Learn the 5 criteria that determine whether you have healthy TCP sessions
As a rule of thumb, healthy TCP sessions are identified and referenced by the following 5 Key Performance Indicators (KPIs) and their thresholds:
- # SYN’s – refers to session starts; these should be twice the number of sessions. This is because of the 3-way handshake SYN | SYN-ACK | ACK (ACK equals Acknowledgment indicating “no errors – please continue”).
- # FIN’s – refers to session endings; these should roughly be twice the number of sessions. This is because with healthy TCP sessi0ns, each session ends with a FIN-ACK initiated and confirmed by both the client and the server side. Since some applications are working with long-lasting sessions, from time-to-time, the amount of FIN’s might actually be lower than twice the number of sessions.
- # RST’s – refers to session restarts; these should be close to zero, indicating a low number of session resets. Depending on the TCP stack, it could also be an indicator of the number of session endings; similar to using a FIN-ACK. In particular, Microsoft applications running over SSL are well-known for this (mis-)behavior.
- # DupAck’s – refers to packets that are delayed or dropped; these should be close to zero indicating a low number of delayed/lost packets.
- # 0-Win’s – refers to a high system utilization; the value is expected to be close to zero indicating that the hosts involved are in good shape for processing incoming packets.