Design

1
Design Development of Protocol Stack with Linux

• Bharat Joshi
• Infosys Technologies Ltd
• (bharat_joshi_at_infosys.com)

2
Agenda

• Protocol Protocol Stack.
• Why Design?
• Designing a Protocol Stack.
• Development with Linux.
• References

3
Protocol Protocol Stack

• A Protocol is an agreement between communicating
  parties on how communication is to proceed.
• A list of protocols used by a certain system ,one
  protocol per layer is called Protocol Stack such
  as TCP/IP, OSI etc.
• Source Computer Networks by Tanenbaum.

4
Why Design?

• Simplicity
• Easy to understand and code.
• Maintainability
• Easy to maintain after the development.
• Portability
• Easily portable to other Platforms
• Can choose the most optimized and efficient design

5
Design

• Pre-requisites for Designing
• Protocol Usage.
• Where, why and how??
• Thorough understanding of the Protocol
  functioning.
• Must understand the RFCs, drafts or other
  available standards for this protocol stack.
• Need to take care about the lower versions
  compatibility.
• Identify interfaces to Layers and Modules.

6
Design Cont..

• Task and module decomposition.
• Protocol Functionality.
• Configuration/Management Interface.
• Interface to Memory Manager.
• Interface to Timer Module.
• Interface to Logging Mechanism.
• Network Buffer Management.
• Network I/O Management

7
Development

• Protocol Functionality
• Divide requirements into functions modules.
• Identify the data structures.
• Identify the Logic.
• Implement the Logic.
• Finite State Machine (FSM)
• A two dimensional Matrix
• State and Event represents the rows and columns.
• The cell has the action to perform and the next
  state.

8
Development

• Protocol Implementation with Linux
• Two ways to do it.
• Kernel Space
• Special Memory Area reserved only for kernel is
  called Kernel Space.
• Processes in Kernel can access Kernel User
  addresses.
• User Space
• Rest of the memory area is called User Space.
• Process in User Space can access only their
  instructions and data.

9
Advantage of User Space

• Malfunctioning of any module would not crash the
  OS.
• Higher Portability.
• It is easier to debug the implementation using
  normal user level debugger.
• It is easier to distribute and install the
  modified daemons.
• Scope for faster development.

10
Advantage of Kernel Space

• Better access to all resources.
• No need to copy buffer/data from Kernel Space to
  User Space or vice versa.
• Context switches between processes in User Space
  are very expensive.

11
Development Modules

• Configuration/Management interface
• To configure and manage the protocol.
• User Space
• A Configuration file
• A Generic CLI implementation
• Kernel Space
• Can use proc file system for the configuration,
  status data related to protocol. Like
  /proc/sys/net/ipv4/conf/ and /proc/net/igmp

12
Modules Cont..

• Interface to Memory Manager
• To allocate,free and manage the memory.
• User Space
• malloc() and free() or new() and delete()
• can write a simple memory pools scheme
• Pools based on modules/structures etc..
• Can track the memory usage per pool/structure
  type.
• Kernel Space
• __get_free_pages() in mm/page_alloc.c
• Kmalloc() kfree() in mm/slab.c
• vmalloc()/vfree() in mm/vmalloc.c

13
Modules Cont..

• Interface to Timer Module
• User Space
• Need to write a timer management module
• Can use select(),hardware interrupt alarm() and
  OS dependent timer like setitimer() for linux.
• Kernel Space
• Kernel timer management is available for use.
• add_timer(), del_timer(), mod_timer() in
  kernel/timer.c

14
Modules Cont..

• Interface to Logging Mechanism
• User Space
• Can write your own logging implementation.
• Must define the priority or levels.
• Can use syslog() function provided by Linux.
• Own implementation can be used for debugging
  during development.
• Kernel Space
• printk() is used to do logging in the kernel.
• Excessive logging may slow down the kernel.
• kernel/printk.c

15
Modules Cont..

• Network buffer management
• Copying of data must be minimized.
• User Space
• Copy the data from one layer to the other.
• Can allocate memory for the whole packet and than
  keep inserting data on different layers.
• Can write a buffer management scheme.
• Kernel Space
• Linux provide sk_buff network buffers to carry
  data from one layer to the other.
  net/core/skbuff.c

16
Modules Cont..

• Network I/O Management
• To receive packets from the network.
• User Space
• Can use raw/packet socket or libpcap library.
• Kernel Space
• Subscribe to Lower and Higher layers.
• inet_add_protocol() can be used to add a
  protocol.
• Need to write receive handler and other
  supporting functions.
• Refer to PIM implementation. net/ipv4/ipmr.c

17
Development

• OS independent Code
• Define interfaces between protocol and different
  modules.
• Just need to change the code in modules.
• Have OS specific code into one module.
• Use Platform Flags (if) in the code.
• User Space code into Kernel-Space
• User-Space Module interfaces APIs similar to
  kernel space function calls.

18
References

• Linux Kernel Source Code 2.4.18
• Lecture Notes from T. Sridhar.
  (Available on Internet)
• IGMP Implementation.
• www.google.com

19
Thank You!

• bharat_joshi_at_infosys.com

20
Finite State Machine

• Example of a Finite State Machine