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Introduction
Vahid Tabatabaee
Fall 2007
ENTS689L: Packet Processing and Switching
Introduction
1
References
 Communication Networks: Fundmantal Concepts and Key
Architectures
Author: A. Leon-Garcia, I. Widjaja
Publisher: McGraw-Hill
 Internetworking with TCP/IP vol. I Principles, Protocols, and
Architecture
Author: Douglas E. Comer
Publisher: Prentice-Hall
 Title: Network Processors Architectures, Protocols, and Platforms
Author: Panos C. Lekkas
Publisher: McGraw-Hill
ENTS689L: Packet Processing and Switching
Introduction
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User view of the internet
 It is a big network that every computer is attached to it
Internet
hosts
ENTS689L: Packet Processing and Switching
Introduction
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Internet view in a networking course:
routers
Physical
net
hosts
ENTS689L: Packet Processing and Switching
Introduction
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Physical Network (Ethernet)
ENTS689L: Packet Processing and Switching
Introduction
Source: http://www.fiber-optics.info/articles/telecom-nets.htm
5
Physical Network (ISP WAN)
Source:
http://www.cisco.com/univercd/cc/td/doc/product/aggr/10000/swconfig/swref/tech-edg/ch1ovw.htm
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Router
In this course we talk about how packets
are processed and forwarded in routers
Line and Switch Cards
SD
Power
CISCO SYSTEMS
Cisco AS5800
SERIES
Source:
http://www.cisco.com/en/US/products/hw/routers/ps167/products_tech_note09186a00801e1dc2.shtml
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In this course
 We discuss:
 Basic components in a router
 The basic processes in a router
 The basic requirements of router
 Some network protocols
 Some example architectures
 We do NOT discuss:
 Hardware design
 Software design
 Router programming
 All other good stuff !
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Introduction
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Review of Networking
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Introduction
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Layering in a Network
Layering simplifies design,
implementation, and testing
Protocol in each layer can be
designed separately
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Introduction
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7-Layer OSI Reference Model
Application
Application
End-to-End Protocols
Application
Layer
Application
Layer
Presentation
Layer
Presentation
Layer
Session
Layer
Session
Layer
Transport
Layer
Transport
Layer
Network
Layer
Network
Layer
Network
Layer
Network
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Physical
Layer
ENTS689L: Packet Processing and Switching
Communicating
Introduction
End Systems
One or More Network Nodes
11
Physical Layer
 Transfers bits across link
 Definition & specification of the physical aspects of a
communications link
 Mechanical: cable, plugs, pins...
 Electrical/optical: modulation, signal strength, voltage levels,
bit times, …
 functional/procedural: how to activate, maintain, and
deactivate physical links…
 Ethernet, DSL, cable modem, telephone modems…
 Twisted-pair cable, coaxial cable optical fiber, radio, infrared, …
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Data Link Layer






Transfers frames across direct connections
Groups bits into frames
Detection of bit errors; Retransmission of frames
Activation, maintenance, & deactivation of data link connections
Medium access control for local area networks
Flow control
Data Link
Layer
Physical
Layer
ENTS689L: Packet Processing and Switching
Introduction
frames
bits
Data Link
Layer
Physical
Layer
13
Network Layer
 Transfers packets across multiple links and/or multiple networks
 Addressing must scale to large networks
 Nodes jointly execute routing algorithm to determine paths across the
network
 Forwarding transfers packet across a node
 Congestion control to deal with traffic surges
 Congestion vs. Flow control
 Connection setup, maintenance, and teardown when connection-based
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Internetworking
Ethernet LAN
 Internetworking is part of network layer and provides transfer of packets
  possibly
   dissimilar networks
across multiple
 Gateways (routers) direct packets across networks
ATM
ATM
Network
Switch
ATM
HSwitch
ATM
Switch
H
G
Net
Net 11
H
Net
Net 33
G
G
G
Net 2
G = gateway
H = host
ENTS689L:
Packet Processing and Switching
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ATM
Switch
Net55
Net
G
Net 4
G
H
15
Transport Layer
 Transfers data (not just packet) end-to-end from process in a machine to
process in another machine
 Reliable stream transfer or quick-and-simple single-block transfer
 Port numbers enable multiplexing
 Message segmentation and reassembly
 Connection setup, maintenance, and release
Transport
Layer
Network
Layer
Transport
Layer
Network
Layer
ENTS689L: Packet Processing and Switching
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Communication
Network
Layer
Network
Layer
16
Network
Headers and trailers
 Each protocol uses a header that carries addresses, sequence
numbers, flag bits, length indicators, etc…
 CRC check bits may be appended for error detection
Application
Application
APP DATA
Application
Layer
AH APP DATA
Application
Layer
Transport
Layer
TH AH APP DATA
Transport
Layer
Network
Layer
NH TH AH APP DATA
Network
Layer
Data Link
Layer
DH NH TH AH APP DATA CRC
Physical
Layer
ENTS689L: Packet Processing and Switching
Introduction
bits
Data Link
Layer
Physical
Layer
17
Why Internetworking
 To build a “network of networks” or internet
 operating over multiple, coexisting, different network
technologies
 providing ubiquitous connectivity through IP packet transfer
 achieving huge economies of scale
H
H
Net51
Net
G
Net53
Net
G
G
G
H
Net52
Net
Net55
Net
G
Net54
Net
G
H
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Why Internetworking
 To provide universal communication services
 independent of underlying network technologies
 providing common interface to user applications
H
H
Net51
Net
G
Net53
Net
G
G
G
H
Net52
Net
Net55
Net
G
Net54
Net
G
H
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Internet Protocol Approach
 IP packets transfer information across Internet
 Host A IP → router→ router…→ router→ Host B IP
 IP layer in each router determines next hop (router)
 Network interfaces transfer IP packets across networks
Host A
Router
Transport
Layer
Internet
Layer
Internet
Layer
Network
Interface
Router
Internet
Layer
Net51
Net
Router
Network
Interface
Network
Interface
Internet
Layer
Net54
Net
Net52
Net
ENTS689L: Packet Processing and Switching
Introduction
Network
Interface
Host B
Transport
Layer
Internet
Layer
Network
Interface
Net53
Net
20
How to create the routing tables
 Need information on state of links
 Link up/down; congested; delay or other metrics
 Need to distribute link state information using a routing protocol
 What information is exchanged? How often?
 Exchange with neighbors; Broadcast or flood
 Need to compute routes based on information
 Single metric; multiple metrics
 Single route; alternate routes
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Routing algorithm requirements
 Responsiveness to changes
 Topology or bandwidth changes, congestion
 Rapid convergence of routers to consistent set of routes
 Freedom from persistent loops
 Optimality
 Resource utilization, path length
 Robustness
 Continues working under high load, congestion, faults,
equipment failures, incorrect implementations
 Simplicity
 Efficient software implementation, reasonable processing load
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Distributed Routing
Routes determined by routers using
distributed algorithm
State information exchanged by routers
Adapts to topology and other changes
Better scalability
ENTS689L: Packet Processing and Switching
Introduction
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Routing Table in Hop-by-Hop routing
 Every router decides where to send the packet next by looking at
the packet header (dest. address, source address, CoS, …)
Node 3
Node 1
Destination
Next node
2
2
3
3
4
4
5
2
6
2
Destination
1
3
4
5
6
Node 2
Next node
1
1
4
5
5
Destination
1
2
4
5
6
Next node
1
4
4
6
6
Destination
1
2
3
5
6
Node 4
Next node
1
2
3
5
3
ENTS689L: Packet Processing and Switching
Introduction
Node 6
Destination
Next node
1
3
2
5
3
3
4
3
5
5
Node 5
Destination
Next node
1
4
2
2
3
4
4
4
6
6
24
Non-Hierarchical Addresses and Routing
 No relationship between addresses & routing proximity
 Routing tables require 16 entries each
0000
0111
1010
1101
1
0001
0100
1011
1110
4
3
R2
R1
5
2
0011
0110
1001
1100
ENTS689L: Packet Processing and Switching
Introduction
0000
0111
1010
…
1
1
1
…
0001
0100
1011
…
4
4
4
…
0011
0101
1000
1111
25
Hierarchical Addresses and Routing
 Prefix indicates network where host is attached
 Routing tables require 4 entries each
 Can we do better than this?
0000
0001
0010
0011
1
0100
0101
0110
0111
4
3
R2
R1
5
2
1000
1001
1010
1011
ENTS689L: Packet Processing and Switching
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00
01
10
11
1
3
2
3
00
01
10
11
3
4
3
5
1100
1101
1110
1111
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Shortest Path Routing
 Many possible paths connect any given source to any given
destination
 Routing involves the selection of the path to be used to accomplish
a given transfer
 Typically it is possible to attach a cost or distance to a link
connecting two nodes
 Routing can then be posed as a shortest path problem
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Routing Metrics
Means for measuring desirability of a path
 Path Length = sum of costs or distances
 Possible metrics
 Hop count: rough measure of resources used
 Reliability: link availability; BER
 Delay: sum of delays along path; complex & dynamic
 Bandwidth: “available capacity” in a path
 Load: Link & router utilization along path
 Cost: $$$
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Shortest Path Approaches
Distance Vector Protocols
 Neighbors exchange list of distances to destinations
 Best next-hop determined for each destination
 Ford-Fulkerson (distributed) shortest path algorithm
Link State Protocols
 Link state information flooded to all routers
 Routers have complete topology information
 Shortest path (& hence next hop) calculated
 Dijkstra (centralized) shortest path algorithm
ENTS689L: Packet Processing and Switching
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Distance Vector Routing
i only has local info
from neighbors
Dj'
San
Jose
j'
Cij'
i
Di
j
Cij
Cij”
j"
ENTS689L: Packet Processing and Switching
Introduction
Dj
Dj"
Pick current
shortest path
30
Why Distance Vector Works
SJ sends
accurate info
3 Hops
From SJ
2 Hops
From SJ
ENTS689L: Packet Processing and Switching
Introduction
1 Hop
From SJ
San
Jose
Hop-1 nodes
Accurate info about SJ calculate current
ripples across network, (next hop, dist), &
Shortest Path Converges
send to neighbors
31
Bellman-Ford Algorithm
 Consider computations for one destination d
 Initialization




Each node table has 1 row for destination d
Distance of node d to itself is zero: Dd=0
Distance of other node j to d is infinite: Dj=, for j d
Next hop node nj = -1 to indicate not yet defined for j  d
 Send Step
 Send new distance vector to immediate neighbors across local link
 Receive Step
 At node j, find the next hop that gives the minimum distance to d,
 Minj { Cij + Dj }
 Replace old (nj, Dj(d)) by new (nj*, Dj*(d)) if new next node or distance
 Go to send step
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Iteration
Node 1
Node 2
Node 3
Node 4
Node 5
Initial
(-1, )
(-1, )
(-1, )
(-1, )
(-1, )
1
2
3
Table entry
@ node 1
for dest SJ
Table entry
@ node 3
for dest SJ
2
3
1
5
San
Jose
1
2
4
3
1
2
ENTS689L: Packet Processing and Switching
Introduction
6
3
4
5
2
33
Iteration
Node 1
Node 2
Node 3
Node 4
Node 5
Initial
(-1, )
(-1, )
(-1, )
(-1, )
(-1, )
1
(-1, )
(-1, )
(6,1)
(-1, )
(6,2)
2
3
D3=D6+1
n3=6
D6=0
3 1
2
1
5
1
2
0
4
3
1
2
D5=D64+2
ENTS689L: Packet Processing and Switching n =6
5
Introduction
6
3
San
Jose
2
5
2
D6=0
34
Iteration
Node 1
Node 2
Node 3
Node 4
Node 5
Initial
(-1, )
(-1, )
(-1, )
(-1, )
(-1, )
1
(-1, )
(-1, )
(6, 1)
(-1, )
(6,2)
2
(3,3)
(5,6)
(6, 1)
(3,3)
(6,2)
3
3
1
2
3
1
5
3
1
2
0
4
3
1
2
6
ENTS689L: Packet Processing and Switching
Introduction
6
3
4
5
San
Jose
2
2
35
Iteration
Node 1
Node 2
Node 3
Node 4
Node 5
Initial
(-1, )
(-1, )
(-1, )
(-1, )
(-1, )
1
(-1, )
(-1, )
(6, 1)
(-1, )
(6,2)
2
(3,3)
(5,6)
(6, 1)
(3,3)
(6,2)
3
(3,3)
(4,4)
(6, 1)
(3,3)
(6,2)
1
3
2
3
1
5
3
1
2
0
4
3
1
2
6 4
ENTS689L: Packet Processing and Switching
Introduction
6
3
4
5
San
Jose
2
2
36
Counting to Infinity Problem
(a)
1
(b)
1
1
1
2
1
2
1
3
3
4
1
4
X
Nodes believe best
path is through each
other
(Destination is node 4)
Update
Node 1
Node 2
Node 3
Before break
(2,3)
(3,2)
(4, 1)
After break
(2,3)
(3,2)
(2,3)
1
(2,3)
(3,4)
(2,3)
2
(2,5)
(3,4)
(2,5)
3
(2,5)
(3,6)
(2,5)
4
(2,7)
(3,6)
(2,7)
5
(2,7)
(3,8)
(2,7)
…
…
…
…
ENTS689L: Packet Processing and Switching
Introduction
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Problem: Bad News Travels Slowly
Remedies
 Split Horizon
 Do not report route to a destination to the neighbor from
which route was learned
 Poisoned Reverse
 Report route to a destination to the neighbor from which route
was learned, but with infinite distance
 Breaks erroneous direct loops immediately
 Does not work on some indirect loops
ENTS689L: Packet Processing and Switching
Introduction
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Split Horizon with Poison Reverse
(a)
(b)
1
1
1
1
2
2
1
1
3
3
1
X
4
4
Nodes believe best
path is through
each other
Update
Node 1
Node 2
Node 3
Before break
(2, 3)
(3, 2)
(4, 1)
After break
(2, 3)
(3, 2)
(-1, )
Node 2 advertizes its route to 4 to node 3 as having distance
infinity; node 3 finds there is no route to 4
1
(2, 3)
(-1, )
(-1, )
Node 1 advertizes its route to 4 to node 2 as having distance
infinity; node 2 finds there is no route to 4
2
(-1, )
(-1, )
(-1, )
Node 1 finds there is no route to 4
ENTS689L: Packet Processing and Switching
Introduction
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Link-State Algorithm
 Basic idea: two step procedure
 Each source node gets a map of all nodes and link metrics (link state)
of the entire network
 Find the shortest path on the map from the source node to all
destination nodes
 Broadcast of link-state information
 Every node i in the network broadcasts to every other node in the
network:
 ID’s of its neighbors: Ni=set of neighbors of i
 Distances to its neighbors: {Cij | j Ni}
 Flooding is a popular method of broadcasting packets
ENTS689L: Packet Processing and Switching
Introduction
40
Dijkstra Algorithm: Finding shortest paths in
order
Closest node to s is 1 hop away
2nd closest node to s is 1 hop
away from s or w”
3rd closest node to s is 1 hop
away from s, w”, or x
Find shortest paths from
source s to all other
destinations
w'
z
w
s
x
w"
ENTS689L: Packet Processing and Switching
Introduction
z'
x'
41
Dijkstra’s algorithm
 N: set of nodes for which shortest path already found
 Initialization: (Start with source node s)
 N = {s}, Ds = 0, “s is distance zero from itself”
 Dj=Csj for all j  s, distances of directly-connected neighbors
 Step A: (Find next closest node i)
 Find i  N such that
 Di = min Dj for j  N
 Add i to N
 If N contains all the nodes, stop
 Step B: (update minimum costs)
 For each node j  N
Minimum distance from
 Dj = min (Dj, Di+Cij)
j through node i in N
 Go to Step A
ENTS689L: Packet Processing and Switching
Introduction
s to
42
Execution of Dijkstra’s algorithm
2
1
5
1
6
5
2
3
3
2
4
4
3
1
2
2
5
N
D2
D3
Initial
{1}
3
2
1
{1,3}
3
2
2
{1,2,3}
3
3
{1,2,3,6}
4
5
6 
2
5
4
Iteration
1
3
2
3
4
1

1
3

2
D4
D5
D6
5


4

3
2
4
7
3
3
2
4
5
3
{1,2,3,4,6}
3
2
4
5
3
{1,2,3,4,5,6}
3
2
4
5
3
ENTS689L: Packet Processing and Switching
Introduction




43
Shortest Paths in Dijkstra’s Algorithm
2
1
2
3
3
2
2
2
3
6
2
4
1
5
4
1
1
2
2
6
2
3
3
ENTS689L: 2Packet Processing and Switching
Introduction
5
4
1
3
5
4
1
2
2
3
2
6
4
1
3
5
5
5
1
1
2
3
2
5
3
4
4
3
1
2
2
2
3
4
6
5
1
4
1
2
3
1
2
5
4
3
6
5
2
1
3
3
4
1
2
1
6
5
1
1
3
2
3
2
4
44
5
Reaction to Failure
 If a link fails,
 Router sets link distance to infinity & floods the network with an update
packet
 All routers immediately update their link database & recalculate their
shortest paths
 Recovery very quick
 But watch out for old update messages




Add time stamp or sequence # to each update message
Check whether each received update message is new
If new, add it to database and broadcast
If older, send update message on arriving link
ENTS689L: Packet Processing and Switching
Introduction
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Why is Link State Better?
 Fast, loopless convergence
 Support for precise metrics, and multiple metrics if necessary
(throughput, delay, cost, reliability)
 Support for multiple paths to a destination
 algorithm can be modified to find best two paths
ENTS689L: Packet Processing and Switching
Introduction
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Open Shortest Path First




OSPF is a well known routing protocol for IP (RFC 2328 (v2))
OSPF is a link state routing protocol
Enables each router to learn complete network topology
Each router monitors the link state to each neighbor and floods the linkstate information to other routers
 Each router builds an identical link-state database
 Allows router to build shortest path tree with router as root
 OSPF typically converges faster than RIP when there is a failure in the
network
ENTS689L: Packet Processing and Switching
Introduction
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Flooding
 Used in OSPF to distribute link state (LS) information
 Forward incoming packet to all ports except where packet came in
 Packet eventually reaches destination as long as there is a path between the
source and destination
 Generates exponential number of packet transmissions
 Approaches to limit # of transmissions:
 Use a TTL at each packet; won’t flood if TTL is reached
 Each router adds its identifier to header of packet before it floods the
packet; won’t flood if its identifier is detected
 Each packet from a given source is identified with a unique sequence
number; won’t flood if sequence number is same
ENTS689L: Packet Processing and Switching
Introduction
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Example OSPF Topology
10.5.1.2
10.5.1.4
10.5.1.1
10.5.1.6
10.5.1.3
10.5.1.5
At steady state:
 All routers have same LS database
 Know how many routers in network
 Interfaces & links between routers
 Cost of each link
 Occasional Hello messages (10 sec) & LS updates sent (30 min)
ENTS689L: Packet Processing and Switching
Introduction
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Exterior Gateway Protocols
 Within each AS, there is a consistent set of routes connecting the
constituent networks
 The Internet is woven into a coherent whole by Exterior Gateway
Protocols (EGPs) that operate between AS’s
 EGP enables two AS’s to exchange routing information about:
 The networks that are contained within each AS
 The AS’s that can be reached through each AS
 EGP path selection guided by policy rather than path optimality
 Trust, peering arrangements, etc
ENTS689L: Packet Processing and Switching
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BGP Example
Only BGP
routers are
shown
R2
R3
AS2
N1 reachable
through AS3
R1
AS1
R4
N1
AS3
•
R4 advertises that network N1 can be reached through AS3
•
R3 examines announcement & applies policy to decide whether it will forward packets
to N1 through R4
•
If yes, routing table updated in R3 to indicate R4 as next hop to N1
•
BGP propagates N1 reachability information through AS2
ENTS689L: Packet Processing and Switching
Introduction
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BGP Example
R2
N1 reachable
through AS2
R3
AS2
R1
R4
N1
AS1
•
BGP routers within an AS, e.g. R3 and R2, are kept consistent
•
Suppose AS2 willing to handle transit packets from AS1 to N1
•
R2 advertises to AS1 the reachability of N1 through AS2
•
R1 applies its policy to decide whether to send to N1 via AS2
ENTS689L: Packet Processing and Switching
Introduction
AS3
52
Peering and Inter-AS connectivity
Peering Centre
Tier 1 ISP (Transit AS)
Tier 1 ISP (Transit AS)
AS
Tier 2 (transit AS)
Tier 2 (transit AS)
AS
AS
AS
Content or
Application Service
Provider (Non-transit)
AS
Tier 2 (transit AS)
AS
AS
•
Non-transit AS’s (stub & multihomed) do not carry transit traffic
•
Tier 1 ISPs peer with each other, privately & peering centers
•
Tier 2 ISPs peer with each other & obtain transit services from Tier 1s; Tier 1’s carry
transit traffic between their Tier 2 customers
•
Client AS’s obtain service from Tier 2 ISPs
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EGP Requirements
 Scalability to global Internet
 Provide connectivity at global scale
 Link-state does not scale
 Should promote address aggregation
 Fully distributed
 EGP path selection guided by policy rather than path optimality
 Trust, peering arrangements, etc
 EGP should allow flexibility in choice of paths
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BGP Features
 BGP is path vector protocol: advertises sequence of AS numbers
to the destination network
 Path vector info used to prevent routing loops
 BGP enforces policy through selection of different paths to a
destination and by control of redistribution of routing information
 Uses CIDR to support aggregation & reduction of routing
information
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iBGP & eBGP
R
eBGP
R
R
R
R
R
iBGP
iBGP
eBGP
iBGP
eBGP
iBGP
iBGP
iBGP
R
eBGP
R
 eBGP to exchange reachability information in different AS’s
 eBGP peers directly connected
 iBGP to ensure net reachability info is consistent among the BGP speakers in the
same AS
 usually not directly connected
 iBGP speakers exchange info learned from other iBGP speakers, and thus fully
meshed
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Path Selection
 Each BGP speaker
 Evaluates paths to a destination from an AS border router
 Selects the best that complies with policies
 Advertises that route to all BGP neighbors
 BGP assigns a preference order to each path & selects path with highest
value; BGP does not keep a cost metric to any path
 When multiple paths to a destination exist, BGP maintains all of the paths,
but only advertises the one with highest preference value
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Introduction
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BGP Policy
 Examples of policy:
 Never use AS X
 Never use AS X to get to a destination in AS Y
 Never use AS X and AS Y in the same path
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