Networking chapter 4

The network layer

network layer

• Responsible for carrying a packet from one computer to another

• Responsible for host-to-host delivery

• Between Transport(gives services to) and Data Link (receives services from) layers

internetworking

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

• Source and destinations are usually referred to as hosts

• A host/router is referred to as a hop

network layer at source

• Receives data from the transport layer

• Adds the universal addresses of sender and receiver

• Makes sure the packet is of correct size for passage through the next link, fragments 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 fragments

• Delivering packet to the transport

addressing

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

multiple addresses

Each address belongs to a single host but a single host can have _________ _______ if it has multiple connections to the internet

internet/IP address

• The identifier used in the network layer of the internet model is called

• Is a 32-bit binary address (in IPv4)

• Unique and universal (one host can have multiple IP addresses)

wastage

With IP addressing being 32-bits there are 2^32 possible hosts, assumed to be too many initially but now not sufficient due to a number of reasons, mainly ______

classful addressing

• Address spacing is divided into 5 classes A-E

• First few bits indicate the class of an address in binary/dotted-decimal notation

classes A-C

• These classes are used for unicast communication

• Hosts need to have at least one unicast address to be able to send and receive

class D

• This class is used for multicast communication (only as a destination)

• If a host belongs to a group/groups, It can have 1+ multicast addresses

class E

• Is a reserved class

• The idea behind this class was to use them for special purposes

netid and hostid

Address in classes A, B, and C are divided into _____(network) and _____(host) of varying length from class to class

problem with classful addressing

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

common IP address notations

• Binary notation

• Dotted-decimal notation

binary IP address notation

One or more spaces inserted between each octet

dotted-decimal IP address notation notation

• More compact

• Easier to read for humans

packetizing

• Encapsulating 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 it in another frame

criteria for format and size for fragmenting

• Received frame: Depends on the protocol used by the physical network from which the frame has just arrived

• Departing frame: Depends on the protocol used by the physical network to which the frame is going

address resolution

Maps an IP address to a MAC address

network layer protocols

• IP

• ARP

• RARP (DHCP)

• ICMP

• IGMP

IP protocol

• Main protocol

• Glue that holds the internet together

• Responsible for host-to-host delivery

• Needs the services of other protocols

reverse address resolution protocol

• Maps a MAC address to an IP address

• Gets used in situations like when a diskless host is booted

• Gets binary image of its OS from a remote file server but doesn’t know its IP address

• Obsolete (replaced by DHCP - Dynamic Host Configuration Protocol)

internet control message protocol

Handles unusual situations such as the occurrence of an error

internet group management protocol

• Built for multicasting

• Used by IP because IP is designed for unicast delivery

IPv4

• Designed with internetworking in mind

• Unreliable and connectionless datagram protocol

• Only detects errors and discards corrupted packets

• Paired with TCP for more reliability

IP datagram

• Consists of a 20-byte header and a text part (optional part with variable length)

big endian order

• IP datagrams are transmitted in this order

• From left to right, with the high-order bit of the version field going first

• SPARC is an example of this order

little endian

• Software conversion is required on machines using this order

• Pentium is an example of this order

version

• 4 bits

• Keeps track of which version of the protocol the datagram belongs (for future transitions between new old versions)

IHL

• Length of the header in 32-bit words

• Minimum value is 5 when no options are present

• Maximum is 15, which limits the header to 60 bytes (optional field to 40 bytes)

type of service

• 8 bits

• Designed to distinguish between different classes of service - for various combination of reliability and speed

• Mostly ignored by current routers

total length

• 16 bits

• Describes size of everything in the data gram (header and data)

identification

• 16 bits

• Used by the destination host to determine which datagram a newly arrived fragment belongs to

DF

• 1 bit

• The destination is incapable of putting the pieces back together

• All machines are required to accept fragments of 576 bytes or less

MF

• 1 bit

• All fragments except the last one have this bit set to know when all fragments of a datagram have arrived

fragment offset

• 13 bits

• Tells where in the current datagram this fragment belongs

• All fragments except the last one must be a multiple of 8 bytes

• A maximum of 8192(2^13) fragments per datagram allowed

• Gives a maximum datagram length of 8×8192 bytes (2^16)

time to live

• A counter used to limit packet lifetimes (max 255 seconds)

• In practice, counts the number of hops

• Set by source host

• Approximately 2 times the max hops between any two hosts

• Decremented on each hop

• Decremented multiple times when queued for a long time in a router

• When 0, is discarded and a warning packet is sent to the source host

protocol

• 8 bits

• Identifies which transport process to give it to (TCP, UDP, ..)

header checksum

• 16 bits

• Verifies header only

• Sum of all fields is assumed to be zero upon arrival

• Recomputed at each hop because at least one field always changes (Time to live)

options

• Allow subsequent versions of the protocol to include information not present in the original design

• Permit experimenters to try out new ideas

• Avoid allocating header bits to information that is rarely needed

types of options

• Security

• Strict source routing

• Loose source routing

• Record route

• Timestamp

security of options

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

• Ignored by all routers in practice

strict source routing of options

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

• Useful for system managers to send emergency packets when routing tables are corrupted/making timing measurements

loose source rooting of options

• Requires the packet to traverse the list of routers specified and order specified

• May pass through other routers on the way

• Most useful for when political/economic considerations dictate passing through/avoiding countries

record route of options

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

• For system managers to track down bugs in routing algorithms

timestamp of optional header of IPv4

• 32-bit

• Used for debugging routing algorithms

class a

• Divided into 128 blocks with unique NetIds

• Block 1 from 0.0.0.0 to 0.255.255.255 - NetId 0

• Block 2 from 1.0.0.0 to 1.255.255.255 - NetId 0

• Last block from 127.0.0.0 to 127.255.255.255 - NetId 126

• These classes addresses were designed for large organizations (up to 16 million hosts (2^24)

network address

Given that an organization is granted a block from class A with a given netid “x”, the first address (x.0.0.0) is called the ________ _________ and is used to identify the organization

126

Total number of organization that can be assigned class A addresses

wastage of granting an organization a block of a class

the number of addresses in each block (16,777,214= 2^24-2) are larger than the needs of almost all organizations

class b

• Divided into 2^14 blocks

• Each block has a different NetId (second portion of the IP address)

• 16 blocks are reserved for private addresses (total number of organization that can be assigned this classes addresses is 16384 - 16

• Designed for midsize organizations

• Many addresses are wasted as there are more than the need of midsize organizations

class c

• Divided into 2^21 blocks with each block having different NetId

• Third portion of IP address

• 256 blocks used for private addresses(total number of organizations that can be assigned in this class is 2,097,152 -256)

• Designed for small organizations

• Limited blocks in this class - most organizations do not want a block in this class

class d

Only one block in this class designed for multicasting

class e

One block in this class designed for use as reserve addresses for research by the Internet Engineering Task Force to develop internet standards

network address

• An address that defines the network itself

• Cannot be assigned to a host

properties of network address

• All hostid bytes are 0s; (different from a netid although this address has netid)

• Defines the network to the rest of the Internet; routing to a host is based on this address

• In classful addressing, the network address is the one that is assigned to the organization

internet corporation for assigned names and numbers

• Manages network addresses to avoid conflicts

• Nonprofit

• Delegates parts of the address space to various regional authorities which in turn allocate IP addresses to ISPs and other companies

special IP addreses

• An IP address with all 0s (hostid and/or netid) - means this is a host/network

• An IP address with all 1s (netid and/or hostid) - means all the hosts on the indicated network for broadcasting

hierarchy of IP addreses

• Site level

• Host level

same netid

All the hosts in a network must have the ____ ___ which does not allow dividing a network into logical groups

subnetwork

• Solution to allow for subdivision of networks into logical groups (internal split, but still act as a single network to the outside world)

• Routing of a datagram now involves three steps: Delivery to the site, delivery to the ________, and delivery to the host

subnetid

With the existence of subnets, IP hostid is divided into _______ and hostid which can be changed later if required by ICANN

mask

32 bit number used for a router to find a network/subnet address

default mask

• Used by routers outside of the organization

• Gives the network address when AND’ed with an address in the block

  • The number of 1s is the same as the number of bits in the netID (8 for A, 16 for B, and 24 for C); the rest are all 0

subnet mask

• Routers inside the organization use this type of mask

• To make this type of mask, change some of the leftmost 0s in the default mask to 1s

2^n

The number of subnets is determined by the number of 1s

classless addressing

• Announced in 1996

• An idea of variable-length blocks that belong to no class was introduced to allocate remaining IP addresses

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

• The whole address space (2^32 addresses) is divided into blocks of different sizes

• The beginning address must be evenly divisible by the number of addresses; if a site needs, say, 2000 addresses it is given a block of 2048 addresses on a 2048 address boundary

• We can still use subnetting if there is a need

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

classless interdomain routing

• New method of forwarding packets for classless addresses

• There is now a single routing table for all networks consisting of an array of (IP address, subnet mask, outgoing line) triples

• 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

• It is possible that multiple entries (with different subnet mask lengths) match, in which case the longest mask is used

IPv5

• Was an experimental real-time stream protocol that was never widely used

• Was designed to coexist with IPv4, not a replacement

major goals of IPv6

• Larger address space

• Inefficient address space

• Better header format

• Better security

• Support for resource allocation

• Allowance for extension

• Coexistence

IPv6 header

• 8 fields (40 bytes)

  • Version (4 bits)

  • Traffic class (4 bits)

  • Flow label (24 bits)

  • Payload length (16 bits)

  • Next header (8 bits)

  • Hop limit (8 bits)

  • Source Address (16 bytes)

  • Destination Address (16 bytes)

IPv6 address notation

• Hexadecimal colon notation (8 groups - 2 bytes each of four hexadecimal digits)

  • Many addresses will have many zeroe

IPv6 address notation abbreviation

• Leading zeros in a group can be omitted

• One or more groups of groups of 16 zero bits can be replaced by a pair of colons (allowed once per address)

• IPv4 can be written as a pair of colons and an old dotted decimal number

categories of addresses in IPv6

• Unicast

• Multicast

• Anycast

anycast category

• Defines a group of computers with addresses that have the same prefix

• Such a packet must be delivered to exactly one of the members of the group

• The closest or the most easily accessible

transition from IPv4 to IPv6

• Cannot happen suddenly, smooth transition is required

• Three strategies designed

  • Dual stack

  • Tunneling

  • Header translation

mapping

• Delivery of a packet of host or router requires two levels of addressing (IP and MAC)

• Two types exist: static and dynamic

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

  • A machine could change its network card

  • In some LANs such as LocalTalk of apple, the MAC address changes every time the computer is turned on

  • A mobile computer can move from one physical network to another

dynamic mapping

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

• Two protocols

  • ARP (maps an IP address to a MAC address)

  • RARP (maps a MAC address to an IP address; obsolete, replaced by DHCP- Dynamic Host Configuration Protocol)

address resolution protocol

• A host or a router looking for a MAC address broadcasts an ARP query packet

  • Includes the MAC (physical) and IP addresses of the sender and the IP address of the receiver

• Only the intended recipient sends back an ARP response packet (it contains the recipient’s IP and physical addresses)

• Is unicast

dynamic host configuration protocol

• A client-server program for assigning network addresses IP addresses, default routers)

• Is an extension of BOOTP (that replaced RARP) that maps IP addresses to Ethernet addresses - it requires manual configuration of the table by an administrator when a new host is added - static

• In contrast with BOOTP, it allows both manual and automatic IP address assignment

DHCP databases

• One that statically binds physical addresses to IP addresses

• The second holds a list of unassigned IP addresses that makes DHCP dynamic

How computers contact the DHCP server

• The server checks its static database; if there is an entry that permanent address is returned

• Otherwise it dynamically assigns an IP address for a fixed period of time

  • This allows sharing of address space - results in more efficient use of address space

DHCP relay agent

Needed on each LAN since the DHCP server may not be reachable by broadcasting

necessity of ICMP

• IP has no error reporting and error correcting mechanisms; if something goes wrong

• It lacks a mechanism for host and management queries (is a router or a host alive?

internet control message protocol

Is a companion to the IP designed to compensate for lack of error reporting and host/management query mechanisms

types of ICMP messages

• Error-reporting

• Query messages

error reporting messages

Always reports error messages to the original source (the only information included in the datagram is the source and destination addresses)

types of error reporting

• Destination unreachable

• Source quench

• Time exceeded

• Parameter problem

• Redirection

destination unreachable

Type of error reporting message received when a router cannot locate the destination or when a packet with the DF bit cannot be delivered because a ‘‘small-packet’’ network stands in the way; the datagram is discarded

source quench

• Type of error-reporting message

• To slow down a source since IP is connectionless and lacks flow control and congestion control

• Rarely used because when congestion occurs, these packets tend to aggravate it

  • Congestion control in the Internet is handled in the transport layer

time exceeded

Type of error-reporting message sent when a packet is dropped because its counter (Time to live) has reached zero (a symptom that packets are looping) or when all fragments that make up a message do not arrive at the destination host within a certain time limit

parameter problem

Type of error-reporting message that indicates that an illegal value has been detected in a header field

redirection

type of error-reporting message sent when a router notices that a packet seems to be routed wrong

routing protocol

A combination of rules and procedures that lets routers in the internet inform one another of changes (to share whatever they know about the internet or their neighborhood)

routing algorithm

• The heart of a routing protocol

• Determines the path for a packet

• Constructs routing tables (mandatory for routing)