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Network Layer 6-1
2011 session 1TELE3118: Network
Technologies
Week 6: Network LayerIntra-Domain Routing Protocols
Some slides have been taken from:Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith Ross. Addison-Wesley, July 2004. All material copyright 1996-2004. J.F Kurose and K.W. Ross, All Rights Reserved.
Network Layer 6-2
IP routing0.0.0.0 0 192.168.1.1
10.0.0.0 8 172.20.4.1
200.23.16.0 20 199.31.18.4
200.23.18.0 23 172.20.4.1
10.20.0.0 24 199.31.18.4
192.168.1.0 24 L 192.168.1.18
172.20.4.0 24 L 172.20.4.253
199.31.18.0 24 L 199.31.18.52
destination mask loca
l
next-hop
LAN
inte
rface
s172.20.4.253/24
192.168.1.18/24199.31.18.52/24
How is the routing table constructed? Static (manual) Dynamic (routing protocol)
Network Layer 6-3
The Internet Network layer
Note on terminology: “routing” vs. “forwarding” “routing table” vs. “forwarding table”
forwardingtable
Routing protocols•path selection•RIP, OSPF, BGP
IP protocol•addressing conventions•datagram format•packet handling conventions
ICMP protocol•error reporting•router “signaling”
Transport layer: TCP, UDP
Link layer
physical layer
Networklayer
Network Layer 6-4
1
23
0111
value in arrivingpacket’s header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
“routing” and “forwarding” tables
Network Layer 6-5
Routing: abstract model
Graph abstraction for routing algorithms:
graph nodes are routers
graph edges are physical links link cost: delay, $
cost, or congestion level
Goal: determine “good” path
(sequence of routers) thru network from source to
dest.
Routing protocol
A
ED
CB
F
2
2
13
1
1
2
53
5
“good” path: typically means
minimum cost path other def’s possible
Network Layer 6-6
Routing algorithm classification
Distance-vector algorithm
Local information: router knows physically-
connected neighbors, link costs to neighbors
2 components: Neighbor routing-table
exchange Bellman-Ford (also
called Ford-Fulkerson) computation
E.g.: RIP
Link-state algorithm Global information:
router knows complete topology and link cost info of entire network
2 components: Reliable flooding Dijkstra shortest-path
tree (SPT) computation
E.g.: OSPF, IS-IS
Network Layer 6-7
Distance vector - RIP
Each node maintains a table of triples.
D
G
A
F
E
B
C
Destination Cost Next-hop
A 1 A
C 1 C
D 2 C
E 2 A
F 2 A
G 3 A
table at B:
Network Layer 6-8
RIP: overview
Iterative, asynchronous, distributed Directly connected neighbors exchange
updates periodically (on the order of several seconds) whenever table changes (called triggered update)
Each update is a vector of distances: (Destination, Cost)
Update local table if receive a “better” route smaller cost came from next-hop
Refresh existing routes; delete if they time out
Network Layer 6-9
RIP: example
Destination Cost Next-hop
B 1 B
C 1 C
D ∞ -
E 1 E
F 1 F
G ∞ -
D
G
A
F
E
B
C
Destination Cost Next-hop
B 1 B
C 1 C
D 2 C
E 1 E
F 1 F
G ∞ -
Destination Cost Next-hop
B 1 B
C 1 C
D 2 C
E 1 E
F 1 F
G 2 F
Initial table at A:
After receiving update from C:
After receiving update from F:
Network Layer 6-10
RIP: recovering from link failure
Dest Cost Nh
A 1 A
B 2 A
C 2 A
D ∞ -
E 2 A
G ∞ -
D
G
A
F
E
B
C
Dest Cost Nh
B 1 B
C 1 C
D 2 C
E 1 E
F 1 F
G ∞ -
Dest Cost Nh
B 1 B
C 1 C
D 2 C
E 1 E
F 1 F
G 3 C
At F:
At A:A receives update from C:
Dest Cost Nh
A 1 A
B 2 A
C 2 A
D 3 A
E 2 A
G 4 A
F receives update from A:
Network Layer 6-11
RIP: link cost decreases
X Z14
12
Y1
X 4 X
Z 1 Z
X 5 Y
Y 1 Y
X 1 X
Z 1 Z
X 5 Y
Y 1 Y
X 1 X
Z 1 Z
X 2 Y
Y 1 Y
At Y:
At Z:
Good news travels fast
Network Layer 6-12
RIP: link cost increases
X Z14
12
Y14
X 4 X
Z 1 Z
X 5 Y
Y 1 Y
X 6 Z
Z 1 Z
X 5 Y
Y 1 Y
X 6 Z
Z 1 Z
X 7 Y
Y 1 Y
At Y:
At Z:
X 8 Z
Z 1 Z
X 7 Y
Y 1 Yand so on
Bad news travels slow “count to infinity” problem loops!
Network Layer 6-13
Breaking the loop …
X Z
14
12
Y14
X 4 X
Z 1 Z
X 5 Y
Y 1 Y
X 14 X
Z 1 Z
X 5 Y
Y 1 Y
X 14 X
Z 1 Z
X 12 X
Y 1 Y
At Y:
At Z:
X 13 Z
Z 1 Z
X 12 X
Y 1 Y
Does this solve the “count to infinity” problem?
If next-hop to D is R: Split Horizon: do not include D
in update to R Split Horizon with Poison
Reverse: include D, but with metric = ∞
Network Layer 6-14
… is not always easy
Dest Cost Nh
B 1 B
C 1 C
D 2 C
E ∞ -
F 1 F
G 2 F
D
G
A
F
E
B
C
Dest Cost Nh
A 1 A
C 1 C
D 2 C
E 3 C
F 2 A
G 3 A
Dest Cost Nh
B 1 B
C 1 C
D 2 C
E 4 B
F 1 F
G 2 F
At A:
B receives update from C:A receives update from B:
Dest Cost Nh
A 1 A
B 1 C
D 1 D
E 5 A
F 2 A
G 2 D
C receives update from A:
Network Layer 6-15
RIPv2 (RFC 2453) details
Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (∞ = 16): why? Distance vectors only exchanged among
neighbors Up to 25 destinations per RIP update message Update-interval is 30 sec:
If too large, convergence is slow If too small, too much traffic
Triggered update whenever change in routing table
Split horizon mandatory, poison reverse optional
Network Layer 6-16
RIPv2 details (contd.)
Updates sent every 30 (+/- 5) seconds
Route not refreshed for 180 sec is timed-out Still included in update
messages Timed-out route is deleted
(garbage-collected) after 120 sec
Triggered update timer set for 1-5 sec Includes only changed routes Suppressed if regular update due
Address of net 2
Distance to net 2
Command Must be zero
Family of net 2 Must be zero
Family of net 1 Must be zero
Address of net 1
Distance to net 1
Version
0 8 16 31
subnet mask of net 1
subnet mask of net 2
next hop of net 1
next hop of net 2
Network Layer 6-17
RIP: where does it run? RIP runs as application-level process (route-d) Updates sent as UDP message (port 520) Multicast IP address 224.0.0.9 (with TTL=1)
physical
link
network forwarding (IP) table
Transprt (UDP)
routed
physical
link
network (IP)
Transprt (UDP)
routed
forwardingtable
Network Layer 6-18
Link State - OSPF
Strategy: each node learns complete topology send information about directly connected links (not
entire routing table) to entire network (not just neighbors)
Link State Advertisement (LSA) include Nodes (routers) and links (networks) Sequence number and age
Reliable flooding Store most recent LSA for each node Send LSA to all nodes except one that sent it Generate LSA periodically (with higher sequence
number) Age out each stored LSA
Network Layer 6-19
A Link-State Routing Algorithm
Notation: c(x,y): link cost from node x
to y; = ∞ if not direct neighbors
D(v): current value of cost of path from source to dest. v
p(v): predecessor node along path from source to v
N': set of nodes whose least cost path definitively known
Dijkstra’s algorithm Given: all nodes know full
topology and link costs Objective: compute least
cost paths from self to all other nodes routing table
iterative: after k iterations, know least cost path to k destinations
distributed: each node computes shortest-path tree from itself
Network Layer 6-20
Dijsktra’s Algorithm
1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = ∞ 7 8 Loop 9 find w not in N' such that D(w) is a minimum 10 add w to N' 11 update D(v) for all v adjacent to w and not in N' : 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N'
Network Layer 6-21
Dijkstra’s algorithm: example
Step012345
N'u
uxuxy
uxyvuxyvw
uxyvwz
D(v),p(v)2,u2,u2,u
D(w),p(w)5,u4,x3,y3,y
D(x),p(x)1,u
D(y),p(y)∞
2,x
D(z),p(z)∞ ∞
4,y4,y4,y
u
yx
wv
z2
2
13
1
1
2
53
5
Network Layer 6-22
Dijkstra’s algorithm, discussionAlgorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n(n+1)/2 comparisons: O(n2) more efficient implementations possible: O(nlogn)
Link Metric Static: link latency, link capacity, … Dynamic: based on load?
e.g.: link cost = amount of carried traffic oscillations!
A
D
C
B1 1+e
e0
e
1 1
0 0
A
D
C
B2+e 0
001+e1
A
D
C
B0 2+e
1+e10 0
A
D
C
B2+e 0
e01+e1
initially… recompute
routing… recompute … recompute
Network Layer 6-23
OSPF details RFC 2328 (244 pages long!) Neighbor up/down detected using “hello” packets LSA reliable flooding over entire AS
LSA includes sequence number and age LSA integrity using checksum (excludes age)
OSPF messages directly over IP (no UDP or TCP) Hierarchical OSPF: allow scaling to larger networks 5 types of LSAs:
1. Router LSA: set of nodes2. Network LSA: set of links3. Summary LSA: inter-area networks4. Summary LSA: area-border-routers5. External LSA: external to AS
Network Layer 6-24
Hierarchical OSPF
Network Layer 6-25
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 AS’s.
Network Layer 6-26
OSPF “advanced” features (not in RIP)
Authentication: prevents malicious intrusion Hierarchy: allows larger domains Load balancing: equal-cost multi-path (ECMP) Extensions to support:
Multicast: MOSPF Traffic-engineering: OSPF-TE
Network Layer 6-27
Comparison of LS and DV algorithms
Messaging DV: entire routing table,
but only exchanged between neighbors
LS: small messages, but flooded in whole network
Speed of Convergence DV: multiple iterations,
each requires recompute and transmit count-to-infinity
problem LS: flood and recalculate,
one shot, faster
Robustness: both LS and DV can be wrecked by one bad router.
In 1997 a bad router in a small ISP advertised a false cost, became flooded with traffic, disconnecting ISPs from most U.S. backbone providers for ~3 hours
Bottom line: No clear winner in terms of
complexity, robustness, etc LS often favored due to
faster convergence