The Application Layer
- 1. COMPUTER NETWORKS The Application Layer By, M.SHARMILADEVI M.Sc(IT)
- 2. Content Introduction DNS – THE DOMAIN NAME SYSTEM The DNS Name Space Domain Resource Records Name Servers Conclusion
- 3. Introduction Application layer is the top most layer in OSI and TCP/IP layered model. The application layer in the OSI model is the closest layer to the end user. It is the layer through which users interact. It provides services to the user.
- 4. In the application layer there is also need for support protocols, to allow the applications to function. DNS which handles naming within the Internet. Three real applications: ◦ Electronic Mail ◦ The World Wide Web ◦ Multimedia
- 5. DNS-THE DOMAIN NAME SYSTEM Web pages, mailboxes, and other resources by using the network (e.g., IP) addresses of the computers on which they are stored, these addresses are hard for people to remember. High-level, readable names were introduced in order to decouple machine names from machine addresses. The company’s Web server might be known as www.cs.washington.edu regardless of its IP address.
- 6. Nevertheless, since the network itself understands only numerical addresses, some mechanism is required to convert the names to network addresses. Way back in the ARPANET days, there was simply a file, hosts.txt, that listed all the computer names and their IP addresses.
- 7. Everyone involved with it realized that this approach could not continue to work forever. ◦ One thing, the size of the file would become too large. ◦ Host name conflicts would occur constantly unless names were centrally managed, something unthinkable in a huge international network due to the load and latency.
- 8. To solve these problems, DNS (Domain Name System) was invented in 1983. The essence of DNS is the invention of a hierarchical, domain-based naming scheme and a distributed database system for implementing this naming scheme. It is primarily used for mapping host names to IP addresses
- 9. Resolver: To map a name onto an IP address, an application program calls a library procedure called the resolver, passing it the name as a parameter. The resolver sends a query containing the name to a local DNS server, which looks up the name and returns a response containing the IP address to the resolver, which then returns it to the caller. The query and response messages are sent as UDP packets.
- 10. The DNS Name Space The top of the naming hierarchy is managed by an organization called ICANN (Internet Corporation for Assigned Names and Numbers). ICANN was created for this purpose in 1998. the Internet is divided into over 250 top-level domains, where each domain covers many hosts. Each domain is partitioned into subdomains, and these are further partitioned, and so on
- 11. All these domains can be represented by a tree A portion of the Internet domain name space
- 12. The top-level domains come in two flavors: generic and countries. The generic domains introduced via applications to ICANN. The country domains include one entry for every country, as defined in ISO 3166. Internationalized country domain names that use non-Latin alphabets were introduced in 2010.
- 13. Getting a second-level domain, such as name-of-company.com, is easy. The top-level domains are run by registrars appointed by ICANN. It requires going to a corresponding registrar (com) to check if the desired name is available and not somebody’s trademark. If there are no problems, the requester pays the registrar a small annual fee and gets the name
- 14. Cybersquatting: The practice of registering a domain only to turn around and sell it off to an interested party at a much higher price even has a name. It is called cybersquatting. Domain names are case insensitive, so edu, Edu, and EDU mean the same thing.
- 15. Domain Resource Records Every domain, whether it is a single host or a top-level domain, can have a set of resource records associated with it. These records are the DNS database. For a single host, the most common resource record is just its IP address, but many other kinds of resource records also exist.
- 16. When a resolver gives a domain name to DNS, what it gets back are the resource records associated with that name. Thus, the primary function of DNS is to map domain names onto resource records. A resource record is a five-tuple. The format we will use is as follows: ◦ Domain_name ◦ Time_to_live ◦ Class ◦ Type ◦ Value
- 17. Domain_name It tells the domain to which this record applies. Time_to_live It gives an indication of how stable the record is. Information that is highly stable is assigned a large value, such as 86400 (the number of seconds in 1 day). Information that is highly volatile is assigned a small value, such as 60 (1 minute).
- 18. Class It is the third field of every resource record For Internet information, it is always IN. Type It tells what kind of record this is There are many kinds of DNS records. The important types are listed
- 19. The principal DNS resource record types
- 20. SOA record: It provides the name of the primary source of information about the name server’s zone, the email address of its administrator, a unique serial number, and various flags and timeouts. A record: The most important record type is the A (Address) record. It holds a 32-bit IPv4 address of an interface for some host. AAAA record: The corresponding AAAA, or ‘‘quad A,’’ record holds a 128-bit IPv6 address.
- 21. MX record: It specifies the name of the host prepared to accept email for the specified domain. NS record: It specifies a name server for the domain or subdomain. CNAME record: It allow aliases to be created. PTR record: It points to another name. PTR is a regular DNS data type whose interpretation depends on the context in which it is found. It is nearly always used to associate a name with an IP address to allow lookups of the IP address and return the name of the corresponding machine. These are called reverse lookups.
- 22. SRV record: It is a newer type of record that allows a host to be identified for a given service in a domain. SPF record: It is also a newer type of record. It lets a domain encode information about what machines in the domain will send mail to the rest of the Internet. This helps receiving machines check that mail is valid.
- 23. TXT records: It were originally provided to allow domains to identify themselves in arbitrary ways. Value field: This field can be a number, a domain name, or an ASCII string. The semantics depend on the record type.
- 24. Name Servers A single name server could contain the entire DNS database and respond to all queries about it. This server would be so overloaded as to be useless. if it ever went down, the entire Internet would be crippled. To avoid the problems associated with having only a single source of information, the DNS name space is divided into nonoverlapping zones. Each circled zone contains some part of the tree.
- 25. Part of the DNS name space divided into zones (which are circled).
- 26. Each zone is also associated with one or more name servers. These are hosts that hold the database for the zone. The process of looking up a name and finding an address is called name resolution. When a resolver has a query about a domain name, it passes the query to a local name server.
- 27. An authoritative record is one that comes from the authority that manages the record and is thus always correct. Authoritative records are in contrast to cached records, which may be out of date. What happens when the domain is remote, such as when flits.cs.vu.nl wants to find the IP address of robot.cs.washington.edu at UW? In this case, and if there is no cached information about the domain available locally, the name server begins a remote query.
- 28. Example of a resolver looking up a remote name in 10 steps.
- 29. Step 1 shows the query that is sent to the local name server. Step 2 the next step is to start at the top of the name hierarchy by asking one of the root name servers. Step 3 it returns the name and IP address for that part of the answer. Steps 4 and 5 the local name server then continues its quest. That name server returns the name server for UW.
- 30. Step 6 the local name server sends the query to the UW name server. Step 7 the query returns the name and IP address of the UW Computer Science name server. Step 8 finally, the local name server queries the UW Computer Science name server. Step 9 this server is authoritative for the domain cs.washington.edu, so it must have the answer. It returns the final answer. Step 10 which the local name server forwards as a response to flits.cs.vu.nl. The name has been resolved.
- 31. When the host flits.cs.vu.nl sends its query to the local name server, that name server handles the resolution on behalf of flits until it has the desired answer to return. It does not return partial answers. This mechanism is called a recursive query.
- 32. The root name server does not recursively continue the query for the local name server. It just returns a partial answer and moves on to the next query. This mechanism is called an iterative query.
- 33. The second point is caching. All of the answers, including all the partial answers returned, are cached. In this way, if another cs.vu.nl host queries for robot.cs.washington.edu the answer will already be known. Using cached answers greatly reduces the steps in a query and improves performance. The original scenario we sketched is in fact the worst case that occurs when no useful information is cached. cached answers are not authoritative, since changes made at cs.washington.edu will not be propagated to all the caches in the world that may know about it.
- 34. The third issue is the transport protocol that is used for the queries and responses. It is UDP. DNS messages are sent in UDP packets with a simple format for queries, answers, and name servers that can be used to continue the resolution.
- 35. Conclusion Even though its purpose is simple, it should be clear that DNS is a large and complex distributed system that is comprised of millions of name servers that work together. It forms a key link between human-readable domain names and the IP addresses of machines. It includes replication and caching for performance and reliability and is designed to be highly robust. Security extensions called DNSSEC have been developed for DNS.