The IP Routing
Process
Introduction
We are going to take a look at what happens when routing
occurs on a network. When I was new to the networking
area, I thought that all you needed was the IP Address
of the machine you wanted to contact but so little did I
know. You actually need a bit more information than just
the IP Address !
The process we are going to explain is fairly simple and
doesn't really change, no matter how big your network
is.
The Example:
In our example, we have 2
networks, Network A
and
Network B. Both networks are
connected via a router (Router A)
which has 2 interfaces: E0
and E1.
These interfaces are just like the
interface on your network card (RJ-45), but built into
the router.
Now, we are going to describe step
by step what happens when
Host A (Network
A) wants to communicate
with Host B
(Network
B) which is on a different
network.
1)
Host A opens a command
prompt and enters >Ping
200.200.200.5.
2) IP works with the Address
Resolution Protocol (ARP)
to determine which network this packet is destined for
by looking at the IP address and the subnet mask of the
Host A.
Since this is a request for a
remote host, which means it is not destined to be sent
to a host on the local network, the packet must be sent
to the router (the gateway for
Network A)
so that it can be routed to the correct remote network
(which is Network B).
3) Now, for
Host A
to send the packet to the router,
it needs to know the hardware address of the router's
interface which is connected to its network (Network
A), in case you didn't
realise, we are talking about the MAC (Media Access
Control) address of interface
E0.
To get the hardware address,
Host A
looks in its
ARP
cache - a memory location where these MAC addresses are
stored for a few seconds .
4) If it doesn't find it in there
it means that either a long time has passed since it
last contacted the router or it simply hasn't resolved
the IP address of the router (192.168.0.1)
to a hardware address (MAC). So it then sends an
ARP
broadcast. This broadcast contains the following "What
is the hardware (MAC) address for IP 192.168.0.1 ?
". The router identifies
that IP address as its own and must answer, so it sends
back to Host A
a reply, giving it the MAC address of its
E0
interface. This is also one of the reasons why sometimes
the first "ping" will timeout. Because it takes some
time for an ARP
to be sent and the requested
machine to respond with its MAC address, by the time all
that happens, the TTL (Time To Live) of the first ping
packet has expired, so it times out !
5) The router responds with the
hardware address of its E0
interface, to which the
192.168.0.1
IP is bound. Host A
now has everything it needs in
order to transmit a packet out on the local network to
the router. Now, the Network Layer hands down to the
Datalink Layer the packet it generated with the ping (ICMP
echo request), along with the hardware address of the
router. This packet includes the source and destination
IP address as well as the ICMP echo request which was
specified in the Network Layer.
6) The Datalink Layer of
Host A
creates a frame, which
encapsulates the packet with the information needed to
transmit on the local network. This includes the source
and destination hardware address (MAC) and the type
field which specifies the Network Layer protocol e.g
IPv4 (that's the IP version we use), ARP. At the end of
the frame, in the FCS
portion of the frame, the Datalink
Layer will stick a Cyclic Redundancy Check (CRC) to make
sure the receiving machine (the router) can figure out
if the frame it received has been corrupted. To learn
more on how the frame is created, visit the
Data Encapsulation - Decapsulation.
7) The Datalink Layer of
Host A
hands the frame to the Physical
layer which encodes the 1s and 0s into a digital signal
and transmits this out on the local physical network.
8)The signal is picked up by the
router's E0
interface and reads the frame. It will first do a CRC
check and compare it with the CRC value Host A added to
this frame, to make sure the frame is not corrupt.
9)After that, the destination
hardware address (MAC) of the received frame is checked.
Since this will be a match, the type field in the frame
will be checked to see what the router should do with
the data packet. IP is in the type field, and the router
hands the packet to the IP protocol running on the
router. The frame is stripped and the original packet
that was generated by Host
A is now in the router's
buffer.
10) IP looks at the
packet's destination IP address to determine if the
packet is for the router. Since the destination IP
address is 200.200.200.5, the router determines from the
routing table that 200.200.200.0 is a directly connected
network on interface
E1.
11) The router places the packet
in the buffer of interface
E1. The router needs to
create a frame to send the packet to the destination
host. First, the router looks in the
ARP
cache to determine whether the hardware address has
already been resolved from a prior communication. If it
is not in the ARP
cache, the router sends an
ARP broadcast out
E1 to
find the hardware address of 200.200.200.5
12)
Host B responds with the
hardware address of its network interface card with an
ARP
reply. The router's
E1
interface now has everything it needs to send the packet
to the final destination.
13)The frame generated from the
router's E1
interface has the source hardware address of
E1
interface and the hardware destination address of
Host B's
network interface card. However, the most important
thing here is that even though the frame's source and
destination hardware address changed at every interface
of the router it was sent to and from, the IP source and
destination addresses never changed. The packet was
never modified at all, only the frame changed.
14)
Host B receives the frame
and runs a CRC. If that checks out, it discards the
frame and hands the packet to IP. IP will then check the
destination IP address. Since the IP destination address
matches the IP configuration of
Host B,
it looks in the protocol field of the packet to
determine the purpose of the packet.
15) Since the packet is an ICMP
echo request, Host B
generates a new
ICMP echo-reply
packet with a source IP address of
Host B
and a destination IP address of
Host A.
The process starts all over again,
except that it goes in the opposite direction. However,
the hardware address of each device along the path is
already known, so each device only needs to look in its
ARP
cache to determine the hardware (MAC)
address of each interface.
And that just about covers our routing analysis. If you
found it confusing, take a break and come back later on
and give it another shot. Its really simple once you
grasp the concept of routing. |
