Network Working Group J. Touch

Network Working Group J. Touch
Request for Comments: 3884 ISI
Category: Informational L. Eggert
NEC
Y. Wang
ISI
September 2004

Use of IPsec Transport Mode for Dynamic Routing

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2004).

IESG Note

This document is not a candidate for any level of Internet Standard.
The IETF disclaims any knowledge of the fitness of this document for
any purpose, and in particular notes that it has not had IETF review
for such things as security, congestion control or inappropriate
interaction with deployed protocols. The RFC Editor has chosen to
publish this document at its discretion. Readers of this document
should exercise caution in evaluating its value for implementation
and deployment.

Abstract

IPsec can secure the links of a multihop network to protect
communication between trusted components, e.g., for a secure virtual
network (VN), overlay, or virtual private network (VPN). Virtual
links established by IPsec tunnel mode can conflict with routing and
forwarding inside VNs because IP routing depends on references to
interfaces and next-hop IP addresses. The IPsec tunnel mode
specification is ambiguous on this issue, so even compliant
implementations cannot be trusted to avoid conflicts. An alternative
to tunnel mode uses non-IPsec IPIP encapsulation together with IPsec
transport mode, which we call IIPtran. IPIP encapsulation occurs as
a separate initial step, as the result of a forwarding lookup of the
VN packet. IPsec transport mode processes the resulting (tunneled) IP
packet with an SA determined through a security association database
(SAD) match on the tunnel header. IIPtran supports dynamic routing

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inside the VN without changes to the current IPsec architecture.
IIPtran demonstrates how to configure any compliant IPsec
implementation to avoid the aforementioned conflicts. IIPtran is
also compared to several alternative mechanisms for VN routing and
their respective impact on IPsec, routing, policy enforcement, and
interactions with the Internet Key Exchange (IKE).

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Document History . . . . . . . . . . . . . . . . . . . . 3
2. Problem Description. . . . . . . . . . . . . . . . . . . . . . 4
2.1. IPsec Overview . . . . . . . . . . . . . . . . . . . . . 5
2.2. Forwarding Example . . . . . . . . . . . . . . . . . . . 6
2.3. Problem 1: Forwarding Issues . . . . . . . . . . . . . . 7
2.4. Problem 2: Source Address Selection . . . . . . . . . . 8
3. IIPtran: IPIP Tunnel Devices + IPsec Transport Mode . . . . . 9
3.1. IIPtran Details . . . . . . . . . . . . . . . . . . . . 10
3.2. Solving Problem 1: Forwarding Issues . . . . . . . . . . 11
3.3. Solving Problem 2: Source Address Selection . . . . . . 12
4. Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1. Other Proposed Solutions . . . . . . . . . . . . . . . . 12
4.1.1. Alternative 1: IPsec with Interface SAs. . . . . 13
4.1.2. Alternative 2: IPsec with Initial
Forwarding Lookup. . . . . . . . . . . . . . . . 13
4.1.3. Alternative 3: IPsec with Integrated
Forwarding . . . . . . . . . . . . . . . . . . . 14
4.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . 14
4.2.1. VN Routing Support and Complexity . . . . . . . 14
4.2.2. Impact on the IPsec Architecture . . . . . . . . 15
4.2.3. Policy Enforcement and Selectors . . . . . . . . 16
4.2.4. IKE Impact . . . . . . . . . . . . . . . . . . . 19
5. Security Considerations . . . . . . . . . . . . . . . . . . . 19
6. Summary and Recommendations . . . . . . . . . . . . . . . . . 20
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.1. Normative References . . . . . . . . . . . . . . . . . . 20
8.2. Informative References . . . . . . . . . . . . . . . . . 21
A. Encapsulation/Decapsulation Issues . . . . . . . . . . . . . . 22
A.1. Encapsulation Issues . . . . . . . . . . . . . . . . . . 22
A.2. Decapsulation Issues . . . . . . . . . . . . . . . . . . 23
A.3. Appendix Summary . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 24
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 25

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1. Introduction

The IP security architecture (IPsec) consists of two modes, transport
mode and tunnel mode [1]. Transport mode is allowed between two end
hosts only; tunnel mode is required when at least one of the
endpoints is a "security gateway" (intermediate system that
implements IPsec functionality, e.g., a router.)

IPsec can be used to secure the links of a virtual network (VN),
creating a secure VN. In a secure VN, trusted routers inside the
network dynamically forward packets in the clear (internally), and
exchange the packets on secure tunnels, where paths may traverse
multiple tunnels. Contrast this to the conventional 'virtual private
network' (VPN), which often assumes that paths tend to traverse one
secure tunnel to resources in a secure core. A general secure VN
allows this secure core to be distributed, composed of trusted or
privately-managed resources anywhere in the network.

This document addresses the use of IPsec to secure the links of a
multihop, distributed VN. It describes how virtual links established
by IPsec tunnel mode can conflict with routing and forwarding inside
the VN, due to the IP routing dependence on references to interfaces
and next-hop IP addresses.

This document proposes a solution called IIPtran that separates the
step of IP tunnel encapsulation from IPsec processing. The solution
combines a subset of the current IPsec architecture with other
Internet standards to arrive at an interoperable equivalent that is
both simpler and has a modular specification.

Later sections of this document compare IIPtran to other proposals
for dynamic routing inside VPNs, focusing on the impact the different
proposals have on the overall IPsec architecture, routing protocols,
security policy enforcement, and the Internet Key Exchange (IKE)
[9][10]. An appendix addresses IP tunnel processing issues in IPsec
related to IPIP encapsulation and decapsulation.

This document assumes familiarity with other Internet standards
[1][2], notably with terminology and numerous acronyms therein.

1.2. Document History

This document was first issued as an Internet Draft on March 10,
2000, entitled "Use of IPSEC Transport Mode for Virtual Networks,"
and was first presented in the IPsec WG at the 47th IETF in Adelaide
in March 2000. It was subsequently revised and presented to the
PPVPN WG at the 51st IETF in London in August 2001, to the IPsec WG
at the 52nd IETF in Salt Lake City in December 2001, and to both the

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IPsec and PPVPN WGs at the 53rd IETF in Minneapolis in March 2002.
Version 04 of this draft was submitted for publication as an
Informational RFC based on suggestions by the IPsec WG in June 2002,
and was under IESG review from then until version 07 was approved for
publication in June 2004. During that time, it was substantively
revised according to feedback from the IESG regarding interactions
with the IPsec specification (RFC 2401 [1]) and other protocols, with
regard to security and compatibility issues.

2. Problem Description

Virtual networks connect subsets of resources of an underlying base
network, and present the result as a virtual network layer to upper-
layer protocols. Similar to a real network, virtual networks consist
of virtual hosts (packet sources and sinks) and virtual routers
(packet transits), both of which can have a number of network
interfaces, and links, which connect multiple network interfaces
together. Virtual links (also called tunnels, especially when
point-to-point) are one-hop links in the VN topology, but are either
direct links or paths (sequences of connected links) in the
underlying base network.

Base network hosts and routers can be part of multiple virtual
networks at the same time, and their role in the base network does
not need to coincide with their role in a virtual network (i.e., base
network hosts may act as VN routers or hosts, as may base network
routers).

It is important to note that this definition of a VN is more general
than some other definitions, where the VN participation of end
systems is limited. Some proposals only allow end systems to be part
of a single VN, or even only allow them to be part of the VN and not
the base network, substituting the VN for the Internet. The
definition above explicitly allows hosts and routers to participate
in multiple, parallel VNs, and allows layered VNs (VN inside VN).

It can be useful for a VN to secure its virtual links [3][4],
resulting in a VPN. This is not equivalent to end-to-end security,
but can be useful when end hosts do not support secure communication
themselves. It can provide an additional level of hop-by-hop network
security to secure routing in the VPN and isolate the traffic of
different VPNs.

The topology of an IPsec VPN commonly consists of IPsec tunnel mode
virtual links, as required by the IPsec architecture when the
communicating peers are gateway pairs, or a host and a gateway [1].
However, this current required use of IPsec tunnel mode can be
incompatible with dynamic routing [3].

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The next section provides a short overview on IPsec transport and
tunnel mode processing, as far as it is relevant for the
understanding of the problem scenarios that follow. The following
sections discuss routing problems in detail, based on a common
example.

2.1. IPsec Overview

There are two modes of IPsec, transport mode and tunnel mode [1].
Transport mode secures portions of the existing IP header and the
payload data of the packet, and inserts an IPsec header between the
IP header and the payload; tunnel mode adds an additional IP header
before performing similar operations. This section gives a short
overview of the relevant processing steps for both modes.

In transport mode, IPsec inserts a security protocol header into
outgoing IP packets between the original IP header and the packet
payload (Figure 1) [5][6][11][12]. The contents of the IPsec header
are based on the result of a "security association" (SA) lookup that
uses the contents of the original packet header (Figure 1, arrow) as
well as its payload (especially transport layer headers) to locate an
SA in the security association database (SAD).

Original Outbound Packet Outbound Packet (IPsec Transport Mode)
+-----------+---------+ +-----------+==============+---------+
| IP Header | Payload | | IP Header | IPsec Header | Payload |
+-----------+---------+ +-----------+==============+---------+
| ^
| |
+-------------+
SA Lookup

Figure 1: Outbound Packet Construction under IPsec Transport Mode

When receiving packets secured with IPsec transport mode, a similar
SA lookup occurs based on the IP and IPsec headers, followed by a
verification step after IPsec processing that checks the contents of
the packet and its payload against the respective SA. The
verification step is similar to firewall processing.

When using tunnel mode, IPsec prepends an IPsec header and an
additional IP header to the outgoing IP packet (Figure 2). In
essence, the original packet becomes the payload of another IP
packet, which IPsec then secures. This has been described [1] as "a
tunnel mode SA is essentially a [transport mode] SA applied to an IP
tunnel." However, there are significant differences between the two,
as described in the remainder of this section.

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In IPsec tunnel mode, the IP header of the original outbound packet
together with its payload (especially transport headers) determines
the IPsec SA, as for transport mode. However, a tunnel mode SA also
contains encapsulation information, including the source and
destination IP addresses for the outer tunnel IP header, which is
also based on the original outbound packet header and its payload
(Figure 2, arrows).

Outbound Packet (IPsec Tunnel Mode)
+==================+==============+-----------------+---------+
| Tunnel IP Header | IPsec Header | Orig. IP Header | Payload |
+==================+==============+-----------------+---------+
^ ^ | |
| | | |
| +--------------+ |
| SA Lookup |
| |
+---------------------------------+
IP Encapsulation

Figure 2: Outbound Packet Construction under IPsec Tunnel Mode

When receiving packets secured with tunnel mode IPsec, an SA lookup
occurs based on the contents of the IPsec header and the outer IP
header. Next, the packet is decrypted or authenticated based on its
IPsec header and the SA, followed by a verification step that checks
the contents of the original packet and its payload (especially the
inner IP header and transport headers) against the respective SA.

2.2. Forwarding Example

Consider a VPN topology with virtual links established by IPsec
tunnel mode SAs, as would be required for compliance with [1]. Such
hop-by-hop security can be useful, for example, to secure VN routing,
and when legacy end systems do not support end-to-end IPsec
themselves.

Virtual routers in a VN need to forward packets the same way regular
Internet routers do: based on the destination IP address and the
forwarding table. These two determine the next hop IP address the
packet should be forwarded to (additional header fields and inner
headers can be used, e.g., in policy routing.)

In Figure 3, traffic arrives at gateway A on virtual link 1, having
come from any of the virtual hosts upstream of that virtual link.
There are two outgoing virtual links for this incoming traffic: out
link 3 going to the VPN next-hop gateway B, and out link 4 going to
the VPN next-hop gateway C.

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For this example, assume the incoming traffic is from a single VPN
source X, going to a single VPN destination Y. Ellipses (...)
represent multiple virtual links in Figure 3.

B ---...---
/ \
/ 3 \
/ \
X ---...--- A D ---...--- Y
1 2 \ /
\ 4 /
\ /
C ---...---

Figure 3: Topology of a Virtual Network

Two problems arise; one is forwarding of VN traffic over IPsec tunnel
mode links, the other is source address selection on VN end systems.

2.3. Problem 1: Forwarding Issues

Assume a packet from source X to destination Y arrives on link 2 at
gateway A. Gateway A now needs to both forward and encrypt the packet
to make progress to the next hop gateway inside the VPN.

Dynamically routed gateways forward packets based on a forwarding
table managed by a routing daemon that exchanges connectivity
information with directly connected peers by communicating on its
local interfaces. Entries in the forwarding table map destination IP
addresses to the IP address of a next-hop gateway and an associated
outbound interface.

The problem is that an intermediate router needs to pick a next hop
gateway for a transit packet based on its destination IP address and
the contents of the forwarding table. However, the IPsec
architecture does not define if and how tunnel mode SAs are
represented in the forwarding table.

The problem occurs when A tries to decide how to forward the packet
X->Y. In a regular IP network, this decision depends on a forwarding
lookup on destination address Y, which indicates the IP address of
the next-hop gateway and an associated outbound interface. In the
case of a VN, forwarding lookups occur on virtual destination
addresses. For the forwarding lookup on such a virtual destination
address to succeed, routes through virtual interfaces (tunnels) must
exist in the forwarding table.

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There are two common implementation scenarios for tunnel mode SAs:
One is based on firewall-like packet matching operations where tunnel
mode SAs are not virtual interfaces, another is tunnel-based, and
treats a tunnel mode SA as a virtual interface. The current IPsec
architecture does not mandate one or the other.

Under the first approach, the presence of IPsec tunnel mode SAs is
invisible to the IP forwarding mechanism. The lookup uses matching
rules in the SA lookup process, closer to firewall matching than
traditional IP forwarding lookups, and independent from existing IP
forwarding tables. The SA lookup determines which virtual link the
packet will be forwarded over, because the tunnel mode SA includes
encapsulation information. This lookup and the subsequent tunnel
mode processing both ignore the contents of the existing IP
forwarding table, whether static or dynamic routing are used. This
type of tunnel mode processing is thus incompatible with dynamically
routed VPNs.

The second approach - requiring tunnel mode SAs to be interfaces -
can be compatible with dynamically routed VPNs (see Section 4)
depending on how it is implemented; however, IIPtran (see Section 3)
has the additional benefit of greatly simplifying the IPsec
architecture and related specifications, and of being compatible with
all IPsec specification compliant implementations.

2.4. Problem 2: Source Address Selection

A second issue is source address selection at the source host. When
an application sends traffic to another host, the host must choose an
IP source address for the IP packets before transmission.

When an end system is connected to multiple networks, it must set the
source address properly to receive return traffic over the correct
network. When a node participates in a virtual network, it is always
connected to two networks, the base network and the VN (more if it
connects to at least two VNs.) The IPsec specification currently does
not define how tunnel mode SAs integrate with source address
selection.

For example, when communication occurs over a virtual network, the
source address must lie inside the VN. When X sends to Y (Figure 3),
the source address must be the IP address of X's local end of tunnel
1. If host A, which has multiple interfaces inside the VN, sends to
Y, the source address must be the IP address of the local end of
either tunnel 3 or 4.

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Most applications do not bind to a specific source IP address, and
instead let the host pick one for their traffic [7]. Rules for
source address selection that depend heavily on the notions of
interfaces and routes.

According to [7], the IP source address of an outbound packet should:
(1) for directly connected networks derive from the corresponding
interface, or (2) derive from existing dynamic or static route
entries to the destination, or finally (3) derive from the interface
attached to a default gateway.

Because IPsec tunnel mode SAs are not required to be interfaces,
rules (1) and (2) may not return a usable source address for a given
packet. Consequently, VN packets will use the IP address of the
local interface connecting to a default gateway as their source
address. Often, a default gateway for a host provides connectivity
in the base network underlying the VN. The outgoing packet will thus
have a source address in the base network, and a destination address
in the VN.

This can result in numerous problems, including applications that
fail to operate at all, firewalls and admission control failures, and
may even lead to compromised security. Consider two cases, one with
IPsec tunnels configured with no wildcard tunnel addresses, the other
using certain wildcards. In both cases, an application whose source
address is set by RFC 1122 [7] rules may send packets (e.g.) with the
source address of that host's base network (via the default route)
and a destination address of the remote tunnel endpoint.

3. IIPtran: IPIP Tunnel Devices + IPsec Transport Mode

This section introduces a solution - called IIPtran - for the two
issues identified above. IIPtran replaces IPsec tunnel mode with a
combination of IPIP tunnel interfaces that support forwarding and
source address selection (as per RFC 2003 [2]), followed by IPsec
transport mode on the encapsulated packet.

The IPsec architecture [1] defines the appropriate use of IPsec
transport mode and IPsec tunnel mode (host-to-host communication for
the former, and all transit communication for the latter). IIPtran
appears to violate this requirement, because it uses IPsec transport
mode for transit communication.

However, for an IPIP tunnel between security gateways, the gateways
themselves source or sink base network traffic when tunneling - they
act as hosts in the base network. Thus, IPsec transport mode is also
appropriate, if not required, for encapsulated traffic, according to
[1].

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As a result, replacing IPsec tunnel mode with IPIP tunnel devices and
IPsec transport mode is consistent with the existing architecture.
Furthermore, this does not compromise the end-to-end use of IPsec,
either inside a VPN or in the base network; it only adds IPsec
protection to secure virtual links.

The next sections will give a short overview of IPIP encapsulation,
and show it combines with IPsec transport mode processing. This
section will then discuss how IIPtran addresses each of the problems
identified above.

3.1. IIPtran Details

IIPtran uses IPIP tunnels (as defined in RFC 2003 [2]), followed by
IPsec transport mode on the encapsulated packet.

RFC 2003 [2] uniquely specifies IPIP encapsulation (placing an IP
packet as payload inside another IP packet.) Originally developed for
MobileIP, it has often been adopted when virtual topologies were
required. Examples include virtual (overlay) networks to support
emerging protocols such as IP Multicast, IPv6, and Mobile IP itself,
as well as systems that provide private networks over the Internet
(X-Bone [3] and PPVPN).

IPIP outbound packet processing, as specified by RFC 2003 [2],
tunnels an existing IP packet by prepending it with another IP header
(Figure 4.)

Outbound Packet (IPIP Tunnel)
+==================+-----------------+---------+
| Tunnel IP Header | Orig. IP Header | Payload |
+==================+-----------------+---------+
^ |
| |
+------------------+
IPIP Encapsulation

Figure 4: Outbound Packet Construction for IPIP Tunnel

IIPtran performs this IPIP processing as a first step, followed by
IPsec transport mode processing on the resulting IPIP packet (Figure
5.)

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Outbound Packet (IPIP Tunnel + IPsec Transport Mode)
+==================+==============+-----------------+---------+
| Tunnel IP Header | IPsec Header | Orig. IP Header | Payload |
+==================+==============+-----------------+---------+
^ | ^ |
| | | |
| +---------------+ |
| SA Lookup |
| |
+----------------------------------+
IPIP Encapsulation

Figure 5: Outbound Packet Construction for IPIP Tunnel with IPsec
Transport Mode

A key difference between Figure 2 and Figure 5 is that in the
proposed solution, the IPsec header is based on the outer IP header,
whereas under IPsec tunnel mode processing, the IPsec header depends
on the contents of the inner IP header and payload (see Section 2.1).

However, the resulting VPN packet (Figure 5) on the wire cannot be
distinguished from a VPN packet generated by IPsec tunnel mode
processing (Figure 2); and the two methods inter-operate, given
appropriate configurations on both ends [3].

A detailed discussion of the differences between IIPtran, IPsec
tunnel mode, and other proposed mechanisms follows in Section 4. The
remainder of this section will describe how IIPtran combines IPIP
tunnel devices with IPsec transport mode to solve the problems
identified in Section 2.

3.2. Solving Problem 1: Forwarding Issues

Section 2.3 described how IP forwarding over IPsec tunnel mode SAs
breaks, because tunnel mode SAs are not required to be network
interfaces. IIPtran uses RFC 2003 IPIP tunnels [2] to establish the
topology of the virtual network. RFC 2003 [2] requires that IPIP
tunnels can be routed to, and have configurable addresses. Thus,
they can be references in node's routing table (supporting static
routing), as well as used by dynamic routing daemons for local
communication of reachability information.

RFC 2003 [2] addressed the issue of inserting an IPsec header between
the two IP headers that are a result of IPIP encapsulation. IIPtran
provides further details on this configuration, and demonstrates how
it enables dynamic routing in a virtual network.

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It is important to note that the RFC 2003 IPIP tunnels [2] already
provide a complete virtual network that can support static or dynamic
routing. The proposed solution of using IPIP tunnel with IPsec
transport mode decouples IPsec processing from routing and
forwarding. IIPtran's use of IPsec is limited to securing the links
of the VN (creating a VPN), because IPsec (rightly) lacks internal
support for routing and forwarding.

3.3. Solving Problem 2: Source Address Selection

Section 2.4 gave an overview of IP source address selection and its
dependence on interfaces and routes.

Using RFC 2003 IPIP tunnel devices [2] for VN links, instead of IPsec
tunnel mode SAs, allows existing multihoming solutions for source
address selection [1] to solve source address selection in this
context as well. As indicated in Section 2.4, according to [1], the
IP source address of an outbound packet is determined by the outbound
interface, which is in turn determined by existing forwarding
mechanism. Because IPIP tunnels are full-fledged interfaces with
associated routes (as in Section 3.2 of [2]), the routes and address
selection as specified in [1] can also operate as desired in the
context of VN links.

4. Comparison

The previous sections described problems when IPsec tunnel mode
provides VPN links, and proposed a solution. This section introduces
a number of proposed alternatives, and compares their effect on the
IPsec architecture, routing, and policy enforcement, among others, to
IIPtran.

4.1. Other Proposed Solutions

This section gives a brief overview of a number of alternative
proposals that aim at establishing support for dynamic routing for
IPsec-secured VNs. The following section then compares these
proposals in detail.

Although some of the alternatives also address the issues identified
above, IIPtran alone also significantly simplifies and modularizes
the IPsec architecture.

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4.1.1. Alternative 1: IPsec with Interface SAs

In the first alternative, each IPsec tunnel mode SA is required to
act as a full-fledged network interface. This SA interface acts as
the outbound interface of the virtual destination's forwarding table
entry. IPsec dynamically updates the SA interface configuration in
response to SAD changes, e.g., caused by IKE negotiation.

This approach supports dynamic routing and existing source address
selection rules, but requires extensions to the IPsec architecture
that define tunnel mode SA interfaces and their associated management
procedures.

It would necessitate recapitulating the definition of the entirety of
RFC 2003 IPIP encapsulation [2], including the association of tunnels
with interfaces, inside IPsec. This defeats the modular architecture
of the Internet, and violates the specification of type 4 IP in IP
packets as being uniquely defined by a single Internet standard (it
is already standardized by [2]).

This solution also requires augmenting the IPsec specification to
mandate an implementation detail, one that may be difficult to
resolve with other IPsec designs, notably the BITS (bump-in-the-
stack) alternative. Although the current IPsec specification is
ambiguous and allows this implementation, an implementation-
independent design is preferable.

4.1.2. Alternative 2: IPsec with Initial Forwarding Lookup

A second alternative is the addition of an extra forwarding lookup
before IPsec tunnel mode processing. This forwarding lookup will
return a "virtual interface" identifier, which indicates how to route
the packet [13]. Due to a lack of concrete documentation of this
alternative at this time, proposed for an update pending to RFC 2401
[1], two variants are presumed possible:

In the first scenario, the extra forwarding lookup indicates the
outbound interface of the final encapsulated tunnel mode packet,
i.e., usually a physical interface in the base network. The tunnel
mode SA lookup following the forwarding lookup will occur in the
per-interface SAD associated with the respective virtual interface.

In the second scenario, the extra forwarding lookup returns an
outbound tunnel SA interface. This solution seems to be equivalent
to the one described above (Section 4.1.1), i.e., all tunnel mode SAs
must be interfaces, and is not discussed separately below.

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4.1.3. Alternative 3: IPsec with Integrated Forwarding

In the third alternative, the routing protocols and forwarding
mechanisms are modified to consult both the routing tables and SADs
to make forwarding decision. To prevent IPsec processing from
interfering with routing, forwarding table lookup must precede SAD
lookup.

This approach supports dynamic routing, but requires changes to
routing mechanisms such that SAD contents are included in the route
exchanges. It is unclear how transport-layer selectors would affect
this approach.

4.2. Discussion

This section compares the three different alternatives and IIPtran
according to a number of evaluation criteria, such as support for VN
forwarding, or impact on the IPsec architecture.

4.2.1. VN Routing Support and Complexity

This section investigates whether the three alternatives and IIPtran
support VN routing, especially dynamic routing based on existing IP
routing protocols.

Both IIPtran (IPIP tunnels + transport mode) and alternative 1 (per-
SA interfaces) establish VN links as full-fledged devices that can be
referred to in the routing table, as well as used for local
communication by dynamic routing protocols. They both support static
and dynamic VN routing.

However, because the current IPsec architecture does not require
tunnel mode SAs to behave similarly to interfaces (some implementers
chose alternative 1, but it is not mandated by the specification),
alternative 1 requires extensions to the current IPsec architecture
that define the exact behavior of tunnel mode SAs. The proposed
solution does not require any such changes to IPsec, and for tunnels
RFC 2003 already specifies those requirements [2]. Furthermore,
addition of those requirements would be redundant and potentially
conflict with RFC 2003 [2].

Alternative 3 supports dynamic VN routing, but requires modifying
routing protocols and forwarding lookup mechanisms to act or
synchronize based on SAD entries. This requires substantial changes
to routing software and forwarding mechanisms in all participating
nodes to interface to the internals of IPsec; this would require
revising a large number of current Internet standards. It is also

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not clear how tunnel mode SAs that specify port selectors would
operate under this scheme, since IP routing has no dependence on
transport-layer fields.

Alternative 2 does not support dynamic VN routing. The additional
forwarding lookup before IPsec processing is irrelevant, because
IPsec tunnel mode SAs are not represented as interfaces, and thus
invisible to IP routing protocols.

Additionally, the forwarding lookup suggested for alternative 2 is
not compatible with a weak ES model described in [1], which requires
both an outbound interface indicator as well as the IP address of the
next-hop gateway. For example, multiple tunnels can use the same
outgoing interface and thus same SAD. The forwarding lookup would
return only the interface; lacking the next-hop gateway, the correct
SAD entry cannot be determined. Given the next-hop gateway would not
help, because the SAD is not indexed by tunnel mode SA encapsulation
destination IP address.

Because alternative 2 fails to support VN routing, it will not be
discussed in the remainder of this section.

4.2.2. Impact on the IPsec Architecture

IIPtran recognizes that encapsulation is already a property of
interface processing, and thus relies on IPIP tunnel devices to
handle the IPIP encapsulation for VN links. Tunnel mode IPsec thus
becomes unnecessary and can potentially be removed from the IPsec
architecture, greatly simplifying the specification.

Alternative 1 requires SAs to be represented as full-fledged
interfaces, for the purpose of routing. SAD changes must furthermore
dynamically update the configuration of these SA interfaces. The
IPsec architecture thus needs extensions that define the operation of
interfaces and their interactions with the forwarding table and
routes.

Additionally, RFC 2401 [1] describes per-interface SADs as a
component of IPsec. When tunnel mode SAs themselves act as
interfaces, the function of per-interface SADs needs clarification as
follows:

First, each tunnel interface SAD must contain exactly one IPsec
tunnel mode SA. Transport mode SAs are prohibited, because they
would not result in IP encapsulation (the encapsulation header is
part of the tunnel mode SA, a transport mode SA would not cause
encapsulation), and thus lead to processing loops. Multiple tunnel
mode SAs are prohibited, because dynamic routing algorithms construct

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topology information based on per-interface communication. Merging
different virtual links (tunnels) into a single SA interface can
cause routing events on one virtual link to apply incorrectly to
other links sharing an SA interface.

Second, only the SAD of physical interfaces may contain IPsec
transport mode SAs; otherwise, the current issues with VN routing
remain unsolved.

In summary, these restrictions cause the SADs of SA interfaces to
contain only tunnel mode SAs, and the SADs of regular interfaces to
contain only transport mode SAs. Thus, tunnel encapsulation
essentially becomes a unique property of the interface, and not
IPsec.

IIPtran already recognizes this property. Consequently, it uses IPIP
tunnels directly, and combines them with transport mode processing.
By eliminating the use of tunnel mode, it removes the need for
additional constraints on the contents of per-interface SAs.

4.2.3. Policy Enforcement and Selectors

On receiving a packet, both IPsec tunnel mode and IIPtran decrypt
and/or authenticate the packet with the same techniques. IPsec
tunnel mode decapsulates and decrypts the packet in a single step,
followed by a policy check of the inner packet and its payload
against the respective IPsec tunnel mode SA. IIPtran uses IPsec
transport mode to decrypt and verify the incoming packet, then passes
the decrypted IPIP packet on to RFC 2003 IPIP processing [2]. At
that point, IIPtran can support selector checks on both the header
and its payload using firewall mechanisms, similar to IPsec tunnel
mode processing.

The primary difference between the two is that IPsec tunnel mode does
not require a separate processing step for validating packets; once
IPsec accepts them during the policy check during decapsulation, they
are accepted. IIPtran requires additional processing on the
decapsulated packets, to validate whether they conform to their
respective IPsec policy.

As noted in Section 5.2 of the IPsec architecture document [1], IPsec
processing should retain information about what SAs matched a given
packet, for subsequent IPsec or firewall processing. To allow for
complex accept policies, it should be possible to reconstruct the
format of the original packet at the time it first entered a machine
based on saved processing context at any time during inbound

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processing. IIPtran accepts incoming VN packets only if they have
arrived over a specific IPIP tunnel that was secured with IPsec
transport mode, but as a separate step following IPIP decapsulation.

Note that IPsec tunnel mode and IIPtran are interoperable [3].
Experiments have verified this interoperability, notably because
there are no differences in the resulting packets on the wire, given
appropriate keys.

4.2.3.1. Selector Expressiveness

When looking up an SA for a given packet, IPsec allows selectors to
match on the contents of the IP header and transport headers.
IIPtran using existing IPsec cannot support transport header matches,
because SA lookup occurs before decapsulation. A small extension to
IPsec can address this issue in a modular way.

RFC 2401 [1] explicitly recognizes that the transport layer header
may be nested several headers deep inside the packet, and allows a
system to (quote) "chain through the packet headers checking the
'Protocol' or 'Next Header' field until it encounters either one it
recognizes as a transport protocol, or until it reaches one that
isn't on its list of extension headers, or until it encounters an ESP
header that renders the transport protocol opaque."

With IIPtran, the SA lookup starts on the outer (tunnel) header, and
selectors including port number information must thus traverse the
inner IP header (and possibly other headers) before they can match on
the transport headers. IIPtran thus requires that IP be a known
IPsec "extension header." This recognizes that with IPIP
encapsulation, IP VNs use the base IP network as a link layer.
Although this small extension to IPsec is not explicitly required, it
is already implied.

Recognizing IP as a valid transport layer over IP also allows
selectors to match on the contents of the inner ("transport") IP
header. Thus, IPsec selectors under IIPtran can express the same set
of policies as conventional IPsec tunnel mode.

Note that in both cases, these policy enforcement rules violate
layering by looking at information other than the outermost header.
This is consistent with IPsec's current use of port-based selectors.
The next section discusses that selectors may not be useful for
virtual networks.

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4.2.3.2. Role of Selectors for VPNs

For secure VN links established via IPsec tunnel mode SAs, the
selectors for the inner (VN) source and destination IP addresses
often need to be wildcarded to support dynamic routing in a VN.
Thus, the limitation described in 4.2.3.1 (without the proposed
extension) may not be important in a VN scenario.

Consider a four-node VN with nodes A, B, C, and N (Figure 6).
Consider the case where N is either a new node joining an existing
VPN, or an existing node that had been disconnected and was just
rediscovered via dynamic routing.

In this example, A has IPsec tunnel mode SAs to B and C. If the
selectors for the virtual source and destination IP addresses for
those SAs are not wildcards, the SA needs to be dynamically modified
to permit packets from N to pass over the tunnels to B and C. This
becomes quickly impractical as VPN sizes grow.

B
/
/
/
N ------ A
\
\
\
C

Figure 6: Topology of a Virtual Network

Thus, IPsec selectors appear much less useful in a VPN scenario than
expected. A consequence might be that IIPtran - even without
extensions to support the full expressiveness of tunnel mode SA
selectors as described above - can still support the majority of VPN
scenarios.

One purpose of selectors matching on transport header content is
policy routing. Different SAs can apply to different applications,
resulting in different apparent virtual topologies. IIPtran supports
policy routing in a more modular way, by having existing policy
routing implementations forward traffic over multiple, parallel VNs.
IIPtran supports arbitrary IP-based policy routing schemes, while
policies are limited by the expressiveness of IPsec's selectors in
the former case.

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4.2.4. IKE Impact

The Internet Key Exchange (IKE) [9][10] is a protocol to negotiate
IPsec keys between end systems dynamically and securely. It is not a
strictly required component of IPsec in the sense that two hosts can
communicate using IPsec without having used IKE to negotiate keys
(through manually keyed SAs, for example). Despite its name, IKE
also acts as a tunnel management protocol (when IPsec tunnel mode SAs
are configured), and negotiates security policies between the peers.

Alternatives 1 and 3 use existing IKE without changes.

One possible approach to use IKE with IIPtran is to negotiate a
tunnel mode SA, and then treat it as a transport mode SA against an
IPIP tunnel when communicating with conventional peers. For policies
that do not specify selectors based on transport-layer information,
this establishes interoperability.

However, since IIPtran eliminates IPsec tunnel mode, it could also
simplify IKE, by limiting it to its original purpose of key exchange.
A new tunnel management protocol (e.g., ATMP [8]) would set up IPIP
tunnels, use an as of yet unspecified second protocol to negotiate
security policy, and then use IKE to exchange keys for use with the
policy.

Current IKE operation would become a modular composition of separate
protocols, similar to how IIPtran modularizes IPsec by combining
existing Internet standards. For example, a VPN link creation could
follow these steps: (1) IKE negotiation in the base network to secure
(2) a subsequent tunnel management exchange [8] in the base network,
followed by (3) IKE exchanges over the established tunnel to create a
secure VPN link.

5. Security Considerations

This document addresses security considerations throughout, as they
are a primary concern of proposed uses of IPsec.

The primary purpose of this document is to extend the use of IPsec to
dynamically routed VPNs, which will extend the use of IPsec and, it
is hoped, increase the security of VPN infrastructures using existing
protocols.

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6. Summary and Recommendations

This document presents a mechanism consistent with the current use of
IPsec which supports dynamic routing inside a virtual network that
uses IPsec to secure its links. It illustrates how current use of
IPsec tunnel mode can fail to support dynamic VN routing (depending
on the implementation), and compares IIPtran with several different
alternatives. It finds that IIPtran, a composite of a subset of
IPsec (i.e., transport mode) together with existing standard IPIP
encapsulation, results in an interoperable, standards-conforming
equivalent that is both simpler and modular.

7. Acknowledgments

The authors would like to thank the members of the X-Bone and
DynaBone projects at USC/ISI for their contributions to the ideas
behind this document, notably (current) Greg Finn and (past) Amy
Hughes, Steve Hotz and Anindo Banerjea.

The authors would also like to thank Jun-ichiro (itojun) Hagino and
the KAME project for bringing IKE implications of this proposal to
our attention, as well as implementing the mechanisms in this
document in the KAME IPv6/IPsec network stack. Members of several
IETF WGs (especially IPsec: Stephen Kent, PPVPN: Eric Vyncke, Paul
Knight, various members of MobileIP) provided valuable input on the
details of IPsec processing in earlier revisions of this document.

Effort sponsored by the Defense Advanced Research Projects Agency
(DARPA) and Air Force Research Laboratory, Air Force Materiel
Command, USAF, under agreements number F30602-98-1-0200 entitled "X-
Bone" and number F30602-01-2-0529 entitled "DynaBone".

8. References

8.1. Normative References

[1] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.

[2] Perkins, C., "IP Encapsulation within IP", RFC 2003, October
1996.

[3] Touch, J., "Dynamic Internet overlay deployment and management
using the X-Bone", Computer Networks Vol. 36, No. 2-3, July
2001.

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RFC 3884 IPsec Transport Mode for Dynamic Routing September 2004

[4] Touch, J., Wang, Y., Eggert, L. and G. Finn, "A Virtual
Internet Architecture", ISI Technical Report ISI-TR-570,
Workshop on Future Directions in Network Architecture (FDNA)
2003, March 2003.

[5] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.

[6] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.

[7] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.

[8] Hamzeh, K., "Ascend Tunnel Management Protocol - ATMP", RFC
2107, February 1997.

8.2. Informative References

[9] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.

[10] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", Work in
Progress, January 2004.

[11] Kent, S., "IP Authentication Header", Work in Progress,
February 2004.

[12] Kent, S., "IP Encapsulating Security Payload (ESP)", Work in
progress, February 2004.

[13] Kent, S., "Personal Communication", November 2002.

[14] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.

[15] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923,
September 2000.

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Appendix A. Encapsulation/Decapsulation Issues

There are inconsistencies between the IPIP encapsulation rules
specified by IPsec [1] and those specified by MobileIP [2]. The
latter specification is standards track, and the IP protocol number
of 4 (payload of an IP packet of type 4) is uniquely specified by RFC
2003 according to IANA [2]. The use of IPIP inside an IPsec
transport packet can be confused with IPsec tunnel mode, because
IPsec does not specify any limits on the types of IP packets that
transport mode can secure.

A.1. Encapsulation Issues

When an IP packet is encapsulated as payload inside another IP
packet, some of the outer header fields can be newly written (and the
inner header determines some others [2].) Among these fields is the
IP DF (do not fragment) flag. When the inner packet DF flag is
clear, the outer packet may copy it or set it; however, when the
inner DF flag is set, the outer header must copy it [2]. IPsec
defines conflicting rules, where that flag and other similar fields
(TOS, etc.) may be copied, cleared, or set as specified by an SA.

The IPsec specification indicates that such fields must be
controlled, to achieve security. Otherwise, such fields could
provide a covert channel between the inner packet header and outer
packet header. However, RFC 2003 [2] requires that the outer fields
not be cleared when the inner ones are set, to prevent MTU discovery
"black holes" [14][15].

To avoid a conflict between these rules, and to avoid security
weaknesses associated with solely copying the fields, it is
recommended that IPsec IPIP encapsulation not permit the clearing of
the outer DF flag. When the SA requires clearing the DF flag, and
the inner packet DF is set, it is proposed that IPsec drop that
packet, rather than violate RFC 2003 processing rules [2]. Similar
rules are being developed for TOS and other similar IP header fields,
to be included in an update of RFC 2003 [2].

Another approach to closing the covert channel is always to set the
DF flag in the outer header (whether or not it is set in the inner
header). Setting the DF flag allows PMTU discovery to operate
normally. The details of this approach are discussed in [2].

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A.2. Decapsulation Issues

Given identical keys, a packet created by IPIP tunnel encapsulation
combined with IPsec transport mode and an IPsec tunnel mode packet
look identical on the wire. Thus, when an IPsec'ed packet arrives
that contains an IPIP inner packet, it is not possible to distinguish
whether the packet was created using IPsec tunnel mode or IPsec
transport mode of an IPIP encapsulated packet. In both cases, the
protocol field of the outer header is IPsec (AH or ESP), and the
"next header" field for the inner data is 4 (IP). IPsec requires the
SA matching a received packet to indicate whether to apply tunnel
mode or transport mode.

Incoming packet processing must check the SAD before determining
whether to decapsulate IPsec packets with inner payload of protocol
type 4. If the SAD indicates that a tunnel mode association applies,
IPsec must decapsulate the packet. If the SAD indicates that a
transport mode association applies, IPsec must not decapsulate the
packet. This requires that the SAD indicate one of these two
options; wildcard SAD entries ("ANY", or "TUNNEL or TRANSPORT")
cannot be supported.

A.3. Appendix Summary

IPsec's use of IPIP encapsulation conflicts with the IPIP standard
[2]. This issue is already being resolved in an update to RFC 2003,
instead of specifying a non-standard conforming variant of IPIP
encapsulation inside IPsec.

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Authors' Addresses

Joe Touch
USC Information Sciences Institute
4676 Admiralty Way
Marina del Rey, CA 90292
US

Phone: +1 310 822 1511
Fax: +1 310 823 6714
EMail: [email protected]
URI: http://www.isi.edu/touch

Lars Eggert
NEC Network Laboratories
Kurfuersten-Anlage 36
Heidelberg 69115
DE

Phone: +49 6221 90511 43
Fax: +49 6221 90511 55
EMail: [email protected]
URI: http://www.netlab.nec.de/

Yu-Shun Wang
USC Information Sciences Institute
4676 Admiralty Way
Marina del Rey, CA 90292
US

Phone: +1 310 822 1511
Fax: +1 310 823 6714
EMail: [email protected]
URI: http://www.isi.edu/yushunwa

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Full Copyright Statement

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