Cell Relay Archive

[Chan Wai Kok <neraisn@po.jaring.my>]

Srikrishna wrote:
> 
> Justwant to clarify a subtle point to the above explanation.
> 
> If the first cell of an IP packet is discarded using the PPD mechanism then I
> would assume that this is same as EPD because all the cells corresponding to
> that IP packet are going to be discarded before they are queued. However there
> is no guarantee that this will be the case. Most probably, some intermediate
> cell of an IP packet may get discarded due to congestion with earlier cells of
> this IP packet already queued in the switch. As a result, junk cells are going
> to be switched unnecessarily  because TCP is going to retransmit the entire IP
> packet again anyway. Hence PPD is less efficient than EPD.
> 
> EPD on the other hand checks to make sure that the entire IP packet (and hence
> all the cells pertaining to this IP packet) has enough buffer resources upfront
> (via athreshold mechanism) before queueing any cell for this IP packet. If so,
> then no cell (starting one or intermediate ones) are discarded as a result of
> which junks cells as mentioned above are not switched. Hence EPD is more
> efficient than PPD.
> 
> However, both PPD and EPD require VC queueing implementation in the switch I
> presume?
> 
> Having said that EPD is more efficient than PPD, note that it is only within
> that switch. If a subsequent switch drops the entire packet, the endpoint TCP
> will retransmit the entire IP packet which makes the first switch look
> inefficient but not the subsequent switch whihc caused the retransmit.
> 
> Any comments or corrections to this is greatly appreciated to ensure what I
> state above is not totally wrong.
> 
> Thanks in advance..
> 
> Srikrishna..

-- 
Regards
Chan Wai Kok
Network Engineer          pager : 603-7048833 pg no :24122
NERA Malaysia Sdn Bhd.    tel: 603-2523211.  fax :- 603-2525171. 
5th Flr, Syed Kechik Foundation Building,
Jln Kapas, Bangsar 59100
Kuala Lumpur Malaysia.
e-mail office :- wkchan@neramal.po.my
e-mail remote (any where) :- neraisn@pop1.jaring.my