Session PEERing for Multimedia M. Haberler
INTerconnect IPA
Internet-Draft M. Hammer
Expires: February 11, 2007 Cisco
O. Lendl
enum.at
August 10, 2006
A Federation based VoIP Peering Architecture
draft-lendl-speermint-federations-02
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Abstract
This document defines the federation concept and proposes a peering
and routing architecture for SIP-based applications. Federations can
be used to establish selective peerings e.g. in the Voice over IP and
Instant Messaging space. Service providers may announce federation
membership as domain attributes. This document contains the policy-
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type definition for federations within the Domain Policy DDDS
Application.
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Federations . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Federation based Routing . . . . . . . . . . . . . . . . . . . 4
4.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. Call Flows . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2.1. Direct Intra-federation calls . . . . . . . . . . . . 6
4.2.2. Single-transit Inter-federation calls . . . . . . . . 6
4.2.3. Multiple-Transit calls . . . . . . . . . . . . . . . . 6
4.3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . 6
4.4. Routing Architecture . . . . . . . . . . . . . . . . . . . 7
4.4.1. Static configuration . . . . . . . . . . . . . . . . . 8
4.4.2. Forward Search . . . . . . . . . . . . . . . . . . . . 8
4.4.3. Route Announcements . . . . . . . . . . . . . . . . . 8
5. Policy-Type template . . . . . . . . . . . . . . . . . . . . . 8
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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1. Terminology
This document uses the terminology as defined in
draft-ietf-speermint-terminology-00 [1].
The acronym VSP will stand for "VoIP Service Provider".
Our definition of VSP encompasses commercial service providers as
well as enterprises and end user operating their own SIP [4] proxy.
2. Introduction
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 [3].
The domain policy DDDS application [2] defines a generic method how a
domain owner may announce the conditions to accept incoming
communications. This documents defines the policy-type for
publishing federation membership.
This document focuses on the use of federations for SIP peering. The
same mechanism may be applied to other application protocols as well.
The difference is in the protocol field of the service parameter in
the NAPTR records.
3. Federations
The proposed method is based upon the concept of a "Federation". A
federation is defined as follows:
A Federation is a group of VoIP service providers which
* agree to accept calls from each other via SIP,
* agree on a set of administrative rules for these calls
(settlement, abuse-handling, ...), and
* agree on rules for the technical details of the
interconnection.
The actual rules are private to the federation and need not be
published. Federation members are expected to know and abide by
these rules.
Federations are identified by URIs. It is RECOMMENDED that
federations use URLs as identifiers which point to documents
describing the federation.
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For the purposes of the domain policy DDDS application, federation
identifiers are opaque strings. The only operations performed on
these identifiers are string comparisons. If the identifier is in
the form of an URL, the document referred to by that URL is never
evaluated during the basic peer discovery process.
The federation named "urn:ietf:rfc:3261" stands for the public
Internet. A SIP service provider who announces his membership in
"urn:ietf:rfc:3261" will accept calls as defined in the generic SIP
RFC [4].
Examples:
o A group of VoIP service providers forms an association and agrees
to accept calls from each other via the public Internet provided
the TLS transport is used for SIP signalling and members present a
valid X.509 cert signed by the association's certificate
authority.
o A group of VoIP service providers build a Layer 3 network for VoIP
peering ("walled garden", e.g. similar to the 3GPP GRX network).
They agree to accept calls from all participants in that network
and settle through a clearinghouse.
o A group of VoIP service providers agree to accept calls
originating from from each other. They use firewall rules to
block calls from all other networks.
o Peering fabric based on SIP: A SIP hub acts as a forwarding proxy
between participants. Intra-federation calls are to be routed
through the SIP hub.
o Peer to Peer SIP clouds: P2P SIP proposes an alternative
resolution method based on distributed hash tables (DHT). The set
participants in each such DHT can be seen as a federation whose
technical rules stipulate the URI resolution via the DHT ring.
4. Federation based Routing
This section outlines how the federations concept relates to the
Speermint routing architecture.
4.1. Assumptions
Many VSPs will prefer not to run open SIP proxies and accept calls
from the public Internet.
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Some VSPs will establish private peerings between each other.
Groups of VSPs will enter into mutual peering agreements. In other
cases, third parties might build such peering fabrics as a service.
Both private peerings and such peering fabrics are federations as
defined by this document.
VSPs might choose to join several federations if it suits their
business strategy. This set of federations defines the range of
destination VSPs reachable with a direct SIP connection.
VSPs which are members of multiple federations may choose to provide
transit services to other VSPs. Such VSPs act as bridges between
federations.
On the other hand, the VSPs who decline to join many federations
might choose to buy transit from VSPs which offer such services.
4.2. Call Flows
To visualize the possible call flows we use the following set of VSPs
and federations:
+-----+
/ FED \
\ 1 /
+-----+
/ | \
/ | \
/ | \
+---+ +---+ +---+
| A | | B | | C | VSPs offering transit services
+---+ +---+ +---+
\ / \ /
\ / \ /
+---+ +---+
/ FED \ / FED \
\ 2 / \ 3 /
+---+ +---+
/ \ / \
/ \ / \
+---+ +---+ +---+
| X | | Y | | Z | VSPs
+---+ +---+ +---+
X, Y, and Z are terminating VSPs which serve as SIP providers for
end-customers. A, B, and C are VSPs offering transit into a
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federation to members of other federations.
4.2.1. Direct Intra-federation calls
Calls from customers of X to customers of Y can be passed directly
according to rules of federation 2. Transit is not required.
Details how X passes traffic to Y are internal to federation 2 - it
could be end-to-end or, for example, through a SIP hub.
4.2.2. Single-transit Inter-federation calls
Calls from X to Z need to traverse a third VSP as X and Z do not
share a common federation. B shares federations with X and Z, thus
it can bridge calls between X and Z. VSP X thus may elect to enlist
the help of B to complete calls to Z.
On a high level this call is the combination of two intra-federation
call legs - one within FED2 from X to B, and one within FED3 from B
to Z. If FED2 and FED3 share the same Layer 3 network, then the RTP
stream may well be end to end (X to Z directly). If not (e.g. FED3
employs a private network), then B needs to provide media relay
service as well.
4.2.3. Multiple-Transit calls
If B is not available, calls from X to Z need to traverse via FED2 to
A, then via FED1 to C, and finally via FED3 to Z. Now there are three
segments in the call.
4.3. Procedures
The basic call flow is as follows (this is an extension to
draft-mahy-speermint-direct-peering):
1. If number-based dialing is used, then the initiating VSP converts
the dial-string to a fully qualified E.164 number and retrieves a
SIP URI through User ENUM and/or Infrastructure ENUM.
2. The initiating VSP performs the Domain Policy DDDS Application
[2] and thus retrieves the set of federation of the target VSP.
If source and destination VSP share a federation then the call is
established according to its rules. The destination VSP can use
non-terminal NAPTRs in his policy announcements to indicate that
he has contracted another VSP to provide transit services towards
him. In that case, the originating VSP repeats this step in the
algorithm in order to find shared federations with the transit
providing VSP.
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3. If no common federation is found, the initiating VSP may choose
to enlist the help of a transit VSP on his side. The call to the
transit VSP follows normal federation rules. See the next
section for details how a suitable transit VSP is selected.
4. For number-based dialing: if no path can be found through either
a common federation or any transit VSP, then the originating VSP
may fall back to PSTN delivery. Thus, the PSTN may be viewed as
just another "default" federation where all VSPs using E.164
numbers and having PSTN connectivity are members.
4.4. Routing Architecture
For the direct intra-federation call, it is sufficient to match the
federation memberships of the initiating and destination VSP. This
matching can be achieved through the domain policy DDDS application.
While direct matching of federations enables direct peering, it does
not solve the universal reachability problem.
In the general case, a routing algorithm is needed: Once the source
VSP does not share a common federation with the destination VSP the
source VSP needs select a transit VSPs. This transit VSP in turn
needs to make a routing decision.
The "next hop" selection is akin to other routing problems, thus
similar approaches can be used. In some way, topology information
beyond the next hop needs to be communicated between VSPs. Other
than in IP (layer 3) routing, announcements need not exclusively be
learned from adjacent nodes and can be published through other means
since IP connectivity can be assumed.
Non-terminal NAPTRs in the Domain Policy DDDS can be used by the
destination VSP to publish a list of VSPs which provides transit
services towards that network. If one assumes that a multi-tier
hierarchy of VSPs will emerge (similar to the current PSTN or
Internet one), then such referrals point up the hierarchy on the
destination side. If the source VSP is a major carrier then climbing
up the hierarchy ladder on the destination side will likely lead to a
known peer.
If the call originates within a small VSP then he might not find a
common federation with one of the major VSPs the destination
(directly or indirectly) refers to. He will need to hand the call to
some larger VSP which he pays to connect him to the major transit
networks.
This document does not propose a routing protocol for that. The
following options are intended to stimulate discussions in the
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SPEERMINT working-group.
4.4.1. Static configuration
For small VSPs this can be simple choice: everything that cannot be
handed off to the destination network directly is relayed to a
default transit provider.
4.4.2. Forward Search
Another option is to follow the referrals up the hierarchy on the
source side, too. Any VSP can do this offline to learn the set of
VSPs and thus the range of federations that are reachable via the
VSPs it has contracted to provide transit service.
Walking the domain policy referrals generates a tree of VSPs which
are all willing to pass calls from/to the root of the tree. Building
these trees both for the source and the destination VSP and then
checking for shared VSPs (or federations) between these two trees
will find a valid path if the top-tier VSPs peer amongst each other.
4.4.3. Route Announcements
SIP messages between federation members could be used to distribute
reachability information. To use the above example:
If X buys transit from B then X might subscribe to a "topology" event
package with B. Using NOTIFIES, B may announce to X its reachable
federations.
The same mechanisms can be used amongst transit VSPs (e.g. in
federation 1) to exchange reachability information. VSP A could
learn through a NOTIFY from C that C is a member of FED3.
5. Policy-Type template
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Policy Type: "fed"
URI Scheme(s): Any URI is allowed.
Functional Specification: The URI acts purely as an identifier
of a federation. If both the sender and the destination
are members of the same federation then they can communicate
using this federation's rules.
Security considerations:
Intended usage: COMMON
Author: Otmar Lendl
6. Examples
The examples show the NAPTR records for some the VSPs from the
diagram from section 4.2. The VSPs shall use domains like vsp-
X.example.com and federations use identifiers like
"http://fed-1.example.org/".
o VSP X is only reachable through FED2, thus:
$ORIGIN vsp-X.example.com
@ IN NAPTR 10 50 "U" "D2P+SIP:fed" (
"!^.*$!http://fed-2.example.org/!" . )
o VSP C is a member of both FED1 and FED3, thus:
$ORIGIN vsp-C.example.com
@ IN NAPTR 10 10 "U" "D2P+SIP:fed" (
"!^.*$!http://fed-1.example.org/!" . )
@ IN NAPTR 20 10 "U" "D2P+SIP:fed" (
"!^.*$!http://fed-3.example.org/!" . )
o The lower order value indicate that C prefers to receive calls via
FED1. B, who is also a member of FED1 and FED3, can choose to
honor that preference and use FED1 when contacting C.
7. Security Considerations
The publishing of the access policy via the DNS RR described in this
draft will reduce the amount of unwanted communication attempts, as
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all well-meaning clients will follow them, but these records cannot
substitute measures to actually enforce the published policy.
8. IANA Considerations
This document registers the policy-type "fed" for the domain policy
DDDS application.
9. Acknowledgements
The author would like to thank Alexander Mayrhofer, Henry Sinnreich,
Eli Katz, Reinaldo Penno, Patrick Melampy, Daryl Malas and Richard
Stastny for their contributions.
10. References
10.1. Normative References
[1] Meyer, D., "SPEERMINT Terminology",
draft-ietf-speermint-terminology-00 (work in progress),
May 2006.
[2] Lendl, O., "The Domain Policy DDDS Application",
draft-lendl-domain-policy-ddds-00 (work in progress),
February 2006.
10.2. Informative References
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] 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.
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Authors' Addresses
Michael Haberler
Internet Foundation Austria
Waehringerstrasse 3/19
Wien A-1090
Austria
Phone: +43 664 4213465
Email: mah@inode.at
URI: http://www.nic.at/ipa/
Mike Hammer
Cisco Systems
13615 Dulles Technology Drive
Herndon VA 20171
USA
Phone: +1-703-484-3069
Email: mhammer@cisco.com
Otmar Lendl
enum.at GmbH
Karlsplatz 1/9
Wien A-1010
Austria
Phone: +43 1 5056416 33
Email: otmar.lendl@enum.at
URI: http://www.enum.at/
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