Session PEERing for Multimedia O. Lendl
INTerconnect enum.at
Internet-Draft Apr 18, 2007
Intended status: Informational
Expires: October 20, 2007
Background and Assumptions of the Speermint WG
draft-lendl-speermint-background-00
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Abstract
This documents provides background for the Speermint WG. It is
intended to spur discussion about the goals of this WG and tries to
provide guidance on what kind of work is needed to facilitate
widespread SIP-based peering of telephony networks.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Interconnection Models . . . . . . . . . . . . . . . . . . . . 3
2.1. The PSTN model . . . . . . . . . . . . . . . . . . . . . . 3
2.2. The email model . . . . . . . . . . . . . . . . . . . . . 4
3. Why is Speermint needed? . . . . . . . . . . . . . . . . . . . 5
3.1. Why the email model failed for SIP . . . . . . . . . . . . 5
3.2. The PSTN model does not fit, either . . . . . . . . . . . 8
4. Core Assumptions . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. The real problem with SPIT . . . . . . . . . . . . . . . . 9
4.2. What is a SIP URI? . . . . . . . . . . . . . . . . . . . . 9
4.3. Peering vs. Reachability . . . . . . . . . . . . . . . . . 10
4.4. The Key to Routing Data . . . . . . . . . . . . . . . . . 10
4.5. Lookups vs. Announcements . . . . . . . . . . . . . . . . 11
4.6. No National Solutions . . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . . . 14
7.2. Informative References . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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1. Introduction
The Speermint WG is chartered to help with the interconnection of SIP
based layer 7 networks. It should not deal with basic IP
connectivity and SIP protocol issues; those are covered by other
working-groups.
Speermint focuses on what guidelines (and perhaps protocol elements)
are needed by service providers and enterprises to move from ad-hoc,
manual peerings to a fully standardized, secure, easy to implement,
and thus widespread SIP based peering setup.
This document aims solely at the telephony network aspects of SIP and
ignores applications like Instant Messaging or Presence which might
also be implemented using SIP. The focus here is on the use of SIP
in PSTN replacement services.
The intent of this document is to advance the discussion in the WG by
providing an alternative look at the problem space. It does not
propose any standard and is not intended for publication as a RFC.
2. Interconnection Models
In order to understand what Speermint is supposed to achieve, it is
necessary to go beyond pure protocol issues and instead also talk
about the ecosystems in which the protocols operate. This section
tries to be purely descriptive and makes no recommendations.
2.1. The PSTN model
The public switched telephone network (PSTN) is built upon the
following fundamental assumptions:
o Global reachability is achieved by interconnecting individual
smaller networks. There is no global lower-layer connectivity:
calls are passed through transit networks on the application
layer.
o There are no ad-hoc connections between networks: all links are
manually configured (physical) lines.
o There is a clear separation between network operators and network
users. This applies both to protocols (e.g. SS7 ISUP versus
ISDN) as well as to the regulatory rules.
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o Routing information is not directly passed from the destination
network to the source network via some global database. Instead,
transit networks communicate to their customers which destinations
they can handle.
o Accounting and settlement are core features.
2.2. The email model
SIP according to RFC 3261/3263 [2][3] follows the email model. It
can be summarized as follows:
o Email and SIP addresses are structured as username@domain. For
routing purposes, only the domain part is relevant. The username
is only interpreted by the machines serving this specific domain.
o The global, public DNS is used to map the domain from the address
to a prioritized set of ingress points which handle incoming
communication requests for this domain. As the DNS is agnostic to
who is querying data stored there, all senders receive the same
set of ingress points.
o In order to achieve worldwide reachability, these ingress points
need to accept incoming requests from the open Internet. If they
reject e.g. incoming packets from China, then there is no backup
path for this communication, and the destination just will not be
reachable from China.
o As anybody on the Internet can contact any destination domain, no
business relationship between sender and destination domain is
required. This implies that there is no settlement: No money is
changing hands because of such a communication.
o There is no inherent distinction between end-users and service
providers hard-coded into the protocol. Any client can do the DNS
lookups himself and directly contact the destination servers.
o Usually clients don't talk directly to each other: On the source
side the email (or SIP INVITE) is handed to a server which
performs the routing algorithm (plus other services like email
retries or NAT traversal), which then contacts his peer on the
destination side which also performs additional services (SIP:
registrar/location lookup, email: virus filtering, spooling)
before handing off the communication to the other end user.
The email model has proved to be extremely successful -- for email.
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3. Why is Speermint needed?
In theory, the Speermint WG is not necessary: The SIP RFCs envision
instant reachability of all SIP devices over the public Internet.
Source networks just need to resolve the domain from the URI
according to RFC 3263 and send the INVITE to the SIP proxy in charge
of that destination domain.
Telephone number (TN) based calling is also supported: RFC 3761 [1]
provides TN to URI mapping and thus reduces the call routing problem
to the already solved case of SIP URI resolution.
* Apparently, the real world did not choose to implement and deploy
SIP and ENUM as initially envisioned by their inventors. In other
words: the motivation for Speermint is the failure of the world to
conform to the original IETF vision of SIP based real-time
communication. *
3.1. Why the email model failed for SIP
Although SIP has won the protocol war against H.323 (just as SMTP won
against X.400), it failed to establish the same sort of ecosystem in
the Internet as SMTP was able to do. The number of SIP users who are
reachable via the open Internet using RFC 3263 is miniscule when
compared to the number of SIP based telephones in operation today.
SIP as a protocol has succeeded, SIP as an ecosystem similar to SMTP
has failed.
The need for Speermint arises from that failure: SIP has seen
widespread deployment within enterprises and service providers, but
the inter-connection part of SIP has not: current deployments usually
do not follow RFC 3263, but use either hard-coded IP-addresses or
private DNS to route calls between VSPs, if they use SIP at all and
not the PSTN.
The same applies to ENUM according to RFC 3761: The technology has
been successful (as the large number of private ENUM trees
demonstrates), but the original vision of ENUM proved to be elusive:
the "golden tree" under e164.arpa contains a fraction of entries
compared to the numbers found in private trees.
Speermint is chartered to provide solutions for the interconnection
problem. It is thus essential to examine why the current standards
have failed. Without this gap-analysis there is little chance that
Speermint will come up with the missing pieces.
As mentioned before, this analysis cannot be restricted to pure
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technological aspects, and will thus touch on the business models
implied by the technical standards. It is the firm believe of the
author that the IETF credo "we don't do business models" has been
implicitly violated by existing standards. One approach is thus to
identify these implications and augment the protocols to be neutral
with respect to ecosystems and business models.
Business model
The email model hard-codes a "sender-keeps-all" settlement regime.
As anybody is able to connect to anybody, no business relationship
is needed between communication partners, thus no termination fees
can be collected.
The economic model of the current carrier landscape in most
countries depends on these charges, and it just does not make
sense for any single carrier to allow anonymous incoming SIP-based
interconnection as that means lost income. If call patterns are
about symmetrical, switching to sender-keeps-all is revenue-
neutral for all carriers. There is no clear path on how such a
fundamental shift in the bedrock of telco settlement could happen.
The end-state might be viable business model, but there is no
incentive for any individual VSP to start the transition.
This argument applies only to VSPs which are substitutes for PSTN
carriers, and not to enterprises operating a SIP infrastructure.
Unwanted calls
Spam over Internet Telephony (SPIT) is another concern. The free
for all nature of the email ecosystem has led to a barrage of
unsolicited email (SPAM) which poses a serious threat to the
usefulness of email.
Email is non-interactive: filters can be deployed to detect spam
by the content of the mail before the recipient is alerted. That
is not possible for SPIT: content is only available after the
recipient has picked up the phone. A number of SPIT mitigation
strategies have been proposed over the last years, their
effectiveness is yet untested.
As of 2007, SPIT is not a problem, mainly because the number of
open reachable SIP devices is so low. Just as SPAM only started
to become a problem after open SMTP servers became common, many
VSPs fear that SPIT will appear if they open up their networks.
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Identity
Traditional telecom services provide reasonably reliable caller
identification. Telcos trust each other's signalling and end-
users have learned to trust caller-id. Such a trust model is not
compatible with the email model of open SIP servers: the INVITE
message can come from any host on the Internet and is thus not
trusted.
Providing a reliable caller identification is also important for
policing: Harassing and abusive calls are more or less under
control, as legal and contractual rules can be enforced by tracing
calls back to the culprit.
The SIP identity standard (RFC 4474) uses a different approach
(certificate-based, end-to-end) than what is current practice in
the telco space (transitive trust).
QoS and Denial of Service
The email model is not suitable for stringent Quality of Service
(QoS) implementations. As there are no pre-arranged relationships
with between all communicating SIP servers, there are no
mechanisms to guarantee neither network performance on the IP
layer for the actual voice transmission, nor can there be
comprehensive tests on SIP layer compatibility.
As the ingress points need to be open to anybody on the Internet,
they are exposed to Denial of Service attacks.
This combination is at odds with the telco mind-set which thrives
on predictable quality and stringent service level guarantees.
Legal Requirements
Operators of public telephony services need to observe a range of
regulatory requirements. These rules were written for the PSTN
scenario with clearly defined boundaries between operators and
users of the telephone network. Changing the interconnection
model make these regulations a bad fit for the email model.
For example, if the user requests CLIR (Calling Line
Identification Restriction) then his VSPs needs to differentiate
in the call handling whether the peering partner is another
commercial VSPs (transmit caller-ID) or an enterprise (suppress
caller-ID). Interconnecting only with other VSPs which operate
under the same rules simplifies compliance.
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3.2. The PSTN model does not fit, either
It is of course possible to rebuild the PSTN based on SIP instead of
SS7. Some might argue that this is what the IMS and NGN efforts are
all about. This is selling SIP and the Internet short: The basic
infrastructure which the Internet offers allows for far more flexible
interconnection arrangements than a simple copy of the PSTN
structure.
Shared Layer 3 Infrastructure
The PSTN is based on trunk lines which connect voice switches.
These trunks are manually established between carriers. Each such
link needs physical ports as well as dedicated bandwidth.
Establishing direct links between carriers is thus only sensible
if call volumes can justify the effort.
In contrast to the point-to-point link world of the PSTN, SIP
assumes an any-to-any IP based network like the Internet. This
has a profound impact on the economics of interconnection: A new
peering is this not a matter of provisioning a new bit-pipe, but
just one of configuring border elements on both sides.
Economic theory states that there must be a optimal number of
peerings per VSP given the costs to establish an interconnection
versus the costs of transit. As the cost structure is
fundamentally different, the mesh density of the optimal SIP based
network will deviate significantly from the current PSTN.
Enterprise Peering
As a corollary: Peering between TDM-based enterprise telephony
systems is usually limited to very high traffic cross-links. As
enterprise to enterprise calls do not require settlement, there is
a huge potential for additional peering in this space.
Dynamic Routing
Worldwide routing in the PSTN is still based mainly on manual
processes. As a consequence, it takes years to a get a new number
range routed in the PSTN. The switch from SS7 to SIP must be
taken as a chance to upgrade the worldwide call routing to a
better routing algorithm.
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4. Core Assumptions
We believe that some of the basic assumptions in the SIP peering
world have to be re-thought as the current orthodoxy stiffles any
progress. Here are some suggestions to that end:
4.1. The real problem with SPIT
SPIT and QoS/DoS issues have often been cited as reason why so few
people (enterprises and commercial VSPs) run an open SIP service.
The IETF has taken on the challenge and tried to develop protocol
extensions which should help with the SIP adoption. These are often
based on the following reasoning:
+------------+ +-------------+ +-------------+
|We want to | |They need to | |They have a |
|interconnect|===(1)===>|run open |===(2)===>|problem with |
|VSPs | |SIP servers | |SPIT and DoS.|
+------------+ +-------------+ +-------------+
The IETF focus has been on step (2). Protocols and procedures have
been proposed to mitigate the exposure of open SIP proxies. These
include the consent framework for SIP, SPIT identification, anti-SPIT
policy rules, the Identity: header, and all the transport layer
security work.
These are all worthwhile proposals which solve certain specific
problems. Regrettably, they don't remove the roadblocks to
widespread SIP-based peering. For that, they tackle the wrong set of
problems. They assume that the email model can be successful and we
just need to make sure that all the associated problems are
addressed.
This assumption is wrong. To move Speermint forward, we need to
tackle step (1) instead.
The question should not be "How can we achieve universal peering?",
but "How can we get VSPs to peer at all?". Instead of "How to keep
out the unwanted connects?" we should focus on "How can we entice
willing partners to a peering?".
4.2. What is a SIP URI?
SIP URIs are used in various contexts: They can specify contact
points (sip:user@10.0.0.1), they can specify next hop information in
a private interconnection setting
(sip:012345678@sbc1.chicago.us.example.net), and they can be public
SIP URIs (sip:alice@example.com) for which the responsible SIP proxy
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can be determined using RFC 3263.
There is yet another interpretation of the SIP URI which may be
relevant for Speermint: The URI as a simple identifier of a telephony
customer without the commonly implied semantic on how that user can
be contacted.
While that is close to the public URI, the difference is important:
RFC 3263 does not apply. There is no simple, globally valid set of
ingress points for calls towards that URI. The default SIP call
routing logic just is not applicable to such URIs.
In other words: It is a very useful concept to use the SIP protocol
and the URIs from RFC 3261 without also adopting RFC 3263, because
the latter more or less implies the email model which has proven to
be unworkable in the Speermint context. It is thus expected that any
SIP URI published in a public infrastructure ENUM will not imply the
applicability of RFC 3263.
In order to place calls, some alternative to RFC 3263 needs to be
developed which accommodates the needs of carriers.
4.3. Peering vs. Reachability
Whatever the interconnection setup, subscribers of a telephony
service expect to be able to call all subscribes of all other VSPs.
When the email model cannot be assumed, this requires the use of
transit networks and thus some sort of routing mechanism to find a
path to the destination VSP.
This is important, but not the focus of Speermint. As the name
states, Speermint is about "peering", i.e. how two (or a group of)
VSPs can interconnect. Speermint is about how one VSP can find out
that a SIP call he is handling can be passed to peering partner and
about details on how the actual call is signalled.
Multi-hop routing is not part of the Speermint scope (yet).
[Note: It is the opinion of the author that the IETF will have to
tackle the complete routing problem in the future. Given the current
state of Speermint, it may be unwise to go beyond peering discovery
at this stage.]
4.4. The Key to Routing Data
Currently, the PSTN side is using telephone numbers (TN) as the key
to the routing information, whereas the RFC 3263 SIP uses the domain
name.
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The TN used to be the perfect identifier for routing as the
hierarchical structure of the number corresponded to the network
topology in the PSTN. The emergence of alternative carriers, number
porting, and service numbers (free-phone, premium rate, ...) changed
that: The telephone number is now primarily a "name" and not any
longer an "address".
Prefix-based routing used to be the way to aggregate routes to
telephone numbers in order to keep the routing tables and their
updates manageable. While that is still useful to encode policies
like "Route all of +43 to Carrier XYZ", within a country number
portability made prefix-based routing increasingly inefficient.
Looking at the routing information from a database design point of
view, it does not make sense to store the set of possible routes
(incl. all meta-data like prices, capacity, quality,...) for every
individual number, as these will be identical for at least all
numbers operated by a single carrier in some area.
Any routing protocol will thus scale by several orders of magnitude
better if it is based on some sort of "Route-ID" which comprises a
carrier identification plus optionally a service or region-specific
tag.
Thus: while the telephone number is the starting point of the routing
information lookup, it is not a good identifier to use as the key for
storing routes.
4.5. Lookups vs. Announcements
Generally speaking, there are two ways how to distribute routing
information:
On Demand
Whenever routing information is needed some external database is
queried.
Example: DNS (including ENUM).
Pro-active Distribution
The information for all possible destinations is distributed
before the first routing decision is made.
Example: BGP, OSPF.
There are of mixed models as well, e.g. when an organization gathers
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routing information pro-actively to load an internal database which
is then queried on an "on-demand" basis by network elements.
Alternatively, some systems might pro-actively fetch the fraction of
the global routing information which covers the most likely
destinations, and only fall back to on-demand queries for the rest.
The on-demand model requires a lower level transport infrastructure
to contact the external database. It's thus clear that Layer 3
routing cannot use that model as this leads to a chicken-and-egg
problem. This is not an issue for the Speermint setting: Basic
Internet connectivity can be assumed.
The pro-active model on the other hand operates under a different
constraint: Distributing all information needed for the routing
decisions to all carriers requires that the aggregate dataset size of
these routing information tables does not exceed sensible size
limits. For example, it is not feasible to replace the MX record
lookup of mail-servers by a routing protocol which replicates the
domain-name to mail-server mapping information to all ISPs and
enterprise mail-servers. There are just too many domains in use and
thus the "mail-routing tables" would exceed all practical limits.
With regards to TN based calls, both options are possible. ENUM
according to RFC 3761 is a clear on-demand approach. On the PSTN
side, downloads of database dumps are a common method to distribute
routing information.
Regarding routing in the Speermint context, we need to scale the
system to the set of all active TN. They number in the billions.
Installing a "full routing table" into a core telephony (soft-)switch
is thus not feasible. Current PSTN implementations cope by crude
aggregation of routes to foreign countries.
The number of reachable IP addresses is roughly of the same order of
magnitude, but the aggregation properties of IP addresses reduces to
global routing table to under 500000 entries without any impact on
the quality of the routing decisions. Telephone numbers don't
aggregate as well which makes a TN-based protocol similar to BGP
infeasible (see TRIP [4]).
The obvious solution is to add an on-demand mapping step ahead of the
routing protocol. That on-demand mapping should include the option
to seed a cache with the most likely destination TNs.
4.6. No National Solutions
Telecom regulation, especially concerning number assignments and
interconnection rules, is a national matter. Calling patterns favor
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local destinations: local and national calls make up the majority of
all calls. It is thus not surprising that number based PSTN (and
some of the emerging VoIP-based) routing exchanges only deal with
numbers from a single country code.
On the other hand, international voice termination markets deal
usually not with individual numbers, but with routes to number
prefixes.
Given the increasingly international footprint of voice operators the
country-specify ways of handling inter-carrier routing is an
anachronism. Just as the Internet routing does not care about
national borders, there is no inherent reason why a single set of TN
mapping and voice routing protocols can't be seamlessly deployed in
an international setting. There should be no need for special
handling per country-code in the routing logic.
Consider the case of a pan-European mobile operator X. If X has
signed a peering agreement with a local Austrian VoIP operator Y,
then Y should pass all calls over that link which terminate in any
GSM network that X operates. Ideally Y should notice when a number
was ported to X's network in Germany and adapt the routing. If X
acquires a new network in, say Bulgaria, then Y should automatically
route all calls to that set of numbers over the peering with X. All
this should happen without Y having to participate in Germany- or
Bulgaria-specific TN database exchanges.
All this has been standard in Internet-based communication: both BGP
as well as application layer protocols like SMTP or HTTP do not care
about national borders. The protocol to resolve a .com name is the
same as the one to resolve within .cn. BGP speakers announce routes
without any regard for national borders. Speermint should strive to
achieve the same level of universality.
This does not preclude local optimizations: e.g. if the mapping from
TN to some sort of routing identifier is done by Infrastructure ENUM,
then it makes sense to pro-actively prime the VSP name-servers with
the data for all local numbers.
5. Security Considerations
Not applicable at this stage of the discussion.
6. IANA Considerations
Not applicable.
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7. References
7.1. Normative References
[1] Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource
Identifiers (URI) Dynamic Delegation Discovery System (DDDS)
Application (ENUM)", RFC 3761, April 2004.
[2] 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.
[3] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002.
7.2. Informative References
[4] Rosenberg, J., Salama, H., and M. Squire, "Telephony Routing
over IP (TRIP)", RFC 3219, January 2002.
Author's Address
Otmar Lendl
enum.at GmbH
Karlsplatz 1/9
Wien A-1010
Austria
Phone: +43 1 5056416 33
Email: otmar.lendl@enum.at
URI: http://www.enum.at/
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