Moderate BGP
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Transcript of Moderate BGP
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8/3/2019 Moderate BGP
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BGP, where are wenow?
John Scudder and David WardMay 2007
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Agenda
• Trivia
• Dynamic behavior
• Convergence properties and problems
• Convergence/stability work items
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Goals and Priorities
• BGP Goal: Maximize connectivity ofInternet
• Convergence and stability are
subsidiary to this• Implication: Priorities
• First: fastest service restoration• Second: minimize peak load on control
plane
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Focus• This talk focuses on performance and
stability• There are other very important aspectsof BGP
• Services
• Operations
• Weird behaviors (wedgies, etc)• Security
• Policy modeling
• …
• But we don’t have all day
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Shalt Not’s
• BGP uses ASes for loop suppression —and nothing else!
• Speaking of “overloading things”… ASes are not locators . No topological significance.
• Auto-aggregation appears to be a non-starter
• Even proxy aggregation is tricky, but that’san operational consideration
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MP-BGP
• BGP carries data for multiple addressfamilies (AFs)
•Plain old IP (v4, v6)
• VPNv4
• Other things
• Not all AFs need to be present on allrouters!
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VPNs
•Often observed that VPN tables largerthan Internet table
• True, in aggregate
• But, not true of any single VPN table
• Inherently parallelizable
• No single PE or RR holds all VPN tables• Operational challenges to managing
• Some tools to do this, e.g. rt-constrain
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BGP dynamic
behavior
• Confusion even among routing experts
• Of course, surprising emergentbehaviors are possible
• … but important to understandbounding conditions
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BGP and TCP
• BGP runs over TCP
• Flow control: important implications fordynamics
•Intuition about TCP is usually wrong…
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BGP under load
• When uncongested, BGP will passupdates as fast as they are received
•Modulo MRAI, dampening
• Degradation mode under (CPU)congestion: state compression
• “Adaptive low-pass filter” behavior emerges
• Things slow down, they typically do not melt
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BGP under load [2]• BGP adapts to speed of peer
• Slow peer gets routes as slow as it wants (withstate compression)
•Fast peer gets routes as fast as it wants
• Implication: One slow peer does not hinderoverall convergence
• Update packing• Low prefix/update ratios when not congested…
but that’s fine!
• High ratios emerge under congestion… whichis when needed
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BGP convergence• At least O(n) in the size of the DFZtable
• Fundamental to how BGP transports routes
• But full convergences don’t happen
often!• At startup (“initial convergence”)
• On rare occasions otherwise
• Hard to “fix” completely — but is itbroke?
• “BGP’s biggest, yet least important,problem.”
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BGP convergence [2]• Techniques to avoid full convergences
• Graceful Restart
• Nonstop Routing
• … or to cover them up
• Different flavors of fast reroute
• … or to pre-converge by advertisingextra routes
• Best-external, multi-path and similar
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Route Reflection
• RRs hide backup paths• Reduce RIB sizes (but less than you think)
•Bad for convergence
• Convergence:
• State reduction/data hiding
• Faster convergence
• Pick one
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Known Algorithmic
Deficiencies
•Path hunting
• Nonconverging policies
• At least O(n) in DFZ size
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Path Hunting
• Well-known amplification effect
• Approaches to reduce
• Root cause notification
• Propagation of backup paths
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Propagation of
Backup Paths
• Transit ASes seldom fully partition fromeach other
• However, when a single AS-AS linkgoes down, border router temporarilyloses routes
• Due to aggressive data hiding by less-preferred border routers and RRs
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Propagation ofBackup Paths [2]
• Speculation: many “path disturbance”events caused by this effect
• Intra-domain backup propagation
feasible today• Cost: some additional RIB state within
AS
• Benefit: faster internal convergence and global stability
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Some Possible Tools**** = under discussion
• As-pathlimit ****
• Aggregate withdraw ****
• Best-external ****
• Better instrumentationreusing WRD infra
• BGP free core (pick yourencap) ****
• Dampening (with better
parameters) ****
• Multi-path ****
• Root cause notification
• BGP - Fast Re-Route ****
• Better UPDATE packingalgorithms/techniques
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Moving Forward• Narrow down (or expand!) “possible tools”
list
• Align costs and benefits
• Those who pay, must benefit, or solution willnever be deployed
• Many examples of existing technically-excellent“solutions” to current problems… but problems
still exist. Example: BCP-38
• Deployment trumps all considerations!
• Focus on behavior under load (or makingload go away!)
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Dampening• Misused in past (we were wrong aboutdefault parameters)
• Heavy contribution of few sites to GHdata suggests very generous
parameters which only penalizeegregious flappers
• Study needed to validate what constitutes
“egregious”• Given parameters, can be turned on
today
• Lower-than-low hanging fruit
• Ali ns costs and benefits
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Punch Line• BGP not in danger of falling over
• Lots of runway
• IDR• Near-term improvements
• Most cause increased use of router resources
• RRG
• Fundamental changes, e.g. new routing and addressingarchitectures
• GROW (recharter)
• Analysis of routing system
• BMWG, IPPM
• Define metrics