[Serusers] cancel and 487 relay

Jiri Kuthan jiri at iptel.org
Mon Oct 18 05:54:30 CEST 2004


At 11:33 PM 10/17/2004, Richard wrote:
>Hi,
>
>I ran into some issues. Hope that someone can enlighten me. When ser does a
>parallel forking, it uses its uac to initiate a new call. The scenario is
>Phone A calls phone B via ser. In ser, it forks into two branches, one for
>phone B and one for phone C. To call phone C, ser's uac initiates the call.
>When Phone B picks up the call, ser sends a CANCEL to phone C and cancesl
>the call. The following diagram illustrates the sip call flow. For
>simplicity, I didn't include any message involving Phone B.
>
>Packet #    Phone A ------- (ser UAC) -------- ser --------- Phone C
>0               |     INVITE ---------------->  |               | 
>1               |               |   INVITE ->   |               |
>2               |     <----------- 100 trying   |               |
>3               |               |               |  INVITE ->    |
>4               |               |               | <- 100 trying |
>5               |               |               |  <- 183       |
>6               |     <-------------- 183       |               |
>7               |               |   CANCEL ->   |               |
>8               |               |<- 200canceling|               |
>9               |               |               |   CANCEL ->   |
>10              |               |               |    <- 487     |
>11              |               |               |  <- 200 OK    |
>12              |               |               |    ACK ->     |
>13              |               |    <- 487     |               |
>14              |               |    ACK ->     |               |
>15              |               |               |    ACK ->     |
>16              |    <---------------  487      |               |
>17              |     ACK --------------->      |               |
>18              |               |               |    ACK ->     |
>
>Per rfc, 487 and its ACK are hop-by-hop, i.e. ACK is generated at each hop.
>Ser correctly does it at Packet 12. The problem happens after packet 14.
>Somehow ser doesn't think this ACK of Packet 14 is an acknowledgement of
>Packet 13.

that's strange. I don't know why it is this way without seeing the
packets. (I have to admit that even if I received them, my current
worload would not allow me to study them.)

> So it is forwarded to Phone C. Not sure if I can use ser.cfg to
>absorb it without forwarding. Anyway, not a big deal for an extra ACK. The
>big problem is that somehow ser relays the 487 back to Phone A. It doesn't
>seem right. In Packet 10, 487 is to cancel the original INVITE at Packet 3
>which has a record-route of Phone A, ser. So 487 has the same "Via" field
>back to Phone A. When this 487 is relayed back to Phone A, it already gets
>the 200 OK from Phone B and has an active conversation.

well that's a race condition -- if caller cancelled, then the 487 should
propagate to UAC. If at the same time one of the called parties answered,
the UAC will see the 200 too. The case that caller hangs up at the same
point of time when called party happens simply happens.

> Depending on the
>implementation, Phone A may choose to close the call because of the 487.
>
>Any help is highly appreciated.
>
>Thanks,
>Richard
>
>
>
>
>
>
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--
Jiri Kuthan            http://iptel.org/~jiri/ 




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