SBC SIP Transparency Implementation Guide

Introduction

This document provides configuration and provisioning guidance to enable SIP transparency on SBC Core systems. In addition to the configuration examples on the Session Border Controller (SBC), this document provides an introduction to key topics related to SIP headers and bodies on the SBC Core platform.

Audience

This document is intended for design engineers, system engineers and operations staff for the purpose of deploying SIP on a SBC Core system. Although this document provides some background on the concepts involved, the reader is expected to have a basic understanding of SIP.

Support

Ribbon Technical Support can be obtained using the following methods:

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

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.

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 SBC is not defined in any IETF standard, it is most closely associated functionally with a SIP B2BUA (RFC 5853, 7092). Unless otherwise specified, this document will use B2BUA and SBC terms interchangeably.

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/SBC. Some key selling points of an SBC highlight its ability to not be transparent:

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

Fundamentally, the Ribbon SBC behaves as a SIP Back-to-Back User Agent (B2BUA) and not as a SIP Proxy. (If SIP Proxy behavior is actually needed then use of the Ribbon PSX Policy Server should be considered as it can be deployed specifically as a SIP Proxy or Redirector.) Unlike a standard SIP Proxy, the Ribbon SBC can provide a wide spectrum of SIP message transparency, from fairly transparent to almost completely non-transparent.

This document describes the SBC SIP transparency controls, how they behave, and how they interact. Some configuration examples using these transparency controls is also provided.

SBC SIP Transparency and Control Mechanisms

Since its inception, the SBC includes two related types of control flags: Relay Flags and Transparency Flags. Relay Flags primarily control SIP at the Request and Response level and are discussed later in a separate section. Transparency Flags control SIP headers and bodies that are generally not modified when received in a SIP message. While these controls are related, there is no direct overlap or precedence between them.

SIP Message Flow

Existing SBC 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 SBC, it was treated as unknown, which meant it, along with any other SBC-unknown header, was controlled by the single unknownHeader flag (or unknownBody).

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 SBC. It also meant that the unknownHeader flag was a very coarse control as it would allow any header that was unknown to the SBC.

The SBC introduced a more robust future-proofing mechanism in release 4.0 called the Transparency Profile. Ribbon SBC version 4.2 extends the Transparency Profile with similar support for SIP message bodies and the flexible ability to explicitly exclude some headers and/or methods.

SIP Transparency Profile

A Transparency Profile is a user-configurable profile allowing the user to transparently pass almost any SIP header/body through the SBC. It is no longer necessary for a user to request SBC to create a specific Transparency Flag for the desired header/body.

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. If a received Content-Type header value matches any “Message Body” entry configured in the Transparency Profile, the SBC transparently passes the corresponding message body in the outgoing message. The SBC supports configuring up to 256 Transparency Profiles.

The following functionality is included:

• Ability to transparently pass "all" SIP headers and message body types.
• Selectively ignore transparency of select headers and message body types (useful when transparency is enabled for all headers and/or message body types).
• Exclude one or more methods for which transparency of headers or message body types is not needed.

When configuring a Transparency Profile for specific SIP headers, Ribbon recommends that the unknownHeader flag be disabled (similarly, when configuring a Transparency Profile for specific SIP message bodies, Ribbon recommends that the unknownBody flag be disabled).

The following transparency is not supported by the SBC:

• ACK messages are not normally sent end-to-end through the SBC. Transparency of ACK messages is not supported even if the endToEndAck flag is enabled in the "IP Signaling Profile".
• In late media scenarios, the SBC does not support transparency of headers and bodies.

For configuration details, see Service Profiles - Transparency Profile (EMA) or Transparency Profile - CLI.

SIP Message Header

The SBC introduced the Transparency Profile in release 4.0 to configure one or more SIP headers in a single profile to be passed transparently through. Release 4.2 extended the abilities of the Transparency Profile further. It now supported transparency for out-of-dialog messages, the ability to exclude specific headers from transparency and the ability to configure transparency on a per-method basis (e.g. INVITE, REGISTER, SUBSCRIBE, REFER, etc...), where specific methods can be excluded from transparency for that header. If no methods are specified to be excluded, then the configured header will be transparent for all methods.

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, Ribbon recommends the configuration of both compact and long forms.

Compact form can be received by the SBC, but the Ribbon SBC never generates the Compact form of any headers.

The Ribbon SBC does not send multiple header instances as a comma separated list; they are always sent as separate headers.

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

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

• RAck

• P-Associated-URI

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).

SIP Message Body

Ribbon SBC version 4.2 extends the Transparency Profile with similar support for SIP message bodies. In addition, both message header and body transparency is configurable on a per-method basis (e.g. INVITE, REGISTER, SUBSCRIBE, REFER, etc...), where specific methods can be excluded from transparency for that body. If no methods are specified to be excluded, then the configured body is transparent for all methods.

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/alternative

Multipart/mixed and multipart/alternative are ignored because the SBC automatically matches each component message body contained within a multipart message independently. For example, if "application/qsig" is configured in a profile, the SBC will match it even if it is contained within a multipart/mixed message with no additional configuration needed.

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 SBC.

See Relay Flags below for details.

Inter-working with IP Signaling Profile

SIP Transparency Profile provides advanced control of the transparency of headers and message bodies. However, customers may continue with the existing (albeit simple) IPSP transparency controls in PSX/e-PSX/ERE.

Using message body transparency as an example:

• If a message body is allowed by the IPSP but configured to be ignored by the Transparency Profile, it is not transparently passed through.
• If a message body is allowed by the IPSP but configured to be excluded by the Transparency Profile for a particular SIP method, it is not transparently passed through for that specific SIP method.
• If specific message bodies are allowed by the IPSP, but transparency of 'all' message bodies is configured in the Transparency Profile, all types of message bodies are transparently passed through.
• If 'Unknown Body' transparency is enabled in the IPSP, but an unknown message body is configured to be ignored (or excluded for a specific SIP method) by the Transparency Profile, it is not transparently passed through.

SIP Header Transparency

Header transparency is based on the headers that are present in the Transparency Profile of the egress trunk group for requests and headers that are present in the ingress trunk group for responses. By default, no headers are present in the Transparency Profile.

Allowed Values for a SIP Header in the Transparency Profile

A 'sipHeader' in the Transparency Profile can be composed of:

• Any string with a maximum length of 31 characters
• Any case, lower/upper/mixed
• Special characters are allowed

SIP Headers not Under Transparency Control in Relay Scenarios

Some headers are not under the control of transparency flags in relay scenarios. These headers can be classified into three categories as shown in the below table:

SIP Headers Not Transparently Passed in Calls

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

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

• RAck

• P-Associated-URI

These SIP headers are entirely added and/or modified by the SBC itself and cannot be transparently passed.

SIP Headers Brought Under Transparency Control in Relay Scenarios

Previously, the following headers were transparently passed by the SBC. From release 4.2 onwards, these headers are controlled using transparency flags.

• Accept
• SIP-ETag
• SIP-If-Match
• Suppress-If-Match

SIP Header Transparency Behaviors

SBC transparency mechanisms control the initial INVITE, its responses, and other requests/responses within the INVITE dialog, as well as REGISTER, BYE, UPDATE, REFER, INFO, PUBLISH, MESSAGE, OPTIONS, SUBSCRIBE, NOTIFY requests and their responses (this assumes that the request method has been allowed by the applicable Relay Flag: INFO, MESSAGE, etc…).

There are some exceptions to the transparency mechanisms. Some SIP Methods and some SIP headers are not affected by any configurable transparency mechanism, while other headers may not be affected by transparency controls in some scenarios (in-dialog vs. out-of-dialog).

Transparently Pass or Block Option Tags/Methods of Allow/Require/Supported Headers

Option tags/methods of the following SIP headers can be transparently passed or blocked by configuring the SIP Param Filter Profile.

• Allow
• Require
• Supported

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 SBC can receive messages within a dialog or outside of a dialog, and treats them differently based upon that relationship (or lack thereof).

If "Session-Expires" header is configured, the SBC passes the "Min-SE" header and their values transparently to the egress remote peer. This behavior is achieved either by configuring “all” header or “Session-Expires” and “Min-SE” header in the Transparency Profile. The Keep-Alive behavior must be disabled by setting “Session-Expires” to “0” in the respective SIP Trunk Group.

For example,

set profiles services transparencyProfile ALL_HEADER sipHeader all 
set profiles services transparencyProfile SIP_HEADER sipHeader Session-Expires ignoreTransparency no
set profiles services transparencyProfile SIP_HEADER sipHeader Min-SE ignoreTransparency no 
set addressContext default zone Zone1 sipTrunkGroup TG1 signaling timers sessionKeepalive 0

If "all" header in Transparency Profile is configured and SBC supports "Session Keep Alive" mechanism, the "Session-Expiry" and "Min-SE" headers are excluded from the Transparency Profile by setting ignoreTransparency as “yes”.

For example,

set profiles services transparencyProfile ALL_HEADER sipHeader all 
set profiles services transparencyProfile ALL_HEADER sipHeader Session-Expires ignoreTransparency yes
set profiles services transparencyProfile ALL_HEADER sipHeader Min-SE ignoreTransparency yes         

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

SIP Message Body Transparency

Message body transparency is based on message bodies that are present in the Transparency Profile of the egress trunk group for requests and content-types/bodies that are present in the ingress trunk group for responses. By default, no message bodies are present in the Transparency Profile.

Allowed Values for a Content Type in the Transparency Profile

The allowed range for a "contentType" in the Transparency Profile includes:

• Any string with a maximum length of 127 characters
• Any case, lower/upper/mixed
• Special characters are allowed
• An existing transparency flag for that Content-Type in IP Signaling Profile (IPSP) is not required

Transparency of Multi-part Message Bodies

The SBC treats constituent parts of a 'multipart/mixed' message body just as it treats any message with a single body. Therefore, a constituent part of a 'multipart/mixed' message body will be transparently passed through only if the Content-Type specified in the MIME envelope of the corresponding part has been configured in the Transparency Profile.

For example, consider a SIP message with content-type 'multipart/mixed' with two parts in its body: the first part is type 'application/foo' and the other type 'application/bar'. If the Transparency Profile is configured to transparently pass 'application/foo', then the first part of the message body is passed transparently in the egress SIP message.

Support for Message Body Transparency Across Ribbon Gateways

This feature will be supported across Ribbon Gateways (using Ribbon Proprietary GW to GW Signalling) only for SIP INVITE messages.

SDP

he SBC supports anchoring the following media types:

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 SBC consumes (hence does not transparently relay) the following attributes that are required to anchor the media:

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

• SBC 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 SBC does not transparently relay the ptime and maxptime attributes in addition to the above mentioned attributes.

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 

SDP Transparency Flag

Make note that the sdpTransparencyState signaling object within the SIP Trunk Group must 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.

Audio Transparency

The SBC for the audio m line allows relaying unknown attributes. The SBC allows transparency for subset of the attributes like rtpmap, fmtp, and T38 fax. Audio transparency functionality is used to manage bandwidth for audio stream in the pass-through calls. By enabling this feature, audio codecs that are unknown to the system are available to establish audio calls or streams.

SBC supports audio transparency for known attributes by relaying attributes and codecs transparently in pass-through scenarios for SIP-SIP calls only. However, the following exceptions require system handling:

• 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

Audio Transparency and Reserve Bandwidth for Preferred Common Codec

By default for pass-through calls, SBC reserves the worst case common audio codec bandwidth on Trunk Groups and IP interfaces, and polices for the same bandwidth. To facilitate pass-through calls scenarios/cases, where media uses the preferred common codec the flag reserveBwForPreferredAudioCommonCodec is added to reserve the bandwidth associated with the preferred common codec (instead of the worst case common codec) on the Trunk Groups and IP interfaces. When this flag is enabled, bandwidth of the first common codec from Answer (SIP) is used for reservation and bandwidth of the heaviest common codec is used for policer.

Media Policer Reservation For Worst Case Codec

By default for pass-through calls, the SBC reserves the worst case common audio codec bandwidth on trunk groups and IP interfaces, and polices for the same bandwidth. To facilitate asymmetric pass-through calls scenarios/cases and to police on the heaviest codec in the offer or answer, the flag policeOnHeaviestAudioCodec is used in the PSP.

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

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, SBC recommends disabling the unknownHeader flag (similarly, when configuring a Transparency Profile for specific SIP message bodies, SBC recommends disabling the unknownBody flag).

There are three modes of using Transparency Profile:

• Transparency Profile with Specific Headers (Positive Enumeration)

• Maximum Transparency Using the Transparency Profile

• Transparency Profile, All Headers with Exceptions (Negative Enumeration)

Transparency Profile with Specific Headers (Positive Enumeration)

In this mode, only headers/bodies explicitly configured in the Transparency Profile are allowed to pass-through.

For example, the following scenario allows only the headers "p-asserted-identity" and "xyzHdr" and message bodies of type "application/simple-message-summary" and "xyzContentType" to pass transparently.

set profiles services transparencyProfile ALLOW_SPECIFIC_HDRS_BODIES sipHeader p-asserted-identity
set profiles services transparencyProfile ALLOW_SPECIFIC_HDRS_BODIES sipHeader xyzHdr
set profiles services transparencyProfile ALLOW_SPECIFIC_HDRS_BODIES sipMessageBody application/simple-message-summary
set profiles services transparencyProfile ALLOW_SPECIFIC_HDRS_BODIES sipMessageBody xyzContentType
set profiles services transparencyProfile ALLOW_SPECIFIC_HDRS_BODIES state enabled
commit

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.

Transparency Profile, All Headers with Exceptions (Negative Enumeration)

All headers/bodies are allowed to pass-through unless they are explicitly disallowed by Transparency Profile configuration. 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.

For example, in the following scenario, the rules are configured and all the headers and message bodies except "history-info" and "application/resource-lists+xml" are passed transparently. The "xyzHeader" is passed transparently in all methods except INFO and REGISTER. The "xyzContentType" is passed transparently in all methods except INVITE.

set profiles services transparencyProfile ALMOST_ALL_TRANSPARENCY sipHeader all
set profiles services transparencyProfile ALMOST_ALL_TRANSPARENCY sipMessageBody all
set profiles services transparencyProfile ALMOST_ALL_HDRS sipHeader history-info ignoreTransparency yes
set profiles services transparencyProfile ALMOST_ALL_HDRS sipMessageBody application/resource-lists+xml ignoreTransparency yes
set profiles services transparencyProfile ALMOST_ALL_HDRS sipHeader xyzHeader excludedMethods info,register
set profiles services transparencyProfile ALMOST_ALL_HDRS sipMessageBody xyzContentType excludedMethods invite
set profiles services transparencyProfile ALMOST_ALL_HDRS state enabled
commit

The excludedMethods parameter indicates the list of methods for which the transparency is not allowed and is common to both header and body entries.

set profiles services transparencyProfile TP1 sipHeader all
set profiles services transparencyProfile TP1 sipHeader all excludedMethods bye,info,notify
commit

Existing Deployment Augmented with a Transparency Profile

Existing deployments will likely utilize Transparency Flags, and those that must pass proprietary or otherwise SBC unsupported SIP headers will most likely make use of the unknownHeader transparency flag in an IP Signaling Profile.

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 SBC transparently pass RFC 4474 identity headers. Prior to the introduction of the Transparency Profile, the user would have had to enable the unknownHeader transparency flag.

Rather than continue to allow all unknown headers through the SBC, the user can configure a Transparency Profile that only allows the RFC 4474 identity headers (configured in standard and compact forms) and disable the unknownHeader transparency flag.

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

SRTP Pass-through

SBC supports SRTP media pass-through for SRTP and SRTCP media streams. SBC does not terminate the SDP security description or SRTP media streams and passes them through without authenticating, decrypting, and encrypting. In this pass-through mode of operation, SBC treats SRTP media as plain text RTP pass-through media.

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, the allowPassthru flag is available from the secureRtpRtcp parameter of the 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.

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 SBC when received in the incoming SIP message.

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 SBC, while the Transparency controls whether a header/body in a SIP request/response is sent through the SBC.

Relaying REFER Request

The SBC is enhanced to relay REFER request, even though the refer relay flag is disabled. To support this enhancement, Conditional Relay Matching criteria is provided by the SBC. Using this criteria, SBC decides whether to relay and process the REFER request message or not.

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, SBC 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 SBC. The REFER request acts as the transferor and the call is forwarded to the Egress SIPSG, resulting in call bridging. In this scenario, SBC sends back a 202 response and proceeds for local processing.

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

Configuring SBC For Enhanced Refer Processing

To configure this feature, perform the following steps:

1. Configure SBC 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.

   Info

   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

   
   
   
   Note

   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.

   Note

   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

   Note

   The corresponding Sip domain group must be configured in SBC.

   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 
   
   

SIP Param Filter Profile

The SBC is enhanced to support SIP Param Filter Profile to allow the operator to create a profile defining a set of SIP header tags and methods to transparently pass or block, and then assign that profile to a trunk group. The SIP headers configured in this profile for pass-through are transparently passed to the Egress trunk group if received in the Ingress SIP message. 

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

SIP Filter Profile

The SIP Filter Profile is a collection of the configurable filter settings of individual SIP headers. Depending on the filter settings of each of the SIP headers in the SIP Filter Profile, the SBC either relays the SIP messages without parsing the header, or parses the headers of the messages.

For every SIP message associated with the ingress leg of the call, the SBC first checks the SIP Filter Profile for the filter setting of the SIP header. If the SIP Filter Profile indicates that a particular SIP header needs filtering, the SBC stores it without parsing.  

The SIP header of the egress leg is populated on the basis of the configuration in the IP Signaling Profile and the Transparency Profile. In the egress leg of a call, the transparency bit mask is set to identify the headers that are transparently passed. If the transparency settings of all unknown headers in the IP Signaling Profile is enabled, all the stored headers (including the ones filtered in the ingress leg), is copied to the SIP header of the egress leg.

If the Transparency Profile attached with the Egress Trunk Group indicates that specific headers are allowed to pass transparently, and those headers are present as filtered headers, they are individually copied to the SIP header of the egress leg. In this case, the transparency bits are enabled, either by the IP Signaling Profile or through the flexible header transparency.

Configuring a SIP Filter Profile

Perform the following:

• Creating a new SIP Filter Profile
• Changing the Transparency Setting of a Header Under a SIP Filter Profile
• Configuring the Zone ID for the Selected Zone
• Configuring the Media IP Interface Group Name for a SIP Trunk Group
• Attaching the SIP Filter Profile to the SIP Trunk Group

Creating a new SIP Filter Profile

set profiles signaling sipFilterProfile doc_FILTER2

Changing the Transparency Setting of a Header Under a SIP Filter Profile

set profiles signaling sipFilterProfile doc_FILTER2 header Also enabled

Configuring the Zone ID for the Selected Zone

set addressContext default zone doc_ZONE_IAD id 10

Configuring the Media IP Interface Group Name for the SIP Trunk Group

set addressContext default zone doc_ZONE_IAD sipTrunkGroup doc_SBX10_IAD media mediaIpInterfaceGroupName LIF1

Attaching the SIP Filter Profile to the SIP Trunk Group

set addressContext default zone doc_ZONE_IAD sipTrunkGroup doc_SBX10_IAD signaling sipFilterProfile doc_FILTER2     

Privacy Transparency

A new flag anonymizeHostIpAddress is introduced for the privacy parameter of the IP Signaling Profile to enable or disable this feature. When this flag is activated, the SBC anonymizes the incoming host IP portion of the private headers (P-AID, P-PID, RPID) by replacing it with the IP address of the SBC, before sending it to the egress leg of a call.

Enabling the transparency Flag

To enable the transparency flag for an ipSignalingProfile, enter the following command:

set profiles signaling ipSignalingProfile DEFAULT_SIP egressIpAttributes privacy transparency enable

Enabling the anonymizeHostIpAddress Flag

To enable the anonymizeHostIpAddress flag to activate this feature, enter the following command:

set profiles signaling ipSignalingProfile DEFAULT_SIP egressIpAttributes privacy anonymizeHostIpPortion enable