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TCP Adaptive Buffers and TCP Buffer Sizes

I have a two folded question:

1. I have noticed during the WAAS boot cycle from a log file that many variants of TCP are loaded by the software (Nevada,High, BIC, etc). From reading and experience I have seen different vendors deploy different variants of TCP based on their implemented TCP stack. These all have different behaviors based on the environment in which they are used - same are ideal for high packet loss, high latency networks, with high bandwidth (LFNs, etc). Are these used interchangeably or in some dynamic manner by the WAAS during specific scenarios? Are there specific application AOs that take advantage of different variants of TCP? In addition, how does the TCP adaptive buffer function work exactly?

2. I normally don't change much on a typical WAAS installation besides custom policies for specific applications. According to what I have read, the TCP parameters are fine for most scenarios. Is there a best practice approach or some type of BDP calculation that engineers should consider during a typical WAAS install to ensure that we are configuring these devices to their maximum performance from a TCP standpoint? Cisco has done such a great job on engineering the defaults into WAAS it doesn't leave a lot for the installer to change unless a specific application or link needs special attention. I want to make sure I am not missing anything.

Your input is greatly appreciated.


Michael Louis

Senior Solutions Architect

CCIE 17082 (R/S)

NWN, Inc

  • Wide Area Application Services (WAAS)
Cisco Employee

Re: TCP Adaptive Buffers and TCP Buffer Sizes

Hi Michael,

Ans 1:  This is purely tcp window sizing and sliding mechanism to deal with latency and bandwidth. In most cases, you do not need to modify the acceleration TCP settings  because your WAAS system automatically configures the acceleration TCP  settings based on the hardware platform of the WAE device. WAAS  automatically configures the settings only under the following  circumstances:

When you first install the WAE device in your network.

When you enter the restore factory-default command on the device.

The WAAS system automatically adjusts the maximum segment size (MSS) to  match the advertised MSS of the client or server for each connection.  The WAAS system uses the lower of 1432 or the MSS value advertised by  the client or server.

If your network has high BDP links, you may need to adjust the default  buffer settings automatically configured for your WAE device.

If you want to adjust the default TCP adaptive buffering settings for your WAE device, see the link below.

Ans 2:  You will find more details here on TCP buffers for different WAAS  models. There is also a formula on how to calculdate the BDP value for  any link and lot of explanation on what the MSS, send buffer size and  receive buffer size will do and how tcp buffering will work.

Hope this helps.


PS: If this answers your question, please mark this as Answered.

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