Internet Engineering Task Force (IETF) V. Smyslov

Internet Engineering Task Force (IETF) V. Smyslov
Request for Comments: 7383 ELVIS-PLUS
Category: Standards Track November 2014
ISSN: 2070-1721

Internet Key Exchange Protocol Version 2 (IKEv2) Message Fragmentation

Abstract

This document describes a way to avoid IP fragmentation of large
Internet Key Exchange Protocol version 2 (IKEv2) messages. This
allows IKEv2 messages to traverse network devices that do not allow
IP fragments to pass through.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7383.

Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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Table of Contents

1. Introduction ....................................................2
1.1. Problem Description ........................................2
1.2. Proposed Solution ..........................................3
1.3. Conventions Used in This Document ..........................4
2. Protocol Details ................................................4
2.1. Overview ...................................................4
2.2. Limitations ................................................4
2.3. Negotiation ................................................5
2.4. Using IKE Fragmentation ....................................5
2.5. Fragmenting Message ........................................6
2.5.1. Selecting Fragment Size .............................8
2.5.2. PMTU Discovery ......................................9
2.5.3. Fragmenting Messages Containing Unprotected
Payloads ...........................................11
2.6. Receiving IKE Fragment Message ............................11
2.6.1. Replay Detection and Retransmissions ...............13
3. Interaction with Other IKE Extensions ..........................14
4. Transport Considerations .......................................14
5. Security Considerations ........................................15
6. IANA Considerations ............................................16
7. References .....................................................16
7.1. Normative References ......................................16
7.2. Informative References ....................................16
Appendix A. Design Rationale ......................................19
Appendix B. Correlation between IP Datagram Size and Encrypted
Payload Content Size ..................................19
Acknowledgements ..................................................20
Author's Address ..................................................20

1. Introduction

1.1. Problem Description

The Internet Key Exchange Protocol version 2 (IKEv2), specified in
[RFC7296], uses UDP as a transport for its messages. Most IKEv2
messages are relatively small, usually below several hundred bytes.
A notable exception is the IKE_AUTH exchange, which requires fairly
large messages, up to several KB, especially when certificates are
transferred. When the IKE message size exceeds the path MTU, it gets
fragmented at the IP level. The problem is that some network
devices, specifically some NAT boxes, do not allow IP fragments to
pass through. This apparently blocks IKE communication and,
therefore, prevents peers from establishing an IPsec Security
Association (SA). Section 2 of [RFC7296] discusses the impact of IP
fragmentation on IKEv2 and acknowledges this problem.

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Widespread deployment of Carrier-Grade NATs (CGNs) introduces new
challenges. [RFC6888] describes requirements for CGNs. It states
that CGNs must comply with Section 11 of [RFC4787], which requires
NATs to support receiving IP fragments (REQ-14). In real life,
fulfillment of this requirement creates an additional burden in terms
of memory, especially for high-capacity devices used in CGNs. It was
found by people deploying IKE that more and more ISPs use equipment
that drops IP fragments, thereby violating this requirement.

Security researchers have found, and continue to find, attack vectors
that rely on IP fragmentation. For these reasons, and also as
articulated in [FRAGDROP], many network operators filter all IPv6
fragments. Also, the default behavior of many currently deployed
firewalls is to discard IPv6 fragments.

In one recent study [BLACKHOLES], two researchers utilized a
measurement network to measure fragment filtering. They sent
packets, fragmented to the minimum MTU of 1280, to 502 IPv6-enabled
and reachable probes. They found that during any given trial period,
ten percent of the probes did not receive fragmented packets.

Thus, this problem is valid for both IPv4 and IPv6 and may be caused
by either deficiency of network devices or operational choice.

1.2. Proposed Solution

The solution to the problem described in this document is to perform
fragmentation of large messages by IKEv2 itself and replace them with
a series of smaller messages. In this case, the resulting IP
datagrams will be small enough so that no fragmentation at the IP
level will take place.

The primary goal of this solution is to allow IKEv2 to operate in
environments that might block IP fragments. This goal does not
assume that IP fragmentation should be avoided completely, but only
in those cases when it interferes with IKE operations. However, this
solution could be used to avoid IP fragmentation in all situations
where fragmentation within IKE is applicable, as recommended in
Section 3.2 of [RFC5405]. Avoiding IP fragmentation would be
beneficial for IKEv2 in general. The Security Considerations section
of [RFC7296] mentions exhaustion of the IP reassembly buffers as one
of the possible attacks on the protocol. In [DOSUDPPROT], several
aspects of attacks on IKE using IP fragmentation are discussed, and
one of the defenses it proposes is to perform fragmentation within
IKE, similar to the solution described in this document.

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1.3. Conventions Used in 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 [RFC2119].

2. Protocol Details

2.1. Overview

The idea of the protocol described in this document is to split large
IKEv2 messages into a set of smaller ones, called IKE Fragment
messages. Fragmentation takes place before the original message is
encrypted and authenticated, so that each IKE Fragment message
receives individual protection. On the receiving side, IKE Fragment
messages are collected, verified, decrypted, and merged together to
get the original message before encryption. See Appendix A for
details on design rationale.

2.2. Limitations

Since IKE Fragment messages are cryptographically protected, SK_a and
SK_e must already be calculated. In general, it means that the
original message can be fragmented if and only if it contains an
Encrypted payload.

This implies that messages of the IKE_SA_INIT exchange cannot be
fragmented. In most cases, this is not a problem because IKE_SA_INIT
messages are usually small enough to avoid IP fragmentation. But in
some cases (advertising a badly structured long list of algorithms,
using large Modular Exponentiation (MODP) groups, etc.), these
messages may become fairly large and get fragmented at the IP level.
In this case, the solution described in this document will not help.

Among existing IKEv2 extensions, messages of an IKE_SESSION_RESUME
exchange, as defined in [RFC5723], cannot be fragmented either. See
Section 3 for details.

Another limitation is that the minimum size of an IP datagram bearing
an IKE Fragment message is about 100 bytes, depending on the
algorithms employed. According to [RFC0791], the minimum IPv4
datagram size that is guaranteed not to be further fragmented is
68 bytes. So, even the smallest IKE Fragment messages could be
fragmented at the IP level in some circumstances. But such extremely
small Path MTU (PMTU) sizes are very rare in real life.

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2.3. Negotiation

The initiator indicates its support for IKE fragmentation and
willingness to use it by including a Notification payload of type
IKEV2_FRAGMENTATION_SUPPORTED in the IKE_SA_INIT request message. If
the responder also supports this extension and is willing to use it,
it includes this notification in the response message.

Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni,
[N(IKEV2_FRAGMENTATION_SUPPORTED)] -->

<-- HDR, SAr1, KEr, Nr, [CERTREQ],
[N(IKEV2_FRAGMENTATION_SUPPORTED)]

The Notify payload is formatted as follows:

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Protocol ID(=0)| SPI Size (=0) | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

o Protocol ID (1 octet) - MUST be 0.

o SPI Size (1 octet) - MUST be 0, meaning no Security Parameter
Index (SPI) is present.

o Notify Message Type (2 octets) - MUST be 16430, the value assigned
for the IKEV2_FRAGMENTATION_SUPPORTED notification.

This notification contains no data.

2.4. Using IKE Fragmentation

IKE fragmentation MUST NOT be used unless both peers have indicated
their support for it. After that, it is up to the initiator of each
exchange to decide whether or not to use it. The responder usually
replies in the same form as the request message, but other
considerations might override this.

The initiator can employ various policies regarding the use of IKE
fragmentation. It might first try to send an unfragmented message
and resend it as fragmented only if no complete response is received
even after several retransmissions. Alternatively, it might choose

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to always send fragmented messages (however, see Section 3), or it
might fragment only large messages and messages that are expected to
result in large responses.

The following general guidelines apply:

o If either peer has information that a part of the transaction is
likely to be fragmented at the IP layer, causing interference with
the IKE exchange, that peer SHOULD use IKE fragmentation. This
information might be passed from a lower layer, provided by
configuration, or derived through heuristics. Examples of
heuristics are the lack of a complete response after several
retransmissions for the initiator, and receiving repeated
retransmissions of the request for the responder.

o If either peer knows that IKE fragmentation has been used in a
previous exchange in the context of the current IKE SA, that peer
SHOULD continue to use IKE fragmentation for the messages that are
larger than the current fragmentation threshold (see
Section 2.5.1).

o IKE fragmentation SHOULD NOT be used in cases where IP-layer
fragmentation of both the request and response messages is
unlikely. For example, there is no point in fragmenting liveness
check messages.

o If none of the above apply, the responder SHOULD respond in the
same form (fragmented or not) as the request message to which it
is responding. Note that the other guidelines might override this
because of information or heuristics available to the responder.

In most cases, IKE fragmentation will be used in the IKE_AUTH
exchange, especially if certificates are employed.

2.5. Fragmenting Message

Only messages that contain an Encrypted payload are subject to IKE
fragmentation. For the purpose of construction of IKE Fragment
messages, the original (unencrypted) content of the Encrypted payload
is split into chunks. The content is treated as a binary blob and is
split regardless of the boundaries of inner payloads. Each of the
resulting chunks is treated as an original content of the Encrypted
Fragment payload and is then encrypted and authenticated. Thus, the
Encrypted Fragment payload contains a chunk of the original content
of the Encrypted payload in encrypted form. The cryptographic
processing of the Encrypted Fragment payload is identical to that

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described in Section 3.14 of [RFC7296], as well as documents updating
such processing for particular algorithms or modes, such as
[RFC5282].

As is the case for the Encrypted payload, the Encrypted Fragment
payload, if present in a message, MUST be the last payload in the
message.

The Encrypted Fragment payload is denoted SKF{...}, and its payload
type is 53. This payload is also called the "Encrypted and
Authenticated Fragment" payload.

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fragment Number | Total Fragments |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initialization Vector |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Encrypted content ~
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Padding (0-255 octets) |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| | Pad Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Integrity Checksum Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Encrypted Fragment Payload

o Next Payload (1 octet) - in the very first fragment (with Fragment
Number equal to 1), this field MUST be set to the payload type of
the first inner payload (the same as for the Encrypted payload).
In the rest of the Fragment messages (with Fragment Number greater
than 1), this field MUST be set to zero.

o Fragment Number (2 octets, unsigned integer) - current Fragment
message number, starting from 1. This field MUST be less than or
equal to the next field (Total Fragments). This field MUST NOT be
zero.

o Total Fragments (2 octets, unsigned integer) - number of Fragment
messages into which the original message was divided. This field
MUST NOT be zero. With PMTU discovery, this field plays an
additional role. See Section 2.5.2 for details.

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The other fields are identical to those specified in Section 3.14 of
[RFC7296].

When prepending the IKE header to the IKE Fragment messages, it MUST
be taken intact from the original message, except for the Length and
Next Payload fields. The Length field is adjusted to reflect the
length of the IKE Fragment message being constructed, and the Next
Payload field is set to the payload type of the first payload in that
message (in most cases, it will be the Encrypted Fragment payload).
After prepending the IKE header and all payloads that possibly
precede the Encrypted payload in the original message (if any; see
Section 2.5.3), the resulting messages are sent to the peer.

Below is an example of fragmenting a message.

HDR(MID=n), SK(NextPld=PLD1) {PLD1 ... PLDN}

Original Message

HDR(MID=n), SKF(NextPld=PLD1, Frag#=1, TotalFrags=m) {...},
HDR(MID=n), SKF(NextPld=0, Frag#=2, TotalFrags=m) {...},
...
HDR(MID=n), SKF(NextPld=0, Frag#=m, TotalFrags=m) {...}

IKE Fragment Messages

2.5.1. Selecting Fragment Size

When splitting the content of an Encrypted payload into chunks, the
sender SHOULD choose their size so that the resulting IP datagrams
will be smaller than some fragmentation threshold. Implementations
may calculate the fragmentation threshold using various sources of
information.

If the sender has information about the PMTU size, it SHOULD use it.
The responder in the exchange may use the maximum size of the
received IKE Fragment message IP datagrams as a threshold when
constructing a fragmented response. Successful completion of
previous exchanges (including those exchanges that cannot employ IKE
fragmentation, e.g., IKE_SA_INIT) may be an indication that the
fragmentation threshold can be set to the size of the largest message
of those messages already sent.

Otherwise, for messages to be sent over IPv6, it is RECOMMENDED that
a value of 1280 bytes as a maximum IP datagram size be used
([RFC2460]). For messages to be sent over IPv4, it is RECOMMENDED
that a value of 576 bytes as a maximum IP datagram size be used. The

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presence of tunnels on the path may reduce these values.
Implementations may use other values if they are appropriate in the
current environment.

According to [RFC0791], the minimum IPv4 datagram size that is
guaranteed not to be further fragmented is 68 bytes, but it is
generally impossible to use such a small value for the solution
described in this document. Using 576 bytes is a compromise -- the
value is large enough for the presented solution and small enough to
avoid IP fragmentation in most situations. Several other UDP-based
protocols (Syslog, DNS, etc.) use 576 bytes as a safe low limit for
IP datagram size.

See Appendix B for correlation between IP datagram size and Encrypted
payload content size.

2.5.2. PMTU Discovery

The amount of traffic that the IKE endpoint produces during the
lifetime of an IKE SA is fairly modest -- it is usually below 100 KB
within a period of several hours. Most of this traffic consists of
relatively short messages -- usually below several hundred bytes. In
most cases, the only time when IKE endpoints exchange messages of
several KB in size is IKE SA establishment, and often each endpoint
sends exactly one such message.

For the reasons articulated above, implementing PMTU discovery in IKE
is OPTIONAL. It is believed that using the values recommended in
Section 2.5.1 as a fragmentation threshold will be sufficient in most
cases. Using these values could lead to suboptimal fragmentation,
but it is acceptable given the amount of traffic IKE produces.
Implementations may support PMTU discovery if there are good reasons
to do it (for example, if they are intended to be used in
environments where the MTU size might be less than the values listed
in Section 2.5.1).

PMTU discovery in IKE follows recommendations given in Section 10.4
of [RFC4821] with some modifications, induced by the distinctive
features of IKE listed above. The difference is that the PMTU search
is performed downward, while in [RFC4821] it is performed upward.
The reason for this change is that IKE usually sends large messages
only when the IKE SA is being established, and in many cases there is
only one such message. If the probing were performed upward, this
message would be fragmented using the smallest allowable threshold,
and usually all other messages are small enough to avoid IP
fragmentation, so continued probing would be of little value.

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It is the initiator of the exchange who performs PMTU discovery.
This is done by probing several values of fragmentation threshold.
Implementations MUST be prepared to probe in every exchange that
utilizes IKE fragmentation to deal with possible changes in path MTU
over time. While doing probes, it MUST start from larger values and
refragment the original message, using the next smaller value of the
threshold if it did not receive a response in a reasonable time after
several retransmissions. The exact number of retransmissions and
length of timeouts are not covered in this specification because they
do not affect interoperability. However, the timeout interval is
supposed to be relatively short, so that unsuccessful probes would
not delay IKE operations too much. Performing a few retries within
several seconds for each probe seems appropriate, but different
environments may require different rules. When starting a new probe,
the node MUST reset its retransmission timers so that if it employs
exponential back-off the timers will start over. After reaching the
smallest allowed value for the fragmentation threshold, an
implementation MUST continue retransmitting until the exchange either
completes or times out using some timeout interval as discussed in
Section 2.4 of [RFC7296].

PMTU discovery in IKE is supposed to be coarse-grained, i.e., it is
expected that a node will try only a few fragmentation thresholds in
order to minimize delays caused by unsuccessful probes. If path MTU
information is not yet available, the endpoint may use the link MTU
size when it starts probing. In subsequent exchanges, the node
should start with the current value of the fragmentation threshold.

If an implementation is capable of receiving ICMP error messages, it
can additionally utilize classic PMTU discovery methods, as described
in [RFC1191] and [RFC1981]. In particular, if the initiator receives
a Packet Too Big error in response to the probe, and it contains a
smaller value than the current fragmentation threshold, then the
initiator SHOULD stop retransmitting the probe and SHOULD select a
new value for the fragmentation threshold that is less than or equal
to the value from the ICMP message and meets the requirements listed
below.

In the case of PMTU discovery, the Total Fragments field is used to
distinguish between different sets of fragments, i.e., the sets that
were created by fragmenting the original message using different
fragmentation thresholds. Since the sender starts from larger
fragments and then makes them smaller, the value in the Total
Fragments field increases with each new probe. When selecting the
next smaller value for the fragmentation threshold, the sender MUST
ensure that the value in the Total Fragments field is really
increased. This requirement should not be a problem for the sender,
because PMTU discovery in IKE is supposed to be coarse-grained, so

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the difference between previous and next fragmentation thresholds
should be significant anyway. The need to distinguish between the
sets is vital for the receiver, since receiving a valid fragment from
a newer set means that it has to start the reassembly process over
and not mix fragments from different sets.

2.5.3. Fragmenting Messages Containing Unprotected Payloads

Currently, there are no IKEv2 exchanges that define messages,
containing both unprotected payloads and payloads, that are protected
by the Encrypted payload. However, IKEv2 does not prohibit such
construction. If some future IKEv2 extension defines such a message
and it needs to be fragmented, all unprotected payloads MUST be
placed in the first fragment (with the Fragment Number field equal to
1), along with the Encrypted Fragment payload, which MUST be present
in every IKE Fragment message and be the last payload in it.

Below is an example of a fragmenting message that contains both
protected and unprotected payloads.

HDR(MID=n), PLD0, SK(NextPld=PLD1) {PLD1 ... PLDN}

Original Message

HDR(MID=n), PLD0, SKF(NextPld=PLD1, Frag#=1, TotalFrags=m) {...},
HDR(MID=n), SKF(NextPld=0, Frag#=2, TotalFrags=m) {...},
...
HDR(MID=n), SKF(NextPld=0, Frag#=m, TotalFrags=m) {...}

IKE Fragment Messages

Note that the size of each IP datagram bearing IKE Fragment messages
should not exceed the fragmentation threshold, including the first
one, that contains unprotected payloads. This will reduce the size
of the Encrypted Fragment payload content in the first IKE Fragment
message to accommodate all unprotected payloads. In an extreme case,
the Encrypted Fragment payload will contain no data, but it still
must be present in the message, because only its presence allows the
receiver to determine that the sender has used IKE fragmentation.

2.6. Receiving IKE Fragment Message

The receiver identifies the IKE Fragment message by the presence of
an Encrypted Fragment payload in it. In most cases, it will be the
first and only payload in the message; however, this may not be true
for some hypothetical IKE exchanges (see Section 2.5.3).

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Upon receiving the IKE Fragment message, the following actions are
performed:

o Check message validity - in particular, check whether the values
in the Fragment Number and the Total Fragments fields in the
Encrypted Fragment payload are valid. The following tests need to
be performed.

* check that the Fragment Number and the Total Fragments fields
contain non-zero values

* check that the value in the Fragment Number field is less than
or equal to the value in the Total Fragments field

* if reassembling has already started, check that the value in
the Total Fragments field is equal to or greater than the Total
Fragments field in the fragments that have already been stored
in the reassembling queue

If any of these tests fail, the message MUST be silently
discarded.

o Check that this IKE Fragment message is new for the receiver and
not a replay. If an IKE Fragment message with the same Message
ID, Fragment Number, and Total Fragments fields is already present
in the reassembling queue, this message is considered a replay and
MUST be silently discarded.

o Verify IKE Fragment message authenticity by checking the Integrity
Check Value (ICV) in the Encrypted Fragment payload. If the ICV
check fails, the message MUST be silently discarded.

o If reassembling is not finished yet and the Total Fragments field
in the received fragment is greater than the Total Fragments field
in those fragments that are in the reassembling queue, the
receiver MUST discard all received fragments and start the
reassembly process over with just the received IKE Fragment
message.

o Store the message in the reassembling queue waiting for the rest
of the fragments to arrive.

When all IKE Fragment messages (as indicated in the Total Fragments
field) are received, the decrypted content of all Encrypted Fragment
payloads is merged together to form the content of the original
Encrypted payload and, therefore, along with the IKE header and

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unprotected payloads (if any), the original message. Then, it is
processed as if it was received, verified, and decrypted as a regular
IKE message.

If the receiver does not get all IKE fragments needed to reassemble
the original message within a timeout interval, it MUST discard all
IKE Fragment messages received so far for the exchange. The next
actions depend on the role of the receiver in the exchange.

o The initiator acts as described in Section 2.1 of [RFC7296]. It
either retransmits the fragmented request message or deems the IKE
SA to have failed and deletes it. The number of retransmits and
length of timeouts for the initiator are not covered in this
specification, since they are assumed to be the same as in a
regular IKEv2 exchange and are discussed in Section 2.4 of
[RFC7296].

o The responder in this case acts as if no request message was
received. It would delete any memory of the incomplete request
message and not treat it as an IKE SA failure. It is RECOMMENDED
that the reassembling timeout for the responder be equal to the
time interval that the implementation waits before completely
giving up when acting as the initiator of an exchange.
Section 2.4 of [RFC7296] gives recommendations for selecting this
interval. Implementations can use a shorter timeout to conserve
memory.

2.6.1. Replay Detection and Retransmissions

According to Section 2.2 of [RFC7296], the Message ID is used, in
particular, to identify retransmissions of IKE messages. Each
request or response message, sent by either side, must have a unique
Message ID, or be considered a retransmission otherwise. This logic
has already been updated by [RFC6311], which deliberately allows any
number of messages with zero Message ID. This document also updates
this logic for those situations where IKE fragmentation is in use.

If an incoming message contains an Encrypted Fragment payload, the
values of the Fragment Number and Total Fragments fields MUST be used
along with the Message ID to detect retransmissions and replays.

If the responder receives a retransmitted fragment of a request when
it has already processed that request and has sent back a response,
that event MUST only trigger a retransmission of the response message
(fragmented or not) if the Fragment Number field in the received
fragment is set to 1; otherwise, it MUST be ignored.

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3. Interaction with Other IKE Extensions

IKE fragmentation is compatible with most IKE extensions, such as IKE
Session Resumption ([RFC5723]), the Quick Crash Detection Method
([RFC6290]), and so on. It neither affects their operation nor is
affected by them. It is believed that IKE fragmentation will also be
compatible with future IKE extensions, if they follow general
principles of formatting, sending, and receiving IKE messages, as
described in [RFC7296].

When IKE fragmentation is used with IKE Session Resumption
([RFC5723]), messages of an IKE_SESSION_RESUME exchange cannot be
fragmented, since they do not contain an Encrypted payload. These
messages may be large due to the ticket size. To avoid IP
fragmentation in this situation, it is recommended that smaller
tickets be used, e.g., by utilizing a "ticket by reference" approach
instead of "ticket by value".

Protocol Support for High Availability of IKEv2/IPsec, described in
[RFC6311], requires special care when deciding whether to fragment an
IKE message or not. Since it deliberately allows any number of
synchronization exchanges to have the same Message ID, namely zero,
standard IKEv2 replay detection logic, based on checking the Message
ID, is not applicable for such messages, and the receiver has to
check message content to detect replays. When implementing IKE
fragmentation along with [RFC6311], IKE Message ID Synchronization
messages MUST NOT be sent fragmented, to simplify the receiver's task
of detecting replays. Fortunately, these messages are small, and
there is no point in fragmenting them anyway.

4. Transport Considerations

With IKE fragmentation, if any single IKE Fragment message gets lost,
the receiver becomes unable to reassemble the original message. So,
in general, using IKE fragmentation implies a higher probability that
the message will not be delivered to the peer. Although in most
network environments the difference will be insignificant, on some
lossy networks it may become noticeable. When using IKE
fragmentation, implementations MAY use longer timeouts and do more
retransmits than usual before considering the peer dead.

Note that Fragment messages are not individually acknowledged. The
response Fragment messages are all sent back together only when all
fragments of the request are received, and the original request
message is reassembled and successfully processed.

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5. Security Considerations

Most of the security considerations for IKE fragmentation are the
same as those for the base IKEv2 protocol described in [RFC7296].
This extension introduces the Encrypted Fragment payload to protect
the content of an IKE Message Fragment. This allows the receiver to
individually check the authenticity of fragments, thus protecting
peers from a DoS attack.

The Security Considerations section of [RFC7296] mentions a possible
attack on IKE where an attacker could prevent an exchange from
completing by exhausting the IP reassembly buffers. The mechanism
described in this document allows IKE to avoid IP fragmentation and
therefore increases its robustness to DoS attacks.

The following attack is possible with IKE fragmentation. An attacker
can initiate an IKE_SA_INIT exchange, complete it, compute SK_a and
SK_e, and then send a large but still incomplete set of IKE_AUTH
fragments. These fragments will pass the ICV check and will be
stored in reassembly buffers, but since the set is incomplete, the
reassembling will never succeed and eventually will time out. If the
set is large, this attack could potentially exhaust the receiver's
memory resources.

To mitigate the impact of this attack, it is RECOMMENDED that the
receiver limit the number of fragments it stores in the reassembling
queue so that the sum of the sizes of Encrypted Fragment payload
contents (after decryption) for fragments that are already placed
into the reassembling queue is less than some value that is
reasonable for the implementation. If the peer sends so many
fragments that the above condition is not met, the receiver can
consider this situation to be either an attack or a broken sender
implementation. In either case, the receiver SHOULD drop the
connection and discard all the received fragments.

This value can be predefined, can be a configurable option, or can be
calculated dynamically, depending on the receiver's memory load.
Some care should be taken when selecting this value because if it is
too small it might prevent a legitimate peer from establishing an IKE
SA if the size of messages it sends exceeds this value. It is NOT
RECOMMENDED for this value to exceed 64 KB because any IKE message
before fragmentation would likely be shorter than that.

If IKE fragments arrive in order, it is possible, but not advised,
for the receiver to parse the beginning of the message that is being
reassembled and extract the already-available payloads before the
reassembly is complete. It can be dangerous to take any action based
on the content of these payloads, because the fragments that have not

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yet been received might contain payloads that could change the
meaning of them (or could even make the whole message invalid), and
this can potentially be exploited by an attacker. It is important to
address this threat by ensuring that all the fragments are received
prior to parsing the reassembled message, as described in
Section 2.6.

6. IANA Considerations

This document defines a new payload in the "IKEv2 Payload Types"
registry:

53 Encrypted and Authenticated Fragment SKF

This document also defines a new Notify Message Type in the "IKEv2
Notify Message Types - Status Types" registry:

16430 IKEV2_FRAGMENTATION_SUPPORTED

7. References

7.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, October 2014,
<http://www.rfc-editor.org/info/rfc7296>.

[RFC6311] Singh, R., Kalyani, G., Nir, Y., Sheffer, Y., and D.
Zhang, "Protocol Support for High Availability of IKEv2/
IPsec", RFC 6311, July 2011,
<http://www.rfc-editor.org/info/rfc6311>.

7.2. Informative References

[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981, <http://www.rfc-editor.org/info/rfc791>.

[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990, <http://www.rfc-editor.org/info/rfc1191>.

[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996,
<http://www.rfc-editor.org/info/rfc1981>.

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[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998,
<http://www.rfc-editor.org/info/rfc2460>.

[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007,
<http://www.rfc-editor.org/info/rfc4787>.

[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007,
<http://www.rfc-editor.org/info/rfc4821>.

[RFC5282] Black, D. and D. McGrew, "Using Authenticated Encryption
Algorithms with the Encrypted Payload of the Internet Key
Exchange version 2 (IKEv2) Protocol", RFC 5282,
August 2008, <http://www.rfc-editor.org/info/rfc5282>.

[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405,
November 2008, <http://www.rfc-editor.org/info/rfc5405>.

[RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
January 2010, <http://www.rfc-editor.org/info/rfc5723>.

[RFC6290] Nir, Y., Wierbowski, D., Detienne, F., and P. Sethi, "A
Quick Crash Detection Method for the Internet Key Exchange
Protocol (IKE)", RFC 6290, June 2011,
<http://www.rfc-editor.org/info/rfc6290>.

[RFC6888] Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
and H. Ashida, "Common Requirements for Carrier-Grade NATs
(CGNs)", BCP 127, RFC 6888, April 2013,
<http://www.rfc-editor.org/info/rfc6888>.

[FRAGDROP] Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
M., and T. Taylor, "Why Operators Filter Fragments and
What It Implies", Work in Progress, draft-taylor-v6ops-
fragdrop-02, December 2013.

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[BLACKHOLES]
De Boer, M. and J. Bosma, "Discovering Path MTU black
holes on the Internet using RIPE Atlas", July 2012,
<http://www.nlnetlabs.nl/downloads/publications/
pmtu-black-holes-msc-thesis.pdf>.

[DOSUDPPROT]
Kaufman, C., Perlman, R., and B. Sommerfeld, "DoS
protection for UDP-based protocols", ACM Conference on
Computer and Communications Security, October 2003.

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Appendix A. Design Rationale

The simplest approach to IKE fragmentation would have been to
fragment a message that is fully formed and ready to be sent.
However, if a message got fragmented after being encrypted and
authenticated, this could make a simple DoS attack possible. The
attacker could infrequently emit forged but valid-looking fragments
into the network, and some of these fragments would be fetched by the
receiver into the reassembling queue. The receiver would not be able
to distinguish forged fragments from valid ones and would only be
able to determine that some of the received fragments were forged
after the whole message was reassembled and its authenticity check
failed.

To prevent this kind of attack and also reduce vulnerability to some
other kinds of DoS attacks, it was decided to perform fragmentation
before applying cryptographic protection to the message. In this
case, each Fragment message becomes individually encrypted and
authenticated; this allows the receiver to determine forged fragments
and not store them in the reassembling queue.

Appendix B. Correlation between IP Datagram Size and Encrypted Payload
Content Size

In the case of IPv4, the content size of the Encrypted Payload is
less than the IP datagram size by the sum of the following values:

o IPv4 header size (typically 20 bytes, up to 60 if IP options are
present)

o UDP header size (8 bytes)

o non-ESP (Encapsulating Security Payload) marker size (4 bytes if
present)

o IKE header size (28 bytes)

o Encrypted payload header size (4 bytes)

o initialization vector (IV) size (variable)

o padding and its size (at least 1 byte)

o ICV size (variable)

The sum may be estimated as 61..105 bytes + IV + ICV + padding.

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In the case of IPv6, the content size of the Encrypted Payload is
less than the IP datagram size by the sum of the following values:

o IPv6 header size (40 bytes)

o IPv6 extension headers (optional; size varies)

o UDP header size (8 bytes)

o non-ESP marker size (4 bytes if present)

o IKE header size (28 bytes)

o Encrypted payload header size (4 bytes)

o IV size (variable)

o padding and its size (at least 1 byte)

o ICV size (variable)

If no extension header is present, the sum may be estimated as
81..85 bytes + IV + ICV + padding. If extension headers are present,
the payload content size is further reduced by the sum of the size of
the extension headers. The length of each extension header can be
calculated as 8 * (Hdr Ext Len) bytes, except for the fragment
header, which is always 8 bytes in length.

Acknowledgements

The author would like to thank Tero Kivinen, Yoav Nir, Paul Wouters,
Yaron Sheffer, Joe Touch, Derek Atkins, Ole Troan, and others for
their reviews and valuable comments. Thanks to Ron Bonica for
contributing text to the Introduction section. Thanks to Paul
Hoffman and Barry Leiba for improving text clarity.

Author's Address

Valery Smyslov
ELVIS-PLUS
PO Box 81
Moscow (Zelenograd) 124460
Russian Federation

Phone: +7 495 276 0211
EMail: [email protected]

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