Re: [core] #409 (block): CoAP over TCP: Multiple Simultaneous TCP Connections

Hi,

The problem seems to be whether CoAP over Multiple Simultaneous TCP 
Connections needs considering in CoAP related drafts.

My suggestion is no.

There are many P2P communications adopts P2P/HTTP over Multiple Simultaneous 
TCP Connections.
It is the P2P software to manage the multiple TCP connections, not HTTP.

Regards,

Gengyu WEI
Network Technology Center
School of Computer
Beijing University of Posts and Telecommunications
-----原始邮件----- 
From: core issue tracker
Sent: Tuesday, May 03, 2016 4:39 PM
To: draft-ietf-core-block@tools.ietf.org ; Hannes.Tschofenig@gmx.net
Cc: core@ietf.org
Subject: [core] #409 (block): CoAP over TCP: Multiple Simultaneous TCP 
Connections

#409: CoAP over TCP: Multiple Simultaneous TCP Connections

CoAP has been designed to support small data transmissions. For device
management it is, however, necessary to also deal with the transmission of
larger payloads as well. Such larger payloads may be, for example,
firmware/software updates.

The maximum payload size of CoAP is determined by the length field of the
length field provided in the UDP header. It is ~64KB. For practical
purposes the usable payload size is, however, much smaller due to
performance issues introduced by fragmentation provided at the IP layer
and/or adaption layers. For this reason the block-wise transfer mechanism
has been defined.

Block-wise transfer is a mechanism that can be used to transfer larger
payloads by chunking them into smaller parts (each part with a maximum
size of 1024 bytes each). The block-wise transfer depends on CoAP and
therefore uses the stop-and-wait defined in RFC 7252 (as a simple
congestion control mechanism). The transfer of large payloads does,
however, not block the transmission of other pending messages since they
can easily be interleaved due to the nature of the block-wise transfer
design.

When large payloads are transferred by CoAP over TCP then a large transfer
blocks any other requests unless multiple TCP connections are opened. The
question is therefore what should the CoAP over TCP state regarding the
use of multiple TCP connections? Using multiple TCP connection increases
RAM requirements; a single TCP connection introduces head-of-line
blocking.

When CoAP over TCP is used with Block-wise transport in combination with
BERT, see https://tools.ietf.org/html/draft-bormann-core-block-bert-00,
then the previously described problem of large transfers that block other
ongoing activities is (partially) mitigated. BERT changes the
interpretation of the length information and changes it as a multiple of
1024 bytes (and thus increasing the size of the chunks).

The question is therefore whether CoAP over TCP should recommend the use
of Block-wise Transfer for large file transfers and incorporate BERT into
the CoAP over TCP draft.

(I would like to thank Achim Kraus for raising this issue during the OMA
face-to-face meeting.)

-- 
-------------------------------------+-------------------------------------
Reporter:                           |      Owner:  draft-ietf-core-
  Hannes.Tschofenig@gmx.net          |  block@tools.ietf.org
     Type:  protocol defect          |     Status:  new
Priority:  major                    |  Milestone:
Component:  block                    |    Version:
Severity:  -                        |   Keywords:
-------------------------------------+-------------------------------------

Ticket URL: <https://trac.tools.ietf.org/wg/core/trac/ticket/409>
core <https://tools.ietf.org/core/>

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