A New Internet-Draft is available from the on-line Internet-Drafts directories.
This draft is a work item of the DNS Extensions Working Group of the IETF.
Title : Revised extension mechanisms for DNS (EDNS0)
Author(s) : P. Vixie
Filename : draft-ietf-dnsext-rfc2671bis-edns0-00.txt
Pages : 9
Date : 2007-12-27
The Domain Name System's wire protocol includes a number of fixed
fields whose range has been or soon will be exhausted and does not
allow clients to advertise their capabilities to servers. This
document describes backward compatible mechanisms for allowing the
protocol to grow.
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1 - Introduction
1.1. DNS (see [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 is
fixed. Many of DNS's protocol limits are too small for uses which are
or which are desired to become common. There is no way for
implementations to advertise their capabilities.
1.2. Unextended agents will not know how to interpret the protocol
extensions detailed here. In practice, these clients will be upgraded
when they have need of a new feature, and only new features will make
use of the extensions. Extended agents must be prepared for behaviour
of unextended clients in the face of new protocol elements, and fall
back gracefully to unextended DNS.
1.3. 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 - Affected Protocol Elements
2.1. The DNS Message Header's (see [RFC1035 4.1.1]) second full 16-bit
word is divided into a 4-bit OPCODE, a 4-bit RCODE, and a number of
1-bit flags. The original reserved Z bits have been allocated to
various purposes, and most of the RCODE values are now in use. More
flags and more possible RCODEs are needed.
2.2. The first two bits of a wire format domain label are used to
denote the type of the label. [RFC1035 4.1.4] allocates two of the
four possible types and reserves the other two. Proposals for use of
the remaining types far outnumber those available. More label types
were needed, and an extension mechanism was proposed in RFC 2671
[RFC2671 Section 3].
2.3. DNS Messages are limited to 512 octets in size when sent over
UDP. While the minimum maximum reassembly buffer size still allows a
limit of 512 octets of UDP payload, most of the hosts now connected to
the Internet are able to reassemble larger datagrams. Some mechanism
must be created to allow requestors to advertise larger buffer sizes
to responders.
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3 - Extended Label Types
[RFC2671 Section 3] reserved label type "0 1" to indicate that an
extended label type followed in the next octet, but gave inadequate
guidance as to how EDNS, as a hop-by-hop signalling method, could be
used to carry a new kind of DNS label. Extended label types might be
addressed in a future specification, perhaps requiring that the EDNS
VERSION be incremented.
4 - OPT pseudo-RR
4.1. One OPT pseudo-RR (RR type 41) MAY be added to the additional
data section of a request, and to responses to such requests. An OPT
is called a pseudo-RR because it pertains to a particular transport
level message and not to any actual DNS data. OPT RRs MUST NOT be
cached, forwarded, or stored in or loaded from master files. The
quantity of OPT pseudo-RRs per message MUST be either zero or one, but
not greater.
4.2. An OPT RR has a fixed part and a variable set of options
expressed as {attribute, value} pairs. The fixed part holds some DNS
meta data and also a small collection of new protocol elements which
we expect to be so popular that it would be a waste of wire space to
encode them as {attribute, value} pairs.
4.3. The fixed part of an OPT RR is structured as follows:
Field Name
Field Type
Description
------------------------------------------------------
NAME
domain name
empty (root domain)
TYPE
u_int16_t
OPT
CLASS
u_int16_t
sender's UDP payload size
TTL
u_int32_t
extended RCODE and flags
RDLEN
u_int16_t
describes RDATA
RDATA
octet stream
{attribute,value} pairs
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4.4. The variable part of an OPT RR is encoded in its RDATA and is
structured as zero or more of the following:
+0 (MSB)
+1 (LSB)
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: |
OPTION-CODE
|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: |
OPTION-LENGTH
|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
4: |
|
/
OPTION-DATA
/
/
/
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
OPTION-CODE
(Assigned by IANA.)
OPTION-LENGTH Size (in octets) of OPTION-DATA.
OPTION-DATA
Varies per OPTION-CODE.
4.4.1. Order of appearance of option tuples is never relevant. Any
option whose meaning is affected by other options is so affected no
matter which one comes first in the OPT RDATA.
4.4.2. Any OPTION-CODE values not understood by a responder or
requestor MUST be ignored. So, specifications of such options might
wish to include some kind of signalled acknowledgement. For example,
an option specification might say that if a responder sees option XYZ,
it SHOULD include option XYZ in its response.
4.5. The sender's UDP payload size (which OPT stores in the RR CLASS
field) is the number of octets of the largest UDP payload that can be
reassembled and delivered in the sender's network stack. Note that
path MTU, with or without fragmentation, may be smaller than this.
4.5.1. Note that a 512-octet UDP payload requires a 576-octet IP
reassembly buffer. Choosing 1280 on an Ethernet connected requestor
would be reasonable. The consequence of choosing too large a value
may be an ICMP message from an intermediate gateway, or even a silent
drop of the response message.
4.5.2. Both requestors and responders are advised to take account of
the path's discovered MTU (if already known) when considering message
sizes.
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4.5.3. The requestor's maximum payload size can change over time, and
therefore MUST NOT be cached for use beyond the transaction in which
it is advertised.
4.5.4. The responder's maximum payload size can change over time, but
can be reasonably expected to remain constant between two sequential
transactions; for example, a meaningless QUERY to discover a
responder's maximum UDP payload size, followed immediately by an
UPDATE which takes advantage of this size. (This is considered
preferrable to the outright use of TCP for oversized requests, if
there is any reason to suspect that the responder implements EDNS, and
if a request will not fit in the default 512 payload size limit.)
4.5.5. Due to transaction overhead, it is unwise to advertise an
architectural limit as a maximum UDP payload size. Just because your
stack can reassemble 64KB datagrams, don't assume that you want to
spend more than about 4KB of state memory per ongoing transaction.
4.6. The extended RCODE and flags (which OPT stores in the RR TTL
field) are structured as follows:
+0 (MSB)
+1 (LSB)
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: |
EXTENDED-RCODE
|
VERSION
|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | DO|
Z
|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
EXTENDED-RCODE Forms upper 8 bits of extended 12-bit RCODE. Note
that EXTENDED-RCODE value "0" indicates that an
unextended RCODE is in use (values "0" through "15").
VERSION
Indicates the implementation level of whoever sets it.
Full conformance with this specification is indicated
by version ``0.'' Requestors are encouraged to set
this to the lowest implemented level capable of
expressing a transaction, to minimize the responder
and network load of discovering the greatest common
implementation level between requestor and responder.
A requestor's version numbering strategy should
ideally be a run time configuration option.
If a responder does not implement the VERSION level of
the request, then it answers with RCODE=BADVERS. All
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