SBC SIP Transparency Implementation Guide

1. Introduction

This document provides configuration and provisioning guidance to enable SIP transparency on

1.1 Audience

This document is intended for design engineers, system engineers and operations staff for the purpose of deploying SIP on a 

• Phone: +1 888-391-3434 (Toll-free) or +1 978-614-8589 (Direct)
• Web: https://doc.rbbn.com/display/PORTAL/Salesforce+Login

1.2 Requirements

This document describes configuration procedures related to version 4.2 of the 

1.3 References

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   product documentation—Go to
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  Documentation and Support Portal at: https://doc.rbbn.com/display/ALLDOC/SBC+5000-7000-SWe+Documentation
• Transparency Profile Configuration Examples

2. SIP Transparency

For some SIP elements, transparency is a frequently-debated topic. When transparency for a SIP header or body is desired, the user may often compare the element against a SIP Proxy which is a typical benchmark for significant transparency. Considered a popular comparison, this topic needs to addressed up front when discussing SIP transparency.

2.1 SIP Proxy vs. SIP B2BUA

The SIP devices that connect most peers and endpoints are typically a SIP Proxy or Back-to-Back User Agent (B2BUA). The most transparent device is the SIP Proxy; its behaviors are primarily specified in RFC 3261 and are very basic in its message processing capabilities. The required transparency of a Proxy is one of its few strengths when compared to a B2BUA.

SIP Transparency Spectrum

Although an

While RFC 3261 goes into detail describing the required behavior of a SIP Proxy, its description for a B2BUA could be considered somewhat terse: "Since it is a concatenation of a UAC [User Agent Client] and UAS [User Agent Server], no explicit definitions are needed for its behavior." This statement notwithstanding, debate and research into the transparency behavior of a B2BUA continued, but seemingly without consensus. An often referenced IETF draft (draft-marjou-sipping-01) submitted to the SIPPING WG was not accepted as a working group document.

Admittedly, complete SIP transparency is not achievable due to the needs and requirements of changing some headers. Even a SIP Proxy is not completely transparent. In many scenarios the ability to control and even minimize transparency is a strength of a B2BUA/

• SIP Normalization (including arbitrary SIP Message Manipulation)
• Topology Hiding
• Protocol Translation
• Codec Transcoding (allowing a non-transparent SDP)

Fundamentally, the 

This document describes the

3. 

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SIP Transparency and Control Mechanisms

Since its inception, the 

3.1 Existing 

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Transparency Mechanisms

Prior to release 4.0, SIP header and body transparency was controlled primarily by the use of individual Transparency Flags, mostly within the IP Signaling Profile (IPSP; ipSignalingProfile > commonIpAttributes > transparencyFlags) and apply on the egress leg of a session (egress relative to the SIP message).

 If a header or body did not have a specific flag on the

When a transparency flag was added for a header, it meant that the header was now known and that the unknownHeader flag no longer controlled it.

This methodology was problematic as headers transitioned from unknown to known on the

Starting in version 4.0, the 

3.2 Transparency Profile

A Transparency Profile is a user-configurable profile allowing the user to transparently pass almost any SIP header/body through the

Both already-known and previously-unknown SIP headers and bodies can be configured in a Transparency Profile. By default, no headers or message bodies are present in a Transparency Profile. Up to 256 unique Transparency Profiles can be created on the 

When a header or body is specified in a Transparency Profile, the profile will take precedence over any applicable Transparency Flag. For headers not specified in a transparency profile, the setting of existing Transparency Flags will continue to determine the transparency of that header. In this way, a Transparency Profile can either override or augment existing Transparency Flag settings. This document will describe some usage scenarios where both mechanisms may be used together.

When configuring a Transparency Profile for specific SIP headers,

3.2.1 SIP Message Header

Starting in version 4.0, the 

set profiles services transparencyProfile <profile> sipHeader <SIP Header>

where <SIP Header> is case insensitive, supports up to 31 characters, and supports an "all" entry to match all headers (see section 3.3 for exceptions).

The ability to exclude specific headers from transparency is primarily intended for use in conjunction with the "all" header option.

SIP headers are also configurable using compact form. When configuring specific headers in a Transparency Profile,

Compact form can be received by the

The 

The following SIP headers are not controlled by the Transparency Profile (or any Transparency Flags), and are ignored if configured in a profile:

• Allow
• Call-ID
• CSeq
• Max-Forwards
• Require
• RSeq
• Supported

If Contact Header is specified in a Transparency Profile, then it is treated as full Contact transparency and it will take precedence over other Contact related flags (such as useZoneLevelDomainNameInContact).

3.2.2 SIP Message Body

set profiles services transparencyProfile <profile> sipMessageBody <Content-Type> 

where <Content-Type> is case insensitive, supports up to 127 characters, and supports an "all" entry to match all message bodies except those described in the below list.

The following Content-Types are not controlled by the Transparency Profile and are ignored if configured in a profile:

• application/sdp
• application/dtmf
• application/dtmf-relay
• application/sonus-media
• application/broadsoft
• application/isup
• multipart/mixed
• multipart/related

Multipart/mixed and multipart/related are ignored because the 

A Transparency Profile cannot control the SDP (application/sdp). The SDP and its controls will be discussed later in this document.

The other exceptions are due to existing Relay Flags (see table below) elsewhere within the 

See RelayFlags below for details.

3.3 SIP Header Transparency Behaviors

3.3.1 

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Non-Transparent Methods and Scenarios

The following SIP methods are not supported by a Transparency Profile (or any Transparency Flags):

• ACK (even if the endToEndAck flag is enabled)
• CANCEL
• PRACK

3.3.2 

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Non-Transparent Headers

The following SIP headers are not supported by a Transparency Profile (or any Transparency Flags):

• Allow

• Call-ID

• CSeq

• Max-Forwards

• Require

• RSeq

• Supported

These SIP headers are entirely added and/or modified by the 

3.3.3 In-Dialog vs. Out-of-Dialog

Some header behaviors vary depending on whether they are received in or out of an existing dialog. While the Transparency Profile has been extended in 4.2 to apply to out-of-dialog messages, there are some specific headers whose behavior is not under the control of a Transparency Profile (or Transparency Flags) when received in out-of-dialog messages.

A Dialog "represents a peer-to-peer SIP relationship between two user agents that persists for some time. The dialog facilitates sequencing of messages between the user agents and proper routing of requests between both of them. The dialog represents a context in which to interpret SIP messages." (reference: RFC 3261)

The 

Out-of-Dialog header behavior irrespective of the Transparency Profile or Flags:

3.4 SDP

The

1. Audio
2. Video Main
3. Video Extended (for Content Share)
4. Binary Floor Control Protocol (UDP and TCP)
5. Far End Camera Control (FECC)
6. Message Session Relay Protocol ( MSRP)

For all the above mentioned media types (with the exception of Audio), the 

• C line
• RTCP attributes
• Media direction (a= sendrecv/sendonly/inactive/recvonly)

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  supports Secure Real-Time Transport Protocol (SRTP) media pass-through for SRTP and Secure Real-Time Transport Control Protocol (SRTCP) media streams.

For Audio, the 

• rtpmap and fmtp – For supported codecs, see Audio Support and Video Support.
• Fax Relay attributes (T38)
• ptime & maxptime

You must enable Video (assign a valid video bandwidth) and Audio transparency to achieve the above described behavior using the below CLI syntax.

% set profiles media packetServiceProfile <packetServiceProfileName> packetToPacketControl transcode transcoderFreeTransparency
% set addressContext <addressContextName> zone <zoneName> sipTrunkGroup <trunkGroupName> media sdpAttributesSelectiveRelay enabled
% set addressContext <addressContextName> zone <zoneName> sipTrunkGroup <trunkGroupName> media lateMediaSupport passthru 

3.4.1 SRTP Pass-through

The following diagram illustrates the media flow for an SRTP pass-through call.

SRTP Packet to Packet Media Call Flow

To control this SRTP media pass-through, a new allowPassthru flag is introduced to the secureRtpRtcp of PSP. When allowPassthru flag is enabled along with the security enableSrtp flag, it allows SBC to pass-through SRTP media without authenticating, decrypting, and encrypting it internally. When selected, this flag prioritizes SRTP pass-through media over terminated SRTP media. When disabled, this flag terminates all SRTP and SRTCP media for authentication, encryption, or decryption. This flag is disabled by default.

3.4.2 SDP Transparency Flag

Make note that the sdpTransparencyState signaling object within the SIP Trunk Group should not be considered a general use parameter.  It is specific to some functionality (mainly ICE) and environments; however, this flag does not apply to all types of call flows.

3.5 Relay Flags

Relay Flags exist mostly within the IP Signaling Profile (IPSP; ipSignalingProfile > commonIpAttributes > relayFlags) and apply on the ingress leg of a session (ingress relative to the SIP message).
Relay Flags are intended mainly for SIP Methods (Requests) and Responses (and some SIP message bodies) that normally get consumed or modified by the 

Albeit imprecise, a good method to contrast Relay Flags and Transparency Flags/Profiles is to consider that Relay controls whether a SIP request/response is sent through the

3.6 Relaying REFER Request

If the refer relay flag is disabled, the Call Control (CC) mechanism forwards the REFER request to Digital Signaling (DS). DS exchanges information with the PSX to check the match criteria set in Conditional Relay Matching. 

The matched criteria includes call parameters such as Username, Directory Number (DN), or Fully Qualified Domain Name (FQDN).

• If the call parameters received with the REFER request match the call routing criteria, 
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  relays the REFER request to Egress SIPSG.
• If the call parameters received with the REFER request do not match the call routing criteria, the REFER request is processed locally by
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  . The REFER request acts as the transferor and the call is forwarded to the Egress SIPSG, resulting in call bridging. In this scenario, 
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  sends back a 202 response and proceeds for local processing.

The following figure shows the enhanced REFER request processing call flow:

3.6.1 Configuring 

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For Enhanced Refer Processing

To configure this feature, perform the following steps:

1. Configure 
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   for regular REFER call Blind Transfer.

2. Create SIP_MSG_TYPE_REFER call parameter filter profile (CPFP) in the PSX. Execute the following command to view the CPFP SIP_MSG_TYPE_REFER. This profile is already present in ERE.

   For more information on creating CPFP, refer to PSX documentation.

   > show table profiles callParameterFilterProfile
   Description:
   Profile used for routing based on SIP message type.
   
   Possible completions:
     SIP_MSG_TYPE_INFO      - SIP Message Type is Info
     SIP_MSG_TYPE_MESSAGE   - SIP Message Type is Message
     SIP_MSG_TYPE_NOTIFY    - SIP Message Type is Notify
     SIP_MSG_TYPE_REFER     - SIP Message Type is Refer
     SIP_MSG_TYPE_REGISTER  - SIP Message Type is Register
     SIP_MSG_TYPE_SUBSCRIBE - SIP Message Type is Subscribe
     none                   - seed data for provisioning support
   
   

   All > Profiles > Call Parameter Filter Profile

   Call Parameter Filter Profile List showing SIP_MSG_TYPE_REFER profile

   

   A new script SONS_SIP_REFER_RELAY is seeded in both ERE and PSX.

3. Disable the Refer relay flag in IPSP.

   % set profiles signaling ipSignalingProfile DEFAULT_SIP commonIpAttributes relayFlags refer disable

4. Enable the Notify relay in Egress side on IPSP to relay REFER for DN/Username/FQDN match.

   % set profiles signaling ipSignalingProfile DEFAULT_SIP commonIpAttributes relayFlags notify enable

   All > Profiles > Signaling > Ip Signaling Profile > Common Ip Attributes > Relay Flags

   Notify and Refer relay flags

   

5. Create a new routing label with the script SONS_SIP_REFER_RELAY to trigger process refer request feature.

   The routing label action must be set as script.

   % set global callRouting routingLabel <routing_label> script SONS_SIP_REFER_RELAY action script

   All > Global > Call Routing > Routing Label

   Creating a Routing Label

   

   Routing Label screen

   

6. Configure a DN criteria in the standard route and attach the SIP_MSG_TYPE_REFER profile to the standard route by executing the following command:

   % set global callRouting route none Sonus_NULL Sonus_NULL standard  <Matched_DN or FQDN> 1 all ALL SIP_MSG_TYPE_REFER Sonus_NULL routingLabel <routing_label>

   For DN (Directory Number) or username

   % set global callRouting route none Sonus_NULL Sonus_NULL standard <Matched_DN or Username> 1 all ALL SIP_MSG_TYPE_REFER Sonus_NULL  routingLabel <routing_label>

   For FQDN with DN or username

   The corresponding Sip domain group must be configured in

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   .

   % set global sipDomain <Matched_domain_name >
    
   % set global callRouting route none Sonus_NULL Sonus_NULL standard <Matched_DN or Username> 1 all ALL SIP_MSG_TYPE_REFER  <Matched_domain_name> routingLabel <routing_label>
   
   

   All > Global > Call Routing > Route

   Creating a Route

   

   Route screen

   

7. Execute the following commands to view the call detail status and call media status.

   > show status global callDetailStatus 
   or
   > show status global callMediaStatus 
   
   

3.7 Transparency Profile Usage

As discussed previously, the Transparency Profile does not deprecate any existing Transparency Flag. Those flags continue to function as designed. When a header/body is specified in a Transparency Profile, then the profile takes precedence over any applicable Transparency Flag. For headers/bodies not specified in a transparency profile, the setting of existing Transparency Flags continues to determine the transparency of that header.

When configuring a Transparency Profile for specific SIP headers,

3.7.1 Maximum Transparency using the Transparency Profile

Complete or maximum transparency is occasionally desired, especially during initial integration testing to determine if specific headers are required for the success of certain call flows.

set profiles services transparencyProfile MAX_TRANSPARENCY sipHeader all
set profiles services transparencyProfile MAX_TRANSPARENCY sipMessageBody all
set profiles services transparencyProfile MAX_TRANSPARENCY state enabled
commit
set addressContext <AC> zone <ZONE> sipTrunkGroup <TG> services transparencyProfile MAX_TRANSPARENCY
commit 

Additional Relay Flags also need to be enabled to maximize the transparency of the Trunk Group for testing. See Relay Flags above.

3.7.2 Transparency Profile, All Headers, with Exceptions

The ignoreTransparency header option within the Transparency Profile is primarily used for excluding one or more specific headers when paired with the "all" header option. In the example below, the user wishes to pass all SIP headers except for the History-Info header.

set profiles services transparencyProfile ALMOST_ALL_HDRS sipHeader all
set profiles services transparencyProfile ALMOST_ALL_HDRS sipHeader History-Info ignoreTransparency yes
set profiles services transparencyProfile ALMOST_ALL_HDRS state enabled
commit
set addressContext <AC> zone <ZONE> sipTrunkGroup <TG> services transparencyProfile ALMOST_ALL_HDRS
commit

3.7.3 Existing Deployment Augmented with a Transparency Profile

Existing deployments will likely utilize Transparency Flags, and those that must pass proprietary or otherwise 

While a Transparency Profile can be configured to completely overlap with any existing Transparency Flags settings, it is not required. A Transparency Profile can be configured to simply augment existing Transparency Flags settings with a more surgical configuration and allowing unknownHeader to be disabled.

For example, a user may wish to have the 

Rather than continue to allow all unknown headers through the

set profiles services transparencyProfile IDENTITY_HDRS sipHeader Identity
set profiles services transparencyProfile IDENTITY_HDRS sipHeader y
set profiles services transparencyProfile IDENTITY_HDRS sipHeader Identity-Info
set profiles services transparencyProfile IDENTITY_HDRS sipHeader n
set profiles services transparencyProfile IDENTITY_HDRS state enabled
commit
set addressContext <AC> zone <ZONE> sipTrunkGroup <TG> services transparencyProfile IDENTITY_HDRS
commit
set profiles signaling ipSignalingProfile <IPSP> commonIpAttributes transparencyFlags unknownHeader disable
commit

3.8 Audio Transparency

• recvonly/sendonly/sendrecv/inactive
• crypto
• X-dmi
• rtcp
• fingerprint
• OMR

Audio Transparency Feature is controlled by two flags:

• Enable Transcoder-Free-Transparency for the session (enable on either of the PSPs).
• Enable Selective-SDP-Transparency on both ingress and egress Trunk Groups that receive the relayed SDP.

Bandwidth (b) lines are transparently relayed and do not play any role in calculating the unknown audio codec bandwidth. The following PSP configuration bits for Audio Transparency feature are included for Unknown audio bandwidth reservation to calculate the Unknown audio bandwidth:

• unknownCodecBitRate

• unknownCodecPacketSize

3.8.1 Audio Transparency and Reserve Bandwidth for Preferred Common Codec

By default for pass-through calls, 

3.8.2 Media Policer Reservation For Worst Case Codec

By default, for pass-through calls the

3.8.3 Configuring Audio Transparency

Configuring the basic audio transparency feature contains:

• Enabling the sdpAttributesSelectiveRelay Parameter on Both Ingress and Egress Trunk Groups
• Configuring the transcoderFreeTransparency Parameter on Packet Service Profile
• Configuring audioTransparecy Parameter on Packet Service Profile

Enabling the sdpAttributesSelectiveRelay Parameter on Both Ingress and Egress Trunk Groups

% set addressContext default zone ZONE1 sipTrunkGroup TG_SBX_INT media sdpAttributesSelectiveRelay enabled
% set addressContext default zone ZONE2 sipTrunkGroup TG_SBX_EXT media sdpAttributesSelectiveRelay enabled

Configuring the transcoderFreeTransparency Parameter on Packet Service Profile

% set profiles media packetServiceProfile PSP_INT packetToPacketControl transcode transcoderFreeTransparency 
% set profiles media packetServiceProfile PSP_EXT packetToPacketControl transcode transcoderFreeTransparency

Configuring audioTransparecy Parameter on Packet Service Profile

% set profiles media packetServiceProfile PSP_INT audioTransparency unknownCodecBitRate 124
% set profiles media packetServiceProfile PSP_EXT audioTransparency unknownCodecBitRate 124

% set profiles media packetServiceProfile PSP_INT audioTransparency unknownCodecPacketSize 10
% set profiles media packetServiceProfile PSP_EXT audioTransparency unknownCodecPacketSize 10

% set profiles media packetServiceProfile PSP_INT flags reserveBwForPreferredAudioCommonCodec enable
% set profiles media packetServiceProfile PSP_EXT flags reserveBwForPreferredAudioCommonCodec enable

3.9 SIP Param Filter Profile

The SIP Param Filter Profile includes the following characteristics:

• This profile takes precedence over existing mechanism/flags when transparently passing Allow/Supported/Require headers, but does not impact corresponding configurations established by the operator. It is operators responsibility to ensure the system is configured properly so that transparently-passed values do not conflict with existing configurations. For example, do not configure 100rel as pass-through if 100rel support fro SIP Trunk Group Signaling is disabled.
• The settings of SIP Param Filter Profiles for both ingress and egress legs dictate the actual pass-through results (see SIP Param Filter Profile Behavior table below for details.)
• Pass-through of individual header values is configurable.
• SIP tags are provided for unknown SIP parameter transparency only. Known SIP parameter transparency is still determined using existing SBC application logic (from Ingress leg to Egress leg) and configurations.

The following table explains the SIP Param Filter Profile behavior when using the Allow, Supported and Require headers.