The Ethernet header structure is shown in the illustration below:

以太網頭部14 bytes

Destination	Source	Len	Data unit + pad	FCS
(6 bytes)	(6 bytes)	(2)	(46-1500 bytes)	(4 bytes)
Ethernet header structure

The IP header structure is as follows:

IP頭部20 bytes

4	8	16		32 bits
Ver.	IHL	Type of service		Total length
Identification				Flags	Fragment offset
Time to live		Protocol		Header checksum
Source address
Destination address
Option + Padding
Data
IP header structure

The TCP header structure is as follows:

TCP頭部20 bytes

16								32 bits
Source port								Destination port
Sequence number
Acknowledgement number
Offset	Resrvd	U	A	P	R	S	F	Window
Checksum								Urgent pointer
Option + Padding
Data
TCP header structure

The UDP header structure is shown as follows:

UDP頭部8 bytes

16	32 bits
Source port	Destination port
Length	Checksum
Data
UDP header structure

一些協議頭部的定義：

struct ip { #if BYTE_ORDER == LITTLE_ENDIANu_char ip_hl:4, /* header length */ip_v:4; /* version */ #endif #if BYTE_ORDER == BIG_ENDIANu_char ip_v:4, /* version */ip_hl:4; /* header length */ #endifu_char ip_tos; /* type of service */short ip_len; /* total length */u_short ip_id; /* identification */short ip_off; /* fragment offset field */u_char ip_ttl; /* time to live */u_char ip_p; /* protocol */u_short ip_sum; /* checksum */struct in_addr ip_src,ip_dst; /* source and dest address */ };struct udphdr {u_short uh_sport; /* source port */u_short uh_dport; /* destination port */short uh_ulen; /* udp length */u_short uh_sum; /* udp checksum */ };struct tcphdr {u_short th_sport; /* source port */u_short th_dport; /* destination port */tcp_seq th_seq; /* sequence number */tcp_seq th_ack; /* acknowledgement number */ #if BYTE_ORDER == LITTLE_ENDIANu_char th_x2:4, /* (unused) */th_off:4; /* data offset */ #endif #if BYTE_ORDER == BIG_ENDIANu_char th_off:4, /* data offset */th_x2:4; /* (unused) */ #endifu_char th_flags;u_short th_win; /* window */u_short th_sum; /* checksum */u_short th_urp; /* urgent pointer */ };

更多網絡協議格式細節參考：http://www.protocols.com/pbook/

轉載于:https://www.cnblogs.com/qq76211822/p/5137043.html

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