website：https://frrouting.org/
github：https://github.com/FRRouting/frr
FRRouting (FRR) is an IP routing protocol suite for Linux and Unix platforms which includes protocol daemons for BGP, IS-IS, LDP, OSPF, PIM, and RIP.
FRR’s seamless integration with the native Linux/Unix IP networking stacks makes it applicable to a wide variety of use cases including connecting hosts/VMs/containers to the network, advertising network services, LAN switching and routing, Internet access routers, and Internet peering.
支持的协议列表：
Protocol IPv4 IPv6
OSPF OSPFv2 OSPFv3
ISIS ISIS IPv4 ISIS IPv6
RIP RIP RIPng
BGP BGP IPv4 BGP IPv6
BGP 4-byte AS BGP IPv4 4-byte AS BGP IPv6 4-byte AS
LDP LDP IPv4
每个守护进程使用socket收发报文，以RIP为例：
1、static int rip_create(void)
/* Create read and timer thread. */
rip_event(RIP_READ, rip->sock);
rip_event(RIP_UPDATE_EVENT, 1);
2、void rip_event(enum rip_event event, int sock)
thread_add_read(master, rip_read, NULL, sock, &rip->t_read);
thread_add_timer(master, rip_update, NULL,
sock ? 2 : rip->update_time + jitter,
&rip->t_update);
3、static int rip_read(struct thread *t)
len = recvfrom(sock, (char *)&rip_buf.buf, sizeof(rip_buf.buf), 0,
(struct sockaddr *)&from, &fromlen);
4、static int rip_send_packet(u_char *buf, int size, struct sockaddr_in *to, struct connected *ifc)
ret = sendto(rip->sock, buf, size, 0, (struct sockaddr *)&sin,
sizeof(struct sockaddr_in));
