Watchguard Firebox X1000, Reference Manual

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CHAPTER 1: Internet Protocol Reference
2 WatchGuard Firebox System
IP header number list
The IP Protocol header contains an 8-bit field that identifies the protocol
for the transport layer for the datagram.
Attribute Size Description
Version 4 bits IP format number (Current version = 4)
IHL 4 bits Header length in 32-bit words (Minimum = 5)
TOS 8 bits Type of service sets routing priorities. It is
generally under-utilized because few application
layers can set it.
Tot_Len 16 bits Total length of packet measured in octets. It is
used in reassembling fragments.
ID 16 bits Packet ID, used for reassembling fragments.
Flags 3 bits Miscellaneous flags
Frag_Off 13 bits Identifies fragment part for this packet.
TTL 8 bits Time to live. It sets the maximum time the
datagram remains alive in the system.
Protocol 8 bits IP protocol number. Indicates which of TCP,
UDP, ICMP, IGMP, or other Transport protocol
is inside.
Check 16 bits Checksum for the IP header
Sour_Addr 32 bits Source IP address
Dest_Addr 32 bits Destination IP address
Options 24 bits IP Options (Present if IHL is 6)
Keyword Number Protocol
0 Reserved
ICMP 1 Internet Control Message
IGMP 2 Internet Group Management
GGP 3 Gateway-to-Gateway
IP 4 IP-within-IP (encapsulation)
ST 5 Stream