6MAN B. Carpenter, Ed.
Internet-Draft Univ. of Auckland
Intended status: Informational T. Chown
Expires: October 13, 2014 Univ. of Southampton
F. Gont
SI6 Networks / UTN-FRH
S. Jiang
Huawei Technologies Co., Ltd
A. Petrescu
CEA, LIST
A. Yourtchenko
cisco
April 11, 2014

Analysis of the 64-bit Boundary in IPv6 Addressing
draft-ietf-6man-why64-00

Abstract

The IPv6 unicast addressing format includes a separation between the prefix used to route packets to a subnet and the interface identifier used to specify a given interface connected to that subnet. Currently the interface identifier is defined as 64 bits long for almost every case, leaving 64 bits for the routing prefix. This document describes the advantages of this fixed boundary and analyses the issues that would be involved in treating it as a variable boundary.

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 October 13, 2014.

Copyright Notice

Copyright (c) 2014 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 (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

IPv6 addresses were originally chosen to be 128 bits long to provide flexibility and new possibilities, rather than simply relieving the IPv4 address shortage by doubling the address size to 64 bits. In particular, the notion of a well-defined interface identifier was added to the IP addressing model. The IPv6 addressing architecture [RFC4291] specifies that a unicast address is divided into n bits of subnet prefix followed by (128-n) bits of interface identifier (IID). The bits in the IID have no meaning and the entire identifier should be treated as an opaque value [RFC7136]. Also, since IPv6 routing is entirely based on variable length subnet masks, there is no basic architectural assumption that n has any particular fixed value. All IPv6 routing protocols support subnet masks of any length up to /128.

The IID is of basic importance in the IPv6 stateless address autoconfiguration (SLAAC) process [RFC4862]. However, it is important to understand that its length is a parameter in the SLAAC process, and it is determined in a separate link-type specific document (see Section 2 of RFC 4862). The SLAAC protocol does not define its length or assume any particular length.

The notion of a /64 boundary in the address was introduced after the initial design of IPv6 was done, following a period when it was expected to be at /80. There were two motivations for setting it at /64. One was the original "8+8" proposal [DRAFT-odell] that eventually led to ILNP [RFC6741], which required a fixed point for the split between local and wide-area parts of the address. The other was the expectation that EUI-64 MAC addresses would become widespread in place of 48-bit addresses, coupled with the plan at that time that auto-configured addresses would normally be based on interface identifiers derived from MAC addresses.

As a result, RFC 4291 describes a method of forming interface identifiers from IEEE EUI-64 hardware addresses [IEEE802] and this specifies that such interface identifiers are 64 bits long. Various other methods of forming interface identifiers also specify a length of 64 bits. The addressing architecture as modified by [RFC7136] states that "For all unicast addresses, except those that start with the binary value 000, Interface IDs are required to be 64 bits long. If derived from an IEEE MAC-layer address, they must be constructed in Modified EUI-64 format." The de facto length of almost all IPv6 interface identifiers is therefore 64 bits. The only documented exception is in [RFC6164], which standardises 127-bit prefixes for point-to-point links between routers, among other things to avoid a loop condition known as the ping-pong problem.

With that exception, and despite the comments above about the routing architecture and the design of SLAAC, using an IID shorter than 64 bits and a subnet prefix longer than 64 bits is outside the current IPv6 specifications, so results may vary.

The question is often asked why the subnet prefix boundary is set rigidly at /64. The first purpose of this document is to explain the advantages of the fixed IID length. Its second purpose is to analyse in some detail the effects of hypothetically varying the IID length. The fixed length limits the practical length of a routing prefix to 64 bits, whereas architecturally, and from the point of view of routing protocols, it could be anything (in theory) between /1 and /128 inclusive. Here, we mainly discuss the question of a shorter IID, which would allow a longer routing prefix. The document makes no proposal for a change to the IID length.

The following three sections describe in turn the advantages of the fixed length IID, some arguments for shorter lengths, and the expected effects of varying the length.

2. Advantages of a fixed identifier length

As mentioned in Section 1, the existence of an IID of a given length is a necessary part of IPv6 stateless address autoconfiguration (SLAAC) [RFC4862]. This length is normally the same for all nodes on a given link that is running SLAAC. Even though this length is a parameter for SLAAC, determined separately for each interface, a globally fixed IID length for all link layer media is the simplest solution, and is consistent with the principles of Internet host configuration described in [RFC5505].

An interface identifier of significant length, clearly separated from the routing prefix, makes it possible to limit the traceability of a host computer by varying the identifier. This is discussed further in Section 4.5.

An interface identifier of significant length guarantees that there are always enough addresses in any subnet to add one or more real or virtual interfaces. There might be other limits, but IP addressing will never get in the way.

The addressing architecture [RFC4291] [RFC7136] sets the IID length at 64 bits for all unicast addresses, and therefore for all media supporting SLAAC. An immediate effect of fixing the IID length at 64 bits is, of course, that it fixes the subnet prefix length also at 64 bits, regardless of the wide-area prefix assigned to the site concerned, which in accordance with [RFC6177] should be /56 or shorter. This situation has various specific advantages:

The remainder of this document describes arguments that have been made against the current fixed IID length and analyses the effects of a possible change. However, the consensus of the IETF is that the benefits of keeping the length fixed at 64 bits, and the practical difficulties of changing it, outweigh the arguments for change.

3. Arguments for shorter identifier lengths

In this section we describe arguments for scenarios where shorter IIDs, implying prefixes longer than /64, have been used or proposed.

3.1. Insufficient address space delegated

A site may not be delegated a sufficiently large prefix from which to allocate a /64 prefix to all of its internal subnets. In this case the site may either determine that it does not have enough address space to number all its network elements and thus, at the very best, be only partially operational, or it may choose to use internal prefixes longer than /64 to allow multiple subnets and the hosts within them to be configured with addresses.

In this case, the site might choose, for example, to use a /80 per subnet, in combination with hosts using either manually configured addressing or DHCPv6.

Scenarios that have been suggested where an insufficient prefix might be delegated include home or small office networks, vehicles, building services and transportation services (road signs, etc.). It should be noted that the homenet architecture text [I-D.ietf-homenet-arch] states that a CPE should consider the lack of sufficient address space to be an error condition, rather than using prefixes longer than /64 internally.

Another scenario occasionally suggested is one where the Internet address registries actually begin to run out of IPv6 prefix space, such that operators can no longer assign reasonable prefixes to users in accordance with [RFC6177]. It is sometimes suggested that assigning a prefix such as /48 or /56 to every user site (including the smallest) as recommended by [RFC6177] is wasteful. In fact, the currently released unicast address space, 2000::/3, contains 35 trillion /48 prefixes ((2**45 = 35,184,372,088,832). With only 2000::/3 currently committed for unicast addressing, we still have most of the address space in reserve. Thus there is no objective risk of prefix depletion by assigning /48 or /56 prefixes even to the smallest sites.

3.2. Hierarchical addressing

Some operators have argued that more prefix bits are needed to allow an aggregated hierarchical addressing scheme within a campus or corporate network. However, flat IGP routing is widely and successfully used within rather large networks, with hundreds of routers and thousands of end systems. Therefore there is no objective need for additional prefix bits to support hierarchy and aggregation within enterprises.

3.3. Audit requirement

Some network operators wish to know and audit which nodes are active on a network, especially those that are allowed to communicate off link or off site. They may also wish to limit the total number of active addresses and sessions that can be sourced from a particular host, LAN or site, in order to prevent potential resource depletion attacks or other problems spreading beyond a certain scope of control. It has been argued that this type of control would be easier if only long network prefixes with relatively small numbers of possible hosts per network were used, reducing the discovery problem. However, such sites most typically operate using DHCPv6, which means that all legitimate hosts are automatically known to the DHCPv6 servers, which is sufficient for audit purposes. Such hosts could, if desired, be limited to a small range of IID values without changing the /64 subnet length. Any hosts inadvertently obtaining addresses via SLAAC can be audited through Neighbor Discovery logs.

3.4. Concerns over ND cache exhaustion

A site may be concerned that it is open to neighbour discovery (ND) cache exhaustion attacks, whereby an attacker sends a large number of messages in rapid succession to a series of (most likely inactive) host addresses within a specific subnet, in an attempt to fill a router's ND cache with ND requests pending completion, in so doing denying correct operation to active devices on the network.

An example would be to use a /120 prefix, limiting the number of addresses in the subnet to be similar to an IPv4 /24 prefix, which should not cause any concerns for ND cache exhaustion. Note that the prefix does need to be quite long for this scenario to be valid. The number of theoretically possible ND cache slots on the segment needs to be of the same order of magnitude as the actual number of hosts. Thus small increases from the /64 prefix length do not have a noticeable impact: even 2^32 potential entries, a factor of two billion decrease compared to 2^64, is still more than enough to exhaust the memory on current routers.

Hosts would likely use DHCPv6, or be manually configured with addresses.

It should be noted that several other mitigations of the ND cache attack are described in [RFC6583], and that limiting the size of the cache and the number of incomplete entries allowed would also defeat the attack. For the specific case of a point-to-point link between routers, this attack is indeed mitigated by a /127 prefix [RFC6164].

4. Effects of varying the interface identifier length

This section of the document analyses the impact and effects of varying the length of an IPv6 unicast IID by reducing it to less than 64 bits.

4.1. Interaction with IPv6 specifications

The precise 64-bit length of the Interface ID is widely mentioned in numerous RFCs describing various aspects of IPv6. It is not straightforward to distinguish cases where this has normative impact or affects interoperability. This section aims to identify specifications that contain an explicit reference to the 64-bit length. Regardless of implementation issues, the RFCs themselves would all need to be updated if the 64-bit rule was changed, even if the updates were small.

First and foremost, the RFCs describing the architectural aspects of IPv6 addressing explicitly state, refer and repeat this apparently immutable value: Addressing Architecture [RFC4291], Reserved Interface Identifiers [RFC5453], ILNP [RFC6741]. Customer Edge routers impose /64 for their interfaces [RFC7084]. Only the IPv6 Subnet Model [RFC5942] refers to the assumption of /64 prefix length as a potential implementation error.

Numerous IPv6-over-foo documents make mandatory statements with respect to the 64-bit length of the Interface ID to be used during the Stateless Autoconfiguration. These documents include [RFC2464] (Ethernet), [RFC2467] (FDDI), [RFC2470] (Token Ring), [RFC2492] (ATM), [RFC2497] (ARCnet), [RFC2590] (Frame Relay), [RFC3146] (IEEE 1394), [RFC4338] (Fibre Channel), [RFC4944] (IEEE 802.15.4), [RFC5072] (PPP), [RFC5121] [RFC5692] (IEEE 802.16), [RFC2529] (6over4), [RFC5214] (ISATAP), [I-D.templin-aerolink] (AERO), [I-D.ietf-6lowpan-btle], [I-D.ietf-6man-6lobac], [I-D.brandt-6man-lowpanz].

To a lesser extent, the address configuration RFCs themselves may in some way assume the 64-bit length of an Interface ID (SLAAC for the link-local addresses, DHCPv6 for the potentially assigned EUI-64-based IP addresses, Default Router Preferences [RFC4191] for its impossibility of Prefix Length 4, Optimistic Duplicate Address Detection [RFC4429] which computes 64-bit-based collision probabilities).

The MLDv2 protocol [RFC3810] mandates all queries be sent with the fe80::/64 link-local source address prefix and subsequently bases the querier election algorithm on the link-local subnet prefix length of length /64.

The IPv6 Flow Label Specification [RFC6437] gives an example of a 20-bit hash function generation which relies on splitting an IPv6 address in two equally-sized 64bit-length parts.

The basic transition mechanisms [RFC4213] refer to IIDs of length 64 for link-local addresses, and other transition mechanisms such as Teredo [RFC4380] assume the use of IIDs of length 64. Similar assumptions are found in 6to4 [RFC3056] and 6rd [RFC5969]. Translation-based transition mechanisms such as NAT64 and NPTv6 have some dependency on prefix length, discussed below.

The proposed method [I-D.ietf-v6ops-64share] of extending an assigned /64 prefix from a smartphone's cellular interface to its WiFi link relies on prefix length, and implicitely on the length of the Interface ID, to be valued at 64.

The CGA and HBA specifications rely on the 64-bit identifier length (see below), as do the Privacy extensions [RFC4941] and some examples in IKEv2bis [RFC5996].

464XLAT [RFC6877] explicitly mentions acquiring /64 prefixes. However, it also discusses the possibility of using the interface address on the device as the endpoint for the traffic, thus potentially removing this dependency.

[RFC2526] reserves a number of subnet anycast addresses by reserving some anycast IIDs. An anycast IID so reserved cannot be less than 7 bits long. This means that a subnet prefix length longer than /121 is not possible, and a subnet of exactly /121 would be useless since all its identifiers are reserved. It also means that half of a /120 is reserved for anycast. This could of course be fixed in the way described for /127 in [RFC6164], i.e., avoiding the use of anycast within a /120 subnet.

The Mobile IP home network models [RFC4887] rely heavily on the /64 subnet length and assume a 64-bit IID.

While preparing this document, it was noted that many other IPv6 specifications refer to mandatory alignment on 64-bit boundaries, 64-bit data structures, 64-bit counters in MIBs, 64-bit sequence numbers and cookies in security, etc. Finally, the number "64" may be considered "magic" in some RFCs, e.g., 64k limits in DNS and Base64 encodings in MIME. None of this has any influence on the length of the IID, but might confuse a careless reader.

4.2. Possible areas of breakage

This section discusses several specific aspects of IPv6 where we can expect operational breakage with subnet prefixes other than /64.

It goes without saying that if prefixes longer than /64 are to be used, all hosts must be capable of generating IIDs shorter than 64 bits, in order to follow the auto-configuration procedure correctly [RFC4862]. There is however the rather special case of the link-local prefix. While RFC 4862 is careful not to define any specific length of link-local prefix within fe80::/10, [RFC4291] does define the link-local IID length to be 64 bits. Operationally there could be a problem unless all hosts on a link use IIDs of the same length to configure a link-local address during reboot. Typically today the choice of 64 bits for the link-local IID length is hard-coded per interface, and systems behave as if the link local prefix was actually fe80::/64. There might be no way to change this except conceivably by manual configuration, which will be impossible if the host concerned has no local user interface.

4.3. Experimental observations

4.3.1. Survey of the processing of Neighbor Discovery options with prefixes other than /64

This section provides a survey of the processing of Neighbor Discovery options which include prefixes that are different than /64.

The behavior of nodes was assessed with respect to the following options:

In the tables below, the following notation is used:

NOT-SUP:

This option is not supported (i.e., it is ignored no matter the prefix length used).
LOCAL:

The corresponding prefix is considered "on-link".
ROUTE

The corresponding route is added to the IPv6 routing table.
IGNORE:

The Option is ignored as an error.

Processing of ND options with prefixes longer than /64
Operating System PIO-A PIO-L PIO-AL RIO
FreeBSD 9.0 IGNORE LOCAL LOCAL NOT-SUP
Linux 3.0.0-15 IGNORE LOCAL LOCAL NOT-SUP
Linux-current IGNORE LOCAL LOCAL NOT-SUP
NetBSD 5.1 IGNORE LOCAL LOCAL NOT-SUP
OpenBSD-current IGNORE LOCAL LOCAL NOT-SUP
Win XP SP2 IGNORE LOCAL LOCAL ROUTE
Win 7 Home Premium IGNORE LOCAL LOCAL ROUTE

Processing of ND options with prefixes shorter than /64
Operating System PIO-A PIO-L PIO-AL RIO
FreeBSD 9.0 IGNORE LOCAL LOCAL NOT-SUP
Linux 3.0.0-15 IGNORE LOCAL LOCAL NOT-SUP
Linux-current IGNORE LOCAL LOCAL NOT-SUP
NetBSD 5.1 IGNORE LOCAL LOCAL NOT-SUP
OpenBSD-current IGNORE LOCAL LOCAL NOT-SUP
Win XP SP2 IGNORE LOCAL LOCAL ROUTE
Win 7 Home Premium IGNORE LOCAL LOCAL ROUTE

The results obtained can be summarized as follows:

  • the "A" bit in the Prefix Information Options is honored only if the prefix length is 64. At least for the case of a prefix longer than /64, this is consistent with [RFC4862], which defines the case where the sum of the link-local prefix length and the IID length is larger than 128 as an error consition.
  • the "L bit in the Prefix Information Options is honored for any arbitrary prefix length (whether shorter or longer than /64).
  • nodes that support the Route Information Option, allow such routes to be specified with prefixes of any arbitrary length (whether shorter or longer than /64)

4.3.2. Other Observations

Participants in the V6OPS working group have indicated that some forwarding devices have been shown to work correctly with long prefix masks such as /80 or /96. Indeed, it is to be expected that longest prefix match based forwarding will work for any prefix length, and no reports of this completely failing have been noted. Also, DHCPv6 is in widespread use without any dependency on the /64 boundary. Reportedly, there are deployments of /120 subnets configured using DHCPv6.

It has been reported that at least one type of switch has a content-addressable memory limited to 144 bits. This means that filters cannot be defined based on 128-bit addresses and two 16-bit port numbers; the longest prefix that could be used in such a filter is /112.

There have been definite reports that some routers have a performance drop-off or even resource exhaustion for prefixes longer than /64, due to design issues. In particular, some routing chip designs allocate much less space for longer prefixes than for prefixes up to /64, for the sake of savings in memory, power and lookup latency. Some devices need special-case code to handle point-to-point links according to [RFC6164].

4.4. Implementation and deployment issues

From an early stage, implementations and deployments of IPv6 assumed the /64 subnet length, even though routing was based on variable-length subnet masks of any length. As shown above, this became anchored in many specifications (Section 4.1) and in important aspects of implementations commonly used in local area networks (Section 4.3). In fact, a programmer might be lulled into assuming a comfortable rule of thumb that subnet prefixes are always /64 and an IID is always of length 64. Apart from the limited evidence in Section 4.3.1, we cannot tell without code inspections or tests whether existing stacks are able to handle a flexible IID length, or whether they would require modification to do so. A conforming implementation of an IPv6-over-foo that specifies a 64 bit IID for foo links will of course only support 64. But in a well designed stack, the IP layer itself will treat that 64 as a parameter, so changing the IID length in the IPv6-over-foo code should be all that is necessary.

The main practical consequence of the existing specifications is that deployments in which longer subnet prefixes are used cannot make use of SLAAC-configured addresses, and require either statically configured addresses or DHCPv6. To reverse this argument, if it was considered desirable to allow auto-configured addresses with subnet prefixes longer than /64, all of the specifications identified above as depending on /64 would have to be modified, with due regard to interoperability with unmodified stacks. In fact [I-D.ietf-6man-stable-privacy-addresses] allows for this possibility. Then modified stacks would have to be developed and deployed. It might be the case that some stacks contain dependencies on the /64 boundary which are not directly implied by the specifications, and any such hidden dependencies would also need to be found and removed.

At least one DHCPv6 client unconditionally installs a /64 prefix as on-link when it configures an interface with an address, although some specific operating system vendors seem to change this default behavior by tweaking a client-side script. It does this (even if technically it violates the protocol) because if there is no router on the link, the hosts effectively would fail to communicate each other with the configured addresses because the "on-link assumption" was removed in [RFC4861]. This is not really about the magic number of 64, but an implementation may sometimes pick a specific value of prefix length due to the removal of the on-link assumption, and the value chosen will most likely be 64.

Typical IP Address Management (IPAM) tools treat /64 as the default subnet length, but allow users to specify longer subnet prefixes if desired. Clearly, all IPAM tools and network management systems would need to be checked in detail.

Finally, IPv6 is already installed on many sites, with a large number of staff trained on the basis of the existing standards, supported by documentation and tools based on those standards. Numerous existing middlebox devices are also based on those standards. These people, documents, tools and devices represent a very large investment that would be seriously impacted by a change in the /64 boundary.

4.5. Privacy issues

The length of the interface identifier has implications for privacy [I-D.ietf-6man-ipv6-address-generation-privacy]. In any case in which the value of the identifier is intended to be hard to guess, whether or not it is cryptographically generated, it is apparent that more bits are better. For example, if there are only 20 bits to be guessed, at most just over a million guesses are needed, today well within the capacity of a low cost attack mechanism. It is hard to state in general how many bits are enough to protect privacy, since this depends on the resources available to the attacker, but it seems clear that a privacy solution needs to resist an attack requiring billions rather than millions of guesses. Trillions would be better, suggesting that at least 40 bits should be available. Thus we can argue that subnet prefixes longer than say /80 might raise privacy concerns by making the IID guessable.

A prefix long enough to limit the number of addresses comparably to an IPv4 subnet, such as /120, would create exactly the same situation for privacy as IPv4 except for the absence of NAT. In particular, a host would be forced to pick a new IID when roaming to a new network, to avoid collisions. An argument could be made that since this reduces traceability, it is a good thing from a privacy point of view.

5. Security Considerations

In addition to the privacy issues mentioned in Section 4.5, and the issues mentioned with CGAs and HBAs in Section 4.2, the length of the subnet prefix affects the matter of defence against scanning attacks [I-D.ietf-opsec-ipv6-host-scanning]. Assuming the attacker has discovered or guessed the prefix length, a longer prefix reduces the space that the attacker needs to scan, e.g., to only 256 addresses if the prefix is /120. On the other hand, if the attacker has not discovered the prefix length and assumes it to be /64, routers can trivially discard attack packets that do not fall within an actual subnet.

However, assume that an attacker finds one valid address A and then starts a scanning attack by scanning "outwards" from A, by trying A+1, A-1, A+2, A-2, etc. This attacker will easily find all hosts in any subnet with a long prefix, because they will have addresses close to A. We therefore conclude that any prefix containing densely packed valid addresses is vulnerable to a scanning attack, without the attacker needing to guess the prefix length. Therefore, to preserve IPv6's advantage over IPv4 in resisting scanning attacks, it is important that subnet prefixes are short enough to allow sparse allocation of identifiers within each subnet. The considerations are similar to those for privacy, and we can again argue that prefixes longer than say /80 might significantly increase vulnerability. Ironically, this argument is exactly converse to the argument for longer prefixes to resist an ND cache attack, as described in Section 3.4.

Denial of service attacks related to Neighbor Discovery are discussed in Section 3.4 and in [RFC6583]. One of the mitigations suggested by that document is "sizing subnets to reflect the number of addresses actually in use", but the fact that this greatly simplifies scanning attacks is not noted. For further discussion of scanning attacks, see [I-D.ietf-opsec-ipv6-host-scanning].

Note that, although not known at the time of writing, there might be other resource exhaustion attacks available, similar in nature to the ND cache attack. We cannot exclude that such attacks might be exacerbated by sparsely populated subnets such as a /64. It should also be noted that this analysis assumes a conventional deployment model with a significant number of end-systems located in a single LAN broadcast domain. Other deployment models might lead to different conclusions.

6. IANA Considerations

This document requests no action by IANA.

7. Acknowledgements

This document was inspired by a vigorous discussion on the V6OPS working group mailing list with at least 20 participants. Later, valuable comments were received from Ran Atkinson, Fred Baker, Steven Blake, Lorenzo Colitti, David Farmer, Bill Fenner, Ray Hunter, Jen Linkova, Philip Matthews, Mark Smith, Tatuya Jinmei, Fred Templin, Stig Venaas, and numerous other participants in the 6MAN working group.

This document was produced using the xml2rfc tool [RFC2629].

8. Change log [RFC Editor: Please remove]

draft-ietf-6man-why64-00: WG adoption, WG comments, including major text reorganisation: 3 main sections describe advantages of fixed length IID, arguments for shorter lengths, and expected effects of varying the length, 2014-04-11.

draft-carpenter-6man-why64-01: WG comments, added experimental results, implementation/deployment text, 2014-02-06.

draft-carpenter-6man-why64-00: original version, 2014-01-06.

9. References

9.1. Normative References

[I-D.ietf-opsec-ipv6-host-scanning] Gont, F. and T. Chown, "Network Reconnaissance in IPv6 Networks", Internet-Draft draft-ietf-opsec-ipv6-host-scanning-00, December 2012.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet Networks", RFC 2464, December 1998.
[RFC2467] Crawford, M., "Transmission of IPv6 Packets over FDDI Networks", RFC 2467, December 1998.
[RFC2470] Crawford, M., Narten, T. and S. Thomas, "Transmission of IPv6 Packets over Token Ring Networks", RFC 2470, December 1998.
[RFC2492] Armitage, G., Schulter, P. and M. Jork, "IPv6 over ATM Networks", RFC 2492, January 1999.
[RFC2497] Souvatzis, I., "Transmission of IPv6 Packets over ARCnet Networks", RFC 2497, January 1999.
[RFC2526] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast Addresses", RFC 2526, March 1999.
[RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4 Domains without Explicit Tunnels", RFC 2529, March 1999.
[RFC2590] Conta, A., Malis, A. and M. Mueller, "Transmission of IPv6 Packets over Frame Relay Networks Specification", RFC 2590, May 1999.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4 Clouds", RFC 3056, February 2001.
[RFC3146] Fujisawa, K. and A. Onoe, "Transmission of IPv6 Packets over IEEE 1394 Networks", RFC 3146, October 2001.
[RFC3306] Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6 Multicast Addresses", RFC 3306, August 2002.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC3956] Savola, P. and B. Haberman, "Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address", RFC 3956, November 2004.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)", RFC 3972, March 2005.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and More-Specific Routes", RFC 4191, November 2005.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006.
[RFC4338] DeSanti, C., Carlson, C. and R. Nixon, "Transmission of IPv6, IPv4, and Address Resolution Protocol (ARP) Packets over Fibre Channel", RFC 4338, January 2006.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through Network Address Translations (NATs)", RFC 4380, 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.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J. and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, September 2007.
[RFC5072] Varada, S., Haskins, D. and E. Allen, "IP Version 6 over PPP", RFC 5072, September 2007.
[RFC5121] Patil, B., Xia, F., Sarikaya, B., Choi, JH. and S. Madanapalli, "Transmission of IPv6 via the IPv6 Convergence Sublayer over IEEE 802.16 Networks", RFC 5121, February 2008.
[RFC5214] Templin, F., Gleeson, T. and D. Thaler, "Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, March 2008.
[RFC5453] Krishnan, S., "Reserved IPv6 Interface Identifiers", RFC 5453, February 2009.
[RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming Shim Protocol for IPv6", RFC 5533, June 2009.
[RFC5535] Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535, June 2009.
[RFC5692] Jeon, H., Jeong, S. and M. Riegel, "Transmission of IP over Ethernet over IEEE 802.16 Networks", RFC 5692, October 2009.
[RFC5942] Singh, H., Beebee, W. and E. Nordmark, "IPv6 Subnet Model: The Relationship between Links and Subnet Prefixes", RFC 5942, July 2010.
[RFC5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4 Infrastructures (6rd) -- Protocol Specification", RFC 5969, August 2010.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y. and P. Eronen, "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC 5996, September 2010.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M. and X. Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052, October 2010.
[RFC6146] Bagnulo, M., Matthews, P. and I. van Beijnum, "Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers", RFC 6146, April 2011.
[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, April 2011.
[RFC6177] Narten, T., Huston, G. and L. Roberts, "IPv6 Address Assignment to End Sites", BCP 157, RFC 6177, March 2011.
[RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix Translation", RFC 6296, June 2011.
[RFC6437] Amante, S., Carpenter, B., Jiang, S. and J. Rajahalme, "IPv6 Flow Label Specification", RFC 6437, November 2011.
[RFC6741] Atkinson,, RJ., "Identifier-Locator Network Protocol (ILNP) Engineering Considerations", RFC 6741, November 2012.
[RFC7084] Singh, H., Beebee, W., Donley, C. and B. Stark, "Basic Requirements for IPv6 Customer Edge Routers", RFC 7084, November 2013.
[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6 Interface Identifiers", RFC 7136, February 2014.

9.2. Informative References

[DRAFT-odell] O'Dell, M., "8+8 - An Alternate Addressing Architecture for IPv6", draft-odell-8+8.00 (work in progress), October 1996.
[I-D.brandt-6man-lowpanz] Brandt, A. and J. Buron, "Transmission of IPv6 packets over ITU-T G.9959 Networks", Internet-Draft draft-brandt-6man-lowpanz-02, June 2013.
[I-D.ietf-6lowpan-btle] Nieminen, J., Savolainen, T., Isomaki, M., Patil, B., Shelby, Z. and C. Gomez, "Transmission of IPv6 Packets over BLUETOOTH Low Energy", Internet-Draft draft-ietf-6lowpan-btle-12, February 2013.
[I-D.ietf-6man-6lobac] Lynn, K., Martocci, J., Neilson, C. and S. Donaldson, "Transmission of IPv6 over MS/TP Networks", Internet-Draft draft-ietf-6man-6lobac-01, March 2012.
[I-D.ietf-6man-ipv6-address-generation-privacy] Cooper, A., Gont, F. and D. Thaler, "Privacy Considerations for IPv6 Address Generation Mechanisms", Internet-Draft draft-ietf-6man-ipv6-address-generation-privacy-00, October 2013.
[I-D.ietf-6man-stable-privacy-addresses] Gont, F., "A method for Generating Stable Privacy-Enhanced Addresses with IPv6 Stateless Address Autoconfiguration (SLAAC)", Internet-Draft draft-ietf-6man-stable-privacy-addresses-03, January 2013.
[I-D.ietf-homenet-arch] Chown, T., Arkko, J., Brandt, A., Troan, O. and J. Weil, "Home Networking Architecture for IPv6", Internet-Draft draft-ietf-homenet-arch-07, February 2013.
[I-D.ietf-v6ops-64share] Byrne, C., Drown, D. and V. Ales, "Extending an IPv6 /64 Prefix from a 3GPP Mobile Interface to a LAN link", Internet-Draft draft-ietf-v6ops-64share-09, October 2013.
[I-D.templin-aerolink] Templin, F., "Transmission of IPv6 Packets over AERO Links", Internet-Draft draft-templin-aerolink-06, February 2014.
[IEEE802] IEEE, "IEEE Standard for Local and Metropolitan Area Networks: Overview and Architecture", IEEE Std 802-2001 (R2007), 2007.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, June 1999.
[RFC4887] Thubert, P., Wakikawa, R. and V. Devarapalli, "Network Mobility Home Network Models", RFC 4887, July 2007.
[RFC5505] Aboba, B., Thaler, D., Andersson, L. and S. Cheshire, "Principles of Internet Host Configuration", RFC 5505, May 2009.
[RFC6583] Gashinsky, I., Jaeggli, J. and W. Kumari, "Operational Neighbor Discovery Problems", RFC 6583, March 2012.
[RFC6741] Atkinson,, RJ., "Identifier-Locator Network Protocol (ILNP) Engineering Considerations", RFC 6741, November 2012.
[RFC6877] Mawatari, M., Kawashima, M. and C. Byrne, "464XLAT: Combination of Stateful and Stateless Translation", RFC 6877, April 2013.

Authors' Addresses

Brian Carpenter (editor) Department of Computer Science University of Auckland PB 92019 Auckland, 1142 New Zealand EMail: brian.e.carpenter@gmail.com
Tim Chown University of Southampton Southampton, Hampshire SO17 1BJ United Kingdom EMail: tjc@ecs.soton.ac.uk
Fernando Gont SI6 Networks / UTN-FRH Evaristo Carriego 2644 Haedo, Provincia de Buenos Aires 1706 Argentina EMail: fgont@si6networks.com
Sheng Jiang Huawei Technologies Co., Ltd Q14, Huawei Campus No.156 Beiqing Road Hai-Dian District, Beijing, 100095 P.R. China EMail: jiangsheng@huawei.com
Alexandru Petrescu CEA, LIST CEA Saclay Gif-sur-Yvette, Ile-de-France 91190 France EMail: Alexandru.Petrescu@cea.fr
Andrew Yourtchenko cisco 7a de Kleetlaan Diegem, 1830 Belgium EMail: ayourtch@cisco.com