1. Static and Dynamic Routing
Chapter 4
Static and Dynamic Routing

2. 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.

3. 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.

4. Routing Table Flags: Destination IP address
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.
Find matching host address
Find matching network address
Find default entry

5. 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

6. 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

7. 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

8. Dynamic Routing Dynamic routing is used to Autonomous System
eliminate loops in paths, and
react to changes in the network topology.
Autonomous System

9. 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).

10. Autonomous Systems (AS)

11. Interdomain and 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)

12. 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

13. 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

14. 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.

15. Example

16. 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

17. 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.

18. 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;
Dn = minw [ Dn, Dw + dwn ];
Update route;
enddo

19. Example (at node 1)
Dijkstra’s algorithm

20. Resulting Routing Tree
The tree is translated into a routing table at node 1: Destination Next Hop

21. 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.

22. 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

23. RIP Packet Format

24. 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.

25. RIPv2
RIPv2 also supports multicast and provides authentication

26. OSPF Open SPF Runs over IP directly, protocol number 89
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

27. 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

28. 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
is reserved for the backbone area
Four types of routers
Internal router, area border router, backbone router, autonomous system boundary router (ASBR)

29. OSPF AS organized into a 2-level hierarchy
(ASBR)
Within each area, border router
responsible for routing outside
the area
Exactly one area
is backbone area
Backbone area contains all area border routers and possibly others

30. OSPF packets Five types of OSPF packets OSPF common header 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

31. 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 ( reserved for backbone area)
Checksum: error detection
Three Authentication related fields: to authenticate OSPF packets

32. Backup designated router
Hello packet
Network mask
Hello interval
Options
Priority
Dead interval
Designated router
Backup designated router
Neighbor 1 …
Neighbor n

33. 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 if there is no designated router
Neighbors: Router ID of each neighbor from whom Hello packets have recently been received

34. Database description packet
Interface MTU
Options
Zero I M S
Database description sequence number
LSA header(s)
LSA header
LSA: Link State Advertisement
Link-state age
Options
Link-state type
Link-state ID
Advertising router
Link-state sequence number
Link-state checksum
Length

35. 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

36. 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

37. Link-state Request/Update/ Acknowledgement
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

38. Router LSA 32 bits LSA header V E B #links Type #TOS Metric per-linkV E B
#links
Type
#TOS
Metric
per-link
fields
Link ID
Link Data
Repeat per-link fields for each link

39. 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

40. 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
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
Link Data: depends upon type of link
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

41. 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.

42. OSPF operations Hello protocol Database synchronization
Propagation of link-state information
Building of routing table

43. 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

44. 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

45. 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

46. 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.

47. 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

48. Example Assume that all RT3 interfaces have 3 as their host ID AREA 1
Backbone
area 3
RT1 N1 1 N3
RT4 3
RT2 N2 1 8 6
RT3
RT6
This is the link metric
- note NOT hostID 2 N
Construct Router LSA for RT3

49. RT3’s router-LSA for Area 1
Link to N4: [COMMENT]
Link ID = ; IP Network number
Link Data = 0xffffff00; Network mask
Type = 3; connects to stub network
# TOS metrics = 0
metric = 2
LS age = 0; true on origination
Options =;
LS type = 1; indicates router-LSA
Link State ID = ; RT3's Router ID
Advertising Router = ; 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 = ; IP address of Desig. Rtr.
Link Data = ; RT3's IP interface to net
Type = 2; connects to transit network
# TOS metrics = 0
metric = 1

50. RT3's router-LSA for the backbone
LS age = 0; always true on origination
Options = ;
LS type = 1; indicates router-LSA
Link State ID = ; RT3's router ID
Advertising Router = ; 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 = ; Neighbor's Router ID
Link Data = ; MIB-II ifIndex of P-P link
Type = 1; connects to router
# TOS metrics = 0
metric = 8

51. 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.

52. 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.

53. 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.

54. 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).

55. 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: