MMUSIC Thomas Zeng
Internet-Draft PacketVideo Network Solutions
Expires: Aug 8, 2004 Jan 8, 2004
Mapping ICE (Interactive Connectivity Establishment) to RTSP
<draft-zeng-mmusic-map-ice-rtsp-00.txt>
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This memo describes a mapping from ICE (Interactive Connectivity
Establishment) to RTSP for the purpose of Network Address
Translator (NAT) traversal for RTSP protocol. In order to become
compatible with ICE, the Transport header in RTSP is extended with
new syntax elements. This memo presents a few examples RTSP
coversations that uses ICE for NAT/firewall traversals.
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1. Introduction
ICE protocol is a proposed framework for NAT and firewall traversal.
In [1], the parameters for various ICE messages are defined in generic
XML syntax. Each multimedia signalling protocol needs to map these
parameters to its own protocol parameters. Section 9 of [1] provides
a mapping for SIP
(Session Initiation Protocol) based on the SDP Offer/Answer model.
This memo provides a mapping for RTSP (Real-Time Streaming Protocol).
Unlike SIP, RTSP is a multimedia signalling protcol
that does not follow the SDP Offer/Answer model defined in RFC3264,
for historical reasons.
It is therefore necessary to extend the Transport header in RTSP
with new syntax elements in order to fully implement ICE features.
The readers of this memo are expected to have read [1]
(especially sections 5 and 9) and have gained a reasonable
understand of ICE framework.
RTSP differs from SIP in that RTSP server and RTSP client are almost
never deployed behind different NAT/firewalls at the same time.
That is, either RTSP server or RTSP client is in the open.
The examples in this memo limit the traversal problem to
1) RTSP server in the open;
2) RTSP client in the open.
In such cases, TURN services are not required for connectivity
establishment between RTSP server and client.
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2. Mapping ICE to RTSP
2.1 How Does ICE Work For RTSP: an overview
The key assumption in ICE is that a signalling entity cannot know,
apriori,
whether the peer it wishes to communicate with is connected to one or
all of the address realms it is in. Therefore, in order to
communicate, it has to try them all, and choose the best one that
works. This assumption is true for RTSP.
As described in figure 1, section 2 of [1], in terms of signalling
model for RTSP, the initiator is the RTSP
client, the responder is the RTSP server, the initiate message is a
SETUP message, and the accept message is a SETUP response. The modify
message is a SETUP message, and the modify acceptance message is a
SETUP response.
It is also an option to treat the DESCRIBE response from RTSP server
to RTSP client as another initiate message in the ICE context. For
RTSP, DESCRIBE response normally carries the session description
in SDP format. It is in this SDP that the RTSP server may use the
SDP extension in section 9 of [1] to inform its client of the
addresses, ports and associated parameters (e.g., user name, password)
that the server has discovered. However, since not all RTSP
sessions begin with DESCRIBE (many rely on ftp or HTTP protocols to
obtain session descriptions out of band), in the rest of this memo,
we will only consider SETUP as the initiate message, even though
starting ICE process with DESCRIBE response can save up to one
round of ICE negotiations.
Here is how ICE would work with RTSP.
Before the RTSP client establishes a session, it obtains as many IP
address and port combinations in as many address realms as it can.
Any protocol that provides a
client with an IP address and port on which the RTSP client
can receive traffic
can be used. These include STUN and even VPN. The RTSP
client also uses any local interface addresses. A dual-stack v4/v6
client will obtain both a v6 and a v4 address/port. The only
requirement is that, across all of these addresses, the RTSP client
can be certain that at least one of them will work for any
responder it might communicate with. This is guaranteed by:
1) The assumption that the RTSP client and server are separated
by at most one level of NAT/firewall;
2) The assumption that co-located STUN servers can be installed
on the media ports in each protocol entity.
The RTSP client then makes a STUN server available on each of the
address/port combinations it has obtained. This STUN server is
running locally, on the initiator. All of these addresses are placed
into the Tranport header of the SETUP request and they are ordered in
terms of preference given in [1]. The SETUP request also conveys
the STUN username and password which are required to gain access to
the STUN server on each address/port combination. Tranport header
extensions are described in the next section to convey username and
password.
The initiate message -- the SETUP request,
is sent to the responder(normally RTSP server)
via the RTSP connection, preferably using
a secure protocol such as TLS.
Once the RTSP server receives the SETUP request,
it sends STUN requests to each alternate address/
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port in the Transport. These STUN requests include the
username and password obtained from the initiate message.
The STUN requests serve two purposes. The first
is to check for connectivity. If a response is received, the
RTSP server knows that it can reach the client at that address. The
second purpose is to obtain more addresses at which the RTSP server
can be contacted. If the client is behind a NAT,
the RTSP server may discover another address through the STUN
responses. In its accept message -- 200 OK Setup response, the
RTSP server includes all
addresses that it can unilaterally determine (just as the client
did), in addition to any that were discovered using the STUN messages
to the RTSP client.
When the accept message arrives at the RTSP client, the client
performs a similar operation. Using STUN, it checks connectivity to
each of the addresses in the accept message. Through the STUN
responses, it may learn of additional addresses that it can use to
receive media. If it does learn any new address, the clinet generates
a modify message to pass
this address to the RTSP server. For RTSP, modify message is
re-SETUP request.
The RTSP server processes the re-SETUP request as a "ICE Modify"
message
and sends a "200 OK" SETUP response as the "ICE Modify response"
message.
At this point, ICE process is complete, or else connection
cannot be established.
2.1 Extending RTSP Transport Header Syntax
In order to convey username and password used to access colocated
STUN servers, it is necessary to extend the RTSP Transport header
definitions in [4].
Transport = "Transport" ":" 1#transport-spec
transport-spec = transport-id *parameter
transport-id = transport-protocol "/" profile
["/" lower-transport]
; no LWS is allowed inside transport-id
transport-protocol = "RTP" / token
profile = "AVP" / token
lower-transport = "TCP" / "UDP" / token
parameter = ";" ( "unicast" / "multicast" )
...
...
/ ";" "dest_addr" "=" addr-list
/ ";" "src_addr" "=" addr-list
/ ";" "username" "=" non-ws-string
/ ";" "password" "=" non-ws-string
; the above two are new parameters for ICE
/ ";" trn-parameter-extension
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3. Terminology
Several new terms are introduced in [1] and elaborated in this memo:
Session Initiator: A software entity that, at the request of a user,
tries to establish communications with another entity, called the
session responder. A session initiator is also called an
initiator. In RTSP context, initiator is normally the RTSP client.
Initiator: Another term for a session initiator.
Session Responder: A software entity that receives a request for
establishment of communications from the session initiator, and
either accepts or declines the request. A session responder is
also called a responder. In RTSP context, a session responder
is normally the RTSP server.
Responder: Another term for a session responder.
Initiate Message: The signaling message used by an initiator to
establish communications. It contains capabilities and other
information needed by the responder to send media to the
initiator.
Accept Message: The signaling message used by a responder to agree to
communications. It contains capabilities and other information
needed by the initiator to send media to the responder.
Modify Message: The signaling message used by either an initiator or
responder to change the capability and other information needed by
the peer for sending media.
Modify Acceptance Message: The signaling message used by a client to
agree to the changes proposed in a modify message, and to present
the capability or other information needed by its peer for sending
media.
Protocol Entity: either side of the media stream. For RTSP, a
protocol entity is either the RTSP server or the RTSP client.
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Terminate Message The signaling message used by a client to terminate
the session and associated media streams.
Transport Address: The combination of an IP address and port.
Local Transport Address: A local transport address is transport
address that has been allocated from the operating system on the
host. This includes transport addresses obtained through VPNs, and
also transport addresses obtained through RSIP (which lives at the
operating system level). Transport addresses are typically
obtained by binding to an interface.
Derived Transport Address: A derived transport address is a transport
address which is associated with, but different from, a local
transport address. The derived transport address is associated
with the local transport address in that packets sent to the
derived transport address are received on the socket bound to that
local transport address. Derived addresses are obtained using
protocols like STUN and TURN, and more generally, any UNSAF
protocol [11].
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4. Example RTSP Conversations
The examples below follow the RTSP ABNF rules in [4]. It is worth
noting that some of the syntax elements, such as "dest_addr" and
"src_addr", are new to [4], and were not in RFC2326.
4.1 Case 1: RTSP server is in the open; client behind cone NAT
The following sample RTSP conversation describes how ICE
traverses a "cone" NAT on behalf of an RTSP client behind NAT. In this
example, RTSP server is in the public address realm, which is
true for most RTSP serive deployment to-date.
Recall from [5] the definition of cone NATs:
Full Cone: A full cone NAT is one where all requests from the same
internal IP address and port are mapped to the same external IP
address and port. Furthermore, any external host can send a packet to
the internal host, by sending a packet to the mapped external
address.
Restricted Cone: A restricted cone NAT is one where all requests from
the same internal IP address and port are mapped to the same external
IP address and port. Unlike a full cone NAT, an external host (with
IP address X) can send a packet to the internal host only if the
internal host had previously sent a packet to IP address X.
We assume that the RTSP clinet behind this cone NAT
obtains its external IP (i.e., 24.2.1.1) and port apriori, using a
public STUN server. The RTSP client's first SETUP request includes
two choices of addresses for RTP/RTCP ports, as shown by the
relevant RTSP conversation below:
C->S SETUP rtsp://foo.com/test.wav/streamid=0 RTSP/1.0
Transport: RTP/AVP/UDP;unicast;src_addr="172.16.1.1:6970"/
"172.16.1.1:6971"; username="foo"; password="x",
RTP/AVP/UDP;unicast;src_addr="24.2.1.1:9970"/
"24.2.1.1:9980"; username="server"; password="s"
CSeq: 2
S->C RTSP/1.0 200 OK
Transport: RTP/AVP/UDP;unicast;dest_addr="24.2.1.1:9970"/
"24.2.1.1:9980"; src_addr="24.2.8.8:5540"/
"24.2.8.8:5541"; username="client"; password="c"
CSeq: 2
Session: 2034820394
Comments: in the first SETUP request message, there are two tranport
specifications, separated by a comma as per [4].
The first transport uses local address and port, while the second uses STUN
discovered public address and port. In the 200 OK response, the presence
of "dest_addr" parameter indicates that RTSP server has completed
its ICE process after successful STUN bindings.
Finally RTSP client performs STUN bindings against the RTSP server
using the "src_addr", username and password in the SETUP request, and
receives STUN responses.
In this example, connectivity is established in only one round of ICE
negotiation, thanks to the fact that STUN binding is performed approri.
A nice benefit is that RTSP conversational
delay is not increased by much. But connectivity may not always be
established in one SETUP / Response cycle.
In the case of symetric NAT, STUN binding must be done
during RTSP conversations, not before, as shown by the next example.
4.2 Case 2: RTSP server is in the open; client behind Symetric NAT
In this case, obtaining external IP address and port
appriori is of no value,
given the symetric nature of the NAT. Therefore, the RTSP client does
not list any public address in its first SETUP request.
C->S SETUP rtsp://foo.com/test.wav/streamid=0 RTSP/1.0
Transport: RTP/AVP/UDP;unicast;src_addr="172.16.1.1:6970"/
"172.16.1.1:6971"; username="foo"; password="x"
CSeq: 2
/* RTSP server cannot reach "172.16.1.1". The server's STUN binding
request will timeout, and it then sends the following response.
The lessen here is that STUN binding timeout should be set to
a fairly short value so as to minimize the impact on RTSP delay. */
S->C RTSP/1.0 200 OK
Transport: RTP/AVP/UDP;unicast; src_addr="24.2.8.8:5540"/
"24.2.8.8:5541"; username="client"; password="c"
CSeq: 2
Session: 2034820394
/* RTSP client now performs STUN bindings and finds its external
address/port pair as, say, "24.2.1.1:6970"/"24.2.1:6971",
it then sends re-SETUP as ICE modify message: */
C->S SETUP rtsp://foo.com/test.wav/streamid=0 RTSP/1.0
Transport: RTP/AVP/UDP;unicast;src_addr="172.16.1.1:6970"/
"172.16.1.1:6971"; username="foo"; password="x"
RTP/AVP/UDP;unicast;src_addr="24.2.1.1:6970"/
"24.2.1.1:6971"; username="server"; password="s"
CSeq: 3
Session: 2034820394
/* RTSP server can reach 24.2.1.1. So it sends the following 200 OK: */
S->C RTSP/1.0 200 OK
Transport: RTP/AVP/UDP;unicast;dest_addr="24.2.1.1:6970"/
"24.2.1.1:6971"; src_addr="24.2.8.8:5540"/
"24.2.8.8:5541"; username="client"; password="c"
CSeq: 3
Session: 2034820394
Comments: RTSP SETUP delay has been increased in this case by two factors,
when compared to case 1:
1) STUN timeout after the first SETUP request is received by RTSP
server.
2) Additoinal SETUP/response round trip.
Case 3: RTSP client is in the open, RTSP server is behind symetric NAT
In this scenario, client has only one address to include in its
SETUP request.
C->S SETUP rtsp://foo.com/test.wav/streamid=0 RTSP/1.0
Transport: RTP/AVP/UDP;unicast;src_addr="24.2.1.1:6970"/
"24.2.1.1:6971"; username="server";
password="x"
CSeq: 2
/* RTSP server's STUN packets can reach "24.2.1.1" and discover its
own external IP/port as 24.2.8.8/5540 and 24.2.8.8/5541 (RTCP). */
S->C RTSP/1.0 200 OK
Transport: RTP/AVP/UDP;unicast; dest_addr="24.2.1.1:6970"/
"24.2.1.1:6971"; src_addr="24.2.8.8:5540"/
"24.2.8.8:5541"; username="client"; password="c"
CSeq: 2
Session: 2034820394
Here no additional RTSP message exchange is needed.
5. Security Considerations
The sections titled "security considerations" in [1] and [4] covers
all the security considerations relevant to this memo. No additional
consideration is deemed necessary.
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Normative References
[1] Rosenberg, J., " Interactive Connectivity Establishment (ICE):
A Methodology for Network Address Translator (NAT) Traversal ",
draft-ietf-mmusic-ice-00, October 2003.
[2] Rosenberg, J., Weinberger, J., Huitema, C. and R. Mahy, "STUN -
Simple Traversal of User Datagram Protocol (UDP) Through Network
Address Translators (NATs)", RFC 3489, March 2003.
[3] Camarillo, G. and J. Rosenberg, "The Alternative Semantics for
the Session Description Protocol Grouping Framework",
draft-camarillo-mmusic-alt-01 (work in progress), June 2003.
[4] H. Schulzrinne, et. al., "Real Time Streaming Protocol (RTSP)",
draft-ietf-mmusic-rfc2326bis-05.txt, Oct 2003
[5] Westlunder, M. and Zeng, T., "How to make Real-Time
Streaming Protocol (RTSP) traverse Network
Address Translators (NAT) and interact with Firewalls",
draft-ietf-mmusic-rtsp-nat-01.txt, May 2003
[6] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235, January 2002.
[7] Borella, M., Lo, J., Grabelsky, D. and G. Montenegro, "Realm
Specific IP: Framework", RFC 3102, October 2001.
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Author's Address
Thomas Zeng
PV Network Solutions,
10350 Science Center Dr., Suite 200
San Diego, CA92127
US
Phone: +1 858 731 5465
EMail: zeng@pv.com
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