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1 Computer)Networks)UDP)UDP UDP:)User)Datagram)Protocol)[RFC)768])• “no)frills,”)“bare)bones”)Internet)transport)protocol)• “best)effort”)service,)UDP)segments)may)be:)– lost)– delivered)out)of)order)to)app)• connec%onless:)– no)handshaking)between)UDP)sender,)receiver)– each)UDP)segment)handled)independently)of)others)Why)is)there)a)UDP?)• no)connecLon)establishment)(which)can)add)delay))• simple:)no)connecLon)state)at)sender,)receiver)• small)segment)header)• no)congesLon)control:)UDP)can)blast)away)as)fast)as)desired)UDP2 UDP:)more)• oOen)used)for)streaming)mulLmedia)apps)– loss)tolerant)– rate)sensiLve)• other)UDP)uses)– DNS)– SNMP)• reliable)transfer)over)UDP:)add)reliability)at)applicaLon)layer)– applicaLonRspecific)error)recovery!)UDP source port # dest port # 32 bits Application data (message) UDP segment format length checksum Length, in bytes of UDP segment, including header UDP)checksum)Sender:)• treat)segment)contents)as)sequence)of)16Rbit)integers)• checksum:)addiLon)(1’s)complement)sum))of)segment)contents)• sender)puts)checksum)value)into)UDP)checksum)field)Receiver:)• compute)checksum)of)received)segment)• check)if)computed)checksum)equals)checksum)field)value:)– NO)R)error)detected)– YES)R)no)error)detected.)But,maybe,errors,nonetheless?)More)later)….)UDP Goal: detect “errors” (e.g., flipped bits) in transmitted segment3 Internet)Checksum)Example)• Note)– When)adding)numbers,)a)carryout)from)the)most)significant)bit)needs)to)be)added)to)the)result)• Example:)add)two)16Rbit)integers)UDP 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1 wraparound sum


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