1. Introduction to Data Link Layer
Reading: Chapter 6
Fall 2004
FSU CIS 5930 Internet Protocols

2. Structure of Data Link Layer
802.1
Overview
Architecture
Management
802.1 Locigal Link Control
802.3
CSMA/CD
802.5
Token Ring
802.11
WirelessLAN
...
Network Layer 2b 2a 1
IEEE 802 standard
Network
adaptersr
devices
Network layer
Protocol instances
Linux network architecture
Drivers
Fall 2004
FSU CIS 5930 Internet Protocols

3. Processes on Data Link Layer
System Calls
NET_RX_SOFTIRQ-
Software-Interrupt
CPU1
CPU2
Layer 1-4
eth0
eth1
Layer 5
Application processes *
* Kernel activities (Interrupts, Tasklets, Timer-Handler etc.)
Hardware-Interrupt
NET_RX-
Soft-IRQ
System- calls
NET_TX-
Fall 2004
FSU CIS 5930 Internet Protocols

4. Path of a packet in data-link layer..
...
arp_rcv
ip_rcv
p8022_rcv
arp_send
ip_queue_xmit
Layer 3
ETH_P_802_2
dev.c
br_input.c
dev.c
net_rx_action
handle_bridge
...
dev_queue_xmit
CONFIG_BRIDGE
dev->qdisc-> enqueue
do_softirq
Scheduler
eth0
eth1
CPU1
CPU2
dev.c
net_tx_action
qdisc_run
softnet_data[cpun].input_pkt_queue
Data-link layer (OSI-Layer 1+2)
dev.c
netif_rx
qdisc_restart
Scheduler
driver.c
eth_type_trans()
dev_alloc_skb()
dev->qdisc->dequeue
net_interrupt
driver.c
dev->hard_start_xmit
Fall 2004
FSU CIS 5930 Internet Protocols

5. FSU CIS 5930 Internet Protocols
Receiving a packet
Adapter triggers interrupt, which is handled by the driver (e.g., net_interrupt)
net_rx if receiving a packet
dev_alloc_skb() into socket buffer
Identifying data type in payload (e.g., eth_type_trans())
netif_rx completes interrupt handling
Enqueueing the buffer (softnet_data[cpu].input_pkt_queue)
Registering soft interrupt
__cpu_raise_softirq(cpu, NET_RX_SOFTIRQ)
CPU resumes the previous activity
Fall 2004
FSU CIS 5930 Internet Protocols

6. Receiving packet (Cont’d)
CPU finishes the current activity
Schedule a new activity (schedule)
Check soft interrupt queue first using do_softirq
NET_RX_SOFTIRQ >net_rx_action()
net_rx_action()
Dequeue packet
Handled by different protocol instance based on protocol’s func()
Here for IP, it is ip_rcv()
Fall 2004
FSU CIS 5930 Internet Protocols

7. Transmitting a packet Fall 2004 FSU CIS 5930 Internet Protocols
dev.c, net/sched/*
softirq.c, netdevice.h
dev->qdisc->enqueue
driver.c
dev->hard_start_xmit
qdisc_run
qdisc_restart
dev->qdisc->dequeue
Scheduler
net_tx_action
dev_queue_xmit
timer_handler
netif_schedule
Timer
cpu_raise_softirq
do_softirq
NET_TX_SOFTIRQ
Fall 2004
FSU CIS 5930 Internet Protocols

8. Transmitting packets (Cont’s)
dev_queue_xmit() (net/core/dev.c)
qdisc_run() (include/net/pkt_sched.h)
qdisc_restart() (net/sched/sch_generic.c)
Fall 2004
FSU CIS 5930 Internet Protocols

9. Managing Layer-3 protocols
type: ETH_P_ARP
dev: NULL
func
data: 1
packet_type
next
arp_rcv()
ptype_base[16] 1
type: ETH_P_IP 16
ip_rcv()
ptype_all
type: ETH_P_ALL
dev
data
. . .
Fall 2004
FSU CIS 5930 Internet Protocols

10. Managing Layer-3 Protocols (Cont’d)
dev_add_pack() (net/core/dev.c)
dev_remove_pack() (net/core/dev.c)
Fall 2004
FSU CIS 5930 Internet Protocols