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
• 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– 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
A
DE
B
C
F
3
42
1 1
4
1
3
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 … E F (prev of F is E)• A… D E F (prev of E is D) • A D E F (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 … E F (prev of F is E)• A… D E F (prev of E is D) • A D E F (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: B D E
B 3 7 9
C 4 7 8
D 9 2 5
E 8 3 4
F 7 4 5
• Red (minimum) indicates which next-hop to take
A
DE
B
C
F
3
42
1 1
4
1
3
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
A
DE
B
C
F
3
42
1 1
4
1
3
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
Host h2
a
b
b
aaC
A
Bd c
A.a
A.c
C.bB.a
cb
Hosth1
Intra-AS routingwithin AS A
Inter-AS routingbetween A and B
Intra-AS routingwithin 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
