Mboned J. Holland
Internet-Draft Akamai Technologies, Inc.
Intended status: Standards Track January 13, 2019
Expires: July 17, 2019
DNS Reverse IP AMT Discovery
draft-jholland-mboned-driad-amt-discovery-03
Abstract
This document defines a new DNS resource record (RR) used to
advertise addresses for Automatic Multicast Tunneling (AMT) relays
capable of receiving multicast traffic from the owner of the RR. The
new AMTRELAY RR makes possible a source-specific method for AMT
gateways to discover appropriate AMT relays, in order to ingest
traffic for source-specific multicast channels into multicast-capable
receiving networks when no multicast connectivity is directly
available between the sending and receiving networks.
Status of This Memo
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This Internet-Draft will expire on July 17, 2019.
Copyright Notice
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document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2.1. Relays and Gateways . . . . . . . . . . . . . . . . . 3
1.2.2. Definitions . . . . . . . . . . . . . . . . . . . . . 4
2. Relay Discovery Operation . . . . . . . . . . . . . . . . . . 5
2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Signaling and Discovery . . . . . . . . . . . . . . . . . 5
2.3. Optimal Relay Selection . . . . . . . . . . . . . . . . . 7
2.4. DNS Configuration . . . . . . . . . . . . . . . . . . . . 8
3. Example Deployments . . . . . . . . . . . . . . . . . . . . . 9
3.1. Example Receiving Networks . . . . . . . . . . . . . . . 9
3.1.1. Tier 3 ISP . . . . . . . . . . . . . . . . . . . . . 9
3.1.2. Small Office . . . . . . . . . . . . . . . . . . . . 10
3.2. Example Sending Networks . . . . . . . . . . . . . . . . 13
3.2.1. Sender-controlled Relays . . . . . . . . . . . . . . 13
3.2.2. Provider-controlled Relays . . . . . . . . . . . . . 14
4. AMTRELAY Resource Record Definition . . . . . . . . . . . . . 15
4.1. AMTRELAY RRType . . . . . . . . . . . . . . . . . . . . . 15
4.2. AMTRELAY RData Format . . . . . . . . . . . . . . . . . . 15
4.2.1. RData Format - Precedence . . . . . . . . . . . . . . 16
4.2.2. RData Format - Discovery Optional (D-bit) . . . . . . 16
4.2.3. RData Format - Type . . . . . . . . . . . . . . . . . 17
4.2.4. RData Format - Relay . . . . . . . . . . . . . . . . 17
4.3. AMTRELAY Record Presentation Format . . . . . . . . . . . 17
4.3.1. Representation of AMTRELAY RRs . . . . . . . . . . . 17
4.3.2. Examples . . . . . . . . . . . . . . . . . . . . . . 18
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
6.1. Record-spoofing . . . . . . . . . . . . . . . . . . . . . 19
6.2. Local Override . . . . . . . . . . . . . . . . . . . . . 19
6.3. Congestion . . . . . . . . . . . . . . . . . . . . . . . 20
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.1. Normative References . . . . . . . . . . . . . . . . . . 20
8.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. New RRType Request Form . . . . . . . . . . . . . . 23
Appendix B. Unknown RRType construction . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
This document defines DNS Reverse IP AMT Discovery (DRIAD), a
mechanism for AMT gateways to discover AMT relays which are capable
of forwarding multicast traffic from a known source IP address.
AMT (Automatic Multicast Tunneling) is defined in [RFC7450], and
provides a method to transport multicast traffic over a unicast
tunnel, in order to traverse non-multicast-capable network segments.
Section 4.1.5 of [RFC7450] explains that relay selection might need
to depend on the source of the multicast traffic, since a relay must
be able to receive multicast traffic from the desired source in order
to forward it.
That section suggests DNS-based queries as a possible solution.
DRIAD is a DNS-based solution, as suggested there. This solution
also addresses the relay discovery issues in the "Disadvantages"
lists in Section 3.3 of [RFC8313] and Section 3.4 of [RFC8313].
The goal for DRIAD is to enable multicast connectivity between
separate multicast-enabled networks when neither the sending nor the
receiving network is connected to a multicast-enabled backbone,
without pre-configuring any peering arrangement between the networks.
1.1. Background
The reader is assumed to be familiar with the basic DNS concepts
described in [RFC1034], [RFC1035], and the subsequent documents that
update them, particularly [RFC2181].
The reader is also assumed to be familiar with the concepts and
terminology regarding source-specific multicast as described in
[RFC4607] and the use of IGMPv3 [RFC3376] and MLDv2 [RFC3810] for
group management of source-specific multicast channels, as described
in [RFC4604].
The reader should also be familiar with AMT, particularly the
terminology listed in Section 3.2 of [RFC7450] and Section 3.3 of
[RFC7450].
1.2. Terminology
1.2.1. Relays and Gateways
When reading this document, it's especially helpful to recall that
once an AMT tunnel is established, the relay receives native
multicast traffic and sends unicast tunnel-encapsulated traffic to
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the gateway, and the gateway receives the tunnel-encapsulated
packets, decapsulates them, and forwards them as native multicast
packets, as illustrated in Figure 1.
Multicast +-----------+ Unicast +-------------+ Multicast
>---------> | AMT relay | >=======> | AMT gateway | >--------->
+-----------+ +-------------+
Figure 1: AMT Tunnel Illustration
1.2.2. Definitions
+------------+------------------------------------------------------+
| Term | Definition |
+------------+------------------------------------------------------+
| (S,G) | A source-specific multicast channel, as described in |
| | [RFC4607]. A pair of IP addresses with a source host |
| | IP and destination group IP. |
| | |
| downstream | Further from the source of traffic. |
| | |
| FQDN | Fully Qualified Domain Name, as described in |
| | [RFC8499] |
| | |
| gateway | An AMT gateway, as described in [RFC7450] |
| | |
| relay | An AMT relay, as described in [RFC7450] |
| | |
| RPF | Reverse Path Forwarding, as described in [RFC5110] |
| | |
| RR | A DNS Resource Record, as described in [RFC1034] |
| | |
| RRType | A DNS Resource Record Type, as described in |
| | [RFC1034] |
| | |
| SSM | Source-specific multicast, as described in [RFC4607] |
| | |
| upstream | Closer to the source of traffic. |
+------------+------------------------------------------------------+
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
[RFC2119] and [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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2. Relay Discovery Operation
2.1. Overview
The AMTRELAY resource record (RR) defined in this document is used to
publish the IP address or domain name of an AMT relay that can
receive, encapsulate, and forward multicast traffic from a particular
sender.
The sender is the owner of the RR, and configures the RR so that it
contains the address or domain name of an AMT relay that can receive
multicast IP traffic from that sender.
This enables AMT gateways in remote networks to discover an AMT relay
that is capable of forwarding traffic from the sender. This in turn
enables those AMT gateways to receive the multicast traffic tunneled
over a unicast AMT tunnel from those relays, and then to pass the
multicast packets into networks or applications that are using the
gateway to subscribe to traffic from that sender.
This mechanism only works for source-specific multicast (SSM)
channels. The source address of the (S,G) is reversed and used as an
index into one of the reverse mapping trees (in-addr.arpa for IPv4,
as described in Section 3.5 of [RFC1035], or ip6.arpa for IPv6, as
described in Section 2.5 of [RFC3596]).
Some detailed example use cases are provided in Section 3, and other
applicable example topologies appear in Section 3.3 of [RFC8313],
Section 3.4 of [RFC8313], and Section 3.5 of [RFC8313].
2.2. Signaling and Discovery
This section describes a typical example of the end-to-end process
for signaling a receiver's join of a SSM channel that relies on an
AMTRELAY RR.
The example in Figure 2 contains 2 multicast-enabled networks that
are both connected to the internet with non-multicast-capable links,
and which have no direct association with each other.
A content provider operates a sender, which is a source of multicast
traffic inside a multicast-capable network.
An end user who is a customer of the content provider has a
multicast-capable internet service provider, which operates a
receiving network that uses an AMT gateway. The AMT gateway is
DRIAD-capable.
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The content provider provides the user with a receiving application
that tries to subscribe to at least one (S,G). This receiving
application could for example be a file transfer system using FLUTE
[RFC6726] or a live video stream using RTP [RFC3550], or any other
application that might subscribe to a SSM channel.
+---------------+
| Sender |
| | | 198.51.100.15 |
| | +---------------+
|Data| |
|Flow| Multicast |
\| |/ Network |
\ / | 5: Propagate RPF for Join(S,G)
\ / +---------------+
\/ | AMT Relay |
| 203.0.113.15 |
+---------------+
| 4: Gateway connects to Relay,
sends Join(S,G) over tunnel
|
Unicast
Tunnel |
^ | 3: --> DNS Query: type=AMTRELAY,
| / 15.100.51.198.in-addr.arpa.
| | / <-- Response:
Join/Leave +-------------+ AMTRELAY=203.0.113.15
Signals | AMT gateway |
| +-------------+
| | 2: Propagate RPF for Join(S,G)
| Multicast |
Network |
| 1: Join(S=198.51.100.15, G)
+-------------+
| Receiver |
| (end user) |
+-------------+
Figure 2: DRIAD Messaging
In this simple example, the sender IP is 198.51.100.15, and the relay
IP is 203.0.113.15.
The content provider has previously configured the DNS zone that
contains the domain name "15.100.51.198.in-addr.arpa.", which is the
reverse lookup domain name for his sender. The zone file contains an
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AMTRELAY RR with the Relay's IP address. (See Section 4.3 for
details about the AMTRELAY RR format and semantics.)
The sequence of events depicted in Figure 2 is as follows:
1. The end user starts the app, which issues a join to the (S,G):
(198.51.100.15, 232.252.0.2).
2. The join propagates with RPF through the multicast-enabled
network with PIM [RFC7761] or another multicast routing
mechanism, until the AMT gateway receives a signal to join the
(S,G).
3. The AMT gateway performs a reverse DNS lookup for the AMTRELAY
RRType, by sending an AMTRELAY RRType query for the FQDN
"15.100.51.198.in-addr.arpa.", using the reverse IP domain name
for the sender's source IP address (the S from the (S,G)), as
described in Section 3.5 of [RFC1035].
The DNS resolver for the AMT gateway uses ordinary DNS recursive
resolution until it has the authoritative result that the content
provider configured, which informs the AMT gateway that the relay
address is 203.0.113.15.
4. The AMT gateway performs AMT handshakes with the AMT relay as
described in Section 4 of [RFC7450], then forwards a Membership
report to the relay indicating subscription to the (S,G).
5. The relay propagates the join through its network toward the
sender, then forwards the appropriate AMT-encapsulated traffic to
the gateway, which decapsulates and forwards it as native
multicast through its downstream network to the end user.
2.3. Optimal Relay Selection
The reverse source IP DNS query of an AMTRELAY RR is a good way for a
gateway to discover a relay that is known to the sender.
However, it is NOT necessarily a good way to discover the best relay
for that gateway to use, because the RR IP will only provide
information about relays known to the source.
If there is an upstream relay in a network that is more local to the
gateway and able to receive and forward multicast traffic from the
sender, that relay is better for the gateway to use, since more of
the network path uses native multicast, allowing more chances for
packet replication. But since that relay is not known to the sender,
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it won't be advertised in the sender's reverse IP DNS record. An
example network with this scenario is outlined in Section 3.1.2.
It's only appropriate for an AMT gateway to discover an AMT relay by
querying an AMTRELAY RR owned by a sender when all of these
conditions are met:
1. The gateway needs to propagate a join of an (S,G) over AMT,
because in the gateway's network, no RPF next hop toward the
source can propagate a native multicast join of the (S,G); and
2. The gateway is not already connected to a relay that forwards
multicast traffic from the source of the (S,G); and
3. The gateway is not configured to use a particular IP address for
AMT discovery, or a relay discovered with that IP is not able to
forward traffic from the source of the (S,G); and
4. The gateway is not able to find an upstream AMT relay with DNS-SD
[RFC6763], using "_amt._udp" as the Service section of the
queries, or a relay discovered this way is not able to forward
traffic from the source of the (S,G)
When the above conditions are met, the gateway has no path within its
local network that can receive multicast traffic from the source IP
of the (S,G).
In this situation, the best way to find a relay that can forward the
required traffic is to use information that comes from the operator
of the sender. When the sender has configured the AMTRELAY RR
defined in this document, gateways can use the DRIAD mechanism
defined in this document to discover the relay information provided
by the sender.
2.4. DNS Configuration
Often an AMT gateway will only have access to the source and group IP
addresses of the desired traffic, and will not know any other name
for the source of the traffic. Because of this, typically the best
way of looking up AMTRELAY RRs will be by using the source IP address
as an index into one of the reverse mapping trees (in-addr.arpa for
IPv4, as described in Section 3.5 of [RFC1035], or ip6.arpa for IPv6,
as described in Section 2.5 of [RFC3596]).
Therefore, it is RECOMMENDED that AMTRELAY RRs be added to reverse IP
zones as appropriate. AMTRELAY records MAY also appear in other
zones, but the primary intended use case requires a reverse IP
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mapping for the source from an (S,G) in order to be useful to most
AMT gateways.
<TBD>
Please can a DNS expert review the following paragraph and perhaps
help construct an equivalent and more clear explanation?
I borrowed the language from https://tools.ietf.org/html/
rfc4025#section-1.2, but I'm not actually sure what "the fashion
usual for PTR records" means, precisely...
PTR gives a domain name, and then we do what, an A/AAAA record
lookup, and then a AMTRELAY lookup on the final name that has a valid
A/AAAA after any CNAME/DNAME chain? - jake 2019-01-13
</TBD>
When the reverse IP mapping has no AMTRELAY RR but does have a PTR
record, the lookup is done in the fashion usual for PTR records. The
IP address' octets (IPv4) or nibbles (IPv6) are reversed and looked
up with the appropriate suffix. Any CNAMEs or DNAMEs found MUST be
followed, and finally the AMTRELAY RR is queried with the resulting
domain name.
See Section 4 and Section 4.3 for a detailed explanation of the
contents for a DNS Zone file.
3. Example Deployments
3.1. Example Receiving Networks
3.1.1. Tier 3 ISP
One example of a receiving network is an ISP that offers multicast
ingest services to its subscribers, illustrated in Figure 3.
In the example network below, subscribers can join (S,G)s with MLDv2
or IGMPv3 as described in [RFC4604], and the AMT gateway in this ISP
can receive and forward multicast traffic from one of the example
sending networks in Section 3.2 by discovering the appropriate AMT
relays with a DNS lookup for the AMTRELAY RR with the reverse IP of
the source in the (S,G).
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Internet
^ ^ Multicast-enabled
| | Receiving Network
+------|------------|-------------------------+
| | | |
| +--------+ +--------+ +=========+ |
| | Border |---| Border | | AMT | |
| | Router | | Router | | gateway | |
| +--------+ +--------+ +=========+ |
| | | | |
| +-----+------+-----------+--+ |
| | | |
| +-------------+ +-------------+ |
| | Agg Routers | .. | Agg Routers | |
| +-------------+ +-------------+ |
| / \ \ / \ |
| +---------------+ +---------------+ |
| |Access Systems | ....... |Access Systems | |
| |(CMTS/OLT/etc.)| |(CMTS/OLT/etc.)| |
| +---------------+ +---------------+ |
| | | |
+--------|------------------------|-----------+
| |
+---+-+-+---+---+ +---+-+-+---+---+
| | | | | | | | | |
/-\ /-\ /-\ /-\ /-\ /-\ /-\ /-\ /-\ /-\
|_| |_| |_| |_| |_| |_| |_| |_| |_| |_|
Subscribers
Figure 3: Receiving ISP Example
3.1.2. Small Office
Another example receiving network is a small branch office that
regularly accesses some multicast content, illustrated in Figure 4.
This office has desktop devices that need to receive some multicast
traffic, so an AMT gateway runs on a LAN with these devices, to pull
traffic in through a non-multicast next-hop.
The office also hosts some mobile devices that have AMT gateway
instances embedded inside apps, in order to receive multicast traffic
over their non-multicast wireless LAN. (Note that the "Legacy
Router" is a simplification that's meant to describe a variety of
possible conditions- for example it could be a device providing a
split-tunnel VPN as described in [RFC7359], deliberately excluding
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multicast traffic for a VPN tunnel, rather than a device which is
incapable of multicast forwarding.)
Internet
(non-multicast)
^
| Office Network
+----------|----------------------------------+
| | |
| +---------------+ (Wifi) Mobile apps |
| | Modem+ | Wifi | - - - - w/ embedded |
| | Router | AP | AMT gateways |
| +---------------+ |
| | |
| | |
| +----------------+ |
| | Legacy Router | |
| | (unicast) | |
| +----------------+ |
| / | \ |
| / | \ |
| +--------+ +--------+ +--------+=========+ |
| | Phones | | ConfRm | | Desks | AMT | |
| | subnet | | subnet | | subnet | gateway | |
| +--------+ +--------+ +--------+=========+ |
| |
+---------------------------------------------+
Figure 4: Small Office (no multicast up)
By adding an AMT relay to this office network as in Figure 5, it's
possible to make use of multicast services from the example
multicast-capable ISP in Section 3.1.1.
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Multicast-capable ISP
^
| Office Network
+----------|----------------------------------+
| | |
| +---------------+ (Wifi) Mobile apps |
| | Modem+ | Wifi | - - - - w/ embedded |
| | Router | AP | AMT gateways |
| +---------------+ |
| | +=======+ |
| +---Wired LAN---| AMT | |
| | | relay | |
| +----------------+ +=======+ |
| | Legacy Router | |
| | (unicast) | |
| +----------------+ |
| / | \ |
| / | \ |
| +--------+ +--------+ +--------+=========+ |
| | Phones | | ConfRm | | Desks | AMT | |
| | subnet | | subnet | | subnet | gateway | |
| +--------+ +--------+ +--------+=========+ |
| |
+---------------------------------------------+
Figure 5: Small Office Example
When multicast-capable networks are chained like this, with a network
like the one in Figure 5 receiving internet services from a
multicast-capable network like the one in Figure 3, it's important
for AMT gateways to reach the more local AMT relay, in order to avoid
accidentally tunneling multicast traffic from a more distant AMT
relay with unicast, and failing to utilize the multicast transport
capabilities of the network in Figure 3.
For this reason, it's RECOMMENDED that AMT gateways by default
perform service discovery using DNS Service Discovery (DNS-SD)
[RFC6763] for _amt._udp.<domain> (with <domain> chosen as described
in Section 11 of [RFC6763]) and use the AMT relays discovered that
way in preference to AMT relays discoverable via the mechanism
defined in this document (DRIAD).
It's also RECOMMENDED that when the well-known anycast IP addresses
defined in Section 7 of [RFC7450] are suitable for discovering an AMT
relay that can forward traffic from the source, that a DNS record
with the AMTRELAY RRType be published for those IP addresses along
with any other appropriate AMTRELAY RRs to indicate the best relative
precedences for receiving the source traffic.
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Accordingly, AMT gateways SHOULD by default discover the most-
preferred relay first by DNS-SD, then by DRIAD as described in this
document (in precedence order, as described in Section 4.2.1), then
with the anycast addresses defined in Section 7 of [RFC7450] (namely:
192.52.193.1 and 2001:3::1) if those IPs weren't listed in the
AMTRELAY RRs. This default behavior MAY be overridden by
administrative configuration where other behavior is more appropriate
for the gateway within its network.
The discovery and connection process for multiple relays MAY operate
in parallel, but when forwarding multicast group membership reports
with new joins from an AMT gateway, membership reports SHOULD be
forwarded to the most-preferred relays first, falling back to less
preferred relays only after failing to receive traffic for an
appropriate timeout, and only after reporting a leave to any more-
preferred connected relays that have failed to subscribe to the
traffic.
It is RECOMMENDED that the default timeout be no less than 3 seconds,
but the value MAY be overridden by administrative configuration,
where known groups or channels need a different timeout for
successful application performance.
3.2. Example Sending Networks
3.2.1. Sender-controlled Relays
When a sender network is also operating AMT relays to distribute
multicast traffic, as in Figure 6, each address could appear as an
AMTRELAY RR for the reverse IP of the sender, or one or more domain
names could appear in AMTRELAY RRs, and the AMT relay addresses can
be discovered by finding an A or AAAA record from those domain names.
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Sender Network
+-----------------------------------+
| |
| +--------+ +=======+ +=======+ |
| | Sender | | AMT | | AMT | |
| +--------+ | relay | | relay | |
| | +=======+ +=======+ |
| | | | |
| +-----+------+----------+ |
| | |
+-----------|-----------------------+
v
Internet
(non-multicast)
Figure 6: Small Office Example
3.2.2. Provider-controlled Relays
When an ISP offers a service to transmit outbound multicast traffic
through a forwarding network, it might also offer AMT relays in order
to reach receivers without multicast connectivity to the forwarding
network, as in Figure 7. In this case it's RECOMMENDED that the ISP
also provide a domain name for the AMT relays for use with the
discovery process defined in this document.
When the sender wishes to use the relays provided by the ISP for
forwarding multicast traffic, an AMTRELAY RR should be configured to
use the domain name provided by the ISP, to allow for address
reassignment of the relays without forcing the sender to reconfigure
the corresponding AMTRELAY RRs.
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+--------+
| Sender |
+---+----+ Multicast-enabled
| Sending Network
+-----------|-------------------------------+
| v |
| +------------+ +=======+ +=======+ |
| | Agg Router | | AMT | | AMT | |
| +------------+ | relay | | relay | |
| | +=======+ +=======+ |
| | | | |
| +-----+------+--------+---------+ |
| | | |
| +--------+ +--------+ |
| | Border |---| Border | |
| | Router | | Router | |
| +--------+ +--------+ |
+-----|------------|------------------------+
| |
v v
Internet
(non-multicast)
Figure 7: Sending ISP Example
4. AMTRELAY Resource Record Definition
4.1. AMTRELAY RRType
The AMTRELAY RRType has the mnemonic AMTRELAY and type code TBD1
(decimal).
4.2. AMTRELAY RData Format
The AMTRELAY RData consists of a 8-bit precedence field, a 1-bit
"Discovery Optional" field, a 7-bit type field, and a variable length
relay field.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| precedence |D| type | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
~ relay ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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4.2.1. RData Format - Precedence
This is an 8-bit precedence for this record. It is interpreted in
the same way as the PREFERENCE field described in Section 3.3.9 of
[RFC1035].
Relays listed in AMTRELAY records with a lower value for precedence
are to be attempted first.
Where there is a tie in precedence, the default choice of relay MUST
be non-deterministic, to support load balancing. The AMT gateway
operator MAY override this default choice with explicit configuration
when it's necessary for administrative purposes.
For example, one network might prefer to tunnel IPv6 multicast
traffic over IPv6 AMT and IPv4 multicast traffic over IPv4 AMT to
avoid routeability problems in IPv6 from affecting IPv4 traffic and
vice versa, while another network might prefer to tunnel both kinds
of traffic over IPv6 to reduce the IPv4 space used by its AMT
gateways. In this example scenario or other cases where there is an
administrative preference that requires explicit configuration, a
receiving network MAY make systematically different precedence
choices among records with the same precedence value.
4.2.2. RData Format - Discovery Optional (D-bit)
The D bit is a "Discovery Optional" flag.
If the D bit is set to 0, a gateway using this RR MUST perform AMT
relay discovery as described in Section 4.2.1.1 of [RFC7450], rather
than directly sending an AMT request message to the relay.
That is, the gateway MUST receive an AMT relay advertisement message
(Section 5.1.2 of [RFC7450]) for an address before sending an AMT
request message (Section 5.1.3 of [RFC7450]) to that address. Before
receiving the relay advertisement message, this record has only
indicated that the address can be used for AMT relay discovery, not
for a request message. This is necessary for devices that are not
fully functional AMT relays, but rather load balancers or brokers, as
mentioned in Section 4.2.1.1 of [RFC7450].
If the D bit is set to 1, the gateway MAY send an AMT request message
directly to the discovered relay address without first sending an AMT
discovery message.
This bit should be set according to advice from the AMT relay
operator. The D bit MUST be set to zero when no information is
available from the AMT relay operator about its suitability.
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4.2.3. RData Format - Type
The type field indicates the format of the information that is stored
in the relay field.
The following values are defined:
o type = 0: The relay field is empty (0 bytes).
o type = 1: The relay field contains a 4-octet IPv4 address.
o type = 2: The relay field contains a 16-octet IPv6 address.
o type = 3: The relay field contains a wire-encoded domain name.
The wire-encoded format is self-describing, so the length is
implicit. The domain name MUST NOT be compressed. (See
Section 3.3 of [RFC1035] and Section 4 of [RFC3597].)
4.2.4. RData Format - Relay
The relay field is the address or domain name of the AMT relay. It
is formatted according to the type field.
When the type field is 0, the length of the relay field is 0, and it
indicates that no AMT relay should be used for multicast traffic from
this source.
When the type field is 1, the length of the relay field is 4 octets,
and a 32-bit IPv4 address is present. This is an IPv4 address as
described in Section 3.4.1 of [RFC1035]. This is a 32-bit number in
network byte order.
When the type field is 2, the length of the relay field is 16 octets,
and a 128-bit IPv6 address is present. This is an IPv6 address as
described in Section 2.2 of [RFC3596]. This is a 128-bit number in
network byte order.
When the type field is 3, the relay field is a normal wire-encoded
domain name, as described in Section 3.3 of [RFC1035]. Compression
MUST NOT be used, for the reasons given in Section 4 of [RFC3597].
4.3. AMTRELAY Record Presentation Format
4.3.1. Representation of AMTRELAY RRs
AMTRELAY RRs may appear in a zone data master file. The precedence,
D-bit, relay type, and relay fields are REQUIRED.
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If the relay type field is 0, the relay field MUST be ".".
The presentation for the record is as follows:
IN AMTRELAY precedence D-bit type relay
4.3.2. Examples
In a DNS resolver that understands the AMTRELAY type, the zone might
contain a set of entries like this:
$ORIGIN 100.51.198.in-addr.arpa.
10 IN AMTRELAY 10 0 1 203.0.113.15
10 IN AMTRELAY 10 0 2 2001:DB8::15
10 IN AMTRELAY 128 1 3 amtrelays.example.com.
This configuration advertises an IPv4 discovery address, an IPv6
discovery address, and a domain name for AMT relays which can receive
traffic from the source 198.51.100.10. The IPv4 and IPv6 addresses
are configured with a D-bit of 0 (meaning discovery is mandatory, as
described in Section 4.2.2), and a precedence 10 (meaning they're
preferred ahead of the last entry, which has precedence 128).
For zone files in resolvers that don't support the AMTRELAY RRType
natively, it's possible to use the format for unknown RR types, as
described in [RFC3597]. This approach would replace the AMTRELAY
entries in the example above with the entries below:
[To be removed (TBD): replace 65280 with the IANA-assigned value
TBD1, here and in Appendix B. ]
10 IN TYPE65280 \# (
6 ; length
0a ; precedence=10
01 ; D=0, relay type=1, an IPv4 address
cb00710f ) ; 203.0.113.15
10 IN TYPE65280 \# (
18 ; length
0a ; precedence=10
02 ; D=0, relay type=2, an IPv6 address
20010db800000000000000000000000f ) ; 2001:db8::15
10 IN TYPE65280 \# (
24 ; length
80 ; precedence=128
83 ; D=1, relay type=3, a wire-encoded domain name
616d7472656c6179732e6578616d706c652e636f6d2e ) ; domain name
See Appendix B for more details.
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5. IANA Considerations
This document updates the IANA Registry for DNS Resource Record Types
by assigning type TBD1 to the AMTRELAY record.
This document creates a new registry named "AMTRELAY Resource Record
Parameters", with a sub-registry for the "Relay Type Field". The
initial values in the sub-registry are:
+-------+---------------------------------------+
| Value | Description |
+-------+---------------------------------------+
| 0 | No relay is present. |
| 1 | A 4-byte IPv4 address is present |
| 2 | A 16-byte IPv6 address is present |
| 3 | A wire-encoded domain name is present |
| 4-255 | Unassigned |
+-------+---------------------------------------+
Values 0, 1, 2, and 3 are further explained in Section 4.2.3 and
Section 4.2.4. Relay type numbers 4 through 255 can be assigned with
a policy of Specification Required (as described in [RFC8126]).
6. Security Considerations
[ TBD: these 3 are just the first few most obvious issues, with just
sketches of the problem. Explain better, and look for trickier
issues. ]
6.1. Record-spoofing
If AMT is used to ingest multicast traffic, providing a false
AMTRELAY record to a gateway using it for discovery can result in
Denial of Service, or artificial multicast traffic from a source
under an attacker's control.
Therefore, it is important to ensure that the AMTRELAY record is
authentic, with DNSSEC [RFC4033] or other operational safeguards that
can provide assurance of the authenticity of the record contents.
6.2. Local Override
The local relays, while important for overall network performance,
can't be secured by DNSSEC.
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6.3. Congestion
Multicast traffic, particularly interdomain multicast traffic,
carries some congestion risks, as described in Section 4 of
[RFC8085]. Network operators are advised to take precautions
including monitoring of application traffic behavior, traffic
authentication, and rate-limiting of multicast traffic, in order to
ensure network health.
7. Acknowledgements
This specification was inspired by the previous work of Doug Nortz,
Robert Sayko, David Segelstein, and Percy Tarapore, presented in the
MBONED working group at IETF 93.
Thanks also to Jeff Goldsmith and Lenny Giuliano for helpful reviews
and feedback.
8. References
8.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
<https://www.rfc-editor.org/info/rfc3376>.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", STD 88,
RFC 3596, DOI 10.17487/RFC3596, October 2003,
<https://www.rfc-editor.org/info/rfc3596>.
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[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
2003, <https://www.rfc-editor.org/info/rfc3597>.
[RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
DOI 10.17487/RFC3810, June 2004,
<https://www.rfc-editor.org/info/rfc3810>.
[RFC4604] Holbrook, H., Cain, B., and B. Haberman, "Using Internet
Group Management Protocol Version 3 (IGMPv3) and Multicast
Listener Discovery Protocol Version 2 (MLDv2) for Source-
Specific Multicast", RFC 4604, DOI 10.17487/RFC4604,
August 2006, <https://www.rfc-editor.org/info/rfc4604>.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
<https://www.rfc-editor.org/info/rfc4607>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[RFC7450] Bumgardner, G., "Automatic Multicast Tunneling", RFC 7450,
DOI 10.17487/RFC7450, February 2015,
<https://www.rfc-editor.org/info/rfc7450>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC4025] Richardson, M., "A Method for Storing IPsec Keying
Material in DNS", RFC 4025, DOI 10.17487/RFC4025, March
2005, <https://www.rfc-editor.org/info/rfc4025>.
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[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.
[RFC5110] Savola, P., "Overview of the Internet Multicast Routing
Architecture", RFC 5110, DOI 10.17487/RFC5110, January
2008, <https://www.rfc-editor.org/info/rfc5110>.
[RFC5507] IAB, Faltstrom, P., Ed., Austein, R., Ed., and P. Koch,
Ed., "Design Choices When Expanding the DNS", RFC 5507,
DOI 10.17487/RFC5507, April 2009,
<https://www.rfc-editor.org/info/rfc5507>.
[RFC6726] Paila, T., Walsh, R., Luby, M., Roca, V., and R. Lehtonen,
"FLUTE - File Delivery over Unidirectional Transport",
RFC 6726, DOI 10.17487/RFC6726, November 2012,
<https://www.rfc-editor.org/info/rfc6726>.
[RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
April 2013, <https://www.rfc-editor.org/info/rfc6895>.
[RFC7359] Gont, F., "Layer 3 Virtual Private Network (VPN) Tunnel
Traffic Leakages in Dual-Stack Hosts/Networks", RFC 7359,
DOI 10.17487/RFC7359, August 2014,
<https://www.rfc-editor.org/info/rfc7359>.
[RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
Multicast - Sparse Mode (PIM-SM): Protocol Specification
(Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
2016, <https://www.rfc-editor.org/info/rfc7761>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8313] Tarapore, P., Ed., Sayko, R., Shepherd, G., Eckert, T.,
Ed., and R. Krishnan, "Use of Multicast across Inter-
domain Peering Points", BCP 213, RFC 8313,
DOI 10.17487/RFC8313, January 2018,
<https://www.rfc-editor.org/info/rfc8313>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
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Appendix A. New RRType Request Form
This is the template for requesting a new RRType recommended in
Appendix A of [RFC6895].
A. Submission Date:
B.1 Submission Type:
[x] New RRTYPE [ ] Modification to RRTYPE
B.2 Kind of RR:
[x] Data RR [ ] Meta-RR
C. Contact Information for submitter (will be publicly posted):
Name: Jake Holland
Email Address: jakeholland.net@gmail.com
International telephone number: +1-626-486-3706
Other contact handles: jholland@akamai.com
D. Motivation for the new RRTYPE application.
It provides a bootstrap so AMT (RFC 7450) gateways can discover
an AMT relay that can receive multicast traffic from a specific source,
in order to signal multicast group membership and receive multicast
traffic over a unicast tunnel using AMT.
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 draft-jholland-mboned-driad-amt-discovery.
F. What existing RRTYPE or RRTYPEs come closest to filling that need
and why are they unsatisfactory?
Some similar concepts appear in IPSECKEY, as described in
Section 1.2 of [RFC4025]. The IPSECKEY RRType is unsatisfactory
because it refers to IPSec Keys instead of to AMT relays, but
the motivating considerations for using reverse IP and for
providing a precedence are similar--an AMT gateway often
has access to a source address for a multicast (S,G), but does
not have access to a relay address that can receive multicast
traffic from the source, without administrative configuration.
Defining a format for a TXT record could serve the need for AMT
relay discovery semantics, but Section 5 of [RFC5507] provides a
compelling argument for requesting a new RRType instead.
G. What mnemonic is requested for the new RRTYPE (optional)?
AMTRELAY
H. Does the requested RRTYPE make use of any existing IANA registry
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or require the creation of a new IANA subregistry in DNS
Parameters?
Yes, IANA is requested to create a subregistry named "AMT Relay
Type Field" in a "AMTRELAY Resource Record Parameters" registry.
The field values are defined in Section 4.2.3 and Section 4.2.4,
and a summary table is given in Section 5.
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:
It may be worth noting that the gateway type field from Section 2.3 of
[RFC4025] and Section 2.5 of [RFC4025] is very similar to the
Relay Type field in this request. I tentatively assume that trying to
re-use that sub-registry is a worse idea than duplicating it, but I'll
invite others to consider the question and voice an opinion, in case
there is a different consensus.
https://www.ietf.org/assignments/
ipseckey-rr-parameters/ipseckey-rr-parameters.xml
Appendix B. Unknown RRType construction
In a DNS resolver that understands the AMTRELAY type, the zone file
might contain this line:
IN AMTRELAY 128 0 3 amtrelays.example.com.
In order to translate this example to appear as an unknown RRType as
defined in [RFC3597], one could run the following program:
<CODE BEGINS>
$ cat translate.py
#!/usr/bin/python3
import sys
name=sys.argv[1]
print(len(name))
print(''.join('%02x'%ord(x) for x in name))
$ ./translate.py amtrelays.example.com.
22
616d7472656c6179732e6578616d706c652e636f6d2e
<CODE ENDS>
The length and the hex string for the domain name
"amtrelays.example.com." are the outputs of this program, yielding a
length of 22 and the above hex string.
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22 is the length of the domain name, so to this we add 2 (1 for the
precedence field and 1 for the combined D-bit and relay type fields)
to get the full length of the RData.
This results in a zone file entry like this:
IN TYPE65280 \# ( 24 ; length
80 ; precedence = 128
03 ; D-bit=0, relay type=3 (wire-encoded domain name)
616d7472656c6179732e6578616d706c652e636f6d2e ) ; domain name
Author's Address
Jake Holland
Akamai Technologies, Inc.
150 Broadway
Cambridge, MA 02144
United States of America
Email: jakeholland.net@gmail.com
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