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TDTS41 Computer Networks
Lecture 4: Network layer I
Claudiu Duma, [email protected]/IDA
Linköpings universitet
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Outline
Introduction Routing algorithms
Link state shortest path first Distance vector Hierarchical routing
Routing in the Internet RIP OSPF BGP
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Network layer Transport data from sending to
receiving host
IP datagram/packet
Network layer protocols in every host, router
H1
H2
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
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Key Network-Layer Functions
1st: routing: determine route taken by packets from source to dest.
2nd: forwarding: move packets from router’s input to appropriate router output
analogy:
process of planning trip from source to dest
process of getting through single interchange
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1
23
0111
value in arrivingpacket’s header
routing algorithm
local forwarding tableheader value output link
0100010101111001
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Interplay between routing and forwarding
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Routing Principles
Minimize the cost of the routing path Scalability
Local changes should not affect globally Administrative issues
Different networks belong to different organizations
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Routing Algorithms
Simple Flooding
Those that minimize costs Link-state shortest-path-first Distance vector
Those that scale and meet administrative needs Intra-/Inter-autonomous systems routing
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Graph abstraction
u
yx
wv
z2
2
13
1
1
2
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5 • Set of nodes {u,v, w, ..}• Set of edges {(u,v), …}• Cost of links, e.g. c(u,v) = 2
• inverse to bandwidth• proportional to congestion• …
Cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp)
Question: What’s the shortest-path between u and w?
Note: “shortest-path” vs. “least-cost-path”
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Link-state shortest-path-first
Routers find, by broadcasts, about all links in the net (costs).
Each router computes locally the shortest-paths from itself to all other routers. Dijkstra algorithm
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Dijkstra’s Algorithm
1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = ∞ 7 8 Loop 9 find w not in N' such that D(w) is a minimum 10 add w to N' 11 update D(v) for all v adjacent to w and not in N' : 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N'
N': set of nodes whose least cost path is known D(v): current value of cost of path from source to dest. v
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Distance Vector Routing
Bellman-Ford Equation (dynamic programming)
Definedx(y) := cost of least-cost path from x to y
Then
dx(y) = min {c(x,v) + dv(y) }
where min is taken over all neighbors v of x
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Bellman-Ford Example
u
yx
wv
z2
2
13
1
1
2
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5dv(z) = 5, dx(z) = 3, dw(z) = 3
du(z) = min { c(u,v) + dv(z), c(u,x) + dx(z), c(u,w) + dw(z) } = min {2 + 5, 1 + 3, 5 + 3} = 4
Node that achieves minimum is nexthop in shortest path ➜ forwarding table
B-F equation says:
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Distance Vector Algorithm (1)
Dx(y) = estimate of least cost from x to y
Each node x knows: Its distance vector (DV): Dx = [Dx(y): y є N ] Cost to each neighbor v: c(x,v) Its neighbors’ distance vectors: Dv
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Distance Vector Algorithm (2)
Basic idea: Each node periodically sends its own distance
vector estimate to neighbors When node x receives new DV estimate from
neighbor, it updates its own DV using B-F equation:
Dx(y) ← minv{c(x,v) + Dv(y)} for each node y ∊ N
Under minor, natural conditions, the estimate Dx(y) converges to the actual least cost dx(y)
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x y z
xyz
0 2 7
∞ ∞ ∞∞ ∞ ∞
from
cost to
x y z
xyz
∞ ∞
∞ ∞ ∞
x y z
xyz
∞ ∞ ∞7 1 0
∞2 0 1
∞ ∞ ∞
x y z
xyz
0 2 3
x y z
xyz
0 2 3
x y z
xyz
0 2 3
2 0 13 1 0
2 0 1
3 1 0
2 0 1
3 1 0
x z12
7
y
node x
node y
node z
x y z
xyz
0 2 7
2 0 17 1 0
x y z
xyz
0 2 7
x y z
xyz
0 2 7
2 0 17 1 0
2 0 17 1 0
Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2
Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3
3
3
Next hop: x y z
x y z
x y z
x y z
x y z
x y z
x y y
x y z
y y z
y
y
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Count to infinity problem
Solution: Poisoned reverse If Z routes through Y to get to X, Z tells Y that Dz(x) =
∞.
A variant of split horizon Do not advertise a route back to the interface from
which you have learned about it!
x z14
50
y60y z
Dz(x) = 5Dy(x) = 6
Dy(x) = 6Dz(x) = 7
Dz(x) = 7Dy(x) = 8
.
.
.Dz(x) = 49Dy(x) = 50
Dy(x) = 8Dz(x) = 7
.
.
. ...
Dy(x) = 50Dz(x) = 50But not through y!
After the cost change occurs …
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Comparison of LS-SPF and DV algorithms
Message complexity LS-SPF: with n nodes, E
links, O(nE) msgs sent DV: exchange between
neighbors only convergence time varies
Speed of Convergence LS-SPF: O(n2) algorithm
requires O(nE) msgs may have oscillations
DV: convergence time varies may have routing loops count-to-infinity problem
Robustness: what happens if router malfunctions?
LS-SPF: node can advertise
incorrect link cost each node computes only
its own table
DV: DV node can advertise
incorrect path cost each node’s table used by
others • error propagate thru
network
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Hierarchical Routing
scale: with 200 million destinations:
can’t store all dest’s in routing tables!
routing table exchange would swamp links!
administrative autonomy
internet = network of networks
each network admin may want to control routing in its own network
Our routing study thus far - idealization all routers identical network “flat”… not true in practice
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3b
1d
3a
1c2aAS3
AS1
AS21a
2c2b
1b
3c
Autonomous systems (AS)
Internal and gateway routers
Two routing protocols: intra-AS routing inter-AS routing
Routers in same AS run same intra-AS routing protocol
Intra-ASRouting algorithm
Inter-ASRouting algorithm
Forwardingtable
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3b
1d
3a
1c2aAS3
AS1
AS21a
2c2b
1b
3c
Inter-AS tasks Suppose a router in
AS1 receives datagram for which dest is outside of AS1 Router should forward
packet towards one of the gateway routers, but which one?
AS1 needs:1. to learn which dests
are reachable through AS2 and which through AS3
2. to propagate this reachability info to all routers in AS1
Job of inter-AS routing!
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Example: Setting forwarding table in router 1d Suppose AS1 learns from the inter-AS protocol that subnet
x is reachable from AS3 (gateway 1c) but not from AS2. Inter-AS protocol propagates reachability info to all internal
routers. Router 1d determines from intra-AS routing info that its
interface I is on the least cost path to 1c. Router 1d puts in forwarding table entry (x,I).
3b
1d
3a
1c2aAS3
AS1
AS21a
2c2b
1b
3c
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Learn from inter-AS protocol that subnet x is reachable via multiple gateways
Use routing infofrom intra-AS
protocol to determine
costs of least-cost paths to each
of the gateways
Hot potato routing:Choose the
gatewaythat has the
smallest least cost
Determine fromforwarding table the interface I that leads
to least-cost gateway. Enter (x,I) in
forwarding table
Example: Choosing among multiple ASes
Now suppose AS1 learns from the inter-AS protocol that subnet x is reachable from AS3 and from AS2.
To configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x.
This is also the job of inter-AS routing protocol! Hot potato routing: send packet towards closest of
two routers.
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Intra-AS Routing
Also known as Interior Gateway Protocols (IGP) Most common Intra-AS routing protocols:
RIP: Routing Information Protocol
OSPF: Open Shortest Path First
IGRP: Interior Gateway Routing Protocol (Cisco proprietary)
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RIP ( Routing Information Protocol)
Distance vector algorithm Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15
hops) RIP advertisements, containing DVs
Exchanged every 30 sec among neighbors Each advertisement list of up to 25 destination
nets within AS If no advertisement heard after 180 sec -->
neighbor/link declared dead
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RIP Table Processing
RIP routing tables managed by application-level process called route-d (daemon)
advertisements sent in UDP packets, periodically repeated (port 520)
physical
link
network forwarding (IP) table
Transprt (UDP)
routed
physical
link
network (IP)
Transprt (UDP)
routed
forwardingtable
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OSPF (Open Shortest Path First)
“open”: publicly available Uses LS-SPF algorithm
LS packet dissemination Topology map at each node Route computation using Dijkstra’s algorithm
Link weights can be configured by net admin OSPF advertisement carries one entry per
neighbor router Advertisements disseminated to entire AS (via
flooding) Carried in OSPF messages directly over IP (rather than
TCP or UDP)
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OSPF “advanced” features (not in RIP)
Security: all OSPF messages authenticated (to prevent malicious intrusion)
Multiple same-cost paths allowed (only one path in RIP)
For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort; high for real time)
Hierarchical OSPF in large domains.
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Internet inter-AS routing: BGP
BGP (Border Gateway Protocol): the de facto standard
BGP provides each AS a means to:1. Obtain subnet reachability information from
neighboring ASs.2. Propagate the reachability information to all
routers internal to the AS.3. Determine “good” routes to subnets based
on reachability information and policy. Allows a subnet to advertise its
existence to rest of the Internet: “I am here”
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BGP basics Pairs of routers (BGP peers) exchange routing info over
semi-permanent (“long-lived”) TCP connections: BGP sessions
When AS2 advertises a prefix to AS1, AS2 is promising it will forward any datagrams destined to that prefix towards the prefix. AS2 can aggregate prefixes in its advertisement
3b
1d
3a
1c2aAS3
AS1
AS21a
2c
2b
1b
3c
eBGP external session
iBGP internal session
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Distributing reachability info Example:
3a sends reach. info to 1c 1c distributes this reach. info to all routers
in AS1 1b can then re-advertise the new reach info
to AS2 over the 1b-to-2a eBGP session
3b
1d
3a
1c2aAS3
AS1
AS21a
2c
2b
1b
3c
eBGP session
iBGP session
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Path attributes & BGP routes
When advertising a prefix, advert includes BGP attributes. prefix + attributes = “route”
Two important attributes: AS-PATH: contains the ASes through which
the advert for the prefix passed: • E.g. AS 67 AS 17
NEXT-HOP: Indicates the specific internal-AS router to next-hop AS.
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Some AS-Paths known by PeakWebHosting's BGP routers AS path: 6453[Teleglobe], 3356[Level3], 2529[Demon UK], 5459[LINX]
AS path: 20248[NetVMG], 3356[Level3], 2529[Demon UK], 5459[LINX]
AS path: 3356[Level3], 2529[Demon UK], 5459[LINX]
AS path: 174[PSI/Cogent], 2914[Verio], 5413[GX Networks], 5459[LINX]
AS path: 2914[Verio], 5413[GX Networks], 5459[LINX]
AS path: 19151[WebUseNet], 3257[Tiscali Backbone], 5459[LINX]
AS path: 6327[Shaw Cable], 4589[Easynet], 5459[LINX]
AS path: 3549[Global Crossing], 5459[LINX]
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BGP route selection
Router may learn about more than 1 route to some prefix. Router must select route.
Elimination rules:1. Local preference value attribute: policy
decision2. Shortest AS-PATH 3. Closest NEXT-HOP router: hot potato
routing4. Additional criteria
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BGP routing policy
Figure 4.5-BGPnew: a simple BGP scenario
A
B
C
W X
Y
legend:
customer network:
provider network
A,B,C are provider networks X,W,Y are customer (of provider networks) X is dual-homed: attached to two networks
X does not want to route from B via X to C .. so X will not advertise to B a route to C
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BGP routing policy (2)
Figure 4.5-BGPnew: a simple BGP scenario
A
B
C
W X
Y
legend:
customer network:
provider network
A advertises to B the path AW B advertises to X the path BAW Should B advertise to C the path BAW?
No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers
B wants to force C to route to w via A B wants to route only to/from its customers!
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Why different Intra- and Inter-AS routing ?
Policy: Inter-AS: admin wants control over how its traffic
routed, who routes through its net. Intra-AS: single admin, so no policy decisions
needed
Scale: hierarchical routing saves table size, reduced
update trafficPerformance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance