| MMUSIC | K. Drage, Ed. |
| Internet-Draft | M. Makaraju |
| Intended status: Standards Track | J. Stoetzer-Bradler |
| Expires: October 12, 2015 | Alcatel-Lucent |
| R. Ejzak | |
| J. Marcon | |
| Unaffiliated | |
| April 10, 2015 |
SDP-based Data Channel Negotiation
draft-ietf-mmusic-data-channel-sdpneg-02
The Real-Time Communication in WEB-browsers (RTCWeb) working group is charged to provide protocols to support direct interactive rich communications using audio, video, and data between two peers' web-browsers. For the support of data communication, the RTCWeb working group has in particular defined the concept of bi-directional data channels over SCTP, where each data channel might be used to transport other protocols, called sub-protocols. Data channel setup can be done using either the internal in-band band (also referred to as 'internal' for the rest of the document) Data Channel Establishment Protocol or some external out-of-band simply referred to as 'external negotiation' in the rest of the document . This document specifies how the SDP offer/answer exchange can be used to achieve such an external negotiation. Even though data channels are designed for RTCWeb use initially they may be used by other protocols like, but not limited to, the CLUE protocol. This document is intended to be used wherever data channels are used.
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Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.
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The RTCWeb working group has defined the concept of bi-directional data channels running on top of SCTP/DTLS. RTCWeb leaves it open for other applications to use data channels and its in-band or out-of-band protocol for creating them. Each data channel consists of paired SCTP streams sharing the same SCTP Stream Identifier. Data channels are created by endpoint applications through the WebRTC API, or other users of data channel like CLUE, and can be used to transport proprietary or well-defined protocols, which in the latter case can be signaled by the data channel "sub-protocol" parameter, conceptually similar to the WebSocket "sub-protocol". However, apart from the "sub-protocol" value transmitted to the peer, RTCWeb leaves it open how endpoint applications can agree on how to instantiate a given sub-protocol on a data channel, and whether it is signaled in-band or out-of-band (or both). In particular, the SDP offer generated by the application includes no channel-specific information.
This document defines SDP-based out-of-band negotiation procedures to establish data channels for transport of well-defined sub-protocols.
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 [RFC2119].
This document uses the following terms:
The mechanism in this specification only applies to the Session Description Protocol (SDP) [RFC4566], when used together with the SDP offer/answer mechanism [RFC3264]. Declarative usage of SDP is out of scope of this document, and is thus undefined.
This section summarizes how data channels work in general. Note that the references to 'browser' here is intentional as in this specific example the data channel user is a Webrtc enabled browser.
A WebRTC application creates a data channel via the data channel API, by providing a number of setup parameters (sub-protocol, label, reliability, order of delivery, priority). The application also specifies if it wants to make use of the in-band negotiation using the DCEP [I-D.ietf-rtcweb-data-protocol], or if the application intends to perform an "external negotiation" using some other in-band or out-of-band mechanism.
In any case, the SDP offer generated by the browser is per [I-D.ietf-mmusic-sctp-sdp]. In brief, it contains one "m" line for the SCTP association on top of which data channels will run, and one attribute per protocol assigned to the SCTP ports:
m=application 54111 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 79.97.215.79
a=max-message-size:100000
a=sctp-port 5000
a=setup:actpass
a=connection:new
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
Note: A WebRTC browser will only use "m" line format "webrtc-datachannel", and will not use other formats in the "m" line for other protocols such as t38. [I-D.ietf-mmusic-sctp-sdp] supports only one SCTP association to be established on top of a DTLS session.
Note: This SDP syntax does not contain any channel-specific information.
Independently from the requested type of negotiation, the application creating a data channel can either pass to the browser the stream identifier to assign to the data channel or else let the browser pick one identifier from the ones unused.
To avoid glare situations, each endpoint can moreover own an exclusive set of stream identifiers, in which case an endpoint can only create a data channel with a stream identifier it owns.
Which set of stream identifiers is owned by which endpoint is determined by convention or other means.
In-band negotiation only provides for negotiation of data channel transport parameters and does not provide for negotiation of sub-protocol specific parameters. External negotiation can be defined to allow negotiation of parameters beyond those handled by in-band negotiation, e.g., parameters specific to the sub-protocol instantiated on a particular data channel. See Section 6.1.2 for an example of such a parameter.
The following procedures are common to all methods of external negotiation, whether in-band (communicated using proprietary means on an already established data channel) or out-of-band (using SDP or some other protocol associated with the signaling channel).
In the case of external negotiation, the endpoint application has the option to fully control the stream identifier assignments. However these assignments have to coexist with the assignments controlled by the data channel stack for the in-band negotiated data channels (if any). It is the responsibility of the application to ensure consistent assignment of stream identifiers.
When the application requests the creation of a new data channel to be set up via external negotiation, the data channel stack creates the data channel locally without sending any DATA_CHANNEL_OPEN message in-band, and sets the data channel state to Connecting if the SCTP association is not yet established, or sets the data channel state to Open if the SCTP association is already established. The side which starts external negotiation creates data channel using underlying data channel stack API and the data channel is put into open state immediately (assuming ICE, SCTP procedures were already done). However, the application can't send data on this data channel until external negotiation is complete with the peer. This is because peer needs to be aware and accept the data channel via external negotiation. The peer after accepting the data channel offer can start sending data immediately. This implies that the offerer may get data channel message before external negotiation is complete and the application should be ready to handle it.
If the peer rejects the data channel part of the offer then it doesn't have to do anything as the data channel was not created using the stack. The offerer on the other hand needs to close the data channel that was opened by invoking relevant data channel stack API procedures.
It is also worth noting that a data channel stack implementation may not provide any API to create and close data channels; instead the data channels are used on the fly as needed just by communicating via external means or by even having some local configuration/assumptions on both the peers.
The application then externally negotiates the data channel properties and sub-protocol properties with the peer's application.
[ASSUMPTION] The peer MUST then symmetrically create a data channel with these negotiated data channel properties. This is the only way for the peer's data channel stack to know which properties to apply when transmitting data on this channel. The data channel stack MUST allow data channel creation with any non-conflicting stream identifier so that both peers can create the data channel with the same stream identifier.
In case the external negotiation is correlated with an SDP offer/answer exchange that establishes the SCTP association, the SCTP initialization completion triggers a callback from the data channel stack to an application on both the ends to change the data channel state from Connecting to Open. The details of this interface is specific to the data channel user application. Browser based applications (could include hybrid apps) will use [WebRtcAPI], while native applications use a compatible API, which is yet to be specified. See Section 6.2.3 for details on when the data channel stack can assume the data channel is open, and on when the application can assume the data channel is open.
When the application requests the closing of an externally negotiated data channel, the data channel stack always performs an in-band SSN reset for this channel.
Depending upon the method used for external negotiation and the sub-protocol associated with the data channel, the closing might in addition be signaled to the peer via external negotiation.
This section defines a method of external negotiation by which two clients can negotiate data channel-specific and sub-protocol-specific parameters, using the out-of-band SDP offer/answer exchange. This SDP extension can only be used with SDP offer/answer model.
Two new SDP attributes are defined to support external negotiation of data channels. The first attribute provides for negotiation of channel-specific parameters. The second attribute provides for negotiation of sub-protocol-specific parameters.
Associated with the SDP "m" line that defines the SCTP association for data channels (defined in Section 4), each SDP offer and answer includes one "a=dcmap:" attribute that defines the data channel parameters for each data channel to be negotiated. Each such attribute line specifies the following parameters for a data channel: SCTP stream identifier, sub-protocol, label, reliability, order of delivery, and priority.
The intention of exchanging these attributes is to create data channels on both the peers with the same set of attributes without actually using [I-D.ietf-rtcweb-data-protocol]. It is assumed that the data channel properties (reliable/partially reliable, ordered/unordered) are suitable per the sub-protocol transport requirements.
Formal Syntax:
Name: dcmap
Value: dcmap-value
Usage Level: media
Charset Dependent: no
Syntax:
dcmap-value = dcmap-stream-id
[ SP dcmap-opt *(";" dcmap-opt) ]
dcmap-opt = ordering-opt / subprotocol-opt / label-opt
/ maxretr-opt / maxtime-opt
; Either only maxretr-opt or maxtime-opt
; is present.
dcmap-stream-id = 1*DIGIT
ordering-opt = "ordered=" ordering-value
ordering-value = "true" / "false"
subprotocol-opt = "subprotocol=" quoted-string
label-opt = "label=" quoted-string
maxretr-opt = "max-retr=" maxretr-value
maxretr-value = <from-Reliability-Parameter of
I-D.ietf-rtcweb-data-protocol>
; number of retransmissions
maxtime-opt = "max-time=" maxtime-value
maxtime-value = <from-Reliability-Parameter of
I-D.ietf-rtcweb-data-protocol>
; milliseconds
quoted-string = DQUOTE *(quoted-char / escaped-char) DQUOTE
quoted-char = SP / quoted-visible
quoted-visible = %21 / %23-24 / %26-7E ; VCHAR without " or %
escaped-char = "%" HEXDIG HEXDIG
DQUOTE = <from-RFC5234>
integer = <from-RFC5234>
Examples:
a=dcmap:0
a=dcmap:1 subprotocol="BFCP";max-time=60000
a=dcmap:2 subprotocol="MSRP";ordered=true;label="MSRP"
a=dcmap:3 label="Label 1";ordered=false;max-retr=5
a=dcmap:4 label="foo%09bar";ordered=true;max-time=15000;max-retr=3
"a=dcmap:" is a media level attribute having following ABNF syntax.
Within an 'a=dcmap' attribute line's 'dcmap-opt' value either only one 'maxretr-opt' parameter or one 'maxtime-opt' parameter is present. Both MUST NOT be present.
The 'dcmap-stream-id' parameter indicates the SCTP stream identifier within the SCTP association used to form the data channel.
The 'label' parameter indicates the name of the channel. It represents a label that can be used to distinguish, in the context of the WebRTC API, an RTCDataChannel object from other RTCDataChannel objects. This parameter maps to the 'Label' parameter defined in [I-D.ietf-rtcweb-data-protocol]. The 'label' parameter is optional. If it is not present, then its value defaults to the empty string.
Note: The empty string may also be explicitly used as 'label' value, such that 'label=""' is equivalent to the 'label' parameter not being present at all. [I-D.ietf-rtcweb-data-protocol] allows the DATA_CHANNEL_OPEN message's 'Label' value to be an empty string.
The 'subprotocol' parameter indicates which protocol the client expects to exchange via the channel. 'Subprotocol' is an optional parameter. If the 'subprotocol' parameter is not present, then its value defaults to the empty string.
This parameter indicates that the data channel is partially reliable. The 'max-retr' parameter indicates the max times a user message will be retransmitted. The max-retr parameter is optional. If the max-retr parameter is not present, then the maximal number of retransmissions is determined as per the generic SCTP retransmission rules as specified in [RFC4960]. This parameter maps to the 'Number of RTX' parameter defined in [I-D.ietf-rtcweb-data-protocol].
This parameter indicates that the data channel is partially reliable. A user message will no longer be transmitted or retransmitted after a specified life-time given in milliseconds in the 'max-time' parameter. The max-time parameter is optional. If the max-time parameter is not present, then the generic SCTP retransmission timing rules apply as specified in [RFC4960]. This parameter maps to the 'Lifetime in ms' parameter defined in [I-D.ietf-rtcweb-data-protocol].
The 'ordered' parameter with value "true" indicates that the receiver MUST dispatch DATA chunks in the data channel to the upper layer while preserving the order. The ordered parameter is optional and takes two values: "true" for ordered and "false" for unordered delivery with "true" as the default value. Any other value is ignored and default "ordered=true" is assumed. In the absence of this parameter "ordered=true" is assumed. This parameter maps to the ordered or unordered data channel types as defined in [I-D.ietf-rtcweb-data-protocol].
In the SDP, each data channel declaration MAY also be followed by other SDP attributes specific to the sub-protocol in use. Each of these attributes is represented by one new attribute line, and it includes the contents of a media-level SDP attribute already defined for use with this (sub)protocol in another IETF specification. Sub-protocol-specific attributes might also be defined for exclusive use with data channel transport, but should use the same syntax described here for other sub-protocol-specific attributes.
Formal Syntax: Name: dcsa Value: dcsa-value Usage Level: media Charset Dependent: no Syntax: dcsa-value = stream-id SP attribute attribute = <from-RFC4566> Example: a=dcsa:2 accept-types:text/plain
Each sub-protocol specific SDP attribute that would normally be used to negotiate the subprotocol using SDP is replaced with an attribute of the form "a=dcsa:stream-id original-attribute", where dcsa stands for "data channel sub-protocol attribute", stream-id is the SCTP stream identifier assigned to this sub-protocol instance, and original-attribute represents the contents of the sub-protocol related attribute to be included.
Note that the above reference to RFC 4566 defines were the attribute definition can be found; it does not provide any limitation on support of attributes defined in other documents in accordance with this attribute definition.
Thus in the example above, the original attribute line "a=accept-types:text/plain" is represented by the attribute line "a=dcsa:2 accept-types:text/plain", which specifies that this instance of MSRP being transported on the sctp association using the data channel with stream id 2 accepts plain text files.
As opposed to the data channel "a=dcmap:" attribute parameters, these parameters are subject to offer/answer negotiation following the procedures defined in the sub-protocol specific documents.
The same syntax applies to any other SDP attribute required for negotiation of this instance of the sub-protocol.
Note: This document does not provide a complete specification of how to negotiate the use of a data channel to transport MSRP. Procedures specific to each sub-protocol such as MSRP will be documented elsewhere. The use of MSRP is only an example of how the generic procedures described herein might apply to a specific sub-protocol.
If an SDP offer / answer exchange (could be the initial or a subsequent one) results in a UDP/DTLS/SCTP or TCP/DTLS/SCTP based media description being accepted, and if this SDP offer / answer exchange results in the establishment of a new SCTP association, then the SDP offerer owns the even SCTP stream ids of this new SCTP association and the answerer owns the odd SCTP stream identifiers. If this "m" line is removed from the signaling session (its port number set to zero), and if usage of this or of a new UDP/DTLS/SCTP or TCP/DTLS/SCTP based "m" line is renegotiated later on, then the even and odd SCTP stream identifier ownership is redetermined as well as described above.
This specification allows simultaneous use of external and internal negotiation. However, a single stream is managed using one method at a time. Stream ids that are not currently used in SDP can be used for internal negotiation. Stream id allocation per SDP based external negotiation may not align with DTLS role based allocation. This could cause glare conditions when one side trying to do external negotiation on a stream id while the other end trying to open a data channel on the same stream id using internal negotiation. To avoid these glare conditions this specification recommends that the data channel stack user always selects stream ids per above described SDP offer / answer rule even when internal negotiation is used. To avoid glare conditions, it is possible to come up with a different stream id allocation scheme, but such schemes are outside the scope of this specification.
Conveying a reliable data channel is achieved by including neither 'max-retr' nor 'max-time' in corresponding SDP offer's or answer's a=dcmap attribute line. Conveying a partially reliable data channel is achieved by including only one of 'max-retr' or 'max-time'. By definition max-retr and max-time are mutually exclusive, so only one of them can be present in a=dcmap. If an SDP offer contains both of these parameters then the receiver of such an SDP offer MUST reject the SDP offer. If an SDP answer contains both of these parameters then the offerer MAY treat it as an error and MAY assume the associated SDP offer/answer failed and MAY take appropriate recovery actions. These recovery options are outside the scope of this specification.
The SDP answer SHALL echo the same subprotocol, max-retr, max-time, ordered parameters, if those were present in the offer, and MAY include a label parameter. They MAY appear in any order, which could be different from the SDP offer, in the SDP answer.
When sending a subsequent offer or an answer, and for as long as the data channel is still open, the sender MUST replicate the same information.
Data channel types defined in [I-D.ietf-rtcweb-data-protocol] are mapped to SDP in the following manner, where "ordered=true" is the default and may be omitted:
DATA_CHANNEL_RELIABLE
ordered=true
DATA_CHANNEL_RELIABLE_UNORDERED
ordered=false
DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT
ordered=true;max-retr=<number of retransmissions>
DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT_UNORDERED
ordered=false;max-retr=<number of retransmissions>
DATA_CHANNEL_PARTIAL_RELIABLE_TIMED
ordered=true;max-time=<lifetime in milliseconds>
DATA_CHANNEL_PARTIAL_RELIABLE_TIMED_UNORDERED
ordered=false;max-time=<lifetime in milliseconds>
The procedure for opening a data channel using external negotiation starts with the agent preparing to send an SDP offer. If a peer receives an SDP offer before getting to send a new SDP offer with data channels that are to be externally negotiated, or loses an SDP offer glare resolution procedure in this case, it MUST wait until the ongoing SDP offer/answer completes before resuming the external negotiation procedure.
The agent that intends to send an SDP offer to create data channels through SDP-based external negotiation performs the following:
The peer receiving such an SDP offer performs the following:
The agent receiving such an SDP answer performs the following:
Any data channels in Connecting state are transitioned to the Open state when the SCTP association is established.
Each agent application MUST wait to send data until it has confirmation that the data channel at the peer is in the Open state. For WebRTC, this is when both data channel stacks have channel parameters instantiated. This occurs:
When the application requests the closing of a data channel that was externally negotiated, the data channel stack always performs an in-band SSN reset for this channel.
It is specific to the sub-protocol whether this closing MUST in addition be signaled to the peer via a new SDP offer/answer exchange.
The intention to close a data channel can be signaled by sending a new SDP offer which excludes the "a=dcmap:" and "a=dcsa:" attribute lines for the data channel. The offerer SHOULD NOT change the port value for the "m" line (e.g. to zero) when closing a data channel (unless all data channels are being closed and the SCTP association is no longer needed), since this would close the SCTP association and impact all of the data channels. If the answerer accepts the SDP offer then the answerer MUST close those data channels whose "a=dcmap:" and "a=dcsa:" attribute lines were excluded from the received SDP offer, unless those data channels were already closed, and the answerer MUST also exclude the corresponding attribute lines in the answer. In addition to that, the SDP answerer MAY exclude other data channels which were closed but not yet communicated to the peer. So, the offerer MUST inspect the answer to see if it has to close other data channels which are now not included in the answer.
If a new SDP offer/answer is used to close data channels then the data channel(s) SHOULD only be closed by the answerer/offerer after a successful SDP answer is sent/received.
If a client receives a data channel close indication (due to inband SSN reset or some other reason) without associated SDP offer then the client SHOULD generate an SDP offer which excludes this closed data channel.
The application MUST also close any data channel that was externally negotiated, for which the stream identifiers are not listed in an incoming SDP offer.
A closed data channel using local close (SCTP reset), without an additional SDP offer/answer to close it, may be reused for a new data channel. This can only be done via new SDP offer/answer, describing the new sub-protocol and its attributes, only after the corresponding data channel close acknowledgement is received from the peer (i.e. SCTP reset of both incoming and outgoing streams is completed). This restriction is to avoid the race conditions between arrival of "SDP offer which reuses stream" with "SCTP reset which closes outgoing stream" at the peer
SDP offer:
m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.1
a=max-message-size:100000
a=sctp-port 5000
a=setup:actpass
a=connection:new
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=dcmap:0 subprotocol="BFCP";label="BFCP"
SDP answer:
m=application 10002 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.2
a=max-message-size:100000
a=sctp-port 5002
a=setup:passive
a=connection:new
a=fingerprint:SHA-1 \
5B:AD:67:B1:3E:82:AC:3B:90:02:B1:DF:12:5D:CA:6B:3F:E5:54:FA
Figure 1: Example 1
In the above example the SDP answerer rejected the data channel with stream id 0 either for explicit reasons or because it does not understand the a=dcmap attribute. As a result the offerer will close the data channel created with the external negotiation option. The SCTP association will still be setup over DTLS. At this point the offerer or the answerer may use internal negotiation to open data channels.
SDP offer:
m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.1
a=max-message-size:100000
a=sctp-port 5000
a=setup:actpass
a=connection:new
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=dcmap:0 subprotocol="BFCP";label="BFCP"
a=dcmap:2 subprotocol="MSRP";label="MSRP"
a=dcsa:2 accept-types:message/cpim text/plain text/
a=dcsa:2 path:msrp://alice.example.com:10001/2s93i93idj;dc
SDP answer:
m=application 10002 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.2
a=max-message-size:100000
a=sctp-port 5002
a=setup:passive
a=connection:new
a=fingerprint:SHA-1 \
5B:AD:67:B1:3E:82:AC:3B:90:02:B1:DF:12:5D:CA:6B:3F:E5:54:FA
a=dcmap:2 subprotocol="MSRP";label="MSRP"
a=dcsa:2 accept-types:message/cpim text/plain
a=dcsa:2 path:msrp://bob.example.com:10002/si438dsaodes;dc
Figure 2: Example 2
In the above example SDP offer contains data channels for BFCP and MSRP sub-protocols. SDP answer rejected BFCP and accepted MSRP. So, the offerer should close the data channel for BFCP and both offerer and answerer may start using MSRP data channel (after SCTP/DTLS association is setup). The data channel with stream id 0 is free and can be used for future internal or external negotiation.
Continuing on the earlier example in Figure 1.
Subsequent SDP offer:
m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.1
a=max-message-size:100000
a=sctp-port 5000
a=setup:actpass
a=connection:existing
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=dcmap:4 subprotocol="MSRP";label="MSRP"
a=dcsa:4 accept-types:message/cpim text/plain
a=dcsa:4 path:msrp://alice.example.com:10001/2s93i93idj;dc
Subsequent SDP answer:
m=application 10002 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.2
a=max-message-size:100000
a=sctp-port 5002
a=setup:passive
a=connection:existing
a=fingerprint:SHA-1 \
5B:AD:67:B1:3E:82:AC:3B:90:02:B1:DF:12:5D:CA:6B:3F:E5:54:FA
a=dcmap:4 subprotocol="MSRP";label="MSRP"
a=dcsa:4 accept-types:message/cpim text/plain
a=dcsa:4 path:msrp://bob.example.com:10002/si438dsaodes;dc
Figure 3: Example 3
The above example is a continuation of the example in Figure 1. The SDP offer now removes the MSRP data channel with stream id 2, but opens a new MSRP data channel with stream id 4. The answerer accepted the entire offer. As a result the offerer closes the earlier negotiated MSRP related data channel and both offerer and answerer may start using new the MSRP related data channel.
No security considerations are envisaged beyond those already documented in [RFC4566]
Registration of new subprotocol identifiers is performed using the existing IANA table "WebSocket Subprotocol Name Registry".
The following text should be added following the title of the table.
"This table also includes subprotocol identifiers specified for usage within a WebRTC data channel."
The following reference should be added to under the heading reference: "RFC XXXX".
This document assigns no new values to this table.
NOTE to RFC Editor: Please replace "XXXX" with the number of this RFC.
The authors wish to acknowledge the borrowing of ideas from other internet drafts by Salvatore Loreto, Gonzalo Camarillo, Peter Dunkley and Gavin Llewellyn, and to thank Christian Groves, Christer Holmberg, Paul Kyzivat, Jonathan Lennox, and Uwe Rauschenbach for their invaluable comments.