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 <kxuan...@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 _______________________________________________ TLS mailing list TLS@ietf.org https://www.ietf.org/mailman/listinfo/tls
