Subject: [tsvwg] Two comments about removing the 16 kB limitation in draft-ietf-tsvwg-dtls-over-sctp-bis-02

From: <[email-masked]>
Date: Tue, 11 Jan 2022 10:10:37 +0800 (CST)

Dear tsvwg members,

Happy new year!

About the current version of draft-ietf-tsvwg-dtls-over-sctp-bis-02, ZTE has two comments as below:

1. Supplementary Note on Fragment reassembly

It is better to add more description in section 4.1 in the current draft.

The description about how to reassemble the fragemented user message in the DTLS layer is not clear enough, and may cause confusion for readers.It is recommended to add further description about it.

For example：

On the receiving side, after getting the user_message’ from SCTP,  DTLS makes use of the length field in the DTLS record header to decrypt the content of the first DTLS record to get m0, then m1......

The length field in DTLS record header is very import for decryption of each DTLS record.According to the length field, the receiving side knows the end of decryption of one DTLS record and the start of the next DTLS record.

Detemining which DTLS record is the last one needs to use the overall length of the user_message’ from SCTP to minus the lenght of each DTLS record.

After the last DTLS record is decrypted, the user_message is reassembled: user_message = m0 | m1 | m2......

2. Consideration of backward compatibility

The issue of backward compatibility needs to be considered, for example,if the server side supports DTLS without user message size limitation, while the client has message size limitation, packet loss may occur on the client side. Therefore, a mechanism is needed to identify whether the peer has the requirement of user message size limitation.

More details welcome further discussion.

Yan

Kind Regards,

李燕 LiYan

RAN产品经营团队/无线规划部/无线产品经营部

RAN Product Operation Team/Wireless Product Planning Dept./Wireless Product Operation Division

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