62 lines
3.6 KiB
Markdown
62 lines
3.6 KiB
Markdown
---
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title: IPv4 Header
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---
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## IPv4 Header Format
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Below is the IPv4 header format according to [RFC791](https://tools.ietf.org/html/rfc791):
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##### Version [4-bits]
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* Internet Protocol (IP) version number (decimal 4, binary 0100, hex 4)
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##### Internet Header Length (IHL) [4-bits]
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* Length of the IPv4 header
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* The value of the IHL is measured in 32-bit words
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* A value of 5 specifies the length of the header is 5 * 32-bits = 160bits = 20 bytes
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* The minimum value of an IPv4 header is 20 bytes
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##### Differentiated Services Code Point (DSCP) [6-bits]
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* This value is used by network devices to determine precedence of an IP packet [[RFC2474](https://tools.ietf.org/html/rfc2474)]
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* Rules can be created on network devices to handle different DSCP values appropriately
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* For example - VoIP traffic may be given a DSCP value of 46 (binary 101110, hex 0x2E) which may inform a network device this packet should be given high priority
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##### Explicit Congestion Notification (ECN) [2-bits]
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* An optional feature that allows communicating end hosts to identify network congestion without necessarily dropping packets.
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* ECN feature is only available if the underlying network infrastructure also supports it.
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##### Total Length [16-bits]
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* Length of the packet, including the header.
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* This value is measured in bytes (8-bits), and has a maximum value of 2^16 - 1 = 65,535 bytes
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##### Identification [16-bits]
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* Assigned by the sending host to help in assembling the fragments of a packet
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##### Flags [3-bits]
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* Used to determine if a packet may be fragmented or to inform the receiving host that additional fragmented packets are inbound
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* The Do Not Fragment (DF) bit is used when a packet may not be fragmented. If a device must fragment the packet because the packet is larger than the Maximum Transmission Unit (MTU) of the link, the packet is dropped
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##### Fragment Offset [13-bits]
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* The calculated offset for a fragmented packet relative to the beginning of the original unfragmented packet
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* Measured in 8-byte blocks
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* Maximum value is (2^13 - 1) * 8 = 65,528 bytes
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##### Time To Live (TTL) [8-bits]
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* The amount of time (in seconds) a packet is allowed to remain in the network
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* This value is increased by one second at each network device encountered
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* When the value reaches zero, the packet is discarded
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##### Protocol [8-bits]
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* Similar to the Next Header field in IPv6, this field specifies the protocol encapsulated in the IPv4 packet
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* For example, TCP Protocol Number 6 would be binary 110, hex 0x6
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##### Header Checksum [16-bits]
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* The checksum is the 16-bit [one's complement](https://www.cs.uaf.edu/2004/fall/cs301/notes/node41.html) of the one's complement sum of all 16-bit words in the header. [[RFC792](https://tools.ietf.org/html/rfc791#page-14)]
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##### Source IP Address [32-bits]
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* The source IP address of the sender of the packet
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##### Destination IP Address [32-bits]
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* The destination IP address for the receiver of the packet
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##### Options [32-bits]
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* Specifies additional options
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* Only present if the IHL is greater than 5, meaning the length is more than 5 * 32-bit words or 160-bits or 20 bytes - the length of a standard IPv4 header without options
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* Options must include additional padding if they do not completely fill a 32-bit word boundary
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#### More Information:
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* IP Option Numbers [[iana.org]](https://www.iana.org/assignments/ip-parameters/ip-parameters.xhtml)
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* List of IP protocol numbers used in the Next Header [[iana.org]](https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml)
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