Transcript Wireless Networking & Mobile Computing Network Layer Overview ECE 256 Romit Roy Choudhury Dept.

Wireless Networking & Mobile Computing
Network Layer Overview
ECE 256
Romit Roy Choudhury
Dept. of ECE and CS
1
Recall Layering
 transport segment from
sending to receiving host
 on sending side encapsulates
segments into datagrams
 on rcving side, delivers
segments to transport layer
 network layer protocols in
every host, router
 Router examines header fields
in all IP datagrams passing
through it
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
2
Routing - Why Difficult ?
 Several algorithmic problems:
 Many many paths - which is the best?
 Each path has changing characteristics
• Queuing time varies, losses happen, router down …
 How do you broadcast (find where someone is)
 How do you multicast (webTV, conference call)
 How do routers perform routing at GBbps scale
 Several management problems:
 How do you detect/diagnose faults
 How do you do pricing, accounting
3
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a router
 4.4 IP: Internet Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet
 RIP
 OSPF
 BGP
 4.7 Broadcast and
multicast routing
4
Key Network-Layer Functions
 forwarding: move
packets from router’s
input to appropriate
router output
 routing: determine
route taken by packets
from source to dest.
 Routing algorithms
analogy:
 routing: process of
planning trip from
source to dest
 forwarding: process of
getting through actual
traffic intersections
5
Interplay between routing and forwarding
routing algorithm
local forwarding table
header value output link
0100
0101
0111
1001
3
2
2
1
value in arriving
packet’s header
0111
1
3 2
6
Two types of Network Architecture
 Connection-Oriented and Connection-Less
Virtual Circuit Switching
Datagram forwarding
Example:ATM, X.25
Analogy: Telephone
Example: IP networks
Analogy: Postal service
7
Virtual circuits: signaling protocols
 used to setup, maintain teardown VC
 used in ATM, frame-relay, X.25
 not used in today’s Internet
application
transport 5. Data flow begins
network 4. Call connected
data link 1. Initiate call
physical
6. Receive data application
3. Accept call transport
2. incoming call network
data link
physical
8
Datagram networks
 No call setup at network layer
 @ routers: no state about end-to-end connections
 no concept of “connection”
 packets forwarded using destination host address
 May take different path for same source-dest pair
application
transport
network
data link 1. Send data
physical
application
transport
2. Receive data network
data link
physical
9
Design Decisions
 Thoughts on why VC isn’t great?
 Thoughts on why dataram may not be great?
 Think of an application that’s better with VC
10
Datagram or VC network: why?
Internet
ATM
 data traffic
 “elastic” service, no strict
timing req.
 evolved from telephony
 Call admission control
 “smart” end computers
 simple network
 complexity at “edge”
 human conversation:
 strict timing, reliability
requirements
 need for guaranteed
service
 many link types
 different characteristics
 uniform service difficult
 “dumb” end systems
 telephones
 complexity inside
network
11
Chapter 4: Network Layer
IP Addressing
12
IP Addressing: introduction
 IP address: 32-bit
identifier for host,
router interface
 interface: connection
between host/router
and physical link
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
 router’s typically have
223.1.3.2
223.1.3.1
multiple interfaces
 host typically has one
interface
 IP addresses associated
223.1.1.1 = 11011111 00000001 00000001 00000001
with each interface
223
1
1
1
13
Subnets
 IP address:
 subnet part (high order
bits)
 host part (low order bits)
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
 What’s a subnet ?
 device interfaces with
same subnet part of IP
address
 can physically reach
each other without
intervening router
223.1.2.9
223.1.3.27
223.1.2.2
subnet
223.1.3.1
223.1.3.2
network consisting of 3 subnets
14
IP addressing: CIDR
CIDR: Classless InterDomain Routing
 subnet portion of address of arbitrary length
 address format: a.b.c.d/x, where x is # bits in subnet
portion of address
subnet
part
host
part
11001000 00010111 00010000 00000000
200.23.16.0/23
15
IP addresses: how to get one?
Q: How does network get subnet part of IP addr?
A: gets allocated portion of its provider ISP’s
address space
ISP's block
11001000 00010111 00010000 00000000
200.23.16.0/20
Organization 0
Organization 1
Organization 2
...
11001000 00010111 00010000 00000000
11001000 00010111 00010010 00000000
11001000 00010111 00010100 00000000
…..
….
200.23.16.0/23
200.23.18.0/23
200.23.20.0/23
….
Organization 7
11001000 00010111 00011110 00000000
200.23.30.0/23
16
Network Address Translation
17
Scalability Problem
 Internet growing very fast
 Many million devices
 Each device needs an address for communication
 Question is
 How do you address each of them
 IP addresing can give you 232
 May not be enough
18
NAT: Network Address Translation
rest of
Internet
local network
(e.g., home network)
10.0.0/24
10.0.0.4
10.0.0.1
10.0.0.2
138.76.29.7
10.0.0.3
All datagrams leaving local
network have same single source
NAT IP address: 138.76.29.7,
different source port numbers
Datagrams with source or
destination in this network
have 10.0.0/24 address for
source, destination (as usual)
19
NAT makes Globally non-routable hosts
 Non-routable
 Means you cannot ping 192.168.0.3 (your home
machines) from Duke Lab
 But, Skype, GotoMyPC, etc. can access / call
your home machine
 How ?
20
An Alternate Approach: IPv6
 Initial motivation: Make space for 64 bit address
space
 How can this be made compatible to IPv4 routers?
 IPv6 not flying
 NAT coping fine with today’s needs
21
Chapter 4: Network Layer
Routing Algorithms
22
Graph abstraction
5
2
u
2
1
Graph: G = (N,E)
v
x
3
w
3
1
5
z
1
y
2
N = set of routers = { u, v, w, x, y, z }
E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }
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
Graph abstraction: costs
5
2
u
v
2
1
x
What factors influence this cost ?
3
w
3
1
z
1
y
Should costs be only on links ?
5
2
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
24
Routing Algorithm classification
2 main classes:
Centralized
 all routers have complete topology, link cost info
 “link state” algorithms
Distributed:
 Each router knows link costs to neighbor routers only
 “distance vector” algorithms
25
A Link-State Routing Algorithm
Dijkstra’s algorithm
 Link costs known to all nodes
 computes least cost paths from one node (‘source”) to all
other nodes
 gives forwarding table for that node
 iterative: after k iterations, know least cost path to k
dest.’s
26
Dijkstra’s Algorithm
Notation:
 c(x,y): link cost from node x to y;
= ∞ if not direct neighbors
 D(v): current value of cost of path
1 Initialization:
from source to dest. v
2 N' = {u}
3 for all nodes v
5
4
if v adjacent to u
5
then D(v) = c(u,v)
3
v
w
5
2
6
else D(v) = ∞
u
7
2
z
1
3
8 Loop
1
2
x
y
9 find w not in N' s.t. D(w) is a minimum
1
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'
27
Dijkstra’s algorithm: example (2)
Resulting shortest-path tree from u:
v
w
u
z
x
y
Resulting forwarding table in u:
destination
link
v
x
(u,v)
(u,x)
y
(u,x)
w
(u,x)
z
(u,x)
28
Distributed: Distance Vector
 To find D, node S asks each neighbor X




How far X is from D
X asks its neighbors … comes back and says C(X,D)
Node S deduces C(S,D) = C(S,X) + C(X,D)
S chooses neighbor Xi that provides min C(S,D)
 Later, Xj may find better route to D
 Xj advertizes C(Xj,D)
 All nodes update their cost to D if new min found
29
Distance Vector Algorithm
Bellman-Ford Equation (dynamic programming)
Define
dx(y) := cost of least-cost path from x to y
v1
Then
x
y
v2
dx(y) = min
{c(x,v) + dv(y) }
v
where min is taken over all neighbors v of x
30
Bellman-Ford example
5
2
u
v
2
1
x
3
w
3
1
5
z
1
y
Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3
2
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
31
Distance Vector: link cost changes
Link cost changes:
 if DV changes, notify neighbors
1
x
4
y
50
1
z
At time t0, y detects the link-cost change, updates its DV,
and informs its neighbors.
At time t1, z receives the update from y and updates its table.
It computes a new least cost to x and sends its neighbors its DV.
At time t2, y receives z’s update and updates its distance table.
y’s least costs do not change and hence y does not send any
message to z.
When can it get complicated ?
32
Distance Vector: link cost changes
Link cost changes:




Y thinks Z’s best cost is 5
Thus C(y,x) = 5 + 1 = 6
Announces this cost
Z thinks C(z,x) = 6 + 1 …
60
x
4
y
50
1
z
Food
for thought
Poissoned
reverse: …
 If Z routes through
to get converge ?
WillYthis
to X :
If Yso,
after
how
 Z tells
its (Z’s)
distance
to many rounds ?
X is infinite (so Y won’t route
can this be solved?
to X via How
Z)
Should
will this completely
solve change from 4 to 60?
Y announce
count to infinity problem?
33
Routing in Internet
 Similar to international FedEx routing
 FedEx figures out best route within country
• Uses google maps say
• This is link state -- All info available
 USA FedEx does not have international map,
also no permission to operate outside USA
 Gets price quote from Germany FedEx, Japan FedEx
etc. to route to India
 Chooses minimum price and handles package to say
Germany (Distance Vector)
 Germany has country map (link state)
 Germany asks for cost from Egypt, South Africa …
34
Internet Routing
 Think of each country FedEx as ISPs
 Routing on internet very similar to prior example
 The link state and DV routing protocols used in
internet routing
 RIP (routing information protocol)
 OSPF (Open shortest path first)
 BGP (Border gateway protocol)
 They utilize the concepts of
 Link state
 Distance vector routing
35
How is this different in wireless?
36
Routing in wireless Mobile Networks
 Imagine hundreds of hosts moving
 Routing algorithm needs to cope up with varying
wireless channel and node mobility
Where’s
RED guy
37
Questions ?
38
Backup Slides
39
Comparison of LS and DV algorithms
Message complexity
 LS: with n nodes, E links, O(nE)
msgs sent
 DV: exchange between neighbors
only
 convergence time varies
Speed of Convergence
 LS: O(n2) algorithm requires
O(nE) msgs
 may have oscillations
 DV: convergence time varies
 may be routing loops
 count-to-infinity problem
Robustness: what happens if
router malfunctions?
LS:
 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
40
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a router
 4.4 IP: Internet Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet
 RIP
 OSPF
 BGP
 4.7 Broadcast and
multicast routing
41
Hierarchical Routing
Our routing study thus far - idealization
 all routers identical
 network “flat”
… not true in practice
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
42
Hierarchical Routing
 aggregate routers into
regions, “autonomous
systems” (AS)
 routers in same AS run
same routing protocol
Gateway router
 Direct link to router in
another AS
 “intra-AS” routing
protocol
 routers in different AS
can run different intra-AS
routing protocol
43
Interconnected ASes
3c
3a
3b
AS3
1a
2a
1c
1d
1b
Intra-AS
Routing
algorithm
2c
AS2
AS1
Inter-AS
Routing
algorithm
Forwarding
table
2b
 Forwarding table is
configured by both
intra- and inter-AS
routing algorithm
 Intra-AS sets entries for
internal dests
 Inter-AS & Intra-As sets
entries for external dests
44
Inter-AS tasks
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!
 Suppose 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?
3c
3b
3a
AS3
1a
2a
1c
1d
1b
2c
AS2
2b
AS1
45
Inter-AS tasks
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!
 Suppose 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?
3c
3b
3a
AS3
1a
2a
1c
1d
1b
2c
AS2
2b
AS1
46
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.
 Puts in forwarding table entry (x,I).
47
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 on inter-AS routing protocol!
 Hot potato routing: send packet towards closest of two
routers.
Learn from inter-AS
protocol that subnet
x is reachable via
multiple gateways
Use routing info
from intra-AS
protocol to determine
costs of least-cost
paths to each
of the gateways
Hot potato routing:
Choose the gateway
that has the
smallest least cost
Determine from
forwarding table the
interface I that leads
to least-cost gateway.
Enter (x,I) in
forwarding table
48
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a router
 4.4 IP: Internet Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet
 RIP
 OSPF
 BGP
 4.7 Broadcast and
multicast routing
49
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)
50
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a router
 4.4 IP: Internet Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet
 RIP
 OSPF
 BGP
 4.7 Broadcast and
multicast routing
51
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”
52
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
3c
3a
3b
AS3
1a
AS1
2a
1c
1d
1b
2c
AS2
2b
eBGP session
iBGP session
53
Distributing reachability info
 With eBGP session between 3a and 1c, AS3 sends prefix
reachability info to AS1.
 1c can then use iBGP do distribute this new prefix reach info to
all routers in AS1
 1b can then re-advertise the new reach info to AS2 over the 1bto-2a eBGP session
 When router learns about a new prefix, it creates an entry for the
prefix in its forwarding table.
3c
3a
3b
AS3
1a
AS1
2a
1c
1d
1b
2c
AS2
2b
eBGP session
iBGP session
54
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.
55
BGP route selection
 Router may learn about more than 1 route to
some prefix. Router must select route.
 Elimination rules:
1.
2.
3.
4.
Local preference value attribute: policy decision
Shortest AS-PATH
Closest NEXT-HOP router: hot potato routing
Additional criteria
56
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
57
BGP routing policy
legend:
B
W
provider
network
X
A
customer
network:
C
Y
Figure 4.5-BGPnew: a simple BGP scenario
 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
58
BGP routing policy (2)
legend:
B
W
provider
network
X
A
customer
network:
C
Y
Figure 4.5-BGPnew: a simple BGP scenario
 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!
59
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
60
Questions ?
61
NAT: Network Address Translation
 Motivation: local network uses just one IP address as far as
outside world is concerned:
 range of addresses not needed from ISP: just one IP
address for all devices
 can change addresses of devices in local network
without notifying outside world
 can change ISP without changing addresses of devices
in local network
 devices inside local net not explicitly addressable,
visible by outside world (a security plus).
62
NAT: Network Address Translation
Implementation: NAT router must:
 outgoing datagrams: replace (source IP address, port #) of every
outgoing datagram to (NAT IP address, new port #)
. . . remote clients/servers will respond using (NAT IP address,
new port #) as destination addr.
 remember (in NAT translation table) every (source IP address, port
#) to (NAT IP address, new port #) translation pair
 incoming datagrams: replace (NAT IP address, new port #) in dest
fields of every incoming datagram with corresponding (source IP
address, port #) stored in NAT table
63
Distance Vector Algorithm
 Dx(y) = estimate of least cost from x to y
 Distance vector: Dx = [Dx(y): y є N ]
 Node x knows cost to each neighbor v: c(x,v)
 Node x maintains Dx = [Dx(y): y є N ]
 Node x also maintains its neighbors’ distance
vectors
 For each neighbor v, x maintains
Dv = [Dv(y): y є N ]
64
Distance vector algorithm (4)
Basic idea:
 Each node periodically sends its own distance vector estimate to
neighbors
 When a 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)
converge to the actual least cost dx(y)
65
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a router
 4.4 IP: Internet Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet
 RIP
 OSPF
 BGP
 4.7 Broadcast and
multicast routing
66
Router Architecture Overview
Two key router functions:
 run routing algorithms/protocol (RIP, OSPF, BGP)
 forwarding datagrams from incoming to outgoing link
67
Input Port Functions
Physical layer:
bit-level reception
Data link layer:
e.g., Ethernet
see chapter 5
Decentralized switching:
 given datagram dest., lookup output port using
forwarding table
 goal: complete input port processing at ‘line
speed’
 queuing: if datagrams arrive faster than
forwarding rate into switch fabric
68
Three types of switching fabrics
69
The Internet Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
Network
layer
IP protocol
•addressing conventions
•datagram format
•packet handling conventions
Routing protocols
•path selection
•RIP, OSPF, BGP
forwarding
table
ICMP protocol
•error reporting
•router “signaling”
Link layer
physical layer
70
Hierarchical addressing: route aggregation
Hierarchical addressing allows efficient advertisement of routing
information:
Organization 0
200.23.16.0/23
Organization 1
200.23.18.0/23
Organization 2
200.23.20.0/23
Organization 7
.
.
.
.
.
.
Fly-By-Night-ISP
“Send me anything
with addresses
beginning
200.23.16.0/20”
Internet
200.23.30.0/23
ISPs-R-Us
“Send me anything
with addresses
beginning
199.31.0.0/16”
71
Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
Organization 0
200.23.16.0/23
Organization 2
200.23.20.0/23
Organization 7
.
.
.
.
.
.
Fly-By-Night-ISP
“Send me anything
with addresses
beginning
200.23.16.0/20”
Internet
200.23.30.0/23
ISPs-R-Us
Organization 1
200.23.18.0/23
“Send me anything
with addresses
beginning 199.31.0.0/16
or 200.23.18.0/23”
72
IP addressing: the last word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers
 allocates addresses
 manages DNS
 assigns domain names, resolves disputes
73
Network layer connection and connection-less
service
 Datagram network provides network-layer
connectionless service
 VC network provides network-layer connection
service
 Analogous to the transport-layer services, but:
 Service: host-to-host
 No choice: network provides one or the other
 Implementation: in the core
74
Virtual circuits
 Call setup, teardown for each call before data can flow
 Each packet carries VC identifier (not destination host
address)
 Every router on source-dest path maintains “state” for
each passing connection
 Link, router resources (bandwidth, buffers) may be
allocated to VC
75
VC implementation
A VC consists of:
1. Path from source to destination
2. VC numbers, one number for each link along path
3. Entries in forwarding tables in routers along path
 Packet belonging to VC carries a VC number.
 VC number must be changed on each link.

New VC number comes from forwarding table
76
Forwarding table
VC number
22
12
1
Forwarding table in
northwest router:
Incoming interface
1
2
3
1
…
2
32
3
interface
number
Incoming VC #
12
63
7
97
…
Outgoing interface
3
1
2
3
…
Outgoing VC #
22
18
17
87
…
Routers maintain connection state information!
77
Datagram Forwarding Table
Destination Address Range
4 billion
possible entries
Link Interface
11001000 00010111 00010000 00000000
through
11001000 00010111 00010111 11111111
0
11001000 00010111 00011000 00000000
through
11001000 00010111 00011000 11111111
1
11001000 00010111 00011001 00000000
through
11001000 00010111 00011111 11111111
2
otherwise
3
78
Longest prefix matching
Prefix Match
11001000 00010111 00010
11001000 00010111 00011000
11001000 00010111 00011
otherwise
Link Interface
0
1
2
3
Examples
DA: 11001000 00010111 00010110 10100001
Which interface?
DA: 11001000 00010111 00011000 10101010
Which interface?
79