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ISP Services

ISP Services. CCNA Discovery 2– Chapter 7. Contents. 7.1 : ISP Services 7.2: TCP / IP Protocols 7.3: DNS 7.4 : Application Layer Protocols. Managed Services.

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ISP Services

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  1. ISP Services CCNA Discovery 2– Chapter 7

  2. Contents • 7.1: ISP Services • 7.2:TCP / IP Protocols • 7.3: DNS • 7.4: Application Layer Protocols

  3. Managed Services • ISPs offer managed services that enable organizations to have access to leading network technologies and applications without having to make large investments in equipment and support • When a company subscribes to a managed service, the ISP manages the network equipment and applications according to the terms of a service level agreement (SLA) • The purpose of an SLA is to outline the management, monitoring, and maintenance of a network

  4. ISP / Client Relationships • 3 Different scenarios for ISP customer relationships: • Scenario 1 - Customer owns and manages all own network equipment and services. • ISP provides reliable Internet connectivity • Scenario 2 - The ISP provides Internet connectivity, and owns and manages the network connecting equipment installed at the customer site. • The ISP sets up, maintains, and administers the equipment for the customer. • The customer monitors the status of the network and the applications • Scenario 3 - The customer owns the network equipment, but ALL applications that the business relies on are hosted by the ISP. • servers are located at the ISP facility and may be owned by the customer or the ISP • Servers often kept in server farms in the (NOC)

  5. ISP Services 2 1 3

  6. Reliability & Availability: • ISPs provide services to customers for a fee and guarantee a level of service in the SLA. • To meet customer expectations, the service offerings have to be reliable and available. • When there is an equipment failure, and the network or service becomes unavailable, it impacts the ability of the ISP to meet the terms of the SLA. • As ISPs offer more critical business services, such as IP telephony or high-volume retail sale transactions, ISPs must meet the higher expectations of their customers.

  7. Reliability • Reliability of Service can be measured in 2 ways • Mean time between failure (MTBF) • MTBF is determined by manufacturers, based on fault tolerance tests • Mean time to repair (MTTR) • MTTR is established by warranty or service agreements. • In order to provide Reliability, ISPs may: • purchase expensive service agreements for critical hardware to ensure rapid manufacturer or vendor response • purchase redundant hardware and keep spare parts on site

  8. Availability • Availability is normally measured in the % of time that a resource is accessible. • A perfect availability percentage is 100% • the system is never down or unreachable • Five-9s standard of availability: 99.999% availability • Telephone services are expected to meet this standard • Only a very small percentage (0.001%) of downtime is acceptable. • ISPs ensure accessibility by: • Doubling up on network devices and servers using technologies designed for high availability. • In a redundant configuration, if one device fails, the other one can take over the functions automatically.

  9. Reliability and Availability

  10. 7.2: TCP / IP Protocols • ISP servers need to be able to support multiple applications for many different customers • They must use functions provided by the 2 TCP/IP transport protocols: TCP and UDP • Common hosted applications, like web serving and email accounts, also depend on underlying TCP/IP protocols to ensure their reliable delivery. • All IP services rely on domain name servers, hosted by the ISPs, to provide the link between the IP addressing structure and the URLs that customers use to access them.

  11. TCP / IP Protocols • Clients and servers use specific protocols and standards when exchanging information • The TCP/IP protocols can be represented using a four-layer model • Many of the services provided to ISP customers depend on protocols that reside at the Application and Transport layers of the TCP/IP model.

  12. TCP/IP Protocol Suite

  13. Networking Models • The TCP/IP model and the OSI model have similarities and differences. • The TCP/IP model is based on actual developed protocols and standards, whereas the OSI model is a theoretical guide for how protocols interact. • Similarities: • Use of layers to visualize the interaction of protocols and services • Comparable Transport and Network layers • Used in the networking field when referring to protocol interaction • Differences • The OSI model breaks the function of the TCP/IP Application Layer into 3 distinct layers (application, presentation, session). • The upper 3 layers of the OSI model have the same function as the Application Layer of the TCP/IP model. • The TCP/IP suite does not specify protocols for the physical network interconnection(OSI Physical Layer) • The 2 lower layers of the OSI model are concerned with access to the physical network and the delivery of bits between hosts on a local network.

  14. OSI vs. TCP / IP

  15. Application Layer protocols • Application Layer protocols specify the format and control the information necessary for many of the common Internet communication functions. • Protocols Include: • Email: smtp, pop3, imap • Web: http, https • Naming: DNS • IP Addressing: DHCP • Remote Access: telnet, ssh • File transfer: ftp, tftp

  16. Application Layer Protocols • DHCP – Dynamic assignment of IP Addressing • Simple Mail Transfer Protocol (SMTP) - Transfers mail messages and attachments between mail servers • POP3: allows clients to retrieve email from a mail server –downloads email from the server • IMAP4: allows clients to retrieve email from a mail server – leaves email on the server • FTP: a reliable protocol for transferring files between hosts over a network • TFTP: an unreliable protocol for transferring files between hosts over a network • Domain Name System (DNS) - Resolves Internet names to IP addresses. • HyperText Transfer Protocol (HTTP) -Transfers files that make up the web pages of the World Wide Web. • Telnet - Terminal emulation protocol that provides remote access to servers and networking devices.

  17. Transport Layer Protocols • It is the job of the Transport Layer to deliver data to the appropriate application. • The 2 primary Transport Layer protocols: TCP and UDP • Different types of data can have unique requirements. • For some applications, communication segments must arrive in a specific sequence to be processed successfully. • In other instances, all the data must be received for any of it to be of use. • Sometimes, an application can tolerate the loss of a small amount of data during transmission over the network. • Different Transport Layer protocols have different rules to enable devices to handle these diverse data requirements. • The lower layers are not aware that there are multiple applications sending data on the network. • Their responsibility is to get the data to the device.

  18. Transport Layer Protocols

  19. TCP • TCP is a connection-oriented Protocol: • Provides reliable, guaranteed-deliveryof data from end to end • Governs the exchange of messages between the source and destination hosts to create a communication session • TCP provides reliable deliver of data using these 4 techniques: • Sequencing of data segments • Acknowledgement of data • Retransmission of improperly transmitted data • Flow control

  20. Transport Layer Protocols • The Transport Layer protocol used by Applications is determined by the type of application data being sent. • Applications, such as databases, web pages, and email, need to have all data arrive at the destination in its original condition, for the data to be useful. • Any missing data can cause the messages to be corrupt or unreadable. • These applications are designed to use TCP due to its reliability • However, TCP requires overhead, which includes extra bandwidth and increased processing, to keep track of the individual conversations between the source and destination hosts and to process acknowledgements and retransmissions. • In some cases, the delays caused by this overhead cannot be tolerated by the application. • These applications are better suited for UDP.

  21. Encapsulation • TCP divides data to be transmitted into segments and then passes them to the Internet Layer, which places each segment into a packet for transmission • This process is known as encapsulation. • At the destination, the process is reversed, and the packets are de-encapsulated. • The enclosed segments are sent through the TCP process, which converts the segments back to a stream of bytes to be passed to the email server application.

  22. TCP Sequencing • TCP specifies how messages are Sequenced at the source host and reassembled at the destination host: • When data is sent using TCP, each segment is identified with a sequence number at the source host • At the destination host, the TCP process stores received segments in a buffer. • By evaluating the segment sequence numbers, the TCP process can confirm that there are no gaps in the received data. • When data is received out of order, TCP can also reorder the segments as necessary.

  23. TCP 3-Way Handshake • Before a TCP session can begin, the source and destination hosts exchange info to set up a connection • They use a 3-Way Handshake to set up the connection. • SYN: The source host sends a Synchronization Message, or SYN, to begin the TCP session establishment process. • Indicates the intention of the source host to establish a connection with the destination host • Synchronizes the TCP sequence numbers between the two hosts, so that each host can keep track of the segments sent and received during the conversation. • SYN-ACK: The destination host replies to the SYN message with a synchronization acknowledgement, or SYN-ACK • ACK: The sending host receives the SYN-ACK and sends an ACK message back to complete the connection setup. • Data segments can now be reliably sent.

  24. TCPAcknowledgement & Retransmission • When a host sends message segments to a destination host using TCP, the TCP process on the source host starts a timer. • The timer allows sufficient time for the message to reach the destination host and for an acknowledgement to be returned. • If the source host does not receive an acknowledgement from the destination within the allotted time, the timer expires, and the source assumes the message is lost. • The portion of the message that was not acknowledged is then re-transmitted.

  25. TCP Acknowledgement

  26. UDP • UDP is a very simple, connectionless protocol: • Provides low overhead data delivery • Considered a connectionless, "best effort" Transport Layer protocol • Does not provide error checking, guaranteed data delivery, or flow control • Considered an unreliable delivery protocol, because there is no guarantee that a message has been received by the destination host. • UDP datagrams may arrive at the destination out of order, or may even be lost all together. • Applications that use UDP can tolerate small amounts of missing data. • Reliability and error checking must be provided by other layers when UDP is in use

  27. UDP • Used by online games, Online Radio, DHCP, DNS, SNMP, TFTP, RIP and VoIP

  28. TCP Segment vs. UDP Datagram • The main differences between TCP and UDP are the specific functions that each protocol implements and the amount of overhead incurred • Each TCP segment has 20 bytes of overhead in the header that encapsulates the Application Layer data. • This overhead is due to the error-checking mechanisms • UDP datagramsare sent as "best effort" and, therefore, only require 8 bytes of overhead. • Data transmitted with TCP will require more transmission time than that sent with UDP, due to the error-checking and delivery verification processes that must occur

  29. TCP Segment vs. UDP Datagram

  30. Supporting Multiple Services • The task of managing multiple simultaneous communication processes is done at the Transport Layer. • The TCP and UDP services keep track of the various applications that are communicating over the network. • Port Numbers are used to differentiate between the segments and datagrams for each application • A source port and destination port are located in the header of each segment or datagram

  31. Port Numbers • In any Internet transaction, there is a source host and a destination host, normally a client and a server. • The TCP processes on the sending and receiving hosts are slightly different. • Clients are active and request connections, while servers are passive, and listen for and accept connections. • Port numbers are assigned in various ways, depending on whether the message is a request or a response.

  32. Destination Port Numbers • Destination Ports: Server processes are usually statically assigned well-known port numbers from 0 to 1023. • Well-known port numbers enable a client application to assign the correct destination port when generating a request for services. • When a Client Requests service from a Service, the Destination Port is the Port Number assigned to the application • Many common applications have default port assignments. • HTTP: Port 80 • SMTP Email servers: Port 25 • FTP: Ports 20, 21 • Telnet: Port 23

  33. Port Numbers

  34. Queuing of Data Segments • As segments are received for a specific port, TCP or UDP places the incoming segments in the appropriate queue. • Example: HTTP requests are received, the TCP process running on a web server places incoming segments in the web server queue. • These segments are then passed up to the HTTP application as quickly as HTTP can accept them. • Transport Layer protocols enable servers at the ISP to host many different applications and services simultaneously.

  35. Queuing of Data Segments

  36. Source Ports • Source ports allow clients to identify the requesting client application • Source Ports are dynamically assigned from the port range 1024 to 65535. • This port assignment acts like a return address for the requesting application. • The Transport Layer protocols keep track of the source port and the application that initiated the request, so that when a response is returned, it can be forwarded to the correct application.

  37. Sockets • Socket: The combination of the Transport Layer port number and the Network Layer IP address of the host that uniquely identifies a particular application process running on an individual host device. • Socket pair: consists of the source and destination IP addresses and port numbers, and identifies the specific conversation between the two hosts. • With the creation of sockets, communication endpoints are known so that data can move from an application on one host to an application on another. • Sockets enable multiple processes running on a client to distinguish themselves from each other, and multiple connections to a server process to be distinguished from each other.

  38. Socket Pair Example • Client socket: 192.168.1.1:7151 • Server Socket: 10.10.10.101:80 • Socket Pair: 192.168.1.1:7151, 10.10.10.101:80

  39. Socket Pair • Destination Port: The data conversation was started by the http application process running on the client (it sent a request to port 80 on the web server) • Source Port: The web server will respond by sending data to port 8547 on the client

  40. 7.3: Network Naming • Communication between source and destination hosts over the Internet requires a valid IP address for each host. • However, numeric IP addresses, especially the hundreds of thousands of addresses assigned to servers available over the Internet, are difficult for humans to remember. • Human-readable domain names, like cisco.com, are easier for people to use. • Network naming systems are designed to translate human-readable names into machine-readable IP addresses that can be used to communicate over the network. • Network naming systems are a human convenience to help users reach the resource they need without having to remember the complex IP address.

  41. HOSTS File • In the early days of the Internet, host names and IP addresses were managed through the use of a single Hosts File • The central HOSTS file contained mappings of host names and IP addresses for every device connected to the early Internet. • Each site could download the HOSTS file and use it to resolve host names on the network. • When a host name was entered, the sending host would check the downloaded HOSTS file to obtain the IP addr • As the Internet grew, so did the number of hosts needing name-to-IP translations. • It became impossible to keep the HOSTS file up to date. • As a result, a new method to resolve host names to IP addresses was developed, DNS, and a centrally administered HOSTS file was no longer needed • All computer systems still maintain a localHOSTS file • It is created when TCP/IP is loaded on a host device. • As part of the name resolution process on a computer system, the HOSTS file is scanned even before the DNS service is queried. • It can be used for troubleshooting or to override records found in a DNS server

  42. HOSTS File

  43. DNS Hierarchy • DNS was created for domain name to address resolution • DNS solves the shortcomings of the HOSTS file. • DNS relies on a hierarchy of decentralized servers to store and maintain records of host name to Ip address mappings • It uses distributed database of host name to IP mappings spread across many DNS servers all over the world. • This is unlike a HOSTS file, which requires all mappings to be maintained on one server. • DNS uses domain names to form the hierarchy. • The naming structure is broken down into small, manageable zones. • Each DNS server maintains a specific database file and is only responsible for managing name-to-IP mappings for that small portion of the entire DNS structure. • When a DNS server receives a request for a name translation that is not within its DNS zone, the DNS server forwards the request to another DNS server within the proper zone for translation. • DNS is scalable because host name resolution is spread across multiple servers.

  44. DNS Hierarchy

  45. Components of DNS • DNS is made up of 3 components. • Resource records and domain namespace • Domain name system servers • Resolvers

  46. Resource Records and Namespace • Resource record: • A data record in the database of a DNS zone server • The resource records contain domain names that the server can resolve, and alternate servers that can also process requests. • It is used to identify a type of host, a host IP address, or a parameter of the DNS database. • Domain namespace : • The hierarchical naming structure for organizing resource records. • It is made up of various domains, or groups, and the resource records within each group.

  47. Domain Name System Servers • Domain name system servers maintain the DNS databases that store resource records and information about the domain namespace structure. • DNS servers attempt to resolve client queries using the domain namespace and resource records it maintains in its zone database files. • If the name server does not have the requested information in its DNS zone database, it uses additional predefined name servers to help resolve the name-to-IP query. • It basically sends the request to another DNS zone server

  48. Resolvers • Resolvers are software protocols that are used to send DNS queries between clients and servers • It can be a stand-alone application or a function built-into the operating system • When a domain name is used, the resolver software queries the DNS server to translate that name to an IP address. • A resolver is loaded onto DNS clients, and is used to create the DNS name query that is sent to a DNS server. • Resolvers are also loaded on DNS servers. • If the DNS server does not have the name-to-IP mapping requested, it uses the resolver to forward the request to another DNS server.

  49. DNS Domain Levels • DNS uses a hierarchical system to provide name resolution. • The hierarchy looks like an inverted tree, with the root at the top and branches below. • At the top of the hierarchy, the root servers maintain records about how to reach the top-level domain servers, which in turn have records that point to the second-level domain servers. • The different top-level domains represent either the type of organization or the country of origin. • .au - Australia • .co - Colombia • .com - a business or industry • .jp - Japan • .org - a nonprofit organization • Under the top-level domains are second-level domain names, and below them are other lower level domains.

  50. Domain Hierarchy Top-Level Domains 2nd Level Domains

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