TDTS41 Computer Networks

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TDTS41 Computer Networks

• Lecture 4 Network layer I
• Claudiu Duma, cladu_at_ida.liu.se
• IISLAB/IDA
• Linköpings universitet

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Network Layer

• Goals
• understand principles behind routing
• routing in the Internet

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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
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Key Network-Layer Functions

• analogy
• process of planning trip from source to dest
• process of getting through single interchange

• 1st routing determine route taken by packets
  from source to dest.
• 2nd forwarding move packets from routers input
  to appropriate router output

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Interplay between routing and forwarding
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• Routing

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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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• Routing algorithms that minimize cost

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Graph abstraction

• 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 Whats 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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Dijkstras 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) 8 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)
• Define
• dx(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
dv(z) 5, dx(z) 3, dw(z) 3
B-F equation says
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 next hop in
shortest path ? forwarding table
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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 ? 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) ? minvc(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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Dx(y) minc(x,y) Dy(y), c(x,z) Dz(y)
min20 , 71 2
node x
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Next hop
y
x y z
node y
x
8
8
8 2 0 1
y
z
8
8
8
x y z
node z
x
8 8 8
y
8
8
8
z
3
7
1
0
y
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Count to infinity problem
After the cost change occurs
y
z
Dz(x) 5 Dy(x) 6
Dy(x) 6 Dz(x) 7
Dz(x) 7 Dy(x) 8

• Solution Poisoned reverse
• If Z routes through Y to get to X, Z tells Y
  that Dz(x) 8.
• A variant of split horizon
• Do not advertise a route back to the interface
  from which you have learned about it!

. . .
Dy(x) 8 Dz(x) 7
. . .
. . .
Dz(x) 49 Dy(x) 50
Dy(x) 50 Dz(x) 50 But not through y!
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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 nodes table used by others
• error propagate thru network

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• Hierarchical routing

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Hierarchical Routing

• Our routing study thus far - idealization
• all routers identical
• network flat
• not true in practice

• scale with 200 million destinations
• cant store all dests 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

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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

3a
3b
2a
AS3
AS2
1a
AS1
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Inter-AS tasks

• AS1 needs
• to learn which dests are reachable through AS2
  and which through AS3
• to propagate this reachability info to all
  routers in AS1
• Job of inter-AS routing!

• 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?

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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).

3a
3b
2a
AS3
AS2
1a
AS1
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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

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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 --gt
  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)

Transprt (UDP)
Transprt (UDP)
network forwarding (IP) table
network (IP)
forwarding table
link
link
physical
physical
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• OSPF

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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 Dijkstras 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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Hierarchical OSPF
Hierarchically structured OSPF autonomous system
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• Inter-AS Routing

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Internet inter-AS routing BGP

• BGP (Border Gateway Protocol) the de facto
  standard
• BGP provides each AS a means to
• Obtain subnet reachability information from
  neighboring ASs.
• Propagate the reachability information to all
  routers internal to the AS.
• 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

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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

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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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ASN examples

• ASN Network
• 3356 Level3
• 3549 Global Crossing
• 2529 Demon UK
• 4589 Easynet
• 5459 LINX

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Some AS-Paths known by PeakWebHosting's BGP
routers

• AS path 6453Teleglobe, 3356Level3,
  2529Demon UK, 5459LINX
• AS path 20248NetVMG, 3356Level3, 2529Demon
  UK, 5459LINX
• AS path 3356Level3, 2529Demon UK, 5459LINX
  
• AS path 174PSI/Cogent, 2914Verio, 5413GX
  Networks, 5459LINX
• AS path 2914Verio, 5413GX Networks,
  5459LINX
• AS path 19151WebUseNet, 3257Tiscali
  Backbone, 5459LINX
• AS path 6327Shaw Cable, 4589Easynet,
  5459LINX
• AS path 3549Global Crossing, 5459LINX

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BGP route selection

• Router may learn about more than 1 route to some
  prefix. Router must select route.
• Elimination rules
• Local preference value attribute policy decision
• Shortest AS-PATH
• Closest NEXT-HOP router hot potato routing
• Additional criteria

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BGP routing policy

• 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)

• 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 Bs 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 traffic
• Performance
• Intra-AS can focus on performance
• Inter-AS policy may dominate over performance

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Summary

• What weve covered
• Network layers functions
• Routing principles
• Hierarchical routing
• Internet routing protocols RIP, OSPF, BGP