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CSE 461: Distance Vector Routing
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Focus
How do we calculate routes for
packets?
Routing is a network layer function
Routing Algorithms
Distance Vector routing (RIP)
Physical
Data Link
Network
Transport
Session
Presentation
Application
IP Addresses and IP Datagram
Forwarding
How the source gets the packet to the destination:
if source is on same network (LAN) as destination, source sends packet
directly to destination host
else source sends data to a router on the same network as the source
router will forward packet to a router on the next network over
and so on…
until packet arrives at router on same network as destination; then,
router sends packet directly to destination host
Requirements
every host needs to know IP address of the router on its LAN
every router needs a routing table to tell it which neighboring network
to forward a given packet on
Forwarding and Routing
Forwarding is the process that each router goes through
for every packet to send it on its way
Involves local decisions
Routing is the process that all routers go through to
calculate the routing tables
Involves global decisions
What’s in a Routing Table?
The routing table at A, for example, lists at a minimum
the next hops for the different destinations
D
G
A
F
E
B
C
F E
G F
E E
D C
C C
B B
Next
Hop
Dest
Kinds of Routing Schemes
Many routing schemes have been proposed/explored!
Distributed or centralized
Hop-by-hop or source-based
Deterministic or stochastic
Single or multi-path
Static or dynamic route selection
Internet is to the left
Routing Questions/Challenges
How to choose best path? What is best path?
How to scale to millions of users?
How to adapt to failures or changes?
Node and link failures, plus message loss
We will use distributed algorithms
Some Pitfalls
Using global knowledge is challenging
Hard to collect
Can be out-of-date
Needs to summarize in a locally-relevant way
Inconsistencies in local /global knowledge can cause:
Loops (black holes)
Oscillations, esp. when adapting to load
Routing is essentially a problem in graph theory
D
G
A
F
E
B
C
=router
=link
Network as a Graph
X
1 =cost
Distance Vector Routing
Assume:
Each router knows only address/cost of neighbors
Goal:
Calculate routing table of next hop information for
each destination at each router
Idea:
Tell neighbors about learned distances to all
destinations
DV Algorithm
Each router maintains a vector of costs to all destinations as
well as routing table
Initialize neighbors with known cost, others with infinity
Periodically send copy of distance vector to neighbors
On reception of a vector, if neighbors path to a destination
plus neighbor cost is better, then switch to better path
- update cost in vector and next hop in routing table
Assuming no changes, will converge to shortest paths
But what happens if there are changes?
DV Example – Initial Table at A
D
G
A
F
E
B
C
F 1 F
G
E 1 E
D
C 1 C
B 1 B
Dest Cost Next
Split Horizon
Solves trivial count-to-infinity problem
Router never advertises the cost of a destination back to
to its next hop – that’s where it learned it from!
Poison reverse: go even further – advertise back infinity
However, DV protocols still subject to the same problem
with more complicated topologies
Many enhancements suggested
Routing Information Protocol (RIP)
DV protocol with hop count as metric
Infinity value is 16 hops; limits network size
Includes split horizon with poison reverse
Routers send vectors every 30 seconds
With triggered updates for link failures
Time-out in 180 seconds to detect failures
RIPv1 specified in RFC
www.ietf.org/rfc/rfc1058.txt
RIPv2 (adds authentication etc.) in RFC
www.ietf.org/rfc/rfc1388.txt
RIP is an “Interior Gateway
Protocol”
Suitable for small- to medium-sized networks
such as within a campus, business, or ISP
Unsuitable for Internet-scale routing
hop count metric poor for heterogeneous links
16-hop limit places max diameter on network
Later, we’ll talk about “Exterior Gateway Protocols”
used between organizations to route across Internet
Key Concepts
Routing is a global process, forwarding is local one
The Distance Vector algorithm and RIP
Simple and distributed exchange of shortest paths.
Weak at adapting to changes (loops, count to infinity)
