Page 299 - DCAP403_Operating System

Operating System

Notes Receiving IP Packets

Each device data structure describes its device and provides a set of callback routines that the
network protocol layers call when they need the network driver to perform work. These functions
are mostly concerned with transmitting data and with the network device’s addresses. When a
network device receives packets from its network it must convert the received data into sk_
buff data structures. These received sk_buff’s are added onto the backlog queue by the network
drivers as they are received.
If the backlog queue grows too large, then the received sk_buff’s are discarded. The network
bottom half is flagged as ready to run as there is work to do.

When the network bottom half handler is run by the scheduler it processes any network packets
waiting to be transmitted before processing the backlog queue of sk_buff’s determining which
protocol layer to pass the received packets to.

As the Linux networking layers were initialized, each protocol registered itself by adding a
packet_type data structure onto either the ptype_all list or into the ptype_base hash table. The
packet_type data structure contains the protocol type, a pointer to a network device, a pointer

to the protocol’s receive data processing routine and, finally, a pointer to the next packet_type
data structure in the list or hash chain. The ptype_all chain is used to snoop all packets being
received from any network device and is not normally used. The ptype_base hash table is hashed

by protocol identifier and is used to decide which protocol should receive the incoming network
packet. The network bottom half matches the protocol types of incoming sk_buff’s against one
or more of the packet_type entries in either table. The protocol may match more than one entry,
for example when snooping all network traffi c, and in this case the sk_buff will be cloned. The
sk_buff is passed to the matching protocol’s handling routine.

Sending IP Packets

Packets are transmitted by applications exchanging data or else they are generated by the network
protocols as they support established connections or connections being established. Whichever
way the data is generated, an sk_buff is built to contain the data and various headers are added
by the protocol layers as it passes through them.
The sk_buff needs to be passed to a network device to be transmitted. First though the protocol,
for example IP, needs to decide which network device to use. This depends on the best route for
the packet. For computers connected by modem to a single network, say via the PPP protocol,
the routing choice is easy. The packet should either be sent to the local host via the loopback
device or to the gateway at the end of the PPP modem connection. For computers connected to an
ethernet the choices are harder as there are many computers connected to the network.

For every IP packet transmitted, IP uses the routing tables to resolve the route for the destination
IP address. Each IP destination successfully looked up in the routing tables returns a rtable data
structure describing the route to use. This includes the source IP address to use, the address of
the network device data structure and, sometimes, a prebuilt hardware header. This hardware

header is network device specific and contains the source and destination physical addresses and

other media specific information. If the network device is an ethernet device, the hardware header
would be as shown in Figure 13 and the source and destination addresses would be physical
ethernet addresses. The hardware header is cached with the route because it must be appended
to each IP packet transmitted on this route and constructing it takes time. The hardware header
may contain physical addresses that have to be resolved using the ARP protocol. In this case the
outgoing packet is stalled until the address has been resolved. Once it has been resolved and the
hardware header built, the hardware header is cached so that future IP packets sent using this
interface do not have to ARP.

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