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SFC WG                                                        R. Khalili
Internet-Draft                                             Z. Despotovic
Intended status: Informational                                 A. Hecker
Expires: September 3, 2018                   Huawei ERC, Munich, Germany
D. Purkayastha
A. Rahman
D. Trossen
InterDigital Communications, LLC
March 2, 2018

Optimized Service Function Chaining
draft-khalili-sfc-optimized-chaining-00

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.

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Task Force (IETF), its areas, and its working groups.  Note that
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and may be updated, replaced, or obsoleted by other documents at any
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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

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Copyright (c) 2018 IETF Trust and the persons identified as the

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
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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.

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  . . . . . . . . . . . . . . . . . . . . . . . .  5
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 . . . . . . . . . . . . . . . . . . . . . . . . 16

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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
[Kumar2017], 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.

In this document, we expand on this possibility by focusing on the
realization of efficient chaining over a single transport network.
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. By a
careful design, such optimization can be achieved without making the
chaining topology dependent, satisfying the requirements specified in
[RFC7665].

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

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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.

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.

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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.

_ _ _ _ _          _ _ _             _ _ _          _ _ _
|          |       |     |           |     |        |      |
|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.

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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)
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

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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 [Wijnands2017]. 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

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.

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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.

[Kumar2017]     S. Kumar, et al., "Service Function Chaining Use
Cases In Data Centers", IETF draft, draft-ietf-sfc-
dc-use-cases-06 (work in progress), February 2017.

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

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

[Wijnands2017]  IJ. Wijnands, et al., "Multicast using Bit Index
Explicit Replication", IETF draft, draft-ietf-bier-
architecture-08 (work in progress), September 2017

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

R. Khalili et al.      Expires September 3, 2018               [Page 16]


INTERNET DRAFT    Optimized Service Function Chaining      March 2, 2018

Email: Debashish.Purkayastha@InterDigital.com

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

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

R. Khalili et al.      Expires September 3, 2018               [Page 17]



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