Re: [TLS] About the contents of Record Layers

[Michael D'Errico <[e-mail]>]

2^(1/2):

You are correct that application data is just a blob that is passed on to
the application.  It is not processed by the TLS "layer" at all, other
than to decrypt it and verify its integrity.

Multiple handshake messages can be combined into one record, or a single
handshake message may span multiple records.  Handling this can be tricky.

Usually only one alert message will be sent over a connection in each
direction, but you have to be prepared to receive several, even in the
same record.  You should also be prepared to receive just the AlertLevel
byte in one record and the AlertDescription in another.

Change cipher spec messages only make sense at specific places in the
handshake message sequence, so you should never receive more than one in
a single record.  If you do receive more than one, it is an error.

Mike



[e-mail] wrote:
> My thanks Juho and Brian for your feedback. On a similar topic, my 
> understanding is that a Record Layer can encapsulate several protocol 
> messages, all of them belonging to the same protocol. This makes sense 
> for Alert and Handshake protocol messages, for they contain all the 
> information necessary to find out where protocol messages of those types 
> end (which may happen only after many Record Layers have been received.) 
> The same applies to ChangeCipherSpec messages, albeit I do not 
> understand why one would want to encapsulate more than one such message 
> in a Record Layer. Now what about Application data messages? I believe 
> that as far as TLS is concerned these messages are just blobs of data; 
> i.e. it's beyond the scope of TLS to know where they end. Thus, is it 
> not the case that a Record Layer can contain one, and only one, 
> Application data message? Obviously, the application can spread its data 
> over many Record Layers, but when TLS receives a Record Layer with 
> Application data it automatically knows that the whole of the Record 
> Layer's payload is a single, structure-less (as far as TLS is concerned) 
> string of bytes. Is this a correct assessment?