1. Introduction

1. Introduction

DNS [RFC1035] specifies a message format, and within such messages there are standard formats for encoding options, errors, and name compression. The maximum allowable size of a DNS message over UDP not using the extensions described in this document is 512 bytes. Many of DNS's protocol limits, such as the maximum message size over UDP, are too small to efficiently support the additional information that can be conveyed in the DNS (e.g., several IPv6 addresses or DNS Security (DNSSEC) signatures). Finally, RFC 1035 does not define any way for implementations to advertise their capabilities to any of the other actors they interact with.

[RFC2671] added extension mechanisms to DNS. These mechanisms are widely supported, and a number of new DNS uses and protocol extensions depend on the presence of these extensions. This memo refines and obsoletes [RFC2671].

Unextended agents will not know how to interpret the protocol extensions defined in [RFC2671] and restated here. Extended agents need to be prepared for handling the interactions with unextended clients in the face of new protocol elements and fall back gracefully to unextended DNS.

EDNS is a hop-by-hop extension to DNS. This means the use of EDNS is negotiated between each pair of hosts in a DNS resolution process, for instance, the stub resolver communicating with the recursive resolver or the recursive resolver communicating with an authoritative server.

[RFC2671] specified extended label types. The only such label proposed was in [RFC2673] for a label type called "Bit-String Label" or "Binary Labels", with this latest term being the one in common use. For various reasons, introducing a new label type was found to be extremely difficult, and [RFC2673] was moved to Experimental. This document obsoletes [RFC2673], deprecating Binary Labels. Extended labels remain defined, but their use is discouraged due to practical difficulties with deployment; their use in the future SHOULD only be considered after careful evaluation of the deployment hindrances.