Internet Engineering Task Force K. Grewal
Internet-Draft Intel Corporation
Intended status: Standards Track G. Montenegro
Expires: December 25, 2008 Microsoft Corporation
June 23, 2008
XESP for Traffic Visibility
draft-grewal-ipsec-traffic-visibility-01
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Abstract
This document describes an ESP encapsulation for IPsec, allowing
intermediate devices to ascertain if ESP-NULL is being employed and
hence inspect the IPsec packets for network monitoring and access
control functions. Currently in the IPsec standard, there is no way
to differentiate between ESP encryption and ESP NULL encryption by
simply examining a packet.
1. Introduction
Use of ESP within IPsec [RFC4303] specifies how ESP packet
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encapsulation is performed. It also specifies that ESP can use NULL
encryption [RFC2410] while preserving data integrity and
authenticity. The exact encapsulation and algorithms employed are
negotiated out-of-band using, for example, IKE [RFC2409] or IKEv2
[RFC4306] and based on policy.
Enterprise environments typically employ numerous security policies
(and tools for enforcing them), as related to access control,
firewalls, network monitoring functions, deep packet inspection,
Intrusion Detection and Prevention Systems (IDS and IPS), scanning
and detection of viruses and worms, etc. In order to enforce these
policies, network tools and intermediate devices require visibility
into packets, ranging from simple packet header inspection to deeper
payload examination. Network security protocols which encrypt the
data in transit prevent these network tools from performing the
aforementioned functions.
When employing IPsec within an enterprise environment, it is
desirable to employ ESP instead of AH [RFC4302], as AH does not work
in NAT environments. Furthermore, in order to preserve the above
network monitoring functions, it is desirable to use ESP-NULL. In a
mixed mode environment some packets containing sensitive data employ
a given encryption cipher suite, while other packets employ ESP-NULL.
For an intermediate device to unambiguously distinguish which packets
are leveraging ESP-NULL, they would require knowledge of all the
policies being employed for each protected session. This is clearly
not practical. Heuristic-based methods can be employed to parse the
packets, but these can be very expensive, containing numerous rules
based on each different protocol and payload. Even then, the parsing
may not be robust in cases where fields within a given encrypted
packet happen to resemble the fields for a given protocol or
heuristic rule. This is even more problematic when different length
Initialization Vectors (IVs), Integrity Check Values (ICVs) and
padding are used for different security associations, making it
difficult to determine the start and end of the payload data, let
alone attempting any further parsing. Furthermore, storage, lookup
and cross-checking a set of comprehensive rules against every packet
adds cost to hardware implementations and degrades performance. In
cases where the packets may be encrypted, it is also wasteful to
check against heuristics-based rules, when a simple exception policy
(e.g., allow, drop or redirect) can be employed to handle the
encrypted packets. Because of the non-deterministic nature of
heuristics-based rules for disambiguating between encrypted and non-
encrypted data, an alternative method for enabling intermediate
devices to function in encrypted data environments needs to be
defined. Enterprise environments typically use both stateful and
stateless packet inspection mechanisms. The previous considerations
weigh particularly heavy on stateless mechanisms such as router ACLs
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and NetFlow exporters.
This document defines a mechanism to prove additional information in
relevant IPsec packets so intermediate devices can efficiently
differentiate between encrypted ESP packets and ESP packets with NULL
encryption.
The document is consistent with the operation of ESP in NAT
environments [RFC3947].
The design principles for this protocol are the following:
o Allow easy identification and parsing of integrity-only IPsec
traffic
o Leverage the existing hardware IPsec parsing engines as much as
possible to minimize additional hardware design costs
o Minimize the packet overhead in the common case
1.1. Requirements Language
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 RFC 2119 [RFC2119].
1.2. Applicability Statement
The document is applicable only to the Extended ESP header defined
below, and does not describe any changes to either ESP [RFC4303] nor
AH [RFC4302].
2. Extended ESP (XESP) Header format
The proposal is to define an Extended ESP protocol number, which
provides additional attributes in each packet. The value of the new
protocol is TBD and the format of the new encapsulation is defined
below.
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0 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 Header | HdrLen | TrailerLen | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Security Parameters Index (SPI) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IV (variable) |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Data |
~ ~
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | TFC Padding * (optional, variable) |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Padding (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Padding (0-255 bytes) |PAD Length | Next Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Integrity Check Value-ICV (variable) |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
XESP Header
Figure 1
Where:
Next Header: next protocol header (encrypted in ESP trailer, but in
the clear in header), providing easy access to a HW parser to
extract the upper layer protocol. Note: For security concerns,
this value may optionally be set to zero, in which case the next
header can be extracted from the ESP trailer.
HdrLen: includes the new header + full ESP header + the IV (if
present). It is an offset to the beginning of the Payload Data.
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TrailerLen: Offset from the end of the packet including the ICV, pad
length, and any padding. It is an offset from the end of the
packet to the last byte of the payload data.
Flags
2 bits: Version
1 bit: IntegrityOnly: Payload Data is not encrypted (ESP-NULL).
5 bits: reserved for future use. These MUST be set to zero per
this specification, but usage may be defined by other
specifications.
As can be seen, this Extended ESP format simply extended the standard
ESP header by the first 4 octets.
2.1. UDP Encapsulation
This section describes a mechanism for running the new packet format
over the existing UDP encapsulation of ESP as defined in RFC 3948.
This allows leveraging the existing IKE negotiation of the UDP port
for NAT-T discovery and usage [RFC3947], as well as preserving the
existing UDP ports for ESP (port 4500). With UDP encapsulation, the
packet format can be depicted as follows.
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0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src Port (4500) | Dest Port (4500) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol Identifier (value = 0x00000001) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | HdrLen | TrailerLen | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Security Parameters Index (SPI) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IV (variable) |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Data |
~ ~
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | TFC Padding * (optional, variable) |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Padding (0-255 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Padding (0-255 bytes) |PAD Length | Next Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Integrity Check Value-ICV (variable) |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
UDP-encapsulated XESP Header
Figure 2
Where:
Source/Destination port (4500) and checksum: describes the UDP
encapsulation header, per RFC3948.
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Protocol Identifier: new field to demultiplex between UDP
encapsulation of IKE, UDP encapsulation of ESP per RFC 3948, and
this proposal.
According to RFC 3948, clause 2.2, a 4 octet value of zero (0)
immediately following the UDP header indicates a Non-ESP marker,
which can be used to assume that the data following that value is an
IKE packet. Similarly, a value of non-zero indicates that the packet
is an ESP packet and the 4-octet value can be treated as the ESP SPI.
However, RFC 4303, clause 2.1 indicates that the values 1-255 are
reserved and cannot be used as the SPI. We leverage that knowledge
and use a value of 1 to indicate that the UDP encapsulated ESP header
contains this new packet format for ESP encapsulation.
The remaining fields in the packet have the same meaning as per
section 2.0 above.
2.2. Tunnel and Transport mode of considerations
This extension is equally applicable for tunnel and transport mode
where the ESP Next Header field is used to differentiate between
these modes, as per the existing IPsec specifications.
2.3. IKE Considerations
In order to negotiate the new format of ESP encapsulation via IKE,
both sides of the security channel need to agree upon using the new
packet format. This can be achieved by proposing a new protocol ID
within the existing IKE proposal structure as defined by RFC 4306,
clause 3.3.1. The existing proposal substructure in this clause
allows negotiation of ESP/AH (among others) by using different
protocol Ids for these protocols. By using the same protocol
substructure in the proposal payload and using a new value (TBD) for
this encapsulation, the existing IKE negotiation can be leverage with
minimal changes to support negotiation of this encapsulation.
Furthermore, because the negotiation is at the protocol level, other
transforms remain valid for this new encapsulation and consistent
with IKEv2 [RFC4306]. Additionally, NAT-T [RFC3948] is wholly
compatible with this extended frame format and can be used as-is,
without any modifications, in environments where NAT is present and
needs to be taken into account.
3. Acknowledgements
The authors would like to acknowledge the following people for their
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feedback on updating the definitions in this document.
David McGrew, Brian Weis, Philippe Joubert, Brian Swander, Yaron
Sheffer, Men Long, David Durham, Prashant Dewan, Marc Millier among
others.
4. IANA Considerations
Reserving an appropriate value for this encapsulation as well as a
new value for the protocol in the IKE negotiation is TBD by IANA.
5. Security Considerations
As this document augments the existing ESP encapsulation format, UDP
encapsulation definitions specified in RFC 3948 and IKE negotiation
of the new encapsulation, the security observations made in those
documents also apply here. In addition, as this document allows
intermediate device visibility into IPsec ESP encapsulated frames for
the purposes of network monitoring functions, care should be taken
not to send sensitive data over connections using definitions from
this document, based on network domain/administrative policy. A
strong key agreement protocol, such as IKE, together with a strong
policy engine should be used to in determining appropriate security
policy for the given traffic streams and data over which it is being
employed.
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.
[RFC2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and
Its Use With IPsec", RFC 2410, November 1998.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
6.2. Informative References
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC3947] Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
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"Negotiation of NAT-Traversal in the IKE", RFC 3947,
January 2005.
[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, January 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
Authors' Addresses
Ken Grewal
Intel Corporation
2111 NE 25th Avenue, JF3-232
Hillsboro, OR 97124
USA
Phone:
Email: ken.grewal@intel.com
Gabriel Montenegro
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
Phone:
Email: gabriel.montenegro@microsoft.com
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