Internet and Intranet Protocols and Applications The Internet: Delay, Errors, Detection February, 2002 Joseph Conron Computer Science Department New York University

Internet: Congestion and Reliability Definition of reliable communication –Packets are delivered without modification. –Packets are delivered in order. –Packets are delivered exactly once. –Packets are delivered within some maximum time. –The sender and receiver(s) agree.

Internet: RTT and Delay Round Trip Time (RTT) –the time measured from send of request to receipt of response (from application view, this is called response time). Good Idea –keep average RTT small! Sources of Delay –Processing delay –Queuing delay –Transmission Delay –Propagation delay

Internet: Delay Processing delay –Time to process packet header and determine disposition Queuing delay –Time packet waits on a queue prior to transmission onto the link Transmission delay –Time for all bits of packet to be transmitted = L/R where L is packet size in bits and R is transmit rate in bits/sec. Propagation delay –Time from first bit is “on the wire” until bit is seen at the receiver = d/s where d is link distance (meters) and s is propagation speed of link (2*10 8 to 3*10 8 meters/sec)

Internet: How big are these delays? Example –NY to CA, T1 Link (1.54*10 6 bps), no routers –d = 5*10 6 m, s = 3*10 8 m/sec –So, propagation delay = 1.67*10 -2 sec = 16.7 msec –What is absolute minimum RTT? –For 1024 byte IP packet: Td = 8*1024/(1.54*10 6 ) = 5.3 msec –So, to send 1 packet and get 1 packet in response takes AT LEAST 44 msec! And that’s with a direct link – no routers!

Internet: detecting trouble Routers can drop packets (why?) But how do packets get out of order? How are they duplicated? How can they be corrupted? And most importantly, how can we detect this?

Internet: blame it on congestion! Routers can drop packets because: –They have no resources (memory) to process packet –The packet time-to-live (TTL) has expired (no route to host) Hhow do packets get out of order? –Packets n and n+1 take different routes –Path for packet n is more congested than path for packet n+1, hence n+1 packet arrives before packet n

Internet: how do duplicates occur! Well, we can only blame congestion indirectly. Suppose sender times out waiting for ACK for packet n? Suppose receiver actually got first packet n and sent ACK, but ACK is dropped due to congestion. Sender resends packet n! Receiver sees it as a duplicate! Can the ACK be duplicated?

Internet: packet corruption How can packets be corrupted? –Typically caused by a transient failure within the transmission medium, but relatively rare these days. –Can a software bug cause corruption? –What about an intruder (malicious tamper)?

Internet: sender/receiver agreement Sender wants to know that receiver has the packet) Most common technique: positive acknowledgment. TCP uses positive ack in sliding window mechanism.

Error detection mechanisms Sequence numbers –Defines order (for all n, n-1 before n) –Implies packet existence ( if receiver gets packet n+1, then there must be a packet n) –Detects duplicates (if all packets up to n have been received, then any packet < n is a duplicate) Timers –Using our knowledge of delays and the application, we can set time limits for responses.

Error detection mechanisms Detecting corruption –Check sums –CRC (Cyclic Redundancy Check) –Reed Solomon Codes –FEC (Forward Error Correction) –Message Digests