Network Working Group S. Vaarala (Ed.)
Internet-Draft Netseal
Expires: December 27, 2003 June 28, 2003
Mobile IPv4 Traversal Across IPsec-based VPN Gateways
draft-ietf-mobileip-vpn-problem-solution-02
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on December 27, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document outlines the proposed solution for the Mobile IPv4 and
IPsec coexistence problem for enterprise users. The solution
consists of an applicability statement for using Mobile IPv4 and
IPsec for session mobility in corporate remote access scenarios, and
a required mechanism for detecting the trusted internal network
securely. The solution requires only changes to the mobile node;
changes to Mobile IPv4 or IPsec are not required.
Vaarala (Ed.) Expires December 27, 2003 [Page 1]
Internet-Draft MIPv4-VPN June 2003
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Related work . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Terms and abbreviations . . . . . . . . . . . . . . . . . . 6
1.5 Requirement levels . . . . . . . . . . . . . . . . . . . . . 7
2. Topology . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Access modes . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 Access mode: 'c' . . . . . . . . . . . . . . . . . . . . . . 11
3.2 Access mode: 'f' . . . . . . . . . . . . . . . . . . . . . . 11
3.3 Access mode: 'cvc' . . . . . . . . . . . . . . . . . . . . . 12
3.4 Access mode: 'fvc' . . . . . . . . . . . . . . . . . . . . . 12
3.5 NAT traversal . . . . . . . . . . . . . . . . . . . . . . . 13
4. Internal network detection . . . . . . . . . . . . . . . . . 14
4.1 Assumptions . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2 Implementation requirements . . . . . . . . . . . . . . . . 15
4.2.1 Connection status change . . . . . . . . . . . . . . . . . . 15
4.2.2 Registration-based internal network detection . . . . . . . 16
4.2.3 Registration-based internal network monitoring . . . . . . . 16
4.2.4 Handling of network interfaces . . . . . . . . . . . . . . . 17
4.3 Proposed algorithm . . . . . . . . . . . . . . . . . . . . . 17
4.4 Implementation issues . . . . . . . . . . . . . . . . . . . 18
4.5 Rationale . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.5.1 Firewall configuration requirements . . . . . . . . . . . . 19
4.5.2 Registration-based internal network monitoring . . . . . . . 19
4.6 Improvements . . . . . . . . . . . . . . . . . . . . . . . . 20
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 21
5.1 Mobile node requirements . . . . . . . . . . . . . . . . . . 21
5.2 VPN device requirements . . . . . . . . . . . . . . . . . . 21
5.3 Home agent requirements . . . . . . . . . . . . . . . . . . 21
6. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 22
6.1 Comparison against guidelines . . . . . . . . . . . . . . . 22
6.2 Packet overhead . . . . . . . . . . . . . . . . . . . . . . 23
6.3 Latency considerations . . . . . . . . . . . . . . . . . . . 24
6.4 Firewall state considerations . . . . . . . . . . . . . . . 25
6.5 Intrusion detection systems (IDSs) . . . . . . . . . . . . . 26
6.6 Implementation of mobile node . . . . . . . . . . . . . . . 26
6.7 Non-IPsec VPN protocols . . . . . . . . . . . . . . . . . . 26
6.8 Shortcomings for enterprise use . . . . . . . . . . . . . . 27
7. Security considerations . . . . . . . . . . . . . . . . . . 28
Vaarala (Ed.) Expires December 27, 2003 [Page 2]
Internet-Draft MIPv4-VPN June 2003
7.1 Internal network detection . . . . . . . . . . . . . . . . . 28
7.2 Mobile IPv4 versus IPsec . . . . . . . . . . . . . . . . . . 28
8. Intellectual property rights . . . . . . . . . . . . . . . . 30
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31
References . . . . . . . . . . . . . . . . . . . . . . . . . 32
Author's Address . . . . . . . . . . . . . . . . . . . . . . 33
A. Packet flow examples . . . . . . . . . . . . . . . . . . . . 34
A.1 Connection setup for access mode 'cvc' . . . . . . . . . . . 34
A.2 Connection setup for access mode 'fvc' . . . . . . . . . . . 38
B. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Full Copyright Statement . . . . . . . . . . . . . . . . . . 42
Vaarala (Ed.) Expires December 27, 2003 [Page 3]
Internet-Draft MIPv4-VPN June 2003
1. Introduction
The Mobile IP working group started a design team to explore the
problem and solution spaces of IPsec and Mobile IP coexistence. The
problem statement and solution requirements for Mobile IPv4 case was
first documented in [2]. The design team then set out to evaluate
solution candidates and ultimately arrive at a solution draft.
The current version of this document outlines the proposed solution
for IPv4. The solution places only requirements on the
implementation of the mobile node.
This document contains two parts:
o a basic solution which is an applicability statement of Mobile
IPv4 and IPsec to provide session mobility between internal and
external networks, intended for enterprise mobile users; and
o a technical specification and a set of requirements for secure
detection of the internal and the external networks.
1.1 Overview
Typical corporate networks consist of three different domains: the
Internet (untrusted external network), the intranet (trusted internal
network), and the DMZ, which connects the two networks. Access to
the internal network is guarded both by a firewall and a VPN device;
access is only allowed if both firewall and VPN security policies are
respected.
Enterprise mobile users benefit from unrestrained seamless session
mobility between subnets, regardless of whether the subnets are part
of the internal or the external network. Unfortunately the current
Mobile IPv4 and IPsec standards alone do not provide such a service
[12].
The proposed solution is to use standard Mobile IPv4 when the mobile
node is in the internal network, and to use the inner address of a
VPN tunnel (VPN-TIA) as a co-located care-of address for Mobile IPv4
registration when outside. IPsec-based VPN tunnels require re-
negotiation after movement; thus, some additional mechanism must deal
with mobility when the MN is outside.
The external mobility is provided by another layer of Mobile IPv4
underneath IPsec, in effect making IPsec unaware of movement. Thus,
the mobile node can freely move in the external network without
disrupting the VPN connection. The downside of this approach is that
Vaarala (Ed.) Expires December 27, 2003 [Page 4]
Internet-Draft MIPv4-VPN June 2003
an external home agent is required, and that the packet overhead is
considerable (see Section 6).
Briefly, when outside, the mobile node:
o detects (securely) that it is outside (Section 4);
o registers its co-located or foreign agent care-of address with the
external home agent;
o establishes a VPN tunnel using e.g. IKE (or IKEv2) if security
associations are not already available;
o registers the VPN tunnel inner address (VPN-TIA) as its co-located
care-of address with the internal home agent; this registration
request is sent inside the IPsec tunnel.
The solution requires some control over the protocol layers in the
mobile node. The mobile node must be capable of (1) detecting
whether it is inside or outside in a secure fashion, and (2) control
the protocol layers accordingly. For instance, if the mobile node is
inside, the IPsec layer needs to become dormant.
Current Mobile IPv4 and IPsec standards, when used in a suitable
combination, are sufficient to implement the solution; no changes are
required to existing VPN devices, home agents, or foreign agents.
Although the basic solution has a number of shortcomings, especially
in terms of overhead and complexity, optimizations that require
changes to Mobile IPv4 or IPsec are out of scope of this document.
These will be pursued as separate work items.
1.2 Scope
This document describes a solution for IPv4 only.
VPN, in this document, refers to an IPsec-based remote access VPN.
Other types of VPNs are out of scope.
1.3 Related work
Related work has been done on Mobile IPv6 in [13] which discusses the
interaction of IPsec and Mobile IPv6 in protecting Mobile IPv6
signalling. The draft also discusses dynamic updating of the IPsec
endpoint based on Mobile IP signaling packets.
The "transient pseudo-NAT" attack, described in [14] and [4], affects
any approach which attempts to provide security of mobility
signalling in conjunction with NAT devices. In many cases, one
Vaarala (Ed.) Expires December 27, 2003 [Page 5]
Internet-Draft MIPv4-VPN June 2003
cannot assume any co-operation from NAT devices which thus have to be
treated as "adversaries" of a sort.
1.4 Terms and abbreviations
co-CoA: co-located care-of address
external network: the untrusted network (i.e. Internet). Note that
a private network (e.g. another corporate network) other than the
mobile node's internal network is considered an external network.
FA-CoA: foreign agent care-of address
internal network: the trusted network; for instance, a physically
secure corporate network where the i-HA is located.
inside: in the internal network; each network interface may be
independently inside or outside
i-FA: Mobile IPv4 foreign agent residing in the internal network
i-HA: Mobile IPv4 home agent residing in the internal network;
typically has a private address [5]
i-HoA: home address of the mobile node in the internal home agent
VPN-TIA: VPN tunnel inner address, the address(es) negotiated during
IKE phase 2 (quick mode), assigned manually, using IPsec-DHCP,
using mode config, or by some other means. Some VPN clients use
their current care-of address as their TIA for architectural
reasons.
VPN tunnel: an IPsec-based tunnel; for instance, IPsec tunnel mode
IPsec connection, or L2TP combined with IPsec transport
connection.
outside: in the external network; each network interface may be
independently inside or outside
x-FA: Mobile IPv4 foreign agent residing in the external network
x-HA: Mobile IPv4 home agent residing in the external network
x-HoA: home address of the mobile node in the external home agent
Vaarala (Ed.) Expires December 27, 2003 [Page 6]
Internet-Draft MIPv4-VPN June 2003
1.5 Requirement levels
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
Vaarala (Ed.) Expires December 27, 2003 [Page 7]
Internet-Draft MIPv4-VPN June 2003
2. Topology
The following figure describes an example network topology
illustrating the relationship between the internal and external
networks, the possible locations of the mobile node ("MN" in
parenthesis). The access modes (described later in Section 3)
available to the mobile node from each location are also shown in
curly braces.
(MN) {fvc} {home} (MN) [i-HA]
! \ /
.--+---. .-+---+-.
( ) ( )
`--+---' [VPN] `--+----'
\ ! !
[R/FA] [x-HA] .--+--. [R]
\ / ( DMZ ) !
.-+-------+--. `--+--' .-----+------.
( ) ! ( )
( external net +---[R]----[FW]----[R]--+ internal net )
( ) ( )
`--+---------' `---+---+----'
/ / \
[DHCP] [R] [DHCP] [R] [R] [i-FA]
\ / \ / \ /
.+--+---. .-+-+--. .--+--+-.
( ) ( ) ( )
`---+---' `--+---' `---+---'
! ! !
(MN) {cvc} (MN) {c} (MN) {f}
Figure: Basic topology, possible MN locations and access modes
The internal network is typically a multi-subnetted network which
uses private addressing [5]. Subnets may contain internal home
agent(s) (typically using private addresses), DHCP server(s), and/or
foreign agent(s). Current IEEE 802.11 wireless LANs are typically
deployed in the external network or the DMZ because of security
concerns.
The external network term used in this document includes the public
Internet, and private networks other than the mobile node's internal
network.
The de-militarized zone (DMZ) is a tiny network typically containing
servers that need to be accessed from both internal and external
Vaarala (Ed.) Expires December 27, 2003 [Page 8]
Internet-Draft MIPv4-VPN June 2003
networks; for instance, VPN devices.
The figure leaves out a few details worth noticing:
o There may be multiple NAT devices anywhere in the diagram.
* When the MN is outside, the NAT devices may be placed between
the MN and the x-HA or the x-HA and the VPN.
* There may be also be NAT(s) between the VPN and the i-HA, or a
NAT integrated into the VPN. In essence, any router in the
figure may be considered to represent zero or more routers,
each possibly performing NAT and/or ingress filtering.
* When the MN is inside, there may be NAT devices between the MN
and the i-HA, although this is not typical.
o Site-to-site VPN tunnels are not shown. Although mostly
transparent, IPsec endpoints may perform ingress filtering as part
of enforcing their policy. (Thus, reverse tunnelling SHOULD
always be used.)
o Trusted foreign agents (in this context referring to foreign
agents connected to the internal network using an IPsec tunnel)
are not shown. Trusted foreign agents are logically part of the
internal network.
o The figure represents a "canonical" topology where each functional
entity is illustrated as a separate device. However, it is
possible that in a physical network several functions are co-
located in a single device (for instance, the x-HA and VPN
functionalities). In fact, all three server components (x-HA,
VPN, and i-HA) may be co-located in a single physical device.
The following issues are also of importance:
o Some firewalls are configured to block ICMP messages and/or
fragments. Such firewalls (routers) cannot be detected reliably.
o Some networks contain transparent application proxies, especially
for the HTTP protocol. Like firewalls, such proxies cannot be
detected reliably in general. IPsec and Mobile IPv4 are
incompatible with such networks.
Vaarala (Ed.) Expires December 27, 2003 [Page 9]
Internet-Draft MIPv4-VPN June 2003
3. Access modes
In every possible location described in Section 2 the mobile node can
establish a connection to its i-HA by using a suitable "access mode".
An access mode is here defined to consist of:
1. a composition of the mobile node networking stack (i-MIP or x-
MIP/VPN/i-MIP); and
2. registration mode(s) of i-MIP and x-MIP (if used); i.e. co-
located care-of address or foreign agent care-of address.
Each possible access mode is encoded as "xyz", where:
o "x" indicates whether the x-MIP layer is used, and if used, the
mode ("f" indicates FA-CoA, "c" indicates co-CoA, absence
indicates not used);
o "y" indicates whether the VPN layer is used ("v" indicates VPN
used, absence indicates not used);
o "z" indicates mode of i-MIP layer ("f" indicates FA-CoA, "c"
indicates co-CoA).
This results in four access modes:
c: i-MIP w/ co-CoA
f: i-MIP w/ FA-CoA
cvc: x-MIP w/ co-CoA, VPN-TIA as i-MIP co-CoA
fvc: x-MIP w/ FA-CoA, VPN-TIA as i-MIP co-CoA
This notation is more useful when optimizations to protocol layers
are considered. The notation is preserved here so that work on the
optimizations can refer to a common notation.
Whenever a mobile node obtains either a co-CoA (using e.g. DHCP) or
a FA-CoA (from a foreign agent advertisement), the following steps
(conceptually) take place:
o The mobile node detects whether the subnet where the care-of
address was obtained belongs to the internal or the external
network using the method described in Section 4 (or a vendor
specific mechanism fulfilling the requirements described).
o The mobile node performs necessary registrations and other
connection setup signalling for the protocol layers (in the
following order):
Vaarala (Ed.) Expires December 27, 2003 [Page 10]
Internet-Draft MIPv4-VPN June 2003
* x-MIP (if used);
* VPN (if used); and
* i-MIP.
Note that these two tasks are intertwined to some extent: detection
of internal network may actually result in a successful registration
to the i-HA, for instance.
The following subsections describe the different access modes and the
requirements for registration and connection setup phase.
3.1 Access mode: 'c'
This access mode is standard Mobile IPv4 [3] with a co-located
address, except that:
o the mobile node MUST detect that it is in the internal network;
and
o the mobile node MUST re-register periodically (with a configurable
interval) to ensure it is still inside the internal network (see
Section 5).
The registration request SHOULD have T-bit reverse tunnelling) set.
Reverse tunnelling allows Mobile IPv4 to be used even in the presence
of ingress filtering. Since some site-to-site VPN tunnels perform
ingress filtering as a side effect of IPsec policy processing,
reverse tunnelling should be used to increase interoperability.
3.2 Access mode: 'f'
This access mode is standard Mobile IPv4 [3] with a foreign agent
care-of address, except that
o the mobile node MUST detect that it is in the internal network;
and
o the mobile node MUST re-register periodically (with a configurable
interval) to ensure it is still inside the internal network (see
Section 5).
The registration request SHOULD request reverse tunnelling (see
Section 3.1).
Vaarala (Ed.) Expires December 27, 2003 [Page 11]
Internet-Draft MIPv4-VPN June 2003
3.3 Access mode: 'cvc'
Steps:
o The mobile node obtains a care-of address from e.g. a DHCP
server.
o The mobile node detects it is outside and registers with the x-HA
(possibly as a side effect of the detection process), where
* D-bit MUST be set (co-located)
* T-bit SHOULD be set (reverse tunnelling)
o If necessary, the mobile node uses IKE to set up an IPsec
connection with the VPN gateway, using the x-HoA as the IP address
for IKE/IPsec communication. The VPN-TIA is assigned in some
manner (or chosen by the MN). VPN capability negotiation is done
at the same time.
o The mobile node sends a MIPv4 RRQ to the i-HA, registering the
VPN-TIA as a co-located care-of address, where
* D-bit MUST be set (co-located)
* T-bit MUST be set (reverse tunnelling)
3.4 Access mode: 'fvc'
Steps:
o The mobile node obtains a foreign agent advertisement from the
local network.
o The mobile node detects it is outside and registers with the x-HA
(possibly as a side effect of the detection process), where
* D-bit MUST NOT be set (foreign agent)
* T-bit SHOULD be set (reverse tunnelling)
o If necessary, the mobile node uses IKE to set up an IPsec
connection with the VPN gateway, using the x-HoA as the IP address
for IKE/IPsec communication. The VPN-TIA is assigned in some
manner (or chosen by the MN). VPN capability negotiation is done
at the same time.
Vaarala (Ed.) Expires December 27, 2003 [Page 12]
Internet-Draft MIPv4-VPN June 2003
o The mobile node sends a MIPv4 RRQ to the i-HA, registering the
VPN-TIA as a co-located care-of address, where
* D-bit MUST be set (co-located)
* T-bit MUST be set (reverse tunnelling)
3.5 NAT traversal
NAT devices may affect each layer independently (and even all three
layers at the same time). Mobile IPv4 NAT traversal MUST be used
for x-MIP and i-MIP layers, while IPsec NAT traversal [6][7] MUST be
used for VPN layer.
Note that NAT traversal for the internal MIPv4 layer may be necessary
even when there is no separate NAT device between the VPN gateway and
the internal network. Some VPN implementations NAT VPN tunnel inner
addresses before routing traffic to the intranet. Sometimes this is
done to make a deployment easier, but in some cases this approach
makes VPN client implementation easier. Mobile IPv4 NAT traversal is
required to establish a MIPv4 session in this case.
Vaarala (Ed.) Expires December 27, 2003 [Page 13]
Internet-Draft MIPv4-VPN June 2003
4. Internal network detection
Secure detection of the internal network is security critical: if the
mechanism fails for some reason, plaintext traffic may be sent to an
untrusted network. In other words, the overall security
(confidentiality and integrity of user data) is a minimum of IPsec
security and the internal network detection mechanism security. For
this reason, a set of requirements relevant to security are described
in this section.
In addition to detecting entry into the internal network, the mobile
node must also detect when it leaves the internal network. Entry
into the internal network is easier security-wise: the mobile node
can take all the time it needs to ensure that it is inside the
internal network before sending any plaintext traffic. Exit from the
internal network is more difficult to detect, and the MN may
accidentally leak plaintext packets if the event is not detected
properly.
Several events cause the mobile node to exit the internal network,
for instance:
o a routing change upstream;
o a reassociation of 802.11 on layer 2 which the mobile node
software does not detect;
o a physical cable disconnect and reconnect which the mobile node
software does not detect.
Whether the mobile node can detect such changes in the current
connection reliably depends on the implementation. For instance,
some mobile nodes may be implemented as pure layer three entities.
Even if the mobile node software has access to layer two information,
such information is not trustworthy security-wise (and depends on the
network interface driver).
If the mobile node does not detect these events properly, it may leak
plaintext traffic into an untrusted network. A number of approaches
can be used to detect exit from the internal network, ranging from
frequent re-registration to the use of layer two information.
A mobile node MUST implement a detection mechanism fulfilling the
requirements described in Section 4.2; this ensures that basic
security requirements are fulfilled. The basic algorithm described
in Section 4.3 is one way to do that, but alternative methods may be
used instead or in conjunction. The assumptions that the
requirements and the proposed mechanism rely upon are described in
Vaarala (Ed.) Expires December 27, 2003 [Page 14]
Internet-Draft MIPv4-VPN June 2003
Section 4.1.
4.1 Assumptions
The firewall MUST be configured to block traffic originating from
external networks going to the i-HA. In other words, if the mobile
node succeeds in registering with the i-HA directly (without using
IPsec), the mobile node may safely infer that it is connected to the
trusted internal network, and may therefore use plaintext traffic.
The firewall MAY be configured to block registration traffic to the
x-HA originating from within the internal network, which makes the
network detection algorithm simpler and more robust. However, as the
registration request is basically UDP traffic, an ordinary firewall
(even a stateful one) would typically allow the registration request
to be sent, and a registration reply to be received through the
firewall.
4.2 Implementation requirements
Any mechanism used to detect the internal network MUST fulfill the
following requirements.
4.2.1 Connection status change
When the mobile node detects that its connection status on a certain
network interface changes, the mobile node MUST:
o immediately stop relaying user data packets;
o detect whether this interface is connected to the internal or the
external network;
o resume data traffic only after the internal network detection and
necessary registrations and VPN tunnel establishment have been
completed.
The mechanism used to detect a connection status change depends on
the mobile node implementation and the access mode. The connection
status is considered to change whenever any of the following happens:
o when the interface is connected to the internal network, the i-HA
can no longer be reached using a re-registration;
o the next hop router is no longer reachable (e.g. ARP fails);
o when using an FA, FA advertisements from the FA used for
registration are no longer received; or
Vaarala (Ed.) Expires December 27, 2003 [Page 15]
Internet-Draft MIPv4-VPN June 2003
o layer two or other such information indicates that the physical
connection status has changed.
The mobile node MUST detect the first event, i.e. failure to re-
register when inside. Detecting the other events is RECOMMENDED.
4.2.2 Registration-based internal network detection
The mobile node MUST NOT infer that an interface is connected to the
internal network unless a successful registration has been completed
through that particular interface and the connection status of the
interface has not changed since.
4.2.3 Registration-based internal network monitoring
Some leak of plaintext packets to a (potentially) untrusted network
cannot always be completely prevented; this depends heavily on the
client implementation. In some cases the client cannot detect such a
change (for instance if the subnet is reconnected to another place in
the network topology in its entirety).
To bound the maximum amount of time that such a leak may persist, the
mobile node MUST fulfill the following requirements when inside:
o When the mobile node is registered directly to the i-HA (i.e. not
using IPsec), the mobile node MUST re-register with the i-HA
periodically to ensure that is still connected to the trusted
internal network.
o This re-registration interval and associated retransmission
parameters MUST be configurable in the mobile node, so that the
maximum exposure time can be reliably controlled.
o The default values MUST ensure that the mobile node will stop
sending plaintext traffic within one minute of the change of i-HA
reachability.
o When the mobile node fails to re-register, it MUST stop sending
and receiving plaintext traffic immediately, to prevent plaintext
traffic from leaking out and untrusted data from leaking in.
The re-registration requirement allows the administrator to determine
the required security level for the particular deployment.
Configuring the re-registration interval to a very small value (i.e.
in the order of few seconds) is not practical; alternative mechanisms
need to be considered if such confidence is required.
Note that this is just the fallback mechanism. If additional
Vaarala (Ed.) Expires December 27, 2003 [Page 16]
Internet-Draft MIPv4-VPN June 2003
information (such as layer two information) is available to the
mobile node, the mobile node SHOULD assume it has moved and restart
the registration process to minimize exposure.
Also note that the re-registration interval only applies when the
mobile node is inside the internal network. When outside, ordinary
Mobile IPv4 re-registration process (based on registration lifetime)
is used.
4.2.4 Handling of network interfaces
The mobile node implementation MUST track each network interface
separately. Successful registration with the i-HA through interface
X does not imply anything about the status of interface Y.
4.3 Proposed algorithm
When the MN detects that it has changed its point of network
attachment (on a certain interface), it issues two simultaneous
registration requests, one to the i-HA and another to the x-HA.
These registration requests are periodically retransmitted if reply
messages are not received.
Registration replies are processed as follows:
o If a response from the x-HA is received, the MN stops
retransmitting its registration request to the x-HA and determines
it is outside. However, the MN MUST keep on retransmitting its
registration to the i-HA for a period of time. The MN MAY
postpone the IPsec connection setup for some period of time
("detection period") while it waits for a response from the i-HA.
o If a response from the i-HA is received, the MN MUST determine
that it is inside. If a previous registration reply from the x-HA
has been received, the MN SHOULD de-register with the x-HA. In
any case, the MN MUST stop retransmitting its registration
requests to both i-HA and x-HA.
o If a response from the x-HA is received while the MN has
successfully registered with the i-HA, the MN SHOULD de-register
with the x-HA.
If the MN ends up detecting that it is inside, it MUST re-register
periodically (regardless of binding lifetime). The re-registration
interval and related parameters (e.g. for retransmission) MUST be
configurable, as they are security related parameters (see Section
4.2.3). If the re-registration fails, the MN MUST stop sending and
receiving plaintext traffic, and MUST restart the detection
Vaarala (Ed.) Expires December 27, 2003 [Page 17]
Internet-Draft MIPv4-VPN June 2003
algorithm.
Plaintext re-registration messages are always addressed either to the
x-HA or the i-HA, not to both. This is because the MN knows, after
initial registration, whether it is inside or outside. (However,
when the mobile node is outside, it re-registers independently with
the x-HA using plaintext, and with the i-HA through the VPN tunnel.)
The "detection period" is OPTIONAL, and may be useful in avoiding
aborted IKE sessions due to timing of i-HA and x-HA registration
reply messages. Aborted IKE sessions may be a problem in some cases
because IKE does not provide a reliable, standardized, and mandatory-
to-implement mechanism for terminating a session cleanly.
If the x-HA is not reachable from inside (i.e. the firewall
configuration is known), a detection period of zero is preferred, as
it minimizes connection setup overhead and causes no timing problems.
Should the assumption have been invalid and a response from the i-HA
received after a response from the x-HA, the MN SHOULD re-register
with the i-HA directly.
Note that it is possible that an i-HA is initially unreachable for
some time, but later becomes reachable (consider e.g. a routing
problem in the internal network). To eventually detect the i-HA, the
MN MAY send periodic registration attempts to the i-HA even after
determining initially that it is outside. The period of such re-
registration attempts should be in the order of minutes (e.g. 10
minutes), and configurable.
4.4 Implementation issues
When the MN uses a parallel detection algorithm and is using an FA,
the MN sends two registration requests through the same FA with the
same MAC address (or equivalent) and possibly even the same home
address. Although this is not in conflict with existing
specifications, it is not a usual scenario; hence some FA
implementations may not work properly in such a situation. However,
practical testing against deployed foreign agents seems to indicate
that a majority of foreign agents handle this situation.
When the x-HA and i-HA addresses are the same, the scenario is even
more difficult for the FA, and it is almost certain that existing FAs
do not deal with the situation correctly. Therefore, it is required
that x-HA and i-HA addresses MUST be different. This requirement is
automatically satisfied if the x-HA has a public address.
The mobile node MAY use the following hints to determine that it is
inside, but MUST verify reachability of the i-HA anyway:
Vaarala (Ed.) Expires December 27, 2003 [Page 18]
Internet-Draft MIPv4-VPN June 2003
o a domain name in a DHCPDISCOVER / DHCPOFFER message;
o a NAI in a foreign agent advertisement;
o a list of default gateway MAC addresses which are known to reside
in the internal network (i.e. configured as such, or have been
previously verified to be inside).
For instance, if the MN has reason to believe it is inside, it MAY
postpone sending of registration request to the x-HA for some time.
Similarly, if the MN has a reason to believe it is outside, it may
start IPsec connection setup immediately after receiving a
registration reply from the x-HA. However, should the MN receive a
registration reply from the i-HA after IPsec connection setup has
been started, the MN SHOULD still switch to using the i-HA directly.
4.5 Rationale
4.5.1 Firewall configuration requirements
The assumption that the i-HA cannot be reached from the external
network is, in practice, unavoidable. Suppose the assumption is not
made, i.e. the i-HA is reachable from some external networks. As a
result, a successful registration with the i-HA (without IPsec)
cannot be used as a secure indication that the mobile node is inside.
A possible solution to the obvious security problem would be to
define and deploy a secure internal network detection mechanism based
on e.g. signed FA advertisement or signed DHCP messages.
However, unless the mechanism is defined for both FA and DHCP
messages and is deployed in every internal network, it has limited
applicability. In other words, the mobile node MUST NOT assume it is
in the internal network unless it receives a signed FA or DHCP
message (regardless of whether it can register directly with the i-HA
or not!). If it receives an unsigned FA or DHCP message, it MUST use
IPsec; otherwise the mobile node can be easily tricked into using
plaintext.
Assuming that all FA and DHCP servers in the internal network are
upgraded to support such a feature does not seem realistic; it is
highly desirable to be able to take advantage of existing DHCP and FA
deployments. Similar analysis seems to apply regardless of what kind
of additional security mechanism is defined.
4.5.2 Registration-based internal network monitoring
This issue also affects IPsec client security. However, as IPsec
specifications take no stand on how and when the client applies
Vaarala (Ed.) Expires December 27, 2003 [Page 19]
Internet-Draft MIPv4-VPN June 2003
IPsec, the issue is out of scope for IPsec. Because this document
describes an algorithm and requirements for (secure) internal network
detection, the issue is in scope of the document.
The current requirement for internal network monitoring was added as
a fallback mechanism. It seems to be the best what can be done with
only layer three mechanisms.
4.6 Improvements
The registration process can be improved in many ways. One simple
way is to make the x-HA detect whether a registration request came
from inside or outside. If it came from inside, the x-HA can simply
drop the registration request, thus effectively "firewalling" the
request.
This approach is feasible without protocol changes in scenarios where
a corporation owns both the VPN and the x-HA. The x-HA can simply
determine based on incoming interface identifier (or the router which
relayed the packet) whether the registration request came from inside
or not.
In other scenarios protocol changes may be needed. Such changes are
out of scope of this document.
Vaarala (Ed.) Expires December 27, 2003 [Page 20]
Internet-Draft MIPv4-VPN June 2003
5. Requirements
5.1 Mobile node requirements
The mobile node MUST implement an internal network detection
algorithm fulfilling the requirements set forth in Section 4.2.
The mobile node MUST support access modes: c, f, cvc, fvc (Section
3).
The mobile node SHOULD support Mobile IPv4 NAT traversal [4] for both
internal and external Mobile IP.
The mobile node SHOULD support IPsec NAT traversal [6][7].
When the mobile node has direct access to the i-HA, it SHOULD use
only the inner Mobile IPv4 layer to minimize firewall and VPN impact.
5.2 VPN device requirements
The VPN security policy MUST allow communication using UDP to the
internal home agent(s), with home agent port 434 and any remote port.
The security policy SHOULD allow IP-IP to internal home agent(s) in
addition to UDP port 434.
The VPN device SHOULD implement the IPsec NAT traversal mechanism
described in [6][7].
5.3 Home agent requirements
The home agent SHOULD implement the Mobile IPv4 NAT traversal
mechanism described in [4]. (This also refers to the i-HA: NAT
traversal is required to support VPNs that NAT VPN tunnel addresses
or block IP-IP traffic.)
Vaarala (Ed.) Expires December 27, 2003 [Page 21]
Internet-Draft MIPv4-VPN June 2003
6. Analysis
This section provides a comparison against guidelines described in
Section 6 of the problem statement [2] and additional analysis of
packet overhead with and without the optional mechanisms.
6.1 Comparison against guidelines
Preservation of existing VPN infrastructure
o The proposed solution does not mandate any changes to existing VPN
infrastructure, other than possibly changes in configuration to
avoid stateful filtering of traffic.
Software upgrades to existing VPN clients and gateways
o The solution described does not require any changes to VPN
gateways or Mobile IPv4 home agents or foreign agents.
IPsec protocol
o Proposed solution does not require any changes to existing IPsec
or key exchange standard protocols, and does not require
implementation of new protocols in the VPN device.
Multi-vendor interoperability
o The proposed solution provides easy multi-vendor interoperability
between server components (VPN device, foreign agents and home
agents). Indeed, these components need not be aware of each
other.
o The mobile node networking stack is somewhat complex to implement,
which may be an issue for multi-vendor interoperability.
MIPv4 protocol
o The solution adheres to the MIPv4 protocol.
o The solution requires the use of two parallel MIPv4 layers.
Handoff overhead
o The solution provides a mechanism to avoid VPN tunnel SA
renegotiation upon movement by using the external MIPv4 layer.
Scalability, availability, reliability, and performance
Vaarala (Ed.) Expires December 27, 2003 [Page 22]
Internet-Draft MIPv4-VPN June 2003
o The solution complexity is linear with the number of MNs
registered and accessing resources inside the intranet.
o Additional overhead is imposed by the solution.
Functional entities
o The solution does not impose any new types of functional entities
or required changes to existing entities. However, an external HA
device is required.
Implications of intervening NAT gateways
o The solution leverages existing MIPv4 NAT traversal [4] and IPsec
NAT traversal [6][7] solutions and does not require any new
functionality to deal with NATs.
Security implications
o The solution requires a new mechanism to detect whether the mobile
node is in the internal or the external network. The security of
this mechanism is critical in ensuring that the security level
provided by IPsec is not compromised by a faulty detection
mechanism.
o When the mobile node is outside, the external Mobile IPv4 layer
may allow some traffic redirection attacks that plain IPsec does
not allow. Other than that, IPsec security is unchanged.
o More security considerations are described in Section 7.
6.2 Packet overhead
The maximum packet overhead depends on access mode as follows:
o f: 0 octets
o c: 20 octets
o fvc: 77 octets
o cvc: 97 octets
The overhead consists of the following:
o IP-IP for i-MIPv4: 20 octets
Vaarala (Ed.) Expires December 27, 2003 [Page 23]
Internet-Draft MIPv4-VPN June 2003
o IPsec ESP: 57 octets total, consisting of: 20 (new IP header),
4+4+8 = 16 (SPI, sequence number, cipher initialization vector),
7+2 = 9 (padding, padding length field, next header field), 12
(ESP authentication trailer)
o IP-IP for x-MIPv4: 20 octets
When IPsec is used, a variable amount of padding is present in each
ESP packet. The figures were computed for a cipher with 64-bit block
size, padding overhead of 9 octets (next header field, padding length
field, and 7 octets of padding, see Section 2.4 of [8]), and ESP
authentication field of 12 octets (HMAC-SHA1-96 or HMAC-MD5-96).
Note that an IPsec implementation MAY pad with more than a minimum
amount of octets.
NAT traversal overhead is not included, and adds 8 octets when IPsec
NAT traversal [6][7] is used and 12 octets when MIP NAT traversal [4]
is used. For instance, when using access mode cvc, the maximum NAT
traversal overhead is 12+8+12 = 32 octets. Thus, the worst case
scenario (with the abovementioned ESP assumptions) is 129 octets for
cvc.
6.3 Latency considerations
The following terms are used:
i-RTT: round trip time to i-HA
x-RTT: round trip time to x-HA
i-TP: total processing time (MN & HA) for one i-HA round trip
x-TP: total processing time (MN & HA) for one x-HA round trip
DP-T: "detection period" when MN is outside
VPN-T: VPN connection setup time
DET-T: time to detect a change in network connection
DHCP-T: time to obtain co-located care-of address using DHCP
FA-T: time to obtain a foreign agent care-of address
In the analysis below, packet loss is ignored. DHCP is used as an
example; any method of obtaining a co-located care-of address is
equivalent. Note that i-RTT and x-RTT always refer to the round trip
time from the current location. Thus, i-RTT is typically "large"
when the mobile node is outside, and "small" when inside.
The basic network detection algorithm has no "memory"; thus
connection setup latency is only dependent on the current access
network, not whether the previous access network was outside or
inside.
When the mobile node is inside, connection setup latency is
Vaarala (Ed.) Expires December 27, 2003 [Page 24]
Internet-Draft MIPv4-VPN June 2003
determined simply by the latency of registration with the i-HA, which
is typically simply (i-RTT + i-TP). When a foreign agent is used to
register a co-located care-of address, and a NAT is detected, the
latency is 2*(i-RTT + i-TP) (see [4] Section 4.11). The "detection
period" does not affect latency because the mobile node SHOULD use
the i-HA directly if the i-HA replies.
When the mobile node is outside, connection setup latency is
typically (x-RTT + x-TP + DP-T + VPN-T + i-RTT + i-TP), where VPN-T
is omitted if an IPsec connection already exists. When a foreign
agent is used to register a co-located care-of address to the x-HA,
and a NAT is detected, the latency is (2*(x-RTT + x-TP) + DP-T + VPN-
T + i-RTT + i-TP). Since each step of the connection setup builds on
the previous one, the steps always proceed in strict sequence and no
parallellism is possible.
The total latency from change in network connection to bi-directional
packet flow is the sum of DET-T, min(DHCP-T, FA-T), and the
connection setup time. For instance, when outside, typically: (DET-T
+ min(DHCP-T, FA-T) + x-RTT + x-TP + DP-T + VPN-T + i-RTT + i-TP).
Because the network detection uses parallel registration to x-HA and
i-HA, there is no considerable latency impact from the parallel
registration as such, except of course the small delay imposed on the
second registration request because sending is sequential in reality.
However, detection period (DP-T) increases total latency directly.
The mobile node may improve latency when outside by two means:
o sending the registration request most likely to succeed first,
thus avoiding the small delay caused by sequential sending; and
o using a detection period of zero.
These two can be done based on heuristics about the network, e.g.
addresses, MAC address of the default gateway (which the mobile node
may remember from previous access), based on the previous access
network (i.e. optimize for inside-inside and outside-outside
movement), etc.
6.4 Firewall state considerations
A separate firewall device or an integrated firewall in the VPN
gateway typically performs stateful inspection of user traffic. The
firewall may, for instance, track TCP session status and block TCP
segments not related to open connections. Other stateful inspection
mechanisms also exist.
Vaarala (Ed.) Expires December 27, 2003 [Page 25]
Internet-Draft MIPv4-VPN June 2003
Firewall state poses a problem when the mobile node moves between the
internal and external networks. The mobile node may, for instance,
initiate a TCP connection while inside, and later go outside while
expecting to keep the connection alive. From the point of view of
the firewall, the TCP connection has not been initiated, as it has
not witnessed the TCP connection setup packets, thus potentially
resulting in connectivity problems.
When the VPN-TIA is registered as a co-located care-of address with
the i-HA, all mobile node traffic appears as IP-IP for the firewall.
Typically firewalls don't continue inspection beyond the IP-IP
tunnel, but it is not inconceivable that some firewalls may do that.
In summary, the firewall must allow traffic coming from and going
into the IPsec connection to be routed, even though they may not have
successfully tracked the connection state. How this is done is out
of scope of this document.
6.5 Intrusion detection systems (IDSs)
Many firewalls incorporate intrusion detection systems, which monitor
traffic for unusual patterns and clear signs of attack. Since
traffic from a mobile node implementing this specification is UDP to
i-HA port 434, and possibly IP-IP traffic to the i-HA address,
existing IDSs may treat the traffic differently than ordinary VPN
remote access traffic. Like firewalls, IDSs are not standardized, so
it is impossible to guarantee interoperability with any particular
IDS system.
6.6 Implementation of mobile node
Implementation of the mobile node requires the use of three
tunnelling layers, which may be used in various configurations
depending on whether that particular interface is inside or outside.
Note that it is possible that one interface is inside and another
interface is outside, which requires a different layering for each
interface at the same time.
For multi-vendor implementation, the IPsec and Mobile IPv4 layers
need to interoperate in the same mobile node. This implies that a
flexible framework for protocol layering (or protocol-specific APIs)
are required.
6.7 Non-IPsec VPN protocols
The proposed solution works also for VPN tunneling protocols that are
not IPsec-based, provided that the mobile node is provided IPv4
connectivity with an address suitable for registration. However,
Vaarala (Ed.) Expires December 27, 2003 [Page 26]
Internet-Draft MIPv4-VPN June 2003
such VPN protocols are not explicitly considered.
6.8 Shortcomings for enterprise use
The proposed solution has the following shortcomings for enterprise
use:
o Networks which provide only HTTP access (sometimes found in
corporate networks) cannot be used for remote access.
o Fragments are filtered by some routers. MIP NAT traversal [4]
solves some, but not all, fragment related issues.
However, these are not part of the problem statement.
Vaarala (Ed.) Expires December 27, 2003 [Page 27]
Internet-Draft MIPv4-VPN June 2003
7. Security considerations
7.1 Internal network detection
If the mobile node mistakenly believes it is in the internal network
and sends plaintext packets, it compromises IPsec security. For this
reason, the overall security (confidentiality and integrity) of user
data is a minimum of (1) IPsec security, and (2) security of the
internal network detection mechanism.
Security of the internal network detection relies on a successful
registration with the i-HA. For standard Mobile IPv4 [3] this means
HMAC-MD5 and Mobile IPv4 replay protection.
When the connection status of an interface changes, an interface
previously connected to the trusted internal network may suddenly be
connected to an untrusted network. Although the same problem is also
relevant to IPsec-based VPN implementations, the problem is
especially relevant in the scope of this specification.
In most cases, mobile node implementations are expected to have layer
two information available, making connection change detection both
fast and robust. To cover cases where such information is not
available (or fails for some reason), the mobile node is required to
periodically re-register with the internal home agent to verify that
it is still connected to the trusted network. It is also required
that this re-registration interval be configurable, thus giving the
administrator a parameter by which potential exposure may be
controlled robustly even for the worst case.
7.2 Mobile IPv4 versus IPsec
MIPv4 and IPsec have different goals and approaches for providing
security services. MIPv4 typically uses a shared secret for
authentication of (only) signalling traffic, while IPsec typically
uses IKE (an authenticated Diffie-Hellman exchange) to set up session
keys. Thus, the overall security properties of a combined MIPv4 and
IPsec system depend on both mechanisms.
In a "dual HA" solution the external MIPv4 layer provides mobility
for IPsec traffic. If the security of MIPv4 is broken in this
context, traffic redirection attacks against the IPsec traffic are
possible. However, such routing attacks do not affect other IPsec
properties (confidentiality, integrity, replay protection, etc),
because IPsec does not consider the network between two IPsec
endpoints to be secure in any way.
Because MIPv4 shared secrets are usually configured manually, they
Vaarala (Ed.) Expires December 27, 2003 [Page 28]
Internet-Draft MIPv4-VPN June 2003
may be weak if easily memorizable secrets are chosen, thus opening up
redirection attacks described above. Note that a weak secret in the
i-HA is fatal to security, as the mobile node can be fooled into
dropping encryption if the i-HA secret is broken.
Assuming the MIPv4 shared secrets have sufficient entropy, there are
still at least the following differences and similarities between
MIPv4 and IPsec worth considering:
o Both IPsec and MIPv4 are susceptible to the "transient pseudo NAT"
attack described in [14] and [4], assuming that NAT traversal is
enabled (which is typically the case).
o When considering a "pseudo NAT" attack against standard IPsec and
standard MIP (with NAT traversal), redirection attacks against MIP
may be easier because:
* MIPv4 re-registrations typically occur more frequently than
IPsec SA setups (although this may not be the case for mobile
hosts).
* It suffices to catch and modify a single registration request,
whereas attacking IKE requires that multiple IKE packets are
caught and modified.
o There may be concerns about mixing of algorithms. For instance,
IPsec may be using HMAC-SHA1-96, while MIP is always using HMAC-
MD5 (RFC 3344) or prefix+suffix MD5 (RFC 2002). Furthermore,
while IPsec algorithms are typically configurable, MIPv4 clients
typically use only HMAC-MD5 or prefix+suffix MD5. Although this
is probably not a security problem as such, it is more difficult
to communicate to users.
o When IPsec is used with a PKI, the key management properties are
superior to those of basic MIPv4. Thus, adding MIPv4 to the
system makes key management more complex.
o In general, adding new security mechanisms increases overall
complexity and makes the system more difficult to understand.
Vaarala (Ed.) Expires December 27, 2003 [Page 29]
Internet-Draft MIPv4-VPN June 2003
8. Intellectual property rights
Birdstep Technology has submitted patent application(s) related to
the dual mobile IP design for VPN gateway traversal. Birdstep's
objective is to seek intellectual property protection for its mobile
IP client implementation of such a design. If any standards arising
from this document are or become protected by one or more patents
assigned to Birdstep Technology, and if any claims of any issued
Birdstep patents are necessary for practicing such a standard, any
party will be able to obtain a license from Birdstep to use any such
patent claims under reasonable, non-discriminatory terms, with
reciprocity, to implement and fully comply with the standard.
Vaarala (Ed.) Expires December 27, 2003 [Page 30]
Internet-Draft MIPv4-VPN June 2003
9. Acknowledgements
This document is a joint work of the contributing authors (in
alphabetical order):
- Farid Adrangi (Intel Corporation)
- Nitsan Baider (Check Point Software Technologies, Inc.)
- Gopal Dommety (Cisco Systems)
- Eli Gelasco (Cisco Systems)
- Dorothy Gellert (Nokia Corporation)
- Espen Klovning (Birdstep)
- Milind Kulkarni (Cisco Systems)
- Henrik Levkowetz (ipUnplugged AB)
- Frode Nielsen (Birdstep)
- Sami Vaarala (Netseal)
- Qiang Zhang (Liqwid Networks, Inc.)
The authors would like to thank MIP/VPN design team, especially Mike
Andrews, Gaetan Feige, Prakash Iyer, Brijesh Kumar, Joe Lau, Kent
Leung, Gabriel Montenegro, Ranjit Narjala, Antti Nuopponen, Alan
O'Neill, Alpesh Patel, Ilkka Pietikainen, Phil Roberts, Hans
Sjostrand, and Serge Tessier for their continuous feedback and
helping us improve this draft. We would also like to thank the
Mobile IP working group chairs (Gabriel Montenegro, Basavaraj Patil,
and Phil Roberts) for important feedback and guidance.
Vaarala (Ed.) Expires December 27, 2003 [Page 31]
Internet-Draft MIPv4-VPN June 2003
References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[2] Adrangi, F., Kulkarni, M., Dommety, G., Gelasco, E., Zhang, Q.,
Vaarala, S., Gellert, D., Baider, N. and H. Levkowetz, "Problem
Statement and Solution Guidelines for Mobile IPv4 Traversal
Across IPsec-based VPN Gateways (draft-ietf-mobileip-vpn-
problem-statement-guide-00e, work in progress)", January 2003.
[3] Perkins, C., "IP Mobility Support for IPv4", RFC 3344, August
2002.
[4] Levkowetz, H. and S. Vaarala, "Mobile IP Traversal of Network
Address Translation (NAT) Devices", RFC 3519, April 2003.
[5] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and E.
Lear, "Address Allocation for Private Internets", RFC 1918, BCP
5, February 1996.
[6] Kivinen, T., Swander, B., Huttunen, A. and V. Volpe,
"Negotiation of NAT-Traversal in the IKE (draft-ietf-ipsec-nat-
t-ike-05, work in progress)", January 2003.
[7] Huttunen, A., Swander, B., Stenberg, M., Volpe, V. and L.
DiBurro, "UDP Encapsulation of IPsec packets (draft-ietf-ipsec-
udp-encaps-06, work in progress)", January 2003.
[8] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[9] Nuopponen, A. and S. Vaarala, "Mobile IPv4 coexistence with
IPsec remote access tunnelling (draft-nuopponen-vaarala-mipvpn-
00, work in progress)", July 2002.
[10] Adrangi, F., Iyer, P., Zhang, Q. and N. Baider, "Mobile IPv4
Traversal Across IPsec-based VPN Gateways (draft-adrangi-
mobileip-mipvpn-traversal, work in progress)", January 2003.
[11] Adrangi, F., Iyer, P., Leung, K., Kulkarni, M., Patel, A.,
Zhang, Q. and J. Lau, "Mobile IPv4 Traversal Across IPsec-based
VPN Gateways (draft-adrangi-mobileip-vpn-traversal-02)", July
2002.
[12] Tessier, S., "Guidelines for Mobile IP and IPsec VPN Usage",
December 2002.
Vaarala (Ed.) Expires December 27, 2003 [Page 32]
Internet-Draft MIPv4-VPN June 2003
[13] Arkko, J., Devarapalli, V. and F. Dupont, "Using IPsec to
Protect Mobile IPv6 Signaling between Mobile Nodes and Home
Agents (draft-ietf-mobileip-mipv6-ha-ipsec-01, work in
progress)", October 2002.
[14] Dupont, F. and J. Bernard, "Transient pseudo-NAT attacks or how
NATs are even more evil than you believed (draft-dupont-
transient-pseudonat-01, work in progress)", December 2002.
Author's Address
Sami Vaarala
Netseal
Niittykatu 6
Espoo 02201
FINLAND
Phone: +358 9 435 310
EMail: sami.vaarala@iki.fi
Vaarala (Ed.) Expires December 27, 2003 [Page 33]
Internet-Draft MIPv4-VPN June 2003
Appendix A. Packet flow examples
A.1 Connection setup for access mode 'cvc'
The following figure illustrates connection setup when the mobile
node is outside and using a co-located care-of address. IKE
connection setup is not shown in full, and involves multiple round
trips (4.5 round trips when using main mode followed by quick mode).
MN-APP MN x-HA VPN i-HA CN
! ! ! ! ! !
! ! -------> ! ! ! !
! ! rrq ! ! ! !
! ! -----------------------------X ! ! rrq not
! ! rrq ! ! ! ! received
! ! ! ! ! ! by i-HA
! ! <------- ! ! ! !
! ! rrp ! ! ! !
! ! ! ! ! !
! [wait for detection period for response from i-HA] !
! [may also retransmit to i-HA, depending on config] ! no rrp
! ! ! ! ! ! from i-HA
! ! ==(1)==> ! ! ! !
! ! ike {1a}! -------> ! ! !
! ! ! ike ! ! !
! ! ! <------- ! ! !
! ! <==(1)== ! ike ! ! !
! ! ike ! ! ! !
: : : : : :
: : : : : :
! ! ! ! ! !
! ! ==(2)==> ! ! ! !
! ! rrq {2a}! ==(1)==> ! ! !
! ! ! rrq {2b}! -------> ! !
! ! ! ! rrq {2c}! !
! ! ! ! <------- ! !
! ! ! <==(1)== ! rrp ! !
! ! <==(2)== ! rrp ! ! !
! ! rrp ! ! ! !
! ! ! ! ! !
[[--- connection setup ok, bidirectional connection up ---]]
! ! ! ! ! !
! -------> ! ! ! ! !
! pkt {3a}! ==(3)==> ! ! ! !
! ! pkt {3b}! ==(2)==> ! ! !
! ! ! pkt {3c}! ==(1)==> ! !
! ! ! ! pkt {3d}! -------> !
! ! ! ! ! pkt {3e}!
Vaarala (Ed.) Expires December 27, 2003 [Page 34]
Internet-Draft MIPv4-VPN June 2003
! ! ! ! ! <------- !
! ! ! ! <==(1)== ! pkt !
! ! ! <==(2)== ! pkt ! !
! ! <==(3)== ! pkt ! ! !
! <------ ! pkt ! ! ! !
! pkt ! ! ! ! !
: : : : : :
: : : : : :
The notation "==(N)==>" or "<==(N)==" indicates that the innermost
packet has been encapsulated N times, using IP-IP, ESP, or MIP NAT
traversal.
Packets marked with {xx} are shown in more detail below. Each area
represents a protocol header (labeled). Source and destination
addresses or ports are shown underneath the protocol name when
applicable. Note that there are no NAT traversal headers in the
example packets.
Packet {1a}
.------------------------------------.
! IP ! IP ! UDP ! IKE !
! co-CoA ! x-HoA ! 500 ! !
! x-HA ! VPN-GW ! 500 ! !
`------------------------------------'
Packet {2a}
.--------------------------------------------------------.
! IP ! IP ! ESP ! IP ! UDP ! MIP RRQ !
! co-CoA ! x-HoA ! ! VPN-TIA ! ANY ! !
! x-HA ! VPN-GW ! ! i-HA ! 434 ! !
`--------------------------------------------------------'
Packet {2b}
.----------------------------------------------.
! IP ! ESP ! IP ! UDP ! MIP RRQ !
! x-HoA ! ! VPN-TIA ! ANY ! !
! VPN-GW ! ! i-HA ! 434 ! !
`----------------------------------------------'
Packet {2c}
.----------------------------.
! IP ! UDP ! MIP RRQ !
! VPN-TIA ! ANY ! !
! i-HA ! 434 ! !
`----------------------------'
Packet {3a}
Vaarala (Ed.) Expires December 27, 2003 [Page 35]
Internet-Draft MIPv4-VPN June 2003
.-------------------.
! IP ! user !
! i-HoA ! protocol !
! CN ! !
`-------------------'
Packet {3b}
.------------------------------------------------------- -
! IP ! IP ! ESP ! IP ! IP ! user \
! co-CoA ! x-HoA ! ! VPN-TIA ! i-HoA ! protocol../
! x-HA ! VPN-GW ! ! i-HA ! CN ! \
`------------------------------------------------------- -
- - -----------------.
\..user ! ESP !
/ protocol ! trailer !
\ ! !
- - -----------------'
Packet {3c}
.--------------------------------------------------------.
! IP ! ESP ! IP ! IP ! user ! ESP !
! x-HoA ! ! VPN-TIA ! i-HoA ! protocol ! trailer !
! VPN-GW ! ! i-HA ! CN ! ! !
`--------------------------------------------------------'
Packet {3d}
.------------------------------.
! IP ! IP ! user !
! VPN-TIA ! i-HoA ! protocol !
! i-HA ! CN ! !
`------------------------------'
Packet {3e}
.-------------------.
! IP ! user !
! i-HoA ! protocol !
! CN ! !
`-------------------'
Packet {3b} with all NAT traversal headers (x-MIP, ESP, and i-MIP) is
shown below for comparison.
Vaarala (Ed.) Expires December 27, 2003 [Page 36]
Internet-Draft MIPv4-VPN June 2003
Packet {3b} (with NAT traversal headers)
.------------------------------------------------- -
! IP ! UDP ! MIP ! IP ! UDP ! ESP.. \
! co-CoA ! ANY ! tunnel ! x-HoA ! 4500 ! /
! x-HA ! 434 ! data ! VPN-GW ! 4500 ! \
`------------------------------------------------- -
<=== external MIPv4 ====> <=== IPsec ESP ======== = =
- - ------------------------------------------------ -
\..ESP ! IP ! UDP ! MIP ! IP ! user \
/ ! VPN-TIA ! ANY ! tunnel ! i-HoA ! protocol../
\ ! i-HA ! 434 ! data ! CN ! \
- - ------------------------------------------------ -
= ===> <==== internal MIPv4 ====> <== user packet == =
- - -----------------.
\..user ! ESP !
/ protocol ! trailer !
\ ! !
- - -----------------'
= = ======> <= ESP =>
The following diagram illustrates what happens when the i-HA response
is delayed beyond detection period (and is received while IKE is on-
going).
MN-APP MN x-HA VPN i-HA CN
! ! -------> ! ! ! !
! ! rrq ! ! ! !
! ! -----------------------------X ! ! rrq not
! ! rrq ! ! ! ! received
! ! ! ! ! ! by i-HA
! ! <------- ! ! ! !
! ! rrp ! ! ! !
! ! ! ! ! !
! [wait for detection period for response from i-HA] !
! [retranmissions to i-HA] ! ! no rrp
! ! ! ! ! ! from i-HA
! ! ==(1)==> ! ! ! !
! ! ike {1a}! -------> ! ! !
! ! ! ike ! ! !
! ! ! <------- ! ! !
! ! <==(1)== ! ike ! ! !
! ! ! ! ! !
: : : : : :
! ! <----------------------------- ! ! late rrp
! ! rrp ! ! ! !
Vaarala (Ed.) Expires December 27, 2003 [Page 37]
Internet-Draft MIPv4-VPN June 2003
! ! ! ! ! !
! [bidirectional connection with i-HA up] ! !
! [abort ike, de-register with x-ha] ! !
! ! ! ! ! !
! ! ! <------- ! ! !
! ! <==(1)== ! ike ! [ike packets may]
! ! ike ! ! [arrive for some time]
! [drop] ! ! ! !
! ! ! [peer not responding] !
! ! ! [retransmit for some time] !
! ! ! ! ! !
! ! -------> ! ! ! !
! ! rrq ! ! ! !
! ! (dereg) ! ! ! !
! ! ! ! ! !
! ! <------- ! ! [after de-reg, x-HA]
! ! rrp ! <------- ! [drops ike packets]
! ! ! ike ! ! !
! ! [drop] ! ! !
! ! ! ! ! !
! ! ==(1)========================> ! !
! ! pkt ! ! ! -------> !
! ! ! ! ! pkt !
! ! ! ! ! !
! ! ! ! ! <------- !
! ! <==(1)======================== ! pkt !
! ! pkt ! ! ! !
: : : : : :
: : : : : :
In the diagram above, the IKE session in the VPN device eventually
times out. Some IKE implementations support aborting a session
(ISAKMP exchange) in some way; if so, the IKE state is dropped
cleanly.
Note that it is possible to receive the registration reply from the
i-HA after a registration request has been sent to the i-HA through
the VPN tunnel (or indeed, even after a reply for the latter
registration has been received). This case is dealt with by ordinary
Mobile IPv4 means.
A.2 Connection setup for access mode 'fvc'
The diagram below illustrates connection setup in access mode fvc.
MN-APP MN x-FA x-HA VPN i-HA CN
! ! ! ! ! ! !
! ! -------> ! ! ! ! !
Vaarala (Ed.) Expires December 27, 2003 [Page 38]
Internet-Draft MIPv4-VPN June 2003
! ! rrq ! -------> ! ! ! !
! ! ! rrq ! ! ! !
! ! ! ! ! ! !
! ! -------> ! ! ! ! !
! ! rrq ! -----------------------------X ! !
! ! ! rrq ! ! ! !
! ! ! ! ! ! !
! ! ! <------- ! ! ! !
! ! <------- ! rrp ! ! ! !
! ! rrp ! ! ! ! !
! ! ! ! ! ! !
! [wait for detection period for response from i-HA] !
! [may also retransmit to i-HA, depending on config] !
! ! ! ! ! ! !
! ! -------> ! ! ! ! !
! ! ike ! ==(1)==> ! ! ! !
! ! ! ike ! -------> ! ! !
! ! ! ! ike ! ! !
! ! ! ! <------- ! ! !
! ! ! <==(1)== ! ike ! ! !
! ! <------- ! ike ! ! ! !
! ! ike ! ! ! ! !
: : : : : : :
: : : : : : :
! ! ! ! ! ! !
! ! ! ! ! ! !
! ! ==(1)==> ! ! ! ! !
! ! rrq ! ==(2)==> ! ! ! !
! ! ! rrq ! ==(1)==> ! ! !
! ! ! ! rrq ! -------> ! !
! ! ! ! ! rrq ! !
! ! ! ! ! <------- ! !
! ! ! ! <==(1)== ! rrp ! !
! ! ! <==(2)== ! rrp ! ! !
! ! <==(1)== ! rrp ! ! ! !
! ! rrp ! ! ! ! !
! ! ! ! ! ! !
! [[--- connection setup ok, bidirectional connection up ---]] !
! ! ! ! ! ! !
! -------> ! ! ! ! ! !
! pkt ! ==(2)==> ! ! ! ! !
! ! pkt ! ==(3)==> ! ! ! !
! ! ! pkt ! ==(2)==> ! ! !
! ! ! ! pkt ! ==(1)==> ! !
! ! ! ! ! pkt ! -------> !
! ! ! ! ! ! pkt !
! ! ! ! ! ! <------- !
! ! ! ! ! <==(1)== ! pkt !
Vaarala (Ed.) Expires December 27, 2003 [Page 39]
Internet-Draft MIPv4-VPN June 2003
! ! ! ! <==(2)== ! pkt ! !
! ! ! <==(3)== ! pkt ! ! !
! ! <==(2)== ! pkt ! ! ! !
! <------- ! pkt ! ! ! ! !
! pkt ! ! ! ! ! !
: : : : : : :
: : : : : : :
Vaarala (Ed.) Expires December 27, 2003 [Page 40]
Internet-Draft MIPv4-VPN June 2003
Appendix B. Changes
Changes from -01 to -02:
o Packet flow examples added.
o Explicit IDS reference added.
o Requirement levels adjusted; NAT traversal requirements changed
from MUST to SHOULD and other changes.
o MN no longer required to use i-HA directly whenever available (in
some cases that may not be desired).
o IPR section revised.
o Latency considerations section added.
o External HA reachability assumption refined; if firewall properly
configured, handover performance can be improved. This is now
mentioned in the detection section.
o Overhead section simplified, only base solution discussed.
o Proposed solutions section removed from appendix.
o Strawmen of optimizations removed from appendix, references to
optimizations removed from text.
Changes from -00 to -01:
o First description of proposed solution based on basic and
optimized dual HA drafts, as well as IPsec endpoint update
mechanism.
o List of proposed solutions in -00 included in appendix.
Vaarala (Ed.) Expires December 27, 2003 [Page 41]
Internet-Draft MIPv4-VPN June 2003
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Vaarala (Ed.) Expires December 27, 2003 [Page 42]
