Packet fragmented after exceeding 1500 length

[Alex Gilson]

Hi,

We have seen that whenever the UDP payload exceeds 1500 in length, the packet is fragmented and it is dropped once it reaches emu. Is there something I have to change in order to increase this limit? I have tried changing the mtu value in trex_astf_profile.py, trex_ns.py and the emu plugin itself.

We are using trex with these options:

 ./t-rex-64 -i --cfg /root/tmp/trex_cfg.yaml --astf --software --emu --tso-disable 16 --bird-server -c 1

Thank you.

Regards,

Alejandro Gilson.

[hanoh haim]

Hi Alex, 

In your command line there are 3 different generators of traffic suggest to check each of them separately to understand what is the limitation 

1. The astf profile is generated by trex- dp cores 

2. The emu service - the emu golang server profile 

3. The bird service - LXC c++ server, for generating routing protocols 

What is the generator that has fragmentation?

Thanks

Hanoh

[Alex Gilson]

Hi Hanoh,

The issue had to do with packets that entered T-rex. My assumption was that it was trex that was fragmenting them once the size exceeded 1500 but now I've seen this was done by one of the routers.

The network stack we are dealing with is ETH -> IP(1) -> UDP -> GTP -> IP(2) -> TCP -> TLS -> DNS

Because T-rex drops packets that are fragmented we decided to reduce the MTU from our traffic generator so that they are fragmented at IP(2) level. We then reassemble them in the emu plugin.

Thank you for your help,

Alejandro.