So let’s roll up our sleeves and get a Lab scenario happening,
To keep things (relatively) simple, I’ve put the eNodeB on the same subnet as the MME and Serving/Packet-Gateway.
So the traffic will flow from the eNodeB to the S/P-GW, via a simple Network Switch (I’m using a Mikrotik).
While life is complicated, I’ll try and keep this lab easy.
Experiment 1: MTU of 1500 everywhere
| Network Element | MTU |
| Advertised MTU in PCO | 1500 |
| eNodeB | 1500 |
| Switch | 1500 |
| Core Network (S/P-GW) | 1500 |
So everything attaches and traffic flows fine. There is no problem right?
Well, not a problem that is immediately visible.
While the PCO advertises the MTU value at 1500 if we look at the maximum payload we can actually get through the network, we find that’s not the case.
This means if our end user on a mobile device tried to send a 1500 byte payload, it’d never get through.
While DNS would work, most TCP traffic would flow fine, certain UDP applications would start to fail if they were sending payloads nearing 1500 bytes.
So why is this?
Well GTP adds overhead.
- 8 bytes for the GTP header
- 8 bytes for the transport UDP header
- 20 bytes for the transport IPv4 header
- 14 bytes if our transport is using Ethernet
For a total of 50 bytes of overhead, assuming we’re not using MPLS, QinQ or anything else funky on our transport network and just using Ethernet.
So we have two options here – We can either lower the MTU advertised in our Protocol Configuration Options, or we can increase the MTU across our transport network. Let’s look at each.
Experiment 2: Lower Advertised MTU in PCO to 1300
Well this works, and looks the same as our previous example, except now we know we can’t handle payloads larger than 1300 without fragmentation.
Experiment 3: Increase MTU across transmission Network
While we need to account for the 50 bytes of overhead added by GTP, I’ve gone the safer option and upped the MTU across the transport to 1600 bytes.
With this, we can transport a full 1500 byte MTU on the UE layer, and we’ve got the extra space by enabling jumbo frames.
Obviously this requires a change on all of the transmission layer – And if you have any hops without support for this, you’ll loose packets.
Conclusions?
Well, fragmentation is bad, and we want to avoid it.
For this we up the MTU across the transmission network to support jumbo frames (greater than 1500 bytes) so we can handle the 1500 byte payloads that users want.