37.1.0.0.4 Packet Number vs TSN Numbering

While ns starts numbering packets at 0, the SCTP module starts numbering DATA chunk TSNs at 1 and assigns undefined TSN values (-1) to control chunks (ie, INIT, SACK, HEARTBEAT, etc). The four packets exchanged during the association establishment are counted in the packet enumeration, but do not show up in graphs. This information is important when doing things like specifying a drop list for the ErrorModel object. For example, packet 2 actually refers to the first SCTP packet with DATA chunk(s).