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SIP Session controllers in an MPLS/VPN environment

 
 
Richard Liu
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      03-03-2004
Using SDP to signal VLAN membership in MPLS/VPN environments

Authors: Richard Liu, Dane Dickens
__________________________________________________ _________________________

This note describes a method for passing VPN membership and private IP
addresses between Session Controllers to enable the direct termination
of media streams in the case where both SIP end points are in the same
VPN.
1. Background
Service providers are now deploying SIP-based VoIP solutions over MPLS
VPNs. Session Controllers (also known as Session Border Controllers)
are often used in such environments to provide firewalling, Network
Address Translation (NAT), Call Admission Control and other functions.
Session Controllers terminate both signalling and media streams across
network borders and domains.

In a MPLS/VPN scenario where the origination and terminating SIP end
points are in the same MPLS VPN, it is desirable for the media streams
to terminate on the actual SIP end points instead of traversing the
Session Controllers.

In many cases, more that one Session Controller may be involved in the
call. This is the case if the service provider has a service that
spans multiple cities with multiple POPs in a national deployment. In
such a scenario, SIP IP phones would register with their nearest
geographically located Session Controller.

Session Controllers are usually attached to MPLS VPNs via 802.1q
VLANS. In most cases, the originating and terminating Session
Controllers are on different Ethernet switches and hence will have
different VLAN to MPLS VPN mappings. Therefore, the terminating
Session Controller will not know the VPN membership or real IP address
of the originating SIP endpoint. In the case where the originating and
terminating SIP end points are in the same MPLS VPN, the terminating
Session Controller can only terminate RTP media streams directly on
the SIP end points if it knows both the VPN membership and real IP
address of the original SIP end point.

The next section describes a method for passing VPN membership and
private IP addresses between Session Controllers to enable the direct
termination of media streams in the case where both SIP end points are
in the same VPN.

2. Signalling between Session Controllers to Pass VPN Membership
and Private IP Addresses
The originating Session Controller will receive a SIP INVITE from the
calling SIP endpoint. This Session Controller, acting as a B2BUA will
terminate the signalling, note which VLAN the calling SIP endpoint is
in and its IP address. It will then start a new call to the call
server agent using the SDP it has just received, but adding a
proprietary "a" line (as specified in RFC 2327) which will include a
tag or identifier which will mark which VPN the caller is in, and the
IP address (this is the IP address before network address
translation).

For example: a=vpn1:10.0.0.2

This would indicate that the calling SIP endpoint belongs to vpn1 and
has an IP address of 10.0.0.2.

When the terminating Session Controller receives a SIP INVITE command,
it will examine the SDP it contains and use the information in the "a"
line to make a decision on how to terminate the media streams. Note
that the terminating Session Controller will have to have some
mechanism that maps VLAN numbers to VPN tags. How this is set up and
maintained is beyond the scope of this note but for small deployments
of Session Controllers it could be maintained manually. If both the
calling and called SIP end points are in the same VPN, the terminating
Session Controller would change the "c" line of the SDP to match the
IP address contained in the "a" line of the SDP. The RTP media stream
would thus flow directly between the SIP end points without traversing
the Session Controllers.
 
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