Networking Chapter 4

Network layer

Network Layer

Responsible for carrying a packet from one computer to another.
Responsible for host-to-host delivery.

Responsible for routing the packets through the routers or switches.

Internetworking

Refers to the logical gluing of heterogeneous physical networks together to look like a single network to the upper transport and application layers.

Hosts

The sources and destinations in internetworking are usually referred to as _______.

Hop

A host or router in internetworking is referred to as a _______.

Network layer at source

• Receives data from the transport layer

• Adds the universal addresses of A and D

• Makes sure the packet is correct size for passage through the next link, fragment it if necessary.

Network layer at router or switch

• Responsible for routing the packet using a routing table

• The packet may go through another fragmentation if need be

Network layer at destination

• Address verification

• Error detection

• reassembling the fragments

• Delivering to the transport layer

Addressing

Used to uniquely and universally identify each device on the internet to allow global communication between all devices.

Each address belongs to a single host, but a single host can have multiple addresses if it has multiple connections to the internet.

Internet address

The identifier used in the network layer of the internet model.

IP address

32-bit binary address

Common address notations

• Binary notation

• Dotted-decimal notation

Binary notation

One or more spaces inserted between each octet(8 bits)

Dotted-decimal notation

Used to make it more compact and easier to read ( for humans)

Packetizing

Encapsulates packets received from upper layer protocols and makes new packets out of them.

Done by the IP protocol in the internet model

Fragmenting

Each router decapsulates the IP datagram from the received frame, processes it, and then encapsulates in another frame.

Received frame

Format and size depends on the protocol used by the physical network from which the frame has just received

Departing frame

Format and size depend on the protocol used by the physical network to which the frame is going.

Other issues of the internet not directly related to the network layer

• Address resolution

• Multicasting

• Routing Protocols

Network layer protocols in TCP/IP

• IP

• ARP

• RARP(DHCP)

• ICMP

• IGMP

IP

• The main protocol

• The glue that holds the whole internet together

• Responsible for host-to-host delivery

• Also needs the services of the other protocols

ARP

Address Resolution Protocol

Maps an IP address to a MAC address (of the next hop)

RARP

• Reverse ARP

• Maps a MAC address to an IP address

• Usually used in some situations such as when a diskless host is booted

• It gets binary image of its operating system from a remote file server but does not know its IP address

• Obsolete, replaced by DHCP - Dynamic Host Configuration Protocol

ICMP

Internet Control Message Protocol

Used to handle unusual situations such as the occurrence of an error

IGMP

Internet Group Management Protocol

Used for multicasting since IP is designed for unicast delivery

IP v4

Connectionless

Unreliable

Best effort delivery

Only detects errors and discards if it is corrupted.

Transmitted in big endian order

IP datagram

header + text

Options

To allow subsequent versions of the protocol to include information not present int the original design.

• To permit experimenters to try new ideas

• To avoid allocating header bits to rarely needed information

Types of options

• Security

• Strict source routing

• Loose source routing

• Record route

• Timestamp

Security option

For a router to specify not to route through certain countries.

Ignored by all routers in practice.

Strict source routing option

Gives the complete path from source to destination as a sequence of IP addresses

• For system managers to send emergency packets when routing tables are corrupted

• for making timing measurements

Loose source routing option

Requires the packet to traverse List of routers specified in the order specified but also may also pass through other routers on the way.

Record route option

Tells routers along the path to append their IP address to the option field.

• For system managers to track down bugs in routing algorithms

Timestamp option

Each router records a 32-bit timestamp

• Mostly for debugging routing algorithms

Classful addressing

Address space is divided into 5 classes ( A - E )

The first few bits indicate the class of an address

Classes for unicast communication

Addresses in classes A, B, and C

A host needs to have at least one unicast address to be able to send and receive.

Classes for multicast communication

Addresses in class D

If a host belongs to a group or groups, it can have one or more multicast addresses.

Reserved classes

Addresses in class E

Used for special purposes

netid, hostid

Addresses in classes A, B, and C are divided into ________ and _________ of varying length from class to class.

Problem with classful addressing

Each class is divided into fixed number of blocks with each block having fixed size.

Class A

Divided into 128 blocks with each block having a different netid.

0 - 127

First bit of byte 1 is always 0

For large organizations

Up to 16 million hosts 2²⁴

Class B

Divided into 16,384 = 2¹⁴ blocks, each block having a different netid.

128 - 191

First two bits of byte 1 are always 10

16 blocks are reserved for private addresses

Class C

Divided into 2,097,152 = 2²¹ blocks, each block having a different netid.

192 - 223

The fist 3 bits of 1 byte are always 10.

256 blocks are used for private addresses

Class D

One block of this address designed for multicasting

224 - 239

Class E

One block of this address designed for use as reserved addresses.

240 - 255

Network address

An address that defines the network itself.

It cannot be assigned to a host.

Network address properties

• All hostid bytes are 0

• Defines the network to the rest of the internet. Routing is based on this.

• In classful addressing, this is the one assigned to the organization.

ICANN

Internet Corporation for Assigned Names and Numbers

A nonprofit that manages network addresses.

IP address with all 0s

It means this network or this host

IP address with all 1s

It means all the hosts on the indicated network for broadcasting.

IP address levels of heirarchy

• Site

• Subnet

• Host

Subnet

A logical sub-division of an IP network.

Subnetting

The practice of dividing a network into two or more networks.

Computers that belong to a subnet are addressed with an identical most significant bit-group in their IP addresses.

Steps of routing of a datagram

• Delivery to the site

• Delivery to the subnetwork

• Delivery to the host

subnetid, hostid

The IP hostid is divided into _________ and _________.

First host number

Reserved to identify the subnet itself.

Last host number

Reserved to broadcast within the subnet

The mask

32-bit number used to find the network address or subnet address.

Default mask

32-bit binary number that gives the network address when ANDed with an address in the block.

The number of 1s is the same as the number of bits in the netid.

Slash notation

An alternative mask notation

A slash followed by the number oof 1s

Subnet mask

Who are your neighbors in the network

Change some of the leftmost 0s in the default mask to 1s.

The number of subnets is determined by the number of 1s( 2ⁿ )

Classless addressing

Variable-length blocks that belong to no class

• The number of addresses in a block must be a power of 2

• The beginning addresses must be evenly divisible by the number of addresses

• We can still use subnetting if needed

• An organization is given the beginning address of the block and a mask in slash notation

CIDR

Classless Inter Domain Routing

Means for forwarding packets in classless addressing.

Uses a single routing table for all networks consisting of an array of IP address, subnet mask, outgoing line triples.

Steps of CIDR

• When a packet comes in, its destination IP address is first extracted.

• The routing table is then scanned entry by entry, masking the destination address and comparing it to the table entry looking for a match

• If there were multiple entries with different subnet mask lengths matching, we use the longest mask.

IPv5

It was an experimental real-time stream protocol that was never widely used.

Was designed to coexist with IP v4, not a replacement.

IPv6

Header has 8 fields ( 40 bytes )

• Version (4 bits) - 6

• Traffic class (4 bits)

• Flow label ( 24 bits)

• Payload length ( 16 bits )

• Next header ( 8 bits )

• Hop limit ( 8 bits )

• Source address ( 16 byte )

• Destination address (16 bytes)

Traffic class

Used to distinguish between packets with different real-time delivery requirements.

Flow label

Used to provide special handling for a particular flow of data.

Major goals of IPv6

• Larger address space

• Better header format

• Better security

• Support for resource allocation

• Allowance for extension

• Coexistence

IPv6 addresses notation

• Hexadecimal colon notation

  • Eight groups ( each with 2 bytes) of four hexadecimal digits with colons between the groups

IPv6 abbreviation methods

• Leading zeros (not the trailing ones) within a group can be omitted

• One

Static mapping

Create a table that associates an IP address with a MAC address, stored in each machine on a network.

Network performance is degraded to update the table periodically.

Dynamic mapping

Each time a machine knows one of the two addresses, it can use a protocol to find the other one.

• ARP ( IP → MAC)

• RARP ( MAC → IP)

ARP

• Broadcasted by a host or a router looking for a MAC address.

• Includes MAC and IP addresses of the sender and IP address of the receiver.

• Unicast

Each computer connected to the internet must have

• Its IP address

• Subnet mask

• IP address of the router

• IP address of a name server

DHCP

A client server program for assigning network addresses.

Allows both manual and automatic IP address assignment.

DHCP server databases

• One statistically binds physical addresses to IP addresses

• Second one holds a list of unassigned IP addresses that makes the DHCP dynamic

leased

Addresses are __________ for some time; when the time expires the client must request renewal allowing the sharing of address space.

DHCP relay agent

Needed on each LAN incase DHCP server not reachable by broadcasting.

ICMP

Internet Control Message Protocol

A companion to the IP designed to compensate the error reporting and host and management query mechanisms the IP lacks

ICMP messages

• Error reporting

• Query messages

ICMP Error reporting

• Destination unreachable - discards datagram

• Source quench - slowing down a source

• Time exceeded

• Parameter problem - detecting an illegal value in header

• Redirection

Routing

Making a decision and choosing one route whenever there are multiple routes.

Requires a router to have a routing table.

Routing protocol

A combination of rules and procedures that let routers inform one another of changes.

Routing algorithm

Main part of routing protocol and determines the path for a packet.

Methods of reducing routing table size

• Next hop routing

• Network specific routing

Next hop routing

The routing table holds only the information that leads to the next hop

Network specific routing

There is only an entry for the address of the network itself instead of an entry for every host connected to it.

Types of routing algorithms

• Nonadaptive ( static )

• Adaptive ( dynamic )

Nonadaptive routing algorithm

The administrator enters the route for each destination into the table

Adaptive

Routing decisions are made periodically

Types of routing protocols

• Interior routing

• Exterior routing

Autonomous system

A group of networks and routers under the authority of a single administration.

Interior routing

Routing inside an autonomous system.

Each AS can choose its own

Exterior routing

Routing between AS

One protocol is chosen to handle routing between AS

Optimality principle

Chose an optimal route from source to destination

Sink tree

The set of the optimal routes from all sources to a given destination form a tree rooted at the destination.

Shortest path routing

For unicast routing

To select a route between two routers, the algorithm finds the shortest path between them on the graph.

Each node labeled with its distance from the source may be tentatively or permanently.

Flooding

For multicast and broadcast routing.

Every incoming packet is sent out on every outgoing line except the one it arrived on.

Not mostly practical