Jump to first page 1 CICS 515 (Part 2) University of British Columbia CICS 515 (Part 2) Computer Networks Lecture 5 – IP (Ch 4) Instructor: Dr. Son T

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University of British Columbia

CICS 515 (Part 2)CICS 515 (Part 2) Computer NetworksLecture 5 – IP (Ch 4)

Instructor: Dr. Son T. VuongEmail: [email protected]

The World Connected

CICS515 Summer 2012 Instructor: Dr. Son Vuong 2

Ch 4: Network Layer and Routing The IP Protocol

IP Format, Addressing, fragmentation, Internet Control Protocols (ICMP) (next lecture)

Routing RIP (Routing Information Protocol) OSPF (Open Shortest Path First) The Interior Gateway Routing Protocol BGP – The Exterior Gateway Routing Protocol

IPv6 Internet Multicasting Mobile IP


ISO Architecture

Application

Presentation

Session

Transport

End host

One or more nodes

within the network

Network

Data link

Physical

Network

Data link

Physical

Network

Data link

Physical

Application

Presentation

Session

Transport

End host

Network

Data link

Physical

IP


Internet Architecture

Defined by Internet Engineering Task Force (IETF) Hourglass Design Application vs Application Protocol (FTP, HTTP)

…

FTP HTTP NV TFTP

TCP UDP

IP

NET1 NET2 NETn

TCP UDP

IP

Network

Application


Design Principles for Internet Make sure it works. Keep it simple. Make clear choices. Exploit modularity. Expect heterogeneity. Avoid static options and parameters. Look for a good design; it need not be perfect. Be strict when sending and tolerant when

receiving. Think about scalability. Consider performance and cost.


Collection of Subnetworks

The Internet = interconnected collection of many networks.


Example TCP/IP internet

R1

ETH FDDI

IPIP

ETH

TCP R2

FDDI PPP

IP

R3

PPP ETH

H1

IP

ETH

TCP

H8

R2

R1

H4

H5

H3H2H1

Network 2 (Ethernet)


H6

Network 3 (FDDI)

Network 4(point-to-point)

H7 R3 H8

IP


IP Service Model Packet Delivery Model

Connectionless (datagram-based) Best-effort delivery (unreliable service)

Loss, out-of-order, duplication long, variable delay

Global Addressing Scheme IP Addresses Routing info provided within header, no set up

phase.

IP runs over any Layer 2/3 network Ethernet, FDDI, ATM, Point to Point, etc.


IP Packet Format

V ersion HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

DestinationIPAddr

Options (variable)Pad

(variable)

0 4 8 16 19 31

Data

SourceIPAddr


The IP Protocol (2)

Some of the IP options.

5-54


IP Packet Details Datagram format

Version (4) - Currently set to 4 (IPv4). We’ll discuss IPv6.

Hlen (4) - Number of 32-bit words in the header (allows for a variable number of options)

TOS (8) - Type of service (not widely used) Length (16) - Number of bytes in this datagram -

Maximum size is 64KB. Ident (16) - Used for fragmentation Flags(3)/Offset(13) (16) - Used for

fragmentation (offset in units of 8 bytes)


IP Packet Details cont.

TTL (8) - Number of hops this datagram can travel (defaults to 64). Originally was intended to count seconds, but impossible without a central clock.

Protocol (8) - Demultiplexing key for higher level protocols (TCP=6, UDP=17)

Checksum (16) - Of the header only, using Internet Checksum method (as in UDP and TCP)

DestAddr & SrcAddr (32) - See later. Options, e.g. timestamp, record route, (strict/loose)

source routing


Fragmentation and Reassembly Each Layer 2/3 network has a Maximum

Transmission Unit (MTU) e.g. Ethernet is 1500, FDDI is 4500

Unreasonable to make all IP packets small enough to fit within all possible MTUs.

Strategy Fragment only when necessary (MTU < Datagram) Try to avoid fragmentation at source host Fragments are self-contained IP datagrams Reassembly of fragments at destination host.


Fragmentation Example

H1 R1 R2 R3 H8

ETH IP (1400) FDDI IP (1400) PPP IP (512)

PPP IP (376)

PPP IP (512)

ETH IP (512)

ETH IP (376)

ETH IP (512)


Fragmentation cont.

If one fragment is lost, discard all other fragments. Higher layers will recover.

The IP header has fields for handling this type of fragmentation. Set the M bit (in flags) to indicate that more

data is coming. Set the offset to indicate where each of the

fragmented blocks starts. Set the ident field to identify related packets.


IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =1480/8

length = 4000 – 2*1480 = 4000 - 2960 = 1040


IP Fragmentation– Peer Instruction – Question 5.1

A 1300-byte IP datagram sent through a network with 500-byte MTU must be fragmented into 3 fragments with the following respective <length, offset> values in the header:

A. <500, 0>, <500, 460>, <500, 920> B. <500, 0>, <500, 480>, <300, 960>. C. <500, 0>, <500, 500, <300, 1000> D. <500, 0>, <500, 60>, <340, 120> E. None of the above


Global Addresses Properties of IP addresses.

Globally unique - No confusion about where to send a packet.

Hierarchical - Network component and host number.

Normally written in “Dot notation” (4 byte values, total 32 bits) 10.3.2.4 128.96.33.81 192.12.69.77 142.103.7.7 (cascade.cs.ubc.ca)


IP Addresses

IP address formats.


Address Notation

Binary 11000000 00000101 00110000 00000011

Hex Colon C0:05:30:03

Dotted Decimal 192.5.48.3


Class Ranges

Dotted Decimal w.x.y.z Class A: w= 0 thru 127 Class B: w= 128 thru 191 Class C: w= 192 thru 223 Class D: w= 224 thru 239 Class E: w= 240 thru 255


Class Formats Class A: 128 Networks, 16777216 hosts each Class B: 16384 Networks, 65536 hosts each Class C: 2097152 Networks, 256 hosts each

The plan was to give each organization (company or university) a network number that is appropriate for their size, and let them allocate host numbers.

Example: UBC has several class B and C addresses. E.g. 142.103.7.7 and 198.162.33.12

In reality, variations on this method are used.


IP Addresses (2)

Special IP addresses.


Subnets

A campus network consisting of LANs for various departments.


Subnets (2)

A class B network subnetted into 64 subnets.


CIDR – Classless InterDomain Routing

A set of IP address assignments

address format: a.b.c.d/x subnet portion of arbitrary length x

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23


NAT – Network Address Translation

Placement and operation of a NAT box.


Datagram Forwarding Using these IP address, how do we route

messages? Strategy

every datagram contains destination's address if directly connected to destination network, then

forward to host if not directly connected to destination network, then

forward to some router forwarding table maps network number into next hop each host has a default router each router maintains a forwarding table

A forwarding table maps network numbers into router addresses.


Example: Forwarding Table for R2

Network

1234

Next Hop

R3R1interface 1interface 0

For Router R2

R2

R1

H4

H5

H3H2H1



H6

Network 3 (FDDI)

Network 4(point-to-point)

H7 R3 H8


Examples

Sending from H1 to H2: Same network, so send an Ethernet frame to

the Ethernet address for H2

Sending from H1 to H8: Send an Ethernet frame from H1 to R1 Send an FDDI packet from R1 to R2 Send a point to point message from R2 to R3 Send an Ethernet frame from R3 to H8


Scalability In reality, it’s not possible to list an appropriate

router for every network on the internet. The table will get too big.

Commonly we’ll have a list of well-known networks, but use a default router for all other networks.

For example: Network 3 could get to Network 2 via R1, and will use R2 for all other networks.

Sometimes, we only have a single default router on each network.


Internet Control Message Protocol (ICMP) If something goes wrong with an IP packet, a

control message is sent back to the sender: Echo (ping) Request/Reply Timestamp Request/Reply Redirect (from router to source host) Source quench Destination unreachable (protocol, port, or host) TTL exceeded (so datagrams don't cycle forever) Checksum failed Reassembly failed Cannot fragment


Summary - What have we covered?

internetworks IP (Layer 3.5) packets and fragmentation addressing and address classes packet forwarding ICMP

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Jump to first page 1 CICS 515 (Part 2) University of British Columbia CICS 515 (Part 2) Computer Networks Lecture 5 – IP (Ch 4) Instructor: Dr. Son T