Transcript Chapter 4 slides

Chapter 4
Network Layer
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Edited by Lindsay, Marga, and Sarah - March 2003
All material copyright 1996-2002
J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking: A
Top Down Approach
Featuring the Internet,
2nd edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2002.
1
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
4.5 Routing in the Internet
 4.5.1 Intra-AS routing: RIP and OSPF
 4.5.2 Inter-AS routing: BGP
4.6 What’s Inside a Router?
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
2
Intra-AS Routing
 Routing within an Autonomous System (AS)
 Also known as Interior Gateway Protocols (IGP)
 Most common Intra-AS routing protocols:

RIP: Routing Information Protocol

OSPF: Open Shortest Path First

IGRP: Interior Gateway Routing Protocol (Cisco
proprietary)
3
RIP ( Routing Information Protocol)
 Distance vector algorithm
 Distance/Cost metric:


Each link has a cost of 1.
Maximum path cost is 15 – limits use of RIP to AS’s with a diameter of
fewer than 15 hops.
 Distance vectors: exchanged among neighbors every 30 sec
via RIP Response Messages (also called advertisements)
 Each advertisement: list of up to 25 destination routers and
their distances from the router
 Each forwarding table has at least one row for forwarding to
networks outside the AS
4
RIP: Example
z
w
x
A
y
D
B
C
Destination Network
Next Router
Num. of hops to dest.
w
y
z
x
A
B
B
--
2
2
7
1
….
….
....
Routing table in D
5
RIP: Example
Dest
w
x
z
….
Next
C
…
w
hops
4
...
Advertisement
from A to D
z
y
x
A
D
B
C
Destination Network
w
y
z
x
….
Next Router
A
B
BA
-….
Routing table in D
Num. of hops to dest.
2
2
75
1
....
6
RIP: Link Failure and Recovery
 If no advertisement heard after 180 sec --> neighbor/link
declared dead
 routes via neighbor invalidated
 new advertisements sent to neighbors
 neighbors in turn send out new advertisements (if tables
changed)
 link failure info quickly propagates to entire net
 poison reverse used to prevent ping-pong loops
 Routers can request info from neighbors about cost to a
given destination using a request message
 Request and response messages are sent over UDP and
UDP Packet is carried in a standard IP packet
7
RIP Table processing
 RIP routing tables managed by application-level process
called routed
routed
routed
Transprt
(UDP)
network
(IP)
link
physical
Transprt
(UDP)
forwarding
table
forwarding
table
network
(IP)
link
physical
8
RIP Table example (continued)
Router: giroflee.eurocom.fr
Destination
-------------------127.0.0.1
192.168.2.
193.55.114.
192.168.3.
224.0.0.0
default
Gateway
Flags Ref
Use
Interface
-------------------- ----- ----- ------ --------127.0.0.1
UH
0 26492 lo0
192.168.2.5
U
2
13 fa0
193.55.114.6
U
3 58503 le0
192.168.3.5
U
2
25 qaa0
193.55.114.6
U
3
0 le0
193.55.114.129
UG
0 143454
 Three attached destination networks (LANs)
 Router only knows routes to attached LANs
 Default router used to go to any network not explicitly
entered in the forwarding table
 Route multicast address: 224.0.0.0
 Loopback interface (for debugging)
9
OSPF (Open Shortest Path First)
 “open”: publicly available
 Uses Link State algorithm



LS packet broadcasting
Topology map (directed graph) at each node
Route computation for forwarding table uses Dijkstra’s algorithm
 OSPF advertisement carries one entry per neighbor router.
 Individual link costs specified by the administrator


If all are set to 1, we have minimum hop routing
Link weights are set to be inversely proportional to link capacity.
This discourages traffic on low bandwidth links
 Advertisements disseminated to entire AS (via flooding)

Advertisements are carried in OSPF messages directly over IP
(rather than TCP or UDP). Must implement RMT
10
OSPF “advanced” features (not in RIP)
 Security: all OSPF messages authenticated (to prevent
malicious intrusion)
 Multiple same-cost paths allowed (only one path in RIP)
 Integrated uni- and multicast support:
 Multicast OSPF (MOSPF) uses same topology data base as
OSPF
 Hierarchical OSPF in large domains.
11
Hierarchical OSPF
 Two-level hierarchy: local area, backbone.
Link-state advertisements only in area
 each node has detailed area topology
 Area border routers: “summarize” distances to nets in own
area, advertise to other Area Border routers.
 Backbone routers:
 run OSPF routing limited to backbone.
 Route between areas in the AS
 Boundary routers:
 in backbone
 all outgoing packets are routed to boundary router if going
to another AS

12
Hierarchical OSPF
13
Inter-AS routing in the Internet: BGP
BGP provides for routing among autonomous systems
R4
R5
R3
BGP
AS1
AS2
(RIP intra-AS
routing)
(OSPF
intra -AS
routing)
BGP
R1
R2
AS3
(OSPF intra -AS
routing)
14
Internet inter-AS routing: BGP
 BGP (Border Gateway Protocol): the de facto standard
 Path Vector protocol:




similar to Distance Vector protocol
each Border Gateway broadcast to neighbors (peers)
entire path (i.e., sequence of AS’s) to destination
BGP routes to networks (ASs), not individual hosts
E.g., Gateway X may send its path to dest. Z:
Path (X,Z) = X,Y1,Y2,Y3,…,Z
15
Internet inter-AS routing: BGP
Suppose: gateway X send its path to peer gateway W
 W may or may not select path offered by X
 cost, policy (don’t route via competitors AS), loop
prevention reasons.
 If W selects path advertised by X, then:
Path (W,Z) = w, Path (X,Z)
 Note: X can control incoming traffic by controlling its route
advertisements to peers:
 e.g., don’t want to route traffic to Z -> don’t advertise any
routes to Z
16
BGP: controlling who routes to you
legend:
B
W
X
A
C
Y
provider
network
customer
network
 X,W,Y are stub networks (all traffic entering them must be
destined for them and all traffic exiting them must have
originated there.)
 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
17
BGP: controlling who routes to you
legend:
B
W
X
A
C
Y
provider
network
customer
network
 A advertises to B the path AW
 B advertises to X the path BAW
 Should B advertise to C the path BAW?



No way! B gets no “revenue” for routing CBAW since neither W nor
C are B’s customers
B wants to force C to route to w via A
B wants to route only to/from its customers!
18
BGP operation
Q: What does a BGP router do?
 Receiving and filtering route advertisements from
directly attached neighbor(s).
 Route selection.
 To route to destination X, which path (of
several advertised) will be taken?
 Sending route advertisements to neighbors.
19
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
20
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
21
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
4.5 Routing in the Internet
4.6 What’s Inside a Router?
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
22
Router Architecture Overview
Two basic router functions:
 run routing algorithms/protocol (RIP, OSPF, BGP)
 switching datagrams from incoming to outgoing link
23
High-Level Router Architecture
 Input Ports:
 Physical layer functionality, terminates incoming
physical link
 Interoperates with the data link layer
 Performs lookup and forwarding functions
 In practice, multiple ports are often gathered together in
a single line card within a router
24
High Level Router Architecture
 Switching Fabric:
 Connects the router’s input ports to its output ports
 Output Ports:
 Stores packets forwarded to it through the switching
fabric
 Routing Processor:
 Executes the routing protocols
 Maintains the routing information and forwarding
tables
 Performs network management functions within the
router
25
Input Port Functions
• Also known as Decentralized Switching
Physical layer:
bit-level reception
Data link layer:
e.g., Ethernet
see chapter 5
 A copy of the forwarding table is stored at
each input port and updated as needed
 The switching decision can be made locally
at each input port
 Decentralized switching avoids a forwarding
bottleneck at a single point within the router
26
Complicating Factors
 Backbone routers must operate at high speeds, so they
therefore must be capable of performing millions of
lookups per second.
 Line speed: a lookup is performed in less than the amount
of time needed to receive a packet at the input port.
 Example: Consider an OC48 link that runs at 2.5 Gbps.
Assuming a packet size of 256 bytes, this implies a lookup
speed of approximately a million lookups per second
performed.
27
Switching Fabrics
Move packets from the input ports to the output ports
28
Switching Via Memory
First generation routers:
 packet copied by system’s (single) CPU
 speed limited by memory bandwidth (2 bus crossings per
datagram)
Input
Port
Memory
Output
Port
System Bus
Modern routers:
 input port processor performs lookup, copies into memory
 Cisco Catalyst 8500
29
Switching Via a Bus
 Datagram moved from input port
memory to output port memory via a
shared bus
 Switching speed limited by bus
bandwidth
 1 Gbps bus, Cisco 1900: sufficient speed
for access and enterprise routers (not
regional or backbone)
30
Switching Via An Interconnection Network
 Overcomes bus bandwidth limitations
 Some interconnection networks were initially
developed to connect processors in a single
multiprocessor
 Advanced design: fragments datagram into fixed length
cells, then switches cells through the fabric.
 Cisco 12000: switches Gbps through the
interconnection network
31
Output Ports
 Transmits the datagrams that have been stored in the
output port’s memory and transports them over the
outgoing link
 Buffering is required when datagrams arrive from fabric
faster than the transmission rate of the output port
 Scheduling discipline is used to choose among queued
datagrams for transmission onto network
32
Queuing at the Output Port
 Buffering occurs when arrival rate via the switching fabric
exceeds output line speed
 Consequently, a delay due to queuing occurs and there is
potential packet loss due to output port buffer overflow
33
Queuing at Input Port
 Switching fabric slower than input ports combined means that
queueing may occur at input ports
 Head-of-the-Line (HOL) blocking: queued datagram at front
of queue prevents others in queue from moving forward
 Consequently, queuing delay and packet loss due to input
buffer overflow
34
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
4.5 Routing in the Internet
4.6 What’s Inside a Router?
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
35
IPv6
The 32-bit address space of IPv4 has begun to cause
Initial Motivation
concern. Why?
for creating IPv6
 32-bit address space means all possible addresses will
be completely allocated by sometime between 2008 and
2018.
 Although there is a lot of time left until the current
address space is exhausted, it will take considerable time
to deploy a new technology on such an extensive scale so
it is important to start now.
IPv6 will have 128 bits for the IP address. This is enough
to allow every grain of sand its own IP address!
36
IPv6
 Additional motivation:
 header format helps speed processing/forwarding
 new “anycast” address: route to “best” of several
replicated servers
 IPv6 datagram format:
 fixed-length 40 byte header
 no fragmentation allowed
 ICMPv6: new version of ICMP
 additional message types, e.g. “Packet Too Big”
 multicast group management functions
37
Header: IPv4 vs IPv6
head. type of
length
len service
fragment
16-bit identifier flgs
offset
upper
time to
Internet
layer
live
checksum
ver
32 bit source IP address
32 bit destination IP address
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
38
IPv6 Header
A closer look at some of the fields:
Priority: identify priority among datagrams in flow
Flow Label: identify datagrams in same “flow.”
(concept of“flow” not well defined).
Next header: identify upper layer protocol for data
Traffic Class: Similar idea to the type of service field in
IPv4
Checksum: Does not exist in IPv6! It was removed entirely
to reduce processing time at each hop
Options: allowed, but outside of header, indicated by
“Next Header” field
39
Transition From IPv4 To IPv6
 Not all routers can be upgraded simultaneously
 no “flag days”
 How will the network operate with mixed IPv4 and IPv6
routers?
 Two proposed approaches:
 Dual Stack: some routers with dual stack (v6, v4) can
“translate” between formats
 Tunneling: IPv6 carried as payload in IPv4 datagram
among IPv4 routers
40
Dual Stack Approach
IPv6 nodes have full IPv4 capabilities as well. When operating
with an IPv4 node, the IPv6 node uses v4 datagrams. The node
will be able to determine the capabilities of the node it is
communicating with by looking at the address returned by the
DNS.
A
B
C
D
E
F
IPv6
IPv6
IPv4
IPv4
IPv6
IPv6
Flow: X
Src: A
Dest: F
Src:A
Dest: F
Src:A
Dest: F
Flow: ??
Src: A
Dest: F
data
data
data
data
B-to-C:
IPv4
D-to-E:
IPv4
E-to-F:
IPv6
A-to-B:
IPv6
41
Tunneling
Logical view:
A
B
IPv6
IPv6
A
B
C
IPv6
IPv6
IPv4
E
F
IPv6
IPv6
D
E
F
IPv4
IPv6
IPv6
tunnel
Physical view:
Flow: X
Src: A
Dest: F
data
A-to-B:
IPv6
Src:B
Dest: E
Src:B
Dest: E
Flow: X
Src: A
Dest: F
Flow: X
Src: A
Dest: F
data
data
B-to-C:
IPv6 inside
IPv4
D-to-E:
IPv6 inside
IPv4
Flow: X
Src: A
Dest: F
data
E-to-F:
IPv6
42