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The document is well written and looks good from a technical perspective.

One small concern, coming from a total outsider perspective, so apologies if this is something obvious. I'm thinking of how flows map to Flex algorithms, and specifically misclassification of flows (through fault or malicious intent). For instance, cases where a large set of flows all map to the same flex algo that has some constraints on the available paths (e.g., exclude paths based on max delay). Can the introduction of these metrics lead to new types of overload scenarios? Would it be the role of a PCE to detect and mitigate such scenarios? Is this something that needs mentioning in this document?
 
A few, mostly editorial comments below. 

The term Elephant flow is used a number of times in the document, for instance in section 3:
"In networks that carry elephant flows, directing an elephant flow down a low-bandwidth link would be catastrophic."
In what sense is it catastrophic? Flows should be congestion controlled and therefore adapt to link conditions. Some applications performance might suffer from slow transfer of bulk data, whereas others aren't noticeably affected.
I think it could be good to have a slightly more clear definition of the term elephant flow, or even better, use a more concrete example of the type of application flow you are considering here.

In section 3 you also have the following text: 
"Delay is an important consideration in High Frequency Trading applications, networks with transparent L2 link recovery, or in satellite networks, where link delay may fluctuate."
There's a bit of an editorial inconsistency here (imo), mixing application types and network types. I assume you want to give an example of an application for which it could make sense to exclude links based on a Max delay constraint.
Perhaps write it something like this:
"Applications, such as High-Frequency Trading are sensitive to link delays and may perform poorly in networks prone to delay variability, such as those with transparent Layer 2 link recovery mechanisms or satellite links."