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A packet transmission diagnostic system transmits diagnostic packets during a communications session in which data packets are transmitted between a source and destination. The diagnostic packets elicit responses from routers along a path between the source and destination. The elicited responses, i

A packet transmission diagnostic system transmits diagnostic packets during a communications session in which data packets are transmitted between a source and destination. The diagnostic packets elicit responses from routers along a path between the source and destination. The elicited responses, in the form of packets, are used by the diagnostic system to produce one or more packet-delivery metrics for one or more of the routers along a packet transmission path between the source and destination.

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What is claimed is: 1. In a packet network communications system, a method of passively analyzing packet-delivery performance during a communications session between a source and destination, the method comprising: detecting by a call signaling exchange the establishment of a communications session

What is claimed is: 1. In a packet network communications system, a method of passively analyzing packet-delivery performance during a communications session between a source and destination, the method comprising: detecting by a call signaling exchange the establishment of a communications session between a source and a destination, the communications session being that period between a call answer signal from the destination and a call termination condition during which data packets are sent between the source and the destination; sending in response to detecting the establishment of the communications session, diagnostic packets from the source during the established communication session, each packet eliciting a response from a router along a path between the source and destination; and analyzing the responses from routers along at least one path to produce a packet-delivery metric for one or more of the routers along one or more paths between the source and the destination, wherein each diagnostic packet is of the same format as data packets delivered during the communications session, each diagnostic packet is of the same length as data packets being transmitted in the communications session, and each diagnostic packet including: the same user datagram protocol (UDP) destination port number as data packets, the same differential services code point as the data packets, and a real time protocol (RTP) header that is valid for the data being transmitted in the communications session. 2. The method of claim 1 further comprising obtaining information from the destination through out-of-band signaling and using the information to determine the number of routers between the source and destination. 3. The method of claim 2 wherein the out of band signaling returns the time to live (TTL) value of an RTP packet at the destination, the difference between the TTL value at the destination and the TTL value at the source indicating the number of routers along a path from the source to the destination. 4. The method of claim 1 further comprising: from the analysis of the responses from the routers, identifying one or more routers whose performance is diminished in comparison to one or more other routers delivering said data packets. 5. The method of claim 1 wherein the response elicited by a diagnostic packet is a returned packet that includes a timestamp. 6. The method of claim 1 wherein each diagnostic packet contains a flag in its header, the value of which is modified by each router it passes through and which elicits a response from a router whenever the received value of the flag reaches a threshold. 7. The method of claim 6 wherein the flag is a TTL flag and each router decrements the value of the flag as a packet passes through the router. 8. The method of claim 6 wherein the elicited response is an error response packet that includes a timestamp. 9. The method of claim 6 wherein the elicited response is an Internet control message protocol (ICMP) time exceeded packet that includes a timestamp and the Internet protocol (IP) address of the router sending the ICMP packet as the source address. 10. The method of claim 1 wherein the data packets are routed on a policy basis and the diagnostic packets include the same policy attributes. 11. The method of claim 10 wherein the policy basis is a quality of service policy. 12. The method of claim 1 wherein the data packets are media packets, video packets, audio packets, real time protocol (RTP) packets send over Internet protocol (IP), Internet protocol (IP) packets, hypertext transfer protocol (HTTP) packets, or any combination thereof. 13. The method of claim 1 wherein the diagnostic packets elicit at least one response from each router along one or more paths between the source and destination. 14. The method of claim 1 further comprising signaling an on-hook condition to close the communications session. 15. The method of claim 1 further comprising transmitting a plurality of diagnostic packets to a router and determining the jitter attributable to the router from timestamps within the elicited packets. 16. The method of claim 1 further comprising determining the delay from timestamps within elicited packets. 17. The method of claim 1 further comprising determining loss from the number of transmitted packets that don't elicit a response. 18. In a packet network communications system, a method of passively analyzing packet-delivery performance during a communications session between a source and destination, the method comprising: determining from a call signaling exchange that a communications session between a source and a destination has been established, the communications session being that period between a call answer signal from the destination and a call termination condition during which data packets are sent between the source and the destination; sending in response to determining the establishment of the communications session, diagnostic packets from the source during the established communication session, each diagnostic packet being of the same length as data packets being transmitted in the communications session, each diagnostic packet including: the same user datagram protocol (UDP) destination port number as data packets, the same differential services code point as the data packets, and a real time protocol (RTP) header that is valid for the data being transmitted in the communications session , each diagnostic packet eliciting a response from a router along a path between the source and destination; analyzing the responses from routers along at least one path to produce a packet-delivery metric for one or more of the routers along one or more paths between the source and the destination; and obtaining information from the destination through out-of-band signaling and using the information to determine the number of routers between the source and destination, the out of band signaling returns the time to live (TTL) value of an RTP packet at the destination, the difference between the TTL value at the destination and the TTL value at the source indicating the number of routers along a path from the source to the destination. 19. An apparatus for passively analyzing packet-delivery performance during a communications session between a source and destination, the apparatus comprising: a monitor configured to detect by a call signaling exchange the establishment of a communications session between a source and a destination, the communications session being the period between a call answer signal from the destination and a call termination condition during which data packets are sent between the source and the destination; a diagnostic packet source configured to send in response to having detected the establishment of the communications session, diagnostic packets from the source during the established communication session, each packet eliciting a response from a router along a path between the source and destination; and a controller device configured to analyze the responses from routers along at least one path to produce a packet-delivery metric for one or more of the routers along one or more paths between the source and the destination; wherein the diagnostic packet source is configured to produce each diagnostic packet in the same format as data packets delivered during the communications session and to produce each diagnostic packet the same length as data packets being transmitted in the communications session, and each diagnostic packet including: the same user datagram protocol (UDP) destination port number as data packets, the same differential services code point as the data packets, and a real time protocol (RTP) header that is valid for the data being transmitted in the communications session. 20. In a packet network communications system, an apparatus for passively analyzing packet-delivery performance during a communications session between a source and destination, the apparatus comprising: a monitor configured to detect by a call signaling exchange the establishment of a packet communications session between a source and a destination, the packet communications session being that period between a call answer signal from the destination and a call termination condition during which data packets are sent between the source and the destination; a diagnostic packet source configured to send in response to detecting the establishment of the packet communications session, diagnostic packets from the source during the established packet communication session, each diagnostic packet being of the same length as data packets being transmitted in the communications session, each diagnostic packet including: the same user datagram protocol (UDP) destination port number as data packets, the same differential services code point as the data packets, and a real time protocol (RTP) header that is valid for the data being transmitted in the packet communications session, each diagnostic packet eliciting a response from a router along a path between the source and destination; a controller device configured to analyze the responses from routers along at least one path to produce a packet-delivery metric for one or more of the routers along one or more paths between the source and the destination; and the controller device also configured to obtain information from the destination through out-of-band signaling and to use the information to determine the number of routers between the source and destination, the difference between the TTL value at the destination and the TTL value at the source indicating the number of routers along a path from the source to the destinationan.