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Chapter 4 Network Layer Chapter 4: Network Layer Chapter goals: understand principles behind network layer services: network layer service models forwarding versus routing how a router works routing (path selection) dealing with scale IPv6 instantiation, implementation in the Internet

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chapter 4 network layer
Chapter 4: Network Layer

Chapter goals:

  • understand principles behind network layer services:
    • network layer service models
    • forwarding versus routing
    • how a router works
    • routing (path selection)
    • dealing with scale
    • IPv6
  • instantiation, implementation in the Internet
chapter 4 network layer3
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

4.5 Routing algorithms

Link state

Distance Vector

Hierarchical routing

4.6 Routing in the Internet

RIP

OSPF

BGP

Chapter 4: Network Layer
network layer
transport segment from sending to receiving host

on sending side encapsulates segments into datagrams

on rcving side, delivers segments to transport layer

network layer protocols in every host and router

router examines header fields in all IP datagrams passing through it

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

application

transport

network

data link

physical

application

transport

network

data link

physical

Network layer
two key network layer functions
Two 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
slide6

routing algorithm

local forwarding table

header value

output link

0100

0101

0111

1001

3

2

2

1

value in arriving

packet’s header

1

0111

2

3

Interplay between routing and forwarding

chapter 4 network layer7
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

4.5 Routing algorithms

Link state

Distance Vector

Hierarchical routing

4.6 Routing in the Internet

RIP

OSPF

BGP

Chapter 4: Network Layer
router architecture overview
Router Architecture Overview

Two key router functions:

  • run routing algorithms/protocol (RIP, OSPF, BGP)
  • forwarding datagrams from incoming to outgoing link
input port functions
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

see chapter 5

output ports
Output Ports
  • Buffering required when datagrams arrive from fabric faster than the transmission rate
  • Scheduling discipline chooses among queued datagrams for transmission
output port queueing
Output port queueing
  • buffering when arrival rate via switch exceeds output line speed
  • queueing (delay) and loss due to output port buffer overflow!
how much buffering

.

RTT C

N

How much buffering?
  • RFC 3439 rule of thumb: average buffering equal to “typical” RTT (say 250 msec) times link capacity C
    • e.g., C = 10 Gps link: 2.5 Gbit buffer
  • Recent recommendation: with N flows, buffering equal to
input port queuing
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!
chapter 4 network layer15
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

4.5 Routing algorithms

Link state

Distance Vector

Hierarchical routing

4.6 Routing in the Internet

RIP

OSPF

BGP

Chapter 4: Network Layer
the internet network layer
Host, router network layer functions:
  • ICMP protocol
  • error reporting
  • router “signaling”
  • IP protocol
  • addressing conventions
  • datagram format
  • packet handling conventions
  • Routing protocols
  • path selection
  • RIP, OSPF, BGP

forwarding

table

The Internet Network layer

Transport layer: TCP, UDP

Network

layer

Link layer

physical layer

chapter 4 network layer17
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

4.5 Routing algorithms

Link state

Distance Vector

Hierarchical routing

4.6 Routing in the Internet

RIP

OSPF

BGP

Chapter 4: Network Layer
ip datagram format

IP protocol version

number

header length

(bytes)

for

fragmentation/

reassembly

“type” of data

max number

remaining hops

(decremented at

each router)

IP datagram format

32 bits

total datagram

length (bytes)

type of

service

head.

len

ver

length

fragment

offset

flgs

16-bit identifier

upper

layer

time to

live

header

checksum

32 bit source IP address

32 bit destination IP address

upper layer protocol

to deliver payload to

E.g. timestamp,

record route

taken, specify

list of routers

to visit. Typically, these are ignored

Options (if any)

how much overhead with TCP?

  • 20 bytes of TCP
  • 20 bytes of IP
  • = 40 bytes + app layer overhead

data

(variable length,

typically a TCP

or UDP segment)

ip fragmentation reassembly
network links have MTU (max.transfer size) - largest possible link-level frame.

different link types, different MTUs

E.g., ethernet allows 1500B frames

802.11 allows 2346B frames

It would be very difficult for the end host to know the correct packet size

Note that larger packets are more efficient (less bandwidth is consumed by the header)

Large IP datagram divided (“fragmented”) within the network

one datagram becomes several datagrams

“reassembled” only at final destination

IP header bits used to identify, order related fragments

IP Fragmentation & Reassembly

fragmentation:

in: one large datagram

out: 3 smaller datagrams

reassembly

ip fragmentation and reassembly

length

=1500

length

=1500

length

=4000

length

=1040

ID

=x

ID

=x

ID

=x

ID

=x

fragflag

=0

fragflag

=1

fragflag

=1

fragflag

=0

offset

=0

offset

=0

offset

=370

offset

=185

One large datagram becomes

several smaller datagrams

IP Fragmentation and Reassembly

Example

  • 4000 byte datagram
  • MTU = 1500 bytes

1480 bytes in data field

offset =

1480/8

stealthy scanning
Stealthy Scanning
  • Before attacking a network, one must learn which hosts are present.
    • That is, which IP addresses have host that are running various services (e.g., listening on various TCP ports)
  • This is done by scanning. For example, sending an ICMP ping message to random IP address or sending TCP-SYN messages
    • What happens if a host receives an TCP-SYN on a port that is not listening
    • It depends on the OS, but the typically, a TCP-RST packet is generated
  • ISPs (e.g., UD) will look for scanners and take action (e.g., disconnect them)
  • So what is an attacker to do?
stealthy scanning22
Stealthy Scanning

victim

If victim exists and port is open: TCP-SYN-ACK

Some machine is confused (it didn’t send a TCP-SYN)

TCP-RST with IP-ID = X + 1

SomeMachine

ICMP echo-request (ping)

TCP-SYN: Dest=Victim, Source=SomeMachine

attacker

ICMP echo reply with IP-ID = X+2

ICMP echo reply with IP-ID = X

Since the IP-ID incremented by 2, the victim must have sent a SYN-ACK.

If the IP-ID only incremented by 1, then the victim is not listening on the port, or does not exist

Attacker records IP-ID=X

chapter 4 network layer23
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

4.5 Routing algorithms

Link state

Distance Vector

Hierarchical routing

4.6 Routing in the Internet

RIP

OSPF

BGP

Chapter 4: Network Layer
ip addressing introduction
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

IP address can be associated with an internal interface (e.g., a primary IP address) when multiple interfaces exist

223.1.1.2

223.1.2.1

223.1.3.27

223.1.3.1

223.1.3.2

223.1.2.2

IP Addressing: introduction

223.1.1.1

223.1.2.9

223.1.1.4

223.1.1.3

223.1.1.1 = 11011111 00000001 00000001 00000001

223

1

1

1

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

Subnets

223.1.1.1

223.1.2.1

223.1.1.2

223.1.2.9

223.1.1.4

223.1.2.2

223.1.1.3

223.1.3.27

subnet

223.1.3.2

223.1.3.1

network consisting of 3 subnets

subnets26
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.

223.1.1.0/24

223.1.2.0/24

223.1.3.0/24

Subnets

Subnet mask: /24

subnets27
How many?Subnets

223.1.1.2

223.1.1.1

223.1.1.4

223.1.1.3

223.1.7.0

223.1.9.2

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

ip addressing cidr
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 part or CIDR-block

host

part

11001000 0001011100010000 00000000

200.23.16.0/23

ip addresses how to get one
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

hierarchical addressing route aggregation

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

200.23.20.0/23

.

.

.

.

.

.

Hierarchical addressing: route aggregation

Hierarchical addressing allows efficient advertisement of routing

information:

Organization 0

Organization 1

“Send me anything

with addresses

beginning

200.23.16.0/20”

Organization 2

ISP1

Border Router

Internet

Organization 7

“Send me anything

with addresses

beginning

199.31.0.0/16”

ISP2

This way, the whole 32 bit address does not need to be examined

hierarchical addressing more specific routes

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

200.23.20.0/23

.

.

.

.

.

.

Hierarchical addressing: more specific routes

ISP2 has a more specific route to Organization 1

Organization 0

“Send me anything

with addresses

beginning

200.23.16.0/20”

Organization 2

ISP1

Border Router

Internet

Organization 7

“Send me anything

with addresses

beginning 199.31.0.0/16

or 200.23.18.0/23”

ISP2

Organization 1

longest prefix matching
Longest prefix matching

Border Router Forwarding Table

Prefix MatchLink Interface

200.23.16.0/20 0

200.23.18.0/23 1

199.31.0.0/16 1

otherwise 2

If a packet with destination address 200.23.18.12 arrives at the boarder router, then is it forwarding to interface 0 or 1?

Since interface 1 has a longer match, it goes to interface 1

ip addressing the last word
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
nat network address translation
NAT: Network Address Translation

rest of

Internet

local network

(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

Datagrams with source or

destination in this network

have 10.0.0/24 address for

source, destination (as usual)

All datagrams leaving local

network have same single source NAT IP address: 138.76.29.7,

different source port numbers

nat network address translation35
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 translation36
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 translation37

3

1

2

4

S: 10.0.0.1, 3345

D: 128.119.40.186, 80

S: 138.76.29.7, 5001

D: 128.119.40.186, 80

1: host 10.0.0.1

sends datagram to

128.119.40.186, 80

2: NAT router

changes datagram

source addr from

10.0.0.1, 3345 to

138.76.29.7, 5001,

updates table

S: 128.119.40.186, 80

D: 10.0.0.1, 3345

S: 128.119.40.186, 80

D: 138.76.29.7, 5001

NAT: Network Address Translation

NAT translation table

WAN side addr LAN side addr

138.76.29.7, 5001 10.0.0.1, 3345

…… ……

10.0.0.1

10.0.0.4

10.0.0.2

138.76.29.7

10.0.0.3

4: NAT router

changes datagram

dest addr from

138.76.29.7, 5001 to 10.0.0.1, 3345

3: Reply arrives

dest. address:

138.76.29.7, 5001

nat network address translation38
NAT: Network Address Translation
  • 16-bit port-number field:
    • 65,000 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
      • The NAT must know about TCP and UDP. What about other transport protocols?
    • address shortage should instead be solved by IPv6
nat traversal problem
NAT traversal problem
  • client wants to connect to server with address 10.0.0.1
    • server address 10.0.0.1 local to LAN (client can’t use it as destination addr)
    • only one externally visible NATted address: 138.76.29.7
  • solution 1: statically configure NAT to forward incoming connection requests at given port to server
    • e.g., (123.76.29.7, port 2500) always forwarded to 10.0.0.1 port 25000

10.0.0.1

Client

?

10.0.0.4

138.76.29.7

NAT

router

nat traversal problem40
NAT traversal problem
  • solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATted host to:
    • learn public IP address (138.76.29.7)
    • add/remove port mappings (with lease times)

i.e., automate static NAT port map configuration

10.0.0.1

IGD

10.0.0.4

138.76.29.7

NAT

router

nat traversal problem41

10.0.0.1

NAT

router

NAT traversal problem
  • solution 3: relaying (used in Skype)
    • NATed client establishes connection to relay
    • External client connects to relay
    • relay bridges packets between to connections

2. connection to

relay initiated

by client

1. connection to

relay initiated

by NATted host

3. relaying

established

Client

138.76.29.7

ip addresses how to get one42
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 as server
    • “plug-and-play”
dhcp dynamic host configuration protocol
DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamically obtain its IP address from network server when it joins network

Can renew its lease on address in use

Allows reuse of addresses (only hold address while connected an “on”)

Support for mobile users who want to join network (more shortly)

DHCP overview:

  • host broadcasts “DHCP discover” msg
  • DHCP server responds with “DHCP offer” msg
  • host requests IP address: “DHCP request” msg
  • DHCP server sends address: “DHCP ack” msg
dhcp client server scenario

E

B

A

DHCP client-server scenario

223.1.2.1

DHCP

223.1.1.1

server

223.1.1.2

223.1.2.9

223.1.1.4

223.1.2.2

arriving DHCP

client needs

address in this

network

223.1.1.3

223.1.3.27

223.1.3.2

223.1.3.1

dhcp client server scenario45

DHCP discover

src : 0.0.0.0, port: 68

dest.: 255.255.255.255, port: 67

yiaddr: 0.0.0.0

transaction ID: 654

DHCP client-server scenario

arriving

client

DHCP server: 223.1.2.5

DHCP offer

src: 223.1.2.5, port: 67

dest: 255.255.255.255, port: 68

yiaddrr: 223.1.2.4

transaction ID: 654

Lifetime: 3600 secs

DHCP request

src: 0.0.0.0, port: 68

dest:: 255.255.255.255, port: 67

yiaddrr: 223.1.2.4

transaction ID: 655

Lifetime: 3600 secs

time

DHCP ACK

src: 223.1.2.5, port: 67

dest: 255.255.255.255, port: 68

yiaddrr: 223.1.2.4

transaction ID: 655

Lifetime: 3600 secs

chapter 4 network layer46
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

4.5 Routing algorithms

Link state

Distance Vector

Hierarchical routing

4.6 Routing in the Internet

RIP

OSPF

BGP

Chapter 4: Network Layer
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

ICMP: Internet Control Message Protocol

TypeCodedescription

0 0 echo reply (ping)

3 0 dest. network unreachable

3 1 dest host unreachable

3 2 dest protocol unreachable

3 3 dest port unreachable

3 6 dest network unknown

3 7 dest host unknown

4 0 source quench (congestion

control - not used)

8 0 echo request (ping)

9 0 route advertisement

10 0 router discovery

11 0 TTL expired

12 0 bad IP header

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 (might) send 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 (might) return ICMP “host unreachable” packet (type 3, code 3)

When source gets this ICMP, stops.

Traceroute and ICMP
icmp ping flood
ICMP ping flood
  • Send many ICMP ping messages to a web server
  • The server will not be able to respond fast enough, and hence not be able to provide is primary service
    • Denial of service attack (DoS)
  • DDoS (distributed DoS). Many hosts send ICMP ping messages to a web server
  • One defense is to filter out messages from hosts that send too many ICMP messages
  • So, attackers send ICMP messages, but with a random source address.
  • Or attackers can send ICMP messages to random hosts but with the source address of the victim
  • One defense is to filter all ICMP messages
chapter 4 network layer50
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

4.5 Routing algorithms

Link state

Distance Vector

Hierarchical routing

4.6 Routing in the Internet

RIP

OSPF

BGP

Chapter 4: Network Layer
slide51
IPv6
  • Initial motivation:32-bit address space soon to be completely allocated.
  • Additional motivation:
    • header format helps speed processing/forwarding
    • header changes to facilitate QoS

IPv6 datagram format:

    • fixed-length 40 byte header
    • no fragmentation allowed
ipv6 header cont
IPv6 Header (Cont)

Priority: identify priority among datagrams in flow, like TOS in IPv4

Flow Label: identify datagrams in same “flow.” (concept of“flow” not well defined).

Next header: identify upper layer protocol for data (like protocol number in IPv4)

128 bit address permits 5×10^28 addressed for each person on the planet

other changes from ipv4
Other Changes from IPv4
  • Checksum:removed entirely to reduce processing time at each hop
  • Fragmentation: removed, but new ICMP messages
  • Options: allowed, but outside of header, indicated by “Next Header” field
  • ICMPv6: new version of ICMP
    • additional message types, e.g. “Packet Too Big”
    • multicast group management functions
transition from ipv4 to ipv6
Transition From IPv4 To IPv6
  • Not all routers can be upgraded simultaneous
    • no “flag days”
    • How will the network operate with mixed IPv4 and IPv6 routers?
  • Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers
tunneling

A

B

E

F

F

E

B

A

tunnel

Logical view:

IPv6

IPv6

IPv6

IPv6

Physical view:

IPv6

IPv6

IPv6

IPv6

IPv4

IPv4

Tunneling
tunneling56

Flow: X

Src: A

Dest: F

data

Flow: X

Src: A

Dest: F

data

Flow: X

Src: A

Dest: F

data

Flow: X

Src: A

Dest: F

data

D

C

F

E

B

A

F

E

B

A

Src:B

Dest: E

Src:B

Dest: E

Tunneling

tunnel

Logical view:

IPv6

IPv6

IPv6

IPv6

Physical view:

IPv6

IPv6

IPv6

IPv6

IPv4

IPv4

A-to-B:

IPv6

E-to-F:

IPv6

B-to-C:

IPv6 inside

IPv4

B-to-C:

IPv6 inside

IPv4

chapter 4 network layer57
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

4.5 Routing algorithms

Link state

Distance Vector

Hierarchical routing

4.6 Routing in the Internet

RIP

OSPF

BGP

4.7 Broadcast and multicast routing

Chapter 4: Network Layer