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djw // CS 561, Spring 2000 L5.2
This Lecture
• First, Clark on Internet design philosophy
• Making routing scale– Landmark paper– Other techniques
• Mobile IP
djw // CS 561, Spring 2000 L5.3
Clark, 1988
• Design philosophy in retrospect; important themes
• Survivability and impact on where to store state• Division into IP + TCP / UDP for QOS
djw // CS 561, Spring 2000 L5.4
Network Service Models
• Datagram delivery: postal service– Also connectionless, best-effort or unreliable service– Network can’t 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
djw // CS 561, Spring 2000 L5.5
“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
djw // CS 561, Spring 2000 L5.6
“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?
djw // CS 561, Spring 2000 L5.7
IPv4 Header Fields …
• Type of Service
• Abstract notion of kind of service, never really worked out– Routers
ignore
• Field will be reused for Diffserv
Version HLen TOS Length
Identifier for Fragments Flags Fragment Offset
TTL Protocol Checksum
Source Address
Destination Address
Options (variable) Pad(variable)
0 4 8 16 19 31
Data
djw // CS 561, Spring 2000 L5.8
Shortcomings
• Some have become increasingly apparent today …
• Accounting• End host control• Distributed management
djw // CS 561, Spring 2000 L5.9
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
djw // CS 561, Spring 2000 L5.10
Landmark Routing
• Area versus landmark hierarchy
• Pros and cons of hierarchy– Routing table size, number of messages, computation– Path length– Forwarding algorithm
djw // CS 561, Spring 2000 L5.11
Backbone service provider
Peeringpoint
Peeringpoint
Large corporation
Large corporation
Smallcorporation
“Consumer ” ISP
“Consumer” ISP
“ Consumer” ISP
You at home
You at work
Structure of the Internet
djw // CS 561, Spring 2000 L5.12
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” doesn’t involve
router– (will make “network” precise later on)
djw // CS 561, Spring 2000 L5.13
Network Host
7 24
0
Network Host
14 16
1 0
Network Host
21 8
1 1 0
IPv4 Address Formats
• 32 bits written in “dotted quad” notation, e.g., 18.31.0.135
Class A
Class B
Class C
djw // CS 561, Spring 2000 L5.14
IPv6 Address Format
• 128 bits written in 16 bit hexadecimal chunks• Still hierarchical, just more levels
SubscriberIDProviderIDRegistryID001 InterfaceIDSubnetID
djw // CS 561, Spring 2000 L5.15
Network number Host number
Class B address
Subnet mask (255.255.255.0)
Subnetted address
111111111111111111111111 00000000
Network number Host IDSubnet ID
Subnetting
• Split up one network number into multiple physical networks
• Internal structure isn’t propagated
• Helps allocation efficiency
djw // CS 561, Spring 2000 L5.16
Subnet mask: 255.255.255.128Subnet number: 128.96.34.0
128.96.34.15128.96.34.1
H1
R1
128.96.34.130 Subnet mask: 255.255.255.128Subnet number: 128.96.34.128
128.96.34.129128.96.34.139
R2H2
128.96.33.1128.96.33.14
Subnet mask: 255.255.255.0Subnet number: 128.96.33.0
H3
Subnet Example
djw // CS 561, Spring 2000 L5.17
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
12
44
7
321
23
1123
djw // CS 561, Spring 2000 L5.18
Inter-Domain Routing
• 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
R1
Autonomous system 1R2
R3
Autonomous system 2R4
R5 R6
AS1
AS2
Border router
Border router
djw // CS 561, Spring 2000 L5.19
NSFNET backboneStanford
BARRNETregional
BerkeleyPARC
NCAR
UA
UNM
Westnetregional
UNL KU
ISU
MidNetregional…
Exterior Gateway Protocol (EGP)
• First major inter-domain routing protocol• Constrained Internet to tree structure; no longer
in use
djw // CS 561, Spring 2000 L5.20
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
djw // CS 561, Spring 2000 L5.21
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
12
44
7
321
23
1123
djw // CS 561, Spring 2000 L5.22
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 doesn’t provide transit for B, or A prefers not to
use X A BX
djw // CS 561, Spring 2000 L5.23
Impact of Policies – Example
• Early Exit / Hot Potato– “if it’s not for you, bail”
• Combination of best local policies not globally best
• Side-effect: asymmetryB
A
djw // CS 561, Spring 2000 L5.24
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– Shouldn’t routing messages be higher priority than
data?
djw // CS 561, Spring 2000 L5.25
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
djw // CS 561, Spring 2000 L5.26
Border gateway(advertises path to11000000000001)
Regional network
Corporation X(11000000000001000001)
Corporation Y(11000000000001000000)
CIDR Example
• X and Y routes can be aggregated
djw // CS 561, Spring 2000 L5.27
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
djw // CS 561, Spring 2000 L5.28
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
djw // CS 561, Spring 2000 L5.29
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 A’s MAC and IP addresses, other nodes refresh
djw // CS 561, Spring 2000 L5.30
Mobile IP Issues in Practice
• Scalable, and backwards compatible• Slow takeoff: quasi-mobile and DHCP• Handoff latencies• Security
djw // CS 561, Spring 2000 L5.31
ARP Example
• To send a message … common case doesn’t require ARP
time
A B
Who-is X?
I-am X
<Message>
