diff --git a/guide/english/network-engineering/ipv4-header/index.md b/guide/english/network-engineering/ipv4-header/index.md index 5cd6a72310..a75aee19ba 100644 --- a/guide/english/network-engineering/ipv4-header/index.md +++ b/guide/english/network-engineering/ipv4-header/index.md @@ -3,13 +3,59 @@ title: IPv4 Header --- ## IPv4 Header Format -This is a stub. Help our community expand it. +Below is the IPv4 header format according to [RFC791](https://tools.ietf.org/html/rfc791): + +![Image of IPv4 Header](https://i.imgur.com/5ys9G3k.png) + + +##### Version [4-bits] + * Internet Protocol (IP) version number (decimal 4, binary 0100, hex 4) +##### Internet Header Length (IHL) [4-bits] + * Length of the IPv4 header + * The value of the IHL is measured in 32-bit words + * A value of 5 specifies the length of the header is 5 * 32-bits = 160bits = 20 bytes + * The minimum value of an IPv4 header is 20 bytes + +##### Differentiated Services Code Point (DSCP) [6-bits] + * This value is used by network devices to determine precedence of an IP packet [[RFC2474](https://tools.ietf.org/html/rfc2474)] + * Rules can be created on network devices to handle different DSCP values appropriately + * 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 + +##### Explicit Congestion Notification (ECN) [2-bits] + * An optional feature that allows communicating end hosts to identify network congestion without necessarily dropping packets. + * ECN feature is only available if the underlying network infrastructure also supports it. +##### Total Length [16-bits] + * Length of the packet, including the header. + * This value is measured in bytes (8-bits), and has a maximum value of 2^16 - 1 = 65,535 bytes +##### Identification [16-bits] + * Assigned by the sending host to help in assembling the fragments of a packet +##### Flags [3-bits] + * Used to determine if a packet may be fragmented or to inform the receiving host that additional fragmented packets are inbound + * 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 +##### Fragment Offset [13-bits] + * The calculated offset for a fragmented packet relative to the beginning of the original unfragmented packet + * Measured in 8-byte blocks + * Maximum value is (2^13 - 1) * 8 = 65,528 bytes +##### Time To Live (TTL) [8-bits] + * The amount of time (in seconds) a packet is allowed to remain in the network + * This value is increased by one second at each network device encountered + * When the value reaches zero, the packet is discarded +##### Protocol [8-bits] + * Similar to the Next Header field in IPv6, this field specifies the protocol encapsulated in the IPv4 packet + * For example, TCP Protocol Number 6 would be binary 110, hex 0x6 +##### Header Checksum [16-bits] + * 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)] +##### Source IP Address [32-bits] + * The source IP address of the sender of the packet +##### Destination IP Address [32-bits] + * The destination IP address for the receiver of the packet +##### Options [32-bits] + * Specifies additional options + * 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 + * Options must include additional padding if they do not completely fill a 32-bit word boundary -This quick style guide will help ensure your pull request gets accepted. - #### More Information: - - - + * IP Option Numbers [[iana.org]](https://www.iana.org/assignments/ip-parameters/ip-parameters.xhtml) + * List of IP protocol numbers used in the Next Header [[iana.org]](https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml)