Why does SChannel TLS limit a message to 32kb - c

Question

I am working on using SChannel to build a client/server program. One of the things I would like to do is have file sharing. I found some example code of a client program using Schannel to communicate and I am wondering why the max size of a message is 32kb. Here is the example function that does the receiving
int tls_handshake(tls_ctx *c, tls_session *s) {
DWORD flags_in, flags_out;
SecBuffer ib[2], ob[1];
SecBufferDesc in, out;
int len;
// send initial hello
if (!tls_hello(c, s)) {
return 0;
}
flags_in = ISC_REQ_REPLAY_DETECT |
ISC_REQ_CONFIDENTIALITY |
ISC_RET_EXTENDED_ERROR |
ISC_REQ_ALLOCATE_MEMORY |
ISC_REQ_MANUAL_CRED_VALIDATION;
c->ss = SEC_I_CONTINUE_NEEDED;
s->buflen = 0;
while (c->ss == SEC_I_CONTINUE_NEEDED ||
c->ss == SEC_E_INCOMPLETE_MESSAGE ||
c->ss == SEC_I_INCOMPLETE_CREDENTIALS)
{
if (c->ss == SEC_E_INCOMPLETE_MESSAGE)
{
// receive data from server
len = recv(s->sck, &s->buf[s->buflen], s->maxlen - s->buflen, 0);
// socket error?
if (len == SOCKET_ERROR) {
c->ss = SEC_E_INTERNAL_ERROR;
break;
// server disconnected?
} else if (len==0) {
c->ss = SEC_E_INTERNAL_ERROR;
break;
}
// increase buffer position
s->buflen += len;
}
// inspect what we've received
//tls_hex_dump(s->buf, s->buflen);
// input data
ib[0].pvBuffer = s->buf;
ib[0].cbBuffer = s->buflen;
ib[0].BufferType = SECBUFFER_TOKEN;
// empty buffer
ib[1].pvBuffer = NULL;
ib[1].cbBuffer = 0;
ib[1].BufferType = SECBUFFER_VERSION;
in.cBuffers = 2;
in.pBuffers = ib;
in.ulVersion = SECBUFFER_VERSION;
// output from schannel
ob[0].pvBuffer = NULL;
ob[0].cbBuffer = 0;
ob[0].BufferType = SECBUFFER_VERSION;
out.cBuffers = 1;
out.pBuffers = ob;
out.ulVersion = SECBUFFER_VERSION;
c->ss = c->sspi->
InitializeSecurityContextA(
&s->cc, &s->ctx, NULL, flags_in, 0,
SECURITY_NATIVE_DREP, &in, 0, NULL,
&out, &flags_out, NULL);
// what have we got so far?
if (c->ss == SEC_E_OK ||
c->ss == SEC_I_CONTINUE_NEEDED ||
(FAILED(c->ss) && (flags_out & ISC_RET_EXTENDED_ERROR)))
{
// response for server?
if (ob[0].cbBuffer != 0 && ob[0].pvBuffer) {
// send response
tls_send(s->sck, ob[0].pvBuffer, ob[0].cbBuffer);
// free response
c->sspi->FreeContextBuffer(ob[0].pvBuffer);
ob[0].pvBuffer = NULL;
}
}
// incomplete message? continue reading
if (c->ss==SEC_E_INCOMPLETE_MESSAGE) continue;
// completed handshake?
if (c->ss==SEC_E_OK) {
s->established = 1;
// If the "extra" buffer contains data, this is encrypted application
// protocol layer stuff and needs to be saved. The application layer
// will decrypt it later with DecryptMessage.
if (ib[1].BufferType == SECBUFFER_EXTRA) {
DEBUG_PRINT(" [ we have extra data after handshake.\n");
memmove(s->pExtra.pvBuffer,
&s->buf[(s->buflen - ib[1].cbBuffer)], ib[1].cbBuffer);
s->pExtra.cbBuffer = ib[1].cbBuffer;
s->pExtra.BufferType = SECBUFFER_TOKEN;
} else {
// no extra data encountered
s->pExtra.pvBuffer = NULL;
s->pExtra.cbBuffer = 0;
s->pExtra.BufferType = SECBUFFER_EMPTY;
}
break;
}
// some other error
if(FAILED(c->ss)) break;
// Copy any leftover data from the "extra" buffer, and go around again.
if(ib[1].BufferType == SECBUFFER_EXTRA) {
memmove(s->buf, &s->buf[(s->buflen - ib[1].cbBuffer)], ib[1].cbBuffer);
s->buflen = ib[1].cbBuffer;
DEBUG_PRINT(" [ we have %i bytes of extra data.\n", s->buflen);
tls_hex_dump(s->buf, s->buflen);
} else {
s->buflen = 0;
}
}
return c->ss==SEC_E_OK ? 1 : 0;
}
The code comes from a Github I found here: https://github.com/odzhan/shells/blob/master/s6/tls.c
Inside one of his header files he defines
#define TLS_MAX_BUFSIZ 32768
I have also read in other places that this is a limit with TLS. Is it possible to increase that limit? What happens if I need to receive more then that? Like a large file?