bidirectional forwarding
TRANSCRIPT
-
8/2/2019 Bidirectional Forwarding
1/19
Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net 1
Bidirectional Forwarding
Detection (BFD)
0DWW0DWW .RORQ.RORQ
$35,&27$35,&27
.XDOD/XPSXU.XDOD/XPSXU
-
8/2/2019 Bidirectional Forwarding
2/19
2YHUYLHZ
S Goals
S Protocol Overview
S Operation
SStatus
S Implementation Details
-
8/2/2019 Bidirectional Forwarding
3/19
3Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
Detecting Forwarding Failures (I)
S For IP, classically a function of the routing protocol
Because formerly, routing = forwarding
Fault resolution in perhaps tens of seconds
Perhaps too slow for anything but best-effort IP
Sometimes there is no routing protocol!
6RIWZDUH
5RXWHU
6RIWZDUH
5RXWHU
5RXWLQJ3URWRFRO5RXWLQJ3URWRFRO
-
8/2/2019 Bidirectional Forwarding
4/19
4Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
Detecting Forwarding Failures (II)
S Interface state can be used, but:
Transient failures are problematic
So are switched environments
6RIWZDUH
5RXWHU
6RIWZDUH
5RXWHU
-
8/2/2019 Bidirectional Forwarding
5/19
5Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
Detecting Forwarding Failures (III)
S In modern architectures: routing forwarding
S Would be nice to have a way to detect forwardingwithout relying on interface state
5RXWLQJ(QJLQH
3DFNHW)RUZDUGLQJ(QJLQH
5RXWLQJ(QJLQH
3DFNHW)RUZDUGLQJ(QJLQH
-
8/2/2019 Bidirectional Forwarding
6/19
6Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
Goals for BFD
S Detection of forwarding plane-to-forwarding plane
connectivity (including links, interfaces, etc.)S Faster convergence of routing protocols, particularly on shared
media like Ethernet
S
Semantic separation of forwarding plane connectivity vs.control plane connectivity (works well in concert with
Graceful Restart)
S A single mechanism that is independent of media, routing
protocol, and data protocol
S No changes to existing protocols
-
8/2/2019 Bidirectional Forwarding
7/19
7Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
BFD Protocol Overview
S At its heart, Yet Another Hello Protocol
S Packets sent at regular intervals; neighbor failure detectedwhen packets stop showing up
S Intended to be implemented in the forwarding plane to the
extent possible (avoiding fate sharing with the control plane)S Context defined by encapsulating protocol (sending inside
IPv4 packets signals IPv4 connectivity; also could be sent
over IPv6, directly over the datalink, or whatever)
S Always unicast, even on shared media
-
8/2/2019 Bidirectional Forwarding
8/19
8Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
BFD Protocol Overview
S Not just for direct links; can also be used over MPLS LSPs,other tunnels
SSufficient context in the protocol to keep track of multipleparallel paths between systems
S Timing is adjustable on-the-fly, allowing for adaptivity toavoid catastrophic collapse due to false failure detection
6HVVLRQ6HVVLRQ
6HVVLRQ6HVVLRQ
6HVVLRQ6HVVLRQ
6HVVLRQ6HVVLRQ
/LQN/LQN
7XQQHO7XQQHO
-
8/2/2019 Bidirectional Forwarding
9/19
9Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
BFD Operation
S BFD Control Packets sent in both directions, providing basic
connectivity check and continuous parameter negotiationS Optional Echo Mode can be negotiated
BFD Echo Packets transmitted addressed to originating
system; other system forwards them back through regularforwarding path
Exercises entire forwarding path in destination system
May not always be possible or desirable, thus it isnegotiated
-
8/2/2019 Bidirectional Forwarding
10/19
10Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
BFD Async Mode
S Control packets flow in each direction
-
8/2/2019 Bidirectional Forwarding
11/19
11Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
BFD Echo Mode
S Control packets flow in each direction
S Echo packets loop through remote system
S Control packet flow is slower
-
8/2/2019 Bidirectional Forwarding
12/19
12Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
BFD Operation
S Combination of continuous negotiation and unicast allowshigh adaptivity
Less capable systems can negotiate slower detectionwithout penalizing faster systems
Transmission rate and detection time can be changed on-
the-fly, allowing for automatic mechanisms for recoveryfrom overly aggressive timers or unusual network events
Echo mode can be enabled or disabled without taking downBFD session
S Three-way handshake on both up and down transitions
S Each system in full control of its own fate
-
8/2/2019 Bidirectional Forwarding
13/19
13Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
BFD OperationS No discovery protocol, handled by control protocol
S
Semantics of BFD session failure are contextual BFD-over-IP implies neighbor failure; IGP neighborshould be torn down
BFD-over-Ethernet implies switch failure; subnet should be
withdrawn from routing protocols
S Works over a wide range of time constants
Timers specified in microseconds, allowing very fast or
very slow detection
Systems specify how quickly they can receive BFD packetsso that boxes of differing abilities can interoperate
-
8/2/2019 Bidirectional Forwarding
14/19
14Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
BFD Operation
S Discriminator field used to demultiplex incoming packets oncesession is up
Each system chooses 32 bit value that is unique among allsessions, puts it in My Discr field
Remote systems discriminator is looped back in Your
Discr field
Your Discr used to choose session for incoming packets
S Initialization is slightly tricky
First packet has to bedemuxed on other info--sourceaddress and interface index, for example
-
8/2/2019 Bidirectional Forwarding
15/19
15Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
BFD Control Message
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version |H| Diagnostic | Detect Mult | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Discr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Your Discr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Desired Min TX Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Required Min RX Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Required Min Echo RX Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-
8/2/2019 Bidirectional Forwarding
16/19
16Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
BFD State Machine
'RZQ'RZQ,QLW,QLW
8S8S)DLOLQJ)DLOLQJ
,+8,+8
,+8,+8
,+8,+8
WLPHRXWWLPHRXW
,+8,+8,+8,+8
-
8/2/2019 Bidirectional Forwarding
17/19
17Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
BFD Applications
S Routing protocol liveness detection (obviously)
S Switch-to-routerliveness detection
S Router-to-hostlivenessdetection (a real answer forthe web farms)
S Virtual link liveness (MPLS, GRE tunnels, etc.--noneed to keep reinventing these)
S Alarm passthrough (receive alarm from concatenated
link; take session admin-down with diagnosticconcatenated link down)
-
8/2/2019 Bidirectional Forwarding
18/19
18Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net
BFD Status
S Protocol jointly developed by Juniper and Cisco
S Internet draft published (draft-katz-ward-bfd-0x.txt)
S Published as an Informational document (so that
progress can be made in a reasonable amount of time
S Rumors of fast track standardization being pursued
-
8/2/2019 Bidirectional Forwarding
19/19
Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net 19
http://www.juniper.net
Thank You!