< draft-ietf-stir-problem-statement-01.txt   draft-ietf-stir-problem-statement-02.txt >
Network Working Group J. Peterson Network Working Group J. Peterson
Internet-Draft NeuStar, Inc. Internet-Draft NeuStar, Inc.
Intended status: Informational H. Schulzrinne Intended status: Informational H. Schulzrinne
Expires: June 10, 2014 Columbia University Expires: July 17, 2014 Columbia University
H. Tschofenig H. Tschofenig
Nokia Siemens Networks Nokia Siemens Networks
December 7, 2013 January 13, 2014
Secure Telephone Identity Problem Statement Secure Telephone Identity Problem Statement
draft-ietf-stir-problem-statement-01.txt draft-ietf-stir-problem-statement-02.txt
Abstract Abstract
Over the past decade, Voice over IP (VoIP) systems based on SIP have Over the past decade, Voice over IP (VoIP) systems based on SIP have
replaced many traditional telephony deployments. Interworking VoIP replaced many traditional telephony deployments. Interworking VoIP
systems with the traditional telephone network has reduced the systems with the traditional telephone network has reduced the
overall security of calling party number and Caller ID assurances by overall security of calling party number and Caller ID assurances by
granting attackers new and inexpensive tools to impersonate or granting attackers new and inexpensive tools to impersonate or
obscure calling party numbers when orchestrating bulk commercial obscure calling party numbers when orchestrating bulk commercial
calling schemes, hacking voicemail boxes or even circumventing multi- calling schemes, hacking voicemail boxes or even circumventing multi-
skipping to change at page 1, line 47 skipping to change at page 1, line 47
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This Internet-Draft will expire on June 10, 2014. This Internet-Draft will expire on July 17, 2014.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3 2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. VoIP-to-VoIP Call . . . . . . . . . . . . . . . . . . . . 6 4.1. VoIP-to-VoIP Call . . . . . . . . . . . . . . . . . . . . 6
4.2. IP-PSTN-IP Call . . . . . . . . . . . . . . . . . . . . . 7 4.2. IP-PSTN-IP Call . . . . . . . . . . . . . . . . . . . . . 7
4.3. PSTN-to-VoIP Call . . . . . . . . . . . . . . . . . . . . 8 4.3. PSTN-to-VoIP Call . . . . . . . . . . . . . . . . . . . . 8
4.4. VoIP-to-PSTN Call . . . . . . . . . . . . . . . . . . . . 9 4.4. VoIP-to-PSTN Call . . . . . . . . . . . . . . . . . . . . 9
4.5. PSTN-VoIP-PSTN Call . . . . . . . . . . . . . . . . . . . 9 4.5. PSTN-VoIP-PSTN Call . . . . . . . . . . . . . . . . . . . 10
4.6. PSTN-to-PSTN Call . . . . . . . . . . . . . . . . . . . . 10 4.6. PSTN-to-PSTN Call . . . . . . . . . . . . . . . . . . . . 11
5. Limitations of Current Solutions . . . . . . . . . . . . . . 10 5. Limitations of Current Solutions . . . . . . . . . . . . . . 11
5.1. P-Asserted-Identity . . . . . . . . . . . . . . . . . . . 11 5.1. P-Asserted-Identity . . . . . . . . . . . . . . . . . . . 12
5.2. SIP Identity . . . . . . . . . . . . . . . . . . . . . . 12 5.2. SIP Identity . . . . . . . . . . . . . . . . . . . . . . 14
5.3. VIPR . . . . . . . . . . . . . . . . . . . . . . . . . . 15 5.3. VIPR . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6. Environmental Changes . . . . . . . . . . . . . . . . . . . . 17 6. Environmental Changes . . . . . . . . . . . . . . . . . . . . 19
6.1. Shift to Mobile Communication . . . . . . . . . . . . . . 17 6.1. Shift to Mobile Communication . . . . . . . . . . . . . . 19
6.2. Failure of Public ENUM . . . . . . . . . . . . . . . . . 18 6.2. Failure of Public ENUM . . . . . . . . . . . . . . . . . 19
6.3. Public Key Infrastructure Developments . . . . . . . . . 18 6.3. Public Key Infrastructure Developments . . . . . . . . . 20
6.4. Pervasive Nature of B2BUA Deployments . . . . . . . . . . 18 6.4. Pervasive Nature of B2BUA Deployments . . . . . . . . . . 20
6.5. Stickiness of Deployed Infrastructure . . . . . . . . . . 19 6.5. Stickiness of Deployed Infrastructure . . . . . . . . . . 20
6.6. Relationship with Number Assignment and Management . . . 19 6.6. Relationship with Number Assignment and Management . . . 21
7. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 20 7. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 21
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 21 10. Security Considerations . . . . . . . . . . . . . . . . . . . 22
11. Informative References . . . . . . . . . . . . . . . . . . . 21 11. Informative References . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction 1. Introduction
In many communication architectures that allow users to communicate In many communication architectures that allow users to communicate
with other users, the need for identifying the originating party that with other users, the need for identifying the originating party that
initiates a call or a messaging interaction arises. The desire for initiates a call or a messaging interaction arises. The desire for
identifying the communication parties in the end-to-end communication identifying the communication parties in the end-to-end communication
attempt derives from the need to implement authorization policies (to attempt derives from the need to implement authorization policies (to
grant or reject call attempts) but has also been utilized for grant or reject call attempts) but has also been utilized for
charging. While there are a number of ways to enable identification charging. While there are a number of ways to enable identification
this functionality has been provided by the Session Initiation this functionality has been provided by the Session Initiation
Protocol (SIP) [2] by using two main types of approaches, namely Protocol (SIP) [2] by using two main types of approaches, namely
using P-Asserted-Identity (PAI) [5] and SIP Identity [1], which are using P-Asserted-Identity (PAI) [5] and SIP Identity [1], which are
described in more detail in Section 5. The goal of these mechanisms described in more detail in Section 5. The goal of these mechanisms
is to validate that originator of a call is authorized to claim an is to validate that originator of a call is authorized to claim an
originating identifier. Protocols, like XMPP, use mechanisms that originating identifier. Protocols, like XMPP, use mechanisms that
are conceptually similar to those offered by SIP. are conceptually similar to those offered by SIP.
Although solutions have been standardized, it turns out that the Although solutions have been standardized, it turns out that the
current deployment situation is unsatisfactory and, even worse, there current deployment situation is unsatisfactory and, even worse, there
is little indication that it will be improved in the future. In [10] is little indication that it will be improved in the future. In [11]
we illustrate what challenges arise. In particular, interworking we illustrate what challenges arise. In particular, interworking
with different communication architectures (e.g., SIP, PSTN, XMPP, with different communication architectures (e.g., SIP, PSTN, XMPP,
RTCWeb) or other forms of mediation breaks the end-to-end semantic of RTCWeb) or other forms of mediation breaks the end-to-end semantic of
the communication interaction and destroys any identification the communication interaction and destroys any identification
capabilities. Furthermore, the use of different identifiers (e.g., capabilities. Furthermore, the use of different identifiers (e.g.,
E.164 numbers vs. SIP URIs) creates challenges for determining who is E.164 numbers vs. SIP URIs) creates challenges for determining who is
able to claim "ownership" for a specific identifier; although domain- able to claim "ownership" for a specific identifier; although domain-
based identifiers (sip:user@example.com) might use certificate or based identifiers (sip:user@example.com) might use certificate or
DNS-related approaches to determine who is able to claim "ownership" DNS-related approaches to determine who is able to claim "ownership"
of the URI, telephone numbers do not yet have any similar mechanism of the URI, telephone numbers do not yet have any similar mechanism
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statement as well as requirements to develop a vision on how to statement as well as requirements to develop a vision on how to
advance the state of the art and to initiate technical work to enable advance the state of the art and to initiate technical work to enable
secure call origin identification. secure call origin identification.
2. Problem Statement 2. Problem Statement
In the classical public-switched telephone network, a limited number In the classical public-switched telephone network, a limited number
of carriers trusted each other, without any cryptographic validation, of carriers trusted each other, without any cryptographic validation,
to provide accurate caller origination information. In some cases, to provide accurate caller origination information. In some cases,
national telecommunication regulation codified these obligations. national telecommunication regulation codified these obligations.
This model worked as long as the number of entities was relatively This model worked as long as the number of entities was relatively
small, easily identified (e.g., through the concept of certificated small, easily identified (e.g., through the concept of certificated
carriers) and subject to effective legal sanctions in case of carriers) and subject to effective legal sanctions in case of
misbehavior. However, for some time, these assumptions have no misbehavior. However, for some time, these assumptions have no
longer held true. For example, entities that are not traditional longer held true. For example, entities that are not traditional
telecommunication carriers, possibly located outside the country telecommunication carriers, possibly located outside the country
whose country code they are using, can act as voice service whose country code they are using, can act as voice service
providers. While in the past, there was a clear distinction between providers. While in the past, there was a clear distinction between
customers and service providers, VoIP service providers can now customers and service providers, VoIP service providers can now
easily act as customers, originating and transit providers. The easily act as customers, originating and transit providers. The
problem is moreover not limited to voice communications, as growth in problem is moreover not limited to voice communications, as growth in
text messaging has made it another vector for bulk unsolicited text messaging has made it another vector for bulk unsolicited
commercial messaging relying on impersonation of a source telephone commercial messaging relying on impersonation of a source telephone
number (sometimes a short code). For telephony, Caller ID spoofing number (sometimes a short code). For telephony, Caller ID spoofing
has become common, with a small subset of entities either ignoring has become common, with a small subset of entities either ignoring
abuse of their services or willingly serving to enable fraud and abuse of their services or willingly serving to enable fraud and
other illegal behavior. other illegal behavior.
For example, recently, enterprises and public safety organizations For example, recently, enterprises and public safety organizations
[16] have been subjected to telephony denial-of-service attacks. In [17] have been subjected to telephony denial-of-service attacks. In
this case, an individual claiming to represent a collections company this case, an individual claiming to represent a collections company
for payday loans starts the extortion scheme with a phone call to an for payday loans starts the extortion scheme with a phone call to an
organization. Failing to get payment from an individual or organization. Failing to get payment from an individual or
organization, the criminal organization launches a barrage of phone organization, the criminal organization launches a barrage of phone
calls, with spoofed numbers, preventing the targeted organization calls, with spoofed numbers, preventing the targeted organization
from receiving legitimate phone calls. Other boiler-room from receiving legitimate phone calls. Other boiler-room
organizations use number spoofing to place illegal "robocalls" organizations use number spoofing to place illegal "robocalls"
(automated telemarketing, see, for example, the FCC webpage [17] on (automated telemarketing, see, for example, the FCC webpage [18] on
this topic). Robocalls is a problem that has been recognized already this topic). Robocalls is a problem that has been recognized already
by various regulators, for example the Federal Communications by various regulators, for example the Federal Tde Commission (FTC)
Commission (FCC) recently organized a robocall competition to solicit recently organized a robocall competition to solicit ideas for
ideas for creating solutions that will block illegal robocalls [18]. creating solutions that will block illegal robocalls [19]. Criminals
Criminals may also use number spoofing to impersonate banks or bank may also use number spoofing to impersonate banks or bank customers
customers to gain access to information or financial accounts. to gain access to information or financial accounts.
In general, number spoofing is used in two ways, impersonation and In general, number spoofing is used in two ways, impersonation and
anonymization. For impersonation, the attacker pretends to be a anonymization. For impersonation, the attacker pretends to be a
specific individual. Impersonation can be used for pretexting, where specific individual. Impersonation can be used for pretexting, where
the attacker obtains information about the individual impersonated, the attacker obtains information about the individual impersonated,
activates credit cards or for harassment, e.g., by causing utility activates credit cards or for harassment, e.g., by causing utility
services to be disconnected, take-out food to be delivered, or by services to be disconnected, take-out food to be delivered, or by
causing police to respond to a non-existing hostage situation causing police to respond to a non-existing hostage situation
("swatting", see [20]). Some voicemail systems can be set up so that ("swatting", see [21]). Some voicemail systems can be set up so that
they grant access to stored messages without a password, relying they grant access to stored messages without a password, relying
solely on the caller identity. As an example, the News International solely on the caller identity. As an example, the News International
phone-hacking scandal [19] has also gained a lot of press attention phone-hacking scandal [20] has also gained a lot of press attention
where employees of the newspaper were accused of engaging in phone where employees of the newspaper were accused of engaging in phone
hacking by utilizing Caller ID spoofing to get access to a voicemail. hacking by utilizing Caller ID spoofing to get access to a voicemail.
For numbers where the caller has suppressed textual caller For numbers where the caller has suppressed textual caller
identification, number spoofing can be used to retrieve this identification, number spoofing can be used to retrieve this
information, stored in the so-called Calling Name (CNAM) database. information, stored in the so-called Calling Name (CNAM) database.
For anonymization, the caller does not necessarily care whether the For anonymization, the caller does not necessarily care whether the
number is in service, or who it is assigned to, and may switch number is in service, or who it is assigned to, and may switch
rapidly and possibly randomly between numbers. Anonymization rapidly and possibly randomly between numbers. Anonymization
facilitates automated illegal telemarketing or telephony denial-of- facilitates automated illegal telemarketing or telephony denial-of-
service attacks, as described above, as it makes it difficult to service attacks, as described above, as it makes it difficult to
blacklist numbers. It also makes tracing such calls much more labor- identify perpetators and craft policies to block them. It also makes
intensive, as each such call has to be identified in each transit tracing such calls much more labor-intensive, as each such call has
carrier hop-by-hop, based on destination number and time of call. to be identified in each transit carrier hop-by-hop, based on
destination number and time of call.
It is insufficient to simply outlaw all spoofing of originating It is insufficient to simply outlaw all spoofing of originating
telephone numbers, because the entities spoofing numbers are already telephone numbers, because the entities spoofing numbers are already
committing other crimes and thus unlikely to be deterred by legal committing other crimes and thus unlikely to be deterred by legal
sanctions. Secure origin identification should prevent impersonation sanctions. Secure origin identification should prevent impersonation
and, to a lesser extent, anonymization. However, if numbers are easy and, to a lesser extent, anonymization. However, if numbers are easy
and cheap to obtain, and if the organizations assigning identifiers and cheap to obtain, and if the organizations assigning identifiers
cannot or will not establish the true corporate or individual cannot or will not establish the true corporate or individual
identity of the entity requesting such identifiers, robocallers will identity of the entity requesting such identifiers, robocallers will
still be able to switch between many different identities. still be able to switch between many different identities.
The problem space is further complicated by a number of use cases The problem space is further complicated by a number of use cases
where entities in the telephone network legitimately send calls on where entities in the telephone network legitimately send calls on
behalf of others. Ultimately, any SIP entity can receive an INVITE behalf of others, including "Find-Me/Follow-Me" services.
and forward it any other entity, and the recipient of a forwarded Ultimately, any SIP entity can receive an INVITE and forward it any
message has little means to ascertain which recipient a call should other entity, and the recipient of a forwarded message has little
legitimately target. Also, in some cases, third parties may need to means to ascertain which recipient a call should legitimately target.
temporarily use the identity of another individual or organization, Also, in some cases, third parties may need to temporarily use the
with full consent of the "owner" of the identifier. For example: identity of another individual or organization, with full consent of
the "owner" of the identifier. For example:
The doctor's office: Physicians calling their patients using their The doctor's office: Physicians calling their patients using their
cell phones would like to replace their mobile phone number with cell phones would like to replace their mobile phone number with
the number of their office to avoid being called back by patients the number of their office to avoid being called back by patients
on their personal phone. on their personal phone.
Call centers: Call centers operate on behalf of companies and the Call centers: Call centers operate on behalf of companies and the
called party expects to see the Caller ID of the company, not the called party expects to see the Caller ID of the company, not the
call center. call center.
3. Terminology 3. Terminology
The following terms are defined in this document: The following terms are defined in this document:
In-band Identity Conveyance: In-band conveyance is the presence of In-band Identity Conveyance: In-band conveyance is the presence of
call origin identification information conveyed within the control call origin identification information conveyed within the control
plane protocol(s) setting up a call. Any in-band solution must plane protocol(s) setting up a call. Any in-band solution must
accommodate prevalence of in-band intermdiaries such as B2BUAs. accommodate prevalence of in-band intermdiaries such as B2BUAs.
Out-of-Band Identity Verification: Out-of-band verification Out-of-Band Identity Verification: Out-of-band verification
determines whether the E.164 number used by the calling party determines whether the telephone number used by the calling party
actually exists, whether the calling entity is entitled to use the actually exists, whether the calling entity is entitled to use the
number and whether a call has recently been made from this phone number and whether a call has recently been made from this phone
number. This approach is needed when the in-band technique does number. This approach is needed because the in-band technique
not work due to intermediaries or due to interworking with PSTN does not work in all cases, as when certain intermediaries are
networks. involved or due to interworking with PSTN networks.
Authority Delegation Infrastructure: This functionality defines how Authority Delegation Infrastructure: This functionality defines how
existing authority over E.164 telephone numbers are used in number existing authority over telephone numbers are used in number
portability and delegation cases. It also describes how the portability and delegation cases. It also describes how the
existing numbering infrastructure is re-used to maintain the existing numbering infrastructure is re-used to maintain the
lifecycle of number assignments. lifecycle of number assignments.
Canonical Telephone Number: In order for either in-band conveyance Canonical Telephone Number: In order for either in-band conveyance
or out-of-band verification to work, entities in this architecture or out-of-band verification to work, entities in this architecture
must be able to canonicalize telephone numbers to arrive at a must be able to canonicalize telephone numbers to arrive at a
common syntactical form. common syntactical form.
4. Use Cases 4. Use Cases
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that offer interconnected VoIP service exchange calls using SIP end- that offer interconnected VoIP service exchange calls using SIP end-
to-end, but may also deliver some calls via circuit-switched to-end, but may also deliver some calls via circuit-switched
facilities, as described in separate use cases below. These service facilities, as described in separate use cases below. These service
providers use telephone numbers as source and destination providers use telephone numbers as source and destination
identifiers, either as the user component of a SIP URI (e.g., identifiers, either as the user component of a SIP URI (e.g.,
sip:12125551234@example.com) or as a tel URI [8]. sip:12125551234@example.com) or as a tel URI [8].
As illustrated in Figure 1, if Alice calls Bob, the call will use SIP As illustrated in Figure 1, if Alice calls Bob, the call will use SIP
end-to-end. (The call may or may not traverse the Internet.) end-to-end. (The call may or may not traverse the Internet.)
+------------+ +------------+
| IP-based | | IP-based |
| SIP Phone |<--+ | SIP Phone |<--+
| of Bob | | | of Bob | |
|+19175551234| | |+19175551234| |
+------------+ | +------------+ |
| |
+------------+ |
+------------+ | | IP-based | |
| IP-based | | | SIP Phone | ------------
| SIP Phone | ------------ | of Alice | / | \
| of Alice | / | \ |+12121234567| // | \\
|+12121234567| // | \\ +------------+ // ,' \\\
+------------+ // ,' \\\ | /// / -----
| /// / ----- | //// ,' \\\\
| //// ,' \\\\ | / ,' \
| / ,' \ | | ,' |
| | ,' | +---->|......: IP-based |
+---->|......: IP-based | | Network |
| Network | \ /
\ / \\\\ ////
\\\\ //// -------------------------
-------------------------
Figure 1: VoIP-to-VoIP Call. Figure 1: VoIP-to-VoIP Call.
4.2. IP-PSTN-IP Call 4.2. IP-PSTN-IP Call
Frequently, two VoIP-based service providers are not directly Frequently, two VoIP-based service providers are not directly
connected by VoIP and use TDM circuits to exchange calls, leading to connected by VoIP and use TDM circuits to exchange calls, leading to
the IP-PSTN-IP use case. In this use case, Dan's VSP is not a member the IP-PSTN-IP use case. In this use case, Dan's VSP is not a member
of the interconnect federation Alice's and Bob's VSP belongs to. As of the interconnect federation Alice's and Bob's VSP belongs to. As
far as Alice is concerned Dan is not accessible via IP and the PSTN far as Alice is concerned Dan is not accessible via IP and the PSTN
is used as an interconnection network. Figure 2 shows the resulting is used as an interconnection network. Figure 2 shows the resulting
exchange. exchange.
-------- --------
//// \\\\ //// \\\\
+--- >| PSTN | +--- >| PSTN |
| | | | | |
| \\\\ //// | \\\\ ////
| -------- | --------
| | | |
| | | |
| | | |
+------------+ +--+----+ | +------------+ +--+----+ |
| IP-based | | PSTN | | | IP-based | | PSTN | |
| SIP Phone | --+ VoIP +- v | SIP Phone | --+ VoIP +- v
| of Alice | / | GW | \ +---+---+ | of Alice | / | GW | \ +---+---+
|+12121234567| // `''''''' \\| PSTN | |+12121234567| // `''''''' \\| PSTN |
+------------+ // | \+ VoIP + +------------+ // | \+ VoIP +
| /// | | GW |\ | /// | | GW |\
| //// | `'''''''\\ +------------+ | //// | `'''''''\\ +------------+
| / | | \ | IP-based | | / | | \ | IP-based |
| | | | | | Phone | | | | | | | Phone |
+---->|---------------+ +------|---->| of Dan | +---->|---------------+ +------|---->| of Dan |
| | |+12039994321| | | |+12039994321|
\ IP-based / +------------+ \ IP-based / +------------+
\\\\ Network //// \\\\ Network ////
------------------------- -------------------------
Figure 2: IP-PSTN-IP Call. Figure 2: IP-PSTN-IP Call.
Note: A B2BUA/Session Border Controller (SBC) exhibits behavior that Note: A B2BUA/Session Border Controller (SBC) exhibits behavior that
looks similar to this scenario since the original call content would, looks similar to this scenario since the original call content would,
in the worst case, be re-created on the call origination side. in the worst case, be re-created on the call origination side.
4.3. PSTN-to-VoIP Call 4.3. PSTN-to-VoIP Call
Consider Figure 3 where Carl is using a PSTN phone and initiates a Consider Figure 3 where Carl is using a PSTN phone and initiates a
call to Alice. Alice is using a VoIP-based phone. The call of Carl call to Alice. Alice is using a VoIP-based phone. The call of Carl
traverses the PSTN and enters the Internet via a PSTN/VoIP gateway. traverses the PSTN and enters the Internet via a PSTN/VoIP gateway.
This gateway attaches some identity information to the call, for This gateway attaches some identity information to the call, for
example based on the information it had received through the PSTN, if example based on the information it had received through the PSTN, if
available. available.
-------- --------
//// \\\\ //// \\\\
+->| PSTN |--+ +->| PSTN |--+
| | | | | | | |
| \\\\ //// | | \\\\ //// |
| -------- | | -------- |
| | | |
| v | v
| +----+-------+ | +----+-------+
+---+------+ |PSTN / VoIP | +-----+ +---+------+ |PSTN / VoIP | +-----+
|PSTN Phone| |Gateway | |SIP | |PSTN Phone| |Gateway | |SIP |
|of Carl | +----+-------+ |UA | |of Carl | +----+-------+ |UA |
+----------+ | |Alice| +----------+ | |Alice|
Invite +-----+ Invite +-----+
| ^ | ^
V | V |
+---------------+ Invite +---------------+ Invite
|VoIP | | |VoIP | |
|Interconnection| Invite +-------+ |Interconnection| Invite +-------+
|Provider(s) |----------->+ | |Provider(s) |----------->+ |
+---------------+ |Alice's| +---------------+ |Alice's|
|VSP | |VSP |
| | | |
+-------+ +-------+
Figure 3: PSTN-to-VoIP Call. Figure 3: PSTN-to-VoIP Call.
4.4. VoIP-to-PSTN Call 4.4. VoIP-to-PSTN Call
Consider Figure 4 where Alice calls Carl. Carl uses a PSTN phone and Consider Figure 4 where Alice calls Carl. Carl uses a PSTN phone and
Alice an IP-based phone. When Alice initiates the call the E.164 Alice an IP-based phone. When Alice initiates the call the E.164
number needs to get translated to a SIP URI and subsequently to an IP number needs to get translated to a SIP URI and subsequently to an IP
address. The call of Alice traverses her VoIP provider where the address. The call of Alice traverses her VoIP provider where the
call origin identification information is added. It then hits the call origin identification information is added. It then hits the
PSTN/VoIP gateway. The gateway must verify that Alice can claim the PSTN/VoIP gateway. It is desirable that the gateway verify that
E.164 number she is using before it populates the corresponding Alice can claim the E.164 number she is using before it populates the
calling party number field in telephone network signaling. Carl's corresponding calling party number field in telephone network
phone must be able to verify that it is receiving a legitimate call signaling. Carl's phone must be able to verify that it is receiving
from the calling party number it will render to Carl. a legitimate call from the calling party number it will render to
Carl.
+-------+ +-----+ -C +-------+ +-----+ -C
|PSTN | |SIP | |a |PSTN | |SIP | |a
|Phone |<----------------+ |UA | |l |Phone |<----------------+ |UA | |l
|of Carl| | |Alice| |l |of Carl| | |Alice| |l
+-------+ | +-----+ |i +-------+ | +-----+ |i
--------------------------- | |n --------------------------- | |n
//// \\\\ | |g //// \\\\ | |g
| PSTN | Invite | | PSTN | Invite |
| | | |P | | | |P
\\\\ //// | |a \\\\ //// | |a
--------------------------- | |r --------------------------- | |r
^ | |t ^ | |t
| v |y | v |y
+------------+ +--------+| +------------+ +--------+|
|PSTN / VoIP |<--Invite----|VoIP ||D |PSTN / VoIP |<--Invite----|VoIP ||D
|Gateway | |Service ||o |Gateway | |Service ||o
+------------+ |Provider||m +------------+ |Provider||m
|of Alice||a |of Alice||a
+--------+|i +--------+|i
-n -n
Figure 4: IP-to-PSTN Call. Figure 4: IP-to-PSTN Call.
4.5. PSTN-VoIP-PSTN Call 4.5. PSTN-VoIP-PSTN Call
Consider Figure 5 where Carl calls Alice. Both users have PSTN Consider Figure 5 where Carl calls Alice. Both users have PSTN
phones but interconnection between the two PSTN networks is phones but interconnection between the two PSTN networks is
accomplished via an IP network. Consequently, Carl's operator uses a accomplished via an IP network. Consequently, Carl's operator uses a
PSTN-to-VoIP gateway to route the call via an IP network to a gateway PSTN-to-VoIP gateway to route the call via an IP network to a gateway
to break out into the PSTN again. to break out into the PSTN again.
skipping to change at page 11, line 7 skipping to change at page 12, line 9
alter or inspect SIP messages and offers no cryptographic validation. alter or inspect SIP messages and offers no cryptographic validation.
As P-Asserted-Identity is used increasingly across multiple networks, As P-Asserted-Identity is used increasingly across multiple networks,
it cannot offer any protection against identity spoofing by it cannot offer any protection against identity spoofing by
intermediaries or entities that allow untrusted entities to set the P intermediaries or entities that allow untrusted entities to set the P
-Asserted-Identity information. An overview of addressing spam in -Asserted-Identity information. An overview of addressing spam in
SIP, and explaining how it differs from simiilar problems with email, SIP, and explaining how it differs from simiilar problems with email,
appeared in [9]. appeared in [9].
Subsequent efforts to prevent calling origin identity spoofing in SIP Subsequent efforts to prevent calling origin identity spoofing in SIP
include the SIP Identity effort (the "long term" identity solution) include the SIP Identity effort (the "long term" identity solution)
[1] and Verification Involving PSTN Reachability (VIPR) [14]. SIP [1] and Verification Involving PSTN Reachability (VIPR) [15]. SIP
Identity attaches a new header field to SIP requests containing a Identity attaches a new header field to SIP requests containing a
signature over the From header field value combined with other signature over the From header field value combined with other
message components to prevent replay attacks. SIP Identity is meant message components to prevent replay attacks. SIP Identity is meant
both to prevent originating calls with spoofed From headers and both to prevent originating calls with spoofed From headers and
intermediaries, such as SIP proxies, from launching man-in-the-middle intermediaries, such as SIP proxies, from launching man-in-the-middle
attacks to alter calls passing through. The VIPR architecture attacks to alter calls passing through. The VIPR architecture
attacked a broader range of problems relating to spam, routing and attacked a broader range of problems relating to spam, routing and
identity with a new infrastructure for managing rendezvous and identity with a new infrastructure for managing rendezvous and
security, which operated alongside of SIP deployments. security, which operated alongside of SIP deployments.
skipping to change at page 12, line 29 skipping to change at page 13, line 31
-Asserted-Identity is only as good as the least trustworthy member of -Asserted-Identity is only as good as the least trustworthy member of
a trust domain. Since the contents of P-Asserted-Identity are not a trust domain. Since the contents of P-Asserted-Identity are not
intended for consumption by end users, end users must trust that intended for consumption by end users, end users must trust that
their service provider participates in an appropriate trust domain, their service provider participates in an appropriate trust domain,
as there will be no direct evidence of the trust domain in SIP as there will be no direct evidence of the trust domain in SIP
signaling that end user devices receive. Since the mechanism is so signaling that end user devices receive. Since the mechanism is so
closely modeled on the traditional telephone network, it is unlikely closely modeled on the traditional telephone network, it is unlikely
to provide a higher level of security than that. to provide a higher level of security than that.
Since [5] was written, the whole notion of P- headers intended for Since [5] was written, the whole notion of P- headers intended for
use in private SIP domains has also been deprecated, largely because use in private SIP domains has also been deprecated (see [10],
of overwhelming evidence that these headers were being used outside largely because of overwhelming evidence that these headers were
of private contexts and leaking into the public Internet. It is being used outside of private contexts and leaking into the public
unclear how many deployments that make use of P-Asserted-Identity in Internet. It is unclear how many deployments that make use of P
fact conform with the Spec-T requirements of RFC3324. -Asserted-Identity in fact conform with the Spec-T requirements of
RFC3324.
P-Asserted-Identity also complicates the question of which URI should P-Asserted-Identity also complicates the question of which URI should
be presented to a user when a call is received. Per RFC3261, SIP be presented to a user when a call is received. Per RFC3261, SIP
user agents would render the contents of the From header field to a user agents would render the contents of the From header field to a
user when receiving an INVITE request, but what if the P-Asserted- user when receiving an INVITE request, but what if the P-Asserted-
Identity contains a more trustworthy URI, and presentation is not Identity contains a more trustworthy URI, and presentation is not
restricted? Subsequent proposals have suggested additional header restricted? Subsequent proposals have suggested additional header
fields to carry different forms of identity related to the caller, fields to carry different forms of identity related to the caller,
including billing identities. As the calling identities in a SIP including billing identities. As the calling identities in a SIP
request proliferate, the question of how to select one to render to request proliferate, the question of how to select one to render to
skipping to change at page 14, line 16 skipping to change at page 15, line 24
provision to identify the assignee of a telephone number. While it provision to identify the assignee of a telephone number. While it
could be the case that the domain name portion of a SIP URI signifies could be the case that the domain name portion of a SIP URI signifies
a carrier (like "att.com") to whom numbers are assigned, the SIP a carrier (like "att.com") to whom numbers are assigned, the SIP
Identity mechanism provides no assurance that a number is assigned to Identity mechanism provides no assurance that a number is assigned to
any carrier. For a tel URI, moreover, it is unclear in [1] what any carrier. For a tel URI, moreover, it is unclear in [1] what
entity should hold a corresponding certificate. A caller may not entity should hold a corresponding certificate. A caller may not
want to reveal the identity of its service provider to the callee, want to reveal the identity of its service provider to the callee,
and may thus prefer tel URIs in the From header field. and may thus prefer tel URIs in the From header field.
This lack of authority gives rise to a whole class of SIP identity This lack of authority gives rise to a whole class of SIP identity
problems when dealing with telephone numbers, as is explored in [12]. problems when dealing with telephone numbers, as is explored in [13].
That document shows how the Identity header of a SIP request That document shows how the Identity header of a SIP request
targeting a telephone number (embedded in a SIP URI) could be dropped targeting a telephone number (embedded in a SIP URI) could be dropped
by an intermediate domain, which then modifies and resigns the by an intermediate domain, which then modifies and resigns the
request, all without alerting the verification service: the request, all without alerting the verification service: the
verification service has no way of knowing which original domain verification service has no way of knowing which original domain
signed the request. Provided that the local authentication service signed the request. Provided that the local authentication service
is complicit, an originator can claim virtually any telephone number, is complicit, an originator can claim virtually any telephone number,
impersonating any chosen Caller ID from the perspective of the impersonating any chosen Caller ID from the perspective of the
verifier. Both of these attacks are rooted in the inability of the verifier. Both of these attacks are rooted in the inability of the
verification service to ascertain a specific certificate that is verification service to ascertain a specific certificate that is
authoritative for a telephone number. authoritative for a telephone number.
As deployed, SIP is moreover highly mediated, and mediated in ways As deployed, SIP is moreover highly mediated, and mediated in ways
that [2] did not anticipate. As request routing commonly depends on that [2] did not anticipate. As request routing commonly depends on
policies dissimilar to [15], requests transit multiple intermediate policies dissimilar to [16], requests transit multiple intermediate
domains to reach a destination; some forms of intermediaries in those domains to reach a destination; some forms of intermediaries in those
domains may effectively re-initiate the session. domains may effectively re-initiate the session.
One of the main reasons that SIP deployments mimic the PSTN One of the main reasons that SIP deployments mimic the PSTN
architecture is because the requirement for interconnection with the architecture is because the requirement for interconnection with the
PSTN remains paramount: a call may originate in SIP and terminate on PSTN remains paramount: a call may originate in SIP and terminate on
the PSTN, or vice versa; and worse still, a PSTN-to-PSTN call may the PSTN, or vice versa; and worse still, a PSTN-to-PSTN call may
transit a SIP network in the middle, or vice versa. This necessarily transit a SIP network in the middle, or vice versa. This necessarily
reduces SIP's feature set to the least common dominator of the reduces SIP's feature set to the least common dominator of the
telephone network, and mandates support for telephone numbers as a telephone network, and mandates support for telephone numbers as a
skipping to change at page 21, line 18 skipping to change at page 22, line 36
Display name: This effort does not consider how the display name of Display name: This effort does not consider how the display name of
the caller might be validated. the caller might be validated.
Response authentication: This effort only considers the problem of Response authentication: This effort only considers the problem of
providing secure telephone identity for requests, not for providing secure telephone identity for requests, not for
responses to requests; no solution is here proposed for the responses to requests; no solution is here proposed for the
problem of determining to which number a call has connected. problem of determining to which number a call has connected.
8. Acknowledgments 8. Acknowledgments
We would like to thank Mike Hammer, Dan York, Andrew Allen, Philippe We would like to thank Fernando Mousinho, David Frankel, Penn Pfautz,
Fouquart, Hadriel Kaplan, Russ Housley, Alissa Cooper, Bernard Aboba, Mike Hammer, Dan York, Andrew Allen, Philippe Fouquart, Hadriel
Kaplan, Richard Shockey, Russ Housley, Alissa Cooper, Bernard Aboba,
Sean Turner, Brian Rosen, Eric Burger, and Eric Rescorla for their Sean Turner, Brian Rosen, Eric Burger, and Eric Rescorla for their
discussion input that lead to this document. discussion input that lead to this document.
9. IANA Considerations 9. IANA Considerations
This memo includes no request to IANA. This memo includes no request to IANA.
10. Security Considerations 10. Security Considerations
This document is about improving the security of call origin This document is about improving the security of call origin
skipping to change at page 22, line 20 skipping to change at page 23, line 42
[7] Elwell, J., "Connected Identity in the Session Initiation [7] Elwell, J., "Connected Identity in the Session Initiation
Protocol (SIP)", RFC 4916, June 2007. Protocol (SIP)", RFC 4916, June 2007.
[8] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC [8] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
3966, December 2004. 3966, December 2004.
[9] Rosenberg, J. and C. Jennings, "The Session Initiation [9] Rosenberg, J. and C. Jennings, "The Session Initiation
Protocol (SIP) and Spam", RFC 5039, January 2008. Protocol (SIP) and Spam", RFC 5039, January 2008.
[10] Cooper, A., Tschofenig, H., Peterson, J., and B. Aboba, [10] Peterson, J., Jennings, C., and R. Sparks, "Change Process
for the Session Initiation Protocol (SIP) and the Real-
time Applications and Infrastructure Area", BCP 67, RFC
5727, March 2010.
[11] Cooper, A., Tschofenig, H., Peterson, J., and B. Aboba,
"Secure Call Origin Identification", draft-cooper-iab- "Secure Call Origin Identification", draft-cooper-iab-
secure-origin-00 (work in progress), November 2012. secure-origin-00 (work in progress), November 2012.
[11] Peterson, J., "Retargeting and Security in SIP: A [12] Peterson, J., "Retargeting and Security in SIP: A
Framework and Requirements", draft-peterson-sipping- Framework and Requirements", draft-peterson-sipping-
retarget-00 (work in progress), February 2005. retarget-00 (work in progress), February 2005.
[12] Rosenberg, J., "Concerns around the Applicability of RFC [13] Rosenberg, J., "Concerns around the Applicability of RFC
4474", draft-rosenberg-sip-rfc4474-concerns-00 (work in 4474", draft-rosenberg-sip-rfc4474-concerns-00 (work in
progress), February 2008. progress), February 2008.
[13] Kaplan, H. and V. Pascual, "Loop Detection Mechanisms for [14] Kaplan, H. and V. Pascual, "Loop Detection Mechanisms for
Session Initiation Protocol (SIP) Back-to- Back User Session Initiation Protocol (SIP) Back-to- Back User
Agents (B2BUAs)", draft-ietf-straw-b2bua-loop-detection-02 Agents (B2BUAs)", draft-ietf-straw-b2bua-loop-detection-03
(work in progress), September 2013. (work in progress), December 2013.
[14] Barnes, M., Jennings, C., Rosenberg, J., and M. Petit- [15] Barnes, M., Jennings, C., Rosenberg, J., and M. Petit-
Huguenin, "Verification Involving PSTN Reachability: Huguenin, "Verification Involving PSTN Reachability:
Requirements and Architecture Overview", draft-jennings- Requirements and Architecture Overview", draft-jennings-
vipr-overview-04 (work in progress), February 2013. vipr-overview-06 (work in progress), December 2013.
[15] Rosenberg, J. and H. Schulzrinne, "Session Initiation [16] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263, June Protocol (SIP): Locating SIP Servers", RFC 3263, June
2002. 2002.
[16] Krebs, B., "DHS Warns of 'TDoS' Extortion Attacks on [17] Krebs, B., "DHS Warns of 'TDoS' Extortion Attacks on
Public Emergency Networks", URL: http:// Public Emergency Networks", URL:
krebsonsecurity.com/2013/04/dhs-warns-of-tdos-extortion- http://krebsonsecurity.com/2013/04/dhs-warns-of-tdos-
attacks-on-public-emergency-networks/, Apr 2013. extortion-attacks-on-public-emergency-networks/, Apr 2013.
[17] FCC, , "Robocalls", URL: [18] FCC, , "Robocalls", URL:
http://www.fcc.gov/guides/robocalls, Apr 2013. http://www.fcc.gov/guides/robocalls, Apr 2013.
[18] FCC, , "FCC Robocall Challenge", URL: [19] FTC, , "FTC Robocall Challenge", URL:
http://robocall.challenge.gov/, Apr 2013. http://robocall.challenge.gov/, Apr 2013.
[19] Wikipedia, , "News International phone hacking scandal", [20] Wikipedia, , "News International phone hacking scandal",
URL: http://en.wikipedia.org/wiki/ URL: http://en.wikipedia.org/wiki/
News_International_phone_hacking_scandal, Apr 2013. News_International_phone_hacking_scandal, Apr 2013.
[20] Wikipedia, , "Don't Make the Call: The New Phenomenon of [21] Wikipedia, , "Don't Make the Call: The New Phenomenon of
'Swatting'", URL: http://www.fbi.gov/news/stories/2008/ 'Swatting'", URL: http://www.fbi.gov/news/stories/2008/
february/swatting020408, Feb 2008. february/swatting020408, Feb 2008.
Authors' Addresses Authors' Addresses
Jon Peterson Jon Peterson
Neustar, Inc. Neustar, Inc.
1800 Sutter St Suite 570 1800 Sutter St Suite 570
Concord, CA 94520 Concord, CA 94520
US US
Email: jon.peterson@neustar.biz Email: jon.peterson@neustar.biz
Henning Schulzrinne Henning Schulzrinne
Columbia University Columbia University
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