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 other network scenarios, some existing or new customers may require connectivity services with advanced characteristics comparing to traditional 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 to deliver IETF network slices . 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 which consists of a set of dedicated or shared network resources allocated from the physical underlay network, and is associated with a logical network topology. VPN+ services can be delivered by mapping one or a group of overlay VPNs to the appropriate VTNs as the underlay, so as to provide the network characteristics 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, VTN and NRP are considered as similar concepts, and NRP 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 VTN-specific packet processing can be performed using that set of resources, which avoids the possible resource competition with services in other VTNs. This is called Resource Independent (RI) VTN. A VTN Resource ID 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 associated with a VTN could be defined using mechanisms such as Multi-Topology , or Flex-Algo , etc. This document proposes a mechanism to carry the VTN resource ID together with other VTN related information in a new Hop-by-Hop option called "VTN option" of IPv6 extension header of IPv6 packet, so that on each network node along the packet forwarding path, the VTN option in the packet is parsed, and the obtained VTN Resource ID is used to instruct the network node to use the set of network resources allocated to the corresponding VTN to process and forward the packet. The procedure for processing the VTN option is also specified. This provides a scalable solution to support a relatively large number of VTNs in an IPv6 network.

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 RFC 2119 RFC 8174 when, and only when, they appear in all capitals, as shown here.

A new Hop-by-Hop option type "VTN" is defined to carry the VTN resource identifier and other VTN related information in an IPv6 packet. Its format is shown as below:

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. The value of Opt Data Len of VTN option SHOULD be set to 8. 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 Resource ID MUST be strictly matched for the processing of the packet. When S flag is set to 1, if the VTN resource ID in the VTN option does not match with any of the VTN resource ID provisioned on the outgoing interface of the network node, the packet MUST be dropped. When S flag is set to 0, if the VTN resource ID in the VTN option does not match with any of the VTN resource ID provisioned on the outgoing interface of the network node, the packet SHOULD be forwarded using the default set of network resource on the outgoing interface. U (Unused): These flags are reserved for future use. They SHOULD be set to 0 on transmission and MUST be ignored on receipt. Reserved: 3-octet field reserved for future use. SHOULD be set to 0 on transmission and MUST be ignored on receipt. VTN Resource ID: 4-octet identifier which uniquely identifies the set of network resources allocated to a VTN. Note that, if a deployment found it useful, the four-octet VTN Resource ID field may be derived from the four-octet Single Network Slice Selection Assistance Information (S-NSSAI) defined in 3GPP .

As the VTN option needs to be processed by each node along the path for VTN-specific forwarding, it MUST be carried in IPv6 Hop-by-Hop options header.

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

On receipt of a packet with the VTN option, each network node which can process the VTN option in fast path MUST use the VTN Resource ID to determine the set of local network resources allocated to the VTN for packet processing. 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 MAY 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 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 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 continue to forward the packet 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. specifies the modified procedures for the processing of IPv6 Hop-by-Hop Options header. Operator needs 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, it is practical 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 other word, packets steered into a VTN MUST NOT be dropped due to the existence of the Hop-by-Hop Options header. It is RECOMMENDED to configure all the network nodes involved in a VTN to process the Hop-by-Hop Options header and the VTN option if there is a nob for this.

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

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 and Eric Vyncke for their review and valuable comments.