Transcript l4-2 - Heyook Lab

Lecture 4: Dynamic routing protocols
Tuesday:
1. Overview of router architecture
2. RIP
Thursday:
1. OSPF, BGP
Review of RIP
• A distance vector routing protocol
2
A router updates its distance vectors using
bellman-ford equation
Bellman-Ford Equation
Define
dx(y) := cost of the least-cost path from x to y
Then
• dx(y) = minv{c(x,v) + dv(y) }, where min is taken over all
neighbors of node x
3
Distance vector algorithm: initialization
• Let Dx(y) be the estimate of least cost from x to y
• Initialization:
– Each node x knows the cost to each neighbor: c(x,v). For
each neighbor v of x, Dx(v) = c(x,v)
– Dx(y) to other nodes are initialized as infinity.
• Each node x maintains a distance vector (DV):
– Dx = [Dx(y): y 2 N ]
4
Distance vector algorithm: updates
• Each node x sends its distance vector to its neighbors,
either periodically, or triggered by a change in its DV.
• When a node x receives a new DV estimate from a neighbor
v, it updates its own DV using B-F equation:
– If c(x,v) + Dv(y) < Dx(y) then
• Dx(y) = c(x,v) + Dv(y)
• Sets the next hop to reach the destination y to the
neighbor v
• Notify neighbors of the change
• The estimate Dx(y) will converge to the actual least cost dx(y)
5
The Count-to-Infinity Problem
1
A
A's Routing Table
to
C
via
(next hop)
C
B's Routing Table
cost
B
1
B
via
to
2
(next hop)
cost
C
C
1
C
-
1
A
3
-
1
now link B-C goes down
C
B
2
C
C
-
C
1
C
C
2
B
C
1
C
4
C
1
3
C
4
C
1
6
Count-to-Infinity
• The reason for the count-to-infinity problem is that each node
only has a “next-hop-view”
• For example, in the first step, A did not realize that its route
(with cost 2) to C went through node B
• How can the Count-to-Infinity problem be solved?
7
Count-to-Infinity
• The reason for the count-to-infinity problem is that each node
only has a “next-hop-view”
• For example, in the first step, A did not realize that its route
(with cost 2) to C went through node B
• How can the Count-to-Infinity problem be solved?
• Solution 1: Always advertise the entire path in an update
message to avoid loops (Path vectors)
– BGP uses this solution
8
Count-to-Infinity
• The reason for the count-to-infinity problem is that each node
only has a “next-hop-view”
• For example, in the first step, A did not realize that its route
(with cost 2) to C went through node B
• How can the Count-to-Infinity problem be solved?
• Solution 2: Never advertise the cost to a neighbor if this
neighbor is the next hop on the current path (Split Horizon)
– Example: A would not send the first routing update to B, since B
is the next hop on A’s current route to C
– Split Horizon does not solve count-to-infinity in all cases!
» You can produce the count-to-infinity problem in Lab 4.
9
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) to all of its neighbors
• RIP always uses 1 as link metric
• Maximum hop count is 15, with “16” equal to “”
• Routes are timeout (set to 16) after 3 minutes if they are not
updated
10
RIP - History
• Late 1960s : Distance Vector protocols were used in the
ARPANET
• Mid-1970s: XNS (Xerox Network system) routing protocol is
the ancestor of RIP in IP (and Novell’s IPX RIP
and Apple’s routing protocol)
• 1982
Release of routed for BSD Unix
• 1988
RIPv1 (RFC 1058)
- classful routing
• 1993
RIPv2 (RFC 1388)
- adds subnet masks with each route entry
- allows classless routing
• 1998
Current version of RIPv2 (RFC 2453)
11
RIPv1 Packet Format
IP header UDP header
RIP Message
1: RIPv1
2: for IP
Command Version
Set to 00...0
address family
Set to 00.00
32-bit address
Unused (Set to 00...0)
Address of destination
Cost (measured in hops)
One RIP message can
have up to 25 route entries
Unused (Set to 00...0)
one route entry
(20 bytes)
1: request
2: response
metric (1-16)
Up to 24 more routes (each 20 bytes)
32 bits
12
RIPv2
• RIPv2 is an extends RIPv1:
– Subnet masks are carried in the route information
– Authentication of routing messages
– Route information carries next-hop address
– Uses IP multicasting
• Extensions of RIPv2 are carried in unused fields of RIPv1
messages
13
RIPv2 Packet Format
IP header UDP header
RIP Message
2: RIPv2
2: for IP
Command Version
Set to 00...0
address family
Set to 00.00
32-bit address
Unused (Set to 00...0)
Address of destination
Cost (measured in hops)
One RIP message can
have up to 25 route entries
Unused (Set to 00...0)
metric (1-16)
one route entry
(20 bytes)
1: request
2: response
Up to 24 more routes (each 20 bytes)
32 bits
14
RIPv2 Packet Format
Used to provide a
method of separating
"internal" RIP routes
(routes for networks
within the RIP routing
domain) from "external"
RIP routes
Subnet mask for IP
address
Identifies a better next-hop
address on the same
subnet than the advertising
router, if one exists
(otherwise 0….0)
RIPv2 Message
Command Version
Set to 00.00
address family
route tag
IP address
Subnet Mask
Next-Hop IP address
metric (1-16)
2: RIPv2
one route entry
(20 bytes)
IP header UDP header
Up to 24 more routes (each 20 bytes)
32 bits
15
RIP Messages
• This is the operation of RIP in routed. Dedicated port for
RIP is UDP port 520.
• Two types of messages:
– Request messages
• used to ask neighboring nodes for an update
– Response messages
• contains an update
16
Routing with RIP
• Initialization: Send a request packet (command = 1, address
family=0..0) on all interfaces:
• RIPv1 uses broadcast if possible,
• RIPv2 uses multicast address 224.0.0.9, if possible
requesting routing tables from neighboring routers
• Request received: Routers that receive above request send their entire
routing table
• Response received: Update the routing table
• Regular routing updates: Every 30 seconds, send all or part of the
routing tables to every neighbor in an response message
• Triggered Updates: Whenever the metric for a route change, send entire
routing table.
17
RIP Security
• Issue: Sending bogus routing updates to a router
• RIPv1: No protection
• RIPv2: Simple authentication scheme
RIPv2 Message
Command Version
Set to 00.00
0xffff
Authentication Type
Password (Bytes 0 - 3)
Password (Bytes 4 - 7)
Password (Bytes 8- 11)
Password (Bytes 12 - 15)
2: plaintext
password
Authetication
IP header UDP header
Up to 24 more routes (each 20 bytes)
32 bits
18
RIP Problems
• RIP takes a long time to stabilize
– Even for a small network, it takes several minutes until the
routing tables have settled after a change
• RIP has all the problems of distance vector algorithms, e.g.,
count-to-Infinity
» RIP uses split horizon to avoid count-to-infinity
• The maximum path in RIP is 15 hops
19
Dynamic Routing Protocols II
OSPF
Relates to Lab 4. This module covers link state
routing and the Open Shortest Path First (OSPF)
routing protocol.
Distance Vector vs. Link State Routing
• With distance vector routing, each node has information only
about the next hop:
•
•
•
•
Node A: to reach F go to B
Node B: to reach F go to D
Node D: to reach F go to E
Node E: go directly to F
• Distance vector routing makes
poor routing decisions if
directions are not completely
correct
(e.g., because a node is down).
A
B
C
D
E
F
• If parts of the directions incorrect, the routing may be incorrect until the
routing algorithms has re-converged.
21
Distance Vector vs. Link State Routing
• In link state routing, each node has a complete map of the
topology
A
• If a node fails, each
node can calculate
the new route
B
C
D
E
A
F
A
• Difficulty: All nodes need to
have a consistent view of the
network
B
C
D
E
A
F
B
C
D
E
C
D
E
B
C
D
E
A
A
B
B
F
C
A
D
F
F
E
B
C
D
E
F
F
22
Link State Routing: Properties
• Each node requires complete topology information
• Link state information must be flooded to all nodes
• Guaranteed to converge
23
Link State Routing: Basic principles
1. Each router establishes a relationship (“adjacency”) with its neighbors
2. Each router generates link state advertisements (LSAs) which are
distributed to all routers
LSA = (link id, state of the link, cost, neighbors of the link)
Each router sends its LSA to all routers in the network (using a method
called reliable flooding)
3. Each router maintains a database of all received LSAs (topological
database or link state database), which describes the network has a
graph with weighted edges
4. Each router uses its link state database to run a shortest path
algorithm (Dijikstra’s algorithm) to produce the shortest path to each
network
24
Link state routing: graphical illustration
b
3
1
a
c
6
a’s view
b’s view 3
6
b
c
d’s view
1
a
c’s view
d
Collecting all pieces yield
a complete view of the network!
b
3
a
2
2
c
c
b
a
1
c
6
d
d
25
Operation of a Link State Routing protocol
Received
LSAs
Link State
Database
Dijkstra’s
Algorithm
IP Routing
Table
LSAs are flooded
to other interfaces
26
Dijkstra’s Shortest Path Algorithm for a Graph
Input: Graph (N,E) with
N the set of nodes and E the set of edges
cvw
link cost (cvw = 1 if (v,w)  E, cvv = 0)
s
source node.
Output: Dn
cost of the least-cost path from node s to node n
M = {s};
for each n  M
Dn = csn;
while (M  all nodes) do
Find w  M for which Dw = min{Dj ; j  M};
Add w to M;
for each neighbor n of w and n  M
Dn = min[ Dn, Dw + cwn ];
Update route;
enddo
27
OSPF
• OSPF = Open Shortest Path First
• The OSPF routing protocol is the most important link state
routing protocol on the Internet (another link state routing
protocol is IS-IS (intermediate system to intermediate system)
• The complexity of OSPF is significant
– RIP (RFC 2453 ~ 40 pages)
– OSPF (RFC 2328 ~ 250 pages)
• History:
–
–
–
–
–
1989: RFC 1131 OSPF Version 1
1991: RFC1247 OSPF Version 2
1994: RFC 1583 OSPF Version 2 (revised)
1997: RFC 2178 OSPF Version 2 (revised)
1998: RFC 2328 OSPF Version 2 (current version)
28
Features of OSPF
• Provides authentication of routing messages
• Enables load balancing by allowing traffic to be split evenly
across routes with equal cost
• Type-of-Service routing allows to setup different routes
dependent on the TOS field
• Supports subnetting
• Supports multicasting
• Allows hierarchical routing
29
Hierarchical OSPF
30
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.
31
Example Network
Router IDs can be
selected
independent of
interface addresses,
but usually chosen to
be the smallest
interface address
4
.2
.2
3
2
3
.6
1
.5
.3
5
.5
.5
10.1.5.0/24
10.1.2.3
.6
10.1.7.0 / 24
.4
.3
•Link costs are called Metric
1
.4
.2
.3
• Metric is in the range [0 ,
.4
10.1.4.0 / 24
10.1.1.0 / 24
.1
2
10.1.7.6
10.1.4.4
10.1.6.0 / 24
.1
10.1.1.2
10.1.3.0 / 24
10.1.1.1
10.1.5.5
216]
• Metric can be asymmetric
32
Link State Advertisement (LSA)
.1
.2
.2
10.1.1.0 / 24
3
2
.2
10.1.7.6
10.1.4.4
.4
10.1.4.0 / 24
.3
.4
10.1.6.0 / 24
4
.1
10.1.1.2
10.1.3.0 / 24
10.1.1.1
.6
10.1.7.0 / 24
.4
.6
.5
.3
.5
•
.5
10.1.5.0/24
10.1.5.5
10.1.2.3
The LSA of router 10.1.1.1 is as follows:
•
Link State ID:
10.1.1.1
•
•
Advertising Router:
Number of links:
10.1.1.1 = Router ID
3 = 2 links plus router itself
•
•
•
Description of Link 1:
Description of Link 2:
Description of Link 3:
Link ID = 10.1.1.2, Metric = 4
Link ID = 10.1.2.3, Metric = 3
Link ID = 10.1.1.1, Metric = 0
.3
= Router ID
33
Network and Link State Database
.1
.2
10.1.1.0 / 24
Each router has a
database which
contains the LSAs
from all other routers
.2
10.1.3.0 / 24
.1
10.1.1.2
.2
.4
10.1.4.0 / 24
.3
.4
.6
10.1.7.0 / 24
.4
.6
.5
.3
.3
10.1.7.6
10.1.4.4
10.1.6.0 / 24
10.1.1.1
.5
.5
10.1.5.0/24
10.1.5.5
10.1.2.3
LS Type
Link StateID
Adv. Router
Checksum
LS SeqNo
LS Age
Router-LSA
10.1.1.1
10.1.1.1
0x9b47
0x80000006
0
Router-LSA
10.1.1.2
10.1.1.2
0x219e
0x80000007
1618
Router-LSA
10.1.2.3
10.1.2.3
0x6b53
0x80000003
1712
Router-LSA
10.1.4.4
10.1.4.4
0xe39a
0x8000003a
20
Router-LSA
10.1.5.5
10.1.5.5
0xd2a6
0x80000038
18
Router-LSA
10.1.7.6
10.1.7.6
0x05c3
0x80000005
1680
34
Link State Database
• The collection of all LSAs is called the link-state database
• Each router has an identical link-state database
– Useful for debugging: Each router has a complete description of
the network
• If neighboring routers discover each other for the first time,
they will exchange their link-state databases
• The link-state databases are synchronized using reliable
flooding
35
OSPF Packet Format
OSPF Message
IP header
OSPF packets are not
carried as UDP payload!
OSPF has its own IP
protocol number: 89
OSPF Message
Header
Body of OSPF Message
Message Type
Specific Data
LSA
LSA
... ...
LSA
TTL: set to 1 (in most cases)
LSA
Header
LSA
Data
Destination IP: neighbor’s IP address or 224.0.0.5
(ALLSPFRouters) or 224.0.0.6 (AllDRouters)
36
OSPF Packet Format
OSPF Message
Header
2: current version
is OSPF V2
version
Message types:
1: Hello (tests reachability)
2: Database description
3: Link Status request
4: Link state update
5: Link state acknowledgement
Standard IP checksum taken
over entire packet
Authentication passwd = 1:
Authentication passwd = 2:
Body of OSPF Message
type
message length
source router IP address
ID of the Area
from which the
packet originated
Area ID
checksum
authentication type
authentication
authentication
32 bits
64 cleartext password
0x0000 (16 bits)
KeyID (8 bits)
Length of MD5 checksum (8 bits)
Nondecreasing sequence number (32 bits)
0: no authentication
1: Cleartext
password
2: MD5 checksum
(added to end
packet)
Prevents replay
attacks
37
OSPF LSA Format
LSA
Link Age
LSA
Header
LSA
Header
LSA
Data
Link Type
Link State ID
advertising router
link sequence number
checksum
length
Link ID
Link 1
Link Data
Link Type #TOS metrics
Metric
Link ID
Link 2
Link Data
Link Type #TOS metrics
Metric
38
Discovery of Neighbors
• Routers multicasts OSPF Hello packets on all OSPF-enabled
interfaces.
• If two routers share a link, they can become neighbors, and
establish an adjacency
10.1.10.1
10.1.10.2
Scenario:
Router 10.1.10.2 restarts
OSPF Hello
OSPF Hello: I heard 10.1.10.2
• After becoming a neighbor, routers exchange their link state
databases
39
Neighbor discovery and
database synchronization
10.1.10.1
Discovery of
adjacency
Scenario:
Router 10.1.10.2 restarts
10.1.10.2
OSPF Hello
OSPF Hello: I heard 10.1.10.2
After neighbors are discovered the nodes exchange their databases
Database Description: Sequence = X
Sends database
description.
(description only
contains LSA
headers)
Acknowledges
receipt of
description
Database Description: Sequence = X, 5 LSA headers =
Router-LSA, 10.1.10.1, 0x80000006
Router-LSA,
10.1.10.2, 0x80000007
Router-LSA,
10.1.10.3, 0x80000003
Router-LSA,
10.1.10.4, 0x8000003a
Router-LSA,
10.1.10.5, 0x80000038
Router-LSA,
10.1.10.6, 0x80000005
Database Description: Sequence = X+1, 1 LSA header=
Router-LSA,
10.1.10.2, 0x80000005
Sends empty
database
description
Database
description of
10.1.10.2
Database Description: Sequence = X+1
40
Regular LSA exchanges
10.1.10.1
Link State Request packets, LSAs =
Router-LSA, 10.1.10.1,
Router-LSA, 10.1.10.2,
Router-LSA, 10.1.10.3,
Router-LSA, 10.1.10.4,
Router-LSA, 10.1.10.5,
Router-LSA, 10.1.10.6,
10.1.10.1 sends
requested LSAs
10.1.10.2
10.1.10.2 explicitly
requests each LSA
from 10.1.10.1
Link State Update Packet, LSAs =
Router-LSA, 10.1.10.1,
0x80000006
Router-LSA, 10.1.10.2, 0x80000007
Router-LSA, 10.1.10.3, 0x80000003
Router-LSA, 10.1.10.4, 0x8000003a
Router-LSA, 10.1.10.5, 0x80000038
Router-LSA, 10.1.10.6, 0x80000005
41
Routing Data Distribution
• LSA-Updates are distributed to all other routers via Reliable
Flooding
• Example: Flooding of LSA from 10.10.10.1
10.1.1.1
10.1.2.2
LSA
ACK
10.1.3.4
LSA
Update
database
Update
database
10.1.1.2
Update
database
LSA
10.1.7.6
LSA
ACK
Update
database
Update
database
10.1.4.5
42
Dissemination of LSA-Update
• A router sends and refloods LSA-Updates, whenever the
topology or link cost changes. (If a received LSA does not
contain new information, the router will not flood the packet)
• Exception: Infrequently (every 30 minutes), a router will flood
LSAs even if there are not new changes.
• Acknowledgements of LSA-updates:
• explicit ACK, or
• implicit via reception of an LSA-Update
• Question: If a new node comes up, it could build the
database from regular LSA-Updates (rather than exchange of
database description). What role do the database description
packets play?
43
Border Gateway protocol (BGP)
BGP
• BGP = Border Gateway Protocol . Currently in version 4,
specified in RFC 1771. (~ 60 pages)
• Interdomain routing protocol for routing between autonomous
systems
• Uses TCP to establish a BGP session and to send routing
messages over the BGP session
• BGP is a path vector protocol. Routing messages in BGP
contain complete routes.
• Network administrators can specify routing policies
45
BGP policy routing
• BGP’s goal is to find any path (not an optimal one). Since the
internals of the AS are never revealed, finding an optimal path
is not feasible.
• Network administrator sets BGP’s policies to determine the
best path to reach a destination network.
46
BGP basics
• A route is defined as a unit of information that pairs a
destination with the attributes of a path to that destination.
• EBGP session is a BGP session between two routers in
different ASes.
• IBGP session is a BGP session between internal routers of an
AS.
47
EBGP and IBGP
128.195.0.0/16 0
128.195.0.0/16 0
R2
R3
AS 1
R1
AS 0
128.195.0.0/16 1 0
R4
R6
R5
AS 2
128.195.0.0/16 2 1 0
R8
•
R7
AS 3
IBGP is organized into a full mesh topology, or IBGP sessions are
relayed using a route reflector.
48
Commonly BGP attributes
• Origin: indicates how BGP learned about a particular route
– IGP
– EGP
– Incomplete
• AS path :
– When a route advertisement passes through an autonomous system, the AS
number is added to an ordered list of AS numbers that the route advertisement
has traversed
• Next hop
• Multi_Exit_Disc (MED, multiple exit discriminator):
-used as a suggestion to an external AS regarding the preferred route into the AS
• Local_pref: is used to prefer an exit point from the local autonomous
system
• Community: apply routing decisions to a group of destinations
49
BGP route selection process
Routes sent
to peers
Routes recved from peers
Input
Decision
Policy
process
Engine
Best
routes
Out
Policy
Enigne
• Input/output engine may filter routes or
manipulate their attributes
50
Best path selection algorithm
1. If next hop is inaccessible, ignore routes
2. Prefer the route with the largest local preference value.
3. If local prefs are the same, prefer route with the shortest AS
path
4. If AS_path is the same, prefer route with lowest origin (IGP <
EGP < incomplete)
5. If origin is the same, prefer the route with lowest MED
6. IF MEDs are the same, prefer EBGP paths to IBGP paths
7. If all the above are the same, prefer the route that can be
reached via the closest IGP neighbor.
8. If the IGP costs are the same, prefer the router with lowest
router id.
51
Summary
• Router architectures
• Dynamic routing protocols: RIP, OSPF, BGP
• RIP uses distance vector algorithm, and converges slow (the
count-to-infinity problem)
• OSPF uses link state algorithm, and converges fast. But it is
more complicated than RIP.
• Both RIP and OSPF finds lowest-cost path.
• BGP uses path vector algorithm, and its path selection
algorithm is complicated, and is influenced by policies.
52