Internet DRAFT - draft-palet-v6ops-p2p-links
draft-palet-v6ops-p2p-links
v6ops J. Palet Martinez
Internet-Draft The IPv6 Company
Intended status: Informational November 4, 2019
Expires: May 7, 2020
IPv6 Point-to-Point Links
draft-palet-v6ops-p2p-links-04
Abstract
This document describes different alternatives for configuring IPv6
point-to-point links, considering the prefix size, numbering choices
and prefix pool to be used.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 7, 2020.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. The Ping-Pong Problem in Point-to-Point Links . . . . . . . . 3
4. Prefix Size Choices . . . . . . . . . . . . . . . . . . . . . 3
4.1. Rationale for using /64 . . . . . . . . . . . . . . . . . 3
4.2. Rationale for using /127 . . . . . . . . . . . . . . . . 4
4.3. Rationale for using /126 and Other Options . . . . . . . 5
4.4. A Possible Middle-Term Choice . . . . . . . . . . . . . . 5
5. Numbering Choices . . . . . . . . . . . . . . . . . . . . . . 5
5.1. GUA (Global Unicast Addresses) . . . . . . . . . . . . . 5
5.2. ULA (Unique Local Addresses) . . . . . . . . . . . . . . 5
5.3. Link-Local Addresses Only . . . . . . . . . . . . . . . . 6
6. Prefix Pool Choices . . . . . . . . . . . . . . . . . . . . . 7
7. /64 from Customer Prefix for point-to-point links . . . . . . 7
7.1. Numbering Interfaces . . . . . . . . . . . . . . . . . . 7
7.2. Routing Aggregation of the Point-to-Point Links . . . . . 8
7.3. DHCPv6 Considerations . . . . . . . . . . . . . . . . . . 9
7.4. Router Considerations . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
There are different alternatives for numbering IPv6 point-to-point
links, and from an operational perspective, there may have different
advantages or disadvantages that need to be taken in consideration
under the scope of each specific network architecture design.
[RFC6164] describes using /127 prefixes for inter-router point-to-
point links, using two different address pools, one for numbering the
point-to-point links and another one for delegating the prefixes at
the end of the point-to-point link. However, this doesn't exclude
other choices.
This document describes alternative approaches, for the prefix size,
the numbering of the link and the prefix pool.
The proposed approaches are suitable for those point-to-point links
connecting ISP to customers, but not limited to those cases, and in
fact, all them are being used by a relevant number of networks
worldwide, in several different scenarios (service providers,
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enterprise networks, etc.).
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. The Ping-Pong Problem in Point-to-Point Links
Some point-to-point links may present the ping-pong problem, (a
forwarding loop). The fundamental root cause of this problem is an
IPv6 implementations not performing full Neighbor Discovery (NS/NA)
on addresses that the prefix says could exist on the link.
IPv6 implementations are assuming that all addresses within the
prefix must exist at the other end of the point-to-point link, and
send the traffic straight onto the link. If the address doesn't
exist, and there is a covering route back in the other direction, the
ping-pong problem occurs.
Full Neighbor Discovery is doing more than just resolving the link-
layer address of an IPv6 address. Neighbor Discovery is also
determining if the address exists. Even if a point-to-point link
doesn't have link-layer addresses to resolve, ND determining if an
address exists on the link is very beneficial because it will prevent
the ping-pong problem occurring entirely regardless of the IPv6
prefix length being used on the link.
4. Prefix Size Choices
[RFC7608] already discusses about the IPv6 prefix length
recommendations for forwarding, and the need for routing and
forwarding implementations to ensure that longest-prefix-match works
on any prefix length. So, in this document, we concentrate in the
most commonly used choices, not excluding other options.
4.1. Rationale for using /64
The IPv6 Addressing Architecture ([RFC4291]) specifies that all the
Interface Identifiers for all the unicast addresses (except for
000/3) are required to be 64 bits long and to be constructed in
Modified EUI-64 format.
The same document also mandates the usage of the predefined subnet-
router anycast address, which has cleared to zero all the bits that
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do not form the subnet prefix.
Using /64 is the most common scenario and currently the best practice
by the number of service providers using this approach compared to
others.
Using a /64 has the advantage of being future proof and avoids
renumbering, in the event that new standards take advantage of the 64
bits for other purposes, or the link becomes a point-to-multipoint,
or there is a need to use more addresses in the link (e.g.,
monitoring equipment, managed bridges).
It has been raised also the issue of some hardware having limitations
in using prefixes longer then /64, for example using extra hardware
resources.
Section 5. of [RFC6164] describes possible issues when using /64 for
the point-to-point links, such as the ping-pong and the neighbor
cache exhaustion. However, it also states that they can be mitigated
by other means, including the latest ICMPv6 [RFC4443] ND [RFC4861].
Indeed, considering the publication date of that document, those
issues should not be any longer a concern. The fact is that many
operators worldwide, today use /64 without any concerns, as vendors
have taken the necessary code updates.
Consequently, we shall conclude that it is a valid approach to use
/64 prefixes for the point-to-point links.
4.2. Rationale for using /127
[RFC6164] already do a complete review of reasons why /127 is a good
approach vs other options. However, it needs to be considered that
it was published a number of years ago, and most of the hardware
today already incorporate mitigations.
It should be noted that, when using a /127 prefix, configuration of
each of the addresses within the /127 prefix, at each respective end
of the link, must be actively validated by the network operator. A
missing /127 address from one end of the link, with a local route
pointing out that end of the link that covers the missing /127
address, such as a default route, causes a "ping-pong" scenario to
exist for the missing /127 address. The link could still be
successfully carrying transit traffic, and IPv6 will not report any
errors, because IPv6 doesn't require or nor check to ensure all
interfaces attached to a link has addresses from all prefixes
assigned to the link, excepting the Link-Local prefix per [RFC4291].
It is a valid approach to use /127 for the point-to-point links,
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however is not future proof considering the comments from the
previous section, and older equipment may not support it.
4.3. Rationale for using /126 and Other Options
/126 was considered by [RFC3627], and despite this document has been
obsoleted, because was considering /127 as harmful, the
considerations in Section 4.3 are still valid.
The same document describes options such as /112 and /120, and all
those are commonly used in worldwide IPv6 deployments [IPv6-Survey],
though in a lesser degree than /64 or /127.
Consequently, we shall conclude that /126, /120 and /112 are valid
approaches for the point-to-point links.
4.4. A Possible Middle-Term Choice
A possible "middle-term" approach, will be to allocate a /64 for each
point-to-point link, but use just one /127 out of it, making it
future proof and at the same time avoiding possible issues indicated
in the previous sections.
5. Numbering Choices
IPv6 provides different unicast addressing scopes which can be
considered when numbering a point-to-point link.
It has been reported that certain hardware may consume resources when
using numbered links. This is a very specific situation that may
need to be consider on a case by case basis.
5.1. GUA (Global Unicast Addresses)
Using GUA is the most common approach. It provides full
functionality for both end-points of the point-to-point link and
consequently, facilitates troubleshooting.
5.2. ULA (Unique Local Addresses)
Some networks use ULAs for numbering the point-to-point links. This
approach may cause numerous problems when carrying Internet traffic
and therefore, is strongly discouraged. For example, if the CE needs
to send an ICMPv6 message to a host outside that network (to the
Internet), the packet with ULA source address will not get thru and
PMTUD will break, which in turn will completely break that IPv6
connection when the MTU is not the same for all the path.
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ULAs are IPv6 private addresses, not intended to be used as source or
destination addresses across the Internet. This issue also exists in
IPv4 when using [RFC1918] addresses on links carrying IPv4 Internet
traffic. [RFC6752] discusses this issue for IPv4, with much of the
discussion applying similarly to IPv6 and ULAs.
However, this approach is valid if, following Section 2.2 of
[RFC4443], and despite using ULA for the point-to-point link, the
router is configured with at least one GUA and the source of the
ICMPv6 messages are always a GUA, per the IPv6 Default Address
Selection algorithm [RFC6724].
5.3. Link-Local Addresses Only
Some networks leave the point-to-point links with only Link-Local
addresses used at both ends of the link. This is sometimes
improperly referred as "unnumbered", because the Link-Local addresses
are also "numbers". Furthermore, [RFC4291] requires that all
interfaces attached to a link have at least a Link-Local "number" or
address from the Link-Local prefix.
[RFC7404] (Using Only Link-Local Addressing inside an IPv6 Network)
discusses pros and cons of this alternative, which in general apply
for the point-to-point links.
While this choice might work if the point-to-point link is terminated
in a router, which typically will get configured with a suitable
routable GUA or ULA, it will not work for devices that can't be
further configured, for example if they do not support DHCPv6-PD.
This is the case for hosts, when the Operating System is not expected
to be a DHCPv6-PD client and are therefore left without any usable
GUA to allow traffic forwarding.
In the case of a router, the route for the assigned prefix is pointed
towards the link-local address on the router WAN port and the default
route on the router is pointed towards the link-local address on the
upstream network equipment port.
This choice seems easier to implement, compared the previous ones,
but it also brings some drawbacks, such as difficulties with
troubleshooting and monitoring. For example, link local addresses do
not appear in traceroute, so it makes more difficult to locate the
exact point of failure.
It is more useful in scenarios where it is known that only a router
will be attached to the point-to-point link, and where the configured
address of the router is known. Non-routers connecting to a network,
which can't initiate DHCPv6-PD might experience problems and will
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stay unnumbered upon connection, if a /64 prefix is not used to
number the link. This may be also the case for routers, which will
not be able to complete the DHCPv6-PD in unnumbered links.
The considerations indicated in the previous section, regarding not
using ULA as source address of ICMPv6 messages, and instead ensuring
there is at least one GUA configured for that, also apply if link-
locals are used for the point-to-point link.
6. Prefix Pool Choices
The logic choice seems to use a dedicated pool of IPv6 addresses, as
this is the way we are "used to" with IPv4. Actually, this is done
often by means of different IPv6 pools at every PoP in a service
provider network.
A possible benefit of using a dedicated IPv6 pool, is that allows
applying security policies without harming the customers. This is
only true if customers always have a CE at their end of the WAN link.
However, the fact that the default IPv6 link size is /64 and commonly
multiple /64's are assigned to a single customer, provides an
interesting alternative approach for combining "best practices"
described in the precedent sections.
The following section depicts this alternative.
7. /64 from Customer Prefix for point-to-point links
Using a /64 from the customer prefix, in addition to the advantages
already indicated when using /64, simplifies the addressing plan.
The use of /64 also facilitates an easier way for routing the shorter
aggregated prefix into the point-to-point link. Consequently it
simplifies the "view" of a more unified addressing plan, providing an
easier path for following up any issue when operating IPv6 networks
and typically, will have a great impact in saving expensive hardware
resources (lower usage of TCAM, typically by half).
This mechanism would not work in broadcast layer two media that rely
on ND, because it will try ND for all the addresses within the
shorter prefix that is being routed thru the point-to-point link.
7.1. Numbering Interfaces
Often, in point-to-point links, hardware tokens are not available, or
there is the need to keep certain bits (u, g) cleared, so the links
can be manually numbered sequentially with most of the bits cleared
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to zero. This numbering makes as well easier to remember the
interfaces, which typically will become numbered as 0 (with 63
leading zero bits) for the provider side and 1 (with 63 leading zero
bits) for the customer side.
Using interface identifiers as 0 and 1 is not only a very simple
approach, but also a very common practice. Other different choices
can as well be used as required in each case.
On the other hand, using the EUI-64, makes it more difficult to
remember and handle the interfaces, but provides an additional degree
of protection against port (actually address) scanning as described
at [RFC7707].
7.2. Routing Aggregation of the Point-to-Point Links
Following this approach and assuming that a shorter prefix is
typically delegated to a customer, for example a /48, it is possible
to simplify the routing aggregation of the point-to-point links.
Towards this, the point-to-point link may be numbered using the first
/64 of the /48 delegated to the customer.
Let's see a practical example:
o A service provider uses the prefix 2001:db8::/32 and is using
2001:db8:aaaa::/48 for a given customer.
o Instead of allocating the point-to-point link from a different
addressing pool, it may use 2001:db8:aaaa::/64 (which is the first
/64 subnet from the 2001:db8:aaaa::/48) to number the link.
o This means that, in the case the non-EUI-64 approach is used, the
point-to-point link may be numbered as 2001:db8:aaaa::1/64 for the
provider side and 2001:db8:aaaa::2/64 for the customer side.
o Note that using the first /64 and interface identifiers 1 and 2 is
a very common practice. However other values may be chosen
according to each case specific needs.
In this way, as the same address pool is being used for both, the
prefix and the point-to-point link, one of the advantages of this
approach is to make very easy the recognition of the point-to-point
link that belongs to a given customer prefix, or in the other way
around, the recognition of the prefix that is linked by a given
point-to-point link.
For example, making a trace-route to debug any issue to a given
address in the provider network, will show a straight view, and it
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becomes unnecessary one extra step to check a database that correlate
an address pool for the point-to-point links and the customer
prefixes, as all they are the same.
Moreover, it is possible to use the shorter prefix as the provider
side numbering for the point-to-point link and keep the /64 for the
customer side. In our example, it will become:
o Point-to-point link at provider side: 2001:db8:aaaa::1/48
o Point-to-point link at customer side: 2001:db8:aaaa::2/64
This provides one additional advantage as in some platforms the
configuration may be easier saving one step for the route of the
delegated prefix (no need for two routes to be configured, one for
the delegated prefix, one for the point-to-point link). It is
possible because the longest-prefix-match rule.
The behavior of this type of configuration has been successfully
deployed in different operator and enterprise networks, using
commonly available implementations with different routing protocols,
including RIP, BGP, IS-IS, OSPF, along static routing, and no
failures or interoperability issues have been reported.
7.3. DHCPv6 Considerations
As stated in [RFC3633], "the requesting router MUST NOT assign any
delegated prefixes or subnets from the delegated prefix(es) to the
link through which is received the DHCP message from the delegating
router", however the approach described in this document is still
useful in other DHCPv6 scenarios or non-DHCPv6 scenarios.
Furthermore, [RFC3633] was updated by Prefix Exclude Option for
DHCPv6-based Prefix Delegation ([RFC6603]), precisely to define a new
DHCPv6 option, which covers the case described by this document.
Moreover, [RFC3769] has no explicit requirement that avoids the
approach described in this document.
7.4. Router Considerations
This approach is being used by operators in both, residential/SOHO
and enterprise networks, so the routers at the customer end for those
networks MUST support [RFC6603] if DHCPv6-PD is used.
In the case of Customer Edge Routers there is a specific requirement
([RFC7084]) WPD-8 (Prefix delegation Requirements), marked as SHOULD
for [RFC6603]. However, in a scenario where the approach described
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in this document is followed, together with DHCPv6-PD, the CE Router
MUST support [RFC6603].
8. Security Considerations
This document does not have any new specific security considerations.
9. IANA Considerations
This document does not have any new specific IANA considerations.
10. Acknowledgements
The author would like to acknowledge the inputs of Mikael
Abrahamsson, Brian Carpenter, Eric Vyncke, Mark Smith and TBD.
Acknowledge is also due to my co-authors of RIPE-690 (Best Current
Operational Practice for Operators: IPv6 prefix assignment for end-
users - persistent vs non-persistent, and what size to choose,
https://www.ripe.net/publications/docs/ripe-690) and global
community, which provided valuable inputs which have been key for
this document.
Acknowledgement to co-authors, Cesar Olvera and Miguel Angel Diaz, of
a previous related document (draft-palet-v6ops-point2point, 2006), as
well as inputs for that version from Alain Durand, Chip Popoviciu,
Daniel Roesen, Fred Baker, Gert Doering, Olaf Bonness, Ole Troan,
Pekka Savola and Vincent Jardin, are also granted.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
DOI 10.17487/RFC3633, December 2003,
<https://www.rfc-editor.org/info/rfc3633>.
[RFC3769] Miyakawa, S. and R. Droms, "Requirements for IPv6 Prefix
Delegation", RFC 3769, DOI 10.17487/RFC3769, June 2004,
<https://www.rfc-editor.org/info/rfc3769>.
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[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC6603] Korhonen, J., Ed., Savolainen, T., Krishnan, S., and O.
Troan, "Prefix Exclude Option for DHCPv6-based Prefix
Delegation", RFC 6603, DOI 10.17487/RFC6603, May 2012,
<https://www.rfc-editor.org/info/rfc6603>.
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<https://www.rfc-editor.org/info/rfc6724>.
[RFC7084] Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
Requirements for IPv6 Customer Edge Routers", RFC 7084,
DOI 10.17487/RFC7084, November 2013,
<https://www.rfc-editor.org/info/rfc7084>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
11.2. Informative References
[IPv6-Survey]
Palet Martinez, J., "IPv6 Deployment Survey (Residential/
Household Services)", January 2018,
<https://indico.uknof.org.uk/event/41/contribution/5/
material/slides/0.pdf>.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
<https://www.rfc-editor.org/info/rfc1918>.
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[RFC3627] Savola, P., "Use of /127 Prefix Length Between Routers
Considered Harmful", RFC 3627, DOI 10.17487/RFC3627,
September 2003, <https://www.rfc-editor.org/info/rfc3627>.
[RFC6164] Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti,
L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter-
Router Links", RFC 6164, DOI 10.17487/RFC6164, April 2011,
<https://www.rfc-editor.org/info/rfc6164>.
[RFC6752] Kirkham, A., "Issues with Private IP Addressing in the
Internet", RFC 6752, DOI 10.17487/RFC6752, September 2012,
<https://www.rfc-editor.org/info/rfc6752>.
[RFC7404] Behringer, M. and E. Vyncke, "Using Only Link-Local
Addressing inside an IPv6 Network", RFC 7404,
DOI 10.17487/RFC7404, November 2014,
<https://www.rfc-editor.org/info/rfc7404>.
[RFC7608] Boucadair, M., Petrescu, A., and F. Baker, "IPv6 Prefix
Length Recommendation for Forwarding", BCP 198, RFC 7608,
DOI 10.17487/RFC7608, July 2015,
<https://www.rfc-editor.org/info/rfc7608>.
[RFC7707] Gont, F. and T. Chown, "Network Reconnaissance in IPv6
Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
<https://www.rfc-editor.org/info/rfc7707>.
Author's Address
Jordi Palet Martinez
The IPv6 Company
Molino de la Navata, 75
La Navata - Galapagar, Madrid 28420
Spain
EMail: jordi.palet@theipv6company.com
URI: http://www.theipv6company.com/
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