Networking Working Group N. Shen
Internet-Draft E. Chen
Intended status: Standards Track Cisco Systems
Expires: September 1, 2017 February 28, 2017
Generalized UDP Source Port for DHCP Relay
draft-ietf-dhc-relay-port-02
Abstract
This document proposes an extension to the DHCP and DHCPv6 protocols
that allows any valid number to be used as the relay agent UDP source
port for DHCP packets.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Changes to DHCP and DHCPv6 Specifications . . . . . . . . . . 3
3.1. Changes to DHCP in RFC 2131 . . . . . . . . . . . . . . . 3
3.2. Changes to DHCPv6 in RFC 3315 . . . . . . . . . . . . . . 4
4. Relay Source Port Sub-option and Option . . . . . . . . . . . 4
4.1. Source Port Sub-option for DHCPv4 . . . . . . . . . . . . 4
4.2. Relay Source Port Option for DHCPv6 . . . . . . . . . . . 5
5. A DHCPv6 Cascaded Relay Example . . . . . . . . . . . . . . . 6
6. Compatibility . . . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
10. Document Change Log . . . . . . . . . . . . . . . . . . . . . 8
10.1. Changes to draft-ietf-dhc-relay-port-02 . . . . . . . . 8
10.2. Changes to draft-ietf-dhc-relay-port-01 . . . . . . . . 9
10.3. Changes to draft-ietf-dhc-relay-port-00 . . . . . . . . 9
11. Normative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
RFC 2131 [RFC2131] and RFC 3315 [RFC3315] specify the use of UDP as
the transport protocol for DHCP and DHCPv6. They also define both
the server side and client side port numbers. The DHCP server port
is UDP number (67) and the client port is UDP number (68); for DHCPv6
the server port is (546) and the client port is (547).
This fixed UDP port of 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
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 deployment, the decision to split the DHCP
functionality into multiple processes on a node may not be purely
based on DHCP relay computational load. But rather DHCP relay could
just be one of the functions in a multi-process implementation.
Although assigning a different IP/IPv6 source address for each DHCP
relay process can be a solution, it would introduce operational and
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network management complexities, especially given the scarceness of
the IPv4 addresses.
This document proposes an extension to relax the fixed UDP source
port requirement for the DHCP relay agents. This extension requires
a DHCP server or relay agent, in the case of cascaded relay agents
[RFC3315], to remember the inbound packet's UDP port number along
with the IP/IPv6 address. The DHCP server when sending back replies
MUST use the UDP port number that the incoming relay agent uses
instead of the fixed DHCP port number.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Terminology
Downstream Device: In the DHCP relay context, it refers to the next
relay agent for forwarding Relay-reply Messages.
Upstream Device: In the DHCP relay context, it refers to the next
relay agent or DHCP server for forwarding Relay-forward
Messages.
Relay Source Port: This is the UDP port that a relay agent uses to
receive Relay-forward Messages from an upstream device.
Downstream Source Port: This is the UDP port that the downstream
device uses when forwarding Relay-forward Messages to this
relay agent device. This UDP port is to be used by this
relay agent device when forwarding the Relay-reply Messages
to that downstream device.
Non-DHCP UDP Port: Any valid UDP port other than port 67 for DHCP
and port 547 for DHCPv6.
3. Changes to DHCP and DHCPv6 Specifications
3.1. Changes to DHCP in RFC 2131
Section 4.1 of RFC 2131 [RFC2131] specifies that:
DHCP uses UDP as its transport protocol. DHCP messages from a
client to a server are sent to the 'DHCP server' port (67), and
DHCP messages from a server to a client are sent to the 'DHCP
client' port (68).
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This specification adds the following extension to the above
paragraph.
DHCP messages from a relay agent to a server are sent to the 'DHCP
server' port (67), and the UDP source port it uses can be any
valid UDP port available in the relay system, including the DHCP
port 67. The default port number is 67 if there is no explicit
configuration for the generalized source UDP port extension for
DHCP relay.
3.2. Changes to DHCPv6 in RFC 3315
Section 5.2 of RFC 3315 [RFC3315] specifies that:
Clients listen for DHCP messages on UDP port 546. Servers and
relay agents listen for DHCP messages on UDP port 547.
This specification adds the following extension to the above
paragraph.
A DHCP relay agent can listen for DHCP messages from a server or
another upstream relay agent device on any valid UDP port
available in the relay system including the DHCP UDP port 547.
The default is port 547 if there is no explicit configuration for
the generalized UDP source port extension for DHCP relay.
4. Relay Source Port Sub-option and Option
Although a DHCP or DHCPv6 server can implicitly determine a UDP
source port when it receives an inbound message from a relay agent,
this sub-option makes the request explicit for the server to use a
non-DHCP UDP port in the reply message. When DHCPv6 cascaded relay
agents are involved, the downstream non-DHCP UDP port needs to be
recorded using the option.
4.1. Source Port Sub-option for DHCPv4
The Relay Agent "Source Port Sub-option" is a new option, and it is
part of the relay-agent-information option for DHCPv4 [RFC3046]. It
SHOULD be used by a relay agent that uses a non-DHCP UDP port
communicating with the DHCP server.
The format of the "Source Port Sub-option" is shown below:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SubOpt Code | Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Where:
SubOpt Code: SUBOPT_RELAY_PORT. 8 bits value, to be assigned by
IANA.
Len: 8 bits value to be set to 0.
When a DHCP server receives a message from a relay agent with the
"Source Port Sub-option", it MUST remember the UDP source port of the
message and use that port number as the UDP destination port when
sending the reply message to the same relay agent.
4.2. Relay Source Port Option for DHCPv6
The "Relay Source Port Option" is a new DHCPv6 option. It SHOULD be
used either by a DHCPv6 relay agent that uses a non-DHCP UDP port
communicating with the DHCP server and the upstream relay agent, or
by a DHCPv6 relay agent that detects the use of a non-DHCP UDP port
by a downstream relay agent.
The format of the "Relay Source Port Option" is shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_RELAY_RELAY_PORT | Option-Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Source Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where:
Option-Code: OPTION_RELAY_RELAY_PORT. 16 bits value, to be
assigned by IANA.
Option-Len: 16 bits value to be set to 2.
Downstream Source Port: 16 bits value. To be set by the DHCPv6
relay either to the downstream relay agent's UDP source
port used for the UDP packet, or to zero if only the
local relay agent uses the non-DHCP UDP port.
The DHCPv6 relay agent SHOULD include the "Relay Source Port Option"
when it uses a non-DHCP UDP port to communicate to a DHCPv6 server or
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an upstream DHCPv6 relay agent. Also when a DHCPv6 relay agent
detects that a downstream relay agent uses a non-DHCP UDP port in the
packet, it MUST record the port number in the "Downstream Source
Port" field of this option. If this option is included to indicate
only the local non-DHCP UDP port usage and there is no downstream
relay agent's non-DHCP UDP port usage, the field Downstream Source
Port field MUST be set to zero.
The DHCPv6 relay agent SHOULD include this option in the following
three cases:
1) The local relay agent uses a non-DHCP UDP port.
2) the downstream relay agent uses a non-DHCP UDP port.
3) the local relay agent and the downstream relay agent both use
non-DHCP UDP ports.
In the first case, the value of the field in "Downstream Source Port"
field is set to zero. In the other two cases, the value of the field
is set to the UDP port number that the downstream relay agent uses.
When a DHCPv6 server receives a Relay-forward message with the "Relay
Source Port Option", it MUST copy the option when constructing the
Relay-reply chain in response to the Relay-forward message. This
option MUST NOT appear in any message other than a Relay-forward or
Relay-reply message. Additionally, the DHCPv6 server MUST check and
use the UDP source port from the UDP packet of the Relay-forward
message in replying to the relay agent.
When a relay agent receives a Relay-reply message with the "Relay
Source Port Option" from a server or from an upstream relay agent, if
the "Downstream Source Port" field in the option is non-zero, it MUST
use this UDP port number to forward the Relay-reply message to the
downstream relay agent.
5. A DHCPv6 Cascaded Relay Example
An example of DHCPv6 cascaded relay agents with the "Relay Source
Port Option" is shown below.
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(forward) (forward) (forward)
Relay1 ----------> Relay2 ----------> Relay3 ----------> Server
(1000) (547) (547)
(reply) (reply) (reply)
<---------- <---------- <----------
In the above diagram, all the DHCPv6 devices support this generalized
UDP source port extension except for Relay3. Relay1 is the only
relay agent device uses a non-DHCP UDP port. Relay2 is the upstream
device of Relay1.
Both Relay1 and Relay2 include the "Relay Source Port Option" in
Relay-forward message. Relay1 sets the "Downstream Source Port"
field in the option to zero. Relay2 notices the "Relay Source Port
Option" is included in the message from Relay1, and it determines
that the UDP source port used by Relay1 is 1000. Relay2 will include
the "Relay Source Port Option" and it sets the "Downstream Source
Port" field in the option to 1000. The DHCPv6 server copies the
"Relay Source Port Option" when replying with the Relay-reply
message.
When Relay2 receives the Relay-reply message with the "Relay Source
Port Option", it finds the "Downstream Source Port" field has the
value of 1000. Relay2 then uses this port number in the UDP packet
when sending the Relay-reply message to Relay1.
When Relay1 receives the Relay-reply message with the "Relay Source
Port Option", it finds that the "Downstream Source Port" field has
the value of zero. Relay1 then uses the normal DHCP port 547 in the
packet sending the Relay-reply message to its downstream relay agent
or uses UDP port 546 to a DHCPv6 client.
6. Compatibility
With this source port generalization for DHCP and DHCPv6, the server
behavior is compatible with the relay agent that uses the normal DHCP
UDP port. The DHCP server will use the well-known UDP port (67 or
547) when sending Relay-reply message back to the relay agent. It is
recommended to upgrade the server side first before using a non-DHCP
UDP port for a relay agent.
The implementation is advised to allow configuration for relay agent
specifying a DHCP relay port number. It can be used to allow the
relay agent either using a normal DHCP UDP port or non-DHCP UDP port.
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7. IANA Considerations
A new sub-option, DHCP Relay Source Port, is defined in this document
within the DHCPv4 Relay Agent Information Option. It needs to be
assigned by IANA from the DHCP Relay Agent sub-options space
[RFC3046].
A new option, DHCPv6 Relay Source Port, is defined in this document
for DHCPv6 and it needs to be assigned by IANA for the DHCPv6 option
code.
8. Security Considerations
If the network uses firewall to block or allow DHCP packets with both
static UDP source and destination port numbers, this may no longer
match the packets from new DHCP relay agent and server software. The
firewall rules need to be modified only to match the DHCP server side
of the UDP port number, and if necessary, IP addresses and other
attributes.
9. Acknowledgments
The authors would like to thank Peter Arberg, Bhanu Gopalasetty,
Andre Kostur, Ted Lemon, and Kishore Seshadri for their review and
comments of this document.
The authors would like to thank Bernie Volz for discussions that lead
to the definition of The Relay Source Port sub-option and DHCPv6
Relay Source Port Option.
The RFC text was produced using Marshall Rose's xml2rfc tool.
10. Document Change Log
10.1. Changes to draft-ietf-dhc-relay-port-02
o Posted the draft in February 2017.
o Added the Terminology section.
o Defined the Sub-option and Option names for DHCPv4 and DHCPv6.
o Added the DHCPv6 cascaded relay agents example.
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10.2. Changes to draft-ietf-dhc-relay-port-01
o Posted the draft in January 2017.
o Change the DHCPv6 Relay Source Port Option, UDP Source Port field
to Downstream Source Port. Add the option handling mechanism for
DHCPv6 server and relay agents.
10.3. Changes to draft-ietf-dhc-relay-port-00
o Posted first version of working group draft in October 2016.
o This draft was renamed from draft-shen-dhc-client-port-03.txt.
11. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<http://www.rfc-editor.org/info/rfc2131>.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, DOI 10.17487/RFC3046, January 2001,
<http://www.rfc-editor.org/info/rfc3046>.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <http://www.rfc-editor.org/info/rfc3315>.
Authors' Addresses
Naiming Shen
Cisco Systems
560 McCarthy Blvd.
Milpitas, CA 95035
US
Email: naiming@cisco.com
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Enke Chen
Cisco Systems
560 McCarthy Blvd.
Milpitas, CA 95035
US
Email: enkechen@cisco.com
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