Cisco AnyConnect speed woes!

James Michael Keller jmkeller at
Tue Dec 16 02:01:56 UTC 2014

On 12/11/2014 04:18 PM, Roy Hirst wrote:
> Confidently based on no knowledge at all -
> *Roy Hirst* | 425-556-5773 | 425-324-0941 cell
> XKL LLC | 12020 113th Ave NE, Suite 100 | Kirkland, WA 98034 | USA
>>>     - We have noticed that in some instances that if a user is on a low
>>>     speed connection that their VPN speed gets cut by about 1/3.  
>>> This doesn't
>>>     seem normal that the VPN would use this much overhead
> No, sure, but are you sure that congestion is not dropping a packet 
> somewhere in the end-to-end? If you offend TCP it will likely cut the 
> sender's packet transmit rate, even if the "possible" VPN rate is much 
> higher.
>>>     - We do not have the issue when connecting to VPN directly on 
>>> our own
>>>     network, only connections from the Internet
> Internet would mean maybe a proxy or firewall then, with too-small 
> buffers or an old-time TCP/IP stack? Just a thought.
>>> If you have any ideas on what we could try net, please let me know!
>>> - Zachary
>> What OS builds?   At one point the code had an 8 packet hard coded 
>> window per tcp flow, which capped ssl over tcp window size to about 
>> 5mbps depending on RTT.     Recent 8 branches raised this to 
>> something more reasonable that capped around 20 mbps. DTLS over udp 
>> and IPSEC tunnels did not have this issue.
> UDP traffic does not have this problem but TCP does? Hmmm...

UDP transport with DTLS or IPSEC in UDP Encapsulation doesn't need to 
deal with tcp window size scaling and the associated packet buffers.


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