CODEWITHSUNDEEP
network-programminglinux-kernelipnetwork-protocols
Andrey G.A
Andrey G.A

Reputation: 305

Understanding the process of receiving network packets

I started to learn Linux Networking and packets filtering. In the iptables documentation it is stated that:

  1. If a packet is destined for this box, the packet passes downwards in the diagram, to the INPUT chain. If it passes this, any processes waiting for that packet will receive it.

So, suppose there're 3 server apps on a host. Servers A and B are TCP servers, and C is UDP server. Is it true, that if we receive an UDP packet, at IP level this packet is to be delivered for apps A, B, C? Or sockets of apps A & B wouldn't receive this packet at all?

Upvotes: 0

Views: 1555

Answers (3)

Andrey G.A
Andrey G.A

Reputation: 305

My question wasn't well formulated, unfortunatelly. I understood it when I had seen the answers. Here is an explanation which I was looking for, it's from http://www.cs.unh.edu/cnrg/people/gherrin/linux-net.html#tth_chAp6:
> When the process scheduler sees that there are networking tasks to do it runs the network bottom-half. This function pops packets off of the backlog queue, matches them to a known protocol (typically IP), and passes them to that protocol's receive function. The IP layer examines the packet for errors and routes it; the packet will go into an outgoing queue (if it is for another host) or up to the transport layer (such as TCP or UDP). This layer again checks for errors, looks up the socket associated with the port specified in the packet, and puts the packet at the end of that socket's receive queue.

Upvotes: 0

Gil Hamilton
Gil Hamilton

Reputation: 12337

TCP servers and UDP servers operate in very different ways.

At most one TCP server will listen on a given TCP port (corner cases ignored for the sake of simplicity). Connection requests (encapsulated in IP packets) destined for that port are "accepted" by exactly one process (more accurately, accepted by a process that has a file descriptor corresponding to exactly one listening endpoint). The combination of [remote_address,remote_port] and [local_address,local_port] is unique. A TCP server doesn't really receive "packets", it receives a stream of data that doesn't have any specific relationship to the underlying packets that carry the data (packet "boundaries" are not directly visible to the receiving process). And a TCP packet that is neither a connection request nor associated with any existing connection would simply be discarded.

With UDP, each UDP datagram is logically independent and may be received by multiple listening processes. That is, more than one process can bind to the same UDP endpoint and receive datagrams sent to it. Typically, each datagram corresponds to a single IP packet though it is possible for a datagram to be broken into multiple packets for transmission.

So, in your example: no, a server that is listening for TCP requests (a "TCP server") will never receive a UDP packet. The port namespaces for TCP and UDP are completely separate.

Upvotes: 3

Arshan
Arshan

Reputation: 744

The delivery of the packet will depend on its destination port.

Lets assume that the servers A, B and C are listening on port 1111, 2222 and 3333 respectively, so when a packet with destination port 2222 is arrived, it will be delivered to server B.

Upvotes: 0

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