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Traffic Accounting in Segment Routing Networks

Document Type Active Internet-Draft (individual)
Authors Zafar Ali , Clarence Filsfils , Ketan Talaulikar , Siva Sivabalan , Martin Horneffer , Robert Raszuk , Stephane Litkowski , Daniel Voyer , Rick Morton
Last updated 2022-05-08
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SPRING Working Group                                      Z. Ali 
Internet-Draft                                       C. Filsfils 
Intended status: Informational                     K. Talaulikar 
Expires: November 8, 2022                    Cisco Systems, Inc.
                                                  Siva Sivabalan  
                                               Ciena Corporation 
                                                    M. Horneffer           
                                                Deutsche Telekom 
                                                       R. Raszuk 
                                         NTT Network Innovations
                                                    S. Litkowski 
                                        Orange Business Services 
                                                        D. Voyer 
                                                       R. Morton 
                                                     Bell Canada 
                                                        G. Dawra
                                                     May 8, 2022 
         Traffic Accounting in Segment Routing Networks

Status of this Memo 

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without warranty as described in the Simplified BSD License. 
Capacity planning is the continuous art of forecasting traffic 
load and failures to evolve the network topology, its capacity, 
and its routing to meet a defined Service-Level Agreement (SLA). 
This document takes a holistic view of network capacity planning 
and identifies the role of traffic accounting in network 
operation and capacity planning, without creating any additional 
states in the SR fabric. 
Table of Contents 

1  Introduction...................................................2 
2  SR Traffic Counters............................................4 
3  SR Traffic Matrix (TM).........................................4 
   3.1  TM Border ................................................4 
   3.1  Choosing TM Border .......................................5 
   3.2  Deriving Demand Matrix ...................................5 
   3.1  Traffic Matrix Counters ..................................5 
4  Internet Protocol Flow Information Export (IPFIX)..............6 
5  Segment Routing Traffic Accounting.............................6 
6  Security Considerations........................................8 
7  IANA Considerations............................................8 
8  References.....................................................8 
   8.1  Normative References .....................................8 
9  Acknowledgments................................................9 
10   Contributors ................................................9 

1  Introduction 
Capacity planning is the continuous art of forecasting traffic 
load and failures to evolve the network topology, its capacity, 
and its routing to meet a defined Service-Level Agreement (SLA). 
This document takes a holistic view of traffic accounting and 
its role in operation and capacity planning in Segment Routing 
(SR) networks.  
One of the main architecture principles of Segment Routing (SR) 
is that it maintains per-flow states only at the ingress nodes 

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to the SR domain. The approach taken in this document respects 
the architecture principles of SR, i.e., this draft does not 
create any additional control and data plane states at the 
ingress, transit or egress node for traffic accounting. Only the 
ingress node of an SR policy maintains per-flow counters for 
traffic accounting, which are also needed for other use-cases 
like billing.  
The Traffic Matrix (TM) is one of the main components of the 
holistic approach to traffic accounting taken in this document. 
A network's traffic matrix is the volume of aggregated traffic 
flows that enter, traverse and leave an arbitrarily defined 
boundary in the network over a given time interval.  The TM 
border defines the arbitrary boundary nodes of a contiguous 
portion of the network across which service providers wish to 
measure traffic flows. The TM border defined for traffic matrix 
collection does not have to be at the edge of the network, e.g., 
it can also be placed at the aggregation layer. Knowledge of the 
traffic matrix is essential to efficient and effective planning, 
design, engineering, and operation of any IP or MPLS network.  
[I-D.draft-ietf-spring-segment-routing-policy] defines the 
traffic matrix counters for accounting at the router. This draft 
describes how these counters simplify traffic matrix collection 
process. [I-D.draft-ietf-spring-segment-routing-policy] also 
specifies policy, prefix-SID and interface counters for 
accounting in an SR network. This document along with the 
traffic counters defined in [I-D.draft-filsfils-spring-segment-
routing-policy] constitute the holistic view of traffic 
accounting in an SR network.  
This document assumes that the routers export the traffic 
counters defined in [I-D.draft-filsfils-spring-segment-routing-
policy] to an external controller. It is also assumed that the 
controller also collects the following information in order to 
get the visibility required for traffic accounting: 
  - Network topology information indicates all the nodes and their inter-
     connecting links (e.g. via BGP-LS [RFC7752]). 
  - SR Policies instantiated at various node and their BSID (e.g. using 
     PCEP as in RFC8231 or BGP-LS as in draft-ietf-idr-te-lsp-distribution). 
  - Aggregate traffic counters and statistics for links that include link 
     utilization, per TC statistics, drop counters, etc. 
  - IPFIX data and the flow accounting information derived from it from an 
     IPFIX collector. 
The methods for collection of this information by the controller 
is beyond the scope of the document.  

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  2  SR Traffic Counters 
[I-D.draft-ietf-spring-segment-routing-policy] specifies SR 
counters that form building blocks on accounting in SR networks. 
Listing all counters in this document is not the goal of this 
draft. Some of those counters are outlined  below:  
- Per-prefix SID egress traffic counter (PSID.E) 
For a remote prefix SID M, this counter accounts for the 
aggregate traffic forwarded towards M.  
- Per-prefix SID per-TC egress traffic counter (PSID.E.TC) 
This counter provides per Traffic Class (TC) breakdown of 
- Per-SR Policy Aggregate traffic counter (POL) 
This counter accounts for both labelled and unlabeled traffic 
steered on an SR policy (P). This counter is only maintained by 
the head-end node.  
Traffic matrix counters are outlined in the traffic matrix 

  3  SR Traffic Matrix (TM)  
A traffic matrix T(N, M) is the amount of traffic entering the 
network at node N and leaving the network at node M, where N and 
M are border nodes at an arbitrarily defined boundary in the 
network. The TM border defines the arbitrary boundary nodes of a 
contiguous portion of the network across which service providers 
wish to measure traffic flows. The traffic matrix (also called 
demand matrix) contains all the demands crossing the TM border. 
It has as many rows as ingress edge nodes and as many columns as 
egress edge nodes at the TM border. The demand D(N, M) is the 
cell of the matrix at row N and column M.  
  3.1 TM Border 
The service provider needs to establish Traffic Matrix (TM) 
border to collect traffic matrix. The TM border defines the 
boundary nodes of a contiguous portion of the network across 

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which the service provider wishes to measure traffic flows. The 
TM border divides the network into two parts: 
-    Internal part: a contiguous part of the network that is 
located within the TM border. 
-    External part: anything outside of the TM border 
The TM border cuts through nodes, resulting in two types of 
interfaces: internal and external interfaces. Interfaces are 
internal if they are located inside the TM border, they are 
external if they are found outside the TM border. 
How a node marks it interfaces as external or internal is an 
implementation matter and beyond the scope of this document.  
  3.1 Choosing TM Border 
An operator can choose where the TM border is located. 
Typically, this will be at the edge of the network, but it can 
also be placed at the aggregation layer. Or an operator can use 
multiple TM borders for each of their network domains, with each 
TM border cutting through different nodes; different TM borders 
cannot cut through the same nodes. 
  3.2 Deriving Demand Matrix 
The goal is to measure the volume of traffic that enters a TM 
border node n through an external interface and leaves through 
an external interface of another TM border node m. This traffic 
volume yields the traffic matrix entry T  . Measuring this for 
every pair of TM border nodes (n,m) results in the complete 
traffic matrix. 
Service providers use various techniques to compute traffic 
matrix, including a combination of collecting link utilization, 
gathering IPFIX data, collect MPLS forwarding statistics, etc.  
A service provider may also use traffic matrix counters defined 
in [I-D.draft-ietf-spring-segment-routing-policy] for this 
purpose. The usefulness and applicability of the Traffic Matrix 
do not depend on the TM collection mechanism.   
  3.1 Traffic Matrix Counters 
Traffic Matrix counters are defined in [I-D.draft-filsfils-
spring-segment-routing-policy]. The TM counters are summarized 
in the following for completeness.   

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When Node N receives a packet, N maintains the following 
-    Per-Prefix SID Traffic Matrix counter (PSID.E.TM) 
For a given remote prefix SID M, this counter accounts for all 
the traffic received on any external interfaces and forwarded 
towards M.  
- Per-Prefix, Per TC SID Traffic Matrix counter (PSID.E.TM.TC) 
This counter provides per Traffic Class (TC) breakdown of 

  4  Internet Protocol Flow Information Export (IPFIX) 
Internet Protocol Flow Information Export (IPFIX) [RFC 7011]- 
[RFC7015] is a standard of export for Internet Protocol flow 
information.  IPFIX is extensively deployed and used by network 
management systems to facilitate services such as measurement, 
security, accounting and billing. IPFIX also plays a vital role 
in traffic accounting in SR network. For example, IPFIX can be 
used for traffic accounting on an SR policy, without requiring 
any change to the SR-MPLS or IPFIX protocols.  

  5  Segment Routing Traffic Accounting  
The SR counters, IPFIX data, Traffic Matrix, network topology 
information, node, and link statistics, SR policies 
configuration and various SR counters described in [I-D.draft-
ietf-spring-segment-routing-policy], etc. constitute 
components of SR traffic accounting. This section describes some 
potential use of this information, but other mechanisms also 
One of the possible uses is centered around the traffic matrix.  
An external controller collects the traffic counters, including 
the traffic matrix, defined in [I-D.draft-filsfils-spring-
segment-routing-policy] from the routers. Using the Traffic 
Matrix TM(N, M), the controller knows the exact traffic is 
entering node N and leaving node M, where node N and M are edge 
node on an arbitrary TM border. The controller also collects 
network topology and SR policies configuration from the network. 

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Using this information, the controller runs local path 
calculation algorithm to map these demands onto the individual 
SR paths. This enables a controller to determine the path that 
would be taken through the network (including ECMP paths) for 
any prefix at any node. Specifically, the controller starts with 
distributing the TM(N, M) equally among all ECMP from node N to 
node M. By repeating the process for all entry and exit nodes in 
the network, the controller predicts how the demands are 
distributed among SR paths in the network. The equal 
distribution of the traffic demand assumption is validated by 
correlating the projected load with the link and node statistics 
and other traffic counters described in [I-D.draft-filsfils-
spring-segment-routing-policy]. Specifically, the various SR 
counters described in [I-D.draft-filsfils-spring-segment-
routing-policy] provide the view of each segment's ingress and 
egress statistics at every node and link in the network, which 
is further supplemented by SR Policies' statistics that are 
available at all head-end nodes. The uses this information to 
adjust the predicted load, accordingly. How such adjustments are 
performed is beyond the scope of this document. The predicted 
traffic mapping to the individual SR path may be used for serval 
purposes. That includes simulating what-if scenarios, develop 
contingency and maintenance plans, manage network latency and to 
anticipate and prevent congestion, etc. For example, if there is 
congestion on the link between two nodes, the controller can 
identify the SR path causing the congestion and how to re-route 
it to relieve it. 
Another possible use is built around the IPFIX data. IPFIX can 
be used for traffic account on an SR policy, without requiring 
any change to the SR-MPLS or IPFIX protocols. It provides a more 
granular visibility of network flows (including SR Policy flows) 
at any point in the network that can be correlated. For example, 
IPFIX may be enabled on the nodes and links at the traffic 
matrix border nodes to analyze the flows entering and leaving a 
specific network region. Additionally, it can be also enabled at 
any node or a specific link within the network for analyzing 
flows through it either on demand or continuously basis. IPFIX 
can also be enabled on the head-end nodes and endpoints of SR 
Policies in the network to analyze flows steered through various 
policies. When traffic is steered on an SR policy, the steering 
is based on a match of the fields of the incoming packet. A 
controller can replicate the matching criteria to account for 
the traffic received at the egress for the given SR policy. The 
policy counters, other traffic counters defined in 
[I-D.draft-ietf-spring-segment-routing-policy], and information of 
packet loss over policy can further supplement the IPFIX based 

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accounting for measuring, accounting, and billing on per policy 
basis. Since IPFIX sampling also includes the MPLS label stack 
on the packet and the underlying payload, the traffic flows for 
a specific SR policy can also be determined at any intermediate 
link or node in the network, if necessary. 
Link level statistics information, derived using the ingress and 
egress counters (including the QoS counters on a per TC basis), 
provides the view of link utilization including for a specific 
class of service at any point. This helps detect congestion for 
the link as a whole or for specific class of service.  
In summary, a controller can use the holistic view of traffic 
accounting provided in this document to predicted traffic 
mapping to the individual SR paths. The aggregate demands on the 
network and their paths can be determined and correlated with 
link utilization to identify the flows causing congestion for 
specific links. Further visibility into all the flows on a link 
can be achieved using the SR counters and supplemented by IPIX 

6  Security Considerations 
This document does not define any new protocol extensions and 
does not impose any additional security challenges.  

7  IANA Considerations 
This document does not define any new protocol or any extension 
to an existing protocol.   

  8  References 
  8.1 Normative References 
     Filsfils, C., et al.,  "Segment Routing Policy for Traffic 
     Engineering", draft-ietf-spring-segment-routing-policy 
     (work in progress), . 
  8.2. Informative References 
[RFC7011] Specification of the IP Flow Information Export (IPFIX) Protocol for 
     the Exchange of Flow Information. B. Claise, Ed., B. Trammell, Ed., 
     P. Aitken. September 2013. (Format: TXT=170852 bytes) (Obsoletes 
     RFC5101) (Also STD0077) (Status: INTERNET STANDARD) (DOI: 
[RFC7012] Information Model for IP Flow Information Export (IPFIX). B. Claise, 

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     Ed., B. Trammell, Ed.. September 2013. (Format: TXT=50237 bytes) 
     (Obsoletes RFC5102) (Status: PROPOSED STANDARD) (DOI: 
[RFC7013] Guidelines for Authors and Reviewers of IP Flow Information Export 
     (IPFIX) Information Elements. B. Trammell, B. Claise. September 
     2013. (Format: TXT=76406 bytes) (Also BCP0184) (Status: BEST CURRENT 
     PRACTICE) (DOI: 10.17487/RFC7013)  
[RFC7014] Flow Selection Techniques. S. D'Antonio, T. Zseby, C. Henke, L. 
     Peluso. September 2013. (Format: TXT=72581 bytes) (Status: PROPOSED 
     STANDARD) (DOI: 10.17487/RFC7014)  
[RFC7015] Flow Aggregation for the IP Flow Information Export (IPFIX) 
     Protocol. B. Trammell, A. Wagner, B. Claise. September 2013. 
     (Format: TXT=112055 bytes) (Status: PROPOSED STANDARD) (DOI: 
[TM] S. Schnitter, T-Systems; M. Horneffer, T-Com. "Traffic 
Matrices for MPLS Networks with LDP Traffic Statistics. " Proc. 
Networks2004, VDE-Verlag 2004.  

  9  Acknowledgments 
The authors would like to thank Kris Michielsen and Jose Liste 
for their contribution to this document.  

  10 Contributors 
Francois Clad 
Cisco Systems, Inc. 
Faisal Iqbal 
Cisco Systems, Inc. 

Authors' Addresses
Zafar Ali 
Cisco Systems, Inc.  

Clarence Filsfils 
Cisco Systems, Inc. 

Internet-Draft        SR Traffic Accounting 

Ketan Talaulikar 
Cisco Systems, Inc.  
Siva Sivabalan 
Cisco Systems, Inc. 

Martin Horneffer 
Deutsche Telekom 
Robert Raszuk 
Bloomberg LP 
Stephane Litkowski 
Orange Business Services 
Daniel Voyer 
Bell Canada 
Rick Morton 
Bell Canada 
Gaurav Dawra