network layer dr. yingwu zhu 4-1. network layer r introduction r datagram networks r ip: internet...

Network Layer

Dr. Yingwu Zhu

4-1

Network Layer

Introduction Datagram networks IP: Internet Protocol

Datagram format IPv4 addressing ICMP

What’s inside a router

Routing algorithms Link state Distance Vector Hierarchical routing

Routing in the Internet RIP OSPF BGP

Broadcast and multicast routing

4-2

Network layer transport segment from sending to receiving

host on sending side encapsulates segments into

datagrams on receiving side, delivers segments to

transport layer network layer protocols in every host, router Router examines header fields in all IP

datagrams passing through it

networkdata linkphysical








application

transportnetworkdata linkphysical

application


4-3

Key Network-Layer Functions

forwarding: move packets from router’s input to appropriate router output

routing: determine route taken by packets from source to dest.

Routing algorithms

analogy:

routing: process of planning trip from source to dest

forwarding: process of getting through single interchange

4-4

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

4-5

Network Layer







4-6

Datagram networks Connection-less service; no call setup at network layer routers: no state about end-to-end connections (why?)

no network-level concept of “connection”

packets forwarded using destination host address packets between same source-dest pair may take different paths

application


application


1. Send data 2. Receive data

4-7

Network Layer







4-8

The Internet Network layer

forwardingtable

Host, router network layer functions:

Routing protocols•path selection•RIP, OSPF, BGP

IP protocol•addressing conventions•datagram format•packet handling conventions

ICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

4-9

The Internet Protocol (IP)

App

Transport

Network

Link

TCP / UDP

IP

Data Hdr

Data Hdr

TCP Segment

IP Datagram

Protocol Stack

4-10

The Internet Protocol (IP)

Characteristics of IP CONNECTIONLESS: mis-sequencing

UNRELIABLE: may drop packets…

BEST EFFORT: … but only if necessary

DATAGRAM: individually routed

A

R1

R2

R4

R3

BSource Destination

D H

D H

•Architecture•Links•Topology

Transparent

4-11

IP datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

Internet checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.

how much overhead with TCP?

20 bytes of TCP 20 bytes of IP = 40 bytes +

app layer overhead

6 for TCP

4-12

IP Fragmentation & Reassembly Problem: A router may

receive a packet larger than the maximum transmission unit (MTU) of the outgoing link. different link types,

different MTUs Solution: large IP datagram

divided (“fragmented”) within net one datagram becomes

several datagrams “reassembled” only at

final destination, why? IP header bits used to

identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

E.g., Ethernet frames carry up to 1500 bytes, frames for some wide-area links carry no more than 576 bytes. (MTU: the max of data a link-layer frame can carry) 4-13

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

Note: the offset value is specified in units of 8-byte chunks!!! 4-14

Fragmentation

Fragments are re-assembled by the destination host; not by intermediate routers.

To avoid fragmentation, hosts commonly use path MTU discovery to find the smallest MTU along the path.

Path MTU discovery involves sending various size datagrams until they do not require fragmentation along the path.

Most links use MTU>=1500bytes today. Try:

traceroute www.berkeley.edu 500 –F andtraceroute www.berkeley.edu 1501

-F: Set the "don't fragment" bit, return error it is too long Bonus: Can you find a destination for which the path

MTU < 1500 bytes?

4-15

Network Layer

Introduction Virtual circuit and

datagram networks What’s inside a

router IP: Internet Protocol

Datagram format IPv4 addressing ICMP IPv6




4-16

IP Addresses IP (Version 4) addresses are 32 bits long Every interface has a unique IP address:

A computer might have two or more IP addresses A router has many IP addresses

IP addresses are hierarchical They contain a network ID and a host ID E.g. SeattleU addresses start with: 172.17…

IP addresses are assigned statically or dynamically (e.g. DHCP)

IP (Version 6) addresses are 128 bits long

4-17

IP AddressesOriginally there were 5 classes:

CLASS “A” 1 7

0 Net ID Host-ID

CLASS “B” 10 Net ID Host-ID

24

2 14 16

CLASS “C” 110

Net ID Host-ID

3 21 8

CLASS “D”

1110

Multicast Group ID

4 28

CLASS “E” 11110 Reserved

5 27

A B C D0 232-1

4-18

IP AddressesExamples

Class “A” address: www.mit.edu18.181.0.31

(18<128 => Class A)

Class “B” address: www.seattleu.edu172.17.72.14

(128<171<128+64 => Class B)

4-19

IP Addressing

Problem: Address classes were too “rigid”. For most

organizations, Class C were too small and Class B too big. Led to inefficient use of address space, and a shortage of addresses.

Organizations with internal routers needed to have a separate (Class C) network ID for each link.

And then every other router in the Internet had to know about every network ID in every organization, which led to large address tables.

Small organizations wanted Class B in case they grew to more than 255 hosts. But there were only about 16,000 Class B network IDs.

4-20

IP Addressing

Two solutions were introduced: Subnetting within an organization to subdivide

the organization’s network ID. Classless Interdomain Routing (CIDR) in the

Internet backbone was introduced in 1993 to provide more efficient and flexible use of IP address space.

CIDR is also known as “supernetting” because subnetting and CIDR are basically the same idea.

4-21

Subnetting

CLASS “B”e.g.

Company

10 Net ID Host-ID

2 14 16

10 Net ID Host-ID

2 14 16

0000

Subnet ID (20) SubnetHost ID (12)

10 Net ID Host-ID

2 14 16

1111


10 Net ID Host-ID

2 14 16

000000


10 Net ID Host-ID

2 14 16

1111011011


e.g. Site

e.g. Dept

4-22

Subnetting

Subnetting is a form of hierarchical routing. Subnets are usually represented via an address

plus a subnet mask or “netmask”. e.g. [zhuy@cs1 ~]$ /sbin/ifconfig eth0 eth0 Link encap:Ethernet HWaddr 00:21:9B:8F:64:6D inet addr:10.124.72.20 Bcast:10.124.72.255

Mask:255.255.255.0

Netmask 255.255.255.0: the first 24 bits are the subnet ID, and the last 8 bits are the host ID.

Can also be represented by a “prefix + length”, e.g. 171.64.15.0/24, or just 171.64.15/24.

4-23

IP Addressing IP address: 32-bit

identifier for host, router interface

interface: connection between host/router and physical link router’s 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

4-24

Subnets IP address:

subnet part (high order bits)

host part (low order bits)

What’s a subnet ? device interfaces

with same subnet part of IP address

can physically reach each other without intervening router

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

network consisting of 3 subnets

subnet

4-25

Subnets 223.1.1.0/24223.1.2.0/24

223.1.3.0/24

Recipe To determine the

subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet. Subnet mask: /24

4-26

SubnetsHow many? 223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

4-27

Classless Interdomain Routing (CIDR)Addressing

The IP address space is broken into line segments. Subnet portion of address of arbitrary length Each line segment is described by a prefix. A prefix is of the form x/y where x indicates the prefix of all

addresses in the line segment, and y indicates the length of the segment.

e.g. The prefix 128.9/16 represents the line segment containing addresses in the range: 128.9.0.0 … 128.9.255.255.

0 232-1

128.9/16

128.9.0.0

216

142.12/19

65/8

128.9.16.14

4-28


0 232-1

128.9/16

128.9.16.14

128.9.16/20128.9.176/20

128.9.19/24

128.9.25/24

Most specific route = “longest matching prefix”

4-29


Prefix aggregation: If a service provider serves two organizations

with prefixes, it can (sometimes) aggregate them to form a shorter prefix. Other routers can refer to this shorter prefix, and so reduce the size of their address table.

E.g. ISP serves 128.9.14.0/24 and 128.9.15.0/24, it can tell other routers to send it all packets belonging to the prefix 128.9.14.0/23.

ISP Choice: In principle, an organization can keep its prefix

if it changes service providers. 4-30

Size of the Routing Table at the core of the Internet

Source: http://www.cidr-report.org/

31

IP addresses: how to get one?

Q: How does host get IP address?

hard-coded by system admin in a file Wintel: control-panel->network->configuration->tcp/ip-

>properties UNIX: /etc/rc.config

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

4-32

IP addresses: how to get one?

Q: How does network get subnet part of IP addr?

A: gets 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

4-33

Hierarchical addressing: route aggregation

“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

...

...

Hierarchical addressing allows efficient advertisement of routing information:

4-34

Hierarchical addressing: more specific routes

ISPs-R-Us has a more specific route to Organization 1

“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 199.31.0.0/16or 200.23.18.0/23”

200.23.20.0/23Organization 2

...

...

Longest prefix match 4-35

IP addressing: the last word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for Assigned

Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes

4-36

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 networkhave 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

4-37


Motivation: local network uses just one IP address as far as outside world is concerned: no need to be allocated range of addresses from

ISP: - just one IP address is used 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).

4-38


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

4-39


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, 80

2

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

4-40


16-bit port-number field: 60,000 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

4-41

Network Layer

Introduction Virtual circuit and

datagram networks What’s inside a

router IP: Internet Protocol

Datagram format IPv4 addressing ICMP IPv6




4-42

ICMP: Internet Control Message Protocol

used by hosts & routers to communicate network-level information error reporting:

unreachable host, network, port, protocol

echo request/reply (used by ping)

network-layer “above” IP: ICMP msgs carried in IP

datagrams ICMP message: type, code

plus first 8 bytes of IP datagram causing error

Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

4-43

An aside:Error Reporting (ICMP) and traceroute

Internet Control Message Protocol: Used by a router/end-host to report some

types of error: E.g. Destination Unreachable: packet can’t

be forwarded to/towards its destination. E.g. Time Exceeded: TTL reached zero, or

fragment didn’t arrive in time. Traceroute uses this error to its advantage.

An ICMP message is an IP datagram, and is sent back to the source of the packet that caused the error.

4-44

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.It would be fun if you design a traceroute tool on your own!

4-45

Network Layer







4-46

Router Architecture Overview

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

4-47

Input Port Functions

Decentralized switching: given datagram dest., lookup output

port using forwarding table in input port memory

goal: complete input port processing at ‘line speed’

queuing: if datagrams arrive faster than forwarding rate into switch fabric

Physical layer:bit-level reception

Data link layer:e.g., Ethernet

4-48

Three types of switching fabrics

4-49

Switching Via MemoryFirst generation routers: traditional computers with switching under direct control of CPUpacket copied to system’s memory speed limited by memory bandwidth (2 bus crossings per datagram)

InputPort

OutputPort

Memory

System Bus

4-50

Switching Via a Bus

datagram from input port memory

to output port memory via a shared bus

bus contention: switching speed limited by bus bandwidth

1 Gbps bus, Cisco 1900: sufficient speed for access and enterprise routers (not regional or backbone)

4-51

Switching Via An Interconnection Network

overcome bus bandwidth limitations Banyan networks, other interconnection nets

initially developed to connect processors in multiprocessor

Advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric.

Cisco 12000: switches Gbps through the interconnection network

4-52

Output Ports

Buffering required when datagrams arrive from fabric faster than the transmission rate

Scheduling discipline chooses among queued datagrams for transmission

4-53

Output port queueing

buffering when arrival rate via switch exceeds output line speed

queueing (delay) and loss due to output port buffer overflow!

4-54

Input Port Queuing

Fabric slower than input ports combined -> queueing may occur at input queues

Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward

queueing delay and loss due to input buffer overflow!

4-55

How a Router Forwards Datagrams

128.9/16128.9.16/20

128.9.176/20

128.9.19/24128.9.25/24

142.12/19

65/8

Prefix Port

3227213

128.17.14.1128.17.14.1

128.17.20.1

128.17.10.1128.17.14.1

128.17.16.1

128.17.16.1

Next-hop

R1

R2

R3

R4

12

3

128.17.20.1

128.17.16.1

e.g. 128.9.16.14 => Port 2

Forwarding/routing table

4-56

How a Router Forwards Datagrams Every datagram contains a destination

address. The router determines the prefix to

which the address belongs, and routes it to the“Network ID” that uniquely identifies a physical network.

All hosts and routers sharing a Network ID share same physical network.

4-57

Forwarding Datagrams

Is the datagram for a host on a directly attached network?

If no, consult forwarding table to find next-hop.

4-58

Inside a router

Link 1, ingress Link 1, egress




ChooseEgress

ChooseEgress

ChooseEgress

ChooseEgress

4-59

Inside a router





ChooseEgress

ChooseEgress

ChooseEgress

ForwardingDecision

ForwardingTable

4-60

Forwarding in an IP Router

• Lookup packet DA in forwarding table.– If known, forward to correct port.– If unknown, drop packet.

• Decrement TTL, update header Checksum.

• Forward packet to outgoing interface.• Transmit packet onto link.

Question: How is the address looked up in a real router?

4-61

Making a Forwarding DecisionClass-based addressing

Class A Class B Class C D

212.17.9.4

Class A

Class B

Class C212.17.9.0 Port 4

Exact Match

Routing Table:

IP Address Space

212.17.9.0

Exact Match: There are many well-known ways to find an exact match in a table.

4-62

Direct Lookup

IP Address

Ad

dre

ss

MemoryD

ata

Next-hop, Port

Problem: With 232 addresses, the memory would require 4 billion entries.

4-63

Associative Lookups“Contents addressable memory” (CAM)

NetworkAddress

PortNumber

AssociativeMemory or CAM

Search Data

32

PortNumber

Hit?

Advantages:• Simple

Disadvantages• Slow• High Power• Small• Expensive

Search data is compared with every entry in parallel

4-64

Hashed Lookups

HashingFunction

Memory

Addre

ss

Data

Search Data

log2N

AssociatedData

Hit?

Address{1632

4-65

Lookups Using HashingAn example

Hashing Function16

#1 #2 #3 #4

#1 #2

#1 #2 #3Linked list of entrieswith same hash key.

Memory

Search Data

AssociatedData

Hit?32

4-66

Lookups Using Hashing

Advantages:

• Simple

• Expected lookup time can be small

Disadvantages• Non-deterministic lookup time

• Inefficient use of memory

4-67

Trees and Tries

Binary Search Tree:

< >

< > < >

log

2 NN entries

Binary Search Trie: (“reTRIEval”)

0 1

0 1 0 1

111010

Requires 32 memory references, regardless of number of addresses.

4-68

Search TriesMultiway tries reduce the number of memory references

16-ary Search Trie

0000, ptr 1111, ptr

0000, 0 1111, ptr

000011110000

0000, 0 1111, ptr

111111111111

Question: Why not just keep increasing the degree of the trie?

4-69

Classless AddressingCIDR

0 232-1

128.9/16

128.9.16.14

128.9.16/20128.9.176/20

128.9.19/24

128.9.25/24

Most specific route = “longest matching prefix”

Question: How can we look up addresses if they are not an exact match?

4-70

Ternary CAMs

10.1.1.32 1

10.1.1.0 2

10.1.3.0 3

10.1.0.0 4

255.255.255.255

255.255.255.0

255.255.255.0

255.255.0.0

255.0.0.010.0.0.0 4

Value Mask

Priority Encoder

Port

Associative Memory

Port

Note: Most specific routes appear closest to top of table

4-71

Longest prefix matches using Binary Tries

Example Prefixes:a) 00001b) 00010c) 00011d) 001e) 0101f) 011g) 10h) 1010i) 111j) 111100

e

f

g

h

i

j

0 1

a b c

d

kk) 11110001

4-72

Lookup Performance Required

Line Line Rate Pkt size=40B Pkt size=240B

T1 1.5Mbps 4.68 Kpps 0.78 Kpps

OC3 155Mbps 480 Kpps 80 Kpps

OC12 622Mbps 1.94 Mpps 323 Kpps

OC48 2.5Gbps 7.81 Mpps 1.3 Mpps

OC192 10 Gbps 31.25 Mpps 5.21 Mpps

4-73

Discussion

Why was the Internet Protocol designed this way? Why connectionless, datagram, best-effort? Why not automatic retransmissions? Why fragmentation in the network?

Must the Internet address be hierarchical? What address does a mobile host have? Are there other ways to design networks?

4-74

network layer dr. yingwu zhu 4-1. network layer r introduction r datagram networks r ip: internet...

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