--- 1/draft-ietf-dnssd-push-09.txt 2017-03-13 17:14:34.355830047 -0700 +++ 2/draft-ietf-dnssd-push-10.txt 2017-03-13 17:14:34.419831583 -0700 @@ -1,105 +1,108 @@ Internet Engineering Task Force T. Pusateri Internet-Draft Seeking affiliation Intended status: Standards Track S. Cheshire -Expires: May 4, 2017 Apple Inc. - October 31, 2016 +Expires: September 14, 2017 Apple Inc. + March 13, 2017 DNS Push Notifications - draft-ietf-dnssd-push-09 + draft-ietf-dnssd-push-10 Abstract The Domain Name System (DNS) was designed to return matching records efficiently for queries for data that is relatively static. When those records change frequently, DNS is still efficient at returning - the updated results when polled. But there exists no mechanism for a - client to be asynchronously notified when these changes occur. This - document defines a mechanism for a client to be notified of such - changes to DNS records, called DNS Push Notifications. + the updated results when polled. But there exists no mechanism + for a client to be asynchronously notified when these changes occur. + This document defines a mechanism for a client to be notified + of such changes to DNS records, called DNS Push Notifications. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." - This Internet-Draft will expire on May 4, 2017. + This Internet-Draft will expire on September 14, 2017. Copyright Notice - Copyright (c) 2016 IETF Trust and the persons identified as the + Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents - 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 - 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3 - 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4 - 4. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 6 - 5. State Considerations . . . . . . . . . . . . . . . . . . . . 6 - 6. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 7 - 6.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 8 - 6.2. DNS Push Notification SUBSCRIBE . . . . . . . . . . . . . 10 - 6.2.1. SUBSCRIBE Request . . . . . . . . . . . . . . . . . . 11 - 6.2.2. SUBSCRIBE Response . . . . . . . . . . . . . . . . . 14 - 6.3. DNS Push Notification Update Messages . . . . . . . . . . 18 - 6.3.1. PUSH Message format . . . . . . . . . . . . . . . . . 18 - 6.4. DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . . 21 - 6.4.1. UNSUBSCRIBE Request . . . . . . . . . . . . . . . . . 22 + 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4 + 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5 + 4. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 7 + 5. State Considerations . . . . . . . . . . . . . . . . . . . . 8 + 6. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 9 + 6.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 10 + 6.2. DNS Push Notification SUBSCRIBE . . . . . . . . . . . . . 12 + 6.2.1. SUBSCRIBE Request . . . . . . . . . . . . . . . . . . 13 + 6.2.2. SUBSCRIBE Response . . . . . . . . . . . . . . . . . 15 + 6.3. DNS Push Notification Updates . . . . . . . . . . . . . . 18 + 6.3.1. PUSH Message . . . . . . . . . . . . . . . . . . . . 19 + 6.3.2. PUSH Response . . . . . . . . . . . . . . . . . . . . 21 + 6.4. DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . . 22 + 6.4.1. UNSUBSCRIBE Request . . . . . . . . . . . . . . . . . 23 6.4.2. UNSUBSCRIBE Response . . . . . . . . . . . . . . . . 24 - 6.5. DNS Session Signaling Push Notification RECONFIRM . . . . 26 - 6.6. Client-Initiated Termination . . . . . . . . . . . . . . 28 - 7. Security Considerations . . . . . . . . . . . . . . . . . . . 28 - 7.1. Security Services . . . . . . . . . . . . . . . . . . . . 29 - 7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 29 - 7.3. TLS Compression . . . . . . . . . . . . . . . . . . . . . 30 - 7.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 30 - 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30 - 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30 - 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 31 - 10.1. Normative References . . . . . . . . . . . . . . . . . . 31 - 10.2. Informative References . . . . . . . . . . . . . . . . . 32 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34 + 6.5. DNS Push Notification RECONFIRM . . . . . . . . . . . . . 26 + 6.5.1. RECONFIRM Request . . . . . . . . . . . . . . . . . . 26 + 6.5.2. RECONFIRM Response . . . . . . . . . . . . . . . . . 28 + 6.6. Client-Initiated Termination . . . . . . . . . . . . . . 30 + 7. Security Considerations . . . . . . . . . . . . . . . . . . . 31 + 7.1. Security Services . . . . . . . . . . . . . . . . . . . . 31 + 7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 31 + 7.3. TLS Compression . . . . . . . . . . . . . . . . . . . . . 32 + 7.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 32 + 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32 + 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32 + 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 33 + 10.1. Normative References . . . . . . . . . . . . . . . . . . 33 + 10.2. Informative References . . . . . . . . . . . . . . . . . 34 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36 1. Introduction DNS records may be updated using DNS Update [RFC2136]. Other - mechanisms such as a Hybrid Proxy [I-D.ietf-dnssd-hybrid] can also - generate changes to a DNS zone. This document specifies a protocol - for DNS clients to subscribe to receive asynchronous notifications of - changes to RRSets of interest. It is immediately relevant in the - case of DNS Service Discovery [RFC6763] but is not limited to that - use case, and provides a general DNS mechanism for DNS record change - notifications. Familiarity with the DNS protocol and DNS packet - formats is assumed [RFC1034] [RFC1035] [RFC6895]. + mechanisms such as a Discovery Proxy [DisProx] can also generate + changes to a DNS zone. This document specifies a protocol for DNS + clients to subscribe to receive asynchronous notifications of changes + to RRSets of interest. It is immediately relevant in the case of DNS + Service Discovery [RFC6763] but is not limited to that use case, and + provides a general DNS mechanism for DNS record change notifications. + Familiarity with the DNS protocol and DNS packet formats is assumed + [RFC1034] [RFC1035] [RFC6895]. 1.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "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" [RFC2119]. 2. Motivation @@ -112,71 +115,45 @@ generally highly tuned and capable of a high rate of query/response traffic, adding a publish/subscribe model for tracking changes to DNS records can result in more timely notification of changes with reduced CPU usage and lower network traffic. Multicast DNS [RFC6762] implementations always listen on a well known link-local IP multicast group, and new services and updates are sent for all group members to receive. Therefore, Multicast DNS already has asynchronous change notification capability. However, when DNS Service Discovery [RFC6763] is used across a wide area network using - Unicast DNS (possibly facilitated via a Hybrid Proxy - [I-D.ietf-dnssd-hybrid]) it would be beneficial to have an equivalent - capability for Unicast DNS, to allow clients to learn about DNS - record changes in a timely manner without polling. - - DNS Long-Lived Queries (LLQ) [I-D.sekar-dns-llq] is an existing - deployed solution to provide asynchronous change notifications. Even - though it can be used over TCP, LLQ is defined primarily as a UDP- - based protocol, and as such it defines its own equivalents of - existing TCP features like the three-way handshake, flow control, and - reliability. This document builds on experience gained with the LLQ - protocol, with an improved design. Instead of using UDP, this - specification uses long-lived TCP connections - [I-D.ietf-dnsop-session-signal], and therefore doesn't need to - reinvent existing TCP functionality. Instead of inventing a new - vocabulary of messages to communicate DNS zone changes, this - specification adopts the syntax and semantics of DNS Update messages - [RFC2136]. - - DNS Push Notifications impose less load on the responding server than - rapid polling would, but Push Notifications do still have a cost, so - DNS Push Notification clients MUST NOT recklessly create an excessive - number of Push Notification subscriptions. A subscription SHOULD - only be active when there is a valid reason to need live data (for - example, an on-screen display is currently showing the results to the - user) and the subscription SHOULD be cancelled as soon as the need - for that data ends (for example, when the user dismisses that - display). Implementations MAY want to implement idle timeouts, so - that if the user ceases interacting with the device, the display - showing the result of the DNS Push Notification subscription is - automatically dismissed after a certain period of inactivity. For - example, if a user presses the "Print" button on their smartphone, - and then leaves the phone showing the printer discovery screen until - the phone goes to sleep, then the printer discovery screen should be - automatically dismissed as the device goes to sleep. If the user - does still intend to print, this will require them to press the - "Print" button again when they wake their phone up. + Unicast DNS (possibly facilitated via a Discovery Proxy [DisProx]) it + would be beneficial to have an equivalent capability for Unicast DNS, + to allow clients to learn about DNS record changes in a timely manner + without polling. - A DNS Push Notification client MUST NOT routinely keep a DNS Push - Notification subscription active 24 hours a day 7 days a week just to - keep a list in memory up to date so that it will be really fast if - the user does choose to bring up an on-screen display of that data. - DNS Push Notifications are designed to be fast enough that there is - no need to pre-load a "warm" list in memory just in case it might be - needed later. + The DNS Long-Lived Queries (LLQ) [I-D.sekar-dns-llq] mechanism is an + existing deployed solution to provide asynchronous change + notifications, used by Apple's Back to My Mac Service [RFC6281]. + Back to My Mac was designed in an era when the data centre operations + staff asserted that it was impossible for a server to handle large + numbers of mostly-idle TCP connections, so LLQ had to defined as a + UDP-based protocol, effectively replicating much of TCP's connection + state management logic in user space, and creating its own poor + imitations of existing TCP features like the three-way handshake, + flow control, and reliability. - Generally, a client SHOULD NOT keep a connection to a server open - indefinitely if it has no active subscriptions on that connection. - After 30 seconds with no active subscriptions the client SHOULD close - the idle connection, and, if needed in the future, open a new - connection. + This document builds on experience gained with the LLQ protocol, with + an improved design. Instead of using UDP, this specification uses + TCP, and therefore doesn't need to reinvent existing TCP + functionality. Using TCP also gives long-lived low-traffic + connections better longevity through NAT gateways without resorting + to excessive keepalive traffic [SessSig]. Instead of inventing a new + vocabulary of messages to communicate DNS zone changes as LLQ did, + this specification adopts the syntax and semantics of DNS Update + messages [RFC2136]. 3. Overview The existing DNS Update protocol [RFC2136] provides a mechanism for clients to add or delete individual resource records (RRs) or entire resource record sets (RRSets) on the zone's server. This specification adopts a simplified subset of these existing syntax and semantics, and uses them for DNS Push Notification messages going in the opposite direction, from server to client, to @@ -216,20 +193,57 @@ comes to an authoritative server informing a client of changes to DNS records. This DNS Push Notification specification includes support for DNS classes, for completeness. However, in practice, it is anticipated that for the foreseeable future the only DNS class in use will be DNS class "IN", as is the reality today with existing DNS servers and clients. A DNS Push Notification server MAY choose to implement only DNS class "IN". + DNS Push Notifications impose less load on the responding server than + rapid polling would, but Push Notifications do still have a cost, so + DNS Push Notification clients MUST NOT recklessly create an excessive + number of Push Notification subscriptions. A subscription SHOULD + only be active when there is a valid reason to need live data (for + example, an on-screen display is currently showing the results to the + user) and the subscription SHOULD be cancelled as soon as the need + for that data ends (for example, when the user dismisses that + display). Implementations MAY want to implement idle timeouts, so + that if the user ceases interacting with the device, the display + showing the result of the DNS Push Notification subscription is + automatically dismissed after a certain period of inactivity. For + example, if a user presses the "Print" button on their smartphone, + and then leaves the phone showing the printer discovery screen until + the phone goes to sleep, then the printer discovery screen should be + automatically dismissed as the device goes to sleep. If the user + does still intend to print, this will require them to press the + "Print" button again when they wake their phone up. + + A DNS Push Notification client MUST NOT routinely keep a DNS Push + Notification subscription active 24 hours a day, 7 days a week, just + to keep a list in memory up to date so that if the user does choose + to bring up an on-screen display of that data, it can be displayed + really fast. DNS Push Notifications are designed to be fast enough + that there is no need to pre-load a "warm" list in memory just in + case it might be needed later. + + Generally, as described in the DNS Session Signaling specification + [SessSig], a client MUST NOT keep a connection to a server open + indefinitely if it has no subscriptions (or other operations) active + on that connection. A client MAY close a connection as soon as it + becomes idle, and then if needed in the future, open a new connection + when required. Alternatively, a client MAY speculatively keep an + idle connection open for some time, subject to the constraint that it + MUST NOT keep a connection open that has been idle for more than the + session's idle timeout (15 seconds by default). + 4. Transport Implementations of DNS Update [RFC2136] MAY use either User Datagram Protocol (UDP) [RFC0768] or Transmission Control Protocol (TCP) [RFC0793] as the transport protocol, in keeping with the historical precedent that DNS queries must first be sent over UDP [RFC1123]. This requirement to use UDP has subsequently been relaxed [RFC7766]. In keeping with the more recent precedent, DNS Push Notification is defined only for TCP. DNS Push Notification clients MUST use TLS @@ -260,58 +274,62 @@ and DANE TLSA records [RFC7673] is strongly encouraged. See below in Section 7.2 for details. 5. State Considerations Each DNS Push Notification server is capable of handling some finite number of Push Notification subscriptions. This number will vary from server to server and is based on physical machine characteristics, network bandwidth, and operating system resource allocation. After a client establishes a connection to a DNS server, - each record subscription is individually accepted or rejected. - Servers may employ various techniques to limit subscriptions to a - manageable level. Correspondingly, the client is free to establish - simultaneous connections to alternate DNS servers that support DNS - Push Notifications for the zone and distribute record subscriptions - at its discretion. In this way, both clients and servers can react - to resource constraints. Token bucket rate limiting schemes are also + each subscription is individually accepted or rejected. Servers may + employ various techniques to limit subscriptions to a manageable + level. Correspondingly, the client is free to establish simultaneous + connections to alternate DNS servers that support DNS Push + Notifications for the zone and distribute subscriptions at its + discretion. In this way, both clients and servers can react to + resource constraints. Token bucket rate limiting schemes are also effective in providing fairness by a server across numerous client requests. 6. Protocol Operation The DNS Push Notification protocol is a session-oriented protocol, - and makes use of DNS Session Signaling - [I-D.ietf-dnsop-session-signal]. + and makes use of DNS Session Signaling [SessSig]. + + For details of the DNS Session Signaling message format refer to the + DNS Session Signaling specification [SessSig]. Those details are not + repeated here. DNS Push Notification clients and servers MUST support DNS Session - Signaling, but the server must not issue any DNS Session Signaling + Signaling, but the server MUST NOT issue any DNS Session Signaling operations until after the client has first initiated a DNS Session Signaling operation of its own. A single server can support DNS Queries, DNS Updates, and DNS Push Notifications (using DNS Session Signaling) on the same TCP port, and until the client has sent at least one DNS Session Signaling operation the server does not know what kind of client has connected to it. Once the client has indicated willingness to use DNS Session Signaling operations by sending one of its own, either side of the connection may then initiate further Session Signaling operations at any time. A DNS Push Notification exchange begins with the client discovering the appropriate server, using the procedure described in Section 6.1, and then making a TLS/TCP connection to it. A typical DNS Push Notification client will immediately issue a DNS - Session Signaling Idle Timeout operation to request a session timeout - longer than the the 30-second default, but this is NOT REQUIRED. A - DNS Push Notification client MAY issue other requests on the - connection first, and only issue a DNS Session Signaling Idle Timeout - operation later if it determines that to be necessary. + Session Signaling Keepalive operation to request a session timeout or + keepalive interval longer than the the 15-second defaults, but this + is NOT REQUIRED. A DNS Push Notification client MAY issue other + requests on the connection first, and only issue a DNS Session + Signaling Keepalive operation later if it determines that to be + necessary. Once the connection is made, the client may then add and remove Push Notification subscriptions. In accordance with the current set of active subscriptions the server sends relevant asynchronous Push Notifications to the client. Note that a client MUST be prepared to receive (and silently ignore) Push Notifications for subscriptions it has previously removed, since there is no way to prevent the situation where a Push Notification is in flight from server to client while the client's UNSUBSCRIBE message cancelling that subscription is simultaneously in flight from client to server. @@ -321,36 +339,37 @@ 6.1. Discovery The first step in DNS Push Notification subscription is to discover an appropriate DNS server that supports DNS Push Notifications for the desired zone. The client MUST also determine which TCP port on the server is listening for connections, which need not be (and often is not) the typical TCP port 53 used for conventional DNS, or TCP port 853 used for DNS over TLS [RFC7858]. - 1. The client begins the discovery by sending a DNS query to the - local resolver with record type SOA [RFC1035] for the name of the - record it wishes to subscribe. + 1. The client begins the discovery by sending a DNS query to its + local resolver, with record type SOA [RFC1035], for the domain + name to which it wishes to subscribe. 2. If the SOA record exists, it MUST be returned in the Answer Section of the response. If not, the local resolver SHOULD include the SOA record for the zone of the requested name in the Authority Section. 3. If no SOA record is returned, the client then strips off the leading label from the requested name. If the resulting name has at least one label in it, the client sends a new SOA query and processing continues at step 2 above. If the resulting name is empty (the root label) then this is a network configuration error and the client gives up. The client MAY retry the operation at a - later time. + later time, of the client's choosing, such after a change in + network attachment. 4. Once the SOA is known (either by virtue of being seen in the Answer Section, or in the Authority Section), the client sends a DNS query with type SRV [RFC2782] for the record name "_dns-push-tls._tcp.", where is the owner name of the discovered SOA record. 5. If the zone in question does not offer DNS Push Notifications then SRV record MUST NOT exist and the SRV query will return a negative answer. @@ -380,168 +399,135 @@ connection, it SHOULD repeat the discovery process in order to determine the preferred DNS server for subscriptions at that time. Note that this repeated discovery step is typically very fast and typically results in no queries on the network. The client device MUST respect the DNS TTL values on records it receives, and store them in its local cache with this lifetime. This means that, as long as the DNS TTL values on the authoritative records were set to reasonable values, repeated application of this discovery process can be completed nearly instantaneously by the client, using only - locally-stored data. + locally-stored cached data. 6.2. DNS Push Notification SUBSCRIBE - After connecting, and requesting a longer idle timeout if necessary, - a DNS Push Notification client then indicates its desire to receive - DNS Push Notifications for a given domain name by sending a SUBSCRIBE - request over the established TLS connection to the server. A - SUBSCRIBE request is encoded in a DNS Session Signaling - [I-D.ietf-dnsop-session-signal] message. This specification defines - a new DNS Session Signaling TLV for DNS Push Notification SUBSCRIBE - Requests/Responses (tentatively Session Signaling Type Code 64). + After connecting, and requesting a longer idle timeout and/or + keepalive interval if necessary, a DNS Push Notification client then + indicates its desire to receive DNS Push Notifications for a given + domain name by sending a SUBSCRIBE request over the established TLS + connection to the server. A SUBSCRIBE request is encoded in a DNS + Session Signaling [SessSig] message. This specification defines a + DNS Session Signaling TLV for DNS Push Notification SUBSCRIBE + Requests/Responses (tentatively Session Signaling Type Code 0x40). - A server may not initiate a SUBSCRIBE request. + A server MUST NOT initiate a SUBSCRIBE request. 6.2.1. SUBSCRIBE Request A SUBSCRIBE request message begins with the standard DNS Session - Signaling 4-byte header [I-D.ietf-dnsop-session-signal], followed by - the SUBSCRIBE TLV. + Signaling 12-byte header [SessSig], followed by the SUBSCRIBE TLV. + The SSOP-DATA for the the SUBSCRIBE TLV is as follows: 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | MESSAGE ID | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - |QR| Opcode | Z | RCODE | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | SSOP-TYPE (SUBSCRIBE) | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | SSOP-LENGTH | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | - \ QNAME \ + \ NAME \ \ \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | QTYPE | + | TYPE | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | QCLASS | + | CLASS | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ Figure 1 The MESSAGE ID field MUST be set to a unique value, that the client is not using for any other active operation on this connection. For the purposes here, a MESSAGE ID is in use on this connection if the client has used it in a request for which it has not yet received a response, or if if the client has used it for a subscription which it has not yet cancelled using UNSUBSCRIBE. In the SUBSCRIBE response the server MUST echo back the MESSAGE ID value unchanged. - In a request the DNS Header QR bit MUST be zero. - - The DNS Header Opcode field holds the Session Signaling Opcode value - (tentatively 6). - - The Z bits MUST be zero on transmission, and MUST be silently ignored - on reception. - - The return code (RCODE) field MUST be set to 0 in a request. - - In the SUBSCRIBE TLV the SSOP-TYPE is SUBSCRIBE (tentatively 64). - The SSOP-LENGTH is the length of the data that follows, which + In the SUBSCRIBE TLV the SSOP-TYPE is SUBSCRIBE (tentatively 0x40). + The SSOP-LENGTH is the length of the SSOP-DATA that follows, which specifies the name, type, and class of the record(s) being sought. - A SUBSCRIBE request MUST contain exactly one question. There is no - QCOUNT field to specify more than one question. Since SUBSCRIBE - requests are sent over TCP, multiple SUBSCRIBE requests can be - concatenated in a single TCP stream and packed efficiently into TCP - segments. + A SUBSCRIBE request MUST contain exactly one question. The SUBSCRIBE + TLV has no QDCOUNT field to specify more than one question. Since + SUBSCRIBE requests are sent over TCP, multiple SUBSCRIBE request + messages can be concatenated in a single TCP stream and packed + efficiently into TCP segments. If accepted, the subscription will stay in effect until the client cancels the subscription using UNSUBSCRIBE or until the connection between the client and the server is closed. SUBSCRIBE requests on a given connection MUST be unique. A client - MUST NOT send a SUBSCRIBE message that duplicates the QNAME, QTYPE - and QCLASS of an existing active subscription on that TLS/TCP - connection. For the purpose of this matching, the established DNS - case-insensitivity for US-ASCII letters applies (e.g., "foo.com" and + MUST NOT send a SUBSCRIBE message that duplicates the NAME, TYPE and + CLASS of an existing active subscription on that TLS/TCP connection. + For the purpose of this matching, the established DNS case- + insensitivity for US-ASCII letters applies (e.g., "foo.com" and "Foo.com" are the same). If a server receives such a duplicate SUBSCRIBE message this is an error and the server MUST immediately - close the TCP connection. + immediately terminate the connection with a TCP RST (or equivalent + for other protocols). DNS wildcarding is not supported. That is, a wildcard ("*") in a SUBSCRIBE message matches only a literal wildcard character ("*") in the zone, and nothing else. Aliasing is not supported. That is, a CNAME in a SUBSCRIBE message matches only a literal CNAME record in the zone, and nothing else. A client may SUBSCRIBE to records that are unknown to the server at the time of the request (providing that the name falls within one of the zone(s) the server is responsible for) and this is not an error. The server MUST accept these requests and send Push Notifications if and when matching records are found in the future. - If neither QTYPE nor QCLASS are ANY (255) then this is a specific - subscription to changes for the given QNAME, QTYPE and QCLASS. If - one or both of QTYPE or QCLASS are ANY (255) then this subscription - matches any type and/or any class, as appropriate. + If neither TYPE nor CLASS are ANY (255) then this is a specific + subscription to changes for the given NAME, TYPE and CLASS. If one + or both of TYPE or CLASS are ANY (255) then this subscription matches + any type and/or any class, as appropriate. NOTE: A little-known quirk of DNS is that in DNS QUERY requests, QTYPE and QCLASS 255 mean "ANY" not "ALL". They indicate that the server should respond with ANY matching records of its choosing, not necessarily ALL matching records. This can lead to some surprising and unexpected results, were a query returns some valid answers but not all of them, and makes QTYPE=ANY queries less useful than people sometimes imagine. - When used in conjunction with SUBSCRIBE, QTYPE and QCLASS 255 should - be interpreted to mean "ALL", not "ANY". After accepting a - subscription where one or both of QTYPE or QCLASS are 255, the server - MUST send Push Notification Updates for ALL record changes that match - the subscription, not just some of them. + When used in conjunction with SUBSCRIBE, TYPE and CLASS 255 should be + interpreted to mean "ALL", not "ANY". After accepting a subscription + where one or both of TYPE or CLASS are 255, the server MUST send Push + Notification Updates for ALL record changes that match the + subscription, not just some of them. 6.2.2. SUBSCRIBE Response Each SUBSCRIBE request generates exactly one SUBSCRIBE response from the server. A SUBSCRIBE response message begins with the standard DNS Session - Signaling 4-byte header [I-D.ietf-dnsop-session-signal], possibly - followed by one or more optional modifier TLVs such as a Terminate - modifier TLV [I-D.ietf-dnsop-session-signal]. - - 1 1 1 1 1 1 - 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | MESSAGE ID | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - |QR| Opcode | Z | RCODE | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - - Figure 2 + Signaling 12-byte header [SessSig], possibly followed by one or more + optional Modifier TLVs, such as a Retry Delay Modifier TLV. The MESSAGE ID field MUST echo the value given in the ID field of the SUBSCRIBE request. This is how the client knows which request is being responded to. - In a response the DNS Header QR bit MUST be one. - If the QR bit is not one the message is not a response. - - The DNS Header Opcode field holds the Session Signaling Opcode value - (tentatively 6). - - The Z bits MUST be zero on transmission, and MUST be silently ignored - on reception. + A SUBSCRIBE response message MUST NOT contain a Session Signaling + Operation TLV. The Session Signaling Operation TLV is NOT copied + from the SUBSCRIBE request. In the SUBSCRIBE response the RCODE indicates whether or not the subscription was accepted. Supported RCODEs are as follows: +------------+-------+----------------------------------------------+ | Mnemonic | Value | Description | +------------+-------+----------------------------------------------+ | NOERROR | 0 | SUBSCRIBE successful. | | FORMERR | 1 | Server failed to process request due to a | | | | malformed request. | @@ -568,39 +554,37 @@ clients MUST be prepared to accept SUBSCRIBE Responses with any RCODE value. If the server sends a nonzero RCODE in the SUBSCRIBE response, either the client is (at least partially) misconfigured or the server resources are exhausted. In either case, the client shouldn't retry the subscription right away. Either end can terminate the connection, but the client may want to try this subscription again or it may have other successful subscriptions that it doesn't want to abandon. If the server sends a nonzero RCODE then it SHOULD append a - Terminate modifier TLV [I-D.ietf-dnsop-session-signal] to the - response specifying a delay before the client attempts this operation - again. Recommended values for the delay for different RCODE values - are given below: + Retry Delay Modifier TLV [SessSig] to the response specifying a delay + before the client attempts this operation again. Recommended values + for the delay for different RCODE values are given below: For RCODE = 1 (FORMERR) the delay may be any value selected by the - implementer. A value of five minutes is RECOMMENDED, to avoid - high load from defective clients. + implementer. A value of five minutes is RECOMMENDED, to reduce + the risk of high load from defective clients. For RCODE = 2 (SERVFAIL), which occurs due to resource exhaustion, the delay should be chosen according to the level of server overload and the anticipated duration of that overload. By default, a value of one minute is RECOMMENDED. For RCODE = 4 (NOTIMP), which occurs on a server that doesn't - implement DNS Session Signaling [I-D.ietf-dnsop-session-signal], - it is unlikely that the server will begin supporting DNS Session - Signaling in the next few minutes, so the retry delay SHOULD be - one hour. + implement DNS Session Signaling [SessSig], it is unlikely that the + server will begin supporting DNS Session Signaling in the next few + minutes, so the retry delay SHOULD be one hour. For RCODE = 5 (REFUSED), which occurs on a server that implements DNS Push Notifications, but is currently configured to disallow DNS Push Notifications, the retry delay may be any value selected by the implementer and/or configured by the operator. This is a misconfiguration, since this server is listed in a "_dns-push-tls._tcp." SRV record, but the server itself is not currently configured to support DNS Push Notifications. Since it is possible that the misconfiguration may be repaired at any time, the retry delay should not be set too high. By default, a @@ -627,84 +611,57 @@ server as appropriate for that error condition. By default, a value of 5 minutes is RECOMMENDED. For RCODE = 9 (NOTAUTH), the time delay applies to requests for other names falling within the same zone. Requests for names falling within other zones are not subject to the delay. For all other RCODEs the time delay applies to all subsequent requests to this server. After sending an error response the server MAY allow the connection - to remain open, or MAY send a DNS Push Notification Terminate Session - operation TLV and then close the TCP connection, as described in the - DNS Session Signaling specification [I-D.ietf-dnsop-session-signal]. - Clients MUST correctly handle both cases. + to remain open, or MAY send a DNS Push Notification Retry Delay + Operation TLV and then close the TCP connection, as described in the + DNS Session Signaling specification [SessSig]. Clients MUST + correctly handle both cases. -6.3. DNS Push Notification Update Messages +6.3. DNS Push Notification Updates Once a subscription has been successfully established, the server - generates PUSH messages to send to the client as appropriate. An - initial PUSH message will be sent immediately in the case that the - answer set was non-empty at the moment the subscription was - established. Subsequent changes to the answer set are then - communicated to the client in subsequent PUSH messages. - -6.3.1. PUSH Message format - - A PUSH message begins with the standard DNS Session Signaling 4-byte - header [I-D.ietf-dnsop-session-signal], followed by the PUSH TLV. - - The format of PUSH messages borrows from the existing DNS Update - [RFC2136] protocol, with some simplifications. - - 1 1 1 1 1 1 - 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | MESSAGE ID | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - |QR| Opcode | Z | RCODE | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | SSOP-TYPE (PUSH) | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | SSOP-LENGTH | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | UPCOUNT | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | | - \ Resource Records... \ - \ \ - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - - Figure 3 - - The MESSAGE ID field MUST be set to zero on transmission, and - silently ignored on reception. A PUSH message could potentially - match more than one subscription, or could relate to a subscription - that the client has just cancelled with an UNSUBSCRIBE message, so - the MESSAGE ID field serves no useful purpose. - - In a PUSH message the DNS Header QR bit MUST be zero. + generates PUSH messages to send to the client as appropriate. In the + case that the answer set was non-empty at the moment the subscription + was established, an initial PUSH message will be sent immediately + following the SUBSCRIBE Response. Subsequent changes to the answer + set are then communicated to the client in subsequent PUSH messages. - The DNS Header Opcode field holds the Session Signaling Opcode value - (tentatively 6). +6.3.1. PUSH Message - The Z bits MUST be zero on transmission, and MUST be silently ignored - on reception. + A PUSH message begins with the standard DNS Session Signaling 12-byte + header [SessSig], followed by the PUSH TLV. - The return code (RCODE) field MUST be set to 0 in a request. + The MESSAGE ID field MUST be set to a unique value, that the server + is not currently using for any other active outgoing request that it + has sent on this connection. The MESSAGE ID in the outgoing PUSH + message is selected by the server and has no relationship to the + MESSAGE ID in any of the client subscriptions it may relate to. In + the PUSH response the client MUST echo back the MESSAGE ID value + unchanged. - In the PUSH message TLV the SSOP-TYPE is PUSH (tentatively 65). The - SSOP-LENGTH is the length of the SSOP-DATA that follows. + In the PUSH TLV the SSOP-TYPE is PUSH (tentatively 0x41). The SSOP- + LENGTH is the length of the SSOP-DATA that follows, which specifies + the changes being communicated. - The SSOP-DATA contains a two-byte count of the number of records that - follow, followed by the records, in customary Resource Record format - (as used in DNS Update [RFC2136] messages). + The SSOP-DATA contains one or more Update records, in customary + Resource Record format, as used in DNS Update [RFC2136] messages. A + PUSH Message MUST contain at least one Update record. If a PUSH + Message is received that contains no Update records this is a fatal + error, and the receiver MUST immediately terminate the connection + with a TCP RST (or equivalent for other protocols). The SSOP-DATA contains the relevant change information for the client, formatted identically to a DNS Update [RFC2136]. To recap: Delete all RRsets from a name: TTL=0, CLASS=ANY, RDLENGTH=0, TYPE=ANY. Delete an RRset from a name: TTL=0, CLASS=ANY, RDLENGTH=0; TYPE specifies the RRset being deleted. @@ -714,174 +671,168 @@ TYPE, RDLENGTH and RDATA specifies the RR being deleted. Add to an RRset: TTL, CLASS, TYPE, RDLENGTH and RDATA specifies the RR being added. When processing the records received in a PUSH Message, the receiving client MUST validate that the records being added or deleted correspond with at least one currently active subscription on that connection. Specifically, the record name MUST match the name given in the SUBSCRIBE request, subject to the usual established DNS case- - insensitivity for US-ASCII letters. If the QTYPE in the SUBSCRIBE + insensitivity for US-ASCII letters. If the TYPE in the SUBSCRIBE request was not ANY (255) then the TYPE of the record must match the - QTYPE given in the SUBSCRIBE request. If the QCLASS in the SUBSCRIBE + TYPE given in the SUBSCRIBE request. If the CLASS in the SUBSCRIBE request was not ANY (255) then the CLASS of the record must match the - QCLASS given in the SUBSCRIBE request. If a matching active + CLASS given in the SUBSCRIBE request. If a matching active subscription on that connection is not found, then that individual record addition/deletion is silently ignored. Processing of other additions and deletions in this message is not affected. The TCP connection is not closed. This is to allow for the unavoidable race condition where a client sends an outbound UNSUBSCRIBE while inbound PUSH messages for that subscription from the server are still in flight. In the case where a single change affects more than one active subscription, only one PUSH message is sent. For example, a PUSH message adding a given record may match both a SUBSCRIBE request with - the same QTYPE and a different SUBSCRIBE request with QTYPE=ANY. It - is not the case that two PUSH messages are sent because the new - record matches two active subscriptions. + the same TYPE and a different SUBSCRIBE request with TYPE=ANY. It is + not the case that two PUSH messages are sent because the new record + matches two active subscriptions. The server SHOULD encode change notifications in the most efficient manner possible. For example, when three AAAA records are deleted from a given name, and no other AAAA records exist for that name, the server SHOULD send a "delete an RRset from a name" PUSH message, not three separate "delete an individual RR from a name" PUSH messages. Similarly, when both an SRV and a TXT record are deleted from a given name, and no other records of any kind exist for that name, the server SHOULD send a "delete all RRsets from a name" PUSH message, not two separate "delete an RRset from a name" PUSH messages. A server SHOULD combine multiple change notifications in a single PUSH message when possible, even if those change notifications apply - to different subscriptions. Conceptually, a PUSH messages is a - connection-level concept, not a subscription-level concept. + to different subscriptions. Conceptually, a PUSH message is a + connection-level mechanism, not a subscription-level mechanism. - Reception of a PUSH message does not directly generate a response - back to the server. (Updates may indirectly generate other - operations; e.g., a Push Notification Update Message declaring the - appearance of a PTR record could lead to a query for the SRV record - named in the rdata of that PTR record [RFC6763].) + Reception of a PUSH message by a client generates a PUSH response + back to the server. The TTL of an added record is stored by the client and decremented as time passes, with the caveat that for as long as a relevant subscription is active, the TTL does not decrement below 1 second. For as long as a relevant subscription remains active, the client SHOULD assume that when a record goes away the server will notify it of that fact. Consequently, a client does not have to poll to verify that the record is still there. Once a subscription is cancelled (individually, or as a result of the TCP connection being closed) record ageing resumes and records are removed from the local cache when their TTL reaches zero. +6.3.2. PUSH Response + + Each PUSH message generates exactly one PUSH response from the + receiver. + + A PUSH response message begins with the standard DNS Session + Signaling 12-byte header [SessSig], possibly followed by one or more + optional Modifier TLVs, such as a Retry Delay Modifier TLV. + + The MESSAGE ID field MUST echo the value given in the ID field of the + PUSH message. + + A PUSH response message MUST NOT contain a Session Signaling + Operation TLV. The Session Signaling Operation TLV is NOT copied + from the PUSH message. + + In a PUSH response the RCODE MUST be zero. Receiving a PUSH response + with a nonzero RCODE is a fatal error, and the receiver MUST + immediately terminate the connection with a TCP RST (or equivalent + for other protocols). + 6.4. DNS Push Notification UNSUBSCRIBE To cancel an individual subscription without closing the entire connection, the client sends an UNSUBSCRIBE message over the established TCP connection to the server. The UNSUBSCRIBE message is - encoded in a DNS Session Signaling [I-D.ietf-dnsop-session-signal] - message. This specification defines a new DNS Session Signaling TLV - for DNS Push Notification UNSUBSCRIBE Requests/Responses (tentatively - Session Signaling Type Code 66). + encoded in a DNS Session Signaling [SessSig] message. This + specification defines a DNS Session Signaling TLV for DNS Push + Notification UNSUBSCRIBE Requests/Responses (tentatively Session + Signaling Type Code 0x42). - A server may not initiate an UNSUBSCRIBE request. + A server MUST NOT initiate an UNSUBSCRIBE request. 6.4.1. UNSUBSCRIBE Request An UNSUBSCRIBE request message begins with the standard DNS Session - Signaling 4-byte header [I-D.ietf-dnsop-session-signal], followed by - the UNSUBSCRIBE TLV. - - 1 1 1 1 1 1 - 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | MESSAGE ID | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - |QR| Opcode | Z | RCODE | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | SSOP-TYPE (UNSUBSCRIBE) | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | SSOP-LENGTH (0) | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ + Signaling 12-byte header [SessSig], followed by the UNSUBSCRIBE TLV. - Figure 4 + In the UNSUBSCRIBE TLV the SSOP-TYPE is UNSUBSCRIBE (tentatively + 0x42). The SSOP-LENGTH is zero. There is no SSOP-DATA for + UNSUBSCRIBE. The MESSAGE ID field MUST match the value given in the ID field of an active SUBSCRIBE request. This is how the server knows which SUBSCRIBE request is being cancelled. After receipt of the UNSUBSCRIBE request, the SUBSCRIBE request is no longer active. If a server receives an UNSUBSCRIBE message where the MESSAGE ID does not - match the ID of an active SUBSCRIBE request this is an error and the - the server MUST return a response containing RCODE = 1 (FORMERR). In - the UNSUBSCRIBE response the server MUST echo back the MESSAGE ID - value unchanged. It is allowable for the client to issue an - UNSUBSCRIBE request for a previous SUBSCRIBE request for which the - client has not yet received a SUBSCRIBE response. This is to allow - for the case where a client starts and stops a subscription in less - than the round-trip time to the server. The client is NOT required - to wait for the SUBSCRIBE response before issuing the UNSUBSCRIBE - request. - - In a request the DNS Header QR bit MUST be zero. - - The DNS Header Opcode field holds the Session Signaling Opcode value - (tentatively 6). - - The Z bits MUST be zero on transmission, and MUST be silently ignored - on reception. - - The return code (RCODE) field MUST be set to 0 in a request. + match the ID of an active SUBSCRIBE request the server MUST return a + response containing RCODE = 3 (NXDOMAIN). - In the UNSUBSCRIBE TLV the SSOP-TYPE is UNSUBSCRIBE (tentatively 66). + It is allowable for the client to issue an UNSUBSCRIBE request for a + previous SUBSCRIBE request for which the client has not yet received + a SUBSCRIBE response. This is to allow for the case where a client + starts and stops a subscription in less than the round-trip time to + the server. The client is NOT required to wait for the SUBSCRIBE + response before issuing the UNSUBSCRIBE request. A consequence of + this is that if the client issues an UNSUBSCRIBE request for an as- + yet unacknowledged SUBSCRIBE request, and the SUBSCRIBE request is + subsequently unsuccessful for some reason, then when the UNSUBSCRIBE + request is eventually processed it will be an UNSUBSCRIBE request for + a nonexistent subscription, which will result NXDOMAIN response. - The SSOP-LENGTH is zero. + Note that when the client issues an UNSUBSCRIBE request for an as-yet + unacknowledged SUBSCRIBE request, at that moment the client will have + two outstanding DNS Session Signaling operations with same MESSAGE + ID, a SUBSCRIBE request and an UNSUBSCRIBE request, which will both + receive responses, in that order. When the client has multiple + outstanding DNS Session Signaling operations with same MESSAGE ID, + care should be taken that when a DNS Session Signaling response + message is received for that MESSAGE ID, it is associated with the + *first* unacknowledged request. 6.4.2. UNSUBSCRIBE Response Each UNSUBSCRIBE request generates exactly one UNSUBSCRIBE response from the server. - An UNSUBSCRIBE response message contains with the standard DNS - Session Signaling 4-byte header [I-D.ietf-dnsop-session-signal]. - - 1 1 1 1 1 1 - 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | MESSAGE ID | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - |QR| Opcode | Z | RCODE | - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - - Figure 5 + An UNSUBSCRIBE response message begins with the standard DNS Session + Signaling 12-byte header [SessSig], possibly followed by one or more + optional Modifier TLVs, such as a Retry Delay Modifier TLV. The MESSAGE ID field MUST echo the value given in the ID field of the UNSUBSCRIBE request. This is how the client knows which request is being responded to. - In a response the DNS Header QR bit MUST be one. - If the QR bit is not one the message is not a response. - - The DNS Header Opcode field holds the Session Signaling Opcode value - (tentatively 6). - - The Z bits MUST be zero on transmission, and MUST be silently ignored - on reception. + An UNSUBSCRIBE response message MUST NOT contain a Session Signaling + Operation TLV. The Session Signaling Operation TLV is NOT copied + from the UNSUBSCRIBE request. In the UNSUBSCRIBE response the RCODE indicates whether or not the unsubscribe request was successful. Supported RCODEs are as follows: +------------+-------+----------------------------------------------+ | Mnemonic | Value | Description | +------------+-------+----------------------------------------------+ | NOERROR | 0 | UNSUBSCRIBE successful. | | FORMERR | 1 | Server failed to process request due to a | | | | malformed request. | + | NXDOMAIN | 3 | Specified subscription does not exist. | | NOTIMP | 4 | Server does not recognize DNS Session | | | | Signaling Opcode. | | SSOPNOTIMP | 11 | UNSUBSCRIBE operation not supported. | +------------+-------+----------------------------------------------+ UNSUBSCRIBE Response codes This document specifies only these RCODE values for UNSUBSCRIBE Responses. Servers sending UNSUBSCRIBE Responses SHOULD use one of these values. However, future circumstances may create situations @@ -890,129 +841,229 @@ RCODE value. Having being successfully revoked with a correctly-formatted UNSUBSCRIBE message (resulting in a response with RCODE NOERROR) the previously referenced subscription is no longer active and the server MAY discard the state associated with it immediately, or later, at the server's discretion. Nonzero RCODE values signal some kind of error. - RCODE value FORMERR indicates an incorrect MESSAGE ID or other - message format error. + RCODE value FORMERR indicates a message format error. + + RCODE value NXDOMAIN indicates a MESSAGE ID that does not correspond + to any active subscription. RCODE values NOTIMP and SSOPNOTIMP should not occur in practice. A server would only generate NOTIMP if it did not support Session Signaling, and if the server does not support Session Signaling then it should not be possible for a client to have an active subscription to cancel. Similarly, a server would only generate SSOPNOTIMP if it did not support Push Notifications, and if the server does not support Push Notifications then it should not be possible for a client to have an active subscription to cancel. - All nonzero RCODE values indicate a serious problem with the client. - After sending an error response, the server SHOULD send a DNS Push - Notification Terminate Session operation TLV and then close the TCP - connection, as described in the DNS Session Signaling specification - [I-D.ietf-dnsop-session-signal]. + Nonzero RCODE values other than NXDOMAIN indicate a serious problem + with the client. After sending an error response other than + NXDOMAIN, the server SHOULD send a DNS Session Signaling Retry Delay + Operation TLV and then close the TCP connection, as described in the + DNS Session Signaling specification [SessSig]. -6.5. DNS Session Signaling Push Notification RECONFIRM +6.5. DNS Push Notification RECONFIRM - Sometimes, particularly when used with a Hybrid Proxy - [I-D.ietf-dnssd-hybrid], a DNS Zone may contain stale data. When a - client encounters data that it believe may be stale (e.g., an SRV - record referencing a target host+port that is not responding to - connection requests) the client can send a RECONFIRM message to - request that the server re-verify that the data is still valid. For - a Hybrid Proxy, this causes it to issue new Multicast DNS requests to - ascertain whether the target device is still present. For other - types of DNS server, the RECONFIRM operation is currently undefined - and SHOULD be silently ignored. + Sometimes, particularly when used with a Discovery Proxy [DisProx], a + DNS Zone may contain stale data. When a client encounters data that + it believe may be stale (e.g., an SRV record referencing a target + host+port that is not responding to connection requests) the client + can send a RECONFIRM request to ask the server to re-verify that the + data is still valid. For a Discovery Proxy, this causes it to issue + new Multicast DNS requests to ascertain whether the target device is + still present. For other types of DNS server, the RECONFIRM + operation is currently undefined, and SHOULD result in a NOERROR + response, but otherwise need not cause any action to occur. Frequent + RECONFIRM operations may be a sign of network unreliability, or some + kind of misconfiguration, so RECONFIRM operations MAY be logged or + otherwise communicated to a human administrator to assist in + detecting, and remedying, such network problems. - A RECONFIRM request is formatted identically to a SUBSCRIBE request, - except that the TLV type is RECONFIRM (tentatively 67) instead of - SUBSCRIBE. Additionally, QTYPE MUST NOT be the value ANY (255) and - QCLASS MUST NOT be the value ANY (255). + If, after receiving a valid RECONFIRM request, the server determines + that the disputed records are in fact no longer valid, then + subsequent DNS PUSH Messages will be generated to inform interested + clients. Thus, one client discovering that a previously-advertised + device (like a network printer) is no longer present has the side + effect of informing all other interested clients that the device in + question is now gone. - Like all DNS Session Signaling [I-D.ietf-dnsop-session-signal] - requests, a RECONFIRM request MUST contain a unique MESSAGE ID, not - currently in use in this session. +6.5.1. RECONFIRM Request - A RECONFIRM request generates exactly one RECONFIRM response from the - server, formatted identically to a SUBSCRIBE response, which echoes - back the unique MESSAGE ID from the RECONFIRM request. + A RECONFIRM request message begins with the standard DNS Session + Signaling 12-byte header [SessSig], followed by the RECONFIRM TLV. + The SSOP-DATA for the the RECONFIRM TLV is as follows: - In the RECONFIRM response the RCODE indicates whether or not the - request was successful. Supported RCODEs are as follows: + 1 1 1 1 1 1 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 + +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ + | | + \ NAME \ + \ \ + +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ + | TYPE | + +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ + | CLASS | + +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ + | RDLEN | + +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ + | | + \ RDATA \ + \ \ + +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ + + Figure 2 + + The MESSAGE ID field MUST be set to a unique value, that the client + is not using for any other active operation on this connection. For + the purposes here, a MESSAGE ID is in use on this connection if the + client has used it in a request for which it has not yet received a + response, or if if the client has used it for a subscription which it + has not yet cancelled using UNSUBSCRIBE. In the RECONFIRM response + the server MUST echo back the MESSAGE ID value unchanged. + + In the RECONFIRM TLV the SSOP-TYPE is RECONFIRM (tentatively 0x43). + The SSOP-LENGTH is the length of the data that follows, which + specifies the name, type, class, and content of the record being + disputed. + + A RECONFIRM request MUST contain exactly one record. The RECONFIRM + TLV has no count field to specify more than one record. Since + RECONFIRM requests are sent over TCP, multiple RECONFIRM request + messages can be concatenated in a single TCP stream and packed + efficiently into TCP segments. + + TYPE MUST NOT be the value ANY (255) and CLASS MUST NOT be the value + ANY (255). + + DNS wildcarding is not supported. That is, a wildcard ("*") in a + RECONFIRM message matches only a literal wildcard character ("*") in + the zone, and nothing else. + + Aliasing is not supported. That is, a CNAME in a RECONFIRM message + matches only a literal CNAME record in the zone, and nothing else. + +6.5.2. RECONFIRM Response + + Each RECONFIRM request generates exactly one RECONFIRM response from + the server. + + A RECONFIRM response message begins with the standard DNS Session + Signaling 12-byte header [SessSig], possibly followed by one or more + optional Modifier TLVs, such as a Retry Delay Modifier TLV. + + The MESSAGE ID field MUST echo the value given in the ID field of the + RECONFIRM request. This is how the client knows which request is + being responded to. + + A RECONFIRM response message MUST NOT contain a Session Signaling + Operation TLV. The Session Signaling Operation TLV is NOT copied + from the RECONFIRM request. + + In the RECONFIRM response the RCODE confirms receipt of the + reconfirmation request. Supported RCODEs are as follows: +------------+-------+----------------------------------------------+ | Mnemonic | Value | Description | +------------+-------+----------------------------------------------+ - | NOERROR | 0 | RECONFIRM successful. | + | NOERROR | 0 | RECONFIRM accepted. | | FORMERR | 1 | Server failed to process request due to a | | | | malformed request. | + | SERVFAIL | 2 | Server failed to process request due to | + | | | resource exhaustion. | + | NXDOMAIN | 3 | NOT APPLICABLE. DNS Push Notification | + | | | servers MUST NOT return NXDOMAIN errors in | + | | | response to RECONFIRM requests. | | NOTIMP | 4 | Server does not recognize DNS Session | | | | Signaling Opcode. | + | REFUSED | 5 | Server refuses to process request for policy | + | | | or security reasons. | + | NOTAUTH | 9 | Server is not authoritative for the | + | | | requested name. | | SSOPNOTIMP | 11 | RECONFIRM operation not supported. | +------------+-------+----------------------------------------------+ RECONFIRM Response codes This document specifies only these RCODE values for RECONFIRM Responses. Servers sending RECONFIRM Responses SHOULD use one of these values. However, future circumstances may create situations where other RCODE values are appropriate in RECONFIRM Responses, so clients MUST be prepared to accept RECONFIRM Responses with any RCODE value. - A correctly-formatted RECONFIRM message results in a response with - RCODE NOERROR. + Nonzero RCODE values signal some kind of error. - Nonzero RCODE values signal some kind of error. If the server sends - a nonzero RCODE then it SHOULD append a Terminate modifier TLV - [I-D.ietf-dnsop-session-signal] to the response specifying a delay - before the client attempts this operation again. The RECOMMENDED - value for the delay is five minutes. For serious errors, after - sending the error response, the server SHOULD send a DNS Push - Notification Terminate Session operation TLV and then close the TCP - connection, as described in the DNS Session Signaling specification - [I-D.ietf-dnsop-session-signal]. + RCODE value FORMERR indicates a message format error, for example + TYPE or CLASS being ANY (255). - If, after receiving a valid RECONFIRM request, the server determines - that the records are in fact no longer valid, then subsequent DNS - PUSH Messages will be generated to inform interested clients. Thus, - one client discovering that a previously-advertised printer is no - longer present has the side effect of informing all other interested - clients that the printer in question is now gone. + RCODE value SERVFAIL indicates that the server is overloaded. + + RCODE values NOTIMP indicates that the server does not support + Session Signaling, and Session Signaling is required for RECONFIRM + requests. + + RCODE value REFUSED indicates that the server supports RECONFIRM + requests but is currently not configured to accept them from this + client. + + RCODE value NOTAUTH indicates that the server is not authoritative + for the requested name, and can do nothing to remedy the apparent + error. Note that there may be future cases in which a server is able + to pass on the RECONFIRM request to the ultimate source of the + information, and in these cases the server should return NOERROR. + + RCODE value SSOPNOTIMP indicates that the server does not support + RECONFIRM requests. + + Similarly, a server would only generate SSOPNOTIMP if it did not + support Push Notifications, and if the server does not support Push + Notifications then it should not be possible for a client to have an + active subscription to cancel. + + Nonzero RCODE values SERVFAIL, REFUSED and SSOPNOTIMP are benign from + the client's point of view. The client may log them to aid in + debugging, but otherwise they require no special action. + + Nonzero RCODE values other than these three indicate a serious + problem with the client. After sending an error response other than + one of these three, the server SHOULD send a DNS Session Signaling + Retry Delay Operation TLV and then close the TCP connection, as + described in the DNS Session Signaling specification [SessSig]. 6.6. Client-Initiated Termination An individual subscription is terminated by sending an UNSUBSCRIBE TLV for that specific subscription, or all subscriptions can be cancelled at once by the client closing the connection. When a client terminates an individual subscription (via UNSUBSCRIBE) or all subscriptions on that connection (by closing the connection) it is signaling to the server that it is longer interested in receiving those particular updates. It is informing the server that the server may release any state information it has been keeping with regards to these particular subscriptions. After terminating its last subscription on a connection via UNSUBSCRIBE, a client MAY close the connection immediately, or it may keep it open if it anticipates performing further operations on that connection in the future. If a client wishes to keep an idle - connection open, it MUST continue to meet its keepalive obligations - [I-D.ietf-dnsop-session-signal] or the server is entitled to close - the connection (see below). + connection open, it MUST respect the maximum idle time required by + the server [SessSig]. If a client plans to terminate one or more subscriptions on a connection and doesn't intend to keep that connection open, then as an efficiency optimization it MAY instead choose to simply close the connection, which implicitly terminates all subscriptions on that connection. This may occur because the client computer is being shut down, is going to sleep, the application requiring the subscriptions has terminated, or simply because the last active subscription on that connection has been cancelled. @@ -1099,46 +1150,41 @@ resumed, the DNS Push Notification server will not have any subscription state and will proceed as with any other new connection. Use of TLS Session Resumption allows a new TLS connection to be set up more quickly, but the client will still have to recreate any desired subscriptions. 8. IANA Considerations This document defines the service name: "_dns-push-tls._tcp". It is only applicable for the TCP protocol. - This name is to be published in the IANA Service Name Registry. + This name is to be published in the IANA Service Name Registry + [RFC6335][SN]. This document defines three DNS Session Signaling TLV types: - SUBSCRIBE with (tentative) value 64, PUSH with (tentative) value 65, - UNSUBSCRIBE with (tentative) value 66, and RECONFIRM with (tentative) - value 67. + SUBSCRIBE with (tentative) value 0x40 (64), PUSH with (tentative) + value 0x41 (65), UNSUBSCRIBE with (tentative) value 0x42 (66), and + RECONFIRM with (tentative) value 0x43 (67). 9. Acknowledgements The authors would like to thank Kiren Sekar and Marc Krochmal for previous work completed in this field. This draft has been improved due to comments from Ran Atkinson, Tim Chown, Mark Delany, Ralph Droms, Bernie Volz, Jan Komissar, Manju Shankar Rao, Markus Stenberg, Dave Thaler, and Soraia Zlatkovic. 10. References 10.1. Normative References - [I-D.ietf-dnsop-session-signal] - Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S., - Mankin, A., and T. Pusateri, "DNS Session Signaling", - draft-ietf-dnsop-session-signal-00 (work in progress), - August 2016. - [I-D.ietf-tls-tls13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", draft-ietf-tls-tls13-18 (work in progress), October 2016. [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980, . [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, @@ -1176,47 +1222,62 @@ [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, . [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011, . + [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. + Cheshire, "Internet Assigned Numbers Authority (IANA) + Procedures for the Management of the Service Name and + Transport Protocol Port Number Registry", BCP 165, + RFC 6335, DOI 10.17487/RFC6335, August 2011, + . + [RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895, April 2013, . [RFC7673] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS- Based Authentication of Named Entities (DANE) TLSA Records with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October 2015, . [RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and D. Wessels, "DNS Transport over TCP - Implementation Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016, . + [SessSig] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S., + Mankin, A., and T. Pusateri, "DNS Session Signaling", + draft-ietf-dnsop-session-signal-02 (work in progress), + March 2017. + + [SN] "Service Name and Transport Protocol Port Number + Registry", . + 10.2. Informative References + [DisProx] Cheshire, S., "Hybrid Unicast/Multicast DNS-Based Service + Discovery", draft-ietf-dnssd-hybrid-06 (work in progress), + March 2017. + [I-D.dukkipati-tcpm-tcp-loss-probe] Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis, "Tail Loss Probe (TLP): An Algorithm for Fast Recovery of Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work in progress), February 2013. - [I-D.ietf-dnssd-hybrid] - Cheshire, S., "Hybrid Unicast/Multicast DNS-Based Service - Discovery", draft-ietf-dnssd-hybrid-03 (work in progress), - February 2016. - [I-D.sekar-dns-llq] Sekar, K., "DNS Long-Lived Queries", draft-sekar-dns- llq-01 (work in progress), August 2006. [IPJ.9-4-TCPSYN] Eddy, W., "Defenses Against TCP SYN Flooding Attacks", The Internet Protocol Journal, Cisco Systems, Volume 9, Number 4, December 2006. [obs] "Observer Pattern", . [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State", RFC 5077, DOI 10.17487/RFC5077, January 2008, . + [RFC6281] Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang, + "Understanding Apple's Back to My Mac (BTMM) Service", + RFC 6281, DOI 10.17487/RFC6281, June 2011, + . + [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, DOI 10.17487/RFC6762, February 2013, . [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, . [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, "TCP Extensions for Multipath Operation with Multiple