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diff --git a/specs/draft-cheshire-dnsext-dns-sd-04.txt b/specs/draft-cheshire-dnsext-dns-sd-04.txt new file mode 100644 index 0000000..3179028 --- /dev/null +++ b/specs/draft-cheshire-dnsext-dns-sd-04.txt @@ -0,0 +1,2205 @@ +Document: draft-cheshire-dnsext-dns-sd-04.txt Stuart Cheshire +Internet-Draft Marc Krochmal +Category: Standards Track Apple Computer, Inc. +Expires 10th February 2007 10th August 2006 + + DNS-Based Service Discovery + + <draft-cheshire-dnsext-dns-sd-04.txt> + +Status of this Memo + + By submitting this Internet-Draft, each author represents that any + applicable patent or other IPR claims of which he or she is aware + have been or will be disclosed, and any of which he or she becomes + aware will be disclosed, in accordance with Section 6 of BCP 79. + For the purposes of this document, the term "BCP 79" refers + exclusively to RFC 3979, "Intellectual Property Rights in IETF + Technology", published March 2005. + + Internet-Drafts are working documents of the Internet Engineering + Task Force (IETF), its areas, and its working groups. Note that + other groups may also distribute working documents as Internet- + Drafts. + + Internet-Drafts are draft documents valid for a maximum of six months + and may be updated, replaced, or obsoleted by other documents at any + time. It is inappropriate to use Internet-Drafts as reference + material or to cite them other than as "work in progress." + + The list of current Internet-Drafts can be accessed at + http://www.ietf.org/1id-abstracts.html + + The list of Internet-Draft Shadow Directories can be accessed at + http://www.ietf.org/shadow.html + + +Abstract + + This document describes a convention for naming and structuring DNS + resource records. Given a type of service that a client is looking + for, and a domain in which the client is looking for that service, + this convention allows clients to discover a list of named instances + of that desired service, using only standard DNS queries. In short, + this is referred to as DNS-based Service Discovery, or DNS-SD. + + + + + + + + + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 1] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + +Table of Contents + + 1. Introduction...................................................3 + 2. Conventions and Terminology Used in this Document..............4 + 3. Design Goals...................................................4 + 4. Service Instance Enumeration...................................5 + 4.1 Structured Instance Names......................................5 + 4.2 User Interface Presentation....................................7 + 4.3 Internal Handling of Names.....................................7 + 4.4 What You See Is What You Get...................................8 + 4.5 Ordering of Service Instance Name Components...................9 + 5. Service Name Resolution.......................................11 + 6. Data Syntax for DNS-SD TXT Records............................12 + 6.1 General Format Rules for DNS TXT Records......................12 + 6.2 DNS TXT Record Format Rules for use in DNS-SD.................13 + 6.3 DNS-SD TXT Record Size........................................14 + 6.4 Rules for Names in DNS-SD Name/Value Pairs....................14 + 6.5 Rules for Values in DNS-SD Name/Value Pairs...................16 + 6.6 Example TXT Record............................................17 + 6.7 Version Tag...................................................17 + 7. Application Protocol Names....................................18 + 7.1 Selective Instance Enumeration................................19 + 7.2 Service Name Length Limits....................................20 + 8. Flagship Naming...............................................22 + 9. Service Type Enumeration......................................23 + 10. Populating the DNS with Information...........................24 + 11. Relationship to Multicast DNS.................................24 + 12. Discovery of Browsing and Registration Domains................25 + 13. DNS Additional Record Generation..............................26 + 14. Comparison with Alternative Service Discovery Protocols.......27 + 15. Real Examples.................................................29 + 16. User Interface Considerations.................................30 + 16.1 Service Advertising User-Interface Considerations.............30 + 16.2 Client Browsing User-Interface Considerations.................31 + 17. IPv6 Considerations...........................................34 + 18. Security Considerations.......................................34 + 19. IANA Considerations...........................................34 + 20. Acknowledgments...............................................35 + 21. Deployment History............................................35 + 22. Copyright Notice..............................................36 + 23. Normative References..........................................37 + 24. Informative References........................................37 + 25. Authors' Addresses............................................38 + + + + + + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 2] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + +1. Introduction + + This document describes a convention for naming and structuring DNS + resource records. Given a type of service that a client is looking + for, and a domain in which the client is looking for that service, + this convention allows clients to discover a list of named instances + of a that desired service, using only standard DNS queries. In short, + this is referred to as DNS-based Service Discovery, or DNS-SD. + + This document proposes no change to the structure of DNS messages, + and no new operation codes, response codes, resource record types, + or any other new DNS protocol values. This document simply proposes + a convention for how existing resource record types can be named and + structured to facilitate service discovery. + + This proposal is entirely compatible with today's existing unicast + DNS server and client software. + + Note that the DNS-SD service does NOT have to be provided by the same + DNS server hardware that is currently providing an organization's + conventional host name lookup service (the service we traditionally + think of when we say "DNS"). By delegating the "_tcp" subdomain, + all the workload related to DNS-SD can be offloaded to a different + machine. This flexibility, to handle DNS-SD on the main DNS server, + or not, at the network administrator's discretion, is one of the + things that makes DNS-SD so compelling. + + Even when the DNS-SD functions are delegated to a different machine, + the benefits of using DNS remain: It is mature technology, well + understood, with multiple independent implementations from different + vendors, a wide selection of books published on the subject, and an + established workforce experienced in its operation. In contrast, + adopting some other service discovery technology would require every + site in the world to install, learn, configure, operate and maintain + some entirely new and unfamiliar server software. Faced with these + obstacles, it seems unlikely that any other service discovery + technology could hope to compete with the ubiquitous deployment + that DNS already enjoys. + + This proposal is also compatible with (but not dependent on) the + proposal outlined in "Multicast DNS" [mDNS]. + + + + + + + + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 3] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + +2. Conventions and Terminology Used in this Document + + The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", + "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this + document are to be interpreted as described in "Key words for use in + RFCs to Indicate Requirement Levels" [RFC 2119]. + + +3. Design Goals + + A good service discovery protocol needs to have many properties, + three of which are mentioned below: + + (i) The ability to query for services of a certain type in a certain + logical domain and receive in response a list of named instances + (network browsing, or "Service Instance Enumeration"). + + (ii) Given a particular named instance, the ability to efficiently + resolve that instance name to the required information a client needs + to actually use the service, i.e. IP address and port number, at the + very least (Service Name Resolution). + + (iii) Instance names should be relatively persistent. If a user + selects their default printer from a list of available choices today, + then tomorrow they should still be able to print on that printer -- + even if the IP address and/or port number where the service resides + have changed -- without the user (or their software) having to repeat + the network browsing step a second time. + + In addition, if it is to become successful, a service discovery + protocol should be so simple to implement that virtually any + device capable of implementing IP should not have any trouble + implementing the service discovery software as well. + + These goals are discussed in more detail in the remainder of this + document. A more thorough treatment of service discovery requirements + may be found in "Requirements for a Protocol to Replace AppleTalk + NBP" [NBP]. That document draws upon examples from two decades of + operational experience with AppleTalk Name Binding Protocol to + develop a list of universal requirements which are broadly + applicable to any potential service discovery protocol. + + + + + + + + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 4] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + +4. Service Instance Enumeration + + DNS SRV records [RFC 2782] are useful for locating instances of a + particular type of service when all the instances are effectively + indistinguishable and provide the same service to the client. + + For example, SRV records with the (hypothetical) name + "_http._tcp.example.com." would allow a client to discover a list of + all servers implementing the "_http._tcp" service (i.e. Web servers) + for the "example.com." domain. The unstated assumption is that all + these servers offer an identical set of Web pages, and it doesn't + matter to the client which of the servers it uses, as long as it + selects one at random according to the weight and priority rules + laid out in RFC 2782. + + Instances of other kinds of service are less easily interchangeable. + If a word processing application were to look up the (hypothetical) + SRV record "_ipp._tcp.example.com." to find the list of IPP printers + at Example Co., then picking one at random and printing on it would + probably not be what the user wanted. + + The remainder of this section describes how SRV records may be used + in a slightly different way to allow a user to discover the names + of all available instances of a given type of service, in order to + select the particular instance the user desires. + + +4.1 Structured Instance Names + + This document borrows the logical service naming syntax and semantics + from DNS SRV records, but adds one level of indirection. Instead of + requesting records of type "SRV" with name "_ipp._tcp.example.com.", + the client requests records of type "PTR" (pointer from one name to + another in the DNS namespace). + + In effect, if one thinks of the domain name "_ipp._tcp.example.com." + as being analogous to an absolute path to a directory in a file + system then the PTR lookup is akin to performing a listing of that + directory to find all the files it contains. (Remember that domain + names are expressed in reverse order compared to path names: An + absolute path name is read from left to right, beginning with a + leading slash on the left, and then the top level directory, then + the next level directory, and so on. A fully-qualified domain name is + read from right to left, beginning with the dot on the right -- the + root label -- and then the top level domain to the left of that, and + the second level domain to the left of that, and so on. If the fully- + qualified domain name "_ipp._tcp.example.com." were expressed as a + file system path name, it would be "/com/example/_tcp/_ipp".) + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 5] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + The result of this PTR lookup for the name "<Service>.<Domain>" is a + list of zero or more PTR records giving Service Instance Names of the + form: + + Service Instance Name = <Instance> . <Service> . <Domain> + + The <Instance> portion of the Service Instance Name is a single DNS + label, containing arbitrary precomposed UTF-8-encoded text [RFC + 3629]. It is a user-friendly name, meaning that it is allowed to + contain any characters, without restriction, including spaces, upper + case, lower case, punctuation -- including dots -- accented + characters, non-roman text, and anything else that may be represented + using UTF-8. DNS recommends guidelines for allowable characters for + host names [RFC 1033][RFC 1034][RFC 1035], but Service Instance Names + are not host names. Service Instance Names are not intended to ever + be typed in by a normal user; the user selects a Service Instance + Name by selecting it from a list of choices presented on the screen. + + Note that just because this protocol supports arbitrary UTF-8-encoded + names doesn't mean that any particular user or administrator is + obliged to make use of that capability. Any user is free, if they + wish, to continue naming their services using only letters, digits + and hyphens, with no spaces, capital letters, or other punctuation. + + DNS labels are currently limited to 63 octets in length. UTF-8 + encoding can require up to four octets per Unicode character, which + means that in the worst case, the <Instance> portion of a name could + be limited to fifteen Unicode characters. However, the Unicode + characters with longer UTF-8 encodings tend to be the more obscure + ones, and tend to be the ones that convey greater meaning per + character. + + Note that any character in the commonly-used 16-bit Unicode space + can be encoded with no more than three octets of UTF-8 encoding. This + means that an Instance name can contain up to 21 Kanji characters, + which is a sufficiently expressive name for most purposes. + + The <Service> portion of the Service Instance Name consists of a pair + of DNS labels, following the established convention for SRV records + [RFC 2782], namely: the first label of the pair is the Application + Protocol Name, and the second label is either "_tcp" or "_udp", + depending on the transport protocol used by the application. + More details are given in Section 7, "Application Protocol Names". + + The <Domain> portion of the Service Instance Name specifies the DNS + subdomain within which the service names are registered. It may be + "local", meaning "link-local Multicast DNS" [mDNS], or it may be + a conventional unicast DNS domain name, such as "apple.com.", + "cs.stanford.edu.", or "eng.us.ibm.com." Because service names are + not host names, they are not constrained by the usual rules for host + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 6] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + names [RFC 1033][RFC 1034][RFC 1035], and rich-text service + subdomains are allowed and encouraged, for example: + + Building 2, 1st Floor.apple.com. + Building 2, 2nd Floor.apple.com. + Building 2, 3rd Floor.apple.com. + Building 2, 4th Floor.apple.com. + + In addition, because Service Instance Names are not constrained by + the limitations of host names, this document recommends that they + be stored in the DNS, and communicated over the wire, encoded as + straightforward canonical precomposed UTF-8, Unicode Normalization + Form C [UAX15]. In cases where the DNS server returns an NXDOMAIN + error for the name in question, client software MAY choose to retry + the query using "Punycode" [RFC 3492] encoding, if possible. + + +4.2 User Interface Presentation + + The names resulting from the PTR lookup are presented to the user in + a list for the user to select one (or more). Typically only the first + label is shown (the user-friendly <Instance> portion of the name). In + the common case, the <Service> and <Domain> are already known to the + user, these having been provided by the user in the first place, by + the act of indicating the service being sought, and the domain in + which to look for it. Note: The software handling the response + should be careful not to make invalid assumptions though, since it + *is* possible, though rare, for a service enumeration in one domain + to return the names of services in a different domain. Similarly, + when using subtypes (see "Selective Instance Enumeration") the + <Service> of the discovered instance my not be exactly the same as + the <Service> that was requested. + + Having chosen the desired named instance, the Service Instance + Name may then be used immediately, or saved away in some persistent + user-preference data structure for future use, depending on what is + appropriate for the application in question. + + +4.3 Internal Handling of Names + + If the <Instance>, <Service> and <Domain> portions are internally + concatenated together into a single string, then care must be taken + with the <Instance> portion, since it is allowed to contain any + characters, including dots. + + Any dots in the <Instance> portion should be escaped by preceding + them with a backslash ("." becomes "\."). Likewise, any backslashes + in the <Instance> portion should also be escaped by preceding them + with a backslash ("\" becomes "\\"). Having done this, the three + components of the name may be safely concatenated. The backslash- + + +Expires 10th February 2007 Cheshire & Krochmal [Page 7] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + escaping allows literal dots in the name (escaped) to be + distinguished from label-separator dots (not escaped). + + The resulting concatenated string may be safely passed to standard + DNS APIs like res_query(), which will interpret the string correctly + provided it has been escaped correctly, as described here. + + +4.4 What You See Is What You Get + + Some service discovery protocols decouple the true service identifier + from the name presented to the user. The true service identifier used + by the protocol is an opaque unique id, often represented using a + long string of hexadecimal digits, and should never be seen by the + typical user. The name presented to the user is merely one of the + ephemeral attributes attached to this opaque identifier. + + The problem with this approach is that it decouples user perception + from reality: + + * What happens if there are two service instances, with different + unique ids, but they have inadvertently been given the same + user-visible name? If two instances appear in an on-screen list + with the same name, how does the user know which is which? + + * Suppose a printer breaks down, and the user replaces it with + another printer of the same make and model, and configures the + new printer with the exact same name as the one being replaced: + "Stuart's Printer". Now, when the user tries to print, the + on-screen print dialog tells them that their selected default + printer is "Stuart's Printer". When they browse the network to see + what is there, they see a printer called "Stuart's Printer", yet + when the user tries to print, they are told that the printer + "Stuart's Printer" can't be found. The hidden internal unique id + that the software is trying to find on the network doesn't match + the hidden internal unique id of the new printer, even though its + apparent "name" and its logical purpose for being there are the + same. To remedy this, the user typically has to delete the print + queue they have created, and then create a new (apparently + identical) queue for the new printer, so that the new queue will + contain the right hidden internal unique id. Having all this hidden + information that the user can't see makes for a confusing and + frustrating user experience, and exposing long ugly hexadecimal + strings to the user and forcing them to understand what they mean + is even worse. + + * Suppose an existing printer is moved to a new department, and given + a new name and a new function. Changing the user-visible name of + that piece of hardware doesn't change its hidden internal unique + id. Users who had previously created print queues for that printer + will still be accessing the same hardware by its unique id, even + + +Expires 10th February 2007 Cheshire & Krochmal [Page 8] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + though the logical service that used to be offered by that hardware + has ceased to exist. + + To solve these problems requires the user or administrator to be + aware of the supposedly hidden unique id, and to set its value + correctly as hardware is moved around, repurposed, or replaced, + thereby contradicting the notion that it is a hidden identifier that + human users never need to deal with. Requiring the user to understand + this expert behind-the-scenes knowledge of what is *really* going on + is just one more burden placed on the user when they are trying to + diagnose why their computers and network devices are not working as + expected. + + These anomalies and counter-intuitive behaviors can be eliminated by + maintaining a tight bidirectional one-to-one mapping between what + the user sees on the screen and what is really happening "behind + the curtain". If something is configured incorrectly, then that is + apparent in the familiar day-to-day user interface that everyone + understands, not in some little-known rarely-used "expert" interface. + + In summary: The user-visible name is the primary identifier for a + service. If the user-visible name is changed, then conceptually + the service being offered is a different logical service -- even + though the hardware offering the service stayed the same. If the + user-visible name doesn't change, then conceptually the service being + offered is the same logical service -- even if the hardware offering + the service is new hardware brought in to replace some old equipment. + + There are certainly arguments on both sides of this debate. + Nonetheless, the designers of any service discovery protocol have + to make a choice between between having the primary identifiers be + hidden, or having them be visible, and these are the reasons that + we chose to make them visible. We're not claiming that there are no + disadvantages of having primary identifiers be visible. We considered + both alternatives, and we believe that the few disadvantages + of visible identifiers are far outweighed by the many problems + caused by use of hidden identifiers. + + +4.5 Ordering of Service Instance Name Components + + There have been questions about why services are named using DNS + Service Instance Names of the form: + + Service Instance Name = <Instance> . <Service> . <Domain> + + instead of: + + Service Instance Name = <Service> . <Instance> . <Domain> + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 9] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + There are three reasons why it is beneficial to name service + instances with the parent domain as the most-significant (rightmost) + part of the name, then the abstract service type as the next-most + significant, and then the specific instance name as the + least-significant (leftmost) part of the name: + + +4.5.1. Semantic Structure + + The facility being provided by browsing ("Service Instance + Enumeration") is effectively enumerating the leaves of a tree + structure. A given domain offers zero or more services. For each + of those service types, there may be zero or more instances of + that service. + + The user knows what type of service they are seeking. (If they are + running an FTP client, they are looking for FTP servers. If they have + a document to print, they are looking for entities that speak some + known printing protocol.) The user knows in which organizational or + geographical domain they wish to search. (The user does not want a + single flat list of every single printer on the planet, even if such + a thing were possible.) What the user does not know in advance is + whether the service they seek is offered in the given domain, or if + so, how many instances are offered, and the names of those instances. + Hence having the instance names be the leaves of the tree is + consistent with this semantic model. + + Having the service types be the terminal leaves of the tree would + imply that the user knows the domain name, and already knows the + name of the service instance, but doesn't have any idea what the + service does. We would argue that this is a less useful model. + + +4.5.2. Network Efficiency + + When a DNS response contains multiple answers, name compression works + more effectively if all the names contain a common suffix. If many + answers in the packet have the same <Service> and <Domain>, then each + occurrence of a Service Instance Name can be expressed using only + the <Instance> part followed by a two-byte compression pointer + referencing a previous appearance of "<Service>.<Domain>". This + efficiency would not be possible if the <Service> component appeared + first in each name. + + +4.5.3. Operational Flexibility + + This name structure allows subdomains to be delegated along logical + service boundaries. For example, the network administrator at Example + Co. could choose to delegate the "_tcp.example.com." subdomain to a + different machine, so that the machine handling service discovery + + +Expires 10th February 2007 Cheshire & Krochmal [Page 10] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + doesn't have to be the same as the machine handling other day-to-day + DNS operations. (It *can* be the same machine if the administrator so + chooses, but the point is that the administrator is free to make that + choice.) Furthermore, if the network administrator wishes to delegate + all information related to IPP printers to a machine dedicated to + that specific task, this is easily done by delegating the + "_ipp._tcp.example.com." subdomain to the desired machine. It is + also convenient to set security policies on a per-zone/per-subdomain + basis. For example, the administrator may choose to enable DNS + Dynamic Update [RFC 2136] [RFC 3007] for printers registering + in the "_ipp._tcp.example.com." subdomain, but not for other + zones/subdomains. This easy flexibility would not exist if the + <Service> component appeared first in each name. + + +5. Service Name Resolution + + Given a particular Service Instance Name, when a client needs to + contact that service, it sends a DNS query for the SRV record of + that name. + + The result of the DNS query is a SRV record giving the port number + and target host where the service may be found. + + The use of SRV records is very important. There are only 65535 TCP + port numbers available. These port numbers are being allocated + one-per-application-protocol at an alarming rate. Some protocols + like the X Window System have a block of 64 TCP ports allocated + (6000-6063). If we start allocating blocks of 64 TCP ports at a time, + we will run out even faster. Using a different TCP port for each + different instance of a given service on a given machine is entirely + sensible, but allocating large static ranges, as was done for X, is a + very inefficient way to manage a limited resource. On any given host, + most TCP ports are reserved for services that will never run on that + particular host. This is very poor utilization of the limited port + space. Using SRV records allows each host to allocate its available + port numbers dynamically to those services running on that host that + need them, and then advertise the allocated port numbers via SRV + records. Allocating the available listening port numbers locally + on a per-host basis as needed allows much better utilization of the + available port space than today's centralized global allocation. + + In some environments there may be no compelling reason to assign + managed names to every host, since every available service is + accessible by name anyway, as a first-class entity in its own right. + However, the DNS packet format and record format still require a host + name to link the target host referenced in the SRV record to the + address records giving the IPv4 and/or IPv6 addresses for that + hardware. In the case where no natural host name is available, the + SRV record may give its own name as the name of the target host, and + then the requisite address records may be attached to that same name. + + +Expires 10th February 2007 Cheshire & Krochmal [Page 11] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + It is perfectly permissible for a single name in the DNS hierarchy + to have multiple records of different type attached. (The only + restriction being that a given name may not have both a CNAME record + and other records at the same time.) + + In the event that more than one SRV is returned, clients MUST + correctly interpret the priority and weight fields -- i.e. Lower + numbered priority servers should be used in preference to higher + numbered priority servers, and servers with equal priority should be + selected randomly in proportion to their relative weights. However, + in the overwhelmingly common case, a single advertised DNS-SD service + instance is described by exactly one SRV record, and in this common + case the priority and weight fields of the SRV record SHOULD both be + set to zero. + + +6. Data Syntax for DNS-SD TXT Records + + Some services discovered via Service Instance Enumeration may need + more than just an IP address and port number to properly identify the + service. For example, printing via the LPR protocol often specifies a + queue name. This queue name is typically short and cryptic, and need + not be shown to the user. It should be regarded the same way as the + IP address and port number -- it is one component of the addressing + information required to identify a specific instance of a service + being offered by some piece of hardware. Similarly, a file server may + have multiple volumes, each identified by its own volume name. A Web + server typically has multiple pages, each identified by its own URL. + In these cases, the necessary additional data is stored in a TXT + record with the same name as the SRV record. The specific nature of + that additional data, and how it is to be used, is service-dependent, + but the overall syntax of the data in the TXT record is standardized, + as described below. Every DNS-SD service MUST have a TXT record in + addition to its SRV record, with same name, even if the service has + no additional data to store and the TXT record contains no more than + a single zero byte. + + +6.1 General Format Rules for DNS TXT Records + + A DNS TXT record can be up to 65535 (0xFFFF) bytes long. The total + length is indicated by the length given in the resource record header + in the DNS message. There is no way to tell directly from the data + alone how long it is (e.g. there is no length count at the start, or + terminating NULL byte at the end). (Note that when using Multicast + DNS [mDNS] the maximum packet size is 9000 bytes, which imposes an + upper limit on the size of TXT records of about 8800 bytes.) + + The format of the data within a DNS TXT record is one or more + strings, packed together in memory without any intervening gaps + or padding bytes for word alignment. + + +Expires 10th February 2007 Cheshire & Krochmal [Page 12] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + The format of each constituent string within the DNS TXT record is a + single length byte, followed by 0-255 bytes of text data. + + These format rules are defined in Section 3.3.14 of RFC 1035, and are + not specific to DNS-SD. DNS-SD simply specifies a usage convention + for what data should be stored in those constituent strings. + + An empty TXT record containing zero strings is disallowed by RFC + 1035. DNS-SD implementations MUST NOT emit empty TXT records. + DNS-SD implementations receiving empty TXT records MUST treat them + as equivalent to a one-byte TXT record containing a single zero byte + (i.e. a single empty string). + + +6.2 DNS TXT Record Format Rules for use in DNS-SD + + DNS-SD uses DNS TXT records to store arbitrary name/value pairs + conveying additional information about the named service. Each + name/value pair is encoded as its own constituent string within the + DNS TXT record, in the form "name=value". Everything up to the first + '=' character is the name. Everything after the first '=' character + to the end of the string (including subsequent '=' characters, if + any) is the value. Specific rules governing names and values are + given below. Each author defining a DNS-SD profile for discovering + instances of a particular type of service should define the base set + of name/value attributes that are valid for that type of service. + + Using this standardized name/value syntax within the TXT record makes + it easier for these base definitions to be expanded later by defining + additional named attributes. If an implementation sees unknown + attribute names in a service TXT record, it MUST silently ignore + them. + + The TCP (or UDP) port number of the service, and target host name, + are given in the SRV record. This information -- target host name and + port number -- MUST NOT be duplicated using name/value attributes in + the TXT record. + + The intention of DNS-SD TXT records is to convey a small amount of + useful additional information about a service. Ideally it SHOULD NOT + be necessary for a client to retrieve this additional information + before it can usefully establish a connection to the service. For a + well-designed TCP-based application protocol, it should be possible, + knowing only the host name and port number, to open a connection + to that listening process, and then perform version- or feature- + negotiation to determine the capabilities of the service instance. + For example, when connecting to an AppleShare server over TCP, the + client enters into a protocol exchange with the server to determine + which version of the AppleShare protocol the server implements, and + which optional features or capabilities (if any) are available. For a + well-designed application protocol, clients should be able to connect + + +Expires 10th February 2007 Cheshire & Krochmal [Page 13] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + and use the service even if there is no information at all in the TXT + record. In this case, the information in the TXT record should be + viewed as a performance optimization -- when a client discovers many + instances of a service, the TXT record allows the client to know some + rudimentary information about each instance without having to open a + TCP connection to each one and interrogate every service instance + separately. Extreme care should be taken when doing this to ensure + that the information in the TXT record is in agreement with the + information retrieved by a client connecting over TCP. + + There are legacy protocols which provide no feature negotiation + capability, and in these cases it may be useful to convey necessary + information in the TXT record. For example, when printing using the + old Unix LPR (port 515) protocol, the LPR service provides no way + for the client to determine whether a particular printer accepts + PostScript, or what version of PostScript, etc. In this case it is + appropriate to embed this information in the TXT record, because the + alternative is worse -- passing around written instructions to the + users, arcane manual configuration of "/etc/printcap" files, etc. + + +6.3 DNS-SD TXT Record Size + + The total size of a typical DNS-SD TXT record is intended to be small + -- 200 bytes or less. + + In cases where more data is justified (e.g. LPR printing), keeping + the total size under 400 bytes should allow it to fit in a single + standard 512-byte DNS message. (This standard DNS message size is + defined in RFC 1035.) + + In extreme cases where even this is not enough, keeping the size of + the TXT record under 1300 bytes should allow it to fit in a single + 1500-byte Ethernet packet. + + Using TXT records larger than 1300 bytes is NOT RECOMMENDED at this + time. + + +6.4 Rules for Names in DNS-SD Name/Value Pairs + + The "Name" MUST be at least one character. Strings beginning with an + '=' character (i.e. the name is missing) SHOULD be silently ignored. + + The characters of "Name" MUST be printable US-ASCII values + (0x20-0x7E), excluding '=' (0x3D). + + Spaces in the name are significant, whether leading, trailing, or in + the middle -- so don't include any spaces unless you really intend + that! + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 14] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + Case is ignored when interpreting a name, so "papersize=A4", + "PAPERSIZE=A4" and "Papersize=A4" are all identical. + + If there is no '=', then it is a boolean attribute, and is simply + identified as being present, with no value. + + A given attribute name may appear at most once in a TXT record. + The reason for this simplifying rule is to facilitate the creation + of client libraries that parse the TXT record into an internal data + structure, such as a hash table or dictionary object that maps from + names to values, and then make that abstraction available to client + code. The rule that a given attribute name may not appear more than + once simplifies these abstractions because they aren't required to + support the case of returning more than one value for a given key. + + If a client receives a TXT record containing the same attribute name + more than once, then the client MUST silently ignore all but the + first occurrence of that attribute. For client implementations that + process a DNS-SD TXT record from start to end, placing name/value + pairs into a hash table, using the name as the hash table key, this + means that if the implementation attempts to add a new name/value + pair into the table and finds an entry with the same name already + present, then the new entry being added should be silently discarded + instead. For client implementations that retrieve name/value pairs by + searching the TXT record for the requested name, they should search + the TXT record from the start, and simply return the first matching + name they find. + + When examining a TXT record for a given named attribute, there are + therefore four broad categories of results which may be returned: + + * Attribute not present (Absent) + + * Attribute present, with no value + (e.g. "Anon Allowed" -- server allows anonymous connections) + + * Attribute present, with empty value (e.g. "Installed PlugIns=" -- + server supports plugins, but none are presently installed) + + * Attribute present, with non-empty value + (e.g. "Installed PlugIns=JPEG,MPEG2,MPEG4") + + Each author defining a DNS-SD profile for discovering instances of a + particular type of service should define the interpretation of these + different kinds of result. For example, for some keys, there may be + a natural true/false boolean interpretation: + + * Present implies 'true' + * Absent implies 'false' + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 15] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + For other keys it may be sensible to define other semantics, such as + value/no-value/unknown: + + * Present with value implies that value. + E.g. "Color=4" for a four-color ink-jet printer, + or "Color=6" for a six-color ink-jet printer. + + * Present with empty value implies 'false'. E.g. Not a color printer. + + * Absent implies 'Unknown'. E.g. A print server connected to some + unknown printer where the print server doesn't actually know if the + printer does color or not -- which gives a very bad user experience + and should be avoided wherever possible. + + (Note that this is a hypothetical example, not an example of actual + name/value keys used by DNS-SD network printers.) + + As a general rule, attribute names that contain no dots are defined + as part of the open-standard definition written by the person or + group defining the DNS-SD profile for discovering that particular + service type. Vendor-specific extensions should be given names of the + form "keyname.company.com=value", using a domain name legitimately + registered to the person or organization creating the vendor-specific + key. This reduces the risk of accidental conflict if different + organizations each define their own vendor-specific keys. + + +6.5 Rules for Values in DNS-SD Name/Value Pairs + + If there is an '=', then everything after the first '=' to the end + of the string is the value. The value can contain any eight-bit + values including '='. Leading or trailing spaces are part of the + value, so don't put them there unless you intend them to be there. + Any quotation marks around the value are part of the value, so don't + put them there unless you intend them to be part of the value. + + The value is opaque binary data. Often the value for a particular + attribute will be US-ASCII (or UTF-8) text, but it is legal for a + value to be any binary data. For example, if the value of a key is an + IPv4 address, that address should simply be stored as four bytes of + binary data, not as a variable-length 7-15 byte ASCII string giving + the address represented in textual dotted decimal notation. + + Generic debugging tools should generally display all attribute values + as a hex dump, with accompanying text alongside displaying the UTF-8 + interpretation of those bytes, except for attributes where the + debugging tool has embedded knowledge that the value is some other + kind of data. + + Authors defining DNS-SD profiles SHOULD NOT convert binary attribute + data types into printable text (e.g. using hexadecimal, Base-64 or UU + + +Expires 10th February 2007 Cheshire & Krochmal [Page 16] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + encoding) merely for the sake of making the data be printable text + when seen in a generic debugging tool. Doing this simply bloats the + size of the TXT record, without actually making the data any more + understandable to someone looking at it in a generic debugging tool. + + +6.6 Example TXT Record + + The TXT record below contains three syntactically valid name/value + pairs. (The meaning of these name/value pairs, if any, would depend + on the definitions pertaining to the service in question that is + using them.) + + --------------------------------------------------------------- + | 0x0A | name=value | 0x08 | paper=A4 | 0x0E | DNS-SD Is Cool | + --------------------------------------------------------------- + + +6.7 Version Tag + + It is recommended that authors defining DNS-SD profiles include an + attribute of the form "txtvers=xxx" in their definition, and require + it to be the first name/value pair in the TXT record. This + information in the TXT record can be useful to help clients maintain + backwards compatibility with older implementations if it becomes + necessary to change or update the specification over time. Even if + the profile author doesn't anticipate the need for any future + incompatible changes, having a version number in the specification + provides useful insurance should incompatible changes become + unavoidable. Clients SHOULD ignore TXT records with a txtvers number + higher (or lower) than the version(s) they know how to interpret. + + Note that the version number in the txtvers tag describes the version + of the TXT record specification being used to create this TXT record, + not the version of the application protocol that will be used if the + client subsequently decides to contact that service. Ideally, every + DNS-SD TXT record specification starts at txtvers=1 and stays that + way forever. Improvements can be made by defining new keys that older + clients silently ignore. The only reason to increment the version + number is if the old specification is subsequently found to be so + horribly broken that there's no way to do a compatible forward + revision, so the txtvers number has to be incremented to tell all the + old clients they should just not even try to understand this new TXT + record. + + If there is a need to indicate which version number(s) of the + application protocol the service implements, the recommended key + name for this is "protovers". + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 17] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + +7. Application Protocol Names + + The <Service> portion of a Service Instance Name consists of a pair + of DNS labels, following the established convention for SRV records + [RFC 2782], namely: the first label of the pair is an underscore + character followed by the Application Protocol Name, and the second + label is either "_tcp" or "_udp". + + Application Protocol Names may be no more than fourteen characters + (not counting the mandatory underscore), conforming to normal DNS + host name rules: Only lower-case letters, digits, and hyphens; must + begin and end with lower-case letter or digit. + + Wise selection of an Application Protocol Name is very important, + and the choice is not always as obvious as it may appear. + + In some cases, the Application Protocol Name merely names and refers + to the on-the-wire message format and semantics being used. FTP is + "ftp", IPP printing is "ipp", and so on. + + However, it is common to "borrow" an existing protocol and repurpose + it for a new task. This is entirely sensible and sound engineering + practice, but that doesn't mean that the new protocol is providing + the same semantic service as the old one, even if it borrows the same + message formats. For example, the local network music playing + protocol implemented by iTunes on Macintosh and Windows is little + more than "HTTP GET" commands. However, that does *not* mean that it + is sensible or useful to try to access one of these music servers by + connecting to it with a standard web browser. Consequently, the + DNS-SD service advertised (and browsed for) by iTunes is "_daap._tcp" + (Digital Audio Access Protocol), not "_http._tcp". Advertising + "_http._tcp" service would cause iTunes servers to show up in + conventional Web browsers (Safari, Camino, OmniWeb, Opera, Netscape, + Internet Explorer, etc.) which is little use since it offers no pages + containing human-readable content. Similarly, browsing for + "_http._tcp" service would cause iTunes to find generic web servers, + such as the embedded web servers in devices like printers, which is + little use since printers generally don't have much music to offer. + + Similarly, NFS is built on top of SUN RPC, but that doesn't mean it + makes sense for an NFS server to advertise that it provides "SUN RPC" + service. Likewise, Microsoft SMB file service is built on top of + Netbios running over IP, but that doesn't mean it makes sense for + an SMB file server to advertise that it provides "Netbios-over-IP" + service. The DNS-SD name of a service needs to encapsulate both the + "what" (semantics) and the "how" (protocol implementation) of the + service, since knowledge of both is necessary for a client to + usefully use the service. Merely advertising that a service was + built on top of SUN RPC is no use if the client has no idea what + the service actually does. + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 18] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + Another common mistake is to assume that the service type advertised + by iTunes should be "_daap._http._tcp." This is also incorrect. + Similarly, a protocol designer implementing a network service that + happens to use Simple Object Access Protocol [SOAP] should not feel + compelled to have "_soap" appear somewhere in the Application + Protocol Name. Part of the confusion here is that the presence of + "_tcp" or "_udp" in the <Service> portion of a Service Instance Name + has led people to assume that the structure of a service name has to + reflect the internal structure of how the protocol was implemented. + This is not correct. All that is required is that the service be + identified by a unique Application Protocol Name. Making the + Application Protocol Name at least marginally descriptive of + what the service does is desirable, though not essential. + + The "_tcp" or "_udp" should be regarded as little more than + boilerplate text, and care should be taken not to attach too much + importance to it. Some might argue that the "_tcp" or "_udp" should + not be there at all, but this format is defined by RFC 2782, and + that's not going to change. In addition, the presence of "_tcp" has + the useful side-effect that it provides a convenient delegation point + to hand off responsibility for service discovery to a different DNS + server, if so desired. + + +7.1. Selective Instance Enumeration + + This document does not attempt to define an arbitrary query language + for service discovery, nor do we believe one is necessary. + + However, there are some circumstances where narrowing the list of + results may be useful. A hypothetical Web browser client that is able + to retrieve HTML documents via HTTP and display them may also be able + to retrieve HTML documents via FTP and display them, but only in the + case of FTP servers that allow anonymous login. For that Web browser, + discovering all FTP servers on the network is not useful. The Web + browser only wants to discover FTP servers that it is able to talk + to. In this case, a subtype of "_ftp._tcp" could be defined. Instead + of issuing a query for "_ftp._tcp.<Domain>", the Web browser issues a + query for "_anon._sub._ftp._tcp.<Domain>", where "_anon" is a defined + subtype of "_ftp._tcp". The response to this query only includes the + names of SRV records for FTP servers that are willing to allow + anonymous login. + + Note that the FTP server's Service Instance Name is unchanged -- it + is still something of the form "The Server._ftp._tcp.example.com." + The subdomain in which FTP server SRV records are registered defines + the namespace within which FTP server names are unique. Additional + subtypes (e.g. "_anon") of the basic service type (e.g. "_ftp._tcp") + serve to narrow the list of results, not to create more namespace. + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 19] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + Subtypes are appropriate when it is desirable for different kinds + of clients to be able to browse for services at two levels of + granularity. In the example above, we hypothesize two classes of + ftp client: clients that can provide username and password when + connecting, and clients that can only do anonymous login. The set of + ftp servers on the network is the same in both cases; the difference + is that the more capable client wants to discover all of them, + whereas the more limited client only wants to find the subset of + those ftp servers that it can talk to. Subtypes are only appropriate + in two-level scenarios such as this one, where some clients want to + find the full set of services of a given type, and at the same time + other clients only want to find some subset. Generally speaking, if + there is no client that wants to find the entire set, then it's + neither necessary nor desirable to use the subtype mechanism. If all + clients are browsing for some particular subtype, and no client + exists that browses for the parent type, then an Application Protocol + Name representing the logical service should be defined, and software + should simply advertise and browse for that particular service type + directly. In particular, just because a particular network service + happens to be implemented in terms of some other underlying protocol, + like HTTP, Sun RPC, or SOAP, doesn't mean that it's sensible for that + service to be defined as a subtype of "_http", "_sunrpc", or "_soap". + That would only be useful if there were some class of client for + which it is sensible to say, "I want to discover a service on the + network, and I don't care what it does, as long as it does it using + the SOAP XML RPC mechanism." + + As with the TXT record name/value pairs, the list of possible + subtypes, if any, are defined and specified separately for each basic + service type. Note that the example given here using "_ftp" is a + hypothetical one. The "_ftp" service type does not (currently) have + any subtypes defined. Subtypes are currently a little-used feature + of DNS-SD, and experience will show whether or not they ultimately + prove to have broad applicability. + + +7.2 Service Name Length Limits + + As described above, application protocol names are allowed to be up + to fourteen characters long. The reason for this limit is to leave + as many bytes of the domain name as possible available for use + by both the network administrator (choosing service domain names) + and the end user (choosing instance names). + + A domain name may be up to 255 bytes long, including the final + terminating root label at the end. Domain names used by DNS-SD + take the following forms: + + <Instance>.<app>._tcp.<servicedomain>.<parentdomain>. + <sub>._sub.<app>._tcp.<servicedomain>.<parentdomain>. + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 20] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + The first example shows a service instance name, i.e. the name of the + service's SRV and TXT records. The second shows a subtype browsing + name, i.e. the name of a PTR record pointing to service instance + names (see "Selective Instance Enumeration"). + + The instance name <Instance> may be up to 63 bytes. Including the + length byte used by the DNS format when the name is stored in a + packet, that makes 64 bytes. + + When using subtypes, the subtype identifier is allowed to be up to + 63 bytes, plus the length byte, making 64. Including the "_sub" + and its length byte, this makes 69 bytes. + + The application protocol name <app> may be up to 14 bytes, plus the + underscore and length byte, making 16. Including the "_udp" or "_tcp" + and its length byte, this makes 21 bytes. + + Typically, DNS-SD service records are placed into subdomains of their + own beneath a company's existing domain name. Since these subdomains + are intended to be accessed through graphical user interfaces, not + typed on a command-line they are frequently long and descriptive. + Including the length byte, the user-visible service domain may be up + to 64 bytes. + + The terminating root label at the end counts as one byte. + + Of our available 255 bytes, we have now accounted for 69+21+64+1 = + 155 bytes. This leaves 100 bytes to accommodate the organization's + existing domain name <parentdomain>. When used with Multicast DNS, + <parentdomain> is "local", which easily fits. When used with parent + domains of 100 bytes or less, the full functionality of DNS-SD is + available without restriction. When used with parent domains longer + than 100 bytes, the protocol risks exceeding the maximum possible + length of domain names, causing failures. In this case, careful + choice of short <servicedomain> names can help avoid overflows. + If the <servicedomain> and <parentdomain> are too long, then service + instances with long instance names will not be discoverable or + resolvable, and applications making use of long subtype names + may fail. + + Because of this constraint, we choose to limit Application Protocol + Names to 14 characters or less. Allowing more characters would not + add to the expressive power of the protocol, and would needlessly + lower the limit on the maximum <parentdomain> length that may be + safely used. + + + + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 21] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + +8. Flagship Naming + + In some cases, there may be several network protocols available + which all perform roughly the same logical function. For example, + the printing world has the LPR protocol, and the Internet Printing + Protocol (IPP), both of which cause printed sheets to be emitted + from printers in much the same way. In addition, many printer vendors + send their own proprietary page description language (PDL) data + over a TCP connection to TCP port 9100, herein referred to as the + "pdl-datastream" protocol. In an ideal world we would have only one + network printing protocol, and it would be sufficiently good that no + one felt a compelling need to invent a different one. However, in + practice, multiple legacy protocols do exist, and a service discovery + protocol has to accommodate that. + + Many printers implement all three printing protocols: LPR, IPP, and + pdl-datastream. For the benefit of clients that may speak only one of + those protocols, all three are advertised. + + However, some clients may implement two, or all three of those + printing protocols. When a client looks for all three service types + on the network, it will find three distinct services -- an LPR + service, an IPP service, and a pdl-datastream service -- all of which + cause printed sheets to be emitted from the same physical printer. + + In the case of multiple protocols like this that all perform + effectively the same function, the client should suppress duplicate + names and display each name only once. When the user prints to a + given named printer, the printing client is responsible for choosing + the protocol which will best achieve the desired effect, without, for + example, requiring the user to make a manual choice between LPR and + IPP. + + As described so far, this all works very well. However, consider some + future printer that only supports IPP printing, and some other future + printer that only supports pdl-datastream printing. The name spaces + for different service types are intentionally disjoint -- it is + acceptable and desirable to be able to have both a file server called + "Sales Department" and a printer called "Sales Department". However, + it is not desirable, in the common case, to have two different + printers both called "Sales Department", just because those printers + are implementing different protocols. + + To help guard against this, when there are two or more network + protocols which perform roughly the same logical function, one of + the protocols is declared the "flagship" of the fleet of related + protocols. Typically the flagship protocol is the oldest and/or + best-known protocol of the set. + + If a device does not implement the flagship protocol, then it instead + creates a placeholder SRV record (priority=0, weight=0, port=0, + + +Expires 10th February 2007 Cheshire & Krochmal [Page 22] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + target host = hostname of device) with that name. If, when it + attempts to create this SRV record, it finds that a record with the + same name already exists, then it knows that this name is already + taken by some entity implementing at least one of the protocols from + the class, and it must choose another. If no SRV record already + exists, then the act of creating it stakes a claim to that name so + that future devices in the same class will detect a conflict when + they try to use it. The SRV record needs to contain the target host + name in order for the conflict detection rules to operate. If two + different devices were to create placeholder SRV records both using a + null target host name (just the root label), then the two SRV records + would be seen to be in agreement so no conflict would be registered. + + By defining a common well-known flagship protocol for the class, + future devices that may not even know about each other's protocols + establish a common ground where they can coordinate to verify + uniqueness of names. + + No PTR record is created advertising the presence of empty flagship + SRV records, since they do not represent a real service being + advertised. + + +9. Service Type Enumeration + + In general, clients are not interested in finding *every* service on + the network, just the services that the client knows how to talk to. + (Software designers may *think* there's some value to finding *every* + service on the network, but that's just wooly thinking.) + + However, for problem diagnosis and network management tools, it may + be useful for network administrators to find the list of advertised + service types on the network, even if those service names are just + opaque identifiers and not particularly informative in isolation. + + For this reason, a special meta-query is defined. A DNS query for + PTR records with the name "_services._dns-sd._udp.<Domain>" yields + a list of PTR records, where the rdata of each PTR record is the + name of a service type. A subsequent query for PTR records with + one of those names yields a list of instances of that service type. + + + + + + + + + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 23] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + +10. Populating the DNS with Information + + How the SRV and PTR records that describe services and allow them to + be enumerated make their way into the DNS is outside the scope of + this document. However, it can happen easily in any of a number of + ways, for example: + + On some networks, the administrator might manually enter the records + into the name server's configuration file. + + A network monitoring tool could output a standard zone file to be + read into a conventional DNS server. For example, a tool that can + find Apple LaserWriters using AppleTalk NBP could find the list + of printers, communicate with each one to find its IP address, + PostScript version, installed options, etc., and then write out a + DNS zone file describing those printers and their capabilities using + DNS resource records. That information would then be available to + DNS-SD clients that don't implement AppleTalk NBP, and don't want to. + + Future IP printers could use Dynamic DNS Update [RFC 2136] to + automatically register their own SRV and PTR records with the DNS + server. + + A printer manager device which has knowledge of printers on the + network through some other management protocol could also use Dynamic + DNS Update [RFC 2136]. + + Alternatively, a printer manager device could implement enough of + the DNS protocol that it is able to answer DNS queries directly, + and Example Co.'s main DNS server could delegate the + _ipp._tcp.example.com subdomain to the printer manager device. + + Zeroconf printers answer Multicast DNS queries on the local link + for appropriate PTR and SRV names ending with ".local." [mDNS] + + +11. Relationship to Multicast DNS + + DNS-Based Service Discovery is only peripherally related to Multicast + DNS, in that the standard unicast DNS queries used by DNS-SD may also + be performed using multicast when appropriate, which is particularly + beneficial in Zeroconf environments [ZC]. + + + + + + + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 24] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + +12. Discovery of Browsing and Registration Domains (Domain Enumeration) + + One of the main reasons for DNS-Based Service Discovery is so that + when a visiting client (e.g. a laptop computer) arrives at a new + network, it can discover what services are available on that network + without manual configuration. This logic that applies to discovering + services without manual configuration also applies to discovering the + domains in which services are registered without requiring manual + configuration. + + This discovery is performed recursively, using Unicast or Multicast + DNS. Five special RR names are reserved for this purpose: + + b._dns-sd._udp.<domain>. + db._dns-sd._udp.<domain>. + r._dns-sd._udp.<domain>. + dr._dns-sd._udp.<domain>. + lb._dns-sd._udp.<domain>. + + By performing PTR queries for these names, a client can learn, + respectively: + + o A list of domains recommended for browsing + + o A single recommended default domain for browsing + + o A list of domains recommended for registering services using + Dynamic Update + + o A single recommended default domain for registering services. + + o The final query shown yields the "legacy browsing" domain. + Sophisticated client applications that care to present choices + of domain to the user, use the answers learned from the previous + four queries to discover those domains to present. In contrast, + many current applications browse without specifying an explicit + domain, allowing the operating system to automatically select an + appropriate domain on their behalf. It is for this class of + application that the "legacy browsing" query is provided, to allow + the network administrator to communicate to the client operating + systems which domain should be used for these applications. + + These domains are purely advisory. The client or user is free to + browse and/or register services in any domains. The purpose of these + special queries is to allow software to create a user-interface that + displays a useful list of suggested choices to the user, from which + they may make a suitable selection, or ignore the offered suggestions + and manually enter their own choice. + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 25] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + The <domain> part of the name may be "local" (meaning "perform the + query using link-local multicast) or it may be learned through some + other mechanism, such as the DHCP "Domain" option (option code 15) + [RFC 2132] or the DHCP "Domain Search" option (option code 119) + [RFC 3397]. + + The <domain> part of the name may also be derived from the host's IP + address. The host takes its IP address, and calculates the logical + AND of that address and its subnet mask, to derive the 'base' address + of the subnet. It then constructs the conventional DNS "reverse + mapping" name corresponding to that base address, and uses that + as the <domain> part of the name for the queries described above. + For example, if a host has address 192.168.12.34, with subnet mask + 255.255.0.0, then the 'base' address of the subnet is 192.168.0.0, + and to discover the recommended legacy browsing domain for devices + on this subnet, the host issues a DNS PTR query for the name + "lb._dns-sd._udp.0.0.168.192.in-addr.arpa." + + Sophisticated clients may perform domain enumeration queries both in + "local" and in one or more unicast domains, and then present the user + with an aggregate result, combining the information received from all + sources. + + +13. DNS Additional Record Generation + + DNS has an efficiency feature whereby a DNS server may place + additional records in the Additional Section of the DNS Message. + These additional records are typically records that the client did + not explicitly request, but the server has reasonable grounds to + expect that the client might request them shortly. + + This section recommends which additional records should be generated + to improve network efficiency for both unicast and multicast DNS-SD + responses. + + +13.1 PTR Records + + When including a PTR record in a response packet, the + server/responder SHOULD include the following additional records: + + o The SRV record(s) named in the PTR rdata. + o The TXT record(s) named in the PTR rdata. + o All address records (type "A" and "AAAA") named in the SRV rdata. + + + + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 26] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + +13.2 SRV Records + + When including an SVR record in a response packet, the + server/responder SHOULD include the following additional records: + + o All address records (type "A" and "AAAA") named in the SRV rdata. + + +13.3 TXT Records + + When including a TXT record in a response packet, no additional + records are required. + + +13.4 Other Record Types + + In response to address queries, or other record types, no additional + records are required by this document. + + +14. Comparison with Alternative Service Discovery Protocols + + Over the years there have been many proposed ways to do network + service discovery with IP, but none achieved ubiquity in the + marketplace. Certainly none has achieved anything close to the + ubiquity of today's deployment of DNS servers, clients, and other + infrastructure. + + The advantage of using DNS as the basis for service discovery is + that it makes use of those existing servers, clients, protocols, + infrastructure, and expertise. Existing network analyzer tools + already know how to decode and display DNS packets for network + debugging. + + For ad-hoc networks such as Zeroconf environments, peer-to-peer + multicast protocols are appropriate. The Zeroconf host profile [ZCHP] + requires the use of a DNS-like protocol over IP Multicast for host + name resolution in the absence of DNS servers. Given that Zeroconf + hosts will have to implement this Multicast-based DNS-like protocol + anyway, it makes sense for them to also perform service discovery + using that same Multicast-based DNS-like software, instead of also + having to implement an entirely different service discovery protocol. + + In larger networks, a high volume of enterprise-wide IP multicast + traffic may not be desirable, so any credible service discovery + protocol intended for larger networks has to provide some facility to + aggregate registrations and lookups at a central server (or servers) + instead of working exclusively using multicast. This requires some + service discovery aggregation server software to be written, + debugged, deployed, and maintained. This also requires some service + discovery registration protocol to be implemented and deployed for + + +Expires 10th February 2007 Cheshire & Krochmal [Page 27] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + clients to register with the central aggregation server. Virtually + every company with an IP network already runs a DNS server, and DNS + already has a dynamic registration protocol [RFC 2136]. Given that + virtually every company already has to operate and maintain a DNS + server anyway, it makes sense to take advantage of this instead of + also having to learn, operate and maintain a different service + registration server. It should be stressed again that using the + same software and protocols doesn't necessarily mean using the same + physical piece of hardware. The DNS-SD service discovery functions + do not have to be provided by the same piece of hardware that + is currently providing the company's DNS name service. The + "_tcp.<Domain>" subdomain may be delegated to a different piece of + hardware. However, even when the DNS-SD service is being provided + by a different piece of hardware, it is still the same familiar DNS + server software that is running, with the same configuration file + syntax, the same log file format, and so forth. + + Service discovery needs to be able to provide appropriate security. + DNS already has existing mechanisms for security [RFC 2535]. + + In summary: + + Service discovery requires a central aggregation server. + DNS already has one: It's called a DNS server. + + Service discovery requires a service registration protocol. + DNS already has one: It's called DNS Dynamic Update. + + Service discovery requires a query protocol + DNS already has one: It's called DNS. + + Service discovery requires security mechanisms. + DNS already has security mechanisms: DNSSEC. + + Service discovery requires a multicast mode for ad-hoc networks. + Zeroconf environments already require a multicast-based DNS-like + name lookup protocol for mapping host names to addresses, so it + makes sense to let one multicast-based protocol do both jobs. + + It makes more sense to use the existing software that every network + needs already, instead of deploying an entire parallel system just + for service discovery. + + + + + + + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 28] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + +15. Real Examples + + The following examples were prepared using standard unmodified + nslookup and standard unmodified BIND running on GNU/Linux. + + Note: In real products, this information is obtained and presented to + the user using graphical network browser software, not command-line + tools, but if you wish you can try these examples for yourself as you + read along, using the command-line tools already available on your + own Unix machine. + +15.1 Question: What FTP servers are being advertised from dns-sd.org? + + nslookup -q=ptr _ftp._tcp.dns-sd.org. + _ftp._tcp.dns-sd.org + name = Apple\032QuickTime\032Files._ftp._tcp.dns-sd.org + _ftp._tcp.dns-sd.org + name = Microsoft\032Developer\032Files._ftp._tcp.dns-sd.org + _ftp._tcp.dns-sd.org + name = Registered\032Users'\032Only._ftp._tcp.dns-sd.org + + Answer: There are three, called "Apple QuickTime Files", + "Microsoft Developer Files" and "Registered Users' Only". + + Note that nslookup escapes spaces as "\032" for display purposes, + but a graphical DNS-SD browser does not. + +15.2 Question: What FTP servers allow anonymous access? + + nslookup -q=ptr _anon._sub._ftp._tcp.dns-sd.org + _anon._sub._ftp._tcp.dns-sd.org + name = Apple\032QuickTime\032Files._ftp._tcp.dns-sd.org + _anon._sub._ftp._tcp.dns-sd.org + name = Microsoft\032Developer\032Files._ftp._tcp.dns-sd.org + + Answer: Only "Apple QuickTime Files" and "Microsoft Developer Files" + allow anonymous access. + +15.3 Question: How do I access "Apple QuickTime Files"? + + nslookup -q=any "Apple\032QuickTime\032Files._ftp._tcp.dns-sd.org." + Apple\032QuickTime\032Files._ftp._tcp.dns-sd.org + text = "path=/quicktime" + Apple\032QuickTime\032Files._ftp._tcp.dns-sd.org + priority = 0, weight = 0, port= 21 host = ftp.apple.com + ftp.apple.com internet address = 17.254.0.27 + ftp.apple.com internet address = 17.254.0.31 + ftp.apple.com internet address = 17.254.0.26 + + Answer: You need to connect to ftp.apple.com, port 21, path + "/quicktime". The addresses for ftp.apple.com are also given. + + +Expires 10th February 2007 Cheshire & Krochmal [Page 29] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + +16. User Interface Considerations + + DNS-Based Service Discovery was designed by first giving careful + consideration to what constitutes a good user experience for service + discovery, and then designing a protocol with the features necessary + to enable that good user experience. This section covers two issues + in particular: Choice of factory-default names (and automatic + renaming behavior) for devices advertising services, and the + "continuous live update" user-experience model for clients + browsing to discover services. + + +16.1 Service Advertising User-Interface Considerations + + When a DNS-SD service is advertised using Multicast DNS [mDNS], + automatic name conflict and resolution will occur if there is already + another service of the same type advertising with the same name. + As described in the Multicast DNS specification [mDNS], upon a + conflict, the service should: + + 1. Automatically select a new name (typically by appending + or incrementing a digit at the end of the name), + 2. try advertising with the new name, and + 3. upon success, record the new name in persistent storage. + + This renaming behavior is very important, because it is the key + to providing user-friendly service names in the out-of-the-box + factory-default configuration. Some product developers may not + have realized this, because there are some products today where + the factory-default name is distinctly unfriendly, containing + random-looking strings of characters, like the device's Ethernet + address in hexadecimal. This is unnecessary, and undesirable, because + the point of the user-visible name is that it should be friendly and + useful to human users. If the name is not unique on the local network + the protocol will rememdy this as necessary. It is ironic that many + of the devices with this mistake are network printers, given that + these same printers also simultaneously support AppleTalk-over- + Ethernet, with nice user-friendly default names (and automatic + conflict detection and renaming). Examples of good factory-default + names are as follows: + + Brother 5070N + Canon W2200 [ Apologies to makers of ] + HP LaserJet 4600 [ DNS-SD/mDNS printers ] + Lexmark W840 [ not listed. Email ] + Okidata C5300 [ the authors and we'll ] + Ricoh Aficio CL7100 [ add you to the list. ] + Xerox Phaser 6200DX + + + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 30] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + To complete the case for why adding long ugly serial numbers to + the end of names is neither necessary nor desirable, consider + the cases where the user has (a) only one network printer, + (b) two network printers, and (c) many network printers. + + (a) In the case where the user has only one network printer, a simple + name like (to use a vendor-neutral example) "Printer" is more + user-friendly than an ugly name like "Printer 0001E68C74FB". + Appending ugly hexadecimal goop to the end of the name to make + sure the name is unique is irrelevant to a user who only has one + printer anyway. + + (b) In the case where the user gets a second network printer, + having it detect that the name "Printer" is already in use + and automatically instead name itself "Printer (2)" provides a + good user experience. For the users, remembering that the old + printer is "Printer" and the new one is "Printer (2)" is easy + and intuitive. Seeing two printers called "Printer 0001E68C74FB" + and "Printer 00306EC3FD1C" is a lot less helpful. + + (c) In the case of a network with ten network printers, seeing a + list of ten names all of the form "Printer xxxxxxxxxxxx" has + effectively taken what was supposed to be a list of user-friendly + rich-text names (supporting mixed case, spaces, punctuation, + non-Roman characters and other symbols) and turned it into + just about the worst user-interface imaginable: a list of + incomprehensible random-looking strings of letters and digits. + In a network with a lot of printers, it would be desirable for + the people setting up the printers to take a moment to give each + one a descriptive name, but in the event they don't, presenting + the users with a list of sequentially-numbered printers is a much + more desirable default user experience than showing a list of raw + Ethernet addresses. + + +16.2 Client Browsing User-Interface Considerations + + Of particular concern in the design of DNS-SD was the dynamic nature + of service discovery in a changing network environment. Other service + discovery protocols have been designed with an implicit unstated + assumption that the usage model is: + + (a) client calls the service discovery code + (b) client gets list of discovered services + as of a particular instant in time, and then + (c) client displays list for user to select from + + Superficially this usage model seems reasonable, but the problem is + that it's too optimistic. It only considers the success case, where + the user successfully finds the service they're looking for. In the + + +Expires 10th February 2007 Cheshire & Krochmal [Page 31] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + case where the user is looking for (say) a particular printer, and + that printer's not turned on or not connected, the user first has + to attempt to remedy the problem, and then has to click a "refresh" + button to retry the service discovery (or, worse, dismiss the + browsing window entirely, and open a new one to initiate a new + network search attempt) to find out whether they were successful. + Because nothing happens instantaneously in networking, and packets + can be lost, necessitating some number of retransmissions, a service + discovery search typically takes a few seconds. A fairly typical user + experience model is: + + (a) display an empty window, + (b) display some animation like a searchlight + sweeping back and forth for ten seconds, and then + (c) at the end of the ten-second search, display + a static list showing what was discovered. + + Every time the user clicks the "refresh" button they have to endure + another ten-second wait, and every time the discovered list is + finally shown at the end of the ten-second wait, the moment it's + displayed on the screen it's already beginning to get stale and + out-of-date. + + The service discovery user experience that the DNS-SD designers had + in mind has some rather different properties: + + 1. Displaying a list of discovered services should be effectively + instantaneous -- i.e. typically 1/10 second, not 10 seconds. + + 2. The list of discovered services should not be getting stale + and out-of-date from the moment it's displayed. The list + should be 'live' and should continue to update as new services + are discovered. Because of the delays, packet losses, and + retransmissions inherent in networking, it is to be expected + that sometimes, after the initial list is displayed showing + the majority of discovered services, a few remaining stragglers + may continue to trickle in during the subsequent few seconds. + Even after this initial stable list has been built and displayed, + the list should remain 'live' and should continue to update. + At any future time, be it minutes, hours, or even days later, + if a new service of the desired type is discovered, it should be + displayed in the list automatically, without the user having to + click a "refresh" button or take any other explicit action to + update the display. + + 3. With users getting to be in the habit of leaving service discovery + windows open, and coming to expect to be able to rely on them + to show a continuous 'live' view of current network reality, + this creates a new requirement for us: deletion of stale services. + When a service discovery list shows just a static snapshot at a + moment in time, then the situation is simple: either a service was + + +Expires 10th February 2007 Cheshire & Krochmal [Page 32] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + discovered and appears in the list, or it was not, and does not. + However, when our list is live and updates continuously with the + discovery of new services, then this implies the corollary: when + a service goes away, it needs to *disappear* from the service + discovery list. Otherwise, the result would be unacceptable: the + service discovery list would simply grow monotonically over time, + and would require a periodic "refresh" (or complete dismissal and + recreation) to clear out old stale data. + + 4. With users getting to be in the habit of leaving service discovery + windows open, these windows need to update not only in response + to services coming and going, but also in response to changes + in configuration and connectivity of the client machine itself. + For example, if a user opens a service discovery window when no + Ethernet cable is connected to the client machine, and the window + appears empty with no discovered services, then when the user + connects the cable the window should automatically populate with + discovered services without requiring any explicit user action. + If the user disconnects the Ethernet cable, all the services + discovered via that network interface should automatically + disappear. If the user switches from one 802.11 wireless base + station to another, the service discovery window should + automatically update to remove all the services discovered + via the old wireless base station, and add all the services + discovered via the new one. + + If these requirements seem to be setting an arbitrary and + unreasonably high standard for service discovery, bear in mind that + while it may have seemed that way to some, back in the 1990s when + these ideas were first proposed, in the years since then Apple and + other companies have shipped multiple implementations of DNS-SD/mDNS + that meet and exceed these requirements. In the years since Apple + shipped Mac OS X 10.2 Jaguar with the Open Source mDNSResponder + daemon, this service discovery "live browsing" paradigm has been + adopted and implemented in a wide range of Apple and third-party + applications, including printer discovery, Safari discovery of + devices with embedded web servers (for status and configuration), + iTunes music sharing, iPhoto photo sharing, the iChat Bonjour buddy + list, SubEthaEdit multi-user document editing, etc. + + With so many different applications demonstrating that the "live + browsing" paradigm is clearly achievable, these four requirements + should not be regarded as idealistic unattainable goals, but + instead as the bare minimum baseline functionality that any + credible service discovery protocol needs to achieve. + + + + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 33] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + +17. IPv6 Considerations + + IPv6 has no significant differences, except that the address of the + SRV record's target host is given by the appropriate IPv6 address + records instead of the IPv4 "A" record. + + +18. Security Considerations + + DNSSEC [RFC 2535] should be used where the authenticity of + information is important. Since DNS-SD is just a naming and usage + convention for records in the existing DNS system, it has no specific + additional security requirements over and above those that already + apply to DNS queries and DNS updates. + + +19. IANA Considerations + + This protocol builds on DNS SRV records [RFC 2782], and similarly + requires IANA to assign unique application protocol names. + Unfortunately, the "IANA Considerations" section of RFC 2782 says + simply, "The IANA has assigned RR type value 33 to the SRV RR. + No other IANA services are required by this document." + Due to this oversight, IANA is currently prevented from carrying + out the necessary function of assigning these unique identifiers. + + This document proposes the following IANA allocation policy for + unique application protocol names: + + Allowable names: + * Must be no more than fourteen characters long + * Must consist only of: + - lower-case letters 'a' - 'z' + - digits '0' - '9' + - the hyphen character '-' + * Must begin and end with a lower-case letter or digit. + * Must not already be assigned to some other protocol in the + existing IANA "list of assigned application protocol names + and port numbers" [ports]. + + These identifiers are allocated on a First Come First Served basis. + In the event of abuse (e.g. automated mass registrations, etc.), + the policy may be changed without notice to Expert Review [RFC 2434]. + + The textual nature of service/protocol names means that there are + almost infinitely many more of them available than the finite set of + 65535 possible port numbers. This means that developers can produce + experimental implementations using unregistered service names with + little chance of accidental collision, providing service names are + chosen with appropriate care. However, this document strongly + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 34] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + advocates that on or before the date a product ships, developers + should properly register their service names. + + Some developers have expressed concern that publicly registering + their service names (and port numbers today) with IANA before a + product ships may give away clues about that product to competitors. + For this reason, IANA should consider allowing service name + applications to remain secret for some period of time, much as US + patent applications remain secret for two years after the date of + filing. + + This proposed IANA allocation policy is not in force until this + document is published as an RFC. In the meantime, unique application + protocol names may be registered according to the instructions at + <http://www.dns-sd.org/ServiceTypes.html>. As of August 2006, there + are roughly 300 application protocols in currently shipping products + that have been so registered as using DNS-SD for service discovery. + + +20. Acknowledgments + + The concepts described in this document have been explored, developed + and implemented with help from Richard Brown, Erik Guttman, Paul + Vixie, and Bill Woodcock. + + Special thanks go to Bob Bradley, Josh Graessley, Scott Herscher, + Roger Pantos and Kiren Sekar for their significant contributions. + + +21. Deployment History + + The first implementations of DNS-Based Service Discovery and + Multicast DNS were initially developed during the late 1990s, + but the event that put them into the media spotlight was Steve Jobs + demonstrating it live on stage in his keynote presentation opening + Apple's annual Worldwide Developers Conference in May 2002, and + announcing Apple's adoption of the technology throughout its hardware + and software product line. Three months later, in August 2002, Apple + shipped Mac OS X 10.2 Jaguar, and millions of end-users got their + first exposure to Zero Configuration Networking with DNS-SD/mDNS + in applications like Safari, iChat, and printer setup. A month later, + in September 2002, Apple released the entire source code for the + mDNS Responder daemon under its Darwin Open Source project, with + code not just for Mac OS X, but also for a range of other platforms + including Windows, VxWorks, Linux, Solaris, FreeBSD, etc. + + Many hardware makers were quick to see the benefits of Zero + Configuration Networking. Printer makers especially were enthusiastic + early adopters, and within a year every major printer manufacturer + was shipping DNS-SD/mDNS-enabled network printers. If you've bought + any network printer at all in the last few years, it was probably one + + +Expires 10th February 2007 Cheshire & Krochmal [Page 35] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + that supports DNS-SD/mDNS, even if you didn't know that at the time. + For Mac OS X users, telling if you have DNS-SD/mDNS printers on your + network is easy because they automatically appear in the "Bonjour" + submenu in the "Print" dialog of every Mac application. Microsoft + Windows users can get a similar experience by installing Bonjour for + Windows (takes about 90 seconds, no restart required) and running the + Bonjour for Windows Printer Setup Wizard [B4W]. + + The Open Source community has produced several independent + implementations of DNS-Based Service Discovery and Multicast DNS, + some in C like Apple's mDNSResponder daemon, and others in a variety + of different languages including Java, Python, Perl, and C#/Mono. + + +22. Copyright Notice + + Copyright (C) The Internet Society (2006). + + This document is subject to the rights, licenses and restrictions + contained in BCP 78, and except as set forth therein, the authors + retain all their rights. For the purposes of this document, + the term "BCP 78" refers exclusively to RFC 3978, "IETF Rights + in Contributions", published March 2005. + + This document and the information contained herein are provided on an + "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS + OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET + ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, + INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE + INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED + WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. + + + + + + + + + + + + + + + + + + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 36] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + +23. Normative References + + [ports] IANA list of assigned application protocol names and port + numbers <http://www.iana.org/assignments/port-numbers> + + [RFC 1033] Lottor, M., "Domain Administrators Operations Guide", + RFC 1033, November 1987. + + [RFC 1034] Mockapetris, P., "Domain Names - Concepts and + Facilities", STD 13, RFC 1034, November 1987. + + [RFC 1035] Mockapetris, P., "Domain Names - Implementation and + Specifications", STD 13, RFC 1035, November 1987. + + [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", RFC 2119, March 1997. + + [RFC 2782] Gulbrandsen, A., et al., "A DNS RR for specifying the + location of services (DNS SRV)", RFC 2782, February 2000. + + [RFC 3629] Yergeau, F., "UTF-8, a transformation format of ISO + 10646", RFC 3629, November 2003. + + [UAX15] "Unicode Normalization Forms" + http://www.unicode.org/reports/tr15/ + + +24. Informative References + + [B4W] Bonjour for Windows <http://www.apple.com/bonjour/> + + [mDNS] Cheshire, S., and M. Krochmal, "Multicast DNS", + Internet-Draft (work in progress), + draft-cheshire-dnsext-multicastdns-06.txt, August 2006. + + [NBP] Cheshire, S., and M. Krochmal, + "Requirements for a Protocol to Replace AppleTalk NBP", + Internet-Draft (work in progress), + draft-cheshire-dnsext-nbp-05.txt, August 2006. + + [RFC 2132] Alexander, S., and Droms, R., "DHCP Options and BOOTP + Vendor Extensions", RFC 2132, March 1997. + + [RFC 2136] Vixie, P., et al., "Dynamic Updates in the Domain Name + System (DNS UPDATE)", RFC 2136, April 1997. + + [RFC 2434] Narten, T., and H. Alvestrand, "Guidelines for Writing + an IANA Considerations Section in RFCs", RFC 2434, + October 1998. + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 37] + +Internet Draft DNS-Based Service Discovery 10th August 2006 + + + [RFC 2535] Eastlake, D., "Domain Name System Security Extensions", + RFC 2535, March 1999. + + [RFC 3007] Wellington, B., et al., "Secure Domain Name System (DNS) + Dynamic Update", RFC 3007, November 2000. + + [RFC 3397] Aboba, B., and Cheshire, S., "Dynamic Host Configuration + Protocol (DHCP) Domain Search Option", RFC 3397, November + 2002. + + [SOAP] Nilo Mitra, "SOAP Version 1.2 Part 0: Primer", + W3C Proposed Recommendation, 24 June 2003 + http://www.w3.org/TR/2003/REC-soap12-part0-20030624 + + [ZC] Williams, A., "Requirements for Automatic Configuration + of IP Hosts", Internet-Draft (work in progress), + draft-ietf-zeroconf-reqts-12.txt, September 2002. + + [ZCHP] Guttman, E., "Zeroconf Host Profile Applicability + Statement", Internet-Draft (work in progress), + draft-ietf-zeroconf-host-prof-01.txt, July 2001. + + +25. Authors' Addresses + + Stuart Cheshire + Apple Computer, Inc. + 1 Infinite Loop + Cupertino + California 95014 + USA + + Phone: +1 408 974 3207 + EMail: rfc [at] stuartcheshire [dot] org + + + Marc Krochmal + Apple Computer, Inc. + 1 Infinite Loop + Cupertino + California 95014 + USA + + Phone: +1 408 974 4368 + EMail: marc [at] apple [dot] com + + + + + + + + +Expires 10th February 2007 Cheshire & Krochmal [Page 38] |