NSIS Working Group M. Stiemerling
Internet-Draft NEC
Expires: September 7, 2006 H. Tschofenig
Siemens
C. Aoun
ENST
E. Davies
Folly Consulting
March 6, 2006
NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
draft-ietf-nsis-nslp-natfw-10
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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 (NATs) and firewalls. This NSLP allows
hosts to signal on the data path for NATs and firewalls to be
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configured according to the needs of the application data flows. It
enables hosts behind NATs to obtain a public reachable address and
hosts behind firewalls to receive data traffic. The overall
architecture is given by the framework and requirements defined by
the Next Steps in Signaling (NSIS) working group. The network
scenarios, the protocol itself, and examples for path-coupled
signaling are given in this memo.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Terminology and Abbreviations . . . . . . . . . . . . . . 7
1.2 Middleboxes . . . . . . . . . . . . . . . . . . . . . . . 9
1.3 General Scenario for NATFW Traversal . . . . . . . . . . . 11
2. Network Deployment Scenarios using the NATFW NSLP . . . . . 13
2.1 Firewall Traversal . . . . . . . . . . . . . . . . . . . . 13
2.2 NAT with two private Networks . . . . . . . . . . . . . . 14
2.3 NAT with Private Network on Sender Side . . . . . . . . . 15
2.4 NAT with Private Network on Receiver Side Scenario . . . . 15
2.5 Both End Hosts behind twice-NATs . . . . . . . . . . . . . 16
2.6 Both End Hosts Behind Same NAT . . . . . . . . . . . . . . 17
2.7 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
3.2.1 Message Types . . . . . . . . . . . . . . . . . . . . 25
3.2.2 Classification of RESPONSE Messages . . . . . . . . . 26
3.2.3 NATFW NSLP Signaling Sessions . . . . . . . . . . . . 26
3.3 Basic Message Processing . . . . . . . . . . . . . . . . . 27
3.4 Calculation of Session Lifetime . . . . . . . . . . . . . 28
3.5 Message Sequencing . . . . . . . . . . . . . . . . . . . . 30
3.6 Session Ownership . . . . . . . . . . . . . . . . . . . . 30
3.7 Authentication, Authorization, and Policy Decisions . . . 31
3.8 Protocol Operations . . . . . . . . . . . . . . . . . . . 32
3.8.1 Creating Sessions . . . . . . . . . . . . . . . . . . 32
3.8.2 Reserving External Addresses . . . . . . . . . . . . . 35
3.8.3 NATFW Session Refresh . . . . . . . . . . . . . . . . 41
3.8.4 Deleting Sessions . . . . . . . . . . . . . . . . . . 42
3.8.5 Reporting Asynchronous Events . . . . . . . . . . . . 43
3.8.6 Tracing Signaling Sessions . . . . . . . . . . . . . . 45
3.8.7 Proxy Mode of Operation . . . . . . . . . . . . . . . 46
3.9 De-Multiplexing at NATs . . . . . . . . . . . . . . . . . 49
3.10 Reacting to Route Changes . . . . . . . . . . . . . . . 51
3.11 Updating Policy Rules . . . . . . . . . . . . . . . . . 52
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4. NATFW NSLP Message Components . . . . . . . . . . . . . . . 53
4.1 NSLP Header . . . . . . . . . . . . . . . . . . . . . . . 53
4.2 NSLP Objects . . . . . . . . . . . . . . . . . . . . . . . 54
4.2.1 Session Lifetime Object . . . . . . . . . . . . . . . 55
4.2.2 External Address Object . . . . . . . . . . . . . . . 55
4.2.3 Extended Flow Information Object . . . . . . . . . . . 56
4.2.4 Information Code Object . . . . . . . . . . . . . . . 57
4.2.5 Nonce Object . . . . . . . . . . . . . . . . . . . . . 60
4.2.6 Message Sequence Number Object . . . . . . . . . . . . 60
4.2.7 Data Terminal Information Object . . . . . . . . . . . 61
4.2.8 Trace Object . . . . . . . . . . . . . . . . . . . . . 62
4.2.9 NI Credential Object . . . . . . . . . . . . . . . . . 63
4.2.10 ICMP Types Object . . . . . . . . . . . . . . . . . 64
4.3 Message Formats . . . . . . . . . . . . . . . . . . . . . 64
4.3.1 CREATE . . . . . . . . . . . . . . . . . . . . . . . . 65
4.3.2 RESERVE-EXTERNAL-ADDRESS (REA) . . . . . . . . . . . . 66
4.3.3 RESPONSE . . . . . . . . . . . . . . . . . . . . . . . 66
4.3.4 NOTIFY . . . . . . . . . . . . . . . . . . . . . . . . 67
4.3.5 TRACE . . . . . . . . . . . . . . . . . . . . . . . . 67
5. Security Considerations . . . . . . . . . . . . . . . . . . 68
5.1 Authorization Framework . . . . . . . . . . . . . . . . . 68
5.2 Peer-to-Peer Relationship . . . . . . . . . . . . . . . . 68
5.3 Intra-Domain Relationship . . . . . . . . . . . . . . . . 69
5.4 End-to-Middle Relationship . . . . . . . . . . . . . . . . 70
5.5 Security Threats and Requirements . . . . . . . . . . . . 71
5.5.1 Data Sender (DS) behind a firewall . . . . . . . . . . 71
5.5.2 Data Sender (DS) behind a NAT . . . . . . . . . . . . 72
5.5.3 Data Receiver (DR) behind a firewall . . . . . . . . . 72
5.5.4 Data Receiver (DR) behind a NAT . . . . . . . . . . . 74
5.5.5 NSLP Message Injection . . . . . . . . . . . . . . . . 75
5.6 Denial-of-Service Attacks . . . . . . . . . . . . . . . . 76
5.6.1 Flooding with CREATE messages from outside . . . . . . 76
5.6.2 Flooding with REA messages from inside . . . . . . . . 77
5.7 Man-in-the-Middle Attacks . . . . . . . . . . . . . . . . 77
5.8 Message Modification by non-NSIS on-path node . . . . . . 78
5.9 Message Modification by malicious NSIS node . . . . . . . 78
5.10 Session Modification/Deletion . . . . . . . . . . . . . 79
5.10.1 Misuse of mobility in NAT handling . . . . . . . . . 79
5.11 Misuse of unreleased sessions . . . . . . . . . . . . . 81
5.12 Data Traffic Injection . . . . . . . . . . . . . . . . . 82
5.13 Eavesdropping and Traffic Analysis . . . . . . . . . . . 84
5.14 Security Framework for the NAT/Firewall NSLP . . . . . . 85
5.14.1 Security Protection between neighboring NATFW
NSLP Nodes . . . . . . . . . . . . . . . . . . . . . 85
5.14.2 Security Protection between non-neighboring NATFW
NSLP Nodes . . . . . . . . . . . . . . . . . . . . . 85
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6. IAB Considerations on UNSAF . . . . . . . . . . . . . . . . 88
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 89
8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 90
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 91
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 92
10.1 Normative References . . . . . . . . . . . . . . . . . . 92
10.2 Informative References . . . . . . . . . . . . . . . . . 92
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 94
A. Selecting Signaling Destination Addresses for REA . . . . . 95
B. Applicability Statement on Data Receivers behind Firewalls . 97
C. Firewall and NAT Resources . . . . . . . . . . . . . . . . . 98
C.1 Wildcarding of Policy Rules . . . . . . . . . . . . . . . 98
C.2 Mapping to Firewall Rules . . . . . . . . . . . . . . . . 98
C.3 Mapping to NAT Bindings . . . . . . . . . . . . . . . . . 99
C.4 NSLP Handling of Twice-NAT . . . . . . . . . . . . . . . . 99
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 internal networks and the
Internet, whereas NATs provide a virtual extension of the IP address
space. Both types of devices may be obstacles to some applications,
since they only allow traffic created by a limited set of
applications to traverse them, typically those that use protocols
with relatively predetermined and static properties (e.g., most HTTP
traffic, and other client/server applications). Other applications,
such as IP telephony and most other peer-to-peer applications, which
have more dynamic properties, create traffic that is unable to
traverse NATs and firewalls unassisted. In practice, the traffic of
many applications cannot traverse autonomous firewalls or NATs, even
when they have additional functionality which attempts to restore the
transparency of the network.
Several solutions to enable applications to traverse such entities
have been proposed and are currently in use. Typically, application
level gateways (ALG) have been integrated with the firewall or NAT to
configure the firewall or NAT dynamically. Another approach is
middlebox communication (MIDCOM). In this approach, ALGs external to
the firewall or NAT configure the corresponding entity via the MIDCOM
protocol [7]. Several other work-around solutions are available,
such as STUN [19]. However, all of these approaches introduce other
problems that are generally hard to solve, such as dependencies on
the type of NAT implementation (full-cone, symmetric, etc), or
dependencies on certain network topologies.
NAT and firewall (NATFW) signaling shares a property with Quality of
Service (QoS) signaling. The signaling of both must reach any device
on the data path that is involved in, respectively, NATFW or QoS
treatment of data packets. This means, that for both, NATFW and QoS,
it is convenient if signaling travels path-coupled, meaning that the
signaling messages follow exactly the same path that the data packets
take. RSVP [13] is an example of a current QoS signaling protocol
that is path-coupled. [27] proposes the use of RSVP as firewall
signaling protocol but does not include NATs.
This memo defines a path-coupled signaling protocol for NAT and
firewall configuration within the framework of NSIS, called the NATFW
NSIS Signaling Layer Protocol (NSLP). The general requirements for
NSIS are defined in [5] and the general framework of NSIS is outlined
in [4]. It introduces the split between an NSIS transport layer and
an NSIS signaling layer. The transport of NSLP messages is handled
by an NSIS Network Transport Layer Protocol (NTLP, with General
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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].
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
rules with an action that allows the data flow packets to be
forwarded and creating NAT bindings. Blocking of data flows requires
the loading of firewalls rules with an action that will deny
forwarding of the data flow packets. A simplified example for
enabling data flows: A source host sends a NATFW NSLP signaling
message towards its data destination. This message follows the data
path. Every NATFW NSLP-enabled NAT/firewall along the data path
intercepts these messages, processes them, and configures itself
accordingly. Thereafter, the actual data flow can traverse all these
configured firewalls/NATs.
It is necessary to distinguish between two different basic scenarios
when operating the NATFW NSLP, independent of the type of 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. This scenario is referred
as to end-to-end mode operation and is used as base assumption if
not otherwise noted.
2. Only one end host or region of the network is NSIS NATFW NSLP
aware, either data receiver or data sender. This scenario is
referred to as proxy mode operation.
NATFW NSLP provides two basic signaling modes which are sufficient to
cope with the 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 to a data receiver.
RESERVE-EXTERNAL-ADDRESS (REA) mode: Used to locate upstream NATs/
firewalls and prime them to expect downstream signaling and at
NATs to pre-allocate a public address. This is used for data
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receivers behind these devices to enable their reachability.
Once there is full deployment of NSIS (i.e., end-to-end mode
operations are possible), the requisite NAT and firewall state can be
created using only CREATE mode. However, if the data receiver
resides in a public addressing realm. 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, a combination of RESERVE-EXTERNAL-ADDRESS and
CREATE modes is used.
During the introduction of NSIS, it is likely that one or other of
the data sender and receiver will not be NSIS aware. In these cases,
the NATFW NSLP can utilize NSIS aware middleboxes on the path between
the data sender and data receiver to provide proxy NATFW NSLP
services (i.e., proxy mode operation). Typically, these boxes will
be at the boundaries of the realms in which the end hosts are
located.
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.
This state is managed using a soft-state mechanism, i.e., it expires
unless it is refreshed from time to time. The NAT bindings and
firewall rules being installed during the state setup are bound to
the particular signaling session. However, the exact local
implementation of the NAT bindings and firewall rules are NAT/
firewall specific.
This memo is structured as follows. Section 2 describes the network
environment for NATFW NSLP signaling. Section 3 defines the NATFW
signaling protocol and Section 4 defines the message components and
the overall messages used in the protocol. The remaining parts of
the main body of the document, covers security considerations
Section 5, IAB considerations on UNilateral Self-Address Fixing
(UNSAF) [15] in Section 6 and IANA considerations in Section 7.
Please note that readers familiar with firewalls and NATs and their
possible location within networks can safely skip Section 2.
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] and [4]. The
following additional terms are used:
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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" [20]. In the context of NSIS
NATFW NSLP, the conditions are the specification of a set of
packets to which the rule is applied. The set of actions always
contains just a single element per rule, and is limited to either
action "deny" or action "allow".
o Reserved policy rule: A policy rule stored at NATs or firewalls
for activation by a later, different signaling exchange. This
type of policy rule is kept in the NATFW NSLP and is not loaded
into the firewall or NAT engine, i.e., it does not affect the data
flow handling.
o Installed policy rule: A policy rule in operation at NATs or
firewalls. This type of rule is kept in the NATFW NSLP and is
loaded into the firewall or NAT engine, i.e., it is affecting the
data flow.
o Remembered policy rule: A policy rule stored at NATs and firewalls
for immediate use, as soon as the signaling exchange is
successfully completed.
o Firewall: A packet filtering device that matches packets against a
set of policy rules and applies the actions. In the context of
NSIS NATFW NSLP we refer to this device as a firewall.
o Network Address Translator: Network Address Translation is a
method by which IP addresses are mapped from one IP address realm
to another, in an attempt to provide transparent routing between
hosts (see [9]). Network Address Translators are devices that
perform this work by modifying packets passing through them.
o Middlebox: "A middlebox is defined as any intermediate device
performing functions other than the normal, standard functions of
an IP router on the datagram path between a source host and a
destination host" [11]. In the context of this document, the term
middlebox refers to firewalls and NATs only. Other types of
middlebox are outside of the scope of this document.
o Data Receiver (DR): The node in the network that is receiving the
data packets of a flow.
o Data Sender (DS): The node in the network that is sending the data
packets of a flow.
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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 with a globally routable IP
address which is reachable from the public Internet.
o Edge-firewall: An edge-firewall is a firewall device that is
located on the border line of an administrative domain.
o Public Network: "A Global or Public Network is an address realm
with unique network addresses assigned by Internet Assigned
Numbers Authority (IANA) or an equivalent address registry. This
network is also referred as external network during NAT
discussions" [9].
o Private/Local Network: "A private network is an address realm
independent of external network addresses. Private network may
also be referred alternately as Local Network. Transparent
routing between hosts in private realm and external realm is
facilitated by a NAT router" [9].
o Public/Global IP address: An IP address located in the public
network according to Section 2.7 of [9].
o Private/Local IP address: An IP address located in the private
network according to Section 2.8 of [9].
o Signaling Destination Address (SDA): An IP address generally taken
from the public/global IP address range, although, the SDA may in
certain circumstances be part of the private/local IP address
range. This address is used in REA signaling message exchanges,
if the data receiver's IP address is unknown.
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
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set of filter rules, and NATs that translate packet addresses from
one address realm to another address realm. Other categories of
middleboxes, such as QoS traffic shapers, are out of scope of this
memo.
The term NAT used in this document is a placeholder for a range of
different NAT flavors. We consider the following types of NATs:
o Traditional NAT (basic NAT and NAPT)
o Bi-directional NAT
o Twice-NAT
o Multihomed NAT
For definitions and a detailed discussion about the characteristics
of each NAT type please see [9].
All types of middleboxes under consideration here, use policy rules
to make a decision on data packet treatment. Policy rules consist of
a flow identifier which selects the packets to which the policy
applies and an associated action; data packets matching the flow
identifier are subjected to the policy rule action. A typical flow
identifier is the 5-tuple selector which matches the following fields
of a packet to configured values:
o Source and destination IP addresses
o Transport protocol number
o Transport source and destination port numbers
Actions for firewalls are usually one or more of:
o Allow: forward data packet
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.
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It should be noted that a middlebox may contain two logical
representations of the policy rule. The policy rule has a
representation within the NATFW NSLP, comprising the message routing
information (MRI) of the NTLP and NSLP information (such as the rule
action). The other representation is the implementation of the NATFW
NSLP policy rule within the NAT and firewall engine of the particular
device. Refer to Appendix C for further details.
1.3 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 of NSIS with NATs and/or Firewalls
For end-to-end NATFW signaling, it is necessary that each firewall
and each NAT along the path between the data sender and the data
receiver implements the NSIS NATFW NSLP. There might be several NATs
and FWs in various possible combinations on a path between two hosts.
Section 2 presents a number of likely scenarios with different
combinations of NATs and firewalls.
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2. Network Deployment Scenarios using the NATFW NSLP
This section introduces several scenarios for middlebox placement
within IP networks. Middleboxes are typically found at various
different locations, including at Enterprise network borders, within
enterprise networks, as mobile phone network gateways, etc. Usually,
middleboxes are placed more towards the edge of networks than in
network cores. Firewalls and NATs may be found at these locations
either alone, or they may be combined; other categories of
middleboxes may also be found at such locations, possibly combined
with the NATs and/or firewalls. Using combined middleboxes typically
reduces the number of network elements needed.
NSIS initiators (NI) send NSIS NATFW NSLP signaling messages via the
regular data path to the NSIS responder (NR). On the data path,
NATFW NSLP signaling messages reach different NSIS nodes that
implement the NATFW NSLP. Each NATFW NSLP node processes the
signaling messages according to Section 3 and, if necessary, installs
policy rules for subsequent data packets.
Each of the following sub-sections introduces a different scenario
for a different set of middleboxes and their ordering within the
topology. It is assumed that each middlebox implements the NSIS
NATFW NSLP signaling protocol.
2.1 Firewall Traversal
This section describes a scenario with firewalls only; NATs are not
involved. Each end host is behind a firewall. The firewalls are
connected via the public Internet. Figure 2 shows the topology. The
part labeled "public" is the Internet connecting both firewalls.
+----+ //----\\ +----+
NI -----| FW |---| |------| FW |--- NR
+----+ \\----// +----+
private public private
FW: Firewall
NI: NSIS Initiator
NR: NSIS Responder
Figure 2: Firewall Traversal Scenario
Each firewall on the data path must provide traversal service for
NATFW NSLP in order to permit the NSIS message to reach the other end
host. All firewalls process NSIS signaling and establish appropriate
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policy rules, so that the required data packet flow can traverse
them.
There are several very different ways to place firewalls in a network
topology. To distinguish firewalls located at network borders, such
as administrative domains, from others located internally, the term
edge-firewall is used. A similar distinction can be made for NATs,
with an edge-NAT fulfilling the equivalent role.
2.2 NAT with two private Networks
Figure 3 shows a scenario with NATs at both ends of the network.
Therefore, each application instance, the NSIS initiator and the NSIS
responder, are behind NATs. The outermost NAT, known as the edge-
NAT, at each side is connected to the public Internet. The NATs are
generically labeled as MB (for middlebox), since those devices
certainly implement NAT functionality, but can implement firewall
functionality as well.
Only two middleboxes MB are shown in Figure 3 at each side, but in
general, any number of MBs on each side must be considered.
+----+ +----+ //----\\ +----+ +----+
NI --| MB |-----| MB |---| |---| MB |-----| MB |--- NR
+----+ +----+ \\----// +----+ +----+
private public private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 3: NAT with two Private Networks Scenario
Signaling traffic from NI to NR has to traverse all the middleboxes
on the path, and all the middleboxes must be configured properly to
allow NSIS signaling to traverse them. The NATFW signaling must
configure all middleboxes and consider any address translation that
will result from this configuration in further signaling. The sender
(NI) has to know the IP address of the receiver (NR) in advance,
otherwise it will not be possible to send any NSIS signaling messages
towards the responder. Note that this IP address is not the private
IP address of the responder. Instead a NAT binding (including a
public IP address) has to be previously installed on the NAT that
subsequently allows packets reaching the NAT to be forwarded to the
receiver within the private address realm. This generally requires
further support from an application layer protocol for the purpose of
discovering and exchanging information. The receiver might have a
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number of ways to learn its public IP address and port number
(including the NATFW NSLP) and might need to signal this information
to the sender using the application level signaling protocol.
2.3 NAT with Private Network on Sender Side
This scenario shows an application instance at the sending node that
is behind one or more NATs (shown as generic MB, see discussion in
Section 2.2). The receiver is located in the public Internet.
+----+ +----+ //----\\
NI --| MB |-----| MB |---| |--- NR
+----+ +----+ \\----//
private public
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 4: NAT with Private Network on Sender Side Scenario
The traffic from NI to NR has to traverse middleboxes only on the
sender's side. The receiver has a public IP address. The NI sends
its signaling message directly to the address of the NSIS responder.
Middleboxes along the path intercept the signaling messages and
configure the policy rules accordingly.
The data sender does not necessarily know whether the receiver is
behind a NAT or not, hence, it is the receiving side that has to
detect whether itself is behind a NAT or not. As described in
Section 3.8.2 NSIS can also provide help for this procedure.
2.4 NAT with Private Network on Receiver Side Scenario
The application instance receiving data is behind one or more NATs
shown as MB (see discussion in Section 2.2).
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//----\\ +----+ +----+
NI ---| |---| MB |-----| MB |--- NR
\\----// +----+ +----+
public private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 5: NAT with Private Network on Receiver Scenario
Initially, the NSIS responder must determine its publicly reachable
IP address at the external middlebox and notify the NSIS initiator
about this address. One possibility is that an application level
protocol is used, meaning that the public IP address is signaled via
this protocol to the NI. Afterwards the NI can start its signaling
towards the NR and therefore 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 entities,
such as a DNS server. The application level entities must handle
requests that are based on symbolic names, and configure the
middleboxes so that data packets are correctly forwarded from NI to
NR. The configuration of those middleboxes may require other
middlebox communication protocols, such as MIDCOM [7]. NSIS
signaling is not required in the twice-NAT only case, since
middleboxes of the twice-NAT type are normally configured by other
means. Nevertheless, NSIS signaling might be useful when there are
also firewalls on the path. In this case NSIS will not configure any
policy rule at twice-NATs, but will configure policy rules at the
firewalls on the path. The NSIS signaling protocol must be at least
robust enough to survive this scenario. This requires that twice-
NATs must implement the NATFW NSLP also and participate in NATFW
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 C.4
2.6 Both End Hosts Behind Same NAT
When NSIS initiator and NSIS responder are behind the same NAT (thus
being in the same address realm, see Figure 7), they are most likely
not aware of this fact. As in Section 2.4 the NSIS responder must
determine its public IP address in advance and transfer it to the
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 public
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 9: Multihomed Network with Two NATs
Depending on the destination, either one or the other middlebox is
used for the data flow. Which middlebox is used, depends on local
policy or routing decisions. NATFW NSLP must be able to handle this
situation properly, see Section 3.8.2 for an extended 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 decides which firewall is
being taken for data flows. Depending on the data flow's direction,
either outbound or inbound, a different firewall could be traversed.
This is a challenge for the REA mode of the NATFW NSLP where the NSIS
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responder is located behind these firewalls within the private
network. The REA mode is used to block a particular data flow on an
upstream firewall. NSIS must route the REA mode message upstream
from NR to NI probably without knowing which path the data traffic
will take from NI to NR (see also Appendix B.
+----+
NR -------| MB |\
\ +----+ \ //---\\
\ -| |-- NI
\ \\---//
\ +----+ |
--| MB |-------+
+----+
private
private public
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 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.
3.1 Policy Rules
Policy rules, bound to a session, are the building blocks of
middlebox devices considered in the NATFW NSLP. For firewalls the
policy rule usually consists of a 5-tuple, source/destination
addresses, transport protocol, and source/destination port numbers,
plus an action, such as allow or deny. For NATs the policy rule
consists of the action 'translate this address' and further mapping
information, that might be, in the simplest case, internal IP address
and external IP address.
The NATFW NSLP carries, in conjunction with the NTLP's Message
Routing Information (MRI), the policy rules to be installed at NATFW
peers. This policy rule is an abstraction with respect to the real
policy rule to be installed at the respective firewall or NAT. It
conveys the initiator's request and must be mapped to the possible
configuration on the particular used NAT and/or firewall in use. For
pure firewalls one or more filter rules must be created and for pure
NATs one or more NAT bindings must be created. In mixed firewall and
NAT boxes, the policy rule must be mapped to filter rules and
bindings observing the ordering of the firewall and NAT engine.
Depending on the ordering, NAT before firewall or vice versa, the
firewall rules must carry public or private IP addresses. However,
the exact mapping depends on the implementation of the firewall or
NAT which is different for each vendor.
The policy rule at the NATFW NSLP level comprises the message routing
information (MRI) part, carried in the NTLP, and the information
available in the NATFW NSLP. The information provided by the NSLP is
stored in the 'extend flow information' (NATFW_EFI) and 'data
terminal information' (NATFW_DTINFO) objects, and the message type,
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. The MRI carries the filter part of
the NAT/firewall-level policy rule that is to be installed.
3.2 Basic Protocol Overview
The NSIS NATFW NSLP is carried over the General Internet Signaling
Transport (GIST, the implementation of the NTLP) defined in [1].
NATFW NSLP messages are initiated by the NSIS initiator (NI), handled
by NSIS forwarders (NF) and received by the NSIS responder (NR). It
is required that at least NI and NR implement this NSLP, intermediate
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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.)
+-------+ +-------+ +-------+ +-------+
| 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 normal sequence of NSLP events is as follows:
o NSIS initiators generate NATFW NSLP request messages and send
these 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
(REA) 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
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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 that has been
included in the prior signaling session setup.
o Once the NI has received a successful response, the data sender
can start sending its data flow to the data receiver.
Because NATFW NSLP signaling follows the data path from DS to DR,
this immediately enables communication between both hosts for
scenarios with only firewalls on the data path or NATs on 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. This application
layer signaling, which is out of scope of the NATFW NSLP, may involve
third parties that assist in exchanging these messages.
The same holds partially true for NRs located behind firewalls that
block all traffic by default. In this case, NR must tell its
upstream firewalls of inbound NATFW NSLP signaling and corresponding
data traffic. Once the NR has informed the upstream firewalls, it
can start its application level signaling to initiate communication
with the NI. This application layer signaling, which is out of scope
of the NATFW NSLP, may involve third parties that assist in
exchanging these messages. This mechanism can be used by machines
hosting services behind firewalls as well. In this case, the NR
informs the upstream firewalls as described, but does not need to
communicate this to the NIs.
NATFW NSLP signaling supports this scenario by using the REA mode of
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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 public addresses is called reservation.
During this process the NR reserves publicly reachable addresses
and ports suitable for further usage in application level
signaling and the publicly reachable address for further NATFW
NSLP signaling. However, the data traffic will not be allowed to
use this address/port initially (see next point).
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 forwarding of
signaling messages, but not data flow packets. Policy rules
allowing forwarding of data flow packets set up by the prior REA
mode signaling will be 'activated' by the signaling from NI
towards NR. The RESERVE-EXTERNAL-ADDRESS (REA) mode of operation
is detailed in Section 3.8.2
+-------+ +-------+ +-------+ +-------+
| 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 fails when NSIS is only deployed at one end of the path.
In this case only one of the receiving or sending side is NSIS aware
and not both at the same time. NATFW NSLP supports this scenario
(i.e., the DR does not support NSIS) by using a proxy mode, as
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described in Section 3.8.7; the proxy mode operation also supports
scenarios with a data sender that does not support NSIS, i.e. the
data receiver must act to enable data flows towards itself.
The basic functionality of the NATFW NSLP provides for opening
firewall pin holes and creating NAT bindings to enable data flows to
traverse these devices. Firewalls are normally expected to work on a
'deny-all' policy, meaning that traffic not explicitly matching any
firewall filter rule will be blocked. Similarly, the normal behavior
of NATs is to block all traffic that does not match any already
configured/installed binding or 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 message with
action set to 'deny' 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 NI involved will have to specifically request a session
extension. An explicit NATFW NSLP state deletion capability is also
provided by the protocol.
If the actions requested by a NATFW NSLP message cannot be carried
out, NFs and the NR should return a failure, such that appropriate
actions can be taken. They can do this either during a the request
message handling (synchronously) by sending an error RESPONSE
message, or at any time (asynchronously) by sending a notification
message.
The next sections define the NATFW NSLP message types and formats,
protocol operations, and policy rule operations.
3.2.1 Message Types
The protocol uses five messages types:
o CREATE: a request message used for creating, changing, refreshing,
and deleting CREATE NATFW NSLP sessions, i.e., open the data path
from DS to DR.
o RESERVE-EXTERNAL-ADDRESS (REA): a request message used for
reserving, changing, refreshing, and deleting REA NATFW NSLP
sessions. REA messages are forwarded to the edge-NAT or edge-
firewall and allow inbound CREATE messages to be forwarded to the
NR. Additionally, REA messages reserve an external address and,
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if applicable, port number at an edge-NAT.
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 NATFW peers about specific events (especially failures).
o RESPONSE: used as a response to CREATE, REA, and TRACE request
messages.
3.2.2 Classification of RESPONSE Messages
RESPONSE messages will be generated synchronously by NSIS Forwarders
and Responders to report success or failure of operations or some
information relating to the session or a node.
All RESPONSE messages MUST carry a NATFW_INFO object which contains a
severity class code and a response code (see Section 4.2.4). This
section defines terms for groups of RESPONSE messages depending on
the severity class.
o Successful RESPONSE: Messages carrying NATFW_INFO with severity
class 'Success' (0x2).
o Informational RESPONSE: Messages carrying NATFW_INFO with severity
class 'Informational' (0x1) (normally only used with NOTIFY
messages).
o Error RESPONSE: Messages carrying NATFW_INFO with severity class
other than 'Success' or 'Informational'.
3.2.3 NATFW NSLP Signaling Sessions
The general idea of signaling sessions is described in [4]. There is
signaling session state stored at the NTLP layer and at the NATFW
NSLP level. The signaling session state for the NATFW NSLP consists
comprises NSLP state and the associated policy rules at a middlebox.
A NATFW NSLP signaling session can conceptually be in different
states, implementations may use other or even more states. The
signaling session can have these states at a node:
o Pending: The signaling session has been created and the node is
waiting for a RESPONSE message to the request message. A
signaling session in state 'Pending' MUST be marked as 'Dead' if
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no corresponding RESPONSE message has been received within the
time of the locally granted session lifetime of the forwarded
request message (as described in Section 3.4).
o Established: The signaling session is established, i.e, the
signaling has been successfully performed. A signaling session in
state 'Established' MUST be marked as 'Dead' if no refresh message
has been received within the time of the locally granted session
lifetime of the RESPONSE message (as described in Section 3.4).
o Dead: The node has received an error RESPONSE message for the
signaling session and the signaling session can be deleted.
o Transit: The node has received an asynchronous message, i.e., a
NOTIFY, and can delete the signaling session if needed. When a
node has received a NOTIFY message (for instance, indicating a
route change) it marks it as 'Transit' and deletes this session if
it is unused for some time specific to the local node. This idle
time does not need to be fixed, since it can depend on the node
local maintenance cycle, i.e., the session could be deleted if the
node runs it garbage collection cycle.
3.3 Basic Message Processing
All NATFW messages are subject to some basic message processing when
received at a node, independent of request or response messages.
Initially, the syntax of the NSLP message is checked and a RESPONSE
message with an appropriate error of class 'Protocol error' (0x1)
code is generated if any problem is detected. If a message is
delivered to the NATFW NSLP, this implies that the NTLP layer has
been able to correlate it with the SID and MRI entries in its
database. There is therefore enough information to identify the
source of the message and routing information to route the message
back to the NI through an established chain of MAs since the NATFW
NSLP always requests reliable delivery resulting in the NTLP using
C-mode. The message is not further forwarded if any error in the
syntax is detected. The specific response codes stemming from the
processing of objects are described in the respective object
definition section (see Section 4). After passing this check, the
NATFW NSLP node MUST first perform the checks defined on session
ownership in Section 3.6 and authentication/authorization in
Section 3.7. Further processing is executed only if these tests have
been successfully passed, otherwise the processing stops and an error
RESPONSE is returned, as described in these sections.
Further message processing stops whenever an error RESPONSE message
is generated, and the request message is discarded.
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3.4 Calculation of Session Lifetime
NATFW NSLP sessions, and the corresponding policy rules which may
have been installed, are maintained via a soft-state mechanism. Each
session is assigned a lifetime and the session is kept alive as long
as the lifetime is valid. After the expiration of the lifetime,
sessions and policy rules MUST be removed automatically and resources
bound to them MUST be freed as well. Session lifetime is handled at
every NATFW NSLP node. The NSLP forwarders and NSLP responder MUST
NOT trigger lifetime extension refresh messages (see Section 3.8.3):
this is the task of the NSIS initiator. This section describes how
the session lifetime is set within a signaling session.
The NSIS initiator MUST choose a session lifetime value (expressed in
seconds) before sending any message, including the initial message
which creates the session, 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 as a multiplier for
R;
The RSVP specification [13] provides an appropriate 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].
2. To avoid premature loss of state, lt (with lt being the session
lifetime) must satisfy lt >= (K + 0.5)*1.5*R, where K is a small
integer. Then in the worst case, K-1 successive messages may be
lost without state being deleted. Currently K = 3 is suggested
as the default. However, it may be necessary to set a larger K
value for hops with high loss rate. Other algorithms could be
used to define the relation between the session lifetime and the
refresh message period; the algorithm provided is only given as
an example.
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This requested lifetime value lt is stored in the NATFW_LT object of
the NSLP message.
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 store the new value in the
'lifetime' object. NFs MUST NOT increase the lifetime value; they
MAY reject the requested lifetime immediately and MUST generate an
error RESPONSE message of class 'Signaling session failures' (0x6)
with error response code 'Requested lifetime is too big' (0x02) upon
rejection. The NSLP request message is forwarded until it reaches
the NSLP responder. The NSLP responder may reject the requested
lifetime value and MUST generate an error RESPONSE message of class
'Signaling session failures' (0x6) with response code 'Requested
lifetime is too big' (0x02) upon rejection. The NSLP responder MAY
also lower the requested lifetime to an acceptable value (based on
its local policies). The NSLP responder generates a successful
RESPONSE 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 MUST accept the granted
lifetime, as long as this value is less than or equal to their
proposed value. For received values greater than the proposed value,
NSLP forwarders MUST generate an RESPONSE message of class 'Signaling
session failures' (0x6) with response code 'Requested lifetime is too
big' (0x02). Figure 13 shows the procedure with an example, where an
initiator requests 60 seconds lifetime in the CREATE message and the
lifetime is shortened along the path by the forwarder to 20 seconds
and by the responder to 15 seconds.
+-------+ CREATE(lt=60s) +-------------+ CREATE(lt=20s) +--------+
| |---------------->| NSLP |---------------->| |
| NI | | forwarder | | NR |
| |<----------------| check 15<20 |<----------------| |
+-------+ RESPONSE(lt=15s)+-------------+ RESPONSE(lt=15s)+--------+
lt = lifetime
Figure 13: Lifetime Setting Example
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3.5 Message Sequencing
NATFW NSLP messages need to carry an identifier so that all nodes
along the path can distinguish messages sent at different points in
time. Messages can be lost along the path or duplicated. So all
NATFW NSLP nodes should be able to identify either old messages that
have been received before (duplicated), or the case that messages
have been lost before (loss). For message replay protection it is
necessary to keep information about messages that have already been
received and requires every NATFW NSLP message to carry a message
sequence number (MSN), see also Section 4.2.6.
The MSN MUST be set by the NI and MUST NOT be set or modified by any
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.
All NATFW NSLP nodes MUST use the algorithm defined in [3] to detect
MSN wrap-arounds.
NSIS forwarders and the responder store the MSN from the initial
CREATE or REA packet which creates the session as the 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
NSLP MUST update its stored state accordingly, if permitted by all
security checks (see Section 3.6 and Section 3.7), and stores the
updated MSN value accordingly.
3.6 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
demonstrating a valid session ownership are processed further.
Within the NATFW NSLP, the NSIS initiator 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.
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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
proof 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.
3.7 Authentication, Authorization, and Policy Decisions
NATFW NSLP nodes receiving signaling messages MUST first check
whether this message is authenticated and authorized to perform the
requested action. The necessary information for these checks can be
carried in the NATFW_CREDENTIAL object. NATFW NSLP nodes requiring
more information than provided MUST generate an error RESPONSE of
class 'Permanent failure' (0x5) with response code 'Authentication
failed' (0x01) or with response code 'Authorization failed' (0x02).
The NATFW NSLP is expected to run in various environments, such as
IP-based telephone systems, enterprise networks, home networks, etc.
The requirements on authentication and authorization are quite
different between these use cases. While a home gateway, or an
Internet cafe, using NSIS may well be happy with a "NATFW signaling
coming from inside the network" policy for authorization of
signaling, enterprise networks are likely to require more strongly
authenticated/authorized signaling. This enterprise scenario may
require the use of an infrastructure and administratively assigned
identities to operate the NATFW NSLP.
Once the NI is authenticated and authorized, another step is
performed. The requested policy rule for the session is checked
against a set of policy rules, i.e., whether the requesting NI is
allowed to request the policy rule to be loaded in the device. If
this fails the NF or NR must send an error RESPONSE of class
'Permanent failure' (0x5) and with response code 'Authorization
failed' (0x02).
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3.8 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.
3.8.1 Creating Sessions
Allowing two hosts to exchange data even in the presence of
middleboxes is realized in the NATFW NSLP by use of the CREATE
request message. The NI (either the data sender or a proxy)
generates a CREATE message as defined in Section 4.3.1 and hands it
to the NTLP. The NTLP forwards the whole message on the basis of the
message routing information 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 14 sketches the
message flow between NI (DS in this example), a NF (e.g., NAT), and
NR (DR in this example).
NI Private Network NF Public Internet NR
| | |
| CREATE | |
|----------------------------->| |
| | |
| | |
| | CREATE |
| |--------------------------->|
| | |
| | RESPONSE |
| RESPONSE |<---------------------------|
|<-----------------------------| |
| | |
| | |
Figure 14: CREATE message flow with success RESPONSE
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There are several processing rules for a NATFW peer when generating
and receiving CREATE messages, since this message type is used for
creating new signaling sessions, updating existing, extending the
lifetime and deleting signaling session. The three latter functions
operate in the same way for all kinds of request message, and are
therefore described in separate sections:
o Extending the lifetime of signaling sessions is described in
Section 3.8.3.
o Deleting signaling sessions is described in Section 3.8.4.
o Updating policy rules is described in Section 3.11.
For an initial CREATE message creating a new NATFW NSLP session, the
processing of CREATE messages is different for every NATFW node type:
o NSLP initiator: An NI only generates CREATE messages and hands
them over to the NTLP. The NI should never receive request
messages and MUST discard it.
o NATFW NSLP forwarder: NFs that are unable to forward the request
message to the next hop MUST generate an error RESPONSE of class
'Permanent failure' (0x6) with response code 'Did not reach the
NR' (0x06). This case may occur if the NTLP layer cannot find an
NATFW NSLP peer, either another NF or the NR, and returns an error
via the GIST API. The NSLP message processing at the NFs depends
on the middlebox type:
* NAT: When the initial CREATE message is received at the public
side of the NAT, it looks for a reservation made in advance, by
using a REA message (see Section 3.8.2). The matching process
considers the received MRI information and the stored MRI
information, as described in Section 3.9. If no matching
reservation can be found, i.e. no reservation has been made in
advance, the NSLP MUST return an error RESPONSE of class
'Signaling session failure' (0x6) with response code 'No
reservation found matching the MRI of the CREATE request'
(0x03) MUST be generated. If there is a matching reservation,
the NSLP stores the data sender's address (and if applicable
port number) as part of the source address of the policy rule
('the remembered policy rule') to be loaded and forwards the
message with the destination address set to the internal
(private in most cases) address of NR. When the initial CREATE
message is received at the private side, the NAT binding is
allocated, but not activated (see also Appendix C.3). The MRI
information is updated to reflect the address, and if
applicable port, translation. The NSLP message is forwarded
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towards the NR with source address set to the NAT's external
address from the newly remembered binding.
* Firewall: When the initial CREATE message is received, the NSLP
just remembers the requested policy rule, but does not install
any policy rule. Afterwards, the message is forwarded towards
the NR.
* Combined NAT and firewall: Processing at combined firewall and
NAT middleboxes is the same as in the NAT case. No policy
rules are installed. Implementations MUST take into account
the order of packet processing in the firewall and NAT
functions within the device. This will be referred to as
'order of functions' and is generally different depending on
whether the packet arrives at the external or internal side of
the middlebox.
o NSLP receiver: NRs receiving initial CREATE messages MUST reply
with a success RESPONSE of class 'Success' (0x2) with response
code set to 'All successfully processed' (0x01), if they accept
the CREATE request message. Otherwise they MUST generate a
RESPONSE message with a suitable response code. RESPONSE messages
are sent back NSLP hop-by-hop towards the NI, irrespective of the
response codes, either success or error.
Remembered policy rules at middleboxes MUST be only installed upon
receiving a corresponding successful RESPONSE message with the same
SID and MSN as the CREATE message that caused them to be remembered.
This is a countermeasure to several problems, for example, wastage of
resources due to loading policy rules at intermediate NFs when the
CREATE message does not reach the final NR for some reason.
Processing of a RESPONSE message is different for every NSIS node
type:
o NSLP initiator: After receiving a successful RESPONSE, the data
path is configured and the DS can start sending its data to the
DR. After receiving an error RESPONSE message, the NI MAY try to
generate the CREATE message again or give up and report the
failure to the application, depending on the error condition.
o NSLP forwarder: NFs install the remembered policy rules, if a
successful RESPONSE message with matching SID and MSN is received.
If an ERROR RESPONSE message with matching SID and MSN is
received, the session is marked as dead, no policy rule is
installed and the remembered rule is discarded.
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o NSIS responder: The NR should never receive RESPONSE messages and
MUST silently drop any such messages received.
3.8.2 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 3.8.1. For scenarios where the data receiver is located
behind a NAT or a firewall and it needs to receive data flows from
outside its own network (usually referred to as inbound flows, see
Figure 5) the problem is more troublesome.
NSIS signaling, as well as subsequent data flows, are directed to a
particular destination IP address that must be known in advance and
reachable. Data receivers must tell the local NSIS infrastructure
(i.e., the upstream firewalls/NATs) about incoming NATFW NSLP
signaling and data flows before they can receive these flows. It is
necessary to differentiate between data receivers behind NATs and
behind firewalls for understanding the further NATFW procedures.
Data receivers that are only behind firewalls already have a public
IP address and they need only to be reachable for NATFW signaling.
Unlike data receivers behind just firewalls, data receivers behind
NATs do not have public IP addresses; consequently they are not
reachable for NATFW signaling by entities outside their addressing
realm.
The preceding discussion addresses the situation where a DR node that
wants to be reachable is unreachable because the NAT lacks a suitable
rule with the 'allow' action which would forward inbound data.
However, in certain scenarios, a node situated behind upstream
firewalls that do not block inbound data traffic (firewalls with
"default to allow") unless requested might wish to prevent traffic
being sent to it from specified addresses. In this case, NSIS NATFW
signaling can be used to achieve this by installing a policy rule
with its action set to 'deny' using the same mechanisms as for
'allow' rules.
The required result is obtained by sending a RESERVE-EXTERNAL-ADDRESS
(REA) message in the upstream direction of the intended data flow.
When using this functionality the NSIS initiator for the 'Reserve
External Address' signaling is typically the node that will become
the DR for the eventual data flow. To distinguish this initiator
from the usual case where the NI is associated with the DS, the NI is
denoted by NI+ and the NSIS responder is similarly denoted by NR+.
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Public Internet Private Address
Space
Edge
NI(DS) NAT/FW NAT NR(DR)
NR+ NI+
| | | |
| | | |
| | | |
| | REA[(DTInfo)] | REA[(DTInfo)] |
| |<----------------------|<----------------------|
| | | |
| |RESPONSE[Success/Error]|RESPONSE[Success/Error]|
| |---------------------->|---------------------->|
| | | |
| | | |
============================================================>
Data Traffic Direction
Figure 15: Reservation message flow for DR behind NAT or firewall
Figure 15 shows the REA message flow for enabling inbound NATFW NSLP
signaling messages. In this case the roles of the different NSIS
entities are:
o The data receiver (DR) for the anticipated data traffic is the
NSIS initiator (NI+) for the 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 It may be necessary to use a signaling destination address (SDA)
as the actual target of the REA message (NR+) if the DR is located
behind a NAT and the address of the DS is unknown. The SDA is an
arbitrary address in the outermost address realm on the other side
of the NAT from the DR. Typically this will be a suitable public
IP address when the 'outside' realm is the public Internet. This
choice of address causes the REA message to be routed through the
NATs towards the outermost realm and would force interception of
the message by the outermost NAT in the network at the boundary
between the private address and the public address realm (the
edge-NAT). It may also be intercepted by other NATs and firewalls
on the path to the edge-NAT.
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Basically, there are two different signaling scenarios. Either
1. the DR behind the NAT/firewall knows the IP address of the DS in
advance,
2. or the address of DS is not known in advance.
Case 1 requires the NATFW NSLP to request the path-coupled message
routing method (PC-MRM) from the NTLP. The REA message MUST be sent
with PC-MRM (see Section 5.8.1 in [1]) with the direction set to
'upstream'. The handling of case 2 depends on the situation of DR:
If DR is solely located behind a firewall, the REA message MUST be
sent with the PC-MRM, direction 'upstream', and data flow source IP
address set to wildcard. If DR is located behind a NAT, the REA
message MUST be sent with the loose-end message routing method (LE-
MRM, see Section 5.8.2 in [1]), the destination-address set to the
signaling destination address (SDA, see also Appendix A). For
scenarios with DR being behind a firewall, special conditions apply
(applicability statement, Appendix B). The data receiver is
challenged to determine whether it is solely located behind firewalls
or NATs, for choosing the right message routing method. This
decision can depend on a local configuration parameter, possibly
given through DHCP, or it could be discovered through other non-NSLP
related testing of the network configuration.
For case 2 with NAT, the NI+ (which could be on the data receiver DR
or on any other host within the private network) sends the REA
message targeted to the signaling destination address. 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 an NSIS session) the signaling
direction does not matter since the data path is modified through
route pinning due to the external IP address at the NAT. Subsequent
NSIS messages (and also data traffic) will travel through the same
NAT boxes. However, this is only valid for the NAT boxes, but not
for any intermediate firewall. That is the reason for having a
separate CREATE message enabling the reservations made with REA at
the NATs and either enabling prior reservations or creating new
pinholes at the firewalls which are encountered on the downstream
path depending on whether the upstream and downstream routes
coincide.
The REA signaling message creates an NSIS NATFW session at any
intermediate NSIS NATFW peer(s) encountered, independent of the
message routing method used. Furthermore, it has to be ensured that
the edge-NAT or edge-firewall device is discovered as part of this
process. The end host cannot be assumed to know this device -
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instead the NAT or firewall box itself is assumed to know that it is
located at the outer perimeter of the network. Forwarding of the REA
message beyond this entity is not necessary, and MUST be prohibited
as it may provide information on the capabilities of internal hosts.
It should be noted, that it is the outermost NAT or firewall that is
the edge-device that must be found during this discovery process.
For instance, when there are a NAT and afterwards a firewall on the
outbound path at the network border, the firewall is the edge-
firewall. All messages must be forwarded to the topology-wise
outermost edge-device, to ensure that this devices knows about the
signaling sessions for incoming CREATE messages. However, the NAT is
still the edge-NAT because it has a public globally routable IP
address on its public side: this is not affected by any firewall
between the edge-NAT and the public network.
Possible edge arrangements are:
Public Net ----------------- Private net -------------------
| Public Net|--|Edge-FW|--|FW|...|FW|--|DR|
| Public Net|--|Edge-FW|--|Edge-NAT|...|NAT or FW|--|DR|
| Public Net|--|Edge-NAT|--|NAT or FW|...|NAT or FW|--|DR|
The edge-NAT or edge-firewall device closest to the public realm
responds to the REA message with a successful RESPONSE message. An
edge-NAT includes an NATFW_EXT_IP object (see Section 4.2.2),
carrying the public reachable IP address, and if applicable port
number.
There are several processing rules for a NATFW peer when generating
and receiving REA messages, since this message type is used for
creating new reserve signaling sessions, updating existing, extending
the lifetime and deleting signaling session. The three latter
functions operate in the same way for all kinds of request message,
and are therefore described in separate sections:
o Extending the lifetime of signaling sessions is described in
Section 3.8.3.
o Deleting signaling sessions is described in Section 3.8.4.
o Updating policy rules is described in Section 3.11.
The NI+ MAY include a NATFW_DTINFO_IPv4 object in the REA message
when using the LE-MRM. The LE-MRM does not include enough
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information for some types of NATs (basically, those NATs which also
translate port numbers) to perform the address translation. This
information is provided in the NATFW_DTINFO_IPv4 (see Section 4.2.7).
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. All other fields
MAY be set by the NI+ to restrict the set of possible NIs. An edge-
NAT will use the information provided in the NATFW_DTINFO_IPv4 object
to allow only NATFW CREATE message with the MRI matching ('src
IPv4/v6 address', 'src port number', 'protocol') to be forwarded. A
NAT requiring information carried in the NATFW_DTINFO_IPv4 can
generate a number of error RESPONSE messages of class 'Signaling
session failures' (0x6):
o 'Requested policy rule denied due to policy conflict' (0x04)
o 'DTINFO object is required' (0x07)
o 'Requested value in sub_ports field in NATFW_EFI not permitted'
(0x08)
o 'Requested IP protocol not supported' (0x09)
o 'Plain IP policy rules not permitted -- need transport layer
information' (0x0A)
o 'source IP address range is too large' (0x0C)
o 'destination IP address range is too large' (0x0D)
o 'source L4-port range is too large' (0x0E)
o 'destination L4-port range is too large' (0x0F)
Processing of REA messages is specific to the NSIS node type:
o NSLP initiator: NI+ only generate REA messages. When the data
sender's address information is known in advance the NI+ MAY
include a NATFW_DTINFO_IPv4 object in the REA message (as
described above). When the data sender's IP address is not known,
the NI+ MUST NOT include a NATFW_DTINFO_IPv4 object. The NI
should never receive request messages and MUST silently discard
it.
o NSLP forwarder: The NSLP message processing at NFs depends on the
middlebox type:
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* 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
MUST generate an error RESPONSE of class 'Protocol error' (0x3)
with response code 'Received REA request message on external
side' (0x0B) MUST be generated. If received at the internal
(private address) and the NATFW_EFI object contains the action
'deny', an error RESPONSE of class 'Protocol error' (0x3) with
response code 'Requested rule action not applicable' (0x06)
MUST be generated. If received at the internal address, an IP
address, and if applicable a port, is reserved. If it is an
edge-NAT and there is no edge-firewall beyond, the NSLP message
is not forwarded any further and a successful RESPONSE message
is generated containing an NATFW_EXT_IP object holding the
translated address, and if applicable port, information 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 or any
other NAT MAY reject REA messages not carrying a
NATFW_DTINFO_IPv4 object or if the address information within
this object is invalid or is not compliant with local policies
(e.g., the information provided relates to a range of addresses
('wildcarded') but the edge-NAT requires exact information
about DS' IP address and port) with the above mentioned
response codes.
* Firewall: Non edge-firewalls remember the requested policy
rule, keep session state, and forward the message. Edge-
firewalls stop forwarding the request message. The policy rule
is immediately loaded if the action in the NATFW_EFI object is
set to 'deny' and the node is an edge-firewall. The policy
rule is remembered, but not activated, if the action in the
NATFW_EFI object is set to 'allow'. In both cases, a
successful RESPONSE message is generated.
* 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 MUST generate an error RESPONSE message of class
'Permanent failure' (0x5) with response code 'No edge-device here'
(0x05).
Processing of a RESPONSE message is different for every NSIS node
type:
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o NSLP initiator: Upon receiving a successful RESPONSE message, the
NI+ can rely on the requested configuration for future inbound
sessions. If the response contains an NATFW_EXT_IP object, the NI
can use IP address and port pairs carried for further application
signaling. After receiving a error RESPONSE message, the NI+ MAY
try to generate the REA message again or give up and report the
failure to the application, depending on the error condition.
o NSLP forwarder: NFs simply forward this message as long as they
keep state for the requested reservation, if the RESPONSE message
contains NATFW_INFO object with class set to 'Success' (0x2). If
the RESPONSE message contains NATFW_INFO object with class set not
to 'Success' (0x2), the session is marked as dead.
o NSIS responder: This type of message should never be received by
any NR+. The NF should never receive response messages and MUST
silently discard it.
Reservations with action 'allow' made with REA MUST be enabled by a
subsequent CREATE message. A reservation made with REA (independent
of selected action) 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 3.8.1 or REA in proxy mode Section 3.8.7 no data
traffic will be forwarded to DR beyond the edge-NAT or edge-firewall.
The only function of REA is to ensure that subsequent CREATE messages
traveling towards the NR will be forwarded across the public-private
boundary towards the DR. Correlation of incoming CREATE messages to
REA reservation states is described in Section 3.9.
3.8.3 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 and REA and the maintenance of this
state must be done by these messages. State created by CREATE must
be maintained by CREATE, state created by REA must be maintained by
REA. Refresh messages, are messages carrying the same session ID as
the initial message and a NATFW_LT lifetime object with a lifetime
greater than zero. Messages with the same SID but carrying a
different MRI are treated as updates of the policy rules and are
processed as defined in Section 3.11. Every refresh request message
MUST be acknowledged by an appropriate response message generated by
the NR. Upon reception by each NSIS forwarder, the state for the
given session ID is extended by the session refresh period, a period
of time calculated based on a proposed refresh message period. The
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lifetime extension of a session is calculated as current local time
plus proposed lifetime value (session refresh period). Section 3.4
defines the process of calculating lifetimes in detail.
NI Public Internet NAT Private address NR
| | space |
| CREATE[lifetime > 0] | |
|----------------------------->| |
| | |
| | |
| | CREATE[lifetime > 0] |
| |--------------------------->|
| | |
| | RESPONSE[Success/Error] |
| RESPONSE[Success/Error] |<---------------------------|
|<-----------------------------| |
| | |
| | |
Figure 16: Successful Refresh Message Flow, CREATE as example
Processing of session refresh CREATE and REA messages is different
for every NSIS node type:
o NSLP initiator: The NI/NI+ can generate session refresh CREATE/REA
messages before the session times out. The rate at which the
refresh CREATE/REA messages are sent and their relation to the
session state lifetime is discussed further in Section 3.4.
o NSLP forwarder: Processing of this message is independent of the
middlebox type and is as described in Section 3.4.
o NSLP responder: NRs accepting a session refresh CREATE/REA message
generate a successful RESPONSE message, including the granted
lifetime value of Section 3.4 in a NATFW_LT object.
3.8.4 Deleting Sessions
NATFW NSLP sessions can be deleted at any time. NSLP initiators can
trigger this deletion by using a CREATE or REA messages with a
lifetime value set to 0, as shown in Figure 17. Whether a CREATE or
REA message type is used, depends on how the session was created.
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NI Public Internet NAT Private address NR
| | space |
| CREATE[lifetime=0] | |
|----------------------------->| |
| | |
| | CREATE[lifetime=0] |
| |--------------------------->|
| | |
Figure 17: Delete message flow, CREATE as example
NSLP nodes receiving this message delete the session immediately.
Policy rules associated with this particular session MUST be also
deleted immediately. This message is forwarded until it reaches the
final NR. The CREATE/REA 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.
NSIS initiators can use CREATE/REA message with lifetime set to zero
in an aggregated way, such that a single request message is
terminating multiple signaling sessions. NIs can follow this
procedure if the like to aggregate session deletion requests: The NI
uses the CREATE or REA request message with the session ID set to
zero and the MRI's source-address set to its used IP address. All
other fields of the respective sessions to be terminated are set as
well, otherwise these fields are completely wildcarded. The NSLP
message is transfered to the NTLP requesting 'explicit routing' as
described in Sections 5.2.1 and 7.1.4. in [1].
The downstream NF receiving such an aggregated request message MUST
reject the request with an error RESPONSE of class 'Permanent
failure' (0x5) with response code 'Authentication failed' (0x01) if
the authentication fails and with an error RESPONSE of class
'Permanent failure' (0x5) with response code 'Authorization failed'
(0x02) if the authorization fails. Per session proof of ownership,
as it is defined in this memo, is not possible anymore when using
this aggregated mode. However, the downstream NF can use the
relationship between the information of the received request message
and the GIST messaging association where the request has been
received. The downstream NF MUST only accept this aggregated request
message through already established GIST messaging associations with
the NI. The downstream NF MUST NOT propagate this aggregated request
message but it MAY generate and forward per session request messages.
3.8.5 Reporting Asynchronous Events
NATFW NSLP forwarders and NATFW NSLP responders must have the ability
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to report asynchronous events to other NATFW NSLP nodes, especially
to allow reporting back to the NATFW NSLP initiator. Such
asynchronous events may be premature session termination, changes in
local policies, route change or any other reason that indicates
change of the NATFW NSLP session state. Currently, asynchronous
session termination, re-authorization required and route change
detected (see Section 3.10) are the only events that are defined, but
other events may be defined in later revisions of this memo.
NFs and NRs may generate NOTIFY messages upon asynchronous events,
with a NATFW_INFO object indicating the reason for event. These
reasons can be carried in the NATFW_INFO object (class MUST be set to
'Informational' (0x1)) within the NOTIFY message. This list shows
the response codes and the associated actions to take at NFs and the
NI:
o 'Route change: possible route change on the downstream path'
(0x01): Follow instructions in Section 3.10.
o 'Re-authentication required' (0x02): The NI should re-send the
authentication.
o 'NATFW node is going down soon' (0x03): The NI and other NFs
should be prepared for a service interruption at any time.
NOTIFY messages are sent hop-by-hop upstream towards NI until they
reach NI.
The initial processing when receiving a NOTIFY message is the same
for all NATFW nodes: NATFW nodes MUST only accept NOTIFY messages
through already established NTLP messaging associations. The further
processing is different for each NATFW NSLP node type and depends on
the events notified:
o NSLP initiator: NIs analyze the notified event and behave
appropriately based on the event type. NIs MUST NOT generate
NOTIFY messages.
o NSLP forwarder: NFs analyze the notified event and behave based
on the above description per response code. NFs SHOULD generate
NOTIFY messages upon asynchronous events and forward them upstream
towards the NI.
o NSLP responder: NRs SHOULD generate NOTIFY messages upon
asynchronous events including a response code based on the
reported event. The NF should never receive NOTIFY messages and
MUST silently discard it.
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NATFW NSLP forwarders, keeping multiple signaling sessions at the
same time, can experience problems when shutting down service
suddenly. This sudden shutdown can be result of node local failure,
for instance, due to a hardware failure. This NF generates NOTIFY
messages for each of the signaling sessions and tries to send them
upstream. Due to the number of NOTIFY messages to be sent, the
shutdown of the node may be unnecessarily prolonged, since not all
messages can be sent at the same time. This case can be described as
a NOTIFY storm, if a multitude of signaling sessions is involved.
To avoid the need of generating per signaling session NOTIFY messages
in such a scenario described or similar cases, NFs SHOULD follow this
procedure: The NF uses the NOTIFY message with the session ID in the
NTLP set to zero, with the MRI completely wildcarded, using the
'explicit routing' as described in Sections 5.2.1 and 7.1.4. in [1].
The upstream NF receiving this type of NOTIFY immediately regards all
signaling sessions from that peer matching the MRI as void. This
message will typically result in multiple NOTIFY messages at the
upstream NF, i.e., the NF can generate per terminated session a
NOTIFY message. However, a NF MAY aggregate again the NOTIFY
messages as described here.
3.8.6 Tracing Signaling Sessions
The NATFW NSLP provides a diagnosis capability to session owners (the
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)] | |
|<-----------------------------| |
| | |
| | |
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Figure 18: 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. The NI
should never receive request messages and MUST silently discard
it.
o NSLP forwarder: NFs solely forward the message if their local
policies permits tracing. A NF MUST generate an error RESPONSE of
class 'Permanent failure' (0x6) with response code 'Tracing is not
allowed' (0x07) if the local policies do not allow tracing.
o NSLP responder: NRs receiving a TRACE request message terminate
the forwarding and reply with a successful RESPONSE message. 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 local processing.
o NSLP forwarder: NFs MAY include their identifier in the
NATFW_TRACE object and increment the 'hop count' field by one.
This memo defines IPv4 and IPv6 IP addresses as possible de
identifier. NFs MUST forward this type of RESPONSE.
o NSLP responder: A NR should never see such a RESPONSE message and
it MUST silently discard it.
3.8.7 Proxy Mode of Operation
Some migration scenarios need specialized support to cope with cases
where NSIS is only deployed in same areas of the Internet. End-to-
end signaling is going to fail without NSIS support at or near both
data sender and data receiver terminals. A proxy mode of operation
is needed. This proxy mode of operation must terminate the NATFW
NSLP signaling as topologically close to the terminal for which it is
proxying and proxy all request and response messages. This NATFW
NSLP node doing the proxying of the signaling messages becomes either
the NI or the NR for the particular signaling session, depending on
whether it is the DS or DR that does not support NSIS. Typically,
the edge-NAT or the edge-firewall would be used to proxy NATFW NSLP
messages.
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This proxy mode operation does not require any new request message
type, but relies on extended CREATE and REA message types. They are
called respectively CREATE-PROXY and REA-PROXY and are distinguished
by setting the P flag in the NSLP header is set to P=1. This flag
instructs edge-NATs and edge-firewalls receiving them to operate in
proxy mode for the session in question. The semantics of the CREATE
and REA message types are not changed and the behavior of the various
node types is as defined in Section 3.8.1 and Section 3.8.2, except
for the proxying node. The following paragraphs describe the proxy
mode operation for data receivers behind middleboxes and data senders
behind middleboxes.
3.8.7.1 Proxying for a Data Sender
The NATFW NSLP gives the NR the ability to install state on the
upstream path towards the data sender for downstream data packets,
even when only the receiving side is running NSIS (as shown in
Figure 19). The goal of the method described is to trigger the edge-
NAT/edge-firewall to generate a CREATE message on behalf of the data
receiver. In this case, a NR can signal towards the network border
as it is performed in the standard REA message handling scenario as
in Section 3.8.2. The message is forwarded until the edge-NAT/
edge-firewall is reached. A public IP address and port number is
reserved at an edge-NAT/edge-firewall. As shown in Figure 19, unlike
the standard REA message handling case, the edge-NAT/edge-firewall is
triggered to send a CREATE message on a new reverse path which
traverse several firewalls or NATs. The new reverse path for CREATE
is necessary to handle routing asymmetries between the edge-NAT/
edge-firewall and DR. It must be stressed that the semantics of the
CREATE and REA request messages is not changed, i.e., each is
processed as described earlier.
DS Public Internet NAT/FW Private address NR
No NI NF space NI+
NR+
| | REA-PROXY[(DTInfo)] |
| |<------------------------- |
| | RESPONSE[Error/Success] |
| | ---------------------- > |
| | CREATE |
| | ------------------------> |
| | RESPONSE[Error/Success] |
| | <---------------------- |
| | |
Figure 19: REA Triggering Sending of CREATE Message
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A NATFW_NONCE object, carried in the REA and CREATE message, is used
to build the relationship between received CREATEs at the message
initiator. An NI+ uses the presence of the NATFW_NONCE object to
correlate it to the particular REA-PROXY request. The absence of an
NONCE object indicates a CREATE initiated by the DS and not by the
edge-NAT. Therefore, these processing rules of REA-PROXY messages
are added to the regular REA processing:
o NSLP initiator (NI+): The NI+ MUST choose a random value and place
it in the NATFW_NONCE object.
o NSLP forwarder being either edge-NAT or edge-firewall: When the
NF accepts a REA_PROXY message, it generates a successful RESPONSE
message as if it were the NR and additionally, it generates a
CREATE message as defined in Section 3.8.1 and includes a
NATFW_NONCE object having the same value as of the received
NATFW_NONCE object. The NF MUST not generate a CREATE-PROXY
message. The NF MUST refresh the CREATE message session only if a
REA-PROXY refresh message has been received first.
The scenario described in this section challenges the data receiver
because it must make a correct assumption about the data sender's
ability to use NSIS NATFW NSLP signaling. It is possible for the DR
to make the wrong assumption in two different ways:
a) the DS is NSIS unaware but the DR assumes the DS to be NSIS
aware and
b) the DS is NSIS aware but the DR assumes the DS to be NSIS
unaware.
Case a) will result in middleboxes blocking the data traffic, since
DS will never send the expected CREATE message. Case b) will result
in the DR successfully requesting proxy mode support by the edge-NAT
or edge-firewall. The edge-NAT/edge-firewall will send CREATE
messages and DS will send CREATE messages as well. Both CREATE
messages are handled as separated sessions and therefore the common
rules per session apply; the NATFW_NONCE object is used to
differentiate CREATE messages generated by the edge-NAT/edge-firewall
from NI initiated CREATE messages. It is the NR's responsibility to
decide whether to teardown the 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.
There is a possible race condition between the RESPONSE message to
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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.
3.8.7.2 Proxying for a Data Receiver
As with data receivers behind middleboxes, data senders behind
middleboxes can require proxy mode support. The issue here is that
there is no NSIS support at the data receiver's side and, by default,
there will be no response to CREATE 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).
DS Private Address NAT/FW Public Internet NR
NI Space NF no NR
| | |
| CREATE-PROXY | |
|------------------------------>| |
| | |
| RESPONSE[SUCCESS/ERROR] | |
|<------------------------------| |
| | |
Figure 20: Proxy Mode CREATE Message Flow
The processing of CREATE-PROXY messages and RESPONSE messages is
similar to Section 3.8.1, except that forwarding is stopped at the
edge-NAT/edge-firewall. The edge-NAT/edge-firewall responds back to
NI according the situation (error/success) and will be the NR for
future NATFW NSLP communication.
The NI can choose the proxy mode of operation although the DR is NSIS
aware. The CREATE-PROXY mode would not configure all NATs and
firewalls along the data path, since it is terminated at the edge-
device. Any device beyond this point will never received any NATFW
NSLP signaling for this flow.
3.9 De-Multiplexing at NATs
Section 3.8.2 describes how NSIS nodes behind NATs can obtain a
public reachable IP address and port number at a NAT and and how the
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resulting mapping rule can be activated by using CREATE messages (see
Section 3.8.1). 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 the local NATFW
NSLP node and the of the policy rule. The policy rule uses the LE-
MRM MRI source-address (see [1]) as the flow destination IP address
and the network-layer-version as IP version. The external IP address
at the NAT is stored as the external flow destination IP address.
All other parameters of the policy rule other than the flow
destination IP address are wildcarded if no NATFW_DTINFO_IPv4 object
is included in the REA request message. The LE-MRM MRI destination-
address MUST NOT be used in the policy rule, since it is solely a
signaling destination address.
If the NATFW_DTINFO_IPv4 object is included in the REA request
message, the policy rule is filled with further information. The
'dst port number' field of the NATFW_DTINFO_IPv4 is stored as the
flow destination port number. The 'protocol' field is stored as the
flow protocol. The 'src port number' field is stored as the flow
source port number. The 'data sender's IPv4 address' is stored as
the flow source IP address. Note that some of these field can
contain wildcards.
When receiving a CREATE message at the NATFW NSLP it uses the flow
information stored in the MRI to do the matching process. This table
shows the parameters to be compared against each others. Note that
not all parameters can be present in a MRI at the same time.
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+-------------------------------+--------------------------------+
| Flow parameter (Policy Rule) | MRI parameter (CREATE message) |
+-------------------------------+--------------------------------+
| IP version | network-layer-version |
| | |
| Protocol | IP-protocol |
| | |
| source IP address (w) | source-address (w) |
| | |
| external IP address | destination-address |
| | |
| destination IP address (n/u) | N/A |
| | |
| source port number (w) | L4-source-port (w) |
| | |
| external port number (w) | L4-destination-port (w) |
| | |
| destination port number (n/u) | N/A |
| | |
| IPsec SPI | ipsec-SPI |
+-------------------------------+--------------------------------+
Table entries marked with (w) can be wildcarded and entries marked
with (n/u) are not used for the matching.
Table 1
3.10 Reacting to Route Changes
The NATFW NSLP needs to react to route changes in the data path.
This assumes the capability to detect route changes, to perform NAT
and firewall configuration on the new path and possibly to tear down
session state on the old path. The detection of route changes is
described in Section 7 of [1] and the NATFW NSLP relies on
notifications about route changes by the NTLP. This notification
will be conveyed by the API between NTLP and NSLP, which is out of
scope of this memo.
A NATFW NSLP node 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, as described in
Section 3.2.3. It is important to consider the transport limitations
of NOTIFY messages as mandated in Section 3.8.5.
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The NI receiving this NOTIFY message MAY generate a new request
CREATE or REA message and sends it respectively downstream or
upstream as for the initial exchange using the same session ID. All
the remaining processing and message forwarding, such as NSLP next
hop discovery, is subject to regular NSLP processing as described in
the particular sections. Normal routing will guide the new request
to the correct NFs along the changed route. NFs that were on the
original path receiving these new request messages (see also
Section 3.11), can use the session ID (session ownership information,
see also Section 3.6) to update the existing session, whereas NFs
that were not on the original path will create new state for this
session. The next section describes how policy rules are updated.
3.11 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 3.4) with modified message routing information
(MRI) as compared with that installed previously, but using the
existing session ID to identify the intended target of the update.
With respect to authorization and authentication, this update request
message is treated in exactly the same way as any initial request.
Therefore, any node along in the signaling session can reject the
update with an error RESPONSE message, as defined in the previous
sections.
The request/response message processing and forwarding is executed as
defined in the those sections. A NF or the NR receiving an update,
simply replaces the installed policy rules installed in the firewall/
NAT. The local procedures on how to update the MRI in the firewall/
NAT is out of scope of this memo.
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4. NATFW NSLP Message Components
A NATFW NSLP message consists of a NSLP header and one or more
objects following the header. The NSLP header is 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 21.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message type |P| reserved | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 21: Common NSLP header
The reserved field MUST be set to zero in the NATFW NSLP header
before sending and MUST be ignored during processing of the header.
The message types identify requests and responses. Defined messages
types are:
o IANA-TBD(1) : CREATE
o IANA-TBD(2) : RESERVE-EXTERNAL-ADDRESS(REA)
o IANA-TBD(3) : TRACE
o IANA-TBD(4) : RESPONSE
o IANA-TBD(5) : NOTIFY
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If a message with another type is received, an error RESPONSE of
class 'Protocol error' (0x3) with response code 'Illegal message
type' (0x01) MUST be generated.
The P flag indicates the usage of proxy mode. If proxy mode is used
it MUST be set to 1. Proxy mode usage is only allowed in combination
with the message types CREATE and REA, P=1 MUST NOT be set with
message types other than CREATE and REA. The P flag MUST be ignored
when processing messages with type RESPONSE. An error RESPONSE
message of class 'Protocol error' (0x3) and type 'Bad flags value'
(0x03) MUST be generated, if the P flag is set in TRACE or NOTIFY
messages.
4.2 NSLP Objects
NATFW NSLP objects use a common header format defined by Figure 22.
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 22: Common NSLP object header
The object length field contains the total length of the object
without the object header. The unit is a word, consisting of 4
octets. The particular values of type and length for each NSLP
object are listed in the subsequent sections that define the NSLP
objects. An error RESPONSE of class 'Protocol error' (0x3) with
response code 'Wrong object length' (0x07) MUST be generated if the
length given for the object in the object header did not match the
length of the object data present. The two leading bits of the NSLP
object header are used to signal the desired treatment for objects
whose treatment has not been defined in this memo (see [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 RESPONSE of
class 'Protocol error' (0x3) with response code 'Unknown object
present' (0x06).
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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 and an error RESPONSE of class
'Protocol error' (0x3) with response code 'Invalid Flag-Field
combination' (0x09) MUST be generated.
The following sections do not repeat the common NSLP object header,
they just name the type and the length.
4.2.1 Session Lifetime Object
The session lifetime object carries the requested or granted lifetime
of a NATFW NSLP session measured in seconds.
Type: NATFW_LT (IANA-TBD)
Length: 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NATFW NSLP session lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 23: Lifetime object
4.2.2 External Address Object
The external address object can be included in RESPONSE messages
(Section 4.3.3) only. It carries the publicly reachable IP address,
and if applicable port number, at an edge-NAT.
Type: NATFW_EXT_IP (IANA-TBD)
Length: 2
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port number | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 24: External Address Object for IPv4 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.
4.2.3 Extended Flow Information Object
In general, flow information is kept in the message routing
information (MRI) of the NTLP. Nevertheless, some additional
information may be required for NSLP operations. The 'extended flow
information' object carries this additional information about the
action of the policy rule for firewalls/NATs and contiguous port .
Type: NATFW_EFI (IANA-TBD)
Length: 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rule action | sub_ports |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 25: Extended Flow Information
This object has two fields, 'rule action' and 'sub_ports'. The 'rule
action' field has these meanings:
o 0x0001: Allow: A policy rule with this action allows data traffic
to traverse the middlebox and the NATFW NSLP MUST allow NSLP
signaling to be forwarded.
o 0x0002: Deny: A policy rule with this action blocks data traffic
from traversing the middlebox and the NATFW NSLP MUST NOT allow
NSLP signaling to be forwarded.
If the 'rule action' field contains neither 0x0001 nor 0x0002, an
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error RESPONSE of class 'Signaling session error' (0x6) with response
code 'Unknown policy rule action' (0x05) MUST be generated.
The 'sub_ports' field contains the number of contiguous transport
layer ports to which this rule applies. The default value of this
field is 0, i.e., only the port specified in the NTLP's MRM is used
for the policy rule. A value of 1 indicates that additionally to the
port specified in the NTLP's MRM (port1), a second port (port2) is
used. This value of port 2 is calculated as: port2 = port1 + 1.
Other values than 0 or 1 MUST NOT be used in this field and an error
RESPONSE of class 'Signaling session error' (0x6) with response code
'Requested value in sub_ports field in NATFW_EFI not permitted'
(0x08) MUST be generated. Further version of this memo may allow
other values for the 'sub_ports' field. This two contiguous port
numbered ports, can be used by legacy voice over IP equipment. This
legacy equipment assumes that two adjacent port numbers for its RTP/
RTCP flows respectively.
4.2.4 Information Code Object
This object carries the response code, which may be indications for
either a successful request or failed request depending on the value
of the 'response code' field.
Type: NATFW_INFO (IANA-TBD)
Length: 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Resv. | Class | Response Code | Object Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 26: Information Code Object
The field 'resv.' is reserved for future extensions and MUST be set
to zero when generating such an object and MUST be ignored when
receiving. The 'Object Type' field contains the type of the object
causing the error. The value of 'Object Type' is set to 0, if no
object is concerned. The 4 bit class field contains the severity
class. The following classes are defined:
o 0x1: Informational (NOTIFY only)
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o 0x2: Success
o 0x3: Protocol error
o 0x4: Transient failure
o 0x5: Permanent failure
o 0x6: Signaling session failures
Within each severity class a number of responses values are defined
o Informational:
* 0x01: Route change: possible route change on the downstream
path.
* 0x02: Re-authentication required.
* 0x03: NATFW node is going down soon.
o Success:
* 0x01: All successfully processed.
o Protocol error:
* 0x01: Illegal message type: the type given in the Message Type
field of the NSLP header is unknown.
* 0x02: Wrong message length: the length given for the message in
the NSLP header does not match the length of the message data.
* 0x03: Bad flags value: an undefined flag or combination of
flags was set in the NSLP header.
* 0x04: Mandatory object missing: an object required in a message
of this type was missing.
* 0x05: Illegal object present: an object was present which must
not be used in a message of this type.
* 0x06: Unknown object present: an object of an unknown type was
present in the message.
* 0x07: Wrong object length: the length given for the object in
the object header did not match the length of the object data
present.
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* 0x08: Unknown object field value: a field in an object had an
unknown value.
* 0x09: Invalid Flag-Field combination: An object contains an
invalid combination of flags and/or fields.
* 0x0A: Duplicate object present.
* 0x0B: Received REA request message on external side.
o Transient failure:
* 0x01: Requested resources temporarily not available.
o Permanent failure:
* 0x01: Authentication failed.
* 0x02: Authorization failed.
* 0x02: Unable to agree transport security with peer.
* 0x03: Internal or system error.
* 0x04: No NAT here.
* 0x05: No edge-device here.
* 0x06: Did not reach the NR.
* 0x07: Tracing is not allowed.
o Signaling session failures:
* 0x01: Session terminated asynchronously.
* 0x02: Requested lifetime is too big.
* 0x03: No reservation found matching the MRI of the CREATE
request.
* 0x04: Requested policy rule denied due to policy conflict.
* 0x05: Unknown policy rule action.
* 0x06: Requested rule action not applicable.
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* 0x07: DTINFO object is required.
* 0x08: Requested value in sub_ports field in NATFW_EFI not
permitted.
* 0x09: Requested IP protocol not supported.
* 0x0A: Plain IP policy rules not permitted -- need transport
layer information.
* 0x0B: ICMP type value not permitted.
* 0x0C: source IP address range is too large.
* 0x0D: destination IP address range is too large.
* 0x0E: source L4-port range is too large.
* 0x0F: destination L4-port range is too large.
4.2.5 Nonce Object
This object carries the nonce value as described in Section 3.8.7.
Type: NATFW_NONCE (IANA-TBD)
Length: 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 27: Nonce Object
4.2.6 Message Sequence Number Object
This object carries the MSN value as described in Section 3.5.
Type: NATFW_MSN (IANA-TBD)
Length: 1
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| message sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 28: Message Sequence Number Object
4.2.7 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_DTINFO_IPv4 (IANA-TBD)
Length: 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|I|P|S| reserved | dest prefix | protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: dst port number | src port number :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: IPsec SPI :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data sender's IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 29: Data Terminal IPv4 Address Object
The flags are:
o I: I=1 means that Protocol should be interpreted.
o I: P=1 means that 'dst port number' and 'src port number' are
present and should be interpreted.
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o S: S=1 means that SPI is present and should be interpreted.
The SPI field is only present if F is set. The port numbers are only
present if P is set. The flags P and F MUST NOT be set at the same
time. An error RESPONSE of class 'Protocol error' (0x3) with
response code 'Invalid Flag-Field combination' (0x09) MUST be
generated if they are both set.
The fields MUST be interpreted according these rules:
o dest prefix: This parameter indicates the prefix length of the
'data sender's 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. This field MUST be interpreted
if I=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 data sender's IPv4 address: The source IP address of the data
sender. This field MUST be set to zero if no IP address is
provided, i.e., a complete wildcard is desired (see dest prefix
field above).
4.2.8 Trace Object
The NATFW_TRACE object may contain zero or more identifiers of
visited NATFW NSLP peers. However, it is only possible to store a
single type of identifier, either IPv4 or IPv6 addresses.
Type: NATFW_TRACE (IANA-TBD)
Length: Variable
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| trace type | hop count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: IP address :
: ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 30: Trace object
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. Other trace types MUST
generate an error RESPONSE of class 'Protocol error' (0x3) with
response code 'Unknown object field value' (0x08). The 'hop count'
field counts the total number of visited NATFW NSLP nodes that are
willing to include their identifier in this object. Each such node
appends its identifier at the end of the object.
4.2.9 NI Credential Object
This object is a container intended to carry credentials provided by
the NI.
Type: NATFW_CREDENTIAL (IANA-TBD)
Length: Variable
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| credential type | credential length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: credential data :
: ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 31: Credential Object
The field 'credential type' field contains one of these values:
o 0x0002: Token
Other trace types MUST generate an error RESPONSE of class 'Protocol
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error' (0x3) with response code 'Unknown object field value' (0x08).
4.2.10 ICMP Types Object
The 'ICMP types' object contains additional information needed to
configure a NAT of firewall with rules to control ICMP traffic. The
object contains a number of values of the ICMP Type field for which a
filter action should be set up:
Type: NATFW_ICMP_TYPES (IANA-TBD)
Length: Variable = ((Number of Types carried + 1) + 3) DIV 4
Where DIV is an integer division.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Count | Type | Type | ........ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ................ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ........ | Type | (Padding) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 32: ICMP Types Object
The fields MUST be interpreted according these rules:
count: 8 bit integer specifying the number of 'Type' entries in
the object.
type: 8 bit field specifying an ICMP Type value to which this rule
applies.
padding: Sufficient 0 bits to pad out the last word so that the
total size of object is an even multiple of words. Ignored on
reception.
4.3 Message Formats
This section defines the content of each NATFW NSLP message type.
The message types are defined in Section 4.1. 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.
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Basically, each message is constructed of NSLP header and one or more
NSLP objects. The order of objects is not defined, meaning that
objects may occur in any sequence. Objects are marked either with
mandatory [M] or optional [O]. Where [M] implies that this
particular object MUST be included within the message and where [O]
implies that this particular object is OPTIONAL within the message.
Objects defined in this memo carry always the flag combination AB=00
in the NSLP object header. An error RESPONSE message of class
'Protocol error' (0x3) with response code 'Mandatory object missing'
(0x02) MUST be generated if a mandatory declared object is missing.
An error RESPONSE message of class 'Protocol error' (0x3) with
response code 'Illegal object present' (0x05) MUST be generated if an
object was present which must not be used in a message of this type.
An error RESPONSE message of class 'Protocol error' (0x3) with
response code 'Duplicate object present' (0x0A) MUST be generated if
an object appears more than once in a message.
Each section elaborates the required settings and parameters to be
set by the NSLP for the NTLP, for instance, how the message routing
information is set.
4.3.1 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 request message carries these objects:
o Lifetime object [M]
o Extended flow information object [M]
o Message sequence number object [M]
o Credential object [O]
o Nonce object [M] if P flag set to 1 in the NSLP header, otherwise
[O]
o ICMP Types Object [O]
The message routing information in the NTLP MUST be set to DS as
source address and DR as destination address. All other parameters
MUST be set according the required policy rule. CREATE messages MUST
be transported by using the path-coupled MRM with direction set to
downstream.
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4.3.2 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.
The REA request message carries these objects:
o Lifetime object [M]
o Message sequence number object [M]
o Extended flow information object [M]
o Credential object [O]
o Data terminal information object [O]
o Nonce object [M if P flag set to 1 in the NSLP header, otherwise
[O]
o ICMP Types Object [O]
The selected message routing method of the REA request message
depends on a number of considerations. Section 3.8.2 describes it
exhaustively how to select the correct method. REA request messages
can be transported via the path-coupled message routing method (PC-
MRM) or via the loose-end message routing method (LE-MRM). In the
case of PC-MRM, the source-address is set to DS' address and the
destination address is set to DR's address, the direction is set to
upstream. In the case of LE-MRM, the destination-address is set to
DR's address or to the signaling destination address. The source-
address is set to DS's address.
4.3.3 RESPONSE
RESPONSE messages are responses to CREATE and REA messages.
The RESPONSE message for the class 'Success' (0x2) carries these
objects:
o Lifetime object [M]
o Message sequence number object [M]
o Information code object [M]
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o External address object [O]
o Trace object [O]
The RESPONSE message for other classes than 'Success' (0x2) carries
these objects:
o Message sequence number object [M]
o Information code object [M]
This message is routed upstream hop-by-hop, using existing NTLP
messaging associations.
4.3.4 NOTIFY
The NOTIFY messages is used to report asynchronous events happening
along the signaled path to other NATFW NSLP nodes.
The NOTIFY message carries this object:
o Information code object [M].
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.
4.3.5 TRACE
The TRACE 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]
o Trace object [M]
TRACE request messages are sent path-coupled (PC-MRM).
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5. Security Considerations
Security is of major concern particularly in case of firewall
traversal. This section provides security considerations for the
NAT/firewall traversal and is organized as follows.
In Section 5.1 we describe the participating entities relate to each
other from a security point of view. This subsection also motivates
a particular authorization model.
Security threats that focus on NSIS in general are described in [8]
and they are applicable to this document as well. Within Section 5.5
we extend this threat investigation by considering NATFW NSLP
specific threats in detail. Based on the investigated security
threats we derive security requirements.
Finally, we illustrate how the security requirements that were
created based on the security threats can be fulfilled by specific
security mechanisms. These aspects will be elaborated in
Section 5.14.
5.1 Authorization Framework
The NATFW NSLP is a protocol which may involve a number of NSIS nodes
and is, as such, not a two-party protocol. Figure 1 and Figure 2 of
[8] already depict the possible set of communication patterns. In
this section we will re-evaluate these communication patters with
respect to the NATFW NSLP protocol interaction.
The security solutions for providing authorization have a direct
impact on the treatment of different NSLPs. As it can be seen from
the QoS NSLP [6] and the corresponding Diameter QoS work [23]
accounting and charging seems to play an important role for QoS
reservations, whereas monetary aspects might only indirectly effect
authorization decisions for NAT and firewall signaling. Hence, there
are differences in the semantic of authorization handling between QoS
and NATFW signaling. A NATFW aware node will most likely want to
authorize the entity (e.g., user or machine) requesting the
establishment of pinholes or NAT bindings. The outcome of the
authorization decision is either allowed or disallowed whereas a QoS
authorization decision might indicate that a different set of QoS
parameters is authorization (see [23] as an example).
5.2 Peer-to-Peer Relationship
Starting with the simplest scenario, it is assumed that neighboring
nodes are able to authenticate each other and to establish keying
material to protect the signaling message communication. An addition
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to authentication the nodes will have to authorize each other. We
use the term 'Security Context' as a placeholder for referring to the
entire security procedure, the necessary infrastructure that needs to
be in place in order for this to work (e.g., key management) and the
established security related state. The required long-term key
(symmetric or asymmetric keys) used for authentication are either
made available using an out-of-band mechanism between the two NSIS
NATFW nodes or they are dynamically established using mechanisms not
further specified in this document. Note that the deployment
environment will most likely have an impact on the choice of
credentials being used. The choice of these credentials used is also
outside the scope of this document.
+------------------------+ +-------------------------+
|Network A | | Network B|
| +---------+ +---------+ |
| +-///-+ Middle- +---///////----+ Middle- +-///-+ |
| | | box 1 | Security | box 2 | | |
| | +---------+ Context +---------+ | |
| | Security | | Security | |
| | Context | | Context | |
| | | | | |
| +--+---+ | | +--+---+ |
| | Host | | | | Host | |
| | A | | | | B | |
| +------+ | | +------+ |
+------------------------+ +-------------------------+
Figure 33: Peer-to-Peer Relationship
Figure 33 shows a possible relationship between participating NSIS
aware nodes. Host A might be, for example, a host in an enterprise
network that has keying material established (e.g., a shared secret)
with the company's firewall (Middlebox 1). The network administrator
of Network A (company network) has created access control lists for
Host A (or whatever identifiers a particular company wants to use).
Exactly the same procedure might also be used between Host B and
Middlebox 2 in Network B. For the communication between Middlebox 1
and Middlebox 2 a security context is also assumed in order to allow
authentication, authorization and signaling message protection to be
successful.
5.3 Intra-Domain Relationship
In larger corporations, for example, a middlebox is used to protect
individual departments. In many cases, the entire enterprise is
controlled by a single (or a small number of) security department,
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which gives instructions to the department administrators. In such a
scenario, a the previously discussed peer-to-peer 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 exchange the authorization
decision with another middlebox within the same trust domain.
Individual middleboxes within an administrative domain may exploit
their relationship instead of requesting authentication and
authorization of the signaling initiator again and again. Figure 34
illustrates a network structure which uses a centralized entity.
+-----------------------------------------------------------+
| Network A |
| +---------+ +---------+
| +----///--------+ Middle- +------///------++ Middle- +---
| | Security | box 2 | Security | box 2 |
| | Context +----+----+ Context +----+----+
| +----+----+ | | |
| | Middle- +--------+ +---------+ | |
| | box 1 | | | | |
| +----+----+ | | | |
| | Security | +----+-----+ | |
| | Context | | Policy | | |
| +--+---+ +-----------+ Decision +----------+ |
| | Host | | Point | |
| | A | +----------+ |
| +------+ |
+-----------------------------------------------------------+
Figure 34: Intra-domain Relationship
The interaction between individual middleboxes and a policy decision
point (or AAA server) is outside the scope of this document.
5.4 End-to-Middle Relationship
The peer-to-peer relationship between neighboring NSIS NATFW NSLP
nodes might not always be sufficient. Network B might require
additional authorization of the signaling message initiator (in
addition to the authorization of the neighboring node). If
authentication and authorization information is not attached to the
initial signaling message then the signaling message arriving at
Middlebox 2 would result in an error message being created, which
indicates the additional authorization requirement. In many cases
the signaling message initiator might already aware of the
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additionally required authorization before the signaling message
exchange is executed.
Figure 35 shows this scenario.
+--------------------+ +---------------------+
| Network A | |Network B |
| | Security | |
| +---------+ Context +---------+ |
| +-///-+ Middle- +---///////----+ Middle- +-///-+ |
| | | box 1 | +-------+ box 2 | | |
| | +---------+ | +---------+ | |
| |Security | | | Security | |
| |Context | | | Context |
| | | | | | |
| +--+---+ | | | +--+---+ |
| | Host +----///----+------+ | | Host | |
| | A | | Security | | B | |
| +------+ | Context | +------+ |
+--------------------+ +---------------------+
Figure 35: End-to-Middle Relationship
5.5 Security Threats and Requirements
This section describes NATFW specific security threats and
requirements.
5.5.1 Data Sender (DS) behind a firewall
+------------------------------+
| |
| +-----+ create +-----+
| | DS | --------------> | FW |
| +-----+ +-----+
| |
+------------------------------+
DS sends a CREATE message to request the traversal of a data flow.
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 requesting an action.
Authentication and authorization of the initiator SHOULD be
provided to NATs and firewalls along the path.
5.5.2 Data Sender (DS) behind a NAT
The case 'DS behind a NAT' is analogous to the case 'DS behind a
firewall'.
Figure 37 illustrates such a scenario:
+------------------------------+
| |
| +------+ CREATE |
| | NI_1 | ------\ +-----+ CREATE +-----+
| +------+ \------> | NAT |-------->| MB |
| +-----+ +-----+
| +------+ |
| | NI_2 | |
| +------+ |
+------------------------------+
Figure 37: 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 non-neighboring middleboxes
SHOULD be provided.
5.5.3 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 | |
+-----+ <------------- +-----+ <------------- +-----+ |
successful RESPONSE | successful 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 successful 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 39 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 39: 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 39. Many operators might not allow NSIS
signaling message to traverse the firewall in Figure 39 without
having the DR to interact with the FW first.
SECURITY REQUIREMENT: No requirements are created by this scenario.
5.5.4 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
(i.e., external address and port) allocated at the NAT possibly by
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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
<------------------------------------------>
/ \
/ +----------------------\-------+
v | REA v |
+-----+ +-----+ <----------- +-----+ |
| DS | -> | NAT | -----------> | DR | |
+-----+ / +-----+ DR_external +-----+ |
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.
5.5.5 NSLP Message Injection
Malicious hosts, located either off-path or on-path, could inject
arbitrary NATFW NSLP messages into the signaling path. 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 NOTIFY, can be misused by
malicious hosts, and therefore MUST be authorized by the
respective NATFW NLSP entities.
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5.6 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.
5.6.1 Flooding with CREATE messages from outside
5.6.1.1 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 could launch a DoS attack with an arbitrary number of
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
establishment of state information.
5.6.1.2 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 exchange
mechanisms.
5.6.1.3 Attacking 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 node MUST be configurable to block
unauthorized signaling message.
5.6.2 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.
5.7 Man-in-the-Middle Attacks
Figure 41 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. Mallory 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.
M will be able to read, modify or block the data traffic (if the end-
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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 41: 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 41 the firewall FW must
perform an authorization with the neighboring entities.
5.8 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.
5.9 Message Modification by malicious NSIS node
Message modification by an NSIS node that became malicious is more
serious. An adversary could easily create arbitrary pinholes or NAT
bindings. For example:
o NATs need to modify the source/destination of the data flow in the
'create session' message.
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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: Malicious NSIS NATs and firewalls will not be
addressed by this specification.
5.10 Session Modification/Deletion
Section 4.10 in [8] describes a threat where an adversary is able to
modify previously installed state information at NATFW NSLP nodes
along the path. An adversary therefore needs to know session
specific information, such as the session identifier and MRI
information.
SECURITY REQUIREMENT: No countermeasure will be provided as part of
this document. The fact that the adversary needs to learn the
randomly generated Session-ID already provides some degree of
protection (although not perfect protection).
5.10.1 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 end host.
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Public Private Address
Internet space
+----------+ +----------+
+----------| NAT |------------------|End host |
| | | |
+----------+ +----------+
|
| +----------+
\--------------------|Malicious |
|End host |
+----------+
data traffic
<========================
Figure 42: 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 42 where an the end host(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:
o Redirecting packets to the attacker as in Figure 43.
o Third party flooding by redirecting packets to arbitrary hosts
o 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 43: Connection Hijacking
SECURITY REQUIREMENT: A NAT/FW signaling message MUST be
authenticated, integrity and replay protected between neighboring
NAT/FW NSLP nodes. The NSIS NATFW NSLP aware NAT MUST authorize
the end host to insure that the messages are indeed belonging to
the previously established session.
5.11 Misuse of unreleased sessions
Assume that DS (N1) initiates NSIS session with DR (N2) through a
series of middleboxes as in Figure 44. 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 45. 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 | |
| | successful RESPONSE | | | | |
| |==============>======| |====>===| | |
+-------+ Data Traffic +---+-----+ +-------+ |
| |
+--------------------------+
Figure 44: Before mobility
Public Internet
+--------------------------+
| |
+-------+ +---+-----+ +-------+ |
| | | | | | |
| N1 |==============>======| FW |====>===| N3 | |
| | Data Traffic | | | | |
+-------+ +---+-----+ +-------+ |
| |
+--------------------------+
Figure 45: 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: In order to allow firewalls to verify that a
legitimate end host indeed transmitted data traffic it is
necessary to provide data origin authentication. This is,
however, outside the scope of this document. Hence, there are no
security requirements imposed by this threat, which will be
addressed by the NATFW NSLP.
5.12 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
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practice by itself represents a security weakness. Using IPy
spoofing a connection, an attacker an adversary is able to reach the
target machines if they match , 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 countermeasures need to be
taken by 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 46. 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 46: Ingress filtering
5.13 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 information to inject its own data traffic due to the IP
spoofing capability already mentioned in Section 5.12. An on-path
adversary could also observe the data traffic and he could conclude
that it is possible to traverse a firewall.
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. This threat is discussed in
the respective document suggesting the usage of authorization token
in the NSIS protocol suite.
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 MAY
be necessary to offer confidentiality protection. The latter
aspect is outside the scope of this document.
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5.14 Security Framework for the NAT/Firewall NSLP
Based on the identified threats a list of security requirements has
been created.
5.14.1 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 security protocol, support for multiple authentication
protocols might 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. Almost all security threats at the NATFW NSLP layer can be
prevented by using a mutually authenticated Transport Layer secured
connection and by relying on authorization by the neighboring NATFW
NSLP entities.
5.14.2 Security Protection between non-neighboring NATFW NSLP Nodes
Based on the security threats and the listed requirements it was
noted that some scenarios threats also demand authentication and
authorization of a NATFW signaling entity (including the initiator)
towards a non-neighboring node. This mechanism mainly demands entity
authentication. Additionally, security protection of certain
payloads MAY is be required between non-neighboring signaling
entities and the Cryptographic Message Syntax (CMS) [17] SHOULD be
used. The most important information exchanged at the NATFW NSLP is
information related to the establishment for firewall pinholes and
NAT bindings. This information can, however, not be protected over
multiple NSIS NATFW NSLP hops since this information might change
depending on the capability of each individual NATFW NSLP node.
Protection using CMS is not described in this document.
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Some scenarios might also benefit from the usage of authorization
tokens. Their purpose is to associate two different signaling
protocols (e.g., SIP and NSIS) and their authorization decision.
These tokens are obtained by non-NSIS protocols, such as SIP or as
part of network access authentication. When a NAT or firewall along
the path receives the token it might be verified locally or passed to
the AAA infrastructure.
Examples of authorization tokens or assertions can be found in RFC
3520 [21] and RFC 3521 [22]. Security Assertion Markup Language
(SAML) is an example for a more recent mechanisms carrying
authorization specific assertions. For details about SAML see [24],
[25] and [26]. Figure 47 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 successfully 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 47: Authorization Token Usage
Threats against the usage of authorization tokens have been mentioned
in [8] and also in Section 5.5. 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.
This document does provide an initial specification of an NATFW NSLP
object for usage of authorization tokens. The NATFW_CREDENTIAL
object can carry authorization token or any other type.
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6. IAB Considerations on UNSAF
UNilateral Self-Address Fixing (UNSAF) is described in [15] as a
process at originating endpoints that attempt to determine or fix the
address (and port) by which they are known to another endpoint.
UNSAF proposals, such as STUN [RFC3489] are considered as a general
class of workarounds for NAT traversal and as solutions for scenarios
with no middlebox communication.
This memo specifies a path-coupled middlebox communication protocol,
i.e., the NSIS NATFW NSLP. NSIS in general and the NATFW NSLP are
not intended as a short-term workaround, but more as a long-term
solution for middlebox communication. In NSIS, endpoints are
involved in allocating, maintaining, and deleting addresses and ports
at the middlebox. However, the full control of addresses and ports
at the middlebox is at the NATFW NSLP daemon located to the
respective NAT.
Therefore, this document addresses the UNSAF considerations in
[RFC3424] by proposing a long-term alternative solution.
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7. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the
NATFW NSLP, in accordance with BCP 26 RFC 2434 [16].
The NATFW NSLP requires IANA to create a number of new registries.
These registries may require further coordination with the registries
of the NTLP [1] and the QoS NSLP [6].
NATFW NSLP Message Type Registry
The NATFW NSLP Message Type is a 8 bit value. The allocation of
values for new message types requires standards action. Updates and
deletion of values from the registry is not possible. This
specification defines five NATFW NSLP message types, which form the
initial contents of this registry. IANA is requested to add these
five NATFW NSLP Message Types: CREATE, REA, TRACE, RESPONSE, and
NOTIFY.
NATFW NSLP Header Flag Registry
NATFW NSLP messages have a messages-specific 8 bit flags/reserved
field in their header. The registration of flags is subject to IANA
registration. The allocation of values for flag types requires
standards action. Updates and deletion of values from the registry
is not possible. This specification defines only one flag, the P
flag in Figure 21.
NSLP Object Type Registry
This document defines 10 objects for the NATFW NSLP: NATFW_LT,
NATFW_EXT_IP, NATFW_EFI, NATFW_INFO, NATFW_NONCE, NATFW_MSN,
NATFW_DTINFO_IPv4, NATFW_TRACE, NATFW_CREDENTIAL, NATFW_ICMP_TYPES.
The allocation of values for new message types requires standards
action. IANA is request to assigned values for them from NSLP Object
Type registry and to replace the corresponding IANA-TBD tags with the
numeric values.
NSLP Response Code Registry
In addition it defines a number of Response Codes for the NATFW NSLP.
These can be found in Section 4.2.4 and are to be assigned values
from NSLP Response Code registry. The allocation of values for
Response Codes Codes requires standards action. IANA is request to
assigned values for them from NSLP Response Code registry.
Furthermore, IANA is requested to add a new value to the NSLP
Identifiers (NSLPID) registry defined in [1] for the the NATFW NSLP.
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8. Open Issues
A more detailed list of open issue can be found at:
https://kobe.netlab.nec.de/roundup/nsis-natfw-nslp/index
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9. Acknowledgments
We would like to thank the following individuals for their
contributions to this document at different stages:
o Marcus Brunner and Henning Schulzrinne for work on work on IETF
drafts which lead us to start with this document,
o Miquel Martin for his help on the initial version of this
document,
o Srinath Thiruvengadam and Ali Fessi work for their work on the
NAT/firewall threats draft,
o Henning Peters for his comments and suggestions,
o and the NSIS working group.
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10. References
10.1 Normative References
[1] 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 Next
Steps in Signaling (NSIS)", RFC 4081, June 2005.
[9] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999.
[10] 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] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
[16] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[17] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369,
August 2002.
[18] 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.
[19] 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.
[20] 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.
[21] Hamer, L-N., Gage, B., Kosinski, B., and H. Shieh, "Session
Authorization Policy Element", RFC 3520, April 2003.
[22] Hamer, L-N., Gage, B., and H. Shieh, "Framework for Session
Set-up with Media Authorization", RFC 3521, April 2003.
[23] Alfano, F., "Diameter Quality of Service Application",
draft-alfano-aaa-qosprot-05 (work in progress), October 2005.
[24] Maler, E., Philpott, R., and P. Mishra, "Bindings and Profiles
for the OASIS Security Assertion Markup Language (SAML) V1.1",
September 2003.
[25] Maler, E., Philpott, R., and P. Mishra, "Assertions and
Protocol for the OASIS Security Assertion Markup Language
(SAML) V1.1", September 2003.
[26] Maler, E. and J. Hughes, "Technical Overview of the OASIS
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Security Assertion Markup Language (SAML) V1.1", March 2004.
[27] 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.
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
Elwyn Davies
Folly Consulting
Soham
UK
Phone: +44 7889 488 335
Email: elwynd@dial.pipex.com
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Appendix A. Selecting Signaling Destination 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'.
1. Public IP address of the data sender:
* Assumption:
+ The data receiver already learned the IP address of the
data sender (e.g., via a third party).
* Problems:
+ The data sender might also be behind a NAT. In this case
the public IP address of the data receiver is the IP
address allocated at this NAT.
+ Due to routing asymmetry it might be possible that the
routes taken by a) the data sender and the application
server b) the data sender and NAT B might be different. As
a consequence it might be necessary to advertise a new (and
different) external IP address 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:
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+ An application server (or a different third party) is
available.
* Problems:
+ If the NSIS signaling message is not terminated at the NAT
of the local network then an NSIS unaware application
server might discard the message.
+ Routing might not be optimal since the route between a) the
data receiver and the application server b) the data
receiver and the data sender might be different.
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Appendix B. Applicability Statement on Data Receivers behind Firewalls
Section 3.8.2 describes how data receivers behind middleboxes can
instruct upstream firewalls/NATs to forward NATFW NSLP signaling
towards them. Finding an upstream edge-NAT in address environment
with NAT'ed addresses is quite easy. It is only required to find
some edge-NAT, as the data traffic will be route-pinned to the NAT,
which is done with the LE-MRM. Locating the appropriate edge-
firewall with the PC-MRM, sent upstream is difficult. For cases with
a single, symmetric route from the Internet to the data receiver, it
is quite easy; simply follow the default route in the upstream
direction.
+------+ Data Flow
+-------| EFW1 +----------+ <===========
| +------+ ,--+--.
+--+--+ / \
NI+-----| FW1 | (Internet )----NR+/NI/DS
NR +--+--+ \ /
| +------+ `--+--'
+-------| EFW2 +----------+
+------+
~~~~~~~~~~~~~~~~~~~~~>
Signaling Flow
Figure 48: Data receiver behind multiple, parallel located firewalls
When a data receiver, and thus NR, is located in a network site that
is multihomed with several independently firewalled connections to
the public Internet (as shown in Figure 48), the specific firewall
through which the data traffic will be routed has to be ascertained.
NATFW NSLP signaling messages sent from the NI+/NR during the REA
request message exchange towards the NR+ must be routed by the NTLP
to the edge-firewall that will be passed by the data traffic as well.
The NTLP would need to be aware about the routing within the Internet
to determine the path between DS and DR. Out of this, the NTLP could
determine which of the edge-firewalls, either EFW1 or EFW2, must be
selected to forward the NATFW NSLP signaling. Signaling to the wrong
edge-firewall, as shown in Figure 48, would install the NATFW NSLP
policy rules at the wrong device. This causes either a blocked data
flow (when the policy rule is 'allow') or an ongoing attack (when the
policy rule is 'deny'). Requiring the NTLP to know all about the
routing within the Internet is definitely a tough challenge and
usually not possible. In such described case, the NTLP must
basically give up and return an error to the NSLP level, indicating
that the next hop discovery is not possible.
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Appendix C. Firewall and NAT Resources
This section gives some examples on how NATFW NSLP policy rules could
be mapped to real firewall or NAT resources. The firewall rules and
NAT bindings are described in a natural way, i.e., in a way one will
find it in common implementation.
C.1 Wildcarding of Policy Rules
The policy rule/MRI to be installed can be wildcarded to some degree.
Wildcarding applies to IP address, transport layer port numbers, and
the IP payload (or next header in IPv6). Processing of wildcarding
splits into the NTLP and the NATFW NSLP layer. The processing at the
NTLP layer is independent of the NSLP layer processing and per layer
constraints apply. For wildcarding in the NTLP see Section 5.8 of
[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.
C.2 Mapping to Firewall Rules
This section describes how a NSLP policy rule signaled with a CREATE
request message is mapped to a firewall rule. The MRI is set as
follows:
o network-layer-version=IPv4
o source-address=192.0.2.100, prefix-length=32
o destination-address=192.0.50.5, prefix-length=32
o IP-protocol=UDP
o L4-source-port=34543, L4-destination-port=23198
The NATFW_EFI object is set to action=allow and sub_ports=0.
The resulting policy rule (firewall rule) to be installed might look
like: allow udp from 192.0.2.100 port=34543 to 192.0.50.5 port=23198
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C.3 Mapping to NAT Bindings
This section describes how a NSLP policy rule signaled with a REA
request message is mapped to a NAT binding. It is assumed that the
REA message is sent by a NI+ being located behind a NAT and does
contain a NATFW_DTINFO object. The MRI is set following using the
signaling destination address, since the IP address of the real data
sender is not known:
o network-layer-version=IPv4
o source-address= 192.168.5.100
o destination-address=SDA
o IP-protocol=UDP
The NATFW_EFI object is set to action=allow and sub_ports=0. The
NATFW_DTINFO object contains these parameters:
o P=1
o dest prefix=0
o protocol=UDP
o dst port number = 20230, src port number=0
o src IP=0.0.0.0
The edge-NAT allocates the external IP 192.0.2.79 and port 45000.
The resulting policy rule (NAT binding) to be installed could look
like: translate from any to 192.0.2.79 port=45000 to 192.168.5.100
port=20230
C.4 NSLP Handling of Twice-NAT
The dynamic configuration of twice-NATs requires application level
support, as stated in Section 2.5. The NATFW NSLP cannot be used for
configuring twice-NATs if application level support is needed.
Assuming application level support performing the configuration of
the twice-NAT and the NATFW NSLP being installed at this devices, the
NATFW NSLP must be able to traverse it. The NSLP is probably able to
traverse the twice-NAT, as any other data traffic, but the flow
information stored in the NTLP's MRI will be invalidated through the
translation of source and destination address. The NATFW NSLP
implementation on the twice-NAT MUST intercept NATFW NSLP and NTLP
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signaling messages as any other NATFW NSLP node does. For the given
signaling flow, the NATFW NSLP node MUST look up the corresponding IP
address translation and modify the NTLP/NSLP signaling accordingly.
The modification results in an updated MRI with respect to the source
and destination IP addresses.
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