When using QoS to regulate data transfer, you can decide whether the corresponding rule applies to the send or receive direction. Of course, whether a particular data transmission is being sent or received is a question of perspective. There are two variants:
- The direction corresponds to the logical connection establishment
- The direction corresponds to the physical data transmission over the respective interface
The differences are made clear when we consider an FTP transfer. A client on the LAN is connected to the Internet via a device.
- In an active FTP session, the client uses the PORT command to inform the server on which port it expects to receive the DATA connection. The server then establishes the connection to the client and sends the data in the same direction. In this case the logical connection and the actual data stream are sent from the server over the interface to the client, so the device considers both to be the receive direction.
- The situation is different with a passive FTP session. Here, the client establishes the connection to the server. Consequently the logical connection is from the client towards the server, but the data transmission over the physical interface is in the reverse direction, from the server to the client.
By default, a device evaluates the send or receive direction based on the logical connection establishment. In some applications this way of seeing things is not so obvious, and an alternative is to switch to considering the physical data stream.
Important: The distinction between the send and receive directions only applies for the setup of maximum bandwidths. For guaranteed minimum bandwidths, fragmentation and PMTU reduction, the direction is always taken to be the physical data transfer over the respective interface.
