Chapter 4: Network Layer

1
Chapter 4 Network Layer

• 4. 1 Introduction
• 4.2 Virtual circuit and datagram networks
• 4.3 Whats inside a router
• 4.4 IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6

• 4.5 Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• 4.6 Routing in the Internet
• RIP
• OSPF
• BGP
• 4.7 Broadcast and multicast routing

2
Recall Subnets
223.1.1.2
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
3
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

Subnet part or CIDR-block
host part
11001000 00010111 00010000 00000000
200.23.16.0/23
4
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
5
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
ISP1
Border Router
Internet
Organization 7
Send me anything with addresses beginning
199.31.0.0/16
ISP2
This way, the whole 32 bit address does not need
to be examined
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Hierarchical addressing more specific routes
ISP2 has a more specific route to Organization 1
Organization 0
Send me anything with addresses beginning
200.23.16.0/20
Organization 2
ISP1
Border Router
Internet
Organization 7
Send me anything with addresses beginning
199.31.0.0/16 or 200.23.18.0/23
ISP2
Organization 1
7
Longest prefix matching
Border Router Forwarding Table
Prefix Match Link
Interface 200.23.16.0/20 0
200.23.18.0/23 1 199.31.0.0/16
1 otherwise
2
If a packet with destination address 200.23.18.12
arrives at the boarder router, then is it
forwarding to interface 0 or 1? Since interface 1
has a longer match, it goes to interface 1
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A Problem with Longest Match and subnetting
In order to improve reliability, organization 7
has a backup link with ISP1. This way, if ISP1
has problems or ISP1s provider has problems,
then organization 7 is still reachable. Will this
work?
Organization 0
Organization 1
Send me anything with addresses beginning
Organization 2
ISP1
Border Router
Internet
Organization 7
Send me anything with addresses beginning ..
ISP2
9
Hierarchical Routing

• Our routing study thus far - idealization
• all routers identical
• network flat
• not true in practice

• administrative autonomy
• internet network of networks
• each network admin wants to control routing in
  its own network
• ATT does not want sprint to know what their
  topology is
• Trade secret
• Improves security
• ATT wants to select a routing protocol and
  parameters without getting sprints permission

• scale with 200 million destinations
• cant store all dests in routing tables!
• Memory for address table must be very fast
• How fast? How long can an address lookup take on
  a 10GBit interface?
• routing table exchange would swamp links!
• There are 1 million links
• If link state was exchanged every 10 seconds and
  each link state is 20B, then each router receives
  and processes 160Mbps in link announcements
• But, perhaps, only changes in link state could be
  distributed.

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Hierarchical Routing

• aggregate routers into regions, autonomous
  systems (AS)
• Single administrative domain
• Routers in the same AS run same routing protocol
• intra-AS routing protocol
• routers in different AS can run different
  intra-AS routing protocol
• An ISP may be made of 1 or more ASs
• ATT-USA 1 AS and ATT-Europe is another
• Some stub networks are an AS
• UD is an AS
• Some companies have routers but are not Ass
• ASs have their own number, assigned by ICANN
• There are 50K ASs

• Gateway router
• Direct link to router in another AS
• Gateway routers run a common inter-networking
  routing protocol
• For inter-AS routing, the destinations are always
  ASs
• Actually, destinations are always ASs. But for
  inter-AS routing, it does not make much sense for
  a destination to be a single end-host.

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Simple example
Connections to other ASs and the rest of the
Internet
AS2
Service provider of AS1 (e.g., AS1UD and
AS2cogent)
E
(Recall that ASs (ISPs) sometiems meet at NAPs.
E.g., google MAE-East) An AS could also meet its
provider at a POP.
The rest of the internet
Stub network (at the edge of the network)
These tables are made with RIP, OSPF, ISIS, etc
Forwarding table
B
Prefix
Next hop
10.1.1.0/23
C
Forwarding table
Prefix
10.1.2.0/22
A
Next hop
10.1.1.0/23
C
AS1
10.1.2.0/22
A
C
10.1.1.0/24
Forwarding table
Prefix
Next hop
10.1.1.0/23
A
C
10.1.1.1/24
10.1.2.0/22
A
10.1.4.0/22
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Q How can routers in AS1 know where to send pkts
with destination not in AS1? A Easy, if a pkt is
for an unknown address, send it to
B. Specifically, B advertises a link to prefix
0.0.0.0/0 This is called a default route, and it
can be statically set (no need for any routing
protocol beside OSPF)
AS2
Service provider of AS1 (e.g., AS1UD and
AS2cogent)
E
The rest of the internet
Stub network (at the edge of the network)
These tables are made with RIP, OSPF, ISIS, etc
Forwarding table
B
Prefix
Next hop
10.1.1.0/23
C
Forwarding table
Prefix
10.1.2.0/22
A
Next hop
10.1.1.0/23
C
0.0.0.0/32
E
AS1
10.1.2.0/22
A
A
0.0.0.0/32
C
10.1.1.0/24
Forwarding table
Prefix
Next hop
10.1.1.0/23
A
C
10.1.1.1/24
10.1.2.0/22
A
10.1.4.0/22
0.0.0.0/32
A
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We need to put prefixes 1.1.0.0/16, 1.2.0.0/16,
2.2.0.0/16 in the forwarding tables Specifically,
B should announce to A that is can reach
1.1.0.0/16 and 1.2.0.0/16, and D should announce
it can reach 2.2.0.0/16

• How to get there?
• B must learn from E that 1.1.0.0/16 and
  1.2.0.0/16 are reachable through E
• A must learn that 2.2.0.0/16 is reachable through
  D
• B and A must distribute this information
  throughout AS1

But 1 and 2 need a exterior inter-networking
routing protocol 3 need interior inter-networking
routing protocol EBGP and IBGP border gateway
routing protocol can accomplish this
1.2.0.0/16
AS2
1.1.0.0/16
E
These tables are made with RIP, OSPF, ISIS, etc
Forwarding table
B
Prefix
Next hop
10.1.1.0/23
C
Forwarding table
A
Prefix
10.1.2.0/22
Next hop
10.1.1.0/23
C
AS1
A
10.1.2.0/22
C
10.1.1.0/24
Forwarding table
Prefix
Next hop
10.1.1.0/23
A
C
10.1.1.1/24
A
10.1.2.0/22
10.1.4.0/22
AS3
D
2.2.0.0/16
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Interconnected ASes

• forwarding table configured by both intra- and
  inter-AS routing algorithm
• intra-AS sets entries for internal dests
• inter-AS intra-As sets entries for external
  dests

15
Example Setting forwarding table in router 1d

• suppose AS1 learns (via inter-AS protocol) that
  subnet x reachable via AS3 (gateway 1c) but not
  via AS2.
• inter-AS protocol propagates reachability info to
  all internal routers.
• router 1d determines from intra-AS routing info
  that its interface I is on the least cost path
  to 1c.
• installs forwarding table entry (x,I)

x
3a
3b
2a
AS3
AS2
1a
AS1
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Example Choosing among multiple ASes

• now suppose AS1 learns from inter-AS protocol
  that subnet x is reachable from AS3 and from AS2.
• to configure forwarding table, router 1d must
  determine towards which gateway it should forward
  packets for dest x.
• this is also job of inter-AS routing protocol!
• If both gateways are equivalent, then the
  intra-AS routing protocol will route packets to
  the best gateway
• This is called hot potato routing send packet
  towards closest of two routers.

x
17
Hot Potato Routing
128.4.0.0/16
AS1
A
B
AS2
Pkt arrives with dest in 124.4.0.0/16
AS2 could give send the pkt to gateway B hot
potato routing. But AS1 would prefer AS2 to carry
its own traffic. So AS1 might require that AS2
gives higher priority to gateway A. In which
case, AS1 could inject traffic into AS2 with
destination in 128.4.0.0/16 at gateway B
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Example Choosing among multiple ASes

• now suppose AS1 learns from inter-AS protocol
  that subnet x is reachable from AS3 and from AS2.
• to configure forwarding table, router 1d must
  determine towards which gateway it should forward
  packets for dest x.
• this is also job of inter-AS routing protocol!
• hot potato routing send packet towards closest
  of two routers.

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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 reachability information to all
  AS-internal routers.
• Determine good routes to subnets based on
  reachability information and policy.
• allows subnet to advertise its existence to rest
  of Internet I am here

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BGP basics

• pairs of routers (BGP peers) exchange routing
  info over semi-permanent TCP connections BGP
  sessions
• BGP sessions need not correspond to physical
  links.
• when AS2 advertises a prefix to AS1
• AS2 promises it will forward datagrams towards
  that prefix.
• AS2 can aggregate prefixes in its advertisement
• But this can cause problems when some prefixes
  have backup links

eBGP session
iBGP session
3a
3b
2a
AS3
AS2
1a
AS1
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Distributing reachability info

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

eBGP session
iBGP session
3a
3b
2a
AS3
AS2
1a
AS1
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Path attributes BGP routes

• advertised prefix includes BGP attributes.
• prefix attributes route
• two important attributes
• AS-PATH contains ASs through which prefix
  advertisement has passed e.g, AS 67, AS 17
• NEXT-HOP indicates specific internal-AS router
  to next-hop AS. (may be multiple links from
  current AS to next-hop-AS)
• when gateway router receives route advertisement,
  uses import policy to accept/decline.

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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

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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

25
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

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BGP routing policy (2)

• A advertises path AW to B
• B advertises path BAW to X
• Should B advertise path BAW to C?
• 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!

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BGP route processing

• BGP advertises and withdraws paths with the
  UPDATE message
• UPDATE has three fields
• Router to withdraw
• Attributes of routes to prefixes in NLRI
• NLRI
• The NLRI is a list of prefixes that the list of
  attributes applies to. If two prefixes have
  different attributes, then these two prefixes
  need to be announced with different UPDATE
  messages.
• In OSPF each path is a list of routes and a total
  cost (two attributes). In BGP, routes have many
  attributes, cost (in AS hops) is but one.

configuration
configuration
routing decision
routing table
input policy engine
output policy engine
from peers
to peers
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RIBs

• Routing information base (RIB) a list of routes
  (attributes and all)
• Adj-RIB-In RIB learned from neighbor (many of
  these)
• Adj-RIB-Out RIB to be sent to neighbor (many of
  these)
• Loc-RIB RIB for local use (only one of these)

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Sample routing environment
AS3

• deny 0/0 from AS1
• Give 192.213.1.0/24 form AS1 better preference
• Accept other routes

AS1

• Do not propagate 0/0
• Do not send 193.214.10.0/24 to AS4
• Give 192.213.1.0/24 with metric 10 to AS3

• 193.214.10.0/24 path(AS5, AS2)
• 192.213.1.0/24 path(AS5, AS1) metric10
• 172.16.10.0/24 path(AS5)

192.213.1.0/24 0/0
input policy engine
decision process
routes
output policy engine

• 172.16.10.0/24 path(AS5)
• 192.213.1.0/24 path(AS5 AS1)

• Use 0/0 from AS2
• Use 192.213.1.0/24 from AS1
• Use 193.214.10.0/24 from AS2
• Use 172.16.10.0/24 from AS5 (this AS)

AS4
AS2
192.213.1.0/24 193.214.10.0/24 0/0
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Fun with BGP

• Routeviews.org collects and archives BGP
  announcements
• One way to use routeviews is with dig
• At the linux prompt
• dig txt 4.128.aspath.routeviews.org
• Outputs various stuff and
• Answer section
• 4.128.aspath.routeviews.org 600 IN TXT 5056 1238
  174 34 128.4.0.0 16
• Syntax ASPath Prefix prefix length
• Now use whois -h whois.arin.net "a ASXX" to learn
  about ASs where XX is an AS number. E.g., whois
  -h whois.arin.net "a AS34" gives information
  about AS34
• Try with some other AS

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Check out a collection of path announcements

• Open bgp030408p39.Partial
• http//www.eecis.udel.edu/bohacek/Classes/ELEG651
  Spring2008/bgp030508p39.Partial
• An old (2003) partial list of BGP announcements
  received by several routers
• Check which ASs peer with UD (ASN 34)

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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