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  • format The question section is used in all kinds of queries other than inverse queries In responses to inverse queries this section may contain multiple entries for all other responses it contains a single entry Each entry has the following format 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 QNAME QTYPE QCLASS where QNAME a variable number of octets that specify a domain name This field uses the compressed domain name format described in the next section of this memo This field can be used to derive a text string for the domain name Note that this field may be an odd number of octets no padding is used QTYPE a two octet code which specifies the type of the query The values for this field include all codes valid for a TYPE field together with some more general codes which can match more than one type of RR For example QTYPE might be A and only match type A RRs or might be MAILA which matches MF and MD type RRs The values for this field are listed in Appendix 2 QCLASS a two octet code that specifies the class of the query For example the QCLASS field is IN for the ARPA Internet CS for the CSNET etc The numerical values are defined in Appendix 2 Mockapetris Page 29 RFC 883 November 1983 Domain Names Implementation and Specification Resource record format The answer authority and additional sections all share the same format a variable number of resource records where the number of records is specified in the corresponding count field in the header Each resource record has the following format 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 NAME TYPE CLASS TTL RDLENGTH RDATA where NAME a compressed domain name to which this resource record pertains TYPE two octets containing one of the RR type codes defined in Appendix 2 This field specifies the meaning of the data in the RDATA field CLASS two octets which specify the class of the data in the RDATA field TTL a 16 bit unsigned integer that specifies the time interval in seconds that the resource record may be cached before it should be discarded Zero values are interpreted to mean that the RR can only be used for the transaction in progress and should not be cached For example SOA records are always distributed with a zero TTL to prohibit caching Zero values can also be used for extremely volatile data Mockapetris Page 30 RFC 883 November 1983 Domain Names Implementation and Specification RDLENGTH an unsigned 16 bit integer that specifies the length in octets of the RDATA field RDATA a variable length string of octets that describes the resource The format of this information varies according to the TYPE and CLASS of the resource record For example the if the TYPE is A and the CLASS is IN the RDATA field is a 4 octet ARPA Internet address Formats for particular resource records are shown in Appendicies 2 and 3 Domain name representation and compression Domain names messages are expressed in terms of a sequence of labels Each label is represented as a one octet length field followed by that number of octets Since every domain name ends with the null label of the root a compressed domain name is terminated by a length byte of zero The high order two bits of the length field must be zero and the remaining six bits of the length field limit the label to 63 octets or less To simplify implementations the total length of label octets and label length octets that make up a domain name is restricted to 255 octets or less Since the trailing root label and its dot are not printed printed domain names are 254 octets or less Although labels can contain any 8 bit values in octets that make up a label it is strongly recommended that labels follow the syntax described in Appendix 1 of this memo which is compatible with existing host naming conventions Name servers and resolvers must compare labels in a case insensitive manner i e A a and hence all character strings must be ASCII with zero parity Non alphabetic codes must match exactly Whenever possible name servers and resolvers must preserve all 8 bits of domain names they process When a name server is given data for the same name under two different case usages this preservation is not always possible For example if a name server is given data for ISI ARPA and isi arpa it should create a single node not two and hence will preserve a single casing of the label Systems with case sensitivity should take special precautions to insure that the domain data for the system is created with consistent case In order to reduce the amount of space used by repetitive domain names the sequence of octets that defines a domain name may be terminated by a pointer to the length octet of a previously specified label string The label string that the pointer Mockapetris Page 31 RFC 883 November 1983 Domain Names Implementation and Specification specifies is appended to the already specified label string Exact duplication of a previous label string can be done with a single pointer Multiple levels are allowed Pointers can only be used in positions in the message where the format is not class specific If this were not the case a name server that was handling a RR for another class could make erroneous copies of RRs As yet there are no such cases but they may occur in future RDATA formats If a domain name is contained in a part of the message subject to a length field such as the RDATA section of an RR and compression is used the length of the compressed name is used in the length calculation rather than the length of the expanded name Pointers are represented as a two octet field in which the high order 2 bits are ones and the low order 14 bits specify an offset from the start of the message The 01 and 10 values of the high order bits are reserved for future use and should not be used Programs are free to avoid using pointers in datagrams they generate although this will reduce datagram capacity However all programs are required to understand arriving messages that contain pointers For example a datagram might need to use the domain names F ISI ARPA FOO F ISI ARPA ARPA and the root Ignoring the other fields of the message these domain names might be represented as Mockapetris Page 32 RFC 883 November 1983 Domain Names Implementation and Specification 20 1 F 22 3 I 24 S I 26 4 A 28 R P 30 A 0 40 3 F 42 O O 44 1 1 20 64 1 1 26 92 0 The domain name for F ISI ARPA is shown at offset 20 The domain name FOO F ISI ARPA is shown at offset 40 this definition uses a pointer to concatenate a label for FOO to the previously defined F ISI ARPA The domain name ARPA is defined at offset 64 using a pointer to the ARPA component of the name F ISI ARPA at 20 note that this reference relies on ARPA being the last label in the string at 20 The root domain name is defined by a single octet of zeros at 92 the root domain name has no labels Organization of the Shared database While name server implementations are free to use any internal data structures they choose the suggested structure consists of several separate trees Each tree has structure corresponding to the domain name space with RRs attached to nodes and leaves Each zone of authoritative data has a separate tree and one tree holds all non authoritative data All of the trees corresponding to zones are managed identically but the non authoritative or cache tree has different management procedures Mockapetris Page 33 RFC 883 November 1983 Domain Names Implementation and Specification Data stored in the database can be kept in whatever form is convenient for the name server so long as it can be transformed back into the format needed for messages In particular the database will probably use structure in place of expanded domain names and will also convert many of the time intervals used in the domain systems to absolute local times Each tree corresponding to a zone has complete information for a pruned subtree of the domain space The top node of a zone has a SOA record that marks the start of the zone The bottom edge of the zone is delimited by nodes containing NS records signifying delegation of authority to other zones or by leaves of the domain tree When a name server contains abutting zones one tree will have a bottom node containing a NS record and the other tree will begin with a tree location containing a SOA record Note that there is one special case that requires consideration when a name server is implemented A node that contains a SOA RR denoting a start of zone will also have NS records that identify the name servers that are expected to have a copy of the zone Thus a name server will usually find itself and possibly other redundant name servers referred to in NS records occupying the same position in the tree as SOA records The solution to this problem is to never interpret a NS record as delimiting a zone started by a SOA at the same point in the tree The sample programs in this memo deal with this problem by processing SOA records only after NS records have been processed Zones may also overlap a particular part of the name space when they are of different classes Other than the abutting and separate class cases trees are always expected to be disjoint Overlapping zones are regarded as a non fatal error The scheme described in this memo avoids the overlap issue by maintaining separate trees other designs must take the appropriate measures to defend against possible overlap Non authoritative data is maintained in a separate tree This tree is unlike the zone trees in that it may have holes Each RR in the cache tree has its own TTL that is separately managed The data in this tree is never used if authoritative data is available from a zone tree this avoids potential problems due to cached data that conflicts with authoritative data The shared database will also contain data structures to support the processing of inverse queries and completion queries if the local system supports these optional features Although many schemes are possible this memo describes a scheme that is based on tables of pointers that invert the database according to key Mockapetris Page 34 RFC 883 November 1983 Domain Names Implementation and Specification Each kind of retrieval has a separate set of tables with one table per zone When a zone is updated these tables must also be updated The contents of these tables are discussed in the Inverse query processing and Completion query processing sections of this memo The database implementation described here includes two locks that are used to control concurrent access and modification of the database by name server query processing name server maintenance operations and resolver access The first lock main lock controls access to all of the trees Multiple concurrent reads are allowed but write access can only be acquired by a single process Read and write access are mutually exclusive Resolvers and name server processes that answer queries acquire this lock in read mode and unlock upon completion of the current message This lock is acquired in write mode by a name server maintenance process when it is about to change data in the shared database The actual update procedures are described under NAME SERVER MAINTENANCE but are designed to be brief The second lock cache queue lock controls access to the cache queue This queue is used by a resolver that wishes to add information to the cache tree The resolver acquires this lock then places the RRs to be cached into the queue The name server maintenance procedure periodically acquires this lock and adds the queue information to the cache The rationale for this procedure is that it allows the resolver to operate with read only access to the shared database and allows the update process to batch cache additions and the associated costs for inversion calculations The name server maintenance procedure must take appropriate precautions to avoid problems with data already in the cache inversions etc This organization solves several difficulties When searching the domain space for the answer to a query a name server can restrict its search for authoritative data to that tree that matches the most labels on the right side of the domain name of interest Since updates to a zone must be atomic with respect to searches maintenance operations can simply acquire the main lock insert a new copy of a particular zone without disturbing other zones and then release the storage used by the old copy Assuming a central table pointing to valid zone trees this operation can be a simple pointer swap Mockapetris Page 35 RFC 883 November 1983 Domain Names Implementation and Specification TTL management of zones can be performed using the SOA record for the zone This avoids potential difficulties if individual RRs in a zone could be timed out separately This issue is discussed further in the maintenance section Query processing The following algorithm outlines processing that takes place at a name server when a query arrives 1 Search the list of zones to find zones which have the same class as the QCLASS field in the query and have a top domain name that matches the right end of the QNAME field If there are none go to step 2 If there are more than one pick the zone that has the longest match and go to step 3 2 Since the zone search failed the only possible RRs are contained in the non authoritative tree Search the cache tree for the NS record that has the same class as the QCLASS field and the largest right end match for domain name Add the NS record or records to the authority section of the response If the cache tree has RRs that are pertinent to the question domain names match classes agree not timed out and the type field is relevant to the QTYPE copy these RRs into the answer section of the response The name server may also search the cache queue Go to step 4 3 Since this zone is the best match the zone in which QNAME resides is either this zone or a zone to which this zone will directly or indirectly delegate authority Search down the tree looking for a NS RR or the node specified by QNAME If the node exists and has no NS record copy the relevant RRs to the answer section of the response and go to step 4 If a NS RR is found either matching a part or all of QNAME then QNAME is in a delegated zone outside of this zone If so copy the NS record or records into the authority section of the response and search the remainder of the zone for an A type record corresponding to the NS reference If the A record is found add it to the additional section Go to step 2 If the node is not found and a NS is not found there is no such name set the Name error bit in the response and exit 4 When this step is reached the answer and authority sections are complete What remains is to complete the additional section This procedure is only possible if the name server Mockapetris Page 36 RFC 883 November 1983 Domain Names Implementation and Specification knows the data formats implied by the class of records in the answer and authority sections Hence this procedure is class dependent Appendix 3 discusses this procedure for Internet class data While this algorithm deals with typical queries and databases several additions are required that will depend on the database supported by the name server QCLASS Special procedures are required when the QCLASS of the query is If the database contains several classes of data the query processing steps above are performed separately for each CLASS and the results are merged into a single response The name error condition is not meaningful for a QCLASS query If the requestor wants this information it must test each class independently If the database is limited to data of a particular class this operation can be performed by simply reseting the authoritative bit in the response and performing the query as if QCLASS was the class used in the database labels in database RRs Some zones will contain default RRs that use to match in cases where the search fails for a particular domain name If the database contains these records then a failure must be retried using in place of one or more labels of the search key The procedure is to replace labels from the left with s looking for a match until either all labels have been replaced or a match is found Note that these records can never be the result of caching so a name server can omit this processing for zones that don t contain RRs with in labels or can omit this processing entirely if never appears in local authoritative data Inverse query processing Name servers that support inverse queries can support these operations through exhaustive searches of their databases but this becomes impractical as the size of the database increases An alternative approach is to invert the database according to the search key For name servers that support multiple zones and a large amount of data the recommended approach is separate inversions for each Mockapetris Page 37 RFC 883 November 1983 Domain Names Implementation and Specification zone When a particular zone is changed during a refresh only its inversions need to be redone Support for transfer of this type of inversion may be included in future versions of the domain system but is not supported in this version Completion query processing Completion query processing shares many of the same problems in data structure design as are found in inverse queries but is different due to the expected high rate of use of top level labels ie ARPA CSNET A name server that wishes to be efficient in its use of memory may well choose to invert only occurrences of ARPA etc that are below the top level and use a search for the rare case that top level labels are used to constrain a completion Mockapetris Page 38 RFC 883 November 1983 Domain Names Implementation and Specification NAME SERVER MAINTENANCE Introduction Name servers perform maintenance operations on their databases to insure that the data they distribute is accurate and timely The amount and complexity of the maintenance operations that a name server must perform are related to the size change rate and complexity of the database that the name server manages Maintenance operations are fundamentally different for authoritative and non authoritative data A name server actively attempts to insure the accuracy and timeliness of authoritative data by refreshing the data from master copies Non authoritative data is merely purged when its time to live expires the name server does not attempt to refresh it Although the refreshing scheme is fairly simple to implement it is somewhat less powerful than schemes used in other distributed database systems In particular an update to the master does not immediately update copies and should be viewed as gradually percolating though the distributed database This is adequate for the vast majority of applications In situations where timliness is critical the master name server can prohibit caching of copies or assign short timeouts to copies Conceptual model of maintenance operations The vast majority of information in the domain system is derived from master files scattered among hosts that implement name servers some name servers will have no master files other name servers will have one or more master files Each master file contains the master data for a single zone of authority rather than data for the whole domain name space The administrator of a particular zone controls that zone by updating its master file Master files and zone copies from remote servers may include RRs that are outside of the zone of authority when a NS record delegates authority to a domain name that is a descendant of the domain name at which authority is delegated These forward references are a problem because there is no reasonable method to guarantee that the A type records for the delegatee are available unless they can somehow be attached to the NS records For example suppose the ARPA zone delegates authority at MIT ARPA and states that the name server is on AI MIT ARPA If a resolver gets the NS record but not the A type record for AI MIT ARPA it might try to ask the MIT name server for the address of AI MIT ARPA Mockapetris Page 39 RFC 883 November 1983 Domain Names Implementation and Specification The solution is to allow type A records that are outside of the zone of authority to be copied with the zone While these records won t be found in a search for the A type record itself they can be protected by the zone refreshing system and will be passed back whenever the name server passes back a referral to the corresponding NS record If a query is received for the A record the name server will pass back a referral to the name server with the A record in the additional section rather than answer section The only exception to the use of master files is a small amount of data stored in boot files Boot file data is used by name servers to provide enough resource records to allow zones to be imported from foreign servers e g the address of the server and to establish the name and address of root servers Boot file records establish the initial contents of the cache tree and hence can be overridden by later loads of authoritative data The data in a master file first becomes available to users of the domain name system when it is loaded by the corresponding name server By definition data from a master file is authoritative Other name servers which wish to be authoritative for a particular zone do so by transferring a copy of the zone from the name server which holds the master copy using a virtual circuit These copies include parameters which specify the conditions under which the data in the copy is authoritative In the most common case the conditions specify a refresh interval and policies to be followed when the refresh operation cannot be performed A name server may acquire multiple zones from different name servers and master files but the name server must maintain each zone separately from others and from non authoritative data When the refresh interval for a particular zone copy expires the name server holding the copy must consult the name server that holds the master copy If the data in the zone has not changed the master name server instructs the copy name server to reset the refresh interval If the data has changed the master passes a new copy of the zone and its associated conditions to the copy name server Following either of these transactions the copy name server begins a new refresh interval Copy name servers must also deal with error conditions under which they are unable to communicate with the name server that holds the master copy of a particular zone The policies that a copy name server uses are determined by other parameters in the conditions distributed with every copy The conditions include a retry interval and a maximum holding time When a copy name server is Mockapetris Page 40 RFC 883 November 1983 Domain Names Implementation and Specification unable to establish communications with a master or is unable to complete the refresh transaction it must retry the refresh operation at the rate specified by the retry interval This retry interval will usually be substantially shorter than the refresh interval Retries continue until the maximum holding time is reached At that time the copy name server must assume that its copy of the data for the zone in question is no longer authoritative Queries must be processed while maintenance operations are in progress because a zone transfer can take a long time However to avoid problems caused by access to partial databases the maintenance operations create new copies of data rather than directly modifying the old copies When the new copy is complete the maintenance process locks out queries for a short time using the main lock and switches pointers to replace the old data with the new After the pointers are swapped the maintenance process unlocks the main lock and reclaims the storage used by the old copy Name server data structures and top level logic The name server must multiplex its attention between multiple activities For example a name server should be able to answer queries while it is also performing refresh activities for a particular zone While it is possible to design a name server that devotes a separate process to each query and refresh activity in progress the model described in this memo is based on the assumption that there is a single process performing all maintenance operations and one or more processes devoted to handling queries The model also assumes the existence of shared memory for several control structures the domain database locks etc The model name server uses the following files and shared data structures 1 A configuration file that describes the master and boot files which the name server should load and the zones that the name server should attempt to load from foreign name servers This file establishes the initial contents of the status table 2 Domain data files that contain master and boot data to be loaded 3 A status table that is derived from the configuration file Each entry in this table describes a source of data Each entry has a zone number The zone number is zero for Mockapetris Page 41 RFC 883 November 1983 Domain Names Implementation and Specification non authoritative sources authoritative sources are assigned separate non zero numbers 4 The shared database that holds the domain data This database is assumed to be organized in some sort of tree structure paralleling the domain name space with a list of resource records attached to each node and leaf in the tree The elements of the resource record list need not contain the exact data present in the corresponding output format but must contain data sufficient to create the output format for example these records need not contain the domain name that is associated with the resource because that name can be derived from the tree structure Each resource record also internal data that the name server uses to organize its data 5 Inversion data structures that allow the name server to process inverse queries and completion queries Although many structures could be used the implementation described in this memo supposes that there is one array for every inversion that the name server can handle Each array contains a list of pointers to resource records such that the order of the inverted quantities is sorted 6 The main and cache queue locks 7 The cache queue The maintenance process begins by loading the status table from the configuration file It then periodically checks each entry to see if its refresh interval has elapsed If not it goes on to the next entry If so it performs different operations depending on the entry If the entry is for zone 0 or the cache tree the maintenance process checks to see if additions or deletions are required Additions are acquired from the cache queue using the cache queue lock Deletions are detected using TTL checks If any changes are required the maintenance process recalculates inversion data structures and then alters the cache tree under the protection of the main lock Whenever the maintenance process modifies the cache tree it resets the refresh interval to the minimum of the contained TTLs and the desired time interval for cache additions If the entry is not zone 0 and the entry refers to a local file the maintenance process checks to see if the file has been modified since its last load If so the file is reloaded using the procedures specified under Name server file Mockapetris Page 42 RFC 883 November 1983 Domain Names Implementation and Specification loading The refresh interval is reset to that specified in the SOA record if the file is a master file If the entry is for a remote master file the maintenance process checks for a new version using the procedure described in Names server remote zone transfer Name server file loading Master files are kept in text form for ease of editing by system maintainers These files are not exchanged by name servers name servers use the standard message format when transferring zones Organizations that want to have a domain but do not want to run a name server can use these files to supply a domain definition to another organization that will run a name server for them For example if organization X wants a domain but not a name server it can find another organization Y that has a name server and is willing to provide service for X Organization X defines domain X via the master file format and ships a copy of the master file to organization Y via mail FTP or some other method A system administrator at Y configures Y s name server to read in X s file and hence support the X domain X can maintain the master file using a text editor and send new versions to Y for installation These files have a simple line oriented format with one RR per line Fields are separated by any combination of blanks and tab characters Tabs are treated the same as spaces in the following discussion the term blank means either a tab or a blank A line can be either blank and ignored a RR or a INCLUDE line If a RR line starts with a domain name that domain name is used to specify the location in the domain space for the record i e the owner If a RR line starts with a blank it is loaded into the location specified by the most recent location specifier The location specifiers are assumed to be relative to some origin that is provided by the user of a file unless the location specifier contains the root label This provides a convenient shorthand notation and can also be used to prevent errors in master files from propagating into other zones This feature is particularly useful for master files imported from other sites An include line begins with INCLUDE starting at the first line position and is followed by a local file name and an optional offset modifier The filename follows the appropriate local conventions The offset is one or more labels that are added to the offset in use for the file that contained the INCLUDE If the offset is omitted the included file is loaded using the Mockapetris Page 43 RFC 883 November 1983 Domain Names Implementation and Specification offset of the file that contained the INCLUDE command For example a file being loaded at offset ARPA might contain the following lines INCLUDE isi data ISI INCLUDE addresses data The first line would be interpreted to direct loading of the file isi data at offset ISI ARPA The second line would be interpreted as a request to load data at offset ARPA Note that INCLUDE commands do not cause data to be loaded into a different zone or tree they are simply ways to allow data for a given zone to be organized in separate files For example mailbox data might be kept separately from host data using this mechanism Resource records are entered as a sequence of fields corresponding to the owner name TTL CLASS TYPE and RDATA components Note that this order is different from the order used in examples and the order used in the actual RRs the given order allows easier parsing and defaulting The owner name is derived from the location specifier The TTL field is optional and is expressed as a decimal number If omitted TTL defaults to zero The CLASS field is also optional if omitted the CLASS defaults to the most recent value of the CLASS field in a previous RR The RDATA fields depend on the CLASS and TYPE of the RR In general the fields that make up RDATA are expressed as decimal numbers or as domain names Some exceptions exist and are documented in the RDATA definitions in Appendicies 2 and 3 of this memo Because CLASS and TYPE fields don t contain any common identifiers and because CLASS and TYPE fields are never decimal numbers the parse is always unique Because these files are text files several special encodings are necessary to allow arbitrary data to be loaded In particular A free standing dot is used to refer to the current domain name A free standing is used to denote the current origin Mockapetris Page 44 RFC 883 November 1983 Domain Names Implementation and Specification Two free standing dots represent the null domain name of the root X where X is any character other than a digit 0 9 is used to quote that character so that its special meaning does not apply For example can be used to place a dot character in a label DDD where each D is a digit is the octet corresponding to the decimal number described by DDD The resulting octet is assumed to be text and is not checked for special meaning Parentheses are used to group data that crosses a line boundary In effect line terminations are not recognized within parentheses Semicolon is used to start a comment the remainder of the line is ignored Name server file loading example A name server for F ISI ARPA serving as an authority for the ARPA and ISI ARPA domains might use a boot file and two master files The boot file initializes some non authoritative data and would be loaded without an origin 9999999 IN NS B ISI ARPA 9999999 CS NS UDEL CSNET B ISI ARPA 9999999 IN A 10 3 0 52 UDEL CSNET 9999999 CS A 302 555 0000 This file loads non authoritative data which provides the identities and addresses of root name servers The first line contains a NS RR which is loaded at the root the second line starts with a blank and is loaded at the most recent location specifier in this case the root the third and fourth lines load RRs at B ISI ARPA and UDEL CSNET respectively The timeouts are set to high values 9999999 to prevent this data from being discarded due to timeout The first master file loads authoritative data for the ARPA domain This file is designed to be loaded with an origin of ARPA which allows the location specifiers to omit the trailing ARPA labels Mockapetris Page 45 RFC 883 November 1983 Domain Names Implementation and Specification IN SOA F ISI ARPA Action E ISI ARPA 20 SERIAL 3600 REFRESH 600

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  • to use partial information neither of these is really acceptable The change is to replace MD and MF with a new type of RR called MX which conveys similar information in a single RR type MX has been assigned a type code of 15 decimal The format of the MX RR is a 16 bit preference value followed by a domain name A node may have multiple MX RRs and multiple MX RRs with the same preference value are allowed at a given node Mockapetris Page 4 RFC 973 January 1986 Domain System Changes and Observations The preference values denote the relative preference that the mail destination places on the mail agents with lower values being better A mailer is expected to at least try the mail agent s with the lowest preference value The significance of particular preference values the units of preference and the linearity of preference values are not defined but left open preference values should only be used to establish relative rankings For example the current RRs MAIL ORG MD HOST1 MD HOST2 MF HOST3 might be replaced by MAIL ORG MX 10 HOST1 MX 10 HOST2 MX 20 HOST3 The values 10 and 20 have no significance other than 10 Y GOV 10000 A and the following to the child zone ISI EDU 10000 NS X ISI EDU 10000 NS Y GOV 50000 SOA X ISI EDU 10000 A Y GOV 10000 A Note the following In both cases the A RR for Y GOV is included even though Y GOV isn t in the EDU or ISI EDU domains This RR isn t authoritative but is included to guarantee that the address of Y GOV is passed with delegations to it Strictly speaking this RR need not be in either zone but its presence is recommended The X ISI EDU A RR is absolutely essential The only time that a server should use the glue RRs is when it is returning the NS RRs and doesn t otherwise have the address of the server For example if the parent server also was authoritative for GOV the glue RR would typically not be consulted However it is still a good idea for it to be present so that the zone is self sufficient Mockapetris Page 6 RFC 973 January 1986 Domain System Changes and Observations The child zone and the parent zone have identical NS RRs for the ISI EDU domain This guarantees that no matter which server is asked about the ISI EDU domain the same set of name servers is returned The child zone and the parent zone have A RRs for the name servers in the NS RRs that delegate the ISI EDU domain This guarantees that in addition to knowing the name servers for the ISI EDU domain the addresses of the servers are known as well The TTLs for the NS RRs that delegate the ISI EDU domain and the A RRs that represent the addresses of the name servers

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  • route disappears an alternative route if available can replace it as the new best route o If the amount of memory this consumes is problematic the routing application must keep SOME alternative routing information say a best route and two alternatives If the router ever has to discard routing information about a route it should note the fact If the routes that have been kept disappear because they have become unreachable the router MUST issue a request on all interfaces to try and obtain discarded alternatives It is recommended that the request is issued BEFORE all routes to a destination have been lost Entries in the routing database can either be permanent or temporary Entries learned from broadcasts on LANs are temporary They will expire if not periodically refreshed by further broadcasts Entries learned from a triggered response on the WAN are permanent They MUST not time out in the normal course of events The entries state MUST be changed to temporary by the following events o The arrival of a routing update containing the entry set to unreachable The normal hold down timer MUST be started after which the entry disappears from the routing database Meyer Page 9 RFC 1582 Demand RIP February 1994 o The arrival of a routing update with the entry absent If the hold down timer is not already running the entry MUST be set to unreachable and the hold down timer started o A message sent from the circuit manager to indicate that it failed to make a connection in normal running The routing table MUST be scanned for all routes via that next hop router Aging of these routing entries MUST commence If the aging timer expires the entry MUST be set to unreachable and the hold down timer started If the hold down timer expires the entry disappears from the routing database o If the interface goes down the circuit manager should indicate that all circuits on that interface have gone down Database timer values are covered in section 7 2 7 New Packet Types To support triggered updates three new packet types MUST be supported TRIGGERED REQUEST A request to the responding system to send all appropriate elements in its routing database A triggered request is retransmitted at periodic intervals until a triggered response is received Routing requests are transmitted in the following circumstances o Firstly when the router is powered on o Secondly when the circuit manager indicates a destination has been in an unreachable circuit down state for an extended period and changes to a reachable circuit up state o Thirdly in the event of all routing update fragments failing to arrive within a set period o It may also send triggered requests at other times to compensate for discarding non optimal routing information Meyer Page 10 RFC 1582 Demand RIP February 1994 TRIGGERED RESPONSE A message containing all appropriate elements of the routing database An appropriate element is an entry NOT learned from the interface to which the routing information is being sent out This is known as split horizon Stability is improved by adding poisoned reverse on routes learned from a destination This consists of also including some routes learned from a destination in routing updates sent back to that destination but setting the routes as unreachable A route is only poisoned if it is the best route rather than an inferior alternative route in the database A triggered response message may be sent in response to a triggered request or it may be an update message issued because of a change in the routing database A triggered response message MUST be sent in response to a triggered request message even if there are no routes to propagate This would be the case for a host which had a WAN interface only but which wished to run the triggered update protocol A triggered response is retransmitted at periodic intervals until a triggered acknowledgement is received TRIGGERED ACKNOWLEDGEMENT A message sent in response to every triggered response packet received Triggered response and triggered acknowledgement packets MUST contain additional fields for a sequence number fragment number and number of fragments If a triggered request or response is not acknowledged after 10 retransmissions routes to the destination should be marked as unreachable for the duration of a hold down timer before being deleted The destination should then be polled at a lower frequency using triggered request packets When a triggered response is received the router should prime the next hop router my sending its routing database through triggered response packets Meyer Page 11 RFC 1582 Demand RIP February 1994 Strictly speaking polling should occur indefinitely to guarantee database integrity However the administrator MAY wish the router to cease polling after a few attempts believing that the lack of response is due to a mis configuration of the next hop router The destination should be marked as NOT supporting the mechanism and no further routing messages should be sent to that destination Before marking the destination as not supporting the mechanism at least 5 triggered request polls without acknowledgement should be sent If a destination marked as not supporting the mechanism subsequently sends a valid triggered message the destination should be marked as supporting the mechanism once more to allow for the next hop router s configuration being changed It should be sent a triggered request and a triggered response to obtain and propagate up to date routing information 2 8 Fragmentation If a routing update is sufficiently large the information MUST be fragmented over several triggered response packets o Each fragment MUST be individually acknowledged with a triggered acknowledgement packet The sender of the routing update MUST periodically retransmit fragments which have not been acknowledged or until the destination is marked as not supporting the mechanism o A router receiving fragments MUST re assemble them before updating its routing database o If all fragments are not received within four times the retransmit period they MUST be discarded A triggered request packet MUST then be sent to the originator of the routing update On receiving the triggered request packet the originator of the routing update MUST retransmit ALL fragments o If a fragment with an updated sequence number is received ALL fragments with the earlier sequence number MUST be discarded An updated sequence number is defined as any sequence number that is different There is no concept of the value of the sequence number conveying its age Meyer Page 12 RFC 1582 Demand RIP February 1994 Fragmentation timer values are covered in section 7 2 9 Preventing Queue Overload In order to prevent too many routing messages being queued at a WAN interface the routing task MAY operate a scheme whereby broadcasting of a triggered request or triggered response to a WAN interface is staggered All routing requests or routing responses are not sent to ALL next hop routers on the interface in a single batch o The routing task should limit the number of outstanding triggered request messages for which a triggered response has not been received o The routing task should limit the number of outstanding triggered response messages for which a triggered acknowledgement has not been received As outstanding messages are appropriately acknowledged further messages can be sent out to other next hop routers until all next hop routers have been sent the message and have acknowledged it The maximum number of outstanding messages transmitted without acknowledgement is a function of the link speed and the number of other routing protocols operating the triggered update mechanism Messages should always be acknowledged immediately even if it causes the limit to be exceeded since a connection is almost certainly available This has the potential benefit of allowing the VC to close sooner on its idle timer Sending all triggered request fragments to a destination at once is also beneficial 3 IP Routing Information Protocol Version 1 This section should be read in conjunction with reference 1 IP RIP is a UDP based protocol which generally sends and receives datagrams on UDP port number 520 To support the mechanism outlined in this proposal the packet format for RIP version 1 1 is modified as shown in Figure 2 Every Routing Information Protocol datagram contains the following Meyer Page 13 RFC 1582 Demand RIP February 1994 COMMAND Commands supported in RIP Version 1 are request 1 response 2 traceon 3 traceoff 4 SUN reserved 5 The fields sequence number fragment number and number of fragments MUST NOT be included in packets with these command values The following new commands with values in brackets are required TRIGGERED REQUEST 6 A request for the responding system to send all of its routing database Only the first 4 octets of the packet format shown in figure 2 are sent since all routing information is implied by this request type TRIGGERED RESPONSE 7 A message containing all of the sender s routing database excluding those entries learned from the interface to which the routing information is being sent This message may be sent in response to a triggered request or it may be an update message resulting from a change in the routing database A triggered response message MUST be sent in response to a triggered request message even if there are no routes to propagate This would be the case for a host which had a WAN interface only but which wished to run the triggered update protocol 0 1 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 command 1 version 1 must be zero 2 The following new fields are inserted for some commands 0 1 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 sequence number 2 fragment 1 no of frags 1 Meyer Page 14 RFC 1582 Demand RIP February 1994 Followed by up to 25 routing entries each 20 octets 0 1 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 address family identifier 2 must be zero 2 IP address 4 must be zero 4 must be zero 4 metric 4 The format of an IP RIP datagram in octets with each tick mark representing one bit All fields are in network order The four octets sequence number 2 fragment number 1 and number of fragments 1 are not present in the original RIP specification They are only present if command takes the values 7 or 8 Figure 2 IP Routing Information Protocol packet format TRIGGERED ACKNOWLEDGEMENT 8 A message sent in response to every triggered response packet received Only the first 8 octets of the packet format shown in figure 2 are sent VERSION In this instance Version 1 SEQUENCE NUMBER This is a new field inserted if command takes the values 7 or 8 The sequence number MUST be incremented every time updated information is sent out on a WAN The sequence number wraps round at 65535 Meyer Page 15 RFC 1582 Demand RIP February 1994 When a triggered acknowledgement is sent the sequence number is set to the same value as the triggered response packet being acknowledged The sequence number MUST be identical over fragments If a fragment is retransmitted the sequence number MUST not change FRAGMENT NUMBER The fragment number is one for the first fragment of a routing update and is incremented for each subsequent fragment A fragment can contain up to 25 routing entries When a triggered acknowledgement is sent the fragment number is set to the same value as the triggered response packet being acknowledged NUMBER OF FRAGMENTS In a triggered response packet this indicates the number of packets required to complete the routing update This field has no relevance for triggered acknowledgement packets so should be set to zero For triggered response packets the rest of the datagram contains a list of destinations with information about each Each entry in this list contains the address family identifier 2 for IP a destination network or host and the metric for it The packet format is intended to allow RIP to carry routing information for several different protocols identifiable by the family identifier The IP address is the usual Internet address stored as 4 octets in network order The metric field contains a value between 1 and 15 inclusive specifying the current metric for the destination or the value 16 representing infinity which indicates that the destination is not reachable Each route sent by a router supersedes any previous route to the same destination from the same router The maximum datagram size is 508 octets excluding UDP and IP headers 4 IP Routing Information Protocol Version 2 An enhancement to IP RIP to include subnetting has recently become available 2 This section only describes differences from that RFC Meyer Page 16 RFC 1582 Demand RIP February 1994 The triggered update mechanism can be supported by including the triggered request 6 triggered response 7 and triggered acknowledgement 8 commands described in the previous section The sequence number fragment number and number of fragments fields are included in triggered response and triggered acknowledgement commands The triggered request packet should also contain the 4 extra octets corresponding to the sequence number fragment number and number of fragments fields but set to zero Because additional security information is included in RIP Version 2 packets this MUST be appended to the triggered request and triggered acknowledgement packets as well as being present in the triggered response packet The version number becomes 2 Other aspects of packet layout follow reference 2 5 Netware Routing Information Protocol This section should be read in conjunction with references 3 since it only describes differences from the specification Netware 3 is the trade name of Novell Research s protocols for computer communication which are derived and extended from Xerox Network System s XNS protocols 4 Netware supports a mechanism that allows routers on an internetwork to exchange routing information using the Routing Information Protocol RIP which runs over the Internetwork Packet Exchange IPX protocol using socket number 453h Netware RIP and IP RIP share a common heritage in that they are both based on XNS RIP but there is some divergence mostly at the packet format level to reflect the differing addressing schemes The triggered update mechanism can be applied to Netware RIP To support the mechanism outlined in this proposal the packet format for Netware RIP is modified as shown in Figure 3 Every datagram contains the following Meyer Page 17 RFC 1582 Demand RIP February 1994 RIP OPERATION Operations supported in standard Netware RIP are request 1 and response 2 The fields sequence number fragment number and number of fragments MUST NOT be included in packets with these operation values The following new operations are required with values chosen to be the same as for IP RIP commands 0 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 operation 2 The following new fields are inserted for some operations 0 1 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 sequence number 2 fragment 1 no of frags 1 Followed by up to 50 routing entries each 8 octets 0 1 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 network number 4 number of hops 2 number of ticks 2 The format of a Netware RIP datagram in octets with each tick mark representing one bit All fields are in network order The four octets sequence number 2 fragment number 1 and number of fragments 1 are not present in the original RIP specification They are only present if operation takes the values 7 or 8 Figure 3 Netware Routing Information Protocol packet format Meyer Page 18 RFC 1582 Demand RIP February 1994 TRIGGERED REQUEST 6 A request for the responding system to send all of its routing database Only the first 2 octets of the packet format shown in figure 3 are sent since all routing information is implied by this request type TRIGGERED RESPONSE 7 A message containing all of the sender s routing database excluding those entries learned from the interface to which the routing information is being sent This message may be sent in response to a triggered request or it may be an update message resulting from a change in the routing database A triggered response message MUST be sent in response to a triggered request message even if there are no routes to propagate This would be the case for a host which had a WAN interface only but which wished to run the triggered update protocol TRIGGERED ACKNOWLEDGEMENT 8 A message sent in response to every triggered response packet received Only the first 6 octets of the packet format shown in figure 3 are sent SEQUENCE NUMBER This is a new field inserted if operation takes the values 7 or 8 The sequence number MUST be incremented every time updated information is sent out on a WAN The sequence number wraps round at 65535 When a triggered acknowledgement is sent the sequence number is set to the same value as the triggered response packet being acknowledged Meyer Page 19 RFC 1582 Demand RIP February 1994 The sequence number MUST be identical over fragments If a fragment is retransmitted the sequence number MUST not change

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  • MODULE IDENTITY OBJECT TYPE Counter32 TimeTicks IpAddress FROM SNMPv2 SMI TEXTUAL CONVENTION RowStatus FROM SNMPv2 TC MODULE COMPLIANCE OBJECT GROUP FROM SNMPv2 CONF mib 2 FROM RFC1213 MIB This MIB module uses the extended OBJECT TYPE macro as defined in 9 rip2 MODULE IDENTITY LAST UPDATED 9407272253Z Wed Jul 27 22 53 04 PDT 1994 ORGANIZATION IETF RIP II Working Group CONTACT INFO Fred Baker Postal Cisco Systems 519 Lado Drive Santa Barbara California 93111 Tel 1 805 681 0115 E Mail fbaker cisco com Postal Gary Malkin Xylogics Inc 53 Third Avenue Burlington MA 01803 Phone 617 272 8140 EMail gmalkin Xylogics COM DESCRIPTION The MIB module to describe the RIP2 Version 2 Protocol mib 2 23 RIP 2 Management Information Base the RouteTag type represents the contents of the Route Domain field in the packet header or route entry The use of the Route Domain is deprecated RouteTag TEXTUAL CONVENTION STATUS current DESCRIPTION the RouteTag type represents the contents of the Route Domain field in the packet header or route entry SYNTAX OCTET STRING SIZE 2 Malkin Baker Page 5 RFC 1724 RIP 2 MIB Extension November 1994 4 1 Global Counters The RIP 2 Globals Group Implementation of this group is mandatory for systems which implement RIP 2 These counters are intended to facilitate debugging quickly changing routes or failing neighbors rip2Globals OBJECT IDENTIFIER rip2 1 rip2GlobalRouteChanges OBJECT TYPE SYNTAX Counter32 MAX ACCESS read only STATUS current DESCRIPTION The number of route changes made to the IP Route Database by RIP This does not include the refresh of a route s age rip2Globals 1 rip2GlobalQueries OBJECT TYPE SYNTAX Counter32 MAX ACCESS read only STATUS current DESCRIPTION The number of responses sent to RIP queries from other systems rip2Globals 2 4 2 RIP Interface Tables RIP Interfaces Groups Implementation of these Groups is mandatory for systems which implement RIP 2 The RIP Interface Status Table rip2IfStatTable OBJECT TYPE SYNTAX SEQUENCE OF Rip2IfStatEntry MAX ACCESS not accessible STATUS current DESCRIPTION A list of subnets which require separate status monitoring in RIP rip2 2 rip2IfStatEntry OBJECT TYPE Malkin Baker Page 6 RFC 1724 RIP 2 MIB Extension November 1994 SYNTAX Rip2IfStatEntry MAX ACCESS not accessible STATUS current DESCRIPTION A Single Routing Domain in a single Subnet INDEX rip2IfStatAddress rip2IfStatTable 1 Rip2IfStatEntry SEQUENCE rip2IfStatAddress IpAddress rip2IfStatRcvBadPackets Counter32 rip2IfStatRcvBadRoutes Counter32 rip2IfStatSentUpdates Counter32 rip2IfStatStatus RowStatus rip2IfStatAddress OBJECT TYPE SYNTAX IpAddress MAX ACCESS read only STATUS current DESCRIPTION The IP Address of this system on the indicated subnet For unnumbered interfaces the value 0 0 0 N where the least significant 24 bits N is the ifIndex for the IP Interface in network byte order rip2IfStatEntry 1 rip2IfStatRcvBadPackets OBJECT TYPE SYNTAX Counter32 MAX ACCESS read only STATUS current DESCRIPTION The number of RIP response packets received by the RIP process which were subsequently discarded for any reason e g a version 0 packet or an unknown command type rip2IfStatEntry 2 rip2IfStatRcvBadRoutes OBJECT TYPE SYNTAX Counter32 MAX ACCESS read only STATUS current Malkin Baker Page 7 RFC 1724 RIP 2 MIB Extension November 1994 DESCRIPTION The number of routes in valid RIP packets which were ignored for any reason e g unknown address family or invalid metric rip2IfStatEntry 3 rip2IfStatSentUpdates OBJECT TYPE SYNTAX Counter32 MAX ACCESS read only STATUS current DESCRIPTION The number of triggered RIP updates actually sent on this interface This explicitly does NOT include full updates sent containing new information rip2IfStatEntry 4 rip2IfStatStatus OBJECT TYPE SYNTAX RowStatus MAX ACCESS read create STATUS current DESCRIPTION Writing invalid has the effect of deleting this interface rip2IfStatEntry 5 The RIP Interface Configuration Table rip2IfConfTable OBJECT TYPE SYNTAX SEQUENCE OF Rip2IfConfEntry MAX ACCESS not accessible STATUS current DESCRIPTION A list of subnets which require separate configuration in RIP rip2 3 rip2IfConfEntry OBJECT TYPE SYNTAX Rip2IfConfEntry MAX ACCESS not accessible STATUS current DESCRIPTION A Single Routing Domain in a single Subnet INDEX rip2IfConfAddress rip2IfConfTable 1 Rip2IfConfEntry SEQUENCE Malkin Baker Page 8 RFC 1724 RIP 2 MIB Extension November 1994 rip2IfConfAddress IpAddress rip2IfConfDomain RouteTag rip2IfConfAuthType INTEGER rip2IfConfAuthKey OCTET STRING SIZE 0 16 rip2IfConfSend INTEGER rip2IfConfReceive INTEGER rip2IfConfDefaultMetric INTEGER rip2IfConfStatus RowStatus rip2IfConfSrcAddress IpAddress rip2IfConfAddress OBJECT TYPE SYNTAX IpAddress MAX ACCESS read only STATUS current DESCRIPTION The IP Address of this system on the indicated subnet For unnumbered interfaces the value 0 0 0 N where the least significant 24 bits N is the ifIndex for the IP Interface in network byte order rip2IfConfEntry 1 rip2IfConfDomain OBJECT TYPE SYNTAX RouteTag MAX ACCESS read create STATUS obsolete DESCRIPTION Value inserted into the Routing Domain field of all RIP packets sent on this interface DEFVAL 0000 h rip2IfConfEntry 2 rip2IfConfAuthType OBJECT TYPE SYNTAX INTEGER noAuthentication 1 simplePassword 2 md5 3 MAX ACCESS read create Malkin Baker Page 9 RFC 1724 RIP 2 MIB Extension November 1994 STATUS current DESCRIPTION The type of Authentication used on this interface DEFVAL noAuthentication rip2IfConfEntry 3 rip2IfConfAuthKey OBJECT TYPE SYNTAX OCTET STRING SIZE 0 16 MAX ACCESS read create STATUS current DESCRIPTION The value to be used as the Authentication Key whenever the corresponding instance of rip2IfConfAuthType has a value other than noAuthentication A modification of the corresponding instance of rip2IfConfAuthType does not modify the rip2IfConfAuthKey value If a string shorter than 16 octets is supplied it will be left justified and padded to 16 octets on the right with nulls 0x00 Reading this object always results in an OCTET STRING of length zero authentication may not be bypassed by reading the MIB object DEFVAL h rip2IfConfEntry 4 rip2IfConfSend OBJECT TYPE SYNTAX INTEGER doNotSend 1 ripVersion1 2 rip1Compatible 3 ripVersion2 4 ripV1Demand 5 ripV2Demand 6 MAX ACCESS read create STATUS current DESCRIPTION What the router sends on this interface ripVersion1 implies sending RIP updates compliant with RFC 1058 rip1Compatible implies broadcasting RIP 2 updates using RFC 1058 route subsumption rules ripVersion2 implies multicasting RIP 2 updates ripV1Demand indicates the use of Demand RIP on a WAN interface under RIP Version 1 rules ripV2Demand indicates the use of Malkin Baker Page 10 RFC 1724 RIP 2 MIB Extension

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    êåéìÝíïõ êáé íá äþóåé óôï ñÞóôç ôç äõíáôüôçôá íá óõíèÝóåé ìßá äçìïóßåõóç ìå åëêõóôéêü êáé êáëëéôå íéêü ôñüðï Åíáò Üëëïò ôñüðïò íá åéóÜãïõìå ìßá åéêüíá óå Ýíá ðñüãñáììá åðéôñáðÝæéïõ åêäïôéêïý óõóôÞìáôïò åßíáé íá Ý ïõìå åðéêïéíùíßá ìå Ýíá scanner Ýíá ðïíôßêé Þ ìßá êÜìåñá ãéá ôçí ðáñáãùãÞ ôçò øçöéáêÞò åéêüíáò Áõôü åßíáé ðéï áðëü ãéá ôïí ñÞóôç áöïý åßíáé äõíáôüí íá åðáíáëáìâÜíåé ãñÞãïñá ôá Ýñãá ó åäßáóçò øçöéïðïßçóçò êáé óýíèåóçò óåëßäùí Åíôïýôïéò ëüãù ôùí ðåñéïñéóìþí ðïõ õðÜñ ïõí ôá ðåñéóóüôåñá ðáêÝôá åðéôñáðÝæéùí åêäïôéêþí óõóôçìÜôùí äåí ðåñéëáìâÜíïõí ëåéôïõñãßåò ïé ïðïßåò íá åëÝã ïõí scanner Þ íá åðéôñÝðïõí óå Ýíá ñÞóôç íá êÜíåé ëåðôïìåñÞ ó åäßáóç Åôóé Ýíá óçìåßï êëåéäß ãéá ôá åðéôñáðÝæéá åêäïôéêÜ óõóôÞìáôá åßíáé ç ìåôáöïñÜ ðëçñïöïñéþí ìåôáîý åéäéêþí ðñïãñáììÜôùí êáé åöáñìïãþí ôá ïðïßá ìðïñïýí íá êÜíïõí ðïëëÝò äéáöïñåôéêÝò åñãáóßåò äïõëåýïíôáò ìáæß ãéá ôçí ðáñáãùãÞ ôïõ ôåëéêïý áðïôåëÝóìáôïò ÁõôÜ üëá åðéêïéíùíïýí ìåôáîý ôïõò äéá ìÝóù áñ åßùí ìå äéáöïñåôéêÞ äéÜôáîç Óôçí ïñãÜíùóç êáé äïìÞ ôùí áñ åßùí ðïõ ñçóéìïðïéïýíôáé óôá åðéôñáðÝæéá åêäïôéêÜ óõóôÞìáôá äßíåôáé ìåãÜëç óçìáóßá Ç äéÜôáîç ôùí áñ åßùí ðñÝðåé íá óõãêñïôåßôáé Ýôóé þóôå íá õðïóôçñßæåé ôá áðáñáßôçôá áñáêôçñéóôéêÜ ðïõ ñåéÜæïíôáé áðü ôéò óõóêåõÝò ðáñáãùãÞò åéêüíáò Åíôïýôïéò ðñÝðåé áêüìá íá ëÜâïõìå õðüøç ìáò üôé ïé åöáñìïãÝò åðéôñáðÝæéùí åêäïôéêþí óõóôçìÜôùí áíáðôýóóïíôáé åõêïëüôåñá åÜí áðáéôïýíôáé ëéãüôåñåò äéáöïñåôéêÝò äéáôÜîåéò áñ åßùí ãéá ôçí õðïóôÞñéîç ôùí åéäéêþí ðñïãñáììÜôùí åöáñìïãÞò Ç äéÜôáîç áñ åßùí åéêüíáò ìå åôéêÝôá TIFF åßíáé ìßá êáéíïýñãéá äéÜôáîç áñ åßùí ç ïðïßá ðëçñåß áõôÝò ôéò ðñïûðïèÝóåéò Ôá TIFF Ý ïõí áñáêôçñéóôéêÜ ôá ïðïßá êáëýðôïõí ôéò ðåñéóóüôåñåò áðáéôÞóåéò ôùí óõóêåýùí åéóüäïõ üðùò scanner ðñïãñÜììáôá ó åäßáóçò êáé êÜìåñåò Ôá TIFF åðßóçò ðåñéëáìâÜíïõí áñáêôçñéóôéêÜ ðïõ äåí õðÜñ ïõí óå Üëëåò äéáôÜîåéò áñ åßùí üðùò ç õðïóôÞñéîç ìßáò ðïéêéëßáò áðü ôå íéêÝò óõìðßåóçò äåäïìÝíùí êáé ç åõåëéîßá íá ðñïóèÝôïíôáé åýêïëá íÝá áñáêôçñéóôéêÜ ìå åëåã üìåíï ôñüðï Ç äéÜôáîç ôùí áñ åßùí TIFF

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    ïñãÜíùóç ç ïðïßá óõ íÜ ñçóéìïðïéåßôáé êáé óôçí ó åäßáóç âÜóåùí äåäïìÝíùí Óôï ó Þìá 1 2 âëÝðïõìå Ýíá äéÜãñáììá ìå ìåñéêÝò ðëçñïöïñßåò åíüò áñ åßïõ TIFF ÊÜèå åôéêÝôá ôïõ áñ åßïõ TIFF åßíáé ìßá åðéêåöáëßäá ç ïðïßá ðåñéãñÜöåé ðëçñïöïñßåò ðïõ ðåñéëáìâÜíïíôáé óôï áñ åßï Ç êÜèå åôéêÝôá ùñßæåôáé óå åðéìÝñïõò ðåäßá ÌÝóá óå Ýíá áñ åßï TIFF ïé åôéêÝôåò äåí Ý ïõí êáèïñéóìÝíåò èÝóåéò ÊÜèå åôéêÝôá áíáãíùñßæåôáé åðåéäÞ ôï ðñþôï ðåäßï ôçò äçëþíåé ôï üíïìá ôçò Ç åôéêÝôá ðåñéëáìâÜíåé êáé Üëëá ðåäßá ôá ïðïßá ðåñéãñÜöïõí ôï ìÞêïò êáé ôï ìÝãåèïò ôçò ðåñéãñáöüìåíçò ðëçñïöïñßáò ÔÝëïò áêïëïõèåß ç ßäéá ç ðëçñïöïñßá Þ ï äåßêôçò ôçò äéåýèõíóçò ôçò ðñáãìáôéêÞò ðëçñïöïñßáò ôçò åôéêÝôáò óôçí ðåñßðôùóç ðïõ ç ðëçñïöïñßá åßíáé ìåãáëýôåñç áðü ôÝóóåñá BYTE ðåñéóóüôåñåò ðëçñïöïñßåò ðÜíù óôéò åôéêÝôåò èá âñåßôå óôéò åðüìåíåò åíüôçôåò Ðáñáäåßãìáôïò Üñçí ãéá íá âñïýìå ôçí áíÜëõóç ôçò åéêüíáò óôïí Üîïíá ñçóéìïðïéåßôáé ç åôéêÝôá ÁÍÁËÕÓÇ ÓÔÏÍ ÁÎÏÍÁ Ç åôéêÝôá åíôïðßæåôáé äéáâÜæïíôáò ôïõò êùäéêïýò üëùí ôùí åôéêåôþí åíþ óôá ôÝóóåñá ôåëåõôáßá ôçò BYTE äåí ðåñéÝ åé ôçí ðñáãìáôéêÞ ðëçñïöïñßá äåäïìÝíá áëëÜ ôçí äéåýèõíóç ðïõ âñßóêåôáé ç ðëçñïöïñßá ìÝóá óôï áñ åßï ÊÜèå ðëçñïöïñßá ðïõ ðåñéãñÜöåé ôçí åéêüíá áðáéôåß ìßá ìïíáäéêÞ åôéêÝôá Ãéá ðáñÜäåéãìá õðÜñ åé ìßá åôéêÝôá ãéá ôçí ÁÍÁËÕÓÇ ÓÔÏÍ

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    åßá äåí ñåéÜæåôáé íá îáíáãñáöôïýí åêôüò êáé åÜí èÝëïõí íá êÜíïõí ñÞóç ôçò íÝáò åôéêÝôáò Ïé åöáñìïãÝò áíÜãíùóçò ìðïñïýí óôéò ðåñéóóüôåñåò ðåñéðôþóåéò íá áãíïÞóïõí ìå áóöÜëåéá ôéò ìç áíáãíùñßóéìåò åôéêÝôåò üðùò ç ÇÌÅÑÏÌÇÍÉÁ Ó ÅÄÉÁÓÇÓ Óå áíôßèåóç ìßá äéÜôáîç ìå óôáèåñÞ ïñãÜíùóç óõíÞèùò áðáéôåß íá îáíáãñáöôïýí üëåò ïé åöáñìïãÝò êÜèå öïñÜ ðïõ ðñïóèÝôïõìå Ýíá íÝï ðåäßï Ç åðáíåããñáöÞ oëüêëçñïõ ôïõ ëïãéóìéêïý åßíáé áðáñáßôçôç áêüìá êáé Ýáí ìåñéêÝò åöáñìïãÝò äåí èÝëïõí íá ñçóéìïðïéÞóïõí ôçí íÝá ðëçñïöïñßá ÌåñéêÝò áëëáãÝò óôï ìÝëëïí üìùò åßíáé äõíáôüí íá åðçñåÜóïõí ìßá õðÜñ ïõóá åöáñìïãÞ Ãéá ðáñÜäåéãìá áí ìåñéêÝò ðñüóèåôåò ôñïðïðïéÞóåéò ðñÝðåé íá ãßíïõí ãéá ôçí õðïóôÞñéîç åíüò íÝïõ åßäïõò äåäïìÝíùí åéêüíáò ìßá õðÜñ ïõóá åöáñìïãÞ åßíáé ðéèáíüí íá ìçí ìðïñåß íá åîçãÞóåé êáôÜëëçëá ôá äåäïìÝíá ôçò åéêüíáò Ïé ðåñéóóüôåñïé ôýðïé äåäïìÝíùí åéêüíùí üðùò ìå áðï ñþóåéò ôïõ ãêñé êáé Ýã ñùìåò Ý ïõí ïñéóôåß þóôå íá êÜíïõí ôç äéÜôáîç TIFF üóï ðéï åõÝëéêôç ãßíåôáé Åíá Üëëï ðëåïíÝêôçìá ôùí TIFF åßíáé üôé ìðïñïýí íá áíôáðïêñéèïýí óå ìéá ìåãÜëç ðïéêéëßá åéêüíùí Ôá TIFF áñ åßá óõìðåñéëáìâÜíïõí åôéêÝôåò ìå ðëçñïöïñßåò ü é ìüíï ãéá ôï ýøïò êáé ôï ðëÜôïò ìßáò åéêüíáò áëëÜ åðßóçò ãéá ôçí áíÜëõóç ôçò åéêüíáò Áêüìá õðïóôçñßæïõí äåäïìÝíá ãéá åéêüíåò ìå áðï ñþóåéò ôïõ ãêñé êáé Ýã ñùìåò åíþ áêüìá ìáò åðéôñÝðïõí íá ñçóéìïðïéïýìå ôüóï ãíùóôïýò üóï êáé éäéùôéêïýò áëãüñéèìïõò óõìðßåóçò äåäïìÝíùí ÕðÜñ ïõí üìùò êáé ìåñéêÜ ìåéïíåêôÞìáôá óôá TIFF ðïõ èá ðñÝðåé íá áíôéìåôùðßæïíôáé Ýîõðíá Ç åõåëéîßá ôùí TIFF èÝôåé Ýíá ðñüâëçìá ôï ïðïßï åßíáé üôé äåí õðÜñ åé Ýíáò êáèïñéóìÝíïò ôýðïò áñ åßïõ TIFF êáé äéÜöïñåò åôáéñßåò ìðïñïýí íá åðéëÝãïõí ôçí õðïóôÞñéîç äéÜöïñùí ðåäßùí åôéêåôþí TIFF Äçë êáíåßò äåí ìáò åããõÜôáé üôé Ýíá ðñüãñáììá åðéôñáðÝæéïõ åêäïôéêïý óõóôÞìáôïò èá äéáâÜæåé ïðïéáäÞðïôå óõëëïãÞ åôéêåôþí TIFF Åôóé õðÜñ åé ìßá óïâáñÞ áðáßôçóç ãéá ðñüóèåôï óõíôïíéóìü ðÝñá áðü ôçí ôÞñçóç ôùí êáíüíùí ôùí TIFF Ãéá áðïôåëåóìáôéêü åéñéóìü áõôïý

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    øçöéáêÜ äåäïìÝíá ôçò åéêüíáò ÃåíéêÜ ç åðéêåöáëßäá äßíåé ôçí Üêñç ôïõ íÞìáôïò ãéá ôçí ðáñáðÝñá åñìçíåßá ôùí ðëçñïöïñéþí ôïõ áñ åßïõ TIFF Ç åðéêåöáëßäá ðåñéÝ åé äåäïìÝíá ãéá ôçí äéÜôáîç ôùí BYTE INTEL Þ MOTOROLA ôïí áñéèìü ôçò âåëôéùìÝíçò Ýêäïóçò êáé ôïí äåßêôç ôïõ ðñþôïõ êáôáëüãïõ äéåýèõíóç Óôï ôìÞìá ôïõ êáôáëüãïõ ðåñéãñÜöåôáé ïôéäÞðïôå åßíáé áðáñáßôçôï ãéá íá ìðïñÝóïõìå íá ñçóéìïðïéÞóïõìå ôïí áêïëïõèüìåíï áðü ôïí êáôÜëïãï Üñôç øçöéáêþí äåäïìÝíùí åéêüíáò Åíáò êáôÜëïãïò áðïôåëåßôáé áðü åôéêÝôåò êáé Ýíá äåßêôç ãéá ôïí åðüìåíï êáôÜëïãï åÜí õðÜñ åé Ç ïñãÜíùóç áñ åßùí TIFF åðéôñÝðåé ðïëëáðëïýò êáôáëüãïõò ìÝóá óå Ýíá áðëü áñ åßï ÊÜèå êáôÜëïãïò áíáöÝñåôáé óå ðëçñïöïñßåò ìéáò îå ùñéóôÞò øçöéáêÞò áðåéêüíéóçò Ãéá ðáñÜäåéãìá ðïëëáðëïß êáôÜëïãïé êáé øçöéáêÝò áðåéêïíßóåéò ìðïñïýí íá ñçóéìïðïéïýíôáé ãéá íá áðïèçêåýïõí ìéá åéêüíá ìå ðëÞñç áíÜëõóç ãéá åêôýðùóç êáé ôçí ßäéá åéêüíá ìå ðåñéïñéóìÝíç áíÜëõóç ãéá óêïðïýò ðáñïõóßáóçò Ðïëëáðëïß åðßóçò êáôÜëïãïé ñçóéìïðïéïýíôáé êáé ãéá ôçí áðïèÞêåõóç ôùí óåëßäùí åéêüíùí åíüò ìåôáâéâáæüìåíïõ åããñÜöïõ ðïëëþí óåëßäùí ìå FAX Ôï ìÞêïò ôçò åôéêÝôáò åßíáé êáô áñ Þí óôáèåñü 12 BYTE áëëÜ üôáí ôá äåäïìÝíá ôçò åôéêÝôáò äåí ùñÜíå óôïí ðñïêáèïñéóìÝíï þñï ôïõ ðåäßïõ äåäïìÝíùí 4 BYTE ôïðïèåôïýíôáé ìå áíÜëïãç äåéêôïðïßçóç óôçí ôñßôç ðåñéï Þ ôïõ áñ åßïõ ôï ôìÞìá Þ ôá ôìÞìáôá õðåñ

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