Information Network I Network layer: the Internet Protocol · Information Network I Network layer:...

Information Network I

Network layer: the Internet Protocol

Youki KadobayashiNAIST

Internet Protocol

• Vinton G. Cerf and Robert E. Kahn, “A Protocol for Packet Network Intercommunication”, IEEE Trans. on Comms, 22(5), May 1974

2017/4/21 Copyright(C) 2016 Youki Kadobayashi, all rights reserved. 2

Internet Protocol


• Jonathan B. Postel, “Internetwork Protocol Approaches”, invited paper, IEEE Trans. on Comms, 28(4), April 1980

Gateway, a.k.a. router

IP packet and IP header

• Internet Protocol defines structure of bits

• It also defines associated semantics of bits– Defined in Internet standards: RFC 791


8 310 4 16

Ver.

Option (if any)

IHL Type ofService Total Length (in Octet)

Identification Flags Fragment Offset

Time to Live Protocol Header Checksum

Source Address

Destination Address

IP version 4 header

IP version 6 header

• Similar, but simplified a lot• Enlarged address, from 32 bit to 128 bits

– 32bit 4,294,967,296– 128bit 340,282,366,920,938,463,463,374,607,431,768,211,456


Ver TrafficClass Flow Label

Payload LengthNext Header Hop Limit

Source Address

Destination Address

How IP works: addressing

• Structure

– Symbolically:IP-address ::= { <Network-prefix>, <Host-number> }

• Semantics– Unicast – transmission to single host– Multicast – transmission to interested hosts– Broadcast – transmission to all hosts in the network


How IP works: sending packets

• Indicate IP version• Write source IP address• Write destination IP address• Write packet length• Write header checksum• Indicate upper layer protocol

• Then transmit to the predefined gateway


How IP works: hop-by-hop inspection

• Decrease “time to live”in order to avoid meltdown from loops– Discard if TTL = 0

• Verify header checksumin order to avoid data corruption

• Forward to next hop

• See RFC 1812: Requirements for IPv4 routers,esp. Section 4


How IP works: receiving packets

• Check IP version• Check source IP address

– Discard if multicast or broadcast• Check destination IP address

– Discard if it’s not mine• Verify header checksum

• See RFC 1122: Requirements for Internet hosts,esp. Section 3


10

Q&A

2017/4/21 Copyright(C) 2016 Youki Kadobayashi, all rights reserved.

IP Address

Before sending anything, you’ll face a simple question:How do we identify communicating peers?


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Identifying nodes with IP address

Characteristics of IP address:•It is global: uniquely identifiable worldwide•It is hierarchical: delegated to regional bodies•It has two parts: network identifier and host identifier•It has several types

Type ofaddress

Subspace of addresses thatare hierarchically delegated

Network Host

0xDD0xA3 0x4A 0x7F

163.221.74.127/24

163 221 74 127

Network address : 24bit

Prefix length

e.g. IPv4 address


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IPv4 address space

• Global and unique address space– 232

• Types of addresses– Class A (begins with 0)– Class B (begins with 10)– Class C (begins with 110)– Class D (begins with 1110) – Multicast.

• Unicast address allocation has been delegated to regional entities– RIPE in Europe, APNIC in Asia-Pacific, ARIN in

America


Unicast

IPv4 address space: minor exceptionsRFC 1918, RFC 3330


The Internet Assigned Numbers Authority (IANA) has reserved thefollowing three blocks of the IP address space for private internets:

10.0.0.0 - 10.255.255.255 (10/8 prefix)172.16.0.0 - 172.31.255.255 (172.16/12 prefix)192.168.0.0 - 192.168.255.255 (192.168/16 prefix)

0.0.0.0/8 – Addresses in this block refer to source hosts on "this” network. 127.0.0.0/8 – loopback address. A datagram sent by a higher level protocol

to an address anywhere within this block should loop back inside the host.

169.254.0.0/16 – "link local" block. It is allocated for communication betweenhosts on a single link. Hosts obtain these addresses by auto-configuration,

192.0.2.0/24 – "TEST-NET" for use in documentation and example code.Addresses within this block should not appear on the public Internet.

224.0.0.0/4 – multicast address. This block, formerly known as the Class Daddress space, is allocated for use in IPv4 multicast address assignments.

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Addressing one or more nodes

• Unicast– Point-to-point communication– Specify a source and a destination address

• Broadcast

• Multicast– Transmission to “interested” hosts– Interested hosts join the group


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Broadcast

• Broadcast– Sends data to all hosts on the same datalink

• Broadcast depends on the same capability in datalink– IP broadcast

should be mapped to:– Link-layer broadcast

• Some datalinks don’t support broadcast

– Some hosts may not be hearing you• Offline hosts can’t receive• Receiver may be busy


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Multicast

• Multicast– Group communication; one-to-many or many-to-many

– Membership:• if a host is not a member of the group, it doesn’t receive

packets to the group– membership management– group management

• Well-defined mapping– IP multicast, to– Link-layer multicast


IPv6 address space


Usage Prefix Ratio

Reserved 00000000 1/256...Global Unicast 001 1/8Unassigned 010 1/8Unassigned 011 1/8Unassigned 100 1/8Unassigned 101 1/8..Link-Local Unicast 11111110 10 1/1024Multicast Address 11111111 1/256

Global Unicast: It’s a pretty big address space of 2^30,large enough to accommodate Internet of Things

IPv6 address format

The community has been discussing optimal value of m and n.• RFC 3177 said m = 16• Today: RFC 6177 supersedes RFC 3177, reflecting community

consensus


Global routing prefix Subnet ID Interface IDn m 128-n-m

RFC 3513 defines address format:

ARIN (American Registry) address allocation policy for end sites:- /64 when it is known that one and only one subnet is needed

(n = 64)- /56 for small sites who need only a few subnets over the next 5 years

(n = 56, m = 8)- /48 for larger sites

(n = 48, m = 16)

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Bootstrapping with broadcast or multicast

• The single most useful example of broadcast/multicast:Automatic configuration.

A B Ca b c Data-link layer

Network layer

• Bootstrap A:l A → all routers:

“who is router”l R → A:

“your router is R”

Rr


21

Q&A


Mapping to Datalink

How do we transmit that IP packet on Ethernet, or Wi-Fi?


Network to datalink: Address Resolution Protocolfor IP version 4

• A → B: “M”– a → all stations: “where is B”– b → a: “B is at b”– a → b: “A → B: “M””



Network layer

RFC 826

Network to datalink: Neighbor Discovery Protocol for IP version 6

• A → B: “M”– A → M(B): “where is B”– B → A: “B is at b”– a → b: “A → B: “M””– M() maps unicast address to solicited-node multicast

address


RFC 4861


Network layer

Network to datalink: the case of bridged networks

• A → C: “M”– a → all stations: “where is C”– c → a: “C is at a”– a → c: “A → C: “M””


A Ba b

C Dc d

Tt

Data-link layerNetwork layer

Network to datalink: the case of routed networks

• A → C: “M”– a → all stations: “where is R”– r → a: “R is at r”– a → r: “A → C: “M””


l r → all stations: “where is C”l c → r: “C is at c”l r → c: “A → C: “M””

A Ba b

C Dc d

Rr

Data-link layerNetwork layer

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Datalink to network layer

• Multiple network layer protocols are multiplexed on a single datalink

• Needed: Multiplexing, de-multiplexing

IPv4 IPv6 ....

Ethernet

IPv4 IPv6 ....

Ethernet Datalink

Network

?


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De-multiplexing with LLC

Source SAP Address Information

1

Control

1 or 2 bytes

Destination SAP Address Source SAP Address

I/G

7 bits1

C/R

7 bits1

I/G = Individual or group addressC/R = Command or response frame

DestinationSAP Address

1 byte

e.g. SAP address :06 IP packetE0 Novell IPXFE OSI packetAA SubNetwork Access protocol (SNAP)


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De-multiplexing with LLC/SNAP

MAC Header FCS

AA AA 03LLC PDU1 1 1

InformationSNAP Header

TypeORG

SNAP PDU

3 2


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A hierarchical perspective

Datalink Layer Datalink Layer

Network Layer

Datalink to Network:

UsingLLC/SNAP

Network to Datalink:

UsingARP (IPv4), ND (IPv6)


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Q&A

• Hands-on Tutorial/Walkthrough

Diving Deeper with (Wire)shark!


Debugging IP

How do I know IP is nicely working over given datalink?How do I know the IP address is correct?


ICMP: internet control message protocol

• Handy for probing the Internetwithout help of transport protocols

• A very simple “debug function” for IP:– Echo request– Echo reply

• Gateways may return diagnostic ICMP messages:• 0 = net unreachable;• 1 = host unreachable;• 2 = protocol unreachable;• 3 = port unreachable;• 4 = fragmentation needed and DF set;• 5 = source route failed.


RFC 792

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End-to-end diagnostics with ICMP

• ICMP echo can be used to:– Check if the destination IP is reachable from source– Identify faulty section in the middle

• By obtaining echo replyor other diagnostic ICMP messages

ApplicationPresentation

SessionTransportNetwork

Data LinkPhysical



Data LinkPhysical


ICMP message

Groupwork Hands-on


“easter_challenge”

• Divide into 4 groups• Find the hidden pcap file in the VM provided.• We have injected 4 ‘Easter eggs’ into the packets• A packet has no more than 1 ‘Easter egg’• Analyze the file using Wireshark• You have 20 minutes to consult with you group

members and find the ‘Easter eggs’• Good luck!


Some tricks in connecting networks

Is it so simple to connect vastly different two networks?


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Fragmentation and reassembly:adapting to diverse datalinks

• Datalinks have different MTUs– MTU: Maximum Transmission Unit

• Fragmentation：– Splits packets in order to fit into smaller MTU

• Reassembly:– Assembles fragmented packets at the receiving host

• NOTE: this applies to IPv4 only

MTU = 1520MTU = 9128


4500 1500 1500 1500

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Fragmentation info in IPv4 header

• Flags = {0, MF, DF}• Fragment offset: 13 bits

8 310 4 16

Ver.

Option (if any)

IHL Type ofService Total Length (in Octet)

Identification Flags Fragment Offset

Time to Live Protocol Header Checksum

Source Address

Destination Address


Path MTU discoverymandatory in IP version 6

• Probe MTU for destination• Remember MTU

• Avoid fragmentation in the middle


RFC 1981



Data LinkPhysical

ICMPpacket too big

90009000 1500

41

Q&A


Assignment No. 3• Imagine you are a Network Architect in a start-up

network operator (no more than 1000 subscribers) and you have been tasked with building the Physical and Data link infrastructure

• Please build an action plan for building the L1/L2 infrastructure– Build a diagram of the L1/L2 infrastructure– Explain why you chose one technology over another by

highlighting the pro/cons• Report format: [name]-[studentID].pdf• Deadline: April 26, 2017 17:00 JST


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