Internet DRAFT - draft-ietf-ipsecme-ikev2-fragmentation
draft-ietf-ipsecme-ikev2-fragmentation
Network Working Group V. Smyslov
Internet-Draft ELVIS-PLUS
Intended status: Standards Track June 10, 2014
Expires: December 12, 2014
IKEv2 Fragmentation
draft-ietf-ipsecme-ikev2-fragmentation-10
Abstract
This document describes the way to avoid IP fragmentation of large
IKEv2 messages. This allows IKEv2 messages to traverse network
devices that do not allow IP fragments to pass through.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on December 12, 2014.
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Problem description . . . . . . . . . . . . . . . . . . . 3
1.2. Proposed solution . . . . . . . . . . . . . . . . . . . . 3
1.3. Conventions Used in This Document . . . . . . . . . . . . 4
2. Protocol details . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Limitations . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Negotiation . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Using IKE Fragmentation . . . . . . . . . . . . . . . . . 6
2.5. Fragmenting Message . . . . . . . . . . . . . . . . . . . 7
2.5.1. Selecting Fragment Size . . . . . . . . . . . . . . . 9
2.5.2. PMTU Discovery . . . . . . . . . . . . . . . . . . . . 10
2.5.3. Fragmenting Messages containing unprotected
Payloads . . . . . . . . . . . . . . . . . . . . . . . 11
2.6. Receiving IKE Fragment Message . . . . . . . . . . . . . . 12
2.6.1. Replay Detection and Retransmissions . . . . . . . . . 14
3. Interaction with other IKE extensions . . . . . . . . . . . . 15
4. Transport Considerations . . . . . . . . . . . . . . . . . . . 16
5. Security Considerations . . . . . . . . . . . . . . . . . . . 17
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.1. Normative References . . . . . . . . . . . . . . . . . . . 21
8.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Design rationale . . . . . . . . . . . . . . . . . . 23
Appendix B. Correlation between IP Datagram size and
Encrypted Payload content size . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
1.1. Problem description
The Internet Key Exchange Protocol version 2 (IKEv2), specified in
[IKEv2], uses UDP as a transport for its messages. Most IKEv2
messages are relatively small, usually below several hundred bytes.
Noticeable exception is IKE_AUTH Exchange, which requires fairly
large messages, up to several kBytes, especially when certificates
are transferred. When IKE message size exceeds path MTU, it gets
fragmented by 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 IPsec SA. Section 2 of [IKEv2]
discusses the impact of IP fragmentation on IKEv2 and acknowledges
this problem.
Widespread deployment of Carrier-Grade NATs (CGN) introduces new
challenges. [RFC6888] describes requirements for CGNs. It states,
that CGNs must comply with Section 11 of [RFC4787], which requires
NAT 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 drop IP fragments, violating this requirement.
Security researchers have found and continue to find attack vectors
that rely on IP fragmentation. For these reasons, and also
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
either by deficiency of network devices or by 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, replacing them by
series of smaller messages. In this case the resulting IP Datagrams
will be small enough so that no fragmentation on IP level will take
place.
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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 it is recommended in
Section 3.2 of [RFC5405]. Avoiding IP fragmentation would be
beneficial for IKEv2 in general. Security Considerations Section of
[IKEv2] mentions exhausting of the IP reassembly buffers as one of
the possible attacks on the protocol. In the paper [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 similarly to the solution described in this
document.
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].
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2. Protocol details
2.1. Overview
The idea of the protocol is to split large IKEv2 message 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 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 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 MODP Groups, etc.) these messages may become fairly large
and get fragmented by IP level. In this case the described solution
will not help.
Among existing IKEv2 extensions, messages of IKE_SESSION_RESUME
Exchange, defined in [RFC5723], cannot be fragmented either. See
Section 3 for details.
Another limitation is that the minimal size of IP Datagram bearing
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 by IP level in
some circumstances. But such extremely small PMTU sizes are very
rare in real life.
2.3. Negotiation
The Initiator indicates its support for the IKE Fragmentation and
willingness to use it by including Notification Payload of type
IKEV2_FRAGMENTATION_SUPPORTED in IKE_SA_INIT request message. If
Responder also supports this extension and is willing to use it, it
includes this notification in response message.
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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 SPI is present.
o Notify Message Type (2 octets) - MUST be xxxxx, the value assigned
for IKEV2_FRAGMENTATION_SUPPORTED notification.
This Notification contains no data.
2.4. Using IKE Fragmentation
The 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
always to send fragmented messages (but 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
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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 the use of 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 it is
responding to. 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 for IKE
Fragmentation. For the purpose of IKE Fragment Messages construction
original (unencrypted) content of the Encrypted Payload is split into
chunks. The content is treated as a binary blob and is split
regardless of inner Payloads boundaries. Each of 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 Section 3.14 of [IKEv2],
as well as documents updating it for particular algorithms or modes,
such as [RFC5282].
The Encrypted Fragment Payload, similarly to the Encrypted 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 XXX (TBA by IANA). This payload is also called the
"Encrypted and Authenticated Fragment" payload.
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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 Payload Type of the
first inner Payload (similarly to the Encrypted Payload). In the
rest fragments MUST be set to zero.
o Fragment Number (2 octets) - current fragment 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) - number of fragments original message
was divided into. This field MUST NOT be zero. With PMTU
discovery this field plays additional role. See Section 2.5.2 for
details.
The other fields are identical to those specified in Section 3.14 of
[IKEv2].
When prepending IKE Header to the IKE Fragment Messages it MUST be
taken intact from the original message, except for the Length and the
Next Payload fields. The Length field is adjusted to reflect the
length of the constructed message and the Next Payload field is set
to the payload type of the first Payload in constructed message (in
most cases it will be Encrypted Fragment Payload). After prepending
IKE Header and all Payloads that possibly precede Encrypted Payload
in 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.
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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 content of Encrypted Payload into chunks sender SHOULD
choose their size so, that resulting IP Datagrams be smaller than
some fragmentation threshold. Implementations may calculate
fragmentation threshold using various sources of information.
If the Sender has information about PMTU size it SHOULD use it. The
Responder in the exchange may use maximum size of received IKE
Fragment Message IP Datagrams as threshold when constructing
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 fragmentation threshold
can be set to the size of the largest of already sent messages.
Otherwise for messages to be sent over IPv6 it is RECOMMENDED to use
value 1280 bytes as a maximum IP Datagram size ([RFC2460]). For
messages to be sent over IPv4 it is RECOMMENDED to use value 576
bytes as a maximum IP Datagram size. Presence of tunnels on the path
may reduce these values. Implementations may use other values if
they are appropriate in 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 small value for 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 protocol
assume the value 576 bytes as a safe low limit for IP Datagrams size
(Syslog, DNS, etc.).
See Appendix B for correlation between IP Datagram size and Encrypted
Payload content size.
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2.5.2. PMTU Discovery
The amount of traffic that IKE endpoint produces during lifetime of
IKE SA is fairly modest - usually it is below one hundred kBytes
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 kBytes 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 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 it is intended to be used in environments
where MTU size is possible to be less that values listed in
Section 2.5.1).
PMTU discovery in IKE follows recommendations given in Section 10.4
of [RFC4821] with the difference, induced by the specialties 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 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 there
would be little value to continue probing.
It is the Initiator of the exchange, who performs PMTU discovery. It
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 of path MTU
over time. While doing probes, it MUST start from larger values and
refragment original message using next smaller value of threshold if
it did not receive 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 few retries within several
seconds for each probe seems appropriate, but different environments
may require different rules. When starting new probe 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
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retransmitting until either exchange completes or times out using
timeout interval from Section 2.4 of [IKEv2].
PMTU discovery in IKE is supposed to be coarse-grained, i.e. it is
expected, that node will try only few fragmentation thresholds, in
order to minimize delays caused by unsuccessful probes. If no
information about path MTU is known yet, endpoint may start probing
from link MTU size. In the following exchanges node should start
from the current value of fragmentation threshold.
If an implementation is capable to receive ICMP error messages it can
additionally utilize classic PMTU discovery methods, described in
[RFC1191] and [RFC1981]. In particular, if the Initiator receives
Packet Too Big error in response to the probe, and it contains
smaller value, than current fragmentation threshold, then the
Initiator SHOULD stop retransmitting the probe and SHOULD select new
value for fragmentation threshold that is less than or equal to the
value from the ICMP message and meets the requirements listed below.
In case of PMTU discovery Total Fragments field is used to
distinguish between different sets of fragments, i.e. the sets that
were created by fragmenting original message using different
fragmentation thresholds. Since sender starts from larger fragments
and then make them smaller, the value in Total Fragments field
increases with each new probe. When selecting next smaller value for
fragmentation threshold, sender MUST ensure that the value in 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 difference between previous and next
fragmentation thresholds should be significant anyway. The necessity
to distinguish between the sets is vital for receiver since receiving
valid fragment from newer set means that it have to start
reassembling over and not to 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, protected by
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 Fragment Number field equal to 1),
along with Encrypted Fragment Payload, which MUST be present in every
IKE Fragment Message and be the last payload in it.
Below is an example of fragmenting message, containing both protected
and unprotected Payloads.
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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 fragmentation threshold, including the first one,
that contains unprotected Payloads. This will reduce the size of
Encrypted Fragment Payload content in the first IKE Fragment Message
to accommodate all unprotected Payloads. In extreme case Encrypted
Fragment Payload will contain no data, but it still must be present
in the message, because only its presence allows receiver to
determine that sender have 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 the only payload in the message, however this may not be
true for some hypothetical IKE exchanges (see Section 2.5.3)
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 Total
Fragments field in the fragments that have already been stored
in the reassembling queue
If any of this tests fails the message MUST be silently discarded.
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o Check, that this IKE Fragment Message is new for the receiver and
not a replay. If IKE Fragment message with the same Message ID,
same Fragment Number and same 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 ICV in
Encrypted Fragment Payload. If ICV check fails message MUST be
silently discarded.
o If reassembling is not finished yet and Total Fragments field in
received fragment is greater than this field in those fragments,
that are in the reassembling queue, receiver MUST discard all
received fragments and start reassembling over with just received
IKE Fragment Message.
o Store message in the reassembling queue waiting for the rest of
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 content of original Encrypted
Payload, and, therefore, along with IKE Header and unprotected
Payloads (if any), original message. Then it is processed as if it
was received, verified and decrypted as regular IKE message.
If 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 receiver in the exchange.
o The Initiator acts as described in Section 2.1 of [IKEv2]. It
either retransmits the fragmented request Message or deems 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 regular
IKEv2 exchange and are discussed in Section 2.4 of [IKEv2].
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. The reassembling
timeout for the Responder is RECOMMENDED to be equal to the time
interval that the implementation waits before completely giving up
when acting as Initiator of exchange. Section 2.4 of [IKEv2]
gives recommendations for selecting this interval.
Implementations can use a shorter timeout to conserve memory.
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2.6.1. Replay Detection and Retransmissions
According to [IKEv2] implementations must reject message with the
same Message ID as it has seen before (taking into consideration
Response bit). 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 the situations, when
IKE Fragmentation is in use.
If incoming message contains Encrypted Fragment Payload, the values
of Fragment Number and Total Fragments fields MUST be used along with
Message ID to detect retransmissions and replays.
If Responder receives retransmitted fragment of request when it has
already processed that request and has sent back a response, that
event MUST only trigger retransmission of the response message
(fragmented or not) if Fragment Number field in received fragment is
set to 1 and MUST be ignored otherwise.
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3. Interaction with other IKE extensions
IKE Fragmentation is compatible with most of IKE extensions, such as
IKE Session Resumption ([RFC5723]), Quick Crash Detection Method
([RFC6290]) and so on. It neither affect 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,
described in [IKEv2].
When IKE Fragmentation is used with IKE Session Resumption
([RFC5723]), messages of IKE_SESSION_RESUME Exchange cannot be
fragmented since they do not contain Encrypted Payload. These
messages may be large due to the ticket size. To avoid IP
Fragmentation in this situation it is recommended to use smaller
tickets, e.g. by utilizing "ticket by reference" approach instead of
"ticket by value".
One exception that requires a special care is Protocol Support for
High Availability of IKEv2/IPsec ([RFC6311]). 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 Message ID is not applicable for such messages, and
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
receiver's task of detecting replays. Fortunately, these messages
are small and there is no point in fragmenting them anyway.
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4. Transport Considerations
With IKE Fragmentation if any single IKE Fragment Message get lost,
receiver becomes unable to reassemble original Message. So, in
general, using IKE Fragmentation implies higher probability for the
Message not to 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 peer dead.
Note that Fragment Messages are not individually acknowledged. The
response Fragment Messages are sent back all together only when all
fragments of request are received, 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 [IKEv2]. This
extension introduces Encrypted Fragment Payload to protect content of
IKE Message Fragment. This allows receiver to individually check
authenticity of fragments, thus protecting peers from DoS attack.
Security Considerations Section of [IKEv2] mentions possible attack
on IKE by exhausting of 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 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
receiver limits the number of fragments it stores in 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 attack or as 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 legitimate peer to establish IKE SA if the
size of messages it sends exceeds this value. It is NOT RECOMMENDED
for this value to exceed 64 Kbytes 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 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 not yet received
fragments might contain payloads that could change the meaning of
them (or could even make the whole message invalid) and this can
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potentially be exploited by an attacker. It is important to address
this threat by ensuring all the fragments are received prior to parse
reassembled message, as it described in Section 2.6.
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6. IANA Considerations
This document defines new Payload in the "IKEv2 Payload Types"
registry:
<TBA> Encrypted and Authenticated Fragment SKF
This document also defines new Notify Message Types in the "Notify
Message Types - Status Types" registry:
<TBA> IKEV2_FRAGMENTATION_SUPPORTED
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7. 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.
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[IKEv2] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", draft-kivinen-ipsecme-ikev2-rfc5996bis-03 (work
in progress), April 2014.
[RFC6311] Singh, R., Kalyani, G., Nir, Y., Sheffer, Y., and D.
Zhang, "Protocol Support for High Availability of IKEv2/
IPsec", RFC 6311, July 2011.
8.2. Informative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[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.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405,
November 2008.
[RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
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January 2010.
[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.
[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.
[FRAGDROP]
Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
M., and T. Taylor, "Why Operators Filter Fragments and
What It Implies", draft-taylor-v6ops-fragdrop-02 (work in
progress), December 2013.
[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 the IKE fragmentation would have been to
fragment message that is fully formed and ready to be sent. But if
message got fragmented after being encrypted and authenticated, this
could open a possibility for a simple Denial of Service attack. The
attacker could infrequently emit forged but valid looking fragments
into the network, and some of these fragments would be fetched by
receiver into the reassembling queue. Receiver could not distinguish
forged fragments from valid ones and could only determine that some
of received fragments were forged when the whole message got
reassembled and check for its authenticity failed.
To prevent this kind of attack and also to reduce vulnerability to
some other kinds of DoS attacks it was decided to make fragmentation
before applying cryptographic protection to the message. In this
case each Fragment Message becomes individually encrypted and
authenticated, that allows receiver to determine forged fragments and
not to store them in the reassembling queue.
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Appendix B. Correlation between IP Datagram size and Encrypted Payload
content size
For IPv4 Encrypted Payload content size is less than 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 marker size (4 bytes if present)
o IKE Header size (28 bytes)
o Encrypted Payload header size (4 bytes)
o IV size (varying)
o padding and its size (at least 1 byte)
o ICV size (varying)
The sum may be estimated as 61..105 bytes + IV + ICV + padding.
For IPv6 Encrypted Payload content size is less than 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 (varying)
o padding and its size (at least 1 byte)
o ICV size (varying)
If no extension header is present, the sum may be estimated as 81..85
bytes + IV + ICV + padding. If extension headers are present, the
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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.
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Author's Address
Valery Smyslov
ELVIS-PLUS
PO Box 81
Moscow (Zelenograd) 124460
Russian Federation
Phone: +7 495 276 0211
Email: svan@elvis.ru
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