| Internet Engineering Task Force | N. Akiya |
| Internet-Draft | C. Pignataro |
| Intended status: Standards Track | D. Ward |
| Expires: February 22, 2014 | Cisco Systems |
| M. Bhatia | |
| Alcatel-Lucent | |
| August 21, 2013 |
Seamless Bidirectional Forwarding Detection (BFD) with MPLS Label Verification Extension
draft-akiya-bfd-seamless-base-01
This document defines a simplified mechanism to use Bidirectional Forwarding Detection (BFD) with large portions of negotiation aspects eliminated, that allows full and partial reachability verification. For MPLS based BFD, extensions to the generic mechanism are defined that allows BFD to perform a level of label verification.
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].
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
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This Internet-Draft will expire on February 22, 2014.
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Bidirectional Forwarding Detection (BFD), [RFC5880] and related documents, has efficiently generalized the failure detection mechanism for multiple protocols and applications. There are some improvements which can be made to better fit existing technologies. There are also possibility of evolving BFD to better fit new technologies. This document focuses on several aspects of BFD in order to further improve efficiency, to expand failure detection coverage and to allow BFD usage for wider scenarios.
This specification provides solutions to above issues by defining a generic mechanism to use Bidirectional Forwarding Detection (BFD) with large portions of negotiation aspects eliminated, that allows full and partial reachability validation. For MPLS based BFD, extensions to the generic mechanism are defined for BFD to perform a level of label verifications. Because the mechanism eliminates much of negotiation aspects of the BFD protocol, "Seamless BFD" has been chosen as the name for this mechanism.
To operate Seamless BFD, set of network entities are first selected. Each network node hosting selected network entities then assigns a special BFD discriminator to each selected local network entity. These network nodes will also create a BFD session instance that listens for incoming BFD control packets with "your discriminator" having local special BFD discriminators. Mappings between selected network entities and corresponding special BFD discriminators are known by other network nodes belonging in the same network. A network node in such network is then able to send a BFD control packet to a particular target with corresponding special BFD discriminator as "your discriminator". Target network node, upon reception of such BFD control packet, will transmit a response BFD control packet back to the sender.
Example: IPv4 address 1.2.3.4 is selected as the Seamless BFD target. Node hosting IPv4 address 1.2.3.4 reserves the BFD discriminator 0x01020304, and creates a BFD session instance in listening mode. Node X sends a BFD control packet with destination IP address 1.2.3.4, source IP address X, "your discriminator"=0x01020304 and "my discriminator"=<locally assigned discriminator>. Node hosting IPv4 address 1.2.3.4 will receive this packet, swaps received "your discriminator"/"my discriminator" and generates a response BFD control packet destined to X.
The reader is expected to be familiar with the BFD, IP, MPLS and SR terminology and protocol constructs. This section describes several new terminology introduced by Seamless BFD.
Number of network entity types (ex: IP address, segment ID) can make use of this mechanism. To differentiate between different network entity types, a value is assigned to each type.
BFD Target Identifier types:
Value BFD Target Identifier Type
------ --------------------------
0 Reserved
1 IP (IPv4 Address and Router ID)
2 Segment Routing Node Segment ID
Further identifier types to be defined as needed basis.
All local network identifiers which are to participate in this mechanism are to have specific BFD discriminators assigned. Assigned BFD discriminators are attached to corresponding identifiers until they are explicitly un-provisioned. BFD discriminators used for this mechanism are considered reserved, and MUST NOT be reused for other BFD sessions.
Some examples of network identifier to BFD discriminator mappings:
It is possible, although foreseen to be extremely rare, for identifiers of different BFD target identifier types to map to same BFD discriminator. How such conflict is to be resolved is outside the scope of this document.
Each network node is responsible for creating and maintaining a table that contains BFD discriminators, BFD target identifier types and BFD target identifiers. Intention of this table is to allow local entities to perform following lookups:
This table is to contain entries for all locally reserved BFD discriminators and corresponding information. This table may need to contain entries from other network nodes, depending on the BFD target identifier type.
Each network node MUST create one or more reflector BFD sessions. This reflector BFD session is a session which transmits BFD control packets in response to received valid locally destined BFD control packets. Specifically, this reflector BFD session is to have following characteristics:
One reflector BFD session can be responsible for handling response to received BFD control packets targeted to all local BFD target identifiers, or few reflector BFD sessions can each be responsible for subset of local BFD target identifiers. This policy is a local matter, and is outside the scope of this document.
Not that incoming BFD control packets destined to certain BFD target identifier types may be IPv4, IPv6 and MPLS based. For those BFD target identifier types, implementations can either allow same reflector BFD session to handle all incoming BFD control packets in address family agnostic fashion, or setup multiple reflector BFD sessions to handle incoming BFD control packets with deviation address family. This policy is again a local matter, and is outside the scope of this document.
Any network node can attempt to perform a full reachability validation to any BFD target identifier on other network nodes, as long as destination BFD target identifier is provisioned to use this mechanism. Transmitted BFD control packet by the initiator is to have "your discriminator" corresponding to destination BFD target identifier.
A node that initiates a BFD control packet can create an active BFD session to periodically send BFD control packet to a target, or BFD control packet can be crafted and sent out on "as needed basis" (ex: BFD ping) without any session presence. In both cases, a BFD instance MUST have unique "my discriminator" value assigned. If a node is to create multiple BFD instances to a same BFD target identifier, then each instance MUST have separate "my discriminator" values assigned. A BFD instance MUST NOT use a discriminator corresponding to one of local BFD target identifiers as "my discriminator". This is to prevent incoming response BFD control packets ("pong" packets) having "your discriminator" as a discriminator corresponding local BFD target identifier.
Below ASCII art describes high level concept of full reachability validations using this mechanism. R2 reserves value XX as BFD discriminator for its BFD target identifier. ASCII art shows that R1 and R4 performing full reachability validation to XX on R2.
-- md=50/yd=XX (BFD ping) -->
<-- md=XX/yd=50 (BFD pong) --
[*]
R1 ---------------------- R2 ----------- R3 ----------- R4
| ^
| |
| + - md=60/yd=XX (BFD ping) --
+ - - -md=XX/yd=60 (BFD pong) -->
[*] Reflector BFD session on R2.
If BFD control packet is to be sent via IP path, then:
If BFD control packet is to be sent via explicit label switching, then:
A network node which receives BFD control packets transmitted by an initiator is referred as responder. Responder, upon reception of BFD control packets, is to perform necessary relevant validations described in [RFC5880]/[RFC5881]/[RFC5883]/[RFC5884]/[RFC5885].
When responder receives a BFD control packet, if "your discriminator" value is not one of local entries in the BFD target identifier table, then this packet MUST NOT be considered for this mechanism. If "your discriminator" value is one of local entries in the BFD target identifier table, then the packet is determined to be handled by a reflector BFD session responsible for specified BFD targeted identifier. If the packet was determined to be processed further for this mechanism, then chosen reflector BFD session is to transmit a response BFD control packet using procedures described in Section 8.2.2, unless prohibited by local administrative or local policy reasons.
BFD target identifier type MUST be used to determine further information on how to reach back to the initiator.
In addition, destination IP address of received BFD control packet MUST be examined to determine how to construct response BFD control packet to send back to the initiator.
If destination IP address of received BFD control packet is not 127/8 for IPv4 or 0:0:0:0:0:FFFF:7F00/104 for IPv6, then:
If destination IP address of received BFD control packet is 127/8 for IPv4 or 0:0:0:0:0:FFFF:7F00/104 for IPv6, then received IP destination MUST be further examined to determine response transport options. If last 23 bits of 127/8 for IPv4 and 0:0:0:0:0:FFFF:7F00/104 for IPv6 is zero, then response SHOULD be label switched but MAY be IP routed. If last 23 bits of 127/8 for IPv4 and 0:0:0:0:0:FFFF:7F00/104 for IPv6 is not zero, then response SHOULD be label switched and SHOULD NOT be IP routed. Description of 23 bits is described in Section 10.
If BFD control packet response is determined to be IP routed, then:
If BFD control packet response is determined to be label switched, then:
Regardless of the response type, BFD control packet being sent by the responder MUST perform following procedures:
In addition, reflector BFD session SHOULD transmit response BFD control packet on the same interface which it received the packet from initiator.
Further details of BFD control packets sent by initiator (ex: active BFD session):
Further details of BFD control packets sent by responder (reflector BFD session):
Diagnostic value in both directions MAY be set to a certain value, to attempt to communicate further information to both ends. However, details of such are outside the scope of this specification.
The Poll sequence MUST operate accordance as [RFC5880].
Control plane independent (C) bit for BFD instances speaking to a reflector BFD session MUST work accordingly to [RFC5880]. Reflector BFD session also MUST work accordingly to [RFC5880]. Specifically, if reflector BFD session implementation does not share fate with control plane, then response BFD control packets transmitted MUST have control plane independent (C) bit set. If reflector BFD session implementation shares fate with control plane, then response BFD control packets transmitted MUST NOT have control plane independent (C) bit set.
Same mechanism as described in "Full Reachability Validations" section will be applied with exception of following differences on initiator.
This section is only applicable to MPLS based sessions using this mechanism.
With full and partial reachability validations, initiator has the ability to determine if target identifier received the packet on any interfaces. This section describes additional mechanism for initiator to determine if target identifier received the packet on a specific interface.
So far for MPLS based sessions, this mechanism makes use of destination IP address of 127/8 range for IPv4 and of 0:0:0:0:0:FFFF:7F00/104 range for IPv6, in both directions. In this section, 127/8 will be used to describe the MPLS label verification mechanism. However, same concept is to be applied to IPv6 range 0:0:0:0:0:FFFF:7F00/104.
When a network node wishes to perform MPLS label verification, BFD control packet will have lower 23 bits of 127/8 destination IP address embedded with non-zero value. One such non-zero value MAY be (label value + EXP) that is used to reach intended target identifier. Receiver of this BFD control packet, if last 23 bits of 127/8 address is not zero, then will embed information reflecting how the packet was received in the lower 23 bits of 127/8 destination IP address in the response BFD control packet. If responder received the BFD control packet on a non-point-to-point interface, source MAC address MAY need to be examined to determine the "RX info" to embed in the returning packet.
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x7F |R| Zero or (label + EXP) or RX info | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiator receiving back a response will know that packet did reach intended identifier. Initiator can also look into lower 23 bits of IP destination address in received BFD control packet to determine if packet sent was received by intended identifier in expected way (ex: expected RX interface).
When (label + EXP) is being encoded, label is specified in higher 20 bits of 23 bits and EXP is specified in lower 3 bits of 23 bits.
If a response BFD control packet is received, then initiator can conclude that a packet has reached intended node correctly. With information embedded in last 23 bits of response BFD control packet from responder, initiator has the ability to perform further verifications on how responded node received BFD control packet.
Last 23 bits of 127/8 for IPv4 and 0:0:0:0:0:FFFF:7F00/104 for IPv6 being non-zero is the trigger for responder to embed RX information in the response. When initiator is performing only reachability validations to target identifiers, then last 23 bits of the localhost address SHOULD be zero. This is to ensure unnecessary processing at responder is eliminated. However, last 23 bits of the localhost address MAY be set to a non-zero value to traverse specific ECMP path if required. Obvious side effect is the additional processing at responder to populate the RX info in response packet.
This mechanism brings forth one noticeable difference in terms of scaling aspect: number of BFD sessions. This specification eliminates the need for egress nodes to have fully active BFD sessions when only side is desired to perform reachability validations. With introduction of reflector BFD concept, egress no longer is required to create any active BFD session per path/LSP basis. Due to this, total number of BFD sessions in a network is reduced.
If traditional BFD technology was used on a network comprised of N nodes, and each node monitored M unidirectional paths/LSPs, then total number of BFD sessions in such network will be:
(((N - 1) x M) x 2)
Assuming that each network node creates one reflector BFD session to handle all local BFD target identifiers, then total number of BFD sessions in same scenario will be:
(((N - 1) x M) + N)
This mechanism has no issues being deployed with traditional BFDs ([RFC5881]/[RFC5883]/[RFC5884]/[RFC5885]) because BFD discriminators which allow this mechanism to function are explicitly reserved.
BFD echo is outside the scope of this document.
Conceptually, Seamless BFD is as a way to perform BFD Echo Mode using BFD control packets. Critical differentiator being that target (ex: egress) is still required to respond. This allows greater control of a session to the initiator while required target (ex: egress) response allows for proper validations.
This section visits each aspect specified in the Introduction (Section 1) and describes how Seamless BFD provides beneficial impacts.
Same security considerations as [RFC5880], [RFC5881], [RFC5883], [RFC5884] and [RFC5885] apply to this document.
Additionally, implementing following measures will strengthen security aspects of this mechanism described by this document.
BFD Target Identifier types:
Value BFD Target Identifier Type
------ --------------------------
0 Reserved
1 IP (IPv4 Address and Router ID)
2 Segment Routing Node Segment ID
Authors would like to thank Marc Binderberger, Mallik Mudigonda, Srihari Raghavan and Vengada Prasad Govindan from Cisco Systems for providing valuable comments.
Tarek Saad
Cisco Systems
Email: tsaad@cisco.com
Siva Sivabalan
Cisco Systems
Email: msiva@cisco.com
Nagendra Kumar
Cisco Systems
Email: naikumar@cisco.com
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC5880] | Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD)", RFC 5880, June 2010. |
| [RFC5881] | Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June 2010. |
| [RFC5883] | Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD) for Multihop Paths", RFC 5883, June 2010. |
| [RFC5884] | Aggarwal, R., Kompella, K., Nadeau, T. and G. Swallow, "Bidirectional Forwarding Detection (BFD) for MPLS Label Switched Paths (LSPs)", RFC 5884, June 2010. |
| [I-D.previdi-filsfils-isis-segment-routing] | Previdi, S., Filsfils, C., Bashandy, A., Horneffer, M., Decraene, B., Litkowski, S., Milojevic, I., Shakir, R., Ytti, S., Henderickx, W. and J. Tantsura, "Segment Routing with IS-IS Routing Protocol", Internet-Draft draft-previdi-filsfils-isis-segment-routing-02, March 2013. |
| [RFC2827] | Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, May 2000. |
| [RFC4379] | Kompella, K. and G. Swallow, "Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures", RFC 4379, February 2006. |
| [RFC5885] | Nadeau, T. and C. Pignataro, "Bidirectional Forwarding Detection (BFD) for the Pseudowire Virtual Circuit Connectivity Verification (VCCV)", RFC 5885, June 2010. |
| [RFC6428] | Allan, D., Swallow Ed. , G. and J. Drake Ed. , "Proactive Connectivity Verification, Continuity Check, and Remote Defect Indication for the MPLS Transport Profile", RFC 6428, November 2011. |
| [I-D.ietf-bfd-on-lags] | Bhatia, M., Chen, M., Boutros, S., Binderberger, M. and J. Haas, "Bidirectional Forwarding Detection (BFD) on Link Aggregation Group (LAG) Interfaces", Internet-Draft draft-ietf-bfd-on-lags-00, May 2013. |