S-BFD for IP and MPLS: Seamless Bidirectional Forwarding Detection (S-BFD) for IPv4, IPv6, and MPLS
The UDP port numbers and packet rules that let Seamless BFD run over IPv4, IPv6 and MPLS.
Pignataro, C., Ward, D., and N. Akiya, "Seamless Bidirectional Forwarding Detection (S-BFD) for IPv4, IPv6, and MPLS", RFC 7881, DOI 10.17487/RFC7881, July 2016, <https://www.rfc-editor.org/info/rfc7881>.
@misc{rfc7881,
series = {Request for Comments},
number = 7881,
howpublished = {RFC 7881},
publisher = {RFC Editor},
doi = {10.17487/RFC7881},
url = {https://www.rfc-editor.org/info/rfc7881},
author = {C. Pignataro and D. Ward and N. Akiya},
title = {{Seamless Bidirectional Forwarding Detection (S-BFD) for IPv4, IPv6, and MPLS}},
pagetotal = 8,
year = 2016,
month = jul,
} RFC text
Contents
Internet Engineering Task Force (IETF) C. Pignataro
Request for Comments: 7881 D. Ward
Category: Standards Track Cisco
ISSN: 2070-1721 N. Akiya
Big Switch Networks
July 2016
Seamless Bidirectional Forwarding Detection (S-BFD)
for IPv4, IPv6, and MPLS
Abstract
This document defines procedures for using Seamless Bidirectional
Forwarding Detection (S-BFD) in IPv4, IPv6, and MPLS environments.
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
http://www.rfc-editor.org/info/rfc7881.
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 ....................................................2
2. S-BFD UDP Port ..................................................2
3. S-BFD Echo UDP Port .............................................3
4. S-BFD Control Packet Demultiplexing .............................3
5. Initiator Procedures ............................................3
5.1. Details of S-BFD Control Packets Sent by SBFDInitiator .....4
5.1.1. Target versus Remote Entity (S-BFD Discriminator) ...4
6. Responder Procedures ............................................5
6.1. Details of S-BFD Control Packets Sent by SBFDReflector .....5
7. Security Considerations .........................................6
8. IANA Considerations .............................................6
9. References ......................................................7
9.1. Normative References .......................................7
9.2. Informative References .....................................7
Acknowledgements ...................................................8
Contributors .......................................................8
Authors' Addresses .................................................8
1. Introduction
Seamless Bidirectional Forwarding Detection (S-BFD) [RFC7880] defines
a generalized mechanism to allow network nodes to seamlessly perform
continuity checks to remote entities. This document defines
necessary procedures for using S-BFD in IPv4, IPv6, and MPLS
environments.
The reader is expected to be familiar with the IP [RFC791] [RFC2460],
BFD [RFC5880], MPLS BFD [RFC5884], and S-BFD [RFC7880] terms and
protocol constructs.
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. S-BFD UDP Port
A new UDP port is defined for use by S-BFD in IPv4, IPv6, and MPLS
environments: 7784.
In S-BFD Control packets from the SBFDInitiator to the SBFDReflector,
the SBFDReflector session MUST listen for incoming S-BFD Control
packets on port 7784. SBFDInitiator sessions MUST transmit S-BFD
Control packets with destination port 7784. The source port of the
S-BFD Control packets transmitted by SBFDInitiator sessions can be
any port, with one exception: it MUST NOT be 7784. The same UDP
source port number MUST be used for all S-BFD Control packets
associated with a particular SBFDInitiator session. The source port
number is unique among all SBFDInitiator sessions on the system.
In S-BFD Control packets from the SBFDReflector to the SBFDInitiator,
the SBFDInitiator session MUST listen for reflected S-BFD Control
packets at its source port.
3. S-BFD Echo UDP Port
The BFD Echo port defined by [RFC5881], port 3785, is used for the
S-BFD Echo function in IPv4, IPv6, and MPLS environments.
SBFDInitiator sessions MUST transmit S-BFD Echo packets with
destination port 3785. The setting of the UDP source port [RFC5881]
and the procedures [RFC7880] for the S-BFD Echo function are outside
the scope of this document.
4. S-BFD Control Packet Demultiplexing
S-BFD Control packet demultiplexing follows the procedure specified
in Section 7.1 of [RFC7880]. A received S-BFD Control packet MUST be
demultiplexed with the destination UDP port field.
This procedure for an S-BFD packet is executed on both the initiator
and the reflector. If the port is 7784 (i.e., an S-BFD packet for
the SBFDReflector), then the packet MUST be looked up to locate a
corresponding SBFDReflector session based on the value from the
Your Discriminator field in the table describing S-BFD
Discriminators. If the port is not 7784 but the packet is
demultiplexed to be for an SBFDInitiator, then the packet MUST be
looked up to locate a corresponding SBFDInitiator session based on
the value from the Your Discriminator field in the table describing
BFD Discriminators. In that case, the destination IP address of the
packet SHOULD be validated to be for itself. If the packet
demultiplexes to a classical BFD session, then the procedures from
[RFC5880] apply.
5. Initiator Procedures
S-BFD Control packets are transmitted with an IP header, UDP header,
and BFD Control packet ([RFC5880]). When S-BFD Control packets are
explicitly label switched (i.e., not IP routed and forwarded over a
Label Switched Path (LSP), but explicitly sent on a specific LSP),
the former is prepended with a label stack. Note that this document
does not make a distinction between a single-hop S-BFD scenario and a
multi-hop S-BFD scenario; both scenarios are supported.
The necessary values in the BFD control headers are described in
[RFC7880]. Section 5.1 describes necessary values in the MPLS
header, IP header, and UDP header when an SBFDInitiator on the
initiator is sending S-BFD Control packets.
5.1. Details of S-BFD Control Packets Sent by SBFDInitiator
o Specifications common to both IP-routed S-BFD Control packets and
explicitly label-switched S-BFD Control packets:
* The Source IP Address field of the IP header MUST be set to a
local IP address that is expected to be routable by the target
(i.e., not an IPv6 link-local address when the target is
multiple hops away).
* The UDP destination port MUST be set to a well-known UDP
destination port assigned for S-BFD, i.e., 7784.
* The UDP source port MUST NOT be set to 7784.
o Specifications for IP-routed S-BFD Control packets:
* The Destination IP Address field of the IP header MUST be set
to an IP address of the target.
* The TTL / Hop Limit field of the IP header SHOULD be set
to 255.
o Specifications for explicitly label-switched S-BFD Control
packets:
* S-BFD Control packets MUST have the label stack that is
expected to reach the target.
* The TTL field of the topmost label SHOULD be 255.
* The destination IP address MUST be chosen from the 127/8 range
for IPv4 and from the 0:0:0:0:0:ffff:7f00:0/104 range for IPv6,
as per [RFC5884].
* The TTL / Hop Limit field of the IP header MUST be set to 1.
5.1.1. Target versus Remote Entity (S-BFD Discriminator)
Typically, an S-BFD Control packet will have the Your Discriminator
field corresponding to an S-BFD Discriminator of the remote entity
located on the target network node defined by the destination IP
address or the label stack. It is, however, possible for an
SBFDInitiator to carefully set the Your Discriminator and TTL fields
to perform a continuity test in the direction towards a target, but
destined to a transit network node and not to the target itself.
Section 5.1 intentionally uses the word "target" instead of "remote
entity" to accommodate this possible S-BFD usage through TTL expiry.
This also requires that S-BFD Control packets not be dropped by the
responder node due to TTL expiry. Thus, implementations on the
responder MUST allow received S-BFD Control packets taking a TTL
expiry exception path to reach the corresponding SBFDReflector
session. This is an existing packet-processing exception practice
for Operations, Administration, and Maintenance (OAM) packets, where
the control plane further identifies the type of OAM by the protocol
and port numbers.
6. Responder Procedures
S-BFD Control packets are IP routed back to the initiator and will
have an IP header, UDP header, and BFD control header. If an
SBFDReflector receives an S-BFD Control packet with a UDP source port
of 7784, the packet MUST be discarded. Necessary values in the BFD
control header are described in [RFC7880]. Section 6.1 describes
necessary values in the IP header and UDP header when an
SBFDReflector on the responder is sending S-BFD Control packets.
6.1. Details of S-BFD Control Packets Sent by SBFDReflector
o The Destination IP Address field of the IP header MUST be copied
from the Source IP Address field of the received S-BFD Control
packet.
o The Source IP Address field of the IP header MUST be set to a
local IP address that the initiator expects to be visible (i.e.,
not an IPv6 link-local address when the initiator is multiple hops
away). The source IP address SHOULD be copied from the
Destination IP Address field of the received S-BFD Control packet,
except when it is from the 127/8 range for IPv4 or from the
0:0:0:0:0:ffff:7f00:0/104 range for IPv6.
o The TTL / Hop Limit field of the IP header MUST be set to 255.
o The UDP destination port MUST be copied from the received UDP
source port.
o The UDP source port MUST be copied from the received UDP
destination port.
7. Security Considerations
Security considerations for S-BFD are discussed in [RFC7880].
Additionally, implementing the following measures will strengthen
security aspects of the mechanism described by this document:
o Implementations MUST provide filtering capability based on source
IP addresses of received S-BFD Control packets; see [RFC2827].
o Implementations MUST NOT act on received S-BFD Control packets
containing source Martian IP addresses (i.e., addresses that, by
application of the current forwarding tables, would not have their
return traffic routed back to the sender).
o Implementations MUST ensure that response S-BFD Control packets
generated by the SBFDReflector and sent to the initiator have a
reachable target (e.g., destination IP address).
8. IANA Considerations
A new port number value, 7784, was allocated from the "Service Name
and Transport Protocol Port Number Registry". The allocated registry
entry is:
Service Name (REQUIRED)
s-bfd
Transport Protocol(s) (REQUIRED)
udp
Assignee (REQUIRED)
IESG <iesg@ietf.org>
Contact (REQUIRED)
IETF Chair <chair@ietf.org>
Description (REQUIRED)
Seamless Bidirectional Forwarding Detection (S-BFD)
Reference (REQUIRED)
RFC 7881
Port Number (OPTIONAL)
7784
9. References
9.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<http://www.rfc-editor.org/info/rfc5880>.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
DOI 10.17487/RFC5881, June 2010,
<http://www.rfc-editor.org/info/rfc5881>.
[RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
Pallagatti, "Seamless Bidirectional Forwarding Detection
(S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
<http://www.rfc-editor.org/info/rfc7880>.
9.2. Informative References
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC791, September 1981,
<http://www.rfc-editor.org/info/rfc791>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <http://www.rfc-editor.org/info/rfc2827>.
[RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
"Bidirectional Forwarding Detection (BFD) for MPLS Label
Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884,
June 2010, <http://www.rfc-editor.org/info/rfc5884>.
Acknowledgements
The authors would like to thank the BFD WG members for helping to
shape the contents of this document. In particular, significant
contributions were made by the following people: Marc Binderberger,
Jeffrey Haas, Santosh Pallagatti, Greg Mirsky, Sam Aldrin, Vengada
Prasad Govindan, Mallik Mudigonda, and Srihari Raghavan.
Contributors
The following are key contributors to this document:
Tarek Saad, Cisco Systems, Inc.
Siva Sivabalan, Cisco Systems, Inc.
Nagendra Kumar, Cisco Systems, Inc.
Authors' Addresses
Carlos Pignataro
Cisco Systems, Inc.
Email: cpignata@cisco.com
Dave Ward
Cisco Systems, Inc.
Email: wardd@cisco.com
Nobo Akiya
Big Switch Networks
Email: nobo.akiya.dev@gmail.com
AI ELI5
On this page
§1Introduction
Seamless Bidirectional Forwarding Detection (S-BFD), defined in RFC 7880, is a generalized mechanism that lets network nodes perform continuity checks to remote entities without the session setup of classical BFD. RFC 7881 supplies the procedures for using it in IPv4, IPv6 and MPLS environments.
The reader is expected to know IP, BFD (RFC 5880), MPLS BFD (RFC 5884) and S-BFD. The document does not distinguish single-hop from multi-hop S-BFD; both are supported.
S-BFD is a quick way to ask 'can I still reach you?'. This document says exactly how to send that question on common kinds of networks.
§2UDP Ports
A new UDP port, 7784, is defined for S-BFD control packets. A SBFDReflector session MUST listen on port 7784, and SBFDInitiator sessions MUST send control packets with destination port 7784. The initiator's source port can be any port except 7784. The same source port MUST be used for all packets of one initiator session, and it is unique among all initiator sessions on the system. The initiator listens for reflected packets at that source port.
The S-BFD Echo function uses the BFD Echo port from RFC 5881, port 3785. Initiators MUST send Echo packets with destination port 3785. The source port and the Echo procedures themselves are outside the scope of this document.
Ports are like door numbers. Questions go to door 7784, and the answer comes back to whichever door the asker used, which is never 7784.
§3Demultiplexing Control Packets
Received S-BFD control packets are sorted using the destination UDP port, following Section 7.1 of RFC 7880. The same procedure runs on both initiator and reflector.
If the port is 7784, the packet is for a reflector, and the node MUST look up a reflector session using the Your Discriminator field in the table of S-BFD discriminators. If the port is not 7784 but the packet is for an initiator, the node MUST look up an initiator session using Your Discriminator in the table of BFD discriminators, and SHOULD check that the destination IP address is its own. If the packet belongs to a classical BFD session, the procedures of RFC 5880 apply.
When a message arrives, the door number says whether it is a question or an answer, and a name tag inside says which exact desk should get it.
§4Initiator Procedures
S-BFD control packets carry an IP header, a UDP header and a BFD control packet. When they are explicitly label switched (sent on a specific LSP rather than IP routed), a label stack is prepended. The BFD control header values come from RFC 7880.
For all packets, the source IP address MUST be a local address expected to be routable by the target (not an IPv6 link-local address when the target is several hops away), the UDP destination port MUST be 7784, and the UDP source port MUST NOT be 7784.
For IP-routed packets, the destination address MUST be an address of the target and the TTL or Hop Limit SHOULD be 255. For explicitly label-switched packets, the packet MUST carry the label stack expected to reach the target, the topmost label's TTL SHOULD be 255, the destination IP address MUST come from 127/8 for IPv4 or 0:0:0:0:0:ffff:7f00:0/104 for IPv6 as per RFC 5884, and the IP TTL or Hop Limit MUST be 1.
The RFC uses the word target rather than remote entity on purpose. An initiator can set Your Discriminator and TTL carefully to test the path toward a target but destined to a transit node, using TTL expiry. For this, responders MUST let received S-BFD control packets on the TTL expiry exception path reach the matching reflector session, as is existing practice for OAM packets.
The asker writes its own address on the envelope, aims at door 7784, and gives it plenty of travel time. If it uses a special tunnel, it uses a special address so the message stays on that tunnel.
- S-BFD ControlUDP destination port 7784; source port is any port except 7784.
- Reflected S-BFD ControlPorts swapped; IP source and destination addresses reversed; TTL 255.
§5Reflector Procedures
The reflector sends its replies back by normal IP routing, with an IP header, a UDP header and a BFD control header. If a reflector receives a packet whose UDP source port is 7784, it MUST discard it.
For each reply, the destination IP address MUST be copied from the source address of the received packet. The source IP address MUST be a local address the initiator expects to see (again not IPv6 link-local if the initiator is several hops away) and SHOULD be copied from the destination address of the received packet, except when that is from 127/8 or 0:0:0:0:0:ffff:7f00:0/104. The TTL or Hop Limit MUST be 255. The UDP ports are swapped: the destination port is copied from the received source port, and the source port from the received destination port.
The answerer flips the envelope: it sends the reply to the sender's address and door, from the door the question came in on.
§6Security Considerations
Security considerations for S-BFD are in RFC 7880. RFC 7881 adds three measures. Implementations MUST be able to filter on the source IP addresses of received S-BFD control packets (see RFC 2827). They MUST NOT act on control packets from source Martian addresses, meaning addresses whose return traffic would not be routed back to the sender by the current forwarding tables. And they MUST make sure reflected packets have a reachable target, such as a reachable destination IP address.
Do not answer letters with fake return addresses, and only reply to addresses you can actually reach.
§7IANA Port Assignment
IANA allocated port 7784 from the Service Name and Transport Protocol Port Number Registry. The entry has service name s-bfd, transport protocol UDP, the IESG as assignee, the IETF Chair as contact, and the description 'Seamless Bidirectional Forwarding Detection (S-BFD)', with RFC 7881 as the reference.
Door number 7784 is now officially reserved for this kind of check.