I have reviewed this document as part of the security directorate's
ongoing effort to review all IETF documents being processed by the
IESG.  These comments were written primarily for the benefit of the
security area directors.  Document editors and WG chairs should treat
these comments just like any other last call comments.

This document proposes an algorithm to make TCP more robust to long connectivity
disruptions.  Currently TCP has no way of distinguishing disruptions due to connectivity
loss from disruptions due to congestion.   Thus, TCP will back off when faced with connectivity
loss, which will lead to further delays.  The proposed algorithm uses the ICMP destination unreachable
messages as indications of a connectivity disruption, and alters the behavior of TCP accordingly.

My impression from reading this draft is that the behavior and utility of this algorithm will depend on
further research and experimentation.  There are a number of situations in which it will still be possible
to confuse congestion and long connectivity disruptions that may need further exploration.  The authors of the document do a good job of pointing
these out, but I would have liked to have seen more evidence that the solutions recommended are the optimal
ones, and under what situations.  This is especially the case for the security issues, although it is not
limited to those.  For example, in the discussion of probing frequency in Section 5.4 the authors make a claim
that in their belief the approach of their algorithm is preferable to others that would give higher probing
frequency, but they need to provide more evidence to back this up.

The security considerations section itself is rather sketchy, and doesn't support that authors' assertions
that the algorithm is "considered to be secure."  The greatest security threat posed by this
algorithm is that an attacker could exploit it to persuade a TCP sender that communication problems
due to congestion are actually due to a connectivity problem, leading the sender to further contribute to the
congestion.  However, the authors mention only one possible attack: forging ICMP destination unreachable
messages, which they present only as an "example" of an attack.   I would recommend a more complete
discussion, considering each of the potential ambiguity cases discussed in the document, and discussing
how an attacker could exploit them and how such exploitation could be prevented or mitigated.  You might
also want to discuss the opposite problem: how an attacker could convince a sender that a connectivity
problem is a congestion problem.  This is less serious, at least for the moment, since in the current
situation that is exactly what happens, but it could be more of a threat further down the line if people come
to rely more on this ability to disambiguate.  


Catherine Meadows
Naval Research Laboratory
Code 5543
4555 Overlook Ave., S.W.
Washington DC, 20375
phone: 202-767-3490
fax: 202-404-7942
email: catherine.meadows@nrl.navy.mil