1. Dan LI CS Department, Tsinghua University
Lecture 4 Routing
Dan LI
CS Department, Tsinghua University
Today we’ll talk about routing.
2018/12/4

2. Today’s Lecture
Inter-Domain Routing
Reading list
2018/12/4

3. Inter-Domain Routing Routing hierarchy Internet structure
External BGP (E-BGP)
Internal BGP (I-BGP)
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4. Routing Hierarchies Flat routing doesn’t scale Key observation
Storage  Each node cannot be expected to store routes to every destination (or destination network)
Convergence times increase
Communication  Total message count increases
Key observation
Need less information with increasing distance to destination
Need lower diameters networks
Solution: area hierarchy
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5. Areas Divide network into areas
Areas can have nested sub-areas
Hierarchically address nodes in a network
Sequentially number top-level areas
Sub-areas of area are labeled relative to that area
Nodes are numbered relative to the smallest containing area
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6. Area Hierarchy Addressing1 2
2.2
1.1
2.1
2.2.2
1.2
2.2.1
1.2.1
1.2.2 3
3.2
3.1
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7. Routing Hierarchy Partition Network into “Areas”
Area-Border
Router
Backbone Areas
Lower-level Areas
Partition Network into “Areas”
Within area
Each node has routes to every other node
Outside area
Each node has routes for other top-level areas only
Inter-area packets are routed to nearest appropriate border router
Constraint: no path between two sub-areas of an area can exit that area
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8. Path Sub-optimality start end 1 2 2.1 2.2 1.1 1.2 2.2.1 3
1.2.1
start
end
3.2.1 3
3 hop red path
vs.
2 hop green path
3.2
3.1
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9. Inter-Domain Routing Routing hierarchy Internet structure
External BGP (E-BGP)
Internal BGP (I-BGP)
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10. Internet’s Area Hierarchy
What is an Autonomous System (AS)?
A set of routers under a single technical administration
Using an interior gateway protocol (IGP) and common metrics to route packets within the AS
Using an exterior gateway protocol (EGP) to route packets to other AS’s
Sometimes AS’s use multiple IGPs and metrics, but appear as single AS’s to other AS’s
Each AS assigned unique ID
AS’s peer exchanges information
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11. AS Numbers (ASNs) ASNs represent units of routing policy
ASNs are 16 bit values
64512 through are “private”
Currently over 15,000 in use
Genuity: 1
MIT: 3
JANET: 786
UC San Diego: 7377
AT&T: 7018, 6341, 5074, …
UUNET: 701, 702, 284, 12199, …
Sprint: 1239, 1240, 6211, 6242, … …
ASNs represent units of routing policy
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12. Example IGP IGP EGP EGP EGP EGP IGP EGP IGP IGP 1 2 2.1 2.2 1.1 1.2
2.2.1
EGP
EGP
EGP 3
4.2
4.1
IGP
EGP 4
IGP 5
3.2
3.1
IGP
5.2
5.1
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13. A Logical View of the Internet
Tier 1 ISP
“Default-free” with global reachability info
Tier 2 ISP
Regional or country-wide
Tier 3 ISP
Local
Tier 3
Tier 2
Tier 2
Customer
Provider
Tier 1
Tier 1
Tier 2
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14. Transit vs. Peering ISP Y ISP Z Transit ($$ 1/2) Transit ($$$) ISP P
ISP X
When a member of one network wants to communicate with a member of another network different mechanisms are required depending on the type of connection between the two networks.
Where the networks share a direct connection, communication takes the path directly between the customers of two networks, a process called peering.
If a third party network (or networks) is required to support the communication, that network is said to provide transit, ie, the communication crosses but does not terminate in the transit network.
In the case of peering both customers use only networks they have paid for to communicate.
It is thus common for peering to occur for "free," IE there is no payment by the networks to each other, the networks are considered paid by their customers for carrying the information, the payment to each network reflects that networks costs for serving their own customer.
The connection costs between the networks are often shared or paid separately by each network (to an Internet Exchange for example) so there is no basis for further remuneration.
Where the communication requires transit to occur, additional payment by the communicating members is often levied. This levy is paid to the transit network(s) since the transit traffic doesn't serve any of the transit network's customers.
Transit ($)
Transit ($$)
Transit ($$)
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15. Customers and Providers
IP traffic
customer
Customer pays provider for access to the Internet
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16. The “Peering” Relationship
Peers provide transit between
their respective customers
Peers do not provide transit
between peers
Peers (often) do not exchange $$$
peer
customer
provider
traffic
allowed
traffic NOT
allowed
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17. Peering Provides Shortcuts
customer
provider
Peering also allows connectivity between
the customers of “Tier 1” providers.
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18. Peering Wars Reduces upstream transit costs
Don’t Peer
Reduces upstream transit costs
Can increase end-to-end performance
May be the only way to connect your customers to some part of the Internet (“Tier 1”)
You would rather have customers
Peers are usually your competitors
Peering relationships may require periodic renegotiation
Peering struggles are by far the most
contentious issues in the ISP world!
Peering agreements are often confidential.
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19. Policy Impact “Valley-free” routing WHY?
Number links as (+1, 0, -1) for provider, peer and customer
In any path should only see sequence of +1, followed by at most one 0, followed by sequence of -1
WHY?
Consider the economics of the situation
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20. Inter-Domain Routing Routing hierarchy Internet structure
External BGP (E-BGP)
Internal BGP (I-BGP)
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21. BGP History Mid-80s: EGP Result: BGP introduced as routing protocol
Reachability protocol (no shortest path)
Did not accommodate cycles (tree topology)
Evolved when all networks connected to NSF backbone
Result: BGP introduced as routing protocol
Latest version = BGP 4
BGP-4 supports CIDR
Primary objective: connectivity not performance
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22. BGP Basics
Use TCP
Border Gateway Protocol (BGP), based on Bellman-Ford path vector
AS’s exchange reachability information through their BGP routers, only when routes change
BGP routing information – a sequence of AS’s indicating the path traversed by a route
General operations of a BGP router:
Learns multiple paths
Picks best path according to its AS policies
Install best pick in IP forwarding tables
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23. Two Types of BGP Neighbor Relationships
External Neighbor (E-BGP) in a different Autonomous Systems
Internal Neighbor (I-BGP) in the same Autonomous System
AS1
I-BGP is routed
using Interior Gateway Protocol
(IGP) such as OSPF/IS-IS
E-BGP
I-BGP
AS2
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24. E-BGP Choices Link state or distance vector?
No universal metric – policy decisions
Problems with distance-vector:
Bellman-Ford algorithm may not converge
Problems with link state:
Metric used by routers not the same – loops
LS database too large – entire Internet
May expose policies to other AS’s
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25. Solution: Distance Vector with Path
Each routing update carries the entire path
Loops are detected as follows:
When AS gets route check if AS already in path
If yes, reject route
If no, add self and (possibly) advertise route further
Advantage:
Metrics are local - AS chooses path, protocol ensures no loops
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26. Interconnecting BGP Peers
BGP uses TCP to connect peers
Advantages:
Simplifies BGP
No need for periodic refresh - routes are valid until withdrawn, or the connection is lost
Incremental updates
Disadvantages
Congestion control on a routing protocol?
Poor interaction during high load
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27. Hop-by-hop Model
BGP advertises to neighbors only those routes that it uses
Consistent with the hop-by-hop Internet paradigm
e.g., AS1 cannot tell AS2 to route to other AS’s in a manner different than what AS2 has chosen (need source routing for that)
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28. Policy with BGP BGP provides capability for enforcing various policies
Policies are not part of BGP: they are provided to BGP as configuration information
BGP enforces policies by choosing paths from multiple alternatives and controlling advertisement to other AS’s
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29. Examples of BGP Policies
A multi-homed AS refuses to act as transit
Limit path advertisement
A multi-homed AS can become transit for some AS’s
Only advertise paths to some AS’s
An AS can favor or disfavor certain AS’s for traffic transit from itself
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30. BGP Messages Open Keep_alive Notification Announces AS ID
Determines hold timer – interval between keep_alive or update messages, zero interval implies no keep_alive
Keep_alive
Sent periodically (but before hold timer expires) to peers to ensure connectivity
Sent in place of an UPDATE message
Notification
Used for error notification
TCP connection is closed immediately after notification
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31. BGP UPDATE Message List of withdrawn routes
Network layer reachability information
List of reachable prefixes
Path attributes
Origin
Path
Metrics
All prefixes advertised in message have same path attributes
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32. Path Selection Criteria
Information based on path attributes
Attributes + external (policy) information
Examples:
Hop count
Policy considerations
Preference for AS
Presence or absence of certain AS
Path origin
Link dynamics
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33. LOCAL PREF
Local (within an AS) mechanism to provide relative priority among BGP routers R5 R1
AS 200 R2
AS 100
AS 300
The local preference is a Local (within an AS) mechanism to provide relative priority among BGP routers. It is used by a BGP speaker to inform other BGP speakers in its own autonomous system of the originating speaker's degree of preference for an advertised route. R3
Local Pref = 500
Local Pref =800 R4
I-BGP
AS 256
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34. LOCAL PREF – Common Uses
Handle routes advertised to multi-homed transit customers
Should use direct connection
Peering vs. transit
Prefer to use peering connection
In general, customer > peer > provider
Use LOCAL PREF to ensure this
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35. AS_PATH List of traversed AS’s AS 200 AS 100 AS 300 AS 500
/16
/16
AS 300 / /
AS 500
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36. Multi-Exit Discriminator (MED)
Hint to external neighbors about the preferred path into an AS
Non-transitive attribute (we will see later why)
Different AS choose different scales
Used when two AS’s connect to each other in more than one place
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37. MED Hint to R1 to use R3 over R4 link
Cannot compare AS40’s values to AS30’s
MED = 50 R1 R2
AS 10
AS 40
For exam, in this picture, AS 30 has two connection with R1, so it use MED to differ this two connection.
It’s tell R1 to use R3 over R4 link.
But as Different AS choose different scales, it cannot compare different AS’s MED.
MED = 120
MED = 200 R3 R4
AS 30
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38. MED MED is typically used in provider/subscriber scenarios
It can lead to unfairness if used between ISP because it may force one ISP to carry more traffic
ISP1 SF
ISP2 NY
ISP1 ignores MED from ISP2
ISP2 obeys MED from ISP1
ISP2 ends up carrying traffic most of the way
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39. Decision Process Processing order of attributes:
Select route with highest LOCAL-PREF
Select route with shortest AS-PATH
Apply MED (if routes learned from same neighbor)
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40. Inter-Domain Routing Routing hierarchy Internet structure
External BGP (E-BGP)
Internal BGP (I-BGP)
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41. Internal vs. External BGP
BGP can be used by R3 and R4 to learn routes
How do R1 and R2 learn routes? R1
E-BGP
AS1 R3 R4
AS2 R2
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42. Internal BGP (I-BGP) Same messages as E-BGP
Different rules about re-advertising prefixes:
Prefix learned from E-BGP can be advertised to I-BGP neighbor and vice-versa, but
Prefix learned from one I-BGP neighbor cannot be advertised to another I-BGP neighbor
Reason: no AS PATH within the same AS and thus danger of looping.
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43. Internal BGP (I-BGP) R3 can tell R1 and R2 prefixes from R4
R3 can tell R4 prefixes from R1 and R2
R3 cannot tell R2 prefixes from R1
R2 can only find these prefixes through a direct connection to R1
Result: I-BGP routers must be fully connected (via TCP)!
contrast with E-BGP sessions that map to physical links R1
E-BGP
AS1 R3 R4
AS2 R2
I-BGP
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44. I-BGP Peers Must be Fully Meshed
E-BGP update
I-BGP updates
I-BGP is needed to avoid routing loops within an AS
Why not injecting external routes into IGP?
Does not scale and causes BGP policy information to be lost
So BGP and IGP are separated
BGP does not provide “shortest path” routing
I-BGP neighbors do not announce
routes received via I-BGP to other I-BGP
neighbors.
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45. Route Selection Summary
Highest Local Preference
Enforce relationships
Shortest ASPATH
Lowest MED
Traffic engineering
i-BGP < e-BGP
Lowest IGP cost
to BGP egress
Throw up hands and
break ties
Lowest router ID
2018/12/4