NSIS Working Group M. Stiemerling
Internet-Draft NEC
Expires: November 19, 2004 H. Tschofenig
Siemens
M. Martin
NEC
C. Aoun
Nortel Networks
May 21, 2004
NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
draft-ietf-nsis-nslp-natfw-02
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on November 19, 2004.
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
network scenarios, problems and solutions for path-coupled Network
Address Translator and Firewall signaling are described. The overall
Stiemerling, et al. Expires November 19, 2004 [Page 1]
Internet-Draft NAT/FW NSIS NSLP May 2004
architecture is given by the framework and requirements defined by
Next Steps in Signaling (NSIS) working group. This is one of two
NSIS Signaling Layer Protocols (NSLPs) the working group will address
during its work.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Terminology and Abbreviations . . . . . . . . . . . . . . 5
1.2 Middleboxes . . . . . . . . . . . . . . . . . . . . . . . 7
1.3 General Scenario for NATFW Traversal . . . . . . . . . . . 8
2. Network Environment . . . . . . . . . . . . . . . . . . . . . 10
2.1 Network Scenarios for Protocol Functionality . . . . . . . 10
2.1.1 Firewall traversal . . . . . . . . . . . . . . . . . . 10
2.1.2 NAT with two private Networks . . . . . . . . . . . . 11
2.1.3 NAT with private network on sender side . . . . . . . 12
2.1.4 NAT with private network on receiver side . . . . . . 12
2.1.5 Both End Hosts behind twice-NATs . . . . . . . . . . . 13
2.1.6 Both End Hosts behind same NAT . . . . . . . . . . . . 14
2.1.7 IPv4/v6 NAT with two private networks . . . . . . . . 15
2.1.8 Multihomed Network with NAT . . . . . . . . . . . . . 16
2.2 Trust Relationship and Authorization . . . . . . . . . . . 17
2.2.1 Peer-to-Peer Trust Relationship . . . . . . . . . . . 17
2.2.2 Intra-Domain Trust Relationship . . . . . . . . . . . 18
2.2.3 End-to-Middle Trust Relationship . . . . . . . . . . . 19
3. Protocol Description . . . . . . . . . . . . . . . . . . . . . 21
3.1 Basic protocol overview . . . . . . . . . . . . . . . . . 21
3.2 Protocol Operations . . . . . . . . . . . . . . . . . . . 23
3.2.1 Creating Sessions . . . . . . . . . . . . . . . . . . 23
3.2.2 Reserving External Addresses . . . . . . . . . . . . . 25
3.2.3 Reserving External Addresses and Create Session . . . 28
3.2.4 Prolonging Sessions . . . . . . . . . . . . . . . . . 28
3.2.5 Deleting Sessions . . . . . . . . . . . . . . . . . . 29
3.2.6 Authorization . . . . . . . . . . . . . . . . . . . . 30
3.2.7 Calculation of Lifetimes . . . . . . . . . . . . . . . 30
3.2.8 Middlebox Resource . . . . . . . . . . . . . . . . . . 31
3.2.9 De-Multiplexing at NATs . . . . . . . . . . . . . . . 31
3.2.10 Selecting Destination IP addresses for REA . . . . . . 32
3.3 NATFW NSLP Messages Components . . . . . . . . . . . . . . 33
3.3.1 NSLP Header . . . . . . . . . . . . . . . . . . . . . 33
3.3.2 NSLP message types . . . . . . . . . . . . . . . . . . 34
3.3.3 NSLP Objects . . . . . . . . . . . . . . . . . . . . . 34
3.3.3.1 Session ID Object . . . . . . . . . . . . . . . . 35
3.3.3.2 Session Lifetime Object . . . . . . . . . . . . . 35
3.3.3.3 External Address Object . . . . . . . . . . . . . 36
3.3.3.4 Extended Flow Information Object . . . . . . . . . 37
Stiemerling, et al. Expires November 19, 2004 [Page 2]
Internet-Draft NAT/FW NSIS NSLP May 2004
3.3.3.5 Error Object . . . . . . . . . . . . . . . . . . . 37
3.4 Message Formats . . . . . . . . . . . . . . . . . . . . . 38
3.4.1 Policy Rules . . . . . . . . . . . . . . . . . . . . . 38
3.4.2 Create Session (CRS) . . . . . . . . . . . . . . . . . 39
3.4.3 Reserve External Address (REA) . . . . . . . . . . . . 39
3.4.4 Reserve-Create (REC) . . . . . . . . . . . . . . . . . 39
3.4.5 Prolong Session (PLS) . . . . . . . . . . . . . . . . 39
3.4.6 Delete Session (DLS) . . . . . . . . . . . . . . . . . 40
3.4.7 Path Succeeded (PS) . . . . . . . . . . . . . . . . . 40
3.4.8 Path Deleted (PD) . . . . . . . . . . . . . . . . . . 40
3.4.9 Return External Address (RA) . . . . . . . . . . . . . 40
3.4.10 Error Response (ER) . . . . . . . . . . . . . . . . . 41
4. NSIS NAT and Firewall transitions issues . . . . . . . . . . . 42
5. Security Considerations . . . . . . . . . . . . . . . . . . . 43
6. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 45
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 46
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
8.1 Normative References . . . . . . . . . . . . . . . . . . . . 47
8.2 Informative References . . . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 49
A. Problems and Challenges . . . . . . . . . . . . . . . . . . . 51
A.1 Missing Network-to-Network Trust Relationship . . . . . . 51
A.2 Relationship with routing . . . . . . . . . . . . . . . . 52
A.3 Affected Parts of the Network . . . . . . . . . . . . . . 53
A.4 NSIS backward compatibility with NSIS unaware NAT and
Firewalls . . . . . . . . . . . . . . . . . . . . . . . . 53
A.5 Authentication and Authorization . . . . . . . . . . . . . 54
A.6 Directional Properties . . . . . . . . . . . . . . . . . . 54
A.7 Addressing . . . . . . . . . . . . . . . . . . . . . . . . 54
A.8 NTLP/NSLP NAT Support . . . . . . . . . . . . . . . . . . 55
A.9 Combining Middlebox and QoS signaling . . . . . . . . . . 55
A.10 Inability to know the scenario . . . . . . . . . . . . . . 55
B. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 57
Intellectual Property and Copyright Statements . . . . . . . . 58
Stiemerling, et al. Expires November 19, 2004 [Page 3]
Internet-Draft NAT/FW NSIS NSLP May 2004
1. Introduction
Firewalls and Network Address Translators (NAT) have been both used
throughout the Internet for many years and they will be present in
future. Using Firewalls brings security to networks and in times of
IPv4 address depletion NATs virtually extend IP address space. In
general, both types may be obstacles to many applications, since they
only allow specific applications to traverse them (i.e., HTTP traffic
or in general client/server applications). Other applications, for
instance, IP telephony or any other peer-to-peer application, with
more dynamic properties suffer from Firewalls and NATs so that they
do not work at all. Therefore, many applications cannot traverse
Firewall or NATs.
Several solutions to enable any application to traverse those boxes
have been proposed and are currently used. Typically, application
level gateways (ALG) have been integrated and so configuring
Firewalls and NATs dynamically. Another approach is middlebox
communication (MIDCOM, currently under standardization at the IETF).
In this approach Firewall and NAT external ALGs configure them via
the MIDCOM protocol [7]. Several other work around solutions are
available as well, see STUN [32] and [31]. However, all of these
approaches introduce other problems that are hard to solve; like
dependencies on certain NAT implementations or dependency on
topology.
NAT and Firewall (NATFW) signaling share a property with Quality of
Service (QoS) signaling, i.e., in both cases it is required to reach
any device on the data path that is involved in QoS or NATFW
treatment of data packets. For both, NATFW and QoS, signaling
travels path-coupled, meaning that the signaling messages follow
exactly the same path as the data packets do. RSVP [14] is an
example for a QoS signaling protocol.
This memo defines a path-coupled signaling protocol in the framework
of NSIS for NAT and Firewall configuration, called the NATFW NSIS
Signaling Layer Protocol (NSLP). The general framework of NSIS is
outlined in [1] and introduces the split between NSIS transport layer
and NSIS signaling layer. The transport of NSLP messages is handled
by NSIS Network Transport Layer Protocol (NTLP, see [3]) and takes
care about NSLP message transport. The signaling logic for QoS and
NATFW signaling is implemented in the different NSLPs. The QoS NSLP
is defined in [4], furthermore the general requirements for NSIS are
defined in [2].
There is a series of related documents to NATFW NSLP discussing
several other aspects of path-coupled NATFW signaling, including
security [20], migration [17], intrarealm signaling [18], and
Stiemerling, et al. Expires November 19, 2004 [Page 4]
Internet-Draft NAT/FW NSIS NSLP May 2004
inter-working with SIP [19].
The NATFW NSLP allows requesting the configuration of NATs and/or
Firewalls along the data path to enable data flows to traverse these
devices without being obstructed. A simplified example: A source
host sends a NATFW NSLP signaling message towards its data
destination. This message follows the data path and every NATFW NSLP
NAT/Firewall along the data path intercepts these messages, processes
it 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 path directed mode
where Firewalls and NATs are configured along the data path as
pointed out in the above example. The second one is the reserve
mode, where NATs are detected by the NSLP/NTLP within the network and
a public reachable IP address and port number are reserved. This
reserve mode enables hosts located behind NATs to receive data
originated in the public Internet on the reverse data path. Both
modes create NATFW NSLP and NTLP state in the network. The NSLP
state is maintained via a soft-state mechanism. State includes not
only signaling state, but as well as NAT bindings and Firewall rules.
This state is maintained via a lifetime and must be kept alive via a
lifetime extension mechanism if needed. Two signaling messages are
used for deleting state explicitly and extending state's lifetime.
In general, all NATFW NSLP signaling messages are exchanged
end-to-end.
Traversal of non NATFW NSLPs or the NTLP is out of scope of this
document. Furthermore, only Firewalls and NATs are considered in
this document, any other device, for instance IPSec security gateway,
is out of scope.
Section 2 describes the network environment for NATFW NSLP signaling
and highlights the required trust relationship/ authorization.
Section 3 defines the NATFW signaling protocol with its message
components, message formats, and protocol operations. The remaining
document refers in Section 4 to transition issues and security
considerations are handled in [20]. Currently unsolved problems and
challenges are listed and discussed in Appendix A. Please note that
readers familiar with possible locations of Firewalls and NAT in
networks can safely skip Section 2.
1.1 Terminology and Abbreviations
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
Stiemerling, et al. Expires November 19, 2004 [Page 5]
Internet-Draft NAT/FW NSIS NSLP May 2004
This document uses terms defined in [2]. Furthermore, these
following terms are used:
o NSIS NAT Forwarding State: The term "NSIS NAT Forwarding State" in
this context refers to a state used to forward the NSIS signaling
message beyond the targeted destination address; that state is
typically used when the NSIS Responder address is not known
o Sender-/Receiver Initiated Signaling
Sender-initiated: NAT bindings and Firewall rules are created
immediately when the "path" message hits the NSIS nodes. With
"path" message we refer to the signaling message traveling from
the data sender towards the data receiver.
Receiver-initiated: NAT bindings and Firewall rules are created
when the "reserve" message returns from the other end. With
"reserve" message we refer to a signaling message on the
reverse path, this means from the receiver to the sender (i.e.
backwards routed).
Note that these definitions have nothing to do with number of
roundtrips, who performs authorization etc.
o Policy rule: In general, 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." [RFC3198]. 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" or action "enable".
o Firewall: A packet filtering device that matches packet against a
set of policy rules and applies the actions. In the context of
NSIS NATFW NSLP we refer to this device as 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 to hosts (see [9]).
Network Address Translators are devices that perform this method.
o Middlebox: from [12]: "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". The term middlebox in context of
this document and in NSIS refers to Firewalls and NATs only.
Other types of middlebox are currently outside the scope.
o Security Gateway: IPsec based gateways.
o NSIS Initiator (NI): the signaling entity, which makes the
resource request, usually as a result of user application request.
o NSIS Responder (NR): the signaling entity , which acts as the
final destination for the signaling and can optionally interact
with applications as well.
o NSIS Forwarder (NF): the signaling entity between an NI and NR
which propagates NSIS signaling further through the network.
Stiemerling, et al. Expires November 19, 2004 [Page 6]
Internet-Draft NAT/FW NSIS NSLP May 2004
o Receiver (DR or R): the node in the network, which is receiving
the data packets of a flow.
o Sender (DS or S): the node in the network, which is sending the
data packets of a flow.
o NATFW NSLP session: 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 is NSLP state and associated policy rules at the middlebox.
o NSIS peer or peer: NSIS node with which a NSIS adjacency has been
created as defined in [3].
o Edge NAT: By edge NAT we refer to the NAT device, which is
reachable from outside and has a globally routable IP address.
o Public Network: Definition according to [8] is "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."
o Private/Local Network: Definition according to [8] is " 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." IP address
space allocation for private networks is recommended in [33]
o Public/Global IP address: An IP address located in the public
network.
o Private/Local IP address: An IP address located in the private
network.
1.2 Middleboxes
The term middlebox raises different expectations about functionality
provided by such a device. Middleboxes in the scope of this memo are
Firewalls that filter data packets against their set of filter rules
and NATs that translate addresses from one address realm to another
address realm. Other types of middleboxes, for instance QoS traffic
shapers and security gateways, are out of scope.
The term NAT used in this document is placeholder for a range of
different NAT flavors. We consider those types of NATs:
o traditional NAT (basic NAT and NAPT)
o Bi-directional NAT
o Twice-NAT
o Multihomed NAT
For a detailed discussion about each NAT type please see [8].
Both types of middleboxes use policy rules for decision on data
packet treatment. Policy rules consist of a 5-tuple and an
associated action. Data packets matching this 5-tuple experience the
Stiemerling, et al. Expires November 19, 2004 [Page 7]
Internet-Draft NAT/FW NSIS NSLP May 2004
policy rule action. A 5-tuple consists of:
o Source IP address and port number
o Destination IP address and port number
o Transport protocol
Actions for Firewalls are usually:
o Allow: forward data packet
o Deny: block data packet and discard it
o Other actions like logging, diverting, etc
Actions for NATs are (amongst many others):
o Change source IP address and port number to a global routeable IP
address and port number.
o Change destination IP address and port number to a private IP
address and port number.
The exact implementation of policy rules and 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 entitled 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. Hence we do not address
these types of devices (referred as security gateways) since per-flow
signaling is rather uncommon in this environment. For a discussion
of network access authentication and associated scenarios the reader
is referred to the PANA working group (see [26]).
Discovering security gateways, which was also mentioned as an
application for NSIS signaling, for the purpose of executing an IKE
to create an IPsec SA, is already solved without requiring NSIS.
In mobility scenarios an often experienced problem is the traversal
of a security gateway at the edge of the corporate network. Network
administrators often rely on the policy that only authenticated data
traffic is allowed to enter the network. A problem statement for the
traversal of these security gateways in the context of Mobile IP can
be found at [25]).
Other proposals for path-coupled NAT and Firewall traversal like RSVP
and CASP are described in [27] and [28].
1.3 General Scenario for NATFW Traversal
The purpose of NSIS NATFW signaling is to enable any communication
between endpoints across networks even in presence of middleboxes.
It is expected that those middleboxes be configured in such a way
that NSIS NATFW signaling messages itself are allowed to traverse
them. NSIS NATFW NSLP signaling is used to install such policy rules
Stiemerling, et al. Expires November 19, 2004 [Page 8]
Internet-Draft NAT/FW NSIS NSLP May 2004
in all 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 endhosts are
located behind middleboxes (NAT/FW in Figure 1). Applications
located at these endhosts try to establish communication between them
and use NSIS NATFW NSLP signaling to establish policy rules on a data
path, which allows the said data to travel from the sender to the
receiver unobstructed. The applications can somehow trigger
middlebox traversal (e.g. via an API call) at the NSIS entity at the
local host.
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 running NATFW signaling it is necessary that each Firewall and
each NAT involved in the signaling communication runs an NSIS NATFW
entity. There might be several NATs and FWs in various possible
combinations on a path between two hosts. The reader is referred to
Section 2.1 where different scenarios are presented.
Stiemerling, et al. Expires November 19, 2004 [Page 9]
Internet-Draft NAT/FW NSIS NSLP May 2004
2. Network Environment
2.1 Network Scenarios for Protocol Functionality
This section introduces several scenarios for middleboxes in the
Internet. Middleboxes are located at different locations, i.e. at
Enterprise network borders, within enterprise networks, mobile phone
network gateways, etc. In general, middleboxes are placed more
towards the edge of networks and less in network cores. Those
middleboxes are not only either Firewall or NAT and any other type of
combination is possible. Thus, combined Firewall and NATs are
available.
NSIS initiators (NI) are sending 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
have the NATFW NSLP implemented. Each NATFW NSLP node processes the
signaling messages according to Section 3 and installs, if necessary,
policy rules for subsequent data packets.
Each following section 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.
2.1.1 Firewall traversal
This section describes a scenario with Firewalls only and NATs are
not involved. Both end hosts are behind a Firewall that is connected
via the public Internet. Figure 2 shows the topology. The part
labeled "public" is the Internet connection 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-path must provide traversal service for NATFW NSLP
Stiemerling, et al. Expires November 19, 2004 [Page 10]
Internet-Draft NAT/FW NSIS NSLP May 2004
in order to permit the NSIS message to reach the other end host. All
Firewalls process NSIS signaling and establish appropriate policy
rules, so that the required data packet flow can traverse them.
2.1.2 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 labeled as MB (for
middlebox), since those devices implement at least NAT-only, but can
implement Firewalling as well.
Only two middleboxes MB are shown in Figure 3 at each side, but in
general more than one MB 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 four middleboxes
on the path and all four middleboxes must be configured properly to
allow NSIS signaling to traverse. The NATFW signaling must configure
all middleboxes and consider any address translation in further
signaling. The sender (NI) has to know the IP address of the
receiver (NR) in advance, otherwise he cannot send a single NSIS
signaling message 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 obtained from the
NAT that subsequently allows packets hitting 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.
Stiemerling, et al. Expires November 19, 2004 [Page 11]
Internet-Draft NAT/FW NSIS NSLP May 2004
2.1.3 NAT with private network on sender side
This scenario shows an application instance at the sending node that
is behind one or more NATs (shown as MB). 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 scenario
The traffic from NI to NR has to traverse only middleboxes 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 3.2.2 NSIS can also provide help for this procedure.
2.1.4 NAT with private network on receiver side
The application instance receiving data is behind one or more NATs.
Stiemerling, et al. Expires November 19, 2004 [Page 12]
Internet-Draft NAT/FW NSIS NSLP May 2004
//----\\ +----+ +----+
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 public reachable IP
address at the external middlebox and notify the NSIS initiator about
this address. One possibility is that an application level protocol
is used, meaning that the public IP address is signaled via this
protocol to the NI. Afterwards the NI can start its signaling
towards the NR and so establishing the path via the both middleboxes
MB.
This scenario describes the use case for the reserve mode of the
NATFW NSLP.
2.1.5 Both End Hosts behind twice-NATs
This is a special case, where the main problem is to detect that both
nodes are logically within the same address space, also behind a
twice-NAT (see [8] for discussion about twice-NAT functionality).
Sender and receiver are both within a private address realm and
potentially have overlapping IP addresses. Figure 6 shows the
ordering of NATs. This is a common configuration in several
networks, particularly after the merging of companies that have used
the same address space, thus having overlapping addresses in many
cases.
Stiemerling, et al. Expires November 19, 2004 [Page 13]
Internet-Draft NAT/FW NSIS NSLP May 2004
public
+----+ +----+ //----\\
NI --| MB |--+--| MB |---| |
+----+ | +----+ \\----//
|
| +----+
+--| MB |------------ NR
+----+
private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 6: NAT to public, sender and receiver behind 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 assistance of application level entities, like
DNS server. Those application level gateways must handle request
that are based on symbolic names and configure the middleboxes so
that data packets are correctly forwarded from NI to NR. The
configuration of those middleboxes may require other middlebox
communication protocols, like MIDCOM [7]. NSIS signaling is not
required in the twice-NAT only case, since the middleboxes of type
twice-NAT are configured by other means. Nevertheless, NSIS
signaling might by useful when there are Firewalls on path. In this
case NSIS will not configure any policy rule at twice-NATs, but will
configure policy rules at the intermediate Firewalls. The NSIS
signaling protocol must be at least robust enough to survive this
scenario.
2.1.6 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 2.1.4 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 two times: from
Stiemerling, et al. Expires November 19, 2004 [Page 14]
Internet-Draft NAT/FW NSIS NSLP May 2004
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, both sender and receiver obtain a public IP address
at the NAT and the communication path is not optimal anymore.
NI public
\ +----+ //----\\
+-| MB |----| |
/ +----+ \\----//
NR
private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 7: NAT to public, both host behind same NAT
NSIS NATFW signaling protocol should support mechanisms to detect
such a scenario. The signaling should directly by exchanged between
NI and NR without involving the middlebox.
2.1.7 IPv4/v6 NAT with two private networks
This scenario combines the usage case mentioned in Section 2.1.2
with the IPv4 to IPv6 transition scenario, i.e. using Network
Address and Protocol Translators (NAT-PT, [11]).
The difference to the other scenarios is the use of IPv6 to IPv4 (and
vice versa) address and protocol translation. Additionally, the base
NTLP must take care of this case for its own functionality of
forwarding messages between NSIS peers.
Stiemerling, et al. Expires November 19, 2004 [Page 15]
Internet-Draft NAT/FW NSIS NSLP May 2004
+----+ +----+ //---\\ +----+ //---\\ +----+ +----+
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 2.1.5 and so those issues of twice-NATs apply
here as well.
2.1.8 Multihomed Network with NAT
The previous chapters sketched network topologies where NAT and
Firewalls are ordered sequentially on the path. This chapter
describes a multihomed scenario with two NATs to the Internet.
+----+
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 the one or the other middlebox is used
for the data flow. Which middlebox is used depends on local routing
decisions. NATFW NSLP must be able to handle this situation proper,
see Section 3.2.2 for a more elaborated discussion of this topic with
Stiemerling, et al. Expires November 19, 2004 [Page 16]
Internet-Draft NAT/FW NSIS NSLP May 2004
respect to NATs.
2.2 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" then authorization is very important due to
the nature of middleboxes.
It is particularly not surprising that different degrees of required
authorization in a QoS signaling environment and middlebox signaling
exist. As elaborated in [23], establishment of a financial
relationship is very important for QoS signaling, whereas for
middlebox signaling is not directly of interest. For middlebox
signaling a stronger or weaker degree of authorization might be
needed.
Different trust relationships that appear in middlebox signaling
environments are described in the subsequent sections. Peer-to-peer
trust relationships are those, which are used in QoS signaling today
and seem to be the simplest. However, there are reasons to believe
that this is not the only type of trust relationship found in today's
networks.
2.2.1 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
(manual configuration) or 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 far distant
node. 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 10 does not illustrate the trust
relationship between the end host and the access network.
Stiemerling, et al. Expires November 19, 2004 [Page 17]
Internet-Draft NAT/FW NSIS NSLP May 2004
+------------------------+ +-------------------------+
| | | |
| Network A | | Network B |
| | | |
| +---------+ +---------+ |
| +-///-+ Middle- +---///////----+ Middle- +-///-+ |
| | | box 1 | Trust | box 2 | | |
| | +---------+ Relationship +---------+ | |
| | | | | |
| | | | | |
| | | | | |
| | Trust | | Trust | |
| | Relationship | | Relationship | |
| | | | | |
| | | | | |
| | | | | |
| +--+---+ | | +--+---+ |
| | Host | | | | Host | |
| | A | | | | B | |
| +------+ | | +------+ |
+------------------------+ +-------------------------+
Figure 10: Peer-to-Peer Trust Relationship
2.2.2 Intra-Domain Trust Relationship
In larger corporations often more than one middlebox is used to
protect 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 to 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 interaction is avoided. This
provides both a performance improvement without a security
disadvantage since a single administrative domain can be seen as a
single entity. Figure 11 illustrates a network structure, which uses
a centralized entity.
Stiemerling, et al. Expires November 19, 2004 [Page 18]
Internet-Draft NAT/FW NSIS NSLP May 2004
+-----------------------------------------------------------+
| |
| Network A |
| |
| |
| +---------+ +---------+
| +----///--------+ Middle- +------///------++ Middle- +---
| | | box 2 | | box 2 |
| | +----+----+ +----+----+
| | | | |
| +----+----+ | | |
| | Middle- +--------+ +---------+ | |
| | box 1 | | | | |
| +----+----+ | | | |
| | | | | |
| - | | | |
| - | +----+-----+ | |
| | | | Policy | | |
| +--+---+ +-----------+ Decision +----------+ |
| | Host | | Point | |
| | A | +----------+ |
| +------+ |
+-----------------------------------------------------------+
Figure 11: Intra-domain Trust Relationship
2.2.3 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 cause an error message to be 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 12 shows the slightly more complex trust relationships in this
scenario.
Stiemerling, et al. Expires November 19, 2004 [Page 19]
Internet-Draft NAT/FW NSIS NSLP May 2004
+----------------------+ +--------------------------+
| | | |
| Network A | | Network B |
| | | |
| | Trust | |
| | Relationship | |
| +---------+ +---------+ |
| +-///-+ Middle- +---///////----+ Middle- +-///-+ |
| | | box 1 | +-------+ box 2 | | |
| | +---------+ | +---------+ | |
| | | | | | |
| |Trust | | | | |
| |Relationship | | | | |
| | | | | Trust | |
| | | | | Relationship| |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
| +--+---+ | | | +--+---+ |
| | Host +----///----+------+ | | Host | |
| | A | |Trust | | B | |
| +------+ |Relationship | +------+ |
+----------------------+ +--------------------------+
Figure 12: End-to-Middle Trust Relationship
Stiemerling, et al. Expires November 19, 2004 [Page 20]
Internet-Draft NAT/FW NSIS NSLP May 2004
3. Protocol Description
The protocol description section defines the NSIS NATFW NSLP with its
messages, objects, and the protocol semantics. Section 3.1
introduces the protocol and Section 3.3 defines the syntax of the
messages and objects. The protocol behavior is defined in Section
3.2.
3.1 Basic protocol overview
The NSIS Signaling Layer Protocol (NSLP) for NAT and FW traversal 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 NF only implement this NSLP when they provide middlebox
functions. Forwarders that do not have any NATFW NSLP functions just
forward these messages; those forwarders implement NTLP and one or
more other NSLPs.
A Data Sender (DS) that is intending to send data to a Data Receiver
(DR) must start its NATFW NSLP signaling. So the NI at the data
sender (DS) starts NSLP signaling towards the address of data
receiver DR (see Figure 13).
+-------+ +-------+ +-------+ +-------+
| 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 13: General NSIS signaling
The NSLP request messages are processed each time a NF with NATFW
Stiemerling, et al. Expires November 19, 2004 [Page 21]
Internet-Draft NAT/FW NSIS NSLP May 2004
NSLP support is passed. Those 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.
NSIS responders will check received messages and process those if
applicable. NSIS responders generate response messages and sent them
back to the NI via the same chain of NFs. The response message is
processed at each NI forwarder implementing NATFW NSLP. The Data
Sender can start sending its data flow to the Data Receiver, when the
signaling was successful, meaning that NI has received a successful
response.
In general, NATFW NSLP signaling follows the data path from DS to DR.
This 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, see also
Section 2.
When Data receiver (DR) and Data Sender (DS) are located in different
address realms and DR is behind a NAT, DS cannot signal to DR
directly. DR is not reachable from DS and thus no NATFW signaling
can be sent to DR's address. Therefore, DR must first determine an
address at a NAT that is reachable for DS, for instance DR must
determine its public IP address. Once DR has determined a public
address it forwards this to DS via a separate mechanism, which may be
application level signaling like SIP. This application level
signaling may involve third parties that assist in exchanging this
information. This separate mechanism is out of scope of NATFW NSLP.
NATFW NSLP signaling supports this public address fixing with this
mechanism:
o First, DR determines a public address by signaling on the reverse
path (DR towards DS) and thus making itself available to other
hosts. This process of determining a public addresses is called
reservation. This way DR reserves publicly reachable addresses
and ports, but this address/port cannot be used by data traffic at
this point of time.
o Second, DS is signaling directly to DR as DS would do if there is
no NAT in between, and so creating policy rules at middleboxes.
Note, that the reservation mode will make reservations only,
which will be "activated" by the signaling from DS towards DR.
The first mode is detailed in the Section 3.2.2
The protocol works on a soft-state basis, meaning that that whatever
state is installed or reserved on a middlebox, it will expire, and
thus be de-installed/ forgotten after a certain period of time. To
prevent this, the involved boxes will have to specifically request a
session extension. An explicit NATFW NSLP state deletion message is
Stiemerling, et al. Expires November 19, 2004 [Page 22]
Internet-Draft NAT/FW NSIS NSLP May 2004
also provided by the protocol.
Middleboxes should report back in case of error, so that appropriate
measures and debugging can be performed.
The next sections define the NATFW NSLP message types and formats,
protocol operations, and policy rule operations.
3.2 Protocol Operations
This section defines the protocol operations, how to create sessions,
maintain them, and how to reserve addresses.
3.2.1 Creating Sessions
Allowing two hosts to exchange data even in the presence of
middleboxes is realized in the NATFW NSLP by the 'create session'
request message. The data sender generates a 'create session'
message as defined in Section 3.4.2 and handles it to the NTLP. The
NTLP forwards the whole message on the basis of the flow routing
information towards DR. Each NSIS forwarders along the path that is
implementing NATFW NSLP process the NSLP message, this is done NSLP
hop-by-hop. Finally, the message is approaching DR, DR can accept
the request or reject it. DR generates a response to the request,
this response is transported hop by hop towards (XXX terminology) DS.
NATFW NSLP forwarders may reject requests at any time. Figure 14
sketches the message flow between NI (DS), a NF (NAT), and NR (DR).
NI Private Network NF Public Internet NR
| | |
| Create | |
|----------------------------->| |
| | |
| Error (if necessary) | |
|<-----------------------------| Create |
| |--------------------------->|
| | |
| | Path Succeeded/Error |
| Path Succeeded/Error |<---------------------------|
|<-----------------------------| |
| | |
| | |
Figure 14: Creation message flow
Stiemerling, et al. Expires November 19, 2004 [Page 23]
Internet-Draft NAT/FW NSIS NSLP May 2004
Processing of 'create session' messages is differently per NSIS node:
o NSLP initiator: NI only generate 'create session' messages and
handle them over to the NTLP. After receiving a 'path succeeded'
the data path is configured and the NI can start sending its data
to NR. After receiving an 'error' message the NI MAY try to
generate the 'create session' message again or give up, depending
on the error condition.
o NSLP forwarder: NSLP forwarders receiving 'create session'
messages MUST first check authentication and authorization before
any further processing is executed. The NF SHOULD check with its
local policies if he can accept the desired policy rule given by
NTLP's flow routing information. Further processing depends on
the middlebox type:
* NAT: When the 'create session' message is received at the
public side a network external node is trying to open a NAT
binding. First, it looks for a reservation made in advance by
means of 'reserve external address' that matches the
destination address/port of the flow routing information
provided by the NTLP. If there is no reservation made in
advance the NSLP SHOULD return an error 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 address of the next NSIS node.
When the 'create session' message is received at the private
side the NAT binding is reserved, but not activated. The NSLP
message is forwarded to next hop with source address set to the
NAT's external address.
* Firewall: When the 'create session' 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.
* 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 care about the
order of Firewall and NAT functions within the device. Order
of functions is to be interpreted as how packets experience the
treatment of those functions.
o NSLP receiver: NRs receiving 'create session' messages MUST reply
with a 'path succeeded' message if they accept the request
message. Otherwise they SHOULD generate an error message. Both
messages are sent back NSLP hop-by-hop towards NI.
Policy rules at middleboxes MUST be only installed upon receiving a
successful response of type 'path succeeded'. This is a
countermeasure to several problems, for instance, loaded policy rules
at intermediate NF without reaching the actual NR.
Stiemerling, et al. Expires November 19, 2004 [Page 24]
Internet-Draft NAT/FW NSIS NSLP May 2004
3.2.2 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 and (covered by Section 3.2.1).
For scenarios where the data receiver is located behind a NAT and it
needs to receive data flows from outside its own network (see Figure
5) it 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 15: The Data Receiver behind NAT problem
Figure 15 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 as
Application Server). The communication with the application server
and the two end points (data sender and data receivers) serves a
number of functions. As one of the most important functions it
Stiemerling, et al. Expires November 19, 2004 [Page 25]
Internet-Draft NAT/FW NSIS NSLP May 2004
enables the two end hosts to learn the IP address of each other. 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 NSIS). NSIS
signaling messages cannot be used to communicate application level
relevant end point identifiers (in the generic case at least) as a
replacement for the 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 (if there
was one 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 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 force end hosts to have this type of topology knowledge.
Public Internet Private Address
Space
Edge
NI(DS) NAT NAT NR(DR)
NR+ NI+
| | | |
| | | |
| | | |
| | Reserve | Reserve |
| |<---------|<----------------|
| | | |
| | Return | ext addr/Error |
| |--------->|---------------->|
| | | |
| | | |
====================================================>
Data Traffic Direction
Figure 16: Reservation message flow
Stiemerling, et al. Expires November 19, 2004 [Page 26]
Internet-Draft NAT/FW NSIS NSLP May 2004
Figure 16 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 actual data receiver (DR) is the NSIS initiator (NI+) for the
'reserved external address' message, but the NSIS responder (NR)
for 'create session' messages following later.
o The actual data sender (DS) will be the NSIS initiator (NI) for
later 'create session' messages and may be the NSIS target of the
signaling (NR+).
o The actual target of the 'reserved external address' message may
be an arbitrary address NR+.
The data receiver DR starts to signal an 'reserve external address'
message into the "wrong direction". By "wrong" we refer to the usual
behavior of path-coupled signaling where the data sender starts
signaling in order to tackle with routing asymmetry. The data
receiver would typically return signaling messages to the data sender
in the reverse direction by utilizing state created at nodes along
the path (i.e. to reverse route signaling messages). In case of
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 signaling target address selection for this message is
discussed in Section 3.2.10.
The signaling message creates NSIS NAT Forwarding State at
intermediate NSIS NAT node(s). 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 has such a capability. Forwarding
of the 'reserve external address' message beyond this entity is not
necessary, and should be prohibited as it provides information on
internal hosts capabilities.
The edge NAT device is responding with a 'return external address'
message containing the public reachable IP address/port number.
Processing of 'reserve external address' messages is differently per
NSIS node:
o NSLP initiator: NI+ only generate 'reserve external address'
messages and should never receive them.
o NSLP forwarder: NSLP forwarders receiving 'reserve external
address' messages MUST first check authentication and
authorization before any further processing is executed. The NF
SHOULD check with its local policies if he can accept the desired
policy rule given by NTLP's flow routing information. Further
processing depends on the middlebox type:
Stiemerling, et al. Expires November 19, 2004 [Page 27]
Internet-Draft NAT/FW NSIS NSLP May 2004
* NAT: NATs check whether the message is received at the public
address or at the private address. If received at the public
address a NF MAY generate an error message of type 'requested
external address from outside'. If received at the private
address, an IP address/port is reserved. In the case it is an
edge-NAT, the NSLP message is not forwarded anymore and a
response of type 'return external address' is generated. If it
is not an edge-NAT, the NSLP message is forwarded further.
* Firewall: Firewalls MUST not change their configuration upon a
'reserve external address' message. They simply MUST forward
the message and MUST keep NTLP state. Firewalls that are
configured as edge-Firewalls (XXX, do definition!) 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 'return external address' messages is differently per
NSIS node:
o NSLP initiator: Upon receiving a 'return external address'
message the NI+ can use the obtained IP address and port number
for further application signaling.
o NSLP forwarder: NFs simply forward this message as long as they
keep state for the requested reservation.
o NSIS responder: This type of message should never be received by
any NR and it SHOULD be discarded silently.
3.2.3 Reserving External Addresses and Create Session
Some migration scenarios need specialized support to cope with the
situation where the receiving side is running NSIS only. End-to-end
signaling is going to fail without NSIS support at both sides. For
this the 'create-reverse' signaling mode is supported. In this case,
a DR can signal towards the DS like in the 'reserve external address'
message scenario. The message is forwarded until it reaches the
edge-NAT and retrieves a public IP address and port number. Unlike
in the 'reserve external address' no 'return external address'
response message is created, the forwarding of the request message
stops and a 'create session' message is generated by the edge-NAT.
This request message is sent towards DR with DS as source address and
follows the regular processing orders as 'create session' messages
do. The exact definition of this mode is to be done.
3.2.4 Prolonging Sessions
NATFW NSLP sessions are maintained on a soft-state base. After a
certain timeout sessions and corresponding policy rules are removed
Stiemerling, et al. Expires November 19, 2004 [Page 28]
Internet-Draft NAT/FW NSIS NSLP May 2004
automatically by the middlebox, if they are not refreshed by a
prolong session message. NI is sending prolong message towards NR
and each NSIS forwarder maintaining state for the given session ID
extends the lifetime of the session. Extending lifetime of a session
is calculated as current local time plus lifetime. Section 3.2.7 is
defining the process of calculating lifetimes in detail.
NI Public Internet NAT Private address NR
| | space |
| Prolong | |
|----------------------------->| |
| | |
| Error (if necessary) | |
|<-----------------------------| Prolong |
| |--------------------------->|
| | |
| | Error (if necessary) |
| Error (if necessary) |<---------------------------|
|<-----------------------------| |
| | |
| | |
Figure 17: Prolongation message flow
Processing of 'prolong session' messages is differently per NSIS
node:
o NSLP initiator: NI can generate 'prolong session' messages before
the session times out.
o NSLP forwarder: NSLP forwarders receiving 'prolong session'
messages MUST first check authentication and authorization before
any further processing is executed. The NF SHOULD check with its
local policies if he 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: NIs accepting this prolong message generate a
'path succeeded' message.
3.2.5 Deleting Sessions
NATFW NSLP sessions may be deleted at any time. NSLP initiators can
trigger this deletion via the 'delete session' message, as shown in
Figure 17.
Stiemerling, et al. Expires November 19, 2004 [Page 29]
Internet-Draft NAT/FW NSIS NSLP May 2004
NI Public Internet NAT Private address NR
| | space |
| Delete | |
|----------------------------->| |
| | |
| | Delete |
| |--------------------------->|
| | |
Figure 18: Delete message flow
NSLP nodes receiving this message MUST delete the session
immediately. Corresponding policy rules to this particular session
MUST be deleted immediately, too. This message is forwarded until it
reaches the final NR. The 'delete' message does not generate any
response, neither positive nor negative, since there is no NSIS state
left at the nodes along the path.
3.2.6 Authorization
Authorization and security issues are currently discussed in a
different document and will be included after reaching consensus (
[20]).
3.2.7 Calculation of Lifetimes
NATFW NSLP sessions, and the corresponding policy rules possibly
installed, are maintained via soft-state. Each session is assigned a
lifetime and they are 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 are not responsible for triggering
lifetime prolongination messages (see Section 3.2.4), this is the
task of the NSIS initiator.
NSIS initiator MUST choose a lifetime value before they can sent any
message (except 'delete session' messages) to other NSLP nodes. This
lifetime value should consider application's needs, i.e., duration in
terms of minutes or hours, and networking needs, i.e., values in the
range less than 30 seconds may not be useful. 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 NSLP forwarders processing the request message along the path
MAY lower the request lifetime given to fit their needs and/or local
Stiemerling, et al. Expires November 19, 2004 [Page 30]
Internet-Draft NAT/FW NSIS NSLP May 2004
policy. NATFW forwarders MUST NOT increase the lifetime value; they
MAY reject the requested lifetime immediately and MUST generate an
error response message of type 'lifetime too big' upon rejection.
The NSLP request message is forwarded until it reaches the NSLP
responder. NSLP responder MAY reject the requested lifetime value
and MUST generate an error response message of type 'lifetime too
big' upon rejection. NSLP responder MAY lower the requested lifetime
as well to a granted lifetime. 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 equal or lower than the requested
lifetime. They MAY reject the lifetime and generate a 'lifetime not
acceptable' error response message. Figure 19 shows the procedure
with an example, where an initiator requests 60 minutes lifetime in
'create session' message and the lifetime is shortened along the path
by the forwarder to 20 minutes and by the responder to 5 minutes.
+-------+ CREATE(lt=60m) +-----------+ CREATE(lt=20m) +--------+
| |---------------->| NSLP |---------------->| |
| NI | | | | NR |
| |<----------------| forwarder |<----------------| |
+-------+ OK(lt=5m) +-----------+ OK(lt=5m) +--------+
lt = lifetime
CREATE = 'create session' message
OK = 'path succeeded' message
Figure 19: Lifetime Calculation Example
3.2.8 Middlebox Resource
This section needs to be done and should describe how to map flow
routing information to middlebox policy rules. Further, this section
should clarify wildcarding. XXX
3.2.9 De-Multiplexing at NATs
Section 3.2.2 describes how NSIS nodes behind NATs can obtain a
Stiemerling, et al. Expires November 19, 2004 [Page 31]
Internet-Draft NAT/FW NSIS NSLP May 2004
public reachable IP address and port number at a NAT. The
information IP address/port number can be transmitted via a signaling
protocol and/or third party to the communication partner that would
like to send data towards. 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.
Two options for de-multiplexing incoming NSLP requests are:
1. Based on flow routing information, like protocol number and TCP
port numbers.
2. Based on NSIS session IDs.
Approach 2) would require that both NSIS ends, initiator and
responder, use the same session ID in NSIS signaling. Since session
IDs are usually generated randomly, application level signaling would
have to be adapted to carry NSIS session IDs used during reservation
to the other end (the NSIS initiator sending the 'create session'
message). This approach SHOULD NOT be used.
Approach 1) uses information stored at NATs (like mapping of public
IP address to private, transport protocol, port numbers) and
information given by NTLP's flow routing information to de-multiplex
NSIS messages. This approach is RECOMMENDED.
3.2.10 Selecting Destination IP addresses for REA
Request messages of type 'reserve external address' do need, as any
other message type as well, a final destination IP address to reach.
But as many applications do not provide a destination IP address at
the first place, there is a need to choose a destination address for
the 'reserve external address' messages. This destination can be the
final target, but for the mentioned type of application, the
destination address can be arbitrary. Taking the "correct"
destination IP address might be difficult and there is no right
answer. [19] shows choices for SIP and this section provides some
hints about choosing a good destination IP address in general.
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.
Stiemerling, et al. Expires November 19, 2004 [Page 32]
Internet-Draft NAT/FW NSIS NSLP May 2004
+ Due to routing asymmetry it might be possible that the
routes taken by a) the data sender and the application
server b) the data sender and NAT B might be different. As
a consequence it might be necessary to advertise a new (and
different) external IP address with SIP 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 at the Application Server:
* Assumption:
+ An application server (or a different third party) is
available.
* Problems:
+ If the NSIS signaling message is not terminated at the NAT
of the local network then an NSIS unaware application
server might discard the message.
+ Routing might not be optimal since the route between a) the
data receiver and the application server b) the data
receiver and the data sender might be different.
3.3 NATFW NSLP Messages 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 bound to 32 bits and objects that do not fall into 32
bits boundaries must be padded to 32 bits.
The whole NSLP message is carried in a NTLP message.
Note that the notation 0x is used to indicate hexadecimal numbers.
3.3.1 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 20.
Stiemerling, et al. Expires November 19, 2004 [Page 33]
Internet-Draft NAT/FW NSIS NSLP May 2004
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSLP message type | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 20: Common NSLP header
The reserved field MUST be set to zero in the NATFW NSLP header
before sending and MUST be ignored during processing the header.
Note that other NSLPs use this field as flag field.
3.3.2 NSLP message types
The message types identify requests and responses. Defined messages
types for requests are:
o 0x0101 : create
o 0x0102 : reserve
o 0x0103 : reserve-create
o 0x0104 : prolong
o 0x0105 : delete
Defined message types for responses are:
o 0x0201 : path_succeed
o 0x0202 : path_deleted
o 0x0203 : ret_ext_addr
o 0x0204 : error
3.3.3 NSLP Objects
NATFW NSLP objects use a common header format defined by Figure 21.
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.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSLP object type | NSLP object length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 21: Common NSLP object header
The length is the total length of the object without the object
Stiemerling, et al. Expires November 19, 2004 [Page 34]
Internet-Draft NAT/FW NSIS NSLP May 2004
header. The unit is bytes. The particular values of type and length
for each NSLP object are listed in the subsequent chapters that
define the NSLP objects.
3.3.3.1 Session ID Object
The session ID object carries an identifier for the session of the
signaled flow. The only field is the session ID of 16 bytes length.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0001 | 16 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// 16 bytes session id //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 22: Session ID object
The session ID is generated in random way by the NSIS initiator.
3.3.3.2 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.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0002 | 4 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NATFW NSLP session lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 23: Lifetime object
Stiemerling, et al. Expires November 19, 2004 [Page 35]
Internet-Draft NAT/FW NSIS NSLP May 2004
3.3.3.3 External Address Object
The external address objects can be included in ret_ext_addr
responses (Section 3.4.9) only. It contains the external IP address
and port number allocated at the edge-NAT. Note that this address/
port may be either reserved or reserve-create. Two fields are
defined, the external IP address, and the external port number. For
IPv4 the object with value 0x0010 is defined. It has a length of 8
bytes and is shown in Figure 24.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0010 | 8 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port number | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 24: External Address Object for IPv4 addresses
For IPv6 the object with value 0x0011 is defined. It has a length of
20 bytes and is shown in Figure 25.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0011 | 20 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port number | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IPv6 address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 25: External Address Object for IPv6 addresses
Stiemerling, et al. Expires November 19, 2004 [Page 36]
Internet-Draft NAT/FW NSIS NSLP May 2004
3.3.3.4 Extended Flow Information Object
In general, flow information is kept at the NTLP level during
signaling. Nevertheless, some additional information may be required
for NSLP operations. The 'extended flow information' object carries
this additional information about number of subsequent port numbers
that should be allocated at middleboxes.
These fields are defined for the policy rule object:
o Number of ports: This field gives the number of ports that should
be allocated beginnig at the port given in NTLP's flow routing
information.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0011 | 4 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| number of ports | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 26: Extended Flow Information
3.3.3.5 Error Object
The error object carries the reason for an error. It has only one
field, the error code, and is 2 bytes long.
0 16 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0002 | 4 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| error code | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 27: Error
TBD: Define error clases and define the error coded. Possible
classes are:
Stiemerling, et al. Expires November 19, 2004 [Page 37]
Internet-Draft NAT/FW NSIS NSLP May 2004
o Policy rule errors
o Authentication and Authorization errors
o NAT
Currently in this memo defined errors:
o lifetime too big
o lifetime not acceptable
o no NAT here
o no reservation found
o requested external address from outside
3.4 Message Formats
This section defines the content of each NATFW NSLP message type.
The message types are defined in Section 3.3.2. 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.
Each section elaborates the required settings and parameters to be
set by the NSLP at the NTLP, for instance, how the flow routing
information is set.
3.4.1 Policy Rules
Policy rules are the building block of middlebox devices considered
in the NATFW NSLP. For Firewalls the policy rule consists usually of
a 5-tuple, source/destination address, transport protocol, and
source/destination port number, plus an action like allow or deny.
Other actions are available depending on the implementation of the
Firewall, but NATFW NSLP uses only allow action, since a default to
deny policy at the middlebox is assumed. For NATs the policy rule
consists of action 'map this another address realm' and further
mapping information, that might be in the most simply 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 filter
specification, an implicit allow action, and additional information.
The filter specification is carried within NTLP's flow routing
information and additional information is carried in NSLP's objects.
Additional information is for instance the lifetime of a policy rule
or session.
Stiemerling, et al. Expires November 19, 2004 [Page 38]
Internet-Draft NAT/FW NSIS NSLP May 2004
3.4.2 Create Session (CRS)
The create session request message is used to create NSLP sessions
and at middleboxes to create policy rules.
The create session messages carries these objects:
o Session ID object
o Lifetime object
The flow 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.
3.4.3 Reserve External Address (REA)
The reserve external address (REA) request message is used to lookup
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 Session ID object
o Lifetime object
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 3.2.10). 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.
3.4.4 Reserve-Create (REC)
XXX This is a proposal for a new message to support the reservation
and simultaneous/implicit create message generation.
The reserve-create message carries these objects:
o Session ID object
o Lifetime object
NTLP issues: TBD.
3.4.5 Prolong Session (PLS)
The prolong request message is used to prolong (extend) the lifetime
of a NATFW NSLP and policy rules at middleboxes.
The prolong session message carries these objects:
Stiemerling, et al. Expires November 19, 2004 [Page 39]
Internet-Draft NAT/FW NSIS NSLP May 2004
o Session ID object
o Lifetime object
The flow 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.
3.4.6 Delete Session (DLS)
The delete request message is used to delete NATFW NSLP sessions.
The delete session message carries these objects:
o Session ID object
The flow 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.
3.4.7 Path Succeeded (PS)
The path succeeded response message is used to acknowledge a
successful create and prolong.
The path succeeded message carries these objects:
o Session ID object
o lifetime object
This message is routed on the reverse path.
3.4.8 Path Deleted (PD)
The path deleted response message is used to acknowledge a successful
delete request message.
The path deleted message carries this object:
o Session ID object
This message is routed on the reverse path.
3.4.9 Return External Address (RA)
The return external address response message is sent back as a
positive result of reserve external address request. It contains the
reserved external IP address and port number.
The path succeeded message carries these objects:
o Session ID object
Stiemerling, et al. Expires November 19, 2004 [Page 40]
Internet-Draft NAT/FW NSIS NSLP May 2004
o Lifetime object
o External address object (either IPv4 or IPv6 type)
This message is routed on the reverse path.
3.4.10 Error Response (ER)
The error response message is sent back by any NSIS node involved in
the session that occurs an error condition.
The error message carries these objects:
o Session ID object
o Error object
This message is routed on the reverse path.
Stiemerling, et al. Expires November 19, 2004 [Page 41]
Internet-Draft NAT/FW NSIS NSLP May 2004
4. NSIS NAT and Firewall transitions 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.
Stiemerling, et al. Expires November 19, 2004 [Page 42]
Internet-Draft NAT/FW NSIS NSLP May 2004
5. Security Considerations
Security is of major concern particularly in case of Firewall
traversal. Generic threats for NSIS signaling have been discussed in
[6] and are applicable here as well. It is necessary to provide
proper signaling message protection and proper authorization. Note
that the NAT is likely to be co-located with a Firewall and might
therefore require packet filters to be changed in order to allow the
signaling message to process and to traverse. This section aims to
raise some items for further discussion and illustrates the problems
the authors faced when creating a security solution for the NAT/
Firewall NSLP.
Installing packet filters provides some security, but 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.
Without doubts there is a dependency on the security provided by the
NTLP. Section Appendix A and Section 2.2 motivates some trust
relationship and authorization scenarios. These scenarios deserve a
discussion since some of them (particularly one with a missing
network-to-network trust relationship) is different to what is know
from QoS signaling. To address some of these trust relationships and
authorization issues requires security mechanisms between
non-neighboring nodes at the NSLP layer. For the group of authors it
seems that peer-to-peer and end-to-middle security needs to be
provided. An NSLP security mechanism between neighboring NSLP peers
might be necessary if security mechanisms at the NTLP do not provide
adequate protection mechanisms. This issue is, however, still in
discussion.
As a design goal it seems to be favorable to reuse existing
mechanisms to the best extend possible. In most cases it is
necessary to carry the objects for end-to-middle as NSLP payloads
since the presence of NATs might prevent direct communication. Three
security mechanisms have to be considered in more detail in a future
version of this document: CMS [21] and Identity Representation for
RSVP [15]. The authors believe that CMS more suitable (since it
provides much more functionality). The details deserve further
Stiemerling, et al. Expires November 19, 2004 [Page 43]
Internet-Draft NAT/FW NSIS NSLP May 2004
discussion and implementation experience.
With regard to signal between two end hosts even though the receiver
is behind a NAT this proposal suggests creating state by the data
receiver first. This allows NSIS signaling messages to traverse a
NAT at the receiver side (due to the established state at this NAT
box) and simplifies security handling. To achieve this behavior it
is required to install NSIS NTLP and NSLP state. Furthermore, it is
envisioned to associate the two signaling parts (one part from the
data sender to the NAT and the other part from the NAT to the data
receiver) with the help of the Session Identifier. As such, the
discussion in [15] is relevant for this document.
Another interesting property of this protocol proposal is to prevent
Denial of Service attacks against NAT boxes whereby an adversary
allocates NAT bindings with the help of data packets. Since these
data packets do not provide any type of authentication and are not
authorized any adversary is able to mount such an attack. This
attack has been mentioned at several places in the literature
already and is particularly harmful if no NAPT functionality is used
(i.e. if a new NAT binding consumes one IP address of a pool of IP
addresses). Using the protocol described in this document additional
security can be achieved and more fairness can be provided.
Stiemerling, et al. Expires November 19, 2004 [Page 44]
Internet-Draft NAT/FW NSIS NSLP May 2004
6. Open Issues
At least the following issues require further discussion:
o Option processing rules in presence of unknown options.
o Terminology w.r.t. the term wrong way.
o NTLP: New object and semantics for REA.
o NTLP and NATFW NSLP interaction
o List of NTLP transport modes per NSLP message
o Routing Change detection
o Query message, definition of semantics needed
o Is there a need for a QoS NSLP RSN like object/mechanism in NATFW
NSLP?
o Add IANA considerations section.
o re-work security considerations.
o Query message: Syntax and semantics.
o Add text about asynchronous messages.
o Anycast address for REA.
o Check common formats with QoS NSLP
o Change length field of objects to long words as unit?
o Variable length for session id?
o Meaning of 0 as session ID.
o Extended flow object: Needs refinement
Stiemerling, et al. Expires November 19, 2004 [Page 45]
Internet-Draft NAT/FW NSIS NSLP May 2004
7. 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.
Stiemerling, et al. Expires November 19, 2004 [Page 46]
Internet-Draft NAT/FW NSIS NSLP May 2004
8. References
8.1 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-00.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.
8.2 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.
Stiemerling, et al. Expires November 19, 2004 [Page 47]
Internet-Draft NAT/FW NSIS NSLP May 2004
[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, Februar 2004.
[21] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369,
August 2002.
[22] Manner, J., Suihko, T., Kojo, M., Liljeberg, M. and K.
Raatikainen, "Localized RSVP", DRAFT draft-manner-lrsvp-00.txt,
November 2002.
[23] Tschofenig, H., Buechli, M., Van den Bosch, S. and H.
Schulzrinne, "NSIS Authentication, Authorization and Accounting
Issues", March 2003.
[24] Amini, L. and H. Schulzrinne, "Observations from router-level
internet traces", DIMACS Workshop on Internet and WWW
Measurement, Mapping and Modelin Jersey) , Februar 2002.
[25] 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.
Stiemerling, et al. Expires November 19, 2004 [Page 48]
Internet-Draft NAT/FW NSIS NSLP May 2004
[26] 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.
[27] Shore, M., "The TIST (Topology-Insensitive Service Traversal)
Protocol", DRAFT draft-shore-tist-prot-00.txt, May 2002.
[28] Tschofenig, H., Schulzrinne, H. and C. Aoun, "A Firewall/NAT
Traversal Client for CASP", DRAFT
draft-tschofenig-nsis-casp-midcom-01.txt, March 2003.
[29] 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.
[30] 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.
[31] 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.
[32] Rosenberg et al, J., "STUN - Simple Traversal of User Datagram
Protocol (UDP) Through Network Address Translators (NATs)", RFC
3489, March 2003.
[33] Rekhter et al, Y., "Address Allocation for Private Internets",
RFC 1918, February 1996.
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:
Stiemerling, et al. Expires November 19, 2004 [Page 49]
Internet-Draft NAT/FW NSIS NSLP May 2004
Hannes Tschoefenig
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
France
EMail: cedric.aoun@nortelnetworks.com
Stiemerling, et al. Expires November 19, 2004 [Page 50]
Internet-Draft NAT/FW NSIS NSLP May 2004
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 10, 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. Hence in the following scenario we assume a somewhat
different message processing and show three possible approaches to
tackle the problem. None of these three approaches is without
drawbacks or constraining assumptions.
Stiemerling, et al. Expires November 19, 2004 [Page 51]
Internet-Draft NAT/FW NSIS NSLP May 2004
+----------------------+ +--------------------------+
| | | |
| 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 28: Missing Network-to-Network Trust Relationship
Figure 28 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
with a certain authorization permission. Hence the solution
alternatives in Section 3.2.2 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).
Stiemerling, et al. Expires November 19, 2004 [Page 52]
Internet-Draft NAT/FW NSIS NSLP May 2004
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
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
Stiemerling, et al. Expires November 19, 2004 [Page 53]
Internet-Draft NAT/FW NSIS NSLP May 2004
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
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
Stiemerling, et al. Expires November 19, 2004 [Page 54]
Internet-Draft NAT/FW NSIS NSLP May 2004
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
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 2.1 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
Stiemerling, et al. Expires November 19, 2004 [Page 55]
Internet-Draft NAT/FW NSIS NSLP May 2004
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.
Stiemerling, et al. Expires November 19, 2004 [Page 56]
Internet-Draft NAT/FW NSIS NSLP May 2004
Appendix B. Acknowledgments
We would like to acknowledge Vishal Sankhla and Joao Girao for their
input to this draft.
Stiemerling, et al. Expires November 19, 2004 [Page 57]
Internet-Draft NAT/FW NSIS NSLP May 2004
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
Stiemerling, et al. Expires November 19, 2004 [Page 58]
Internet-Draft NAT/FW NSIS NSLP May 2004
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Stiemerling, et al. Expires November 19, 2004 [Page 59]
