Set IPv4 ID on all non-atomic datagrams

RFC 6864 imposes various restrictions on the uniqueness of the IPv4
Identification field for non-atomic datagrams, defined as an IP datagram that
either can be fragmented (DF=0) or is already a fragment (MF=1 or positive
fragment offset). In order to be compliant, the ID field is assigned for all
non-atomic datagrams.

Add a TCP unit test that induces retransmissions and checks that the IPv4
ID field is unique every time. Add basic handling of the IP_MTU_DISCOVER
socket option so that the option can be used to disable PMTU discovery,
effectively setting DF=0. Attempting to set the sockopt to anything other
than disabled will fail because PMTU discovery is currently not implemented,
and the default behavior matches that of disabled.

PiperOrigin-RevId: 320081842

1 files changed, 3 insertions, 48 deletions

diff --git a/test/syscalls/linux/raw_socket_hdrincl.cc b/test/syscalls/linux/raw_socket_hdrincl.cc
index 0a27506aa..16cfc1d75 100644
--- a/test/syscalls/linux/raw_socket_hdrincl.cc
+++ b/test/syscalls/linux/raw_socket_hdrincl.cc
@@ -273,52 +273,7 @@ TEST_F(RawHDRINCL, SendAndReceive) {
   // The network stack should have set the source address.
   EXPECT_EQ(src.sin_family, AF_INET);
   EXPECT_EQ(absl::gbswap_32(src.sin_addr.s_addr), INADDR_LOOPBACK);
-  // The packet ID should be 0, as the packet is less than 68 bytes.
-  struct iphdr iphdr = {};
-  memcpy(&iphdr, recv_buf, sizeof(iphdr));
-  EXPECT_EQ(iphdr.id, 0);
-}
-
-// Send and receive a packet with nonzero IP ID.
-TEST_F(RawHDRINCL, SendAndReceiveNonzeroID) {
-  int port = 40000;
-  if (!IsRunningOnGvisor()) {
-    port = static_cast<short>(ASSERT_NO_ERRNO_AND_VALUE(
-        PortAvailable(0, AddressFamily::kIpv4, SocketType::kUdp, false)));
-  }
-
-  // IPPROTO_RAW sockets are write-only. We'll have to open another socket to
-  // read what we write.
-  FileDescriptor udp_sock =
-      ASSERT_NO_ERRNO_AND_VALUE(Socket(AF_INET, SOCK_RAW, IPPROTO_UDP));
-
-  // Construct a packet with an IP header, UDP header, and payload. Make the
-  // payload large enough to force an IP ID to be assigned.
-  constexpr char kPayload[128] = {};
-  char packet[sizeof(struct iphdr) + sizeof(struct udphdr) + sizeof(kPayload)];
-  ASSERT_TRUE(
-      FillPacket(packet, sizeof(packet), port, kPayload, sizeof(kPayload)));
-
-  socklen_t addrlen = sizeof(addr_);
-  ASSERT_NO_FATAL_FAILURE(sendto(socket_, &packet, sizeof(packet), 0,
-                                 reinterpret_cast<struct sockaddr*>(&addr_),
-                                 addrlen));
-
-  // Receive the payload.
-  char recv_buf[sizeof(packet)];
-  struct sockaddr_in src;
-  socklen_t src_size = sizeof(src);
-  ASSERT_THAT(recvfrom(udp_sock.get(), recv_buf, sizeof(recv_buf), 0,
-                       reinterpret_cast<struct sockaddr*>(&src), &src_size),
-              SyscallSucceedsWithValue(sizeof(packet)));
-  EXPECT_EQ(
-      memcmp(kPayload, recv_buf + sizeof(struct iphdr) + sizeof(struct udphdr),
-             sizeof(kPayload)),
-      0);
-  // The network stack should have set the source address.
-  EXPECT_EQ(src.sin_family, AF_INET);
-  EXPECT_EQ(absl::gbswap_32(src.sin_addr.s_addr), INADDR_LOOPBACK);
-  // The packet ID should not be 0, as the packet was more than 68 bytes.
+  // The packet ID should not be 0, as the packet has DF=0.
   struct iphdr* iphdr = reinterpret_cast<struct iphdr*>(recv_buf);
   EXPECT_NE(iphdr->id, 0);
 }
@@ -368,10 +323,10 @@ TEST_F(RawHDRINCL, SendAndReceiveDifferentAddress) {
   // The network stack should have set the source address.
   EXPECT_EQ(src.sin_family, AF_INET);
   EXPECT_EQ(absl::gbswap_32(src.sin_addr.s_addr), INADDR_LOOPBACK);
-  // The packet ID should be 0, as the packet is less than 68 bytes.
+  // The packet ID should not be 0, as the packet has DF=0.
   struct iphdr recv_iphdr = {};
   memcpy(&recv_iphdr, recv_buf, sizeof(recv_iphdr));
-  EXPECT_EQ(recv_iphdr.id, 0);
+  EXPECT_NE(recv_iphdr.id, 0);
   // The destination address should be localhost, not the bad IP we set
   // initially.
   EXPECT_EQ(absl::gbswap_32(recv_iphdr.daddr), INADDR_LOOPBACK);