Bandwidth/speed issues over ATM OC3

Unanswered Question
Jun 15th, 2010
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Scenario:


Point to point OC3 ATM circuit between two sites that are thousands of miles apart.  3845 on one side with a PA-A3-OC3SMI and a 7206 on the other side with a NM-1A-OC3-POM.


Problem:


Using iPerf, with a window size of 4MB, I can sustain 120Mbps of TCP traffic in one direction, but the other direction will not sustain a steady rate... it goes up to 70Mbps then down to 2Mbps, then back up to 80Mbps, then down to 5Mbps, etc...  UDP traffic in both directions works perfect, at near OC3 rates.


Point to point transfers using HTTP, FTP, SMB, etc... from one side to the other will never go above 15Mbps.


Question:


I don't know a lot about ATM, so I am confused as to why I am having issues pushing data over this link.  What am I missing?


No errors on any of the ATM interfaces, or the layer 2 path on the LAN side of the routers.  I am about as close as I can get to the router on the LAN side without plugging directly into it.


Configs:


7200:


interface ATM2/0

no ip address

no ip redirects

no ip unreachables

no ip proxy-arp

ip flow ingress

no atm ilmi-keepalive

!

interface ATM2/0.1 point-to-point

bandwidth 2048

ip address 192.168.158.153 255.255.255.252

no ip redirects

no ip unreachables

no ip proxy-arp

ip flow ingress

ip ospf network point-to-point

snmp trap link-status

pvc 0/33

  vbr-nrt 149760 149760

3800:
interface ATM2/0
no ip address
no ip redirects
no ip unreachables
no ip proxy-arp
no atm ilmi-keepalive
!
interface ATM2/0.1 point-to-point
bandwidth 2048
ip address 192.168.158.154 255.255.255.252
no ip redirects
no ip unreachables
no ip proxy-arp
ip flow ingress
ip flow egress
ip ospf network point-to-point
ip ospf priority 2
ip ospf mtu-ignore
no snmp trap link-status
pvc 0/33
  vbr-nrt 149760 149760
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Giuseppe Larosa Tue, 06/15/2010 - 13:52
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Hello Gvb,


>> I don't know a lot about ATM, so I am confused as to why I am having issues pushing data over this link.  What am I missing?


Segmentation and Reassemby


to carry an IP packet of 1500 bytes ATM puts adds to the packet a SNAP header and puts it inside an AAL5 PDU the resulting PDU is segmented into 32 ATM cells the reverse process happens on the other side.


You should try to use multiple parallel TCP sessions with iperf, I had similar problem in testing an ATM 155 Mbps in a lab with 10 meters of fiber.

I was using netperf to test and I couldn't get more then 40 Mbps of  TCP throughput with a single session.


Hope to help

Giuseppe

gvb Tue, 06/15/2010 - 13:59
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But this 15Mbps per TCP session that I am getting between two machines (HTTP, FTP, etc..) WITHOUT tweaking window size, is normal?

Giuseppe Larosa Tue, 06/15/2010 - 14:14
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Hello Gvb,


given the distance there is a consistent propagation delay and default TCP window is only 64K bytes


increasing the TCP window using the extended window is the workaround for this.


see

http://www.psc.edu/networking/projects/tcptune/


it can be possible we had complains over 700 km of fiber by server people (with no ATM in the middle) when using standard TCP window


Hope to help

Giuseppe

gvb Tue, 06/15/2010 - 14:22
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This is all endpoint tuning though... is there any way to optimize this at the network level?

Giuseppe Larosa Tue, 06/15/2010 - 14:30
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Hello Gvb,


>> This is all endpoint tuning though... is there any way to optimize this at the network level?


no because it is a bottleneck at application layer if you would use a traffic generator at the two ends sending IP packets you could demonstrate you can reach better speeds


of course C3845 could be upgraded to a more powerful router but the problem for TCP traffic would be still there with standard TCP window


Hope to help

Giuseppe

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