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IP

Protocol​

IP Header

Service Model​

  • Prevent packets looping forever(TTL/time to live field in header): if TTL gets decreased to zero, then drop this datagram.
  • Limit fragment packets size(Packet ID, Flags, Fragment Offset).
  • Reduce changes of wrong destination(Checksum, Destination Address).
  • 操作系统协议栈的 IP 模块会检查 IP 报文头部:
    • 判断本机是否为接收方.
    • 判断网络包是否经过分片.
    • 将包转交给 TCP 模块或 UDP 模块.
IP Layer

IP 本身不负责包的传输, 而是委托各种通信技术将包传输到下一个路由器 (纯粹路由器). 极简的 IP Layer 设计, 可以根据需要灵活运用各种通信技术 (以太网, 无线网 etc), 从而构建出互联网这一规模巨大的网络.

Router​

  • 路由器的各个端口都具有 MAC 地址和 IP 地址.
  • 路由器会忽略主机号, 只匹配网络号. 通过路由聚合, 减少路由表记录数, 合并子网.

IPv4​

Address structure:

  • 32 bits long: a.b.c.d.
  • Historical:
    • Class A: 0-network(7 bits)-host(24 bits).
    • Class B: 10-network(14 bits)-host(16 bits).
    • Class C: 110-network(21 bits)-host(8 bits).
  • Today: 171.64.0.0/16 means 171.64.0.0 to 171.64.255.255, A/24 describes 256 addresses, A/20 describes 4096 addresses.
  • Longest prefix matching and netmask (A/16): e.g. 0.0.0.0/0 => matching all addresses.

Packet Format​

  • Network is big-endian.
  • In x86 processor, use htons()/ntohs()/htonl()/ntohl() host: network short/long helper function to transform format.
#include <arpa/inet.h>

uint16_t http_port = 80;
uint16_t packet_port = ntohs(packet->port);

if (packet_port == http_port) {
    // OK
}

ICMP​

  • Report error conditions back.
  • Help diagnose problems.
  • Site above IP: ICMP 报文包含在 IP 数据报中, 属于 IP 的一个用户. 一个 ICMP 报文包括 IP 头部 + ICMP 头部 + ICMP 报文. IP 头部的 Protocol 值为 1 表示这是一个 ICMP 报文.

ICMP Header

  • 0 type 0 code: echo reply(by ping)
  • 3 type 0 code: destination network unreachable
  • 3 type 1 code: destination host unreachable
  • 3 type 3 code: destination port unreachable
  • 8 type 0 code: echo request(by ping)
  • 11 type 0 code: TTL(time to live) Expired(by ping)