How is a connection kept opened at the physical layer level

Question

I'm having a difficult time understanding the concept of persisent connection.

If we take as an example the Http protocol using the keep-alive header, after tcp/ip performing the handshake along with the necessary validations, the connection is kept alive for x ms, allowing the user to send multiple requests using the same connection/port. The impact of a persistent connection is clear to me, at least at this level of understanding, overhead is reduced.

• However, what I cannot seem to understand is how does this connection is kept alive at physical level. Is it at logical or physical level and how ?
• If it is at logical level, the message to be sent will already contain the headers with information from the performed handshakes and validations so it will skip through the steps involved in finding the target physical address and other router related actions on the packet including http handshake and so on ?
• While a connection is alive, will there be any control (ping) messages sent between source and destination to see if they are still there ?

Regards

Answer 1

The physical layer doesn't know any "open connection". It works on bits or packets alone and just transports bits from one end of a link to the other.

A logical connection becomes only possible much higher up the stack where TCP (or a similar protocol) uses the lower layers to simulate a connection that really isn't there in packet-switched networks.

Within a single logical connection, completely different routes = physical paths can be used without the transport layer even noticing.

Whether or not a transport layer protocol (or application layer protocol for that matter) uses some kind of keep-alive or not is entirely up to the protocol. Most often, a connection is closed when there's been no transmission for a certain period (timeout) so you don't end up with a bunch of zombie sockets.