Network Working Group V. Smyslov
Internet-Draft ELVIS-PLUS
Intended status: Informational October 15, 2012
Expires: April 18, 2013
IKEv2 Fragmentation
draft-smyslov-ipsecme-ikev2-fragmentation-00
Abstract
This document describes the way to avoid IP fragmentation of large
IKEv2 messages. This allows IKEv2 messages to traverse network
devices that don't 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.
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This Internet-Draft will expire on April 18, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used in This Document . . . . . . . . . . . . 3
2. Protocol details . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Limitations . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Negotiation . . . . . . . . . . . . . . . . . . . . . . . 4
2.4. Activation . . . . . . . . . . . . . . . . . . . . . . . . 5
2.5. Fragmenting Message . . . . . . . . . . . . . . . . . . . 6
2.5.1. Fragment size . . . . . . . . . . . . . . . . . . . . 7
2.5.2. Fragmenting Messages containing unencrypted
Payloads . . . . . . . . . . . . . . . . . . . . . . . 8
2.6. Receiving IKE Fragment Message . . . . . . . . . . . . . . 9
2.6.1. Replay Protection . . . . . . . . . . . . . . . . . . 9
3. Security Considerations . . . . . . . . . . . . . . . . . . . 10
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Normative References . . . . . . . . . . . . . . . . . . . 12
5.2. Informative References . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
The Internet Key Exchange Protocol version 2 (IKEv2), specified in
[RFC5996], uses UDP as a transport for its messages. When IKE
message size exceed path MTU, it gets fragmented by IP level. The
problem is that some network devices, specifically some NAT boxes,
don't allow IP fragments to pass through. This apparently blocks IKE
communication and, therefore, prevents peers from establishing IPsec
SA.
The solution to the problem described in this document is to perform
fragmentation of large messages by IKE 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.
1.1. 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 IKE message into the set
of smaller ones, calling Fragment Messages. On the receiving side
Fragment Messages are collected and merged together to get original
message. In general this approach increases receiver's vulnerability
to Denial of Service attack. To reduce this vulnerability Fragment
Messages are individually encrypted and authenticated. This implies
that message cannot be fragmented until shared secret is calculated.
This take place once IKE_SA_INIT exchange has completed.
2.2. Limitations
In general, original message can be fragmented if and only if it
contains Encrypted Payload. That said, messages in IKE_SA_INIT
Exchange cannot be fragmented. In most cases this is not a problem,
since IKE_SA_INIT messages are usually small enough to avoid IP
fragmentations. But in some cases (advertising a badly structured
long list of algorithms, using large MODP Groups, etc.) those
messages may become fairly large and get fragmented by IP level. In
these cases the described solution won't help.
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 IP 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
Initiator MAY indicate its support for IKE Fragmentation and
willingness to use it by including Notification Payload of type
IKE_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.
Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni,
[N(IKE_FRAGMENTATION_SUPPORTED)] -->
<-- HDR, SAr1, KEr, Nr, [CERTREQ],
[N(IKE_FRAGMENTATION_SUPPORTED)]
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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 IKE_FRAGMENTATION_SUPPORTED by IANA.
This Notification contains no data.
2.4. Activation
Once support for IKE Fragmentation is negotiated, any peer MAY
activate it. Activation is performed simply by sending IKE Fragment
Messages instead of original IKE Message. Until any IKE Fragment
Message appeared on the wire, IKE Fragmentation is considered
inactive and behavior of the peers is identical to described in
[RFC5996].
Activation MUST be done by Initiator of Exchange. This is not
necessary to be Original Initiator of the IKE SA. There my be two
reasons to activate IKE Fragmentation:
o Initiator didn't receive response message after sending
retransmissions several times. In this case Initiator may suspect
that either request or response message get fragmented by IP level
and some of those fragments get lost. In this case it MAY try to
use IKE Fragmentation on further retransmissions.
o Initiator knows beforehand (probably by some administrative means)
that IKE Fragmentation is necessary to communicate with particular
peer. In this case there is no additional delay in completing
Exchange if IP fragments are dropped, but some constant overhead
is present even if no IP fragmentation takes place or IP fragments
successfully pass through.
Activation may be done in any Exchange. In most cases it will be
IKE_AUTH Exchange, because its messages may be fairly large due to
certificates inclusion. Once activated IKE Fragmentation cannot de
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deactivated until IKE SA dies.
2.5. Fragmenting Message
Sender decides to fragment outgoing message if IKE fragmentation is
active and message size exceeds some fragmentation threshold. In
some cases message may be sent as IKE Fragment Message even if its
size less than threshold. In particular, this may be necessary when
activating IKE Fragmentation. In this case it is possible that
request message reaches responder, but response message got
fragmented and doesn't reach initiator. In this case initiator need
to send IKE Fragment Message to activate IKE Fragmentation even if
original message size doesn't exceed fragmentation threshold.
Message to be fragmented MUST contain Encrypted Payload. For the
purpose of IKE Fragment Messages construction original (unencrypted)
content of Encrypted Payload is broken down into parts. It is
treated as a binary blob and is broken down regardless of inner
Payloads boundaries. Each of resulting parts is treated as a content
for Encrypted Fragment Payload.
The Encrypted Fragment Payload, denoted SKF{...}, contains other
payloads in encrypted form. The Encrypted Fragment Payload, as well
as Encrypted Payload from [RFC5996], if present in a message, MUST be
the last payload in the message.
The payload type for an Encrypted Fragment payload is XXX (TBA by
IANA).
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 |
| (length is block size for encryption algorithm) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Encrypted content ~
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Padding (0-255 octets) |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| | Pad Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Integrity Checksum Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Encrypted Fragment Payload
o Next Fragment (1 octet) - in the very first fragment MUST be set
to Payload Type of the first inner Payload (as in Encrypted
Payload). In the rest fragments MUST be set to zero.
o Fragment Number (2 octets) - current fragment number starting from
1.
o Total Fragments (2 octets) - number of fragments original message
was divided into.
Other fields are identical to those specified in Section 3.14 of
[RFC5996].
When prepending IKE Header, Length field MUST be adjusted to reflect
the length of constructed message and Next Payload field MUST reflect
payload type of the first Payload in the constructed message (that in
most cases will be Encrypted Fragment Payload). All newly
constructed messages MUST retain the same Message ID as original
message. After prepending IKE Header and possibly any of Payloads
that precedes Encrypted Payload in original message (see
Section 2.5.2), the resulting messages are sent to the peer.
Below is an example of fragmenting 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. Fragment size
When breaking content of Encrypted Payload down into parts sender
SHOULD chose size of those parts so, that resulting message sizes not
exceed fragmentation threshold - be small enough to avoid IP
fragmentation.
If sender has some knowledge about PMTU size it MAY use it.
Otherwise for messages to be sent over IPv6 it is RECOMMENDED to use
value 1280 bytes as a maximum message size ([RFC2460]). For messages
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to be sent over IPv4 it is RECOMENDED to use value 576 bytes as a
maximum message size.
According to [RFC0791] the minimum IP 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. Sender MAY use other values if it
is appropriate.
2.5.2. Fragmenting Messages containing unencrypted Payloads
Currently no one of IKEv2 Exchanges defines messages, containing both
unencrypted payloads and payloads, protected by Encrypted Payload.
But IKEv2 doesn't forbid such messages. So, if some future IKEv2
extension defines such a message and it needs to be fragmented, all
unprotected payloads (if any) MUST be in the first fragment, along
with Encrypted Fragment Payload, which MUST be present in any IKE
Fragment Message.
Below is an example of fragmenting Message, containing both encrypted
and unencrypted 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 size each of IKE Fragment Messages SHOULD not exceed
fragmentation threshold, including the very first, which contains
unprotected Payloads. This will reduce size of Encrypted Fragment
Payload content in the first IKE Fragment Message to accommodate
unprotected Payloads. In extreme cases Encrypted Fragment Payload
will contain no data, but it is still MUST be present in the message,
because only its presence allows receiver to distinguish IKE Fragment
Message from IKE Message.
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2.6. Receiving IKE Fragment Message
Receiver identifies IKE Fragment Message by the presence of Encrypted
Fragment Payload in it. Note, that it is possible for this payload
to be not the first payload in message (see Section 2.5.2). But for
all currently defined IKEv2 exchanges this payload will be the first
and the only payload in the message.
Upon receiving IKE Fragment Message the following actions are
performed:
o check message validity - in particular, check whether values of
Fragment Number and Total Fragments in Encrypted Fragment Payload
make sense. If not - message MUST be silently discarded.
o check, that this IKE Fragment Message is new for the receiver and
not replay. If message with the same Message ID and same Fragment
Number in Encrypted Fragment Payload was already received and
processed, this message 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 store message in the list waiting for the rest of fragments to
arrive.
When all IKE Fragment Messages (as indicated in the field Total
Fragments) are received, content of their Encrypted Fragment Payloads
is decrypted and merged together to form original message, which is
then processed as regular unfragmented message.
2.6.1. Replay Protection
According to [RFC5996] IKEv2 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 Message ID zero.
This document also updates this logic: if message contains Encrypted
Fragment Payload, the value of Fragment Number field from this
payload MUST be used along with Message ID to detect retransmissions
and replays. In other words, to consider message as replay or
retransmission 2-tuple of Message ID and Fragment Number must be met
before in context of this particular SA.
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3. Security Considerations
Most of the security considerations for IKE Fragmentation are the
same as those for base IKEv2 protocol described in [RFC5996]. This
extension introduces Encrypted Fragment Payload to protect content of
IKE Message Fragment. This allows receiver to individually check
authenticity of fragments, thus protecting itself from Denial of
Service attack.
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4. IANA Considerations
This document defines new Payload in the "IKEv2 Payload Types"
registry:
<TBA> Encrypted Fragment Payload SKF
This document also defines new Notify Message Types in the "Notify
Messages Types - Status Types" registry:
<TBA> IKE_FRAGMENTATION_SUPPORTED
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5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
5.2. Informative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC6311] Singh, R., Kalyani, G., Nir, Y., Sheffer, Y., and D.
Zhang, "Protocol Support for High Availability of IKEv2/
IPsec", RFC 6311, July 2011.
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Author's Address
Valery Smyslov
ELVIS-PLUS
PO Box 81
Moscow (Zelenograd) 124460
RU
Phone: +7 495 276 0211
Email: svan@elvis.ru
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