Internet Engineering Task Force (IETF) S. Jiang, Ed.
Request for Comments: 8992 Huawei Technologies Co., Ltd
Category: Informational Z. Du
ISSN: 2070-1721 China Mobile
B. Carpenter
Univ. of Auckland
Q. Sun
China Telecom
May 2021
Autonomic IPv6 Edge Prefix Management in Large-Scale Networks
Abstract
This document defines two autonomic technical objectives for IPv6
prefix management at the edge of large-scale ISP networks, with an
extension to support IPv4 prefixes. An important purpose of this
document is to use it for validation of the design of various
components of the Autonomic Networking Infrastructure.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8992.
Copyright Notice
Copyright (c) 2021 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Terminology
3. Problem Statement
3.1. Intended User and Administrator Experience
3.2. Analysis of Parameters and Information Involved
3.2.1. Parameters Each Device Can Define for Itself
3.2.2. Information Needed from Network Operations
3.2.3. Comparison with Current Solutions
3.3. Interaction with Other Devices
3.3.1. Information Needed from Other Devices
3.3.2. Monitoring, Diagnostics, and Reporting
4. Autonomic Edge Prefix Management Solution
4.1. Behavior of a Device Requesting a Prefix
4.2. Behavior of a Device Providing a Prefix
4.3. Behavior after Successful Negotiation
4.4. Prefix Logging
5. Autonomic Prefix Management Objectives
5.1. Edge Prefix Objective Option
5.2. IPv4 Extension
6. Prefix Management Parameters
6.1. Example of Prefix Management Parameters
7. Security Considerations
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Deployment Overview
A.1. Address and Prefix Management with DHCP
A.2. Prefix Management with ANI/GRASP
Acknowledgements
Authors' Addresses
1. Introduction
The original purpose of this document was to validate the design of
the Autonomic Networking Infrastructure (ANI) for a realistic use
case. It shows how the ANI can be applied to IP prefix delegation,
and it outlines approaches to build a system to do this. A fully
standardized solution would require more details, so this document is
informational in nature.
This document defines two autonomic technical objectives for IPv6
prefix management in large-scale networks, with an extension to
support IPv4 prefixes. The background to Autonomic Networking is
described in [RFC7575] and [RFC7576]. The GeneRic Autonomic
Signaling Protocol (GRASP) is specified by [RFC8990] and can make use
of the technical objectives to provide a solution for autonomic
prefix management. An important purpose of the present document is
to use it for validation of the design of GRASP and other components
of the ANI as described in [RFC8993].
This document is not a complete functional specification of an
autonomic prefix management system, and it does not describe all
detailed aspects of the GRASP objective parameters and Autonomic
Service Agent (ASA) procedures necessary to build a complete system.
Instead, it describes the architectural framework utilizing the
components of the ANI, outlines the different deployment options and
aspects, and defines GRASP objectives for use in building the system.
It also provides some basic parameter examples.
This document is not intended to solve all cases of IPv6 prefix
management. In fact, it assumes that the network's main
infrastructure elements already have addresses and prefixes. This
document is dedicated to how to make IPv6 prefix management at the
edges of large-scale networks as autonomic as possible. It is
specifically written for Internet Service Provider (ISP) networks.
Although there are similarities between ISPs and large enterprise
networks, the requirements for the two use cases differ. In any
case, the scope of the solution is expected to be limited, like any
Autonomic Network, to a single management domain.
However, the solution is designed in a general way. Its use for a
broader scope than edge prefixes, including some or all
infrastructure prefixes, is left for future discussion.
A complete solution has many aspects that are not discussed here.
Once prefixes have been assigned to routers, they need to be
communicated to the routing system as they are brought into use.
Similarly, when prefixes are released, they need to be removed from
the routing system. Different operators may have different policies
regarding prefix lifetimes, and they may prefer to have centralized
or distributed pools of spare prefixes. In an Autonomic Network,
these are properties decided upon by the design of the relevant ASAs.
The GRASP objectives are simply building blocks.
A particular risk of distributed prefix allocation in large networks
is that over time, it might lead to fragmentation of the address
space and an undesirable increase in the size of the interior routing
protocol tables. The extent of this risk depends on the algorithms
and policies used by the ASAs. Mitigating this risk might even
become an autonomic function in itself.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses terminology defined in [RFC7575].
3. Problem Statement
The Autonomic Networking use case considered here is autonomic IPv6
prefix management at the edge of large-scale ISP networks.
Although DHCPv6-PD (DHCPv6 Prefix Delegation) [RFC8415] supports
automated delegation of IPv6 prefixes from one router to another,
prefix management still largely depends on human planning. In other
words, there is no basic information or policy to support autonomic
decisions on the prefix length that each router should request or be
delegated, according to its role in the network. Roles could be
defined separately for individual devices or could be generic (edge
router, interior router, etc.). Furthermore, IPv6 prefix management
by humans tends to be rigid and static after initial planning.
The problem to be solved by Autonomic Networking is how to
dynamically manage IPv6 address space in large-scale networks, so
that IPv6 addresses can be used efficiently. Here, we limit the
problem to assignment of prefixes at the edge of the network, close
to access routers that support individual fixed-line subscribers,
mobile customers, and corporate customers. We assume that the core
infrastructure of the network has already been established with
appropriately assigned prefixes. The Autonomic Networking approach
discussed in this document is based on the assumption that there is a
generic discovery and negotiation protocol that enables direct
negotiation between intelligent IP routers. GRASP [RFC8990] is
intended to be such a protocol.
3.1. Intended User and Administrator Experience
The intended experience is, for the administrators of a large-scale
network, that the management of IPv6 address space at the edge of the
network can be run with minimum effort, as devices at the edge are
added and removed and as customers of all kinds join and leave the
network. In the ideal scenario, the administrators only have to
specify a single IPv6 prefix for the whole network and the initial
prefix length for each device role. As far as users are concerned,
IPv6 prefix assignment would occur exactly as it does in any other
network.
The actual prefix usage needs to be logged for potential offline
management operations, including audit and security incident tracing.
3.2. Analysis of Parameters and Information Involved
For specific purposes of address management, each edge device will
implement several parameters. (Some of them can be preconfigured
before they are connected.) They include the following:
* Identity, authentication, and authorization of this device. This
is expected to use the Autonomic Networking secure bootstrap
process [RFC8995], following which the device could safely take
part in autonomic operations.
* Role of this device. Some example roles are discussed in
Section 6.1.
* An IPv6 prefix length for this device.
* An IPv6 prefix that is assigned to this device and its downstream
devices.
The network as a whole will implement the following parameters:
* Identity of a trust anchor, which is a certification authority
(CA) maintained by the network administrators, used during the
secure bootstrap process.
* Total IPv6 address space available for edge devices. It is a pool
of one or several IPv6 prefixes.
* The initial prefix length for each device role.
3.2.1. Parameters Each Device Can Define for Itself
This section identifies those of the above parameters that do not
need external information in order for the devices concerned to set
them to a reasonable default value after bootstrap or after a network
disruption. They are as follows:
* Default role of this device.
* Default IPv6 prefix length for this device.
* Cryptographic identity of this device, as needed for secure
bootstrapping [RFC8995].
The device may be shipped from the manufacturer with a preconfigured
role and default prefix length, which could be modified by an
autonomic mechanism. Its cryptographic identity will be installed by
its manufacturer.
3.2.2. Information Needed from Network Operations
This section identifies those parameters that might need operational
input in order for the devices concerned to set them to a non-default
value.
* Non-default value for the IPv6 prefix length for this device.
This needs to be decided based on the role of this device.
* The initial prefix length for each device role.
* Whether to allow the device to request more address space.
* The policy regarding when to request more address space -- for
example, if the address usage reaches a certain limit or
percentage.
3.2.3. Comparison with Current Solutions
This section briefly compares the above use case with current
solutions. Currently, the address management is still largely
dependent on human planning. It is rigid and static after initial
planning. Address requests will fail if the configured address space
is used up.
Some autonomic and dynamic address management functions may be
achievable by extending the existing protocols -- for example,
extending DHCPv6-PD [RFC8415] to request IPv6 prefixes according to
the device role. However, defining uniform device roles may not be a
practical task, as some functions cannot be configured on the basis
of role using existing prefix delegation protocols.
Using a generic autonomic discovery and negotiation protocol instead
of specific solutions has the advantage that additional parameters
can be included in the autonomic solution without creating new
mechanisms. This is the principal argument for a generic approach.
3.3. Interaction with Other Devices
3.3.1. Information Needed from Other Devices
This section identifies those of the above parameters that need
external information from neighbor devices (including the upstream
devices). In many cases, two-way dialogue with neighbor devices is
needed to set or optimize them.
* Information regarding the identity of a trust anchor is needed.
* The device will need to discover another device from which it can
acquire IPv6 address space.
* Information regarding the initial prefix length for the role of
each device is needed, particularly for its own downstream
devices.
* The default value of the IPv6 prefix length may be overridden by a
non-default value.
* The device will need to request and acquire one or more IPv6
prefixes that can be assigned to this device and its downstream
devices.
* The device may respond to prefix delegation requests from its
downstream devices.
* The device may require the assignment of more IPv6 address space
if it used up its assigned IPv6 address space.
3.3.2. Monitoring, Diagnostics, and Reporting
This section discusses what role devices should play in monitoring,
fault diagnosis, and reporting.
* The actual address assignments need to be logged for potential
offline management operations.
* In general, the usage situation regarding address space should be
reported to the network administrators in an abstract way -- for
example, statistics or a visualized report.
* A forecast of address exhaustion should be reported.
4. Autonomic Edge Prefix Management Solution
This section introduces the building blocks for an autonomic edge
prefix management solution. As noted in Section 1, this is not a
complete description of a solution, which will depend on the detailed
design of the relevant Autonomic Service Agents (ASAs). It uses the
generic discovery and negotiation protocol defined by [RFC8990]. The
relevant GRASP objectives are defined in Section 5.
The procedures described below are carried out by an ASA in each
device that participates in the solution. We will refer to this as
the PrefixManager ASA.
4.1. Behavior of a Device Requesting a Prefix
If the device containing a PrefixManager ASA has used up its address
pool, it can request more space according to its requirements. It
should decide the length of the requested prefix and request it via
the mechanism described in Section 6. Note that although the
device's role may define certain default allocation lengths, those
defaults might be changed dynamically, and the device might request
more, or less, address space due to some local operational heuristic.
A PrefixManager ASA that needs additional address space should
firstly discover peers that may be able to provide extra address
space. The ASA should send out a GRASP Discovery message that
contains a PrefixManager Objective option (see Section 2 of [RFC8650]
and Section 5.1) in order to discover peers also supporting that
option. Then, it should choose one such peer, most likely the first
to respond.
If the GRASP Discovery Response message carries a Divert option
pointing to an off-link PrefixManager ASA, the requesting ASA may
initiate negotiation with that ASA-diverted device to find out
whether it can provide the requested length of the prefix.
In any case, the requesting ASA will act as a GRASP negotiation
initiator by sending a GRASP Request message with a PrefixManager
Objective option. The ASA indicates in this option the length of the
requested prefix. This starts a GRASP negotiation process.
During the subsequent negotiation, the ASA will decide at each step
whether to accept the offered prefix. That decision, and the
decision to end the negotiation, are implementation choices.
The ASA could alternatively initiate GRASP discovery in rapid mode
with an embedded negotiation request, if it is implemented.
4.2. Behavior of a Device Providing a Prefix
At least one device on the network must be configured with the
initial pool of available prefixes mentioned in Section 3.2. Apart
from that requirement, any device may act as a provider of prefixes.
A device that receives a Discovery message with a PrefixManager
Objective option should respond with a GRASP Response message if it
contains a PrefixManager ASA. Further details of the discovery
process are described in [RFC8990]. When this ASA receives a
subsequent Request message, it should conduct a GRASP negotiation
sequence, using Negotiate, Confirm Waiting, and Negotiation End
messages as appropriate. The Negotiate messages carry a
PrefixManager Objective option, which will indicate the prefix and
its length offered to the requesting ASA. As described in [RFC8990],
negotiation will continue until either end stops it with a
Negotiation End message. If the negotiation succeeds, the ASA that
provides the prefix will remove the negotiated prefix from its pool,
and the requesting ASA will add it. If the negotiation fails, the
party sending the Negotiation End message may include an error code
string.
During the negotiation, the ASA will decide at each step how large a
prefix to offer. That decision, and the decision to end the
negotiation, are implementation choices.
The ASA could alternatively negotiate in response to GRASP discovery
in rapid mode, if it is implemented.
This specification is independent of whether the PrefixManager ASAs
are all embedded in routers, but that would be a rather natural
scenario. In a hierarchical network topology, a given router
typically provides prefixes for routers below it in the hierarchy,
and it is also likely to contain the first PrefixManager ASA
discovered by those downstream routers. However, the GRASP discovery
model, including its redirection feature, means that this is not an
exclusive scenario, and a downstream PrefixManager ASA could
negotiate a new prefix with a device other than its upstream router.
A resource shortage may cause the gateway router to request more
resources in turn from its own upstream device. This would be
another independent GRASP discovery and negotiation process. During
the processing time, the gateway router should send a Confirm Waiting
message to the initial requesting router, to extend its timeout.
When the new resource becomes available, the gateway router responds
with a GRASP Negotiate message with a prefix length matching the
request.
The algorithm used to choose which prefixes to assign on the devices
that provide prefixes is an implementation choice.
4.3. Behavior after Successful Negotiation
Upon receiving a GRASP Negotiation End message that indicates that an
acceptable prefix length is available, the requesting device may use
the negotiated prefix without further messages.
There are use cases where the ANI/GRASP-based prefix management
approach can work together with DHCPv6-PD [RFC8415] as a complement.
For example, the ANI/GRASP-based method can be used intra-domain,
while the DHCPv6-PD method works inter-domain (i.e., across an
administrative boundary). Also, ANI/GRASP can be used inside the
domain, and DHCP/DHCPv6-PD can be used on the edge of the domain to
clients (non-ANI devices). Another similar use case would be ANI/
GRASP inside the domain, with RADIUS [RFC2865] providing prefixes to
client devices.
4.4. Prefix Logging
Within the autonomic prefix management system, all prefix assignments
are done by devices without human intervention. It may be required
that all prefix assignment history be recorded -- for example, to
detect or trace lost prefixes after outages or to meet legal
requirements. However, the logging and reporting process is out of
scope for this document.
5. Autonomic Prefix Management Objectives
This section defines the GRASP technical objective options that are
used to support autonomic prefix management.
5.1. Edge Prefix Objective Option
The PrefixManager Objective option is a GRASP Objective option
conforming to the GRASP specification [RFC8990]. Its name is
"PrefixManager" (see Section 8), and it carries the following data
items as its value: the prefix length and the actual prefix bits.
Since GRASP is based on CBOR (Concise Binary Object Representation)
[RFC8949], the format of the PrefixManager Objective option is
described in the Concise Data Definition Language (CDDL) [RFC8610] as
follows:
objective = ["PrefixManager", objective-flags, loop-count,
[length, ?prefix]]
loop-count = 0..255 ; as in the GRASP specification
objective-flags /= ; as in the GRASP specification
length = 0..128 ; requested or offered prefix length
prefix = bytes .size 16 ; offered prefix in binary format
The use of the "dry run" mode of GRASP is NOT RECOMMENDED for this
objective, because it would require both ASAs to store state
information about the corresponding negotiation, to no real benefit
-- the requesting ASA cannot base any decisions on the result of a
successful dry-run negotiation.
5.2. IPv4 Extension
This section presents an extended version of the PrefixManager
objective that supports IPv4 by adding an extra flag:
objective = ["PrefixManager", objective-flags, loop-count, prefval]
loop-count = 0..255 ; as in the GRASP specification
objective-flags /= ; as in the GRASP specification
prefval /= pref6val
pref6val = [version6, length, ?prefix]
version6 = 6
length = 0..128 ; requested or offered prefix length
prefix = bytes .size 16 ; offered prefix in binary format
prefval /= pref4val
pref4val = [version4, length4, ?prefix4]
version4 = 4
length4 = 0..32 ; requested or offered prefix length
prefix4 = bytes .size 4 ; offered prefix in binary format
Prefix and address management for IPv4 is considerably more difficult
than for IPv6, due to the prevalence of NAT, ambiguous addresses
[RFC1918], and address sharing [RFC6346]. These complexities might
require further extending the objective with additional fields that
are not defined by this document.
6. Prefix Management Parameters
An implementation of a prefix manager MUST include default settings
of all necessary parameters. However, within a single administrative
domain, the network operator MAY change default parameters for all
devices with a certain role. Thus, it would be possible to apply an
intended policy for every device in a simple way, without traditional
configuration files. As noted in Section 4.1, individual autonomic
devices may also change their own behavior dynamically.
For example, the network operator could change the default prefix
length for each type of role. A prefix management parameters
objective, which contains mapping information of device roles and
their default prefix lengths, MAY be flooded in the network, through
the Autonomic Control Plane (ACP) [RFC8994]. The objective is
defined in CDDL as follows:
objective = ["PrefixManager.Params", objective-flags, any]
loop-count = 0..255 ; as in the GRASP specification
objective-flags /= ; as in the GRASP specification
The "any" object would be the relevant parameter definitions (such as
the example below) transmitted as a CBOR object in an appropriate
format.
This could be flooded to all nodes, and any PrefixManager ASA that
did not receive it for some reason could obtain a copy using GRASP
unicast synchronization. Upon receiving the prefix management
parameters, every device can decide its default prefix length by
matching its own role.
6.1. Example of Prefix Management Parameters
The parameters comprise mapping information of device roles and their
default prefix lengths in an autonomic domain. For example, suppose
an IPRAN (IP Radio Access Network) operator wants to configure the
prefix length of a Radio Network Controller Site Gateway (RSG) as 34,
the prefix length of an Aggregation Site Gateway (ASG) as 44, and the
prefix length of a Cell Site Gateway (CSG) as 56. This could be
described in the value of the PrefixManager.Params objective as:
[
[["role", "RSG"],["prefix_length", 34]],
[["role", "ASG"],["prefix_length", 44]],
[["role", "CSG"],["prefix_length", 56]]
]
This example is expressed in JSON [RFC8259], which is easy to
represent in CBOR.
An alternative would be to express the parameters in YANG [RFC7950]
using the YANG-to-CBOR mapping [CORE-YANG-CBOR].
For clarity, the background of the example is introduced below and
can also be regarded as a use case for the mechanism defined in this
document.
An IPRAN is used for mobile backhaul, including radio stations, RNCs
(Radio Network Controllers) (in 3G) or the packet core (in LTE), and
the IP network between them, as shown in Figure 1. The eNB (Evolved
Node B) entities, the RNC, the SGW (Serving Gateway), and the MME
(Mobility Management Entity) are mobile network entities defined in
3GPP. The CSGs, ASGs, and RSGs are entities defined in the IPRAN
solution.
The IPRAN topology shown in Figure 1 includes Ring1, which is the
circle following ASG1->RSG1->RSG2->ASG2->ASG1; Ring2, following
CSG1->ASG1->ASG2->CSG2->CSG1; and Ring3, following
CSG3->ASG1->ASG2->CSG3. In a real deployment of an IPRAN, there may
be more stations, rings, and routers in the topology, and normally
the network is highly dependent on human design and configuration,
which is neither flexible nor cost-effective.
+------+ +------+
| eNB1 |---| CSG1 |\
+------+ +------+ \ +-------+ +------+ +-------+
| \ | ASG1 |-------| RSG1 |-----------|SGW/MME|
| Ring2 +-------+ +------+ \ /+-------+
+------+ +------+ / | | \ /
| eNB2 |---| CSG2 | \ / | Ring1 | \/
+------+ +------+ \ Ring3| | /\
/ \ | | / \
+------+ +------+ / \ +-------+ +------+/ \+-------+
| eNB3 |---| CSG3 |--------| ASG2 |------| RSG2 |---------| RNC |
+------+ +------+ +-------+ +------+ +-------+
Figure 1: IPRAN Topology Example
If ANI/GRASP is supported in the IPRAN, the network nodes should be
able to negotiate with each other and make some autonomic decisions
according to their own status and the information collected from the
network. The prefix management parameters should be part of the
information they communicate.
The routers should know the role of their neighbors, the default
prefix length for each type of role, etc. An ASG should be able to
request prefixes from an RSG, and a CSG should be able to request
prefixes from an ASG. In each request, the ASG/CSG should indicate
the required prefix length, or its role, which implies what length it
needs by default.
7. Security Considerations
Relevant security issues are discussed in [RFC8990]. The preferred
security model is that devices are trusted following the secure
bootstrap procedure [RFC8995] and that a secure Autonomic Control
Plane (ACP) [RFC8994] is in place.
It is RECOMMENDED that DHCPv6-PD, if used, should be implemented
using DHCPv6 authentication or Secure DHCPv6.
8. IANA Considerations
This document defines two new GRASP Objective option names:
"PrefixManager" and "PrefixManager.Params". The IANA has added these
to the "GRASP Objective Names" registry defined by [RFC8990].
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[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>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
[RFC8990] Bormann, C., Carpenter, B., Ed., and B. Liu, Ed., "GeneRic
Autonomic Signaling Protocol (GRASP)", RFC 8990,
DOI 10.17487/RFC8990, May 2021,
<https://www.rfc-editor.org/info/rfc8990>.
[RFC8994] Eckert, T., Ed., Behringer, M., Ed., and S. Bjarnason, "An
Autonomic Control Plane (ACP)", RFC 8994,
DOI 10.17487/RFC8994, May 2021,
<https://www.rfc-editor.org/info/rfc8994>.
[RFC8995] Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
May 2021, <https://www.rfc-editor.org/info/rfc8995>.
9.2. Informative References
[CORE-YANG-CBOR]
Veillette, M., Ed., Petrov, I., Ed., and A. Pelov, "CBOR
Encoding of Data Modeled with YANG", Work in Progress,
Internet-Draft, draft-ietf-core-yang-cbor-15, 24 January
2021, <https://tools.ietf.org/html/draft-ietf-core-yang-
cbor-15>.
[DHCP-YANG-MODEL]
Liu, B., Ed., Lou, K., and C. Chen, "Yang Data Model for
DHCP Protocol", Work in Progress, Internet-Draft, draft-
liu-dhc-dhcp-yang-model-07, 12 October 2018,
<https://tools.ietf.org/html/draft-liu-dhc-dhcp-yang-
model-07>.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
J., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
February 1996, <https://www.rfc-editor.org/info/rfc1918>.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, DOI 10.17487/RFC2865, June 2000,
<https://www.rfc-editor.org/info/rfc2865>.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, DOI 10.17487/RFC3046, January 2001,
<https://www.rfc-editor.org/info/rfc3046>.
[RFC6221] Miles, D., Ed., Ooghe, S., Dec, W., Krishnan, S., and A.
Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221,
DOI 10.17487/RFC6221, May 2011,
<https://www.rfc-editor.org/info/rfc6221>.
[RFC6346] Bush, R., Ed., "The Address plus Port (A+P) Approach to
the IPv4 Address Shortage", RFC 6346,
DOI 10.17487/RFC6346, August 2011,
<https://www.rfc-editor.org/info/rfc6346>.
[RFC7575] Behringer, M., Pritikin, M., Bjarnason, S., Clemm, A.,
Carpenter, B., Jiang, S., and L. Ciavaglia, "Autonomic
Networking: Definitions and Design Goals", RFC 7575,
DOI 10.17487/RFC7575, June 2015,
<https://www.rfc-editor.org/info/rfc7575>.
[RFC7576] Jiang, S., Carpenter, B., and M. Behringer, "General Gap
Analysis for Autonomic Networking", RFC 7576,
DOI 10.17487/RFC7576, June 2015,
<https://www.rfc-editor.org/info/rfc7576>.
[RFC8650] Voit, E., Rahman, R., Nilsen-Nygaard, E., Clemm, A., and
A. Bierman, "Dynamic Subscription to YANG Events and
Datastores over RESTCONF", RFC 8650, DOI 10.17487/RFC8650,
November 2019, <https://www.rfc-editor.org/info/rfc8650>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
[RFC8993] Behringer, M., Ed., Carpenter, B., Eckert, T., Ciavaglia,
L., and J. Nobre, "A Reference Model for Autonomic
Networking", RFC 8993, DOI 10.17487/RFC8993, May 2021,
<https://www.rfc-editor.org/info/rfc8993>.
Appendix A. Deployment Overview
This appendix includes logical deployment models and explanations of
the target deployment models. Its purpose is to help in
understanding the mechanism described in this document.
This appendix includes two subsections: Appendix A.1 for the two most
common DHCP deployment models and Appendix A.2 for the PD deployment
model described in this document. It should be noted that these are
just examples, and there are many more deployment models.
A.1. Address and Prefix Management with DHCP
Edge DHCP server deployment requires every edge router connecting to
a Customer Premises Equipment (CPE) device to be a DHCP server
assigning IPv4/IPv6 addresses to CPEs -- and, optionally, IPv6
prefixes via DHCPv6-PD for IPv6-capable CPEs that are routers and
have LANs behind them.
edge
dynamic, "NETCONF/YANG" interfaces
<---------------> +-------------+
+------+ <- telemetry | edge router/|-+ ----- +-----+
|config| .... domain ... | DHCP server | | ... | CPE |+ LANs
|server| +-------------+ | ----- +-----+| (---| )
+------+ +--------------+ DHCP/ +-----+
DHCPv6-PD
Figure 2: DHCP Deployment Model without a Central DHCP Server
This requires various coordination functions via some backend system
(depicted as the "config server" in Figure 2): the address prefixes
on the edge interfaces should be slightly larger than required for
the number of CPEs connected so that the overall address space is
best used.
The config server needs to provision edge interface address prefixes
and DHCP parameters for every edge router. If prefixes that are too
fine-grained are used, this will result in large routing tables
across the domain shown in the figure. If prefixes that are too
coarse-grained are used, address space is wasted. (This is less of a
concern for IPv6, but if the model includes IPv4, it is a very
serious concern.)
There is no standard that describes algorithms for how configuration
servers would best perform this ongoing dynamic provisioning to
optimize routing table size and address space utilization.
There are currently no complete YANG data models that a config server
could use to perform these actions (including telemetry of assigned
addresses from such distributed DHCP servers). For example, a YANG
data model for controlling DHCP server operations is still being
developed [DHCP-YANG-MODEL].
Due to these and other problems related to the above model, the more
common DHCP deployment model is as follows:
+------+ edge
|config| initial, "CLI" interfaces
|server| ----------------> +-------------+
+------+ | edge router/|-+ ----- +-----+
| .... domain ... | DHCP relay | | ... | CPE |+ LANs
+------+ +-------------+ | ----- +-----+| (---| )
|DHCP | +--------------+ DHCP/ +-----+
|server| DHCPv6-PD
+------+
Figure 3: DHCP Deployment Model with a Central DHCP Server
Dynamic provisioning changes to edge routers are avoided by using a
central DHCP server and reducing the edge router from DHCP server to
DHCP relay. The "configuration" on the edge routers is static. The
DHCP relay function inserts an "edge interface" and/or subscriber-
identifying options into DHCP requests from CPEs (e.g., [RFC3046]
[RFC6221]), and the DHCP server has complete policies for address
assignments and prefixes usable on every edge router / interface /
subscriber group. When the DHCP relay sees the DHCP reply, it
inserts static routes for the assigned address / address prefix into
the routing table of the edge router; these routes are then to be
distributed by the IGP (or BGP) inside the domain to make the CPE and
LANs reachable across the domain shown in the figure.
There is no comprehensive standardization of these solutions. For
example, [RFC8415], Section 19.1.3 simply refers to "a [non-defined]
protocol or other out-of-band communication to configure routing
information for delegated prefixes on any router through which the
client may forward traffic."
A.2. Prefix Management with ANI/GRASP
Using the ANI and prefix management ASAs (PM-ASAs) using GRASP, the
deployment model is intended to look as follows:
|<............ ANI domain / ACP............>| (...) ........->
Roles
|
v "Edge routers"
GRASP parameter +----------+
Network-wide | PM-ASA | downstream
parameters/policies | (DHCP | interfaces
| |functions)| ------
v "central device" +----------+
+------+ ^ +--------+
|PM-ASA| <............GRASP .... .... | CPE |-+ (LANs)
+------+ . v |(PM-ASA)| | ---|
. +........+ +----------+ +--------+ |
+...........+ . PM-ASA . | PM-ASA | ------ +---------+
.DHCP server. +........+ | (DHCP | SLAAC/
+...........+ "intermediate |functions)| DHCP/DHCP-PD
router" +----------+
Figure 4: Deployment Model Using ANI/GRASP
The network runs an ANI domain with an ACP [RFC8994] between some
central device (e.g., a router or an ANI-enabled management device)
and the edge routers. ANI/ACP provides a secure, zero-touch
communication channel between the devices and enables the use of
GRASP [RFC8990] not only for peer-to-peer communication but also for
distribution/flooding.
The central devices and edge routers run software in the form of ASAs
to support this document's autonomic IPv6 edge prefix management.
PM-ASAs as discussed below together comprise the Autonomic Prefix
Management Function.
Edge routers can have different roles based on the type and number of
CPEs attaching to them. Each edge router could be an RSG, ASG, or
CSG in mobile aggregation networks (see Section 6.1). Mechanisms
outside the scope of this document make routers aware of their roles.
Some considerations related to the deployment model are as follows.
1. In a minimum prefix management solution, the central device uses
the PrefixManager.Params GRASP objective introduced in this
document to disseminate network-wide, per-role parameters to edge
routers. The PM-ASA uses the parameters that apply to its own
role to locally configure preexisting addressing functions.
Because the PM-ASA does not manage the dynamic assignment of
actual IPv6 address prefixes in this case, the following options
can be considered:
1.a The edge router connects via downstream interfaces to each
(host) CPE that requires an address. The PM-ASA sets up for
each such interface a DHCP requesting router (according to
[RFC8415]) to request an IPv6 prefix for the interface. The
router's address on the downstream interface can be another
parameter from the GRASP objective. The CPEs assign
addresses in the prefix via Router Advertisements (RAs), or
the PM-ASA manages a local DHCPv6 server to assign addresses
to the CPEs. A central DHCP server acting as the DHCP
delegating router (according to [RFC8415]) is required. Its
address can be another parameter from the GRASP objective.
1.b The edge router also connects via downstream interfaces to
(customer managed) CPEs that are routers and act as DHCPv6
requesting routers. The need to support this could be
derived from role or GRASP parameters, and the PM-ASA sets
up a DHCP relay function to pass on requests to the central
DHCP server as in point 1.a.
2. In a solution without a central DHCP server, the PM-ASA on the
edge routers not only learns parameters from PrefixManager.Params
but also utilizes GRASP to request/negotiate actual IPv6 prefix
delegation via the GRASP PrefixManager objective, as described in
more detail below. In the simplest case, these prefixes are
delegated via this GRASP objective from the PM-ASA in the central
device. This device must be provisioned initially with a large
pool of prefixes. The delegated prefixes are then used by the
PM-ASA on the edge routers to configure prefixes on their
downstream interfaces to assign addresses via RA/SLAAC to host
CPEs. The PM-ASA may also start local DHCP servers (as in point
1.a) to assign addresses via DHCP to the CPEs from the prefixes
it received. This includes both host CPEs requesting IPv6
addresses and router CPEs that request IPv6 prefixes. The PM-ASA
needs to manage the address pool(s) it has requested via GRASP
and allocate sub-address pools to interfaces and the local DHCP
servers it starts. It needs to monitor the address utilization
and accordingly request more address prefixes if its existing
prefixes are exhausted, or return address prefixes when they are
unneeded.
This solution is quite similar to the previous IPv6 DHCP
deployment model without a central DHCP server, and ANI/ACP/GRASP
and the PM-ASA do provide the automation to make this approach
work more easily than is possible today.
3. The address pools from which prefixes are allocated do not all
need to be taken from one central location. An edge-router
PM-ASA that received a big (short) prefix from a central PM-ASA
could offer smaller sub-prefixes to a neighboring edge-router
PM-ASA. GRASP could be used in such a way that the PM-ASA would
find and select the objective from the closest neighboring
PM-ASA, therefore allowing aggregation to be maximized: a PM-ASA
would only request further smaller prefixes when it exhausts its
own pool (from the central location) and cannot get further large
prefixes from that central location anymore. Because the
overflow prefixes taken from a topologically nearby PM-ASA, the
number of longer prefixes that have to be injected into the
routing tables is limited and the topological proximity increases
the chances that aggregation of prefixes in the IGP can most
likely limit the geography in which the longer prefixes need to
be routed.
4. Instead of peer-to-peer optimization of prefix delegation, a
hierarchy of PM-ASAs can be built (indicated in Figure 4 via a
dotted intermediate router). This would require additional
parameters in the PrefixManager objective to allow the creation
of a hierarchy of PM-ASAs across which the prefixes can be
delegated.
5. In cases where CPEs are also part of the ANI domain (e.g.,
"managed CPEs"), then GRASP will extend into the actual customer
sites and can also run a PM-ASA. All the options described in
points 1 to 4 above would then apply to the CPE as the edge
router, with the major changes being that (a) a CPE router will
most likely not need to run DHCPv6-PD itself, but only DHCP
address assignment and (b) the edge routers to which the CPE
connects would most likely become ideal places on which to run a
hierarchical instance of PD-ASAs, as outlined in point 1.
Acknowledgements
Valuable comments were received from William Atwood, Fred Baker,
Michael Behringer, Ben Campbell, Laurent Ciavaglia, Toerless Eckert,
Joel Halpern, Russ Housley, Geoff Huston, Warren Kumari, Dan
Romascanu, and Chongfeng Xie.
Authors' Addresses
Sheng Jiang (editor)
Huawei Technologies Co., Ltd
Q14, Huawei Campus
No. 156 Beiqing Road
Hai-Dian District, Beijing
100095
China
Email: jiangsheng@huawei.com
Zongpeng Du
China Mobile
32 Xuanwumen West St
Xicheng District, Beijing
100053
China
Email: duzongpeng@chinamobile.com
Brian Carpenter
University of Auckland
School of Computer Science
PB 92019
Auckland 1142
New Zealand
Email: brian.e.carpenter@gmail.com
Qiong Sun
China Telecom
118 Xizhimennei St
Beijing
100035
China
Email: sunqiong@chinatelecom.cn
