Computer Networks

1
Computer Networks

• Network layer (Part 3)

2
Network layer (part 3)

• 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 addressing, IP routing

3
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)

4
NL NAT Illustration
Pool of global IP addresses
Destination
Source
P
G
Global Internet
Private Network
NAT
Dg
Sp
Data
Dg
Sg
Data

• 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

5
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

6
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

7
NL IP routing

• Who provides the functionality?
• Internet area hierarchy
• IP route lookups
• Original route lookup
• CIDR address aggregation
• Specific IP routing protocols
• Intra-AS routing
• Inter-AS routing

8
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

9
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

10
NL Source Routing Example
Packet
2
2
Sender
R1
R1
3
1
3
1
4
4
2
R2
Receiver
1
3
4
11
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)

12
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

13
NL Virtual Circuits Examples
Packet
2
2
Sender
R1
R1
1,7 ? 4,2
3
1
3
1
4
4
1,5 ? 3,7
2
R2
Receiver
1
3
4
2,2 ? 3,6
14
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

15
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?

16
NL IP Datagrams on Virtual Circuits

• Traffic pattern
• Few long lived flows
• Flow set of data packets from source to
  destination
• Large percentage of packet traffic
• Improving forwarding performance by using virtual
  circuits for these flows
• Other traffic uses normal IP forwarding

17
NL Network routers (Global IP addresses)

• 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

18
NL Global Address Example
Packet
2
2
Sender
R1
R1
R ? 4
3
1
3
1
4
4
R ? 3
2
R2
Receiver
1
3
4
R ? 3
19
NL 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

20
NL Global Addresses

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

21
NL Comparison
Source Routing
Global Addresses
Virtual Circuits
Header Size
Worst
OK Large address
OK (larger than global if IP payload)
Router Table Size
None
Number of hosts (prefixes)
Number of circuits
Forward Overhead
Best
Prefix matching
Good (table index)
Setup Overhead
None
None
Connection Setup
Error Recovery
Tell all hosts
Tell all routers
Tell all routers, Tear down circuit and re-route
22
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

23
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
24
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
25
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
26
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)

27
NL AS categories example
AS3
AS1
AS1
AS2
AS3
AS1
AS2
Transit
Stub
AS2
Multi-homed
28
NL IP route lookups

• Original IP Route Lookup
• Address classes
• 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

29
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

30
NL Getting a datagram from source to dest.
routing table in A

• Classful routing example
• IP datagram

• datagram remains unchanged, as it travels source
  to destination
• addr fields of interest here

31
NL Getting a datagram from source to dest.
misc fields
data
223.1.1.1
223.1.1.3

• Starting at A, given IP datagram addressed to B
• look up net. address of B
• find B is on same net. as A
• link layer will send datagram directly to B
  inside link-layer frame
• B and A are directly connected

32
NL Getting a datagram from source to dest.
misc fields
data
223.1.1.1
223.1.2.3

• Starting at A, dest. E
• look up network address of E
• E on different network
• A, E not directly attached
• routing table next hop router to E is 223.1.1.4
• link layer sends datagram to router 223.1.1.4
  inside link-layer frame
• datagram arrives at 223.1.1.4
• continued..

33
NL Getting a datagram from source to dest.
misc fields
data
223.1.1.1
223.1.2.3

• Arriving at 223.1.4, destined for 223.1.2.2
• look up network address of E
• E on same network as routers interface 223.1.2.9
  
• router, E directly attached
• link layer sends datagram to 223.1.2.2 inside
  link-layer frame via interface 223.1.2.9
• datagram arrives at 223.1.2.2!!! (hooray!)

34
NL CIDR

• Supernets
• Classless routing (CIDR)
• Assign adjacent net addresses to same org
• Combine routing table entries whenever all nodes
  with same prefix share same hop

35
NL CIDR and IP route lookups

• Network provider is 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
• Replaces 8 class C routing entries with 1
  combined entry
• Routing protocols carry prefix with destination
  network address
• Longest prefix match for forwarding
• More on this when we talk about implementations.

36
NL CIDR example
ISP X given 16 class C networks 200.23.16. to
200.23.31. (or 200.23.16/20)
ISP X
Adjacent ISP router
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
Large company 200.23.16.0/21
Medium company 200.23.24.0/22
Small company 200.23.28.0/23
Tiny company 200.23.30.0/24
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
37
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
38
NL Another CIDR example

• Routing to the network
• Packet to 10.1.1.3 arrives
• Path is R2 R1 H1 H2

10.1.1.2 10.1.1.4
10.1.1.3
H2
H1
10.1.1/24
10.1.0.2
10.1.0.1 10.1.1.1 10.1.2.2
H3
R1
10.1.0/24
10.1.2/23
10.1/16
10.1.8/24
R2
Provider
10.1.8.1 10.1.2.1 10.1.16.1
H4
10.1.8.4
39
NL Another CIDR example

• Subnet Routing
• Packet to 10.1.1.3
• Matches 10.1.0.0/23

10.1.1.2 10.1.1.4
10.1.1.3
H2
H1
10.1.1/24
10.1.0.2
10.1.0.1 10.1.1.1 10.1.2.2
H3
R1
Routing table at R2
10.1.0/24
10.1.2/23
Destination
Next Hop
Interface
10.1/16
10.1.8/24
R2
127.0.0.1
127.0.0.1
lo0
10.1.8.1 10.1.2.1 10.1.16.1
Default or 0/0
provider
10.1.16.1
H4
10.1.8.0/24
10.1.8.1
10.1.8.1
10.1.8.4
10.1.2.0/23
10.1.2.1
10.1.2.1
10.1.0.0/23
10.1.2.2
10.1.2.1
40
NL Another CIDR example

• Subnet Routing
• Packet to 10.1.1.3
• Matches 10.1.1.1/31
• Longest prefix match

10.1.1.2 10.1.1.4
10.1.1.3
H2
H1
10.1.1/24
10.1.0.2
10.1.0.1 10.1.1.1 10.1.2.2
H3
R1
10.1.0/24
Routing table at R1
10.1.2/23
10.1/16
10.1.8/24
R2
Destination
Next Hop
Interface
10.1.8.1 10.1.2.1 10.1.16.1
127.0.0.1
127.0.0.1
lo0
H4
Default or 0/0
10.1.2.1
10.1.2.2
10.1.8.4
10.1.0.0/24
10.1.0.1
10.1.0.1
10.1.1.0/24
10.1.1.1
10.1.1.4
10.1.2.0/23
10.1.2.2
10.1.2.2
10.1.1.2/31
10.1.1.2
10.1.1.2
41
NL Another CIDR example

• Subnet Routing
• Packet to 10.1.1.3
• Direct route
• Longest prefix match

10.1.1.2 10.1.1.4
10.1.1.3
H2
H1
10.1.1/24
10.1.0.2
10.1.0.1 10.1.1.1 10.1.2.2
H3
R1
10.1.0/24
Routing table at H1
10.1.2/23
10.1/16
10.1.8/24
R2
Destination
Next Hop
Interface
10.1.8.1 10.1.2.1 10.1.16.1
127.0.0.1
127.0.0.1
lo0
H4
Default or 0/0
10.1.1.1
10.1.1.2
10.1.8.4
10.1.1.0/24
10.1.1.2
10.1.1.1
10.1.1.3/31
10.1.1.2
10.1.1.2
42
NL CIDR Shortcomings

• Customer selecting a new provider
• Renumbering required

199.31.0.0/16
201.10.0.0/21
Provider 1
Provider 2
201.10.0.0/22
201.10.4.0/24
201.10.5.0/24
201.10.6.0/23
43
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
44
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

45
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

46
NL Intra-AS Distance Vector Protocols

• GGP Gateway-to-Gateway Protocol (1970s)
• RIP Routing Information Protocol (1982)
• 30 sec update with triggered updates
• Split horizon with poisonous reverse
• RIP-2 in 1993 adds prefix mask for CIDR
• IGRP Interior Gateway Routing Protocol (1988)
• Ciscos successor to RIP
• 90 sec update with triggered updates
• Split horizon
• V1 path holddown
• V2 route poisoning
• Composite metric, multiple paths
• EIGRP adds prefix mask and DUAL for CIDR

47
NL RIP (Routing Information Protocol)

• Included in BSD-UNIX Distribution in 1982
• Distance metric of hops (max 15 hops)
• Can you guess why?
• Distance vectors exchanged every 30 sec via
  Response Message (also called advertisement)
• Each advertisement route to up to 25 destination
  nets

48
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)

49
NL RIP Table processing

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

50
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

• Three attached class C networks (LANs)
• Router only knows routes to attached LANs
• Default router used to go up
• Route multicast address 224.0.0.0
• Loopback interface (for debugging)

51
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)
• uses TCP to exchange routing updates
• Loop-free routing via Distributed Updating Alg.
  (DUAL) based on diffused computation

52
NL Intra-AS Link State Protocols

• OSPF
• Hierarchical OSPF

53
NL OSPF (Open Shortest Path First)

• open publicly available
• 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)

54
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.

55
NL Hierarchical OSPF
56
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.

57
NL Inter-AS routing
58
NL 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

59
NL History

• Mid-80s EGP (Exterior Gateway Protocol)
• Used in original ARPAnet
• Reachability protocol (no shortest path)
• Single bit for reachability information
• Did not accommodate cycles (topology restricted
  to a tree)
• ARPA-managed packet switches at top of tree
• Unacceptable once Internet grew to multiple
  independent backbones
• Result BGP development

60
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

61
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

62
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

63
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

64
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

65
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

66
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

67
NL IP summary

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

68
NL IPv6

• Redefine functions of IP (version 4)
• Remove ancillary functionality
• Add missing, but essential functionality
• Recall, functions of IPv4
• What changes should be made in.
• IP addressing
• IP delivery semantics
• IP quality of service
• IP security
• IP routing
• IP fragmentation
• IP error detection

69
NL IPv6

• Initial motivation 32-bit address space
  completely allocated by 2008.
• Additional motivation
• header format helps speed processing/forwarding
• header changes to facilitate QoS
• new anycast address route to best of several
  replicated servers
• IPv6 datagram format
• fixed-length 40 byte header
• no fragmentation allowed

70
NL IPv6 Header
0
4
16
24
32
12
19
Version
Class
Flow Label
Payload Length
Next Header
Hop Limit
Source Address (128 bits)
Destination Address (128 bits)
71
NL IPv6 Changes

• Scale addresses are 128bit
• Header size?
• Simplification
• Removes infrequently used parts of header
• 40 byte fixed size vs. 20 byte variable
• IPv6 removes checksum
• Relies on upper layer protocols to provide
  integrity
• Reduces processing time at each hop
• IPv6 eliminates fragmentation
• Requires path MTU discovery

72
NL IPv6 Changes

• TOS replaced with traffic class octet
• Flow
• Help soft state systems
• Maps well onto TCP connection or stream of UDP
  packets on host-port pair
• Easy configuration
• Provides auto-configuration using hardware MAC
  address to provide unique base
• Additional requirements
• Support for security
• Support for mobility

73
NL IPv6 Changes

• Protocol field replaced by next header field
• Support for protocol demultiplexing as well as
  option processing
• Option processing
• Options allowed, but only outside of header,
  indicated by Next Header field
• Options header does not need to be processed by
  every router
• Large performance improvement
• Makes options practical/useful
• ICMPv6 new version of ICMP
• additional message types, e.g. Packet Too Big
• multicast group management functions

74
NL Transition From IPv4 To IPv6

• Not all routers can be upgraded simultaneous
• no flag days
• How will the network operate with mixed IPv4 and
  IPv6 routers?
• Two proposed approaches
• Dual Stack some routers with dual stack (v6, v4)
  can translate between formats
• Tunneling IPv6 carried as payload in an IPv4
  datagram among IPv4 routers

75
NL Dual Stack Approach
76
NL Tunneling
IPv6 inside IPv4 where needed