Ensuring correct behaviour of a distributed real-time function requires bounds on the network traversal time. The network calculus is a theory designed to compute such bounds, used to certify the A380 AFDX backbone. In the PEGASE project, some enhancements have been done to the theory, and a software implementation has been developed. To ensure correctness of the software, instead of a process based on tests and code coverage, the "proof by instance" approach is used. The tool generates a proof of correctness of the result, that can be checked by a proof assistant.