diff options
| -rw-r--r-- | host/lib/usrp/mpmd/mpmd_link_if_ctrl_udp.cpp | 42 |
1 files changed, 42 insertions, 0 deletions
diff --git a/host/lib/usrp/mpmd/mpmd_link_if_ctrl_udp.cpp b/host/lib/usrp/mpmd/mpmd_link_if_ctrl_udp.cpp index fe7d2b670..92d2ae99c 100644 --- a/host/lib/usrp/mpmd/mpmd_link_if_ctrl_udp.cpp +++ b/host/lib/usrp/mpmd/mpmd_link_if_ctrl_udp.cpp @@ -138,6 +138,46 @@ std::vector<std::string> get_addrs_from_mb_args(const uhd::device_addr_t& mb_arg return addrs; } +/*! Run a plausibility check on a detected MTU, and return a value that passes + * custom constraints. + * + * This function forcibly overrides the detected MTU value using hardcoded + * heuristics/rules, even if the detected MTU is actually correct! + * These rules should thus be chosen very carefully, and should only coerce down + * (i.e., the return value should be smaller than argument). + */ +size_t run_mtu_plausibility_check(const size_t detected_mtu) +{ + // 1 GbE MTU check: We have observed that the detected path MTU for 1 GbE + // devices can come out a few bytes too high over 1 GbE. This is most likely + // due to some drivers being a little more tolerant with larger-than-MTU + // packets, which is not helpful for us. When the MTU detection errs on the + // large side, it can happen that either packets going from UHD to the + // device get fragmented (this is bad, the USRP can't defragment) or that + // packets coming from the device won't get accepted by our NIC/driver, + // causing drops (this is the rarer case). We avoid this by detecting typical + // 1 GbE MTU sizes and coercing them to 1472 bytes. When using a NIC MTU of + // 1500, we have observed detected MTUs of 1476 up to 1488 bytes, when they + // should be 1472 bytes instead. + { + constexpr size_t DEFAULT_1GBE_MTU = 1472; // bytes + constexpr size_t MIN_1GBE_MTU_COERCE_VALUE = 1472; // bytes + constexpr size_t MAX_1GBE_MTU_COERCE_VALUE = 1500; // bytes + if (detected_mtu > MIN_1GBE_MTU_COERCE_VALUE + && detected_mtu < MAX_1GBE_MTU_COERCE_VALUE) { + UHD_LOG_DEBUG("MPMD", + "MTU discovery detected " + << detected_mtu + << " bytes. This may be due to a faulty MTU discovery. Coercing to " + << DEFAULT_1GBE_MTU << " bytes."); + return DEFAULT_1GBE_MTU; + } + } // End 1 GbE MTU check. + + // If no one raises any red flags, we let the detected MTU slide. + return detected_mtu; +} + /*! Do a binary search to discover MTU * * Uses the MPM echo service to figure out MTU. We simply send a bunch of @@ -233,6 +273,8 @@ size_t discover_mtu(const std::string& address, max_frame_size = len; } } + + min_frame_size = run_mtu_plausibility_check(min_frame_size); UHD_LOG_DEBUG("MPMD", "Path MTU for address " << address << ": " << min_frame_size); return min_frame_size; } |