Delivery & Routing IP packets

04/11/23 Ms.Sonali Borse, CS Dept. 1

Unit-5Delivery & Routing of IP Packets


Delivery Forwarding & Routing of IP Packets Delivery:- refers to the way a packet is

handled by the underlying networks under the control of network layer

Forwarding:- refers to the way a packet is delivered to the next station or stations

Routing:- refers to the way routing tables are created to help in forwarding


Delivery Connection Types:- delivery of IP packets

in network layer is accomplished using 2 types of connections:-

a) Connection-oriented service b) Connectionless service Direct versus indirect delivery:- the

delivery of IP packet to its final destination is accomplished using 2 different methods:-

a) Direct delivery b) Indirect Delivery


Connection-oriented Service

In connection oriented service, the local network layer protocol first makes a connection with the network layer protocol at the remote site before sending data packet

When the connection is established, a sequence of packets from the source to the destination can be sent one after another



In connection oriented service there is relationship between the packets

They are sent on the same path in sequential order.

A packet is logically connected to the packet travelling before it & to the packet traveling after it.

When all packets of a message have been delivered, the connection is terminated



In a connection-oriented service, the decision about the route for sequence of packets with the same source and destination addresses can be made only once, when the connection is established.

Routers do not recalculate the route for each individual packet


Connectionless Service In connectionless service, the network layer protocol

treats each packet independently, with each packet having no relationship to any other packet

The packet in a message may or may not travel the same path to their destination

In connectionless service, the decision about the route of a packet is made individually by each router

The IP protocol is a connectionless protocol; it provides a connectionless service


Direct Delivery In a direct delivery, the final destination of

the packet is a host connected to the same physical network as the deliverer

Direct delivery occurs when the source & destination of the packet are located on the same physical network or if the delivery is between the last router & the destination host


Direct Delivery In direct delivery, sender can easily determine

destination IP address (can extract network address of destination using mask) & compare this address with the addresses of the networks to which it is connected

In direct delivery, the sender uses the destination IP address to find the destination physical address

Then IP software gives the destination IP address with the destination physical address to DLL for actual delivery

This process is called mapping the IP address to physical address

This mapping is done by ARP dynamically.


Direct Delivery


Indirect Delivery If the destination host is not on the same network as

the deliverer, the packet is delivered indirectly In an indirect delivery, the packet goes from router to

router until it reaches the one connected to the same physical network as its final destination

In an indirect delivery, the sender uses the destination IP address & a routing table to find the IP address of the next router to which the packet should be delivered

Then the sender uses ARP to find the physical address of the next router


Indirect Delivery


Forwarding/Routing methods

o Next hop routingo Network specific routingo Host specific routingo Default routing


Next hop routing

Technique to reduce contents of routing table

Table holds only address of next hop

Entries of routing table must be consistent with each other


Next hop routing


Network specific routing

Simplify searching process Instead of having entry for every

destination host connected to the same physical network, only one entry is there that defines the address of destination network itself.

All hosts connected on network are considered as single entity.


Network specific routing


Host specific routing

Destination host address is given in the routing table.

Not efficient to put the host address in routing table, when administrator wants to have more control over routing.

Used for purposes such as checking the route or providing security measures.


Host specific routing


Default routing


Static Routing Table A static routing table contains information

entered manually. The administrator enters the route for each destination into the table

When the table is created, it cannot update automatically when there is change in the internet

The table must be manually altered by the administrator

Static routing table can be used in a small internet that does not change very often


Static Routing Advantage Of Static routing :- Minimal CPU Processing. Easier For Administrator to understand. Easy To Configure.

Disadvantages Of Static routing:- Configuration and Maintenance are time confusing. Configuration is Error Prone especially in large network. Administrator intervention is required to maintain

changing route information. Does not scale well in growing networks; Maintenance

becomes cumbersome. Require complete knowledge of entire network for proper

implementation.


Dynamic Routing Table A dynamic routing table is updated

periodically using one of the dynamic routing protocols such as RIP, OSPF, or BGP

Whenever there is change in the Internet, such as shutdown of a router or breaking of a link, the dynamic routing protocols update all of the tables in the routers automatically

The routers in a big internet such as Internet need to be updated dynamically for efficient delivery of IP packets


Dynamic RoutingAdvantage of Dynamic routing Administrator has less work in maintaining the

configuration when adding or deleting the networks. Protocols automatically react to topology changes. Configuration is less error prone. More Scalable; growing the network usually does not

present a problem.

Disadvantage Router resources are used (CPU cycles, memory, link

bandwidth). More administrator knowledge is required for

configuration, verification, and troubleshooting.


Routing Table

Common fields in Routing Table:-


Routing Tables- Fields Mask:- Defines mask applied for the entry Network address:- defines the network address to

which the packet is finally delivered Next-hop address:- This field defines the address of

the next hop router to which the packet is delivered Interface:- Shows the name of the interface Reference count:-gives the number of users that are

using this route at the moment Use:- shows the number of packets transmitted

through this router for the corresponding destination


Routing Tables- Fields Flags:- flags are on/off switches that signify

either presence or absence. The 5 flags are:-1. U (up):- it indicates that the router is up &

running. If this flag is not present, it means that the router is down. The packet can not be forwarded & discarded

2. G (gateway):- the G flag means that the destination is in another network. The packet is delivered to the next-hop router for delivery. If this flag is missing, it means the destination is in this network


Routing Tables- Fields

3. H (Host-specific):- it indicates that the entry in the network address field is a host-specific address

4. D (added by redirection):- it indicates that routing information for this destination has been added to the host routing table by a redirection message

5. M (modified by redirection) :- this M flag indicates that the routing information for this destination has been modified by a redirection message from ICMP


Routing in classful & classless address environment

Delivery & Routing IP packets

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