Re: [TLS] [DTLS1.3]About the retransmission of Handshake records

Xuan k <kxuanobj@gmail.com> Mon, 02 December 2019 11:21 UTC

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From: Xuan k <kxuanobj@gmail.com>
Date: Mon, 2 Dec 2019 19:20:43 +0800
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To: Benjamin Kaduk <bkaduk@akamai.com>
Cc: Eric Rescorla <ekr@rtfm.com>, "<tls@ietf.org>" <tls@ietf.org>
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Subject: Re: [TLS] [DTLS1.3]About the retransmission of Handshake records
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Sorry for my mistake.

The HelloVerifyRequest in the previous email should be replace with
"HelloRetryRequest".


On Mon, Dec 2, 2019 at 7:17 PM Xuan k <kxuanobj@gmail.com>; wrote:

> Hi Ben,
>
> Thanks for your reply. But we understand may not be the same.
>
> Since ACK messages are not retransmitted, I think they should not consume
> message_seqs.
> If an ACK really use a message_seq and it is lost in network, new
> handshake records will
> always use a larger message_seq than the peer expected (next_receive_seq).
> The peer will unable to continue the handshake process.
>
> And I think the ServerHello in the figure should start with 1, as the
> figure shows.
> The message_seq = 0 is used by HelloVerifyRequest, which is also been
> processed by client
> and been fed into its transcript hash function.
>
> So I think the HelloVerifyRequest should be counted and new ServerHello in
> the figure
> should start with message_seq = 1.
>
> Thanks,
>
> Zhai Zhaoxuan
>
> On Mon, Dec 2, 2019 at 3:54 PM Benjamin Kaduk <bkaduk@akamai.com>; wrote:
>
>> That might be an artifact from Ack having at one point (the -00) been
>> a handshake message.  Though, it seems that the Server's message_seq
>> space should also start at 0 with ServerHello, if I am understanding
>> correctly.
>>
>> -Ben
>>
>> On Thu, Nov 28, 2019 at 03:15:38PM +0800, Xuan k wrote:
>> >  Hi Ekr,
>> >
>> > Thanks for your help.
>> >
>> > I have another question about the "message_seq" in section "6. Example
>> of
>> > Handshake with Timeout and Retransmission".
>> > Could you please explain it?
>> >
>> > In the secion 6, the Client send message_seq = 0 in Record 0 and
>> > message_seq = 2,3,4 in Record 2.
>> >
>> > Why message_seq = 1 is skipped by Client?
>> > I think the ClientHello in the figure should begin with message_seq=1.
>> The
>> > message_seq = 0 is used by the first ClientHello which is the one
>> without
>> > cookies.
>> >
>> >
>> > Thanks,
>> > Zhai Zhaoxuan
>> >
>> > On 11/28/19 5:37 AM, Eric Rescorla wrote:
>> >
>> >
>> >
>> > On Tue, Nov 26, 2019 at 10:05 PM Xuan k <kxuanobj@gmail.com>; wrote:
>> >
>> > > Hi all,
>> > >
>> > > I'm trying to implement a DTLS1.3 library for embedded devices. But It
>> > > seems something weird about retransmissions and ACKs.
>> > >
>> > > In the section "5.2. DTLS Handshake Message Format":
>> > >
>> > >    The first message each side transmits in each association always
>> has
>> > >    message_seq = 0.  Whenever a new message is generated, the
>> > >    message_seq value is incremented by one.  When a message is
>> > >    retransmitted, the old *message_seq value is re-used*, i.e., not
>> > >    incremented.  From the perspective of the DTLS record layer, the
>> > >    retransmission is a new record.  This record will have a *new*
>> > > *   DTLSPlaintext.sequence_number* value.
>> > >
>> > >
>> > > In the section "7. ACK Message", the ACK message use the
>> record_numbers
>> > > (corresponds to *DTLSPlaintext.sequence_number*).
>> > >
>> > > For my understanding, the "message_seq" belongs to "Handshake" and the
>> > > "sequence_number" or "record_numbers" belongs to
>> > > record layer.
>> > >
>> >
>> > Yes.
>> >
>> > The retransmission detection is done by "Handshake" using "message_seq",
>> > > but the "acknowledge" is done by "record layer" using
>> "record_numbers".
>> > > It is so weird.
>> > >
>> >
>> > Hmm... I don't think that this is particularly weird. This is, for
>> > instance, how QUIC stream acknowledgement and retransmission works.
>> >
>> >
>> > The retransmission, retransmission detection and acknowledge should be
>> done
>> > > in handshake process, but we need the record layer passing the
>> > > record_numebrs to the handshake process.
>> > >
>> > > Since a new "sequence_number" is used for retransmission, we have to
>> > > maintain a "record_numbers" to "message_seq" map with dynamic size.
>> > > Each retransmission attempt creates a new relationship between a new
>> > > "record_numbers" to an old "message_seq".
>> > >
>> >
>> > Yes, that's how it works in NSS.
>> >
>> >
>> > Since ACK is only used with Handshake messages, is it possible that we
>> use
>> > > "message_seq" in ACK messages?
>> > >
>> > Or we use *old* "sequence_number" for retransmission,
>> > >
>> >
>> > Both of these give you strictly less information about the network. One
>> of
>> > the cool innovations in QUIC is to label each packet separately so you
>> can
>> > determine whether an ACK is an ACK of the original packet or a
>> retransmit.
>> > We are trying to inherit tha there
>> >
>> > -Ekr
>> >
>> > so we do not need maintain the dynamic map. And if replay detection is
>> > > implemented, the retransmitted
>> > > record can be dropped by record layer (by replay detection), the
>> > > "Handshake Protocol" do not need to do retransmission detection.
>> > >
>> > > Thanks
>> > > Zhai Zhaoxuan
>> > > _______________________________________________
>> > > TLS mailing list
>> > > TLS@ietf.org
>> > > https://www.ietf.org/mailman/listinfo/tls
>> > >
>>
>> > _______________________________________________
>> > TLS mailing list
>> > TLS@ietf.org
>> > https://www.ietf.org/mailman/listinfo/tls
>>
>>