Method and apparatus for monitoring packet networks
A packet probe for a packet network accurately generates and monitors packets within the network. The packet probe supports packet generation and packet transmission. When a packet is ready for transmission, a hardware-based time stamp unit affixes a time stamp to the packet reflecting an actual transmission time. The packet probe also supports receiving, filtering, and time stamping received packets. When a packet is received, a packet filter determines whether the received packet should be stored in memory along with a time stamp reflecting an actual reception time.
1. A method of transmitting a packet onto a physical layer of a packet network, comprising the steps of:
generating a packet template having a plurality of data fields including a time stamp data field;
updating the data fields of the packet template including the time stamp data field in hardware, the time stamp data field being updated last; and
transmitting the packet template with all of the data fields updated as a completed packet,
wherein the time stamp data field is updated based on a start time of the step of transmitting and a time offset, and
wherein the time offset is an estimate of a delay between the start time of the step of transmitting and an actual time the completed packet is transmitted onto a physical layer of the packet network.
2. The method according to claim 1 , wherein the completed packet is transmitted to a media access control unit for subsequent transmission onto a physical layer of the packet network and the time stamp data field is updated based on a start time of the step of transmitting and an estimated delay through the media access control unit.
3. The method according to claim 1 , further comprising the steps of scheduling packets for transmission at a programmed interval and transmitting the packets one at a time at the programmed interval, wherein successively transmitted packets have time stamps that differ from each other by the programmed interval.
4. The method according to claim 1 , further comprising the steps of scheduling packets for transmission at a programmed interval and transmitting the packets one at a time at the programmed interval unless a packet is already in transmission, in which case, delaying the transmission of the subsequent packets, wherein the time stamp data field of a delayed packet is not updated until the delayed packet is ready for transmission.
5. The method according to claim 1 , further comprising the steps of scheduling packets for burst mode transmission according to which packets are transmitted at uneven time intervals that have a repeating pattern.
6. A method of processing a packet received from a physical layer of a packet network, comprising the steps of:
receiving packets through a media access control unit; and
storing at least one received packet in memory with an associated time stamp, wherein the associated time stamp is generated in hardware and has a time value equal to a reference time at the time the received packet is stored in memory minus an estimate of a delay through the media access control unit.
7. The method according to claim 6 , further comprising the step of filtering the received packets based on packet type.
8. The method according to claim 7 , wherein the step of filtering includes:
examining a data field of a received packet; and
determining if a bit pattern in said data field matches a predetermined bit pattern.
9. The method according to claim 7 , wherein the received packets that are filtered are discarded.
10. The method according to claim 7 , wherein the received packets that are not filtered are stored in memory with an associated time stamp.
11. A packet probe comprising:
a transmit module coupled to a reference time source and configured to time stamp a transmit packet in hardware with a first time value that is offset from a time indicated by the reference time source at the time the transmit packet is transmitted to a transmit media access control unit for subsequent transmission onto a physical layer of a packet network; and
a receive module coupled to the reference time source and configured to time stamp a receive packet in hardware with a second time value that is offset from a time indicated by the reference time source at the time the receive packet is received from the physical layer of the packet network through a receive media access control unit.
12. The packet probe according to claim 11 , wherein the amount of offset in the first time value is representative of a delay through the transmit media access control unit.
13. The packet probe according to claim 11 , wherein the amount of offset in the second time value is representative of a delay through the receive media access control unit.