CSE524: Lecture 9

1
CSE524 Lecture 9

• Network Layer (Part 3)

2
Administrative

• Approved project descriptions due Wednesday
• Homework 3 due next Monday 11/3/03

3
Roadmap

• Last two classes
• Network layer functionality
• IP network layer implementation
• IP security, IP error detection, IP delivery
  semantics, IP quality of service, IP
  fragmentation
• This class
• Finish IP network layer implementation
• IP addressing, IP routing

4
NL IP Addressing

• IP address fixed-length, 32-bit identifier for
  host, router interface
• semantics getting fuzzy, though (more later)
• interface connection between host, router and
  physical link
• routers typically have multiple interfaces
• host may have multiple interfaces
• IP addresses associated with interface, not host,
  router

223.1.1.1
223.1.2.9
223.1.1.4
223.1.1.3
223.1.1.1 11011111 00000001 00000001 00000001
223
1
1
1
5
NL IP Addressing

• IP address
• network part (high order bits)
• host part (low order bits)
• Whats a network ?
• all device interfaces with same network part of
  IP address
• all interfaces that can physically reach each
  other without intervening router

223.1.1.1
223.1.2.1
223.1.1.2
223.1.2.9
223.1.1.4
223.1.2.2
223.1.1.3
223.1.3.27
LAN
223.1.3.2
223.1.3.1
network consisting of 3 IP networks (for IP
addresses starting with 223, first 24 bits are
network address)
6
NL IP Addressing
223.1.1.2

• How to find the networks?
• Detach each interface from router, host
• create islands of isolated networks

223.1.1.1
223.1.1.4
223.1.1.3
223.1.7.0
223.1.9.2
223.1.9.1
223.1.7.1
223.1.8.0
223.1.8.1
223.1.2.6
Interconnected system consisting of six networks
223.1.2.1
223.1.2.2
7
NL Classful IP Addressing (1981)

• Total IP address size 4 billion
• Initially one large class (8-bit network, 24-bit
  host)
• Classful addressing for smaller networks (LANs)
• Class A 128 networks, 16M hosts
• Class B 16K networks, 64K hosts
• Class C 2M networks, 256 hosts

High Order Bits 0 10 110
Format 7 bits of net, 24 bits of host 14 bits of
net, 16 bits of host 21 bits of net, 8 bits of
host
Class A B C
8
NL IP address classes
8
16
32
24
Class A
1.0.0.0 to 127.255.255.255
Class B
Host ID
Network ID
128.0.0.0 to 191.255.255.255
Class C
Host ID
Network ID
192.0.0.0 to 223.255.255.255
Class D
224.0.0.0 to 239.255.255.255
Class E
9
NL Special IP Addresses

• Private addresses
• http//www.rfc-editor.org/rfc/rfc1918.txt
• Class A 10.0.0.0 - 10.255.255.255 (10/8 prefix)
• Class B 172.16.0.0 - 172.31.255.255 (172.16/12
  prefix)
• Class C 192.168.0.0 - 192.168.255.255
  (192.168/16 prefix)
• 127.0.0.1 local host (a.k.a. the loopback
  address)
• 255.255.255.255
• IP broadcast to local hardware that must not be
  forwarded
• http//www.rfc-editor.org/rfc/rfc919.txt
• Same as network broadcast if no subnetting
• IP of network broadcastNetworkID(all 1s for
  HostID)
• 0.0.0.0
• IP address of unassigned host (BOOTP, ARP, DHCP)
• Default route advertisement

10
NL IP Addressing Problem 1 (1984)

• Inefficient use of address space
• Class A (rarely given out, not many of them given
  out by IANA)
• Class B 64k hosts
• Very few LANs have close to 64K hosts
• Electrical/LAN limitations, performance or
  administrative reasons
• e.g., class B net allocated enough addresses for
  64K hosts, even if only 2K hosts in that network
• Need simple/address-efficient way to get multiple
  networks
• Reduce the total number of addresses that are
  assigned, but not used
• Subnet addressing
• http//www.rfc-editor.org/rfc/rfc917.txt
• Split up single large network address ranges into
  multiple smaller ones (subnet)

11
NL Subnetting

• Variable length subnet masks
• Subnet a class B address space into several chunks

Network
Host
Network
Host
Subnet
1111..
00000000
..1111
Mask
12
NL Subnetting Example

• Assume an organization was assigned address
  150.100
• Assume lt 100 hosts per subnet
• How many host bits do we need? Seven
• What is the network mask?
• 11111111 11111111 11111111 10000000
• 255.255.255.128

13
NL IP Address Problem 2 (1991)

• Address space depletion
• In danger of running out of classes A and B
• Class A
• very few in number, IANA frugal in giving them
  out
• Class B
• subnetting only applied to new allocations of
  class B
• existing class B networks sparsely populated
• people refuse to give it back
• Class C
• plenty available, but too small for most domains
• giving out multiple class C to a domain explodes
  of routes
• Supernetting
• Assign multiple consecutive class C blocks as
  one block
• http//www.rfc-editor.org/rfc/rfc1338.txt

14
NL CIDR

• Evolved into Classless Inter-Domain Routing
  (CIDR)
• http//www.rfc-editor.org/rfc/rfc1518.txt
• http//www.rfc-editor.org/rfc/rfc1519.txt

15
NL IP addressing CIDR

• Original classful addressing
• Use class structure (A, B, C) to determine
  network ID for route lookup
• CIDR Classless InterDomain Routing
• Do not use classes to determine network ID
• network portion of address of arbitrary length
• address format a.b.c.d/x, where x is bits in
  network portion of address

16
NL CIDR

• Assign any range of addresses to network
• Use common part of address as network number
• e.g., addresses 192.4.16. to 192.4.31. have the
  first 20 bits in common. Thus, we use this as the
  network number
• netmask is /20, /xx is valid for almost any xx
• 192.4.16.0/20
• Enables more efficient usage of address space
  (and router tables)
• More on how this impacts routing later.

17
NL IP addressing How are they allocated?

• Hosts (host portion)
• From organization via static configuration or
  DHCP
• Network (network portion)
• Organization gets from ISPs assigned address
  space
• ISPs get it from ICANN Internet Corporation for
  Assigned Names and Numbers

ISP's block 11001000 00010111 00010000
00000000 200.23.16.0/20 Organization 0
11001000 00010111 00010000 00000000
200.23.16.0/23 Organization 1 11001000
00010111 00010010 00000000 200.23.18.0/23
Organization 2 11001000 00010111 00010100
00000000 200.23.20.0/23 ...
..
. . Organization
7 11001000 00010111 00011110 00000000
200.23.30.0/23
18
NL IP addressing and NAT

• Network Address Translation (NAT)
• Alternate solution to address space depletion
  problem
• Kludge (but useful)
• Sits between your network and the Internet
• Translates local, private, network layer
  addresses to global IP addresses
• Has a pool of global IP addresses (less than
  number of hosts on your network)

19
NL NAT Illustration
Pool of global IP addresses
Destination
Source
P
G

• Operation Source (S) wants to talk to
  Destination (D)
• Create Sg-Sp mapping
• Replace Sp with Sg for outgoing packets
• Replace Sg with Sp for incoming packets

20
NL Problems with NAT

• What if we only have few (or just one) IP
  address?
• Use NAPT (Network Address Port Translator)
• NAPT translates
• Translates Paddr flow info to Gaddr new flow
  info
• Uses TCP/UDP port numbers
• Potentially thousands of simultaneous connections
  with one global IP address

21
NL Problems with NAT

• Hides the internal network structure
• Some consider this an advantage
• Multiple NAT hops must ensure consistent mappings
• Some protocols carry addresses
• e.g., FTP carries addresses in text
• What is the problem?
• Encryption
• No inbound connections

22
NL IP routing

• Who provides the functionality?
• Internet area hierarchy
• IP route lookups
• Specific IP routing protocols

23
NL Who handles IP routing functions?

• Source (IP source routing)
• Packet carries path
• Network edge devices
• Map IP route into label, wavelength, or circuit
  at edges
• Switch on label, wavelength, or circuit in the
  core
• ATM
• MPLS
• lambda switching
• Network routers
• Hop-by-hop forwarding based on destination IP
  carried by packet
• Routers keep next hop for destination
• IP route table calculated in network routers
• Most common

24
NL Source Routing

• IP source route option
• List entire path (strict) or partial path (loose)
  in packet
• Attach list of IP addresses within header
• Router processing
• Examine first step in directions
• Increment pointer offset in header
• Forward to step
• Copy entire source route header on fragmentation

25
NL Source Routing Example
Packet
2
2
3
1
3
1
4
4
2
1
3
4
26
NL Source Routing

• Advantages
• Switches can be very simple and fast
• Disadvantages
• Variable (unbounded) header size
• Sources must know or discover topology (e.g.,
  failures)
• Typical use
• Ad-hoc networks (DSR)
• Machine room networks (Myrinet)

27
NL Network edge devices

• Virtual circuits, tag switching
• Connection setup phase
• IP route lookup at edges to generate appropriate
  label, wavelength, circuit
• Switch on label, wavelength, circuit ID in core
• Router processing
• Lookup flow ID simple table lookup
• Potentially replace flow ID with outgoing flow ID
• Forward to output port

28
NL Virtual Circuits Examples
Packet
2
2
3
1
3
1
4
4
2
1
3
4
29
NL Virtual Circuits

• Advantages
• More efficient lookup (simple table lookup)
• More flexible (different path for each flow)
• Can reserve bandwidth at connection setup
• Easier for hardware implementations
• Disadvantages
• Still need to route connection setup request
• More complex failure recovery must recreate
  connection state
• Typical uses
• ATM combined with fix sized cells
• MPLS tag switching for IP networks

30
NL IP Datagrams on Virtual Circuits

• Challenge when to setup connections
• At bootup time permanent virtual circuits (PVC)
• Large number of circuits
• For every packet transmission
• Connection setup is expensive
• For every connection
• What is a connection?
• How to route connectionless traffic?
• Based on traffic
• VC for long-lived flows
• Normal IP forwarding for all other flows

31
NL Network routers (Global IP addresses)

• Most prevalent way to route on the Internet
• Each packet has destination IP address
• Each router has forwarding table of..
• destination IP ? next hop IP address
• Distributed routing algorithm for calculating
  forwarding tables

32
NL Global Address Example
Packet
2
2
3
1
3
1
4
4
2
1
3
4
33
NL Issues in Router Table Size

• One entry for every host on the Internet
• 100M entries,doubling every year
• One entry for every LAN
• Every host on LAN shares prefix
• Still too many, doubling every year
• One entry for every organization
• Every host in organization shares prefix
• Requires careful address allocation

34
NL Global Addresses

• Advantages
• Stateless simple error recovery
• Disadvantages
• Every router knows about every destination
• Potentially large tables
• All packets to destination take same route

35
NL Comparison
36
NL IP route lookups

• Original IP Route Lookup
• In the early days, address classes made it easy
• A 0 7 bit network 24 bit host (16M each)
• B 10 14 bit network 16 bit host (64K)
• C 110 21 bit network 8 bit host (255)
• Address would specify prefix for forwarding table
• Simple lookup

37
NL Original IP Route Lookup Example

• www.ogi.edu address 129.95.5.30
• Class B address class network is 129.95
• Lookup 129.95 in forwarding table
• Prefix part of address that really matters for
  routing
• Forwarding table contains
• List of prefix entries
• A few fixed prefix lengths (8/16/24)
• Large tables
• 2 Million class C networks
• Sites with multiple class C networks have
  multiple route entries at every router

38
NL Getting a datagram from source to dest.

• Classful routing example
• IP datagram

39
NL Getting a datagram from source to dest.
misc fields
data
223.1.1.1
223.1.1.3
40
NL Getting a datagram from source to dest.
misc fields
data
223.1.1.1
223.1.2.3
41
NL Getting a datagram from source to dest.
misc fields
data
223.1.1.1
223.1.2.3
42
NL IP route lookup and CIDR

• Recall Classless routing (CIDR)
• Advantages
• Saves space in route tables
• Makes more efficient use of address space
• ISP allocated 8 class C chunks, 201.10.0.0 to
  201.10.7.255
• Allocation uses 3 bits of class C space
• Remaining 21 bits are network number, written as
  201.10.0.0/21
• Replace 8 class C entries with 1 combined entry
• Routing protocols carry prefix length with
  destination network address
• But....Makes route lookup more complex
• No longer separate class A/B/C route tables each
  with O(1) lookup
• One table containing many prefix lengths
• Must match against all routes simultaneously via
  longest prefix match

43
NL CIDR example
ISP X given 16 class C networks 200.23.16. to
200.23.31. (or 200.23.16/20)
Route Interface 200.23.16/21
2 200.23.24/22 3 200.23.28/23
4 200.23.30/24 5
1
1
2
5
Route Interface 200.23.16/20
1
3
4
200.23.16.0/24, 200.200.17.0/24 200.23.18.0/24,
200.200.19.0/24 200.23.20.0/24,
200.200.21.0/24 200.23.22.0/24, 200.200.23.0/24
200.23.24.0/24 200.23.25.0/24 200.23.26.0/24 200.2
3.27.0/24
200.23.28.0/24 200.23.29.0/24
44
NL CIDR, hierarchical addressing, route
aggregation
Hierarchical addressing allows efficient
advertisement of routing information
Organization 0
Organization 1
Send me anything with addresses beginning
200.23.16.0/20
Organization 2
Fly-By-Night-ISP
Internet
Organization 7
Send me anything with addresses beginning
199.31.0.0/16
ISPs-R-Us
45
NL Another CIDR example
10.1.1.2/31
10.1.1.3
10.1.1.2 10.1.1.4
10.1.1/24
10.1.3.2
10.1.1.1 10.1.2.2 10.1.3.1
10.1.3/24
10.1.2/24
10.1.16/24
10.1.8/24
10.1.8.1 10.1.2.1 10.1.16.1
10.1.8.4
46
NL Another CIDR example
10.1.1.2/31
10.1.1.3
10.1.1.2 10.1.1.4
10.1.1/24
10.1.3.2
10.1.1.1 10.1.2.2 10.1.3.1
Routing table at R2
10.1.3/24
10.1.2/24
10.1.16/24
10.1.8/24
10.1.8.1 10.1.2.1 10.1.16.1
10.1.8.4
47
NL Another CIDR example
10.1.1.2/31
10.1.1.3
10.1.1.2 10.1.1.4
10.1.1/24
10.1.3.2
10.1.1.1 10.1.2.2 10.1.3.1
Routing table at R1
10.1.3/24
10.1.2/24
10.1.16/24
10.1.8/24
10.1.8.1 10.1.2.1 10.1.16.1
10.1.8.4
10.1.1.3 matches both routes, use longest prefix
match
48
NL Another CIDR example
10.1.1.2/31
10.1.1.3
10.1.1.2 10.1.1.4
10.1.1/24
10.1.3.2
10.1.1.1 10.1.2.2 10.1.3.1
10.1.3/24
Routing table at H1
10.1.2/24
10.1.16/24
10.1.8/24
10.1.8.1 10.1.2.1 10.1.16.1
10.1.8.4
10.1.1.2
10.1.1.2
10.1.1.3 matches both routes, use longest prefix
match
49
NL CIDR Shortcomings

• Customer selecting a new provider
• Renumbering required

199.31.0.0/16
201.10.0.0/21
201.10.0.0/22
201.10.4.0/24
201.10.5.0/24
201.10.6.0/23
50
NL CIDR Shortcomings

• Multi-homing

ISPs-R-Us has a more specific route to
Organization 1
Organization 0
Send me anything with addresses beginning
200.23.16.0/20
Organization 2
Fly-By-Night-ISP
Internet
Organization 7
Send me anything with addresses beginning
199.31.0.0/16 or 200.23.18.0/23
ISPs-R-Us
Organization 1
51
NL Hierarchical routing in the Internet

• Area routing
• aggregate routers into regions, autonomous
  systems (AS)
• administrative autonomy
• routers in same AS run same routing protocol
• intra-AS routing protocol or interior gateway
  protocol (IGP)
• routers in different AS can run different
  intra-AS routing protocol

• special routers in AS
• run intra-AS routing protocol with all other
  routers in AS
• also responsible for routing to destinations
  outside AS
• run inter-AS routing protocol or exterior gateway
  protocol (EGP) with other gateway routers in
  other ASs

52
NL Example 1
1
2
IGP
2.1
2.2
IGP
EGP
1.1
2.2.1
1.2
EGP
EGP
EGP
3
4.2
4.1
IGP
EGP
4
IGP
5
3.2
3.1
IGP
5.2
5.1
53
NL Example 2

• Gateways
• perform inter-AS routing amongst themselves
• perform intra-AS routers with other routers in
  their AS

b
a
a
C
B
d
A
network layer
inter-AS, intra-AS routing in gateway A.c
link layer
physical layer
54
NL Path Sub-optimality
1
2
2.1
2.2
1.1
2.2.1
1.2
1.2.1
start
end
3.2.1
3
3 hop red path vs. 2 hop green path
3.2
3.1
55
NL AS Categories

• Stub an AS that has only a single connection to
  one other AS - carries only local traffic.
• Multi-homed an AS that has connections to more
  than one AS, but does not carry transit traffic
• Transit an AS that has connections to more than
  one AS, and carries both transit and local
  traffic (under certain policy restrictions)

56
NL AS categories example
Transit
Stub
Multi-homed
57
NL Specific IP routing protocols

• Intra-AS routing protocols (interior routing
  protocols)
• GGP
• RIP
• IGRP
• OSPF
• Inter-AS routing protocols (exterior routing
  protocols)
• EGP
• BGP

58
NL Intra-AS Routing

• Generate Intra-AS routing tables
• Also known as Interior Gateway Protocols (IGP)
• Most common IGPs
• Distance vector protocols
• RIP Routing Information Protocol
• IGRP Interior Gateway Routing Protocol (Cisco
  propr.)
• Link state protocols
• OSPF Open Shortest Path First

59
NL Intra-AS Distance Vector Protocols

• GGP Gateway-to-Gateway Protocol (1970s)
• RIP Routing Information Protocol (1982)
• IGRP Interior Gateway Routing Protocol (1988)

60
NL RIP (Routing Information Protocol)

• Included in BSD-UNIX Distribution in 1982
• Distance metric of hops (max 15 hops)
• Vectors exchanged every 30 sec and when triggered
• Static update period leads to synchronization
  problems
• Split horizon with poisonous reverse
• RIP-2 in 1993 adds prefix mask for CIDR

61
NL RIP Link Failure and Recovery

• If no advertisement heard after 180 sec --gt
  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)

62
NL RIP Table processing

• RIP routing tables managed by application-level
  process called route-d (daemon)
• advertisements sent in UDP packets, periodically
  repeated

63
NL RIP Table example (continued)

• Router giroflee.eurocom.fr

Destination Gateway
Flags Ref Use Interface
-------------------- -------------------- -----
----- ------ --------- 127.0.0.1
127.0.0.1 UH 0 26492 lo0
192.168.2. 192.168.2.5 U
2 13 fa0 193.55.114.
193.55.114.6 U 3 58503 le0
192.168.3. 192.168.3.5 U
2 25 qaa0 224.0.0.0
193.55.114.6 U 3 0 le0
default 193.55.114.129 UG
0 143454
64
NL IGRP (Interior Gateway Routing Protocol)

• CISCO proprietary successor of RIP (mid 80s)
• Distance Vector, like RIP
• several cost metrics (delay, bandwidth,
  reliability, load etc)
• 90 sec update with triggered updates
• Split horizon
• V1 path holddown
• V2 route poisoning
• multiple path support
• uses TCP to exchange routing updates
• EIGRP
• Loop-free routing via DUAL (based on diffused
  computation)
• CIDR support

65
NL Intra-AS Link State Protocols

• OSPF
• Hierarchical OSPF

66
NL OSPF (Open Shortest Path First)

• Uses Link State algorithm
• LS packet dissemination
• Topology map at each node
• Route computation using Dijkstras algorithm
• OSPF advertisement carries one entry per neighbor
  router
• Advertisements disseminated to entire AS (via
  flooding)

67
NL OSPF advanced features (not in RIP)

• Security all OSPF messages authenticated (to
  prevent malicious intrusion) TCP connections
  used
• Multiple same-cost paths allowed (only one path
  in RIP)
• For each link, multiple cost metrics for
  different TOS (eg, 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.

68
NL Hierarchical OSPF
69
NL 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.
• 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 ASs.

70
NL Inter-AS routing
71
NL Why different Intra- and Inter-AS routing ?

• Policy vs. Performance
• Inter-AS
• ISPs want control over how its traffic routed,
  who routes through its net
• policy and monetary factors dominate over
  performance
• Intra-AS
• single administrative policy
• performance dominates

72
NL History

• Mid-80s EGP (Exterior Gateway Protocol)
• Used in original ARPAnet
• Reachability protocol (no shortest path)
• Single bit for reachability information
• Topology restricted to a tree (no cycles allowed)
• ARPA-managed packet switches at top of tree
• Unacceptable once Internet grew to multiple
  independent backbones
• Result BGP development

73
NL BGP

• Link state or distance vector?
• Problems with distance-vector
• Bellman-Ford algorithm may not converge
• Problems with link state
• Metric used by routers not the same loops
• No universal routing metric
• Policy drives routing decisions
• LS database too large entire Internet
• May expose policies to other ASs

74
NL BGP

• BGP (Border Gateway Protocol) the de facto
  standard
• Path Vector protocol
• similar to Distance Vector protocol
• each Border Gateway broadcast to neighbors
  (peers) entire path (I.e, sequence of ASs) to
  destination
• E.g., Gateway X sends its path to dest. Z
• Path (X,Z) X,Y1,Y2,Y3,,Z
• When AS gets route check if AS already in path
• If yes, reject route
• If no, add self and (possibly) advertise route
  further
• Allows for policy application (different metrics)
  
• Metrics are local - AS chooses path, protocol
  ensures no loops
• Supports CIDR aggregation (BGP4)
• Supports alternative routes

75
NL Path Selection Criteria

• Path attributes external (policy) information
• Examples
• Hop count
• Policy considerations
• Preference for AS
• Presence or absence of certain AS
• Path origin
• Link dynamics
• Early-exit
• Hot-potato routing for transit packets

76
NL Policy with BGP

• BGP provides capability for enforcing various
  policies
• Policies are not part of BGP they are provided
  to BGP as configuration information
• BGP enforces policies by choosing paths from
  multiple alternatives and controlling
  advertisement to other ASs

77
NL Examples of BGP Policies

• A multi-homed AS refuses to act as transit
• Limit path advertisement
• A multi-homed AS can become transit for some ASs
• Only advertise paths to some ASs
• An AS can favor or disfavor certain ASs for
  traffic transit from itself

78
NL Interconnecting BGP Peers

• BGP uses TCP to connect peers
• Advantages
• Simplifies BGP
• No need for periodic refresh - routes are valid
  until withdrawn, or the connection is lost
• Incremental updates
• Disadvantages
• Congestion control on a routing protocol?
• Poor interaction during high load

79
NL Internet inter-AS routing BGP

• 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

80
NL IP summary

• Security
• Error detection
• Delivery semantics
• Quality-of-service
• Fragmentation
• Addressing
• Routing