NSIS A. Pashalidis
Internet-Draft H. Tschofenig
Expires: April 27, 2006 Siemens
October 24, 2005
NAT Traversal for GIST
draft-pashalidis-nsis-gimps-nattraversal-01.txt
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Abstract
This document describes how different types of Network Address
Translator (NAT) interact with the General Internet Signalling
Transport (GIST) protocol. The purpose of this interaction is for
signalling traffic to traverse the NATs in a way that preserves its
semantics with respect to the data flows it corresponds to.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 6
4. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Transparent NAT traversal for GIST . . . . . . . . . . . . . . 13
5.1. NI-side NSLP-unaware GaNATs . . . . . . . . . . . . . . . 13
5.2. NR-side NSLP-unaware GaNATs . . . . . . . . . . . . . . . 18
5.3. NSLP-aware GaNATs . . . . . . . . . . . . . . . . . . . . 21
5.4. Combination of NSLP-aware and NSLP-unaware GaNATs . . . . 25
6. Non-transparent NAT traversal . . . . . . . . . . . . . . . . 26
6.1. NI-side NSLP-unaware GaNATs . . . . . . . . . . . . . . . 26
6.2. NR-side NSLP-unaware GaNATs . . . . . . . . . . . . . . . 30
6.3. GIST peer processing . . . . . . . . . . . . . . . . . . . 34
7. GIST-unaware NATs . . . . . . . . . . . . . . . . . . . . . . 36
8. Security Considerations . . . . . . . . . . . . . . . . . . . 39
8.1. Service Denial Attacks . . . . . . . . . . . . . . . . . . 39
8.2. Network Intrusions . . . . . . . . . . . . . . . . . . . . 40
9. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 42
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 43
11. Normative References . . . . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 44
Intellectual Property and Copyright Statements . . . . . . . . . . 45
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1. Introduction
Network Address Translators (NATs) modify certain fields in the IP
header of the packets that traverse them. In the context of
signalling as specified by the General Internet Signalling Transport
(GIST) protocol [1], this behaviour may lead to the installation of
state at network nodes that may be inconsistent and meaningless with
respect to the actual traffic that traverses these nodes.
This document describes how GIST signalling messages traverse NATs in
a way that preserves the consistency of state that is installed in
the network with respect to the data flows to which the signalling
messages refer. As the mechanisms that are described in this
document exclusively operate at the GIST layer, they are transparent
to signalling applications. The document is organised as follows.
The next section introduces the terminology that is used throughout
this document. Section 3 provides a detailed discussion of the NAT
traversal problem and highlights certain design decisions that have
to be taken when addressing the problem. Section 4 lists the
assumptions on which the proposed mechanisms are based. Section 5
presents the proposed mechanisms for "transparent" NAT traversal, and
??? presents the proposed mechanisms where such protection is
required.
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2. Terminology
The terminology, abbreviations and notational conventions that are
used throughout the document are as follows.
o DR: Data Responder, as defined in [1]
o DS: Data Sender, as defined in [1]
o GaNAT: GIST-aware NAT - a GaNAT MAY implement a number of NSLPs.
o GIST: General Internet Messaging Protocol for Signalling [1]
o NAT: Network Address Translator
o NI: NSIS Initiator, as defined in [1]
o NR: NSIS Responder, as defined in [1]
o NSIS: Next Steps in Signalling: The name of the IETF working group
that specified the family of signalling protocols of which this
specification is also a member. The term NSIS is also used to
refer to this family of signalling protocols as a whole.
o GIST-aware: Implements GIST and MAY also implement a number of
NSLPs.
o GIST-unaware: GIST-unaware, does not implement any NSLP.
o NSLP: NSIS Signalling Layer Protocol, as defined in [1]
o downstream: as defined in [1]
o upstream: as defined in [1]
o MRI: Message Routing Information, as defined in [1]
o NLI.IA: Interface Address field of the Network Layer Information
header field, as defined in [1]
o <- : Assignment operator. The quantity to the right of the
operator is assigned to the variable to its left.
o A.B: Element B of structure A. Example: [IP
header].SourceIPAddress denotes the source IP address of an IP
header.
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o [data item]: This notation indicates that "data item" is a single
identifier of a data structure. (Square brackets do not denote
optional arguments in this document.)
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3. Problem Statement
According to [1], all GIST messages between two peers carry IP
addresses in order to define the data flow to which the signalling
refers. Moreover, certain GIST messages also carry the IP address of
the sending peer, in order to enable the receiving peer to address
subsequent traffic to the sender. Packets that cross an addressing
boundary, say from addressing space S1 to S2, have the IP addresses
in the IP header translated from space S1 to S2 by the NAT; if GIST
payloads are not translated in a consistent manner, the MRI in a GIST
packet that crosses the boundary, e.g. from address space S1 to S2,
refers to a flow that does not exist in S2. In fact, the flow may be
invalid in S2 because at the IP address that belongs to S1 may not be
routable or invalid in S2. Moreover, the IP address of the sending
peer may also be not routable or invalid in the addressing space of
the receiving peer. The purpose of this document is to describe a
way for GIST messages to be translated in a way that is consistent
with the translation that NATs apply to the IP headers of both
signalling and data traffic.
A NAT may either be GIST-unaware or GIST-aware. We denote a GIST-
aware NAT as a GaNAT in the sequel. A GaNAT MAY also support at
least one NSLP. Note that there exists an NSLP, namely the NATFW
NSLP [2], that specifically addresses NAT traversal for data flows.
Inevitably, the NATFW NSLP also provides the necessary mechanisms for
the related signalling to traverse the NATs involved. Consider a
GaNAT that supports both the NATFW NSLP, and the NAT traversal
extension to GIST that is specified in this document. Suppose now
that a GIST QUERY message arrives at this GaNAT that contains the
NSLP identifier (NSLPID) of the NATFW NSLP. A question that arises
is whether the GaNAT should use the GIST NAT traversal extension
defined in this document, or the NATFW NSLP mechanism, in order to
provide "NAT traversal" for both the signalling message and the data
flow to which it refers. The answer to this question is that such a
GaNAT MUST implement a policy according to which method is used in
preference to the other. Note, however, that, should the GaNAT
prefer the GIST NAT traversal to NATFW NSLP, then it will happen, if
no on-path GaNATs exist that prefer the NATFW NSLP, that the
downsteam NATFW peer will be unable to discover any downstream NATFW
NSLP peers. This may make the entire NATFW session obsolete.
Clearly, if a GaNAT does not implement the NATFW NSLP, then the only
way for the signalling and the data flow to traverse that GaNAT, is
for the necessary mechanisms to operate on the GIST layer. The same
holds when the NSLP that is being signalled is an NSLP other than the
NATFW NSLP. Enabling NAT traversal in presicely these circumstances
is the motivation of this specification.
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In general, a given data flow between a data sender (DS) and a data
receiver (DR) may have to traverse a number of NATs, some of which
may be GIST-and-NATFW-aware, some may be GIST-aware, and some may be
GIST-unaware. Additionally, NSLP signalling for such a data flow may
be required to traverse through a subset of those NATs. Whether or
not the routing inftrastructure and state of the network causes the
signalling for such a data flow to traverse the same NATs as the flow
depends, among other things, on which NSLP is being signalled. While
signalling of the QoS NSLP, for example, might not traverse any of
the NATs that are traversed by the data flow, the signalling of the
NATFW NSLP traverses at least those NATs that implement the NATFW
NSLP (otherwise the signalling path would no longer be coupled to the
data path, as this coupling is defined by the GIST QUERY/RESPONSE
discovery mechanism for the "path coupled" Message Routing Method).
It is desirable that the NAT traversal extension for GIST provides
NAT traversal for every possible combination of NATs, either on the
data or the signalling path, in a secure manner.
Due to the GIST QUERY/RESPONSE discovery mechanism (according to
which QUERY messages are simply forwarded if the current node does
not support the required NSLP), two GIST nodes typically identify
themselves as NSLP peers only if they both implement the same NSLP.
This means that, if one or more NATs that are unaware of this NSLP
are between them, then the two NSLP peers are not able to discover
each other at all. This is because, even in the unlikely event that
the NAT bindings that are necessary for the GIST traffic to traverse
the in-between NAT(s) exist, the NLI.IA field included in the
RESPONSE message sent by the downstream GIST peer is invalid (or the
IP address is unreachable) in the address space of the upstream GIST
peer. In order to overcome this limitation, either the two peers
need to cope with the in-between NAT(s), or, if the NAT(s) are
GaNATs, they (the GaNATs) need to apply additional processing in
order to transparently create and maintain consistency between the
information in the header of GIST signalling messages and the
information in the IP header of the data traffic. Additionally, if
NSLP-aware NATs are on the data path, then these NATs should process
NSLP traffic in a way that preserves consistency after address
translation. This processing deviates from the processing of NSLP-
aware non-NAT nodes. In the following sections we specify mechanisms
that aim to overcome the limitation of two adjacent GIST peers not
being able to execute an NSLP in the presence of in-between NAT(s).
Note that a number of different situations has to be dealt with,
depending on the level of NSIS support by a NAT. The following
combinations of NATs that are located between two adjacent GIST peers
are considered.
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o all NAT(s) are GIST-unaware
o all NAT(s) are (NSLP-unaware) GaNAT(s)
o all NAT(s) are NSLP-aware
o a combination of the above NAT types.
The approach taken in this document is to propose separate mechanisms
for the traversal of each of the above type of NAT. If NATs that
belong to multiple types exist on the path between two adjacent NSLP
peers, the proposed mechanisms should work in combination. Thus,
traversal of multiple NATs of different types should not require
further specification from a functional perspective. However,
security issues that arise due to the combination of NAT types may
have to be considered.
A GIST-unaware NAT cannot tell data and signalling traffic apart.
The installation of the NAT binding for the signalling traffic in
such a NAT occurs typically independently from the installation of
the NAT binding for the data traffic. Furthermore, as the NAT cannot
associate the signalling and the data traffic in any way, it cannot
indicate that an association exists between the two NAT bindings.
Therefore, in the presence of such a NAT, GIST nodes that are located
on either side of the NAT have to cope with the NAT without
assistance from the NAT. This would typically require initially
discovering the NAT and subsequently establishing an association
between between the MRI in the signalling messages and the translated
IP header in the data traffic. Due to the variety of behaviours that
a GIST-unaware NAT may exhibit, establishing this association is a
non-trivial task. Therefore, traversal of such (i.e. GIST-unaware)
NATs is considered a special case and is further discussed in
Section 7.
Traversal of GaNAT(s) is comperatively more straightforward. This is
because, based on the MRI in a given incoming GIST message, a GaNAT
can identify the data flow to which the message refers. It can then
check its NAT binding cache and determine the translation that is
(or, if no NAT binding for the flow exists yet, will be) applied to
the IP header of the data flow. The GaNAT can then include
sufficient information about this translation into the signalling
message, such that its receiver (i.e. the GIST peer that receives the
data traffic after network address translation has been applied) can
map the signalling message to the data flow.
There exist a variety of ways for a GaNAT to encode the
abovementioned information into signalling messages. In this
document the following two ways are considered.
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1. Non-transparent apprach: The GaNAT includes an additional "NAT
Traversal" payload (see section A.3.8 of [1]) into the GIST
header of the GIST QUERY message. This "NAT Traversal" payload
is echoed by the GIST responder on the other side of the NAT.
Both peers use the information in this payload in order to map
subsequent signalling messages to the data flows they refer to.
2. Transparent approach: The GaNAT replaces GIST header fields in a
way that is consistent with the translation it applies to the
data traffic, as necessary. The GaNAT does this GIST QUERY and
RESPONSE messages, for D-mode as well as for C-mode messages
throughout the duration of the signalling session.
The second approach being "transparent" means that a GaNAT that
follows this approach remains completely transparent to the GIST
peers that are located either side of it. Thus, this approach works
even if these GIST peers do not support the NAT traversal object for
GIST (as described in section 7.2 of [1]). Unfortunately though, the
transparent approach does not work if the GaNAT is NSLP-unaware and
if signalling traffic is to be cryptographically protected between
the two GIST peers that are located either side of the GaNAT. If,
however, the GaNAT is NSLP-aware, then cryptographic protection is
terminated at the GaNAT (i.e. the GaNAT is a GIST peer itself). In
this scenario, it is clearly preferable for the GaNAT to follow the
transparent approach, rather than to include a NAT Traversal object.
Thus, if a GaNAT acts as a GIST peer for a signalling session, it
MUST follow the transparent approach, as described in Section 5.3.
However, due to the fact that the transparent approach does not work
if signalling is to be cryptographically protected, a GaNAT MUST also
implement the non-transparent approach (for the case where an NSLP is
signalled that the GaNAT does not support), unless the GaNAT is going
to be used only in deployments where cryptographic protection of
signalling traffic is not a requirement.
Note that a GaNAT MAY implement both approaches. If such a GaNAT is
NSLP-unaware, it can then adopt the correct behaviour, based on
whether or not cryptographic protection is required for the
signalling traffic between two GIST peers. If such protection is
required, the GaNAT MUST adopt the mechanisms that follow the non-
transparent approach; if it is not, it MAY follow the mechanisms
implementing the transparent approach. The GaNAT can tell whether or
not cryptographic protection is required from the stack proposals
that are exchanged in the GIST QUERY and RESPONSE messages; inclusion
of IPsec or TLS proposals amounts to cryptographic protection being
required.
Regardless of which approach is taken, after a GIST response passes
through an NSLP-unaware GaNAT, the GaNAT should expect a messaging
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association to be set up between the two involved GIST peers. For
this to happen, the GaNAT has to allow traffic to traverse it. From
a security perspective, the GaNAT should allow the minimum necessary
traffic types to traverse. Additionally, the amount of per
signalling session state that is allocated at a GaNAT should be
minimised for reasons of efficiency and resistance to service denial
attacks. For these reasons, it makes sense for the GaNAT to be able
to predict with certainty which of the GIST responder's stack
proposal will be used by the initiator for the establishement of a
messaging association. This can be accomplished by having the GaNAT
modify the stack configuration object in the GIST RESPONSE as it
passes though it such that the initiator is forced to use a
particular stack proposal and, if applicable, a particular transport
layer destination port.
The reason why it is preferable for the GaNAT to modify only the
stack configuration data object (as opposed to the stack proposal
object) is that the GIST responder expects its original stack
proposal to be included in the GIST CONFIRM message. The initiator
would not be able to echo that object as it would not have seen it
and, if signalling messages are cryptographically protected, then the
GaNAT cannot replace the stack proposal in the GIST CONFIRM message
with the one the responder expects, without causing cryptographic
checks to fail at the responder. Thus, the approach taken in this
document is for the GaNAT to render invalid all but one stack
configuration data objects in the GIST RESPONSE message. How this is
done depends on the format of this object. If, for example, it
contains transport layer port numbers, it is rendered invalid by
setting these numbers to zero.
A question arises due to the fact that the stack proposal carried by
a GIST QUERY may include multiple proposals of which some provide
cryptographic protection for signalling traffic and some do not.
Which behaviour should a GaNAT that supports both approaches assume
in this case? In order to provide maximise the probability that
cryptographic protection is going to used, the GaNAT MUST follow the
non-transparent approach in this case.
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4. Assumptions
The discussion in this document is based on the following
assumptions.
1. No IP addresses and port numbers are carried in the payloads of
an NSLP.
2. The path taken by the signalling traffic between those GIST peers
that have GaNATs in between is such that the responses to packets
that a GaNAT sends on given interface arrive on the same
interface (if such responses are sent at all).
3. The path taken by signalling traffic remains fixed between the
two GIST peers, as far as the in-between GaNATs are concerned.
That is, we assume that signalling traffic traverses the same
GaNAT(s) until at least one of the following conditions is met.
* The NSIS state that is installed at the two GIST peers
expires.
* The NSIS state that is installed at the two GIST peers is
refreshed using a GIST QUERY.
* A new GIST QUERY/RESPONSE exchange takes place due to other
reasons, e.g. a detected route change.
Note that this assumption is not necessarily met by "normal" data
path coupled signalling. This is because, under "normal" data
path coupled signalling, the signalling traffic is "coupled" to
the data traffic at nodes that decide to act as GIST peers.
Thus, under "normal" path coupled signalling, it is not an error
condition (e.g. a reason to trigeer a "route change"), for
example, if the set of on-path nodes, which do not act as GIST
peers, changes, as long as adjacent GIST peers remain the same.
4. The data flow traverses the same set of GaNATs as the signalling
traffic. By assumption 3, this set of GaNATs is fixed until the
next GIST QUERY/RESPONSE procedure is executed.
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+-----+
+----+GaNAT|-----+
| | A | |
| +-----+ |
+------+ +------+ +--+---+ +------+
+--+ | GIST | | IP | | IP | | GIST | +--+
|DS+-+peer 1+--+router| |router+--+peer 2+-+DR|
+--+ +------+ +---+--+ +--+---+ +------+ +--+
| +-----+ |
| |GaNAT| |
+----+ B +-----+
+-----+
Figure 1: Network with more than one NAT at an addressing boundary
Figure 1 illustrates the importance of assumptions (3) and (4). With
regard to that figure, suppose that a (D-mode) signalling session has
been setup between the two adjacent GIST peers 1 and 2 and that both
signalling and data traffic follows the path GIST peer 1 -> IP router
-> GaNAT A -> IP router -> GIST peer 2. Suppose now that, after some
time, GIST peer 1 decides to set up a C-mode connection with peer 2.
Suppose moreover that the left IP router decides to forward the
C-mode signalling traffic on the link towards GaNAT B. Thus,
signalling traffic now follows the alternative path GIST peer 1 -> IP
router -> GaNAT B -> IP router -> GIST peer 2. Note that this change
in forwarding between the two adjacent GIST peers does not trigger a
"route change" at the GIST layer because (a) it does not necessarily
destroy the adjacency of peer 1 and 2 and (b) it does not necassarily
destroy the coupling of the path taken by signalling traffic to that
taken by data traffic (at GIST nodes). Nevertheless, assumptions (3)
and (4) mandate that this situation does not occur. However, even if
such a situation occurs, the mechanisms described in this document
may still work as state expires after a certain timeout period.
If assumption (1) does not hold, the NSLP has to provide additional
mechanisms for the traversal of (Ga)NATs. These mechanisms must be
compatible with the mechanisms employed by GIST. Assumptions (2),
(3) and (4) hold if, at an addressing boundary, only one NAT exists.
Due to security and management reasons, this is likely to be the case
in many settings.
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5. Transparent NAT traversal for GIST
This section describes the operation of GaNATs that implement the
transparent approach listed in Section 3. Note that the GaNAT may
follow this approach only if it is unaware of the NSLP that is being
signalled, and when no cryptographic protection of signalling data is
requested by the two NSLP peers.
Note that we have to deal with two types of GaNATs, namely those that
are located at the NSIS initiator (NI-side), and those that are
located at the NSIS responder (NR-side). This distinction arises due
to the fact that NI-side and NR-side GaNATs obtain the destination IP
address for forwarded packets in different ways.
5.1. NI-side NSLP-unaware GaNATs
This section describes the "transparent" operation of an NI-side,
NSLP-unaware GaNAT.
For every arriving IP packet P, an NSLP-unaware, NI-side GaNAT
executes the following algorithm.
1. If P has a RAO followed by the GIST header with an NSLP ID that
is not supported, it is identified as a GIST QUERY. In this case
the GaNAT performs the following.
1. We denote P as GQ. It looks at the stack proposal ST in GQ.
If it indicates that cryptographic protection is required,
the mechanism that is applies is the one described in ???.
2. The GaNAT remembers GQ along with the interface on which it
arrived. We call this interface the "upstream link".
3. It searches its table of existing NAT bindings against
entries that match the GQ.MRI. A matching entry means that
the data flow, to which the signalling refers, already
exists.
1. If a matching entry is found, the GaNAT looks at which
link the packets of the data flow are forwarded; we call
this link the "downstream" link. Further, the GaNAT
checks how the headers of the data flow (IP addresses,
port numbers, etc) are translated according to this NAT
binding. We denote [IP header].SourceIPAddress used on
the downstream link as IPGaNATds, and the source port
number used to forward the data traffic as SPNDTGaNATds.
The NAT may also use a different source port number when
forwarding signalling traffic. This port number is
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denoted as SPNSTGaNATds.
2. If no matching entry is found, the GaNAT determines,
based on its routing table, the link on which packets
that match GQ.MRI (excluding GQ.MRI.SourceIPAddress)
would be forwarded. We call this link the "downstream
link". Then, the GaNAT acquires an IP address for itself
on the downstream link. (This address could be dynamic
or static.) This address will be used to forward both
signalling and data traffic on the downstream link. If
it also performs port translation, the GaNAT also
acquires a source port number for the data traffic on the
downstream link. This will be used with the NAT binding,
if such a binding will be established for the data
traffic at a later stage, and is denoted as SPNDTGaNATds.
The signalling traffic packets may also be forwarded
using the a different source port number as the incoming
packets. We denote the acquired IP address as IPGaNATds
and the source port number for the signalling traffic as
SPNSTGaNATds.
Issues: The reason why the GaNAT may also assign a
different source port number to the signalling traffic,
is to enable the GaNAT to demultiplex (i.e. forward to
the correct internal address) the signalling responses
that arrive from downstream. Of course, a GaNAT does not
need to actually change the source port of signalling
traffic; it can always use SPNSTGaNATds the same port as
in the incoming packet. Such a GaNAT may use the GIST
session ID in order to demultiplex the traffic that
arrives from the downstream direction. It is unclear
which of the two approaches is preferable.
4. It creates a new GIST QUERY packet GQ', as follows.
1. GQ' <- GQ
2. GQ'.MRI.SourceIPAddress <- IPGaNATds
3. GQ'.MRI.SourcePortNumber <- SPNDTGaNATds
4. GQ'.[IP header].SourceIPAddress <- IPGaNATds
5. GQ'.[UDP HEADER].SourcePort <- SPNSTGaNATds
6. GQ'.NLI.IA <- IPGaNATds
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7. GQ'.S <- true
5. It remembers GQ and GQ', the fact that they are associated,
and the associated upstream and downstream links. (Note: The
GaNAT does not have to remember the entire packets; for
simplicity of exposition, however, we assume it does. An
implementation SHOULD discard at this point all information
that is not used later.)
6. It forwards GQ' on the downstream link.
2. Otherwise, if P carries an [IP header].DestinationIPAddress that
belongs to the GaNAT, and if it is identified as a GIST response
in D-mode with an NSLP ID that is not supported, the GaNAT does
the following (P is denoted as GR).
1. It searches for a matching GQ' in its buffer. A GR is said
to match a GQ' if they carry the same cookie value. If none
is found, GR is discarded. Otherwise, the GaNAT may also
perform further consistency checks on a matching GR/GQ' pair,
such as checking that they contain the same session IDs,
MRIs, NSLP IDs. If consistency checks succeed, the GaNAT
constructs a new GIST response GR', as follows.
1. GR' <- GR
2. GR'.MRI <- GQ.MRI, where GQ is the packet associated with
GQ'(as remembered previously), and GQ' is the packet that
matches the received GR.
3. GR'.[IP header].SourceIPAddress <- IPGaNATus, where
IPGaNATus = GQ.[IP header].DestinationIPAddress.
4. GR'.[IP header].DestinationIPAddress <- GQ.NLI.IA
5. GP'.S <- true.
6. It inspects the stack proposals in GR' and the
corresponding GQ' to see if the upstream GIST peer has a
choice of more than one possible stack. If such a choice
exists, the GaNAT removes as many stack proposals from
GR' as necessary, until only one stack can be chosen by
the upstream peer for the messaging association. We
denote this stack as ST. The GaNAT remembers this ST and
its association with GQ, GQ', GR, GR'. We say that, in
this case, the GaNAT "installs" the ST.
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2. It forwards GR' on the upstream link.
3. If no NAT binding for the data traffic was found in step
1.3.2, the GaNAT now installs a NAT binding (for the
unidirectional data traffic) which says that "a packet K that
arrives on the upstream link and for which it holds that
+ K.[IP
header].DestinationIPAddress=GQ.MRI.DestinationIPAddress,
+ K.[IP header].Protocol=GQ.MRI.Protocol, and
+ K.[TCP/UDP header].SourcePort=GQ.MRI.SourcePort
should be forwarded on the downstream link, with [IP
header].SourceIPAddress = IPGaNATds.
Issues: there is a question of whether this NAT binding
should also enable data traffic in the opposite direction to
traverse the NAT; in order to be able to demultiplex upstream
traffic that carries data that belongs to different flows,
the GaNAT should keep the necessary per-flow state. From a
signalling point of view, however, upstream data traffic that
corresponds (on the application level) to the downstream flow
to which this GIST session refers, is a separate flow for
which, depending on the application, there may or there may
not exist a signalling session. If such a signalling session
exists, then the GaNAT acts as an NR-side GaNAT for this
session. Thus, during the processing of this signalling care
has to be taken not to establish a NAT binding for a flow for
which a NAT binding already exists. Finally, security issues
arise when traffic, for which no signalling exists, is
allowed to traverse a GaNAT.
Another issue is about refreshing the NAT binding. A NAT
binding that was established as a result of GIST signalling
should remain in place for as long as the associated GIST
state in the GaNAT remains valid. If GIST signalling refers
to a NAT binding that already exists, then the timeout of the
NAT binding should occur according to the NAT policy, in a
manner independent from GIST processing. (If signalling
persists after the deletion of a NAT binding, then the NAT
binding may be re-installed and then timed out together with
GIST state).
3. Otherwise, if P.[IP header].DestinationIPAddress belongs to the
GaNAT, and if P carries the transport protocol and port numbers
indicated by some stack proposal ST that has previously been
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installed by the GaNAT, and if P has arrived on either the
upstream or the downstream interface that is associated with ST,
then P is said to "match" ST. For such a packet, the GaNAT does
the following. If P is expected to contain a GIST header, then
the GaNAT check whether or not the bits where the GIST header is
expected, consitute a valid GIST header. If not, P is silently
discarded. Otherwise, the GaNAT constructs an outgoing packet P'
as follows (the variables used below refer to those stored
together with ST).
1. P' <- P
2. If P has arrived on the upstream link, then
1. P'.[IP header].SourceIPAddress <- IPGaNATds
2. P'.MRI <- GQ'.MRI
3. P'.NLI.IA <- IPGaNATus
4. The GaNAT forwards P' on the downstream link.
3. else (if P has arrived on the downstream link)
1. P'.[IP header].SourceIPAddress <- IPGaNATus
2. P'.MRI <- GQ.MRI
3. P'.NLI.IA <- IPGaNATus
4. The GaNAT forwards P' on the upstream link.
Note that the GaNAT can determine the location in a packet
where a GIST header is expected. If, for example, the packet
is a UDP packet, then the GIST header should follow
immediately after the UDP header. If the packet is a TCP
packet, then the GaNAT can determine the location where the
GIST heaer should start by counting the number of NSLP
payload bits that followed the end of the previous GIST
header. The start of the next GIST header is expected at the
position where the previous GIST message, including NSLP
payload, ends. The GaNAT can tell where this message ends
from the LENGTH field inside the previous GIST header. It
should be noted here that, in order to correctly count the
bits, the GaNAT may have to keep track of TCP sequence
numbers, and thereby be aware of the correct ordering of
packets. However, the GaNAT only has to keep buffers that
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are as long as the LENGTH field inside the previous GIST
header (and possibly up to one MTU size more than that).
Also note that some TCP packets P may not be expected to
contain any GIST header (this happens when the NSLP payload
from a previous packet stretches over several packets). For
those packets, the GaNAT only applies the transformation in
the IP header. Finally, note that a GIST header may start a
packet but finish in another. If such a packet is received
by the GaNAT, it MUST buffer the entire packet, until the
packet is received where the GIST header completes. It can
then apply the required processing and forward both packets.
4. Otherwise, if P matches a (data) NAT binding, the GaNAT applies
normal NAT processing and forwards the packet on the
corresponding link.
5. Otherwise, P is silently discarded.
Brief discussion of the algorithm: The fact that the GaNAT replaces
the NSLP peer's NLI.IA with its own IP address (in both directions),
causes the two GIST peers to send subsequent signalling messages to
the GaNAT, in the belief that they talk to the their adjacent peer.
The GaNAT transparently forwards the signalling traffic and
appropriately translates the fields in the GIST header, in a way that
is consistent with the translation it applies to the data traffic.
Note that, according to this mechanism, the size of an outgoing GIST
message is always the same as the size of its corresponding incoming
GIST message. Finally note that the MRI that the NR sees indicates
as destination address the IP address of the DR (as expected), but as
source address it indicates the is IPGaNATds of the GaNAT that is
closest to the NR.
5.2. NR-side NSLP-unaware GaNATs
The case of NR-side GaNATs is more subtle, since, in this setting,
the DS does not learn the IP address of the DR (which is assumed to
be on the same side of the GaNATs as the NR) and the NI does not
learn the address of the NR. In this setting we assume that each NR-
side GaNAT that is in between two GIST peers, a priori knows a
routable IP address of the downstream GaNAT. The last GaNAT of this
chain is assumed to know the IP address of the DR. In order to
clarify this assumption, see, for example, Figure 2. In this figure,
GaNAT A is assumed to know the IP address of GaNAT B, GaNAT B is
assumed to know the IP address of GaNAT C, and GaNAT C is assumed to
know the IP address of the DR. A given GaNAT that knows such an
address, in effect anticipates to receive a signalling message from
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the upstream direction that refers to a data flow that terminates in
a downstream node. In other words, such a GaNAT may typically have
already a NAT binding in place for the data traffic. We call the IP
address of the next downstream GaNAT (or, if the GaNAT is the last in
the chain, the address of the DR) the "pending" IP address. In the
following description it is denoted by IPNext. How IPNext is made
known to each GaNAT (e.g. how the NAT binding for the data traffic is
installed in the GaNAT) is outside the scope of this document.
+--+ +------+ +-----+ +-----+ +-----+ +------+ +--+ +--+
+NI+--+ GIST +---+GaNAT+---+GaNAT+---+GaNAT+---+ GIST +--+NR+--+DR|
+--+ |peer 1| | A | | B | | C | |peer 2| +--+ +--+
+------+ +-----+ +-----+ +-----+ +------+
Figure 2: Network with NR-side GaNATs (the public Internet is assumed
to be between NI and GIST peer 1)
For every arriving IP packet P, an NSLP-unaware, NR-side GaNAT
executes the following algorithm.
1. If P has a RAO followed by the GIST header with the NSLP ID
indicates an unsupported NSLP, it is identified as a GIST QUERY.
In this case the GaNAT does the following.
1. We denote P as GQ. The GaNAT looks at the stack proposal ST
in GQ. If it indicates that cryptographic protection is
required, the algorithm that is executed is the one described
in section Section 6 below.
2. The GaNAT remembers GQ along with the link on which it
arrived. We call this link the "upstream" link.
3. The GaNAT determines whether or not this GIST QUERY is
anticipated, i.e. if a pending IPNext exists that matches
this GIST QUERY. A pending IPNext is said to "match" a GIST
QUERY, if [this condition is an open issue!] If no pending
IPNext is also matching, P is discarded (it is a question
whether or not an error message should be sent). Otherwise,
additional checks may be performed (e.g. a DSInfo object may
have to be checked against the GQ). If these checks fail, P
is discarded. Otherwise, the GaNAT performs the following.
4. It searches its table of existing NAT bindings against
entries that match the GQ.MRI. A matching entry means that
the data flow, to which the signalling refers, already
exists.
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+ If a matching entry is found, the GaNAT looks at which
link the packets of the data flow are forwarded; we call
this link the "downstream" link. Further, the GaNAT
checks how the headers of the data flow (IP addresses,
port numbers, etc) are translated according to this NAT
binding. We denote [IP header].SourceIPAddress used on
the downstream link as IPGaNATds, and the source port
number as SPNDTGaNATds. Note that the [IP
header].DestinationIPAddress of this NAT binding should be
equal to IPNext. If it is not, this should be handled as
an auditive error condition. The GaNAT may also assign a
new source port number to signalling traffic, which is
denoted as SPNSTGaNATds.
+ If no matching entry is found, the GaNAT determines, based
on its routing table, the link on which packets that match
GQ.MRI (excluding GQ.MRI.SourceIPAddress and where
GQ.MRI.DestinationIPAddress is replaced with IPNext) would
be forwarded. We call this link the "downstream" link.
Then, the GaNAT acquires an IP address for itself on the
downstream link. (This address could be dynamic or
static.) Depending on its type, the GaNAT may also
acquire source port numbers for the translation of data
traffic. We denote the acquired IP address as IPGaNATds
and the source port numbers for data and signalling
traffic as SPNDTGaNATds and SPNSTGaNATds respectively.
5. It creates a new GIST QUERY packet GQ', as follows.
1. GQ' <- GQ
2. GQ'.MRI.SourceIPAddress <- IPGaNATds
3. GQ'.MRI.DestinationIPAddress <- IPNext.
4. GQ'.MRI.SourcePort <- SPNDTGaNATds.
5. GQ'.[IP header].SourceIPAddress <- IPGaNATds
6. GQ'.[UDP HEADER].SourcePort <- SPNSTGaNATds
7. GQ'.[IP header].Destination_IP_Address <- IPNext
8. GQ'.NLI.IA <- IPGaNATds.
9. GQ'.S <- true
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6. It remembers GQ, GQ' the fact that they are associated, and
the associated upstream and downstream links (interfaces).
7. It forwards GQ' on the downstream link.
Steps 2,3, 4 and 5 of the algorithm are analogous to the
corresponding steps of the algorithm executed by NSLP-unaware, NI-
side GaNATs, which was described in Section 5.1.
5.3. NSLP-aware GaNATs
The difference of NSLP-aware GaNATs and NSLP-unaware GaNATs is that
the former perform NSLP processing in addition to the processing of
the NSLP-unaware GaNATs. Another way to see this is by observing
that NSLP-aware GaNATs should provide an "MRI translation service"
(MRITS) in addition to normal GIST and NSLP processing. The
motivation behind the MRITS is for GIST to hide from the NSLP that
signalling messages traverse an addressing boundary. In other words,
the purpose of the MRITS is to make the NSLP believe that it is
operating in a single IP addressing space. When and how the MRITS is
invoked for a particular packet depends on (i) the direction of the
packet (i.e. downstream or upstream) and (ii) the location of the
GaNAT (i.e. NI-side or NR-side). It should also be noted that
certain NSLP layer tasks must be carried out in consistency with the
placement of the MRITS. This is to prevent events triggered by the
NSLP to cause installation of inconsistent state. In order to
clarify this, consider the scenario of the QoS NSLP running in a
GaNAT that operates according to the mechanisms described in this
section. Since the GaNAT only presents a single addressing space to
the NSLP (say, the internal addressing space), the packet classifier
of the GaNAT's QoS provisioning subsystem should classify packets
based on internal addresses only (i.e. it should first translate
packets that carry external addresses and then classify them).
Whether the MRITS presents internal-only or external-only addresses
to the NSLP is not significant, as long as NSLP layer operations are
carried out consistently. In the remainder of this section we
present the case where internal addresses are presented to the NSLP.
The MRITS is obviously invoked only on GIST packets that carry an
NSLP identifier that corresponds to an NSLP that the GaNAT actually
supports. Also, for non-GIST packets, normal NAT behaviour applies.
Although the MRITS is part of GIST processing, in order to clarify
our discussion, we view it as a somewhat separate processing step
(i.e. like a subroutine). For NI-side, NSLP-aware GaNATs, it holds
that
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o for a GIST/NSLP packet that is to be forwarded on the downstream
link of an NI-side GaNAT, the MRITS is invoked after the packet
has been processed by the NSLP and before it is given to GIST, and
o for a GIST/NSLP packet that is received on the downstream link,
the MRITS is invoked after GIST processing and before the packet
is given to the NSLP.
The converse holds for NR-side NSLP-aware GaNATs. In particular,
o for a GIST/NSLP packet that is to be forwarded on the upstream
link of an NI-side GaNAT, the MRITS is invoked after the packet
has been processed by the NSLP and before it is given to GIST, and
o for a GIST/NSLP packet that is received on the upstream link, the
MRITS is invoked after GIST processing and before NSLP processing.
Figure 3 illustrates this idea.
+----------------+ +----------------+
| +------+ | | +------+ |
| | NSLP | | | | NSLP | |
| +-+---++ | | +-+--+-+ |
| | | | | | | |
| | +-+---+ | | +----++ | |
| | |MRITS| | | |MRITS| | |
| | +---+-+ | | ++----+ | |
| | | | | | | |
| +-+-----+-+ | | ++------+-+ |
| | GIST | | | | GIST | |
u/s | +-+-----+-+ | d/s u/s | ++------+-+ | d/s
-----+----+ +-----+----- -----+---+ +-----+-----
link +----------------+ link link +----------------+ link
NI-side NR-side
NSLP-aware NSLP-aware
GaNAT GaNAT
Figure 3: Operation of the MRI Translation Service
The reason for this construction is to give the NSLP the impression
that it works only with flows that originate and terminate in the
internal address space. We now describe the operation of the MRITS
and GIST in NSLP-aware GaNATs. An NI-side NSLP-aware GaNAT operates
according to the following rules.
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1. When the NSLP asks for a message to be sent towards the
downstream GIST peer, the MRITS does the following (IPGaNATds and
SPNDTGaNATds are obtained similarly to the case of an NSLP-
unaware GaNAT).
1. MRI.SourceIPAddress <- IPGaNATds
2. MRI.SourcePort <- SPNDTGaNATds
2. Additionally, GIST performs the following on the resulting packet
before it is forwarded on the downstream link (SPNSTGaNATds is
obtained similarly to the case of an NSLP-unaware GaNAT).
1. [IP header].SourceIPAddress <- IPGaNATds
2. [UDP/TCP header].SourcePort <- SPNSTGaNATds
3. NLI.IA <- IPGaNATds
4. S <- true
3. If a message is received on the downstream link, the MRITS does
the following before the NSLP is invoked.
1. MRI.SourceIPAddress <- IPflow
2. MRI.SourcePort <- SPNDTGaNATus, where IPflow is the IP
address of the DS (as seen by the GaNAT) and SPNDTGaNATus is
the destination port number used in the original MRI.
4. If, after NSLP processing, a message is to be forwarded on the
upstream link, GIST performs the following processing (note that
no MRITS processing takes place in this case).
1. [IP header].SourceIPAddress <- IPGaNATus
2. [IP header].DestinationIPAddress <- IPpeer
3. NLI.IA <- IPGaNATus
4. S <- true, where IPGaNATus is the GaNATs IP address for the
upstream link, IPpeer is the IPaddress of the NI (or the next
GaNAT in the upstream direction), and IPflow is the IP
address of the DS (as seen by the GaNAT). The GaNAT is
assumed to determine the correct IPGaNATus and IPpeer from
previous communications and in cooperation with GIST.
[Issue: how exactly should IPGaNATus, IPpeer and IPflow be
resolved; i.e. what exactly should the GaNAT remember?]
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An NR-side NSLP-aware GaNAT operates according to the following
rules.
1. If the packet is received on the upstream link, the MRITS does
the following, before the NSLP is notified.
1. P.MRI.SourceIPAddress <- IPGaNATds
2. P.MRI.DestinationIPAddress <- IPNext, where IPGaNATds is the
GaNAT's IP address for the downstream link and IPNext is the
address of the DR. IPNext is obtained in a way similar to
the case of an NSLP-unaware GaNAT.
2. If, after NSLP processing, a message is to be forwarded on the
downstream link, GIST performs the following processing (note
that no MRITS processing takes place in this case).
1. [IP header].SourceIPAddress <- IPGaNATds
2. [IP header].DestinationIPAddress <- IPNext
3. NLI.IA <- IPGaNATds
4. S <- true, where IPGaNATds is the GaNATs IP address for the
downstream link, IPNext is the IP address of the DR (or the
next GaNAT in the downstream direction). The GaNAT is
assumed to determine the correct IPNext in a way similar to
the case of an NSLP-unaware GaNAT.
3. When the NSLP asks for a message to be sent towards the upstream
GIST peer, the MRITS does the following.
1. MRI.SourceIPAddress <- IPflow
2. MRI.Destination_IP_Address <- IPGaNATus
4. Additionally, GIST performs the following on the resulting packet
before it is forwarded on the downstream link.
1. [IP header].SourceIPAddress <- IPGaNATus
2. [IP header].DestinationIPAddress <- IPpeer
3. NLI.IA <- IPGaNATus
4. S <- true, where IPGaNATus is the GaNATs IP address for the
upstream link, IPpeer is the IP address of the NI (or the
next GaNAT in the upstream direction), and IPflow is the IP
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address of the DS. The GaNAT is assumed to determine the
correct IPGaNATus and IPpeer fields from previous
communications and in cooperation with GIST. [question: how
exactly should IPGaNATus and IPpeer be resolved; i.e. what
exactly should the GaNAT remember]?
5.4. Combination of NSLP-aware and NSLP-unaware GaNATs
In the absence of an adversary, a combination of NSLP-aware and NSLP-
unaware GaNATs should work without further specification. However,
in the presence of an adversary, additional security issues may arise
from the combination. These issues may introduce opportunities for
attack that do not exist in setting where the on-path GaNATs are
either all NSLP-aware or all NSLP-unaware.
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6. Non-transparent NAT traversal
This section discusses the "non-transparent" operation for GaNAT
traversal at the GIST layer, i.e. the first approach listed in
Section 3. For this approach, the behaviour of both the GaNAT and
the GIST peers is defined. As with the transparent approach, the
case of the in-between GaNAT(s) being located at the NI-side is
different from that of NR-side GaNATs. Note that the mechanisms in
this section apply only to NSLP-unware GaNATs.
The GaNAT informs the NSLP peers about its presence during the GIST
discovery process. This information enables the NSLP peers to map
the translated data flow to the signalling messages, and to
consistently translate the MRI, so that the NSLP only "sees" the
correct MRI. Cryptographic protection of signalling messages can be
supported with this approach because the GaNAT only modifies the GIST
QUERY and REPONSE messages, which are never cryptographically
protected in their entirety.
In this approach, the GaNAT embeds a new GIST payload type into the
GIST QUERY. This payload encodes the aforementioned information, and
we call this payload type the "NAT Traversal Object" (NTO). The NTO
is an optional payload in the GIST header of a GIST QUERY, and is
added, and processed, by the GaNAT(s) through which the QUERY
traverses. The information in the NTO must enable the two NSLP peers
to locally translate the MRI in the same way as if it were
consistently and transparently translated by the in-between GaNAT(s).
Note that there may be more than one GaNAT between the two NSLP
peers. The format of the NTO follows the format of the object in the
GIST common header. In particular, the NTO is preceded by a TLV
common header, as defined in [1]. The A and B flags are both set to
0 in this header, indicating that support for the NTO is mandatory.
The type value is TBD. The NTO is defined as in section A.3.8 of
[1].
6.1. NI-side NSLP-unaware GaNATs
For every arriving IP packet P, an NSLP-unaware, NI-side GaNAT
executes an algorithm that is equivalent to the following.
1. If P has a RAO followed by the GIST header with an NSLP ID that
is not supported, it is identified as a GIST QUERY. In this case
the GaNAT does the following.
1. We denote P as GQ. The GaNAT looks at the stack proposal ST
in GQ. If it does not include a proposal with cryptographic
protection, the GaNAT MAY choose to follow the approach
described in Section 5.1 above.
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2. We call the link on which GQ arrived the "upstream" link.
3. The GaNAT searches its table of existing NAT bindings against
entries that match the GQ.MRI. A matching entry means that
the data flow, to which the signalling refers, already
exists.
+ If a matching entry is found, the GaNAT looks at which
link the packets of the data flow are forwarded; we call
this link the "downstream" link. Further, the GaNAT
checks how the headers of the data flow (IP addresses and
port numbers) are translated according to this NAT
binding.
+ If no matching entry is found, the GaNAT determines, based
on its routing table, the link on which packets that match
GQ.MRI (excluding GQ.MRI.SourceIPAddress) would be
forwarded. We call this link the "downstream" link.
Then, the GaNAT acquires an IP address and source port for
itself on the downstream link. (This address and port
could be dynamic or static.)
4. We denote [IP header].SourceIPAddress used on the downstream
link as IPGaNATds, and the source port number for the data
and signalling traffic as SPNDTGaNATds and SPNSTGaNATds
respectively.
5. It creates a new GIST QUERY packet GQ', as follows.
1. GQ' <- GQ
2. GQ'.MRI.SourceAddress <- IPGaNATds.
3. GQ'.MRI.SourcePort <- SPNDTGaNATds.
4. GQ'.NLI.IA.<- IPGaNATds.
5. GQ'.[IP header].SourceIPAddress <- IPGaNATds.
6. GQ'.[UDP header].SourcePort <- SPNSTGaNATds.
7. GQ'.S <- true.
8. It checks whether or not an NTO was included in GQ.
- If none was included, it creates a new NTO as follows
and adds it to GQ'.
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o NTO.[NAT Count] <- 1.
o NTO.MRI <- GQ.MRI.
o NTO.[List of translated objects] <- [type of MRI],
[type of NLI], [type of SCD]
o NTO.opaque information for NAT 1 <- GQ.[IP
header].SourceAddress, [link identifier of upstream
link].
- If one was included, it replaces certain fields and
appends new fields into the NTO, as follows, and adds
the resulting object to GQ'.
o NTO.[NAT Count] <- i, where i is the current [NAT
count] value increased by one.
o NTO.[List of translated objects] <- [type of MRI],
[type of NLI], [type of STD]
o NTO.[opaque information replaced by NAT i] <-
GQ.[IP header].SourceAddress, [LinkID of upstream
link].
9. It forwards GQ' on the downstream link. Note: The
encoding of the information that the GaNAT encodes into
the NTO.[opaque information replaced by NAT i] field is a
local implementation issue.
2. Otherwise, if P carries an [IP header].DestinationIPAddress that
belongs to the GaNAT, and if it is identified as a GIST RESPONSE
packet in datagram mode with an NSLP ID that is not supported,
the GaNAT does the following (P is denoted as GR).
1. If P does not contain an NTO, the GaNAT forwards it as usual
without further processing. Otherwise, the GaNAT selects the
information encoded by it in the [opaque information replaced
by NAT] field of the embedded NTO, denoted by IPAddressToSend
and LinkID. If multiple [opaque information replaced by NAT]
fields are present in the NTO, the GaNAT uses the last one in
the list. It then constructs a new GIST response GR', as
follows (note that no changes are made to the MRI).
1. GR' <- GR
2. GR'.[IP header].DestinationIPAddress <- IPAddressToSend.
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3. GR'.NTO.[NAT Count] <- current value minus one.
4. Remove the last [opaque information replaces by NAT i]
field from GR'.NTO
5. GR'.S <- true.
2. It forwards GR' on the upstream link, i.e. the link
identified by LinkID.
3. The GaNAT SHOULD now invalidate all but one stack
configuration objects in the stack proposal in GR'. This is
done so that the quering node can only chose that one
proposal, and that therefore only one NAT binding must be
installed for the signalling traffic to traverse the GaNAT.
The GaNAT SHOULD keep valid the strongest, in terms of
security, stack proposal. We denote this proposal as PR.
4. The GaNAT now installs a NAT binding for the signalling
traffic which says that "a packet K that arrives on the
upstream link and for which it holds that
+ K.[IP header].SourceIPAddress=IPAddressToSend,
+ K.[IP header].Protocol=PR.Protocol, and
+ K.[TCP/UDP header].DestinationPort=PR.[Destination Port]
should be forwarded on the downstream link (i.e. on the link
on which GR arrived), with [IP header].SourceIPAddress =
IPGaNATds.
5. If no NAT binding for the data traffic was found in step
1.3.2, the GaNAT now installs a NAT binding (for the
unidirectional data traffic) which says that "a packet K that
arrives on the upstream link and for which it holds that
+ K.[IP header].DestinationIPAddress=GQ'.NTO.MRI.Destination
IPAddress,
+ K.[IP header].Protocol=GQ'.NTO.MRI.Protocol, and
+ K.[TCP/UDP header].PortNumbers=GQ'.NTO.MRI.PortNumbers
should be forwarded on the downstream link (i.e. the link on
which GR arrived), with [IP header].SourceIPAddress =
IPGaNATds and [UDP/TCP header].SourcePort=SPDTGaNATds.
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Issues: there is a question of whether this NAT binding
should also enable data traffic in the opposite direction to
traverse the NAT; in order to be able to demultiplex upstream
traffic that carries data that belongs to different flows,
the GaNAT should keep the necessary per-flow state. From a
signalling point of view, however, upstream data traffic that
corresponds (on the application level) to the downstream flow
to which this GIST session refers, is a separate flow for
which, dependent on the application, there may or there may
not exist a signalling session. If such a signalling session
exists, then the GaNAT acts as an NR-side GaNAT for this
session. Thus, during the processing of this signalling,
care has to be taken not to establish a NAT binding for a
flow for which a NAT binding already exists. Finally,
security issues may arise when traffic, for which no
signalling exists, is allowed to traverse a GaNAT.
3. Otherwise, if P matches an existing NAT binding, normal NAT
processing is applied.
4. Otherwise, P is silently discarded.
6.2. NR-side NSLP-unaware GaNATs
As is the case with NR-side NSLP-unaware GaNATs that follow the
"transparent" approach, an NR-side NSLP-unaware GaNAT that follows
the "non-transparent" approach must know a "pending" IP address and,
depending on the scenario, also a destination port number, as
described in Section 5.2. This IP address and destination port
number are denoted as IPNext and PortNext respectively. How they are
made known to the GaNAT is outside the scope of this document. Note,
however, that a typical scenario would be that the GaNAT has an
existing NAT binding for the data flow in place from where this
information can be derived.
For every arriving IP packet P, an NSLP-unaware, NR-side GaNAT
executes the following algorithm.
1. If P has a RAO followed by a GIST header with an unsupported
NSLPID, and is identified as a GIST QUERY, the GaNAT does the
following.
1. We denote P as GQ. The GaNAT looks at the stack proposal ST
in GQ. If it indicates that no cryptographic protection is
required, the GaNAT MAY choose to follow the "transparent"
approach as described in Section 5.2 above.
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2. If GQ.[IP header].[Destination Address] is not bound to the
link on which GQ arrived, the GaNAT silently discards the
packet.
3. The GaNAT determines whether or not this GIST QUERY is
anticipated, i.e. if a pending IPNext and PortNext exists.
One way of determining whether or not a pending IPNext and
PortNext exists is checking whether or not a NAT binding for
the data traffic, as this is defined by the MRI in the GIST
QUERY, exists in the NAT binding cache. If one exists, then
IPNext and PortNext is the address and port number to which
the data traffic is sent after translation. If no pending
IPNext is found, GQ is discarded (it is an open issue whether
or not an error message should be sent). Otherwise,
additional checks may be performed (e.g. a DSInfo object may
have to be checked against the GQ). If these checks fail, GQ
is discarded. Otherwise, the GaNAT performs the following.
4. We call the link on which GQ arrived the "upstream" link.
The GaNAT aquires an IP address for itself for the upstream
link (this could be a static or a dynamic IP address). This
address is denoted IPGaNATus. The GaNAT will use this
address as a source IP address in order to send subsequent
signalling messages to the upstream direction. If the GaNAT
is an edge NAT, IPGaNATus will typically coincide with the
destination IP address of the (original) MRI in the GIST
QUERY.
5. The GaNAT searches its table of existing NAT bindings against
entries that match the GQ.MRI. A matching entry means that
the data flow, to which the signalling refers, already
exists.
+ If a matching entry is found, the GaNAT determines the
link on which the packets of the data flow are forwarded;
we call this link the "downstream" link. Further, the
GaNAT checks how the headers of the data flow (IP
addresses, port numbers) are translated according to this
NAT binding. Note that the [IP
header].DestinationIPAddress of this NAT binding should be
equal to IPNext. If it is not, this should be handled as
an auditive error condition. (This check is done as a
consistency check.)
+ If no matching entry is found, the GaNAT determines, based
on its routing table, the link on which packets that match
GQ.MRI (where GQ.MRI.DestinationIPAddress is replaced with
IPNext) would be forwarded. We call this link the
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"downstream" link.
6. It creates a new GIST QUERY packet GQ', as follows.
1. GQ' <- GQ
2. GQ'.MRI.DestinationAddress <- IPNext.
3. GQ'.MRI.DestinationPort <- PortNext.
4. GQ'.[IP header].DestinationIPAddress <- IPNext.
5. It checks whether or not an NTO was included in GQ.
- If none was included, it creates a new NTO as follows
and adds it to GQ'.
o NTO.[NAT Count] <- 1.
o NTO.MRI <- GQ.MRI.
o NTO.[List of translated objects] <- [type of MRI],
[type of NLI], [type of SCD]
o NTO.opaque information for NAT 1 <- IPGaNATus,
[link identifier of upstream link].
- If one was included, it replaces certain fields and
appends new fields into the NTO, as follows, and adds
the resulting object to GQ'.
o NTO.[NAT Count] <- i, where i is the current [NAT
count] value increased by one.
o NTO.[List of translated objects] <- [type of MRI],
[type of NLI], [type of SCD]
o NTO.[opaque information replaced by NAT i] <-
IPGaNATus, [LinkID of upstream link].
6. It forwards GQ' on the downstream link. Note: The
encoding of the information that the GaNAT encodes into
the NTO.[opaque information replaced by NAT i] field is a
local implementation issue.
2. Otherwise, if P is identified as a GIST RESPONSE packet in
datagram mode with an NSLP ID that is not supported, the GaNAT
does the following (P is denoted as GR).
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1. If P does not contain an NTO, the GaNAT forwards it as usual
without further processing. Otherwise, the GaNAT selects the
information encoded by it in the [opaque information replaced
by NAT] field of the embedded NTO, denoted by IPGaNATus and
LinkID. If multiple [opaque information replaced by NAT]
fields are present in the NTO, the GaNAT uses the last one in
the list. The GaNAT then constructs a new GIST response GR',
as follows (note that no changes are made to the MRI).
1. GR' <- GR
2. GR'.[IP header].SourceIPAddress <- IPGaNATus.
3. GR'.MRI.NLI.IA <- IPGaNATus.
4. Remove the last [opaque information replaced by NAT i]
field from GR'.NTO.
5. GR'.S <- true.
2. The GaNAT SHOULD now invalidate all but one stack
configuration objects in the stack proposal in GR'. This is
done so that the quering node can only chose that one
proposal, and that therefore only one NAT binding must be
installed for the signalling traffic to traverse the GaNAT.
The GaNAT SHOULD keep valid the strongest, in terms of
security, stack proposal. We denote this proposal as PR. If
PR.[Destination Port] is already used by the GaNAT as a port
in order to demultiplex an existing signalling flow, the
GaNAT reserves a port SIGPORT (that it will use as a source
port for UDP/TCP signalling traffic that it will send on the
upstream link) and replaces PR.[Destination Port] with
SIGPORT. Otherwise it sets SIGPORT=PR.[Destination Port].
It then sets GR'.[UDP header].SourcePort <- SIGPORT.
3. It forwards GR' on the upstream link, i.e. the link
identified by LinkID.
4. The GaNAT now installs a NAT binding for the signalling
traffic which says that "a packet K that arrives on the
upstream link and for which it holds that
+ K.[IP header].DestinationIPAddress=IPGaNATus,
+ K.[IP header].Protocol=PR.Protocol, and
+ K.[TCP/UDP header].DestinationPort=SIGPORT
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should be forwarded on the downstream link (i.e. on the link
on which GR arrived), with [IP header].DestinationIPAddress =
GR.MRI.NLI.IA and [UDP/TCP
header].DestinationPort=PR.[Destination Port].
5. If no NAT binding for the data traffic was found in step
1.3.2, the GaNAT may now install a NAT binding (for the
unidirectional data traffic) which says that "a packet L that
arrives on the upstream link and for which it holds that
+ L.[IP header].DestinationIPAddress=GR'.NTO.MRI.Destination
IPAddress,
+ L.[IP header].Protocol=GR'.NTO.MRI.Protocol, and
+ L.[TCP/UDP
header].DestinationPortNumbers=GR'.NTO.MRI.DestinationPort
should be forwarded on the downstream link, with [IP
header].DestinationIPAddress = IPNext and [UDP/TCP
header].DestinationPort=PortNext.
Issues: there is a question of whether this NAT binding
should also enable data traffic in the opposite direction to
traverse the NAT; in order to be able to multiplex upstream
traffic that carries data that belongs to different flows,
the GaNAT should keep the necessary per-flow state. From a
signalling point of view, however, upstream data traffic that
corresponds (on the application level) to the downstream flow
to which this GIST session refers, is a separate flow for
which, dependent on the application, there may or there may
not exist a signalling session. If such a signalling session
exists, then the GaNAT acts as an NI-side GaNAT for this
session. Thus, during the processing of this signalling care
has to be taken not to establish a NAT binding for a flow for
which a NAT binding already exists. Finally, security issues
may arise when traffic, for which no signalling exists, is
allowed to traverse a GaNAT.
3. Otherwise, if P matches an existing NAT binding, normal NAT
processing is applied.
4. Otherwise, P is silently discarded.
6.3. GIST peer processing
In the presence of GaNATs on the signalling path between two NSLP
peers, and if the GaNATs follow the "non-transparent" approach (which
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they have to follow in the context of cryptographically protected
signalling), the consistent translation of the GIST header fields
must be carried out by the NSLP peers. The GIST processing that
performs this task, is described in section 7.2 of [1].
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7. GIST-unaware NATs
The following may serve as indications for the existence of an GIST-
unaware NAT between two GIST peers. These indications can only be
detected by the receiver of a GIST message. The first occasion these
indications may be detected is with the reception of a GIST QUERY,
typically by the downstream peer. (Note that != denotes inequality).
o The MRI.SourceIPAddress does not belong to the addressing space of
the receiving peer.
o The MRI.DestinationIPAddress does not belong to the addressing
space of the receiving peer.
o The IP address in the NLI.IA field does not belong to the
addressing space of the receiving peer.
o The D flag of a received GIST packet denotes downstream direction
and the S flag is not set and [IP header].SourceIPAddress !=
MRI.SourceIPAddress.
o The D flag of a received GIST packet denotes upstream direction
and the S flag is not set and [IP header].SourceIPAddress !=
MRI.DestinationIPAddress.
o This is a GIST QUERY and [IP header].DestinationIPAddress !=
MRI.DestinationIPAddress.
Note that these are only indications. In the presence of an
adversary, a GIST peer may be tricked into believing that an GIST-
unaware NAT exists between itself and one of its neighbouring peers,
while in reality this may not be the case.
When a downstream GIST peer detects such an indication, it may notify
the upstream peer about the error. It may include additional
information that enables the upstream peer to construct a GIST packet
in such a way that, after it traverses the GIST-unaware NAT, the IP
addresses in the MRI field and the NLI.IA field are consistent with
those in the IP header (which match the addressing space of the
receiving peer). However, this requires the specification of new
data structures and formats, processing rules, and requires the peers
to maintain additional state.
Unfortunately, this approach is likely to fail in many circumstances.
In order to see this, consider the behaviour of an GIST-unaware NAT
when it receives an IP packet. The packet either
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1. matches an existing NAT binding in which case its IP header is
translated and the packet it is forwarded on another link, or
2. matches an existing policy rule which causes a new binding to be
established and then (1) happens, or
3. is discarded because neither (1) nor (2) applies.
With GIST-unaware NATs it is a matter of local policy (i.e. the rules
that exist in case (2) above) whether or not traffic will be allowed
to traverse the NAT. This obviously applies to both signalling and
data traffic, as an GIST-unaware NAT is unable to distinguish the two
types of traffic. It may be the case that GIST node A is unable to
contact GIST node B which is "behind" a NAT, even if communication in
from B to A may be possible because such communication would match a
policy rule; typically, in a scenarios where A is towards the NI and
B is towards the NR, the NAT would have this behaviour.
Another approach to deal with GIST-unaware NATs is similar to the NAT
traversal approach taken by IKEv2, i.e. by encapsulating GIST
messages into UDP datagrams, rather than directly into IP datagrams.
This technique requires the inclusion of additional fields into a
GIST QUERY, as follows. The sender adds (a hash of) its own IP
address and the IP address of what it believes to be the DR into the
GIST payload. The receiver of this GIST messages compares these
addresses to the [IP header].SourceIPAddress and the [IP
header].DestinationIPAddress respectively. If at least one of them
is unequal, the receiver deduces that a NAT is between sender and
receiver. After the detection of a NAT, the remainder of the
communication is encapsulated into UDP datagrams that are addressed
to a specified port.
Unfortunately, the IKEv2 NAT traversal mechanism cannot be used "as
is" for NAT traversal in GIST. This is because of a number of
reasons, including the following.
o The NAT may use an IP address for the forwarding of data traffic
that is different from the IP address it uses to forward GIST
traffic. Since the NAT is GIST-unaware it cannot update the MRI
in the GIST messages such that it matches the translation applies
to the data traffic. Moreover, neither the GIST sending, nor the
GIST receiving peer can perform this update; the sending peer
cannot predict the translation that the NAT will apply, and the
receiving peer does not have enough information to associate data
flows to signalling messages.
o It is unclear whether or not the IKEv2 NAT traversal mechanism
supports cascades of NATs.
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o It seems to be inappropriate to use UDP encapsulation for certain
C-mode scenarios. For example, using UDP encapsulation for TCP
C-mode would result in GIST to appear in TCP over UDP over IP.
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8. Security Considerations
The mechanisms proposed in this document give rise to a number of
threats that must be considered. In the following, a subset of these
threats is mentioned.
8.1. Service Denial Attacks
As described above, NSLP-unaware GaNATs create some state whenever
they receive a GIST QUERY message. This state is necessary in order
for the GaNAT to be able to map a GIST RESPONSE that arrives from the
downstream direction to the corresponding GIST QUERY and thereby to
perform the required translation.
The threat here is an attacker flooding the GaNAT with maliciously
constructed GIST QUERIES with the aim of exhausting the GaNAT's
memory. The attacker might use a variety of methods to construct
such GIST QUERIES, including the following.
1. Use as [IP header].SourceIPAddress the address of some other node
or an unallocated IP address. This method is also known as IP
spoofing.
2. Use an invalid NSLPID, in order to make sure that all on-path
GaNAT(s) will behave like NSLP-unaware GaNATs.
3. For each packet, use a different value for the cookie field.
4. For each packet, use a different value for the session ID field.
5. Combinations of the above.
How vulnerable a GaNAT is to the above service denial attack depends
on a variaty of factors, including the following.
o The amount of state allocated at the receipt of a GIST QUERY.
This amount may vary depending on whether or not the data flow to
which the signalling refers, already exists (i.e. whether or not
the GaNAT already maintains a NAT binding for it).
o The mechanism that the GaNAT uses to map RESPONSEs to QUERIEs.
o Whether or not the GaNAT acriques dynamic IP addresses and ports
for the downstream link.
In order to decrease the exposure of a GaNAT to service denial
attacks, the following recommendations are made.
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o The GaNAT should perform ingress filtering. This limits the
amount of locations from which an attacker can perform IP spoofing
without being detected.
o The GaNAT should allocate the minimum amount of state required at
the reception of a GIST QUERY.
o All state allocated by the GaNAT should timeout according to a
local policy. If the GaNAT detects heavy loads (which may
indicate a service denial attack in progress), the GaNAT should
timeout the state allocated as a result of a received GIST QUERY
quicker, proportionally to the experienced load.
o The installation of a NAT binding for the data traffic (if such a
binding does not exist prior to signalling) should be postponed
until the correct GIST REPONSE traverses the NAT.
The service denial threats mentioned in this section do not apply to
an NSLP-aware GaNAT, as such a GaNAT is required, in accordance with
its local policy, to verify the validity of the cookie(s) before
allocating any state, including the state required by the mechanisms
in this document.
8.2. Network Intrusions
Although the primary goal of a NAT is to perform address translation
between two addressing spaces, NATs are sometimes also used to
provide a security service similar to the security service provided
by firewalls. That is, a NAT can be configured so that it does not
forward packets from the external into the internal network, unless
it determines that the packets belong to a communication session that
was originally initiated from an internal node and are, as such,
solicited.
If an NSLP-unaware GaNAT performs the above security-relevant
function in addition to address translation, then the presence of
GIST signalling and, in particular the mechanisms described in this
document, might allow an adversary cause the installation of NAT
bindings in the GaNAT using these mechansisms. These NAT bindings
would then enable the adversary to inject unsolicited traffic into
the internal network, a capability that it may not have in the
absence of the mechanisms described in this document.
The administrator of an NSLP-unaware GaNAT should therefore make
security-concious decisions regarding the operation of the GaNAT. An
NSLP-aware GaNAT, on the other hand, follows an NSLP policy which
indicates the required security mechanisms. This policy should
account for the fact that this NSLP-aware node performs also NAT and
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the associated packet filtering.
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9. IAB Considerations
None.
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10. Acknowledgements
The authors would like to thank Cedric Aoun, Christian Dickmann,
Robert Hancock, and Martin Stiemerling for their insightful comments.
Furthermore, we would like to mention that this document builds on
top of a previous document regarding migration scenarios.
11. Normative References
[1] Schulzrinne, H. and R. Hancock, "GIST: General Internet
Signalling Transport", draft-ietf-nsis-ntlp-06 (work in
progress), May 2005.
[2] Stiemerling, M., Tschofenig, H., and C. Aoun, "NAT/Firewall NSIS
Signaling Layer Protocol (NSLP)", draft-ietf-nsis-nslp-natfw-06
(work in progress), May 2005.
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Authors' Addresses
Andreas Pashalidis
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Andreas.Pashalidis@siemens.com
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Hannes.Tschofenig@siemens.com
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