IPv4 Packet Header Fields: Identify the major header fields in the IPv4 protocol and describe each field's role in transporting packets
- IHL (Header Length): The size of the packet
header. Necessary because the options field means the header size can vary and
the protocol needs to know where the header ends and the data starts when
processing the packet.
- Type of Service: Data QoS priority: Enable
router to give priority to voice and network route information over regular
data.
- Packet Length: Size of entire packet, including
header and data, in byes. The minimum length packet is 20 byes (20 byes header
+ 0 byes data) and the maximum is 65,535 – the maximum value that this 16-bit
field can have.
- Identification: Uniquely identified fragments of
an original IP packet.
- Flag: These 3bits represent control flags, such
as DF (Don't Fragment) and MF (Fragment Offset).
o
MF = 0: When a receiving host receives a frame
with the MF = 0 and a non-zero value in the Fragment offset, it places that
fragment as the last part of the reconstructed packet.
o
MF = 1: The More Fragments flag bit is set, it
means that it is not the last fragment of a packet. When a receiving host sees
a packet arrive with the MF = 1, it examines the Fragment Offset to see where
this fragment is to be placed in the reconstructed packet.
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DF = 1: The Don't Fragment (DF) flag is a single
bit in the Flag field that indicates that fragmentation of the packet is not
allowed. If the Don't Fragment flag bit is set, then fragmentation of this
packet is NOT permitted. If a router needs to fragment a packet to allow it to
be passed downward to the Data Link layer but the DF bit is set to 1, then the
router will discard this packet.
- Fragment Offset: These 13 bits allow a receive
to determine the place of a particular fragment in the original IP datagram
- Time to Live: Number of hops before packet is
dropped: this value is decremented at each hop to prevent packets being passed
around the network in routing loops.
- Protocol: The data payload protocol type:
Denotes if the data is UDP datagram or TCP segment because these Transport
layer protocols manage the receipt of their PDUs differently.
- Header Checksum: Used for error checking the
packet header. At each hop, the checksum of the header must be compared to the
value of this field. If header checksum value does not match the calculated
checksum then the packet is discarded. At each hop the TTL field is decremented
and fragmentation is also possible, so the checksum has to be recalculated at
each hop. Note: this checksum only applies to the header not the encapsulated
data.
- Source Address: IPv4 address of host sending the
packet: Remains unchanged throughout the passage of the packet across the
internetwork. Enables the destination host to respond to the source if
required.
- Destination Address: IPv4 address of host to
receive the packet: Remains unchanged throughout the passage of the packet
across the internetwork. Enables routers at each hop to forward the packet
towards the destination.
- Options: Additional fields header to provide other services; rarely used.
- Options: Additional fields header to provide other services; rarely used.
Typical IP Packet
The figure represents a complete IP packet with typical header field values.
Ver = 4; IP version.
IHL = 5; size of header in 32 bit words (4 bytes). This header is 5*4 = 20 bytes, the minimum valid size.
Total Length = 472; size of packet (header and data) is 472 bytes.
Identification = 111; original packet identifier (required if it is later fragmented).
Flag = 0; denotes packet can be fragmented if required.
Fragment Offset = 0; denotes that this packet is not currently fragmented (there is no offset).
Time to Live = 123; denotes the Layer 3 processing time in seconds before the packet is dropped (decremented by at least 1 every time a device processes the packet header).
Protocol = 6; denotes that the data carried by this packet is a TCP segment .
