Spring J. Brzozowski
Internet-Draft J. Leddy
Intended status: Informational Comcast
Expires: September 7, 2014 I. Leung
Rogers Communications
S. Previdi
M. Townsley
C. Martin
C. Filsfils
R. Maglione
Cisco Systems
March 6, 2014

IPv6 Segment Routing Use Cases


Segment Routing (SR) leverages the source routing paradigm. A node steers a packet through a controlled set of instructions, called segments, by prepending the packet with an SR header. A segment can represent any instruction, topological or service-based. A segment can have a local semantic to an SR node or global within an SR domain. SR allows to enforce a flow through any topological path and service chain while maintaining per-flow state only at the ingress node to the SR domain.

The objective of this document is to illustrate some use cases that would benefit from an IPv6 Segment Routing data-plane architecture.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on September 7, 2014.

Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

1. Introduction

Segment Routing (SR) leverages the source routing paradigm. An ingress node steers a packet through a controlled set of instructions, called segments, by prepending the packet with an SR header. A segment can represent any instruction, topological or service-based. A segment can represent a local semantic on an SR node, or a global semantic within an SR domain. Segment Routing allows one to enforce a flow through any topological path and service chain while maintaining per-flow state only at the ingress node to the Segment Routing domain.

The Segment Routing architecture is described in [I-D.filsfils-rtgwg-segment-routing]. The Segment Routing control plane is agnostic to the dataplane, thus it can be applied to both MPLS and IPv6. In case of MPLS the (list of) segment identifiers are carried in the MPLS label stack, while for the IPv6 dataplane, a new type of routing extension header is required.

The details of the new routing extension header are not in scope of this document and will be published on a separate draft which also will cover the security considerations and the aspects related to the deprecation of the IPv6 Type 0 Routing Header described in [RFC5095].

2. IPv6 Segment Routing use cases

In today's networks, source routing is typically accomplished by encapsulating IP packets in MPLS LSPs that are signaled via RSVP-TE. Therefore, there are scenarios where it may be possible to run IPv6 on top of MPLS, and as such, the MPLS Segment Routing architecture described in [I-D.filsfils-spring-segment-routing-mpls] could be leveraged to provide Segment Routing capabilities in an IPv6/MPLS environment.

However, there are other cases and/or specific network environments where MPLS may not be available or deployable for lack of support on network elements or for an operator’s design choice. In such scenarios a non-MPLS based solution would be required.

Specifically, there are a class of use cases that motivate an IPv6 data plane. We identify some fundamental scenarios that, when recognized in conjunction, strongly indicate an IPv6 data plane:

  1. There is a need or desire to impose source-routing semantics within an application or at the edge of a network (for example, a CPE or home gateway)
  2. There is a strict lack of an MPLS dataplane
  3. There is a need or desire to remove routing state from any node other than the source, such that the source is the only node that knows and will know the path a packet will take, a priori
  4. There is a need to connect millions of addressable segment endpoints, thus high routing scalability is a requirement. IPv6 addresses are inherently summarizable: a very large operator could scale by summarizing IPv6 subnets at various internal boundaries. This is very simple and is a basic property of IP routing. MPLS node segments are not summarizable. To reach the same scale, an operator would need to introduce additional complexity, such as mechanisms described in [I-D.ietf-mpls-seamless-mpls]

In any environment with requirements such as those listed above, an IPv6 data plane provides a powerful combination of capabilities for a network operator to realize benefits in explicit routing, protection and restoration, high routing scalability, traffic engineering, service chaining, service differentiation and application flexibility via programmability.

This section will describe some scenarios where MPLS may not be present and it will highlight how an IPv6 Segment Routing solution could be used to address such use cases, particularly, when an MPLS data plane is neither present nor desired.

The use cases described in the section do not constitute an exhaustive list of all the possible scenarios; this section only includes some of the most common envisioned deployment models for IPv6 Segment Routing.

In addition to the use cases described in this document the IPv6 Segment Routing architecture can be applied to all the use cases described in [I-D.filsfils-rtgwg-segment-routing-use-cases] for the Segment Routing MPLS data plane, when an IPv6 data plane is present.

2.1. IPv6 Segment Routing in the Home Network

An IPv6-enabled home network provides ample globally routed IP addresses for all devices in the home. An IPv6 home network with multiple egress points and associated provider-assigned prefixes will, in turn, provide multiple IPv6 addresses to hosts. A homenet performing Source and Destination Routing ([I-D.troan-homenet-sadr]) will ensure that packets exit the home at the appropriate egress based on the associated delegated prefix for that link.

An IPv6 Segment Routing enabled home provides the possibility for imposition of a Segment List by end-hosts in the home, or a customer edge router in the home. The semantic of the data included in the Segment List is translated into an IPv6 address. If the Segment List is enabled at the customer edge router, that router is responsible for classifying traffic and inserting the appropriate Segment List. If hosts in the home have explicit source selection rules (see [I-D.lepape-6man-prefix-metadata]), classification can be based on source address or associated network egress point, avoiding the need for DPI-based implicit classification techniques. If the Segment List is inserted by the host itself, it is important to know which networks can interpret the SR extension header. This information can be provided as part of host configuration as a property of the configured IP address (see [I-D.bhandari-dhc-class-based-prefix]).

The ability to steer traffic to an appropriate egress or utilize a specific type of media (e.g., low-power, WIFI, wired, femto-cell, bluetooth, MOCA, HomePlug, etc.) within the home itself are obvious cases which may be of interest to an application running within a home network.

Steering to a specific egress point may be useful for a number of reasons, including:

Information included in the Segment List, whether imposed by the end-host itself, a customer edge router, or within the access network of the ISP, may be of use at the far ends of the data communication as well. For example, an application running on an end-host with application-support in a data center can utilize the Segment List as a channel to include information that affects its treatment within the data center itself, allowing for application-level steering and load-balancing without relying upon implicit application classification techniques at the data-center edge. Further, as more and more application traffic is encrypted, the ability to extract (and include in the Segment List) just enough information to enable the network and data center to load-balance and steer traffic appropriately becomes more and more important.

2.2. IPv6 Segment Routing in the Access Network

Access networks deliver a variety of types of traffic from the service provider's network to the home environment and from the home towards the service provider's network.

For bandwidth management or related purposes, the service provider may want to associate certain types of traffic to specific physical or logical downstream capacity pipes.

This mapping is not the same thing as classification and scheduling. In the Cable access network, each of these pipes are represented at the DOCCIS layer as different service flows, which are better identified as differing data links. As such, creating this separation allows an operator to differentiate between different types of content and perform a variety of differing functions on these pipes, such as egress vectoring, byte capping, regulatory compliance functions, and billing.

In a cable operator's environment, these downstream pipes could be a specific QAM, a DOCSIS service flow or a service group.

Similarly, the operator may want to map traffic from the home sent towards the service provider's network to specific upstream capacity pipes. Information carried in a packet's SR header could provide the target pipe for this specific packet. The access device would not need to know specific details about the packet to perform this mapping; instead the access device would only need to know how to map the SR SID value to the target pipe.

2.3. IPv6 Segment Routing in the Data Center

A key use case for SR is to cause a packet to follow a specific path through the network. One can think of the service performed at each SR node to be forwarding. Forwarding is one such service provided by an SR node. More complex services could be applied to the packet by an SR node including accounting, IDS, load balancing, and fire walling. "Service chaining" is the name given to the mechanism where these more complicated services are executed in a specific order for a target set of packets. A service provider may choose to have these services performed external to the routing infrastructure, specifically on either dedicated physical servers or within VMs running on a virtualization platform.

To support service chaining, an SR header could then be used to detail the set of forwarding or services to be applied to the packet by creating an SR header with the desired sequence of service IDs to be applied to the packet.

Note that a service, operating on a physical server or within a VM, might not be directly connected to an SR aware router. In fact multiple non-SR aware routers might exist between the service and the nearest SR router. Encoding the SIDs as ipv6 addresses allows benefiting from SID SR header compaction.

When a DC offers infrastructure as a service to multiple tenants, maintaining tenant traffic separation is a key requirement. This can be supported without requiring the DC to run a flat layer 2 network segmented with VLANs or to build an overlay like solution (e.g. VXLAN). Instead, multi-tenant separation can be performed using an SR header where the outer IPv6 DA is the remote hypervisor IP and the SR header contains an identifier of the virtual interface on that hypervisor that logically connects to the target remote VM.

2.4. IPv6 Segment Routing in the Content Delivery Networks

The rise of online video applications and new, video-capable IP devices has led to an explosion of video traffic traversing network operator infrastructures. In the drive to reduce the capital and operational impact of the massive influx of online video traffic, as well as to extend traditional TV services to new devices and screens, network operators are increasingly turning to Content Delivery Networks (CDNs).

Several studies showed the benefits of connecting caches in a hierarchical structure following the hierarchical nature of the Internet. In a cache hierarchy one cache establishes peering relationships with its neighbor caches. There are two types of relationship: parent and sibling. A parent cache is essentially one level up in a cache hierarchy. A sibling cache is on the same level. Multiple levels of hierarchy are commonly used in order to build efficient caches architecture.

In an environment, where each single cache system can be uniquely identified by its own IPv6 address, a Segment List containing a sequence of the caches in a hierarchy can be built. At each node (cache) present in the Segment List a TCP session to port 80 is established and if the requested content is found at the cache (cache hits scenario) the sequence ends, even if there are more nodes in the list.

To achieve the behavior described above, in addition to the Segment List, which specifies the path to be followed to explore the hierarchic architecture, a way to instruct the node to take a specific action is required. The function to be performed by a service node can be carried into a new header called Network Service Header (NSH) defined in [I-D.quinn-sfc-nsh]. A Network Service Header (NSH) is metadata added to a packet that is used to create a service plane. The service header is added by a service classification function that determines which packets require servicing, and correspondingly which service path to follow to apply the appropriate service.

In the above example the service to be performed by the service node was to establish a TCP session to port 80, but in other scenarios different functions may be required. Another example of action to be taken by the service node is the capability to perform transformations on payload data, like real-time video transcode option (for rate and/or resolution).

The use of Segment Routing together with the NSH allows building flexible service chains where the topological information related to the path to be followed is carried into the Segment List while the "service plane related information" (function/action to be performed) is encoded in the metadata, carried into the NSH. The details about using Segment Routing together with NSH will be described in a separate document.

2.5. IPv6 Segment Routing in the Core networks

MPLS is a well-known technology widely deployed in many IP core networks. However there are some operators that do not run MPLS everywhere in their core network today, thus moving forward they would prefer to have an IPv6 native infrastructure for the core network.

While the overall amount of traffic offered to the network continues to grow and considering that multiple types of traffic with different characteristics and requirements are quickly converging over single network architecture, the network operators are starting to face new challenges.

Many operators are looking at the possibility to setup an explicit path based on the IPv6 source address for specific types of traffic in order to efficiently use their network infrastructure. In case of IPv6 some operators are currently assigning or plan to assign IPv6 prefix(es) to their IPv6 customers based on regions/geography, thus the subscriber's IPv6 prefix could be used to identify the region where the customer is located. In such environment the IPv6 source address could be used by the Edge nodes of the network to steer traffic and forward it through a specific path other than the optimal path.

The need to setup a source-based path, going through some specific middle/intermediate points in the network may be related to different requirements:

All these scenarios would require a form of traffic engineering capabilities in IP core networks not running MPLS and not willing to run it.

IPv4 protocol does not provide such functionalities today and it is not the intent of this document to address the IPv4 scenario, both because this may create a lot of backward compatibility issues with currently deployed networks and for the security issues that may raise.

The described use cases could be addressed with the SR architecture applied to the ipv6 data-plane and by having the Edge nodes of network to impose a Segment List on specific traffic flows, based on certain classification criteria that would include source IPv6 address.

3. Acknowledgements

The authors would like to thank Brian Field for his valuable comments and inputs to this document.

4. IANA Considerations

This document does not require any action from IANA.

5. Security Considerations

There are a number of security concerns with source routing at the IP layer [RFC5095]. The new IPv6-based routing header will be defined in way that blind attacks are never possible, i.e., attackers will be unable to send source routed packets that get successfully processed, without being part of the negations for setting up the source routes or being able to eavesdrop legitimate source routed packets. In some networks this base level security may be complemented with other mechanisms, such as packet filtering, cryptographic security, etc.

6. Informative References

[I-D.bhandari-dhc-class-based-prefix] Systems, C., Halwasia, G., Gundavelli, S., Deng, H., Thiebaut, L., Korhonen, J. and I. Farrer, "DHCPv6 class based prefix", Internet-Draft draft-bhandari-dhc-class-based-prefix-05, July 2013.
[I-D.filsfils-rtgwg-segment-routing] Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., Ytti, S., Henderickx, W., Tantsura, J. and E. Crabbe, "Segment Routing Architecture", Internet-Draft draft-filsfils-rtgwg-segment-routing-01, October 2013.
[I-D.filsfils-rtgwg-segment-routing-use-cases] Filsfils, C., Francois, P., Previdi, S., Decraene, B., Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., Ytti, S., Henderickx, W., Tantsura, J., Kini, S. and E. Crabbe, "Segment Routing Use Cases", Internet-Draft draft-filsfils-rtgwg-segment-routing-use-cases-02, October 2013.
[I-D.filsfils-spring-segment-routing-mpls] Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., Ytti, S., Henderickx, W., Tantsura, J. and E. Crabbe, "Segment Routing with MPLS data plane", Internet-Draft draft-filsfils-spring-segment-routing-mpls-00, October 2013.
[I-D.ietf-mpls-seamless-mpls] Leymann, N., Decraene, B., Filsfils, C., Konstantynowicz, M. and D. Steinberg, "Seamless MPLS Architecture", Internet-Draft draft-ietf-mpls-seamless-mpls-06, February 2014.
[I-D.lepape-6man-prefix-metadata] Pape, M., Systems, C. and I. Farrer, "IPv6 Prefix Meta-data and Usage", Internet-Draft draft-lepape-6man-prefix-metadata-00, July 2013.
[I-D.quinn-sfc-nsh] Quinn, P., Guichard, J., Fernando, R., Surendra, S., Smith, M., Yadav, N., Agarwal, P., Manur, R., Chauhan, A., Elzur, U., McConnell, B. and C. Wright, "Network Service Header", Internet-Draft draft-quinn-sfc-nsh-02, February 2014.
[I-D.troan-homenet-sadr] Troan, O. and L. Colitti, "IPv6 Multihoming with Source Address Dependent Routing (SADR)", Internet-Draft draft-troan-homenet-sadr-01, September 2013.
[RFC5095] Abley, J., Savola, P. and G. Neville-Neil, "Deprecation of Type 0 Routing Headers in IPv6", RFC 5095, December 2007.

Authors' Addresses

John Brzozowski Comcast EMail: john_brzozowski@cable.comcast.com
John Leddy Comcast EMail: John_Leddy@cable.comcast.com
Ida Leung Rogers Communications 8200 Dixie Road Brampton, ON L6T 0C1 CANADA EMail: Ida.Leung@rci.rogers.com
Stefano Previdi Cisco Systems Via Del Serafico, 200 Rome, 00142 Italy EMail: sprevidi@cisco.com
Mark Townsley Cisco Systems EMail: townsley@cisco.com
Christian Martin Cisco Systems EMail: martincj@cisco.com
Clarence Filsfils Cisco Systems Brussels, BE EMail: cfilsfil@cisco.com
Roberta Maglione Cisco Systems 181 Bay Street Toronto, M5J 2T3 Canada EMail: robmgl@cisco.com