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TDC 365TDC 365Network Interconnections Network Interconnections

TechnologiesTechnologies

Lecture #4 NotesLecture #4 NotesSpring, 2009Spring, 2009

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Lecture 4 Topics Lecture 4 Topics

IP Protocol and Operations (Chapter 8)IP Protocol and Operations (Chapter 8) IP Header and packet formatIP Header and packet format Packet FragmentationPacket Fragmentation optionsoptions checksumchecksum

Internet Control Message Protocol (ICMP) Internet Control Message Protocol (ICMP) (Chapter 9)(Chapter 9)

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Internet ProtocolInternet Protocol

Network layer protocol (layer 3)Network layer protocol (layer 3) The Internet is named after this protocolThe Internet is named after this protocol ““Unreliable” best effort serviceUnreliable” best effort service ConnectionlessConnectionless Packets in IP layer are called “Datagrams”Packets in IP layer are called “Datagrams”

a variable-length packet consisting of two parts: header a variable-length packet consisting of two parts: header and data. and data.

The header is 20 to 60 bytes in length and contains The header is 20 to 60 bytes in length and contains information essential to routing and delivery.information essential to routing and delivery.

Header has a fixed part (20 bytes) and variable part of options Header has a fixed part (20 bytes) and variable part of options (maximum 40 bytes)(maximum 40 bytes)

Each datagram is handled independently and can follow Each datagram is handled independently and can follow different paths to the destinationdifferent paths to the destination

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Position of IP Position of IP in TCP/IP protocol suitein TCP/IP protocol suite

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IP Field FormatIP Field Format

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IP Header Fields IP Header Fields

Version: IP protocol version. Version: IP protocol version. We are currently on IP version 4. Soon the We are currently on IP version 4. Soon the

Internet may be moved to IP version 6.Internet may be moved to IP version 6. 4 bit field – value 01004 bit field – value 0100

IP Header Length(HLEN) IP Header Length(HLEN) 4 bit field4 bit field Length of IP header in 32-bit words. Length of IP header in 32-bit words.

Min. value = 5 (20 bytes) Min. value = 5 (20 bytes) Max. value = 15 (60 bytes)Max. value = 15 (60 bytes)

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Total Length FieldTotal Length Field

Total Length field: Length of IP packet in bytes (including header) maximum 65535 (216 -1) bytes

Why do we need the Total Length Field? For Ethernet to work properly, this portion must be at least 46 bytes long. We need the Total Length field in IP to know the actual length of data, since

IP datagram can be less than 46 bytes, and would require padding.

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Protocol FieldProtocol Field

Protocol fieldProtocol field IP carries PDUs of multiple protocols (e.g. TCP, UDP, IP carries PDUs of multiple protocols (e.g. TCP, UDP,

ICMP) ICMP) The Protocol field contains a code that tells them apartThe Protocol field contains a code that tells them apart

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Not all finalized, Read yourself (pg 183)

Service Type Service Type or Differentiated Servicesor Differentiated Services

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Type of ServiceType of Service

Indicates packet Indicates packet precedence and path precedence and path optimization type (currently optimization type (currently ignored by ALL routers):ignored by ALL routers): 3 bit precedence - 0 normal, 3 bit precedence - 0 normal,

7 control packet (has never 7 control packet (has never been used in v4)been used in v4)

1000 D bit – Minimize Delay 1000 D bit – Minimize Delay 0100 T bit – Maximize 0100 T bit – Maximize

Throughput Throughput 0010 R bit – Maximize 0010 R bit – Maximize

ReliabilityReliability0001 C bit – Minimize Cost0001 C bit – Minimize Cost

Default Type of Service

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Packet Size Calculationexample 1

Question:Question:In an IP packet, the value of HLEN is 1000 in binary. How In an IP packet, the value of HLEN is 1000 in binary. How

many bytes of options are being carried by this packet?many bytes of options are being carried by this packet?

SolutionSolutionThe HLEN value is 8, which means the total number of The HLEN value is 8, which means the total number of

bytes in the header is 8 × 4 or 32 bytes. The first 20 bytes in the header is 8 × 4 or 32 bytes. The first 20 bytes are the base header, the next 12 bytes are the bytes are the base header, the next 12 bytes are the options.options.

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Example 3Packet Size Calculation

example 2 Question:Question:

In an IP packet, the value of HLEN is 516 In an IP packet, the value of HLEN is 516 and the value of the total length field is 002816 . Howand the value of the total length field is 002816 . Howmany bytes of data are being carried by this packet?many bytes of data are being carried by this packet?

SolutionSolution The HLEN value is 5, which means the total number of The HLEN value is 5, which means the total number of

bytes in the header is 5 × 4 or 20 bytes (no options). The bytes in the header is 5 × 4 or 20 bytes (no options). The total length is 40 bytes, which means the packet is carrying total length is 40 bytes, which means the packet is carrying 20 bytes of data (40 − 20).20 bytes of data (40 − 20).

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IP Header Fields (con’t)IP Header Fields (con’t)

Identification / Fragment Offset: Identification / Fragment Offset: used to identify fragments that are formed when an IP packet used to identify fragments that are formed when an IP packet

needs to be broken up into smaller packetsneeds to be broken up into smaller packets Time to Live: Time to Live:

Max. number of routers this IP packet may pass through. This field Max. number of routers this IP packet may pass through. This field is decremented by each router the packet passes through. If it is decremented by each router the packet passes through. If it reaches zero, packet will be discarded.reaches zero, packet will be discarded.

Header Checksum: Header Checksum: Allows error checking of IP packetsAllows error checking of IP packets

Source Address: Source Address: 4-byte IP source address for this packet4-byte IP source address for this packet

Destination Address: Destination Address: 4-byte IP destination address for this packet4-byte IP destination address for this packet

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An IP packet has arrived with the first few hexadecimal digits as shown below:

45000028000100000102 . . .

How many hops can this packet travel before being dropped? The data belong to what upper layer protocol?

SolutionTo find the time-to-live field, we skip 8 bytes (16 hexadecimal digits). The time-to-live field is the ninth byte, which is 01. This means the packet can travel only one hop. The protocol field is the next byte (02), which means that the upper layer protocol is IGMP (see slide 7).

Example 4

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IP Packet Fragmentation IP Packet Fragmentation

The format and size of a frame The format and size of a frame depends on the protocol used by the depends on the protocol used by the physical network. Each layer 2 physical network. Each layer 2 protocol has a maximum protocol has a maximum transmission unit (MTU) size (max. transmission unit (MTU) size (max. frame data size)frame data size)

If a router determines that an IP If a router determines that an IP packet length exceeds the MTU for packet length exceeds the MTU for an outgoing port, it will fragment the an outgoing port, it will fragment the packet (that is, break it into smaller packet (that is, break it into smaller packets)packets)

Fragments are reassembled by IP Fragments are reassembled by IP software at destination host.software at destination host.

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IP Header Fields Used for IP Header Fields Used for Fragmentation Fragmentation

Identification: Identification: This is a random value chosen by the sender. If a packet is fragmented, the This is a random value chosen by the sender. If a packet is fragmented, the

same ID value is copied into all fragments.same ID value is copied into all fragments. Flags:Flags:

Don’t Fragment (DF) bit - Set to 1 if this packet should not be fragmented, 0 Don’t Fragment (DF) bit - Set to 1 if this packet should not be fragmented, 0 otherwiseotherwise

More Fragments (MF) bit – Set to 1 if packet has been fragmented and this is More Fragments (MF) bit – Set to 1 if packet has been fragmented and this is not the last fragment, 0 otherwise.not the last fragment, 0 otherwise.

Fragment Offset: Fragment Offset: 13 bit field shows relative position of this fragment with respect to the whole 13 bit field shows relative position of this fragment with respect to the whole

datagramdatagram Highest value can be 213-1 = 8191Highest value can be 213-1 = 8191 If this is a fragment packet, this is set to (byte offset of this packet data in the If this is a fragment packet, this is set to (byte offset of this packet data in the

original packet data) / 8original packet data) / 8 Note that this means that all fragments (except possibly last) must contain an Note that this means that all fragments (except possibly last) must contain an

exact multiple of 8 bytes of data.exact multiple of 8 bytes of data. This means that we can fragment a datagram of maximum length of 65535 This means that we can fragment a datagram of maximum length of 65535

(8192 * 8)(8192 * 8)

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Flags field

L2

(Mainly for diagnostic purpose)

= 0 this is the last fragment

= 1 more fragments to follow

MTU (Maximum Transfer Unit)MTU (Maximum Transfer Unit)

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IP Fragmentation ExampleIP Fragmentation Example

Question:Question: Assume a router receives an IP packet containing 4000 Assume a router receives an IP packet containing 4000

bytes of data (not including IP header). The Routing table bytes of data (not including IP header). The Routing table indicates this should be forwarded onto a subnet with MTU indicates this should be forwarded onto a subnet with MTU of 1420 bytes. How should this this packet be fragmented of 1420 bytes. How should this this packet be fragmented to enable L2 subnet transmission?to enable L2 subnet transmission?

Solution:Solution: The 4000 bytes is broken into fragments as follows (IP The 4000 bytes is broken into fragments as follows (IP

header size not included):header size not included): Fragment #1: 1400 bytes of dataFragment #1: 1400 bytes of data Fragment #2: 1400 bytes of dataFragment #2: 1400 bytes of data Fragment #3: 1200 bytes of dataFragment #3: 1200 bytes of data

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Fragmentation Example

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IP Fragmentation ExampleIP Fragmentation Example

Further assume that Fragment #2 (containing Further assume that Fragment #2 (containing 1400 data bytes) is later sent over an L2 1400 data bytes) is later sent over an L2 subnet with MTU of 820 bytes.subnet with MTU of 820 bytes. Fragment #2.1 contains 800 data bytesFragment #2.1 contains 800 data bytes Fragment #2.2 contains 600 data bytesFragment #2.2 contains 600 data bytes

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Total lengthIdentification

Fragment OffsetFlag

More Fragment

Detailed fragmentation

example

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Categories of OptionsCategories of Options

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IP Options FormatIP Options Format

Used for testing and debugging

Record Route Add router address to the

header Strict Source Routing

Gives complete path Loose Source Routing

Traverse the list of routers in order

Timestamp Add time stamp as well as

router address

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End of option

No-Op and End-OpNo-Op and End-Op

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Determines how many addresses to be recorded

Byte number of the first empty entry

1st 3 bytes of overhead

1st entry starts at 4

Total 15 bytes

Record Route

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Source RoutingSource Routing

Normally IP routing is dynamic where each router Normally IP routing is dynamic where each router makes its own decision about the next hop routermakes its own decision about the next hop router

Applications have no control, nor are concernedApplications have no control, nor are concerned In source routing, on the other hand, the sender In source routing, on the other hand, the sender

specifies the routespecifies the route Strict source routingStrict source routing

Sender specifies exact path – ICMP error if unspecified router is Sender specifies exact path – ICMP error if unspecified router is visitedvisited

Loose source routingLoose source routing Sender specifies a list of IP addresses that the datagram must Sender specifies a list of IP addresses that the datagram must

traverse, but the datagram can also pass thru other routerstraverse, but the datagram can also pass thru other routers

The destination address changes at each hop!!The destination address changes at each hop!!

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Strict Source RoutingStrict Source Routing

When an application receives When an application receives data that was source routed, it data that was source routed, it should supply a reversed route should supply a reversed route by fetching the value of the by fetching the value of the received routereceived route

STEP 1 Router compares STEP 1 Router compares destination IP withits incoming destination IP withits incoming IPIP

STEP 2: Router swaps IP address withoutgoing IP and forwards packet

STEP 3: Router also changes dest addrin IP header

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Figure 8-21

•To record the time at each router, expressed in ms from midnight, Universal time•Estimate, since router clocks may not be synchronized

Timestamp

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IP Header ChecksumIP Header Checksum

Sender IP Checksum calculation:Sender IP Checksum calculation: Split IP header into 16-bit fields Split IP header into 16-bit fields Add fields together (set Checksum to 0)Add fields together (set Checksum to 0) Truncate result to 16 bitsTruncate result to 16 bits 1-s Complement result1-s Complement result Put into Checksum field, send packetPut into Checksum field, send packet

Receiver Checksum check:Receiver Checksum check: Perform same calculation (with nonzero Perform same calculation (with nonzero

checksum)checksum) If result is not zero, discard IP packetIf result is not zero, discard IP packet

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Checksum ConceptChecksum Concept

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1’s complement

Checksum ExampleChecksum Example

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Read Yourself..Read Yourself..

Section 8.5 – IP PackageSection 8.5 – IP Package Especially the algorithmsEspecially the algorithms

Processing ModuleProcessing Module Fragmentation ModuleFragmentation Module Reassembly ModuleReassembly Module

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ICMP ICMP (Internet Control Message Protocol)(Internet Control Message Protocol) Companion to IPCompanion to IP

Network layer protocol, but encapsulated in IP Network layer protocol, but encapsulated in IP datagramsdatagrams

What happens when errors occur in delivering What happens when errors occur in delivering IP packets?IP packets? ICMP messages are used for Error ReportingICMP messages are used for Error Reporting

What if a host needs more information about What if a host needs more information about the network?the network? ICMP messages are used for Status QueriesICMP messages are used for Status Queries

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ICMP DesignICMP Design

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General format of ICMP messages

ICMP Encapsulation

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Error Report EncapsulationError Report Encapsulation

First 8 bytes of data are added contains the port

information from transport layer (TCP or UDP)

So that source can inform the transport protocols about the error

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Error-reporting messages

Query messages

ICMP Messages

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ICMP messagesICMP messagesICMP messages

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ICMP Error ReportsICMP Error Reports

Read more yourself –pgs 214-216Read more yourself –pgs 214-216 Sent by router (typically) or host back to the Source Address on data Sent by router (typically) or host back to the Source Address on data

packet when error occurs – includes header of error-causing packet.packet when error occurs – includes header of error-causing packet. Destination UnreachableDestination Unreachable

IP packet destination does not exist or cannot be reachedIP packet destination does not exist or cannot be reached One case is Fragmentation required, but DF bit is setOne case is Fragmentation required, but DF bit is set Another is source routing cannot be accomplishedAnother is source routing cannot be accomplished

Source QuenchSource Quench informs the source that a datagram has been discarded due to congestion in a router informs the source that a datagram has been discarded due to congestion in a router

or the destination hostor the destination host Used for limited flow controlUsed for limited flow control

Time ExceededTime Exceeded Whenever a router decrements a datagram with a time-to-live (no. of hops) value to Whenever a router decrements a datagram with a time-to-live (no. of hops) value to

zero, it discards the datagram and sends a time-exceeded message zero, it discards the datagram and sends a time-exceeded message All fragments did not arrive before fragment reassembly timeoutAll fragments did not arrive before fragment reassembly timeout

Parameter ProblemParameter Problem Error in header parameters in datagramError in header parameters in datagram

Redirection Redirection A better route to destination is knownA better route to destination is known

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‘Wrong’ router‘Correct’ router

RedirectionRedirection

Hosts do not take part in routing update process (ch. 14) used by routers

The static routing table of the host might not be efficient ‘Wrong’ router forwards datagram to correct router and

sends a redirection message to host in the local network

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ICMP Query MessagesICMP Query Messages

Sent by host (typically) to diagnose networkSent by host (typically) to diagnose network Echo Request / ReplyEcho Request / Reply

Used for “ping”Used for “ping” Time Stamp Request / ReplyTime Stamp Request / Reply

““ping” with timestamps (ping –s)ping” with timestamps (ping –s) Address Mask Request / ReplyAddress Mask Request / Reply

Asks for IP Subnet Mask for current subnetAsks for IP Subnet Mask for current subnet Router Solicitation / AdvertisementRouter Solicitation / Advertisement

To find local routerTo find local router

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Ping and TraceroutePing and Traceroute

PingPing To find if a host is alive and respondingTo find if a host is alive and responding -R for record route-R for record route

TracerouteTraceroute Trace route from source to destinationTrace route from source to destination -g for loose source routing, -G for strict source -g for loose source routing, -G for strict source

routingrouting

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A sends to B a UDP msg with TTL=1

R1 decrements TTL to 0, discards the packet, sends ‘time exceeded’ ICMP msg to A with its own IP

A then sends to B a UDP msg with TTL=2

R2 decrements TTL to 0, discards the packet, sends ‘time exceeded’ msg to A with its own IP

A sends to B a UDP msg with TTL = n andunsupported port

B decrements TTL to 0, sends ‘destination unreachable’ msg to A with its own IP

Traceroute Operation