Re: [TLS] Suggestions in draft-mavrogiannopoulos-tls-cid

"Fossati, Thomas (Nokia - GB/Cambridge, UK)" <> Fri, 07 July 2017 19:01 UTC

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From: "Fossati, Thomas (Nokia - GB/Cambridge, UK)" <>
To: Raja ashok <>, "" <>, "" <>, "" <>
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Thread-Topic: Suggestions in draft-mavrogiannopoulos-tls-cid
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Subject: Re: [TLS] Suggestions in draft-mavrogiannopoulos-tls-cid
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Hi Ashok,

Thanks very much for your comments.

See inline.

Cheers, t

On 07/07/2017, 14:44, "Raja ashok" <<>> wrote:

Hi Nikos, Hannes & Thomas,

This ConnectionID concept is really a useful feature for a client node which faces a change in transport and network layer. I am having few suggestions in the proposed draft, which are listed below.

1) In section 3 of this draft, explains about the modification in "DTLSCiphertext" structure. I feel instead of modifying existing DTLS and TLS record header, we can directly introduce a new record type (ContentType) for app data (application_data_with_cid(25)).

[tf] Why do you think a new record type would be better than the present construction?  (BTW, we would also need a new alert_with_cid, I guess.)

For this new record type, we can propose a modified record header for both TLS and DTLS.

[tf] If you are already modifying the record header, I am not sure why you also need to add new content type(s)?

2) More over section 3 says modification only in "DTLSCiphertext", not for "TLSCiphertext". I hope this CID mechanism should be used for both TLS and DTLS. Because this transport layer change problem is there for TLS based applications (HTTPS) also in Smartphone(when it switches from Wifi to LTE). But this TLS application problem is solved by MPTCP, but I dont think all webservers are supporting this MPTCP. So I feel CID is required for both TLS and DTLS.

[tf] In principle I’m not opposed to open the solution space to TLS, but we need to work out the implications.

3) As part of defining new record type, we can remove some unused fields like version, epoch. After removing epoch we can mandate that  both entity should not start renegotiation. Sample design for new record header with CID is mentioned below
    struct {
        uint8_t ContentType;
        uint8_t CID_len;
        CID cid;        /* Length varies from 4 to 8 (or 16) */
        uint48 sequence_number;
        uint16_t record_length;
    } DTLSRecordHeader;
    opaque CID <4..8>;

    struct {
        uint8_t ContentType;
        uint8_t CID_len;
        CID cid;        /* Length varies from 4 to 8 (or 16) */
        uint16_t record_length;
    } TLSRecordHeader;
    opaque CID <4..8>;

4) Another option is we can keep CID_len as 4 bit to represent a CID of size. And this 4 bit can be placed in the MSB of the CID field.
        uint8_t CID_len:4;
        CID cid;        /* Length varies from 28bit to 60 bit (or 124bit) */

5) Section 3.1 and 3.2 talks about the new extensions for negotiating this feature support. But I feel no need of new extensions to negotiate this, we can make client to add new SCSV cipher in its cipher list. If server accepts then after handshake the first application data send by server should be of type application_data_with_cid(25), which should hold the new record header with CID. The same CID should be used by client in the further messasge. If client sends the 1st application data after handshake, then it can send application data with default record type (application_data(23)). If the first application data record received from server is not of application_data_with_cid(25) means client should understand that server has not accepted the SCSV proposed. And client should continue app transfer with default record type (application_data(23)).

[tf] What is the problem with using a new extension?  How would negotiating parameters (e.g., CID type, CID length) work in your proposal?

Please provide your comments on this suggestion.



Raja Ashok V K
Huawei Technologies
Bangalore, India
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