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Rivier College
CS575: Advanced LANs
Chapter 13: Internetworking & Routers
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Internetworking & Routers Overview
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Internetworking Protocol Architecture
Design Issues
Autonomous Systems
Routing Border Gateway (BGP) Protocol
Routing Open Shortest Path First (OSPF) Protocol
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The Router Functions
0 Provide a link between networks.
0 Provide for the routing and delivery of data between processes on
end systems attached to different networks.
0 Provide these functions in such a way as not to require
modifications of the networking architecture of any of the attached
subnetworks.
0 These functions are provided by the Internet Protocol, which is
implemented in all end systems and routers.
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Routers
0 Router connects dissimilar networks and operates at layer 3 of the
OSI model
0 Differences among networks include the following:
- Addressing schemes (e.g., IEEE 802 LAN uses either 16-bit or
48-bit binary addresses for each attached device; an X.25 public
frame-switching network uses 12-digit decimal addresses
[encoded as 4-bit per digit for a 48-bit address])
- Maximum frame sizes (e.g., Ethernet imposes 1500-byte frame;
X.25 – 1000-byte frame)
- Interfaces (the concept of a router is independent of interfaces)
- Reliability
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Protocol Architecture for Router
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Internet Protocol Operations (X.25 packet-switched WAN)
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IP-controlled internet: Design Issues
0 Addressing: a unique address is associated with each End System (e.g.,
workstation or server) and each intermediate system (e.g., router) in a
configuration [it is a network-level address]. In the case of the TCP/IP
architecture, this is referred to as an IP (internet) address. In the case of the OSI
architecture, this is referred to as a network service access point (NSAP).
0 Each application and each concurrent user of an application, is assigned
a unique identifier, referred to as a port in the TCP/IP architecture and as
a Service Access Point (SAP) in the OSI architecture. A unique SAP can
be assigned to each level of the OSI architecture.
0 The IP (internet) address and Service Access Point (SAP) are global
addresses with following key characteristics:
* Global nonambiguity: a global address identifies a unique system. Synonyms
are permitted.
* Global applicability: it is possible at any global address to identify any other
global address in any system.
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TCP/IP Concepts
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Other Design Issues: Routing & Datagram Lifetime
0 Routing: accomplished by maintaining a (static or dynamic) routing table in
each end system and router that gives, for each possible destination network,
the next router to which the internet datagram should be sent.
0 A static routing table could contain alternative routes if a router is
unavailable. A dynamic table is more flexible in responding to both error
and congestion conditions. They may also be used to support other
internetworking services (e.g., security and priority).
0 Source routing is another routing technique. The source station specifies the
route by including a sequential list of routers in the datagram.
0 Route recording is a service related to routing for testing and debugging
purposes. To record a route, each router appends its internet address to a list
of addresses in the datagram.
0 Datagram lifetime is calculated (e.g. by a hop count or a true measure of
time), and used to avoid areas of congestion.
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Other Design Issues: Fragmentation and Reassembly
0 In IP, datagram fragments are reassembled at the destination end system. The
following fields are used in the IP header:
* Data Unit Identifier (ID); Data Length; Offset; More Flag.
0 To fragment a long datagram, an IP module in a router performs the tasks:
1. Create two new dataframs and copy the header fields of the incoming
datagram into both.
2. Divide the incoming user data field into two approximately equal portions,
placing one portion in each new datagram. The first portion must be a multiple of
64 bits.
3. Set the Data Length of the first new datagram to the length of the inserted data,
and set More Flag to 1 (TRUE). The Offset field is unchanged.
4. Set the Data Length of the second new datagram to the length of the inserted
data, and add the length of the first data portion divided by 8 to the Offset field.
The More Flag remains the same.
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Fragmentation Example
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Routing
0 Routing Information about the topology and delays of the internet.
0 Routing Algorithm is used to make a routing decision for a
particular datagram, based on current routing information.
0 Autonomous System (AS) exhibits the following characteristics:
1. An AS consists of a group of routers exchanging information via a
common routing protocol.
2. An AS is a set of routers and networks managed by a single
organization.
3. There is a path between any two pair of nodes.
0 Interior Router Protocol (IRP) passes routing information between
routers within an Autonomous System (e.g., OSPF).
0 Exterior Router Protocol (ERP) passes routing information between
routers in different Autonomous Systems (e.g., BGP).
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Application of Exterior and Interior Routing Protocols
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Border Gateway Protocol (BGP)
0 BGP was developed for use with the TCP/IP protocol suite.
0 BGP has become the standardized exterior router protocol for the
Internet.
0 The BGP protocol operates in terms of messages, which are sent
over TCP connections.
0 Three functional procedures are involved in BGP:
* Neighbor acquisition
* Neighbor reachability
* Network reachability
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BGP-4 Messages
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BGP Message Formats
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BGP Message Formats (continued)
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Open Shortest Path First (OSPF) Protocol
Example: a Sample Autonomous System
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Directed Graph of the Autonomous System
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The Shirt-Path-First (SPF) Tree for Router R6
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Routing Table for Router R6
OSPF Algorithm proposed by Dijkstra (1959)
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Example Directed Graph 2
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Example of Least-Cost Routing Algorithms (see Graph 2)
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Example of Least-Cost Routing Algorithms (see Graph 2)
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