compute-branch

[Huang Hai <thumahh@263.net>]

Hi,

In metord
    CtrMcastComp instproc compute-branch { src group member }
(~ns/tcl/ctr-mcast/CtrMcastComp.tcl), I found the following lines.

 1      if {$tmp == $target} {
 2          if {$treetype($group) == $SPT || $tmp == $src} {
 3              #when the member is also the source
 4              set iif -2
 5          } else {
 6              #when member is at RP, find iif from RP to source
 7              set upstreamtmp [$ns upstream-node $tmp $src]
 8              set iilink [$ns RPF-link $src [$upstreamtmp id] $tmp]
 9              set iif [[$iilink set dest_] id]
10          }
11      } else {
12          set upstreamtmp [$ns upstream-node $tmp $target]
13          set iilink [$ns RPF-link $target [$upstreamtmp id] $tmp]
14          set iif [[$iilink set dest_] id]
15      }

I think it's not adequate to use upstream-node (line7) to find the node
upstream. According to "ns Notes and Documentation", the root of the shared
tree is now $target, and all packets from $src are unicast to $target. In
my opinion, we should find the upstream node downwards from $src but not
upwards from $target ($tmp), though the results are the same when all the
costs of links are the same. When the upward cost and downward cost are
different, line 7 will give an error answer. All other upstream-node deserve
no changes. Becasue in real world, multicast tree is built upwards, just in
the same way with this method. I substituted line 7 and 8 with the following
lines (ugly codes :-)).

        set hh_tmp $src
        while {$hh_tmp!=$tmp} {
            set upstreamtmp [$ns set Node_($hh_tmp)]
            set hh_tmp [[$ns upstream-node $hh_tmp $tmp] id]
        }
        set iilink [$ns set link_([$upstreamtmp id]:$tmp)]

After substituting, ns can run smoothly even there is only a simplex link
from $src to $target. This property can be used to simulate one-way
satellite broadcast.

Another problem is that switch-treetype can only change the type to SPT
whatever the former type is, while in "ns Notes and Documentation", it says
"or vice-versa".

Any discussion is welcome.

Best regards,
 Huang                          mailto:thumahh@263.net