Advanced Computer Networks1 Efficient Policies for Carrying Traffic Over Flow-Switched Networks Anja Feldmann, Jenifer Rexford, and Ramon Caceres Presenters:

Advanced Computer Networks 1

Efficient Policies for Carrying Traffic Over Flow-Switched

Networks

Anja Feldmann, Jenifer Rexford, and Ramon Caceres

Presenters: Tauhid, Reji and Murshed


Objective

• Increase the efficiency of the switching technique

• Reduce the overhead for establishing and maintaining the dedicated route or shortcut for flow


Introduction

A possible solution could be grouping series of related data packets into flows and sending the flows through a shortcut path

Benefits: improve performance and capitalizing on QoS of the switchesDrawbacks: consume network resources to create and maintain dedicated

path for the flow

Different definition and criteria are necessary for flow construction and shortcut creation and maintenance


Introduction (Contd.)

Three parameters determine the flow and

shortcut decisions:

• aggregation: level for traffic combination• timeout: time interval of any flow• trigger: traffic quantity or number of packets for

constituting a flow


Introduction (Contd.)

The effects of varying these parameters were

explored on three metrics of interest:

• percentage of traffic following the shortcut• shortcut setup rate• number of simultaneous shortcut


Trace Collection of Packets

The Extensive packet-level trace collection was carried out on T1 line of AT&T Research Lab. Several aspects were considered as:

-the traffic trace reflects the dominance of WWW

-long continuous traffic traces were observed

-the endpoint machines were taken for consideration

-all the flow and shortcut parameters were evaluated

on various parameters


Trace Collection of Packets (Contd.)

•The T1 line connects the AT&T Lab to the external

internet. The Ethernet segment carries all traffic to and

from the T1 line at a speed of 10-Mbits/s

The Sniffer was equipped with tcdump to collect information of 100 00 packets in raw binary format.



• Tcdump read each packet and generated an ASCII log

• Perl script classified each packet into flows

• Splus function processed all the log files on different performance metrics



From the optional field of the web request messages, the operating system of the client machine was identified. Linux and/or Windows systems were classified as personal computers

IP address associated with multiple types of machines such as Windows and SunOS are classifies as Proxy Servers


Hourly average throughput of theEthernet segment


The packet size distribution of the network traffic



• Half of the outgoing traffic consists of 40 bytes TCP acknowledgement packets, so the average packet size became 123 bytes • More than 60% of the incoming packets have 552, 567, or 1500 bytes, that correspond to common maximum transfer unit size on the Internet• Using tcpreduce tool, the FTP, HTTP, SMTP, DNS, etc. data transfer were measured


Flow Size Distributions

• Mixture of flows between 100 and 100,000 bytes

• Majority of flows within range of 50 to 1000 bytes

• 1/3 of flows between 1000 – 10,000 bytes. 1/3 between 100 – 1000 bytes


Cumulative Distribution


Derivative of Cumulative Distribution


Density of the logarithm of the flow sizes


Flow sizes of Web Traffic

• SMTP (email) – large concentration of 1000 byte flows, remaining flows around 3000 bytes

• Telnet – Bytes per flow is typically less than 100 bytes

• HTTP has the largest bytes per flow. Flows fall into 3 main categories– 1. Less than 150 bytes, from failed TCP sessions and

error messages. – 2. 150 – 300 bytes, from cache hit messages.– 3. Greater than 300 bytes, actual web page transfers.


Flow sizes of Traffic


Flow duration of Web Traffic

• Duration of SMTP traffic falls between 0.01 seconds and 100 seconds, with high concentration between .01 and 1 second

• Telnet flows typically have the longest duration. Data shows duration up to 17 minutes. Usually around 100 seconds

• HTTP Flow duration typically around 1 second, but can last up to 100 seconds


Flow duration of Traffic


HTTP Flows by Machine Type

• End point machines have unique flow characteristics

• Flow duration for modem connection is longer• Traffic to proxy servers tend to be a mixture of

shorter and longer flows• Larger transfers have higher throughputs since

TCP window grows


Flow duration for different machines


Combining Multiple Web Responses

• 3 different levels of end-point aggregation– Port-to-port: Combines packets with the same

IP address and port numbers at both end-points.– Host-level: Combines different TCP session

from the same Web server to the same Web client.

– Net-to-Net: Aggregating hosts that share the first 16 bits of the IP address.


Flow sizes for web responses


Flow sizes for web responses


Proportion of Shortcut Traffic

• With a trigger of 10 – 20 packets, network can avoid establishing shortcuts for short-lived flows.

• X-packet trigger less effective for host-to-host using packet trigger since aggregation increases number of packets thus triggering too early


Percent of shortcut traffic (port-to-port)


Percent of shortcut traffic (host-to-host)


Shortcut Setup Rate


Shortcut Setup Rate (Contd.)


Shortcut Setup Rate (Contd.)

• Setup rate is lower for larger packet size

• Setup rate is lower for higher timeout


Simultaneous Shortcut Connections


Simultaneous Shortcut Connections (Contd.)

• 0-packet trigger 75.8 shortcuts in average• 10-packet trigger reduces the average no of

simultaneous shortcuts by a factor of 3.3 (75.8 to 23.6)– Reason: higher packet trigger causes less

shortcut connections• For stable operation during heavy load, coarser

aggregation and larger triggers may be necessary





• 12 sec timeout causes 4.3 shortcuts• 300 sec timeout causes 44.5 shortcuts• Larger timeout increases no of shortcuts, as a

result it reduces signaling load• Establishment of shortcuts is limited by no of

connections, or the signaling capacity of the switch, or both

• Substantial network overhead reduction is possible by timeouts, triggers, end-point aggregation


Traffic Flows Along Partial Routes

• As the hop count increases, the numbers increases exponentially. ( EX: first seven hops have 26, 71, 137, 267, 409, 916, 1508 different outcomes, respectively.)


Partial Route Aggregation


Partial Route Aggregation (Contd.)

• Aggregating traffic along a portion of the routes decreases both the setup rate and the number of simultaneous connection– Seven hop: Setup rate = 20%, No. of Shortcuts =

11%– Three hop: Setup rate = 37%, No of Shortcuts =

16%

• Percent of Byte in Shortcuts– End-to-End: 92.2%– Seven hop: 94.0%– Three hop: 96.0%


Partial Route Aggregation (Contd.)

• Partial-route aggregation reduces network overheads while increasing the proportion of shortcut traffic

• Partial-route aggregation combines transforms from replicas of the same web site, as well as related servers at the same institution

• Partial route flows benefit more from larger timeout values– As seen in Table III for 60 sec and 300 sec.


Summary

• Flow characteristics vary with endpoint type (modem, personal computer, compute servers, proxy servers)

• Aggregating consecutive and concurrent transfers from the same web server yields substantial benefits


Summary (Contd.)

• Aggregation and triggers both reduce overhead, but the reductions are not multiplicative

• Variability in traffic load changes with time scale• Aggregating traffic along portions of the route

between the source and destination yields additional reduction in network overhead


Future Work

• To evaluate specific new policies that balance the short-term tradeoffs between processor and network load

• To investigate the policy and performance implications of combining traffic along a portion of the route

• To study more detailed breakdown of web traffic by content type, as well as the implications of push technology and the new features in the emerging HTTP standards

Documents

Advanced Computer Networks1 Efficient Policies for Carrying Traffic Over Flow-Switched Networks Anja Feldmann, Jenifer Rexford, and Ramon Caceres Presenters: