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The Via Header transparency is achieved by combining Replace Host On Via Header and VIA Header Transparency. Via Header is configurable on either the Embedded Routing Engine (ERE) or centralized PSX. The SBC sends the IP address or host name specified in the Via Header using the SMM Header rule.

The SBC provides the following Via Header functionality:

• Enabling VIA Header Transparency, whereby all the Via headers received in original INVITE or REGISTER are passed transparently.
  
  Example:

  Incoming REQUEST (INVITE or REGISTER)	Outgoing REQUEST (INVITE or REGISTER)
  Via: 1.2.3.4:9000;branch=xyz	Via: 1.2.3.4:5060;branch=gkSBC5200createdbranchid
  Via: 5.6.7.8:10000;branch=abcxyz

  Span

• Enabling Replace Host, whereby the IP address of the top-most header on the outgoing request with IP address /hostname present in the top most header received is replaced.
  
  Example:

  Incoming REQUEST (INVITE or REGISTER)	Outgoing REQUEST (INVITE or REGISTER)
  Via: 1.2.3.4:9000;branch=xyz	Via: 10.7.1.3:5060;branch=gkSBC5200createdbranchid Via: 1.2.3.4:9000;branch=xyz Via: 5.6.7.8:9000;branch=xyz
  Via: 5.6.7.8:9000;branch=xyz

  Span

• If transparency and Replace host on Via are enabled, then the top-most via header's host portion replaces the top most via header on the outgoing request. The rest of the via headers if any will be transparently passed through in the request.
  
  Example:

  Incoming REQUEST (INVITE or REGISTER)	Outgoing REQUEST (INVITE or REGISTER)
  Via: 1.2.3.4:9000;branch=xyz	Via: 1.2.3.4:5060;branch=gkSBC5200createdbranchid Via: 5.6.7.8:9000;branch=xyz
  Via: 5.6.7.8:9000;branch=xyz

  Span

• The top most Via header has the local SIP port number while receiving the responses.