Network Working Group                                      J. Hadi Salim
Internet-Draft                                         Mojatatu Networks
Expires: December 6, 2009                                       K. Ogawa
                                                         NTT Corporation
                                                            June 4, 2009

      SCTP based TML (Transport Mapping Layer) for ForCES protocol

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   This document defines the SCTP based TML (Transport Mapping Layer)
   for the ForCES protocol.  It explains the rationale for choosing the

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   SCTP (Stream Control Transmission Protocol) [RFC4960] and also
   describes how this TML addresses all the requirements described in
   [RFC3654] and the ForCES protocol [FE-PROTO] draft.

Table of Contents

   1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Protocol Framework Overview  . . . . . . . . . . . . . . . . .  3
     3.1.  The PL . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  The TML  . . . . . . . . . . . . . . . . . . . . . . . . .  5
       3.2.1.  TML and PL Interfaces  . . . . . . . . . . . . . . . .  5
       3.2.2.  TML Parameterization . . . . . . . . . . . . . . . . .  6
   4.  SCTP TML overview  . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Rationale for using SCTP for TML . . . . . . . . . . . . .  8
     4.2.  Meeting TML requirements . . . . . . . . . . . . . . . . .  8
       4.2.1.  SCTP TML Channels  . . . . . . . . . . . . . . . . . .  9
       4.2.2.  Satisfying TML Requirements  . . . . . . . . . . . . . 14
   5.  SCTP TML Channel work  . . . . . . . . . . . . . . . . . . . . 16
     5.1.  SCTP TML Channel Initialization  . . . . . . . . . . . . . 16
     5.2.  Channel work scheduling  . . . . . . . . . . . . . . . . . 16
       5.2.1.  FE Channel work scheduling . . . . . . . . . . . . . . 17
       5.2.2.  CE Channel work scheduling . . . . . . . . . . . . . . 17
     5.3.  SCTP TML Channel Termination . . . . . . . . . . . . . . . 18
     5.4.  SCTP TML NE level channel scheduling . . . . . . . . . . . 18
   6.  Service Interface  . . . . . . . . . . . . . . . . . . . . . . 19
     6.1.  TML Boot-strapping . . . . . . . . . . . . . . . . . . . . 19
     6.2.  TML Shutdown . . . . . . . . . . . . . . . . . . . . . . . 21
     6.3.  TML Sending and Receiving  . . . . . . . . . . . . . . . . 21
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
     8.1.  IPsec Usage  . . . . . . . . . . . . . . . . . . . . . . . 24
       8.1.1.  SAD and SPD setup  . . . . . . . . . . . . . . . . . . 24
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 25
     10.2. Informative References . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26

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1.  Definitions

   The following definitions are taken from [RFC3654]and [RFC3746]:

   ForCES Protocol -- The protocol used at the Fp reference point in the
   ForCES Framework in [RFC3746].

   ForCES Protocol Layer (ForCES PL) -- A layer in ForCES protocol
   architecture that defines the ForCES protocol architecture and the
   state transfer mechanisms as defined in [FE-PROTO].

   ForCES Protocol Transport Mapping Layer (ForCES TML) -- A layer in
   ForCES protocol architecture that specifically addresses the protocol
   message transportation issues, such as how the protocol messages are
   mapped to different transport media (like SCTP, IP, ATM, Ethernet,
   etc), and how to achieve and implement reliability, security, etc.

2.  Introduction

   The ForCES (Forwarding and Control Element Separation) working group
   in the IETF defines the architecture and protocol for separation of
   Control Elements(CE) and Forwarding Elements(FE) in Network
   Elements(NE) such as routers.  [RFC3654] and [RFC3746] respectively
   define architectural and protocol requirements for the communication
   between CE and FE.  The ForCES protocol layer specification
   [FE-PROTO] describes the protocol semantics and workings.  The ForCES
   protocol layer operates on top of an inter-connect hiding layer known
   as the TML.  The relationship is illustrated in Figure 1.

   This document defines the SCTP based TML for the ForCES protocol
   layer.  It also addresses all the requirements for the TML including
   security, reliability, etc as defined in [FE-PROTO].

3.  Protocol Framework Overview

   The reader is referred to the Framework document [RFC3746], and in
   particular sections 3 and 4, for an architectural overview and
   explanation of where and how the ForCES protocol fits in.

   There is some content overlap between the ForCES protocol draft
   [FE-PROTO] and this section (Section 3) in order to provide basic
   context to the reader of this document.

   The ForCES protocol layering constitutes two pieces: the PL and TML
   layer.  This is depicted in Figure 1.

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               |                    CE PL                     |
               |                    CE TML                    |
                           ForCES PL  | messages
               |                   FE TML                      |
               |                   FE PL                       |

      Figure 1: Message exchange between CE and FE to establish an NE

   The PL is in charge of the ForCES protocol.  Its semantics and
   message layout are defined in [FE-PROTO].  The TML is necessary to
   connect two ForCES end-points as shown in Figure 1.

   Both the PL and TML are standardized by the IETF.  While only one PL
   is defined, different TMLs are expected to be standardized.  The TML
   at each of the nodes (CE and FE) is expected to be of the same
   definition in order to inter-operate.

   When transmitting from a ForCES end-point, the PL delivers its
   messages to the TML.  The TML then delivers the PL message to the
   destination TML(s).

   On reception of a message, the TML delivers the message to its
   destination PL level (as described in the ForCES header).

3.1.  The PL

   The PL is common to all implementations of ForCES and is standardized
   by the IETF [FE-PROTO].  The PL level is responsible for associating
   an FE or CE to an NE.  It is also responsible for tearing down such

   An FE may use the PL level to asynchronously send packets to the CE.
   The FE may redirect via the PL (from outside the NE) various control
   protocol packets (e.g.  OSPF, etc) to the CE.  Additionally, the FE
   delivers various events that CE has subscribed-to via PL [FE-MODEL].

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   The CE and FE may interact synchronously via the PL.  The CE issues
   status requests to the FE and receives responses via the PL.  The CE
   also configures the associated FE's LFBs' components using the PL

3.2.  The TML

   The TML level is responsible for transport of the PL level messages.
   [FE-PROTO] section 5 defines the requirements that need to be met by
   a TML specification.  The SCTP TML specified in this document meets
   all the requirements specified in [FE-PROTO] section 5.
   Section 4.2.2 describes how the TML requirements are met.

3.2.1.  TML and PL Interfaces

   There are two interfaces to the PL and TML, both of which are out of
   scope for ForCES.  The first one is the interface between the PL and
   TML and the other is the CE Manager (CEM)/FE Manager (FEM)[RFC3746]
   interface to both the PL and TML.  Both interfaces are shown in
   Figure 2.

                      |  +----------------------+  |
                      |  |                      |  |
     +---------+      |  |       PL Layer       |  |
     |         |      |  +----------------------+  |
     |FEM/CEM  |<---->|             ^              |
     |         |      |             |              |
     +---------+      |             |TML API       |
                      |             |              |
                      |             V              |
                      |  +----------------------+  |
                      |  |                      |  |
                      |  |       TML Layer      |  |
                      |  |                      |  |
                      |  +----------------------+  |

                      Figure 2: The TML-PL interface

   Figure 2 also shows an interface referred to as CEM/FEM[RFC3746]
   which is responsible for bootstrapping and parameterization of the
   TML.  In its most basic form the CEM/FEM interface takes the form of
   a simple static config file which is read on startup in the pre-
   association phase.

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   Section 6 discusses in more details the service interfaces.

3.2.2.  TML Parameterization

   It is expected that it should be possible to use a configuration
   reference point, such as the FEM or the CEM, to configure the TML.

   Some of the configured parameters may include:

   o  PL ID

   o  Connection Type and associated data.  For example if a TML uses
      IP/SCTP then parameters such as SCTP ports and IP addresses need
      to be configured.

   o  Number of transport connections

   o  Connection Capability, such as bandwidth, etc.

   o  Allowed/Supported Connection QoS policy (or Congestion Control

4.  SCTP TML overview

   SCTP [RFC4960] is an end-to-end transport protocol that is equivalent
   to TCP, UDP, or DCCP in many aspects.  With a few exceptions, SCTP
   can do most of what UDP, TCP, or DCCP can achieve.  SCTP as well can
   do most of what a combination of the other transport protocols can
   achieve (e.g.  TCP and DCCP or TCP and UDP).

   Like TCP, it provides ordered, reliable, connection-oriented, flow-
   controlled, congestion controlled data exchange.  Unlike TCP, it does
   not provide byte streaming and instead provides message boundaries.

   Like UDP, it can provide unreliable, unordered data exchange.  Unlike
   UDP, it does not provide multicast support

   Like DCCP, it can provide unreliable, ordered, congestion controlled,
   connection-oriented data exchange.

   SCTP also provides other services that none of the 3 transport
   protocols mentioned above provide.  These include:

   o  Multi-homing
      An SCTP connection can make use of multiple destination IP
      addresses to communicate with its peer.

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   o  Runtime IP address binding
      With the SCTP Dynamic Address Reconfiguration ([RFC5061]) feature,
      a new IP address can be bound at runtime.  This allows for
      migration of endpoints without restarting the association
      (valuable for high availability).

   o  A range of reliability shades with congestion control
      SCTP offers a range of services from full reliability to none, and
      from full ordering to none.  With SCTP, on a per message basis,
      the application can specify a message's time-to-live.  When the
      expressed time expires, the message can be "skipped".

   o  Built-in heartbeats
      SCTP has built-in heartbeat mechanism that validate the
      reachability of peer addresses.

   o  Multi-streaming
      A known problem with TCP is head of line (HOL) blocking.  If you
      have independent messages, TCP enforces ordering of such messages.
      Loss at the head of the messages implies delays of delivery of
      subsequent packets.  SCTP allows for defining up to 64K
      independent streams over the same socket connection, which are
      ordered independently.

   o  Message boundaries with reliability
      SCTP allows for easier message parsing (just like UDP but with
      reliability built in) because it establishes boundaries on a PL
      message basis.  On a TCP stream, one would have to use techniques
      such peeking into the message to figure the boundaries.

   o  Improved SYN DOS protection
      Unlike TCP, which does a 3 way connection setup handshake, SCTP
      does a 4 way handshake.  This improves against SYN-flood attacks
      because listening sockets do not set up state until a connection
      is validated.

   o  Simpler transport events
      An application (such as the TML) can subscribe to be notified of
      both local and remote transport events.  Events that can be
      subscribed-to include indication of association changes,
      addressing changes, remote errors, expiry of timed messages, etc.
      These events are off by default and require explicit subscription.

   o  Simplified replicasting
      Although SCTP does not allow for multicasting it allows for a
      single message from an application to be sent to multiple peers.
      This reduces the messaging that typically crosses different memory
      domains within a host (example in a kernel to user space domain of

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      an operating system).

4.1.  Rationale for using SCTP for TML

   SCTP has all the features required to provide a robust TML.  As a
   transport that is all-encompassing, it negates the need for having
   multiple transport protocols in order to satisfy the TML requirements
   ([FE-PROTO] section 5).  As a result it allows for simpler coding and
   therefore reduces a lot of the interoperability concerns.

   SCTP is also very mature and widely used making it a good choice for
   ubiquitous deployment.

4.2.  Meeting TML requirements

                  |                      |
                              |   TML API
                   TML        |
                  |           |          |
                  |    +------+------+   |
                  |    |  TML core   |   |
                  |    +-+----+----+-+   |
                  |      |    |    |     |
                  |    SCTP socket API   |
                  |      |    |    |     |
                  |      |    |    |     |
                  |    +-+----+----+-+   |
                  |    |    SCTP     |   |
                  |    +------+------+   |
                  |           |          |
                  |           |          |
                  |    +------+------+   |
                  |    |      IP     |   |
                  |    +-------------+   |

                     Figure 3: The TML-SCTP interface

   Figure 3 details the interfacing between the PL and SCTP TML and the
   internals of the SCTP TML.  The core of the TML interacts on its
   north-bound interface to the PL (utilizing the TML API).  On the

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   south-bound interface, the TML core interfaces to the SCTP layer
   utilizing the standard socket interface[SCTP-API] There are three
   SCTP socket connections opened between any two PL endpoints (whether
   FE or CE).

4.2.1.  SCTP TML Channels

                  |                    |
                  |     TML   core     |
                  |                    |
                    |       |        |
                    |   Med prio,    |
                    |  Semi-reliable |
                    |    channel     |
                    |       |      Low prio,
                    |       |      Unreliable
                    |       |      channel
                    |       |        |
                    ^       ^        ^
                    |       |        |
                    Y       Y        Y
          High prio,|       |        |
           reliable |       |        |
            channel |       |        |
                    Y       Y        Y
                 |                     |
                 |        SCTP         |
                 |                     |

                      Figure 4: The TML-SCTP channels

   Figure 4 details further the interfacing between the TML core and
   SCTP layers.  There are 3 channels used to separate and prioritize
   the different types of ForCES traffic.  Each channel constitutes a
   socket interface.  It should be noted that all SCTP channels are
   congestion aware (and for that reason that detail is left out of the
   description of the 3 channels).  SCTP port 6700, 6701, 6702 are used
   for the higher, medium and lower priority channels respectively.

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   SCTP allows up to 64K streams to be sent over a single socket
   interface.  The authors initially envisioned using a single socket
   for all three channels (mapping a channel to an SCTP stream).  This
   simplifies programming of the TML as well as conserves use of SCTP

   Further analysis revealed head of line blocking issues with this
   initial approach.  Lower priority packets not needing reliable
   delivery could block higher priority packets (needing reliable
   delivery) under congestion situation for an indeterminate period of
   time (depending on how many outstanding lower priority packets are
   pending).  For this reason, we elected to go with mapping each of the
   three channels to a different SCTP socket (instead of a different
   stream within a single socket).  Higher Priority, Reliable channel

   The higher priority (HP) channel uses a standard SCTP reliable socket
   on port 6700.  It is used for CE solicited messages and their

   1.  ForCES configuration messages flowing from CE to FE and responses
       from the FE to CE.

   2.  ForCES query messages flowing from CE to FE and responses from
       the FE to the CE.

   It is recommended that PL priorities 4-7 be used for this channel and
   that the following PL messages use the HP channel for transport:

   o  Association Setup

   o  Association Setup Response

   o  Association Teardown

   o  Config

   o  Config Response

   o  Query

   o  Query Response

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Internet-Draft               ForCES SCTP TML                   June 2009  Medium Priority, Semi-Reliable channel

   The medium priority (MP) channel uses SCTP-PR on port 6701.  Time
   limits on how long a message is valid are set on each outgoing
   message.  This channel is used for events from the FE to the CE that
   are obsoleted over time.  Events that are accumulative in nature and
   are recoverable by the CE (by issuing a query to the FE) can tolerate
   lost events and therefore should use this channel.  For example, a
   generated event which carries the value of a counter that is
   monotonically incrementing fits to use this channel.

   It is recommended that PL priorities 2-3 be used for this channel and
   that the following PL messages use the MP channel for transport:

   o  Event Notification  Lower Priority, Unreliable channel

   The lower priority (LP) channel uses SCTP port 6702.  This channel
   also uses SCTP-PR with lower timeout values than the MP channel.  The
   reason an unreliable channel is used for redirect messages is to
   allow the control protocol at both the CE and its peer-endpoint to
   take charge of how the end-to-end semantics of the said control
   protocol's operations.  For example:

   1.  Some control protocols are reliable in nature, therefore making
       this channel reliable introduces an extra layer of reliability
       which could be harmful.  So any end-to-end retransmits will
       happen from remote.

   2.  Some control protocols may desire to have obsolescence of
       messages over retransmissions; making this channel reliable
       contradicts that desire.

   Given ForCES PL level heartbeats are traffic sensitive, sending them
   over the LP channel also makes sense.  If the other end is not
   processing other channels it will eventually get heartbeats; and if
   it is busy processing other channels heartbeats will be obsoleted
   locally over time (and it does not matter if they did not make it).

   It is recommended that PL priorities 0-1 be used for this channel and
   that that the following PL messages use the LP channel for transport:

   o  Packet Redirect

   o  Heartbeats

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Internet-Draft               ForCES SCTP TML                   June 2009  Scheduling of The 3 Channels

   Strict priority work-conserving scheduling is used to process both on
   sending and receiving (of the PL messages) by the TML Core as shown
   in Figure 5.

   This means that the HP messages are always processed first until
   there are no more left.  The LP channel is processed only if a
   channel that is higher priority than itself has no more messages left
   to process.  This means that under congestion situation, a higher
   priority channel with sufficient messages that occupy the available
   bandwidth would starve lower priority channel(s).

   The design intent of the SCTP TML is to tie prioritization as
   described in Section and transport congestion control to
   provide implicit node congestion control.  This is further detailed
   in Section 5.2.

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       SCTP channel            +----------+
       Work available          |   DONE   +---<--<--+
           |                   +---+------+         |
           Y                                        ^
           |         +-->--+         +-->---+       |
   +-->-->-+         |     |         |      |       |
   |       |         |     |         |      |       ^
   |       ^         ^     Y         ^      Y       |
   ^      / \        |     |         |      |       |
   |     /   \       |     ^         |      ^       ^
   |    / Is  \      |    / \        |     / \      |
   |   / there \     |   /Is \       |    /Is \     |
   ^  / HP work \    ^  /there\      ^   /there\    ^
   |  \    ?    /    | /MP work\     |  /LP work\   |
   |   \       /     | \    ?  /     |  \   ?   /   |
   |    \     /      |  \     /      |   \     /    ^
   |     \   /       ^   \   /       ^    \   /     |
   |      \ /        |    \ /        |     \ /      |
   ^       Y-->-->-->+     Y-->-->-->+      Y->->->-+
   |       |    NO         |    NO          |  NO
   |       |               |                |
   |       Y               Y                Y
   |       | YES           | YES            |
   ^       |               |                |
   |       Y               Y                Y
   |  +----+------+    +---|-------+   +----|------+
   |  |- process  |    |- process  |   |- process  |
   |  |  HP work  |    |  MP work  |   | LP work   |
   |  +------+----+    +-----+-----+   +-----+-----+
   |         |               |               |
   ^         Y               Y               Y
   |         |               |               |
   |         Y               Y               Y

               Figure 5: SCTP TML Strict Priority Scheduling  SCTP TML Parameterization

   The following is a list of parameters needed for booting the TML.  It
   is expected these parameters will be extracted via the FEM/CEM
   interface for each PL ID.

   1.  The IP address or a resolvable DNS/hostname of the CE/FE.

   2.  Whether to use IPsec or not.  If IPsec is used, how to
       parameterize the different required ciphers, keys etc as

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       described in Section 8.1

   3.  The HP SCTP port, as discussed in Section  The default
       HP port value is 6700 (Section 7).

   4.  The MP SCTP port, as discussed in Section  The default
       MP port value is 6701 (Section 7).

   5.  The LP SCTP port, as discussed in Section  The default
       LP port value is 6702 (Section 7).

4.2.2.  Satisfying TML Requirements

   [FE-PROTO] section 5 lists requirements that a TML needs to meet.
   This section describes how the SCTP TML satisfies those requirements.  Satisfying Reliability Requirement

   As mentioned earlier, a shade of reliability ranges is possible in
   SCTP.  Therefore this requirement is met.  Satisfying Congestion Control Requirement

   Congestion control is built into SCTP.  Therefore, this requirement
   is met.  Satisfying Timeliness and Prioritization Requirement

   By using 3 sockets in conjunction with the partial-reliability
   feature, both timeliness and prioritization can be achieved.  Satisfying Addressing Requirement

   There are no extra headers required for SCTP to fulfil this
   requirement.  SCTP can be told to replicast packets to multiple
   destinations.  The TML implementation will need to translate PL level
   addresses, to a variety of unicast IP addresses in order to emulate
   multicast and broadcast PL addresses.  Satisfying HA Requirement

   Transport link resiliency is one of SCTP's strongest point.  Failure
   detection and recovery is built in, as mentioned earlier.

   o  The SCTP multi-homing feature is used to provide path diversity.
      Should one of the peer IP addresses become unreachable, the
      other(s) are used without needing lower layer convergence
      (routing, for example) or even the TML becoming aware.

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   o  SCTP heartbeats and data transmission thresholds are used on a per
      peer IP address to detect reachability faults.  The faults could
      be a result of an unreachable address or peer, which may be caused
      by a variety of reasons, like interface, network, or endpoint
      failures.  The cause of the fault is noted.

   o  With the ADDIP feature, one can migrate IP addresses to other
      nodes at runtime.  This is not unlike the VRRP[RFC3768] protocol
      use.  This feature is used in addition to multi-homing in a
      planned migration of activity from one FE/CE to another.  In such
      a case, part of the provisioning recipe at the CE for replacing an
      FE involves migrating activity of one FE to another.  Satisfying Node Overload Prevention Requirement

   The architecture of this TML defines three separate channels, one per
   socket, to be used within any FE-CE setup.  The scheduling design for
   processing the TML channels (Section is strict priority.  A
   fundamental desire of the strict prioritization is to ensure that
   more important work always gets node resources such as CPU and
   bandwidth over lesser important work.

   When a ForCES node CPU is overwhelmed because the incoming packet
   rate is higher than it can keep up with, the channel queues grow and
   transport congestion subsequently follows.  By virtue of using SCTP,
   the congestion is propagated back to the source of the incoming
   packets and eventually alleviated.

   The HP channel work gets prioritized at the expense of the MP which
   gets prioritized over LP channels.  The preferential scheduling only
   kicks in when there is node overload regardless of whether there is
   transport congestion.  As a result of the preferential work
   treatment, the ForCES node achieves a robust steady processing
   capacity.  Refer to Section 5.2 for details on scheduling.

   For an example of how the overload prevention works: consider a
   scenario where an overwhelming amount redirected packets (from
   outside the NE) coming into the NE may overload the FE while it has
   outstanding config work from the CE.  In such a case, the FE, while
   it is busy processing config requests from the CE ignores processing
   the redirect packets on the LP channel.  If enough redirect packets
   accumulate, they are dropped either because the LP channel threshold
   is exceeded or because they are obsoleted.  If on the other hand, the
   FE has successfully processed the higher priority channels and their
   related work, then it can proceed and process the LP channel.  So as
   demonstrated in this case, the TML ties transport and node overload
   implicitly together.

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   There is no extra encapsulation added by the SCTP TML.

   In the future, should the need arise, a new SCTP extension/chunk can
   be defined to meet newer ForCES requirements [RFC4960].

5.  SCTP TML Channel work

   There are two levels of TML channel work within an NE when a ForCES
   node (CE or FE) is connected to multiple other ForCES nodes:

   1.  NE-level I/O work where a ForCES node (CE or FE) needs to choose
       which of the peer nodes to process.

   2.  Node-level I/O work where a ForCES node, handles the three SCTP
       TML channels separately for each single ForCES endpoint.

   NE-level scheduling definition is left up to the implementation and
   is considered out of scope for this document.  Section 5.4 discuss
   briefly some constraints that an implementor needs to worry about.

   This document and in particular Section 5.1, Section 5.2 and
   Section 5.3 discuss details of node-level I/O work.

5.1.  SCTP TML Channel Initialization

   It is recommended that the FE SHOULD do socket connections to the CE
   in the order of incrementing priorities i.e.  LP socket first,
   followed by MP and ending with HP socket connection.  The CE,
   however, MUST NOT assume that there is ordering of socket connections
   from any FE.  Section 6.1 has more details on the expected
   initialization of SCTP channel work.

5.2.  Channel work scheduling

   This section provides high level details of the scheduling view of
   the SCTP TML core (Section 4.2.1).  A practical scheduler
   implementation takes care of many little details (such as timers,
   work quanta, etc) not described in this document.  The implementor is
   left to take care of those details.

   The CE(s) and FE(s) are coupled together in the principles of the
   scheduling scheme described here to tie together node overload with
   transport congestion.  The design intent is to provide the highest
   possible robust work throughput for the NE under any network or
   processing congestion.

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5.2.1.  FE Channel work scheduling

   The FE scheduling, in priority order, needs to I/O process:

   1.  The HP channel I/O in the following priority order:

       1.  Transmitting back to the CE any outstanding result of
           executed work via the HP channel transmit path.

       2.  Taking new incoming work from the CE which creates ForCES
           work to be executed by the FE.

   2.  ForCES events which result in transmission of unsolicited ForCES
       packets to the CE via the MP channel.

   3.  Incoming Redirect work in the form of control packets that come
       from the CE via LP channel.  After redirect processing, these
       packets get sent out on external (to the NE) interface.

   4.  Incoming Redirect work in the form of control packets that come
       from other NEs via external (to the NE) interfaces.  After some
       processing, such packets are sent to the CE.

   It is worth emphasizing at this point again that the SCTP TML
   processes the channel work in strict priority.  For example, as long
   as there are messages to send to the CE on the HP channel, they will
   be processed first until there are no more left before processing the
   next priority work (which is to read new messages on the HP channel
   incoming from the CE).

5.2.2.  CE Channel work scheduling

   The CE scheduling, in priority order, needs to deal with:

   1.  The HP channel I/O in the following priority order:

       1.  Process incoming responses to requests of work it made to the

       2.  Transmitting any outstanding HP work it needs for the FE(s)
           to complete.

   2.  Incoming ForCES events from the FE(s) via the MP channel.

   3.  Outgoing Redirect work in the form of control packets that get
       sent from the CE via LP channel destined to external (to the NE)
       interface on FE(s).

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   4.  Incoming Redirect work in the form of control packets that come
       from other NEs via external (to the NE) interfaces on the FE(s).

   It is worth to repeat for emphasis again that the SCTP TML processes
   the channel work in strict priority.  For example, if there are
   messages incoming from an FE on the HP channel, they will be
   processed first until there are no more left before processing the
   next priority work which is to transmit any outstanding HP channel
   messages going to the FE.

5.3.  SCTP TML Channel Termination

   Section 6.2 describes a controlled disassociation of the FE from the

   It is also possible for connectivity to be lost between the FE and CE
   on one or more sockets.  In cases where SCTP multi-homing features
   are used for path availability, the disconnection of a socket will
   only occur if all paths are unreachable; otherwise, SCTP will ensure
   reachability.  In the situation of a total connectivity loss of even
   one SCTP socket, it is recommended that the FE and CE SHOULD assume a
   state equivalent to ForCES Association Teardown being issued and
   follow the sequence described in Section 6.2.

   A CE could also disconnect sockets to an FE to indicate an "emergency
   teardown".  The "emergency teardown" may be necessary in cases when a
   CE needs to disconnect an FE but knows that an FE is busy processing
   a lot of outstanding commands (some of which the FE hasn't got around
   to processing yet).  By virtue of the CE closing the connections, the
   FE will immediately be asynchronously notified and will not have to
   process any outstanding commands from the CE.

5.4.  SCTP TML NE level channel scheduling

   In handling NE-level I/O work, an implementation needs to worry about
   being both fair and robust across peer ForCES nodes.

   Fairness is desired so that each peer node makes progress across the
   NE.  For the sake of illustration consider two FEs connected to a CE;
   whereas one FE has a few HP messages that need to be processed by the
   CE, another may have infinite HP messages.  The scheduling scheme may
   decide to use a quota scheduling system to ensure that the second FE
   does not hog the CE cycles.

   Robustness is desired so that the NE does not succumb to a DoS attack
   from hostile entities and always achieves a maximum stable workload
   processing level.  For the sake of illustration consider again two
   FEs connected to a CE.  Consider FE1 as having a large number of HP

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   and MP messages and FE2 having a large number of MP and LP messages.
   The scheduling scheme needs to ensure that while FE1 always gets its
   messages processed, at some point we allow FE2 messages to be
   processed.  A promotion and preemption based scheduling could be used
   by the CE to resolve this issue.

6.  Service Interface

   This section provides high level service interface between FEM/CEM
   and TML, the PL and TML, and between local and remote TMLs.  The
   intent of this interface discussion is to provide general guidelines.
   The implementer is expected to worry about details and even follow a
   different approach if needed.

   The theory of operation for the PL-TML service is as follows:

   1.  The PL starts up and bootstraps the TML.  The end result of a
       successful TML bootstrap is that the CE TML and the FE TML
       connect to each other at the transport level.

   2.  Sending and reception of the PL level messages commences after a
       successful TML bootstrap.  The PL uses send and receive PL-TML
       interfaces to communicate to its peers.  The TML is agnostic to
       the nature of the messages being sent or received.  The first
       message exchanges that happen are to establish ForCES
       association.  Subsequent messages maybe either unsolicited events
       from the FE PL, control message redirects from/to the CE to/from
       FE, and configuration from the CE to the FE and their responses
       flowing from the FE to the CE.

   3.  The PL does a shutdown of the TML after terminating ForCES

6.1.  TML Boot-strapping

   Figure 6 illustrates a flow for the TML bootstrapped by the PL.

   When the PL starts up (possibly after some internal initialization),
   it boots up the TML.  The TML first interacts with the FEM/CEM and
   acquires the necessary TML parameterization (Section  Next
   the TML uses the information it retrieved from the FEM/CEM interface
   to initialize itself.

   The TML on the FE proceeds to connect the 3 channels to the CE.  The
   socket interface is used for each of the channels.  The TML continues
   to re-try the connections to the CE until all 3 channels are
   connected.  It is advisable that the number of connection retry

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   attempts and the time between each retry is also configurable via the
   FEM.  On failure to connect one or more channels, and after the
   configured number of retry thresholds is exceeded, the TML will
   return an appropriate failure indicator to the PL.  On success (as
   shown in Figure 6), a success indication is presented to the TML.

   FE PL      FE TML           FEM  CEM        CE TML              CE PL
     |            |             |    |            |                    |
     |            |             |    |            |      Bootup        |
     |            |             |    |            |<-------------------|
     |  Bootup    |             |    |            |                    |
     |----------->|             |    |get CEM info|                    |
     |            |get FEM info |    |<-----------|                    |
     |            |------------>|    ~            ~                    |
     |            ~             ~    |----------->|                    |
     |            |<------------|                 |                    |
     |            |                               |-initialize TML     |
     |            |                               |-create the 3 chans.|
     |            |                               | to listen to FEs   |
     |            |                               |                    |
     |            |-initialize TML                |Bootup success      |
     |            |-create the 3 chans. locally   |------------------->|
     |            |-connect 3 chans. remotely     |                    |
     |            |------------------------------>|                    |
     |            ~                               ~ - FE TML connected ~
     |            ~                               ~ - FE TML info init ~
     |            | channels connected            |                    |
     |            |<------------------------------|                    |
     | Bootup     |                               |                    |
     | succeeded  |                               |                    |
     |<-----------|                               |                    |
     |            |                               |                    |

                     Figure 6: SCTP TML Bootstrapping

   On the CE things are slightly different.  After initializing from the
   CEM, the TML on the CE side proceeds to initialize the 3 channels to
   listen to remote connections from the FEs.  The success or failure
   indication is passed on to the CE PL level (in the same manner as was
   done in the FE).

   Post boot-up, the CE TML waits for connections from the FEs.  Upon a
   successful connection by an FE, the CE TML level keeps track of the
   transport level details of the FE.  Note, at this stage only
   transport level connection has been established; ForCES level

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   association follows using send/receive PL-TML interfaces (refer to
   Section 6.3 and Figure 8).

6.2.  TML Shutdown

   Figure 7 shows an example of an FE shutting down the TML.  It is
   assumed at this point that the ForCES Association Teardown has been
   issued by the CE.

   When the FE PL issues a shutdown to its TML for a specific PL ID, the
   TML releases all the channel connections to the CE.  This is achieved
   by closing the sockets used to communicate to the CE.

   FE PL      FE TML                      CE TML              CE PL
     |            |                         |                    |
     |  Shutdown  |                         |                    |
     |----------->|                         |                    |
     |            |-disconnect 3 chans.     |                    |
     |            |------------------------>|                    |
     |            |                         |                    |
     |            |                         |-FE TML info cleanup|
     |            |                         |-optionally tell PL |
     |            |                         |------------------->|
     |            |- clean up any state of  |                    |
     |            | channels disconnected   |                    |
     |            |                         |                    |
     |            |<------------------------|                    |
     | Shutdown   |                         |                    |
     | succeeded  |                         |                    |
     |<-----------|                         |                    |
     |            |                         |                    |

                        Figure 7: FE Shutting down

   On the CE side, a TML level disconnection would result in possible
   cleanup of the FE state.  Optionally, depending on the
   implementation, there may be need to inform the PL about the TML

6.3.  TML Sending and Receiving

   The TML is agnostic to the nature of the PL message it delivers to
   the remote TML (which subsequently delivers the message to its PL).
   Figure 8 shows an example of a message exchange originated at the FE
   and sent to the CE (such as a ForCES association message) which

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   illustrates all the necessary service interfaces for sending and

   When the FE PL sends a message to the TML, the TML is expected to
   pick one of HP/MP/LP channels and send out the ForCES message.

   FE PL       FE TML           CE TML                CE PL
      |            |              |                      |
      |PL send     |              |                      |
      |----------->|              |                      |
      |            |              |                      |
      |            |-Format msg.  |                      |
      |            |-pick channel |                      |
      |            |-TML  Send    |                      |
      |            |------------->|                      |
      |            |              |-TML Receive on chan. |
      |            |              |-decapsulate          |
      |            |              |- mux to PL/PL recv   |
      |            |              |--------------------->|
      |            |              |                      ~
      |            |              |                      ~ PL Process
      |            |              |                      ~
      |            |              |  PL send             |
      |            |              |<---------------------|
      |            |              |-Format msg. for send |
      |            |              |-pick chan to send on |
      |            |              |-TML send             |
      |            |<-------------|                      |
      |            |-TML Receive  |                      |
      |            |-decapsulate  |                      |
      |            |-mux to PL    |                      |
      | PL Recv    |              |                      |
      |<---------- |              |                      |
      |            |              |                      |

                       Figure 8: Send and Recv Flow

   When the CE TML receives the ForCES message on the channel it was
   sent on, it demultiplexes the message to the CE PL.

   The CE PL, after some processing (in this example dealing with the
   FE's association), sends to the TML the response.  And as in the case
   of FE PL, the CE TML picks the channel to send on before sending.

   The processing of the ForCES message upon arriving at the FE TML and
   delivery to the FE PL is similar to the CE side equivalent as shown

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   above in Section 6.3.

7.  IANA Considerations

   This document makes request of IANA to reserve SCTP ports 6700, 6701,
   and 6702.

8.  Security Considerations

   The SCTP TML provides the following security services to the PL

   o  A mechanism to authenticate ForCES CEs and FEs at transport level
      in order to prevent the participation of unauthorized CEs and
      unauthorized FEs in the control and data path processing of a
      ForCES NE.

   o  A mechanism to ensure message authentication of PL data and
      headers transferred from the CE to FE (and vice-versa) in order to
      prevent the injection of incorrect data into PL messages.

   o  A mechanism to ensure the confidentiality of PL data and headers
      transferred from the CE to FE (and vice-versa), in order to
      prevent disclosure of PL level information transported via the

   Security choices provided by the TML are made by the operator and
   take effect during the pre-association phase of the ForCES protocol.
   An operator may choose to use all, some or none of the security
   services provided by the TML in a CE-FE connection.

   When operating under a secured environment, or for other operational
   concerns (in some cases performance issues) the operator may turn off
   all the security functions between CE and FE.

   IP Security Protocol (IPsec) [RFC4301] is used to provide needed
   security mechanisms.

   IPsec is an IP level security scheme transparent to the higher-layer
   applications and therefore can provide security for any transport
   layer protocol.  This gives IPsec the advantage that it can be used
   to secure everything between the CE and FE without expecting the TML
   implementation to be aware of the details.

   The IPsec architecture is designed to provide message integrity and
   message confidentiality outlined in the TML security requirements

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   ([FE-PROTO]).  Mutual authentication and key exchange protocol are
   provided by Internet Key Exchange (IKE)[RFC4109].

8.1.  IPsec Usage

   A ForCES FE or CE MUST support the following:

   o  Internet Key Exchange (IKE)[RFC4109] with certificates for
      endpoint authentication.

   o  Transport Mode Encapsulating Security Payload (ESP)[RFC4303].

   o  HMAC-SHA1-96 [RFC2404] for message integrity protection

   o  AES-CBC with 128-bit keys [RFC3602] for message confidentiality.

   o  Replay protection[RFC4301].

   It is expected to be possible for the CE or FE to be operationally
   configured to negotiate other cipher suites and even use manual

8.1.1.  SAD and SPD setup

   To minimize the operational configuration it is recommended that only
   the IANA issued SCTP protocol number(132) be used as a selector in
   the Security Policy Database (SPD) for ForCES.  In such a case only a
   single SPD and SAD entry is needed.

   It should be straightforward to extend such a policy to alternatively
   use the 3 SCTP TML port numbers as SPD selectors.  But as noted above
   this choice will require increased number of SPD entries.

   In scenarios where multiple IP addresses are used within a single
   association, and there is desire to configure different policies on a
   per IP address, then it is recommended to follow [RFC3554]

9.  Acknowledgements

   The authors would like to thank Joel Halpern, Michael Tuxen, Randy
   Stewart and Evangelos Haleplidis for engaging us in discussions that
   have made this draft better.

10.  References

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10.1.  Normative References

   [RFC2404]  Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within
              ESP and AH", RFC 2404, November 1998.

   [RFC3554]  Bellovin, S., Ioannidis, J., Keromytis, A., and R.
              Stewart, "On the Use of Stream Control Transmission
              Protocol (SCTP) with IPsec", RFC 3554, July 2003.

   [RFC3602]  Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
              Algorithm and Its Use with IPsec", RFC 3602,
              September 2003.

   [RFC4109]  Hoffman, P., "Algorithms for Internet Key Exchange version
              1 (IKEv1)", RFC 4109, May 2005.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061,
              September 2007.

10.2.  Informative References

              Halpern, J. and J. Hadi Salim, "ForCES Forwarding Element
              Model", October 2008.

              Doria (Ed.), A., Haas (Ed.), R., Hadi Salim (Ed.), J.,
              Khosravi (Ed.), H., M. Wang (Ed.), W., Dong, L., and R.
              Gopal, "ForCES Protocol Specification", November 2008.

   [RFC3654]  Khosravi, H. and T. Anderson, "Requirements for Separation
              of IP Control and Forwarding", RFC 3654, November 2003.

   [RFC3746]  Yang, L., Dantu, R., Anderson, T., and R. Gopal,
              "Forwarding and Control Element Separation (ForCES)
              Framework", RFC 3746, April 2004.

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   [RFC3768]  Hinden, R., "Virtual Router Redundancy Protocol (VRRP)",
              RFC 3768, April 2004.

              Stewart, R., Poon, K., Tuexen, M., Yasevich, V., and P.
              Lei, "Sockets API Extensions for Stream Control
              Transmission Protocol (SCTP)", Feb. 2009.

Authors' Addresses

   Jamal Hadi Salim
   Mojatatu Networks
   Ottawa, Ontario


   Kentaro Ogawa
   NTT Corporation
   3-9-11 Midori-cho
   Musashino-shi, Tokyo  180-8585


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