AWS Direct Connect: Troubleshooting Dropped Packets with Jumbo Frames

Troubleshooting Dropped Packets with Jumbo Frames

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Question

You are trying to send packets from an EC2 Instance to an on-premises server over AWS Direct Connect using Public Virtual Interface.

You have set Jumbo frames due to the size of the packets being sent.

But the packets are being dropped.

What needs to be done to ensure that the packets don't get dropped?

Answers

Explanations

Click on the arrows to vote for the correct answer

A. B. C. D.

Answer - C.

The AWS Documentation mentions the following.

Jumbo frames allow more than 1500 bytes of data by increasing the payload size per packet and thus increasing the percentage of the packet that is not packet overhead.

Fewer packets are needed to send the same amount of usable data.

However, outside of a given AWS region (EC2-Classic), a single VPC, or a VPC peering connection, you will experience a maximum path of 1500 MTU.

VPN connections and traffic sent over an Internet gateway are limited to 1500 MTU.

If packets are over 1500 bytes, they are fragmented, or they are dropped if the Don't Fragment flag is set in the IP header.

Option B is invalid because this would lead to dropping frames.

Option A is invalid because this is equivalent to Jumbo frames.

Option D is invalid because this would not resolve the issue.

For more information on Network MTU, please refer to the below URL.

https://docs.aws.amazon.com/AWSEC2/latest/WindowsGuide/network_mtu.html

When sending packets from an EC2 Instance to an on-premises server over AWS Direct Connect using Public Virtual Interface, the size of the packets being sent can be an issue. Jumbo frames can be used to mitigate this problem, but sometimes packets can be dropped despite this. In such cases, there are several steps that can be taken to prevent the packets from being dropped.

Option A suggests ensuring that the MTU is set to 9001. This is a valid solution because the Maximum Transmission Unit (MTU) determines the maximum size of a packet that can be sent over a network. By default, the MTU for Ethernet frames is 1500 bytes, which is insufficient for large packets. Jumbo frames have an MTU of 9000 bytes or more, and therefore, setting the MTU to 9001 should ensure that the packets are not dropped.

Option B suggests ensuring that the "Do Not Fragment" flag is set in the IP header. This is not the correct solution as setting the "Do Not Fragment" flag in the IP header prevents the packet from being fragmented if the MTU of a network link is smaller than the packet size. This can cause packets to be dropped if they are larger than the MTU of the network link.

Option C suggests ensuring that the "Do Not Fragment" flag is not set in the IP header. This is also not the correct solution as mentioned above, not setting the "Do Not Fragment" flag can cause packets to be fragmented and dropped if the MTU of the network link is smaller than the packet size.

Option D suggests enabling Enhanced Networking on the instance. While this is a good practice, it may not necessarily solve the issue of dropped packets due to the packet size. Enhanced Networking can improve network performance by using single root I/O virtualization (SR-IOV) to provide high-performance networking capabilities. However, it may not be sufficient to prevent packets from being dropped due to large packet sizes.

In conclusion, the correct answer to ensure that packets don't get dropped when sending Jumbo frames from an EC2 Instance to an on-premises server over AWS Direct Connect using Public Virtual Interface is to set the MTU to 9001, which is option A.