Static PW MAC Withdrawal: Media Access Control (MAC) Address Withdrawal over Static Pseudowire
How a pseudowire endpoint tells its peers to forget MAC addresses when signaling over LDP is not available.
Sivabalan, S., Boutros, S., Shah, H., Aldrin, S., and M. Venkatesan, "Media Access Control (MAC) Address Withdrawal over Static Pseudowire", RFC 7769, DOI 10.17487/RFC7769, February 2016, <https://www.rfc-editor.org/info/rfc7769>.
@misc{rfc7769,
series = {Request for Comments},
number = 7769,
howpublished = {RFC 7769},
publisher = {RFC Editor},
doi = {10.17487/RFC7769},
url = {https://www.rfc-editor.org/info/rfc7769},
author = {S. Sivabalan and S. Boutros and H. Shah and S. Aldrin and M. Venkatesan},
title = {{Media Access Control (MAC) Address Withdrawal over Static Pseudowire}},
pagetotal = 10,
year = 2016,
month = feb,
} RFC text
Contents
Internet Engineering Task Force (IETF) S. Sivabalan
Request for Comments: 7769 S. Boutros
Category: Standards Track Cisco Systems, Inc.
ISSN: 2070-1721 H. Shah
Ciena Corp.
S. Aldrin
Google Inc.
M. Venkatesan
Comcast
February 2016
Media Access Control (MAC) Address Withdrawal over Static Pseudowire
Abstract
This document specifies a mechanism to signal Media Access Control
(MAC) address withdrawal notification using a pseudowire (PW)
Associated Channel (ACH). Such notification is useful when
statically provisioned PWs are deployed in a Virtual Private LAN
Service (VPLS) or Hierarchical Virtual Private LAN Service (H-VPLS)
environment.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7769.
Copyright Notice
Copyright (c) 2016 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
(http://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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. MAC Withdraw OAM Message . . . . . . . . . . . . . . . . . . 4
4. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Operation of Sender . . . . . . . . . . . . . . . . . . . 6
4.2. Operation of Receiver . . . . . . . . . . . . . . . . . . 7
5. Security Consideration . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. MPLS G-Ach Type . . . . . . . . . . . . . . . . . . . . . 8
6.2. Sequence Number TLV . . . . . . . . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
An LDP-based MAC address withdrawal mechanism is specified in
[RFC4762] to remove dynamically learned MAC addresses when the source
of those addresses can no longer forward traffic. This is
accomplished by sending an LDP Address Withdraw Message with a MAC
List TLV containing the MAC addresses to be removed from all other
Provider Edge nodes over the LDP sessions. [RFC7361] describes an
optimized MAC withdrawal mechanism that can be used to remove only
the set of MAC addresses that need to be relearned in H-VPLS
networks. [RFC7361] also describes optimized MAC withdrawal
operations in PBB-VPLS networks.
A PW can be signaled via the LDP or can be statically provisioned.
In the case of a static PW, an LDP-based MAC withdrawal mechanism
cannot be used. This is analogous to the problem and solution
described in [RFC6478] where a PW OAM (Operations, Administration,
and Maintenance) message has been introduced to carry the PW status
TLV using the in-band PW Associated Channel. In this document, we
use a PW OAM message to withdraw MAC address(es) learned via a static
PW.
Thus, MAC withdraw signaling for static PW reuses the following
concepts:
- in-band signaling mechanisms used by static PW status signaling
and
- MAC withdrawal mechanisms described by [RFC4762] and [RFC7361].
MAC withdraw signaling is a best effort scheme. It is an attempt to
optimize network convergence by reducing blackholes caused by PW
failover for protected PWs. The protocol defined in this document
addresses possible loss of the MAC withdraw signal due to network
congestion, but does not guarantee delivery, as is the case for the
LDP-based MAC withdraw signaling. In the event that MAC withdraw
signaling does not reach the intended target, the fallback to MAC
re-learning due to bi-directional traffic or as a last resort aging
out of MAC addresses in the absence of frames from the sources, will
resume the traffic via new PW path. Such fallbacks would cause
temporary blackouts but does not render a network permanently
unusable.
2. Terminology
The following terminology is used in this document:
ACK: Acknowledgement for MAC withdraw message
LDP: Label Distribution Protocol
MAC: Media Access Control
MPLS: Multiprotocol Label Switching
PW: Pseudowire
PW OAM: PW Operations, Administration, and Maintenance
TLV: Type, Length, and Value
VPLS: Virtual Private LAN Services
In addition, 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
[RFC2119].
3. MAC Withdraw OAM Message
LDP provides reliable packet transport for control plackets for
dynamic PWs. This can be contrasted with static PWs that rely on
retransmission and acknowledgments (ACKs) for reliable OAM packet
delivery as described in [RFC6478]. The proposed solution for MAC
withdrawal over a static PW also relies on retransmissions and ACKs.
However, an ACK is mandatory. A given MAC withdrawal notification is
sent as a PW OAM message, and the sender retransmits the message a
configured number of times in the absence of an ACK response for the
sequence-numbered message. The receiver removes the MAC address(es)
for a given sequence-number MAC withdraw signaling message and sends
the ACK response. The receipt of the same or lower sequence-number
message is responded to with an ACK but does not cause removal of MAC
addresses. A new TLV to carry the sequence number has been defined.
The format of the MAC address withdraw OAM message is shown in Figure
1. The MAC withdraw PW OAM message follows the same guidelines used
in [RFC6478], whereby the first 4 bytes of the OAM message header are
followed by a message-specific field and a set of TLVs relevant for
the message. Since the MAC withdrawal PW OAM message is not
refreshed forever, a MAC address withdraw OAM message MUST contain a
"Sequence Number TLV"; otherwise, the entire message is dropped. It
MAY contain the MAC Flush Parameter TLV defined in [RFC7361] when
static PWs are deployed in H-VPLS and PBB-VPLS scenarios. The first
2 bits of the sequence-number TLV are reserved and MUST be set to 0
on transmit and ignored on receipt.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1|Version| Reserved | MAC Withdraw OAM Msg (0x28) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | TLV Length |A|R| Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Res| Sequence No. TLV (0x1) | Sequence Number TLV Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MAC List TLV |
~ MAC Flush Parameter TLV (optional) ~
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: MAC Address Withdraw PW OAM Packet Format
In this section, the MAC List TLV and MAC Flush Parameter TLV are
collectively referred to as "MAC TLV(s)". The definition and
processing rules of the MAC List TLV are described by [RFC4762], and
the corresponding rules of the MAC Flush Parameter TLV are governed
by [RFC7361].
"TLV Length" is the total length of all TLVs in the message, and
"Sequence Number TLV Length" is the length of the Sequence Number
field.
A single bit (called "A-bit") is set by a receiver to acknowledge
receipt and processing of a MAC Address Withdraw OAM Message. In the
acknowledge message, with the A-bit set, the MAC TLVs are excluded.
A single bit (called "R-bit") is set to indicate if the sender is
requesting reset of the sequence numbers. The sender sets this bit
when the pseudowire is restarted and has no local record of previous
send and expected receive sequence numbers.
The Sequence Number TLV MUST be the first TLV in the message.
The lack of a reliable transport protocol for the in-band OAM
necessitates a presence of sequencing and acknowledgement scheme so
that the receiver can recognize newer message from retransmitted
older messages. [RFC4385] describes the details of sequence-number
handling, which includes overflow detection for a Sequence Number
field size of 16 bits. This document leverages the same scheme with
the two exemptions:
- the Sequence Number field is of size 32 bits.
- overflow detection is simplified such that a sequence number
that exceeds 2,147,483,647 (0x7FFFFFFF) is considered an
overflow and reset to 1.
4. Operation
This section describes how the initial MAC Withdraw OAM Messages are
sent and retransmitted, as well as how the messages are processed and
retransmitted messages are identified.
4.1. Operation of Sender
Each PW is associated with a counter to keep track of the sequence
number of the transmitted MAC withdrawal messages. Whenever a node
sends a new set of MAC TLVs, it increments the transmitted sequence-
number counter and includes the new sequence number in the message.
The transmit sequence number is initialized to 1 at the onset, after
the wrap and after the sequence number reset request receipt. Hence
the transmit sequence number is set to 2 in the first MAC withdraw
message sent after the sequence number is initialized to 1.
The sender expects an ACK from the receiver within a time interval we
call "Retransmit Time", which can be either a default (1 second) or a
configured value. If the ACK does not arrive within the Retransmit
Time, the sender retransmits the message with the same sequence
number as the original message. The retransmission MUST cease when
an ACK is received. In order to avoid continuous retransmissions in
the absence of acknowledgements, a method of suppressing
retransmissions MUST be implemented. A simple and well-used approach
is to cease retransmission after a small number of transmissions. In
the absence of an ACK response, a one second retransmission with two
retries is RECOMMENDED. However, both the interval and the number of
retries are a local matter that present no interworking issues; thus,
the operator MAY configure different values. Alternatively, an
increasing backoff delay with a larger number of retries MAY be
implemented to improve scaling issues. Whilst there are no
interworking issues with any of these methods, the implementer must
be mindful to not introduce network congestion and must take into
account the decaying value of the delayed MAC withdraw signaling
against possible relearning due to bidirectional traffic or MAC
timeout.
During the period of retransmission, if a need to send a new MAC
withdraw message with updated sequence number arises, then
retransmission of the older unacknowledged withdraw message MUST be
suspended and retransmit time for the new sequence number MUST be
initiated. In essence, a sender engages in retransmission logic only
for the most recently sent withdraw message for a given PW.
In the event that a pseudowire is deleted and re-added or the router
is restarted with configuration, the local node may lose information
about the previously sent sequence number. This becomes problematic
for the remote peer as it will continue to ignore the received MAC
withdraw messages with lower sequence numbers. In such cases, it is
desirable to reset the sequence numbers at both ends of the
pseudowire. The reset R-bit is set in the first MAC withdraw to
notify the remote peer to reset the send and receive sequence
numbers. The R-bit must be cleared in subsequent MAC withdraw
messages after the acknowledgement is received.
4.2. Operation of Receiver
Each PW is associated with a register to keep track of the expected
sequence number of the MAC withdrawal message and is initialized to
1. Whenever a MAC withdrawal message is received, and if the
sequence number on the message is greater than the value in the
register, the MAC addresses contained in the MAC TLVs are removed,
and the register is updated with the received sequence number. The
receiver sends an ACK whose sequence number is the same as that in
the received message.
If the sequence number in the received message is smaller than or
equal to the value in the register, the MAC TLVs are not processed.
However, an ACK with the received sequence number MUST be sent as a
response. The receiver processes the ACK message as an
acknowledgement for all the MAC withdraw messages sent up to the
sequence number present in the ACK message and terminates
retransmission.
The handling of the sequence number is described in Section 3.
A MAC withdraw message with the R-bit set MUST be processed by
resetting the send and receive sequence number first. The rest of
MAC withdraw message processing is performed as described above. The
acknowledgement is sent with the R-bit cleared.
5. Security Consideration
The security measures described in [RFC4447], [RFC5085], and
[RFC6073] are adequate for the proposed mechanism.
6. IANA Considerations
6.1. MPLS G-Ach Type
IANA has assigned a new channel type (0x0028) from the "MPLS
Generalized Associated Channel (G-ACh) Types (including Pseudowire
Associated Channel Types)" registry. The description of the new
channel type is "MAC Withdraw OAM Message".
6.2. Sequence Number TLV
IANA has assigned a new TLV Type (0x0001) from the existing LDP "TLV
Type Name Space" registry. The description for the new TLV Type is
"Sequence Number TLV".
7. 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>.
[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385,
February 2006, <http://www.rfc-editor.org/info/rfc4385>.
[RFC4447] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
G. Heron, "Pseudowire Setup and Maintenance Using the
Label Distribution Protocol (LDP)", RFC 4447,
DOI 10.17487/RFC4447, April 2006,
<http://www.rfc-editor.org/info/rfc4447>.
[RFC4762] Lasserre, M., Ed., and V. Kompella, Ed., "Virtual Private
LAN Service (VPLS) Using Label Distribution Protocol (LDP)
Signaling", RFC 4762, DOI 10.17487/RFC4762, January 2007,
<http://www.rfc-editor.org/info/rfc4762>.
[RFC5085] Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire
Virtual Circuit Connectivity Verification (VCCV): A
Control Channel for Pseudowires", RFC 5085,
DOI 10.17487/RFC5085, December 2007,
<http://www.rfc-editor.org/info/rfc5085>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC6073] Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
Aissaoui, "Segmented Pseudowire", RFC 6073,
DOI 10.17487/RFC6073, January 2011,
<http://www.rfc-editor.org/info/rfc6073>.
[RFC6478] Martini, L., Swallow, G., Heron, G., and M. Bocci,
"Pseudowire Status for Static Pseudowires", RFC 6478,
DOI 10.17487/RFC6478, May 2012,
<http://www.rfc-editor.org/info/rfc6478>.
[RFC7361] Dutta, P., Balus, F., Stokes, O., Calvignac, G., and D.
Fedyk, "LDP Extensions for Optimized MAC Address
Withdrawal in a Hierarchical Virtual Private LAN Service
(H-VPLS)", RFC 7361, DOI 10.17487/RFC7361, September 2014,
<http://www.rfc-editor.org/info/rfc7361>.
Authors' Addresses
Siva Sivabalan
Cisco Systems, Inc.
2000 Innovation Drive
Kanata, Ontario K2K 3E8
Canada
Email: msiva@cisco.com
Sami Boutros
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134
United States
Email: sboutros@cisco.com
Himanshu Shah
Ciena Corp.
3939 North First Street
San Jose, CA 95134
United States
Email: hshah@ciena.com
Sam Aldrin
Google Inc.
Email: aldrin.ietf@gmail.com
Mannan Venkatesan
Comcast
1800 Bishops Gate Blvd
Mount Laurel, NJ 08075
United States
Email: mannan_venkatesan@cable.comcast.com
AI ELI5
On this page
§1Why Static PWs Need Their Own Withdrawal
RFC 4762 defines an LDP-based MAC address withdrawal. A node sends an LDP Address Withdraw Message with a MAC List TLV, and other Provider Edge nodes remove those dynamically learned MAC addresses. RFC 7361 adds an optimized form that removes only the addresses that need to be relearned in H-VPLS networks.
A PW can be signaled with LDP or statically provisioned. On a static PW the LDP mechanism cannot be used. This RFC follows the approach of RFC 6478, which carries PW status over the in-band PW Associated Channel, and uses a PW OAM message to withdraw MAC addresses learned over a static PW.
Some pathways have a built-in loudspeaker for announcements, and some do not. This RFC gives the ones without a loudspeaker a way to pass notes instead.
§2Best Effort, With Fallbacks
MAC withdraw signaling is a best effort scheme. It tries to speed up network convergence by reducing blackholes caused by PW failover for protected PWs. The protocol handles possible loss of the signal due to congestion, but does not guarantee delivery, just as with LDP-based signaling.
If the signal never arrives, traffic resumes by other means: MAC relearning from bi-directional traffic, or, as a last resort, aging out of MAC addresses when no frames arrive from those sources. These fallbacks cause temporary blackouts but do not leave the network permanently unusable.
If your note gets lost, people will eventually notice the kid moved anyway. It just takes longer, and some mail goes to the wrong desk in the meantime.
§3The MAC Withdraw OAM Message
Unlike LDP, static PWs have no reliable transport, so this design relies on retransmissions and acknowledgements. Here the ACK is mandatory. The message follows the RFC 6478 guidelines: the first 4 bytes of the OAM header, then a message-specific field and a set of TLVs.
The message MUST contain a Sequence Number TLV, and it MUST be the first TLV; otherwise the whole message is dropped. It MAY also carry the MAC Flush Parameter TLV from RFC 7361 for H-VPLS and PBB-VPLS. The MAC List TLV follows the rules of RFC 4762. The channel type is 0x28 and the Sequence Number TLV type is 0x1.
Two flag bits matter. The A-bit is set by a receiver to acknowledge a message, and an acknowledgement leaves out the MAC TLVs. The R-bit asks the peer to reset sequence numbers; a sender sets it when the PW has restarted and has no record of earlier sequence numbers.
Each note has a number on it, a box to tick that says "this is my reply", and a box that says "let's start counting from the beginning again".
§3.1Sequence Numbers
Because the OAM channel has no reliable transport, sequence numbers let the receiver tell a new message from a retransmitted old one. The scheme comes from RFC 4385 with two differences: the Sequence Number field is 32 bits, and overflow detection is simpler. A sequence number above 2,147,483,647 (0x7FFFFFFF) counts as an overflow and is reset to 1.
Numbering the notes lets you spot a copy of an old note you already read. If the counter gets too big, it starts again at 1.
§4Sender and Receiver Behavior
The sender keeps a transmit counter per PW. Each new set of MAC TLVs increments it and the new number goes in the message. The counter starts at 1, so the first withdraw message sent after initialization carries 2. The sender waits for an ACK for the Retransmit Time, 1 second by default or a configured value, then resends with the same sequence number. Retransmission MUST stop when an ACK arrives, and a way to suppress endless retransmission MUST be implemented. One second with two retries is RECOMMENDED, but intervals, retry counts and an increasing backoff are local matters. If a newer withdraw message is needed, the sender MUST drop retransmission of the older one and only retries the latest.
The receiver keeps an expected-sequence register per PW, initialized to 1. If an arriving sequence number is greater than the register, it removes the MAC addresses and updates the register. It then sends an ACK carrying the same sequence number. If the number is smaller than or equal to the register, the MAC TLVs are not processed, but an ACK with that number MUST still be sent. An ACK counts for all messages up to that sequence number.
If a PW is deleted and re-added or the router restarts, the node may forget its sequence number and the peer would ignore its messages. The first MAC withdraw then sets the R-bit. A receiver MUST reset its send and receive sequence numbers first, then process the message, and send the ACK with the R-bit cleared.
The sender keeps knocking until someone says "got it", but only for its latest message. The listener answers every knock, even repeats, but only acts on a note it has not seen before.
- MAC withdraw, seq 2Sent with the MAC TLVs.
- MAC withdraw, seq 2Resent after the Retransmit Time passes with no ACK.
- ACK, seq 2 (A-bit)No MAC TLVs. The sender stops retransmitting.
§5Security and IANA
The security section is a single sentence: the measures described in RFC 4447, RFC 5085 and RFC 6073 are adequate for this mechanism.
IANA assigned channel type 0x0028 ("MAC Withdraw OAM Message") from the MPLS Generalized Associated Channel (G-ACh) Types registry, and TLV type 0x0001 ("Sequence Number TLV") from the LDP TLV Type Name Space registry.
The authors say the safety rules already used for these pathways are good enough. Two new labels were registered so nobody else reuses them.