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TCP/IP Essentials
A Lab-Based Approach
Chapter 4
Static and Dynamic
Routing
Shivendra Panwar, Shiwen Mao
Jeong-dong Ryoo, and Yihan Li
Routing


Transfer packets from a source to a destination using
network layer protocol information.
Two activities:



Determine optimal routing paths
Transport packets through an internetwork
Routing table



records optimal routes.
Is consulted when a forwarding decision is to be made.
Can be set manually, updated by an ICMP message received, or
by using dynamic routing protocols.
Panwar, Mao, Ryoo, Li: TCP/IP Essentials
2
Next-hop Routing


Direct delivery: send datagram directly when the
source and the destination are on the same
network.
Indirect delivery: not on the same network
 Send
datagram to a router.
 Consult the routing table to determine the next hop
router.
 Only one hop on the path is listed in the routing table.
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Routing Table

Flags:


U: route is up
G: route is to a gateway (router); if flag is not set, destination is directly
connected
 H: route is to a host, I.e., destination address is the complete host
address; if flag is not set, route is to a network and destination address
is netID or subnetID
 D: route created by redirect
 M: route modified by redirect




Destination IP address
The IP address of the next-hop router, or of a directly connected
network.
The host’s network interface
For each IP packet, there is one routing table lookup.
1.
2.
3.
Find matching host address
Find matching network address
Find default entry
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Statically Setting IP Routing Tables

There are several ways for setting IP routing tables
without a routing protocol (Static Routing)
 Automatic
creation of entry during initialization of a
local interface (with ifconfig)
 During bootstrap with route command
 Via ICMP redirect messages
 Via ICMP router advertisement/router discovery
messages
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ICMP Redirect


Based on routing data in host, it does an arp for router 1 and sends
packet to router 1
When router 1 detects that an IP datagram should have gone to a
different router, the router:



forwards the IP datagram to the correct router
sends an ICMP redirect message to the host
Host uses ICMP message to update its routing table
(1) IP datagram
(3) ICMP Redirect
(2) IP datagram
Router
1 Essentials
Panwar, Mao, Ryoo,
Li: TCP/IP
Router 2
6
ICMP Router Discovery




Configure the default route for a host when it bootstraps.
After bootstrapping a host broadcasts an ICMP router solicitation
message.
In response, routers send an ICMP router advertisement message
Also, routers periodically broadcast ICMP router advertisement
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Dynamic Routing

Dynamic routing is used to
 eliminate
loops in paths, and
 react to changes in the network topology.

Autonomous System
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Autonomous Systems



An autonomous system is a region of the Internet that is
administered by a single entity.
Examples of autonomous regions are:
 Poly’s campus network
 MCI’s backbone network
 Regional Internet Service Provider
Routing is done differently within an autonomous system
(intradomain routing) and between autonomous system
(interdomain routing).
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Autonomous Systems (AS)
Ethernet
Router
Autonomous
System 1
Ethernet
Ethernet
Router
Router
Router
Ethernet
Router
Autonomous
System 2
Ethernet
Router
Ethernet
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Interdomain and Intradomain
Routing




Intradomain Routing
Routing within an AS
Ignores the Internet outside the
AS
Protocols for Intradomain
routing are also called Interior
Gateway Protocols or IGP’s.
Popular protocols are
 RIP (simple, old)
 OSPF (better)
Interdomain Routing





Routing between AS’s
Assumes that the Internet
consists of a collection of
interconnected AS’s
Normally, there is one dedicated
router in each AS that handles
interdomain traffic.
Protocols for interdomain routing
are also called Exterior
Gateway Protocols or EGP’s.
Routing protocols:

EGP
 BGP (more recent)
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Distance Vector Routing



Each node knows the distance (=cost) to its
directly connected neighbors
A node sends a list to its neighbors with the
current distances to all nodes.
If all nodes update their distances, the routing
tables eventually converge
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Link State Routing



Each node knows the distance to its neighbors
The distance information (=link state) is
broadcast to all nodes in the network
Each node calculates the routing tables
independently
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Routing Algorithms

Goal: Given a network where each link between two
nodes i and j is assigned a cost. Find the path with the
least cost between nodes s and d.

Parameters:
dij
Cost of link between node i and node j;
dij = , if nodes i and j are not connected;
dii = 0.
N
Set of nodes.
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Example
5
2
3
3
5
2
1
3
1
2
1
6
2
4
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1
d56 = 2
5
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Distance Vector

Each node maintains two tables:
 Distance Table: Cost to each node via each outgoing
link.
 Routing Table: Minimum cost to each node and next
hop node.

Nodes exchange messages that contain information on
the cost of a route
Reception of messages triggers recalculation of routing
table

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Discussion of Distance Vector
Routing



Entries of routing tables can change while a
packet is being transmitted. This can lead to a
single datagram visiting the same node more
than once (Looping).
If the period for updating the routing tables is too
short, routing table entries are changed before
convergence (from the previous updates) is
achieved.
Example: ARPANET used a Distance Vector
algorithm with an update period of <1 sec. This
resulted in instability of routing.
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Link State Route Calculations
• Calculate shortest path for node s
Dijkstra’s Algorithm:
s
source node.
Dn
cost of the least-cost path from node s to node n
M = {s};
for each n  M
Dn = dsn;
while (M  all nodes) do
Find w  M for which Dw = min{Dj ; j  M};
Add w to M;
for each n  M
Dn = minw [ Dn, Dw + dwn ];
Update route;
enddo
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Example (at node 1)

Dijkstra’s algorithm
0
1
2
3
4
M
{1}
{1,4}
{1,4,2,5}
{1,4,2,5,3}
{1,4,2,5,3,6}
D1
0
0
0
0
0
Panwar, Mao, Ryoo, Li: TCP/IP Essentials
D2
2
2
2
2
2
D3
5
4
3
3
3
D4
1
1
1
1
1
D5
inf
2
2
2
2
D6
inf
inf
4
4
4
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Resulting Routing Tree
2
2
3
1
1
1
4
1
5
6
2
•The tree is translated into a routing table at node 1:
Destination
2
3
4
5
6
Panwar, Mao, Ryoo, Li: TCP/IP Essentials
Next Hop
2
4
4
4
4
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Link State Discussion




Each node requires complete topology
information.
Link state information must be flooded to all
nodes. Guaranteed to converge.
Each node must maintain a global database.
Convergence of the algorithm is guaranteed.
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RIP - Routing Information
Protocol





A simple intradomain protocol
Straightforward implementation of Distance Vector
Routing
Each router advertises its distance vector every 30
seconds (or whenever its routing table changes)
RIP always uses the hop-count as link metric. Maximum
hop count is 15, with “16” equal to “”.
Routes timeout after 3 minutes if they are not updated.
Route metric is set to  (16) and marked for deletion
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RIP Packet Format
RIP Message
IP header UDP header
1: request
2: reply
3, 4: unused
5: poll
6: poll entry
addr. family: 2 for IP
0
7 8
1516
Command Version
(1-6)
(1)
address family
31
Set to 00...0
Set to 00.00
32-bit address
20 bytes
long
IP address for which a
route is requested
Unused (Set to 00...0)
Unused (Set to 00...0)
metric (1-16)
Up to 24 more routes (each 20 bytes)
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Routing with RIP






This is the operation of RIP in routed. Dedicated port for RIP is
UDP port 520.
Initialization: Broadcast a request packet (command = 1,
metric=16; address family=0, metric=16) on the interfaces
requesting current routing tables from routers.
Request received: Routers that receive above request send their
entire routing table.
Response received: Update the routing table (see distance vector
algorithm).
Regular routing updates: Every 30 seconds, send all or part of the
routing tables to every neighbor.
Triggered Updates: Whenever the metric for a route changes, send
data that has changed.
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RIPv2
RIPv2 Message
IP header UDP header
Process ID of
routing daemon
Command Version
(1-6)
(=2)
address family
routing domain
route tag
32-bit address
Subnet Mask (32 bits)
Support of EGP
and BGP
Subnet Mask of IP address
(RIP version 1 is not aware of
subnet masks)
Next-Hop IP address (32 bits)
Metric (1-16)
Identifies next hop; value of 0 means packets
Identifies next hop: value of 0 means packets should be
should be sent to node sending this RIP message
sent to node sending this RIP message
Up to 24 more routes (each 20 bytes)
•RIPv2 also supports multicast and provides authentication
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OSPF

Open
 Developed

by IETF IGP working group, RFC2328
SPF
 Each
router floods link-state information through its
neighbors to other routers
 Based on the flooded link-state information, each
router maintains a complete link-state database
 Based on the link-state database, a routing table is
constructed using SPF (e.g., Dijkstra’s) algorithm

Runs over IP directly, protocol number 89
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Features of OSPF







Use flexible metrics instead of only hop count
Supports variable-length subnetting
Allows load balancing among equal-cost paths
Supports multiple routes; one for each IP type of
service (ToS)
Authenticates route exchanges
Quick convergence
Uses multicast rather than broadcast of its
messages to reduce network load
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Hierarchical OSPF

AS is organized as two-level hierarchy
 AS
is partitioned into self-contained areas
 Areas are interconnected by a backbone area
 Areas are identified by a 32-bit area ID
 0.0.0.0 is reserved for the backbone area

Four types of routers
 Internal
router, area border router, backbone router,
autonomous system boundary router (ASBR)
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OSPF AS organized into a 2-level
hierarchy
Within each area, border router
responsible for routing outside
the area
Backbone
area
contains
all area
border
routers
and
possibly
others
Panwar, Mao, Ryoo, Li: TCP/IP Essentials
(ASBR)
Exactly one area
is backbone area
29
OSPF packets

Five types of OSPF packets
 Hello(1)
 Database
description(2)
 Link-State Request(3)/Update(4)/Acknowledgement(5)

OSPF common header
Version
Type (1-5)
Packet Length
Router ID
Area ID
Checksum
Authentication Type
Authentication
Authentication
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OSPF common header fields







Version number: 2
Type: Type of OSPF packet
Packet length: in bytes, includes OSPF header
Router ID: 32-bit number assigned to each OSPF
running router – uniquely identifies router within AS
Area ID: any four-byte number (0.0.0.0 reserved for
backbone area)
Checksum: error detection
Three Authentication related fields: to authenticate OSPF
packets
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Hello packet
Network mask
Hello interval
Options
Priority
Dead interval
Designated router
Backup designated router
Neighbor 1
…
Neighbor n
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Hello packet fields







Network mask: Subnet mask of the interface the packet is sent on
Hello interval: Number of seconds between Hello packets
Options: optional capabilities supported by the router
Priority: of the router – used in election of designated router
Dead interval: Number of seconds before declaring a nonresponding
neighbor down
Designated router/Backup: Every broadcast network with at least
two routers has a designated router. This sends network LSAs for
the broadcast network. This field is 0.0.0.0 if there is no designated
router
Neighbors: Router ID of each neighbor from whom Hello packets
have recently been received
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Database description packet

Database description packet
Interface MTU
Options
Zero
I
M
M
S
Database description sequence number
LSA header(s)

LSA: Link State Advertisement
LSA header
Link-state age
Options
Link-state type
Link-state ID
Advertising router
Link-state sequence number
Link-state checksum
Panwar, Mao, Ryoo, Li: TCP/IP Essentials
Length
34
Database description packet
fields






Interface MTU: MTU of the associated interface
Options: optional capabilities supported by the router
I bit: Init bit set to 1 if this packet is the first in the
sequence of database description packets
M bit: More bit is set to 1 if more database description
packets are to follow
MS bit: Master/Slave bit
Database description seq. no.: identifies the packet
number sequentially so that a receiver can detect a
missing packet
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LSA header fields








Link-state age: time since LSA generation
Options: optional capabilities supported by the router
Link-state type: router LSA, network LSA, summary LSA for IP
networks, summary LSA for ASB routers, AS-external LSAs
Link-state ID: describes routing domain for the LSA; depends on
LSA type
Advertising router: router ID of the router that generated the LSA
Link-state sequence number: Numbers LSAs sequentially to identify
old and duplicate LSAs
Link-state checksum: entire contents of LSA except link-state age
Length: in bytes of LSA including LSA header
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Link-state Request/Update/
Acknowledgement

Link-state Request
Link-state type
Link-state ID
Advertising router
… (above 3 fields repeated for each link)

Link-state Update
Number of LSAs
LSA 1
…
LSA n

Link-state Acknowledgement
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Router LSA
32 bits
LSA header
0
V
E B
Type
0
#links
#TOS
per-link
fields
Metric
Link ID
Link Data
Repeat per-link fields for each link
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LSA fields





bit V (Virtual): 1 when router is a virtual link endpoint
bit E (External): 1 when router is an ASBR
bit B (Border): 1 when router is an area border router
# links: # of links described in this LSA
Per link:

Type:




Point-to-point connection to another router: 1
Connection to a transit network: 2
Connection to a stub network: 3
Virtual link: 4
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LSA fields contd.

Per-link:


#TOS: number of TOS metrics other than the required link metric
given to this link; if none, 0
Link ID: Identifies the type of node connected to this link





Link Data: depends upon type of link




If type 1, Neighboring router's Router ID
If type 2, IP address of Designated Router
If type 3, IP network/subnet number
If type 4, Neighboring router's Router ID
For links to stub networks: Network's IP address mask
For point-to-point links: the interface's MIB-II ifIndex value.
For the other link types: the router interface's IP address.
Metric: cost of using this link
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Link state updates

Each Link State Update packet carries a
set of new link state advertisements (LSAs)
one hop further away from their point of
origination. A single Link State Update
packet may contain the LSAs of several
routers. Each LSA is tagged with the ID of
the originating router and a checksum of
its link state contents.
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OSPF operations
Hello protocol
 Database synchronization
 Propagation of link-state information
 Building of routing table

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Hello Protocol
Hello packets are transmitted to all
interfaces periodically
 Discover neighbors, establish and
maintain neighbor adjacency relationships
 Elect Designated Router (DR) if there are
multiple routers in a broadcast network

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Database synchronization
Two neighboring routers exchange
database description packets to
synchronize their link-state databases.
 Database description includes only a list of
LSA headers. New or more up-to-date
LSAs will be requested later
 Packets sent by master are acknowledged
by slave

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Propagation of link-state information

Link-state request sent
 When
a router wants to update parts of its link-state
database

Link-state update sent
 When
a link state is requested, or
 When a link-state changes, or
 Periodically

Link-state acknowledgement sent in response to
a link-state update
 Link-state
updates retransmitted periodically until
acknowledged
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Flooding LSAs


A node receiving a link-state update selectively
first installs each LSA in the update into its LSA
database
Then it decides on which of its other links to
flood the LSA
 it
may decide not to flood an LSA out a particular
interface if there is a high probability that the attached
neighbors have already received the LSA.
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Building of routing table
Router S has knowledge of the entire area
topology (complete link-state database)
 Some algorithm such as Dijkstra’s is used
to generate shortest path tree, rooted at
router S
 Only the next hop will be used in the
routing table

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Example
AREA 1
192.1.2
N1
3
RT1
1
N3
192.1.1
N2
3
Backbone
area
RT2
Assume
that all RT3
interfaces have
3 as their host ID
RT4
18.10.0.6
1
RT3
192.1.3
8
2
N4 192.1.4
6
RT6
This is the link metric
- note NOT hostID
Construct Router LSA for RT3
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RT3’s router-LSA for Area 1









LS age = 0; true on origination
Options =;
LS type = 1; indicates router-LSA
Link State ID = 192.1.1.3; RT3's Router ID
Advertising Router = 192.1.1.3; RT3's Router ID
bit E = 0; not an AS boundary router
bit B = 1; area border router
#links = 2
Link to N3: [COMMENT]

Link ID = 192.1.1.4 ; IP address of Desig. Rtr.
Link Data = 192.1.1.3; RT3's IP interface to net
Type = 2; connects to transit network
# TOS metrics = 0

metric = 1



Panwar, Mao, Ryoo, Li: TCP/IP Essentials

Link to N4: [COMMENT]

Link ID = 192.1.4.0 ; IP Network number
Link Data = 0xffffff00; Network mask
Type = 3; connects to stub network
# TOS metrics = 0

metric = 2



49
RT3's router-LSA for the
backbone









LS age = 0; always true on origination
Options =
;
LS type = 1; indicates router-LSA
Link State ID = 192.1.1.3; RT3's router ID
Advertising Router = 192.1.1.3; RT3's router ID
bit E = 0; not an AS boundary router
bit B = 1; area border router
#links = 1
Link to RT6





Link ID = 18.10.0.6; Neighbor's Router ID
Link Data = 0.0.0.3; MIB-II ifIndex of P-P link
Type = 1; connects to router
# TOS metrics = 0
metric = 8
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Classless Interdomain Routing
(CIDR)
Routing table are getting longer with the
exponential growth of the Internet.
 CIDR uses supernetting to summarize
multiple routing entries into a smaller
number of entries.
 CIDR is supported in almost all new
routing.

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CIDR-type Address

IP address in CIDR


Not classified into classes
Two components of an IP address



Network prefix ranging from 13 to 27 bits
Host ID using the remaining bits
Slashed-notation
A dotted-decimal IP address + / + Number of bits used for the network prefix



Network address are assigned in a hierarchical manner.
In the core network, routing entries for networks with the
same higher level prefix can be summarized into one entry.
The longest-prefix-matching rule is used in table lookups.
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52
Multi-Protocol Label Switching
(MPLS)


MPLS is a virtual-circuit packet switching technology
overlaid on the datagram packet-switched Internet.
Attach short labels to network packets that describe how
to forward them through the network.




Located after the layer two header and before the IP header.
Used to make forwarding decisions
Independent of any routing protocol.
provides a mechanism for engineering network traffic
patterns.
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MPLS Traffic Engineering (TE)


OSPF always chooses the shortest path, which
may be over used and congested.
MPLS TE
 Label Switched Path (LSP)
 Allows path selection without
adjusting link OSPF
cost, so that flows can be moved from congested links
to alternate links with larger costs.
 Creates paths for each traffic type.
 Allows network service provider to set up IP tunnels
for virtual private networks (VPNs).
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Traceroute



Helps determine all the routers in an end-to-end path
Uses the Time-to-Live (TTL) field in the IP header and
the ICMP protocol.
Traceroute operation:
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