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CSE 561

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Each packet from a host is routed independently. Example: IP. Virtual circuit models: telephone ... Some have become increasingly apparent today ... Accounting ... – PowerPoint PPT presentation

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Title: CSE 561


1
CSE 561 Scalable Routing
  • David Wetherall
  • djw_at_cs.washington.edu
  • Spring 2000

2
This Lecture
  • First, Clark on Internet design philosophy
  • Making routing scale
  • Landmark paper
  • Other techniques
  • Mobile IP

3
Clark, 1988
  • Design philosophy in retrospect important themes
  • Survivability and impact on where to store state
  • Division into IP TCP / UDP for QOS

4
Network Service Models
  • Datagram delivery postal service
  • Also connectionless, best-effort or unreliable
    service
  • Network cant guarantee delivery of the packet
  • Each packet from a host is routed independently
  • Example IP
  • Virtual circuit models telephone
  • Also connection-oriented service
  • Signaling connection establishment, data
    transfer, teardown
  • All packets from a host are routed the same way
  • Example ATM, Frame Relay, X.25

5
Survivability
  • What state goes where?
  • Router versus end system
  • What does survivability entail?
  • Application versus network perspective
  • Problem loss of state held at routers could
    disrupt application even though a path existed
  • Solution fate sharing
  • Follow-on flows and soft-state

6
Types of Service
  • Application requirements
  • Impact of reliability
  • The TCP/IP split, with UDP
  • Quality of service
  • What is provided by the TCP/UDP distinction?
  • What is provided by other mechanisms?

7
IPv4 Header Fields
0
4
8
16
19
31
  • Type of Service
  • Abstract notion of kind of service, never really
    worked out
  • Routers ignore
  • Field will be reused for Diffserv

V
ersion
HLen
TOS
Length
Identifier for Fragments
Flags
Fragment Offset
TTL
Protocol
Checksum
Source Address
Destination Address
Pad
Options (variable)
(variable)
Data
8
Shortcomings
  • Some have become increasingly apparent today
  • Accounting
  • End host control
  • Distributed management

9
Scalability Concerns
  • Routing burden grows with size of an internetwork
  • Size of routing tables
  • Volume or routing messages
  • Amount of routing computation
  • RIP/OSPF do not scale to the size of the Internet
  • We must apply further techniques
  • Careful address allocation
  • Use of hierarchy
  • Route aggregation

10
Landmark Routing
  • Area versus landmark hierarchy
  • Pros and cons of hierarchy
  • Routing table size, number of messages,
    computation
  • Path length
  • Forwarding algorithm

11
Structure of the Internet
You at work
Large corporation


Consumer
ISP
Peering
point
Backbone service provider
Peering
point
Consumer

ISP


Consumer

ISP
Large corporation
Small
corporation
You at home
12
IP Addresses
  • Reflect location in topology used for scalable
    routing
  • Unlike flat Ethernet addresses
  • Interfaces on same network share prefix
  • Prefix administratively assigned (IANA or ISP)
  • Addresses globally unique
  • Routing only advertises entire networks
  • Local delivery within a single network doesnt
    involve router
  • (will make network precise later on)

13
IPv4 Address Formats
7
24
  • 32 bits written in dotted quad notation, e.g.,
    18.31.0.135

Class A
Network
Host
0
14
16
Class B
Network
Host
1
0
21
8
Class C
Network
Host
1
1
0
14
IPv6 Address Format
  • 128 bits written in 16 bit hexadecimal chunks
  • Still hierarchical, just more levels

SubscriberID
ProviderID
RegistryID
001
InterfaceID
SubnetID
15
Subnetting
  • Split up one network number into multiple
    physical networks
  • Internal structure isnt propagated
  • Helps allocation efficiency

Network number
Host number
Class B address
111111111111111111111111
00000000
Subnet mask (255.255.255.0)
Network number
Host ID
Subnet ID
Subnetted address
16
Subnet Example
Subnet mask 255.255.255.128
Subnet number 128.96.34.0
128.96.34.15
128.96.34.1
H1
R1
Subnet mask 255.255.255.128
128.96.34.130
Subnet number 128.96.34.128
128.96.34.139
128.96.34.129
H2
R2
H3
128.96.33.1
128.96.33.14
Subnet mask 255.255.255.0
Subnet number 128.96.33.0
17
Inter-Domain Routing
  • Network comprised of many Autonomous Systems
    (ASes) or domains
  • To scale, use hierarchy separate inter-domain
    and intra-domain routing
  • Also called interior vs exterior gateway
    protocols (IGP/EGP)
  • IGP RIP, OSPF
  • EGP EGP, BGP

23
12
7
44
321
1123
18
Inter-Domain Routing
AS1
  • Border routers summarize and advertise internal
    routes to external neighbors and vice-versa
  • Border routers apply policy
  • Internal routers can use notion of default routes
  • Core is default-free routers must have a route
    to all networks in the world

Border router
Border router
AS2
19
Exterior Gateway Protocol (EGP)
  • First major inter-domain routing protocol
  • Constrained Internet to tree structure no longer
    in use

NSFNET backbone
Stanford
ISU
BARRNET
MidNet

regional
regional
W
estnet
regional
Berkeley
P
ARC
UNL
KU
UNM
NCAR
UA
20
Border Gateway Protocol (BGP-4)
  • EGP used in the Internet backbone today
  • Features
  • Path vector routing
  • Application of policy
  • Operates over reliable transport (TCP)
  • Works with CIDR

21
Path Vectors
  • Similar to distance vector, except send entire
    paths
  • e.g. 321 hears 7,12,44
  • stronger avoidance of loops
  • supports policies (later)
  • Modulo policy, shorter paths are chosen in
    preference to longer ones
  • Reachability only no metrics

23
12
7
44
321
1123
22
Policies
  • Choice of routes may depend on owner, cost, AUP,
  • Business considerations
  • Local policy dictates what route will be chosen
    and what routes will be advertised!
  • e.g., X doesnt provide transit for B, or A
    prefers not to use X

A
B
X
23
Impact of Policies Example
  • Early Exit / Hot Potato
  • if its not for you, bail
  • Combination of best local policies not globally
    best
  • Side-effect asymmetry

A
B
24
Operation over TCP
  • Most routing protocols operate over UDP/IP
  • BGP uses TCP
  • TCP handles error control reacts to congestion
  • Allows for incremental updates
  • Issue Data vs. Control plane
  • Shouldnt routing messages be higher priority
    than data?

25
CIDR (Supernetting)
  • CIDR Classless Inter-Domain Routing
  • Aggregate advertised network routes
  • e.g., ISP has class C addresses 192.4.16 through
    192.4.31
  • Really like one larger 20 bit address class
  • Advertise as such (network number, prefix length)
  • Reduces size of routing tables
  • But IP forwarding is more involved
  • Based on Longest Matching Prefix operation

26
CIDR Example
  • X and Y routes can be aggregated

Corporation X
(11000000000001000001)
Border gateway
Regional network
(advertises path to
11000000000001)
Corporation Y
(11000000000001000000)
27
Mobile IP
  • Problem addresses tied to topology and so change
    as a node moves this disrupts communications
  • Approaches
  • Change DNS
  • Mobile IP (level of indirection at network level)
  • Link-layer mobility
  • Mobile IP Solution

28
Getting an IP address
  • Old fashioned way sysadmin configured each
    machine
  • Dynamic Host Configuration Protocol (DHCP)
  • One DHCP server with the bootstrap info
  • Host address, gateway address, subnet mask,
  • Find it using broadcast
  • Addresses may be leased renew periodically
  • Stateless Autoconfiguration (in IPv6)
  • Get rid of server reuse Ethernet addresses for
    lower portion of address (uniqueness) and learn
    higher portion from routers

29
Address Resolution Protocol (ARP)
  • On a single link, need Ethernet addresses to send
    a frame source is a given, but what about
    destination?
  • Requires mapping from IP to MAC addresses
  • ARP is a dynamic approach to learn mapping
  • Node A sends broadcast query for IP address X
  • Node B with IP address X replies with its MAC
    address M
  • A caches (X, M) old information is timed out
    (15 mins)
  • Also B caches As MAC and IP addresses, other
    nodes refresh

30
Mobile IP Issues in Practice
  • Scalable, and backwards compatible
  • Slow takeoff quasi-mobile and DHCP
  • Handoff latencies
  • Security

31
ARP Example
  • To send a message common case doesnt require
    ARP

Who-is X?
time
I-am X
ltMessagegt
A
B
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