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Network Models The OSI Standard Model of ISO Figure 2.3 The interaction between layers in the OSI model 2.7 Figure 2.4 An exchange using the OSI model 2.8 -- 7 layers proto stack, with 7 corresponding protocols. -- Peer to peer processes at each layer in different machines. -- What is a "layer"? What is a layer's "protocol"? --Why do we need the OSI stack of layered protocols, i.e., Network Software? 1) Physical Layer: PDU N/A, bit stream. Figure 2.5 Physical layer 2.10 ** The Physical Layer moves bit sequence over a physical link. ** Links/Media high quality/reliability play a major factor of the design complexity of upper layers' protocols, some layers might be significantly reduced or even finished. Defines the following: a) Physical characteristics of EIA (Electronic Industries Alliances) 422/485 balanced mode interfaces and medium. b) Bit representation: encoding/decoding, electrical/optical. c) Data rate: (b/s) bit TX duration. d) Bits synch: sender and receiver clock synch and same data rate. e) Line configuration: Point-to-point, Multipoint f) Physical Topology: Mesh, ring, bus, and hybrid. g) Transfer mode: Simplex, F/D, and H/D. h) Physical Media: Coaxial, TP, Fiber, Wireless. 2) Data Link Layer: PDU frame with header/trailer , Address Physical MAC address Figure 2.6 Data link layer 2.12 Two Sublayers: 1) Logic Link Control (LLC): **Source-Destination DL-PDUs (frames) delivery. a. Framing/Deframing. b. Physical Addressing: Sender/receiver addresses in the frame header. c. Flow Control: To prevent fast sender from flooding a slower receiver with frames. d. Error Control: To Increase physical layer reliability by adding mechanism to detect and ReTx damages and lost frames. (Trailer) 2) Medium Access Control (MAC): **Control the access to the shared and limited physical medium among all connected devices.(Ethernet, FDDI, 802.11, TR,TB, ...). [?] Inter-LANing (Bridging)- **Moving frames between different LANs with different MACs' protocols, resolving all conflicts due to the different protocol syntax (frame format) and semantics (frame interpretation and control procedure/steps). Figure 2.7 Hop-to-hop delivery 2.14 3) Network Layer: PDUpacket Address logical address (IP) Figure 2.8 Network layer 2.15 Figure 2.9 Source-to-destination delivery 2.17 Two Major Functions:(Sublayers??? not yet!!?) 1- Routing: To route packets over the subnet (cloud of routers and switches), makes the optimal routing decisions (src/destination). Source to destination host system delivery, utilizing the data link layer for peer-to-peer delivery. Physical addresses at the D.L are not enough; we need to add logical addressing in the packet header, of the sender and receiver. 2- Internetworking: resolve any Network protocol conflicts while moving in the subnet. (IPX-Netware, IP, X.25, AFP-Apple talk, etc) 4) Transport Layer: PDUsegment , Addressing "Service Access Point (SAP)" Figure 2.10 Transport layer 2.18 The most important layer since it abstracts the complicated details of the subnet to the user, allowing for QoS over the Network protocol. It communicates a meaningful data unit called message (group of related packets) between users over the subnet trying for the most optimal utilization of the subnet. Responsibilities (ISO TP-4): a) Service Access Point Addressing: (SAP) The network logical address (i.e., IP) is for src_system to dest_system not src_user_process to destination_user_process, hence we need another addressing mechanism => SAP addresses (port numbers) within the same system for message delivery between user's processes, where each process assigned to do specific service is tied to certain port number. b) Segmentation and Reassembly: segment <=> packets A process (user) message is divided, at the sender side into segments (if needed), each with a segment sequence number to aid in assembly (in a correct order) of related segments into the original user’s message, at the receiver side. c) Connection Control: 1) Connection reliable service. "Connection oriented" TP guarantees delivery in order with ACK'ment of segments back from the receiver to the sender. 2) Connectionless unreliable service (no ACK, no safe arrival guarantee) d) Flow Control: same mechanism as in the Data-Link Layer (DLL), but applied at the message level between “end-users” processes. e) Error Control: Like the DLL, but process_to_process delivery of messages. Errors(damaged, lost or duplicate) cause retransmission of messages. We should state that there is another major TL functions remains such as Gatewaying to resolve conflicts when moving between different TL protocols. Figure 2.11 Reliable process-to-process delivery of a message 2.20 5) Session Layer: a. Dialog Controls H/D or F/D. b. Synchronization: Checkpoints are added to data streams for dividing into units of independent ACK. Communication robustness in case of crashes. 6) Presentation Layer: a. Translation: ASCII,--.EBCDIC. Abstract syntax notation (ASN). b. Encryption: To secure information Tx for privacy c. Compression: For efficient utilization of bandwidth. 7) Application Layer: 1) Virtual terminal”putty”to allow remote logins (emulations) 2) File transfer 3) Mail Service, 4) Directory service. SMTP, HTTP, DNS, SNMP, TELNET. TCP/IP Protocol Suite (Stack) Figure 2.16 TCP/IP and OSI model 2.29 Application STMP FTP HTTP DNS SNMP TELNET Presentation Session SCTP TCP UDP Transport ICMP IGMP Network (internet) IP RARP/ARP DataLink Physical Host-to-network (LAN/MAN/WAN) Figure 2.17 Addresses in TCP/IP 2.31 Figure 2.18 Relationship of layers and addresses in TCP/IP 2.32 1) Physical Layer: Intentionally left very vague!?! It can be a LAN, MAN, or WAN with Synchronous Optical NETork (SONET), Asynchronous Transfer Mode (ATM), all-fiber Ultra Dense Wave Division Muxing (UDWDM) physical carriers technologies. 2) Network Layer: Best-Effort-Delivery Internet Protocol (IP) Unreliable connectionless datagram protocol (no end control flow). At the network layer there are other companion protocols to help the poor performance IP: A) Internet Control Message Protocol (ICMP): It is an IP companion that helps in reporting any failure/congestion in subnet, via host-management queries which is not part of the IP protocol, hence alleviating the IP deficiencies. With the ICMP, a host/admin can enquire the status of another host or router in the subnet cloud. Such knowledge can aid in the efficient management of the subnet utilization, identifying subnet problems via the ICMP query messages. Ping and traceroute (UNIX)/tracert (Windows) utilize ICMP. traceroute & tracert - print the route packets take to network host destination. ICMPv6: (IPv6-- 128 bit address): (Reference: http://www.tcpipguide.com/free/t_ICMPv6RouterRenumberingMessages.htm) ICMPv6 informational messages of which are used in matching pairs. Echo Request and Echo Reply messages-- used for network connectivity testing. Router Advertisement and Router Solicitation messages-- used to let hosts discover local routers and learn necessary parameters from them. Neighbor Advertisement and Neighbor Solicitation messages-- used for various communications between hosts on a local network, including IPv6 address resolution. Redirect messages-- which let routers inform hosts of better first-hop routers, and Router Renumbering messages-- With the huge 12-bit addressing, a network administrator (NA) has the potential to make large network migrations and merges much simpler. A NA selects a machine to generate one or more Router Renumbering Command messages. These messages provide a list of prefixes of routers that are to be renumbered. Each router processes these messages to see if the addresses on any of their interfaces match the specified prefixes. If so, they change the matched prefixes to the new ones specified in the message. B) Internet Group Message Protocol (IGMP): It is an IP companion that aids in expanding the IP to have multicasting capability to a group of destinations, by managing group membership to help routing at the network's multicast routers. Address Resolutions: C) Address Resolution Protocol (ARP): Logical to Physical The destination IP address (known at a sender from the DNS) is not enough to get to the destination machine (physical at the MAC). Hence the sender will seek the help of ARP (via query packet) to get the destination's MAC physical address (needed to form the MAC frame's destination address), i.e., to map: IP (logical) MAC (physical) destination address. ARP proxy -- It represents a set of hosts, providing its own MAC for any of those hosts’ query, i.e., temporary destination, then when receiving any of the hosts’ actual IP packets, it directs it to the intended host MAC sublayer, via a mapping table, IPMAC. Security Issue: Read about ARP Spoofing (Poisoning) & Mitigation Techniques. Attacks: (stemmed from the fact that ARP DOES NOT authenticate) (Reference: http://www.watchguard.com/infocenter/editorial/135324.asp) Denial of Service-- Falsely associating your network's router (NR) IP address to the intruder machine MAC address (IMAC); hence all of the attacked network users' traffic is gone into a black hole sink machine!, Man in the Middle-- 1) The attacker issues an ARP reply (with no previous ARP request) to your NR, falsely associating the IMAC to your IP address, i.e., playing you to the NR! 2) The hacker then sends a false ARP reply to your machine associating the IMAC to the NR's IP address, playing NR to you. 3) The hacker uses the IP forwarding OS utility to forward your traffic (after sniffing/hijacking it) to the NR! MAC
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