Ethernet device MTU setting implications in packet ingress path

What is the behavior of an ethernet device in the packet ingress path?

If the sender is sending a larger-than-MTU frame, then:

1) does the receiver's device drop it directly in hardware,

2) or accept it and send it up for the kernel's IP stack to handle it?

3) when is ICMP frag-required sent?

4) does it ma开发者_开发问答ke a difference if the ethernet device is say on a intermediate router vs a end host?

---

It is impossible to answer 1), 2) definitively for all devices and networking stacks. The Ethernet standard defines an MTU of 1500 bytes so that is all you can rely on and in general you should expect frames with larger MTUs to be dropped.

However in reality it is likely in an end host that if the network interface hardware doesn't drop the oversized frame (which is typically referred to as giant) then it will make it's way up the software stack and be processed. Even though the stack may not drop the oversized frame due to it being over the MTU it may still get dropped for other reasons, e.g. due to an internal queue exhaustion.

Although the maximum MTU of an Ethernet frame has remained constant the maximum size of an Ethernet frame has grown over time to encompass features like 802.1Q VLAN single and double tags. MPLS further increases the frame size to include label stacks. This means intermediate switches are usually tolerant of frames that exceed the interface MTU by some amount. One major vendor effectively tolerates a max MTU of 2000 bytes by default in their current switches. Older switches may be less tolerant.

To get a definitive answer you'll need to do some research into the specific hardware and software that you care about.