What does "Outbound Data Transfer" entail?

Lord Ratte asked:

When calculating pricing for ECS on https://calculator.aws/#/createCalculator there is a distinction between the pricing of inbound and outbound traffic. I am confused about what the precise difference between those two is.

For instance, let’s imagine two roughly identical ec2 instances. They are identical except that they both have a different cronjob on them.

Instance A makes a REST request to a public endpoint every hour and receives a 1 KB payload as a response every time.

Instance B makes a REST request to the same public endpoint every hour with a different parameter and receives a 1 GB download as a response.

The two things I can see happening based on the information provided by Amazon are:

  1. Both get basically the same quantity billed for Outbound Data Transfer but Instance B gets a significantly larger quantity under "Inbound".
  2. Instance B has a high quantity under Outbound Data Transfer because the responses are considered part of the outbound request.

An auxiliary question is this: if the above assumes TCP, will the answer be consistent over UDP?

My answer:

This is extremely simple. Inbound is traffic that arrives at AWS and outbound is traffic that leaves AWS. Thus the first scenario you pose is correct. There is no possibility that inbound traffic would be counted as outbound as in your second scenario. And the network protocol is irrelevant.

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