[openssl-users] How to know maximum sendable fragment size?

Matt Caswell matt at openssl.org
Tue Nov 7 10:28:29 UTC 2017

On 07/11/17 03:56, J Decker wrote:
> I've been developing this NodeJS plugin, it implements HTTPS server and
> now client.  I was having an issue with HTTPS request getting ECONNRESET
> for no apparent reason; so I implemented my own request, and ran into
> the same sort of issue.  What I was requesting was some .js files from
> the server, and apparently my most recent changes to those files made
> them larger than some magic number greater than 4096 but less than
> 6561.  The server was sending using OpenSSL (statically linked in the
> NodeJS executable) on CentOS, and it was sending the full length of the
> file as one buffer.  I'm using memory BIO to interact with the SSL
> object; The buffer was transmitted as one block. With my own client,
> (where I could add debugging) was receiving the full count of bytes from
> the server but in two blocks, the first 4096 and the second
> 2472(something like that).  Because my network read buffer was
> only 4096.... So the first read was short, and caused SSL_read to fail,

What do you mean this cause SSL_read() to fail? You got a <= 0 return
value? This should not happen. It is perfectly valid to read less bytes
than OpenSSL has available.

> which I had initially treated as a failure and terminated my
> connection.  I then
> found I could (almost) check using SSL_pending before getting an
> error (really I ended up doing SSL_read( ssl, NULL, 0 ) and then
> SSL_pending(ssl)
> ).  But after receiving the full count of bytes and having nothing else
> to receive, the message never completed (read return -1, and error 2,
> pending
> returned 0 ).

I'm not sure sure what you mean by "the message never completed". Do you
mean you were expecting more bytes but they never came?

> I manually broke up the transmission to 4356 (3*1452 -29)
> bytes so it ends up sending in 3 full tcp buffer units, and that works. 
> (it's http protocol so it's got higher level gathering for the
> fragments).  It also works if I revert to using the NodeJS HTTPS request
> object instead of my own.
> So - how do I know what the maximum amount of data I can send is?

TLS is a stream protocol. There is no maximum amount of data you can
send in one go. Internally the protocol breaks up the data into a number
of records. The maximum amount of plaintext data sent in a single record
is SSL3_RT_MAX_PLAIN_LENGTH (16384) bytes. This can be changed by your
application (to something smaller - not larger) but you have to
explicitly do that. However changing this should have no impact on the
functional behaviour of your application.

> Node TLS object (on which HTTPS is based)
> has tlsSocket.setMaxSendFragment(size)(which defaults to 16384)  but
> that's about sending, not receiving, so I really have no idea how big
> the receive buffer is actually.... (same as SSL send fragment default)
> I did
> find https://www.openssl.org/docs/man1.1.0/ssl/SSL_CTX_set_split_send_fragment.html
> <https://www.openssl.org/docs/man1.1.0/ssl/SSL_CTX_set_split_send_fragment.html> but
> there's no get_for fragment size I could find.  (this would be on
> the server side that I need to know how much I can send).
> how do I set how big of a fragment I can receive?

The TLS protocol defines the size of the maximum fragment so no well
behaved implementation will ever send more than that. The OpenSSL
buffers are sized accordingly. In normal operation you should never need
to play around with these settings. Currently it is not possible to
change the size of the receive fragment - the sender is always allowed
to send any size up the to the maximum, so the receive buffers must
always be at least as big as that. In 1.1.1 (in development) there is a
feature which allows the client and server to negotiate a smaller
fragment size if they wish - but this is typically only useful in
resource constrained environments.

>  Like what if I tried
> to send 100's of Meg as a single fragment?   (I guess it should auto
> fragment to like 16k)

If you send a large amount of data in an SSL_write() call then OpenSSL
will automatically break that up into a series of records containing the
maximum amount of data until all of the data has been sent.

> I guess there (will be) SSL_CTX_set_default_read_buffer_len() (1.1.0)

Unless you are doing pipelining with an engine that can support it then
you do not need to call this function.


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