Interfaces : Ethernet

1
Interfaces Ethernet

• 2005. 5. 23(?)
• ? ? ?
• icebyung_at_hufs.ac.kr

2
Introduction

• how the Ethernet device driver operates
• Network device drivers are accessed through the
  seven function pointers in ifnet structure

ifnet Ethernet Description
if_init if_output if_start if_done if_ioctl if_reset if_watchdog leinit ether_output lestart leioctl lereset hardware initialization accept and queue frame for transmission begin transmission of frame output complete(unused) handle ioctl commands from a process reset the device to a known state watch the device for failures r collect statistics
3
Ethernet Interface

• Net/3 Ethernet device drivers all follow the same
  general design
• This is common for most Unix device drivers
• We will refer to the LANCE driver to illustrate
  the design

4
Ethernet encapsulation of an IP packet
source address
destination address
type
data
CRC
6 bytes
2
6 bytes
46 1500 bytes
4 bytes
type 0800
IP packet
46 1500 bytes

• Ethernet frame consist of 48-bit destination and
  source address
• 16-bit type field identifies the format of the
  data carried by the frame
• Type field for IP packet is 0x0800
• Frame is terminated with a 32-bit CRC

5
Ethernet device driver
6
leintr Function

• leintr is called when the interface generates an
  interrupt
• call leread to transfer the frame from the
  interface to a chain of mbufs
• mbuf chain doesnt include the Ethernet header
• Ethernet header is passed as a separate argument
  to ether_input
• Ethernet CRC is not generally available
• computed and checked by the interface hardware

struct ether_header u_char ether_dhost6 u_
char ether_shost6 u_short ether_type
7
leread Function

• leread function
• start with a contiguous buffer of memory passed
  to by leintr
• construct an ether_header structure
• construct a chain of mbufs
• use three arguments leread(unit, buf, len)
• unit identifies the particular interface card
  that received a frame
• buf points to the received frame
• len the number of bytes in the frame

8
leread Function

• constructs the ether_header by pointing et to
  the front of the buf
• converting the ethernet value to host byte order

• destination address is examined to determine

9
leread Function

• If the interface is tapped by BPF,
• the frame is passed directly to BPF

• copies the data from the buffer passed to leread
  to an mbuf chain

10
ether_input Function

• ether_input function
• examines the ether_header structure to determine
  the type of data has been received
• queues the received packet for processing
• arguments to ether_input
• ether_input(ifp, eh, m)
• ifp pointer to the receiveing interfaces ifnet
  structure
• eh pointer to the Ethernet header of the
  received packet
• m pointer to the received packet (excluding the
  Ethernet header)

11
ether_input Function

• broadcast and multicast recognition

• Link-Level demultiplexing
• determine the type of data

• the packet is placed on the selected queue
• if queue is full, packet is discarded

12
ether_input Function

• Link-Level demultiplexing
• ether_input jumps according to the Ethernet type
  field
• For an IP packet,schednetisr schedules an IP
  software interrupt and the IP input queue,
  ipintrq, is selected
• For an ARP packet,the ARP software interrupt is
  scheduled and arpintrq is selected
• The default case processes unrecognized Ethernet
  types or packets that are encapsulated according
  to the 802.3 standard

13
ether_output Function

• ether_output function
• if_output function for all Ethernet is
  ether_output
• ether_output takes the data portion of an
  Ethernet frame and places it on the interfaces
  send queue
• describe in four parts
• verification
• protocol-specific processing
• frame construction
• interface queueing
• arguments to ether_output
• ether_output(ifp, m0, dst, rt0)
• ifp points to the outgoing interfaces ifnet
  structure
• m0 the packet to send
• dst the destination address of the packet
• rt0 routing information

14
ether_output Function

• Host route

• Gateway route

• discard packets when the destination is not
  responding to ARP request

15
ether_output Function

• ether_output jumps according to sa_family in the
  destination address

• check the ARP cache to determine the Ethernet
  Addr corresponding IP Addr

/ OSI Code/

• It isnt necessary to call arpresolve
• Ethernet destination address has been provided
• explicilty by the caller

• unrecognized address families

16
ether_output Function

• At this point, the mbuf contains a complete
  Ethernet frame except for 32-bit CRC

• If output queue is full ether_output discards the
  frame

17
lestart Function

• lestart function dequeues frames from the
  interface output queue
• Arranges for them to be transmitted by the LANCE
  Ethernet card
• Interface must be initialized
• Dequeue frame from output queue
• Repeat if device is ready for more frames

18
lestart Function

• If the interface is not initialized.
• lestart returns immediately

/ device-specific code /
/ device-specific code /
/ device-specific code /