PowerPoint Presentation ITECH1102 Networking and Security Topic 4 – The Data Link Layer (Layer 2) [Network Interface] 1 ITECH1102 Networking & Security 2 Last week Last week we looked at: The...

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PowerPoint Presentation ITECH1102 Networking and Security Topic 4 – The Data Link Layer (Layer 2) [Network Interface] 1 ITECH1102 Networking & Security 2 Last week Last week we looked at: The Application Layer Application layer programs are able to interact by virtue of computer networks. Examples: Web Browser (client application) can communicate with a Web Server (Server program) FTP client (user application) downloading/uploading files from/to an FTP Server (Server program) Email client accessing Email from an Email server. Phone App accessing Domino’s Pizza online. We saw many other categories of Client (user applications) communicating with other application layer programs, including: BitTorrent NTP POP & others. 2 ITECH1102 Networking & Security 3 Application to Application communications Client Application Transport Network Data Link Physical Server Application Transport Network Data Link Physical Actual communications proceeds down through each layer, across the network then up through each server layer. ITECH1102 Networking & Security 4 Last week (continued) Last week we also saw: There are many Application protocols used for data exchange Examples: HTTP is used for web traffic FTP for FTP file transfers POP for access to email Protocols define the rules by which communications can take place, for example: How data is formatted Who speaks first, then next etc. etc. ITECH1102 Networking & Security 5 Visualization of client HTTP request from Firefox. Application Transport Network Data Link User Program (Firefox) Physical Step 1: User opens browser and enters URL to a web site. HTTP request is created at the Application layer. Step 2: HTTP request is passed to the Transport layer. Transport layer information (TCP) is added. Step 3: The process continues. IP information is added at the Network layer. Step 4: Ethernet information is added at the Data Link Layer. Then the resulting frame that is sent to the network media. The reverse process occurs at the server end. Headers are removed at each layer as the data moves up through the network layers to the application layer. This is how layers on the source device communicates with its peer layer on the destination device. The process of adding headers at each Layer is called encapsulation. Each layer encapsulates the Protocol Data Unit (PDU) from the previous layer with a header. Headers pass Addressing and other information between peer layers ITECH1102 Networking & Security 6 This week – Overview of content The Data Link: The role of the Data Link Data Link in the Internet Model of networking The two parts of the Data Link The role of the Network Interface Card (NIC) Identification of NIC’s (MAC addresses) Network cards are smart devices Media Access Methods Local Address Resolution (using ARP) ITECH1102 Networking & Security 7 Role of the Data Link Layer The Data Layer only communicates with devices on its local network. Example: Another PC on your home network A Printer on your local network The Router on your local network This is often described as Hop to Hop communication. Question. Wide Area Networks often involve significant distances through fibre optic or satellite connections. Would communication between two satellite interfaces be considered local networking ? Yes!!! Why? Because the two interfaces are on the same Network. This applies to other Wide area network links for instance between Melbourne & Sydney. The Data Link is only responsible for communication between two machines that are connected to the same network. This network could be your home WiFi network, it could be a section of the university network (for instance all computers in labs 1 & 2), or it could be two machines that are connected at each end of a long fibre optic or satellite link. Hence the idea of local when discussing networks at the data link has more to do with the connection to a single network than it does to distance. Two computers connected at each end of a 1 metre cable are considered local as is two routers connected at each end of a geostationary satellite link. 7 ITECH1102 Networking & Security 8 Data Link Layer in the Internet Model The Internet Model Application (Layer 5) Transport (Layer 4) Internet (Network) (Layer 3) Network Interface (Data Link) (Layer 2) Physical (Layer 1) Data from Application programs passes down through the Transport and Internet layers. The Data Link Layer is the Interface between the Network layer and the Physical network (Cable, Fibre, WiFi, Satellite etc.) Data from the Network media passes up through the network layers to the Application on the receiving host ITECH1102 Networking & Security 9 The two parts of the Data Link layer Logical Link Layer (LLC) Media Access Control (MAC) The LLC is the interface with the upper layer. It allows IPv4, IPv6, IPX and other protocol transactions concurrently. (More on this next week) The Media Access Control layer sends traffic to the network media and receives traffic from the network media. This is the Data Link Layer ITECH1102 Networking & Security 10 The role of the Network Interface Card (NIC) The Network Interface Card is the connection to the Network. Notebooks and other devices often have multiple connections: Ethernet connection (via blue Cat 5 or Cat 6 cable) WiFi connection Bluetooth connection Network interfaces are also built into Printers, Network scanners Routers and other network devices. Routers have one NIC connected to the local network and one or more connected to other networks. ITECH1102 Networking & Security 11 Network Cards have a unique MAC (Media Access Control) address. All Network Interface cards have a unique 48 bit MAC address. The MAC address is built into each interface by the manufacturer. All Data Link data exchanges use the source and destination MAC addresses for any communication. MAC addresses consist of two parts: OUI (Organisation unique Identifier) [ first 24 bits ] Unique 24 bit number [ last 24 bits ] Hence MAC address are globally unique! ITECH1102 Networking & Security 12 How to determine your MAC address On a Windows system running the command ipconfig /all will display network setting such as: IPv4 address IPv6 address MAC address (Also called the Physical Address) ITECH1102 Networking & Security 13 Determining the MAC address on a Linux System The Linux command to determine network settings is ifconfig The output displays: MAC address (HWaddr) IPv4 address (inet addr) IPv6 address inet6 addr ITECH1102 Networking & Security 14 Network cards (NICs) are smart devices Network cards filter traffic Networks cards inspect every packet they see on the local network. Packets that are for other machines are discarded. Packets for the host device (PC, Router etc) are delivered to the host device. (To do this the NIC Interrupts the host and passes the network packet to it.) Benefits: The host machine only deals with important network traffic, not packets that other devices must process. Hence the filtering of traffic by the NIC reduces processing by the host machine and hence does not impede host performance. ITECH1102 Networking & Security 15 The NIC also checks for errors How the Cyclic Redundancy Check (CRC) works Prior to sending a network packet a NIC performs a 32 bit CRC calculation that characterises the contents of the network packet. The 32 bit CRC is appended to the end of the packet then sent to the destination. When the packet arrives at the destination its NIC does the same CRC calculation on the received data. If the two CRC values are different then the destination knows the packet is corrupt and so discards it. This type of error checking is common in networking. ITECH1102 Networking & Security 16 Ethernet Frame Format There is a strict format for Ethernet Frames. A Ethernet frame contains the payload from all upper layers which consists of headers and the upper layer protocol data (Eg. HTTP). Example: We will refer to this as DATA. A 32 bit Frame Check Sequence is attached to the end of the frame. The Destination (first) and Source MAC addresses are also required to specify where the packet came from and its destination. ITECH1102 Networking & Security 17 Format of Ethernet Frames The Destination MAC address is at the front of an Ethernet frame so the destination NIC can drop unwanted frames quickly. The DATA field is often the largest part of the frame, it contains the application payload (HTTP, FTP or other protocol information) plus any headers from upper layer protocols. ITECH1102 Networking & Security 18 A network packet captured in Wireshark This is a Ping packet. It is 74 bytes in length. The first 14 are the Ethernet frame. These 6 bytes are the Ethernet destination MAC address These 6 bytes are the Source MAC address. Ethernet Type is IPv4 (08 00) ITECH1102 Networking & Security 19 Broadcast addressing If the destination MAC address in an Ethernet frame is all binary 1’s the frame must be processed by all machines on the local network. Such broadcast packets are a requirement of many network protocols. (Example ARP discussed later). When expressed in Hexadecimal a broadcast address is: FF FF FF FF FF FF (that is 48 bits) ITECH1102 Networking & Security 20 Media Access Control Media Access Control is concerned with arbitrating access to the network. For shared media (for example WiFi) there are two categories of media access: Controlled Access Contention Based Access In controlled access only one station can transmit at a time. Devices wanting to transmit must wait their turn. Token Ring & FDDI are two examples that use this type of media access control. Both are known as deterministic protocols because they are guaranteed a minimum level of access to the media. ITECH1102 Networking & Security 21 Media Access Control (continued) Contention based Media Access In shared networks the most common type of media access control is contention based. One issue with contention based protocols is collisions can occur. Protocols used in such network are: CSMA/CD for Ethernet networks CSMA/CA for 802.11 wireless networks CSMA (Carrier Sense multiple Access) CD Collision detect | CA Collision Avoidance. ITECH1102 Networking & Security 22 CSMA/CD protocol Carrier Sense Multiple Access with collision detect (CSMA/CD) This is the Media Access Protocol used by Ethernet. Ethernet is the most widely used Data Link method for connecting computers and other devices to a network. This media access method allows multiple computers to all use a common network media. One issue is that collisions can occur when two machines try to send an Ethernet packet simultaneously. ITECH1102 Networking & Security 23 How CSMA/CD works Carrier Sense: NIC listens to the network media to see if another device is transmitting. If it is it waits for the network to become available. Multiple Access: Multiple devices can be connected to the same network. Collision Detect: After transmitting a station will listen to the network to determine if there has been a collision. ITECH1102 Networking & Security 24 How CSMA/CD works Collisions: If a collision does occur, both stations will detect the collision. Both will stop transmitting. Both will wait a random period of time then attempt a retransmission. In shared networks (networks with hubs) collisions can occur. In switched networks collisions are less of a problem. ITECH1102 Networking