SIPPING B. Stucker
Internet-Draft Nortel
Intended status: Informational October 18, 2006
Expires: April 21, 2007
Coping with Early Media in the Session Initiation Protocol (SIP)
draft-stucker-sipping-early-media-coping-03
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 21, 2007.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Several mechanisms for early media have been proposed in the past,
each attacking a different aspect of the problem. A good example of
this is RFC-3960 which talks about two models of early media: the
gateway model, and the application model. The gateway model uses a
series of offer/answer exchanges to control the rendering of early
media, but breaks down in the presence of forking (as mentioned in
section 3 of RFC-3960). The application model relies on the UAS to
know when it is generating early media and use RFC-3959 to keep early
Stucker Expires April 21, 2007 [Page 1]
Internet-Draft Coping w/ Early Media in SIP October 2006
media and regular media streams separate to avoid clipping. Even in
the presence of the recommendations in RFC-3960 some problems exist
within SIP in the area of early media. Although some of these
challenges are likely to never be overcome, for example when
interworking with a PSTN gateway that does not take into account CPG
or ACM messages (in the case of ISUP). However, the potential to
improve on what is already there does exist. This document attempts
to go into more detail around early media where RFC-3960 left off,
what sorts of mechanisms are in use today in existing implementations
to deal with the challenges at hand, derives requirements and a
possible mechanism to improve upon the current model. In addition,
the document goes into other areas that can complicate or be
complicated by the presence of early media (especially with forking)
such as SRTP keying and media flow authorization.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Types of Early Media . . . . . . . . . . . . . . . . . . . . . 5
3.1. Pre-routing early media . . . . . . . . . . . . . . . . . 5
3.2. Pre-presentation early media . . . . . . . . . . . . . . . 6
3.3. Post-presentation early media . . . . . . . . . . . . . . 6
3.4. Non-SDP early media . . . . . . . . . . . . . . . . . . . 7
4. Current common coping mechanisms for early media . . . . . . . 7
4.1. Problems with current coping mechanisms . . . . . . . . . 8
4.1.1. Proxy-side coping mechanisms . . . . . . . . . . . . . 8
4.1.1.1. Proxy SDP stripping . . . . . . . . . . . . . . . 8
4.1.1.2. Proxy SDP weighting . . . . . . . . . . . . . . . 9
4.1.2. Client-side coping mechanisms . . . . . . . . . . . . 9
4.1.2.1. Client detection of forking . . . . . . . . . . . 9
4.1.2.2. Client slow-start INVITE . . . . . . . . . . . . . 10
4.1.2.3. Client Usage of Gateway Model . . . . . . . . . . 10
4.1.2.4. Client Usage of Application Server Model . . . . . 10
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Deprecation of forking . . . . . . . . . . . . . . . . . . 11
5.2. Deprecation of early media . . . . . . . . . . . . . . . . 11
5.3. Originating UA's to render early media . . . . . . . . . . 12
5.4. Downstream signaling of acceptance . . . . . . . . . . . . 12
5.5. Upstream signaling of importance . . . . . . . . . . . . . 13
5.6. Universal backward-compatibility . . . . . . . . . . . . . 13
5.7. Recursive forking . . . . . . . . . . . . . . . . . . . . 13
5.8. Media Gating . . . . . . . . . . . . . . . . . . . . . . . 14
6. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Early Media Classification and Prioritization . . . . . . 14
6.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 14
6.1.1.1. Early-Media Classifications . . . . . . . . . . . 15
Stucker Expires April 21, 2007 [Page 2]
Internet-Draft Coping w/ Early Media in SIP October 2006
6.2. Early Media Flow Negotiation . . . . . . . . . . . . . . . 16
6.2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 16
6.2.2. SDP parameters . . . . . . . . . . . . . . . . . . . . 16
6.2.3. Usage of emflow with offer/answer . . . . . . . . . . 17
6.2.3.1. Meaning of a=emflow:none . . . . . . . . . . . . . 17
6.2.3.2. Meaning of a=emflow:send . . . . . . . . . . . . . 17
6.2.3.3. Meaning of a=emflow:recv . . . . . . . . . . . . . 18
6.2.3.4. Meaning of a=emflow:sendrecv . . . . . . . . . . . 18
6.2.3.5. Usage of RTP-SSRC-Value . . . . . . . . . . . . . 18
6.2.4. Option tag for emflow . . . . . . . . . . . . . . . . 19
6.2.5. Example . . . . . . . . . . . . . . . . . . . . . . . 19
6.3. Early Media and SRTP . . . . . . . . . . . . . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Normative References . . . . . . . . . . . . . . . . . . . 22
9.2. Informational References . . . . . . . . . . . . . . . . . 22
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 23
Intellectual Property and Copyright Statements . . . . . . . . . . 24
Stucker Expires April 21, 2007 [Page 3]
Internet-Draft Coping w/ Early Media in SIP October 2006
1. Introduction
One of the mechanisms within SIP [RFC3261] that has caused much
consternation (and interesting service scenarios) is forking,
especially forking of INVITE requests. This is where a SIP INVITE
request sent to a SIP proxy is resolved into two or more destinations
which are signaled in parallel or sequentially by the proxy. When
this occurs, multiple downstream parties will receive similar INVITE
requests to initiate a SIP session from a given originating SIP user
agent (UA). This creates the possibility of race conditions where
the ordering of the provisional and final responses to this request,
as observed by the originating SIP UA, may potentially arrive in any
order, or not at all.
Another mechanism in SIP that looks simple, but causes difficult
interactions, was introduced to handle SIP to PSTN interworking.
Because the PSTN has a specific set of behaviors which require that
only one endpoint in the PSTN network (typically the last PSTN switch
reached) may generate media back to the originator of a PSTN call,
generation of early media (media produced prior to the intended
terminator of a call answering the call) is relatively straight-
forward. In SIP, this PSTN interaction with early media was handled
by allowing any endpoint that has received an SDP offer as part of
setting up a session to be able to immediately generate media back to
the to SDP offerer. Further, the SDP offerer was obligated to be
prepared to render any media received at the location specified in
the SDP offer at any time as long as the session was in a setup or
stable state.
Each of these mechanisms, taken separately, can create complex
signaling flows and difficult service interactions to resolve.
Together, however, they compound the effects of one another to create
an area of study that has been open within the SIP design community
for some time. Several extensions to [RFC3261] have been proposed to
handle some of the various effects that early media suffers from,
most notably [RFC3959] and [RFC3960]. However, none have fully
attacked a few key areas of interest:
o Controlling the order and timing of early media stream rendering
at the originating SIP UAC.
o Knowing under what general conditions early media flows are
potentially being sent to the originating SIP UAC.
This document seeks to capture the salient requirements for these
areas, and propose a mechanism for handling these early media
interactions in a more predictable manner.
Stucker Expires April 21, 2007 [Page 4]
Internet-Draft Coping w/ Early Media in SIP October 2006
2. Terminology
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 BCP 14, RFC 2119 [3].
3. Types of Early Media
Not all early media is created equal, some types are more problematic
than others. There are four generic types of early media within SIP:
1. Pre-routing early media - This is early media that is conveyed
via SDP and is presented to the originator by a proxy before
routing on the URI is started.
2. Pre-presentation early media - This is early media that is
presented to the originator, conveyed via SDP, by a proxy after
the URI has been routed upon, but before any forwarding of the
INVITE request has occurred.
3. Post-presentation early media - This is early media that is
presented to the originator, conveyed via SDP, by either a
forking proxy or any subsequent hop after the INVITE request has
been forwarded from the proxy.
4. Non-SDP early media - This is early media that may be presented
to the originator at any time through means other than SDP, such
as the Alert-Info header as defined in [RFC3261]
3.1. Pre-routing early media
Pre-routing early media is typically generated and characterized by a
proxy that has an associated media resource. An example of this type
of early media would be a brief 'branding' message that is played to
the originator thanking them for using the service provider
associated with the originator's local outbound proxy. When the
message ends, the media resource signals this to the proxy and
routing of the request continues per [RFC3261]
This type of early media typically does not pose the originator's
local outbound proxy any issues unless the client is using one of the
mechanisms defined in Section 4.1.2 or something similar. This is
because the proxy is in complete control over the pace at which the
terminator will be routed to relative to the media stream being
presented. If the proxy attempting to present pre-routing early
media to the originator is a subsequent proxy from the originator's
local outbound proxy, then the service may not work due to upstream
proxies employing one of the mechanisms described in Section 4.1.1
Stucker Expires April 21, 2007 [Page 5]
Internet-Draft Coping w/ Early Media in SIP October 2006
3.2. Pre-presentation early media
Pre-presentation early media is similar to pre-routing early media
except that it may take into account the routes that the proxy is
about to route the INVITE request to in its decision of what to play.
This may allow the proxy to employ one of the proxy-side early media
coping mechanisms defined in Section 4.1.1. Likewise, the proxy may
inject its own SDP answer into the signaling to the originator to
kick off services like colorful ringback tone (CRBT) where the
originator is hearing a recording (typically music) selected by the
terminator while the network attempts to reach the terminating party.
Pre-presentation early media also differs from non-SDP early media in
that the proxy or proxies are manipulating the SDP offer/answer
rather than SIP headers such as Alert-Info (as defined in [RFC3261])
to signify what media the originator should be rendering. There are
several potential reasons why the Alert-Info header is not used in
this case: the service may be interactive, requiring two-way media in
order to work (such as digit collection for a credit card number), or
may not want to rely on the originator's ability to render the
information in the Alert-Info header to the end user (such as a call
originating from the PSTN through a SIP gateway).
3.3. Post-presentation early media
Post-presentation early media is most typically characterized by the
ugly interactions that arise between it and forking. Since this is
early media that has come about after the proxy has potentially
caused multiple endpoints to be contacted, and therefore the
possibility that multiple early media streams may have been
triggered, it is commonly considered to be the worst-case scenario
with early media.
To compound the basic issue at play, the presence of forking can
confuse the type of early media being presented to the originator. A
downstream proxy that has received a forked request may not be aware
that the INVITE has forked as a B2BUA may have forked the request.
As a result, the proxy may be acting as if it is the only proxy to
handle the request from the originator, and operate in a pre-routing,
pre-presentation, or non-SDP early media mode despite the fact that
the early media reaching the originator is post-presentation.
Therefore, unless the proxy is the originator's edge proxy, it cannot
necessarily determine what kind of early media it may actually be
sending to the originator.
Stucker Expires April 21, 2007 [Page 6]
Internet-Draft Coping w/ Early Media in SIP October 2006
3.4. Non-SDP early media
Non-SDP early media is typically characterized by the presence of an
Alert-Info header [RFC3261]. The Alert-Info header specifies a URI
that the originator may go to in order to receive a file or stream
that contains information (such as a wave recording) about the
ringback tone the terminator wishes the originator to hear. It is
somewhat simpler in that it is not part of the offer/answer model,
and that it is not trying to create a two-way media stream. The
interaction between inband ringback, client generated (local)
ringback, and other forms of early media is spelled out in [RFC3960].
It is worth noting that rendering the Alert-Info header contents
should only be done when the origin of the header is trusted (per
[RFC3261]), so this may limit its usefulness to a considerable
degree. The remainder of this document assumes that the UAC and UAS
follows the advice in [RFC3960] with respect to interactions with
early media.
Although non-SDP early media is for future study, it is envisioned
that this document would clarify the behavioral interactions between
non-SDPearly media and other types of early media.
4. Current common coping mechanisms for early media
A number of mechanisms exist for coping with early media. They all
rely, generally, on 'fixing' the early media problem by 'breaking'
the behaviors specified in other RFCs (or at least bending the spirit
of them to some extent):
1. Proxy SDP stripping - If a proxy detects that it is about to fork
an INVITE, it keeps track of this fact in its processing state
for the INVITE transaction. Any SDP answers in provisional
responses are stripped before being forwarded upstream. The SDP
answer may be added into a 200 response upstream from last
provisional SDP answer received if SDP is not already present in
the message to ensure that the offer/answer exchange is
completed. This effectively turns off early media.
2. Proxy SDP weighting - If a proxy detects that it had previously
forked the INVITE to which it is now receiving a provisional
response it may allow a particular provisional response to retain
the SDP answer in the message body and strip other SDP answers in
provisional responses per the proxy SDP stripping methodology.
This mechanism is used to favor SDP that the proxy may have some
control over. For instance, if the proxy knows that one forked
leg is to a media server streaming CRBT media to the originator,
it may allow that SDP answer to flow back, but block all other
SDP answers on other legs in the meantime.
Stucker Expires April 21, 2007 [Page 7]
Internet-Draft Coping w/ Early Media in SIP October 2006
3. Client detection of forking - Clients may start out playing local
ringback to the originator until the first SDP answer is
received. When the first SDP answer is received, the client may
switch to playing the media for that SDP answer. However, upon
detecting that the INVITE forked through subsequent provisionals
being received (reception of two or more distinct SDP answers or
[RFC3261] 'TO' header tags) the client may irrevocably return to
playing local ringback. At this point, the client is likely to
continue to playing local ringback until the call is answered, or
an error condition arises.
4. Client slow-start - Clients may wish to simply not include any
SDP in their initial INVITE message in order to accumulate a set
of SDP offers from their prospective terminating endpoints. Such
INVITEs are known as 'slow-start' INVITEs, because the SDP offer/
answer exchange gets off to a 'slow start'. These may also be
used in protocol interworkings (notably H.323 to SIP) with no
intent as to managing early media. The client can either use
PRACK or UPDATE to respond to offers received in provisional
responses at the point in time the originating client wishes to
stage early media streams.
4.1. Problems with current coping mechanisms
4.1.1. Proxy-side coping mechanisms
4.1.1.1. Proxy SDP stripping
This is a very common mechanism, perhaps second only to the two
client mechansims mentioned above. When a proxy employs this
mechanism, it remembers when forking has occurred and removes any SDP
in provisional responses as a result. This means that if the
originator supports reliable provisional responses (100rel) as
defined in [RFC3262], that this option tag must be removed by the
proxy before forwarding the INVITE to each forked leg. Otherwise it
may be forced to potentially handle SDP in negotiation within a PRACK
transaction for the originating client with little or no information
about the originating client's capabilities. In the case that the
originator requires support for PRACK the proxy may have to fail the
call setup, handle very complex negotiation signaling in the case
that the call forks, or simply not fork the call.
Additionally, this mechanism also completely breaks any early media
services or announcements, some of which may be critical to proper
completion or billing disposition of the call upon answer. For
instance, the call may fork to a PSTN gateway that is trying to tell
the originator that it is about to bill them $500 to complete the
current call. With proxy SDP stripping this announcement would not
be heard by the originator.
Stucker Expires April 21, 2007 [Page 8]
Internet-Draft Coping w/ Early Media in SIP October 2006
4.1.1.2. Proxy SDP weighting
Proxy weighting of SDP can be useful in situations where the proxy
knows what is going on with the call routing for each leg. However,
lack of information as to why downstream elements are sending SDP in
provisional responses can cause proxies to weight the SDP
incorrectly. Further, if multiple proxies are traversed, the SDP
that is accepted for delivery to the originating UA may not be the
SDP selected at any given proxy. There is no indication to
downstream network clients as to what has happened with their SDP as
it traverses proxys back upstream towards the originator. Likewise,
the $500 warning announcement presented in the previous section may
or may not be heard.
4.1.2. Client-side coping mechanisms
4.1.2.1. Client detection of forking
This mechanism is where a client may play audible ringback while
waiting for an initial provisonal or final response to an INVITE
message it originated. When the first provisional response with SDP
is received, it may switch from playing audible ringback to rendering
the media stream defined in the SDP. If a subsequent provisional
response is received from a different endpoint (identifiable by a
different to-tag in the 'TO' header as defined in [RFC3261]) it stops
rendering any early media media packets it is receiving and typically
returns to audible ringback. Upon receiving a non-3xx final
response, the UA switches media appropriately to the response. For
3xx responses, the client continues to play audible ringback if that
was what is currently being rendered, or switches (typically) to
ringback again if it was rendering media packets. This mechanism is
used by client devices for a number of reasons:
o What gets presented to the end user is predictable.
o Does not rely on the set of proxies handling any given INVITE
request to do anything special.
o Is easy to implement.
The problem with this approach is that it often causes early media to
break altogether. If a leg that the call was forked to is awaiting
media from the originating client (such as prompting for digit
collection, like a credit card number or extension) that leg's early
media may fail due to other provisional responses sent to the
originator by other call legs.
Network and terminating services that utilize early media are likely
to fail or work erratically (due to race conditions between messages)
when an originating client behaves in this manner. What's worse, is
Stucker Expires April 21, 2007 [Page 9]
Internet-Draft Coping w/ Early Media in SIP October 2006
that there's no indication as to what the originating client is doing
to the downstream network elements.
Due to the client switching to locally generated playback, and
ignoring early media RTP streams prior to receiving a final response
to the INVITE, there is the opportunity for clipping to occur is the
SIP signaling path latentcy lags the media path latentcy.
4.1.2.2. Client slow-start INVITE
Slow-start INVITEs circumvent the problem of having to immediately
render media packets from an unknown set of terminating endpoints by
not giving those endpoints anywhere to send the media to. However,
this mechanism has some serious drawbacks, most notably guaranteed
clipping (potentially severe if the SDP offer is not received from
the other end until a 200 response is received) and the potential for
an increased number of messaging round-trips to setup a call.
Due to some service designs and protocol interworking slow-start
INVITEs will continue to be seen, but due to the clipping problems
associated with slow-start INVITEs this coping mechanism is
considered to be incomplete.
4.1.2.3. Client Usage of Gateway Model
Clients typically do not use the [RFC3960] gateway model because of
the limitations presented in the RFC around early media and forking
with the gateway model in section 3.1.
4.1.2.4. Client Usage of Application Server Model
The application server model defined in [RFC3960], along with
[RFC3959] define an improved mechanism over the gateway model in that
early media is negotiated separately from regular media to reduce
media clipping issues. However, there still are problems with UASs
that generate early media packets upon receiving the SDP offer from
the UAC that cannot currently be distinguished from other media in
all situations, and the UAC has no feedback from the various UASs
that are generating early media as to which ones are of importance or
otherwise. UASs typically do adhere to the request in [RFC3960]
section 4.1 that they not generate superfluous early media streams to
assist the UAC with early media rendering.
5. Requirements
The following requirements are considered to be the starting point in
more formally discussing improvements to SIP for early media
Stucker Expires April 21, 2007 [Page 10]
Internet-Draft Coping w/ Early Media in SIP October 2006
interactions:
R1: Deprecation of forking within the [RFC3261] is considered to be
out-of-scope of the possible solutions (sorry Dean).
R2: Deprecation of early media from within [RFC3261] is considered
to be out-of-scope of the possible solutions (sorry again,
Dean).
R3: SIP UAs that are attempting to create a new SIP dialog using the
INVITE method should no longer be obligated to blindly render
media packets that are delivered to them as a result of an SDP
offer sent in the INVITE.
R4: A mechanism should exist by which an originating SIP UA can
signal to a downstream SIP endpoint that it is now willing to
accept media packets.
R5: A mechanism should exist by which a terminating SIP UA can
signal to an upstream SIP endpoint what type of early media (if
known) it wishes to present to the originating UA, if it
requires one-way, or two-way media flows, and the relative
importance of the early media.
R6: Universal backwards-compatability is a secondary goal. Where
possible, backwards-compatability with clients that do not
implement recommendations in this draft should be preserved.
R7: The mechanism must be able to deal with recursive forking
scenarios. This is where an INVITE passes two or more proxies
that both choose to fork the request to two or more endpoints at
each proxy in parallel.
R8: The mechanism must not require exchange of packets on the media
path to identify or coordinate early media streams as this may
not interoperate with common network media gating mechanisms.
5.1. Deprecation of forking
Deprecation of forking from SIP [RFC3261] is considered to be out of
scope. This is due to the heavy deployment of forking in existing
implementations for key routing services. Changes of this nature are
considered by the author (and others) to be of too large a scope
relative to the problem at hand and are subsequently excluded from
this draft in favor of searching for less radical solutions.
5.2. Deprecation of early media
Deprecation of early media from SIP [RFC3261] is considered to be out
of scope. Early media is required in order to handle certain PSTN
interactions as defined in RFC-3398 [RFC3398] and elsewhere. In
addition, the desire to provide announcements and other media prior
to the terminating party answering the call is considered desirable
and must therefore use some form of "pre-answer" media (currently
known as early media).
Stucker Expires April 21, 2007 [Page 11]
Internet-Draft Coping w/ Early Media in SIP October 2006
5.3. Originating UA's to render early media
Currently, section 5.1 of the offer/answer model [RFC3264] states
that the offerer in an SDP offer/answer exchange must be prepared to
receive media from media streams described in the offer as being
'recvonly' or 'sendrecv'. Further, in section 6.1 of [RFC3264] it
states that the answerer in an SDP offer/answer exchange may
immediately send media to media streams that are described in the
answer as being 'sendrecv' (note: [RFC3264] does not explicitly state
as much, but it is assumed that media streams that are 'sendonly' in
the SDP answer can also have media immediately sent to them by the
SDP offerer).
These statements, taken together, create an obligation upon the
originating UA to render any early media sent to them by anyone to
whom their SDP offer was delivered (unless the media stream was
defined to be 'sendonly' or 'inactive'). This is useful in resolving
the PSTN interactions in [RFC3398], especially as noted in the
example call flows and ACM message processing in section 7 of that
document. This obligation on the part of the originating UA has
subsequently been used in the absence of actual PSTN interworking to
provide services that mimic the PSTN network (such as providing far-
end announcements), or provide other services such as colorful
ringback tones (CRBT) in which media is streamed to the originator
while the terminator is being located/alerted.
The argument can, and has, been made that simply because a service
exists in the PSTN world, that it does not mean that it must exist
within SIP. However, given the prevalence of services that utilize
early media, and the number of RFCs that talk about dealing with
various aspects of early media, this particular train appears to have
long ago left the station. It is not the intent of this document to
pass judgement upon these services, but to find a way to cope with
them in a more robust manner than currently is available.
The obvious downside to this property of [RFC3264] is that while the
offerer may have limited control over the delivery of their SDP
offer, they have an obligation to render anything sent to them. This
severely restricts the policies that the offerer (as the originator)
may use to decide to render early media, which needs to be augmented.
5.4. Downstream signaling of acceptance
An INVITE with SDP should serve two simple purposes: establish the
path by which all signaling shall follow to/from the originator and
the set of terminating clients, and to let each terminating party
know what sort of communications the originator can and will engage
in. Currently, SDP offers also imply tacit acceptance of any and all
Stucker Expires April 21, 2007 [Page 12]
Internet-Draft Coping w/ Early Media in SIP October 2006
media that might be generated in the reverse path upstream towards
the originator. This should not necessarily always be the case, and
a mechanism whereby the originator may assert that it is further
ready to receive media packets is needed. The originator may wish to
imply a combination of early and final media acceptance or denial in
order to prevent unruly early media interactions and clipping of
final media.
5.5. Upstream signaling of importance
A provisional response from a terminating party currently implies
that the terminating party is listening to the SIP signaling it is
receiving, and (if an SDP answer is present) the type of
communications that the terminator wishes to engage in (if any).
What is missing is a way for the terminating party to tell upstream
entities what sort of demands it has upon the originator for
rendering of its early media, and the relative importance associated
with the media that it generates towards the originator. This helps
the originator decide what is important and what is not when choosing
which media stream it should render (if it wishes to, see
Section 4.1.2.1).
5.6. Universal backward-compatibility
There are scenarios in which there is no way to cope appropriately
with early media streams. An example would be a call that forks to
an ISUP PSTN gateway as defined in [RFC3398] that is ignorant of the
content of early media it is generating. There is no reliable
indication in ISUP CPG or ACM messages as to what the other end might
be doing for early media. It is possible that a cause code is
present in the CPG in some ISUP to legacy platform interworking
scenarios, but these are not present generally in ISUP signaling
flows, and therefore cannot be relied upon. Mechansims to deal with
these types of devices is currently for future study and not explored
further here at this time.
5.7. Recursive forking
The mechanism should be able to deal with recursive forking
scenarios. This would be where two or more independent proxies fork
a given INVITE request from an originating client. In this case, the
proxies are normally not coordinated in their operations. As a
result, the mechanism proposed should be robust enough to allow for
both end-to-middle and end-to-end negotiation of early media.
Stucker Expires April 21, 2007 [Page 13]
Internet-Draft Coping w/ Early Media in SIP October 2006
5.8. Media Gating
In many network environments, it is common for the media flow to be
'gated' in some way. Gating refers to a practice whereby an element
in the network is examining the signaling (SIP and SDP) being
exchanged by UAs and is sending instructions to a middlebox as to
when media packets are authorized to flow between UAs. This gating
behavior is typically used to prevent theft of service. As a result
of this gating behavior, any mechanism used to identify or coordinate
early media should not employ media packet exchanges. It is
allowable for early media itself to be marked as such in the media
packets, however, because gating behavior does not interact
negatively with such a mechanism. Operations that require early
media packet behavior by the UAC may fail in the presence of gating.
6. Recommendations
The following sections include recommendations that create a
framework that is capable of both identifying/prioritizing the type
of early media being presented to the originator, and giving the
originating client a means by which it can control the order in which
early media flows are presented to it.
6.1. Early Media Classification and Prioritization
6.1.1. Overview
Regardless of the mechanism that is used to control the presentation
of early media, if at any point more than one endpoint is attempting
to stream early media to the originator a few problems arise:
o Nobody upstream of the device attempting to stream early media to
the originator is aware of what exactly it is that the early media
generator is generating. Is it advertising? Is it an important
message? Who knows. This is important not only for the
originating client (see Section 4.1.2.1), but proxies as well
since they may be employing a weighting mechanism as described in
Section 4.1.1.2.
o The device generating the early media may have no idea how many
other devices that are peer to it or downstream from it are also
trying to generate early media. Again, this is important if the
client is using the client-side detection of forking mechanism
defined in Section 4.1.2.1.
o Multiple streams may be included in the offer, not all of which
are suitable or intended for early media. For instance, an offer
may include video and audio streams. Early media may only be
streamed to the audio port during call setup. Another example
Stucker Expires April 21, 2007 [Page 14]
Internet-Draft Coping w/ Early Media in SIP October 2006
would be the inclusion of RTP and SRTP streams where only the RTP
stream is intended for early media. Therefore, the UAC may not
wish to apply early-media coping mechanisms to all streams
offered.
In order to rectify this situation, proper classification of the
possible early media to be sent after completion of the SDP exchange
is needed and a specific linkage of that classification to particular
streams is highly desirable. This can be handled either by inclusion
of SIP headers in the message carrying SDP sent towards the
originator or by inclusion in the SDP itself. If the classification
is handled in the SDP itself, this limits the ability of
intermediaries to use this information to update the SDP as the
message body may covered by an integrity protection mechanism or may
be otherwise unavailable (for example, the SDP could be encrypted).
If the classification is handled in the SIP headers, then it may be
unclear as to which SDP stream the classification applies to. If
classification is handled via a SIP header (previous revisions of
this document referred to an 'Early-Media-Class' header), then it is
recommended that the SIP header only apply to SDP covered by an
Early-Session content disposition as defined in [RFC3959]. This
allows the UAC to clearly understand which streams the classification
applies to. In either case, via SIP or SDP, upon answer of the
INVITE, all processing of media streams and SDP should revert to
[RFC3261]RFC-3261 rules as the call is answered and no media from
this point on should be considered 'early'.
6.1.1.1. Early-Media Classifications
The following list is given to show a possible set of common early-
media classifications. Each class is given in increasing order of
importance.
1. RFC-3264 - The default behaivor defined in RFC-3264 is requested.
2. Advertisement - A non-critical advertisement.
3. Warning - A non-critical announcement.
4. Two-way - The endpoint presenting early media wishes to establish
a two-way early media session before completing the call.
5. Critical - A critical announcement, such as: "We're about to bill
you for $10k".
6. Unknown - The nature of the early media being presented to the
originator is unknown (such as from a PSTN gateway receiving a
generic announcement.)
Early media classified as "Unknown" must unfortunately be considered
of the highest importance: there's no indication given that qualifies
it to be of lower importance. It is recommended that unclassified
early media would be treated as RFC-3264. This is to prevent network
Stucker Expires April 21, 2007 [Page 15]
Internet-Draft Coping w/ Early Media in SIP October 2006
elements that do not classify their early media from overriding
elements that are more forthcoming. An additional q-value, such as
that defined in section 20.10 of [RFC3261], can be used to break ties
between classifications.
6.2. Early Media Flow Negotiation
The following sections take the requirements from Section 5 and tries
to create a mechanism that can satisfy them. This mechanism is built
along similar lines as the SIP preconditions framework [RFC3312].
6.2.1. Overview
A simple mechanism is introduced that tells terminators what the
originator expects to have happen with respect to early media. This
information may also be of use to intermediate nodes that also wish
to generate early media. The mechanism differs from the SDP
[RFC2327] 'a=recvonly', 'a=sendonly', 'a=sendrecv', and 'a=inactive'
attributes in that the final media flow mode can be negotiated and
ready upon answer without further messaging, and from the
preconditions [RFC3959] SDP attributes in that QoS can be negotiated
separately as well.
6.2.2. SDP parameters
The following media-level parameters are defined:
early-media-flow-status = "a=emflow:" direction-tag [ COMMA rtp-
ssrc-value ]
direction-tag = ("none" | "send" | "recv" | "sendrecv")
rtp-ssrc-value = 1 * 8hex
The early-media-flow-status 'a=emflow' denotes two things:
o The current state of the early media from the perspective of the
originating party of the call as specified by the direction-tag.
o The RTP SSRC for a given early media stream (as defined in section
8 of [RFC3550]) to facilitate correlation of RTP packets with a
particular early media session. It is possible for this value to
be the same for two different early media stream. The intent of
this is to give the UAC a starting point to work from.
ISSUE: What should the UAC do if it sees that the RTP SSRC in
two or more early media flows collides?
ISSUE: How stable are RTP SSRC values during call setup?
It is expected that the directionality indicators defined in
[RFC2327] as 'a=sendrecv', 'a=sendonly', 'a=recvonly', and
'a=inactive' are otherwise unaffected. Likewise, preconditions, as
defined in [RFC3312] are likewise unaffected. The emflow values may
Stucker Expires April 21, 2007 [Page 16]
Internet-Draft Coping w/ Early Media in SIP October 2006
be changed in subsequent offer/answer exchanges to allow the
originator to properly stage multiple early media streams according
to the Early-Media-Class header values. For example, an originator
may specify 'a=emflow:none' initially to suppress all early media
flows, and then send an UPDATE with a new SDP offer to an endpoint
the originator received an early media indication from with
'a=emflow:recv' to denote that the originator is now willing to
receive early media.
Regardless of the value of this parameter, both endpoints may
immediately begin exchanging media packets upon answer according to
[RFC3261], [RFC3264] and [RFC2327].Intermediate proxies should honor
this indication, and adjust their behavior accordingly, potentally
causing them to divert from their normal early media coping
mechanisms.
6.2.3. Usage of emflow with offer/answer
6.2.3.1. Meaning of a=emflow:none
If the emflow value of 'none' is set in an the SDP offer, it
indicates that the endpoint generating the offer will not accept
early media and that anyone accepting this SDP offer MUST NOT send
early media. If the emflow value in the SDP offer was 'none', then
the emflow value in the SDP answer MUST be set to 'none' as well.
If the emflow value of 'none' is set in an SDP answer, it indicates
that the endpoint generating the answer will not generate early
media. The SDP offeror can take this indication to mean that they
should not expect early media packets from this endpoint per
[RFC3264], and that any received prior to answer from this source MAY
be discarded.
6.2.3.2. Meaning of a=emflow:send
If the emflow value of 'send' is set in an the SDP offer, it
indicates that the endpoint generating the offer may send early media
packets, but will not accept early media. Anyone accepting this SDP
offer MUST NOT send early media, but SHOULD process received early
media packets if it is appropriate to the device receiving packets to
do so. If the emflow value in the SDP offer was 'send', then the
emflow value in the SDP answer MUST be set to 'none' or 'recv'
depending on whether the application intends to process the early
media packets that the offeror may send to it.
If the emflow value of 'send' is set in an SDP answer, it indicates
that the endpoint generating the answer may generate early media but
will not process any sent to it. Any early media sent to it per
Stucker Expires April 21, 2007 [Page 17]
Internet-Draft Coping w/ Early Media in SIP October 2006
[RFC3264] MAY be discarded. The SDP offeror can take this indication
to mean that they should expect early media packets from this
endpoint and behave appropriately.
6.2.3.3. Meaning of a=emflow:recv
If the emflow value of 'recv' is set in an the SDP offer, it
indicates that the endpoint generating the offer may be sent early
media packets, but will not generate early media. Anyone accepting
this SDP offer MAY send early media, but SHOULD NOT expect to receive
early media from the SDP offeror, and that any media packets received
prior to answer from this the offeror may safely be discarded. If
the emflow value in the SDP offer was 'recv', then the emflow value
in the SDP answer MUST be set to 'none' or 'send' depending on
whether the application intends to send the early media packets to
the offeror or not.
If the emflow value of 'recv' is set in an SDP answer, it indicates
that the endpoint generating the answer will accept early media but
will not generate any.The SDP offeror can take this indication to
mean that they should not expect early media packets from this
endpoint and may safely discard any received prior to answer.
6.2.3.4. Meaning of a=emflow:sendrecv
If the emflow value of 'sendrecv' is set in an the SDP offer, it
indicates that the endpoint generating the offer may send and receive
early media packets. Anyone accepting this SDP offer MAY send early
media, and SHOULD process received early media packets if it is
appropriate to the device receiving packets to do so. If the emflow
value in the SDP offer was 'sendrecv', then the emflow value in the
SDP answer MAY be set to any value. The value set in the SDP answer
depends on if the endpoint answering the SDP offer intends to send
and/or receive early media packets.
If the emflow value of 'sendrecv' is set in an SDP answer, it
indicates that the endpoint generating the answer may generate and
receive early media and behave appropriately.
6.2.3.5. Usage of RTP-SSRC-Value
The RTP-SSRC value is useful in helping endpoints correlate incoming
RTP packets with SDP offer/answer exchanges. The value used in this
tag is the SSRC value used in the header portion of an RTP packet as
defined in [RFC3550]. The SSRC value in an RTP packet is used to
define a means for an endpoint to synchronize RTP packets sent from a
particular source. As such, the SSRC value must be unique for a
given RTP stream.
Stucker Expires April 21, 2007 [Page 18]
Internet-Draft Coping w/ Early Media in SIP October 2006
The worst-case expectation for uniqueness of an SSRC value during the
offer/answer SDP phase of RTP resource allocation is given in Section
8 of [RFC3550] as 10^(-4) if there are 1000 different RTP streams
being offered. As the number of RTP streams typically used in a call
setup, even with significant forking involved, is likely to be O(10)
or fewer, the likelihood of each RTP stream getting a unique SSRC
number early on is good. If a collision is detected, then [RFC3550]
defines a mechanism for detecting this and reselecting a unique SSRC
value. This re-selection does not require another SDP exchange
today, but if necessary, an SDP exchange could be initiated through a
target refresh of the INVITE dialog to update the SDP offer and/or
answer with the update SSRC value in the emflow parameter.
6.2.4. Option tag for emflow
The option tag "emflow" is defined for use in the Require and
Supported header fields [RFC3261]. An offerer MAY include this tag
in a Require header if they wish to ensure that any endpoint reached
supports this extension (typically when 'a=emflow:' is not set to
'sendrecv'). Then if the party generating an SDP offer or answer
supports this extension it MUST include this tag in a Supported
header if it is not already in a Require header of any message
containing SDP. This allows the other party or parties involved in
the signaling flow to know that the other end is processing their
emflow values.
6.2.5. Example
The following figures show a simple offer/answer exchange in which
the UAC does not wish to receive early media automatically. The UAS
then answers indicating that it has a warning announcement it would
like to play as early media. The UAC then updates the emflow value
to allow the warning announcement to proceed.
v=0
o=alice 2890844526 2890844526 IN IP4 uac.anywhere.com
s=
c=IN IP4 uac.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=emflow: none, 1e4f381
Figure 1: An SDP offer with no early media allowed and SSRC 1e4f381.
Stucker Expires April 21, 2007 [Page 19]
Internet-Draft Coping w/ Early Media in SIP October 2006
...
Early-Media-Class: Warning; q=1.0
v=0
o=bob 2890844730 2890844730 IN IP4 uas.example.com
s=
c=IN IP4 uas.example.com
t=0 0
m=audio 49920 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=emflow: none, 23a73c01
Figure 2: A SIP early media answer to the offer with SSRC of
23a73c01.
v=0
o=alice 2890844526 2890844526 IN IP4 uac.anywhere.com
s=
c=IN IP4 uac.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=emflow: recv, 1e4f381
Figure 3: An SDP offer with early media allowed towards the UAC.
v=0
o=bob 2890844730 2890844730 IN IP4 uas.example.com
s=
c=IN IP4 uas.example.com
t=0 0
m=audio 49920 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=emflow: send, 23a73c01
Figure 4: An SDP answer to the offer acknowledging that early media
will be sent using RTP SSRC 23a73c01.
6.3. Early Media and SRTP
One of the challenges in dealing with SRTP is the initial key
exchanges required to support it. The draft
[I-D.wing-rtpsec-keying-eval] discusses a number of keying
mechanisms, concentrating on their interaction with SIP. Given the
amount of effort likely involved to establish a secure media flow, it
is undesirable to require that early media be secure. After all, an
Stucker Expires April 21, 2007 [Page 20]
Internet-Draft Coping w/ Early Media in SIP October 2006
attacker can likely surmise from a 180 Ringing response to an INVITE
that the originator is probably hearing ringback. It is the
conversation that typically seeks to be protected, therefore securing
early media in many situations is likely wasteful.
Additionally, there may be issues where the early media coping
mechanisms mentioned in Section 4 are employed that prevents SRTP
keying exchanges from taking place in a timely manner. This can
cause a number of potentially poor outcomes, especially when SDP is
stripped or otherwise manipulated during call setup by a network
element.
Finally, network elements that wish to generate early media typically
serve many endpoints simultaneously. This means that they do not
have the computational power available to support key exchange and
encryption without an undesirable reduction in the amount of traffic
that they can handle. Therefore, it is recommended that if a client
is offering an SRTP stream, that they also offer a regular RTP stream
as well for purposes of early media. This gives the network a
separate playground to work with for purposes of establishing early
media to the UAC. If the SDP for the early media stream were
separated in the SDP offer (possibly using [RFC3959]) it is
conceivable that network elements that employ the mechanisms
described in Section 4 would simply leave the SRTP portion of a UAC's
offer alone, thereby improving the observed behavior of SRTP and
early media by the user and SIP network administrator.
ISSUE: The interaction between the mechanisms outlined in this
draft and SRTP clearly warrants more investigation.
7. Security Considerations
This document is a work in progress. Security considerations will be
added as various recommendations become more concrete.
8. IANA Considerations
This document defines the SDP media type of "emflow" and the
direction-tag values of "none", "send", "recv", and "sendrecv" which
will require IANA registration.
9. References
Stucker Expires April 21, 2007 [Page 21]
Internet-Draft Coping w/ Early Media in SIP October 2006
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
9.2. Informational References
[RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[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.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3312] Camarillo, G., Marshall, W., and J. Rosenberg,
"Integration of Resource Management and Session Initiation
Protocol (SIP)", RFC 3312, October 2002.
[RFC3398] Camarillo, G., Roach, A., Peterson, J., and L. Ong,
"Integrated Services Digital Network (ISDN) User Part
(ISUP) to Session Initiation Protocol (SIP) Mapping",
RFC 3398, December 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3959] Camarillo, G., "The Early Session Disposition Type for the
Session Initiation Protocol (SIP)", RFC 3959,
December 2004.
[RFC3960] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing
Tone Generation in the Session Initiation Protocol (SIP)",
RFC 3960, December 2004.
[I-D.wing-rtpsec-keying-eval]
Stucker Expires April 21, 2007 [Page 22]
Internet-Draft Coping w/ Early Media in SIP October 2006
Audet, F. and D. Wing, "Evaluation of SRTP Keying with
SIP", draft-wing-rtpsec-keying-eval-01 (work in progress),
June 2006.
Author's Address
Brian Stucker
Nortel
2201 Lakeside
Richardson, TX 75082
US
Phone: +1 972 685 7724
Email: bstucker@nortel.com
URI: http://www.nortel.com/
Stucker Expires April 21, 2007 [Page 23]
Internet-Draft Coping w/ Early Media in SIP October 2006
Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
Stucker Expires April 21, 2007 [Page 24]