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24 25 RFC 5973
NSIS Working Group M. Stiemerling
Internet-Draft NEC
Expires: April 25, 2005 H. Tschofenig
Siemens
M. Martin
NEC
C. Aoun
Nortel Networks
October 25, 2004
NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
draft-ietf-nsis-nslp-natfw-04
Status of this Memo
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patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
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This Internet-Draft will expire on April 25, 2005.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This memo defines the NSIS Signaling Layer Protocol (NSLP) for
Network Address Translators and Firewalls. This NSLP allows hosts to
signal along a data path for Network Address Translators and
Firewalls to be configured according to the data flow needs. The
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network scenarios, problems and solutions for path-coupled Network
Address Translator and Firewall signaling are described. The overall
architecture is given by the framework and requirements defined by
the Next Steps in Signaling (NSIS) working group.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Terminology and Abbreviations . . . . . . . . . . . . . . 7
1.2 Middleboxes . . . . . . . . . . . . . . . . . . . . . . . 8
1.3 Non-Goals . . . . . . . . . . . . . . . . . . . . . . . . 9
1.4 General Scenario for NATFW Traversal . . . . . . . . . . . 9
2. Network Deployment Scenarios using NATFW NSLP . . . . . . . . 11
2.1 Firewall Traversal . . . . . . . . . . . . . . . . . . . . 11
2.2 NAT with two private Networks . . . . . . . . . . . . . . 12
2.3 NAT with Private Network on Sender Side . . . . . . . . . 12
2.4 NAT with Private Network on Receiver Side Scenario . . . . 13
2.5 Both End Hosts behind twice-NATs . . . . . . . . . . . . . 14
2.6 Both End Hosts Behind Same NAT . . . . . . . . . . . . . . 15
2.7 IPv4/v6 NAT with two Private Networks . . . . . . . . . . 15
2.8 Multihomed Network with NAT . . . . . . . . . . . . . . . 16
3. Protocol Description . . . . . . . . . . . . . . . . . . . . . 18
3.1 Policy Rules . . . . . . . . . . . . . . . . . . . . . . . 18
3.2 Basic protocol overview . . . . . . . . . . . . . . . . . 18
3.3 Protocol Operations . . . . . . . . . . . . . . . . . . . 20
3.3.1 Creating Sessions . . . . . . . . . . . . . . . . . . 21
3.3.2 Reserving External Addresses . . . . . . . . . . . . . 23
3.3.3 NATFW Session refresh . . . . . . . . . . . . . . . . 27
3.3.4 Deleting Sessions . . . . . . . . . . . . . . . . . . 28
3.3.5 Reporting Asynchronous Events . . . . . . . . . . . . 29
3.3.6 QUERY capabilities within the NATFW NSLP protocol . . 30
3.3.7 QUERY Message semantics . . . . . . . . . . . . . . . 31
3.3.8 Reserving External Addresses and triggering Create
messages . . . . . . . . . . . . . . . . . . . . . . . 32
3.3.9 Using CREATE messages to Trigger Reverse Path
CREATE Messages . . . . . . . . . . . . . . . . . . . 34
3.4 Calculation of Session Lifetime . . . . . . . . . . . . . 36
3.5 Firewall and NAT Resources . . . . . . . . . . . . . . . . 38
3.6 De-Multiplexing at NATs . . . . . . . . . . . . . . . . . 38
3.7 Selecting Opportunistic Addresses for REA . . . . . . . . 38
4. NATFW NSLP NTLP Requirements . . . . . . . . . . . . . . . . . 40
5. NATFW NSLP Message Components . . . . . . . . . . . . . . . . 41
5.1 NSLP Header . . . . . . . . . . . . . . . . . . . . . . . 41
5.2 NSLP message types . . . . . . . . . . . . . . . . . . . . 41
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5.3 NSLP Objects . . . . . . . . . . . . . . . . . . . . . . . 42
5.3.1 Session Lifetime Object . . . . . . . . . . . . . . . 42
5.3.2 External Address Object . . . . . . . . . . . . . . . 43
5.3.3 Extended Flow Information Object . . . . . . . . . . . 44
5.3.4 Response Code Object . . . . . . . . . . . . . . . . . 45
5.3.5 Response Type Object . . . . . . . . . . . . . . . . . 45
5.3.6 Message Sequence Number Object . . . . . . . . . . . . 46
5.3.7 Scoping Object . . . . . . . . . . . . . . . . . . . . 46
5.3.8 Bound Session ID Object . . . . . . . . . . . . . . . 47
5.3.9 Notify Target Object . . . . . . . . . . . . . . . . . 47
5.4 Message Formats . . . . . . . . . . . . . . . . . . . . . 48
5.4.1 CREATE . . . . . . . . . . . . . . . . . . . . . . . . 48
5.4.2 RESERVE-EXTERNAL-ADDRESS (REA) . . . . . . . . . . . . 49
5.4.3 TRIGGER . . . . . . . . . . . . . . . . . . . . . . . 49
5.4.4 RESPONSE . . . . . . . . . . . . . . . . . . . . . . . 49
5.4.5 QUERY . . . . . . . . . . . . . . . . . . . . . . . . 50
5.4.6 NOTIFY . . . . . . . . . . . . . . . . . . . . . . . . 50
6. NSIS NAT and Firewall Transition Issues . . . . . . . . . . . 51
7. Security Considerations . . . . . . . . . . . . . . . . . . . 52
7.1 Trust Relationship and Authorization . . . . . . . . . . . 52
7.1.1 Peer-to-Peer Trust Relationship . . . . . . . . . . . 53
7.1.2 Intra-Domain Trust Relationship . . . . . . . . . . . 54
7.1.3 End-to-Middle Trust Relationship . . . . . . . . . . . 55
8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 57
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 58
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 59
10.1 Normative References . . . . . . . . . . . . . . . . . . . . 59
10.2 Informative References . . . . . . . . . . . . . . . . . . . 59
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 62
A. Problems and Challenges . . . . . . . . . . . . . . . . . . . 63
A.1 Missing Network-to-Network Trust Relationship . . . . . . 63
A.2 Relationship with routing . . . . . . . . . . . . . . . . 64
A.3 Affected Parts of the Network . . . . . . . . . . . . . . 64
A.4 NSIS backward compatibility with NSIS unaware NAT and
Firewalls . . . . . . . . . . . . . . . . . . . . . . . . 64
A.5 Authentication and Authorization . . . . . . . . . . . . . 65
A.6 Directional Properties . . . . . . . . . . . . . . . . . . 65
A.7 Addressing . . . . . . . . . . . . . . . . . . . . . . . . 66
A.8 NTLP/NSLP NAT Support . . . . . . . . . . . . . . . . . . 66
A.9 Combining Middlebox and QoS signaling . . . . . . . . . . 66
A.10 Inability to know the scenario . . . . . . . . . . . . . . 67
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B. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 68
Intellectual Property and Copyright Statements . . . . . . . . 69
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1. Introduction
Firewalls and Network Address Translators (NAT) have both been used
throughout the Internet for many years, and they will remain present
for the foreseeable future. Firewalls are used to protect networks
against certain types of attacks from the outside, and in times of
IPv4 address depletion, NATs virtually extend the IP address space.
Both types of devices may be obstacles to many applications, since
they only allow traffic created by a limited set of applications to
traverse them (e.g., most HTTP traffic, and client/server
applications), due to the relatively static properties of the
protocols used. Other applications, such as IP telephony and most
other peer-to-peer applications, which have more dynamic properties,
create traffic which is unable to traverse NATs and Firewalls
unassisted. In practice, the traffic from many applications cannot
traverse autonomous Firewalls or NATs, even when they have added
functionality which attempts to restore the transparency of the
network.
Several solutions to enable applications to traverse such entities
have been proposed and are currently in use. Typically, application
level gateways (ALG) have been integrated with the Firewall or NAT to
configure the Firewall or NAT dynamically. Another approach is
middlebox communication (MIDCOM, currently under standardization at
the IETF). In this approach, ALGs external to the Firewall or NAT
configure the corresponding entity via the MIDCOM protocol [7].
Several other work-around solutions are available, including STUN
[35] and TURN [37]. However, all of these approaches introduce other
problems that are generally hard to solve, such as dependencies on
the type of NAT implementation (full-cone, symmetric, ...), or
dependencies on a certain network topology.
NAT and Firewall (NATFW) signaling shares a property with Quality of
Service (QoS) signaling. The signaling of both must reach any device
on the data path that is involved in QoS or NATFW treatment of data
packets. This means, that for both, NATFW and QoS, it is convenient
if signaling travels path-coupled, meaning that the signaling
messages follow exactly the same path that the data packets take.
RSVP [14] is an example of a current QoS signaling protocol that is
path-coupled.
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 [2]. The general framework of NSIS is outlined
in [1]. 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 GIMPS [3]
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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 [4], while the NATFW NSLP is defined in
this document.
The NATFW NSLP is designed to request the dynamic configuration of
NATs and/or Firewalls along the data path to enable data flows to
traverse these devices without being obstructed. Here is a
simplified example: A source host sends a NATFW NSLP signaling
message towards its data destination. This message follows the data
path. Every NATFW NSLP NAT/Firewall along the data path intercepts
these messages, processes them, and configures itself accordingly.
Afterwards, the actual data flow can traverse every configured
Firewall/NAT.
NATFW NSLP runs in two different modes, one is the CREATE mode in
which state at firewalls and NATs is created. In the above example,
this takes place in the direction from the data sender to the data
receiver. The other mode is the RESERVE mode. In this mode, NATs
are discovered by the NSLP/NTLP signaling messages, and a publicly
reachable IP address and a port number are reserved at each NAT.
This mode enables hosts located in a private addressing realm
delimited by a NAT to subsequently receive data originated in the
public network. Both modes create NATFW NSLP and NTLP state in
network entities. NTLP state allows signaling messages to travel in
the forward (downstream ) and the reverse (upstream) direction along
the path between an NAT/Firewall NSLP sender and a corresponding
receiver. NAT bindings and firewall rules are NAT/Firewall specific
state. This state is managed using a soft-state mechanism, i.e., it
expires unless it is refreshed every now and then by a CREATE
message. If state is to be deleted explicitly before it
automatically expires, another message can be used for that. To find
out what state is currently installed in NSIS NAT/Firewall nodes, a
QUERY message can be used at any time.
Section 2 describes the network environment for NATFW NSLP signaling,
highlighting the trust relationships and authorization required.
Section 3 defines the NATFW signaling protocol. Section 5 defines
the messages and and message components. In the remaining parts of
the main body of the document, Section 6 covers transition issues,
while Section 7 addresses security considerations, with more
extensive discussions of security issues currently being contained in
[20]. Currently unsolved problems and challenges are listed and
discussed in Appendix A. Please note that readers familiar with
Firewalls and NATs and their possible location within networks can
safely skip Section 2.
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1.1 Terminology and Abbreviations
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
This document uses a number of terms defined in [2]. The following
additional terms are used:
o NSIS NAT Forwarding State: This term refers to a state used to
forward the NSIS signaling message towards the targeted
destination address.
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" [38]. In the context
of NSIS NATFW NSLP, the condition is a specification of a set of
packets to which rules are applied. The set of actions always
contains just a single element per rule, and is limited to either
action "reserved", "deny" or action "allow".
o Firewall: A packet filtering device that matches packets against a
set of policy rules and applies the actions. In the context of
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 realm to another,
in an attempt to provide transparent routing between hosts (see
[8]). Network Address Translators are devices that perform this
work.
o Middlebox: "A middlebox is defined as any intermediate device
performing functions other than the normal, standard functions of
an IP router on the datagram path between a source host and a
destination host" [12]. In the context of this document, the term
middlebox refers to Firewalls and NATs only. Other types of
middlebox are currently outside the scope of this document.
o Security Gateway: IPsec based gateways.
o (Data) Receiver (DR or R): The node in the network that is
receiving the data packets of a flow.
o (Data) Sender (DS or S): The node in the network that is sending
the data packets of a flow.
o NATFW NSLP session: An application layer flow of information for
which some network control state information is to be manipulated
or monitored (as defined in [1]). 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 [3].
o Edge NAT: An edge NAT is a NAT device that is reachable from the
public Internet and that has a globally routable IP address.
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o Edge Firewall: An edge Firewall is a Firewall device that is
located on the demarcation line of an administrative domain.
o Public Network: "A Global or Public Network is an address realm
with unique network addresses assigned by Internet Assigned
Numbers Authority (IANA) or an equivalent address registry. This
network is also referred as External network during NAT
discussions" [8].
o Private/Local Network: "A private network is an address realm
independent of external network addresses. Private network may
also be referred alternately as Local Network. Transparent
routing between hosts in private realm and external realm is
facilitated by a NAT router" [8]. IP address space allocation for
private networks is recommended in [36]
o Public/Global IP address: An IP address located in the public
network according to Section 2.7 of [8].
o Private/Local IP address: An IP address located in the private
network according to Section 2.8 of [8].
o Initial CREATE: A CREATE message creating a new session.
1.2 Middleboxes
The term middlebox covers a range of devices which intercept the flow
of packets between end hosts and perform actions other than standard
forwarding expected in an IP router. As such, middleboxes fall into
a number of categories with a wide range of functionality, not all of
which is pertinent to the NATFW NSLP. Middlebox categories in the
scope of this memo are Firewalls that filter data packets against a
set of filter rules, and NATs that translate packet addresses from
one address realm to another address realm. Other categories of
middleboxes, such as QoS traffic shapers and security gateways, are
out of scope.
The term NAT used in this document is a placeholder for a range of
different NAT flavors. We consider the following types of NATs:
o traditional NAT (basic NAT and NAPT)
o Bi-directional NAT
o Twice-NAT
o Multihomed NAT
For definitions and a detailed discussion about the characteristics
of each NAT type please see [8].
Both types of middleboxes under consideration here use policy rules
to make a decision on data packet treatment. Policy rules consist of
a flow identifier which selects the packets to which the policy
applies and an associated action; data packets matching the flow
identifier are subjected to the policy rule action. A typical flow
identifier is the 5-tuple selector which matches the following fields
of a packet to configured values:
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o Source and destination IP addresses
o Transport protocol number
o Transport source and destination port numbers
For further examples of flow identifiers see Section 5.1 of [3].
Actions for Firewalls are usually one or more of:
o Allow: forward data packet
o Deny: block data packet and discard it
o Other actions such as logging, diverting, duplicating, etc
Actions for NATs include (amongst many others):
o Change source IP address and transport port number to a globally
routeable IP address and associated port number.
o Change destination IP address and transport port number to a
private IP address and associated port number.
1.3 Non-Goals
Traversal of non-NSIS and non-NATFW NSLP aware NATs and Firewalls
is outside the scope of this document.
Only Firewalls and NATs are considered in this document, any other
types of devices, for instance IPSec security gateway, are out of
scope.
The exact implementation of policy rules and their mapping to
firewall rule sets and NAT bindings or sessions at the middlebox
is an implementation issue and thus out of scope of this document.
Some devices categorized as firewalls only accept traffic after
cryptographic verification (i.e., IPsec protected data traffic).
Particularly for network access scenarios, either link layer or
network layer data protection is common. We do not address these
types of devices (referred to as security gateways) since per-flow
signaling is typically not used in this environment.
Another application, for which NSIS signaling has been proposed
but which is out of scope for this document, is discovering
security gateways, for the purpose of executing IKE to create an
IPsec SA.
In mobility scenarios, a common problem is the traversal of a
security gateway at the edge of a corporate network. Network
administrators allow only authenticated data to enter the network.
A problem statement for the traversal of these security gateways
in the context of Mobile IP can be found in [28]). This topic is
not within the scope of the present document.
1.4 General Scenario for NATFW Traversal
The purpose of NSIS NATFW signaling is to enable communication
between endpoints across networks even in the presence of NAT and
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Firewall middleboxes. It is assumed that these middleboxes will be
statically configured in such a way that NSIS NATFW signaling
messages themselves are allowed to traverse them. NSIS NATFW NSLP
signaling is used to dynamically install additional policy rules in
all NATFW middleboxes along the data path. Firewalls are configured
to forward data packets matching the policy rule provided by the NSLP
signaling. NATs are configured to translate data packets matching
the policy rule provided by the NSLP signaling.
The basic high-level picture of NSIS usage is that end hosts are
located behind middleboxes, meaning that there is a middlebox on the
data path from the end host in a private network and the external
network (NAT/FW in Figure 1). Applications located at these end
hosts try to establish communication with corresponding applications
on other such end hosts. They trigger the NSIS entity at the local
host to provide for middlebox traversal along the prospective data
path (e.g., via an API call). The NSIS entity in turn uses NSIS
NATFW NSLP signaling to establish policy rules along the data path,
allowing the data to travel from the sender to the receiver
unobstructed.
Application Application Server (0, 1, or more) Application
+----+ +----+ +----+
| +------------------------+ +------------------------+ |
+-+--+ +----+ +-+--+
| |
| NSIS Entities NSIS Entities |
+-+--+ +----+ +-----+ +-+--+
| +--------+ +----------------------------+ +-----+ |
+-+--+ +-+--+ +--+--+ +-+--+
| | ------ | |
| | //// \\\\\ | |
+-+--+ +-+--+ |/ | +-+--+ +-+--+
| | | | | Internet | | | | |
| +--------+ +-----+ +----+ +-----+ |
+----+ +----+ |\ | +----+ +----+
\\\\ /////
sender NAT/FW (1+) ------ NATFW (1+) receiver
Figure 1: Generic View on NSIS in a NAT / Firewall case
For end-to-end NATFW signaling, it is necessary that each firewall
and each NAT along the path between the data sender and the data
receiver implement the NSIS NATFW NSLP. There might be several NATs
and FWs in various possible combinations on a path between two hosts.
Section 2 presents a number of likely scenarios with different
combinations of NATs and firewalls.
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2. Network Deployment Scenarios using NATFW NSLP
This section introduces several scenarios for middlebox placement
within IP networks. Middleboxes are typically found at various
different locations, including at Enterprise network borders, within
enterprise networks, as mobile phone network gateways, etc. Usually,
middleboxes are placed more towards the edge of networks than in
network cores. Firewalls and NATs may be found at these locations
either alone, or they may be combined; other categories of
middleboxes may also be found at such locations, possibly combined
with the NATs and/or Firewalls. To reduce the number of network
elements needed, combined Firewall and NATs have been made available.
NSIS initiators (NI) send NSIS NATFW NSLP signaling messages via the
regular data path to the NSIS responder (NR). On the data path,
NATFW NSLP signaling messages reach different NSIS peers 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.
2.2 NAT with two private Networks
Figure 3 shows a scenario with NATs at both ends of the network.
Therefore, each application instance, NSIS initiator and NSIS
responder, are behind NATs. The outermost NAT 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
number of ways to learn its public IP address and port number and
might need to signal this information to the sender using the
application level signaling protocol.
2.3 NAT with Private Network on Sender Side
This scenario shows an application instance at the sending node that
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is behind one or more NATs (shown as generic MB, see discussion in
Section 2.2). The receiver is located in the public Internet.
+----+ +----+ //----\\
NI --| MB |-----| MB |---| |--- NR
+----+ +----+ \\----//
private public
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 4: NAT with Private Network on Sender Side Scenario
The traffic from NI to NR has to traverse middleboxes only on the
sender's side. The receiver has a public IP address. The NI sends
its signaling message directly to the address of the NSIS responder.
Middleboxes along the path intercept the signaling messages and
configure the policy rules accordingly.
Note that the data sender does not necessarily know whether the
receiver is behind a NAT or not, hence, it is the receiving side that
has to detect whether itself is behind a NAT or not. As described in
Section 3.3.2 NSIS can also provide help for this procedure.
2.4 NAT with Private Network on Receiver Side Scenario
The application instance receiving data is behind one or more NATs
shown as MB (see discussion in Section 2.2).
//----\\ +----+ +----+
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
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this protocol to the NI. Afterwards the NI can start its signaling
towards the NR and so establish the path via the middleboxes in the
receiver side private network.
This scenario describes the use case for the RESERVE mode of the
NATFW NSLP.
2.5 Both End Hosts behind twice-NATs
This is a special case, where the main problem arises from the need
to detect that both end hosts are logically within the same address
space, but are also in two partitions of the address realm on either
side of a twice-NAT (see [8] for a discussion of twice-NAT
functionality).
Sender and receiver are both within a single private address realm
but the two partitions potentially have overlapping IP address
ranges. Figure 6 shows the arrangement of NATs. This is a common
configuration in networks, particularly after the merging of
companies that have used the same private address space, resulting in
overlapping address ranges.
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, at private and public
interfaces.
This scenario requires the assistance of application level entities,
such as a DNS server. The application level gateways must handle
requests that are based on symbolic names, and configure the
middleboxes so that data packets are correctly forwarded from NI to
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NR. The configuration of those middleboxes may require other
middlebox communication protocols, such as MIDCOM [7]. NSIS
signaling is not required in the twice-NAT only case, since
middleboxes of the twice-NAT type are normally configured by other
means. Nevertheless, NSIS signaling might by useful when there are
also Firewalls on path. In this case NSIS will not configure any
policy rule at twice-NATs, but will configure policy rules at the
Firewalls on the path. The NSIS signaling protocol must be at least
robust enough to survive this scenario.
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
signaling messages to the responder's public IP address. During this
process, a public IP address will be allocated for the NSIS initiator
at the same middlebox as for the responder. Now, the NSIS signaling
and the subsequent data packets will traverse the NAT twice: from
initiator to public IP address of responder (first time) and from
public IP address of responder to responder (second time). This is
the worst case in which both sender and receiver obtain a public IP
address at the NAT, and the communication path is certainly not
optimal in this case.
NI public
\ +----+ //----\\
+-| MB |----| |
/ +----+ \\----//
NR
private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 7: NAT to Public, Both Hosts Behind Same NAT
The NSIS NATFW signaling protocol should support mechanisms to detect
such a scenario. The signaling should be exchanged directly between
NI and NR without involving the middlebox.
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
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translation, i.e., using Network Address and Protocol Translators
(NAT-PT, [11]).
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
This scenario needs the same type of application level support as
described in Section 2.5, and so the issues relating to twice-NATs
apply here as well.
2.8 Multihomed Network with NAT
The previous sub-sections sketched network topologies where several
NATs and/or Firewalls are ordered sequentially on the path. This
section describes a multihomed scenario with two NATs placed on
alternative paths to the public network.
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+----+
NI -------| MB |\
\ +----+ \ //---\\
\ -| |-- NR
\ \\---//
\ +----+ |
--| MB |-------+
+----+
private
private public
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 9: Multihomed Network with Two NATs
Depending on the destination or load balancing requirements, either
one or the other middlebox is used for the data flow. Which
middlebox is used depends on local policy or routing decisions.
NATFW NSLP must be able to handle this situation properly, see
Section 3.3.2 for an expanded discussion of this topic with respect
to NATs.
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3. Protocol Description
This section defines messages, objects, and protocol semantics for
the NATFW NSLP. Section 3.1 introduces the base element of a NSLP
session , the policy rule. Section 3.2 introduces the protocol and
the protocol behavior is defined in Section 3.3. Section 5 defines
the syntax of the messages and objects.
3.1 Policy Rules
Policy rules, bound to a session, are the building block of middlebox
devices considered in the NATFW NSLP. For Firewalls the policy rule
usually consists of a 5-tuple, source/destination addresses,
transport protocol, and source/destination port numbers, plus an
action such as allow or deny. For NATs the policy rule consists of
action 'translate this address' and further mapping information, that
might be, in the simplest case, internal IP address and external IP
address.
Policy rules are usually carried in one piece in signaling
applications. In NSIS the policy rule is divided into the flow
identifier, an allow or deny action, and additional information. The
filter specification is carried within NTLP's message routing
information (MRI) and additional information, including the
specification of the action, is carried in NSLP's objects.
Additional information is, for example, the lifetime of a policy rule
or session.
3.2 Basic protocol overview
The NSIS NATFW NSLP is carried over the NSIS Transport Layer Protocol
(NTLP) defined in [3]. NATFW NSLP messages are initiated by the NSIS
initiator (NI), handled by NSIS forwarders (NF) and finally processed
by the NSIS responder (NR). It is required that at least NI and NR
implement this NSLP, intermediate NFs only implement this NSLP when
they provide relevant middlebox functions. NSIS forwarders that do
not have any NATFW NSLP functions just forward these packets when
they have no interest.
A Data Sender (DS), intending to send data to a Data Receiver (DR)
must first initiate NATFW NSLP signaling. This causes the NI
associated with the data sender (DS) to launch NSLP signaling towards
the address of data receiver DR (see Figure 10). Although it is
expected that the DS and the NATFW NSLP NI will normally reside on
the same host, this specification does not rule out scenarios where
the DS and NI reside on different hosts.
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+-------+ +-------+ +-------+ +-------+
| DS/NI |<~~~| MB1/ |<~~~| MB2/ |<~~~| DR/NR |
| |--->| NF1 |--->| NF2 |--->| |
+-------+ +-------+ +-------+ +-------+
========================================>
Data Traffic Direction
---> : NATFW NSLP request signaling
~~~> : NATFW NSLP response signaling
DS/NI : Data sender and NSIS initiator
DR/NR : Data receiver and NSIS responder
MB1 : Middlebox 1 and NSIS forwarder 1
MB2 : Middlebox 2 and NSIS forwarder 2
Figure 10: General NSIS signaling
The sequence of NSLP events is as follows:
o NSLP request messages are processed each time a NF with NATFW NSLP
support is traversed. These nodes process the message, check
local policies for authorization and authentication, possibly
create policy rules, and forward the signaling message to the next
NSIS node. The request message is forwarded until it reaches the
NSIS responder.
o NSIS responders will check received messages and process them if
applicable. NSIS responders generate response messages and send
them hop-by-hop back to the NI via the same chain of NFs
(traversal of the same NF chain is guaranteed through the
established reverse message routing state in the NTLP).
o The response message is processed at each NF implementing the
NATFW NSLP.
o Once the NI has received a successful response, the Data Sender
can start sending its data flow to the Data Receiver.
Because NATFW NSLP signaling follows the data path from DS to DR,
this immediately enables communication between both hosts for
scenarios with only Firewalls on the data path or NATs on sender
side. For scenarios with NATs on the receiver side certain problems
arise, as described in Section 2.
When the NR and the NI are located in different address realms and
the NR is behind a NAT, the NI cannot signal to the NR directly. The
DR and NR are not reachable from the NIs using the private address of
the NR and thus NATFW signaling messages cannot be sent to the NR/
DR's address. Therefore, the NR must first obtain a NAT binding that
provides an address that is reachable for the NI. Once the NR has
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acquired a public IP address, it forwards this information to the DS
via a separate protocol (such as SDP within SIP). This application
layer signaling, which is out of scope of the NATFW NSLP, may involve
third parties that assist in exchanging these messages.
NATFW NSLP signaling supports this scenario by using the RESERVE mode
of operation
1. The NR acquires a public address by signaling on the reverse path
(NR towards NI) and thus making itself available to other hosts.
This process of acquiring a public addresses is called
reservation. During this process the DR reserves publicly
reachable addresses and ports suitable for NATFW NSLP signaling,
but data traffic will not be allowed to use this address/port
initially.
2. The NI signals directly to the NR as the NI would do if there is
no NAT in between, and creates policy rules at middleboxes.
Note, that the reservation mode will only allow the forwarding
of signaling messages but not data flow packets. Data flow
packets will be 'activated' by the signaling from NI towards NR.
The RESERVE mode of operation is detailed in Section 3.3.2
The protocol works on a soft-state basis, meaning that whatever state
is installed or reserved on a middlebox will expire, and thus be
de-installed/ forgotten after a certain period of time. To prevent
this, the NATFW nodes involved will have to specifically request a
session extension. An explicit NATFW NSLP state deletion capability
is also provided by the protocol.
Middleboxes should return an error in case of a failure, such that
appropriate actions can be taken; this ability would allow debugging
and error recovery. Error messages could be sent upstream (for
errors related to received messages as well as asynchronous error
notification messages) towards the NI as well as downstream towards
the NR (in the case of asynchronous error notification messages).
The next sections define the NATFW NSLP message types and formats,
protocol operations, and policy rule operations.
3.3 Protocol Operations
This section defines the protocol operations, how to create sessions,
maintain them, and how to reserve addresses. All the NATFW NSLP
protocol messages require C-mode handling by the NTLP and cannot be
piggybacked into D-mode NTLP messages used during the NTLP path
discovery/refresh phase. The usage of the NTLP by protocol messages
is described in more details in Section 5.
The protocol uses six messages:
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o CREATE: a request message used for creating, changing, refreshing
and deleting NATFW NSLP sessions.
o RESERVE-EXTERNAL-ADDRESS (REA): a request message used for
reserving an external address
o RESPONSE: used as a response to CREATE, REA and QUERY messages
with Success or Error information
o QUERY: a request message used by authorized NATFW NEs for querying
installed NATFW states
o NOTIFY: an asynchronous message used by NATFW NEs to alert
upstream and/or downstream NATFW NEs about specific events
(especially failures).
o TRIGGER: a message sent upstream to trigger the sending of CREATE
messages
The following sections will present the semantics of these messages
by exhibiting their impact on the protocol state machine.
3.3.1 Creating Sessions
Allowing two hosts to exchange data even in the presence of
middleboxes is realized in the NATFW NSLP by the CREATE request
message. The data sender generates a CREATE message as defined in
Section 5.4.1 and hands it to the NTLP. The NTLP forwards the whole
message on the basis of the message routing information towards the
NR. Each NSIS forwarder along the path that implements NATFW NSLP,
processes the NSLP message. Forwarding is thus managed NSLP
hop-by-hop but may pass transparently through NSIS forwarders which
do not contain NATFW NSLP functionality and non-NSIS aware routers
between NSLP hop waypoints. 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
11 sketches the message flow between NI (DS), a NF (NAT), and NR
(DR).
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NI Private Network NF Public Internet NR
| | |
| CREATE | |
|----------------------------->| |
| | |
| RESPONSE[Error](if necessary)| |
|<-----------------------------| CREATE |
| |--------------------------->|
| | |
| | RESPONSE[Success/Error] |
| RESPONSE[Success/Error] |<---------------------------|
|<-----------------------------| |
| | |
| | |
Figure 11: Creation message flow
Since the CREATE message is used for several purposes within the
lifetime of a session, there are several processing rules for NATFW
NEs when generating and receiving CREATE messages. The different
processing methods depend not only on the function which the CREATE
is performing (to create, modify, refresh or delete a session) but
also on the node at which the processing happens. For an initial
CREATE message the processing of CREATE messages is different for
every NSIS node type:
o NSLP initiator: NI only generates initial CREATE messages and
hands them over to the NTLP. After receiving a successful
response, the data path is configured and the DS can start
sending its data to the DR. After receiving an 'error' response
message the NI MAY try to generate the CREATE message again or
give up and report the failure to the application, depending on
the error condition.
o NATFW NSLP forwarder: NFs receiving an initial CREATE message
MUST first check authentication and authorization before any
further processing is executed. The NF SHOULD check with its
local policies if it can accept the desired policy rule given the
combination of the NTLP's 'Message-Routing-Information' (MRI) [3]
(the flow description information) and the CREATE payload
(behavior to be enforced on the packet stream). An initial CREATE
is distinguished from subsequent CREATE messages by the presence
of existing NSLP session state related to the same session ID or
the same MRI. The NSLP message processing depends on the
middlebox type:
* NAT: When the initial CREATE message is received at the public
side of the NAT, it looks for a reservation made in advance, by
using a REA message Section 3.3.2, that matches the destination
address/port of the MRI provided by the NTLP. If no
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reservation had been made in advance the NSLP SHOULD return an
error response message of type 'no reservation found' and
discard the request. If there is a reservation, NSLP stores
the data sender's address as part of the policy rule to be
loaded and forwards the message with the address set to the
internal (private in most cases) address of the next NSIS node.
When the initial CREATE message, for a new session, is received
at the private side the NAT binding is reserved, but not
activated. The NSLP message is forwarded to the next NSIS hop
with source address set to the NAT's external address from the
newly reserved binding.
* Firewall: When the initial CREATE message is received the NSLP
just remembers the requested policy rule, but does not install
any policy rule. Afterwards, the message is forwarded to the
next NSLP hop. There is a difference between requests from
trusted (authorized NIs) and un-trusted (un-authorized NIs);
requests from trusted NIs will be pre-authorized, whereas
requests from un-trusted NIs will not be pre-authorized. This
difference is required to speed-up the protocol operations as
well as for proxy mode usage (please refer to Section 3.3.8 and
[17]).
* Combined NAT and Firewall: Processing at combined Firewall and
NAT middleboxes is the same as in the NAT case. No policy
rules are installed. Implementations MUST take into account
the order of packet processing in the Firewall and NAT
functions within the device. This will be referred to as
'order of functions' and is generally different depending on
whether the packet arrives at the external or internal side of
the middlebox.
o NSLP receiver: NRs receiving initial CREATE messages MUST reply
with a 'success' (response object has success information)
RESPONSE message if they accept the CREATE request message.
Otherwise they SHOULD generate a RESPONSE message with an error
code. RESPONSE messages are sent back NSLP hop-by-hop towards the
NI, independently of the response codes, either success or error.
Policy rules at middleboxes MUST be only installed upon receiving a
successful response. This is a countermeasure to several problems,
for example wastage of resources due to loading policy rules at
intermediate NF when the CREATE message does not reach the final NR
for some reason.
3.3.2 Reserving External Addresses
NSIS signaling is intended to travel end-to-end, even in the presence
of NATs and Firewalls on-path. This works well in cases where the
data sender is itself behind a NAT as described in Section 3.3.1.
For scenarios where the data receiver is located behind a NAT and
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needs to receive data flows from outside its own network (see Figure
5) the problem is more troublesome. NSIS signaling, as well as
subsequent data flows, are directed to a particular destination IP
address that must be known in advance and reachable.
+-------------+ AS-Data Receiver Communication
+-------->| Application |<-----------------------------+
| | Server | |
| +-------------+ |
| IP(R-NAT_B) |
| NSIS Signaling Message +-------+--+
| +------------------------------------------>| NAT/NAPT |
| | | B |
| | +-------+--+
| | |
AS-Data| | |
Receiver| | +----------+ |
Comm.| | | NAT/NAPT | |
| | | A | |
| | +----------+ |
| | |
| | |
| | |
| | |
v | IP(R) v
+--------+ +---------+
| Data | | Data |
| Sender | | Receiver|
+--------+ +---------+
Figure 12: The Data Receiver behind NAT problem
Figure 12 describes a typical message communication in a peer-to-peer
networking environment whereby the two end points learn of each
others existence with the help of a third party (referred to as an
Application Server). Communication between the application server
and each of the two end points (data sender and data receiver)
enables the two end hosts to learn each other's IP addresses. The
approach described in this memo supports this peer-to-peer approach,
but is not limited to it.
Some sort of communication between the data sender/data receiver and
a third party is typically necessary (independently of whether NSIS
is in use). NSIS signaling messages cannot be used to communicate
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the relevant application level end point identifiers (in the generic
case at least) as a replacement for communication with the
application server.
If the data receiver is behind a NAT then an NSIS signaling message
will be addressed to the IP address allocated at the NAT (assuming
one had already been allocated). If no corresponding NSIS NAT
Forwarding State at NAT/NAPT B exists (binding IP(R-NAT B) <-> IP(R))
then the signaling message will terminate at the NAT device (most
likely without generating a proper response message). The signaling
message transmitted by the data sender cannot install the NAT binding
or NSIS NAT Forwarding State "on-the-fly" since this would assume
that the data sender knows the topology at the data receiver side
(i.e., the number and the arrangement of the NAT and the private IP
address(es) of the data receiver). The primary goal of path-coupled
middlebox communication was not to avoid end hosts learning and
preserving this type of topology knowledge.
Public Internet Private Address
Space
Edge
NI(DS) NAT NAT NR(DR)
NR+ NI+
| | | |
| | | |
| | | |
| | REA | REA |
| |<----------------------|<----------------------|
| | | |
| |RESPONSE[Success/Error]|RESPONSE[Success/Error]|
| |---------------------->|---------------------->|
| | | |
| | | |
============================================================>
Data Traffic Direction
Figure 13: Reservation message flow
Figure 13 shows the message flow for reserving an external address/
port at a NAT. In this case the roles of the different NSIS entities
are:
o The data receiver (DR) for the anticipated data traffic is the
NSIS initiator (NI+) for the RESERVE-EXTERNAL-ADDRESS (REA)
message, but becomes the NSIS responder (NR) for following CREATE
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messages.
o The actual data sender (DS) will be the NSIS initiator (NI) for
later CREATE messages and may be the NSIS target of the signaling
(NR+).
o The actual target of the REA message, the Opportunistic Address
(OA) an arbitrary address, that would force the message to get
intercepted by the far outmost NAT in the network.
The NI+ agent (could be on the data receiver DR or on any other host
within the private network) sends a the REA message targeted to the
Opportunistic Address (OA). The OA selection for this message is
discussed in Section 3.7. The message routing for the REA message is
in the reverse direction to the normal message routing used for
path-coupled signaling where the signaling is sent downstream (as
opposed to upstream in this case). When establishing NAT bindings
(and NSIS NAT Forwarding State) the direction does not matter since
the data path is modified through route pinning due to the external
NAT address. Subsequent NSIS messages (and also data traffic) will
travel through the same NAT boxes.
The REA signaling message creates NSIS NAT Forwarding State at any
intermediate NSIS NAT node(s) encountered. Furthermore it has to be
ensured that the edge NAT device is discovered as part of this
process. The end host cannot be assumed to know this device -
instead the NAT box itself is assumed to know that it is located at
the outer perimeter of the private network addressing realm.
Forwarding of the REA message beyond this entity is not necessary,
and should be prohibited as it provides information on the
capabilities of internal hosts.
The edge NAT device responds to the REA message with a RESPONSE
message containing a success object carrying the public reachable IP
address/port number.
Processing of REA messages is specific to the NSIS node type:
o NSLP initiator: NI+ only generate REA messages and should never
receive them.
o NSLP forwarder: NSLP forwarders receiving REA messages MUST first
check authentication and authorization before any further
processing is executed. The NF SHOULD check with its local
policies if it can accept the desired policy rule given by NTLP's
message routing information (MRI). Further processing depends on
the middlebox type:
* NAT: NATs check whether the message is received at the
external (public in most cases) address or at the internal
(private) address. If received at the external address a NF
MAY generate a RESPONSE message with an error of type 'REA
received from outside'. If received at the internal address,
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an IP address/port is reserved. In the case it is an edge-NAT,
the NSLP message is not forwarded any further and a RESPONSE
message with the external address and port information is
generated. If it is not an edge-NAT, the NSLP message is
forwarded further with the translated IP address/port (if
required by the NI+).
* Firewall: Firewalls MUST not change their configuration upon a
REA message. They simply MUST forward the message and MUST
keep NTLP state. Firewalls that are configured as
edge-Firewalls MAY return an error of type 'no NAT here'.
* Combined NAT and Firewall: Processing at combined Firewall and
NAT middleboxes is the same as in the NAT case.
o NSLP receiver: This type of message should never be received by
any NR+ and it SHOULD be discarded silently.
Processing of a RESPONSE message with an external address object is
different for every NSIS node type:
o NSLP initiator: Upon receiving a RESPONSE message with an
external address object, the NI+ can use the IP address and port
pairs carried for further application signaling.
o NSLP forwarder: NFs simply forward this message as long as they
keep state for the requested reservation.
o NSIS responder: This type of message should never be received by
an NR and it SHOULD be discarded silently.
o Edge-NATs: This type of message should never be received by any
Edge-NAT and it SHOULD be discarded silently.
3.3.3 NATFW Session refresh
NATFW NSLP sessions are maintained on a soft-state basis. After a
specified timeout, sessions and corresponding policy rules are
removed automatically by the middlebox, if they are not refreshed.
The protocol uses a CREATE message to refresh sessions. Even if used
for refresh purposes the CREATE message requires that a response
message is generated by the NR. This response message is routed back
towards the NI, to allow the intermediate NFs to propose a refresh
period that would align with their local policies. The NI sends
CREATE messages destined for the NR. Upon reception by each NSIS
forwarder, the state for the given session ID is extended by the
session refresh period, a period of time calculated based on a
proposed refresh message period. The lifetime extension of a session
is calculated as current local time plus proposed lifetime value
(session refresh period). Section 3.4 defines the process of
calculating lifetimes in detail.
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NI Public Internet NAT Private address NR
| | space |
| CREATE[lifetime > 0] | |
|----------------------------->| |
| | |
| RESPONSE[Error] (if needed) | |
|<-----------------------------| CREATE[lifetime > 0] |
| |--------------------------->|
| | |
| | RESPONSE[Success/Error] |
| RESPONSE[Success/Error] |<---------------------------|
|<-----------------------------| |
| | |
| | |
Figure 14: State Refresh Message Flow
Processing of session refresh CREATE messages is different for every
NSIS node type:
o NSLP initiator: The NI can generate session refresh CREATE
messages before the session times out. The rate at which the
refresh CREATE messages are sent and their relation to the session
state lifetime are further discussed in Section 3.4. The message
routing information and the extended flow information object MUST
be set equal to the values of the initial CREATE request message.
o NSLP forwarder: NSLP forwarders receiving session refresh messages
MUST first check authentication and authorization before any
further processing is executed. The NF SHOULD check with its
local policies if it can accept the desired lifetime extension for
the session referred by the session ID. Processing of this
message is independent of the middlebox type.
o NSLP responder: NRs accepting a session refresh CREATE message
generate a RESPONSE message with response object set to success.
3.3.4 Deleting Sessions
NATFW NSLP sessions may be deleted at any time. NSLP initiators can
trigger this deletion by using a CREATE messages with a lifetime
value set to 0, as shown in Figure 15.
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NI Public Internet NAT Private address NR
| | space |
| CREATE[lifetime=0] | |
|----------------------------->| |
| | |
| | CREATE[lifetime=0] |
| |--------------------------->|
| | |
Figure 15: Delete message flow
NSLP nodes receiving this message MUST delete the session
immediately. Policy rules associated with this particular session
MUST be deleted immediately. This message is forwarded until it
reaches the final NR. The CREATE request message with a lifetime
value of 0, does not generate any response, neither positive nor
negative, since there is no NSIS state left at the nodes along the
path.
3.3.5 Reporting Asynchronous Events
NATFW NSLP forwarders and NATFW NSLP responders must have the ability
to report asynchronous events to other NATFW NSLP nodes, especially
to allow reporting back to the NATFW NSLP initiator. Such
asynchronous events may be premature session termination, changes in
local policies, routing 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 are the only events that are defined, but other events may
be defined in later versions of this memo. One or several events
could be reported within the NOTIFY message.
NFs and NRs may generate NOTIFY messages upon asynchronous events,
with a response object indicating the reason of the event. There are
two suggested mode of operations:
1. NOTIFY messages are sent hop-by-hop upstream towards NI. Those
NOTIFY messages may be sent downstream towards NR, if generated
by a NF, if needed. Indication to send NOTIFY messages
downstream to NR is indicated within the CREATE message.
2. During session creation, via CREATE or REA, NIs may insert a
special 'notify address' object into the NSLP message, indicating
a node's address that should be notified about this event. When
this address is used, NOTIFY messages are not sent to the NR.
Note: Should this mode include the first one?, in that case NR
could be reached by providing its address as the 'notify address'
this would use 32 or 128 bits instead of a flag bit as done in
the first mode.
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Processing is different for every NATFW NSLP node type and depends on
the notified events :
o NSLP initiator: NIs receiving NOTIFY messages MUST first check for
authentication and authorization. After successfully doing so,
NIs analyze the notified event(s) and behave appropriately based
on the event type. Section 5.3.4 discusses the required behavior
for each notified event. NIs MUST NOT generate NOTIFY messages.
o NSLP forwarder: NFs receiving NOTIFY messages MUST first check for
authentication and authorization. After successfully doing so,
NFs analyze the notified event(s) and behave based on the notified
events defined in Section 5.3.4. NFs occurring an asynchronous
event generate NOTIFY messages and set the response object(s) code
based on the reported event(s). NOTIFY messages are sent
hop-by-hop upstream towards NI (This depends on the design choice
mentioned above).
o NSLP responder: NRs may generate NOTIFY messages. NRs receiving
NOTIFY messages MUST first check for authentication and
authorization. After successfully doing so, NRs analyze the
notified event(s) and behave appropriately based on the event
type. Section 5.3.4 discusses the required behavior for each
notified event. NRs occurring an asynchronous event generate
NOTIFY messages and set the response object(s) code based on the
reported event(s). NOTIFY messages are sent hop-by-hop upstream
towards NI (This depends on the design choice mentioned above).
3.3.6 QUERY capabilities within the NATFW NSLP protocol
The NATFW NSLP provides query capabilities that could be used by a
session owner to track the session state. This would be used for
diagnostic purposes when no data packets were received and the policy
rule was supposed to have been created on the NATFW NFs.
The QUERY message could be used to query the following session
information: session id, flow source, destination and status of the
state listed in best status to worst status: up, high traffic (used
to detect DOS attack or unexpected traffic rate), pending, down. The
status of the policy rule will probably provide sufficient diagnostic
information; in case more diagnostic information is required it could
be provided by the NATFW NF logs. Session status is only provided by
an NF if no session status was provided in the QUERY message or the
NF's session status is worse than the one provided by the queried
upstream NEs. The Session information could be retrieved by sending
a QUERY against a specific session id, a flow source and destination
or user identifier with session id or flow source and destination.
QUERY message processing is different for every NATFW NSLP node type:
o NSLP initiator: NIs only generate QUERY messages, but never with
session status information, so that received QUERY messages MUST
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be discarded.
o NSLP forwarder: NFs receiving QUERY messages MUST first check for
authentication and authorization. After successfully doing so,
NFs will behave differently depending on the QUERY. If the QUERY
is about a specific session: if it contains a session status the
NF compares it to the current local session status; if no session
status is provided in the QUERY message the NF will insert its own
session status in the QUERY message. If the current local session
status is worse, it will incorporate its own session status field
in the QUERY message. Every NF will provide the flow description
in case it was not inside the QUERY. Once the message processing
is done, if the message was not scoped then NF will forward the
QUERY message to the next downstream node.
o NSLP responder: NRs (any node being the destination of the
message) receiving QUERY messages MUST first check for
authentication and authorization. After successfully doing so,
NRs must process the message as the NFs and respond with a
RESPONSE message to the NI. The RESPONSE message will travel
along the established reverse path given by the message routing
state.
Responses to QUERY messages are processed differently for every NATFW
NSLP node type:
o NSLP initiator: NIs receiving RESPONSEs to QUERY messages MUST
first check for authentication and authorization. After
successfully doing so, the objects within the RESPONSE messages
are provided up to the application layers and the session state
remains as it was unless the application triggers NATFW NSLP state
changes.
o NSLP forwarder: NFs receiving RESPONSEs to QUERY messages MUST
first check for authentication and authorization. After
successfully doing so, NFs forward the message upstream without
any interpretation.
o NSLP responder: if an NR receives a RESPONSE to QUERY message it
MUST discard it.
3.3.7 QUERY Message semantics
From a semantics perspective, the QUERY messages may require the
following information incorporated within the messages:
o Session ID
o Flow source (address and port) and destination (address and port),
in case the flow doesn't use a transport protocol a protocol
number would be used with another identifier (SPI for IPsec)
QUERY responses should provide the following information:
o List of active sessions
o Editor's note: next version will discuss in which form the list
publishes the active sessions (by session id or session ID and
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flow description or other formats)
o Information related to a session (when the query is specific to
one session): session ID, flow description and policy rule state
information
3.3.8 Reserving External Addresses and triggering Create messages
Some migration scenarios need specialized support to cope with cases
where only the receiving side is running NSIS. End-to-end signaling
is going to fail without NSIS support at both data sender and data
receiver, unless the NATFW NSLP also gives the NR the ability to
install sessions. The goal of the described method is to trigger the
network to generate a CREATE message at the edge NAT. In this case,
a NR can signal towards the Opportunistic Address as is done in the
standard REA message handling scenario Section 3.3.2. The message is
forwarded until it reaches the edge-NAT and retrieves a public IP
address and port number. As shown in Figure 16, unlike the standard
REA message handling case, the edge-NAT is triggered to send a CREATE
message on a new reverse path to handle asymmetric routes, which
could go through internal firewalls or NATs. This behavior requires
within the REA message an indication to the edge NAT if either the
standard behavior is required or a CREATE message is required to be
sent by the edge-NAT. In addition when a CREATE message need to be
sent by the edge-NAT, the REA message includes the data sender
address if available to the NR.
DS Public Internet NAT Private address NR
No NI | space
| | REA[CREATE] |
| |<------------------------- |
| | RESPONSE[Error/Success] |
| | ---------------------- > |
| | CREATE |
| | ------------------------> |
| | RESPONSE[Error/Success] |
| | <---------------------- |
| | |
| | |
Figure 16: REA Triggering Sending of CREATE Message on Separate
Reverse Path
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If a CREATE message is received from the far end NI and relates to an
installed session (correlation being made with the message routing
information, MRI), that CREATE message would have precedence over the
previous CREATE. The CREATE sent by the NI could specify a more
granular policy rule as only the data sender (the triggered CREATE
may not have that information) could send data whereas in the REA
triggered CREATE message any data source can send packets to the data
receiver. The edge NAT is not aware of the applications context for
which the CREATE messages were required. Hence it is up to the NR to
inform the Edge NAT if there was a possibility to reduce the number
of accepted data sources to the real data sender, as well as to
inform the Edge NAT to refresh the established session.
For that purpose the NR will send TRIGGER messages, to the edge NAT
that responded to the REA message. These messages are sent upon
reception, from the user application, of further information on the
Data Sender (either explicit information or implied information such
as far end-host data sender address is equivalent to data reception
address and same for the transport port). The TRIGGER messages would
be sent periodically to the Edge NAT that responded to the REA. The
TRIGGER messages would be sent until either a CREATE message is
received from the far-end or when the user application no longer
needs the NSIS session. Figure 17 shows how TRIGGER messages would
be used after the message sequences of Figure 16. Triggered CREATE
message follow the path pinned down by the earlier CREATE messages
triggered by the REA message in Figure 16. If a CREATE message is
received from the far end NI and relates to an installed session,
that CREATE message would have precedence over the triggered CREATE
messages. TRIGGER messages do not require to be responded back with
a RESPONSE message on the existing established reverse path. In case
normal CREATE messages (i.e. not sent by the NI) where not received
by the NR and the session is no longer needed, the NR could use the
TRIGGER message indicating that the session should be removed (CREATE
message will be sent by edge NAT with lifetime=0). The benefits of
using REA triggering a CREATE and then using the TRIGGER messages are
that an end-host does not need to know if the far-end host support
the NSIS protocol.
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Foo.com Public Internet Bar.com
DS NAT Firewall NR
No NI | | TRIGGER[DSinfo]
TRIGGER[DSinfo]<-------------|
<-------------| |
|CREATE |
|----------->|CREATE |
| |-------------->|
| | RESPONSE[SUCCESS]
| | <-------------|
RESPONSE[SUCCESS] |
|<-----------| |
Refresh period expiry |
or updates to Data Sender information
| |
| | TRIGGER[DSinfo]
TRIGGER[DSinfo]<-------------|
<-------------| |
|CREATE |
|----------->|CREATE |
| |-------------->|
| | RESPONSE[SUCCESS]
| | <-------------|
RESPONSE[SUCCESS] |
|<-----------| |
Figure 17: TRIGGER message usage
3.3.9 Using CREATE messages to Trigger Reverse Path CREATE Messages
In certain network deployments, where the NATFW NSLP might not be
available on the end-host (Figure 18) or the NSIS messages are scoped
(Figure 19) implicitly or explicitly with a scoping object, a CREATE
message could be used to trigger another CREATE message sent by the
last NF terminating the CREATE message. To handle routing asymmetry
issues, the triggered CREATE messages will be sent on a newly pinned
down reverse path.
In Figure 18, the first CREATE indicates that if the message can not
reach its destination (NoNR indication), a CREATE message should be
sent back to the NI by the last reached NATFW NE. As in Section
3.3.8 this mode of operation requires that the CREATE message
indicate the type of required response behavior which in this case is
a triggered CREATE message. However this response behavior is
subject to a condition: it should be sent only if the NR can not
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respond. This conditional behavior requires a specific flag to
indicate it (NoNR indication in Figure 18).
Foo.com Public Internet Bar.com
2-RESPONSE1
/-------------|---------------------
/ --> FW1-NF --------------------- \
V / 1-CREATE1[CREATE,NoNR] | \ \
Host Foo/ | | NF3-NF Host Bar
NI/NR ^ | | |^
\ \ | 3-CREATE2 | ||
\ \--- FW2-NF --------------------------|
\----/ \--------------------------
| 4-RESPONSE2 |
Figure 18: CREATE Triggering Sending of CREATE Message at last NSLP
and Using Separate Reverse path
To minimize the asymmetric route problem, the node responding with a
CREATE message would request the NTLP to rediscover the reverse path.
A RESPONSE message would be sent on the existing pinned down reverse
path (Step 2 in Figure 18), and a CREATE would be sent on a newly
discovered reverse path (Step 3 in Figure 18). Upon reception of the
latter message, the initiating NI will respond with a RESPONSE
message (Step 4 in Figure 18) as is done for the normal CREATE
message operations (Section 3.3.1). The CREATE message would need to
indicate to the last NATFW NF that a CREATE must be sent on a
separately discovered path and that a RESPONSE message needs to be
sent on the established pinned down reverse path. The new CREATE
message need to indicate to the NI that this session is bound to the
previous session. In addition the first message should indicate that
the last available NATFW NF will need to terminate the message and
start the above procedures.
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Foo.com Public Internet Bar.com
2-RESPONSE1
/-------------|---------------------
/ --> FW1-NF --------------------- \
V / 1-CREATE1[CREATE,scope] | \ \
Host Foo/ | | NF3-NF Host Bar
NI/NR ^ | | |^
\ \ | 3-CREATE2 | ||
\ \--- FW2-NF --------------------------|
\----/ \--------------------------
| 4-RESPONSE2 |
Figure 19: CREATE Triggering Sending of CREATE Message with Scoping
and Using Separate Reverse path
In Figure 19, the first CREATE indicates that once the end of the
scope is reached, the last NATFW NSLP will send a CREATE message (if
the first CREATE request was successful). As in Section 3.3.8, this
mode of operation requires that the CREATE message indicate the
required type of response behavior which in this case is a CREATE
message. As the CREATE needs to terminate at a scope end, the scope
need to be provided within the CREATE message.
3.4 Calculation of Session Lifetime
NATFW NSLP sessions, and the corresponding policy rules which may
have been installed, are maintained via soft-state mechanism. Each
session is assigned a lifetime and the session is kept alive as long
as the lifetime is valid. After the expiration of the lifetime,
sessions and policy rules MUST be removed automatically and resources
bound to them should be freed as well. Session lifetime is kept at
every NATFW NSLP node. The NSLP forwarders and NSLP responder
(except when TRIGGER is used) are not responsible for triggering
lifetime extension refresh messages (see Section 3.3.3): this is the
task of the NSIS initiator.
NSIS initiator MUST choose a session lifetime (expressed in seconds)
value before sending any message (except 'delete session' messages)
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 ([21]). Session
hijacking is made easier when the NI does not explicitly remove
the session.
o The user application's data exchange duration, in terms of
seconds, minutes or hours and networking needs. This duration is
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modeled as M x R, with R the message refresh period (in seconds)
and M a multiple of R.
As opposed to the NTLP Message Routing state [3] lifetime, the NSLP
session lifetime is not required to have a small value since the NSLP
state refresh is not handling routing changes but security related
concerns. [14] provides a good algorithm to calculate the session
lifetime as well as how to avoid refresh message synchronization
within the network. [14] recommends:
1. The refresh message timer to be randomly set to a value in the
range [0.5R, 1.5R].
2. To avoid premature loss of state, L (with L being the session
lifetime) must satisfy L >= (K + 0.5)*1.5*R, where K is a small
integer. Then in the worst case, K-1 successive messages may be
lost without state being deleted. Currently K = 3 is suggested
as the default. However, it may be necessary to set a larger K
value for hops with high loss rate. Other algorithms could be
used to define the relation between the session lifetime and the
refresh message period, the algorithm provided is only given as
an example.
This requested lifetime value is placed in the 'lifetime' object of
the NSLP message and messages are forwarded to the next NATFW NSLP
node.
NATFW NFs processing the request message along the path MAY change
the requested lifetime to fit their needs and/or local policy. If an
NF changes the lifetime value it must also indicate the corresponding
refresh message period. NFs MUST NOT increase the lifetime value
unless the lifetime value was below their acceptable range; they MAY
reject the requested lifetime immediately and MUST generate an error
response message of type 'lifetime too big' upon rejection. The NSLP
request message is forwarded until it reaches the NSLP responder.
NSLP responder MAY reject the requested lifetime value and MUST
generate an error response message of type 'lifetime too big' upon
rejection. The NSLP responder MAY also lower the requested lifetime
to an acceptable value (based on its local policies). NSLP
responders generate their appropriate response message for the
received request message, sets the lifetime value to the above
granted lifetime and sends the message back hop-by-hop towards NSLP
initiator.
Each NSLP forwarder processes the response message, reads and stores
the granted lifetime value. The forwarders SHOULD accept the granted
lifetime, as long as the value is within the tolerable lifetime range
defined in their local policies. They MAY reject the lifetime and
generate a 'lifetime not acceptable' error response message. Figure
20 shows the procedure with an example, where an initiator requests
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60 seconds lifetime in the CREATE message and the lifetime is
shortened along the path by the forwarder to 20 seconds and by the
responder to 15 seconds.
+-------+ CREATE(lt=60s) +-----------+ CREATE(lt=20s) +--------+
| |---------------->| NSLP |---------------->| |
| NI | | | | NR |
| |<----------------| forwarder |<----------------| |
+-------+ RESPONSE(lt=15s +-----------+ RESPONSE(lt=15s +--------+
MRR=3s) MRR=3s)
lt = lifetime
MRR = Message Refresh Rate
Figure 20: Lifetime Calculation Example
3.5 Firewall and NAT Resources
TBD: This section needs to be done and will describe how to map flow
routing information to middlebox policy rules. Further, this section
should clarify wildcarding.
3.6 De-Multiplexing at NATs
Section 3.3.2 describes how NSIS nodes behind NATs can obtain a
publicly reachable IP address and port number at a NAT. The
information IP address/port number can then be transmitted via a
signaling protocol and/or third party to the communication partner
that would like to send data towards hosts 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. 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 requests.
The de-multiplexing method uses information stored at NATs (such as
mapping of public IP address to private, transport protocol, port
numbers) and information given by NTLP's message routing information.
3.7 Selecting Opportunistic Addresses for REA
REA do need, as with all other message types, a final destination IP
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address to reach. 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'. [19] shows
choices for SIP and this section provides some hints about choosing a
good destination IP address.
1. Public IP address of the data sender:
* Assumption:
+ The data receiver already learned the IP address of the
data sender (e.g., via a third party).
* Problems:
+ The data sender might also be behind a NAT. In this case
the public IP address of the data receiver is the IP
address allocated at this NAT.
+ Due to routing asymmetry it might be possible that the
routes taken by a) the data sender and the application
server b) the data sender and NAT B might be different,
this could happen in a network deployment such as in Figure
12. 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 (allocated at NAT B):
* Assumption:
+ The data receiver already learned his externally visible IP
address (e.g., based on the third party communication).
* Problems:
+ Communication with a third party is required.
3. IP address of the Application Server:
* Assumption:
+ An application server (or a different third party) is
available.
* Problems:
+ If the NSIS signaling message is not terminated at the NAT
of the local network then an NSIS unaware application
server might discard the message.
+ Routing might not be optimal since the route between a) the
data receiver and the application server b) the data
receiver and the data sender might be different.
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4. NATFW NSLP NTLP Requirements
The NATFW NSLP requires the following capabilities from the NTLP:
o Ability to detect that the NSIS Responder does not support NATFW
NSLP. This capability is key to launching the proxy mode behavior
as described in Section 3.3.8 and [17].
o Detection of NATs and their support of the NSIS NATFW NSLP. If
the NTLP discovers that the NSIS host is behind an NSIS aware NAT,
the NR will send REA messages to the opportunistic address. If
the NTLP discovers that the NSIS host is behind a NAT that does
not support NSIS then the NSIS host will need to use a separate
NAT traversal mechanism.
o Message origin authentication and message integrity protection
o Detection of routing changes
o Protection against malicious announcement of fake path changes,
this is needed to mitigate a threat discussed in section 7 of [21]
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5. 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.
5.1 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSLP message type | 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.
Note that other NSLPs use this field as a flag field.
5.2 NSLP message types
The message types identify requests and responses. Defined messages
types for requests are:
o 0x0101 : CREATE
o 0x0102 : RESERVE-EXTERNAL-ADDRESS(REA)
o 0x0103 : QUERY
o 0x0104 : NOTIFY
o 0x0105 : RESPONSE
o 0x0106 : TRIGGER
Defined message types for responses are (TBD):
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o TBD
5.3 NSLP Objects
NATFW NSLP objects use a common header format defined by Figure 22.
Objects are Type-Length-Value (TLV) encoded using big endian (network
ordered) binary data representations. The object header contains two
fields, the NSLP object type and the object length. Its total length
is 32 bits.
Note that all objects MUST be padded to a length which is a multiple
of 32 bits.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSLP object type | NSLP object length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 22: Common NSLP object header
The length is the total length of the object without the object
header. The unit is a word, consisting of 4 octets. The particular
values of type and length for each NSLP object are listed in the
subsequent sections that define the NSLP objects.
TBD: Processing of unknown options is currently subject to
discussions within the working group. It is proposed to extend the
NSLP object header with some bits that indicate treatment of unknown
options. The compatibility bits (CP) are coded into two 2 bits and
determine the action to take upon receiving an unknown option. The
applied behavior based on the CP bits is:
00 - Abort processing and report error
01 - Ignore object and do not forward
10 - Ignore object and do forward
All other combinations MUST NOT be set and objects carrying these
other CP bit combinations MUST discarded.
5.3.1 Session Lifetime Object
The session lifetime object carries the requested or granted lifetime
of a NATFW NSLP session measured in seconds. The object consists
only of the 4 bytes lifetime field.
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0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OID_NATFW_LT | 0x0001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NATFW NSLP session lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 23: Lifetime object
5.3.2 External Address Object
The external address objects can be included in RESPONSE messages
(Section 5.4.4) only. It contains the external IP address and port
number allocated at the edge-NAT. Two fields are defined, the
external IP address, and the external port number. For IPv4 the
object with value OID_NATFW_IPv4 is defined. It has a length of 8
bytes and is shown in Figure 24.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OID_NATFW_IPv4 | 0x0002 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port number | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 24: External Address Object for IPv4 addresses
For IPv6 the object with value OID_NATFW_IPv6 is defined. It has a
length of 20 bytes and is shown in Figure 25.
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0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OID_NATFW_IPv6 | 0x0005 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port number | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IPv6 address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 25: External Address Object for IPv6 addresses
5.3.3 Extended Flow Information Object
In general, flow information is kept at the NTLP level during
signaling. The message routing information of the NTLP carries all
necessary information. Nevertheless, some additional information may
be required for NSLP operations. The 'extended flow information'
object carries this additional information about action to be taken
on the installed policy rules and subsequent numbers of policy rules.
These fields are defined for the policy rule object:
o Rule action: This field indicates the action for the policy rule
to be activated. Allow values are 'allow' (0x01) and 'deny'
(0x02)
o Number of ports: This field gives the number of ports that should
be allocated beginning at the port given in NTLP's message routing
information.
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0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OID_NATFW_FLOW | 0x0001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rule action | number of ports |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 26: Extended Flow Information
5.3.4 Response Code Object
This object carries the response code, which may be indications for
either a successful request or failed request depending on the value
of the 'response code' field.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OID_NATFW_RESPONSE | 0x0001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| response code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 27: Response Code Object
TBD: Define response classes, success codes and error codes.
Possible error classes are:
o Policy rule errors
o Authentication and Authorization errors
o NAT
Currently errors defined in this memo are:
o lifetime too big
o lifetime not acceptable
o no NAT here
o no reservation found
o requested external address from outside
o re-authorization needed
o routing change detected
5.3.5 Response Type Object
The response type object indicates that a specific response is needed
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to the NSLP responder.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OID_NATFW_RESP_TYPE | 0x0001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|S|L| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 28: Response Type Object
If the C bit is set to 1 the required response is a CREATE request
message, otherwise a RESPONSE message. If the S bit is set to 1 the
scoping object MUST be used. If the L bit is set to 1 the CREATE
request message is ONLY sent if the message does not reach its
target, even though the if the C bit is set.
The source IP address is optional and may be set to a zero IP address
or to a real IP address. If set to a real address, NATFW NSLP uses
this address as assumed data sender's address.
5.3.6 Message Sequence Number Object
TBD.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OID_NATFW_MSN | 0x0001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| message sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 29: Message Sequence Number Object
5.3.7 Scoping Object
The scoping object determines the allowed scope for the particular
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message.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OID_NATFW_SCOPE | 0x0001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| message scope |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 30: Scoping Object
These possible message scope values are: region, single hop.
5.3.8 Bound Session ID Object
This object carries a session ID and is used for QUERY messages only.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OID_NATFW_BID | 0x0001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| bound session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 31: Bound Session ID Object
5.3.9 Notify Target Object
This object carries the IP address of the notify target node. TBD:
Details on this, like IPv6 version etc.
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0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OID_NATFW_NOTIFY_TGT | 0x0001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| notify nodes' IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 32: Notify Target Object
5.4 Message Formats
This section defines the content of each NATFW NSLP message type.
The message types are defined in Section 5.2. First, the request
messages are defined with their respective objects to be included in
the message. Second, the response messages are defined with their
respective objects to be included.
Basically, each message is constructed of NSLP header and one or more
NSLP objects. The order of objects is not defined, meaning that
objects may occur in any sequence. Objects are marked either with
mandatory [M] or optional [O]. Where [M] implies that this
particular object MUST be included within the message and where [O]
implies that this particular object is OPTIONAL within the message.
Each section elaborates the required settings and parameters to be
set by the NSLP for the NTLP, for instance, how the message routing
information is set.
5.4.1 CREATE
The CREATE request message is used to create NSLP sessions and to
create policy rules. Furthermore, CREATE messages are used to
refresh sessions and to delete them.
The CREATE message carries these objects:
o Lifetime object [M]
o Extended flow information object [M]
o Message sequence number object [M]
o Respose type object [O]
o Scoping object[O]
o Notify target [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. When the CREATE
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messages is received by a node operating in proxy mode Section 3.3.8
the NI address is the NR address from the message that triggered the
CREATE to be sent, if that address is not valid (wildcarded) the
proxy node address is used instead. The NR address would be the NI's
address provided by the message routing information of the message
that triggered the CREATE.
5.4.2 RESERVE-EXTERNAL-ADDRESS (REA)
The RESERVE-EXTERNAL-ADDRESS (REA) request message is used to target
a NAT and to allocated an external IP address and possibly port
number, so that the initiator of the REA request has a public
reachable IP address/port number.
The REA request message carries these objects:
o Lifetime object [M]
o Message sequence number object [M]
o Response type object [M]
o Scoping object [M]
o Extended flow information [O]
The REA message needs special NTLP treatment. First of all, REA
messages travel the wrong way, from the DR towards DS. Second, the
DS' address used during the signaling may be not the actual DS (see
Section 3.7). Therefore, the NTLP flow routing information is set to
DR as initiator and DS as responders, a special field is given in the
NTLP: The signaling destination.
5.4.3 TRIGGER
The TRIGGER request message is used in proxy mode operation.
The TRIGGER request message carries these objects:
o Lifetime object [M]
o Message sequence number object [M]
o Response type object [M]
o Scoping object [M]
o Extended flow information [O]
NTLP settings to be defined.
5.4.4 RESPONSE
RESPONSE messages are responses to CREATE, REA, and QUERY messages.
The RESPONSE message carries these objects:
o Lifetime object [M]
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o Response object [M]
o External address object ([M] for success responses to REA)
This message is routed upstream.
5.4.5 QUERY
QUERY messages are used for diagnosis purposes.
The QUERY message carries these objects:
o Response object [M]
o Message sequence number object [M]
o Scoping object [M]
o Bound session ID [O]
This message is routed downstream.
5.4.6 NOTIFY
The NOTIFY messages is used to report asynchronous events happening
along the signaled path to other NATFW NSLP nodes.
The NOTIFY message carries this object:
o Response code object with NOTIFY code [M].
The message routing information in the NTLP MUST be set to DS as
source address and DR as destination address, forwarding direction is
upstream (Note that Section 5.4.6 discusses some design options
regarding the message transport). The session id object must be set
to the corresponding session that is effected by this asynchronous
event.
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6. NSIS NAT and Firewall Transition Issues
NSIS NAT and Firewall transition issues are premature and will be
addressed in a separate draft (see [17]). An update of this section
will be based on consensus.
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7. Security Considerations
Security is of major concern particularly in case of Firewall
traversal. Security threats for NSIS signaling in general have been
described in [6] and they are applicable to this document. [21]
extends this threat investigtion by considering NATFW NSLP specific
threats. Based on the identified threats a list of security
requirements have been defined. As an important requirement for
security protection it is necessary to provide
o data origin authentication
o replay protection
o integrity protection and
o optionally confidentiality protection
between neighboring NATFW NSLP nodes.
To consider the aspect of authentication and key exchange we aim to
reuse the mechanisms provided in [3] between neighboring nodes.
Some scenarios also demand security between non-neighboring nodes but
this aspect is still in discussions. A possible commonality with the
QoS NSLP has been identified and CMS [24] has been investigated as a
possible candidate for security protection between non-neighboring
entities. Note that this aspect also includes some more
sophisticated user authentication mechanisms, as described in [23].
With regard to end-to-end security the need for a binding between an
NSIS signaling session and application layer session has been
described in Section 3.3 of [6].
In order to solicit feedback from the IETF community on some hard
security problems for path-coupled NATFW signaling a more detailed
description in [22] is available.
The NATFW NSLP is a protocol which may involve a number of NSIS nodes
and is, as such, not a two-party protocol. This fact requires more
thoughts about scenarios, trust relationships and authorization
mechanisms. This section lists a few scenarios relevant for security
and illustrates possible trust reationships which have an impact to
authorization. More problematic scenarios are described in Appendix
A.
7.1 Trust Relationship and Authorization
Trust relationships and authorization are very important for the
protocol machinery. Trust and authorization are closely related to
each other in the sense that a certain degree of trust is required to
authorize a particular action. For any action (e.g. "create/delete
/prolong policy rules), authorization is very important due to the
nature of middleboxes.
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Different types of trust relationships may affect different
categories of middleboxes. As explained in [26], establishment of a
financial relationship is typically very important for QoS signaling,
whereas financial relationships are less directly of interest for
NATFW middlebox signaling. It is therefore not particularly
surprising that there are differences in the nature and level of
authorization likely to be required in a QoS signaling environment
and in NATFW middlebox signaling. For NATFW middlebox signaling, a
stronger or weaker degree of authorization might be needed.
Typically NATFW signaling requires authorization to configure and
traverse particular nodes or networks which may derive indirectly
from a financial relationship. This is a more 'absolute' situation
either the usage is allowed or not, and the effect on both network
owner and network user is 'binary'.
Different trust relationships that appear in middlebox signaling
environments are described in the subsequent sub-sections. QoS
signaling today uses peer-to-peer trust relationships. They are
simplest kind of trust relationships. However, there are reasons to
believe that this is not the only type of trust relationship found in
today's networks.
7.1.1 Peer-to-Peer Trust Relationship
Starting with the simplest scenario, it is assumed that neighboring
nodes trust each other. The required security association to
authenticate and to protect a signaling message is either available
(after manual configuration), or has been dynamically established
with the help of an authentication and key exchange protocol. If
nodes are located closely together, it is assumed that security
association establishment is easier than establishing it between
distant nodes. It is, however, difficult to describe this
relationship generally due to the different usage scenarios and
environments. Authorization heavily depends on the participating
entities, but for this scenario, it is assumed that neighboring
entities trust each other (at least for the purpose of policy rule
creation, maintenance, and deletion). Note that Figure 33 does not
illustrate the trust relationship between the end host and the access
network.
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+------------------------+ +-------------------------+
| | | |
| Network A | | Network B |
| | | |
| +---------+ +---------+ |
| +-///-+ Middle- +---///////----+ Middle- +-///-+ |
| | | box 1 | Trust | box 2 | | |
| | +---------+ Relationship +---------+ | |
| | | | | |
| | | | | |
| | | | | |
| | Trust | | Trust | |
| | Relationship | | Relationship | |
| | | | | |
| | | | | |
| | | | | |
| +--+---+ | | +--+---+ |
| | Host | | | | Host | |
| | A | | | | B | |
| +------+ | | +------+ |
+------------------------+ +-------------------------+
Figure 33: Peer-to-Peer Trust Relationship
7.1.2 Intra-Domain Trust Relationship
In larger corporations, often more than one middlebox is used to
protect or serve different departments. In many cases, the entire
enterprise is controlled by a security department, which gives
instructions to the department administrators. In such a scenario, a
peer-to-peer trust-relationship might be prevalent. Sometimes it
might be necessary to preserve authentication and authorization
information within the network. As a possible solution, a
centralized approach could be used, whereby an interaction between
the individual middleboxes and a central entity (for example a policy
decision point - PDP) takes place. As an alternative, individual
middleboxes could exchange the authorization decision with another
middlebox within the same trust domain. Individual middleboxes
within an administrative domain should exploit their trust
relationship instead of requesting authentication and authorization
of the signaling initiator again and again. Thereby complex protocol
interactions are avoided. This provides both a performance
improvement without a security disadvantage since a single
administrative domain can be seen as a single entity. Figure 34
illustrates a network structure which uses a centralized entity.
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+-----------------------------------------------------------+
| |
| Network A |
| |
| |
| +---------+ +---------+
| +----///--------+ Middle- +------///------++ Middle- +---
| | | box 2 | | box 2 |
| | +----+----+ +----+----+
| | | | |
| +----+----+ | | |
| | Middle- +--------+ +---------+ | |
| | box 1 | | | | |
| +----+----+ | | | |
| | | | | |
| - | | | |
| - | +----+-----+ | |
| | | | Policy | | |
| +--+---+ +-----------+ Decision +----------+ |
| | Host | | Point | |
| | A | +----------+ |
| +------+ |
+-----------------------------------------------------------+
Figure 34: Intra-domain Trust Relationship
7.1.3 End-to-Middle Trust Relationship
In some scenarios, a simple peer-to-peer trust relationship between
participating nodes is not sufficient. Network B might require
additional authorization of the signaling message initiator. If
authentication and authorization information is not attached to the
initial signaling message then the signaling message arriving at
Middlebox 2 would result in an error message being created, which
indicates the additional authorization requirement. In many cases
the signaling message initiator is already aware of the additionally
required authorization before the signaling message exchange is
executed. Replay protection is a requirement for authentication to
the non-neighboring middlebox, which might be difficult to accomplish
without adding additional roundtrips to the signaling protocol (e.g.,
by adding a challenge/response type of message exchange).
Figure 35 shows the slightly more complex trust relationships in this
scenario.
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+----------------------+ +--------------------------+
| | | |
| Network A | | Network B |
| | | |
| | Trust | |
| | Relationship | |
| +---------+ +---------+ |
| +-///-+ Middle- +---///////----+ Middle- +-///-+ |
| | | box 1 | +-------+ box 2 | | |
| | +---------+ | +---------+ | |
| | | | | | |
| |Trust | | | | |
| |Relationship | | | | |
| | | | | Trust | |
| | | | | Relationship| |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
| +--+---+ | | | +--+---+ |
| | Host +----///----+------+ | | Host | |
| | A | |Trust | | B | |
| +------+ |Relationship | +------+ |
+----------------------+ +--------------------------+
Figure 35: End-to-Middle Trust Relationship
Finally it should be noted that installing packet filters provides
some security, but also has some weaknesses, which heavily depend on
the type of packet filter installed. A packet filter cannot prevent
an adversary to inject traffic (due to the IP spoofing capabilities).
This type of attack might not be particular helpful if the packet
filter is a standard 5 tuple which is very restrictive. If packet
filter installation, however, allows specifying a rule, which
restricts only the source IP address, then IP spoofing allows
transmitting traffic to an arbitrary address. NSIS aims to provide
path-coupled signaling and therefore an adversary is somewhat
restricted in the location from which attacks can be performed. Some
trust is therefore assumed from nodes and networks along the path.
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8. Open Issues
The NATFW NSLP has a series of related documents discussing several
other aspects of path-coupled NATFW signaling, including security
[22], migration (i.e., traversal of nsis unaware NATs) [17],
intra-realm signaling [18], and inter-working with SIP [19].
Summaries of the outcomes from these documents may be added,
depending on WG feedback, to a later version of this draft.
A more detailed list of open issue can be found at:
http://nsis.srmr.co.uk/cgi-bin/roundup.cgi/nsis-natfw-issues/index
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9. Contributors
A number of individuals have contributed to this draft. Since it was
not possible to list them all in the authors section, it was decided
to split it and move Marcus Brunner and Henning Schulzrinne into the
contributors section. Separating into two groups was done without
treating any one of them better (or worse) than others.
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10. References
10.1 Normative References
[1] Hancock et al, R., "Next Steps in Signaling: Framework", DRAFT
draft-ietf-nsis-fw-05.txt, October 2003.
[2] Brunner et al., M., "Requirements for Signaling Protocols",
DRAFT draft-ietf-nsis-req-09.txt, October 2003.
[3] Schulzrinne, H. and R. Hancock, "GIMPS: General Internet
Messaging Protocol for Signaling", DRAFT
draft-ietf-nsis-ntlp-02.txt, October 2003.
[4] Van den Bosch, S., Karagiannis, G. and A. McDonald, "NSLP for
Quality-of-Service signaling", DRAFT
draft-ietf-nsis-qos-nslp-03.txt, May 2004.
[5] IANA, "Special-Use IPv4 Addresses", RFC 3330, September 2002.
[6] Tschofenig, H. and D. Kroeselberg, "Security Threats for NSIS",
DRAFT draft-ietf-nsis-threats-01.txt, January 2003.
[7] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A. and A.
Rayhan, "Middlebox communication architecture and framework",
RFC 3303, August 2002.
10.2 Informative References
[8] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations, RFC 2663", August 1999.
[9] Srisuresh, P. and M. Holdrege, "Network Address Translator
(NAT)Terminology and Considerations, RFC 2663".
[10] Srisuresh, P. and E. Egevang, "Traditional IP Network Address
Translator (Traditional NAT), RFC 3022", January 2001.
[11] Tsirtsis, G. and P. Srisuresh, "Network Address Translation -
Protocol Translation (NAT-PT), RFC 2766", February 2000.
[12] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues",
RFC 3234, February 2002.
[13] Srisuresh, P., Tsirtsis, G., Akkiraju, P. and A. Heffernan,
"DNS extensions to Network Address Translators (DNS_ALG)", RFC
2694, September 1999.
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[14] Braden, B., Zhang, L., Berson, S., Herzog, S. and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", September 1997.
[15] Yadav, S., Yavatkar, R., Pabbati, R., Ford, P., Moore, T.,
Herzog, S. and R. Hess, "Identity Representation for RSVP", RFC
3182, October 2001.
[16] Tschofenig, H., Schulzrinne, H., Hancock, R., McDonald, A. and
X. Fu, "Security Implications of the Session Identifier", June
2003.
[17] Aoun, C., Brunner, M., Stiemerling, M., Martin, M. and H.
Tschofenig, "NAT/Firewall NSLP Migration Considerations", DRAFT
draft-aoun-nsis-nslp-natfw-migration-01.txt, Februar 2004.
[18] Aoun, C., Brunner, M., Stiemerling, M., Martin, M. and H.
Tschofenig, "NATFirewall NSLP Intra-realm considerations",
DRAFT draft-aoun-nsis-nslp-natfw-intrarealm-00.txt, Februar
2004.
[19] Martin, M., Brunner, M. and M. Stiemerling, "SIP NSIS
Interactions for NAT/Firewall Traversal", DRAFT
draft-martin-nsis-nslp-natfw-sip-00.txt, Februar 2004.
[20] Martin, M., Brunner, M., Stiemerling, M., Girao, J. and C.
Aoun, "A NSIS NAT/Firewall NSLP Security Infrastructure", DRAFT
draft-martin-nsis-nslp-natfw-security-01.txt, February 2004.
[21] Fessi, A., Brunner, M., Stiemerling, M., Thiruvengadam, S.,
Tschofenig, H. and C. Aoun, "Security Threats for the NAT/
Firewall NSLP", DRAFT draft-fessi-nsis-natfw-threats-01.txt,
July 2004.
[22] Tschofenig, H., "Path-coupled NAT/Firewall Signaling Security
Problems", draft-tschofenig-nsis-natfw-security-problems-00.txt
(work in progress), July 2004.
[23] Tschofenig, H. and J. Kross, "Extended QoS Authorization for
the QoS NSLP", draft-tschofenig-nsis-qos-ext-authz-00.txt (work
in progress), July 2004.
[24] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369,
August 2002.
[25] Manner, J., Suihko, T., Kojo, M., Liljeberg, M. and K.
Raatikainen, "Localized RSVP", DRAFT draft-manner-lrsvp-00.txt,
November 2002.
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[26] Tschofenig, H., Buechli, M., Van den Bosch, S. and H.
Schulzrinne, "NSIS Authentication, Authorization and Accounting
Issues", March 2003.
[27] Amini, L. and H. Schulzrinne, "Observations from router-level
internet traces", DIMACS Workshop on Internet and WWW
Measurement, Mapping and Modelin Jersey) , Februar 2002.
[28] Adrangi, F. and H. Levkowetz, "Problem Statement: Mobile IPv4
Traversal of VPN Gateways",
draft-ietf-mobileip-vpn-problem-statement-req-02.txt (work in
progress), April 2003.
[29] Ohba, Y., Das, S., Patil, P., Soliman, H. and A. Yegin,
"Problem Space and Usage Scenarios for PANA",
draft-ietf-pana-usage-scenarios-06 (work in progress), April
2003.
[30] Shore, M., "The TIST (Topology-Insensitive Service Traversal)
Protocol", DRAFT draft-shore-tist-prot-00.txt, May 2002.
[31] Tschofenig, H., Schulzrinne, H. and C. Aoun, "A Firewall/NAT
Traversal Client for CASP", DRAFT
draft-tschofenig-nsis-casp-midcom-01.txt, March 2003.
[32] 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.
[33] Brunner, M., Stiemerling, M., Martin, M., Tschofenig, H. and H.
Schulzrinne, "NSIS NAT/FW NSLP: Problem Statement and
Framework", DRAFT draft-brunner-nsis-midcom-ps-00.txt, June
2003.
[34] Ford, B., Srisuresh, P. and D. Kegel, "Peer-to-Peer(P2P)
communication Network Address Translators(NAT)", DRAFT
draft-ford-midcom-p2p-02.txt, March 2004.
[35] Rosenberg et al, J., "STUN - Simple Traversal of User Datagram
Protocol (UDP) Through Network Address Translators (NATs)", RFC
3489, March 2003.
[36] Rekhter et al, Y., "Address Allocation for Private Internets",
RFC 1918, February 1996.
[37] Rosenberg, J., "Traversal Using Relay NAT (TURN)",
draft-rosenberg-midcom-turn-05 (work in progress), July 2004.
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[38] 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.
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:
Miquel Martin
Network Laboratories, NEC Europe Ltd.
Kurfuersten-Anlage 36
Heidelberg 69115
Germany
Phone: +49 (0) 6221 905 11 16
EMail: miquel.martin@netlab.nec.de
URI:
Cedric Aoun
Nortel Networks/ENST Paris
France
EMail: cedric.aoun@nortelnetworks.com
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Appendix A. Problems and Challenges
This section describes a number of problems that have to be addressed
for NSIS NAT/Firewall. Issues presented here are subject to further
discussions. These issues might be also of relevance to other NSLP
protocols.
A.1 Missing Network-to-Network Trust Relationship
Peer-to-peer trust relationship, as shown in Figure 33, is a very
convenient assumption that allows simplified signaling message
processing. However, it might not always be applicable, especially
between two arbitrary access networks (over a core network where
signaling messages are not interpreted). Possibly peer-to-peer trust
relationship does not exist because of the large number of networks
and the unwillingness of administrators to have other network
operators to create holes in their Firewalls without proper
authorization.
+----------------------+ +--------------------------+
| | | |
| Network A | | Network B |
| | | |
| +---------+ Missing +---------+ |
| +-///-+ Middle- | Trust | Middle- +-///-+ |
| | | box 1 | Relation- | box 2 | | |
| | +---------+ ship +---------+ | |
| | | or | | |
| | | Authorization| | |
| | | | | |
| | Trust | | Trust | |
| | Relationship | | Relationship | |
| | | | | |
| | | | | |
| | | | | |
| +--+---+ | | +--+---+ |
| | Host | | | | Host | |
| | A | | | | B | |
| +------+ | | +------+ |
+----------------------+ +--------------------------+
Figure 36: Missing Network-to-Network Trust Relationship
Figure 36 illustrates a problem whereby an external node is not
allowed to manipulate (create, delete, query, etc.) packet filters at
a Firewall. Opening pinholes is only allowed for internal nodes or
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with a certain authorization permission. Hence the solution
alternatives in Section 3.3.2 focus on establishing the necessary
trust with cooperation of internal nodes.
A.2 Relationship with routing
The data path is following the "normal" routes. The NAT/FW devices
along the data path are those providing the service. In this case
the service is something like "open a pinhole" or even more general
"allow for connectivity between two communication partners". The
benefit of using path-coupled signaling is that the NSIS NATFW NSLP
does not need to determine what middleboxes or in what order the data
flow will go through.
Creating NAT bindings modifies the path of data packets between two
end points. Without NATs involved, packets flow from endhost to
endhost following the path given by the routing. With NATs involved,
this end-to-end flow is not directly possible, because of separated
address realms. Thus, data packets flow towards the external IP
address at a NAT (external IP address may be a public IP address).
Other NSIS NSLPs, for instance QoS NSLP, which do not interfere with
routing - instead they only follow the path of the data packets.
A.3 Affected Parts of the Network
NATs and Firewalls are usually located at the edge of the network,
whereby other signaling applications affect all nodes along the path.
One typical example is QoS signaling where all networks along the
path must provide QoS in order to achieve true end-to-end QoS. In
the NAT/Firewall case, only some of the domains/nodes are affected
(typically access networks), whereas most parts of the networks and
nodes are unaffected (e.g., the core network).
This fact raises some questions. Should an NSIS NTLP node intercept
every signaling message independently of the upper layer signaling
application or should it be possible to make the discovery procedure
more intelligent to skip nodes. These questions are also related to
the question whether NSIS NAT/FW should be combined with other NSIS
signaling applications.
A.4 NSIS backward compatibility with NSIS unaware NAT and Firewalls
Backward compatibility is a key for NSIS deployments, as such the
NSIS protocol suite should be sufficiently robust to allow traversal
of none NSIS aware routers (QoS gates, Firewalls, NATs, etc ).
NSIS NATFW NSLP's backward compatibility issues are different than
the NSIS QoS NSLP backward compatibility issues, where an NSIS
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unaware QoS gate will simply forward the QoS NSLP message. An NSIS
unaware Firewall rejects NSIS messages, since Firewalls typically
implement the policy "default to deny".
The NSIS backward compatibility support on none NSIS aware Firewall
would typically consist of configuring a static policy rule that
allows the forwarding of the NSIS protocol messages (either protocol
type if raw transport mode is used or transport port number in case a
transport protocol is used).
For NATs backward compatibility is more problematic since signaling
messages are forwarded (at least in one direction), but with a
changed IP address and changed port numbers. The content of the NSIS
signaling message is, however, unchanged. This can lead to
unexpected results, both due to embedded unchanged local scoped
addresses and none NSIS aware Firewalls configured with specific
policy rules allowing forwarding of the NSIS protocol (case of
transport protocols are used for the NTLP). NSIS unaware NATs must
be detected to maintain a well-known deterministic mode of operation
for all the involved NSIS entities. Such a "legacy" NAT detection
procedure can be done during the NSIS discover procedure itself.
Based on experience it was discovered that routers unaware of the
Router Alert IP option [RFC 2113] discarded packets, this is
certainly a problem for NSIS signaling.
A.5 Authentication and Authorization
For both types of middleboxes, Firewall and NAT security is a strong
requirement. Authentication and authorization means must be
provided.
For NATFW signaling applications it is partially not possible to do
authentication and authorization based on IP addresses. Since NATs
change IP addresses, such an address based authentication and
authorization scheme would fail.
A.6 Directional Properties
There two directional properties that need to be addressed by the
NATFW NSLP:
o Directionality of the data
o Directionality of NSLP signaling
Both properties are relevant to NATFW NSLP aware NATs and Firewalls.
With regards to NSLP signaling directionality: As stated in the
previous sections, the authentication and authorization of NSLP
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signaling messages received from hosts within the same trust domain
(typically from hosts located within the security perimeter delimited
by Firewalls) is normally simpler than received messages sent by
hosts located in different trust domains.
The way NSIS signaling messages enters the NSIS entity of a Firewall
(see Figure 2) might be important, because different policies might
apply for authentication and admission control.
Hosts deployed within the secured network perimeter delimited by a
Firewall, are protected from hosts deployed outside the secured
network perimeter, hence by nature the Firewall has more restrictions
on flows triggered from hosts deployed outside the security
perimeter.
A.7 Addressing
A more general problem of NATs is the addressing of the end-point.
NSIS signaling message have to be addressed to the other end host to
follow data packets subsequently sent. Therefore, a public IP
address of the receiver has to be known prior to sending an NSIS
message. When NSIS signaling messages contain IP addresses of the
sender and the receiver in the signaling message payloads, then an
NSIS entity must modify them. This is one of the cases, where a NSIS
aware NATs is also helpful for other types of signaling applications
e.g., QoS signaling.
A.8 NTLP/NSLP NAT Support
It must be possible for NSIS NATs along the path to change NTLP and/
or NSLP message payloads, which carry IP address and port
information. This functionality includes the support of providing
mid-session and mid-path modification of these payloads. As a
consequence these payloads must not be reordered, integrity protected
and/or encrypted in a non peer-to-peer fashion (e.g., end-to-middle,
end-to-end protection). Ideally these mutable payloads must be
marked (e.g., a protected flag) to assist NATs in their effort of
adjusting these payloads.
A.9 Combining Middlebox and QoS signaling
In many cases, middlebox and QoS signaling has to be combined at
least logically. Hence, it was suggested to combine them into a
single signaling message or to tie them together with the help of
some sort of data connection identifier, later on referred as Session
ID. This, however, has some disadvantages such as:
- NAT/FW NSLP signaling affects a much small number of NSIS nodes
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along the path (for example compared to the QoS signaling).
- NAT/FW signaling might show different signaling patterns (e.g.,
required end-to-middle communication).
- The refresh interval is likely to be different.
- The number of error cases increase as different signaling
applications are combined into a single message. The combination of
error cases has to be considered.
A.10 Inability to know the scenario
In Section 2 a number of different scenarios are presented. Data
receiver and sender may be located behind zero, one, or more
Firewalls and NATs. Depending on the scenario, different signaling
approaches have to be taken. For instance, data receiver with no
NAT and Firewall can receive any sort of data and signaling without
any further action. Data receivers behind a NAT must first obtain a
public IP address before any signaling can happen. The scenario
might even change over time with moving networks, ad-hoc networks or
with mobility.
NSIS signaling must assume the worst case and cannot put
responsibility to the user to know which scenario is currently
applicable. As a result, it might be necessary to perform a
"discovery" periodically such that the NSIS entity at the end host
has enough information to decide which scenario is currently
applicable. This additional messaging, which might not be necessary
in all cases, requires additional performance, bandwidth and adds
complexity. Additional, information by the user can provide
information to assist this "discovery" process, but cannot replace
it.
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Appendix B. Acknowledgments
We would like to acknowledge: Vishal Sankhla and Joao Girao for their
input to this draft; and Reinaldo Penno for his comments on the
initial version of the document. Furthermore, we would like thank
Elwyn Davis for his valuable help and input.
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