ccna-notes

IPv4 Header

Fields

• Version
• IHL
• DSCP
• ECN
• Total Length
• Identification
• Flags
• Fragment Offset
• Time To Live
• Protocol
• Header Checksum
• Source and Destination IP Addresses
• 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 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