RFR 8211018: Session Resumption without Server-Side State

Xuelei Fan xuelei.fan at oracle.com
Thu Jun 6 00:37:34 UTC 2019

On 6/5/2019 4:57 PM, Jamil Nimeh wrote:
> I think what it's saying is that what was explicitly called out in 4507 
> (where there is both the extension_data length bytes AND the opaque 
> vector length) is not how deployed implementations did it. It implies 
> that deployed implementations do what you laid out below where you just 
> have 2 bytes of ID and 2 bytes of length.  And I believe that is 
> compatible with what 5077 specifies.
Hm, I agreed with you.

> So the potential problem is if one endpoint or the other happens to 
> implement 4507 to the letter, extra length bytes and all.  But the 
> authors of 5077 say that no known implementations do this.  That's good 
> for us I think, because in the two-ish years between 4507 and 5077 
> nobody did straight 4507, or they maybe did and fixed it by the time 
> 5077 came around.
I may view it differently.

If an implementation encoded per the format:
          00 23      Extension type 35
          00 02      Length of extension contents
          00 00      Length of ticket

Just as your analysis previous, a RFC 5077 server will just ignore the 
extension.  No real hurt actually.

But if an implementation encoded empty SessionTicket extension per the 
format (the known implementation):
          00 23      Extension type 35
          00 00      Length of extension contents

The server could read it as RFC 5077, and use stateless implementation. 
When the ServerHello extension sent back.  No matter the RFC 4057 client 
accept it or not, there are interop issues.

If the client does not accept it (unlikely), the connection cannot be 

If the client accept it, the resumption session will use the negotiated 
ticket, and then non-empty SessionTicket extension is encoded as:
         00 23          Ticket Extension type 35
         01 02          Length of extension contents
         01 00          Length of ticket
         FF FF .. ..    Actual ticket

The server would have to handle it (RFC 4507 format) if it want the 
session resumption works.  Here come the interop issues.


> I dug up a few pcaps I've kept around during testing of other TLS 
> features over the past few years.  I had Chrome, Mozilla and OpenSSL 
> s_client pcaps and they all appear to follow the 5077 format.  Of 
> course, anything after 2008 is more likely to do 5077 than 4507.
> --Jamil
> On 6/5/2019 4:35 PM, Xuelei Fan wrote:
>> I'm not sure I understand the following words in page 17, RFC 5077.
>> "  An error in the encoding caused the specification to differ from
>>    deployed implementations.  At the time of this writing there are no
>>    known implementations that follow the encoding specified in RFC 4507.
>> "
>> Is it means that the known implementation encode empty SessionTicket 
>> extension as?
>>         00 23      Extension type 35
>>         00 00      Length of extension contents
>> Xuelei
>> On 6/5/2019 4:26 PM, Xuelei Fan wrote:
>>> On 6/5/2019 3:37 PM, Jamil Nimeh wrote:
>>>> I think we're overstating the "otherwise" case.  A client that uses 
>>>> this strict 4507 format would initially send a ticket that looks 
>>>> like { 00 23 00 02 00 00 } to which our server would reject this 
>>>> extension (since the final 00 00 would be interpreted as a ticket 
>>>> when the client did not intend it to be so).  The result of this 
>>>> SHOULD be that the server responds with a ServerHello that doesn't 
>>>> have the SessionTicket extension.
>>>> That doesn't mean that resumption cannot happen.  It just means that 
>>>> resumption happens using the usual session ID lookup approach and 
>>>> the server is caching the session rather than letting the client do 
>>>> the work.  Given that this is a degenerate case for what (I hope) is 
>>>> a small subset of older clients, I think using server-cached 
>>>> sessions is OK.
>>>> I don't believe we should ever find ourselves in a case where the 
>>>> strict-4507 client actually gets a real ticket from our server, and 
>>>> in turn should never hand us a ticket thinking that resumption could 
>>>> actually take place via said ticket.
>>> I'm not very sure if I read the Appendix A of RFC 5077 correctly. I 
>>> think it is trying to explain that client does not use strict-4507 
>>> for the empty extension and then result in the interop issues.
>>> Page 18, RFC 5077:
>>> "   Note that the encoding of an empty SessionTicket extension was
>>>     ambiguous in RFC 4507.  An RFC 4507 implementation may have encoded
>>>     it as:
>>>          00 23      Extension type 35
>>>          00 02      Length of extension contents
>>>          00 00      Length of ticket
>>>     or it may have encoded it the same way as this update:
>>>          00 23      Extension type 35
>>>          00 00      Length of extension contents
>>> "
>>>> On the client side, we cannot know ahead of time that the server is 
>>>> strict-4057, so we have to send a 5077 style SessionTicket 
>>>> extension. The server will probably not like that and not assert 
>>>> SessionTicket in its server hello.  So our client should fall back 
>>>> to using the session ID to support resumption, and the server should 
>>>> follow suit by caching the session.
>>> I agreed.  We should stick to the RFC 5077 format in client side.
>>> Thanks,
>>> Xuelei
>>>> --Jamil
>>>> On 6/5/2019 2:28 PM, Xuelei Fan wrote:
>>>>> I don’t know if there are any deployment of RFC 4507.  If not, we 
>>>>> are safe; otherwise there are interop problems for session resumption.
>>>>> Xuelei
>>>>>> On Jun 5, 2019, at 2:19 PM, Jamil Nimeh <jamil.j.nimeh at oracle.com> 
>>>>>> wrote:
>>>>>> Hi Xuelei,
>>>>>> Given that 4507 is obsoleted in favor of 5077 is there really that 
>>>>>> much value to supporting this older/broken extension format?  Do 
>>>>>> we know of clients that still adhere to 4507?  Otherwise it seems 
>>>>>> better to stick to 5077 and the approach in TLS 1.3 and not try to 
>>>>>> go back and support an earlier obsoleted approach to this feature.
>>>>>>> These lines took me to the cooperation behaviors between RFC 5077 
>>>>>>> and RFC 4507.  It looks like we don't support RFC 4507 format of 
>>>>>>> SessionTicket extension.  As RFC 5077 and RFC 4507 use the same 
>>>>>>> extension ID for different extension format. There are potential 
>>>>>>> compatibility issues, and make session resumption impossible.  I 
>>>>>>> would like to have a workaround to accept both formats.  For 
>>>>>>> example, using the a cookie at the beginning of the ticket, as 
>>>>>>> described in appendix-A of RFC 5077.
>>>>>>> I will review the rest of this class in the afternoon or tomorrow.
>>>>>>> Thanks,
>>>>>>> Xuelei

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