SFC Working Group D. Migault, Ed.
Internet-Draft Ericsson
Intended status: Informational C. Pignataro
Expires: October 6, 2016 T. Reddy
C. Inacio
April 4, 2016

SFC environment Security requirements


This document provides environment security requirements for the SFC architecture. Environment security requirements are independent of the protocols used for SFC – such as NSH for example. As a result, the requirements provided in this document are intended to provide good security practices so SFC can be securely deployed and operated. These security requirements are designated as environment security requirements as opposed to the protocol security requirements.

Status of This Memo

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This Internet-Draft will expire on October 6, 2016.

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Table of Contents

1. Requirements notation

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 [RFC2119].

2. Introduction

This document provides environment security requirements for the SFC architecture [I-D.ietf-sfc-architecture]. Environment security requirements are independent of the protocols used for SFC – such as NSH [I-D.ietf-sfc-nsh]. As a result, the requirements provided in this document are intended to provide good security practice so SFC can be securely deployed and operated. These security requirements are designated as environment security requirements as opposed to the protocol security requirements. This document is built as follows. Section 4 provides an overall description of the SFC environment with the introduction of the different planes (SFC Control Plane, the SFC Management Plane, the Tenant’s user Plane and the SFC Data Plane). Section 6 lists environment security requirements for the SFC. These requirements are intended to prevent attacks, as well as network and SFC misconfigurations. When such events happens, the security recommendations also aim at detecting and identifying the threats or misconfiguration as well as limiting their impact. Recommendations also may apply differently depending on the infrastructure. For example trusted environment may enforce lighter security recommendations than public and open SFC infrastructures. However, one should also consider future evolution of their infrastructure, and consider the requirements as a way to maintain the SFC architecture stable during its complete life cycle. For each requirement this document attempts to provide further guidance on the reasons to enforce it as well as what should be considered while enforcing it or the associated risks of not enforcing it.

This document assumes the reader is familiar with the SFC architecture defined in [I-D.ietf-sfc-architecture].

3. Terminology and Acronyms

- Tenant:
A tenant is one organization that is using SFC. A tenant may use SFC on one’s own private infrastructure or on a shared infrastructure.
- Tenant's User Data Plane:
The tenant may be using SFC to provide service to its customers or users. The communication of these users is designated as Tenant’s user Data Plane and includes all communications involving the tenant’s users. As a result, if a user is communicating with a server or a user from another domain, the communication with that tenant’s user is part of the Tenant’s Users Data Plane.

4. SFC Environment Overview

This section provides an overview of SFC. It is not in the scope to this document to provide an explicit description of SFC. Instead, the reader is expected to read [I-D.ietf-sfc-architecture], [I-D.ietf-sfc-control-plane] and other SFC related documents.

Service Function Chaining (SFC) architecture is defined in [I-D.ietf-sfc-architecture]. This section briefly illustrates the main concepts of the SFC architecture and positions the architecture within an environment.

                    SFC Control Plane
                                         ^             ^    ^
            |         C1 |            C3 |          C2 |    | C4
            |            |            +-----+          |    |
            |            |            |  SF |          |    |
            |            |            +-----+          |    |
            |            v               |             |    |
            |       +------------+     +-----+      +-----+ |
            |    +->|    SFC     |---->| SFF |----->| SFF |------+
 SFC        |    |  | Classifier |<----|     |<-----|     | |    |
 Management |    |  +------------+     +-----+      +-----+ |    |
 Plane      |    |         ^                           |    |    |
            |    |         |                     +-----------+   |
            |    |         |                     | SFC Proxy |   |
            |    |         |                     +-----------+   |
            |    |         |                           |         |
            |    |         |                        +------+     |
            |    |         v                        |  SF  |     |
            |    |    +---------+                   +------+     |
            |    |    |  SFC 2  | <------------ SFC 1 ------->   |
            |    |    +---------+                                |
            |    |            SFC Data Plane                     |
    SFC incoming |                                  SFC outgoing |
    Data traffic |                                  Data traffic v

                      SFC Tenant’s Users Data Plane

SFC Environment Overview

SFC defined a Service Function Path (SFP) which is an ordered set of Service Functions (SF) applied to packets. SFP is defined at the SF level. A SF may be performed by different instances of SF located at different positions. As a result, a specific packet may pass through different instances of SFC. The ordered set of SF instances a packet goes through is called the Rendered SF Path (RSFP).

Upon the receipt of an incoming packet from the tenant’s user, the SFC Classifier determines, according to Classifiers, which SFP is associated to that packet. The packet is forwarded from Service Function Forwarders (SFF) to SFF. SFF are then in charge of forwarding the packet to the next SFF or to a SF. Forwarding decisions may be performed using SFP information provided by the SFC Encapsulation. As described in [I-D.ietf-sfc-nsh] the SFC Encapsulation contains SFP information such as the SFP ID and Service Index and eventually (especially for the MD-2 in NSH) some additional metadata. SF may be SFC aware or not. In the case the SFC functions are not SFC aware, a SFC Proxy performs the SFC Decapsulation (resp. SFC Encapsulation) before forwarding the packet to the SF (resp. after receiving the packet from the SF).

The environment associated to SFC may be separated into three main planes:

SFC Management Plane and Control Plane are defined in [I-D.ietf-sfc-control-plane].
SFC Data Plane consists in all SF components as well as the data exchanged between the SF components. Communications between SF components includes the packet themselves, their associated metadata, the routing logic - similar to RIB - or SF logic, i.e. what they retuned values are for example.
SFC Tenant’s Users Data Plane consists in the traffic data provided by the different users of the tenants. When a user is communicating with a server or another user -eventually from another administrative domain - , the communication belongs to the SFC Tenant’s Users Data Plane whenever packets are provided by the server of by the user.

5. Threat Analysis

This section describes potential threats the SFC Data Plane may be exposed. The list of threats is not expected to be complete. More especially, the threats mentioned are provided to illustrate some security requirements for the SFC architecture. For simplicity, this document mostly considers that security breaches are performed by an attacker. However, such breaches may also be non-intentional and may result from misconfiguration for example.

Attacks may be performed from inside the SFC Data Plane or from outside the SFC Data plane, in which case, the attacker is in at least one of the following planes: SFC Control Plane, SFC Management Plane or SFC Tenants’ Users Plane. Some most sophisticated attacks may involve a coordination of attackers in multiple planes.

5.1. Attacks performed from the SFC Control Plane

Attacks related to the control plane have been detailed in section 5 of [I-D.ietf-sfc-control-plane].

5.2. Attacks performed from the SFC Management Plane

Attacks performed on the SFC Management Plane are similar to those performed from the SFC Control Plane. The main difference is that the SFC Management Plan provides usually a greater control of the SFC component that the SFC Control Plane.

In addition, the actions performed by the SFC Management Plane have fewer restrictions, which means it may be harder to enforce strong control access policies.

5.3. Attacks performed from the Tenant’s Users Plane

The SFC Tenant’s User Plane is not expected to have fine access control policies on the packets sent or received by users. Unless they are filtered, all packets are good candidate to the SFC Classifier. This provides the user some opportunities to test the behavior of the SFC.

In addition, the Tenant’s Users Plane is not controlled by the SFC Tenant, and users may initiate communications where both ends – the client and the server- are under the control of the same user. Such communications may be seen as user controlled communications (UCC).

UCC may enable any user to monitor and measure the health of the SFC. This may be an useful information to infer information on the tenant’s activity or to define when a DoS attack may cause more damage. One way to measure the health or load of the tenant’s SFC is to regularly send a packet and measure the time it takes to be received, in order to estimate the processing time within the SFC.

UCC may enable any user to test the consistency of the SFC. One example of inconsistency could be that SFC decapsulation is not performed – or inconsistently performed - before leaving the SFC, which could leak some metadata with private information. For example, a user may send spoofed packet. Suppose for example, that a request HTTP GET video.example.com/movie is received with some extra header information such as CLIENT_ID: 1234567890, or CLIENT_EMAIL: client@example.foo. If these pieces of information are derived from the source IP address, the attacker may collect them by changing the IP address for example. In this case, the spoofed packets as used to collect private and confidential information of the tenant’s users. Note that such threat is not specific to SFC, and results from the combination of spoofed IP and non-authenticated IP address are used to identify a user. What is specific to SFC is that metadata are likely to carry multiple pieces of information - potentially non-authenticated - associated to the user. In the case above, meta-data is carried over the HTTP header. Inserting the metadata in the HTTP header may be performed by a SF that takes its input from the SFC encapsulation. In addition, SFC encapsulation may also leak this information directly to a malicious node if that node belongs to the SFC plane. In this later case, the user builds on the top of and intrusion to the SFC Data Plane that is detailed later.

In some case, spoofed packet may impersonate other’s tenants. Suppose for example that the same infrastructure is used by multi tenants, and which are identified by the IP address of their users. In this case, spoofing an IP address associated to another tenant may be sufficient to collect the information confidential and private information.

Similarly, UCC may enable any user to infer packet has been dropped or is in a loop. Suppose a user send a spoofed packet and receives no response. The attacker may infer that the packet has been dropped or is in a loop. A loop is expect to load the system and sending a “well known packet” over the UCC and measuring the response time may determine whether the packet has been dropped or is in a loop.

Correlation of time measurement and spoofed packet over a UCC may provide various type information that could be used by an attacker.

The attacker may correlate spoofed packet and time measurement in order discover the SFC topology or the logic of the SFC Classifier. Typically, it may infer when new SFs are placed in the SFC for example. In addition, as metadata are placed in band, the time response may also provide an indication of the size of the metadata associated to the packet. The combination of these pieces of information may help an attacker to orchestrate a future attack on a specific SF either to maximize the damages or to collect some metadata – like identification credentials.
The attacker may also define the type of packets that require the SFC the more processing. Additional processing may be due a large set of additional metadata that require fragmentation, some packets that are not treated in a coherent and consistent manner within the SFC. Such information may be used for example to optimize a DoS attack. In addition, it could also be used in order to artificially increase the necessary resource of the Tenant in order to increase the cost of operation for running its service.

Time measurement and spoofed packet in combination with variable query rate over a UCC may provide information on the orchestration of the SFC itself. For example, the user may be able to detect when elasticity mechanisms are triggered.

An attacker may be able to leverage the knowledge that SFC is in use by specific carriers to effect the processing of data using the SFC system as a processor in the attack. This leads to a number of potential weaknesses in the Internet ecosystem.

An attacker may be able to characterize the type of client platforms using a web site by carefully crafting data streams that will be modified by the SFC system versus client systems that would view web data unmodified. For example, leveraging SFC and carefully crafted data, a malicious web site operator may be able to create a particularly formatted common file that when modified by a cellular operator for bandwidth savings creates a file that may crash, (creating a DoS attack) on a select set of clients. Clients not accessing that web site using the same RSFP would not experience any issues. Additionally, external examination of the malicious site would not demonstrate any malicious content, relying on the SF to modify the content.

A well crafted site could potentially leverage the variances of functionality from different RSFPs in order to GEO locate a user. An example would be creating an image file which when recompressed creates image artifacts rendering the image unusable, but allowing the user to respond to such an event, thereby letting the web site operator know the user has potentially moved from a higher to lower bandwidth network location within the area of a specific network operator.

5.4. Attacks performed from the SFC Data Plane

This section considers an attacker has been able to take control of an SFC component. As a result, the attacker may become able to modify the traffic and perform, on-path attacks, it may also be able to generate traffic, or redirect traffic to perform some kind of Man-in-the-middle attacks. This is clearly a fault, and security policies should be set to avoid this situation. This section analyses in case this intrusion occurs, the potential consequences on the SFC. As mentioned earlier, this section assumes all these actions are performed by an attacker. However, what is designated by an attack may also result from misconfigurations at various layer. A SF or a SFF may become inadvertently configured or programed which may result in similar outcomes as an attack. Whatever result in what we designate as an attack, the purpose of security requirements will be to detect, to analyse and mitigate such security breaches.

The traffic within the SFC Data Plane is composed of multiple layers. The traffic is composed of communications between SFC components. The transport between the SFC component is the transport protocol and is not considered in the SFC. It can typically be a L2 transport layer, or an L3 transport layer using various encapsulation techniques (vLAN, VxLAN, GRE, IPsec tunnels for example). The transport layer carries SFC Encapsulated that are composed of an SFC Encapsulation envelope that carries metadata and a SFC payload that is the actual packet exchanged between the two end points.

As a result, attacker may use the traffic to perform attacks at various layers. More specifically, attacks may be performed at the transport layer, the SFC Encapsulation layer or the SFC payload layer.

Attacks performed at the transport layer may be related to SFC in the sense that illegitimate SFC traffic could be provided to the SF. Typically, a malicious node that is not expected to communicate with that SF may inject packets into the SFC, such malicious node may eventually spoof the IP address of legitimate SF, so the receiving SF may not be able to detect the packet is not legitimate. Threats related to IP spoofing are described in [RFC6959] and may be addresses by authenticated traffic (e.g. using IPsec). Such threats are not related to SFC even though they may impact a given SF.
the SFC Encapsulation as well as the SFC payload are usually considered as input by a SF. As such they may represent efficient vector of attacks for the SF. Attacks performed through SFC payload are similar as the ones described in the Tenant’s Users Data Plane section. As a result, such attacks are not considered in this section, and this section mostly considers attacks based on the SFC Encapsulation and malicious metadata.

When an attacker is within the SFC Data Plane, it may have a full or partial control of one SF component in which case, the attacker is likely to compromise the associated SFCs. It could for example, modify the expected operation of the SFC. Note that in this case, the SFC may be appropriately provisioned and set, however, the SFC does not operate as expected this may only be detected by monitoring and auditing the SFC Data Plane.

Although traffic authentication may be performed at various layers L2 L3 or at the SFC Encapsulation layer, this section considers the SFC traffic. As a result, the SFC traffic is authenticated if the SF is able to authenticate the incoming SFC packet.

When SFC traffic is not authenticated, an attacker may inject spoofed packet in any SFC component. The attacker may use spoofed packet to discover the logic of the SFC. On the other hand, the attacker may also inject packet in order to perform DoS attack via reflection. In fact, some SF may carry large metadata, which may provide a vector for amplification within the SFC Data Plane and thus either load the network or the next SF. Note that amplification may be generated by metadata, the SFC payload, and the attacker may replay packets or completely craft new packets. In addition, the attacker may choose a spoofed packet to increase the CPU load on the SFC components. For example, it could insert additional metadata to generate fragmentation. Similarly, it may also insert unnecessary metadata that may need to be decapsulated and analyzed even though they may not be considered for further actions. Spoofed packet may not only be generated to attack the SFC component at the SFC layer. In fact spoofed packet may also target applications of the SF. For example an attacker may also forge packet for HTTP based application – like a L7 firewall – in order to perform a slowloris [SLOWLORIS] like attack. Note that in this case, such attacks are addressed in the Tenant’s Users Data Plane section. The specificity here is that the attacker has a more advanced understanding of the processing of the SFC, and can thus be more efficient.

When SFC traffic is not authenticated, an attacker may also modify on-path the packet. By changing some metadata contained in the SFC Encapsulation, the attacker may test and discover the logic of the SFF. Similarly, when the attacker is aware of the logic of a SFC component, the attacker may modify some metadata in order to modify the expected operation of the SFC. Such example includes for example redirection to a SF which could result in overloading the SF and overall affect the complete SFC. Similarly, the attacker may also create loops within the SFC. Note that redirection may not occur only in a given SFC. In fact, the attacker may use SFC branching to affect other SFC. Another example would also include a redirection to a node owned by the attacker and which is completely outside the SFC. Motivation for such redirection would be that the attacker has full administrator privileges on that node, whereas it only has limited capabilities on the corrupted node. Such attack is a man-in-the-middle attack. The important thing to note is that in this case the traffic is brought outside the legitimate SFC domain. In fact, performing a man-in-the-middle attack as described above means that the SFC domain has been extended. This can be easily performed in case all node of the data center or the tenant’s virtual network is likely to host a SFC component. A similar scenario may also consider that the traffic could be redirected outside the data center or the tenant’s virtual network if the routing of firewall rule enables such policies.

A direct consequence is that a corrupted SFC component may affect the whole SFC. This also means that the trust of a given SFC decreases with the number of SF involved as each SF presents a surface of attack.

An attacker may also perform passive attacks by listening to traffic exchanged throughout the SFC Data Plane. Such attacks are described in [RFC7258]. Metadata are associated to each packet. These metadata are additional pieces of information not carried in the packet and necessary for each SF to operate. As a result, metadata may contain private information such as identifiers or credentials. In addition, observing the traffic may provide information on the tenant’s activity. Note that encryption only may not prevent such attacks, as activity may be inferred by the traffic load.

6. Security Requirements

This section aims at providing environment security requirements. These requirements are derived from the threat analysis provided in Section 5

Although the security requirements are derived from described threats, the scope of security should be understood in a much broader way than addressing threats. In fact the primary purpose of the security requirements is to ensure the SFC architecture can remain robust and stable.

Protecting the SFC architecture from attacker is one goal of the security requirements. Some could easily argue that such requirements are not needed for example in a private SFC deployment where SFC components may be considered in a trusted environment and administrated by a single entity. However, even in a single administrative domain, inside attacks are possible. (e.g. inside attacker sniffing the SFC metadata, sending spoofed packets etc.). Then, the trusted domain assumption may not remain valid over time. Suppose, for example, that the SFC architecture is now interconnected with some third party SF or SFF. Such SFC component is now outside the initial trusted domain which has several security implications. Similarly, a single trusted domain with one tenant may evolve over time and become multitenants and share a SFC platform. These tenants, may be trusted as in the case for example where each tenant represents a different department of a single company. Authentication is not sufficient, and relying only on a access control presents some risks. If the tenants are not strongly isolated - with physical or logical networks isolation, they may share a common SFF and one tenant may update the SFP of the other tenant. Such misconfiguration has similar impact as a redirecting attack. This document provide guidance that result in limiting such risks and improve detection for further mitigation.

As a result, the goal of this document is to provide some security requirements that should be checked against any evolution of the SFC architecture. The requirements should be understand and the risks of not following them should be evaluated with the current deployment as well as the foreseen evolutions.

Similarly, the document provides means to evaluate the consequences of a security breach, as well as means to detect them.

The motivations for the security requirements are:

Preventing attacks
Preventing misconfigurations - as far as stability and security of the SFC architecture is concerned.
Providing means to evaluate the consequences of a security breach
Making possible to audit, and detect any misbehavior that may affect stability and security of the SFC.

6.1. Plane Isolation Requirements

Plane Isolation consists in limiting the surface of attack of the SFC Data Plane by controlling the interfaces between the SFC Data Plane and the other planes.

Complete isolation of the planes is not possible, as there are still some communications that must be enabled in order to benefit from the benefits of SFC. As a result, isolation should be understood as enabling communications between planes in a controlled way.

This section lists the recommendations so communication between planes can be controlled. This involves controlling communications between planes as well as controlling communication within a plane.

The requirements listed below applies to all planes, whereas the following subsection are more specific to each plane, providing recommendations on the interface with the SFC Data Plane.

In order to increase isolation every plane that communicates with another plane SHOULD use a dedicated interface. In our case, the SFC Management Plane, the SFC Control Plane and the SFC Data Plane SHOULD use dedicated networks and dedicated interfaces. Isolation of inter-plane communication may be enforced using different ways. How isolation is enforced depends on the type of traffic, the network environment for example, and within a given SFC architecture different techniques may be used for the different planes. One way to isolate communications is to use completely different network on dedicated NICS. On the other hand, depending on the required level of isolation, a logical isolation may be performed using different IP addresses or ports with network logically isolated - that is using for example different VxLAN, or GRE tunnels. In this case, isolation relies on the trust associated to the different switches and router. In case of a lake of trust on the on-path elements, authenticated encryption may be used to provide a logical isolation. With authenticated encryption, trust is placed on the end points. Note also that encryption can also be used in combination of other isolation mechanisms in order to increase the level of isolation.
Activity on each interface between planes SHOULD be monitored and regularly audited.
Each interface between planes SHOULD be provided means to filter traffic or rate-limit the traffic. Filtering and rate-limiting policies may be finer grained and may apply for a subset of traffic.

The above requirements mostly corresponds to the architecture best current practice. Isolation is mostly motivated to avoid the planes to interact on each other. For example the load on the SFC Data Plane should not affect the SFC Control Plane and SFC Management Plane communications. The purpose of the encryption is to prevent the communication to be visible outside the SFC components involved and to be authenticated.

The two remaining requirements provides means to be informed and to to protect the planes at their boundaries. Such requirements are also current best practices.

Such recommendations are thus strongly recommended even in the case the two planes are considered to belong to trusted environments.

6.1.1. SFC Control Plane Isolation

In order to limit the risks of an attack from the SFC Control Plane, effort should be made in order to restrict the capabilities and the information provided by the SFC Data Plane to the SFC Control Plane to the authorized tenants only. In this case the authorized tenants are the users or organizations responsible for the SFC domain.

Tenants of the SFC Control Plane SHOULD authenticate in order to prevent tenant’s usurpation or communication hijacking.
Communications between SFC Control Plane and the SFC Data Plane MUST be authenticated and encrypted in order to preserve privacy. The purpose of encryption in this case prevents an attacker to be aware of the action performed by the SFC Control Plane. Such information may be used to orchestrate an attack – especially when SFC component report their CPU/network load.
Strong access control policies SHOULD be enforced. Control SHOULD be performed on the engaged resource (e.g. CPU, memory, disk access for example) and SHOULD be associated explicitly to authorized tenants. By default, a tenant SHOULD be denied any access to resource, and access SHOULD be explicit.

Given the SFC Control Plane traffic load that is expected to be light - at least compared to the SFC Tenant's Users Data Plane or the SFC Data Plane. As a result, encryption is not expect to impact the performances of the SFC architecture. Given the effort to migrate from an non authenticated (and non protected) communications to a protected communication, we recommend these requirements to be considered even in trusted environments.

Access Control policies can be enforced in various ways. One way could be to consider the systems of the SF to limit the resources associated to each tenants. Other ways include the use of API in order to limit the scope of possible interactions between the SFC Control Plane and the SFC Data Plane. This is one way to limit the possibilities of the tenants. In addition, each of these actions should be associated an authorized tenant, as well as authorized parameters. The use of API belongs to best practices and so is strongly recommended even in trusted environments.

Audit SHOULD be performed regularly to check access control policies are still up-to-date and prevent non-authorized users to control the SFC Data Plane.

The purpose of audits is to provide evidences when something went wrong. As a result, audit facilities are expected to be provided even in trusted environments.

6.1.2. SFC Management Plane Isolation

The requirements for the SFC Control Plane and SFC Management Plane are similar. The main difference of the interfaces between the SFC Management Plane and the SFC Control Plane is that it is less likely that APIs could be used to configure the different SFC components. As a result, users of the SFC Management Plane are likely to have a broader and wider control over the SFC component.

it is RECOMMENDED to enforce stronger authentication mechanisms (for example relying on hardware tokens or keys) and to limit the scope of administrative roles on a per component basis.
SFC Control Plane and SFC Management Plane may present some overlap. Each SFC component MUST have clear policies in case these two planes enter in conflict.

6.1.3. Tenant’s Users Data Plane Isolation

The Tenant’s Users Data Plane is supposed to have less restricted access control than the other SFC Management Plane and SFC Control Planes. A typical use case could be that each tenant are controlling and managing the SFC in order to provide services to their associated users. The number of users interacting with the SFC Data Plane is expected to be larger than the number of tenants interacting with the SFC Control and SFC Management Planes. In addition, the scope of communications initiated or terminating at the user end points is likely to be unlimited compared to the scope of communications between the tenants and the SFC Control Plane or SFC Management Plane. In such cases, the tenant may be provided two roles. One to grant access to the SFC, and another one to control and manage the SFC. These two roles should be able to interact and communicate.

Users SHOULD be authenticated, and only being granted access to the SFC if authorized. Authorization may be provided by the SFC itself or outside the SFC.
Filtering policies SHOULD prevent access to a user, or traffic when a malicious behavior is noticed. A malicious activity may be noticed once a given behavioral pattern is detected or when unexpected load is monitored in the SFC Data Plane.
Tenant’s User Plane SHOULD be monitored, in order to detect malicious behaviors.
When SFC is used by multiple tenants, each tenant’s traffic SHOULD be isolated based on authenticated information. More specifically, the use of a Classifier that can easily be spoofed like an IP address SHOULD NOT be used.

It is RECOMMENDED that user's access authorization be performed outside the SFC. In fact granting access and treating the traffic are two different functions, and we RECOMMEND they remain separated. Then, splitting these two functions makes it possible for a tester to perform tests of an potential attacker, without any contextual information. More specifically, having a traffic identified as associated to test by the SFC reduces the scope of the tests simply because an attacker will not be considered as a tester. For that reason, we RECOMMEND authorization is performed outside the SFC, and SFC deployment may not be designed to authenticate end users.

The remaining requirements are associated to monitoring the network and providing interactions between the access and the SFC. This interaction may be provided outside SFC itself.

6.2. SFC Data Plane Requirements

This section provides requirements and recommendation for the SFC Data Plane.

Communications within the SFC Data Plane SHOULD be authenticated in order to prevent the traffic to be modified by an attacker. As a result, authentication includes the SFC Encapsulation as well as the SFC payload.
Communication MUST NOT reveal privacy sensitive metadata.
The metadata provided in the communication MUST be limited in in term of volume as to limit the amplification factor as well as fragmentation.
Metadata SHOULD NOT be considered by the SFF for forwarding decision. In fact, the inputs considered for switching the packet to the next SFF or a SF should involve a minimum processing operation to be read. More specifically, these inputs are expected fixed length value fields in the SFC Encapsulation header rather than any TLV format.
When multiple tenants share a given infrastructure, the traffic associated to each tenant MUST be authenticated and respective Tenant's Users Planes MUST remain isolated. More specifically, if for example, a SFC Classifier is shared between multiple tenants. The Classifier used to associate the SFC MUST be authenticated. This is to limit the use of spoofed Classifiers. In any case, the SFC component that receives traffic from multiple tenants is assumed to be trusted.
Being a member of a SFC domain SHOULD be explicitly mentioned by the node and means should be provided so the SFC domain the node belongs to may be checked. Such requirement intends to prevent a packet to go outside a SFC domain, for example in the case of a man-in-the-middle attacks, where a redirection occurs outside the SFC domain. It is expected that most deployment will rely on border / port mechanisms that prevent outsider users from injecting packets with spoofed metadata. Although such mechanisms are strongly recommended to deploy, in case of failure, they do not prevent man-in-the-middle attack outside the SFC domain.

Authentication of the traffic within the SFC Data Plane is particularly recommended in an open environment where third party SF or SFF are involved. It can also be recommended when a strong isolation of the traffic is crucial for the infrastructure or to meet some level of certification. In addition, authentication may also be performed using various techniques. The whole packet may be authenticated or limited to some parts (like the flow ID). Authenticating the traffic and how or what to authenticate is a trade off between the risk associated and the cost of encryption. When possible we recommend to authenticate, but we also consider that the price may be too high in controlled and small trusted environment.

Metadata is an important part of the SFC architecture, and their impact on security should be closely evaluated. It is the responsibility of the SFC administrator to evaluate the privacy associated by the metadata - section 5.2.2 of [RFC6973] - and according to this evaluation to consider appropriated mechanisms to prevent the privacy leakage. Mechanisms should be provided even though they may not be activated.

As a general guidance exposing privacy sensitive metadata in any communications between two any SFC component should be avoided. [One way, for example to avoid exposing privacy sensitive metadata is to include a reference to the metadata instead of the metadata itself. Another way could be to encrypt the metadata itself - but that is part of the solution space.] Applying this principle prevents any private oriented data to be leaked. This requirement is mandatory when the SFC is not deployed in a trusted environment.

When exposition of the privacy sensitive metadata cannot be avoided and you are in a trusted domain, then exposing privacy sensitive metadata may be considered as long as they do not leak outside the boundaries of the trusted environment. In this case, the security is delegated to the security policies of the trusted environment boundaries, that may be outside the scope of SFC. More especially, the security policies may be for example enforced by a firewall. In this specific case, the trusted environment MUST prevent leakage of the metadata out of the trusted environment and MUST ensure that untrusted node cannot access in any way the communications within the trusted environment.

The reason this requirement is set to MUST is to specify that if one does not follow the requirement it is at its your own risk and must provide the necessary means to prevent any leak - in our case enforcing the necessary security policies that your environment / deployment needs.

Similarly, it is the responsibility of the administrator to define what an acceptable size for metadata is. Even in trusted environment, we recommend the SFC administrator be able to define and change this level.

Processing metadata by the SFF seems also expensive, and it is the responsibility of the SFC administrator to evaluate whether processing metadata by the SFF may impact the SFC architecture. In addition, metadata are expected to be associated to SF as opposed to the forwarding information that are associated to the SFF. These inputs have different functions, are associated to different processing rules, and may be administrated by different parties. It is thus part of the general good practise to split these functionalities. Optimization may require to combine the analysis of metadata and forwarding information, but this should be handled cautiously.

Assertion of belonging to a security domain, is especially recommended in open environments. This may also partly be addressed by node authenticating.

In addition, the following operational requirements have been identified:

SFC components SHOULD be uniquely identified and have their own cryptographic material. In other words the use of a shared secret for all nodes SHOULD NOT be considered as one corrupted node would be able to impersonate any node of the SFC Data Plane. This is especially useful for audit.
Activity in the SFC Data Plane MUST be monitored and audited regularly. Audit and log analysis is especially useful to check that SFC architecture assessments. They can be useful to detect a security breach for example before it is being discovered and exploited by a malicious user. Monitoring the system is also complementary in order to provide alarms when a suspicious activity is detected. Monitoring enables the system to react to unexpected behaviors in a dynamic way. Both activities are complementary as monitoring enables to counter suspicious behavior and audit may detect misconfiguration or deep causes of a malicious behavior. For these reasons, audit and monitoring facilities are expected even in trusted environment.
Isolate the Plane with border and firewall to restrict access of SFC components to legitimate traffic. More specifically, SFC components are supposed to be accessed only via dedicated interfaces. Outside these interfaces, inbound or outbound traffic SHOULD be rejected.

6.3. Additional Requirements

SFC Encapsulation SHOULD carry some identification so it can be associated to the appropriated SFP as well as its position within the SFC or SFP. Indicating the SFP ID may be sufficient as long as a SFP can uniquely be associated to a single SFC. Otherwise, the SFC should be also somehow indicated. This is especially useful for audit and to avoid traffic coming from one SFC to mix with another SFC. Authentication of the SFP ID is one way to enforce SFP ID uniqueness. This may not be mandatory, but large deployment or deployment that are involving multiple parties are expected enforce this. In fact assuming SFP ID will have no collision is an hypothesis that may be hard to fulfill over time.
It is RECOMMENDED that SFF and SF keep separate roles. SFF should be focused on SF forwarding. As a result, they are expected to access a limited information from the packet - mostly fixed size information. SF on the other hand are service oriented, and are likely to access all SFC information which includes metadata for example. The reasons to keep these functions are clearly different and may involve different entities. For example, SF management or SF configuration may involve different administrators as those orchestrating the SFC.
SFC Encapsulation SHOULD be integrity protected to prevent attackers from modifying the SFP ID. See Data Plane communication Requirements and considerations)

7. Security Considerations

8. Privacy Considerations

9. IANA Considerations

10. Acknowledgments

The authors would like to thank Joel Halpern, Mohamed Boucadair and Linda Dunbar for their valuable comments.

11. References

11.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC6959] McPherson, D., Baker, F. and J. Halpern, "Source Address Validation Improvement (SAVI) Threat Scope", RFC 6959, DOI 10.17487/RFC6959, May 2013.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., Morris, J., Hansen, M. and R. Smith, "Privacy Considerations for Internet Protocols", RFC 6973, DOI 10.17487/RFC6973, July 2013.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 2014.

11.2. Informative References

[I-D.ietf-sfc-nsh] Quinn, P. and U. Elzur, "Network Service Header", Internet-Draft draft-ietf-sfc-nsh-01, July 2015.
[I-D.ietf-sfc-architecture] Halpern, J. and C. Pignataro, "Service Function Chaining (SFC) Architecture", Internet-Draft draft-ietf-sfc-architecture-11, July 2015.
[I-D.ietf-sfc-control-plane] Li, H., Wu, Q., Huang, O., Boucadair, M., Jacquenet, C., Haeffner, W., Lee, S., Parker, R., Dunbar, L., Malis, A., Halpern, J., Reddy, T. and P. Patil, "Service Function Chaining (SFC) Control Plane Components & Requirements", Internet-Draft draft-ietf-sfc-control-plane-00, August 2015.
[SLOWLORIS] Wikipedia, "Slowloris"

Authors' Addresses

Daniel Migault (editor) Ericsson 8400 boulevard Decarie Montreal, QC , H4P 2N2 Canada Phone: +1 514-452-2160 EMail: daniel.migault@ericsson.com
Carlos Pignataro Cisco Systems, Inc. 7200-12 Kit Creek Road Research Triangle Park, NC 27709 USA Phone: +1 919-392-7428 EMail: cpignata@cisco.com
Tirumaleswar Reddy Cisco Systems, Inc. Cessna Business Park, Varthur Hobli Bangalore, Karnataka 560103 India Phone: +91 9886 EMail: tireddy@cisco.com
Christopher Inacio CERT, Software Engineering Institute, Carnegie Mellon University 4500 5th Ave Pittsburgh, PA 15213 USA Phone: +1 412-268-3098 EMail: inacio@cert.org