Internet Protocol. IP overview Service model Addressing Forwarding (Routing later)

Internet Protocol

IP overview

Service model Addressing Forwarding (Routing later)

Layer reminder

Bridges - emulate single link Everything broadcast Same collision domain

Switches - emulate single network Flat addressing Broadcast supported

Internet - connect multiple networks Hierarchical addressing No broadcast Highly scalable

IP service model

Service provided to transport layer (TCP, UDP) Global name space Host-to-host connectivity (connectionless) Best-effort packet delivery

Not in IP service model Delivery guarantees on bandwidth, delay or loss

Delivery failure modes Packet delayed for a very long time Packet loss Packet delivered more than once Packets delivered out of order

IP comparison with ATM

Network Architecture

Service Model

Guarantees Congestion Feedback

Bandwidth Loss Order Timing

Internet best effort

none no no no no

ATM CBR constant yes yes yes no congestion

ATM VBR guaranteed rate

yes yes yes no congestion

ATM ABR guaranteed minimum

no yes no yes

ATM UBR none no yes no no

IP addressing

Ethernet address space Flat Assigned at manufacture time

IP address space Hierarchical Assigned at configuration time

IP Addressing: introduction

IP address: 32-bit identifier for host, router interface

interface: connection between host/router and physical link routers typically have

multiple interfaces host typically has one

interface IP addresses

associated with each interface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

IP networks

Address has 2 components Network (high-order

bits) Host (low-order bits)

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

IPv4 Address Model

Class Network ID Host ID # of Addresses

# of Networks

A 0 + 7 bit 24 bit 224-2 126

B 10 + 14 bit 16 bit 65,536 - 2 214

C 110 + 21 bit 8 bit 256 - 2 221

D 1110 + Multicast Address IP Multicast

E Future Use

0 Network (7 bits)

Network (14 bits)

1 1 0

1 0

Network (21 bits)

Host (24 bits)

Host (16 bits)

Host (8 bits)

Class A:

Class B:

Class C:

IP networks

Class A network: 18.0.0.0 (MIT) www.mit.edu has address 18.7.22.83

Class B network: 128.174.0.0 (UIUC) www.cs.uiuc.edu has address 128.174.252.84

Class C network: 216.125.249.0 (Parkland) www.parkland.edu has address 216.125.249.97

CIDR

3-class model too inflexible CIDR: Classless InterDomain Routing

Arbitrary number of bits to specify network

Address format: a.b.c.d/x, where x is # bits in network portion

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

Classless Domains

Internet Archive - 207.241.224.0/20 4K hosts 207.241.224.0 - 207.241.239.255

AT&T - 204.127.128.0/18 16K hosts 204.127.128.0 - 204.127.191.255

UUNET - 63.64.0.0/10 4M hosts 63.64.0.0 - 63.127.255.255

IP forwarding

Forwarding table has: Network number Interface

Avoid having to store 4 billion entries But there are still 2 million class C’s …and perhaps more CIDR networks

Hierarchical Networks

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us “Send me anythingwith addresses beginning 199.31.0.0/16”

200.23.20.0/23Organization 2

...

...

Subnetting

UIUC - 130.126.0.0/16 130.126.0.0 - 130.126.255.255

CRHC - 130.126.136.0/21 130.126.136.0 - 130.126.143.255

EWS - 130.126.160.0/21 130.126.160.0 - 130.126.167.255

Forwarding Tables

130.126.136.0/21 if1130.126.160.0/21 if2130.126.0.0/16 if30.0.0.0/0 if4

Most specific rule is used Most hosts outside of the core have

default rules

CRHC

EWS

UIUC

Internet

if1

if2if4

if3

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or destination in this network

have 10.0.0/24 address for source, destination (as usual)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers

NAT: Network Address Translation

Motivation: local network uses just one IP address as far as outside world is concerned: range of addresses not needed from ISP: just one IP

address for all devices can change addresses of devices in local network

without notifying outside world can change ISP without changing addresses of devices

in local network devices inside local net not explicitly addressable,

visible by outside world (a security plus).

NAT: Network Address Translation

Implementation: NAT router must: outgoing datagrams: replace (source IP address, port #)

of every outgoing datagram to (NAT IP address, new port #)

. . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr.

remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair

incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

1

10.0.0.4

138.76.29.7

1: host 10.0.0.1 sends datagram to 128.119.40.186, 80

NAT translation tableWAN side addr LAN side addr

138.76.29.7, 5001 10.0.0.1, 3345…… ……

S: 128.119.40.186, 80 D: 10.0.0.1, 3345 4

S: 138.76.29.7, 5001D: 128.119.40.186, 802

2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table

S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3

3: Reply arrives dest. address: 138.76.29.7, 5001

4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345

NAT: Network Address Translation

16-bit port-number field: 60K simultaneous connections with a single

LAN-side address! NAT is controversial:

routers should only process up to layer 3 violates end-to-end argument

NAT possibility must be taken into account by app designers, eg, P2P applications

address shortage should instead be solved by IPv6

IPv4 Address Translation support

IP addresses to LAN physical addresses Problem

An IP route can pass through many physical networks

Data must be delivered to destination’s physical network

Hosts only listen for packets marked with physical interface names Each hop along route Destination host

IP to Physical Address Translation

Hard-coded Encode physical address in IP address Ex: Map Ethernet addresses to IP addresses

Makes it impossible to associate address with topology Fixed table

Maintain a central repository and distribute to hosts Bottleneck for queries and updates

Automatically generated table Use ARP to build table at each host Use timeouts to clean up table

ARP

Check table for physical address If address not present

Broadcast a query, include host’s translation Wait for a response

Upon receipt of ARP query/response Targeted host responds with address translation If address already present

Refresh entry and reset timeout If address not present

Add entry for requesting host Ignore for other hosts

Timeout and discard entries after O(10) minutes

ARP Packet

TargetHardwareAddr (bytes 2 – 5)

TargetProtocolAddr (bytes 0 – 3)

SourceProtocolAddr (bytes 2 – 3)

Hardware type = 1 ProtocolType = 0x0800

SourceHardwareAddr (bytes 4 – 5)

TargetHardwareAddr (bytes 0 – 1)

SourceProtocolAddr (bytes 0 – 1)

HLEN = 48 PLEN = 32 Operation

SourceHardwareAddr (bytes 0 – 3)

0 8 16 31

IP Packet Format

Version HLen TOS LengthIdent Flags Offset

TTL Protocol Checksum

SourceAddrDestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

IP Packet Format

4-bit version IPv4 = 4, IPv6 = 6

4-bit header length Counted in words, minimum of 5

8-bit type of service field (TOS) Mostly unused

16-bit data length Counted in bytes

IP Packet Format

Fragmentation support 16-bit packet ID

All fragments from the same packet have the same ID 3-bit flags

1-bit to mark last fragment 13-bit fragment offset into packet

Counted in 8-byte words

8-bit time-to-live field (TTL) Hop count decremented at each router Packet is discard if TTL = 0

IP Packet Format

8-bit protocol field TCP = 6, UDP = 17

16-bit IP checksum on header 32-bit source IP address 32-bit destination IP address Options

Variable size Source-based routing Record route

Padding Fill to 32-bit boundaries

IP Packet Size

Problem Different physical layers provide different

limits on frame length Maximum transmission unit (MTU)

Source host does not know minimum value Especially along dynamic routes

IP Fragmentation and Reassembly

Solution When necessary, split IP packet into

acceptably sized packets prior to sending over physical link

Questions Where should reassembly occur? What happens when a fragment is

damaged/lost?

IP Fragmentation and Reassembly

Fragments are self-contained IP datagrams Reassemble at destination to minimize

refragmentation Drop all fragments in packet if one or more

fragments are lost Avoid fragmentation at source host

Transport layer should send packets small enough to fit into one MTU of local physical network Must consider IP header

Note: MTU in ATM is based on CS-PDU size

IP Fragmentation and Reassembly

ETH IP (1400)

FDDI IP (1400)

PPP IP (376)

PPP IP (512)

PPP IP (512)

ETH IP (376)

ETH IP (512)

ETH IP (512)

Start of headerIdent = x 0 Offset 0

Rest of header

1400 data bytes

Start of headerIdent = x 1 Offset 0

Rest of header

512 data bytes

Start of headerIdent = x 1 Offset 512

Rest of header

512 data bytes

Start of headerIdent = x 0 Offset 1024

Rest of header

376 data bytes

H1 R1 R2 R3 H2

ETH FDDI PPP ETH

Internet Control Message Protocol (ICMP)

IP companion protocol Handles error and control messages

ModemATMFDDIEthernet

FTP TFTPNVHTTP

TCP UDP

IP ICMP

ICMP

Error Messages Host unreachable Reassembly failed IP checksum failed TTL exceeded (packet dropped) Invalid header

Control Messages Echo/ping request and reply Echo/ping request and reply with timestamps Route redirect

Traceroute and ICMP

Source sends series of UDP segments to dest

First has TTL =1 Second has TTL=2, etc. Unlikely port number

When nth datagram arrives to nth router:

Router discards datagram And sends to source an

ICMP message (type 11, code 0)

Message includes name of router& IP address

When ICMP message arrives, source calculates RTT

Traceroute does this 3 times

Stopping criterion UDP segment eventually

arrives at destination host Destination returns ICMP

“host unreachable” packet (type 3, code 3)

When source gets this ICMP, stops.

Host Configuration

Plug new host into network How much information must be known? What new information must be assigned? How can process be automated?

Some answers Host needs an IP address (must know it) Host must also

Send packets out of physical (direct) network Thus needs physical address of router

Host Configuration

Reverse Address Resolution Protocol (RARP) Translate physical address to IP address Used to boot diskless hosts Host broadcasts request to boot RARP server tells host the host’s own IP address

Boot protocol (BOOTP) Use UDP packets for same purpose as RARP Allows boot requests to traverse routers IP address of BOOTP server must be known Also returns file server IP, subnet mask, and default router

for host

Dynamic Host Configuration Protocol (DHCP)

A simple way to automate configuration information Network administrator does not need to

enter host IP address by hand Good for large and/or dynamic networks

Dynamic Host Configuration Protocol (DHCP)

New machine sends request to DHCP server for assignment and information

Server receives Directly if new machine given server’s IP address Through broadcast if on same physical network Via DHCP relay nodes that forward requests onto the server’s

physical network Server assigns IP address and provides other info Can be made secure (present signed request or just a “valid”

physical address)

DHCP

DHCP Server

Host AHost A

broadcasts DHCPDISCOVER

messageHost A

broadcasts DHCP request

Host B

DHCP Server

DHCP Relay

Other Networks

Other Networks

Relay unicasts DHCP request

to serverServer

responds with host’s IP address