ZyXEL Communications ZyWall 110 User Manual page 497

Figure 335 IP v4 Packet Headers
The header fields are discussed in the following table.
Table 206 IP v4 Packet Headers
HEADER
Version
IHL
Type of Service
Total Length
Identification
Flags
Fragment Offset
Time To Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options
Padding
Chapter 30 IDP
DESCRIPTION
The value 4 indicates IP version 4.
IP Header Length is the number of 32 bit words forming the total length of the
header (usually five).
The Type of Service, (also known as Differentiated Services Code Point (DSCP)) is
usually set to 0, but may indicate particular quality of service needs from the
network.
This is the size of the datagram in bytes. It is the combined length of the header
and the data.
This is a 16-bit number, which together with the source address, uniquely
identifies this packet. It is used during reassembly of fragmented datagrams.
Flags are used to control whether routers are allowed to fragment a packet and to
indicate the parts of a packet to the receiver.
This is a byte count from the start of the original sent packet.
This is a counter that decrements every time it passes through a router. When it
reaches zero, the datagram is discarded. It is used to prevent accidental routing
loops.
The protocol indicates the type of transport packet being carried, for example, 1 =
ICMP; 2= IGMP; 6 = TCP; 17= UDP.
This is used to detect processing errors introduced into the packet inside a router
or bridge where the packet is not protected by a link layer cyclic redundancy
check. Packets with an invalid checksum are discarded by all nodes in an IP
network.
This is the IP address of the original sender of the packet.
This is the IP address of the final destination of the packet.
IP options is a variable-length list of IP options for a datagram that define IP
Security Option, IP Stream Identifier, (security and handling restrictions for
the military), Record Route (have each router record its IP address), Loose
Source Routing (specifies a list of IP addresses that must be traversed by the
datagram), Strict Source Routing (specifies a list of IP addresses that must
ONLY be traversed by the datagram), Timestamp (have each router record its IP
address and time), End of IP List and No IP Options.
Padding is used as a filler to ensure that the IP packet is a multiple of 32 bits.
ZyWALL/USG Series User's Guide
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