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Versions: (draft-khalili-optimized-service-function-chaining) 00 01 02 03

SFC WG                                                        R. Khalili
Internet-Draft                                             Z. Despotovic
Intended status: Informational                                 A. Hecker
Expires: March 2, 2020                       Huawei ERC, Munich, Germany
                                                          D. Purkayastha
                                                               A. Rahman
                                                              D. Trossen
                                        InterDigital Communications, LLC
                                                         August 30, 2019


                  Optimized Service Function Chaining
                draft-khalili-sfc-optimized-chaining-03

Abstract

   This draft investigates possibilities to use so-called 'transport-
   derived service function forwarders' (tSFFs) that uses existing
   transport information for explicit service path information. The
   draft discusses two such possibilities, focusing on realization of
   efficient chaining over single transport networks. In the first one,
   the transport network is SDN-based. The second one introduces and
   explains a specific service request routing (SRR) function to support
   URL-level routing of service requests.

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
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/1id-abstracts.html

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

Copyright and License Notice




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   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document. Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



Table of Contents

   1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1  Terminology . . . . . . . . . . . . . . . . . . . . . . . .  4
   2 SFC Forwarding Solutions . . . . . . . . . . . . . . . . . . . .  5
     2.1 Edge classification and network forwarding aggregation . . .  5
       2.1.1 Example  . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.2 SRR  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3 Optimized SFC Chaining . . . . . . . . . . . . . . . . . . . . .  9
     3.1 Utilizing Transport-derived SFFs . . . . . . . . . . . . . .  9
       3.1.1 Hierarchical addressing for service chaining . . . . . .  9
       3.1.2 Edge classification and service chains . . . . . . . . . 10
     3.2 Pre-Warming SFP Information for SRR-based Chaining . . . . . 11
   4 Applicability  . . . . . . . . . . . . . . . . . . . . . . . . . 14
   5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   6 Informative References . . . . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17



















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1 Introduction

   The delivery of end-to-end network services often requires steering
   traffic through a sequence of individual service functions.
   Deployment of these functions and particularly creation of a
   composite service from them, i.e. steering traffic through them, had
   traditionally been coupled to the underlying network topology and as
   such awkward to configure and operate [RFC7498].

   To remedy the problems identified by [RFC7498], [RFC7665] defines
   architecture for service function chaining that is topology
   independent. The architecture is predicated on a service indirection
   layer composed of architectural elements such as service functions
   (SF), service function forwarders (SFF), classifiers, etc. SFFs are
   the key architectural element as they connect the attached SFs and
   thus create a service plane.

   [RFC7665] proposes SFC encapsulation as a means for service plane
   elements to communicate. The SFC encapsulation serves essentially two
   purposes. It provides path identification in the service plane (which
   is the primary and mandatory usage of the encapsulation) and serves
   as a placeholder for metadata transferred among SFs. [RFC8300]
   defines NSH as a particular realization of the SFC encapsulation.

   Standalone SFC encapsulation such as NSH is the mainstream SFC
   forwarding method with the intention to work over multiple transport
   networks. However, SFC has been identified as a suitable methodology
   to chain services within single transport networks or, as outlined in
   [Guichard2018], even in data centers. In such cases, [RFC7665] points
   at the possibility of utilizing so-called 'transport-derived service
   function forwarders' (tSFFs) that ignore the SFC encapsulation, using
   existing transport information for explicit service path
   information.

   [Farrel2019] has discussed how the NSH can be logically represented
   in MPLS label stacks, to enable service function chaining in MPLS
   networks without relying on NSH header. In this document, we expand
   on this possibility by focusing on the realization of efficient
   chaining over SDN and HTTP transport networks. The chaining and the
   transport network configuration in such setting can be optimized to
   reduce the initial request latency. This is specifically important in
   networks with tight latency requirements, such as data centers, and
   for latency sensitive services such as URLLC, Ultra-Reliable Low-
   Latency Communication, defined in 5G [TS22.261]. By a careful design,
   such optimization can be achieved without making the chaining
   topology dependent, satisfying the requirements specified in
   [RFC7665]. As also stated in [Farrel2019], the goal here is not to
   replace NSH header but to demonstrate the benefits of using tSFFs in



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   such transport networks.

   In our first solution, said transport network is an SDN-based one
   where we represent the SFP (service function path) through a vector
   of aggregated flow identifiers. This solution is positioned as a tSFF
   between two or more SFs with no need for this solution to be SFC
   encapsulation aware. Hence, it can also be applied in cases where NSH
   encapsulation is not feasible. In our second solution, we refer to
   [Purka2018] which uses a specific service request routing (SRR)
   function to support URL-level routing of service requests. Chaining
   more than one SRR-connected SFs can be optimized for reducing the
   initial request latency, while supporting at least three different
   tSFFs, including the flow aggregation one presented as the first
   solution.

1.1  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].































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2 SFC Forwarding Solutions

2.1 Edge classification and network forwarding aggregation

   Assume we are free to choose network locators (routable addresses in
   the considered network) for edge nodes in a network. Besides, assume
   that routers (switches in SDN terminology) in that network can
   forward packets based on wildcard matching on bit-fields in the
   header. For example, a switch somewhere in the network can forward a
   packet by following this logic: "the packet should be sent out the
   port k, because the bits 15, 16, and 17 of the destination address
   are 1, 0, and 1, respectively." This is possible with SDN deployments
   compliant e.g. with OpenFlow v1.3 and higher.

   One can then come up with a multi-level classification of edge nodes,
   which leads to an assignment of locators to the edge nodes such that
   for every switch of the network the following holds:

        The switch has as many forwarding rules as it has ports

        For switch port k, the rule takes the form: when the destination
        address of the incoming packet contains a bit-field of a
        specific form, forward the packet to port k . For example, if
        the packet has 1 in the bit p of the destination address,
        forward to port 4.


   When this is done, the network essentially becomes a fabric that
   delivers a packet arriving at one of its inports to the appropriate
   outport. It does that while maintaining the minimum internal state.
   [Khalili2016] explains details of the approach. In particular, it
   shows that large networks and networks with particular topologies
   require a large ID space. With that in mind, [Khalili2016] proposes
   an approximate method that trades node state for ID (address) space
   and shows that a small increase of the node state brings a large
   reduction or the address space (additional forwarding rules that
   don't follow the above form). It is this approximate method that we
   refer to in the rest of this section.

   We showed in [khalili2018] that applying such aggregation at core can
   radically simplify procedures such as flow setup and mobility
   management: as the core switches are proactively configured,
   providing connectivity among all edge switches, the individual flows
   can be processed at the ingress edge switches, from where they are
   forwarded through pre-configured fabric pipes to the egress edge
   switches. This minimizes the set of switches to be updated for new
   user initiated, or mobile, flows to a constant number,  providing
   guarantees on latency requirements (e.g. below 10ms in large



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   settings) while still supporting fine-grained flow management. In
   this draft, we extend the context to service function chaining
   procedure.


2.1.1 Example

   Consider a simple network with an ingress (classifier) and an egress
   node, two transport switches/routers C1 and C2, and two service
   function forwarders, SFF1 and SFF2 (as depicted in Figure 1). Service
   functions SF1 and SF2 are attached to SFF1 and SF3 and SF4 are
   attached to SFF2. In this example, we assume that edge nodes are
   SFF1, SFF2, and the egress node.

   The ASC algorithm proposed in [Khalili2016] assigns to an edge node
   in the network an ID of the form (v(1), v(2), ..., v(K)), where v(j),
   j\in[1, K], being 1 if there is a path crossing link j that ends in
   the corresponding edge node, and 0 otherwise. K is the size of IDs
   assigned to edge nodes and is an output of the algorithm.

   Applying ASC algorithm to our example, we have:

        IDSFF1 = (0, 1, 1, 0, 0),

        IDSFF2 = (1, 0, 0, 1, 0),

        IDEgress = (1, 0, 0, 0, 1).

   Assuming that the destination-edge IDs are embedded in the header of
   the packets, e.g. via encapsulation, the forwarding rules at C1 and
   C2 can be aggregated by matching on bits of these IDs:

        At C1: if 1st bit is 1, forward over port 3; if 2nd bit is 1,
        forward over port 2.

        At C2: if 3rd bit is 1, forward over port 1, if 4th bit is 1,
        forward over port 2, if 5th bit is 1, forward over port 3.


   Note from this example that each edge node has a unique ID and that
   we put no limitation on how SFCs are defined.










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       _ _ _ _ _          _ _ _             _ _ _          _ _ _
      |          |       |     |           |     |        |      |
      |classifier|-----1-| C1  |-3-------1-| C2  |-3------|Egress|
      |_ _ _ _ _ |       |_ _ _|           |_ _ _|        |_ _ _ |
                            |                 |
                            2                 2
                            |                 |
                           _|_               _|_
                          |    |            |    |
                          |SFF1|            |SFF2|
                          | _ _|            | _ _|
                           /  \              /  \
                          /    \            /    \
                        _/_    _\_        _/_    _\_
                       |SF1|  |SF2|      |SF3|  |SF4|
                       |_ _|  |_ _|      |_ _|  |_ _|


   Figure 1: A simple topology with two SFFs and two transport
   switches/routers.



2.2 SRR

   In [Purka2018], an extension to the Service Function Chaining (SFC)
   concept is being proposed for a flexible chaining of service
   functions in an SFC environment, where a number of virtual instances
   for a single service function might exist. Hence, instead of
   explicitly (re-)chaining a given SFC in order to utilize a new
   virtual instance for an existing SF, a special service function
   called SRR (service request routing) is utilized to direct the
   requests via a URL-based abstraction (here, www.foo.com) for the SF
   address. As a first step, the work in [Purka2018] proposes to extend
   the notion of the service function path (SFP) to include such URLs in
   addition to already defined Ethernet or IP addresses. This is shown
   in Figure 2. Here the SFP includes the URLs of the service functions
   1 to N (i.e., www.foo.com to www.fooN.com) as well as link-local IP
   addresses being used for forwarding at the local access (here shown
   as simple 192.168.x.x IP addresses). The creation of a suitable SFP
   is assumed to be part of an orchestration process, which is not
   within the scope of the SFC framework per se.

   The SRR service function in Figure 2 can be further divided into sub-
   functions for realizing the dynamic chaining capabilities, as shown
   in [Purka2018]. Here, the service functions (such as clients and SF1
   in Figure 2) communicate with local NAPs (network attachment points),
   while the latter communicate with the PCE (path computation element)



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   to realize the IP and HTTP-level communication. In this case, the
   incoming NAP is denoted as the client NAP (cNAP) and the outgoing NAP
   as server NAP (sNAP). The Layer 2 transport is realized via the tSFF1
   function (transport-derived service function forwarder). Here we
   assume that each service function is connected to an own NAP (via
   link-local IP communication) although one or more service functions
   could also reside at a single NAP.

                                          +--------+
                                          |        |
        |-------------------|-------------+  SRR   + <-------------|
        |                   |             |        |               |
        |                   |             +---/|\--+               |
        |                   |                  |                   |
    +--\|/--+   +----+   +-\|/-+   +----+   +--+--+   +----+   +--+--+
    |       |   |    |   |     |   |    |   |     |   |    |   |     |
    +Client +-->+SRR +-->+ SF1 +-->+SRR +-->+ SF2 +-->+SRR |-->| SFn |
    |       |   |    |   |     |   |    |   |     |   |    |   |     |
    +-------+   +----+   +-----+   +----+   +-----+   +----+   +-----+

   Figure 2: Dynamic Chaining SFC, as proposed in [Purka2018]. SFP:
   192.168.x.x -> www.foo.com -> 192.168.x.x -> www.foo2.com -> ... ->
   www.fooN.com


   As presented in [Purka2018], the hierarchical addressing presented in
   Section 3.1.1 can be utilized for the realization of said tSFF1,
   while other realizations could utilize SDN-based transport networks
   or a BIER routing layer [RFC8279]. With this, the SRR service
   function is placed in-between specific tSFFs (the three
   aforementioned ones) and general service functions to be chained.




















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3 Optimized SFC Chaining

3.1 Utilizing Transport-derived SFFs

   Our model retains the architectural behavior of the SFC architecture
   of [RFC7665]. Yet, the SFC and the transport encapsulation are merged
   into the transport header. Thus everything, both transport and
   service plane forwarding, is happening based on transport
   encapsulation bits. The model builds on the edge node classification
   presented in Section 2.1 and comes in two flavors. The first one
   (Section 3.1.1) treats SFFs as edge nodes. The second one (Section
   3.1.2) assigns fictitious edge nodes to entire service chains. In
   both cases, the key points are how we identify the service chains,
   and related to that, how we embed these identifiers into the
   available address space.

3.1.1 Hierarchical addressing for service chaining

   This approach treats SFFs as edge nodes. The set of SFFs, as points
   of attachment of SFs, is normally static, known in advance in a
   network. In that sense, SFFs do not impose any stronger requirements
   than edge nodes, so the approach presented next looks viable.

   The hierarchical service chain addressing works with the address
   structure (IDSFF.IDSF.SPI), in which IDSFF identifies an SFF in the
   network, IDSF identifies an SF attached to that SFF, while SPI is the
   Service Path Identifier as defined in [RFC8300]. Note that SFs can be
   attached to multiple SFFs, i.e. the approach is not limiting in this
   sense. It is rather obvious that multiple SFs can be attached to a
   single SFF.

    _ _ _ _ _        _ _       _ _        _ _       _ _        _ _
   |          |     /   \     |    |     /   \     |    |     /   \
   |classifier|----| net |----|SFF1|----| net |----|SFF2|----| net |
   |_ _ _ _ _ |     \_ _/     |_ _ |     \_ _/     |_ _ |     \_ _/
                               /  \                 /  \
                              /    \               /    \
                            _/_    _\_           _/_    _\_
                           |SF1|  |SF2|         |SF3|  |SF4|
                           |_ _|  |_ _|         |_ _|  |_ _|


   Figure 3: a service chain of SF1-SF2-SF3 is considered in this
   example.


   See Figure 3 for an example. Assume that the edge nodes in the shown
   network are SFF1 and SFF2 (with possibly many other nodes which are



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   not shown), while the service functions SF1, ..., SF4 are considered
   end nodes, i.e. they are not edge nodes as such do not underlie
   classification. (Note that this will be changed in Section 3.1.2.).
   Assume that the classification yields the locators (IDs) IDSFF1 and
   IDSFF2 for SFF1, respectively SFF2, and that SPI is assigned to the
   service path. The service chain SF1-SF2-SF3 can operate as follows.

   The classifier first adds the outer transport header with the
   destination address (IDSFF1.IDSF1.SPI). The network uses the IDSFF1
   bitfield to route the packet to SFF1. SFF1 uses the middle part of
   the address, IDSF1, to deliver the packet to SF1. SF1, being SFC-
   aware, strips off the transport header and saves it, then processes
   the packet and, after restoring the saved transport header, sends it
   back to SFF1. SFF1 changes the transport header destination address
   to (IDSFF1.IDSF2.SPI) and forwards the packet to SF2. SF2 performs
   similar steps as SF1 and returns the packet to SFF1. SFF1 changes the
   transport header to (IDSFF2.IDSF3.SPI) and sends the packet towards
   SFF2. (In an SDN network, switches can manipulate with the headers by
   means of suitable flow rules, which should match on the (IDSF.SPI)
   fraction of the destination address. A second pass through the SDN
   processing stack will select the appropriate port to send the packet
   towards SFF2.). SFF2 performs the very same sequence of steps to
   deliver the packet to the correct SF and then further to the
   network.

   Note the role of the (SPI) part of the address. It serves to
   differentiate between different service chains that pass a single SF.
   For example, if in addition to SF1-SF3 there is a service chain SF1-
   SF5, where SF5 is attached to SFF3, SFF1 will use the (SPI) to
   forward packets coming from SF1. For chains such as SF1-SF2-SF1 and
   SF1-SF2-SF2, where a specific SF is visited multiple times, SI
   (Service Index as defined in [RFC8300]) must be included in the
   address. SFFs should then match on (IDSF2.SPI.SI) part of the address
   to determine appropriate action after receiving the packet back from
   an SF. (Note that SI should be modified after being processed by an
   SF, either by the SF or by the SFF).

3.1.2 Edge classification and service chains

   Continuing with the classification discussion from Section 3.1.1, let
   us assign a fictitious edge node to a service chain under
   consideration. More precisely, let us assign one such node to every
   subsequence of the chain that starts at each possible position in the
   chain and goes until its end. For example, for a chain SF1-SF2-SF3,
   define three such nodes for sub-chains SF1-SF2-SF3, SF2-SF3 and SF3.
   Let locators of these fictitious edge nodes be the SFs that start the
   corresponding sub-chains. So, in the example, the locators are SF1,
   SF2 and SF3. If we had another chain that goes over SF1, then we



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   would simply add another node, say SF1', and attach it to SFF1, next
   to SF1. This is to indicate that we need a distinct locator for each
   chain that goes over SF1. So we now have the starting network and
   additional imaginary edge nodes which topologically coincide with
   existing service functions but require additional, separate
   classification vectors.

   Assume that, after the classification as described in Section 3.1.1,
   we generate locators (classification vectors) IDSF1, IDSF2 and IDSF3
   for the chain SF1-SF2-SF3 and setup rules (e.g. OpenFlow compliant)
   that:

   At SFF1:

        Forward to SF1 packets with destination IDSF1, that come from
        the network.

        Replace IDSF1 with IDSF2 and then forward to SF2 packet that
        arrive from SF1.

        Replace IDSF2 with IDSF3 and then forward to SFF2.

   At SFF2:

        Forward to SF3 packets with destination IDSF3 that come from the
        network.

   We can distinguish between the packets that are received from the
   network and those received from SFs by using the inport information.

3.2 Pre-Warming SFP Information for SRR-based Chaining

   One issue when chaining service functions utilizing the SRR function
   is the initial delay incurred through the necessary path computation
   for a new service segment along the overall service function path.
   For instance, when the service function 'client' residing at the
   first SRR in Figure 2 issues a request to foo.com, i.e., the URL for
   the second service function, the NAP sub-function will trigger a PCE
   request for path resolution within the Layer 2 transport network.
   Such PCE request incurs said delay for the initial request while all
   subsequent requests along the same path are likely going to use
   locally cached information at the SRR function (we here assume but do
   not detail suitable path information update procedures being
   implemented by the SRR sub-functions in case of path changes to
   another service function).

   It is reasonable to assume that SFPs can be established across the
   realm of more than one PCE, e.g., each administering one



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   administrative domain. However, in the case of a single PCE across a
   number of SRR functions, Figure 2 can be redrawn as follows.


                                            +--------+
                                 -----------| SRR    |-------------
                                |          +--------+             |
                                 |             |                   |
                                 |             |                   |
                             +------+     +------+    +-----+   +-----+
                             | SF1  |     | SF2  |----| SRR |---| SFn |
                             +------+     +------+    +-----+   +-----+
                                 |             |
             +-------------------|-------------|---------+
             |                   |             |         |
  +-------+  |  +-------+    +--------+     +--------+   |
  |Client |-----| NAP1  |    | NAP2   |     | NAP3   |   |
  +-------+  |  +-------+    +--------+     +--------+   |
             |     \              |          /           |
             |      \             |         /            |
             |       \        +-------+    /             |
             |        \-------| tSFF1 |----              |
             |                +-------+                  |
             |                    |                      |
             |                    |                      |
             |                +-------+                  |
             |                | PCE   |                  |
             |                +-------+                  |
             +-------------------------------------------+

   Figure 4. Decomposed Dynamic Chaining SFC across two or more SFCs.


   Here, two SRR functions utilize the same PCE, e.g., within a single
   transport network. In this case, we propose to reduce such initial
   chaining delay by virtue of a 'pre-warming' of the SRR sub-functions,
   specifically the incoming NAP at the suitable SRR along the SFP. For
   this, we require a communication of the NSH and therefore the SFP
   information to the PCE - such communication is subject to a
   standardized protocol based on a trigger that led to the formation of
   said SFP, as shown in Figure 4. Once such SFP information has been
   received by the PCE, it then executes the following procedure.

   FOR ALL SF requests routed via an SRR served by the PCE:

        1. Determine the incoming NAP of the first SF request, e.g.,
        192.168.x.x in Figure 2.




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        2. Determine the outgoing NAP of the service endpoint address at
        the outgoing SF, e.g., www.foo.com in Figure 2.

        3. Compute path between incoming NAP and outgoing NAP - path
        computation might include a policy constraint, such as shortest
        path or shortest delay.

        4. Deliver path information to incoming NAP.

   END FOR

   Figure 5 outlines the messages being exchanged between the joint PCE
   and the various NAPs of the SRR function. The exact nature of the
   messages is subject to standardization and not shown at this stage of
   the draft.


           Trigger         PCE        NAP1        NAP2      NAP3
               |             |          |           |         |
               |             |<----a----|           |         |
               |             |<-----------a---------|         |
               |             |<-----------------------a-------|
               |             |          |           |         |
         --b-->|\            |          |           |         |
               | c           |          |           |         |
               |/            |          |           |         |
               |------d----->|\         |           |         |
               |             | e        |           |         |
               |             |/         |           |         |
               |             |----f---->|           |         |
               |             |------------f-------->|         |
               |             |--------------------------f---->|
               |             |          |           |         |


   Figure 5. Message Sequence Chart Resulting in Pre-Warming of Routing
   Entries. a) subscribe to pre-warming information, b)initiate service
   chaining based on external mgmt. trigger, c) compute SFP, d) send
   SFP, e) map SFP information onto paths from incoming to ongoing NAPs,
   f) push path information with forwarding/path identifier and URL.











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4 Applicability

   This draft investigates whether transport encapsulation can be used
   for service function chaining. The main message it delivers is that
   this seems possible. This was demonstrated on an example of
   underlying SDN network. However, we are not normative here with
   respect to what transport encapsulation and which bits thereof are
   used for service function chaining, i.e. which existing transport
   encapsulations give us the needed features (e.g. said assignment of
   transport identifiers and their handling at transport nodes) to
   successfully incorporate service chaining. This will be a subject of
   future investigations.







































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5 Discussion

   Transport-derived SFC forwarding is related to a number of
   advantages. In particular, easier deployment of service chaining, as
   SFs and SFFs in a transport-derived chaining do not have to be SFC
   encapsulation aware. Moreover, the transport network configuration
   and service chaining can be optimized to reduce initial request
   latency.

   For the SDN-based solution, the transport network was pre-configured,
   to provide communication among the edge nodes. It performs as an
   fabric switch that transfers a packet received on one port to another
   port. To serve a chain, we therefore need to only provide
   policy/mapping information at edge nodes (e.g. SFFs in the solution
   proposed in Section 3.1.1), reducing the initial request latency.
   Pre-warming approaches, as explained in Section 3.2, can be applied
   to further reduce this latency when SRR-based chaining is used.

   Furthermore, in this draft, we assume that the SFP for a packet is
   already known (e.g. decided by the central controller in an SDN
   setting) and hence the question is how to realize such path. Our
   solutions however can be extended to a more general setting, where
   the SFP is not known 'a priori' but decided step by step by SFFs
   along the path. The policy is therefore performed in a distributed
   manner. [Despotovic19] has provided some insight to such system and
   possible benefits.

























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6 Informative References

   [RFC7498]       P. Quinn, et al., "Problem Statement for Service
                   Function Chaining", RFC 7498 (INFORMATIONAL), April
                   2015.

   [RFC7665]       Joel Halpern, et al., "Service Function Chaining
                   (SFC) Architecture", RFC 7665 (INFORMATIONAL),
                   October 2015.

   [RFC8300]       P. Quinn, et al., "Network Service Header", RFC 8300,
                   January 2018.

   [Guichard2018]  J. Guichard, et al., "Network Service Header (NSH) MD
                   Type 1: Context Header Allocation (Data Center)",
                   IETF draft, draft-ietf-sfc-nsh-dc-allocation-02 (work
                   in progress), September 2018.

   [Khalili2016]   R. Khalili, et al., "Reducing State of OpenFlow
                   Switches in Mobile Core Networks by Flow Rule
                   Aggregation" IEEE ICCCN 2016.

   [khalili2018]   R. Khalili, et al., "Flow setup latency in SDN
                   networks", at  IEEE Journal on Selected Areas in
                   Communications, Volume: 36 , Issue: 12 , Dec. 2018

   [Purka2018]     Purkayastha, e. al., "Alternative Handling of Dynamic
                   Chaining and Service Indirection", IETF draft, draft-
                   purkayastha-sfc-service-indirection-02 (work in
                   progress), March 2018.

   [RFC8279]       IJ. Wijnands, et al., "Multicast using Bit Index
                   Explicit Replication", RFC8279, November 2017

   [Farrel2019]    A. Farrel et al., "An MPLS-Based Forwarding Plane for
                   Service Function Chaining", IETF draft,  draft-ietf-
                   mpls-sfc-07 (work in progress), March 2019

   [TS22.261]      3GPP, "Service requirements for the 5G system; Stage
                   1," 3GPP, Technical Specification (TS) 22.261,
                   September 2017, version 15.2.0.

   [Despotovic19]  Z. Despotovic, et al. "Dynamic and Scalable Control
                   as a Foundation for Future Networks", Book chapter,
                   Emerging Automation Techniques for the Future
                   Internet, p. 208-230, 2019





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Authors' Addresses

   Ramin Khalili
   Huawei ERC
   Munich, Germany
   Email: Ramin.khalili@huawei.com

   Zoran Despotovic
   Huawei ERC
   Munich, Germany
   Email: Zoran.Despotovic@huawei.com

   Artur Hecker
   Huawei ERC
   Munich, Germany
   Email: Artur.Hecker@huawei.com

   Debashish Purkayastha
   InterDigital Communications, LLC
   Conchoken, USA
   Email: Debashish.Purkayastha@InterDigital.com

   Akbar Rahman
   InterDigital Communications, LLC
   Montreal, Canada
   Email: Akbar.Rahman@InterDigital.com

   Dirk Trossen
   InterDigital Communications, LLC
   London, United Kingdom
   Email: Dirk.Trossen@InterDigital.com




















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