All,
Working primarily from the HTML diff, towards the end of Section 3,
the draft -03 text says in part:
"The IPsec community generally prefers ESP with NULL encryption
over AH, but AH is required in some protocols; further, AH is
more appropriate when there are security-sensitive options in
the IP header"
The 1st part of this assumes that IP-layer options aren't in the
packet (which most commonly is true) and that the deployment threat
model does not consider option insertion attacks to be a major threat
(a widely held belief for the commercial portions of the Internet).
The 2nd part of this is vague, unclear, and a bit misleading. It
could be read as indicating that ESP with NULL encryption is able to
protect IP header options, but AH is preferred for that case.
In fact, ESP is *NEVER CAPABLE* of (A) protecting IPv4 options or
(B) of protecting IPv6 options that are seen & processed by
intermediate systems (e.g. routers, security gateways, other
middleboxes). ESP also NEVER can prevent option insertion attacks.
Lastly, it is ALWAYS possible for an intermediate system (e.g. router
with ACLs) to parse past the AH to see transport-layer headers,
but even the best of the heuristics for parsing past an ESP header
are not 100% reliable.
[IMPORTANT NOTE: A previous employer of mine shipped IPv4/IPv6 routers
with forwarding silicon that could parse AH and any other IPv4/IPv6
options - at wire-speed for 10 Gbps interfaces - 10 years ago. I am
aware of 2 other very widely deployed implementations with the same
ability to parse past AH to read TCP/UDP ports and apply ACLs at
wire-speed. So this is a widely deployed capability, rather than
theory. :-]
We owe it to readers to be crisp, clear, complete, and accurate
with the text in this draft.
Candidate new text:
"When no IPv4 options or IPv6 optional headers are present, and
in environments without concerns about attacks based on option
insertion (e.g. inserting a source routing header to facilitate
pervasive eavesdropping), the IPsec community generally prefers
ESP with NULL encryption over AH. However, some protocols
require AH. Also, AH always protects all IPv4 options and IPv6
optional headers, while ESP NULL is unable to protect any IPv4
options or to protect IPv6 options that are seen & processed by
intermediate systems (e.g. routers, security gateways, other
middle-boxes). Some IP-layer options, for example IPSO [RFC-1108]
and CALIPSO [RFC-5570], today are deployed in some environments
while using AH to provide both integrity protection & authentication.
Further, deployed routers from multiple vendors are able to parse
past an AH header in order to read upper-layer protocol
(e.g. TCP) header information (e.g. to apply port-based router
ACLs) at wire-speed. By contrast, there is no 100% reliable way
to parse past an ESP header, although some ESP header parsing
heuristics have been documented [RFC-5879] that work in some cases.
Yours,
Ran Atkinson
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