Internet DRAFT - draft-smyslov-ipsecme-ikev2-fragmentation
draft-smyslov-ipsecme-ikev2-fragmentation
Network Working Group V. Smyslov
Internet-Draft ELVIS-PLUS
Intended status: Informational April 10, 2013
Expires: October 12, 2013
IKEv2 Fragmentation
draft-smyslov-ipsecme-ikev2-fragmentation-01
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 October 12, 2013.
Copyright Notice
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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. Using IKE Fragmentation . . . . . . . . . . . . . . . . . 5
2.5. Fragmenting Message . . . . . . . . . . . . . . . . . . . 6
2.5.1. Selecting Fragment Size . . . . . . . . . . . . . . . 7
2.5.2. Fragmenting Messages containing unencrypted
Payloads . . . . . . . . . . . . . . . . . . . . . . . 8
2.6. Receiving IKE Fragment Message . . . . . . . . . . . . . . 9
2.6.1. Changes in Replay Protection Logic . . . . . . . . . . 10
3. Interaction with other IKE extensions . . . . . . . . . . . . 11
4. Security Considerations . . . . . . . . . . . . . . . . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Normative References . . . . . . . . . . . . . . . . . . . 14
6.2. Informative References . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
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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.
2.2. Limitations
In general, original message can be fragmented if and only if it
contains Encrypted Payload. It means that 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
fragmentation. 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)]
The Notify payload is formatted as follows:
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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. Using IKE Fragmentation
After IKE Fragmentation is negotiated, it is up to Initiator of each
Exchange, whether to use it or not. In most cases IKE Fragmentation
will be used in IKE_AUTH Exchange, especially if certificates are
employed. Initiator may first try to send unfragmented message and
resend it fragmented only if it didn't receive response after several
retransmissions, or it may always send messages fragmented (but see
Section 3), or it may fragment only large messages and messages
causing large responses.
In general the following guidelines are applicable:
o Initiator MAY fragment outgoing message if it suspects that either
request or response message may be fragmented by IP level.
o Initiator SHOULD fragment outgoing message if it suspects that
either request or response message may be fragmented by IP level
and IKE Fragmentation was already used in one of previous
Exchanges in the context of the current IKE SA.
o Initiator SHOULD NOT fragment outgoing message if both request and
response messages of the Exchange are small enough not to cause
fragmentation on IP level (for example, there is no point in
fragmenting Liveness Check messages).
Responder MUST send response message in the same form (fragmented or
not) as corresponded request message. If it received unfragmented
request message, responded with unfragmented response message and
then received fragmented retransmission of the same request, it MUST
resend its response back to Initiator fragmented.
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2.5. Fragmenting Message
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. Its content
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Encrypted content ~
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Padding (0-255 octets) |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| | Pad Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Integrity Checksum Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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. This field MUST be less than or equal to the next field, Total
Fragments.
o Total Fragments (2 octets) - number of fragments original message
was divided into. This field MUST NOT be zero.
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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 some 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 breaking content of Encrypted Payload down into parts sender
SHOULD chose size of those parts so, that resulting IP Datagram size
not exceed some fragmentation threshold - be small enough to avoid IP
fragmentation.
If sender has some knowledge about PMTU size it MAY use it. If
sender is a Responder in the Exchange and it has received fragmented
request, it MAY use maximum size of received IKE Fragment Message IP
Datagrams as threshold when constructing fragmented response.
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.
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)
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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 this estimation is difficult as there may be varying IPv6
Extension headers included.
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. Sender MAY use other values if
they are appropriate.
Initiator MAY try to discover path MTU by using several values of
fragmentation threshold, provided that it starts with larger values
and fragments message again with next smaller value if it doesn't
receive response in a reasonable time after several retransmissions.
In this case using next smaller value MUST result in increasing Total
Fragments field.
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. If some future IKEv2
extension defines such a message and it needs to be fragmented, all
unprotected payloads 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}
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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 very first,
which contains unprotected Payloads. This will reduce the 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 regular IKE message.
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 (and the only) payload in the 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
are valid. If not - message MUST be silently discarded.
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 was already
received and successfully processed, this message is considered a
replay and MUST be 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 isn't finished yet and Total Fragments field in
received IKE Fragment Message is greater than this field in
previously received fragments, receiver MUST discard all received
fragments and start reassembling over with just received IKE
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Fragment Message.
o Store message in the list waiting for the rest of fragments to
arrive.
When all IKE Fragment Messages (as indicated in the Total Fragments
field) are received, content of their Encrypted Fragment Payloads is
decrypted and merged together to form content of original Encrypted
Payload, and, therefore, along with IKE Header, original message.
Then it is processed as if it was received, verified and decrypted as
as regular unfragmented message.
2.6.1. Changes in Replay Protection Logic
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 zero Message ID.
This document also updates this logic: if message contains Encrypted
Fragment Payload, the values of Fragment Number and Total Fragments
fields from this payload MUST be used along with Message ID to detect
retransmissions and replays.
If Responder receives IKE Fragment Message after it received,
successfully verified and processed regular message with the same
Message ID, it means that response message didn't reach Initiator and
it activated IKE Fragmentation. If Fragment Number in Encrypted
Fragment Payload in this message is equal to 1, Responder MUST
fragment its response and retransmit it back to Initiator in
fragmented form.
If Responder receives a replay IKE Fragment Message for already
reassembled, verified and processed fragmented message, it MUST
retransmit response back to Initiator, but only if Fragment Number
field in Encrypted Fragment Payload is equal to 1 and MUST silently
discard received message otherwise.
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3. Interaction with other IKE extensions
IKE Fragmentation is compatible with most of defined IKE extensions,
like 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 most future IKE extensions, if they follow general
principles of formatting, sending and receiving IKE messages,
described in [RFC5996].
The notable exception that requires a special care is [RFC6311] -
Protocol Support for High Availability of IKEv2. As it deliberately
allows any number of synchronization Exchanges to have the same
Message ID - zero, standard 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. 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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5. 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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6. References
6.1. 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.
[RFC6311] Singh, R., Kalyani, G., Nir, Y., Sheffer, Y., and D.
Zhang, "Protocol Support for High Availability of IKEv2/
IPsec", RFC 6311, July 2011.
6.2. 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.
[RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
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.
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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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