Internet Engineering Task Force (IETF) K. Watsen
Request for Comments: 9643 Watsen Networks
Category: Standards Track M. Scharf
ISSN: 2070-1721 Hochschule Esslingen
September 2024
YANG Groupings for TCP Clients and TCP Servers
Abstract
This document presents three YANG 1.1 modules to support the
configuration of TCP clients and TCP servers. The modules include
basic parameters of a TCP connection relevant for client or server
applications, as well as client configuration required for traversing
proxies. The data models defined by these modules may be used
directly (e.g., to define a specific TCP-client or TCP-server) or in
conjunction with the configuration defined for higher level protocols
that depend on TCP (e.g., SSH, TLS, etc.). Examples of higher level
protocol configuration designed to be used in conjunction with this
configuration are in RFCs 9644 and 9645.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9643.
Copyright Notice
Copyright (c) 2024 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
(https://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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Relation to Other RFCs
1.2. Specification Language
1.3. Adherence to the NMDA
1.4. Conventions
2. The "ietf-tcp-common" Module
2.1. Data Model Overview
2.2. Example Usage
2.3. YANG Module
3. The "ietf-tcp-client" Module
3.1. Data Model Overview
3.2. Example Usage
3.3. YANG Module
4. The "ietf-tcp-server" Module
4.1. Data Model Overview
4.2. Example Usage
4.3. YANG Module
5. Security Considerations
5.1. Considerations for the "ietf-tcp-common" YANG Module
5.2. Considerations for the "ietf-tcp-client" YANG Module
5.3. Considerations for the "ietf-tcp-server" YANG Module
6. IANA Considerations
6.1. The IETF XML Registry
6.2. The YANG Module Names Registry
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
This document defines three YANG 1.1 [RFC7950] modules to support the
configuration of TCP clients and TCP servers (TCP is defined in
[RFC9293]). The data models defined by these modules may be used
directly (e.g., to define a specific TCP-client or TCP-server) or in
conjunction with the configuration defined for higher level protocols
that depend on TCP (e.g., SSH, TLS, etc.). Examples of higher level
protocol configuration designed to be used in conjunction with this
configuration are in [RFC9644] and [RFC9645].
The modules focus on three different types of base TCP parameters
that matter for TCP-based applications: First, the modules cover
fundamental configuration of a TCP client or TCP server application,
such as addresses and port numbers. Second, a reusable grouping
enables modification of application-specific parameters for TCP
connections, such as use of TCP keepalives. And third, client
configuration for traversing proxies is included as well. In each
case, the modules have a very narrow scope and focus on a minimum set
of required parameters.
Please be advised that while this document presents support for some
TCP proxy techniques, there are other TCP proxy techniques that are
not part of this document but could be added by augmenting the YANG
module.
1.1. Relation to Other RFCs
This document presents three YANG modules [RFC7950] that are part of
a collection of RFCs that work together to ultimately support the
configuration of both the clients and servers of both the Network
Configuration Protocol (NETCONF) [RFC6241] and RESTCONF [RFC8040].
The dependency relationship between the primary YANG groupings
defined in the various RFCs is presented in the below diagram. In
some cases, a document may define secondary groupings that introduce
dependencies not illustrated in the diagram. The labels in the
diagram are shorthand names for the defining RFCs. The citation
references for shorthand names are provided below the diagram.
Please note that the arrows in the diagram point from referencer to
referenced. For example, the "crypto-types" RFC does not have any
dependencies, whilst the "keystore" RFC depends on the "crypto-types"
RFC.
crypto-types
^ ^
/ \
/ \
truststore keystore
^ ^ ^ ^
| +---------+ | |
| | | |
| +------------+ |
tcp-client-server | / | |
^ ^ ssh-client-server | |
| | ^ tls-client-server
| | | ^ ^ http-client-server
| | | | | ^
| | | +-----+ +---------+ |
| | | | | |
| +-----------|--------|--------------+ | |
| | | | | |
+-----------+ | | | | |
| | | | | |
| | | | | |
netconf-client-server restconf-client-server
+========================+==========================+
| Label in Diagram | Reference |
+========================+==========================+
| crypto-types | [RFC9640] |
+------------------------+--------------------------+
| truststore | [RFC9641] |
+------------------------+--------------------------+
| keystore | [RFC9642] |
+------------------------+--------------------------+
| tcp-client-server | RFC 9643 |
+------------------------+--------------------------+
| ssh-client-server | [RFC9644] |
+------------------------+--------------------------+
| tls-client-server | [RFC9645] |
+------------------------+--------------------------+
| http-client-server | [HTTP-CLIENT-SERVER] |
+------------------------+--------------------------+
| netconf-client-server | [NETCONF-CLIENT-SERVER] |
+------------------------+--------------------------+
| restconf-client-server | [RESTCONF-CLIENT-SERVER] |
+------------------------+--------------------------+
Table 1: Label in Diagram to RFC Mapping
1.2. Specification Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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.
1.3. Adherence to the NMDA
This document is compliant with the Network Management Datastore
Architecture (NMDA) [RFC8342]. It does not define any protocol
accessible nodes that are "config false".
1.4. Conventions
Various examples in this document use the XML [W3C.REC-xml-20081126]
encoding. Other encodings, such as JSON [RFC8259], could
alternatively be used.
2. The "ietf-tcp-common" Module
This section defines a YANG 1.1 module called "ietf-tcp-common". A
high-level overview of the module is provided in Section 2.1.
Examples illustrating the module's use are provided in Section 2.2
("Example Usage"). The YANG module itself is defined in Section 2.3.
2.1. Data Model Overview
This section provides an overview of the "ietf-tcp-common" module in
terms of its features and groupings.
2.1.1. Model Scope
This document presents a common "grouping" statement for basic TCP
connection parameters that matter to applications. It is "common" in
that this grouping is used by both the "ietf-tcp-client" and "ietf-
tcp-server" modules. In some TCP stacks, such parameters can also
directly be set by an application using system calls, such as the
sockets API. The base YANG data model in this document focuses on
modeling TCP keepalives. This base model can be extended as needed.
2.1.2. Features
The following diagram lists all the "feature" statements defined in
the "ietf-tcp-common" module:
Features:
+-- keepalives-supported
The diagram above uses syntax that is similar to but not defined in
[RFC8340].
2.1.3. Groupings
The "ietf-tcp-common" module defines the following "grouping"
statement:
* tcp-common-grouping
This grouping is presented in the following subsection.
2.1.3.1. The "tcp-common-grouping" Grouping
The following tree diagram [RFC8340] illustrates the "tcp-common-
grouping" grouping:
grouping tcp-common-grouping:
+-- keepalives! {keepalives-supported}?
+-- idle-time? uint16
+-- max-probes? uint16
+-- probe-interval? uint16
Comments:
* The "keepalives" node is a "presence" container so that the
mandatory descendant nodes do not imply that keepalives must be
configured.
* The "idle-time", "max-probes", and "probe-interval" nodes have the
common meanings. Please see the YANG module in Section 2.3 for
details.
2.1.4. Protocol-Accessible Nodes
The "ietf-tcp-common" module defines only "grouping" statements that
are used by other modules to instantiate protocol-accessible nodes.
Thus, this module, when implemented, does not itself define any
protocol-accessible nodes.
2.1.5. Guidelines for Configuring TCP Keepalives
Network stacks may include keepalives in their TCP implementations,
although this practice is not universally implemented. If keepalives
are included, [RFC9293] mandates that the application MUST be able to
turn them on or off for each TCP connection and that they MUST
default to off.
Keepalive mechanisms exist in many protocols. Depending on the
protocol stack, TCP keepalives may only be one out of several
alternatives. Which mechanism(s) to use depends on the use case and
application requirements. If keepalives are needed by an
application, it is RECOMMENDED that the liveness check happens only
at the protocol layers that are meaningful to the application.
A TCP keepalive mechanism SHOULD only be invoked in server
applications that might otherwise hang indefinitely and consume
resources unnecessarily if a client crashes or aborts a connection
during a network failure [RFC9293]. TCP keepalives may consume
significant resources both in the network and in endpoints (e.g.,
battery power). In addition, frequent keepalives risk network
congestion. The higher the frequency of keepalives, the higher the
overhead.
Given the cost of keepalives, parameters have to be configured
carefully:
* The default idle interval (leaf "idle-time") is two hours, i.e.,
7200 seconds [RFC9293]. A lower value MAY be configured, but idle
intervals SHOULD NOT be smaller than 15 seconds. Longer idle
intervals SHOULD be used when possible.
* The maximum number of sequential keepalive probes that can fail
(leaf "max-probes") trades off responsiveness and robustness
against packet loss. ACK segments that contain no data are not
reliably transmitted by TCP. Consequently, if a keepalive
mechanism is implemented, it MUST NOT interpret failure to respond
to any specific probe as a dead connection [RFC9293]. Typically,
a single-digit number should suffice.
* TCP implementations may include a parameter for the number of
seconds between TCP keepalive probes (leaf "probe-interval"). In
order to avoid congestion, the time interval between probes MUST
NOT be smaller than one second. Significantly longer intervals
SHOULD be used. It is important to note that keepalive probes (or
replies) can get dropped due to network congestion. Sending
further probe messages into a congested path after a short
interval, without backing off timers, could cause harm and result
in a congestion collapse. Therefore, it is essential to pick a
large, conservative value for this interval.
2.2. Example Usage
This section presents an example showing the "tcp-common-grouping"
grouping populated with some data.
<!-- The outermost element below doesn't exist in the data model. -->
<!-- It simulates if the "grouping" were a "container" instead. -->
<tcp-common xmlns="urn:ietf:params:xml:ns:yang:ietf-tcp-common">
<keepalives>
<idle-time>7200</idle-time>
<max-probes>9</max-probes>
<probe-interval>75</probe-interval>
</keepalives>
</tcp-common>
2.3. YANG Module
The "ietf-tcp-common" YANG module references [RFC6991] and [RFC9293].
<CODE BEGINS> file "ietf-tcp-common@2024-04-04.yang"
module ietf-tcp-common {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-tcp-common";
prefix tcpcmn;
organization
"IETF NETCONF (Network Configuration) Working Group and the
IETF TCP Maintenance and Minor Extensions (TCPM) Working Group";
contact
"WG Web: https://datatracker.ietf.org/wg/netconf
https://datatracker.ietf.org/wg/tcpm
WG List: NETCONF WG list <mailto:netconf@ietf.org>
TCPM WG list <mailto:tcpm@ietf.org>
Authors: Kent Watsen <mailto:kent+ietf@watsen.net>
Michael Scharf
<mailto:michael.scharf@hs-esslingen.de>";
description
"This module define a reusable 'grouping' that is common
to both TCP clients and TCP servers. This grouping statement
is used by both the 'ietf-tcp-client' and 'ietf-tcp-server'
modules.
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
are to be interpreted as described in BCP 14 (RFC 2119)
(RFC 8174) when, and only when, they appear in all
capitals, as shown here.
Copyright (c) 2024 IETF Trust and the persons identified
as authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with
or without modification, is permitted pursuant to, and
subject to the license terms contained in, the Revised
BSD License set forth in Section 4.c of the IETF Trust's
Legal Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC 9643
(https://www.rfc-editor.org/info/rfc9643); see the RFC
itself for full legal notices.";
revision 2024-04-04 {
description
"Initial version.";
reference
"RFC 9643: YANG Groupings for TCP Clients and TCP Servers";
}
// Features
feature keepalives-supported {
description
"Indicates that keepalives are supported.";
}
// Groupings
grouping tcp-common-grouping {
description
"A reusable grouping for configuring TCP parameters common
to TCP connections as well as the operating system as a
whole.";
container keepalives {
if-feature "keepalives-supported";
presence "Indicates that keepalives are enabled, aligning to
the requirement in Section 3.8.4 of RFC 9293 that
states keepalives are off by default.";
description
"Configures the keepalive policy to proactively test the
aliveness of the TCP peer. An unresponsive TCP peer is
dropped after approximately (idle-time + max-probes *
probe-interval) seconds. Further guidance can be found
in Section 2.1.5 of RFC 9643.";
reference
"RFC 9293: Transmission Control Protocol (TCP)";
leaf idle-time {
type uint16 {
range "1..max";
}
units "seconds";
default "7200";
description
"Sets the amount of time after which a TCP-level probe
message will be sent to test the aliveness of the TCP
peer if no data has been received from the TCP peer.
Two hours (7200 seconds) is a safe value, per Section
3.8.4 of RFC 9293.";
reference
"RFC 9293: Transmission Control Protocol (TCP)";
}
leaf max-probes {
type uint16 {
range "1..max";
}
default "9";
description
"Sets the maximum number of sequential keepalive probes
that can fail to obtain a response from the TCP peer
before assuming the TCP peer is no longer alive.";
}
leaf probe-interval {
type uint16 {
range "1..max";
}
units "seconds";
default "75";
description
"Sets the time interval between failed probes. The
interval SHOULD be significantly longer than one second
in order to avoid harm on a congested link.";
}
} // container keepalives
} // grouping tcp-common-grouping
}
<CODE ENDS>
3. The "ietf-tcp-client" Module
This section defines a YANG 1.1 module called "ietf-tcp-client". A
high-level overview of the module is provided in Section 3.1.
Examples illustrating the module's use are provided in Section 3.2
("Example Usage"). The YANG module itself is defined in Section 3.3.
3.1. Data Model Overview
This section provides an overview of the "ietf-tcp-client" module in
terms of its features and groupings.
3.1.1. Features
The following diagram lists all the "feature" statements defined in
the "ietf-tcp-client" module:
Features:
+-- local-binding-supported
+-- tcp-client-keepalives
+-- proxy-connect
+-- socks4-supported {proxy-connect}?
+-- socks4a-supported {proxy-connect}?
+-- socks5-supported {proxy-connect}?
+-- socks5-gss-api {socks5-supported}?
+-- socks5-username-password {socks5-supported}?
Comments:
* The "local-binding-supported" feature indicates that the server
supports configuring local bindings (i.e., the local address and
local port) for TCP clients.
* The "tcp-client-keepalives" feature indicates that TCP keepalive
parameters are configurable for TCP clients on the server
implementing this feature.
* The "proxy-connect" feature indicates the TCP client supports
connecting through TCP proxies.
* The "socks4-supported" feature indicates the TCP client supports
Socks4-based proxies.
* The "socks4a-supported" feature indicates the TCP client supports
Socks4a-based proxies. The difference between Socks4 and Socks4a
is that Socks4a enables the "remote-address" to be specified using
a hostname, in addition to an IP address.
* The "socks5-supported" feature indicates the TCP client supports
Socks5-based proxies.
* The "socks5-gss-api" feature indicates that the server, when
acting as a TCP client, supports authenticating to a SOCKS Version
5 proxy server using Generic Security Service Application Program
Interface (GSS-API) credentials.
* The "socks5-username-password" feature indicates that the server,
when acting as a TCP client, supports authenticating to a SOCKS
Version 5 proxy server using "username" and "password"
credentials."
The diagram above uses syntax that is similar to but not defined in
[RFC8340].
3.1.2. Groupings
The "ietf-tcp-client" module defines the following "grouping"
statement:
* tcp-client-grouping
This grouping is presented in the following subsection.
3.1.2.1. The "tcp-client-grouping" Grouping
The following tree diagram [RFC8340] illustrates the "tcp-client-
grouping" grouping:
grouping tcp-client-grouping:
+-- remote-address inet:host
+-- remote-port? inet:port-number
+-- local-address? inet:ip-address
| {local-binding-supported}?
+-- local-port? inet:port-number
| {local-binding-supported}?
+-- proxy-server! {proxy-connect}?
| +-- (proxy-type)
| +--:(socks4) {socks4-supported}?
| | +-- socks4-parameters
| | +-- remote-address inet:ip-address
| | +-- remote-port? inet:port-number
| +--:(socks4a) {socks4a-supported}?
| | +-- socks4a-parameters
| | +-- remote-address inet:host
| | +-- remote-port? inet:port-number
| +--:(socks5) {socks5-supported}?
| +-- socks5-parameters
| +-- remote-address inet:host
| +-- remote-port? inet:port-number
| +-- authentication-parameters!
| +-- (auth-type)
| +--:(gss-api) {socks5-gss-api}?
| | +-- gss-api
| +--:(username-password)
| {socks5-username-password}?
| +-- username-password
| +-- username string
| +---u ct:password-grouping
+---u tcpcmn:tcp-common-grouping
Comments:
* The "remote-address" node, which is mandatory, may be configured
as an IPv4 address, an IPv6 address, or a hostname.
* The "remote-port" node is not mandatory, but its default value leaf is defined with neither a "default" nor a
"mandatory" statement. YANG models using this grouping SHOULD
refine the invalid value "0", thus forcing grouping with a "default" statement, usually with the consuming data model to
refine it in order
port allocated by IANA, or a "mandatory" statement, if the ports
needs to provide it an appropriate default value. always be present.
* The "local-address" node, which is enabled by the "local-binding-
supported" feature (Section 3.1.1), may be configured as an IPv4
address, an IPv6 address, or a wildcard value.
* The "local-port" node, which is enabled by the "local-binding-
supported" feature (Section 3.1.1), is not mandatory. Its default
value is "0", indicating that the operating system can pick an
arbitrary port number.
* The "proxy-server" node is enabled by a "feature" statement and,
for servers that enable it, is a "presence" container so that the
descendant "mandatory true" choice node does not imply that the
proxy-server node must be configured. The proxy-server node uses
a "choice" statement to allow one of several types of proxies to
be configured. The choices presented in this document include
Socks4, Socks4a, and Socks5, each enabled by a YANG feature (see
Section 3.1.1). Other proxy types may be added by future work.
* This grouping uses the "password-grouping" grouping discussed in
[RFC9640].
* This grouping uses the "tcp-common-grouping" grouping discussed in
Section 2.1.3.1.
3.1.3. Protocol-Accessible Nodes
The "ietf-tcp-client" module defines only "grouping" statements that
are used by other modules to instantiate protocol-accessible nodes.
Thus, this module, when implemented, does not itself define any
protocol-accessible nodes.
3.2. Example Usage
This section presents two examples showing the "tcp-client-grouping"
grouping populated with some data. This example shows a TCP client
configured to not connect via a proxy:
<!-- The outermost element below doesn't exist in the data model. -->
<!-- It simulates if the "grouping" were a "container" instead. -->
<tcp-client xmlns="urn:ietf:params:xml:ns:yang:ietf-tcp-client">
<remote-address>www.example.com</remote-address>
<remote-port>8443</remote-port>
<local-address>192.0.2.2</local-address>
<local-port>12345</local-port>
<keepalives>
<idle-time>7200</idle-time>
<max-probes>9</max-probes>
<probe-interval>75</probe-interval>
</keepalives>
</tcp-client>
This example shows a TCP client configured to connect via a proxy.
<!-- The outermost element below doesn't exist in the data model. -->
<!-- It simulates if the "grouping" were a "container" instead. -->
<tcp-client xmlns="urn:ietf:params:xml:ns:yang:ietf-tcp-client">
<remote-address>www.example.com</remote-address>
<remote-port>8443</remote-port>
<local-address>192.0.2.2</local-address>
<local-port>12345</local-port>
<proxy-server>
<socks5-parameters>
<remote-address>proxy.example.com</remote-address>
<remote-port>1080</remote-port>
<authentication-parameters>
<username-password>
<username>foobar</username>
<cleartext-password>secret</cleartext-password>
</username-password>
</authentication-parameters>
</socks5-parameters>
</proxy-server>
<keepalives>
<idle-time>7200</idle-time>
<max-probes>9</max-probes>
<probe-interval>75</probe-interval>
</keepalives>
</tcp-client>
3.3. YANG Module
The "ietf-tcp-client" YANG module references [SOCKS], [SOCKS_4A],
[RFC1928], [RFC1929], [RFC2743], [RFC6991], [RFC9293], and [RFC9640].
<CODE BEGINS> file "ietf-tcp-client@2024-04-04.yang"
module ietf-tcp-client {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-tcp-client";
prefix tcpc;
import ietf-inet-types {
prefix inet;
reference
"RFC 6991: Common YANG Data Types";
}
import ietf-crypto-types {
prefix ct;
reference
"RFC 9640: YANG Data Types and Groupings for Cryptography";
}
import ietf-tcp-common {
prefix tcpcmn;
reference
"RFC 9643: YANG Groupings for TCP Clients and TCP Servers";
}
organization
"IETF NETCONF (Network Configuration) Working Group and the
IETF TCP Maintenance and Minor Extensions (TCPM) Working Group";
contact
"WG Web: https://datatracker.ietf.org/wg/netconf
https://datatracker.ietf.org/wg/tcpm
WG List: NETCONF WG list <mailto:netconf@ietf.org>
TCPM WG list <mailto:tcpm@ietf.org>
Authors: Kent Watsen <mailto:kent+ietf@watsen.net>
Michael Scharf
<mailto:michael.scharf@hs-esslingen.de>";
description
"This module defines reusable groupings for TCP clients that
can be used as a basis for specific TCP client instances.
Copyright (c) 2024 IETF Trust and the persons identified
as authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with
or without modification, is permitted pursuant to, and
subject to the license terms contained in, the Revised
BSD License set forth in Section 4.c of the IETF Trust's
Legal Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC 9643
(https://www.rfc-editor.org/info/rfc9643); see the RFC
itself for full legal notices.";
revision 2024-04-04 {
description
"Initial version.";
reference
"RFC 9643: YANG Groupings for TCP Clients and TCP Servers";
}
// Features
feature local-binding-supported {
description
"Indicates that the server supports configuring local
bindings (i.e., the local address and local port) for
TCP clients.";
}
feature tcp-client-keepalives {
description
"TCP keepalive parameters are configurable for
TCP clients on the server implementing this feature.";
reference
"RFC 9293: Transmission Control Protocol (TCP)";
}
feature proxy-connect {
description
"Indicates the TCP client supports connecting through
TCP proxies.";
}
feature socks4-supported {
if-feature "proxy-connect";
description
"Indicates the TCP client supports Socks4-based proxies.";
reference
"SOCKS Proceedings: 1992 Usenix Security Symposium";
}
feature socks4a-supported {
if-feature "proxy-connect";
description
"Indicates the TCP client supports Socks4a-based proxies.";
reference
"OpenSSH message:
SOCKS 4A: A Simple Extension to SOCKS 4 Protocol
<https://www.openssh.com/txt/socks4a.protocol>";
}
feature socks5-supported {
if-feature "proxy-connect";
description
"Indicates the TCP client supports Socks5-based proxies.";
reference
"RFC 1928: SOCKS Protocol Version 5";
}
feature socks5-gss-api {
if-feature "socks5-supported";
description
"Indicates that the server, when acting as a TCP client,
supports authenticating to a SOCKS Version 5 proxy server
using GSS-API credentials.";
reference
"RFC 1928: SOCKS Protocol Version 5";
}
feature socks5-username-password {
if-feature "socks5-supported";
description
"Indicates that the server, when acting as a TCP client,
supports authenticating to a SOCKS Version 5 proxy server
using 'username' and 'password' credentials.";
reference
"RFC 1928: SOCKS Protocol Version 5";
}
// Groupings
grouping tcp-client-grouping {
description
"A reusable grouping for configuring a TCP client.
Note that this grouping uses fairly typical descendant
node names such that a stack of 'uses' statements will
have name conflicts. It is intended that the consuming
data model will resolve the issue (e.g., by wrapping
the 'uses' statement in a container called
'tcp-client-parameters'). This model purposely does
not do this itself so as to provide maximum flexibility
to consuming models.";
leaf remote-address {
type inet:host;
mandatory true;
description
"The IP address or hostname of the remote peer to
establish a connection with. If a domain name is
configured, then the DNS resolution should happen on
each connection attempt. If the DNS resolution
results in multiple IP addresses, the IP addresses
are tried according to local preference order until
a connection has been established or until all IP
addresses have failed.";
}
leaf remote-port {
type inet:port-number;
description
"The IP port number for of the remote peer to establish a
connection with. Importing modules should 'refine' in
an appropriate default port number value."; TCP server.";
}
leaf local-address {
if-feature "local-binding-supported";
type inet:ip-address;
description
"The local IP address/interface to bind to for when
connecting to the remote peer. INADDR_ANY ('0.0.0.0') or
INADDR6_ANY ('0:0:0:0:0:0:0:0' a.k.a. '::') MAY be used to
explicitly indicate the implicit default, which the server
can bind to any IPv4 or IPv6 address.";
}
leaf local-port {
if-feature "local-binding-supported";
type inet:port-number;
default "0";
description
"The local IP port number to bind to for when connecting
to the remote peer. The port number '0', which is the
default value, indicates that any available local port
number may be used.";
}
container proxy-server {
if-feature "proxy-connect";
presence "Indicates that a proxy connection has been
configured. Present so that the mandatory
descendant nodes do not imply that this node
must be configured.";
choice proxy-type {
mandatory true;
description
"Selects a proxy connection protocol.";
case socks4 {
if-feature "socks4-supported";
container socks4-parameters {
leaf remote-address {
type inet:ip-address;
mandatory true;
description
"The IP address of the proxy server.";
}
leaf remote-port {
type inet:port-number;
default "1080";
description
"The IP port number for the proxy server.";
}
description
"Parameters for connecting to a TCP-based proxy
server using the SOCKS4 protocol.";
reference
"SOCKS Proceedings: 1992 Usenix Security Symposium";
}
}
case socks4a {
if-feature "socks4a-supported";
container socks4a-parameters {
leaf remote-address {
type inet:host;
mandatory true;
description
"The IP address or hostname of the proxy server.";
}
leaf remote-port {
type inet:port-number;
default "1080";
description
"The IP port number for the proxy server.";
}
description
"Parameters for connecting to a TCP-based proxy
server using the SOCKS4a protocol.";
reference
"SOCKS Proceedings:
1992 Usenix Security Symposium
OpenSSH message:
SOCKS 4A: A Simple Extension to SOCKS 4 Protocol
<https://www.openssh.com/txt/socks4a.protocol>";
}
}
case socks5 {
if-feature "socks5-supported";
container socks5-parameters {
leaf remote-address {
type inet:host;
mandatory true;
description
"The IP address or hostname of the proxy server.";
}
leaf remote-port {
type inet:port-number;
default "1080";
description
"The IP port number for the proxy server.";
}
container authentication-parameters {
presence "Indicates that an authentication mechanism
has been configured. Present so that the
mandatory descendant nodes do not imply that
this node must be configured.";
description
"A container for SOCKS Version 5 authentication
mechanisms.
A complete list of methods is defined at:
<https://www.iana.org/assignments/socks-methods>.";
reference
"RFC 1928: SOCKS Protocol Version 5";
choice auth-type {
mandatory true;
description
"A choice amongst supported SOCKS Version 5
authentication mechanisms.";
case gss-api {
if-feature "socks5-gss-api";
container gss-api {
description
"Contains GSS-API configuration. Defines
as an empty container to enable specific
GSS-API configuration to be augmented in
by future modules.";
reference
"RFC 1928: SOCKS Protocol Version 5
RFC 2743: Generic Security Service
Application Program Interface
Version 2, Update 1";
}
}
case username-password {
if-feature "socks5-username-password";
container username-password {
leaf username {
type string;
mandatory true;
description
"The 'username' value to use for client
identification.";
}
uses ct:password-grouping {
description
"The password to be used for client
authentication.";
}
description
"Contains username/password configuration.";
reference
"RFC 1929: Username/Password Authentication
for SOCKS V5";
}
}
}
}
description
"Parameters for connecting to a TCP-based proxy server
using the SOCKS5 protocol.";
reference
"RFC 1928: SOCKS Protocol Version 5";
}
}
}
description
"Proxy server settings.";
}
uses tcpcmn:tcp-common-grouping {
refine "keepalives" {
if-feature "tcp-client-keepalives";
description
"An 'if-feature' statement so that implementations
can choose to support TCP client keepalives.";
}
}
}
}
<CODE ENDS>
4. The "ietf-tcp-server" Module
This section defines a YANG 1.1 module called "ietf-tcp-server". A
high-level overview of the module is provided in Section 4.1.
Examples illustrating the module's use are provided in Section 4.2
("Example Usage"). The YANG module itself is defined in Section 4.3.
4.1. Data Model Overview
This section provides an overview of the "ietf-tcp-server" module in
terms of its features and groupings.
4.1.1. Features
The following diagram lists all the "feature" statements defined in
the "ietf-tcp-server" module:
Features:
+-- tcp-server-keepalives
The diagram above uses syntax that is similar to but not defined in
[RFC8340].
4.1.2. Groupings
The "ietf-tcp-server" module defines the following "grouping"
statement:
* tcp-server-grouping
This grouping is presented in the following subsection.
4.1.2.1. The "tcp-server-grouping" Grouping
The following tree diagram [RFC8340] illustrates the "tcp-server-
grouping" grouping:
grouping tcp-server-grouping:
+-- local-bind* [local-address]
| +-- local-address inet:ip-address
| +-- local-port? inet:port-number
+---u tcpcmn:tcp-common-grouping
Comments:
* The "local-address" node, which is mandatory, may be configured as
an IPv4 address, an IPv6 address, or a wildcard value.
* The "local-port" node is not mandatory, but its default value leaf is defined with neither a "default" nor a
"mandatory" statement. YANG models using this grouping SHOULD
refine the invalid value "0", thus forcing grouping with a "default" statement, usually with the consuming data model to
refine it in order
port allocated by IANA, or a "mandatory" statement, if the port
needs to provide it an appropriate default value. always be present.
* This grouping uses the "tcp-common-grouping" grouping discussed in
Section 2.1.3.1.
4.1.3. Protocol-Accessible Nodes
The "ietf-tcp-server" module defines only "grouping" statements that
are used by other modules to instantiate protocol-accessible nodes.
Thus, this module, when implemented, does not itself define any
protocol-accessible nodes.
4.2. Example Usage
This section presents an example showing the "tcp-server-grouping"
grouping populated with some data.
<!-- The outermost element below doesn't exist in the data model. -->
<!-- It simulates if the "grouping" were a "container" instead. -->
<tcp-server xmlns="urn:ietf:params:xml:ns:yang:ietf-tcp-server">
<local-bind>
<local-address>192.0.2.2</local-address>
<local-port>49152</local-port>
</local-bind>
<keepalives>
<idle-time>7200</idle-time>
<max-probes>9</max-probes>
<probe-interval>75</probe-interval>
</keepalives>
</tcp-server>
4.3. YANG Module
The "ietf-tcp-server" YANG module references [RFC6991] and [RFC9293].
<CODE BEGINS> file "ietf-tcp-server@2024-04-04.yang"
module ietf-tcp-server {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-tcp-server";
prefix tcps;
import ietf-inet-types {
prefix inet;
reference
"RFC 6991: Common YANG Data Types";
}
import ietf-tcp-common {
prefix tcpcmn;
reference
"RFC 9643: YANG Groupings for TCP Clients and TCP Servers";
}
organization
"IETF NETCONF (Network Configuration) Working Group and the
IETF TCP Maintenance and Minor Extensions (TCPM) Working Group";
contact
"WG Web: https://datatracker.ietf.org/wg/netconf
https://datatracker.ietf.org/wg/tcpm
WG List: NETCONF WG list <mailto:netconf@ietf.org>
TCPM WG list <mailto:tcpm@ietf.org>
Authors: Kent Watsen <mailto:kent+ietf@watsen.net>
Michael Scharf
<mailto:michael.scharf@hs-esslingen.de>";
description
"This module defines reusable groupings for TCP servers that
can be used as a basis for specific TCP server instances.
Copyright (c) 2024 IETF Trust and the persons identified
as authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with
or without modification, is permitted pursuant to, and
subject to the license terms contained in, the Revised
BSD License set forth in Section 4.c of the IETF Trust's
Legal Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC 9643
(https://www.rfc-editor.org/info/rfc9643); see the RFC
itself for full legal notices.";
revision 2024-04-04 {
description
"Initial version.";
reference
"RFC 9643: YANG Groupings for TCP Clients and TCP Servers";
}
// Features
feature tcp-server-keepalives {
description
"TCP keepalive parameters are configurable for
TCP servers on the server implementing this feature.";
reference
"RFC 9293: Transmission Control Protocol (TCP)";
}
// Groupings
grouping tcp-server-grouping {
description
"A reusable grouping for configuring a TCP server.
Note that this grouping uses fairly typical descendant
node names such that a stack of 'uses' statements will
have name conflicts. It is intended that the consuming
data model will resolve the issue (e.g., by wrapping
the 'uses' statement in a container called
'tcp-server-parameters'). This model purposely does
not do this itself so as to provide maximum flexibility
to consuming models.";
list local-bind {
key "local-address";
min-elements 1;
description
"A list of bind (listen) points for this server
instance. A server instance may have multiple
bind points to support, e.g., the same port in
different address families or different ports
in the same address family.";
leaf local-address {
type inet:ip-address;
description
"The local IP address to listen on for incoming
TCP client connections. To configure listening
on all IPv4 addresses, the value must be '0.0.0.0'
(INADDR_ANY). To configure listening on all IPv6
addresses, the value must be '::' (INADDR6_ANY).";
}
leaf local-port {
type inet:port-number;
default "0";
description
"The local port number to listen on for incoming TCP
client connections. An invalid default value (0)
is used (instead of 'mandatory true') so that an
application-level data model may 'refine' it with
an application-specific default port number value."; connections.”;
}
}
uses tcpcmn:tcp-common-grouping {
refine "keepalives" {
if-feature "tcp-server-keepalives";
description
"An 'if-feature' statement so that implementations
can choose to support TCP server keepalives.";
}
}
}
}
<CODE ENDS>
5. Security Considerations
The three YANG modules in this document define groupings and will not
be deployed as standalone modules. Their security implications may
be context-dependent based on their use in other modules. The
designers of modules that import these groupings must conduct their
own analysis of the security considerations.
5.1. Considerations for the "ietf-tcp-common" YANG Module
This section is modeled after the template defined in Section 3.7.1
of [RFC8407].
The "ietf-tcp-common" YANG module defines "grouping" statements that
are designed to be accessed via YANG-based management protocols, such
as NETCONF [RFC6241] and RESTCONF [RFC8040]. Both of these protocols
have mandatory-to-implement secure transport layers (e.g., Secure
Shell (SSH) [RFC4252], TLS [RFC8446], and QUIC [RFC9000]) and
mandatory-to-implement mutual authentication.
The Network Configuration Access Control Model (NACM) [RFC8341]
provides the means to restrict access for particular users to a
preconfigured subset of all available protocol operations and
content.
Please be aware that this YANG module uses groupings from other YANG
modules that define nodes that may be considered sensitive or
vulnerable in network environments. Please review the security
considerations for dependent YANG modules for information as to which
nodes may be considered sensitive or vulnerable in network
environments.
None of the readable data nodes defined in this YANG module are
considered sensitive or vulnerable in network environments. The NACM
"default-deny-all" extension has not been set for any data nodes
defined in this module.
None of the writable data nodes defined in this YANG module are
considered sensitive or vulnerable in network environments. The NACM
"default-deny-write" extension has not been set for any data nodes
defined in this module.
This module does not define any RPCs, actions, or notifications, and
thus, the security considerations for such are not provided here.
5.2. Considerations for the "ietf-tcp-client" YANG Module
This section is modeled after the template defined in Section 3.7.1
of [RFC8407].
The "ietf-tcp-client" YANG module defines "grouping" statements that
are designed to be accessed via YANG-based management protocols, such
as NETCONF [RFC6241] and RESTCONF [RFC8040]. Both of these protocols
have mandatory-to-implement secure transport layers (e.g., Secure
Shell (SSH) [RFC4252], TLS [RFC8446], and QUIC [RFC9000]) and
mandatory-to-implement mutual authentication.
The Network Configuration Access Control Model (NACM) [RFC8341]
provides the means to restrict access for particular users to a
preconfigured subset of all available protocol operations and
content.
Please be aware that this YANG module uses groupings from other YANG
modules that define nodes that may be considered sensitive or
vulnerable in network environments. Please review the security
considerations for dependent YANG modules for information as to which
nodes may be considered sensitive or vulnerable in network
environments.
One readable data node defined in this YANG module may be considered
sensitive or vulnerable in some network environments. This node is
as follows:
* The "proxy-server/socks5-parameters/authentication-parameters/
username-password/password" node:
The "password" node defined in the "tcp-client-grouping"
grouping is defined using the "password-grouping" grouping
presented in [RFC9640]. This grouping enables both cleartext
and encrypted passwords to be configured. As the referenced
document states, configuration of cleartext passwords is NOT
RECOMMENDED. However, in the case cleartext values are
configured, this node is additionally sensitive to read
operations such that, in normal use cases, it should never be
returned to a client. For this reason, the NACM "default-deny-
all" extension has been applied to it.
None of the writable data nodes defined in this YANG module are
considered sensitive or vulnerable in network environments. The NACM
"default-deny-write" extension has not been set for any data nodes
defined in this module.
This module does not define any RPCs, actions, or notifications, and
thus, the security considerations for such are not provided here.
Implementations are RECOMMENDED to implement the "local-binding-
supported" feature for cryptographically secure protocols so as to
enable more granular ingress/egress firewall rule bases. It is NOT
RECOMMENDED to implement this feature for unsecure protocols, as per
[RFC6056].
5.3. Considerations for the "ietf-tcp-server" YANG Module
This section is modeled after the template defined in Section 3.7.1
of [RFC8407].
The "ietf-tcp-server" YANG module defines "grouping" statements that
are designed to be accessed via YANG-based management protocols, such
as NETCONF [RFC6241] and RESTCONF [RFC8040]. Both of these protocols
have mandatory-to-implement secure transport layers (e.g., Secure
Shell (SSH) [RFC4252], TLS [RFC8446], and QUIC [RFC9000]) and
mandatory-to-implement mutual authentication.
The Network Configuration Access Control Model (NACM) [RFC8341]
provides the means to restrict access for particular users to a
preconfigured subset of all available protocol operations and
content.
Please be aware that this YANG module uses groupings from other YANG
modules that define nodes that may be considered sensitive or
vulnerable in network environments. Please review the security
considerations for dependent YANG modules for information as to which
nodes may be considered sensitive or vulnerable in network
environments.
None of the readable data nodes defined in this YANG module are
considered sensitive or vulnerable in network environments. The NACM
"default-deny-all" extension has not been set for any data nodes
defined in this module.
None of the writable data nodes defined in this YANG module are
considered sensitive or vulnerable in network environments. The NACM
"default-deny-write" extension has not been set for any data nodes
defined in this module.
This module does not define any RPCs, actions, or notifications, and
thus, the security considerations for such are not provided here.
6. IANA Considerations
6.1. The IETF XML Registry
IANA has registered the following URI in the "ns" registry of the
"IETF XML Registry" [RFC3688].
URI: urn:ietf:params:xml:ns:yang:ietf-tcp-common
Registrant Contact: The IESG
XML: N/A; the requested URI is an XML namespace.
URI: urn:ietf:params:xml:ns:yang:ietf-tcp-client
Registrant Contact: The IESG
XML: N/A; the requested URI is an XML namespace.
URI: urn:ietf:params:xml:ns:yang:ietf-tcp-server
Registrant Contact: The IESG
XML: N/A; the requested URI is an XML namespace.
6.2. The YANG Module Names Registry
IANA has registered the following three YANG modules in the "YANG
Module Names" registry [RFC6020].
Name: ietf-tcp-common
Namespace: urn:ietf:params:xml:ns:yang:ietf-tcp-common
Prefix: tcpcmn
Reference: RFC 9643
Name: ietf-tcp-client
Namespace: urn:ietf:params:xml:ns:yang:ietf-tcp-client
Prefix: tcpc
Reference: RFC 9643
Name: ietf-tcp-server
Namespace: urn:ietf:params:xml:ns:yang:ietf-tcp-server
Prefix: tcps
Reference: RFC 9643
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4252] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252,
January 2006, <https://www.rfc-editor.org/info/rfc4252>.
[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC6020, October 2010,
<https://www.rfc-editor.org/info/rfc6020>.
[RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types",
RFC 6991, DOI 10.17487/RFC6991, July 2013,
<https://www.rfc-editor.org/info/rfc6991>.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8341] Bierman, A. and M. Bjorklund, "Network Configuration
Access Control Model", STD 91, RFC 8341,
DOI 10.17487/RFC8341, March 2018,
<https://www.rfc-editor.org/info/rfc8341>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/info/rfc9293>.
[RFC9640] Watsen, K., "YANG Data Types and Groupings for
Cryptography", RFC 9640, DOI 10.17487/RFC9640, September
2024, <https://www.rfc-editor.org/info/rfc9640>.
7.2. Informative References
[HTTP-CLIENT-SERVER]
Watsen, K., "YANG Groupings for HTTP Clients and HTTP
Servers", Work in Progress, Internet-Draft, draft-ietf-
netconf-http-client-server-23, 15 August 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-netconf-
http-client-server-23>.
[NETCONF-CLIENT-SERVER]
Watsen, K., "NETCONF Client and Server Models", Work in
Progress, Internet-Draft, draft-ietf-netconf-netconf-
client-server-37, 14 August 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-netconf-
netconf-client-server-37>.
[RESTCONF-CLIENT-SERVER]
Watsen, K., "RESTCONF Client and Server Models", Work in
Progress, Internet-Draft, draft-ietf-netconf-restconf-
client-server-38, 14 August 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-netconf-
restconf-client-server-38>.
[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
L. Jones, "SOCKS Protocol Version 5", RFC 1928,
DOI 10.17487/RFC1928, March 1996,
<https://www.rfc-editor.org/info/rfc1928>.
[RFC1929] Leech, M., "Username/Password Authentication for SOCKS
V5", RFC 1929, DOI 10.17487/RFC1929, March 1996,
<https://www.rfc-editor.org/info/rfc1929>.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743,
DOI 10.17487/RFC2743, January 2000,
<https://www.rfc-editor.org/info/rfc2743>.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<https://www.rfc-editor.org/info/rfc3688>.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
DOI 10.17487/RFC6056, January 2011,
<https://www.rfc-editor.org/info/rfc6056>.
[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,
<https://www.rfc-editor.org/info/rfc6241>.
[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[RFC8340] Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
<https://www.rfc-editor.org/info/rfc8340>.
[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,
<https://www.rfc-editor.org/info/rfc8342>.
[RFC8407] Bierman, A., "Guidelines for Authors and Reviewers of
Documents Containing YANG Data Models", BCP 216, RFC 8407,
DOI 10.17487/RFC8407, October 2018,
<https://www.rfc-editor.org/info/rfc8407>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC9641] Watsen, K., "A YANG Data Model for a Truststore",
RFC 9641, DOI 10.17487/RFC9641, September 2024,
<https://www.rfc-editor.org/info/rfc9641>.
[RFC9642] Watsen, K., "A YANG Data Model for a Keystore", RFC 9642,
DOI 10.17487/RFC9642, September 2024,
<https://www.rfc-editor.org/info/rfc9642>.
[RFC9644] Watsen, K., "YANG Groupings for SSH Clients and SSH
Servers", RFC 9644, DOI 10.17487/RFC9644, September 2024,
<https://www.rfc-editor.org/info/rfc9644>.
[RFC9645] Watsen, K., "YANG Groupings for TLS Clients and TLS
Servers", RFC 9645, DOI 10.17487/RFC9645, September 2024,
<https://www.rfc-editor.org/info/rfc9645>.
[SOCKS] Koblas, D. and M. Koblas, "SOCKS", USENIX UNIX Security
Symposium III, September 1992, <https://www.usenix.org/leg
acy/publications/library/proceedings/sec92/full_papers/
koblas.pdf>.
[SOCKS_4A] Lee, Y., "SOCKS 4A: A Simple Extension to SOCKS 4
Protocol", <https://www.openssh.com/txt/socks4a.protocol>.
[W3C.REC-xml-20081126]
Bray, T., Paoli, J., Sperberg-McQueen, C.M., Maler, E.,
and F. Yergeau, "Extensible Markup Language (XML) 1.0
(Fifth Edition)", World Wide Web Consortium
Recommendation REC-xml-20081126, November 2008,
<https://www.w3.org/TR/2008/REC-xml-20081126/>.
Acknowledgements
The authors would like to thank the following for lively discussions
on list and in the halls (ordered by first name): Éric Vyncke, Joe
Clarke, Jürgen Schönwälder, Ladislav Lhotka, Mallory Knodel, Martin
Duke, Michael Tüxen, Mohamed Boucadair, Nancy Cam-Winget, Nick
Hancock, Per Andersson, Rob Wilton, Roman Danyliw, Tom Petch, and Wim
Henderickx.
Authors' Addresses
Kent Watsen
Watsen Networks
Email: kent+ietf@watsen.net
Michael Scharf
Hochschule Esslingen
University of Applied Sciences
Kanalstr. 33
73728 Esslingen am Neckar
Germany
Email: michael.scharf@hs-esslingen.de