Network Working Group G. Montenegro
Request for Comments: 3104 Sun Microsystems, Inc.
Category: Experimental M. Borella
October 2001 RSIP Support for End-to-end IPsec
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2001). All Rights Reserved.
The IESG notes that the set of documents describing the RSIP
technology imply significant host and gateway changes for a complete
implementation. In addition, the floating of port numbers can cause
problems for some applications, preventing an RSIP-enabled host from
interoperating transparently with existing applications in some cases
(e.g., IPsec). Finally, there may be significant operational
complexities associated with using RSIP. Some of these and other
complications are outlined in section 6 of the RFC 3102, as well as
in the Appendices of RFC 3104. Accordingly, the costs and benefits
of using RSIP should be carefully weighed against other means of
relieving address shortage.
This document proposes mechanisms that enable Realm Specific IP
(RSIP) to handle end-to-end IPsec (IP Security).
Table of Contents
1. Introduction .................................................. 22. Model ......................................................... 23. Implementation Notes .......................................... 34. IKE Handling and Demultiplexing ............................... 45. IPsec Handling and Demultiplexing ............................. 56. RSIP Protocol Extensions ...................................... 66.1 IKE Support in RSIP ....................................... 66.2 IPsec Support in RSIP ..................................... 77. IANA Considerations ........................................... 108. Security Considerations ....................................... 109. Acknowledgements .............................................. 10
References ....................................................... 11
Authors' Addresses ............................................... 12
Appendix A: On Optional Port Allocation to RSIP Clients .......... 13
Appendix B: RSIP Error Numbers for IKE and IPsec Support ......... 14
Appendix C: Message Type Values for IPsec Support ................ 14
Appendix D: A Note on Flow Policy Enforcement .................... 14
Appendix E: Remote Host Rekeying ................................. 14
Appendix F: Example Application Scenarios ........................ 15
Appendix G: Thoughts on Supporting Incoming Connections .......... 17
Full Copyright Statement ......................................... 191. Introduction
This document specifies RSIP extensions to enable end-to-end IPsec.
It assumes the RSIP framework as presented in [RSIP-FW], and
specifies extensions to the RSIP protocol defined in [RSIP-P]. Other
terminology follows [NAT-TERMS].
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.
For clarity, the discussion below assumes this model:
RSIP client RSIP server Host
Xa Na Nb Yb
+------------+ Nb1 +------------+
[X]------| Addr space |----[N]-----| Addr space |-------[Y]
| A | Nb2 | B |
+------------+ ... +------------+
Hosts X and Y belong to different address spaces A and B,
respectively, and N is an RSIP server. N has two addresses: Na on
address space A, and Nb on address space B. For example, A could be
a private address space, and B the public address space of the
general Internet. Additionally, N may have a pool of addresses in
address space B which it can assign to or lend to X.
This document proposes RSIP extensions and mechanisms to enable an
RSIP client X to initiate IKE and IPsec sessions to a legacy IKE and
IPsec node Y. In order to do so, X exchanges RSIP protocol messages
with the RSIP server N. This document does not yet address IKE/IPsec
session initiation from Y to an RSIP client X. For some thoughts on
this matter see Appendix G.
The discussion below assumes that the RSIP server N is examining a
packet sent by Y, destined for X. This implies that "source" refers
to Y and "destination" refers to Y's peer, namely, X's presence at N.
This document assumes the use of the RSAP-IP flavor of RSIP (except
that port number assignments are optional), on top of which SPI
values are used for demultiplexing. Because of this, more than one
RSIP client may share the same global IP address.
3. Implementation Notes
The RSIP server N is not required to have more than one address on
address space B. RSIP allows X (and any other hosts on address space
A) to reuse Nb. Because of this, Y's SPD SHOULD NOT be configured to
support address-based keying. Address-based keying implies that only
one RSIP client may, at any given point in time, use address Nb when
exchanging IPsec packets with Y. Instead, Y's SPD SHOULD be
configured to support session-oriented keying, or user-oriented
keying [Kent98c]. In addition to user-oriented keying, other types
of identifications within the IKE Identification Payload are equally
effective at disambiguating who is the real client behind the single
address Nb [Piper98].
Because it cannot rely on address-based keying, RSIP support for
IPsec is similar to the application of IPsec for remote access using
dynamically assigned addresses. Both cases impose additional
requirements which are not met by minimally compliant IPsec
Note that a minimally-compliant IKE implementation (which only
implements Main mode with Pre-shared keys for Phase I
authentication) cannot be used on a remote host with a dynamically
assigned address. The IKE responder (gateway) needs to look up
the initiator's (mobile node's) pre-shared key before it can
decrypt the latter's third main mode message (fifth overall in
Phase I). Since the initiator's identity is contained in the
encrypted message, only its IP address is available for lookup and
must be predictable. Other options, such as Main mode with
digital signatures/RSA encryption and Aggressive mode, can
accommodate IKE peers with dynamically assigned addresses.
IKE packets are typically carried on UDP port 500 for both source and
destination, although the use of ephemeral source ports is not
precluded [ISAKMP]. IKE implementations for use with RSIP SHOULD
employ ephemeral ports, and should handle them as follows [IPSEC-MSG]:
IKE implementations MUST support UDP port 500 for both source and
destination, but other port numbers are also allowed. If an
implementation allows other-than-port-500 for IKE, it sets the
value of the port numbers as reported in the ID payload to 0
(meaning "any port"), instead of 500. UDP port numbers (500 or
not) are handled by the common "swap src/dst port and reply"
It is important to note that IPsec implementations MUST be aware of
RSIP, at least in some peripheral sense, in order to receive assigned
SPIs and perhaps other parameters from an RSIP client. Therefore,
bump-in-the-stack (BITS) implementations of IPsec are not expected to
work "out of the box" with RSIP.
4. IKE Handling and Demultiplexing
If an RSIP client requires the use of port 500 as its IKE source,
this prevents that field being used for demultiplexing. Instead, the
"Initiator Cookie" field in the IKE header fields must be used for
this purpose. This field is appropriate as it is guaranteed to be
present in every IKE exchange (Phase 1 and Phase 2), and is
guaranteed to be in the clear (even if subsequent IKE payloads are
encrypted). However, it is protected by the Hash payload in IKE
[IKE]. Because of this, an RSIP client and server must agree upon a
valid value for the Initiator Cookie.
Once X and N arrive at a mutually agreeable value for the Initiator
Cookie, X uses it to create an IKE packet and tunnels it the RSIP
server N. N decapsulates the IKE packet and sends it on address
The minimum tuple negotiated via RSIP, and used for demultiplexing
incoming IKE responses from Y at the RSIP server N, is:
- IKE destination port number
- Initiator Cookie
- Destination IP address
One problem still remains: how does Y know that it is supposed to
send packets to X via Nb? Y is not RSIP-aware, but it is definitely
IKE-aware. Y sees IKE packets coming from address Nb. To prevent Y
from mistakenly deriving the identity of its IKE peer based on the
source address of the packets (Nb), X MUST exchange client
identifiers with Y:
- IDii, IDir if in Phase 1, and
- IDci, IDcr if in Phase 2.
The proper use of identifiers allows the clear separation between
those identities and the source IP address of the packets.
5. IPsec Handling and Demultiplexing
The RSIP client X and server N must arrive at an SPI value to denote
the incoming IPsec security association from Y to X. Once N and X
make sure that the SPI is unique within both of their SPI spaces, X
communicates its value to Y as part of the IPsec security association
establishment process, namely, Quick Mode in IKE [IKE] or manual
This ensures that Y sends IPsec packets (protocols 51 and 50 for AH
and ESP, respectively) [Kent98a,Kent98b] to X via address Nb using
the negotiated SPI.
IPsec packets from Y destined for X arrive at RSIP server N. They
are demultiplexed based on the following minimum tuple of
- protocol (50 or 51)
- destination IP address
If N is able to find a matching mapping, it tunnels the packet to X
according to the tunneling mode in effect. If N cannot find an
appropriate mapping, it MUST discard the packet.
6. RSIP Protocol Extensions
The next two sections specify how the RSIP protocol [RSIP-P] is
extended to support both IKE (a UDP application) and the IPsec-
defined AH and ESP headers (layered directly over IP with their own
If a server implements RSIP support for IKE and IPsec as defined in
this document, it MAY include the RSIP Method parameter for RSIP with
IPsec in the REGISTER_RESPONSE method sent to the client. This
method is assigned a value of 3:
3 RSIP with IPsec (RSIPSEC)
Unless otherwise specified, requirements of micro and macro flow-
based policy are handled according to [RSIP-P].
6.1 IKE Support in RSIP
As discussed above, if X's IPsec implementation allows use of an
ephemeral source port for IKE, then incoming IKE traffic can be
demultiplexed by N based on the destination address and port tuple.
This is the simplest and most desirable way of supporting IKE, and
IPsec implementations that interact with RSIP SHOULD allow it.
However, if X must use source port 500 for IKE, there are two
techniques with which X and N can arrive at a mutually unique
- Trial and error.
- Negotiation via an extension of the RSIP protocol.
The trial and error technique consists of X first obtaining resources
with which to use IPsec (via ASSIGN_REQUEST_RSIPSEC, defined below),
and then randomly choosing an Initiator Cookie and transmitting the
first packet to Y. Upon arrival at N, the RSIP server examines the
Initiator Cookie for uniqueness per X's assigned address (Nb). If
the cookie is unique, N allows the use of this cookie for this an all
subsequent packets between X and Y on this RSIP binding. If the
cookie is not unique, N drops the packet.
When an IKE packet is determined to be lost, the IKE client will
attempt to retransmit at least three times [IKE]. An RSIP-aware IKE
client SHOULD use different Initiator Cookies for each of these
The probability of an Initiator Cookie collision at N and subsequent
retransmissions by X, is infinitesimal given the 64-bit cookie space.
According to the birthday paradox, in a population of 640 million
RSIP clients going through the same RSIP server, the chances of a
first collision is just 1%. Thus, it is desirable to use the trial
and error method over negotiation, for these reasons:
- Simpler implementation requirements
- It is highly unlikely that more than one round trip between X
and N will be necessary.
6.2 IPsec Support in RSIP
This section defines the protocol extensions required for RSIP to
support AH and ESP. The required message types are
ASSIGN_REQUEST_RSIPSEC and ASSIGN_RESPONSE_RSIPSEC:
The ASSIGN_REQUEST_RSIPSEC message is used by an RSIP client to
request IPsec parameter assignments. An RSIP client MUST request
an IP address and SPIs in one message.
If the RSIP client wishes to use IPsec to protect a TCP or UDP
application, it MUST use the port range parameter (see Appendix
A). Otherwise, it MUST set the port parameters to the "don't
need" value. This is accomplished by setting the length field to
0, and by omitting both the number field and the port field. This
informs the server that the client does not actually need any port
The client may initialize the SPI parameter to the "don't care"
value (see below). In this case, it is requesting the server to
assign it a valid SPI value to use.
Alternatively, the client may initialize the SPI parameter to a
value it considers valid. In this case, it is suggesting that
value to the server. Of course, the server may choose to reject
that suggestion and return an appropriate error message.
The format of this message is:
<ASSIGN_REQUEST_RSIPSEC> ::= <Version>
The following message-specific error conditions exist. The error
behavior of ASSIGN_REQUEST_RSIP_IPSEC follows that of
ASSIGN_REQUEST_RSAP-IP for all non-IPsec errors.
- If the client is not allowed to use IPsec through the server,
the server MUST respond with an ERROR_RESPONSE containing the
- If the SPI parameter is a "don't care" value and the RSIP
server cannot allocate ANY SPIs, the RSIP server MUST respond
with an ERROR_RESPONSE containing the IPSEC_SPI_UNAVAILABLE
- If an SPI parameter is not a "don't care" value and the RSIP
server cannot allocate it because the requested address and SPI
tuple is in use, the RSIP server MUST respond with an
ERROR_RESPONSE containing the IPSEC_SPI_INUSE error.
The ASSIGN_RESPONSE_RSIPSEC message is used by an RSIP server to
assign parameters to an IPsec-enabled RSIP client.
The format of this message is:
<ASSIGN_RESPONSE_RSIPSEC> ::= <Version>
[Address (tunnel endpoint)]
If the port parameters were set to the "don't need" value in the
request (see above), the RSIP server must do the same in the
Additionally, RSIP support for IPsec requires the following new
Code Length Number SPI SPI
| 22 | 2 | 2 bytes | 4 bytes | ... | 4 bytes |
Sent by the RSIP client in ASSIGN_REQUEST_RSIPSEC messages to ask for
a particular number of SPIs to be assigned. Also sent by the RSIP
server to the client in ASSIGN_RESPONSE_RSIPSEC messages.
The "SPI" fields encode one or more SPIs. When a single SPI is
specified, the value of the number field is 1 and there is one SPI
field following the number field. When more than one SPI is
specified, the value of the number field will indicate the total
number of SPIs contained, and the parameter may take one of two
forms. If there is one SPI field, the SPIs specified are considered
to be contiguous starting at the SPI number specified in the SPI
field. Alternatively, there may be a number of SPI fields equal to
the value of the number field. The number of SPI fields can be
extrapolated from the value of the length field.
In some cases, it is necessary to specify a "don't care" value for
one or more SPIs. This is accomplished by setting the length field
to 2 (to account for the 2 bytes in the Number field), setting the
number field to the number of SPIs necessary, and omitting all SPI
fields. The value of the number field MUST be greater than or equal
7. IANA Considerations
All of the designations below are tentative.
- RSIP IPsec error codes (see below).
- ASSIGN_REQUEST_RSIP_IPSEC message type code.
- SPI parameter code.
8. Security Considerations
This document does not add any security issues to those already posed
by NAT, or normal routing operations. Current routing decisions
typically are based on a tuple with only one element: destination IP
address. This document just adds more elements to the tuple.
Furthermore, by allowing an end-to-end mode of operation and by
introducing a negotiation phase to address reuse, the mechanisms
described here are more secure and less arbitrary than NAT.
A word of caution is in order: SPI values are meant to be semi-
random, and, thus serve also as anti-clogging tokens to reduce off-
the-path denial-of-service attacks. However, RSIP support for IPsec,
renders SPI's a negotiated item: in addition to being unique values
at the receiver X, they must also be unique at the RSIP server, N.
Limiting the range of the SPI values available to the RSIP clients
reduces their entropy slightly.
Many thanks to Bernard Aboba, Vipul Gupta, Jeffrey Lo, Dan Nessett,
Gary Jaszewski and Prakash Iyer for helpful discussions.
[Gupta] Gupta, V., "Secure Remote Access over the Internet using
IPSec", Work in Progress.
[IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[ISAKMP] Maughan, D., Schertler, M., Schneider, M. and J. Turner,
"Internet Security Association and Key Management
Protocol (ISAKMP)", RFC 2408, November 1998.
[IPSEC-MSG] Ted Ts'o, message to the IETF's IPsec mailing list,
November 23, 1999.
[Jenkins] Jenkins, T., "IPsec Rekeying Issues", Work in Progress.
[Kent98a] Kent, S. and R. Atkinson, "IP Encapsulating Payload", RFC
2406, November 1998.
[Kent98b] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
2402, November 1998.
[Kent98c] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[Piper98] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP", RFC 2407, November 1998.
[NAPT] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
[NAT-TERMS] Srisuresh, P. and M. Holdredge, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.
[RSIP-FW] Borella, M., Lo, J., Grabelsky, D. and G. Montenegro,
"Realm Specific IP: A Framework", RFC 3102, October 2001.
[RSIP-P] Borella, M., Grabelsky, D., Lo, J. and K. Taniguchi,
"Realm Specific IP: Protocol Specification", RFC 3103,
Gabriel E. Montenegro
29, chemin du Vieux Chene
Phone: +33 476 18 80 45
3800 Golf Rd.
Rolling Meadows IL 60008
Phone: (847) 262-3083
Appendix A: On Optional Port Allocation to RSIP Clients
Despite the fact that SPIs rather than ports are used to
demultiplex packets at the RSIP server, the RSIP server may
still allocate mutually exclusive port numbers to the RSIP
clients. If this does not happen, there is the possibility that
two RSIP clients using the same IP address attempt an IPsec
session with the same server using the same source port
| RSIP client |
| X1 +--+
| | | +-------------+
+-------------+ | | |Nb
+---------+ RSIP server +----------------
+-------------+ | | N |
| RSIP client | | +-------------+
| X2 +--+ private public
| | | network network
For example, consider hosts X1 and X2 depicted above. Assume that
they both are using public address Nb, and both are contacting an
external server Y at port 80. If they are using IPsec but are not
allocated mutually exclusive port numbers, they may both choose the
same ephemeral port number to use when contacting Y at port 80.
Assume client X1 does so first, and after engaging in an IKE
negotiation begins communicating with the public server using IPsec.
When Client X2 starts its IKE session, it sends its identification to
the public server. The latter's SPD requires that different
identities use different flows (port numbers). Because of this, the
IKE negotiation will fail. Client X2 will be forced to try another
ephemeral port until it succeeds in obtaining one which is currently
not in use by any other security association between the public
server and any of the RSIP clients in the private network.
Each such iteration is costly in terms of round-trip times and CPU
usage. Hence --and as a convenience to its RSIP clients--, an RSIP
server may also assign mutually exclusive port numbers to its IPsec
Despite proper allocation of port numbers, an RSIP server cannot
prevent their misuse because it cannot examine the port fields in
packets that have been encrypted by the RSIP clients. Presumably, if
the RSIP clients have gone through the trouble of negotiating ports
numbers, it is in their best interest to adhere to these assignments.
Appendix B: RSIP Error Numbers for IKE and IPsec Support
This section provides descriptions for the error values in the RSIP
error parameter beyond those defined in [RSIP-P].
401: IPSEC_UNALLOWED. The server will not allow the client
to use end-to-end IPsec.
402: IPSEC_SPI_UNAVAILABLE. The server does not have an SPI
available for client use.
403: IPSEC_SPI_INUSE. The client has requested an SPI that
another client is currently using.
Appendix C: Message Type Values for IPsec Support
This section defines the values assigned to RSIP message types beyond
those defined in [RSIP-P].
Appendix D: A Note on Flow Policy Enforcement
An RSIP server may not be able to enforce local or remote micro-flow
policy when a client uses ESP for end-to-end encryption, since all
TCP/UDP port numbers will be encrypted. However, if AH without ESP
is used, micro-flow policy is enforceable. Macro-flow policy will
always be enforceable.
Appendix E: Remote Host Rekeying
Occasionally, a remote host with which an RSIP client has established
an IPsec security association (SA) will rekey [Jenkins]. SA rekeying
is only an issue for RSIP when IKE port 500 is used by the client and
the rekey is of ISAKMP phase 1 (the ISAKMP SA). The problem is that
the remote host will transmit IKE packets to port 500 with a new
initiator cookie. The RSIP server will not have a mapping for the
cookie, and SHOULD drop the the packets. This will cause the ISAKMP
SA between the RSIP client and remote host to be deleted, and may
lead to undefined behavior given that current implementations handle
rekeying in a number of different ways.
If the RSIP client uses an ephemeral source port, rekeying will not
be an issue for RSIP. If this cannot be done, there are a number of
RSIP client behaviors that may reduce the number of occurrences of
this problem, but are not guaranteed to eliminate it.
- The RSIP client's IKE implementation is given a smaller ISAKMP
SA lifetime than is typically implemented. This would likely
cause the RSIP client to rekey the ISAKMP SA before the remote
host. Since the RSIP client chooses the Initiator Cookie,
there will be no problem routing incoming traffic at the RSIP
- The RSIP client terminates the ISAKMP SA as soon as the first
IPsec SA is established. This may alleviate the situation to
some degree if the SA is coarse-grained. On the other hand,
this exacerbates the problem if the SA is fine-grained (such
that it cannot be reused by other application-level
connections), and the remote host needs to initialize sockets
back to the RSIP client.
Note that the unreliability of UDP essentially makes the ephemeral
source approach the only robust solution.
Appendix F: Example Application Scenarios
This section briefly describes some examples of how RSIP may be used
to enable applications of IPsec that are otherwise not possible.
The SOHO (small office, home office) scenario
|client X1 +--+
| | | +-------------+ +-------+
+----------+ | |NAPT gateway | |public |
+--+ and +--.......---+IPsec |
+----------+ | |RSIP server | |peer Y |
|RSIP | | +-------------+ +-------+
|client X2 +--+ private public
| | | "home" Internet
+----------+ | network
Suppose the private "home" network is a small installation in
somebody's home, and that the RSIP clients X1 and X2 must use the
RSIP server N as a gateway to the outside world. N is connected via
an ISP and obtains a single address which must be shared by its
clients. Because of this, N has NAPT, functionality. Now, X1 wishes
to establish an IPsec SA with peer Y. This is possible because N is
also an RSIP server augmented with the IPsec support defined in this
document. Y is IPsec-capable, but is not RSIP aware. This is
perhaps the most typical application scenario.
The above is equally applicable in the ROBO (remote office, branch
The Roadwarrior scenario
+---------+ +------------+ +----------+
|RSIP | |Corporate | | IPsec |
|client X +--..........--+Firewall +---+ peer Y |
| | public | and | | (user's |
+---------+ Internet |RSIP server | | desktop) |
| N | | |
In this example, a remote user with a laptop gains access to the
Internet, perhaps by using PPP or DHCP. The user wants to access its
corporation private network. Using mechanisms not specified in this
document, the RSIP client in the laptop engages in an RSIP
authentication and authorization phase with the RSIP server at the
firewall. After that phase is completed, the IPsec extensions to
RSIP defined here are used to establish an IPsec session with a peer,
Y, that resides within the corporation's network. Y could be, for
example, the remote user's usual desktop when at the office. The
corporate firewall complex would use RSIP to selectively enable IPsec
traffic between internal and external systems.
Note that this scenario could also be reversed in order to allow an
internal system (Y) to initiate and establish an IPsec session with
an external IPsec peer (X).
Appendix G: Thoughts on Supporting Incoming Connections
Incoming IKE connections are much easier to support if the peer Y can
initiate IKE exchanges to a port other than 500. In this case, the
RSIP client would allocate that port at the RSIP server via
ASSIGN_REQUEST_RSAP-IP. Alternatively, if the RSIP client is able to
allocate an IP address at the RSIP server via ASSIGN_REQUEST_RSA-IP,
Y could simply initiate the IKE exchange to port 500 at that address.
If there is only one address Nb that must be shared by the RSIP
server and all its clients, and if Y can only send to port 500, the
problem is much more difficult. At any given time, the combination
of address Nb and UDP port 500 may be registered and used by only one
RSIP system (including clients and server).
Solving this issue would require demultiplexing the incoming IKE
connection request based on something other than the port and address
combination. It may be possible to do so by first registering an
identity with a new RSIP command of LISTEN_RSIP_IKE. Note that the
identity could not be that of the IKE responder (the RSIP client),
but that of the initiator (Y). The reason is that IKE Phase 1 only
allows the sender to include its own identity, not that of the
intended recipient (both, by the way, are allowed in Phase 2).
Furthermore, the identity must be in the clear in the first incoming
packet for the RSIP server to be able to use it as a demultiplexor.
This rules out all variants of Main Mode and Aggressive Mode with
Public Key Encryption (and Revised Mode of Public Key Encryption),
since these encrypt the ID payload.
The only Phase 1 variants which enable incoming IKE sessions are
Aggressive Mode with signatures or with pre-shared keys. Because
this scheme involves the RSIP server demultiplexing based on the
identity of the IKE initiator, it is conceivable that only one RSIP
client at a time may register interest in fielding requests from any
given peer Y. Furthermore, this precludes more than one RSIP client'
s being available to any unspecified peer Y.
Once the IKE session is in place, IPsec is set up as discussed in
this document, namely, by the RSIP client and the RSIP server
agreeing on an incoming SPI value, which is then communicated to the
peer Y as part of Quick Mode.
The alternate address and port combination must be discovered by the
remote peer using methods such as manual configuration, or the use of
KX (RFC2230) or SRV (RFC2052) records. It may even be possible for
the DNS query to trigger the above mechanisms to prepare for the
incoming and impending IKE session initiation. Such a mechanism
would allow more than one RSIP client to be available at any given
time, and would also enable each of them to respond to IKE
initiations from unspecified peers. Such a DNS query, however, is
not guaranteed to occur. For example, the result of the query could
be cached and reused after the RSIP server is no longer listening for
a given IKE peer's identity.
Because of the limitations implied by having to rely on the identity
of the IKE initiator, the only practical way of supporting incoming
connections is for the peer Y to initiate the IKE session at a port
other than 500.
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