NSIS Working Group M. Stiemerling
Internet-Draft NEC
Expires: August 16, 2004 H. Tschofenig
Siemens
M. Martin
NEC
C. Aoun
Nortel Networks
February 16, 2004
NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
draft-ietf-nsis-nslp-natfw-01
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This memo defines the NSIS Signaling Layer Protocol (NSLP) for
Network Address Translators and Firewalls. The network scenarios,
problems and solutions for path-coupled Network Address Translator
and Firewall signaling are described. The overall architecture is
given by the framework and requirements defined by Next Steps in
Signaling (NSIS) working group. This is one of two NSIS Signaling
Layer Protocols (NSLPs) the working group will address during its
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work.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Terminology and Abbreviations . . . . . . . . . . . . . . 6
1.2 Middleboxes . . . . . . . . . . . . . . . . . . . . . . . 7
1.3 General Scenario for NATFW Traversal . . . . . . . . . . . 9
2. Network Environment . . . . . . . . . . . . . . . . . . . 11
2.1 Network Scenarios for Protocol Functionality . . . . . . . 11
2.1.1 Firewall traversal . . . . . . . . . . . . . . . . . . . . 11
2.1.2 NAT with two private Networks . . . . . . . . . . . . . . 12
2.1.3 NAT with private network on sender side . . . . . . . . . 13
2.1.4 NAT with private network on receiver side . . . . . . . . 13
2.1.5 Both End Hosts behind twice-NATs . . . . . . . . . . . . . 14
2.1.6 Both End Hosts behind same NAT . . . . . . . . . . . . . . 15
2.1.7 IPv4/v6 NAT with two private networks . . . . . . . . . . 16
2.2 Trust Relationship and Authorization . . . . . . . . . . . 17
2.2.1 Peer-to-Peer Trust Relationship . . . . . . . . . . . . . 17
2.2.2 Intra-Domain Trust Relationship . . . . . . . . . . . . . 18
2.2.3 End-to-Middle Trust Relationship . . . . . . . . . . . . . 19
3. Problems and Challenges . . . . . . . . . . . . . . . . . 21
3.1 Missing Network-to-Network Trust Relationship . . . . . . 21
3.2 End-to-end significance . . . . . . . . . . . . . . . . . 22
3.3 Relationship with routing . . . . . . . . . . . . . . . . 22
3.4 Dynamic state installation and maintenance . . . . . . . . 22
3.5 Affected Parts of the Network . . . . . . . . . . . . . . 22
3.6 NSIS backward compatibility with NSIS unaware NAT and
Firewalls . . . . . . . . . . . . . . . . . . . . . . . . 23
3.7 Authentication and Authorization . . . . . . . . . . . . . 24
3.8 Directional Properties . . . . . . . . . . . . . . . . . . 24
3.9 Routing Asymmetry . . . . . . . . . . . . . . . . . . . . 24
3.10 Addressing . . . . . . . . . . . . . . . . . . . . . . . . 25
3.11 NTLP/NSLP NAT Support . . . . . . . . . . . . . . . . . . 25
3.12 Route changes . . . . . . . . . . . . . . . . . . . . . . 25
3.13 Combining Middlebox and QoS signaling . . . . . . . . . . 26
3.14 Difference between sender- and receiver-initiated
signaling . . . . . . . . . . . . . . . . . . . . . . . . 26
3.15 Inability to know the scenario . . . . . . . . . . . . . . 26
4. NSIS NAT Handling Solution . . . . . . . . . . . . . . . . 28
4.1 Problem Description . . . . . . . . . . . . . . . . . . . 28
4.2 Solution Overview . . . . . . . . . . . . . . . . . . . . 31
4.2.1 Destination IP address Selection . . . . . . . . . . . . . 33
5. Protocol Description . . . . . . . . . . . . . . . . . . . 35
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5.1 Basic protocol overview . . . . . . . . . . . . . . . . . 35
5.2 NATFW NSLP Header . . . . . . . . . . . . . . . . . . . . 37
5.3 NATFW NSLP Objects . . . . . . . . . . . . . . . . . . . . 37
5.3.1 NATFW NSLP Object Header . . . . . . . . . . . . . . . . . 37
5.3.2 NATFW Session ID Object . . . . . . . . . . . . . . . . . 38
5.3.3 Lifetime Object . . . . . . . . . . . . . . . . . . . . . 38
5.3.4 Policy Rule Object . . . . . . . . . . . . . . . . . . . . 38
5.3.5 External Address Object . . . . . . . . . . . . . . . . . 39
5.4 Request Message Formats . . . . . . . . . . . . . . . . . 39
5.4.1 Create Session . . . . . . . . . . . . . . . . . . . . . . 40
5.4.2 Prolong Session . . . . . . . . . . . . . . . . . . . . . 40
5.4.3 Delete Session . . . . . . . . . . . . . . . . . . . . . . 40
5.4.4 Reserve External Address . . . . . . . . . . . . . . . . . 40
5.5 Response Messages . . . . . . . . . . . . . . . . . . . . 41
5.5.1 Return External Address Response . . . . . . . . . . . . . 41
5.5.2 Path Succeeded Response . . . . . . . . . . . . . . . . . 42
5.5.3 Error Response Messages . . . . . . . . . . . . . . . . . 42
5.6 Protocol Operations . . . . . . . . . . . . . . . . . . . 42
5.6.1 Message Handling Overview . . . . . . . . . . . . . . . . 42
5.6.1.1 Reserving Addresses . . . . . . . . . . . . . . . . . . . 44
5.6.1.2 Creating Sessions . . . . . . . . . . . . . . . . . . . . 46
5.6.1.3 Prolonging Session . . . . . . . . . . . . . . . . . . . . 47
5.6.1.4 Deleting Sessions . . . . . . . . . . . . . . . . . . . . 48
6. Solution examples . . . . . . . . . . . . . . . . . . . . 50
6.1 Firewall traversal . . . . . . . . . . . . . . . . . . . . 50
6.2 NAT with private network on sender side . . . . . . . . . 51
6.3 NAT with private network on receiver side . . . . . . . . 52
6.4 Both end hosts are in same private network behind NATs . . 56
6.5 IPv4/v6 NAT with two private networks . . . . . . . . . . 58
6.6 Full example for NAT/FW with two private networks . . . . 58
7. NSIS NAT and Firewall transitions issues . . . . . . . . . 65
8. Security Considerations . . . . . . . . . . . . . . . . . 66
9. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 68
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . 69
Normative References . . . . . . . . . . . . . . . . . . . 70
Informative References . . . . . . . . . . . . . . . . . . 71
Authors' Addresses . . . . . . . . . . . . . . . . . . . . 72
A. Inter-working of SIP with NSIS NATFW NSLP . . . . . . . . 74
A.1 The Session Initiation Protocol . . . . . . . . . . . . . 74
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A.2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . 79
B. Ad-Hoc networks . . . . . . . . . . . . . . . . . . . . . 80
C. Interworking of Security Mechanisms and NSIS NATFW NSLP . 81
D. Solution approaches in case of missing authorization . . . 82
D.1 Solution Approach: Local authorization from both end
points . . . . . . . . . . . . . . . . . . . . . . . . . . 82
D.2 Solution Approach: Access Network-Only Signaling . . . . . 83
D.3 Solution Approach: Authorization Tokens . . . . . . . . . 83
E. Acknowledgments . . . . . . . . . . . . . . . . . . . . . 86
Intellectual Property and Copyright Statements . . . . . . 87
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1. Introduction
Firewalls and Network Address Translators (NAT) have been both used
throughout the Internet for many years and they will be present in
future. Using firewalls brings security to networks and in times of
IPv4 address depletion NATs virtually extend IP address space. In
general, both types are obstacles to many applications, since they
only allow specific applications to traverse them (i.e. HTTP
traffic). Other applications, as for instance IP telephony, with
more dynamic properties suffer from firewalls and NATs so that they
don't work at all. Therefore, many applications cannot traverse any
kind of firewall or NAT.
Several solutions to enable any application to traverse those boxes
have been proposed and are currently used. Typically, application
level gateways (ALG) have been integrated and so configuring
firewalls and NATs dynamically. Another approach is middlebox
communication (MIDCOM, currently under standardization at the IETF).
In this approach firewall and NAT external ALGs configure them via
the MIDCOM protocol [6]. Several other work around solutions are
available as well. Anyway, all of these approaches introduce other
problems that are hard to solve; one of them is dependency on
topology issues.
NAT and firewall (NATFW) signaling share a property with Quality of
Service (QoS) signaling, i.e. in both cases it is needed to reach any
device on the data path that is involved in QoS or NATFW treatment of
data packets. Currently, RSVP [13] is used for QoS signaling, but
the conception of a new IP signaling protocol is under work in the
Next Step of Signaling (NSIS) working group. This new signaling
protocol is path-coupled, like RSVP is, and its primary use is QoS
signaling, but NATFW signaling is considered as well.
This memo defines this NATFW path-coupled protocol. The NATFW
signaling protocol is carried over the NSIS Network Transport Layer
Protocol (NTLP, [3]) as NATFW NSIS Signaling Layer Protocol (NSLP).
This NATFW NSLP is used to open pin-holes in firewalls and create NAT
address mappings along the data path, so that subsequent data packets
can traverse those devices.
Traversal of non NATFW NSLPs or the NTLP is out of scope of this
document. Furthermore, only firewalls and NATs are considered in
this document, any other device, for instance IPSec security gateway,
is out of scope.
Section 2 describes the network environment for NATFW NSLP signaling
and highlights the trust relationship/ authorization. Problems and
challenges are listed in section 3, whereas a NSIS NAT handling
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solution is described in section 4. Section 5 describes the protocol
itself and section 6 gives some usage examples.
Readers are recommended to read the NSIS framework [1] and
requirements documents beforehand [2]).
1.1 Terminology and Abbreviations
This document uses terms defined in [2]. Furthermore, these following
terms are used:
o NSIS NAT Forwarding State: The term "NSIS NAT Forwarding State" in
this context refers to a state used to forward the NSIS signaling
message beyond the targeted destination address; that state is
typically used when the NSIS Responder address is not known
o Sender-/Receiver Initiated Signaling
Sender-initiated: NAT bindings and firewall rules are created
immediately when the "path" message hits the NSIS nodes. With
"path" message we refer to the signaling message traveling from
the data sender towards the data receiver.
Receiver-initiated: NAT bindings and firewall rules are created
when the "reserve" message returns from the other end. With
"reserve" message we refer to a signaling message on the
reverse path, this means from the receiver to the sender (i.e.
backwards routed).
Note that these definitions have nothing to do with number of
roundtrips, who performs authorization etc.
o Policy rule: In general, a policy rule is "a basic building block
of a policy-based system. It is the binding of a set of actions
to a set of conditions - where the conditions are evaluated to
determine whether the actions are performed." [RFC3198]. In the
context of NSIS NATFW NSLP the condition is a specification of a
set of packets to which rules are applied. The set of actions
always contains just a single element per rule, and is limited to
either action "reserved" or action "enable".
o Firewall: A packet filtering device that matches packet against a
set of policy rules and applies the actions. In the context of
NSIS NATFW NSLP we refer to this device as firewall.
o Network Address Translator: Network Address Translation is a
method by which IP addresses are mapped from one realm to another,
in an attempt to provide transparent routing to hosts (see [8]).
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Network Address Translator are devices that perform this method.
o Middlebox: from [11]: "A middlebox is defined as any intermediate
device performing functions other than the normal, standard
functions of an IP router on the datagram path between a source
host and a destination host". The term middlebox in context of
this document and in NSIS refers to firewalls and NATs only.
Other types of middlebox are currently outside the scope.
o Security Gateway: IPsec based gateways.
o NSIS Initiator (NI): the signaling entity which makes the resource
request, usually as a result of user application request.
o NSIS Responder (NR): the signaling entity that acts as the final
destination for the signaling and can optionally interact with
applications as well.
o NSIS Forwarder (NF): the signaling entity between an NI and NR
which propagates NSIS signaling further through the network.
o Receiver (DR or R): the node in the network which is receiving the
data packets of a flow.
o Sender (DS or S): the node in the network which is sending the
data packets of a flow.
o NATFW NSLP session: Application layer flow of information for
which some network control state information is to be manipulated
or monitored (as defined in [1]). The control state for NATFW NSLP
is NSLP state and associated policy rules at the middlebox.
o NSIS peer or peer: NSIS node with which a NSIS adjacency has been
created as defined in [3].
o Edge NAT: By edge NAT we refer to the NAT device which is
reachable from outside and has a globally routable IP address.
1.2 Middleboxes
The term middlebox raises different expectations about functionality
provided by such a device. Middleboxes in the scope of this memo are
firewalls that filter data packets against their set of filter rules
and NATs that translate addresses from one address realm to another
address realm. Other types of middleboxes, for instance QoS traffic
shapers and security gateways, are out of scope.
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The term NAT used in this document is placeholder for a range of
different NAT flavors. We consider those types of NATs:
o traditional NAT (basic NAT and NAPT)
o Bi-directional NAT
o Twice-NAT
o Multihomed NAT
For a detailed discussion about each NAT type please see [7].
Both types of middleboxes use policy rules for decision on data
packet treatment. Policy rules consist of a 5-tuple and an
associated action; Data packets matching this 5-tuple experience the
policy rule action. A 5-tuple consists of:
o Source IP address and port number
o Destination IP address and port number
o Transport protocol
Actions for firewalls are usually:
o Allow: forward data packet
o Deny: block data packet and discard it
o Other actions like logging, diverting, etc
Actions for NATs are (amongst many others):
o Change source IP address and port number to an global routeable IP
address and port number.
o Change destination IP address and port number to a private IP
address and port number.
The exact implementation of policy rules and mapping to firewall rule
sets and NAT bindings or sessions at the middlebox is an
implementation issue and thus out of scope of this document.
Some devices entitled as firewalls only accept traffic after
cryptographic verification (i.e. IPsec protected data traffic).
Particularly for network access scenarios either link layer or
network layer data protection is common. Hence we do not address
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these types of devices (referred as security gateways) since per-flow
signaling is rather uncommon in this environment. For a discussion
of network access authentication and associated scenarios the reader
is referred to the PANA working group (see [22]).
Discovering security gateways, which was also mentioned as an
application for NSIS signaling, for the purpose of executing an IKE
to create an IPsec SA, is already solved without requiring NSIS.
In mobility scenarios an often experienced problem is the traversal
of a security gateway at the edge of the corporate network. Network
administrators often rely on the policy that only authenticated data
traffic is allowed to enter the network. A problem statement for the
traversal of these security gateways in the context of Mobile IP can
be found at [21]).
Other proposals for path-coupled NAT and firewall traversal like RSVP
and CASP are described in [23] and [24].
1.3 General Scenario for NATFW Traversal
The purpose of NSIS NATFW signaling is to enable any communication
between endpoints across networks even in presence of middleboxes. It
is expected that those middleboxes are configured in such a way that
NSIS NATFW signaling messages itself are allowed to traverse them.
NSIS NATFW NSLP signaling is used to install such policy rules in all
middleboxes along the data path. Firewalls are configured to forward
data packets matching the policy rule provided by the NSLP signaling.
NATs are configured to translate data packets matching the policy
rule provided by the NSLP signaling.
The basic high-level picture of NSIS usage is that endhosts are
located behind middleboxes (NAT/FW in Figure 1). Applications located
at these endhosts try to establish communication between them and use
NSIS NATFW NSLP signaling to establish policy rules on a data path,
which allows the said data to travel from the endhost to the endpoint
unobstructed. The applications can somehow trigger middlebox
traversal (e.g. via an API call) at the NSIS agent at the local host.
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Application Application Server (0, 1, or more) Application
+----+ +----+ +----+
| +------------------------+ +------------------------+ |
+-+--+ +----+ +-+--+
| |
| NSIS Agents |
+-+--+ +----+ +-----+ +-+--+
| +--------+ +----------------------------+ +-----+ |
+-+--+ +-+--+ +--+--+ +-+--+
| | ------ | |
| | //// \\\\\ | |
+-+--+ +-+--+ |/ | +-+--+ +-+--+
| | | | | Internet | | | | |
| +--------+ +-----+ +----+ +-----+ |
+----+ +----+ |\ | +----+ +----+
\\\\ /////
sender NAT/FW (1+) ------ NATFW (1+) receiver
Figure 1: Generic View on NSIS in a NAT / Firewall case
For running NATFW signaling it is necessary that each firewall and
each NAT involved in the signaling communication runs an NSIS NATFW
agent. There might be several NATs and FWs in various possible
combinations on a path between two hosts. The reader is referred to
Section 2.1 where different scenarios are presented.
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2. Network Environment
2.1 Network Scenarios for Protocol Functionality
This section introduces several scenarios for middleboxes in the
Internet. Middleboxes are located in different locations, i.e. at
Enterprise network borders, within enterprise networks, mobile phone
network gateways, etc. In general, middleboxes are place more
towards the edge of networks and less in network cores. Those
middleboxes are not only either firewall or NAT, but any type of
combination is possible. Thus, combined firewall and NATs are
available.
NSIS NATFW NSLP signaling messages are sent by the NSIS initiator
(NI) via the regular data path to the NSIS responder (NR). On the
data path NATFW NSLP signaling messages reach different NSIS peers
that have the NATFW NSLP implemented. Each NATFW NSLP node processes
the signaling messages according to Section 5 and installs, if
necessary, policy rules for subsequent data packets.
Each following section introduces a different scenario for a
different set of middleboxes and their ordering within the topology.
It is assumed that each middlebox implements the NSIS NATFW NSLP
signaling protocol.
2.1.1 Firewall traversal
This section describes a scenario with firewalls only, no NATs are
involved. Both end hosts are behind a firewall that are connected
via the public Internet. Figure 2 shows the topology. The part
labeled "public" is the Internet connection both firewalls.
+----+ //----\\ +----+
NI -----| FW |---| |------| FW |--- NR
+----+ \\----// +----+
private public private
FW: Firewall
NI: NSIS Initiator
NR: NSIS Responder
Figure 2: Firewall Traversal Scenario
Each firewall on-path must provide traversal service for NATFW NSLP
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in order to permit the NSIS message to reach the other end host. All
firewalls process NSIS signaling and establish appropriate policy
rules, so that the required data packet flow can traverse them.
The difference between this scenario and the following is that only
firewalls are on the path, but no NATs. This has specific
implication concerning the used destination address for path-coupled
signaling message sent by the NSIS initiator to an NSIS responder
hosted behind a NAT.
2.1.2 NAT with two private Networks
Figure 3 shows a scenario with NATs at both ends of the network.
Therefore, each application instance, NSIS initiator and NSIS
responder are behind NATs. The outermost NAT at each side is
connected to the public Internet. The NATs are labeled as MB (for
middlebox), since those devices implement at least NAT-only, but can
implement firewalling as well.
Only two middleboxes MB are shown in Figure 3 at each side, but in
general more than one MB on each side must be considered.
+----+ +----+ //----\\ +----+ +----+
NI --| MB |-----| MB |---| |---| MB |-----| MB |--- NR
+----+ +----+ \\----// +----+ +----+
private public private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 3: NAT with two private networks Scenario
Signaling traffic from NI to NR has to traverse all four middleboxes
on the path and all four middleboxes must be configured properly to
allow NSIS signaling to traverse. The NATFW signaling must configure
all middleboxes and consider any address translation in further
signaling. The sender (NI) has to know the IP address of the receiver
(NR) in advance, otherwise he cannot send a single NSIS signaling
message towards the responder. Note that this IP address is not the
private IP address of the responder. Instead a NAT binding
(including a public IP address) has to be obtained from the NAT which
subsequently allows packets hitting the NAT to be forwarded to the
receiver within the private address realm. This generally requires
further support from an application layer protocol for the purpose of
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discovering and exchanging information. The receiver might have a
number of ways to learn its public IP address and port number and
might need to signal this information to the sender using the
application level signaling protocol.
2.1.3 NAT with private network on sender side
This scenario shows an application instance at the sending node which
is behind one or more NATs (shown as MB). The receiver is located in
the public Internet.
+----+ +----+ //----\\
NI --| MB |-----| MB |---| |--- NR
+----+ +----+ \\----//
private public
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 4: NAT with private network on sender scenario
The traffic from NI to NR has to traverse only middleboxes on the
sender's side. The receiver has a public IP address. The NI sends
its signaling message directly to the address of the NSIS responder.
Middleboxes along the path intercept the signaling messages and
configure the policy rules accordingly.
Note that the data sender does not necessarily know whether the
receiver is behind a NAT or not, hence, it is the receiving side that
has to detect the whether it is behind a NAT or not. As described in
Section 4 NSIS can also provide help for this procedure.
2.1.4 NAT with private network on receiver side
The application instance receiving data is behind one or more NATs.
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//----\\ +----+ +----+
NI ---| |---| MB |-----| MB |--- NR
\\----// +----+ +----+
public private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 5: NAT with private network on receiver Scenario
First, the sender must determine the public IP address of the
receiver. One possibility is that an application level protocol is
used. In this case, the receiver must first fix its public IP
addresses at the middlebox on its side. This information about IP
address and port number could be signaled via an application level
protocol to the actual sender directly or indirectly via a third
party (e.g. proxy). In the scenario, this means the receiver has
first to determine its public IP address (NAT binding) and register
this address with a third party.
The NSIS initiator can start NSIS signaling after he has received
information about the receiver's public IP address and port number.
2.1.5 Both End Hosts behind twice-NATs
This is a special case, where the main problem is to detect that both
nodes are logically within the same address space, also behind a
twice-NAT (see [7] for discussion about twice-NAT functionality).
This scenario primarily addresses performance aspects.
Sender and receiver are both within a private address realm and
potentially have overlapping IP addresses. Figure 6 shows the
ordering of NATs. This is a common configuration in several
networks, particularly after the merging of companies that have use
the same address space, thus having overlapping addresses in many
cases.
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public
+----+ +----+ //----\\
NI --| MB |--+--| MB |---| |
+----+ | +----+ \\----//
|
| +----+
+--| MB |------------ NR
+----+
private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 6: NAT to public, sender and receiver behind twice-NAT
Scenario
The middleboxes shown in Figure 6 are twice-NATs, i.e. they map IP
addresses and port numbers on both sides, at private and public
interfaces.
This scenario requires assistance of application level entities, like
DNS server. Those application level gateways must handle request
that are based on symbolic names and configure the middleboxes so
that data packets are correctly forwarded from NI to NR. The
configuration of those middleboxes may require other middlebox
communication protocols, like MIDCOM [6]. NSIS signaling is not
required in the twice-NAT only case, since the middleboxes of type
twice-NAT are configured by other means. Nevertheless, NSIS
signaling might by useful when there are firewalls in between. In
this case NSIS will not configure any policy rule at twice-NATs, but
will configure policy rules at the intermediate firewalls. The NSIS
signaling protocol must be at least robust enough to survive this
scenario.
2.1.6 Both End Hosts behind same NAT
When NSIS initiator and NSIS responder are behind the same NAT (thus
being in the same address realm, see Figure 7) , they are most likely
not aware of this fact. As in Section 2.1.4 the NSIS responder must
determine its public IP address in advance and transfer it to the
NSIS initiator. Afterwards, the NSIS initiator can start sending the
signaling messages to the responder's public IP address. During this
process, a public IP address will be allocated for the NSIS initiator
at the same middlebox as for the responder. Now, the NSIS signaling
and the subsequent data packets will traverse the NAT twice: from
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initiator to public IP address of responder (first time) and from
public IP address of responder to responder (second time). This is
the worst case, both sender and receiver obtain a public IP address
at the NAT and the communication path is not optimal anymore.
NI public
\ +----+ //----\\
+-| MB |----| |
/ +----+ \\----//
NR
private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 7: NAT to public, both host behind same NAT
NSIS NATFW signaling protocol should support mechanisms to detect
such a scenario. The signaling should directly by exchanged between
NI and NR without involving the middlebox.
2.1.7 IPv4/v6 NAT with two private networks
This scenario combines the usage case mentioned in Section 2.1.2
with the IPv4 to IPv6 transition scenario, i.e. using Network Address
and Protocol Translators (NAT-PT, [10]).
The difference to the other scenarios is the use of IPv6 to IPv4 (and
vice versa) address and protocol translation. Additionally, the base
NTLP must take care of this case for its own functionality of
forwarding messages between NSIS peers.
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+----+ +----+ //---\\ +----+ //---\\ +----+ +----+
NI --| MB |--| MB |--| |--| MB |-| |--| MB |--| MB |-- NR
+----+ +----+ \\---// +----+ \\---// +----+ +----+
private public public private
IPv4 IPv6
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 8: IPv4/v6 NAT with two private networks
This scenario needs the same type of application level support as
described in Section 2.1.5 and so those issues of twice-NATs apply
here as well.
2.2 Trust Relationship and Authorization
Trust relationships and authorization are very important for the
protocol machinery. Trust and authorization are closely related to
each other in the sense that a certain degree of trust is required to
authorize a particular action. For any action (e.g. "create/delete /
prolong policy rules" then authorization is very important due to the
nature of middleboxes.
It is particularly not surprising that different degrees of required
authorization in a QoS signaling environment and middlebox signaling
exist. As elaborated in [19], establishment of a financial
relationship is very important for QoS signaling, whereas for
middlebox signaling is not directly of interest. For middlebox
signaling a stronger or weaker degree of authorization might be
needed.
Different trust relationships that appear in middlebox signaling
environments are described in the subsequent sections. Peer-to-peer
trust relationships are those, which are used in QoS signaling today
and seem to be the simplest. However, there are reasons to believe
that this is not the only type of trust relationship found in today's
networks.
2.2.1 Peer-to-Peer Trust Relationship
Starting with the simplest scenario it is assumed that neighboring
nodes trust each other. The required security association to
authenticate and to protect a signaling message is either available
(manual configuration) or dynamically established with the help of an
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authentication and key exchange protocol. If nodes are located
closely together it is assumed that security association
establishment is easier than establishing it between far distant
node. It is, however, difficult to describe this relationship
generally due to the different usage scenarios and environments.
Authorization heavily depends on the participating entities but for
this scenario it is assumed that neighboring entities trust each
other (at least for the purpose of policy rule creation, maintenance
and deletion). Note that Figure 9 does not illustrate the trust
relationship between the end host and the access network.
+------------------------+ +-------------------------+
| | | |
| Network A | | Network B |
| | | |
| +---------+ +---------+ |
| +-///-+ Middle- +---///////----+ Middle- +-///-+ |
| | | box 1 | Trust | box 2 | | |
| | +---------+ Relationship +---------+ | |
| | | | | |
| | | | | |
| | | | | |
| | Trust | | Trust | |
| | Relationship | | Relationship | |
| | | | | |
| | | | | |
| | | | | |
| +--+---+ | | +--+---+ |
| | Host | | | | Host | |
| | A | | | | B | |
| +------+ | | +------+ |
+------------------------+ +-------------------------+
Figure 9: Peer-to-Peer Trust Relationship
2.2.2 Intra-Domain Trust Relationship
In larger corporations often more than one middlebox is used to
protect different departments. In many cases the entire enterprise is
controlled by a security department, which gives instructions to the
department administrators. In such a scenario a peer-to-peer
trust-relationship might be prevalent. Sometimes it might be
necessary to preserve authentication and authorization information
within the network. As a possible solution a centralized approach
could be used whereby an interaction between the individual
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middleboxes and a central entity (for example a policy decision point
- PDP) takes place. As an alternative individual middleboxes could
exchange the authorization decision to another middlebox within the
same trust domain. Individual middleboxes within an administrative
domain should exploit their trust relationship instead of requesting
authentication and authorization of the signaling initiator again and
again. Thereby complex protocol interaction is avoided. This
provides both a performance improvement without a security
disadvantage since a single administrative domain can be seen as a
single entity. Figure 10 illustrates a network structure which uses
a centralized entity.
+-----------------------------------------------------------+
| |
| Network A |
| |
| |
| +---------+ +---------+
| +----///--------+ Middle- +------///------++ Middle- +---
| | | box 2 | | box 2 |
| | +----+----+ +----+----+
| | | | |
| +----+----+ | | |
| | Middle- +--------+ +---------+ | |
| | box 1 | | | | |
| +----+----+ | | | |
| | | | | |
| - | | | |
| - | +----+-----+ | |
| | | | Policy | | |
| +--+---+ +-----------+ Decision +----------+ |
| | Host | | Point | |
| | A | +----------+ |
| +------+ |
+-----------------------------------------------------------+
Figure 10: Intra-domain Trust Relationship
2.2.3 End-to-Middle Trust Relationship
In some scenarios a simple peer-to-peer trust relationship between
participating nodes is not sufficient. Network B might require
additional authorization of the signaling message initiator. If
authentication and authorization information is not attached to the
initial signaling message then the signaling message arriving at
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Middlebox 2 would cause an error message to be created, which
indicates the additional authorization requirement. In many cases the
signaling message initiator is already aware of the additionally
required authorization before the signaling message exchange is
executed. Replay protection is a requirement for authentication to
the non-neighboring middlebox which might be difficult to accomplish
without adding additional roundtrips to the signaling protocol (e.g.
by adding a challenge/response type of message exchange).
Figure 11 shows the slightly more complex trust relationships in this
scenario.
+----------------------+ +--------------------------+
| | | |
| Network A | | Network B |
| | | |
| | Trust | |
| | Relationship | |
| +---------+ +---------+ |
| +-///-+ Middle- +---///////----+ Middle- +-///-+ |
| | | box 1 | +-------+ box 2 | | |
| | +---------+ | +---------+ | |
| | | | | | |
| |Trust | | | | |
| |Relationship | | | | |
| | | | | Trust | |
| | | | | Relationship| |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
| +--+---+ | | | +--+---+ |
| | Host +----///----+------+ | | Host | |
| | A | |Trust | | B | |
| +------+ |Relationship | +------+ |
+----------------------+ +--------------------------+
Figure 11: End-to-Middle Trust Relationship
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3. Problems and Challenges
This section describes a number of problems which have to be
addressed for NSIS NAT/Firewall. These might be also of relevance to
other NSLP protocols.
3.1 Missing Network-to-Network Trust Relationship
Peer-to-peer trust relationship, as shown in Figure 9, is a very
convenient assumption that allows simplified signaling message
processing. However, it might not always be applicable, especially
between two arbitrary access networks (over a core network where
signaling messages are not interpreted). Possibly peer-to-peer
trust relationship does not exist because of the large number of
networks and the unwillingness of administrators to have other
network operators to create holes in their firewalls without proper
authorization. Hence in the following scenario we assume a somewhat
different message processing and show three possible approaches to
tackle the problem. None of these three approaches is without
drawbacks or constraining assumptions.
+----------------------+ +--------------------------+
| | | |
| Network A | | Network B |
| | | |
| +---------+ Missing +---------+ |
| +-///-+ Middle- | Trust | Middle- +-///-+ |
| | | box 1 | Relation- | box 2 | | |
| | +---------+ ship +---------+ | |
| | | or | | |
| | | Authorization| | |
| | | | | |
| | Trust | | Trust | |
| | Relationship | | Relationship | |
| | | | | |
| | | | | |
| | | | | |
| +--+---+ | | +--+---+ |
| | Host | | | | Host | |
| | A | | | | B | |
| +------+ | | +------+ |
+----------------------+ +--------------------------+
Figure 12: Missing Network-to-Network Trust Relationship
Figure 12 illustrates a problem whereby an external node is not
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allowed to manipulate (create, delete, query, etc.) packet filters at
a firewall. Opening pinholes is only allowed for internal nodes or
with a certain authorization permission. Hence the solution
alternatives in Section 4 focus on establishing the necessary trust
with cooperation of internal nodes.
3.2 End-to-end significance
In the case of NAT/firewalls traversal, the NSIS signaling messages
need to be sent all they way from the DS and DR or vice versa. This
is so because a middlebox does not know whether the remaining path to
the destination is clear of potentially obstructing middleboxes or
not.
3.3 Relationship with routing
The data path is following the "normal" routes. The NAT/FW devices
along the data path are those providing the service. In this case
the service is something like "open a pinhole" or even more general
"allow for connectivity between two communication partners". The
benefit of using path-coupled signaling is that the NSIS NATFW NSLP
does not need to determine what middleboxes or in what order the data
flow will go through.
Creating NAT bindings modifies the path of data packets between two
end points. Without NATs involved, packets flow from endhost to
endhost following the path given by the routing. With NATs involved,
this end-to-end flow is not directly possible, because of separated
address realms. Thus, data packets flow towards the external IP
address at a NAT (external IP address may be a public IP address).
Other NSIS NSLPs, for instance QoS NSLP, which do not interfere with
routing - instead they only follow the path of the data packets.
3.4 Dynamic state installation and maintenance
For NAT/Firewall traversal, the lifetime of a NAT binding or a packet
filter is maintained through periodic refresh. For short-lived
flows, having unpredictable filters, signaling for dynamically policy
rules is preferable as opposed to statically configured policy rules
requested for long duration in time.
For static state other mechanisms than an NSIS signaling protocol
might be preferable; such mechanisms would include a management
protocols such as SNMP or command line interfaces.
3.5 Affected Parts of the Network
NATs and Firewalls are usually located at the edge of the network,
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whereby other signaling applications affect all nodes along the path.
One typical example is QoS signaling where all networks along the
path must provide QoS in order to achieve true end-to-end QoS. In
the NAT/Firewall case, only some of the domains/nodes are affected
(typically access networks), whereas most parts of the networks and
nodes are unaffected (e.g. the core network).
This fact raises some questions. Should an NSIS NTLP node intercept
every signaling message independently of the upper layer signaling
application or should it be possible to make the discovery procedure
more intelligent to skip nodes. These questions are also related to
the question whether NSIS NAT/FW should be combined with other NSIS
signaling applications.
3.6 NSIS backward compatibility with NSIS unaware NAT and Firewalls
Backward compatibility is a key for NSIS deployments, as such the
NSIS protocol suite should be sufficiently robust to allow traversal
of none NSIS aware routers (QoS gates, Firewalls, NATs, etc ).
NSIS NATFW NSLP's backward compatibility issues is different than the
NSIS QoS NSLP backward compatibility issues, where an NSIS unaware
QoS gate will simply forward the QoS NSLP message. An NSIS unaware
firewall rejects NSIS messages, since firewalls typically implement
the policy "default to deny".
The NSIS backward compatibility support on none NSIS aware firewall
would typically consist of configuring a static policy rule that
allows the forwarding of the NSIS protocol messages (either protocol
type if raw transport mode is used or transport port number in case a
transport protocol is used).
For NATs backward compatibility is more problematic since signaling
messages are forwarded (at least in one direction), but with a
changed IP address and changed port numbers. The content of the NSIS
signaling message is, however, unchanged. This can lead to
unexpected results, both due to embedded unchanged local scoped
addresses and none NSIS aware firewalls configured with specific
policy rules allowing forwarding of the NSIS protocol (case of
transport protocols are used for the NTLP). NSIS unaware NATs must
be detected to maintain a well known deterministic mode of operation
for all the involved NSIS entities. Such a "legacy" NAT detection
procedure can be done during the NSIS discover procedure itself.
Based on experience it was discovered that routers unaware of the
Router Alert IP option [RFC 2113] discarded packets, this is
certainly a problem for NSIS signaling.
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3.7 Authentication and Authorization
For both types of middleboxes, firewall and NAT security is a strong
requirement. Authentication and authorization means must be
provided.
For NATFW signaling applications it is partially not possible to do
authentication and authorization based on IP addresses. Since NATs
change IP addresses, such a address based authentication and
authorization scheme would fail.
3.8 Directional Properties
There two directional properties that need to be addressed by the
NATFW NSLP:
o Directionality of the data
o Directionality of NSLP signaling
Both properties are relevant to NATFW NSLP aware NATs and Firewalls.
With regards to NSLP signaling directionality: As stated in the
previous sections, the authentication and authorization of NSLP
signaling messages received from hosts within the same trust domain
(typically from hosts located within the security perimeter delimited
by firewalls) is normally simpler than received messages sent by
hosts located in different trust domains.
The way NSIS signaling messages enters the NSIS agent of a firewall
(see Figure 2) might be important, because different policies might
apply for authentication and admission control.
Hosts deployed within the secured network perimeter delimited by a
firewall, are protected from hosts deployed outside the secured
network perimeter, hence by nature the firewall has more restrictions
on flows triggered from hosts deployed outside the security
perimeter.
3.9 Routing Asymmetry
Routing asymmetry [20] is a general problem for path-coupled
signaling, especially when installed states on NSIS forwarders are
related to bi-directional flows.
Path state, on an NSIS forwarder, including the next NSIS hop (for
packets sent from the NR to NI), is used to handle routing asymmetry
for NSIS messages, but not for data flows (i.e. no route pinning for
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data flows).
Similarly to path-coupled QoS signaling, middlebox signaling also has
to be aware of the routing asymmetry when bi-directional flows
relevant states need to be installed on NSIS aware nodes, although
the routing asymmetry might not be significant within the local
networks where firewalls are typically located. For signaling NAT
bindings this issue comes with a different flavor since an
established NAT binding changes the path of the data packets. Hence a
data receiver might still be able to send NSIS signaling messages to
create a NAT binding, although they travel the previously "wrong"
path.
3.10 Addressing
A more general problem of NATs is the addressing of the end-point.
NSIS signaling message have to be addressed to the other end host to
follow data packets subsequently sent. Therefore, a public IP address
of the receiver has to be known prior to sending an NSIS message.
When NSIS signaling messages contain IP addresses of the sender and
the receiver in the signaling message payloads, then an NSIS agent
must modify them. This is one of the cases, where a NSIS aware NATs
is also helpful for other types of signaling applications e.g. QoS
signaling.
3.11 NTLP/NSLP NAT Support
It must be possible for NSIS NATs along the path to change NTLP and/
or NSLP message payloads , which carry IP address and port
information. This functionality includes the support of providing
mid-session and mid-path modification of these payloads. As a
consequence these payloads must not be reordered, integrity protected
and/or encrypted in a non peer-to-peer fashion (e.g. end-to-middle,
end-to-end protection). Ideally these mutable payloads must be marked
(e.g. a protected flag) to assist NATs in their effort of adjusting
these payloads.
3.12 Route changes
The effect of route changes are more severe than in other signaling
applications since a firewall pinhole and NAT binding is needed
before further communication can take place. This is true for both
NSIS signaling and for subsequent data traffic. If a route changes
and NSIS signaling messages do not configure NSIS NATs and firewalls
along the new path then the communication is temporarily interrupted.
This is naturally a big problem for networks where routes frequently
change e.g. ad-hoc networks or in case of fast mobility. In these
cases state refresh messages have to provide a mechanism for fast
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reaction.
3.13 Combining Middlebox and QoS signaling
In many cases, middlebox and QoS signaling has to be combined at
least logically. Hence, it was suggested to combine them into a
single signaling message or to tie them together with the help of
some sort of data connection identifier, later on referred as Session
ID. This, however, has some disadvantages such as:
- NAT/FW NSLP signaling affects a much small number of NSIS nodes
along the path (for example compared to the QoS signaling).
- NAT/FW signaling might show different signaling patterns (e.g.
required end-to-middle communication).
- The refresh interval is likely to be different.
- The number of error cases increase as different signaling
applications are combined into a single message. The combination of
error cases has to be considered.
3.14 Difference between sender- and receiver-initiated signaling
For NAT/FW signaling there seems to be little difference between
sender- and receiver-initiated signaling messages. Some other
characteristics of QoS signaling protocols are not applicable (e.g.
the adspec object) to the NAT/FW context. It seems that a full
roundtrip is always required if the protocol aims to be generic
enough.
3.15 Inability to know the scenario
In Section 2.1 a number of different scenarios are presented. Data
receiver and sender may be located behind zero, one, or more
firewalls and NATs. Depending on the scenario, different signaling
approaches have to be taken. For instance, data receiver with no
NAT and firewall can receive any sort of data and signaling without
any further action. Data receivers behind a NAT must first obtain a
public IP address before any signaling can happen. The scenario might
even change over time with moving networks, ad-hoc networks or with
mobility.
NSIS signaling must assume the worst case and cannot put
responsibility to the user to know which scenario is currently
applicable. As a result, it might be necessary to perform a
"discovery" periodically such that the NSIS agent at the end host has
enough information to decide which scenario is currently applicable.
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This additional messaging, which might not be necessary in all cases,
requires additional performance, bandwidth and adds complexity.
Additional, information by the user can provide information to assist
this "discovery" process but cannot replace it.
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4. NSIS NAT Handling Solution
This section describes a mechanism for allowing NSIS signaling
messages to travel end-to-end in the presence of NATs at the
receiving side. This requires to establish state information at the
NSIS-aware NAT device.
Note: The discussed mechanism only creates state relevant for NSIS
message handling. It does not create NAT bindings for data traffic.
4.1 Problem Description
NSIS signaling messages follow the data path from the data sender to
the data receiver. To provide this property of being path-coupled a
discovery process sends signaling messages along the same route as
taken by subsequent data packets. The NSIS messages are directed to
a particular destination IP address and hence the destination address
needs to be known in advance before NSIS signaling can start.
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+-------------+ AS-Data Receiver Communication
+-------->| Application |<-----------------------------+
| | Server | |
| +-------------+ |
| IP(R-NAT_B) |
| NSIS Signaling Message +-------+--+
| +------------------------------------------>| NAT/NAPT |
| | | B |
| | +-------+--+
| | |
AS-Data| | |
Receiver| | +----------+ |
Comm.| | | NAT/NAPT | |
| | | A | |
| | +----------+ |
| | |
| | |
| | |
| | |
v | IP(R) v
+--------+ +---------+
| Data | | Data |
| Sender | | Receiver|
+--------+ +---------+
Figure 13: The Data Receiver behind NAT problem
Figure 13 describes a typical message communication in a peer-to-peer
networking environment whereby the two end points learn of each
others existence with the help of a third party (referred as
Application Server). The communication with the application server
and the two end points (data sender and data receivers) serves a
number of functions. As one of the most important functions it
enables the two end hosts to learn the IP address of each other.
Without the proposed mechanism it would not be possible to establish
a NAT binding end-to-end in all scenarios.
Some sort of communication between the end hosts and a third party is
typically necessary (independently of NSIS). NSIS signaling messages
cannot be used to communicate application level relevant end point
identifiers (in the generic case at least) as a replacement for the
communication with the application server.
If the data receiver is behind a NAT then an NSIS signaling message
will be addressed to the IP address allocated at the NAT (if there
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was one allocated). If no corresponding NSIS NAT Forwarding State at
NAT/NAPT B exists (binding IP(R-NAT B) <-> IP(R)) then the signaling
message will terminate at the NAT device (most likely without proper
response message). The signaling message transmitted by the data
sender cannot install the NAT binding or NSIS NAT Forwarding State
"on-the-fly" since this would assume that the data sender knows the
topology at the data receiver side (i.e. the number and the
arrangement of the NAT and the private IP address(es) of the data
receiver). The primary goal of path-coupled middlebox communication
was not to force end hosts to have this type of topology knowledge.
A number of solutions exist to allow nodes behind a NAT to establish
a NAT binding to allow the receiver to receive IP packets. These
solutions can at best be labeled as hacks (see [NATP2P]) and they
have their drawbacks:
o They assume a certain behavior of NAT boxes.
o They work in some environments whereas in others they do not
properly function.
o They only allow NAT bindings for UDP traffic to be established.
o They often fail.
Some other solutions assume that both nodes are registered in the DNS
directory (see [12]).
The requirements for an NSIS solution are two-fold:
1. NSIS signaling messages must be able to travel end-to-end
(between data sender and data receiver) - if desired. This is
important for a number of NSIS NSLPs
2. NSIS relies on a generic solution which works in all scenarios
(see section 5 of [26]).
Since the NSIS signaling messages are intercepted at each NSIS
device, the NAT solution depends on the properties of the NTLP. In
particular, multiplexing capability is important. Two possible
options are feasible:
1. Multiplexing with the help of transport layer information (i.e.
port information)
2. Multiplexing at the NSIS application layer (e.g. based on session
identifier)
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We describe the second approach although we believe that alternatives
are possible.
Enough information has to be available to convert IP address
information of an incoming signaling message to different IP
addresses of an outgoing NSIS message. Finally the signaling message
must reach the data receiver.
It seems that the session identifier can be used to associate state
information of the two independent signaling exchanges. The two
exchanges (as described in Section 4.2) are:
1. Signaling exchange from the data receiver (NR) to the NAT(s)
2. End-to-end NSIS signaling message exchange from the NI to the NR.
If the session identifier is used for this purpose then it is
necessary to communicate the session ID from the data receiver (NR)
to the NI. Communicating the IP address information instead (as an
alternative solution approach) is easier since this functionality is
already provided by SIP whereas securely exchanging (e.g.
confidentiality protected) the Session Identifier is not available.
4.2 Solution Overview
The data receiver starts to signal an NSIS Create-NAT-Binding message
into the "wrong direction". By "wrong" we refer to the usual
behavior of path-coupled signaling where the data sender starts
signaling in order to tackle with routing asymmetry. The data
receiver would typically return signaling messages to the data sender
in the reverse direction by utilizing state created at nodes along
the path (i.e. to reverse route signaling messages). The concept of
path-coupled or path-decoupled signaling is, however, no relevant for
this special type of signaling communication. In case of
establishing NAT bindings (and NSIS NAT Forwarding State) the
direction does not matter since routing is modified. Subsequent NSIS
messages (and also data traffic) will travel through the same NAT
boxes.
The proposed solution requires two NSIS signaling messages:
1. Reserve External Address Request
2. Reserve External Address Acknowledgment
The semantics of the two messages will be described in detail in this
section.
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The data receiver sends a Reserve External Address NSIS signaling
message into the local network (before the data sender starts NSIS
signaling). In Section Section 4.2.1 we will discuss where to address
this signaling message (i.e. which destination IP address to use).
The signaling message creates NSIS NAT Forwarding State at
intermediate NSIS NAT node(s). Furthermore it has to be ensured that
the edge NAT device is discovered as part of this process. The end
host cannot be assumed to know this device - instead the NAT box
itself is assumed to know that it has such a capability. Forwarding
of the Reserve External Address NSIS message beyond this entity is
not necessary, and should be prohibited as it provides information on
internal hosts capabilities.
Reserve External Address Request
+-------+ +-------+ +-------+ +---------+
| NAT X |<---| NAT Y |<---| NAT Z |<---| Data |
| |--->| |--->| |--->| Receiver|
+-------+ +-------+ +-------+ +---------+
Reserve External Address Response
========================================>
Data Traffic Direction
Figure 14: Reserve External Address NSIS Signaling Message
The goal of this signaling message exchange is:
o to create one (or more) NAT binding(s)
o to allow the data receiver to learn its global routable IP address
(for communication with NSIS)
o not to require the data receiver to learn topology information.
Figure 14 shows a number of NAT devices at the data receivers network
side. NSIS NAT Forwarding State is established at these network
elements.
The Reserve External Address Request message triggers the state
creation and the discovery. The message carries information where the
sender expects incoming NSIS signaling messages.
The Reserve External Address Response message confirms the state
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creation and allows to return information about the NATs and the
topology to the end host (for informational purposes). As a result
the end host will learn the public IP address which can be used by
the data sender to address NSIS signaling messages.
4.2.1 Destination IP address Selection
The Reserve External Address Request message has to be addressed to a
specific destination IP address. Since there is no natural candidate
a few alternatives might be considered. The discussed options refer
to entities of Figure 13
Possible options are:
1. Public IP address of the data sender
2. Public IP address of the data receiver (allocated at NAT B)
3. IP address at the Application Server
Actually, there is no "correct" answer to this question and from a
theoretical point of view it does not really matter as long as Host A
learns an IP address where he has to send the NSIS signaling message.
From a performance point of view there is, however, a difference
since it would be desirable to create an "optimal" routing path.
1. Public IP address of the data sender:
* Assumption:
+ The data receiver already learned the IP address of the
data sender (e.g. via a third party).
* Problems:
+ The data sender might also be behind a NAT. In this case
the public IP address of the data receiver is the IP
address allocated at this NAT.
+ Due to routing asymmetry it might be possible that the
routes taken by a) the data sender and the application
server b) the data sender and NAT B might be different. As
a consequence it might be necessary to advertise a new (and
different) external IP address with SIP after using NSIS to
establish a NAT binding.
2. Public IP address of the data receiver (allocated at NAT B):
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* Assumption:
+ The data receiver already learned his externally visible IP
address (e.g. based on the third party communication).
* Problems:
+ Communication with a third party is required.
3. IP address at the Application Server:
* Assumption:
+ An application server (or a different third party) is
available.
* Problems:
+ If the NSIS signaling message is not terminated at the NAT
of the local network then an NSIS unaware application
server might discard the message.
+ Routing might not be optimal since the route between a) the
data receiver and the application server b) the data
receiver and the data sender might be different.
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5. Protocol Description
5.1 Basic protocol overview
The NSIS Signaling Layer Protocol (NSLP) for NAT and FW traversal is
carried over the NSIS Transport Layer Protocol (NTLP) defined in [3].
NATFW NSLP messages are initiated by the NSIS initiator, (NI) handled
by NSIS forwarders (NF) and finally processed by the NSIS responder
(NR). It is required that at least NI and NR implement this NSLP,
intermediate NF only implement this NSLP when they provide middlebox
functions. Forwarders that do not have any NATFW NSLP functions just
forward these messages; those forwarders implement NTLP and one or
more other NSLPs.
A Data Sender (DS) that intents to send data to a Data Receiver (DR)
must start its NATFW NSLP signaling. So the NI at the data sender
(DR) starts NSLP signaling towards the address of data receiver DR
(see Figure 15).
+-------+ +-------+ +-------+ +-------+
| DS/NI |<~~~| MB1/ |<~~~| MB2/ |<~~~| DR/NR |
| |--->| NF1 |--->| NF2 |--->| |
+-------+ +-------+ +-------+ +-------+
========================================>
Data Traffic Direction
---> : NATFW NSLP request signaling
~~~> : NATFW NSLP response signaling
DS/NI : Data sender and NSIS initiator
DR/NR : Data receiver and NSIS responder
MB1 : Middlebox 1 and NSIS forwarder 1
MB2 : Middlebox 2 and NSIS forwarder 2
Figure 15: General NSIS signaling
The NSLP request messages are processed each time a NF with NATFW
NSLP support is passed. Those nodes process the message, check local
policies for authorization and authentication, possibly create policy
rules, and forward the signaling message to the next NSIS node. The
request message is forwarded until it reaches the NSIS responder.
NSIS responders will check received messages and process those if
applicable. NSIS responders generate response messages and sent them
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back to the NI via the same chain of NFs. The response message is
processed at each NI forwarder implementing NATFW NSLP. The Data
Sender can start sending its data flow to the Data Receiver, when the
signaling was successful, meaning that NI has received a successful
response.
In general, NATFW NSLP signaling follows the data path from DS to DR.
This enables communication between both hosts for scenarios with only
firewalls on the data path or NATs on sender side. For scenarios
with NATs on the receiver side certain problems arise.
When Data receiver (DR) and Data Sender (DS) are located in different
address realms and DR is behind a NAT, DS cannot signal to DR. DR is
not reachable from DS and thus no NATFW signaling can be sent to DR's
address. Therefore, DR must first fix a address at a NAT that is
reachable for DS, for instance DR must determine its public IP
address. Once DR has fixed a public address it forwards this to DS
via a separate mechanism, which may be application level signaling
like SIP. This application level signaling may involve third parties
that assist in exchanging this information. This separate mechanism
is out of scope of NATFW NSLP.
NATFW NSLP signaling supports this public address fixing with this
mechanism:
o First, DR fixes a public address by signaling on the reverse path
(DR towards DS) and thus making itself available to other hosts.
This process of fixing public addresses is called reservation.
This way DR reserves publicly reachable addresses and ports.
o Second, DS is signaling directly to DR, creating policy rules at
middleboxes. Note, that the reservation mode will usually make
reservations only, which will be "activated" by the signaling from
DS towards DR. The first mode is detailed in the Section 4
The protocol is intended to work on a soft-state basis. This means,
that whatever state is installed or reserved on a middlebox, will
expire, and thus be de-installed/ forgotten after a certain period of
time. To prevent this the involved boxes will have to specifically
request a session prolongation. An explicit NATFW NSLP state
deletion message is also provided by the protocol.
Middleboxes should report back in case of error, so that appropriate
measures and debugging can be performed.
The next sections define the NATFW NSLP message types and formats,
protocol operations, and policy rule operations.
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5.2 NATFW NSLP Header
The NATFW NSLP header is common to all messages and follows directly
the NTLP header. A NSLP node can distinguish based on this header
whether a request or response message is passed in the packet. It is
followed by a series of objects.
The NSIS NATFW NSLP header contains:
o version: NSIS NATFW NSLP protocol version number.
o header_len: length of the NSLP payload in bytes, including NSLP
header
o obj_count: number of objects that follow after the NSIS header.
o message type: The type of the NSLP message, request or response.
Sub-types are encoded in this field as well.
Message type indicates whether the NSLP packet is a request or a
response. For request messages, four sub-types are defined:
o Create Session
o Prolong Session
o Delete Session
o Reserve Session
For response messages, three sub-types are defined:
o Return External Address
o Path Succeeded
o Error
The next sections define which objects are included in which message
type. For each message type the allowed combination of objects is
described.
5.3 NATFW NSLP Objects
5.3.1 NATFW NSLP Object Header
NATFW NSLP objects carry the actual information about policy rules,
lifetimes and error conditions. All objects share the same object
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header. An object header is followed by the object data, whereas the
format of the object data depends on the object type. A NATFW NSLP
payload may contain several objects.
The object header has the following format:
o obj_len: total length of the object, including object header
o obj_type: type of NATFW NSLP object. Identifies the data that
follows.
For the moment four object types are defined in the next sections.
Other objects can be defined later on. These four objects each
describe a message request type.
5.3.2 NATFW Session ID Object
The NATFW Session ID is the handle to the NATFW session at a
particular NSIS node. It is randomly generated by the NSIS
initiator.
5.3.3 Lifetime Object
The lifetime object indicates the lifetime of a NATFW NSLP session.
The real lifetime at a NSIS peer is the current time plus the
lifetime value of this object.
5.3.4 Policy Rule Object
The policy rule objects contains the flow information for the data
traffic from DS to DR. The information contained in this object will
change as soon as NATs are involved.
The policy rule object has these fields:
o Source address: The IP address where the data will come from. If
it is DS sending data to DR, the source address is either DS or
the closest NAT in the route from DS to the middlebox that gets
the packet; That is, the address where each middlebox will see the
packet come from.
o Destination address: The IP address where the data is headed. If
it is DS sending data to DR, the destination address is either DR
or the public address DR reserved itself.
o Protocol: The protocol carried in the IP data packet. Currently
TCP, UDP and IP is defined.
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o Source Port: The transport layer port the data will come from
o Destination Port: The transport layer port the data will go to.
o IPv flow label: The IPv6 flow label (Editor's note: needs further
in-depth discussion).
Note: you might want to leave the source address or port set to ANY,
to accept any source address port. This makes the pinhole not so pin
like, but might be necessary at the integration with certain NAT/FW
types. Whether this loose pinhole is authorized or not by the
middlebox, is a policy decision based on the middlebox configuration.
5.3.5 External Address Object
This object contains the reserved external address and if applicable
port number.
The object has these fields:
o External IP address: The reserved external IP address at the NAT.
o External port number: The reserved external port number at the
NAT.
5.4 Request Message Formats
This section defines the message types and their format for the NATFW
NSLP. Note, that at the moment of writing this document, no final
decision has been reached on the details of the NTLP. Thus, message
types and formats may change in future revisions of this document.
Currently, the NATFW NSLP header and 4 request messages are defined.
Furthermore, three response message types are defined. All those
messages are explained in this chapter.
The NATFW payload of a NSIS NTLP packet consists of a NATFW NSLP
header that is common to all request, response and error messages.
Several NATFW NSLP objects follow the NSLP header, depending on the
message type.
NOTE: Any bit-level definition of messages and headers are to be done
in future revision of this memo. Furthermore, any order of object
fields below is not mandating their order in the actual bit-level
definition.
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5.4.1 Create Session
The create session request message is used to create policy rules on
middleboxes. Middleboxes receiving this message type will, if
authenticated and authorized, enable the requested policy rules, so
that data packets of the specified data flow can traverse.
The create session messages carries these objects:
o Session ID object: A newly generated session ID
o Policy Rule object: The description of the data flow
o Lifetime Object: A request lifetime for this NSIS NATFW NSLP
session
5.4.2 Prolong Session
The prolong session request message is used to extend the lifetime of
a NATFW NSLP session. The NSIS initiator requests a certain lifetime
extension.
The prolong session message carries these objects:
o Session ID object: Session to be prolonged
o Lifetime Object: Requested new lifetime
5.4.3 Delete Session
The delete session request message is used to delete NATFW NSLP
session.
The delete session object carries this object:
o Session ID: The session to be deleted.
5.4.4 Reserve External Address
The reserve external address request message is used in the case that
a Data Receiver (DR) is located behind a NAT. The DR needs to
received data and so uses this request message to reserve an external
IP address at a NAT.
The reserve external address message carries these objects:
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o Session ID: The session ID for the reservation. Note that this
session ID is only valid for the reservation. Create messages
using the reservation will use their own generate session ID.
o Lifetime: The lifetime of the reservation
o Policy Rule: In the reserve external address message the policy
rule object must be set accordingly:
Source address: The source address of the data flow. This is
the destination of the NATFW reserve address packet. The way
of NSLP signaling is in the reverse way of the data flow.
Source port: The source port of the data flow.
Destination address: The internal IP address to where data
flow will be destined. This is the source address of the NATFW
reserve address packet.
Destination port: The destination port of the data flow
Protocol: Expected protocol
The direction of NSIS NATFW NSLP signaling is reverse to the reserved
data flow. The source address of the expected data flow is the
destination of the signaling. Vice versa, the destination address of
the expected data flow is the source of the signaling (see section
Section 4).
Note that no state, be it a firewall rule or a NAT binding, is
installed as a result of this message. The state is only remembered,
and might be later installed by a create message.
5.5 Response Messages
The following messages are responses messages that are generate
either by any NF or NR. Currently, three different types of request
messages are defined.
5.5.1 Return External Address Response
The return external address response message is sent as a successful
reply to a reserve external address request.
Return external address message contains these objects:
o Session ID: The session this packet is replying to.
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o External address object: Contains reserved external IP address
and port number
o Lifetime: The minimum granted lifetime for this reservation.
5.5.2 Path Succeeded Response
The path succeeded response message is sent as a successful reply to
a create session request.
The Path succeeded response message contains these objects:
o Session ID: The session ID for which a path was successfully
installed
o Lifetime: The minimum granted lifetime of this session
5.5.3 Error Response Messages
Any NATFW NSLP error occurring at NF or NR is reported via the error
response message towards the NI.
The error message contains these objects:
o Session ID: The session id of the object that generated the error
o Error code: The error to report.
Possible error code classes are:
o Policy rule errors
o Authentication and Authorization errors
o NATFW NSLP protocol errors
5.6 Protocol Operations
This section defines the message flow and protocol operation for all
message types
5.6.1 Message Handling Overview
When a NSIS NATFW peer receives an NSIS message, it might take an
action based on the message type, the nature of the middlebox
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function, its configuration and local security policies.
As a summary, here's the behavior of the boxes, depending on message
type and configuration parameters:
NAT FW NAT+FW DS DR
reserve 5 - 5 + +
ret_ext_addr - - - + 8
create 1 2 3 + 4
prolong 6 6 6 + 4
delete 7 7 7 + 4
path_succeed 9 9 9 8 +
ret_ext_addr: Return External Address response message
1: Remember the policy rule, but do not install. Rewriting either
the source or destination address depending on whether the packet
comes from the external_address or not. Always forward.
2: Remember policy rule, but do not install. Always forward.
3: 1+2. The order depends on whether it comes from the outside
address (NAT, then FW) or the inside one (FW, then NAT)
4: If it fits one of its requests, send a path_succeeded packet
back. Otherwise, drop the packet.
5: Make a reservation. If middlebox is an edge NAT is set, send
back the reserved external address and do not forward the message
further. Otherwise, forward and do not send anything back.
6: Prolong the session. Always forward.
7: Terminate the session. Always forward.
8: hand it over to upper layers, and stop processing.
9: If it fits a prior request, enable policy rule that has been
remembered only before.
-: ignore and forward.
+: ignore and drop.
Note that policy rule ordering at middlebox is important, when it
comes to combined NAT and firewall middleboxes, because the filter
rules have to be set up according to the packet they will see.
Source NAT is done at the end so it does not disturb routing
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decisions, meaning that filter sees the original packets.
Destination NAT, on the other hand, is done at the beginning, so it
can be routed properly, and so the filter sees the modified packets.
Note also that for each action, the host might demand a certain
degree of authorization, and thus refuse to take the action, sending
an error message back instead.
The details of protocol operations for each request type is defined
in the next sections. Each section describes the exact handling for
each type of middlebox.
5.6.1.1 Reserving Addresses
As explained in section Section 4, data receivers located behind a
NAT must first reserve an external address and port number (if
applicable) before any NSIS message can be send towards them.
With the reserved external address message exchange NSIS peers can
obtain this required external address and port number at a NAT.
Therefore, NI sets the policy rule object and sends the signaling
message to an address chosen on its own (see Section 4.2.1. The
reserve message is sent in this way:
Public Internet Private Address
Space
Edge
DS NAT NAT NI(DR)
| | | |
| | | |
| | | |
| | Reserve | Reserve |
| |<---------|<----------------|
| | | |
| | Return | ext addr/Error |
| |--------->|---------------->|
| | | |
| | | |
Handling of reserve external address messages depends on the
middlebox type and NSIS peer:
o NAT Box:
When a NAT box gets a Reserve external address message, it checks
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whether it arrived on the public address, or the private one. If
it arrived in the public one, an error message of the type:
"Requested an external address from the outside" is sent back.
If it arrived on the private side, an entry is made in the
internal reservation list with the packet information. If the box
is an edge NAT (either by configuring it to true, or just for that
connection if it is set to auto), it drops the message, and
replies with a return external address message containing the
allocated address port pair. If it is not an edge NAT, it forwards
the packet on.
o Firewall Box:
Reserve messages are silently ignored in Firewall boxes. They are
simply forwarded on.
o NAT+FW Box:
When a box that integrates both a NAT and a Firewall gets a
reserve message, it will hand it to its NAT part. Its firewall
part will simple ignore it.
o Data Sender:
The message should never get here. It should be ignored and
dropped.
o Data Receiver:
The message should never get here. It should be ignored and
dropped.
Response messages are handled differently depending on NSIS peer
type:
o NAT Box, Firewall Box and NAT+Firewall Box:
When one of these boxes gets a Return external address message, it
must simply ignore it and let it traverse.
o Data Sender:
The message should never get here. It should be ignored and
dropped.
o Data Receiver:
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A return external address message in the Data receiver, has
reached its destination. It must be dropped, and it's information
handed to superior layers.
5.6.1.2 Creating Sessions
Creating sessions enables communication between DS and DR. Both are
enabled to exchange data packets even with middleboxes on path. DS
generates a create session message with a chosen session ID, the
policy rule object set to the requested flow, and a requested
lifetime. DS sends the create session message towards DR. The
message flow is sketched in the next figure.
DS Public Internet NAT Private address DR
| | space |
| Create | |
|----------------------------->| |
| | |
| Error (if necessary) | |
|<-----------------------------| Create |
| |--------------------------->|
| | |
| | Path Succeeded/Error |
| Path Succeeded/Error |<---------------------------|
|<-----------------------------| |
| | |
| | |
Create session messages are processed differently at each NSIS peer:
o NAT Box:
When a NAT box gets a create message, it first checks if it
arrived on the public address or not.
If it came from the public side, it means an external box will try
to send data. It then looks for a reservation in its reservation
list, that matches the dst_addr and dst_port of policy rule
included in the create message. If it does not find it, it
returns an error message of the type "No reservation found". If it
finds it, it fills in the reservation with the data from the
packet, and remembers the given rule. It then changes the dst_addr
and dst_port fields of the create packet and forwards it to the
tgt_addr of the reservation.
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If it came from the private side, it installs the NAT rule with
the information in the packet. It then changes the src_addr and
src_port of the create message to its own external address and
port.
o Firewall Box:
When a firewall box gets a create message, it simply remembers the
rule specified in the message and forwards the packet.
o NAT+FW Box:
When a box that integrates both a NAT and a Firewall gets a create
message, it first checks whether it arrived on the public address
or not.
If it arrived on the public side, the NAT part of the box takes
care of the packet first, as said in the NAT Box case. Afterwards,
the modified packet is handed to the firewall part, where it is
handled as in the Firewall Box case.
If it arrived on the private side, the message is handed to the
firewall part first, and then to the NAT one.
o Data Sender:
The message should never get here. It should be ignored and
dropped.
o Data Receiver:
If the data receiver gets a create message, it means all the boxes
on the way accepted it, and so the signaling succeeded. All it
does is drop the packet, and send back a Path Succeeded message to
the IP packet source address.
As described above, DRs return a path succeeded when the create
message arrived at DR. The path succeeded message is returned along
all NSIS forwarders. Each NSIS forwarder enables the prior
remembered policy rules and forwards the message to next NSIS hop.
Forwarding of the path succeeded messages is terminated at the DS.
5.6.1.3 Prolonging Session
NATFW NSLP sessions are maintained on a soft-state base. After a
certain timeout they are removed automatically by the middlebox, if
they are not refreshed by a prolong session message. DS is sending
prolong message towards DR and each NSIS forwarder maintaining state
for the given session ID extends the lifetime of the session.
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Extending lifetime of a session is calculated as current local time
plus lifetime.
DS Public Internet NAT Private address DR
| | space |
| Prolong/Delete | |
|----------------------------->| |
| | |
| Error (if necessary) | |
|<-----------------------------| Prolong/Delete |
| |--------------------------->|
| | |
| | Error (if necessary) |
| Error (if necessary) |<---------------------------|
|<-----------------------------| |
| | |
| | |
Figure 19: Prolongation message flow
o NAT Box, Firewall Box and NAT+Firewall Box:
When one of these boxes gets a Prolong session message, the
expiration time of the session should be changed to the time of
reception plus the configured session lifetime.
o Data Sender:
As in the create session message, this packet is sent from the DS
to the DR, and should never arrive at the DS. Again, it should be
ignored and dropped.
o Data Receiver:
The same behavior as in the case of a Delete session message on
the DR should be applied.
5.6.1.4 Deleting Sessions
Deleting sessions is done via the delete session message. DS can
request the deletion of a session at any time by sending this
message. Processing of these messages at:
o NAT Box, Firewall Box and NAT+Firewall Box:
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When one of these boxes gets a Delete session message, it erases
the session referred in the message.
o Data Sender:
This packet should never get to the DS, so it is to be ignored and
dropped.
o Data Receiver:
As in the create session message, this is the final destination of
the message. DR erases its session. Message forwarding stops
here.
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6. Solution examples
6.1 Firewall traversal
DS wants to send data traffic to DR through tight firewalls, as seen
in Figure 20. To do that, it will have to signal using NSIS, on the
data path.
+-----+ +-----+ //----\\ +-----+ +-----+
DS --| FW1 |-----| FW2 |---| |---| FW3 |-----| FW4 |--- DR
+-----+ +-----+ \\----// +-----+ +-----+
private public private
Data Flow
===============================>
Figure 20: Firewall Traversal Scenario
Therefore, DS initiates signaling to DR by sending a create object to
the IP address of DR. Note that DS already knows its source address
and port (say, 1111), and the destination address of DR. The
destination port (let's say 9999) has been send to DS by DR via
application layer messages, possibly, but not necessarily involving a
third party. The message looks like:
o dst_addr = DR
o dst_port = 9999
o src_addr = DS
o src_port = 1111
This message is received by FW1, which installs the state that reads:
"Any packet coming from DS:1111 headed for DR:9999 will be allowed
traversal"
FW2, FW3 and FW4 do exactly the same, and forward the packet to each
other, until it finally reaches DR. At this point, the data path is
open, and DR sends back a Path succeeded message to DS, which can now
start sending traffic.
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6.2 NAT with private network on sender side
In the example in Figure 21, DS is in a private network and wants to
send data to DR, out in the public internet. To do so, DS will have
to initiate NSIS signaling towards DR
+------+ +------+ //----\\
DS --| NAT1 |-----| NAT2 |---| |--- DR
+------+ +------+ \\----//
private public
Figure 21: NAT with private network on sender scenario
Apparently, the normal NAT functionality will take care of sending
the data from DS out into the public internet, and route back the
replies from DR. This is indeed true, but doesn't give NSIS control
on what the source address or port is, as it is usually assigned
dynamically by the NAT. Moreover, the NSLP would have no information
on this hops, and could not install proper pinholes, as it would set
DS as the source address, and not that of the last NAT.
DS builds a create packet with the information he has, which is the
same as that in Section 6.1. It looks like this:
o dst_addr = DR
o dst_port = 9999
o src_addr = DS
o src_port = 1111
NAT1 is the first to get the packet; It is not coming from its
configured "nat external address", and so, it knows it will have to
rewrite the information on the source, and not that of the
destination. NAT1 then picks a free port (incidentally 1011) and
installs a nat rule that reads: "Whatever packet comes from DS:111,
heading for DR:9999 will be rewritten so that the source address
looks like NAT1:1011".
It then rewrites the packet it received as follows:
o dst_addr = DR
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o dst_port = 9999
o src_addr = NAT1
o src_port = 1011
And forwards the packet.
NAT2 gets it now, and does exactly the same. Port 2022 is chosen, and
the rule: "Whatever packet comes from NAT1:1011, heading for DR:9999
will be rewritten so that the source address looks like NAT2:2022" is
installed. The packet gets modified as follows:
o dst_addr = DR
o dst_port = 9999
o src_addr = NAT2
o src_port = 2022
And is forwarded. It eventually reaches DR, who sends back a path
succeeded message. Data flow from DS:1111 to DR:9999 is now possible.
6.3 NAT with private network on receiver side
In this example, DS wants to send data to DR over the network in
Figure 22:
//----\\ +------+ +------+
DS ---| |---| NAT1 |-----| NAT2 |--- DR
\\----// +------+ +------+
public private
Figure 22: NAT with private network on receiver Scenario
The problem, of course, is that DR is not publicly reachable. Because
of that, DR will have to signal on the data path, in the opposite
direction (DR->DS) to get itself a public address it can use. This
method is described in Section 4
To get an external address, DR sends a packet to DS. It could
actually send it to anything in the public internet, as it would
force it to traverse what NATs are on its way. In the case of
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multihomed environments, though, more than one NAT to the outside is
possible, so the better we "aim" the more the chances we go out the
right NATs and get more optimal routes.
The said packet is an NSIS reserve_addr object which looks like this:
o tgt_addr = DR
o tgt_port = 9999
o src_addr = 0.0.0.0
o src_port = 0
Notice that this is a really loose pinhole, since any src_addr and
port is allowed.
NAT2 gets the packet and looks for a free port (say, 2022, for
clarity's sake). It then adds an entry to its reservation list. The
entry looks like this:
o src_addr = 0.0.0.0
o src_port = 0
o dst_addr = NAT2
o dst_port = 2022
o tgt_addr = DR
o tgt_port = 9999
This means simply that packets coming from any source, destined to
the public address we just reserved, should be targeted to the
internal box DR, on port 9999
It then rewrites the packet so that it looks like:
o tgt_addr = NAT2
o tgt_port = 2022
o src_addr = 0.0.0.0
o src_port = 0
Because it is not an edge NAT, it forwards the modified packet and
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does not sent a return_external_addr object to DR. Note that no NAT
binding is installed so far in NAT2, although the state is now
reserved.
NAT1 now gets the packet, picks free port 1011 and adds the following
entry to its reservation list:
o src_addr = 0.0.0.0
o src_port = 0
o dst_addr = NAT1
o dst_port = 1011
o tgt_addr = NAT2
o tgt_port = 2022
As it turns out, NAT1 IS an edge_nat, so it doesn't forward the
packet. Instead, it replies to DR sending back a return external
address packet on the same connection, so it finds its way back
through the NATs:
o ext_addr = NAT2
o ext_port = 2022
By using some application layer protocol, and possibly, although not
necessarily, using a third party box, DR sends it's freshly allocated
external address and port to DS.
DS now knows who to signal, so it sends a create message:
o dst_addr = NAT1
o dst_port = 1011
o src_addr = DS
o src_port = 1111
When it reaches NAT1, it does so through NAT1 external address. It
realizes it is being asked to forward the traffic from some outside
box towards the inside. It then looks up its reservation list,
looking for a session that has the external address and port
NAT1:1011 assigned. It finds this:
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o src_addr = 0.0.0.0
o src_port = 0
o dst_addr = NAT1
o dst_port = 1011
o tgt_addr = NAT2
o tgt_port = 2022
Using the information in the create object, it then fills in this
structure to:
o src_addr = DS
o src_port = 1111
o dst_addr = NAT1
o dst_port = 1011
o tgt_addr = NAT2
o tgt_port = 2022
This IS a tight pinhole. NAT1 installs the rules now, which say:
"Whatever packet comes from DS:1111 heading for NAT1:1011, should
have its destination address changed to NAT2:2022, and be forwarded".
The packet is also rewritten into this:
o src_addr = DS
o src_port = 1111
o dst_addr = NAT2
o dst_port = 2022
And is forwarded to NAT2. Upon arrival, a similar process issues.
NAT2 finds its reservation entry:
o src_addr = 0.0.0.0
o src_port = 0
o dst_addr = NAT2
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o dst_port = 2022
o tgt_addr = DR
o tgt_port = 9999
Fills it in accordingly:
o src_addr = DS
o src_port = 1111
o dst_addr = NAT2
o dst_port = 2022
o tgt_addr = DR
o tgt_port = 9999
Rewrites the packet:
o src_addr = DS
o src_port = 1111
o dst_addr = DR
o dst_port = 2222
And forwards it to DR. Once there, DR acknowledges it by sending back
a path succeeded message in reply, back to DS.
The path is now open and data transmission from DS:1111->DR:9999 can
commence.
6.4 Both end hosts are in same private network behind NATs
In this example (see Figure 23), DS, in a private address space,
wants to send data to DR, in another private address space. The point
marked "%" is yet another private address space. Notice that since
NAT1 and NAT3 have addresses in the same address space, NAT3 might
want to consider itself an edge NAT. We will consider both
situations.
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public
+------+ % +------+ //----\\
DS --| NAT1 |--+--| NAT2 |---| |
+------+ | +------+ \\----//
|
| +------+
+--| NAT3 |------------ DR
+------+
private
Figure 23: NAT to public, receiver in same private network Scenario
We will first assume that NAT3 has the edge_nat option set to false.
In this case, the connection is a combination of Section 6.3 and
Section 6.2.
Firstly DR will signal against on the data path, against the data
flow, with a reserve external address object. NAT3 will reserve the
address and forward the packet on to NAT2, who IS an edge NAT in all
cases. NAT2 will reply with the external address, and the connection
goes on just as in Section 6.2, except for the fact the topology
becomes:
public
+------+ +------+
DS --| NAT1 |-----o------o---+
+------+ | | |
| NAT2 |---+
| | |
+--o------o---+
| +------+
|
| +------+
+--| NAT3 |------------ DR
+------+
private
Figure 24: New topology due to the non optimal edge nat parameter
decision
This is not optimal, but the connection does succeed, and data flow
can commence.
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Let us now solve the case in which NAT3 has edge_nat set to auto.
Back in Figure 23, NAT3 will decide it IS an edge_nat if the
destination we pick up for the reserve address packet is in the
address space marked as "%", and will NOT consider itself an edge_nat
if we point it anywhere else. This is an optimization issue such as
the one pointed out in Section 6.3.
Well so, if it doesn't consider itself an edge NAT, we already saw
what the topological equivalent is, and how it proceeds. If it IS an
edge NAT, the topological equivalent would be:
+------+
DS --| NAT1 |--+
+------+ |
|
| +------+
+--| NAT3 |------------ DR
+------+
private
Figure 25: A good edge nat decision brings an optimal route
And we would proceed in the same way, only on a more optimal route.
6.5 IPv4/v6 NAT with two private networks
TBD
6.6 Full example for NAT/FW with two private networks
The NAT's have the nat_capabilities variable set to true. NAT+FW3 and
NAT+FW5 have the edge_nat variable set to true. The rest of boxes
have it set to false.
Let's now suppose that DR wants to get a data stream from DS in
Figure 26. For that, we need some way for B to get messages from A,
be it through some third party application server or some publicly
reachable proxy, perhaps made public through a NAT binding.
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+-----+
+--------| FW4 |--------+
| +-----+ |
+---------+ +---------+
| NAT+FW3 | | NAT+FW5 |
+---------+ +---------+
| |
+---------+ +---------+
| NAT3 | | NAT6 |
+---------+ +---------+
| |
+---------+ +---------+
| FW1 | | FW7 |
+---------+ +---------+
| |
+---------+ +---------+
| DS | | DR |
+---------+ +---------+
Data Flow
==========================>
Figure 26: Example network topology
DR wants a data stream from DS, which means that the direction of the
data is DS->DR. A will have to make itself publicly reachable by
signaling its NATs and firewalls as necessary. This is a step by step
guide to the whole process.
In steps 1 to 4, DR makes itself publicly reachable. From 5 and on,
DS is signaling on the data path towards DR.
1. DR wants to get data from DS, so it sends a reserve_addr object to
a target in the public internet. The closest this target is, the more
the chances that the resulting route is optimal, but any will work.
The reserve_addr obj looks like this:
o tgt_addr = DR
o tgt_port = 888
o src_addr = 0.0.0.0
o src_port = 0
Notice that this is a really loose pinhole, since any src_addr and
port is allowed.
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2. FW7 gets the packet, ignores its contents and forwards it.
Firewalls always ignore reserve_addr objects.
3. NAT6 gets the packet, and looks for a free port (say, 666, for
clarity's sake). It then adds an entry to its reservation list. The
entry looks like this:
o src_addr = 0.0.0.0
o src_port = 0
o dst_addr = NAT6
o dst_port = 666
o tgt_addr = DR
o tgt_port = 888
It then rewrites the packet so that it looks like:
o tgt_addr = NAT6
o tgt_port = 666
o src_addr = 0.0.0.0
o src_port = 0
Because it is not an edge NAT (edge_nat=false), it does not sent a
return_external_addr object to DR, but rather forwards the modified
packet. Note that no NAT binding is installed so far in NAT6,
although the state is now reserved.
4. NAT+FW5 receives the packet. The firewall part gets the object,
but, being as it is an address reservation only object, it ignores
it. The NAT part gets it next. Because it is a NAT, it binds a free
port, which is thus reserved. An entry to the reservation list is
added:
o src_addr = 0.0.0.0
o src_port = 0
o dst_addr = NAT+FW5
o dst_port = 555
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o tgt_addr = NAT6
o tgt_port = 666
Because it is an edge_nat, it sends a return_external_addr packet
with address NAT+FW5 and port 555 back to DR. It does so by simply
sending it back to the source IP address in the IP header of the
packet. In this case, it is NAT6. The standard capabilities of NAT6
will send it back to DR, since we are always working on the same
connection. Because it is an edge_nat and this is a
reserve_external_addr packet, it does not forward the packet.
At this stage, the end host DR has learned what its (reserved)
external address is, even if it can not be used. It is now publicly
reachable, and path-coupled NSIS signaling in direction DS->DR can
start.
5. Firstly, DR tells DS about it's freshly reserved outside address
through some higher layer protocol, using the third-party box.
6. DS now initiates signaling to DR by sending a create object to the
brand new public address of DR. It looks like:
o dst_addr = NAT+FW5
o dst_port = 555
o src_addr = DS
o src_port = 111
7. The firewall FW1 gets it, and installs the requested pinhole.
(Note this IS a tight pinhole with well defined source and
destination). It then forwards the packet.
8. NAT2 gets the packet. Because it is NOT coming from it's external
address, it realizes it is being asked to forward DS's future data
packets, and so, it will have to rewrite it's source address. To do
so, NAT2 picks a random free port (which turns out to be 222), and
installs a NAT rule that says: "Whatever packet comes from DS:111,
heading for NAT+FW5:555 will be rewritten so that the source address
looks like NAT2:222". That is usually known as Source NAT. The NSIS
create request is then rewritten to look like:
o dst_addr = NAT+FW5
o dst_port = 555
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o src_addr = NAT2
o src_port = 222
Because it is not an edge NAT, it simply forwards the modified
packet.
9. NAT+FW3 gets the packet next. Because it is NOT coming from the
extarnal_addr of the NAT+FW, The firewall part gets it first, and
installs the filter rule that says: "Allow traversal of packets going
from NAT2:222 towards NAT+FW5:555". It then hands it to the NAT part.
The NAT part gets it then. It is not coming from its external
address, and so, it does as NAT2, binding a port (333) and installing
a rule that says: "Whatever packet comes from NAT2:222, heading for
NAT+FW5:555, will be rewritten so that the source address looks like
NAT+FW3:333". It will then rewrite the create object to:
o dst_addr = NAT+FW5
o dst_port = 555
o src_addr = NAT+FW3
o src_port = 333
Note that the box won't send a packet back to DS informing it of its
external address, because DS will never need that.
10. FW4 gets the create object, and installs the rule "Allow
traversal of packets going from NAT+FW3:333 towards NAT+FW5:555" It
then forwards the object.
11. NAT+FW5 gets the create object. It arrived at its external
address, so it realizes it doesn't have to change the source address
of the future data packets of DS, but rather its destination. It also
means that the NAT part will have to handle it first. It then tries
to find out where it has to re-destined it to, by looking up its
reservation tables. It finds the previous reservation, by matching it
with their dst_addr and dst_port of the create object:
o src_addr = 0.0.0.0
o src_port = 0
o dst_addr = NAT+FW5
o dst_port = 555
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o tgt_addr = NAT6
o tgt_port = 666
And proceeds to fill it in with the information of the create object
(src_addr and src_port):
o src_addr = NAT+FW3
o src_port = 333
o dst_addr = NAT+FW5
o dst_port = 555
o tgt_addr = NAT6
o tgt_port = 666
It then installs a NAT rule with that information. It reads:
"Whatever packet comes from NAT+FW3:333, heading for NAT+FW5:555 will
be rewritten, so that its destination address looks like NAT6:666".
The reservation is erased and the rule starts working. The NAT
binding becomes thus usable.
The object is modified, so that it now looks like:
o dst_addr = NAT+FW3
o dst_port = 333
o src_addr = NAT6
o src_port = 666
The FW part now gets the object, and installs the rule: "Allow
traversal of whatever packet that comes from NAT+FW3:333 heading for
NAT6:666". The packet is then forwarded.
12. NAT6 gets the packet. As it comes from the external address, it
does as NAT+FW5, looking up the reservation list and filling it in
with:
o src_addr = NAT+FW3
o src_port = 333
o dst_addr = NAT6
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o dst_port = 666
o tgt_addr = DR
o tgt_port = 888
It then installs the rule: "Whatever packet comes from NAT+FW3:333,
heading for NAT6:666 will be rewritten, so that its destination
address looks like DR:888". The rule reservation is erased, and the
NAT binding becomes active. The object is rewritten as:
o src_addr = NAT+FW3
o src_port = 333
o dst_addr = DR
o dst_port = 888
The object is thus forwarded.
13. FW7 gets the packet now, and installs the rule: "Allow traversal
of whatever packet that comes from NAT+FW3:333 heading for DR:888".
It forwards the packet.
14: DR gets (finally) the packet. It realizes it is a create object
headed for him, to the port which he expected, and so it sees
everything went well. A reply to the packet is send, and the NAT's on
the way, knowing the already established connection, will route it to
DS. The packet is a path_succesful message, which simply means
"Everything is fine, send data whenever you want".
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7. NSIS NAT and Firewall transitions issues
NSIS NAT and Firewall transition issues are premature and will be
addressed in a separate draft (see [16]). An update of this section
will be based on consensus.
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8. Security Considerations
Security is of major concern particularly in case of firewall
traversal. Generic threats for NSIS signaling have been discussed in
[5] and are applicable here as well. It is necessary to provide
proper signaling message protection and proper authorization. Note
that the NAT is likely to be co-located with a firewall and might
therefore require packet filters to be changed in order to allow the
signaling message to process and to traverse. This section aims to
raise some items for further discussion and illustrates the problems
the authors faced when creating a security solution for the NAT/
Firewall NSLP.
Installing packet filters provides some security, but has some
weaknesses, which heavily depend on the type of packet filter
installed. A packet filter cannot prevent an adversary to inject
traffic (due to the IP spoofing capabilities). This type of attack
might not be particular helpful if the packet filter is a standard 5
tuple which is very restrictive. If packet filter installation,
however, allows specifying a rule, which restricts only the source IP
address, then IP spoofing allows transmitting traffic to an arbitrary
address. NSIS aims to provide path-coupled signaling and therefore an
adversary is somewhat restricted in the location from which attacks
can be performed. Some trust is therefore assumed from nodes and
networks along the path.
Without doubts there is a dependency on the security provided by the
NTLP. Section Section 3 and Section 2.2 motivates some trust
relationship and authorization scenarios. These scenarios deserve a
discussion since some of them (particularly one with a missing
network-to-network trust relationship) is different to what is know
from QoS signaling. To address some of these trust relationships and
authorization issues requires security mechanisms between
non-neighboring nodes at the NSLP layer. For the group of authors it
seems that peer-to-peer and end-to-middle security needs to be
provided. An NSLP security mechanism between neighboring NSLP peers
might be necessary if security mechanisms at the NTLP do not provide
adequate protection mechanisms. This issue is, however, still in
discussion.
As a design goal it seems to be favorable to reuse existing
mechanisms to the best extend possible. In most cases it is
necessary to carry the objects for end-to-middle as NSLP payloads
since the presence of NATs might prevent direct communication. Three
security mechanisms have to be considered in more detail in a future
version of this document: CMS [17] and Identity Representation for
RSVP [14]. The authors believe that CMS more suitable (since it
provides much more functionality). The details deserve further
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discussion and implementation experience.
With regard to signal between two end hosts even though the receiver
is behind a NAT this proposal suggests creating state by the data
receiver first. This allows NSIS signaling messages to traverse a
NAT at the receiver side (due to the established state at this NAT
box) and simplifies security handling. To achieve this behavior it
is required to install NSIS NTLP and NSLP state. Furthermore, it is
envisioned to associate the two signaling parts (one part from the
data sender to the NAT and the other part from the NAT to the data
receiver) with the help of the Session Identifier. As such, the
discussion in [14] is relevant for this document.
Another interesting property of this protocol proposal is to prevent
Denial of Service attacks against NAT boxes whereby an adversary
allocates NAT bindings with the help of data packets. Since these
data packets do not provide any type of authentication and are not
authorized any adversary is able to mount such an attack. This attack
has been mentioned at several places in the literature already and
is particularly harmful if no NAPT functionality is used (i.e. if a
new NAT binding consumes one IP address of a pool of IP addresses).
Using the protocol described in this document additional security can
be achieved and more fairness can be provided.
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9. Open Issues
At least the following issues require further discussion:
o Message format: The exact message format is still to be
determined, both in regards of bit level details and on
parameters, such as the need for an object header length, since,
until now, that is a constant.
o Message type numbering
o Error codes: error codes have to be defined still. Among others,
we will need: missing authorization, out of resources, unable to
understand the packet, or maximum resources for that individual
already allocated.
o middlebox default policies: allow for the configuration of the
default policies of the box. For a NAT+Firewall box, for instance,
the firewall default policy might be "accept", and so, no packet
filters would have to be installed on that regard (we would still
need the NAT bindings, though).
o IPv6 flow label usage
o Stacking
o Edit Section 6 "Solution Examples"
o Edit Security Consideration section
o Edit Appendix A.
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10. Contributors
A number of individuals have contributed to this draft. Since it was
not possible to list them all in the authors section, it was decided
to split it and move Marcus Brunner and Henning Schulzrinne into the
contributors section. Separating into two groups was done without
treating any one of them better (or worse) than others.
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Normative References
[1] Hancock et al, R., "Next Steps in Signaling: Framework", DRAFT
draft-ietf-nsis-fw-05.txt, October 2003.
[2] Brunner et al., M., "Requirements for Signaling Protocols",
DRAFT draft-ietf-nsis-req-09.txt, October 2003.
[3] Schulzrinne, H. and R. Hancock, "GIMPS: General Internet
Messaging Protocol for Signaling", DRAFT
draft-ietf-nsis-ntlp-00.txt, October 2003.
[4] IANA, "Special-Use IPv4 Addresses", RFC 3330, September 2002.
[5] Tschofenig, H. and D. Kroeselberg, "Security Threats for NSIS",
DRAFT draft-ietf-nsis-threats-01.txt, January 2003.
[6] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A. and A.
Rayhan, "Middlebox communication architecture and framework",
RFC 3303, August 2002.
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Informative References
[7] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations, RFC 2663", August 1999.
[8] Srisuresh, P. and M. Holdrege, "Network Address Translator
(NAT)Terminology and Considerations, RFC 2663".
[9] Srisuresh, P. and E. Egevang, "Traditional IP Network Address
Translator (Traditional NAT), RFC 3022", January 2001.
[10] Tsirtsis, G. and P. Srisuresh, "Network Address Translation -
Protocol Translation (NAT-PT), RFC 2766", February 2000.
[11] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues",
RFC 3234, February 2002.
[12] Srisuresh, P., Tsirtsis, G., Akkiraju, P. and A. Heffernan,
"DNS extensions to Network Address Translators (DNS_ALG)", RFC
2694, September 1999.
[13] Braden, B., Zhang, L., Berson, S., Herzog, S. and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", September 1997.
[14] Yadav, S., Yavatkar, R., Pabbati, R., Ford, P., Moore, T.,
Herzog, S. and R. Hess, "Identity Representation for RSVP", RFC
3182, October 2001.
[15] Tschofenig, H., Schulzrinne, H., Hancock, R., McDonald, A. and
X. Fu, "Security Implications of the Session Identifier", June
2003.
[16] Aoun and others...., C., "NAT/Firewall NSLP migration, routing,
NTLP requirements and off-path Considerations", October 2003.
[17] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369,
August 2002.
[18] Manner, J., Suihko, T., Kojo, M., Liljeberg, M. and K.
Raatikainen, "Localized RSVP", DRAFT draft-manner-lrsvp-00.txt,
November 2002.
[19] Tschofenig, H., Buechli, M., Van den Bosch, S. and H.
Schulzrinne, "NSIS Authentication, Authorization and Accounting
Issues", March 2003.
[20] Amini, L. and H. Schulzrinne, "Observations from router-level
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internet traces", DIMACS Workshop on Internet and WWW
Measurement, Mapping and Modelin Jersey) , Februar 2002.
[21] Adrangi, F. and H. Levkowetz, "Problem Statement: Mobile IPv4
Traversal of VPN Gateways",
draft-ietf-mobileip-vpn-problem-statement-req-02.txt (work in
progress), April 2003.
[22] Ohba, Y., Das, S., Patil, P., Soliman, H. and A. Yegin,
"Problem Space and Usage Scenarios for PANA",
draft-ietf-pana-usage-scenarios-06 (work in progress), April
2003.
[23] Shore, M., "The TIST (Topology-Insensitive Service Traversal)
Protocol", DRAFT draft-shore-tist-prot-00.txt, May 2002.
[24] Tschofenig, H., Schulzrinne, H. and C. Aoun, "A Firewall/NAT
Traversal Client for CASP", DRAFT
draft-tschofenig-nsis-casp-midcom-01.txt, March 2003.
[25] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[26] Brunner, M., Stiemerling, M., Martin, M., Tschofenig, H. and H.
Schulzrinne, "NSIS NAT/FW NSLP: Problem Statement and
Framework", DRAFT draft-brunner-nsis-midcom-ps-00.txt, June
2003.
Authors' Addresses
Martin Stiemerling
Network Laboratories, NEC Europe Ltd.
Kurfuersten-Anlage 36
Heidelberg 69115
Germany
Phone: +49 (0) 6221 905 11 13
EMail: stiemerling@netlab.nec.de
URI:
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Hannes Tschoefenig
Siemens AG
Otto-Hahn-Ring 6
Munich 81739
Germany
Phone:
EMail: Hannes.Tschofenig@siemens.com
URI:
Miquel Martin
Network Laboratories, NEC Europe Ltd.
Kurfuersten-Anlage 36
Heidelberg 69115
Germany
Phone: +49 (0) 6221 905 11 16
EMail: miquel.martin@netlab.nec.de
URI:
Cedric Aoun
Nortel Networks
France
EMail: cedric.aoun@nortelnetworks.com
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Appendix A. Inter-working of SIP with NSIS NATFW NSLP
This document aims at pinpointing the problems of using SIP in
nowadays networks, focusing on the problems derived of NAT's,
Firewalls and multi-path communications. It is intended to fit in a
scenario description that shows the necessity of NSIS, as well as
depicting it's requirements. However, note that there are a number of
other solutions available. For example the IETF Midcom working group
is working on [6].
A.1 The Session Initiation Protocol
[25] describes the Session Initiation Protocol, an application-layer
control protocol that can establish, modify, and terminate multimedia
sessions. This often involves several flows for video and voice,
which are transported over new connections. These use of dynamically
allocated ports which results in protocol complexity which can not be
handled by nowadays NAT's and Firewalls.
Session initiation when one or both of the users is behind a NAT is
also not possible, given the impossibility to address a private IP
over the internet. Moreover, network deployments often allow for
different paths per connection and direction, making the setup of the
middleboxes even more complicated.
The following figure depicts a typical SIP connection:
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Ernie(192.0.2.1) Bert(192.0.2.2)
| |
| 1# SIP INVITE |
+--------------------------------------->|
|
| 2# SIP Ringing |
|<---------------------------------------+
| |
| 3# SIP OK | <-- Call accepted
|<---------------------------------------+
| |
| 4# SIP ACK |
+--------------------------------------->|
| |
| 5# DATA |
|=======================================>|
|<=======================================|
| |
1# SIP Invite (192.0.2.1:? -> 192.0.2.2:SIP): I Listen on
192.0.2.1:1000 Ernie invites Bert to the conference, and informs
it's awaiting media data on port 1000.
2# SIP Ringing (192.0.2.2:SIP -> 192.0.2.1:?): Ringing Bert's
phone The ringing simply implies that there's something sip aware
on Bert's side, and that it's ringing his phone
3# SIP OK (192.0.2.2:SIP -> 192.0.2.1:?): Call accepted, I listen
on 192.0.2.2:2000 This OK means that the Bert took the phone off
hook, and thus accepted the call. It also informs Ernie that Bert
is awaiting his media data at port 2000
4# SIP ACK (192.0.2.1:? -> 192.0.2.2:SIP): All is fine, start
transmitting. ACK means the ports are accepted and the call can
start in the selected data ports on both sides.
5# DATA (192.0.2.1:? -> 192.0.2.2:2000 and 192.0.2.2:? ->
192.0.2.1:1000): Voice,image, video.. This is the actual data
being transmitted.
In the above example, SIP is used successfully to establish a
communication, which includes negotiating the data ports for the
actual transmission. Unfortunately, this scheme will not work for
more complex setups.
Let's now consider one firewall in the data path, be it on Ernie's or
Bert's network, or elsewhere in the middle. We assume that the
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firewall is allowing traffic directed to the SIP port. As to the rest
of the ports, a typical setup involves outgoing connections being
allowed, and incoming connections being dropped, except for those
already established. That is, we allow packets to go out and their
replies to come in, but disable all other traffic.
In this case, the connection is as follows, for the case of a
firewall on Ernie's network:
Ernie(192.0.2.1) FW Bert(192.0.2.2)
| | |
| 1# SIP INVITE | |
+--------------------------------------->|
| | |
| | 2# SIP Ringing |
|<---------------------------------------+
| | |
| | 3# SIP OK | <-- Call accepted
|<---------------------------------------+
| | |
| 4# SIP ACK | |
+--------------------------------------->|
| | |
| 5# DATA | |
|=======================================>|
| |<=======================|
| | |
Notice how the SIP messages #1 and #4 traverse the firewall, because
they are outbound, and how 2# and 3# traverse it too, because they
are replies to the connection established at 1#.
Notice now how 5# can go outwards, but Bert can not go through the
firewall to reach Ernie's port 1000. The reason is the connection is
a new one, and the firewall won't allow it through.
Bert will now get media from Ernie, but Ernie is never going to get
anything from Bert. The call is thus considered unsuccessful. The
reason is that the application level port negotiation is never
acknowledge by the network-transport layer firewall, which doesn't
know what to expect. We would still face the same problem if the
connection used a SIP Proxy, for it would only translate names into
IP addresses.
Let us now assume that we indeed have an application layer firewall,
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be it by design, or because we load some sort of SIP module to it.
The previous case would now work, since the firewall can now
understand the packets going through it and open the necessary ports.
Still, we cannot assume that SIP signalization packets and the actual
data follow the same path. The following figure shows a likely setup.
FW+ stands for one or more firewalls:
SIP Signalization Path +-----+
/---------------->-----------| FW+ |-------\
| +-----+ |
+------+ +------+ +-----+
|Ernie |----|Router| |Bert |
+------+ +------+ +-----+
| Data Path +-----+ |
\---------------->-----------| FW+ |-------/
+-----+
The SIP packets with the information about the listening ports now
travels on the SIP Signalization path, and so the firewalls on that
path can read them. The Data, though, is traveling through the Data
path, and the firewalls in that path never get to see the Invite and
Ok packets. They are thus unable to open the ports.
Two issues are arisen here: first, we need on-path signalization
unless we already know the path our packets will take; a highly
unlikely situation in today's internet. Second, if we patch the
firewalls to understand SIP, we will provide any caller with a
hole-puncher for the firewall, since SIP is not provisioned with
proper authentication mechanism.
It is now clear that tight firewalls prevent SIP from successfully
working. There is still another obstacle: NATs.
NATs provide for a link between two different address spaces,
typically connecting a private range network to a public range one.
As a consequence, connections going from the inside (usually the
private range) are translated using the NAT's public interface
address, and the replies are routed back. The public side of the
network can only see the NATs public interface, and know nothing of
the private network inside. This means computers outside the NAT
won't be able to address computers inside the NAT.
Let us analyse the SIP example when Ernie is behind a NAT. The
following figure depicts a typical session:
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Ernie(10.0.0.2) (10.0.0.1) NAT (192.0.2.1) Bert(192.0.2.2)
| | |
| 1# SIP INVITE | |
+--------------------------\ |
| |----------------->|
| | |
| | 2# SIP Ringing |
| /------------------+
|<-------------------------| |
| | |
| | 3# SIP OK | <-- Call accepted
| /------------------+
|<-------------------------| |
| | |
| 4# SIP ACK | |
+--------------------------\ |
| |----------------->|
| | |
| 5# DATA | |
|==========================\ |
| |=================>|
| | ?<=============|
| | |
The communication is analogous to the one in the previous examples,
except for the fact the NAT is rewriting the source address of the
packets as they traverse it.
For instance, packet 1# is going from 10.0.0.2:? towards
192.0.2.2:SIP. The NAT box intercepts the message and puts
192.0.2.1:? as the source address and port, with ? being a
dynamically picked port, which might be different from the original
one 1# used.
On the way back, Bert is replying to the source of the IP packet,
that is, 192.0.2.1, and so, when 2# reaches 192.0.2.1, the NAT know
it is a reply from 1#, because it established a NAT binding, and this
replaces the destination address, 192.0.2.1:? with 10.0.0.2:? and
forwards the packet inside the NAT.
As a result, Ernie never knows there is a NAT in his communication
path, since he sends and receives packets from 192.0.2.2 normally.
This means that the INVITE packet will tell Bert to send data back to
10.0.0.2, a private IP. Once the signalization is finished, and the
actual DATA transmission starts, Bert tries to connect to 10.0.0.2, a
private IP address, from the internet; The routers don't know how to
route this, and the packet is eventually dropped.
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One possible solution would be for Ernie to know the NAT exists, and
already indicate that it listens on 192.0.2.1, and not 10.0.0.2.
That, still would not work, since the NAT binding is not performed at
the NAT box.
A.2 Conclusions
The above examples display the inability to use standard SIP through
tight firewalls or NATs, and points at the necessity of a secure
on-path protocol to negotiate firewall pinholes and NAT bindings.
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Appendix B. Ad-Hoc networks
Some forms of ad-hoc networks exist where trust in the network is not
justified. Figure Figure 31 mainly illustrates the problems of
malicious NSIS entities graphically:
+------------------------------------------+ +--------//
| Ad-Hoc | | ISP
| Network | |
| regular data | |
| traffic by +---------+ | |
| node A |Malicious| | +-+--------+
| +-------------->+ Node +-----+///-->+ Firewall +-//
| ^ | 3 |===========>| 1 |
| | +---------+ injected +-+--------+
| | data traffic |
| | | |
| | | |
| +---+-----+ +---------+ | |
| + Node | | Node | | |
| | 1 | | 2 | | |
| +---------+ +---------+ | |
| ^ | +--------//
| | |
+----------+-------------------------------+
|
+--+---+
| Node |
| A |
+------+
Figure 31: Limits of packet filter security
An ad-hoc network consists of a number of nodes between the end host
(Node A) and the ISP to which Node A wants to get access. Although
Node A uses an authentication and key exchange protocol to create a
policy rule at the firewall 1 it is still possible for an untrusted
node (in this case Node 3) to inject data traffic which will pass
Firewall 1 since the data traffic is not authenticated. To prevent
this type of threat two approaches are possible. First, a restrictive
packet filter limits the capabilities of an adversary. Finally, there
is always the option of using data traffic protection.
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Appendix C. Interworking of Security Mechanisms and NSIS NATFW NSLP
TBD
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Appendix D. Solution approaches in case of missing authorization
D.1 Solution Approach: Local authorization from both end points
The first approach makes use of local authorization from both end
points. If Host A sends a signaling message toward the destination to
Middlebox 1 the message will perform the desired action in Network A.
Middlebox 1 establishes some state information and forwards the
signaling message towards Host B. Signaling message protection
between the two access networks might be difficult. A missing trust
relationship does not necessarily mean that no security association
establishment is possible. The lacking trust disallows Middlebox 1
(or indirectly Host A where the signaling message was initiated) to
create packet filters at Middlebox 2. We assume that the NSIS
signaling message is allowed to pass the firewall then it finally
reaches Host B. Due to the missing authorization no packet filter
specific state is created. The filters will be installed later after
receiving an authorization from Host B. When Host B returns a
confirmation or acknowledgement then Middlebox 2 treats it as an
authorization and finally triggers filter creation. The message is
then forwarded to Middlebox 1, where filters are either already
installed or require an additional confirmation. Finally the
signaling message is forwarded to Host A, which can be assured that
subsequent data traffic can be transmitted end-to-end from Host A to
Host B. The same procedure has to be applied again to signal
information for the other direction (Host B to Host A).
The following behavior has to be assumed in order for this approach
to be applicable:
1. Signaling messages must be allowed to pass firewalls along the
path.
2. NSIS signaling must operate in the described manner which could
be described as: Install where you have authorization - delay and
forward where you have no authorization.
This approach suffers from the following drawbacks:
1. Firewalls which block NSIS signaling from external networks or
nodes prevent a successful operation.
2. A full roundtrip is required to signal packet filter information.
The NSIS signaling message must therefore provide the capability
to route signaling message in both direction which might either
require state installation at nodes along the path (route
pinning) or a stateless version via record-route. Some risk of
DoS protection might exist.
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D.2 Solution Approach: Access Network-Only Signaling
The next approach is based on signaling packet filter information by
both hosts into the local access network only. An NSIS allows
specifying such a behavior by indicating the signaling endpoint with
the help of scoping (for example with domain name or a "local network
only" flag). Scoping means that the signaling message although
addressed to a particular destination IP address terminates somewhere
along the path. If packet filters for both directions have to be
installed then the signaling messages have to make packet filter
installations up- and downstream along the data path. Similar to
proposals in the area of QoS signaling some problems are likely to
occur. One such problem is that downstream signaling in general
causes problems because of asymmetric routes. In particular it is
difficult to determine the firewall where the downstream data traffic
will enter a network. The problem of triggering downstream
reservations is for example described in [18] . Another problem for
example is the placement of a firewall or NAT along the path other
than in the access network. This would prevent a successful data
exchange.
The following behavior has to be assumed in order for this approach
to be applicable:
1. It must be possible to trigger a signaling message exchange for a
downstream signaling message exchange at the firewall where the
data traffic enters the network.
2. No other firewalls or NATs are present along the path other than
in the access network.
This approach suffers from the following drawbacks:
1. To signal policy rules only within the access network (by both
end-points) has a number of disadvantage and challenges (see for
example [18] ). The complex message processing caused by this
approach strongly argues against it although it might sound
simple (and even might be simple in restricted environments).
2. Complex topologies might lead to ineffective policy rules (i.e.
data traffic hits firewalls hits wrong firewalls).
D.3 Solution Approach: Authorization Tokens
The last approach is based on some exchanged authorization tokens
which are created by an authorized entity (such as the PDP) or by a
trusted third party. Both end hosts need to exchange these tokens
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with protocols such as SIP or HTTP since these protocols are likely
to be allowed to bypass the firewall. The basic idea of this approach
is to provide an end host, which requests access to the network, with
credentials (referred as authorization tokens). These tokens have to
possess some properties, namely:
1. They have to be restrictive by including lifetimes, source and
destination identifiers, usage indication and more.
2. They have to provide basic replay protection to prevent
unauthorized reuse.
3. The have be cryptographically protected to prevent manipulations.
4. There has to be a mechanism to dynamically create them for a
specific reason and to distribute them to the end points.
5. It has to be possible to exchange tokens via a trusted third part
in cases where no direct communication between the end hosts is
possible (due to NAT).
6. The token can be created locally at the network or by a trusted
third party.
An example of a possible signaling communication could have the
following structure: After exchanging the tokens between the two end
hosts. Host A would include the received authorization token to the
signaling message for Network B. When the signaling message arrives
at Middlebox 2 then the token is verified by the token-creating
entity. In order to prevent parties from reusing the token timestamps
(e.g. token creation, token lifetime, etc.) have to be included.
Adding IP address information about Host A would create difficulties
in relationship with NATs. Information about Host B might be possible
to include in order to limit attacks where a token is lost and reused
by a different host for a different purpose. The goal is to restrict
the usage of the token for a specific session. The content of the
token only needs to be verified by the originator of the token since
it only has to be verified locally. Since authorization needs to be
linked to the authorized actions, which have to be performed on the
packets matching the packet filter, the token may include the
associated action or a reference to it. The following behavior has to
be assumed in order for this approach to be applicable:
1. The exchange of authorization tokens between end-systems must be
possible. These protocols must be allowed to pass the firewalls.
2. An end-system must be able to request such an authorization token
at some entity in the local network or at a trusted third party.
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This approach suffers from the following drawback:
1. Possibly an additional protocol is required for an end host to
request an authorization token from an entity in the local
network.
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Appendix E. Acknowledgments
We would like to acknowledge Vishal Sankhla and Joao Girao for their
input to this draft.
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