OpenFlow Protocol

Openflow version 1.0: The controller establishes a secure channel (Sceure Channel) with the switch through the Openflow protocol, and issues the flow table.

Version 1.3 Openflow: multi-controller, multi-flow table.
A series of standard terms are defined for implementing the communication process between Controller and Switch.
Defines how the Controller controls the Switch and how the Switch feeds the Controller.
Defines the message type and format of the communication process between Controller and Switch .

It is equivalent to the routing table and CAM table of the traditional network.
The table of the traditional network is dead, just follow the above to find and forward.
SDN flow table, many sheets, programmable.

A collection of data packets that pass through the same network and have the same attributes at the same time .
The definition of flow can be different for different situations. Generally based on port and source and destination IP.
In the SDN system, all data is processed in units of "streams".

When the flow comes, look up the table (based on the serial number lookup).
Each flow table has detailed flow table entries.
There are corresponding parameters in each flow table entry, and corresponding actions are made according to the parameters (either transfer or discard, neither transfer nor discard, the next table will be processed ).
Although it is necessary to look up the table recursively, the calculation time is saved, and the forwarding speed is accelerated in general.

The flow entries of different versions of Openflow are somewhat different. The following are the flow entries of Openflow 1.0.
It includes three parts: header field, counter, and action.

In addition to the incoming interface, the traditional addressing information of layers 2-4 appears in the header field (MAC, IP, PORT).
Compared with traditional network switches (routers), Openflow switches are a vague concept and no longer distinguish between the two. Therefore, Openflow switches can be broadly understood as Openflow forwarding devices (switches, routers, and firewalls).

The counter mainly counts each table, each port, each flow, etc., which is convenient for traffic monitoring.
It lays the foundation for traffic visualization .

To process the matched flow, in the traditional network, it is either forwarded or discarded, and there is no third option. Openflow1.0 specifies the required actions (Required Actions) and optional actions (Optional Actions).
Required Action - Forward:
- ALL: Forward to all egress (excluding ingress).
- CONTROLLER: Encapsulate and forward to the controller.
- LOCAL: Forward to the local network stack.
- TABLE: Performs the actions in the flow table on the packets to be emitted.
- IN_PORT: Emitted from the portal.
Necessary Action - Drop
- All matched packets are discarded by default if there is no table entry that specifies a processing action.
Optional Action - Forward
- NORMAL: Forwarding processing is performed according to Layer 2 or Layer 3 of traditional switches.
- FLOOD: Floods from egress via minimum spanning tree , not including ingress. Traditional networks that only flood when the CAM table is empty or full are now more flexible.
Optional Action - Enqueue
- Forward packets to a queue bound to a port. (Flow Control)
Optional Action - Modify-field
- Modify the header content.
- This action is the biggest difference from the traditional network, Openflow can modify the packet header! (The information of layers 2-4 can be modified).