Cell Relay Archive

[Kaustabh Duorah <duorah@txc.com>]

William Stallings wrote:
>    Partial packet discard (PPD) operates as follows: if a cell from an IP
> datagram is dropped from a switch buffer, the subsequent cells in that
> datagram are also discarded. For a switch to recognize, at the ATM level,
> which cells belong to a given IP datagram, PPD must operate on a
> per-virtual-channel basis. When a switch discards a cell on a particular
> VC, it then discards all subsequent cells on that VC until it sees a cell
> with the SDU type bit set to one in the ATM cell header. This cell marks
> the end of an AAL5 PDU and therefore the end of an IP datagram. This final
> cell is not dropped. Because AAL5 does not support the multiplexing of
> cells from different PDUs, the SDU parameter can be used successfully to
> delimit IP datagram boundaries.

>    A more effective scheme is early packet discard (EPD). In this case,
> when a switch buffer reaches a threshold level, but before it is actually
> required to discard any cells due to buffer overflow, an entire IP
> datagram is dropped. Thus, when the switch senses that congestion is
> beginning and that cell discard may soon be necessary, it preemptively
> discards all of the cells, starting with the first, of an IP datagram. For
> this purpose, the switch looks for the first incoming cell on a VC with an
> SDU bit of 0 that immediately follows a cell on the same VC with an SDU
> bit of 1. This cell marks the beginning of a new datagram, and the
> discarding begins with this cell. In effect, the EPD strategy emulates a
> packet-switching network in which entire packets are dropped.

I cannot seem to find a reason for the difference in performance. If as
you say, that PPD discards only half a PDU on average, the same should
hold for EPD. Because, statistically, in EPD the switch should hit the
threshold after half a PDU on average and so discard only half the PDU.
For complete PDUs after congestion beginning, there is no difference. In
both cases, full PDUs will be discarded.
Where is the discrepancy?

Kaustabh
-- 
Opinions expressed here are my own
-----------------------------------------------------------------
Kaustabh Duorah, SMTS                      TranSwitch Corporation
203-929-8810(W)                                3 Enterprise Drive
203-926-9453(Fax)                                Shelton CT 06484
duorah@txc.com
-----------------------------------------------------------------