Covert timing channels are used to transmit information through computer networks in a seamless or secret way. Inter-packet gap and packet reordering are among techniques of encoding information in covert channels. Several reports on evaluating the capacity and the robustness of covert channels are published in which the channel is evaluated using real environment field tests or by means of computational methods. Real environment field experiment lacks the repeatability property of the test and computational method is normally too simple to enable one to evaluate the channel under desired circumstances. In this paper, a hybrid coding schema is proposed compounding inter-packet gap and packet reordering techniques, and also a model for an end to end communication channel in the Internet is proposed using Coloured Petri net. The model is able to inject different types of noises such as packet loss, packet reordering, and jitter to a given traffic. The model was used to implement a controlled hybrid covert timing channel under desired noise levels. Then, the capacity and robustness of the proposed covert channel was measured using different noise scenarios. The evaluation results demonstrate that capacity and robustness of the proposed covert channel is improved compared with the previous reports. Also it is proved that the proposed model is effective for further covert channel evaluation in given circumstances.