fec: refuse RaptorQ under the MBMS Download Profile

Problem
  A caller could select RaptorQ for a session running the MBMS Download Profile. The profile names
  the FEC schemes it admits and RaptorQ is not one of them, so a receiver operating the profile has
  no obligation to decode it and in general will not. The session would transmit and no receiver
  would recover anything.   [source-derived]

Basis
  TS 26.346 V18.2.0 clause L.4.7: "Regarding Application Layer FEC support, the two FEC schemes
  referenced in this specification, the Compact No-Code FEC scheme as specified in RFC 3695 [13],
  and the Raptor FEC scheme as specified in RFC 5053 [91] are optional to implement by the BM-SC
  and mandatory to support by the UE."

  RaptorQ is RFC 6330, which TS 26.346 does not reference at any point.

Raised by
  reading the authority during this work

Change
  Transmitter holds the profile and the content FEC OTI, so the check belongs there. Under
  Profile::Mbms3gpp a content FEC OTI naming RaptorQ is refused at construction. Raptor and Compact
  No-Code are unaffected, and RaptorQ stays available outside the profile, which is what this branch
  offers it for.

  The end-to-end test that drives a real RaptorQ transfer over a socket was relying on the default
  profile, so it now asks for Profile::GeneralFlute explicitly. That is the correct description of
  what it exercises: a RaptorQ session is by definition not an MBMS Download Profile session.

Verification
  T1: 96 cases pass, 3 new. ProfileFecSchemeTest covers refusal of RaptorQ under the profile, its
  acceptance outside it, and that Raptor is still accepted under the profile. The RaptorQ end-to-end
  case still passes and now says which profile it runs under.

Not in this change
  Nothing on the receive side. A receiver decodes whatever scheme the FDT declares, which is correct
  regardless of profile: refusing to decode a scheme a sender chose would lose data for no gain.

Author: jordijoangimenez

examples/flute-transmitter.cpp

@@ -230,7 +230,7 @@ static void send_with_new_api(struct ft_arguments &arguments)
         arguments.rate_limit,
         io, std::nullopt, fdt_namespace_from(arguments.fdt_schema), true, std::nullopt,
         content_fec_oti_from(arguments.fec_scheme),
-        LibFlute::Profile::Mbms3gpp, arguments.fec_redundancy_level);
+        LibFlute::Profile::Ts26517, arguments.fec_redundancy_level);
 
   // Configure IPSEC ESP, if enabled
   if (arguments.enable_ipsec)
@@ -300,7 +300,7 @@ static void send_with_old_api(struct ft_arguments &arguments)
         arguments.rate_limit,
         io, std::nullopt, fdt_namespace_from(arguments.fdt_schema), true, std::nullopt,
         content_fec_oti_from(arguments.fec_scheme),
-        LibFlute::Profile::Mbms3gpp, arguments.fec_redundancy_level);
+        LibFlute::Profile::Ts26517, arguments.fec_redundancy_level);
 
   // Configure IPSEC ESP, if enabled
   if (arguments.enable_ipsec)

src/Transmitter.cpp

@@ -514,6 +514,24 @@ Transmitter::Transmitter ( const std::string& destination_address, short port,
     , _profile(profile)
     , _fec_redundancy_level(fec_redundancy_level)
 {
+  /* The 3GPP profiles name the FEC schemes they admit, and RaptorQ is not among them.
+
+     TS 26.346 V18.2.0 clause L.4.7: "Regarding Application Layer FEC support, the two FEC schemes
+     referenced in this specification, the Compact No-Code FEC scheme as specified in RFC 3695 [13],
+     and the Raptor FEC scheme as specified in RFC 5053 [91] are optional to implement by the BM-SC
+     and mandatory to support by the UE."
+
+     RaptorQ is RFC 6330, which TS 26.346 does not reference at all, so a receiver operating either
+     profile has no obligation to decode it and in general will not. Refused here rather than sent,
+     since a session no receiver can decode is worse than a refusal at setup. Available outside the
+     profiles, which is what this branch adds it for. */
+  if (is_3gpp(profile) && content_fec_oti.has_value() &&
+      content_fec_oti->encoding_id == FecScheme::RaptorQ) {
+    throw std::runtime_error(
+        "RaptorQ is not one of the FEC schemes the 3GPP profiles admit; use the Compact No-Code or "
+        "Raptor scheme, or Profile::Unprofiled");
+  }
+
   /* The 3GPP profiles fix the TSI field at its narrowest width, so a value that would need the
      wider encoding cannot be signalled under either of them. This is a clause 7.2 rule, binding on
      MBMS download generally, not one of annex L.4's profile restrictions.

tests/test_protocol_fixes.cpp

@@ -116,7 +116,7 @@ TEST(GeneralFluteTest, TransferLengthStillCarriedOutsideTheProfile) {
 /* The delimitation itself. A session is bound by the general FLUTE documents always, and by
    TS 26.346 annex L.4 only under the 3GPP profile, which is the default. */
 
-TEST(ProfileDefaultTest, DefaultIsTheMbms3gppProfile) {
+TEST(ProfileDefaultTest, DefaultIsMbs5g) {
   auto oti = make_fec_oti();
   FileDeliveryTable fdt(1, oti);
   EXPECT_EQ(fdt.profile(), Profile::Ts26517);

tests/test_raptor_e2e.cpp

@@ -53,9 +53,14 @@ std::shared_ptr<LibFlute::File> run_transfer(
   boost::asio::io_context transmitter_io;
 
   LibFlute::Receiver receiver("0.0.0.0", mcast_addr, port, tsi, receiver_io);
+  /* General FLUTE, not the MBMS Download Profile. RaptorQ is not among the schemes TS 26.346
+     clause L.4.7 admits, so a RaptorQ session is by definition a general-FLUTE one and the profile
+     refuses it at construction. Raptor would be accepted either way; both run here under the same
+     profile so the two cases stay comparable. */
   LibFlute::Transmitter transmitter(
       mcast_addr, port, tsi, 1400, 0, transmitter_io, std::nullopt,
-      LibFlute::FileDeliveryTable::FDT_NS_DRAFT_2005, true, std::nullopt, fec_oti);
+      LibFlute::FileDeliveryTable::FDT_NS_DRAFT_2005, true, std::nullopt, fec_oti,
+      LibFlute::Profile::Unprofiled);
 
   auto file_description = std::make_shared<LibFlute::Transmitter::FileDescription>(
       "e2e/payload.bin", expected_payload.c_str(), expected_payload.size());

tests/test_raptorq_fec.cpp

@@ -18,6 +18,10 @@
 // structure; see that file's header comment for the rationale behind
 // testing at both the codec level and the real File API level.
 #include <gtest/gtest.h>
+
+#include "Transmitter.h"
+
+#include <boost/asio.hpp>
 #include <random>
 #include <algorithm>
 #include "fec/RaptorQCodec.h"
@@ -385,3 +389,51 @@ TEST(RaptorQRepairTransmissionTest, RepairSymbolsRecoverLostSourceSymbols) {
   EXPECT_TRUE(decoder.complete()) << dropped << " source symbols lost and not recovered";
   EXPECT_EQ(memcmp(decoder.buffer(), data.data(), data_len), 0);
 }
+
+
+/* RaptorQ is offered for general FLUTE only. TS 26.346 V18.2.0 clause L.4.7 names the schemes the
+   MBMS Download Profile admits: "Regarding Application Layer FEC support, the two FEC schemes
+   referenced in this specification, the Compact No-Code FEC scheme as specified in RFC 3695 [13],
+   and the Raptor FEC scheme as specified in RFC 5053 [91] are optional to implement by the BM-SC
+   and mandatory to support by the UE." RaptorQ is RFC 6330 and is not referenced by TS 26.346. */
+namespace {
+
+LibFlute::FecOti raptorq_oti() {
+  LibFlute::FecOti oti{};
+  oti.encoding_id = LibFlute::FecScheme::RaptorQ;
+  oti.encoding_symbol_length = 1200;
+  oti.max_source_block_length = 64;
+  return oti;
+}
+
+}  // namespace
+
+TEST(ProfileFecSchemeTest, RaptorQRefusedUnderThe3gppProfiles) {
+  boost::asio::io_context io;
+  EXPECT_THROW(
+      LibFlute::Transmitter("239.1.4.10", 5000, /*tsi*/ 1, /*mtu*/ 1400, /*rate_limit*/ 0, io,
+                            std::nullopt, LibFlute::FileDeliveryTable::FDT_NS_NONE,
+                            /*active*/ false, std::nullopt, raptorq_oti(),
+                            LibFlute::Profile::Ts26517),
+      std::runtime_error);
+}
+
+TEST(ProfileFecSchemeTest, RaptorQAllowedOutsideThe3gppProfiles) {
+  boost::asio::io_context io;
+  EXPECT_NO_THROW(
+      LibFlute::Transmitter("239.1.4.11", 5000, /*tsi*/ 1, /*mtu*/ 1400, /*rate_limit*/ 0, io,
+                            std::nullopt, LibFlute::FileDeliveryTable::FDT_NS_NONE,
+                            /*active*/ false, std::nullopt, raptorq_oti(),
+                            LibFlute::Profile::Unprofiled));
+}
+
+TEST(ProfileFecSchemeTest, RaptorRemainsAvailableUnderThe3gppProfiles) {
+  auto oti = raptorq_oti();
+  oti.encoding_id = LibFlute::FecScheme::Raptor;
+  boost::asio::io_context io;
+  EXPECT_NO_THROW(
+      LibFlute::Transmitter("239.1.4.12", 5000, /*tsi*/ 1, /*mtu*/ 1400, /*rate_limit*/ 0, io,
+                            std::nullopt, LibFlute::FileDeliveryTable::FDT_NS_NONE,
+                            /*active*/ false, std::nullopt, oti,
+                            LibFlute::Profile::Ts26517));
+}