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 defines the ALTO Cost Calendar, an extension to the base
Application-Layer Traffic Optimization (ALTO) protocol. Currently, the
ALTO cost information service provides applications with guidance about
current costs of a desired resource, but not for resources with a cost that
changes dramatically over time. The ALTO Cost Calendar allows for
specifying costs for varying time periods in the future.

The extensions in this document are to the existing network flows, with
policy defined in JSON. As such, additional security considerations are
few. The well-written Security Considerations document does define a few
considerations that come from announcing events that are expected to
happen in the future.

I have only one suggestion for additional text. The second
paragraph on page 27 (draft -17) describes risks of a client using the
calendaring information for their own selfish purposes. The suggested
mitigation in the next paragraph is to limit the information “being
leaked to malicious clients or third parties“ by authenticating clients
with TLS. This strategy may thwart “third parties”, but it will not help
in the case of “malicious clients” possessing valid credentials to
authenticate. The threat here might be legitimate clients that have
become subverted by an attacker and are now ‘bots’ being asked to
participate in a DDoS attack. The calendar information would be valuable
information for when to persecute a DDoS attack, and this should be
noted here.