Abstract of CCNA study guide-7 - tcp/ip 2

Continue the series of  Abstract CCNA study guide book .

The Internet Layer Protocols

In the DoD model, there are two main reasons for the Internet layer’s existence: routing and providing a single network interface to the upper layers.

And all the other protocols at this layer, as well as all those at the upper layers, use it. Never forget that. All paths through the DoD model go through IP.

The following sections describe the protocols at the Internet layer:

-Internet Protocol (IP)

-Internet Control Message Protocol (ICMP)

-Address Resolution Protocol (ARP)

-Reverse Address Resolution Protocol (RARP)

-Proxy ARP

Internet Protocol (IP)

Internet Protocol (IP) is the Internet layer. The other protocols found here merely exist to support it.IP looks at each packet’s address. Then, using a routing table, it decides where a packet is to be sent next, choosing the best path.

IP receives segments from the Host-to-Host layer and fragments them into datagrams (packets). IP then reassembles datagrams back into segments on the receiving side.

 Each datagram is assigned the IP address of the sender and of the recipient. Each router (layer 3 device) that receives a datagram makes routing decisions based on the packet’s destination IP address.

Next Figure shows an IP header.

The following fields make up the IP header:

Version IP version number.

Header length Header length in 32-bit words.

Priority and Type of Service Type of Service tells how the datagram should be handled. The first 3 bits are the priority bits.

Total length Length of the packet including header and data.

Identification Unique IP-packet value.

Flags Specifies whether fragmentation should occur.

Fragment offset Provides fragmentation and reassembly if the packet is too large to put in a frame.

Time to Live The time to live is set into a packet when it is originally generated. If it doesn’t get to where it wants to go before the TTL expires, it’s gone.

Protocol Port of upper-layer protocol (TCP is port 6 or UDP is port 17 [hex]).

Header checksum Cyclic redundancy check (CRC) on header only.

Source IP address 32-bit IP address of sending station.

Destination IP address 32-bit IP address of the station this packet is destined for.

Options Used for network testing, debugging, security, and more.

Data  the upper-layer data.

Internet Control Message Protocol (ICMP)

Internet Control Message Protocol (ICMP) works at the Network layer and is used by IP for many different services. ICMP is a management protocol and messaging service provider for IP. Its messages are carried as IP datagrams.

ICMP packets have the following characteristics:

- They can provide hosts with information about network problems.

- They are encapsulated within IP datagrams.

The following are some common events and messages that ICMP relates to:

Destination Unreachable If a router can’t send an IP datagram any further, it uses ICMP to send a message back to the sender. For example, take a look at Figure 2.8, which shows that interface E0 of the Lab_B router is down.

When Host A sends a packet destined for Host B, the Lab_B router will send an ICMP destination unreachable message back to the sending device (Host A in this example).

Buffer Full If a router’s memory buffer for receiving incoming datagrams is full, it will use ICMP to send out this message.

Hops Each IP datagram has a certain number of routers, called hops, to pass through. If it reaches its limit of hops before arriving at its destination, the last router deletes datagram and uses ICMP to send message, informing the sending machine of the demise of its datagram.

Ping Packet Internet Groper (Ping) uses ICMP echo request and reply messages to check the physical and logical connectivity of machines on an internetwork.

Traceroute Using ICMP time-outs, Traceroute is used to discover the path a packet takes as it traverses an internetwork.

Address Resolution Protocol (ARP)

Address Resolution Protocol (ARP) finds the hardware address of a host from a known IP address.

Here’s how it works: When IP has a datagram to send, it must inform a Network Access protocol, such as Ethernet or Token Ring, of the destination’s hardware address on the local network. (It has already been informed by upper-layer protocols of the destination’s IP address.) If IP doesn’t find the destination host’s hardware address in the ARP cache, it uses ARP to find this information.

ARP interrogates the local network by sending out a broadcast asking the machine with the specified IP address to reply with its hardware address. So basically, ARP translates the software (IP) address into a hardware address

Reverse Address Resolution Protocol (RARP)

When an IP machine happens to be a diskless machine, it has no way of initially knowing its IP address. But it does know its MAC address. Reverse Address Resolution Protocol (RARP) discovers the identity of the IP address for diskless machines by sending out a packet that includes its MAC address and a request for the IP address assigned to that MAC address. A RARP server, responds with the answer.

Proxy Address Resolution Protocol (Proxy ARP)

On a network, your hosts can’t have more then one default gateway configured.

What if the default gateway (router) happens to go down? The host won’t just start sending to another router automatically—you’ve got to reconfigure that host. But Proxy ARP can actually help machines on a subnet reach remote subnets without configuring routing or even a default gateway.

One advantage of using Proxy ARP is that it can be added to a single router on a network without disturbing the routing tables of all the other routers that live there too. But there’s a serious downside to using Proxy ARP. Using Proxy ARP will definitely increase the amount of traffic on your network segment, and hosts will have a larger ARP table than usual in order to handle all the IP-to-MAC-address mappings. And Proxy ARP is Configured on all Cisco routers by default