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Transport Level Passive Ping Monitoring Tool

Pollere believes the Internet community needs more passive diagnostic tools. To that end, we've been giving talks at the IRTF MAPRG, the Stanford Networking Seminar and Google to emphasize the fun and utility of extracting information from real Internet traffic. In response to feedback at those talks and to help people new to passive network measurement get up to speed, we coded and put out an open source example for others to build on.

Measuring Round-Trip Delay in the Internet

Ping has been an essential tool for network engineers and users from the beginning. It computes the round-trip time to any host by sending it a packet that requires a response and measuring the time until that response is returned or "reflected" from the host. This resemblance to active sonar gives ping its name. Ping uses the lack of a response to determine reachability and the measured reflection time to determine delay, but only for the packets that it injects into a network.

Ping's key enabler, the ability to reflect a packet off a host, was designed into IP's control message protocol, ICMP, for diagnostic purposes. Similar packet "reflections" exist in other parts of the Internet protocol stack notably, TCP ACKs are a receiver's reflection of a sender's data packet. The ACK stream allows senders to continuously estimate a connection's round-trip time, critical for efficient loss recovery. This reflection mechanism only works for the sender and requires relatively complex sequence number tracking. Another reflection mechanism was introduced in RFC1323, a TCP "timestamp" option and this can be used by senders, receivers and intermediaries to monitor connection behavior.

Passive Ping (pping)

When the timestamp option is used in a TCP connection, it provides a naturally occurring reflected signal that allows calculation of a "passive ping" (think of passive sonar) that measures the round trip delay application packets experience relative to any capture point on the connection's path. Passive ping delays are collected per TCP connection with packets of the connection's outbound (relative to the monitoring point) flow providing the signal and packets of the inbound flow the reflection. The inbound flow IP source is the passively pinged host and round trip times are reported relative to the packet capture point.

Unlike ping, pping measures the delay of two different round-trips from the packets it monitors, i.e. the round-trip times to the applications at both ends of a TCP connection. If pping is co-located with one of the applications, it measures delay through the local protocol stack and the application as well as the round-trip time to the communicating host. If located along the path, e.g. at a WiFi router or a cable modem, delay between the host endpoints can be bifurcated with respect to the location of the packet capture. Also unlike ping, pping captures all of the flows through it, giving visibility to to the various Internet connections to the host. Unlike some other passive metrics, pping monitoring can start at any time in the lifetime of a connection, i.e., it does not require SYN packets.

Our C++ code for a basic passive ping utility, pping, is available. It can be utilized as-is or as a basis for other monitoring utilities and works on end hosts as well as on in-network elements.

Any transport protocol with a suitable reflected signal can be used by pping, In particular, RTP and other conferencing protocols could be used but the necessary header information is usually encrypted. Unencrypted protocols with reflected tags could be added to the libtins library pping uses and that protocol could also be used by pping.

Further Information on Passive Monitoring at Pollere

Slides from a talk on passive monitoring that includes use of passive ping deployed in a transparent bridge can be found at Listening to Networks. These include results from Pollere's Transport Segment Delay Estimator (TSDE), developed under an SBIR grant from the Department of Energy. Other talks on measurements using TSDE and on passive monitoring are available on our Presentations page.

Passive Monitoring with a Transparent Bridge

For more information, contact info@pollere.net.



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