Internet DRAFT - draft-bhatia-ipsecme-avoiding-ah
draft-bhatia-ipsecme-avoiding-ah
Network Working Group M. Bhatia
Internet-Draft Alcatel-Lucent
Intended status: BCP January 2, 2012
Expires: July 5, 2012
Avoiding Authentication Header (AH)
draft-bhatia-ipsecme-avoiding-ah-00
Abstract
This document recommends retiring Authentication Header (AH) and
discusses the reasons for doing so. It recommends that AH must not
be used for new applications and protocols, since Encapsulating
Security Payload (ESP) using NULL encryption algorithm provides the
same level of security in most real deployments.
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].
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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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on July 5, 2012.
Copyright Notice
Copyright (c) 2012 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
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publication of this document. Please review these documents
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described in the Simplified BSD License.
1. Introduction
IPsec uses two protocols to provide traffic security services --
Authentication Header (AH) and Encapsulating Security Payload (ESP).
Both protocols are described in detail in their respective RFCs
[RFC4302] [RFC4303]
[RFC4301] recommends IPsec implementations to MUST support ESP and
MAY support AH. Support for AH was downgraded to MAY because
experience has shown that there are very few contexts in which ESP
cannot provide the requisite security services. Note that ESP can be
used to provide only integrity, without confidentiality, making it
comparable to AH in most contexts.
AH offers integrity and data origin authentication, with optional (at
the discretion of the receiver) anti-replay features. ESP, on the
other hand, offers the same set of services, and also additionally
offers confidentiality.
These protocols may be applied individually or in combination with
each other to provide IPv4 and IPv6 security services. However, most
security requirements can be met through the use of ESP by itself.
Each protocol supports two modes of use: transport mode and tunnel
mode. In transport mode, AH and ESP provide protection primarily for
next layer protocols; in tunnel mode, AH and ESP are applied to
tunneled IP packets. [RFC4301] describes detailed differences
between these two modes.
There is no particular security problem with using AH. It lives up
to its security claims. Its just that its completely redundant with
ESP, since ESP will NULL encryption algorithm (ESP-NULL) [RFC2410]
can provide the same functionality and the world can do with one less
protocol.
Retiring AH doesn't mean that people have to stop using AH right now.
It only means that in the opinion of the community there are now
better alternatives. This will discourage new protocols to mandate
the use of AH. It however, does not preclude the possibility of new
work to IETF that will require or enhance AH. It just means that the
authors will have to explain why that solution is really needed and
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the reason why ESP with NULL encryption algorithm cannot be used
instead.
2. AH and ESP
It is alleged that AH provides more security than ESP in the
transport mode as AH also authenticates the IP header fields. This
argument is however moot as ESP in the tunnel mode can provide the
same level of security since the payload now includes the original IP
header. It is also believed by many that securing the IP header isnt
really very important [Schneier].
It is commonly believed that AH is quite useful in securing the IPv6
extension headers. AH protects most of the basic IPv6 header, the
non-mutable extension headers after the AH, and the IP payload.
Protection for the IPv6 header excludes the mutable fields: DSCP,
ECN, Flow Label, and Hop Limit. ESP, on the other hand, doesn't
protect the immutable parts of the IPv6 header nor those of any
extension header. This can however be fixed by putting the IPv6
extension headers that are required to be protected after the ESP
header. Hop-by-Hop options are not an issue, as the intermediate
hops do not have keys to verify the message authentication code so
they cannot really be protected anyways.
AH breaks Network Address Translators (NATs). This is because AH
relies on the sanctity of the IP header so that any tamperings, even
by a NAT, get detected and packets get discarded. Solving this issue
requires another device that fixes the NAT translation back to the
original one (a specific case of Double NAT). ESP, on the other
hand, fixes this problem by encapsulating ESP packets inside UDP
packets for traversing NATs [RFC3948].
Firewalls in the enterprise environments often require visibility
into packets, ranging from simple packet header inspection to deeper
payload examination. Routers also often need to deep inspect control
traffic to prioritize certain protocol packets over the others. This
was initially difficult with ESP since was impossible to know whether
an ESP packet was integrity protected or encrypted by merely
inspecting the packet. This was easy with AH since the payload was
transmitted in clear. This problem however has been solved by
introducing WESP [RFC5840] which defines a mechanism to provide
additional information in relevant IPsec packets so intermediate
devices can efficiently differentiate between encrypted and
integrity-only ESP packets.
ESP with NULL encryption algorithm seems to do everything useful that
can be done with AH without the side effects of AH. Given this, it
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makes sense to retire AH so that newer applications and protocols
dont mandate or propose extensions that rely on AH to be supported or
extended.
3. Security Considerations
Its argued that ESP in the tunnel mode is equivalent to the AH in the
transport mode. It should however be noted that ESP tunnel mode SA
applied to an IPv6 flow results in at least 50 bytes of additional
overhead per packet. This additional overhead may be undesirable for
many bandwidth-constrained wireless and/or satellite communications
networks, as these types of infrastructure are not overprovisioned.
Packet overhead is particularly significant for traffic profiles
characterized by small packet payloads (e.g., various voice codecs).
If these small packets are afforded the security services of an IPsec
tunnel mode SA, the amount of per-packet overhead is increased.
This issue will perhaps be alleviated by header compression schemes
defined in [RFC5856] [RFC5857] and [RFC5858].
4. IANA Considerations
This document includes no request to IANA.
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.
[RFC2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and
Its Use With IPsec", RFC 2410, November 1998.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
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5.2. Informative References
[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, January 2005.
[RFC5840] Grewal, K., Montenegro, G., and M. Bhatia, "Wrapped
Encapsulating Security Payload (ESP) for Traffic
Visibility", RFC 5840, April 2010.
[RFC5856] Ertekin, E., Jasani, R., Christou, C., and C. Bormann,
"Integration of Robust Header Compression over IPsec
Security Associations", RFC 5856, May 2010.
[RFC5857] Ertekin, E., Christou, C., Jasani, R., Kivinen, T., and C.
Bormann, "IKEv2 Extensions to Support Robust Header
Compression over IPsec", RFC 5857, May 2010.
[RFC5858] Ertekin, E., Christou, C., and C. Bormann, "IPsec
Extensions to Support Robust Header Compression over
IPsec", RFC 5858, May 2010.
[Schneier]
Ferguson, N. and B. Schneier, "A Cryptographic Evaluation
of IPsec", December 2003,
<http://www.schneier.com/paper-ipsec.pdf>.
Author's Address
Manav Bhatia
Alcatel-Lucent
Email: manav.bhatia@alcatel-lucent.com
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