Transcript TCP, IGP, EGP etc. - La Salle University

TCP (Cont.)
Based on Chapter 24 in
Computer Networks and
Internets, Comer
CSIT 220 (Blum)
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TCP Provides

Reliability: Data will be delivered exactly as sent.
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Full Duplex Communication: The conversation is two way
without either side having to wait its turn.
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Stream Interface: until the session ends, all of the data is
treated the same; it belongs to one stream.

Reliable Connection Startup: sessions are not confused with
simultaneous or previous sessions.

Graceful Connection Shutdown: both participants are aware
of the ending of a session.
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TCP
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TCP is called an end-to-end service since it
guarantees a connection between applications
on different computers (the endpoints of the
route).
TCP connections are called virtual since they
are built entirely from software.
TCP messages are encapsulated in IP packets
and sent across the network. TCP treats IP as a
packet delivery system. IP treats TCP messages
as data to be delivered.
Only the endpoints need TCP software.
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Routers don’t need to know TCP
Delivery is taken care of IP and lower layers. A packet doesn’t
have to pass through the TCP layer except at source and
destination.
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Achieving Reliability
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TCP must handle various problems, such as
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Packets are dropped, delayed or duplicated by the
unreliable underlying communication system (IP).
One computer in a session is rebooted.
TCP must guarantee that
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If a computer establishes multiple sessions between 2
computers, each has its own uniquely defined connection.
Duplicate messages should be rejected.
In case of reboot, delayed messages from any previous
session between two computers will not be accepted.
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Retransmission
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The primary technique of ensuring
reliability is retransmission
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When data is sent, a timer is set.
When the destination receives the data, it
sends an acknowledged to the source.
If the timer set by the source expires before an
acknowledgment arrives, the sender
retransmits the data.
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Retransmission
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How much time?
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How much time should a computer wait before
retransmitting a message?
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That depends on whom you’re talking to.
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The time to send a message and receive an
acknowledgment will be short (a few milliseconds) if the
two nodes are on the same LAN.

The time to send a message halfway across the world
using a long-distance satellite connection will be longer.
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Setting delay time
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If the delay time were set to a reasonable time for
LANs, the long-distance communication would be
retransmitted many times over before it could
reach its destination let alone be acknowledged.
If the delay time were set to a reasonable time for
long-distance communication, communication
within the LAN would be delayed unnecessarily.
Unnecessary delay wastes the bandwidth.
Unnecessary retransmission clogs the system.
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Adaptive Retransmission
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At the beginning of the session (the threeway handshake), TCP already has some
data on the round trip time.
TCP maintains statistics (average and
deviation) on each session and adjusts
the retransmission times to a little over the
average (how much over depends on the
deviation).
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Adaptive Retransmission
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Adaptive retransmission sets the retransmission
delay on a session-by-session basis using the
statistics on the session to set a delay time
appropriate for it.
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Local communication will have a short delay.
Long-distance communication will have a longer delay.
This method can adjust to changes in internet traffic.
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Adaptive Retransmission
Long distance
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Local
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Flow Control
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TCP uses windows to control data flow.
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Each connection allocates the same buffer size to
hold data. This is called the window size.
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The window size is not negotiated, it is determined
solely by the receiver. But the receiver must let the
sender know how much room it has.

Receivers send window advertisements (size) with
each acknowledgement. Based on the window
advertisement size, the sender will increase,
decrease or halt transmission of buffers.
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Window Advertisement
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Congestion Control
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If a packet is dropped, necessitating
retransmission, the sender slows down
significantly.
Otherwise congestion would cause delays, which
in turn lead to retransmission, which causes
more congestion, and so on. This is called
congestion collapse.
To avoid this, TCP uses packet loss as a
measure of congestion.
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Congestion Control
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If TCP losses a packet, it will reduce the rate at
which it retransmits data.
If an ACK is received for a message TCP doubles its
transmission rate.
If a second ACK is received, TCP double the rate
again. (exponential increase)
It continues to double the rate until the number of
sending packets is 1/2 the receiver’s window size.
At this time TCP slows down the rate of increase.
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Closing a session
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A session is closed in a manner similar to the
three-way handshake that started the
session.
Each party sends a closing segment and
receives an acknowledgment of its closing
segment before the session ends (gracefully).
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Socket Number
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An application on a computer is identified by
the “socket number” which is a combination
of the computers IP address and the
application’s port number.
A session is identified by a pair of socket
numbers.
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Routing Again
Based on Chapter 27 in
Computer Networks and
Internets by Comer
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So I lied

We said previously that routers do not need to know
about the TCP layer; however, routers need to
exchange information among themselves so that
they can build efficient and accurate routing tables.
And if they want reliability in this exchange of
information, they will use TCP.
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Making a routing table
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There are two basic approaches
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Static: The network administrator enters and edits the
routing table
 Pro: simple, low overhead

Assuming a “simple” network
Con: not adaptable
Dynamic: routers exchange information and a program
generates the table from that
 Pro: can adapt to changes in the traffic pattern
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

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scalable
Con: more expensive software, uses more CPU, etc.
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The edge of the cloud
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Most routers sit at the edge of a cloud, e.g.
connecting a LAN to the Internet cloud.
Such a router needs only distinguish between
traffic destined for the LAN and traffic
destined for beyond the LAN.
In this case, a static routing table is sufficient.
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Simple static routing scenario
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Scaling
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As one enters the cloud, the complexity of
generating an efficient routing table grows
quickly and dynamic routing is used.
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Routers broadcast messages which are
responded to by other routers.
In this way the routers learn about their
neighbors.
They pass this information to their neighbors and
they pass it on to their neighbors and so on.
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Scaling again
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Even automated routing tables cannot handle
the problem of generating a routing table if all
the routers connected to the Internet are to
be considered in the calculation.
In order to handle this scaling problem,
another level of hierarchy is introduced.
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Groups
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A school of fish
A herd of elephants
A pride of lions
A flock of sheep
A gaggle of geese
An autonomous system of routers
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Autonomous Systems
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AS
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An autonomous system (AS) is a group of
routers adhering to a single router “policy.”
It may be a single network or a group of
networks that is overseen by a one network
administrator or group of administrators who
agree on a single policy.
Typically the AS is operated by some entity
such as a university or a business.
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AS (Cont.)
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An autonomous system is sometimes called a
“routing domain.”
An autonomous system is assigned a globally
unique number, sometimes called an
Autonomous System Number (ASN).
Networks within an autonomous system
communicate routing information to each other
using an Interior Gateway Protocol (IGP).
Some use the term IGP as a generic term, a
category of protocols, others use the term as a
specific protocol.
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IGP used within an AS
IGP to communicate within an AS.
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AS (Cont.)
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AS’s communicate using Border Gateway Protocol
(BGP) or Exterior Gateway Protocol (EGP).
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Eventually, the BGP is expected to be replaced by the OSI InterDomain Routing Protocol (IDRP).
Some use the term EGP as a generic term, a category of
protocols, others use the term as a specific protocol
The Internet's protocol guideline defines an
autonomous system as:
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“… a connected group of one or more Internet Protocol
prefixes run by one or more network operators which has a
SINGLE and CLEARLY DEFINED routing policy.”
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EGP or BGP used among AS’s
BGP or
Communication between AS’s uses EGP or BGP
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IGP
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An Interior Gateway Protocol is a set of rules for
exchanging routing information between routers
(gateways) within an autonomous system.
This information is then used by routers to generate
routing tables.
There are two commonly used IGPs:
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Routing Information Protocol (RIP)
Open Shortest Path First (OSPF) protocol.
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RIP
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Routing Information Protocol is a set of rules
defined by RFC (Request for Comments) 1058 that
specifies how routers exchange information.
RIP uses a “distance vector algorithm” in which
routers periodically exchange their entire routing
tables.
A router compares the entries of the table it receives
with its own and replaces any paths with ones found
to be cheaper.
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RFC 1058
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RIP Packet
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RIP
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RIP uses UDP (user datagram protocol)
instead of TCP to exchange information, so it
is fast but unreliable.
Because RIP is inefficient (exchanging entire
tables), it is gradually being replaced by
Open Shortest Path First (OSPF).
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It remains fairly common.
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OSPF
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Open Shortest Path First is set of rules for routers
to exchange information and generate routing
tables within an autonomous system.
It is based on the “shortest path first” or “link-state”
algorithm.
Routers do not send tables, instead they send
information about their nearest neighbors and the
cost thereto.
The shortest path to each node is then calculated
(e.g. using Dijkstra’s algorithm).
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OSPF
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The advantage of OSPF is that it results in smaller
more frequent updates throughout the AS.
It converges quickly yielding a stable network.
OSPF does require more CPU and memory.
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But the advantages out weigh the disadvantages.
OSPF Version 2 is defined in RFC 1583.
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RFC 1583
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IS-IS
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IS-IS: Intermediate System-to-Intermediate
System protocol,
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Intermediate system refers to a router as opposed
to an end system (host).
IS-IS is the OSI version OSPF, that is, a set of
rules for routers to exchange Link-State
information.
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RFC 1142
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IGRP
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Interior Gateway Routing Protocol (IGRP) is a
proprietary network protocol, developed by Cisco
Systems.
IGRP is a distance-vector routing protocol used for
routers communicating within an AS.
Think of IGRP as a new and improved RIP. While RIP
uses only one criteria (hops) to determine the best
path. IGRP uses five factors:
 the link's speed, delay, packet size, loading and
reliability.
Network administrators weight these factors as they
see fit.
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EIGRP
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A new and improved, new and improved RIP.
Enhanced Interior Gateway Routing Protocol
is a newer version of IGRP that addresses
the demands of large-scale internets and the
developments in network technology since
IGRP was developed.
IGRP and EIGP can work together.
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EGP
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Exterior Gateway Protocol is a set of rules for
exchanging routing information between neighboring
AS’s.
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The routing table contains a list of known routers in
other AS’s and the addresses they can reach.
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Since different AS’s may use different measures of
cost, the cost measure is based mainly on the
number of hops.
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RFC 904
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EGP (Cont.)
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Each router polls its neighbor at intervals
between 120 to 480 seconds and the
neighbor responds by sending its complete
routing table.
EGP-2 is the latest version of EGP. A more
recent exterior gateway protocol, the Border
Gateway Protocol (BGP), provides additional
capabilities.
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BGP
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Border Gateway Protocol is a set of rules
for AS’s to exchange routing information, so
that efficient routes can be found between
AS’s.
BGP is commonly used within and between
Internet Service Providers (ISPs) and is
defined in RFC 1771.
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RFC 1771
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BGP (Cont.)
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The routing table contains a list of known routers,
the addresses they can reach, and a cost for each
path.
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BGP uses TCP to exchange information.
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They only do so when a change is detected. Only
the affected part of the routing table is sent.
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BGP-4 allows administrators to configure costs
based on policy statements.
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BGP (Cont.)
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BGP can be used to communicate within an
AS and among AS’s.
Within an AS, it uses Internal BGP (IBGP)
since BGP doesn't work well with IGP.
The routers inside the autonomous network
thus maintain two routing tables: one for the
interior gateway protocol and one for IBGP.
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IGMP
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Internet Group Management Protocol is defined in
RFC 1112 as the standard for IP multicasting in the
Internet.
It is used to establish host memberships in particular
multicast groups on a single network.
A host tells the local router (using Host Membership
Reports) that it wants to receive messages
addressed to a specific multicast group.
It is the routers responsibility from there on.
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Hot potato routing
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In hot potato routing, or deflection routing, the
routers have no buffer to store packets in before
forwarded on to their final predetermined
destination.
In normal routing, when many packets are destined
for a given outgoing channel, packets are either
buffered or dropped. But in hot potato routing, each
packet is constantly transferred until it reaches its
final destination because the routers can not support
more than one packet at a time.
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Hot potato (Cont.)
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The packet is bounced around like a "hot potato,"
sometimes moving further away from its
destination because it has to keep moving
through the network. This technique allows
multiple packets to reach their destinations
without being dropped. This is in contrast to
"store and forward" routing where the network
allows temporary storage at intermediate
locations. Hot potato routing has applications in
optical networks where messages made from
light do not have to be stored in any medium.
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Other References
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http://www.webopedia.com
http://www.whatis.com
http://www.scit.wlv.ac.uk/~jphb/comms/tcp.ht
ml
Computer Dictionary, Shnier
http://www.wildpackets.com/compendium/IP/I
CM-Redr.html
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