Re: [Anima-bootstrap] bootstrap over CoAP

Carsten Bormann <cabo@tzi.org> Sat, 09 July 2016 19:40 UTC

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Date: Sat, 09 Jul 2016 21:40:10 +0200
From: Carsten Bormann <cabo@tzi.org>
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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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>> 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.

Grüße, Carsten