Network Working Group H. Kaplan
Internet Draft Acme Packet
Intended status: Informational C. Pons
Expires: April 24, 2012 KPN
P. Gorman
Sprint Nextel
October 24, 2011
Routing SIP Requests with ENUM
draft-kaplan-enum-sip-routing-04
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Abstract
A common ENUM use-case is for hop-by-hop or domain-by-domain
"routing" of SIP requests, using private DNS trees and servers.
This document describes this use-case, and a mechanism for a source-
based query/answer mechanism for such.
Table of Contents
1. Terminology.................................................2
2. Introduction................................................3
2.1. Background.............................................3
3. The Problem.................................................5
4. The Proposed Solution.......................................6
4.1. Generating the ENUM-based DNS Query with Source URI....6
5. Source URI Details..........................................6
5.1. Providing Trunk Group Information in Source URI........7
6. Examples....................................................8
6.1. Basic SIP Scenario.....................................8
6.2. Peering SIP Scenario...................................9
6.3. Transit SIP Scenario..................................10
6.4. Basic PSTN Scenario...................................11
7. Security Considerations....................................11
8. IANA Considerations........................................12
9. Acknowledgments............................................12
10. References.................................................12
10.1. Normative References..................................12
10.2. Informative References................................12
Authors' Addresses...............................................13
Appendix A. Why ENUM-DNS vs. Other Protocols....................13
Appendix B. Alternative Solutions...............................13
1. 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 RFC 2119. The
terminology in this document conforms to RFC 2828, "Internet
Security Glossary".
For the purposes of this document and sake of simplicity, only the
ENUM/DNS NAPTR URI result for a SIP URI is discussed, but it applies
to Tel-URI, and potentially other URIs as well.
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Source URI: the URI encoded in the EDNS0 Option data field, as
described in [draft-edns0-source-info].
ENUM Client: a SIP device or PSTN Gateway which generates a DNS
query for E.164 number resolution, per [RFC3761]
ENUM Server: a DNS server which processes DNS queries for E.164
number resolution, per [RFC3761], and is deployed specifically for
that purpose
Prefix: in this document, the term "prefix" is just some arbitrary
number of the leading digits of an E.164 number, such as the country
code, or country plus region, or even including the local exchange
portion. It does not mean additional pre-pended digits used only
for internal routing but which are not part of the called/calling
number, for which the term "prefix" is also commonly used.
SSP: SIP Service Provider, as per [RFC5486].
2. Introduction
The E.164 number to URI DDDS (ENUM) application provides a mapping
from E.164-based "names" to various URIs, including SIP, H.323, and
others, as defined in [RFC3761]. The reader is assumed to be
familiar with ENUM and its normative documents.
The goal of this document is to describe one of the common uses of
ENUM today: SIP Request Routing. SIP Routing using ENUM generally
works very well, but it is still missing one important capability:
source-based queries and results, whereby the resultant routes are
based on the source of the SIP requests or PSTN calls. This source-
based routing problem is described in this document, as well as the
solution, which has been implemented by multiple vendors and is in
use.
2.1. Background
When it was originally created, ENUM DNS entries were intended to be
under the authority of the entity or person identified by the E.164
number, and be something the end user could populate. For example,
for SIP the resultant URI would be the user's global SIP Address-of-
Record URI, or even a specific SIP Contact URI of the user's SIP
User-Agent host. This model is sometimes called "End User ENUM" or
"Public ENUM". In practice, this model has seen fairly limited
deployment or use, for numerous reasons which will not be enumerated
in this document.
Another model called "Infrastructure ENUM" or "Carrier ENUM",
described in [RFC5526], changes the authority model of the ENUM DNS
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entries to make the registrant be the carrier-of-record, as opposed
to the end user. In the Infrastructure ENUM model, the returned URI
was intended to represent a "point of interconnection" into the
carrier-of-record's SIP domain, such as an SBE.
While there are deployments of Infrastructure ENUM, in practice it
is not often deployed as originally defined. The public DNS
database cannot reasonably be usable for URIs which represent
specific points of interconnection or ingress, because such URIs are
rarely usable in a global context; only carriers with direct access
to the interconnection points can use such URIs to reach the
carrier-of-record, and even then the interconnection points would be
different per originating carrier.
One could use specific DNS "views" for Infrastructure ENUM, to
return different answers per querying carrier IP Address range, but
that is difficult to accomplish in the public DNS, in a manageable,
scalable manner. A more reasonable URI to return from the public
DNS database would be a globally reachable SIP Address-of-Record,
but one for which the carrier-of-record is the registrant.
Unfortunately, even that type of URI is difficult to use; both
because many carriers do not wish to publish such data in a public
database, and because in practice few Address-of-Records are
actually globally, directly, and publicly reachable over the
Internet.
An alternative model, often called "Private ENUM", is widely
deployed. Private ENUM uses the DNS Protocol, but not the public
DNS Database. Instead, the database either uses a private domain
suffix/apex reserved for this purpose and known to all participants,
or is provided by local DNS servers which do not tie into the public
IANA-based tree, or more commonly both privacy tactics are used.
The Private ENUM DNS servers typically reside in a private or
restricted IP network, and are only accessible to specific clients.
Such Private ENUM clients are typically constrained to be ones owned
and managed by the carrier, such as SIP Proxies, Application
Servers, PSTN Gateways, Soft-switches, and Session Border
Controllers.
Unlike Infrastructure ENUM, Private ENUM DNS database entries are
not registered and populated by the carrier-of-record for a given
E.164 number. Instead, the private database's administrator (the
local carrier) directly provisions the entries for all E.164 numbers
it cares about, based on various indirect information data sources,
and sets the entry URI values relative to their specific "view".
In some cases the resolved URI still does not represent a point of
interconnection, such as when it is just used for Number Portability
or Calling Name resolution; in other cases it represents a specific
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interconnection point: either for the peering SBE(s) or tandem PSTN
Gateway(s). The interconnection URI identifies either a host, or
possibly also a Trunk Group. When Private ENUM is used for local
interconnection point resolution for SIP requests, it is typically
described as providing an "ENUM Routing" service, or as "SIP Routing
using ENUM", because the private DNS database represents a call
routing database.
3. The Problem
SIP routing based on private-ENUM resolution has been gaining ground
in some large SIP operator networks. However, a need has arisen to
respond with different ENUM responses based on the source
originating number or domain of the SIP request. There are various
reasons for this need, a non-exhaustive list of which are itemized
as follows:
. It is often cheaper to route calls from local source prefix
numbers to other local prefixes numbers in a given region
directly, whereas out-of-region sources going to the same
destination numbers of the same carrier may be cheaper or even
legally required to be sent through an interexchange or transit
provider, or even the PSTN.
. For interconnection traffic between carriers, where calls
coming from a specific region or originating peer need to be
routed through specific routes or border elements to the
terminating or next-peer, usually due to billing and
commercially-related reasons.
. For specific destination numbers, such as premium rate numbers,
where calls towards these specific destination numbers need to
be routed based on the originating region or ingress border
element, to a specific destination node or a specific border
element towards a next-peer, usually due to operational and
capacity management issues.
. For Emergency Services destination numbers, where the
originating information may affect the chosen emergency center
for a call.
. To provide "near-end" or "hot-potato" routing, whereby the
nearest transit inter-exchange point is selected, instead of
the farthest point (which is "far-end" or "cold-potato"
routing).
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4. The Proposed Solution
The proposed solution uses a new EDNS0 Option code, defined in
[draft-edns0-source-info], to add the source SIP/PSTN URI
information into the ENUM DNS query. For example, the Source URI
could be based on the P-Asserted-Identity URI of the SIP request to
be routed, possibly including [RFC4904] Trunk-group parameters from
the received trunk group, if applicable. This Source URI info would
be used by the responding DNS server to "filter" its response based
on the source information as appropriate.
4.1. Generating the ENUM-based DNS Query with Source URI
When an ENUM client such as a SIP Proxy or PSTN Gateway generates an
ENUM-based DNS query following this document's mechanism, it follows
[RFC3761] and transforms the target E.164 number into a reverse-
number dotted-format domain name for the DNS query key. The domain-
name suffix is defined by local policy, but SHOULD NOT be
"e164.arpa" because this mechanism is for purely private use.
The ENUM client then appends an EDNS0 OPT pseudo-RR to the query,
with the sender's maximum UDP length it can handle, as defined in
[RFC2761], as well as the EDNS0 Option-Code reserved by [draft-
ends0-source-info]. Within the Option-Data field it encodes a SIP
or TEL URI, based on the source information of the received SIP
request or PSTN call, as defined in the next section.
If the ENUM client needs to recursively resolve the name, by issuing
the DNS query multiple times to different servers, it MUST add the
same Source URI to each repeated query.
5. Source URI Details
In general, the Source URI can contain whatever the administrator
wishes it to, since this mechanism is defined for private use only.
However, to aid in multi-vendor interoperability, this section
provides guidance for reasonable default behavior. Local policy MAY
override behavior defined in this section.
The Source URI for the EDNS0 Option data field conveys source
originator and transit information to the ENUM Server(s), and from a
logical perspective the Source URI is a brand new URI constructed by
the ENUM client. In order to construct the Source URI, the client
SHOULD use relevant information from the received SIP or PSTN
message fields, as described next.
If the ENUM client received a SIP request which triggered the ENUM
query, and the SIP request contained a P-Asserted-Identity header
value that it trusts to be accurate, the Source URI SHOULD be based
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on the SIP or TEL URI information in the received P-Asserted-
Identity header value. If there was no P-Asserted-Identity header
value in the request, or it does not trust the URI to be accurate,
then the Source URI MAY be based on local policy; for example, it
may be a statically defined or default URI representing the peer, or
the policy may be to not use the Source URI mechanism in such a
case.
If the ENUM client received a PSTN message which triggered the ENUM
query, such as an IAM, the Source URI SHOULD be a TEL URI of the
calling party number.
The Source URI MAY contain additional information providing routing
number (RN) in an 'rn' parameter, and/or carrier identification code
(CIC) in a 'cic' parameter, as defined in [RFC4694]. Note that for a
SIP URI these would be user parameters, not URI parameters. Such
fields would be used to identify the porting information of the
originating number, or the originating carrier.
Although the most common case will be that the Source URI user name
is a phone-number, it need not be the only case and ENUM servers
supporting this mechanism MUST support non-phone-number cases as
valid ENUM queries. In other words, it is not a DNS protocol
failure to receive such a query, even though the ENUM server may not
have an appropriate answer for the query given such source
information, and thus return a DNS Not Found response (as it could
for phone number Source URI cases that it does not have any entries
for).
If the Source URI's SIP URI user name is a phone number, or it is a
TEL URI, the ENUM client MUST NOT encode any visual-separators (the
tokens named visual-separator in [RFC3966]) in it. A Source URI as
a SIP URI of a phone number SHOULD contain a 'user' URI parameter of
the value 'phone' (i.e., a ";user=phone"); however an ENUM server
MAY treat any Source URI user portion as a phone number if it
follows the syntax of a TEL URI for such.
ENUM servers supporting this mechanism MUST ignore unknown fields in
the Source URI, such as unknown parameters or embedded headers.
5.1. Providing Trunk Group Information in Source URI
The Source URI MAY contain trunk group and context information,
encoded as the "tgrp" and "trunk-context" parameters within the URI
per [RFC4904]. Note that for a SIP URI these would be URI user
parameters, not URI parameters.
By definition, trunk group names are scoped to their trunk-context,
and are not global in nature. In particular, a trunk group name
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used by one SSP typically has no meaning to another SSP, and since
it defines a specific trunk of the SSP it is not usable by another
SSP. In order for such information in a Source URI to be usable, it
needs to be what the local SSP's ENUM server can understand and have
policies for. Therefore, the trunk group information SHOULD
identify the PSTN trunk or direct SIP peer trunk from which the
SIP/PSTN request was received, and not any trunk group information
in the received SIP request. In other words, the trunk group
information conveys the local SSP's defined trunk, not what the
previous carrier's trunk name was.
Note that the previous carrier may be a transit carrier rather than
the originating carrier, and thus the trunk group information
conveys the transit provider not originating provider.
6. Examples
The following examples are designed to show various Source URI usage
possibilities. These are not the only possibilities, however.
6.1. Basic SIP Scenario
In the following example, a SIP UA in the local SSP generates an
INVITE to Proxy-1, which performs a private ENUM query using this
document's mechanism.
Local
Domain
ssp.example.com
+--------+
| ENUM |
+---+ +-------+ /| Server |
|SIP|-->| SIP |/ +--------+
|UA | |Proxy-1|
+---+ +-------+
SIP Proxy-1 receives:
INVITE sip:+17815551212@ssp.example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.10.10:5060;branch=z9hG4bK74b43
Max-Forwards: 69
To: <sip:+17815551212@ssp.example.com>
From: Jenny <sip:7818675309@ssp.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511
CSeq: 1 INVITE
Contact: <sip:jenny@192.0.1.1>
Content-Type: application/sdp
Content-Length: ... [SDP omitted from example]
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Although no P-Asserted-Identity header is received in the request,
Proxy-1 authenticates the UA to be authorized for the identity
"sip:+17818675309@ssp.example.com", and thus generates a DNS query
for the domain:
2.1.2.1.5.5.5.1.8.7.1.priv-enum.ssp.example.com
with an EDNS0 OPT RR with the appropriate Option-Code for Source
URI, and the Option-Data Source URI of:
sip:+17818675309@ssp.example.com;user=phone
The ENUM server looks up the query key for the domain, filters the
response based on the Source URI information, and returns the
appropriate NAPTR entries.
6.2. Peering SIP Scenario
In the following example, a SIP INVITE request originates in
orig.example.com from a SIP UA for the destination phone number
+17815551212, with a P-Asserted-Identity of
"sip:+17818675309@orig.example.com", and reaches the local SSP's
Proxy-2. Proxy-2 receives the request over trunk group "tg1-orig-
ssp" in context "ssp.example.com".
|
Originating | Local
Domain | Domain
orig.example.com | ssp.example.com
| +--------+
| | ENUM |
+---+ +-------+ | +-------+ /| Server |
|SIP|->| SIP +------>| SIP |/ +--------+
|UA | |Proxy-1| | |Proxy-2|
+---+ +-------+ | +-------+
SIP Proxy-2 receives:
INVITE sip:+17815551212@ssp.example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.10.10:5060;branch=z9hG4bK74b43
Max-Forwards: 69
To: <sip:+17815551213@ssp.example.com>
From: Jenny <sip:7818675309@orig.example.com>;tag=9fxced76sl
P-Asserted-Identity: "T. Tutone" <sip:+17818675309@orig.example.com>
Call-ID: 3848276298220188511
CSeq: 1 INVITE
Contact: sip:jenny;tgrp=s1;trunk-context=orig.example.net@192.0.1.1
Content-Type: application/sdp
Content-Length: ... [SDP omitted from example]
Proxy-2 generates a DNS query for the domain:
2.1.2.1.5.5.5.1.8.7.1.priv-enum.ssp.example.com
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with an EDNS0 OPT RR with the appropriate Option-Code for Source
URI, and the Option-Data Source URI of:
sip:+17818675309;tgrp=tg1-orig-ssp;
trunk-context=ssp.example.com@orig.example.com;user=phone
Note that although the received Contact URI contains trunk group
information, this is not what Proxy-2 inserts in the Source URI,
since it identifies Proxy-1's trunk info instead of Proxy-2's.
6.3. Transit SIP Scenario
In the following example, a SIP INVITE request originates in
orig.example.com from a SIP UA for the destination phone number
+17815551212, with a P-Asserted-Identity of
"sip:+17818675309@orig.example.com", goes through a transit carrier
trans.example.net, and reaches the local SSP's Proxy-3. Proxy-3
receives the request over trunk "tg1-strans-ssp" in context
"ssp.example"com".
| |
Originating | Transit | Local
Domain | Domain | Domain
orig.example.com |trans.example.net| ssp.example.com
| | +--------+
| | | ENUM |
+---+ +-------+ | +-------+ | +-------+ /| Server |
|SIP|->| SIP +------->| SIP +------->| SIP |/ +--------+
|UA | |Proxy-1| | |Proxy-2| | |Proxy-3|
+---+ +-------+ | +-------+ | +-------+
SIP Proxy-3 receives:
INVITE sip:+17815551212@ssp.example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.10.10:5060;branch=z9hG4bK74b43
Max-Forwards: 69
To: <sip:+17815551213@ssp.example.com>
From: Jenny <sip:7818675309@orig.example.com>;tag=9fxced76sl
P-Asserted-Identity: "T. Tutone" <sip:+17818675309@orig.example.com>
Call-ID: 3848276298220188511
CSeq: 1 INVITE
Contact: sip:jenny;tgrp=s1;trunk-context=trans.example.net@192.0.1.1
Content-Type: application/sdp
Content-Length: ... [SDP omitted from example]
Proxy-3 generates a DNS query for the domain:
2.1.2.1.5.5.5.1.8.7.1.priv-enum.ssp.example.com
with an EDNS0 OPT RR with the appropriate Option-Code for Source
URI, and the Option-Data Source URI of:
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sip:+17818675309;tgrp=tg1-trans-ssp;
trunk-context=ssp.example.com@orig.example.com;user=phone
Note that although the received Contact URI contains trunk group
information, this is not what Proxy-3 inserts in the Source URI,
since it identifies Proxy-2's trunk info instead of Proxy-3's.
6.4. Basic PSTN Scenario
In the following example, a PSTN Gateway in the local SSP receives
an IAM message, and performs a private ENUM query using this
document's mechanism.
Local
Domain
ssp.example.com
+--------+
| ENUM |
+-------+ /| Server |
| PSTN |/ +--------+
|Gateway|
+-------+
PSTN Gateway receives an IAM with a Called Party Number parameter
indicating the international number 17815551212, and a Calling Party
Number parameter indicating the international number of 17818675309,
over PSTN trunk "tg1-pri" in context "ssp.example.com".
The Gateway generates a DNS query for the domain:
2.1.2.1.5.5.5.1.8.7.1.priv-enum.ssp.example.com
with an EDNS0 OPT RR with the appropriate Option-Code for Source
URI, and the Option-Data Source URI of:
tel:+17818675309;tgrp=tg1-pri;trunk-context=ssp.example.com
The ENUM server looks up the query key for the domain, filters the
response based on the Source URI information, and returns the
appropriate NAPTR entries.
7. Security Considerations
Conveying source information in DNS queries exposes the source
information to eavesdropping and modification by intermediaries.
Furthermore, DNS has no default authorization nor authentication
mechanism for client queries, and thus any device can issue such
queries, using any source information it wishes to generate.
Therefore, the mechanism described in this document MUST only be
used in controlled, restricted environments. It is not appropriate
for the general Internet, and will not function correctly with
public Internet DNS servers.
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8. IANA Considerations
This document makes no request of IANA.
9. Acknowledgments
Thanks to Tom Creighton (Comcast), James Yu (Neustar), Nick Russell
(Vodafone), and Timothy Dwight (Verizon) for their feedback and
support. Funding for the RFC Editor function is provided by the
IETF Administrative Support Activity (IASA).
10. References
10.1. Normative References
[RFC2761] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
[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.
[RFC3761] Faltstrom, P., Mealling, M., "The E.164 to Uniform
Resource Identifiers (URI) Dynamic Delegation Discovery System
(DDDS) Application (ENUM)", RFC 3761, April 2004.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
3966, December 2004.
10.2. Informative References
[RFC4904] Gurbani, V., Jennings, C., "Representing Trunk Groups in
tel/sip Uniform Resource Identifiers (URIs)", RFC 4904, June 2007.
[RFC4694] Yu, J., "Number Portability Parameters for the 'tel' URI",
RFC 4694, October 2006.
[RFC5486] Malas, D., Meyer, D., "Session Peering for Multimedia
Interconnect (SPEERMINT) Terminology", RFC 5486, March 2009.
[draft-edns0-source-info] Kaplan, H., Walter, R., Gorman, P.,
Maharishi, M., "EDNS Option Code for SIP and PSTN Source Reference
Info", draft-kaplan-dnsext-enum-sip-source-ref-opt-code-02, March
2011.
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Authors' Addresses
Hadriel Kaplan
Acme Packet
Email: hkaplan@acmepacket.com
Colin Pons
KPN
Email: colin.pons@kpn.com
Pierce Gorman
Sprint Nextel Corporation
Email: sprint.gorman@sprint.com
Appendix A. Why ENUM-DNS vs. Other Protocols
A common question raised in the IETF regarding using DNS for SIP
routing is why use DNS to begin with, instead of another database
query protocol or even SIP itself (through SIP redirect servers).
In the authors' opinions, the following are the major advantages of
DNS which are driving the market for Private-ENUM:
1. Performance - DNS queries and responses are single-transaction,
compact size, efficiently parsed, and can be used over a
connectionless transport (UDP). Compared to SIP Redirect and
other protocols, the performance gains can be drastic.
2. Scalability - DNS has an underlying database scalability model
built into the protocol itself, and taken advantage of by
ENUM's naming scheme, which does not natively exist in other
database protocols nor for SIP Redirect
3. Resiliency - since DNS uses a single request-response
transaction model and runs over UDP, it can be deployed using
an anycast addressing model
4. Interoperability - DNS is a simple, well-understood protocol
with significant maturity and developer experience
5. Time-to-market - most SIP devices and PSTN gateways already
have DNS libraries, and can leverage them for ENUM use fairly
quickly
Appendix B. Alternative Solutions
Today such source-based routing with ENUM is performed through
various means, which are usually cumbersome and error-prone. These
mechanisms typically require the Private ENUM clients and servers to
agree on a common scheme, and thus require every SIP Proxy to know
and use the same proprietary scheme, which leads to interoperability
problems when multiple vendors are used.
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A common example is where the SIP Proxies performing the lookup
change the ENUM base domain name suffix based on the source E.164
number leading digits, and thus the ENUM-DNS servers have a separate
zone per source prefix. Such a scheme needs to be fixed and common;
for example that a 7-digit prefix length always be used for the name
suffix, instead of for only specific source or destination numbers;
the relevant source prefix cannot be a different length for
different numbers, prefixes, or call flows.
Another example is where the ENUM server returns all possible NAPTR
entries in the DNS response, with proprietary indicators in the
NAPTR URIs for the client SIP Proxy to choose from, using the SIP
source information it has. The problem with this approach is that
the same selection algorithm needs to be supported by all clients,
and the DNS response size can grow very, very large. For example,
some routing tables in North America need to have entries for
hundreds of source North American Numbering Plan (NANP) area codes
and local-exchange prefixes, for the same destination number.
Kaplan, et al Expires - April 2011 [Page 14]