Re: [Anima-bootstrap] bootstrap over CoAP

Brian E Carpenter <brian.e.carpenter@gmail.com> Sun, 10 July 2016 06:33 UTC

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To: Carsten Bormann <cabo@tzi.org>
References: <3A2F4C70-4960-4592-9314-6EC53B53CC94@cisco.com> <5d5623cd-fe4b-e443-da5d-6a43ffb9b5c6@gmail.com> <57810029.2070408@tzi.org> <17d1c08e-e9c6-d017-58ba-85989d56273d@gmail.com> <5781531A.1080702@tzi.org>
From: Brian E Carpenter <brian.e.carpenter@gmail.com>
Organization: University of Auckland
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Date: Sun, 10 Jul 2016 18:33:09 +1200
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Cc: "Max Pritikin \(pritikin\)" <pritikin@cisco.com>, "anima-bootstrap@ietf.org" <anima-bootstrap@ietf.org>
Subject: Re: [Anima-bootstrap] bootstrap over CoAP
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On 10/07/2016 07:40, Carsten Bormann wrote:
>>> I'm not sure I understood that part of the draft*), but generally CoAP
>>> is designed so you can avoid fragmentation 
>>
>> That isn't what the draft seems to say, though. However, it's clearly talking
>> about application-layer fragmentation to avoid IP fragmentation.
> 
> (that's segmentation...  I know.)
> 
>>> (and use the segmentation
>>> provided by draft-ietf-core-block instead); the latter has per-segment
>>> reliability (acknowledgements and retransmits).   DTLS may require
>>> fragmentation during its handshake; this is mitigated if you can use a
>>> PSK (symmetric) or, if you need asymmetric, ECC-based ciphersuite, which
>>> allows the packets to stay well below 1280 bytes.
>>
>> Right. So the app layer chops the message into <1280 byte pieces, and one
>> of them is lost...?
> 
> Each of those segments ("blocks") is (actually, can be, but you do want
> to do this with the block-wise protocol) transmitted reliably, so it
> doesn't get lost.

OK, so I think that a couple of sentences in draft-pritikin-coap-bootstrap
could clarify all that.

Thanks
    Brian

> Grüße, Carsten
>