RFC 7296
Internet Key Exchange Protocol Version 2 (IKEv2)
Pages: 142
Internet Standard: 79
→ Errata
Obsoletes: 5996
Updated by: 7427 7670 8247 8983 9370
Internet Standard: 79
→ Errata
Obsoletes: 5996
Updated by: 7427 7670 8247 8983 9370
Part 2 of 6 – Pages 17 to 46
1.4. The INFORMATIONAL Exchange
At various points during the operation of an IKE SA, peers may desire to convey control messages to each other regarding errors or notifications of certain events. To accomplish this, IKE defines an INFORMATIONAL exchange. INFORMATIONAL exchanges MUST ONLY occur after the initial exchanges and are cryptographically protected with the negotiated keys. Note that some informational messages, not exchanges, can be sent outside the context of an IKE SA. Section 2.21 also covers error messages in great detail. Control messages that pertain to an IKE SA MUST be sent under that IKE SA. Control messages that pertain to Child SAs MUST be sent under the protection of the IKE SA that generated them (or its successor if the IKE SA was rekeyed). Messages in an INFORMATIONAL exchange contain zero or more Notification, Delete, and Configuration payloads. The recipient of an INFORMATIONAL exchange request MUST send some response; otherwise, the sender will assume the message was lost in the network and will
retransmit it. That response MAY be an empty message. The request
message in an INFORMATIONAL exchange MAY also contain no payloads.
This is the expected way an endpoint can ask the other endpoint to
verify that it is alive.
The INFORMATIONAL exchange is defined as:
Initiator Responder
-------------------------------------------------------------------
HDR, SK {[N,] [D,]
[CP,] ...} -->
<-- HDR, SK {[N,] [D,]
[CP,] ...}
The processing of an INFORMATIONAL exchange is determined by its
component payloads.
1.4.1. Deleting an SA with INFORMATIONAL Exchanges
ESP and AH SAs always exist in pairs, with one SA in each direction.
When an SA is closed, both members of the pair MUST be closed (that
is, deleted). Each endpoint MUST close its incoming SAs and allow
the other endpoint to close the other SA in each pair. To delete an
SA, an INFORMATIONAL exchange with one or more Delete payloads is
sent listing the SPIs (as they would be expected in the headers of
inbound packets) of the SAs to be deleted. The recipient MUST close
the designated SAs. Note that one never sends Delete payloads for
the two sides of an SA in a single message. If there are many SAs to
delete at the same time, one includes Delete payloads for the inbound
half of each SA pair in the INFORMATIONAL exchange.
Normally, the response in the INFORMATIONAL exchange will contain
Delete payloads for the paired SAs going in the other direction.
There is one exception. If, by chance, both ends of a set of SAs
independently decide to close them, each may send a Delete payload
and the two requests may cross in the network. If a node receives a
delete request for SAs for which it has already issued a delete
request, it MUST delete the outgoing SAs while processing the request
and the incoming SAs while processing the response. In that case,
the responses MUST NOT include Delete payloads for the deleted SAs,
since that would result in duplicate deletion and could in theory
delete the wrong SA.
Similar to ESP and AH SAs, IKE SAs are also deleted by sending an
INFORMATIONAL exchange. Deleting an IKE SA implicitly closes any
remaining Child SAs negotiated under it. The response to a request
that deletes the IKE SA is an empty INFORMATIONAL response.
Half-closed ESP or AH connections are anomalous, and a node with auditing capability should probably audit their existence if they persist. Note that this specification does not specify time periods, so it is up to individual endpoints to decide how long to wait. A node MAY refuse to accept incoming data on half-closed connections but MUST NOT unilaterally close them and reuse the SPIs. If connection state becomes sufficiently messed up, a node MAY close the IKE SA, as described above. It can then rebuild the SAs it needs on a clean base under a new IKE SA.1.5. Informational Messages outside of an IKE SA
There are some cases in which a node receives a packet that it cannot process, but it may want to notify the sender about this situation. o If an ESP or AH packet arrives with an unrecognized SPI. This might be due to the receiving node having recently crashed and lost state, or because of some other system malfunction or attack. o If an encrypted IKE request packet arrives on port 500 or 4500 with an unrecognized IKE SPI. This might be due to the receiving node having recently crashed and lost state, or because of some other system malfunction or attack. o If an IKE request packet arrives with a higher major version number than the implementation supports. In the first case, if the receiving node has an active IKE SA to the IP address from whence the packet came, it MAY send an INVALID_SPI notification of the wayward packet over that IKE SA in an INFORMATIONAL exchange. The Notification Data contains the SPI of the invalid packet. The recipient of this notification cannot tell whether the SPI is for AH or ESP, but this is not important because in many cases the SPIs will be different for the two. If no suitable IKE SA exists, the node MAY send an informational message without cryptographic protection to the source IP address, using the source UDP port as the destination port if the packet was UDP (UDP- encapsulated ESP or AH). In this case, it should only be used by the recipient as a hint that something might be wrong (because it could easily be forged). This message is not part of an INFORMATIONAL exchange, and the receiving node MUST NOT respond to it because doing so could cause a message loop. The message is constructed as follows: there are no IKE SPI values that would be meaningful to the recipient of such a notification; using zero values or random values are both acceptable, this being the exception to the rule in Section 3.1 that prohibits zero IKE Initiator SPIs. The Initiator
flag is set to 1, the Response flag is set to 0, and the version flags are set in the normal fashion; these flags are described in Section 3.1. In the second and third cases, the message is always sent without cryptographic protection (outside of an IKE SA), and includes either an INVALID_IKE_SPI or an INVALID_MAJOR_VERSION notification (with no notification data). The message is a response message, and thus it is sent to the IP address and port from whence it came with the same IKE SPIs and the Message ID and Exchange Type are copied from the request. The Response flag is set to 1, and the version flags are set in the normal fashion.1.6. Requirements Terminology
Definitions of the primitive terms in this document (such as Security Association or SA) can be found in [IPSECARCH]. It should be noted that parts of IKEv2 rely on some of the processing rules in [IPSECARCH], as described in various sections of this document. 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 [MUSTSHOULD].1.7. Significant Differences between RFC 4306 and RFC 5996
This document contains clarifications and amplifications to IKEv2 [IKEV2]. Many of the clarifications are based on [Clarif]. The changes listed in that document were discussed in the IPsec Working Group and, after the Working Group was disbanded, on the IPsec mailing list. That document contains detailed explanations of areas that were unclear in IKEv2, and is thus useful to implementers of IKEv2. The protocol described in this document retains the same major version number (2) and minor version number (0) as was used in RFC 4306. That is, the version number is *not* changed from RFC 4306. The small number of technical changes listed here are not expected to affect RFC 4306 implementations that have already been deployed at the time of publication of this document. This document makes the figures and references a bit more consistent than they were in [IKEV2]. IKEv2 developers have noted that the SHOULD-level requirements in RFC 4306 are often unclear in that they don't say when it is OK to not obey the requirements. They also have noted that there are MUST- level requirements that are not related to interoperability. This
document has more explanation of some of these requirements. All non-capitalized uses of the words SHOULD and MUST now mean their normal English sense, not the interoperability sense of [MUSTSHOULD]. IKEv2 (and IKEv1) developers have noted that there is a great deal of material in the tables of codes in Section 3.10.1 in RFC 4306. This leads to implementers not having all the needed information in the main body of the document. Much of the material from those tables has been moved into the associated parts of the main body of the document. This document removes discussion of nesting AH and ESP. This was a mistake in RFC 4306 caused by the lag between finishing RFC 4306 and RFC 4301. Basically, IKEv2 is based on RFC 4301, which does not include "SA bundles" that were part of RFC 2401. While a single packet can go through IPsec processing multiple times, each of these passes uses a separate SA, and the passes are coordinated by the forwarding tables. In IKEv2, each of these SAs has to be created using a separate CREATE_CHILD_SA exchange. This document removes discussion of the INTERNAL_ADDRESS_EXPIRY configuration attribute because its implementation was very problematic. Implementations that conform to this document MUST ignore proposals that have configuration attribute type 5, the old value for INTERNAL_ADDRESS_EXPIRY. This document also removed INTERNAL_IP6_NBNS as a configuration attribute. This document removes the allowance for rejecting messages in which the payloads were not in the "right" order; now implementations MUST NOT reject them. This is due to the lack of clarity where the orders for the payloads are described. The lists of items from RFC 4306 that ended up in the IANA registry were trimmed to only include items that were actually defined in RFC 4306. Also, many of those lists are now preceded with the very important instruction to developers that they really should look at the IANA registry at the time of development because new items have been added since RFC 4306. This document adds clarification on when notifications are and are not sent encrypted, depending on the state of the negotiation at the time. This document discusses more about how to negotiate combined-mode ciphers.
In Section 1.3.2, "The KEi payload SHOULD be included" was changed to be "The KEi payload MUST be included". This also led to changes in Section 2.18. In Section 2.1, there is new material covering how the initiator's SPI and/or IP is used to differentiate if this is a "half-open" IKE SA or a new request. This document clarifies the use of the critical flag in Section 2.5. In Section 2.8, "Note that, when rekeying, the new Child SA MAY have different Traffic Selectors and algorithms than the old one" was changed to "Note that, when rekeying, the new Child SA SHOULD NOT have different Traffic Selectors and algorithms than the old one". The new Section 2.8.2 covers simultaneous IKE SA rekeying. This document adds the restriction in Section 2.13 that all pseudorandom functions (PRFs) used with IKEv2 MUST take variable- sized keys. This should not affect any implementations because there were no standardized PRFs that have fixed-size keys. Section 2.18 requires doing a Diffie-Hellman exchange when rekeying the IKE_SA. In theory, RFC 4306 allowed a policy where the Diffie- Hellman exchange was optional, but this was not useful (or appropriate) when rekeying the IKE_SA. Section 2.21 has been greatly expanded to cover the different cases where error responses are needed and the appropriate responses to them. Section 2.23 clarified that, in NAT traversal, now both UDP- encapsulated IPsec packets and non-UDP-encapsulated IPsec packets need to be understood when receiving. Added Section 2.23.1 to describe NAT traversal when transport mode is requested. Added Section 2.25 to explain how to act when there are timing collisions when deleting and/or rekeying SAs, and two new error notifications (TEMPORARY_FAILURE and CHILD_SA_NOT_FOUND) were defined. In Section 3.6, "Implementations MUST support the "http:" scheme for hash-and-URL lookup. The behavior of other URL schemes is not currently specified, and such schemes SHOULD NOT be used in the absence of a document specifying them" was added.
In Section 3.15.3, a pointer to a new document that is related to configuration of IPv6 addresses was added. Appendix C was expanded and clarified.1.8. Differences between RFC 5996 and This Document
Clarified in the Abstract and the Introduction section that the status of this document is Internet Standard. The new Section 2.9.2 covers Traffic Selectors in rekeying. Added reference to RFC 6989 when reusing Diffie-Hellman exponentials (Section 2.12). Added name "Last Substruc" for the Proposal Substructure and Transform Substructure header (Sections 3.3.1 and 3.3.2) for the 0 (last) or 2/3 (more) field. Added reference to RFC 6989 when using groups that are not Sophie Germain Modular Exponentiation (MODP) groups (Section 3.3.2). Added reference to RFC 4945 in the Identification Payloads section (Section 3.5). Deprecated Raw RSA public keys in Section 3.6. There is new work in progress adding a more generic format for raw public keys. Fixed Sections 3.6 and 3.10 as specified in the errata for RFC 5996 (RFC Errata IDs 2707 and 3036). Added a note in the IANA Considerations section (Section 6) about deprecating the Raw RSA Key, and removed the old contents (which was already done during RFC 5996 processing). Added a note that IANA should update all references to RFC 5996 to point to this document.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 request 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. One type of DoS attack on the initiator of an IKE SA can be avoided if the initiator takes 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 response as potentially legitimate, respond to each one, 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, 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 through 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'.2.9.2. Traffic Selectors in Rekeying
Rekeying is used to replace an existing Child SA with another. If the new SA would be allowed to have a narrower set of selectors than the original, traffic that was allowed on the old SA would be dropped in the new SA, thus violating the idea of "replacing". Thus, the new SA MUST NOT have narrower selectors than the original. If the rekeyed SA would ever need to have a narrower scope than the currently used SA, that would mean that the policy was changed in a way such that the currently used SA is against the policy. In that case, the SA should have been already deleted after the policy change took effect. When the initiator attempts to rekey the Child SA, the proposed Traffic Selectors SHOULD be either the same as, or a superset of, the Traffic Selectors used in the old Child SA. That is, they would be the same as, or a superset of, the currently active (decorrelated) policy. The responder MUST NOT narrow down the Traffic Selectors narrower than the scope currently in use. Because a rekeyed SA can never have a narrower scope than the one currently in use, there is no need for the selectors from the packet, so those selectors SHOULD NOT be sent.
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