HotNets Public Review of "Cross-layer Visibility as a Service" 
             Public reviewer: Bengt Ahlgren

This paper is about network management, in particular on how to
improve the operator's knowledge and control over the topology of the
network at various layers. Are these two IP links really independent?
Or do they really run on the same physical fiber? To answer questions
like this, the authors argue that the needed information from each
layer should be gathered via a management plane service on the side,
rather than through wider inter-layer interfaces.  They also argue
that the management system should be designed around the information
it need as input, rather than the information each box can provide (as
is the case with SNMP today).

The reason I like this paper lies more in the overall presented
architecture, rather than in the main argument of fatter interfaces
vs. a cross-layer service interface on the side. The outlined
management system combines information from different sources, ranging
from completely automatic to completely manual, in order to distill an
accurate cross-layer view of the network where lower level components
(fibers, conduits and the like) are correlated with the higher level
topology (routers and IP links). This kind of management system can
better analyse the cause of a fault, and is a better aid in planning
and maintaining a network.

The main argument in the paper, I however find a little weak. I don't
find that the presented architecture really depends on it. The same
kind of management system should be able to use fatter cross-layer
interfaces instead of the management interfaces on the side. For both
alternatives, additional information is needed, information that is
not present within the protocol layers in the first place and
therefore cannot be provided regardless of the kind of interface.

There is another twist that I would like to discuss.  Cross-layer
information is a buzz-word in wireless communication which is used in
approximately the same way as the authors do, but in the context of
providing better communication performance over wireless links. In
this context, the vision is that by propagating information about the
low-layer radio properties to higher layers, the latter can take the
right decision depending on what really happened, rather than basing
decisions on guessing. The prime example is TCP congestion control
which can't distinguish between a packet loss caused by congestion
from loss caused by radio interference.  If we agree that we need
cross-layer information for both network management and for providing
better performance over wireless links, wouldn't it be useful to have
one infrastructure that could accommodate both? The information needed
is at some level of similar sorts. In the wireless scenario, the IP
layer would benefit from information about how the radio channels
interfere with each other, that is, information about the physical
radio space. But perhaps the information has completely different time
properties for this to be feasible?