



Internet Engineering Task Force                          G. D'Amore, Ed.
Internet-Draft                                            March 18, 2015
Intended status: Informational
Expires: September 19, 2015


                Surveyor/Respondent Scalability Protocol
                             sp-surveyor-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.

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   This Internet-Draft will expire on September 19, 2015.

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   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 responses 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 cancellation.  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@damore.org



















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