Cheers,
Brad

Technologies in the middle can and do have an impact on the overall TCP window size. For instance , if you were to look at an “legacy” implemantation of site to site VPN across a 6500 using a VPN IPSEC module you would most certainly find tcp window/mtu manipulation on the router that will impact Windows Servers and clients. This will also without a doubt be application dependent. For instance don’t assume that all applications operate under the same conditions with windows sizes, etc. Your browser may operate a certain throughput and MS outlook at another throughput level.

If you are using MPLS to connect your site don’t always assume that all the carrier gear is setup the same way to handle a particular MTU size from end to end. For instance Carrier “X” may have legacy devices that do in fact impact the MTU size and therefore TCP stack is impacted indirectly.

Granted I am not stating that IP MTU and TCP window sizes are the same animal. But what you will find is there is a very close relationship and the MTU will in fact impact TCP applications.
Most the Cisco VPN Documentation will refer to this a MSS size , but a closer look will revel it’s window sizing.

I would just caution that you need to look at the underlying technologies that connect the locations and see if they could potentially impact tcp windows size, througput , etc.

I would highly suggest you get a software package that tests throughput from end to end on server/client before suggesting results. As over time you will see the TCP window sizes and throughput began to degrade on networks.

Thats my two cents.
Regards,

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THROUGHPUT from BDP:
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What’s my throughput then,

RWIN (66,780 bytes) / RTT (.032) = 2,086,875 bytes (or 16.7Mbps)

Umm, my DSL is only up to 2Mbps, so that is NOT my throughput nor correct

so my questions are :
1- is it really that server sense that i can download with 16 M , but i will drop the rest as my physical speed is 2M ( that means i will drop 14 M)?

2- what about if my speed was 24 M , is that means my max peed will be 16 M ??

i hope that i could elaborated my questions correctly

thanks again for your effor ,
Mina

1) The routers are operating at Layer 3 of the OSI model and therefore pass IP packets paying no attention the upper layer TCP information. A standard router never changes window sizes etc. TCP windowing is managed by the end hosts participating in the TCP exchange.

2) Correct. The TCP throughput calculations discussed in this article are for the purposes of calculating the throughput potential of an individual TCP session. If you have multiple TCP sessions, each session would add its own bandwidth load to the link. If the link does not have enough bandwidth to carry the potential bandwidth of all the TCP sessions, congestion will occur, packets will get dropped, TCP will detect that and dynamically scale back the window size in half, then slowly increase the window size until packet loss happens again, and the cycle repeats. The result of this behavior is all TCP flows evenly and fairly balancing throughput on the link.

1.As I understand Windows systems on the LAN uses a TCP window size of 17K+ bytes. In your example, each server has a default setting of 17K+ bytes. Now while replicating the data between the two servers in remote sites, will the TCP window size be changed by the routers to 65K ? Or will it remain to be 17K+ through the entire path ? Assume we are not using any bandwidth optimizers.

2. Assume there are two servers at the source site replicating to two servers at the remote site via a common router pair. Each replication context will then constitue a seperate stream and hence we will get double the bandwidth over the WAN link [assuming the WAN link to be a fat pipe]. Is my understanding correct ?

Ashwin

Thanks for the very interesting explanation here! I have a question and hope you can help.

We want to perform a data consolidation for some of our application whereby we already know the number of server and we will be able to get the volume of traffic. However, we are not sure how much WAN bandwidth needed in the data center that we will do the data consolidation.

Let’s say, I have the following parameters:
- 120GB of aggregated traffic over the LAN (this is the server/client traffic)
- WAN latency is 75ms (RTD-150ms)

Objective: To know the WAN bandwidth to be used

Thanks…Slimer