The IP Datagram

I wanted to know and now you can too.

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ipv				ihl				tos								total length
identification																flags			frag offs
ttl								proto								header checksum
source address
destination address
(options)																								(padding)
(data)

1. ip version (4 bits)
   The ip protocol version. Currently 4 as of 1979 (rfc 791).

2. ip header length (4 bits)
   Total header length in 32 bit words. Usually five (5) (since you hardly ever use options). Sort of silly because values of 0-4 cannot be used. Perhaps these could be used as some sort of biased value to extend the length of the header?

3. type of service (8 bits)
   Describes how the packet should be handled in transit (speed vs. reliability vs. throughput). Bits arranged as follows:

   0	1	2	3	4	5	6	7
   precedence			d	t	r	reserved

   bits 0-2: (precedence)	000 - routine traffic 001 - priority 010 - immediate 011 - flash 100 - flash override 101 - critic/ecp 110 - internetwork control 111 - network control
   bit 3: (d)	normal(0)/low(1) delay
   bit 4: (t)	normal(0)/high(1) throughput
   bit 5: (r)	normal(0)/high(1) reliability
   bits 6-7:	reserved for future use

4. total packet length (16 bits)
   Packet length in bytes up to 65535 (includes the bytes in the header). You won't probably won't see many packets this big because ethernet framing doesn't let you have anything more than 1500 and modem links are usually set much lower (about 296 perhaps?) for responsiveness. So larger poackets would always get fragemented and that's something to avoid. If possible.

5. identification (16 bits)
   An identifying number to be used to reassemble fragmented packets.

6. fragmentation flags (3 bits)
   Flags controlling whether a packet is fragmented and/or may be fragmented. Structured as follows:

   0	1	2
   reserved	df	lf

   where the df bit (don't fragment) prevents packets from being fragmented if set, and the lf bit (last fragment) is set in the last packet in the sequence of fragments.

7. fragmentation offset (13 bits)
   Measured in units of 8 octets (or bytes) is simply a sequence number for the bytes in this packet when reassembling. The first offset will be zero.

8. time to live (8 bits)
   Maximum life of a packet. If this field is zero, the packet is destroyed. Usually decremented by one when passed from one hop to the next (unless bridging).

9. protocol (8 bits)
   The protocol used in the data portion (see "assigned numbers" rfc 790 or 1010 or below).

10. header checksum (16 bits)
    A sixteen bit checksum on the ip header only. The ttl field changes at each hop so this needs to be recomputed on each hop. "The checksum field is the 16 bit one's complement of the one's complement sum of all 16 bit words in the header. For purposes of computing the checksum, the value of the checksum field is zero." (At least, according to rfc 791). My interpretation: one's complement each sixteen bit word in the header, add all these quantities (drop carries) and then one's complement that sum.

11. source address &
    
12. destination address (32 bits each)
    The ip addresses of the ultimate sending and receiving hosts of the packet. In network byte order (of course) which is just another way of saying big endian, but it's more politically correct.

13. options (variable, up to 40 bytes)
    The first byte (if any options bytes are present) is the option type byte laid out as follows:

    0	1	2	3	4	5	6	7
    copy flag	option class		option number

    If the copy flag is set, the option is copied into all fragments upon fragmentation. Option classes are: 0) control, 1) reserved, 2) debugging and measurement, and 3) reserved. A few option types are defined:

    class	number	length	description
    0	0	-	end of option list
    0	1	-	no operation
    0	2	11	security
    0	3	var.	loose source routing
    0	9	var.	strict source routing
    0	7	var.	record route
    0	8	4	stream id
    2	4	var.	internet timestamp

    The next option byte (if required by the option type) is usually a length byte (in bytes, not 32 bit words).

14. padding (variable, fills out the 32 bit words)
    I'm pretty sure this is ignored so it probably doesn't matter but just make it all zeroes anyway, okay? It just makes the header an even 32 bit length.

15. data (variable, up to 65535 - header bytes)
    This is where all your data goes. Whatever ip can package (see the protocol field) goes here. Take your pick (from rfc 790 or 1010)...

    protocol number			protocol name		rfcs listed
    decimal	octal	hexadecimal	keyword	meaning	790	1010
    0	000	00		(reserved)	*	*
    1	001	01	ICMP	internet control message protocol	*	*
    2	002	02	IGMP	Internet Gateway Management Protocol		*
    3	003	03	GGP	Gateway-to-Gateway	*	*
    4	004	04		(unassigned)	CMCC	(unassigned)
    5	005	05	ST	Stream	*	*
    6	006	06	TCP	Transmission Control Protocol	*	*
    7	007	07	UCL	UCL	*	*
    8	010	08	EGP	External Gateway Protocol		*
    9	011	09	IGP	any private interior gateway	SECURE	IGP
    10	012	0a	BNN-RCC-MON	BBN RCC Monitoring	*	*
    11	013	0b	NVP-II	Network Voice Protocol	NVP	NVP-II
    12	014	0c	PUP	PUP	*	*
    13	015	0d	ARGUS	ARGUS	PLURIBUS	ARGUS
    14	016	0e	EMCON	EMCON	TELENET	EMCON
    15	017	0f	XNET	Cross Net Debugger	*	*
    16	020	10	CHAOS	Chaos	*	*
    17	021	11	UDP	User Datagram Protocol	*	*
    18	022	12	MUX	Multiplexing	*	*
    19	023	13	DCN-MEAS	DCN Measurement Subsystems	DCN	DCN-MEAS
    20	024	14	HMP	Host Monitoring	TAC	HMP
    21	025	15	PRM	Packet Radio Measurement		*
    22	026	16	XNS-IDP	XEROX NS IDP		*
    23	027	17	TRUNK-1	Trunk-1		*
    24	030	18	TRUNK-2	Trunk-2		*
    25	031	19	LEAF-1	Leaf-1		*
    26	032	1a	LEAF-2	Leaf-2		*
    27	033	1b	RDP	Reliable Data Prococol		*
    28	034	1c	IRTP	Internet Reliable Transaction		*
    29	035	1d	ISO-TP4	ISO Transport Protocol Class 4		*
    30	036	1e	NETBLT	Bulk Data Transfer Protocol		*
    31	037	1f	MFE-NSP	MFE Network Services Protocol		*
    32	040	20	MERIT-INP	MERIT Internodal Protocol		*
    33	041	21	SEP	Sequential Exchange Protocol		*
    34-60	042-074	22-3c		(unassigned)	*	*
    61	075	3d		any host internal procotol		*
    62	076	3e	CFTP	CFTP		*
    63	077	3f		any local network
    64	100	40	SAT-EXPAK	SATNET and backroom EXPAK	*	*
    65	101	41	MIT-SUBNET	MIT subnet support
    66	102	42	RVD	MIT Remote Virtual Disk Protocol		*
    67	103	43	IPPC	Internet Pluribus Packet Core		*
    68	104	44		any distributed file system		*
    69	105	45	SAT-MON	SATNET Monitoring	*	*
    70	106	46		(unassigned)	*	*
    71	107	47	IPCV	Internet Packet Core Utility	*	*
    72-75	110-113	48-4b		(unassigned)	*	*
    76	114	4c	BR-SAT-MON	Backroom SATNET Monitoring	*	*
    77	115	4d		(unassigned)	*	*
    78	116	4e	WB-MON	WIDEBAND Monitoring	*	*
    79	117	4f	WB-EXPAK	WIDEBAND EXPAK	*	*
    80-254	120-376	50-fe		(unassigned)	*	*
    255	377	ff		(reserved)	*	*

    or try reading your local /etc/protocols file.