CS 453 Computer Networks

CS 453CS 453Computer NetworksComputer Networks

Lecture 20Lecture 20

Layer 3Network LayerLayer 3Network Layer

Network Layer of the InternetNetwork Layer of the Internet

Internet – Network Layer

The Internet is a world-wide economic, social, educational and cultural force

It has literally changed the we conduct our daily lives

IP protocol is the foundation of the Internet It’s the glue that makes it all work


Design principles behind the IP protocol It has to work – build and test prototypes before

committing the standards Simplicity – don’t add unnecessary features and keep

the “necessary” ones to a minimum Don’t create options – have a way to do something,

not several ways Modularity – keep design in modules Deal with heterogeneity – complex networks are

going to mixes of technology – design for this


Design principles behind the IP protocol Negotiable parameters – allow devices to

negotiate parameters, don’t have fixed parameters

Don’t shoot for perfect – don’t try to deal with every possible wrinkle, strange requirements

Sender adheres to standard, receiver tries to adapt

Scalability – design must be scalable Cost/performance – must be within

acceptable limits


From: Tanenbaum, 2003, pg 433

Internet

See previous diagram from Tanenbaum Interconnection of network

Across organizationsAcross countriesAcross continentsAcross oceans

IP Protocol is the common thread IP protocol was designed from its inception to

deal with networks of networks IP stands for internet protocol – with a little i

0.0001% of the Internet– according to Wikipedia.org

From:http://en.wikipedia.org/wiki/Internet

From: www.internet2.org

IP ProtocolIPv4 Header

From:http://en.wikipedia.org/wiki/IPv4

IPv4 Header

Header has 20 bytes of fixed fields

+ a variable length option part

Transmitted in Big-endian order

IPv4 HeaderHeader fields Version – 4 bits – identify the version of

datagram – lets routers know how to handle the packet

Header length – 4 bits because header has optional part length can vary, must declare header length – in N of 4 byte words – so max header length = 60 bytes

Type Of Service (TOS) – allows for differentiated services – low delay, high throughput – allows routes to decide what to do

IPv4 HeaderHeader fields Datagram length – length of entire datagram –

16 bits so max datagram length = 65,535, but usually 1500 or less (why?)

Identifier – 16 bits – ids the datagram, so devices will know which datagram fragments belong to

Flags – 3 bits – fragmentation flagsBit 16 = 0Bit 17 = DF (don’t fragment)Bit 18 = MF (more fragments coming)

IPv4 HeaderHeader fields Fragment offset – 13 bits – defines the

fragment’s slot in the datagram (for reassembly)

In 8 byte slots

8192 slots = max datagram = 65,536 Time to Live (TTL) – measures life of

datagram in router hops – each hop TTL-1If TTL reaches 0 datagram trashed and warning sent to source

IPv4 HeaderHeader fields Transport Layer Protocol – what transport

layer process should receive the datagram (TCP, UDP, …)

Header Checksum – calculated check sum but just for the header portion of the packet

Must be recalculated on each router hop Source address – 32 bit address of packet

source Destination address – 32 bit address of

packet destination recipient

IPv4 HeaderHeader fields

Option – variable length– intended to allow things not in the design Some predefined options originally – list has grown To see list go to

http://www.iana.org/assignments/ip_parameters

Option Description

Security Says routers should use secret routes

Strict source routing Define route to take

Loose source routing Defines part of route to take

Record route Record in route hops in options field

Timestamp Each router adds its address and timestamp

IPv4

Then the payload

IP Addressing

All communications has a source and a destination (or more)

IP address scheme defines source and destination

IP address in IP Packet, not in frame

Each device on Internet has an IP address

Each address is unique – in theory (but not really)

IP Addressing

Each address represents a network interface …not a host

IP address has 32 bits…Represented (to us) in Dotted quad notation Dotted decimal notation

157.182.95.120

IP Addressing

But really it a string of bits157.182.95.120

So this is –

10011101.10110110.0101111.01111000

Which internally is –

1001110110110110010111101111000

IP AddressingClass addressing

Originally IP address were defined in terms of classes Since the 32 address defines all hosts/interfaces in

the Internet… … the Internet is a network of networks Fixed portion of the IP address were defined to

represent a network… …i.e. the first x bits The network address was defined on even octet

boundries --- 8 bits, 16 bits, 24 bitThe left x bits is the network addressThe right y bits is the host address x+y=32


From: Tanenbaum, 2003, pg. 437


Special Addresses

From: Tanenbaum, 2003, pg. 438

IP AddressingClass addressing …was a bit of problem If WVU was assigned a Class B address (as it

was sometime ago……then the first 16 bits was the network address…the right 16 bits was for the address of all of the hosts in the network…and every host at WVU was on the the same network, right?…maybe for a few days

IP AddressingClass addressing We need someway of defining smaller

networks (subnets) for departments, buildings, etc.

And be able to aggregate these into the designated network (left x bits)

That is called classless addressing

IP AddressingClassless addressing Generalize the idea of classes, … but make them hierarchical …and arbitrary size ( number of bits) For example, suppose the left 16 bits

represent a network… …the next 8 bits represent network addresses

within that network – or subnets Since 8 bits can be left for the host address

part of the address There can be ? Hosts in such a subnet

IP AddressingClassless addressing At any given level an IP address needs to look

like a network address and a host address 157.182.95.120

At the highest network level –

157.182 is the network address

… and 95.120 is the host address…

But…

IP AddressingClassless addressing subnet mask A string of bits that acts as a bitmap Left x bits are set to 1 to say this many bit is

the network address Right x bit is set to 0 to say this many bits is

the host address…

11111111.11111111.11111111.00000000

Or…

255.255.255.0

IP AddressingClassless addressing This subnetting process can be done

recursively University gets a network

College creates a subnet

Department creates a subsubnet

Lab creates a subsubnet

IP AddressingClassless addressing Subnet mask is used as a bit mask ANDed with address to see if Address in in local network or .. Needs to be routed to another network

IP Network Address Translation NAT

NAT


NAT

From: Tanenbaum, 2003, 446


NAT NAT really irks a lot in the IP community Violates the IP unique address rule IP is connectionless, NAT creates a

connectionMust track state

Violates protocol layer conventionDips into IP header (port address)

Breaks on some protocols (payload imbedded addresses

Might not be able to scale to large subnet

CS 453 Computer Networks

Documents

Transcript of CS 453 Computer Networks