I’ve written (starting the from the req/rep RFC) a new Surveyor RFC.
Major highlights are:
1. Protocol numbers changed from 96 & 97 to 98 & 99. (To avoid
incompatible communications.)
2. Back traces in the headers following REQ/REP convention.
3. Backpressure reporting for SURVEYOR is now optional. (REQ is
mandatory.) For RESPONDENT there is no backpressure reporting (just
drop).
4. Loop avoidance as per REQ/REP (8 hops max).
Note that this protocol is necessary to allow a respondent to reply to
multiple different surveyors. There are real use cases for this. The
backtraces are also mandatory to prevent loops. (In fact, all the
other protocols we have that *don’t* have headers are subject to loop
conditions. At a minimum a hop count is required if there is no
backtrace. Fixing this will be the subject of some follow up work.)
Heres the new RFC in ASCII (the input file is in XML, this is the
output from xml2rfc). I’d appreciate any constructive feedback.
- Garrett
Internet Engineering Task Force G. D'Amore,
Ed.
Internet-Draft
Intended status: Informational February 19,
2015
Expires: August 23, 2015
Surveyor/Respondent Scalability Protocol
sp-request-reply-01
Abstract
This document defines a scalability protocol used for performing
surveys and collecting responses amongst a number of stateless
processing nodes, and returning the results of those surveyors.
This
protocol can be used for solving such problems as voting (consensus
algorithms), presence detection, and peer discovery.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six
months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 23, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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1. Introduction
A fairly common problem in building distributed applications is peer
discovery -- or how do you find your peers. For example, imagine an
internet chat type application, where server wants to determine the
presence of all peers, including perhaps some information such as
their unique social networking handle.
Another similar problem involves voting algorithms, where a survey
of
all connected peers is required to arrive to some solution to a
problem. This is common with distributed consensus algorithms.
One of the most common problems in distributed applications is how
to
delegate a work to another processing node and get the result back
to
the original node. In other words, the goal is to utilise the CPU
power of a remote node.
It turns out that these problems are very similar. We can assume
potential participants will register with a central process. Once
that is done, the central process can send out a survey request to
the participants when it wants to perform a survey.
Also, note that it is reasonable and possible for a participant to
decline to participate (i.e. decline to respond.) This can happen
due to loss of network connectivity, or can represent a conscious
decision on the part of the respondent.
For example, a real-world example of this would be asking audience
members to raise their hands if they like the color red. The act of
raising one's hand can be thought of as responding.
As a consequence, taken generally, the surveyor should not infer any
thing about parties it doesn't get a response from. Perhaps the
respondent simply didn't hear the question, or perhaps she declines
to self-identify.
This measn that surveying should be thought of as a best-effort
service. Applications which need more resilience may repeat their
inquiries. It is common in other networking protocols to do so
periodically, and only "expire" the response from a peer that is
non-
responsive after it has missed several successive surveys.
Furthermore, the act of asking a question has to be time bounded.
This is particularly important if multiple surveys are to be issued.
Sufficient time for responses from the first survey to occur must
pass before starting a new one, unless some other identifying
content
is present to distinguish the results from one survey from another.
(Going back to our raised hands, imagine two questions asked in
rapid
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succession, one if you like the color red, the other if you like the
color blue. If only one hand is used, and there is not sufficient
time between the questions, it becomes impossible to distinguish
which color is preferred. Of course, if one uses two hands -- a
distinguishing identifier, now we can have two surveys running in
parallel. Fortunately we usually have more bits available for
conveying this kind of information in network protocols.)
In all cases the act of surveying and replying can be thought of as
state-less. In otherwords, a given response should not depend upon
the content of any prior surveys. Ideally, because of the best-
effort nature of this, it is also beneficial if surveying is itself
idempotent, i.e. the act of responding to a survey should not itself
change state on the respondent.
Generally there are few common scenarios that come up with
real-world
situations. Here are some of them.
1. One surveyor issues one survey, and then zero, one or many
responders reply. The surveyor collects then these responses
over a period of time before issuing a new survey.
2. One surveyor issues multiple surveys, distinguishing which
replies are to which survey based on some identifying content.
For example, this can be thought of like ARP, where multiple
requests can be outstanding.
3. Multiple surveyors issue surveys, but one each at a time.
Responders reply to each of these as appropriate. For example,
imagine a network with two print clients and a number of
networked printers. Both clients may occasionally desire to
inquire as supply levels, and since they don't talk to each
other, the replies may go to either system.
4. Multiple surveyors issuing multiple surveys concurrently. This
is the combination of the second and third cases above.
2. Underlying protocol
The surveyor/respondent protocol can be run on top of any SP
mapping,
such as, for example, SP TCPmapping [SPoverTCP].
Also, given that SP protocols describe the behaviour of entire
arbitrarily complex topology rather than of a single node-to-node
communication, several underlying protocols can be used in parallel.
For example, a client may send a request via WebSocket, then, on the
edge of the company network an intermediary node may retransmit it
using TCP etc.
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+---+ WebSocket +---+ TCP +---+
| |-------------| |-----------| |
+---+ +---+ +---+
| |
+---+ IPC | | SCTP +---+ DCCP +---+
| |---------+ +--------| |-----------| |
+---+ +---+ +---+
3. Overview of the algorithm
Surveyor/respondent protocol defines two different endpoint types:
The SURVEYOR and the replier or RESPONDENT.
A SURVEYOR endpoint can be connected only to a RESPONDENT endpoint,
and vice versa. If the underlying protocol indicates that there's
an
attempt to create a channel to an incompatible endpoint, the channel
MUST NOT be used. In the case of TCP mapping, for example, the
underlying TCP connection MUST be closed.
When creating more complex topologies, SURVEYOR and RESPONDENT
endpoints are paired in the intermediate nodes to form a forwarding
component, so called "device". Device receives requests from the
SURVEYOR endpoint and forwards them to the RESPONDENT endpoint. At
the same time it receives replies from the RESPONDENT endpoint and
forwards them to the SURVEYOR endpoint:
--- surveys -->
+----------+ +------------+----------+ +------------+
| |-->| | |-->| |
| SURVEYOR | | RESPONDENT | SURVEYOR | | RESPONDENT |
| |<--| | |<--| |
+----------+ +------------+----------+ +------------+
<-- responses ---
Using devices, arbitrary complex topologies can be built. The rest
of this section explains how are the requests routed through a
topology towards processing nodes and how are responses routed back
from processing nodes to the original clients.
Because the delivery of both surveys and responses is handled on a
best-effort basis, when the transport is faced with pushback, it is
acceptable for the implementation to drop the message.
Applications expecting resilience in the face of such events should
expect to perform multiple surveys over time; a failure to respond
to
a survey shall not be taken as a critical fault.
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As for delivering replies back to the clients, it should be
understood that the client may not be directly accessible (say using
TCP/IP) from the processing node. It may be beyond a firewall, have
no static IP address etc. Furthermore, the client and the
processing
may not even speak the same transport protocol -- imagine client
connecting to the topology using WebSockets and processing node via
SCTP.
Given the above, it becomes obvious that the replies must be routed
back through the existing topology rather than directly. In fact,
surveyor/respondent topology may be thought of as an overlay network
on the top of underlying transport mechanisms.
As for routing replies within the surveyor/respondent topology, it
is
designed in such a way that each reply contains the whole routing
path, rather than containing just the address of destination node,
as
is the case with, for example, TCP/IP.
The downside of the design is that surveys and responses are a
little
bit longer. Also this assumes symmetric connectivity in the
underlying transports.
The upside, on the other hand, is that the nodes in the topology
don't have to maintain any routing tables beside the simple table of
adjacent channels along with their IDs. There's also no need for
any
additional protocols for distributing routing information within the
topology.
The most important reason for adopting the design though is that
there's no propagation delay and any nodes becomes accessible
immediately after it is started. Given that some nodes in the
topology may be extremely short-lived this is a crucial requirement.
Imagine a database client that sends a survey, gets a single
response, and then immediately answers. (Think of a simple question
like "is anyone here?" A single reply is sufficies to answer the
question.) It makes no sense to delay the whole process until the
routing tables are synchronised between the client and the server.
The algorithm thus works as follows: When a survey is routed from
the
client to the processing node, every RESPONDENT endpoint determines
which channel it was received from and adds the ID of the channel to
the survey. Thus, when the survey arrives at the ultimate
respondent
it already contains a full backtrace stack, which in turn contains
all the info needed to route a message back to the original
surveyor.
After processing the survey, the responding node attaches the
backtrace stack from the survey to the response and sends it back to
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the topology. At that point every RESPONDENT endpoint can check the
traceback and determine which channel it should send the reply to.
In addition to routing, surveyor/respondent protocol takes care of
matching responses and surveys. That is, it can ensure that a given
response cannot be mismatched to a different survey.
In order to avoid confusion, after the surveyor has received all the
responses it expects to (typically when a period of time has
passed),
it should discard further stray responses.
The surveyor thus adds an unique request ID to the survey. The ID
gets copied from the survey to the response by the responding node.
When the response gets back to the surveyor, it can simply check
whether the survey in question is still being outstanding and if not
so, it can ignore the response.
To implement all the functionality described above, messages (both
surveys and responses have the following format:
+-+------------+-+------------+ +-+------------+-------------+
|0| Channel ID |0| Channel ID |...|1| Request ID | payload |
+-+------------+-+------------+ +-+------------+ ------------+
The payload of the message is preceded by a stack of 32-bit tags.
The most significant bit of each tag is set to 0 except for the very
last tag. That allows the algorithm to find out where the tags end
and where the message payload begins.
As for the remaining 31 bits, they are either survey ID (in the last
tag) or a channel ID (in all the remaining tags). The first channel
ID is added and processed by the RESPONDENT endpoint closest to the
processing node. The last channel ID is added and processed by the
RESPONDENT endpoint closest to the client.
Following picture shows an example of request saying "Hello" being
routed from the client through two intermediate nodes to the
processing node and the reply "World" being routed back. It shows
what messages are passed over the network at each step of the
process:
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client
Hello | World
| +------------+ ^
| | SURVEYOR | |
V +------------+ |
1|823|Hello | 1|823|World
| +------------+ ^
| | RESPONDENT | |
| +------------+ |
| | SURVEYOR | |
V +------------+ |
0|299|1|823|Hello | 0|299|1|823|World
| +------------+ ^
| | RESPONDENT | |
| +------------+ |
| | SURVEYOR | |
V +------------+ |
0|446|0|299|1|823|Hello | 0|446|0|299|1|823|World
| +------------+ ^
| | RESPONDENT | |
V +------------+ |
Hello | World
server
4. Hop-by-hop vs. End-to-end
All endpoints implement so called "hop-by-hop" functionality. It's
the functionality concerned with sending messages to the immediately
adjacent components and receiving messages from them.
To make an analogy with the TCP/IP stack, IP provides hop-by-hop
functionality, i.e. routing of the packets to the adjacent node,
while TCP implements end-to-end functionality such resending of lost
packets.
As a rule of thumb, raw hop-by-hop endpoints are used to build
devices (intermediary nodes in the topology) while end-to-end
endpoints are used directly by the applications.
To prevent confusion, the specification of the endpoint behaviour
below will discuss hop-by-hop and end end-to-end functionality in
separate chapters.
5. Hop-by-hop functionality
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5.1. SURVEYOR endpoint
The SURVEYOR endpoint is used by the user to send surveyor to the
responding nodes and receive the responses afterwards.
When user asks the SURVEYOR endpoint to send a request, the endpoint
should send it to ALL of the associated outbound channels (TCP
connections or similar). The request sent is exactly the message
supplied by the user. SURVEYOR sockets MUST NOT modify an outgoing
survey in any way.
If there's no channel to send the survey to, the survey is merely
discarded. The endpoint MAY report the backpressure condition to
the
user as well.
If there are associated channels but none of them is available for
sending, i.e. all of them are already reporting backpressure, the
endpoint won't send the message and MAY report the backpressure
condition to the user. The actual survey is discarded.
If the channel is not capable of reporting backpressure (e.g. DCCP)
the endpoint SHOULD consider it as always available for sending new
request.
When there are multiple channels available for sending the survey
endpoint MUST deliver the survey to all of them.
As for incoming messages, i.e. responses, SURVEYOR endpoints MUST
fair-queue them. In other words, if there are replies available on
several channels, they MUST receive them in a round-robin fashion.
They must also take care not to compromise the fairness when new
channels are added or old ones removed.
In addition to providing basic fairness, the goal of fair-queueing
is
to prevent DoS attacks where a huge stream of fake responses from
one
channel would be able to block the real replies coming from
different
channels. Fair queueing ensures that messages from every channel
are
received at approximately the same rate. That way, DoS attack can
slow down the system but it can't entirely block it.
Incoming responses MUST be handed to the user exactly as they were
received. SURVEYOR endpoints MUST not modify the responses in any
way.
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5.2. RESPONDENT endpoint
RESPONDENT endpoints are used to receive surveys from the clients
and
send resopnses back to the clients.
First of all, each RESPONDENT socket is responsible for assigning
unique 31-bit channel IDs to the individual associated channels.
The first ID assigned MUST be random. Next is computed by adding 1
to the previous one with potential overflow to 0.
The implementation MUST ensure that the random number is different
each time the endpoint is re-started, the process that contains it
is
restarted or similar. So, for example, using pseudo-random
generator
with a constant seed won't do.
The goal of the algorithm is to the spread of possible channel ID
values and thus minimise the chance that a response is routed to an
unrelated channel, even in the face of intermediate node failures.
When receiving a message, RESPONDENT endpoints MUST fair-queue among
the channels available for receiving. In other words they should
round-robin among such channels and receive one request from a
channel at a time. They MUST also implement the round-robin
algorithm is such a way that adding or removing channels doesn't
break the fairness.
In addition to guaranteeing basic fairness in access to computing
resources the above algorithm makes it impossible for a malevolent
or
misbehaving client to completely block the processing of requests
from other clients by issuing steady stream of surveys.
After receiving the survey, the RESPONDENT socket should prepend it
by 32 bit value, consisting of 1 bit set to 0 followed by the 31-bit
ID of the channel the request was received from. The extended
survey
will be then handed to the user.
The goal of adding the channel ID to the response is to be able to
route the response back to the original channel later on. Thus,
when
the user sends a response, endpoint strips first 32 bits off and
uses
the value to determine where it is to be routed.
If the response is shorter than 32 bits, it is malformed and the
endpoint MUST ignore it. Also, if the most relevant bit of the
32-bit value isn't set to 0, the response is malformed and MUST be
ignored.
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Otherwise, the endpoint checks whether its table of associated
channels contains the channel with a corresponding ID. If so, it
sends the response (with first 32 bits stripped off) to that
channel.
If the channel is not found, the response MUST be dropped. If the
channel is not available for sending, i.e. it is applying
backpressure, the response MUST be dropped.
Note that when the response is unroutable two things might have
happened. Either there was some kind of network disruption, in
which
case the survey may be re-sent later on, or the original client have
failed or been shut down. In such case the survey won't be resent,
however, it doesn't really matter because there's no one to deliver
the response to any more anyway.
Unlike surveys, there's never pushback applied to the responses;
they
are simply dropped. If the endpoint blocked and waited for the
channel to become available, all the subsequent replies, possibly
destined for different unblocked channels, would be blocked in the
meantime. That allows for a DoS attack simply by firing a lot of
surveys and not receiving the responses.
6. End-to-end functionality
End-to-end functionality is built on top of hop-to-hop
functionality.
Thus, an endpoint on the edge of a topology contains all the hop-by-
hop functionality, but also implements additional functionality of
its own. This end-to-end functionality acts basically as a user of
the underlying hop-by-hop functionality.
6.1. SURVEYOR endpoint
End-to-end functionality for SURVEYOR sockets is concerned with
matching the responses to surveys, and with filtering out stray or
outdated responses.
To be able to do this, the endpoint must tag the survey with unique
31-bit survey IDs. First survey ID is picked at random. All
subsequent survey IDs are generated by adding 1 to the last survey
ID
and possibly overflowing to 0.
To improve robustness of the system, the implementation MUST ensure
that the random number is different each time the endpoint, the
process or the machine is restarted. Pseudo-random generator with
fixed seed won't do.
When user asks the endpoint to send a message, the endpoint prepends
a 32-bit value to the message, consisting of a single bit set to 1
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followed by a 31-bit survey ID and passes it on in a standard
hop-by-
hop way.
If the hop-by-hop layer reports pushback condition, the end-to-end
layer considers the survey unsent and MAY report pushback condition
to the user.
If the survey is successfully sent, the endpoint stores the survey
including its survey ID, so that it can be resent later on if
needed.
At the same time it sets up a timer to receive all of the responses.
The user MUST be allowed to specify the timeout interval. The
default timeout interval must be 60 seconds.
When a response is received from the underlying hop-by-hop
implementation, the endpoint should strip off first 32 bits from the
response to check whether it is a valid reply.
If the response is shorter than 32 bits, it is malformed and the
endpoint MUST ignore it. If the most significant bit of the 32-bit
value is set to 0, the reply is malformed and MUST be ignored.
Otherwise, the endpoint should check whether the survey ID in the
response matches any of the survey IDs of the surveys being
processed
at the moment. If not so, the response MUST be ignored. It is
either a stray message or a too-long delayed response.
Please note that the endpoint can support either one or more surveys
being processed in parallel. Which one is the case depends on the
API exposed to the user and is not part of this specification.
If the ID in the response matches one of the surveys in progress,
the
response MUST be passed to the user (with the 32-bit prefix stripped
off).
A SURVEYOR endpoint MUST make it possible for the user to cancel a
particular survey in progress. What it means technically is
deleting
the stored copy of the survey and cancelling the associated timer.
Thus, once the response arrives, it will be discarded by the
algorithm above.
Finally, when the timeout for a survey expires, then the survey must
be canceled in a manner similar to user-initiated cancelation. That
is, the stored copy of the survey must be deleted, the timer
removed,
and any further responses received with the same survey ID are
subsequently discarded.
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6.2. RESPONDENT endpoint
End-to-end functionality for RESPONDENT endpoints is concerned with
turning surveys into corresponding responses.
When user asks to receive a survey, the endpoint gets next request
from the hop-by-hop layer and splits it into the traceback stack and
the message payload itself. The traceback stack is stored and the
payload is returned to the user.
The algorithm for splitting the survey is as follows: Strip 32 bit
tags from the message in one-by-one manner. Once the most
significant bit of the tag is set, we've reached the bottom of the
traceback stack and the splitting is done. If the end of the
message
is reached without finding the bottom of the stack, the survey is
malformed and MUST be ignored.
Note that the payload produced by this procedure is the same as the
survey payload sent by the original client.
Once the user processes the survey and sends the response, the
endpoint prepends the response with the stored traceback stack and
sends it on using the hop-by-hop layer. At that point the stored
traceback stack MUST be deallocated.
Additionally, RESPONDENT endpoints MUST support cancelling any
survey
being processed at the moment. What it means, technically, is that
state associated with the survey, i.e. the traceback stack stored by
the endpoint is deleted and reply to that particular survey is never
sent.
The most important use of cancellation is allowing the service
instances to ignore surveys (whether due to malformation or for
other
application specific reasons.) In such case the reply is never
sent.
Of course, if application wants to send an application-specific
error
massage back to the client it can do so by not cancelling the survey
and sending a regular response.
7. Loop avoidance
It may happen that a request/reply topology contains a loop. It
becomes increasingly likely as the topology grows out of scope of a
single organisation and there are multiple administrators involved
in
maintaining it. Unfortunate interaction between two perfectly
legitimate setups can cause loop to be created.
With no additional guards against the loops, it's likely that
requests will be caught inside the loop, rotating there forever,
each
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message gradually growing in size as new prefixes are added to it by
each RESPONDENT endpoint on the way. Eventually, a loop can cause
congestion and bring the whole system to a halt.
To deal with the problem SURVEYOR endpoints MUST check the depth of
the traceback stack for every outgoing request and discard any
requests where it exceeds certain threshold. The threshold SHOULD
be
defined by the user. The default value is suggested to be 8.
8. IANA Considerations
New SP endpoint types SURVEYOR and RESPONDENT should be registered
by
IANA. For now, value of 98 should be used for SURVEYOR endpoints
and
value of 99 for RESPONDENT endpoints. (An earlier similar protocol
without the backtrace headers used protocol numbers 96 and 97.)
9. Security Considerations
The mapping is not intended to provide any additional security to
the
underlying protocol. DoS concerns are addressed within the
specification.
10. References
[SPoverTCP]
Sustrik, M., "TCP mapping for SPs", August 2013.
Author's Address
Garrett D'Amore (editor)
Email: garrett@xxxxxxxxxx
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