Re: [TLS] Maximum Fragment Length negotiation

[Raja ashok <[e-mail]>]

Hi Thomas



Your idea of defining a new similar extension is the only choice for us. Because as per existing max_fragment_length extension in RFC 6066, client should fail if it receives different value from server.

And also your idea of making the new extension as mandatory for TLS1.3 is good, as it will be more useful for constraint server.



Earlier in our team also we were discussing about defining new extension specially for constraint client and server. I suggest we should include the below points for new fragment length extension

      1) As per RFC 6066, if 512 is negotiated then both entity should keep buffer of size 805 bytes (5 byte - record header, 512 bytes - data, 256 bytes - padding, 32 bytes - MAC). I think we should change this in our new fragment extension. If 512 is negotiated then both entity should not send any [D]TLS record of size more than that (includes record header and payload).  Because the control overhead of 256 bytes padding and 32 bytes MAC are not applicable for recent AEAD algorithms. That too in AES_CCM there is no need of padding.

      2) Currently least value supported by max_fragment_length is 512. I prefer we should add support for 256 and 128 also. If AES_CCM_8 is used, the control overhead on application record is 21 bytes (5 byte - record header, 8 byte - IV and 8 byte - MIC). If its DTLS record, overhead is 29 bytes. So if max fragment length is 128, we get 99 bytes for data. This is more than sufficient for a constraint protocol like CoAP.

      Note : Existing max_fragment_length extension cannot be extended to support new values like 128 and 256.

      3) If a client sends both old and new extension, then priority should be given to new extension. Server MUST not send both the extension.



I feel the current IoT world needs this kind of new extension. This is the time to do.



Regards,

Ashok



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Raja Ashok VK
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-----Original Message-----
From: TLS [mailto:[e-mail]] On Behalf Of Thomas Pornin
Sent: 30 November 2016 00:25
To: Fossati, Thomas (Nokia - GB)
Cc: [e-mail]
Subject: Re: [TLS] Maximum Fragment Length negotiation



On Thu, Nov 24, 2016 at 09:10:00PM +0000, Fossati, Thomas (Nokia - GB)

wrote:

> I like your proposal, but I'm not convinced that overloading the

> semantics of an already existing extension when used in combination

> with a specific version of the protocol is necessarily the best

> strategy.  Besides, I'd like to be able to deploy a similar mechanism

> in 1.2.



Defining a new extension is certainly possible. However, it would then require deciding on the intended behaviour when both that new extension and the RFC 6066 extension are present.



Tentatively, one could try this:



  - The new extension documents the maximum record length supported

    by whoever sends it. Encoding is as in RFC 6066: one byte of

    value x for a maximum record plaintext length of 2^(x+8) bytes).

    We extend that to the whole 1..8 range so that larger records

    may be used by implementations who can afford them and obtain

    some performance increase by doing so (actual maximum plaintext

    length will be slightly less than 65535 bytes becose the length

    header is 16-bit and there must be some room for the MAC).



  - If a client sends both the RFC 6066 extension and the new extension,

    and the server supports the new extension, then the RFC 6066

    extension is ignored and only the new extension is used. A server

    MUST NOT send both extensions.



  - All implementations that support the extension MUST have the

    ability to apply a shorter size limit than their maximum limit

    (this is for _sending_ records).



  - The length sent by the server is the one that will be applied to

    subsequent records on the connection, in both directions. This

    applies to the whole connection, including subsequent handshakes

    (renegotiations), unless both client and server send the new

    extension again in a renegotiation (in which case the new length

    appplies).



  - If using TLS 1.3, then the following extra rules apply:



     - All TLS 1.3 implementations MUST support the extension.



     - If the client does not send the new extension, then this is

       equivalent to the client sending the new extension with a

       value of 6 (i.e. maximum plaintext length is 2^14 = 16384 bytes).

       In particular, this allows the server to send the extension.



     - If the server does not send the new extension, then this is

       equivalent to the server sending the new extension with the

       same value as the one from the client. (So, if neither sends

       the extension, then the usual 16384-byte limit applies.)



  - If using TLS 1.2 or a previous version, then there is no implicit

    usage:



     - The server MUST NOT send the new extension unless the client sent

       it.



     - The maximum plaintext limit shall be enforced only if the server

       sent the extension; that limit is the one defined by the server's

       extension.



     - If the client and/or the server does not send the extension, then

       the maximum plaintext length is the one that was in force at that

       point, i.e. 16384 bytes for a new connection, or whatever was

       used before the new handshake in case of renegotiation.



Some noteworthy points:



  * TLS 1.3 has no renegotiation, so the renegotiation behaviour is

    for TLS 1.2 and previous. This avoids tricky issues with the

    TLS 1.3 implicit behaviour in case of renegotiation.



  * A client SHOULD send the new extension in all ClientHello if it

    is ready to use TLS 1.2 or previous, so that a non-1.3-aware

    server may have the possibility to negotiate a shorter maximum

    plaintext length.



  * The initial ClientHello may use records larger than what the server

   is willing to accept, and before the server has any chance to

    advertise its own maximum record size. However, since the initial

    records are unprotected, implementations may be able to process

    partial records, and thus could accept un-MACed records larger

    than their incoming buffer (at least BearSSL can do that).





The "implicit" behaviour (both for client and server) with TLS 1.3 is a way to make the extension free (with regards to network usage) in the common case. It cannot be applied unless the extension support is made mandatory for TLS 1.3. Making it mandatory is also an important feature, since otherwise such an extension would likely remain unimplemented by "big" clients (e.g. Web browsers).





Any comments?

I can try to write the corresponding text for inclusion in the TLS 1.3 draft. What is the process for submitting such text?





      --Thomas Pornin



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