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Transparent Bridges in Internet Protocols

Explore the basics of transparent bridges, including MAC layers, forwarding functions, and learning mechanisms in Internet Protocols. Dive into implementation details and data structures for efficient bridge operation.

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Transparent Bridges in Internet Protocols

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  1. Transparent Bridges Reading: Chapter 12 FSU CIS 5930 Internet Protocols

  2. Basics of Transparent bridges • Work at Data-Link Layer • Different MAC layers can be used on different subnet (port) of a bridge • Transparent • Stations/routers need not to know the existence of a bridge • Forwarding and filtering • Only forward packet to necessary subnet FSU CIS 5930 Internet Protocols

  3. Linux bridge LAN 1802.3 LAN 2 802.3u Station A Station B LAN 3 802.11 An example FSU CIS 5930 Internet Protocols

  4. Forwarding function • Forwarding table • MAC address • If destination MAC is in table • Forward to the corresponding subnet ONLY • Otherwise • Flooding to all ports except the incoming port FSU CIS 5930 Internet Protocols

  5. A A B B Station A Station B Station C LAN 1 LAN 2 LAN 3 Forwarding function (example 1) FSU CIS 5930 Internet Protocols

  6. LAN 1 LAN 2 LAN 3 A C Station A Station B Station C Forwarding (example 2) FSU CIS 5930 Internet Protocols

  7. LAN 1 LAN 2 LAN 3 A A A E E E Station A Station B Station C Forwarding (example 3) FSU CIS 5930 Internet Protocols

  8. Learning Function • How the forwarding table is populated • Static configuration • Self-learning • Backward learning • Learning the address of someone when he is talking • MAC address, no hierarchical structure • Time stamp to delete “old” entries FSU CIS 5930 Internet Protocols

  9. Implementation • Architecture • Data structures • Forwarding function • Learning new MAC address FSU CIS 5930 Internet Protocols

  10. Higher layers dev.c br_fdb.c, br_if.c, br_stp.c, br_stp_bpdu, ... br_ioctl.c netif_rx SpanningTree Protocol ForwardingDataBase br_ioctl br_input.c Bridge Configuraion br_handle_stp_pdu br_pass_frame_up br_send_tcn_bpdu(..) br_send_config_bpdu(..) br_fdb_get(..) br_forward.c br_handle_frame br_forward br_flood dev.c dev.c dev_queue_xmit net_rx_action Architecture of Bridge Implementation FSU CIS 5930 Internet Protocols

  11. bridge_list net_bridge net_bridge next lock port_list net_bridge_fdb_entry net_bridge_fdb_entry 0 dev next_hash statistics pprev_hash 1 hash_lock use_counter 2 hash[BR_HASH_SIZE] addr Spanning Tree ... ... dst timer1 aging_timer forward_delay_timer is_local hold_timer is_static message_age_timer BR_HASH-SIZE -1 net_bridge_fdb_entry net_bridge_fdb_entry next_hash pprev_hash use_counter net_bridge_port addr net_bridge_port next dst net_bridge aging_timer net_device is_local port_no is_static Spanning Tree ... forward_delay_timer hold_timer message_age_timer Data structures FSU CIS 5930 Internet Protocols

  12. Bridge forwarding function • net_rx_action() when soft IRQ is handled • For a bridge, it calls br_handle_frame_hook(), which points to br_handle_frame() • Check if the packet needs to be forwarded • By br_fdb_get() • If so, br_forward() • Learning the position of the source by br_fdb_put() • If the packet is for ourselves, br_pass_frame_up() FSU CIS 5930 Internet Protocols

  13. Forwarding fuctions • br_handle_frame() • br_forward() • br_flood() • br_pass_frame_up() • br_fdb_get() FSU CIS 5930 Internet Protocols

  14. Learning MAC addresses • br_fdb_insert() • br_mac_hash() • br_fdb_cleanup() • gc_timer • gc_interval FSU CIS 5930 Internet Protocols

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