Traffic-shape for incoming traffic

[michael.dolan]

We have a 1MB connection to the internet.

We wish to divide the traffic is such a way that LAN users only have 256k of the 1mb. Internet users browsing sites hosted on our local servers (inbound traffic) should have unrestrcited bandwidth.

I have followed the GTS guide and recommendations from other posts. Here is a example of our configuration:

interface Serial0

traffic-shape group 110 256000 7936 7936 1000

interface FastEthernet0

traffic-shape group 110 256000 7936 7936 1000

access-list 110 permit ip host x.x.x.x any

access-list 110 permit ip any host x.x.x.x

What i am unsure of this exactly how it is possible to restrict incoming traffic considering the shaping is happening once the packets have already passed over the leased line?. I cannot seem to get a definitive answer.

Does TCP/IP adapt (i.e lowering its windows size due to unacknowledged packets? or something similar) or does it just retransmit the dropped packets??

Thanks in advance

[michael.dolan]

I just picked up my good old TCP/IP book and from what i have read i understand the following:

Incoming packets will be routed from the ISP down the serial line. They will arrive at a rate of 1mb (128k/sec).

The local router configured with traffic shaping (see above) will only transmit traffic at a rate of 256k (32k/sec). This would probably cause packets to either be dropped or acks to be sent to the sender too late.The problem i assume is that the sender will just retransmit the data segment and the serial line will still be fully congested as the shaping is happening after the data has already traversed the link.

However from what i have read:

If an ACK is not received for a data segment in a specific period of time (RTO/RTT), the sender assumes that a router has dropped the packet due to congestion. The sender doubles the time to wait for its next ACK(RTO/RTT). It then retransmits the segment but uses the Slow start and congestion avoidance algorithms to control the windows sizes (sending of data) and consequently limiting the bandwdith consumed on the leased line.