The Time Delay of TCP/IP and Related Protocols

Zhen Song

Motivation: why study TCP/IP It is popular and reliable. Low cost. (Both software and hardware) Scalable. (WAN + LAN) Platform independent. (Wire/Wireless,

Connection/Connectionless, Different OS, Various of hardware)

Potential applications: LonWorks, Teleoperation.

TCP/IP stack layout

Lan adapters

Lan drivers

IP

ICMP Routing

ARP

TCPUDP Other protocols

applicationsapplications applications

Wire/fiber

Other protocols

From http://netlab1.usu.edu/pub/bsuk99/tcp.ppt

TCP/IP vs. OSI

OSI: Open System Interconnection

Related Protocols

Figure from A.S. Tanenbaum “Computer Networks” 3ed Edition

For most PC systemsData link: Ethernet, ISDN/ADSL/Telephone/Internet.Network: IP, ARP.Transport: TCP, UDP.Application: WWW, TELNET, FTP, SMTP, DNS.

Delay of The Ethernet (IEEE 802.3)

Data Link layer, Medium Access sublayer. Performance

If there are k stations ready for transmit and the probability for each station transmits during a contention slot is p. The probability A that some station acquires the channel in that slot is A=k p(1-p)k-1. A is maximized when p=1/k, with A -> 1/e as k->Inf, so the mean number of slots per contention is

Each slot has a duration 2t, the mean contention interval, w, is 2t/A. Assuming optimal p, the mean number of contention slots is never more than e, so w is at most .

0

1 1)1(

j

j

AAjA

tte 4.52

Cont. If the mean frame takes P sec to transmit, when many stations have frames to

send,Channel efficiency =

Larger frames are more efficient, but others have to wait longer before they can get access to the channel. 1024 bytes is a round off between performance and “fairness”.

The trouble maker for controller: the “mean” not “worst” case.

Atp

p

/2

Channel Efficiency

1024 byte frames

512 byte frames

256 byte frames

128 byte frames

64 byte frames

Number of stations trying to send

Cha

nnel

eff

icie

ncy

Figure from A.S. Tanenbaum “Computer Networks” 3ed Edition

Collision of The Ethernet For a mean interval of 2t/A, collision happens

and Binary Expenential Backoff (BEB) procedure starts.

BEB: after i collisions, a random number between 0 and 2i-1 is chosen as the number of slots to skip. The maximum number of the skipped slots is 1023.

The possibility of collision after the last round of BEB is negligible for a LAN, but significant delay might be introduced.

Go to WAN ARP: Ethernet address (MAC) to Internet address (IP).

(Defined in RFC826)(Defined in RFC826) Why: Every computer on the Internet has a unique IP address, which is not the address of Ethernet. So there must be a mechanism to translate between them.How: Broadcast “Who owns the IP address 129.123.85.29” Acknowledge “I have, and my MAC address is …..”Note: ARP is not routable.

The commercial implementations are optimized by various way, including ARP cache. So the delay of ARP introduced only once.

ARP is not mandatory. A system with static database does not need ARP. ARP was designed to reduce the work load of people.

IP Routing Simple IP routing

The Netmask can judge if a specific IP is in the associate subnet. If (((their_IP ^ my_IP) & netmask) )!=0 use_gateway( ); /* not in this subnet */ else go_direct( ); /* on the same wire */

This job would be done by the router, with unknown delay. Depends on the CPU, software, load, etc of the router.

More complex routing The interior gateway routing protocol:OSPF

RFC1247. OSPF represents Open Shortest Path First.Original Internet interior gateway protocol was a distance vector protocol (RIP) based on the Bellman-Ford algorithm. It worked well in small systems, but less well on larger ones. OSPF became a standard in 1990, and it will become the major interior gateway routing protocol soon. Unknown delay

The exterior gateway routing protocol:BGPRFC1265BGP: Border Gateway ProtocolNot just the shortest path, It considers more about politics. Traffic starting or ending at Netscape should not transit Microsoft.

Congestion Control in The Network Layer John Nagle mode. “Congestion Control in IP/TCP” RFC896

If there are infinite memory on the router, or the CPU of router is too slow, congestion may happen.

General principle of congestion control: using “control theory point of view”, “close loop”.

There are many algorithms, such asNagle mode, Leaky bucket, Token bucket, etc.

DNS From domain name to IP address

www.yahoo.com -> 126.115.102.75 -> 126.115.102.76www.sohu.com -> 61.135.131.13www.virtualhost.com -> 159.226.35.24www.anothervirtual.com -> 159.226.35.24

TCP congestion control Jacobson algorithm TCP updates an estimate of the average

RTT (round trip time) a by

gmaga )1(

,where g is a ‘gain’ (0<g<1) that should be related to the signal-to-noise ratio of m.

Measuring The Delay Is Simple

Just Ping

Conclusion Consider the worst case for either LAN, such as

Ethernet, or WAN, is very difficult. There is possibility mathematic model for the

collision of the Ethernet. But some parameters for the formula cannot be calculated, rather, they are measured on the real system. Not surprisingly, different systems might have quite different delay.

Cont. The delay is very easy to measure but hard to predict. The more you know about the delay about other

computers, the easier to predict the delay of your system.

Though it has not been proved yet, the delay, even of LAN, must be uncontrollable, if other computer involved in the same network.

High performance indicates small delay. For a reliable medium like twist par 100M Ethernet, large frames will have better performance.

Cont. Unfortunately, control systems prefer small

frame. So we should not put heavy load on the network. Efficiency theory gives us a reference on selection the working load.

At the beginning, researchers assumes the traffic of the Ethernet is Poisson, but it now appears the network traffic is self-similar (Paxson and Floyd 1994; and Willinger et al., 1997)

Internet tracking software.http://www.sane.com/ http://www.deepmetrix.com/

References Classics

352 citations Wide-Area Traffic: The Failure of Poisson Modeling254 citations Self Similarity Through High-Variability: Statistical Analysis of Ethernet LAN Traffic at the Source Level

148 citations End-to-End Routing Behavior in the Internet A good presentation

Why is the internet traffic self-similar? 30 citations

Forecasting network performance to support dynamic scheduling using the network weather service

Papers on 24th Conference on Local Computer Networks17 - 20 October, 1999 Lowell, MassachusettsTime Series Models for Internet Data TrafficC. You and K. ChandraOn the Distribution of Round-Trip Delays in TCP/IP NetworksT. Élteto and S. MolnárEfficient and Accurate Ethernet SimulationJ. Wang and S. KeshavThe Worst-Case Scenario for Transmission of Synchronous Traffic in an FDDI NetworkS. Zhang and E. Lee

Cont. Papers on frequency domain.

Modeling Spectral Features in TCP trafficG. Olowoyeye, B. Kim and K. Chandra 1998 Network traffic modeling using a multifractal wavelets model

Non-linear time-series models for Ethernet traffic K. Chandra, C. You, G. Olowoyeye and C. Thompson , June 1998

Analyzing Stability in Wide-Area Network Performance Measuring link bandwidths using a deterministic model of packet delay, Characteristics of wide-area TCP/IP conversations,