coap: ensure DTLS buffer is at least 200 bytes

[benpicco]

Contribution description

The initial DTLS handshake can be larger than a block in CoAP block-wise.
#19892 reduced the DTLS buffer to a CoAP block size + 36 bytes.
This has proven to be too little, breaking DTLS clients.

Bump DTLS_MAX_BUF to be at least 200 bytes as this has proven to work well in the past.

Testing procedure

First, create the tap interfaces by running sudo dist/tools/tapsetup/tapsetup.

In one terminal, run the GCoAP DTSL server:

make -C examples/gcoap_dtls PORT=tap1 all term

2024-02-26 20:18:43,184 # main(): This is RIOT! (Version: 2024.04-devel-327-gb8d58-DTLS_MAX_BUF-fix)
2024-02-26 20:18:43,184 # gcoap example app
2024-02-26 20:18:43,185 # All up, running the shell now

2024-02-26 20:22:23,060 # > ifconfig
2024-02-26 20:22:23,060 # Iface  7  HWaddr: 5A:1A:98:23:2D:6C 
2024-02-26 20:22:23,061 #           L2-PDU:1500  MTU:1500  HL:64  Source address length: 6
2024-02-26 20:22:23,061 #           Link type: wired
2024-02-26 20:22:23,062 #           inet6 addr: fe80::581a:98ff:fe23:2d6c  scope: link  VAL
2024-02-26 20:22:23,063 #           inet6 group: ff02::1
2024-02-26 20:22:23,063 #           inet6 group: ff02::1:ff23:2d6c

In another terminal, run the nanoCoAP DTLS client:

make -C tests/net/nanocoap_cli PORT=tap0 all term

2024-02-26 20:23:47,051 # main(): This is RIOT! (Version: 2024.04-devel-326-gf04871)
2024-02-26 20:23:47,052 # nanocoap test app
2024-02-26 20:23:47,052 # All up, running the shell now

2024-02-26 20:23:52,901 # > ncget coaps://[fe80::581a:98ff:fe23:2d6c]/.well-known/core -
2024-02-26 20:23:52,905 # </cli/stats>;ct=0;rt="count";obs,</riot/board>

In master this would yield

2024-02-26 20:25:18,617 # main(): This is RIOT! (Version: 2024.04-devel-326-gf04871)
2024-02-26 20:25:18,618 # nanocoap test app
2024-02-26 20:25:18,618 # All up, running the shell now

2024-02-26 20:25:19,605 # > ncget coaps://[fe80::581a:98ff:fe23:2d6c]/.well-known/core -
2024-02-26 20:25:19,606 # cannot send ClientHello
2024-02-26 20:25:20,606 # cannot send ClientHello
2024-02-26 20:25:22,607 # cannot send ClientHello

Issues/PRs references

follow-up to #19892

[riot-ci]

Murdock results

✔️ PASSED

11b4564 sys/net/gcoap: ensure DTLS buffer is at least 200 bytes

Success	Failures	Total	Runtime
10027	0	10027	11m:29s

Artifacts

• Documentation preview

[Teufelchen1]

*ding dong it's release' o'clock * Ping @benpicco

[benpicco]

I now use a shell call to calculate DTLS_MAX_BUF which gives much more sane values.

[benpicco]

@Teufelchen1 can this still get into the release? It fixes a regression.