Internet DRAFT - draft-smith-6man-form-func-anycast-addresses
draft-smith-6man-form-func-anycast-addresses
Internet Engineering Task Force M.R. Smith
Internet-Draft 22 February 2024
Updates: RFC4291, RFC4443, RFC6724 (if approved)
Intended status: Standards Track
Expires: 25 August 2024
IPv6 Formal Anycast Addresses and Functional Anycast Addresses
draft-smith-6man-form-func-anycast-addresses-02
Abstract
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
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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 25 August 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Drawbacks of Informal Anycast Addresses . . . . . . . . . . . 5
4. Formal Anycast Addresses . . . . . . . . . . . . . . . . . . 6
4.1. Address Format . . . . . . . . . . . . . . . . . . . . . 6
4.2. Address Fields . . . . . . . . . . . . . . . . . . . . . 6
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 . . . . . . . . . . . . . . . . . 7
4.3. Anycast Address Registration Protocol . . . . . . . . . . 7
4.4. Network Service Provider Visible Scope . . . . . . . . . 7
4.5. Link-Local Visible Scope . . . . . . . . . . . . . . . . 8
4.6. ICMPv6 Destination Unreachable Message . . . . . . . . . 9
4.7. Default Address Selection . . . . . . . . . . . . . . . . 9
4.7.1. Formal Anycast Scope Comparison . . . . . . . . . . . 9
4.7.2. Source Address Selection . . . . . . . . . . . . . . 10
4.7.3. Destination Address Selection . . . . . . . . . . . . 10
4.8. Non-Local Anycast Forwarding . . . . . . . . . . . . . . 10
4.9. Advice on Structuring the Anycast Identifier Field
Values . . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Functional Anycast Addresses . . . . . . . . . . . . . . . . 13
5.1. Features . . . . . . . . . . . . . . . . . . . . . . . . 13
5.2. Address Format . . . . . . . . . . . . . . . . . . . . . 14
5.3. Assignment of Anycast Function Identifiers . . . . . . . 17
5.4. Assigned Anycast Function Identifiers . . . . . . . . . . 18
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 . . . . . . . . . . . . . . . . . . . . 21
5.7.1. Devices Factory Configured with NTP Functional Anycast
Addresses . . . . . . . . . . . . . . . . . . . . . . 21
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 . . . . . . . . . . . 26
5.7.5. Microservices Architecture Applications . . . . . . . 27
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5.7.6. Global Time Distribution Network . . . . . . . . . . 28
5.7.7. Multipath Transport Layer Protocols . . . . . . . . . 28
6. Security Considerations . . . . . . . . . . . . . . . . . . . 29
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
9. Change Log [RFC Editor please remove] . . . . . . . . . . . . 30
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
10.1. Normative References . . . . . . . . . . . . . . . . . . 31
10.2. Informative References . . . . . . . . . . . . . . . . . 31
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:
* using parts of the existing address space
* 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 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.
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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.
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
removed.
2. Terminology
Anycast Domain
Formal Anycast Address
Functional Anycast Address
Informal Anycast Address
Semi-Formal Anycast Address
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3. Drawbacks of Informal Anycast Addresses
There are drawbacks and limitions of the existing IPv6 Informal
Anycast addresses:
* 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
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.
* 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.
* 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.
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4. Formal Anycast Addresses
4.1. Address Format
The following diagram shows the structure of an IPv6 Formal Anycast
address.
DIAGRAM TO COME
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. 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.
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
addresses.
(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.)
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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.
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:
* A manually configured entry in the host's Prefix List [RFC4861].
* 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
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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.
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, 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.
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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
address and a unicast address, the Formal Anycast address should 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 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.
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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
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:
* When being sent by the node, the packet is not delivered
internally to the node's own instance of the Formal Anycast
address.
* 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
path.
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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
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.
* 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.
* 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.
* 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.
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* 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.
* 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
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].
* 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.
* Provides a minimum of 24 bits for use as identifiers for anycast
functions or services, supporting more than 16 million values.
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* 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.
* 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
also correspond to routing protocol scaling boundaries, such as
OSPFv3 areas [RFC5340] or IS-IS level [RFC5308] 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:
* 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.
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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.
* Reserved - a 2 bit reserved field.
Set to zero upon transmission, ignored upon receipt.
* 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.
(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.)
* Flags - A 8 bit flag field.
The lower 6 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.
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The second most high order flag is the 'LI' or Last Instance flag.
When a network supports hop-by-hop processing of packets using
anycast addressing, this flag indicates whether anycast processing
of the packet should cease at the first anycast instance
encountered (LI=0), even though there may be further candidate
anycast hops that suit, or whether the packet should continue to
be sent to any subsequent anycast hops until the Last Instance of
the matching anycast address is encountered (LI=1).
* 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.
* 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.
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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
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
protocol.
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.
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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
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.
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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
resolvers.
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
(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.
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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
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.
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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
[RFC8633].
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
follows:
* 8 bit Formal Anycast Prefix value (0xaa proposed).
* 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.
* 4 bit Anycast Identifier Format value of 0x0, corresponding to the
Functional Anycast format.
* 112 bit Anycast Identifier in the Functional Anycast format:
* - 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.
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- 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):
* aa80::4440
* aa80::4441
* aa80::4442
* 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
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
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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:
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
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.
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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
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].
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[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.
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
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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. 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
Internet.
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.
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
network.
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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
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
[RFC4193].
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.
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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.
* 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
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.
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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.
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
services.
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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 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.
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,
2018-10-22
draft-smith-6man-form-func-anycast-addresses-01, updates, 2019-11-03
* Fixed scope location error in addresses used throughout
* Added section on the idea of an anycast address registration
protocol
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* Reference updates
draft-smith-6man-form-func-anycast-addresses-02, updates, 2024-02-23
* Last Instance flag
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,
<https://www.rfc-editor.org/info/rfc8200>.
10.2. 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>.
[RFC0318] Postel, J., "Telnet Protocols", RFC 318,
DOI 10.17487/RFC0318, April 1972,
<https://www.rfc-editor.org/info/rfc318>.
[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>.
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[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>.
[RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710,
DOI 10.17487/RFC2710, October 1999,
<https://www.rfc-editor.org/info/rfc2710>.
[RFC2941] Ts'o, T., Ed. and J. Altman, "Telnet Authentication
Option", RFC 2941, DOI 10.17487/RFC2941, September 2000,
<https://www.rfc-editor.org/info/rfc2941>.
[RFC2946] Ts'o, T., "Telnet Data Encryption Option", RFC 2946,
DOI 10.17487/RFC2946, September 2000,
<https://www.rfc-editor.org/info/rfc2946>.
[RFC3286] Ong, L. and J. Yoakum, "An Introduction to the Stream
Control Transmission Protocol (SCTP)", RFC 3286,
DOI 10.17487/RFC3286, May 2002,
<https://www.rfc-editor.org/info/rfc3286>.
[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>.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
DOI 10.17487/RFC3633, December 2003,
<https://www.rfc-editor.org/info/rfc3633>.
[RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
DOI 10.17487/RFC3810, June 2004,
<https://www.rfc-editor.org/info/rfc3810>.
[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>.
[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>.
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[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,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
[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>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
DOI 10.17487/RFC5308, October 2008,
<https://www.rfc-editor.org/info/rfc5308>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC5505] Aboba, B., Thaler, D., Andersson, L., and S. Cheshire,
"Principles of Internet Host Configuration", RFC 5505,
DOI 10.17487/RFC5505, May 2009,
<https://www.rfc-editor.org/info/rfc5505>.
[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>.
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[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>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/info/rfc5952>.
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<https://www.rfc-editor.org/info/rfc6724>.
[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,
<https://www.rfc-editor.org/info/rfc6824>.
[RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing
of IPv6 Extension Headers", RFC 7045,
DOI 10.17487/RFC7045, December 2013,
<https://www.rfc-editor.org/info/rfc7045>.
[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>.
[RFC7608] Boucadair, M., Petrescu, A., and F. Baker, "IPv6 Prefix
Length Recommendation for Forwarding", BCP 198, RFC 7608,
DOI 10.17487/RFC7608, July 2015,
<https://www.rfc-editor.org/info/rfc7608>.
[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>.
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[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,
<https://www.rfc-editor.org/info/rfc7872>.
[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>.
[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,
<https://www.rfc-editor.org/info/rfc8415>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
[RFC8633] Reilly, D., Stenn, H., and D. Sibold, "Network Time
Protocol Best Current Practices", BCP 223, RFC 8633,
DOI 10.17487/RFC8633, July 2019,
<https://www.rfc-editor.org/info/rfc8633>.
Author's Address
Mark Smith
PO BOX 521
HEIDELBERG VIC 3084
Australia
Email: markzzzsmith@gmail.com
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