LWIG Working Group A. Castellani
Internet-Draft University of Padova
Intended status: Informational March 26, 2012
Expires: September 27, 2012
Learning CoAP separate responses by examples
draft-castellani-lwig-coap-separate-responses-00
Abstract
This draft aims at providing interesting examples of CoAP separate
responses that are useful to aid CoAP implementers on understanding
possible rare situation incurring.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Taxonomy of cases . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Request lost . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Request ACK lost . . . . . . . . . . . . . . . . . . . . . 3
2.3. Response lost . . . . . . . . . . . . . . . . . . . . . . . 4
2.4. Response ACK lost . . . . . . . . . . . . . . . . . . . . . 4
3. Client perspective . . . . . . . . . . . . . . . . . . . . . . 5
3.1. No request ACK received . . . . . . . . . . . . . . . . . . 5
3.2. Response ACK lost . . . . . . . . . . . . . . . . . . . . . 6
4. Server perspective . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Request ACK lost . . . . . . . . . . . . . . . . . . . . . 7
4.2. Response lost . . . . . . . . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
6. Normative References . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
To be done if interest is shown (TBDIIIS).
2. Taxonomy of cases
In the following sections all the possible situations are briefly
described.
2.1. Request lost
C S
| CON MID=0x1234 |
| PUT /increment |
|-------->X |
Figure 1: Example of request lost
As shown in Figure 1, this case includes all the situations where the
request, including all its retransmissions, has never got through the
network up to the server.
2.2. Request ACK lost
C S
| CON MID=0x1234 |
| PUT /increment |
|--------------->|
| ACK MID=0x1234 |
| X<----------|
| |
Figure 2: Example of request ACK lost
As shown in Figure 2, this case includes all the situations where the
request has got to the server, but the corresponding ACK has never
got through the network up to the client.
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2.3. Response lost
C S
| CON MID=0x1234 |
| PUT /increment |
|--------------->|
| ACK MID=0x1234 |
|<---------------|
| |
| CON MID=0xfefe |
| 2.04 "Done" |
| X<----------|
Figure 3: Example of response lost
As shown in Figure 3, this case includes all the situations where the
response and its ACK have not been lost, but the corresponding
separate response, including all its retransmissions, has never got
through the network up to the client.
2.4. Response ACK lost
C S
| CON MID=0x1234 |
| PUT /increment |
|--------------->|
| ACK MID=0x1234 |
|<---------------|
| |
| CON MID=0xfefe |
| 2.04 "Done" |
|<---------------|
| ACK MID=0xfefe |
|----------->X |
Figure 4: Example of response ACK lost
As shown in Figure 3, this case includes all the situations where the
response, its ACK, and the corresponding separate response have not
been lost by the network, but the last ACK sent by the client has
been lost.
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3. Client perspective
In this section interesting situations incurring to a client
implementation are discussed.
3.1. No request ACK received
The client cannot distinguish between the first two cases request
lost (see Section 2.1) and request ACK lost (see Section 2.1). We
call this state C_NO_ACK.
C S
| CON MID=0x1234 |
| PUT /increment |
|--------------->|
| ACK MID=0x1234 |
| X<----------|
| |
| CON MID=0xfefe |
| 2.04 "Done" |
|<---------------|
Figure 5: Response without ACK
Figure 5 shows an example where the client is in the C_NO_ACK state
and it receives a CON response in the same session (i.e. matching
[loc-host, loc-port, rem-host, rem-port, token]). A realistic but
optimistic implementation might think that this response is related
to the previous not acked request; a pessimistic but paranoid
implementation could decide that the response is NOT related to the
previous response.
Client implementations supporting only the empty Token (no Token
support) are encouraged to randomly select local UDP source port at
each new request; this implementation shrewdness smoothly resolves
confusion.
Always having the Token Option set to a random value realistically
resolves any possible confusion in this case, at the obvious cost of
its added complexity in the client implementation and network
overhead.
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C S
| CON MID=0x1234 |
| PUT /increment |
|--------------->| server processing starts.
| ACK MID=0x1234 |
| X<----------|
| |
| CON MID=0x1234 |
| PUT /increment |
|---------->X |
| |
| CON MID=0x1234 |
| PUT /increment |
|--------------->|
| ACK MID=0x1234 |
| X<----------|
| |
| | client gives up
| |
| CON MID=0x1235 | client issues a new request
| PUT /decrement |
|-------->X |
| |
| CON MID=0xfefe | server processing ends.
| 2.04 "Done" |
|<---------------|
| ACK MID=0xfefe |
|--------------->|
| | inconsistency!
Figure 6: Naive client
Figure 6 shows an incident occurring to a naive client implementation
using the empty Token and a static local UDP port. This leads to the
indication that a client should in general avoid reusing the same
session , i.e., [loc-host, loc-port, rem-host, rem-port, token], even
if it has failed.
3.2. Response ACK lost
After sending the ACK to the response, a provident client
implementation keeps the request session up for some time. This
avoids issuing new requests in the same session (i.e. [loc-host, loc-
port, rem-host, rem-port, token]), and allows smoothly responding
with an empty ACK to response retransmissions received by the server.
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C S
| CON MID=0x1234 |
| PUT /increment |
|--------------->|
| ACK MID=0x1234 |
|<---------------|
| |
| CON MID=0xfefe |
| 2.04 "Done" |
|<---------------|
| ACK MID=0xfefe |
|---------->X |
| |
| CON MID=0x1235 |
| PUT /decrement | client issues a new request
|---------->X |
| |
| CON MID=0xfefe | server retransmits the response
| 2.04 "Done" |
|<---------------|
| ACK MID=0xfefe | client deduplication did not work
|--------------->|
Figure 7: Inexperienced client
Figure 7 shows an incident occurring to an inexperienced client not
having robust deduplication in place and reusing the same session.
4. Server perspective
TBDIIIS
4.1. Request ACK lost
TBDIIIS
4.2. Response lost
The server is said to be in S_NO_ACK when no empty ack to the
response is received by the server.
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C S
| CON MID=0x1234 |
| PUT /increment |
|--------------->|
| ACK MID=0x1234 |
|<---------------|
| |
| CON MID=0xfefe |
| 2.04 "Done" |
|<---------------|
| ACK MID=0xfefe |
|---------->X |
| |
| CON MID=0xfefe |
| 2.04 "Done" |
|<---------------|
| RST MID=0xfefe |
|--------------->|
Figure 8: Forgetful client
Figure 8 shows the realistic case of a server in state S_NO_ACK, that
deals with a client that do not mantains a successful session open
after receiving the response. An optimistic server implementation
might think that the client has received the response even if it has
replied with a RST.
C S
| CON MID=0x1234 |
| PUT /increment |
|--------------->|
| ACK MID=0x1234 |
|<---------------|
| | client goes down for reboot
| CON MID=0xfefe |
| 2.04 "Done" |
| X<----------|
| | client is up again
| CON MID=0xfefe |
| 2.04 "Done" |
|<---------------|
| RST MID=0xfefe |
|--------------->|
Figure 9: Rebooting client
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However as Figure 9 shows that the very same result might be happen
if the server is interacting with a rebooting client.
Open issue: Should the server change its behaviour depending on the
fact that it received a RST instead of an ACK? (e.g., Should it apply
the actual change to the resource only after an ACK to the response
has been received?)
5. Acknowledgements
Special thanks to Carsten Bormann for substantial contributions on
this topic that significantly aided me in compiling this document, to
Mattia Gheda for the long discussion had on this topic, and to Jeroen
Hoebeke that publicly started discussion on this topic in the CoRE
mailing list.
Thanks to Zach Shelby, Salvatore Loreto and Guido Moritz for helpful
comments and discussions that have shaped the document.
6. Normative References
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-09 (work in progress), March 2012.
Author's Address
Angelo P. Castellani
University of Padova
Via Gradenigo 6/B
Padova 35131
Italy
Email: angelo@castellani.net
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