How Routers Handle IPX Type 20 NetBIOS Packets (149270)

SUMMARY

NetBIOS provides session-layer services like packet assembly-disassembly and named addressing for Microsoft's NWLink protocol. NetBIOS, like SPX, also provides guaranteed packet delivery and packet sequencing on the Transport layer. For NetBIOS to operate within an NWLink environment, routers must be able to propagate NetBIOS packets across the network. This is done by defining a specific IPX packet for NetBIOS. Since IPX is a derivative of XNS's IDP protocol, it follows the assigned packet types given by Xerox as shown below:

   Protocol                 Packet type (hex)
   ------------------------------------------
   Unknown                            00
   Routing information                01
   Echo                               02
   Error                              03
   Packet exchange protocol (PEP)     04
   Sequence packet exchange (SPX)     05
   Experimental                       10
   NetWare core protocol              11
   NetBIOS                            14
				

Packet type 20 (0x14) identifies a NetBIOS packet and is designated as a propagated packet. The Destination Node field in the IPX header is set to 0xFFFFFFFFFFFF. When this packet is received by a router, the router checks the IPX header Packet Type field. If the Packet Type is 20 (0x14), the router examines the Transport Control field. If the Transport Control field is more than 8, the router will discard the packet. (A type 20 packet is propagated over a maximum of 8 networks.) The router then compares the value in the Network Number field with the network number of the segment it received the packet from. If these match, the router discards the packet to prevent it from being sent over the same segment twice. The router then puts the network number of the segment it received the packet from in the next available Network Number field and increments the Transport Control field before broadcasting the packet to all directly connected network segments not represented in the Network Number fields.