INTERNET PROTOCOL IP

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INTERNET PROTOCOL (IP)

• ARUNA THAKUR
• aruna_at_jack.see.plymouth.ac.uk

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INTERNET ARCHITECTURE.
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THE PROTOCOL STACK

• Network device is the lowest layer in the
  protocol stack.
• They use the link layer (usually) Ethernet
• to communicate with other devices.
• to receive and communicate with other computer
  and devices.
• Network layer.
• Transport layer.

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INTERNET PROTOCOL (IP)

• IP is the standard network layer protocol
• It checks to see if the incoming packet is for
  the host computer or they need to be forwarded.
• Fragments/de-fragments the packets if required.
• It maintains a database of routes for outgoing
  packets.
• It address and fragments packets before sending
  them down to the link layer.
• It resides on the third layer of the protocol
  stack.

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IP Packet Format
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REAL TIME TRANSPORT PROTOCOL (RTP) - HEADER
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H.261/RTP HEADER
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EFFECT OF CHANGING PACKET HEADER IN H261/RTP
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SOCKET BUFFER (sk_buff)

• sk_buff provides a mechanism for adding and
  removing protocol headers and tails.
• sk_buff - Advantages of using a common packet
  data structure is that
• The payload data of a packet is copied only
  twice.
• Once from the user to the kernel space.
• Second time from kernel space to the output
  medium.
• It contains pointer and length fields that allow
  each protocol layer to manipulate the application
  data via standard functions or "methods".
• The skbuff files can be found
• /usr/src/linux-2.6.5-1.358/include/linux/skbuff.h
• /sur/src/linux-2.6.5-1.358/net/core/skbuff.c

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SK_BUFF PACKET STRUCTURE
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SOCKET BUFFER (sk_buff)

• All the network layers uses sk_buff.
• sk_buffs are the buffers in which the linux
  kernel handles network packets in an efficient
  way.
• The packet is received by the network card
• Packets are put into a sk_buff
• and then passed to the network stack which uses
  the sk_buff till the time packet reaches the
  right place in the protocol stack.

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JOURNEY OF NETWORK PACKETS THROUGH LINUX KERNEL

• An application generates packets.
• Packets are sent through socket to
• Transport layer (TCP/UCP).
• Network layer (IP).
• In the network layer
• Kernel looks for the host destination in routing
  cache or Forward Information Base (FIB).
• When the kernel finds that the packet is for
  another computer it addresses it
• and sends to the link layer output interface
  (Ethernet Device).
• link layer finally sends the packet to the
  physical medium.

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JOURNEY OF NETWORK PACKETS THROUGH LINUX KERNEL
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SENDING A PACKET USING IP
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SENDING A PACKET USING IP APPLICATION LAYER
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SENDING A PACKET USING IP APPLICATION LAYER

• /usr/src/linx-2.6.5-1.358/net/socket.c
• This is an implementation of the socket network
  access protocol.
• Writes data to a socket
• sock_write() static ssize_t_aio_write() line
  649
• Calls socki_lookup() in order to associate socket
  with the fd/file inode.
• Creates and fills message header with data size
  and adreesses
• return sock_sendmsg
• sock_sendmsg() Used to send the message
• inet_sendmsg() Present in /usr/src/linux/2.6.5-1
  .358/net/ipv4/af_inet.c
• af_inet.c This file deals with an
  implementation of TCP/IP protocol suite for
  LINUX.

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SENDING A PACKET USING IP TRANSPORT LAYER
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SENDING A PACKET USING IP TRANSPORT LAYER

• tcp_do_sendmsg() Function found in the file
• /usr/src/linux-2.6.5-1.358/net/ip4/tcp.c
• It waits for a connection.
• If ok then starts sending message.
• Add data to the packet.
• Allocates memory space for skb.
• Copies the packet and do checksum.
• Finally calls a function tcp_send_skb().
• Tcp_send_skb() -
• It call function skb_queue_tail() to add packet
  into the queue.
• Calls tcp_transmitskb().

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SENDING A PACKET USING IP TRANSPORT LAYER -CONT

• tcp_transmit_skb()
• Found in the file /usr/src/linux-2.6.5-1.358/net/i
  pv4
• It builds TCP header and adds checksum.
• It check ACK and SYN
• tcp_v4_sendmsg()
• Found in the file /usr/src/linux-2.6.5-1.358/net/i
  pv4
• Checks for IP address type.
• Opens a connection.
• Port etc.
• udp_get_frag() net/ipv4/udp.c
• Copies and checksums a buffer.
• udp_sendmsg()
• It checks length, flag, etc.
• Sets UDP header.
• Etc.

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SENDING A PACKET USING IP NETWORK LAYER
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SENDING A PACKET USING IP NETWORK LAYER - CONT

• ip_build_xmit - /net/ipv4/ip_output.c
• Creates memory for skb().
• Sets skb header.
• Ip_queue_xmit() net/ipv4/ip_output.c
• Looks up for a route for the IP packet.
• Builds IP header.
• Fragments the packet if required.
• Adds checksum.

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RECEIVING A PACKET USING IP
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RECEIVING A PACKET USING IP - CONT

• An interrupt is issued to tell the device that an
  incoming message is ready.
• The device then allocates space for the incoming
  message.
• The packet is then passed to the link layer
• This layer puts the packet into the backlog
  queue.
• And marks the network flag for the next
  bottom-half run.
• Then packets are taken out the backlog queue,
  matches them to a known protocol (typically IP)
  passes them to the protocol receive function.
• IP layers checks for any packet error and routes
  (packet will traverse internally or externally).
• Checks for the socket associated with the port
  specified in the packet.
• Finally puts the packet at the end of the socket
  receive queue.

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SOME IMPORTANT FILES AND FUNCTIONS USED TO
RECEIVE PACKET

• DEVICE_rx device dependent drivers/net/DEVICE.
  c
• Calls dev_alloc_skb to reserve space for the
  packet.
• Calls eth_type_trans() to find the protocol.
• inet_recvmsg() net/ipv4/af_inet.c
• Checks the socket to check if it is accepting.
• Verifies the protocol pointer.

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SOME IMPORTANT FILES AND FUNCTIONS USED TO
RECEIVE PACKET

• ip_rcv() net/ipv4/ip_input.c
• It examines packet for error
• Invalid length too short or long.
• Incorrect version.
• Invalid checksum.
• It de-fragments the packet.
• Calls ip_route_input() Routes the packet.
• Sock_queue_rcv_skb() include/net/sock.h
• Calls skb_queue_tail() to put packet in the
  socket to receive queue.

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LINKS FOR FURTHER READING

• http//kernelnewbies.org/documents/ipnetworking/li
  nuxipnetworking.html