]> Energy Sciences Network

buraglio@forwardingplane.net

Huawei

tal.mizrahi.phd@gmail.com

China Unicom

tongt5@chinaunicom.cn

Telefonica

luismiguel.contrerasmurillo@telefonica.com

SI6 Networks

fgont@si6networks.com

Routing Source Packet Routing in Networking Internet-Draft SRv6 is a traffic engineering, encapsulation and steering mechanism utilizing IPv6 addresses to identify segments in a pre-defined policy. This document discusses security considerations in SRv6 networks, including the potential threats and the possible mitigation methods. The document does not define any new security protocols or extensions to existing protocols. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Source Packet Routing in Networking Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction Segment Routing (SR) utilizing an IPv6 data plane is a source routing model that leverages an IPv6 underlay and an IPv6 extension header called the Segment Routing Header (SRH) to signal and control the forwarding and path of packets by imposing an ordered list of segments that are processed at each hop along the signaled path. SRv6 is fundamentally bound to the IPv6 protocol and introduces a new extension header. There are security considerations which must be noted or addressed in order to operate an SRv6 network in a reliable and secure manner. Specifically, some primary properties of SRv6 that affect the security considerations are:

• SRv6 may use the SRH which is a type of Routing Extension Header defined by . Security considerations of the SRH are discussed section 7, and were based in part on security considerations of the deprecated routing header 0 as discussed in section 5.
• SRv6 uses the IPv6 data-plane, and therefore security considerations of IPv6 are applicable to SRv6 as well. Some of these considerations are discussed in Section 10 of and in .
• While SRv6 uses what appear to be typical IPv6 addresses, the address space is processed differently by segment endpoints. A typical IPv6 unicast address is comprised of a network prefix, host identifier. A typical SRv6 segment identifier (SID) is comprised of a locator, a function identifier, and optionally, function arguments. The locator must be routable, which enables both SRv6 capable and incapable devices to participate in forwarding, either as normal IPv6 unicast or SRv6 segment endpoints. The capability to operate in environments that may have gaps in SRv6 support allows the bridging of islands of SRv6 devices with standard IPv6 unicast routing.

This document describes various threats to SRv6 networks and also presents existing approaches to avoid or mitigate the threats.

Scope of this Document The following IETF RFCs were selected for security assessment as part of this effort:

• : "Segment Routing Architecture"
• : "IPv6 Segment Routing Header (SRH)"
• : "Segment Routing over IPv6 (SRv6) Network Programming"
• : "YANG Data Model for Segment Routing"
• : "Segment Routing Policy Architecture"
• : "Integration of the Network Service Header (NSH) and Segment Routing for Service Function Chaining (SFC)"
• : "Segment Routing Replication for Multipoint Service Delivery"

We note that SRv6 is under active development and, as such, the above documents might not cover all protocols employed in an SRv6 deployment.

Conventions and Definitions

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Terminology

• HMAC TLV: Hashed Message Authentication Code Type Length Value
• SID: Segment Identifier
• SRH: Segment Routing Header
• SRv6: Segment Routing over IPv6

Threat Model This section introduces the threat model that is used in this document. The model is based on terminology from the Internet threat model , as well as some concepts from , , [RFC7835] and . Details regarding inter-domain segment routing (SR) are out of scope for this document.

Internal vs. External:

An internal attacker in the context of SRv6 is an attacker who is located within an SR domain. Specifically, an internal attacker either has access to a node in the SR domain, or is located on an internal path between two nodes in the SR domain. External attackers, on the other hand, are not within the SR domain.

On-path vs. Off-path:

On-path attackers are located in a position that allows interception, modification or dropping of in-flight packets, as well as insertion (generation) of packets. Off-path attackers can only attack by insertion of packets.

Data plane vs. control plane vs. Management plane:

Attacks can be classified based on the plane they target: data, control, or management. The distinction between on-path and off-path attackers depends on the plane where the attack occurs. For instance, an attacker might be off-path from a data plane perspective but on-path from a management plane perspective.

The following figure depicts an example of an SR domain with six attacker types, labeled 1-6. For instance, attacker 2 is located along the path between the SR ingress node and SR endpoint 1, and is therefore an on-path attacker both in the data plane and in the control plane. Thus, attacker 2 can listen, insert, delete, modify or replay data plane and/or control plane packets in transit. Off-path attackers, such as attackers 4 and 5, can insert packets, and in some cases can passively listen to some traffic, such as multicast transmissions. Attacker 3 is internal an on-path attacker in the management plane, as it is located along the path between the Network Management System (NMS) and SR endpoint 1.

Threat Model Taxonomy X------>O--->X---------->O------->O-------------->O----> ^\ ^ /^\ /^ | \___/\_ /\_ | _/\__/ | \___/\______/ | | \__/ | | | | | | | SR SR SR SR ingress endpoint 1 endpoint 2 egress node node ]]>

As defined in , SR operates within a "trusted domain". Therefore, in the current threat model the SR domain defines the boundary that distinguishes internal from external threats. Specifically, an attack on one domain that is invoked from within a different domain is considered an external attack in the context of the current document.

Impact One of the important aspects of a threat analysis is the potential impact of each threat. SRv6 allows for the forwarding of IPv6 packets via predetermined SR policies, which determine the paths and the processing of these packets. An attack on SRv6 may cause packets to traverse arbitrary paths and to be subject to arbitrary processing by SR endpoints within an SR domain. This may allow an attacker to perform a number of attacks on the victim networks and hosts that would be mostly unfeasible for a non-SRv6 environment. The threat model in classifies threats according to their potential impact, defining six categories. For each of these categories we briefly discuss its applicability to SRv6 attacks.

• Unauthorized Access: an attack that results in unauthorized access might be achieved by having an attacker leverage SRv6 to circumvent security controls as a result of security devices being unable to enforce security policies. For example, this can occur if packets are directed through paths where packet filtering policies are not enforced, or if some security policies are not enforced in the presence of IPv6 Extension Headers.
• Masquerade: various attacks that result in spoofing or masquerading are possible in IPv6 networks. However, these attacks are not specific to SRv6, and are therefore not within the scope of this document.
• System Integrity: attacks on SRv6 can manipulate the path and the processing that the packet is subject to, thus compromising the integrity of the system. Furthermore, an attack that compromises the control plane and/or the management plane is also a means of impacting the system integrity.
• Communication Integrity: SRv6 attacks may cause packets to be forwarded through paths that the attacker controls, which may facilitate other attacks that compromise the integrity of user data. Integrity protection of user data, which is implemented in higher layers, avoids these aspects, and therefore communication integrity is not within the scope of this document.
• Confidentiality: as in communication integrity, packets forwarded through unintended paths may traverse nodes controlled by the attacker. Since eavesdropping to user data can be avoided by using encryption in higher layers, it is not within the scope of this document. However, eavesdropping to a network that uses SRv6 allows the attacker to collect information about SR endpoint addresses, SR policies, and network topologies, is a specific form of reconnaissance
• Denial of Service: the availability aspects of SRv6 include the ability of attackers to leverage SRv6 as a means for compromising the performance of a network or for causing Denial of Service (DoS). Compromising the availability of the system can be achieved by sending multiple SRv6-enabled packets to/through victim nodes, where the SRv6-enabled packets result in a negative performance impact of the victim systems (see for further details). Alternatively, an attacker might achieve attack amplification by causing packets to "bounce" multiple times between a set of victim nodes, with the goal of exhausting processing resources and/or bandwidth (see for a discussion of this type of attack).

discusses specific implementations of these attacks, and possible mitigations are discussed in .

Attacks

Attack Abstractions Packet manipulation and processing attacks can be implemented by performing a set of one or more basic operations. These basic operations (abstractions) are as follows:

• Passive listening: an attacker who reads packets off the network can collect information about SR endpoint addresses, SR policies and the network topology. This information can then be used to deploy other types of attacks.
• Packet replaying: in a replay attack the attacker records one or more packets and transmits them at a later point in time.
• Packet insertion: an attacker generates and injects a packet to the network. The generated packet may be maliciously crafted to include false information, including for example false addresses and SRv6-related information.
• Packet deletion: by intercepting and removing packets from the network, an attacker prevents these packets from reaching their destination. Selective removal of packets may, in some cases, cause more severe damage than random packet loss.
• Packet modification: the attacker modifies packets during transit.

This section describes attacks that are based on packet manipulation and processing, as well as attacks performed by other means. While it is possible for packet manipulation and processing attacks against all the fields of the IPv6 header and its extension headers, this document limits itself to the IPv6 header and the SRH.

Modification Attack

Overview An on-path internal attacker can modify a packet while it is in transit in a way that directly affects the packet's segment list and other extension headers.

Header Modification	Impact
source address	spoof another source
destination address	modify the segment list active segment and arguments, including next segments like binding SIDs Binding SID or compressed segments
SRH	insert or delete the SRH modifying the previous and next segments in the segment list
SRH segment list	insert, delete or modify the previous or next segments in the segment list
SRH TLV	insert, delete or modify TLVs in the SRH
SRH flags and tag	modify tags and flags

An on-path internal attacker can also modify, insert or delete other extension headers but these are outside the scope of this document.

Scope An SR modification attack can be performed by on-path attackers. If filtering is deployed at the domain boundaries as described in , the ability to implement SR modification attacks is limited to on-path internal attackers.

Impact The SR modification attack allows an on-path internal attacker to change the segment list in the packet, i.e. the SR policy that the packet is steered through, and thus to manipulate the path and the processing that the packet is subject to. Specifically, the SR modification attack can impact the network and the forwarding behavior of packets in one or more of the following ways:

Avoiding a specific node or path:

An attacker can manipulate the DA and/or SRH in order to avoid a specific node or path. This approach can be used, for example, for bypassing the billing service or avoiding access controls and security filters.

Preferring a specific path:

The packet can be manipulated to divert packets to a specific path. This attack can result in allowing various unauthorized services such as traffic acceleration. Alternatively, an attacker can divert traffic to be forwarded through a specific node that the attacker has access to, thus facilitating more complex on-path attacks such as passive listening, recon and various man-in-the-middle attacks. It is noted that the SR modification attack is performed by an on-path attacker who has access to packets in transit, and thus can implement these attacks directly. However, SR modification is relatively easy to implement and requires low processing resources by an attacker, while it facilitates more complex on-path attacks by averting the traffic to another node that the attacker has access to and has more processing resources.

Forwarding through a path that causes the packet to be discarded:

SR modification may cause a packet to be forwarded to a point in the network where it can no longer be forwarded, causing the packet to be discarded.

Manipulating the SRv6 network programming:

An attacker can trigger a specific endpoint behavior by modifying the destination address and/or SIDs in the segment list. This attack can be invoked in order to manipulate the path or in order to exhaust the resources of the SR endpoint.

Availability:

An attacker can add SIDs to the segment list in order to increase the number hops that each packet is forwarded through and thus increase the load on the network. For example, a set of SIDs can be inserted in a way that creates a forwarding loop (, ) and thus loads the nodes along the loop. Network programming can be used in some cases to manipulate segment endpoints to perform unnecessary functions that consume processing resources. TLV fields such as the HMAC TLV can be maliciously added to the SRH in order to consume processing resources. Path inflation, malicious looping and unnecessary instructions and TLVs have a common outcome, resource exhaustion, which may in severe cases cause Denial of Service (DoS).

Passive Listening

Overview An on-path internal attacker can passively listen to packets and specifically listen to the SRv6-related information that is conveyed in the IPv6 header and the SRH. This approach can be used for reconnaissance, i.e., for collecting segment lists.

Scope A reconnaisance attack is limited to on-path internal attackers. If filtering is deployed at the domain boundaries (), it prevents any leaks of explicit SRv6 routing information through the boundaries of the administrative domain. In this case external attackers can only collect SRv6-related data in a malfunctioning network in which SRv6-related information is leaked through the boundaries of an SR domain.

Impact While the information collected in a reconnaisance attack does not compromise the confidentiality of the user data, it allows an attacker to gather information about the network which in turn can be used to enable other attacks.

Packet Insertion

Overview In a packet insertion attack packets are inserted (injected) into the network with a segment list. The attack can be applied either by using synthetic packets or by replaying previously recorded packets.

Scope Packet insertion can be performed by either on-path or off-path attackers. In the case of a replay attack, recording packets in-flight requires on-path access and the recorded packets can later be injected either from an on-path or an off-path location. If filtering is deployed at the domain boundaries (), insertion attacks can only be implemented by internal attackers.

Impact The main impact of this attack is resource exhaustion which compromises the availability of the network, as described in .

Control and Management Plane Attacks

Overview Depending on the control plane protocols used in a network, it is possible to use the control plane as a way of compromising the network. For example, an attacker can advertise SIDs in order to manipulate the SR policies used in the network. Known IPv6 control plane attacks (e.g., overclaiming) are applicable to SRv6 as well. A compromised management plane can also facilitate a wide range of attacks, including manipulating the SR policies or compromising the network availability.

Scope The control plane and management plane may be either in-band or out-of-band, and thus the on-path and off-path taxonomy of is not necessarily common between the data plane, control plane and management plane. As in the data plane, on-path attackers can be implement a wide range of attacks in order to compromise the control and/or management plane, including selectively removing legitimate messages, replaying them or passively listening to them. However, while an on-path attacker in the data plane is potentially more harmful than an off-path attacker, effective control and/or management plane attacks can be implemented off-path rather than by trying to intercept or modify traffic in-flight, for example by exchanging malicious control plane messages with legitimate routers, by spoofing an SDN (Software Defined Network) controller, or by gaining access to an NMS (Network Management System). SRv6 domain boundary filtering can be used for mitigating potential control plane and management plane attacks from external attackers. Segment routing does not define any specific security mechanisms in existing control plane or management plane protocols. However, existing control plane and management plane protocols use authentication and security mechanisms to validate the authenticity of information.

Impact A compromised control plane or management plane can impact the network in various possible ways. SR policies can be manipulated by the attacker to avoid specific paths or to prefer specific paths, as described in . Alternatively, the attacker can compromise the availability, either by defining SR policies that load the network resources, as described in , or by blackholing some or all of the SR policies. A passive attacker can use the control plane or management plane messages as a means for recon, similarly to .

Other Attacks Various attacks which are not specific to SRv6 can be used to compromise networks that deploy SRv6. For example, spoofing is not specific to SRv6, but can be used in a network that uses SRv6. Such attacks are outside the scope of this document. Because SRv6 is completely reliant on IPv6 for addressing, forwarding, and fundamental networking basics, it is potentially subject to any existing or emerging IPv6 vulnerabilities , however, this is out of scope for this document.

Mitigation Methods This section presents methods that can be used to mitigate the threats and issues that were presented in previous sections. This section does not introduce new security solutions or protocols.

Trusted Domains and Filtering

Overview As specified in : Following the spirit of , the current document assumes that SRv6 is deployed within a trusted domain. Traffic MUST be filtered at the domain boundaries. Thus, most of the attacks described in this document are limited to within the domain (i.e., internal attackers).

SRH Filtering Filtering on presence of an SRH is possible but not useful for two reasons: 1. The SRH is optional for SID processing as described in section 3.1 and 4.1. 2. A packet containing an SRH may not be destined to the SR domain, it may be simply transiting the domain. For these reasons SRH filtering is not a useful method of mitigation, and thus filtering can only be applied based on the address range, as described below.

Address Range Filtering The IPv6 destination address can be filtered at the SR ingress node and at all nodes implementing SRv6 SIDs within the SR domain in order to mitigate external attacks. Section 5.1 of describes this in detail, it's summarized here as: 1. At ingress nodes, any packet entering the SR domain and destined to a SID within the SR domain is dropped. 2. At every SRv6 enabled node, any packet destined to a SID instantiated at the node from a source address outside the SR domain is dropped. In order to apply such a filtering mechanism the SR domain needs to have an infrastructure address range for SIDs, and an infrastructure address range for source addresses, that can be detected and enforced. Some examples of an infrastructure address range for SIDs are: 1. ULA addresses 2. The prefix defined in [RFC9602]. 3. GUA addresses Many operators reserve a /64 block for all loopback addresses and allocate /128 for each loopback interface. This simplifies the filtering of permitted source addresses. Failure to implement address range filtering at ingress nodes is mitigated with filtering at SRv6 enabled node. Failure to implement both filtering mechanisms could result in a "fail open" scenario, where some attacks by internal attackers described in this document may be launched by external attackers.

Encapsulation of Packets Packets steered in an SR domain are often encapsulated in an IPv6 encapsulation. This mechanism allows for encapsulation of both IPv4 and IPv6 packets. Encapsulation of packets at the SR ingress node and decapsulation at the SR egress node mitigates the ability of external attackers to attack the domain.

Hashed Message Authentication Code (HMAC) The SRH can be secured by an HMAC TLV, as defined in . The HMAC is an optional TLV that secures the segment list, the SRH flags, the SRH Last Entry field and the IPv6 source address. A pre-shared key is used in the generation and verification of the HMAC. Using an HMAC in an SR domain can mitigate some of the SR Modification Attacks (). For example, the segment list is protected by the HMAC. The following aspects of the HMAC should be considered:

• The HMAC TLV is OPTIONAL.
• While it is presumed that unique keys will be employed by each participating node, in scenarios where the network resorts to manual configuration of pre-shared keys, the same key might be reused by multiple systems as an (incorrect) shortcut to keeping the problem of pre-shared key configuration manageable.
• When the HMAC is used there is a distinction between an attacker who becomes internal by having physical access, for example by plugging into an active port of a network device, and an attacker who has full access to a legitimate network node, including for example encryption keys if the network is encrypted. The latter type of attacker is an internal attacker who can perform any of the attacks that were described in the previous section as relevant to internal attackers.
• An internal attacker who does not have access to the pre-shared key can capture legitimate packets, and later replay the SRH and HMAC from these recorded packets. This allows the attacker to insert the previously recorded SRH and HMAC into a newly injected packet. An on-path internal attacker can also replace the SRH of an in-transit packet with a different SRH that was previously captured.

Implications on Existing Equipment

Limitations in Filtering Capabilities provides recommendations on the filtering of IPv6 packets containing IPv6 extension headers at transit routers. However, this class of filtering is shown to not be useful and can be ignored. Filtering on prefixes has been shown to be useful, specifically 's description of packet filtering. There are no known limitations with filtering on infrastructure addresses, and expands on the concept with control plane filtering.

Middlebox Filtering Issues When an SRv6 packet is forwarded in the SRv6 domain, its destination address changes constantly, the real destination address is hidden. Security devices on SRv6 network may not learn the real destination address and fail to take access control on some SRv6 traffic. The security devices on SRv6 networks need to take care of SRv6 packets. However, the SRv6 packets usually use loopback address of the PE device as a source address. As a result, the address information of SR packets may be asymmetric, resulting in improper filter traffic problems, which affects the effectiveness of security devices. For example, along the forwarding path in SRv6 network, the SR-aware firewall will check the association relationships of the bidirectional VPN traffic packets. And it is able to retrieve the final destination of SRv6 packet from the last entry in the SRH. When the <source, destination> tuple of the packet from PE1 to PE2 is <PE1-IP-ADDR, PE2-VPN-SID>, and the other direction is <PE2-IP-ADDR, PE1-VPN-SID>, the source address and destination address of the forward and backward VPN traffic are regarded as different flow. Eventually, the legal traffic may be blocked by the firewall. SRv6 is commonly used as a tunneling technology in operator networks. To provide VPN service in an SRv6 network, the ingress PE encapsulates the payload with an outer IPv6 header with the SRH carrying the SR Policy segment List along with the VPN service SID. The user traffic towards SRv6 provider backbone will be encapsulated in SRv6 tunnel. When constructing an SRv6 packet, the destination address field of the SRv6 packet changes constantly and the source address field of the SRv6 packet is usually assigned using an address on the originating device, which may be a host or a network element depending on configuration. This may affect the security equipment and middle boxes in the traffic path. Because of the existence of the SRH, and the additional headers, security appliances, monitoring systems, and middle boxes could react in different ways if do not incorporate support for the supporting SRv6 mechanisms, such as the IPv6 Segment Routing Header (SRH) . Additionally, implementation limitations in the processing of IPv6 packets with extension headers may result in SRv6 packets being dropped ,.

Limited capability hardware In some cases, access control lists capabilities are a resource shared with other features across a given hardware platform. Filtering capabilities should be considered along with other hardware reliant functions such as VLAN scale, route table size, MAC address table size, etc. Filtering both at the control and data plane may or may not require shared resources. For example, some platforms may require allocating resources from route table size in order to accommodate larger numbers of access lists. Hardware and software configurations should be considered when designing the filtering capabilities for an SRv6 control and data plane.

Security Considerations The security considerations of SRv6 are presented throughout this document.

IANA Considerations This document has no IANA actions.

Topics for Further Consideration This section lists topics that will be discussed further before deciding whether they need to be included in this document, as well as some placeholders for items that need further work.

• Add tables for attack section
• The following references may be used in the future: RFC9256
• SRH compression
• Spoofing
• Path enumeration
• host to host scenario involving a WAN and/or a data center fabric.
• Terms that may be used in a future version: Locator Block, FRR, uSID
• L4 checksum: specifies that when the Routing header is present the L4 checksum is computed by the originating node based on the IPv6 address of the last element of the Routing header. When compressed segment lists are used, the last element of the Routing header may be different than the Destination Address as received by the final destination. Furthermore, compressed segment lists can be used in the Destination Address without the presence of a Routing header, and in this case the IPv6 Destination address can be modified along the path. As a result, some existing middleboxes which verify the L4 checksum might miscalculate the checksum. This issue is currently under discussion in the SPRING WG.
• Segment Routing Header figure: the SRv6 Segment Routing Header (SRH) is defined in .

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Segment Routing (SR) leverages the source routing paradigm. A node steers a packet through an ordered list of instructions, called "segments". A segment can represent any instruction, topological or service based. A segment can have a semantic local to an SR node or global within an SR domain. SR provides a mechanism that allows a flow to be restricted to a specific topological path, while maintaining per-flow state only at the ingress node(s) to the SR domain. SR can be directly applied to the MPLS architecture with no change to the forwarding plane. A segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels. The segment to process is on the top of the stack. Upon completion of a segment, the related label is popped from the stack. SR can be applied to the IPv6 architecture, with a new type of routing header. A segment is encoded as an IPv6 address. An ordered list of segments is encoded as an ordered list of IPv6 addresses in the routing header. The active segment is indicated by the Destination Address (DA) of the packet. The next active segment is indicated by a pointer in the new routing header. Segment Routing can be applied to the IPv6 data plane using a new type of Routing Extension Header called the Segment Routing Header (SRH). This document describes the SRH and how it is used by nodes that are Segment Routing (SR) capable. The Segment Routing over IPv6 (SRv6) Network Programming framework enables a network operator or an application to specify a packet processing program by encoding a sequence of instructions in the IPv6 packet header. Each instruction is implemented on one or several nodes in the network and identified by an SRv6 Segment Identifier in the packet. This document defines the SRv6 Network Programming concept and specifies the base set of SRv6 behaviors that enables the creation of interoperable overlays with underlay optimization. This document defines three YANG data models. The first is for Segment Routing (SR) configuration and operation, which is to be augmented by different Segment Routing data planes. The next is a YANG data model that defines a collection of generic types and groupings for SR. The third module defines the configuration and operational states for the Segment Routing MPLS data plane. Segment Routing (SR) allows a node to steer a packet flow along any path. Intermediate per-path states are eliminated thanks to source routing. SR Policy is an ordered list of segments (i.e., instructions) that represent a source-routed policy. Packet flows are steered into an SR Policy on a node where it is instantiated called a headend node. The packets steered into an SR Policy carry an ordered list of segments associated with that SR Policy. This document updates RFC 8402 as it details the concepts of SR Policy and steering into an SR Policy. This document describes the integration of the Network Service Header (NSH) and Segment Routing (SR), as well as encapsulation details, to efficiently support Service Function Chaining (SFC) while maintaining separation of the service and transport planes as originally intended by the SFC architecture. Combining these technologies allows SR to be used for steering packets between Service Function Forwarders (SFFs) along a given Service Function Path (SFP), whereas the NSH is responsible for maintaining the integrity of the service plane, the SFC instance context, and any associated metadata. This integration demonstrates that the NSH and SR can work cooperatively and provide a network operator with the flexibility to use whichever transport technology makes sense in specific areas of their network infrastructure while still maintaining an end-to-end service plane using the NSH. This document describes the Segment Routing Replication segment for multipoint service delivery. A Replication segment allows a packet to be replicated from a replication node to downstream nodes. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References All RFCs are required to have a Security Considerations section. Historically, such sections have been relatively weak. This document provides guidelines to RFC authors on how to write a good Security Considerations section. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. A DetNet (deterministic network) provides specific performance guarantees to its data flows, such as extremely low data loss rates and bounded latency (including bounded latency variation, i.e., "jitter"). As a result, securing a DetNet requires that in addition to the best practice security measures taken for any mission-critical network, additional security measures may be needed to secure the intended operation of these novel service properties. This document addresses DetNet-specific security considerations from the perspectives of both the DetNet system-level designer and component designer. System considerations include a taxonomy of relevant threats and attacks, and associations of threats versus use cases and service properties. Component-level considerations include ingress filtering and packet arrival-time violation detection. This document also addresses security considerations specific to the IP and MPLS data plane technologies, thereby complementing the Security Considerations sections of those documents. As time and frequency distribution protocols are becoming increasingly common and widely deployed, concern about their exposure to various security threats is increasing. This document defines a set of security requirements for time protocols, focusing on the Precision Time Protocol (PTP) and the Network Time Protocol (NTP). This document also discusses the security impacts of time protocol practices, the performance implications of external security practices on time protocols, and the dependencies between other security services and time synchronization. Poor selection of transient numerical identifiers in protocols such as the TCP/IP suite has historically led to a number of attacks on implementations, ranging from Denial of Service (DoS) or data injection to information leakages that can be exploited by pervasive monitoring. Due diligence in the specification of transient numeric identifiers is required even when cryptographic techniques are employed, since these techniques might not mitigate all the associated issues. This document formally updates RFC 3552, incorporating requirements for transient numeric identifiers, to prevent flaws in future protocols and implementations. The ability for a node to specify a forwarding path, other than the normal shortest path, that a particular packet will traverse, benefits a number of network functions. Source-based routing mechanisms have previously been specified for network protocols but have not seen widespread adoption. In this context, the term "source" means "the point at which the explicit route is imposed"; therefore, it is not limited to the originator of the packet (i.e., the node imposing the explicit route may be the ingress node of an operator's network). This document outlines various use cases, with their requirements, that need to be taken into account by the Source Packet Routing in Networking (SPRING) architecture for unicast traffic. Multicast use cases and requirements are out of scope for this document. This document presents real-world data regarding the extent to which packets with IPv6 Extension Headers (EHs) are dropped in the Internet (as originally measured in August 2014 and later in June 2015, with similar results) and where in the network such dropping occurs. The aforementioned results serve as a problem statement that is expected to trigger operational advice on the filtering of IPv6 packets carrying IPv6 EHs so that the situation improves over time. This document also explains how the results were obtained, such that the corresponding measurements can be reproduced by other members of the community and repeated over time to observe changes in the handling of packets with IPv6 EHs. This document summarizes the operational implications of IPv6 extension headers specified in the IPv6 protocol specification (RFC 8200) and attempts to analyze reasons why packets with IPv6 extension headers are often dropped in the public Internet. The functionality provided by IPv6's Type 0 Routing Header can be exploited in order to achieve traffic amplification over a remote path for the purposes of generating denial-of-service traffic. This document updates the IPv6 specification to deprecate the use of IPv6 Type 0 Routing Headers, in light of this security concern. [STANDARDS-TRACK] This document analyzes the security implications of IPv6 Extension Headers and associated IPv6 options. Additionally, it discusses the operational and interoperability implications of discarding packets based on the IPv6 Extension Headers and IPv6 options they contain. Finally, it provides advice on the filtering of such IPv6 packets at transit routers for traffic not directed to them, for those cases where such filtering is deemed as necessary. Knowledge and experience on how to operate IPv4 networks securely is available, whether the operator is an Internet Service Provider (ISP) or an enterprise internal network. However, IPv6 presents some new security challenges. RFC 4942 describes security issues in the protocol, but network managers also need a more practical, operations-minded document to enumerate advantages and/or disadvantages of certain choices. This document analyzes the operational security issues associated with several types of networks and proposes technical and procedural mitigation techniques. This document is only applicable to managed networks, such as enterprise networks, service provider networks, or managed residential networks. This document specifies version 6 of the Internet Protocol (IPv6). It obsoletes RFC 2460. China Mobile Cisco Systems, Inc. Huawei Technologies Orange Cisco Systems, Inc. Segment Routing over IPv6 (SRv6) is the instantiation of Segment Routing (SR) on the IPv6 dataplane. This document specifies new flavors for the SRv6 endpoint behaviors defined in RFC 8986, which enable the compression of an SRv6 segment list. Such compression significantly reduces the size of the SRv6 encapsulation needed to steer packets over long segment lists. This document updates RFC 8754 by allowing a Segment List entry in the Segment Routing Header (SRH) to be either an IPv6 address, as specified in RFC 8754, or a REPLACE-CSID container in packed format, as specified in this document. n.d.

Acknowledgments The authors would like to acknowledge the valuable input and contributions from Zafar Ali, Andrew Alston, Dale Carder, Bruno Decraene, Dhruv Dhody, Joel Halpern, Bruno Hassanov, Alvaro Retana, Eric Vyncke, and Russ White.