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Elements of Transport Protocols Addressing Connection Establishment Connection Release Flow Control and Buffering Multiplexing Crash Recovery "Section Break(N€1t Page)•••••••.

Router Transport Protocol Router Physical communication channel Subnet Host (a) Environment of the data link layer. (b) Environment of the transport layer. • "Section Bruk(Next Page)".

I Over point-to-point links such as wires or optical fiber, it is usually not necessary for a router to specify which router it wants to talk to-each outgoing line leads directly to a particular router. In the transport layer, explicit addressing of destinations is required. 2 The process of establishing a connection over the wire of Fig(a) is simple: the other end is always there (unless it has crashed, in which case it is not there). Either way, there is not much to do. Even on wireless links the process is not much different. Just sending a message is sufficient to have it reach all other destinations. If the message is not acknowledged due to an error, it can be resent. In the transport layer, initial connection establishment is complicated, as we will see. 3 Another (exceedingly annoying) difference between the data link layer and the transport layer is the potential existence of storage capacity in the network. The consequences of the network's ability to delay and duplicate packets can sometimes be disastrous and can require the use of special protocols to correctly transport information. 4. Buffering and flow control are needed in both layers, but the presence in the transport layer of a large and varying number of connections with bandwidth that fluctuates as the connections compete with each other may require a different approach than we.

Addressing When an application (e.g., a user) process wishes to set up a connection to a remote application process, it must specify which one to connect to. (Connectionless transport has the same problem: to whom should each message be sent?) The method normally used is to define transport addresses to which processes can listen for connection requests. In the Internet, these endpoints are called ports. We will use the generic term TSAP (Transport Service Access Point) to mean a specific endpoint in the transport layer. The analogous endpoints in the network layer (i.e., network layer addresses) are not-surprisingly called NSAPs (Network Service Access Points). IP addresses are examples of NSAPs. ••••••••••SectiorBreak(NextPage)•••••••.

Addressing Host 1 Host 2 Sewer 1 Server 2 Application TSAP 1208 Application process layer Transpc,d : Transport layer SAP1522\ TSAP1836 connection I NSAP Network 1 NSAP Data Iirk layer Physe layer TSAPs, NSAPs and transport connections • Break(NutPager•••••••.

A possible scenario for a transport connection is as follows: l. A mail server process attaches itself to TSAP 1522 on host 2 to wait for an incoming call. A call such as our LISTEN might be used, for example. 2. An application process on host 1 wants to send an email message, so it attaches itself to TSAP 1208 and issues a CONNECT request. The request specifies TSAP 1208 on host 1 as the source and TSAP 1522 on host 2 as the destination. This action ultimately results in a transport connection being established between the application process and the server. 3. The application process sends over the mail message. 4. The mail server responds to say that it will deliver the message. 5. The transport connection is released. special process called a portmapper.

CONNECTION ESTABLISHMENT Establishing a connection sounds easy, but it is actually surprisingly tricky. At first glance, it would seem sufficient for one transport entity to just send a CONNECTION REQUEST segment to the destination and wait for a CONNECTION ACCEPTED reply. The problem occurs when the network can lose, delay, corrupt, and duplicate packets. This behavior causes serious complications To solve this specific DUPLICATES) Tomlinson (1975) introduced the three-way handshake. This establishment protocol involves one peer checking with the other that the connection request is indeed current. The normal setup procedure when host I initiates is shown in Fig. (a). Host I chooses a sequence number, x, and sends a CONNECTION REQUEST segment containing it to host 2. Host 2 replies with an ACK segment acknowledging x and announcing its own initial sequence number, y. Finally, host 1 acknowledges host 2's choice of an initial sequence number in the first data segment that it sends. -Section Break(NextPage)•••-.

Connection Establishment Host 1 Old dupicate REdEcr(ACK Host 1 Host2 (a) Od dugtate Odduphcate (Act * Three protocol scenarios for establishing a connection using a three-way handshake. CR denotes CONNECTION REQUEST. (a) Normal operation, (b) Old CONNECTION REQUEST appearing out of nowhere. (c) Duplicate CONNECTION REQUEST and duplicate ACK..

In Fig.(b), the first segment is a delayed duplicate CONNECTION REQUEST from an old connection. This segment arrives at host 2 without host I's knowledge. Host 2 reacts to this segment by sending host I an ACK segment, in effect asking for verification that host 1 was indeed trying to set up a new connection. When host 1 rejects host 2's attempt to establish a connection, host 2 realizes that it was tricked by a delayed duplicate and abandons the connection. In this way, a delayed duplicate does no damage The worst case is when both a delayed CONNECTION REQUEST and an ACK are floating around in the subnet. This case is shown in Fig. (c). As in the previous example, host 2 gets a delayed CONNECTION REQUEST and replies to it. At this point, it is crucial to realize that host 2 has proposed using y as the initial sequence number for host 2 to host 1 traffic, knowing full well that no segments containing sequence number y or acknowledgements to y are still in existence. When the second delayed segment arrives at host 2, the fact that z has been acknowledged rather than y tells host 2 that this, too, is an old duplicate. The important thing to realize here is that there is no combination of old segments that can cause the protocol to fail and have a connection set up by accident when no one wants it..

Connection Release Host 1 Host 2 DATA DATA 08 No data are delivered after a disconnect request Abrupt disconnection with loss of data..

there are two styles of terminating a connection: asymmetric release and symmetric release Asymmetric release is the way the telephone system works: when one party hangs up, the connection is broken. Symmetric release treats the connection as two separate unidirectional connections and requires each one to be released separately Asymmetric release is abrupt and may result in data loss. Consider the scenario of Fig. After the connection is established, host 1 sends a segment that arrives properly at host 2. Then host 1 sends another segment. Unfortunately, host 2 issues a DISCONNECT before the second segment arrives. The result is that the connection is released and data are lost. Clearly, a more sophisticated release protocol is needed to avoid data loss. One way is to use symmetric release, in which each direction is released independently of the other one. Here, a host can continue to receive data even after it has sent a DISCONNECT segment. Symmetric release does the job when each process has a fixed amount of data to send and clearly knows when it has sent it, One can envision a protocol in which host I says til am done. Are you done too?' If host 2 responds: "I am done too. Goodbye, the connection can be safely released." •-•Section.

Connection Release The two-army problem. • Pager-- Blue army White army army.

Connection Release Hostl Send DR + start timer Release connection Send ACK 08 ACK Host 2 Send DR + start timer Release connection Host 1 Send DR + start timer Release connection Send ACK ACK Host 2 Send DR + start timer Lost (Tldout) release connection (b) Four protocol scenarios for releasing a connection. (a) Normal case of a three-way handshake, (b) final ACK lost..

Connection Release Host I Send DR + start timer ( Timeout) send DR + start timer Release connection Send ACK Lost ACK Host 2 Send DR & start timer Send & start timer Release connection Host 1 Send + start timer Lost ( Timeout) send DR + start timer (N Tirieouts) release connection Lost Host 2 Send DR & start timer (Tim%ut) release connection (c) Response lost. (d) Response lost and subsequent DRS lost. "Section Break(Next.

In Fig. (a), we see the normal case in which one of the users sends a DR (DISCONNECTION REQUEST) segment to initiate the connection release. When it arrives, the recipient sends back a DR segment and starts a timer, just in case its DR is lost. When this DR arrives, the original sender sends back an ACK segment and releases the connection. Finally, when the ACK segment arrives, the receiver also releases the connection. If the final ACK segment is lost, as shown in Fig.(b), the situation is saved by the timer. When the timer expires, the connection is released anyway. Now consider the case of the second DR being lost. The user initiating the disconnection will not receive the expected response, will time out, and will start all overagain. In Fig.(c), we see how this works, assuming that the second time no segments are lost and all segments are delivered correctly and on time. Last scenario, Fig.(d), is the same as Fig. (c) except that now we assume all the repeated attempts to retransmit the DR also fail due to lost segments. After N retries, the senderjust gives up.

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