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Versions: 00 01

Internet Engineering Task Force                                 M. Smith
Internet-Draft                                          November 3, 2019
Updates: RFC4291, RFC4443, RFC6724 (if
Intended status: Standards Track
Expires: May 6, 2020

     IPv6 Formal Anycast Addresses and Functional Anycast Addresses


   Currently, 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 would be to
   have a special class of addresses for use as anycast addresses.  This
   memo proposes a distinct general anycast addressing class for IPv6,
   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 May 6, 2020.

Copyright Notice

   Copyright (c) 2019 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 . . . . . . . . . . . . . . . .   6
       4.2.2.  Visible Scope . . . . . . . . . . . . . . . . . . . .   6
       4.2.3.  Anycast Identifier Format . . . . . . . . . . . . . .   6
       4.2.4.  Anycast Identifier  . . . . . . . . . . . . . . . . .   6
     4.3.  Anycast Address Registration Protocol . . . . . . . . . .   6
     4.4.  Network Service Provider Visible Scope  . . . . . . . . .   7
     4.5.  Link-Local Visible Scope  . . . . . . . . . . . . . . . .   7
     4.6.  ICMPv6 Destination Unreachable Message  . . . . . . . . .   8
     4.7.  Default Address Selection . . . . . . . . . . . . . . . .   9
       4.7.1.  Formal Anycast Scope Comparison . . . . . . . . . . .   9
       4.7.2.  Source Address Selection  . . . . . . . . . . . . . .   9
       4.7.3.  Destination Address Selection . . . . . . . . . . . .   9
     4.8.  Non-Local Anycast Forwarding  . . . . . . . . . . . . . .  10
     4.9.  Advice on Structuring the Anycast Identifier Field Values  11
   5.  Functional Anycast Addresses  . . . . . . . . . . . . . . . .  13
     5.1.  Features  . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.2.  Address Format  . . . . . . . . . . . . . . . . . . . . .  14
     5.3.  Assignment of Anycast Function Identifiers  . . . . . . .  16
     5.4.  Assigned Anycast Function Identifiers . . . . . . . . . .  17
     5.5.  Sources of Inspiration for Anycast Function Identifiers .  18
     5.6.  Global Scope Functional Anycast Addresses on the Internet  19
     5.7.  Example Use Cases . . . . . . . . . . . . . . . . . . . .  20
       5.7.1.  Devices Factory Configured with NTP Functional
               Anycast Addresses . . . . . . . . . . . . . . . . . .  20
       5.7.2.  Branch Office DNS Resolvers . . . . . . . . . . . . .  22
       5.7.3.  Automatic eBGP Session Establishment  . . . . . . . .  23
       5.7.4.  An ISP's Anycast DNS Resolvers  . . . . . . . . . . .  25
       5.7.5.  Microservices Architecture Applications . . . . . . .  27
       5.7.6.  Global Time Distribution Network  . . . . . . . . . .  27
       5.7.7.  Multipath Transport Layer Protocols . . . . . . . . .  27
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  29
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30
   9.  Change Log [RFC Editor please remove] . . . . . . . . . . . .  30
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  30

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     10.2.  Informative References . . . . . . . . . . . . . . . . .  30
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  35

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

   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 well known value that indicates they are anycast
   addresses, regardless of context.

   In terms of a spectrum of packet delivery, ranging from delivery to a
   single destination (unicast), through to delivery to multiple
   destinations (multicast), 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 host, however that host is a member 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 a packet delivery spectrum.  Packet delivery to an
   anycast address shares characteristics of both unicast and multicast
   address packet delivery.

   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.

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   This memo proposes a distinct 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, allowing
   other semantics to be encoded in the anycast address.  Following the
   definition of the Formal Anycast address class, this memo then
   proposes the first sub-class, called "Functional Anycast" addresses.

   There are some existing reserved and well known anycast addresses
   within the existing Informal Anycast address space, that have been
   assigned by IANA [IANA-IPV6ANYC].  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.

   "MRS:" comments - points to consider further, to eventually be

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 the existing 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 or
      prefix appearing in a route table with different 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 address or prefix routes are Informal Anycast

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      routes i.e., the address or prefix from within the unicast address
      space is being used an anycast address or anycast prefix.  The
      duplicate routes and the addresses or prefixes 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 local network scope
      (Unique Local Address) or having global Internet scope (Global
      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.

   o  Some transport layer protocols, such as SCTP [RFC3286] and
      Multipath TCP [RFC6824], and some applications, deal directly with
      IPv6 addresses, and supply them to their communications peer or
      peers.  Currently, if these transport layer protocols or
      applications are dealing with both unicast and anycast addresses,
      and wish to provide only unicast or anycast types of address or
      set of addresses to their peer or peers during their
      communications transactions, it would be necessary to manually
      configure the transport protocol or application so that it can
      distinguish between unicast and anycast addresses.  A well known
      address class that identifies anycast addresses would allow these
      transport layer protocols or applications to identify the
      different address types without any manual configuration.

4.  Formal Anycast Addresses

4.1.  Address Format

   The following diagram shows the structure of an IPv6 Formal Anycast


4.2.  Address Fields

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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

   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.

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.  Anycast Address Registration Protocol

   Rather than having to manually configure a network's routing protocol
   to distribute a host's anycast address, or have a host participate in
   the network's routing protocol, a protocol for hosts to automatically
   register an anycast address for routing protocol distribution would
   be beneficial.

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   Development of this protocol is left for a later memo, however as the
   requirements of such an anycast address registration protocol are
   very similar to that of hosts' multicast address registration with a
   network, it is likely that an anycast address registration protocol
   could be modelled on and derived from the IPv6 Multicast Listener
   Discovery protocol [RFC2710][RFC3810].

4.4.  Network Service Provider Visible Scope

   A network service provider, such as an Internet Service Provider, may
   wish to use an anycast address to provide a service with a
   visibilility limited to all of its direct customers.

   When using a Formal Anycast address for this service, that reuses
   IPv6 multicast scopes, this means the address needs to have a scope
   that is greater than the Organization-Local scope, yet smaller than
   the unlimited Global scope.

   This memo defines a new scope called the Network Service Provider
   (NSP) scope, that falls between the Organization-Local and Global
   IPv6 multicast scopes.  This NSP scope's hexidecimal value is B.
   This scope can be used with both Formal Anycast and IPv6 multicast

   (MRS: perhaps this scope shouldn't be at B, but instead hard up
   against the Organization-Local scope i.e. at value 9?  Could there be
   a need for any other future scopes between Organization-Local and
   Network Service Provider - which would be scopes within the Network
   Service Provider's network.)

4.5.  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 aa20::/12 to be on-link.

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   Unlike the unicast Link-Local prefix, updated IPv6 implementations
   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 constraint, and more generally
   the idea of host automatically generated dynamic anycast addresses,
   rather than either having well known or system administrator chosen
   and configured anycast addresses.  If there is an anycast address
   registration protocol that hosts can use (suggested above), then
   hosts could possibly dynamically generate anycast addresses and then
   register them.)

   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.

   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].

   o  A dynamic update to nodes' Prefix Lists using a Router
      Advertisement Prefix Information Option (PIO) [RFC4861]for the
      Formal Anycast Link-Local Visible Scope prefix of aa20::/12, 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.

4.6.  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.

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   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]

   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, saving router control
   plane resources.

4.7.  Default Address Selection

4.7.1.  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 also used with Formal Anycast
   address scopes.

4.7.2.  Source Address Selection

   As mentioned in Appendix B. of [RFC6724], anycast addresses are
   candidates during source address selection.

4.7.3.  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 by the destination is an active and conscious one.
   Therefore, when a choice needs to be made between a Formal Anycast

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   address and a unicast address, the Formal Anycast address should be

   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 easily identified and ignored using
   the well known 8 bit Formal Anycast prefix.

4.8.  Non-Local Anycast Forwarding

   (MRS:This section is being left here for the moment, however this
   idea should probaby be moved to a different memo, as it is describing
   forwarding that isn't unicast forwarding (unicast forwarding is used
   by conventional anycast))

   One possible use for Formal Anycast addresses is to represent a
   function that is performed on the packet by the network that is
   beyond conventional destination address based unicast IPv6
   forwarding, as the packet traverses the path towards its final
   delivery point.

   Currently, hop-by-hop processing of an IPv6 packet as it traverses
   the network is indicated using the Hop-by-Hop Options (Extension)
   Header [RFC8200].

   Drawbacks of using the Hop-by-Hop Option Header are that some high
   speed routers ignore them [RFC7045], and that packets with the Hop-
   by-Hop Options Header may be dropped by transit Autonomous System
   (AS) networks [RFC7872].  The dropping of these types of packets by
   transit ASes may be due to a default deny policy for Extention Header
   types other than those of TCP, UDP, ICMPv6 and possibly IPsec ESP.

   Encoding the intent of hop-by-hop processing of a packet as an
   anycast IPv6 destination address has the advantage of the packet
   always being processed by all router implementations, including high
   speed implementations, as processing a packet's IPv6 destination
   address is required to perform IPv6 destination address based

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   forwarding.  As there is no explicit Hop-by-Hop Options Header in the
   packet, a transit AS is less likely to drop the packet, unless it
   explicitly implements IPv6 Destination Address packet filters that
   drop packets with Formal Anycast addresses.

   Another advantage is that there may now be no need for the addition
   of an Extension Header to the IPv6 packet for hop-by-hop processing,
   increasing the packet's effective payload size.

   Conventional unicast IPv6 forwarding based on destination address
   prioritises a node's local addresses over all others.  This means
   that when a node originates a packet with one its own addresses as
   the destination, the node will deliver the packet internally for
   local processing, rather than sending it out one of the node's
   network interfaces.

   The functional requirements for Non-Local Anycast Forwarding are:

   o  When being sent by the node, the packet is not delivered
      internally to the node's own instance of the Formal Anycast

   o  After being submitted to the network for forwarding, the packet
      must not be sent back towards its source, as this would
      potentially cause the packet to follow a loop in its forwarding

4.9.  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
   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

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   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 guildeline should not
      take 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 type.  A shorter address to enter
      naturally reduces the opportunities for and likelihood of errors

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      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 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

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      also correspond to routing protocol scaling boundaries, such as
      OSPFv3 areas [RFC5340] or IS-IS level [RFC5308] boundaries, when

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:


   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 specific Anycast
      Function Identifier provider (MRS: Need to think about more about
      using Link-Local prefix, as it really isn't specific - perhaps
      either prohibit, or make it all zeros "current" equivalent).
      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.

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      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

      (MRS: This field is inspired by the equivlent field when IPv6
      multicast addresses contain a unicast prefix per [RFC3306].  I'm
      not entirely sure it is necessary, as we don't embed prefix length
      in unicast addresses, and unicast routing protocols, also used for
      anycast, carry prefix length information separately.)

   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
      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.

      (MRS: is there a way to avoid this flag field?  Some of the flags
      that are used in IPv6 multicast addresses could instead be
      accomodated by defining an entirely new 112 bit Anycast Identifier
      Format.  Avoiding this field and the Prefix-Length field frees up
      16 bits which makes this Anycast Identifier Format simpler and
      would provide 16 more bits for the following fields, which may be

   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

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      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

      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 [RFC0854] had been the
   most popular "Network Virtual Terminal" protocol.  In more recent
   times, the SSH protocol [RFC4252] has become the de facto network
   virtual terminal protocol.  Accessing the network virtual terminal
   service has either been referred to as "TELNETting in" or "SSHing in"
   to the host providing the service, using the protocol being used to
   refer to the service being accessed.

   In either case, TELNET and SSH protocols are being used to access a
   remote network virtual terminal service.  Functionally, from the
   perspective of network 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 authentication [RFC2941] and encryption [RFC2946]
   of the data stream is optional, 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

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   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 (NVT) [RFC0318]
   service, 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

   Another example is the domain name service.  Currently domain name
   resolution commonly takes place using the Domain Name Service
   protocol [RFC1035], carried directly over UDP and TCP, using port 53.
   More recently, work has been taking place to operate DNS over TLS
   [RFC7858] and HTTPS [RFC8484] 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,
   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

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   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 [RFC8415] 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 seperately
   verifiable DNS resolver services from up to 3 different resolvers,
   and allowing the client to distribute requests across all 3

   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

   While using these possible source of inspiration, the recommendation
   to choose protocol agnostic function or service identifiers still

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   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

   (MRS: Need to fully review this idea and section.  An idea to help
   overcome the /48 prefix length constraint would be to have all Formal
   Anycast addresses that are going to be used on the Internet come from
   an IANA reserved prefix within the existing GUA address space e.g.
   20e0::/12 (i.e. operators would accept a prefix announcement of
   length of up to /80 within 20e0::/12). aa::/8 would still be used for
   smaller than global scope anycast, because a prefix of "aa" is very
   helpful to identify anycast addresses.)

   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.  These Functional Anycast prefixes SHOULD BE aggregate
   announcements with the aggregation boundary occurring directly after
   the Anycast Domain Prefix.

   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

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   limit the number of IPv6 routes in the Internet 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
   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 aae0::/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 [RFC5905], to facilitate meeting
   the NTP best practice of having a minimum of 4 NTP time sources

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   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.

   The corresponding Functional Anycast address is constructed as

   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

   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:


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

      *  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  aa80::4440

   o  aa80::4441

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   o  aa80::4442

   o  aa80::4443

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

   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


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   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:

   1.  aa80:2001:db8:2554::5301

   2.  aa80:2001:db8:2554::5302

   3.  aa80:2001: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

   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

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   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
   subsequent BGP announcement [RFC3633], meaning it can act as a much
   simpler and pure DHCPv6 relay [RFC8415].

   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

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   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.

   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

   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

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

   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.  An ISP's Anycast DNS Resolvers

   ISPs' IPv4 DNS resolvers have been the target of Distributed Denial
   of Service attacks (DDoS) [REF], with the attacks launched from the

   These attacks have been possible because ISPs' have assigned their
   DNS resolvers public IPv4 addresses, with the public IPv4 addresses
   being both globally unique and globally reachable.

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   The need for global uniqueness comes from the requirement for the DNS
   resolvers to have an addresses that are not present within the ISP's
   customers' networks.  Inherent with global uniqueness of a public
   IPv4 address is global reachability.  To protect the DNS resolvers
   from DDoS attacks, an ISP has to actively configure protection
   mechanisms such as packet filters that discard traffic from the
   Internet, while continuing to allow the ISP's customers to use the
   DNS resolver.

   There are two drawbacks of having to use purposely configured
   protection mechanisms such as packet filters once the DNS resolver
   has a publically unique and reachable address.  Firstly, as the
   public IPv4 address is normally reachable, an error in configuring
   the packet filter means a "fail unsafe" consequence.

   Secondly, packet filters can only be applied within the local
   network.  This means that the large volume of DDoS traffic will reach
   the local network before it can be discarded.  This discarded DDoS
   traffic consumes network capacity on paths towards the network that
   should instead be available to legitimate traffic towards the

   Ideally, the ISP could assign its DNS resolvers addresses that should
   be unique within both the ISP's network and all of its customers'
   networks.  The addresses should be reachable from the customers'
   networks while not being reachable from the Internet.  Inherently,
   these customer and ISP only visible addresses would protect the DNS
   resolvers from Internet launched DDoS attacks.  There would be no
   need for the ISP to configure packet filters to protect the DNS
   resolvers from the Internet, and as the DNS resolvers addresses are
   unreachable from the Internet, it would not be possible to send large
   volumes of DDoS traffic towards them.  The ISP's Internet transit
   capacity would be more available for legitimate traffic.

   Unique Local Addresses (ULA) [RFC4193] would appear to be addresses
   that could be used for this purpose.  They are intended to be
   globally unique, due to their embedded 41 bit random number, meaning
   that they should not collide with any of the ISP's customers
   network's addresses.  They are also not intended to be reachable
   globally across the Internet.

   However, the ISP's ULA addresses assigned to its DNS resolvers would
   be unlikely be realiably reachable from all of its customers'
   networks.  The IPv6 source address selection algorithm tries to pick
   source addresses that have high order prefix address bits that match
   those of the destination [RFC6724].  Consequently, to reach an ISP's
   ULA addressed DNS resolvers, customers' hosts would pick their ULA
   source addresses, should they have them.  These packets may reach the

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   ISP's DNS resolvers, due to customers' default routes, however the
   DNS response return packets are unlikely to reach the customers'
   hosts as the ISP is unlikely to know customers' ULA routing prefixes,
   due to trading of ULA routing prefixes being prohibited by default

   So the source addresses that customers' hosts use when sending to the
   ISP's DNS resolvers need to be of greater scope and reachability than
   ULA addresses, while the ISP's DNS resolvers need to have addresses
   that have a greater scope and reachability than ULA addresses, yet
   are not IPv6 Globally Unique Addresses [RFC4291].

   Customers' GUA addresses would meet this customer host source address
   requirement, while IPv6 Functional Anycast addresses with a Network
   Service Provider Visibility Scope would meet the ISP's DNS resolver
   address requirements.

5.7.5.  Microservices Architecture Applications

   (MRS: Any possible application here?  Perhaps service redundancy and
   service anycast service addresses.

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  aae0:2001:db8:0:0:2000::101

5.7.7.  Multipath Transport Layer Protocols

   Multipath transport layer protocols, such as MPTCP [RFC6824] and SCTP
   [RFC3286], establish a full multipath session between hosts in three
   stages, using multiple connections.

   Stage one involves establishing an initial connection between the
   hosts.  During stage two, the hosts' remaining set of IP addresses
   are exchanged.  Finally, in stage three, the full multipath session

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   is established, with the hosts establishing further connections
   between the alternative IP addresses exchanged during stage two.
   Note that during the multipath session, any of the connections could
   fail or could be purposely torn down, and as long as at least one
   connection persists, the multipath session continues.

   When multipath transport protocols are used for client-server
   applications, a single common IPv6 anycast address could be used by
   multiple servers, and then for the initial connection between the
   clients and servers during stage one.

   During stage two, the server limits the set of alternative IP
   addresses it supplies to clients to its unicast addresses, excluding
   its one or more anycast addresses.

   Subsequent connections that establish the full multipath session
   during stage three would then be limited to only being established
   between the unicast addresses of the clients and server.

   Once any of these stage three unicast address connections is
   established, the server could actively tear down the initial
   connection to its anycast address, meaning that all of the now
   remaining connections are established with the individual server's
   unicast address or addresses.

   Switching to using only unicast connections for the remainder of the
   multipath session overcomes one of the significant limitations of the
   use of anycast with connection oriented transport layer protocols
   [RFC1546]; the limitation that where the anycast address instance's
   packets are delivered to depends on the network's forwarding domains
   topology, and if the network topology changes, packets may be
   delivered to a different anycast instance that is unaware of and has
   not previously been involved in the transport layer connection.

   With this use of both anycast and unicast addresses in combination
   with multipath transport protocols, the effects of this anycast
   limitation are reduced to the time between establishment of the
   initial client-server connection to the server's anycast address, and
   when the first client-server connection is established using
   exclusively unicast addresses.  This is in contrast to this risk
   possibility existing for the entire duration of a single path
   transport layer protocol connection to an anycast address.

   The benefit of the above use of anycast in combination with multipath
   transport layer protocols applies to all types of anycast addresses
   discussed in this memo.

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   There is a further advantage if Formal Anycast addresses are used.
   This is that as a Formal Anycast address is easily identified due to
   its well known prefix, the multipath transport layer protocol
   implementation does not have to be configured with which of the
   server's IPv6 addresses are its anycast addresses, so they can be
   excluded from the IPv6 address exchange that occurs during stage two.

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

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

   IANA are requested to update the use of the IPv6 multicast scopes
   registry to also record use with Formal Anycast IPv6 addresses.

   IANA are requested to record a new IPv6 multicast and Formal Anycast
   scope named the "Network Service Provider" scope, with a scope value
   of B in hexidecimal.

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

   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.

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8.  Acknowledgements

   Gavin Owen prompted the lunch time conversation where the author
   joined together and immedidately recognised the benefits of using of
   anycast addresses in combination with multipath transport layer
   protocols to provide more robust anycast services.

   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,

   draft-smith-6man-form-func-anycast-addresses-01, updates, 2019-11-03

   o  Fixed scope location error in addresses used throughout

   o  Added section on the idea of an anycast address registration

   o  Reference updates

10.  References

10.1.  Normative References

   [RFC0854]  Postel, J. and J. Reynolds, "Telnet Protocol
              Specification", STD 8, RFC 854, DOI 10.17487/RFC0854, May
              1983, <https://www.rfc-editor.org/info/rfc854>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

10.2.  Informative References

              "Internet Protocol Version 6 (IPv6) Anycast Addresses",

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              "IPv6 Multicast Address Space Registry",

              "IPv6 Multicast Address Space Registry",

   [RFC0318]  Postel, J., "Telnet Protocols", RFC 318,
              DOI 10.17487/RFC0318, April 1972,

   [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,

   [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710,
              DOI 10.17487/RFC2710, October 1999,

   [RFC2941]  Ts'o, T., Ed. and J. Altman, "Telnet Authentication
              Option", RFC 2941, DOI 10.17487/RFC2941, September 2000,

   [RFC2946]  Ts'o, T., "Telnet Data Encryption Option", RFC 2946,
              DOI 10.17487/RFC2946, September 2000,

   [RFC3286]  Ong, L. and J. Yoakum, "An Introduction to the Stream
              Control Transmission Protocol (SCTP)", RFC 3286,
              DOI 10.17487/RFC3286, May 2002,

   [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>.

   [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>.

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   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              DOI 10.17487/RFC3633, December 2003,

   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,

   [RFC4252]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252,
              January 2006, <https://www.rfc-editor.org/info/rfc4252>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,

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

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,

   [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,

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

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   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
              DOI 10.17487/RFC5308, October 2008,

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,

   [RFC5505]  Aboba, B., Thaler, D., Andersson, L., and S. Cheshire,
              "Principles of Internet Host Configuration", RFC 5505,
              DOI 10.17487/RFC5505, May 2009,

   [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,

   [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,

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952,
              DOI 10.17487/RFC5952, August 2010,

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,

   [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", RFC 7045,
              DOI 10.17487/RFC7045, December 2013,

   [RFC7094]  McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
              "Architectural Considerations of IP Anycast", RFC 7094,
              DOI 10.17487/RFC7094, January 2014,

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   [RFC7346]  Droms, R., "IPv6 Multicast Address Scopes", RFC 7346,
              DOI 10.17487/RFC7346, August 2014,

   [RFC7608]  Boucadair, M., Petrescu, A., and F. Baker, "IPv6 Prefix
              Length Recommendation for Forwarding", BCP 198, RFC 7608,
              DOI 10.17487/RFC7608, July 2015,

   [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>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC7872]  Gont, F., Linkova, J., Chown, T., and W. Liu,
              "Observations on the Dropping of Packets with IPv6
              Extension Headers in the Real World", RFC 7872,
              DOI 10.17487/RFC7872, June 2016,

   [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,

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [RFC8633]  Reilly, D., Stenn, H., and D. Sibold, "Network Time
              Protocol Best Current Practices", BCP 223, RFC 8633,
              DOI 10.17487/RFC8633, July 2019,

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Author's Address

   Mark Smith
   PO BOX 521

   Email: markzzzsmith@gmail.com

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