Post on 20-Dec-2015
transcript
Slide -1-February, 2006
Interdomain Routing
Gordon WilfongDistinguished Member of Technical Staff
Algorithms Research DepartmentMathematical and Algorithmic Sciences Center
Bell Laboratories
February, 2006
Slide -2-February, 2006
Current Internet Routing
Autonomous Systems (ASes)– Subnetwork under single administrative control
• Each AS has its own economic incentives to:
– cooperate so as to achieve connectivity and to abide by economic contracts (SLAs)
– to hide information about its internal network
Routing between ASes achieved by protocol where– AS selectively announces to neighbors its chosen route to
destinations
– AS selects highest ranked route announced to it
Slide -3-February, 2006
Border Gateway Protocol (BGP)
“I routedirectly to d”
AS1 AS2
“The route I use to d goes through AS0”
AS0
d
“The route I use to d goes throughAS1 then AS0”
AS3
Data packets
Slide -4-February, 2006
Routing Protocol Requirements
Protocol must scale
– Routing table entries (destinations): ~150,000 and increasing
– Autonomous Systems: ~20,000 and increasing
Administrators require expressiveness for routing policies
– Local AS policy (based on SLAs, traffic engineering etc) determines
• route selection (effects where AS sends packets)
• route announcements (effects how/if packets from other ASes traverse it)
Slide -5-February, 2006
External BGP (E-BGP) is the mode of BGP that propagates routes between autonomous systems.
E-BGP
Slide -6-February, 2006
Stable Paths Problem
1
3
2
3 2 03 1 03 2 1 0
1 01 2 0
0
2 1 02 0
1 0210
310
0
Slide -7-February, 2006
1 2 01 0
Independent policies can lead to nonconvergence
0
1
23 2 3 02 0
3 1 03 0
Bad Gadget
Each AS chooses“favorite” availableroute.
0
Slide -8-February, 2006
May Be Multiple Solutions
First solution
1
0
2
1 2 01 0
1
0
2
1
0
2
2 1 02 0
1 2 01 0
2 1 02 0
1 2 01 0
2 1 02 0
Second solutionDISAGREE
Slide -9-February, 2006
What can be done?
Modify protocol
– Design a protocol that has provably good properties. e.g., [Griffin, W.]
Constrain topology and/or policy
– Prove good protocol behavior assuming constrained configurations. e.g., [Gao, Rexford]
• These constraints give rise to interesting “valley-free” routing problems (from customer to local provider to regional provider to national provider and then down chain again to another customer). e.g. [Erlebach et al]
Slide -10-February, 2006
Possible Research Directions
Complexity issues
– What is the complexity of deciding if a configuration is “safe” (i.e., will it always converge)?
• NP-complete to determine if there is a solution it can converge to [Griffin, W.]
Extending model
– Model assumes destinations can be considered independently
• Aggregation has not been taken into consideration
Trade-offs between protocol expressivity and safety
– Formal model to study such trade-offs
Slide -11-February, 2006
Internal BGP (I-BGP) is the protocol used to propagate external routes within an autonomous system.
I-BGP
Slide -12-February, 2006
Fully Meshed
RR
RR
RR
RR
RR
RR
A router only announcesits own routes
Slide -13-February, 2006
Route Reflectors
RR
RR
RR
RR
RR
RR
Route Reflectors must be fullymeshed
Route Reflectors pass along updatesto client routers
Slide -14-February, 2006
Route Selection Summary
Highest Local Preference
Shortest ASPATH
Lowest MED (if same next AS)
I-BGP < E-BGP
Lowest IGP cost to BGP egress
Lowest router ID
Traffic engineering
Enforce relationships
Throw up hands andbreak ties
Slide -15-February, 2006
What Are MEDs?
MED=2MED=0
AS1
AS2
CP
Client
“Hot Potato Routing” vs.“Cold Potato Routing”
MED=1
AS3
Slide -16-February, 2006
Persistent Route Oscillations
135.104.54.4
5040
120
1
MED=1
MED=2MED=1
r1
r2
r3
A B
Slide -17-February, 2006
Oscillations in I-BGP
135.104.54.4
1
115 5 5
RR1 RR2 RR3
C1 C2 C3
P1 P2 P3I-BGP connection
Physical link
AS0
Slide -18-February, 2006
Deflections
135.104.54.4
3
1
1
P
Q
RR
A B
Packets can be diverted out of a network unexpectedly.
AS0
Slide -19-February, 2006
Routing Loops
135.104.54.4
5
11
1
RR1RR2
C1 C2
Badly configurednetworks can alsoexperience routingloops
AS0
Slide -20-February, 2006
A Real-world MED Oscillation Example
• A functioning network breaks into a state of persistent route oscillations when a BGP session goes down• First thought to be a hardware problem• Analysis shows that route oscillations caused by the use of the MED attribute• The example was actually predicted by the theoretical analysis [Griffin, W.]
Slide -21-February, 2006
Initial State
AS 4
A B
E F
AS 3
AS 1
AS 2
reflector reflector
I-BGP
E-BGP
(1)(2)
C
MEDs
IGP12
D
21
(2)
70000
Only AS 2 sends MEDs to AS 4
Slide -22-February, 2006
Initial Routing
AS 4
A B
E F
AS 3
AS 1
AS 2
reflector reflector
I-BGP
E-BGP
(1)
C
IGP12
D
21
(2)MEDs (2)
70000
D prefers AS 2path due to router IDtie breaking
Slide -23-February, 2006
B Changes Its Route
AS 4
A B
E F
AS 3
AS 1
AS 2
reflector reflector
I-BGP
E-BGP
(1)
C
IGP12
D
21
sessi
on down!
The AS 4 AS 3 BGP Session is dropped
(2)MEDs (2)
70000
Slide -24-February, 2006
A Changes Its Route
AS 4
A B
E F
AS 3
AS 1
AS 2
reflector reflector
I-BGP
E-BGP
(1)
C
IGP12
D
21
sessi
on down!
(2)MEDs (2)
The MED 1 route from B beats the MED 2 routesthat A sees from its clients….
70000
CALL THISSTATE ZERO
Slide -25-February, 2006
C & D Change Routes
AS 4
A B
E F
AS 3
AS 1
AS 2
reflector reflector
I-BGP
E-BGP
(1)
C
IGP12
D
21
sessi
on down!
(2)MEDs (2)
The MED 1 route from A knocks both MED 2 routesout of the picture for C & D …
70000
Slide -26-February, 2006
A Changes Route Again
AS 4
A B
E F
AS 3
AS 1
AS 2
reflector reflector
I-BGP
E-BGP
(1)
C
IGP12
D
21
sessi
on down!
(2)MEDs (2)
A now sees the route from D through AS 3, and it is closer IGP-wise than the route from B…
70000
Slide -27-February, 2006
C&D Return to Initial Routes
AS 4
A B
E F
AS 3
AS 1
AS 2
reflector reflector
I-BGP
E-BGP
(1)
C
IGP12
D
21
sessi
on down!
(2)MEDs (2)
C & D no longer see MED 1 route from A, so they return to the eBGP routes with MED 2…
70000
Slide -28-February, 2006
Back to State Zero!
AS 4
A B
E F
AS 3
AS 1
AS 2
reflector reflector
I-BGP
E-BGP
(1)
C
IGP12
D
21
sessi
on down!
(2)MEDs (2)
A switches back to MED 1 route through B.
70000
Slide -29-February, 2006
What Can Be Done?
Possible Approaches:
Use only configurations that guarantee no problems
No modification to BGP required
Previous example shows this might be difficult
Prevent problems for any configuration
Modification to BGP required, e.g. [Basu et al]
Minimize overhead (number of messages, memory requirements, …)
Slide -30-February, 2006
• BGP is extremely flexible, allowing operators to easily make obscure errors that are difficult to find and correct.
• Policy based routing is difficult to get right.
• Defining a formal model of path vector protocols helps to understand what features can cause problems and what can be done to avoid errors.
Conclusions
Slide -31-February, 2006
• BGP is the routing protocol in today’s internet
• What about the future?• new Internet architectures• new architectures within an AS (e.g., Routing Control Platform [J. Rexford, et al])
• In any case, a formal theoretical approach is essential to getting it right.
What about the future?
Slide -32-February, 2006
Extras
Slide -33-February, 2006
Persistent Route Oscillations
135.104.54.4
5040
120
1
MED=1
MED=2MED=1
r1
r2
r3
A B
r2r1r3
r1r3
r1r2r3
r3r2
r1r2r3
r1r3
r2r13
r1r3
r2r1rr3
r3r2