Network Layer
Routing
IS250
Spring 2010
chuang@ischool.berkeley.edu
Outline
 Introduction
- Router operation
- Metric vs. policy based routing
 Metric-based routing
- Shortest path computation
- Distance vector and link-state algorithms and
protocols
 Policy-based routing
- Border Gateway Protocol (BGP)
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Two Basic Functions in
Packet Switching
 Routing
- Learn the best route to (or best next-hop for) any
given destination
- Exchange local link status or destination reachability
information
- Compute best path to destinations
• Algorithms for computing shortest paths (e.g., Dijkstra’s)
• Policies that reflect business agreements (use BGP)
- Update the routing table
 Packet forwarding
- For each packet received on an incoming link,
forward it to an outgoing link according to the routing
table
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Routing of Atoms
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Routing of Bits
Backbone
MAN
Ethernet
Local loop
Host A
Host B
12.2.14.60
128.32.226.87
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Packet Switch (Router)
 Special-purpose computer system
-
CPU
Memory
I/O interfaces
Software
 Connects to
- Other packet switches
- End hosts
 Store-and-forward paradigm
-
“Stores” packet in memory
Examines packet’s destination address
Looks up next-hop in routing table
“Forwards” packets toward destination (hop-by-hop)
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Packet Switch in Action
Incoming links
Packet
Outgoing links
Data Plane
Destination Outgoing
Address
Link
w.x.y.z
C
Routing table
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Control Plane
Control plane of a router communicates with its counterparts
at other routers using routing protocols (e.g., RIP, OSPF, BGP)
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What is the Best Route?
 Route selection may be based on:
-
Latency/hop-count
Bandwidth capacity
Loss rate
Cost
Security requirements
Business relationships (e.g., transit or peering
agreements)
- …
 Two flavors of routing: metric-based and policybased
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Metric-Based Routing and
Policy-Based Routing
 Both used in practice for routing on the Internet
- Metric-based routing typically employed within a network
domain
- Policy-based routing typically employed across network domains
 A network domain, also known as an autonomous
system (AS), is a collection of IP networks and routers,
usually under the control of a single administrative
entity, that presents a common routing policy to the
Internet
- Examples: ISPs, corporations, universities
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Outline
 Introduction
- Router operation
- Metric vs. policy based routing
 Metric-based routing
- Shortest path computation
- Distance vector and link-state algorithms and
protocols
 Policy-based routing
- Border Gateway Protocol (BGP)
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Routing: A Graph Theoretic
Perspective
 Note that each node has different view of the
network, and therefore different routing table
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Shortest Path Computation
 Algorithm from graph theory
 Distributed computation (no central authority)
 A switch
- Must learn route to all destinations
- While communicating only with directly connected
neighbors
 Example:
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Shortest Path Algorithms
 Distance vector
- Switches exchange routing table info
 Link state
- Switches exchange link status info
 Both used in practice
- e.g., RIP (RFC 1058, 1388, 1723) is a
distance vector routing protocol, OSPF (RFC
1131, 1247) is a link state routing protocol
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Link-State Routing
 Pair of switches periodically
- Test link between them
- Broadcast link status message to all switches in
network
 Switch
- Receives link status messages
- Computes new routes (using Dijkstra’s algorithm)
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Dijkstra’s Algorithm




Shortest Path First (SPF) algorithm
Greedy algorithm
Input: graph with nodes and weighted edges
Output: shortest paths from source node i to every
other node; cost of each path
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Dijkstra’s Algorithm
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Source: Doug Comer
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Algorithm Intuition
 Start at source node
 Move outward
 At each step:
- Find node u that
- has not been considered before; and
- is closest to source
- Compute:
- Distance from u to each neighbor v
- If distance shorter, make path to v go through u
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Dijkstra’s Algorithm Example
Distance
Predecessor
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Node A’s Routing Table
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Destination
Address
B
Next Hop
(Cost)
B (2)
C
D (3)
D
D (1)
E
D (2)
F
D (4)
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Routing Dynamics
 Dijkstra’s Algorithm has a run-time of O(|V|2+|E|), but
the routers run the algorithm continuously to keep their
routing tables updated
 Link states constantly updated
- New links can be added to network
- Latency can change continuously
- if link goes down, cost = infinity
 Routing tables may have transient inconsistency
- Link state messages take time to propagate across network
- Routers not synchronized in running route computation
algorithm
- Transient routing loops or routing black-holes
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Outline
 Introduction
- Router operation
- Metric vs. policy based routing
 Metric-based routing
- Shortest path computation
- Distance vector and link-state algorithms and
protocols
 Policy-based routing
- Border Gateway Protocol (BGP)
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Internet Routing Protocols
 Interior Gateway Protocols (IGPs)
-
Intra-AS or intra-domain
Metric-based routing (e.g., hops, latency, cost)
AS free to choose IGP and metric
Examples: RIP, OSPF, IBGP
 Exterior Gateway Protocols (EGPs)
- Inter-AS or inter-domain
- Policy-based routing
- All AS’s have to use the same EGP (currently BGP v4)
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Route Advertisement
 Instead of computing shortest paths based on DV or LS
information, inter-domain routing is based on
advertisements of routes
 Example: ISP1 sends route advertisement to ISP2
- ISP1 is willing to receive packets, destined to end-hosts within
its domain, from ISP2
- ISP2 will enter this information into its routing table
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Network Interconnection
transit
Backbone
AS 1 A
Backbone
AS 3 C
AS 2 B
Backbone
peering
Transit
 AS3 is a transitprovider of AS1:
- For a payment, AS3
agrees to advertise
route to AS1 to rest of
Internet
 AS2 and AS3 are
peers:
- Exchange route
advertisements
(without any
payments)
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Stub, Transit, and Multi-homed
 Three different types of AS:
- Stub AS
- Transit AS
Backbone A
Backbone A
- Multi-homed AS
Backbone A
Backbone C
AS 3 B
Backbone
Backbone A
Backbone C
AS 4 B
Backbone
Transit
Backbone C
AS 1 B
Backbone
AS 5 B
Backbone
Backbone C
Backbone A
Transit
Transit
AS 2 B
Backbone
Transit
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Border Gateway Protocol (BGP)




Version 4: RFC 1771 (March 1995)
Routing among Autonomous Systems
No routing metrics
Provisions for policies
- Individual AS’s control advertisement of routes
 Facilities for transit routing
- transit vs. stub domains
- Multi-homing
 Reliable transport
- Two BGP peers establish TCP session
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BGP Attributes
 Community attribute (no-advertise, no-export,
internet) controls propagation of advertisement
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Community
Attribute
No Advertise
No Export
Internet
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BGP Attributes
 Community attribute (no-export, no-advertise,
internet) controls propagation of advertisement
 Other attributes used for selecting best path
among multiple paths to same destination:
-
Weight (local to a router)
Local preference (propagated throughout AS)
Multi-exit discriminator (suggestion to external AS)
Origin (IGP, EGP, Incomplete)
AS_Path (AS hop count, loop detection)
Next hop (IP Address)
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BGP Path Selection
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
If the path specifies a next hop that is inaccessible, drop the update.
Prefer the path with the largest weight.
If the weights are same, prefer the path with largest local preference.
If the local preferences are same, prefer the path originated by BGP
running on this router.
If no route was originated, prefer the route that with shortest AS_path.
If all paths have the same AS_path length, prefer the path with the
lowest origin type (where IGP < EGP < incomplete).
If the origin codes are same, prefer the path with lowest MED attribute.
If the paths have same MED, prefer external over internal path.
If the paths are same, prefer path through the closest IGP neighbor.
Prefer path with lowest IP address, as specified by the BGP router ID.
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Summary
 Packet switching as key network layer
responsibility
 Packet switching = routing + packet forwarding
 Internet uses combination of metric-based
routing (intra-domain) and policy-based routing
(inter-domain)
- Metric-based routing: shortest path computation
based on distance vector or link state algorithms
- Policy-based routing: route advertisements
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