Network stack overview
Drivers & ifnet
Netisr(9) is generic network dispatch service. It allows you to determine how exactly packets for given L3 protocol are processed in terms of flow grouping and queuing to different queues (to enable parallel processing on SMP). Since every network forwarding element should avoid reordering for the packets inside single flow (for example, TCP session) we need some way to determine which flow packets belongs to. Typically this is done using some kind of hashing function that uses L3+L4 headers to make u16 value which is the same for single unidirectional flow. Different protocols require different flow handling techniques, so, quoting manual:
netisr supports multi-processor execution of handlers, and relies on a combination of source ordering and protocol-specific ordering and work- placement policies to decide how do distribute work across one or more worker threads. Registering protocols will declare one of three poli‐ cies: NETISR_POLICY_SOURCE netisr should maintain source ordering without advice from the protocol. netisr will ignore any flow IDs present on mbuf headers for the purposes of work placement. NETISR_POLICY_FLOW netisr should maintain flow ordering as defined by the mbuf header flow ID field. If the protocol implements nh_m2flow, then netisr will query the protocol in the event that the mbuf doesn't have a flow ID, falling back on source ordering. NETISR_POLICY_CPU netisr will entirely delegate all work placement decisions to the protocol, querying nh_m2cpuid for each packet.
Netisr supports 2 (3) different dispatch models: "direct" (e.g.
Base netisr entry point lives in net/if_ethersubr.c:ether_demux() function. However, many places on stack need to perform packet recirculation (various tunneling, etc..) so they call netisr directly using netisr_dispatch(_ETHERTYPE_, ..) function.
Software interrupts, SWI_NET. Used by netisr(9) and pf(4).
- Current implementation and agreements
IPv4 address assignment
- "inet" and "alias"
- Under the hood
IPv6 address assignment
- Under the hood