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Versions: 00 01                                                         
Network Working Group                                      M. Kuehlewind
Internet-Draft                                               B. Trammell
Intended status: Informational                                ETH Zurich
Expires: January 4, 2016                                    July 3, 2015


                             SPUD Use Cases
                   draft-kuehlewind-spud-use-cases-00

Abstract

   The Substrate Protocol for User Datagrams (SPUD) BoF session at the
   IETF 92 meeting in Dallas in March 2015 identified the potential need
   for a UDP-based encapsulation protocol to allow explicit cooperation
   with middleboxes while using new, encrypted transport protocols.
   This document summarizes the use cases discuss at the BoF and thereby
   proposes a structure for the description of further use cases.

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 January 4, 2016.

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




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Firewall Traversal  . . . . . . . . . . . . . . . . . . . . .   3
   3.  State Lifetime Discovery  . . . . . . . . . . . . . . . . . .   5
   4.  Low-Latency Service . . . . . . . . . . . . . . . . . . . . .   8
   5.  Application-Limited Flows . . . . . . . . . . . . . . . . . .  10
   6.  Service Multiplexing  . . . . . . . . . . . . . . . . . . . .  13
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   10. Informative References  . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   This document describe use cases for a common Substrate Protocol for
   User Datagrams (SPUD) that could be used by an overlaying transport
   or application to explicitely expose information to middleboxes or
   request information from (SPUD-aware) middleboxes.

   For each use case, we first describe a problem that can not be solved
   with current protocols, or only solved inefficiently.  We then
   discuss which information should be exposed by which party to help
   the described problem.  We also discuss potential mechanisms to use
   that exposed information at middleboxes and/or endpoints, in order to
   demonstrate the feasibility of using the exposed information to the
   given use case.  The described mechanisms are not necessarily
   proposals for moving forward, nor do they necessarily represent the
   best approach for applying the exposed information, but should
   illustrate and motivate the applicability of the exposed information.

   In this document we assume that there is no pre-existing trust
   relationship between the communication endpoints and any middlebox on
   the path.  Therefore we must always assume that information that is
   exposed can be wrong or nobody will actually act based on the exposed
   information.  However, for the described use cases there should still
   be a benefit, e.g if otherwise no information would be available.

   Based on each mechanism, we discuss deployment incentives of each
   involved party.  There must be clear incentives for each party to
   justify the proposed information exposure and at best an incremental
   deployment strategy.  Finally, we discuss potential privacy concerns
   regarding the information to be exposed, as well as potential
   security issues of the proposed mechanisms.



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2.  Firewall Traversal

2.1.  Problem Statement

   Today UDP is often blocked by firewalls, or only enabled for a few
   well-known applications.  However, this makes it hard to deploy new
   services on top of UDP.

   For a long time UDP has not been used much for high volume traffic
   and therefore it was assumed that most UDP traffic is spam or attack
   traffic.  This is not true anymore.  The volume of (good) UDP traffic
   is growing, mostly due to voice and video (real-time) services, e.g.
   RTCWEB uses UDP for data and media, where TCP is not suitable anyway.

   Even if firewall administrators are willing to implement new rules
   for UDP services, it is hard to track session state for UDP traffic.
   As UDP is unidirectional, it is unknown whether the receiver is
   willing to accept the connection.  Further there is no way to figure
   how long state must be maintained once established.  To efficiently
   establish state along the path we need an explicit contract, as is
   done implicitly with TCP today.

2.2.  Information Exposure

   To maintain state in the network, it must be possible to easily
   assign each packet to a session that is passing a certain network
   node.  This state should be bound to something beyond the five-tuple
   to link packets together.  In [I-D.trammell-spud-req] propose the use
   of identifiers "tubes".  This allows for differential treatment of
   different packets within one five-tuple flow, presuming the
   application has control over segmentation and can provide
   requirements on a per-tube basis.  Tube IDs must be hard to guess: a
   tube ID in addition to a five-tuple as an identifier, given
   significant entropy in the tube ID, provides an additional assurance
   that only devices along the path or devices cooperating with devices
   along the path can send packets that will be recognized by
   middleboxes and endpoints as valid.

   Further, to maintain state, the sender must explicitly indicate the
   start and end of a tube to the path, while the receiver must confirm
   connection establishment.  This, together with the first packet
   following the confirmation, provides a guarantee of return
   routability; i.e. that the sender is actually at the address it says
   it is.  This impies all SPUD tubes must be bidirectional, or at least
   support a feedback channel for this confirmation.  Even though UDP is
   not a bidirectional transport protocol, often services on top of UDP
   are bidirectional anyway.  Even if not, we only require one packet to
   acknowledge a new connection.  This is low overhead for this basic



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   security feature.  This connection set-up should not impose any
   additional start-up latency, so the sender must be also able to send
   payload data in the first packet.

   If a firewall blocks a SPUD packet, it can be beneficial for the
   sender to know why the packet was blocked.  Therefore a SPUD-aware
   middlebox should be able to send error messages.  Such an error
   message can either be sent directly to the sender itself, or
   alternatively to the receiver that can decide to forward the error
   message to a sender or not.

2.3.  Mechanism

   A firewall or middlebox can use the tube ID as an identifier for its
   session state information.  If the tube ID is large enough it will be
   hard for a non-eavesdropping attacker to guess the ID.

   If a firewall receives a SPUD message that signals the start of a
   connection, it can decide to establish new state for this tube.
   Alternatively, it can also forward the packet to the receiver and
   wait if the connection is wanted before establishing state.  To not
   require forwarding of unknown payload, a firewall might want to
   forward the initial SPUD packet without payload and only send the
   full packet if the connection has be accepted by the receiver.

   The firewall must still maintain a timer to delete the state of a
   tube if no packets were received for a while.  However, if a end
   signal is received the firewall can remove the state information
   faster.

   If a firewall receives a SPUD message which does not indicate the
   start of a new tube and no state is available for this tube, it may
   decide to block the traffic.  This can happen if the state has
   already timed out or if the traffic was rerouted.  In addition a
   firewall may send an error message to the sender or the receiver
   indicatng that no state information are available.  If the sender
   receives such a message it can resend a start signal (potentially
   together with other tube state information) and continue its
   transmission.

2.4.  Deployment Incentives

   It is not expected that the provided SPUD information will enable all
   generic UDP-based services to safely pass firewalls , however, for
   new services that a firewall administrator is willing to allow, it
   makes state handling easier.





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   For application developers that actually would like to use a new
   transport services, there are today often only two choices;
   encapsulation over UDP or over TCP.  SPUD already provides
   encapsulation over UDP as well as maintains (a few) additional
   information about the network state.  This shim layer can support
   application developers to more easily implement new services.

2.5.  Trust and Privacy

   We proposed to limit the scope of the tube ID to the five-tuple.
   While this makes the tube ID useless for session mobility, it does
   mean that the valid ID space is sufficiently sparse to maintain the
   "hard to guess" property, and prevents tube IDs from being misused to
   track flows from the same endpoint across multiple addresses.  This
   limitation may need further discussion.

   By providing information on the connection start up, SPUD only
   exposes information that are often already given in the higher layer
   semantics.  Thus it does not expose additional information, it only
   makes the information explicit and accessible without specific
   higher-layer/application-level knowledge.

3.  State Lifetime Discovery

3.1.  Problem Statement

   Even if the transport protocol implements a close-down mechanism or
   SPUD explicitly provides an end of tube signal, a network device
   cannot assume that these signals are provided reliably.  Therefore
   each network device that holds per-flow/per-tube state must implement
   a mechanism to remove the state if no traffic that is matching this
   state information has been observer for a while.  Usually this is
   realized by maintaining a timeout since the last observed packet.

   An endpoint that wants to keep a connection open even if it is not
   sending any data for a while might need to send heartbeat packets to
   keep state alive that potentially is store somewhere on the network
   path.  However, the timeout period of the network device storing this
   information is unknow to the endpoint.  Therefore it has to send
   heartbeat fairly rapidly, or might assume a default value of 150ms
   that is commonly used today.

3.2.  Information Exposure

   SPUD can be used to request the timeout used by a middlebox.  As
   SPUD-enabled endpoint therefore sends a path-to-endpoint option that
   is initialized with an non-valid value (e.g. 0) and midpoints can
   update this information to the timeout value that is used to maintain



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   per-tube state.  As multiple network devices might be on a path that
   maintain per-tube state, the timeout information should only be
   updated to the minimum value.  A sender could also initial the
   timeout value to the minimum heartbeat frequency it will use or the
   maximum idle period (if known).

   [Editor's note: Would it be necessary/useful to get a (separate)
   confirmation from each middlebox that has understood and read this
   SPUD information?  Alternatively, it would maybe be useful signal the
   proposed heartbeat period separately, however that's also complicated
   because the endpoint might adapt it's heartbeat period based on the
   timeout information...]

3.3.  Mechanism

   If a network device that uses a timeout to remove per-tube state
   receives a SPUD timeout information request, it should expose its own
   timeout value if smaller than the one already given in the SPUD
   header.  Alternatively, if a value is already given, it might decide
   to use the given value as timeout for the state information of this
   tube.

   A SPUD sender can request the timeout used by network devices on path
   to maintain state.  If a minimum heartbeat frequency is used or the
   maximum idle period is known, the sender might pre-set this value.
   If the pre-set value is not changed, the sender does not know if
   there is at least one SPUD-aware middlebox on the path that
   understands the time-out information.  In any case a sender must
   always assume that there could be additional non-SPUD aware middlebox
   that has a smaller timeout.  Therefore even if the proposed timeout
   is used for heartbeating, traffic can still be blocked due to removed
   state.  This is also the case if a middlebox did not correctly
   indicate its timeout value, e.g. when the value is dynamically
   changed to a smaller value if more state needs to be maintained.
   However, usually the number of middleboxes on the path that hold per-
   flow/tube state is low.  Therefore the chance that the received
   feedback indicates the right timeout value is high.

   [Editor's note: Do we need a SPUD message that can be initialized by
   the middlebox to let the endpoint know that the time has changed?]

   A SPUD endpoint receiving a SPUD header with timeout information
   should reflect this information to the sender with the next packet
   that it will be sent (or after a short timeout).  Therefore this
   information should be requested with the first packet, that should
   immediately trigger the receiver to at least send one packet.  In
   addition SPUD-aware nodes on the backward path are able to also
   signal their timeout.



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   [Editor's note: Is it necessary to have an explicit SPUD heartbeat
   packet, that should also be reflected by the receiver to keep state
   on the backwards path alive..? And then request timeouts for the
   forward and backward path separately?]

3.4.  Deployment Incentives

   Initially, if not widely deployed, there will be not much benefit to
   using this extension.  However, an endpoint can never be sure that
   all middleboxes on the path that maintain state information based on
   a timeout will expose this information (correctly).  An endpoint must
   always be prepared that traffic can be blocked (after an idle period)
   and the connection must be restarted.  This is the same today if
   heartbeats are used.  Therefore, SPUD will not help to simplify the
   implementation but it will also no make it much more complicated as
   only the heartbeat interval might be changed.

   However, under the assumption that there are usually only a small
   number of middbleboxes on one network path that hold (per-tube) state
   information, it is likely that if information is exposed by a
   middlebox, this information is correct and can be used.

   The more SPUD gets deployed, the more often endpoints will be able to
   set the heartbeat interval correctly.  This will reduce the number of
   unnecessary reconnects that cause additional latency.  Further, an
   endpoint might be able to request a higher timeout by pre-setting the
   value.

   Network nodes that understand the SPUD timeout information and expose
   their timeouts are able to handle timeouts more flexibly, e.g.
   announcing lower timeout values if space is sparse.  Further if an
   endpoint announces a low pre-set value because the endpoint knows
   that it will only have short idle periods, the timeout interval could
   be reduced.

3.5.  Trust, Privacy and Security

   [Editor's note: no trust needed here as discussed above... right?
   And I currently don't see privacy issues here...?']

   [Editor's note: Make sure this is not a vector for simplified state
   exhaustion attacks...? Don't think it's worse than TCP...? Any other
   attacks?]








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4.  Low-Latency Service

4.1.  Problem Statement

   Networks are often optimized for low loss rates and high throughput
   by providing large buffers that can absorb traffic spikes or rate
   variations and always holding enough data to keep the link full.
   This is beneficial for applications like high-priority bulk transfer,
   where only the total transfer time is of interest.  (High volume)
   interactive application, such as video calls, however, have very
   different requirements.  Usually these application can tolerate
   high(er) loss rates, as they anyway cannot wait for missing data to
   be retransmitted, while having hard latency requirements necessary to
   make their service work.

   Large network buffers may induce high queuing delays due to greedy
   cross traffic using loss-based congestion control that periodically
   fills the buffer.  In loss-based congestion control the sending rate
   is periodically increased until a loss is observed to probe for
   available bandwidth.  Unfortunately, the queuing delay that is
   indices by this probing can downgrade the quality of experience for
   competing interactive applications or even make them simply unusable.
   Further, to co-exist with greedy flows that use loss-based congestion
   control, one has to react based on the same feedback signal (loss)
   and implement about the same aggressiveness than these competing
   flows.

4.2.  Information Exposure

   While large buffers that are able to absorb traffic spikes that are
   often induced by short bursts are beneficial for some applications,
   the queuing delay that might be induced by these large buffers is
   very harmful to other applications.  We therefore propose an explicit
   indication of loss- vs. latency-sensitivity per SPUD tube.  This
   indication does not prioritize one kind of traffic over the other:
   while loss-sensitive traffic might face larger buffer delay but lower
   loss rate, latency-sensitive traffic has to make exactly the opposite
   tradeoff.

   Further, an application can indicate a maximum acceptable single-hop
   queueing delay per tube, expressed in milliseconds.  While this
   mechanism does not guarantee that sent packets will experience less
   than the requested delay due to queueing delay, it can significantly
   reduce the amount of traffic uselessly sitting in queues, since at
   any given instance only a small number of queues along a path
   (usually only zero or one) will be full.





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4.3.  Mechanism

   A middlebox may use the loss-/latency-sensitive signal to assign
   packet to the appropriate service if different services are
   implemented at this middlebox.  Today's traffic, that does not
   indicate a low loss or low latency preference, would still be
   assigned to today's best-effort service, while a new low latency
   service would be introduced in addition.

   The simplest implementation of such a low latency service (without
   disturbing existing traffic) is to manage traffic with the latency-
   sensitive flag set in a separate queue.  This queue either, in
   itself, provides only a short buffer which induces a hard limit for
   the maximum (per-queue) delay or uses an AQM (such as PIE/ CoDel)
   that is configured to keep the queuing delay low.

   In such a two-queue system the network provider must decides about
   bandwidth sharing between both services, and might or might not
   expose this information.  Initially there will only be a few flows
   that indicate low latency preference.  Therefore at the beginning
   this service might have a low maximum bandwidth share assigned in the
   scheduler.  However, the sharing ratio should be adopted to the
   traffic load/number of flows in each service class over time.  This
   can be done manually by a network administrator or in an automated
   way.

   Applications and endpoints setting the latency sensitivity flag on a
   tube must be prepared to experience relatively higher loss rates on
   that tube, and might use techniques such as Forward Error Correction
   (FEC) to cope with these losses.

   If in addition the maximum per-hop delay is indicated by the sender,
   a SPUD-aware router might drop any packet which would be placed in a
   queue that has more than the maximum single-hop delay at that point
   in time before queue admission.  Thereby the overall congestion can
   be reduced early instead of withdrawing the packet at the receiver
   after it has blocked network resources for other traffic.
   Alternatively, a SPUD-aware node might only remove the payload and
   add a SPUD error message, to report what the problem is.

   An endpoint indicating the maximum per-hop delay must be aware that
   is might face higher loss rates under congestion than competing
   traffic on the same bottleneck.  Especially, packets might be dropped
   due to the maximium per-hop delay indication before any congestion
   notification is given to any other competing flows on the same
   bottleneck.  This should considered in the congestion reaction as any
   loss should be consider as a sign for congestion.




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4.4.  Deployment Incentives

   Application developers go to a great deal of effort to make latency-
   sensitive traffic work over today's Internet.  However, if large
   delays are induced by the network, an application at the endpoint
   cannot do much.  Therefore applications can benefit from further
   support by the network.

   Network operators have already realized a need to better support low
   latency services.  However, they want to avoid any service
   degradation for existing traffic as well as risking stability due to
   large configuration changes.  Introducing an additional service for
   latency-sensitive traffic that can exist in parallel to today's
   network service (or potentially fully replace today's service at some
   point in future...) helps this problem.

4.5.  Trust and Privacy

   An application does not benefit from wronly indicating loss- or
   latency-sensitivity as it has to make a tradeoff between low loss and
   potential high delay or low delay and potential high loss.  Therefore
   there is no incentive for lying.  A simple classification of traffic
   in loss-sensitive and latency-sensitive does not expose privacy-
   critical information about the user's behavior.

5.  Application-Limited Flows

5.1.  Problem Statement

   Today, there are a large number of flows that are mostly application-
   limited, where the application can adapt this limit to changing
   traffic conditions.  An example is unicast streaming video where the
   coding rate can be adapted based on detected congestion or changing
   link characteristics.  This adaptation is difficult, since cross-
   traffic (much of which uses TCP congestion control) will often probe
   for available bandwidth more aggressively than the application's
   control loop.  Further complicating the situation is the fact that
   rate adaptation may have negative effects on the user's quality of
   experience, and should therefore be done infrequently.

5.2.  Information Exposure

   With SPUD, the sender can provide an explicit indication of the
   maximum data rate that the current encoding needs.  This can provide
   useful information to the bottleneck to decide how to correctly treat
   the corresponding tube, e.g. setting a rate limit or scheduling
   weight if served from its own queue.




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   Further, a network node that imposes rate shaping could expose the
   rate limit to the sender if requested.  This would help the sender to
   choose the right encoding and simplifies probing.  If the rate
   limited is changed the network node might want to signal this change
   without being requested for it.

   In addition, both the endpoint as well as a middlebox could announce
   sudden changes in bandwidth demand/offer.  While for the endpoint it
   might be most important to indicate that the bandwidth demand has
   increased, a middlebox could indicate if more bandwidth is
   (currently) available.  Note that this information should only be
   indicated if the network node was previously the bottleneck/the out-
   going link is fully loaded.  Further, if the information that
   bandwidth is available is provided to multiple endpoints at the same
   time, there is a higher risk of overloading the network as all
   endpoints might increase their rate at the same time.

   [Editor's note: Should a middlebox even indicate how much capacity is
   available.. or 1/n of the available capacity if indicated to n
   endpoints?  But there might be a new bottleneck now...]

5.3.  Mechanism

   If the maximum sending rate of a flow is exposed this information
   could be used to make routing decision, if e.g. two paths are
   available that have different link capacity and average load
   characteristics.

   Further, a network nodes, that receives an indication of the maximum
   rate limit for a certain tube, might decide to threat this flow in an
   own queue and prioritize this flow in order to keep the delay low as
   long as the indicated rate limit is not exceeded.  This should only
   be done if there is sufficient capacity on the link (the average load
   over a previous time period has be low enough to serve an additional
   maximum traffic load as indicated by the rate limit) or the flow is
   known to have priority, e.g. based on additional out-of-band
   signaling.  If the link, however, is currently congested, a middlebox
   might choose to ignore this information or indicate a lower rate
   limit.

   If a network node indicates rate shaping, this information can be
   used by the sender to choose its current data/coding rate
   appropriately.  However, a sender should still implement a mechanism
   to probe ifor available bandwidth to verify the provided information.
   As a certain rate limit is expected the sender should probe carefully
   around this rate.





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   A network node might further indicate a different/lower rate limit
   during the transmission.  However, in this case, it might be easy for
   an attacker to send a wrong rate limit, therefore an endpoint should
   not change its data rate immediately, but might be prepared to see
   higher losses rates instead.

   If a sender receives an indication that more bandwidth is available
   it should not just switch to a higher rate but probe carefully.
   Therefore it might step-wise increase its coding rate or first add
   additional FEC information which will increase the traffic rate on
   the link and at the same time provide additional protection as soon
   as the new capacity limit is reached.

   A network node that receives an indication that a flow will increase
   its rate abruptly, might prioritize this flow for a certain (short)
   time to enable a smoother transition.  [Editor's node: Need to figure
   out if high loss/delay when the coding rate is increased is actually
   a problem and if so further evaluate if short-term prioritization
   helps.]

5.4.  Deployment Incentives

   By indicating a maximum sending rate a network operator might be able
   to better handle/schedule the current traffic.  Therefore the network
   operator might be willing to support these kind of flows explicitly
   by trying to serve the flow with the requested rate.  This can
   benefit the service quality and increase the user's satisfaction with
   the provided network service.

   If the maximum sending rate is known by the application, the
   application might be willing to expose this information if there is a
   chance that the network will try to support this flow by providing
   sufficient capacity.

   Currently application have no good indication when to change their
   coding rate.  Especially, increasing the rate is hard.  Further, it
   should be avoided to change the rate (forth and back) too often.  An
   indication if and how much bandwidth is available, is therefore
   helpful for the application and can simplify probing (even though
   there will still and always be an additional control loop needed to
   react to congestion and for probing).

5.5.  Trust, Privacy and Security

   [TBD] [Editor's note: is there an attack possible by indicating a low
   limit (from or to the application)?  Note, that the application
   should not rely on this information and still probe for more capacity
   (if needed) and react to congestion!]



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6.  Service Multiplexing

6.1.  Problem Statement

   Many services rewuire multiple parallel transmissions to transfer
   different kinds of data which usually have a clear priority between
   each other.  One example is WebRTC where the audio is most important
   and should be higher prioritized than the video, while control
   traffic might have the lowest priority.  Further, some packets within
   one flow might be more important than others within the same flow/
   tube, e.g. such as I-frames in video transmissions.  However, today a
   network will treat all packets the same in case of congestion and
   might e.g. drop audio packets while video and control traffic are
   still transmitted.

6.2.  Information Exposure

   A SPUD sender may indicate a lower priority relative to another tube
   that is used in the same 5-tuple.

   Similarly, a lower packet priority within one flow/tube could be
   indicated to give one packet a low priority than other packets with
   the same tube ID.  This information can be used to preferentially
   drop less inportant packets e.g. carrying information that could be
   recovered by FEC or where missing data can be easily concealed.

   Further, with a stronger integration of codec and transport
   technology SPUD could even indicate more even finer grained priority
   levels to provide automatic graceful degradation of service within
   the network itself.

   [Editor's note: do we want to also provide per-packet information
   over spud?  Or would all lower priority packets of one flow simply
   below to a different tube?  In this case can we send a SPUD start
   message with more than on tube ID?]

6.3.  Mechanism

   Preferential dropping can be implemented by a router queue in case
   packets need to be dropped due to congestion.  In this case the
   router might not drop the incoming packet but look for a packet with
   the same tube ID that is already in the queue and has a lower
   priority than to actual packet that should have been dropped.  Note
   that a middlebox should only drop a different packet if there is
   currently a lower priority packet in the queue, because it otherwise
   does not know whether it will every see a lower priority packet for
   this flow.  This could cause unfairness issues.  Therefore a
   middlebox might need to hold additional state, e.g. keeping position



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   of the last low priority packet of each tube in a separate table.
   The chance that a low priority packet of the same or corresponding
   tube currently sits in the queue, is lower the smaller the buffer is.
   Therefore for low-latency, real-time services, there is a tradeoff.

   Alternatively, the middlebox might queue the lower priority traffic
   in a different queue.  Using a different queue might be suitable for
   lower flow priority but should not be used for lower priority packets
   within the same flow as this can also lead to other issues such as
   high reordering.  Further, using a lower priority queue will not only
   give higher priority to the traffic belong to the same service/sender
   but also to all other competing flows.  This is usually not the
   intention.

   [Editor's note: Does it makes sense to, in addition, rate-limit the
   higher prirority flows to their current rate to make sure that the
   bottleneck is not further overloaded...?]

   If a sender has indicated lower priority to certain tubes and only
   experiences losses/congestion for the lower priority tubes, the
   sender should still not increase its sending for the higher priority
   tube and might even consider to decrease the sending rate for the
   higher prioroty tubes as well.  Potentially a (delay-based) mechanism
   for shared bottleneck detection should be used to ensure that all
   transmissions actually share the same bottleneck.

6.4.  Deployment Incentives

   [Editor's note: similar as above -> support of interactive services
   increases costumer satisfaction...]

6.5.  Trust and Privacy

   As only lower priority should be indicated, it is harder to use this
   information for an attack.

   [Editor's note: Do not really see any trust or privacy concerns
   here...?]

7.  Acknowledgements

   This document grew in part out of discussions of initial use cases
   for middlebox cooperation at the IAB SEMI Workshop and the IETF 92
   SPUD BoF; thanks to the participants.







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8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   Security and privacy considerations for each use case are given in
   the corresponding subsection.

10.  Informative References

   [I-D.trammell-spud-req]
              Trammell, B. and M. Kuehlewind, "Requirements for the
              design of a Substrate Protocol for User Datagrams (SPUD):
              draft-trammell-stackevo-spud-req-00 (To be published
              soon)", 2015.

Authors' Addresses

   Mirja Kuehlewind
   ETH Zurich
   Zurich, Switzerland

   Email: mirja.kuehlewind@tik.ee.ethz.ch


   Brian Trammell
   ETH Zurich
   Zurich, Switzerland

   Email: ietf@trammell.ch




















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