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)
• Crypto—
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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. 
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  does not terminate the Session Description Protocol (SDP) security description or SRTP media streams and passes them through without authenticating, decrypting, and encrypting. In this pass-through mode of operation, 
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  treats SRTP media as plain text RTP pass-through media. on a leg by leg basis

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 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 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,

3.6.1 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.

3.6.2 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

3.6.3 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

3.7 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

3.7.1 Audio Transparency and Reserve Bandwidth for Preferred Common Codec

By default for pass-through calls, 

3.7.2 Media Policer Reservation For Worst Case Codec

By default, for pass-through calls the

3.8 Allow/Supported/Require Transparency Profiles

To support this functionality the following profiles are added:

• Allow
• Supported
• Require

This feature provides SIP tags only for unknown SIP parameter transparency. Known SIP parameter transparency is still determined using existing 

These profile have precedence of existing mechanism/flags when it comes to passing through content of Allow/Supported/Require headers but it does not impact corresponding semantics executed by

The profile both for Ingress and Egress legs determines the actual pass-through result. It is possible to control pass-through for individual values for each header.

Allow/Supported/Require Profiles related procedures are executed after Ingress SMM and before other Ingress processing on the Ingress leg and after other Egress processing and before Egress SMM on the Egress leg.

The following table explains the semantics of the Allow/Supported/Require Profiles.

reserveBwForPreferredAudioCommonCodec