Flow control in data link layer

Do we need flow control in datalink layer ?

May be we should have. The example given in switch book is like this: You may be travelling to watch a cricket match and the stadium has enough parking space.. but the road in btw is jammed due to traffic. So even if the parking space is there, it does not help us are we are stuck in traffic.

Similarly TCP has end-end flow control. But it does not detect link layer flow issues fast. Many techniques are used in ethernet for flow control.

In half duplex ethernet some of the techniques used are:
1. send jam signal in the link where u want to throttle the flow
2. or send preamble signal in the link where u want to throttle the flow
3. or start sending the packets before the interframe gap is finished ( so that ur output queue gets priority )

For full duplex ethernet, IEEE has designed MAC control frames - A PAUSE frame is send to start/stop flow.

The link layer techniques to control flow are handy. But it sometimes confuses the upper layer protocols. TCP may not detect the flow issues and will be sending packets.., if link layer flow controls are not there, TCP may detect flow control issues faster and can take remedial actions..

Full duplex Ethernet - without CS, MA and CD

Always we used to ask in interviews.. what is the media access protocol in ethernet ? - we always needed the answer CSMA/CD ... CSMA/CD was and is ethernet...

But in full duplex ethernet ???

In full duplex ethernet there is no multiple access... Rx of Host is connected directly to Tx of Switch and vice versa... No multiple access ( no MA).

So no need for career sensing ( no CS )

No chances of collision... so loopback connection also not needed ( no CD)

So full duplex ethernet is half duplex ethernet without CSMA/CD... Only the frame formats and related stuff is retained.. and full duplex ethernet does not add anything more to the protocol. It just makes it simple...

WAN technologies

Going through the WAN technologies topics.....

Frame Relay
ISDN
T1/E1
PPP
ATM

etc etc.....

Will post interesting stuff as i learn through it.....

Switch

The Complete Guide to LAN Switching Technology - Switch book by Rich Siefert

Switched Lan sniffing!!!!!!!

I wanted to sniff packets from all ports in a switch, and the first thing I tried to do was to attach a PC to one port and run a netywork analyser software like ethreal in it. Then suddenly I realized my blunder. Unlike shared lan, a port in a switch receives only the traffic destined for that port....!

So I was curious on how sniffing was done in switches and found these technologies:
1. Port Mirroring
2. Switch Mirroring
In Port Mirroring, you map one monitored port to another mirrored port. So the traffic in monitored port will get reflected in mirrored port and can be used for analysis by a network analyser.
In Switch Mirroring the traffic from all ( or the configured ones) can be reflected in the mirrored port. The mirrored port shuld hav greater bandwidth to handle, this, but this mode is useful as all traffic can be seen in the mirrored port.

Another solution would be for Switch to support RMON MIBs. These MIBs give necessary information about the data transfer statistics in switch.

Magic Quadrant for Campus LANs report

Evaluation of Enterprise Switches: Report:
Cisco remains the leader in the Campus LAN Magic Quadrant; however, cracks are showing, and viable competitors, such as HP ProCurve and Foundry Networks, are rapidly increasing share.

- http://mediaproducts.gartner.com/reprints/hp/153760.html

IETF Workgroups

Started following IETF workgroups, and became a member of DHC workgroup - http://www.ietf.org/html.charters/dhc-charter.html

DNS Configuration Option via DHCP - RFCs of interests

RFCs which are of interested to implement DNS configuration options through DHCP:

RFC2132 - DHCP Options and BOOTP Vendor Extensions
RFC3397 - Dynamic Host Configuration Protocol (DHCP) Domain Search Option
RFC3646 - DNS Configuration options for Dynamic Host Configuration Protocol for IPv6(DHCPv6)
RFC2131 - Dynamic Host Configuration Protocol
RFC3315 - Dynamic Host Configuration Protocol for IPv6 (DHCPv6)

Restart blogging

Planning to restart blogging. Will note down things that I come across which are useful to me.