TCPM                                                           M. Scharf
Internet-Draft                                      Hochschule Esslingen
Intended status: Standards Track                               V. Murgai
Expires: January 8, 2020                               Cisco Systems Inc
                                                            July 7, 2019

  YANG Groupings for Transmission Control Protocol (TCP) Configuration


   This document specifies a YANG model for TCP on devices that are
   configured by network management protocols.  The YANG model defines
   groupings for fundamental parameters that can be modified in many TCP
   implementations.  The model extends a base model for TCP clients and
   servers [I-D.ietf-netconf-tcp-client-server].

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
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   This Internet-Draft will expire on January 8, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Model Overview  . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Modeling Scope  . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Basic TCP Configuration Parameters  . . . . . . . . . . .   5
     3.3.  Model Design  . . . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Tree Diagram  . . . . . . . . . . . . . . . . . . . . . .   7
   4.  TCP Configuration YANG Model  . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  10
   Appendix B.  Changes compared to previous versions  . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The Transmission Control Protocol (TCP) [RFC0793] is used by many
   applications in the Internet, including control and management
   protocols.  Therefore, TCP is implemented on network elements that
   can be configured via network management protocols such as NETCONF
   [RFC6241] or RESTCONF [RFC8040].  This document specifies a YANG
   model [RFC6020][RFC7950] for configuring TCP on network elements that
   support YANG data models.  This document extends a base model for TCP
   clients and servers [I-D.ietf-netconf-tcp-client-server].  The model
   focuses on fundamental and standard TCP functions that are widely
   implemented.  The model can be augmented to address more advanced or
   implementation-specific TCP features.  Operational state and
   statistics are outside the scope of this memo.

   Many protocol stacks on Internet hosts use other methods to configure
   TCP, such as operating system configuration or policies.  Many TCP/IP
   stacks cannot be configured by network management protocols such as
   NETCONF or RESTCONF and they do not use YANG data models.  Yet, such
   TCP implementations often also have means to configure the parameters
   listed in this document.  All parameters defined in this document are

   This specification is orthogonal to a Management Information Base
   (MIB) for the Transmission Control Protocol (TCP) that has been
   standardized [RFC4022].  A MIB providing extended statistics for TCP

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   is also available [RFC4898], and there are also MIBs for UDP
   [RFC4113] and SCTP [RFC3873].  It is possible to translate a MIB into
   a YANG model, for instance using the translation described in
   [RFC6643].  However, this approach is not used in this document, as
   such a translated model would not be up-to-date.

   There are also other related YANG models.  Examples are:

   o  Application protocol models may include TCP parameters, for
      example in case of BGP [I-D.ietf-idr-bgp-model].

   o  TCP header attributes are modeled in other models, such as

   o  TCP-related configuration of a NAT is defined in

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Model Overview

3.1.  Modeling Scope

   TCP is implemented on many different system architectures.  As a
   result, there are may different and often implementation-specific
   ways to configure parameters of the TCP protocol engine.  In
   addition, in many TCP/IP stacks configuration exists for different

   o  Global configuration: Many TCP implementations have configuration
      parameters that affect all TCP connections.  Typical examples
      include the enabling or disabling optional protocol features.

   o  Interface configuration: It can be useful to use different TCP
      parameters on different interfaces, e.g., different device ports
      or IP interfaces.  In that case, TCP parameters can be part of the
      interface configuration.  Typical examples are the Maximum Segment
      Size (MSS) or configuration related to hardware offloading.

   o  Connection parameters: Many implementations have means to
      influence the behavior of each TCP connection, e.g., on the
      programming interface used by applications.  A typical example are

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      socket options in the socket API, such as disabling the Nagle
      algorithm by TCP_NODELAY.  In an application uses such an
      interface, it is possible that the configuration of the
      application or application protocol includes TCP-related
      parameters.  An example is the YANG model for BGP configuration

   o  Policies: Setting of TCP parameters can also be part of system
      policies, templates, or profiles.  An example would be the
      preferences defined in the TAPS interface

   There is no ground truth for setting certain TCP parameters, and
   traditionally different implementation have used different modeling
   approaches.  For instance, one implementation may define a given
   configuration parameter globally, while another one uses per-
   interface settings, and both approaches work well for the
   corresponding use cases.  Also, different systems may use different
   default values.

   In addition to configuration of the TCP protocol engine, a TCP
   implementation typically also offers access to operational state and
   statistics.  This includes amongst others:

   o  Statistics: Counters for the number of active/passive opens, sent
      and received segments, errors, and possibly other detailed
      debugging information

   o  TCP connection table: Access to status information for all TCP

   o  TCP listener table: Tnformation about all TCP listening endpoints

   This document focuses solely on modeling basic TCP configuration
   state.  Operational state (see [RFC8342]) is outside the scope of
   this specification.

   The YANG model defined in this document extends a base model for TCP
   clients and servers [I-D.ietf-netconf-tcp-client-server].  Similar to
   the base model, this specification only defines YANG groupings.  This
   allows reuse of these groupings in different YANG data models.  It is
   intended that these groupings will be used either standalone or for
   TCP-based protocols as part of a stack of protocol-specific
   configuration models.

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3.2.  Basic TCP Configuration Parameters

   There are a number of basic system parameters that are configurable
   on many TCP implementations, even if not all TCP implementations may
   indeed have exactly all these settings.  Also, the syntax, semantics
   and scope (e.g., global or interface-specific) can be different in
   different system architectures.

   The following list of fundamental parameters considers both TCP
   implementations on hosts and on routers:

   o  Keepalives (see also [I-D.ietf-netconf-tcp-client-server])

      *  Idle-time (in seconds): integer

      *  Probe-interval (in seconds): integer

      *  Max-probes: integer

   o  Maximum MSS (in byte): integer

   o  FIN timeout (in seconds): integer

   o  SACK (disable/enable): boolean

   o  Timestamps (disable/enable): boolean

   o  Path MTU Discovery (disable/enable): boolean

   o  ECN

      *  Enabling (disable/passive/active): enumeration

   Some other parameters are also common but not ubiquitously supported,
   or modeled in very different ways.  Therefore, the following
   attributes are not considered in this document:

   o  Delayed ACK timeout (in ms)

   o  Initial RTO value (in ms)

   o  Maximum number of retransmissions

   o  Window scaling

   o  Maximum number of connections

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   TCP can be implemented in different ways and design choices by the
   protocol engine often affect configuration options.  In a number of
   areas there are major differences between different software
   architectures.  As a result, there are not many commonalities in the
   corresponding configuration parameters:

   o  Window size: TCP stacks can either store window state variables
      (such as the congestion window) in segments or in bytes.

   o  Buffer sizes: The memory management depends on the operating
      system.  As the size of buffers can vary over several orders of
      magnitude, very different implementations exist.  This typically
      influences TCP flow control.

   o  Timers: Timer implementation is another area in which TCP stacks
      may differ.

   o  Congestion control algorithms: Many congestion control algorithms
      have configuration parameters, but except for fundamental
      properties they often tie into the specific implementation.

   This document only models fundamental system parameters that are
   configurable on many TCP implementations, and for which the
   configuration is reasonably similar.

3.3.  Model Design

   [[Editor's node: This section requires further work.]]

   This document extends the YANG model "ietf-tcp-common" defined in
   [I-D.ietf-netconf-tcp-client-server].  The exact modeling is TBD.
   The intention is to define YANG groupings for all parameters so that
   they can be used in different YANG models.

   As an example, enabling the support of Selective Acknowledgements
   (SACK) can be modelled as follows:

     grouping tcp-sack-grouping {
       description "Support of Selective Acknowledgements (SACK)";

       leaf sack {
         type boolean;
         default "true";

           "Enable support of Selective Acknowledgements (SACK)";

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   A YANG model could then, for instance, import the YANG model "ietf-
   tcp-common" as well as the model defined in this document as follows:

     grouping example-tcp-config {
       description "Example TCP stack configuration";

       uses tcp-common-grouping;
       uses tcp-sack-grouping;

3.4.  Tree Diagram

   [[Editor's node: This section will be completed in follow-up versions
   of this document.]]

   This section provides a tree diagram [RFC8340] for the YANG module
   defined in this document.

4.  TCP Configuration YANG Model

   [[Editor's node: This section is TBD.]]

5.  IANA Considerations

   [[Editor's node: This section will be completed in follow-up versions
   of this document.]]

6.  Security Considerations

   The YANG module specified in this document defines a schema for data
   that is designed to be accessed via network management protocols such
   as NETCONF [RFC6241] or RESTCONF [RFC8040].  The lowest NETCONF layer
   is the secure transport layer, and the mandatory-to-implement secure
   transport is Secure Shell (SSH) [RFC6242].  The lowest RESTCONF layer
   is HTTPS, and the mandatory-to-implement secure transport is TLS

   The Network Configuration Access Control Model (NACM) [RFC8341]
   provides the means to restrict access for particular NETCONF or
   RESTCONF users to a preconfigured subset of all available NETCONF or
   RESTCONF protocol operations and content.

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7.  References

7.1.  Normative References

              Watsen, K. and M. Scharf, "YANG Groupings for TCP Clients
              and TCP Servers", draft-ietf-netconf-tcp-client-server-02
              (work in progress), July 2019.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,

   [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
              Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,

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   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
              Access Control Model", STD 91, RFC 8341,
              DOI 10.17487/RFC8341, March 2018,

   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore Architecture
              (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

7.2.  Informative References

              Jethanandani, M., Patel, K., and S. Hares, "BGP YANG Model
              for Service Provider Networks", draft-ietf-idr-bgp-
              model-06 (work in progress), June 2019.

              Jethanandani, M., Agarwal, S., Huang, L., and D. Blair,
              "Network Access Control List (ACL) YANG Data Model",
              draft-ietf-netmod-acl-model-21 (work in progress),
              November 2018.

              Boucadair, M., Sivakumar, S., Jacquenet, C., Vinapamula,
              S., and Q. Wu, "A YANG Module for Network Address
              Translation (NAT) and Network Prefix Translation (NPT)",
              draft-ietf-opsawg-nat-yang-17 (work in progress),
              September 2018.

              Trammell, B., Welzl, M., Enghardt, T., Fairhurst, G.,
              Kuehlewind, M., Perkins, C., Tiesel, P., and C. Wood, "An
              Abstract Application Layer Interface to Transport
              Services", draft-ietf-taps-interface-03 (work in
              progress), March 2019.

   [RFC3873]  Pastor, J. and M. Belinchon, "Stream Control Transmission
              Protocol (SCTP) Management Information Base (MIB)",
              RFC 3873, DOI 10.17487/RFC3873, September 2004,

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   [RFC4022]  Raghunarayan, R., Ed., "Management Information Base for
              the Transmission Control Protocol (TCP)", RFC 4022,
              DOI 10.17487/RFC4022, March 2005,

   [RFC4113]  Fenner, B. and J. Flick, "Management Information Base for
              the User Datagram Protocol (UDP)", RFC 4113,
              DOI 10.17487/RFC4113, June 2005,

   [RFC4898]  Mathis, M., Heffner, J., and R. Raghunarayan, "TCP
              Extended Statistics MIB", RFC 4898, DOI 10.17487/RFC4898,
              May 2007, <>.

   [RFC6643]  Schoenwaelder, J., "Translation of Structure of Management
              Information Version 2 (SMIv2) MIB Modules to YANG
              Modules", RFC 6643, DOI 10.17487/RFC6643, July 2012,

Appendix A.  Acknowledgements

   Michael Scharf is supported by the project, which is
   funded by the European Commission under the Horizon 2020 Programme.

Appendix B.  Changes compared to previous versions

   Changes compared to draft-scharf-tcpm-yang-tcp-01

   o  Alignment with [I-D.ietf-netconf-tcp-client-server]

   o  Removing backward-compatibility to the TCP MIB

   o  Additional co-author

   Changes compared to draft-scharf-tcpm-yang-tcp-00

   o  Editorial improvements

Authors' Addresses

   Michael Scharf
   Hochschule Esslingen - University of Applied Sciences
   Flandernstr. 101
   Esslingen  73732


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   Vishal Murgai
   Cisco Systems Inc


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