[Opencdn-users] OpenCDN edge surrogate determination by a latency measure

[Alessandro F. <aa...@li...>]

Release 0.7.6 (http://sourceforge.net/project/showfiles.php?
group_id=121628) of OpenCDN supports a method for determination of the
LastHop node which is nearest to the client, based on an objective
latency measure. This allows an effective use of "Global LastHop" nodes,
i.e. LastHop nodes which advertise a Footprint with global coverage (see
README.routing, point 6). 
 
Earlier OpenCDN releases, only tackled the LastHop selection by using
static configuration of Footprints, whose feasibility is questionable
for global nodes 
 
1 Theory of Operation 
------------------- 
 
* every OpenCDN Node, at registration time, communicates to the Control
Entity the name and port of an image file, which can be accessed through
an embedded HTTP server 
 
* the page built at the portal includes references to image files
located at LastHop nodes 
 
* when the viewer's browser downloads these images form the HTTP server
which runs inside of nodes, a Round Trip Time (RTT) delay is evaluated,
measuring the proximity in between the viewer and every LastHop. Such a
value is stored in association to the viewer IP address. 
 
* upon reception of UDP probe requests (see third paragraph of point 5
in README.routing), the LastHop informs the prober (the Control Entity
or a Transit node) about the RTT toward the Viewer IP address, which is
communicated back, inside of the probe response. 
 
* after complete reception of all the responses to the UDP probe scan,
the prober decides which node is the nearest to the viewer, by comparing
the reported RTT.

-- 
Alessandro Falaschi