RFC3104 - RSIP Support for End-to-end IPsec

Network Working Group G. Montenegro

Request for Comments: 3104 Sun Microsystems, Inc.

Category: EXPerimental M. Borella

CommWorks

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.

IESG Note

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 RFC3102, as well as

in the Appendices of RFC3104. Accordingly, the costs and benefits

of using RSIP should be carefully weighed against other means of

relieving address shortage.

Abstract

This document proposes mechanisms that enable Realm Specific IP

(RSIP) to handle end-to-end IPsec (IP Security).

Table of Contents

1. IntrodUCtion .................................................. 2

2. Model ......................................................... 2

3. Implementation Notes .......................................... 3

4. IKE Handling and Demultiplexing ............................... 4

5. IPsec Handling and Demultiplexing ............................. 5

6. RSIP Protocol Extensions ...................................... 6

6.1 IKE Support in RSIP ....................................... 6

6.2 IPsec Support in RSIP ..................................... 7

7. IANA Considerations ........................................... 10

8. Security Considerations ....................................... 10

9. 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 ......................................... 19

1. 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 RFC2119.

2. Model

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

implementations [Gupta]:

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"

method.

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

space B.

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

assignment.

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

demultiplexing fields:

- protocol (50 or 51)

- SPI

- 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

protocol numbers).

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

Initiator Cookie.

- 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

retransmissions.

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:

ASSIGN_REQUEST_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

assignments.

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>

<SPI>

[Message Counter]

[Lease Time]

[Tunnel Type]

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

IPSEC_UNALLOWED parameter.

- 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

error.

- 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.

ASSIGN_RESPONSE_RSIPSEC

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>

<SPI>

[Address (tunnel endpoint)]

[Message Counter]

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

response.

Additionally, RSIP support for IPsec requires the following new

parameter:

SPI

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

to one.

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.

9. Acknowledgements

Many thanks to Bernard Aboba, Vipul Gupta, Jeffrey Lo, Dan Nessett,

Gary Jaszewski and Prakash Iyer for helpful discussions.

References

[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)", RFC2409, November 1998.

[ISAKMP] Maughan, D., Schertler, M., Schneider, M. and J. Turner,

"Internet Security Association and Key Management

Protocol (ISAKMP)", RFC2408, November 1998.

[IPSEC-MSG] Ted Ts'o, message to the IETF's IPsec mailing list,

Message-Id:<199911232216.RAA01932@trampoline.thunk.org>,

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", RFC2401, November 1998.

[Piper98] Piper, D., "The Internet IP Security Domain of

Interpretation for ISAKMP", RFC2407, November 1998.

[NAPT] Srisuresh, P. and K. Egevang, "Traditional IP Network

Address Translator (Traditional NAT)", RFC3022, January

2001.

[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", RFC3102, October 2001.

[RSIP-P] Borella, M., Grabelsky, D., Lo, J. and K. Taniguchi,

"Realm Specific IP: Protocol Specification", RFC3103,

October 2001.

Authors' Addresses

Gabriel E. Montenegro

Sun Microsystems

Laboratories, Europe

29, chemin du Vieux Chene

38240 Meylan

FRANCE

Phone: +33 476 18 80 45

EMail: gab@sun.com

Michael Borella

CommWorks

3800 Golf Rd.

Rolling Meadows IL 60008

Phone: (847) 262-3083

EMail: mike_borella@commworks.com

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

numbers.

+-------------+

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

RSIP clients.

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].

22 ASSIGN_REQUEST_RSIPSEC

23 ASSIGN_RESPONSE_RSIPSEC

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

server.

- 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

---------------------------------------------

+----------+

RSIP

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

office) scenario.

The Roadwarrior scenario

------------------------

+---------+ +------------+ +----------+

RSIP Corporate IPsec

client X +--..........--+Firewall +---+ peer Y

public and (user's

+---------+ Internet RSIP server desktop)

+------------+ +----------+

private corporate

network

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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