Unit 7: Delivery, Forwarding, Unicast Routing Protocols, Multicast Routing protocols 6 Hours

Delivery: refers to the way a packet - is handled by the underlying networks - under the control of the network layer

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

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

Routing protocols: used to continuously - update the routing tables - that are consulted for forwarding and routing

Delivery

Network layer: supervises the handling of the packets by - underlying physical networks - this handling, defined as the

delivery of a packet

Direct vs. Indirect delivery

Two different methods of delivery: used to delivery packet to its final destination

1. Direct delivery

2. Indirect delivery

1. Direct delivery

Final destination of the packet: is a host connected to the same physical network - as the deliverer

Occurs: when the source and destination of the packet - are located on the same physical network or when the delivery

is between the last router and the destination host

Sender can easily: determine if the delivery is direct

Can extract - network address of the destination (using the mask) and compare this address with the addresses of the

networks to which it is connected - If a match is found, delivery is direct

2. Indirect delivery

Delivery in which: destination host is not on the same network as the deliverer

Packets are delivered - indirectly

Packets - goes from router to router - until it reaches the one connected to the same physical network - as its final

destination

Note: Delivery always involves: 1 direct delivery and 0 or more indirect deliveries

Last delivery is always a direct delivery

Forwarding

Means - to place the packet in its route to its destination

Requires - a host or a router to have a routing table

When a host - has a packet to send or when a router has received - a packet - to be forwarded - it looks at this table to

find the route to the final destination

This simple solution: impossible in an inter-network - like Internet - because, number of entries needed in the routing

table would make table lookups inefficient

Forwarding techniques

Techniques for: size of routing table manageable and handle issues such as security

Techniques are:

1. Next-hop method vs. route method

Method to reduce contents of a routing table: next-hop method

Next-hop method: routing table holds - only the address of the next hop

Route method: routing table holds - the information about the complete route

Entries of routing table: must be consistent with one another

Fig. shows how routing tables can be simplified by using this technique

2. Network-specific method vs. host-specific method

Network-specific method: reduce the routing tale and simplify the searching process

Has only one entry - that defines the address of the destination network itself

Host-specific method: has entries for every destination host connected to the same physical network

All hosts: connected to the same network - treated as one single entity

Ex.: If 1000 hosts are attached the same network: only one entry exist in the routing table instead of 1000

Host-specific routing: used for purposes such as checking the route or providing security measures

3. Default method

Another method to simplify routing

Fig. shows host A is connected to a network with 2 routers

Router R1: routes the packets to hosts connected to network N2

For the rest of the Internet: router R2 is used - so, instead of listing all networks in the entire Internet - host A can just

have one entry called Default (normally, with network address 0.0.0.0)

Forwording process

Assumption: hosts and routers use classless addressing (because, classful addressing can be treated as special case of

classless addressing)

Classless addressing:

Routing table: needs to have one row of information for each block involved

Needs to be searched based on the network address (first address in the block)

Unfortunately - destination address in the packet -gives no clue about he network address

Solution: need to include the mask (/n) in the table

Need to have an extra column - that includes the mask for the corresponding block

Fig. shows simple forwarding module for classless addressing

Note: Classless addressing in: at least 4 columns in the routing table - usually, there are more

Address aggregation

Classless addressing in: number of routing table entries will increase - because, intent of classless addressing is to

divide up the whole address space into manageable blocks

Increased size of table: results in an increase in the amount of time needed ot search the table

Solution: usage of address aggregation

Fig. has 2 routers

Router R1: connected to networks of 4 organizations - that each use 64 addresses

Router R2: is far from R1

R1: has a longer routing table - because, each packet must be correctly routed to the appropriate organization

R2: can have a very small routing table

Any packet with destination 140.24.7.0 to 140.24.7.255 - is sent out from interface m0 - regardless of the organization

number - this is called address aggregation - because, blocks of address for 4 organizations are aggregated into one

larger block

Would have a longer routing table- if each organization - had addresses that could not be aggregated into one block

Note: Idea of address aggregation and idea of sub-netting appears same - but has differences

Address aggregation: do not have a common site and network for each organization is independent - can have several

levels of aggregation

Longest mask matching: If one of the organization in fig. is in geographically close to the other 3 – ex. if organization

4 - cannot be connected to router R1 - for some reason - still use of the address aggregation and assigning block

140.24.7.192/26 to organization 4

Solution: yes - because, routing in classless addressing - uses, another principle called longest mask matching

Principle: routing table is sorted from the longest mask to the shortest mask

If there are 3 masks: /27,/26, and /24 - the mask /27 must be the first entry and /24 must be last

Consider the situation: organization 4 is separated from the other 3 organizations - Fig. shows the situation

Let a packet arrives for organization 4 - with destination address 140.24.7.200

First mask at router R2 is applied - it gives the network address 140.24.7.192

Packet is routed correctly - from interface m1 - and reaches organization 4

If however - routing table was not stored with the longest prefix first - applying the /24 mask - would result in the

incorrect routing of the packet to router R1

Hierarchical routing

Solution for the gigantic routing tables: routing tables in - a sense of hierarchy can be created

Ex.: Internet

Internet today - has a series of hierarchy

Internet is divided into international and national ISPs

National ISPs: are divided into regional ISPs

Regional ISPs: are divided into local ISPs

If the routing table ahs a sense of hierarchy: like the Internet architecture - the routing table can decrease in size

Consider: Local ISP case

Local ISP: can be assigned a single - but, large block of addresses with a certain prefix length

Local ISP: can divide this block into smaller blocks of different sizes and can assign these to individual susers and

organizations, both large and small

If the block - assigned to the local ISP - starts with a.b.c.d/n - the ISP can create blocks starting with e.f.g.h/m - where,

m may vary for each customer and is greater than n

Routing table reduction: rest of the Internet: does not have to be aware of this division

All customers of the local ISP: are defined as a.b.c.d/n to the rest of the Internet

Every packet - destined for one of the addresses in this large block is routed to the local ISP

There is only one entry in every router in the world - for all these customers - they all belong to the same group

Inside the local ISP: router must recognize the sub-blocks and route the packet to the destined customer

If one of the customers is a large organization: it also can create another level of hierarchy by sub-netting and dividing

its sub-block into smaller sub-blocks (or sub sub-blocks)

Classless routing in: the levels of hierarchy are unlimited - so, long as classless addressing rules are followed

Geographical routing

Need of geographical routing technique:

1. Size of the routing table decrease to

2. Need to extend hierarchical routing to include geographical routing

Must divide the entire address space into a few large blocks

Assigning a block to North America - a block to Europe - a block to Asia - a block to Africa, and so on

Routers of ISPs: outside Europe - will have only one entry for packets to Europe - in their routing tables

Routers of ISPs: outside North America will have only one entry for packets to North America in their routing tables,

and so on

Routing table

A host or a router: has a routing table with an entry for each destination or a combination of destinations - to route IP

packets

Types of routing table - can be either static or dynamic

1. Static routing table

Contains information entered manually

Administrator enters - the route for each destination into the table

Table when is crated: it cannot update automatically - when there is a change in the Internet - table must be manually

altered by the administrator

Can be used in a small internet - that does not change very often, or in an experimental internet for troubleshooting

Poor strategy it is to use static routing table - in a big internet such as the Internet

2. Dynamic routing table

Updated periodically - by using one of the dynamic routing protocols - like RIP, OSPF, or BGP

Change whenever in the Internet: like shutdown of a router or breaking of a link - dynamic routing protocols

Update all the tables in the routers (and so, in the host) automatically

Routers in a big internet like Internet: need to be updated dynamically for efficient delivery of the IP packets

Format

Routing table for classless addressing has - minimum of 4 columns - today's routers have more than 4 columns

Number of columns is vendor-dependent - and not all columns can be found in all routers

Fig. shows some common fields in today's routers

1. Mask: field defines masks applied for the entry

2. Network address: defines the network address to which the packet is finally delivered

3. Host-specific routing: in this field defines the address of destination host

4. Next-hop address: Defines the address of the next-hop router to which the packet is delivered

5. Interface: shows the name of the interface

6. Flags: defines up to five flags - Flags are on/off switches that signify either presence or absence

Five flags: U (up), G (gateway), H (host-specific), D (added by redirection), and M (modified by redirection)

a. U (up): indicates the router is up and running - if not present: router is down - and packets cannot be forwarded and

is discarded

b. G (gateway): destination is in another network - packet is delivered to the next-hop router for delivery (indirect

delivery) - when no flag, destination is in this network (direct delivery)

c. H (host-specific): indicates that the entry in the network address field is a host-specific address - when no flag,

address is only the network address of the destination

d. D (added by redirection): indicates that routing information of this destination has been added to the host routing

table by a redirection message from ICMP

e. M (modified by redirection): indicates that the routing information for this destination has been modified by a

redirection message from ICMP

7. Reference count: gives the number of users of this router at the moment

Ex.: if 5 people at the same time are connecting to the same host from this router - value of this column is 5

Use: shows the number of packets transmitted through this router for the corresponding destination

8. Utilities: Several utilities that can be used: to find the routing information and the contents of a routing table - netstat

and ifconfig, others

Unicast routing protocols

Multicast routing protocols