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COMS W4995-1
Lecture 6
Dynamic routing protocols II
1. Dynamic Routing Protocols: Link State Routing 2. Intra-Domain Routing Protocols: OSPF & BGP
Dynamic Routing Protocols
Link State Routing
The Gang of Four
Link State Vectoring
EGP
IGP
BGP
RIPIS-IS
OSPF
Link State Routing
Based on Dijkstra’ s Shortest-Path-First algorithm.
Each router starts by knowing: Prefixes of its attached networks. Links to its neighbors.
Each router advertises to the entire network (flooding): Prefixes of its directly connected networks. Active links to its neighbors.
Each router learns: A complete topology of the network (routers, links).
Each router computes shortest path to each destination.
In a stable situation, all routers have the same graph, and compute the same paths.
Dijkstra’s Shortest Path Algorithm for a Graph
Input: Graph (N,E) with N the set of nodes and E the set of edges
cvw link cost (cvw = 1 if (v,w) E, cvv = 0)
s source node.
Output: Dn cost of the least-cost path from node s to node n
M = {s};
for each n M Dn = csn;
while (M all nodes) do Find w M for which Dw = min{Dj ; j M};Add w to M;for each neighbor n of w and n M
Dn = min[ Dn, Dw + cwn ];Update route;
end for end whileend for
Link state routing: graphical illustration
a
b
c d
3 1
62
a
36
b
c
a’s view:
a
b
c
3 1b’s view: c d2
d’s view:
Collecting all views yield a global & complete view of the network!
Global view:
a
b
c d
1
6
c’s view:
2
Operation of a Link State Routing protocol
ReceivedLSAs
IP Routing Table
Dijkstra’s
Algorithm
Link StateDatabase
LSAs are flooded to other interfaces
Link State Routing: Properties
Each node requires complete topology information
Link state information must be flooded to all nodes
Guaranteed to converge
Distance Vector vs. Link State Routing
With distance vector routing, each node has information only about the next hop:
Node A: to reach F go to B Node B: to reach F go to D Node D: to reach F go to E Node E: go directly to F
Distance vector routing makespoor routing decisions if directions are not completelycorrect (e.g., because a node is down).
If parts of the directions incorrect, the routing may be incorrect until the routing algorithms has re-converged.
AA BB CC
DD EE FF
Distance Vector vs. Link State Routing
In link state routing, each node has a complete map of the topology
If a node fails, each node can calculate the new route
Difficulty: All nodes need to have a consistent view of the network
AA BB CC
DD EE FF
A B C
D E F
A B C
D E F
A B C
D E F
A B C
D E F
A B C
D E F
A B C
D E F
• Topology information is flooded within the routing domain
• Best end-to-end paths are computed locally at each router.
• Best end-to-end paths determine next-hops.
• Based on minimizing some notion of distance
• Works only if policy is shared and uniform
• Examples: OSPF, IS-IS
Distance Vector vs. Link State Routing
• Each router knows little about network topology
• Only best next-hops are chosen by each router for each destination network.
• Best end-to-end paths result from composition of all next-hop choices
• Does not require any notion of distance
• Does not require uniform policies at all routers
• Examples: RIP, BGP
Link State Vectoring
Dynamic Routing Protocols
Open Shortest Path First
OSPF = Open Shortest Path First The OSPF routing protocol is the most important link state
routing protocol on the Internet (another link state routing protocol is IS-IS (intermediate system to intermediate system)
The complexity of OSPF is significant RIP (RFC 2453 ~ 40 pages) OSPF (RFC 2328 ~ 250 pages)
History: 1989: RFC 1131 OSPF Version 1 1991: RFC1247 OSPF Version 2 1994: RFC 1583 OSPF Version 2 (revised) 1997: RFC 2178 OSPF Version 2 (revised) 1998: RFC 2328 OSPF Version 2 (current version)
OSPF
Features of OSPF
Provides authentication of routing messages
Enables load balancing by allowing traffic to be split evenly across routes with equal cost
Type-of-Service routing allows to setup different routes dependent on the TOS field
Supports subnetting
Supports multicasting
Allows hierarchical routing
Hierarchical OSPF
Hierarchical OSPF
Two-level hierarchy: local area, backbone. Link-state advertisements only in area each nodes has detailed area topology; only know
direction (shortest path) to nets in other areas.
Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.
Backbone routers: run OSPF routing limited to backbone.
Example Network
Router IDs can be selected independent of interface addresses, but usually chosen to be the smallest interface address
3
4 2
5
1
1
32
• Link costs are called Metric
• Metric is in the range [0 , 216]
• Metric can be asymmetric
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10.1.1.2 10.1.4.4 10.1.7.6
10.1.2.3 10.1.5.5
Link State Advertisement (LSA)
The LSA of router 10.1.1.1 is as follows:
Link State ID: 10.1.1.1 = Router ID
Advertising Router: 10.1.1.1 = Router ID Number of links: 3 = 2 links plus router itself
Description of Link 1: Link ID = 10.1.1.2, Metric = 4 Description of Link 2: Link ID = 10.1.2.2, Metric = 3 Description of Link 3: Link ID = 10.1.1.1, Metric = 0
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10.1.1.2 10.1.4.4 10.1.7.6
10.1.2.3 10.1.5.5
4
3 2
Network and Link State Database
Each router has a database which contains the LSAs from all other routers
LS Type Link StateID Adv. Router Checksum LS SeqNo LS Age
Router-LSA 10.1.1.1 10.1.1.1 0x9b47 0x80000006 0
Router-LSA 10.1.1.2 10.1.1.2 0x219e 0x80000007 1618
Router-LSA 10.1.2.3 10.1.2.3 0x6b53 0x80000003 1712
Router-LSA 10.1.4.4 10.1.4.4 0xe39a 0x8000003a 20
Router-LSA 10.1.5.5 10.1.5.5 0xd2a6 0x80000038 18
Router-LSA 10.1.7.6 10.1.7.6 0x05c3 0x80000005 1680
10.1.1.0 / 24
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10.1.1.2 10.1.4.4 10.1.7.6
10.1.2.3 10.1.5.5
Link State Database
The collection of all LSAs is called the link-state database
Each router has an identical link-state database Useful for debugging: Each router has a complete description of the
network
If neighboring routers discover each other for the first time, they will exchange their link-state databases
The link-state databases are synchronized using reliable flooding
OSPF Packet Format
OSPF MessageIP header
Body of OSPF MessageOSPF MessageHeader
Message TypeSpecific Data
LSA LSALSA ...
LSAHeader
LSAData
...
Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters)
TTL: set to 1 (in most cases)
OSPF packets are not carried as UDP payload!OSPF has its own IP protocol number: 89
OSPF Packet Format
source router IP address
authentication
authentication
32 bits
version type message length
Area ID
checksum authentication type
Body of OSPF MessageOSPF MessageHeader
2: current version is OSPF V2
Message types:1: Hello (tests reachability)2: Database description3: Link Status request4: Link state update5: Link state acknowledgement
ID of the Area from which the packet originated
Standard IP checksum taken over entire packet
0: no authentication1: Cleartext password2: MD5 checksum(added to end packet)
Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits)
KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits)
Prevents replay attacks
OSPF LSA Format
Link State ID
link sequence number
advertising router
Link Age Link Type
checksum length
Link ID
Link Data
Link Type Metric#TOS metrics
LSA
LSAHeader
LSAData
Link ID
Link Data
Link Type Metric#TOS metrics
LSA Header
Link 1
Link 2
Discovery of Neighbors
Routers multicasts OSPF Hello packets on all OSPF-enabled interfaces.
If two routers share a link, they can become neighbors, and establish an adjacency
After becoming a neighbor, routers exchange their link state databases
OSPF Hello
OSPF Hello: I heard 10.1.10.2
10.1.10.1 10.1.10.2
Scenario:Router 10.1.10.2 restarts
Neighbor discovery and database synchronization
OSPF Hello
OSPF Hello: I heard 10.1.10.2
Database Description: Sequence = X
10.1.10.1 10.1.10.2
Database Description: Sequence = X, 5 LSA headers = Router-LSA, 10.1.10.1, 0x80000006 Router-LSA, 10.1.10.2, 0x80000007 Router-LSA, 10.1.10.3, 0x80000003 Router-LSA, 10.1.10.4, 0x8000003a Router-LSA, 10.1.10.5, 0x80000038 Router-LSA, 10.1.10.6, 0x80000005
Database Description: Sequence = X+1, 1 LSA header= Router-LSA, 10.1.10.2, 0x80000005
Database Description: Sequence = X+1
Sends empty database description
Scenario:Router 10.1.10.2 restarts
Discovery of adjacency
Sends database description. (description only contains LSA headers)
Database description of 10.1.10.2
Acknowledges receipt of description
After neighbors are discovered the nodes exchange their databases
Regular LSA exchanges
10.1.10.2 explicitly requests each LSA from 10.1.10.1
10.1.10.1 sends requested LSAs
10.1.10.1 10.1.10.2
Link State Request packets, LSAs =
Router-LSA, 10.1.10.1,
Router-LSA, 10.1.10.2,
Router-LSA, 10.1.10.3,
Router-LSA, 10.1.10.4,
Router-LSA, 10.1.10.5,
Router-LSA, 10.1.10.6,
Link State Update Packet, LSAs =
Router-LSA, 10.1.10.1, 0x80000006
Router-LSA, 10.1.10.2, 0x80000007
Router-LSA, 10.1.10.3, 0x80000003
Router-LSA, 10.1.10.4, 0x8000003a
Router-LSA, 10.1.10.5, 0x80000038
Router-LSA, 10.1.10.6, 0x80000005
Dissemination of LSA-Update
A router sends and refloods LSA-Updates, whenever the topology or link cost changes. (If a received LSA does not contain new information, the router will not flood the packet)
Exception: Infrequently (every 30 minutes), a router will flood LSAs even if there are not new changes.
Acknowledgements of LSA-updates: explicit ACK, or implicit via reception of an LSA-Update
Question: If a new node comes up, it could build the database from regular LSA-Updates (rather than exchange of database description). What role do the database description packets play?
Dynamic Routing Protocols (Inter-domain)
Border Gateway Protocol
BGP Quick View
BGP = Border Gateway Protocol . Currently in version 4, specified in RFC 1771. (~ 60 pages)
Note: In the context of BGP, a gateway is nothing else but an IP router that connects autonomous systems.
Interdomain routing protocol for routing between autonomous systems
Uses TCP to establish a BGP session and to send routing messages over the BGP session
BGP is a path vector protocol. Routing messages in BGP contain complete routes.
Network administrators can specify routing policies
BGP Policy-based Routing
Each node is assigned an AS number (ASN)
BGP’s goal is to find any AS-path (not an optimal one). Since the internals of the AS are never revealed, finding an optimal path is not feasible.
Network administrator sets BGP’s policies to determine the best path to reach a destination network.
How Many ASNs are there today?
Thanks to Geoff Huston. http://bgp.potaroo.net on October 9, 2005
20,570
14,588origin only (notransit)
Autonomous Routing Domains Don’t Always Need BGP or an ASN
Qwest
Yale University
Nail up default routes 0.0.0.0/0pointing to Qwest
Nail up routes 130.132.0.0/16pointing to Yale
130.132.0.0/16
Static routing is the most common way of connecting anautonomous routing domain to the Internet. This helps explain why BGP is a mystery to many …
ARDs versus ASes
ASNs Can Be “Shared” (RFC 2270)
AS 701UUNet
ASN 7046 is assigned to UUNet. It is used byCustomers single homed to UUNet, but needing BGP for some reason (load balancing, etc..) [RFC 2270]
AS 7046Crestar Bank
AS 7046 NJIT
AS 7046HoodCollege
128.235.0.0/16
ARDs and ASes: Summary
Most ARDs have no ASN (statically routed at Internet edge)
Some unrelated ARDs share the same ASN (RFC 2270)
Some ARDs are implemented with multiple ASNs (example: Worldcom)
ASes are just an implementation detail of Inter-domain routing
How many prefixes today?
Thanks to Geoff Huston. http://bgp.potaroo.net on October 9, 2005
221,002
33.3%
23%
IPv4 Address space covered
Policy-Based vs. Distance-Based Routing?
ISP1
ISP2
ISP3
Cust1
Cust2Cust3
Host 1
Host 2
Minimizing “hop count” can violate commercial relationships thatconstrain inter-domain routing.
YES
NO
Thanks to Tim Griffin http://www.cl.cam.ac.uk/users/tgg22
Customer versus Provider
Customer pays provider for access to the Internet
provider
customer
IP trafficprovider customer
Regional ISP1
Regional ISP2
Regional ISP3
Cust1Cust3 Cust2
National ISP1
National ISP2
YES
NO
Shortest path routing is not compatible with commercial relations
Why not minimize “AS hop Count”?
peer peer
customerprovider
Peers provide transit between their respective customers
Peers do not provide transit between peers
Peers (often) do not exchange $$$trafficallowed
traffic NOTallowed
The “Peering” Relationship
Peering also allows connectivity betweenthe customers of “Tier 1” providers.
peer peer
customerprovider
Peering Provides Shortcuts
Peering Wars
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 competition
Peering relationships may require periodic renegotiation
Peering struggles are by far the most contentious issues in the ISP world!
Peering agreements are often confidential.
Peer Don’t Peer
BGP = RFC 1771
+ “optional” extensionsRFC 1997 (communities) RFC 2439 (damping) RFC 2796 (reflection) RFC3065 (confederation) …
+ routing policy configurationlanguages (vendor-specific)
+ Current Best Practices in management of Interdomain Routing
BGP was not DESIGNED. It EVOLVED.
The Border Gateway Protocol (BGP)
BGP Route Processing
Best Route Selection
Apply Import Policies
Best Route Table
Apply Export Policies
Install forwardingEntries for bestRoutes.
ReceiveBGPUpdates
BestRoutes
TransmitBGP Updates
Apply Policy =filter routes & tweak attributes
Based onAttributeValues
IP Forwarding Table
Apply Policy =filter routes & tweak attributes
Open ended programming.Constrained only by vendor configuration language
BGP Attributes
Value Code Reference----- --------------------------------- --------- 1 ORIGIN [RFC1771] 2 AS_PATH [RFC1771] 3 NEXT_HOP [RFC1771] 4 MULTI_EXIT_DISC [RFC1771] 5 LOCAL_PREF [RFC1771] 6 ATOMIC_AGGREGATE [RFC1771] 7 AGGREGATOR [RFC1771] 8 COMMUNITY [RFC1997] 9 ORIGINATOR_ID [RFC2796] 10 CLUSTER_LIST [RFC2796] 11 DPA [Chen] 12 ADVERTISER [RFC1863] 13 RCID_PATH / CLUSTER_ID [RFC1863] 14 MP_REACH_NLRI [RFC2283] 15 MP_UNREACH_NLRI [RFC2283] 16 EXTENDED COMMUNITIES [Rosen] ... 255 reserved for development
From IANA: http://www.iana.org/assignments/bgp-parameters
Mostimportantattributes
Not all attributesneed to be present inevery announcement
AS7018135.207.0.0/16AS Path = 6341
AS 1239Sprint
AS 1755Ebone
AT&T
AS 3549Global Crossing
135.207.0.0/16AS Path = 7018 6341
135.207.0.0/16AS Path = 3549 7018 6341
AS 6341
135.207.0.0/16
AT&T Research
Prefix Originated
AS 12654RIPE NCCRIS project
AS 1129Global Access
135.207.0.0/16AS Path = 7018 6341
135.207.0.0/16AS Path = 1239 7018 6341
135.207.0.0/16AS Path = 1755 1239 7018 6341
135.207.0.0/16AS Path = 1129 1755 1239 7018 6341
ASPATH Attribute
In fairness: could you do this “right” and still scale?
Exporting internalstate would dramatically increase global instability and amount of routingstate
AS 4
AS 3
AS 2
AS 1
Mr. BGP says that path 4 1 is better than path 3 2 1
Duh!
Shorter Doesn’t Always Mean Shorter
Thanks to Han Zheng
Routing Example 1
Thanks to Han Zheng
Routing Example 2
Tweak Tweak Tweak (TE)
For inbound traffic Filter outbound routes Tweak attributes on
outbound routes in the hope of influencing your neighbor’s best route selection
For outbound traffic Filter inbound routes Tweak attributes on
inbound routes to influence best route selection
outboundroutes
inboundroutes
inboundtraffic
outboundtraffic
In general, an AS has more control over outbound traffic
Forces outbound traffic to take primary link, unless link is down.
AS 1
primary link backup link
Set Local Pref = 100for all routes from AS 1 AS 65000
Set Local Pref = 50for all routes from AS 1
Backup Links with Local Preference (Outbound Traffic)
Forces outbound traffic to take primary link, unless link is down.
AS 1
primary link backup link
Set Local Pref = 100for all routes from AS 1
AS 2
Set Local Pref = 50for all routes from AS 3
AS 3provider provider
Multihomed Backups (Outbound Traffic)
Prepending will (usually) force inbound traffic from AS 1to take primary linkAS 1
192.0.2.0/24ASPATH = 2 2 2
customerAS 2
provider
192.0.2.0/24
backupprimary
192.0.2.0/24ASPATH = 2
Yes, this is a Glorious Hack …
Shedding Inbound Traffic with ASPATH Prepending
AS 1
192.0.2.0/24ASPATH = 2 2 2 2 2 2 2 2 2 2 2 2 2
customerAS 2
provider
192.0.2.0/24
192.0.2.0/24ASPATH = 2
AS 3provider
AS 3 will sendtraffic on “backup”link because it prefers customer routes and localpreference is considered before ASPATH length!
Padding in this way is oftenused as a form of loadbalancing
backupprimary
… But Padding Does Not Always Work
AS 1
customerAS 2
provider
192.0.2.0/24
192.0.2.0/24ASPATH = 2
AS 3provider
backupprimary
192.0.2.0/24ASPATH = 2 COMMUNITY = 3:70
Customer import policy at AS 3:If 3:90 in COMMUNITY then set local preference to 90If 3:80 in COMMUNITY then set local preference to 80If 3:70 in COMMUNITY then set local preference to 70
AS 3: normal customer local pref is 100,peer local pref is 90
COMMUNITY Attribute to the Rescue!
BGP Issues - What is a BGP Wedgie?
BGP policies make sense locally Interaction of local policies allows
multiple stable routings Some routings are consistent with
intended policies, and some are not If an unintended routing is
installed (BGP is “wedged”), then manual intervention is needed to change to an intended routing
When an unintended routing is installed, no single group of network operators has enough knowledge to debug the problem
¾ wedgie
Full wedgie
Dynamic Routing Protocols: Summary
Dynamic routing protocols: RIP, OSPF, BGP
RIP uses distance vector algorithm, and converges slow (the count-to-infinity problem)
OSPF uses link state algorithm, and converges fast. But it is more complicated than RIP.
Both RIP and OSPF finds lowest-cost path.
BGP uses path vector algorithm, and its path selection algorithm is complicated, and is influenced by policies.
BGP has its own problems see WIDGI by Tim Griffin
More Readings (Optional)
BGP Wedgies: Bad Routing Policy Interactions that Cannot be Debugged
JI’s Intro to interdomain routing.
"Interdomain Setting of PlanetLab Nodes." PlanetLab Meeting, May 14, 2004.
Understanding the Border Gateway Protocol (BGP) ICNP 2002 Tutorial Session
References
[VGE1996, VGE2000] Persistent Route Oscillations in Inter-Domain Routing. Kannan Varadhan, Ramesh Govindan, and Deborah Estrin. Computer Networks, Jan. 2000. (Also USC Tech Report, Feb. 1996)
[GW1999] An Analysis of BGP Convergence Properties. Timothy G. Griffin, Gordon Wilfong. SIGCOMM 1999
[GSW1999] Policy Disputes in Path Vector Protocols. Timothy G. Griffin, F. Bruce Shepherd, Gordon Wilfong. ICNP 1999
[GW2001] A Safe Path Vector Protocol. Timothy G. Griffin, Gordon Wilfong. INFOCOM 2001
[GR2000] Stable Internet Routing without Global Coordination. Lixin Gao, Jennifer Rexford. SIGMETRICS 2000
[GGR2001] Inherently safe backup routing with BGP. Lixin Gao, Timothy G. Griffin, Jennifer Rexford. INFOCOM 2001
– [GW2002a] On the Correctness of IBGP Configurations. Griffin and Wilfong.SIGCOMM 2002.
– [GW2002b] An Analysis of the MED oscillation Problem. Griffin and Wilfong. ICNP 2002.