Transcript LWIP TCP/IP Stack

LWIP TCP/IP Stack
김백규
What is LWIP?
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An implementation of the TCP/IP protocol
stack.
The focus of the lwIP stack is to reduce
memory usage and code size
 suitable for embedded systems.
uses a tailor made API that does not
require any data copying.
Features of TCP/IP stack
(Traditional version)
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Designing in a layered
fashion leads to…
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communication
overhead between
layers
Network communication
is similar to IPC or file
I/O
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APP can’t aware of the
buffer mechanisms.
(e.g. reuse buffers with
frequently used data.)
<Layered model>
Features of TCP/IP stack
(LWIP version)
Do not maintain a strict layer.
 More relaxed scheme for communication between
layers.
(By means of shared memory)
- APP layer can use the buffer handling mechanisms
used by the lower layers.
APP can more efficiently reuse buffers.
 Application process can use the same memory as the
networking code
 App can read and write directly to the internal buffers.
 Saving the expense of performing a copy
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Process model of LWIP
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All protocols reside in a single process thus
are separated from the OS kernel.
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Allow to be portable across different OS.
APP may either reside in the LWIP process or
be in separate processes.
Communicate are done by function calls.
 Or a more abstract API.
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The operating system emulation layers
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OS specific function calls and data structures
are not used directly in the code.
 The operating system emulation layer is used.
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The OS emulation layer provides
 Timers, process synchronization, message
passing mechanisms, and so on.
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Porting to a different OS
 Only need the operating system emulation layer.
Buffer and memory management
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Packet buffers – pbufs
 LWIP’s internal representation of a packet,
 Designed for the special needs of the minimal
stack.
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Types of pbufs
 PBUF_RAM, PBUF_ROM, PBUF_POOL
 A pbuf chain may consist of multiple types of
pbufs.
PBUF_RAM pbuf
has the packet data stored in memory managed by the pbuf
subsystem.
 used when an application sends data that is dynamically
generated.
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PBUF_ROM pbuf
Used when an application sends data that is located in
memory managed by the application.
 The main use is when the data is located in ROM
 Header that are prepended to the data in a PBUF_ROM
pbuf are stored in a PBUF_RAM pbuf.
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PBUF_POOL
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Consist of fixed size pbufs allocated from a pool of fixed size
pbufs.
Mainly used by network device drivers since the operation of
allocating a single pbuf is fast and is therefore suitable for use
in an interrupt handler
Network interfaces
The network interfaces are
kept on a global linked list.
Reflect the kind of H/W
Ex) Bluetooth => bt
WLAN => wl
The function the device driver
should call when a packet has
been received.
The function in the device driver
that transmits a packet on the
physical network and it is called
by the IP layer when a packet is
to be sent.
Points to device driver specific
state for the network interface and
is set by the device driver.
IP processing(1/3)
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Receiving packets
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Network device driver calls ip_input() function.
 Checking IP version, header length
 Computing the header checksum
 Checking destination address.
Sending packets
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Handled by the function ip_output()
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Find the appropriate network interface.
All IP header fields are filled.
IP header checksum is computed.
The source and destination address are passed.
IP processing(2/3)
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Forwarding packets
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The packet should be forwarded…
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When none of the network interfaces has the
same IP address as an incoming packet’s
destination address.
This is done by the function ip_forward()
ttl field is decreased.
 If ttl reaches zero, an ICMP error message is
sent.
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IP processing(3/3)
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ICMP processing
This is for ICMP ECHO message.
Just swapping the IP destination
and source address of the incoming
packet.
UDP processing(1/2)
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The udp_pcb structure
The UDP PCBs are kept on a linked list
which is searched for a match when a UDP
datagram arrives.
Called when a
datagram is
received.
UDP processing(2/2)
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UDP processing
TCP processing(1/2)
Function to call when a
listener has been
connected.
Next sequence
number
Receiver’s
window
Timer for
TIME-WAIT
state
Used when passing
received data to the
application layer.
TCP processing(2/2)
Application Program Interface
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The BSD socket API
Require data that is to be sent to be copied from
application program to internal buffers in the TCP/IP
stack.
 Since the two layers reside in different protection
domains.
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The LWIP socket API
Utilizes knowledge of the internal structure of LWIP
to achieve effectiveness.
 Does not require that data is copied.
 Since the application program can manipulate the
internal buffers directly.
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