(With contributions by Jonathan Rosenberg and others.)
- Does SIP support the standard telephone
- Yes. SIP supports, among others:
- call forwarding unconditional, busy, ...
- call transfer (call control spec)
- caller ID
- call hold
- 3-way conferences and multiparty conferencing (call control spec)
- call return ("*69")
- call park (with NOTIFY)
- call waiting
- IVR systems
- multiple line presences
- call waiting
- camp on
- call queueing
- automatic call distribution
- do not disturb
Some services, like repetitive dialing, station speed dialing, last
number redial, and distinctive ringing, are implemented purely in the
end system and require no support from the signaling protocol.
The Telecommunications Industry Association (TIA) is working on a recommendation for
business PBX-style services and other Internet phone requirements.
- How does SIP support caller ID?
- Caller-ID is provided by the From SIP header containing
the caller's name and "number". The number would most likely be placed
in the user field of a SIP URL or appear in a tel: URL.
Since the callee generally does not know or trust the callee's
server, only cryptographic signatures can be used to ensure that the
information is valid. For example, the outgoing proxy might be operated
by an ISP, enterprise or phone company and sign for the identity of the
caller, using the signedby parameter, with the identity of
the company verified by a public key certificate similar to those used
by web sites.
- Should SIP be used to join a conference from a web
- It is possible to embed a SIP URL in a web page, including a session
description. Clicking on that link triggers an invitation for the
conference listed to the address contained in the URL. Unfortunately,
the current standard browsers (Netscape and Internet Explorer) make it
difficult or impossible to add support for another URL type.
Until SIP URIs are implemented in standard browsers, data: URLs can be used to implement
similar functionality, albeit less elegantly.
If it is desired that following the link directly adds the user to an
existing conference, e.g., for a conference "TV guide"-style directory,
URL is more appropriate.
- Can a SIP-initiated session have zero or one
- SIP-initiated sessions can have no or just one participant.
Examples of a session with no participants include an invitation to a
multicast group with no members (beyond the invited party). Also, SDP
sessions can start at a future time relative to the invitation.
- How do I charge/bill for Internet telephony
- This depends on whether you plan to charge for SIP services like
directory look-ups, call processing or mobility, for gateway services to
the PSTN, or for carrying media data:
- SIP services
- The Authorization header can be used to indicate a
customer identity that associates a SIP request with a billable entity.
Examples of possibly chargeable SIP services include:
SIP server operations can be charged based on server logs or, for
real-time billing, via AAA.
- Directory services such as SIP proxy/redirect lookups;
- Customer profile management;
- Media services
- Media services include retrieving and storing voice mail,
as well as transcoding of media streams. They are not initiated by SIP,
but, for example, via RTSP.
- Gateway services
- Similar to SIP services. Care has to be taken to stop billing when
(say) RTP voice data is no longer flowing through the gateway. The
gateway will generate call detail records (CDRs) either directly or
- Transport (network services)
- It seems unlikely that voice calls carried over a best-effort
service will generate per-minute charges. When reserving bandwidth or
guaranteeing other quality-of-service parameters, the resource
reservation protocol or differentiated services are the appropriate
mechanism for including charging. These reservation protocols will
likely be used in applications that are not initiated by SIP, for
example, audio/video on demand or VPNs. Actual accounting records may
be generated by AAA protocols (e.g., by policy enforcement points (PEP)
or policy decision points (PDP)) or log files.
Under some circumstances, a SIP proxy server may be useful to
initiate such reservations or differentiated services treatment on
behalf of a call, since it may be easier to authenticate the SIP request
than the lower-layer reservation request or the end system may not be
capable of making reservations or marking packets. In those cases, the
SIP proxy would initiate a resource reservation and "charge back" the
caller identified by the SIP request.
Dean Willis wrote with
regards to billing for SIP services:
Why can't service providers make a living providing (at a fixed cost)
access to "free services"? Do carriers do per HTTP-transfer billing
now? How much should they charge for an email? For a call, what
parameters might be used? Bandwidth, duration, distance -- the Big
Factors of the POTS bill -- are not issues that SIP is concerned with.
- How do prepaid calling cards work in a
- Note that, in general, prepaid calling cards only make sense in an
IP network if there is a special-purpose VoIP internet, calls traverse a
IP-to-PSTN gateway or VoIP packets receive special treatment. The SIP
requests are forced to traverse a stateful proxy, which controls the
Internet telephony gateway, router QOS function or firewall, depending
on the architecture. When the time is used up, the proxy or gateway
issues a BYE request to both parties, using the existing
call ID. It also disables the gateway connection, turns of any special
QOS treatment for the RTP packets or closes the firewall for that
stream. This requires no additions to either caller or callee. Relying
on SIP BYE itself only suffices the end systems can be
trusted by the network provider not to keep sending packets.
- Does SIP carry DTMF?
- There are at least two options for carrying DTMF and similar signals
in a VoIP network using SIP. First, DTMF can be transported as an RTP
payload (RFC 2833).
This has the advantage that it provides accurate timing and alignment
with the speech RTP packets. Also, media gateways are the most likely to
detect and generate tones, so that making it part of the media stream is
appropriate. However, under some circumstances, it may be necessary for
signaling entities to know about DTMF signals. Currently, there is no
standardized solution within SIP, but it has been proposed to carry
DTMF information in SIP INFO messages, either encoded as simple text or
using the RFC 2833 format. The latter is more complex, but offers
duration and timing information.
- What does the [H14.17] in RFC 2453 stand
- This is explained in Section 3 of RFC 2543. It refers to the section
number in the HTTP/1.1 specification.
- Do callers need to know the location of the
- The caller doesn't interact with the location server directly. A
redirect or proxy server asks the location server (which may be
co-resident with the SIP server or not) for "advice". The location
server is just a logical abstraction to indicate where the SIP server
gets its information from. The protocol between SIP server and location
server is beyond the scope of SIP. Examples of location servers include
- whois, whois++;
- ph and other local directories;
- shared file systems with registration on login;
- local SQL databases reached through TCP.
Also, callers don't register with the location server.
- Which parts of SIP are case-sensitive and which
|Header field name ||CI
|Encoding name (PCMU, L16, etc.) ||CI
- What is the difference between a call leg and a
- A call leg refers to the one-to-one signaling relationship between
two user agents (UAs). The Call-ID is an identifier, carried
in the SIP messages, that refers to the call. A call is a collection of
call legs. A UAC starts by sending an INVITE; because of
forking, it may receive multiple 200 OKs from different UAs. Each
corresponds to a different call leg within the same call. Call is thus
a grouping of call legs. In the call control spec, additional call legs
are created through the Also header.
Call legs refer to end-to-end connections between user agents, rather
than any relationship with proxies. Within a call leg, there are
numerous transactions in both directions.
The request URI is not used in call leg identification.
The To and From field relate to local and
remote in the following way. When Alice sends a request on a call leg
to Bob, the From field contains the local address (Alice),
and the To field the remote address (Bob). When a request is
received by Bob, the To field is matched to Bob's local
address, and the From field to the remote address (Alice).
The CSeq spaces in the two directions of a call leg are
independent. Within a single direction, the sequence number is
incremented for each transaction.
- What is the difference between tag and
- Branch IDs allow proxies to match responses to forked requests.
Without them, a proxy wouldn't be able to tell which branch a response
corresponds to. Tags, in To headers, are of no help here since they are
not known until responses arrive. Tags are used by the UAC to
distinguish multiple final responses from different UAS.
A UAS has no reliable way of determining if the request has been forked or
not. Thus, to be safe it needs to add a tag. Proxies only insert
tags into the final responses they generate themselves; they never
insert tags into requests or responses they forward.
Since a request can be forked several times on its way to UAS, a
single "tag" (or whatever you like to call it) added to the request by
one of the proxies is not sufficient for the next forking proxy along
the chain to match responses on its own branches; every proxy that
forked the request would need to add its own unique IDs to the branches
it created. This is precisely what's being achieved by the branch
parameter in the Via header. (Igor Slepchin)
- How can one recognize a retransmitted,
duplicate or looped request?
||same, but tag may have been added
||must be local host; check for branch parameter to identify which
Looped request are recognized by one or more of the following:
- The server finds itself in the request's Via list,
including any branch parameter. (The server should compute the
branch parameter so that it depends on the request URI.)
- The server is about to proxy the request to one of the hosts listed
in the Via list. The same
- The Max-Forward count is decremented to zero.
- The Expires time has elapsed.
- What is the relationship between the
From, Contact, Via and
- All these headers determine how requests and responses are routed
in a network of SIP proxy servers. Roughly, the distinction is:
- Used for subsequent requests if there is no Contact or
Record-Route header. E.g., if Alice makes a call with
From: Alice <email@example.com> to Bob, an INVITE request
from Bob to Alice would use firstname.lastname@example.org as the To header
- Determines the destination placed in the Request-URI for subsequent
requests and can be used to bypass proxies not enumerated in a
Record-Route header. Also used in responses by redirect servers and in
REGISTER requests and responses.
- The Record-Route header is inserted into requests by proxies that
want to be in the path of subsequent requests for the same call-id. It
is then used by the user agent to route subsequent requests.
The mechanism is similar to a source-route, copying the Record-Route
information into a set of Route headers. The Request-URI is set to the
first Route header.
- Via headers are inserted by servers into requests to detect loops
and to allow responses to find their way back to the client.
They have no influence on the routing of future requests (or responses).
Generally, in short, requests should be sent to Route if
present, Contact if there is no Route,
From if there is no Contact.
- How are URLs compared?
- Two SIP URLs are compared for equality according to the following rules:
- the display name is ignored;
- tags must match;
- user, password, host, port and parameters of the URI must match. If
a component is omitted, it matches based on its default value.
- string comparisons are case-insensitive;
- Characters other than those in the "reserved" and "unsafe" sets (see
RFC 2396) are equivalent to their ""%" HEX HEX" encoding.
- An IP address that is the result of a DNS lookup on a hostname does
not match that hostname.
- Does SIP do admission control?
- Since this offers no real security (calls could always bypass a
servr), admission control is not supported by SIP. If an "outbound
proxy" is used for outgoing calls, that proxy may control the firewall
and thus restrict outgoing calls.
- Does SIP administer bandwidth?
- No, that is the role of a resource reservation protocol. There is no
reason to assume that any Internet telephony signaling server (such as a
proxy) would know the available bandwidth in real networks. Having such
a central server would not scale. Administering bandwidth separately for
each application is also likely to be difficult and inefficient.
There is a proposal for an SDP extension that allows SIP INVITE
requests and responses to indicate that resource reservation must
succeed before the callee is alerted.
- What's the difference between the request URIs
tel:+12125551212 and sip:email@example.com?
- Non-SIP URLs, such as tel:+12125551212 for a telephone number, may
be used as request URIs in SIP INVITE requests. This only
makes sense if all outbound calls are handled by a proxy server. In the
case of a tel: URL, the proxy server would then translate the request
URL to a SIP URL of a gateway server, if it is not handling the gateway
duty itself. The proxy server might use the Gateway Location Protocol
(GLP) to find the appropriate next-hop SIP server. The To
header may always be a tel: URL even if the Request-URI is a SIP URL,
although that breaks with the common practice that Request-URI and
To start out the same.
- Do I always need a proxy or redirect
- No, two SIP servers can contact each other directly.
- How does a caller find its local
- The local registrar is either manually configured or, more likely,
the SIP client issues a multicast registration request to the
sip.mcast.net standard multicast address, which all registrars
- Is the domain of the request-URI and the
To header always the same?
- The Request-URI names the destination of the registration request,
i.e., the domain of the registrar. The user name must be empty.
Generally, the domains in the Request-URI and the To header field have
the same value; however, it is possible to register as a "visitor",
while maintaining one's name. For example, a traveler
sip:firstname.lastname@example.org (To) might register under the
Request-URI sip:atlanta.hiayh.org, with the former as the
To header field and the latter as the Request-URI. Note,
however, that requests for a user at acme.com are not likely to
arrive at the atlanta.hiayah.org server; special purpose
routing logic will generally need to be established in order for
requests for email@example.com to go to the
atlanta.hiayh.org server. In the vast majority of cases, the
domains in the request URI and To field will match. The
REGISTER request is no longer forwarded once it has reached
the server whose authoritative domain is the one listed in the
- How do I ensure registrar
- There are several techniques that can be used to minimize the impact
of registrar/proxy server failures for a server in a local area network:
- Run several servers that all respond to the same multicast
registration address ("warm standby"). As long as multicast requests are
mostly reliable, this ensures a consistent registration picture.
- If a registration server is rebooted and does not have complete
knowledge of the local UA population, it could multicast any incoming
For servers separated from their client by a wide-area network, use
of multicast is not appropriate, so that these servers have to rely on
traditional backup techniques to achieve reliability. For example, the
designated registrar could multicast registration updates within its
local network to keep standby servers synchronized.
- Are ACK requests
- No. An ACK is sent when a response retransmission is
received. Reliability is achieved because the response is retransmitted
until an ACK arrives, and the ACK is retransmitted
on response retransmissions. ACK is only used for
- How are BYE requests routed?
- Since a Contact header MUST be present in
INVITE and 200, the BYE will go directly to the user
agent if there is no Record-Route header. If there is a
Record-Route, it will traverse the list of proxies indicated
If the caller decides to send a BYE before receiving a 200
from the callee, the BYE is be handled by the proxies just as
the corresponding INVITE was handled, i.e., it may be forked.
- Can I CANCEL requests other than
the first INVITE?
- Yes, any request can be cancelled before it has been executed by the
UAS. However, it is likely that this will only make sense in practice
for the initial INVITE and subsequent "re"INVITE.
In the latter case, the call remains, just any changes requests are
- How does a caller find its proxy
- Calls typically proceed directly to the callee's
domain. For example, when calling firstname.lastname@example.org, the
INVITE request would be sent to the SIP server for the domain
example.com, found via DNS.
If a "local" (outbound) proxy is needed for outgoing calls, it
currently needs to be manually configured, similar to the configuration
of web proxies in browsers. Extensions to (for example) use a
REGISTER response or DHCP are under discussion.
- What's the difference between a stateless and
a stateful proxy server?
- Stateless proxies forget about the SIP request once it has been
forwarded. Stateful proxies remember the request after it has been
forwarded, so they can associate the response with some internal state.
In other words, stateful proxies maintain transaction state.
Stateful implies transaction state, not call state.
Stateless proxies scale very well, and can be very fast. They are
good for network cores. Stateful proxies can do more (they can fork,
for example, see the next question) and can provide services stateless
ones can't (call forward busy, for example). They don't scale as much
as stateless ones. An admininstrator gets to decide which to use.
These are also logical entities; a physical proxy is likely to act as a
stateless proxy for some calls, stateful for others, and as a redirect
server for even others.
Neither stateful nor stateless proxies need to maintain call state,
although they can, but will need to make sure that they are part of
subsequent transactions via the Record-Route header.
Proxies must be stateful if one of the following conditions hold:
- uses TCP,
- uses multicast,
- Why can a forking SIP proxy not be
- A forking SIP proxy cannot be stateless because it needs to perform
a filtering operation, returning (in general) one response out of the
many it receives. For example, a forking proxy with three branches,
that receives a 200-class, 400-class, and 500-class response on each
branch respectively, should return only the 200-class response upstream.
If the proxy were stateless, it would end up returning all three of the
responses upstream (since it won't remember that it had received prior
responses when it gets another one). The result of this is (1) response
implosion at the client, and (2) inconsistent responses at the client.
(In this example, depending on the order the responses would be
received, the client would think that the call failed, just to get a
success indication some time later.) Thus, a forking proxy must be
Also note that a proxy that uses TCP must be stateful as well, whether
it forks or not. This has to do with reliability issues.
Why do you want state in a proxy? Certain services (like forking)
simply require it. A sequential search proxy requires state; sequential
search is the heart of services like follow-me and personal mobility.
It's at the discretion of the implementor whether to use a stateful or
stateless proxy. You can even be "super stateful", and use the
Record-Route header to allow a proxy to be on the signaling path of all
subsequent exchanges. This allows a stateful proxy to maintain call
state in addition to transaction state.
- How does a caller find the remote SIP client of
- The process is similar to the delivery of email: The caller uses
the SIP host name to look up the destination host, first trying a SRV
record and then "regular" DNS, just like an email client (MTA) looks up
the MX record. (SRV records are generalized MX records applicable to
any network service, including, but not limited to, SIP and RTSP.) For
example, when contacting email@example.com, the client
finds a SRV record pointing to erlang.cs.columbia.edu as
the SIP server for the domain cs.columbia.edu. As for
email, a single domain name can resolve to multiple servers, allowing
load sharing and redundancy.
The server located in this manner can then proxy or forward the call
to another server.
- How does SIP get through a firewall?
- There are several possible approaches to SIP-capable firewalls. One
of the difficulties is that, unlike for, say, HTTP, connections are
originated both by hosts inside and outside the firewall. A likely
arrangement is that a SIP proxy sits "on" the firewall and relays SIP
requests between the Internet and the intranet. Thix proxy would also
open up the necessary ports in the firewall to let audio and video flow
through, for example using Socks V5.
As an alternative, if a firewall or NAT allows outgoing TCP
connections, the inside client can open up a TCP connection to an
outside proxy. All outgoing and incoming calls would then be handled by
that TCP connection. (The client would still have to use SOCKS or
similar mechanism to convince the firewall to let RTP packets through.)
- How does SIP do "call progress tones" or
- The SIP server being called, such as an Internet telephony gateway,
can return any number of provisional status messages that indicate call
progress. Typically, this is just 100 (Trying) followed by 180
(Ringing), but a server could produce elaborate feedback such as
100 Message received
100 Looking up number
100 Found number, looking up carrier according to profile
100 Finding cheapest carrier which doesn't do animal testing
100 Found carrier "AT&T"
100 Dialing number
182 Queued, 3 people in front of you
182 Queued, 2 people in front of you
The language of the status message should be determined based on the
Accept-Language request header in the call.
A 183 (Session Progress) status response will appear in RFC2543bis.
It can be used for both progress tones as well as error messages.
One would use the 183 only if you:
- Are able to determine that the audio being generated
is something other than ringing (e.g. "comfort tone" or
"pay tone" as defined in E.18x), or
- Are unable to definitively determine that alerting
is occuring. This really should only happen with older
CAS protocols. ISUP and ISDN have sufficient information
to determine what is happening on the far end.
One can also use 183 if the gateway is able to determine that an
error has occured, but that there is a tone or announcement accompanying
it (e.g., an ACM with a cause code present). In that case, the gateway
can send a 183 to set up the media for the announcement (ideally with
the announcement text as the text string), wait for a timer (on the
order of 30 seconds), and then send an appropriate SIP error message.
However, this should only be done if the caller is likely a human
being, as sending 183 would otherwise only delay failure handling.
- Does SIP do keep-alive?
- SIP itself does not have a keep-alive mechanism during the call. It
was felt that loss of connectivity would be detected rapidly by the
absence of media packets, typically sent at a much higher rate than any
signaling keep-alive messages could be sent. In addition, the signaling
path is not needed during the conversation and may well be completely
different (due to proxy and redirect servers) than the media path, so
that keep-alives have a limited functionality. If it is desired to test
the liveness of a signaling server, it is always possible to send either
OPTIONS or (re)INVITE messages.
- Why does SIP not have a
- The Content-Transfer-Encoding header was primarily meant
to allow message bodies to be transformed into formats that could be
transferred on channels that were not 8 bit clean. HTTP, which makes
use of many of the MIME headers, is 8 bit clean, and thus did not need
Content-Transfer-Encoding. SIP followed suit, and so does
not use it either. Content-Encoding is used for things like
compression, which is different. (J. Rosenberg)
See also RFC 2616 (HTTP/1.1), Section 19.4.5.
- I want SIP to be more compact. What can
- First, one should realize that in general, SIP exchanges are only
going to be a tiny fraction of the overall session bandwidth. A typical
SIP call setup takes less than 1000 bytes, or the equivalent of one
second of highly compressed (G.729) audio. Some additional space
savings can be realized by using short headers. (A realistic example
for an audio call setup takes a total of
about 640 bytes, of which about 69 bytes are SIP headers.)
In general, more substantive savings are possible by using either
payload compression (RFC
2393) or link-layer compression, e.g., at the PPP layer. For the
example above, the total size is reduced to about 520 bytes with gzip
- Does SIP do conference control?
- SIP leaves conference control, such as the election of a chair or
floor control, to other protocols. SIP can be used for non-conferencing
applications and floor control may be used outside the scope of
SIP-initiated calls, so it seemed best to separate the functionality.
However, SDP may be used to indicate which media are subject to floor
control and what tools and protocols are to be used. Unfortunately,
there is no IETF-standardized floor control protocol.
- What is the relationship between MGCP and
- The details of combining the two in a system are still being fleshed
out. MGCP is a device control protocol, where a slave (gateway
(MG)) is controlled by a master (media gateway controller (MGC), call
agent). SIP may be used between controllers, in a peer-to-peer
relationship. Note that to the SIP side, the MGC looks like a node with
a large number of connections, but otherwise the same as a "native" SIP
device. Similarly, the MG is completely unaware that the call between
MGCs is established via SIP. Only the MGC needs to understand both
protocols. Additional details.
- What is SIP+ and how does it relate to SIP
- SIP+ was a proposal by Level3 on how to extend SIP to interconnect
two MGCs. This functionality is now being provided by various
orthogonal SIP extensions, including the carriage of multipart MIME
types, the INFO method and others. These are being documented in a BCP
draft. The name SIP+ is obsolete and should not be used to avoid
- How does SIP compare to H.323?
- See H.323 comparison.
- Can H.323 and SIP be used together?
- Yes. SIP can locate the called party and determine its
capabilities, including H.323. H.323 is then used to connect the two
Unfortunately, there is currently no specification on translating
between the two. Conversion is made more difficult by the multiple
versions of H.323 (v1, v2, v3). However, there is at least one product
(Lucent PacketStar IP) that allows SIP and H.323 terminals to call each
- How do I interconnect Q.931 (ISDN signaling) and
- A gateway that initiates an ISDN call based on a SIP call or
vice versa is reasonably straightforward, as sketched in this figure.
- How do I interconnect ISUP (SS7 signaling) and
- SIP can be used either between SS7 nodes or to trigger a phone
call in an SS7 network. While all the details have not been worked out,
the basic call flow is similar to the
- What are the different addresses in
- SIP INVITE requests involve three addresses:
- The host address where the request came from. Responses are sent
back to the same host address, regardless of what the From
header indicates. Note that different requests for the same call can
come from different hosts.
- The From address contains the logical source of the
request. It remains unmodified as a SIP request traverses proxies, for
example. The From address may not be the same as the host
address that generated the SIP request, although that's the typical
- The session description (e.g., SDP) contains one or more addresses
where the caller expects media data (audio, video) to be sent. For some
services, this address may not be the same as the From
- Can SIP be used for Internet telephony
- Yes, in two ways. First, it can indicate to the Internet-based
caller that the callee is reachable via an ITG, via the
Contact header. Secondly, two ITGs connecting parties on
the PSTN can signal new calls to each other, with the destination phone
number contained in the request URL.
- How do I put a call on hold?
- The party wishing to put the other party on hold sends a (re)INVITE,
with a session description containing a null (0.0.0.0) address. When
used with SDP, the ``c'' address field of one or more media types is set
- What is sip-cgi and how does it relate to
- Both are viewed as different approaches for creating VoIP services.
Both are written offline, and both are executed when messages arrive in
order to execute features.
CPL is an XML-based language, while sip-cgi is a mechanism for
invoking scripts or programs written in any language. sip-cgi is very
similar to web cgi scripts.
In its current version, CPL is only invoked when INVITE
requests and responses arrive, while sip-cgi can intercept any request.
sip-cgi is designed to be used by SIP, while CPL can probably be used
by a number of signaling protocols such as Q.931 or H.323.
CPL and sip-cgi differ in their applicability. CPL is designed for
end user service creation. It is intentionally limited in capabilities
and is not a general purpose programming language. Its execution on a
server is generally very fast. CGI is more powerful - you can do nearly
anything. It is programming language independent. It incurs a
process-spawning overhead, so its less efficient than CPL. (CPL is
usually executed in the same process as the server). As a service
provider, I would not want to execute CGI scripts sent to me by end
users. However, I would prefer to use CGI to develop my own services.
Note that CGI may be used as the execution environment for a CPL
script. (Jonathan Rosenberg)
- Is there a SIP interoperability certification?
How can I test interoperability with others?
- There currently is no certification that attests to the
functionality and compatibility of a SIP implementation. However, there
are regular SIP bake-offs where
implementors can test their work. Also, some sites have set up public
- Where can I find more information about SIP?
- The SIP home page contains additional
information about SIP.
Until September 1999, discussion about SIP took place on the MMUSIC
mailing list. The mailing list
also has a HTML
archive. The working group and mailing list is still the appropriate
place for discussions related to SDP, SAP and RTSP, among other topics.
SIP standardization has now moved to the SIP working
group, with its own mailing list and