Generalized UDP Source Port for DHCP Relay
RFC 8357

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Internet Engineering Task Force (IETF)                           N. Shen
Request for Comments: 8357                                       E. Chen
Category: Standards Track                                  Cisco Systems
ISSN: 2070-1721                                               March 2018

               Generalized UDP Source Port for DHCP Relay

Abstract

   This document defines an extension to the DHCP protocols that allows
   a relay agent to use any available source port for upstream
   communications.  The extension also allows inclusion of a DHCP option
   that can be used to statelessly route responses back to the
   appropriate source port on downstream communications.

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/rfc8357.

Copyright Notice

   Copyright (c) 2018 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Shen & Chen                  Standards Track                    [Page 1]
RFC 8357                 DHCP Relay Source Port               March 2018

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     2.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Changes to DHCP Specifications  . . . . . . . . . . . . . . .   4
     3.1.  Additions to DHCPv4 in RFC 2131 . . . . . . . . . . . . .   4
     3.2.  Additions to DHCPv6 in RFC 3315 . . . . . . . . . . . . .   4
   4.  Relay Source Port Sub-option and Option . . . . . . . . . . .   4
     4.1.  Source Port Sub-option for DHCPv4 . . . . . . . . . . . .   5
     4.2.  Relay Source Port Option for DHCPv6 . . . . . . . . . . .   5
   5.  Relay Agent and Server Behavior . . . . . . . . . . . . . . .   6
     5.1.  DHCPv4  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  DHCPv6  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.3.  Compatibility . . . . . . . . . . . . . . . . . . . . . .   7
     5.4.  Deployment Considerations . . . . . . . . . . . . . . . .   7
   6.  Example of an IPv6-Cascaded Relay . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   RFC 2131 [RFC2131] and RFC 3315 [RFC3315] specify the use of UDP as
   the transport protocol for DHCP.  They also define both the server-
   and client-side port numbers.  The IPv4 server port is UDP number
   (67) and the client port is UDP number (68); for IPv6, the server
   port is (547) and the client port is (546).

   The fixed UDP port combinations for the DHCP protocol scheme creates
   challenges in certain DHCP relay operations.  For instance, in a
   large-scale DHCP relay implementation on a single-switch node, the
   DHCP relay functionality may be partitioned among multiple relay
   processes.  All of these DHCP relay processes may share the same IP
   address of the switch node.  If the UDP source port has to be a fixed
   number as currently specified, the transport socket operation of DHCP
   packets would need to go through a central entity or process, which
   would defeat the purpose of distributing DHCP relay functionality.

   In some large-scale deployments, the decision to split the DHCP
   functionality into multiple processes on a node may not be purely
   based on DHCP relay computational load.  Rather, DHCP relay could
   just be one of the functions in a multi-process implementation.

Shen & Chen                  Standards Track                    [Page 2]