- Flow chart of the
IP forwarding algorithm:
- Explanation:
- Node (computer or router)
receives an IP packet
(inside an Ethernet frame !!!)
- It will
first extract
the IP packet and
- Examine the destination IP address (in the IP protocol header)
- It will
first extract
the IP packet and
- First, it check whether the
destination IP address is
one of the IP addresses
in the IP forwarding table
- If so, the IP packet is
sent of the corresponding
network interface
(Because the IP destination is on one of the IP networks attached to the node)
- If so, the IP packet is
sent of the corresponding
network interface
- Otherwise, check whether the
destination IP address is
one of the IP addresses
in the routing table
- If so, the IP packet is sent of the corresponding router
- Otherwise, check if the
routing table has a
default entry
- If so, the IP packet is sent of the corresponding default router
- Final option: drop the packet.... (cannot find a route to the destination)
- Node (computer or router)
receives an IP packet
(inside an Ethernet frame !!!)
- Example 1:
- IP host/node x7
sends
a message to IP destination host/node
y9:
- x7 runs the
IP forwarding algorithm:
- Destination y9's
network ID is
y
- Look up y in the
forwarding table
- Network ID
y is
not
in the forwarding table....
So it will check the routing table
- Network ID
y is
not
in the forwarding table....
- Look up y in the
routing table
- Network ID
y is
not
in the routing table
So it try to use the default route
- Network ID
y is
not
in the routing table
- Host x7 will use the default route: (default, A) for routing
Result: x7 sends the message to A
- Destination y9's
network ID is
y
- Router A runs the
IP forwarding algorithm:
- Destination y9's
network ID is
y
- Look up y in the
forwarding table
- Network ID
y is
not
in the forwarding table....
So it will check the routing table
- Network ID
y is
not
in the forwarding table....
- Look up y in the
routing table
- Network ID
y is
not
in the routing table
So it try to use the default route
- Network ID
y is
not
in the routing table
- Router A will use the default route: (default, B) for routing
Result: router A sends the message to router B
- Destination y9's
network ID is
y
- Router B runs the
IP forwarding algorithm:
- Destination y9's
network ID is
y
- Look up y in the
forwarding table
- Network ID y is found in the forwarding table !!!!
Result: router B sends the message directly on the network interface #3:
- Destination y9's
network ID is
y
- IP host/node x7
sends
a message to IP destination host/node
y9:
- Example 2:
node z9 sends
a message to node
x7:
- We will start
looking at this example when the
IP packet has
arrived
at router B:
- The following diagram
summarizes the
routing decisions:
(Shorten) explanation:
- IP forwarding algorithm
run at router B
- Destination x7's
network ID is
x
x is not in the forwarding table so it uses the routing table
- Router B will use the entry: (Net x, A) for routing
Result: B sends the message to A
- Destination x7's
network ID is
x
- IP forwarding algorithm
run at router A:
- Destination x9's
network ID is
x
x is found in the forwarding table !!!
Result: router A sends the message to the network attached to its port #1
- Destination x9's
network ID is
x
- IP forwarding algorithm
run at router B
- Forwarding example 3:
x7 sends
a message to
z9
- The following diagram
summarizes the
routing decisions:
Notes:
- IP forwarding algorithm
run at x7
- Destination z9's
network ID is
z
z is not in the forwarding table so it uses the routing table
- Host x7 will use the entry: (default, A) for routing
Result: x7 sends the message to A
- Destination z9's
network ID is
z
- IP forwarding algorithm
run at router A:
- Destination z9's
network ID is
z
z is not in the forwarding table, so it uses the routing table
- Router A
will use the entry:
(default, B)
for routing
Result: router A sends the message to router B
- Destination z9's
network ID is
z
- IP forwarding algorithm
run at router B:
- Destination z9's
network ID is
z
z is not in the forwarding table, so it uses the routing table
- Router B will use the entry: (default, C) for routing
Result: router B sends the message to C
- Destination z9's
network ID is
z
The message will eventuall get to D and then to the network z....
- IP forwarding algorithm
run at x7
- Key points:
- A node
(computer or router)
can always tell if the
destination of an IP packet
is located on its own
network by the
network IDs
in its
Forwarding Table
- If the destination is on one of its own networks, it will send the IP-packet directly to the destination in a physical (Ethernet or Token bus) frame.
- If the destination is
not on its own network,
the node will
send the IP-packet to
a neighboring router
(a router that is connected to node)
- This neighboring router is always given in the routing table.
- A node
(computer or router)
can always tell if the
destination of an IP packet
is located on its own
network by the
network IDs
in its
Forwarding Table
- Note:
- Some text book considers the IP forwarding table as as special section of the IP routing table