Virtual circuit and datagram networks

1
Network Layer

• Introduction
• Virtual circuit and datagram networks
• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Whats inside a router
• Input ports
• Switching ports
• Output ports
• Queuing

• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

2
The Internet Network layer

• Host, router network layer functions

Transport layer TCP, UDP
Network layer
Link layer
physical layer
3
Network Layer

• Introduction
• Virtual circuit and datagram networks
• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Whats inside a router
• Input ports
• Switching ports
• Output ports
• Queuing

• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

4
IP datagram format

• how much overhead with TCP?
• 20 bytes of TCP
• 20 bytes of IP
• 40 bytes app layer overhead

5
IP Fragmentation Reassembly

• network links have MTU (max.transfer size) -
  largest possible link-level frame.
• different link types, different MTUs
• large IP datagram divided (fragmented) within
  net
• one datagram becomes several datagrams
• reassembled only at final destination
• IP header bits used to identify, order related
  fragments

fragmentation in one large datagram out 3
smaller datagrams
reassembly
6
IP Fragmentation and Reassembly

• Example
• 4000 byte datagram
• MTU 1500 bytes

1480 bytes in data field
offset 1480/8
7
Network Layer

• Introduction
• Virtual circuit and datagram networks
• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Whats inside a router
• Input ports
• Switching ports
• Output ports
• Queuing

• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

8
IP Addressing introduction
223.1.1.1

• IP address 32-bit identifier for host, router
  interface
• interface connection between host/router and
  physical link
• routers typically have multiple interfaces
• host typically has one interface
• IP addresses associated with each interface

223.1.2.9
223.1.1.4
223.1.1.3
223.1.1.1 11011111 00000001 00000001 00000001
223
1
1
1
9
Subnets
223.1.1.1

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

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
subnet
223.1.3.2
223.1.3.1
network consisting of 3 subnets
10
Subnets

• Recipe
• To determine the subnets, detach each interface
  from its host or router, creating islands of
  isolated networks. Each isolated network is
  called a subnet.

Subnet mask /24
11
Subnets
223.1.1.2

• How many?

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
223.1.3.27
223.1.2.1
223.1.2.2
223.1.3.2
223.1.3.1
12
IP addressing CIDR

• CIDR Classless InterDomain Routing
• subnet portion of address of arbitrary length
• address format a.b.c.d/x, where x is bits in
  subnet portion of address

13
IP addresses how to get one?

• Q How does host get IP address?
• hard-coded by system admin in a file
• Wintel control-panel-gtnetwork-gtconfiguration-gttcp
  /ip-gtproperties
• UNIX /etc/rc.config
• DHCP Dynamic Host Configuration Protocol
  dynamically get address from as server
• plug-and-play
• (more in next chapter)

14
IP addresses how to get one?

• Q How does network get subnet part of IP addr?
• A gets allocated portion of its provider ISPs
  address space

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
15
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
16
Hierarchical addressing more specific routes
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
17
IP addressing the last word...

• Q How does an ISP get block of addresses?
• A ICANN Internet Corporation for Assigned
• Names and Numbers
• allocates addresses
• manages DNS
• assigns domain names, resolves disputes

18
Private Addresses

• 10.0.0.0/8
• 172.16.0.0-172.31.0.0/12
• 192.168.0.0/16

19
Network Layer

• Introduction
• Virtual circuit and datagram networks
• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Whats inside a router
• Input ports
• Switching ports
• Output ports
• Queuing

• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

20
ICMP Internet Control Message Protocol

• used by hosts routers to communicate
  network-level information
• error reporting unreachable host, network, port,
  protocol
• echo request/reply (used by ping)
• network-layer above IP
• ICMP msgs carried in IP datagrams
• ICMP message type, code plus first 8 bytes of IP
  datagram causing error

Type Code description 0 0 echo
reply (ping) 3 0 dest. network
unreachable 3 1 dest host
unreachable 3 2 dest protocol
unreachable 3 3 dest port
unreachable 3 6 dest network
unknown 3 7 dest host unknown 4
0 source quench (congestion
control - not used) 8 0
echo request (ping) 9 0 route
advertisement 10 0 router
discovery 11 0 TTL expired 12 0
bad IP header
21
Traceroute and ICMP

• Source sends series of UDP segments to dest
• First has TTL 1
• Second has TTL2, etc.
• Unlikely port number
• When nth datagram arrives to nth router
• Router discards datagram
• And sends to source an ICMP message (type 11,
  code 0)
• Message includes name of router IP address

• When ICMP message arrives, source calculates RTT
• Traceroute does this 3 times
• Stopping criterion
• UDP segment eventually arrives at destination
  host
• Destination returns ICMP host unreachable
  packet (type 3, code 3)
• When source gets this ICMP, stops.

22
Network Layer

• Introduction
• Virtual circuit and datagram networks
• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Whats inside a router
• Input ports
• Switching ports
• Output ports
• Queuing

• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

23
IPv6

• Initial motivation 32-bit address space soon to
  be completely allocated.
• Additional motivation
• header format helps speed processing/forwarding
• header changes to facilitate QoS
• IPv6 datagram format
• fixed-length 40 byte header
• no fragmentation allowed

24
IPv6 Header (Cont)
Priority identify priority among datagrams in
flow Flow Label identify datagrams in same
flow. (concept offlow
not well defined). Next header identify upper
layer protocol for data
25
Other Changes from IPv4

• Checksum removed entirely to reduce processing
  time at each hop
• Options allowed, but outside of header,
  indicated by Next Header field
• ICMPv6 new version of ICMP
• additional message types, e.g. Packet Too Big
• multicast group management functions

26
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?
• Tunneling IPv6 carried as payload in IPv4
  datagram among IPv4 routers

27
Tunneling
28
Tunneling
tunnel
Logical view
IPv6
IPv6
IPv6
IPv6
Physical view
IPv6
IPv6
IPv6
IPv6
IPv4
IPv4
A-to-B IPv6
E-to-F IPv6
B-to-C IPv6 inside IPv4
B-to-C IPv6 inside IPv4
29
Network Layer

• Introduction
• Virtual circuit and datagram networks
• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Whats inside a router
• Input ports
• Switching ports
• Output ports
• Queuing

• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

30
Intra-AS Routing

• Also known as Interior Gateway Protocols (IGP)
• Most common Intra-AS routing protocols
• RIP Routing Information Protocol
• OSPF Open Shortest Path First
• IGRP Interior Gateway Routing Protocol (Cisco
  proprietary)

31
Network Layer

• Introduction
• Virtual circuit and datagram networks
• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Whats inside a router
• Input ports
• Switching ports
• Output ports
• Queuing

• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

32
RIP ( Routing Information Protocol)

• Distance vector algorithm
• Included in BSD-UNIX Distribution in 1982
• Distance metric of hops (max 15 hops)

From router A to subsets
33
RIP advertisements

• Distance vectors exchanged among neighbors every
  30 sec via Response Message (also called
  advertisement)
• Each advertisement list of up to 25 destination
  nets within AS

34
RIP Example
z
w
x
y
A
D
B
C
Destination Network Next Router Num. of
hops to dest. w A 2 y B 2
z B 7 x -- 1 . . ....
Routing table in D
35
RIP Example
Dest Next hops w - 1 x -
1 z C 4 . ...
Advertisement from A to D
Destination Network Next Router Num. of
hops to dest. w A 2 y B 2 z B
A 7 5 x -- 1 . . ....
Routing table in D
36
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)

37
RIP Table processing

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

Transprt (UDP)
Transprt (UDP)
network forwarding (IP) table
network (IP)
forwarding table
link
link
physical
physical
38
Network Layer

• Introduction
• Virtual circuit and datagram networks
• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Whats inside a router
• Input ports
• Switching ports
• Output ports
• Queuing

• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

39
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)
• Carried in OSPF messages directly over IP (rather
  than TCP or UDP

40
OSPF advanced features (not in RIP)

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

41
Hierarchical OSPF
42
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.

43
Network Layer

• Introduction
• Virtual circuit and datagram networks
• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Whats inside a router
• Input ports
• Switching ports
• Output ports
• Queuing

• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

44
Internet inter-AS routing BGP

• BGP (Border Gateway Protocol) the de facto
  standard
• BGP provides each AS a means to
• Obtain subnet reachability information from
  neighboring ASs.
• Propagate the reachability information to all
  routers internal to the AS.
• Determine good routes to subnets based on
  reachability information and policy.
• Allows a subnet to advertise its existence to
  rest of the Internet I am here

45
BGP basics

• Pairs of routers (BGP peers) exchange routing
  info over semi-permanent TCP conctns BGP
  sessions
• Note that BGP sessions do not correspond to
  physical links.
• When AS2 advertises a prefix to AS1, AS2 is
  promising it will forward any datagrams destined
  to that prefix towards the prefix.
• AS2 can aggregate prefixes in its advertisement

46
eBGP and iBGP

• eBGP is BGP between two ASes
• Advertises prefixes to other ASes
• Implements routing policy, (whether or not
  re-advertise prefixes received from BGP peers)
• eBGP peers are directly connected
• iBGP
• Advertises some or all prefixes learnt from eBGP
• Advertises ISPs customer prefixes (For example
  a network that is recently allocated to some
  customer)
• These are not directly connected but form a fully
  connected logical graph

47
Distributing reachability info

• With eBGP session between 3a and 1c, AS3 sends
  prefix reachability info to AS1.
• 1c can then use iBGP do distribute this new
  prefix reach info to all routers in AS1
• 1b can then re-advertise the new reach info to
  AS2 over the 1b-to-2a eBGP session
• When router learns about a new prefix, it creates
  an entry for the prefix in its forwarding table.

48
Path attributes BGP routes

• When advertising a prefix, advert includes BGP
  attributes.
• prefix attributes route
• Two important attributes
• AS-PATH contains the ASs through which the
  advert for the prefix passed AS 67 AS 17
• NEXT-HOP Indicates the specific internal-AS
  router to next-hop AS. (There may be multiple
  links from current AS to next-hop-AS.)
• When gateway router receives route advert, uses
  import policy to accept/decline.

49
eBGP Next Hop
50
BGP route selection

• Router may learn about more than 1 route to some
  prefix. Router must select route.
• Elimination rules
• Local preference value attribute policy decision
• Shortest AS-PATH
• Closest NEXT-HOP router hot potato routing
• Additional criteria

51
BGP messages

• 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

52
BGP routing policy

• A,B,C are provider networks
• X,W,Y are customer (of provider networks)
• X is dual-homed attached to two networks
• X does not want to route from B via X to C
• .. so X will not advertise to B a route to C

53
BGP routing policy (2)

• A advertises to B the path AW
• B advertises to X the path BAW
• Should B advertise to C the path BAW?
• No way! B gets no revenue for routing CBAW
  since neither W nor C are Bs customers
• B wants to force C to route to w via A
• B wants to route only to/from its customers!

54
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

55
Network Layer

• Introduction
• Virtual circuit and datagram networks
• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Whats inside a router
• Input ports
• Switching ports
• Output ports
• Queuing

• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

56
Broadcast Routing

• Deliver packets from source to all other nodes
• Source duplication is inefficient

• Source duplication how does source determine
  recipient addresses?

57
In-network duplication

• Flooding when node receives brdcst pckt, sends
  copy to all neighbors
• Problems cycles broadcast storm
• Controlled flooding node only brdcsts pkt if it
  hasnt brdcst same packet before
• Node keeps track of pckt ids already brdcsted
• Or reverse path forwarding (RPF) only forward
  pckt if it arrived on shortest path between node
  and source
• Spanning tree
• No redundant packets received by any node

58
Spanning Tree

• First construct a spanning tree
• Nodes forward copies only along spanning tree

59
Spanning Tree Creation

• Center node
• Each node sends unicast join message to center
  node
• Message forwarded until it arrives at a node
  already belonging to spanning tree

3
4
2
5
1

1. Stepwise construction of spanning tree

(b) Constructed spanning tree
60
Multicast Routing Problem Statement

• Goal find a tree (or trees) connecting routers
  having local mcast group members
• tree not all paths between routers used
• source-based different tree from each sender to
  rcvrs
• shared-tree same tree used by all group members

Shared tree
61
Approaches for building mcast trees

• Approaches
• source-based tree one tree per source
• shortest path trees
• reverse path forwarding
• group-shared tree group uses one tree
• minimal spanning (Steiner)
• center-based trees

we first look at basic approaches, then specific
protocols adopting these approaches
62
Shortest Path Tree

• mcast forwarding tree tree of shortest path
  routes from source to all receivers
• Dijkstras algorithm

S source
LEGEND
R1
R4
router with attached group member
R2
router with no attached group member
R5
link used for forwarding, i indicates order
link added by algorithm
R3
R7
R6
63
Reverse Path Forwarding

• rely on routers knowledge of unicast shortest
  path from it to sender
• each router has simple forwarding behavior

• if (mcast datagram received on incoming link on
  shortest path back to center)
• then flood datagram onto all outgoing links
• else ignore datagram

64
Reverse Path Forwarding example
S source
LEGEND
R1
R4
router with attached group member
R2
router with no attached group member
R5
datagram will be forwarded
R3
R7
R6
datagram will not be forwarded

• result is a source-specific reverse SPT
• may be a bad choice with asymmetric links

65
Reverse Path Forwarding pruning

• forwarding tree contains subtrees with no mcast
  group members
• no need to forward datagrams down subtree
• prune msgs sent upstream by router with no
  downstream group members

LEGEND
S source
R1
router with attached group member
R4
router with no attached group member
R2
P
P
R5
prune message
links with multicast forwarding
P
R3
R7
R6
66
Shared-Tree Steiner Tree

• Steiner Tree minimum cost tree connecting all
  routers with attached group members
• problem is NP-complete
• excellent heuristics exists
• not used in practice
• computational complexity
• information about entire network needed
• monolithic rerun whenever a router needs to
  join/leave

67
Center-based trees

• single delivery tree shared by all
• one router identified as center of tree
• to join
• edge router sends unicast join-msg addressed to
  center router
• join-msg processed by intermediate routers and
  forwarded towards center
• join-msg either hits existing tree branch for
  this center, or arrives at center
• path taken by join-msg becomes new branch of tree
  for this router

68
Center-based trees an example
Suppose R6 chosen as center
LEGEND
R1
router with attached group member
R4
3
router with no attached group member
R2
2
1
R5
path order in which join messages generated
R3
1
R7
R6