System information

The Network Layer
Book Title
14-302
Figure 14-1 The IDP Packet Format
The fields of the IDP packet are as follows:
• Checksum—A 16-bit field that helps gauge the integrity of the packet after it traverses the
internetwork.
• Length—A 16-bit field that carries the complete length (including checksum) of the current
datagram.
• Transport control—An 8-bit field that contains hop count and maximum packet lifetime (MPL)
subfields. The hop count subfield is initialized to zero by the source and incremented by one as
the datagram passes through a router. When the hop count field reaches 16, the datagram is
discarded on the assumption that a routing loop is occurring. The MPL subfield provides the
maximum amount of time, in seconds, that a packet can remain on the internetwork.
• Packet type—An 8-bit field that specifies the format of the data field.
• Destination network number—A 32-bit field that uniquely identifies the destination network in
an internetwork.
• Destination host number—A 48-bit field that uniquely identifies the destination host.
• Destination socket number—A 16-bit field that uniquely identifies a socket (process) within the
destination host.
• Source network number—A 32-bit field that uniquely identifies the source network in an
internetwork.
• Source host number—A 48-bit field that uniquely identifies the source host.
• Source socket number—A 16-bit field that uniquely identifies a socket (process) within the
source host.
IEEE 802 addresses are equivalent to host numbers, so a host that is connected to more than one
IEEE 802 network has the same address on each segment. This makes network numbers redundant,
but nevertheless useful for routing. Certain socket numbers are well known, meaning that the service
performed by the software using them is statically defined. All other socket numbers are reusable.
XNS supports Ethernet Version 2.0 encapsulation for Ethernet and three types of encapsulation for
Token Ring: 3Com, Subnet Access Protocol (SNAP), and Ungermann-Bass.
XNS supports unicast (point-to-point), multicast, and broadcast packets. Multicast and broadcast
addresses are further divided into directed and global types. Directed multicasts deliver packets to
members of the multicast group on the network specified in the destination multicast network
address. Directed broadcasts deliver packets to all members of a specified network. Global
A B C D J DataGE HF I
2211 2 0-54624466
Field length,
in bytes
A - Checksum
B - Length
C - Transport control
D - Packet type
E - Destination network number
F - Destination host number
G - Destination socket number
H - Source network number
I - Source host number
J - Source socket number