1. Network Layer
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3. Layer 3 Functionalities
The Network layer provides services to exchange the individual pieces of data over the network between identified end devices.
To accomplish this end-to-end transport, Layer 3 uses four basic processes:
Addressing
Encapsulation
Routing
De-capsulation
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4. Layer 3 Functionalities
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5. Addressing & Routing Addressing Routing
Network layer must provide a mechanism for addressing end devices.
Routing
The packet might have to travel through many different networks
Network layer must direct packets to their destination host
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6. Encapsulation & De-capsulation
Layer 3 receives the Layer 4 PDU and adds a Layer 3 header to create the Layer 3 PDU
the packet is sent down to the Data Link layer to be prepared for transportation over the media
Operating without regard to the application data carried in each packet allows the Network layer to carry packets for multiple types of communications between multiple hosts
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7. Examples of Network Layer Protocols
Internet Protocol version 4 (IPv4)
Most widely used protocol
Internet Protocol version 6 (IPv6)
Novell Internetwork Packet Exchange (IPX)
AppleTalk
Connectionless Network Service (CLNS/DECNet)
an OSI Network Layer service that is not used on the Internet
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8. Basic Characteristics of IPv4
Connectionless
No connection is established before sending data packets.
Best Effort (unreliable)
No overhead is used to guarantee packet delivery.
Media Independent
Operates independently of the medium (copper or fiber) carrying the data.
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9. IPv6
IP version 6 (IPv6) is developed and being implemented in some areas.
IPv6 will operate alongside IPv4 and may replace it in the future
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10. Connectionless Service
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11. Connectionless Service
Recall how TCP operates?
Because IP is connectionless …
it requires no initial exchange of control information to establish an end-to-end connection before packets are forwarded
nor does it require additional fields in the PDU header to maintain this connection.
This process greatly reduces the overhead of IP.
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12. Connectionless Service
Connectionless packet delivery may result in packets arriving at the destination out of sequence.
If out-of-order or missing packets create problems for the application using the data, then upper layer services will have to resolve these issues.
Does TCP take care of this?
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13. Best Effort Service (unreliable)
IP is often referred to as an unreliable protocol.
Unreliable in this context does not mean that IP works properly sometimes and does not function well at other times.
Nor does it mean that it is unsuitable as a data communications protocol.
Unreliable means simply that IP does not have the capability to manage, and recover from, undelivered or corrupt packets.
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14. Best Effort Service
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15. Unreliable
Since protocols at other layers can manage reliability, IP is allowed to function very efficiently at the Network layer.
If we included reliability overhead in our Layer 3 protocol, then …
communications that do not require connections or reliability would be burdened with the bandwidth consumption and delay produced by this overhead.
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16. Unreliable?
The key point is to leave the decision in providing reliable or unreliable services to the upper layer
E.g., TCP, or … YOU!
Network layer can concentrate on what it is designed to do …
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17. Media Independent
IPv4 and IPv6 operate independently of the media that carry the data at lower layers of the protocol stack
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18. Media Independent
There is one major characteristic of the media that the Network layer needs to consider:
Maximum Transmission Unit (MTU) : maximum size of PDU each medium can transport
The Data Link layer passes the MTU upward so that the Network layer can determine how large to create the packets.
An intermediary device - usually a router - will need to split up a packet when forwarding it from one media to a media with a smaller MTU.
This process is called fragmenting the packet or fragmentation.
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19. Details of IP protocol …
Encapsulation & De-capsulation
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20. Encapsulating IPv4 packages
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21. IPv4 Header
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22. Key fields of IPv4 Header
IP Address
Source & Destination Address
Time-to-Live (TTL)
Type-of-Service (ToS)
Protocol
Fragment Offset
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23. Time-to-Live
The Time-to-Live (TTL) is an 8-bit binary value that indicates the remaining "life" of the packet.
TTL value is decreased by at least one each time the packet is processed by a router (that is, each hop).
When the value becomes zero, the router discards or drops the packet
This mechanism prevents packets that cannot reach their destination from being forwarded indefinitely between routers in a routing loop. (e.g., routing loops)
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24. Protocol
This 8-bit binary value indicates the data payload type that the packet is carrying.
enables the Network layer to pass the data to the appropriate upper-layer protocol.
Example values are:
01 ICMP
06 TCP
17 UDP
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25. Type-of-Service
The field contains an 8-bit binary value that is used to determine the priority of each packet.
This value enables a Quality-of-Service (QoS) mechanism to be applied to high priority packets, such as those carrying telephony voice data.
The router can be configured to decide which packet it is to forward first based on the Type-of-Service value.
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26. Fragmentation-related Fields
Fragment Offset, 13-bit
Flag
More Fragments flag (MF), 1-bit
Don't Fragment flag, 1-bit
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27. Fragment Offset
A router may have to fragment a packet when forwarding it from one medium to another medium that has a smaller MTU.
When it occurs, the IPv4 packet uses the Fragment Offset field and the MF flag to reconstruct the packet when it arrives at the destination host.
The field identifies the order in which to place the packet fragment in the reconstruction.
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28. More Fragments flag
The flag (MF) is used with the Fragment Offset for the fragmentation and reconstruction of packets.
MF = 1
it examines the Fragment Offset to see where this fragment is to be placed in the reconstructed packet.
MF = 0 and a non-zero value in the Fragment offset
it places that fragment as the last part of the reconstructed packet.
An un-fragmented packet has all zero fragmentation information (MF = 0, fragment offset =0).
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29. Don't Fragment flag
The flag (DF) indicates that fragmentation of the packet is not allowed.
If the Don't Fragment flag bit is set (=1), then fragmentation of this packet is NOT permitted.
If a router needs to fragment a packet to allow it to be passed downward to the Data Link layer but the DF bit is set to 1, then the router will discard this packet.
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30. Other IPv4 Header Fields
Version - Contains the IP version number (4).
Header Length (IHL) - Specifies the size of the packet header.
Packet Length - This field gives the entire packet size, including header and data, in bytes.
Identification - This field is primarily used for uniquely identifying fragments of an original IP packet.
Header Checksum - The checksum field is used for error checking the packet header.
Options - There is provision for additional fields in the IPv4 header to provide other services but these are rarely used.
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31. Example of IPv4 Packet header length (in 32-byte unit) packet length
size (in byte)
Example of IPv4 Packet
TTL
TCP
original packet identifier (required for fragmented)
denotes packet can be
fragmented if required
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32. Details of IP protocol …
Addressing & Grouping of networks
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33. Networks – separating hosts into common hosts
One of the major roles of the Network layer - provide a mechanism for addressing hosts
As the number of hosts on the network grows, more planning is required to manage and address the network.
Rather than having all hosts everywhere connected to one vast global network, it is more practical and manageable to group hosts into specific networks.
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34. Dividing Networks
IP-based networks have their roots as one large network.
As this single network grew, the large network was separated into smaller networks that were interconnected.
These smaller networks are often called subnetworks or subnets.
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35. Dividing Networks
Networks can be grouped based on factors that include:
Geographic location
Purpose (e.g., 部門)
Ownership
etc
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36. Why separating networks?
Performance
Security
Address management
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37. Why separating networks?  Performance
Compare this …
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38. Why separating networks?  Performance
… and this.
broadcast blocking
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39. Why separating networks?  Security
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40. Why separating networks?  Address Management
Reduces the unnecessary overhead of all hosts needing to know all addresses
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41. Hierarchical Addressing & Grouping of Networks
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42. Details of IP protocol …
Routing
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43. Gateway
As a part of its configuration, a host has a default gateway address defined.
This gateway address is the address of a router interface that is connected to the same network as the host.
To communicate with a device on another network, a host uses the address of this gateway, or default gateway, to forward a packet outside the local network.
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44. Default Gateway
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45. Use “ipconfig” to see your IP settings
In Unix, use “ifconfig”
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46. Gateway enables communications between networks
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47. Gateway
The router also needs a route that defines where to forward the packet next.
This is called the next-hop address.
If a route is available to the router, the router will forward the packet to the next-hop router that offers a path to the destination network.
Routing
See next few slides …
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50. which next hop to send? routing table
How does router know
which next hop to send? routing table
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51. Routing Router
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52. Routing Router
Default Route
In case a packet is destined for , it will be
forwarded to
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53. Routing Table @ End host (“netstat –r” printout)
In Unix, use
“route PRINT”
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54. Packets Routing Process
De-capsulation
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55. What if router has no entry for destined network?
Default route configured
Router forwards packet according to default route setting
No default route configured
Router drops the packet
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56. How do routers learn & build their routing table?
through Routing protocols
Protocols that share routes information among routers
Routing protocols can be:
Static routing
Dynamic routing
Routing Information Protocol (RIP)
Enhanced Interior Gateway Routing Protocol (EIGRP)
Open Shortest Path First (OSPF)
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57. Static Routing
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58. Dynamic Routing
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59. Dynamic vs Static Dynamic routing overhead Cost of static routing
Consumes network bandwidth
Consumes CPU processing capacity
Cost of static routing
Administrative cost
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60. The reality is …
In many internetworks, a combination of static, dynamic, and default routes are used to provide the necessary routes.
The configuration of routing protocols on routers will be covered extensively by a later course.
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