1CMPE 150- Introduction to Computer Networks

CMPE 150

Fall 2005Lecture 28

Introduction to Computer Networks

2CMPE 150- Introduction to Computer Networks

Announcements• Homework 5 posted.

– Due, Mon, 12.05.• Final exam: December 7th. 4-7pm

– In class.– Closed books/notes.– Comprehensive.

• Discussion session:– Monday, 12.05 @ 2pm.– Jack’s lounge.– Questions by e-mail.

• Course evaluation.– Today.

• CE 151 will be offered in Winter 06!– Likely TTh 4-5:45.

3CMPE 150- Introduction to Computer Networks

Today

• Transport Layer (Cont’d).– Chapter 6 of Tanenbaum.

• Application layer.

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TCP Congestion Control

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Network dynamics

load

throughput

load

delay

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TCP Self-Clocking

PrPb

Ar

Ab

receiversender

As

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Congestion Control Design

• Avoidance or detection/recovery? – Avoidance keeps system at knee of curve

• Requires some congestion signal.

– Detection/recovery responds to loss after the fact.

• TCP– Which is TCP?

• Congestion detection/recovery.

– How does TCP do it?• Slow start, congestion avoidance, exponential backoff.

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TCP congestion control basics

• Like, flow control, also window based.– Sender keeps congestion window (cwin).

– Each sender keeps 2 windows: receiver’s advertised window and congestion window.

– Sender’s maximum window: min(advertised window, cwin).

• Sender’s actual window:– Max window - unacknowledged segments.

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TCP congestion control mechanisms

• Collection of interrelated mechanisms:– Slow start.

– Congestion avoidance.

– Accurate retransmission timeout estimation.

– Fast retransmit.

– Fast recovery.

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TCP congestion window

• New congestion window cwnd.– What the network can handle,

– versus flow control window (wnd): what the other end can handle.

• Actual congestion window:– min (wnd, cwnd).

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How to adjust window?

• When to increase/decrease?

• A control theory problem:– Observe network.

– Reduce window if congested.

– Increase window if not congested.

• Constraints:– Efficiency.

– Fairness.

– Stability (too much oscillation is bad).

– Out-of-date info• RTT is fundamental

limit to how quickly you can react

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Slow start [Jacobson 1988]

• How do we get this self-clocking behavior to start?– Initialize cwnd = 1.– Upon receipt of every ack, cwnd = cwnd + 1

• Implications– Window actually increases in RTT * log2W (exponential

increase).– Can overshoot window and cause packet loss.

Connection’s congestion window starts at 1 segment.

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Slow start example

1

one RTT

one pkt time

0R

21R

3

42R

567

83R

91011

1213

1415

1

2 3

4 5 6 7

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When to End Slow-Start?

• Want to end when the pipe is full.– When cwnd > ssthresh.– Start with large ssthresh, but then refine it.

• On packet loss.– cwnd=1 and go back to slow start.– ssthresh = cwnd / 2

• assume that pipe size was somewhere between last good window (cwnd/2) and current window (cwnd).

• Eventually, ssthresh is right and transition to congestion avoidance without packet loss.

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Congestion avoidance

Upon receiving ACK Increase cwnd by 1/cwnd. This is additive increase.

Why not multiplicative increase? Growing too fast in equilibrium =>

oscillations

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TCP Congestion Control: Operation

threshold

timeout

threshold

cwin

time

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TCP Retransmission Timer

• When segment sent, retransmission timer starts.

– If segment ACKed, timer stops.

– If time out, segment retransmitted and timer starts again.

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How to set timer?

• Based on round-trip time: time between a segment is sent and ACK comes back.

• If timer is too short, unnecessary retransmissions.

• If timer is too long, long retransmission delay.

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Jacobson’s Algorithm

• Determining the round-trip time:– TCP keeps RTT variable.

– When segment sent, TCP measures how long it takes to get ACK back (M).

– RTT = alpha*RTT + (1-alpha)M.

– alpha: smoothing factor; determines weight given to previous estimate.

– Typically, alpha=7/8.

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Jacobson’s Algorithm (Cont’d)

• Determining timeout value:– Measure RTT variation, or |RTT-M|.

– Keeps smoothed value of cumulative variation D=alpha*D+(1-alpha)|RTT-M|.

– Alpha may or may not be the same as value used to smooth RTT.

– Timeout = RTT+4*D.

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Karn’s Algorithm

• How to account for ACKs of retransmitted segments?

– Count it for first or second transmission?

– Karn proposed not to update RTT on any retransmitted segment.

– Instead RTT is doubled on each failure until segments get through.

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Application Layer

• Tanenbaum, Chapter 7.

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DNS

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What’s DNS?

• Domain Name System.• What does it do?

– Maps host names to IP addresses, and

– Vice-versa.

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Some History

• Original approach (ARPANET, 1970’s):– File hosts.txt listed all hosts and their IP

addresses.

– Every night every host fetches file from central repository.

– OK for a few hundred hosts.

– Scalability?

• File size.• Centrally managed.

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DNS

• Hierarchical name space.

• Distributed database.

• RFCs 1034 and 1035.

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How is it used?

• Client-server model.– Client DNS (running on client hosts), or “resolver”.

– Application calls “resolver” with name.

– “Resolver” contacts local DNS server (using UDP) passing the name.

– Server returns corresponding IP address.

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DNS Name Space

• A portion of the Internet domain name space.

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Name Space Structure

• Top-level domains:– Generic.– Countries.

• Leaf domains: no sub-domains.• In practice all US organizations are under a

generic domain, while everything outside the US is under the corresponding country domain.

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DNS Names

• Domain names:– Concatenation of all domain names starting from

its own all the way to the root separated by “.”.

– Refers to a tree node and all names under it.

– Case insensitive.

– Components up to 63 characters.

– Full name less than 255 characters.

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Name Space Management

• Domains are autonomous.– Organizational boundaries.

– Each domain manages its own name space independently of other domains.

• Delegation:– When creating new domain: register with parent

domain.

• For name uniqueness.• For name resolution.

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Name Servers

• Entire database in a single name server.– Practical?

– Why?

• DNS database is partitioned into zones.

• Each zone contains part of the DNS tree.

• Zone <-> name server.– Each zone may be served by more than 1 server.

– A server may serve multiple zones.

• Primary and secondary name servers.

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Name Servers

• Part of the DNS name space showing the division into zones.

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Name Resolution • Application wants to resolve name.• Resolver sends query to local name server.

– Resolver configured with list of local name servers.– Select servers in round-robin fashion.

• If name is local, local name server returns matching authoritative RRs.– Authoritative RR comes from authority managing

the RR and is always correct.– Cached RRs may be out of date.

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Name Resolution (Cont’d)• If information not available locally (not even cached), local NS will have to ask someone

else.– It asks the server of the top-level domain of the name requested.

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More on Name Resolution

• How a resolver looks up a remote name in eight steps.

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Recursive Resolution

• Recursive query:– Each server that doesn’t have info forwards it to

someone else.– Response finds its way back.

• Alternative:– Name server not able to resolve query, sends

back the name of the next server to try.– Some servers use this method.– More control for clients.

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Example

• Suppose resolver on flits.cs.vu.nl wants to resolve linda.cs.yale.edu.

– Local NS, cs.vu.nl, gets queried but cannot resolve it.– It then contacts .edu server.– .edu server forwards query to yale.edu server.– yale.edu contacts cs.yale.edu, which has the authoritative

RR.– Response finds its way back to originator.– cs.vu.nl caches this info.

• Not authoritative (since may be out-of-date).• RR TTL determines how long RR should be

cached.

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Resource Records

• Entry in the DNS database.• Several types of entries or RRs.• Example: RR “A” contains IP address.• Name <-> several resource records.• RR format: five-tuple.

– Name.– TTL (in seconds).– Class (usually “IN” for Internet info).– Type: type of RR.– Value.

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Resource Records

• The principal DNS resource records types.

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The Web and HTTP

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Some History Started in 1989 at CERN, European

center for nuclear research, in Switzerland.

Original motivation: need for scientists around the world to collaborate and share multi-media information.

Tim Berners-Lee came up with initial proposal of a web of linked documents

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More History… First text-based prototype demo in

12.91. Release of first graphical interface,

Mosaic, in 02.93 at NCSA by M. Andreessen.

In 1994, Andreessen creates Netscape. In 1994, CERM and MIT set up the

WWW Consortium to further develop the Web. www.w3.org for more information.

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The Web

• WWW, or the world-wide web is a resource discovery service.– Resource space is organized hierarchically,

and resources are linked to one another according to some relation.

– Hypertext organization: link “granularity”; allows links within documents.

– Graphical user interface.

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The client side

• Users perceive the Web as a vast collection of information.– Page is the Web’s information transfer unit.

– Each page may contain links to other pages.

– Users follow links by clicking on them which takes them to the corresponding page.

– This process can go on indefinetly, traversing several pages located in different places.

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The browser

• Program running on client that retrieves and displays pages.– Interacts with server of page.

– Interprets commands and displays page.

• Examples: Mosaic, Netscape’s Navigator and Communicator, Microsoft Internet Explorer.

• Other features: back, forward, bookmark, caching, handle multimedia objects.

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The Server Side

• Web site has Web server running that answers requests for pages locally served.– Web server listens to port 80 for requests.

– When request from client arrives, connection is set up.

– Server replies.

– Connection released.

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Example

• User clicked on www.w3.org/hypertext/WWW/TheProject.html.

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Example (cont’d)

• Browser asks DNS to resolve www.w3.org.• DNS replies with 18.23.0.23.• Browser sets up connection to 18.23.0.23 port 80.• Browser sends GET /hypertext/WWW/TheProject.html.• www.w3.org server sends TheProject.html file.• Connection released.• Browser displays TheProject.html, fetching and displaying

all embedded objects (images, etc).

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Observations

• Many browsers display status information at bottom of the screen.

• For each embedded object (in-line image like icon, picture, etc), browser establishes new connection.– Performance hit.– Revisions to protocol (HTTP) address

this.

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More Observations…

• Server’s response specifies object type (using MIME) followed by object body.

• For example: – Content-Type: Image/GIF

– Content-Type: Text/html