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draft-ietf-tram-turnbis-28.txt
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draft-ietf-tram-turnbis-28.txt
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TRAM WG T. Reddy, Ed.
Internet-Draft McAfee
Obsoletes: 5766, 6156 (if approved) A. Johnston, Ed.
Intended status: Standards Track Villanova University
Expires: January 21, 2020 P. Matthews
Alcatel-Lucent
J. Rosenberg
jdrosen.net
July 20, 2019
Traversal Using Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)
draft-ietf-tram-turnbis-27
Abstract
If a host is located behind a NAT, then in certain situations it can
be impossible for that host to communicate directly with other hosts
(peers). In these situations, it is necessary for the host to use
the services of an intermediate node that acts as a communication
relay. This specification defines a protocol, called TURN (Traversal
Using Relays around NAT), that allows the host to control the
operation of the relay and to exchange packets with its peers using
the relay. TURN differs from other relay control protocols in that
it allows a client to communicate with multiple peers using a single
relay address.
The TURN protocol was designed to be used as part of the ICE
(Interactive Connectivity Establishment) approach to NAT traversal,
though it also can be used without ICE.
This document obsoletes RFC 5766 and RFC 6156.
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 https://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."
Reddy, et al. Expires January 21, 2020 [Page 1]
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This Internet-Draft will expire on January 21, 2020.
Copyright Notice
Copyright (c) 2019 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
(https://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 . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 8
3.1. Transports . . . . . . . . . . . . . . . . . . . . . . . 11
3.2. Allocations . . . . . . . . . . . . . . . . . . . . . . . 12
3.3. Permissions . . . . . . . . . . . . . . . . . . . . . . . 14
3.4. Send Mechanism . . . . . . . . . . . . . . . . . . . . . 15
3.5. Channels . . . . . . . . . . . . . . . . . . . . . . . . 17
3.6. Unprivileged TURN Servers . . . . . . . . . . . . . . . . 19
3.7. Avoiding IP Fragmentation . . . . . . . . . . . . . . . . 19
3.8. RTP Support . . . . . . . . . . . . . . . . . . . . . . . 21
3.9. Happy Eyeballs for TURN . . . . . . . . . . . . . . . . . 21
4. Discovery of TURN server . . . . . . . . . . . . . . . . . . 22
4.1. TURN URI Scheme Semantics . . . . . . . . . . . . . . . . 22
5. General Behavior . . . . . . . . . . . . . . . . . . . . . . 23
6. Allocations . . . . . . . . . . . . . . . . . . . . . . . . . 25
7. Creating an Allocation . . . . . . . . . . . . . . . . . . . 26
7.1. Sending an Allocate Request . . . . . . . . . . . . . . . 26
7.2. Receiving an Allocate Request . . . . . . . . . . . . . . 28
7.3. Receiving an Allocate Success Response . . . . . . . . . 33
7.4. Receiving an Allocate Error Response . . . . . . . . . . 34
8. Refreshing an Allocation . . . . . . . . . . . . . . . . . . 36
8.1. Sending a Refresh Request . . . . . . . . . . . . . . . . 37
8.2. Receiving a Refresh Request . . . . . . . . . . . . . . . 37
8.3. Receiving a Refresh Response . . . . . . . . . . . . . . 38
9. Permissions . . . . . . . . . . . . . . . . . . . . . . . . . 38
10. CreatePermission . . . . . . . . . . . . . . . . . . . . . . 40
10.1. Forming a CreatePermission Request . . . . . . . . . . . 40
10.2. Receiving a CreatePermission Request . . . . . . . . . . 40
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10.3. Receiving a CreatePermission Response . . . . . . . . . 41
11. Send and Data Methods . . . . . . . . . . . . . . . . . . . . 41
11.1. Forming a Send Indication . . . . . . . . . . . . . . . 41
11.2. Receiving a Send Indication . . . . . . . . . . . . . . 42
11.3. Receiving a UDP Datagram . . . . . . . . . . . . . . . . 42
11.4. Receiving a Data Indication . . . . . . . . . . . . . . 43
11.5. Receiving an ICMP Packet . . . . . . . . . . . . . . . . 43
11.6. Receiving a Data Indication with an ICMP attribute . . . 44
12. Channels . . . . . . . . . . . . . . . . . . . . . . . . . . 45
12.1. Sending a ChannelBind Request . . . . . . . . . . . . . 47
12.2. Receiving a ChannelBind Request . . . . . . . . . . . . 48
12.3. Receiving a ChannelBind Response . . . . . . . . . . . . 49
12.4. The ChannelData Message . . . . . . . . . . . . . . . . 49
12.5. Sending a ChannelData Message . . . . . . . . . . . . . 50
12.6. Receiving a ChannelData Message . . . . . . . . . . . . 50
12.7. Relaying Data from the Peer . . . . . . . . . . . . . . 51
13. Packet Translations . . . . . . . . . . . . . . . . . . . . . 51
13.1. IPv4-to-IPv6 Translations . . . . . . . . . . . . . . . 52
13.2. IPv6-to-IPv6 Translations . . . . . . . . . . . . . . . 53
13.3. IPv6-to-IPv4 Translations . . . . . . . . . . . . . . . 55
14. UDP-to-UDP relay . . . . . . . . . . . . . . . . . . . . . . 55
15. TCP-to-UDP relay . . . . . . . . . . . . . . . . . . . . . . 57
16. UDP-to-TCP relay . . . . . . . . . . . . . . . . . . . . . . 59
17. STUN Methods . . . . . . . . . . . . . . . . . . . . . . . . 60
18. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 60
18.1. CHANNEL-NUMBER . . . . . . . . . . . . . . . . . . . . . 61
18.2. LIFETIME . . . . . . . . . . . . . . . . . . . . . . . . 61
18.3. XOR-PEER-ADDRESS . . . . . . . . . . . . . . . . . . . . 61
18.4. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . 61
18.5. XOR-RELAYED-ADDRESS . . . . . . . . . . . . . . . . . . 62
18.6. REQUESTED-ADDRESS-FAMILY . . . . . . . . . . . . . . . . 62
18.7. EVEN-PORT . . . . . . . . . . . . . . . . . . . . . . . 62
18.8. REQUESTED-TRANSPORT . . . . . . . . . . . . . . . . . . 63
18.9. DONT-FRAGMENT . . . . . . . . . . . . . . . . . . . . . 63
18.10. RESERVATION-TOKEN . . . . . . . . . . . . . . . . . . . 63
18.11. ADDITIONAL-ADDRESS-FAMILY . . . . . . . . . . . . . . . 64
18.12. ADDRESS-ERROR-CODE . . . . . . . . . . . . . . . . . . . 64
18.13. ICMP . . . . . . . . . . . . . . . . . . . . . . . . . . 65
19. STUN Error Response Codes . . . . . . . . . . . . . . . . . . 65
20. Detailed Example . . . . . . . . . . . . . . . . . . . . . . 66
21. Security Considerations . . . . . . . . . . . . . . . . . . . 74
21.1. Outsider Attacks . . . . . . . . . . . . . . . . . . . . 74
21.1.1. Obtaining Unauthorized Allocations . . . . . . . . . 74
21.1.2. Offline Dictionary Attacks . . . . . . . . . . . . . 74
21.1.3. Faked Refreshes and Permissions . . . . . . . . . . 75
21.1.4. Fake Data . . . . . . . . . . . . . . . . . . . . . 75
21.1.5. Impersonating a Server . . . . . . . . . . . . . . . 76
21.1.6. Eavesdropping Traffic . . . . . . . . . . . . . . . 76
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21.1.7. TURN Loop Attack . . . . . . . . . . . . . . . . . . 77
21.2. Firewall Considerations . . . . . . . . . . . . . . . . 78
21.2.1. Faked Permissions . . . . . . . . . . . . . . . . . 79
21.2.2. Blacklisted IP Addresses . . . . . . . . . . . . . . 79
21.2.3. Running Servers on Well-Known Ports . . . . . . . . 79
21.3. Insider Attacks . . . . . . . . . . . . . . . . . . . . 79
21.3.1. DoS against TURN Server . . . . . . . . . . . . . . 80
21.3.2. Anonymous Relaying of Malicious Traffic . . . . . . 80
21.3.3. Manipulating Other Allocations . . . . . . . . . . . 80
21.4. Tunnel Amplification Attack . . . . . . . . . . . . . . 80
21.5. Other Considerations . . . . . . . . . . . . . . . . . . 82
22. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 82
23. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 83
24. Changes since RFC 5766 . . . . . . . . . . . . . . . . . . . 84
25. Updates to RFC 6156 . . . . . . . . . . . . . . . . . . . . . 85
26. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 85
27. References . . . . . . . . . . . . . . . . . . . . . . . . . 85
27.1. Normative References . . . . . . . . . . . . . . . . . . 85
27.2. Informative References . . . . . . . . . . . . . . . . . 87
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 92
1. Introduction
A host behind a NAT may wish to exchange packets with other hosts,
some of which may also be behind NATs. To do this, the hosts
involved can use "hole punching" techniques (see [RFC5128]) in an
attempt discover a direct communication path; that is, a
communication path that goes from one host to another through
intervening NATs and routers, but does not traverse any relays.
As described in [RFC5128] and [RFC4787], hole punching techniques
will fail if both hosts are behind NATs that are not well behaved.
For example, if both hosts are behind NATs that have a mapping
behavior of "address-dependent mapping" or "address- and port-
dependent mapping" (Section 4.1 in [RFC4787]), then hole punching
techniques generally fail.
When a direct communication path cannot be found, it is necessary to
use the services of an intermediate host that acts as a relay for the
packets. This relay typically sits in the public Internet and relays
packets between two hosts that both sit behind NATs.
In many enterprise networks, direct UDP transmissions are not
permitted between clients on the internal networks and external IP
addresses. To permit media sessions in such a situation to use UDP
and to avoid forcing the media sessions through TCP, an Enterprise
Firewall can be configured to allow UDP traffic relayed through an
Enterprise relay server. This scenario is required to be supported
Reddy, et al. Expires January 21, 2020 [Page 4]
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by the WebRTC requirements (Section 2.3.5.1 in [RFC7478]). In
addition, in a SIP or WebRTC call, if the user wants IP location
privacy from the peer then the client can select a relay server
offering IP location privacy and only convey the relayed candidates
to the peer for ICE connectivity checks (see Section 4.2.4 in
[I-D.ietf-rtcweb-security]).
This specification defines a protocol, called TURN, that allows a
host behind a NAT (called the TURN client) to request that another
host (called the TURN server) act as a relay. The client can arrange
for the server to relay packets to and from certain other hosts
(called peers) and can control aspects of how the relaying is done.
The client does this by obtaining an IP address and port on the
server, called the relayed transport address. When a peer sends a
packet to the relayed transport address, the server relays the
transport protocol data from the packet to the client. The client
knows the peer from which the transport protocol data was relayed by
the server. If the server receives an ICMP error packet, the server
also relays certain layer 3/4 header fields from the ICMP header to
the client. When the client sends a packet to the server, the server
relays the transport protocol data from the packet to the intended
peer using the relayed transport address as the source.
A client using TURN must have some way to communicate the relayed
transport address to its peers, and to learn each peer's IP address
and port (more precisely, each peer's server-reflexive transport
address, see Section 3). How this is done is out of the scope of the
TURN protocol. One way this might be done is for the client and
peers to exchange email messages. Another way is for the client and
its peers to use a special-purpose "introduction" or "rendezvous"
protocol (see [RFC5128] for more details).
If TURN is used with ICE [RFC8445], then the relayed transport
address and the IP addresses and ports of the peers are included in
the ICE candidate information that the rendezvous protocol must
carry. For example, if TURN and ICE are used as part of a multimedia
solution using SIP [RFC3261], then SIP serves the role of the
rendezvous protocol, carrying the ICE candidate information inside
the body of SIP messages [I-D.ietf-mmusic-ice-sip-sdp]. If TURN and
ICE are used with some other rendezvous protocol, then ICE provides
guidance on the services the rendezvous protocol must perform.
Though the use of a TURN server to enable communication between two
hosts behind NATs is very likely to work, it comes at a high cost to
the provider of the TURN server, since the server typically needs a
high-bandwidth connection to the Internet. As a consequence, it is
best to use a TURN server only when a direct communication path
cannot be found. When the client and a peer use ICE to determine the
Reddy, et al. Expires January 21, 2020 [Page 5]
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communication path, ICE will use hole punching techniques to search
for a direct path first and only use a TURN server when a direct path
cannot be found.
TURN was originally invented to support multimedia sessions signaled
using SIP. Since SIP supports forking, TURN supports multiple peers
per relayed transport address; a feature not supported by other
approaches (e.g., SOCKS [RFC1928]). However, care has been taken to
make sure that TURN is suitable for other types of applications.
TURN was designed as one piece in the larger ICE approach to NAT
traversal. Implementors of TURN are urged to investigate ICE and
seriously consider using it for their application. However, it is
possible to use TURN without ICE.
TURN is an extension to the STUN (Session Traversal Utilities for
NAT) protocol [I-D.ietf-tram-stunbis]. Most, though not all, TURN
messages are STUN-formatted messages. A reader of this document
should be familiar with STUN.
The TURN specification was originally published as [RFC5766], which
was updated by [RFC6156] to add IPv6 support. This document
supersedes and obsoletes both [RFC5766] and [RFC6156].
2. Terminology
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 BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.
Readers are expected to be familiar with [I-D.ietf-tram-stunbis] and
the terms defined there.
The following terms are used in this document:
TURN: The protocol spoken between a TURN client and a TURN server.
It is an extension to the STUN protocol [I-D.ietf-tram-stunbis].
The protocol allows a client to allocate and use a relayed
transport address.
TURN client: A STUN client that implements this specification.
TURN server: A STUN server that implements this specification. It
relays data between a TURN client and its peer(s).
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Peer: A host with which the TURN client wishes to communicate. The
TURN server relays traffic between the TURN client and its
peer(s). The peer does not interact with the TURN server using
the protocol defined in this document; rather, the peer receives
data sent by the TURN server and the peer sends data towards the
TURN server.
Transport Address: The combination of an IP address and a port.
Host Transport Address: A transport address on a client or a peer.
Server-Reflexive Transport Address: A transport address on the
"external side" of a NAT. This address is allocated by the NAT to
correspond to a specific host transport address.
Relayed Transport Address: A transport address on the TURN server
that is used for relaying packets between the client and a peer.
A peer sends to this address on the TURN server, and the packet is
then relayed to the client.
TURN Server Transport Address: A transport address on the TURN
server that is used for sending TURN messages to the server. This
is the transport address that the client uses to communicate with
the server.
Peer Transport Address: The transport address of the peer as seen by
the server. When the peer is behind a NAT, this is the peer's
server-reflexive transport address.
Allocation: The relayed transport address granted to a client
through an Allocate request, along with related state, such as
permissions and expiration timers.
5-tuple: The combination (client IP address and port, server IP
address and port, and transport protocol (currently one of UDP,
TCP, DTLS/UDP or TLS/TCP) used to communicate between the client
and the server. The 5-tuple uniquely identifies this
communication stream. The 5-tuple also uniquely identifies the
Allocation on the server.
Transport Protocol: The protocols above IP that carries TURN
Requests, Responses, and Indications as well as providing
identifiable flows using a 5-tuple. In this specification, UDP
and TCP are defined as transport protocols, as well as their
combination with a security layer using DTLS and TLS respectively.
Channel: A channel number and associated peer transport address.
Once a channel number is bound to a peer's transport address, the
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client and server can use the more bandwidth-efficient ChannelData
message to exchange data.
Permission: The IP address and transport protocol (but not the port)
of a peer that is permitted to send traffic to the TURN server and
have that traffic relayed to the TURN client. The TURN server
will only forward traffic to its client from peers that match an
existing permission.
Realm: A string used to describe the server or a context within the
server. The realm tells the client which username and password
combination to use to authenticate requests.
Nonce: A string chosen at random by the server and included in the
server response. To prevent replay attacks, the server should
change the nonce regularly.
(D)TLS: This term is used for statements that apply to both
Transport Layer Security [RFC8446] and Datagram Transport Layer
Security [RFC6347].
3. Overview of Operation
This section gives an overview of the operation of TURN. It is non-
normative.
In a typical configuration, a TURN client is connected to a private
network [RFC1918] and through one or more NATs to the public
Internet. On the public Internet is a TURN server. Elsewhere in the
Internet are one or more peers with which the TURN client wishes to
communicate. These peers may or may not be behind one or more NATs.
The client uses the server as a relay to send packets to these peers
and to receive packets from these peers.
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Peer A
Server-Reflexive +---------+
Transport Address | |
192.0.2.150:32102 | |
| /| |
TURN | / ^| Peer A |
Client's Server | / || |
Host Transport Transport | // || |
Address Address | // |+---------+
198.51.100.2:49721 192.0.2.15:3478 |+-+ // Peer A
| | ||N| / Host Transport
| +-+ | ||A|/ Address
| | | | v|T| 203.0.113.2:49582
| | | | /+-+
+---------+| | | |+---------+ / +---------+
| || |N| || | // | |
| TURN |v | | v| TURN |/ | |
| Client |----|A|----------| Server |------------------| Peer B |
| | | |^ | |^ ^| |
| | |T|| | || || |
+---------+ | || +---------+| |+---------+
| || | |
| || | |
+-+| | |
| | |
| | |
Client's | Peer B
Server-Reflexive Relayed Transport
Transport Address Transport Address Address
192.0.2.1:7000 192.0.2.15:50000 192.0.2.210:49191
Figure 1
Figure 1 shows a typical deployment. In this figure, the TURN client
and the TURN server are separated by a NAT, with the client on the
private side and the server on the public side of the NAT. This NAT
is assumed to be a "bad" NAT; for example, it might have a mapping
property of "address-and-port-dependent mapping" (see [RFC4787]).
The client talks to the server from a (IP address, port) combination
called the client's HOST TRANSPORT ADDRESS. (The combination of an
IP address and port is called a TRANSPORT ADDRESS.)
The client sends TURN messages from its host transport address to a
transport address on the TURN server that is known as the TURN SERVER
TRANSPORT ADDRESS. The client learns the TURN server transport
address through some unspecified means (e.g., configuration), and
this address is typically used by many clients simultaneously.
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Since the client is behind a NAT, the server sees packets from the
client as coming from a transport address on the NAT itself. This
address is known as the client's SERVER-REFLEXIVE transport address;
packets sent by the server to the client's server-reflexive transport
address will be forwarded by the NAT to the client's host transport
address.
The client uses TURN commands to create and manipulate an ALLOCATION
on the server. An allocation is a data structure on the server.
This data structure contains, amongst other things, the RELAYED
TRANSPORT ADDRESS for the allocation. The relayed transport address
is the transport address on the server that peers can use to have the
server relay data to the client. An allocation is uniquely
identified by its relayed transport address.
Once an allocation is created, the client can send application data
to the server along with an indication of to which peer the data is
to be sent, and the server will relay this data to the intended peer.
The client sends the application data to the server inside a TURN
message; at the server, the data is extracted from the TURN message
and sent to the peer in a UDP datagram. In the reverse direction, a
peer can send application data in a UDP datagram to the relayed
transport address for the allocation; the server will then
encapsulate this data inside a TURN message and send it to the client
along with an indication of which peer sent the data. Since the TURN
message always contains an indication of which peer the client is
communicating with, the client can use a single allocation to
communicate with multiple peers.
When the peer is behind a NAT, then the client must identify the peer
using its server-reflexive transport address rather than its host
transport address. For example, to send application data to Peer A
in the example above, the client must specify 192.0.2.150:32102 (Peer
A's server-reflexive transport address) rather than 203.0.113.2:49582
(Peer A's host transport address).
Each allocation on the server belongs to a single client and has
exactly one or two relayed transport addresses that is used only by
that allocation. Thus, when a packet arrives at a relayed transport
address on the server, the server knows for which client the data is
intended.
The client may have multiple allocations on a server at the same
time.
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3.1. Transports
TURN, as defined in this specification, always uses UDP between the
server and the peer. However, this specification allows the use of
any one of UDP, TCP, Transport Layer Security (TLS) over TCP or
Datagram Transport Layer Security (DTLS) over UDP to carry the TURN
messages between the client and the server.
+----------------------------+---------------------+
| TURN client to TURN server | TURN server to peer |
+----------------------------+---------------------+
| UDP | UDP |
| TCP | UDP |
| TLS-over-TCP | UDP |
| DTLS-over-UDP | UDP |
+----------------------------+---------------------+
If TCP or TLS-over-TCP is used between the client and the server,
then the server will convert between these transports and UDP
transport when relaying data to/from the peer.
Since this version of TURN only supports UDP between the server and
the peer, it is expected that most clients will prefer to use UDP
between the client and the server as well. That being the case, some
readers may wonder: Why also support TCP and TLS-over-TCP?
TURN supports TCP transport between the client and the server because
some firewalls are configured to block UDP entirely. These firewalls
block UDP but not TCP, in part because TCP has properties that make
the intention of the nodes being protected by the firewall more
obvious to the firewall. For example, TCP has a three-way handshake
that makes in clearer that the protected node really wishes to have
that particular connection established, while for UDP the best the
firewall can do is guess which flows are desired by using filtering
rules. Also, TCP has explicit connection teardown; while for UDP,
the firewall has to use timers to guess when the flow is finished.
TURN supports TLS-over-TCP transport and DTLS-over-UDP transport
between the client and the server because (D)TLS provides additional
security properties not provided by TURN's default digest
authentication; properties that some clients may wish to take
advantage of. In particular, (D)TLS provides a way for the client to
ascertain that it is talking to the correct server, and provides for
confidentiality of TURN control messages. If (D)TLS transport is
used between the TURN client and the TURN server, the cipher suites,
server certificate validation and authentication of TURN server are
discussed in Section 6.2.3 of [I-D.ietf-tram-stunbis]. The guidance
given in [RFC7525] MUST be followed to avoid attacks on (D)TLS. TURN
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does not require (D)TLS because the overhead of using (D)TLS is
higher than that of digest authentication; for example, using (D)TLS
likely means that most application data will be doubly encrypted
(once by (D)TLS and once to ensure it is still encrypted in the UDP
datagram).
There is an extension to TURN for TCP transport between the server
and the peers [RFC6062]. For this reason, allocations that use UDP
between the server and the peers are known as UDP allocations, while
allocations that use TCP between the server and the peers are known
as TCP allocations. This specification describes only UDP
allocations.
In some applications for TURN, the client may send and receive
packets other than TURN packets on the host transport address it uses
to communicate with the server. This can happen, for example, when
using TURN with ICE. In these cases, the client can distinguish TURN
packets from other packets by examining the source address of the
arriving packet: those arriving from the TURN server will be TURN
packets. The algorithm of demultiplexing packets received from
multiple protocols on the host transport address is discussed in
[RFC7983].
3.2. Allocations
To create an allocation on the server, the client uses an Allocate
transaction. The client sends an Allocate request to the server, and
the server replies with an Allocate success response containing the
allocated relayed transport address. The client can include
attributes in the Allocate request that describe the type of
allocation it desires (e.g., the lifetime of the allocation). Since
relaying data has security implications, the server requires that the
client authenticate itself, typically using STUN's long-term
credential mechanism or the STUN Extension for Third-Party
Authorization [RFC7635], to show that it is authorized to use the
server.
Once a relayed transport address is allocated, a client must keep the
allocation alive. To do this, the client periodically sends a
Refresh request to the server. TURN deliberately uses a different
method (Refresh rather than Allocate) for refreshes to ensure that
the client is informed if the allocation vanishes for some reason.
The frequency of the Refresh transaction is determined by the
lifetime of the allocation. The default lifetime of an allocation is
10 minutes -- this value was chosen to be long enough so that
refreshing is not typically a burden on the client, while expiring
allocations where the client has unexpectedly quit in a timely
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manner. However, the client can request a longer lifetime in the
Allocate request and may modify its request in a Refresh request, and
the server always indicates the actual lifetime in the response. The
client must issue a new Refresh transaction within "lifetime" seconds
of the previous Allocate or Refresh transaction. Once a client no
longer wishes to use an allocation, it should delete the allocation
using a Refresh request with a requested lifetime of 0.
Both the server and client keep track of a value known as the
5-TUPLE. At the client, the 5-tuple consists of the client's host
transport address, the server transport address, and the transport
protocol used by the client to communicate with the server. At the
server, the 5-tuple value is the same except that the client's host
transport address is replaced by the client's server-reflexive
address, since that is the client's address as seen by the server.
Both the client and the server remember the 5-tuple used in the
Allocate request. Subsequent messages between the client and the
server use the same 5-tuple. In this way, the client and server know
which allocation is being referred to. If the client wishes to
allocate a second relayed transport address, it must create a second
allocation using a different 5-tuple (e.g., by using a different
client host address or port).
NOTE: While the terminology used in this document refers to
5-tuples, the TURN server can store whatever identifier it likes
that yields identical results. Specifically, an implementation
may use a file-descriptor in place of a 5-tuple to represent a TCP
connection.
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TURN TURN Peer Peer
client server A B
|-- Allocate request --------------->| | |
| (invalid or missing credentials) | | |
| | | |
|<--------------- Allocate failure --| | |
| (401 Unauthenticated) | | |
| | | |
|-- Allocate request --------------->| | |
| (valid credentials) | | |
| | | |
|<---------- Allocate success resp --| | |
| (192.0.2.15:50000) | | |
// // // //
| | | |
|-- Refresh request ---------------->| | |
| | | |
|<----------- Refresh success resp --| | |
| | | |
Figure 2
In Figure 2, the client sends an Allocate request to the server with
invalid or missing credentials. Since the server requires that all
requests be authenticated using STUN's long-term credential
mechanism, the server rejects the request with a 401 (Unauthorized)
error code. The client then tries again, this time including
credentials. This time, the server accepts the Allocate request and
returns an Allocate success response containing (amongst other
things) the relayed transport address assigned to the allocation.
Sometime later, the client decides to refresh the allocation and thus
sends a Refresh request to the server. The refresh is accepted and
the server replies with a Refresh success response.
3.3. Permissions
To ease concerns amongst enterprise IT administrators that TURN could
be used to bypass corporate firewall security, TURN includes the
notion of permissions. TURN permissions mimic the address-restricted
filtering mechanism of NATs that comply with [RFC4787].
An allocation can have zero or more permissions. Each permission
consists of an IP address and a lifetime. When the server receives a
UDP datagram on the allocation's relayed transport address, it first
checks the list of permissions. If the source IP address of the
datagram matches a permission, the application data is relayed to the
client, otherwise the UDP datagram is silently discarded.
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A permission expires after 5 minutes if it is not refreshed, and
there is no way to explicitly delete a permission. This behavior was
selected to match the behavior of a NAT that complies with [RFC4787].
The client can install or refresh a permission using either a
CreatePermission request or a ChannelBind request. Using the
CreatePermission request, multiple permissions can be installed or
refreshed with a single request -- this is important for applications
that use ICE. For security reasons, permissions can only be
installed or refreshed by transactions that can be authenticated;
thus, Send indications and ChannelData messages (which are used to
send data to peers) do not install or refresh any permissions.
Note that permissions are within the context of an allocation, so
adding or expiring a permission in one allocation does not affect
other allocations.
3.4. Send Mechanism
There are two mechanisms for the client and peers to exchange
application data using the TURN server. The first mechanism uses the
Send and Data methods, the second mechanism uses channels. Common to
both mechanisms is the ability of the client to communicate with
multiple peers using a single allocated relayed transport address;
thus, both mechanisms include a means for the client to indicate to
the server which peer should receive the data, and for the server to
indicate to the client which peer sent the data.
The Send mechanism uses Send and Data indications. Send indications
are used to send application data from the client to the server,
while Data indications are used to send application data from the
server to the client.
When using the Send mechanism, the client sends a Send indication to
the TURN server containing (a) an XOR-PEER-ADDRESS attribute
specifying the (server-reflexive) transport address of the peer and
(b) a DATA attribute holding the application data. When the TURN
server receives the Send indication, it extracts the application data
from the DATA attribute and sends it in a UDP datagram to the peer,
using the allocated relay address as the source address. Note that
there is no need to specify the relayed transport address, since it
is implied by the 5-tuple used for the Send indication.
In the reverse direction, UDP datagrams arriving at the relayed
transport address on the TURN server are converted into Data
indications and sent to the client, with the server-reflexive
transport address of the peer included in an XOR-PEER-ADDRESS
attribute and the data itself in a DATA attribute. Since the relayed
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transport address uniquely identified the allocation, the server
knows which client should receive the data.
Some ICMP (Internet Control Message Protocol) packets arriving at the
relayed transport address on the TURN server may be converted into
Data indications and sent to the client, with the transport address
of the peer included in an XOR-PEER-ADDRESS attribute and the ICMP
type and code in a ICMP attribute. ICMP attribute forwarding always
uses Data indications containing the XOR-PEER-ADDRESS and ICMP
attributes, even when using the channel mechanism to forward UDP
data.
Send and Data indications cannot be authenticated, since the long-
term credential mechanism of STUN does not support authenticating
indications. This is not as big an issue as it might first appear,
since the client-to-server leg is only half of the total path to the
peer. Applications that want end-to-end security should encrypt the
data sent between the client and a peer.
Because Send indications are not authenticated, it is possible for an
attacker to send bogus Send indications to the server, which will
then relay these to a peer. To partly mitigate this attack, TURN
requires that the client install a permission towards a peer before
sending data to it using a Send indication. The technique to fully
mitigate the attack is discussed in Section 21.1.4.
TURN TURN Peer Peer
client server A B
| | | |
|-- CreatePermission req (Peer A) -->| | |
|<-- CreatePermission success resp --| | |
| | | |
|--- Send ind (Peer A)-------------->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<-------------- Data ind (Peer A) --| | |
| | | |
| | | |
|--- Send ind (Peer B)-------------->| | |
| | dropped | |
| | | |
| |<== data ==================|
| dropped | | |
| | | |
Figure 3
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In Figure 3, the client has already created an allocation and now
wishes to send data to its peers. The client first creates a
permission by sending the server a CreatePermission request
specifying Peer A's (server-reflexive) IP address in the XOR-PEER-
ADDRESS attribute; if this was not done, the server would not relay
data between the client and the server. The client then sends data
to Peer A using a Send indication; at the server, the application
data is extracted and forwarded in a UDP datagram to Peer A, using
the relayed transport address as the source transport address. When
a UDP datagram from Peer A is received at the relayed transport
address, the contents are placed into a Data indication and forwarded
to the client. Later, the client attempts to exchange data with Peer
B; however, no permission has been installed for Peer B, so the Send
indication from the client and the UDP datagram from the peer are
both dropped by the server.
3.5. Channels
For some applications (e.g., Voice over IP), the 36 bytes of overhead
that a Send indication or Data indication adds to the application
data can substantially increase the bandwidth required between the
client and the server. To remedy this, TURN offers a second way for
the client and server to associate data with a specific peer.
This second way uses an alternate packet format known as the
ChannelData message. The ChannelData message does not use the STUN
header used by other TURN messages, but instead has a 4-byte header
that includes a number known as a channel number. Each channel
number in use is bound to a specific peer and thus serves as a
shorthand for the peer's host transport address.
To bind a channel to a peer, the client sends a ChannelBind request
to the server, and includes an unbound channel number and the
transport address of the peer. Once the channel is bound, the client
can use a ChannelData message to send the server data destined for
the peer. Similarly, the server can relay data from that peer
towards the client using a ChannelData message.
Channel bindings last for 10 minutes unless refreshed -- this
lifetime was chosen to be longer than the permission lifetime.
Channel bindings are refreshed by sending another ChannelBind request
rebinding the channel to the peer. Like permissions (but unlike
allocations), there is no way to explicitly delete a channel binding;
the client must simply wait for it to time out.
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TURN TURN Peer Peer
client server A B
| | | |
|-- ChannelBind req ---------------->| | |
| (Peer A to 0x4001) | | |
| | | |
|<---------- ChannelBind succ resp --| | |
| | | |
|-- (0x4001) data ------------------>| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<------------------ (0x4001) data --| | |
| | | |
|--- Send ind (Peer A)-------------->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<------------------ (0x4001) data --| | |
| | | |
Figure 4
Figure 4 shows the channel mechanism in use. The client has already
created an allocation and now wishes to bind a channel to Peer A. To
do this, the client sends a ChannelBind request to the server,
specifying the transport address of Peer A and a channel number
(0x4001). After that, the client can send application data
encapsulated inside ChannelData messages to Peer A: this is shown as
"(0x4001) data" where 0x4001 is the channel number. When the
ChannelData message arrives at the server, the server transfers the
data to a UDP datagram and sends it to Peer A (which is the peer
bound to channel number 0x4001).
In the reverse direction, when Peer A sends a UDP datagram to the
relayed transport address, this UDP datagram arrives at the server on
the relayed transport address assigned to the allocation. Since the
UDP datagram was received from Peer A, which has a channel number
assigned to it, the server encapsulates the data into a ChannelData
message when sending the data to the client.
Once a channel has been bound, the client is free to intermix
ChannelData messages and Send indications. In the figure, the client
later decides to use a Send indication rather than a ChannelData