IPv4 | Works well |
+----------------+-----------+-------------------+
| Unlabeled IPv4 | IPv6 | Next-hop issues |
+----------------+-----------+-------------------+
| Unlabeled IPv6 | IPv4 | Next-hop issues |
+----------------+-----------+-------------------+
| Unlabeled IPv6 | IPv6 | Works well |
+----------------+-----------+-------------------+
| | | |
+----------------+-----------+-------------------+
| Labeled IPv4 | IPv4 | Works well |
+----------------+-----------+-------------------+
| Labeled IPv4 | IPv6 | Next-hop issues |
+----------------+-----------+-------------------+
| Labeled IPv6 | IPv4 | (6PE) Works well |
+----------------+-----------+-------------------+
| Labeled IPv6 | IPv6 | ??? |
+----------------+-----------+-------------------+
| | | |
+----------------+-----------+-------------------+
| VPN IPv4 | IPv4 | Works well |
+----------------+-----------+-------------------+
| VPN IPv4 | IPv6 | Next-hop issues |
+----------------+-----------+-------------------+
| VPN IPv6 | IPv4 | (6VPE) Works well |
+----------------+-----------+-------------------+
| VPN IPv6 | IPv6 | ??? |
+----------------+-----------+-------------------+

The first column lists various route families, where "unlabeled"
means SAFI 1, "labeled" means SAFI 4, and "VPN" means SAFI 128. The
second column lists the protocol used to transport the BGP session,
frequently specified by giving either an IPv4 or IPv6 address in the
"neighbor" statement.

The third column comments on the combination in the first two
columns:

o Operators For combinations marked "Works well", these combinations are
widely supported and their tools are used generally recommended.

o For combinations marked "Next-hop issues", these combinations are
less-widely supported and when supported, often have next-hop
issues. That is, the next-hop address is typically a v4-mapped
IPv6 address, which is based on some IPv4 address on the sending
router. This v4-mapped IPv6 address is often not reachable by
default using IPv6 routing. One common solution to referring this problem
is to eBGP sessions
by use routing policy to change the next-hop to a different
IPv6 address.

o For combinations marked as "???", it is believed that these
combinations will not be supported until MPLS over IPv6 becomes
available. [Need to Confirm].

Also, it is important to note that changing the set of address only,
families being carried over a BGP session requires the BGP session to
be reset (unless something that like [I-D.ietf-idr-dynamic-cap] or
[I-D.ietf-idr-bgp-multisession] is not possible in use). This is generally more
of an issue with link-local
addresses.

o If eBGP sessions than iBGP sessions: for iBGP sessions
it is common practice for a router to have to two iBGP sessions, one
to each member of a route reflector pair, and so one can change the
set of address families on first one session and then the other.

The following subsections discuss specific scenarios in more detail.

2.4.1. BGP Sessions for Unlabeled Routes

Unlabeled routes are commonly carried on eBGP sessions, as well as on
iBGP sessions in networks where Internet traffic is carried unlabeled
across the network. In these scenarios, operators today most
commonly use two BGP sessions: one session is configuring parallel eBGP sessions for transported over IPv4
and IPv6 carries the unlabeled IPv4 routes, then using link-local addresses for while the IPv6 second session
introduces an extra difference between is
transported over IPv6 and carries the two sessions which
could otherwise be avoided. unlabeled IPv6 routes.

There are several reasons for this choice:

o On some products, an eBGP session using a link-local address is
more complex to configure than a session that use It gives a global
address. clean separation between IPv4 and IPv6.

o If hardware or other issues cause one to move This avoids the cable to a
different local interface, then reconfiguration is required at
both ends: at next-hop problem described in note 1 above.

o The status of the local end because routes follows the interface has changed (and
with link-local addresses, status of the interface must always be specified
along with underlying
transport. If, for example, the address), and at IPv6 data path between the remote end because two
BGP speakers fails, then the link-
local address has likely changed. (Contrast this with using
global addresses, where less re-configuration is required at IPv6 session between the
local end, two speakers
will fail and no reconfiguration is required at the remote end).

o Finally, a strict interpretation of RFC 2545 can IPv6 routes will be seen as
forbidding running eBGP between link-local addresses, as RFC 2545
requires withdrawn, which will allow
the BGP next-hop field to contain at least a global
address.

For these reasons, most operators today choose traffic to have their eBGP
sessions use global addresses.

2.6. IGP Choice

One of be re-routed elsewhere. By contrast, if the main decisions for an IPv6 implementor is the choice of
IGP (Interior Gateway Protocol) within the network. The primary
choices are
routes were transported over IPv4, then the IETF protocols failure of RIP [RFC2080], OSPF [RFC2328]
[RFC5340] the IPv6
data path might leave a working IPv4 data path, so the BGP session
would remain up and IS-IS [RFC5120] [RFC5308], though some operators may
consider non-IETF protocols. Here we limit our discussion to the
pros IPv6 routes would not be withdrawn, and cons of OSPF vs. IS-IS.

Considering just OSPF vs. IS-IS,
thus the options are:

a. Use OSPFv2 for IPv6 traffic would be sent into a black hole.

o It avoids resetting the BGP session when adding IPv6 to an
existing session, or when removing IPv4 and OSPFv3 for IPv6, OR

b. Use OSPFv3 from an existing session.

2.4.2. BGP sessions for Labeled or VPN Routes

In these scenarios, it is most common today to carry both the IPv4
and IPv6, OR

c. Use OSPFv2 for IPv4, and IS-IS for IPv6, OR

d. Use IS-IS for IPv4 and IPv6, OR

e. Use IS-IS IPv6 routes over sessions transported over IPv4. This can be
done with either: (a) one session carrying both route families, or
(b) two sessions, one for IPv4 and OSPFv3 each family.

Using a single session is usually appropriate for IPv6.

Note an iBGP session
going to a route reflector handling both route families. Using a
single session here usually means that options (a), (c), and (e) involve the BGP session will reset
when changing the set of address families, but as noted above, this
is usually not a problem when redundant route reflectors are
involved.

In eBGP situations, two sessions are usually more appropriate.

2.4.3. eBGP Endpoints: Global or Link-Local Addresses?

When running eBGP over IPv6, there are two different
routing protocols, while options (b) and (d) involve running just one
routing protocol.

o A big factor in for the addresses
to use at each end of the eBGP session (or more properly, the
underlying TCP session):

a. Use link-local addresses for the eBGP session, OR

b. Use global addresses for the eBGP session.

Note that the choice here is the protocol addresses to use for the operator eBGP
sessions, and not whether the link itself has global (or unique-
local) addresses. In particular, it is
currently using quite possible for routing IPv4. Option (e) is unlikely the eBGP
session to be a
good choice for an operator currently using OSPF use link-local addresses even when the link has global
addresses.

The big attraction for IPv4 routing,
and similarly option (a) is unlikely to be a good choice for security: an
operator currently eBGP session using IS-IS.

o A pro for options (a), (c), and (e), which use two routing
protocols,
link-local addresses is that they give impossible to attack from a hard separation between IPv4 device that is
off-link. This provides very strong protection against TCP RST and
IPv6 routing. Thus a problem with one protocol
similar attacks. Though there are other ways to get an equivalent
level of security (e.g. GTSM [RFC5082], MD5 [RFC5925], or one set ACLs),
these other ways require additional configuration which can be
forgotten or potentially mis-configured.

However, there are a number of
routes is unlikely small disadvantages to affect the other. using link-
local addresses:

o There Using link-local addresses only works for single-hop eBGP
sessions; it does not work for multi-hop sessions.

o One must use "next-hop self" at both endpoints, otherwise re-
advertising routes learned via eBGP into iBGP will not work.
(Some products enable "next-hop self" in this situation
automatically).

o Operators and their tools are two cons used to referring to eBGP sessions
by address only, something that is not possible with link-local
addresses.

o If one is configuring parallel eBGP sessions for options (a), (c), IPv4 and (e). One con is that
two sets of all IPv6
routes, then using link-local addresses for the protocol mechanisms need to IPv6 session
introduces extra operational differences between the two sessions
which could otherwise be maintained. avoided.

o On some products, an eBGP session using a larger modern router, this link-local address is unlikely
more complex to be configure than a problem, but on
some edge devices this might be an issue. The second con is session that
some operational staff use a global
address.

o If hardware or other issues cause one to move the cable to a
different local interface, then reconfiguration is required at
both ends: at the local end because the interface has changed (and
with link-local addresses, the interface must always be familiar specified
along with both protocols. For
many routing problems, the protocols are sufficiently similar that
they can be considered identical, but some problems require a
detailed knowledge of address), and at the remote end because the link-
local address has likely changed. (Contrast this with using
global addresses, where less re-configuration is required at the
local end, and no reconfiguration is required at the differences. remote end).

o Option (b) Finally, a strict interpretation of RFC 2545 can be seen as
forbidding running eBGP between link-local addresses, as RFC 2545
requires the use of new protocol extensions that allow
OSPFv3 BGP next-hop field to also route IPv4. At the time of writing, contain at least a global
address.

For these
extensions are still quite new. reasons, most operators today choose to have their eBGP
sessions use global addresses.

3. General Observations

There are two themes that run though many of the design choices in
this document. This section presents some general discussion on
these two themes.

3.1. Use of Link-Local Addresses

The proper use of link-local addresses is a common theme in the IPv6
network design choices. Link-layer addresses are, of course, always
present in an IPv6 network, but current network design practice
mostly ignores them, despite efforts such as
[I-D.ietf-opsec-lla-only].

There are three main reasons for this current practice:

o Network operators are concerned about the volatility of link-local
addresses based on MAC addresses, despite the fact that this
concern can be overcome by manually-configuring link-local
addresses;

o It is impossible to ping a link-local address from a device that
is not on the same subnet. This is a troubleshooting
disadvantage, though it can also be viewed as a security
advantage.

o Most operators are currently running networks that carry both IPv4
and IPv6 traffic, and wish to harmonize their IPv4 and IPv6 design
and operational practices where possible.

3.2. Separation of IPv4 and IPv6

Currently, most operators are running or planning to run networks
that carry both IPv4 and IPv6 traffic. Hence the question: To what
degree should IPv4 and IPv6 be kept separate? As can be seen above,
this breaks into two sub-questions: To what degree should IPv4 and
IPv6 traffic be kept separate, and to what degree should IPv4 and
IPv6 routing information be kept separate?

The general consensus around the first question is that IPv4 and IPv6
traffic should generally be mixed together. This recommendation is
driven by the operational simplicity of mixing the traffic, plus the
general observation that the service being offered to the end user is
Internet connectivity and most users do not know or care about the
differences between IPv4 and IPv6. Thus it is very desirable to mix
IPv4 and IPv6 on the same link to the end user. On other links,
separation is possible but more operationally complex, though it does
occasionally allow the operator to work around limitations on network
devices. The situation here is roughly comparable to IP and MPLS
traffic: many networks mix the two traffic types on the same links
without issues.

By contrast, there is more of an argument for carrying IPv6 routing
information over IPv6 transport, while leaving IPv4 routing
information on IPv4 transport. By doing this, one gets fate-sharing
between the control and data plane for each IP protocol version: if
the data plane fails for some reason, then often the control plane
will too.

4. IANA Considerations

This document makes no requests of IANA.

5. Security Considerations

(TBD)

6. Acknowledgements

Many, many people in the V6OPS working group provided comments and
suggestions that made their way into this document. A partial list
includes: Rajiv Asati, Fred Baker, Michael Behringer, Marc Blanchet,
Ron Bonica, Randy Bush, Cameron Byrne, Brian Carpenter, KK
Chittimaneni, Tim Chown, Lorenzo Colitti, Gert Doering, Bill Fenner,
Kedar K Gaonkar, Chris Grundemann, Steinar Haug, Ray Hunter, Joel
Jaeggli, Victor Kuarsingh, Ivan Pepelnjak, Alexandru Petrescu, Rob
Shakir, Mark Smith, Jean-Francois Tremblay, Tina Tsou, Dan York, and
Xuxiaohu.

The authors would also like to thank Pradeep Jain and Alastair
Johnson for helpful comments on a very preliminary version of this
document.

7. Informative References

[I-D.ietf-idr-bgp-multisession]
Scudder, J., Appanna, C., and I. Varlashkin, "Multisession
BGP", draft-ietf-idr-bgp-multisession-07 (work in
progress), September 2012.

[I-D.ietf-idr-dynamic-cap]
Ramachandra, S. and E. Chen, "Dynamic Capability for
BGP-4", BGP-
4", draft-ietf-idr-dynamic-cap-14 (work in progress),
December 2011.

[I-D.ietf-opsec-lla-only]
Behringer, M. and E. Vyncke, "Using Only Link-Local
Addressing Inside an IPv6 Network", draft-ietf-opsec-lla-
only-07
only-10 (work in progress), February July 2014.

[I-D.ietf-v6ops-enterprise-incremental-ipv6]
Chittimaneni, K., Chown, T., Howard, L., Kuarsingh, V.,
Pouffary, Y., and E. Vyncke, "Enterprise IPv6 Deployment
Guidelines", draft-ietf-v6ops-enterprise-incremental-
ipv6-05
ipv6-06 (work in progress), January July 2014.

[RFC2080] Malkin, G. and R. Minnear, "RIPng for IPv6", RFC 2080,
January 1997.

[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

[RFC4472] Durand, A., Ihren, J., and P. Savola, "Operational
Considerations and Issues with IPv6 DNS", RFC 4472, April
2006.

[RFC4852] Bound, J., Pouffary, Y., Klynsma, S., Chown, T., and D.
Green, "IPv6 Enterprise Network Analysis - IP Layer 3
Focus", RFC 4852, April 2007.

[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.

[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., and C.
Pignataro, "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, October 2007.

[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120, February 2008.

[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, October
2008.

[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.

[RFC5375] Van de Velde, G., Popoviciu, C., Chown, T., Bonness, O.,
and C. Hahn, "IPv6 Unicast Address Assignment
Considerations", RFC 5375, December 2008.

[RFC5838] Lindem, A., Mirtorabi, S., Roy, A., Barnes, M., and R.
Aggarwal, "Support of Address Families in OSPFv3", RFC
5838, April 2010.

[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.

[RFC5963] Gagliano, R., "IPv6 Deployment in Internet Exchange Points
(IXPs)", RFC 5963, August 2010.

[RFC6180] Arkko, J. and F. Baker, "Guidelines for Using IPv6
Transition Mechanisms during IPv6 Deployment", RFC 6180,
May 2011.

[RFC6342] Koodli, R., "Mobile Networks Considerations for IPv6
Deployment", RFC 6342, August 2011.

[RFC6782] Kuarsingh, V. and L. Howard, "Wireline Incremental IPv6",
RFC 6782, November 2012.

[RFC6883] Carpenter, B. and S. Jiang, "IPv6 Guidance for Internet
Content Providers and Application Service Providers", RFC
6883, March 2013.

Authors' Addresses

Philip Matthews
Alcatel-Lucent
600 March Road
Ottawa, Ontario K2K 2E6
Canada

Phone: +1 613-784-3139
Email: philip_matthews@magma.ca

Victor Kuarsingh
Dyn
150 Dow Street
Manchester, NH 03101
USA

Email: victor@jvknet.com