IP Version 6

• Architecture
• Network Devices
• Device Drivers

Layering

• a packet on the wire has a sequence of headers followed by the payload, possibly followed by a sequence of trailers
• headers and trailers are properly nested: each layer will encapsulate as its own payload the header, payload, and trailer of the layer (transparent exception for fragmentation)
• if a layer can have more than one layer above it, the header must contain one or more demultiplexing fields
• layers communicate by:
  • sending frames/packets/buffers
  • receiving frames/packets/buffers
  • establishing and clearing connections

Upcalls and Queues

• in sending, the data can be transferred from one layer to another as parameter of a call
• in receiving, the data can be transferred from one layer to another as:
  • parameter of an upcall
  • return value of a downcall
• a downcall requires synchronization to handle:
  • the downcall being ready before the data (blocking, context switch)
  • the data being ready before the downcall (queueing)
• an upcall may also require synchronization

Upcalls and Downcalls

• upcalls are simpler to implement and generally more efficient (less synchronization for each packet)
• upcalls are harder to fit into an overall program (multiple threads of control, requiring possibly pervasive synchronization)
• Linux (version 2.0) uses 2 queues:
  • the interrupt hander is an upcall. It does device handling (described later), and puts the received packet in a device-specific queue
  • the network code reads the queue, and when packets are received, upcalls through the entire network stack, then puts the packet(s) in a socket-specific queue
  • the system call reads the queue

Network Interface Devices

• Networking Hardware, NIC
• UART/USART, Universal [Synchronous /] Asynchronous Receiver Transmitter
• UART is a single-chip device accepts or returns data depending on the address on the computer's bus
• the hardware device has bits which control its operation (e.g. a bit to decide whether to interrupt the host). These are grouped into one or more control registers
• the hardware device has bits which record events (e.g. a bit to record whether a character was received, but not given to the host). These are grouped into one or more status registers

Programmed Input and Output

• input:
  • interrupt handler is called to signal availability of data
  • program checks the status register to see if everything is OK
  • program reads the data register to load the data
• output:
  • program checks the status register to see if it is OK to send
  • (if not, next step done by interrupt handler)
  • program writes the data register to send the data

Direct Memory Access

• input:
  • program sets up buffers for incoming data, gives pointer into control register
  • device copies data into buffer
  • device interrupts when buffer(s) are full
• output:
  • program sets up buffers for outgoing data
  • device copies data from buffers onto network
  • device interrupts when buffers are sent

Device Drivers

• at boot time, must do device initialization: discovery (probing), data structure setup
• at interrupt time, the device must put a byte on the (shared) bus to identify who the interrupt is from (interrupt vector)
• the system interrupt handler uses this information to decide which device interrupt handler to call
• the device interrupt handler ("lower half") must read the status register and internal data structures and decide what to do
• the device driver ("top half") handles downcalls