Cell Relay Archive

[anouch_f@my-deja.com]

Hi,

 I've posted this question on the frame relay discussion group but since
this is more a
 Nortel Passport (7K/15K) ATM switch issue, here is the question and one
answer I got from Ken
 Agress (plus my reply):


 > >Hi,
 > >
 > >Does anyone know how to get DE0 frames of a low priority queue
 > >transmited before DE1 frames of a high priority queue on a Nortel
 > >Passport 7K/15K ? (by DE0/1 frames I also mean CLP0/1 cells)
 >
 > I can't help with the commands, but I'd be careful doing this.  If
 you
 > transmit frames from a single host-to-host connection using this
 > scheme, you create the possibility of packets arriving out of
 > sequence, which far too many protocol stacks will treat as packet
 > loss.  If packet 2000 is DE=1 and 2001 is DE=0 and they're in the
 > queues at the same time, 2001 will be delivered first.
 >
 > Ken Agress
 >
 I'm sorry Ken,

 I forgot to say that frames going to these two priority queues are, of
course, from two
 different connections so there is no sequencing problem. I also forgot
to say that this is
 only useful when there is congestion.

 Suppose two connections: connection1=premium FR service
 and connection2=lite FR service
 Connection1' frames/cells goes to high priority transmit queue and
connection2'
 frames/cells goes to low priority transmit queue.
 (Connection1 may for example transport compressed voice while
 connection2 transports pure data LAN packets)

 The scheduler serves all frames/cells from hight priority queue before
serving
 (sending) frames/cells in lower priority queue (as a general rule). In
this way
 frames/cells with DE=1/CLP=1 from connection1 are sent BEFORE
 frames/cells with
 DE=0/CLP=0 from connection2.

 And what I want to do is to send in order:
 DE=0/CLP=0 frames/cells from connection1 (its CIR) then
 DE=0/CLP=0 frames/cells from connection2 (its CIR) then
 DE=1/CLP=1 frames/cells from connection1 (its EIR) then
 DE=1/CLP=1 frames/cells from connection2 (its EIR) .

 Of course the scheduler can't send a DE=0/CLP=0 frame/cell before a
DE=1/CLP=1
 frame/cell if the latter is queued before the former, since the
scheduler picks up the
 frame/cell at the head of the connection's queue.

 By giving a Minimum Bandwidth Guaranty (say 5% of the time) to the
lower priority
 queue (connection2), its frames/cells can "sometimes" be sent BEFORE
those on the
 high priority queue (connection1). And this is what I'm looking for
when there is
 CONGESTION. In the event of no congestion I don't care if all
 frames/cells (even tagged
 one) from connection1 are sent first.
 MBG is one way to do that. The other ways are Port Aggregation and the
Free List
 (frame/cell descriptor block) congestion state that I don't understand
yet.
 Please also note that there are a set of "Discard Priorities" (DP) in
the switch which
 determine the "importance" of frames/cells and are mapped at connection
time. The
 DPs are used in various packet discard algorithms (Early Packet
Discard, PPD, LPD,
 W-RED). DPs are numbered 0 through 3 with DP0 the most important.

 In my case the traffic is mapped as follows:

 DE=0/CLP=0 frames/cells from connection1 : DP2 (high priority Q)
DE=0/CLP=0
 frames/cells from connection2 : DP2 (low priority Q)
 DE=1/CLP=1 frames/cells from connection1 : DP3 (high priority Q)
DE=1/CLP=1
 frames/cells from connection2 : DP3 (low priority Q)

 And the problem is even tagged frames/cells from connection1 are in the
high priority Q
 (otherwise there would be sequencing problems).

 Thanks
 Anouch


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