RFC 5855 BEST CURRENT PRACTICE DNS · REVERSE DNS · OPERATIONS
Reverse DNS Nameservers: Nameservers for IPv4 and IPv6 Reverse Zones
Stable, dedicated names for the servers that answer reverse DNS lookups for IPv4 and IPv6.
RFC EDITOR
Abley, J. and T. Manderson, "Nameservers for IPv4 and IPv6 Reverse Zones", BCP 155, RFC 5855, DOI 10.17487/RFC5855, May 2010, <https://www.rfc-editor.org/info/rfc5855>.
BIBTEX
@misc{rfc5855,
series = {Request for Comments},
number = 5855,
howpublished = {RFC 5855},
publisher = {RFC Editor},
doi = {10.17487/RFC5855},
url = {https://www.rfc-editor.org/info/rfc5855},
author = {J. Abley and T. Manderson},
title = {{Nameservers for IPv4 and IPv6 Reverse Zones}},
pagetotal = 12,
year = 2010,
month = may,
} RFC text
Contents
Internet Engineering Task Force (IETF) J. Abley
Request for Comments: 5855 T. Manderson
BCP: 155 ICANN
Category: Best Current Practice May 2010
ISSN: 2070-1721
Nameservers for IPv4 and IPv6 Reverse Zones
Abstract
This document specifies a stable naming scheme for the nameservers
that serve the zones IN-ADDR.ARPA and IP6.ARPA in the DNS. These
zones contain data that facilitate reverse mapping (address to name).
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5855.
Copyright Notice
Copyright (c) 2010 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
2. Nameservers for IN-ADDR.ARPA ....................................3
3. Nameservers for IP6.ARPA ........................................3
4. IAB Statement ...................................................4
5. IANA Considerations .............................................4
6. Security Considerations .........................................4
7. References ......................................................4
7.1. Normative References .......................................4
7.2. Informative References .....................................5
Appendix A. Existing NS RRSets ....................................6
Appendix B. Performance Characteristics ...........................7
B.1. Label Compression ..........................................7
B.2. Query Patterns .............................................9
B.2.1. QNAME under IN-ADDR.ARPA ..............................10
B.2.2. QNAME under IP6.ARPA ..................................10
1. Introduction
The Domain Name System (DNS) is described in [RFC1034] and [RFC1035].
The DNS currently supports keyed data retrieval using three
namespaces -- domain names, IPv4 addresses, and IPv6 addresses.
Mapping of IPv4 addresses to names is accomplished using data
published in the IN-ADDR.ARPA zone. For IPv6, the IP6.ARPA zone is
used (see [RFC3596]). The process of mapping an address to a name is
generally known as a "reverse lookup", and the IN-ADDR.ARPA and
IP6.ARPA zones are said to support the "reverse DNS".
The secure and stable hosting of the IN-ADDR.ARPA and IP6.ARPA zones
is critical to the operation of the Internet, since many applications
rely upon timely responses to reverse lookups to be able to operate
normally.
At the time of this writing, the IN-ADDR.ARPA zone is served by a
subset of the DNS root servers, and IP6.ARPA by servers operated by
APNIC, ARIN, ICANN, LACNIC, and the RIPE NCC (see Appendix A).
This document specifies a dedicated and stable set of nameserver
names for each of the IN-ADDR.ARPA and IP6.ARPA zones.
The naming scheme specified in this document allows IN-ADDR.ARPA and
IP6.ARPA to be delegated to two different sets of nameservers, to
facilitate operational separation of the infrastructure used to serve
each zone. This separation might help ensure that an operational
failure of IN-ADDR.ARPA servers does not impact IPv6 reverse lookups
as collateral damage, for example.
The choice of operators for individual nameservers is beyond the
scope of this document and is an IANA function that falls under the
scope of Section 4 of the Memorandum of Understanding (MoU) between
the IETF and ICANN [RFC2860].
2. Nameservers for IN-ADDR.ARPA
This document specifies the following naming scheme for servers that
host the IN-ADDR.ARPA zone:
A.IN-ADDR-SERVERS.ARPA
B.IN-ADDR-SERVERS.ARPA
C.IN-ADDR-SERVERS.ARPA
D.IN-ADDR-SERVERS.ARPA
E.IN-ADDR-SERVERS.ARPA
F.IN-ADDR-SERVERS.ARPA
...
The IN-ADDR-SERVERS.ARPA zone has been delegated to the same set of
servers as IN-ADDR.ARPA. IPv4 and IPv6 glue records for each of
those servers has been added to the ARPA zone.
The IN-ADDR-SERVERS.ARPA and IN-ADDR.ARPA zones are delegated to the
same servers, since they are both dedicated for a single purpose and
hence can reasonably share fate.
All servers in the set are named under the same domain to facilitate
label compression. Since glue for all servers exist in the ARPA
zone, the use of a single domain does not present a practical single
point of failure.
3. Nameservers for IP6.ARPA
This document specifies the following nameserver set for the IP6.ARPA
zone:
A.IP6-SERVERS.ARPA
B.IP6-SERVERS.ARPA
C.IP6-SERVERS.ARPA
D.IP6-SERVERS.ARPA
E.IP6-SERVERS.ARPA
F.IP6-SERVERS.ARPA
...
The IP6-SERVERS.ARPA zone has been delegated to the same set of
servers as IP6.ARPA. IPv4 and IPv6 glue records for each of those
servers has been added to the ARPA zone.
4. IAB Statement
In its capacity as the body that provides technical guidance to ICANN
for the administration of the ARPA top-level domain as described in
[RFC3172], the IAB has reviewed this proposal and supports it as an
operational change that is in line with the respective roles of ICANN
and the IAB.
5. IANA Considerations
With due consideration to the approval of the IAB (see Section 4),
the IANA has delegated:
1. IN-ADDR-SERVERS.ARPA to the nameservers listed in Section 2;
2. IP6-SERVERS.ARPA to the nameservers listed in Section 3.
Additionally, IANA has installed IPv4 and IPv6 glue records for the
nameservers concerned in the ARPA zone.
The choice of operators for all nameservers concerned is beyond the
scope of this document and is an IANA function that falls under the
scope of Section 4 of the MoU between the IETF and ICANN [RFC2860].
6. Security Considerations
This document introduces no additional security risks for the
Internet.
7. References
7.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC3172] Huston, G., Ed., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, September 2001.
7.2. Informative References
[RFC2860] Carpenter, B., Baker, F., and M. Roberts, "Memorandum of
Understanding Concerning the Technical Work of the
Internet Assigned Numbers Authority", RFC 2860,
June 2000.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
October 2003.
Appendix A. Existing NS RRSets
The NS RRSet for the IN-ADDR.ARPA zone at the time of this writing is
as follows:
IN-ADDR.ARPA. 86400 IN NS A.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS B.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS C.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS D.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS E.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS F.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS G.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS H.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS I.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS K.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS L.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS M.ROOT-SERVERS.NET.
The NS RRSet for the IP6.ARPA zone at the time of this writing is as
follows:
IP6.ARPA. 84600 IN NS NS-SEC.RIPE.NET.
IP6.ARPA. 86400 IN NS SEC1.APNIC.NET.
IP6.ARPA. 86400 IN NS NS2.LACNIC.NET.
IP6.ARPA. 86400 IN NS NS.ICANN.ORG.
IP6.ARPA. 86400 IN NS TINNIE.ARIN.NET.
For completeness, the NS RRSet for the ARPA zone at the time of this
writing is as follows:
ARPA. 86400 IN NS A.ROOT-SERVERS.NET.
ARPA. 86400 IN NS B.ROOT-SERVERS.NET.
ARPA. 86400 IN NS C.ROOT-SERVERS.NET.
ARPA. 86400 IN NS D.ROOT-SERVERS.NET.
ARPA. 86400 IN NS E.ROOT-SERVERS.NET.
ARPA. 86400 IN NS F.ROOT-SERVERS.NET.
ARPA. 86400 IN NS G.ROOT-SERVERS.NET.
ARPA. 86400 IN NS H.ROOT-SERVERS.NET.
ARPA. 86400 IN NS I.ROOT-SERVERS.NET.
ARPA. 86400 IN NS K.ROOT-SERVERS.NET.
ARPA. 86400 IN NS L.ROOT-SERVERS.NET.
ARPA. 86400 IN NS M.ROOT-SERVERS.NET.
Appendix B. Performance Characteristics
B.1. Label Compression
The choice of names for the respective NS RRSets of the IN-ADDR.ARPA
and IP6.ARPA zones have a relatively minor impact on the delegation
response sizes from their parent zones, given other anticipated
contributors such as DNSSEC. However, it is still considered good
practice to use a naming scheme that is reasonably compressible:
doing so for frequently queried zones such as these is likely to have
at least measurable impact on aggregate DNS traffic in the Internet
as a whole, and has potential transport benefits to clients whose
queries will not result in secure replies.
The naming schemes described in Sections 2 and 3 are highly
compressible. That is, once a single nameserver name has been
encoded in a DNS message, subsequent nameservers can be specified
with substantially smaller encoding.
In the DNS, a complete encoding of an a-label involves a one-byte
length field, plus a one-byte-per-character encoding of the a-label
itself. A domain name's encoding consists of one or more a-labels,
so-encoded, plus a single terminating zero byte. Where a terminating
series of a-labels has already been encoded as described above,
subsequent terminating references to the same series can be made
using a two-byte pointer to that full encoding.
The non-compressed representation of the nameserver A.IN-ADDR-
SERVERS.ARPA fills (1 + 1) + (15 + 1) + (4 + 1) + 1 = 24 bytes.
The non-compressed representation of A.IP6-SERVERS.ARPA fills
(1 + 1) + (10 + 1) + (4 + 1) + 1 = 19 bytes.
Subsequent nameservers under either domain are encoded with the
initial label, plus two bytes for a pointer to the repeated domain
elsewhere in the message, i.e., (1 + 1) + 2 = 4 bytes.
The encoded size of the a-labels in a twelve-record NS RRSet named
according to Section 2 for IN-ADDR.ARPA is as follows:
+------------------------+---------------------------------------+
| Nameserver | Encoded Size |
+------------------------+---------------------------------------+
| A.IN-ADDR-SERVERS.ARPA | (1 + 1) + (15 + 1) + (4 + 1) + 1 = 24 |
| | |
| B.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| C.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| D.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| E.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| F.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| G.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| H.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| I.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| J.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| K.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| L.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| Total | 68 bytes |
+------------------------+---------------------------------------+
The encoded size of the a-labels in a six-record NS RRSet named
according to Section 3 for IP6.ARPA is, hence, as follows:
+--------------------+---------------------------------------+
| Nameserver | Encoded Size |
+--------------------+---------------------------------------+
| A.IP6-SERVERS.ARPA | (1 + 1) + (10 + 1) + (4 + 1) + 1 = 19 |
| | |
| B.IP6-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| C.IP6-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| D.IP6-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| E.IP6-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| F.IP6-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| Total | 39 bytes |
+--------------------+---------------------------------------+
By way of comparison, the encoded size of the labels in the NS RRSet
for IP6.ARPA (shown in Appendix A) is as follows:
+-----------------+--------------------------------------+
| Nameserver | Encoded Size |
+-----------------+--------------------------------------+
| NS-SEC.RIPE.NET | (6 + 1) + (4 + 1) + (3 + 1) + 1 = 17 |
| | |
| SEC1.APNIC.NET | (4 + 1) + (5 + 1) + 2 + 1 = 14 |
| | |
| NS2.LANIC.NET | (3 + 1) + (6 + 1) + 2 + 1 = 14 |
| | |
| NS.ICANN.ORG | (2 + 1) + (5 + 1) + (3 + 1) + 1 = 14 |
| | |
| TINNIE.ARIN.NET | (6 + 1) + (4 + 1) + 2 + 1 = 15 |
| | |
| Total | 74 bytes |
+-----------------+--------------------------------------+
B.2. Query Patterns
A brief description of likely query patterns for an empty cache with
the existing and new NS RRSets follows.
B.2.1. QNAME under IN-ADDR.ARPA
Consider the IN-ADDR.ARPA NS RRSet (described in Appendix A) and a
QNAME that is delegated beneath the IN-ADDR.ARPA zone:
1. Query sent to root server that is also authoritative for
IN-ADDR.ARPA; response is a referral from the IN-ADDR.ARPA zone.
In the case where the initial query is sent to the J root server:
1. Query sent to J.ROOT-SERVERS.NET (which is not authoritative for
the IN-ADDR.ARPA zone); response is a referral to an ARPA server
with additional-section glue.
2. Query sent to an ARPA server (all of which are also authoritative
in this case for IN-ADDR.ARPA); response is a referral from the
IN-ADDR.ARPA zone.
Consider the same query with the IN-ADDR.ARPA NS RRSet (described in
Section 2):
1. Query sent to a root server that is also authoritative for ARPA;
response is a referral to an IN-ADDR.ARPA server, with additional-
section glue.
2. Query sent to an IN-ADDR.ARPA server; response is a referral from
the IN-ADDR.ARPA zone.
In the case where the first query is sent to the J root server:
1. Query sent to J.ROOT-SERVERS.NET (which is not authoritative for
ARPA); response is a referral to an ARPA server, with additional-
section glue.
2. Query sent to an ARPA server; response is a referral to an
IN-ADDR.ARPA server, with additional-section glue.
3. Query sent to an IN-ADDR.ARPA server; response is a referral from
the IN-ADDR.ARPA zone.
B.2.2. QNAME under IP6.ARPA
Consider the IP6.ARPA NS RRSet (described in Appendix A) and a QNAME
that is delegated beneath the IP6.ARPA zone:
1. Query sent to root server that is also authoritative for ARPA;
response is a referral from the ARPA zone to an IP6.ARPA server
with no additional-section glue.
2. A recursive lookup for one of the nameservers specified in the
referral must now be performed in order to obtain an address for
an IP6.ARPA server. In all cases, three queries are required.
Successive recursive lookups may be performed in the event that a
server is unresponsive.
3. Query sent to IP6.ARPA server; response is a referral from the
IP6.ARPA zone.
In the case where the first query is sent to the J root server:
1. Query sent to J.ROOT-SERVERS.NET; response is a referral to an
ARPA server with additional-section glue.
2. Query sent to an ARPA server; response is a referral from the ARPA
zone to an IP6.ARPA server with no additional-section glue.
3. A recursive lookup for one of the nameservers specified in the
referral must now be performed in order to obtain an address for
an IP6.ARPA server. In all cases, three queries are required.
Successive recursive lookups may be performed in the event that a
server is unresponsive.
4. Query sent to IP6.ARPA server; response is a referral from the
IP6.ARPA zone.
Consider the same query with the IP6.ARPA NS RRSet (described in
Section 3):
1. Query sent to a root server that is also authoritative for ARPA;
response is a referral to an IP6.ARPA server, with additional-
section glue.
2. Query sent to an IP6.ARPA server; response is a referral from the
IP6.ARPA zone.
In the case where the first query is sent to the J root server:
1. Query sent to J.ROOT-SERVERS.NET (which is not authoritative for
ARPA); response is a referral to an ARPA server, with additional-
section glue.
2. Query sent to an ARPA server; response is a referral to an
IP6.ARPA server with additional-section glue.
3. Query sent to an IP6.ARPA server; response is a referral from the
IP6.ARPA zone.
Authors' Addresses
Joe Abley
ICANN
4676 Admiralty Way, Suite 330
Marina del Rey, CA 90292
USA
Phone: +1 310 463 9062
EMail: joe.abley@icann.org
Terry Manderson
ICANN
4676 Admiralty Way, Suite 330
Marina del Rey, CA 90292
USA
Phone: +61 4 1127 5673
EMail: terry.manderson@icann.org
AI ELI5
On this page
PROBLEMThe IN-ADDR.ARPA and IP6.ARPA zones, which map addresses back to names, were served by servers named in other, unrelated domains, so there was no stable naming scheme for them.
MECHANISMEach zone gets its own dedicated set of nameserver names under IN-ADDR-SERVERS.ARPA and IP6-SERVERS.ARPA, with the zones delegated to the same servers and glue records added to the ARPA zone.
WHY IT MATTERSIN-ADDR.ARPA and IP6.ARPA can use two separate sets of servers, names compress well in DNS messages, and IPv6 reverse lookups need fewer queries.
§1Reverse Lookups and the ARPA Zones
The DNS supports keyed lookup using three namespaces: domain names, IPv4 addresses and IPv6 addresses. Mapping an IPv4 address to a name uses data in the IN-ADDR.ARPA zone, and IPv6 uses the IP6.ARPA zone. This address-to-name mapping is called a reverse lookup, and the zones support the reverse DNS.
The RFC calls the secure and stable hosting of these zones critical, since many applications rely on timely reverse lookup answers. When it was written, IN-ADDR.ARPA was served by a subset of the DNS root servers, and IP6.ARPA by servers operated by APNIC, ARIN, ICANN, LACNIC and the RIPE NCC. This document specifies a dedicated, stable set of nameserver names for each zone.
Looking up a name from a number is like finding who owns a phone number. This RFC is about giving the helpers who answer that question proper, stable names.
§2The IN-ADDR-SERVERS.ARPA Names
Servers for IN-ADDR.ARPA are named A.IN-ADDR-SERVERS.ARPA, B.IN-ADDR-SERVERS.ARPA and so on through F and beyond. The IN-ADDR-SERVERS.ARPA zone is delegated to the same servers as IN-ADDR.ARPA, with IPv4 and IPv6 glue records for each server added to the ARPA zone.
Both zones share servers because they are dedicated to a single purpose and can reasonably share fate. All servers sit under one domain so that names compress well. Because glue exists in the ARPA zone for every server, using one domain is not a practical single point of failure.
The helpers for the IPv4 phone book are named A, B, C and so on, all under one family name. That makes them easy to list.
§3The IP6-SERVERS.ARPA Names and Separation
IP6.ARPA gets the same treatment: A.IP6-SERVERS.ARPA, B.IP6-SERVERS.ARPA and so on. The IP6-SERVERS.ARPA zone is delegated to the same servers as IP6.ARPA, and IPv4 and IPv6 glue records are added to the ARPA zone.
The scheme allows the two reverse zones to be delegated to two different sets of nameservers. That operational separation could keep a failure of the IN-ADDR.ARPA servers from hurting IPv6 reverse lookups as collateral damage. Choosing the operators of individual servers is out of scope; it is an IANA function under Section 4 of the MoU between the IETF and ICANN.
The IPv6 phone book gets its own set of helpers with their own names, so if one set has a bad day the other keeps working.
§6Performance: Compression and Query Counts
Appendix B argues the names are highly compressible. A full domain name encoding uses a one-byte length plus one byte per character for each label, and a terminating zero byte. Once one name is encoded, later names under the same domain need only their first label plus a two-byte pointer, which is 4 bytes. The first name, A.IN-ADDR-SERVERS.ARPA, takes 24 bytes and A.IP6-SERVERS.ARPA takes 19.
For a twelve-record IN-ADDR.ARPA set the total is 68 bytes. For a six-record IP6.ARPA set it is 39 bytes, against 74 bytes for the existing IP6.ARPA set shown in Appendix A.
The appendix also walks through query patterns with an empty cache. With the old IP6.ARPA servers, the referral carries no glue, so a recursive lookup is needed to find a server address, and three queries are required in all cases. With the new names, the referral comes with glue, so no recursive lookup is needed.
Names that share a family name can be written in a short way after the first one. That makes messages smaller and lookups quicker.
| NS set | Records | Total encoded size |
|---|---|---|
| IN-ADDR-SERVERS.ARPA names THIS RFC | 12 | 68 bytes |
| IP6-SERVERS.ARPA names THIS RFC | 6 | 39 bytes |
| Existing IP6.ARPA servers | 5 | 74 bytes |
§5What IANA Did and the IAB Statement
In its role of giving technical guidance to ICANN on the ARPA top-level domain (see RFC 3172), the IAB reviewed the proposal and supports it as an operational change in line with the roles of ICANN and the IAB.
IANA has delegated IN-ADDR-SERVERS.ARPA to the nameservers listed in Section 2 and IP6-SERVERS.ARPA to those in Section 3, and installed IPv4 and IPv6 glue records for those nameservers in the ARPA zone. The RFC says the new scheme introduces no additional security risks.
The group that looks after the internet's address books said yes, and the record keeper set up the new names.
§6References
[RFC1034]RFC 1034Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987.November 1987[RFC1035]RFC 1035Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.November 1987[RFC3172]RFC 3172Huston, G., Ed., "Management Guidelines & Operational Requirements for the Address and Routing Parameter Area Domain (\"arpa\")", BCP 52, RFC 3172, September 2001.September 2001[RFC2860]RFC 2860Carpenter, B., Baker, F., and M. Roberts, "Memorandum of Understanding Concerning the Technical Work of the Internet Assigned Numbers Authority", RFC 2860, June 2000.June 2000[RFC3596]RFC 3596Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, "DNS Extensions to Support IP Version 6", RFC 3596, October 2003.October 2003