NSIS Working Group M. Stiemerling
Internet-Draft NEC
Intended status: Standards Track H. Tschofenig
Expires: May 22, 2008 NSN
C. Aoun
E. Davies
Folly Consulting
November 19, 2007
NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
draft-ietf-nsis-nslp-natfw-16.txt
Status of this Memo
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Copyright (C) The IETF Trust (2007).
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Abstract
This memo defines the NSIS Signaling Layer Protocol (NSLP) for
Network Address Translators (NATs) and firewalls. This NSLP allows
hosts to signal on the data path for NATs and firewalls to be
configured according to the needs of the application data flows. It
enables hosts behind NATs to obtain a public reachable address and
hosts behind firewalls to receive data traffic. The overall
architecture is given by the framework and requirements defined by
the Next Steps in Signaling (NSIS) working group. The network
scenarios, the protocol itself, and examples for path-coupled
signaling are given in this memo.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Scope and Background . . . . . . . . . . . . . . . . . . . 5
1.2. Terminology and Abbreviations . . . . . . . . . . . . . . 8
1.3. Middleboxes . . . . . . . . . . . . . . . . . . . . . . . 10
1.4. General Scenario for NATFW Traversal . . . . . . . . . . . 11
2. Network Deployment Scenarios using the NATFW NSLP . . . . . . 13
2.1. Firewall Traversal . . . . . . . . . . . . . . . . . . . . 13
2.2. NAT with two private Networks . . . . . . . . . . . . . . 14
2.3. NAT with Private Network on Sender Side . . . . . . . . . 15
2.4. NAT with Private Network on Receiver Side Scenario . . . . 15
2.5. Both End Hosts behind twice-NATs . . . . . . . . . . . . . 16
2.6. Both End Hosts Behind Same NAT . . . . . . . . . . . . . . 17
2.7. Multihomed Network with NAT . . . . . . . . . . . . . . . 18
2.8. Multihomed Network with Firewall . . . . . . . . . . . . . 19
3. Protocol Description . . . . . . . . . . . . . . . . . . . . . 20
3.1. Policy Rules . . . . . . . . . . . . . . . . . . . . . . . 20
3.2. Basic Protocol Overview . . . . . . . . . . . . . . . . . 21
3.2.1. Message Types . . . . . . . . . . . . . . . . . . . . 25
3.2.2. Classification of RESPONSE Messages . . . . . . . . . 25
3.2.3. NATFW NSLP Signaling Sessions . . . . . . . . . . . . 26
3.3. Basic Message Processing . . . . . . . . . . . . . . . . . 27
3.4. Calculation of Signaling Session Lifetime . . . . . . . . 27
3.5. Message Sequencing . . . . . . . . . . . . . . . . . . . . 30
3.6. Authentication, Authorization, and Policy Decisions . . . 31
3.7. Protocol Operations . . . . . . . . . . . . . . . . . . . 32
3.7.1. Creating Signaling Sessions . . . . . . . . . . . . . 32
3.7.2. Reserving External Addresses . . . . . . . . . . . . . 35
3.7.3. NATFW NSLP Signaling Session Refresh . . . . . . . . . 42
3.7.4. Deleting Signaling Sessions . . . . . . . . . . . . . 43
3.7.5. Reporting Asynchronous Events . . . . . . . . . . . . 44
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3.7.6. Proxy Mode of Operation . . . . . . . . . . . . . . . 46
3.8. De-Multiplexing at NATs . . . . . . . . . . . . . . . . . 49
3.9. Reacting to Route Changes . . . . . . . . . . . . . . . . 51
3.10. Updating Policy Rules . . . . . . . . . . . . . . . . . . 51
4. NATFW NSLP Message Components . . . . . . . . . . . . . . . . 53
4.1. NSLP Header . . . . . . . . . . . . . . . . . . . . . . . 53
4.2. NSLP Objects . . . . . . . . . . . . . . . . . . . . . . . 54
4.2.1. Signaling Session Lifetime Object . . . . . . . . . . 55
4.2.2. External Address Object . . . . . . . . . . . . . . . 55
4.2.3. Extended Flow Information Object . . . . . . . . . . . 56
4.2.4. Information Code Object . . . . . . . . . . . . . . . 57
4.2.5. Nonce Object . . . . . . . . . . . . . . . . . . . . . 60
4.2.6. Message Sequence Number Object . . . . . . . . . . . . 60
4.2.7. Data Terminal Information Object . . . . . . . . . . . 61
4.2.8. ICMP Types Object . . . . . . . . . . . . . . . . . . 62
4.3. Message Formats . . . . . . . . . . . . . . . . . . . . . 63
4.3.1. CREATE . . . . . . . . . . . . . . . . . . . . . . . . 64
4.3.2. EXTERNAL (EXT) . . . . . . . . . . . . . . . . . . . . 64
4.3.3. RESPONSE . . . . . . . . . . . . . . . . . . . . . . . 65
4.3.4. NOTIFY . . . . . . . . . . . . . . . . . . . . . . . . 65
5. Security Considerations . . . . . . . . . . . . . . . . . . . 67
5.1. Authorization Framework . . . . . . . . . . . . . . . . . 67
5.1.1. Peer-to-Peer Relationship . . . . . . . . . . . . . . 67
5.1.2. Intra-Domain Relationship . . . . . . . . . . . . . . 68
5.1.3. End-to-Middle Relationship . . . . . . . . . . . . . . 69
5.2. Security Framework for the NAT/Firewall NSLP . . . . . . . 70
5.2.1. Security Protection between neighboring NATFW NSLP
Nodes . . . . . . . . . . . . . . . . . . . . . . . . 70
5.2.2. Security Protection between non-neighboring NATFW
NSLP Nodes . . . . . . . . . . . . . . . . . . . . . . 71
6. IAB Considerations on UNSAF . . . . . . . . . . . . . . . . . 73
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 74
8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 76
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 77
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 78
10.1. Normative References . . . . . . . . . . . . . . . . . . . 78
10.2. Informative References . . . . . . . . . . . . . . . . . . 78
Appendix A. Selecting Signaling Destination Addresses for EXT . . 80
Appendix B. Applicability Statement on Data Receivers behind
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Firewalls . . . . . . . . . . . . . . . . . . . . . . 81
Appendix C. Firewall and NAT Resources . . . . . . . . . . . . . 83
C.1. Wildcarding of Policy Rules . . . . . . . . . . . . . . . 83
C.2. Mapping to Firewall Rules . . . . . . . . . . . . . . . . 83
C.3. Mapping to NAT Bindings . . . . . . . . . . . . . . . . . 84
C.4. NSLP Handling of Twice-NAT . . . . . . . . . . . . . . . . 84
Appendix D. Assigned Numbers for Testing . . . . . . . . . . . . 86
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 87
Intellectual Property and Copyright Statements . . . . . . . . . . 88
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1. Introduction
1.1. Scope and Background
Firewalls and Network Address Translators (NAT) have both been used
throughout the Internet for many years, and they will remain present
for the foreseeable future. Firewalls are used to protect networks
against certain types of attacks from internal networks and the
Internet, whereas NATs provide a virtual extension of the IP address
space. Both types of devices may be obstacles to some applications,
since they only allow traffic created by a limited set of
applications to traverse them, typically those that use protocols
with relatively predetermined and static properties (e.g., most HTTP
traffic, and other client/server applications). Other applications,
such as IP telephony and most other peer-to-peer applications, which
have more dynamic properties, create traffic that is unable to
traverse NATs and firewalls unassisted. In practice, the traffic of
many applications cannot traverse autonomous firewalls or NATs, even
when they have additional functionality which attempts to restore the
transparency of the network.
Several solutions to enable applications to traverse such entities
have been proposed and are currently in use. Typically, application
level gateways (ALG) have been integrated with the firewall or NAT to
configure the firewall or NAT dynamically. Another approach is
middlebox communication (MIDCOM). In this approach, ALGs external to
the firewall or NAT configure the corresponding entity via the MIDCOM
protocol [7]. Several other work-around solutions are available,
such as STUN [15]. However, all of these approaches introduce other
problems that are generally hard to solve, such as dependencies on
the type of NAT implementation (full-cone, symmetric, etc), or
dependencies on certain network topologies.
NAT and firewall (NATFW) signaling shares a property with Quality of
Service (QoS) signaling. The signaling of both must reach any device
on the data path that is involved in, respectively, NATFW or QoS
treatment of data packets. This means, that for both, NATFW and QoS,
it is convenient if signaling travels path-coupled, meaning that the
signaling messages follow exactly the same path that the data packets
take. RSVP [11] is an example of a current QoS signaling protocol
that is path-coupled. [21] proposes the use of RSVP as firewall
signaling protocol but does not include NATs.
This memo defines a path-coupled signaling protocol for NAT and
firewall configuration within the framework of NSIS, called the NATFW
NSIS Signaling Layer Protocol (NSLP). The general requirements for
NSIS are defined in [5] and the general framework of NSIS is outlined
in [4]. It introduces the split between an NSIS transport layer and
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an NSIS signaling layer. The transport of NSLP messages is handled
by an NSIS Network Transport Layer Protocol (NTLP, with General
Internet Signaling Transport (GIST) [2] being the implementation of
the abstract NTLP). The signaling logic for QoS and NATFW signaling
is implemented in the different NSLPs. The QoS NSLP is defined in
[6].
The NATFW NSLP is designed to request the dynamic configuration of
NATs and/or firewalls along the data path. Dynamic configuration
includes enabling data flows to traverse these devices without being
obstructed, as well as blocking of particular data flows at inbound
firewalls. Enabling data flows requires the loading of firewall
rules with an action that allows the data flow packets to be
forwarded and creating NAT bindings. Blocking of data flows requires
the loading of firewalls rules with an action that will deny
forwarding of the data flow packets. A simplified example for
enabling data flows: A source host sends a NATFW NSLP signaling
message towards its data destination. This message follows the data
path. Every NATFW NSLP-enabled NAT/firewall along the data path
intercepts these messages, processes them, and configures itself
accordingly. Thereafter, the actual data flow can traverse all these
configured firewalls/NATs.
It is necessary to distinguish between two different basic scenarios
when operating the NATFW NSLP, independent of the type of the
middleboxes to be configured.
1. Both, data sender and data receiver, are NSIS NATFW NSLP aware.
This includes the cases where the data sender is logically
decomposed from the NSIS initiator or the data receiver logically
decomposed from the NSIS receiver, but both sides support NSIS.
This scenario assumes deployment of NSIS all over the Internet,
or at least at all NATs and firewalls. This scenario is used as
base assumption, if not otherwise noted.
2. Only one end host or region of the network is NSIS NATFW NSLP
aware, either data receiver or data sender. This scenario is
referred to as proxy mode.
The NATFW NSLP has two basic signaling messages which are sufficient
to cope with the various possible scenarios likely to be encountered
before and after widespread deployment of NSIS:
CREATE message: The basic message for configuring a path outbound
from a data sender to a data receiver.
EXTERNAL (EXT) message: Used to locate inbound NATs/firewalls and
prime them to expect outbound signaling and at NATs to pre-
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allocate a public address. This is used for data receivers behind
these devices to enable their reachability.
CREATE and EXT messages are sent by the NSIS initiator (NI) towards
the NSIS responder (NR). Both type of messages are acknowledged by a
subsequent RESPONSE message. This RESPONSE message is generated by
the NR if the requested configuration can be established, otherwise
the NR or any of the NSIS forwarders (NFs) can also generate such a
message if an error occurs. NFs and the NR can also generate
asynchronous messages to notify the NI, the so called NOTIFY
messages.
If the data receiver resides in a private addressing realm or
firewall, and needs to preconfigure the edge-NAT/edge-firewall to
provide a (publicly) reachable address for use by the data sender, a
combination of EXTERNAL and CREATE messages is used.
During the introduction of NSIS, it is likely that one or other of
the data sender and receiver will not be NSIS aware. In these cases,
the NATFW NSLP can utilize NSIS aware middleboxes on the path between
the data sender and data receiver to provide proxy NATFW NSLP
services (i.e., the proxy mode operation). Typically, these boxes
will be at the boundaries of the realms in which the end hosts are
located.
The CREATE and EXT messages create NATFW NSLP and NTLP state in NSIS
entities. NTLP state allows signaling messages to travel in the
forward (outbound) and the reverse (inbound) direction along the path
between a NAT/firewall NSLP sender and a corresponding receiver.
This state is managed using a soft-state mechanism, i.e., it expires
unless it is refreshed from time to time. The NAT bindings and
firewall rules being installed during the state setup are bound to
the particular signaling session. However, the exact local
implementation of the NAT bindings and firewall rules are NAT/
firewall specific.
This memo is structured as follows. Section 2 describes the network
environment for NATFW NSLP signaling. Section 3 defines the NATFW
signaling protocol and Section 4 defines the message components and
the overall messages used in the protocol. The remaining parts of
the main body of the document, covers security considerations
Section 5, IAB considerations on UNilateral Self-Address Fixing
(UNSAF) [12] in Section 6 and IANA considerations in Section 7.
Please note that readers familiar with firewalls and NATs and their
possible location within networks can safely skip Section 2.
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1.2. Terminology and Abbreviations
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [1].
This document uses a number of terms defined in [5] and [4]. The
following additional terms are used:
o Policy rule: 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" [16]. In the context of NSIS
NATFW NSLP, the conditions are the specification of a set of
packets to which the rule is applied. The set of actions always
contains just a single element per rule, and is limited to either
action "deny" or action "allow".
o Reserved policy rule: A policy rule stored at NATs or firewalls
for activation by a later, different signaling exchange. This
type of policy rule is kept in the NATFW NSLP and is not loaded
into the firewall or NAT engine, i.e., it does not affect the data
flow handling.
o Installed policy rule: A policy rule in operation at NATs or
firewalls. This type of rule is kept in the NATFW NSLP and is
loaded into the firewall or NAT engine, i.e., it is affecting the
data flow.
o Remembered policy rule: A policy rule stored at NATs and firewalls
for immediate use, as soon as the signaling exchange is
successfully completed.
o Firewall: A packet filtering device that matches packets against a
set of policy rules and applies the actions. In the context of
NSIS NATFW NSLP we refer to this device as a firewall.
o Network Address Translator: Network Address Translation is a
method by which IP addresses are mapped from one IP address realm
to another, in an attempt to provide transparent routing between
hosts (see [9]). Network Address Translators are devices that
perform this work by modifying packets passing through them.
o Middlebox: "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" [10]. In the context of this document, the term
middlebox refers to firewalls and NATs only. Other types of
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middlebox are outside of the scope of this document.
o Data Receiver (DR): The node in the network that is receiving the
data packets of a flow.
o Data Sender (DS): The node in the network that is sending the data
packets of a flow.
o NATFW NSLP peer or peer: An NSIS NATFW NSLP node with which an
NSIS adjacency has been created as defined in [2].
o NATFW NSLP signaling session or signaling session: A signaling
session defines an association between the NI, NFs, and the NR
related to a data flow. All the NATFW NSLP peers on the path,
including the NI and the NR, use the same identifier to refer to
the state stored for the association. The same NI and NR may have
more than one signaling session active at any time. The state for
NATFW NSLP consists of NSLP state and associated policy rules at a
middlebox.
o Edge-NAT: An edge-NAT is a NAT device with a globally routable IP
address which is reachable from the public Internet.
o Edge-firewall: An edge-firewall is a firewall device that is
located on the border line of an administrative domain.
o Public Network: "A Global or Public Network is an address realm
with unique network addresses assigned by Internet Assigned
Numbers Authority (IANA) or an equivalent address registry. This
network is also referred as external network during NAT
discussions" [9].
o Private/Local Network: "A private network is an address realm
independent of external network addresses. Private network may
also be referred alternately as Local Network. Transparent
routing between hosts in private realm and external realm is
facilitated by a NAT router" [9].
o Public/Global IP address: An IP address located in the public
network according to Section 2.7 of [9].
o Private/Local IP address: An IP address located in the private
network according to Section 2.8 of [9].
o Signaling Destination Address (SDA): An IP address generally taken
from the public/global IP address range, although, the SDA may in
certain circumstances be part of the private/local IP address
range. This address is used in EXT signaling message exchanges,
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if the data receiver's IP address is unknown.
1.3. Middleboxes
The term middlebox covers a range of devices which intercept the flow
of packets between end hosts and perform actions other than standard
forwarding expected in an IP router. As such, middleboxes fall into
a number of categories with a wide range of functionality, not all of
which is pertinent to the NATFW NSLP. Middlebox categories in the
scope of this memo are firewalls that filter data packets against a
set of filter rules, and NATs that translate packet addresses from
one address realm to another address realm. Other categories of
middleboxes, such as QoS traffic shapers, are out of scope of this
memo.
The term NAT used in this document is a placeholder for a range of
different NAT flavors. We consider the following types of NATs:
o Traditional NAT (basic NAT and NAPT)
o Bi-directional NAT
o Twice-NAT
o Multihomed NAT
For definitions and a detailed discussion about the characteristics
of each NAT type please see [9].
All types of middleboxes under consideration here, use policy rules
to make a decision on data packet treatment. Policy rules consist of
a flow identifier which selects the packets to which the policy
applies and an associated action; data packets matching the flow
identifier are subjected to the policy rule action. A typical flow
identifier is the 5-tuple selector which matches the following fields
of a packet to configured values:
o Source and destination IP addresses
o Transport protocol number
o Transport source and destination port numbers
Actions for firewalls are usually one or more of:
o Allow: forward data packet
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o Deny: block data packet and discard it
o Other actions such as logging, diverting, duplicating, etc
Actions for NATs include (amongst many others):
o Change source IP address and transport port number to a globally
routeable IP address and associated port number.
o Change destination IP address and transport port number to a
private IP address and associated port number.
It should be noted that a middlebox may contain two logical
representations of the policy rule. The policy rule has a
representation within the NATFW NSLP, comprising the message routing
information (MRI) of the NTLP and NSLP information (such as the rule
action). The other representation is the implementation of the NATFW
NSLP policy rule within the NAT and firewall engine of the particular
device. Refer to Appendix C for further details.
1.4. General Scenario for NATFW Traversal
The purpose of NSIS NATFW signaling is to enable communication
between endpoints across networks, even in the presence of NAT and
firewall middleboxes that have not been specially engineered to
facilitate communication with the application protocols used. This
removes the need to create and maintain application layer gateways
for specific protocols that have been commonly used to provide
transparency in previous generations of NAT and firewall middleboxes.
It is assumed that these middleboxes will be statically configured in
such a way that NSIS NATFW signaling messages themselves are allowed
to reach the locally installed NATFW NSLP daemon. NSIS NATFW NSLP
signaling is used to dynamically install additional policy rules in
all NATFW middleboxes along the data path that will allow
transmission of the application data flow(s). 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. An
additional capability, that is an exception to the primary goal of
NSIS NATFW signaling, is that the NATFW nodes can request blocking of
particular data flows instead of enabling these flows at inbound
firewalls.
The basic high-level picture of NSIS usage is that end hosts are
located behind middleboxes, meaning that there is a middlebox on the
data path from the end host in a private network and the external
network (NATFW in Figure 1). Applications located at these end hosts
try to establish communication with corresponding applications on
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other such end hosts. They trigger the NSIS entity at the local host
to control provisioning for middlebox traversal along the prospective
data path (e.g., via an API call). The NSIS entity in turn uses NSIS
NATFW NSLP signaling to establish policy rules along the data path,
allowing the data to travel from the sender to the receiver
unobstructed.
Application Application Server (0, 1, or more) Application
+----+ +----+ +----+
| +------------------------+ +------------------------+ |
+-+--+ +----+ +-+--+
| |
| NSIS Entities NSIS Entities |
+-+--+ +----+ +-----+ +-+--+
| +--------+ +----------------------------+ +-----+ |
+-+--+ +-+--+ +--+--+ +-+--+
| | ------ | |
| | //// \\\\\ | |
+-+--+ +-+--+ |/ | +-+--+ +-+--+
| | | | | Internet | | | | |
| +--------+ +-----+ +----+ +-----+ |
+----+ +----+ |\ | +----+ +----+
\\\\ /////
sender NATFW (1+) ------ NATFW (1+) receiver
Note that 1+ refers to one or more NATFW nodes.
Figure 1: Generic View of NSIS with NATs and/or Firewalls
For end-to-end NATFW signaling, it is necessary that each firewall
and each NAT along the path between the data sender and the data
receiver implements the NSIS NATFW NSLP. There might be several NATs
and FWs in various possible combinations on a path between two hosts.
Section 2 presents a number of likely scenarios with different
combinations of NATs and firewalls.
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2. Network Deployment Scenarios using the NATFW NSLP
This section introduces several scenarios for middlebox placement
within IP networks. Middleboxes are typically found at various
different locations, including at enterprise network borders, within
enterprise networks, as mobile phone network gateways, etc. Usually,
middleboxes are placed more towards the edge of networks than in
network cores. Firewalls and NATs may be found at these locations
either alone, or they may be combined; other categories of
middleboxes may also be found at such locations, possibly combined
with the NATs and/or firewalls. Using combined middleboxes typically
reduces the number of network elements needed.
NSIS initiators (NI) send NSIS NATFW NSLP signaling messages via the
regular data path to the NSIS responder (NR). On the data path,
NATFW NSLP signaling messages reach different NSIS nodes that
implement the NATFW NSLP. Each NATFW NSLP node processes the
signaling messages according to Section 3 and, if necessary, installs
policy rules for subsequent data packets.
Each of the following sub-sections 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. Firewall Traversal
This section describes a scenario with firewalls only; NATs are not
involved. Each end host is behind a firewall. The firewalls are
connected via the public Internet. Figure 2 shows the topology. The
part labeled "public" is the Internet connecting 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 the data path must provide traversal service for
NATFW NSLP in order to permit the NSIS message to reach the other end
host. All firewalls process NSIS signaling and establish appropriate
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policy rules, so that the required data packet flow can traverse
them.
There are several very different ways to place firewalls in a network
topology. To distinguish firewalls located at network borders, such
as administrative domains, from others located internally, the term
edge-firewall is used. A similar distinction can be made for NATs,
with an edge-NAT fulfilling the equivalent role.
2.2. NAT with two private Networks
Figure 3 shows a scenario with NATs at both ends of the network.
Therefore, each application instance, the NSIS initiator and the NSIS
responder, are behind NATs. The outermost NAT, known as the edge-NAT
(MB2 and MB3), at each side is connected to the public Internet. The
NATs are generically labeled as MBX (for middlebox No. X), since
those devices certainly implement NAT functionality, but can
implement firewall functionality as well.
Only two middleboxes MB are shown in Figure 3 at each side, but in
general, any number of MBs on each side must be considered.
+----+ +----+ //----\\ +----+ +----+
NI --| MB1|-----| MB2|---| |---| MB3|-----| MB4|--- 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 the middleboxes
on the path (MB1 to MB4, in this order), and all the middleboxes must
be configured properly to allow NSIS signaling to traverse them. The
NATFW signaling must configure all middleboxes and consider any
address translation that will result from this configuration in
further signaling. The sender (NI) has to know the IP address of the
receiver (NR) in advance, otherwise it will not be possible to send
any NSIS signaling messages towards the responder. Note that this IP
address is not the private IP address of the responder but the NAT's
public IP address (here MB3's IP address). Instead a NAT binding
(including a public IP address) has to be previously installed on the
NAT MB3. This NAT binding subsequently allows packets reaching the
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NAT to be forwarded to the receiver within the private address realm.
The receiver might have a number of ways to learn its public IP
address and port number (including the NATFW NSLP) and might need to
signal this information to the sender using the application level
signaling protocol.
2.3. NAT with Private Network on Sender Side
This scenario shows an application instance at the sending node that
is behind one or more NATs (shown as generic MB, see discussion in
Section 2.2). 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 Side Scenario
The traffic from NI to NR has to traverse middleboxes only 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.
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 whether itself is behind a NAT or not.
2.4. NAT with Private Network on Receiver Side Scenario
The application instance receiving data is behind one or more NATs
shown as MB (see discussion in Section 2.2).
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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
Initially, the NSIS responder must determine its publicly reachable
IP address at the external middlebox and notify the NSIS initiator
about this address. One possibility is that an application level
protocol is used, meaning that the public IP address is signaled via
this protocol to the NI. Afterwards the NI can start its signaling
towards the NR and therefore establish the path via the middleboxes
in the receiver side private network.
This scenario describes the use case for the EXTERNAL message of the
NATFW NSLP.
2.5. Both End Hosts behind twice-NATs
This is a special case, where the main problem arises from the need
to detect that both end hosts are logically within the same address
space, but are also in two partitions of the address realm on either
side of a twice-NAT (see [9] for a discussion of twice-NAT
functionality).
Sender and receiver are both within a single private address realm
but the two partitions potentially have overlapping IP address
ranges. Figure 6 shows the arrangement of NATs.
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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 on either side of a
twice-NAT Scenario
The middleboxes shown in Figure 6 are twice-NATs, i.e., they map IP
addresses and port numbers on both sides, meaning the mapping of
source and destination address at the private and public interfaces.
This scenario requires the assistance of application level entities,
such as a DNS server. The application level entities must handle
requests 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, such as MIDCOM [7]. NSIS
signaling is not required in the twice-NAT only case, since
middleboxes of the twice-NAT type are normally configured by other
means. Nevertheless, NSIS signaling might be useful when there are
also firewalls on the path. In this case NSIS will not configure any
policy rule at twice-NATs, but will configure policy rules at the
firewalls on the path. The NSIS signaling protocol must be at least
robust enough to survive this scenario. This requires that twice-
NATs must implement the NATFW NSLP also and participate in NATFW
signaling sessions but they do not change the configuration of the
NAT, i.e., they only read the address mapping information out of the
NAT and translate the Message Routing Information (MRI, [2]) within
the NSLP and NTLP accordingly. For more information see Appendix C.4
2.6. 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 2.4 the NSIS responder must
determine its public IP address in advance and transfer it to the
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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
initiator to public IP address of responder (first time) and from
public IP address of responder to responder (second time).
NI public
\ +----+ //----\\
+-| MB |----| |
/ +----+ \\----//
NR
private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 7: NAT to Public, Both Hosts Behind Same NAT
2.7. Multihomed Network with NAT
The previous sub-sections sketched network topologies where several
NATs and/or firewalls are ordered sequentially on the path. This
section describes a multihomed scenario with two NATs placed on
alternative paths to the public network.
+----+ //---\\
NI -------| MB |---| |
\ +----+ \\-+-//
\ |
\ +----- NR
\ |
\ +----+ //-+-\\
--| MB |---| |
+----+ \\---//
private public
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
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Figure 8: Multihomed Network with Two NATs
Depending on the destination, either one or the other middlebox is
used for the data flow. Which middlebox is used, depends on local
policy or routing decisions. NATFW NSLP must be able to handle this
situation properly, see Section 3.7.2 for an extended discussion of
this topic with respect to NATs.
2.8. Multihomed Network with Firewall
This section describes a multihomed scenario with two firewalls
placed on alternative paths to the public network (Figure 9). The
routing in the private and public network decides which firewall is
being taken for data flows. Depending on the data flow's direction,
either outbound or inbound, a different firewall could be traversed.
This is a challenge for the EXT message of the NATFW NSLP where the
NSIS responder is located behind these firewalls within the private
network. The EXT message is used to block a particular data flow on
an inbound firewall. NSIS must route the EXT message inbound from NR
to NI probably without knowing which path the data traffic will take
from NI to NR (see also Appendix B).
+----+
NR -------| MB |\
\ +----+ \ //---\\
\ -| |-- NI
\ \\---//
\ +----+ |
--| MB |-------+
+----+
private
private public
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 9: Multihomed Network with two Firewalls
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3. Protocol Description
This section defines messages, objects, and protocol semantics for
the NATFW NSLP.
3.1. Policy Rules
Policy rules, bound to a NATFW NSLP signaling session, are the
building blocks of middlebox devices considered in the NATFW NSLP.
For firewalls the policy rule usually consists of a 5-tuple, source/
destination addresses, transport protocol, and source/destination
port numbers, plus an action, such as allow or deny. For NATs the
policy rule consists of the action 'translate this address' and
further mapping information, that might be, in the simplest case,
internal IP address and external IP address.
The NATFW NSLP carries, in conjunction with the NTLP's Message
Routing Information (MRI), the policy rules to be installed at NATFW
peers. This policy rule is an abstraction with respect to the real
policy rule to be installed at the respective firewall or NAT. It
conveys the initiator's request and must be mapped to the possible
configuration on the particular used NAT and/or firewall in use. For
pure firewalls one or more filter rules must be created and for pure
NATs one or more NAT bindings must be created. In mixed firewall and
NAT boxes, the policy rule must be mapped to filter rules and
bindings observing the ordering of the firewall and NAT engine.
Depending on the ordering, NAT before firewall or vice versa, the
firewall rules must carry public or private IP addresses. However,
the exact mapping depends on the implementation of the firewall or
NAT which is different for each vendor.
The policy rule at the NATFW NSLP level comprises the message routing
information (MRI) part, carried in the NTLP, and the information
available in the NATFW NSLP. The information provided by the NSLP is
stored in the 'extend flow information' (NATFW_EFI) and 'data
terminal information' (NATFW_DTINFO) objects, and the message type.
Additional information, such as the external IP address and port
number, stored in the NAT or firewall, will be used as well. The MRI
carries the filter part of the NAT/firewall-level policy rule that is
to be installed.
The NATFW NSLP specifies two actions for the policy rules: deny and
allow. A policy rule with action set to deny will result in all
packets matching this rule to be dropped. A policy rule with action
set to allow will result in all packets matching this rule to be
forwarded.
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3.2. Basic Protocol Overview
The NSIS NATFW NSLP is carried over the General Internet Signaling
Transport (GIST, the implementation of the NTLP) defined in [2].
NATFW NSLP messages are initiated by the NSIS initiator (NI), handled
by NSIS forwarders (NF) and received by the NSIS responder (NR). It
is required that at least NI and NR implement this NSLP, intermediate
NFs only implement this NSLP when they provide relevant middlebox
functions. NSIS forwarders that do not have any NATFW NSLP functions
just forward these packets as they have no interest in them.
A Data Sender (DS), intending to send data to a Data Receiver (DR)
has to start NATFW NSLP signaling. This causes the NI associated
with the data sender (DS) to launch NSLP signaling towards the
address of data receiver (DR) (see Figure 10). Although it is
expected that the DS and the NATFW NSLP NI will usually reside on the
same host, this specification does not rule out scenarios where the
DS and NI reside on different hosts, the so-called proxy mode (see
Section 3.7.6.)
+-------+ +-------+ +-------+ +-------+
| DS/NI |<~~~| MB1/ |<~~~| MB2/ |<~~~| DR/NR |
| |--->| NF1 |--->| NF2 |--->| |
+-------+ +-------+ +-------+ +-------+
========================================>
Data Traffic Direction (outbound)
---> : 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 10: General NSIS signaling
The following list shows the normal sequence of NSLP events without
detailing the interaction with the NTLP and the interactions on the
the NTLP level.
o NSIS initiators generate NATFW NSLP CREATE/EXT messages and send
these towards the NSIS responder. This CREATE/EXT message is the
initial message which creates a new NATFW NSLP signaling session.
The NI and the NR will most likely already share an application
session before they start the NATFW NSLP signaling session. Note
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the difference between both sessions.
o NSLP CREATE/EXT messages are processed each time a NF with NATFW
NSLP support is traversed. Each NF that is intercepting a CREATE/
EXT message and is accepting it for further treatment is joining
the particular NATFW NSLP signaling session. These 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.
o NSIS responders will check received messages and process them if
applicable. NSIS responders generate RESPONSE messages and send
them hop-by-hop back to the NI via the same chain of NFs
(traversal of the same NF chain is guaranteed through the
established reverse message routing state in the NTLP). The NR is
also joining the NATFW NSLP signaling session if the CREATE/EXT
message is accepted.
o The RESPONSE message is processed at each NF that has been
included in the prior NATFW NSLP signaling session setup.
o If the NI has received a successful RESPONSE message and if the
signaling NATFW NSLP session started with a CREATE message, the
data sender can start sending its data flow to the data receiver.
If the Ni has received a successful RESPONSE message and if the
signaling NATFW NSLP session started with a EXT message, the data
receiver is ready to receive further CREATE messages.
Because NATFW NSLP signaling follows the data path from DS to DR,
this immediately enables communication between both hosts for
scenarios with only firewalls on the data path or NATs on the sender
side. For scenarios with NATs on the receiver side certain problems
arise, as described in Section 2.4.
When the NR and the NI are located in different address realms and
the NR is located behind a NAT, the NI cannot signal to the NR
address directly. The DR/NR are not reachable from the NIs using the
private address of the NR and thus NATFW signaling messages cannot be
sent to the NR/DR's address. Therefore, the NR must first obtain a
NAT binding that provides an address that is reachable for the NI.
Once the NR has acquired a public IP address, it forwards this
information to the DS via a separate protocol. This application
layer signaling, which is out of scope of the NATFW NSLP, may involve
third parties that assist in exchanging these messages.
The same holds partially true for NRs located behind firewalls that
block all traffic by default. In this case, NR must tell its inbound
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firewalls of inbound NATFW NSLP signaling and corresponding data
traffic. Once the NR has informed the inbound firewalls, it can
start its application level signaling to initiate communication with
the NI. This application layer signaling, which is out of scope of
the NATFW NSLP, may involve third parties that assist in exchanging
these messages. This mechanism can be used by machines hosting
services behind firewalls as well. In this case, the NR informs the
inbound firewalls as described, but does not need to communicate this
to the NIs.
NATFW NSLP signaling supports this scenario by using the EXT message
1. The DR acquires a public address by signaling on the reverse path
(DR towards DS) and thus making itself available to other hosts.
This process of acquiring public addresses is called reservation.
During this process the DR reserves publicly reachable addresses
and ports suitable for further usage in application level
signaling and the publicly reachable address for further NATFW
NSLP signaling. However, the data traffic will not be allowed to
use this address/port initially (see next point). In the process
of reservation the DR becomes the NI for the messages necessary
to obtain the publicly reachable IP address, i.e., the NI for
this specific NATFW NSLP signaling session.
2. Now on the side of DS, the NI creates a new NATFW NSLP signaling
session and signals directly to the public IP address of DR.
This public IP address is used as NR's address, as the NI would
do if there is no NAT in between, and creates policy rules at
middleboxes. Note, that the reservation will only allow
forwarding of signaling messages, but not data flow packets.
Policy rules allowing forwarding of data flow packets set up by
the prior EXT message signaling will be activated when the
signaling from NI towards NR is confirmed with a positive
RESPONSE message. The EXTERNAL (EXT) message is described
inSection 3.7.2.
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+-------+ +-------+ +-------+ +-------+
| DS/NI |<~~~| MB1/ |<~~~| NR | | DR |
| |--->| NF1 |--->| | | |
+-------+ +-------+ +-------+ +-------+
========================================>
Data Traffic Direction (outbound)
---> : 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 11: A NSIS proxy mode signaling
The above usage assumes that both ends of a communication support
NSIS, but fails when NSIS is only deployed at one end of the path.
In this case only one of the receiving or sending side is NSIS aware
and not both at the same time. NATFW NSLP supports this scenario
(i.e., the DR does not support NSIS) by using a proxy mode, as
described in Section 3.7.6; the proxy mode operation also supports
scenarios with a data sender that does not support NSIS, i.e. the
data receiver must act to enable data flows towards itself.
The basic functionality of the NATFW NSLP provides for opening
firewall pin holes and creating NAT bindings to enable data flows to
traverse these devices. Firewalls are normally expected to work on a
'deny-all' policy, meaning that traffic not explicitly matching any
firewall filter rule will be blocked. Similarly, the normal behavior
of NATs is to block all traffic that does not match any already
configured/installed binding or NATFW NSLP session. However, some
scenarios require support of firewalls having 'allow-all' policies,
allowing data traffic to traverse the firewall unless it is blocked
explicitly. Data receivers can utilize NATFW NSLP's EXT message with
action set to 'deny' to install policy rules at inbound firewalls to
block unwanted traffic.
The protocol works on a soft-state basis, meaning that whatever state
is installed or reserved on a middlebox will expire, and thus be de-
installed or forgotten after a certain period of time. To prevent
premature removal of state that is needed for ongoing communication,
the NATFW NI involved will have to specifically request a NATFW NSLP
signaling session extension. An explicit NATFW NSLP state deletion
capability is also provided by the protocol.
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If the actions requested by a NATFW NSLP message cannot be carried
out, NFs and the NR must return a failure, such that appropriate
actions can be taken. They can do this either during a the request
message handling (synchronously) by sending an error RESPONSE
message, or at any time (asynchronously) by sending a notification
message.
The next sections define the NATFW NSLP message types and formats,
protocol operations, and policy rule operations.
3.2.1. Message Types
The protocol uses four messages types:
o CREATE: a request message used for creating, changing, refreshing,
and deleting NATFW NSLP signaling sessions, i.e., open the data
path from DS to DR.
o EXTERNAL (EXT): a request message used for reserving, changing,
refreshing, and deleting EXT NATFW NSLP signaling sessions. EXT
messages are forwarded to the edge-NAT or edge-firewall and allow
inbound CREATE messages to be forwarded to the NR. Additionally,
EXT messages reserve an external address and, if applicable, port
number at an edge-NAT.
o NOTIFY: an asynchronous message used by NATFW peers to alert
inbound NATFW peers about specific events (especially failures).
o RESPONSE: used as a response to CREATE and EXT request messages.
3.2.2. Classification of RESPONSE Messages
RESPONSE messages will be generated synchronously to CREATE and EXT
messages by NSIS Forwarders and Responders to report success or
failure of operations or some information relating to the NATFW NSLP
signaling session or a node. RESPONSE messages MUST NOT be generated
for any other message, such as NOTIFY and RESPONSE.
All RESPONSE messages MUST carry a NATFW_INFO object which contains a
severity class code and a response code (see Section 4.2.4). This
section defines terms for groups of RESPONSE messages depending on
the severity class.
o Successful RESPONSE: Messages carrying NATFW_INFO with severity
class 'Success' (0x2).
o Informational RESPONSE: Messages carrying NATFW_INFO with severity
class 'Informational' (0x1) (only used with NOTIFY messages).
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o Error RESPONSE: Messages carrying NATFW_INFO with severity class
other than 'Success' or 'Informational'.
3.2.3. NATFW NSLP Signaling Sessions
A NATFW NSLP signaling session defines an association between the NI,
NFs, and the NR related to a data flow. This association is created
when the initial CREATE or EXT message is successfully received at
the NFs or the NR. There is signaling NATFW NSLP session state
stored at the NTLP layer and at the NATFW NSLP level. The NATFW NSLP
signaling session state for the NATFW NSLP comprises NSLP state and
the associated policy rules at a middlebox.
The NATFW NSLP signaling session is identified by the session ID
(plus other information at the NTLP level). The session ID is
generated by the NI before the initial CREATE or EXT message is sent.
The value of the session ID MUST generated in a random way, i.e., the
output MUST NOT be easily guessable by third parties. The session ID
is not stored in any NATFW NSLP message but passed on to the NTLP.
A NATFW NSLP signaling session can conceptually be in different
states, implementations may use other or even more states. The
signaling session can have these states at a node:
o Pending: The NATFW NSLP signaling session has been created and the
node is waiting for a RESPONSE message to the CREATE or EXT
message. A NATFW NSLP signaling session in state 'Pending' MUST
be marked as 'Dead' if no corresponding RESPONSE message has been
received within the time of the locally granted NATFW NSLP
signaling session lifetime of the forwarded CREATE or EXT message
(as described in Section 3.4).
o Established: The NATFW NSLP signaling session is established, i.e,
the signaling has been successfully performed and the lifetime of
NATFW NSLP signaling session is counted from now on. A NATFW NSLP
signaling session in state 'Established' MUST be marked as 'Dead'
if no refresh message has been received within the time of the
locally granted NATFW NSLP signaling session lifetime of the
RESPONSE message (as described in Section 3.4).
o Dead: Either the NATFW NSLP signaling session is timed out or the
node has received an error RESPONSE message for the NATFW NSLP
signaling session and the NATFW NSLP signaling session can be
deleted.
o Transit: The node has received an asynchronous message, i.e., a
NOTIFY, and can delete the NATFW NSLP signaling session if needed.
When a node has received a NOTIFY message (for instance,
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indicating a route change) it marks it as 'Transit' and deletes
this NATFW NSLP signaling session if it is unused for some time
specific to the local node. This idle time does not need to be
fixed, since it can depend on the node local maintenance cycle,
i.e., the NATFW NSLP signaling session could be deleted if the
node runs it garbage collection cycle.
3.3. Basic Message Processing
All NATFW messages are subject to some basic message processing when
received at a node, independent of message type. Initially, the
syntax of the NSLP message is checked and a RESPONSE message with an
appropriate error of class 'Protocol error' (0x1) code is generated
if any problem is detected. If a message is delivered to the NATFW
NSLP, this implies that the NTLP layer has been able to correlate it
with the SID and MRI entries in its database. There is therefore
enough information to identify the source of the message and routing
information to route the message back to the NI through an
established chain of NTLP messaging associations. The message is not
further forwarded if any error in the syntax is detected. The
specific response codes stemming from the processing of objects are
described in the respective object definition section (see
Section 4). After passing this check, the NATFW NSLP node performs
authentication/authorization related checks described in Section 3.6.
Further processing is executed only if these tests have been
successfully passed, otherwise the processing stops and an error
RESPONSE is returned.
Further message processing stops whenever an error RESPONSE message
is generated, and the EXT or CREATE message is discarded.
3.4. Calculation of Signaling Session Lifetime
NATFW NSLP signaling sessions, and the corresponding policy rules
which may have been installed, are maintained via a soft-state
mechanism. Each signaling session is assigned a signaling session
lifetime and the signaling session is kept alive as long as the
lifetime is valid. After the expiration of the signaling session
lifetime, signaling sessions and policy rules MUST be removed
automatically and resources bound to them MUST be freed as well.
Signaling session lifetime is handled at every NATFW NSLP node. The
NSLP forwarders and NSLP responder MUST NOT trigger signaling session
lifetime extension refresh messages (see Section 3.7.3): this is the
task of the NSIS initiator.
The NSIS initiator MUST choose a NATFW NSLP signaling session
lifetime value (expressed in seconds) before sending any message,
including the initial message which creates the NATFW NSLP signaling
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session, to other NSLP nodes. The NATFW NSLP signaling session
lifetime value is calculated based on:
o the number of lost refresh messages that NFs should cope with;
o the end-to-end delay between the NI and NR;
o network vulnerability due to NATFW NSLP signaling session
hijacking ([8]), NATFW NSLP signaling session hijacking is made
easier when the NI does not explicitly remove the NATFW NSLP
signaling session);
o the user application's data exchange duration, in terms of time
and networking needs. This duration is modeled as M x R, with R
the message refresh period (in seconds) and M as a multiplier for
R;
o the load on the signaling plane. Short lifetimes imply more
frequent signaling messages.
o the acceptable time for a NATFW NSLP signaling session to be
present after it is no longer actually needed. For example, if
the existence of the NATFW NSLP signaling session implies a
monetary cost and teardown cannot be guaranteed, shorter lifetimes
would be preferable.
o the lease time of the NI's IP address. The chosen NATFW NSLP
signaling session lifetime must be larger than the lease time,
otherwise the IP address can be re-assigned to a different node.
This node may receive unwanted traffic, although it never has
requested a NAT/firewall configuration, which might be an issue in
mobile environments.
The RSVP specification [11] provides an appropriate algorithm for
calculating the NATFW NSLP signaling session lifetime as well as
means to avoid refresh message synchronization between NATFW NSLP
signaling sessions. [11] recommends:
1. The refresh message timer to be randomly set to a value in the
range [0.5R, 1.5R].
2. To avoid premature loss of state, lt (with lt being the NATFW
NSLP signaling session lifetime) must satisfy lt >= (K +
0.5)*1.5*R, where K is a small integer. Then in the worst case,
K-1 successive messages may be lost without state being deleted.
Currently K = 3 is suggested as the default. However, it may be
necessary to set a larger K value for hops with high loss rate.
Other algorithms could be used to define the relation between the
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NATFW NSLP signaling session lifetime and the refresh message
period; the algorithm provided is only given as an example.
This requested NATFW NSLP signaling session lifetime value lt is
stored in the NATFW_LT object of the NSLP message.
NSLP forwarders can execute the following behavior with respect to
the lifetime handling:
Requested signaling session lifetime acceptable:
No changes to the NATFW NSLP signaling session lifetime values are
needed. The CREATE or EXT message is forwarded.
Signaling session lifetime can be lowered:
The NSLP responder MAY also lower the requested NATFW NSLP
signaling session lifetime to an acceptable value (based on its
local policies). If an NF changes the NATFW NSLP signaling
session lifetime value, it MUST store the new value in the
NATFW_LT object. The CREATE or EXT message is forwarded.
Requested signaling session lifetime is too big:
The NSLP responder MAY reject the requested NATFW NSLP signaling
session lifetime value as being too big and MUST generate an error
RESPONSE message of class 'Signaling session failures' (0x6) with
response code 'Requested lifetime is too big' (0x02) upon
rejection. Lowering the lifetime is preferred instead of
generating an error message.
Requested signaling session lifetime is too small:
The NSLP responder MAY reject the requested NATFW NSLP signaling
session lifetime value as being to small and MUST generate an
error RESPONSE message of class 'Signaling session failures' (0x6)
with response code 'Requested lifetime is too small' (0x10) upon
rejection.
NFs MUST NOT increase the NATFW NSLP signaling session lifetime
value. Messages can be rejected on the basis of the NATFW NSLP
signaling session lifetime being too long when a NATFW NSLP signaling
session is first created and also on refreshes.
The NSLP responder generates a successful RESPONSE for the received
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CREATE or EXT message, sets the NATFW NSLP signaling session lifetime
value in the NATFW_LT object to the above granted lifetime and sends
the message back towards NSLP initiator.
Each NSLP forwarder processes the RESPONSE message, reads and stores
the granted NATFW NSLP signaling session lifetime value. The
forwarders MUST accept the granted NATFW NSLP signaling session
lifetime, as long as this value is less than or equal to the
acceptable value. The acceptable value refers to the value accepted
by the NSLP forwarder when processing the CREATE or EXT message. For
received values greater than the acceptable value, NSLP forwarders
MUST generate a RESPONSE message of class 'Signaling session
failures' (0x6) with response code 'Requested lifetime is too big'
(0x02). For received values lower than the values acceptable by the
node local policy, NSLP forwarders MUST generate a RESPONSE message
of class 'Signaling session failures' (0x6) with response code
'Requested lifetime is too small' (0x10). Figure 12 shows the
procedure with an example, where an initiator requests 60 seconds
lifetime in the CREATE message and the lifetime is shortened along
the path by the forwarder to 20 seconds and by the responder to 15
seconds. When the NSLP forwarder receives the RESPONSE message with
a NATFW NSLP signaling session lifetime value of 15 seconds it checks
whether this value is lower or equal to the acceptable value.
+-------+ CREATE(lt=60s) +-------------+ CREATE(lt=20s) +--------+
| |---------------->| NSLP |---------------->| |
| NI | | forwarder | | NR |
| |<----------------| check 15<20 |<----------------| |
+-------+ RESPONSE(lt=15s)+-------------+ RESPONSE(lt=15s)+--------+
lt = lifetime
Figure 12: Signaling Session Lifetime Setting Example
3.5. Message Sequencing
NATFW NSLP messages need to carry an identifier so that all nodes
along the path can distinguish messages sent at different points in
time. Messages can be lost along the path or duplicated. So all
NATFW NSLP nodes should be able to identify either old messages that
have been received before (duplicated), or the case that messages
have been lost before (loss). For message replay protection it is
necessary to keep information about messages that have already been
received and requires every NATFW NSLP message to carry a message
sequence number (MSN), see also Section 4.2.6.
The MSN MUST be set by the NI and MUST NOT be set or modified by any
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other node. The initial value for the MSN MUST be generated randomly
and MUST be unique only within the NATFW NSLP signaling session for
which it is used. The NI MUST increment the MSN by one for every
message sent. Once the MSN has reached the maximum value, the next
value it takes is zero. All NATFW NSLP nodes MUST use the algorithm
defined in [3] to detect MSN wrap-arounds.
NSIS forwarders and the responder store the MSN from the initial
CREATE or EXT packet which creates the NATFW NSLP signaling session
as the start value for the NATFW NSLP signaling session. NFs and NRs
MUST include the received MSN value in the corresponding RESPONSE
message that they generate.
When receiving a CREATE or EXT message, a NATFW NSLP node uses the
MSN given in the message to determine whether the state being
requested is different to the state already installed. The message
MUST be discarded if the received MSN value is equal to or lower than
the stored MSN value. Such a received MSN value can indicate a
duplicated and delayed message or replayed message. If the received
MSN value is greater than the already stored MSN value, the NATFW
NSLP MUST update its stored state accordingly, if permitted by all
security checks (see Section 3.6), and stores the updated MSN value
accordingly.
3.6. Authentication, Authorization, and Policy Decisions
NATFW NSLP nodes receiving signaling messages MUST first check
whether this message is authenticated and authorized to perform the
requested action. NATFW NSLP nodes requiring more information than
provided MUST generate an error RESPONSE of class 'Permanent failure'
(0x5) with response code 'Authentication failed' (0x01) or with
response code 'Authorization failed' (0x02).
The NATFW NSLP is expected to run in various environments, such as
IP-based telephone systems, enterprise networks, home networks, etc.
The requirements on authentication and authorization are quite
different between these use cases. While a home gateway, or an
Internet cafe, using NSIS may well be happy with a "NATFW signaling
coming from inside the network" policy for authorization of
signaling, enterprise networks are likely to require more strongly
authenticated/authorized signaling. This enterprise scenario may
require the use of an infrastructure and administratively assigned
identities to operate the NATFW NSLP.
Once the NI is authenticated and authorized, another step is
performed. The requested policy rule for the NATFW NSLP signaling
session is checked against a set of policy rules, i.e., whether the
requesting NI is allowed to request the policy rule to be loaded in
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the device. If this fails the NF or NR must send an error RESPONSE
of class 'Permanent failure' (0x5) and with response code
'Authorization failed' (0x02).
3.7. Protocol Operations
This section defines the protocol operations including, how to create
NATFW NSLP signaling sessions, maintain them, and how to reserve
addresses.
3.7.1. Creating Signaling Sessions
Allowing two hosts to exchange data even in the presence of
middleboxes is realized in the NATFW NSLP by use of the CREATE
message. The NI (either the data sender or a proxy) generates a
CREATE message as defined in Section 4.3.1 and hands it to the NTLP.
The NTLP forwards the whole message on the basis of the message
routing information (MRI) towards the NR. Each NSIS forwarder along
the path that implements NATFW NSLP, processes the NSLP message.
Forwarding is thus managed NSLP hop-by-hop but may pass transparently
through NSIS forwarders which do not contain NATFW NSLP functionality
and non-NSIS aware routers between NSLP hop way points. When the
message reaches the NR, the NR can accept the request or reject it.
The NR generates a response to CREATE and this response is
transported hop-by-hop towards the NI. NATFW NSLP forwarders may
reject requests at any time. Figure 13 sketches the message flow
between NI (DS in this example), a NF (e.g., NAT), and NR (DR in this
example).
NI Private Network NF Public Internet NR
| | |
| CREATE | |
|----------------------------->| |
| | |
| | |
| | CREATE |
| |--------------------------->|
| | |
| | RESPONSE |
| RESPONSE |<---------------------------|
|<-----------------------------| |
| | |
| | |
Figure 13: CREATE message flow with success RESPONSE
There are several processing rules for a NATFW peer when generating
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and receiving CREATE messages, since this message type is used for
creating new NATFW NSLP signaling session, updating existing,
extending the lifetime and deleting NATFW NSLP signaling session.
The three latter functions operate in the same way for all kinds of
CREATE message, and are therefore described in separate sections:
o Extending the lifetime of NATFW NSLP signaling sessions is
described in Section 3.7.3.
o Deleting NATFW NSLP signaling sessions is described in
Section 3.7.4.
o Updating policy rules is described in Section 3.10.
For an initial CREATE message creating a new NATFW NSLP signaling
session, the processing of CREATE messages is different for every
NATFW node type:
o NSLP initiator: An NI only generates CREATE messages and hands
them over to the NTLP. The NI should never receive CREATE
messages and MUST discard it.
o NATFW NSLP forwarder: NFs that are unable to forward the CREATE
message to the next hop MUST generate an error RESPONSE of class
'Permanent failure' (0x6) with response code 'Did not reach the
NR' (0x07). This case may occur if the NTLP layer cannot find an
NATFW NSLP peer, either another NF or the NR, and returns an error
via the GIST API. The NSLP message processing at the NFs depends
on the middlebox type:
* NAT: When the initial CREATE message is received at the public
side of the NAT, it looks for a reservation made in advance, by
using a EXT message (see Section 3.7.2). The matching process
considers the received MRI information and the stored MRI
information, as described in Section 3.8. If no matching
reservation can be found, i.e. no reservation has been made in
advance, the NSLP MUST return an error RESPONSE of class
'Signaling session failure' (0x6) with response code 'No
reservation found matching the MRI of the CREATE request'
(0x03) MUST be generated. If there is a matching reservation,
the NSLP stores the data sender's address (and if applicable
port number) as part of the source address of the policy rule
('the remembered policy rule') to be loaded and forwards the
message with the destination address set to the internal
(private in most cases) address of NR. When the initial CREATE
message is received at the private side, the NAT binding is
allocated, but not activated (see also Appendix C.3). The MRI
information is updated to reflect the address, and if
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applicable port, translation. The NSLP message is forwarded
towards the NR with source address set to the NAT's external
address from the newly remembered binding.
* Firewall: When the initial CREATE message is received, the NSLP
just remembers the requested policy rule, but does not install
any policy rule. Afterwards, the message is forwarded towards
the NR.
* Combined NAT and firewall: Processing at combined firewall and
NAT middleboxes is the same as in the NAT case. No policy
rules are installed. Implementations MUST take into account
the order of packet processing in the firewall and NAT
functions within the device. This will be referred to as
'order of functions' and is generally different depending on
whether the packet arrives at the external or internal side of
the middlebox.
o NSLP receiver: NRs receiving initial CREATE messages MUST reply
with a success RESPONSE of class 'Success' (0x2) with response
code set to 'All successfully processed' (0x01), if they accept
the CREATE message. Otherwise they MUST generate a RESPONSE
message with a suitable response code. RESPONSE messages are sent
back NSLP hop-by-hop towards the NI, irrespective of the response
codes, either success or error.
Remembered policy rules at middleboxes MUST be only installed upon
receiving a corresponding successful RESPONSE message with the same
SID and MSN as the CREATE message that caused them to be remembered.
This is a countermeasure to several problems, for example, wastage of
resources due to loading policy rules at intermediate NFs when the
CREATE message does not reach the final NR for some reason.
Processing of a RESPONSE message is different for every NSIS node
type:
o NSLP initiator: After receiving a successful RESPONSE, the data
path is configured and the DS can start sending its data to the
DR. After receiving an error RESPONSE message, the NI MAY try to
generate the CREATE message again or give up and report the
failure to the application, depending on the error condition.
o NSLP forwarder: NFs install the remembered policy rules, if a
successful RESPONSE message with matching SID and MSN is received.
If an ERROR RESPONSE message with matching SID and MSN is
received, the NATFW NSLP session is marked as dead, no policy rule
is installed and the remembered rule is discarded.
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o NSIS responder: The NR should never receive RESPONSE messages and
MUST silently drop any such messages received.
3.7.2. Reserving External Addresses
NSIS signaling is intended to travel end-to-end, even in the presence
of NATs and firewalls on-path. This works well in cases where the
data sender is itself behind a NAT or a firewall as described in
Section 3.7.1. For scenarios where the data receiver is located
behind a NAT or a firewall and it needs to receive data flows from
outside its own network (usually referred to as inbound flows, see
Figure 5) the problem is more troublesome.
NSIS signaling, as well as subsequent data flows, are directed to a
particular destination IP address that must be known in advance and
reachable. Data receivers must tell the local NSIS infrastructure
(i.e., the inbound firewalls/NATs) about incoming NATFW NSLP
signaling and data flows before they can receive these flows. It is
necessary to differentiate between data receivers behind NATs and
behind firewalls for understanding the further NATFW procedures.
Data receivers that are only behind firewalls already have a public
IP address and they need only to be reachable for NATFW signaling.
Unlike data receivers behind just firewalls, data receivers behind
NATs do not have public IP addresses; consequently they are not
reachable for NATFW signaling by entities outside their addressing
realm.
The preceding discussion addresses the situation where a DR node that
wants to be reachable is unreachable because the NAT lacks a suitable
rule with the 'allow' action which would forward inbound data.
However, in certain scenarios, a node situated behind inbound
firewalls that do not block inbound data traffic (firewalls with
"default to allow") unless requested might wish to prevent traffic
being sent to it from specified addresses. In this case, NSIS NATFW
signaling can be used to achieve this by installing a policy rule
with its action set to 'deny' using the same mechanisms as for
'allow' rules.
The required result is obtained by sending a EXTERNAL (EXT) message
in the inbound direction of the intended data flow. When using this
functionality the NSIS initiator for the 'Reserve External Address'
signaling is typically the node that will become the DR for the
eventual data flow. To distinguish this initiator from the usual
case where the NI is associated with the DS, the NI is denoted by NI+
and the NSIS responder is similarly denoted by NR+.
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Public Internet Private Address
Space
Edge
NI(DS) NAT/FW NAT NR(DR)
NR+ NI+
| | | |
| | | |
| | | |
| | EXT[(DTInfo)] | EXT[(DTInfo)] |
| |<----------------------|<----------------------|
| | | |
| |RESPONSE[Success/Error]|RESPONSE[Success/Error]|
| |---------------------->|---------------------->|
| | | |
| | | |
============================================================>
Data Traffic Direction
Figure 14: Reservation message flow for DR behind NAT or firewall
Figure 14 shows the EXT message flow for enabling inbound NATFW NSLP
signaling messages. In this case the roles of the different NSIS
entities are:
o The data receiver (DR) for the anticipated data traffic is the
NSIS initiator (NI+) for the EXTERNAL (EXT) message, but becomes
the NSIS responder (NR) for following CREATE messages.
o The actual data sender (DS) will be the NSIS initiator (NI) for
later CREATE messages and may be the NSIS target of the signaling
(NR+).
o It may be necessary to use a signaling destination address (SDA)
as the actual target of the EXT message (NR+) if the DR is located
behind a NAT and the address of the DS is unknown. The SDA is an
arbitrary address in the outermost address realm on the other side
of the NAT from the DR. Typically this will be a suitable public
IP address when the 'outside' realm is the public Internet. This
choice of address causes the EXT message to be routed through the
NATs towards the outermost realm and would force interception of
the message by the outermost NAT in the network at the boundary
between the private address and the public address realm (the
edge-NAT). It may also be intercepted by other NATs and firewalls
on the path to the edge-NAT.
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Basically, there are two different signaling scenarios. Either
1. the DR behind the NAT/firewall knows the IP address of the DS in
advance,
2. or the address of DS is not known in advance.
Case 1 requires the NATFW NSLP to request the path-coupled message
routing method (PC-MRM) from the NTLP. The EXT message MUST be sent
with PC-MRM (see Section 5.8.1 in [2]) with the direction set to
'upstream' (inbound). The handling of case 2 depends on the
situation of DR: If DR is solely located behind a firewall, the EXT
message MUST be sent with the PC-MRM, direction 'upstream' (inbound),
and data flow source IP address set to wildcard. If DR is located
behind a NAT, the EXT message MUST be sent with the loose-end message
routing method (LE-MRM, see Section 5.8.2 in [2]), the destination-
address set to the signaling destination address (SDA, see also
Appendix A). For scenarios with DR being behind a firewall, special
conditions apply (applicability statement, Appendix B). The data
receiver is challenged to determine whether it is solely located
behind firewalls or NATs, for choosing the right message routing
method. This decision can depend on a local configuration parameter,
possibly given through DHCP, or it could be discovered through other
non-NSLP related testing of the network configuration.
For case 2 with NAT, the NI+ (which could be on the data receiver DR
or on any other host within the private network) sends the EXT
message targeted to the signaling destination address. The message
routing for the EXT message is in the reverse direction to the normal
message routing used for path-coupled signaling where the signaling
is sent outbound (as opposed to inbound in this case). When
establishing NAT bindings (and an NATFW NSLP signaling session) the
signaling direction does not matter since the data path is modified
through route pinning due to the external IP address at the NAT.
Subsequent NSIS messages (and also data traffic) will travel through
the same NAT boxes. However, this is only valid for the NAT boxes,
but not for any intermediate firewall. That is the reason for having
a separate CREATE message enabling the reservations made with EXT at
the NATs and either enabling prior reservations or creating new
pinholes at the firewalls which are encountered on the outbound path
depending on whether the inbound and outbound routes coincide.
The EXT signaling message creates an NSIS NATFW signaling session at
any intermediate NSIS NATFW peer(s) encountered, independent of the
message routing method used. Furthermore, it has to be ensured that
the edge-NAT or edge-firewall device is discovered as part of this
process. The end host cannot be assumed to know this device -
instead the NAT or firewall box itself is assumed to know that it is
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located at the outer perimeter of the network. Forwarding of the EXT
message beyond this entity is not necessary, and MUST be prohibited
as it may provide information on the capabilities of internal hosts.
It should be noted, that it is the outermost NAT or firewall that is
the edge-device that must be found during this discovery process.
For instance, when there are a NAT and afterwards a firewall on the
outbound path at the network border, the firewall is the edge-
firewall. All messages must be forwarded to the topology-wise
outermost edge-device, to ensure that this devices knows about the
NATFW NSLP signaling sessions for incoming CREATE messages. However,
the NAT is still the edge-NAT because it has a public globally
routable IP address on its public side: this is not affected by any
firewall between the edge-NAT and the public network.
Possible edge arrangements are:
Public Net ----------------- Private net --------------
| Public Net|--|Edge-FW|--|FW|...|FW|--|DR|
| Public Net|--|Edge-FW|--|Edge-NAT|...|NAT or FW|--|DR|
| Public Net|--|Edge-NAT|--|NAT or FW|...|NAT or FW|--|DR|
The edge-NAT or edge-firewall device closest to the public realm
responds to the EXT message with a successful RESPONSE message. An
edge-NAT includes an NATFW_EXT_IP object (see Section 4.2.2),
carrying the public reachable IP address, and if applicable port
number.
There are several processing rules for a NATFW peer when generating
and receiving EXT messages, since this message type is used for
creating new reserve NATFW NSLP signaling sessions, updating
existing, extending the lifetime and deleting NATFW NSLP signaling
session. The three latter functions operate in the same way for all
kinds of CREATE and EXT messages, and are therefore described in
separate sections:
o Extending the lifetime of NATFW NSLP signaling sessions is
described in Section 3.7.3.
o Deleting NATFW NSLP signaling sessions is described in
Section 3.7.4.
o Updating policy rules is described in Section 3.10.
The NI+ MUST include a NATFW_DTINFO object in the EXT message when
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using the LE-MRM. The LE-MRM does not include enough information for
some types of NATs (basically, those NATs which also translate port
numbers) to perform the address translation. This information is
provided in the NATFW_DTINFO (see Section 4.2.7). This information
MUST include at least the 'dst port number' and 'protocol' fields, in
the NATFW_DTINFO object as these may be required by en-route NATs,
depending on the type of the NAT. All other fields MAY be set by the
NI+ to restrict the set of possible NIs. An edge-NAT will use the
information provided in the NATFW_DTINFO object to allow only NATFW
CREATE message with the MRI matching ('src IPv4/v6 address', 'src
port number', 'protocol') to be forwarded. A NAT requiring
information carried in the NATFW_DTINFO can generate a number of
error RESPONSE messages of class 'Signaling session failures' (0x6):
o 'Requested policy rule denied due to policy conflict' (0x04)
o 'NATFW_DTINFO object is required' (0x07)
o 'Requested value in sub_ports field in NATFW_EFI not permitted'
(0x08)
o 'Requested IP protocol not supported' (0x09)
o 'Plain IP policy rules not permitted -- need transport layer
information' (0x0A)
o 'source IP address range is too large' (0x0C)
o 'destination IP address range is too large' (0x0D)
o 'source L4-port range is too large' (0x0E)
o 'destination L4-port range is too large' (0x0F)
Processing of EXT messages is specific to the NSIS node type:
o NSLP initiator: NI+ only generate EXT messages. When the data
sender's address information is known in advance the NI+ can
include a NATFW_DTINFO object in the EXT message, if not anyway
required to do so (see above). When the data sender's IP address
is not known, the NI+ MUST NOT include a NATFW_DTINFO object. The
NI should never receive EXT messages and MUST silently discard it.
o NSLP forwarder: The NSLP message processing at NFs depends on the
middlebox type:
* NAT: NATs check whether the message is received at the external
(public in most cases) address or at the internal (private)
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address. If received at the external an NF MUST generate an
error RESPONSE of class 'Protocol error' (0x3) with response
code 'Received EXT request message on external side' (0x0B).
If received at the internal (private address) and the NATFW_EFI
object contains the action 'deny', an error RESPONSE of class
'Protocol error' (0x3) with response code 'Requested rule
action not applicable' (0x06) MUST be generated. If received
at the internal address, an IP address, and if applicable one
or more ports, are reserved. If it is an edge-NAT and there is
no edge-firewall beyond, the NSLP message is not forwarded any
further and a successful RESPONSE message is generated
containing an NATFW_EXT_IP object holding the translated
address, and if applicable port, information from the binding
reserved as a result of the EXT message. The RESPONSE message
is sent back towards the NI+. If it is not an edge-NAT, the
NSLP message is forwarded further using the translated IP
address as signaling source address in the LE-MRM and
translated port in the NATFW_DTINFO object in the field 'DR
port number', i.e., the NATFW_DTINFO object is updated to
reflect the translated port number. The edge-NAT or any other
NAT MUST reject EXT messages not carrying a NATFW_DTINFO object
or if the address information within this object is invalid or
is not compliant with local policies (e.g., the information
provided relates to a range of addresses ('wildcarded') but the
edge-NAT requires exact information about DS' IP address and
port) with the above mentioned response codes.
* Firewall: Non edge-firewalls remember the requested policy
rule, keep NATFW NSLP signaling session state, and forward the
message. Edge-firewalls stop forwarding the EXT message. The
policy rule is immediately loaded if the action in the
NATFW_EFI object is set to 'deny' and the node is an edge-
firewall. The policy rule is remembered, but not activated, if
the action in the NATFW_EFI object is set to 'allow'. In both
cases, a successful RESPONSE message is generated. If the
action is 'allow', and the NATFW_DTINFO object is included, and
the MRM is set to LE-MRM in the request, additionally an
NATFW_EXT_IP object is included in the RESPONSE message,
holding the translated address, and if applicable port,
information. This information is obtained from the
NATFW_DTINFO object's 'DR port number' and the source-address
of the LE-MRM.
* Combined NAT and firewall: Processing at combined firewall and
NAT middleboxes is the same as in the NAT case.
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o NSLP receiver: This type of message should never be received by
any NR+ and it MUST generate an error RESPONSE message of class
'Permanent failure' (0x5) with response code 'No edge-device here'
(0x06).
Processing of a RESPONSE message is different for every NSIS node
type:
o NSLP initiator: Upon receiving a successful RESPONSE message, the
NI+ can rely on the requested configuration for future inbound
NATFW NSLP signaling sessions. If the response contains an
NATFW_EXT_IP object, the NI can use IP address and port pairs
carried for further application signaling. After receiving a
error RESPONSE message, the NI+ MAY try to generate the EXT
message again or give up and report the failure to the
application, depending on the error condition.
o NSLP forwarder: NFs simply forward this message as long as they
keep state for the requested reservation, if the RESPONSE message
contains NATFW_INFO object with class set to 'Success' (0x2). If
the RESPONSE message contains NATFW_INFO object with class set not
to 'Success' (0x2), the NATFW NSLP signaling session is marked as
dead.
o NSIS responder: This type of message should never be received by
any NR+. The NF should never receive response messages and MUST
silently discard it.
Reservations with action 'allow' made with EXT MUST be enabled by a
subsequent CREATE message. A reservation made with EXT (independent
of selected action) is kept alive as long as the NI+ refreshes the
particular NATFW NSLP signaling session and it can be reused for
multiple, different CREATE messages. An NI+ may decide to teardown a
reservation immediately after receiving a CREATE message. This
implies that a new NATFW NSLP signaling session must be created for
each new CREATE message. The CREATE message does not re-use the
NATFW NSLP signaling session created by REA.
Without using CREATE Section 3.7.1 or EXT in proxy mode Section 3.7.6
no data traffic will be forwarded to DR beyond the edge-NAT or edge-
firewall. The only function of EXT is to ensure that subsequent
CREATE messages traveling towards the NR will be forwarded across the
public-private boundary towards the DR. Correlation of incoming
CREATE messages to EXT reservation states is described in
Section 3.8.
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3.7.3. NATFW NSLP Signaling Session Refresh
NATFW NSLP signaling sessions are maintained on a soft-state basis.
After a specified timeout, sessions and corresponding policy rules
are removed automatically by the middlebox, if they are not
refreshed. Soft-state is created by CREATE and EXT and the
maintenance of this state must be done by these messages. State
created by CREATE must be maintained by CREATE, state created by EXT
must be maintained by EXT. Refresh messages, are messages carrying
the same session ID as the initial message and a NATFW_LT lifetime
object with a lifetime greater than zero. Messages with the same SID
but carrying a different MRI are treated as updates of the policy
rules and are processed as defined in Section 3.10. Every refresh
CREATE or EXT message MUST be acknowledged by an appropriate response
message generated by the NR. Upon reception by each NSIS forwarder,
the state for the given session ID is extended by the NATFW NSLP
signaling session refresh period, a period of time calculated based
on a proposed refresh message period. The lifetime extension of a
NATFW NSLP signaling session is calculated as current local time plus
proposed lifetime value (NATFW NSLP signaling session refresh
period). Section 3.4 defines the process of calculating lifetimes in
detail.
NI Public Internet NAT Private address NR
| | space |
| CREATE[lifetime > 0] | |
|----------------------------->| |
| | |
| | |
| | CREATE[lifetime > 0] |
| |--------------------------->|
| | |
| | RESPONSE[Success/Error] |
| RESPONSE[Success/Error] |<---------------------------|
|<-----------------------------| |
| | |
| | |
Figure 16: Successful Refresh Message Flow, CREATE as example
Processing of NATFW NSLP signaling session refresh CREATE and EXT
messages is different for every NSIS node type:
o NSLP initiator: The NI/NI+ can generate NATFW NSLP signaling
session refresh CREATE/EXT messages before the NATFW NSLP
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signaling session times out. The rate at which the refresh
CREATE/EXT messages are sent and their relation to the NATFW NSLP
signaling session state lifetime is discussed further in
Section 3.4.
o NSLP forwarder: Processing of this message is independent of the
middlebox type and is as described in Section 3.4.
o NSLP responder: NRs accepting a NATFW NSLP signaling session
refresh CREATE/EXT message generate a successful RESPONSE message,
including the granted lifetime value of Section 3.4 in a NATFW_LT
object.
3.7.4. Deleting Signaling Sessions
NATFW NSLP signaling sessions can be deleted at any time. NSLP
initiators can trigger this deletion by using a CREATE or EXT
messages with a lifetime value set to 0, as shown in Figure 17.
Whether a CREATE or EXT message type is used, depends on how the
NATFW NSLP signaling session was created.
NI Public Internet NAT Private address NR
| | space |
| CREATE[lifetime=0] | |
|----------------------------->| |
| | |
| | CREATE[lifetime=0] |
| |--------------------------->|
| | |
Figure 17: Delete message flow, CREATE as example
NSLP nodes receiving this message delete the NATFW NSLP signaling
session immediately. Policy rules associated with this particular
NATFW NSLP signaling session MUST be also deleted immediately. This
message is forwarded until it reaches the final NR. The CREATE/EXT
message with a lifetime value of 0, does not generate any response,
neither positive nor negative, since there is no NSIS state left at
the nodes along the path.
NSIS initiators can use CREATE/EXT message with lifetime set to zero
in an aggregated way, such that a single CREATE or EXT message is
terminating multiple NATFW NSLP signaling sessions. NIs can follow
this procedure if the like to aggregate NATFW NSLP signaling session
deletion requests: The NI uses the CREATE or EXT message with the
session ID set to zero and the MRI's source-address set to its used
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IP address. All other fields of the respective NATFW NSLP signaling
sessions to be terminated are set as well, otherwise these fields are
completely wildcarded. The NSLP message is transferred to the NTLP
requesting 'explicit routing' as described in Sections 5.2.1 and
7.1.4. in [2].
The outbound NF receiving such an aggregated CREATE or EXT message
MUST reject it with an error RESPONSE of class 'Permanent failure'
(0x5) with response code 'Authentication failed' (0x01) if the
authentication fails and with an error RESPONSE of class 'Permanent
failure' (0x5) with response code 'Authorization failed' (0x02) if
the authorization fails. Per NATFW NSLP signaling session proof of
ownership, as it is defined in this memo, is not possible anymore
when using this aggregated way. However, the outbound NF can use the
relationship between the information of the received CREATE or EXT
message and the GIST messaging association where the request has been
received. The outbound NF MUST only accept this aggregated CREATE or
EXT message through already established GIST messaging associations
with the NI. The outbound NF MUST NOT propagate this aggregated
CREATE or EXT message but it MAY generate and forward per NATFW NSLP
signaling session CREATE or EXT messages.
3.7.5. Reporting Asynchronous Events
NATFW NSLP forwarders and NATFW NSLP responders must have the ability
to report asynchronous events to other NATFW NSLP nodes, especially
to allow reporting back to the NATFW NSLP initiator. Such
asynchronous events may be premature NATFW NSLP signaling session
termination, changes in local policies, route change or any other
reason that indicates change of the NATFW NSLP signaling session
state.
NFs and NRs may generate NOTIFY messages upon asynchronous events,
with a NATFW_INFO object indicating the reason for event. These
reasons can be carried in the NATFW_INFO object (class MUST be set to
'Informational' (0x1)) within the NOTIFY message. This list shows
the response codes and the associated actions to take at NFs and the
NI:
o 'Route change: possible route change on the outbound path' (0x01):
Follow instructions in Section 3.9. This MUST be sent inbound.
o 'Re-authentication required' (0x02): The NI should re-send the
authentication. This MUST be sent inbound.
o 'NATFW node is going down soon' (0x03): The NI and other NFs
should be prepared for a service interruption at any time. This
message MAY be sent inbound and outbound.
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o 'NATFW signaling session lifetime expired' (0x04): The NATFW
signaling session has been expired and the signaling session is
invalid now. NFs MUST mark the signaling session as 'Dead'. This
message MAY be sent inbound and outbound.
NOTIFY messages are always sent hop-by-hop inbound towards NI until
they reach NI or outbound towards the NR as indicated in the list
above.
The initial processing when receiving a NOTIFY message is the same
for all NATFW nodes: NATFW nodes MUST only accept NOTIFY messages
through already established NTLP messaging associations. The further
processing is different for each NATFW NSLP node type and depends on
the events notified:
o NSLP initiator: NIs analyze the notified event and behave
appropriately based on the event type. NIs MUST NOT generate
NOTIFY messages.
o NSLP forwarder: NFs analyze the notified event and behave based on
the above description per response code. NFs SHOULD generate
NOTIFY messages upon asynchronous events and forward them inbound
towards the NI or outbound towards the NR, depending on the
received direction, i.e., inbound messages MUST be forwarded
further inbound and outbound messages MUST be forwarded further
inbound. NFs MUST silently discard NOTIFY messages that have been
received outbound but are only allowed to be sent inbound, e.g.
'Re-authentication required' (0x02).
o NSLP responder: NRs SHOULD generate NOTIFY messages upon
asynchronous events including a response code based on the
reported event. The NR MUST silently discard NOTIFY messages that
have been received outbound but are only allowed to be sent
inbound, e.g. 'Re-authentication required' (0x02),
NATFW NSLP forwarders, keeping multiple NATFW NSLP signaling sessions
at the same time, can experience problems when shutting down service
suddenly. This sudden shutdown can be result of node local failure,
for instance, due to a hardware failure. This NF generates NOTIFY
messages for each of the NATFW NSLP signaling sessions and tries to
send them inbound. Due to the number of NOTIFY messages to be sent,
the shutdown of the node may be unnecessarily prolonged, since not
all messages can be sent at the same time. This case can be
described as a NOTIFY storm, if a multitude of NATFW NSLP signaling
sessions is involved.
To avoid the need of generating per NATFW NSLP signaling session
NOTIFY messages in such a scenario described or similar cases, NFs
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SHOULD follow this procedure: The NF uses the NOTIFY message with the
session ID in the NTLP set to zero, with the MRI completely
wildcarded, using the 'explicit routing' as described in Sections
5.2.1 and 7.1.4. in [2]. The inbound NF receiving this type of
NOTIFY immediately regards all NATFW NSLP signaling sessions from
that peer matching the MRI as void. This message will typically
result in multiple NOTIFY messages at the inbound NF, i.e., the NF
can generate per terminated NATFW NSLP signaling session a NOTIFY
message. However, a NF MAY aggregate again the NOTIFY messages as
described here.
3.7.6. Proxy Mode of Operation
Some migration scenarios need specialized support to cope with cases
where NSIS is only deployed in same areas of the Internet. End-to-
end signaling is going to fail without NSIS support at or near both
data sender and data receiver terminals. A proxy mode of operation
is needed. This proxy mode of operation must terminate the NATFW
NSLP signaling as topologically close to the terminal for which it is
proxying and proxy all messages. This NATFW NSLP node doing the
proxying of the signaling messages becomes either the NI or the NR
for the particular NATFW NSLP signaling session, depending on whether
it is the DS or DR that does not support NSIS. Typically, the edge-
NAT or the edge-firewall would be used to proxy NATFW NSLP messages.
This proxy mode operation does not require any new CREATE or EXT
message type, but relies on extended CREATE and EXT message types.
They are called respectively CREATE-PROXY and EXT-PROXY and are
distinguished by setting the P flag in the NSLP header to P=1. This
flag instructs edge-NATs and edge-firewalls receiving them to operate
in proxy mode for the NATFW NSLP signaling session in question. The
semantics of the CREATE and EXT message types are not changed and the
behavior of the various node types is as defined in Section 3.7.1 and
Section 3.7.2, except for the proxying node. The following
paragraphs describe the proxy mode operation for data receivers
behind middleboxes and data senders behind middleboxes.
3.7.6.1. Proxying for a Data Sender
The NATFW NSLP gives the NR the ability to install state on the
inbound path towards the data sender for outbound data packets, even
when only the receiving side is running NSIS (as shown in Figure 18).
The goal of the method described is to trigger the edge-NAT/
edge-firewall to generate a CREATE message on behalf of the data
receiver. In this case, an NR can signal towards the network border
as it is performed in the standard EXT message handling scenario as
in Section 3.7.2. The message is forwarded until the edge-NAT/
edge-firewall is reached. A public IP address and port number is
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reserved at an edge-NAT/edge-firewall. As shown in Figure 18, unlike
the standard EXT message handling case, the edge-NAT/edge-firewall is
triggered to send a CREATE message on a new reverse path which
traverse several firewalls or NATs. The new reverse path for CREATE
is necessary to handle routing asymmetries between the edge-NAT/
edge-firewall and DR. It must be stressed that the semantics of the
CREATE and EXT messages is not changed, i.e., each is processed as
described earlier.
DS Public Internet NAT/FW Private address NR
No NI NF space NI+
NR+
| | EXT-PROXY[(DTInfo)] |
| |<------------------------- |
| | RESPONSE[Error/Success] |
| | ---------------------- > |
| | CREATE |
| | ------------------------> |
| | RESPONSE[Error/Success] |
| | <---------------------- |
| | |
Figure 18: EXT Triggering Sending of CREATE Message
A NATFW_NONCE object, carried in the EXT and CREATE message, is used
to build the relationship between received CREATEs at the message
initiator. An NI+ uses the presence of the NATFW_NONCE object to
correlate it to the particular EXT-PROXY. The absence of a NONCE
object indicates a CREATE initiated by the DS and not by the edge-
NAT. Therefore, these processing rules of EXT-PROXY messages are
added to the regular EXT processing:
o NSLP initiator (NI+): The NI+ MUST choose a random value and place
it in the NATFW_NONCE object.
o NSLP forwarder being either edge-NAT or edge-firewall: When the NF
accepts a EXT_PROXY message, it generates a successful RESPONSE
message as if it were the NR and additionally, it generates a
CREATE message as defined in Section 3.7.1 and includes a
NATFW_NONCE object having the same value as of the received
NATFW_NONCE object. The NF MUST NOT generate a CREATE-PROXY
message. The NF MUST refresh the CREATE message signaling session
only if a EXT-PROXY refresh message has been received first. This
also includes tearing down signaling sessions, i.e., the NF must
teardown the CREATE signaling session only if a EXT-PROXY message
with lifetime set to 0 has been received first.
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The scenario described in this section challenges the data receiver
because it must make a correct assumption about the data sender's
ability to use NSIS NATFW NSLP signaling. It is possible for the DR
to make the wrong assumption in two different ways:
a) the DS is NSIS unaware but the DR assumes the DS to be NSIS
aware and
b) the DS is NSIS aware but the DR assumes the DS to be NSIS
unaware.
Case a) will result in middleboxes blocking the data traffic, since
DS will never send the expected CREATE message. Case b) will result
in the DR successfully requesting proxy mode support by the edge-NAT
or edge-firewall. The edge-NAT/edge-firewall will send CREATE
messages and DS will send CREATE messages as well. Both CREATE
messages are handled as separated NATFW NSLP signaling sessions and
therefore the common rules per NATFW NSLP signaling session apply;
the NATFW_NONCE object is used to differentiate CREATE messages
generated by the edge-NAT/edge-firewall from NI initiated CREATE
messages. It is the NR's responsibility to decide whether to
teardown the EXT-PROXY signaling sessions in the case where the data
sender's side is NSIS aware, but was incorrectly assumed not to be so
by the DR. It is RECOMMENDED that a DR behind NATs uses the proxy
mode of operation by default, unless the DR knows that the DS is NSIS
aware. The DR MAY cache information about data senders which it has
found to be NSIS aware in past NATFW NSLP signaling sessions.
There is a possible race condition between the RESPONSE message to
the EXT-PROXY and the CREATE message generated by the edge-NAT. The
CREATE message can arrive earlier than the RESPONSE message. An NI+
MUST accept CREATE messages generated by the edge-NAT even if the
RESPONSE message to the EXT-PROXY was not received.
3.7.6.2. Proxying for a Data Receiver
As with data receivers behind middleboxes, data senders behind
middleboxes can require proxy mode support. The issue here is that
there is no NSIS support at the data receiver's side and, by default,
there will be no response to CREATE messages. This scenario requires
the last NSIS NATFW NSLP aware node to terminate the forwarding and
to proxy the response to the CREATE message, meaning that this node
is generating RESPONSE messages. This last node may be an edge-NAT/
edge-firewall, or any other NATFW NSLP peer, that detects that there
is no NR available (probably as a result of GIST timeouts but there
may be other triggers).
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DS Private Address NAT/FW Public Internet NR
NI Space NF no NR
| | |
| CREATE-PROXY | |
|------------------------------>| |
| | |
| RESPONSE[SUCCESS/ERROR] | |
|<------------------------------| |
| | |
Figure 19: Proxy Mode CREATE Message Flow
The processing of CREATE-PROXY messages and RESPONSE messages is
similar to Section 3.7.1, except that forwarding is stopped at the
edge-NAT/edge-firewall. The edge-NAT/edge-firewall responds back to
NI according the situation (error/success) and will be the NR for
future NATFW NSLP communication.
The NI can choose the proxy mode of operation although the DR is NSIS
aware. The CREATE-PROXY mode would not configure all NATs and
firewalls along the data path, since it is terminated at the edge-
device. Any device beyond this point will never receive any NATFW
NSLP signaling for this flow.
3.8. De-Multiplexing at NATs
Section 3.7.2 describes how NSIS nodes behind NATs can obtain a
public reachable IP address and port number at a NAT and and how the
resulting mapping rule can be activated by using CREATE messages (see
Section 3.7.1). The information about the public IP address/port
number can be transmitted via an application level signaling protocol
and/or third party to the communication partner that would like to
send data toward the host behind the NAT. However, NSIS signaling
flows are sent towards the address of the NAT at which this
particular IP address and port number is allocated and not directly
to the allocated IP address and port number. The NATFW NSLP
forwarder at this NAT needs to know how the incoming NSLP CREATE
messages are related to reserved addresses, meaning how to de-
multiplex incoming NSIS CREATE messages.
The de-multiplexing method uses information stored at the local NATFW
NSLP node and the of the policy rule. The policy rule uses the LE-
MRM MRI source-address (see [2]) as the flow destination IP address
and the network-layer-version as IP version. The external IP address
at the NAT is stored as the external flow destination IP address.
All other parameters of the policy rule other than the flow
destination IP address are wildcarded if no NATFW_DTINFO object is
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included in the EXT message. The LE-MRM MRI destination-address MUST
NOT be used in the policy rule, since it is solely a signaling
destination address.
If the NATFW_DTINFO object is included in the EXT message, the policy
rule is filled with further information. The 'dst port number' field
of the NATFW_DTINFO is stored as the flow destination port number.
The 'protocol' field is stored as the flow protocol. The 'src port
number' field is stored as the flow source port number. The 'data
sender's IPv4 address' is stored as the flow source IP address. Note
that some of these field can contain wildcards.
When receiving a CREATE message at the NATFW NSLP it uses the flow
information stored in the MRI to do the matching process. This table
shows the parameters to be compared against each others. Note that
not all parameters can be present in a MRI at the same time.
+-------------------------------+--------------------------------+
| Flow parameter (Policy Rule) | MRI parameter (CREATE message) |
+-------------------------------+--------------------------------+
| IP version | network-layer-version |
| | |
| Protocol | IP-protocol |
| | |
| source IP address (w) | source-address (w) |
| | |
| external IP address | destination-address |
| | |
| destination IP address (n/u) | N/A |
| | |
| source port number (w) | L4-source-port (w) |
| | |
| external port number (w) | L4-destination-port (w) |
| | |
| destination port number (n/u) | N/A |
| | |
| IPsec SPI | ipsec-SPI |
+-------------------------------+--------------------------------+
Table entries marked with (w) can be wildcarded and entries marked
with (n/u) are not used for the matching.
Table 1
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3.9. Reacting to Route Changes
The NATFW NSLP needs to react to route changes in the data path.
This assumes the capability to detect route changes, to perform NAT
and firewall configuration on the new path and possibly to tear down
NATFW NSLP signaling session state on the old path. The detection of
route changes is described in Section 7 of [2] and the NATFW NSLP
relies on notifications about route changes by the NTLP. This
notification will be conveyed by the API between NTLP and NSLP, which
is out of scope of this memo.
A NATFW NSLP node other than the NI or NI+ detecting a route change,
by means described in the NTLP specification or others, generates a
NOTIFY message indicating this change and sends this inbound towards
NI. Intermediate NFs on the way to the NI can use this information
to decide later if their NATFW NSLP signaling session can be deleted
locally, if they do not receive an update within a certain time
period, as described in Section 3.2.3. It is important to consider
the transport limitations of NOTIFY messages as mandated in
Section 3.7.5.
The NI receiving this NOTIFY message MAY generate a new CREATE or EXT
message and sends it towards the NATFW NSLP signaling session's NI as
for the initial message using the same session ID. All the remaining
processing and message forwarding, such as NSLP next hop discovery,
is subject to regular NSLP processing as described in the particular
sections. Normal routing will guide the new CREATE or EXT message to
the correct NFs along the changed route. NFs that were on the
original path receiving these new CREATE or EXT messages (see also
Section 3.10), can use the session ID to update the existing NATFW
NSLP signaling session, whereas NFs that were not on the original
path will create new state for this NATFW NSLP signaling session.
The next section describes how policy rules are updated.
3.10. Updating Policy Rules
NSIS initiators can request an update of the installed/reserved
policy rules at any time within a NATFW NSLP signaling session.
Updates to policy rules can be required due to node mobility (NI is
moving from one IP address to another), route changes (this can
result in a different NAT mapping at a different NAT device), or the
wish of the NI to simply change the rule. NIs can update policy
rules in existing NATFW NSLP signaling sessions by sending an
appropriate CREATE or EXT message (similar to Section 3.4) with
modified message routing information (MRI) as compared with that
installed previously, but using the existing session ID to identify
the intended target of the update. With respect to authorization and
authentication, this update CREATE or EXT message is treated in
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exactly the same way as any initial message. Therefore, any node
along in the NATFW NSLP signaling session can reject the update with
an error RESPONSE message, as defined in the previous sections.
The message processing and forwarding is executed as defined in the
particular sections. A NF or the NR receiving an update, simply
replaces the installed policy rules installed in the firewall/NAT.
The local procedures on how to update the MRI in the firewall/NAT is
out of scope of this memo
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4. NATFW NSLP Message Components
A NATFW NSLP message consists of a NSLP header and one or more
objects following the header. The NSLP header is carried in all
NATFW NSLP message and objects are Type-Length-Value (TLV) encoded
using big endian (network ordered) binary data representations.
Header and objects are aligned to 32 bit boundaries and object
lengths that are not multiples of 32 bits must be padded to the next
higher 32 bit multiple.
The whole NSLP message is carried as payload of a NTLP message.
Note that the notation 0x is used to indicate hexadecimal numbers.
4.1. NSLP Header
All GIST NSLP-Data objects for the NATFW NSLP MUST contain this
common header as the first 32 bits of the object (this is not the
same as the GIST Common Header). It contains two fields, the NSLP
message type and a reserved field. The total length is 32 bits. The
layout of the NSLP header is defined by Figure 20.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message type |P| reserved | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 20: Common NSLP header
The reserved field MUST be set to zero in the NATFW NSLP header
before sending and MUST be ignored during processing of the header.
The defined messages types are:
o IANA-TBD(1) : CREATE
o IANA-TBD(2) : EXTERNAL(EXT)
o IANA-TBD(3) : RESPONSE
o IANA-TBD(4) : NOTIFY
If a message with another type is received, an error RESPONSE of
class 'Protocol error' (0x3) with response code 'Illegal message
type' (0x01) MUST be generated.
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The P flag indicates the usage of proxy mode. If proxy mode is used
it MUST be set to 1. Proxy mode usage is only allowed in combination
with the message types CREATE and EXT, P=1 MUST NOT be set with
message types other than CREATE and EXT. The P flag MUST be ignored
when processing messages with type RESPONSE. An error RESPONSE
message of class 'Protocol error' (0x3) and type 'Bad flags value'
(0x03) MUST be generated, if the P flag is set in NOTIFY messages.
4.2. NSLP Objects
NATFW NSLP objects use a common header format defined by Figure 21.
The object header contains two fields, the NSLP object type and the
object length. Its total length is 32 bits.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|B|r|r| Object Type |r|r|r|r| Object Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 21: Common NSLP object header
The object length field contains the total length of the object
without the object header. The unit is a word, consisting of 4
octets. The particular values of type and length for each NSLP
object are listed in the subsequent sections that define the NSLP
objects. An error RESPONSE of class 'Protocol error' (0x3) with
response code 'Wrong object length' (0x07) MUST be generated if the
length given for the object in the object header did not match the
length of the object data present. The two leading bits of the NSLP
object header are used to signal the desired treatment for objects
whose treatment has not been defined in this memo (see [2], Section
A.2.1), i.e., the Object Type has not been defined. NATFW NSLP uses
a subset of the categories defined in GIST:
o AB=00 ("Mandatory"): If the object is not understood, the entire
message containing it MUST be rejected with an error RESPONSE of
class 'Protocol error' (0x3) with response code 'Unknown object
present' (0x06).
o AB=01 ("Optional"): If the object is not understood, it should be
deleted and then the rest of the message processed as usual.
o AB=10 ("Forward"): If the object is not understood, it should be
retained unchanged in any message forwarded as a result of message
processing, but not stored locally.
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The combination AB=11 MUST NOT be used and an error RESPONSE of class
'Protocol error' (0x3) with response code 'Invalid Flag-Field
combination' (0x09) MUST be generated.
The following sections do not repeat the common NSLP object header,
they just list the type and the length.
4.2.1. Signaling Session Lifetime Object
The signaling session lifetime object carries the requested or
granted lifetime of a NATFW NSLP signaling session measured in
seconds.
Type: NATFW_LT (IANA-TBD)
Length: 1
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NATFW NSLP signaling session lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 22: Signaling Session Lifetime object
4.2.2. External Address Object
The external address object can be included in RESPONSE messages
(Section 4.3.3) only. It carries the publicly reachable IP address,
and if applicable port number, at an edge-NAT.
Type: NATFW_EXT_IP (IANA-TBD)
Length: 2
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port number | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 23: External Address Object for IPv4 addresses
Please note that the field 'port number' MUST be set to 0 if only an
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IP address has been reserved, for instance, by a traditional NAT. A
port number of 0 MUST be ignored in processing this object.
4.2.3. Extended Flow Information Object
In general, flow information is kept in the message routing
information (MRI) of the NTLP. Nevertheless, some additional
information may be required for NSLP operations. The 'extended flow
information' object carries this additional information about the
action of the policy rule for firewalls/NATs and contiguous port .
Type: NATFW_EFI (IANA-TBD)
Length: 1
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rule action | sub_ports |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 24: Extended Flow Information
This object has two fields, 'rule action' and 'sub_ports'. The 'rule
action' field has these meanings:
o 0x0001: Allow: A policy rule with this action allows data traffic
to traverse the middlebox and the NATFW NSLP MUST allow NSLP
signaling to be forwarded.
o 0x0002: Deny: A policy rule with this action blocks data traffic
from traversing the middlebox and the NATFW NSLP MUST NOT allow
NSLP signaling to be forwarded.
If the 'rule action' field contains neither 0x0001 nor 0x0002, an
error RESPONSE of class 'Signaling session error' (0x6) with response
code 'Unknown policy rule action' (0x05) MUST be generated.
The 'sub_ports' field contains the number of contiguous transport
layer ports to which this rule applies. The default value of this
field is 0, i.e., only the port specified in the NTLP's MRM or
NATFW_DTINFO object is used for the policy rule. A value of 1
indicates that additionally to the port specified in the NTLP's MRM
(port1), a second port (port2) is used. This value of port 2 is
calculated as: port2 = port1 + 1. Other values than 0 or 1 MUST NOT
be used in this field and an error RESPONSE of class 'Signaling
session error' (0x6) with response code 'Requested value in sub_ports
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field in NATFW_EFI not permitted' (0x08) MUST be generated. Further
version of this memo may allow other values for the 'sub_ports'
field. This two contiguous port numbered ports, can be used by
legacy voice over IP equipment. This legacy equipment assumes that
two adjacent port numbers for its RTP/RTCP flows respectively.
4.2.4. Information Code Object
This object carries the response code, which may be indications for
either a successful or failed CREATE or EXT message depending on the
value of the 'response code' field.
Type: NATFW_INFO (IANA-TBD)
Length: 1
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Resv. | Class | Response Code |r|r|r|r| Object Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 25: Information Code Object
The field 'resv.' is reserved for future extensions and MUST be set
to zero when generating such an object and MUST be ignored when
receiving. The 'Object Type' field contains the type of the object
causing the error. The value of 'Object Type' is set to 0, if no
object is concerned and the leading fours bits marked with 'r' are
always set to zero and ignored. The 4 bit class field contains the
severity class. The following classes are defined:
o 0x1: Informational (NOTIFY only)
o 0x2: Success
o 0x3: Protocol error
o 0x4: Transient failure
o 0x5: Permanent failure
o 0x6: Signaling session failures
Within each severity class a number of responses values are defined
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o Informational:
* 0x01: Route change: possible route change on the outbound path.
* 0x02: Re-authentication required.
* 0x03: NATFW node is going down soon.
o Success:
* 0x01: All successfully processed.
o Protocol error:
* 0x01: Illegal message type: the type given in the Message Type
field of the NSLP header is unknown.
* 0x02: Wrong message length: the length given for the message in
the NSLP header does not match the length of the message data.
* 0x03: Bad flags value: an undefined flag or combination of
flags was set in the NSLP header.
* 0x04: Mandatory object missing: an object required in a message
of this type was missing.
* 0x05: Illegal object present: an object was present which must
not be used in a message of this type.
* 0x06: Unknown object present: an object of an unknown type was
present in the message.
* 0x07: Wrong object length: the length given for the object in
the object header did not match the length of the object data
present.
* 0x08: Unknown object field value: a field in an object had an
unknown value.
* 0x09: Invalid Flag-Field combination: An object contains an
invalid combination of flags and/or fields.
* 0x0A: Duplicate object present.
* 0x0B: Received EXT request message on external side.
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o Transient failure:
* 0x01: Requested resources temporarily not available.
o Permanent failure:
* 0x01: Authentication failed.
* 0x02: Authorization failed.
* 0x03: Unable to agree transport security with peer.
* 0x04: Internal or system error.
* 0x05: No NAT here.
* 0x06: No edge-device here.
* 0x07: Did not reach the NR.
o Signaling session failures:
* 0x01: Session terminated asynchronously.
* 0x02: Requested lifetime is too big.
* 0x03: No reservation found matching the MRI of the CREATE
request.
* 0x04: Requested policy rule denied due to policy conflict.
* 0x05: Unknown policy rule action.
* 0x06: Requested rule action not applicable.
* 0x07: NATFW_DTINFO object is required.
* 0x08: Requested value in sub_ports field in NATFW_EFI not
permitted.
* 0x09: Requested IP protocol not supported.
* 0x0A: Plain IP policy rules not permitted -- need transport
layer information.
* 0x0B: ICMP type value not permitted.
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* 0x0C: source IP address range is too large.
* 0x0D: destination IP address range is too large.
* 0x0E: source L4-port range is too large.
* 0x0F: destination L4-port range is too large.
* 0x10: Requested lifetime is too small.
4.2.5. Nonce Object
This object carries the nonce value as described in Section 3.7.6.
Type: NATFW_NONCE (IANA-TBD)
Length: 1
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 26: Nonce Object
4.2.6. Message Sequence Number Object
This object carries the MSN value as described in Section 3.5.
Type: NATFW_MSN (IANA-TBD)
Length: 1
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| message sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 27: Message Sequence Number Object
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4.2.7. Data Terminal Information Object
The 'data terminal information' object carries additional information
possibly needed during EXT operations. EXT messages are transported
by the NTLP using the Loose-End message routing method (LE-MRM). The
LE-MRM contains only DR's IP address and a signaling destination
address (destination address). This destination address is used for
message routing only and is not necessarily reflecting the address of
the data sender. This object contains information about (if
applicable) DR's port number (the destination port number), DS' port
number (the source port number), the used transport protocol, the
prefix length of the IP address, and DS' IP address.
Type: NATFW_DTINFO (IANA-TBD)
Length: variable. Maximum 3.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|I|P|S| reserved | sender prefix | protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: DR port number | DS port number :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: IPsec SPI :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data sender's IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 28: Data Terminal IPv4 Address Object
The flags are:
o I: I=1 means that 'protocol' should be interpreted.
o P: P=1 means that 'dst port number' and 'src port number' are
present and should be interpreted.
o S: S=1 means that SPI is present and should be interpreted.
The SPI field is only present if S is set. The port numbers are only
present if P is set. The flags P and S MUST NOT be set at the same
time. An error RESPONSE of class 'Protocol error' (0x3) with
response code 'Invalid Flag-Field combination' (0x09) MUST be
generated if they are both set. If either P or S is set, I MUST be
set as well and the protocol field MUST carry the particular
protocol. An error RESPONSE of class 'Protocol error' (0x3) with
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response code 'Invalid Flag-Field combination' (0x09) MUST be
generated if S or P is set but I is not set.
The fields MUST be interpreted according these rules:
o (data) sender prefix: This parameter indicates the prefix length
of the 'data sender's IP address' in bits. For instance, a full
IPv4 address requires 'sender prefix' to be set to 32. A value of
0 indicates an IP address wildcard.
o protocol: The IPv4 protocol field. This field MUST be interpreted
if I=1, otherwise it MUST be set to 0 and MUST be ignored.
o DR port number: The port number at the data receiver (DR), i.e.,
the destination port. A value of 0 indicates a port wildcard,
i.e., the destination port number is not known. Any other value
indicates the destination port number.
o DS port number: The port number at the data sender (DS), i.e., the
source port. A value of 0 indicates a port wildcard, i.e., the
source port number is not known. Any other value indicates the
source port number.
o data sender's IPv4 address: The source IP address of the data
sender. This field MUST be set to zero if no IP address is
provided, i.e., a complete wildcard is desired (see dest prefix
field above).
4.2.8. ICMP Types Object
The 'ICMP types' object contains additional information needed to
configure a NAT of firewall with rules to control ICMP traffic. The
object contains a number of values of the ICMP Type field for which a
filter action should be set up:
Type: NATFW_ICMP_TYPES (IANA-TBD)
Length: Variable = ((Number of Types carried + 1) + 3) DIV 4
Where DIV is an integer division.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Count | Type | Type | ........ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ................ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ........ | Type | (Padding) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 29: ICMP Types Object
The fields MUST be interpreted according these rules:
count: 8 bit integer specifying the number of 'Type' entries in
the object.
type: 8 bit field specifying an ICMP Type value to which this rule
applies.
padding: Sufficient 0 bits to pad out the last word so that the
total size of object is an even multiple of words. Ignored on
reception.
4.3. Message Formats
This section defines the content of each NATFW NSLP message type.
The message types are defined in Section 4.1.
Basically, each message is constructed of NSLP header and one or more
NSLP objects. The order of objects is not defined, meaning that
objects may occur in any sequence. Objects are marked either with
mandatory (M) or optional (O). Where (M) implies that this
particular object MUST be included within the message and where (O)
implies that this particular object is OPTIONAL within the message.
Objects defined in this memo carry always the flag combination AB=00
in the NSLP object header. An error RESPONSE message of class
'Protocol error' (0x3) with response code 'Mandatory object missing'
(0x02) MUST be generated if a mandatory declared object is missing.
An error RESPONSE message of class 'Protocol error' (0x3) with
response code 'Illegal object present' (0x05) MUST be generated if an
object was present which must not be used in a message of this type.
An error RESPONSE message of class 'Protocol error' (0x3) with
response code 'Duplicate object present' (0x0A) MUST be generated if
an object appears more than once in a message.
Each section elaborates the required settings and parameters to be
set by the NSLP for the NTLP, for instance, how the message routing
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information is set.
4.3.1. CREATE
The CREATE message is used to create NATFW NSLP signaling sessions
and to create policy rules. Furthermore, CREATE messages are used to
refresh NATFW NSLP signaling sessions and to delete them.
The CREATE message carries these objects:
o Signaling Session Lifetime object (M)
o Extended flow information object (M)
o Message sequence number object (M)
o Nonce object (M) if P flag set to 1 in the NSLP header, otherwise
(O)
o ICMP Types Object (O)
The message routing information in the NTLP MUST be set to DS as
source address and DR as destination address. All other parameters
MUST be set according the required policy rule. CREATE messages MUST
be transported by using the path-coupled MRM with direction set to
'downstream' (outbound).
4.3.2. EXTERNAL (EXT)
The EXTERNAL (EXT) message is used to a) reserve an external IP
address/port at NATs, b) to notify firewalls about NSIS capable DRs,
or c) to block incoming data traffic at inbound firewalls.
The EXT message carries these objects:
o Signaling Session Lifetime object (M)
o Message sequence number object (M)
o Extended flow information object (M)
o Data terminal information object (M)
o Nonce object [M if P flag set to 1 in the NSLP header, otherwise
(O)
o ICMP Types Object (O)
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The selected message routing method of the EXT message depends on a
number of considerations. Section 3.7.2 describes it exhaustively
how to select the correct method. EXT messages can be transported
via the path-coupled message routing method (PC-MRM) or via the
loose-end message routing method (LE-MRM). In the case of PC-MRM,
the source-address is set to DS' address and the destination address
is set to DR's address, the direction is set to inbound. In the case
of LE-MRM, the destination-address is set to DR's address or to the
signaling destination address. The source-address is set to DS's
address.
4.3.3. RESPONSE
RESPONSE messages are responses to CREATE and EXT messages. RESPONSE
messages MUST NOT be generated for any other message, such as NOTIFY
and RESPONSE.
The RESPONSE message for the class 'Success' (0x2) carries these
objects:
o Signaling Session Lifetime object (M)
o Message sequence number object (M)
o Information code object (M)
o External address object (O)
The RESPONSE message for other classes than 'Success' (0x2) carries
these objects:
o Message sequence number object (M)
o Information code object (M)
This message is routed towards the NI hop-by-hop, using existing NTLP
messaging associations. The MRM used for this message MUST be the
same as MRM used by the corresponding CREATE or EXT message.
4.3.4. NOTIFY
The NOTIFY messages is used to report asynchronous events happening
along the signaled path to other NATFW NSLP nodes.
The NOTIFY message carries this object:
o Information code object (M).
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The NOTIFY message is routed towards the NI hop-by-hop using the
existing inbound node messaging association entry within the node's
Message Routing State table. The MRM used for this message MUST be
the same as MRM used by the corresponding CREATE or EXT message.
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5. Security Considerations
Security is of major concern particularly in case of firewall
traversal. This section provides security considerations for the
NAT/firewall traversal and is organized as follows.
In Section 5.1 we describe the participating entities relate to each
other from a security point of view. This subsection also motivates
a particular authorization model.
Security threats that focus on NSIS in general are described in [8]
and they are applicable to this document as well.
Finally, we illustrate how the security requirements that were
created based on the security threats can be fulfilled by specific
security mechanisms. These aspects will be elaborated in
Section 5.2.
5.1. Authorization Framework
The NATFW NSLP is a protocol which may involve a number of NSIS nodes
and is, as such, not a two-party protocol. Figure 1 and Figure 2 of
[8] already depict the possible set of communication patterns. In
this section we will re-evaluate these communication patters with
respect to the NATFW NSLP protocol interaction.
The security solutions for providing authorization have a direct
impact on the treatment of different NSLPs. As it can be seen from
the QoS NSLP [6] and the corresponding Diameter QoS work [19]
accounting and charging seems to play an important role for QoS
reservations, whereas monetary aspects might only indirectly effect
authorization decisions for NAT and firewall signaling. Hence, there
are differences in the semantic of authorization handling between QoS
and NATFW signaling. A NATFW aware node will most likely want to
authorize the entity (e.g., user or machine) requesting the
establishment of pinholes or NAT bindings. The outcome of the
authorization decision is either allowed or disallowed whereas a QoS
authorization decision might indicate that a different set of QoS
parameters is authorization (see [19] as an example).
5.1.1. Peer-to-Peer Relationship
Starting with the simplest scenario, it is assumed that neighboring
nodes are able to authenticate each other and to establish keying
material to protect the signaling message communication. The nodes
will have to authorize each other, additionally to the
authentication. We use the term 'Security Context' as a placeholder
for referring to the entire security procedure, the necessary
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infrastructure that needs to be in place in order for this to work
(e.g., key management) and the established security related state.
The required long-term key (symmetric or asymmetric keys) used for
authentication are either made available using an out-of-band
mechanism between the two NSIS NATFW nodes or they are dynamically
established using mechanisms not further specified in this document.
Note that the deployment environment will most likely have an impact
on the choice of credentials being used. The choice of these
credentials used is also outside the scope of this document.
+------------------------+ +-------------------------+
|Network A | | Network B|
| +---------+ +---------+ |
| +-///-+ Middle- +---///////----+ Middle- +-///-+ |
| | | box 1 | Security | box 2 | | |
| | +---------+ Context +---------+ | |
| | Security | | Security | |
| | Context | | Context | |
| | | | | |
| +--+---+ | | +--+---+ |
| | Host | | | | Host | |
| | A | | | | B | |
| +------+ | | +------+ |
+------------------------+ +-------------------------+
Figure 30: Peer-to-Peer Relationship
Figure 30 shows a possible relationship between participating NSIS
aware nodes. Host A might be, for example, a host in an enterprise
network that has keying material established (e.g., a shared secret)
with the company's firewall (Middlebox 1). The network administrator
of Network A (company network) has created access control lists for
Host A (or whatever identifiers a particular company wants to use).
Exactly the same procedure might also be used between Host B and
Middlebox 2 in Network B. For the communication between Middlebox 1
and Middlebox 2 a security context is also assumed in order to allow
authentication, authorization and signaling message protection to be
successful.
5.1.2. Intra-Domain Relationship
In larger corporations, for example, a middlebox is used to protect
individual departments. In many cases, the entire enterprise is
controlled by a single (or a small number of) security department,
which gives instructions to the department administrators. In such a
scenario, the previously discussed peer-to-peer relationship might be
prevalent. Sometimes it might be necessary to preserve
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authentication and authorization information within the network. As
a possible solution, a centralized approach could be used, whereby an
interaction between the individual middleboxes and a central entity
(for example a policy decision point - PDP) takes place. As an
alternative, individual middleboxes exchange the authorization
decision with another middlebox within the same trust domain.
Individual middleboxes within an administrative domain may exploit
their relationship instead of requesting authentication and
authorization of the signaling initiator again and again. Figure 31
illustrates a network structure which uses a centralized entity.
+-----------------------------------------------------------+
| Network A |
| +---------+ +---------+
| +----///--------+ Middle- +------///------++ Middle- +---
| | Security | box 2 | Security | box 2 |
| | Context +----+----+ Context +----+----+
| +----+----+ | | |
| | Middle- +--------+ +---------+ | |
| | box 1 | | | | |
| +----+----+ | | | |
| | Security | +----+-----+ | |
| | Context | | Policy | | |
| +--+---+ +-----------+ Decision +----------+ |
| | Host | | Point | |
| | A | +----------+ |
| +------+ |
+-----------------------------------------------------------+
Figure 31: Intra-domain Relationship
The interaction between individual middleboxes and a policy decision
point (or AAA server) is outside the scope of this document.
5.1.3. End-to-Middle Relationship
The peer-to-peer relationship between neighboring NSIS NATFW NSLP
nodes might not always be sufficient. Network B might require
additional authorization of the signaling message initiator (in
addition to the authorization of the neighboring node). If
authentication and authorization information is not attached to the
initial signaling message then the signaling message arriving at
Middlebox 2 would result in an error message being created, which
indicates the additional authorization requirement. In many cases
the signaling message initiator might already be aware of the
additionally required authorization before the signaling message
exchange is executed.
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Figure 32 shows this scenario.
+--------------------+ +---------------------+
| Network A | |Network B |
| | Security | |
| +---------+ Context +---------+ |
| +-///-+ Middle- +---///////----+ Middle- +-///-+ |
| | | box 1 | +-------+ box 2 | | |
| | +---------+ | +---------+ | |
| |Security | | | Security | |
| |Context | | | Context |
| | | | | | |
| +--+---+ | | | +--+---+ |
| | Host +----///----+------+ | | Host | |
| | A | | Security | | B | |
| +------+ | Context | +------+ |
+--------------------+ +---------------------+
Figure 32: End-to-Middle Relationship
5.2. Security Framework for the NAT/Firewall NSLP
The following list of security requirements has been created to
ensure proper secure operation of the NATFW NSLP.
5.2.1. Security Protection between neighboring NATFW NSLP Nodes
Based on the analyzed threats it is RECOMMENDED to provide, between
neighboring NATFW NSLP nodes, the following mechanism:
o data origin authentication
o replay protection
o integrity protection and
o optionally confidentiality protection
It is RECOMMENDED to use the authentication and key exchange security
mechanisms provided in [2] between neighboring nodes when sending
NATFW signaling messages. The proposed security mechanisms of GIST
provide support for authentication and key exchange in addition to
denial of service protection. Depending on the chosen security
protocol, support for multiple authentication protocols might be
provided. If security between neighboring nodes is desired than the
usage of C-MODE for the delivery of data packets and the usage of
D-MODE only to discover the next NATFW NSLP aware node along the path
is highly RECOMMENDED. Almost all security threats at the NATFW NSLP
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layer can be prevented by using a mutually authenticated Transport
Layer secured connection and by relying on authorization by the
neighboring NATFW NSLP entities.
The NATFW NSLP relies on an established security association between
neighboring peers to prevent unauthorized nodes to modify or delete
installed state. Between non-neighboring nodes the session ID (SID)
carried in the NTLP is used to show ownership of a NATFW NSLP
signaling session. The session ID MUST be generated in a random way
and thereby prevent an off-path adversary to mount targeted attacks.
Hence, an adversary would have to learn the randomly generated
session ID to perform an attack. In a mobility environment a former
on-path node that is now off-path can perform an attack. Messages
for a particular NATFW NSLP signaling session are handled by the NTLP
to the NATFW NSLP for further processing. Messages carrying a
different session ID not associated with any NATFW NSLP are subject
to the regular processing for new NATFW NSLP signaling sessions.
5.2.2. Security Protection between non-neighboring NATFW NSLP Nodes
Based on the security threats and the listed requirements it was
noted that some threats also demand authentication and authorization
of a NATFW signaling entity (including the initiator) towards a non-
neighboring node. This mechanism mainly demands entity
authentication. Additionally, security protection of certain
payloads may be required between non-neighboring signaling entities
and the Cryptographic Message Syntax (CMS) [14] might be a potential
solution. Payload protection using CMS is not described in this
document. The most important information exchanged at the NATFW NSLP
is information related to the establishment for firewall pinholes and
NAT bindings. This information can, however, not be protected over
multiple NSIS NATFW NSLP hops since this information might change
depending on the capability of each individual NATFW NSLP node.
Some scenarios might also benefit from the usage of authorization
tokens. Their purpose is to associate two different signaling
protocols (e.g., SIP and NSIS) and their authorization decision.
These tokens are obtained by non-NSIS protocols, such as SIP or as
part of network access authentication. When a NAT or firewall along
the path receives the token it might be verified locally or passed to
the AAA infrastructure. Examples of authorization tokens can be
found in RFC 3520 [17] and RFC 3521 [18]. Figure 33 shows an example
of this protocol interaction.
An authorization token is provided by the SIP proxy, which acts as
the assertion generating entity and gets delivered to the end host
with proper authentication and authorization. When the NATFW
signaling message is transmitted towards the network, the
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authorization token is attached to the signaling messages to refer to
the previous authorization decision. The assertion verifying entity
needs to process the token or it might be necessary to interact with
the assertion granting entity using HTTP (or other protocols). As a
result of a successfully authorization by a NATFW NSLP node, the
requested action is executed and later a RESPONSE message is
generated.
+----------------+ Trust Relationship +----------------+
| +------------+ |<.......................>| +------------+ |
| | Protocol | | | | Assertion | |
| | requesting | | HTTP, SIP Request | | Granting | |
| | authz | |------------------------>| | Entity | |
| | assertions | |<------------------------| +------------+ |
| +------------+ | Artifact/Assertion | Entity Cecil |
| ^ | +----------------+
| | | ^ ^|
| | | . || HTTP,
| | | Trust . || other
| API Access | Relationship. || protocols
| | | . ||
| | | . ||
| | | v |v
| v | +----------------+
| +------------+ | | +------------+ |
| | Protocol | | NSIS NATFW CREATE + | | Assertion | |
| | using authz| | Assertion/Artifact | | Verifying | |
| | assertion | | ----------------------- | | Entity | |
| +------------+ | | +------------+ |
| Entity Alice | <---------------------- | Entity Bob |
+----------------+ RESPONSE +----------------+
Figure 33: Authorization Token Usage
Threats against the usage of authorization tokens have been mentioned
in [8]. Hence, it is required to provide confidentiality protection
to avoid allowing an eavesdropper to learn the token and to use it in
another NATFW NSLP signaling session (replay attack). The token
itself also needs to be protected against tempering.
To harmonize the usage of authorization tokens in NSLPs a separate
document is available, see [20].
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6. IAB Considerations on UNSAF
UNilateral Self-Address Fixing (UNSAF) is described in [12] as a
process at originating endpoints that attempt to determine or fix the
address (and port) by which they are known to another endpoint.
UNSAF proposals, such as STUN [15] are considered as a general class
of workarounds for NAT traversal and as solutions for scenarios with
no middlebox communication.
This memo specifies a path-coupled middlebox communication protocol,
i.e., the NSIS NATFW NSLP. NSIS in general and the NATFW NSLP are
not intended as a short-term workaround, but more as a long-term
solution for middlebox communication. In NSIS, endpoints are
involved in allocating, maintaining, and deleting addresses and ports
at the middlebox. However, the full control of addresses and ports
at the middlebox is at the NATFW NSLP daemon located to the
respective NAT.
Therefore, this document addresses the UNSAF considerations in [12]
by proposing a long-term alternative solution.
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7. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the
NATFW NSLP, in accordance with BCP 26 RFC 2434 [13].
The NATFW NSLP requires IANA to create a number of new registries.
These registries may require further coordination with the registries
of the NTLP [2] and the QoS NSLP [6].
NATFW NSLP Message Type Registry
The NATFW NSLP Message Type is a 8 bit value. The allocation of
values for new message types requires standards action. Updates and
deletion of values from the registry is not possible. This
specification defines four NATFW NSLP message types, which form the
initial contents of this registry. IANA is requested to add these
four NATFW NSLP Message Types: CREATE, EXT, RESPONSE, and NOTIFY.
NATFW NSLP Header Flag Registry
NATFW NSLP messages have a messages-specific 8 bit flags/reserved
field in their header. The registration of flags is subject to IANA
registration. The allocation of values for flag types requires
standards action. Updates and deletion of values from the registry
is not possible. This specification defines only one flag, the P
flag in Figure 20.
NSLP Object Type Registry
[Delete this part if already done by another NSLP:
A new registry is to be created for NSLP Message Objects. This is a
12-bit field (giving values from 0 to 4095). This registry is shared
between a number of NSLPs. Allocation policies are as follows:
0-1023: Standards Action
1024-1999: Specification Required
2000-2047: Private/Experimental Use
2048-4095: Reserved
When a new object is defined, the extensibility bits (A/B) must also
be defined.]
This document defines 8 objects for the NATFW NSLP: NATFW_LT,
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NATFW_EXT_IP, NATFW_EFI, NATFW_INFO, NATFW_NONCE, NATFW_MSN,
NATFW_DTINFO, NATFW_ICMP_TYPES. IANA is request to assigned values
for them from NSLP Object Type registry and to replace the
corresponding IANA-TBD tags with the numeric values.
NSLP Response Code Registry
In addition it defines a number of Response Codes for the NATFW NSLP.
These can be found in Section 4.2.4 and are to be assigned values
from NSLP Response Code registry. The allocation of values for
Response Codes Codes requires standards action. IANA is request to
assigned values for them from NSLP Response Code registry.
Furthermore, IANA is requested to add a new value to the NSLP
Identifiers (NSLPID) registry defined in [2] for the NATFW NSLP.
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8. Open Issues
A more detailed list of open issue can be found at:
https://kobe.netlab.nec.de/roundup/nsis-natfw-nslp/index
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9. Acknowledgments
We would like to thank the following individuals for their
contributions to this document at different stages:
o Marcus Brunner and Henning Schulzrinne for work on work on IETF
drafts which lead us to start with this document,
o Miquel Martin for his help on the initial version of this
document,
o Srinath Thiruvengadam and Ali Fessi work for their work on the
NAT/firewall threats draft,
o Henning Peters for his comments and suggestions,
o and the NSIS working group.
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10. References
10.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Schulzrinne, H. and R. Hancock, "GIST: General Internet
Signalling Transport", draft-ietf-nsis-ntlp-14 (work in
progress), July 2007.
[3] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
August 1996.
10.2. Informative References
[4] Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
Bosch, "Next Steps in Signaling (NSIS): Framework", RFC 4080,
June 2005.
[5] Brunner, M., "Requirements for Signaling Protocols", RFC 3726,
April 2004.
[6] Manner, J., "NSLP for Quality-of-Service Signaling",
draft-ietf-nsis-qos-nslp-15 (work in progress), July 2007.
[7] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A.
Rayhan, "Middlebox communication architecture and framework",
RFC 3303, August 2002.
[8] Tschofenig, H. and D. Kroeselberg, "Security Threats for Next
Steps in Signaling (NSIS)", RFC 4081, June 2005.
[9] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999.
[10] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues",
RFC 3234, February 2002.
[11] Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997.
[12] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
[13] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
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Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[14] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852,
July 2004.
[15] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN
- Simple Traversal of User Datagram Protocol (UDP) Through
Network Address Translators (NATs)", RFC 3489, March 2003.
[16] Westerinen, A., Schnizlein, J., Strassner, J., Scherling, M.,
Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry, J., and
S. Waldbusser, "Terminology for Policy-Based Management",
RFC 3198, November 2001.
[17] Hamer, L-N., Gage, B., Kosinski, B., and H. Shieh, "Session
Authorization Policy Element", RFC 3520, April 2003.
[18] Hamer, L-N., Gage, B., and H. Shieh, "Framework for Session
Set-up with Media Authorization", RFC 3521, April 2003.
[19] Zorn, G., "Diameter Quality of Service Application",
draft-ietf-dime-diameter-qos-01 (work in progress), July 2007.
[20] Manner, J., "Authorization for NSIS Signaling Layer Protocols",
draft-manner-nsis-nslp-auth-03 (work in progress), March 2007.
[21] Roedig, U., Goertz, M., Karten, M., and R. Steinmetz, "RSVP as
firewall Signalling Protocol", Proceedings of the 6th IEEE
Symposium on Computers and Communications, Hammamet,
Tunisia pp. 57 to 62, IEEE Computer Society Press, July 2001.
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Appendix A. Selecting Signaling Destination Addresses for EXT
As with all other message types, EXT messages need a reachable IP
address of the data sender on the GIST level. For the path-coupled
MRM the source-address of GIST is the reachable IP address (i.e., the
real IP address of the data sender, or a wildcard). While this is
straight forward, it is not necessarily so for the loose-end MRM.
Many applications do not provide the IP address of the communication
counterpart, i.e., either the data sender or both a data sender and
receiver. For the EXT messages, the case of data sender is of
interest only. The rest of this section is giving informational
guidance about determining a good destination-address of the LE-MRM
in GIST for EXT messages.
This signaling destination address (SDA, the destination-address in
GIST) can be the data sender, but for applications which do not
provide an address upfront, the destination address has to be chosen
independently, as it is unknown at the time when the NATFW NSLP
signaling has to start. Choosing the 'correct' destination IP
address may be difficult and it is possible that there is no 'right
answer' for all applications relying on the NATFW NSLP.
Whenever possible it is RECOMMENDED to chose the data sender's IP
address as SDA. It necessary to differentiate between the received
IP addresses on the data sender. Some application level signaling
protocols (e.g., SIP) have the ability to transfer multiple contact
IP addresses of the data sender. For instance, private IP address,
public IP address at NAT, and public IP address at a relay. It is
RECOMMENDED to use all non-private IP addresses as SDAs.
A different SDA must be chosen, should the IP address of the data
sender be unknown. This can have multiple reasons: The application
level signaling protocol cannot determine any data sender IP address
at this point of time or the data receiver is server behind a NAT,
i.e., accepting inbound packets from any host. In this case, the
NATFW NSLP can be instructed to use the public IP address of an
application server or any other node. Choosing the SDA in this case
is out of the scope of the NATFW NSLP and depends on the
application's choice. The local network can provide a network-SDA,
i.e., a SDA which is only meaningful to the local network. This will
ensure that GIST packets with destination-address set to this
network-SDA are going to be routed to a edge-NAT or edge-firewall.
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Appendix B. Applicability Statement on Data Receivers behind Firewalls
Section 3.7.2 describes how data receivers behind middleboxes can
instruct inbound firewalls/NATs to forward NATFW NSLP signaling
towards them. Finding an inbound edge-NAT in address environment
with NAT'ed addresses is quite easy. It is only required to find
some edge-NAT, as the data traffic will be route-pinned to the NAT,
which is done with the LE-MRM. Locating the appropriate edge-
firewall with the PC-MRM, sent inbound is difficult. For cases with
a single, symmetric route from the Internet to the data receiver, it
is quite easy; simply follow the default route in the inbound
direction.
+------+ Data Flow
+-------| EFW1 +----------+ <===========
| +------+ ,--+--.
+--+--+ / \
NI+-----| FW1 | (Internet )----NR+/NI/DS
NR +--+--+ \ /
| +------+ `--+--'
+-------| EFW2 +----------+
+------+
~~~~~~~~~~~~~~~~~~~~~>
Signaling Flow
Figure 34: Data receiver behind multiple, parallel located firewalls
When a data receiver, and thus NR, is located in a network site that
is multihomed with several independently firewalled connections to
the public Internet (as shown in Figure 34), the specific firewall
through which the data traffic will be routed has to be ascertained.
NATFW NSLP signaling messages sent from the NI+/NR during the EXT
message exchange towards the NR+ must be routed by the NTLP to the
edge-firewall that will be passed by the data traffic as well. The
NTLP would need to be aware about the routing within the Internet to
determine the path between DS and DR. Out of this, the NTLP could
determine which of the edge-firewalls, either EFW1 or EFW2, must be
selected to forward the NATFW NSLP signaling. Signaling to the wrong
edge-firewall, as shown in Figure 34, would install the NATFW NSLP
policy rules at the wrong device. This causes either a blocked data
flow (when the policy rule is 'allow') or an ongoing attack (when the
policy rule is 'deny'). Requiring the NTLP to know all about the
routing within the Internet is definitely a tough challenge and
usually not possible. In such described case, the NTLP must
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basically give up and return an error to the NSLP level, indicating
that the next hop discovery is not possible.
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Appendix C. Firewall and NAT Resources
This section gives some examples on how NATFW NSLP policy rules could
be mapped to real firewall or NAT resources. The firewall rules and
NAT bindings are described in a natural way, i.e., in a way one will
find it in common implementation.
C.1. Wildcarding of Policy Rules
The policy rule/MRI to be installed can be wildcarded to some degree.
Wildcarding applies to IP address, transport layer port numbers, and
the IP payload (or next header in IPv6). Processing of wildcarding
splits into the NTLP and the NATFW NSLP layer. The processing at the
NTLP layer is independent of the NSLP layer processing and per layer
constraints apply. For wildcarding in the NTLP see Section 5.8 of
[2].
Wildcarding at the NATFW NSLP level is always a node local policy
decision. A signaling message carrying a wildcarded MRI (and thus
policy rule) arriving at an NSLP node can be rejected if the local
policy does not allow the request. For instance, a MRI with IP
addresses set (not wildcarded), transport protocol TCP, and TCP port
numbers completely wildcarded. Now the local policy allows only
requests for TCP with all ports set and not wildcarded. The request
is going to be rejected.
C.2. Mapping to Firewall Rules
This section describes how a NSLP policy rule signaled with a CREATE
message is mapped to a firewall rule. The MRI is set as follows:
o network-layer-version=IPv4
o source-address=192.0.2.100, prefix-length=32
o destination-address=192.0.50.5, prefix-length=32
o IP-protocol=UDP
o L4-source-port=34543, L4-destination-port=23198
The NATFW_EFI object is set to action=allow and sub_ports=0.
The resulting policy rule (firewall rule) to be installed might look
like: allow udp from 192.0.2.100 port=34543 to 192.0.50.5 port=23198
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C.3. Mapping to NAT Bindings
This section describes how a NSLP policy rule signaled with a EXT
message is mapped to a NAT binding. It is assumed that the EXT
message is sent by a NI+ being located behind a NAT and does contain
a NATFW_DTINFO object. The MRI is set following using the signaling
destination address, since the IP address of the real data sender is
not known:
o network-layer-version=IPv4
o source-address= 192.168.5.100
o destination-address=SDA
o IP-protocol=UDP
The NATFW_EFI object is set to action=allow and sub_ports=0. The
NATFW_DTINFO object contains these parameters:
o P=1
o dest prefix=0
o protocol=UDP
o dst port number = 20230, src port number=0
o src IP=0.0.0.0
The edge-NAT allocates the external IP 192.0.2.79 and port 45000.
The resulting policy rule (NAT binding) to be installed could look
like: translate from any to 192.0.2.79 port=45000 to 192.168.5.100
port=20230
C.4. NSLP Handling of Twice-NAT
The dynamic configuration of twice-NATs requires application level
support, as stated in Section 2.5. The NATFW NSLP cannot be used for
configuring twice-NATs if application level support is needed.
Assuming application level support performing the configuration of
the twice-NAT and the NATFW NSLP being installed at this devices, the
NATFW NSLP must be able to traverse it. The NSLP is probably able to
traverse the twice-NAT, as any other data traffic, but the flow
information stored in the NTLP's MRI will be invalidated through the
translation of source and destination address. The NATFW NSLP
implementation on the twice-NAT MUST intercept NATFW NSLP and NTLP
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signaling messages as any other NATFW NSLP node does. For the given
signaling flow, the NATFW NSLP node MUST look up the corresponding IP
address translation and modify the NTLP/NSLP signaling accordingly.
The modification results in an updated MRI with respect to the source
and destination IP addresses.
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Appendix D. Assigned Numbers for Testing
NOTE: This section MUST be removed before publication.
This section defines temporarily used values of the NATFW NSLP for
testing the different implementations.
Values for the NATFW NSLP message types:
o CREATE: 0x01
o EXT: 0x02
o RESPONSE: 0x03
o NOTIFY: 0x04
Values for the NSLP object types
o NATFW_LT: 0x00F1
o NATFW_EXT_IP: 0x00F2
o NATFW_EFI: 0x00F3
o NATFW_INFO: 0x00F4
o NATFW_NONCE: 0x00F5
o NATFW_MSN: 0x00F6
o NATFW_DTINFO: 0x00F7
o NATFW_ICMP_TYPES: 0x00F9
1345
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Authors' Addresses
Martin Stiemerling
NEC Europe Ltd. and University of Goettingen
Kurfuersten-Anlage 36
Heidelberg 69115
Germany
Phone: +49 (0) 6221 4342 113
Email: stiemerling@netlab.nec.de
URI: http://www.stiemerling.org
Hannes Tschofenig
Nokia Siemens Networks
Otto-Hahn-Ring 6
Munich 81739
Germany
Phone:
Email: Hannes.Tschofenig@nsn.com
URI: http://www.tschofenig.com
Cedric Aoun
Paris
France
Email: cedric@caoun.net
Elwyn Davies
Folly Consulting
Soham
UK
Phone: +44 7889 488 335
Email: elwynd@dial.pipex.com
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