A DNS Resource Record for Service Descriptions
draft-lear-httpbis-svcinfo-rr-00

Abstract

Certain application protocols are highly transactional and require substantial care when dealing with latency. Queries for versioning information are, in these circumstances, costly. In addition, there is a desire to allow for a means to specify a lightweight means to alternative transport protocols, such as making use of SCTP instead of TCP. This memo specifies a new DNS RR with an eye toward the least necessary roundtrips to determine various protocols, port numbers, and options for a given service instance.

Note

[[NOTE: For httpbis, the first goal is to focus down on requirements. Consider all this draft through the lens of "http" where one sees the words "application protocol" or "service".]]

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 June 08, 2013.

Copyright Notice

Copyright (c) 2012 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
• 1.1. Design Requirements
• 1.2. Related Work
• 1.2.1. The DNS SRV Record
• 1.2.2. NAPTR
• 1.2.3. The URI resource record
• 1.2.4. Happy Eyeballs
• 1.3. Overall approach
• 1.4. Terminology
• 2. The SVCINFO Record Format and Use
• 2.1. Usage
• 2.2. Notes
• 3. Backward Compabitibility and Other Interactions
• 4. Examples
• 5. Security Considerations
• 6. IANA Considerations
• 7. Acknowledgments
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Appendix A. SRV example
• Appendix B. Changes
• Author's Address

1. Introduction

For an application protocol to survive over time, it must include version information, and usually must have some sort of capability statement or exchange. How this is done depends on a number of characteristics of the protocol, such as whether it is half-duplex and what performance characteristics it has. When using many transactional connections, end-to-end latency will occur before new capabilities can be used.

In addition, traditionally application protocol specifications have indicated the transport protocol to be used. It is useful to be able to declare in advance what protocol(s) a service instance runs atop. For instance, it might be possible to a service instance on both TCP [RFC0792] and SCTP [RFC4960].

1.1. Design Requirements

We have the following five design requirements for any solution:

1. Application protocol version information must be discoverable within the DNS.
2. Transport protocol version information for a service must be discoverable within the DNS.
3. Performance of the application must not be unduly impacted. Some additional latency may be tolerated.
4. It should be possible for multiple instances of an application to be advertised on different ports by the same host. This is particularly common with HTTP [RFC2616].
5. While it may be possible to modify URI schema definitions, all such modifications MUST be 100% backward compatible.

One additional potential requirement would be host-level redirection. The benefit of host-level redirection in the DNS is that it would allow for a virtual server to securely offer TLS for multiple domains without the need for multiple IP addresses, as different ports are offered for different virtual hosts instead of different IP addresses.

1.2. Related Work

There are a number of existing capabilities within the network that can address some or all of the requirements above.

1.2.1. The DNS SRV Record

It has been possible to make use of a new service name and query the DNS for a SRV resource record [RFC2782], again perhaps running a race. SRV provides a mechanism to locate one or more server and port for a given service. There are two concerns with SRV. First, one must indicate the transport protocol as part of the QNAME. This means that discovery of multiple transport protocols requires multiple queries.

In addition, common enterprise deployments create a _TCP zone for purposes such as load balancing of SRV responses separate from a parent domain. Keeping in mind that the goal is to reduce the number of queries to determine version and protocol parameters, multiple DNS queries perform no better than an in-path application protocol exchange. Additional information also cannot always be provided or be trusted, because the authoritative name server for the service name may not also be authoritative for the target domain. A detailed example of the SRV record's performance is given in Appendix A.

1.2.2. NAPTR

Another record that could be considered is NAPTR [RFC3402]. NAPTR is a very powerful means for DNS clients to apply search-and-replace rules to a given URI. Building upon the SRV record, NAPTR provides a means to specify use of alternate services and transport protocols. Because NAPTR may return either an A record or SRV record, one or more follow-on query may be needed. In addition, this leaves us without protocol version information.

1.2.3. The URI resource record

The URI resource record provides for a mapping from hostname to URI. This record can be used to map a domain to multiple URIs, in fact. What it lacks, however, is version information about the application protocol.

1.2.4. Happy Eyeballs

One final approach is to run a race by initiating the protocol exchange using two alternatives, and pursuing the alternative that indicates success first, the assumption being that the service exists. An example of this model is Happy Eyeballs [RFC6555], where different versions of IP are tested. This work specifies that when an AAAA record is specified, a race may be performed. DNS cannot in general be used to determine reachability. Hence, a race may yet be appropriate in some circumstances, when a service is advertised.

1.3. Overall approach

To allow hosts to advertise a service using multiple versions of application protocols or multiple transport protocols, a method is needed to efficiently advertise that there exists an equivalence. To accomplish this, we define a new RRtype that states each way to connect to the service by using as an initial index the port and service that the application intends to connect to, and then providing an additional index known as an "InstanceId" that then establishes an equivalence between the instant record and any other record returned in the query with the same InstanceId. Examples of this approach are found in Section 4. This additional index is useful in circumstances where multiple applications making use of the same service (such as HTTP) are running on a single host. The InstanceId can be used to indicate to the client which application is instantiated through multiple protocols (such as a database application making use of both HTTP 1.1 and HTTP 2.0).

1.4. Terminology

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 RFC 2119 [RFC2119].

2. The SVCINFO Record Format and Use

The SVCINFO RR is queried with DNS type code TBD. The format of the SVCINFO resource record is as follows:

	    
  domain TTL Class SVCINFO InstanceId Priority Proto Port Version

	  

The fields are defined as follows:

Domain, TTL, Class:

These are specified in [RFC1035].

SVCINFO:

This is the resource record type.

InstanceId:

This 16-bit number identifies an instance of a service. If the same InstanceId appears in multiple SVCINFO records, then the services on those ports atop those protocols are said by the service owner to be equivalent.

Priority:

This eight-bit number has similar meaning to "Priority" as defined in [RFC2782]. Its use and interaction with other fields is described below.

Proto:

This field indicates the textual name found in the IANA "Assigned Internet Protocol Numbers" database. Records MUST contain only transport protocols that directly make use of service to port mappings.

Port:

This is a port number associated with the service running on a host. The value must be a legal value for the assocated transport protocol.

Version:

This field indicates a version number and any additional capabilities for a given service. Note that the interpretation of this field is to be defined by individual services.

[[DISCUSS: An alternative approach would be to combine proto and version into a single "profile" field. This would recognize the tight binding between application protocol and transport protocol. While it reduces the number of fields, it abstracts information out of DNS.]]

2.1. Usage

When a host offers an equivalent service in two different ways it will advertise those ways in the DNS with the SVCINFO record, using the same InstanceId. When a host wants to make use of a service, it then queries the DNS with QNAME=domain, QCLASS=IN, and QTYPE=SVCINFO. It processes the information as follows:

[[DISCUSS: Is class IN really correct?]]

1. If the reply is NOERROR and ANCOUNT=0 is returned, the host attempts to connect using either the configured port and transport protocol or the port associated with the service in the IANA registry.
2. If the reply is NOERROR and ANCOUNT > 0, the host searches for the port and transport protocol that it would expect to connect to, absent the SVCINFO record. Upon a match, it notes the InstanceId of that record, and any matching InstanceId of other SVCINFO records returned in the answer.
3. The host then attempts to connect to the protocol and port with the lowest number priority that matches the protocol and version that it understands. Two records that have the same priority may be tried in any order, or raced a'la Happy EyeBalls.

2.2. Notes

Zones MUST NOT publish multiple SVCINFO records for the same domain that use the same protocol AND port. Resolvers SHOULD ignore such SVCINFO records.

A client resolver MUST parse all RRs in the reply in order to properly determine priority. Accordingly, clients MUST handle truncated responses using the rules described in [RFC2181].

In all cases when an SVCINFO record is returned, all A and AAAA record for the domain SHOULD be returned in the additional information section. This eliminates excess queries without adding additional risk of cache poisoning.

When the application loses communication with the other side, it SHOULD re-apply the rules above in attempting to re-establish connectivity. When doing so, applications making use of the SVCINFO record MAY cache results, but MUST observe TTLs.

3. Backward Compabitibility and Other Interactions

There are several side effects of using SVCINFO. The first, is that by its nature, SVCINFO requires backward compatibility. A service must always run on a port that is advertised, be that as an IANA-assigned port, through specification within a URI, or some other means. At the same time, when SVCINFO *is* used by the client, an observer may not see the client connect to a server on a port specified by a given URI.

4. Examples

Case 1: Different versions of HTTP, same transport protocol

         www.example.com 86400 in svcinfo 1 10 tcp 80 1.1
         www.example.com 86400 in svcinfo 1 5  tcp 49080 2.0

         Additional Information:
         www.example.com 86400  in a    192.0.2.1
         www.example.com 86400  in aaaa 2001:db8::1
         example.com     604800 in ns   ns1.example.com            
	  

Consider the case of host www.example.com, which is running an HTTP1.1 server on port 80 and an HTTP2.0 server on port 49080, both using TCP, both serving the same content. A records for this service might appear as follows:

Case two: Same service running on multiple transport protocols

         www.example.com 86400 in svcinfo 1 10 tcp 80 1.1
         www.example.com 86400 in svcinfo 1 5  tcp 49080 2.0
         www.example.com 86400 in svcinfo 1 1  sctp 80 2.0

         Additional Information:
         www.example.com 86400  in a    192.0.2.1
         www.example.com 86400  in aaaa 2001:db8::1
         example.com     604800 in ns   ns1.example.com            
	  

In the next case, we change the case slightly from above by allowing for the idea that http2.0 will make use of SCTP in addition to TCP. Therefore the records might appear as follows:

Case 3: Multiple services

         www.example.com 86400 in svcinfo 1 10 tcp 80 1.1
         www.example.com 86400 in svcinfo 1 5  tcp 49080 2.0
         www.example.com 86400 in svcinfo 1 1  sctp 80 2.0
         www.example.com 86400 in svcinfo 2 10 tcp 8080 1.1
         www.example.com 86400 in svcinfo 2 5  tcp 8081 2
         www.example.com 86400 in svcinfo 2 1  sctp 8080 2

         Additional Information:
         www.example.com 86400  in a    192.0.2.1
         www.example.com 86400  in aaaa 2001:db8::1
         example.com     604800 in ns   ns1.example.com            
	  

In this case the client is attempting to connect to http://www.example.com:8080, where www.example.com runs multiple http servers. The DNS configuration appears as follows:

5. Security Considerations

Absent the use of DNSSEC [RFC4035], it is important that alternatives offered by this service have the same security properties, lest a downgrade attack be introduced.

When published to the world, this record would divulge that the likely presence of services running on a particular set of ports. It may not be all that difficult to divine what is running, either based on a simple probe, or the version number in the response.

6. IANA Considerations

A. Submission Date:

  B. Submission Type:
     [X] New RRTYPE
     [ ] Modification to existing RRTYPE

  C. Contact Information for submitter (will be publicly posted):
        Name: TBD
        Email Address: TBD
        International telephone number: TBD
        Other contact handles: TBD

  D. Motivation for the new RRTYPE application.
     Please keep this part at a high level to inform the Expert and
     reviewers about uses of the RRTYPE.  Most reviewers will be DNS
     experts that may have limited knowledge of your application
     space.

     Please see Section 1 of this document.

  E. Description of the proposed RR type.
     This description can be provided in-line in the template, as an
     attachment, or with a publicly available URL.

     Please see Section 1 of this document.

  F. What existing RRTYPE or RRTYPEs come closest to filling that
  need and why are they unsatisfactory?

     Please see Section 1 of this document.

  G. What mnemonic is requested for the new RRTYPE (optional)?
     Note: this can be left blank and the mnemonic decided after the
     template is accepted.
    
     SVCINFO
      
  H. Does the requested RRTYPE make use of any existing IANA registry
     or require the creation of a new IANA sub-registry in DNS
     Parameters?  If so, please indicate which registry is to be used
     or created.  If a new sub-registry is needed, specify the
     allocation policy for it and its initial contents.  Also include
     what the modification procedures will be.

     This RRType refers to the IANA Assigned Internet Protocol
     Numbers registry, but requires no allocations from it.

  I. Does the proposal require/expect any changes in DNS
     servers/resolvers that prevent the new type from being processed
     as an unknown RRTYPE (see [RFC3597])?

     No.

  J. Comments:

     None.

	  

The IANA is requested to allocate a DNS RRTYPE with the following information:

7. Acknowledgments

This work largely builds upon the experience gathered with SRV records, as originally defined by Paul Vixie. SRV records are still appropriate in many, if not most, circumstances. Thanks also go to Joe Hildebrand, Dan Wing, and Mark Nottingham for their reviews.

8. References

8.1. Normative References

8.2. Informative References

Appendix A. SRV example

Consider the fictional case where http version 2 requires querying the DNS for _http2._tcp.example.com to get to example.com. The SRV record might look something like this:

_http2._tcp.example.com 86400 in srv 10 10 49080 www.example.com

To fully resolve http://example.com to the necessary components of an ip address,transport protocol, and a port, a full service resolver must make the following queries:

1. A query for _http2._tcp.example.com to name servers for example.com will either return the SRV record or name servers for _tcp.example.com.
2. A query is made where there is a zone cut for _tcp.example.com. We now have the SRV record. We may also have an A record for www.example.com if it appears in an additional information section in the same zone.
3. If we do not have the A record for www.example.com, we will then query for it separately.

Step 2 is a common step that is necessary today in many enterprise deployments, even if the client being served is not within that enterprise. The last step may actually be several steps if the target domain is not in the same zone as the name found in the QNAME. A check of one highly optimized common news site found ten separate and distinct domains. Risking an average query response time of 60ms, use of SRV records could inject 600ms on the initial start up for that site.

What we conclude is that the fundamental issue with SRV (and any record where a zone split is likely) is that the service name requires additional A records for a host to connect to the service that may not be possible to provide in a single query.

Appendix B. Changes

This section to be removed prior to publication.

• 00 Initial Revision.

Author's Address