NSIS Working Group M. Stiemerling
Internet-Draft NEC
Expires: January 19, 2006 H. Tschofenig
Siemens
C. Aoun
ENST
July 18, 2005
NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
draft-ietf-nsis-nslp-natfw-07
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo defines the NSIS Signaling Layer Protocol (NSLP) for
Network Address Translators and Firewalls. This NSLP allows hosts to
signal along a data path for Network Address Translators and
Firewalls to be configured according to the data flow needs. The
network scenarios, problems and solutions for path-coupled Network
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Address Translator and Firewall signaling are described. The overall
architecture is given by the framework and requirements defined by
the Next Steps in Signaling (NSIS) working group.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Terminology and Abbreviations . . . . . . . . . . . . . . 7
1.2 Middleboxes . . . . . . . . . . . . . . . . . . . . . . . 9
1.3 Non-Goals . . . . . . . . . . . . . . . . . . . . . . . . 10
1.4 General Scenario for NATFW Traversal . . . . . . . . . . . 11
2. Network Deployment Scenarios using 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 IPv4/v6 NAT with two Private Networks . . . . . . . . . . 18
2.8 Multihomed Network with NAT . . . . . . . . . . . . . . . 19
2.9 Multihomed Network with Firewall . . . . . . . . . . . . . 20
3. Protocol Description . . . . . . . . . . . . . . . . . . . . 21
3.1 Policy Rules . . . . . . . . . . . . . . . . . . . . . . . 21
3.2 Basic protocol overview . . . . . . . . . . . . . . . . . 21
3.3 Protocol Operations . . . . . . . . . . . . . . . . . . . 25
3.3.1 Creating Sessions . . . . . . . . . . . . . . . . . . 25
3.3.2 Reserving External Addresses . . . . . . . . . . . . . 28
3.3.3 NATFW Session refresh . . . . . . . . . . . . . . . . 32
3.3.4 Deleting Sessions . . . . . . . . . . . . . . . . . . 34
3.3.5 Reporting Asynchronous Events . . . . . . . . . . . . 35
3.3.6 Query and diagnosis capabilities within the NATFW
NSLP protocol . . . . . . . . . . . . . . . . . . . . 36
3.3.7 Proxy Mode for Data Receiver behind NAT . . . . . . . 39
3.3.8 Proxy Mode for Data Sender behind Middleboxes . . . . 42
3.3.9 Proxy Mode for Data Receiver behind Firewall . . . . . 43
3.4 Calculation of Session Lifetime . . . . . . . . . . . . . 45
3.5 Message Sequencing . . . . . . . . . . . . . . . . . . . . 47
3.6 De-Multiplexing at NATs . . . . . . . . . . . . . . . . . 48
3.7 Selecting Opportunistic Addresses for REA . . . . . . . . 49
3.8 Session Ownership . . . . . . . . . . . . . . . . . . . . 50
3.9 Authentication and Authorization . . . . . . . . . . . . . 53
3.10 Reacting to Route Changes . . . . . . . . . . . . . . . 54
4. NATFW NSLP Message Components . . . . . . . . . . . . . . . 55
4.1 NSLP Header . . . . . . . . . . . . . . . . . . . . . . . 55
4.2 NSLP message types . . . . . . . . . . . . . . . . . . . . 55
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4.3 NSLP Objects . . . . . . . . . . . . . . . . . . . . . . . 56
4.3.1 Session Lifetime Object . . . . . . . . . . . . . . . 57
4.3.2 PBK Public Key . . . . . . . . . . . . . . . . . . . . 57
4.3.3 External Address Object . . . . . . . . . . . . . . . 58
4.3.4 Extended Flow Information Object . . . . . . . . . . . 59
4.3.5 Response Code Object . . . . . . . . . . . . . . . . . 59
4.3.6 Proxy Support Object . . . . . . . . . . . . . . . . . 60
4.3.7 Nonce Object . . . . . . . . . . . . . . . . . . . . . 60
4.3.8 Message Sequence Number Object . . . . . . . . . . . . 61
4.3.9 Bound Session ID Object . . . . . . . . . . . . . . . 61
4.3.10 Data Sender Information Object . . . . . . . . . . . 62
4.3.11 NATFW NF Hop Count Object . . . . . . . . . . . . . 62
4.3.12 Maximum Hops Object . . . . . . . . . . . . . . . . 63
4.3.13 Session Status object . . . . . . . . . . . . . . . 63
4.3.14 QDRQ type . . . . . . . . . . . . . . . . . . . . . 64
4.3.15 QDRQ Response object . . . . . . . . . . . . . . . . 64
4.4 Message Formats . . . . . . . . . . . . . . . . . . . . . 64
4.4.1 CREATE . . . . . . . . . . . . . . . . . . . . . . . . 65
4.4.2 RESERVE-EXTERNAL-ADDRESS (REA) . . . . . . . . . . . . 65
4.4.3 RESPONSE . . . . . . . . . . . . . . . . . . . . . . . 66
4.4.4 QDRQ . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.4.5 NOTIFY . . . . . . . . . . . . . . . . . . . . . . . . 67
4.4.6 UCREATE . . . . . . . . . . . . . . . . . . . . . . . 67
5. NATFW NSLP NTLP Requirements . . . . . . . . . . . . . . . . 68
6. NSIS NAT and Firewall Transition Issues . . . . . . . . . . 69
7. Security Considerations . . . . . . . . . . . . . . . . . . 70
7.1 Trust Relationship and Authorization . . . . . . . . . . . 70
7.1.1 Peer-to-Peer Trust Relationship . . . . . . . . . . . 71
7.1.2 Intra-Domain Trust Relationship . . . . . . . . . . . 71
7.1.3 End-to-Middle Trust Relationship . . . . . . . . . . . 72
7.2 Security Threats and Requirements . . . . . . . . . . . . 73
7.2.1 Attacks related to authentication and authorization . 73
7.2.2 Denial-of-Service Attacks . . . . . . . . . . . . . . 80
7.2.3 Man-in-the-Middle Attacks . . . . . . . . . . . . . . 81
7.2.4 Message Modification by non-NSIS on-path node . . . . 82
7.2.5 Message Modification by malicious NSIS node . . . . . 82
7.2.6 Session Modification/Deletion . . . . . . . . . . . . 83
7.2.7 Misuse of unreleased sessions . . . . . . . . . . . . 86
7.2.8 Data traffic injection . . . . . . . . . . . . . . . . 87
7.2.9 Eavesdropping and traffic analysis . . . . . . . . . . 89
7.3 Security Framework for the NAT/Firewall NSLP . . . . . . . 90
7.3.1 Security Protection between neighboring NATFW NSLP
Nodes . . . . . . . . . . . . . . . . . . . . . . . . 90
7.3.2 Security Protection between non-neighboring NATFW
NSLP Nodes . . . . . . . . . . . . . . . . . . . . . . 90
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7.3.3 End-to-End Security . . . . . . . . . . . . . . . . . 92
8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 93
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 94
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 95
10.1 Normative References . . . . . . . . . . . . . . . . . . 95
10.2 Informative References . . . . . . . . . . . . . . . . . 95
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 98
A. Firewall and NAT Resources . . . . . . . . . . . . . . . . . 99
A.1 Wildcarding of Policy Rules . . . . . . . . . . . . . . . 99
A.2 Mapping to Firewall Rules . . . . . . . . . . . . . . . . 100
A.3 Mapping to NAT Bindings . . . . . . . . . . . . . . . . . 100
A.4 Mapping for combined NAT and Firewall . . . . . . . . . . 100
A.5 NSLP Handling of Twice-NAT . . . . . . . . . . . . . . . . 100
B. Problems and Challenges . . . . . . . . . . . . . . . . . . 101
B.1 Missing Network-to-Network Trust Relationship . . . . . . 101
B.2 Relationship with routing . . . . . . . . . . . . . . . . 102
B.3 Affected Parts of the Network . . . . . . . . . . . . . . 102
B.4 NSIS backward compatibility with NSIS unaware NAT and
Firewalls . . . . . . . . . . . . . . . . . . . . . . . . 102
B.5 Authentication and Authorization . . . . . . . . . . . . . 103
B.6 Directional Properties . . . . . . . . . . . . . . . . . . 103
B.7 Addressing . . . . . . . . . . . . . . . . . . . . . . . . 104
B.8 NTLP/NSLP NAT Support . . . . . . . . . . . . . . . . . . 104
B.9 Combining Middlebox and QoS signaling . . . . . . . . . . 104
B.10 Inability to know the scenario . . . . . . . . . . . . . 105
C. Object ID allocation for testing . . . . . . . . . . . . . . 106
D. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 107
Intellectual Property and Copyright Statements . . . . . . . 108
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1. Introduction
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 the outside, and in times of
IPv4 address depletion, NATs virtually extend 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 (e.g., most HTTP traffic, and client/server
applications), due to the relatively static properties of the
protocols used. Other applications, such as IP telephony and most
other peer-to-peer applications, which have more dynamic properties,
create traffic which is unable to traverse NATs and Firewalls
unassisted. In practice, the traffic from many applications cannot
traverse autonomous Firewalls or NATs, even when they have added
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, currently under standardization at
the IETF). In this approach, ALGs external to the Firewall or NAT
configure the corresponding entity via the MIDCOM protocol [6].
Several other work-around solutions are available, including STUN
[25] and TURN [28]. 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, ...), 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 QoS or NATFW 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 [12] is an example of a current QoS signaling protocol that is
path-coupled. [35] 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 [4]. The general framework of NSIS is outlined
in [3]. It introduces the split between an NSIS transport layer and
an NSIS signaling layer. The transport of NSLP messages is handled
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by an NSIS Network Transport Layer Protocol (NTLP, with GIMPS [1]
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 [5], while the NATFW NSLP is defined in
this memo.
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 upstream
firewalls. Enabling data flows requires the loading of firewall pin
holes (loading of firewall rules with action allow) and creating NAT
bindings. Blocking of data flows requires the loading of firewalls
rules with action deny/drop. 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 NAT/Firewall along the data path intercepts these
messages, processes them, and configures itself accordingly.
Afterwards, the actual data flow can traverse every configured
Firewall/NAT.
It is necessary to distinguish between two different basic scenarios
when operating the NATFW NSLP, independent of the type of middlebox
to be configured.
1. Both data sender and data receiver of the network 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.
2. Only one end host is NSIS NATFW NSLP aware, either data receiver
or data sender.
NATFW NSLP provides three modes to cope with various possible
scenarios likely to be encountered before and after widespread
deployment of NSIS. Once there is full deployment of NSIS (in the
sense that both end hosts support NATFW NSLP signaling), the
requisite NAT and firewall state can be created using either just
CREATE mode if the data receiver resides in a public addressing
realm, or a combination of RESERVE-EXTERNAL-ADDRESS and CREATE modes
if the data receiver resides in a private addressing realm and needs
to preconfigure the boundary NAT to provide a publicly reachable
address for use by the data sender. During the introduction of NSIS,
it is likely that one or other of the data sender and receiver will
not be NSIS capable. In these cases the NATFW NSLP can utilize NSIS
aware middleboxes on the path between the sender and receiver to
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provide proxy NATFW NSLP services. Typically these boxes will be at
the boundaries of the realms in which the end hosts are located. If
the data receiver is NSIS unaware, the normal modes can be employed
but the NSIS signaling terminates at the NSIS aware node
topologically closest to the receiver which then acts as a proxy for
the receiver. If the data sender is unaware a variant of the
RESERVE-EXTERNAL-ADDRESS mode can be used by a data receiver behind a
NAT and the specialized UCREATE mode can be used by a data receiver
behind a firewall.
All modes of operation create NATFW NSLP and NTLP state in NSIS
entities. NTLP state allows signaling messages to travel in the
forward (downstream) and the reverse (upstream) direction along the
path between a NAT/Firewall NSLP sender and a corresponding receiver.
NAT bindings and firewall rules are NAT/Firewall specific state.
This state is managed using a soft-state mechanism, i.e., it expires
unless it is refreshed from time to time.
Section 2 describes the network environment for NATFW NSLP signaling,
highlighting the trust relationships and authorization required.
Section 3 defines the NATFW signaling protocol. Section 4 defines
the messages and and message components. In the remaining parts of
the main body of the document, Section 6 covers transition issues and
Section 7 addresses security considerations. Currently unsolved
problems and challenges are listed and discussed in Appendix B.
Please note that readers familiar with Firewalls and NATs and their
possible location within networks can safely skip Section 2.
1.1 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 RFC 2119.
This document uses a number of terms defined in [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" [27]. In the context of NSIS
NATFW NSLP, the condition is a specification of a set of packets
to which rules are applied. The set of actions always contains
just a single element per rule, and is limited to either action
"reserved", "deny" or action "allow".
o Firewall: A packet filtering device that matches packets against a
set of policy rules and applies the actions. In the context of
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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 [8]). Network Address Translators are devices that
perform this work.
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
middlebox are currently outside of the scope of this document.
o Security Gateway: IPsec based gateways.
o (Data) Receiver (DR or R): The node in the network that is
receiving the data packets of a flow.
o (Data) Sender (DS or S): The node in the network that is sending
the data packets of a flow.
o NATFW NSLP session: An application layer flow of information for
which some network control state information is to be manipulated
or monitored (as defined in [3]). The control state for NATFW
NSLP consists of NSLP state and associated policy rules at a
middlebox.
o NSIS peer or peer: An NSIS node with which an NSIS adjacency has
been created as defined in [1].
o Edge-NAT: An edge-NAT is a NAT device that is reachable from the
public Internet and that has a globally routable IP address.
o Edge-Firewall: An edge-Firewall is a Firewall device that is
located on the demarcation 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" [8].
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
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facilitated by a NAT router" [8]. IP address space allocation for
private networks is recommended in [26]
o Public/Global IP address: An IP address located in the public
network according to Section 2.7 of [8].
o Private/Local IP address: An IP address located in the private
network according to Section 2.8 of [8].
o Initial CREATE: A CREATE message creating a new session.
1.2 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 and security gateways, are
out of scope.
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 [8].
Both 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:
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o Source and destination IP addresses
o Transport protocol number
o Transport source and destination port numbers
For further examples of flow identifiers see Section 5.2.2 of [1].
Actions for Firewalls are usually one or more of:
o Allow: forward data packet
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.
1.3 Non-Goals
Traversal of non-NSIS and non-NATFW NSLP aware NATs and Firewalls
is outside the scope of this document.
Only Firewalls and NATs are considered in this document, any other
types of devices, for instance IPSec security gateway, are out of
scope.
The exact implementation of policy rules and their mapping to
firewall rule sets and NAT bindings or sessions at the middlebox
is an implementation issue and thus out of scope of this document.
Some devices categorized as firewalls only accept traffic after
cryptographic verification (i.e., IPsec protected data traffic).
Particularly for network access scenarios, either link layer or
network layer data protection is common. We do not address these
types of devices (referred to as security gateways) since per-flow
signaling is typically not used in this environment.
Another application, for which NSIS signaling has been proposed
but which is out of scope for this document, is discovering
security gateways, for the purpose of executing IKE to create an
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IPsec SA.
In mobility scenarios, a common problem is the traversal of a
security gateway at the edge of a corporate network. Network
administrators allow only authenticated data to enter the network.
A problem statement for the traversal of these security gateways
in the context of Mobile IP can be found in [23]). This topic is
not within the scope of the present document.
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. It is assumed that these middleboxes will be
statically configured in such a way that NSIS NATFW signaling
messages themselves are allowed to traverse them. NSIS NATFW NSLP
signaling is used to dynamically install additional policy rules in
all NATFW middleboxes along the data path. Firewalls are configured
to forward data packets matching the policy rule provided by the NSLP
signaling. NATs are configured to translate data packets matching
the policy rule provided by the NSLP signaling. However, there is an
exception to the primary goal of NSIS NATFW signaling, NSIS NATFW
nodes can request blocking of particular data flows instead of
enabling these flows at upstream 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 (NAT/FW in Figure 1). Applications located at these end
hosts try to establish communication with corresponding applications
on other such end hosts. They trigger the NSIS entity at the local
host to provide 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.
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Application Application Server (0, 1, or more) Application
+----+ +----+ +----+
| +------------------------+ +------------------------+ |
+-+--+ +----+ +-+--+
| |
| NSIS Entities NSIS Entities |
+-+--+ +----+ +-----+ +-+--+
| +--------+ +----------------------------+ +-----+ |
+-+--+ +-+--+ +--+--+ +-+--+
| | ------ | |
| | //// \\\\\ | |
+-+--+ +-+--+ |/ | +-+--+ +-+--+
| | | | | Internet | | | | |
| +--------+ +-----+ +----+ +-----+ |
+----+ +----+ |\ | +----+ +----+
\\\\ /////
sender NAT/FW (1+) ------ NATFW (1+) receiver
Figure 1: Generic View on NSIS in a NAT / Firewall case
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 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. To reduce the number of network
elements needed, combined Firewall and NATs have been made available.
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.
Placing firewalls in a network topology can be done in several very
different ways. 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, NSIS initiator and NSIS
responder, are behind NATs. The outermost NAT, called edge-NAT, at
each side is connected to the public Internet. The NATs are
generically labeled as MB (for middlebox), 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 --| MB |-----| MB |---| |---| MB |-----| MB |--- NR
+----+ +----+ \\----// +----+ +----+
private public private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 3: NAT with two Private Networks Scenario
Signaling traffic from NI to NR has to traverse all the middleboxes
on the path, 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. Instead a NAT binding (including a
public IP address) has to be previously installed on the NAT that
subsequently allows packets reaching the NAT to be forwarded to the
receiver within the private address realm. This generally requires
further support from an application layer protocol for the purpose of
discovering and exchanging information. The receiver might have a
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number of ways to learn its public IP address and port number and
might need to signal this information to the sender using the
application level signaling protocol.
2.3 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.
Note that the data sender does not necessarily know whether the
receiver is behind a NAT or not, hence, it is the receiving side that
has to detect whether itself is behind a NAT or not. As described in
Section 3.3.2 NSIS can also provide help for this procedure.
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 so establish the path via the middleboxes in the
receiver side private network.
This scenario describes the use case for the RESERVE-EXTERNAL-ADDRESS
mode 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 [8] 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. This is a common
configuration in networks, particularly after the merging of
companies that have used the same private address space, resulting in
overlapping address ranges.
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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 gateways 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 [6]. 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 by useful when there are
also Firewalls on 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
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, [1])within the NSLP
and NTLP accordingly. For more information see Appendix A.5
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
NSIS initiator. Afterwards, the NSIS initiator can start sending the
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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). This is
the worst case in which both sender and receiver obtain a public IP
address at the NAT, and the communication path is certainly not
optimal in this case.
NI public
\ +----+ //----\\
+-| MB |----| |
/ +----+ \\----//
NR
private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 7: NAT to Public, Both Hosts Behind Same NAT
The NSIS NATFW signaling protocol should support mechanisms to detect
such a scenario.
2.7 IPv4/v6 NAT with two Private Networks
This scenario combines the use case described in Section 2.2 with the
IPv4 to IPv6 transition scenario involving address and protocol
translation, i.e., using Network Address and Protocol Translators
(NAT-PT, [9]).
The difference from the other scenarios is the use of IPv6 to IPv4
(and vice versa) address and protocol translation. Additionally, the
base NTLP must support transport of messages in mixed IPv4 and IPv6
networks where some NSIS peers provide translation.
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+----+ +----+ //---\\ +----+ //---\\ +----+ +----+
NI --| MB |--| MB |--| |--| MB |-| |--| MB |--| MB |-- NR
+----+ +----+ \\---// +----+ \\---// +----+ +----+
private public public private
IPv4 IPv6
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 8: IPv4/v6 NAT with two Private Networks
This scenario needs the same type of application level support as
described in Section 2.5, and so the issues relating to twice-NATs
apply here as well.
2.8 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
private public
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 9: Multihomed Network with Two NATs
Depending on the destination or load balancing requirements, 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 3.3.2 for an expanded discussion of this topic with respect
to NATs.
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2.9 Multihomed Network with Firewall
This section describes a multihomed scenario with two firewalls
placed on alternative paths to the public network (Figure 10). The
routing in the private and public network decided 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 a certain mode of the NATFW NSLP where the
NSIS responder is located behind these firewalls within the private
network: the UCREATE mode. The UCREATE mode is used to block a
particular data flow on an upstream firewall. NSIS must route the
UCREATE mode message upstream from NR to NI without probably knowing
the data traffic's subsequent path will take from NI to NR.
+----+
NR -------| MB |\
\ +----+ \ //---\\
\ -| |-- NI
\ \\---//
\ +----+ |
--| MB |-------+
+----+
private
private public
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 10: Multihomed Network with Two Firewalls
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3. Protocol Description
This section defines messages, objects, and protocol semantics for
the NATFW NSLP. Section 3.1 introduces the base element of a NSLP
session , the policy rule. Section 3.2 introduces the protocol and
the protocol behavior is defined in Section 3.3. Section 4 defines
the syntax of the messages and objects.
3.1 Policy Rules
Policy rules, bound to a session, are the building block 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
action 'translate this address' and further mapping information, that
might be, in the simplest case, internal IP address and external IP
address.
Policy rules are usually carried in one piece in signaling
applications. In NSIS the policy rule is divided into the flow
identifier, an allow or deny action, and additional information. The
filter specification is carried within NTLP's message routing
information (MRI) and additional information, including the
specification of the action, is carried in NSLP's objects.
Additional information is, for example, the lifetime of a policy rule
or session.
3.2 Basic protocol overview
The NSIS NATFW NSLP is carried over the NSIS Transport Layer Protocol
(NTLP) defined in [1]. The interworking with the NTLP and other
components is shown in Figure 60. NATFW NSLP messages are initiated
by the NSIS initiator (NI), handled by NSIS forwarders (NF) and
finally processed by the NSIS responder (NR). It is required that at
least NI and NR implement this NSLP, intermediate 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 when they have no interest.
A Data Sender (DS), intending to send data to a Data Receiver (DR)
must first initiate 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 11). 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 1.)
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+-------+ +-------+ +-------+ +-------+
| DS/NI |<~~~| MB1/ |<~~~| MB2/ |<~~~| DR/NR |
| |--->| NF1 |--->| NF2 |--->| |
+-------+ +-------+ +-------+ +-------+
========================================>
Data Traffic Direction
---> : NATFW NSLP request signaling
~~~> : NATFW NSLP response signaling
DS/NI : Data sender and NSIS initiator
DR/NR : Data receiver and NSIS responder
MB1 : Middlebox 1 and NSIS forwarder 1
MB2 : Middlebox 2 and NSIS forwarder 2
Figure 11: General NSIS signaling
+-------+ +-------+ +-------+ +-------+
| DS/NI |<~~~| MB1/ |<~~~| NR | | DR |
| |--->| NF1 |--->| | | |
+-------+ +-------+ +-------+ +-------+
========================================>
Data Traffic Direction
---> : NATFW NSLP request signaling
~~~> : NATFW NSLP response signaling
DS/NI : Data sender and NSIS initiator
DR/NR : Data receiver and NSIS responder
MB1 : Middlebox 1 and NSIS forwarder 1
MB2 : Middlebox 2 and NSIS forwarder 2
Figure 12: A NSIS proxy mode signaling
The sequence of NSLP events is as follows:
o NSIS initiators generate NATFW NSLP request messages and send
those towards the NSIS responder. Note, that the NSIS initiator
may not necessarily be the data sender but may be the data
receiver, for instance, when using the RESERVE-EXTERNAL-ADDRESS
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message.
o NSLP request messages are processed each time a NF with NATFW NSLP
support is traversed. 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). Note,
that NSIS responder may not necessarily be the data receiver but
may be any intermediate NSIS node that terminates the forwarding,
for example, in a proxy mode case where an edge-NAT is replying to
requests
o The response message is processed at each NF implementing the
NATFW NSLP.
o Once the NI has received a successful response, the data sender
can start sending its data flow to the data receiver.
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 sender
side. For scenarios with NATs on the receiver side certain problems
arise, as described in Section 2.
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
directly. The DR and 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 (such as SDP within
SIP). This application layer signaling, which is out of scope of the
NATFW NSLP, may involve third parties that assist in exchanging these
messages.
NATFW NSLP signaling supports this scenario by using the RESERVE-
EXTERNAL-ADDRESS mode of operation
1. The NR acquires a public address by signaling on the reverse path
(NR towards NI) and thus making itself available to other hosts.
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This process of acquiring a public addresses is called
reservation. During this process the DR reserves publicly
reachable addresses and ports suitable for NATFW NSLP signaling,
but data traffic will not be allowed to use this address/port
initially.
2. The NI signals directly to the NR as the NI would do if there is
no NAT in between, and creates policy rules at middleboxes.
Note, that the reservation mode will only allow the forwarding
of signaling messages but not data flow packets. Data flow
packets will be 'activated' by the signaling from NI towards NR.
The RESERVE-EXTERNAL-ADDRESS mode of operation is detailed in
Section 3.3.2
The above usage assumes that both ends of a communication support
NSIS but fail when NSIS is only deployed at one end of the network.
In this case only the receiving or sending side are NSIS aware and
not both at the same time (see also Section 1). NATFW NSLP supports
this scenario by using a proxy mode, as described in Section 3.3.7
and Section 3.3.8.
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 expected to work on a deny-all
policy, meaning that traffic that does not explicitly match any
firewall filter rule will be blocked. In contrast, the normal
behavior of NATs is to block all traffic that does not match any
already configured/installed binding or session. However, in some
scenarios it is required to support firewalls having allow-all
policies, allowing data traffic to traverse unless it is blocked
explicitly. Data receivers can utilize NATFW NSLP's UCREATE message
to install policy rules at upstream 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/ forgotten after a certain period of time. To prevent
this, the NATFW nodes involved will have to specifically request a
session extension. An explicit NATFW NSLP state deletion capability
is also provided by the protocol.
Middleboxes should return an error in case of a failure, such that
appropriate actions can be taken; this ability would allow debugging
and error recovery. Error messages could be sent upstream (for
errors related to received messages as well as asynchronous error
notification messages) towards the NI as well as downstream towards
the NR (in the case of asynchronous error notification messages).
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The next sections define the NATFW NSLP message types and formats,
protocol operations, and policy rule operations.
3.3 Protocol Operations
This section defines the protocol operations including, how to create
sessions, maintain them, and how to reserve addresses. All the NATFW
NSLP protocol messages require C-mode handling by the NTLP and cannot
be piggybacked into D-mode NTLP messages used during the NTLP path
discovery/refresh phase. The usage of the NTLP by protocol messages
is described in detail in Section 4.
The protocol uses six messages:
o CREATE: a request message used for creating, changing, refreshing
and deleting NATFW NSLP sessions.
o RESERVE-EXTERNAL-ADDRESS (REA): a request message used for
reserving an external address and probably port number, depending
on the type of NAT.
o Query and Diagnosis ReQuest (QDRQ): a request message used by
authorized NATFW NEs for querying and diagnosing installed NATFW
states
o NOTIFY: an asynchronous message used by NATFW NEs to alert
upstream and/or downstream NATFW NEs about specific events
(especially failures).
o UCREATE: a request message used by data receivers to instruct
upstream firewalls to block data traffic.
o RESPONSE: used as a response to CREATE, REA, UCREATE and QDRQ
messages with Success or Error information
3.3.1 Creating Sessions
Allowing two hosts to exchange data even in the presence of
middleboxes is realized in the NATFW NSLP by the CREATE request
message. The data sender generates a CREATE message as defined in
Section 4.4.1 and hands it to the NTLP. The NTLP forwards the whole
message on the basis of the message routing information 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
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accept the request or reject it. The NR generates a response to the
request 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), a NF (NAT), and NR (DR).
NI Private Network NF Public Internet NR
| | |
| CREATE | |
|----------------------------->| |
| | |
| RESPONSE[Error](if necessary)| |
|<-----------------------------| CREATE |
| |--------------------------->|
| | |
| | RESPONSE[Success/Error] |
| RESPONSE[Success/Error] |<---------------------------|
|<-----------------------------| |
| | |
| | |
Figure 13: Creation message flow
Since the CREATE message is used for several purposes within the
lifetime of a session, there are several processing rules for NATFW
NEs when generating and receiving CREATE messages. The different
processing methods depend not only on the function which the CREATE
is performing (to create, modify, refresh or delete a session) but
also on the node at which the processing happens. For an initial
CREATE message, the CREATE message creating a new NSIS session, the
processing of CREATE messages is different for every NSIS node type:
o NSLP initiator: NI only generates initial CREATE messages and
hands them over to the NTLP. 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 NATFW NSLP forwarder: NFs receiving an initial CREATE message
MUST first perform the checks defined in Section 3.8 and
Section 3.9, if applicable, before any further processing is
executed. The NF SHOULD check with its local policies if it can
accept the desired policy rule given the combination of the NTLP's
'Message-Routing-Information' (MRI) [1] (the flow description
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information) and the CREATE payload (behavior to be enforced on
the packet stream). An initial CREATE is distinguished from
subsequent CREATE messages by the absence of existing NSLP session
state related to the same session ID or the same MRI. The NSLP
message processing 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 REA message Section 3.3.2, that matches the destination
address/port of the MRI provided by the NTLP. If no
reservation had been made in advance the NSLP MAY return an
error response message of type 'no reservation found' and
discard the request. If there is a reservation, NSLP stores
the data sender's address as part of the policy rule to be
loaded and forwards the message with the address set to the
internal (private in most cases) address of the next NSIS node.
When the initial CREATE message, for a new session, is received
at the private side the NAT binding is reserved, but not
activated. The NSLP message is forwarded to the next NSIS hop
with source address set to the NAT's external address from the
newly reserved 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 to the
next NSLP hop. There is a difference between requests from
trusted (authorized NIs) and un-trusted (un-authorized NIs);
requests from trusted NIs will be pre-authorized, whereas
requests from un-trusted NIs will not be pre-authorized. This
difference is required to speed-up the protocol operations as
well as for proxy mode usage (please refer to Section 3.3.7 and
[14]).
* 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 object has success information)
RESPONSE message if they accept the CREATE request message and the
defined in Section 3.8 and Section 3.9, if applicable, have been
successful executed. Otherwise they SHOULD generate a RESPONSE
message with an error code. RESPONSE messages are sent back NSLP
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hop-by-hop towards the NI, independently of the response codes,
either success or error.
Policy rules at middleboxes MUST be only installed upon receiving a
successful response. This is a countermeasure to several problems,
for example wastage of resources due to loading policy rules at
intermediate NF when the CREATE message does not reach the final NR
for some reason.
3.3.2 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 as described in Section 3.3.1.
For scenarios where the data receiver is located behind a NAT and
needs to receive data flows from outside its own network (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.
+-------------+ AS-Data Receiver Communication
+-------->| Application |<-----------------------------+
| | Server | |
| +-------------+ |
| IP(R-NAT_B) |
| NSIS Signaling Message +-------+--+
| +------------------------------------------>| NAT/NAPT |
| | | B |
| | +-------+--+
| | |
AS-Data| | |
Receiver| | +----------+ |
Comm.| | | NAT/NAPT | |
| | | A | |
| | +----------+ |
| | |
| | |
| | |
| | |
v | IP(R) v
+--------+ +---------+
| Data | | Data |
| Sender | | Receiver|
+--------+ +---------+
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Figure 14: The Data Receiver behind NAT problem
Figure 14 describes a typical message communication in a peer-to-peer
networking environment whereby the two end points learn of each
others existence with the help of a third party (referred to as an
Application Server). Communication between the application server
and each of the two end points (data sender and data receiver)
enables the two end hosts to learn each other's IP addresses. The
approach described in this memo supports this peer-to-peer approach,
but is not limited to it.
Some sort of communication between the data sender/data receiver and
a third party is typically necessary (independently of whether NSIS
is used). NSIS signaling messages cannot be used to communicate the
relevant application level end point identifiers (in the generic case
at least) as a replacement for communication with the application
server.
If the data receiver is behind a NAT then an NSIS signaling message
will be addressed to the IP address allocated at the NAT (assuming
one had already been allocated). If no corresponding NSIS NAT
Forwarding State at NAT/NAPT B exists (binding IP(R-NAT B) <-> IP(R))
then the signaling message will terminate at the NAT device (most
likely without generating a proper response message). The signaling
message transmitted by the data sender cannot install the NAT binding
or NSIS NAT Forwarding State "on-the-fly" since this would assume
that the data sender knows the topology at the data receiver side
(i.e., the number and the arrangement of the NAT and the private IP
address(es) of the data receiver). The primary goal of path-coupled
middlebox communication was not to avoid end hosts learning and
preserving this type of topology knowledge. Data receivers behind a
NAT must first reserve an external IP address (probably port number
too).
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Public Internet Private Address
Space
Edge
NI(DS) NAT NAT NR(DR)
NR+ NI+
| | | |
| | | |
| | | |
| | REA | REA |
| |<----------------------|<----------------------|
| | | |
| |RESPONSE[Success/Error]|RESPONSE[Success/Error]|
| |---------------------->|---------------------->|
| | | |
| | | |
============================================================>
Data Traffic Direction
Figure 15: Reservation message flow
Figure 15 shows the message flow for reserving an external address/
port at a NAT. 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 RESERVE-EXTERNAL-ADDRESS (REA)
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 The actual target of the REA message, the Opportunistic Address
(OA) is an arbitrary address, that would force the message to get
intercepted by the far outmost NAT in the network. The
Opportunistic Address is shown as NR+.
The NI+ (could be on the data receiver DR or on any other host within
the private network) sends a the REA message targeted to the
Opportunistic Address (OA defined earlier). The OA selection for
this message is discussed in Section 3.7. The message routing for
the REA message is in the reverse direction to the normal message
routing used for path-coupled signaling where the signaling is sent
downstream (as opposed to upstream in this case). When establishing
NAT bindings (and NSIS session state) the direction does not matter
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since the data path is modified through route pinning due to the
external NAT address. Subsequent NSIS messages (and also data
traffic) will travel through the same NAT boxes.
NI+ may include a data sender's address information object (DSInfo)
if they are aware about the data sender. The DSInfo object is used
by the edge-NAT to limit the possible NI addresses to one address. A
NI+ can specify a specific IP address and port from where the
subsequent NSIS signaling must be originated.
The REA signaling message creates NSIS NAT session state at any
intermediate NSIS NAT peer(s) encountered. Furthermore it has to be
ensured that the edge-NAT device is discovered as part of this
process. The end host cannot be assumed to know this device -
instead the NAT box itself is assumed to know that it is located at
the outer perimeter of the private network addressing realm.
Forwarding of the REA message beyond this entity is not necessary,
and should be prohibited as it provides information on the
capabilities of internal hosts.
The edge-NAT device responds to the REA message with a RESPONSE
message containing a success object carrying the public reachable IP
address/port number.
Processing of REA messages is specific to the NSIS node type:
o NSLP initiator: NI+ only generate REA messages and should never
receive them. When the data sender's address information is known
in advance the NI+ MAY include a DSInfo object in the REA message.
When the data sender's IP address is not known, NI+s MUST NOT
include a DSInfo object.
o NSLP forwarder: NSLP forwarders receiving REA messages MUST first
perform the checks defined in Section 3.8 and Section 3.9, if
applicable, before any further processing is executed. The NF
SHOULD check with its local policies if it can accept the desired
policy rule given by NTLP's message routing information (MRI).
Further processing 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) address. If received at the external address a NF
MAY generate a RESPONSE message with an error of type 'REA
received from outside'. If received at the internal address,
an IP address/port is reserved. In the case it is an edge-NAT,
the NSLP message is not forwarded any further and a RESPONSE
message with the external address and port information is
generated. If it is not an edge-NAT, the NSLP message is
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forwarded further with the translated IP address/port. The
edge-NAT MAY reject REA messages not carrying a DSInfo object
or if the address information within this object is invalid or
too much wildcarded.
* Firewall: Firewalls MUST not change their configuration upon a
REA message. They simply MUST forward the message and MUST
keep NTLP state. Firewalls that are configured as edge-
Firewalls MAY return an error of type 'no NAT here'.
* Combined NAT and Firewall: Processing at combined Firewall and
NAT middleboxes is the same as in the NAT case.
o NSLP receiver: This type of message should never be received by
any NR+ and it SHOULD be discarded silently.
Processing of a RESPONSE message with an external address object is
different for every NSIS node type:
o NSLP initiator: Upon receiving a RESPONSE message with an
external address object, the NI+ can use the IP address and port
pairs carried for further application signaling.
o NSLP forwarder: NFs simply forward this message as long as they
keep state for the requested reservation.
o NSIS responder: This type of message should never be received by
an NR and it SHOULD be discarded silently.
o Edge-NATs: This type of message should never be received by any
Edge-NAT and it SHOULD be discarded silently.
Reservations made with REA MUST be enabled by a subsequent CREATE
message. A reservation made with REA is kept alive as long as the
NI+ refreshes the particular 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.
Without using CREATE (Section 3.3.1 or REA in proxy mode
Section 3.3.7 no data traffic will be forwarded to DR beyond the
edge-NAT. REA is just taking care about enabling the forwarding of
subsequent CREATE messages traveling towards the NR. Correlation of
incoming CREATE messages to REA reservation states is described in
Section 3.6
3.3.3 NATFW Session refresh
NATFW NSLP sessions are maintained on a soft-state basis. After a
specified timeout, sessions and corresponding policy rules are
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removed automatically by the middlebox, if they are not refreshed.
Soft-state is created by CREATE, REA, and UCREATE and the maintenance
of this state must be done by these messages. State created by
CREATE must be maintained by CREATE, state created by REA must be
maintained by REA, and state created by UCREATE must be maintained by
UCREATE. Refresh messages, either CREATE/REA/UCREATE, are messages
carrying the exact MRI and session ID as the initial message and a
lifetime object with a lifetime greater than zero. Every refresh
request message MUST be acknowledged by an appropriate response
message generated by the NR. This response message is routed back
towards the NI, to allow the intermediate NFs to propose a refresh
period that would align with their local policies. The NI sends
refresh messages destined for the NR. Upon reception by each NSIS
forwarder, the state for the given session ID is extended by the
session refresh period, a period of time calculated based on a
proposed refresh message period. The lifetime extension of a session
is calculated as current local time plus proposed lifetime value
(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] | |
|----------------------------->| |
| | |
| RESPONSE[Error] (if needed) | |
|<-----------------------------| CREATE[lifetime > 0] |
| |--------------------------->|
| | |
| | RESPONSE[Success/Error] |
| RESPONSE[Success/Error] |<---------------------------|
|<-----------------------------| |
| | |
| | |
Figure 16: State Refresh Message Flow, CREATE as example
Processing of session refresh CREATE/REA/UCREATE messages is
different for every NSIS node type:
o NSLP initiator: The NI can generate session refresh CREATE/REA/
UCREATE messages before the session times out. The rate at which
the refresh CREATE/REA/UCREATE messages are sent and their
relation to the session state lifetime are further discussed in
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Section 3.4. The message routing information and the extended
flow information object MUST be set equal to the values of the
initial request message.
o NSLP forwarder: NSLP forwarders receiving session refresh messages
MUST first perform the checks defined in Section 3.8 and
Section 3.9, if applicable, before any further processing is
executed. The NF SHOULD check with its local policies if it can
accept the desired lifetime extension for the session referred by
the session ID. Processing of this message is independent of the
middlebox type.
o NSLP responder: NRs accepting a session refresh CREATE/REA/UCREATE
message generate a RESPONSE message with response object set to
success. NRs MUST perform the checks defined in Section 3.8 and
Section 3.9, if applicable.
3.3.4 Deleting Sessions
NATFW NSLP sessions may be deleted at any time. NSLP initiators can
trigger this deletion by using a CREATE, REA, or UCREATE messages
with a lifetime value set to 0, as shown in Figure 17.
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 MUST first perform the checks
defined in Section 3.8 and Section 3.9, if applicable, and afterwards
MUST delete the session immediately. Policy rules associated with
this particular session MUST be deleted immediately. This message is
forwarded until it reaches the final NR. The CREATE/REA/UCREATE
request 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.
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3.3.5 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 session termination, changes in
local policies, route change or any other reason that indicates
change of the NATFW NSLP session state. Currently, asynchronous
session termination, re-authorization required and route change
detected (see Section 3.10) are the only events that are defined, but
other events may be defined in later versions of this memo. One or
several events could be reported within the NOTIFY message.
NFs and NRs may generate NOTIFY messages upon asynchronous events,
with a response object indicating the reason of the event and a
corresponding session ID. NOTIFY messages are sent hop-by-hop
upstream towards NI until they reach NI.
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 receiving NOTIFY messages MUST first perform
the checks defined in Section 3.8 and Section 3.9, if applicable.
After successfully doing so, NIs analyze the notified event(s) and
behave appropriately based on the event type. Section 4.3.5
discusses the required behavior for each notified event. NIs MUST
NOT generate NOTIFY messages.
o NSLP forwarder: NFs receiving NOTIFY messages MUST first perform
the checks defined in Section 3.8 and Section 3.9, if applicable,
and MUST only accept NOTIFY messages from downstream peers. After
successfully doing so, NFs analyze the notified event(s) and
behave based on the notified events defined in Section 4.3.5. NFs
occurring an asynchronous event generate NOTIFY messages and set
the response object(s) code based on the reported event(s).
NOTIFY messages are sent further hop-by-hop upstream towards the
NI. NFs SHOULD generate NOTIFY messages upon asynchronous events
and forward them upstream towards the NI.
o NSLP responder: NRs SHOULD generate NOTIFY messages upon
asynchronous events. NRs receiving NOTIFY messages MUST ignore
this message and discard it. NOTIFY messages are sent hop-by-hop
upstream towards NI
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3.3.6 Query and diagnosis capabilities within the NATFW NSLP protocol
The NATFW NSLP provides query and diagnosis capabilities that could
be used by a session(s) owner to monitor the state of those sessions.
This would be used for:
o Diagnostic purposes when no data packets were received (or the
packet stream is subject to significant packet loss) and NATFW
NSLP signaling was supposed to have created appropriate policy
rules on the NATFW NFs along the data path.
o Discover the number of NATFW NSLP Hops between the NI and the NR
(or the last NATFW NE responding to the QDRQ)
o Collecting session states owned by a specific NI, this is required
in case the NI loses its sessions' information (mainly due to node
system issues).
The QDRQ message can be used to query and diagnose the following
session information: session id, the number of NE hops (between the
NI and the last NE responding to the QDRQ) and the following
session's status ordered from best to worst: up, high traffic (used
to detect DOS attack or unexpected traffic rate), pending, down. The
status of the policy rule will probably provide sufficient diagnostic
information in most cases;if more diagnostic information is required
it could be provided by the NATFW NF logs. QDRQ messages may include
an optional maximum hop count number value provided by the NI, when
the hop count value reaches the maximum hop count the receiving NF
should stop propagating the QDRQ and generate a response message to
be sent back upstream to the NI. A QDRQ message usage is shown in
Figure 18 where downstream NF increments the hop counter (except when
they are the responding NF) and when the session's state is not "UP"
(or "UP" but a QDRQ of type LIST is sent) they insert a session
status value and their IP address. The Session information could be
retrieved by sending a QDRQ against a specific session id or a QDRQ
type equal to LIST (this is only applicable when the NI's identity is
available and identical to the one used during the session's
establishment process). In the message sequences shown in Figure 18,
the QDRQ message (which a QDRQ type value of SINGLE) is sent for a
single session ID (provided through the NTLP API), the traversed NAT
didn't have any issues to report for the session however the
Firewall's (FW) traffic meters reported that the flow has exceeded
the maximum number of packets provisioned against the flow, hence in
addition to the session status the firewall provides its address. As
the firewall is the last hop (it is configured to proxy and respond
to QDRQ messages) it does not increment the hop counter and responds
hop by hop back to the NI.
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NI Private address NAT FW
| | |
|QDRQ(SINGLE,HOPCNT=0)| |
|-------------------->| |
| |QDRQ(SINGLE,HOPCNT=1) |
| |---------------------> |
| | |
| |RESPONSE(SINGLE, |
| |HOPCNT=1, |
| | [HIGH_PPS,FW@] |
| |<--------------------- |
|RESPONSE(SINGLE, | |
|HOPCNT=1,[HIGH_PPS, | |
| FW@]) | |
|<--------------------| |
Figure 18: Query and Diagnosis operation
QDRQ message processing is dependent on the NATFW NSLP node type:
o NSLP initiator: NIs only generate QDRQ messages, while inserting:
* a HOPCNT object with a zero value
* a QDRQ type to indicate if the QDRQ is for a single session
(QDRQ type would be SINGLE) or to gather information on all the
sessions initiated by the NI (QDRQ type would be LIST)
* When required (i.e. this is optional) a maximum hop count value
* A SID (embedded in the Bound SIP object) when the QDRQ is not
related to the session handled within the Message Routing State
used to route the QDRQ.
An NI MUST discard received QDRQ messages.
o NSLP forwarder: NFs receiving QDRQ messages MUST first
authenticate and authorize the message source. After successfully
doing so, NFs will behave differently depending if the QDRQ is
specific to one session and whether the NF co-hosts a NAT engine
or not.
* If the QDRQ is about a single session:
1. the NF checks first if the QDRQ includes a maximum hop
count, if the current hop counter is smaller (else the
procedures continues as defined in 2) and the NF was
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previously able to forward NSLP messages downstream for the
same session (else the procedure continues as defined in
2), the NF increments the hop counter. Furthermore if the
NF's session status is not "UP", the NF will insert a
session status object, which includes the session's status
and the node's IP address, as defined in Section 4.3.13.
In case the NF was co-hosting a NAT engine, the NF needs to
ensure the validity of the session status object's embedded
IP address and modify the address based on the local NAT
bind entry. After completing these operations the NF
forwards the message downstream.
2. In addition to the conditions discussed above, this
procedure is applied when the QDRQ message is scoped by the
receiving NF. The NF responds, with a RESPONSE message,
hop by hop back to the NI while copying the hop counter,
the bound session ID if it was present and a series of
session status objects.
* If the QDRQ is of type LIST then the following procedures are
applied:
1. the NF checks first if the QDRQ includes a maximum hop
counter, if the current hop counter is smaller (else the
procedures continues as defined in 2), the NF creates one
or several QDRQ response objects which include a bound
session ID (session ID created by the NI before it lost all
its session states), a flow descriptor and the session
status object (session state and NF's IP address). This is
only performed if the NF is able to get the NI's proof of
ownership on stored sessions within the node. In case the
NF was co-hosting a NAT engine, the NF needs to ensure the
validity of all embedded IP addresses includes in QDRQ
objects and modify the addresses based on the local NAT
bind entry. After completing these operations the NF
increments the hop counter and forwards the message
downstream, if there were no downstream nodes then the hop
counter is decremented and the procedure continues as
described below in step 2.
2. In addition to the conditions discussed above, this
procedure is applied when the QDRQ message is scoped by the
receiving NF. The NF responds, with a RESPONSE message,
hop by hop back to the NI while copying the hop counter and
the series of QDRQ response objects which would include in
addition to the session status objects, bound session ID as
discussed in Section 4.3.15.
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o NSLP responder: NRs (any node being the destination of the
message) receiving QDRQ messages MUST first perform the checks
defined in Section 3.8 and Section 3.9, if applicable. After
successfully doing so, NRs must process the message as the NFs
when responding with a RESPONSE message to the NI. The RESPONSE
message would include a copy of all the received objects within
the QDRQ message. The RESPONSE message will travel along the
established reverse path given by the message routing state.
Responses to QDRQ messages are processed differently depending on
theNATFW NSLP node type:
o NSLP initiator: NIs receiving RESPONSEs to QDRQ messages MUST
first perform the checks defined in Section 3.8 and Section 3.9,
if applicable. After successfully doing so, the objects within
the RESPONSE messages are provided up to the application layers
and the session state remains as it was unless the application
triggers NATFW NSLP state changes.
o NSLP forwarder: NFs receiving RESPONSEs to QDRQ messages MUST
first perform the checks defined in Section 3.8 and Section 3.9,
if applicable. After successfully doing so, NFs forward the
message upstream without any interpretation.
o NSLP responder: if an NR receives a RESPONSE to QDRQ message it
MUST discard it.
QDRQ messages are mainly sent for debugging and outage recovery and
hence should be sent within a trusted network infrastructure, this
could either be achieved by implicitly scoping QDRQ messages at the
edge of the trusted network infrastructure or using the maximum hop
count counter.
3.3.7 Proxy Mode for Data Receiver behind NAT
Some migration scenarios need specialized support to cope with cases
where only the receiving side is running NSIS. End-to-end signaling
is going to fail without NSIS support at both data sender and data
receiver, unless the NATFW NSLP also gives the NR the ability to
install state on the upstream path towards the data sender for
downstream data packets. The goal of the described method is to
trigger the network to generate a CREATE message at the edge-NAT on
behalf of the data receiver. In this case, a NR can signal towards
the Opportunistic Address as is performed in the standard REA message
handling scenario for NATs as in Section 3.3.2. The message is
forwarded until the edge-NAT is reached. A public IP address and
port number is reserved at an edge-NAT. As shown in Figure 19,
unlike the standard REA message handling case, the edge-NAT is
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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 and
DR. This behavior requires an indication to the edge-NAT within the
REA message if either the standard behavior (as defined in
Section 3.3.2) is required or a CREATE message is required to be sent
by the edge-NAT. In addition when a CREATE message needs to be sent
by the edge-NAT, the REA message may include the data sender's
address (DSInfo), if available to the data receiver. Figure 19 shows
this proxy mode REA as REA-PROXY.
DS Public Internet NAT Private address NR
No NI NF space NI+
NR+
| | REA-PROXY[(DSInfo)] |
| |<------------------------- |
| | RESPONSE[Error/Success] |
| | ---------------------- > |
| | CREATE |
| | ------------------------> |
| | RESPONSE[Error/Success] |
| | <---------------------- |
| | |
| | |
Figure 19: REA Triggering Sending of CREATE Message on Separate
Reverse Path
The processing of REA-PROXY messages is different for every NSIS
entity:
o NSLP initiator (NI+): When the data sender's address information
is known in advance the NI+ MAY include a DSInfo object in the
REA-PROXY request message. When the data sender's address is not
known, NI+'s MUST NOT include a DSInfo object. The NI+ MUST
choose a random value and include it in the NONCE object. NI+
only generate REA-PROXY messages and should never receive them.
o NSLP forwarder: NSLP forwarders receiving REA-PROXY messages MUST
first perform the checks defined in Section 3.8 and Section 3.9,
if applicable, before any further processing is executed. The NF
SHOULD check with its local policies if it can accept the desired
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policy rule given by NTLP's message routing information (MRI).
Further processing 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) address. If received at the external address a NF
MAY generate a RESPONSE message with an error of type 'REA
received from outside' and stop forwarding. If received at the
internal address, an IP address/port is reserved. If it is not
an edge-NAT, the NSLP message is forwarded further with the
translated IP address/port. In the case it is an edge-NAT, the
NSLP message is not forwarded any further. The edge-NAT checks
whether it is willing to send CREATE messages on behalf on NI+
and if so it checks the DSInfo object. The edge-NAT MAY reject
the REA-PROXY request if there is no DSInfo object or if the
address information within DSInfo is not valid or too much
wildcarded. If accepted a RESPONSE message with the external
address and port information is generated. When the edge-NAT
accepts it generates a CREATE message as defined in
Section 3.3.1 and includes a NONCE object having the same value
as of the received NONCE object. The edge-NAT MUST not
generate a CREATE-PROXY message. The edge-NAT MUST refresh the
CREATE message session only if a REA-PROXY refresh message has
been received first.
* Firewall: Firewalls MUST not change their configuration upon a
REA message. They simply MUST forward the message and MUST
keep NTLP state. Edge-Firewalls SHOULD reply with an error
RESPONSE indicating 'no egde-NAT here'.
* Combined NAT and Firewall: Processing at combined Firewall and
NAT middleboxes is the same as in the NAT case.
o NSLP receiver: This type of message should never be received by
any NR+ and it SHOULD be discarded silently.
Processing of a RESPONSE message with an external address object is
different for every NSIS node type:
o NSLP initiator: Upon receiving a RESPONSE message with an
external address object, the NI+ can use the IP address and port
pairs carried for further application signaling.
o NSLP forwarder: NFs simply forward this message as long as they
keep state for the requested reservation.
o NSIS responder: This type of message should never be received by
an NR and it SHOULD be discarded silently.
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o Edge-NATs/edge-Firewall: This type of message should never be
received by any Edge-NAT/edge-Firewall and it SHOULD be discarded
silently.
The scenario described in this chapter challenges the data receiver
in a way that it must make a correct assumption about the data
sender's ability to use NSIS NATFW NSLP signaling. There are two
cases a) DS is NSIS unaware and DR assumes DS to be NSIS aware and b)
DS is NSIS aware but DR assumes 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. The
edge-NAT will send CREATE messages and DS will send CREATE messages
too. Both CREATE messages are handled as separated sessions and
therefore the common rules per session apply. It is up to the NR's
responsibility to decide whether to teardown the REA-PROXY sessions
in the case of the data sender's side being NSIS aware. 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 NONCE object is used to build the relationship between received
CREATEs and the message initiator. An NI+ uses the presence of the
NATFW_NONCE object to correlate it to the particular REA-PROXY
request. The absence of an NATFW_NONCE object indicates a CREATE
initiated by the DS and not by the edge-NAT.
There is a possible race condition between the RESPONSE message to
the REA-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 REA-PROXY request was not received.
3.3.8 Proxy Mode for Data Sender behind Middleboxes
As with data receivers behind middleboxes in Section 3.3.7 also data
senders behind middleboxes require proxy mode support as well. The
problem here is that there is no NSIS support at the data receiver's
side and, by default, there will be no response to CREATE request
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
through GIMPS timeouts but not limited to). This proxy mode handles
data senders behind a middlebox only; for receivers behind a NAT see
Section 3.3.7.
NIs being aware about a NSIS unaware DR, send a CREATE message
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towards DR with a proxy support object. Intermediate NFs can use
this additional information to decide whether to terminate the
message forwarding or not. This proxy support object is an implicit
scoping of the CREATE message. Termination of CREATE-PROXY request
messages with proxy support object included MUST only be done by
egde-NATs/edge-Firewalls; future revisions of this document may
change this behavior.
DS Private Address FW Public Internet NR
NI Space NF no NR
| | |
| CREATE-PROXY | |
|------------------------------>| |
| | |
| RESPONSE[SUCCESS/ERROR] | |
|<------------------------------| |
| | |
Figure 20: Proxy Mode Create Message Flow
The processing of CREATE-PROXY messages and RESPONSE messages is
similar to Section 3.3.1, 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.
3.3.9 Proxy Mode for Data Receiver behind Firewall
Data receivers behind firewalls would like to use a similar sort of
proxy mode operation to those behind NATs. While finding the
upstream edge-NAT is quite easy, it is only required to find an edge-
NAT but not a very specific one and then the data traffic is route
pinned to the NAT, the location of the appropriate edge-Firewall is
more difficult. Data receivers that are located behind several
firewalls that are placed topology-wise in parallel (multi-homed
network), must find out the one firewall the data traffic will
traverse. This feature of locating the right firewall can be used
for proxy mode support and for blocking certain incoming data
traffic. Proxy mode support is similar to Section 3.3.7 where the DR
is behind one or more NATs and installs "allow" policy rules.
Blocking incoming data traffic requires that the NATFW NSLP locates
the appropriate firewall in order to install a deny policy rule.
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The upstream CREATE (UCREATE) message is used to locate upstream
firewalls and to request installation of deny policy rules. The goal
of the method described is to trigger the network to generate a
CREATE message at the edge-Firewall on behalf of the data receiver.
In this case, a NR can signal towards the data sender's address as in
the standard REA message handling scenario for NATs Section 3.3.2.
The message is forwarded until it reaches the edge-Firewall. As
shown in Figure 21, the edge-Firewall is triggered to send a CREATE
message on a new reverse path which could go through internal
firewalls or NATs. The new reverse path for CREATE is necessary to
handle routing asymmetries between the edge-Firewall and DR. UCREATE
does not install any policy rule but the subsequent CREATE message
initiated by the edge-Firewall does.
DS Public Internet FW Private address NR
No NI NF space NI+
NR+
| | UCREATE |
| |<------------------------- |
| | RESPONSE[Error/Success] |
| | ---------------------- > |
| | CREATE |
| | ------------------------> |
| | RESPONSE[Error/Success] |
| | <---------------------- |
| | |
| | |
Figure 21: UCREATE Triggering Sending of CREATE Message on Separate
Reverse Path
The processing of UCREATE messages is different for every NSIS
entity:
o NSLP initiator (NI+): NI+ MUST always direct UCREATE message to
the address of DS. NI+ only generates UCREATE messages and should
never receive them.
o NSLP forwarder: NSLP forwarders receiving UCREATE messages MUST
first perform the checks defined in Section 3.8 and Section 3.9,
if applicable, before any further processing is executed. The NF
SHOULD check with its local policies if it can accept the desired
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policy rule given by NTLP's message routing information (MRI).
Further processing 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) address. If received at the internal interface, NATs
allocated a public IP address and port and forward the message
further. Edge-NATs receiving UCREATE SHOULD response with
error RESPONSE indicating 'no edge-Firewall'
* Firewall: Non edge-Firewalls simply forward the message. Edge-
Firewalls stop forwarding the check for performing the checks
defined in Section 3.8 and Section 3.9, if applicable. If the
message is accepted, load the specified policy rule and
generate CREATE messages back towards the DR as defined in
Section 3.3.1.
* Combined NAT and Firewall: Processing at combined Firewall and
NAT middleboxes is the same as in the Firewall case.
o NSLP receiver: This type of message should never be received by
any NR+ and it SHOULD be discarded silently.
Processing of a RESPONSE message with an external address object is
different for every NSIS node type:
o NSLP initiator (NI+): Upon receiving a RESPONSE message NI+
should await incoming corresponding CREATE messages.
o NSLP forwarder: NFs simply forward this message as long as they
keep state for the requested reservation.
o NSIS responder: This type of message should never be received by
an NR and it SHOULD be discarded silently.
o Edge-NATs/edge-Firewall: This type of message should never be
received by any Edge-NAT/edge-Firewall and it SHOULD be discarded
silently.
EDITOR's NOTE: The protocol behavior of UCREATE needs a refinement,
see also issue no. 38.
3.4 Calculation of Session Lifetime
NATFW NSLP sessions, and the corresponding policy rules which may
have been installed, are maintained via soft-state mechanism. Each
session is assigned a lifetime and the session is kept alive as long
as the lifetime is valid. After the expiration of the lifetime,
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sessions and policy rules MUST be removed automatically and resources
bound to them should be freed as well. Session lifetime is kept at
every NATFW NSLP node. The NSLP forwarders and NSLP responder are
not responsible for triggering lifetime extension refresh messages
(see Section 3.3.3): this is the task of the NSIS initiator.
The NSIS initiator MUST choose a session lifetime (expressed in
seconds) value before sending any message (lifetime is set to zero
for deleting sessions) to other NSLP nodes. The 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 session hijacking ([7]). Session
hijacking is made easier when the NI does not explicitly remove
the session.
o The user application's data exchange duration, in terms of
seconds, minutes or hours and networking needs. This duration is
modeled as M x R, with R the message refresh period (in seconds)
and M a multiplier for R.
As opposed to the NTLP Message Routing state [1] lifetime, the NSLP
session lifetime is not required to have a small value since the NSLP
state refresh is not handling routing changes but security related
concerns. [12] provides a good algorithm to calculate the session
lifetime as well as how to avoid refresh message synchronization
within the network. [12] 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, L (with L being the session
lifetime) must satisfy L >= (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 session lifetime and the
refresh message period, the algorithm provided is only given as
an example.
This requested lifetime value is placed in the 'lifetime' object of
the NSLP message and messages are forwarded to the next NATFW NSLP
node.
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NATFW NFs processing the request message along the path MAY change
the requested lifetime to fit their needs and/or local policy. If an
NF changes the lifetime value it must also indicate the corresponding
refresh message period. NFs MUST NOT increase the lifetime value;
they MAY reject the requested lifetime immediately and MUST generate
an error response message of type 'lifetime too big' upon rejection.
The NSLP request message is forwarded until it reaches the NSLP
responder. NSLP responder MAY reject the requested lifetime value
and MUST generate an error response message of type 'lifetime too
big' upon rejection. The NSLP responder MAY also lower the requested
lifetime to an acceptable value (based on its local policies). NSLP
responders generate their appropriate response message for the
received request message, sets the lifetime value to the above
granted lifetime and sends the message back hop-by-hop towards NSLP
initiator.
Each NSLP forwarder processes the response message, reads and stores
the granted lifetime value. The forwarders SHOULD accept the granted
lifetime, as long as the value is within the tolerable lifetime range
defined in their local policies. They MAY reject the lifetime and
generate a 'lifetime not acceptable' error response message.
Figure 22 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.
+-------+ CREATE(lt=60s) +-----------+ CREATE(lt=20s) +--------+
| |---------------->| NSLP |---------------->| |
| NI | | | | NR |
| |<----------------| forwarder |<----------------| |
+-------+ RESPONSE(lt=15s +-----------+ RESPONSE(lt=15s +--------+
MRR=3s) MRR=3s)
lt = lifetime
MRR = Message Refresh Rate
Figure 22: Lifetime Calculation 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 of
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time. Messages can be lost along the path, delayed, 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 already received
messages and requires every NATFW NSLP message to carry a message
sequence number (MSN), see also Section 4.3.8.
The MSN MUST be set by the NI and MUST no be set or modified by any
other node. The initial value for the MSN MUST be generated randomly
and MUST be only unique within the used session. The NI MUST
increment the MSN for every message sent. Once the MSN has reached
the maximum value, the next value it takes is zero.
NSIS forwarders and the responder store the with the initial packet
received MSN as start value. NFs and NRs include the received MSN
value in their response messages.
When receiving a request 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 lower or equal than the stored
MSN value. This 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 3.8 and Section 3.9, and stores the updated MSN value
accordingly.
These semantics applied to a CREATE message exchange mean that the
first CREATE (initial CREATE to setup the path and session) carries
the initial, randomly generated, MSN. All nodes along the path store
this value and the NR includes the received value in its response
(assuming that the CREATE message reaches the NR). Subsequent CREATE
messages, updating the request policy rule or lifetime, carry an
incremented MSN value, so that intermediate nodes can recognize the
requested update.
3.6 De-Multiplexing at NATs
Section 3.3.2 describes how NSIS nodes behind NATs can obtain a
public reachable IP address and port number at a NAT and how it can
be activated by using CREATE messages (see Section 3.3.1)". 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
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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 requests are related to
reserved addresses, meaning how to de-multiplex incoming NSIS
requests.
The de-multiplexing method uses information stored at NATs (such as
mapping of public IP address to private, transport protocol, port
numbers), information given by NTLP's message routing information and
further authentication credentials.
3.7 Selecting Opportunistic Addresses for REA
As with all other message types, REA messages need a reachable final
destination IP address. But as many applications do not provide a
destination IP address in the first place, there is a need to choose
a destination address for REA messages. This destination address can
be the final target, but for applications which do not provide an
upfront address, the destination address has to be chosen
independently. Choosing the 'correct' destination IP address may be
difficult and it is possible there is no 'right answer'. [16] shows
choices for SIP and this section provides some hints about choosing a
good destination IP address.
1. Public IP address of the data sender:
* Assumption:
+ The data receiver already learned the IP address of the
data sender (e.g., via a third party).
* Problems:
+ The data sender might also be behind a NAT. In this case
the public IP address of the data receiver is the IP
address allocated at this NAT.
+ Due to routing asymmetry it might be possible that the
routes taken by a) the data sender and the application
server b) the data sender and NAT B might be different,
this could happen in a network deployment such as in
Figure 14. As a consequence it might be necessary to
advertise a new (and different) external IP address within
the application (which may or may not allow that) after
using NSIS to establish a NAT binding.
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2. Public IP address of the data receiver:
* Assumption:
+ The data receiver already learned his externally visible IP
address (e.g., based on the third party communication).
* Problems:
+ Communication with a third party is required.
3. IP address of the Application Server:
* Assumption:
+ An application server (or a different third party) is
available.
* Problems:
+ If the NSIS signaling message is not terminated at the NAT
of the local network then an NSIS unaware application
server might discard the message.
+ Routing might not be optimal since the route between a) the
data receiver and the application server b) the data
receiver and the data sender might be different.
3.8 Session Ownership
Prove of session ownership is a fundamental part of the NATFW NSLP
signaling protocol. It is used to validate the origin of a request,
i.e. invariance of the message sender. Within the NATFW NSLP, the
NSIS initiator (the NI and the NI+) is the ultimate session owner
for all request messages. The request messages are CREATE, REA,
QDRQ, and UCREATE. A prove of ownership MUST be provided for any
request message sent downstream. All intermediate NATFW NSLP nodes
MUST use this prove of ownership to validate the message's origin.
All NATFW nodes along the path must be able to verify that the sender
of a request is the same entity that initially created the session.
As such, the path spans different administrative domains and cannot
rely on different authentication protocols. This requirement demands
a cryptographic scheme independent of the local authentication scheme
in use and administrative requirements being enforced. Relying on a
public key infrastructure (PKI) for the purpose of prove of session
ownership is not reasonable due to deployment problems of a global
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PKI.
As a solution, the NATFW NSLP uses purpose-built keys (PBK [2]) to
provide session ownership. A Purpose-built key is an ephemeral
public/private key pair aiming to ensure the sameness principle,
i.e., once an initial exchange of keying material has taken place
successive messages in the communication come from the same source.
Note that the usage of Purpose-built keys does not replace the usage
of hop-by-by security between two neighboring NATFW NSLP nodes. The
usage of PBKs only aims to provider sender invariance and cannot
provide user authentication and the ability for a NATFW node to
authorize the request based on the authenticated identity. A number
of security requirements discussed in this document require the usage
of hop-by-hop security independently of the sender invariance
property. The NATFW NSLP uses purpose-built keys (PBK, [2]) to prove
the session ownership. A Purpose-built key is a public/private key
pair generated per session. For every new session, the NI generates
a new public/private key pair and uses a collision-resistant hash
function to compute the hash of the public key. The hash value is
used as session ID and may be truncated to fit the session ID
object's size. The public key is used to sign certain parts of the
signaling message, including the message sequence number (MSN)
[EDITOR's note: objects to be included in the signature need to be
listed]. The combination of signature and MSN mitigates replay
attacks (see also Section 3.5). NATFW NSLP nodes receiving a request
message can use the public key (if distributed along the path, see
later) to verify the session ID and the signature.
The public key must be distributed amongst participating NATFW NSLP
nodes down the path. The absence of a deployed key distribution
system forces distribution of the public key in-band with the NATFW
NSLP signaling. The public key will be carried in a NATFW_PK object
and needs to be included in the signaling message exchange at an
early point in each session signaling message sequence. Selecting
the point in the sequence for distributing the public key depends on
a design choice to be made here. There are two design choices:
1. The first message exchange from NI towards NR (either CREATE,
REA, or UCREATE) creating a new session does not carry a
signature nor the public key. The second message exchange from
NI towards NR carries the signature and the public key. The
public key is distributed with the second message exchange to all
participating nodes. All further message exchanges must carry
the signature but do not need to repeat the public key.
Figure 23 shows an example with four NATFW NSLP nodes using the
here proposed scheme.
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+----+ +-----+ +-----+ +----+
| NI |-------| NF1 |-------| NF2 |---------| NR |
+----+ +-----+ +-----+ +----+
| | | |
| CREATE(sid) | | |
t0 |------------->|------------>|------------->|
| | | OK(sid) |
t1 |<-------------|<------------|--------------|
| | | |
| CREATE(sid,sig,pubkey) | |
t2 |------------->|------------>|------------->|
| | | OK(sid) |
t3 |<-------------|<------------|<-------------|
| | | |
| CREATE(sid) | | |
t4 |------------->|------------>|------------->|
| | | OK(sid) |
t5 |<-------------|<------------|--------------|
| | | |
Figure 23: Key Distribution Scheme 1
This scheme first establishes the complete signaling path
ultimately using the NTLP C-MODE with TLS support to secure the
links hop-by-hop (Points t0 to t1). At point t2, the
distribution of the public key along the path is started.
2. The signature and the public key are included in the first
message exchange from NI towards NR (either CREATE, REA, or
UCREATE) and distributed amongst all nodes. All further message
exchanges must carry the signature only but do not need to repeat
the public key. Figure 24 shows an example with four NATFW NSLP
nodes using the here proposed scheme.
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+----+ +----+ +----+ +----+
| NI |--------| NF |--------| NF |---------| NR |
+----+ +----+ +----+ +----+
| | | |
| CREATE(sid,sig,pubkey) | |
t0 |------------->|------------>|------------->|
| | | OK(sid) |
t1 |<-------------|<------------|--------------|
| | | |
| CREATE(sid) | | |
t2 |------------->|------------>|------------->|
| | | OK(sid) |
t3 |<-------------|<------------|--------------|
| | | |
| CREATE(sid) | | |
t4 |------------->|------------>|------------->|
| | | OK(sid) |
t5 |<-------------|<------------|--------------|
| | | |
Figure 24: Key Distribution Scheme 2
In this scheme, the distribution of the public key is started
with the initial CREATE (point t0) and completed after the first
completed message exchanged (point t1). The first CREATE (at
point t0) is probably exchanged via the unsecured D-MODE.
EDITOR's note: To be done: It is needed to define the hashing
functions as well as the to be used public/private key method.
3.9 Authentication and Authorization
NATFW NSLP nodes receiving signaling messages MUST first check
whether this message is authenticated and authorized to perform the
requested action.
The NATFW NSLP is expected to run in various environments, such as IP
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 a stronger authenticated/authorized
signaling. This enterprise scenario may require the use of an
infrastructure and administratively assigned identities to operate
the NATFW NSLP.
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EDITOR's note: It is still not clear what are the requirements for
authentication and authorization in the NATFW case. This is going to
be discussed at the next IETF meeting.
3.10 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
session state on the old path. The detection of route changes is
described in Section 7 of [1] 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 detecting a route change, by means described in the
NTLP specification or others, generates a NOTIFY message indicating
this change and sends this upstream towards NI. Intermediate NFs on
the way to the NI can use this information to decide later if their
session can be deleted locally if they do not receive an update
within a certain time period.
The NI receiving this NOTIFY message SHOULD generate an update
message and sends it downstream as for the initial exchange. 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. Merge points, NFs receiving update CREATEs, can
easily use the session ID and signature information (session
ownership information) to update the session state.
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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 common for all
NSLPs 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
The NSLP header is common to all NSLPs and is the first part of all
NSLP messages. 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 25.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSLP message type | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 25: 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.
Note that other NSLPs use this field as a flag field.
4.2 NSLP message types
The message types identify requests and responses. Defined messages
types are:
o 0x0101 : CREATE
o 0x0102 : RESERVE-EXTERNAL-ADDRESS(REA)
o 0x0104 : UCREATE
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o 0x0108 : QDRQ
o 0x0201 : RESPONSE
o 0x0301 : NOTIFY
4.3 NSLP Objects
NATFW NSLP objects use a common header format defined by Figure 26.
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 26: Common NSLP object header
The length is 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. 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 [1], Section 3.2), i.e., the Object Type has not been
defined. NATFW NSLP uses a subset of the categories defined in
GIMPS:
o AB=00 ("Mandatory"): If the object is not understood, the entire
message containing it must be rejected with an error indication.
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.
The combination AB=11 ("Refresh") MUST NOT be used since the NATFW
NSLP refreshes its state end-to-end and not locally. Fields marked
with 'r' are reserved for future use.
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The following sections do not repeat the common NSLP object header,
they just state the type and the length.
4.3.1 Session Lifetime Object
The session lifetime object carries the requested or granted lifetime
of a NATFW NSLP session measured in seconds. The Message refresh
rate value is set by default to 0xFFFF and only set to a specific
value when an intermediate node changes the message lifetime and
informs the upstream node about the recommended message refresh rate.
Type: NATFW_LT
Length: 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NATFW NSLP session lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NATFW NSLP message refresh rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 27: Lifetime object
4.3.2 PBK Public Key
The PBK public key object carries the public key used for session
ownership as described in Section 3.8. EDITOR's note: The key length
needs to be defined.
Type: NATFW_LT
Length: tbd.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Public Key //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 28: PBK public key object
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4.3.3 External Address Object
The external address object can be included in RESPONSE messages
(Section 4.4.3) only.
Type: NATFW_EXT_IPv4
Length: 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port number | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 29: External Address Object for IPv4 addresses
Type: NATFW_EXT_IPv6
Length: 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port number | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IPv6 address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 30: External Address Object for IPv6 addresses
Please note that the field 'port number' MUST be set to 0 if only an
IP address has been reserved, for instance, by a traditional NAT. A
port number of 0 MUST be ignored in processing this object.
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4.3.4 Extended Flow Information Object
In general, flow information is kept at the NTLP level during
signaling. The message routing information of the NTLP carries all
necessary information. Nevertheless, some additional information may
be required for NSLP operations. The 'extended flow information'
object carries this additional information about action to be taken
on the installed policy rules.
Type: NATFW_EXT_FLOW
Length: 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rule action | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 31: Extended Flow Information
These fields are defined for the policy rule object:
o Rule action: This field indicates the action for the policy rule
to be activated. Allowed values are 'allow' (0x01) and 'deny'
(0x02)
4.3.5 Response Code Object
This object carries the response code, which may be indications for
either a successful request or failed request depending on the value
of the 'response code' field.
Type: NATFW_RESPONSE
Length: 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| response code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 32: Response Code Object
TBD: Define response classes, success codes and error codes.
Possible error classes are:
o Policy rule errors
o Authentication and Authorization errors
o NAT
Currently errors defined in this memo are:
o lifetime too big
o lifetime not acceptable
o no NAT here
o no reservation found
o requested external address from outside
o re-authorization needed
o routing change detected
4.3.6 Proxy Support Object
This object indicates that proxy mode support is required. Either in
a REA message or CREATE message.
Type: NATFW_PROXY
Length: 0
4.3.7 Nonce Object
Type: NATFW_NONCE
Length: 1
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 33: Nonce Object
4.3.8 Message Sequence Number Object
This object carries the MSN value as described in Section 3.5.
Type: NATFW_RESP_MSN
Length: 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| message sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 34: Message Sequence Number Object
4.3.9 Bound Session ID Object
This object carries a session ID and is used for QDRQ messages only.
Type: NATFW_BSID
Length: 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| bound session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 35: Bound Session ID Object
This object is used when a session owner queries multiple session,
every session would be indicated with the bound session ID object.
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4.3.10 Data Sender Information Object
Type: NATFW_DSINFO_IPv4
Length: 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port number | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 36: Data Sender's IPv4 Address Object
Type: NATFW_DSINFO_IPv6
Length: 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port number | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IPv6 address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 37: Data Sender's IPv6 Address Object for IPv6 addresses
4.3.11 NATFW NF Hop Count Object
Type: NATFW_NF_HOPCNT
Length: 1
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NATFW NF HOP COUNT |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 38: NATFW NF Hop Count Object
Editor note next revision will include Hop count, maximum hops and
QDRQ type in the same object to minimize the overhead since 4 bits
would be sufficient for the counters.
4.3.12 Maximum Hops Object
Type: NATFW_NF_MAX_HOPCNT
Length: 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NATFW NF MAX HOP COUNT |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 39: NATFW NF Maximum Hop Count Object
4.3.13 Session Status object
The session status object is inserted within the QDRQ message and
copied in a response message. It embeds the current local node's
session status and the node's IP address
Type: SESSION_STS
Length: 2 or 5
SESSION STATUS:
Length 1, Possible values: UP(0),HIGH_PPS(1), PENDING(2), DOWN(3)
Reserved bits: RRR
Length 3 bits
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IP version: V bit
Length 1 bit: IPv4(0), IPv6(1)
NODE IP ADDRESS:
Length 1 or 4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SESSION STATUS |R|R|R|V|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NODE IP ADDRESS (1 or 4 words) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 40: Session Status Object
4.3.14 QDRQ type
Type: QDRQ_TYPE
Length: 1
Possible values: SINGLE(0), LIST(1)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QDRQ TYPE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 41: QDRQ TYPE object
4.3.15 QDRQ Response object
TBC for next version: includes an optional bound session id, an
optional flow descriptor (used when a LIST QDRQ type is used) and a
mandatory session status
4.4 Message Formats
This section defines the content of each NATFW NSLP message type.
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The message types are defined in Section 4.2. First, the request
messages are defined with their respective objects to be included in
the message. Second, the response messages are defined with their
respective objects to be included.
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.
Each section elaborates the required settings and parameters to be
set by the NSLP for the NTLP, for instance, how the message routing
information is set.
4.4.1 CREATE
The CREATE request message is used to create NSLP sessions and to
create policy rules. Furthermore, CREATE messages are used to
refresh sessions and to delete them.
The CREATE message carries these objects:
o Lifetime object [M]
o Extended flow information object [M]
o Message sequence number object [M]
o Proxy support object [O]
o Nonce object [M if CREATE-PROXY message]
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.
4.4.2 RESERVE-EXTERNAL-ADDRESS (REA)
The RESERVE-EXTERNAL-ADDRESS (REA) request message is used to target
a NAT and to allocated an external IP address and possibly port
number, so that the initiator of the REA request has a public
reachable IP address/port number.
The REA request message carries these objects:
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o Lifetime object [M]
o Message sequence number object [M]
o Extended flow information object [M]
o Proxy support object [O]
o Nonce object [M if proxy support object is included]
o Data sender information object [O]
The REA message needs special NTLP treatment. First of all, REA
messages travel the wrong way, from the DR towards DS. Second, the
DS' address used during the signaling may be not the actual DS (see
Section 3.7). Therefore, the NTLP flow routing information is set to
DR as initiator and DS as responders, a special field is given in the
NTLP: The signaling destination.
4.4.3 RESPONSE
RESPONSE messages are responses to CREATE, REA, UCREATE, and QDRQ
messages.
The RESPONSE message carries these objects:
o Lifetime object [M]
o Message sequence number object [M]
o Response code object [M]
o External address object [O]([M] for success responses to REA)
This message is routed upstream.
EDITOR's note: Text says that this section is defining the behavior
depending on the response type.
4.4.4 QDRQ
QDRQ messages are used for query and diagnosis purposes.
The QDRQ message carries these objects:
o Response object [M]
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o Message sequence number object [M]
o NATFW NSLP Hop Count [M]
o Maximum Hop Count value [O]
o Bound session ID [O]
o Session status [O]
o QDRQ type [O]
This message is routed downstream.
4.4.5 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 Response code object with NOTIFY code [M].
The message routing information in the NTLP MUST be set with the NI's
address being the destination address and the node's address as
source address. The message is forwarded upstream hop by hop using
the existing upstream node address entry within the node's Message
Routing State table. The session id object must be set to the
corresponding session that is effected by this asynchronous event.
4.4.6 UCREATE
TBD: XYX.
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5. NATFW NSLP NTLP Requirements
The NATFW NSLP requires the following capabilities from the NTLP:
o Ability to detect that the NSIS Responder does not support NATFW
NSLP. This capability is key to launching the proxy mode behavior
as described in Section 3.3.7 and [14].
o Detection of NATs and their support of the NSIS NATFW NSLP. If
the NTLP discovers that the NSIS host is behind an NSIS aware NAT,
the NR will send REA messages to the opportunistic address. If
the NTLP discovers that the NSIS host is behind a NAT that does
not support NSIS then the NSIS host will need to use a separate
NAT traversal mechanism.
o Message origin authentication and message integrity protection
o Detection of routing changes
o Protection against malicious announcement of fake path changes,
this is needed to mitigate a threat discussed in Section 7 of [7]
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6. NSIS NAT and Firewall Transition Issues
NSIS NAT and Firewall transition issues are premature and will be
addressed in a separate draft (see [14]). An update of this section
will be based on consensus.
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7. 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:
Section 7.1 describes the framework assumptions with regard to the
assumed trust relationships between the participating entities. This
subsection also motivates a particular authorization model.
Security threats that focus on NSIS in general are described in [7]
and they are applicable to this document. Within Section 7.2 we
extend this threat investigation by considering NATFW NSLP specific
threats. Based on the security threats we list security
requirements.
Finally we illustrate how the security requirements that were created
based on the security threats can be fullfilled by specific security
mechanisms. These aspects will be elaborated in Section 7.3.
7.1 Trust Relationship and Authorization
The NATFW NSLP is a protocol which may involve a number of NSIS nodes
and is, as such, not a two-party protocol. This fact requires more
thoughts about scenarios, trust relationships and authorization
mechanisms. Trust relationships and authorization are very important
for the protocol machinery and they are closely related to each other
in the sense that a certain degree of trust is required to authorize
a particular action. For any action (e.g. create/delete pinholes),
authorization is very important due to the nature of middleboxes.
More problematic scenarios are described in Appendix B.
Different types of trust relationships may affect different
categories of middleboxes. As explained in [22], establishment of a
financial relationship is typically very important for QoS signaling,
whereas financial relationships are less directly of interest for
NATFW middlebox signaling. It is therefore not particularly
surprising that there are differences in the nature and level of
authorization likely to be required in a QoS signaling environment
and in NATFW middlebox signaling. Typically NATFW signaling requires
authorization to configure firewalls or to modify NAT bindings. The
outcome of the authorization is either allowed or disallowed whereas
QoS signaling might just indicate that a lower QoS reservation is
allowed.
Different trust relationships that appear in middlebox signaling
environments are described in the subsequent sub-sections. As a
comparison with other NSIS signaling application it might be
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interesting to mention that QoS signaling relies on peer-to-peer
trust relationships and authorization between neighboring nodes or
neighboring networks. These type of trust relationships turn out to
be simpler for a protocol. However, there are reasons to believe
that this is not the only type of trust relationship found in today's
networks.
7.1.1 Peer-to-Peer Trust Relationship
Starting with the simplest scenario, it is assumed that neighboring
nodes trust each other. The required security association to
authenticate and to protect a signaling message is either available
(after manual configuration), or has been dynamically established
with the help of an authentication and key exchange protocol. If
nodes are located closely together, it is assumed that security
association establishment is easier than establishing it between
distant nodes. It is, however, difficult to describe this
relationship generally due to the different usage scenarios and
environments. Authorization heavily depends on the participating
entities, but for this scenario, it is assumed that neighboring
entities trust each other (at least for the purpose of policy rule
creation, maintenance, and deletion). Note that Figure 42 does not
illustrate the trust relationship between the end host and the access
network.
+------------------------+ +-------------------------+
|Network A | | Network B|
| +---------+ +---------+ |
| +-///-+ Middle- +---///////----+ Middle- +-///-+ |
| | | box 1 | Trust | box 2 | | |
| | +---------+ Relationship +---------+ | |
| | Trust | | Trust | |
| | Relationship | | Relationship | |
| | | | | |
| +--+---+ | | +--+---+ |
| | Host | | | | Host | |
| | A | | | | B | |
| +------+ | | +------+ |
+------------------------+ +-------------------------+
Figure 42: Peer-to-Peer Trust Relationship
7.1.2 Intra-Domain Trust Relationship
In larger corporations, often more than one middlebox is used to
protect or serve different departments. In many cases, the entire
enterprise is controlled by a security department, which gives
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instructions to the department administrators. In such a scenario, a
peer-to-peer trust-relationship might be prevalent. Sometimes it
might be necessary to preserve authentication and authorization
information within the network. As a possible solution, a
centralized approach could be used, whereby an interaction between
the individual middleboxes and a central entity (for example a policy
decision point - PDP) takes place. As an alternative, individual
middleboxes could exchange the authorization decision with another
middlebox within the same trust domain. Individual middleboxes
within an administrative domain should exploit their trust
relationship instead of requesting authentication and authorization
of the signaling initiator again and again. Thereby complex protocol
interactions are avoided. This provides both a performance
improvement without a security disadvantage since a single
administrative domain can be seen as a single entity. Figure 43
illustrates a network structure which uses a centralized entity.
+-----------------------------------------------------------+
| Network A |
| +---------+ +---------+
| +----///--------+ Middle- +------///------++ Middle- +---
| | | box 2 | | box 2 |
| | +----+----+ +----+----+
| +----+----+ | | |
| | Middle- +--------+ +---------+ | |
| | box 1 | | | | |
| +----+----+ | | | |
| | | +----+-----+ | |
| | | | Policy | | |
| +--+---+ +-----------+ Decision +----------+ |
| | Host | | Point | |
| | A | +----------+ |
| +------+ |
+-----------------------------------------------------------+
Figure 43: Intra-domain Trust Relationship
7.1.3 End-to-Middle Trust Relationship
In some scenarios, a simple peer-to-peer trust relationship between
participating nodes is not sufficient. Network B might require
additional authorization of the signaling message initiator. If
authentication and authorization information is not attached to the
initial signaling message then the signaling message arriving at
Middlebox 2 would result in an error message being created, which
indicates the additional authorization requirement. In many cases
the signaling message initiator is already aware of the additionally
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required authorization before the signaling message exchange is
executed. Replay protection is a requirement for authentication to
the non-neighboring middlebox, which might be difficult to accomplish
without adding additional roundtrips to the signaling protocol (e.g.,
by adding a challenge/response type of message exchange).
Figure 44 shows the slightly more complex trust relationships in this
scenario.
+--------------------+ +---------------------+
| Network A | Trust |Network B |
| | Relationship | |
| +---------+ +---------+ |
| +-///-+ Middle- +---///////----+ Middle- +-///-+ |
| | | box 1 | +-------+ box 2 | | |
| | +---------+ | +---------+ | |
| |Trust | | | Trust | |
| |Relationship | | | Relationship| |
| | | | | | |
| +--+---+ | | | +--+---+ |
| | Host +----///----+------+ | | Host | |
| | A | |Trust | | B | |
| +------+ |Relationship | +------+ |
+--------------------+ +---------------------+
Figure 44: End-to-Middle Trust Relationship
7.2 Security Threats and Requirements
This section describes NATFW specific security threats and
requirements.
7.2.1 Attacks related to authentication and authorization
The NSIS message which installs policy rules at a middlebox is the
CREATE message. The CREATE message travels from the Data Sender (DS)
toward the Data Receiver (DR). The packet filter or NAT binding is
marked as pending by the middleboxes along the path. If it is
confirmed with a success RESPONSE message from the DR, the requested
policy rules on the middleboxes are installed to allow the traversal
of a data flow.
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+-----+ +-----+ +-----+
| DS | | MB | | DR |
+-----+ +-----+ +-----+
| | |
| CREATE | CREATE |
|-------------------->+-------------------->|
| | |
| Succeeded/Error | Succeeded/Error |
|<--------------------+<--------------------|
| | |
==========================================>
Direction of data traffic
Figure 45: CREATE Mode
In this section we will consider some simple scenarios for middlebox
configuration:
o Data Sender (DS) behind a firewall
o Data Sender (DS) behind a NAT
o Data Receiver (DR) behind a firewall
o Data Receiver (DR) behind a NAT
A real-world scenario could include a combination of these firewall/
NAT placements, such as, a DS and/or a DR behind a chain of NATs and
firewalls.
Figure 46 shows one possible scenario:
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+-------------------+ +--------------------+
| Network A | | Network B |
| | | |
| +-----+ | //-----\\ | +-----+ |
| | MB2 |--------+----| INET |----+--------| MB3 | |
| +-----+ | \\-----// | +-----+ |
| | | | | |
| +-----+ | | +-----+ |
| | MB1 | | | | MB4 | |
| +-----+ | | +-----+ |
| | | | | |
| +-----+ | | +-----+ |
| | DS | | | | DR | |
| +-----+ | | +-----+ |
+-------------------+ +--------------------+
MB: Middle box (NAT or Firewall or a combination)
DS: Data Sender
DR: Data Receiver
Figure 46: Several middleboxes per network
7.2.1.1 Data Sender (DS) behind a firewall
+------------------------------+
| |
| +-----+ create +-----+
| | DS | --------------> | FW |
| +-----+ +-----+
| |
+------------------------------+
DS sends a CREATE message to request the traversal of a data flow.
It is up to network operators to decide how far they can trust users
inside their networks. However, there are several reasons why they
should not.
The following attacks are possible:
o DS could open a firewall pinhole with a source address different
from its own host.
o DS could open firewall pinholes for incoming data flows that are
not supposed to enter the network.
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o DS could request installation of any policy rules and allow all
traffic go through.
SECURITY REQUIREMENT: The middlebox MUST authenticate and authorize
the neighboring NAT/FW NSLP node which requests an action.
Authentication and authorization of the initiator SHOULD be
provided to NATs and Firewalls along the path.
7.2.1.2 Data Sender (DS) behind a NAT
The case 'DS behind a NAT' is analogous to the case 'DS behind a
firewall'.
Figure 48 illustrates such a scenario:
+------------------------------+
| |
| +------+ CREATE |
| | NI_1 | ------\ +-----+ CREATE +-----+
| +------+ \------> | NAT |-------->| MB |
| +-----+ +-----+
| +------+ |
| | NI_2 | |
| +------+ |
+------------------------------+
Figure 48: Several NIs behind a NAT
In this case the middlebox MB does not know who is the NSIS Initiator
since both NI_1 and NI_2 are behind a NAT (which is also NSIS aware).
Authentication needs to be provided by other means such as the NSLP
or the application layer.
SECURITY REQUIREMENT: The middlebox MUST authenticate and ensure that
the neighboring NAT/FW NSLP node is authorized to request an
action. Authentication and authorization of the initiator (which
is the DR in this scenario) to the middleboxes (via another NSIS
aware middlebox) SHOULD be provided.
7.2.1.3 Data Receiver (DR) behind a firewall
In this case a CREATE message comes from an entity DS outside the
network towards the DR inside the network.
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+------------------------------+
| |
+-----+ CREATE +-----+ CREATE +-----+ |
| DS | -------------> | FW | -------------> | DR | |
+-----+ <------------- +-----+ <------------- +-----+ |
success RESPONSE | success RESPONSE |
| |
+------------------------------+
Since policy rules at middleboxes must only be installed after
receiving a successful response it is necessary that the middlebox
waits until the Data Receiver DR confirms the request of the Data
Sender DS with a success RESPONSE message. This is, however, only
necessary
o if the action requested with the CREATE message cannot be
authorized and
o if the middlebox is still forwarding the signaling message towards
the end host (without state creation/deletion/modification).
This confirmation implies that the data receiver is expecting the
data flow.
At this point we differentiate two cases:
1. DR knows the IP address of the DS (for instance because of some
previous application layer signaling) and is expecting the data
flow.
2. DR might be expecting the data flow (for instance because of some
previous application layer signaling) but does not know the IP
address of the Data Sender DS.
For the second case, Figure 50 illustrates a possible attack: an
adversary Mallory M could be sniffing the application layer signaling
and thus knows the address and port number where DR is expecting the
data flow. Thus it could pretend to be DS and send a CREATE message
towards DR with the data flow description (M -> DR). Since DR does
not know the IP address of DS, it is not able to recognize that the
request is coming from the "wrong guy". It will send a success
RESPONSE message back and the middlebox will install policy rules
that will allow Mallory M to inject its data into the network.
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Application Layer signaling
<------------------------------------>
/ \
/ +-----------------\------------+
/ | \ |
+-----+ +-----+ +-----+ |
| DS | -> | FW | | DR | |
+-----+ / +-----+ +-----+ |
CREATE / | |
+-----+ / +-------------------------------+
| M |----------
+-----+
Figure 50: DR behind a firewall with an adversary
Network administrators will probably not rely on a DR to check the IP
address of the DS. Thus we have to assume the worst case with an
attack such as in Figure 50. Many operators might not allow NSIS
signaling message to traverse the firewall in Figure 50 without
proper authorization. In this case the threat is not applicable.
SECURITY REQUIREMENT: A binding between the application layer and the
NSIS signaling SHOULD be provided.
7.2.1.4 Data Receiver (DR) behind a NAT
When a data receiver DR behind a NAT sends a RESERVE-EXTERNAL-ADDRESS
(REA) message to get a public reachable address that can be used as a
contact address by an arbitrary data sender if the DR was unable to
restrict the future data sender. The NAT reserves an external
address and port number and sends them back to DR. The NAT adds an
address mapping entry in its reservation list which links the public
and private addresses as follows:
(DR_ext <=> DR_int) (*).
The NAT sends a RESPONSE message with the external address' object
back to the DR with the address DR_ext. DR informs DS about the
public address that it has recently received, for instance, by means
of application layer signaling.
When a data sender sends a CREATE message towards DR_ext then the
message will be forwarded to the DR. The data sender might want to
update the NAT binding stored at the edge-NAT to make it more
restrictive.
We assume that the adversary Mallory M obtains the contact address
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(i.e., external address and port) allocated at the NAT possibly by
eavesdropping on the application layer signaling and sends a CREATE
message. As a consequence Mallory would be able to communicate with
DR (if M is authorized by the edge-NAT and if the DR accepts CREATE
and returns a RESPONSE.
Application Layer signaling
<------------------------------------>
/ \
/ +-----------------\------------+
/ | REA \ |
+-----+ +-----+ <----------- +-----+ |
| DS | -> | NAT | -----------> | DR | |
+-----+ / +-----+ rtn_ext_addr +-----+ |
CREATE / | |
+-----+ / +-------------------------------+
| M |----------
+-----+
SECURITY REQUIREMENT: The DR MUST be able to specify which data
sender are allowed to traverse the NAT in order to be forwarded to
DRs address.
7.2.1.5 NSLP Message Injection
Malicious hosts, located either off-path or on-path, could inject
arbitrary NATFW NSLP messages into the signaling path, causing
several problems. These problems apply when no proper authorization
and authentication scheme is available.
By injecting a bogus CREATE message with lifetime set to zero, a
malicious host could try to teardown NATFW NSLP session state
partially or completely on a data path, causing a service
interruption.
By injecting a bogus responses or NOTIFY message, for instance,
timeout, a malicious host could try to teardown NATFW NSLP session
state as well. This could affect the data path partially or totally,
causing a service interruption.
SECURITY REQUIREMENT: Messages, such as TRIGGER, can be misused by
malicious hosts, and therefore need to be authorized.
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7.2.2 Denial-of-Service Attacks
In this section we describe several ways how an adversary could
launch a Denial of service (DoS) attack on networks running NSIS for
middlebox configuration to exhaust their resources.
7.2.2.1 Flooding with CREATE messages from outside
7.2.2.1.1 Attacks due to NSLP state
A CREATE message requests the NSLP to store state information such as
a NAT binding or a policy rule.
The policy rules requested in the CREATE message will be installed at
the arrival of a confirmation from the Data Receiver with a success
RESPONSE message. A successful RESPONSE message includes the session
ID. So the NSLP looks up the NSIS session and installs the requested
policy rules.
An adversary from outside could launch a DoS attack with arbitrary
CREATE messages. For each of these messages the middlebox needs to
store state information such as the policy rules to be loaded, i.e.,
the middlebox could run out of memory. This kind of attack is also
mentioned in [7] Section 4.8.
SECURITY REQUIREMENT: A NAT/FW NSLP node MUST authorize the 'create-
session' message before storing state information.
7.2.2.1.2 Attacks due to authentication complexity
This kind of attack is possible if authentication is based on
mechanisms that require computing power, for example, digital
signatures.
For a more detailed treatment of this kind of attack, the reader is
encouraged to see [7].
SECURITY REQUIREMENT: A NAT/FW NSLP node MUST NOT introduce new
denial of service attacks based on authentication or key
management mechanisms.
7.2.2.1.3 Attacks to the endpoints
The NATFW NSLP requires firewalls to forward NSLP messages, a
malicious node may keep sending NSLP messages to a target. This may
consume the access network resources of the victim, drain the battery
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of the victim's terminal and may force the victim to pay for the
received although undesired data.
This threat may be more particularly be relevant in networks where
access link is a limited resource, for instance in cellular networks,
and where the terminal capacities are limited.
SECURITY REQUIREMENT: A NATFW NSLP aware firewall or NAT MUST be able
to block unauthorized signaling message, if this threat is a
concern.
7.2.2.2 Flooding with REA messages from inside
Although we are more concerned with possible attacks from outside the
network, we need also to consider possible attacks from inside the
network.
An adversary inside the network could send arbitrary RESERVE-
EXTERNAL-ADDRESS messages. At a certain point the NAT will run out
of port numbers and the access for other users to the outside will be
disabled.
SECURITY REQUIREMENT: The NAT/FW NSLP node MUST authorize state
creation for the RESERVE-EXTERNAL-ADDRESS message. Furthermore,
the NAT/FW NSLP implementation MUST prevent denial of service
attacks involving the allocation of an arbitrary number of NAT
bindings or the installation of a large number of packet filters.
7.2.3 Man-in-the-Middle Attacks
Figure 52 illustrates a possible man-in-the-middle attack using the
RESERVE-EXTERNAL-ADDRESS (REA) message. This message travels from DR
towards the public Internet. The message might not be intercepted
because there are no NSIS aware middleboxes.
Imagine such an NSIS signaling message is then intercepted by an
adversary Mallory (M). M returns a faked RESPONSE message whereby
the adversary pretends that a NAT binding was created. This NAT
binding is returned with the RESPONSE message. Malory might insert
it own IP address in the response, the IP address of a third party or
the address of a black hole. In the first case, the DR thinks that
the address of Mallory M is its public address and will inform the DS
about it. As a consequence, the DS will send the data traffic to
Mallory M.
The data traffic from the DS to the DR will re-directed to Mallory M.
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M will be able to read, modify or block the data traffic (if the end-
to-end communication itself does not experience protection).
Eavesdropping and modification is only possible if the data traffic
is itself unprotected.
+-----+ +-----+ +-----+
| DS | | M | | DR |
+-----+ +-----+ +-----+
| | |
| | REA |
| | <------------------ |
| | |
| | RESPONSE |
| | ------------------> |
| | |
| data traffic | |
|===============>| data traffic |
| |====================>|
Figure 52: MITM attack using the RESERVE-EXTERNAL-ADDRESS message
SECURITY REQUIREMENT: Mutual authentication between neighboring NATFW
NSLP MUST be provided. To ensure that only legitimate nodes along
the path act as NSIS entities the initiator MUST authorize the
responder. In the example in Figure 52 the firewall FW must
perform an authorization with the neighboring entities.
7.2.4 Message Modification by non-NSIS on-path node
An unauthorized on-path node along the path towards the destination
could easily modify, inject or just drop an NSIS message. It could
also hijack or disrupt the communication.
SECURITY REQUIREMENT: Message integrity, replay protection and data
origin authentication between neighboring NAT/FW NSLPs MUST be
provided.
7.2.5 Message Modification by malicious NSIS node
Message modification by a NSIS node that became malicious is more
serious. An adversary could easily create arbitrary pinholes or NAT
bindings. For example:
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o NATs need to modify the source/destination of the data flow in the
'create session' message.
o Each middlebox along the path may change the requested lifetime in
the CREATE message to fit their needs and/or local policy.
SECURITY REQUIREMENT: None. Malicious NSIS NATs and Firewalls will
not be addressed.
7.2.6 Session Modification/Deletion
The Session ID is included in signaling messages as a reference to
the established state. If an adversary is able to obtain the Session
Identifier for example by eavesdropping on signaling messages, it
would be able to add the same Session Identifier to a new signaling
message and effect some modifications.
Consider the scenario described in Figure 53. Here an adversary
pretends to be 'DS in mobility'. The signaling messages start from
the DS and go through a series of routers towards the DR. We assume
that an off-path adversary is connected to one of the routers along
the old path (here Router 3). We also assume that the adversary
knows the Session ID of the NSIS session initiated by the DS.
Knowing the Session ID, the adversary now sends signaling messages
towards the DR. When the signaling message reaches Router3 then
existing state information can be modified or even deleted. The
adversary can modify or delete the established reservation causing
unexpected behavior for the legitimate user. The source of the
problem is that the Router 3 (cross-over router) is unable to decide
whether the new signaling message was initiated from the owner of the
session. In this scenario, the adversary need not even be located in
the DS-DR path. This problem and the solution approaches are
described in more detail in [24].
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Session ID(SID-x)
+--------+ +--------+
+-------->--------+ Router +-------->+ DR |
Session ID(SID-x)| | 4 | | |
+---+----+ +--------+ +--------+
| Router |
+------+ 3 +*******
| +---+----+ *
| *
| Session ID(SID-x) * Session ID(SID-x)
+---+----+ +---+----+
| Access | | Access |
| Router | | Router |
| 1 | | 2 |
+---+----+ +---+----+
| *
| Session ID(SID-x) * Session ID(SID-x)
+----+------+ +----+------+
| DS | | Adversary |
| | | |
+-----------+ +-----------+
Figure 53: State Modification by off-path adversary
As a summary, an off-path adversary's knowledge of Session-ID could
cause session modification/deletion.
SECURITY REQUIREMENT: The initiator MUST be able to demonstrate
ownership of the session it wishes to modify.
7.2.6.1 Misuse of mobility in NAT handling
Another kind of session modification is related to mobility
scenarios. NSIS allows end hosts to be mobile, it is possible that
an NSIS node behind a NAT needs to update its NAT binding in case of
address change. Whenever a host behind a NAT initiates a data
transfer, it is assigned an external IP and port number. In typical
mobility scenarios, the DR might also obtain a new address according
to the topology and it should convey its new IP address to the NAT.
The NAT is assumed to modify these NAT bindings based on the new IP
address conveyed by the endhost.
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Public Private Address
Internet space
+----------+ +----------+
+----------| NAT |------------------|End host |
| | | |
+----------+ +----------+
|
| +----------+
\--------------------|Malicious |
|End host |
+----------+
data traffic
<========================
Figure 54: Misuse of mobility in NAT binding
A NAT binding can be changed with the help of NSIS signaling. When a
DR moves to a new location and obtains a new IP address, it sends an
NSIS signaling message to modify the NAT binding. It would use the
Session-ID and the new flow-id to update the state. The NAT updates
the binding and the DR continues to receive the data traffic.
Consider the scenario in Figure 54 where an the endhost(DR) and the
adversary are behind a NAT. The adversary pretending that it is the
end host could generate a spurious signaling message to update the
state at the NAT. This could be done for these purposes:
Connection hijacking by redirecting packets to the attacker as in
Figure 55
Third party flooding by redirecting packets to arbitrary hosts
Service disruption by redirecting to non-existing hosts
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+----------+ +----------+ +----------+
| NAT | |End host | |Malicious |
| | | | |End host |
+----------+ +----------+ +----------+
| | |
| Data Traffic | |
|--------->----------| |
| | Spurious |
| | NAT binding update |
|---------<----------+--------<------------|
| | |
| Data Traffic | |
|--------->----------+-------->------------|
| | |
Figure 55: Connection Hijacking
SECURITY REQUIREMENT: A NAT/FW signaling message MUST be
authenticated, authorized, integrity protected and replay
protected between neighboring NAT/FW NSLP nodes.
7.2.7 Misuse of unreleased sessions
Assume that DS (N1) initiates NSIS session with DR (N2) through a
series of middleboxes as in Figure 56. When the DS is sending data
to DR, it might happen that the DR disconnects from the network
(crashes or moves out of the network in mobility scenarios). In such
cases, it is possible that another node N3 (which recently entered
the network protected by the same firewall) is assigned the same IP
address that was previously allocated to N2. The DS could take
advantage of the firewall policies installed already, if the refresh
interval time is very high. The DS can flood the node (N3), which
will consume the access network resources of the victim forcing it to
pay for unwanted traffic as shown in Figure 57. Note that here we
make the assumption that the data receiver has to pay for receiving
data packets.
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Public Internet
+--------------------------+
| |
+-------+ CREATE +---+-----+ +-------+ |
| |-------------->------| |---->---| | |
| N1 |--------------<------| FW |----<---| N2 | |
| | success RESPONSE | | | | |
| |==============>======| |====>===| | |
+-------+ Data Traffic +---+-----+ +-------+ |
| |
+--------------------------+
Figure 56: Before mobility
Public Internet
+--------------------------+
| |
+-------+ +---+-----+ +-------+ |
| | | | | | |
| N1 |==============>======| FW |====>===| N3 | |
| | Data Traffic | | | | |
+-------+ +---+-----+ +-------+ |
| |
+--------------------------+
Figure 57: After mobility
Also, this threat is valid for the other direction as well. The DS
which is communicating with the DR may disconnect from the network
and this IP address may be assigned to a new node that had recently
entered the network. This new node could pretend to be the DS and
send data traffic to the DR in conformance with the firewall policies
and cause service disruption.
SECURITY REQUIREMENT: Data origin authentication is needed to
mitigate this threat. In order to allow firewalls to verify that
a legitimate end host transmitted the data traffic data origin
authentication is required. This is, however, outside the scope
of this document. Hence, there are no security requirements
imposed by this section which will be addressed by the NATFW NSLP.
7.2.8 Data traffic injection
In some environments, such as enterprise networks, it is still common
to perform authorization for access to a service based on the source
IP address of the service requester. There is no doubt that this by
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itself represents a security weakness. Hence by spoofing a
connection, an attacker is able to reach the target machines, using
the existing firewall rules.
The adversary is able to inject its own data traffic in conformance
with the firewall policies simultaneously along with the genuine DS.
SECURITY REQUIREMENT: Since IP spoofing is a general limitation of
non-cryptographic packet filters no security requirement needs to
be created for the NAT/FW NSLP. Techniques such as ingress
filtering (described below) and data origin authentication (such
as provided with IPsec based VPNs) can help mitigate this threat.
This issue is, however, outside the scope of this document.
Ingress Filtering: Consider the scenario shown in Figure 58. In this
scenario the DS is behind a router (R1) and a malicious node (M) is
behind another router (R2). The DS communicates with the DR through
a firewall (FW). The DS initiates NSIS signaling and installs
firewall policies at FW. But the malicious node is also able to send
data traffic using DS's source address. If R2 implements ingress
filtering, these spoofed packets will be blocked. But this ingress
filtering may not work in all scenarios. If both the DS and the
malicious node are behind the same router, then the ingress filter
will not be able to detect the spoofed packets as both the DS and the
malicious node are in the same address range.
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+-----------------------------------+
| +------------------+ |
| | +-------+ +---+---+ |
| | | DS +>--+ R1 +->+ |
| | | | | | | |
| | +-------+ +---+---+ | |
| | | | |
| +------------------+ | +---+---+ +-------+
| | | | | |
| +---+ FW +-->--| DR |
| +------------------+ ****| |*****| |
| | | * +---+---+ +-------+
| | +-------+ +---+---+ * |
| | | M | | R2 | * |
| | | |***| |*** |
| | +-------+ +---+---+ |
| +------------------+ |
+-----------------------------------+
---->---- = genuine data traffic
********* = spoofed data traffic
Figure 58: Ingress filtering
7.2.9 Eavesdropping and traffic analysis
By collecting NSLP messages, an adversary is able to learn policy
rules for packet filters and knows which ports are open. It can use
this to inject its own data traffic due to the IP spoofing capability
as already mentioned in Section 7.2.8.
An adversary could learn authorization tokens included in CREATE
messages and use them to launch replay-attacks or to create a session
with its own address as source address. (cut-and-paste attack)
As shown in Section 4.3 of [24] one possible solution for the session
ownership problem is confidentiality protection of signaling messages
SECURITY REQUIREMENT: The threat of eavesdropping itself does not
mandate the usage of confidentiality protection since an adversary
can also eavesdrop on data traffic. In the context of a
particular security solutions (e.g., authorization tokens) it
might be necessary to offer confidentiality protection.
Confidentiality protection also needs to be offered to the refresh
period.
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7.3 Security Framework for the NAT/Firewall NSLP
Based on the identified threats a list of security requirements has
been created.
7.3.1 Security Protection between neighboring NATFW NSLP Nodes
Based on the analyzed threats it is necessary to provide, between
neighboring NATFW NSLP nodes, the following mechanism: provide
o data origin authentication
o replay protection
o integrity protection and
o optionally confidentiality protection
To consider the aspect of authentication and key exchange the
security mechanisms provided in [1] between neighboring nodes MUST be
enabled when sending NATFW signaling messages. The proposed security
mechanisms at GIMPS provide support for authentication and key
exchange in addition to denial of service protection. Depending on
the chosen protocol, support for flexible authentication protocols
could be provided. The mandatory support for security, demands 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.
7.3.2 Security Protection between non-neighboring NATFW NSLP Nodes
Based on the security threats and the listed requirements it was
noted that some scenarios 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) [18] SHOULD be used. CMS
can be used
o This might be, for example, useful to authenticate and authorize a
user towards a middlebox and vice versa.
o If objects have to be protected between certain non-neighboring
NATFW NSLP nodes.
Details about the identifiers, replay protection and the usage of a
dynamic key management with the help of CMS is for further study. In
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some scenarios it is also required to use authorization token. 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 or assertions can be found in RFC
3520 [30] and RFC 3521 [31]. More recent work on authorization token
alike mechanisms is Security Assertion Markup Language (SAML). For
details about SAML see [32], [33] and [34]. Figure 59 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 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 successful authorization
by a NATFW NSLP node, the requested action is executed and later a
RESPONSE message is generated.
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+----------------+ 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 59: Authorization Token Usage
Threats against the usage of authorization tokens have been mentioned
in [7] and also in Section 7.2. Hence, it is required to provide
confidentiality protection to avoid allowing an eavesdropper to learn
the token and to use it in another session (replay attack). The
token itself also needs to be protected against tempering.
7.3.3 End-to-End Security
As part of the threat analysis we concluded that end-to-end security
is not required and, if used, would be difficult to deploy.
Furthermore, it might be difficult to use the suitable identifiers
and to establish the necessary infrastructure for this propose.
The only reasonable end-to-end security protection needed within NSIS
seems to be a binding between an NSIS signaling session and
application layer session. This aspect is, however, for further
study.
In order to solicit feedback from the IETF community on some hard
security problems for path-coupled NATFW signaling a more detailed
description in [21] is available.
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8. Open Issues
The NATFW NSLP has a series of related documents discussing several
other aspects of path-coupled NATFW signaling, including security
[21], migration (i.e., traversal of NSIS unaware NATs) [14], intra-
realm signaling [15], and inter-working with SIP [16]. Summaries of
the outcomes from these documents may be added, depending on WG
feedback, to a later version of this draft.
A more detailed list of open issue can be found at:
https://kobe.netlab.nec.de/roundup/nsis-natfw-nslp/index
It is intended to add an overview figure for all NATFW NSLP building
blocks into the next version of this memo. Figure 60 sketches the
overview
+------------------+
|Security Policies |
| Server |
+--------^---------+
|
+--------------------------------|----------------------+
| +---------+ +-----------V----+ +-------+|
| |Firewall |<-----> | |<------>| NAT ||
| |Engine | | Security policy| | Engine||
| +----^----+ | Table/Cache | +-^-----+|
| | | ^ | | |
| | +---- --------|--+ | |
| +--|---------------------------|-------------|--+ |
| | V NATFW NSLP V V | |
| | | |
| +-----------------------------------------------+ |
| +--------------------------------------------------+|
| | GIMPS ||
| | ||
| +--------------------------------------------------+|
| +---------+ +-------+ +------+ +-------+ +------+|
| | TCP | | UDP | | DCCP | | SCTP | | ICMP ||
| +---------+ +-------+ +------+ +-------+ +------+|
| +-----------------------------+ +--------------------|
| | IPv4 | | IPv6 |
| +-----------------------------+ +--------------------|
+-------------------------------------------------------+
Figure 60: NATFW NSLP Building Blocks
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9. Contributors
We would like to thank the following individuals for their
contributions to this document:
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 Elywn Davies for his help to make this document more readable,
o and the NSIS working group.
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10. References
10.1 Normative References
[1] Schulzrinne, H. and R. Hancock, "GIMPS: General Internet
Messaging Protocol for Signaling", draft-ietf-nsis-ntlp-06 (work
in progress), May 2005.
10.2 Informative References
[2] Bradner, S., Mankin, A., and J. Schiller, "A Framework for
Purpose-Built Keys (PBK)", draft-bradner-pbk-frame-06 (work in
progress), June 2003.
[3] Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
Bosch, "Next Steps in Signaling (NSIS): Framework", RFC 4080,
June 2005.
[4] Brunner, M., "Requirements for Signaling Protocols", RFC 3726,
April 2004.
[5] Bosch, S., Karagiannis, G., and A. McDonald, "NSLP for Quality-
of-Service signaling", draft-ietf-nsis-qos-nslp-06 (work in
progress), February 2005.
[6] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A.
Rayhan, "Middlebox communication architecture and framework",
RFC 3303, August 2002.
[7] Tschofenig, H. and D. Kroeselberg, "Security Threats for NSIS",
draft-ietf-nsis-threats-06 (work in progress), October 2004.
[8] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999.
[9] Tsirtsis, G. and P. Srisuresh, "Network Address Translation -
Protocol Translation (NAT-PT)", RFC 2766, February 2000.
[10] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues",
RFC 3234, February 2002.
[11] Srisuresh, P., Tsirtsis, G., Akkiraju, P., and A. Heffernan,
"DNS extensions to Network Address Translators (DNS_ALG)",
RFC 2694, September 1999.
[12] Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997.
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[13] Yadav, S., Yavatkar, R., Pabbati, R., Ford, P., Moore, T.,
Herzog, S., and R. Hess, "Identity Representation for RSVP",
RFC 3182, October 2001.
[14] Aoun, C., Brunner, M., Stiemerling, M., Martin, M., and H.
Tschofenig, "NAT/Firewall NSLP Migration Considerations",
draft-aoun-nsis-nslp-natfw-migration-02 (work in progress),
July 2004.
[15] Aoun, C., "NATFirewall NSLP Intra-realm considerations",
draft-aoun-nsis-nslp-natfw-intrarealm-01 (work in progress),
July 2004.
[16] Martin, M., "SIP NSIS Interactions for NAT/Firewall Traversal",
draft-martin-nsis-nslp-natfw-sip-01 (work in progress),
July 2004.
[17] Tschofenig, H., "Extended QoS Authorization for the QoS NSLP",
draft-tschofenig-nsis-qos-ext-authz-00 (work in progress),
July 2004.
[18] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369,
August 2002.
[19] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[20] Ohba, Y., "Problem Statement and Usage Scenarios for PANA",
draft-ietf-pana-usage-scenarios-06 (work in progress),
April 2003.
[21] Tschofenig, H., "Path-coupled NAT/Firewall Signaling Security
Problems",
DRAFT draft-tschofenig-nsis-natfw-security-problems-00.txt,
July 2004.
[22] Tschofenig, H., Buechli, M., Van den Bosch, S., and H.
Schulzrinne, "NSIS Authentication, Authorization and Accounting
Issues", March 2003.
[23] Adrangi, F. and H. Levkowetz, "Problem Statement: Mobile IPv4
Traversal of VPN Gateways",
DRAFT draft-ietf-mobileip-vpn-problem-statement-req-02.txt,
April 2003.
[24] Tschofenig, H., "Security Implications of the Session
Identifier", draft-tschofenig-nsis-sid-00 (work in progress),
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June 2003.
[25] 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.
[26] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E.
Lear, "Address Allocation for Private Internets", BCP 5,
RFC 1918, February 1996.
[27] 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.
[28] Rosenberg, J., "Traversal Using Relay NAT (TURN)",
draft-rosenberg-midcom-turn-07 (work in progress),
February 2005.
[29] Tschofenig, H., "Using SAML for SIP",
draft-tschofenig-sip-saml-02 (work in progress), December 2004.
[30] Hamer, L-N., Gage, B., Kosinski, B., and H. Shieh, "Session
Authorization Policy Element", RFC 3520, April 2003.
[31] Hamer, L-N., Gage, B., and H. Shieh, "Framework for Session
Set-up with Media Authorization", RFC 3521, April 2003.
[32] Maler, E., Philpott, R., and P. Mishra, "Bindings and Profiles
for the OASIS Security Assertion Markup Language (SAML) V1.1",
September 2003.
[33] Maler, E., Philpott, R., and P. Mishra, "Assertions and
Protocol for the OASIS Security Assertion Markup Language
(SAML) V1.1", September 2003.
[34] Maler, E. and J. Hughes, "Technical Overview of the OASIS
Security Assertion Markup Language (SAML) V1.1", March 2004.
[35] 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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Authors' Addresses
Martin Stiemerling
Network Laboratories, NEC Europe Ltd.
Kurfuersten-Anlage 36
Heidelberg 69115
Germany
Phone: +49 (0) 6221 905 11 13
Email: stiemerling@netlab.nec.de
URI: http://www.stiemerling.org
Hannes Tschofenig
Siemens AG
Otto-Hahn-Ring 6
Munich 81739
Germany
Phone:
Email: Hannes.Tschofenig@siemens.com
URI: http://www.tschofenig.com
Cedric Aoun
Ecole Nationale Superieure des Telecommunications
Paris
France
Email: cedric@caoun.net
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Appendix A. Firewall and NAT Resources
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. For pure
firewalls a filter rule must be created and for pure NATs a NAT
binding must be created. In mixed firewall and NAT boxes, the policy
rule must be mapped in 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
private or public IP addresses. However, the exact mapping depends
on the implementation of the firewall or NAT which is different for
each vendor. The remainder of this section gives thus only an
abstract mapping of NATFW NSLP policy rules to firewall rules and NAT
bindings, without going into the specifics on single configuration
parameter possibilities.
A policy rule consists out of the message routing information (MRI),
carried in the NTLP, and information available in the NATFW NSLP.
The information of the NSLP is stored in the extend flow information
object and the message type, in particular the flow direction.
Additional information, such as the external IP address and port
number, stored in the NAT or firewall will be used as well.
A.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 7.2 of
[1].
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.
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A.2 Mapping to Firewall Rules
EDITOR's NOTE: This section is to be done (CREATE, UCREATE).
A.3 Mapping to NAT Bindings
EDITOR's NOTE: This section is to be done (CREATE, REA).
A.4 Mapping for combined NAT and Firewall
EDITOR's NOTE: This section is to be done.
A.5 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
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 B. Problems and Challenges
This section describes a number of problems that have to be addressed
for NSIS NAT/Firewall. Issues presented here are subject to further
discussions. These issues might be also of relevance to other NSLP
protocols.
B.1 Missing Network-to-Network Trust Relationship
Peer-to-peer trust relationship, as shown in Figure 42, is a very
convenient assumption that allows simplified signaling message
processing. However, it might not always be applicable, especially
between two arbitrary access networks (over a core network where
signaling messages are not interpreted). Possibly peer-to-peer trust
relationship does not exist because of the large number of networks
and the unwillingness of administrators to have other network
operators to create holes in their Firewalls without proper
authorization.
+----------------------+ +--------------------------+
| | | |
| Network A | | Network B |
| | | |
| +---------+ Missing +---------+ |
| +-///-+ Middle- | Trust | Middle- +-///-+ |
| | | box 1 | Relation- | box 2 | | |
| | +---------+ ship +---------+ | |
| | | or | | |
| | | Authorization| | |
| | | | | |
| | Trust | | Trust | |
| | Relationship | | Relationship | |
| | | | | |
| | | | | |
| | | | | |
| +--+---+ | | +--+---+ |
| | Host | | | | Host | |
| | A | | | | B | |
| +------+ | | +------+ |
+----------------------+ +--------------------------+
Figure 61: Missing Network-to-Network Trust Relationship
Figure 61 illustrates a problem whereby an external node is not
allowed to manipulate (create, delete, query, etc.) packet filters at
a Firewall. Opening pinholes is only allowed for internal nodes or
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with a certain authorization permission. Hence the solution
alternatives in Section 3.3.2 focus on establishing the necessary
trust with cooperation of internal nodes.
B.2 Relationship with routing
The data path is following the "normal" routes. The NAT/FW devices
along the data path are those providing the service. In this case
the service is something like "open a pinhole" or even more general
"allow for connectivity between two communication partners". The
benefit of using path-coupled signaling is that the NSIS NATFW NSLP
does not need to determine what middleboxes or in what order the data
flow will go through.
Creating NAT bindings modifies the path of data packets between two
end points. Without NATs involved, packets flow from endhost to
endhost following the path given by the routing. With NATs involved,
this end-to-end flow is not directly possible, because of separated
address realms. Thus, data packets flow towards the external IP
address at a NAT (external IP address may be a public IP address).
Other NSIS NSLPs, for instance QoS NSLP, which do not interfere with
routing - instead they only follow the path of the data packets.
B.3 Affected Parts of the Network
NATs and Firewalls are usually located at the edge of the network,
whereby other signaling applications affect all nodes along the path.
One typical example is QoS signaling where all networks along the
path must provide QoS in order to achieve true end-to-end QoS. In
the NAT/Firewall case, only some of the domains/nodes are affected
(typically access networks), whereas most parts of the networks and
nodes are unaffected (e.g., the core network).
This fact raises some questions. Should an NSIS NTLP node intercept
every signaling message independently of the upper layer signaling
application or should it be possible to make the discovery procedure
more intelligent to skip nodes. These questions are also related to
the question whether NSIS NAT/FW should be combined with other NSIS
signaling applications.
B.4 NSIS backward compatibility with NSIS unaware NAT and Firewalls
Backward compatibility is a key for NSIS deployments, as such the
NSIS protocol suite should be sufficiently robust to allow traversal
of none NSIS aware routers (QoS gates, Firewalls, NATs, etc ).
NSIS NATFW NSLP's backward compatibility issues are different than
the NSIS QoS NSLP backward compatibility issues, where an NSIS
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unaware QoS gate will simply forward the QoS NSLP message. An NSIS
unaware Firewall rejects NSIS messages, since Firewalls typically
implement the policy "default to deny".
The NSIS backward compatibility support on none NSIS aware Firewall
would typically consist of configuring a static policy rule that
allows the forwarding of the NSIS protocol messages (either protocol
type if raw transport mode is used or transport port number in case a
transport protocol is used).
For NATs backward compatibility is more problematic since signaling
messages are forwarded (at least in one direction), but with a
changed IP address and changed port numbers. The content of the NSIS
signaling message is, however, unchanged. This can lead to
unexpected results, both due to embedded unchanged local scoped
addresses and none NSIS aware Firewalls configured with specific
policy rules allowing forwarding of the NSIS protocol (case of
transport protocols are used for the NTLP). NSIS unaware NATs must
be detected to maintain a well-known deterministic mode of operation
for all the involved NSIS entities. Such a "legacy" NAT detection
procedure can be done during the NSIS discover procedure itself.
Based on experience it was discovered that routers unaware of the
Router Alert IP option [RFC 2113] discarded packets, this is
certainly a problem for NSIS signaling.
B.5 Authentication and Authorization
For both types of middleboxes, Firewall and NAT security is a strong
requirement. Authentication and authorization means must be
provided.
For NATFW signaling applications it is partially not possible to do
authentication and authorization based on IP addresses. Since NATs
change IP addresses, such an address based authentication and
authorization scheme would fail.
B.6 Directional Properties
There two directional properties that need to be addressed by the
NATFW NSLP:
o Directionality of the data
o Directionality of NSLP signaling
Both properties are relevant to NATFW NSLP aware NATs and Firewalls.
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With regards to NSLP signaling directionality: As stated in the
previous sections, the authentication and authorization of NSLP
signaling messages received from hosts within the same trust domain
(typically from hosts located within the security perimeter delimited
by Firewalls) is normally simpler than received messages sent by
hosts located in different trust domains.
The way NSIS signaling messages enters the NSIS entity of a Firewall
(see Figure 2) might be important, because different policies might
apply for authentication and admission control.
Hosts deployed within the secured network perimeter delimited by a
Firewall, are protected from hosts deployed outside the secured
network perimeter, hence by nature the Firewall has more restrictions
on flows triggered from hosts deployed outside the security
perimeter.
B.7 Addressing
A more general problem of NATs is the addressing of the end-point.
NSIS signaling message have to be addressed to the other end host to
follow data packets subsequently sent. Therefore, a public IP
address of the receiver has to be known prior to sending an NSIS
message. When NSIS signaling messages contain IP addresses of the
sender and the receiver in the signaling message payloads, then an
NSIS entity must modify them. This is one of the cases, where a NSIS
aware NATs is also helpful for other types of signaling applications
e.g., QoS signaling.
B.8 NTLP/NSLP NAT Support
It must be possible for NSIS NATs along the path to change NTLP
and/or NSLP message payloads, which carry IP address and port
information. This functionality includes the support of providing
mid-session and mid-path modification of these payloads. As a
consequence these payloads must not be reordered, integrity protected
and/or encrypted in a non peer-to-peer fashion (e.g., end-to-middle,
end-to-end protection). Ideally these mutable payloads must be
marked (e.g., a protected flag) to assist NATs in their effort of
adjusting these payloads.
B.9 Combining Middlebox and QoS signaling
In many cases, middlebox and QoS signaling has to be combined at
least logically. Hence, it was suggested to combine them into a
single signaling message or to tie them together with the help of
some sort of data connection identifier, later on referred as Session
ID. This, however, has some disadvantages such as:
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- NAT/FW NSLP signaling affects a much small number of NSIS nodes
along the path (for example compared to the QoS signaling).
- NAT/FW signaling might show different signaling patterns (e.g.,
required end-to-middle communication).
- The refresh interval is likely to be different.
- The number of error cases increase as different signaling
applications are combined into a single message. The combination of
error cases has to be considered.
B.10 Inability to know the scenario
In Section 2 a number of different scenarios are presented. Data
receiver and sender may be located behind zero, one, or more
Firewalls and NATs. Depending on the scenario, different signaling
approaches have to be taken. For instance, data receiver with no
NAT and Firewall can receive any sort of data and signaling without
any further action. Data receivers behind a NAT must first obtain a
public IP address before any signaling can happen. The scenario
might even change over time with moving networks, ad-hoc networks or
with mobility.
NSIS signaling must assume the worst case and cannot put
responsibility to the user to know which scenario is currently
applicable. As a result, it might be necessary to perform a
"discovery" periodically such that the NSIS entity at the end host
has enough information to decide which scenario is currently
applicable. This additional messaging, which might not be necessary
in all cases, requires additional performance, bandwidth and adds
complexity. Additional, information by the user can provide
information to assist this "discovery" process, but cannot replace
it.
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Appendix C. Object ID allocation for testing
TBD.
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Appendix D. Acknowledgments
We would like to acknowledge: Vishal Sankhla and Joao Girao for their
input to this draft; and Reinaldo Penno for his comments on the
initial version of the document. Furthermore, we would like to
especially thank Elwyn Davies for his valuable help and input.
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