3.1 Sending Packets

Figure 2: A user program sends a packet by Sendto()

This section describes what happens when a user program sends a packet using the UDP transport protocol. The steps look similar when using the TCP transport protocol. Figure 2 depicts the functions involved. First, the user program calls Sendto() to send a packet. The simulated kernel invokes the corresponding socket layer handler k_sendto().

One important task of k_sendto() is to convert the data (passed in by the user using the pointer argument) into a pbuf data structure (or a chain of pbuf's). This is because, inside your kernel, a packet is passed around using the pbuf structure. The pbuf structure is very similar to the mbuf structure used in real BSD-style networking stacks. It is described in detail in the simulator handout. For now, just remember that, inside the kernel, a packet is represented by a pointer to a pbuf.

After verifying arguments and converting the data into pbuf's, k_sendto() invokes the UDP-layer interface function udp_usrreq_send(), which invokes the UDP function udp_send(). udp_send() is the function that actually performs the send operation of the UDP layer, for example, adding the UDP header, doing the checksum, etc. There are corresponding functions for TCP.

After udp_send() finishes its work, ip_output() is invoked. ip_output() will construct the IP header, calculate the checksum, query the routing table (by calling rt_lookup_dest()) to find the output interface, and so on. When ip_output() finishes constructing the packet, it invokes the link-layer function if_start() on the appropriate interface, which then sends out the packet. In this figure, if_start() is provided by our simulated kernel, and you will implement the functionality provided by ip_output(). Details of how to interact with the link layer are described in the simulator handout.

The output function ip_output() is passed a pointer to a pbuf, which contains the data packet to be sent. You can assume that space for the IP header has already been allocated in this pbuf and that the p_data member of the passed pbuf structure points to this IP header.