[c-nsp] Is this throughput on ATM DS3 in 3640 normal?

[Lincoln Dale]

Peter,

others haven't said that it is reasonable but i do.

rationale is as follows:

you say that "throughput is 40Mbps when traffic is pushed in one 
direction" but with duplex traffic "it can't get higher than 34Mbps in 
one direction and 27Mbps in the other".

the mismatch of 34 vs 27 is a little odd - but could be a side-effect of 
TCP.

lets touch the 40Mbps first...
for a DS3 ATM link, you'll likely have a cell-rate of 100k cells/sec 
(perhaps a little more).  based on cell header overhead, you lose ~10% 
of that bandwidth.  90% of 45Mbps =~ 40Mbps ... exactly what you see 
"one way".

however, while you have said its "one way" traffic, my guess is you mean 
that its all throughput in one _direction_ but its TCP traffic.  even 
with selective ACK on TCP you have return-traffic at the rate of one 
SACK packet per two TCP segments.
40Mbps of 1500-byte IP packets is a little over 3300 packets/second.  
TCP ACK traffic will be just over 1650 packets/second in the return 
direction, most of these will be 0-byte payload but will be 40-byte 
packets due to 20-byte TCP header and 20-byte IP header.

alas unfortunately those 40-byte packets won't fit into a single 53-byte 
ATM cell since you have both a 5-byte ATM header on the cell and each IP 
packet will have a AAL5 header (8 bytes) and SNAP header (5 bytes).  [i 
think it will -- not sure what encoding you are running].
20-byte IP header + 20-byte TCP header + 5-byte ATM cell header + 8-byte 
AAL5 header + 5-byte SNAP header = 58 bytes -- which requires two cells.

two cells for every TCP ACK packet =

thus for 40Mbps of TCP traffic in one direction, you are actually also 
consuming ~3300 cells of traffic in the return direction (without TCP 
SACK its twice that -- or -- 6600 cells/sec.

...


now, trying to push traffic in both directions, you can see what will 
happen .. best-case is your available rates drop.
doing a quick calculation of best-case throughput of 100k cells/sec 
minus 6600 cells/sec of TCP ACK traffic results in 93400 cells/sec 
available.
93400 cells/sec x 48-byte payload/cell = 35 Mbps .. or .. pretty close 
to what you are seeing.


still can't explain the 27 vs 34 difference - BUT - you should talk to 
your telco and ask them if THEY are seeing any drops.
the primary idea of ATM is that _you_ shape rather than having your 
service provider shape/police your traffic.
if you end up above the policy they have set then they don't necessarily 
drop what you want - in fact - they may drop one cell in the middle of 
an AAL5 stream rendering up to 31 other cells as useless with 1500-byte MTU.


cheers,

lincoln.

Peter Olsson wrote:

>One of our customers has offices in two towns. They have a
>T3 link between these offices. They use the equipment below,
>with lots of free RAM and only using about 60-70% of the cpu
>during traffic peaks. There are no errors on the interfaces
>and although there are some output drops they seem to few to
>be of relevance (one router has 2545 output drops in 7 weeks
>and the other has 8750 output drops in 1 week).
>
>They get about 40 Mbps when they only push traffic in one direction.
>When they try duplex traffic they can't get higher than about 34 Mbps
>in one direction and about 27 Mbps in the other.
>Are these numbers normal?
>
>Both routers run ip cef and ospf. No policy routing or access lists or
>anything else, just plain routing.
>
>The router in town A has two active FastEthernet, two active Serial
>plus the DS3. The router in town B has one active FastEthernet plus
>the DS3.
>
>Town A:
>cisco 3640 (R4700) processor (revision 0x00) with 123904K/7168K bytes of memory.
>System image file is "flash:c3640-is-mz.123-15"
>* NM-2FE2W Port adapter, 2 ports
>* Serial 2T (12in1)
>* Serial 1T WAN daughter card
>* ATM DS3 Port adapter, 1 port
> 	Hardware Revision        : 1.0
> 	Board Revision           : D0
> 	Product (FRU) Number     : NM-ATM-DS3
>
>Town B:
>cisco 3640 (R4700) processor (revision 0x00) with 123904K/7168K bytes of memory.
>System image file is "flash:c3640-is-mz.123-15"
>* NM-1FE2W Port adapter, 1 port
>* ATM DS3 Port adapter, 1 port
> 	Hardware Revision        : 1.0
> 	Board Revision           : D0
> 	Product (FRU) Number     : NM-ATM-DS3
>
>Thanks!
>
>  
>