Reliable Distributed Systems

1. Reliable Distributed Systems Naming(Communication Basics Part II) Slide set based on one by Prof. Paul Francis, Cornell University. Updated by Bina Ramamurthy

2. “Any problem in computer science can be solved with another layer of indirection” David Wheeler

3. Naming is a layer of indirection • What problems does it solve? • Makes objects human readable • Hides complexity and dynamics • Multiple lower-layer objects can have one name • Changes in lower-layer objects hidden • Allows an object to be found in different ways • One object can have multiple names

4. Names map to objects through a resolution service Name Distributed Name Resolution Service Object

5. Identifiers and Locators • A name is always an identifier to a greater or lesser extent • Can be persistent or non-persistent • Can be globally unique, locally unique, or even non-unique • If a name has structure that helps the resolution service, then the name is also a locator

6. Naming in networks Address Name Route

7. DNS names map into addresses Domain Name System (DNS) Address Many-to-many Name Route • Hierarchical • User-friendly • Location independent • But not org independent Domain Name (www.cnn.com)

8. Addresses map into routes IP address (128.94.2.17) Routing algorithm (BGP, OSPF, RIP) Address One-to-many Name • Hierarchical • Location Dependent • Non-unique • Can change often • Refers to an interface, not a host Route

9. Routes get packets to interfaces Address Name Route • A path • Source dependent • Can change often

10. DNS names and IP addresses are identifiers and locators • Both are typically non-persistent • Private IP addresses identify only in the context of an IP realm • Domain names are good identifiers • woodstock.cs.cornell.edu identifies a host • www.cnn.com identifies a service • URLs are good identifiers

11. Domain Name System (DNS) • Distributed directory service • Hierarchical name space • Each level separated by ‘.’ • Analogous to ‘/’ separator in file systems • One global root • Replicated across <20 root servers! • There have been Denial of Service (DoS) attacks on these root servers, none real successful • Because of caching, queries to root servers relatively rare • DNS maybe only global directory service???

12. DNS is the core of the Internet • Global name space • Can be the core of a naming or identifying scheme • Global directory service • Can resolve a name to nearly every computer on the planet

13. DNS tree structure NS RR “pointers” . edu. com. jp. us. cornell.edu. cmu.edu. mit.edu. cs.cornell.edu. eng.cornell.edu. foo.cs.cornell.edu A 10.1.1.1 bar.cs.cornell.edu A 10.1.1.1

14. LDAP is another popular distributed directory service • Richer and more general than DNS • Has generalized attribute/value scheme • Can search on attribute, not just name • Simpler and more efficient than a full relational database • Not a global directory service, though namespace is global • Its predecessor, X.500, was meant to be • But “local” LDAP services can point to each other • Commonly used for personnel RR databases, subscriber databases

15. URL • Consists of: <scheme>:<scheme-specific-part>

16. URL • Consists of: <scheme>:<scheme-specific-part> A protocol Information the protocol needs

17. URL examples • HTTP (web) • http://www.cnn.com/news/story.html • Email • mailto://francis@cs.cornell.edu • Newsgroups • news:cornell/class/cs514 • SIP (Session Initiation Protocol) • sip://service@phone.verizon.com

18. Note the central role of DNS • HTTP (web) • http://www.cnn.com/news/story.html • Email • mailto://francis@cs.cornell.edu • Newsgroups • news:cornell/class/cs514 • SIP (Session Initiation Protocol) • sip://service@phone.verizon.com

19. Locating mobile entities (section 4.2, AT/MvS) • What is a mobile entity? • From naming perspective, it is an entity whose address changes often • This doesn’t require physical mobility! • Every time you dial up, you may get a new address • So, “mobility” existed well before laptops became common • Though laptops create more mobility

20. Is mobility a problem for DNS? • Not really • Even though DNS was designed with relatively stable IP addresses in mind • Because mobility only effects leaf DNS servers • Recall: A RR TTL is short, but NS RR TTL is long • Note: non-mobile web server’s A RRs often have very short TTLs • To allow quick failover to another web server

21. Is mobility a problem at all? • Less than you’d think • Most mobile systems are clients; servers are rarely mobile • Clients are initiators of connections, not recipients • Therefore, there is not a client locating problem • What about email, instant messaging, and VoIP (Voice over IP)? • Clients receive emails, instant messages, and phone calls

22. Identifiers must be made hard to spoof • That is why driver’s licenses have pictures and credit cards have signatures • In networking, two ways: • Identifier is also a locator • Reverse routability • Some kind of secret-protected signature

23. Summary of Lecture Introduction to Naming • Naming basics: • Names, Addresses, Routes • Identifiers and Locators • DNS is the global directory service • LDAP is a popular local directory service • URLs build on DNS • Identifiers must be hard to spoof • Reverse routability, cryptographic signatures