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Versions: 00 01

Internet Engineering Task Force                                 M. Smith
Internet-Draft                                          October 22, 2018
Updates: RFC4291, RFC4443, RFC6724 (if
         approved)
Intended status: Standards Track
Expires: April 25, 2019


     IPv6 Formal Anycast Addresses and Functional Anycast Addresses
            draft-smith-6man-form-func-anycast-addresses-00

Abstract

   IPv6 anycast addresses are chosen from within the existing IPv6
   unicast address space, with the addresses nominated as anycast
   addresses through configuration.  An alternative scheme is to have a
   special class of addresses for use as anycast addresses.  This memo
   proposes a distinct general anycast addressing class and a more
   specific scheme for functional anycast addresses.

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 https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 25, 2019.

Copyright Notice

   Copyright (c) 2018 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
   (https://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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Drawbacks of Informal Anycast Addresses . . . . . . . . . . .   4
   4.  Formal Anycast Addresses  . . . . . . . . . . . . . . . . . .   5
     4.1.  Address Format  . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Address Fields  . . . . . . . . . . . . . . . . . . . . .   5
       4.2.1.  Formal Anycast Prefix . . . . . . . . . . . . . . . .   5
       4.2.2.  Visible Scope . . . . . . . . . . . . . . . . . . . .   5
       4.2.3.  Anycast Identifier Format . . . . . . . . . . . . . .   5
       4.2.4.  Anycast Identifier  . . . . . . . . . . . . . . . . .   6
     4.3.  Link-Local Visible Scope  . . . . . . . . . . . . . . . .   6
     4.4.  ICMPv6 Destination Unreachable Message  . . . . . . . . .   7
     4.5.  Default Address Selection . . . . . . . . . . . . . . . .   8
       4.5.1.  Destination Address Selection . . . . . . . . . . . .   8
       4.5.2.  Formal Anycast Scope Comparison . . . . . . . . . . .   8
     4.6.  Advice on Structuring the Anycast Identifier Field Values   8
   5.  Functional Anycast Addresses  . . . . . . . . . . . . . . . .  10
     5.1.  Features  . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Address Format  . . . . . . . . . . . . . . . . . . . . .  11
     5.3.  Assignment of Anycast Function Identifiers  . . . . . . .  12
     5.4.  Assigned Anycast Function Identifiers . . . . . . . . . .  13
     5.5.  Sources of Inspiration for Anycast Function Identifiers .  14
     5.6.  Global Scope Functional Anycast Addresses on the Internet  15
     5.7.  Example Use Cases . . . . . . . . . . . . . . . . . . . .  16
       5.7.1.  Devices Factory Configured with NTP Functional
               Anycast Addresses . . . . . . . . . . . . . . . . . .  16
       5.7.2.  Branch Office DNS Resolvers . . . . . . . . . . . . .  18
       5.7.3.  Automatic eBGP Session Establishment  . . . . . . . .  19
       5.7.4.  ISP's Anycast DNS Resolvers . . . . . . . . . . . . .  21
       5.7.5.  Microservices Architecture Applications . . . . . . .  22
       5.7.6.  Global Time Distribution Network  . . . . . . . . . .  22
       5.7.7.  Example3  . . . . . . . . . . . . . . . . . . . . . .  22
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  23
   9.  Change Log [RFC Editor please remove] . . . . . . . . . . . .  23
   10. Informative References  . . . . . . . . . . . . . . . . . . .  23
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  25







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1.  Introduction

   [RFC1546] was the first description of host anycast services, and
   proposed two ways of supporting them in terms of addressing:

   o  using parts of the existing address space

   o  create a special class of addresses for anycast use

   The first method of supporting anycast addresses, by using parts of
   the existing (unicast) address space, could be described as informal.
   From the address itself, it is not possible to determine that the
   apparent unicast address is actually being used as an anycast
   address.

   As the second method would create a special class of addresses for
   anycast use, it could be described as formal.  Encoded within the
   addresses would be a value and perhaps other attributes that
   indicates they are anycast addresses, regardless of context.

   In terms of a spectrum of delivery ranging from to a single or to
   multiple destinations, anycast addresses are a distinct class of
   addresses when compared to unicast and multicast addresses.  Packets
   sent to a unicast destination are intended to be delivered to one and
   only one unique destination host.  Packets sent to a multicast
   destination are intended to be delivered to a group of interested
   destination hosts, with the interested group consisting of one or
   more members, and the packets being duplicated by the network when
   and where necessary.  Packets sent to an anycast destination are
   intended to be delivered to only one of a set of hosts sharing the
   same anycast address.  As a type of address, anycast addresses can be
   imagined to fall between unicast and multicast address types on the
   delivery spectrum.

   IPv6 anycast addresses [RFC4291] are currently from within the
   existing unicast address address space.  Therefore, this memo gives
   these IPv6 anycast addresses the name "Informal Anycast" addresses.

   This memo proposes a special and formal class of IPv6 addresses for
   anycast use, calling them "Formal Anycast" addresses.

   The described IPv6 Formal Anycast address class can support a total
   of 16 sub-classes of anycast address formats and structures.
   Following the description of Formal Anycast address class, this memo
   then proposes the first sub-class called "Functional Anycast"
   addresses.





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   There are some existing reserved and well known anycast addresses
   within the existing Informal Anycast address space.  While well
   known, they do not have any of the formal attributes that the
   proposed formal Functional Anycast addresses have, other than having
   specified and well known values; they could be described as semi-
   formal.  Well known Functional Anycast addresses are proposed that
   correspond to these existing semi-formal anycast addresses.

2.  Terminology

   Anycast Domain

   Formal Anycast Address

   Functional Anycast Address

   Informal Anycast Address

   Semi-Formal Anycast Address

3.  Drawbacks of Informal Anycast Addresses

   There are drawbacks and limitions of IPv6 Informal Anycast addresses:

   o  As mentioned in the Introduction, there is nothing specifically
      encoded in an Informal Anycast address to distinguish it from a
      unicast address, such as being from within a well known address
      prefix.  In some situations, this unintentional obfuscation may be
      of use, however in others, such as while troubleshooting, it can
      be detrimental.  For example, duplicate routes for an address
      appearing in a route table with different and distinct announcing
      routers may be a fault if unintentional, meaning it is a duplicate
      unicast address assignment.  Alternatively, it may be the intended
      configuration if the routes are Informal Anycast routes i.e., the
      address from within the unicast address space is being used an
      anycast address.  The duplicate routes and the addresses
      themselves provide no indication of whether the configuration is
      intentional or not.

   o  Constraining the visibility and reachability of an anycast
      provided service or function may be useful for security reasons,
      which can be fundamentally enforced by encoding and limiting the
      scope of or domain where packets are intended and able to be
      forwarded.  Informal Anycast addresses can only have one of three
      fundamental forwarding scopes encoded in the address, matching
      those of the three types of IPv6 unicast addresses - limited to a
      link scope (Link-Local Address), limited to a network scope
      (Unique Local Address) or having global Internet scope (Global



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      Unicast Address) [RFC4291][RFC4193].  These scopes are coarse.
      More fine grained scopes, such as those used in IPv6 multicast
      [RFC7346], would provide much more control over anycast service or
      function visibility.

4.  Formal Anycast Addresses

4.1.  Address Format

   The following diagram shows the structure of an IPv6 Formal Anycast
   address.

4.2.  Address Fields

4.2.1.  Formal Anycast Prefix

   A 8 bit prefix value of TBD (aa00::/8 preferred, indicating a Anycast
   Address; fa00::/8 an alternative, indicating a Formal Anycast
   address), identifying this address as a Formal Anycast address.

4.2.2.  Visible Scope

   A 4 bit Visible Scope field used to express and enforce visibility
   and assumed reachability of the Formal Anycast address.  The values
   and meanings of the values of this field are the same as those for
   the IPv6 multicast address scope field [RFC4291][RFC7346].

   When packets with a Formal Anycast destinaton address are being
   forwarded, this field's value takes precedence over a non-zero Hop
   Limit field value, meaning the packet MUST be discarded at the edge
   of the indicated visibility domain even though it may have a non-zero
   Hop Limit value.  A specific ICMPv6 Destination Unreachable [RFC4443]
   message, described below, SHOULD be generated and returned to the
   packet's sender indicating the packet was discarded as it reached the
   edge of its Visible Scope.

4.2.3.  Anycast Identifier Format

   A 4 bit field identifying the format of the following Anycast
   Identifier field, holding digits in the range of 0x0 through 0xf in
   hexidecimal.

   The first assigned value is 0, specifying that the following Anycast
   Identifier Format is that of a Functional Anycast addresses, which is
   described later in this memo.

   Other values will be assigned by IANA as future Anycast Identifier
   Formats are specified.



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4.2.4.  Anycast Identifier

   A 112 bit field holding the Anycast Identifier value.  The format and
   structure of this field is encoded in the previous Anycast Identifier
   Format field.

4.3.  Link-Local Visible Scope

   One of the possible Visible Scope values is the Link-Local scope,
   specifying that the Formal Anycast address's visibility is limited to
   a link that the host is directly attached to.

   Nodes on the link will need to consider Formal Anycast addresses with
   a Link-Local Visible Scope on-link, so that they perform Neighbor
   Discovery [RFC4861] for these addresses.

   Similar to the unicast Link-Local prefix [RFC5942], IPv6
   implementations SHOULD BE updated to consider the Formal Anycast
   prefix with a Link-Local Visible Scope on-link.  Using the
   (preferred, IANA TBA) aa00::/8 Formal Anycast prefix, this means IPv6
   implementations will consider aa02::/16 to be on-link.

   Unlike the unicast Link-Local prefix, updated IPv6 implemenations
   MUST NOT use SLAAC [RFC4862] to generate an automatic address from
   within this Formal Anycast Link-Local Visible Scope prefix (MRS: Need
   to further consider this, is there a case I've missed where this
   could be useful?  Hmm, what about, for example, the router subnet
   anycast address?  Perhaps anycast providing applications could/should
   resister their anycast addresses similar to multicast.  Hmm, there
   really needs to be any "Anycast Listener Discovery" protocol similar
   or same as MLD.).

   Note that unlike the unicast Link-Local prefix, IPv6 nodes may not
   and typically would not have an address from within the Formal
   Anycast Link-Local Visible Scope prefix.  One of the node's Link-
   Local addresses on the same link should be used as a source address
   when sending to a Formal Anycast Link-Local Visible Scope
   destination.  This does not preclude using other greater scope
   unicast source addresses.  (MRS: probably need to update source/
   destinaton address selection RFC - I think that is already somewhere
   in this draft?)

   In the interrim IPv6 implementation update period, IPv6 nodes can be
   informed that the Formal Anycast Link-Local Visible Scope prefix is
   on-link in one of two ways:

   o  A manually configured entry in the host's Prefix List [RFC4861].




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   o  A dynamic update to nodes' Prefix Lists using a Router
      Advertisement Prefix Information Option (PIO) for the Formal
      Anycast Link-Local Visible Scope prefix, with the 'L' or on-link
      flag set to on, and the 'A' or autonomous address-configuration
      flag set to off (as this prefix MUST NOT be used by the node to
      automatically generate an address for its use within this prefix).
      The Valid and Preferred Lifetimes for the Formal Anycast Link-
      Local Visible Scope prefix in the PIO are set to infinity.

4.4.  ICMPv6 Destination Unreachable Message

   As mentioned prevously, if a packet with a Formal Anycast destination
   address reaches the edge of the Visible Scope for the address, a
   ICMPv6 Destination Unreachable [RFC4443] message SHOULD be generated
   and sent back to trigger packet's sender.

   Note that if the router at the edge of the visibility domain is also
   assigned the Formal Anycast address, the packet is host processed
   locally rather than being discarded, and an ICMPv6 Destination
   Unreachable message IS NOT generated and returned to the packet's
   sender.

   When a router implementation formally supports Formal Anycast
   addresses, the ICMPv6 Code for the Destination Unreachble message is
   IANA-TBD, indicating that the Edge of the Visible Scope [was]
   Reached.

   If a router implementation does not formally support Formal Anycast
   addresses an operator should use packet filters to enforce the
   Visible Scope boundary.  A packet failing to pass the packet filter
   should cause the router to generate a Destination Unreachable
   Communication with destination administratively prohibited message
   [RFC4443] (Code 1) message, which is semantically similar to the
   formal Edge of Visibile Scope Reached message.

   Note that ICMPv6 messages are not sent reliably, so Formal Anycast
   packet senders will need to be able to handle not receiving a ICMPv6
   Destination Unreachable message in response to a packet reaching the
   edge of the visibility domain.

   There may be situations where silently discarding Formal Anycast
   packets at the Visible Scope boundary may be preferred.  In this
   case, a packet discard route, covering the Visible Scope prefix can
   be installed in a router's forwarding table.







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4.5.  Default Address Selection

4.5.1.  Destination Address Selection

   An IPv6 implementation may be presented with a candidate set of
   destination addresses that consists of both Formal Anycast and
   unicast addresses.  The implementation needs to make a choice or
   choices as to which of these candidate addresses to attempt to use.

   The decision to use a Formal Anycast address instead of a unicast
   address is an active and conscious one.  Therefore, when a choice
   needs to be made between a Formal Anycast address and a unicast
   address, the Formal Anycast address should always be preferred.

   In terms of the Destinaton Address Selection algorithm described in
   [RFC6724], this preference of Formal Anycast over unicast addresses
   introduces a new rule between Rule 1 ("Avoid unusable destinations")
   and Rule 2 ("Prefer matching scope"), specifically (using "1.5" here
   to indicate the position):

      Rule 1.5: Prefer Formal Anycast addresses.

      If DA is a unicast address, and DB is a Formal Anycast address,
      then prefer DB.

   Note that there may be instances where an application would prefer to
   use a unicast address over a Formal Anycast address.  In this case,
   Formal Anycast addresses can be identified and ignored using the well
   known 8 bit Formal Anycast prefix.

4.5.2.  Formal Anycast Scope Comparison

   As the Formal Anycast address scopes are defined to be the same as
   Multicast address scopes, the same Multicast scope comparison
   methods, described in [RFC6724], are used with Formal Anycast address
   scopes.

4.6.  Advice on Structuring the Anycast Identifier Field Values

   The Anycast Identifier Format field within a Formal Anycast address
   specifies the format and structure of the 112 bit Anycast Identifier
   field.  The following is advice and guidelines to use when developing
   a new Anycast Identifier field format and structure.

   Forwarding towards anycast addresses is the same as forwarding
   towards unicast addresses, which uses the longest match rule BCP 198
   [RFC7608].  Longest match forwarding facilitates summarisation of
   forwarding information, where a single more general forwarding route



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   can summarise a number of more specific forwarding routes.
   Summarisation saves entries in forwarding tables outside of the
   summarised forwarding domain, provides simpler destination based
   filtering for security purposes, and facilitates easier destination
   address based traffic analysis.

   The use of route summarisation with anycast addresses effectively
   creates an anycast domain that is being identified and summarised by
   the anycast summary route.  Outside of the anycast domain, a single
   summary route exists, covering all anycast addresses within the
   domain.  Within the anycast domain, individual routes for individual
   anycast addresses exist.

   When designing a new Anycast Identifier field format and structure,
   the following guidelines should be followed.  These guidelines should
   allow a set of more specific anycast routes to be summarised as well
   as improving operator usability.

   o  The order of fields within the Anycast Identifier field, should be
      from the most general to most specific, in the direction following
      the high order to low order bits of the Anycast Identifier field
      and the broader IPv6 address.

   o  The order of bits within fields should also be from the most
      general to most specific, matching the direction of high order to
      low order of bits within the Anycast Identifier field.

   o  The bits in fields holding bits that are matched exactly, such as
      flag fields or fields holding numeric values that are matched
      exactly, can be orded to suit the field's use and application.
      However, a hierarchial order, from most significant to least
      significant bit, following Anycast Identifier field bit order, is
      suggested.  Although initially defined hierarchially, the order of
      flags in flag fields may later deviate from this recommendation
      due to later flag bit definition.

   o  End-users of the functions and services being provided using
      Formal Anycast addresses are unlikely and ideally should never see
      these addresses.  However, operators of these functions and
      services will deal with these addresses during planning,
      configuration and troubleshooting.  Where possible, fields and
      their values should be ordered and structured to assist with these
      tasks.  Field boundaries within the Anycast Identifier field
      should align with 16 bit word, 8 bit octet or 4 bit nibble
      positions within the whole IPv6 address.  For example, if part of
      an IPv6 prefix is included in the Anycast Identifier, it should
      start at a 16 bit "piece" boundary, where colons appear [RFC4291],
      within the IPv6 address.  Note that this guildline should not take



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      precedence over any previous measures to faciliate more specific
      anycast route summarisation.

   o  A further address usability recommendation is to set field and bit
      values to zero for the likely most common or likely most secure
      meaning for these fields or bit values.  In IPv6 addresses zero
      field values can be compressed [RFC5952], resulting in a shorter
      address for an operator to enter.  A shorter address to enter
      naturally reduces the opportunities for and likelihood of errors
      in the address, and reduces the possibilities of security issues
      caused by errors in the Formal Anycast address..

5.  Functional Anycast Addresses

   The first defined sub-class or sub-format of Formal Anycast addresses
   is the Functional Anycast address sub-class.

5.1.  Features

   The following are the features of Functional Anycast addresses.  In
   many cases they're inspired by and mirror IPv6 multicast address
   features [RFC4291][RFC3306].

   o  Provides separate globally well known and local network defined
      anycast function or service identifier spaces.  Globally well
      known identifiers can be encoded in applications during their
      development as constants, avoiding the need for them to be
      specified and configured during deployment of the application.

   o  Provides a minimum of 24 bits for use as identifiers for anycast
      functions or services, supporting more than 16 million values..

   o  Provides 8 bits for the identification of up to 256 local
      instances, versions or revisions of the same function or service,
      assisting with function or service deployment or maintenance.
      These 8 bits can also be used to increase the size of the function
      or service identifier space to 32 bits where useful, increasing
      the range of values to more than 4 billion.

   o  Identifies an anycast function or service identifier space, known
      as an anycast domain, using an IPv6 unicast address prefix of up
      to 64 bits in length.  A network can create multiple distinct
      anycast domains by using multiple of its IPv6 prefixes, from its
      Global [RFC4291] or Unique Local [RFC4193] address spaces (the
      Link-Local prefix could be used to create a distinct anycast
      domain, however it can only be used once, despite the network
      having many instances of the Link-Local prefix (as many as it has
      links)).  A "unspecified" anycast domain is supported using an all



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      zeros 64 bit IPv6 prefix.  External to the anycast domain, the
      identifying 64 bit prefix can be used to create a single summary
      route for the anycast function or service identifier space, which
      will help routing scaling for anycast functions or services.  The
      anycast domain boundary could also correspond to routing protocol
      scaling boundaries, such as OSPF areas [RFCxxxx] or IS-IS level
      [RFCxxxx] boundaries, when useful.

5.2.  Address Format

   The format of Functional Anycast addresses is modelled on the IPv6
   multicast address format [RFC4291].

   The format of an Functional Anycast address is as follows:

   [diagram to come]

   The address fields are as follows:

   o  Anycast Domain Prefix - a 64 bit field holding a IPv6 unicast
      address prefix identifying the anycast domain that is either the
      provider and possible authority for the following Anycast Function
      Identifier space.  The length of the prefix is specified in the
      following Prefix-Length field, with the remaining bits of the
      field set to zero.  An all zero Anycast Domain Prefix means an
      unspecified Anycast Function Identifier provider.  An all zeros
      Prefix in effect means "this" provider, with "this" meaning the
      current anycast domain.  Link-Local, Unique-Local [RFC4193],
      Global and possible future other unicast prefixes [RFC4291]
      identify a Anycast Function Identifier provider.  Within an
      anycast function domain, this allows multiple anycast function
      sub-domains to be created, identified by different unicast
      Prefixes in this field.

   o  Reserved - a 2 bit reserved field.  Set to zero upon transmission,
      ignored upon receipt.

   o  Prefix-Length - An 6 bit field specifying the length of the
      previous Anycast Domain Prefix.  A value of zero means a 64 bit
      length prefix, while prefix lengths of 1 through 63 (0x01 through
      0x3f) are encoded natively.  The unspecified Anycast Domain Prefix
      of all zeros is considered to be 64 bits in length, meaning a
      Prefix-Length value of 0 for this prefix.  This is an
      informational field to assist with operation and troubleshooting.

   o  Flags - A 8 bit flag field.  The lower 7 flags are reserved and
      must be set to zero upon transmission, and ignored upon receipt.
      The high order flag is the 'T' or Transient flag.  T=0 indicates



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      that the later Anycast Function Identifier is well known and
      assigned by the Internet Assigned Numbers Authority (IANA).  T=1
      indicates that the Anycast Function Identifier is transient or
      dyamically assigned, and has been assigned by the Functonal
      Anycast domain's local authority.

   o  Local Instance - An 8 bit field holding a identifier of the
      instance, version or revision of the function identified by the
      following Anycast Function Identifier field, local to the current
      anycast domain.  The default value of this field is zero,
      indicating the default and first instance of the anycast function.
      Non-default values are chosen by the local anycast domain
      operator, even when the following Anycast Function Identifier is
      using a well-known IANA Anycast Function Identifier value.  An
      anycast domain operator may chose to assign other semantics to
      this field, as long as they're both less significant than the
      previous fields and more sigificant than the following Anycast
      Function Identifier field.  When the 'T' bit in the Flags field is
      set to 1, meaning transient Anycast Function Identifiers, this
      field could be used to effectively increase the size of the
      following Anycast Function Identifier field to 32 bits, increasing
      the value range of Anycast Function Identifiers from in the order
      of 16 million to in the order of 4 billion.

   o  Anycast Function Identifier - A 24 bit field identifying the
      anycast function to be performed on the packet when it arrives at
      a host that has been configured with the Functional Anycast
      address.  When the 'T' bit in the Flags field is set to zero, the
      Anycast Function Identifiers values are from a well known Anycast
      Function Identifier registry maintained by IANA, with initial
      entries specified later in this memo.  When the 'T'bit in the
      Flags field is set to 1, the values in the Anycast Function
      Identifier field are local to and assigned by the authority
      identified in the Prefix field in any manner that suits their
      purposes and requirements.

5.3.  Assignment of Anycast Function Identifiers

   In the history of the Internet, it has been common to conflate a
   function or service with a protocol.

   For example, historically, the telnet protocol [RFCxxxx] had been the
   most popular remote virtual terminal protocol.  In more recent times,
   the SSH protocol [RFCxxx] has become the de facto remote virtual
   terminal protocol.  Accessing the remote virtual terminal service has
   either been referred to as "telnet in" or "SSH in" to the host
   providing the service, using the protocol being used to refer to the
   service being accessed.



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   In either case, telnet and SSH protocols are being used to access a
   remote virtual terminal service.  Functionally, from the perspective
   of remote virtual terminal access, the differences are relatively
   minor; data security and integrity via encryption and authentication
   is where the primary differences between telnet and SSH are - in
   telnet encryption of the data stream is optional [RFCxxx], where as
   with SSH it is mandatory.

   When both IANA and local anycast domain operators assign Anycast
   Function Identifiers, it is recommended that they're allocated and
   identified by protocol agnostic function or service type rather than
   to a specific protocol that provides that function or service.  As
   the particular protocol being used to access the function or service
   will be encoded in the upper transport layer protocols and ports in
   the IPv6 packet, service or function based Anycast Function
   Identifers can support and stay constant across the use and evolution
   of different function or service access protocols.

   For example, with a well-known Anycast Function Identifier
   specifically allocated to a Network Virtual Terminal service (NVT)
   [RF852?], the hosts providing the NVT service could initially support
   both telnet (assuming telnet is considered secure enough) and SSH.
   If both telnet and SSH become deprecated, and a new NVT access
   protocol is developed, the same Anycast Function Identifier for the
   NVT service could be used to reach a node supporting this new access
   protocol.

   Another example is the domain name service.  Currently domain name
   resolution takes place using the Domain Name Service protocol
   [RFCxxx], natively carried over UDP and TCP, using port 53.  More
   recently, work has been taking place to operate DNS over TLS [RFCxxx]
   and HTTP [RFCxxx] to enhance the security of the domain name
   resolution function.  The use of multiple protocols to access
   fundamentally the same domain name resolution function suggests a
   protocol agnostic domain name resolution Anycast Function Identifier.

   This doesn't preclude Anycast Function Identifiers being used to
   support and identify specific protocols (examples of this occur
   later).  There may be current and future cases where the allocation
   and use of an Anycast Function Identifier for a specific protocol is
   the better choice.  This should be considered and evaluated on a case
   by case basis.

5.4.  Assigned Anycast Function Identifiers

   A number of past RFCs have reserved anycast addresses and
   identifiers.  These addresses and indentifiers are mapped to the
   following corresponding and well known Anycast Function Identifiers,



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   and are to be listed in the IANA Anycast Function Address Identifier
   registery if it is created.

   [RFC2526] reserves the highest 127 values of a subnet prefix
   Interface Identifier for anycast addresses.  The equivalent values
   for Functional Anycast addresses are the highest 127 values of the 32
   bit Anycast Function Identifier, a range of (in IPv6 address format,
   excluding the high order 96 bits) :ffff:fff8 through :ffff:ffff.  The
   IANA Internet Protocol Version 6 (IPv6) Anycast Addresses registry
   [IANA-IPV6ANYC] records assignments for subnet prefix anycast
   addresses within the Interface Identifier space.  The current and
   future values of these anycast subnet prefix Interface Identifier
   values are to also be recorded in the Anycast Function Address
   Identifier registry.

   [RFC4291] reserves an Interface Identifer of all zeros within a
   unicast prefix as the Subnet-Router anycast address.  The equivalent
   32 bit Anycast Function Identifier value for Functional Anycast
   addresses is also all-zeros.

   [RFC7723] reserves the IPv6 address 2001:1::1/128 for the use as the
   Port Control Protocol Anycast address.  The equivalent 32 bit Anycast
   Function Identifier value is (in IPv6 address format, excluding the
   high order 96 bits) :0001:0001.

   [RFC8155] reserves the IPv6 address 2001:1::2/128 for the use as the
   Traversal Using Relays around NAT Anycast address.  The equivalent 32
   bit Anycast Function Identifier value is (in IPv6 address format,
   excluding the high order 96 bits) :0001:0002.

5.5.  Sources of Inspiration for Anycast Function Identifiers

   A future possible source of inspiraton for well known assigned
   Anycast Function Identifiers could be DHCPv6 [RFC3315] options that
   encode IPv6 addresses for services.  A number of these options encode
   multiple IPv6 addresses as candidates for access to the service (for
   example, the SIP Servers IPv6 Address List option [RFC3310]).  The
   use of anycast for service resiliance would allow a single Anycast
   Function Identifier value to provide equivalent service, although
   this wouldn't preclude defining multiple different Anycast Function
   Identifiers to the service to provide the service client concurrent
   access to multiple service instances.  For example, 3 Functional
   Anycast addresses could be allocated for DNS resolvers, providing a
   client with verifiable DNS resolver services from up to 3 different
   resolvers, and allowing the client to distribute requests across all
   3 resolvers.





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   Another source of inspiration for well known assigned Anycast
   Function Identifiers could be the IANA IPv6 Multicast Address Space
   [IANA-IPV6MCAST] registry, where some of the multicast addresses
   represent services that could also be useful when provided via
   anycast.

   While using these possible source of inspiration, the recommendation
   to choose protocol agnostic function or service identifiers still
   stands.  DHCPv6 or multicast groups can be used to inspire more
   generic function or service identifiers.

5.6.  Global Scope Functional Anycast Addresses on the Internet

   Functonal Anycast addresses could be used to provide anycast services
   across the Internet, by using the the Global scope.

   When being used on the Internet, many of the possible values of the
   Prefix field are ambiguous, meaning that they wouldn't unambiguously
   identify the party using the Functional Anycast address to provide
   the service or function.  Examples of ambiguous prefixes are the all-
   zeros unspecified prefix, any ULA [RFC4193] prefixes, and the Link-
   Local [RFC4291] prefix.  Other ambiguous prefixes are those in the
   IPv6 reserved address registry [IANA-IPV6RESA] that are not valid on
   the Internet.

   To overcome this ambiguity, if Global scope Functional Addresses are
   used over the Internet, the Prefix field MUST be set to a GUA
   [RFC4291] prefix value assigned to the party providing the anycast
   service to Internet clients.  A network either accepting or
   originating a Global scope Functional Address prefix for announcement
   from a downstream stub autonomous system for announcement onto the
   Internet MUST only accept or originate a route announcement for a
   Global scope Functional Anycast prefix that includes an explicitly
   identified GUA prefix.  All other Global scope Functional Anycast
   prefix announcements with ambiguous or non-explicitly identified GUA
   prefixes MUST be ignored.

   As forwarding towards anycast addresses is functionally the same as
   forwarding towards unicast addresses, Functional Anycast prefixes
   would be announced into the Internet's unicast forwarding route
   table.

   It is common practice today to limit the prefix length of unicast
   IPv6 Internet routes accepted to a length of no more than 48 bits
   i.e. a /48.  This is a blunt and simple way to attempt to somewhat
   limit the number of IPv6 routes in the Internt route table.  It is
   imposing this limit by enforcing a minimal level of aggregation at
   the /48 boundary.  Networks using prefix lengths longer than /48 are



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   expected to aggregate those networks into a route advertisement that
   is /48 or shorter.

   This practice of limiting advertised unicast route prefix lengths to
   48 bits will limit the size of the Prefix included in Global scope
   Functional Anycast announcements to 32 bits, as the high order 16
   bits of the Functional Anycast prefix are used to encode the
   Functional Address type prefix and address scope.  This would limit
   the use of Global scope Functional Anycast addresses to provide
   global Internet anycast services to those organisations who have a
   /32 or shorter assignment from an RIR.

   As Functional Anycast addresses are a separate class of addresses,
   and are all identified by a unique /8 prefix, this /48 prefix length
   limit could be specifically relaxed for Functional Anycast routes.  A
   /48 prefix, when included in a Functional Anycast, results in a
   Functional Anycast prefix length of /64.  Imposing a /64 prefix
   length limit on Functional Anycast routes, identified by a high order
   prefix of aa0e::/16, and a GUA anycast domain Prefix, would achieve
   the same outcome of attempting to reducing the number of entries in
   the IPv6 Internet route table.

   Wide acceptance of Functional Anycast prefixes of up to 64 bits in
   length on the Internet may take same time to occur.  Use of Global
   scope Functional Anycast addresses by organisations who have RIR /32
   assignments, which will be accepted by unicast /48 prefix filters
   present today, would raise awareness of Functional Anycast addresses.
   This increased awareness could be leveraged to motivate the changing
   of prefix filters to accept Global scope Functional Anycast prefixes
   up to 64 bits in length.

5.7.  Example Use Cases

   This section provides some example use cases of Functional Anycast
   addresses that would suit the described scenarios.

5.7.1.  Devices Factory Configured with NTP Functional Anycast Addresses

   Assume IANA has allocated a set of well-known Anycast Function
   Identifier values of 0x004440, 0x004441, 0x004442 and 0x004443 for
   use with the Network Time Protocol [RFCxxxx], to facilitate meeting
   the NTP best practice of having a minimum of 4 NTP time sources [NTP
   best practices ID].

   A device manufacturer uses this set of well-known Anycast Function
   Identifier to set factory default Functional Anycast addresses for
   access to a device customer's NTP servers.




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   The corresponding Functional Anycast address is constructed as
   follows:

   o  8 bit Formal Anycast Prefix value (0xaa proposed).

   o  4 bit Visible Scope value of 0x8, corresponding to Organization-
      Local [RFC7346], as the manufacturer is unlikely to have any
      knowledge of device customers' use of or preference for smaller
      scopes.

   o  4 bit Anycast Identifier Format value of 0x0, corresponding to the
      Functional Anycast format.

   o  112 bit Anycast Identifier in the Functional Anycast format:

   o

      *  64 bit Anycast Domain Prefix value of the all zeros unspecified
         prefix.

      *  2 bit reserved field set to zero.

      *  6 bit Prefix-Length field set to zero, meaning a 64 bit length
         Anycast Domain Prefix value.

      *  8 bit Flags field set to all zeros.  The upper 7 bits are zero
         as they're served, while the lowest 'T' or Transcient flag is
         set to zero indicating an IANA assigned well known Anycast
         Function Identifier.

      *  8 bit Local Instance flag set to the default value of zero.

      *  24 bit Anycast Function Identifier field set to either
         0x004440, 0x004441, 0x00442 or 0x004443; one of the IANA
         assigned well known Anycast Function Identifier values for the
         NTP protocol.

   All of the above mean that the NTP server Functional Anycast
   addresses the device manufacturer sets as the defaults would be (in
   IPv6 address compressed format):

   o  aa08::4440

   o  aa08::4441

   o  aa08::4442

   o  aa08::4443



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5.7.2.  Branch Office DNS Resolvers

   An enterprise network operator decides to use Functional Anycast
   addresses to provide DNS resolver service to end-user devices,
   located in various corporate offices.

   Specifically for a single mid-sized branch office, with in the order
   of 200 staff, the operator decides to provide two DNS resolvers
   located in the office.  This will provide lower latency DNS
   resolution through DNS caching, reducing perceiveable application
   response time [NORMNEIL].  Access to a third, geographically close,
   off-site DNS resolver is provided for redundancy.  This off-site DNS
   resolver will be one of the other branch office's local DNS
   resolvers.

   All three DNS resolvers will provide their services to clients via
   Functional Anycast addresses.  Different clients will receive the on-
   site DNS resolver addresses in alternating order, both before the
   off-site DNS resolver address.  This provides rudimentry on-site DNS
   resolver load balancing and keeps both DNS resolvers' lookup caches
   populated to reduce the client visible performance impact of the
   fail-over to the remaining on-site DNS resolver, should its sibling
   fail.  The on-site DNS resolvers will watch each others'
   availablility, taking over its sibling's Functional Anycast address
   if the sibling becomes unavailable.  Should both on-site DNS
   resolvers become unavailable, clients will resort to using the
   remaining off-site DNS resolver.

   Assume IANA have allocated the well known Anycast Function Identifier
   values of 5300, 5301 and 5302 for use with anycast DNS resolvers.

   The operator allocates decimal identifiers of 703, 9556, 4739, 38809
   and 2764 to the corporate offices, with the order reflecting
   geographic proximity.  Each office will have its own unicast /48 from
   within a globally unique address space of 2001:db8::/32, meaning that
   the office prefixes are 2001:db8:2bf::/48, 2001:db8:2554::/48,
   2001:db8:9779::/48 and 2001:db8:acc::/48.

   The corresponding Functional Anycast address is constructed as
   follows:

   o

   The Visibiliity Scope for for these DNS resolvers' Functional Anycast
   addresses will be Organization-Local (8).

   For office 9556, using offic 4739 as an off-site backup, the
   Functional Anycast addresses for the three DNS resolvers will be:



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   1.  aa08:2001:db8:2554::5301

   2.  aa08:2001:db8:2554::5302

   3.  aa08:2001:db8:9779::5303

5.7.3.  Automatic eBGP Session Establishment

   Assume IANA has allocated a well-known Anycast Function Identifier
   value of 0x000179 for use with automatic eBGP [RFC4271?] session
   establishement.

   Smaller, stub site routers are preconfigured with a Functional
   Anycast address to attempt to automatically establish an eBGP session
   with a one or two upstream eBGP peer aggregation routers over one or
   two different designated ("WAN") links upon initialisation.

   The eBGP Functional Anycast address would be a Link-Local Visible
   scope address.  The stub router would use its link's, SLAAC generated
   [RFC4861] and link unique Link-Local address [RFC4291] as the source
   address to reach the eBGP Functional Anycast address.  Using Link-
   Local scope Formal Anycast and unicast addresses for this eBGP
   session would provide a basic level of eBGP access security.

   The stub site router will need to also be preconfigured or somehow
   automatically generate an Autonomous System Number (ASN) to use for
   establishing the eBGP session or sessions.  How the ASN is
   preconfigured or generated is out of scope for this memo, and is left
   to future work.

   Once the eBGP session is established, the peer eBGP routers trade
   routes.  These traded routes could include the upstream eBGP
   providing a default route or other more specific routes, and the
   downstream stub site router providing a route to its downstream
   prefix or prefixes.

   The downstream prefix or prefixes could be those the stub site router
   has learned via DHCPv6 Prefix Delegation (DHCPv6-PD) [RFC3633].  An
   advantage of having the stub site router inject DHCPv6-PD prefixes
   into the BGP routing domain is that the route information for this or
   these prefixes within the BGP routing domain would more accurately
   reflect the state and therefore the availability of the prefixes at
   the site they've been assigned and are being used it.  Stub site
   routers announcing their own prefixes would also distribute the
   announcement processing load across the stub site routers rather than
   concentrating it at the upstream aggregation router(s).  This also
   avoids the upstream aggregation router having to process the
   DHCPv6-PD response to determine the assigned delegated prefix for



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   subsequent BGP announcement [RFC about doing this?], meaning it can
   act as a much simpler and pure DHCPv6 relay [RFC3315].

   The upstream link(s) that a stub site is attached to does not have to
   be limitied to being a true link-layer point-to-point link, meaning
   that the link only supports a single router pair of a stub and
   upstream aggregation router.  The link could be a multi-access link,
   with the single link supporting many stub site routers and a number
   of upstream aggregation routers.

   As the eBGP Functional Anycast address is a Link-Local Visible Scope
   address, the address is configured as an anycast address on the
   upstream aggregation routers' stub site facing network interface.
   This results in the receiver of the Neighbor Advertisements for this
   address using the information received in the first received Neighbor
   Advertisement to update its neighbor cache, rather than the last and
   most recently received Neighbor Advertisement.  These types of
   Neighbor Advertisements are known as "Anycast Neighbor
   Advertisements" in [RFC4861].

   [RFC4861] says that Anycast Neighbor Advertisements should be delayed
   a random amount of time between 0 and MAX_ANYCAST_DELAY_TIME, a
   variable with a default value of 1 second.  This random delay is to
   reduce the probability of loss of the Neighbor Advertisement due to
   network congestion.

   Specific to this eBGP use case, the Anycast Neighbor Advertisements
   delay could include other metrics in the calculation to more
   intelligently distribute the eBGP sessions across the set of upstream
   aggregation routers.  For example, the number of existing eBGP
   sessions could be a metric, where an upstream aggregation router
   delays its Anycast Neighbor Advertisement longer when it has more
   established eBGP sessions.

   An operator set router preference metric could be considered,
   allowing the operator to more gracefully phase out a legacy upstream
   aggregation router by setting it preference lower than the newer
   upstream aggregation routers.  The operator would then manually
   terminate the eBGP sessions individually on the legacy upstream
   aggregation router, at a rate of something like one ever 3 seconds,
   causing them to be restablished on the higher preference and newer
   upstream aggregation routers.  This would be more graceful than
   terminating all eBGP sessions at once on the legacy upstream
   aggregation router by, for example, switching it off.

   Branch office stub router, automatically attempts to establish a BGP
   session with a well-known functional anycast address "out of the box"
   over the default WAN interface.



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   IANA assigned well-known BGP Anycast Function Identifier

   Link-local scope Functional Anycast address.  Provides a minimum
   level of security, as only possible to establish BGP sessions between
   direct link peers.

   Use unspecified prefix (comment that fe80::/64 could be used,
   although unnecessary)

   Well-known AS Number used by all stub routers.  This makes the BGP
   sessions eBGP sessions.  Routers will reject routes from other stub
   routers using the same ASN, however this is both fine and ideal as
   this is a stub router - default only plus announcing its local
   prefix(es).

   Sub router acquires a delegated prefix via DHCPv6-PD

   The delegated prefix is announced via the BGP session.  Stops the
   upstream aggregation router needing to observe a DHCPv6 server's
   relayed response to then announce the delegated prefix into the
   network.

   Upstrem router accepts and establishes BGP sessions with any link-
   local address from the well known ASN, to the Functional Anycast BGP
   address.

   There are potential trust issues here.  BGPsec?  Use the first BGP
   session to bootstrap connectivity to then establish a more trusted
   connection of some sort via PKI.  Requirement for being link-local
   peers adds a minimal level of security and trust, but not much.

5.7.4.  ISP's Anycast DNS Resolvers

   ISP DNS anycast resolvers

   Don't want it to be globally reachable across the Internet to
   mitigate DDoS attacks on the DNS resolver i.e. don't use GUA
   address(es)

   Can't use a prefix that has the possibility of colliding with any of
   the customers' prefixes.  ULA addresses would meet this requirement.

   However, per [RFC4193], ULAs have a site scope.  The boundaries of
   customers' networks correspond to site boundaries, and therefore the
   ISP's ULA prefix is not reachable from customers' networks unless
   customers' network boundary routers are explicitly configured to
   provide ISP ULA prefix access.




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   Organisation Local Functional Anycast address meets all of the
   requirements.

5.7.5.  Microservices Architecture Applications

5.7.6.  Global Time Distribution Network

   We Have The Time Company (WHTT) are an enterprise who specialise in
   providing accurate time to global clients across the Internet, via
   the Network Time Protocol.

   To provide robust time across they Internet, they provide access to
   their NTP servers via Functional Anycast addresses.

   WHTT have a GUA /32 assignment from their Regional Internet Registry.
   They provide time to clients via the following Global scope
   Functional Anycast address, with a Global scope, an anycast domain
   Prefix of 2001:db8::/32, a Prefix Length of 32 (0x20), and the well
   known NTP Anycast Function Identifier of 0x101.

   o  aa0e:2001:db8:0:0:2000::101

5.7.7.  Example3

6.  Security Considerations

   Functional Anycast addresses should not introduce any new security
   concerns in comparison to the use of addresses from within the
   unicast address space as anycast addresses.  [RFC7094] provides
   considerable anycast related security discussion and references.

   The ability to identify a Functional Anycast address using its well
   known 8 bit prefix, and the inclusion of forwarding scopes in the
   addresses, provide opportunies to enhance security of anycast
   services.

7.  IANA Considerations

   IANA are requested to register the aa00::/8 prefix in the Internet
   Protocol Version 6 Address Space registry for use with Formal Anycast
   addresses.  If aa00::/8 is not chosen, then fa00::/8 is a proposed
   alternative.

   IANA are requested to register a new ICMPv6 Destination Unreachble
   code for Edge of Visible Scope Reached.






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   IANA are requested to establish a registry for the Flags field of
   Functional Anycast addresses, and to reserve the T flag to indicate
   transient Anycast Function Identifiers.

   IANA are requested to establish a registry for well known Anycast
   Function Identifiers, and to reserve the values described previously
   in the "Assigned Anycast Function Identifers" section of this memo.

8.  Acknowledgements

   Review and comments were provided by YOUR NAME HERE!

   This memo was prepared using the xml2rfc tool.

9.  Change Log [RFC Editor please remove]

   draft-smith-6man-form-func-anycast-addresses-00, initial version,
   2017-02-XX

10.  Informative References

   [IANA-IPV6ANYC]
              "Internet Protocol Version 6 (IPv6) Anycast Addresses",
              <https://www.iana.org/assignments/ipv6-anycast-addresses/
              ipv6-anycast-addresses.xhtml>.

   [IANA-IPV6MCAST]
              "IPv6 Multicast Address Space Registry",
              <https://www.iana.org/assignments/ipv6-multicast-
              addresses/ipv6-multicast-addresses.xhtml>.

   [IANA-IPV6RESA]
              "IPv6 Multicast Address Space Registry",
              <https://www.iana.org/assignments/ipv6-multicast-
              addresses/ipv6-multicast-addresses.xhtml>.

   [RFC1546]  Partridge, C., Mendez, T., and W. Milliken, "Host
              Anycasting Service", RFC 1546, DOI 10.17487/RFC1546,
              November 1993, <https://www.rfc-editor.org/info/rfc1546>.

   [RFC2526]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
              Addresses", RFC 2526, DOI 10.17487/RFC2526, March 1999,
              <https://www.rfc-editor.org/info/rfc2526>.

   [RFC3306]  Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
              Multicast Addresses", RFC 3306, DOI 10.17487/RFC3306,
              August 2002, <https://www.rfc-editor.org/info/rfc3306>.




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   [RFC3310]  Niemi, A., Arkko, J., and V. Torvinen, "Hypertext Transfer
              Protocol (HTTP) Digest Authentication Using Authentication
              and Key Agreement (AKA)", RFC 3310, DOI 10.17487/RFC3310,
              September 2002, <https://www.rfc-editor.org/info/rfc3310>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <https://www.rfc-editor.org/info/rfc3315>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
              December 2006, <https://www.rfc-editor.org/info/rfc4786>.

   [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>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/info/rfc5905>.

   [RFC5942]  Singh, H., Beebee, W., and E. Nordmark, "IPv6 Subnet
              Model: The Relationship between Links and Subnet
              Prefixes", RFC 5942, DOI 10.17487/RFC5942, July 2010,
              <https://www.rfc-editor.org/info/rfc5942>.

   [RFC7094]  McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
              "Architectural Considerations of IP Anycast", RFC 7094,
              DOI 10.17487/RFC7094, January 2014,
              <https://www.rfc-editor.org/info/rfc7094>.

   [RFC7346]  Droms, R., "IPv6 Multicast Address Scopes", RFC 7346,
              DOI 10.17487/RFC7346, August 2014,
              <https://www.rfc-editor.org/info/rfc7346>.






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   [RFC7723]  Kiesel, S. and R. Penno, "Port Control Protocol (PCP)
              Anycast Addresses", RFC 7723, DOI 10.17487/RFC7723,
              January 2016, <https://www.rfc-editor.org/info/rfc7723>.

   [RFC8155]  Patil, P., Reddy, T., and D. Wing, "Traversal Using Relays
              around NAT (TURN) Server Auto Discovery", RFC 8155,
              DOI 10.17487/RFC8155, April 2017,
              <https://www.rfc-editor.org/info/rfc8155>.

Author's Address

   Mark Smith
   PO BOX 521
   HEIDELBERG, VIC  3084
   AU

   Email: markzzzsmith@gmail.com


































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