Internet DRAFT - draft-detienne-dmvpn
draft-detienne-dmvpn
IPSECME Working Group F. Detienne
Internet-Draft M. Kumar
Updates: 2332 (if approved) M. Sullenberger
Intended status: Standards Track Cisco
Expires: June 23, 2014 December 20, 2013
Flexible Dynamic Mesh VPN
draft-detienne-dmvpn-01
Abstract
The purpose of a Dynamic Mesh VPN (DMVPN) is to allow IPsec/IKE
Security Gateways administrators to configure the devices in a
partial mesh (often a simple star topology called Hub-Spokes) and let
the Security Gateways establish direct protected tunnels called
Shortcut Tunnels. These Shortcut Tunnels are dynamically created
when traffic flows and are protected by IPsec.
To achieve this goal, this document extends NHRP ([RFC2332]) into a
routing policy feed and integrates GRE tunneling with IKEv2 and IPsec
to provide the necessary cryptographic security.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 23, 2014.
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
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publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Tunnel Types . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Initial Connectivity . . . . . . . . . . . . . . . . . . 6
4.2. Initial Routing Table Status . . . . . . . . . . . . . . 8
4.3. Indirection Notification . . . . . . . . . . . . . . . . 9
4.4. Node Discovery via Resolution Request . . . . . . . . . . 10
4.5. Resolution Request Forwarding . . . . . . . . . . . . . . 11
4.6. Egress node NHRP cache and Tunnel Creation . . . . . . . 12
4.7. Resolution Reply format and processing . . . . . . . . . 13
4.8. From Hub and Spoke to Dynamic Mesh . . . . . . . . . . . 14
4.9. Remote Access Clients . . . . . . . . . . . . . . . . . . 15
4.10. Node mutual authentication . . . . . . . . . . . . . . . 16
5. NHRP Extension Format . . . . . . . . . . . . . . . . . . . . 17
5.1. NHRP Traffic Indication . . . . . . . . . . . . . . . . . 17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. Compliance against ADVPN requirements . . . . . . . . . . . . 19
9. Design Considerations . . . . . . . . . . . . . . . . . . . . 26
9.1. Routing Policy and RFC4301 Security . . . . . . . . . . . 26
9.2. Using Configuration Attributes . . . . . . . . . . . . . 28
9.3. NAT Support . . . . . . . . . . . . . . . . . . . . . . . 30
10. Acknowldegements . . . . . . . . . . . . . . . . . . . . . . 30
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
11.1. Normative References . . . . . . . . . . . . . . . . . . 30
11.2. Informative References . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
1. Introduction
This document describes a Dynamic Mesh VPN (DMVPN), in which an
initial partial mesh expands to create direct connections called
Shortcut Tunnels between endpoints that need to exchange data but are
not directly connected in the initial mesh.
The approach used in the design of this specification gives DMVPN the
following advantages:
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o Can run with routing protocol or with IKEv2 policies (CP_Exchange)
making the specification suitable for complex gateways and remote
access clients alike.
o Can handle virtually infinite number of prefixes (including mobile
prefixes) thanks to routing protocol.
o The tunnel approach allows load balancing over multiple Transport
networks and multicast to work natively.
o Routing policy can apply more complex peer selection than 5-tuple
traffic selector.
o The layered approach allows evolution of other specifications used
over DMVPN without having to rewrite or modify DMVPN.
o Non-IP protocols such as ISIS, MPLS, plain ethernet... are
natively supported (e.g. for Data Center Interconnection).
In a generic manner, DMVPN topologies initialize as Hub-Spoke
networks where Spoke Security Gateway nodes S* connect to Hub
Security Gateway nodes H* over a public transport network (such as
the Internet) considered insufficiently secure so as to mandate the
use of IPsec and IKE. For scalability and redundancy reasons, there
may be multiple hubs; the Hubs would then be connected together
through the DMVPN. The diagram Figure 1 depicts this situation.
DC1 DC2
| |
[H1]-----[H2]
| | | |
+-+ | | +-+
| | | |
[S1] [S2] [S3] [S4]
| | |
D1 D2 D3
Figure 1: Hub and Spoke, multiple hubs, multiple spokes
Initially, the Security Gateway nodes (S*) are configured to build
tunnels secured with IPsec to the Security Gateway node (H*) in a hub
and spoke style network (any partial mesh will do, but Hub-Spoke is
common and easily understood). This initial network is then used
when traffic starts flowing between the protected networks D*. DMVPN
uses NHRP as a signaling mechanism over the S*-H* and H*-H* tunnels
to trigger the spokes (S*) to discover each other and build dynamic,
direct Shortcut Tunnels. The Shortcut Tunnels allow those spokes to
communicate directly with each other without forwarding traffic
through the hub, essentially creating a dynamic mesh.
The spokes can be either routers or firewalls playing the role of
Security Gateways or hosts such as computers, mobile phones,etc.
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protecting their own traffic. Nodes S1, S2 and S3 above are routers
while S4 is a host implementation.
This document describes how NHRP is modified and augmented to allow
the rapid creation of dynamic IPsec tunnels between two devices.
Throughout this document, we will call these devices participating in
the DMVPN "nodes".
In the context of this document, the nodes protect a topologically
dispersed Private, Overlay Network address space. The nodes allow
the devices in the Overlay Network to communicate securely with each
other via GRE tunnels secured by IPsec using dynamic tunnels
established between the nodes over the (presumably insecure)
Transport network. I.e. the protected tunnel packets are forwarded
over this Transport network.
The NBMA Next Hop Resolution Protocol (NHRP) as described in
[RFC2332] allows an ingress node to determine the internetworking
layer address and NBMA address of an egress node. The servers in
such an NBMA network provide the functionality of address resolution
based on a cache which contains protocol layer address to NBMA
subnetwork layer address resolution information. This can be used to
create a virtual network where dynamic virtual circuits can be
created on an as needed basis. In this document, we will depart the
underlying notion of a centralized NHS.
All data traffic, NHRP frames and other control traffic needed by
this DMVPN MUST be protected by IPsec. In order to efficiently
support Layer 2 based protocols, all packets and frames MUST be
encapsulated in GRE ([RFC2784]) first; the resulting GRE packet then
MUST be protected by IPsec. IPsec transport mode MUST be supported
while IPsec tunnel mode MAY be used. The usage of a GRE
encapsulation protected by IPsec is described in [RFC4301].
Implementations SHOULD strongly link GRE and IPsec SA's through some
form of connection latching as described in [RFC5660].
2. Terminology
The NHRP semantic is used throughout this document however some
additional terminology is used to better fit to the context.
o Protected Network, Private Network: a network hosted by one of the
nodes. The protected network IP addresses are those that are
resolved by NHRP into an NBMA address.
o Overlay Network: the entire network composed with the Protected
Networks and the IP addresses installed on the Tunnel interfaces
instantiating the DMVPN.
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o Transport Network, Public Network: the network transporting the
GRE/IPsec packets.
o Nodes: the devices connected by the DMVPN that implement NHRP, GRE
/IPsec and IKE.
o Ingress Node: The NHRP node that takes data packets from off of
the DMVPN and injects them into the DMVPN on either a multi-hop
tunnel path (initially) or single hop shortcut tunnel. Also the
node that will send an NHRP Resolution Request and receive an NHRP
Resolution Reply to build a short-cut tunnel.
o Egress Node: The NHRP node that extracts data packets from the
DMVPN and forwards them off of the DMVPN. Also the node that
answers an NHRP Resolution Request and send an NHRP Resolution
Reply.
o Intermediate Node: An NHRP node that is in the middle of multi-hop
tunnel path between an Ingress and Egress Node. For the
particular data traffic in question the Intermediate node will
receive packets from the DMVPN and resend them (hair-pin) them
back onto the DMVPN.
Note, a particular node in the DMVPN, may at the same time be an
Ingress, Egress and Intermediate node depending on the data traffic
flow being looked at.
In general, DMVPN nodes make extensive use of the Local Address
Groups (LAG) and Logically Independent Subnets (LIS)models as
described in [RFC2332]. A compliant implementation MUST support the
LAG model and SHOULD support the LIS model.
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
[RFC2119].
3. Tunnel Types
The tunnels described in this document are of type GRE/IPsec. GRE/
IPsec allows a single pair of IPsec SA's to be negotiated between the
DMVPN nodes. From an IPsec aggregation standpoint, this means less
negotiation, cleaner use of expensive resources and less
reprogramming of the data plane by the IKE control plane as
additional networks are discovered between any two peers.
In the remainder of this document, GRE and GRE/IPsec will be used
interchangeably depending on the focused layer but always imply "GRE
protected by IPsec"
Taking advantage of the GRE encapsulation, and while NHRP could be
forwarded over IP, the RFC recommended Layer 2 NHRP frames have been
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retained in order to simplify the security policies (packet filters
do not have to be augmented to allow NHRP through, no risk of
mistakenly propagating frames where they should not, etc.).
Compliant implementations MUST support L2 NHRP frames.
DMVPN can be implemented in a number of ways and this document places
no restriction on the actual implementation. This section covers
what the authors believe are the important implementation
recommendations to construct a scalable implementation.
The authors recommend using a logical interface construct to
represent the GRE tunnels. These interfaces are called Tunnel
Interfaces or simply Interfaces from here onward.
In the remainder of this document, we will assume the implementation
uses point-to-point Tunnel Interfaces; routes to prefixes in the
Overlay network are in the Routing Table (aka Routing Information
Base). These routes forward traffic toward the tunnel interfaces.
Point-to-Multipoint GRE interfaces (aka multipoint interfaces for
short) can also be used. In that case there is by construction only
one tunnel source NBMA address and the interface has multiple tunnel
endpoints. In this case NHRP registration request and reply
messages, [RFC2332], are used to pass the tunnel address to tunnel
NBMA address mapping from the NHC (S*) to the NHS (H*). The NHRP
registration request and reply MAY be restricted to a single direct
tunnel hop between the NHC (S*) and NHS (H*).
For didactic reasons, and an easier understanding of the LAG support,
we will use the point-to-point construct to highlight the protocol
behavior in the remainder of this document. An implementation can
use different models (point-to-point, multipoint, bump in the
stack,...) but MUST comply to the external (protocol level) behavior
described in this document.
4. Solution Overview
4.1. Initial Connectivity
We assume the following scenario where nodes (S1, S2, H1, H2)
depicted in Figure 2 supporting GRE, IPsec/IKE and NHRP establish
connections instantiated by GRE tunnels. Those GRE tunnels SHOULD be
protected by IPsec/IKE. These tunnels will be used to secure all the
data traffic as well as the NHRP control frames. In general, routing
protocols (and possibly other control protocols such as NHRP or IKE)
will also be protected by IPsec or IKE.
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DC1
|
[H1]
| | ]
+-+ +-+ ] GRE/IPsec tunnels over Transport network
| | ]
[S1] [S2]
| |
D1 D2
Figure 2: Hub and Spoke Initial Connectivity
It is assumed that S1, H1 and S2 are connected via a shared Transport
network (typically a Public, NBMA network) and there is connectivity
between the nodes over that transport network.
The nodes possess multiple interfaces; each of which has a dedicated
IP address:
o a public interface IntPub connected to the transport network; IP
address: Pub{node}
o one or several tunnel interface Tunnel0,1,.. (GRE/IPsec)
connecting to peers; IP address: Tun{i}{node}
o a private interface IntPriv facing the private network of the
node; IP address: Priv{node}
e.g. node S1 owns the following addresses: PubS1, TunS1 and PrivS1
The networks D1, D2, DC1 and also the tunnel address Tun{i} can and
are presumed to be private in the sense that their address space is
kept independent from the transport network address space. Together,
they form the Overlay network. For the transport network, the
address family is either IPv4 or IPv6. In the context of this
document, for the overlay network, the address family is IPv4 and/or
IPv6.
Initially, nodes S1 and S2 create a connection to node H1.
Optionally, S1 and S2 MAY register to H1 via NHRP. Typically the GRE
tunnels between S* and H1 will be protected by IPsec. A compliant
implementation MUST support IPsec protected GRE tunnels and SHOULD
support unprotected GRE tunnels.
At the end of this section, a dynamic tunnel will be set up between
S1 and S2 and traffic will flow directly through S1 and S2 without
going through H1.
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4.2. Initial Routing Table Status
In the context of this document, the authors make no assumption about
how the routing tables are initially populated but one can assume
that routing protocols exchange information between H1 and S1 and
between H1 and S2.
In this diagram, we assume each node has routes (summarized or
specific) for networks D1, D2, DC1 which are IP networks. We assume
the summary prefix SUM to encompass all the private networks depicted
on this diagram. We assume the communication between those networks
needs to be protected and therefore, the routes point to tunnels.
I.e. S1 knows a route summarizing all the Overlay subnets and this
route points to the GRE/IPsec tunnel leading to H1. Note, the the
summary prefix is a network design choice and it can be replaced by a
prefix summary manifold or individual non-summarized routes.
Example 1: Node S1 has the following routing table:
o TunH1 => Tunnel0
o SUM => TunH1 on Tunnel0
o 0.0.0.0/0 => IntPub
o D1 => IntPriv
Example 2: Node H1 has the following routing table:
o TunS1 => Tunnel1
o TunS2 => Tunnel2
o D1 => TunS1 on Tunnel1
o D2 => TunS2 on Tunnel2
o 0.0.0.0/0 => IntPub
o DC1 => IntPriv
The exact format of the routing table is implementation dependent but
the node discovery principle MUST be enforced and the implementation
MUST be compatible with an implementation using the routing tables
outlined above.
This document does not specify how the routes are installed but it
can be assumed that the routes (1) and (2) in the tables above are
exchanged between S* and H* nodes after the S*-H* connections have
been duly authenticated. In a DMVPN solution, it is typical that the
routes are exchanged by a route exchange protocol (e.g. BGP or IKE as
shown in Section 9.2) or are installed statically (usually a mix of
both). It is important that routing updates be filtered in order to
prevent a node from advertising improper routes to another node.
This filtering is out of the scope of this document as most routing
protocol implementations are already capable of such filtering. In
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order to meet these criteria, an implementation SHOULD offer
identity-based policies to filter those routes on a per peer basis.
When a device Ds on network D1 needs to connect to a device Dd on
network D2
o a data packet ip(Ds, Dd) is sent and reaches S1 on IntPriv
o the data packet is routed by S1 via Tunnel0 toward H1; S1
encapsulates, protects and forwards this packet out IntPub via the
transport network to H1
o H1 receives the protected packet on IntPub; H1 decrypts and
decapsulates this packet; the resulting data packet looks to the
IP stack on H1 as if it arrived on interface Tunnel1
o the data packet is routed by H1 via Tunnel2 toward S2; H1
encapsulates, protects and forwards this out IntPub via the
transport network to S2
o S2 receives the protected packet on IntPub; S2 decrypts and
decapsulates this packet; the resulting data packet looks to the
IP stack as if it arrived on interface Tunnel0
o S2 routes the data packet out of its IntPriv interface to the
destination Dd
4.3. Indirection Notification
Considering the packet flow seen in {previous section}. When H1
(Intermediate Node) receives a packet from the ingress node S1 and
forwards it to the Next Node S2, it technically re-injects the packet
back into the DMVPN.
At this point H1 SHOULD an Indirection Notification message to S1.
The Indirection Notification is a dedicated NHRP message indicating
the ingress node that it sent an IP packet that had to be forwarded
via the intermediate node to another node. The Indirection
Notification MUST contain the first 64 bytes of the clear text IP
packet that was forwarded to the next node. The exact format of this
message is detailed in the section [PACKET_FORMAT].
The Indirection Notification MUST be sent back to the ingress node
through the same GRE/IPsec tunnel upon which the hair-pinned IP
packet was received and MUST be rate limited.
This message is a hint that a direct tunnel SHOULD be built between
the end-nodes, bypassing intermediate nodes. This tunnel is called a
"Shortcut Tunnel".
Compliant implementations MUST be able to send and accept the
Indirection Notification, however implementations MUST continue to
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accept traffic over the spoke-hub-spoke path during spoke-spoke path
establishment (Shortcut Tunnel).
When a node receives such a notification, it MUST perform the
following:
o parse and accept the message
o extract the source address of the original protected IP packet
from the 64 bytes available
o perform a route lookup on this source address
* If the routing to this source address is also via the DMVPN
network upon which it received the Indirect Notification then
this node is an intermediate node on the tunnel path from the
ingress node (injection point) to the egress node (extraction
point). In this case this intermediate node MUST silently drop
the Indirect Notification that it received. Note that if the
node is an intermediate node, it is likely that it has
generated and sent an Indirect Notification about this same
protected IP packet to its tunnel neighbor on the tunnel path
back towards the ingress node (injection point). This is
correct behavior.
o if the previous step did succeed, extract the destination IP
address (Dd) of the original protected IP packet from the 64 bytes
available.
The ingress node MAY also extract additional information from those
64 bytes such as the protocol type, port numbers etc.
In steady state, Indirection Notifications MUST be accepted and
processed as above from any trusted peer with which the node has a
direct connection.
4.4. Node Discovery via Resolution Request
After processing the information in the Indirection Notify, the
ingress node local policy SHOULD determine whether a shortcut tunnel
needs to be established. Assuming the local policy requests a
shortcut tunnel, the ingress node MUST emit a Resolution Request for
the destination IP address Dd.
More specifically, the NHRP Resolution Request emitted by S1 to
resolve Dd will contain the following fields:
o Fixed Header
* ar$op.version = 1
* ar$op.type = 1
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o Common Header (Mandatory Header)
* Source NBMA Address = PubS1
* Source Protocol Address = TunS1
* Destination Protocol Address = Dd
The resolution request is routed by S1 to H1 over the GRE/IPsec
tunnel. If an intermediate node has a valid (authoritative) NHRP
mapping in its cache, it MAY respond. An intermediate node SHOULD
NOT answer Resolution Requests in any other case.
Note that a Resolution Request can be voluntarily emitted by Security
Gateway and is not strictly limited to a response to the Indirection
Notify message. Such cases and policies are out of the scope of the
document.
The sending of Resolution Requests by a ingress node MUST be rate
limited.
4.5. Resolution Request Forwarding
The Resolution Request can be sent by S1 to an explicit or implicit
next-hop-server. In the explicit scenario, the NHS is defined in the
node configuration. In the implicit case, the node can infer the NHS
to use. Similarly, an intermediate node that cannot answer a
Resolution Request SHOULD forward the Resolution Request to an
implicit or explicit NHS in the same manner unless local policy
forbids resolution forwarding between Spokes. There can be an
undetermined number of intermediate node.
A DMVPN compliant implementation MUST be able to infer the NHS from
its routing table in the following way:
o the address Dd to be resolved is looked up in the routing table
(other parameters can be considered by the ingress node but these
will not be available to intermediate nodes)
o the best route for Dd is selected (longest prefix match)
* if several routes match (same prefix length) only the routes
pointing to a DMVPN Tunnel interface are kept. This SHOULD NOT
occur in practice.
o if the best route found points to a DMVPN Tunnel interface, the
next-hop address MUST be used as NHS
o if the best route found does not point to a DMVPN Tunnel interface
the forwarding of the packet stops and the matching prefix P and
prefix len (Plen) is kept temporarily. Very often, P/Plen == D2/
D2len (this is the case in the diagram used in this document) but
this may not always be true depending on the structure of the
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networks protected by S2. The associated prefix length (Plen) is
also preserved.
If the Resolution Request forwarding stops at the ingress node (at
emission), the Resolution Request process MUST be stopped with an
error for address Dd. If the lookup succeeds, the next-hop's NBMA
address is used as destination address of the GRE encapsulation.
Before forwarding, each intermediate node MUST add a Forward Transit
Extension record to the NHRP Resolution Request.
Any intermediate nodes SHOULD NOT cache any information while
forwarding Resolution Requests. In the case an intermediate node
implementation caches information, it MUST NOT assume that other
intermediate nodes will also cache that information.
Thanks to the forwarding model described in this document and due to
the absence of intermediate caching, Server Cache Synchronization is
not needed and even recommended against. Therefore, a DMVPN
compliant implementation MUST NOT rely on such a synchronization
which would have adverse effects on the scalability of the entire
system.
If the TTL of the request drops to zero or the current node finds
itself on a Forward Transit Extension record then the NHRP Resolution
Request MUST be dropped and an NHRP error message sent to the source.
When the Resolution Request eventually reaches a node where the
route(s) to the destination would take it out through a non-DMVPN
interface, the Resolution Request process MUST be stopped and this
node becomes the egress node. The egress node is typically (by
virtue of network design) the topologically closest node to the
resolved address Dd.
The egress node must then prepare itself for replying with a
Resolution Reply.
4.6. Egress node NHRP cache and Tunnel Creation
When a node declares itself an egress node while attempting to
forward a Resolution Request, it MUST evaluate the need for
establishing a shortcut tunnel according to a user policy. Note that
an implementation is not mandated to support a user policy but then
the implicit policy MUST request the shortcut establishment. If
policies are supported, one of the possible policies MUST be shortcut
establishment.
If a shortcut is required, the egress node MUST perform the following
operations:
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o the source NBMA address (PubS1) is extracted from the NHRP
Resolution Request
o if a GRE/IPsec tunnel already exists between PubS2 and PubS1, this
tunnel is selected (assuming interface TunnelX)
o otherwise, a new GRE shortcut tunnel is created between PubS2 and
PubS1 (assuming interface TunnelX); the GRE tunnel SHOULD be
protected by IPsec and the SA's immediately negotiated by IKE
o an NHRP cache entry is created for TunS1 => PubS1. The entry
SHOULD NOT remain in the cache for more than the specified Hold
Time (from the NHRP Resolution Request). This NHRP cache entry
may be 'refreshed' for another hold time period prior to expiry by
receipt of another matching NHRP Resolution Request or by sending
an NHRP Resolution Request and receiving an NHRP Resolution Reply.
o a route is inserted into the RIB: TunS1/32 => PubS1 on TunnelX
(assuming IPv4)
Regardless how the shortcut tunnel is created a node SHOULD NOT try
to establish more than one tunnel with a remote node. If there are
other tunnels not managed by DMVPN, the tunnel selectors (source,
destination, tunnel key) MUST NOT interfere with the DMVPN shortcut
tunnels.
If a tunnel has to be created and SA's established, a node SHOULD
wait for the tunnel to be in place before proceeding with further
operations. Regardless of how those operations are timed in the
implementation, a node SHOULD avoid dropping data packets during the
cache and SA installation. The order of operations SHOULD ensure
continuous forwarding.
4.7. Resolution Reply format and processing
After the operations described in the previous section are completed,
a Resolution Reply MUST be emitted by the egress node. Instead of
strictly answering with just the host address being looked up, the
Reply will contain the entire prefix (P/Plen) that was found during
the stopped Resolution Request forwarding phase.
The Resolution Reply main fields MUST be populated as follows:
o Fixed Header
* ar$op.version = 1
* ar$op.type = 2
o Common Header (Mandatory Header)
* Source NBMA Address = PubS1
* Source Protocol Address = TunS1
* Destination Protocol Address = Dd
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o CIE-1
* Prefix-len = Plen
* Client NBMA Address = PubS2
* Client Protocol Address = TunS2
The Destination Protocol address remains the address being resolved
(Dd) while the CIE actually contains the remainder of the response
(Plen via NBMA PubS2, Protocol TunS2). The Resolution Reply MUST be
forwarded to the ingress node S1 either through the shortcut tunnel
or via the Hub.
If the address family of the resolved address Dd is IPv6, the
Resolution Reply SHOULD be augmented with a second CIE containing the
egress node's link local address.
If a node decides to block the resolution process, it MAY simply drop
the Resolution Request or avoid sending a Resolution Reply. A node
MAY also send a NACK Resolution Reply.
When the Resolution Reply is received by the ingress node, a new
tunnel TunnelY MUST be created pointing to PubS2 if one does not
already exist (which depends on whether the Resolution Reply was
routed via the Hub(s) or directly on the shortcut tunnel). The
ingress node MUST process the reply in the following way:
o Validate that this Resolution Reply corresponds to a Request
emitted by S1. If not, issue an error and stop processing the
Reply.
o An NHRP Cache entry is created for TunS2 => PubS2
o Two routes are added to the routing table:
* TunS2 => TunnelY
* P/Plen => TunS2 on TunnelY
Though implementations may be entirely different, a compliant
implementation MUST exhibit a functional behavior strictly equivalent
to the one described above. I.e. IP packets MUST eventually be
forwarded according to the above implementation.
DMVPN compliant implementations MUST support providing and receiving
aggregated address resolution information.
4.8. From Hub and Spoke to Dynamic Mesh
At the end of the resolution process, the overlay topology will be as
follows:
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DC1
|
[H1]
| | ]
+-+ +-+ ] GRE/IPsec tunnels over Transport network
| | ]
[S1]===[S2]
| |
D1 D2
Shortcut tunnel established
Where the tunnel depicted with = is a GRE/IPsec shortcut tunnel
created by NHRP. The Routing Table on S1 will now look as follows:
o TunH1 => Tunnel0
o SUM => TunH1 on Tunnel0
o 0.0.0.0/0 => IntPub
o D1 => IntPriv
o TunS2 => TunnelY
o P/Plen => TunS2 on TunnelY
It is easy to see that traffic from D1 to D2 will follow the shortcut
path under the assumption that P == D2 or D2 is a subnet included in
P.
The tunnels between S* and H* are actually tunnels created
automatically to bootstrap the DMVPN. In practice the initial
topology will be a static star (aka Hub and Spoke) topology between
S* and H* that will evolve into a dynamic mesh between the nodes S*.
From the spokes (S*) standpoint, the bootstrap tunnels can be
established with a node H1 statically defined or discovered by DNS.
The problem of finding the initial hubs in a DMVPN is not different
than finding regular hubs in a traditional Hub and Spoke network.
For scalability reasons, it is expected that the NHRP Indirection/
Resolution is the only way by which routes are exchanged between S*
nodes. While this does not fall in the context of this document, it
is worth mentioning that actual implementations SHOULD NOT establish
a routing protocol adjacency directly over the shortcut tunnels.
4.9. Remote Access Clients
The specification in this document allows a node to not protect any
private network. I.e. in a degenerate case, it MUST be possible for
a node S1 to not have a D1 network attached to it. Instead, S1 only
owns a PubS1? and TunS1? address. This would typically the case of a
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remote access client (PC, mobile device,...) that only has a tunnel
address and an NBMA address.
DC1
|
[H1]
| | ]
+-+ +-+ ] GRE/IPsec tunnels over Transport network
| | ]
[S1]===[S2]
|
D2
Remote Access Client
On the diagram above, S1 is actually a simple PC or mobile node that
is not protecting any other network other than its own tunnel
address.
These nodes may fully participate in a DMVPN network, including
building spoke-spoke tunnels as long as they support GRE, NHRP, IPsec
/IKE, and have a way to separate tunneled traffic (virtual
interfaces) and be able to update a local routing table to associate
networks with different next-hops out either their IntTun (data
traffic going over the tunnel) or (IntPub) (tunnel packets themselves
and/or non-tunneled data traffic). They may not need to run a
routing protocol since they can rely on the Configuration Payload
Exchange described in Section 9.2.
4.10. Node mutual authentication
Nodes authenticate each other using the IKE protocol, while they
attempt to establish a tunnel. Because the system is by nature
extremely distributed, it is recommended to use X.509 certificates
for authentication. Internet Public Key Infrastructure is described
in [RFC5280]
The structured names and various fields in the certificate can be
useful for filtering undesired connectivity in large administrative
domains or when two domains are being partially merged. It is indeed
easy for a system administrator to define filters to prevent
connectivity between nodes that are not supposed to communicate
directly (e.g. filtering based on the O or OU fields).
Though nodes may be blocked from building a direct tunnel by the
above means they may or may not be allowed to communicate via a
spoke-hub-spoke path. Allowing or blocking communication via the
spoke-hub-spoke path is outside the scope of this document.
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5. NHRP Extension Format
As described in [RFC2332], an NHRP packet consists of a fixed part, a
mandatory part and an extensions part. The Fixed Part is common to
all NHRP packet types. The Mandatory Part MUST be present, but
varies depending on packet type. The Extensions Part also varies
depending on packet type, and need not be present. This section
describes the packet format of the new messages introduced as well as
extensions to the existing packet types.
5.1. NHRP Traffic Indication
The fixed part of an NHRP Traffic Indication packet picks itself
directly from the standard NHRP fixed part and all fields pick up the
same meaning as in [RFC2332] unless otherwise explicitly stated.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ar$afn | ar$pro.type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ar$pro.snap |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ar$pro.snap | ar$hopcnt | ar$pktsz |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ar$chksum | ar$extoff |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ar$op.version | ar$op.type | ar$shtl | ar$sstl |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Traffic Indication Fixed Header
o ar$op.type With ar$op.version = 1, this is an NHRP packet.
Further, [RFC2332] uses the numbers 1-7 for standard NHRP
messages. When ar$op.type = 8, this indicates a traffic
indication packet.
The mandatory part of the NHRP Traffic Indication packet is slightly
different from the NHRP Resolution/Registration/Purge Request/Reply
packets and bears a much closer resemblance with the mandatory part
of NHRP Error Indication packet. The mandatory part of an NHRP
Traffic Indication has the following format
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src Proto Len | Dst Proto Len | unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Traffic Code | unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source NBMA Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source NBMA Subaddress (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Contents of Data Packet in traffic (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Traffic Indication Mandatory Part
o Src Proto Len: This field holds the length in octets of the Source
Protocol Address.
o Dst Proto Len: This field holds the length in octets of the
Destination Protocol Address.
o Traffic Code: A code indicating the type of traffic indication
message, chosen from the following list
* 0: NHRP Traffic Redirect/Indirection message.This indirection
is an indication,to the receiver, of the possible existence of
a 'better' path in the NBMA network.
o Source NBMA Address: The Source NBMA address field is the address
of the station which generated the traffic indication.
o Source NBMA SubAddress: The Source NBMA subaddress field is the
address of the station generated the traffic indication. If the
field's length as specified in ar$sstl is 0 then no storage is
allocated for this address at all.
o Source Protocol Address: This is the protocol address of the
station which issued the Traffic Indication packet.
o Destination Protocol Address: This is the destination IP address
from the packet which triggered the sending of this Traffic
Indication message.
Note that unlike NHRP Resolution/Registration/Purge messages, Traffic
Indication message doesn't have a request/reply pair nor does it
contain any CIE though it may contain extension records.
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6. Security Considerations
The use of NHRP and its protocol extensions described in this
document do not open a direct security hole. The peers are duly
authenticated with each other by IKE and the traffic is protected by
IPsec. The only risk may come from inside the network itself; this
is not different from static meshes.
Implementers must be diligent in offering all the control and data
plane filtering options that an administrator would need to secure
the communication inside the overlay network.
7. IANA Considerations
The following values are used experimentally:
o The ar$op.type value of 8 representing Traffic Indication
o Traffic Code value of 0 indicating a Traffic Indirection message.
Full standardization would require official IANA numbers to be
assigned.
8. Compliance against ADVPN requirements
This section compares the adequacy of DMVPN to the requirement list
stated in [RFC7018].
8.1. Requirement 1: minimize configuration change
There are three requirements from requirement 1 from [RFC7018] which
reads:
"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.
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."
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The three requirements are (1a) all hub change, (1b) connected hub
change, and (1c) other spokes.
(1a), (1b), and (1c) are met by the ability for tunnels transport
addresses to be dynamically discovered by NHRP and the tunnels
dynamically created and configured by IKE when the authentication
succeeds.
8.2. Requirement 2: IPsec without config change, even with peer address
change
There is one requirement from Requirement 2 of [RFC7018] which reads:
"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."
Each proposal meets this requirement as described below:
This requirement is met, and uses the Summary route from Hub
(Section 4.2), then method of Indirection (Section 4.3) then a
Resolution Request (Section 4.4) and finally a Resolution reply
(Section 4.7) to identify the overlay address to transport address
mapping in a dynamic manner.
8.3. Requirement 3: Tunnel, Routing, and no Additional Configuration
There are two requirements from requirement 3 of [RFC7018] which
reads:
"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."
The two requirements are: (3a) minimal or no configuration impact
incurred by the tunneling protocols between spokes, (3b) minimal
configuration impact incurred by routing protocols operating over the
spoke-to-spoke tunnels.
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Requirement (3a) is met as dynamic tunnels are dynamically created at
the same time as the IKE SA is authenticated.
Requirement (3b) is met as routing protocols do not operate over
spoke-to-spoke tunnels; only NHRP is responsible for exchanging
prefixes between spokes and NHRP is entirely dynamic.
8.4. Requirement 4: Spoke-to-Spoke Optimization
There are two requirements from requirement 4 of [RFC7018] which
reads:
"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."
The two requirements are: (4a) in full-mesh and dynamic full-mesh
topologies, allow direct spoke-to-spoke communication and (4b) in
star topology, disallow direct spoke-to-spoke communication.
Requirement (4a) is met by the Resolution Request/Reply mechanism
described from Section 4.4 to Section 4.7.
Requirement (4b) is met by disabling the NHRP protocol handler from a
tunnel pointing to a remote peer. As NHRP is disabled, NHRP messages
to and from that peer will be dropped and the peer will be unable to
forge a new dynamic endpoint with any other spoke. It is sufficient
to disable NHRP to that spoke at the hub level to impede the
Resolution mechanism causing the spoke-spoke optimization.
Requirement (4b) can be applied globally (for all spokes) or
individually (for selected spokes) the activation or deactivation of
NHRP on a given peer to peer tunnel can be driven by static
configuration or on a per-identity basis. Additionally, peers can
filter NHRP Resolution Requests or Replies if partial meshing is
allowed to specific prefixes only. Additional identity and
certificate filters can be imposed to further restrict which devices
can connect to others. For instance, Certificates Subject Names
fields such as Organization or Organization Unit are frequently used
to that effect.
8.5. Requirement 5: Credentials and Compromise
There are three requirements from requirement 5 of [RFC7018] which
reads:
"ADVPN Peers MUST NOT have a way to get the long-term
authentication credentials for any other ADVPN Peers. The
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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."
The three requirements are: (5a) no way to get the long-term
authentication credentials from any other ADVPN peers, (5b)
compromise of an endpoint does not affect security of communications
with other peers, and (5c) compromise of a gateway does not affect
the security of communications between ADVPN peers not associated
with that gateway.
Requirement (5a) and is met by Section 4.10 which recommends PKI.
Requirement (5b) is met with mutual authentication. If an endpoint
is compromised, its corresponding certificate will be revoked and it
will be impossible for this endpoint to create any new connection to
any new peer.
Requirement (5c), is met by the same mechanism as (5b).
8.6. Requirement 6: Handoff and Roaming
There are two requirements from requirement 6 of [RFC7018] which
reads:
"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)."
The two requirements are (6a) gateways allow for seamless handoff of
sessions when clients roaming (6b) even if they cross policy
boundaries.
Requirement (6a) is met by the fact that tunnels can be established
dynamically but will not be available for traffic until the IPsec SA
is fully available. This is ensured by the fact that NHRP does not
install prefixes into the routing policy until the SA's are fully
negotiated, as described in Section 4.6
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Requirement (6b) is met because DMVPN is agnostic to policy
boundaries or domains.
8.7. Requirement 7: Easy handoff and Migration
The are two requirements from requirement 7 of [RFC7018] which reads:
"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."
The two requiremets are: (7a) Easy handoff os a session to another
gateway to optimize requirements based on policy, and (7b) ability to
migrate from one gateway to another.
Requirement (7a) can be achieved by using IKEv2 Redirect ([RFC5685])
to redirect a peer entirely to another gateway. Specific Indirection
Notification can be used to redirect specific networks or peers.
Requirement (7b) is met because IKEv2 Redirect, Resolution Request
and Indirection Notification can be sent on a voluntary basis by any
device (hub or spoke) which means than a source node or an egress
node can be of any type (hub or spoke). In practice, this is an
unusual mode of operation (seldom desirable) but it is legitimate.
8.8. Requirement 8: NAT
There are three requirements from requirement 8 of [RFC7018] which
reads:
"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."
The three requirements are: (8a) Gateways and endpoints MUST have the
capability to participate in an ADVPN even when they are located
behind NAT boxes. (8b) When the two endpoints are both behind
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separate NAT boxes. (8c) Shortcuts should continue to work seamlessly
when NAT prevents direct spoke-spoke connectivity.
All requirements (8a,8b) are met by the use of NAT Traversal to
detect NAT devices within the network. If a hub is deployed behind a
NAT address, the spokes need to point their tunnel destination
towards the public address of the Hub,as described in Section 9.3
Requirement (8c) is met since NHRP does not install prefixes into the
routing policy until the SA's are fully negotiated, as described in
Section 4.6.
8.9. Requirement 9: Changes reported
There is one requirement from requirement 9 of [RFC7018] which reads:
"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."
Requirement (9) is met by taking advantage of the various MIB's
defined in existing documents such as [RFC2677], [RFC4292], etc.
There is no standard IPsec MIB but various vendors have developed a
proprietary MIB (typically based on draft-ietf-ipsec-flowmon-mib and
draft-ietf-ipsec-mib) that implementations of this specification can
use. Traps can be triggered as tunnel interfaces come up and down
dynamically as defined in [RFC2863] section 3.1.9. Additional
logging message can be triggered at various levels of the
implementation.
8.10. Requirement 10: Federation between organisations
There is one requirement from requirement 10 of [RFC7018] which
reads:
"To support allied and federated environments, endpoints and
gateways from different organizations SHOULD be able to connect to
each other."
Requirement (10) is met is met by the use of PKI ([RFC5280]),
described in Section 4.6. NHRP can resolve networks across multiple
domains as long as those domains are somehow initially connected to
the topology.
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8.11. Requirement 11: Configuration of star, full-mesh, or partial
full-mesh topologies
There is one requirement from requirement 11 of [RFC7018] which
reads:
"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."
Requirement (11) is met by the same principle as Requirement (4b).
8.12. Requirement 12: Scale for Multicast
There is one requirement from requirement 12 of [RFC7018] which
reads:
"The ADVPN solution SHOULD be able to scale for multicast
traffic."
Requirement (12) is met by the use of a full tunneling interface as
described in Section 1. All multicast control protocols such as PIM
([RFC4601]) or IGMP ([RFC4604]) or even MLD ([RFC3810]) will work
seamlessly on the overlay medium (GRE/IPsec tunnels).
8.13. Requirement 13: Monitoring and Reporting
There is one requirement from requirement 13 of [RFC7018] which
reads:
"The ADVPN solution SHOULD allow for easy monitoring, logging, and
reporting of the dynamic changes to help with troubleshooting such
environments."
Requirement (13) is met by the use of multiple existing technologies
(IPsec, IKE, NHRP, GRE, interfaces) which all generate their own
monitoring, logging, and reporting.
8.14. Requirement 14: L3 VPNs
There is one requirement from requirement 14 of [RFC7018] which
reads:
"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."
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Requirement (14) is met by the use of GRE to encapsulate all traffic
which allows for L2 headers to be transported over DMVPN providing
L3VPN functionality. L3VPN labels can be exchanged by running a
routing protocol over the tunnels.
In accordance to Requirements (1) and (2) about minimal
configuration, the tunnel interfaces only need to activate MPLS as a
supported encapsulation format. This activation can be performed
globally for all tunnels or can be performed for individual tunnels
based on the peer identity.
8.15. Requirement 15: QoS
There is one requirement from requirement 15 of [RFC7018] which
reads:
"The ADVPN solution SHOULD allow the enforcement of per-peer QoS
in both the star and full-mesh topologies."
Requirement (15) is met by applying a QoS policy on the point-to-
point (GRE/IPsec) tunnels, allowing the policy to only parse traffic
that is destined to a specific remote peer.
8.16. Requirement 16: Hub Redundancy
There is one requirement from requirement 16 of [RFC7018] which
reads:
"The ADVPN solution SHOULD take care of not letting the hub be a
single point of failure."
Requirement (16) is met by the ability to use multiple Hubs and an
overlay routing protocol as described in Sections 1 and 4.2. This
method allows a routing based resiliency. Additionally, a spoke can
define multiple addresses or a DNS names to be used as a backup hub.
9. Design Considerations
This section contains a number of points that do not augment the
specification explained so far but instead clarify its use.
9.1. Routing Policy and RFC4301 Security
The notion of routing policy is extensively used throughout this
document. This routing policy is a mechanism used to lookup which
peer or node the packet should be sent to. The exact representation
of a Routing Policy is left to the implementer. It may represent but
is not limited to a unique routing table, a manifold of routing
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table, a policy route or any other mechanism that can take a
forwarding decision.
A key conceptual difference between a Routing Policy and a plain SADB
or a routing table is that packets can be routed to a peer based on
complex rules that may be more complex than just the usual
destination prefix of a RIB or the 3- or 5-tuple (source/destination
IP, source/destination port, protocol) of the SADB.
Most systems can take forwarding decisions that are more elaborate
than that. This includes policy-based-routing, application based
forwarding, multi-topology routing, etc. that are used to evaluate
packets before they optionally undergo the basic routing table or
SADB.
A notable example of a Routing Policy is a manifold of Routing Tables
in the context of VPN Instances (see [RFC4026]); these dedicated
tables are called VRF's. In this example, a dedicated VRF that we
will call VRF Red is associated to the overlay network and
exclusively routes protected packets. In effect, the private
interfaces and the tunnel interfaces are considered Red Interfaces
and exclusively make use of VRF Red as a routing table. Packets
entering the system on a Red interface undergo a VRF Red lookup and
can only leave the device on a Red interface (which tunnels are part
of).
Another routing table called VRF Black is associated to the transport
network (or NBMA network) and exclusively routes traffic to and from
Unprotected Network. This means the physical interfaces facing the
transport network are Black interfaces and traffic entering that
interface is driven by the Black VRF routing table. GRE/IPsec
packets entering the Unprotected interface are such packets.
As noted earlier in this document, GRE tunnels request to be IPsec
protected through a crypto socket as explained in [RFC5660]. A
corresponding SPD and SADB will be created by that socket.
Plain GRE packets will be discarded as they were not duly protected
and no SPD covers that traffic flow ([RFC4301], section 5).
IPsec packets will be accepted by the IPsec stack, their SPI looked
up, get validated (hash, anti-replay) and decrypted. The clear text
packet undergoes the SADB check and MUST be a GRE packet. If it is
not a GRE packet of adequate source/destination, the packet is
discarded. In the light of [RFC5660], the packets will be given to
their application without further intermediate lookup; in this case,
the application is the corresponding GRE Tunnel Interface.
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The protected/overlay packet is now in the clear, ready to be
processed by the GRE Input Features. In particular, security
features can be applied on the clear text, overlay packet (access-
filter, Unicast Reverse Path Forwarding, Layer 7 inspection via
Firewall or Intrusion Prevention system,...). Those policies can be
applied on the fly at IKE negotiation time when the remote peer
identity is known. The clear text packet, should it survive the
security policies, will be forwarded to another Red Interface
according to the VRF Red table.
In the egress direction, clear text packets enter a VRF Red
Interface, get forwarded to a Tunnel interface according to VRF Red.
The packet undergoes output features on the output interface (this
may include filters, firewalling etc.) and is encapsulated into GRE.
The GRE encapsulation function passes the packet to IPsec for
protection through the crypto socket. The packet is now an ESP or AH
packet and can be routed out the public interface according to the
VRF Black table.
For compliance with [RFC4301], explicit leaks may be configured
between VRF's to allow specific traffic to bypass IPsec encryption or
other security policies if necessary but by default, the Red and
Black VRF's are absolutely compartmented.
Various operating systems such as Linux do support VRFs but also have
other methods of implementing a routing policy (e.g. iproute2) that
they can use to their advantage to achieve beyond-routing or beyond-
SADB policy enforcement.
9.2. Using Configuration Attributes
As outlined earlier in this document, this specification lets any
administratively authorized control protocol set up the routing
policy of the base topology. This section explains how IKEv2 can
perform that task.
IKEv2 natively features Configuration Attributes exchanged in
Configuration Payloads ([RFC5996], section 3.15). These payloads can
be used to exchange prefix between peers. The exchange looks like
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Spoke1 Hub
HDR, SK {IDi, [CERT,]
[CERTREQ,] [IDr,] AUTH,
CP(CFG_REQUEST), SAi2,
TSi, TSr} -->
<-- HDR, SK {IDr, [CERT,] AUTH,
CP(CFG_REPLY), SAr2,
TSi, TSr}
HDR, SK {CP(CFG_SET)}
-->
<-- HDR, SK {CP(CFG_ACK)}
Config Exchange
In accordance to the previous notation, the config payloads and
attributes in order to set up the routing table depicted in
Section 4.2 looks as follows:
CP(CFG_REQUEST)=
INTERNAL_ADDRESS()
CP(CFG_REPLY)=
INTERNAL_ADDRESS(TunS1)
INTERNAL_NETMASK(255.255.255.255)
INTERNAL_SUBNET(SUM/SUM_MASK)
... (other INTERNAL_SUBNETs if necessary)
CP(CFG_SET)=
INTERNAL_SUBNET(D1/D1_MASK)
... (other INTERNAL_SUBNETs if necessary)
CP(CFG_ACK)
Config Exchange
The information exchange can be achieved by both sides requesting and
responding solely using CFG_REQUEST and CFG_ACK but it has been
expanded to showcase the conformance to the IKEv2 protocols.
Due to limited packet size and issues caused by fragmentation, the
number of prefixes exchanged by CP exchange is expected to be limited
in practice. This mechanism is not meant to transfer a large number
of prefixes. Should the prefix count be high, the authors strongly
recommend the use of a routing protocol instead.
A peer receiving INTERNAL_SUBNET attributes from another peer MUST be
free to ignore or otherwise interpret that INTERNAL_SUBNET in
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accordance to a security policy. This is necessary in accordance to
[RFC4301] PAD and a recommended practice.Interpretation of that
INTERNAL_SUBNET includes plain rejection (ignore), modification of
the received subnet, logging a warning message and/or termination of
the connection.
9.3. NAT Support
IKEv2 supports NAT Traversal natively. Since GRE provides the
tunneling capability, GRE itself can be protected by IPsec Transport
Mode. See [RFC5996], sections 2.23 for NAT support and 2.23.1 in
particular for NAT Traversal in Transport Mode for the protocol
details.
If a hub is deployed behind a NAT address, the spokes need to point
their tunnel destination towards the public address of the Hub,
assuming the hub is reachable via a well known NAT translation
(static mapping or dynamic public address published via DNS for
instance).
10. Acknowldegements
The authors would like to thank Graham Bartlett, Brian Weis, Mark
Comeadow and Mark Jackson from Cisco for their help in publishing and
reviewing this document. We would also like to acknowledge the
historical DMVPN team, in particular Jan Vilhuber and Pratima Sethi.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2332] Luciani, J., Katz, D., Piscitello, D., Cole, B., and N.
Doraswamy, "NBMA Next Hop Resolution Protocol (NHRP)", RFC
2332, April 1998.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
March 2000.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC4026] Andersson, L. and T. Madsen, "Provider Provisioned Virtual
Private Network (VPN) Terminology", RFC 4026, March 2005.
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[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5660] Williams, N., "IPsec Channels: Connection Latching", RFC
5660, October 2009.
[RFC5685] Devarapalli, V. and K. Weniger, "Redirect Mechanism for
the Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
5685, November 2009.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
5996, September 2010.
[RFC7018] Manral, V. and S. Hanna, "Auto-Discovery VPN Problem
Statement and Requirements", RFC 7018, September 2013.
11.2. Informative References
[RFC2677] Greene, M., Cucchiara, J., and J. Luciani, "Definitions of
Managed Objects for the NBMA Next Hop Resolution Protocol
(NHRP)", RFC 2677, August 1999.
[RFC2863] McCloghrie, K. and F. Kastenholz, "The Interfaces Group
MIB", RFC 2863, June 2000.
[RFC4292] Haberman, B., "IP Forwarding Table MIB", RFC 4292, April
2006.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC4604] Holbrook, H., Cain, B., and B. Haberman, "Using Internet
Group Management Protocol Version 3 (IGMPv3) and Multicast
Listener Discovery Protocol Version 2 (MLDv2) for Source-
Specific Multicast", RFC 4604, August 2006.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
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Authors' Addresses
Frederic Detienne
Cisco
De Kleetlaan 7
Diegem 1831
Belgium
Email: fd@cisco.com
Manish Kumar
Cisco
Mail Stop BGL14/G/
SEZ Unit, Cessna Business Park
Varthur Hobli, Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560 103
India
Email: manishkr@cisco.com
Mike Sullenberger
Cisco
Mail Stop SJCK/3/1
225 W. Tasman Drive
San Jose, California 95134
United States
Email: mls@cisco.com
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