Chapter 4  Naming 1 Naming Chapter 4

Chapter 4  Naming 2 Why Naming?  Names are needed to o Identify entities o Share resources o Refer to locations, etc.  It is necessary to resolve names o What does human-friendly name correspond to  Need a naming system  In distributed systems, naming system itself is often distributed

Chapter 4  Naming 3 Outline of Chapter  General naming issues o Human-friendly names  Naming and mobility o Such as mobile telephony  Unreferenced objects o How to remove unused names

Chapter 4  Naming 4 Names, Identifiers, Addresses  A name is a string of bits that refers to an entity  An entity is “practically anything” o Entities can be operated on  To operate on entity, must know an access point  Access point is just another entity!  Name of an access pt is an address

Chapter 4  Naming 5 Names, Identifiers, Addresses  Example o Telephone is an access point o Telephone number is address  Distributed systems example o Server is an access point o IP address/port number is address  Mobility creates special problems…

Chapter 4  Naming 6 Names, Identifiers, Addresses  Address is a special kind of name o For an access point  Access point associated with an entity  Why not use address as entity name? o Entities may change access points o For example, IP address vs MAC address o Entity may offer more than one access point o For example, Web service with multiple servers  An entity name that is independent of its address is location independent

Chapter 4  Naming 7 Names, Identifiers, Addresses  An identifier is a special type of name o Identifier refers to at most one entity o Entity referred to by at most one identifier o Identifier is never reused  That is, identifiers are unambiguous o “Mark Stamp” is not an identifier o Telephone number is not an identifier  Examples of identifiers?

Chapter 4  Naming 8 Names, Identifiers, Addresses  Human-friendly names? o File names o o Other?  Human unfriendly names? o Memory locations o Other?

Chapter 4  Naming 9 Name Spaces  Organization of names in distributed system is a name space o Represented as a labeled directed graph o Usually restricted to directed acyclic graphs  Leaf node is a named entity o Has no outgoing edge o Leaf node stores address and/or state  Directory node has a table o Can have many outgoing edges  Root node has no incoming edges o Usually only one

Chapter 4  Naming 10 Name Spaces  Naming graph with single root node n0  Complete path is a path name o Begins with n0, then it’s an absolute path o Otherwise, a relative path

Chapter 4  Naming 11 Name Spaces  Names organized in a name space o Implies that a name is defined relative to a directory node  Global name denotes same entity no matter where the name is used  Local name depends on where it’s used o In other words, a local name is a relative name whose directory is known

Chapter 4  Naming 12 Name Spaces  Example: UNIX file system o A single root node o File directory is a directory node o File is a leaf node

Chapter 4  Naming 13 Name Resolution  Given path name, must be able to access the specified node o This process is name resolution  How does this work? o Short answer: traverse the directed graph o Long answer: see the book  But, must know where to start o For example, how to resolve ? o First, must know it’s a phone number o I.e., must know root node of appropriate graph

Chapter 4  Naming 14 Linking and Mounting  An alias is another name for something  Can have multiple paths to same node, or…  A symbolic link in naming graph (as above)

Chapter 4  Naming 15 Linking and Mounting  Combine different name spaces o Need directory node in other name space  Node storing node identifier of foreign name space is mount point o Directory node in foreign name space is mounting point (usually root node)  Important in distributed systems!

Chapter 4  Naming 16 Linking and Mounting  In distributed system…  To mount foreign name space, need o Name of access protocol o Name of server o Name of mounting point  Name resolution required

Chapter 4  Naming 17 Linking and Mounting  Consider Network File System (NFS) o Distributed file system o Discussed in detail in chapter 10  Access a file by NFS URL o For example, nfs://flits.cs.vu.nl//home/steen o File (directory) /home/steen o On server flits.cs.vu.nl o Accessed using NFS protocol o Access protocol, server, mounting point?

Chapter 4  Naming 18 Linking and Mounting  Consider (machine A) /remote/vu/mbox  On A, nfs://flits.cs.vu.nl//home/steen  Then to machine B…

Chapter 4  Naming 19 Linking and Mounting  DEC Global Name Service (GNS)  Insert a new root node  Existing names change  Can avoid changing names o See book

Chapter 4  Naming 20 Name Space Implementation  Naming service o To add, remove, lookup names o Implemented on name server(s) o In distributed system, naming service itself may be distributed  Name space is heart of naming service o Name space distribution (organization) o Name resolution

Chapter 4  Naming 21 Name Space Distribution  Usually organized hierarchically  Assume one root  Three logical layers o Global layer  root and nearby (very stable) o Administrational layer  directory nodes in one organization (relatively stable) o Managerial layer  typically, hosts in one network (not stable)  For example, DNS

Chapter 4  Naming 22 Name Space Distribution  DNS name space in three layers  Subtle performance issues o Different requirements at each layer

Chapter 4  Naming 23 Name Space Distribution  Comparison of name servers SometimesYes Is client-side caching applied? NoneNone or fewManyNumber of replicas Immediate LazyUpdate propagation ImmediateMillisecondsSecondsResponsiveness to lookups Vast numbersManyFewTotal number of nodes DepartmentOrganizationWorldwideGeographical scale of network ManagerialAdministrationalGlobalItem

Chapter 4  Naming 24 Name Resolution  Two approaches…  Iterative name resolution o Server sends result back to client o More work for client  Recursive name resolution o Server contacts next name server o More work for servers

Chapter 4  Naming 25 Iterative Name Resolution  Caching limited to client

Chapter 4  Naming 26 Recursive Name Resolution  Caching is more effective (next slide)  More efficient communication (slide after next)

Chapter 4  Naming 27 Name Resolution  Recursive name resolution of  Name servers cache intermediate results Server for node Should resolve Looks up Passes to child Receives and caches Returns to requester cs # -- # vu # # # # ni # # # # # # root # # # # # # # #

Chapter 4  Naming 28 Name Resolution  Recursive versus iterative name resolution o Comparing communication costs San Jose Netherlands

Chapter 4  Naming 29 Examples  DNS  traditional naming service o Hierarchical, rooted tree o Like a white pages service for Internet o You should be familiar with this o Read it!  X.500  directory service o Find an entity that fits a description o Like a yellow pages service

Chapter 4  Naming 30 DNS Name Space  Hierarchical, rooted tree  Each node has 1 incoming edge (except the root) o Incoming edge used as name of node  A subtree is a domain  Path name is a domain name o Can be relative or absolute  Node contains resource records

Chapter 4  Naming 31 DNS Name Space  Most important types of resource records Type of record Associated entity Description SOAZoneHolds information on the represented zone AHostContains an IP address of the host this node represents MXDomainRefers to a mail server to handle mail addressed to this node SRVDomainRefers to a server handling a specific service NSZoneRefers to a name server that implements the represented zone CNAMENodeSymbolic link with the primary name of the represented node PTRHostContains the canonical name of a host HINFOHostHolds information on the host this node represents TXTAny kindContains any entity-specific information considered useful

Chapter 4  Naming 32 DNS Implementation  DNS includes o Global layer o Administrational layer  Managerial layer not formally in DNS  Read the details

Chapter 4  Naming 33 DNS Implementation  Excerpt from DNS database for the zone cs.vu.nl

Chapter 4  Naming 34 DNS Implementation  Description for vu.nl domain o This domain contains cs.vu.nl domain NameRecord typeRecord value cs.vu.nlNISsolo.cs.vu.nl A

Chapter 4  Naming 35 X.500 Name Space  Each record consists of o (attribute, value) pairs o Attribute can have multiple values  Directory Information Base (DIB) o All entries in X.500 directory service  Each attribute is a Relative Distinguished Name (RDN) o Complete record is globally unique o So it can be looked up

Chapter 4  Naming 36 X.500 Name Space  Example of X.500 directory entry o Unique name: Country, Organization, OrganizationalUnit o /C=NL/O=Vrije Universiteit/OU=Math. & Comp. Sc. o Analogous to DNS: nl.vu.cs AttributeAbbr.Value CountryCNL LocalityLAmsterdam OrganizationLVrije Universiteit OrganizationalUnitOUMath. & Comp. Sc. CommonNameCNMain server Mail_Servers , , FTP_Server WWW_Server

Chapter 4  Naming 37 X.500 Name Space  Globally unique names form hierarchy o Directory Information Tree (DIT) o The naming graph in X.500  Node can act as directory o More than one child o See next slide

Chapter 4  Naming 38 X.500 Name Space  Part of directory information tree  “N” acts as directory…  …and as a node

Chapter 4  Naming 39 X.500 Name Space  Two entries with Host_Name as RDN AttributeValueAttributeValue CountryNLCountryNL LocalityAmsterdamLocalityAmsterdam OrganizationVrije UniversiteitOrganizationVrije Universiteit OrganizationalUnitMath. & Comp. Sc.OrganizationalUnitMath. & Comp. Sc. CommonNameMain serverCommonNameMain server Host_NamestarHost_Namezephyr Host_Address Host_Address

Chapter 4  Naming 40 X.500 Implementation  Like DNS o But more lookup operations to search DIB  For example, can search for all “main servers” at Vrije Universiteit o See example in book  But, need to access many leaf nodes o Leaf nodes might be distributed  This could be expensive!

Chapter 4  Naming 41 X.500 in the Real World  X.500 o Uses Directory Access Protocol (DAP) o Runs over OSI o Therefore, it is “heavyweight”  What if you like X.500…  …but need to use it in the real world?  Need something lightweight…

Chapter 4  Naming 42 LDAP  Lightweight Directory Access Protocol  Application level protocol  Implemented on top of TCP  Lookup, update, passed as strings o No separate encoding required  LDAP is defacto standard  We used LDAP at my startup company

Chapter 4  Naming 43 Mobility  What is different in mobile case? o Names change frequently  Why is this an issue?  Consider DNS o Global layer and admin. layers assume names change infrequently o So replication and caching are used  For mobile, something else is needed… o But what?

Chapter 4  Naming 44 Mobility  Consider DNS o ftp.cs.vu.nl o Local cache probably has cs.vu.nl o One request to find desired address  Now spse ftp server moves o If it stays in cs.vu.nl, only local changes o What if it moves to ftp.cs.unisa.edu.au ?ftp.cs.unisa.edu.au

Chapter 4  Naming 45 Mobility  Spse ftp.cs.vu.nl moves to ftp.cs.unisa.edu.auftp.cs.vu.nlftp.cs.unisa.edu.au  What to do?  Forget about cs.vu.nl o Users won’t be happy  Record new address under cs.vu.nl o If it moves locally, update is not local  Turn cs.vu.nl into a symbolic link o In effect, 2 lookups o But it gets no worse if it moves again  Either way, name can never change

Chapter 4  Naming 46 Mobility  Better idea o Give up on DNS-like approach  Add an intermediate step o Assign non-human-friendly identifier  Then o Name service converts human-friendly name into identifier o Location service converts identifier to current address

Chapter 4  Naming 47 Naming versus Locating a) DNS-like mapping between name and address b) Two-level mapping using identifiers

Chapter 4  Naming 48 Mobility  But this begs the question o How to build location service?  Simple solutions o Broadcasting and multicasting o Forwarding pointers  Complicated (?) solutions o Home-based approaches o Hierarchical approaches

Chapter 4  Naming 49 Broadcasting and Multicasting  Spse mobility restricted to LAN o Broadcasting is efficient on LAN o Use ARP to locate entity o Does not scale well  Can do similar thing at network layer o Use multicasting o Mobile computer gets dynamic IP address and joins multicast group o Multicast group acts as location service

Chapter 4  Naming 50 Forwarding Pointers  Another simple approach  Forwarding pointers  When moving from A to B, leave a pointer at A to new location B o Naming service still points to A  Simple, yes, but… o Chain might get long o Every link in chain must be maintained

Chapter 4  Naming 51 Forwarding Pointers  Forwarding pointers as (proxy, skeleton)

Chapter 4  Naming 52 Forwarding Pointers  Redirecting a forwarding pointer o To shortcut a chain o Subsequent communication is faster o But some skeletons are left unreferenced

Chapter 4  Naming 53 Home-Based Approaches  Home location --- always knows current location of mobile guy o Can be used with forwarding pointers o Or with Mobile IP  Mobile IP on next slide…

Chapter 4  Naming 54 Home-Based Approaches  Mobile IP  Suppose host A is mobile o Host A has a fixed IP address o Host A has a home agent o Home agent of A is at A’s fixed IP address o Host A requests temp address at new location o Care-of address is current location of A o Home agent knows A’s care-of address

Chapter 4  Naming 55 Home-Based Approaches  Mobile IP

Chapter 4  Naming 56 Hierarchical Approaches  Home-based approach is 2 tiered  Can be generalized  Network divided into domains o Like layers in DNS  Top level domain spans network  Lowest level is leaf domain  Directory node for each domain

Chapter 4  Naming 57 Hierarchical Approaches  Hierarchy of location service domains  Each domain has associated directory node

Chapter 4  Naming 58 Hierarchical Approaches  Let Dir(D) be directory for domain D  Location record in dir(D) for each entity currently in D  Suppose entity E is in D  Then chain of pointers to E thru higher level directories of D  E can have more than one address o Due to replication

Chapter 4  Naming 59 Hierarchical Approaches  Entity with two addresses in different leaf domains

Chapter 4  Naming 60 Hierarchical Approaches  Looking up location of E o Efficient way to find current location of E

Chapter 4  Naming 61 Hierarchical Approaches a) Find first node that knows about E b) Create chain of forwarding pointers to new node  To create a replica of E in domain D…

Chapter 4  Naming 62 Pointer Caches  Caching: good if data seldom changed  If mobile, addresses change  But if move within a domain… o Then pointers at higher nodes does not change o It might make sense to cache such info  How to find the right domain? o Travel regularly between LA and SJ (next slide)  When to invalidate cache entry? o Travel to NY or moved there (next next slide)

Chapter 4  Naming 63 Pointer Caches  Caching reference to directory node of lowest-level domain

Chapter 4  Naming 64 Pointer Caches  Cache entry that needs to be invalidated because… o Entry returns a nonlocal address o A local address is available

Chapter 4  Naming 65 Scalability  The biggest issue with hierarchical approach is scalability  Root has to know about everybody! o Storage may be an issue o Lookup is likely bottleneck  Possible solutions (read the book) o Partitioning and/or uniform placement of subnodes

Chapter 4  Naming 66 Scalability Issues  Uniform placement of subnodes

Chapter 4  Naming 67 Unreferenced Entities  How to remove references to entities that are no longer referenced? o A distributed garbage collection problem  Consider remote objects o Recall state is remote o Client-side proxy o Server-side skeleton  Assume that an object can be accessed only if a remote reference exists  If no reference exists, then garbage

Chapter 4  Naming 68 Unreferenced Objects  Other garbage collection issues

Chapter 4  Naming 69 Partial Solutions  Reference counting o Keep running count of no. of references o When count reaches 0, remove object  Reference listing o Skeleton maintains explicit list of proxies that know about it  Tracing o Above methods do not deal with loops, etc. o Tracing: follow all paths from root

Chapter 4  Naming 70 Reference Counting  Keep track each time reference is added or deleted  Easy in non-distributed systems  But unreliable communication causes problems

Chapter 4  Naming 71 Reference Counting  If communication is unreliable o How to maintain accurate reference count?

Chapter 4  Naming 72 Reference Counting a) Incrementing the counter too late (example of a race condition) b) A solution to this problem ???

Chapter 4  Naming 73 Advanced Reference Counting  Weighted reference counting o Object has a fixed total weight o And variable partial weight o Only decrement operation allowed  Weight is split up (partial weights) when new references are created  Delete: decrement by partial weight o Skeleton’s total weight is decremented

Chapter 4  Naming 74 Advanced Reference Counting a) Initial assignment of weights b) Weight assignment for a new reference

Chapter 4  Naming 75 Advanced Reference Counting c) Weight assignment when copying reference

Chapter 4  Naming 76 Advanced Reference Counting  Problems?  Requires reliable communication o Ditto for reference counting  Does not deal with loops o Ditto ditto  Only a limited number of references o We can finesse this…

Chapter 4  Naming 77 Advanced Reference Counting  When partial weight has reached 1 o Use indirection to add more weight

Chapter 4  Naming 78 Advanced Reference Counting  Don’t like indirection? Fine!  Generation reference counting (no weights) o Skeleton maintains array G o G[i] is number of copies at generation i o When deleting, send msg to skeleton  Proxy’s generation no., k, and no. of copies made, n o Skeleton decrements its G[k] by 1 o Skeleton increments its G[k+1] by n o When all G[i] are 0, the object is deleted

Chapter 4  Naming 79 Advanced Reference Counting  Generation reference counting o Still requires reliable communication o Can add references w/o contacting skeleton

Chapter 4  Naming 80 Tracing  Reference counting does not help with loops and such  One approach is mark and sweep o Follow all paths from root o Mark all places reached o Sweep through everything o Remove everything not marked  Can be done in dist. Systems o But requires “stop the world” synchronization

Chapter 4  Naming 81 Tracing  More practical for dist. systems is o Tracing in groups  Group is a collection of processes o For scalability  Algorithm o Mark skeletons o Propagate marks from skeletons to proxies o Propagate marks from proxies to skeletons o Repeat previous 2 steps on larger group o Garbage reclamation

Chapter 4  Naming 82 Tracing in Groups  Initial marking of skeletons

Chapter 4  Naming 83 Tracing in Groups  After local propagation in each process

Chapter 4  Naming 84 Tracing in Groups  Final marking

Chapter 4  Naming 85 Summary  Naming is a serious issue!  Types of names o Address o Identifier o Human-friendly  Naming/naming graph  Human-friendly is not mobile-friendly

Chapter 4  Naming 86 Summary  Mobility o Broadcasting/multicasting o Forwarding pointers o Home location o Hierarchical search tree  Unreferenced objects o Reference counting o Tracing