Internet DRAFT - draft-boucadair-mmusic-altc
draft-boucadair-mmusic-altc
Network Working Group M. Boucadair
Internet-Draft France Telecom
Intended status: Informational H. Kaplan
Expires: August 2, 2013 Acme Packet
R. Gilman
Independent
S. Veikkolainen
Nokia
January 29, 2013
Session Description Protocol (SDP) Alternate Connectivity (ALTC)
Attribute
draft-boucadair-mmusic-altc-09
Abstract
This document proposes a mechanism which allows to carry multiple IP
addresses, of different address families (e.g., IPv4, IPv6), in the
same SDP offer. The proposed attribute solves the backward
compatibility problem which plagued ANAT (Alternative Network Address
Types), due to its syntax.
The proposed solution is applicable to scenarios where connectivity
checks are not required. If connectivity checks are required, ICE
(RFC 5245) provides such a solution.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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 http://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."
Boucadair, et al. Expires August 2, 2013 [Page 1]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
This Internet-Draft will expire on August 2, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Boucadair, et al. Expires August 2, 2013 [Page 2]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Overall Context . . . . . . . . . . . . . . . . . . . . . 4
1.2. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Overview of the ALTC Mechanism . . . . . . . . . . . . . . . . 7
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Rationale for the Chosen Syntax . . . . . . . . . . . . . 8
4. Alternate Connectivity Attribute . . . . . . . . . . . . . . . 8
4.1. ALTC Syntax . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Usage and Interaction . . . . . . . . . . . . . . . . . . 10
4.2.1. Usage . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2.2. Usage of ALTC in an SDP Answer . . . . . . . . . . . . 11
4.2.3. Interaction with ICE . . . . . . . . . . . . . . . . . 11
4.2.4. Interaction with SDP-Cap-Neg . . . . . . . . . . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. ALTC Use Cases . . . . . . . . . . . . . . . . . . . 15
A.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 15
A.2. Multicast Use Case . . . . . . . . . . . . . . . . . . . . 16
A.3. Introducing IPv6 into SIP-based Architectures . . . . . . 17
A.3.1. Avoid Crossing CGN Devices . . . . . . . . . . . . . . 17
A.3.2. Basic Scenario for IPv6 SIP Service Delivery . . . . . 17
A.3.3. Avoid IPv4/IPv6 Interworking . . . . . . . . . . . . . 18
A.3.4. DBE Bypass Procedure . . . . . . . . . . . . . . . . . 20
A.3.5. Direct Communications Between IPv6-enabled User
Agents . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
Boucadair, et al. Expires August 2, 2013 [Page 3]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
1. Introduction
1.1. Overall Context
Due to the IPv4 address exhaustion problem, IPv6 deployment is
becoming an urgent need, along with the need to properly handle IPv6
and IPv4 co-existence. The reality of IPv4-IPv6 co-existence
introduces heterogeneous scenarios with combinations of IPv4 and IPv6
nodes, some of which are capable of supporting both IPv4 and IPv6
dual-stack (DS) and some of which are capable of supporting only IPv4
or only IPv6. In this context, Session Initiation Protocol (SIP
[RFC3261]) User Agents (UAs) need to be able to indicate their
available IP capabilities in order to increase the ability to
establish successful SIP sessions, and also to avoid invocation of
adaptation functions such as Application Layer Gateways (ALGs) and
IPv4-IPv6 interconnection functions (e.g., NAT64 [RFC6146]), and to
avoid using private IPv4 addresses through consumer NATs or Carrier
Grade NATs (CGN, [I-D.ietf-behave-lsn-requirements]).
In the meantime, service providers are investigating scenarios to
upgrade their service offering to be IPv6-capable. The current
strategies involve either offering IPv6 only, for example to mobile
devices, or providing both IPv4 and IPv6 but with private IPv4
addresses which are NAT'ed by CGNs. In the latter case the end
device may be using "normal" IPv4 and IPv6 stacks and interfaces, or
it may tunnel the IPv4 packets though a DS-Lite stack integrated into
the host [RFC6333]; in either case the device has both address
families available from a SIP and media perspective.
Regardless of the IPv6 transition strategy being used, it is obvious
that there will be a need for dual-stack SIP devices to communicate
with IPv4-only legacy UAs, and IPv6-only UAs, and other dual-stack
UAs. It may not, for example, be possible for a dual-stack UA to
communicate with an IPv6-only UA unless the dual-stack UA had a means
of providing the IPv6-only UA with its IPv6 local address for media,
while clearly it needs to provide a legacy IPv4-only device its local
IPv4 address. The communication must be possible in a backwards-
compatible fashion, such that IPv4-only SIP devices need not support
the new mechanism to communicate with dual-stack UAs.
The current means by which multiple address families can be
communicated are through ANAT [RFC4091] or ICE [RFC5245]. ANAT has
serious backwards-compatibility problems as described in [RFC4092],
which effectively make it unusable, and it is deprecated by the IETF
[RFC5245]. ICE at least allows interoperability with legacy devices,
by not doing ICE in such cases, but it is a complicated and
processing intensive mechanism, and has seen limited deployment and
implementation in SIP applications.
Boucadair, et al. Expires August 2, 2013 [Page 4]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
ALTC has been implemented as reported in
[I-D.boucadair-pcp-nat64-experiments]; no issue has been reported in
that document.
1.2. Purpose
This document proposes a new alternative: a backwards-compatible
syntax for indicating multiple media connection addresses and ports
in an SDP offer, which can immediately be selected from and used in
an SDP answer.
The proposed mechanism is independent of the model described in
[RFC5939] and does not require implementation of sdp-capabilities-
negotiations (a.k.a., SDPCapNeg) to function.
It should be noted that "backwards-compatible" in this document
generally refers to working with legacy IPv4-only devices. The
choice has to be made, one way or the other, because to interoperate
with legacy devices requires constructing SDP bodies which they would
understand and support, such that they detect their local address
family in the SDP connection line. It is not possible to support
interworking with both legacy IPv4-only and legacy IPv6-only devices
with the same SDP offer. Clearly, there are far more legacy IPv4-
only devices in existence, and thus those are the ones assumed in
this document. However, the syntax allows for a UA to choose which
address family to be backwards-compatible with, in case it has some
means of determining it.
Furthermore, even for cases where both sides support the same address
family, there should be a means by which the "best" address family
transport is used, based on what the UAs decide. The address family
which is "best" for a particular session cannot always be known a
priori. For example, in some cases the IPv4 transport may be better,
even if both UAs support IPv6.
The proposed solution provides the following benefits:
o Allows a UA to signal more than one IP address (type) in the same
SDP offer/answer;
o Is backwards compatible. No parsing or semantic errors will be
experienced by a legacy UA or intermediary SIP nodes which do not
understand this new mechanism;
o Is as lightweight as possible to achieve the goal, while still
allowing and interoperating with nodes which support other similar
or related mechanisms;
o Is easily deployable in managed networks;
Boucadair, et al. Expires August 2, 2013 [Page 5]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
o Requires minimal increase of the length of the SDP offer (I.e.,
minimizes fragmentation risks).
ALTC may also be useful for the multicast context (e.g., Section 3.4
of [I-D.venaas-behave-v4v6mc-framework] or Section 3.3 of
[I-D.ietf-mboned-multrans-addr-acquisition]).
More detailed information about ALTC use cases is provided in
Appendix A.
1.3. Scope
This document proposes an alternative scheme, as replacement to the
ANAT procedure [RFC4091], to carry several IP address types in the
same SDP offer/answer while preserving backward compatibility.
While clearly two UAs communicating directly at a SIP layer need to
be able to support the same address family for SIP itself, current
SIP deployments almost always have Proxy Servers or B2BUA's in the
SIP signaling path, which can provide the necessary interworking of
the IP address family at the SIP layer (e.g., [RFC6157]). SIP-layer
address family interworking is out of scope of this document.
Instead, this document focuses on the problem of communicating media
address family capabilities in a backwards-compatible fashion. Since
media can go directly between two UAs, without a priori knowledge by
the UAC of which address family the far-end UAS supports, it has to
offer both, in a backwards-compatible fashion.
2. Use Cases
The ALTC mechanism defined in this document is primary meant for
managed networks. In particular, the following use cases were
explicitly considered:
o A dual-stack UAC initiating a SIP session without knowing the
address family of the ultimate target UAS.
o A UA receiving a SIP session request with SDP offer and wishes to
avoid using IPv4, or to avoid IPv6.
o An IPv6-only UA wishes to avoid using a NAT64 [RFC6146].
o A SIP UA behind a Dual-Stack Lite CGN [RFC6333].
o A SIP Service Provider or Enterprise domain of IPv4-only and/or
IPv6-only UA, which provides interworking by invoking IPv4-IPv6
media relays, wishes to avoid invoking such functions and let
media go end-to-end as much as possible.
o A SIP Service Provider or Enterprise domain of a UA, which
communicates with other domains and wishes to either avoid
invoking IPv4-IPv6 interworking or let media go end-to-end as much
Boucadair, et al. Expires August 2, 2013 [Page 6]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
as possible.
o A SIP Service Provider providing transit peering services for SIP
sessions, which may need to modify SDP in order to provide IPv4-
IPv6 interworking, but would prefer to avoid such interworking or
avoid relaying media in general, as much as possible.
o SIP sessions using the new mechanism crossing legacy SDP-aware
middleboxes which may not understand this new mechanism.
3. Overview of the ALTC Mechanism
3.1. Overview
The ALTC mechanism relies solely on the SDP offer/answer mechanism,
with specific syntax to indicate alternative connection addresses.
The basic concept is to use a new SDP attribute "altc", to indicate
the IP addresses for potential alternative connection addresses. The
address which is most likely to get chosen for the session is in the
normal 'c=' line. Typically in current operational networks this
would be an IPv4 address. The "a=altc" lines contain the alternative
addresses offered for this session. This way, a dual-stack UA might
encode its IPv4 address in the "c=" line, while possibly preferring
to use an IPv6 address by explicitly indicating the preference order
in the corresponding "a=altc" line. One of the "a=altc" lines
duplicates the address contained in the "c=" line, for reasons
explained in Section 3.2). The SDP answerer would indicate its
chosen address, by simply using that address family in the "c=" line
of its response.
An example of an SDP offer using this mechanism is as follows when
IPv4 is considered most likely to be used for the session, but IPv6
is preferred:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 12340 RTP/AVP 0 8
a=altc:1 IP6 2001:db8::1 45678
a=altc:2 IP4 192.0.2.1 12340
If IPv6 was considered most likely to be used for the session, the
SDP offer would be as follows:
Boucadair, et al. Expires August 2, 2013 [Page 7]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
v=0
o=- 25678 753849 IN IP6 2001:db8::1
s=
c=IN IP6 2001:db8::1
t=0 0
m=audio 45678 RTP/AVP 0 8
a=altc:1 IP6 2001:db8::1 45678
a=altc:2 IP4 192.0.2.1 12340
Since an alternative address is likely to require an alternative TCP/
UDP port number as well, the new "altc" attribute includes both an IP
address and a receive transport port number (or multiple port
numbers). The ALTC mechanism does not itself support offering a
different transport type (i.e., UDP vs. TCP), codec, nor any other
attribute. It is only intended for offering an alternative IP
address and port number.
3.2. Rationale for the Chosen Syntax
The use of an 'a=' attribute line is, according to [RFC4566], the
primary means for extending SDP and tailoring it to particular
applications or media. A compliant SDP parser will ignore the
unsupported attribute lines.
The rationale for encoding the same address and port in the "a=altc"
line as in the "m=" and "c=" lines is to provide detection of legacy
SDP-changing middleboxes. Such systems may change the connection
address and media transport port numbers, but not support this new
mechanism, and thus two UAs supporting this mechanism would try to
connect to the wrong addresses. Therefore, the rules detailed in
this document require the SDP processor to check for matching altc
and connection line addresses and media ports, before choosing one of
the alternatives.
4. Alternate Connectivity Attribute
4.1. ALTC Syntax
The altc attribute adheres to the [RFC4566] "attribute" production.
The ABNF syntax [RFC5234] of altc is provided below:
altc-attr = "altc" ":" att-value
att-value = altc-num SP addrtype SP connection-address SP port ["/" rtcp-port]
altc-num = 1*DIGIT
rtcp-port = port
Boucadair, et al. Expires August 2, 2013 [Page 8]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
Figure 1: Connectivity Capability Attribute ABNF
The meaning of the fields are listed hereafter:
o altc-num: digit to uniquely refer to an address alternative. It
must be in preference order; "1" indicates the most preferred
address.
o addrtype: the addrtype field as defined in [RFC4566] for
connection data.
o connection-address: a network address as defined in [RFC4566]
corresponding to the address type specified by addrtype.
o port: the port number to be used, as defined in [RFC4566].
Distinct port numbers may be used per IP address type. If the
specified address type does not require a port number, a value
defined for that address type should be used.
o rtcp-port: Including an RTCP port is optional. An RTCP port may
be indicated in the alternative "c=" line when the RTCP port can
not be derived from the RTP port.
The "altc" attribute is only applicable in an SDP offer. The "altc"
attribute is a media-level-only attribute, and MUST NOT appear at the
SDP session level (since it defines a port number, it is inherently
tied to the media level). There MUST NOT be more than one "altc"
attribute per addrtype within each media description. This
restriction is necessary in order that the addrtype of the reply may
be used by the offerer to determine which alternative was accepted.
The <addrtype>'s of the altc MUST correspond to the <nettype> of the
current connection (c=) line.
A media description MUST contain two "altc" attributes: the
alternative address and an alternative port as well as an address and
port which "duplicates" the address/port information from the current
'c=' and 'm=' lines. Each media level MUST contain at least one such
duplicate altc attribute, of the same IP address family, address, and
transport port number as those in the SDP connection and media lines
of its level. In particular, if a 'c=' line appears within a media
description, the addr-type and connection-address from that 'c=' line
MUST be used in the duplicate "altc" attribute for that media
description. If a 'c=' line appears only at the session level and a
given media description does not have its own connection line, then
the duplicate "altc" attribute for that media description MUST be the
same as the session-level address information.
The "altc" attributes appearing within a media description MUST be
prioritized; the explicit preference order is indicated in each line
("1" is used to indicate the address with the highest priority).
Given this rule, and the requirement that the address information
Boucadair, et al. Expires August 2, 2013 [Page 9]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
provided in the "m=" line and "o=" line must be provided in an "altc"
attribute as well, it is possible that the address in the "m=" line
and "o=" line are not the preferred choice.
If the addrtype of an "altc" attribute is not compatible with the
transport protocol or media format specified in the media
description, that altc attribute MUST be ignored.
Note that "a=altc" lines describe alternative connection addresses,
NOT addresses for parallel connections. When several altc lines are
present, multiple sessions establishment MUST be avoided. Only one
session is to be maintained with the remote party for the associated
media description.
If no port number is indicated for the alternative address, the same
port number is used for all address families.
4.2. Usage and Interaction
4.2.1. Usage
In an SDP offer/answer model, the SDP offer includes "altc"
attributes to indicate alternative connection information (i.e.,
address type, address and port number(s)), including the "duplicate"
connection information already identified in the 'c=' and 'm=' lines.
Additional, subsequent offers MAY include "altc" attributes again,
and may change the IP address, port numbers, and order of preference;
but they MUST include a duplicate "altc" attribute for the connection
and media lines in that specific subsequent offer. In other words,
every offered SDP media description with an alternative address offer
with an "altc" attribute has two of them:
- one duplicating the 'c=' and 'm=' line information for that
media description, and
- one for the alternative,
even though these need not be the same as the original SDP offer.
The purpose of encoding a duplicate "altc" attribute is to allow
receivers of the SDP offer to detect if a legacy SDP-changing middle
box has modified the 'c=' and/or 'm=' line address/port information.
If the SDP answerer does not find a duplicate "altc" attribute value
for which the address and port match exactly those in the 'c=' line
and 'm=' line, the SDP answerer MUST ignore the "altc" attributes and
use the 'c=' and 'm=' offered address/ports for the entire SDP
instead, as if no "altc" attributes were present. The rationale for
this is that many SDP-changing middleboxes will end the media
Boucadair, et al. Expires August 2, 2013 [Page 10]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
sessions if they do not detect media flowing through them; if a
middlebox modified the SDP addresses, media MUST be sent using the
modified information.
Note that for RTCP, if applicable for the given media types, each
side would act as if the chosen "altc" attribute's port number was in
the 'm=' media line. Typically, this would mean RTCP is sent to the
odd +1 of the port number, unless some other attribute determines
otherwise. For example the RTP/RTCP multiplexing mechanism defined
in [RFC5761] can still be used with ALTC, such that if both sides
support multiplexing they will indicate so using the 'a=rtcp-mux'
attribute as defined in [RFC5761]; but the IP connection address and
port they use may be chosen using the ALTC mechanism.
If the SDP offerer wishes to use the RTCP attribute defined in
[RFC3605], a complication can arise since it may not be clear which
address choice the 'a=rtcp' attribute applies to, relative the
choices offered by ALTC. Technically RFC 3605 allows indicating the
address for RTCP explicitly in the 'a=rtcp' attribute itself, but
this is optional and rarely used. For this reason, this document
recommends using 'a=rtcp' attribute to be for the address choice
encoded in the "m=" line, and include an alternate RTCP port in the
'a=altc' attribute corresponding to the alternative address. In
other words, if the 'a=rtcp' attribute explicitly encodes an address
in its attribute, then that applies for ALTC as per [RFC3605]; if it
does not, then ALTC assumes the 'a=rtcp' attribute is for the address
in the "m=" line, and the alternative "altc" attribute include an
RTCP alternate port number.
4.2.2. Usage of ALTC in an SDP Answer
The SDP answer SHOULD NOT contain "altc" attributes, as the answer's
'c=' line implicitly and definitively chooses the address family from
the offer and includes it in "c=" and "m=" lines of the answer.
Furthermore, this avoids establishing several sessions simultaneously
between the participating peers.
Any solution requiring the use of ALTC in SDP answer SHOULD document
its usage, in particular how sessions are established between the
participating peers.
4.2.3. Interaction with ICE
Since ICE [RFC5245] also includes address and port number information
in its candidate attributes, a potential problem arises: which one
wins. Since ICE also includes specific ICE attributes in the SDP
answer, the problem is easily avoided: if the SDP offerer supports
both ALTC and ICE, it may include both sets of attributes in the same
Boucadair, et al. Expires August 2, 2013 [Page 11]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
SDP offer. A legacy ICE-only answerer will simply ignore the ALTC
attributes, and use ICE. An ALTC-only answerer will ignore the ICE
attributes and reply without them. An answerer which supports both
MUST choose one and only one of the mechanisms to use: either ICE or
ALTC (unless the 'm=' or 'c=' lines were changed by a middlebox, in
which case the rules for both ALTC and ICE would make the answerer
revert to basic SDP semantics).
4.2.4. Interaction with SDP-Cap-Neg
The ALTC mechanism is orthogonal to SDPCapNeg [RFC5939]. If the
offerer supports both ALTC and SDPCapNeg, it may offer both.
5. IANA Considerations
This document requests the following new SDP attribute:
SDP Attribute ("att-field"):
Attribute name altc
Long form Alternate Connectivity
Type of name att-field
Type of attribute Media level only
Subject to charset No
Purpose See Section 1.2, Section 3
Specification Section 4
The contact person for this registration is Mohamed Boucadair (email:
mohamed.boucadair@orange.com; phone: +33 2 99 12 43 71).
6. Security Considerations
The security implications for ALTC are effectively the same as they
are for SDP in general [RFC4566].
7. Acknowledgements
Many thanks to T. Taylor, F. Andreasen and G. Camarillo for their
review and comments.
8. References
Boucadair, et al. Expires August 2, 2013 [Page 12]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605,
October 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010.
8.2. Informative References
[I-D.boucadair-pcp-nat64-experiments]
Abdesselam, M., Boucadair, M., Hasnaoui, A., and J.
Queiroz, "PCP NAT64 Experiments",
draft-boucadair-pcp-nat64-experiments-00 (work in
progress), September 2012.
[I-D.ietf-behave-lsn-requirements]
Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
and H. Ashida, "Common requirements for Carrier Grade NATs
(CGNs)", draft-ietf-behave-lsn-requirements-10 (work in
progress), December 2012.
[I-D.ietf-mboned-64-multicast-address-format]
Boucadair, M., Qin, J., Lee, Y., Venaas, S., Li, X., and
M. Xu, "IPv6 Multicast Address With Embedded IPv4
Multicast Address",
draft-ietf-mboned-64-multicast-address-format-04 (work in
progress), August 2012.
[I-D.ietf-mboned-multrans-addr-acquisition]
Tsou, T., Clauberg, A., Boucadair, M., Venaas, S., and Q.
Sun, "Address Acquisition For Multicast Content When
Source and Receiver Support Differing IP Versions",
Boucadair, et al. Expires August 2, 2013 [Page 13]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
draft-ietf-mboned-multrans-addr-acquisition-01 (work in
progress), January 2013.
[I-D.ietf-mboned-v4v6-mcast-ps]
Jacquenet, C., Boucadair, M., Lee, Y., Qin, J., Tsou, T.,
and Q. Sun, "IPv4-IPv6 Multicast: Problem Statement and
Use Cases", draft-ietf-mboned-v4v6-mcast-ps-01 (work in
progress), November 2012.
[I-D.venaas-behave-v4v6mc-framework]
Venaas, S., Li, X., and C. Bao, "Framework for IPv4/IPv6
Multicast Translation",
draft-venaas-behave-v4v6mc-framework-03 (work in
progress), June 2011.
[RFC2871] Rosenberg, J. and H. Schulzrinne, "A Framework for
Telephony Routing over IP", RFC 2871, June 2000.
[RFC4091] Camarillo, G. and J. Rosenberg, "The Alternative Network
Address Types (ANAT) Semantics for the Session Description
Protocol (SDP) Grouping Framework", RFC 4091, June 2005.
[RFC4092] Camarillo, G. and J. Rosenberg, "Usage of the Session
Description Protocol (SDP) Alternative Network Address
Types (ANAT) Semantics in the Session Initiation Protocol
(SIP)", RFC 4092, June 2005.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC5853] Hautakorpi, J., Camarillo, G., Penfield, R., Hawrylyshen,
A., and M. Bhatia, "Requirements from Session Initiation
Protocol (SIP) Session Border Control (SBC) Deployments",
RFC 5853, April 2010.
[RFC5939] Andreasen, F., "Session Description Protocol (SDP)
Capability Negotiation", RFC 5939, September 2010.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, April 2011.
[RFC6157] Camarillo, G., El Malki, K., and V. Gurbani, "IPv6
Transition in the Session Initiation Protocol (SIP)",
RFC 6157, April 2011.
Boucadair, et al. Expires August 2, 2013 [Page 14]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
[RFC6333] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4
Exhaustion", RFC 6333, August 2011.
[RFC6406] Malas, D. and J. Livingood, "Session PEERing for
Multimedia INTerconnect (SPEERMINT) Architecture",
RFC 6406, November 2011.
Appendix A. ALTC Use Cases
A.1. Terminology
The following terms are used:
o SBE (Signaling Path Border Element) denotes a functional element,
located at the boundaries of an ITAD (IP Telephony Administrative
Domain, [RFC2871]), which is responsible for intercepting
signaling flows received from User Agents and relay them to the
core service platform. A SBE may be located at the access segment
(i.e., be the service contact point for User Agents) or be located
at the interconnection with adjacent domains ([RFC6406]). A SBE
controls one or more DBEs. SBE and DBE may be located in the same
device (e.g., SBC [RFC5853]) or be separated.
o DBE (Data Path Border Element) denotes a functional element,
located at the boundaries of an ITAD, which is responsible for
intercepting media/data flows received from User Agents and relay
them to another DBE (or media servers, e.g., announcement server
or IVR). An example of DBE is a media gateway intercepting RTP
flows. SBE may be located at the access segment (i.e., be the
service contact point for User Agents) or be located at the
interconnection with adjacent domains ([RFC6406]).
o Core service platform is a macro functional block including
session routing, interfaces to advanced services and access
control. Figure 2 provides an overview of the overall
architecture including SBE, DBE and Core service platform.
Boucadair, et al. Expires August 2, 2013 [Page 15]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
+----------+
| Core SIP |
+--------->| SPF |<---------+
| SIP +----------+ SIP |
v v
+-----------+ +-----------+
+-----+ SIP | SBE | | SBE | SIP
| S |<----->| | | |<----->
| I | +-----------+ +-----------+
| P | || ||
| | +-----------+ +-----------+
| U | RTP | DBE | RTP | DBE | RTP
| A |<----->| |<----------------->| | <----->
+-----+ +-----------+ +-----------+
SIP UA can be embedded in the CPE or in a host behind the CPE
Figure 2: Service Architecture: Overview
A.2. Multicast Use Case
Recently, a significant effort has been undertaken within IETF to
specify new mechanisms to interconnect IPv6-only hosts to IPv4-only
servers (e.g., [RFC6146]). This effort covered exclusively unicast
transfer mode. An ongoing initiative, called multrans, has been
launched to cover multicast issues to be encountered during IPv6
transition. The overall problem statement is documented in
[I-D.ietf-mboned-v4v6-mcast-ps].
A particular issue encountered in the context of IPv4/IPv6 co-
existence and IPv6 transition of multicast services is the discovery
of multicast group and source (refer to Section 3.4 of
[I-D.ietf-mboned-v4v6-mcast-ps]):
1. An IPv6-only receiver requesting multicast content generated by
an IPv4-only source:
(1.1) An ALG is required to help an IPv6 receiver to select the
appropriate IP address when only the IPv4 address is
advertised (e.g., using SDP); otherwise the access to the
IPv4 multicast content can not be offered to the IPv6
receiver. The ALG may be located downstream the receiver.
As such, the ALG does not know in advance whether the
receiver is dual-stack or IPv6-only. The ALG may be tuned
to insert both the original IPv4 address and corresponding
IPv6 multicast address using for instance the ALTC SDP
attribute.
Boucadair, et al. Expires August 2, 2013 [Page 16]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
(1.2) In order to avoid involving an ALG in the path, an IPv4-
only source can advertise both its IPv4 address and IPv4-
embedded IPv6 multicast address
[I-D.ietf-mboned-64-multicast-address-format] using for
instance the ALTC SDP attribute.
2. A dual-stack source sending its multicast content over IPv4 and
IPv6: both IPv4 and IPv6 addresses need to be inserted in the SDP
part. A means (e.g, ALTC) is needed for this purpose.
A.3. Introducing IPv6 into SIP-based Architectures
A.3.1. Avoid Crossing CGN Devices
Some service providers are in the process of enabling DS-Lite
[RFC6333] as a means to continue delivering IPv4 services to their
customers. To avoiding crossing four levels of NAT when placing a
media session (2 NAT in DS-Lite AFTR + 2 NAT in the DBE), it is
recommended to enable IPv6 functions in some SBEs/DBEs. Therefore
DS-Lite AFTRs won't be crossed for DS-Lite serviced customers if
their UA is IPv6-enabled:
o For SIP UA embedded in the CPE, this is easy to implement since
the SIP UA [RFC3261] can be tuned to behave as IPv6-only UA when
DS-Lite is enabled. No ALTC is required for that use case.
o But for SIP User Agents located behind the CPE, a solution to
indicate both IPv4 and IPv6 (e.g., ALTC) is required in order to
avoid crossing the DS-Lite CGN.
A.3.2. Basic Scenario for IPv6 SIP Service Delivery
A basic solution to deliver SIP-based services using IPv4-only core
service platform to IPv6-enabled UA is to enabled IPv4/IPv6
interworking function in SBE/DBE. Signaling and media between two
SBEs and DBEs is maintained over IPv4. IPv6 is used between an IPv6-
enabled UA and a SBE/DBE.
Figure 3 shows the results of session establishment between UAs. In
this scenario, IPv4/IPv6 interworking function is invoked even when
both involved UAs are IPv6-enabled.
Boucadair, et al. Expires August 2, 2013 [Page 17]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
+----------+
| Core SIP |
+--->|SPF (IPv4)|<---+
IPv4 SIP | +----------+ |IPv4 SIP
v v
+-----------+ +-----------+
| SBE | | SBE | SIP
+------->|IPv4/v6 IWF| | |<-------+
|IPv6 +-----------+ +-----------+ IPv4|
| SIP SIP|
+----+ | +-----------+ +-----------+ | +----+
|IPv6|-+IPv6 RTP| DBE |IPv4 RTP| DBE |IPv4 RTP+-|IPv4|
| UA |<-------->|IPv4/v6 IWF|<------>| |<-------->| UA |
+----+ +-----------+ +-----------+ +----+
+----------+
| Core SIP |
+--->|SPF (IPv4)|<---+
IPv4 SIP | +----------+ |IPv4 SIP
v v
+-----------+ +-----------+
| SBE | | SBE | SIP
+------->|IPv4/v6 IWF| |IPv4/v6 IWF|<-------+
|IPv6 +-----------+ +-----------+ IPv6|
| SIP SIP|
+----+ | +-----------+ +-----------+ | +----+
|IPv6|-+IPv6 RTP| DBE |IPv4 RTP| DBE |IPv6 RTP+-|IPv6|
| UA |<-------->|IPv4/v6 IWF|<------>|IPv4/v6 IWF|<-------->| UA |
+----+ +-----------+ +-----------+ +----+
Figure 3: Basic scenario
Solutions to avoid redundant IPv4/IPv6 NAT and involving several DBEs
may be valuable to consider by service providers.
A.3.3. Avoid IPv4/IPv6 Interworking
For services providers wanting:
1. Means to promote the invocation of IPv6 transfer capabilities can
be enabled while no parsing error is to be experienced by core
service nodes legacy nodes
2. Optimize cost related to IPv4-IPv6 translation licenses
3. Reduce the dual-stack lifetime
Boucadair, et al. Expires August 2, 2013 [Page 18]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
4. Maintain an IPv4-only core
5. Only a set of SBE/DBE are IPv6-enabled
A solution to indicate both IPv4 and IPv6 addresses is required.
This section provides an overview of this procedure:
When a SBE receives an INVITE, it instantiates in its DBE an IPv6-
IPv6 context and an IPv6-IPv4 context. Both an IPv6 address and an
IPv4 address are returned together with other information such as
port numbers. SBE builds an SDP offer including both IPv4 and IPv6-
related information using ALTC attribute. IPv6 is indicated as
preferred connectivity type.
o=- 25678 753849 IN IP4 192.0.2.2
c=IN IP4 192.0.2.2
m=audio 12340 RTP/AVP 0 8
a=altc:1 IP6 2001:db8::2 6000
a=altc:2 IP4 192.0.2.2 12340
Figure 4: SDP offer updated by the SBE
The request is then forwarded to the core SPF which in its turn
forwards the requests to the terminating SBE.
o If this SBE is a legacy one, then it will ignore ALTC attributes
and use "c" line.
o If the terminating SBE is IPv6-enabled:
* If the called UA is IPv4-only, then an IPv6-IPv4 context is
created in the corresponding DBE.
* If the called UA is IPv6-enabled, then an IPv6-IPv6 context is
created in the corresponding DBE.
Figure 5 shows the result of the procedure when placing a session
between an IPv4 and IPv6 UAs while Figure 6 shows the results of
establishing a session between two IPv6-enabled UAs. The result is
still not optima since redundant NAT66 is required (Appendix A.3.4).
Boucadair, et al. Expires August 2, 2013 [Page 19]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
+----------+
| Core SIP |
+--->|SPF (IPv4)|<---+
IPv4 SIP | +----------+ |IPv4 SIP
v v
+-----------+ +-----------+
| SBE | | SBE | SIP
+------->|IPv4/v6 IWF| |IPv4/v6 IWF|<-------+
|IPv6 +-----------+ +-----------+ IPv4|
| SIP SIP|
+----+ | +-----------+ +-----------+ | +----+
|IPv6|-+IPv6 RTP| DBE |IPv6 RTP| DBE |IPv4 RTP+-|IPv4|
| UA |<-------->| NAT66 |<------>|IPv4/v6 IWF|<-------->| UA |
+----+ +-----------+ +-----------+ +----+
2001:db8::2
Figure 5: Session establishement between IPv4 and IPv6 UAs
+----------+
| Core SIP |
+--->|SPF (IPv4)|<---+
IPv4 SIP | +----------+ |IPv4 SIP
v v
+-----------+ +-----------+
| SBE | | SBE | SIP
+------->|IPv4/v6 IWF| |IPv4/v6 IWF|<-------+
|IPv6 +-----------+ +-----------+ IPv6|
| SIP SIP|
+----+ | +-----------+ +-----------+ | +----+
|IPv6|-+IPv6 RTP| DBE |IPv6 RTP| DBE |IPv6 RTP+-|IPv6|
| UA |<-------->| NAT66 |<------>| NAT66 |<-------->| UA |
+----+ +-----------+ +-----------+ +----+
2001:db8::2
Figure 6: Session establishement between IPv6 UAs
A.3.4. DBE Bypass Procedure
For service providers wanting to involve only one DBE in the media
path, when not all SBE/DBE and UAs are IPv6-enabled, a means to
indicate both IPv4 and IPv6 addresses without inducing session
failures is required. Below is proposed an example of a proposed
procedure using ALTC attribute.
When the originating SBE receives an INVITE from an IPv6-enabled UA,
it instantiates in its DBE an IPv6-IPv6 context and an IPv6-IPv4
context. Both an IPv6 address and an IPv4 address are returned
together with other information such as port numbers. SBE builds an
Boucadair, et al. Expires August 2, 2013 [Page 20]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
SDP offer including both IPv4 and IPv6-related information using ALTC
attribute (Figure 7). IPv6 is indicated as preferred connectivity
type.
o=- 25678 753849 IN IP4 192.0.2.2
c=IN IP4 192.0.2.2
m=audio 12340 RTP/AVP 0 8
a=altc:1 IP6 2001:db8::2 6000
a=altc:2 IP4 192.0.2.2 12340
Figure 7: SDP offer updated by the SBE
The request is then forwarded to the core SPF which in its turn
forwards the requests to the terminating SBE:
o If the destination UA is IPv6 or reachable with a public IPv4
address, the SBEs only forwards the request without altering the
SDP offer. No parsing error is experienced by core service nodes
since ALTC is backward compatible.
o If the terminating SBE does not support ALTC, it will ignore this
attribute an uses the legacy procedure.
As a consequence, only one DBE is maintained in the path when one of
the involved parties is IPv6-enabled. Figure 8 shows the overall
procedure when involved UAs are IPv6-enabled.
+----------+
| Core SIP |
+--->|SPF (IPv4)|<---+
IPv4 SIP | +----------+ |IPv4 SIP
v v
+-----------+ +-----------+
| SBE | | SBE | SIP
+------->|IPv4/v6 IWF| |IPv4/v6 IWF|<-------+
|IPv6 +-----------+ +-----------+ IPv6|
| SIP SIP|
+----+ | +-----------+ | +----+
|IPv6|-+IPv6 RTP| DBE | IPv6 RTP +-|IPv6|
| UA |<-------->| NAT66 |<----------------------------->| UA |
+----+ +-----------+ +----+
2001:db8::1 2001:db8::2
Figure 8: DBE Bypass Overview
The main advantages of such solutions are as follows:
o DBE resources are optimized
Boucadair, et al. Expires August 2, 2013 [Page 21]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
o No redundant NAT is maintained in the path when IPv6-enabled UAs
are involved
o End-to-end delay is optimized
o The robustness of the service is optimized since the delivery of
the service relies on fewer nodes
o The signaling path is also optimized since no communication
between the SBE (through SPDF in TISPAN/IMS context) and DBE at
the terminating side is required for some sessions.
A.3.5. Direct Communications Between IPv6-enabled User Agents
For service providers wanting to allow direct IPv6 communications
between IPv6-enabled UAs, when not all SBE/DBE and UA are IPv6-
enabled, a means to indicate both IPv4 and IPv6 addresses without
inducing session failures is required. Below is proposed an example
of a proposed procedure using ALTC attribute.
At the SBE originating side, when the SBE receives an INVITE from the
calling IPv6 UA (Figure 9), it uses ALTC to indicate two IP
addresses:
1. An IPv4 address belonging to its controlled DBE
2. The same IPv6 address and port as received in the initial offer
made by the calling IPv6
Figure 10 shows an excerpt example of the SDP offer generated by the
originating SBE.
o=- 25678 753849 IN IP6 2001:db8::1
c=IN IP6 2001:db8::1
m=audio 6000 RTP/AVP 0 8
Figure 9: SDP offer of the calling UA
o=- 25678 753849 IN IP4 192.0.2.2
c=IN IP4 192.0.2.2
m=audio 12340 RTP/AVP 0 8
a=altc:1 IP6 2001:db8::1 6000
a=altc:2 IP4 192.0.2.2 12340
Figure 10: SDP offer updated by the SBE
The INVITE message will be routed appropriately to the destination
SBE:
1. If the SBE is a legacy device (i.e., IPv4-only); it will ignore
IPv6 addresses and contacts its DBE to instantiate an IPv4-IPv4
context.
Boucadair, et al. Expires August 2, 2013 [Page 22]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
2. If the SBE is IPv6-enabled, it will only forwards the INVITE to
the address of contact of the called party:
A. If the called party is IPv6-enabled, the communication will
be placed using IPv6. As such no DBE is involved in the data
path as illustrated in Figure 11.
B. If not, IPv4 will be used between the originating DBE and
called UA.
+----------+
| Core SIP |
+--->|SPF (IPv4)|<---+
IPv4 SIP | +----------+ |IPv4 SIP
v v
+-----------+ +-----------+
| SBE | | SBE | SIP
+------->|IPv4/v6 IWF| |IPv4/v6 IWF|<-------+
|IPv6 +-----------+ +-----------+ IPv6|
| SIP SIP|
+----+ | | +----+
|IPv6|-+ IPv6 RTP +-|IPv6|
| UA |<---------------------------------------------------->| UA |
+----+ +----+
2001:db8::1
Figure 11: Direct IPv6 communication
Authors' Addresses
Mohamed Boucadair
France Telecom
Rennes 35000
France
Email: mohamed.boucadair@orange.com
Hadriel Kaplan
Acme Packet
71 Third Ave.
Burlington, MA 01803
USA
Email: hkaplan@acmepacket.com
Boucadair, et al. Expires August 2, 2013 [Page 23]
Internet-Draft SDP Alternate Connectivity Attribute January 2013
Robert R Gilman
Independent
Email: bob_gilman@comcast.net
URI:
Simo Veikkolainen
Nokia
Email: Simo.Veikkolainen@nokia.com
URI:
Boucadair, et al. Expires August 2, 2013 [Page 24]
