Internet DRAFT - draft-arumuganainar-rtgwg-dps-requirements



INTERNET-DRAFT                                  Arunkumar Arumuga Nainar
Intended Status: Informational draft             Tata Communications Ltd
Expires: April 5, 2014                                   October 2, 2014

                  Dynamic Path Selection Requirements


   The high availability puzzle can be resolved by building in
   resiliency to network designs. Whilst active/backup routing schemes
   are sufficient to create redundancy with low convergence times the
   following deficiencies and customer demands are not addressed

   1. IP routing is essentially best path based. This will lead to
   underutilized or over utilized links.

   2. WAN application performance could be adversely impacted due to
   congestion whilst the backup link remains idle. Techniques such as
   DiffServ QoS do address the problem effectively, but those approaches
   address only the symptoms and not the root cause.

   3. Half of the network resources that the end customer has paid for,
   always remains unused. This is a matter of huge concern for small and
   mid-size customers as WAN circuit costs are very high and recurring.

   Hence there is genuine requirement to offload some of the traffic to
   an alternate path while the primary path is still active and running.
   This document defines this requirement in detail.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

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   The list of Internet-Draft Shadow Directories can be accessed at

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Table of Contents

   1  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1 Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  3
   2. Basic assumption  . . . . . . . . . . . . . . . . . . . . . . .  3
   3.Problem statement :- . . . . . . . . . . . . . . . . . . . . . .  5
   4. DPS Routing Requirements  . . . . . . . . . . . . . . . . . . .  5
   5. Dynamic Path selection Framework  . . . . . . . . . . . . . . .  7
   6. Packet flow in DPS Environment  . . . . . . . . . . . . . . . . 10
   8. Implementation Details and Gap Analysis.  . . . . . . . . . . . 10
     8.1 DPS Routing domain:- . . . . . . . . . . . . . . . . . . . . 10
     8.2 DPS Signaling:-  . . . . . . . . . . . . . . . . . . . . . . 12
     8.3 Profile based Filter . . . . . . . . . . . . . . . . . . . . 13
   7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   8  Security Considerations . . . . . . . . . . . . . . . . . . . . 15
   9  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 15
   10  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     10.1  Normative References . . . . . . . . . . . . . . . . . . . 15
     10.2  Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15


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1  Introduction

   In a large enterprise environment, network locations are often
   distributed over large geographical areas (often it spans different
   countries and continents). In such a case enterprise network managers
   often buy virtual private networks from service providers. Such a
   network is built over either MPLS or Internet/IPSEC extension to MPLS

   Generally the enterprise end user location connects in to the service
   provider using last mile connections. These last mile circuits comes
   in various technology options and bandwidth capacities. While the
   service provider core is very resilient and virtually has infinite
   capacity in terms of bandwidth, the weak link in the enterprise
   network is indeed the last mile.

   Last miles are the costliest component for the enterprise and most of
   the times it contributes to 50-60% of total network costs. Yet these
   are the weakest link in the network. They constitute single point of
   failures and do fail frequently. Hence in order to increase the
   network availability the network managers buy redundant links.

   With redundant links in place, the availability puzzle is fully
   resolved, however, it does pose a significant question. Half of the
   costliest resource that they have purchased will remain idle through
   out the life of this network. This happens while network managers
   work over time to resolve the issues that arise out of congestion on
   the primary circuit.

   There is a genuine need to identify few select application and off
   load them on to a path that is considered to be the best path by the
   routing protocols. This draft defines and documents this requirement.
   This draft also describes the high level framework based on which
   such application off loading could be accomplished. The scope of
   draft also includes the evaluation of the existing techniques and
   their gap analysis.

1.1 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2. Basic assumption

   This draft addresses a problem faced in an enterprise network.
   Typical enterprises comprise of collection of sites/Local Area

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   Networks (LANs) that are spread across a large geographical area
   (across cities, countries and continents). These sites are
   interconnected via a virtual private network of some form. Such
   enterprises typically go to single or even multiple service providers
   to establish this connectivity.

   In such a case, the service provider will interconnect the site at
   their POP location via a last mile circuit. The choice of last mile
   technology could be diverse such as Ethernet, Frame-Relay, ATM, DSL,
   T1/E1/T3/E3, etc. Bandwidth capacity also varies from location to
   location (as low as 512 Kbps DSL to as high as 10 Gbps Ethernet). In
   cases where VPN connectivity is not possible, the VPN network can be
   extended in to a remote location via Internet/IPSEC technologies as
   well. Due to the diverse nature of the connectivity option, any frame
   work should be agonistic to technologies and choice of providers.

           Router 1--------(POP1)------|
                                       |-----Customer VPN
                                       |   (SP Managed Core)
           Router 2--------(POP2)------|

   This draft assumes availability is very important for the target
   enterprises, hence there will be at least two last mile circuits that
   link them to the service provider core. Again, technology and the
   last mile provider can be chosen independently. Often enterprises
   selects a good or premium primary circuit and cheaper option for the
   secondary connectivity. For example

   1) Primary: 10 Mbps Ethernet to MPLS; Secondary:- 10 Mbps Ethernet to

   2) Primary: 10 Mbps Ethernet to MPLS; secondary:- 2 MBPS E1 circuit
   to MPLS

   3) Primary: 1.5 Mbps T1; Secondary:- ADSL 2+ Internet/IPSEC

   The above samples are for illustration taken from typical examples
   found in the field.

   Routing schemes usually designate the premium circuit option as
   primary. Routing protocol metrics will hence be better for the link
   and all the traffic goes over it as long as they are active. Traffic
   shift to the secondary if the primary circuit goes down. This kind of
   routing scheme is called classical routing or Active-passive routing


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3.Problem statement :-

   In the classical routing scheme, all traffic goes over the primary
   circuit. Sizing of the circuit will depend on the bandwidth
   requirements of all the applications running on the LAN side which
   need remote connectivity. Last mile circuits generally constitute a
   large bulk of the wan network cost. In some networks that are
   geographically well spread this could constitute 50-60% of total WAN
   network costs, hence over sizing the network is not an option.

   As an alternative, the Network Administrator could resort to Quality
   of Service. This will ensure applications that are critical to the
   business get enough bandwidth, but as collateral damage, applications
   that are not so critical could be starved of the bandwidth. This
   starvation could happen even while secondary circuit is active and
   contains lots of unused bandwidth.

   Key challenge here is to find a way to select certain applications
   that are considered not-so critical for the business and off load
   them on to secondary circuit. This would ensure applications that
   would otherwise be starved of bandwidth in the primary circuit will
   have lots of room in the secondary circuit. This kind of active-
   active routing scheme will hence forth be called as DPS ( Dynamic
   Path Selection) routing. This when achieved, will improve the overall
   user experience for the not-so-critical applications without
   compromising the critical applications. In majority of the situations
   this will result in a slightly better experience for the critical

   It should be noted that during the event of a primary circuit
   failure, both critical and non-critical applications will go over the
   secondary circuit. Hence the QoS mechanism will have to be applied
   even on the secondary circuit so as to punish the not-so-critical
   applications. Though this is a real problem, the occurrence of such
   an event is very rare. SLA for a premium circuit such as Carrier or
   Metro Ethernet could be as high as 99.5% . With this SLA in place
   network administrators can guarantee acceptable level of performance
   for all applications 99.5% of the time whilst for critical
   applications the experience can be guaranteed 99.999% of times. 

   If this DPS routing can be achieved, then it will directly translate
   in to greater user experience for users with out compromising the
   cost that the Network managers will have to spend on the WAN

4. DPS Routing Requirements


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   The DPS requirements can be summarized in 5 simple steps.

   1) Any two sites participating in the DPS routing, will have to agree
   on off loading patterns. This pattern is referred to as DPS Profiles.
   Example of a profile is illustrated below.

   Profile 1: Critical Application: SAP, Citrix, VOIP ,Telnet; Non-
   Critical: remaining applications

   Profile 2: Critical Application: SAP, Citrix, VOIP, Telnet, SMTP,
   HTTP; Non-Critical: remaining applications

   Profile 3: Critical Application: All non-Internet traffic; Non-
   Critical: Internet-traffic

   If three sites wanted to participate in DPS routing, they need to
   exchange the profile information  and agree on a common profile to
   off load. It should be possible that the first site will negotiate to
   use profile 1 for the second site, and profile 2 with the third. The
   underling requirement is if two sites communicate they should be able
   to agree on a common profile. 

   2) Traffic hitting the input interface of router will have to be
   first classified based on application. Here the application can be
   defined as follows:

   a) A specific known TCP port number

   b) Combination of TCP Port and Server's IP Address

   c) A known Layer 7 Signature

   For example: 

      a) Traffic sent to TCP Port (80) could be classified as HTTP

      b) Traffic sent to TCP Port 80 and Server IP (ex: proxy server)
      can be classified as Internet

      c) Traffic sent to TCP Port 80 and Server IP (ex. proxy
      server)sent to the URL can be classified as
      "EXAMPLE.COM APP". Here the classification was based on the Layer
      7 Signature.

   3) If the profile is agreed and classification is done, then the
   participating router should be able to filter the traffic and send it

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   across available paths.

   4) During step 2, if the traffic is pushed to the secondary circuit
   (not the best path as per the routing protocol)it should stick with
   the circuit end to end. For example an application that has been off
   loaded will have to enter the remote site via the secondary circuit.
   Return path for that flow will have to take the secondary path at
   both locations so as to prevent asymmetric routing.

   As the quality of the link does vary largely at any given site
   asymmetric routing  will invariably create a application performance
   issues and hence should be avoided at all times.Hence, path symmetry
   should be honored even during positive and negative events on the
   network. For example if the secondary circuit fails, either locally
   or at the remote site, it is treated as a negative event and during
   this time path symmetry should not be compromised.

5. Dynamic Path selection Framework

         +-------------+                         +----------------+     
         | DPS Routing |   Fault in DPS Domain   | Normal Routing |     
         | DOMAIN      | -------------------->   |   Domain       |     
         +-------------+                         +----------------+     
               ^                                        ^               
               |                                        |               
               |                                        |               
               |                                        |               
               ----------------   -----------------------               
                               |  |                                     
                               |  |                                     
                          +-------------+      +----------------+       
                          |Profile based|<-----|DPS Signaling   |       
                          |    Filter   |      +----------------+       

   The objective of DPS is to achieve end-to-end application separation
   with out introducing asymmetric routing within the network. In order
   to ensure the above objectives, we should have a comprehensive
   mechanism to achieve the following tasks.

   Task 1: Any two sites participating in DPS will have to agree on a
   common set of applications that will be off loaded to secondary path
   (also referred to as DPS path on this draft).

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   Task 2:  At the time of forwarding the packets, packet should be
   filtered based on agreed application set. Packets should then be
   pushed in to appropriate paths. 

   Task 3:  If the packet is pushed on to a DPS path, it should always
   use the secondary link end to end. .

   Task 4:  A comprehensive fault detection mechanism should be put in
   place to detect the faults in the DPS domain. In such a case, the DPS
   traffic should be re-routed via the normal routing domain. 

   To achieve the above four tasks, this draft recommends four
   functional blocks as described in the above diagram. These functional
   blocks are individually designed and fine tuned to achieve various
   level of sophistication. Details of the building blocks are described
   in the below section.

   Normal Routing Domain:

   This is the Active-Passive or classical routing as described in
   section 2. Any site that is not running DPS (DPS unaware), will have
   only this module. Here, the routing protocol will configured in a
   traditional fashion. The primary circuit will have a best metric and
   will be preferred and the secondary will be acting as backup.

   DPS Routing domain:

   This will be overlay routing domain that will be built over the
   existing Normal Routing domain. However some of the paths will not be
   considered while building the domain. For example Primary Circuits
   can be ignored while building the overlay domain. Alternatively
   overlay domain can be built in such a way that it prefers secondary
   circuit as a path with better metric and hence primarily route over
   secondary path.

   It is mandatory that all the LAN routes that are available on the
   normal routing domain is fully injected in to the overlay DPS Routing
   domain. However, in order to separate the traffic, the overlay DPS
   Routing domain will be put into a separate VRF. Once a packet is
   pushed in to the overlay DSP Routing domain, the Normal Routing
   domain is never consulted, until the time the fault tolerance
   mechanism pushes traffic out of the overlay DPS Routing domain.

   DPS Signaling

   One of the core objective of DPS is to avoid asymmetry while routing.
   Hence, if a particular application is off loaded, then the remote
   network should also off load the same application. The signaling

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   module will provide a mechanism for participating sites to
   communicate with each other and agree about the applications that
   need to be off loaded.

   During the signaling, sites will exchange the Application Profiles as
   described in section 4. Sites could be able to support more than one
   profile, however when they communicate a common profile must be
   chosen. For example, profiles can be defined as follows:

   Profile 1: Critical Application: SAP, Citrix, VoIP, Telnet; Non-
   Critical: remaining applications

   Profile 2: Critical Application: SAP, Citrix, VoIP, Telnet, SMTP,
   HTTP; Non-Critical: remaining applications

   Site A & B can support both profile 1 and profile 2 Site C can
   support profile 1 Site D can support Profile 2

   When Site A talks to Site C, applications will be off loaded as per
   definition of Profile 1

   When Site A talks to Site C, applications will be off loaded as per
   definition of Profile 2

   When Site A talks with Site B, off loading can be done either by
   choosing one profile or by choosing a combination of both profiles.
   One of the profiles can be dynamically chosen based on the network
   conditions prevailing at the time.

   For example Site A could signal to Site B its preference for profile
   1 when the link utilization level is very high. At all other times it
   can prefer profile 2 when dealing with Site B. Such a constraint
   based profile selection should be explicitly acknowledged and agreed
   by site B. If agreement could not be reached then it will default to
   normal routing.

   It should be noted that the profile definition as such is static. All
   of the application sets will have to be individually defined by the
   network administrator. But selection of profiles as such can be
   dynamic and it could depend on a) local or remote network conditions
   and b) capability of the site of which the target IP address belongs

   Profile Based Filtering Module

   Once off loaded applications are defined, the framework should
   support a classification and filtering engine. Classification could
   be done via variety of application signatures. For example an

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   application signature can be defined as:

   1) specific TCP port or an Server IP address

   2) A combination of TCP port and IP Address

   3) A Layer 7 Signature (eg: specific URL like

   The profile based filter should be able to classify the packets and
   then push the off loaded application traffic in to the DPS Routing
   domain. Other traffic should be routed normally.

6. Packet flow in DPS Environment

   If a site is deployed conforming to the above specifications, the
   following things will happen when the packet hits router.

   1) When the packet hits the input interface, it gets classified based
   on the Application Signature.

   2)If signaling happens in the forwarding plane, then the Signaling
   Module will communicate with the remote site and agree on the profile
   that will be used. If the signaling is implemented in the control
   plane, then the profile negotiation would have already taken place.
   In both the scenarios, the profile information are fed in to the
   profile filter.

   3) Once the profile and application signature is determined, the
   profile based filter will push the packet in to the appropriate
   routing domain.

   4) In the case of the packet being pushed in to the DPS Routing
   domain, the forwarding happens based on information available in the
   DPS Routing domain.

   5) Failures might happen in the network and this might result in loss
   of routing information in the DPS routing domain. In such a case the
   DPS routing domain will push the packet out of the domain and in to
   the Normal Routing domain. Under no circumstances should the packet
   be dropped due to non-availability of routing information in the DPS
   routing domain.

8. Implementation Details and Gap Analysis.

8.1 DPS Routing domain:-

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   The main constraint is that provider devices should be transparent to
   DPS Domain routing. This adds more complexity to the design. The
   following techniques were considered for implementation:

   a) Multi-Topology routing:

   Multi-Topology Routing was considered but found unsuitable for DPS
   routing. This is because it requires dependency on providers
   supporting the DPS Routing scheme. Also, it forces packets to be
   marked only with two DSCP values (1 for normal routing and another
   other for DPS routing). Some implementations mandate usage of the
   same or similar DSCP/IP Precedence values for traffic traveling
   through both routing domains. 

   And hence it was found that MTR as not suitable for this purpose.

   b) A separate VPN for DPS:

   This does work fine for the DPS Routing domain. Here we extend a
   second sub-interface to the same POP via the secondary last mile
   circuit. The second sub-interface could be put on a separate VPN.

   Technically this is feasible. There few implementations in the field
   of this type. However this is not a preferred option. Any new VPN on
   a MPLS network will come with extra cost and most of the time, the
   costs are prohibitively high.

   c) Overlay VPN Using Dynamic Mesh VPN tunnels:

   Here the overlay network is built using DMVPN (Dynamic Mesh VPN).
   Details of DMVPN is described in detail in the draft-detienne-dmvpn.
   There is a vendor implementation that exists today that supports a
   multi-point overlay tunnel as described in the draft. This could be
   used to create an overlay DPS Routing domain. A standard routing
   protocol can be run over this overlay network.

   This technique is the widely deployed technique used in field
   deployments today. In the production environment, several
   implementations were done with largest one consisting of 300 sites &
   2000 prefixes. 

   It should be noted that the proposed draft draft-detienne-dmvpn
   suggests that binding IPSEC encryption on to the DMVPN tunnel is
   mandatory. DPS Routing does not require IPSEC. Current
   implementations were all done without encryption. This draft
   recommends the DMVPN draft to be amended so that IPSEC binding
   requirement is de-linked from the DMVPN standard. 


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8.2 DPS Signaling:-

   DPS signaling can be implemented in both forwarding plane and also in
   the control plane.

   Control plane implementations are done via BGP. Here, BGP will be
   used as a routing protocol for the Normal Routing domain. Profile
   information can be exchanged via standard community. For example:

   1) Profile 1 can be represented by community 100:1 

   2) Profile 2 can be represented by community 100:2

   If a given site supports only profile 1, it will advertise site
   prefixes with a community 100:1. If the same site wanted to support
   both the profiles then the prefix would be sent with both
   communities, 100:1 and 100:2. At the remote end, the site will
   understand the capabilities based on the communities associated with
   the prefix. 

   For example Site 1 supports both profile 1 and profile 2 and hence it
   advertise the prefix with community 100:1 and 100:2. But site 2
   supports only profile 1 (100:1). The common profile between both the
   sites is 100:1 and hence profile 1 will be used. It should noted that
   both site 1 and site 2 with full certainty will use profile 1. Under
   such a scenario asymmetric routing will not happen.

   Currently this scheme has been implemented using one vendor specific
   implementation. In the current implementation, the router will colour
   the packet with IP Precedence values. There will be one to one
   correspondence between the profile chosen and the IP Precedence
   values used for colouring. Once the packet is coloured, the profile
   based filter can applied. Filtering will involve inspecting both the
   colouring done by the Signaling Module and the application to which
   the packet belongs to.

   While this works perfectly well on large scale network, a couple of
   short comings remains.

   1) The trust model could not be supported when DPS is enabled. This
   is because the DPS Signaling Module will remark the TOS bits during
   the colouring operation. Hence an alternate mechanism needs to
   developed to color the packet with out modifying the TOS bits.

   2)Because profiles are exchanged via BGP, the router negotiates the
   off loaded applications list at the time of exchanging the BGP
   routing information. This would mean routers will not be able to
   react to local or remote network condition. For example, the choice

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   of profiles can not be changes if the link utilization is very high.
   The draft draft-arumuganainar-rtgwg-DPS-L4-path-preference-
   negotiation-00  suggests an alternate mechanism that will allow
   profile negotiation to happen when the TCP connection gets
   established. This will enable link preference to be negotiated for
   each TCP connection and hence such a mechanism will be more dynamic
   then the BGP one

8.3 Profile based Filter

   Each profile is associated with a pre-determined list of
   applications. These applications are determined by the network
   administrator as an application that can be offloaded. 

   The administrator can define multiple profiles and associate this
   with one or more sites. When two sites communicate, they signal each
   other and agree on a single common profile. The Signaling Module will
   then colour the packet indicating which profile to be used while
   deciding the application to be off loaded.

   Once the steps are complete, the profile based filter can be applied.
   The filter will look for the colour and then matches the protocol
   with corresponding off loaded application. If a match is found the
   packet will be pushed in to the DPS Routing domain, otherwise the
   packet will be normally routed. 

   Current implementations are done using Policy Based Routing (PBR).
   Here, filtering is done using a special PBR policy. This will check
   the colour of the packet and the pre-determined list of applications.
   It could then push the packet in to the appropriate routing domain.

   While in theory PBR works perfectly fine, PBR is a processor
   intensive mechanism and hence when it is applied, the throughput of
   the router is adversely impacted. Hence, router scalability should be
   kept in mind while sizing the router for any particular site. In the
   future a light weight mechanism optimised for profile based filtering
   could be developed as well.

7. Summary

      To summarise, by adopting the DPS frame work, true end to end
   application based routing scheme could be achieved. The DPS framework
   has the following advantages:

      1. Give lots of room for Network Manager to determine which path
      should be used for which application. 

      2. DPS also provides a scalable frame work for achieving the above

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      3. By modularizing the DPS components, advanced development of
      individual components becomes possible. Troubleshooting will also
      get greatly simplified. 

      4. DPS capable sites can co-exist with non-DPS sites and this
      capability provides enough room for a phased migration. Thus DPS
      technology adoption is easy and simple. 

      5. It should be noted that DPS frame work and signaling, needs to
      be understood only by edge devices and any other devices in the
      path such as provider routers need not be aware of DPS.

      Definitions and code {
        line 1
        line 2

   Special characters examples:

   The characters  , , , 
   However, the characters \0, \&, \%, \" are displayed.

   .ti 0  is displayed in text instead of used as a directive. 
   .\"  is displayed in document instead of being treated as a comment

   C:\dir\subdir\file.ext  Shows inclusion of backslash "\".


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8  Security Considerations


9  IANA Considerations


10  References

10.1  Normative References

10.2  Informative References

   11 Acknowledgements

    The authors would like to thank Hesham Moussa for his review and

Authors' Addresses

   Arunkumar Arumuga Nainar
   Tata Communications (UK) Limited
   1st Floor
   20 Old Bailey
   London EC4M 7AN
   United Kingdom


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