Keywords: IPsec, Overlay, SDN, IKE
Internet Engineering Task Force (IETF) V. Manral
Request for Comments: 7018 HP
Category: Informational S. Hanna
ISSN: 2070-1721 Juniper
September 2013
Auto-Discovery VPN Problem Statement and Requirements
Abstract
This document describes the problem of enabling a large number of
systems to communicate directly using IPsec to protect the traffic
between them. It then expands on the requirements for such a
solution.
Manual configuration of all possible tunnels is too cumbersome in
many such cases. In other cases, the IP addresses of endpoints
change, or the endpoints may be behind NAT gateways, making static
configuration impossible. The Auto-Discovery VPN solution will
address these requirements.
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 a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7018.
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Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
1.1. Terminology ................................................3
1.2. Conventions Used in This Document ..........................4
2. Use Cases .......................................................4
2.1. Use Case 1: Endpoint-to-Endpoint VPN .......................4
2.2. Use Case 2: Gateway-to-Gateway VPN .........................5
2.3. Use Case 3: Endpoint-to-Gateway VPN ........................6
3. Inadequacy of Existing Solutions ................................6
3.1. Exhaustive Configuration ...................................6
3.2. Star Topology ..............................................6
3.3. Proprietary Approaches .....................................7
4. Requirements ....................................................7
4.1. Gateway and Endpoint Requirements ..........................7
5. Security Considerations ........................................11
6. Acknowledgements ...............................................11
7. Normative References ...........................................12
1. Introduction
IPsec [RFC4301] is used in several different cases, including
tunnel-mode site-to-site VPNs and remote access VPNs. Both tunneling
modes for IPsec gateways and host-to-host transport mode are
supported in this document.
The subject of this document is the problem presented by large-scale
deployments of IPsec and the requirements on a solution to address
the problem. These may be a large collection of VPN gateways
connecting various sites, a large number of remote endpoints
connecting to a number of gateways or to each other, or a mix of the
two. The gateways and endpoints may belong to a single
administrative domain or several domains with a trust relationship.
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Section 4.4 of RFC 4301 describes the major IPsec databases needed
for IPsec processing. It requires extensive configuration for each
tunnel, so manually configuring a system of many gateways and
endpoints becomes infeasible and inflexible.
The difficulty is that a lot of configuration mentioned in RFC 4301
is required to set up a Security Association. The Internet Key
Exchange Protocol (IKE) implementations need to know the identity and
credentials of all possible peer systems, as well as the addresses of
hosts and/or networks behind them. A simplified mechanism for
dynamically establishing point-to-point tunnels is needed. Section 2
contains several use cases that motivate this effort.
1.1. Terminology
Auto-Discovery Virtual Private Network (ADVPN) - A VPN solution that
enables a large number of systems to communicate directly, with
minimal configuration and operator intervention, using IPsec to
protect communication between them.
Endpoint - A device that implements IPsec for its own traffic but
does not act as a gateway.
Gateway - A network device that implements IPsec to protect traffic
flowing through the device.
Point-to-Point - Communication between two parties without active
participation (e.g., encryption or decryption) by any other
parties.
Hub - The central point in a star topology/dynamic full-mesh
topology, or one of the central points in the full-mesh style VPN,
i.e., a gateway to which multiple other hubs or spokes connect.
The hubs usually forward traffic coming from encrypted links to
other encrypted links, i.e., there are no devices connected to
them in the clear.
Spoke - The endpoint in a star topology/dynamic full-mesh topology
or gateway that forwards traffic from multiple cleartext devices
to other hubs or spokes, and some of those other devices are
connected to it in the clear (i.e., it encrypts data coming from
cleartext devices and forwards it to the ADVPN).
ADVPN Peer - Any member of an ADVPN, including gateways, endpoints,
hubs, or spokes.
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Star Topology - Topology in which there is direct connectivity only
between the hub and spoke, and where communication between the 2
spokes happens through the hub.
Allied and Federated Environments - Environments where we have
multiple different organizations that have close associations and
need to connect to each other.
Full-Mesh Topology - Topology in which there is direct connectivity
between every spoke to every other spoke, without the traffic
between the spokes having to be redirected through an intermediate
hub device.
Dynamic Full-Mesh Topology - Topology in which direct connections
exist in a hub-and-spoke manner but dynamic connections are
created/removed between the spokes on an as-needed basis.
Security Association (SA) - Defined in [RFC4301].
1.2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Use Cases
This section presents the key use cases for large-scale
point-to-point VPNs.
In all of these use cases, the participants (endpoints and gateways)
may be from a single organization (administrative domain) or from
multiple organizations with an established trust relationship. When
multiple organizations are involved, products from multiple vendors
are employed, so open standards are needed to provide
interoperability. Establishing communications between participants
with no established trust relationship is out of scope for this
effort.
2.1. Use Case 1: Endpoint-to-Endpoint VPN
Two endpoints wish to communicate securely via a point-to-point SA.
The need for secure endpoint-to-endpoint communications is often
driven by a need to employ high-bandwidth, low-latency local
connectivity instead of using slow, expensive links to remote
gateways. For example, two users in close proximity may wish to
place a direct, secure video or voice call without needing to send
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the call through remote gateways, as the remote gateways would add
latency to the call, consume precious remote bandwidth, and increase
overall costs. Such a use case also enables connectivity when both
users are behind NAT gateways. Such a use case ought to allow for
seamless connectivity even as endpoints roam and even if they are
moving out from behind a NAT gateway, from behind one NAT gateway to
behind another, or from a standalone position to behind a NAT
gateway.
In a star topology, when two endpoints communicate, they need a
mechanism for authentication such that they do not expose themselves
to impersonation by the other spoke endpoint.
2.2. Use Case 2: Gateway-to-Gateway VPN
A typical Enterprise traffic model follows a star topology, with the
gateways connecting to each other using IPsec tunnels.
However, for voice and other rich media traffic that require a lot of
bandwidth or is performance sensitive, the traffic tromboning (taking
a suboptimal path) to the hub can create traffic bottlenecks on the
hub and can lead to an increase in cost. A fully meshed solution
would make best use of the available network capacity and
performance, but the deployment of a fully meshed solution involves
considerable configuration, especially when a large number of nodes
are involved. It is for this purpose that spoke-to-spoke tunnels are
dynamically created and torn down. For the reasons of cost and
manual error reduction, it is desired that there be minimal
configuration on each gateway.
The solution ought to work in cases where the endpoints are in
different administrative domains that have an existing trust
relationship (for example, two organizations that are collaborating
on a project may wish to join their networks while retaining
independent control over configuration). It is highly desirable that
the solution works for the star, full-mesh, and dynamic full-mesh
topologies.
The solution ought to also address the case where gateways use
dynamic IP addresses.
Additionally, the routing implications of gateway-to-gateway
communication need to be addressed. In the simple case, selectors
provide sufficient information for a gateway to forward traffic
appropriately. In other cases, additional tunneling (e.g., Generic
Routing Encapsulation (GRE)) and routing (e.g., Open Shortest Path
First (OSPF)) protocols are run over IPsec tunnels, and the
configuration impact on those protocols needs to be considered.
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There is also the case where Layer 3 Virtual Private Networks
(L3VPNs) operate over IPsec tunnels.
When two gateways communicate, they need to use a mechanism for
authentication such that they do not expose themselves to the risk of
impersonation by the other entities.
2.3. Use Case 3: Endpoint-to-Gateway VPN
A mobile endpoint ought to be able to use the most efficient gateway
as it roams in the Internet.
A mobile user roaming on the Internet may connect to a gateway that,
because of roaming, is no longer the most efficient gateway to use
(reasons could be cost, efficiency, latency, or some other factor).
The mobile user ought to be able to discover and then connect to the
current, most efficient gateway in a seamless way without having to
bring down the connection.
3. Inadequacy of Existing Solutions
Several solutions exist for the problems described above. However,
none of these solutions is adequate, as described here.
3.1. Exhaustive Configuration
One simple solution is to configure all gateways and endpoints in
advance with all the information needed to determine which gateway or
endpoint is optimal and to establish an SA with that gateway or
endpoint. However, this solution does not scale in a large network
with hundreds of thousands of gateways and endpoints, especially when
multiple administrative domains are involved and things are rapidly
changing (e.g., mobile endpoints). Such a solution is also limited
by the smallest endpoint/gateway, as the same exhaustive
configuration is to be applied on all endpoints/gateways. A more
dynamic, secure, and scalable system for establishing SAs between
gateways is needed.
3.2. Star Topology
The most common way to address a part of this problem today is to use
what has been termed a "star topology". In this case, one or a few
gateways are defined as "hub gateways", while the rest of the systems
(whether endpoints or gateways) are defined as "spokes". The spokes
never connect to other spokes. They only open tunnels with the hub
gateways. Also, for a large number of gateways in one administrative
domain, one gateway may be defined as the hub, and the rest of the
gateways and remote access clients connect only to that gateway.
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This solution, however, is complicated by the case where the spokes
use dynamic IP addresses and DNS with dynamic updates needs to be
used. It is also desired that there is minimal to no configuration
on the hub as the number of spokes increases and new spokes are added
and deleted randomly.
Another problem with the star topology is that it creates a high load
on the hub gateways, as well as on the connection between the spokes
and the hub. This load impacts both processing power and network
bandwidth. A single packet in the hub-and-spoke scenario can be
encrypted and decrypted multiple times. It would be much preferable
if these gateways and clients could initiate tunnels between them,
bypassing the hub gateways. Additionally, the path bandwidth to
these hub gateways may be lower than that of the path between the
spokes. For example, two remote access users may be in the same
building with high-speed WiFi (for example, at an IETF meeting).
Channeling their conversation through the hub gateways of their
respective employers seems extremely wasteful, given that a more
optimal direct path exists.
The challenge is to build large-scale IPsec-protected networks that
can dynamically change with minimal administrative overhead.
3.3. Proprietary Approaches
Several vendors offer proprietary solutions to these problems.
However, these solutions offer no interoperability between equipment
from one vendor and another. This means that they are generally
restricted to use within one organization, and it is harder to move
away from such solutions, as the features are not standardized.
Besides, multiple organizations cannot be expected to all choose the
same equipment vendor.
4. Requirements
This section defines the requirements on which the solution will be
based.
4.1. Gateway and Endpoint Requirements
1. For any network topology (star, full mesh, and dynamic full
mesh), when a new gateway or endpoint is added, removed, or
changed, configuration changes are minimized as follows. Adding
or removing a spoke in the topology MUST NOT require
configuration changes to hubs other than where the spoke was
connected and SHOULD NOT require configuration changes to the
hub to which the spoke was connected. The changes also MUST NOT
require configuration changes in other spokes.
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Specifically, when evaluating potential proposals, we will
compare them by looking at how many endpoints or gateways must
be reconfigured when a new gateway or endpoint is added,
removed, or changed and how substantial this reconfiguration is,
in addition to the amount of static configuration required.
This requirement is driven by use cases 1 and 2 and by the
scaling limitations pointed out in Section 3.1.
2. ADVPN Peers MUST allow IPsec tunnels to be set up with other
members of the ADVPN without any configuration changes, even
when peer addresses get updated every time the device comes up.
This implies that Security Policy Database (SPD) entries or
other configuration based on a peer IP address will need to be
automatically updated, avoided, or handled in some manner to
avoid a need to manually update policy whenever an address
changes.
3. In many cases, additional tunneling protocols (e.g., GRE) or
routing protocols (e.g., OSPF) are run over the IPsec tunnels.
Gateways MUST allow for the operation of tunneling and routing
protocols operating over spoke-to-spoke IPsec tunnels with
minimal or no configuration impact. The ADVPN solution SHOULD
NOT increase the amount of information required to configure
protocols running over IPsec tunnels.
4. In the full-mesh and dynamic full-mesh topologies, spokes MUST
allow for direct communication with other spoke gateways and
endpoints. In the star topology mode, direct communication
between spokes MUST be disallowed.
This requirement is driven by use cases 1 and 2 and by the
limitations of a star topology as pointed out in Section 3.2.
5. ADVPN Peers MUST NOT have a way to get the long-term
authentication credentials for any other ADVPN Peers. The
compromise of an endpoint MUST NOT affect the security of
communications between other ADVPN Peers. The compromise of a
gateway SHOULD NOT affect the security of the communications
between ADVPN Peers not associated with that gateway.
This requirement is driven by use case 1. ADVPN Peers
(especially spokes) become compromised fairly often. The
compromise of one ADVPN Peer SHOULD NOT affect the security of
other unrelated ADVPN Peers.
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6. Gateways SHOULD allow for seamless handoff of sessions in cases
where endpoints are roaming, even if they cross policy
boundaries. This would mean the data traffic is minimally
affected even as the handoff happens. External factors like
firewalls and NAT boxes that will be part of the overall
solution when ADVPN is deployed will not be considered part of
this solution.
Such endpoint roaming may affect not only the endpoint-to-
endpoint SA but also the relationship between the endpoints and
gateways (such as when an endpoint roams to a new network that
is handled by a different gateway).
This requirement is driven by use case 1. Today's endpoints are
mobile and transition often between different networks (from 4G
to WiFi and among various WiFi networks).
7. Gateways SHOULD allow for easy handoff of a session to another
gateway, to optimize latency, bandwidth, load balancing,
availability, or other factors, based on policy.
This ability to migrate traffic from one gateway to another
applies regardless of whether the gateways in question are hubs
or spokes. It even applies in the case where a gateway (hub or
spoke) moves in the network, as may happen with a vehicle-based
network.
This requirement is driven by use case 3.
8. Gateways and endpoints MUST have the capability to participate
in an ADVPN even when they are located behind NAT boxes.
However, in some cases they may be deployed in such a way that
they will not be fully reachable behind a NAT box. It is
especially difficult to handle cases where the hub is behind a
NAT box. When the two endpoints are both behind separate NATs,
communication between these spokes SHOULD be supported using
workarounds such as port forwarding by the NAT or detecting when
two spokes are behind uncooperative NATs, and using a hub in
that case.
This requirement is driven by use cases 1 and 2. Endpoints are
often behind NATs, and gateways sometimes are. IPsec SHOULD
continue to work seamlessly regardless, using ADVPN techniques
whenever possible and providing graceful fallback to hub-and-
spoke techniques as needed.
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9. Changes such as establishing a new IPsec SA SHOULD be reportable
and manageable. However, creating a MIB or other management
technique is not within scope for this effort.
This requirement is driven by manageability concerns for all the
use cases, especially use case 2. As IPsec networks become more
dynamic, management tools become more essential.
10. To support allied and federated environments, endpoints and
gateways from different organizations SHOULD be able to connect
to each other.
This requirement is driven by demand for all the use cases in
federated and allied environments.
11. The administrator of the ADVPN SHOULD allow for the
configuration of a star, full-mesh, or partial full-mesh
topology, based on which tunnels are allowed to be set up.
This requirement is driven by demand for all the use cases in
federated and allied environments.
12. The ADVPN solution SHOULD be able to scale for multicast
traffic.
This requirement is driven by use case 2, where the amount of
rich media multicast traffic is increasing.
13. The ADVPN solution SHOULD allow for easy monitoring, logging,
and reporting of the dynamic changes to help with
troubleshooting such environments.
This requirement is driven by demand for all the use cases in
federated and allied environments.
14. There is also the case where L3VPNs operate over IPsec tunnels,
for example, Provider-Edge-based VPNs. An ADVPN MUST support
L3VPNs as applications protected by the IPsec tunnels.
This requirement is driven by demand for all the use cases in
federated and allied environments.
15. The ADVPN solution SHOULD allow the enforcement of per-peer QoS
in both the star and full-mesh topologies.
This requirement is driven by demand for all the use cases in
federated and allied environments.
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16. The ADVPN solution SHOULD take care of not letting the hub be a
single point of failure.
This requirement is driven by demand for all the use cases in
federated and allied environments.
5. Security Considerations
This is a problem statement and requirements document for the
ADVPN solution and in itself does not introduce any new security
concerns. The solution to the problems presented in this document
may involve dynamic updates to databases defined by RFC 4301,
such as the Security Policy Database (SPD) or the Peer Authorization
Database (PAD).
RFC 4301 is silent about the way these databases are populated, and
it is implied that these databases are static and preconfigured by a
human. Allowing dynamic updates to these databases must be thought
out carefully because it allows the protocol to alter the security
policy that the IPsec endpoints implement.
One obvious attack to watch out for is stealing traffic to a
particular site. The IP address for www.example.com is 192.0.2.10.
If we add an entry to an IPsec endpoint's SPD that says that traffic
to 192.0.2.10 is protected through peer Gw-Mallory, then this allows
Gw-Mallory to either pretend to be www.example.com or proxy and read
all traffic to that site. Updates to this database require a clear
trust model.
Hubs can be a single point of failure that can cause loss of
connectivity of the entire system; this can be a big security issue.
Any ADVPN solution design should take care of these concerns.
6. Acknowledgements
Many people have contributed to the development of this problem
statement. While we cannot thank all contributors, some have played
an especially prominent role. Yoav Nir, Yaron Sheffer, Jorge Coronel
Mendoza, Chris Ulliott, and John Veizades wrote the document upon
which this specification was based. Geoffrey Huang, Toby Mao, Suresh
Melam, Praveen Sathyanarayan, Andreas Steffen, Brian Weis, Lou
Berger, and Tero Kivinen provided essential input.
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7. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
Authors' Addresses
Vishwas Manral
Hewlett-Packard Co.
3000 Hanover St.
Palo Alto, CA 94304
USA
EMail: vishwas.manral@hp.com
Steve Hanna
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
USA
EMail: shanna@juniper.net
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