The hardware ringbuffer Understanding the RTL-8139 mechanism for packet reception

Implementing a ringbuffer Start addressBuffer length datum HEAD TAIL Buffer storage datum TAIL = where to insert the next datum HEAD = where to remove the next datum

Some ringbuffer details When HEAD == TAIL the buffer is empty The HEAD and TAIL pointers advance to the end of the buffer, then ‘wrap around’ Some storage always remains unused When sizes of stored data-items are not uniform, then the size of each item must be stored somewhere – probably with the item itself

RealTek’s implementation data Receive Buffer Start AddressReceive Configuration Register (includes size of the buffer) CAPR Current Address of Packet to Read (minus 16 bytes) CBR Current Buffer Register (4-byte aligned) = packet header (4-bytes) = packet data (size varies) Buffer storage

Packet-Header Format MARMAR PAMPAM BARBAR reserved ISEISE RUNTRUNT LONGLONG CRCCRC FAEFAE ROKROK Packet Status (least significant 16 bits) Packet Length (most significant 16 bits) LEGENDROK = Received OK MAR (Multicast Address Received)FAE = Frame Alignment Error PAM (Physical Address Matched) CRC = CRC Error BAR (Broadcast Address Received)LONG = Packet length exceeds 4KB RUNT = Packet size below 64 bytes ISE = Invalid Symbol Error

Receive Algorithm Reset controller (bit #4 in register CR) Allocate memory and program RBSTART Enable reception (bit #3 in register CR) Configure reception (RXCONFIG register) Adjust CAPR for size of allocated buffer Optionally unmask the receive interrupts Await received packet(s) (bit #0 in CR)

Recall ‘RXCONFIG’ layout 32-bit register (offsets 0x44-0x47) reserved Early Receive Threshold reserved Mult ER INT Rx ERR 8 Rx FIFO Threshold Rx Buf Length Max DMA Burse Size WRAPWRAP 0 LONGLONG RUNTRUNT BCASTBCAST MCASTMCAST MACMAC ALLALL

How to configure Rx Buffer length is the most crucial parameter –8KB (=00)with or without ‘wrap’ –16KB (=01)with or without ‘wrap’ –32KB (=10)with or without ‘wrap’ –64KB (=11) (‘wrap’ is automatic) Next is which packets to receive: –Broadcast, Multicast and MAC-Address Match –(You do NOT want to receive ALL packets)

What about thresholds? The Linux driver offers us guidance –Can use ‘none’ as Early Rx Threshold –Can use ‘none’ as Rx FIFO Threshold –Can use a ‘reserved’ DMA Burst size So the Linux driver used: 0x0000E70E But you can experiment with other values

In-class exercise #1 Install our ‘user8139.c’ device-driver, and take note of its buffer’s physical address Install our ‘dram.c’ device-driver, and use ‘fileview’ to look at the NIC Receive Buffer Then modify ‘nicstudy.cpp’ so that it will enable (and configure) packet-recption Use ‘fileview’ to look at received packets

In-class exercise #2 Now enhance your modified version of the ‘nicstudy’ application so that it will display a succession of received packets (using both hexadecimal and ascii formats), in a manner similar to our ‘rxtester.cpp’ demo Be sure you ‘disable’ the packet reception before you remove ‘user8139.ko’ from the kernel – or risk a system ‘crash’ (Why?)

The ‘/etc/ethers’ file This file describes the list of associations of Ethernet-addresses with IP-addresses It’s a text file, created with a text editor Each line describes one correspondence: xx:xx:xx:xx:xx:xx But comments start with a #-character A sample ‘ethers’ file is on our website

Dynamic Address Resolution A standard Internet Protocol service exists which provides a dynamic way for stations to discover the Ethernet address that goes with a given IP-address It is called ‘Address Resolution Protocol’

ARP request AB CDE request Station ‘A’ wants to know the Ethernet Address for station ‘B’ So ‘A’ broadcasts an ARP request-packet to all other stations

ARP reply AB CDE reply Station ‘B’ recognizes that the request is for its Ethernet Address. So ‘B’ replies directly to ‘A’, and other stations ignore the request.

ARP packet-format ARP packet-data0806Dest’n MACSource MAC packet-header PTYPE=0008HTYPE=0001 OPER=0001/0002PLEN=04HLEN (01) source hardware address (6 bytes) source protocol address (4 bytes) destination hardware address (6 bytes) destination protocol address (4 bytes) All zeros for an ARP request broadcast address for ARP request