ALTO L. M. Contreras, Ed.
Internet-Draft Telefonica
Intended status: Informational S. Randriamasy
Expires: 14 September 2023 Nokia Bell Labs
X. Liu
IBM Corporation
13 March 2023
ALTO extensions for handling Service Functions
draft-lcsr-alto-service-functions-02
Abstract
This document proposes the usage of ALTO (and its extensions) to
provide information about service functions to clients (e.g.,
external systems) that could consume such information for decisions
requiring network information (service composition, traffic steering
to service chains, etc).
Status of This Memo
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Service Function information . . . . . . . . . . . . . . . . 3
2.1. Cases relevant for exposing information related to Service
Functions . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Retrieval of Service Function information . . . . . . . . 5
3. Usage of ALTO for retrieving information relative to service
functions . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Information of interest . . . . . . . . . . . . . . . . . 5
3.2. ALTO mechanisms to support the requests about service
functions . . . . . . . . . . . . . . . . . . . . . . . . 6
4. ALTO architecture for service function information
retrieval . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Proposed ALTO extensions . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Informative References . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Network services are commonly formed by means of the concatenation of
several atomic service functions (SF), resulting in a connected graph
of functions. Those functions can be topologically spread across the
network. In addition to that, there will be typically more than one
instance of any particular atomic service function in the network for
different purposes such as load balancing, redundancy, traffic
optimization, etc.
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During the definition phase of a network service there will be a
process for defining the type of service functions needed to
implement a given network service, as well as the way in which they
should be connected to steer the traffic flows through them. The
type of a SF can be for instance a User Plane Function (UPF) of the
mobile packet core, a cache of a Content Delivery Network (CDN), etc.
Thus when having multiple instances of a function (i.e., multiple
UPFs or multiple caches as in the example before), a decision process
should be in place to determine the particular instances for each
type of service function (i.e., what instance of UPF and CDN cache)
that will be part of the realization of the network service, that we
refer to as network service instance in this document.
Information on the SFs and their instances is available from entities
such as Service Orchestrators. Examples of such information are SF
type, service instance ID, SF locator, etc. Information about a
network service instance can be more comprehensively defined in terms
of performace and resources efficiency if this information is
complemented with network capabilities. This draft proposes to
complement information available on SF or their instances with
information on the paths that interconnect them.
At this point of network service realization, having timely
information of the characteristics of the interconnection paths among
SFs can be crucial. Aspects such as number of hops, associated
performance metrics, etc., can enrich (or even determine) the
decision of which instances of the service function consider as final
election.
This document proposes the usage of ALTO [RFC7285] and its extensions
to provide information about service functions or their
interconnection paths to clients (e.g., external systems) that could
consume such information for decisions requiring network information.
2. Service Function information
2.1. Cases relevant for exposing information related to Service
Functions
Several initiatives in IETF deal with the interconnection of service
functions.
[RFC7665] defines the Service Function Chain (SFC) architecture.
There, the traffic is steered through SFC domains with the objective
of making the flow passing through a number of service functions to
run a service. When entering the domain, the traffic is classified
and assigned to an SFC Path. Specific information is added to the
packet flows within the domain, being this SFC encapsulation
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containing metadata and contextual information useful for the
processing of the flows by the service functions and other components
in the architecture. In all this process, there is no explicit
identification of the service function to direct the traffic to, as
it is implicit in the definition of a specific SFC Path.
Similarly, in [RFC8986] the Segment Identifiers of SRv6 structures
the 128 bits of the IPv6 address in the form LOC:FUNCT:ARG. LOC is
the locator used to route a packet to the endpoint and encoded in the
L most significant bits of the Segment Identifier (SID). FUNCT
represents a Function ID and uses F bits. ARG represents optional
parameters to be interpreted by the function, and uses A bits.
Furthermore, [I-D.ietf-spring-sr-service-programming] defines data
plane functionalities required to implement service segments, in a
similar way as [RFC7665] for SFC.
Moreover, [I-D.ietf-teas-sf-aware-topo-model] defines a YANG data
model able to integrate both network topology and service location on
the same traffic engineering topology. In this model, the service
functions are represented by service-function-id and sf-connection-
point-id.
Finally, [I-D.ldbc-cats-framework] proposes a framework in which the
metrics associated to different service instances of the same service
function type are used for taking traffic steering decisions using
overlay connections. The metrics considered in such decision are
expected to be a combination of networking and computing metrics.
In all these previous cases, the information relative to the service
functions can be limited, if present. Richer information could be
needed for an integration between the control systems responsible for
the service operation and the control systems responsible for the
network actions that could optimize the delivery of services relying
on network information (that is, acting in an integrated fashion).
For instance, taking as example OpenStack [OpenStack], a network
service relies on descriptors providing information about Virtual
Deployment Units (VDUs), Connection Points (CPs) and Virtual Links
(VLs).
A VDU describes the properties of the virtual construct that hosts
the service function. Important information is the function
identifier and its type. The CPs contain the IP and MAC addresses
for such function, showing the binding as well between a VDU and a
VL. Finally, the VL identifies the connectivity between VDUs.
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The level of information is not the same in all the solutions
overviewed, however a solution like ALTO could help to reconcile all
these different approaches by mapping and matching information on the
service and the network planes.
2.2. Retrieval of Service Function information
Different options can be considered, in some cases being
complementary and in some other cases being actual alternatives.
One option is the retrieval of information as provided by Service
Function Orchestration systems, as described by [OpenStack] or
similar solutions.
In addition to that, or as an alternative, different routing protocol
extensions can provide information about existing Service Functions
in the network. For instance [RFC9015] can advertise using BGP the
Service Function Type and Service Function Instances in a network.
Furthermore, [I-D.xu-lsr-isis-service-function-adv] and
[I-D.xu-lsr-ospf-service-function-adv] can advertise at IGP level the
Service Function Identifier as unique identifier representing a
service function within a domain.
3. Usage of ALTO for retrieving information relative to service
functions
ALTO can expose combined information of the service overlay together
with the underlying network characteristics. This section details
the potential usage of ALTO in this respect.
3.1. Information of interest
There can be several kinds of ALTO exposure information in response
to client requests that can be taken into consideration. Some
examples are listed below:
* Path characteristics, from a PID, to any instance of a service
function type.
* Path characteristics, from a PID, to a specific instance of a
service function type.
* Path characteristics among any instance of a service function type
X to any other instance of a service function type Y.
* Path characteristics among a specific instance of a service
function type X to any other instance of a service function type
Y.
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* Path characteristics, from a PID, to a chain of service functions.
* Path characteristics, from a PID, to a chain of specific instances
of service functions.
Other type of requests could be further identified. The information
to be exposed can be a combination of network and service function
related characteristics.
An ALTO server could be able to provide information for a limited set
of requests. Thus, some indication of the possible requests to be
served should be in place when interacting with the client.
3.2. ALTO mechanisms to support the requests about service functions
ALTO can determine the path characteristics between two endpoints as
described in [RFC7285]. ALTO also can provide the view of chain of
functions by leveraging on the path vector concept developed in
[RFC9275], where the endpoints considered represent service
functions.
[RFC9275] introduces the concept of Abstract Network Element (ANE) to
specify a component or an aggregation of components sharing some
characteristics in a network. Furthermore, [RFC9240] generalizes the
concept of endpoint properties to entity properties, where entities
may be defined in semantic domains such as as IPv4 or IPv6, or PIDs
or ANEs.
This draft makes use of these capabilities to support the retrieval
of information relative to service functions.
4. ALTO architecture for service function information retrieval
The following logical architecture defines the usage of ALTO for the
retrieval of information about service functions or interconnection
of service functions.
Figure 1 represents the information exchange of an ALTO Server for
providing information about service functions to ALTO clients (e.g.,
external systems) that require combined information of network and
service functions.
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+--------+ Topological +---------+
| | Information | e.g., |
| |<--------------->| BGP, |
ALTO | | | BGP-LS |
+--------+ protocol | | | |
| Client |<----------->| ALTO | +---------+
+--------+ | Server |
| | Service Function+---------+
| | Information | e.g., |
| |<--------------->| Service |
| | | Function|
| | |Orchestr.|
+--------+ +---------+
Figure 1. Information exchange
The network topological information will be complemented with
information relative to the service functions as provided by the
orchestration system managing and controlling that part.
The ALTO server will integrate the information of the service
functions based on some parameters, such as the IP address of the
service functions.
5. Proposed ALTO extensions
As proposed extension to existing ALTO specifications, the following
aspects are considered:
* Extension to ALTO protocol to allow ALTO clients to express
detailed requests in line with the information of interest
described in Section 3.1.
* Extensions to ALTO in order to combine and expose both service and
network information, in line with the architecture depicted in
Section 3.3. These extensions can involve particularizations of
both [RFC9275] and [RFC9240].
* Identification of mechanisms to be used by ALTO for collecting
service functions' information.
Further extensions could be required.
Next iterations of this draft will further analyze the gap between
existing ALTO features and requirements to support the provisioning
of infrastructure information needed to perform efficient SF
management.
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6. Security Considerations
To be provided.
7. Informative References
[I-D.ietf-spring-sr-service-programming]
Clad, F., Xu, X., Filsfils, C., Bernier, D., Li, C.,
Decraene, B., Ma, S., Yadlapalli, C., Henderickx, W., and
S. Salsano, "Service Programming with Segment Routing",
Work in Progress, Internet-Draft, draft-ietf-spring-sr-
service-programming-07, 15 February 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
sr-service-programming-07>.
[I-D.ietf-teas-sf-aware-topo-model]
Bryskin, I., Liu, X., Lee, Y., Guichard, J., Contreras, L.
M., Ceccarelli, D., Tantsura, J., and D. Shytyi, "SF Aware
TE Topology YANG Model", Work in Progress, Internet-Draft,
draft-ietf-teas-sf-aware-topo-model-11, 12 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-teas-sf-
aware-topo-model-11>.
[I-D.ldbc-cats-framework]
Li, C., Du, Z., Boucadair, M., Contreras, L. M., Drake,
J., Huang, D., and G. S. Mishra, "A Framework for
Computing-Aware Traffic Steering (CATS)", Work in
Progress, Internet-Draft, draft-ldbc-cats-framework-01, 10
March 2023, <https://datatracker.ietf.org/doc/html/draft-
ldbc-cats-framework-01>.
[I-D.xu-lsr-isis-service-function-adv]
Xu, X., Huang, H., Shah, H. C., and L. M. Contreras,
"Advertising Service Functions Using IS-IS", Work in
Progress, Internet-Draft, draft-xu-lsr-isis-service-
function-adv-00, 9 March 2023,
<https://datatracker.ietf.org/doc/html/draft-xu-lsr-isis-
service-function-adv-00>.
[I-D.xu-lsr-ospf-service-function-adv]
Xu, X., Huang, H., Shah, H. C., and L. M. Contreras,
"Advertising Service Functions Using OSPF", Work in
Progress, Internet-Draft, draft-xu-lsr-ospf-service-
function-adv-00, 9 March 2023,
<https://datatracker.ietf.org/doc/html/draft-xu-lsr-ospf-
service-function-adv-00>.
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[OpenStack]
"VNF Descriptor (VNFD) Template Guide,
https://docs.openstack.org/tacker/latest/contributor/
vnfd_template_description.html", n.d..
[RFC7285] Alimi, R., Ed., Penno, R., Ed., Yang, Y., Ed., Kiesel, S.,
Previdi, S., Roome, W., Shalunov, S., and R. Woundy,
"Application-Layer Traffic Optimization (ALTO) Protocol",
RFC 7285, DOI 10.17487/RFC7285, September 2014,
<https://www.rfc-editor.org/info/rfc7285>.
[RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
Chaining (SFC) Architecture", RFC 7665,
DOI 10.17487/RFC7665, October 2015,
<https://www.rfc-editor.org/info/rfc7665>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
[RFC9015] Farrel, A., Drake, J., Rosen, E., Uttaro, J., and L.
Jalil, "BGP Control Plane for the Network Service Header
in Service Function Chaining", RFC 9015,
DOI 10.17487/RFC9015, June 2021,
<https://www.rfc-editor.org/info/rfc9015>.
[RFC9240] Roome, W., Randriamasy, S., Yang, Y., Zhang, J., and K.
Gao, "An Extension for Application-Layer Traffic
Optimization (ALTO): Entity Property Maps", RFC 9240,
DOI 10.17487/RFC9240, July 2022,
<https://www.rfc-editor.org/info/rfc9240>.
[RFC9275] Gao, K., Lee, Y., Randriamasy, S., Yang, Y., and J. Zhang,
"An Extension for Application-Layer Traffic Optimization
(ALTO): Path Vector", RFC 9275, DOI 10.17487/RFC9275,
September 2022, <https://www.rfc-editor.org/info/rfc9275>.
Authors' Addresses
Luis M. Contreras (editor)
Telefonica
Ronda de la Comunicacion, s/n
28050 Madrid
Spain
Email: luismiguel.contrerasmurillo@telefonica.com
URI: http://lmcontreras.com
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Sabine Randriamasy
Nokia Bell Labs
Email: sabine.randriamasy@nokia-bell-labs.com
Xufeng Liu
IBM Corporation
Email: xufeng.liu.ietf@gmail.com
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