ajout 10 IPv4 Header
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shango-wk
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10. IPv4 Header.md
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10. IPv4 Header.md
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id: 1777370971-DYNV
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aliases:
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- IPv4 Header
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tags: []
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---
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# IPv4 Header
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## OSI Model - [[PDU]]s
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Application Data
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↓
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Data + L4 Header = Segment
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↓
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Segment + L3 Header = Packet
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↓
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Packet + L2 Header + L2 Trailer = Frame
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We will focus on the Packet for this lesson
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## IPV4 header
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### Version Field
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length: 4 bits (half of on octet)
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- identifies the version of IP used.
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- IPv4 = 4 (0b0100)
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- IPv4 = 6 (0b0110)
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### Internet Header Length (IHL)
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length: 4 bits (half of on octet)
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- The final field of the IPv4 header (Options) is variable in length, so this field
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is necessary to indicate the total length of the header
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- identifies the lenght of the header in **4 byte increments**
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- Value of 5 = 5*4 bytes = 20 bytes
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- minimum value is 5 (= 20 bytes)
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- **Minimum IPv4 Header Length = 20 bytes**
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- Maximum value is 15 (15*4 bytes = 60 bytes)
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- **Maximum IPv4 Header Length = 60 bytes**
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### DSCP field
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- Length: 6 bits
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- Differentiated Services Code Point
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- Used for Qos (Quality of Service)
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- Used to prioritize delay-sensitive dta (streaming voice, video, etc..)
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### ECN field
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Explicit Congestion Notification
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- Length: 2 bits
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- Provides end-to-end (between two endpoints) notification of network congestion
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**without dropping packets**
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- Optional feature that requires both endpoints, as well as the underlying network
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infrastructure, to support it.
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### Total Length field
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Length: 16 bits
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- Indicates the total lenght of the packet (L3 header+ L4 segment)
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- Measured in bytes (**not 4-byte increments like IHL**)
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- Minimum value of 20 (=IPv4 header with no encapsulated data)
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- Maximum value of 65,535 (maximum 16-bit value)
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### Identification field
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Length: 16 bits
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- if a packet is fragmented due to being too large, this field is used to identify
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which packet the fragment belongs to
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- All fragments of the same packet will have their own [[IPv4]]
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header with the same value in this field
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- Packets are fragmented if larger than the MTU (Maximum Transmission Unit)
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- The MTU is usually 1500 bytes
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- Fragments are reassembled by the receiving host
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### Flags field
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Length: 3bits
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- Used to control/identify fragments.
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- Bit 0: Reserved, always set to 0
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- Bit 1: Don't Fragment (DF bit), used to indicate a packet that should not be fragmented
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- Bit 2: More Fragments (MF bit), set to 1 if there are more fragments in the packet,
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set to 0 for the last fragment
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*Unfragemented packets will always have their MF bit set to 0*
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### Fragment Offset field
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Length: 13 bits
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- Used to indicate the position of the fragment within the original, unfragmented IP packet
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- Allows fragmented packets to be reassembled even if the fragments arrive out of order
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### Time to live field
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Length: 8 bits
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- A router will drop a packet with a TTL of -
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- Used to prevent infinite loops
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- Originlly designed to indicte the packet's maximum lifetime in second
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- In practice, indicates a 'Hop count': each time the packet arrives at a router,
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the router decreases the TTL by 1
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*Recommended default TTL is 64*
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### Protocol field
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Length: 8 bits
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- Indicates the protocol of the encapsulated L4PDU
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- Value of 6: [[TCP]]
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- Value of 17: [[UDP]]
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- Value of 1: [[ICMP]]
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- Value of 89 : [[OSPF]] dynamic routing protocol
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### Header Checksum field
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Lenght 16bits
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- A calculaed checksum used to check for errors in the IPv4 header.
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- When a router receives a packet, it calculates the checksum of the header
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and compares it to the one in this field of the header
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- if they do not match the router drops the packet
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- Used to check for errors only in the IPV4 header.
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- IP relies on the encapsulated protocol to detect errors in the encapsulated data
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- Both TCP and UDP have their own checksum fields to detect errors in the encapsulated data
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### Source/Destination IP address fields
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Length 32 bits(each)
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- Source IP Address = IPv4 address of the sender of the packet
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- Destination IP Address = IPv4 address of the intended receiver of the packet
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### Options fields
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Length: 0 - 320 bits
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- Rarely used.
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- If the IHL field is greater than 5, it means that Options are present
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## Review
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- IPv4 Packet structure
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- Fields of the IPv4 header
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