GGF-NMWG: "Hop" definition?

["McKee, Shawn" <smckee@umich.edu>]

Hi Everyone,

I was requested to try to write-up a broadly applicable definition of
"hop", differentiating it from "path" and "link".  

"path:" A ordered set of 1 or more "hops" in a given measurement
context.  For example, the ordered list of IP addresses from traceroute
between two IP end nodes defines a path in a layer-3 context.   Note it
is possible to have the "path" = "hop" if the two IP end nodes are
directly connected routers.

"link:" A direct layer-2 connection between layer-2 devices.  As an
example, the ports and cable connecting two switches, A and B,
represents a "link".  Note that a "link" could be subdivided further at
layer-1 via repeaters or media converters (100Base-FX to 100Base-TX)

"hop:" The indivisible component which makes up a "path" in a given
measurement context.  For eaxmple, a layer-3 measurement (traceroute) of
the path between two nodes is a set of layer-3 "hops".  Note that this
doesn't preclude a layer-3 "hop" from being further subdivided IN A
DIFFERENT CONTEXT.  A trivial example is the new set of "hops" ("link"s
in this case) which may exist at layer-2 for a SINGLE layer-3 "hop".  A
tricker example is a tool which is sensitive to non-router "layer-3"
devices (NAT, VPN, etc.), as shown below.

It would be useful to discuss how this definition of "hop" would work in
the VPN case (or NAT, etc.)

This represents the physical "path" between two hosts:
<HOST>----<SWITCH>----<VPN>-----<ROUTER>-----<ROUTER>----<VNP>---<SWITCH
>----<HOST>

What are the "hops"?  This is, of course, context dependent:

At layer 2 we have 7 hops:
 
--------------------------------"path1"---------------------------------
--
<HOST>----<SWITCH>----<VPN>-----<ROUTER>-----<ROUTER>----<VNP>---<SWITCH
>----<HOST>
     "hop1"     "hop2"    "hop3"       "hop4"       "hop5"  "hop6"
"hop7"


At layer 3 we have 3 hops:
 
--------------------------------"path2"---------------------------------
--
<HOST>----<SWITCH>----<VPN>-----<ROUTER>-----<ROUTER>----<VNP>---<SWITCH
>----<HOST>
     ------------"hop1"------------  ---"hop2"----
-----------"hop3"----------

If we have some layer 3 application, senstive to the VPN devices we have
5 hops:
 
--------------------------------"path3"---------------------------------
--
<HOST>----<SWITCH>----<VPN>-----<ROUTER>-----<ROUTER>----<VNP>---<SWITCH
>----<HOST>
     -------"hop1"------ --"hop2"--  ---"hop3"--- --"hop4"--
-------"hop5"------

Thus, "hop", and therefore "path", is well defined ONLY for a given
context.  We have avoided the layer-1 possibilities for "hops", like
repeaters and media converters, but those could utilize the same
definition.

Please comment!

Shawn