Telechat Review of draft-ietf-intarea-ipv4-id-update-
I 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, "Updated Specification of the IPv4 ID Field" is a
update of RFCs 791, 1122 and 2003. The primary motivation for the
update is a recognition that the uniqueness requirement imposed on
the field values (on a per host pair and protocol basis) would limit
"connections" to about 6.4 Mb/s (for typical 1500 byte packets), an
unrealistically low data rate today. This document updates the cited
RFCs to reflect current practice and to more closely match IPv6.
Specifically, the field value is defined only when a datagram is
The Security Considerations section is very brief, only three
paragraphs. It notes that removing the prior constraints on ID field
generation (MSL uniqueness) make it easier to use this field as a
covert channel. It suggests that rewriting the field is a possible
countermeasure. This advice is presented with the context of
datagrams not protected using AH. Because AH is no longer a mandatory
to implement element of the IPsec suite, I suggested an edit to avoid
suggesting that AH use if common.
The text goes on to discuss how removing the MSL uniqueness
requirement reduces the entropy associated with the IPv4 header. It
fails to explain why this might be significant. There is no
indication that modern encryption algorithms used IETF security
protocols are harmed by this reduction in entropy. Thus the paragraph
devoted to this issue seems extraneous, possibly confusing to
The final paragraph in this section notes that the proposed ID field
conventions may make it more difficult to count the number of
distinct devices behind a NAT or similar device. I agree with the
author's observation that this side effect of the current ID field
requirements is not a security feature per se and thus not a concern.
Earlier sections of this document do a good job explaining how this
change may impact various forms of middleboxes. The author should
note in the SCC whether the change proposed in this document may
adversely affect availability, if these devices are not updated to
account for this change.
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