The UDP/IP stub will primarily consist of two user level daemons, one which sends UDP messages to desired destination and the other which receives UDP messages.  A user-level implementation of UDP/IP stub seems the best for the following reasons

• The messages will need to be fragmented and de-fragmented over the ethernet interface. In a user level implementation, this will be taken care of transparently by the existing protocol stack.
• Debugging the system is easier since we dont have to worry about errors in sending messages during initial stages of coding other subsystems.
• Performance is not an issue for this module.

This daemon receives datagrams over a UDP socket. In the actual myrinet driver implementation, the driver queues the packet from within interrupt service routine and marks the appropriate CSKM/SSKM's bottom half to execute. To emulate this behaviour, after receiving a packet over UDP socket, the receive daemon makes a system call process_packet( received_packet, source_of_packet). received_packet is assumed to be a fully formed myrinet packet. This system call does the following

• Looks at the packet structure to determine whether the packet is intended for CSKM or SSKM
• Correspondingly calls cskm_rcv() or sskm_rcv(),which in turn queue the packet in CSKM/SSKM receive queues and directly call the routine (cskm_bh() or sskm_bh() ) that is intended to do the bottom half processing . ( This emulates the behaviour of actual bottom half executing immediately after completion of interrupt service routine.

This daemon's job is to send the messages handed over by CSKM/SSKM. The send daemon is a process which loops at the user level calling  pick_packet_to_send(packet, destination_of_packet) system call in every iteration.

• The system call waits for messages from CSKM/SSKM in the kernel if none are available.
• CSKM and SSKM will call

myri_send_datagram(int receiver_id, int receiver_module, MSG *payload)

to queue the packets they want to send to some destination. This function is a modified version of the actual interface provided by myrinet HSN. The difference between myrinet version and udp-ip version is that  in the latter, apart from enqueuing the packet in a send queue, the act of enqueuing will wake up the send daemon.
• The woken up send daemon, which was earlier waiting in pick_packet_to_send(), now dequeues a packet and returns to the user mode with the packet.
• This packet is sent to the destination over the UDP socket.

• A unified send queue  in which both CSKM and SSKM enqueue packets to send by calling myri_send_datagram().
• CSKM receive queue : Managed by CSKM itself, here the UDP/IP stub enqueues messages received for CSKM by calling cskm_rcv() routine.
• SSKM receive queue : Managed by SSKM itself, here the  UDP/IP stub enqueues messages received for SSKM by calling sskm_rcv() routine.