LLC sub layer( ethernet) and control field

Answered Question
May 22nd, 2009
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Hi every body!

I hope you guys are doing great. It has been a while since i posted on forum.


I was reading about LLc layer defined by 802.2.

According to my book, LLC provides following functions:

1) multiplexing ( identify data by type)

2) acts as interface between between mac layer and network layer.

3) flow control.


In LLC header, one of the field is control field which is one byte long.

What does this control do? is there any sub field in control field?


Thanks a lot and have a nice weekend!


Correct Answer by Jon Marshall about 8 years 1 month ago

Sarah


Just to add to Giuseppe's post.


TCP and UDP work across routed networks. LLC by definition doesn't and can't because it works at L2.


So TCP has it's own flow control and congestion mechanisms for end to end delivery of packets eg. sequence numbers, windiw sizes, acknowledgements etc and it does this by using a connection oriented service.


UDP as you say does not provide a connection oriented service so if the developer of a UDP application wants to guarantee delivery etc. they have to write the code for that in their application.


Jon

Correct Answer by Giuseppe Larosa about 8 years 1 month ago

Hello Sarah,

good note.

I agree TCP/IP does not use connection oriented LLC services.

But connection oriented LLC services exist and are used by non IP stacks like Source Route Bridging, IBM networking.


Actually, DLSW+ with TCP encapsulation emulates an end-to-end LLC connection-oriented session to allow transport of non routable connection-oriented protocols over IP networks.

In the banks there are still a lot of routers used for DLSW+ for example.

Also in big enterprises to interface with mainframes there is still some of them.


Hope to help

Giuseppe


Correct Answer by Giuseppe Larosa about 8 years 1 month ago

Hello Sarah,

try again the answer is another one but this is probably the only values to take care of unless you want to know the values used by

STP IEEE 802.1D

IS-IS, CLNS


(hint: SNAP follows and then eventually IP)


Hope to help

Giuseppe


Correct Answer by Jon Marshall about 8 years 1 month ago

Sarah


Good to see you back, how is the studying going ?


Rather than me write it all out attached is a link to a Cisco doc on LLC that covers the control field and what it does -


http://www.cisco.com/en/US/tech/tk331/tk336/technologies_tech_note09186a0080094777.shtml


Jon

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Correct Answer
Jon Marshall Fri, 05/22/2009 - 10:21
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Sarah


Good to see you back, how is the studying going ?


Rather than me write it all out attached is a link to a Cisco doc on LLC that covers the control field and what it does -


http://www.cisco.com/en/US/tech/tk331/tk336/technologies_tech_note09186a0080094777.shtml


Jon

sarahr202 Fri, 05/22/2009 - 11:14
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Thanks Jon!


I just took a break from study as my dad was sick. He is doing great now ,thanks God for that.


Jon Marshall Fri, 05/22/2009 - 11:17
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Sarah


"He is doing great now"


Very glad to hear that.


Jon

Giuseppe Larosa Fri, 05/22/2009 - 11:54
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Hello Sarah,

very good news that your father is better now.


This time I put you a question


What follows when LLC header has the following values ?


AA AA 03 ...


Best Regards

Giuseppe


sarahr202 Fri, 05/22/2009 - 12:00
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Thanks Giuseppe !

I will get back to you after i am done reading the link Jon has forwarded to me.

You have a nice weekend Giuseppe!

sarahr202 Fri, 05/22/2009 - 12:11
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What follows when LLC header has the following values ?


AA AA 03 ...


answer: If DSAP ,SSAP have AA ,AA respectively and Control field has 03, then ip packet is encapsulated in ethernet frame.



Thanks a lot Giuseppe for making me think which i seldom do!

Correct Answer
Giuseppe Larosa Fri, 05/22/2009 - 12:23
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Hello Sarah,

try again the answer is another one but this is probably the only values to take care of unless you want to know the values used by

STP IEEE 802.1D

IS-IS, CLNS


(hint: SNAP follows and then eventually IP)


Hope to help

Giuseppe


sarahr202 Fri, 05/22/2009 - 12:38
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Thanks Giuseppe for correcting me.


you made me read my CCNA book again and i thank you for that. The book says if DSAP and SSAP have AA values,then it means SNAP header will follow which also include 2 byte type field. Now if this type field is 0800, then ip packet follows.


I don't see any mention of control field with value=03. The link Jon has forwarded to me discusses different values and their meanings but does not mention value " 03" for control field.


thanks a lot and have good day!

sarahr202 Fri, 05/22/2009 - 12:22
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Thanks Giuseppe for your kind remarks.


sarahr202 Fri, 05/22/2009 - 13:03
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Hi Jon and Giuseppe!

Just want to confirm my understanding.


In tcp/ip, tcp is used when connection- oriented service is required and udp is used to provide connectionless service.

A data link layer protocol could be any protocol such as ppp, ethernet, 802.11 etc.

For example if ethernet or 802.11 is being used as data link protocol, then LLC only provides multiplexing i.e identifying the data in the frame. Connection-oriented and connectionless services are not provided by LLC in tcp/ip environment as udp and tcp will provide those services.

Is my understanding correct?


thanks a lot!

Correct Answer
Giuseppe Larosa Fri, 05/22/2009 - 13:10
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Hello Sarah,

good note.

I agree TCP/IP does not use connection oriented LLC services.

But connection oriented LLC services exist and are used by non IP stacks like Source Route Bridging, IBM networking.


Actually, DLSW+ with TCP encapsulation emulates an end-to-end LLC connection-oriented session to allow transport of non routable connection-oriented protocols over IP networks.

In the banks there are still a lot of routers used for DLSW+ for example.

Also in big enterprises to interface with mainframes there is still some of them.


Hope to help

Giuseppe


Correct Answer
Jon Marshall Fri, 05/22/2009 - 13:35
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Sarah


Just to add to Giuseppe's post.


TCP and UDP work across routed networks. LLC by definition doesn't and can't because it works at L2.


So TCP has it's own flow control and congestion mechanisms for end to end delivery of packets eg. sequence numbers, windiw sizes, acknowledgements etc and it does this by using a connection oriented service.


UDP as you say does not provide a connection oriented service so if the developer of a UDP application wants to guarantee delivery etc. they have to write the code for that in their application.


Jon

sarahr202 Fri, 05/22/2009 - 13:48
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Thanks Jon and Giuseppe!

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