2.  IKE Protocol Details and Variations

   IKE normally listens and sends on UDP port 500, though IKE messages
   may also be received on UDP port 4500 with a slightly different
   format (see Section 2.23).  Since UDP is a datagram (unreliable)
   protocol, IKE includes in its definition recovery from transmission
   errors, including packet loss, packet replay, and packet forgery.
   IKE is designed to function so long as (1) at least one of a series
   of retransmitted packets reaches its destination before timing out;
   and (2) the channel is not so full of forged and replayed packets so
   as to exhaust the network or CPU capacities of either endpoint.  Even
   in the absence of those minimum performance requirements, IKE is
   designed to fail cleanly (as though the network were broken).

   Although IKEv2 messages are intended to be short, they contain
   structures with no hard upper bound on size (in particular, digital
   certificates), and IKEv2 itself does not have a mechanism for

   fragmenting large messages.  IP defines a mechanism for fragmentation
   of oversized UDP messages, but implementations vary in the maximum
   message size supported.  Furthermore, use of IP fragmentation opens
   an implementation to denial-of-service (DoS) attacks [DOSUDPPROT].
   Finally, some NAT and/or firewall implementations may block IP
   fragments.

   All IKEv2 implementations MUST be able to send, receive, and process
   IKE messages that are up to 1280 octets long, and they SHOULD be able
   to send, receive, and process messages that are up to 3000 octets
   long.  IKEv2 implementations need to be aware of the maximum UDP
   message size supported and MAY shorten messages by leaving out some
   certificates or cryptographic suite proposals if that will keep
   messages below the maximum.  Use of the "Hash and URL" formats rather
   than including certificates in exchanges where possible can avoid
   most problems.  Implementations and configuration need to keep in
   mind, however, that if the URL lookups are possible only after the
   Child SA is established, recursion issues could prevent this
   technique from working.

   The UDP payload of all packets containing IKE messages sent on port
   4500 MUST begin with the prefix of four zeros; otherwise, the
   receiver won't know how to handle them.

2.1.  Use of Retransmission Timers

   All messages in IKE exist in pairs: a request and a response.  The
   setup of an IKE SA normally consists of two exchanges.  Once the IKE
   SA is set up, either end of the Security Association may initiate
   requests at any time, and there can be many requests and responses
   "in flight" at any given moment.  But each message is labeled as
   either a request or a response, and for each exchange, one end of the
   Security Association is the initiator and the other is the responder.

   For every pair of IKE messages, the initiator is responsible for
   retransmission in the event of a timeout.  The responder MUST never
   retransmit a response unless it receives a retransmission of the
   request.  In that event, the responder MUST ignore the retransmitted
   request except insofar as it causes a retransmission of the response.
   The initiator MUST remember each request until it receives the
   corresponding response.  The responder MUST remember each response
   until it receives a request whose sequence number is larger than or
   equal to the sequence number in the response plus its window size
   (see Section 2.3).  In order to allow saving memory, responders are
   allowed to forget the response after a timeout of several minutes.
   If the responder receives a retransmitted request for which it has
   already forgotten the response, it MUST ignore the request (and not,
   for example, attempt constructing a new response).

   IKE is a reliable protocol: the initiator MUST retransmit a request
   until it either receives a corresponding response or deems the IKE SA
   to have failed.  In the latter case, the initiator discards all state
   associated with the IKE SA and any Child SAs that were negotiated
   using that IKE SA.  A retransmission from the initiator MUST be
   bitwise identical to the original request.  That is, everything
   starting from the IKE header (the IKE SA initiator's SPI onwards)
   must be bitwise identical; items before it (such as the IP and UDP
   headers) do not have to be identical.

   Retransmissions of the IKE_SA_INIT request require some special
   handling.  When a responder receives an IKE_SA_INIT request, it has
   to determine whether the packet is a retransmission belonging to an
   existing "half-open" IKE SA (in which case the responder retransmits
   the same response), or a new request (in which case the responder
   creates a new IKE SA and sends a fresh response), or it belongs to an
   existing IKE SA where the IKE_AUTH request has been already received
   (in which case the responder ignores it).

   It is not sufficient to use the initiator's SPI and/or IP address to
   differentiate between these three cases because two different peers
   behind a single NAT could choose the same initiator SPI.  Instead, a
   robust responder will do the IKE SA lookup using the whole packet,
   its hash, or the Ni payload.

   The retransmission policy for one-way messages is somewhat different
   from that for regular messages.  Because no acknowledgement is ever
   sent, there is no reason to gratuitously retransmit one-way messages.
   Given that all these messages are errors, it makes sense to send them
   only once per "offending" packet, and only retransmit if further
   offending packets are received.  Still, it also makes sense to limit
   retransmissions of such error messages.

2.2.  Use of Sequence Numbers for Message ID

   Every IKE message contains a Message ID as part of its fixed header.
   This Message ID is used to match up requests and responses and to
   identify retransmissions of messages.  Retransmission of a message
   MUST use the same Message ID as the original message.

   The Message ID is a 32-bit quantity, which is zero for the
   IKE_SA_INIT messages (including retries of the message due to
   responses such as COOKIE and INVALID_KE_PAYLOAD), and incremented for
   each subsequent exchange.  Thus, the first pair of IKE_AUTH messages
   will have an ID of 1, the second (when EAP is used) will be 2, and so
   on.  The Message ID is reset to zero in the new IKE SA after the IKE
   SA is rekeyed.

   Each endpoint in the IKE Security Association maintains two "current"
   Message IDs: the next one to be used for a request it initiates and
   the next one it expects to see in a request from the other end.
   These counters increment as requests are generated and received.
   Responses always contain the same Message ID as the corresponding
   request.  That means that after the initial exchange, each integer n
   may appear as the Message ID in four distinct messages: the nth
   request from the original IKE initiator, the corresponding response,
   the nth request from the original IKE responder, and the
   corresponding response.  If the two ends make a very different number
   of requests, the Message IDs in the two directions can be very
   different.  There is no ambiguity in the messages, however, because
   the Initiator and Response flags in the message header specify which
   of the four messages a particular one is.

   Throughout this document, "initiator" refers to the party who
   initiated the exchange being described.  The "original initiator"
   always refers to the party who initiated the exchange that resulted
   in the current IKE SA.  In other words, if the "original responder"
   starts rekeying the IKE SA, that party becomes the "original
   initiator" of the new IKE SA.

   Note that Message IDs are cryptographically protected and provide
   protection against message replays.  In the unlikely event that
   Message IDs grow too large to fit in 32 bits, the IKE SA MUST be
   closed or rekeyed.

2.3.  Window Size for Overlapping Requests

   The SET_WINDOW_SIZE notification asserts that the sending endpoint is
   capable of keeping state for multiple outstanding exchanges,
   permitting the recipient to send multiple requests before getting a
   response to the first.  The data associated with a SET_WINDOW_SIZE
   notification MUST be 4 octets long and contain the big endian
   representation of the number of messages the sender promises to keep.
   The window size is always one until the initial exchanges complete.

   An IKE endpoint MUST wait for a response to each of its messages
   before sending a subsequent message unless it has received a
   SET_WINDOW_SIZE Notify message from its peer informing it that the
   peer is prepared to maintain state for multiple outstanding messages
   in order to allow greater throughput.

   After an IKE SA is set up, in order to maximize IKE throughput, an
   IKE endpoint MAY issue multiple requests before getting a response to
   any of them, up to the limit set by its peer's SET_WINDOW_SIZE.
   These requests may pass one another over the network.  An IKE
   endpoint MUST be prepared to accept and process a request while it

   has a request outstanding in order to avoid a deadlock in this
   situation.  An IKE endpoint may also accept and process multiple
   requests while it has a request outstanding.

   An IKE endpoint MUST NOT exceed the peer's stated window size for
   transmitted IKE requests.  In other words, if the responder stated
   its window size is N, then when the initiator needs to make a request
   X, it MUST wait until it has received responses to all requests up
   through request X-N.  An IKE endpoint MUST keep a copy of (or be able
   to regenerate exactly) each request it has sent until it receives the
   corresponding response.  An IKE endpoint MUST keep a copy of (or be
   able to regenerate exactly) the number of previous responses equal to
   its declared window size in case its response was lost and the
   initiator requests its retransmission by retransmitting the request.

   An IKE endpoint supporting a window size greater than one ought to be
   capable of processing incoming requests out of order to maximize
   performance in the event of network failures or packet reordering.

   The window size is normally a (possibly configurable) property of a
   particular implementation, and is not related to congestion control
   (unlike the window size in TCP, for example).  In particular, what
   the responder should do when it receives a SET_WINDOW_SIZE
   notification containing a smaller value than is currently in effect
   is not defined.  Thus, there is currently no way to reduce the window
   size of an existing IKE SA; you can only increase it.  When rekeying
   an IKE SA, the new IKE SA starts with window size 1 until it is
   explicitly increased by sending a new SET_WINDOW_SIZE notification.

   The INVALID_MESSAGE_ID notification is sent when an IKE Message ID
   outside the supported window is received.  This Notify message MUST
   NOT be sent in a response; the invalid request MUST NOT be
   acknowledged.  Instead, inform the other side by initiating an
   INFORMATIONAL exchange with Notification data containing the four-
   octet invalid Message ID.  Sending this notification is OPTIONAL, and
   notifications of this type MUST be rate limited.

2.4.  State Synchronization and Connection Timeouts

   An IKE endpoint is allowed to forget all of its state associated with
   an IKE SA and the collection of corresponding Child SAs at any time.
   This is the anticipated behavior in the event of an endpoint crash
   and restart.  It is important when an endpoint either fails or
   reinitializes its state that the other endpoint detect those
   conditions and not continue to waste network bandwidth by sending
   packets over discarded SAs and having them fall into a black hole.

   The INITIAL_CONTACT notification asserts that this IKE SA is the only
   IKE SA currently active between the authenticated identities.  It MAY
   be sent when an IKE SA is established after a crash, and the
   recipient MAY use this information to delete any other IKE SAs it has
   to the same authenticated identity without waiting for a timeout.
   This notification MUST NOT be sent by an entity that may be
   replicated (e.g., a roaming user's credentials where the user is
   allowed to connect to the corporate firewall from two remote systems
   at the same time).  The INITIAL_CONTACT notification, if sent, MUST
   be in the first IKE_AUTH request or response, not as a separate
   exchange afterwards; receiving parties MAY ignore it in other
   messages.

   Since IKE is designed to operate in spite of DoS attacks from the
   network, an endpoint MUST NOT conclude that the other endpoint has
   failed based on any routing information (e.g., ICMP messages) or IKE
   messages that arrive without cryptographic protection (e.g., Notify
   messages complaining about unknown SPIs).  An endpoint MUST conclude
   that the other endpoint has failed only when repeated attempts to
   contact it have gone unanswered for a timeout period or when a
   cryptographically protected INITIAL_CONTACT notification is received
   on a different IKE SA to the same authenticated identity.  An
   endpoint should suspect that the other endpoint has failed based on
   routing information and initiate a request to see whether the other
   endpoint is alive.  To check whether the other side is alive, IKE
   specifies an empty INFORMATIONAL message that (like all IKE requests)
   requires an acknowledgement (note that within the context of an IKE
   SA, an "empty" message consists of an IKE header followed by an
   Encrypted payload that contains no payloads).  If a cryptographically
   protected (fresh, i.e., not retransmitted) message has been received
   from the other side recently, unprotected Notify messages MAY be
   ignored.  Implementations MUST limit the rate at which they take
   actions based on unprotected messages.

   The number of retries and length of timeouts are not covered in this
   specification because they do not affect interoperability.  It is
   suggested that messages be retransmitted at least a dozen times over
   a period of at least several minutes before giving up on an SA, but
   different environments may require different rules.  To be a good
   network citizen, retransmission times MUST increase exponentially to
   avoid flooding the network and making an existing congestion
   situation worse.  If there has only been outgoing traffic on all of
   the SAs associated with an IKE SA, it is essential to confirm
   liveness of the other endpoint to avoid black holes.  If no
   cryptographically protected messages have been received on an IKE SA
   or any of its Child SAs recently, the system needs to perform a
   liveness check in order to prevent sending messages to a dead peer.
   (This is sometimes called "dead peer detection" or "DPD", although it

   is really detecting live peers, not dead ones.)  Receipt of a fresh
   cryptographically protected message on an IKE SA or any of its Child
   SAs ensures liveness of the IKE SA and all of its Child SAs.  Note
   that this places requirements on the failure modes of an IKE
   endpoint.  An implementation needs to stop sending over any SA if
   some failure prevents it from receiving on all of the associated SAs.
   If a system creates Child SAs that can fail independently from one
   another without the associated IKE SA being able to send a delete
   message, then the system MUST negotiate such Child SAs using separate
   IKE SAs.

   There is a DoS attack on the initiator of an IKE SA that can be
   avoided if the initiator takes the proper care.  Since the first two
   messages of an SA setup are not cryptographically protected, an
   attacker could respond to the initiator's message before the genuine
   responder and poison the connection setup attempt.  To prevent this,
   the initiator MAY be willing to accept multiple responses to its
   first message, treat each as potentially legitimate, respond to it,
   and then discard all the invalid half-open connections when it
   receives a valid cryptographically protected response to any one of
   its requests.  Once a cryptographically valid response is received,
   all subsequent responses should be ignored whether or not they are
   cryptographically valid.

   Note that with these rules, there is no reason to negotiate and agree
   upon an SA lifetime.  If IKE presumes the partner is dead, based on
   repeated lack of acknowledgement to an IKE message, then the IKE SA
   and all Child SAs set up through that IKE SA are deleted.

   An IKE endpoint may at any time delete inactive Child SAs to recover
   resources used to hold their state.  If an IKE endpoint chooses to
   delete Child SAs, it MUST send Delete payloads to the other end
   notifying it of the deletion.  It MAY similarly time out the IKE SA.
   Closing the IKE SA implicitly closes all associated Child SAs.  In
   this case, an IKE endpoint SHOULD send a Delete payload indicating
   that it has closed the IKE SA unless the other endpoint is no longer
   responding.

2.5.  Version Numbers and Forward Compatibility

   This document describes version 2.0 of IKE, meaning the major version
   number is 2 and the minor version number is 0.  This document is a
   replacement for [IKEV2].  It is likely that some implementations will
   want to support version 1.0 and version 2.0, and in the future, other
   versions.

   The major version number should be incremented only if the packet
   formats or required actions have changed so dramatically that an
   older version node would not be able to interoperate with a newer
   version node if it simply ignored the fields it did not understand
   and took the actions specified in the older specification.  The minor
   version number indicates new capabilities, and MUST be ignored by a
   node with a smaller minor version number, but used for informational
   purposes by the node with the larger minor version number.  For
   example, it might indicate the ability to process a newly defined
   Notify message type.  The node with the larger minor version number
   would simply note that its correspondent would not be able to
   understand that message and therefore would not send it.

   If an endpoint receives a message with a higher major version number,
   it MUST drop the message and SHOULD send an unauthenticated Notify
   message of type INVALID_MAJOR_VERSION containing the highest
   (closest) version number it supports.  If an endpoint supports major
   version n, and major version m, it MUST support all versions between
   n and m.  If it receives a message with a major version that it
   supports, it MUST respond with that version number.  In order to
   prevent two nodes from being tricked into corresponding with a lower
   major version number than the maximum that they both support, IKE has
   a flag that indicates that the node is capable of speaking a higher
   major version number.

   Thus, the major version number in the IKE header indicates the
   version number of the message, not the highest version number that
   the transmitter supports.  If the initiator is capable of speaking
   versions n, n+1, and n+2, and the responder is capable of speaking
   versions n and n+1, then they will negotiate speaking n+1, where the
   initiator will set a flag indicating its ability to speak a higher
   version.  If they mistakenly (perhaps through an active attacker
   sending error messages) negotiate to version n, then both will notice
   that the other side can support a higher version number, and they
   MUST break the connection and reconnect using version n+1.

   Note that IKEv1 does not follow these rules, because there is no way
   in v1 of noting that you are capable of speaking a higher version
   number.  So an active attacker can trick two v2-capable nodes into
   speaking v1.  When a v2-capable node negotiates down to v1, it should
   note that fact in its logs.

   Also, for forward compatibility, all fields marked RESERVED MUST be
   set to zero by an implementation running version 2.0, and their
   content MUST be ignored by an implementation running version 2.0 ("Be
   conservative in what you send and liberal in what you receive" [IP]).
   In this way, future versions of the protocol can use those fields in
   a way that is guaranteed to be ignored by implementations that do not

   understand them.  Similarly, payload types that are not defined are
   reserved for future use; implementations of a version where they are
   undefined MUST skip over those payloads and ignore their contents.

   IKEv2 adds a "critical" flag to each payload header for further
   flexibility for forward compatibility.  If the critical flag is set
   and the payload type is unrecognized, the message MUST be rejected
   and the response to the IKE request containing that payload MUST
   include a Notify payload UNSUPPORTED_CRITICAL_PAYLOAD, indicating an
   unsupported critical payload was included.  In that Notify payload,
   the notification data contains the one-octet payload type.  If the
   critical flag is not set and the payload type is unsupported, that
   payload MUST be ignored.  Payloads sent in IKE response messages MUST
   NOT have the critical flag set.  Note that the critical flag applies
   only to the payload type, not the contents.  If the payload type is
   recognized, but the payload contains something that is not (such as
   an unknown transform inside an SA payload, or an unknown Notify
   Message Type inside a Notify payload), the critical flag is ignored.

   Although new payload types may be added in the future and may appear
   interleaved with the fields defined in this specification,
   implementations SHOULD send the payloads defined in this
   specification in the order shown in the figures in Sections 1 and 2;
   implementations MUST NOT reject as invalid a message with those
   payloads in any other order.

2.6.  IKE SA SPIs and Cookies

   The initial two eight-octet fields in the header, called the "IKE
   SPIs", are used as a connection identifier at the beginning of IKE
   packets.  Each endpoint chooses one of the two SPIs and MUST choose
   them so as to be unique identifiers of an IKE SA.  An SPI value of
   zero is special: it indicates that the remote SPI value is not yet
   known by the sender.

   Incoming IKE packets are mapped to an IKE SA only using the packet's
   SPI, not using (for example) the source IP address of the packet.

   Unlike ESP and AH where only the recipient's SPI appears in the
   header of a message, in IKE the sender's SPI is also sent in every
   message.  Since the SPI chosen by the original initiator of the IKE
   SA is always sent first, an endpoint with multiple IKE SAs open that
   wants to find the appropriate IKE SA using the SPI it assigned must
   look at the Initiator flag in the header to determine whether it
   assigned the first or the second eight octets.

   In the first message of an initial IKE exchange, the initiator will
   not know the responder's SPI value and will therefore set that field
   to zero.  When the IKE_SA_INIT exchange does not result in the
   creation of an IKE SA due to INVALID_KE_PAYLOAD, NO_PROPOSAL_CHOSEN,
   or COOKIE (see Section 2.6), the responder's SPI will be zero also in
   the response message.  However, if the responder sends a non-zero
   responder SPI, the initiator should not reject the response for only
   that reason.

   Two expected attacks against IKE are state and CPU exhaustion, where
   the target is flooded with session initiation requests from forged IP
   addresses.  These attacks can be made less effective if a responder
   uses minimal CPU and commits no state to an SA until it knows the
   initiator can receive packets at the address from which it claims to
   be sending them.

   When a responder detects a large number of half-open IKE SAs, it
   SHOULD reply to IKE_SA_INIT requests with a response containing the
   COOKIE notification.  The data associated with this notification MUST
   be between 1 and 64 octets in length (inclusive), and its generation
   is described later in this section.  If the IKE_SA_INIT response
   includes the COOKIE notification, the initiator MUST then retry the
   IKE_SA_INIT request, and include the COOKIE notification containing
   the received data as the first payload, and all other payloads
   unchanged.  The initial exchange will then be as follows:

   Initiator                         Responder
   -------------------------------------------------------------------
   HDR(A,0), SAi1, KEi, Ni  -->
                                <--  HDR(A,0), N(COOKIE)
   HDR(A,0), N(COOKIE), SAi1,
       KEi, Ni  -->
                                <--  HDR(A,B), SAr1, KEr,
                                         Nr, [CERTREQ]
   HDR(A,B), SK {IDi, [CERT,]
       [CERTREQ,] [IDr,] AUTH,
       SAi2, TSi, TSr}  -->
                                <--  HDR(A,B), SK {IDr, [CERT,]
                                         AUTH, SAr2, TSi, TSr}

   The first two messages do not affect any initiator or responder state
   except for communicating the cookie.  In particular, the message
   sequence numbers in the first four messages will all be zero and the
   message sequence numbers in the last two messages will be one.  'A'
   is the SPI assigned by the initiator, while 'B' is the SPI assigned
   by the responder.

   An IKE implementation can implement its responder cookie generation
   in such a way as to not require any saved state to recognize its
   valid cookie when the second IKE_SA_INIT message arrives.  The exact
   algorithms and syntax used to generate cookies do not affect
   interoperability and hence are not specified here.  The following is
   an example of how an endpoint could use cookies to implement limited
   DoS protection.

   A good way to do this is to set the responder cookie to be:

   Cookie = <VersionIDofSecret> | Hash(Ni | IPi | SPIi | <secret>)

   where <secret> is a randomly generated secret known only to the
   responder and periodically changed and | indicates concatenation.
   <VersionIDofSecret> should be changed whenever <secret> is
   regenerated.  The cookie can be recomputed when the IKE_SA_INIT
   arrives the second time and compared to the cookie in the received
   message.  If it matches, the responder knows that the cookie was
   generated since the last change to <secret> and that IPi must be the
   same as the source address it saw the first time.  Incorporating SPIi
   into the calculation ensures that if multiple IKE SAs are being set
   up in parallel they will all get different cookies (assuming the
   initiator chooses unique SPIi's).  Incorporating Ni in the hash
   ensures that an attacker who sees only message 2 can't successfully
   forge a message 3.  Also, incorporating SPIi in the hash prevents an
   attacker from fetching one cookie from the other end, and then
   initiating many IKE_SA_INIT exchanges all with different initiator
   SPIs (and perhaps port numbers) so that the responder thinks that
   there are a lot of machines behind one NAT box that are all trying to
   connect.

   If a new value for <secret> is chosen while there are connections in
   the process of being initialized, an IKE_SA_INIT might be returned
   with other than the current <VersionIDofSecret>.  The responder in
   that case MAY reject the message by sending another response with a
   new cookie or it MAY keep the old value of <secret> around for a
   short time and accept cookies computed from either one.  The
   responder should not accept cookies indefinitely after <secret> is
   changed, since that would defeat part of the DoS protection.  The
   responder should change the value of <secret> frequently, especially
   if under attack.

   When one party receives an IKE_SA_INIT request containing a cookie
   whose contents do not match the value expected, that party MUST
   ignore the cookie and process the message as if no cookie had been
   included; usually this means sending a response containing a new
   cookie.  The initiator should limit the number of cookie exchanges it
   tries before giving up, possibly using exponential back-off.  An

   attacker can forge multiple cookie responses to the initiator's
   IKE_SA_INIT message, and each of those forged cookie replies will
   cause two packets to be sent: one packet from the initiator to the
   responder (which will reject those cookies), and one response from
   responder to initiator that includes the correct cookie.

   A note on terminology: the term "cookies" originates with Karn and
   Simpson [PHOTURIS] in Photuris, an early proposal for key management
   with IPsec, and it has persisted.  The Internet Security Association
   and Key Management Protocol (ISAKMP) [ISAKMP] fixed message header
   includes two eight-octet fields called "cookies", and that syntax is
   used by both IKEv1 and IKEv2, although in IKEv2 they are referred to
   as the "IKE SPI" and there is a new separate field in a Notify
   payload holding the cookie.

2.6.1.  Interaction of COOKIE and INVALID_KE_PAYLOAD

   There are two common reasons why the initiator may have to retry the
   IKE_SA_INIT exchange: the responder requests a cookie or wants a
   different Diffie-Hellman group than was included in the KEi payload.
   If the initiator receives a cookie from the responder, the initiator
   needs to decide whether or not to include the cookie in only the next
   retry of the IKE_SA_INIT request, or in all subsequent retries as
   well.

   If the initiator includes the cookie only in the next retry, one
   additional round trip may be needed in some cases.  An additional
   round trip is needed also if the initiator includes the cookie in all
   retries, but the responder does not support this.  For instance, if
   the responder includes the KEi payloads in cookie calculation, it
   will reject the request by sending a new cookie.

   If both peers support including the cookie in all retries, a slightly
   shorter exchange can happen.

   Initiator                   Responder
   -----------------------------------------------------------
   HDR(A,0), SAi1, KEi, Ni -->
                           <-- HDR(A,0), N(COOKIE)
   HDR(A,0), N(COOKIE), SAi1, KEi, Ni  -->
                           <-- HDR(A,0), N(INVALID_KE_PAYLOAD)
   HDR(A,0), N(COOKIE), SAi1, KEi', Ni -->
                           <-- HDR(A,B), SAr1, KEr, Nr

   Implementations SHOULD support this shorter exchange, but MUST NOT
   fail if other implementations do not support this shorter exchange.

2.7.  Cryptographic Algorithm Negotiation

   The payload type known as "SA" indicates a proposal for a set of
   choices of IPsec protocols (IKE, ESP, or AH) for the SA as well as
   cryptographic algorithms associated with each protocol.

   An SA payload consists of one or more proposals.  Each proposal
   includes one protocol.  Each protocol contains one or more transforms
   -- each specifying a cryptographic algorithm.  Each transform
   contains zero or more attributes (attributes are needed only if the
   Transform ID does not completely specify the cryptographic
   algorithm).

   This hierarchical structure was designed to efficiently encode
   proposals for cryptographic suites when the number of supported
   suites is large because multiple values are acceptable for multiple
   transforms.  The responder MUST choose a single suite, which may be
   any subset of the SA proposal following the rules below.

   Each proposal contains one protocol.  If a proposal is accepted, the
   SA response MUST contain the same protocol.  The responder MUST
   accept a single proposal or reject them all and return an error.  The
   error is given in a notification of type NO_PROPOSAL_CHOSEN.

   Each IPsec protocol proposal contains one or more transforms.  Each
   transform contains a Transform Type.  The accepted cryptographic
   suite MUST contain exactly one transform of each type included in the
   proposal.  For example: if an ESP proposal includes transforms
   ENCR_3DES, ENCR_AES w/keysize 128, ENCR_AES w/keysize 256,
   AUTH_HMAC_MD5, and AUTH_HMAC_SHA, the accepted suite MUST contain one
   of the ENCR_ transforms and one of the AUTH_ transforms.  Thus, six
   combinations are acceptable.

   If an initiator proposes both normal ciphers with integrity
   protection as well as combined-mode ciphers, then two proposals are
   needed.  One of the proposals includes the normal ciphers with the
   integrity algorithms for them, and the other proposal includes all
   the combined-mode ciphers without the integrity algorithms (because
   combined-mode ciphers are not allowed to have any integrity algorithm
   other than "none").

2.8.  Rekeying

   IKE, ESP, and AH Security Associations use secret keys that should be
   used only for a limited amount of time and to protect a limited
   amount of data.  This limits the lifetime of the entire Security
   Association.  When the lifetime of a Security Association expires,
   the Security Association MUST NOT be used.  If there is demand, new

   Security Associations MAY be established.  Reestablishment of
   Security Associations to take the place of ones that expire is
   referred to as "rekeying".

   To allow for minimal IPsec implementations, the ability to rekey SAs
   without restarting the entire IKE SA is optional.  An implementation
   MAY refuse all CREATE_CHILD_SA requests within an IKE SA.  If an SA
   has expired or is about to expire and rekeying attempts using the
   mechanisms described here fail, an implementation MUST close the IKE
   SA and any associated Child SAs and then MAY start new ones.
   Implementations may wish to support in-place rekeying of SAs, since
   doing so offers better performance and is likely to reduce the number
   of packets lost during the transition.

   To rekey a Child SA within an existing IKE SA, create a new,
   equivalent SA (see Section 2.17 below), and when the new one is
   established, delete the old one.  Note that, when rekeying, the new
   Child SA SHOULD NOT have different Traffic Selectors and algorithms
   than the old one.

   To rekey an IKE SA, establish a new equivalent IKE SA (see
   Section 2.18 below) with the peer to whom the old IKE SA is shared
   using a CREATE_CHILD_SA within the existing IKE SA.  An IKE SA so
   created inherits all of the original IKE SA's Child SAs, and the new
   IKE SA is used for all control messages needed to maintain those
   Child SAs.  After the new equivalent IKE SA is created, the initiator
   deletes the old IKE SA, and the Delete payload to delete itself MUST
   be the last request sent over the old IKE SA.

   SAs should be rekeyed proactively, i.e., the new SA should be
   established before the old one expires and becomes unusable.  Enough
   time should elapse between the time the new SA is established and the
   old one becomes unusable so that traffic can be switched over to the
   new SA.

   A difference between IKEv1 and IKEv2 is that in IKEv1 SA lifetimes
   were negotiated.  In IKEv2, each end of the SA is responsible for
   enforcing its own lifetime policy on the SA and rekeying the SA when
   necessary.  If the two ends have different lifetime policies, the end
   with the shorter lifetime will end up always being the one to request
   the rekeying.  If an SA has been inactive for a long time and if an
   endpoint would not initiate the SA in the absence of traffic, the
   endpoint MAY choose to close the SA instead of rekeying it when its
   lifetime expires.  It can also do so if there has been no traffic
   since the last time the SA was rekeyed.

   Note that IKEv2 deliberately allows parallel SAs with the same
   Traffic Selectors between common endpoints.  One of the purposes of
   this is to support traffic quality of service (QoS) differences among
   the SAs (see [DIFFSERVFIELD], [DIFFSERVARCH], and Section 4.1 of
   [DIFFTUNNEL]).  Hence unlike IKEv1, the combination of the endpoints
   and the Traffic Selectors may not uniquely identify an SA between
   those endpoints, so the IKEv1 rekeying heuristic of deleting SAs on
   the basis of duplicate Traffic Selectors SHOULD NOT be used.

   There are timing windows -- particularly in the presence of lost
   packets -- where endpoints may not agree on the state of an SA.  The
   responder to a CREATE_CHILD_SA MUST be prepared to accept messages on
   an SA before sending its response to the creation request, so there
   is no ambiguity for the initiator.  The initiator MAY begin sending
   on an SA as soon as it processes the response.  The initiator,
   however, cannot receive on a newly created SA until it receives and
   processes the response to its CREATE_CHILD_SA request.  How, then, is
   the responder to know when it is OK to send on the newly created SA?

   From a technical correctness and interoperability perspective, the
   responder MAY begin sending on an SA as soon as it sends its response
   to the CREATE_CHILD_SA request.  In some situations, however, this
   could result in packets unnecessarily being dropped, so an
   implementation MAY defer such sending.

   The responder can be assured that the initiator is prepared to
   receive messages on an SA if either (1) it has received a
   cryptographically valid message on the other half of the SA pair, or
   (2) the new SA rekeys an existing SA and it receives an IKE request
   to close the replaced SA.  When rekeying an SA, the responder
   continues to send traffic on the old SA until one of those events
   occurs.  When establishing a new SA, the responder MAY defer sending
   messages on a new SA until either it receives one or a timeout has
   occurred.  If an initiator receives a message on an SA for which it
   has not received a response to its CREATE_CHILD_SA request, it
   interprets that as a likely packet loss and retransmits the
   CREATE_CHILD_SA request.  An initiator MAY send a dummy ESP message
   on a newly created ESP SA if it has no messages queued in order to
   assure the responder that the initiator is ready to receive messages.

2.8.1.  Simultaneous Child SA Rekeying

   If the two ends have the same lifetime policies, it is possible that
   both will initiate a rekeying at the same time (which will result in
   redundant SAs).  To reduce the probability of this happening, the
   timing of rekeying requests SHOULD be jittered (delayed by a random
   amount of time after the need for rekeying is noticed).

   This form of rekeying may temporarily result in multiple similar SAs
   between the same pairs of nodes.  When there are two SAs eligible to
   receive packets, a node MUST accept incoming packets through either
   SA.  If redundant SAs are created though such a collision, the SA
   created with the lowest of the four nonces used in the two exchanges
   SHOULD be closed by the endpoint that created it.  "Lowest" means an
   octet-by-octet comparison (instead of, for instance, comparing the
   nonces as large integers).  In other words, start by comparing the
   first octet; if they're equal, move to the next octet, and so on.  If
   you reach the end of one nonce, that nonce is the lower one.  The
   node that initiated the surviving rekeyed SA should delete the
   replaced SA after the new one is established.

   The following is an explanation on the impact this has on
   implementations.  Assume that hosts A and B have an existing Child SA
   pair with SPIs (SPIa1,SPIb1), and both start rekeying it at the same
   time:

   Host A                            Host B
   -------------------------------------------------------------------
   send req1: N(REKEY_SA,SPIa1),
       SA(..,SPIa2,..),Ni1,..  -->
                                <--  send req2: N(REKEY_SA,SPIb1),
                                         SA(..,SPIb2,..),Ni2
   recv req2 <--

   At this point, A knows there is a simultaneous rekeying happening.
   However, it cannot yet know which of the exchanges will have the
   lowest nonce, so it will just note the situation and respond as
   usual.

   send resp2: SA(..,SPIa3,..),
        Nr1,..  -->
                                -->  recv req1

   Now B also knows that simultaneous rekeying is going on.  It responds
   as usual.

                               <--  send resp1: SA(..,SPIb3,..),
                                        Nr2,..
   recv resp1 <--
                               -->  recv resp2

   At this point, there are three Child SA pairs between A and B (the
   old one and two new ones).  A and B can now compare the nonces.
   Suppose that the lowest nonce was Nr1 in message resp2; in this case,
   B (the sender of req2) deletes the redundant new SA, and A (the node
   that initiated the surviving rekeyed SA), deletes the old one.

   send req3: D(SPIa1) -->
                                <--  send req4: D(SPIb2)
                                -->  recv req3
                                <--  send resp3: D(SPIb1)
   recv req4 <--
   send resp4: D(SPIa3) -->

   The rekeying is now finished.

   However, there is a second possible sequence of events that can
   happen if some packets are lost in the network, resulting in
   retransmissions.  The rekeying begins as usual, but A's first packet
   (req1) is lost.

   Host A                            Host B
   -------------------------------------------------------------------
   send req1: N(REKEY_SA,SPIa1),
       SA(..,SPIa2,..),
       Ni1,..  -->  (lost)
                                <--  send req2: N(REKEY_SA,SPIb1),
                                         SA(..,SPIb2,..),Ni2
   recv req2 <--
   send resp2: SA(..,SPIa3,..),
       Nr1,.. -->
                                -->  recv resp2
                                <--  send req3: D(SPIb1)
   recv req3 <--
   send resp3: D(SPIa1) -->
                                -->  recv resp3

   From B's point of view, the rekeying is now completed, and since it
   has not yet received A's req1, it does not even know that there was
   simultaneous rekeying.  However, A will continue retransmitting the
   message, and eventually it will reach B.

   resend req1 -->
                                -->  recv req1

   To B, it looks like A is trying to rekey an SA that no longer exists;
   thus, B responds to the request with something non-fatal such as
   CHILD_SA_NOT_FOUND.

                                <--  send resp1: N(CHILD_SA_NOT_FOUND)
   recv resp1 <--

   When A receives this error, it already knows there was simultaneous
   rekeying, so it can ignore the error message.

2.8.2.  Simultaneous IKE SA Rekeying

   Probably the most complex case occurs when both peers try to rekey
   the IKE_SA at the same time.  Basically, the text in Section 2.8
   applies to this case as well; however, it is important to ensure that
   the Child SAs are inherited by the correct IKE_SA.

   The case where both endpoints notice the simultaneous rekeying works
   the same way as with Child SAs.  After the CREATE_CHILD_SA exchanges,
   three IKE SAs exist between A and B: the old IKE SA and two new IKE
   SAs.  The new IKE SA containing the lowest nonce SHOULD be deleted by
   the node that created it, and the other surviving new IKE SA MUST
   inherit all the Child SAs.

   In addition to normal simultaneous rekeying cases, there is a special
   case where one peer finishes its rekey before it even notices that
   other peer is doing a rekey.  If only one peer detects a simultaneous
   rekey, redundant SAs are not created.  In this case, when the peer
   that did not notice the simultaneous rekey gets the request to rekey
   the IKE SA that it has already successfully rekeyed, it SHOULD return
   TEMPORARY_FAILURE because it is an IKE SA that it is currently trying
   to close (whether or not it has already sent the delete notification
   for the SA).  If the peer that did notice the simultaneous rekey gets
   the delete request from the other peer for the old IKE SA, it knows
   that the other peer did not detect the simultaneous rekey, and the
   first peer can forget its own rekey attempt.

   Host A                      Host B
   -------------------------------------------------------------------
   send req1:
        SA(..,SPIa1,..),Ni1,.. -->
                             <-- send req2: SA(..,SPIb1,..),Ni2,..
                             --> recv req1
                             <-- send resp1: SA(..,SPIb2,..),Nr2,..
   recv resp1 <--
   send req3: D() -->
                             --> recv req3

   At this point, host B sees a request to close the IKE_SA.  There's
   not much more to do than to reply as usual.  However, at this point
   host B should stop retransmitting req2, since once host A receives
   resp3, it will delete all the state associated with the old IKE_SA
   and will not be able to reply to it.

                             <-- send resp3: ()

   The TEMPORARY_FAILURE notification was not included in RFC 4306, and
   support of the TEMPORARY_FAILURE notification is not negotiated.

   Thus, older peers that implement RFC 4306 but not this document may
   receive these notifications.  In that case, they will treat it the
   same as any other unknown error notification, and will stop the
   exchange.  Because the other peer has already rekeyed the exchange,
   doing so does not have any ill effects.

2.8.3.  Rekeying the IKE SA versus Reauthentication

   Rekeying the IKE SA and reauthentication are different concepts in
   IKEv2.  Rekeying the IKE SA establishes new keys for the IKE SA and
   resets the Message ID counters, but it does not authenticate the
   parties again (no AUTH or EAP payloads are involved).

   Although rekeying the IKE SA may be important in some environments,
   reauthentication (the verification that the parties still have access
   to the long-term credentials) is often more important.

   IKEv2 does not have any special support for reauthentication.
   Reauthentication is done by creating a new IKE SA from scratch (using
   IKE_SA_INIT/IKE_AUTH exchanges, without any REKEY_SA Notify
   payloads), creating new Child SAs within the new IKE SA (without
   REKEY_SA Notify payloads), and finally deleting the old IKE SA (which
   deletes the old Child SAs as well).

   This means that reauthentication also establishes new keys for the
   IKE SA and Child SAs.  Therefore, while rekeying can be performed
   more often than reauthentication, the situation where "authentication
   lifetime" is shorter than "key lifetime" does not make sense.

   While creation of a new IKE SA can be initiated by either party
   (initiator or responder in the original IKE SA), the use of EAP
   and/or Configuration payloads means in practice that reauthentication
   has to be initiated by the same party as the original IKE SA.  IKEv2
   does not currently allow the responder to request reauthentication in
   this case; however, there are extensions that add this functionality
   such as [REAUTH].

2.9.  Traffic Selector Negotiation

   When an RFC4301-compliant IPsec subsystem receives an IP packet that
   matches a "protect" selector in its Security Policy Database (SPD),
   the subsystem protects that packet with IPsec.  When no SA exists
   yet, it is the task of IKE to create it.  Maintenance of a system's
   SPD is outside the scope of IKE, although some implementations might
   update their SPD in connection with the running of IKE (for an
   example scenario, see Section 1.1.3).

   Traffic Selector (TS) payloads allow endpoints to communicate some of
   the information from their SPD to their peers.  These must be
   communicated to IKE from the SPD (for example, the PF_KEY API [PFKEY]
   uses the SADB_ACQUIRE message).  TS payloads specify the selection
   criteria for packets that will be forwarded over the newly set up SA.
   This can serve as a consistency check in some scenarios to assure
   that the SPDs are consistent.  In others, it guides the dynamic
   update of the SPD.

   Two TS payloads appear in each of the messages in the exchange that
   creates a Child SA pair.  Each TS payload contains one or more
   Traffic Selectors.  Each Traffic Selector consists of an address
   range (IPv4 or IPv6), a port range, and an IP protocol ID.

   The first of the two TS payloads is known as TSi (Traffic Selector-
   initiator).  The second is known as TSr (Traffic Selector-responder).
   TSi specifies the source address of traffic forwarded from (or the
   destination address of traffic forwarded to) the initiator of the
   Child SA pair.  TSr specifies the destination address of the traffic
   forwarded to (or the source address of the traffic forwarded from)
   the responder of the Child SA pair.  For example, if the original
   initiator requests the creation of a Child SA pair, and wishes to
   tunnel all traffic from subnet 198.51.100.* on the initiator's side
   to subnet 192.0.2.* on the responder's side, the initiator would
   include a single Traffic Selector in each TS payload.  TSi would
   specify the address range (198.51.100.0 - 198.51.100.255) and TSr
   would specify the address range (192.0.2.0 - 192.0.2.255).  Assuming
   that proposal was acceptable to the responder, it would send
   identical TS payloads back.

   IKEv2 allows the responder to choose a subset of the traffic proposed
   by the initiator.  This could happen when the configurations of the
   two endpoints are being updated but only one end has received the new
   information.  Since the two endpoints may be configured by different
   people, the incompatibility may persist for an extended period even
   in the absence of errors.  It also allows for intentionally different
   configurations, as when one end is configured to tunnel all addresses
   and depends on the other end to have the up-to-date list.

   When the responder chooses a subset of the traffic proposed by the
   initiator, it narrows the Traffic Selectors to some subset of the
   initiator's proposal (provided the set does not become the null set).
   If the type of Traffic Selector proposed is unknown, the responder
   ignores that Traffic Selector, so that the unknown type is not
   returned in the narrowed set.

   To enable the responder to choose the appropriate range in this case,
   if the initiator has requested the SA due to a data packet, the
   initiator SHOULD include as the first Traffic Selector in each of TSi
   and TSr a very specific Traffic Selector including the addresses in
   the packet triggering the request.  In the example, the initiator
   would include in TSi two Traffic Selectors: the first containing the
   address range (198.51.100.43 - 198.51.100.43) and the source port and
   IP protocol from the packet and the second containing (198.51.100.0 -
   198.51.100.255) with all ports and IP protocols.  The initiator would
   similarly include two Traffic Selectors in TSr.  If the initiator
   creates the Child SA pair not in response to an arriving packet, but
   rather, say, upon startup, then there may be no specific addresses
   the initiator prefers for the initial tunnel over any other.  In that
   case, the first values in TSi and TSr can be ranges rather than
   specific values.

   The responder performs the narrowing as follows:

   o  If the responder's policy does not allow it to accept any part of
      the proposed Traffic Selectors, it responds with a TS_UNACCEPTABLE
      Notify message.

   o  If the responder's policy allows the entire set of traffic covered
      by TSi and TSr, no narrowing is necessary, and the responder can
      return the same TSi and TSr values.

   o  If the responder's policy allows it to accept the first selector
      of TSi and TSr, then the responder MUST narrow the Traffic
      Selectors to a subset that includes the initiator's first choices.
      In this example above, the responder might respond with TSi being
      (198.51.100.43 - 198.51.100.43) with all ports and IP protocols.

   o  If the responder's policy does not allow it to accept the first
      selector of TSi and TSr, the responder narrows to an acceptable
      subset of TSi and TSr.

   When narrowing is done, there may be several subsets that are
   acceptable but their union is not.  In this case, the responder
   arbitrarily chooses one of them, and MAY include an
   ADDITIONAL_TS_POSSIBLE notification in the response.  The
   ADDITIONAL_TS_POSSIBLE notification asserts that the responder
   narrowed the proposed Traffic Selectors but that other Traffic
   Selectors would also have been acceptable, though only in a separate
   SA.  There is no data associated with this Notify type.  This case
   will occur only when the initiator and responder are configured
   differently from one another.  If the initiator and responder agree
   on the granularity of tunnels, the initiator will never request a
   tunnel wider than the responder will accept.

   It is possible for the responder's policy to contain multiple smaller
   ranges, all encompassed by the initiator's Traffic Selector, and with
   the responder's policy being that each of those ranges should be sent
   over a different SA.  Continuing the example above, the responder
   might have a policy of being willing to tunnel those addresses to and
   from the initiator, but might require that each address pair be on a
   separately negotiated Child SA.  If the initiator didn't generate its
   request based on the packet, but (for example) upon startup, there
   would not be the very specific first Traffic Selectors helping the
   responder to select the correct range.  There would be no way for the
   responder to determine which pair of addresses should be included in
   this tunnel, and it would have to make a guess or reject the request
   with a SINGLE_PAIR_REQUIRED Notify message.

   The SINGLE_PAIR_REQUIRED error indicates that a CREATE_CHILD_SA
   request is unacceptable because its sender is only willing to accept
   Traffic Selectors specifying a single pair of addresses.  The
   requestor is expected to respond by requesting an SA for only the
   specific traffic it is trying to forward.

   Few implementations will have policies that require separate SAs for
   each address pair.  Because of this, if only some parts of the TSi
   and TSr proposed by the initiator are acceptable to the responder,
   responders SHOULD narrow the selectors to an acceptable subset rather
   than use SINGLE_PAIR_REQUIRED.

2.9.1.  Traffic Selectors Violating Own Policy

   When creating a new SA, the initiator needs to avoid proposing
   Traffic Selectors that violate its own policy.  If this rule is not
   followed, valid traffic may be dropped.  If you use decorrelated
   policies from [IPSECARCH], this kind of policy violations cannot
   happen.

   This is best illustrated by an example.  Suppose that host A has a
   policy whose effect is that traffic to 198.51.100.66 is sent via host
   B encrypted using AES, and traffic to all other hosts in
   198.51.100.0/24 is also sent via B, but must use 3DES.  Suppose also
   that host B accepts any combination of AES and 3DES.

   If host A now proposes an SA that uses 3DES, and includes TSr
   containing (198.51.100.0-198.51.100.255), this will be accepted by
   host B.  Now, host B can also use this SA to send traffic from
   198.51.100.66, but those packets will be dropped by A since it
   requires the use of AES for this traffic.  Even if host A creates a
   new SA only for 198.51.100.66 that uses AES, host B may freely
   continue to use the first SA for the traffic.  In this situation,

   when proposing the SA, host A should have followed its own policy,
   and included a TSr containing ((198.51.100.0-
   198.51.100.65),(198.51.100.67-198.51.100.255)) instead.

   In general, if (1) the initiator makes a proposal "for traffic X
   (TSi/TSr), do SA", and (2) for some subset X' of X, the initiator
   does not actually accept traffic X' with SA, and (3) the initiator
   would be willing to accept traffic X' with some SA' (!=SA), valid
   traffic can be unnecessarily dropped since the responder can apply
   either SA or SA' to traffic X'.

2.10.  Nonces

   The IKE_SA_INIT messages each contain a nonce.  These nonces are used
   as inputs to cryptographic functions.  The CREATE_CHILD_SA request
   and the CREATE_CHILD_SA response also contain nonces.  These nonces
   are used to add freshness to the key derivation technique used to
   obtain keys for Child SA, and to ensure creation of strong
   pseudorandom bits from the Diffie-Hellman key.  Nonces used in IKEv2
   MUST be randomly chosen, MUST be at least 128 bits in size, and MUST
   be at least half the key size of the negotiated pseudorandom function
   (PRF).  However, the initiator chooses the nonce before the outcome
   of the negotiation is known.  Because of that, the nonce has to be
   long enough for all the PRFs being proposed.  If the same random
   number source is used for both keys and nonces, care must be taken to
   ensure that the latter use does not compromise the former.

2.11.  Address and Port Agility

   IKE runs over UDP ports 500 and 4500, and implicitly sets up ESP and
   AH associations for the same IP addresses over which it runs.  The IP
   addresses and ports in the outer header are, however, not themselves
   cryptographically protected, and IKE is designed to work even through
   Network Address Translation (NAT) boxes.  An implementation MUST
   accept incoming requests even if the source port is not 500 or 4500,
   and MUST respond to the address and port from which the request was
   received.  It MUST specify the address and port at which the request
   was received as the source address and port in the response.  IKE
   functions identically over IPv4 or IPv6.

2.12.  Reuse of Diffie-Hellman Exponentials

   IKE generates keying material using an ephemeral Diffie-Hellman
   exchange in order to gain the property of "perfect forward secrecy".
   This means that once a connection is closed and its corresponding
   keys are forgotten, even someone who has recorded all of the data
   from the connection and gets access to all of the long-term keys of

   the two endpoints cannot reconstruct the keys used to protect the
   conversation without doing a brute force search of the session key
   space.

   Achieving perfect forward secrecy requires that when a connection is
   closed, each endpoint MUST forget not only the keys used by the
   connection but also any information that could be used to recompute
   those keys.

   Because computing Diffie-Hellman exponentials is computationally
   expensive, an endpoint may find it advantageous to reuse those
   exponentials for multiple connection setups.  There are several
   reasonable strategies for doing this.  An endpoint could choose a new
   exponential only periodically though this could result in less-than-
   perfect forward secrecy if some connection lasts for less than the
   lifetime of the exponential.  Or it could keep track of which
   exponential was used for each connection and delete the information
   associated with the exponential only when some corresponding
   connection was closed.  This would allow the exponential to be reused
   without losing perfect forward secrecy at the cost of maintaining
   more state.

   Whether and when to reuse Diffie-Hellman exponentials are private
   decisions in the sense that they will not affect interoperability.
   An implementation that reuses exponentials MAY choose to remember the
   exponential used by the other endpoint on past exchanges and if one
   is reused to avoid the second half of the calculation.  See [REUSE]
   for a security analysis of this practice and for additional security
   considerations when reusing ephemeral Diffie-Hellman keys.

2.13.  Generating Keying Material

   In the context of the IKE SA, four cryptographic algorithms are
   negotiated: an encryption algorithm, an integrity protection
   algorithm, a Diffie-Hellman group, and a pseudorandom function (PRF).
   The PRF is used for the construction of keying material for all of
   the cryptographic algorithms used in both the IKE SA and the Child
   SAs.

   We assume that each encryption algorithm and integrity protection
   algorithm uses a fixed-size key and that any randomly chosen value of
   that fixed size can serve as an appropriate key.  For algorithms that
   accept a variable-length key, a fixed key size MUST be specified as
   part of the cryptographic transform negotiated (see Section 3.3.5 for
   the definition of the Key Length transform attribute).  For
   algorithms for which not all values are valid keys (such as DES or
   3DES with key parity), the algorithm by which keys are derived from
   arbitrary values MUST be specified by the cryptographic transform.

   For integrity protection functions based on Hashed Message
   Authentication Code (HMAC), the fixed key size is the size of the
   output of the underlying hash function.

   It is assumed that PRFs accept keys of any length, but have a
   preferred key size.  The preferred key size MUST be used as the
   length of SK_d, SK_pi, and SK_pr (see Section 2.14).  For PRFs based
   on the HMAC construction, the preferred key size is equal to the
   length of the output of the underlying hash function.  Other types of
   PRFs MUST specify their preferred key size.

   Keying material will always be derived as the output of the
   negotiated PRF algorithm.  Since the amount of keying material needed
   may be greater than the size of the output of the PRF, the PRF is
   used iteratively.  The term "prf+" describes a function that outputs
   a pseudorandom stream based on the inputs to a pseudorandom function
   called "prf".

   In the following, | indicates concatenation. prf+ is defined as:

   prf+ (K,S) = T1 | T2 | T3 | T4 | ...

   where:
   T1 = prf (K, S | 0x01)
   T2 = prf (K, T1 | S | 0x02)
   T3 = prf (K, T2 | S | 0x03)
   T4 = prf (K, T3 | S | 0x04)
   ...

   This continues until all the material needed to compute all required
   keys has been output from prf+.  The keys are taken from the output
   string without regard to boundaries (e.g., if the required keys are a
   256-bit Advanced Encryption Standard (AES) key and a 160-bit HMAC
   key, and the prf function generates 160 bits, the AES key will come
   from T1 and the beginning of T2, while the HMAC key will come from
   the rest of T2 and the beginning of T3).

   The constant concatenated to the end of each prf function is a single
   octet.  The prf+ function is not defined beyond 255 times the size of
   the prf function output.

2.14.  Generating Keying Material for the IKE SA

   The shared keys are computed as follows.  A quantity called SKEYSEED
   is calculated from the nonces exchanged during the IKE_SA_INIT
   exchange and the Diffie-Hellman shared secret established during that
   exchange.  SKEYSEED is used to calculate seven other secrets: SK_d
   used for deriving new keys for the Child SAs established with this

   IKE SA; SK_ai and SK_ar used as a key to the integrity protection
   algorithm for authenticating the component messages of subsequent
   exchanges; SK_ei and SK_er used for encrypting (and of course
   decrypting) all subsequent exchanges; and SK_pi and SK_pr, which are
   used when generating an AUTH payload.  The lengths of SK_d, SK_pi,
   and SK_pr MUST be the preferred key length of the PRF agreed upon.

   SKEYSEED and its derivatives are computed as follows:

   SKEYSEED = prf(Ni | Nr, g^ir)

   {SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr }
                   = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr )

   (indicating that the quantities SK_d, SK_ai, SK_ar, SK_ei, SK_er,
   SK_pi, and SK_pr are taken in order from the generated bits of the
   prf+). g^ir is the shared secret from the ephemeral Diffie-Hellman
   exchange. g^ir is represented as a string of octets in big endian
   order padded with zeros if necessary to make it the length of the
   modulus.  Ni and Nr are the nonces, stripped of any headers.  For
   historical backward-compatibility reasons, there are two PRFs that
   are treated specially in this calculation.  If the negotiated PRF is
   AES-XCBC-PRF-128 [AESXCBCPRF128] or AES-CMAC-PRF-128 [AESCMACPRF128],
   only the first 64 bits of Ni and the first 64 bits of Nr are used in
   calculating SKEYSEED, but all the bits are used for input to the prf+
   function.

   The two directions of traffic flow use different keys.  The keys used
   to protect messages from the original initiator are SK_ai and SK_ei.
   The keys used to protect messages in the other direction are SK_ar
   and SK_er.

2.15.  Authentication of the IKE SA

   When not using extensible authentication (see Section 2.16), the
   peers are authenticated by having each sign (or MAC using a padded
   shared secret as the key, as described later in this section) a block
   of data.  In these calculations, IDi' and IDr' are the entire ID
   payloads excluding the fixed header.  For the responder, the octets
   to be signed start with the first octet of the first SPI in the
   header of the second message (IKE_SA_INIT response) and end with the
   last octet of the last payload in the second message.  Appended to
   this (for the purposes of computing the signature) are the
   initiator's nonce Ni (just the value, not the payload containing it),
   and the value prf(SK_pr, IDr').  Note that neither the nonce Ni nor
   the value prf(SK_pr, IDr') are transmitted.  Similarly, the initiator
   signs the first message (IKE_SA_INIT request), starting with the
   first octet of the first SPI in the header and ending with the last

   octet of the last payload.  Appended to this (for purposes of
   computing the signature) are the responder's nonce Nr, and the value
   prf(SK_pi, IDi').  It is critical to the security of the exchange
   that each side sign the other side's nonce.

   The initiator's signed octets can be described as:

   InitiatorSignedOctets = RealMessage1 | NonceRData | MACedIDForI
   GenIKEHDR = [ four octets 0 if using port 4500 ] | RealIKEHDR
   RealIKEHDR =  SPIi | SPIr |  . . . | Length
   RealMessage1 = RealIKEHDR | RestOfMessage1
   NonceRPayload = PayloadHeader | NonceRData
   InitiatorIDPayload = PayloadHeader | RestOfInitIDPayload
   RestOfInitIDPayload = IDType | RESERVED | InitIDData
   MACedIDForI = prf(SK_pi, RestOfInitIDPayload)

   The responder's signed octets can be described as:

   ResponderSignedOctets = RealMessage2 | NonceIData | MACedIDForR
   GenIKEHDR = [ four octets 0 if using port 4500 ] | RealIKEHDR
   RealIKEHDR =  SPIi | SPIr |  . . . | Length
   RealMessage2 = RealIKEHDR | RestOfMessage2
   NonceIPayload = PayloadHeader | NonceIData
   ResponderIDPayload = PayloadHeader | RestOfRespIDPayload
   RestOfRespIDPayload = IDType | RESERVED | RespIDData
   MACedIDForR = prf(SK_pr, RestOfRespIDPayload)

   Note that all of the payloads are included under the signature,
   including any payload types not defined in this document.  If the
   first message of the exchange is sent multiple times (such as with a
   responder cookie and/or a different Diffie-Hellman group), it is the
   latest version of the message that is signed.

   Optionally, messages 3 and 4 MAY include a certificate, or
   certificate chain providing evidence that the key used to compute a
   digital signature belongs to the name in the ID payload.  The
   signature or MAC will be computed using algorithms dictated by the
   type of key used by the signer, and specified by the Auth Method
   field in the Authentication payload.  There is no requirement that
   the initiator and responder sign with the same cryptographic
   algorithms.  The choice of cryptographic algorithms depends on the
   type of key each has.  In particular, the initiator may be using a
   shared key while the responder may have a public signature key and
   certificate.  It will commonly be the case (but it is not required)
   that, if a shared secret is used for authentication, the same key is
   used in both directions.

   Note that it is a common but typically insecure practice to have a
   shared key derived solely from a user-chosen password without
   incorporating another source of randomness.  This is typically
   insecure because user-chosen passwords are unlikely to have
   sufficient unpredictability to resist dictionary attacks and these
   attacks are not prevented in this authentication method.
   (Applications using password-based authentication for bootstrapping
   and IKE SA should use the authentication method in Section 2.16,
   which is designed to prevent off-line dictionary attacks.)  The pre-
   shared key needs to contain as much unpredictability as the strongest
   key being negotiated.  In the case of a pre-shared key, the AUTH
   value is computed as:

   For the initiator:
      AUTH = prf( prf(Shared Secret, "Key Pad for IKEv2"),
                       <InitiatorSignedOctets>)
   For the responder:
      AUTH = prf( prf(Shared Secret, "Key Pad for IKEv2"),
                       <ResponderSignedOctets>)

   where the string "Key Pad for IKEv2" is 17 ASCII characters without
   null termination.  The shared secret can be variable length.  The pad
   string is added so that if the shared secret is derived from a
   password, the IKE implementation need not store the password in
   cleartext, but rather can store the value prf(Shared Secret,"Key Pad
   for IKEv2"), which could not be used as a password equivalent for
   protocols other than IKEv2.  As noted above, deriving the shared
   secret from a password is not secure.  This construction is used
   because it is anticipated that people will do it anyway.  The
   management interface by which the shared secret is provided MUST
   accept ASCII strings of at least 64 octets and MUST NOT add a null
   terminator before using them as shared secrets.  It MUST also accept
   a hex encoding of the shared secret.  The management interface MAY
   accept other encodings if the algorithm for translating the encoding
   to a binary string is specified.

   There are two types of EAP authentication (described in
   Section 2.16), and each type uses different values in the AUTH
   computations shown above.  If the EAP method is key-generating,
   substitute master session key (MSK) for the shared secret in the
   computation.  For non-key-generating methods, substitute SK_pi and
   SK_pr, respectively, for the shared secret in the two AUTH
   computations.