CMPT 471: Networking II�Network Layer: Overview��Mohamed Hefeeda

An IP Packet Journey

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An IP Packet Journey

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An IP Packet Journey

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An IP Packet Journey

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An IP Packet Journey

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An IP Packet Journey

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An IP Packet Journey

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What happens between two routers

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Data Plane

VAN

EDM

Data Plane

IF 1

IF 2

IF 3

IF 4

IF 1

IF 2

IF 3

IF 4

Redundancy

What happens between two routers

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Data Plane

VAN

EDM

Data Plane

IF 1

IF 2

IF 3

IF 4

IF 1

IF 2

IF 3

IF 4

What happens between two routers

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Data Plane

VAN

EDM

Data Plane

IF 1

IF 2

IF 3

IF 4

IF 1

IF 2

IF 3

IF 4

What happens between two routers

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In the current Internet, forwarding happens by:

• examining the destination address, and
• matching it with a local forwarding table

But, who calculates the forwarding tables?

This is called Packet Forwarding

• moving packets from router’s input to appropriate router output
• done by the data-plane component

Routers Have “Brains”

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Control Plane

Control Plane

Data Plane

Data Plane

This brain is called the Control Plane

VAN

EDM

Routers Have “Brains”

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Control Plane

Control Plane

The control plane runs a routing algorithm to:

• find routes, and
• fill the tables

VAN

EDM

Routing algorithm

Routing algorithm

Control Plane: Two Approaches

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Control Plane

Control Plane

VAN

EDM

Routing algorithm

Routing algorithm

Control Plane

REG

Routing algorithm

Distributed Approach: routers exchange messages with each other to calculate tables

• Examples: OSPF, ISIS

…

…

Control Plane: Two Approaches

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Agent

Agent

VAN

Agent

REG

Centralized Approach: routers exchange messages with an external software

• Software-defined networking (SDN)
• Examples: OpenFlow

…

…

Control Plane

Forwarding vs Routing

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Network layer (or routers) has two functions:

• Forwarding
• Routing

Forwarding vs Routing

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Network layer (or routers) has two functions:

• Forwarding
• Routing

Network Layer: Overview

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IP is the waist of the “hourglass”

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• Multiple higher-layer protocols
• Transport and Application

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• Multiple lower-layer protocols
• Link and Physical

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• Single Internet protocol

🡪 No need to update routers and hosts every time we have new: service, device, or link technology, …!

HTTP, FTP, DNS, SMTP, …

TCP, UDP, …

IP

Copper, fiber, radio

Ethernet, PPP, …

CSMA, SONET, …

At every router/host

IPv4 Datagram Format

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Fragmentation

Header & data

Addressing

E.g., TCP segment

ICMP 0x01

TCP 0x06

UDP 0x11

IPv6 0x29

Size: 20 bytes (min)

IPv4 Fragmentation

• Different link layer protocols have different MTU
• MTU: Maximum transmission unit

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• A router can break a datagram into fragments
• If MTU of outgoing link is less than pkt size

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• Destination reassembles IP fragments
• To be delivered to transport layer
• Why is reassembly done at destination?
• Reduce load on routers. Packet can be fragmented multiple times on the path

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IPv4 Fragmentation

• Example:

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Router A

Router B

MTU = 4000

MTU = 1500

4000 bytes

1500 bytes

1500 bytes

1040 bytes

Fragment 1

Fragment 2

Fragment 3

4000 bytes

IPv4 Fragmentation

• Example:

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3980 B

20 B

IPv4 Fragmentation

• Issues
• All fragments must be delivered to destination 🡪 increases likelihood of packet dropping!
• Last fragment may have non-optimal size 🡪 wasting router resources
• Destination needs to hold IP fragments in memory
• Only first datagram contains TCP/UDP header
• Firewalls and other network functions may not work well with IP fragments (some needs to keep state about TCP segments)

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• In the current Internet, fragmentation is not recommended
• IPv6 does not support fragmentation

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Time-to-live (TTL)

• Max. number of traversed hops
• Before a datagram is dropped (Why?)

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• TTL value is set by the source
• Linux/Mac 64
• Windows 128
• Solaris, Cisco IOS 255

​

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Often used in OS Fingerprinting tools

Time-to-live (TTL)

• When a router receives an IP datagram:
• Decrement TTL by 1
• If TTL is 0 🡪 drop pkt
• Else forward pkt

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• Does router need to recalculate checksum?
• Yes. (TTL is part of the IP header)

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IPv4 Addressing

• IP address: 32-bit identifier for an interface
• Interface: connection between host/router and physical link

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• A router typically has multiple (many) interfaces
• A host typically has one or two interfaces

​

​

​

An IP addresses is associated with each interface

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IPv4 Addressing

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IPv4 Addressing: Subnets

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subnet

Subnets

• IP address:
• subnet part: high order bits
• host part: low order bits

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• What’s a subnet ?
• interfaces that can reach each other without intervening router

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Subnet

Subnets

• How many subnets?
• 6

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• Recipe
• to determine the subnets, detach each interface from its host or router, creating islands of isolated networks
• each isolated network is a subnet

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223.1.9.2

223.1.9.1

223.1.7.0

223.1.7.1

223.1.8.0

223.1.8.1

IPv4 Addressing: CIDR

• CIDR: Classless Inter Domain Routing
• IP address is composed of
• a subnet part (or prefix)
• a host part (or suffix)
• Address format: a.b.c.d/x, where x is # bits in subnet portion (called mask)

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11001000 00010111 00010000 00000000

200.23.16.0/24

IPv4 Addressing: CIDR

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11001000.00010111.00010000. 00000000 200.23.16.0

200.23.16.0/24

/24 bits means that we have 8 bits to use for hosts

Subnet part (Prefix) Host part (Suffix) IP address

11001000.00010111.00010000. 00000001 200.23.16.1

11001000.00010111.00010000. 00000010 200.23.16.2

11001000.00010111.00010000. 11111110 200.23.16.254

11001000.00010111.00010000. 11111111 200.23.16.255

…

IPv4 Addressing: CIDR

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11001000.00010111.00010000. 00000000 200.23.16.0

Subnet part (Prefix) Host part (Suffix) IP address

11001000.00010111.00010000. 11111111 200.23.16.255

In practice, the first and last IP addresses of a prefix are reserved

🡪 /24 can support up to 254 (=256-2) hosts

How to get an IP address?

How does a host get IP address?

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• Hard-coded by system admin in a file
• DHCP: Dynamic Host Configuration Protocol
• dynamically get address from a server

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How to get an IP address?

How does a network get IP address space?

• Gets allocated portion of its provider ISP’s address space

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How to get an IP address?

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ISP A block 11001000 00010111 00010000 00000000 200.23.16.0/20

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Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23

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Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23

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Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23

…

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

Example: Given an ISP network called A with address 200.23.16.0/20.

How can it allocate IP addresses for 8 customer networks?

Hierarchical IP Addressing

• IP addresses are hierarchical

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200.23.16.0/20

200.23.16.0/23

200.23.18.0/23

200.23.20.0/23

200.23.30.0/23

…

This is ISP A

The eight customer networks

Hierarchical IP Addressing

• Hierarchical addressing allows efficient advertisement of routing information:

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“Send me anything

with addresses

beginning

200.23.16.0/20”

ISP A

Organization 0

Organization 7

Internet

Organization 1

Organization 2

200.23.16.0/20

Hierarchical IP Addressing

• Hierarchical addressing allows efficient advertisement of routing information:
• 🡺 forwarding tables have 100s of thousands entries instead of billions

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“Send me anything

with addresses

beginning

200.23.16.0/20”

ISP A

Organization 0

Organization 7

Internet

Organization 1

ISP B

“Send me anything

with addresses

beginning

199.31.0.0/16

or 200.23.18.0/23”

Organization 2

200.23.16.0/20

199.31.0.0/16

Organization 1 moves to ISP B

Hierarchical IP Addressing

• Routers forward a packet to its destination based on the subnet part, not the host part
• use longest address prefix that matches destination address
• This is called the longest prefix matching

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Longest prefix match

Hierarchical IP Addressing: Summary

• Scalable forwarding tables

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• Adding/removing hosts without modifying forwarding table

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• Small prefix advertisement overhead

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IPv6

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

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• Additional motivation:
• header format helps speed processing/forwarding
• header changes to facilitate QoS

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• IPv6 datagram format:
• fixed-length 40-byte header
• no fragmentation allowed

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IPv6 Datagram Format

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Priority/Traffic Class: identify priority among datagrams in flow

Flow Label: identify datagrams in same “flow”

Next header: identify upper layer protocol for data

Other Changes

• Checksum: removed entirely to reduce processing time at each hop
• Options: allowed, but outside of header, indicated by “Next Header” field
• No Fragmentation:
• Packet is dropped if its size is larger than outgoing link MTU
• An error message is sent to the sender

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IPv4 🡪 IPv6

• Not all routers can be upgraded simultaneously
• how will network operate with mixed IPv4 and IPv6 routers?

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• Tunneling:
• IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers

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IPv4 🡪 IPv6: Tunneling

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IPv4 tunnel

connecting IPv6 routers

A

B

E

F

IPv6

IPv6

IPv6

IPv6

Logical View

Physical View

A

B

E

F

IPv6

IPv6

IPv6

IPv6

C

D

IPv4

IPv4

IPv6 Deployment

• It is hard to change the network-layer protocols!
• IPv6 was first introduced in 1995!

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Percentage of users accessing Google using IPv6: Source

Network Layer--Data Plane: Summary

• Network layer: forwarding vs. routing
• Control plane vs data plane
• Distributed control plane (traditional) vs centralized control plane (new)
• IPv4, IPv6
• Tunnelling to allow incremental deployment of IPv6
• Subnets and hierarchical addressing
• Allows routing at large scale (much fewer entries in routing tables)

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