414Lecture20

414Lecture20 - Lecture 20 Link Layer PPP, ATM, MPLS ECSE...

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Lecture 20 Link Layer PPP, ATM, MPLS ECSE 414 – Fall 2010
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ECSE 414, Lecture 20 2 Announcements Homework Assignment #4 due today Project Presentations During usual class time Starting Tuesday, November 25 2010 Michael Rabbat
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ECSE 414, Lecture 20 3 Point to Point Data Link Control one sender, one receiver, one link: easier than broadcast link: no Media Access Control no need for explicit MAC addressing e.g., dialup link, ISDN line popular point-to-point DLC protocols: PPP (point-to-point protocol) HDLC: High level data link control 2010 Michael Rabbat
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ECSE 414, Lecture 20 4 PPP Design Requirements [RFC 1557] packet framing: encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time ability to demultiplex upwards bit transparency: must carry any bit pattern in the data field error detection (no correction) connection liveness: detect, signal link failure to network layer network layer address negotiation: endpoint can learn/configure each other’s network address 2010 Michael Rabbat
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ECSE 414, Lecture 20 5 PPP non-requirements no error correction/recovery no flow control out of order delivery OK no need to support multipoint links (e.g., polling) Error recovery, flow control, data re-ordering all relegated to higher layers! 2010 Michael Rabbat
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ECSE 414, Lecture 20 6 PPP Data Frame Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible multiple control fields Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc) 2010 Michael Rabbat
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ECSE 414, Lecture 20 7 PPP Data Frame info: upper layer data being carried check: cyclic redundancy check for error detection 2010 Michael Rabbat
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ECSE 414, Lecture 20 8 Byte Stuffing “data transparency” requirement: data field must be allowed to include flag pattern <01111110> Q: is received <01111110> data or flag? Sender: adds (“stuffs”) escape byte < 01111101> before each <01111110> data byte Also stuffs <01111101> before <01111101> Receiver: two 01111101 bytes in a row: discard first byte, continue data reception single 01111110: flag byte 2010 Michael Rabbat
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9 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data 2010 Michael Rabbat
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414Lecture20 - Lecture 20 Link Layer PPP, ATM, MPLS ECSE...

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