NSIS Working Group M. Stiemerling
Internet-Draft NEC
Expires: August 5, 2006 H. Tschofenig
Siemens
C. Aoun
ENST
February 1, 2006
NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
draft-ietf-nsis-nslp-natfw-09
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Copyright Notice
Copyright (C) The Internet Society (2006).
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 . . . . . . . . . . . . 19
3. Protocol Description . . . . . . . . . . . . . . . . . . . . 21
3.1 Policy Rules . . . . . . . . . . . . . . . . . . . . . . 21
3.2 Basic Protocol Overview . . . . . . . . . . . . . . . . 21
3.3 Basic Message Processing . . . . . . . . . . . . . . . . 25
3.4 Protocol Operations . . . . . . . . . . . . . . . . . . 25
3.4.1 Creating Sessions . . . . . . . . . . . . . . . . . 26
3.4.2 Reserving External Addresses . . . . . . . . . . . . 28
3.4.3 NATFW Session Refresh . . . . . . . . . . . . . . . 35
3.4.4 Deleting Sessions . . . . . . . . . . . . . . . . . 37
3.4.5 Reporting Asynchronous Events . . . . . . . . . . . 37
3.4.6 Tracing Signaling Sessions . . . . . . . . . . . . . 38
3.4.7 Proxy Mode for Data Receiver behind NAT . . . . . . 40
3.4.8 Proxy Mode for Data Sender behind Middleboxes . . . 43
3.4.9 Proxy Mode for Data Receiver behind Firewall . . . . 44
3.5 Calculation of Session Lifetime . . . . . . . . . . . . 47
3.6 Message Sequencing . . . . . . . . . . . . . . . . . . . 49
3.7 De-Multiplexing at NATs . . . . . . . . . . . . . . . . 50
3.8 Selecting Opportunistic Addresses for REA . . . . . . . 50
3.9 Session Ownership . . . . . . . . . . . . . . . . . . . 52
3.10 Authentication and Authorization . . . . . . . . . . . . 52
3.11 Reacting to Route Changes . . . . . . . . . . . . . . . 53
3.12 Updating Policy Rules . . . . . . . . . . . . . . . . . 53
4. NATFW NSLP Message Components . . . . . . . . . . . . . . . 55
4.1 NSLP Header . . . . . . . . . . . . . . . . . . . . . . 55
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4.2 NSLP Message Types . . . . . . . . . . . . . . . . . . . 55
4.3 NSLP Objects . . . . . . . . . . . . . . . . . . . . . . 56
4.3.1 Session Lifetime Object . . . . . . . . . . . . . . 57
4.3.2 External Address Object . . . . . . . . . . . . . . 57
4.3.3 Extended Flow Information Object . . . . . . . . . . 58
4.3.4 Response Code Object . . . . . . . . . . . . . . . . 59
4.3.5 Proxy Support Object . . . . . . . . . . . . . . . . 60
4.3.6 Nonce Object . . . . . . . . . . . . . . . . . . . . 60
4.3.7 Message Sequence Number Object . . . . . . . . . . . 61
4.3.8 Data Terminal Information Object . . . . . . . . . . 61
4.3.9 Trace Object . . . . . . . . . . . . . . . . . . . . 63
4.4 Message Formats . . . . . . . . . . . . . . . . . . . . 63
4.4.1 CREATE . . . . . . . . . . . . . . . . . . . . . . . 64
4.4.2 RESERVE-EXTERNAL-ADDRESS (REA) . . . . . . . . . . . 64
4.4.3 RESPONSE . . . . . . . . . . . . . . . . . . . . . . 65
4.4.4 NOTIFY . . . . . . . . . . . . . . . . . . . . . . . 65
4.4.5 REA-F . . . . . . . . . . . . . . . . . . . . . . . 66
4.4.6 TRACE . . . . . . . . . . . . . . . . . . . . . . . 66
5. NATFW NSLP NTLP Requirements . . . . . . . . . . . . . . . . 67
6. NSIS NAT and Firewall Transition Issues . . . . . . . . . . 68
7. Security Considerations . . . . . . . . . . . . . . . . . . 69
7.1 Trust Relationship and Authorization . . . . . . . . . . 69
7.1.1 Peer-to-Peer Trust Relationship . . . . . . . . . . 70
7.1.2 Intra-Domain Trust Relationship . . . . . . . . . . 70
7.1.3 End-to-Middle Trust Relationship . . . . . . . . . . 71
7.2 Security Threats and Requirements . . . . . . . . . . . 72
7.2.1 Attacks related to authentication and authorization 72
7.2.2 Denial-of-Service Attacks . . . . . . . . . . . . . 79
7.2.3 Man-in-the-Middle Attacks . . . . . . . . . . . . . 80
7.2.4 Message Modification by non-NSIS on-path node . . . 81
7.2.5 Message Modification by malicious NSIS node . . . . 81
7.2.6 Session Modification/Deletion . . . . . . . . . . . 82
7.2.7 Misuse of unreleased sessions . . . . . . . . . . . 85
7.2.8 Data Traffic Injection . . . . . . . . . . . . . . . 86
7.2.9 Eavesdropping and traffic analysis . . . . . . . . . 88
7.3 Security Framework for the NAT/Firewall NSLP . . . . . . 89
7.3.1 Security Protection between neighboring NATFW NSLP
Nodes . . . . . . . . . . . . . . . . . . . . . . . 89
7.3.2 Security Protection between non-neighboring NATFW
NSLP Nodes . . . . . . . . . . . . . . . . . . . . . 89
7.3.3 End-to-End Security . . . . . . . . . . . . . . . . 91
8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 92
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 93
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10. References . . . . . . . . . . . . . . . . . . . . . . . . 94
10.1 Normative References . . . . . . . . . . . . . . . . . . 94
10.2 Informative References . . . . . . . . . . . . . . . . . 94
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 97
A. Firewall and NAT Resources . . . . . . . . . . . . . . . . . 98
A.1 Wildcarding of Policy Rules . . . . . . . . . . . . . . 98
A.2 Mapping to Firewall Rules . . . . . . . . . . . . . . . 99
A.3 Mapping to NAT Bindings . . . . . . . . . . . . . . . . 99
A.4 Mapping for combined NAT and firewall . . . . . . . . . 99
A.5 NSLP Handling of Twice-NAT . . . . . . . . . . . . . . . 99
B. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 100
Intellectual Property and Copyright Statements . . . . . . . 101
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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 provide a virtual extension of the IP
address space. Both types of devices may be obstacles to some
applications, since they only allow traffic created by a limited set
of applications to traverse them, typically those that use protocols
with relatively predetermined and static properties (e.g., most HTTP
traffic, and other client/server applications). Other applications,
such as IP telephony and most other peer-to-peer applications, which
have more dynamic properties, create traffic that is unable to
traverse NATs and firewalls unassisted. In practice, the traffic of
many applications cannot traverse autonomous firewalls or NATs, even
when they have additional functionality which attempts to restore the
transparency of the network.
Several solutions to enable applications to traverse such entities
have been proposed and are currently in use. Typically, application
level gateways (ALG) have been integrated with the firewall or NAT to
configure the firewall or NAT dynamically. Another approach is
middlebox communication (MIDCOM, currently under standardization at
the IETF). In this approach, ALGs external to the firewall or NAT
configure the corresponding entity via the MIDCOM protocol [7].
Several other work-around solutions are available, including STUN
[26] and TURN [29]. 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 [13] is an example of a current QoS signaling protocol that is
path-coupled. [36] proposes the use of RSVP as firewall signaling
protocol but does not include NATs.
This memo defines a path-coupled signaling protocol for NAT and
firewall configuration within the framework of NSIS, called the NATFW
NSIS Signaling Layer Protocol (NSLP). The general requirements for
NSIS are defined in [5]. The general framework of NSIS is outlined
in [4]. 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 General
Internet Signaling Transport (GIST) [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
[6], 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.
Thereafter, the actual data flow can traverse all these configured
firewalls/NATs.
It is necessary to distinguish between two different basic scenarios
when operating the NATFW NSLP, independent of the type of middlebox
to be configured.
1. Both, data sender and data receiver, are NSIS NATFW NSLP aware.
This includes the cases where the data sender is logically
decomposed from the NSIS initiator or the data receiver logically
decomposed from the NSIS receiver, but both sides support NSIS.
This scenario assumes deployment of NSIS all over the Internet,
or at least at all NATs and firewalls.
2. Only one end host or region of the network is NSIS NATFW NSLP
aware, either data receiver or data sender.
NATFW NSLP provides two basic modes to cope with various possible
scenarios likely to be encountered before and after widespread
deployment of NSIS:
CREATE mode: The basic mode for configuring a path downstream from
a data sender
RESERVE-EXTERNAL-ADDRESS (REA) mode: Used to prime upstream NATs/
firewalls to expect downstream signaling and at NATs to pre-
allocate a public address.
Once there is full deployment of NSIS (in the sense that both end
hosts support NATFW NSLP signaling), the requisite NAT and firewall
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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
edge-NAT/edge-firewall 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 aware. In these cases the NATFW NSLP can utilize NSIS aware
middleboxes on the path between the sender and receiver to provide
proxy NATFW NSLP services ("proxy mode" 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 or 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. 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 [2].
This document uses a number of terms defined in [5]. 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" [28]. In the context of NSIS
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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
"deny" or action "allow".
o NSLP (rule) directive: Instruction to a NATFW NSLP node as to how
it should treat the associated policy rule. Directive 'reserve'
requests the middlebox to remember the rule and pre-allocate
addresses where necessary but not install the rule. Directive
'install' requests the middle to install an active rule.
o Firewall: A packet filtering device that matches packets against a
set of policy rules and applies the actions. In the context of
NSIS NATFW NSLP we refer to this device as a firewall.
o Network Address Translator: Network Address Translation is a
method by which IP addresses are mapped from one IP address realm
to another, in an attempt to provide transparent routing between
hosts (see [9]). Network Address Translators are devices that
perform this work.
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" [11]. 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 or signaling session: An application layer flow
of information for which some network control state information is
to be manipulated or monitored (as defined in [4]). 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.
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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" [9].
o Private/Local Network: "A private network is an address realm
independent of external network addresses. Private network may
also be referred alternately as Local Network. Transparent
routing between hosts in private realm and external realm is
facilitated by a NAT router" [9]. IP address space allocation for
private networks is recommended in [27]
o Public/Global IP address: An IP address located in the public
network according to Section 2.7 of [9].
o Private/Local IP address: An IP address located in the private
network according to Section 2.8 of [9].
o Opportunistic Address (OA) or Signaling Destination Address (SDA):
An IP address out of the public/global IP address range. The OA/
SDA may in certain circumstances be part of the private/local IP
address range.
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
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o Twice-NAT
o Multihomed NAT
For definitions and a detailed discussion about the characteristics
of each NAT type please see [9].
All types of middleboxes under consideration here use policy rules to
make a decision on data packet treatment. Policy rules consist of a
flow identifier which selects the packets to which the policy applies
and an associated action; data packets matching the flow identifier
are subjected to the policy rule action. A typical flow identifier
is the 5-tuple selector which matches the following fields of a
packet to configured values:
o Source and destination IP addresses
o Transport protocol number
o Transport source and destination port numbers
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, QoS gateways, are out of scope.
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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 examples are given in Appendix A
1.4 General Scenario for NATFW Traversal
The purpose of NSIS NATFW signaling is to enable communication
between endpoints across networks even in the presence of NAT and
firewall middleboxes that have not been specially engineered to
facilitate communication with the application protocols used. This
removes the need to create and maintain application layer gateways
for specific protocols that have been commonly used to provide
transparency in previous generations of NAT and firewall middleboxes.
It is assumed that these middleboxes will be statically configured in
such a way that NSIS NATFW signaling messages themselves are allowed
to reach the locally installed NATFW NSLP daemon. NSIS NATFW NSLP
signaling is used to dynamically install additional policy rules in
all NATFW middleboxes along the data path that will allow
transmission of the application data flow(s). Firewalls are
configured to forward data packets matching the policy rule provided
by the NSLP signaling. NATs are configured to translate data packets
matching the policy rule provided by the NSLP signaling. An
additional capability, that is an exception to the primary goal of
NSIS NATFW signaling, is that the NATFW nodes can request blocking of
particular data flows instead of enabling these flows at 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 (NATFW 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 control provisioning for middlebox traversal along the prospective
data path (e.g., via an API call). The NSIS entity in turn uses NSIS
NATFW NSLP signaling to establish policy rules along the data path,
allowing the data to travel from the sender to the receiver
unobstructed.
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Application Application Server (0, 1, or more) Application
+----+ +----+ +----+
| +------------------------+ +------------------------+ |
+-+--+ +----+ +-+--+
| |
| NSIS Entities NSIS Entities |
+-+--+ +----+ +-----+ +-+--+
| +--------+ +----------------------------+ +-----+ |
+-+--+ +-+--+ +--+--+ +-+--+
| | ------ | |
| | //// \\\\\ | |
+-+--+ +-+--+ |/ | +-+--+ +-+--+
| | | | | Internet | | | | |
| +--------+ +-----+ +----+ +-----+ |
+----+ +----+ |\ | +----+ +----+
\\\\ /////
sender NATFW (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. Using combined middleboxes typically
reduces the number of network elements needed.
NSIS initiators (NI) send NSIS NATFW NSLP signaling messages via the
regular data path to the NSIS responder (NR). On the data path,
NATFW NSLP signaling messages reach different NSIS nodes that
implement the NATFW NSLP. Each NATFW NSLP node processes the
signaling messages according to Section 3 and, if necessary, installs
policy rules for subsequent data packets.
Each of the following sub-sections introduces a different scenario
for a different set of middleboxes and their ordering within the
topology. It is assumed that each middlebox implements the NSIS
NATFW NSLP signaling protocol.
2.1 Firewall Traversal
This section describes a scenario with firewalls only; NATs are not
involved. Each end host is behind a firewall. The firewalls are
connected via the public Internet. Figure 2 shows the topology. The
part labeled "public" is the Internet connecting both firewalls.
+----+ //----\\ +----+
NI -----| FW |---| |------| FW |--- NR
+----+ \\----// +----+
private public private
FW: Firewall
NI: NSIS Initiator
NR: NSIS Responder
Figure 2: Firewall Traversal Scenario
Each firewall on the data path must provide traversal service for
NATFW NSLP in order to permit the NSIS message to reach the other end
host. All firewalls process NSIS signaling and establish appropriate
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policy rules, so that the required data packet flow can traverse
them.
There are several very different ways to place firewalls in a network
topology. To distinguish firewalls located at network borders, such
as administrative domains, from others located internally, the term
edge-firewall is used. A similar distinction can be made for NATs,
with an edge-NAT fulfilling the equivalent role.
2.2 NAT with two private Networks
Figure 3 shows a scenario with NATs at both ends of the network.
Therefore, each application instance, the NSIS initiator and the NSIS
responder, are behind NATs. The outermost NAT, known as the edge-
NAT, at each side is connected to the processing 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 3.4.2.1 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 [9] for a discussion of twice-NAT
functionality).
Sender and receiver are both within a single private address realm
but the two partitions potentially have overlapping IP address
ranges. Figure 6 shows the arrangement of NATs. 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 gateway,
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 [7]. NSIS
signaling is not required in the twice-NAT only case, since
middleboxes of the twice-NAT type are normally configured by other
means. Nevertheless, NSIS signaling might 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
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, [10]).
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.
+----+ +----+ //---\\ +----+ //---\\ +----+ +----+
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
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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.
Note that the current form of IPv4/v6 NAT known as the Network
Address Translator - Protocol Translator (NAT-PT) [10] is being
removed from the set of recommended mechanisms for general usage in
IPv4/IPv6 transitions. This scenario is therefore not expected to be
commonly seen.
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 3.4.2.1 for an expanded discussion of this topic with respect
to NATs.
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.
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This is a challenge for the REA-F mode of the NATFW NSLP where the
NSIS responder is located behind these firewalls within the private
network. The REA-F mode is used to block a particular data flow on
an upstream firewall. NSIS must route the REA-F mode message
upstream from NR to NI probably without knowing which path the data
traffic 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 3.4. 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 the NATFW 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 54. 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 as they have no interest in them.
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 (downstream)
---> : 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
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
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 the 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.
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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 (see
Figure 11), this immediately enables communication between both hosts
for scenarios with only firewalls on the data path or NATs on the
sender side. For scenarios with NATs on the receiver side certain
problems arise, as described in Section 2.
When the NR and the NI are located in different address realms and
the NR is located behind a NAT, the NI cannot signal to the NR
address directly. The DR 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.
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 3.4.2.1
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+-------+ +-------+ +-------+ +-------+
| DS/NI |<~~~| MB1/ |<~~~| NR | | DR |
| |--->| NF1 |--->| | | |
+-------+ +-------+ +-------+ +-------+
========================================>
Data Traffic Direction (downstream)
---> : 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 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 3.4.7
and Section 3.4.8. Figure 12 sketches the proxy mode operation for a
data sender behind a middlebox.
The basic functionality of the NATFW NSLP provides for opening
firewall pin holes and creating NAT bindings to enable data flows to
traverse these devices. Firewalls are normally expected to work on a
deny-all policy, meaning that traffic that does not explicitly match
any firewall filter rule will be blocked. Similarly, the normal
behavior of NATs is to block all traffic that does not match any
already configured/installed binding or session. However, some
scenarios require support of firewalls having allow-all policies,
allowing data traffic to traverse the firewall unless it is blocked
explicitly. Data receivers can utilize NATFW NSLP's REA-F 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 or forgotten after a certain period of time. To prevent
premature removal of state that is needed for ongoing communication,
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
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appropriate actions can be taken; this ability would allow debugging
and error recovery.
The next sections define the NATFW NSLP message types and formats,
protocol operations, and policy rule operations.
3.3 Basic Message Processing
All NATFW messages are subject to a basic message processing when
received at a node, independent of request or response messages.
Initially, the syntax of the NSLP message is checked and a RESPONSE
message with error code is generated if any problem is detected (for
instance, the message header could not be read). After passing this
check, the NATFW NSLP node MUST first perform the checks defined in
Section 3.9 and Section 3.10, if applicable, before any further
processing is executed.
This section should state this ugly sentence out of all protocol
operations sections on authentication and authorization. So get it
rid of it there. This section opens an interesting question: What
happens if a NSLP nodes receives a malformed response message?
3.4 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 MUST be transported via C-mode handling by the
NTLP and MUST NOT 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 CREATE NATFW NSLP sessions.
o RESERVE-EXTERNAL-ADDRESS (REA): a request message used for
reserving an external address and (if applicable) port number,
depending on the type of NAT. REA messages are used to change,
refresh, and delete REA NATFW NSLP sessions.
o REA-F: a request message used by data receivers located behind
firewalls to instruct upstream firewalls to allow or block
incoming data traffic. REA-F is also used to inform upstream
firewalls about incoming NATFW NSLP signaling messages. REA-F
messages are used to change, refresh, and delete REA-F NATFW NSLP
sessions.
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o TRACE: a request message to trace all involved NATFW NSLP nodes in
a particular signaling session.
o NOTIFY: an asynchronous message used by NATFW peers to alert
upstream and/or downstream NATFW peers about specific events
(especially failures).
o RESPONSE: used as a response to CREATE, REA, and REA-F messages
with Success or Error information.
3.4.1 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
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 (e.g., 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
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Since the CREATE message is used for several purposes within the
lifetime of a session, there are several processing rules for NATFW
peers 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 that creates 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 3.9 and
Section 3.10, 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) (the flow description
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
related to the same session ID. 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 (see Section 3.4.2.1), that matches the
destination address/port of the MRI provided by the NTLP. If
no reservation has been made in advance the NSLP SHOULD 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 (and if applicable port number) 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
is received at the private side, the NAT binding is allocated,
but not activated (see also Appendix A.3). 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
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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 3.4.7).
* 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
defined in Section 3.9 and Section 3.10, if applicable, have been
successful executed. Otherwise they SHOULD generate a RESPONSE
message with an error code. The calculation of session lifetime
applies here as well (see Section 3.5). RESPONSE messages are
sent back NSLP 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.
3.4.2 Reserving External Addresses
NSIS signaling is intended to travel end-to-end, even in the presence
of NATs and firewalls on-path. This works well in cases where the
data sender is itself behind a NAT or a firewall as described in
Section 3.4.1. For scenarios where the data receiver is located
behind a NAT or a firewall and it needs to receive data flows from
outside its own network (see Figure 5) the problem is more
troublesome. NSIS signaling, as well as subsequent data flows, are
directed to a particular destination IP address that must be known in
advance and reachable. Data receivers must tell the local NSIS
infrastructure (i.e., the upstream firewalls/NATs) about incoming
NATFW NSLP signaling and data flows before they can receive these
flows. It is necessary to discriminate between data receivers behind
NATs and behind firewalls for understanding the further NATFW
procedures. Data receivers that are just behind firewalls already
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have a public IP address and they need only to be reachable for NATFW
signaling. Data receivers behind NATs do not a have a public IP
address and are not reachable for NATFW signaling. We first discuss
the DR behind a NAT case.
3.4.2.1 Reserving External Addresses at NATs
Figure 14 describes a typical message sequence 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.
+-------------+ AS-Data Receiver Communication
+-------->| Application |<-----------------------------+
| | Server (AS) | |
| +-------------+ |
| 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|
+--------+ +---------+
IP(R): Private IP Address of Data Receiver
IP(R-NAT_B): Public IP Address for Data Receiver
provided by binding in NAT/NAPT B
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Figure 14: The Data Receiver behind NAT Problem
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). A primary goal of path-coupled
middlebox communication was to avoid end hosts having to discover and
use this type of topology knowledge. Data receivers behind a NAT
must first reserve an external IP address (and, in many cases, a port
number as well).
Public Internet Private Address
Space
Edge
NI(DS) NAT NAT NR(DR)
NR+ NI+
| | | |
| | | |
| | | |
| | REA[DTInfo] | REA[DTInfo] |
| |<----------------------|<----------------------|
| | | |
| |RESPONSE[Success/Error]|RESPONSE[Success/Error]|
| |---------------------->|---------------------->|
| | | |
| | | |
============================================================>
Data Traffic Direction
Figure 15: Reservation message flow for DR behind NAT
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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 outermost NAT in the network at the
boundary between the private address and the public address realm.
The Opportunistic Address is shown as NR+. REA messages for NATs
MUST be transported by using the loose-end message routing method
(LE-MRM) of the NTLP. Note that REA messages for firewalls (the
firewall-REA) must be transported by using the path-coupled
message routing method (PC-MRM), see Section 3.4.2.2.
The NI+ (could be on the data receiver DR or on any other host within
the private network) sends the REA message targeted to the
Opportunistic Address (OA defined earlier). The OA selection for
this message is discussed in Section 3.8. 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 a NSIS session) the signaling direction does not
matter 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.
The NI+ MUST include a 'data terminal information' object (DTInfo) in
the REA message and fill it in appropriately (see Section 4.3.8).
This information SHOULD include at least the 'dst port number' and
'protocol' fields, in the DTInfo object as these may be required by
en-route NATs, depending on the type of the NAT. These two fields
are most likely required by NAPTs to perform the address and port
translation. All other fields MAY be set by the NI+ to restrict the
set of possible NIs. An edge-NAT will use the information provided
within the DTInfo object ('src IPv4/v6 address', 'src port number',
'protocol') to only allow hosts falling within the specified range to
originate NATFW NSLP messages. The possible range is given by the
'src port number' field and the combination of 'dst prefix' and 'src
IP address' (see also Section 4.3.8).
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The REA signaling message creates a NSIS NATFW session at any
intermediate NSIS NATFW 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 addressing realm. Forwarding of
the REA message beyond this entity is not necessary, and MUST 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 in an 'external address' object (see
Section 4.3.2).
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 DTInfo object in the REA message.
When the data sender's IP address is not known, NI+s MUST NOT
include a DTInfo object.
o NSLP forwarder: NSLP forwarders receiving REA messages MUST first
perform the checks defined in Section 3.9 and Section 3.10, 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. If it is an edge-NAT, the NSLP
message is not forwarded any further and a RESPONSE message is
generated containing an 'external address' object (either IPv4
or IPv6 version, as appropriate) holding the translated address
port information in the binding reserved as a result of the REA
message. The RESPONSE message is sent back towards the NI+.
If it is not an edge-NAT, the NSLP message is forwarded further
using the translated IP address as signaling source address and
including the translated IP address/port in the MRI. The edge-
NAT MAY reject REA messages not carrying a DTInfo object or if
the address information within this object is invalid or is not
comliant with local policies (e.g., the information provided is
wildcarded but the edge-NAT requires full information about DS'
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IP address and port).
* Firewall: Firewalls MUST not change their configuration on
receiving a REA message. They MUST simply forward the message
and MUST keep NTLP state. Firewalls that are configured as
edge-firewalls SHOULD 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
any NR+, unless it also the edge-NAT. In any other case, it
SHOULD be discarded silently (EDITOR's note: It can be appropriate
the return an error message).
o Edge-NATs: This type of message should never be received by any
Edge-NAT and it SHOULD be discarded silently. (EDITOR's note: It
can be appropriate the return an error message, btw what means
drop silenty? What happens to the NTLP session?)
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 3.4.1 or REA in proxy mode Section 3.4.7
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 3.7.
3.4.2.2 Signaling Reservation for Firewalls
Data receivers behind firewalls can experience two basic policy
settings of their upstream firewalls. Either the firewall is set to
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a 'deny all by default' or 'allow all by default' policy. In the
'deny all' case, no traffic, neither plain data nor NATFW NSLP
signaling,is allowed to traverse the firewall. Vice versa in the
'allow all' case, all traffic is allowed to traverse. For 'deny all'
firewalls, data receivers must be able to notify upstream firewalls
about their willingness to receive NATFW NSLP signaling (this is
similar to REA for NATs). For 'allow all' firewalls, data receivers
must be able to notify upstream firewalls about unwanted traffic that
should be blocked. Data receivers use the RESERVE-EXTERNAL-ADDRESS
(REA) request message to either allow incoming NATFW NSLP signaling
messages or to block incoming data traffic, as shown in Figure 16.
See also the proxy mode of operation for REA-F in Section 3.4.9.
Public Internet Protected Address
Space
Edge
NI(DS) FW FW NR(DR)
NR+ NI+
| | | |
| | | |
| | | |
| | REA-F[NATFW_EFI] | REA-F[NATFW_EFI] |
| |<----------------------|<----------------------|
| | | |
| |RESPONSE[Success/Error]|RESPONSE[Success/Error]|
| |---------------------->|---------------------->|
| | | |
| | | |
============================================================>
Data Traffic Direction
Figure 16: Signaling reservation message flow
The processing of REA for firewalls (REA-F) messages is different for
every NSIS entity:
o NSLP initiator (NI+): NI+ MUST always direct REA-F message to the
address of DS. NI+ only generates REA-F messages and should never
receive them.
o NSLP forwarder: NSLP forwarders receiving REA-F messages MUST
first perform the checks defined in Section 3.9 and Section 3.10,
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 REA-F SHOULD response with error
RESPONSE indicating 'no edge-firewall'.
* Firewall: Non edge-firewalls keep session state and forward the
message. Edge-firewalls stop forwarding the check for
performing the checks defined in Section 3.9 and Section 3.10,
if applicable. If the message is accepted, load the specified
policy rule and generate RESPONSE messages back towards the DR.
* 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+): The NI+ is ready to received signaling or
data traffic when receiving a RESPONSE message.
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: This section does not explain the operation of the
NATFW_EFI object.
3.4.3 NATFW Session Refresh
NATFW NSLP sessions are maintained on a soft-state basis. After a
specified timeout, sessions and corresponding policy rules are
removed automatically by the middlebox, if they are not refreshed.
Soft-state is created by CREATE, REA, and REA-F and the maintenance
of this state must be done by these messages. State created by
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CREATE must be maintained by CREATE, state created by REA must be
maintained by REA, and state created by REA-F must be maintained by
REA-F. Refresh messages, either CREATE/REA/REA-F, 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 3.5 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 17: State Refresh Message Flow, CREATE as example
Processing of session refresh CREATE/REA/REA-F messages is different
for every NSIS node type:
o NSLP initiator: The NI can generate session refresh CREATE/REA/
REA-F messages before the session times out. The rate at which
the refresh CREATE/REA/REA-F messages are sent and their relation
to the session state lifetime are further discussed in
Section 3.5. The message routing information and the extended
flow information object MUST be set equal to the values of the
initial request message.
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o NSLP forwarder: NSLP forwarders receiving session refresh messages
MUST first perform the checks defined in Section 3.9 and
Section 3.10, 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/REA-F
message generate a RESPONSE message with response object set to
success. NRs MUST perform the checks defined in Section 3.9 and
Section 3.10, if applicable.
3.4.4 Deleting Sessions
NATFW NSLP sessions may be deleted at any time. NSLP initiators can
trigger this deletion by using a CREATE, REA, or REA-F messages with
a lifetime value set to 0, as shown in Figure 18.
NI Public Internet NAT Private address NR
| | space |
| CREATE[lifetime=0] | |
|----------------------------->| |
| | |
| | CREATE[lifetime=0] |
| |--------------------------->|
| | |
Figure 18: Delete message flow, CREATE as example
NSLP nodes receiving this message MUST first perform the checks
defined in Section 3.9 and Section 3.10, 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/REA-F 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.
3.4.5 Reporting Asynchronous Events
NATFW NSLP forwarders and NATFW NSLP responders must have the ability
to report asynchronous events to other NATFW NSLP nodes, especially
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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 3.11) are the only events that are defined, but
other events may be defined in later revisions of this memo.
NFs and NRs may generate NOTIFY messages upon asynchronous events,
with a response object indicating the reason of the event. NOTIFY
messages are sentEhop-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 analyze the notified event and behave
appropriately based on the event type. Section 4.3.4 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 3.9 and Section 3.10, if applicable,
and MUST only accept NOTIFY messages from downstream peers via an
already existing NTLP messaging association. After successfully
doing so, NFs analyze the notified event and behave based on the
notified events defined in Section 4.3.4. NFs SHOULD generate
NOTIFY messages upon asynchronous events and forward them upstream
towards the NI. NOTIFY messages are sent further hop-by-hop
upstream towards the NI.
o NSLP responder: NRs SHOULD generate NOTIFY messages upon
asynchronous events with 'response object(s)' code based on the
reported event(s). NRs receiving NOTIFY messages MUST ignore this
message and discard it. NOTIFY messages are sent hop-by-hop
upstream towards NI
EDITOR's note: The current semantics can result in NOTIFICATION
storms. There is a better semantics needed, how to avoid those
storms and how NOTIFY messages are handled along the path. Elwyn
noted: What to do if more than one node detects a failure condition
along the path. What happens than?
3.4.6 Tracing Signaling Sessions
The NATFW NSLP provides a diagnosis capability to session owners (the
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NI or NI+). Session owners are able to trace the NSIS nodes being
involved in a particular signaling session. The TRACE request
message is used to trace the involved NSIS nodes along the signaling
session and to return their identifiers.
NI Public Internet NAT Private address NR
| | space |
| TRACE | |
|----------------------------->| |
| | |
| | TRACE |
| |--------------------------->|
| | |
| | RESPONSE[IP(NR)] |
| |<---------------------------|
| RESPONSE[IP(NR),IP(NAT)] | |
|<-----------------------------| |
| | |
| | |
Figure 19: Example for tracing the signaling session path
The processing when receiving a TRACE message is the different for
each type of NATFW node:
o NSLP initiator: NI generates TRACE request messages.
o NSLP forwarder: NFs keep session state and forward the message.
o NSLP responder: NRs receiving a TRACE request message terminate
the forwarding and reply with a RESPONSE message including the
NATFW_TRACE object. The NATFW_TRACE object MAY be filled by the
NR with its IP address.
Processing of a RESPONSE message to a TRACE request message is
different for every NSIS node type:
o NSLP initiator: The NI terminates the forwarding and checks the
response message for further internal processing.
o NSLP forwarder: NFs MAY include their identifier in the
NATFW_TRACE object and increment the hop counter by one. This
memo defines IPv4 and IPv6 IP addresses as possible node
identifier. NFs MUST forward this type of RESPONSE.
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o NSLP responder: A NR should never see such a RESPONSE message. It
MUST discard the message and reply with an error message.
3.4.7 Proxy Mode for Data Receiver behind NAT
Comment from Elwyn: The next three sections would benefit from a an
introductory section . Some of the common text about NATFW_PROXY
object and the naming of the xx-PROXY messages, etc., could be
factored out, reducing the size of the text and making the whole
thing clearer. Also the sections 3.3.7-3.3.9 should be reordered
more logically.
Some migration scenarios need specialized support to cope with cases
whereonly 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 method described 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 3.4.2.1. 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 20,
unlike the standard REA message handling case, the edge-NAT is
triggered to send a CREATE message on a new reverse path which
traverse several firewalls or NATs. The new reverse path for CREATE
is necessary to handle routing asymmetries between the edge-NAT and
DR. This behavior requires an indication to the edge-NAT within the
REA message if either the standard behavior (as defined in
Section 3.4.2.1) is required or a CREATE message is required to be
sent by the edge-NAT. This indication is that the REA message
contains a NATFW_PROXY object. We distinguish a REA message
containing a NATFW_PROXY object by calling it a REA-PROXY message.
In addition when a CREATE message needs to be sent by the edge-NAT,
the REA message may include the data sender's address (DTInfo), if
available to the data receiver. Figure 20 shows this proxy mode REA
as REA-PROXY.
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DS Public Internet NAT Private address NR
No NI NF space NI+
NR+
| | REA-PROXY[(DTInfo)] |
| |<------------------------- |
| | RESPONSE[Error/Success] |
| | ---------------------- > |
| | CREATE |
| | ------------------------> |
| | RESPONSE[Error/Success] |
| | <---------------------- |
| | |
| | |
Figure 20: 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 DTInfo object in the
REA-PROXY request message. When the data sender's address is not
known, NI+'s MUST NOT include a DTInfo 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 3.9 and Section 3.10,
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
SHOULD 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. If 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 DTInfo object. The edge-NAT MAY
reject the REA-PROXY request if there is no DTInfo object or if
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the address information within DTInfo is not valid or too much
wildcarded. If accepted, a RESPONSE message is generated
containing an External Address Object (either IPv4 or IPv6
version, as appropriate) holding the translated address port
information in the binding reserved as a result of the REA
message. The RESPONSE message is sent back towards the NI+.
When the edge-NAT accepts, it generates a CREATE message as
defined in Section 3.4.1 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 (see below xref
target="proxy_sender"/>). 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.
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 section challenges the data receiver
because it must make a correct assumption about the data sender's
ability to use NSIS NATFW NSLP signaling. It is possible for the DR
to make the wrong assumption in two different ways:
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a) DS is NSIS unaware but 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 the NR's
responsibility to decide whether to teardown the REA-PROXY sessions
in the case where the data sender's side is NSIS aware but was
incorrectly assumed not to be so by the DR. It is RECOMMENDED that a
DR behind NATs uses the proxy mode of operation by default, unless
the DR knows that the DS is NSIS aware. The DR MAY cache information
about data senders which it has found to be NSIS aware in past
sessions.
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 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.
3.4.8 Proxy Mode for Data Sender behind Middleboxes
As with data receivers behind middleboxes in Section 3.4.7 data
senders behind middleboxes require proxy mode support. The issue
here is that there is no NSIS support at the data receiver's side
and, by default, there will be no response to CREATE 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 as a
result of GIST timeouts but there may be other triggers). This proxy
mode handles only data senders behind a middlebox; for receivers
behind a NAT see Section 3.4.7 and for receivers behind a firewall
see Section 3.4.9.
NIs being aware about a NSIS unaware DR, send a CREATE message
towards DR with a proxy support object (NATFW_PROXY). We distinguish
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a CREATE message containing a NATFW_PROXY object by calling it a
CREATE-PROXY message. 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 the outermost egde-NATs/
edge-firewalls.
DS Private Address FW Public Internet NR
NI Space NF no NR
| | |
| CREATE-PROXY | |
|------------------------------>| |
| | |
| RESPONSE[SUCCESS/ERROR] | |
|<------------------------------| |
| | |
Figure 21: Proxy Mode Create Message Flow
The processing of CREATE-PROXY messages and RESPONSE messages is
similar to Section 3.4.1, 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.
3.4.9 Proxy Mode for Data Receiver behind Firewall
Data receivers behind firewalls would like to use a similar sort of
proxy mode operation compared to those behind NATs. While finding an
upstream edge-NAT is quite easy (it is only required to find some
edge-NAT as the data traffic will be route-pinned to the NAT),
locating the appropriate edge-firewall is difficult. Where a data
receiver is located in a site network that is multihomed with several
independently firewalled connections to the public Internet, the
specific firewall through which the data traffic will be routed has
to be ascertained. With this knowledge, proxy mode support that is
similar to Section 3.3.7 can be used to install appropriate "allow"
rules in the firewall through which the data traffic will be routed.
Being able to identify the firewall through which data from a given
source address will be routed is also essential for implementing the
capability to install a blocking rule for incoming traffic in a
firewall which defaults to "allow all". In the first case the
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downstream data path must be fully enabled by signaling from the
edge-firewall towards the data receiver in case there are additional
firewalls along the path. This additional signaling is not needed in
the blocking case as the intention is prevent traffic entering the
site.
The REA-F (firewall-REA) is used to locate upstream firewalls and to
request installation of the appropriate 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 when this
is needed for an 'allow' rule. Provided the data sender's IP address
is known, a NR can signal towards the data sender's address as in the
standard REA-F message handling scenario for firewalls
Section 3.4.2.2. The message is forwarded until it reaches the edge-
firewall. As shown in Figure 22, the edge-firewall is triggered to
send a CREATE message on a new reverse path which traverses through
internal firewalls or NATs. The new reverse path for CREATE is
necessary to handle routing asymmetries between the edge-firewall and
DR. REA-F does not install any policy rule but the subsequent CREATE
message initiated by the edge-firewall does.
EDITOR's note: The above paragraph describes just the allow case.
The proxy thing is not needed if a 'deny' rule is requested.
DS Public Internet FW Private address NR
No NI NF space NI+
NR+
| | REA-F-PROXY |
| |<------------------------- |
| | RESPONSE[Error/Success] |
| | ---------------------- > |
| | CREATE |
| | ------------------------> |
| | RESPONSE[Error/Success] |
| | <---------------------- |
| | |
| | |
Figure 22: REA-F Triggering Sending of CREATE Message on Separate
Reverse Path
The processing of REA-F-PROXY messages is different for every NSIS
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entity:
o NSLP initiator (NI+): NI+ MUST always direct REA-F-PROXY message
to the address of DS. NI+ only generates REA-F messages and
should never receive them.
o NSLP forwarder: NSLP forwarders receiving REA-F messages MUST
first perform the checks defined in Section 3.9 and Section 3.10,
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 internal interface, NATs
allocated a public IP address and port and forward the message
further. Edge-NATs receiving REA-F-PROXY SHOULD response with
error RESPONSE indicating 'no edge-firewall'
* Firewall: Non edge-firewalls keep session state and forward the
message. Edge-firewalls stop forwarding the check for
performing the checks defined in Section 3.9 and Section 3.10,
if applicable. If the message is accepted, load the specified
policy rule and if the policy rule action is "allow", generate
CREATE messages back towards the DR as defined in
Section 3.4.1. In any case generate a RESPONSE message
indicating success or failure and send it back towarsd the NI+.
* 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 is different for every NSIS node
type:
o NSLP initiator (NI+): Upon receiving a RESPONSE message NI+
should await incoming corresponding CREATE messages if the
UCREATE-PROXY message was sent with an "allow" rule
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.
There is a possible race condition between the RESPONSE message to
the REA-F-PROXY and the CREATE message generated by the edge-
firewall. The CREATE message can arrive earlier than the RESPONSE
message. An NI+ MUST accept CREATE messages generated by the edge-
firewall even if the RESPONSE message to the REA-F-PROXY request was
not received.
3.5 Calculation of Session Lifetime
NATFW NSLP sessions, and the corresponding policy rules which may
have been installed, are maintained via soft-state mechanisms. 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,
sessions and policy rules MUST be removed automatically and resources
bound to them should be freed as well. Session lifetime is handled
at every NATFW NSLP node. The NSLP forwarders and NSLP responder are
not responsible for triggering lifetime extension refresh messages
(see Section 3.4.3): this is the task of the NSIS initiator.
The NSIS initiator MUST choose a session lifetime (expressed in
seconds) value before sending any message including the initial
message which creates the session (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 ([8]). 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 time
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.
The RSVP specification [13] provides an approriate algorithm for
calculating the session lifetime as well as means to avoid refresh
message synchronization between sessions. [13] recommends:
1. The refresh message timer to be randomly set to a value in the
range [0.5R, 1.5R].
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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.
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 23 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.
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+-------+ 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 23: Lifetime Calculation Example
3.6 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
time. Messages can be lost along the path or duplicated. So all
NATFW NSLP nodes should be able to identify either old messages that
have been received before (duplicated), or the case that messages
have been lost before (loss). For message replay protection it is
necessary to keep information about messages that have already been
received and requires every NATFW NSLP message to carry a message
sequence number (MSN), see also Section 4.3.7.
The MSN MUST be set by the NI and MUST NOT be set or modified by any
other node. The initial value for the MSN MUST be generated randomly
and MUST be unique only within the session for which it is used. The
NI MUST increment the MSN by one for every message sent. Once the
MSN has reached the maximum value, the next value it takes is zero.
EDITOR's note: Is it needed to apply this: All NATFW NSLP nodes MUST
use the algorithm defined in [3] to detect MSN wrap arounds.
NSIS forwarders and the responder store the MSN from the initial
CREATE/REA/UCREATE packet which creates the session as start value
for the session. NFs and NRs MUST include the received MSN value in
the corresponding RESPONSE message that they generate.
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 equal to or lower than the
stored MSN value. Such a received MSN value can indicate a
duplicated and delayed message or replayed message. If the received
MSN value is greater than the already stored MSN value, the NATFW
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NSLP MUST update its stored state accordingly, if permitted by all
security checks (see Section 3.9 and Section 3.10), and stores the
updated MSN value accordingly.
For, example, applying these semantics to a CREATE message exchange
mean that the first CREATE and 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.
3.7 De-Multiplexing at NATs
Section 3.4.2.1 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 3.4.1). The
information about the public IP address/port number can be
transmitted via an application level signaling protocol and/or third
party to the communication partner that would like to send data
toward the host behind the NAT. However, NSIS signaling flows are
sent towards the address of the NAT at which this particular IP
address and port number is allocated and not directly to the
allocated IP address and port number. The NATFW NSLP forwarder at
this NAT needs to know how the incoming NSLP 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.
3.8 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'. [17] shows
choices for SIP and this section provides some hints about choosing a
good destination IP address.
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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.
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
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receiver and the data sender might be different.
3.9 Session Ownership
Proof 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. Only request messages
showing a valid session ownership are processed further. Within the
NATFW NSLP, the NSIS initiator (the NI or the NI+) is the ultimate
session owner for all request messages. A proof of ownership MUST be
provided for any request message sent downstream or upstream. All
intermediate NATFW NSLP nodes MUST use this proof 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.
Generally, the path taken spans different administrative domains and
cannot rely on using a common authentication scheme. This
requirement demands a 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 PKI.
The NATFW NSLP relies on the session ID (SID) carried in the NTLP for
prove of session ownership. The session ID MUST be generated in a
random way. Messages for a particular session are handled by the
NTLP to the NATFW NSLP for further processing. Messages carrying a
different session ID not associated with any NATFW NSLP are subject
to the regular processing for new NATFW NSLP sessions.
3.10 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.
3.11 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 other than the NI or NI+ detecting a route change,
by means described in the NTLP specification or others, generates a
NOTIFY message indicating this change and sends this 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 (EDITOR's note:
what should be the default value for this time period?). It is
important to consider the transport limitations of NOTIFY messages as
mandate in Section 3.4.5. NOTIFY messages and therefore route change
notifications and only accept from downstream peers via existing NTLP
messaging associations. (EDITOR's note: double check this measure!
It might be appropriate to allow NOTIFY messages to be sent up- and
downstream and just to mandate the MA transport).
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 request
messages (see also Section 3.12), can easily use the session ID
(session ownership information, see also Section 3.9) to update the
session.
3.12 Updating Policy Rules
NSIS initiators can request an update of the installed/reserved
policy rules at any time within a signaling session. Updates to
policy rules can be required due to node mobility (NI is moving from
one IP address to another), route changes (this can result in a
different NAT mapping at a different NAT device), or the wish of the
NI to simply change the rule. NIs can update policy rules in
existing signaling sessions by sending an appropriate request message
(similar to Section 3.5) with a different message routing information
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(MRI) than installed before. This update request message is treated
with respect to authorization and authentication exactly as any
initial request. Therefore, any node along in the signaling session
can reject the update with an error response. A node rejecting the
update MUST reply with an error message indicating the error reason .
The request/response message processing and forwarding is executed as
defined in the particular sections. The local procedures on how to
update the MRI in the firewall/NAT is out of scope of this memo.
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4. NATFW NSLP Message Components
A NATFW NSLP message consists of a NSLP header and one or more
objects following the header. The NSLP header is 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 24.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSLP message type | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 24: 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 : REA-F
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o 0x0201 : RESPONSE
o 0x0301 : NOTIFY
4.3 NSLP Objects
NATFW NSLP objects use a common header format defined by Figure 25.
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 25: 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 A.2.1), i.e., the Object Type has not been
defined. NATFW NSLP uses a subset of the categories defined in GIST:
o AB=00 ("Mandatory"): If the object is not understood, the entire
message containing it must be rejected with an error 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 MUST NOT be used.
The following sections do not repeat the common NSLP object header,
they just state the type and the length.
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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 26: Lifetime object
4.3.2 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 27: External Address Object for IPv4 addresses
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Type: NATFW_EXT_IPv6
Length: 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port number | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IPv6 address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 28: 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.
4.3.3 Extended Flow Information Object
In general, flow information is kept in the message routing
information (MRI) of the NTLP. Nevertheless, some additional
information may be required for NSLP operations. The 'extended flow
information' object carries this additional information about the
policy rule's action for firewalls/NATs.
Type: NATFW_EFI
Length: 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rule action | sub_ports |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 29: Extended Flow Information
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This object has two fields, 'rule action' and 'sub_ports'. The 'rule
action' field has these meanings:
o Allow: A policy rule with this action allows data traffic to
traverse the middlebox and the NATFW NSLP MUST allow NSLP
signaling to be forwarded.
o Deny: A policy rule with this action blocks data traffic from
traversing the middlebox and the NATFW NSLP MUST NOT allow NSLP
signaling to be forwarded.
o Accept: A policy rule with this action blocks data traffic from
traversing the middlebox and the NATFW NSLP MUST allow NSLP
signaling to be forwarded.
The 'sub_ports' field contains the number of subsequent transport
layer ports. The default value of this field is 0, i.e., only the
port specified in the NTLP's MRM is used for the policy rule. A
value of 1 indicates that additionally to the port specified in the
NTLP's MRM (port1), a second port (port2) is used. This port's value
is calculated as: port2 = port1 + 1. Other values than 0 or 1 MUST
NOT be used in this field, but further version of this memo may allow
other values. This two subsequent port numbers feature can be used
by legacy voice over IP equipment. This legacy equipment assumes two
subsequent port numbers for its RTP/RTCP flows.
4.3.4 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 30: Response Code Object
TBD: Define response classes, success codes and error codes.
Possible error classes are:
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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
4.3.5 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
4.3.6 Nonce Object
Type: NATFW_NONCE
Length: 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 31: Nonce Object
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4.3.7 Message Sequence Number Object
This object carries the MSN value as described in Section 3.6.
Type: NATFW_RESP_MSN
Length: 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| message sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 32: Message Sequence Number Object
4.3.8 Data Terminal Information Object
The 'data terminal information' object carries additional information
possibly needed during REA operations. REA messages are transported
by the NTLP using the Loose-End message routing method (LE-MRM). The
LE-MRM contains only DR's IP address and a signaling destination
address (destination address). This destination address is used for
message routing only and is not necessarily reflecting the address of
the data sender. This object contains information about (if
applicable) DR's port number (the destination port number), DS' port
number (the source port number), the used transport protocol, the
prefix length of the IP address, and DS' IP address.
Type: NATFW_DSINFO_IPv4
Length: 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P| reserved | dest prefix | protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| dst port number | src port number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| src IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 33: Data Terminal IPv4 Address Object
Type: NATFW_DSINFO_IPv6
Length: 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P| reserved | dest prefix | protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| dst port number | src port number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ src IPv6 address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 34: Data Terminal IPv6 Address Object for IPv6 addresses
The fields MUST be interpreted according these rules:
o dest prefix: This parameter indicates the prefix length of the
'src IP address' in bits. For instance, a full IPv4 address
requires 'dest prefix' to be set to 32. A value of 0 indicates an
IP address wildcard.
o protocol: The IPv4 protocol field or the last IPv6 header. This
field MUST be interpreted if P=1, otherwise it MUST be set to 0
and MUST be ignored.
o dst port number: A value of 0 indicates a port wildcard, i.e., the
destination port number is not known. Any other value indicates
the destination port number.
o src port number: A value of 0 indicates a port wildcard, i.e., the
source port number is not known. Any other value indicates the
source port number.
o src IPv4/IPv6 address: The source IP address of the data sender.
This field MUST be set to zero if no IP address is provided, i.e.,
a complete wildcard is desired (see dest prefix field above).
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4.3.9 Trace Object
la
Type: NATFW_TRACE
Length: Variable
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| trace type | hop count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: IP address :
: ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 35: External Address Object for IPv4 addresses
The NATFW_TRACE object may contain zero or more identifiers. The
type of identifier is given by the value of 'trace type' field. This
memo is defining the values for the 'trace type' field: 0x01 for IPv4
addresses and 0x02 for IPv6 addresses. The 'hop count' field counts
the total number of visited NATFW NSLP nodes that are willing to
include their identifier in this object. Each node is appending its
identifier at the end of the object.
4.4 Message Formats
This section defines the content of each NATFW NSLP message type.
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.
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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, otherwise O]
The message routing information in the NTLP MUST be set to DS as
source address and DR as destination address. All other parameters
MUST be set according the required policy rule. CREATE messages MUST
be transported by using the path-coupled MRM.
4.4.2 RESERVE-EXTERNAL-ADDRESS (REA)
The RESERVE-EXTERNAL-ADDRESS (REA) request message is used to a)
reserve an external IP address/port at NATs, b) to notify firewalls
about NSIS capable DRs, or c) to block incoming data traffic at
upstream firewalls. All case can be used in proxy mode operations.
The REA for NATs (case a)) request message carries these objects:
o Lifetime object [M]
o Message sequence number object [M]
o Data terminal information object [M]
o Proxy support object [O]
o Nonce object [M if proxy support object is included, otherwise O]
The REA for NATs message needs special NTLP treatment. First of all,
REA for NATs 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 3.8). REA for NATs messages MUST be
transported by using the loose-end MRM defined in Section 5.8.2. of
[1].
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The REA for firewalls (case b) and c)) request message carries these
objects:
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, otherwise O]
The REA for firewalls message needs path-coupled MRM NTLP treatment
but with messages being sent upstream towards the DS.
4.4.3 RESPONSE
RESPONSE messages are responses to CREATE, REA, and REA-F 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 hop-by-hop.
EDITOR's note: Text says that this section is defining the behavior
depending on the response type.
4.4.4 NOTIFY
The NOTIFY messages is used to report asynchronous events happening
along the signaled path to other NATFW NSLP nodes.
The NOTIFY message carries this object:
o Response code object with NOTIFY code [M].
The NOTIFY message is forwarded upstream hop-by-hop using the
existing upstream node messaging association entry within the node's
Message Routing State table.
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4.4.5 REA-F
The REA-F request message is used to install policy rules at upstream
firewalls
The REA-F request message carries these objects:
o Lifetime object [M]
o Message sequence number object [M]
o Extended flow information object [M]
o Proxy support object [O]
REA-F messages MUST be sent by using the path-coupled MRM upstream
towards the data sender's address.
4.4.6 TRACE
The TRACEF request message is used to trace the involved NATFW NSLP
nodes along a signal session.
The TRACE request message carries these objects:
o Message sequence number object [M]
TRACE request messages are sent path-coupled (PC-MRM).
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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 3.4.7 and [15].
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 [8]
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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 [15]). 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 [8]
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.
Different types of trust relationships may affect different
categories of middleboxes. As explained in [23], 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
interesting to mention that QoS signaling relies on peer-to-peer
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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 36 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 36: 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
instructions to the department administrators. In such a scenario, a
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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 37
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 37: 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
required authorization before the signaling message exchange is
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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 38 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 38: 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 39: 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 40 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 40: 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 42 illustrates such a scenario:
+------------------------------+
| |
| +------+ CREATE |
| | NI_1 | ------\ +-----+ CREATE +-----+
| +------+ \------> | NAT |-------->| MB |
| +-----+ +-----+
| +------+ |
| | NI_2 | |
| +------+ |
+------------------------------+
Figure 42: 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 44 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 44: 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 44. Many operators might not allow NSIS
signaling message to traverse the firewall in Figure 44 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 [8] 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 [8].
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 46 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 46: 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 46 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 47. 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 [25].
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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 47: 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 48: 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 48 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 49
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 49: 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 50. 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 51. 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 50: Before mobility
Public Internet
+--------------------------+
| |
+-------+ +---+-----+ +-------+ |
| | | | | | |
| N1 |==============>======| FW |====>===| N3 | |
| | Data Traffic | | | | |
+-------+ +---+-----+ +-------+ |
| |
+--------------------------+
Figure 51: 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 52. 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 52: 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 [25] 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 GIST 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) [19] 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 [31] and RFC 3521 [32]. More recent work on authorization token
alike mechanisms is Security Assertion Markup Language (SAML). For
details about SAML see [33], [34] and [35]. Figure 53 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 53: Authorization Token Usage
Threats against the usage of authorization tokens have been mentioned
in [8] 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 [22] 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
[22], migration (i.e., traversal of NSIS unaware NATs) [15], intra-
realm signaling [16], and inter-working with SIP [17]. 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 54 sketches the
overview
+------------------+
|Security Policies |
| Server |
+--------^---------+
|
+--------------------------------|----------------------+
| +---------+ +-----------V----+ +-------+|
| |firewall |<-----> | |<------>| NAT ||
| |Engine | | Security policy| | Engine||
| +----^----+ | Table/Cache | +-^-----+|
| | | ^ | | |
| | +---- --------|--+ | |
| +--|---------------------------|-------------|--+ |
| | V NATFW NSLP V V | |
| | | |
| +-----------------------------------------------+ |
| +--------------------------------------------------+|
| | GIST ||
| | ||
| +--------------------------------------------------+|
| +---------+ +-------+ +------+ +-------+ +------+|
| | TCP | | UDP | | DCCP | | SCTP | | ICMP ||
| +---------+ +-------+ +------+ +-------+ +------+|
| +-----------------------------+ +--------------------|
| | IPv4 | | IPv6 |
| +-----------------------------+ +--------------------|
+-------------------------------------------------------+
Figure 54: 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, "GIST: General Internet
Signaling Transport", draft-ietf-nsis-ntlp-08 (work in
progress), September 2005.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
August 1996.
10.2 Informative References
[4] Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
Bosch, "Next Steps in Signaling (NSIS): Framework", RFC 4080,
June 2005.
[5] Brunner, M., "Requirements for Signaling Protocols", RFC 3726,
April 2004.
[6] Bosch, S., "NSLP for Quality-of-Service signalling",
draft-ietf-nsis-qos-nslp-08 (work in progress), October 2005.
[7] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A.
Rayhan, "Middlebox communication architecture and framework",
RFC 3303, August 2002.
[8] Tschofenig, H. and D. Kroeselberg, "Security Threats for NSIS",
draft-ietf-nsis-threats-06 (work in progress), October 2004.
[9] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999.
[10] Tsirtsis, G. and P. Srisuresh, "Network Address Translation -
Protocol Translation (NAT-PT)", RFC 2766, February 2000.
[11] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues",
RFC 3234, February 2002.
[12] Srisuresh, P., Tsirtsis, G., Akkiraju, P., and A. Heffernan,
"DNS extensions to Network Address Translators (DNS_ALG)",
RFC 2694, September 1999.
[13] Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
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Specification", RFC 2205, September 1997.
[14] Yadav, S., Yavatkar, R., Pabbati, R., Ford, P., Moore, T.,
Herzog, S., and R. Hess, "Identity Representation for RSVP",
RFC 3182, October 2001.
[15] 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.
[16] Aoun, C., "NATFirewall NSLP Intra-realm considerations",
draft-aoun-nsis-nslp-natfw-intrarealm-01 (work in progress),
July 2004.
[17] Martin, M., "SIP NSIS Interactions for NAT/Firewall Traversal",
draft-martin-nsis-nslp-natfw-sip-01 (work in progress),
July 2004.
[18] Tschofenig, H., "Extended QoS Authorization for the QoS NSLP",
draft-tschofenig-nsis-qos-ext-authz-00 (work in progress),
July 2004.
[19] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369,
August 2002.
[20] 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.
[21] Ohba, Y., "Problem Statement and Usage Scenarios for PANA",
draft-ietf-pana-usage-scenarios-06 (work in progress),
April 2003.
[22] Tschofenig, H., "Path-coupled NAT/firewall Signaling Security
Problems",
DRAFT draft-tschofenig-nsis-natfw-security-problems-00.txt,
July 2004.
[23] Tschofenig, H., Buechli, M., Van den Bosch, S., and H.
Schulzrinne, "NSIS Authentication, Authorization and Accounting
Issues", March 2003.
[24] 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.
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[25] Tschofenig, H., "Security Implications of the Session
Identifier", draft-tschofenig-nsis-sid-00 (work in progress),
June 2003.
[26] 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.
[27] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E.
Lear, "Address Allocation for Private Internets", BCP 5,
RFC 1918, February 1996.
[28] 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.
[29] Rosenberg, J., "Traversal Using Relay NAT (TURN)",
draft-rosenberg-midcom-turn-08 (work in progress),
September 2005.
[30] Tschofenig, H., "Using SAML for SIP",
draft-tschofenig-sip-saml-04 (work in progress), July 2005.
[31] Hamer, L-N., Gage, B., Kosinski, B., and H. Shieh, "Session
Authorization Policy Element", RFC 3520, April 2003.
[32] Hamer, L-N., Gage, B., and H. Shieh, "Framework for Session
Set-up with Media Authorization", RFC 3521, April 2003.
[33] Maler, E., Philpott, R., and P. Mishra, "Bindings and Profiles
for the OASIS Security Assertion Markup Language (SAML) V1.1",
September 2003.
[34] Maler, E., Philpott, R., and P. Mishra, "Assertions and
Protocol for the OASIS Security Assertion Markup Language
(SAML) V1.1", September 2003.
[35] Maler, E. and J. Hughes, "Technical Overview of the OASIS
Security Assertion Markup Language (SAML) V1.1", March 2004.
[36] 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, REA-F).
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. 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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