8. Linux Networking
8.1 How Linux networking is managed?
There exists a device driver for each kind of NIC. Inside it, Linux will ALWAYS call a standard high level routing: "netif_rx [net/core/dev.c]", which will controls what 3 level protocol the frame belong to, and it will call the right 3 level function (so we'll use a pointer to the function to determine which is right).
8.2 TCP example
We'll see now an example of what happens when we send a TCP packet to Linux, starting from ''netif_rx [net/core/dev.c]'' call.
Interrupt management: "netif_rx"
|netif_rx
|__skb_queue_tail
|qlen++
|* simple pointer insertion *
|cpu_raise_softirq
|softirq_active(cpu) |= (1 << NET_RX_SOFTIRQ) // set bit NET_RX_SOFTIRQ in the BH vector
Functions:
- __skb_queue_tail [include/linux/skbuff.h]
- cpu_raise_softirq [kernel/softirq.c]
Post Interrupt management: "net_rx_action"
Once IRQ interaction is ended, we need to follow the next part of the frame life and examine what NET_RX_SOFTIRQ does.
We will next call ''net_rx_action [net/core/dev.c]'' according to "net_dev_init [net/core/dev.c]".
|net_rx_action
|skb = __skb_dequeue (the exact opposite of __skb_queue_tail)
|for (ptype = first_protocol; ptype < max_protocol; ptype++) // Determine
|if (skb->protocol == ptype) // what is the network protocol
|ptype->func -> ip_rcv // according to ''struct ip_packet_type [net/ipv4/ip_output.c]''
**** NOW WE KNOW THAT PACKET IS IP ****
|ip_rcv
|NF_HOOK (ip_rcv_finish)
|ip_route_input // search from routing table to determine function to call
|skb->dst->input -> ip_local_deliver // according to previous routing table check, destination is local machine
|ip_defrag // reassembles IP fragments
|NF_HOOK (ip_local_deliver_finish)
|ipprot->handler -> tcp_v4_rcv // according to ''tcp_protocol [include/net/protocol.c]''
**** NOW WE KNOW THAT PACKET IS TCP ****
|tcp_v4_rcv
|sk = __tcp_v4_lookup
|tcp_v4_do_rcv
|switch(sk->state)
*** Packet can be sent to the task which uses relative socket ***
|case TCP_ESTABLISHED:
|tcp_rcv_established
|__skb_queue_tail // enqueue packet to socket
|sk->data_ready -> sock_def_readable
|wake_up_interruptible
*** Packet has still to be handshaked by 3-way TCP handshake ***
|case TCP_LISTEN:
|tcp_v4_hnd_req
|tcp_v4_search_req
|tcp_check_req
|syn_recv_sock -> tcp_v4_syn_recv_sock
|__tcp_v4_lookup_established
|tcp_rcv_state_process
*** 3-Way TCP Handshake ***
|switch(sk->state)
|case TCP_LISTEN: // We received SYN
|conn_request -> tcp_v4_conn_request
|tcp_v4_send_synack // Send SYN + ACK
|tcp_v4_synq_add // set SYN state
|case TCP_SYN_SENT: // we received SYN + ACK
|tcp_rcv_synsent_state_process
tcp_set_state(TCP_ESTABLISHED)
|tcp_send_ack
|tcp_transmit_skb
|queue_xmit -> ip_queue_xmit
|ip_queue_xmit2
|skb->dst->output
|case TCP_SYN_RECV: // We received ACK
|if (ACK)
|tcp_set_state(TCP_ESTABLISHED)
Functions can be found under:
- net_rx_action [net/core/dev.c]
- __skb_dequeue [include/linux/skbuff.h]
- ip_rcv [net/ipv4/ip_input.c]
- NF_HOOK -> nf_hook_slow [net/core/netfilter.c]
- ip_rcv_finish [net/ipv4/ip_input.c]
- ip_route_input [net/ipv4/route.c]
- ip_local_deliver [net/ipv4/ip_input.c]
- ip_defrag [net/ipv4/ip_fragment.c]
- ip_local_deliver_finish [net/ipv4/ip_input.c]
- tcp_v4_rcv [net/ipv4/tcp_ipv4.c]
- __tcp_v4_lookup
- tcp_v4_do_rcv
- tcp_rcv_established [net/ipv4/tcp_input.c]
- __skb_queue_tail [include/linux/skbuff.h]
- sock_def_readable [net/core/sock.c]
- wake_up_interruptible [include/linux/sched.h]
- tcp_v4_hnd_req [net/ipv4/tcp_ipv4.c]
- tcp_v4_search_req
- tcp_check_req
- tcp_v4_syn_recv_sock
- __tcp_v4_lookup_established
- tcp_rcv_state_process [net/ipv4/tcp_input.c]
- tcp_v4_conn_request [net/ipv4/tcp_ipv4.c]
- tcp_v4_send_synack
- tcp_v4_synq_add
- tcp_rcv_synsent_state_process [net/ipv4/tcp_input.c]
- tcp_set_state [include/net/tcp.h]
- tcp_send_ack [net/ipv4/tcp_output.c]
Description:
- First we determine protocol type (IP, then TCP)
- NF_HOOK (function) is a wrapper routine that first manages the network filter (for example firewall), then it calls ''function''.
- After we manage 3-way TCP Handshake which consists of:
SERVER (LISTENING) CLIENT (CONNECTING)
SYN
<-------------------
SYN + ACK
------------------->
ACK
<-------------------
3-Way TCP handshake
- In the end we only have to launch "tcp_rcv_established [net/ipv4/tcp_input.c]" which gives the packet to the user socket and wakes it up.
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