OSI model: Does any traffic occur without Layer 7 initiation?

Question

Of course a lot of network traffic occurs at lower layers, but does anything happen without a specific Layer 7 request?

Answer 1

There is plenty of overhead traffic used solely by the network equipment. ARP requests, routing table exchanges and such. Keep in mind though that OSI is just a reference model and not an actual protocol stack. What is considered an application layer in one situation (http for web requests) can act as a transport for a different protocol in other situations (e.g. SOAP).

Answer 2

Certainly. A DNS lookup uses UDP on layer 4 directly, not involving layers 5-7 at all. I suppose you could call DNS an "application layer" request, but there's no session or state, it's really too trivial to count as such in my opinion.

Update:

Okay, I think I see what you're asking. How about DHCP broadcasts when a host joins a network? Technically it's the dhcp client application doing the request, but it's not (necessarily) initiated by a user.

Also, when an IPv4 interface comes online, it will do an ARP probe to make sure the IP address it's configured with is not already in use.

Even deeper, routers and switches will exchange BGP messages when they come online and on a regular basis, and I believe this occurs even if no endpoints are even connected, e.g., two routers connected together and to nothing else.

Answer 3

Even within a single TCP session there can be lower-layer traffic that's not directly triggered by higher-layer events. The best example of this is if TCP Keepalives are enabled, which will cause TCP traffic (transport layer) triggered by a simple timeout even if there is no application-layer activity.