Manual Chapter : Response message routing (insert-via and honor-via settings)

Applies To:

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BIG-IP LTM

  • 16.0.0, 15.1.0, 15.0.1, 15.0.0, 14.1.2, 14.1.0, 14.0.1, 14.0.0, 13.1.3, 13.1.1, 13.1.0
Manual Chapter

Response message routing (insert-via and honor-via settings)

SIP can be configured to route responses twice using two different methods.
The first method attempts to route the message to the connection that originated the request message. This is enabled via the 'insert-via' attribute of the sipsession profile. If set, the request message will have a new via header inserted into the message. This via header will identify the IP and port that the next SIP device should route the response to. The response message should contain all the via headers included in the request message. Each SIP device will remove the via header it inserted as the response passes through. An example inserted via is as follows:
Via: SIP/2.0/UDP 10.10.10.5:5060;rport;branch=z9hG4bKPjlL6pbh49PLliE2ZNBsASKyO7EBckaoQt
When a response is received and insert-via is enabled, the top most via will be removed, and the message will be forwarded to the connection identified by setting the nexthop meta-data field of the message. This can be observed by logging the message's nexthop field during MR_INGRESS event as follows:
when MR_INGRESS { if {[SIP::response code] ne ""} { log local0. "Response: nexthop [MR::message nexthop]" } }
If the request's originating connection no longer exists, the MRF proxy will return the message to the connection that received the response. The MR_FAILED event will be raised. Upon completion of the MR_FAILED logic, the message will be returned the the SIP filter. The SIP filter will use the fallback response routing mechanism if the 'honor-via' attribute is enabled. The fallback response routing used the IP and port of the second topmost VIA header of the received response (now the topmost after deleting the inserted one). This is the via header that was topmost when the request message was received. This header should contain the IP and port that the device which sent the request to the BIG-IP.
The MRF SIP filter will clear the message's nexthop field and instead set the message's route meta-data field to route the message to the IP and port of the device which sent the request to the BIG-IP. Once the route field has been set, the message will again be forwarded to the MRF proxy for routing and MR_INGRESS event will be raised.
The route command will specify the transport of the connection that received the response as the transport to use when creating the connection to the source of the request. If this is not desired, the route field can be modified during the subsequent MR_INGRESS event.