1. 2017 session 1 TELE3118: Network Technologies Week 6: Network Layer Control Plane Inter-Domain Routing Protocols
Some slides have been adapted from:
Computer Networking: A Top Down Approach, 7th (global) edition. Jim Kurose, Keith Ross. Pearson, April All material copyright J.F Kurose and K.W. Ross, All Rights Reserved.
Computer Networks, 5th edition. Andrew S. Tanenbaum, David J. Wetherall, Pearson, 2010.
Network Layer

2. Making routing scalable
our routing study thus far - idealized
all routers identical
network “flat”
… not true in practice
scale: with billions of destinations:
can’t store all destinations in routing tables!
routing table exchange would swamp links!
administrative autonomy
internet = network of networks
each network admin may want to control routing in its own network
Network Layer: Control Plane

3. Internet approach to scalable routing
aggregate routers into regions known as “autonomous systems” (AS) (a.k.a. “domains”)
intra-AS routing
routing among hosts, routers in same AS (“network”)
all routers in AS must run same intra-domain protocol
routers in different AS can run different intra-domain routing protocol
gateway router: at “edge” of its own AS, has link(s) to router(s) in other AS’es
inter-AS routing
routing among AS’es
gateways perform inter-domain routing (as well as intra-domain routing)
Network Layer: Control Plane

4. Interconnected ASes 3c 3a 2c 3b 2a AS3 2b 1c 1a 1b AS1 1d
Intra-AS
Routing
algorithm
Inter-AS
Forwarding
table 3c
forwarding table configured by both intra- and inter-AS routing algorithm
intra-AS routing determine entries for destinations within AS
inter-AS & intra-AS determine entries for external destinations
Network Layer: Control Plane

5. Inter-AS tasks AS1 must:
learn which dests are reachable through AS2, which through AS3
propagate this reachability info to all routers in AS1
job of inter-AS routing!
suppose router in AS1 receives datagram destined outside of AS1:
router should forward packet to gateway router, but which one? 3c 3a 3b 2c
AS3
other
networks
AS1 1c 1a 1d 1b 2a 2b
other
networks
AS2
Network Layer: Control Plane

6. Internet inter-AS routing: BGP
BGP (Border Gateway Protocol): the de facto inter-domain routing protocol
“glue that holds the Internet together”
BGP provides each AS a means to:
eBGP: obtain subnet reachability information from neighboring ASes
iBGP: propagate reachability information to all AS-internal routers.
determine “good” routes to other networks based on reachability information and policy
allows subnet to advertise its existence to rest of Internet: “I am here”
Network Layer: Control Plane

7. BGP basics
BGP session: two BGP routers (“peers”) exchange BGP messages over semi-permanent TCP connection:
advertising paths to different destination network prefixes (BGP is a “path vector” protocol)
when AS3 gateway router 3a advertises path AS3,X to AS2 gateway router 2c:
AS3 promises to AS2 it will forward datagrams towards X
AS 3 1b 1d 1c 1a 3b 3d 3c 3a
AS 1 2b 2d 2c 2a
BGP advertisement:
AS3, X
AS 2 X
Network Layer: Control Plane

8. Path attributes and BGP routes
advertised prefix includes BGP attributes
prefix + attributes = “route”
two important attributes:
AS-PATH: list of ASes through which prefix advertisement has passed
NEXT-HOP: indicates specific internal-AS router to next-hop AS
Policy-based routing:
gateway receiving route advertisement uses import policy to accept/decline path (e.g., never route through AS Y).
AS policy also determines whether to advertise path to other other neighboring ASes
Network Layer: Control Plane

9. BGP path advertisement
AS3 1b 1d 1c 1a 3b 3d 3c 3a
AS1 2b 2d 2c 2a
AS3,X
AS2,AS3,X
AS2 X
AS2 router 2c receives path advertisement AS3,X (via eBGP) from AS3 router 3a
Based on AS2 policy, AS2 router 2c accepts path AS3,X, propagates (via iBGP) to all AS2 routers
Based on AS2 policy, AS2 router 2a advertises (via eBGP) path AS2, AS3, X to AS1 router 1c
Network Layer: Control Plane

10. BGP path advertisement
AS3 1b 1d 1c 1a 3b 3d 3c 3a
AS1
AS3,X 2b 2d 2c 2a
AS3,X
AS2,AS3,X
AS2 X
gateway router may learn about multiple paths to destination:
AS1 gateway router 1c learns path AS2,AS3,X from 2a
AS1 gateway router 1c learns path AS3,X from 3a
Based on policy, AS1 gateway router 1c chooses path AS3,X, and advertises path within AS1 via iBGP
Network Layer: Control Plane

11. BGP messages BGP messages exchanged between peers over TCP connection
OPEN: opens TCP connection to remote BGP peer and authenticates sending BGP peer
UPDATE: advertises new path (or withdraws old)
KEEPALIVE: keeps connection alive in absence of UPDATES; also ACKs OPEN request
NOTIFICATION: reports errors in previous msg; also used to close connection
Network Layer: Control Plane

12. BGP attribute: AS-path
Prevents looping!
Prefix /16, AS1  AS2: AS-path = AS1
AS2  AS3: AS-path = AS2-AS1
AS3  AS1: AS-path = AS3-AS2-AS1
AS1 detects loop, and can reject the route
Partition healing:
rare case where AS1 may
accept “loop” route:
AS 3
AS 1
AS 3
(b)
AS 1
AS 2
(a)
AS 2
/16
/16
Network Layer

13. BGP attribute: Multi-Exit-Discriminator
Used when two AS connect to each other in more than one place
Used by AS to advertise degree of preference of each link to reach a particular prefix
Example:
AS1 and AS2 have 2 BGP sessions: one on each link
AS2 advertises prefixes of AS3 to AS1 on both links
MED advertised on link A better than MED advertised on link B
AS 1
Link A
AS 2
AS 3
AS 4
Link B
Network Layer

14. MED (contd.) ISP-1 and ISP-2 connect in New York and San Francisco
ISP-1 has customer-1 in San Francisco
ISP-2 has customer-2 in New York
What happens if:
Case A: Both ISPs set and accept MED?
Case B: Both ISP-1 and ISP-2 ignore MED?
Case C: ISP-1 accepts MED but ISP-2 ignores MED?
Cust 2
Case A:
ISP 2
ISP 1
Cust 1
Network Layer

15. BGP attribute: Local-Pref
Most commonly used attribute
Determines local (i.e. within AS) preference of use of received route
E.g.: say AS3 provides better service than AS2 to AS4
AS4 can configure local-pref of routes from AS3 to be higher (better) than those heard from AS2
AS1 advertises prefix to AS2 and AS3
AS4 receives the prefix from both, but chooses the AS3-AS1 path since it has better local-pref
AS 3
AS 1
AS 4
AS 2
/16
Network Layer

16. BGP policies
Can be complex, yet are key to flexibility and control of inter-AS routing
Examples:
Avoid using competitor’s network
avoid routes with AS-n in AS-Path
Avoid transit service, i.e. do not carry any traffic that does not have source or destination within AS
Do not advertise any non-local routes to peers
Let another ISP carry most cross-country load
Use of MED was shown earlier
More examples in subscriber-ISP connection next
Network Layer

17. Subscriber connection: singly-homed
Easy case! Possible options:
Static configuration: easiest
Customer has default route via R2
ISP configures static route to customer’s prefix
Include customer in ISP’s IGP (too risky!)
Run a small IGP (say RIP) on R1-R2 link, leak that into BGP
Run a single BGP session
customer will still likely use a default route or a small set of filtered routes and not absorb the entire Internet routing table
AS2
BGP session
ISP
AS1
customer R1 R2
/23
Network Layer

18. Multi-homed subscriber
Multiple customer links to one or more ISPs
Why?
Reliability (redundancy)
Performance (load-sharing)
Challenging
Static routing often doesn’t suffice (why?)
Want to minimize routing prefixes injected into customer network
BGP configuration requires thought and planning, taking into account both traffic directions (to and from the customer)
ISP-1
ISP-2
customer
Network Layer

19. Multi-homing to a single provider
Example 1: same router in ISP, different routers in customer
ISP to customer traffic: customer sets MED
Customer to ISP traffic: 2 default routes!
Example 2: different routers in ISP, same router in customer
ISP to customer traffic: as before
Customer to ISP traffic: customer may have to get BGP prefixes from ISP
ISP R1 R2 R3
customer
138.39/16
204.70/16
ISP R1 R2 R3
customer
138.39/16
204.70/16
Network Layer

20. Multi-homing to multiple providers
ISP3
ISP1
ISP2
138.39/16
204.70/16
customer
Options for customer address space:
Exclusively from ISP1 (or from ISP2)
E.g.: customer uses /24 and advertises this prefix to ISP2
ISP3 gets prefixes /16 from ISP1 and from ISP2
ISP3 traffic to customer will go via ISP2 (longest prefix match)
Aggregation is pushing traffic away?!
From both ISP1 and ISP2
E.g.: customer uses /24 and /24
Good load-sharing if traffic to these prefixes is about the same
Independently from address registry
Can manipulate load-sharing better, but bad for aggregation!
Bottom line: it all depends on the traffic patterns!
Network Layer

21. Interaction among routing protocols
Every routing protocol is computing its own routes: how does it all fit?
Question: do they interact with each other? Yes!
Question: which route is inserted in the forwarding tables? If conflict, priority mechanism is used
Question: how does OSPF/RIP fill its routing table?
Direct routes: directly-connected interfaces
Static routes: user configured
Question: How does BGP fill it routing table?
Learns AS local networks from OSPF/RIP
Network Layer

22. Configuring routing
In your organization you have to install a new PC in a server-farm. The PC is multi-homed on two LANs. What static routes do you need to configure on the PC for shortest-path routing to all destinations? Assume:
The PC is not routing between LANs
The PC is not running any routing protocols
Pick any IP addresses for the router interfaces consistent with the LAN subnets
LAN
/28 R1 R2
ISP
LAN
/28
LAN
/24
LAN
/28
server
farm
Network Layer
new PC

23. Configuring routing (contd.)
Now suppose your organization gets a second link to the ISP via a new router R3. Your PC now has 3 LAN interfaces, and your organization has two links to the Internet. Can you suggest ways of load-balancing traffic to/from your organization?
LAN
/28 R1 R2
ISP
LAN
/28
LAN
/24
LAN
/28
server
farm R3
LAN
/24
new PC
Network Layer

24. Why different Intra-, Inter-AS routing ?
policy:
inter-AS: admin wants control over how its traffic routed, who routes through its net.
intra-AS: single admin, so no policy decisions needed
scale:
hierarchical routing saves table size, reduced update traffic
performance:
intra-AS: can focus on performance
inter-AS: policy may dominate over performance
Network Layer: Control Plane