Artisoft Technical Bulletin Dated: 10.05.93 RRR IPX Bridging & Routing with LANtastic 5.0 Page 1 of 5 컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴 The following is a brief description of IPX Routing and how it functions through bridges and routers. THE STRUCTURE OF THE MAC FRAME IEEE 802.3 Ethernet (Media Access Control - MAC) Frame 旼컴컴컴컫컴컴컴쩡컴컴컫컴쩡컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴쩡컴커 쿛reamble쿏est 쿞ource쿗n� (IPX Header) DATA 쿎RC � 읕컴컴컴컨컴컴컴좔컴컴컨컴좔컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴좔컴켸 8bytes 6bytes 6bytes 2bytes 46-1500 bytes 4bytes IEEE 802.3 Ethernet (MAC) Frame 旼쩡쩡쩡쩡쩡쩡쩡� Preamble 8 bytes 쳐탠탠탠탠탠탠좔� Destination Address 6 bytes 쳐탠탠탠탠탠� Source Address 6 bytes 쳐탠탠좔좔좔� Length 2 bytes 읕좔� Encapsulated Data 46-1500 bytes 旼쩡쩡쩡� 읕좔좔좔�CRC 4 bytes IPX Header - Encapsulated in Data Section of IEEE802.3 Frame 旼컫컴쩡쩡쩡컴컴컫컴컴쩡컫컴컴컴쩡컴컫컴� � � � � � � � � � � � 읕컨컴좔좔좔컴컴컨컴컴좔컨컴컴컴좔컴컨컴� Checksum Source Address Length Source Network Transport control Source Socket Packet Type Destination Address Destination Network Destination Socket IPX Header - Encapsulated in Data section of IEEE802.3 Frame 旼쩡� Checksum - 2 bytes 쳐탠� Length - 2 bytes 쳐탠� Transport Control - 1 byte 쳐� Packet Type - 1 byte 쳐탠쩡쩡쩡쩡� Destination Node Address - 6 bytes 쳐탠탠탠탠좔� Destination Network Address - 4 bytes 쳐탠탠좔� Destination Socket - 2 bytes 쳐탠탠쩡쩡쩡� Source Node Address - 6 bytes 쳐탠탠탠탠좔� Source Network Address - 4 bytes 쳐탠탠좔� Source Socket - 2 bytes 읕좔� Encapsulated Data 16-1470 bytes What happens when a workstation logs into the network When a workstation logs onto a server it establishes a NetBIOS session. To establish a session a workstation sends out a multicast datagram with a call command. The packet sent out contains the ethernet address of the calling adapter card and a multicast address of the destination computer in the MAC frame. The multicast address is FF FF FF FF FF FF. (BUZZWORD ALERT - Multicast vs Broadcast. A multicast address is defined as an address where the most significant of the high order byte is set to 1; i.e., 01 00 23 1D 5F 3C would be a multicast address because of the 01. A broadcast address is all FF's and so meets the above criteria. All adapters receive broadcast addresses, a multicast address must be enabled on the adapter card in order for it to receive on that address. A multicast address is derived from the NetBIOS name of the computer. LANtastic normally uses multicast addressed rather than broadcast addresses to reduce load on the system. With IPX routing broadcast packets are sent as shown below: Workstation during Login Source Node Address=00006E123456 Destination Node Address=FF FF FF FF FF FF When the packet arrives at the server with the NetBIOS name specified in the NetBIOS call packet, the server responds with a answering packet. This packet's header contains the servers ethernet address as the source address and the workstations ethernet address as the destination address. Server's answer to Workstations Login Source Address=0000006E654321 Destination Address=00006E123456 The destination and source then each send out acknowledge packets so each side is sure that a session has been established. Graphically it would appear something like. Fred - WS Joe - Server Establish Name Establish name Call ------------> <------------ Answer call <------------ Session Established Session Established ------------> <------------ Established Session Established session ------------> Once a session has been established each MAC frame header will contain both a source and a destination address. BRIDGING When a workstation is connected to a server via one or more bridges the process is the same. The workstation sends out a multicast packet, each bridge in line passes the packet on down the line. When a server with the NetBIOS name specified in the workstation's call receives the packet it sends out a reply. The source and destination address are the same as in the local login example. The main difference between a bridge and a repeater is the ability of a bridge to filter the packets based on the source and destination addresses only allowing certain packets through the bridge. This allows the bridge to minimize traffic on each section. A bridge is transparent to the network, the network does not know it is there. This places a great strain on the bridge as it must receive and act on every packet transmitted on both sides of the bridge. This necessarily limits the intelligence of a bridge, since it must receive and act on every packet. A bridge acts on the MAC or transport protocol layer the "FRAME", it does not need to know about the underlying network protocol. ROUTING When a workstation is connected to a server via one or more routers the process is very different. Since the router does NOT act on the data in the ethernet frame the network protocol must contain addressing information. NetBIOS normally does not contain this information and so is considered inherently unroutable. To get around this Artisoft has added a 30 byte IPX header to the front of the data field of each packet. Since IPX and NetBIOS use the same ethernet IEEE802.3 frame a router will view the packet as an IPX packet and route accordingly. 1. The workstation sends our a broadcast datagram just is it does on a local network. The source address is the ethernet address of the adapter card in the workstation and the destination address is all FF's in the MAC frame. Inside the data of the packet is an IPX header. In this header the workstation inserts the following information. IPX Header - Encapsulated in Data section of IEEE802.3 Frame 旼쩡� 쳐탠� Checksum FF 쳐탠� Length 53 쳐� Transport Control 00 쳐탠쩡쩡쩡쩡� Packet Type 0x14 쳐탠탠탠탠좔� Destination Node Address 000000000000 쳐탠탠좔� Destination Network Address 0000 쳐탠탠쩡쩡쩡� Destination Socket 0x573 쳐탠탠탠탠좔� Source Node Address 00006E123456 쳐탠탠좔� Source Network Address 0000 읕좔� Source Socket 0x573 2. When this broadcast datagram hits the router it strips the IEEE802.3 frame from the packet, increments the Transport Control field by 1, inserts it's local network number into the source network address (if it is 000000000000) and forwards the packet to another router or the other side of a local router. The changed part of the packet would look like: IPX header changes: Transport Control 1 Source Network Address: 0002 3. When the router receives the packet it builds a packet frame around it, and INSERTS the ROUTERS ethernet address as the MAC source address and leaves the the MAC destination node address as FF's (broadcast datagram). The Transport control field in the IPX header is again incremented. MAC Frame Changes: Ethernet Address: 00006E46A7F4 [Router's ethernet address] IPX header changes: Transport Control 2 4. Steps 2-3 are repeated until the packet arrives at the destination server or the hop count in the Transport Control field exceeds 8 (the packet is discard at this time). 5. When the server named receives the packet a reply packet is sent. The Mac layer of the reply packet contains the ethernet address of the server as the source address and the ethernet address of the LAST router as the destination address. In the IPX header, the transport control is set back to 0, the source node address is set the the ethernet address of the server, the destination node address is set to the address of the original workstation, the destination network address is set to the network address added in step 2 above and the packet is transmitted. MAC Frame Changes: Source Address: 00006E20A35B [Server's ethernet address] Destination Address: 00006E46A7F6 [last routers ethernet address] IPX header changes: Transport Control 0 Source Node Address 00006E20A35B Destination Node Address 00006E123456 Destination Network Address 0001 6. When the packet arrives at the router that is ADDRESSED (note the system is no longer broadcasting), the IEEE MAC frame is stripped, the SOURCE address is added to the IPX header if the source is 0's and the packet is forwarded via the stored route to the network specified in the ipx header. 7. When the router at the remote site receives the packet it builds a MAC frame and transmits the packet to the destination node.