IRTF D. King
Internet-Draft Lancaster University
Intended status: Informational A. Farrel
Expires: May 12, 2022 Old Dog Consulting
November 8, 2021
Challenges for the Internet Routing Infrastructure Introduced by
Semantic Routing
draft-king-irtf-challenges-in-routing-04
Abstract
Historically, the meaning of an IP address has been to identify an
interface on a network device. Routing protocols were developed
based on the assumption that a destination address had this semantic.
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.
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".
This document describes the challenges to the existing routing system
that are introduced by Semantic Routing. It then summarizes the
opportunities for research into new or modified routing protocols to
make use of new or additional semantics.
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.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Current Challenges to IP Routing . . . . . . . . . . . . . . 4
3. What is Semantic Routing? . . . . . . . . . . . . . . . . . . 6
3.1. Architectural Considerations . . . . . . . . . . . . . . 7
4. Challenges for Internet Routing Research . . . . . . . . . . 8
4.1. Research Principles . . . . . . . . . . . . . . . . . . . 9
4.2. Routing Research Questions to be Addressed . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 13
9. Informative References . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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.
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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.
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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
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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
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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.
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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:
o Address consumption caused by lower address utility rate. The
wastage mainly comes from aligning finite allocation for semantic
address blocks.
o Encoding too many semantics into prefixes will require evaluation
of which to prioritize.
o Risk of privacy/information leakage.
o Lack of visibility of the semantic routing information when end-
to-end or edge-to-edge encryption is used.
o Burdening the user, application, or prefix assignment node.
o Source address spoofing preventing mechanisms may be required.
o Overloading of routing protocols causing stability and scaling
problems.
o 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.
o Backwards compatibility with existing IP-based networking.
3.1. Architectural Considerations
Semantic data may be applied in a number of ways to integrate with
existing routing architectures. An overlay can be built such that
semantic routing is used to route between nodes in the overlay, but
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regular IP is used in the underlay. The application of semantics may
also be constrained to within a limited domain. In some cases, such
a domain will use IP, but be disconnected from Internet. In other
cases, traffic from within the domain is exchanged with other domains
that are connected together across an IP-based network using tunnels
or via application gateways. And in still another case traffic from
the domain is routed across the Internet to other nodes and this
requires backward-compatible routing approaches.
Isolated Domains: Some IP network domains are entirely isolated from
the Internet and other IP-based networks. In these cases, there
is no risk to external networks from any semantic routing schemes
carried out within the domain. Thus, the challenges are limited
to enabling the desired function within the domain.
Bridged Domains: In some deployments, it will be desirable to
connect together a number of isolated domains to build a larger
network. These domains may be connected (or bridged) over an IP
network or even over the Internet possibly using tunnels. An
alternative to tunneling is achieved using gateway functionality
where packets from a domain are mapped at the domain boundary to
produce regular IP packets that are sent across the IP network.
Semantic Prefix Domains: A semantic prefix [RFC8799] domain is a
portion of the Internet over which a consistent set of semantic-
based policies are administered in a coordinated fashion. This is
achieved by assigning a routable address prefix (or a set of
prefixes) for use with semantic routing so that packets may be
routed through the regular IP network (or the Internet). Once
delivered to the semantic prefix domain, a packet can be subjected
to whatever semantic routing is enabled in the domain.
Further discussion of architectures for semantic routing can be found
in [I-D.farrel-irtf-introduction-to-semantic-routing].
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.
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4.1. Research Principles
Research into semantic routing should be founded on regular
scientific research principles [royalsoc]. Given the importance of
the Internet today, it is critical that research is targeted,
rigorous, and reproducible.
The most valuable research will go beyond an initial hypothesis, a
report of the work done, and the results observed. Although that is
a required foundation, networking research needs to be independently
reproducible so that claims can be verified or falsified. Further,
the networks on which the research is carried out need to both
reflect the characteristics that are being explicitly tested, and
reproduce the variety of real networks that constitute the Internet.
Thus, when conducting experiments and research to address the
questions in Section 4.2, attention should be given to how the work
is documented and how meaningful the test environment is, with a
strong emphasis on making it possible for others to reproduce and
validate the work.
4.2. Routing Research Questions to be Addressed
As research into the scenarios and possible uses of semantic routing
progresses, a number of questions need to be answered. These
questions go beyond "Why do we need this function?" and "What could
we achieve by carrying additional semantic in an IP address?" The
questions are also distinct from issues of how the additional
semantics can be encoded within an IP address. All of those issues
are, of course, important considerations in the debate about semantic
routing, but they form only part of the essential groundwork of
research into semantic routing itself.
This section sets out some of the concerns about how the wider
routing system might be impacted by the use of semantic routing.
These questions need to be answered in separate research work or
folded into the discussion of each semantic routing proposal.
1. What is the scope of the semantic routing proposal? This
question may be answered as:
Global: It is intended to apply to all uses of IP.
Backbone: It is intended to apply to IP-based network
connectivity.
Overlay: It is to be used as an overlay network over previous
uses of IP or other underlay technologies using tunneling.
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Gateway: The semantic routing will be used within a limited
domain, and communications with the wider Internet will be
handled by a protocol or application gateway.
Domain: The use of the semantic routing is entirely limited to
within a domain or private network.
Underlying this question is a broader question about the
boundaries of the use of IP, and the limit of "the Internet". If
a limited domain is used, is it a semantic prefix domain
[RFC8799] where a part of the IP address space identifies the
domain so that an address is routable to the domain, but the
additional semantics are used only within the domain, or is the
address used exclusively within the domain so that the external
impact of the routability of the address and the additional
semantics is not important?
2. What will be the impact on existing routing systems? What would
happen if a packet carrying additional semantics was subjected to
normal routing operations? How would the existing routing
systems react if such a packet escaped (accidentally or
maliciously) from the planned scope of the proposal? For
example: how are cryptographically generated addresses made
routable?; how are the semantic parts of an address distinguished
from the routable parts?; is there an impact on the size and
maintenance of routing tables due to the addition of semantics?
3. What path characteristics are needed for the routed paths? Since
one of the purposes of adding semantics to the IP packets is to
cause special processing by routers, it is important to
understand what behaviors are wanted. Such path characteristics
include (but are not limited to):
Quality: Expressed in terms of throughput, latency, jitter, drop
precedence, etc.
Resilience: Expressed in terms of survival of network failures
and delivery guarantees
Destination: How is a destination address to be interpreted if
it encodes a choice of actual destinations?
Security: What choices of path reduce the vulnerability of the
traffic to security or privacy attacks?
In these cases, how do the routers utilize the additional
semantics to determine the desired characteristics? What
additional information about the network do the routing protocols
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need to gather? What changes to the routing algorithm is needed
to deliver packets according to the desired characteristics?
4. Can we solve these routing challenges with existing routing tools
and methods? We can break this question into a set of more
detailed questions.
* Is new hardware needed? Existing deployed hardware has
certain assumptions about how forwarding is carried out based
on IP addresses and routing tables. But hardware is
increasingly programmable so that it may be possible to
instruct the forwarding components to act on a variety of
elements of the packets.
* Do we need new routing protocols? We might ask some
subsidiary questions:
+ Can we make do with existing protocols, possibly by tuning
configuration parameters or using them out of the box?
+ Can we make simple backward-compatible modifications to
existing protocols such that they work for today's IP
addresses as well as enhanced-semantics?
+ Do we need entirely new protocols or radical evolutions of
existing protocols in order to deliver the functions that
we need?
+ Should we focus on the benefits of optimized routing
solutions, or should we attempt to generalize to enable
wider applicability?
Do we need new management tools and techniques? Management of
the routing system (especially diagnostic management) is a
crucial and often neglected part of the problem space.
5. What is the scalability impact for routing systems? Scalability
can be measured as:
* Routing table size. How many entries need to be maintained in
the routing table? Some approaches to semantic routing may be
explicitly intended to address this problem.
* Routing performance. Routing performance may be considered in
terms of the volume of data that has to be exchanged both to
establish and to maintain the routing tables at the
participating routers. It may also be measured in terms of
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how much processing is required to derive new routes when
there is a change in the network routing information.
* Routing convergence is the time that it takes for a routing
protocol to discover changes (especially faults) in the
network, to distribute the information about any changes to
the network, and to reach a stable state across the network
such that packets are routed consistently.
For all questions of routing scalability, research that presents
real numbers based on credible example networks is highly
desirable. Similar questions may be asked about the amount of
forwarding state that has to be maintained in the routers.
6. To what extent can multicast be developed:
* To support programmable SDN systems such as P4 [P4]?
* To satisfy end-to-end applications?
* To apply per-packet multicasting to develop new services?
* As a separate network layer distinct from IP or by encoding
group destinations into IP addresses?
7. What aspects need to be standardized? It is really important to
understand the necessity of standardization within this research.
What degree of interoperability is expected between devices and
networks? Is the limited domain so constrained (for example, to
a single equipment vendor) that standardization would be
meaningless? Is the application so narrow (for example, in niche
hardware environments) such that interoperability is best handled
by agreements among small groups of vendors such as in industry
consortia?
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
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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).
8. Contributors
Joanna Dang
Email: dangjuanna@huawei.com
9. Informative References
[I-D.farrel-irtf-introduction-to-semantic-routing]
Farrel, A. and D. King, "An Introduction to Semantic
Routing", draft-farrel-irtf-introduction-to-semantic-
routing-00 (work in progress), November 2021.
[I-D.king-irtf-semantic-routing-survey]
King, D. and A. Farrel, "A Survey of Semantic Internet
Routing Techniques", draft-king-irtf-semantic-routing-
survey-02 (work in progress), June 2021.
[P4] P4 and ONF, "P4 Open Source Programming Language", Web
page, Programming Protocol-independent Packet Processors
(P4), 2021, .
[RFC4984] Meyer, D., Ed., Zhang, L., Ed., and K. Fall, Ed., "Report
from the IAB Workshop on Routing and Addressing",
RFC 4984, DOI 10.17487/RFC4984, September 2007,
.
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[RFC8799] Carpenter, B. and B. Liu, "Limited Domains and Internet
Protocols", RFC 8799, DOI 10.17487/RFC8799, July 2020,
.
[royalsoc]
The Royal Society, "Evidence synthesis : Principles", Web
page, Principles for good evidence synthesis, September
2018, .
Authors' Addresses
Daniel King
Lancaster University
UK
Email: d.king@lancaster.ac.uk
Adrian Farrel
Old Dog Consulting
UK
Email: adrian@olddog.co.uk
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