Message Sequence Numbers

Every packet has a "first" sequence number in the header. This is the number associated with the first message in the packet. Each subsequent message in the packet has a sequence number one greater than the previous message. The next packet will have a starting sequence number one more than the last message in the previous packet except for the start of the trading day's session and in the event of a Cboe system failure. When the session is started the sequence number will be reset to 1. During a Cboe system failure the sequence number can reset to a lower value than was previously seen prior to the failure. In either case, the sequence number will again increase by one for each message.

Each channel of a feed has its own sequence number associated with it starting with sequence number 1. Verification of message sequence numbering must be done for each individual channel.

Firms must ensure that the sequence numbers maintain continuity. Any deviation from an expected sequence number must be considered as an error condition. Firms are required to take appropriate recovery action any time an unexpected sequence number is detected.