add ipv4 header notes

Author: laaraujo

README.md

@@ -11,6 +11,8 @@ Personal notes for learning [CCNA](https://www.cisco.com/site/us/en/learn/traini
 * [Ethernet LAN Switching](./contents/ethernet_lan_switching/)
 * [IPv4 Addressing](./contents/ipv4_addressing/)
 * [Switch Interfaces](./contents/switch_interfaces/)
+* Routing
+    * [IPv4 Header](./contents/ipv4_header/)
 
 
 ## Network node icons

contents/ipv4_header/README.md

@@ -0,0 +1,120 @@
+# IPv4 Header
+
+![](docs/ipv4_header.png)
+
+## Fields
+* [Version](#version)
+* [IHL](#ihl)
+* [DSCP](#dscp)
+* [ECN](#ecn)
+* [Total Length](#total-length)
+* [Identification](#identification)
+* [Flags](#flags)
+* [Fragment Offset](#fragment-offset)
+* [Time To Live](#time-to-live)
+* [Protocol](#protocol)
+* [Header Checksum](#header-checksum)
+* [Source and Destination IP Addresses](#source-and-destination-ip-addresses)
+* [Options](#options)
+
+## Version
+
+* `4 bits`
+* identifies the version of IP used
+    * IPv4 = 4 (`0100`)
+    * IPv6 = 6 (`0110`)
+
+## IHL
+
+Internet Header Length
+
+* `4 bits`
+* indicates total length of the header ([options field](#options) if variable)
+* 4-byte increments
+    * `5 = 5 * 4-bytes = 20 bytes`
+* `5 <= IHL <= 15`
+
+## DSCP
+Differentiated Services Codepoint
+
+* `6 bits`
+* used for QoS (Quality of Service)
+* used to prioritize delay-sensitive data (streaming audio, video, etc)
+
+## ECN
+
+Explicit Congestion Notification
+
+* `2 bits`
+* provides e2e notification of network congestion without dropping packets
+* optional field that requires both endpoints, as well as the network infra, to support it
+
+## Total Length
+
+* `16 bits`
+* indicates the total length of the packet (L3 header + L4 segment)
+* measured in bytes
+    * `20 = 20 bytes`
+* `20 <= Total Length <= 65535`
+
+## Identification
+
+* `16 bits`
+* if a packet is fragmented due to being too large, this field is used to identify which packet the fragment belongs to
+* all fragments of the same packet will have their own IPv4 header with the same value in this field
+* packets are fragmented if larger than the MTU (Maximum Transmission Unit)
+* MTU is usually `1500 bytes`
+    * same as max size of ethernet frame payload
+* fragments are reassembled by the receiving host
+
+## Flags
+
+* `3 bits`
+* used to control/identify fragments
+* `bit 0` is reserved and always set to 0
+* `bit 1` is the **don't fragment** (DF bit), used to indifcate a packet should not be fragmented
+* `bit 2` is the **more fragments** (MF bit)
+    * `1` if there are more fragments in the packet
+    * `0` for the last fragment
+
+## Fragment Offset
+
+* `13 bits`
+* indicates position of the fragment within the original unfragmented IP packet
+* allows fragmented packets to be reassembled even if the fragments arrive out of order
+
+## Time To Live
+
+* `8 bits`
+* a router will drop a packet with a TTL of 0
+* used to prevent infinite loops and congestion
+* originally designed to indicate the packets maximum lifetime in seconds
+* in practice, indicates a *hop count*: each time the packet arrives at a router, the router decreases the TTL by 1
+* recommended default TTL is `64`
+
+## Protocol
+
+* `8 bits`
+* indicates the protocol of the encapsulated LAYER 4 (Transport) PDU
+* `6` = TCP
+* `17` = UDP
+* `1` = ICMP (ping)
+* `89` = OSPF (Open Shortest Path First, dynamic routing protocol)
+
+## Header Checksum
+
+* `16 bits`
+* calculated checksum used to check for errors in the IPv4 header
+* router receives the packet, calculates the checksum and compares it to the one in this field.
+* if they do not match the router drops the package 
+
+## Source and Destination IP Addresses
+
+* `32 bits` each
+* IPv4 Addresses of the sender and intended receiver of the package
+
+## Options
+
+* `0 - 320 bits`
+* rarely used
+* if IHL field is greater than `5`, it means that Options are present