draft-ietf-avt-app-rtp-keepalive-09.txt   draft-ietf-avt-app-rtp-keepalive-10.txt 
Network Working Group X. Marjou Network Working Group X. Marjou
Internet-Draft A. Sollaud Internet-Draft A. Sollaud
Intended status: Standards Track France Telecom Orange Intended status: Standards Track France Telecom Orange
Expires: March 28, 2011 September 24, 2010 Expires: September 5, 2011 March 4, 2011
Application Mechanism for keeping alive the Network Address Translator Application Mechanism for keeping alive the Network Address Translator
(NAT) mappings associated to RTP flows. (NAT) mappings associated to RTP/RTCP flows.
draft-ietf-avt-app-rtp-keepalive-09 draft-ietf-avt-app-rtp-keepalive-10
Abstract Abstract
This document lists the different mechanisms that enable applications This document lists the different mechanisms that enable applications
using Real-time Transport Protocol (RTP) to maintain their RTP using Real-time Transport Protocol (RTP) and RTP control protocol
Network Address Translator (NAT) mappings alive. It also makes a (RTCP) to maintain their RTP Network Address Translator (NAT)
recommendation for a preferred mechanism. This document is not mappings alive. It also makes a recommendation for a preferred
applicable to Interactive Connectivity Establishment (ICE) agents. mechanism. This document is not applicable to Interactive
Connectivity Establishment (ICE) agents.
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 28, 2011. This Internet-Draft will expire on September 5, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. List of Alternatives for Performing RTP Keepalive . . . . . . 5 4. List of Alternatives for Performing RTP Keepalive . . . . . . 5
4.1. Transport Packet of 0-byte . . . . . . . . . . . . . . . . 5 4.1. Transport Packet of 0-byte . . . . . . . . . . . . . . . . 5
4.2. RTP Packet with Comfort Noise Payload . . . . . . . . . . 5 4.2. RTP Packet with Comfort Noise Payload . . . . . . . . . . 5
4.3. RTCP Packets Multiplexed with RTP Packets . . . . . . . . 6 4.3. RTCP Packets Multiplexed with RTP Packets . . . . . . . . 5
4.4. STUN Indication Packet . . . . . . . . . . . . . . . . . . 6 4.4. STUN Indication Packet . . . . . . . . . . . . . . . . . . 6
4.5. RTP Packet with Incorrect Version Number . . . . . . . . . 6 4.5. RTP Packet with Incorrect Version Number . . . . . . . . . 6
4.6. RTP Packet with Unknown Payload Type . . . . . . . . . . . 6 4.6. RTP Packet with Unknown Payload Type . . . . . . . . . . . 6
5. Recommended Solution for Keepalive Mechanism . . . . . . . . . 7 5. Recommended Solution for Keepalive Mechanism . . . . . . . . . 7
6. Media Format Exceptions . . . . . . . . . . . . . . . . . . . 7 6. Media Format Exceptions . . . . . . . . . . . . . . . . . . . 7
7. Timing and Transport Considerations . . . . . . . . . . . . . 7 7. Timing and Transport Considerations . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 8. RTCP Flow Keepalive . . . . . . . . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative references . . . . . . . . . . . . . . . . . . . 8 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.2. Informative references . . . . . . . . . . . . . . . . . . 9 12.1. Normative references . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 12.2. Informative references . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction 1. Introduction
Documents [RFC4787] and [RFC5382] describe Network Address Translator Documents [RFC4787] and [RFC5382] describe Network Address Translator
(NAT) behaviors and point out that two key aspects of NAT are (NAT) behaviors and point out that two key aspects of NAT are
mappings (a.k.a. bindings) and keeping them refreshed. This mappings (a.k.a. bindings) and keeping them refreshed. This
introduces a derived requirement for applications engaged in a introduces a derived requirement for applications engaged in a
multimedia session involving NAT traversal: they need to generate a multimedia session involving NAT traversal: they need to generate a
minimum of flow activity in order to create NAT mappings and maintain minimum of flow activity in order to create NAT mappings and maintain
them. them.
skipping to change at page 3, line 48 skipping to change at page 3, line 48
o Some RTP payload formats, such as the payload format for text o Some RTP payload formats, such as the payload format for text
conversation [RFC4103], may send packets so infrequently that the conversation [RFC4103], may send packets so infrequently that the
interval exceeds the NAT binding timeouts. interval exceeds the NAT binding timeouts.
To solve these problems, an agent therefore needs to periodically To solve these problems, an agent therefore needs to periodically
send keepalive data within the outgoing RTP session of an RTP media send keepalive data within the outgoing RTP session of an RTP media
stream regardless of whether the media stream is currently inactive, stream regardless of whether the media stream is currently inactive,
sendonly, recvonly or sendrecv, and regardless of the presence or sendonly, recvonly or sendrecv, and regardless of the presence or
value of the bandwidth attribute. value of the bandwidth attribute.
It is important to note that the above examples also require the It is important to note that NAT traversals constraints also usually
agents to use symmetric RTP [RFC4961] in addition to RTP keepalive. require the agents to use Symmetric RTP / RTP Control Protocol (RTCP)
[RFC4961] in addition to RTP keepalive.
This document first states the requirements that must be supported to This document first states the requirements that must be supported to
perform RTP keepalives (Section 3). In a second step, the document perform RTP keepalives (Section 3). In a second step, the document
reports the different mechanisms to overcome this problem reports the different mechanisms to overcome this problem
(Section 4). Section 5 finally states the recommended solution for (Section 4). Section 5 finally states the recommended solution for
RTP keepalive. RTP keepalive. Section 6 discusses some media format exceptions.
Section 7 adds details about timing and transport considerations.
Section 8 documents how to maintain NAT bindings for RTCP.
This document is not applicable to Interactive Connectivity This document is not applicable to Interactive Connectivity
Establishment (ICE) [RFC5245] agents. Indeed, the ICE protocol Establishment (ICE) [RFC5245] agents. Indeed, the ICE protocol
together with Session Traversal Utilities for NAT (STUN) [RFC5389] together with Session Traversal Utilities for NAT (STUN) [RFC5389]
and Traversal Using Relays around NAT (TURN) [RFC5766] solve the and Traversal Using Relays around NAT (TURN) [RFC5766] solve the
overall Network Address Translator (NAT) traversal mechanism of media overall Network Address Translator (NAT) traversal mechanism of media
streams. In the context of RTP media streams, some agents may not streams. In the context of RTP media streams, some agents may not
require all ICE functionalities and may only need a keepalive require all ICE functionalities and may only need a keepalive
mechanism. This document thus applies to such agents, and does not mechanism. This document thus applies to such agents, and does not
apply to agents implementing ICE. apply to agents implementing ICE.
The scope of the draft is also limited to RTP flows. In particular,
this document does not address keepalive activity related to:
o Session signaling flows, such as the Session Initiation Protocol
(SIP).
o RTP Control Protocol (RTCP) flows.
Recall that [RFC3550] recommends a minimum interval of 5
seconds and that "on hold" procedures of [RFC3264] do not
impact RTCP transmissions. Therefore, when in use, there is
always some RTCP flow activity.
Note that if a given media uses a codec that already integrates a Note that if a given media uses a codec that already integrates a
keepalive mechanism, no additional keepalive mechanism is required at keepalive mechanism, no additional keepalive mechanism is required at
the RTP level. the RTP level.
As mentionned in Section 3.5 of [RFC5405] "It is important to note As mentioned in Section 3.5 of [RFC5405] "It is important to note
that keep-alive messages are NOT RECOMMENDED for general use -- they that keepalive messages are NOT RECOMMENDED for general use -- they
are unnecessary for many applications and can consume significant are unnecessary for many applications and can consume significant
amounts of system and network resources." amounts of system and network resources."
2. Terminology 2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED", In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 and "OPTIONAL" are to be interpreted as described in RFC 2119
[RFC2119]. [RFC2119].
skipping to change at page 6, line 9 skipping to change at page 5, line 49
o Comfort Noise needs to be supported by the remote peer. o Comfort Noise needs to be supported by the remote peer.
o Comfort Noise needs to be signalled in SDP offer/answer. o Comfort Noise needs to be signalled in SDP offer/answer.
o The peer is likely to render comfort noise at the other side, so o The peer is likely to render comfort noise at the other side, so
the content of the payload (the noise level) needs to be carefully the content of the payload (the noise level) needs to be carefully
chosen. chosen.
4.3. RTCP Packets Multiplexed with RTP Packets 4.3. RTCP Packets Multiplexed with RTP Packets
The application sends RTCP packets in the RTP media path itself (i.e. The application sends RTCP packets in the RTP media path itself (i.e.
same tuples for both RTP and RTCP packets) [RFC5761]. RTCP packets same tuples for both RTP and RTCP packets) [RFC5761]. RTCP packets
therefore maintain the NAT mappings open. therefore maintain the NAT mappings open as long as the requirements
on parameter selection are fulfilled as discussed in Section 8.
Note: "on hold" procedures of [RFC3264] do not impact RTCP
transmissions.
Cons: Cons:
o Multiplexing RTP and RTCP must be supported by the remote peer. o Multiplexing RTP and RTCP must be supported by the remote peer.
o Some RTCP monitoring tools expect that RTCP packets are not o Some RTCP monitoring tools expect that RTCP packets are not
multiplexed. multiplexed.
o RTCP must be configured so that Tmin value [RFC3550] is lower or
equal to the Tr interval.
4.4. STUN Indication Packet 4.4. STUN Indication Packet
The application sends a STUN [RFC5389] Binding Indication packet as The application sends a STUN [RFC5389] Binding Indication packet as
specified in ICE [RFC5245]. specified in ICE [RFC5245].
Thanks to the RTP validity check, STUN packets will be ignored by the Thanks to the RTP validity check, STUN packets will be ignored by the
RTP stack. RTP stack.
Cons: Cons:
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not understand". not understand".
Cons: Cons:
o [RFC4566] and [RFC3264] mandate not to send media with inactive o [RFC4566] and [RFC3264] mandate not to send media with inactive
and recvonly attributes, however this is mitigated as no real and recvonly attributes, however this is mitigated as no real
media is sent with this mechanism. media is sent with this mechanism.
o [RFC3550] does not preclude examination of received packets by the o [RFC3550] does not preclude examination of received packets by the
peer in an attempt to determine if it is under attack. peer in an attempt to determine if it is under attack.
o The statement "RTP Packet with Unknown Payload Type" of RFC3550 is o The statement "RTP Packet with Unknown Payload Type" of RFC3550 is
not always observed in real life. not always observed in real life.
o There is no RTCP reporting for the keepalive packets as RFC3550
mandates to ignore "RTP Packet with Unknown Payload Type".
o Some RTP payload formats do not handle gaps in RTP sequence number
well.
5. Recommended Solution for Keepalive Mechanism 5. Recommended Solution for Keepalive Mechanism
The RECOMMENDED mechanism is the "RTCP packets multiplexed with RTP The RECOMMENDED mechanism is the "RTCP packets multiplexed with RTP
packets" (Section 4.3). This mechanism is desirable because it packets" (Section 4.3). This mechanism is desirable because it
reduces the number of ports when RTP and RTCP are used. It also has reduces the number of ports when RTP and RTCP are used. It also has
the advantage of taking into account RTCP aspects, which is not the the advantage of taking into account RTCP aspects, which is not the
case of other mechanisms. case of other mechanisms.
Other mechanisms (Section 4.1, Section 4.2, Section 4.4, Section 4.5, Other mechanisms (Section 4.1, Section 4.2, Section 4.4, Section 4.5,
skipping to change at page 8, line 18 skipping to change at page 8, line 19
for keepalive, Tr MUST comply with the RTCP timing rules of for keepalive, Tr MUST comply with the RTCP timing rules of
[RFC3550]. [RFC3550].
Keepalive packets within a particular RTP session MUST use the tuple Keepalive packets within a particular RTP session MUST use the tuple
(source IP address, source TCP/UDP ports, target IP address, target (source IP address, source TCP/UDP ports, target IP address, target
TCP/UDP Port) of the regular RTP packets. TCP/UDP Port) of the regular RTP packets.
The agent SHOULD only send RTP keepalive when it does not send The agent SHOULD only send RTP keepalive when it does not send
regular RTP packets. regular RTP packets.
8. Security Considerations 8. RTCP Flow Keepalive
RTCP packets are sent periodically and can thus normally maintain the
NAT mappings open as long as they are sent frequently enough. There
are two conditions for that. First RTCP needs to be used bi-
directionally and in a symmetric fashion, as described in [RFC4961].
Secondly, RTCP needs to be sent frequently enough. However, there
are certain configurations that can break this latter assumption.
There are two factors that need to be considered to ensure that RTCP
is sent frequently enough. First the RTCP bandwidth needs to be
sufficiently large so that transmission will occur more frequently
than the longest acceptable packet transmission interval (Tr). The
worst case RTCP interval (Twc) can be calculated using this formula
by inserting the max value of the following parameters:
o Maximum RTCP packet size (avg_rtcp_size_max)
o Maximum number of participants (members_max)
o RTCP receiver bandwidth (rtcp_bw)
The RTCP bandwidth value to use here is for a worst case, which will
be the receiver proportion when all members are not senders except
one. This can be approximated to be all members. Thus for sessions
where RR and RS values are used, then rtcp_bw shall be set to RR.
For sessions where the [RFC3550] defines proportions of 1/4 for
sender and 3/4 for receivers are used, then rtcp_bw will be 5% of 3/4
of the AS value in bits per second.
Twc = 1.5 / 1.21828 * members_max * rtcp_bw / avg_rtcp_size_max * 8
The second factor is the minimum RTCP interval Tmin defined in
[RFC3550]. Its base value is 5 seconds, but it might also be scaled
to 360 divided by the session bandwidth in kbps. The Extended RTP
Profile for Real-time Transport Control Protocol (RTCP)-Based
Feedback (RTP/AVPF) [RFC4585] also allows for the setting of a trr-
int parameter which is a minimal RTCP interval for regular RTCP
packets. It is also used as the Tmin value in the regular Td
calculation. An analysis of the algorithm gives that the longest
possible regular RTCP interval possible are:
RTCP_int_max = trr-int * 1.5 + Td * 1.5 / 1.21828
And as long as the there is sufficient bandwidth according to
criteria 1, then this can be simplified by setting Td = trr-int
giving
RTCP_int_max = trr-int * (1.5 + 1.5 / 1.21828) = 2.73123 * trr-int
Thus the requirements on the RTCP parameters are the following for
functioning keepalive:
1. Ensure that sufficient RTCP bandwidth is provided by calculating
Twc and ensure that this is less than or equal to Tr.
2. If AVP or SAVP is used the Tmin value can't be greater that Tr
divided by 1.5 / (e-3/2).
3. If AVPF or SAVPF is to be used trr-min must not be set to a
greater value than Tr / 3.
9. Security Considerations
The RTP keepalive packets are sent on the same path as regular RTP The RTP keepalive packets are sent on the same path as regular RTP
media packets and may be perceived as an attack by a peer. However, media packets and may be perceived as an attack by a peer. However,
[RFC3550] mandates a peer to "ignore packets with payload types that [RFC3550] mandates a peer to "ignore packets with payload types that
it does not understand". A peer that does not understand the it does not understand". A peer that does not understand the
keepalive message will thus appropriately drop the received packets. keepalive message will thus appropriately drop the received packets.
9. IANA Considerations 10. IANA Considerations
None. None.
10. Acknowledgements 11. Acknowledgements
Jonathan Rosenberg provided the major inputs for this draft via the Jonathan Rosenberg provided the major inputs for this draft via the
ICE specification. In addition, thanks to Alfred E. Heggestad, Colin ICE specification. Magnus Westerlund provided the text for the RTCP
Perkins, Dan Wing, Gunnar Hellstrom, Hadriel Kaplan, Magnus flow keepalive section. In addition, thanks to Alfred E. Heggestad,
Westerlund, Randell Jesup, Remi Denis-Courmont, Robert Sparks, and Colin Perkins, Dan Wing, Gunnar Hellstrom, Hadriel Kaplan, Randell
Steve Casner for their useful inputs and comments. Jesup, Remi Denis-Courmont, Robert Sparks, and Steve Casner for their
useful inputs and comments.
11. References 12. References
11.1. Normative references 12.1. Normative references
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003. Applications", STD 64, RFC 3550, July 2003.
[RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)", [RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)",
BCP 131, RFC 4961, July 2007. BCP 131, RFC 4961, July 2007.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines [RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405, for Application Designers", BCP 145, RFC 5405,
November 2008. November 2008.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010. Control Packets on a Single Port", RFC 5761, April 2010.
11.2. Informative references 12.2. Informative references
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261, Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002. June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, with Session Description Protocol (SDP)", RFC 3264,
June 2002. June 2002.
[RFC3389] Zopf, R., "Real-time Transport Protocol (RTP) Payload for [RFC3389] Zopf, R., "Real-time Transport Protocol (RTP) Payload for
Comfort Noise (CN)", RFC 3389, September 2002. Comfort Noise (CN)", RFC 3389, September 2002.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text [RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, June 2005. Conversation", RFC 4103, June 2005.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006. Description Protocol", RFC 4566, July 2006.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation [RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127, (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007. RFC 4787, January 2007.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT) (ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, Traversal for Offer/Answer Protocols", RFC 5245,
April 2010. April 2010.
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P. [RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
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