xdp-project-bpf-examples/pping/README.md

# PPing using XDP and TC-BPF
A re-implementation of [Kathie Nichols' passive ping
(pping)](https://github.com/pollere/pping) utility using XDP (on ingress) and
TC-BPF (on egress) for the packet capture logic.

## Simple description
Passive Ping (PPing) is a simple tool for passively measuring per-flow RTTs. It
can be used on endhosts as well as any (BPF-capable Linux) device which can see
both directions of the traffic (ex router or middlebox). Currently it only works
for TCP traffic which uses the TCP timestamp option, but could be extended to
also work with for example TCP seq/ACK numbers, the QUIC spinbit and ICMP
echo-reply messages. See the [TODO-list](./TODO.md) for more potential features
(which may or may not ever get implemented).

The fundamental logic of pping is to timestamp a pseudo-unique identifier for
outgoing packets, and then look for matches in the incoming packets. If a match
is found, the RTT is simply calculated as the time difference between the
current time and the timestamp.

This tool, just as Kathie's original pping implementation, uses TCP timestamps
as identifiers. For outgoing packets, the TSval (which is a timestamp in and off
itself) is timestamped. Incoming packets are then parsed for the TSecr, which
are the echoed TSval values from the receiver. The TCP timestamps are not
necessarily unique for every packet (they have a limited update frequency,
appears to be 1000 Hz for modern Linux systems), so only the first instance of
an identifier is timestamped, and matched against the first incoming packet with
the identifier. The mechanism to ensure only the first packet is timestamped and
matched differs from the one in Kathie's pping, and is further described in
[SAMPLING_DESIGN](./SAMPLING_DESIGN.md).

## Design and technical description
!["Design of eBPF pping](./eBPF_pping_design.png)

### Files:
- **pping.c:** Userspace program that loads and attaches the BPF programs, pulls
  the perf-buffer `rtt_events` to print out RTT messages and periodically cleans
  up the hash-maps from old entries. Also passes user options to the BPF
  programs by setting a "global variable" (stored in the programs .rodata
  section).
- **pping_kern.c:** Contains the BPF programs that are loaded on tc (egress) and
  XDP (ingress), as well as several common functions, a global constant `config`
  (set from userspace) and map definitions. The tc program `pping_egress()`
  parses outgoing packets for identifiers. If an identifier is found and the
  sampling strategy allows it, a timestamp for the packet is created in
  `packet_ts`. The XDP program `pping_ingress()` parses incomming packets for an
  identifier. If found, it looks up the `packet_ts` map for a match on the
  reverse flow (to match source/dest on egress). If there is a match, it
  calculates the RTT from the stored timestamp and deletes the entry. The
  calculated RTT (together with the flow-tuple) is pushed to the perf-buffer
  `rtt_events`.
- **bpf_egress_loader.sh:** A shell script that's used by `pping.c` to setup a
  clsact qdisc and attach the `pping_egress()` program to egress using
  tc. **Note**: Unless your iproute2 comes with libbpf support, tc will use
  iproute's own loading mechanism when loading and attaching object files
  directly through the tc command line. To ensure that libbpf is always used to
  load `pping_egress()`, `pping.c` actually loads the program and pins it to
  `/sys/fs/bpf/pping/classifier`, and tc only attaches the pinned program.
- **functions.sh and parameters.sh:** Imported by `bpf_egress_loader.sh`.
- **pping.h:** Common header file included by `pping.c` and
  `pping_kern.c`. Contains some common structs used by both (are part of the
  maps).

### BPF Maps:
- **flow_state:** A hash-map storing some basic state for each flow, such as the
  last seen identifier for the flow and when the last timestamp entry for the
  flow was created. Entries are created by `pping_egress()`, and can be updated
  or deleted by both `pping_egress()` and `pping_ingress()`. Leftover entries
  are eventually removed by `pping.c`. Pinned at `/sys/fs/bpf/pping`.
- **packet_ts:** A hash-map storing a timestamp for a specific packet
  identifier. Entries are created by `pping_egress()` and removed by
  `pping_ingress()` if a match is found. Leftover entries are eventually
  removed by `pping.c`. Pinned at `/sys/fs/bpf/pping`.
- **rtt_events:** A perf-buffer used by `pping_ingress()` to push calculated RTTs
  to `pping.c`, which continuously polls the map the print out the RTTs.

## Similar projects
Passively measuring the RTT for TCP traffic is not a novel concept, and there
exists a number of other tools that can do so. A good overview of how passive
RTT calculation using TCP timestamps (as in this project) works is provided in
[this paper](https://doi.org/10.1145/2523426.2539132) from 2013.

- [pping](https://github.com/pollere/pping): This project is largely a
  re-implementation of Kathie's pping, but by using BPF and XDP as well as
  implementing some filtering logic the hope is to be able to create a always-on
  tool that can scale well even to large amounts of massive flows.
- [ppviz](https://github.com/pollere/ppviz): Web-based visualization tool for
  the "machine-friendly" output from Kathie's pping tool. If/when we implement a
  similar machine readable output option it should hopefully work with this
  implementation as well.
- [tcptrace](https://github.com/blitz/tcptrace): A post-processing tool which
  can analyze a tcpdump file and among other things calculate RTTs based on
  seq/ACK numbers (`-r` or `-R` flag).
- **Dapper**: A passive TCP data plane monitoring tool implemented in P4 which
  can among other things calculate the RTT based on the matching seq/ACK
  numbers. [Paper](https://doi.org/10.1145/3050220.3050228). [Unofficial
  source](https://github.com/muhe1991/p4-programs-survey/tree/master/dapper).
- [P4 Tofino TCP RTT measurement](https://github.com/Princeton-Cabernet/p4-projects/tree/master/RTT-tofino): 
  A passive TCP RTT monitor based on seq/ACK numbers implemented in P4 for
  Tofino programmable switches. [Paper](https://doi.org/10.1145/3405669.3405823).