Cisco IOS Firewall TCP Handling
{"Image DocumentName": "Cisco IOS Firewall TCP Handling", "Image ImageDescription": "When the first packet from a TCP flow is received by the router (TCP SYN), the inbound ACL on the inside secured interface is checked. If the packet is permitted, a dynamic session entry is created. The session is described by endpoint addresses, port numbers, sequence numbers, and flags. All subsequent packets belonging to this session will be checked against the current state and discarded if the packets are invalid. This graphic illustrates the three-way handshake that TCP uses. The first packet contains a random sequence number and sets the TCP SYN flag. The second packet contains a random sequence number that the responding host generates, an acknowledgment sequence number that is the received sequence number incremented by one, and the TCP SYN and ACK flags set. The third packet acknowledges the received packet by incrementing the packet sequence number in the acknowledgment sequence, raising the sequence number by the appropriate number of transmitted octets, and setting the ACK flag. All subsequent segments increment their sequence numbers by the number of transmitted octets and acknowledge the last received segment by an increment of 1, according to the TCP state machine. After the three-way handshake, all packets will have the ACK flag set until the session is terminated.", "Image XResolution": "37", "Image YResolution": "37", "Image ResolutionUnit": "Pixels/Centimeter", "Image Software": "SlidePlayer", "Image YCbCrPositioning": "Centered"}