Network Layer (part 1)
CPSC 363 Computer Networks
Ellen Walker
Hiram College
(Includes figures from Computer Networking by Kurose & Ross, © Addison Wesley
2002)
Network Layer
• Implements host-to-host communication
• Distributed throughout hosts and routers
– No central routing mechanism!
• Network layer functions
– Path determination (routing)
– Forwarding (to appropriate output link)
– Call setup (ATM, not IP)
Routing vs. Forwarding
• Routing is a network-wide process that
determines end-to-end paths
– Even though it is not done in a centralized way!
• Forwarding is a router-only algorithm that
sends each packet out the “right” interface
– Uses tables determined by routing algorithm
– Reads addresses in datagram headers
Virtual Circuit
• Setup: Build routing information tables (including
unique VC numbers) in each router
• Data transfer: look at each packet, determine output
interface and VC number based on input interface
and VC number
• Teardown: inform both ends and all internal routers
that the VC is no longer needed; routers remove
information from their tables
• Setup & teardown done by “signaling messages” that
cause VC tables to be modified
• Used by ATM networks (basis in telephony) -guaranteed bandwidth (constant or min) and ordering
Datagram
• Every packet contains entire address (sufficient
information to be routed to destination)
• No signaling messages needed
• Each router maintains a forwarding table to indicate
the “direction” to each destination
– Forwarding tables can change at any time
– Therefore no guarantee all packets in message take the
same route!
• Today’s Internet provides (only) Datagram service also called “best effort”
Advantages of Internet Model
• We can build more reliable transfer models
(e.g. TCP) on top of it
• Easy to interconnect networks with different
link-layer technologies (satellite vs. copper
vs. radio, etc.)
• Network doesn’t need to understand
application packets to route them; new
applications can always be added on hosts
Routing
• Each host attached directly to a “first-hop”
router (e.g. in the network card)
• Goal is to find the “least cost” path from
source (router) to destination (router)
– Assume every link has a numerical cost
– Add up all link costs to get a path cost
Find Least Cost Path from A to F
Routing Algorithms
• Dynamic
– Results change as network connectivity changes
• Global
– Works with complete information about all nodes and link
costs
– Also called link state algorithms
• Decentralized
– Each router makes its decisions based on partial information
about nodes and link costs
– More complete routing information evolves by information
exchange between neighbors
– Example: distance vector algorithm
Link State Algorithm
• Initialization: All nodes broadcast the
identities and costs of their neighboring links
until every node has an identical table, then
every node can run its own copy of the
algorithm
• Dijkstra’s Algorithm (all shortest paths)
– Builds a list of shortest (known) distance to every
(other) node
– When complete, all links have been considered
Dijkstra’s Algorithm Initialization
• N = {A} // the only node we know is the
source
• For every node v
– If v is adjacent to A, then
–
D(v) = link cost from A to v
–
Prev(v) = A
– Else D(v) = infinity
Dijkstra’s Algorithm Loop
• Find a node w (not in N) such that D(w) is a
minimum
• Add w to N
• For each v adjacent to w and not in N
– //update distance to be distance through w if it’s
less
– D(v) = min(D(v), D(w)+ link cost from w to v
– If D(w)+link cost is shorter, set Prev(v)=w
• (This loop is repeated until all nodes are in N)
Dijkstra’s Algorithm Example
• Source node is A
– A:0 B:3 C:x D:2 E:4 F:x
B
3
A
1
C
• Choose node D
– A:0 B:3 C:x D:2 E:3 F:x
• Choose node B
– A:0 B:3 C:4 D:2 E:3 F:7
• Choose node E
2
D
4
4
E
1
1
3
F
– A:0 B:3 C:4 D:2 E:3 F:4
• Choose node C
– A:0 B:3 C:4 D:2 E:3 F:4
• Choose node F
– A:0 B:3 C:4 D:2 E:3 F:4
Final “Previous” table
A:A B:A C:B D:A E:D F:E
(Node chosen when v got
its final value to the left)
Result of Dijkstra’s Algorithm
• Each node knows its previous node along the
least cost path from A
• Example
– Previous table is [ A:A B:A C:B D:A E:D F:E ]
– Path from A to F
• A…EF
• A… D E F
• ADEF
(prev of F is E)
(prev of E is D)
(prev of D is A)
• Use this information to make a “next hop”
table
Result of Dijkstra’s Algorithm
Example
• Example
– Previous table is [ A:A B:A C:B D:A E:D F:E ]
– Path from A to F
• A…EF
• A… D E F
• ADEF
(prev of F is E)
(prev of E is D)
(prev of D is A)
• In node A, next-hop(F) is D
• In node D, next-hop(F) is E
• In node E, next-hop(F) is F
Comments on Dijkstra’s Algorithm
• Time cost is O(N2)
– Roughly N searches through N possible next nodes each
time
– Note that the number of links is also O(N2) for most
networks!
• Traffic considerations can cause oscillation
– Make costs not depend on traffic (not practical)
– Avoid running algorithm simultaneously in all nodes (add
random delay)
Distance Vector Algorithm
• Each node…
– Receives information from its neighbors
– Calculates
– Distributes new information to its neighbors
• … until no new information is received
• This algorithm is..
– Asynchronous
– Distributed
– Iterative
Distance Table
• Each row represents a destination
• Each column represents a “next hop”
• Thus a cell indicates the current best estimate
cost from “me” to “row” via “column”
• Example (in node A):
– Row B, col C = c(A,C)+ C’s estimate of (C,B)
Example Distance Table
• In A:
BDE
B379
C478
D925
E834
F745
• Red (minimum) indicates
which next-hop to take
B
3
A
C
4
4
2
D
1
E
1
1
3
F
Distance Algorithm
• Initialize all values where row=col to direct distance
to that col. (e.g. B,B=3 and D,D=2)
• Initialize all other values to infinity
• Send min distance for each destination to all
neighbors
• Wait for a change (either message or link cost)
– If cost to neighbor v changes by d, add d to each value in
column v
– If update from v (cost from v to Y has changed), recalculate
distance to y (link to v + cost from v to Y)
– If change caused a new min, send new D value for that min
to all neighbors
Example:
• Assuming no initial
knowledge, generate
distance table (and next
hop)
• Set AD to 5 and let
network re-adjust
• Set AB to 100 and try
again
B
3
A
C
4
4
2
D
1
E
1
1
3
F
Comments on Distance Vector
Algorithm
• With no changes, settles fairly quickly
– As soon as information propagates from one edge of the
network to the other (network diameter)
• “Good news” spreads quickly
– Reduction in link distance
• “Bad news” takes longer (“count to infinity”)
– When a link is broken
– Because the higher entry in the table might reflect old info
• “Poisoned reverse”
– If Z goes through Y to get to X, then Z tells Y that Z’s
distance to X is infinite
– This prevents routes with short (but not long) loops
Link State vs. Distance Vector
Routing Algorithms
• LS requires enough messages that every node
knows the cost of every link, before the algorithm
starts. DV sends messages as needed.
• When a link cost changes, LS informs every node,
DV only sends messages where it matters (new least
cost paths)
• LS is guaranteed to converge in O(N2) DV can be
faster, but can also “count to infinity”
• Bad cost table in LS hurts only the node; bad cost
table in DV can hurt the whole network (broadcasting
incorrect information)
Networks Are Hierarchical
• Total number of nodes is too big to keep track of all
of them
• Organizations like to keep track of their own
networks, presenting only one gateway to the
“outside world”
• Result:
– Networks divided into Autonomous Systems (AS)
– Gateway router responsible for routing all packets to/from
outside world
– Most routing is Gateway to Gateway (think local roads &
interstates)
Inter and Intra-AS Routing
C.b
a
Host
h1
C
b
A.a
Inter-AS
routing
between
A and B
A.c
a
d
c
b
A
Intra-AS routing
within AS A
B.a
a
c
B
Host
h2
b
Intra-AS routing
within AS B
Necessary Routing Information
• Non-gateway router (intra-AS only)
– Holds complete routing information within AS
– Sends all extra-AS packets to gateway router (as if
that were their address)
– “Hot potato” routing – get packet to “closest”
gateway
• Gateway router (inter-AS and intra-AS)
– Holds complete routing information within AS
– Also hold routing information to other gateways
– Routing protocols include BGP (later)
Summary so far (Sec. 4.5)
• Every router has a “next hop” table to route packets
to the right interface (output)
• Routing algorithms determine these tables
– Global
– Distributed
• In the Internet, hosts are arranged hierarchically, and
inter-gateway routing is separated from intra-gateway
routing
• Next look at network-layer transmission units
(datagrams) and their headers, then return to routing