BSP Developer's Guide

VxWorks 5.5
BSP Developer’s Guide
A protocol writer has to deal only with calls to the MUX. Everything
device-specific is handled in the drivers of the data link layer, the layer below the
MUX.
Protocol Startup
Each protocol that wants to receive packets must first attach itself to the MUX. To
do this, the protocol calls muxBind( ). The returned function value is a “cookie”
that identifies the network interface driver to which the MUX has bound the
protocol. The protocol must save this cookie for use in subsequent calls to the
MUX.
As input to the muxBind( ), you must specify the name of a network device (for
example, ln and 0, ln and 1, ei and 0, and so on), the appropriate receive, restart,
and shutdown functions for the protocol, a protocol type (from RFC 1700), and a
name for the attaching protocol.
The MUX uses the protocol type to prioritize the protocols. This priority
determines which protocol sees packets first. When a driver passes a packet up to
the MUX, the driver includes a pointer to its END_OBJ structure. Included in this
structure is the member, protocols, the head of the list of protocols (a list of
NET_PROTOCOL structures) listening to this device. The order of the protocols in
this list determines the order in which the protocols see the packet. Maintaining
this list is the responsibility of the MUX.
If a protocol specifies a type of MUX_PROTO_SNARF in its muxBind( ), the MUX
adds the protocol to the top of the list in END_OBJ.protocols. At any given
moment, the MUX allows only one active protocol of type MUX_PROTO_SNARF.If
a protocol specifies a type of MUX_PROTO_PROMISC in its muxBind( ), the MUX
adds the protocol to the bottom of the list in END_OBJ.protocols. If a protocol
specifies any other type in its muxBind( ), the MUX adds the protocol to the list just
after the MUX_PROTO_SNARF protocol (or the top of the list if there is no protocol
of type MUX_PROTO_SNARF).
This shuffling of protocols ensures that lower priority protocols cannot steal or
damage a packet before a higher priority protocol gets to see the packet.
Snarfing Protocols Block Packets to Lower Priority Protocols
If the receive routine for a protocol returns TRUE, and, if the protocol is not of type
MUX_PROTO_PROMISC, the MUX lets the protocol snarf the packet (that is, the
MUX lets the protocol take delivery of the packet). If a protocol takes delivery of
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