IPv4 Protocol Notes
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Version
Only 4 bits, this field determines the how the rest of the packet is treated
Default Value: None
HLen
HLen is the header length and is specified as the number of 32 bit words. Thus the header is at most 480 bits (2^4 - 1) * 32.
Default Value: 5, because we don't specify any options varibles.
TOS
This field has had a number of different definitions over the years and is generally used to define the values used on routers etc.
Default Value: None
Length
This field gives the length of the entire datagram in bytes rather than words. Thus the maximum length of a IP datagram is 65535
NOTE: THE NEXT WORD DEALS WITH FRAGMENTATION
Ident
Strictly an ID chosen by the sender meant to be unique among all the datagrams that might arrive at the destination from this source (in a reasonable amount of time - the time it takes to wrap around this number).
Flags
Three bits long:
Reserved bit
Don't Fragment set to 1 if you don't want routers to fragment this packet
M(ore) Bit set to 1 means that there are more fragments. Therefore the last datagram will not have the M(ore) bit set
Offset
Offset is the number of 8-byte data chunks that should be before this one. If the value of this field is X then X*8 is the number of bytes before the data in this fragmented datagram. Therefore the first fragment will always have a value of 0.
Default: 0
TTL
The intent is to limit the time (in seconds) a packet can wander around an internet in loops before it is discarded. Since most routers do not synchronize their clocks, each router just decrements the value by one and discards it when it gets to 0.
Default Value: 64
Protocol
This field defines a multiplexing key that determines which higher level protocol to pass the data to. On Linux systems see /etc/protocols. TCP = 6, UDP = 17.
Checksum
The checksum is calculated by considering the entire IP header as a sequence of 16-bit words added using one's complement arithmetic, and taking the ones complemnt result. The checksum is calculated using the IpCheckSum algorithm. This is not as good as the CyclicRedundancyCheck (CRC), but it's faster and easier to implement.
SourceAddr
IP Address of the source
DestinationAddr
IP Address of the destination
Options(Variable)
The presence or absence of options may be determined by examining the HLen field. While these are rare a complete IP implmentation must handle them all.
Pad (Variable)
Padding to fill out the length of the header.
Data
Payload of the IP datagram.
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