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1DT066 Distributed Information System

Chapter 4

Network Layer

CHAPTER 4: Chapter goals:

¢ Understand principles behind network layer services: — network layer service models — forwarding vs routing — how a router works — routing (path selection) — dealing with scale — advanced topics: IPv6, mobility ¢ Implementation in the Internet

1 CHAPTER 4: NETWORK LAYER

¢ 4. 1 Introduction ¢ 4.2 Virtual circuit and networks ¢ 4.3 What’s inside a router ¢ 4.4 IP: — Datagram format — IPv4 addressing — IPv6

ESSENCE OF NETWORKING LAYER

A B

Data Link

Physical

2 NETWORK LAYER

application ¢ transport segment from transport network sending to receiving host physical

network network

¢ on sending side data link data link

network LayerNetwork physical physical encapsulates segments into data link physical network network data link data link physical physical ¢ on receiver side, delivers network network data link data link segments to physical physical network ¢ network layer protocols in data link physical application every host, router transport network network data link network data link ¢ router examines header network physical data link physical data link physical fields in all IP datagrams physical passing through it

TWO KEY NETWORK-LAYER FUNCTIONS

¢ forwarding: move packets from router’s input to correct router output

¢ routing: determine route taken by packets from source to destination.

— routing algorithms (e.g., OSPF, BGP)

3 Interplay of forwarding and routing

Value in arriving packet’s header

1 0111

2 3 routing algorithm

local forwarding table header output value link 0100 3 0101 2 0111 2 1001 1

CHAPTER 4: NETWORK LAYER

¢ 4. 1 Introduction ¢ 4.2 Virtual Circuit and Datagram networks ¢ 4.3 What’s inside a router ¢ 4.4 IP: Internet Protocol — Datagram format — IPv4 addressing — ICMP — IPv6

4 NETWORK LAYER CONNECTION AND CONNECTION-LESS SERVICE

¢ Datagram network provides network-layer connectionless service

¢ VC network provides network-layer connection service

VIRTUAL CIRCUITS “source-to-dest path behaves like a telephone circuit” — performance benefits — network actions along source-to-dest path

¢ each packet carries VC identifier (not destination host address) ¢ every router on source-dest path maintains “state” for each passing connection ¢ link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service)

5 VC FORWARDING TABLE VC number

12 22 32 1 3 2

Forwarding table in Interface northwest router: number

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 3 22 2 63 1 18 3 7 2 17 1 97 3 87 … … … …

Routers maintain connection state information!

VIRTUAL CIRCUITS: SIGNALING PROTOCOLS

¢ used in ATM, frame-relay, X.25 ¢ not used in today’s Internet

application 6. Receive data application transport 5. Data flow begins transport network 4. Call connected 3. Accept call network data link 1. Initiate call 2. incoming call data link physical physical

6 DATAGRAM NETWORKS ¢ no at network layer ¢ routers: no state about end-to-end connections — no network-level concept of “connection” ¢ packets forwarded using destination host address — packets between same source-dest pair may take different paths

application application transport transport network network data link 1. Send data 2. Receive data data link physical physical

4 billion FORWARDING TABLE possible entries!

Destination Address Range Link Interface

11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000 through 1 11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000 through 2 11001000 00010111 00011111 11111111

Otherwise 3

7 LONGEST PREFIX MATCHING

Prefix Match Link Interface 11001000 00010111 00010______0 11001000 00010111 00011000 ______1 11001000 00010111 00011______2 Otherwise 3

Examples: DEST: 11001000 00010111 00010110 10100001 Which interface?

DEST: 11001000 00010111 00011000 10101010 Which interface?

CHAPTER 4: NETWORK LAYER

¢ 4. 1 Introduction ¢ 4.2 Virtual circuit and datagram networks ¢ 4.3 What’s inside a router ¢ 4.4 IP: Internet Protocol — Datagram format — IPv4 addressing — IPv6

8 ROUTER ARCHITECTURE OVERVIEW

Two key router functions: ¢ run routing algorithms/protocol (RIP, OSPF, BGP) ¢ forwarding datagrams from incoming to outgoing link

CHAPTER 4: NETWORK LAYER

¢ 4. 1 Introduction ¢ 4.2 Virtual circuit and datagram networks ¢ 4.3 What’s inside a router ¢ 4.4 IP: Internet Protocol — Datagram format — IPv4 addressing — IPv6

9 THE INTERNET NETWORK LAYER Host, router network layer functions:

Transport layer: TCP, UDP

Routing protocols IP protocol • path selection • addressing conventions Network • RIP, OSPF, BGP • datagram format layer • packet handling conventions forwarding table ICMP protocol • error reporting • router “signaling”

Link layer

Physical layer

CHAPTER 4: NETWORK LAYER

¢ 4. 1 Introduction ¢ 4.2 Virtual circuit and datagram networks ¢ 4.3 What’s inside a router ¢ 4.4 IP: Internet Protocol — Datagram format — IPv4 addressing — IPv6

10 IP DATAGRAM FORMAT

IP protocol version 32 bits number total datagram header length head. type of length (bytes) ver length (bytes) len service fragmentation/ fragment “type” of data reassembly 16-bit identifier flgs offset max number time to upper header remaining hops live layer checksum (decremented at each router) 32 bit source IP address 32 bit destination IP address upper layer protocol to deliver payload to Options (if any) Data (variable length, typically a TCP or UDP segment)

IP FRAGMENTATION & REASSEMBLY

¢ Network links have MTU (max.transfer size) — largest possible link-level frame. fragmentation: ¢ Large IP datagram divided in: 1 large (“fragmented”) within net out: 3 small — one datagram becomes several datagrams — “reassembled” only at final destination — IP header bits used to identify, order related fragments

reassembly

11 IP FRAGMENTATION AND REASSEMBLY

length ID fragflag offset Example =4000 =x =0 =0

❒ 4000 byte LayerNetwork One large datagram becomes datagram several smaller datagrams ❒ MTU = 1500 bytes

length ID fragflag offset =1500 =x =1 =0 1480 bytes in data field length ID fragflag offset =1500 =x =1 =185 offset = 1480/8 length ID fragflag offset =1040 =x =0 =370

4-2 3

CHAPTER 4: NETWORK LAYER

¢ 4. 1 Introduction ¢ 4.2 Virtual circuit and datagram networks ¢ 4.3 What’s inside a router ¢ 4.4 IP: Internet Protocol — Datagram format — IPv4 addressing — IPv6

12 IP ADDRESSING: INTRODUCTION 223.1.1.1 ¢ IP address: 32-bit 223.1.2.1 identifier for host, 223.1.1.2 router interface 223.1.1.4 223.1.2.9

¢ 223.1.2.2 interface: connection 223.1.1.3 223.1.3.27 between host/router and physical link — router’s typically have 223.1.3.1 223.1.3.2 multiple interfaces — host typically has one interface 223.1.1.1 = 11011111 00000001 00000001 00000001 — IP addresses associated 223 with each interface 1 1 1

SUBNETS

¢ IP address: — subnet part (high order bits) — host part (low order bits) ¢ What is a subnet ? — device interfaces with same subnet part of IP address — can physically reach each other without intervening router

subnet host part part 11001000 00010111 00010000 00000000

200.23.16.0/24

13 223.1.1.0/24 223.1.2.0/24 SUBNETS

¢ To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet. 223.1.3.0/24

Subnet mask: /24

223.1.1.2 SUBNETS How many? 223.1.1.1 223.1.1.4

223.1.1.3

223.1.9.2 223.1.7.0

223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0

223.1.2.6 223.1.3.27

223.1.2.1 223.1.2.2 223.1.3.1 223.1.3.2

14 IP ADDRESSING: CIDR CIDR: Classless InterDomain Routing — Subnet portion of address of arbitrary length — address format: a.b.c.d/x, where x is # bits in subnet portion of address

subnet host part part 11001000 00010111 00010000 00000000

200.23.16.0/23

IP ADDRESSES: HOW TO GET ONE?

Q: How does a host get IP address?

¢ Hard-coded by system admin in a file — Windows: control-panel->network->configuration->tcp/ip- >properties — UNIX: /etc/rc.config

¢ DHCP: Dynamic Host Configuration Protocol: dynamically get address from a server — “plug-and-play”

15 DHCP: DYNAMIC HOST CONFIGURATION PROTOCOL Goal: allow host to dynamically obtain its IP address from network server when it joins network — Allows reuse of addresses

223.1.2.1 A 223.1.1.1 DHCP server 223.1.1.2 223.1.1.4 223.1.2.9 B 223.1.2.2 arriving DHCP 223.1.1.3 223.1.3.27 E client needs address in this 223.1.3.2 223.1.3.1 network

DHCP CLIENT-SERVER SCENARIO arriving DHCP server: 223.1.2.5 DHCP discover client src : 0.0.0.0, 68 dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654 Network LayerNetwork DHCP offer src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 654

Lifetime: 3600 secs DHCP request src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddrr: 223.1.2.4 transaction ID: 655 time Lifetime: 3600 secs

DHCP ACK src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 655 4-3 Lifetime: 3600 secs 2

16 IP ADDRESSES: HOW TO GET ONE? Q: How does network get subnet part of IP addr? A: It’s allocated portion of its provider ISP’s address space

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

HIERARCHICAL ADDRESSING: ROUTE AGGREGATION Hierarchical addressing allows efficient advertisement of routing information: Network Layer

Organization 0 200.23.16.0/23 Organization 1 “Send me anything 200.23.18.0/23 with addresses Organization 2 beginning . ISP Inc. 200.23.16.0/20” 200.23.20.0/23 . . . . Internet Organization 7 . 200.23.30.0/23 “Send me anything ISPs-R-Us with addresses beginning 199.31.0.0/16”

17 NAT: NETWORK ADDRESS TRANSLATION

rest of local network Internet (e.g., home network) 10.0.0/24 10.0.0.1

10.0.0.4 10.0.0.2 138.76.29.7

10.0.0.3

All datagrams leaving local Datagrams with source or network have same single source destination in this network NAT IP address: 138.76.29.7, have 10.0.0/24 address for different source port numbers source, destination (as usual)

4-35

NAT: NETWORK ADDRESS TRANSLATION

¢ Motivation: local network uses just one IP address as far as outside world is concerned: Network LayerNetwork

— Only one IP address for all devices

— Can change addresses of devices in LAN 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).

4-3 6

18 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 #) LayerNetwork . . . 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

4-3 7

NAT: NETWORK ADDRESS TRANSLATION NAT translation table 1: host 10.0.0.1 2: NAT router WAN side addr LAN side addr changes datagram sends datagram to 138.76.29.7, 5001 10.0.0.1, 3345 source addr from 128.119.40.186, Network Layer 80 …… …… 10.0.0.1, 3345 to

138.76.29.7, 5001, S: 10.0.0.1, 3345 updates table D: 128.119.40.186, 80 10.0.0.1 1 S: 138.76.29.7, 5001 2 D: 128.119.40.186, 80 10.0.0.4 10.0.0.2

138.76.29.7 S: 128.119.40.186, 80 D: 10.0.0.1, 3345 4 S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3 10.0.0.3 4: NAT router 3: Reply arrives changes datagram dest. address: dest addr from 138.76.29.7, 5001 138.76.29.7, 5001 to 10.0.0.1, 3345

19 CHAPTER 4: NETWORK LAYER

¢ 4. 1 Introduction

¢ 4.2 Virtual circuit and datagram networks LayerNetwork ¢ 4.3 What’s inside a router ¢ 4.4 IP: Internet Protocol

— Datagram format — IPv4 addressing — IPv6

IPV6

¢ Initial motivation: 32-bit address space soon to be completely allocated.

¢ Additional motivation: — Header format helps speed processing/forwarding — In-built DHCP — Header changes to facilitate QoS IPv6 datagram format: — Fixed-length 40 byte header — No fragmentation allowed

20 IPV6 HEADER (CONT) Priority: identify priority among datagrams in flow Flow Label: identify datagrams in same “flow.” (concept of“flow” not well defined). Next header: identify upper layer protocol for data

CHAPTER 4: SUMMARY

¢ 4. 1 Introduction ¢ 4.2 Virtual circuit and datagram networks ¢ 4.3 What’s inside a router ¢ 4.4 IP: Internet Protocol — Datagram format — IPv4 addressing — IPv6

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