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Network Layer 2017/5/25 1 2017/5/25 2 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: – – – – 2017/5/25 Addressing Encapsulation Routing De-capsulation 3 Layer 3 Functionalities 2017/5/25 4 Addressing & Routing • Addressing – 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 2017/5/25 5 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 2017/5/25 6 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 2017/5/25 7 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. 2017/5/25 8 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 2017/5/25 9 Connectionless Service 2017/5/25 10 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. 2017/5/25 11 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? 2017/5/25 12 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. 2017/5/25 13 Best Effort Service 2017/5/25 14 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. 2017/5/25 15 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 … 2017/5/25 16 Media Independent IPv4 and IPv6 operate independently of the media that carry the data at lower layers of the protocol stack 2017/5/25 17 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. 2017/5/25 18 Details of IP protocol … Encapsulation & De-capsulation 2017/5/25 19 Encapsulating IPv4 packages 2017/5/25 20 IPv4 Header 2017/5/25 21 Key fields of IPv4 Header • IP Address – Source & Destination Address • • • • 2017/5/25 Time-to-Live (TTL) Type-of-Service (ToS) Protocol Fragment Offset 22 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) 2017/5/25 23 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 2017/5/25 24 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 Typeof-Service value. 2017/5/25 25 Fragmentation-related Fields • Fragment Offset, 13-bit • Flag – More Fragments flag (MF), 1-bit – Don't Fragment flag, 1-bit 2017/5/25 26 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. 2017/5/25 27 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). 2017/5/25 28 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. 2017/5/25 29 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. 2017/5/25 30 header length (in 32-byte unit) Example of IPv4 Packet TTL original packet identifier (required for fragmented) 2017/5/25 packet length size (in byte) TCP denotes packet can be fragmented if required 31 Details of IP protocol … Addressing & Grouping of networks 2017/5/25 32 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. 2017/5/25 33 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. 2017/5/25 34 Dividing Networks • Networks can be grouped based on factors that include: – Geographic location – Purpose (e.g., 部門) – Ownership – etc 2017/5/25 35 Why separating networks? • Performance • Security • Address management 2017/5/25 36 Why separating networks? Performance Compare this … 2017/5/25 37 … and this. Why separating networks? Performance broadcast blocking 2017/5/25 38 Why separating networks? Security 2017/5/25 39 Why separating networks? Address Management Reduces the unnecessary overhead of all hosts needing to know all addresses 2017/5/25 40 Hierarchical Addressing & Grouping of Networks 2017/5/25 41 Details of IP protocol … Routing 2017/5/25 42 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. 2017/5/25 43 Default Gateway 2017/5/25 44 Use “ipconfig” to see your IP settings In Unix, use “ifconfig” 2017/5/25 45 Gateway enables communications between networks 2017/5/25 46 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 … 2017/5/25 47 2017/5/25 48 2017/5/25 49 How does router know which next hop to send? routing table 2017/5/25 50 Routing Table @ Router 2017/5/25 51 Routing Table @ Router Default Route In case a packet is destined for 10.1.2.100, it will be forwarded to 192.168.2.2 2017/5/25 52 Routing Table @ End host (“netstat –r” printout) In Unix, use “route PRINT” 2017/5/25 53 Packets Routing Process De-capsulation 2017/5/25 54 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 2017/5/25 55 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) 2017/5/25 56 Static Routing 2017/5/25 57 Dynamic Routing 2017/5/25 58 Dynamic vs Static • Dynamic routing overhead – Consumes network bandwidth – Consumes CPU processing capacity • Cost of static routing – Administrative cost 2017/5/25 59 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. 2017/5/25 60