Add some notes to README

README.md

 ```
 
 
+## Antireplay cache
+
+In order to prevent replay attacks, we have internal storage of first
+frames messages for connected clients. These frames are generated
+randomly by design and we have negligible possibility of duplication
+(probability is 1/(2^64)) but it could be quite effective in order to
+prevent replays.
+
+
 ## Environment variables
 
 It is possible to configure this tool using environment variables. You
 can configure any flag but not secret or adtag. Here is the list of
 supported environment variables:
 
-| Environment variable     | Corresponding flags    | Default value                     | Description                                                                                                                                                                                                                                                                |
-|--------------------------|------------------------|-----------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
-| `MTG_DEBUG`              | `-d`, `--debug`        | `false`                           | Run in debug mode. Usually, you need to run in this mode  only if you develop this tool or its maintainer is asking you to provide  logs with such verbosity.                                                                                                              |
-| `MTG_VERBOSE`            | `-v`, `--verbose`      | `false`                           | Run in verbose mode. This is way less chatty than debug mode.                                                                                                                                                                                                              |
-| `MTG_IP`                 | `-b`, `--bind-ip`      | `127.0.0.1`                       | Which IP should we bind to. As usual, `0.0.0.0` means that we want to listen on all interfaces. Also, 4 zeroes will bind to both IPv4 and IPv6.                                                                                                                            |
-| `MTG_PORT`               | `-p`, `--bind-port`    | `3128`                            | Which port should we bind to (listen on).                                                                                                                                                                                                                                  |
-| `MTG_IPV4`               | `-4`, `--public-ipv4`  | [Autodetect](https://ifconfig.co) | IPv4 address of this proxy. This is required if you NAT your proxy or run it in a docker container. In that case, you absolutely need to specify public IPv4 address of the proxy, otherwise either URLs will be broken or proxy could not access Telegram middle proxies. |
-| `MTG_IPV4_PORT`          | `--public-ipv4-port`   | Value of `--bind-port`            | Which port should be public of IPv4 interface. This affects only generated links and should be changed only if you NAT your proxy or run it in a docker container.                                                                                                         |
-| `MTG_IPV6`               | `-6`, `--public-ipv6`  | [Autodetect](https://ifconfig.co) | IPv6 address of this proxy. This is required if you NAT your proxy or run it in a docker container. In that case, you absolutely need to specify public IPv6 address of the proxy, otherwise either URLs will be broken or proxy could not access Telegram middle proxies. |
-| `MTG_IPV6_PORT`          | `--public-ipv6-port`   | Value of `--bind-port`            | Which port should be public of IPv6 interface. This affects only generated links and should be changed only if you NAT your proxy or run it in a docker container.                                                                                                         |
-| `MTG_STATS_IP`           | `-t`, `--stats-ip`     | `127.0.0.1`                       | Which IP should we bind the internal statistics HTTP server.                                                                                                                                                                                                               |
-| `MTG_STATS_PORT`         | `-q`, `--stats-port`   | `3129`                            | Which port should we bind the internal statistics HTTP server.                                                                                                                                                                                                             |
-| `MTG_STATSD_IP`          | `--statsd-ip`          |                                   | IP/host addresses of statsd service. No defaults, by defaults we do not send anything there.                                                                                                                                                                               |
-| `MTG_STATSD_PORT`        | `--statsd-port`        | `8125`                            | Which port should we use to work with statsd.                                                                                                                                                                                                                              |
-| `MTG_STATSD_NETWORK`     | `--statsd-network`     | `udp`                             | Which protocol should we use to work with statsd. Possible options are `udp` and `tcp`.                                                                                                                                                                                    |
-| `MTG_STATSD_PREFIX`      | `--statsd-prefix`      | `mtg`                             | Which bucket prefix we should use. For example, if you set `mtg`, then metric `traffic.ingress` would be send as `mtg.traffic.ingress`.                                                                                                                                    |
-| `MTG_STATSD_TAGS_FORMAT` | `--statsd-tags-format` |                                   | Which tags format we should use. By default, we are using default vanilla statsd tags format but if you want to send directly to InfluxDB or Datadog, please specify it there. Possible options are `influxdb` and `datadog`.                                              |
-| `MTG_STATSD_TAGS`        | `--statsd-tags`        |                                   | Which tags should we send to statsd with our metrics. Please specify them as `key=value` pairs.                                                                                                                                                                            |
-| `MTG_PROMETHEUS_PREFIX`  | `--prometheus-prefix`  | `mtg`                             | Which namespace should be used for prometheus metrics.                                                                                                                                                                                                                     |
-| `MTG_BUFFER_WRITE`       | `-w`, `--write-buffer` | `65536`                           | The size of TCP write buffer in bytes. Write buffer is the buffer for messages which are going from client to Telegram.                                                                                                                                                    |
-| `MTG_BUFFER_READ`        | `-r`, `--read-buffer`  | `131072`                          | The size of TCP read buffer in bytes. Read buffer is the buffer for messages from Telegram to client.                                                                                                                                                                      |
-| `MTG_SECURE_ONLY`        | `-s`, `--secure-only`  | `false`                           | Support only clients with secure mode (i.e only clients with dd-secrets).                                                                                                                                                                                                  |
+| Environment variable          | Corresponding flags         | Default value                     | Description                                                                                                                                                                                                                                                                |
+|-------------------------------|-----------------------------|-----------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `MTG_DEBUG`                   | `-d`, `--debug`             | `false`                           | Run in debug mode. Usually, you need to run in this mode  only if you develop this tool or its maintainer is asking you to provide  logs with such verbosity.                                                                                                              |
+| `MTG_VERBOSE`                 | `-v`, `--verbose`           | `false`                           | Run in verbose mode. This is way less chatty than debug mode.                                                                                                                                                                                                              |
+| `MTG_IP`                      | `-b`, `--bind-ip`           | `127.0.0.1`                       | Which IP should we bind to. As usual, `0.0.0.0` means that we want to listen on all interfaces. Also, 4 zeroes will bind to both IPv4 and IPv6.                                                                                                                            |
+| `MTG_PORT`                    | `-p`, `--bind-port`         | `3128`                            | Which port should we bind to (listen on).                                                                                                                                                                                                                                  |
+| `MTG_IPV4`                    | `-4`, `--public-ipv4`       | [Autodetect](https://ifconfig.co) | IPv4 address of this proxy. This is required if you NAT your proxy or run it in a docker container. In that case, you absolutely need to specify public IPv4 address of the proxy, otherwise either URLs will be broken or proxy could not access Telegram middle proxies. |
+| `MTG_IPV4_PORT`               | `--public-ipv4-port`        | Value of `--bind-port`            | Which port should be public of IPv4 interface. This affects only generated links and should be changed only if you NAT your proxy or run it in a docker container.                                                                                                         |
+| `MTG_IPV6`                    | `-6`, `--public-ipv6`       | [Autodetect](https://ifconfig.co) | IPv6 address of this proxy. This is required if you NAT your proxy or run it in a docker container. In that case, you absolutely need to specify public IPv6 address of the proxy, otherwise either URLs will be broken or proxy could not access Telegram middle proxies. |
+| `MTG_IPV6_PORT`               | `--public-ipv6-port`        | Value of `--bind-port`            | Which port should be public of IPv6 interface. This affects only generated links and should be changed only if you NAT your proxy or run it in a docker container.                                                                                                         |
+| `MTG_STATS_IP`                | `-t`, `--stats-ip`          | `127.0.0.1`                       | Which IP should we bind the internal statistics HTTP server.                                                                                                                                                                                                               |
+| `MTG_STATS_PORT`              | `-q`, `--stats-port`        | `3129`                            | Which port should we bind the internal statistics HTTP server.                                                                                                                                                                                                             |
+| `MTG_STATSD_IP`               | `--statsd-ip`               |                                   | IP/host addresses of statsd service. No defaults, by defaults we do not send anything there.                                                                                                                                                                               |
+| `MTG_STATSD_PORT`             | `--statsd-port`             | `8125`                            | Which port should we use to work with statsd.                                                                                                                                                                                                                              |
+| `MTG_STATSD_NETWORK`          | `--statsd-network`          | `udp`                             | Which protocol should we use to work with statsd. Possible options are `udp` and `tcp`.                                                                                                                                                                                    |
+| `MTG_STATSD_PREFIX`           | `--statsd-prefix`           | `mtg`                             | Which bucket prefix we should use. For example, if you set `mtg`, then metric `traffic.ingress` would be send as `mtg.traffic.ingress`.                                                                                                                                    |
+| `MTG_STATSD_TAGS_FORMAT`      | `--statsd-tags-format`      |                                   | Which tags format we should use. By default, we are using default vanilla statsd tags format but if you want to send directly to InfluxDB or Datadog, please specify it there. Possible options are `influxdb` and `datadog`.                                              |
+| `MTG_STATSD_TAGS`             | `--statsd-tags`             |                                   | Which tags should we send to statsd with our metrics. Please specify them as `key=value` pairs.                                                                                                                                                                            |
+| `MTG_PROMETHEUS_PREFIX`       | `--prometheus-prefix`       | `mtg`                             | Which namespace should be used for prometheus metrics.                                                                                                                                                                                                                     |
+| `MTG_BUFFER_WRITE`            | `-w`, `--write-buffer`      | `65536`                           | The size of TCP write buffer in bytes. Write buffer is the buffer for messages which are going from client to Telegram.                                                                                                                                                    |
+| `MTG_BUFFER_READ`             | `-r`, `--read-buffer`       | `131072`                          | The size of TCP read buffer in bytes. Read buffer is the buffer for messages from Telegram to client.                                                                                                                                                                      |
+| `MTG_SECURE_ONLY`             | `-s`, `--secure-only`       | `false`                           | Support only clients with secure mode (i.e only clients with dd-secrets).                                                                                                                                                                                                  |
+| `MTG_ANTIREPLAY_MAXSIZE`      | `anti-replay-max-size`      | `128`                             | Max size of antireplay cache in megabytes.                                                                                                                                                                                                                                 |
+| `MTG_ANTIREPLAY_EVICTIONTIME` | `anti-replay-eviction-time` | `168h`                            | Eviction time for antireplay cache entries.                                                                                                                                                                                                                                |
 
 Usually you want to modify only read/write buffer sizes. If you feel
 that proxy is slow, try to increase both sizes giving more priority to