Information Technology Reference
In-Depth Information
Figure13-5 The VIP Packet Format
2
2
1
1
4
2
4
2
Variable
Trans-
port
control
Destination
network
number
Destination
subnetwork
number
Source
network
number
Source
subnetwork
number
Field
length,
in bytes
Check-
sum
Packet
length
Protocol
type
Data
The fields of a VIP packet are as follows:
Checksum
—Used to detect packet corruption.
•
Packet length
—Indicates the length of the entire VIP packet.
•
Transport control
—Consists of several subfields. If the packet is a broadcast packet, two subfields
are provided: class (bits 1 through 3) and hop-count (bits 4 through 7). If the packet is not a
broadcast packet, four subfields are provided: error, metric, redirect, and hop count. The class
subfield specifies the type of node that should receive the broadcast. For this purpose, nodes are
broken into various categories having to do with the type of node and the type of link the node is on.
By specifying the type of nodes to receive broadcasts, the class subfield reduces the disruption
caused by broadcasts. The hop count subfield represents the number of hops (router traversals) the
packet has been through. The error subfield specifies whether the ICP protocol should send an
exception notification packet to the packet's source if a packet turns out to be unroutable. The metric
subfield is set to 1 by a transport entity when it needs to learn the routing cost of moving packets
between a service node and a neighbor. The redirect subfield specifies whether the router should
generate a redirect (when appropriate).
•
Protocol type
—Indicates the network- or transport-layer protocol for which the metric or exception
notification packet is destined.
•
Destination network number
,
Destination subnetwork number
,
Source network number
, and
Source subnetwork number
—Provide VIP address information.
•
RTP
RTP distributes network topology information. Routing update packets are broadcast periodically by
both client and service nodes. These packets inform neighbors of a node's existence and indicate whether
the node is a client or a service node. Service nodes also include, in each routing update packet, a list of
all known networks and the cost factors associated with reaching those networks.
Two routing tables are maintained: a table of all known networks and a table of neighbors. For service
nodes, the table of all known networks contains an entry for each known network except the service
node's own network. Each entry contains a network number, a routing metric, and a pointer to the entry
for the next hop to the network in the table of neighbors. The table of neighbors contains an entry for
each neighbor service node and client node. Entries include a network number, a subnetwork number,
the media-access protocol (for example, Ethernet) used to reach that node, a local-area network (LAN)
address (if the medium connecting the neighbor is a LAN), and a neighbor metric.
RTP specifies four packet types:
Routing update
—Issued periodically to notify neighbors of an entity's existence.
•
Routing request
—Exchanged by entities when they need to learn the network's topology quickly.
•
Routing response
—Contains topological information and is used by service nodes to respond to
routing request packets.
•
Routing redirect
—Provides better path information to nodes using inefficient paths.
•
