= Bridging Details =

----

== Initialization ==


 1. The initialization and clean up routines are br_init() and br_uninit()
    respectively.
 2. Initialization consists of
		1. allocating memory for '''''net_bridge_fdb_entry''''' structures
		1. Initialization of function pointer '''br_ioctl_hook''' to the routine
		1. Initialization of Function pointer '''br_handle_frame_hook''' to the routine to process ingress BPDU.

== Creating a Bridge ==

 1. Each Bridge Device can have up to a maximum of 1024 ports.
 2. Bridge devices are created and removed using '''br_add_bridge()''' and '''br_del_bridge()'''
 3. Ports are added to a bridge device with '''br_add_if()''' and removed with '''br_del_if()'''
 4. The above routines execute with NETLINK routing lock held.The routines '''rtnl_lock()''' and '''rtnl_unlock()''' help in acquiring and releasing the locks.
 5. '''br_add_bridge()''' and '''br_del_bridge()''' take care of locking on their own.
 6. '''br_add_if()''' and '''br_del_if()''' uses '''dev_ioctl()''' to take care of locking/unlocking.

== Bridge Device Creation ==

 1. The '''new_bridge_dev(const char* name_of_the_bridge)''' function takes care of creating a new bridge.
	1. It initializes the net_device data structure
	1. Initializes the private data structure i.e (network device's private data '''''netdev->priv''''') 
	1. Initializes the per-bridge timers with '''br_stp_timer_init(struct net_bridge *br)'''
	 
== Bridge Device Setup ==

 1. The Bridges use the '''br_dev_setup(struct net_device* netdev)''' routine to set up the bridge.The '''br_dev_setup()''' 
    routine is invoked in '''new_bridge_dev()''' function by calling allocate_dev() to which '''br_dev_setup()''' is given as
    function pointer.
 2. The kernel distinguishes the bridge from other devices if the '''IFF_EBRIDGE'''flag is set in '''''struct net_device'''''.
 3. The function '''br_dev_ioctl(struct net_device *dev, struct ifreq *rq, int cmd)''' processes some of the ioctl commands 
    thats issued in the bridge devices.
 4. The bridging device driver initializes the '''br_dev_xmit(struct sk_buff *skb,struct net_device *dev)''' which 
    is invoked in '''br_dev_setup()''' routine by initializing '''''br_dev_xmit''''' to '''''hard_start_xmit''''' function 
    pointer available in '''''net_device''''' data structure.


== Deleting a Bridge ==

 1. To remove a bridge,'''br_del_bridge()''' routine invokes '''del_br()''' which does as
    follows
	1. Removes all the bridge ports
	1. For each port,removes all the corresponding entries in the forwarding database.
	1. Removes the bridge device directory in /sys/class/net directory.
	1. De registers the device using the routine unregister_netdevice

== Forwarding Database ==

 1. Each Bridge instance has its own forwarding database used regardless whether
    STP is run or not.
 2. The Forwarding database is placed in the net_bridge data structure and defined as hash table.
 3. An instance of the net_bridge_fdb_entry data structure is added to the database for each MAC address learnt on the bridge ports.

== Lookups ==

 1. '''struct net_bridge_fdb_entry *fdb_find(struct hlist_head *head,const unsigned char *addr)'''
	1. Is used for querying net_bridge_fdb_entry for a given MAC address.
	1. Not used for forwarding data traffic.

2. '''struct net_bridge_fdb_entry *__br_fdb_get(struct net_bridge *br,const unsigned char *addr)'''
	 1. Similar to '''fdb_find''' but used to forward traffic, called by bridging code ('''br_dev_xmit()''' routine)

3. Adding/Updating/Removing Entries
	1. The '''''net_bridge_fdb_entry''''' data structure is populated with the device MAC
	  address using the '''br_fdb_insert(struct net_bridge *br, struct net_bridge_port *source, const unsigned char *addr)''' 
          which is called by the '''br_add_if(struct net_bridge *br, struct net_device *dev)''' routine
	  which is called by '''add_del_if()''' routine called by '''br_dev_ioctl()''' routine
	  which processes the ioctl commands by the user.

== Removing ==

	*. The net_bridge_fdb_entry entries are removed using 
	  '''void fdb_delete(struct net_bridge_fdb_entry *f)''' routine.


== Handling Ingress Traffic ==

	1. Reciept of frame is handled by '''netif_receive_skb()''' which calls
	   '''handle_bridge()'''. '''handle_bridge()''' is defined as NULL Pointer if the
	   kernel does not have support for bridging.
	2. If the kernel has support for bridging,'''handle_bridge()''' processes the
	   frame with br_handle_frame_hook. 
        3. The '''''br_handle_frame_hook''''' is initialized with '''''br_handle_frame''''' in the routine during the 
           bridge module initialization.When the packet is received,the '''netif_receive_skb()''' calls
	   '''handle_bridge()''' which calls the br_handle_frame_hook.


== Transmitting on a Bridge Device ==

	1. The '''''hard_start_xmit''''' in net_device is initialized with
		'''br_dev_xmit(struct sk_buff *skb, struct net_device *dev)''' routine which
		has the logic used by the bride to transmit data.


== Overall Process ==

 1. When a NIC is configured as a bridge port, the '''''br_port''''' member of the
    '''''net_device''''' is initialized.
 1. Receipt of frame is handled by '''netif_receive_skb()''' which calls
    handle_bridge()
 1. To transmit the frame, '''dev_queue_xmit()''' function is called which invokes the
    '''hard_start_xmit()''' routine provided by the device driver.
 1. The bridging forwarding database is searched for destination MAC address.
 1. In case of a success, the frame is sent to the bridge port by making a call
    to '''br_forward()'''
 1. If the MAC lookup is a failure, the frame is flooded using '''br_flood()'''

Following are the Data Structures
	1. '''''net_bridge_fdb_entry''''' - Entry of the Forwarding Database.There is one for each MAC address learned by the bridge.
	1. '''''net_bridge''''' - Information about the bridge.

= Routing/Forwarding Subsystem =

----

 1. Routing table is implemented using the '''''struct fib_table''''' data structure.
 1. There are two routing tables by default
	* Local FIB Table
	* Main FIB Table
 1. Reading entries in the routing table is done by calling the '''fib_lookup()''' function.
 1. Routing tables are consulted using the '''ip_route_input_slow()''' and '''ip_route_output_slow()''' functions.
 1. Two versions of '''fib_lookup()''' exist,one used when the kernel has support for policy routing and the other when the support is not included.
    The selection is made at compile time.
 1. All routing table lookups regardless the direction of the traffic, is done using the '''fn_hash_lookup()''' function.This function's lookup 
    algorithm uses the LPM algorithm.
 1.The routing cache is built using '''''rtable''''' elements.


== Routing Lookup ==


 1. The '''ip_route_input()''' is called and makes a cache lookup. If the cache lookup
    results in a HIT then the packet is delievered using '''ip_local_deliver()''' or
    '''ip_forward()''' 
 1. If the cache lookup results in a MISS then call to '''fib_lookup()''' is made and
    it in turn queries the LOCAL FIB Table.If the route exists the packet is
    delivered using the '''ip_local_deliver()''' or '''ip_forward()''' function.
 1. If the route does not exist in the LOCAL FIB, the MAIN FIB is looked up and
    if the route exists in the main table the packet is forwarded as above else
    the packet is dropped.

== Packet Forwarding ==

Forwarding is split into two functions 
	* '''ip_forward()'''
	* '''ip_forward_finish()''' 

 1. '''int ip_forward(struct sk_buff* skbuff)'''
	* Forwards the packet
	* Decreases the TTL in the IP header
	* if TTL is <=1, send ICMP message and packet is dropped.
	* Calls '''NF_HOOK(NF_HOOK(PF_INET,NF_IP_FORWARD, skb, skb->dev, 
	rt->u.dst.dev, ip_forward_finish)'''

 1. '''ip_forward_finish()''' 
	* Sends the packet out by calling '''dst_output(skb)'''

 1. '''dst_output(skb)'''
	* is a wrapper which in turn calls '''''skb->dst->output(skb)'''''