PCP D. Wing
Internet-Draft T. Reddy
Intended status: Standards Track P. Patil
Expires: March 17, 2014 R. Penno
Cisco
September 13, 2013
PCP Extension for Third Party Authorization
draft-wing-pcp-third-party-authz-01
Abstract
It is often desirable for an application server to permit a flow
across a firewall, as happens today when a firewall includes an
Application Layer Gateway (ALG) function. However, an ALG has
several weaknesses.
This document describes a cryptographic technique for an application
server to permit a flow across a firewall. This technique uses OAuth
and a new PCP option.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on March 17, 2014.
Copyright Notice
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document authors. All rights reserved.
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
4. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 5
5. Obtaining a Token Using OAuth . . . . . . . . . . . . . . . . 7
5.1. Generating the ACCESS_TOKEN option . . . . . . . . . . . . 10
5.2. PCP server processing ACCESS_TOKEN option . . . . . . . . 11
5.3. Processing the PCP response . . . . . . . . . . . . . . . 12
6. PCP Server and Proxy behavior . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1. Normative References . . . . . . . . . . . . . . . . . . . 13
10.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
It is desirable for a third party to permit flows across a firewall.
A typical use-case is a SIP proxy (which is aware of legitimate
calls) which is not co-located with a firewall. Today, this
functionality is provided by a firewall implementing a SIP-aware
Application Layer Gateway function, which examines the SIP signaling
to that SIP proxy and opens the appropriate pinholes for the RTP
media. This has disadvantages, as described in detail in section
Section 3.
This document addresses requirement "Third Party Authorization"
explained in section 4 of [I-D.reddy-pcp-auth-req].
This document proposes that a PCP [RFC6887] client communicate with
an OAuth Authorization Server to obtain a cryptographic token for its
media flow. That token is included in the PCP request and validated
by the PCP server.
Note: There is no relationship with the THIRD_PARTY option defined in
[RFC6887], which serves a different purpose. THIRD_PARTY Option for
MAP and PEER Opcodes described in [RFC6887] is only applicable when
all entities i.e the PCP client, PCP server and Application Server,
are deployed within the same administrative domain. Since PCP server
does not listen on a public interface, an Application Server outside
the site will not be able to use THIRD_PARTY option to request
services on behalf of the client.
2. Notational Conventions
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 [RFC2119].
WebRTC Server: A web server that supports WebRTC
[I-D.ietf-rtcweb-overview].
3. Problem Statement
To protect networks using real-time communications, firewalls or
session border controllers [RFC5853] are typically deployed.
Firewalls usually implement Application Layer Gateway functionality,
which intercepts and analyzes session signaling traffic such as
Session Initiation Protocol (SIP) [RFC3261] messages and creates a
dynamic mapping to permit the corresponding media traffic. In
particular, a firewall extracts media transport addresses, transport
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protocol and ports from session description and creates a dynamic
mapping for media to flow through. This model will not work in the
following cases:
1. Session signaling is end-to-end encrypted (say, using TLS).
2. Firewall does not understand the session signaling protocol, or
extensions to the protocol, used by the endpoints.
3. Session signaling and media traverse different firewalls (e.g.,
signaling exits a network via one firewall whereas media exits a
network via a different firewall)
When an enterprise deploys WebRTC, the above problems are relevant
because:
1. Session signaling between WebRTC application running in a browser
and a web server will use TLS.
2. WebRTC does not enforce a particular session signaling protocol;
therefore, a firewall is unlikely to understand the signaling
protocol.
3. Session signaling and peer-to-peer media may traverse different
firewalls.
As a result firewalls block media traffic.
A mitigation to the problems above is for an enterprise to deploy a
TURN server in the DMZ and have WebRTC clients use the TURN server.
The use-case explained in Section 4.2.5.1 of
[I-D.ietf-rtcweb-use-cases-and-requirements] refers to deploying a
TURN [RFC5766] server to audit all media sessions from inside the
company premises to any external peer.
However, using TURN for all such communication causes some problems
for an enterprise network administrator :
o Enterprise firewalls would typically have granular policies to
permit calls initiated using selected WebRTC servers (Dr. Good) it
trusts and block the rest (Dr. Evil).
o A TURN server just provides a 5-tuple (source IP address,
destination IP address, protocol number, source port number, and
destination port number) for auditing and no other details of the
WebRTC or SIP server being used to establish the call.
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o A TURN server could increase media latency as explained in section
4.1.2.2 of [RFC5245].
o A TURN server could either be located in the DMZ of the enterprise
network or located in the public Internet. If the TURN server is
located in the public Internet it comes at a high cost to the
provider of the TURN server, since the server typically needs a
high-bandwidth connection to the Internet as explained in the
Introduction of [RFC5766]. As a consequence, it is best to use a
TURN server only when a direct communication path cannot be found.
When the client and a peer use ICE to determine communication
path, ICE will use hole punching techniques to search for a direct
path first and only use a TURN server when a direct path cannot be
found.
o Other limitations of TURN are explained in section 2.6 of
[RFC5766]. For example the value of Diffserv field may not be
preserved, Explicit Congestion Notification (ECN) field may be
reset etc.
4. Solution Overview
In the below topology, the main functional elements involved are :
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=========================
| WebRTC Server |
=========================
| 3rd Party Network
|
|
==================
| WAN |-----+-+----+---+----+-+---
================== |
| |
| |
| |
+-------+-------+ |
| Firewall - | |
| PCP Server | |
+-------+-------+ |
| |
| |
Branch office | | Mobile Network
-+-+-----+-----------+-+-----+-------- ----+-+-------+------
| |
+-+------+ +--------+
| Alice | | Bob |
+--------+ +--------+
Users : Alice, Bob
Figure 1: WebRTC server in a different administrative domain
In the topology, a WebRTC Server is deployed in a third party network
trusted by the Enterprise. For the two endpoints to successfully
establish media sessions, a firewall needs to permit ICE [RFC5245]
connectivity checks and subsequent media traffic.
In such a scenario this specification proposes that a PCP client
follows the steps described below:
1. The PCP client makes a PCP request without any authorization. If
the PCP server returns an AUTHORIZATION_REQUIRED error message,
the PCP client concludes that the PCP server is mandating the use
of third party authorization.
2. The PCP client then obtains a cryptographic token from an OAuth
2.0 server.
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3. The PCP client sends a PCP request including the cryptographic
token in the TOKEN_ACCESS option, defined below. Alternatively,
the PCP client could first obtain a cryptographic token from the
OAuth 2.0 server and send the PCP request with the TOKEN_ACCESS
option by default.
4. The PCP server uses the TOKEN_ACCESS option to perform third
party authorization.
This technique can also be used by a PCP-capable NAT to permit a MAP
request from the PCP client so that the client can learn the External
IP Addresses and Ports using MAP request/response. If server
reflexive candidates learnt using STUN [RFC5389] and External IP
addresses/Ports learnt using PCP are different then the candidates
learnt via PCP are encoded in the ICE offer and answer just like the
server reflexive candidates.
This technique can be used by any other Application Function trusted
by the network to permit time-bound, encrypted, peer-to-peer traffic.
5. Obtaining a Token Using OAuth
This section explains OAuth 2.0 authorization framework [RFC6749] to
solve the "Third Party Authorization" requirement explained in
section 4 of [I-D.reddy-pcp-auth-req].
The following mapping of OAuth concepts to PCP is used :
+----------------------+----------------------------+
| OAuth | PCP |
+======================+============================+
| Client | PCP Client |
+----------------------+----------------------------+
| Resource owner | Authorization Server. For |
| | example the WebRTC server |
+----------------------+----------------------------+
| Authorization server | Authorization server. |
+----------------------+----------------------------+
| Resource server | PCP Server |
+----------------------+----------------------------+
Figure 2: OAuth terminology mapped to PCP terminology
Using the OAuth 2.0 authorization framework, a PCP client (third-
party application) obtains limited access to a PCP server (resource
server) on behalf of the WebRTC server (resource owner or
authorization server). The PCP client requests access to resources
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controlled by the resource owner (WebRTC server) and hosted by the
resource server (PCP server). The PCP client obtains an access
token, lifetime, and other access attributes like the PCP options and
opcodes that the PCP client is permitted to use from the
authorization server. The PCP client conveys the token in the PCP
ACESS_TOKEN option to access the protected resources hosted by the
resource server (PCP server). The PCP server validates the token and
takes appropriate action e.g., allows the PCP request to create
mappings on the PCP server.
+---------------+
^| +-------+
+--------->| Authorization | |
| | Server | |
| |(WebRTC Server)| | Authorization
| | |<--+ | (e.g Permit MAP/PEER)
(1) | +---------------+ | | (4)
Access | | |
Token | Get Token | |
Request | Resource | |
| (3) | |
| | |
| | V
+-------+---+ +-+----=-----+
| | (2) | |
| | PCP Request + Access | |
| PCP | Token | PCP |
| Client |---------------------->| Server |
| (Alice) | | (Firewall) |
| | | |
+-----------+ +------------+
User : Alice
Figure 3: Interactions
OAuth in [RFC6749] defines four grant types. This specification uses
the OAuth grant type "Implicit" explained in section 1.3.2 of
[RFC6749] where the PCP client is issued an access token directly.
The scope of the access token explained in section 3.3 of [RFC6749]
MUST be PCP.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Option Code=TBD| Reserved | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Domain Name Length | Reserved1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Domain Name |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| key id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| access token length | Reserved3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| access token |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: PCP ACCESS_TOKEN Option
The fields are described below:
Option Length: 16 bits. Indicates the length of the enclosed data,
in octets.
Domain Name Length: Length of the 'Domain Name' field in octets.
Server Domain Name: The domain name of the Authorized Server that
generated the access token.
Timestamp: 64-bit unsigned integer field containing a timestamp.
The value indicates the time since January 1, 1970, 00:00 UTC, by
using a fixed point format. In this format, the integer number of
seconds is contained in the first 48 bits of the field, and the
remaining 16 bits indicate the number of 1/64K fractions of a
second (Native format - Unix).
Lifetime: The lifetime of the access token since the response was
generated, in seconds. For example, the value 3600 indicates one
hour. The Lifetime value SHOULD be equal to the "expires_in"
parameter defined in section 4.2.2 of [RFC6749].
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key id: key id, which is an identifier generated by the
authorization server. It generates this key id by computing a
hash over the access token using SHA-1 and truncating the hash to
96 bits (retaining the left most bits).
access token length: Length of the access token field in octets.
OAuth does not impose any limitation on the length of the access
token but since PCP messages cannot exceed 1100 octets (Section 7
of [RFC6887]), access token length needs to be restricted to fit
within the maximum PCP message size. The access token is defined
in section 1.4 of [RFC6749]. TBD : what is the recommended/
maximum token length for PCP. We need a discussion of this
maximum length and analysis of what that means
access token: The access token issued by the authorization server.
Option Name: ACCESS_TOKEN
Number: TBA in the mandatory-to-process range (IANA)
Purpose: This option conveys the token granted by the authorization
server for third party authorization.
Valid for Opcodes: MAP, PEER
Length: Variable, but MUST NOT be 0.
May appear in : request. May appear in response only if it appeared
in the associated request.
Maximum occurrences : 1
5.1. Generating the ACCESS_TOKEN option
The mechanism used by an OAuth client to obtain a token from the
OAuth authorization server is outside the scope of this document.
The OAuth client could obtain the token via in-band signaling or an
exclusive out-of-band protocol. This specification uses the token
type Handle described in [RFC6819]. A handle token is a reference to
some internal data structure within the OAuth authorization server;
the internal data structure contains the attributes of the token such
as allowed PCP Opcode or PCP Option, etc. The PCP client, after
receiving the access token from the OAuth authorization server,
generates the ACCESS_TOKEN option which is included in the PCP
request to the PCP server.
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5.2. PCP server processing ACCESS_TOKEN option
A PCP server performs processing in the order described below.
When a PCP server receives a PCP request with an ACCESS_TOKEN option,
it will verify that the access token is valid. To address replay
attacks, the PCP server MUST perform the following check :
When a PCP request with an ACCESS_TOKEN Option is received, the
received timestamp (TSnew in the Timestamp field) is checked and the
cryptographic token is accepted if the timestamp is recent enough to
the reception time of the PCP request, RDnew :
Lifetime + Delta > abs(RDnew - TSnew)
The RECOMMENDED value for the allowed Delta is 5 seconds. If the
timestamp is NOT within the boundaries then discard the PCP request
with AUTHORIZATION_INVALID error response.
After the validation described above, the PCP server communicates
with the authorization server in order to validate the token and
obtain token-bound data. The mechanism for communication is outside
the scope of this document. The PCP server makes a request to the
authorization server to validate the token but produces no other data
with the request. If the token is successfully validated, the
authorization server just returns the token bound authorization data
in the response. The PCP server then matches this authorization data
with what is requested in the PCP request sent by the PCP client. If
the authorization sets match, the PCP server honors the PCP request
made by the PCP client.
If the token is invalid or the request exceeds what is authorized by
the token then the PCP server generates an AUTHORIZATION_INVALID
error response. An example might be that an OAuth authorization
server permits creating 5 mappings, and the PCP request made by the
client is trying to create a 6th mapping.
Handle token type was selected for the following reasons :
1. The Authorization Server can inform the PCP server to revoke the
access token after the call is terminated. This mechanism
ensures that even if the PCP client does not close the dynamic
mapping created, the PCP server based on the revocation
notification from the Authorization Server can close the dynamic
mapping.
2. A PCP-controlled Firewall with restrictive policies may also want
to validate with the Authorization Server if the selected
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candidate pairs in the final offer/answer match the 5-tuple {dest
addr, source addr, protocol, dest port, source port} sessions
traversing the Firewall. This validation ensures that the PCP
client is using the token only to send and receive the media
streams finalized in the call to the remote peer. Thus the PCP
server can make sure that the token cannot be used for anything
else.
3. If PCP authentication [I-D.ietf-pcp-authentication] is used then
the PCP server may also validate with the authorization server if
the access token is issued and used by the same user or not.
Another approach, not discussed in this document, is a self-contained
token where all the information necessary to authenticate the
validity of the token is contained within the token itself. This
approach has the benefit of avoiding a protocol between the PCP
server and the OAuth authentication server for token validation, thus
reducing latency. However, this approach has the drawback of needing
a large PCP packet to accommodate the token and requiring the
authorization server to generate its message integrity over exactly
the PCP fields, in the same order, that will be sent by the PCP
client. Because PCP messages are limited to 1100 octets, using the
handle approach is more flexible and the trade-off for additional
latency is reasonable. The other disadvantages of self-contained
tokens, such as difficulties with revocation etc., are discussed
in[RFC6819].
5.3. Processing the PCP response
Upon receiving a PCP response, the PCP client performs the normal
processing described in Section 8.3 of [RFC6887]. If the PCP
response was SUCCESS (0), the PCP server has determined that the
token is valid. If the PCP response was AUTHORIZATION_INVALID, it
indicates that the token could be invalid, expired or the PCP request
exceeded what is authorized by the token.
6. PCP Server and Proxy behavior
The ACCESS_TOKEN option is mandatory-to-process (its most significant
bit is clear). Thus, per existing behavior described in [RFC6887], a
PCP server receiving this option MUST return the error
MALFORMED_OPTION if the option contents are malformed, or
UNSUPP_OPTION if the option is unrecognized, unimplemented, or
disabled, or if the client is not authorized to use the option.
A PCP Proxy MUST follow the rules mentioned in section of 7 of
[I-D.ietf-pcp-proxy] when the processing the ACCESS_TOKEN option.
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7. Security Considerations
Security considerations discussed in [RFC6887] and PCP authentication
[I-D.ietf-pcp-authentication] are to be taken into account.
A PCP server will delete explicit dynamic mappings after the lifetime
of the cryptographic token expires. The PCP client must obtain a new
cryptographic token from the authorization server before the current
token becomes invalid or expires. The PCP client must propagate the
new cryptographic token to the PCP server to refresh lifetime of
mappings before the current token becomes invalid or expires. The
PCP server in addition to timestamp checking can also maintain a
cache of used kid as an effective countermeasure against replay
attacks.
Discussion: If the additional latency needs to be avoided and it is
permissible to create a PCP mapping briefly for PCP clients, an
implementation could create PCP mappings while the token is being
validated. The PCP server could create a mapping immediately, send a
PCP response and in parallel start verification of the token. If the
verification request times out or returns a failure response, the PCP
mapping can be destroyed and a PCP mapping update is sent to the PCP
client. The PCP server while waiting for the validation response to
arrive from Authorization server can either drop or buffer the
traffic matching the mapping created.
8. IANA Considerations
We request IANA register the PCP option ACCESS_TOKEN and the result
codes AUTHORIZATION_REQUIRED and AUTHORIZATION_INVALID in
[pcp-registry].
9. Acknowledgements
Authors would like to thank Dave Thaler, Charles Eckel and Paul Jones
for their comments and review.
10. References
10.1. Normative References
[I-D.ietf-pcp-authentication]
Wasserman, M., Hartman, S., and D. Zhang, "Port Control
Protocol (PCP) Authentication Mechanism",
draft-ietf-pcp-authentication-01 (work in progress),
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October 2012.
[I-D.ietf-pcp-proxy]
Boucadair, M., Penno, R., and D. Wing, "Port Control
Protocol (PCP) Proxy Function", draft-ietf-pcp-proxy-04
(work in progress), July 2013.
[I-D.reddy-pcp-auth-req]
Reddy, T., Patil, P., Wing, D., and R. Penno, "PCP
Authentication Requirements", draft-reddy-pcp-auth-req-04
(work in progress), July 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC6407] Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
of Interpretation", RFC 6407, October 2011.
[RFC6749] Hardt, D., "The OAuth 2.0 Authorization Framework",
RFC 6749, October 2012.
[RFC6887] Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)", RFC 6887,
April 2013.
[pcp-registry]
IANA, "Port Control Protocol (PCP) Parameters", May 2013,
<http://www.iana.org/assignments/pcp-parameters/
pcp-parameters.xml>.
10.2. Informative References
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for Brower-
based Applications", draft-ietf-rtcweb-overview-08 (work
in progress), September 2013.
[I-D.ietf-rtcweb-use-cases-and-requirements]
Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
Time Communication Use-cases and Requirements",
draft-ietf-rtcweb-use-cases-and-requirements-11 (work in
progress), June 2013.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
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A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC5853] Hautakorpi, J., Camarillo, G., Penfield, R., Hawrylyshen,
A., and M. Bhatia, "Requirements from Session Initiation
Protocol (SIP) Session Border Control (SBC) Deployments",
RFC 5853, April 2010.
[RFC6342] Koodli, R., "Mobile Networks Considerations for IPv6
Deployment", RFC 6342, August 2011.
[RFC6819] Lodderstedt, T., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", RFC 6819,
January 2013.
Authors' Addresses
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
USA
Email: dwing@cisco.com
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: tireddy@cisco.com
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Prashanth Patil
Cisco Systems, Inc.
Bangalore
India
Email: praspati@cisco.com
Reinaldo Penno
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, 95134
USA
Phone:
Email: repenno@cisco.com
URI:
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