Internet Engineering Task Force G. Montenegro
INTERNET DRAFT V. Gupta
Sun Microsystems, Inc.
September 13, 1996
Firewall Support for Mobile IP
draft-montenegro-firewall-sup-00.txt
Status of This Memo
This document is a submission to the Mobile IP Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
either to the author, or to the mobile-ip@SmallWorks.COM mailing
list.
Distribution of this memo is unlimited.
This document is an Internet-Draft. Internet-Drafts are working
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Abstract
The Mobile IP specification establishes the mechanisms that enable a
mobile host to maintain and use the same IP address as it changes
its point of attachment to the network. Mobility implies higher
security risks than static operation, because the traffic may at
times take unforeseen network paths with unknown or unpredictable
security characteristics. The Mobile IP specification uses
cryptographic techniques to authenticate the parties involved in the
registration protocol. However, it makes no provisions for securing
data traffic. The mechanisms described in this document allow a
mobile node out on a public sector of the network to negotiate
access past a SKIP firewall, and construct a secure channel into its
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home network.
1. Introduction
This document specifies what support is required at a firewall to
allow Mobile IP [1] hosts access into a private network from the
Internet. For example, a company employee could attach his/her
laptop to some Internet access point by:
a) Dialing into a PPP/SLIP account on an Internet service
provider's network.
b) Connecting into a 10Base-T or similar LAN network available
at, for example, an IETF terminal room, a local university,
or another company's premises.
Notice that in these examples, the mobile node's relevant interface
(PPP or 10Base-T) is configured with an IP address different from
that which it uses "normally" (i.e. at the office). Furthermore, the
IP address used is not necessarily a fixed assignment. It may be
assigned temporarily and dynamically at the beginning of the session
(e.g. by IPCP in the PPP case, or DHCP in the 10Base-T case).
The following discussion assumes a network configuration consisting
of a private network separated by a firewall from the general
Internet or public network. The systems involved are:
Private Network
A protected network separated from the Internet by hosts
enforcing access restrictions (firewalls).
Public Network
The Internet at large. Hosts are able to communicate with each
other throughout the public network without firewall-imposed
restrictions.
Mobile Node (MN)
Its permanent address falls within the range of the private
network. The user may remove the system from its home
network, and connects it to the Internet at another point.
The mechanisms outlined in this discussion render this
mobility transparent: the mobile node continues accessing
its home network and its resources exactly as if it were
still within it. Notice that when the mobile node leaves its
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home network, it may migrate both within and outside of the
private network's boundaries. As defined by Mobile IP [1], a
mobile node uses a care-of address while roaming.
Pop-up
A mobile node whose care-of address is an address associated
to one of its own interfaces. This address may be a temporary
address acquired dynamically (e.g. by means of DHCP or PPP's
IPCP), or through manual intervention. It may also be a
permanent address assigned to one of the mobile node's
interfaces (e.g. a permanent PPP address). Since pop-ups do
not require a separate foreign agent, they can operate in
foreign nets that lack Mobile IP support.
Home Agent (HA) for the mobile node
Serves as a location registry and router as described in the
Mobile IP IETF draft.
Foreign Agent (FA)
Serves as a registration relayer and care of address for the
mobile node as described in the Mobile IP IETF draft.
Correspondent Host (CH)
A system that is exchanging data packets with the mobile
node. Its communication with the mobile node does not change
regardless of the latter's current point of attachment to the
network.
Firewall (FW)
The system (or collection of systems) that enforces access
control between the private network and the general Internet.
It may do so by a combination of functions such as application
gatewaying, packet filtering and cryptographic techniques.
The mechanisms described in this document allow a mobile node out on
a public sector of the network to negotiate access past a SKIP
firewall, and construct a secure channel into its home network.
This enables it to communicate with correspondent hosts that belong
to the private network, and, if bi-directional tunnels are used,
with external hosts that are reachable when the mobile node is at
home.
This document does not address the scenarion in which the mobile
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node attempts to access its private network, while within another
private network.
Sections 2 and 3 provide an overview of the environment being
considered and the restrictions it imposes. Sections 4 examines
firewall technologies. Section 5 discusses the best mode of
operation of the participating entities from the point of view of
Mobile IP. Section 6 discusses possible configuration for the
secure channel. Finally, packet formats are the topic of of
sections 7 and 8.
2. Mobility without a Firewall
Suppose the mobile node is roaming throughout the general Internet,
but its home network is not protected by a firewall. This is
typically found in academic environment as opposed to corporate
networks.
This works as prescribed by Mobile IP [1]. The only proviso is that
the mobile node would most probably operate as a pop-up instead of
using a separate foreign agent's care-of address. This is because,
at least in the near term, it is far more likely to be able to
secure a temporary care-of-address than it is to find a foreign
agent already deployed at the site you are visiting. For example:
- Internet Service Provider (ISP): pre-assigns customers IP
addresses, or hands them out dynamically via PPP's address
negotiation.
- IETF terminal room: typically pre-assigns addresses for your use
- other places would probably offer DHCP services
3. Restrictions imposed by a Firewall
The firewall imposes restrictions on packets entering or leaving the
private network. Packets are not allowed through unless they conform
to a filtering specification, or unless there is a negotiation
involving some sort of authentication.
Another restriction is imposed by the separation between private
addresses and general Internet addresses. Strictly speaking, this is
not imposed by a firewall, but by the characteristics of the private
network. For example, if a packet destined to an internal address
originates in the general Internet, it will probably not be
delivered. It is not that the firewall drops it. Rather, the
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Internet's routing fabric is unable to process it. This elicits an
ICMP host unreachable packet sent back to the originating node.
Because of this, the firewall must be explicitly targeted as the
destination node by outside packets seeking to enter the private
network. The routing fabric in the general Internet will only see
the public address of the firewall and route accordingly. Once the
packet arrives at the firewall, the real packet destined to a
private address must be recovered.
4. Two Firewall Options: Application relay and IP Security
Before delving into any details, lets examine two technologies which
may provide firewall support for mobile nodes:
- application relaying or proxying, or
- IP Security
To understand the implications, let's examine two specific schemes
to accomplish the above: SOCKS version 5 and SKIP.
4.1 SOCKS version 5 [5]
There is an effort within the authenticated firewall traversal WG
(aft) of the IETF to provide a common interface for application
relays.
The solution being proposed is a revised specification of the SOCKS
protocol. Version 5 has been extended to include UDP services as
well. The SOCKS solution requires that the mobile node -- or
another node on its behalf -- establish a TCP session to exchange
UDP traffic with the FW. It also has to use the SOCKS library to
encapsulate the traffic meant for the FW. The steps required by a
SOCKS solution are:
- TCP connection established to port 1080 (1.5 round trips)
- version identifier/method selection negotiation (1 round trip)
- method-dependent negotiation. For example, the
Username/Password Authentication [6] requires 1 round trip:
1. client sends a Username/Password request
2. FW (server) responds
The GSS-API negotiation [7] requires at least 3 round
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trips:
1. client context establishment (at least 1 round trip)
2. client initial token/server reply (1 round trip)
3. message protection subnegotiation (at least 1 round trip)
- (finally) SOCKS request/reply (1 round trip)
This is a minimum of 4 (6 with GSS-API) round-trips before the
client is able to pass data through the FW using the following
header:
+----+------+------+----------+----------+----------+
|RSV | FRAG | ATYP | DST.ADDR | DST.PORT | DATA |
+----+------+------+----------+----------+----------+
| 2 | 1 | 1 | Variable | 2 | Variable |
+----+------+------+----------+----------+----------+
Bear in mind that the above must be done each time the mobile
registers a new care-of address. In addition to this inefficiency,
this scheme requires that we use UDP to encapsulate IP datagrams.
There is at least one commercial network that does this, but it is
not the best solution.
This header contains the relay information needed by all parties
involved to reach those not directly reachable.
4.2 SKIP [4]
Alternatively, traffic from the mobile node to the firewall could be
encrypted and authenticated using a session-less IP security
mechanism like SKIP. This obviates the need to set up a TCP session
just to exchange UDP traffic with the firewall.
A solution based on SKIP is very attractive in this scenario, as no
round trip times are involved before the mobile node and the
firewall achieve mutual trust: the firewall can start relaying
packets for the mobile node as soon as it receives the first one.
This, of course, implies that SKIP is being used with AH [10] so
that authentication information is contained in each packet.
Encryption by using ESP [9] is also assumed in this scenario, since
the Internet at large is considered a hostile environment. An ESP
transform that provides both authentication and encryption could be
used, in which case the AH header need not be included.
The firewall and the mobile node must both be previously configured
with the authenticated Diffie-Hellman public values for each other.
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Strictly speaking, they could obtain them in real-time, using any of
the mechanisms defined by the SKIP protocol (online certificate
directory service or certificate discovery protocol). However, in
order to avoid round-trip times, it is preferable to manually
distribute each other's public keys to the the mobile node and the
SKIP firewall. This is part of the administrative process of
enabling a mobile node to roam in the general Internet. Home agents
and the firewall must also have access to each others public keys.
There are other proposals besides SKIP to achieve IP layer
security. However, they are session-oriented key management
solutions, and typically imply negotiations spanning several
round-trip times before cryptographically secure communications are
possible. In this respect they raise similar concerns to those
outlined previously in the discussion on SOCKS-based solutions.
Others have arrived at similar conclusions regarding the importance
of session-less key management for Mobile IP applications [12].
Another advantage of SKIP is its support for nomadic applications.
Typically, two hosts communicating via a secure IP layer channel use
the IP source and destination addresses on incoming packets to
arrive at the appropriate security association. The SKIP header can
easily supersede this default mechanism by including the key ID the
recipient must use to obtain the right certificate. The access
control entry for a nomadic host at a SKIP firewall is a so-called
"nomadic" entry, which is filtered by key ID, instead of by IP
source address, as is the usual case. It basically translates to
"allow access from "any IP source address" if "keyID=<fixed
value>". Incoming packets MUST have an AH header, so that after
properly authenticating them, the firewall establishes a "current
address" for the nomadic host. This information determines which
key should be used when encrypting outgoing packets [13].
Notice that this supports Mobile IP, because the mobile node always
initiates contact, so the SKIP firewall always has a chance to learn
the "current address" to use when encrypting an outgoing packet.
However, this precludes the use of simultaneous bindings by a mobile
node. At the firewall, the last Registration Request sent by the
mobile node replaces the association between its permanent address
and any prior care-of address. In order to support simultaneous
bindings the firewall must be able to interpret Mobile IP
registration messages.
If in addition to registration messages, the firewall understands
the Mobile IP encapsulation process, it is possible to use Unsigned
Diffie-Hellman public values [14]. Doing so greatly reduces SKIP's
infrastructure requirements, because there is no need for a
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Certificate Authority. Of course, for this to be possible the
principals' names MUST be securely communicated.
Section 7.2.2 discusses another advantage of making the firewall
understand Mobile IP packet formats.
In what follows we assume a SKIP-based solution.
5. Supporting Mobile IP: Agents and Mobile Node Configurations
The Mobile IP protocol specifies two ways that a mobile node can
register a mobility binding with its home agent, depending on which
address it uses as its tunnel endpoint:
a) an address advertised for that purpose by the foreign agent
b) an address belonging to one of the mobile node's interfaces
(i.e. pop-up operation).
From the firewall's point of view, the main difference between these
two cases hinges on which node prepares the outermost encrypting
encapsulation. The firewall must be able to obtain the public keys
of the node that creates the outermost SKIP header in an incoming
packet. This is only possible to guarantee in case "b", because the
mobile node and the firewall both belong to the same administrative
domain. The problem is even more apparent when the mobile node
attempts a registration request. Here, the foreign agent is not just
a relayer, it actually examines the packet sent by the mobile node,
and modifies its agent services accordingly. In short, assuming the
current specification of Mobile IP and the current lack of trust in
the internet at large, only case "b" is possible. Case "a" would
require an extension (e.g. a "relay" registration request), and
modifying code at the home agent, the firewall and the foreign
agent.
Assuming that the firewall offers a secure relay service (i.e.
decapsulation and forwarding of packets), the mobile node can reach
addresses internal to the private network by encapsulating the
packets in a SKIP header and directing them to the firewall.
Therefore, It is simplest to assume that the mobile node operates as
a pop-up.
6. Supporting Mobile IP: Secure Channel Configurations
The mobile node participates in two different types of traffic:
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Mobile IP registration protocol and regular data. For the sake of
simplicity, the following discussion evaluates different secure
channel configurations by examining the initial registration request
sent by the mobile node to its home agent.
Assuming the mobile node operates as a pop-up, it can talk directly
to the firewall. The latter is able to reach the home agent in the
private network. Also, the firewall must be able to authenticate the
mobile node.
The following channel configurations assume the mobile node is
operating in pop-up mode. The region between the HA (home agent) and
the FW (firewall) is a private network. The region between the FW
and the MN (mobile node) is the outside or public network.
6.1 Option 1: Encryption only Outside of Private Network
HA FW MN
<=====================> SKIP (AH + ESP)
<-----------------------------------> Data path
The traffic is only encrypted between the mobile node out on the
general Internet, and the firewall's external interface. This is
minimum required. It is the most desirable configuration as the more
expensive encrypted channel is only used where it is necessary: on
the public network.
6.2 Option 2: End-to-End Encryption
Another possible configuration extends the encrypted tunnel through
the FW:
HA FW MN
<===================================> SKIP (AH + ESP)
<-----------------------------------> Data path
This limits the FW to perform a simple packet relay or gatewaying
function. Even though this could be accomplished by using the proper
destination NSID in the packet, in practice it is probably
unrealizable. The reason is that this alternative is probably not
very popular with computer security personnel, as the authentication
functions are now being carried out by the HA, whose security is
potentially much weaker than the FW operated by computer security
personnel.
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6.3 Option 3: End-to-End Encryption, Intermediate Authentication
A third alternative is to allow the FW to be party to the security
association with the mobile node for *authentication* purposes only
(AH header), and then forward the encrypted packet (ESP hdr) to the
HA.
HA FW MN
<+++++++++++++++++++++> SKIP authentication
<===================================> SKIP encryption
<-----------------------------------> Data path
The SKIP specification refers to this as "Intermediate
Authentication with End-to-End security using SKIP" [4]:
"Using SKIP, intermediate authentication of end-to-end
protected IP traffic MAY be realized, if participating
principals can share their long-term private keys with the
intermediate node. This may not be desirable if the long-term
keys belong to individual users, because of privacy related
concerns,..."
Whereas Option 2 above is probably not agreeable to security and
system administration personnel, option 3 is unsavory to the end
user.
6.4 Option 4: Encryption Inside and Outside
HA FW MN
<============><=====================> SKIP (AH + ESP)
<-----------------------------------> Data path
Traffic is encrypted on the public as well as on the private
network. On the public network, encryption is dictated by a security
association between the mobile node and the firewall. On the
private network, it is dictated by a security association between
the home agent and the firewall.
6.5 Choosing a Secure Channel Configuration
A potential problem in both options 2 and 3 is that their end-to-end
channel components assume that the mobile node and the home agent
have reachability to each other. This is generally not the case, as
the Internet routing fabric may not have routes to addresses that
belong to private networks, and the private routing fabric may
ignore how to route to public addresses -- or doing so may be
administratively restricted. Therefore, it is necessary for packets
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to be addressed directly to the firewall, and indirectly -- via some
tunneling or relaying capability -- to the real destination on the
other side of the firewall.
Options 1 and 4 are essentially equivalent. The latter may be
considered overkill, because it uses encryption even within the
private network, and this is generally not necessary. What is
necessary even within the private network is for the home agent to
add an encapsulation (not necessarily encrypted) so as to direct
datagrams to the mobile node via the firewall. How this
encapsulation is achieved is the difference between options 1 and
4. Option 4 uses SKIP, while option 1 uses a cleartext
encapsulation mechanism. This is obtainable by, for example, using
IP in IP encapsulation [2], or by use of a currently undefined null
transform in the SKIP header.
Options 1 and 4 are mostly interchangeable, except in pathologically
paranoid private networks. For example, option 1 allows a malicious
node operation from within the private network to launch a chosen
plaintext attack, by sending data through the firewall.
In the interest of being conservative, in what follows we assume
option 4 (i.e. traffic is encrypted on the general Internet, as well
as within the private network.
Since the firewall is party to the security associations governing
encryption on both the public and private networks, it is always
able to inspect the traffic being exchanged by the home agent and
the mobile node. If this is of any concern, the home agent and
mobile node could set up a bi-directional tunnel [8] and encrypt
it.
7. Mobile IP Registration Procedure with a SKIP Firewall
When roaming within a private network, a mobile node sends
registration requests directly to its home agent. On the public
Internet, it MUST encapsulates the original registration request in
a SKIP packet destined to the firewall. The mobile node MUST
distinguish between "inside" and "outside" addresses. This could be
accomplished by a set of rules defining the address ranges.
Nevertheless, actual installations may present serious difficulties
in defining exactly what is a private address and what is not.
Because of this, errors in judgement are to be expected.
Accordingly, the firewall SHOULD be configured such that it will
still perform its relaying duties even if they are unnecessarily
required by a mobile node with an inside care-of address.
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Upon arriving at a foreign net and acquiring a care-of address, the
mobile node must first -- before any data transfer is possible --
initiate a registration procedure. This consists of an authenticated
exchange by which the mobile node informs its home agent of its
current whereabouts (i.e. its current care-of address), and receives
an acknowledgement. This first step of the protocol is very
convenient, because the SKIP firewall can use it to dynamically
configure its packet filter.
The remainder if this section shows the packet formats used.
Section 7.1 discusses how a mobile node sends a Registration Request
to its home agent via the SKIP firewall. Section 7.2 discusses how
the home agent send the corresponding Registration Reply to the
mobile node.
7.1. Registration Request from the Mobile Node to the Home Agent via
the SKIP Firewall
The mobile node arrives at a foreign net, and using mechanisms
defined by Mobile IP, discovers it has moved away from home. It
acquires a local address at the foreign site, and composes a
registration request meant for its HA. It must decide whether this
packet needs to be processed by SKIP or not.
This is not a simple rule triggered by a given destination address.
It must be applied whenever the following conditions are met:
a) the mobile node is using a care-of address that does not
belong to the private network (i.e. the mobile node is
"outside" its private network), and
b) either of:
b1) the source address of the packet is the mobile node's
home address, or
b2) the source address of the packet is the care-of
address and the destination address belongs to the
private network
Since the above conditions are mobility related, it is best for the
Mobile IP function in the node to evaluate them, and then -- using a
special API -- request the appropriate security services from SKIP.
The SKIP module must use the firewall destination address and the
firewall's certificate in order to address and encrypt the packet.
It encrypts it using SKIP combined with the ESP [9] protocol and
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possibly the AH [10] protocol. The SKIP header's NSID fields
indicate that the Master Key-ID for the source is that of the mobile
node's home address, even though the packet's source address
corresponds to the care-of address -- an address whose corresponding
public key is unknown to the firewall.
The SKIP Firewall's dynamic packet filtering uses this information
to establish a dynamic mapping between the care-of address and the
mobile node's Master Key-ID.
The destination NSID field is zero, prompting the firewall to
process the SKIP header and recover the internal packet. It then
delivers the original packet to another outbound interface, because
it is addressed to the home agent (an address within the private
network). Assuming secure channel configuration number 4, the
firewall will encrypt the packet using SKIP before forwarding to the
home agent.
PACKET FORMAT 1:
+---------------+----------+----+-----+--------------+--------------+
| IP Hdr (SKIP) | SKIP Hdr | AH | ESP | Inner IP Hdr | Reg. Request |
+---------------+----------+----+-----+--------------+--------------+
IP Hdr (SKIP):
Source mobile node's care-of address
Destination public (outside) address on the firewall
SKIP Hdr:
Source NSID = 1
Master Key-ID = IPv4 address of the mobile node
Destination NSID = 0
Master Key-ID = none
Inner IP Hdr:
Source mobile node's care-of address
Destination home agent's address
7.2. Registration Reply from the Home Agent to the Mobile Node via the
SKIP Firewall
The home agent processes the registration request, and composes a
registration reply. Before responding, it examines the care-of
address reported by the mobile node, and determines whether or not
it corresponds to an outside address. If so, the home agent needs
to send all traffic back through the firewall. The home agent can
accomplish this by encapsulating the original registration reply in
a SKIP packet destined to the firewall (i.e. we assume secure
channel configuration number 4).
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7.2.1. On the Inside (Private) Network
The packet from the home agent to the mobile node via the SKIP
Firewall has the same format as shown above. The relevant fields
are:
PACKET FORMAT 2:
+---------------+----------+----+-----+--------------+------------+
| IP Hdr (SKIP) | SKIP Hdr | AH | ESP | Inner IP Hdr | Reg. Reply |
+---------------+----------+----+-----+--------------+------------+
IP Hdr (SKIP):
Source home agent's address
Destination private (inside) address on the firewall
SKIP Hdr:
Source NSID = 0
Master Key-ID = none
Destination NSID = 0
Master Key-ID = none
Inner IP Hdr:
Source home agent's address
Destination mobile node's care-of address
7.2.2. On the Outside (Public) Network
The SKIP Firewall recovers the original registration reply packet
and looks at the destination address: the mobile node's care-of
address.
The SKIP Firewall's dynamic packet filtering used the initial
registration request (Secton 7.1) to establish a dynamic mapping
between the care-of address and the mobile node's Master Key-ID.
Hence, before forwarding the registration reply, it encrypts it
using the mobile node's public key.
This dynamic binding capability and the use of tunneling mode ESP
obviate the need to extend the Mobile IP protocol with a "relay
registration request". However, it requires that the Registration
Reply exit the private network through the same firewall that
forwarded the corresponding Registration Request.
Instead of obtaining the mobile node's permanent address from the
dynamic binding, a Mobile IP aware firewall could also obtain it
from the Registration Reply itself. This renders the firewall
stateless, and lets Registration Requests and Replies traverse the
periphery of the private network through different firewalls.
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PACKET FORMAT 3:
+---------------+----------+----+-----+--------------+------------+
| IP Hdr (SKIP) | SKIP Hdr | AH | ESP | Inner IP Hdr | Reg. Reply |
+---------------+----------+----+-----+--------------+------------+
IP Hdr (SKIP):
Source firewall's public (outside) address
Destination mobile node's care-of address
SKIP Hdr:
Source NSID = 0
Master Key-ID = none
Destination NSID = 1
Master Key-ID = IPv4 addr of the mobile node
Inner IP Hdr:
Source home agent's address
Destination mobile node's care-of address
8. Data Transfer
Data transfer proceeds along lines similar to the registration
request outlined above. Section 8.1 discusses data traffic sent by
a mobile node to a correspondent host. Sections 8.2, 8.3 and 8.4
show three alternative packet formats for the reverse traffic from
the home agent to the mobile node.
8.1. Data Packet From the Mobile Node to a Correspondent Host
The mobile node composes a packet destined to a correspondent host
located within the private network.
The Mobile IP function in the mobile node examines the Inner IP
header, and determines that it satisfies conditions "a" and "b1"
from Section 7.1. The mobile node requests the proper encryption and
encapsulation services from SKIP.
Thus, the mobile node in pop-up mode sends encrypted traffic to the
firewall, using the following format:
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PACKET FORMAT 4:
+---------------+----------+----+-----+--------------+------+
| IP Hdr (SKIP) | SKIP Hdr | AH | ESP | Inner IP Hdr | ULP |
+---------------+----------+----+-----+--------------+------+
IP Hdr (SKIP):
Source mobile node's care-of address
Destination public (outside) address on the firewall
SKIP Hdr:
Source NSID = 1
Master Key-ID = IPv4 address of the mobile node
Destination NSID = 0
Master Key-ID = none
Inner IP Hdr:
Source mobile node's home address
Destination correspondent host's address
The SKIP Firewall intercepts this packet, decrypts the Inner IP Hdr
and upper-layer packet (ULP) and checks the destination address.
Since the packet is destined to a correspondent host in the private
network, the "Inner" IP datagram is delivered internally. Once the
SKIP firewall injects this packet into the private network, it is
routed independently of its source address.
As this last assumption is not always true, the mobile node may
construct a bi-directional tunnel [8] with its home agent. Doing
so, guarantees that the "Inner IP Hdr" is:
Inner IP Hdr:
Source mobile node's home address
Destination home agent address
When at home, communication between the the mobile node and certain
external correspondent hosts might need to go through
application-specific firewalls or proxies, different from the SKIP
firewall. When on the public network, the mobile node's
communication with these hosts, MUST use a bi-directional tunnel.
8.2. Data Packet Tunneled by the Home Agent to the Mobile Node
The home agent intercepts a packet from a correspondent host to the
mobile node. It encapsulates it such that the Mobile IP header's
source and destination addresses are the home agent and care-of
addresses, respectively. This would suffice for delivery within the
private network. Since the current care-of address of the mobile
node is not within the private network, this packet must be sent via
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the firewall. The home agent can accomplish this by encapsulating
the datagram in a SKIP packet destined to the firewall (i.e. we
assume secure channel configuration number 4).
8.2.1 Within the Inside (Private) Network
From the home agent to the private (inside) address of the
firewall the packet format is:
PACKET FORMAT 5:
+--------+------+----+-----+--------+--------+-----+
| IP Hdr | SKIP | AH | ESP | mobip | Inner | ULP |
| (SKIP) | Hdr | | | IP Hdr | IP Hdr | |
+--------+------+----+-----+--------+--------+-----+
IP Hdr (SKIP):
Source home agent's address
Destination private (inside) address on the firewall
SKIP Hdr:
Source NSID = 0
Master Key-ID = none
Destination NSID = 0
Master Key-ID = none
Mobile-IP IP Hdr:
Source home agent's address
Destination care-of address
Inner IP Hdr:
Source correspondent host's address
Destination mobile node's address
ULP: upper-layer packet
The SKIP firewall intercepts the packet and recovers the
Mobile IP encapsulated packet. Before sending out this
packet, the dynamic packet filter configured by the original
registration request above triggers encryption of this packet,
this time by the SKIP firewall for consumption by the mobile node.
The packet format above does not require the firewall to have a
dynamic entry. The association between the mobile node's
permanent address and it care-of address can be deduced from the
contents of the "Mobile-IP IP Hdr" and the "Inner IP Hdr".
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The home agent MAY eliminate the Mobile IP header if it
discriminates between mobile nodes that are registered within the
private network, and those that are outside. In this case, the
resultant packet is:
PACKET FORMAT 6:
+---------------+----------+----+-----+--------------+-----+
| IP Hdr (SKIP) | SKIP Hdr | AH | ESP | Inner IP Hdr | ULP |
+---------------+----------+----+-----+--------------+-----+
IP Hdr (SKIP):
Source home agent's address
Destination private (inside) address on the firewall
SKIP Hdr:
Source NSID = 0
Master Key-ID = none
Destination NSID = 0
Master Key-ID = none
Inner IP Hdr:
Source correspondent host's address
Destination mobile node's address
The SKIP firewall decrypts the packet, and recovers the original
datagram. Before forwarding it, it examines its ruleset. It finds a
dynamic rule which was added in response to the Registration Request
sent by the mobile node. Accordingly, the SKIP firewall encrypts the
packet again before forwarding to the mobile node's care-of
address.
This optimization requires that the firewall keep some state in the
form of the aforementioned dynamic entry for the mobile node.
8.2.2. On the Outside (Public) Network
The SKIP firewall intercepts the packet, and--assuming packet format
5--recovers the Mobile IP encapsulated datagram. Before sending it
out, the dynamic packet filter configured by the original
Registration Request triggers encryption of this packet, this time
by the SKIP firewall for consumption by the mobile node. The
resultant packet is:
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PACKET FORMAT 7:
+--------+------+----+-----+--------+--------+-----+
| IP Hdr | SKIP | AH | ESP | mobip | Inner | ULP |
| (SKIP) | Hdr | | | IP Hdr | IP Hdr | |
+--------+------+----+-----+--------+--------+-----+
IP Hdr (SKIP):
Source firewall's public (outside) address
Destination mobile node's care-of address
SKIP Hdr:
Source NSID = 0
Master Key-ID = none
Destination NSID = 1
Master Key-ID = IPv4 address of the mobile node
Mobile-IP IP Hdr:
Source home agent's address
Destination care-of address
Inner IP Hdr:
Source correspondent host's address
Destination mobile node's address
If the firewall receives a packet in format 6, the outgoing
datagram to the mobile node is:
PACKET FORMAT 8:
+---------------+----------+----+-----+--------------+-----+
| IP Hdr (SKIP) | SKIP Hdr | AH | ESP | Inner IP Hdr | ULP |
+---------------+----------+----+-----+--------------+-----+
IP Hdr (SKIP):
Source public (outside) address on the firewall
Destination mobile node's care-of address
SKIP Hdr:
Source NSID = 0
Master Key-ID = none
Destination NSID = 1
Master Key-ID = IPv4 address of the mobile node
Inner IP Hdr:
Source correspondent host's address
Destination mobile node's address
As an optimization, the firewall MAY produce packet format 8 even if
the packet it receives from the home agent is in format 5. This is
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possible even if the firewall lacks state in the form of a dynamic
binding.
At the mobile node, SKIP processes the packets sent by the
firewall. Eventually, the inner IP header and the upper-layer
packet (ULP) are retrieved and passed on.
9. Security Considerations
The topic of this document is security. Nevertheless, it is
imperative to point out the perils involved in allowing a flow of IP
packets through a firewall. In essence, the mobile host itself MUST
also take on responsibility for securing the private network,
because it extends its periphery. This does not mean it stops
exchanging unencrypted IP packets with hosts on the public network.
For example, it MAY have to do so in order to satisfy billing
requirements imposed by the foreign site, or to renew its DHCP
lease. In the latter case it might filter not only on IP source
address, but also on protocol and port numbers.
Therefore, it MUST have some firewall capabilities, otherwise, any
malicious individual that gains access to it will have gained access
to the private network as well.
Acknowledgements
Ideas in this document have benefited from discussions with at
least the following people: Bill Danielson, Martin Patterson, Tom
Markson, Rich Skrenta, Atsushi Shimbo, Behfar Razavi, Avinash
Agrawal, Tsutomu Shimomura and Don Hoffman.
References
[1] C. Perkins. IP Mobility Support. Internet Draft -- work in
progress, May 1996.
[2] C. Perkins. IP Encapsulation within IP. Internet Draft --
work in progress, May 1996.
[3] C. Perkins. Minimal Encapsulation within IP. Internet Draft
-- work in progress, May 1996.
[4] A. Aziz, T. Markson, H. Prafullchandra. Simple Key-Management
For Internet Protocols (SKIP). Internet Draft -- work in
progress, August 14, 1996.
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INTERNET DRAFT Firewall Support for Mobile IP September 1996
[5] M. Leech, M. Ganis, Y. Lee, R. Kuris, D. Koblas and . Jones.
SOCKS Protocol Version 5. RFC 1928, March 1996.
[6] M. Leech. Username/Password Authentication for SOCKS V5. RFC
1929, March 1996.
[7] P V McMahon. GSS-API Authentication Method for SOCKS Version
5. Internet Draft -- work in progress, July 1995.
[8] G. Montenegro. Bi-directional Tunneling for Mobile IP.
Internet Draft -- work in progress, September 1996.
[9] R. Atkinson. IP Encapsulating Payload. RFC 1827, August 1995
[10] R. Atkinson. IP Authentication Header. RFC 1826, August
1995.
[11] A. Aziz, M. Patterson. "Design and Implementation of SKIP".
Internet Commerce Group white paper ICG-95-004, June 28, 1995.
[12] Stephen Kent, message to the IETF's IPSEC mailing list,
Message-Id: <v02130500ae569a3e904e@[128.89.30.29]>, September
6, 1996.
[13] Tom Markson, private communication, June 12, 1996.
[14] A. Aziz, T. Markson, H. Prafullchandra. Encoding of an Unsigned
Diffie-Hellman Public Value. Internet Draft -- work in
progress, August 14, 1996.
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Author's Address
Gabriel E. Montenegro
Sun Microsystems, Inc.
2550 Garcia Avenue
Mailstop UMPK 15-214
Mountain View, California 94043-1100
Tel: (415)786-6288
Fax: (415)786-6445
gabriel.montenegro@Eng.Sun.COM
Vipul Gupta
Sun Microsystems, Inc.
2550 Garcia Avenue
Mailstop UMPK 15-214
Mountain View, California 94043-1100
Tel: (415)786-3614
Fax: (415)786-6445
vipul.gupta@Eng.Sun.COM
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