Carrying Virtual Transport Network (VTN) Information in IPv6 Extension Header

Virtual Private Networks (VPNs) provide different customers with logically isolated connectivity over a common network infrastructure. With the introduction and evolvement of 5G and also in some existing network scenarios, some customers may require network connectivity services with advanced features comparing to conventional VPNs, such as resource isolation from other services or guaranteed performance. Such kind of network service is called enhanced VPN (VPN+). VPN+ service requires the coordination and integration between the overlay VPNs and the capability and resources of the underlay network. VPN+ can be used, for example, to deliver IETF network slice services . describes a framework and the candidate component technologies for providing VPN+ services. It also introduces the concept of Virtual Transport Network (VTN). A VTN is a virtual underlay network that is associated with a network topology, and is allocated with a set of dedicated or shared resources from the underlay physical network. A VPN+ service is realized by integrating a VPN in the overlay and a VTN in the underlay, so as to provide the network features required by the customers. In packet forwarding, traffic of different VPN+ services needs to be processed separately based on the network resources and the logical topology associated with the corresponding VTN. In the context of network slicing , a network construct called Network Resource Partition (NRP) is introduced, which can be seen as an instantiation of VTN. describes the scalability considerations and the possible optimizations for providing a relatively large number of VTNs for VPN+ services. One approach to improve the data plane scalability of VTN is to introduce a dedicated VTN Resource Identifier (VTN Resource ID) in the data packet to identify the set of network resources allocated to a VTN, so that packets in a VTN can be processed and forwarded using the VTN-specific set of resources, which could avoid possible resource competition with services in other VTNs. A VTN Resource ID is a VTN ID with network resource semantics, which represents a subset of the resources (e.g. bandwidth, buffer and queuing resources) allocated on a given set of links and nodes which constitute a logical network topology. The logical topology of a VTN could be defined and identified using mechanisms such as Multi-Topology , or Flex-Algo . This document specifies a mechanism to carry the VTN related information in a new IPv6 Hop-by-Hop option (Section 4.3 of ) called "VTN option". The VTN option is parsed by every intermediate node along the forwarding path, and the obtained VTN Resource ID is used to invoke VTN-specific packet processing and forwarding using the set of VTN-specific resources. This provides a scalable solution to support a relatively large number of VTNs in an IPv6 network . Although in this document the application of the VTN option is to carry the VTN Resource ID information, the VTN option is considered as a generic mechanism to convey network wide VTN ID and information with different semantics to meet the possible use cases in the future. Some considerations about generalization are described in Section 5.

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 BCP14 when, and only when, they appear in all capitals, as shown here.

A new Hop-by-Hop option (Section 4.3 of ) type "VTN" is defined to carry the VTN related information. Its format is shown in Figure 1.

Option Type: 8-bit identifier of the type of option. The type of VTN option is to be assigned by IANA. The bits of the type field are defined as below: BB 00 The highest-order 2 bits are set to 00 to indicate that a node which does not recognize this type will skip over it and continue processing the header. C 0 The third highest-order bit is set to 0 to indicate this option does not change en route. TTTTT To be assigned by IANA. Opt Data Len: 8-bit unsigned integer indicates the length of the option Data field of this option, in octets. Flags: 8-bit flags field. The most significant bit is defined in this document.

S (Strict Match): The S flag is used to indicate whether the VTN ID MUST be strictly matched for the processing of the packet. When S flag is set to 1, if the VTN ID in the VTN option does not match with any of the VTN ID provisioned on the network node, the packet MUST be dropped. When S flag is set to 0, if the VTN ID does not match with any of the VTN ID provisioned on the network node, the packet MUST be forwarded using the default behavior as if the VTN option does not exist. U (Unused): These flags are reserved for future use. They MUST be set to 0 on transmission and MUST be ignored on receipt. Context Type (CT): One-octet field used to indicate the semantics and length of the VTN ID carried in the option. The context value defined in this document is as follows: CT=0: The VTN ID is a network-wide 4-octet resource ID, which is used to identify the subset of network resources allocated to the VTN on the involved network nodes and links. Reserved: 2-octet field reserved for future use. They MUST be set to 0 on transmission and MUST be ignored on receipt. VTN ID: The identifier of a Virtual Transport Network, the semantics and length of the ID is determined by the Context Type. Note that, in the context of 5G network slicing, if a deployment found it useful, the four-octet VTN ID field may be derived from the four-octet Single Network Slice Selection Assistance Information (S-NSSAI) defined in 3GPP .

This section describes the procedures for VTN option processing when the Context Type in the VTN option is set to 0. The processing procedures for VTN option with other Context Types are out of the scope of this document and will be specified in separate documents which introduce those Context Types.

When an ingress node of an IPv6 domain receives a packet, according to the traffic classification and mapping policy, the packet needs to be steered into one of the VTNs in the network, then the packet MUST be encapsulated in an outer IPv6 header with the source and destination addresses set according to the policy, and the Resource ID of the VTN which the packet is mapped to according to the policy MUST be carried in the VTN option of the Hop-by-Hop Options header, which is associated with the outer IPv6 header.

On receipt of a packet with the VTN option, each network node which can process the Hop-by-Hop Options header and the VTN option in fast path MUST use the VTN Resource ID to determine the set of local network resources which are allocated to the VTN. The packet forwarding behavior is based on both the destination IP address and the VTN Resource ID. More specifically, the destination IP address is used to determine the next-hop and the outgoing interface, and VTN Resource ID is used to determine the set of network resources on the outgoing interface which are allocated to the VTN for processing and sending the packet. If the VTN Resource ID does not match with any of the VTN Resource ID provisioned on the outgoing interface, the S flag in the VTN option is used to determine whether the packet is dropped or forwarded using the default set of network resources of the outgoing interface. The Traffic Class field of the outer IPv6 header can be used to provide differentiated treatment for packets which belong to the same VTN. The egress node of the IPv6 domain MUST decapsulate the outer IPv6 header and the Hop-by-Hop Options header which includes the VTN option. In the forwarding plane, there can be different approaches of partitioning the local network resources and allocating them to different VTNs. For example, on one physical interface, a subset of the forwarding plane resources (e.g. bandwidth and the associated buffer and queuing resources) can be allocated to a particular VTN and represented as a virtual sub-interface or a data channel with reserved bandwidth resource. In packet forwarding, the IPv6 destination address of the received packet is used to identify the next-hop and the outgoing layer-3 interface, and the VTN Resource ID is used to further identify the virtual sub-interface or the data channel on the outgoing interface which is associated with the VTN. Network nodes which do not support the processing of Hop-by-Hop Options header SHOULD ignore the Hop-by-Hop options header and forward the packet only based on the destination IP address. Network nodes which support Hop-by-Hop Options header, but do not support the VTN option SHOULD ignore the VTN option and forward the packet only based on the destination IP address. The network node MAY process the rest of the Hop-by-Hop options in the Hop-by-Hop Options header.

As described in , network nodes may be configured to ignore the Hop-by-Hop Options header, drop packets containing a Hop-by-Hop Options header, or assign packets containing a Hop-by-Hop Options header to a slow processing path. In networks with such network nodes, it is important that packets of a VTN are not dropped due to the existence of the Hop-by-Hop Options header. Operators need to make sure that all the network nodes involved in a VTN can either process the Hop-by-Hop Options header in the fast path, or ignore the Hop-by-Hop Options header. Since a VTN is associated with a logical network topology, one practical approach is to ensure that all the network nodes involved in that logical topology support the processing of the Hop-by-Hop Options header and the VTN option in the fast path, and constrain the packet forwarding path to the logical topology of the VTN. specifies the modified procedures for the processing of IPv6 Hop-by-Hop Options header, with the purpose of making the Hop-by-Hop Options header useful. Network nodes complying with will not drop packets with Hop-by-Hop Options header and the VTN option.

During the discussion of this document in the 6MAN WG, one of the suggestions received is to make the VTN option more generic in terms of semantics and encoding. This section gives some analysis about to what extent the semantics of VTN could be generalized, and how the generalization could be achieved with the proposed encoding. Based on the VTN definition in , the concept of VTN could be extended as: a virtual transport network which is associated with a set of network-wide attributes and states maintained on each participating network node. The attributes associated with an VTN may include but not limited to: network resource attributes, network topology attributes, and network function attributes etc. The network resource can refer to various type of data plane resources, including link bandwidth, bufferage and queueing resources. The network topology can be multipoint-to-multipoint, point-to-point, point-to-multipoint or multipoint-to-point. The network functions may include both data forwarding actions and other types network functions which can be executed on data packets mapped to a VTN. This shows the semantics of VTN can be quite generic. Although generalization is something good to have, it would be important to understand and identify the boundary of generalization. In this document, It is anticipated that for one network attribute to be included in VTN, it needs to be a network-wide attribute rather than a node-specific attribute. Thus whether a network-wide view can be provided or not could be considered as one prerequisite of making one attribute part of the VTN option. The format of the VTN option contains the Flags field, the Context Type field and the Reserved field, which provide the capability for future extensions. That said, since the VTN option needs to be processed by network nodes in the fast path, the capability of network devices need to be considered when new semantics and encoding are introduced.

This document requests IANA to assign a new option type from "Destination Options and Hop-by-Hop Options" registry .

This document requests IANA to create a new registry for the "VTN Option Context Type" under the "Internet Protocol Version 6 (IPv6) Parameters" registry. The allocation policy of this registry is "Standards Action". The initial codepoints are assigned by this document as follows:

The security considerations with IPv6 Hop-by-Hop Options header are described in , , and . This document introduces a new IPv6 Hop-by-Hop option which is either processed in the fast path or ignored by network nodes, thus it does not introduce additional security issues.

The authors would like to thank Juhua Xu, James Guichard, Joel Halpern, Tom Petch, Aijun Wang, Zhenqiang Li, Tom Herbert, Adrian Farrel, Eric Vyncke, Erik Kline and Mohamed Boucadair for their review and valuable comments.