UDP Frames & Fragmentation

[neteng1]

I performed an OTA capture and noticed a lot of the traffic I was interested in had frame size ~3KB. It is all UDP. Could this be problematic with MSS of 1.5KB? Particularly when channel utilization gets high?

[Flavio Miranda]

Hi

 MSS means Maximum Segment Size and is related to TCP and not UDP.  TCP MSS  must be between 536 and 1363 bytes on Cisco WLC.

 High channel utilization can cause retransmission and yes, it get worst when packet is smaller.  

[neteng1]

Thanks. I am wondering about the frame size. I believe it will get fragmented as the traffic is routed which could be bad for UDP performance? High channel utilization is not good for any traffic but I’m guessing larger frames are more susceptible to failures.

[balaji.bandi]

Can you share the information you captured and explain the environment to understand better here/

[neteng1]

I'm working with a developer that has a real time bi-directional video application. We have Cisco 9120 APs configured for 40Mhz channels. We observe 70-80% channel utilization when ~8 clients are on the same AP. The app begins to experience jitter as this occurs. Notice frame size and timestamps in the screenshot below.

[MHM Cisco World]

After analysis your OTA please find below
A-MPDU is enable and it normal have packet with more than 2000 bytes.

 

https://www.cisco.com/en/US/docs/wireless/controller/7.4/configuration/guides/system_management/config_system_management_chapter_0101.html

[Rich R]

@MHM Cisco World  A-MPDU helps to improve efficiency at the wireless layer (introduced in 802.11n).

Rasika explained A-MPDU nicely here: https://mrncciew.com/2013/04/11/a-mpdu-a-msdu/

That doesn't change the fact that the IP interface for the WiFi on the device (and the ethernet interface at the server end) will usually have an interface MTU of 1500 bytes and THAT is primarily what determines when the IP packets get fragmented.  BUT if there are lower MTU's (eg IPSEC, GRE tunnels) in the path between the endpoints then the effective MTU of the end to end path can end up being significantly smaller than 1500.

[MHM Cisco World]

...

[Rich R · Accepted solution]

@MHM Cisco World that is for the CAPWAP tunnel path MTU between AP and WLC.

The user endpoints (wifi user and the server they connect to) have zero visibility of that MTU.

If the app sends 3K packets they'll get fragmented even before the device sends them and then could get fragmented again by the AP.  Every time they get fragmented that degrades performance and increases jitter.  PMTUD was supposed to be the solution to this:

https://en.wikipedia.org/wiki/Path_MTU_Discovery

But in reality because of firewalls blocking ICMP, PMTUD often doesn't work.

One of many discussions you can find if you search https://blog.cloudflare.com/ip-fragmentation-is-broken/

Bottom line - make the app use smaller packets which won't need fragmentation if you want reliable and consistent performance.

The max value you can use without needing fragmentation depends on exactly what is between your endpoints but you can test by setting DF (do not fragment) on the packets and see what the max value is that gets through.  Ping is the easy way to do that.