More on TCP/IP Module A Panko’s Business Data Networks and Telecommunications, 6th edition...

More on Internet Layer Processes

Mask Operations

Masks were introduced in Chapter 3

IP addresses alone do not tell you the size of their network or subnet parts

Network Mask Has 1s in the network part Has 0s in the remaining bits

Subnet Mask Has 1s in the network plus subnet parts Has 0s in the remaining bits

Mask Operations

Based on Logical AND Both must be true for the result to be true

Example 1010101010 Data

1111100000 Mask

1010100000 Result

Mask Operations

Based on Logical AND If mask bit is 1, get back original data

If mask bit is 0, bet back zero

Example 1010101010 Data

1111100000 Mask

1010100000 Result

Mask Operations

IP packet arrives at a router Router sees destination IP address 11111111 01000000 10101010 00000000

Compares to each router forwarding table row Address Part in First Entry 11111111 01000000 00000000 00000000

Mask in First Entry 11111111 11100000 00000000 00000000

Mask Operations

Mask the IP destination Address 11111111 01000000 10101010 00000000 (IP address)

11111111 11100000 00000000 00000000 (mask)

11111111 01000000 00000000 00000000 (result)

Compare Result with First Entry Address part 11111111 01000000 00000000 00000000 (address part)

11111111 01000000 00000000 00000000 (result)

The Entry is a Match!

Mask Operations

Recap Read destination IP address of incoming IP packet

For each entry in the router forwarding tableRead the mask (prefix)Mask the incoming IP addressCompare the result with the entry’s IP address

partDo they match or not?

Mask Operations

Simple for Computers Computers have circuitry to AND to numbers

Computers have circuitry to COMPARE two numbers to see if they are equal or not

Very computer-friendly, so used on routers

Difficult for people, unfortunately

Current version of the Internet Protocol is Version 4 (v4) Earlier versions were not implemented

The next version will be Version 6 (v6) No v5 was implemented Informally called IPng (Next Generation)

IPv6 is Already Defined Continuing improvements in v4 may delay its

adoption

IPv6 will raise the size of the internet address from 32 bits to 128 bits Now running out of IP addresses

Will solve the problem

But current work-arounds are delaying the need for IPv6 addresses

Improved Security But, through IPsec, v4 is being upgraded in security

as well

Improved Quality of Service (QoS) But under IETF Differentiated Services (diffserv)

initiative, IPv4 is being upgraded in this area as well

Extension Headers IPv4 Headers are complex IPv6 basic header is simple Extension headers for options

Basic Header

Extension Header 1

Extension Header 2

Extension Headers Basic header has 8-bit Next Header field

Identifies first extension header or says that payload follows

Basic Header

Extension Header 1

Extension Header 2

Extension Headers Each extension header also has 8-bit Next Header

Identifies next extension header or says that payload follows

Basic Header

Extension Header 1

Extension Header 2

Extension Headers Next header field is an elegant way to allow options

Easy to add new extension headers for new needs

Basic Header

Extension Header 1

Extension Header 2

IP Fragmentation

Maximum Transmission Unit (MTU) Largest IP packet a network will accept

Arriving IP packet may be larger

IP Packet

IP Fragmentation

If IP packet is longer than the MTU, the router breaks packet into smaller packets Called IP fragments

Fragments are still IP packets

Earlier in Mod A, fragmentation in TCP

IP Packet 2 1

IP PacketsFragmentation

IP Fragmentation

What is Fragmented? Only the original data field

New headers are created

IP Packet 2 1

IP Fragmentation

What Does the Fragmentation? The router

Not the subnet

IP Packet 2 1

Multiple Fragmentations

Original packet may be fragmented multiple times along its route

DestinationHost

InternetProcess

SourceHost

InternetProcess

Fragmentation

Defragmentation

Internet layer process on destination host defragments, restoring the original packet

IP Defragmentation only occurs once

DestinationHost

InternetProcess

Defragmentation

SourceHost

InternetProcess

Fragmentation and IP Fields

More Fragments field (1 bit) 1 if more fragments 0 if not Source host internet process sets to 0 If router fragments, sets More Fragments field in

last fragment to 0 In all other fragments, sets to 1

0 0 1 1

Original IP Packet Fragments

Identification Field

IP packet has a 16-bit Identification field

Time to Live (8)

Options (if any)

Version(4)

Hdr Len(4) TOS (8)

Source IP Address

Destination IP Address

Data Field

IP packet has a 16-bit Identification field Source host internet process places a number in the

Identification field

Different for each IP packet

Time to Live (8)

Version(4)

Hdr Len(4) TOS (8)

IP packet has a 16-bit Identification field If router fragments, places the original Identification

field value in the Identification field of each fragment

47 47 47 47

Purpose Allows receiving host’s internet layer process know

what fragments belong to each original packet

Works even if an IP packet is fragmented several times

47 47 47 47

Fragment Offset Field

Fragment offset field (13 bits) is used to reorder fragments with the same Identification field

Contains the data field’s starting point (in octets) from the start of the data field in the original IP packet

Total Length in bytes (16)Version

(4)Hdr Len

(4) TOS (8)

Receiving host’s internet layer process assembles fragments in order of increasing fragment offset field value

This works even if fragments arrive out of order!

Works even if fragmentation occurs multiple times

0212730

Fragmentation: Recap

IP Fragmentation

Data field of a large IP packet is fragmented

The fragments are sent into a series of smaller IP packets fitting a network’s MTU

Fragmentation is done by routers

Fragmentation may be done multiple times along the route

Defragmentation: Recap

IP Defragmentation

Defragmentation (reassembly) is done once, by destination host’s internet layer process

Defragmentation: Recap

All IP packets resulting from the fragmentation of the same original IP packet have the same Identification field value

Destination host internet process orders all IP packets from the same original on the basis of their Fragment Offset field values

More Fragments field tells whether there are no more fragments coming

Dynamic Routing Protocols

Why Dynamic Routing Protocols? Each router acts independently, based on

information in its router forwarding table

Dynamic routing protocols allow routers to share information in their router forwarding tables

RouterForwardingTable Data

Routing Information Protocol (RIP)

Routing Information protocol (RIP) is the simplest dynamic routing protocol Each router broadcasts its entire routing table

frequently

Broadcasting makes RIP unsuitable for large networks

RoutingTable

RIP is the simplest dynamic routing protocol Broadcasts go to hosts as well as to routers

RIP interrupts hosts frequently, slowing them down; Unsuitable for large networks

RoutingTable

RIP is Limited RIP routing table has a field to indicate the number

of router hops to a distant host

The RIP maximum is 15 hops

Farther networks are ignored

Unsuitable for very large networks

Hop Hop

Routing Information Protocol

Is a Distance Vector Protocol “New York” starts, announces itself with a RIP

broadcast

“Chicago” learns that New York is one hop away

Passes this on in its broadcasts

New York Chicago Dallas

NY is 1

Learning Routing Information “Dallas” receives broadcast from Chicago

Already knows “Chicago” is one hop from Dallas

So New York must be two hops from Dallas

Places this information in its routing table

New York Chicago Dallas

1 hop 1 hop

NY is 1

NY is 2

Slow Convergence Convergence is getting correct routing tables after a

failure in a router or link

RIP converges very slowly

May take minutes

During that time, many packets may be lost

Encapsulation Carried in data field of UDP datagram

Port number is 520

UDP is unreliable, so RIP messages do not always get through

A single lost RIP message does little or no harm

UDPHeader

UDP Data FieldRIP Message

OSPF Routing Protocol

Link State Protocol Link is connection between two routers

OSPF routing table stores more information about each link than just its hop count: cost, reliability, etc.

Allows OSPF routers to optimize routing based on these variables

OSPF Routers

Network is Divided into Areas Each area has a designated router

AreaDesignated

Router

OSPF Routers

When a router senses a link state change Sends this information to the designated router

AreaDesignated

Router

Notice ofLink State Change

OSPF Routers

Designed Router Notifies all Routers Within its area

AreaDesignated

Router

OSPF Routers

Efficient Only routers are informed (not hosts)

Usually only updates are transmitted, not whole tables

AreaDesignated

Router

Fast Convergence When a failure occurs, a router transmits the notice

to the designated router

Designated router send the information back out to other routers immediately

Encapsulation Carried in data field of IP packet

Protocol value is 89

IP is unreliable, so OSPF messages do not always get through

A single lost OSPF message does little or no harm

IPHeader

IP Data FieldOSPF Message

Selecting RIP or OSPF

Within a network you control, it is your choice Your network is an autonomous system

Select RIP or OSPF based on your needs

Interior routing protocol

RIP is fine for small networks Easy to implementing

15 hops is not a problem

Broadcasting, interrupting hosts are not too important

OSPF is Scalable

Works with networks of any size

Management complexities are worth the cost in large networks

Border Gateway Protocol (BGP)

To connect different autonomous systems Must standardized cross-system routing information

exchanges

BGP is most popular today

Gateway is the old name for router

Exterior routing protocol

AutonomousSystem

AutonomousSystemBGP

Distance vector approach Number of hops to a distant system is stored in the

router forwarding table

Normally only sends updates

AutonomousSystem

AutonomousSystemBGP

Encapsulation BGP uses TCP for delivery

Reliable

TCP is only for one-to-one connections

If have several external routers, must establish a TCP and BGP connection to each

AutonomousSystem

AutonomousSystemBGP

Address Resolution Protocol (ARP)

Internet and Data Link Layer Addresses

Each host and router on a subnet needs a data link layer address to specify its address on the subnet This address appears in the data link layer frame

sent on a subnet

For instance, 48-bit 802.3 MAC layer frame addresses for LANs

Subnet DADL Frame for Subnet

Addresses

Each host and router also needs an IP address at the internet layer to designate its position in the overall Internet

Subnet

Subnet128.171.17.13

Internet and Data Link Addresses Serve Different Purposes

IP address To guide delivery to destination host across the

Internet (across multiple networks)

Subnet Address To guide delivery between two hosts, two routers,

and a host and router within a single subnet

Same LAN, Frame Relay network, etc.

Analogy

In company, each person has a company-wide ID number (like IP address)

In company, person also has a local office number in a building

Paychecks are made out to ID numbers

For delivery, also need to know office number

Address Resolution

Problem

Router knows that destination host is on its subnet based on the IP address of an arriving packet

Does not know the destination host’s subnet address, so cannot deliver the packet across the subnet

Subnet128.171.17.13

subnet address?

Destination Host

Router creates an ARP Request message to be sent to all hosts on the subnet. Address resolution protocol message asks “Who has

IP address 128.171.17.13?”

Passes ARP request to data link layer process for delivery

Subnet

ARP Request

Data link process of router broadcasts the ARP Request message to all hosts on the subnet. On a LAN, MAC address of 48 ones tells all stations

to pay attention to the frame

Subnet

ARP Request

Host with IP address 128.171.17.13 responds Internet process creates an ARP response message

Contains the destination host’s subnet address (48-bit MAC address on a LAN)

Subnet

ARP Response

Router delivers the IP packet to the destination host Places the IP packet in the subnet frame

Puts the destination host’s subnet address in the destination address field of the frame

Subnet

Deliver IP Packetwithin a subnet frame

Address Resolution Protocol

ARP Requests and Responses are sent between the internet layer processes on the router and the destination host

InternetProcess

Router

InternetProcess

Destination HostARP

Request

ARPResponse

Address Resolution Protocol

However, the data link processes deliver these ARP packets Router broadcasts the ARP Request

Destination host sends ARP response to the subnet source address found in the broadcast frame

InternetProcess

Router

InternetProcess

Destination Host

Broadcast ARP Request

Direct ARP Response

Data LinkProcess

IP Address Classes

How large is the network part in an IP address?

Today we use network masks to tell

Originally, IP had address classes with fixed numbers of bits in the network part Class A: 8 bits (24 bits in local part)

Class B: 16 bits (16 bits in local part)

Class C: 24 bits (8 bits in local part)

Class A IP Address

IP address begins with 0

7 remaining bits in network part Only 128 possible Class A networks

24 bits in local part Over 16 million hosts per Class A network!

All Class A network parts are assigned or reserved

Class B IP Address

IP address begins with 10 (1st zero in 2nd position)

14 remaining bits in network part Over 16,000 possible Class B networks

16 bits in local part Over 65,000 possible hosts

A good trade-off between number of networks and hosts per network

Most have been assigned

Class C IP Address

IP address begins with 110 (1st zero in 3d position)

21 more bits in network part Over 2 million possible Class C networks!

8 bits in local part Only 256 possible hosts per Class C network!

Unpopular, because large firms must have several

Class D IP Address

IP address begins with 1110

Used for multicasting, not defining networks Sending message to group of hosts

Not just to one (unicasting)

Not ALL hosts (broadcasting)

Say to send a videoconference stream to a group of receivers

Class D IP Address

All hosts in a multicast group listen for this multicast address as well as for their specific own host IP address

Packets toMulticast Address

Not in GroupReject

In GroupAccept

Multicasting

Traditionally, unicasting and broadcasting Unicasting: send to one host Broadcasting: send to ALL hosts

Multicasting Send to SOME hosts 500 stations viewing a video course 50 computers getting software upgrades Standards exist and are improving Not widely implemented yet

Why Multicasting

Do not need to send an IP packet to each host Routers split when needed

Reduces traffic

SinglePacket

MultiplePackets

Mobile IP

IP addresses are associated with fixed physical locations

Mobile IP is needed for notebooks, other portable equipment

Computer still gets a permanent IP address

When travels, also gets a temporary IP address at its location

This is linked dynamically to its permanent IP address

MultiProtocol Label Switching (MPLS)

When a packet arrives, routers must consider all possible routes, then select the best one

This is extremely expensive

MPLS adds a tag to each packet

An MPLS router examines the tag and passes the packet back out

This is fast and inexpensive

This is similar to virtual circuits in ATM

In addition, packets with similar priority or other characteristics can be given the same label and so be handled in the same way.

TaggedPacket

MPLS Table for Q

Label110…

Port1…

NHRR…

RoutingDecision

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