What are space and time? These are them:
in [Logic]
|
Prev: parallels of finite-number versus infinite-number and direct vs. indirect Euclid IP #4.07 Correcting Math
Next: How to Hornswoggle Chalmer's Hard Problem
From: sci.math on 22 Jul 2010 20:06 On Jul 22, 4:55 pm, John Stafford <n...(a)droffats.ten> wrote: > In article > <4a067370-2f31-4aa8-9c73-0b41271b7...(a)d8g2000yqf.googlegroups.com>, > > Huang <huangxienc...(a)yahoo.com> wrote: > > In mathematics things are proved. > > Or they are not proved. > > > The reason you can do this is > > because everything exists very nicely and the whole stupid thing fits > > together like Lego building blocks, and ever piece fits perfect. That > > is mathematics. > > Accepted for the moment - mathematical proofs build upon each other, and > that is why proofs are so important - so that later posits do not > collapse into a pile of.. well, legos as you put it. > > > Conjecture is diferent. You begin by saying not "what exists", but > > "what might exist". Conjectures are NEVER proved to be true because > > they are and must remain conjectural. > > No. Some conjectures have been proven. Your logic tumbles into the > dumpster with that. > > > But you CAN show that > > conjectures are consistent, and so all of these conjectures fit > > together like Lego building blocks as well. In fact, for every > > mathematical statement there is a corresponding conjectural statement > > and vice versa. > > IOW, for every conjecture there is an infinite supply of poorly informed > guesswork and wholly impressionistic objection which has nothing to do > with the mathematics. I suspect you are exercising the same. > > > There is no mathematical way to transform back and > > forth between the two, such operations are currently under study but > > to be sure - I do know what math is and what it is not. I also believe > > that there are tools other than math which can accomplish the same > > things that math does. > > Exactly what is this 'back and forth' you write of? > [...] > > > Ok - there are many ways to do this depending on how precise you want > > to make it. If you want an exact derivation you'll never get it > > because it's not calculable, would require too much computing power > > which does not exist at this time and probably never will. > > You must tell us WHY this is so. A declaration is not sufficient. > > > However, if we allow (for brevity) to model objects more coarsely we > > can come up with some decent models. Instead of considering every > > individual atom, just consider a planet as a whole and skip all of the > > fine structure. > > So you are presuming our planet, earth, without considering what you > posited above which suggests differences among other planets. (In other > words, speculative impressionistic ideas about distant systems which > might not have the same physics humans experience. That's a > panthromorpic view.) > > > A planet may then be regarded (in my model) as a gradient. The > > gradient is comprised of a potential, and to each point in space we > > assign a potential that the point exists. That gives rise to this > > gradient. Consider that the nucleus of the planet is enriched, and the > > areas in it's outer shells are rarified. A planet (or atom) is nothing > > more than an imbalance as described. [...] Document: draft-cheshire-dnsext-multicastdns-11.txt Stuart Cheshire Internet-Draft Marc Krochmal Category: Standards Track Apple Inc. Expires: 23 September 2010 23 March 2010 Multicast DNS <draft-cheshire-dnsext-multicastdns-11.txt> Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. 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/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on 23rd September 2010. Abstract As networked devices become smaller, more portable, and more ubiquitous, the ability to operate with less configured infrastructure is increasingly important. In particular, the ability to look up DNS resource record data types (including, but not limited to, host names) in the absence of a conventional managed DNS server is becoming essential. Multicast DNS (mDNS) provides the ability to do DNS-like operations on the local link in the absence of any conventional unicast DNS server. In addition, mDNS designates a portion of the DNS namespace to be free for local use, without the need to pay any annual fee, and without the need to set up delegations or otherwise configure a conventional DNS server to answer for those names. The primary benefits of mDNS names are that (i) they require little or no administration or configuration to set them up, (ii) they work when no infrastructure is present, and (iii) they work during infrastructure failures. Expires 23rd September 2010 Cheshire & Krochmal [Page 1] Internet Draft Multicast DNS 23rd March 2010 Table of Contents 1. Introduction.................................................... 3 2. Conventions and Terminology Used in this Document............... 3 3. Multicast DNS Names............................................. 5 4. Reverse Address Mapping......................................... 6 5. Querying........................................................ 7 6. Duplicate Suppression.......................................... 12 7. Responding..................................................... 14 8. Probing and Announcing on Startup.............................. 21 9. Conflict Resolution............................................ 27 10. Resource Record TTL Values and Cache Coherency................. 28 11. Source Address Check........................................... 34 12. Special Characteristics of Multicast DNS Domains............... 35 13. Multicast DNS for Service Discovery............................ 36 14. Enabling and Disabling Multicast DNS........................... 36 15. Considerations for Multiple Interfaces......................... 37 16. Considerations for Multiple Responders on the Same Machine..... 38 17. Multicast DNS Character Set.................................... 40 18. Multicast DNS Message Size..................................... 41 19. Multicast DNS Message Format................................... 42 20. Summary of Differences Between Multicast DNS and Unicast DNS... 46 21. IPv6 Considerations............................................ 47 22. Security Considerations........................................ 47 23. IANA Considerations............................................ 48 24. Acknowledgments................................................ 50 25. Copyright Notice............................................... 50 26. Normative References........................................... 51 27. Informative References......................................... 51 28. Authors' Addresses............................................. 53 Appendix A. Design Rationale for Choice of UDP Port Number......... 54 Appendix B. Design Rationale for Not Using Hashed Mcast Addresses.. 55 Appendix C. Design Rationale for Max Multicast DNS Name Length..... 56 Appendix D. Benefits of Multicast Responses........................ 58 Appendix E. Design Rationale for Encoding Negative Responses....... 59 Appendix F. Use of UTF-8...........................................60 Appendix G. Governing Standards Body............................... 60 Appendix H. Private DNS Namespaces................................. 61 Appendix I. Deployment History..................................... 62 Expires 23rd September 2010 Cheshire & Krochmal [Page 2] Internet Draft Multicast DNS 23rd March 2010 1. Introduction Multicast DNS and its companion technology DNS Service Discovery [DNS-SD] were created to provide IP networking with the ease-of-use and autoconfiguration for which AppleTalk was well known [ATalk]. When reading this document, familiarity with the concepts of Zero Configuration Networking [Zeroconf] and automatic link-local addressing [RFC 2462] [RFC 3927] is helpful. This document specifies no change to the structure of DNS messages, no new operation codes or response codes, and new resource record types. This document describes how clients send DNS-like queries via IP multicast, and how a collection of hosts cooperate to collectively answer those queries in a useful manner. 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]. When this document uses the term "Multicast DNS", it should be taken to mean: "Clients performing DNS-like queries for DNS-like resource records by sending DNS-like UDP query and response packets over IP Multicast to UDP port 5353." The design rationale for selecting UDP port 5353 is discussed in Appendix A. This document uses the term "host name" in the strict sense to mean a fully qualified domain name that has an IPv4 or IPv6 address record. It does not use the term "host name" in the commonly used but incorrect sense to mean just the first DNS label of a host's fully qualified domain name. A DNS (or mDNS) packet contains an IP TTL in the IP header, which is effectively a hop-count limit for the packet, to guard against routing loops. Each Resource Record also contains a TTL, which is the number of seconds for which the Resource Record may be cached. This document uses the term "IP TTL" to refer to the IP header TTL (hop limit), and the term "RR TTL" or just "TTL" to refer to the Resource Record TTL (cache lifetime). DNS-format messages contain a header, a Question Section, then Answer, Authority, and Additional Record Sections. The Answer, Authority, and Additional Record Sections all hold resource records in the same format. Where this document describes issues that apply equally to all three sections, it uses the term "Resource Record Sections" to refer collectively to these three sections. Expires 23rd September 2010 Cheshire & Krochmal [Page 3] Internet Draft Multicast DNS 23rd March 2010 This document uses the terms "shared" and "unique" when referring to resource record sets: A "shared" resource record set is one where several Multicast DNS Responders may have records with that name, rrtype, and rrclass, and several Responders may respond to a particular query. A "unique" resource record set is one where all the records with that name, rrtype, and rrclass are conceptually under the control or ownership of a single Responder, and it is expected that at most one Responder should respond to a query for that name, rrtype, and rrclass. Before claiming ownership of a unique resource record set, a Responder MUST probe to verify that no other Responder already claims ownership of that set, as described in Section 8.1 "Probing". (For fault-tolerance and other reasons it is permitted sometimes to have more than one Responder answering for a particular "unique" resource record set, but such cooperating Responders MUST give answers containing identical rdata for these records. If they do not give answers containing identical rdata then the probing step will reject the data as being inconsistent with what is already being advertised on the network for those names.) Strictly speaking the terms "shared" and "unique" apply to resource record sets, not to individual resource records, but it is sometimes convenient to talk of "shared resource records" and "unique resource records". When used this way, the terms should be understood to mean a record that is a member of a "shared" or "unique" resource record set, respectively. Expires 23rd September 2010 Cheshire & Krochmal [Page 4] Internet Draft Multicast DNS 23rd March 2010 3. Multicast DNS Names This document specifies that the DNS top-level domain ".local." is a special domain with special semantics, namely that any fully- qualified name ending in ".local." is link-local, and names within this domain are meaningful only on the link where they originate. This is analogous to IPv4 addresses in the 169.254/16 prefix, which are link-local and meaningful only on the link where they originate. Any DNS query for a name ending with ".local." MUST be sent to the mDNS multicast address (224.0.0.251 or its IPv6 equivalent FF02::FB). The design rationale for using a fixed multicast address instead of selecting from a range of multicast addresses using a hash function is discussed in Appendix B. It is unimportant whether a name ending with ".local." occurred because the user explicitly typed in a fully qualified domain name ending in ".local.", or because the user entered an unqualified domain name and the host software appended the suffix ".local." because that suffix appears in the user's search list. The ".local." suffix could appear in the search list because the user manually configured it, or because it was received via DHCP [RFC 2132], or via any other mechanism for configuring the DNS search list. In this respect the ".local." suffix is treated no differently to any other search domain that might appear in the DNS search list. DNS queries for names that do not end with ".local." MAY be sent to the mDNS multicast address, if no other conventional DNS server is available. This can allow hosts on the same link to continue communicating using each other's globally unique DNS names during network outages which disrupt communication with the greater Internet. When resolving global names via local multicast, it is even more important to use DNSSEC or other security mechanisms to ensure that the response is trustworthy. Resolving global names via local multicast is a contentious issue, and this document does not discuss it in detail, instead concentrating on the issue of resolving local names using DNS packets sent to a multicast address. A host that belongs to an organization or individual who has control over some portion of the DNS namespace can be assigned a globally unique name within that portion of the DNS namespace, such as, "cheshire.example.com." For those of us who have this luxury, this works very well. However, the majority of home computer users do not have easy access to any portion of the global DNS namespace within which they have the authority to create names as they wish. This leaves the majority of home computers effectively anonymous for practical purposes. To remedy this problem, this document allows any computer user to elect to give their computers link-local Multicast DNS host names of the form: "single-dns-label.local." For example, a laptop computer Expires 23rd September 2010 Cheshire & Krochmal [Page 5] Internet Draft Multicast DNS 23rd March 2010 may answer to the name "MyPrinter.local." Any computer user is granted the authority to name their computer this way, provided that the chosen host name is not already in use on that link. Having named their computer this way, the user has the authority to continue using that name until such time as a name conflict occurs on the link which is not resolved in the user's favor. If this happens, the computer (or its human user) SHOULD cease using the name, and may choose to attempt to allocate a new unique name for use on that link. These conflicts are expected to be relatively rare for people who choose reasonably imaginative names, but it is still important to have a mechanism in place to handle them when they happen. This document recommends a single flat namespace for dot-local host names, (i.e. the names of DNS "A" and "AAAA" records, which map names to IPv4 and IPv6 addresses), but other DNS record types (such as those used by DNS Service Discovery [DNS-SD]) may contain as many labels as appropriate for the desired usage, up to a maximum of 255 bytes, not including the terminating zero byte at the end. Name length issues are discussed further in Appendix C. Enforcing uniqueness of host names is probably desirable in the common case, but this document does not mandate that. It is permissible for a collection of coordinated hosts to agree to maintain multiple DNS address records with the same name, possibly for load balancing or fault-tolerance reasons. This document does not take a position on whether that is sensible. It is important that both modes of operation are supported. The Multicast DNS protocol allows hosts to verify and maintain unique names for resource records where that behavior is desired, and it also allows hosts to maintain multiple resource records with a single shared name where that behavior is desired. This consideration applies to all resource records, not just address records (host names). In summary: It is required that the protocol have the ability to detect and handle name conflicts, but it is not required that this ability be used for every record. 4. Reverse Address Mapping Like ".local.", the IPv4 and IPv6 reverse mapping domains are also defined to be link-local: Any DNS query for a name ending with "254.169.in-addr.arpa." MUST be sent to the mDNS multicast address 224.0.0.251. Since names under this domain correspond to IPv4 link-local addresses, it is logical that the local link is the best place to find information pertaining to those names. Likewise, any DNS query for a name within the reverse mapping domains for IPv6 Link-Local addresses ("8.e.f.ip6.arpa.", "9.e.f.ip6.arpa.", "a.e.f.ip6.arpa.", and "b.e.f.ip6.arpa.") MUST be sent to the IPv6 mDNS link-local multicast address FF02::FB. Expires 23rd September 2010 Cheshire & Krochmal [Page 6] Internet Draft Multicast DNS 23rd March 2010 5. Querying There are three kinds of Multicast DNS Queries, one-shot queries of the kind made by conventional DNS clients, one-shot queries accumulating multiple responses made by multicast-aware DNS clients, and continuous ongoing Multicast DNS Queries used by IP network browser software. Except in the rare case of a Multicast DNS Responder that is advertising only shared resources records and no unique records, a Multicast DNS Responder MUST also implement a Multicast DNS Querier so that it can first verify the uniqueness of those records before it begins answering queries for them. 5.1 One-Shot Multicast DNS Queries The most basic kind of Multicast DNS client may simply send its DNS queries blindly to 224.0.0.251:5353, without necessarily even being aware of what a multicast address is. This change can typically be implemented with just a few lines of code in an existing DNS resolver library. Any time the name being queried for falls within one of the reserved mDNS domains (see Section 12 "Special Characteristics of Multicast DNS Domains") rather than using the configured unicast DNS server address, the query is instead sent to 224.0.0.251:5353 (or its IPv6 equivalent [FF02::FB]:5353). Typically the timeout would also be shortened to two or three seconds. It's possible to make a minimal mDNS client with only these simple changes. These queries are typically done using a high-numbered ephemeral UDP source port, but regardless of whether they are sent from a dynamic port or from a fixed port, these queries SHOULD NOT be sent using UDP source port 5353, since using UDP source port 5353 signals the presence of a fully-compliant Multicast DNS client, as described below. A simple DNS client like this will typically just take the first response it receives. It will not listen for additional UDP responses, but in many instances this may not be a serious problem. If a user types "http://MyPrinter.local." into their web browser and gets to see the status and configuration web page for their printer, then the protocol has met the user's needs in this case. While a basic DNS client like this may be adequate for simple host name lookup, it may not get ideal behavior in other cases. Additional refinements that may be adopted by more sophisticated clients are described below. Expires 23rd September 2010 Cheshire & Krochmal [Page 7] Internet Draft Multicast DNS 23rd March 2010 5.2 One-Shot Queries, Accumulating Multiple Responses A compliant Multicast DNS client, which implements the rules specified in this document, MUST send its Multicast DNS Queries from UDP source port 5353 (the well-known port assigned to mDNS), and MUST listen for Multicast DNS Replies sent to UDP destination port 5353 at the mDNS multicast address (224.0.0.251 and/or its IPv6 equivalent FF02::FB). As described above, there are some cases, such as looking up the address associated with a unique host name, where a single response is sufficient, and moreover may be all that is expected. However, there are other DNS queries where more than one response is possible and useful, and for these queries a more advanced Multicast DNS client should include the ability to wait for an appropriate period of time to collect multiple responses. A naive DNS client retransmits its query only so long as it has received no response. A more advanced Multicast DNS client is aware that having received one response is not necessarily an indication that it might not receive others, and has the ability to retransmit its query until it is satisfied with the collection of responses it has gathered. When retransmitting, the interval between the first two queries SHOULD be at least one second, and the intervals between successive queries SHOULD increase by at least a factor of two. A Multicast DNS client that is retransmitting a query for which it has already received some responses MUST implement Known Answer Suppression, as described below in Section 6.1 "Known Answer Suppression". This indicates to Responders who have already replied that their responses have been received, and they don't need to send them again in response to this repeated query. 5.3 Continuous Multicast DNS Querying In One-Shot Queries, with either single or multiple responses, the underlying assumption is that the transaction begins when the application issues a query, and ends when the desired responses have been received. There is another type of operation which is more akin to continuous monitoring. Imagine some hypothetical software which allows users to manage their digital music collections, with a graphical user interface which includes a sidebar down the left side of the window, which shows other sources of shared music the software has discovered on the local network. It would be convenient for the user if they could rely on this list of shared music sources displayed in the window sidebar to stay up to date as music sources come and go, rather than displaying out-of-date stale information, and requiring the user explicitly to click a "refresh" button any time they want to see accurate information (which, from the moment it is displayed, is Expires 23rd September 2010 Cheshire & Krochmal [Page 8] Internet Draft Multicast DNS 23rd March 2010 itself already beginning to become out-of-date and stale). If we are to to display a continuously-updated live list like this, we need to be able to do it efficiently, without naive constant polling which would be an unreasonable burden on the network. Therefore, when retransmitting mDNS queries to implement this kind of continuous monitoring, the interval between the first two queries SHOULD be at least one second, the intervals between successive queries SHOULD increase by at least a factor of two, and the querier MUST implement Known Answer Suppression, as described below in Section 6.1. When the interval between queries reaches or exceeds 60 minutes, a querier MAY cap the interval to a maximum of 60 minutes, and perform subsequent queries at a steady-state rate of one query per hour. To avoid accidental synchronization when for some reason multiple clients begin querying at exactly the same moment (e.g. because of some common external trigger event), a Multicast DNS Querier SHOULD also delay the first query of the series by a randomly-chosen amount in the range 20-120ms. When a Multicast DNS Querier receives an answer, the answer contains a TTL value that indicates for how many seconds this answer is valid. After this interval has passed, the answer will no longer be valid and SHOULD be deleted from the cache. Before this time is reached, a Multicast DNS Querier which has clients with an active interest in the state of that record (e.g. a network browsing window displaying a list of discovered services to the user) SHOULD re-issue its query to determine whether the record is still valid. To perform this cache maintenance, a Multicast DNS Querier should plan to re-query for records after at least 50% of the record lifetime has elapsed. This document recommends the following specific strategy: The Querier should plan to issue a query at 80% of the record lifetime, and then if no answer is received, at 85%, 90% and 95%. If an answer is received, then the remaining TTL is reset to the value given in the answer, and this process repeats for as long as the Multicast DNS Querier has an ongoing interest in the record. If after four queries no answer is received, the record is deleted when it reaches 100% of its lifetime. A Multicast DNS Querier MUST NOT perform this cache maintenance for records for which it has no clients with an active interest. If the expiry of a particular record from the cache would result in no net effect to any client software running on the Querier device, and no visible effect to the human user, then there is no reason for the Multicast DNS Querier to waste network bandwidth checking whether the record remains valid. To avoid the case where multiple Multicast DNS Queriers on a network all issue their queries simultaneously, a random variation of 2% of the record TTL should be added, so that queries are scheduled to be performed at 80-82%, 85-87%, 90-92% and then 95-97% of the TTL. Expires 23rd September 2010 Cheshire & Krochmal [Page 9] Internet Draft Multicast DNS 23rd March 2010 An additional efficiency optimization SHOULD be performed when a Multicast DNS response is received containing a unique answer (as indicated by the cache flush bit being set, described in Section 10.3, "Announcements to Flush Outdated Cache Entries"). In this case, there is no need for the querier to continue issuing a stream of queries with exponentially-increasing intervals, since the receipt of a unique answer is a good indication that no other answers will be forthcoming. In this case, the Multicast DNS Querier SHOULD plan to issue its next query for this record at 80-82% of the record's TTL, as described above. 5.4 Multiple Questions per Query Multicast DNS allows a querier to place multiple questions in the Question Section of a single Multicast DNS query packet. The semantics of a Multicast DNS query packet containing multiple questions is identical to a series of individual DNS query packets containing one question each. Combining multiple questions into a single packet is purely an efficiency optimization, and has no other semantic significance. 5.5 Questions Requesting Unicast Responses Sending Multicast DNS responses via multicast has the benefit that all the other hosts on the network get to see those responses, and can keep their caches up to date, and can detect conflicting responses. However, there are situations where all the other hosts on the network don't need to see every response. Some examples are a laptop computer waking from sleep, or the Ethernet cable being connected to a running machine, or a previously inactive interface being activated through a configuration change. At the instant of wake-up or link activation, the machine is a brand new participant on a new network. Its Multicast DNS cache for that interface is empty, and it has no knowledge of its peers on that link. It may have a significant number of questions that it wants answered right away to discover information about its new surroundings and present that information to the user. As a new participant on the network, it has no idea whether the exact same questions may have been asked and answered just seconds ago. In this case, triggering a large sudden flood of multicast responses may impose an unreasonable burden on the network. To avoid large floods of potentially unnecessary responses in these cases, Multicast DNS defines the top bit in the class field of a DNS question as the "unicast response" bit. When this bit is set in a question, it indicates that the Querier is willing to accept unicast responses instead of the usual multicast responses. These questions requesting unicast responses are referred to as "QU" questions, to Expires 23rd September 2010 Cheshire & Krochmal [Page 10] Internet Draft Multicast DNS 23rd March 2010 distinguish them from the more usual questions requesting multicast responses ("QM" questions). A Multicast DNS Querier sending its initial batch of questions immediately on wake from sleep or interface activation SHOULD set the "QU" bit in those questions. When a question is retransmitted (as described in Section 5.3 "Continuous Multicast DNS Querying") the "QU" bit SHOULD NOT be set in subsequent retransmissions of that question. Subsequent retransmissions SHOULD be usual "QM" questions. After the first question has received its responses, the querier should have a large known-answer list (see "Known Answer Suppression" below) so that subsequent queries should elicit few, if any, further responses. Reverting to multicast responses as soon as possible is important because of the benefits that multicast responses provide (see Appendix D). In addition, the "QU" bit SHOULD be set only for questions that are active and ready to be sent the moment of wake from sleep or interface activation. New questions issued by clients afterwards should be treated as normal "QM" questions and SHOULD NOT have the "QU" bit set on the first question of the series. When receiving a question with the "unicast response" bit set, a Responder SHOULD usually respond with a unicast packet directed back to the querier. If the Responder has not multicast that record recently (within one quarter of its TTL), then the Responder SHOULD instead multicast the response so as to keep all the peer caches up to date, and to permit passive conflict detection. In the case of answering a probe question with the "unicast response" bit set, the Responder should always generate the requested unicast response, but may also send a multicast announcement too if the time since the last multicast announcement of that record is more than a quarter of its TTL. Except when defending a unique name against a probe from another host, unicast replies are subject to all the same packet generation rules as multicast replies, including the cache flush bit (see Section 10.3, "Announcements to Flush Outdated Cache Entries") and randomized delays to reduce network collisions (see Section 7, "Responding"). 5.6 Direct Unicast Queries to port 5353 In specialized applications there may be rare situations where it makes sense for a Multicast DNS Querier to send its query via unicast to a specific machine. When a Multicast DNS Responder receives a query via direct unicast, it SHOULD respond as it would for a "QU" query, as described above in Section 5.5 "Questions Requesting Unicast Responses". Since it is possible for a unicast query to be received from a machine outside the local link, Responders SHOULD check that the source address in the query packet matches the local subnet for that link, and silently ignore the packet if not. Expires 23rd September 2010 Cheshire & Krochmal [Page 11] Internet Draft Multicast DNS 23rd March 2010 There may be specialized situations, outside the scope of this document, where it is intended and desirable to create a Responder that does answer queries originating outside the local link. Such a Responder would need to ensure that these non-local queries are always answered via unicast back to the Querier, since an answer sent via link-local multicast would not reach a Querier outside the local link. 6. Duplicate Suppression A variety of techniques are used to reduce the amount of redundant traffic on the network. 6.1 Known Answer Suppression When a Multicast DNS Querier sends a query to which it already knows some answers, it populates the Answer Section of the DNS query message with those answers. A Multicast DNS Responder MUST NOT answer a Multicast DNS Query if the answer it would give is already included in the Answer Section with an RR TTL at least half the correct value. If the RR TTL of the answer as given in the Answer Section is less than half of the true RR TTL as known by the Multicast DNS Responder, the Responder MUST send an answer so as to update the Querier's cache before the record becomes in danger of expiration. Because a Multicast DNS Responder will respond if the remaining TTL given in the known answer list is less than half the true TTL, it is superfluous for the Querier to include such records in the known answer list. Therefore a Multicast DNS Querier SHOULD NOT include records in the known answer list whose remaining TTL is less than half their original TTL. Doing so would simply consume space in the packet without achieving the goal of suppressing responses, and would therefore be a pointless waste of network bandwidth. A Multicast DNS Querier MUST NOT cache resource records observed in the Known Answer Section of other Multicast DNS Queries. The Answer Section of Multicast DNS Queries is not authoritative. By placing information in the Answer Section of a Multicast DNS Query the querier is stating that it *believes* the information to be true. It is not asserting that the information *is* true. Some of those records may have come from other hosts that are no longer on the network. Propagating that stale information to other Multicast DNS Queriers on the network would not be helpful. Expires 23rd September 2010 Cheshire & Krochmal [Page 12] Internet Draft Multicast DNS 23rd March 2010 6.2 Multi-Packet Known Answer Suppression Sometimes a Multicast DNS Querier will already have too many answers to fit in the Known Answer Section of its query packets. In this case, it should issue a Multicast DNS Query containing a question and as many Known Answer records as will fit. It MUST then set the TC (Truncated) bit in the header before sending the Query. It MUST then immediately follow the packet with another query packet containing no questions, and as many more Known Answer records as will fit. If there are still too many records remaining to fit in the packet, it again sets the TC bit and continues until all the Known Answer records have been sent. A Multicast DNS Responder seeing a Multicast DNS Query with the TC bit set defers its response for a time period randomly selected in the interval 400-500ms. This gives the Multicast DNS Querier time to send additional Known Answer packets before the Responder responds. If the Responder sees any of its answers listed in the Known Answer lists of subsequent packets from the querying host, it SHOULD delete that answer from the list of answers it is planning to give (provided that no other host on the network has also issued a query for that record and is waiting to receive an answer). If the Responder receives additional Known Answer packets with the TC bit set, it SHOULD extend the delay as necessary to ensure a pause of 400-500ms after the last such packet before it sends its answer. This opens the potential risk that a continuous stream of Known Answer packets could, theoretically, prevent a Responder from answering indefinitely. In practice answers are never actually delayed significantly, and should a situation arise where significant delays did happen, that would be a scenario where the network is so overloaded that it would be desirable to err on the side of caution. The consequence of delaying an answer may be that it takes a user longer than usual to discover all the services on the local network; in contrast the consequence of incorrectly answering before all the Known Answer packets have been received would be wasting bandwidth sending unnecessary answers on an already overloaded network. In this (rare) situation, sacrificing speed to preserve reliable network operation is the right trade-off. 6.3 Duplicate Question Suppression If a host is planning to send a query, and it sees another host on the network send a QM query containing the same question, and the Known Answer Section of that query does not contain any records which this host would not also put in its own Known Answer Section, then this host should treat its own query as having been sent. When multiple clients on the network are querying for the same resource records, there is no need for them to all be repeatedly asking the same question. Expires 23rd September 2010 Cheshire & Krochmal [Page 13] Internet Draft Multicast DNS 23rd March 2010 6.4 Duplicate Answer Suppression If a host is planning to send an answer, and it sees another host on the network send a response packet containing the same answer record, and the TTL in that record is not less than the TTL this host would have given, then this host SHOULD treat its own answer as having been sent, and not also send an identical answer itself. When multiple Responders on the network have the same data, there is no need for all of them to respond. This feature is particularly useful when Multicast DNS Proxy Servers are in use, where there could be more than one proxy on the network giving Multicast DNS answers on behalf of some other host (e.g. because that other host is currently asleep and is not itself responding to queries). 7. Responding When a Multicast DNS Responder constructs and sends a Multicast DNS response packet, the Resource Record Sections of that packet must contain only records for which that Responder is explicitly authoritative. These answers may be generated because the record answers a question received in a Multicast DNS query packet, or at certain other times that the Responder determines than an unsolicited announcement is warranted. A Multicast DNS Responder MUST NOT place records from its cache, which have been learned from other Responders on the network, in the Resource Record Sections of outgoing response packets. Only an authoritative source for a given record is allowed to issue responses containing that record. The determination of whether a given record answers a given question is done using the standard DNS rules: The record name must match the question name, the record rrtype must match the question qtype unless the qtype is "ANY" (255) or the rrtype is "CNAME" (5), and the record rrclass must match the question qclass unless the qclass is "ANY" (255). A Multicast DNS Responder MUST only respond when it has a positive non-null response to send, or it authoritatively knows that a particular record does not exist. For unique records, where the host has already established sole ownership of the name, it MUST return negative answers to queries for records that it knows not to exist. For example, a host with no IPv6 address, that has claimed sole ownership of the name "host.local." for all rrtypes, MUST respond to AAAA queries for "host.local." by sending a negative answer indicating that no AAAA records exist for that name. See Section 7.1 "Negative Responses". For shared records, which are owned by no single host, the nonexistence of a given record is ascertained by the failure of any machine to respond to the Multicast DNS query, not by any explicit negative response. NXDOMAIN and other error responses MUST NOT be sent. Expires 23rd September 2010 Cheshire & Krochmal [Page 14] Internet Draft Multicast DNS 23rd March 2010 Multicast DNS Responses MUST NOT contain any questions in the Question Section. Any questions in the Question Section of a received Multicast DNS Response MUST be silently ignored. Multicast DNS Queriers receiving Multicast DNS Responses do not care what question elicited the response; they care only that the information in the response is true and accurate. A Multicast DNS Responder on Ethernet [IEEE 802] and similar shared multiple access networks SHOULD have the capability of delaying its responses by up to 500ms, as determined by the rules described below. If a large number of Multicast DNS Responders were all to respond immediately to a particular query, a collision would be virtually guaranteed. By imposing a small random delay, the number of collisions is dramatically reduced. On a full-sized Ethernet using the maximum cable lengths allowed and the maximum number of repeaters allowed, an Ethernet frame is vulnerable to collisions during the transmission of its first 256 bits. On 10Mb/s Ethernet, this equates to a vulnerable time window of 25.6us. On higher-speed variants of Ethernet, the vulnerable time window is shorter. In the case where a Multicast DNS Responder has good reason to believe that it will be the only Responder on the link that will send a response (i.e. because it is able to answer every question in the query packet, and for all of those answer records it has previously verified that the name, rrtype and rrclass are unique on the link) it SHOULD NOT impose any random delay before responding, and SHOULD normally generate its response within at most 10ms. In particular, this applies to responding to probe queries with the "unicast response" bit set. Since receiving a probe query gives a clear indication that some other Responder is planning to start using this name in the very near future, answering such probe queries to defend a unique record is a high priority and needs to be done without delay. A probe query can be distinguished from a normal query by the fact that a probe query contains a proposed record in the Authority Section which answers the question in the Question Section (for more details, see Section 8.2, "Simultaneous Probe Tie-Breaking"). Responding without delay is appropriate for records like the address record for a particular host name, when the host name has been previously verified unique. Responding without delay is *not* appropriate for things like looking up PTR records used for DNS Service Discovery [DNS-SD], where a large number of responses may be anticipated. In any case where there may be multiple responses, such as queries where the answer is a member of a shared resource record set, each Responder SHOULD delay its response by a random amount of time selected with uniform random distribution in the range 20-120ms. The reason for requiring that the delay be at least 20ms is to accommodate the situation where two or more query packets are sent Expires 23rd September 2010 Cheshire & Krochmal [Page 15] Internet Draft Multicast DNS 23rd March 2010 back-to-back, because in that case we want a Responder with answers to more than one of those queries to have the opportunity to aggregate all of its answers into a single response packet. In the case where the query has the TC (truncated) bit set, indicating that subsequent known answer packets will follow, Responders SHOULD delay their responses by a random amount of time selected with uniform random distribution in the range 400-500ms, to allow enough time for all the known answer packets to arrive, as described in Section 6.2 "Multi-Packet Known Answer Suppression". The source UDP port in all Multicast DNS Responses MUST be 5353 (the well-known port assigned to mDNS). Multicast DNS implementations MUST silently ignore any Multicast DNS Responses they receive where the source UDP port is not 5353. The destination UDP port in all Multicast DNS Responses MUST be 5353 and the destination address must be the multicast address 224.0.0.251 or its IPv6 equivalent FF02::FB, except when a unicast response has been explicitly requested: * via the "unicast response" bit, * by virtue of being a Legacy Query (Section 7.6), or * by virtue of being a direct unicast query. The benefits of sending Responses via multicast are discussed in Appendix D. To protect the network against excessive packet flooding due to software bugs or malicious attack, a Multicast DNS Responder MUST NOT (except in the one special case of answering probe queries) multicast a record on a given interface until at least one second has elapsed since the last time that record was multicast on that particular interface. A legitimate client on the network should have seen the previous transmission and cached it. A client that did not receive and cache the previous transmission will retry its request and receive a subsequent response. In the special case of answering probe queries, because of the limited time before the probing host will make its decision about whether or not to use the name, a Multicast DNS Responder MUST respond quickly. In this special case only, when responding via multicast to a probe, a Multicast DNS Responder is only required to delay its transmission as necessary to ensure an interval of at least 250ms since the last time the record was multicast on that interface. 7.1 Negative Responses In the early design of Multicast DNS it was assumed that explicit negative responses would never be needed. Hosts can assert the existence of the set of records which that host claims to exist, Expires 23rd September 2010 Cheshire & Krochmal [Page 16] Internet Draft Multicast DNS 23rd March 2010 and the union of all such sets on a link is the set of Multicast DNS records that exist on that link. Asserting the non-existence of every record in the complement of that set -- i.e. all possible Multicast DNS records that could exist on this link but do not at this moment -- was felt to be impractical and unnecessary. The non-existence of a record would be ascertained by a client querying for it and failing to receive a response from any of the hosts currently attached to the link. However, operational experience showed that explicit negative responses can sometimes be valuable. One such case is when a client is querying for a AAAA record, and the host name in question has no associated IPv6 addresses. In this case the responding host knows it currently has exclusive ownership of that name, and it knows that it currently does not have any IPv6 addresses, so an explicit negative response is preferable to the client having to retransmit its query multiple times and eventually give up with a timeout before it can conclude that a given AAAA record does not exist. A Multicast DNS Responder indicates the nonexistence of a record by using a DNS NSEC record [RFC 3845]. In the case of Multicast DNS the NSEC record is not being used for its usual DNSSEC security properties, but simply as a way of expressing which records do or do not exist with a given name. Implementers working with devices with sufficient memory and CPU resources may choose to implement code to handle the full generality of the DNS NSEC record [RFC 3845], including bitmaps up to 65,536 bits long. To facilitate use by clients with limited memory and CPU resources, Multicast DNS clients are only required to be able to parse a restricted form of the DNS NSEC record. All compliant Multicast DNS clients MUST at least correctly handle the restricted DNS NSEC record format described below: o The 'Next Domain Name' field contains the record's own name. When used with name compression, this means that the 'Next Domain Name' field always takes exactly two bytes in the packet. o The Type Bit Map block number is 0. o The Type Bit Map block length byte is a value in the range 1-32. o The Type Bit Map data is 1-32 bytes, as indicated by length byte. Because this restricted form of the DNS NSEC record is limited to Type Bit Map block number zero, it cannot express the existence of rrtypes above 255. Because of this, if a Multicast DNS Responder were to have records with rrtypes above 255, it MUST NOT generate these restricted-form NSEC records for those names, since to do so would imply that the name has no records with rrtypes above 255, which would be false. In such cases a Multicast DNS Responder MUST either Expires 23rd September 2010 Cheshire & Krochmal [Page 17] Internet Draft Multicast DNS 23rd March 2010 (a) emit no NSEC record for that name, or (b) emit a full NSEC record containing the appropriate Type Bit Map block(s) with the correct bits set for all the record types that exist. In practice this is not a significant limitation, since rrtypes above 255 are not currently in widespread use. If a Multicast DNS implementation receives an NSEC record where the 'Next Domain Name' field is not the record's own name, then the implementation SHOULD ignore the 'Next Domain Name' field and process the remainder of the NSEC record as usual. In Multicast DNS the 'Next Domain Name' field is not currently used, but it could be used in a future version of this protocol, which is why a Multicast DNS implementation MUST NOT reject or ignore an NSEC record it receives just because it finds an unexpected value in the 'Next Domain Name' field. If a Multicast DNS implementation receives an NSEC record containing more than one Type Bit Map, or where the Type Bit Map block number is not zero, or where the block length is not in the range 1-32, then the Multicast DNS implementation MAY silently ignore the entire NSEC record. A Multicast DNS implementation MUST NOT ignore an entire packet just because that packet contains one or more NSEC record(s) that the Multicast DNS implementation cannot parse. This provision is to allow future enhancements to the protocol to be introduced in a backwards-compatible way that does not break compatibility with older Multicast DNS implementations. To help differentiate these synthesized NSEC records (generated programmatically on-the-fly) from conventional Unicast DNS NSEC records (which actually exist in a signed DNS zone) the synthesized Multicast DNS NSEC records MUST NOT have the 'NSEC' bit set in the Type Bit Map, whereas conventional Unicast DNS NSEC records do have the 'NSEC' bit set. The TTL of the NSEC record indicates the intended lifetime of the negative cache entry. In general, the TTL given for an NSEC record SHOULD be the same as the TTL that the record would have had, had it existed. For example, the TTL for address records in Multicast DNS is typically 120 seconds, so the negative cache lifetime for an address record that does not exist should also be 120 seconds. A Responder should only generate negative responses to queries for which it has legitimate ownership of the name/rrtype/rrclass in question, and can legitimately assert that no record with that name/rrtype/rrclass exists. A Responder can assert that a specified rrtype does not exist for one of its names only if it previously claimed unique ownership of that name using probe queries for rrtype "ANY". (If it were to use probe queries for a specific rrtype, then it would only own the name for that rrtype, and could not assert that other rrtypes do not exist.) On receipt of a question for a particular name/rrtype/rrclass which a Responder knows not to exist Expires 23rd September 2010 Cheshire & Krochmal [Page 18] Internet Draft Multicast DNS 23rd March 2010 by virtue of previous successful probing, the Responder MUST send a response packet containing the appropriate NSEC record, if it can do so using the restricted form of the NSEC record described above. If a legitimate restricted-form NSEC record cannot be created (because rrtypes above 255 exist for that name) the Responder MAY emit a full NSEC record, or it MAY emit no NSEC record, at the implementer's discretion. The design rationale for this mechanism for encoding Negative Responses is discussed further in Appendix E. 7.2 Responding to Address Queries In Multicast DNS, whenever a Responder places an IPv4 or IPv6 address record (rrtype "A" or "AAAA") into a response packet, it SHOULD also place the corresponding other address type into the additional section, if there is space in the packet. This is to provide fate sharing, so that all a device's addresses are delivered atomically in a single packet, to reduce the risk that packet loss could cause a querier to receive only the IPv4 addresses and not the IPv6 addresses, or vice versa. In the event that a device has only IPv4 addresses but no IPv6 addresses, or vice versa, then the appropriate NSEC record SHOULD be placed into the additional section, so that queriers can know with certainty that the device has no addresses of that kind. Some Multicast DNS Responders treat a physical interface with both IPv4 and IPv6 address as a single interface with two addresses. Other Multicast DNS Responders treat this case as logically two interfaces, each with one address, but Responders that operate this way MUST NOT put the corresponding automatic NSEC records in replies they send (i.e. a negative IPv4 assertion in their IPv6 responses, and a negative IPv6 assertion in their IPv4 responses) because this would cause incorrect operation in Responders on the network that work the former way. 7.3 Responding to Multi-Question Queries Multicast DNS Responders MUST correctly handle DNS query packets containing more than one question, by answering any or all of the questions to which they have answers. Any (non-defensive) answers generated in response to query packets containing more than one question SHOULD be randomly delayed in the range 20-120ms, or 400-500ms if the TC (truncated) bit is set, as described above. (Answers defending a name, in response to a probe for that name, are not subject to this delay rule and are still sent immediately.) Expires 23rd September 2010 Cheshire & Krochmal [Page 19] Internet Draft Multicast DNS 23rd March 2010 7.4 Response Aggregation When possible, a Responder SHOULD, for the sake of network efficiency, aggregate as many responses as possible into a single Multicast DNS response packet. For example, when a Responder has several responses it plans to send, each delayed by a different interval, then earlier responses SHOULD be delayed by up to an additional 500ms if that will permit them to be aggregated with other responses scheduled to go out a little later. 7.5 Wildcard Queries (qtype "ANY" and qclass "ANY") When responding to queries using qtype "ANY" (255) and/or qclass "ANY" (255), a Multicast DNS Responder MUST respond with *ALL* of its records that match the query. This is subtly different to how qtype "ANY" and qclass "ANY" work in Unicast DNS. A common misconception is that a Unicast DNS query for qtype "ANY" will elicit a response containing all matching records. This is incorrect. If there are any records that match the query, the response is required only to contain at least one of them, not necessarily all of them. This somewhat surprising behavior is commonly seen with caching (i.e. "recursive") name servers. If a caching server receives a qtype "ANY" query for which it has at least one valid answer, it is allowed to return only those matching answers it happens to have already in its cache, and is not required to reconsult the authoritative name server to check if there are any more records that also match the qtype "ANY" query. For example, one might imagine that a query for qtype "ANY" for name "host.example.com" would return both the IPv4 (A) and the IPv6 (AAAA) address records for that host. In reality what happens is that it depends on the history of what queries have been previously received by intervening caching servers. If a caching server has no records for "host.example.com" then it will consult another server (usually the authoritative name server for the name in question) and in that case it will typically return all IPv4 and IPv6 address records. If however some other host has recently done a query for qtype "A" for name "host.example.com", so that the caching server already has IPv4 address records for "host.example.com" in its cache, but no IPv6 address records, then it will return only the IPv4 address records it already has cached, and no IPv6 address records. Multicast DNS does not share this property that qtype "ANY" and qclass "ANY" queries return some undefined subset of the matching records. When responding to queries using qtype "ANY" (255) and/or qclass "ANY" (255), a Multicast DNS Responder MUST respond with *ALL* of its records that match the query. Expires 23rd September 2010 Cheshire & Krochmal [Page 20] Internet Draft Multicast DNS 23rd March 2010 7.6 Legacy Unicast Responses If the source UDP port in a received Multicast DNS Query is not port 5353, this indicates that the client originating the query is a simple client that does not fully implement all of Multicast DNS. In this case, the Multicast DNS Responder MUST send a UDP response directly back to the client, via unicast, to the query packet's source IP address and port. This unicast response MUST be a conventional unicast response as would be generated by a conventional unicast DNS server; for example, it MUST repeat the query ID and the question given in the query packet. In addition, the "cache flush" bit described in Section 10.3 "Announcements to Flush Outdated Cache Entries" is specific to Multicast DNS, and MUST NOT be set in legacy unicast responses. The resource record TTL given in a legacy unicast response SHOULD NOT be greater than ten seconds, even if the true TTL of the Multicast DNS resource record is higher. This is because Multicast DNS Responders that fully participate in the protocol use the cache coherency mechanisms described in Section 10 "Resource Record TTL Values and Cache Coherency" to update and invalidate stale data. Were unicast responses sent to legacy clients to use the same high TTLs, these legacy clients, which do not implement these cache coherency mechanisms, could retain stale cached resource record data long after it is no longer valid. Having sent this unicast response, if the Responder has not sent this record in any multicast response recently, it SHOULD schedule the record to be sent via multicast as well, to facilitate passive conflict detection. "Recently" in this context means "if the time since the record was last sent via multicast is less than one quarter of the record's TTL". 8. Probing and Announcing on Startup Typically a Multicast DNS Responder should have, at the very least, address records for all of its active interfaces. Creating and advertising an HINFO record on each interface as well can be useful to network administrators. Whenever a Multicast DNS Responder starts up, wakes up from sleep, receives an indication of an Ethernet "Link Change" event, or has any other reason to believe that its network connectivity may have changed in some relevant way, it MUST perform the two startup steps below: Probing (Section 8.1) and Announcing (Section 8.3). Expires 23rd September 2010 Cheshire & Krochmal [Page 21] Internet Draft Multicast DNS 23rd March 2010 8.1 Probing The first startup step is that for all those resource records that a Multicast DNS Responder desires to be unique on the local link, it MUST send a Multicast DNS Query asking for those resource records, to see if any of them are already in us |