| Network Working Group | D. Purkayastha |
| Internet-Draft | A. Rahman |
| Intended status: Informational | D. Trossen |
| Expires: January 2, 2018 | InterDigital Communications, LLC |
| July 1, 2017 |
USE CASE FOR HANDLING DYNAMIC CHAINING AND SERVICE INDIRECTION
draft-purkayastha-sfc-service-indirection-00
Many stringent requirements are imposed on today’s network, such as low latency, high availability and reliability in order to support several use cases such as IoT, Gaming, Content distribution, Robotics etc. Networks need to be flexible and dynamic in terms of allocation of services and resources. Network Operators should be able to reconfigure the composition of a service and steer users towards new service end points as user move or resource availability changes. SFC allows network operators to easily create and reconfigure service function chains dynamically in response to changing network requirements. We discuss a use case where Service Function Chain can adapt or self-organize as demanded by the network condition without requiring SPI re-classification. This can be achieved, for example, by decoupling the service consumer and service endpoint by a new service function proposed in this draft. We describe few requirements for this service function to enable dynamic switching between consumer and end point.
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The requirements on today’s networks are very diverse, enabling multiple use cases such as IoT, Content Distribution, Gaming, Network functions such as Cloud RAN. Every use case imposes certain requirements on the network. These requirements vary from one extreme to other and often they are in a divergent direction. Network operator and service providers are pushing many functions towards the edge of the network in order to be closer to the users. This reduces latency and backhaul traffic, as user request can be processed locally.
It becomes more challenging for the network when user mobility as well as non-deterministic availability of compute and storage resources are considered. The impact is felt most in the edge of the network because as the users move, their point of attachment changes frequently, which results in (at least partially) relocating the service as well as the service endpoint. Furthermore, network functions are pushed more and more towards the edge, where compute and storage resources are constrained and availability is non-deterministic. Also, storage resources may need to be moved where the user concentration is more in case of content delivery applications.
Take the following video orchestration service example from ETSI MEC Requirements document [ETSI_MEC]. The proposed use case of edge video orchestration suggests a scenario where visual content can be produced and consumed at the same location close to consumers in a densely populated and clearly limited area. Such a case could be a sports event or concert where a remarkable number of consumers are using their handheld devices to access user select tailored content. The overall video experience is combined from multiple sources, such as local recording devices, which may be fixed as well as mobile, and master video from central production server. The user is given an opportunity to select tailored views from a set of local video sources.
In such a dynamic network environment, the capability to dynamically compose new services from available services as well as move a service instance in response to user mobility or resource availability is desirable. SFC allows network operators as well as service providers to compose new services by chaining individual service functions towards the composed new service. In a dynamic network environment where service functions move frequently because of user movement, load balancing or resource modification, service function chains and the service end points need to be created and recreated frequently. SFC, as defined in IETF, is capable of modifying the service chain dynamically in response to network conditions.
In this document we address this dynamicity by introducing a special Service Function, called SRR (service request routing). We describe the problems associated with today’s network and Layer 3 based approach to handle dynamicity in the network. We then discuss how such new Service Function with certain capabilities can handle the dynamicity better than these conventional methods.
[RFC7498] captures the problems associated with existing service deployments that are problematic. High level problems are listed below.
These factors provide motivation for a simplified and flexible service insertion model that addresses many of the current shortcomings and provides new, much needed functionality to enable service deployments in modern network environments. Service chaining accomplishes this by considering service functions as resources, with associated attributes, available for scheduled consumption. Selective traffic, subject to policy, may then be “steered” to the requisite service resources, along with any “extra” information referred to as metadata. This metadata is used for policy enforcement.
A basic form of service chaining may be realized using existing transport encapsulations. This method of chaining relies upon the tunneling of selected data between service functions. Although this form of service chaining achieves some level of abstraction from the underlying topology, it does not truly create a service plane. NSH [I-D.ietf-sfc-nsh] is a distinct identifiable plane that can be used across all transports to create a service chain and exchange metadata along the chain.
We revisit the dynamic service chain creation capability of NSH. NSH defines a new service plane protocol [I-D.ietf-sfc-nsh]. A Network Service Header (NSH) contains service path information and optionally metadata that are added to a packet or frame and used to create a service plane. A control plane is required in order to exchange NSH values with participating nodes, and to provision the same nodes with requisite information such as service path ID to overlay mapping.
The Network Service Header has three parts, Base header, Service Path Header and Context Header. NSH Service Path Header is a 4-byte service path header follows the base header and defines two fields used to construct a service path:
The following figure depicts the service path header.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Path ID | Service Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: NSH Path Header
The service path identifier (SPI) is used to identify the service path that interconnects the needed service functions. It allows nodes to utilize the identifier to select the appropriate network transport protocol and forwarding techniques. The service index (SI) identifies the location of a packet within a service path. As packets traverse a service path, the SI is decremented post-service.
SPI represents the service path and altering the path identifier results in a change of a service path. A change in SPI value is a result of re-classification. It means a node in the service path determined, based on policy, that the initial classification was incorrect or incomplete. If the updated classification results in the necessity of a new service path, the node updates the SPI and SI fields accordingly. The new identifier is then used to select the appropriate overlay topology. This allows service functions to alter the path of a packet without having to participate in the network topology and its associated control plane(s). The method to determine that an existing classification is incorrect and how to determine the new classification is not defined.
The emerging trend in today’s network is to deploy network functions, services and applications at the edge of the network to support latency requirements, computational offload, traffic optimization etc. As users are moving, application or services being used by users, may need to be moved closer to the user’s new location. This implies another instance of the service function may need to be instantiated close to the user’s new location. It may result in re-establishing service path from the newly instantiated service function to other service instances. It is also possible that the newly instantiated service function may be redirected to a new service end point (e.g. Application Server) for various reasons, such as incomplete content, proximity to data store, load balancing etc. In another scenario, a single instance of the service function may not handle all users. A single service function may be instantiated more than once to balance user load. As the number of instances increase and along with mobility, the complexity of service routing increases. It is anticipated that there may be a constant action of function chaining, re-chaining occurring in the network.
The challenge of dynamic indirection may be better described by analyzing the working of CDNs, which dynamically (re-)direct user-initiated requests towards the most appropriate content instance. This task becomes more difficult if granularity of the instance placement increases. For instance, in case of a CDN being realized close to end users, specifically in edge of the network, the specific content instance might need to be selected dynamically. After initial selection, the instance may change during service execution.
In a conventional network, an instance of a service is found and selected using DNS. The subsequent service request is then routed through the network between the client and the service. If the user is doing a DNS lookup to access content served by a CDN then the DNS service will maintain a list of IP addresses that can be returned for a given domain name and will try to return an IP address of a node geographically close to the client. Should the service provider want to replace an instance of their service with another one at a different IP address (and potentially a different physical location for various reasons such as load balancing, reliability etc.) then the DNS tables must be updated, i.e., the service needs to be (re-)registered quickly. This is done by updating the local authoritative DNS server which then propagates the new mapping to DNS services across the world. DNS propagation can take up to 48 hours so fast and dynamic switching from one service instance to another is not possible in conventional networks. When relying on many surrogate service endpoints to exist in the edge network, there is a clear issue of certain resources not being available in one surrogate instance while existing in another so that changes in redirection might be desirable, while also changes in local load drive the need for such change in redirection.
The other issue in conventional network lies with mobility management procedure. These procedures use an anchor point, which terminates a session at the network edge. As user moves around, traffic is redirected from the anchor point to the new point of attachment. Relying on typical mobility management approaches found in IP networks, usually leads to inefficient ‘triangular’ routing of requests through this common ‘anchor’ point. This triangular routing increases the latency in reaching the new service function or service end points as users move.
Traffic steering is a common procedure in managed networks, particularly at the edge, due to desired subscriber-centric traffic policies (e.g., related to pricing structures), resource requirements (e.g., related to using particular paths in the network) or mobility (e.g., users moving in a cellular network). Today’s methods for traffic steering include anchor-based mobility management as well as traffic classification, for instance, in packet gateways of cellular systems (using, e.g., deep packet inspection as well as port and address classification). While the former leads to inefficient ‘triangular’ traffic forwarding, the latter often requires additional state in the forwarders to differentiate traffic from one user to another.
The analysis of CDN network shows that dynamic indirection is a necessary requirement, which needs to be supported by the networks. The goal for this indirection is to provide user applications lowest possible latency. But as discussed above, relying on today’s technique, does not help in guaranteeing same latency to user applications. On the other hand, there is a high possibility that latency may increase if we rely on Layer 3 based service redirection techniques.
SFC handles indirection through the use of SPI. A packet needs to be reclassified and the intermediate node changes the SPI. Following are the typical steps that happens in order to implement the indirection.
The indirection mechanism in SFC involves certain steps to process policy information and change the SPI in the packet header, making it suitable to handle dynamic indirection requirements. Our proposed SF in this document provides an additional method to handle dynamic indirection of service requests, not relying on the reclassification mechanism. Combining these two techniques may provide flexibility and improvement over single method.
In order to route the service requests to service end points in a dynamic manner, we identify the following desirable features:
The following diagram shows the application of the new proposed SRR service function in an example of media clients connecting to media servers. There may be more than one media functions to support CDN like architecture, Surrogate servers to handle mobility and load balancing.
+--------+
| |
|-------------------------|------------------+ SRR +
| | | |
| | +---/|\--+
| | |
+---\|/--+ +---------+ +--\|/--+ +------+ +----+---+
| | | | | | | | | |
+ Client +-->+ IP +-->+ Media +-->+ SRR +-->+ Media +
| | | Routing | | Fn1 | | | | Fn2 |
+--------+ +---------+ +-------+ +------+ +--------+
Figure 2: Use of SRR function
The clients are connected to media functions through frontend routed network, e.g., relying on standard IP routing, while media functions are chained via the new proposed service request routing (SRR) function. Alternatively, we also envision to utilize the SRR function directly between client SF and media function SF, as outlined in the figure below
+--------+
| |
|-------------------------|------------------+ SRR +
| | | |
| | +---/|\--+
| | |
+---\|/--+ +---------+ +--\|/--+ +------+ +----+---+
| | | | | | | | | |
+ Client +-->+ SRR +-->+ Media +-->+ SRR +-->+ Media +
| | | | | Fn1 | | | | Fn2 |
+--------+ +---------+ +-------+ +------+ +--------+
Figure 3: SRR function between Client and Media Function
The SRR service function decouples clients from media functions. This brings in flexibility in routing service requests from client to service end points. In the edge network, where users are moving and service end points may also change, having flexibility to decide and steer service requests directly helps in guaranteeing the same latency to user applications. Clearly, that is achieved by reducing the switching time from SF to another. As service end point changes, the routing functions makes instantaneous decision to route the request to the appropriate media server.
The possible improvements of using SRR within an SFC framework are listed below:
Does the WG see value in supporting the requirements for SFC to enable routing of service requests between service consumers and service endpoints in a dynamic manner as outlined in Section 4?
This document requests no IANA actions.
TBD.
| [ETSI_MEC] | ETSI, "Mobile Edge Computing (MEC), Technical Requirements", GS MEC 002 1.1.1, March 2016. |
| [I-D.ietf-sfc-nsh] | Quinn, P. and U. Elzur, "Network Service Header", Internet-Draft draft-ietf-sfc-nsh-13, June 2017. |
| [RFC7498] | Quinn, P. and T. Nadeau, "Problem Statement for Service Function Chaining", RFC 7498, DOI 10.17487/RFC7498, April 2015. |