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23/4-07 Datakommunikation - Jonny Pettersson, UmU
Network Layer
Goals:
understand principles
behind network layer services:
forwarding routing (path selection) dealing with scale how a router works advanced topics: IPv6,
multicast instantiation and
implementation in the Internet
Overview:
last time
network layer services virtual circuit and datagram
networks
what’s inside a router? IP: Internet Protocol
today
routing algorithms
- Link state
- Distance Vector
- Hierarchical routing
routing in the Internet
- RIP
- OSPF
- BGP
broadcast and multicast
routing
23/4-07
1
2 3
0111
value in arriving packet’s header
routing algorithm local forwarding table header value output link
0100 0101 0111 1001 3 2 2 1
Interplay between routing and forwarding
23/4-07 Datakommunikation - Jonny Pettersson, UmU
u y
x
w v
z
2 2 1 3 1 1 2 5 3 5 Graph: G = (N,E) N = set of routers = { u, v, w, x, y, z } E = set of links ={ (u,v), (u,x), (u,w), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }
Graph abstraction
Remark: Graph abstraction is useful in other network contexts Example: P2P, where N is set of peers and E is set of TCP connections
23/4-07 Datakommunikation - Jonny Pettersson, UmU
Graph abstraction: costs
u y
x
w v
z
2 2 1 3 1 1 2 5 3 5
- c(x,x’) = cost of link (x,x’)
- e.g., c(w,z) = 5
- cost could always be 1, or
inversely related to bandwidth,
- r inversely related to
congestion Cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp) Question: What’s the least-cost path between u and z ?
Routing algorithm: algorithm that finds least-cost path
23/4-07 Datakommunikation - Jonny Pettersson, UmU
Routing Algorithm classification
Global or decentralized information?
Global:
all routers have complete
topology, link cost info
“link state” algorithms
Decentralized:
router knows physically-
connected neighbors, link costs to neighbors
iterative process of
computation, exchange of info with neighbors
“distance vector” algorithms
Static or dynamic?
Static:
routes change slowly
- ver time
Dynamic:
routes change more
quickly
periodic update in response to link
cost changes
23/4-07 Datakommunikation - Jonny Pettersson, UmU
Link State
Förutsättning
Varje nod kan kolla om närmsta granne är
uppe/nere och kan kostnaden för varje länk Basidé
Alla noder vet hur de ska nå närmsta granne, låt
alla veta det (alla får komplett bild)
Beror av två mekanismer
- Tillförlitlig spridning av link-state information
- Beräkning av vägar från summan av all ackumulerad