Challenges for the Internet Routing Infrastructure Introduced by Semantic Routing

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Table of Contents

1. Introduction

Historically, the meaning of an IP address has been to identify an interface on a network device. Routing protocols were developed to determine paths through the network toward destination addresses so that IP packets with a common destination address converged on that destination. Anycast and multicast addresses were also defined and those address semantics necessitated variations to the routing protocols and the development of new protocols.¶

Over time, routing decisions were enhanced to route packets according to additional information carried within the packets and dependent on policy coded in, configured at, or signaled to the routers. Perhaps the most obvious example is Equal-Cost Multipath (ECMP) where a router makes a consistent choice for forwarding packets over a number of parallel links or paths based on the values of a set of fields in the packet header.¶

Many proposals have been made to add semantics to IP packets by placing additional information existing fields, by adding semantics to IP addresses, or by adding fields to the packets. The intent is to facilitate enhanced routing decisions based on these additional semantics in order to provide differentiated paths for different packet flows distinct from simple shortest path first routing. We call this approach "Semantic Routing".¶

There are many approaches to adding semantics to packet headers. These range from assigning an address prefix to have a special purpose and meaning (such as is done for multicast addressing) through allowing the owner of a prefix to use the low-order bits of an address for their own purposes. Some proposals suggest variable address lengths, others offer hierarchical addresses, and some introduce a structure to addresses so that they can carry additional information in a common way. Other approaches perform routing decisions on fields in the packet header (such as the IPv6 Flow Label, or the Traffic Class field), overload packet fields, or add new information to packet headers.¶

A survey of ways in which routing decisions have been made based on additional information carried in packets can be found in [I-D.king-irtf-semantic-routing-survey].¶

Some Semantic Routing proposals are intended to be deployed in limited domains [RFC8799] (networks) that are IP-based, while other proposals are intended for use across the Internet. The impact the proposals have on routing systems may require clean-slate solutions, hybrid solutions, extensions to existing routing protocols, or potentially no changes at all.¶

This document describes some of the key challenges to routing that are present in today's IP networks. It then defines the concept of "Semantic Routing" and presents some of the challenges to the existing routing system that Semantic Routing may present. Finally, this document presents a list of related research questions that offer opportunities for future research into new or modified routing protocols that make use of Semantic Routing.¶

In this document, the focus is on routing and forwarding at the IP layer. It is possible that a variety of overlay mechanisms exist to perform service or path routing at higher layers, and that those approaches may be based on similar extensions to packet semantics, but that is out of scope for this document. Similarly, it is possible that Semantic Routing can be applied in a number of underlay network technologies, and that, too, is out of scope for this document.¶

This document is presented as study to support further research into clarifying and understanding the issues. It does not pass comment on the advisability or practicality of any of the proposals and does not define any technical solutions.¶

2. Current Challenges to IP Routing

Today's IP routing faces several significant challenges which are a consequence of architectural design decisions and the continued exponential growth. These challenges include mobility, multihoming, programmable paths, scalability, and security, and were not the focus of the original design of the Internet. Nevertheless, IP-based networks have, in general, coped well in an incremental manner as each new challenge has evolved. This list is presented to give context to the continuing requirements that routing protocols must meet as new semantics are applied to the routing process.¶

• Mobility - Mobility introduces several challenges, including maintaining a relationship between a sender and a receiver in cases where the sender or receiver changes their point of network attachment. The network must always be informed about the mobile node's current location, to allow continuity of services. Mobility users may also consume network resources, while physically moving. The mobile user's service instances and attachments will also change due to varying load or latency, e.g., in Multi-access Edge Computing (MEC) scenarios.¶
• Multihoming - Multihomed stations or multihomed networks are connected to the Internet via more than one access circuit or access network and, therefore, may be assigned multiple IP addresses from different pools of addresses. There are challenges concerning how traffic is routed back to the source if the source has originated its traffic using the wrong address for a particular connection, or if one of the connections to the Internet is degraded.¶
• Multi-path - The Internet was initially designed to find the single, "best" path to a destination using a distributed routing algorithm. Current, IP-based network topologies facilitate multiple paths each with different characteristics and with different failure likelihoods. It may be beneficial to send traffic over multiple paths to achieve reliability and enhance throughput, and it may be desirable to select one path or another in order to provide delivery qualities or to avoid transiting specific areas of an IP-based network. However, the way in which packets are routed using the best or shortest path means that distinguishing these alternate paths and directing traffic to them can be hard. Further, problems concerning scalability, commercial agreements among Service Providers, and the design of BGP make the utilization of multi-path techniques difficult for inter-domain routing. (Note that this discussion is distinct from Equal Cost Multi-path (ECMP) where packets are directed onto two "parallel" paths of identical least cost using a hash algorithm operated on some of the packets' header fields.)¶
• Multicast - Delivering the same packet to multiple destinations can place considerable load on a network. Solutions that replicate the packet at the source or at the network edge may obviously cause multiple copies of the packet to flow along the same network links. Solutions that move replication into the network to make more optimal use of the network resources can be complex to set up and manage requiring sophisticated protocols that can determine the best multicast delivery topologies, as well as hardware that can replicate packets within the network. In order that packets can be addressed to a group of destinations and not be routed using the normal unicast approaches, parts of the addressing space (that is, address prefixes) have been reserved to indicate multicast.¶
• Programmable Paths - The ability to decouple IP-based network paths from routing protocols and agreements between Service Providers could allow users and applications to configure and select network paths themselves, based on the required path (that is, traffic-delivery) characteristics. Currently, user and application packets follow the path selected by routing protocols and the way traffic is routed through a network is under the exclusive control of the Service Provider that owns the network.¶
• End-Point Selection - As compute resources and content storage move closer to the edge of the network, there are often multiple points in the network that can satisfy user requests. In order to make best use of these distributed services and so as to not overload parts of the network, user traffic needs to be steered to appropriate servers or data centres. In many cases, this function may be achieved in the application layer (such as through DNS) or in the transport layer (such as using ALTO). The challenge is to balance higher-layer decisions about which application layer resources to use with information from the lower layers about the availability and load of network resources.¶
• Scalability - There are many scaling concerns that pose critical challenges to the Internet. Not least among these challenges is the size of the routing tables that routers in an IP-based network must maintain and exchange with their peers. As the number of devices attached to the network grows, so the number of addresses in use also grows, and because of the address allocation schemes, the mobility of devices, and the various connectivity options between networks, the routing table sizes also grow and are not amenable to aggregation. This problem exists even in limited domains (such as IoT), where, as more devices are added to the network, the size of the routing table may be a gating factor in there applicability of certain routing protocols. It may be noted that scaling issues are exacerbated by multihoming practices if a host that is multihomed is allocated a different address for each point of attachment.¶
• Security - Issues of security and privacy have been largely overlooked within the routing systems. However, there is increasing concern that attacks on routing systems can not only be disruptive (for example, causing traffic to be dropped), but may cause traffic to be routed via inspection points that can breach the security or privacy of the payloads.¶

Some of the challenges outlined here were previously considered within the IETF by the IABs "Routing and Addressing Workshop" held in Amsterdam, The Netherlands on October 18-19, 2006 [RFC4984]. Several architectures and protocols have since been developed and worked on within and outside the IETF, and these are examined in [I-D.king-irtf-semantic-routing-survey].¶

3. What is Semantic Routing?

Semantic Routing is the term applied to routing in an IP-based network that enhances decisions by considering information present in the packet and configured or programmed into the routers in addition to the routable part of the destination IP address (the prefix). Semantic Routing includes mechanisms such as "Preferential Routing", "Policy-based Routing", and "Flow steering".¶

In semantic routing, a packet forwarding engine may examine a variety of fields in a packet and match them against forwarding instructions. Those forwarding instructions may be installed by routing protocols, configured through management protocols or as part of a software defined networking (SDN) system, or derived by a software component on the router that considers network conditions and traffic loads. The packet fields concerned may be the normal fields of the IP header, those same fields but with additional semantics, elements of the packet payload, or new fields defined for inclusion in the packet header. In the the case of additional semantics included in existing packet header fields, the approach implies some "overloading" of those fields to include meaning beyond the original definition. In all cases, a well-known definition of the encoding of the additional information is required to enable consistent interpretation within the network.¶

A more detailed description of semantic routing can be found in [I-D.farrel-irtf-introduction-to-semantic-routing] and a survey of semantic routing proposals and research projects can be found in [I-D.king-irtf-semantic-routing-survey].¶

Several technical challenges exist for semantic routing in IP-based network depending on which approach is taken. These include:¶

• Address consumption caused by lower address utility rate. The wastage mainly comes from aligning finite allocation for semantic address blocks.¶
• Encoding too many semantics into prefixes will require evaluation of which to prioritize.¶
• Risk of privacy/information leakage.¶
• Lack of visibility of the semantic routing information when end-to-end or edge-to-edge encryption is used.¶
• Burdening the user, application, or prefix assignment node.¶
• Source address spoofing preventing mechanisms may be required.¶
• Overloading of routing protocols causing stability and scaling problems.¶
• Depending on encoding mechanisms, there may be challenges for data planes to scale the processes of finding, reading, and looking up semantic data in order to forward packets at line speed.¶
• Backwards compatibility with existing IP-based networking.¶

4. Challenges for Internet Routing Research

It may not be possible to embrace all emerging scenarios with a single approach or solution. Requirements such as 5G mobility, near-space-networking, and networking for outer-space, may need to be handled using separate network technologies. Improving IP-based network capabilities and capacity to scale, and address a set of growing requirements presents significant research challenges, and will require contributions from the networking research community. Solutions need to be both economically feasible and have the support of the networking equipment vendors as well as the network operators.¶

5. Security Considerations

Research into semantic routing must give full consideration to the security and privacy issues that are introduced by these mechanisms. Placing additional information into packet header fields might reveal details of what the packet is for, what function the user is performing, who the user is, etc. Furthermore, in-flight modification of the additional information might not directly change the destination of the packet, but might change how the packet is handled within the network and at the destination.¶

It should also be considered how packet encryption techniques that are increasingly popular for end-to-end or edge-to-edge security may obscure the semantic information carried in some fields of the packet header or found deeper in the packet. This may render some semantic routing techniques impractical and may dictate other methods of carrying the necessary information to enable semantic routing.¶

6. IANA Considerations

This document makes no requests for IANA action.¶

7. Acknowledgements

Thanks to Stewart Bryant for useful conversations. Luigi Iannone, Robert Raszuk, Dirk Trossen, Ron Bonica, Marie-Jose Montpetit, Yizhou Li, Toerless Eckert, Tony Li, Joel Halpern, Stephen Farrell, Carsten Bormann, and Greg Mirsky made helpful suggestions.¶

This work is partially supported by the European Commission under Horizon 2020 grant agreement number 101015857 Secured autonomic traffic management for a Tera of SDN flows (Teraflow).¶

Authors' Addresses