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Csci 232 – Computer Networks Routing and Network Layer Part II 1
Routing: Network Layer Part II
• Routing Algorithms: – Link state vs. Distance Vector
• Routing in the Internet– Intra-AS vs. Inter-AS routing– Intra-AS: RIP and OSPF– Inter-AS: BGP and Policy Routing
• MPLS Readings: Textbook: Chapter 4: Sections 4.2-4.3, 4.5-4.6
Csci 232 – Computer Networks Routing and Network Layer Part II 2
Routing & Forwarding:Logical View of a Router
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Csci 232 – Computer Networks 3
Forwarding Process
IP Forwarding Table Router
1. Remove a packet from an input queue
3. Match packet’s destination to a table entry
2. Check for sanity, decrement TTL field
4. Place packet on correct output queue
If queuesget full, just
drop packets!
If queuesget full, just
drop packets!
IP Forwarding Process
Csci 232 – Computer Networks 4
Destination Next Hop Interface
Net ANet BNet C, Host 3
Router 1DirectRouter 2Router 1
INT 7
INT 7INT 3INT 4
A destination is usuallya network. May also be a host, or a “gateway of last resort” (default)
The next hop is either a directly connected network or a router on a directly connected network
A physical interface
Net C
IP Forwarding Table
Csci 232 – Computer Networks 5
Statically Dynamically
Routers exchange network reachability information using ROUTING PROTOCOLS. Routers use this to compute best routes
Administrator manually configuresforwarding table entries
In practice : a mix of these.Static routing mostly at the “edge”
+ More control+ Not restricted to destination-based forwarding - Doesn’t scale- Slow to adapt to network failures
+ Can rapidly adapt to changes in network topology+ Can be made to scale well- Complex distributed algorithms- Consume CPU, Bandwidth, Memory- Debugging can be difficult- Current protocols are destination-based
How Are Forwarding Tables Populated to Implement
Routing?
Csci 232 – Computer Networks Routing and Network Layer Part II 6
Dynamic Routing: Intra- vs. Inter-AS
AS 1
AS 2
BGP
EGP = Exterior Gateway Protocol
IGP = Interior Gateway Protocol
Metric based: OSPF, IS-IS, RIP, EIGRP (cisco)
Policy based: BGP
The Routing Domain of BGP is the entire Internet
OSPF
EIGRP
Csci 232 – Computer Networks Routing and Network Layer Part II 7
Internet AS Hierarchy
border (exterior gateway) routers
interior routers
Csci 232 – Computer Networks Routing and Network Layer Part II 8
Intra-AS vs. Inter-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
Csci 232 – Computer Networks Routing and Network Layer Part II 9
Intra-AS and Inter-AS Routing
physical layer
“Gateways”:•perform inter-AS routing amongst themselves•perform intra-AS routing with other routers in their AS
inter-AS, intra-AS routing in
gateway A.c
network layer
link layer
a
b
b
aaC
A
Bd
A.aA.c
C.bB.a
cb
c
Csci 232 – Computer Networks 10
Forwarding Table
OSPFDomain
RIPDomain
BGPOSPF Process
OSPF Routing tables
RIP Process
RIP Routing tables
BGP Process
BGP Routing tables
Forwarding Table Manager
Where Does Forwarding Table Come From?
Csci 232 – Computer Networks Routing and Network Layer Part II 11
Routing• Goal: determine a “good” path
through the network from source to destination– Good means usually the shortest path
• Network modeled as a graph– Routers nodes– Link edges
• Edge cost: delay, congestion level,…
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Csci 232 – Computer Networks Routing and Network Layer Part II 12
Basic Routing Problem• Assume
– A network with N nodes, where each edge is associated a cost
– A node knows only its neighbors and the cost to reach them
• How does each node learn how to reach every other node along the shortest path? A
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Csci 232 – Computer Networks Routing and Network Layer Part II 13
Routing: Issues
• How are routing tables determined?• Who determines table entries?• What info is used in determining table
entries?• When do routing table entries change?• Where is routing info stored?• How to control routing table size?Answer these questions, we are done!
Csci 232 – Computer Networks Routing and Network Layer Part II 14
Routing Paradigms
• Hop-by-hop Routing– Each packet contains destination address– Each router chooses next-hop to destination
• routing decision made at each (intermediate) hop!• packets to same destination may take different paths!
– Example: IP’s default datagram routing
• Source Routing– Sender selects the path to destination precisely– Routers forward packet to next-hop as specified
• Problem: if specified path no longer valid due to link failure!
– Example: • IP’s loose/strict source route option • virtual circuit setup phase in ATM (or MPLS)
Csci 232 – Computer Networks Routing and Network Layer Part II 15
Routing Algorithms/ProtocolsIssues Need to Be Addressed:• Route selection may depend on different
criteria– Performance: choose route with the smallest delay– Policy: choose a route that doesn’t cross .gov
network
• Adapt to changes in network topology or condition– Self-healing: little or no human intervention
• Scalability– Must be able to support a large number of hosts,
routers
Csci 232 – Computer Networks Routing and Network Layer Part II 16
Centralized vs. Distributed Routing Algorithms
Centralized:• A centralized route server collects routing information
and network topology, makes route selection decisions, then distributes them to routers
Distributed:• Routers cooperate using a distributed protocol
– to create mutually consistent routing tables• Two standard distributed routing algorithms
– Link State (LS) routing – Distance Vector (DV) routing
Csci 232 – Computer Networks Routing and Network Layer Part II 17
Link State vs Distance Vector
• Both assume that– The address of each neighbor is known– The cost of reaching each neighbor is known
• Both find global information– By exchanging routing info among neighbors
• Differ in the information exchanged and route computation– LS: tells every other node its distances to neighbors– DV: tells neighbors its distance to every other node
Csci 232 – Computer Networks Routing and Network Layer Part II 18
Link State Algorithm
• Basic idea: Distribute link state packet to all routers– Topology of the network
• Cost of each link in the network
• Each router independently computes optimal paths– From itself to every destination– Routes are guaranteed to be loop free if
• Each router sees the same cost for each link• Uses the same algorithm to compute the best path
Csci 232 – Computer Networks Routing and Network Layer Part II 19
Link State: Control Traffic• Each node floods its local information to every
other node in the network• Each node ends up knowing the entire network
topology use Dijkstra to compute the shortest path to every other node
Host A
Host BHost E
Host D
Host C
N1 N2
N3
N4
N5
N7N6
Csci 232 – Computer Networks Routing and Network Layer Part II 20
Link State: Node State
Host A
Host BHost E
Host D
Host C
N1 N2
N3
N4
N5
N7N6
A
BE
DC
A
BE
DC A
BE
DC
A
BE
DC
A
BE
DC
A
BE
DC
A
BE
DC
Csci 232 – Computer Networks Routing and Network Layer Part II 21
Topology Dissemination
• Each router creates a set of link state packets (LSPs)– Describing its links to neighbors– LSP contains
• Router id, neighbor’s id, and cost to its neighbor
• Copies of LSPs are distributed to all routers– Using controlled flooding
• Each router maintains a topology database– Database containing all LSPs
Csci 232 – Computer Networks Routing and Network Layer Part II 22
A
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Topology Database: Example
link state database
Csci 232 – Computer Networks Routing and Network Layer Part II 23
Constructing Routing Table:Dijkstra’s Algorithm
• Given the network topology– How to compute the shortest path to each destination?
• Some notation– X: source node– N: set of nodes to which shortest paths are known so
far• N is initially empty
– D(V): the cost of the known shortest path from source X to V
– C(U,V): cost of link U to V• C(U,V) = if not neighbors
Csci 232 – Computer Networks Routing and Network Layer Part II 24
Algorithm (at Node X)
• Initialization– N = {X}– For all nodes V
• If V adjacent to X, D(V) = C(X,V) else D(V) =
• Loop– Find U not in N such that D(U) is the smallest– Add U into set N– Update D(V) for all V not in N
• D(V) = min{D(V), D(U) + C(U,V)}
– Until all nodes in N
Csci 232 – Computer Networks Routing and Network Layer Part II 25
Example: Dijkstra’s AlgorithmStep
012345
start NA
D(B),p(B)2,A
D(C),p(C)5,A
D(D),p(D)1,A
D(E),p(E) D(F),p(F)
A
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2
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1 Initialization: 2 N = {A};3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(A,v); 6 else D(v) = ;…
Csci 232 – Computer Networks Routing and Network Layer Part II 26
Example: Dijkstra’s AlgorithmStep
012345
start NA
AD
D(B),p(B)2,A
D(C),p(C)5,A4,D
D(D),p(D)1,A
D(E),p(E)
2,D
D(F),p(F)
A
ED
CB
F
2
2
13
1
1
2
53
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…8 Loop 9 find w not in N s.t. 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 until all nodes in N;
Csci 232 – Computer Networks Routing and Network Layer Part II 27
Example: Dijkstra’s AlgorithmStep
012345
start NA
ADADE
D(B),p(B)2,A
D(C),p(C)5,A4,D3,E
D(D),p(D)1,A
D(E),p(E)
2,D
D(F),p(F)
4,E
A
ED
CB
F
2
2
13
1
1
2
53
5…8 Loop 9 find w not in N s.t. 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 until all nodes in N;
Csci 232 – Computer Networks Routing and Network Layer Part II 28
Example: Dijkstra’s AlgorithmStep
012345
start NA
ADADE
ADEB
D(B),p(B)2,A
D(C),p(C)5,A4,D3,E
D(D),p(D)1,A
D(E),p(E)
2,D
D(F),p(F)
4,E
A
ED
CB
F
2
2
13
1
1
2
53
5…8 Loop 9 find w not in N s.t. 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 until all nodes in N;
Csci 232 – Computer Networks Routing and Network Layer Part II 29
Example: Dijkstra’s AlgorithmStep
012345
start NA
ADADE
ADEBADEBC
D(B),p(B)2,A
D(C),p(C)5,A4,D3,E
D(D),p(D)1,A
D(E),p(E)
2,D
D(F),p(F)
4,E
A
ED
CB
F
2
2
13
1
1
2
53
5…8 Loop 9 find w not in N s.t. 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 until all nodes in N;
Csci 232 – Computer Networks Routing and Network Layer Part II 30
Example: Dijkstra’s AlgorithmStep
012345
start NA
ADADE
ADEBADEBC
ADEBCF
D(B),p(B)2,A
D(C),p(C)5,A4,D3,E
D(D),p(D)1,A
D(E),p(E)
2,D
D(F),p(F)
4,E
A
ED
CB
F
2
2
13
1
1
2
53
5…8 Loop 9 find w not in N s.t. 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 until all nodes in N;
Csci 232 – Computer Networks Routing and Network Layer Part II 31
Dijkstra’s Algorithm: In a Nutshell
Step012345
start NA
ADADE
ADEBADEBC
ADEBCF
D(B),p(B)2,A2,A2,A
D(C),p(C)5,A4,D3,E3,E
D(D),p(D)1,A
D(E),p(E)infinity
2,D
D(F),p(F)infinityinfinity
4,E4,E4,E
A
ED
CB
F
2
2
13
1
1
2
53
5
Csci 232 – Computer Networks Routing and Network Layer Part II 32
A
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3
5
Routing Table Computation
dest next
B BC DD DE DF D
Csci 232 – Computer Networks Routing and Network Layer Part II 33
Distance Vector Routing• A router tells neighbors its distance to every
router– Communication between neighbors only
• Based on Bellman-Ford algorithm– Computes “shortest paths”
• Each router maintains a distance table– A row for each possible destination– A column for each neighbor
• DX(Y,Z) : distance from X to Y via Z
• Exchanges distance vector (the table) with neighbors– Distance vector: current least cost to each
destination
Csci 232 – Computer Networks Routing and Network Layer Part II 34
Distance Vector: Control Traffic• When the routing table of a node changes, the
node sends its table to its neighbors• A node updates its table with information
received from its neighbors
Host A
Host BHost E
Host D
Host C
N1 N2
N3
N4
N5
N7N6
Csci 232 – Computer Networks Routing and Network Layer Part II 35
Distance Table: Example
A
E D
CB6
8
1
2
1
2
D ()
A
B
C
D
A
1
7
8
10
B
14
8
9
11
D
11
5
4
2
Ecost to destination via
des
tin
atio
n
Csci 232 – Computer Networks Routing and Network Layer Part II 36
Distance Table to Routing Table
D ()
A
B
C
D
A
1
7
6
4
B
14
8
9
11
D
5
5
4
2
Ecost to destination via
des
tin
atio
n
A
B
C
D
A,1
D,5
D,4
D,2
Outgoing link to use, cost
des
tin
atio
n
Distance table Routing table
Csci 232 – Computer Networks Routing and Network Layer Part II 37
Distance Vector Routing Algorithm
iterative:• continues until no
nodes exchange info.• self-terminating: no
“signal” to stop
asynchronous:• nodes need not
exchange info/iterate in lock step!
distributed:• each node talks only
with directly-attached neighbors
Distance Table data structure
• each node has its own• row for each possible destination• column for each directly-attached
neighbor to node• example: in node X, for dest. Y via
neighbor Z:
D (Y,Z)X
distance from X toY, via Z as next hop
c(X,Z) + min {D (Y,w)}Z
w
=
=
Csci 232 – Computer Networks Routing and Network Layer Part II 38
Distance Vector Routing: Overview
Iterative, asynchronous: each iteration caused by:
• local link cost change • message from neighbor:
its least cost path change from neighbor
Distributed:
• each node notifies neighbors only when its least cost path to any destination changes– neighbors then notify
their neighbors if necessary
wait for (change in local link cost or msg from neighbor)
recompute distance table
if least cost path to any
dest has changed, notify neighbors
Each node:
Csci 232 – Computer Networks Routing and Network Layer Part II 39
Distance Vector Algorithm: Example
X Z12
7
Y
D (Y,Z)X
c(X,Z) + min {D (Y,w)}w=
= 7+1 = 8
Z
D (Z,Y)X
c(X,Y) + min {D (Z,w)}w=
= 2+1 = 3
Y
Csci 232 – Computer Networks Routing and Network Layer Part II 40
Distance Vector Algorithm: Example
X Z12
7
Y
Csci 232 – Computer Networks Routing and Network Layer Part II 41
Convergence of DV Routing• router detects local link cost change • updates distance table• if cost change in least cost path,
notify neighborsX Z
14
50
Y1
algorithmterminates
“goodnews travelsfast”
Csci 232 – Computer Networks Routing and Network Layer Part II 42
Problems with DV RoutingLink cost changes:• good news travels fast • bad news travels slow
– “count to infinity” problem!
X Z14
50
Y60
algorithmcontinues
on!
Csci 232 – Computer Networks Routing and Network Layer Part II 43
Count-to-Infinity Problem
X Y Z1 1
2
Csci 232 – Computer Networks Routing and Network Layer Part II 44
“Fixes” to Count-to-Infinity Problem
• Split horizon– A router never advertises the cost of a
destination to a neighbor• If this neighbor is the next hop to that destination
• Split horizon with poisonous reverse– If X routes traffic to Z via Y, then
• X tells Y that its distance to Z is infinity– Instead of not telling anything at all
– Accelerates convergence
Csci 232 – Computer Networks Routing and Network Layer Part II 45
Split Horizon with Poisoned Reverse
If Z routes through Y to get to X :• Z tells Y its (Z’s) distance to X is
infinite (so Y won’t route to X via Z) X Z14
50
Y60
algorithmterminates
Csci 232 – Computer Networks Routing and Network Layer Part II 46
Count-to-Infinity Problem Revisited
X Y Z
W
Csci 232 – Computer Networks Routing and Network Layer Part II 47
Link State vs Distance Vector
• Tells everyone about neighbors
• Controlled flooding to exchange link state
• Dijkstra’s algorithm• Each router computes
its own table• May have oscillations• Open Shortest Path
First (OSPF)
• Tells neighbors about everyone
• Exchanges distance vectors with neighbors
• Bellman-Ford algorithm
• Each router’s table is used by others
• May have routing loops
• Routing Information Protocol (RIP)
Csci 232 – Computer Networks Routing and Network Layer Part II 48
Link State vs. Distance Vector (cont’d)
Message complexity• LS: O(n2*e) messages
– n: number of nodes – e: number of edges
• DV: O(d*n*k) messages– d: node’s degree– k: number of rounds
Time complexity• LS: O(n*log n) • DV: O(n)Convergence time
• LS: O(1)• DV: O(k)
Robustness: what happens if router malfunctions?
• LS: – node can advertise
incorrect link cost– each node computes only
its own table
• DV:– node can advertise
incorrect path cost– each node’s table used by
others; error propagate through network
Csci 232 – Computer Networks Routing and Network Layer Part II 49
Routing in the Real World
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”
How to do routing in the Internet• scalability and policy issues
Csci 232 – Computer Networks Routing and Network Layer Part II 50
Routing in the Internet• The Global Internet consists of Autonomous
Systems (AS) interconnected with each other hierarchically:– Stub AS: small corporation: one connection to other AS’s– Multihomed AS: large corporation (no transit): multiple
connections to other AS’s– Transit AS: provider, hooking many AS’s together
• Two-level routing: – Intra-AS: administrator responsible for choice of
routing algorithm within network– Inter-AS: unique standard for inter-AS routing: BGP
Csci 232 – Computer Networks Routing and Network Layer Part II 51
Internet Architecture
LANs
International lines
ISP ISPcompany university
national network
regionalnetwork
NAPInternic
on-line services
companyaccess via
modem
Internet: “networks of networks”!
Csci 232 – Computer Networks Routing and Network Layer Part II 52
Internet AS Hierarchy
Inter-AS border (exterior gateway) routers
Intra-AS interior (gateway) routers
Csci 232 – Computer Networks Routing and Network Layer Part II 53
Intra-AS vs. Inter-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
Csci 232 – Computer Networks Routing and Network Layer Part II 54
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, update trafficPerformance: • Intra-AS: can focus on performance• Inter-AS: policy may dominate over performance
Csci 232 – Computer Networks Routing and Network Layer Part II 55
Intra-AS and Inter-AS Routing
physical layer
“Gateways”:•perform inter-AS routing amongst themselves•perform intra-AS routers with other routers in their AS
inter-AS, intra-AS routing in
gateway A.c
network layer
link layer
a
b
b
aaC
A
Bd
A.aA.c
C.bB.a
cb
c
Csci 232 – Computer Networks Routing and Network Layer Part II 56
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– IS-IS: Intermediate System to Intermediate
System (OSI Standard)– EIGRP: Extended Interior Gateway Routing
Protocol (Cisco proprietary)
Csci 232 – Computer Networks Routing and Network Layer Part II 57
RIP ( Routing Information Protocol)
• Distance vector algorithm• Included in BSD-UNIX Distribution in 1982• Distance metric: # of hops (max = 15 hops)
Number of hops from source router A to various subnets:
DC
BA
u v
w
x
yz
destination hops u 1 v 2 w 2 x 3 y 3 z 2
Csci 232 – Computer Networks Routing and Network Layer Part II 58
RIP advertisements
• Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement)
• Each advertisement: list of up to 25 destination nets within AS
Csci 232 – Computer Networks Routing and Network Layer Part II 59
RIP: Example
Destination Network Next Router Num. of hops to dest. w A 2
y B 2 z B 7
x -- 1…. …. ....
w x y
z
A
C
D B
Routing table in D
Csci 232 – Computer Networks Routing and Network Layer Part II 60
RIP: Example
Destination Network Next Router Num. of hops to dest. w A 2
y B 2 z B A 7 5
x -- 1…. …. ....Routing table in D
w x y
z
A
C
D B
Dest Next hops w - - x - - z C 4 …. … ...
Advertisementfrom A to D
Csci 232 – Computer Networks Routing and Network Layer Part II 61
RIP: Link Failure and Recovery If no advertisement heard after 180 sec -->
neighbor/link declared dead– routes via neighbor invalidated– new advertisements sent to neighbors– neighbors in turn send out new advertisements (if tables
changed)– link failure info quickly propagates to entire net– poison reverse used to prevent ping-pong loops (infinite
distance = 16 hops)
Csci 232 – Computer Networks Routing and Network Layer Part II 62
RIP Table processing
• RIP routing tables managed by application-level process called route-d (daemon)
• advertisements sent in UDP packets, periodically repeated
physical
link
network forwarding (IP) table
Transprt (UDP)
routed
physical
link
network (IP)
Transprt (UDP)
routed
forwardingtable
Csci 232 – Computer Networks Routing and Network Layer Part II 63
OSPF (Open Shortest Path First)• “open”: publicly available• Uses Link State algorithm
– LS packet dissemination– Topology map at each node– Route computation using Dijkstra’s algorithm
• 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
Csci 232 – Computer Networks Routing and Network Layer Part II 64
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)
• Integrated uni- and multicast support: – Multicast OSPF (MOSPF) uses same topology data base
as OSPF
• Hierarchical OSPF in large domains.
Csci 232 – Computer Networks Routing and Network Layer Part II 65
Hierarchical OSPF
Csci 232 – Computer Networks Routing and Network Layer Part II 66
Hierarchical OSPF• Two-level hierarchy: local area, backbone.
– Link-state advertisements only in area – each nodes has detailed area topology; only know
direction (shortest path) to nets in other areas.– Communications between areas via backbone
• Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.
• Backbone routers: run OSPF routing limited to backbone.
• Boundary routers: connect to other AS’s.
Csci 232 – Computer Networks Routing and Network Layer Part II 67
Inter-AS Routing in the Internet: BGP
Figure 4.5.2-new2: BGP use for inter-domain routing
AS2 (OSPF
intra-AS routing)
AS1 (RI P intra-AS
routing) BGP
AS3 (OSPF intra-AS
routing)
BGP
R1 R2
R3
R4
R5
Csci 232 – Computer Networks Routing and Network Layer Part II 68
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”
Csci 232 – Computer Networks Routing and Network Layer Part II 69
BGP basics• Pairs of routers (BGP peers) exchange routing info over
semi-permanent TCP conctns: BGP sessions• Note that BGP sessions do not correspond to physical
links.• 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 session
iBGP session
Csci 232 – Computer Networks Routing and Network Layer Part II 70
Distributing reachability info• With eBGP session between 3a and 1c, AS3 sends prefix
reachability info to AS1.• 1c can then use iBGP to distribute this new prefix 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• When router learns about a new prefix, it creates an
entry for the prefix in its forwarding table.
3b
1d
3a
1c2aAS3
AS1
AS21a
2c
2b
1b
3c
eBGP session
iBGP session
Csci 232 – Computer Networks Routing and Network Layer Part II 71
Path attributes & BGP routes
• When advertising a prefix, advert includes BGP attributes. – prefix + attributes = “route”
• Two important attributes:– AS-PATH: contains the ASs through which the advert for
the prefix passed: AS 67 AS 17 – NEXT-HOP: Indicates the specific internal-AS router to next-
hop AS. (There may be multiple links from current AS to next-hop-AS.)
• When gateway router receives route advert, uses import policy to accept/decline.
Csci 232 – Computer Networks Routing and Network Layer Part II 72
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
Csci 232 – Computer Networks Routing and Network Layer Part II 73
BGP messages• BGP messages exchanged using TCP.• BGP messages:
– OPEN: opens TCP connection to peer and authenticates sender
– UPDATE: advertises new path (or withdraws old)– KEEPALIVE keeps connection alive in absence of
UPDATES; also ACKs OPEN request– NOTIFICATION: reports errors in previous msg; also used
to close connection
Csci 232 – Computer Networks Routing and Network Layer Part II 74
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
Csci 232 – Computer Networks Routing and Network Layer Part II 75
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!
Csci 232 – Computer Networks Routing and Network Layer Part II 76
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
Csci 232 – Computer Networks Routing and Network Layer Part II 77
Multi-Protocol Label Switching (MPLS)
• initial goal: speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding – borrowing ideas from Virtual Circuit (VC) approach– but IP datagram still keeps IP address!
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Csci 232 – Computer Networks Routing and Network Layer Part II 78
MPLS Capable Routers• a.k.a. label-switched router• forwards packets to outgoing interface
based only on label value (don’t inspect IP address)– MPLS forwarding table distinct from IP forwarding tables
• signaling protocol needed to set up forwarding– RSVP-TE, LDP– forwarding possible along paths that IP alone would not
allow (e.g., least cost path routing) !!– use MPLS for traffic engineering
• must co-exist with IP-only routers
Csci 232 – Computer Networks Routing and Network Layer Part II 79
R1R2
D
R3R4R5
0
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 0 12 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS Forwarding Tables
Csci 232 – Computer Networks Routing and Network Layer Part II 80
Why Mobile IP?
• Need a protocol which allows network connectivity across host movement
• Protocol to enable mobility must not require massive changes to router software, etc.
• Must be compatible with large installed base of IPv4 networks/hosts
• Confine changes to mobile hosts and a few support hosts which enable mobility
Csci 232 – Computer Networks Routing and Network Layer Part II 81
Internet Protocol (IP)
• Network layer, "best-effort" packet delivery• Supports UDP and TCP (transport layer protocols)• IP host addresses consist of two parts
– network id + host id
• By design, IP host address is tied to home network address– Hosts are assumed to be wired, immobile– Intermediate routers look only at network address– Mobility without a change in IP address results in
un-route-able packets
Csci 232 – Computer Networks Routing and Network Layer Part II 82
IP Routing Breaks Under Mobility
Why this hierarchical approach? Answer: Scalability!Millions of network addresses, billions of hosts!
137.30.2.*
.50 .52 .53
router
router
139.20.3.*
.200
Csci 232 – Computer Networks Routing and Network Layer Part II 83
Mobile IP: Basics
• Proposed by IETF (Internet Engineering Task Force)– Standards development body for the Internet
• Mobile IP allows a mobile host to move about without changing its permanent IP address
• Each mobile host has a home agent on its home network
• Mobile host establishes a care-of address when it's away from home
Csci 232 – Computer Networks Routing and Network Layer Part II 84
Mobile IP: Basics, Cont.• Correspondent host is a host that wants to
send packets to the mobile host• Correspondent host sends packets to the
mobile host’s IP permanent address• These packets are routed to the mobile host’s
home network• Home agent forwards IP packets for mobile
host to current care-of address• Mobile host sends packets directly to
correspondent, using permanent home IP as source IP
Csci 232 – Computer Networks Routing and Network Layer Part II 85
Mobile IP: Basics, Cont.
home agentcorrespondent host
Csci 232 – Computer Networks Routing and Network Layer Part II 86
Mobile IP: Care-of Addresses
• Whenever a mobile host connects to a remote network, two choices:– care-of can be the address of a foreign agent on the
remote network• foreign agent delivers packets forwarded from home
agent to mobile host– care-of can be a temporary, foreign IP address obtained
through, e.g., DHCP• home agent tunnels packets directly to the temporary
IP address
• Regardless, care-of address must be registered with home agent
Csci 232 – Computer Networks Routing and Network Layer Part II 87
IP header
IP-in-IP Tunneling• Packet to be forwarded is
encapsulated in a new IP packet• In the new header:
– Destination = care-of-address– Source = address of home agent– Protocol number = IP-in-IP
IP headerdata IP header
data
Csci 232 – Computer Networks Routing and Network Layer Part II 88
At the Other End...
• Depending on type of care-of address:– Foreign agent or– Mobile host
• … strips outer IP header of tunneled packet, which is then fed to the mobile host
• Aside: Any thoughts on advantages of foreign agent vs. co-located (foreign IP) address?
Csci 232 – Computer Networks Routing and Network Layer Part II 89
Routing Inefficiency
Mobile host and correspondent hostmight even be on the samenetwork!!
home agentcorrespondent host
Csci 232 – Computer Networks Routing and Network Layer Part II 90
Route Optimizations• Possible Solution:
– Home agent sends current care-of address to correspondent host
– Correspondent host caches care-of address– Future packets tunneled directly to care-of address
• But!– An instance of the cache consistency problem arises...– Cached care-of address becomes stale when the mobile
host moves– Potential security issues with providing care-of address to
correspondent
Csci 232 – Computer Networks Routing and Network Layer Part II 91
Possible Route Optimization
Csci 232 – Computer Networks Routing and Network Layer Part II 92
Network Layer Part II Summary• Network Layer Routing
– Basic Issues– Distributed Routing Algorithms: LS vs. DV– Link State (LS): How does it work?– Distance Vector (DV): How does it work? Issues? – Mobile IP: how does it work? Issues?– MPLS
• Routing in the Internet– Intra-AS vs. Inter-AS routing– Intra-AS: RIP and OSPF– Inter-AS: BGP and Policy Routing
• Things we didn’t cover: VPN, IP Multicast, IPv6 (but please read by yourself!)