Subject: Need Information Urgently

From: "R.S.Arkesh Kumar" <[email-masked]>
Date: Fri, 30 Oct 1998 21:07:50 +0500 (GMT+0500)

Hi
	We have started work on a project "Internet Access via Satellite".
The basic architecture is that of a TDM/TDMA, with the interface to the
Internet at the Hub being a standard Router and a Internt Server(a
Workstation).
	I have the following doubts regarding TCP/IP implementation over
the Satellite :
1. Why is TCP/IP implemented over the Satellite ? Cannot something like a
"Store and Forward" policy be adopted, i.e., all the Internet datagrams
which land at the Hub are buffered by the server, and it itself generates
the ACK packet to the source.  The server will then encapsulate the data
in a proprietary format and sends it to the appropriate VSAT(remote
terminal).  Any data lost in the satellite link will be taken care by the
server at the Hub by retransmission independent of the original source.
The same policy is used for the IP datagrams generated by the VSAT.
	Wouldn't the above method be a better implementation, rather than
running TCP/IP over the satellite ?
	Can the same TCP/IP Protocol stack as used for terrestrial
networks be used in this case ?

b. Is a back channel of 64 kbps which will be shared by a number of VSATs
on a TDMA be sufficient for the ACK packets generated by TCP ? Will a
smaller data rate affect the overall throughput ? Where will I get more
details in this regard ?

c. Can a Router be configured for a network MTU manually ? / How does a
Router learn the MTU of a connected network ?  

Reply ASAP,
With regards,
Arkesh
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R. S. ARKESH KUMAR
RESEARCH ENGINEER
SATELLITE COMMUNICATION
CENTRE FOR DEVELOPMENT OF TELEMATICS
71/1, MILLER ROAD
BANGALORE - 560 052

Other E-Mail address : [email-masked]

PHONE : 91-80-2261529
FAX:91-80-2263256

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From: Ketan Bajaj <[email-masked]>
Date: Sat, 31 Oct 1998 21:34:54 +0530 (IST)

Hi Doug,

The Tcp at sender end relies on incoming acks for two major purposes :
1). Reliable delivery at receiver
    If the hub sends an ack to sender, the sender end interprets that the
packet has reached the receiver, though it could have been lost over the
satellite link. So, it would actually mis-interpret the ack and would not
attempt to recover the lost packet
2). RTT calculations
    The sender TCP depends upon the incoming ack to calculate the RTT
values. An ack from the hub would lead to a wrong calculation of RTT,
which in turn would cause a whole lot of other problems. 

ketan

Ketan Bajaj
(M.Tech student)
Deptt. of Computer Sci. & Engg.
Indian Institute of Technology,
New Delhi

On Sat, 31 Oct 1998, Doug Dillon wrote:

> 
>     I'd be interested to know what semantics are lost
> by intermediate acknowledgements. When I look at the
> WinSock socket interface, I don't get any notice after I do a send
> of whether the data I sent actually arrived. This has to happen
> (I think) at the application layer.
> 
>     Doug.............