1. Field of the Invention
The invention relates to network transmission using the TCP protocol.
2. Description of the Related Art
A storage area network (SAN) may be implemented as a high-speed, special purpose network that interconnects different kinds of data storage devices with associated data servers on behalf of a large network of users. Typically, a storage area network includes high performance switches as part of the overall network of computing resources for an enterprise. The storage area network is usually clustered in close geographical proximity to other computing resources, such as mainframe computers, but may also extend to remote locations for backup and archival storage using wide area network carrier technologies. Fibre Channel networking is typically used in SANs although other communications technologies may also be employed, including Ethernet and IP-based storage networking standards (e.g., iSCSI, FCIP (Fibre Channel over Internet Protocol), etc.).
As used herein, the term “Fibre Channel” refers to the Fibre Channel (FC) family of standards (developed by the American National Standards Institute (ANSI)) and other related and draft standards. In general, Fibre Channel defines a transmission medium based on a high speed communications interface for the transfer of large amounts of data via connections between varieties of hardware devices.
FC standards have defined limited allowable distances between FC switch elements. Fibre Channel over IP (FCIP) refers to mechanisms that allow the interconnection of islands of FC SANs over IP-based (internet protocol-based) networks to form a unified SAN in a single FC fabric, thereby extending the allowable distances between FC switch elements to those allowable over an IP network. For example, FCIP relies on IP-based network services to provide the connectivity between the SAN islands over local area networks (LANs), metropolitan area networks (MANs), and wide area networks (WANs). Accordingly, using FCIP, a single FC fabric can connect physically remote FC sites allowing remote disk access, tape backup, and live mirroring.
In an FCIP implementation, FC traffic is carried over an IP network through a logical FCIP tunnel. Each FCIP entity on either side of the IP network works at the session layer of the OSI model. The FC frames from the FC SANs are encapsulated in IP packets and transmission control protocol (TCP) segments and transported in accordance with the TCP layer in one or more TCP sessions. For example, an FCIP tunnel is created over the IP network and a TCP session is opened in the FCIP tunnel.
One common problem in TCP/IP networks is packet loss. Each packet must be acknowledged. Usually this is done sequentially as the packets arrive, but in certain cases packets may be lost or corrupted and following packets received correctly.
While Standard TCP has fast recovery mechanisms to quickly recover from packet loss on a network, they have some limitations when multiple packet loss has occurred. Multiple packet loss is defined as when the first transmission of a packet has been lost and then when one or more of the subsequent retransmissions of the same packet are also lost. With Standard TCP if the fast recovery mechanism fails to recover in the multiple loss scenario, it will resort to a slow recovery mechanism. The slow recovery mechanism will dramatically reduce the overall throughput of the connection.
One approach to address this issue is a mechanism called re-SACK. The re-SACK mechanism tries to describe multiple loss (transmitter to receiver) to the transmitter with the order of information in the Standard TCP SACK optional header. This mechanism is reliant, however, on this information not being lost in the opposite direction (receiver to transmitter). If this re-SACK information packet is lost as well, there will be no recovery of this information and slow recovery is the last resort. This is described in more detail in U.S. patent application Ser. No. 12/972,713, entitled “Repeated Lost Packet Retransmission in a TCP/IP Network,” filed Dec. 20, 2010, hereby incorporated by reference.
Once in Fast Recovery, Extended Fast Recovery operation starts a timer on the retransmission of each packet. The time expires in one adjusted round trip time. If there has not been an acknowledgement for the retransmitted packet and the Extended Fast Recovery timer expires, it is assumed that the retransmitted packet was lost and must be retransmitted again. Extended Fast Recovery operation keeps retransmitting the packet, once every adjusted round trip time, until an acknowledgement is received or the slow recovery timer expires. Segment Timing is an addition to Extended Fast Recovery where every sent packet is timed separately from the time of first transmission, not just retransmitted packets.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an implementation of apparatus and methods consistent with the present invention and, together with the detailed description, serve to explain advantages and principles consistent with the invention.
The IP gateway device 104 encapsulates FC packets received from the source nodes 106, 108, and 110 in TCP segments and IP packets and forwards the TCP/IP-packet-encapsulated FC frames over the IP network 102. The IP gateway device 118 receives these encapsulated FC frames from the IP network 102, “de-encapsulates” them (i.e., extracts the FC frames from the received IP packets and TCP segments), and forwards the extracted FC frames through the FC fabric 120 to their appropriate destination nodes 112, 114, and 116. It should be understood that each IP gateway device 104 and 118 can perform the opposite role for traffic going in the opposite direction (e.g., the IP gateway device 118 doing the encapsulating and forwarding through the IP network 102 and the IP gateway device 104 doing the de-encapsulating and forwarding the extracted FC frames through an FC fabric). In other configurations, an FC fabric may or may not exist on either side of the IP network 102. As such, in such other configurations, at least one of the IP gateway devices 104 and 118 could be a tape extender, an Ethernet NIC, etc.
Each IP gateway device 104 and 118 includes an IP interface, which appears as an end station in the IP network 102. Each IP gateway device 104 and 118 also establishes a logical FCIP tunnel through the IP network 102. The IP gateway devices 104 and 118 implement the FCIP protocol and rely on the TCP layer to transport the TCP/IP-packet-encapsulated FC frames over the IP network 102. Each FCIP tunnel between two IP gateway devices connects two TCP end points in the IP network 102. Viewed from the FC perspective, pairs of switches export virtual E_PORTs or virtual EX_PORTs (collectively referred to as virtual E_PORTs) that enable forwarding of FC frames between FC networks, such that the FCIP tunnel acts as an FC InterSwitch Link (ISL) over which encapsulated FC traffic flows.
The FC traffic is carried over the IP network 102 through the FCIP tunnel between the IP gateway device 104 and the IP gateway device 118 in such a manner that the FC fabric 102 and all purely FC devices (e.g., the various source and destination nodes) are unaware of the IP network 102. As such, FC datagrams are delivered in such time as to comply with applicable FC specifications.
To accommodate multiple levels of priority, the IP gateway devices 104 and 118 create distinct TCP sessions for each level of priority supported, plus a TCP session for a class-F control stream. In one implementation, low, medium, and high priorities are supported, so four TCP sessions are created between the IP gateway devices 104 and 118, although the number of supported priority levels and TCP sessions can vary depending on the network configuration. The control stream and each priority stream is assigned its own TCP session that is autonomous in the IP network 102, getting its own TCP stack and its own settings for VLAN Tagging (IEEE 802.1Q), quality of service (IEEE 802.1P) and Differentiated Services Code Point (DSCP). Furthermore, the traffic flow in each per priority TCP session is enforced in accordance with its designated priority by an algorithm, such as but not limited to a deficit weighted round robin (DWRR) scheduler. All control frames in the class-F TCP session are strictly sent on a per service interval basis.
The FC host 208 couples to an FC port 212 of the IP gateway device 200. The coupling may be made directly between the FC port 212 and the FC host 208 or indirectly through an FC fabric (not shown). The FC port 212 receives FC frames from the FC host 208 and forwards them to an Ethernet port 214, which includes an FCIP virtual E_PORT 216 and a TCP/IP interface 218 coupled to the IP network 204. The FCIP virtual E_PORT 216 acts as one side of the logical ISL formed by the FCIP tunnel 206 over the IP network 204. An FCIP virtual E_PORT 220 in the IP gateway device 202 acts as the other side of the logical ISL. The Ethernet port 214 encapsulates each FC frame received from the FC port 212 in a TCP segment belonging to the TCP session for the designated priority and an IP packet shell and forwards them over the IP network 204 through the FCIP tunnel 206.
The FC target 210 couples to an FC port 226 of the IP gateway device 202. The coupling may be made directly between the FC port 226 and the FC host 210 or indirectly through an FC fabric (not shown). An Ethernet port 222 receives TCP/IP-packet-encapsulated FC frames over the IP network 204 from the IP gateway device 200 via a TCP/IP interface 224. The Ethernet port 222 de-encapsulates the received FC frames and forwards them to an FC port 226 for communication to the FC target device 210.
It should be understood that data traffic can flow in either direction between the FC host 208 and the FC target 210. As such, the roles of the IP gateway devices 200 and 202 may be swapped for data flowing from the FC target 210 and the FC host 208.
Tunnel manager modules 232 and 234 (e.g., circuitry, firmware, software or some combination thereof) of the IP gateway devices 200 and 202 set up and maintain the FCIP tunnel 206. Either IP gateway device 200 or 202 can initiate the FCIP tunnel 206, but for this description, it is assumed that the IP gateway device 200 initiates the FCIP tunnel 206. After the Ethernet ports 214 and 222 are physically connected to the IP network 204, data link layer and IP initialization occur. The TCP/IP interface 218 obtains an IP address for the IP gateway device 200 (the tunnel initiator) and determines the IP address and TCP port numbers of the remote IP gateway device 202. The FCIP tunnel parameters may be configured manually, discovered using Service Location Protocol Version 2 (SLPv2), or designated by other means. The IP gateway device 200, as the tunnel initiator, transmits an FCIP Special Frame (FSF) to the remote IP gateway device 202. The FSF contains the FC identifier and the FCIP endpoint identifier of the IP gateway device 200, the FC identifier of the remote IP gateway device 202, and a 64-bit randomly selected number that uniquely identifies the FSF. The remote IP gateway device 202 verifies that the contents of the FSF match its local configuration. If the FSF contents are acceptable, the unmodified FSF is echoed back to the (initiating) IP gateway device 200. After the IP gateway device 200 receives and verifies the FSF, the FCIP tunnel 206 can carry encapsulated FC traffic.
The TCP/IP interface 218 creates multiple TCP sessions through the single FCIP tunnel 206. In the illustrated implementation, three or more TCP sessions are created in the single FCIP tunnel 206. One TCP connection is designated to carry control data (e.g., class-F data), and the remaining TCP sessions are designated to carry data streams having different levels of priority. For example, considering a three priority QoS scheme, four TCP sessions are created in the FCIP tunnel 206 between the IP gateway device 200 and the IP gateway device 202, one TCP session designated for control data, and the remaining TCP sessions designated for high, medium, and low priority traffic, respectively. Note: It should be understood that multiple TCP sessions designated with the same level of priority may also be created (e.g., two high priority TCP sessions) within the same FCIP tunnel.
The FCIP tunnel 206 maintains frame ordering within each priority TCP flow. The QoS enforcement engine may alter the egress transmission sequence of flows relative to their ingress sequence based on priority. However, the egress transmission sequence of frames within an individual flow will remain in the same order as their ingress sequence to that flow. Because the flows are based on FC initiator and FC target, conversational frames between two FC devices will remain in proper sequence. A characteristic of TCP is to maintain sequence order of bytes transmitted before deliver to upper layer protocols. As such, the IP gateway device at the remote end of the FCIP tunnel 206 is responsible for reordering data frames received from the various TCP sessions before sending them up the communications stack to the FC application layer. Furthermore, in one implementation, each TCP session can service as a backup in the event a lower (or same) priority TCP session fails. Each TCP session can be routed and treated independently of others via autonomous settings for VLAN and Priority Tagging and/or DSCP.
In addition to setting up the FCIP tunnel 206, the IP gateway device 200 may also set up TCP trunking through the FCIP tunnel 206. TCP trunking allows the creation of multiple FCIP connections within the FCIP tunnel 206, with each FCIP connection connecting a source-destination IP address pair. In addition, each FCIP connection can maintain multiple TCP sessions, each TCP session being designated for different priorities of service. As such, each FCIP connection can have different attributes, such as IP addresses, committed rates, priorities, etc., and can be defined over the same Ethernet port or over different Ethernet ports in the IP gateway device. The trunked FCIP connections support load balancing and provide failover paths in the event of a network failure, while maintaining in-order delivery. For example, if one FCIP connection in the TCP trunk fails or becomes congested, data can be redirected to a same-priority TCP session of another FCIP connection in the FCIP tunnel 206. The IP gateway device 202 receives the TCP/IP-packet-encapsulated FC frames and reconstitutes the data streams in the appropriate order through the FCIP virtual E_PORT 220. These variations are described in more detail below.
Each IP gateway device 200 and 202 includes an FCIP control manager (see FCIP control managers 228 and 230), which generate the class-F control frames for the control data stream transmitted through the FCIP tunnel 206 to the FCIP control manager in the opposing IP gateway device. Class-F traffic is connectionless and employs acknowledgement of delivery or failure of delivery. Class-F is employed with FC switch expansion ports (E PORTS) and is applicable to the IP gateway devices 200 and 202, based on the FCIP virtual E_PORT 216 and 220 created in each IP gateway device. Class-F control frames are used to exchange routing, name service, and notifications between the IP gateway devices 200 and 202, which join the local and remote FC networks into a single FC fabric. However, the described technology is not limited to combined single FC fabrics and is compatible with FC routed environments.
The IP gateway devices 200 and 202 emulate raw FC ports (e.g., VE_PORTs or VEX_PORTs) on both of the FCIP tunnel 206. For FC I/O data flow, these emulated FC ports support ELP (Exchange Link Parameters), EFP (Exchange Fabric Parameters, and other FC-FS (Fibre Channel-Framing and Signaling) and FC-SW (Fibre Channel-Switched Fabric) protocol exchanges to bring the emulated FC E_PORTs online. After the FCIP tunnel 206 is configured and the TCP sessions are created for an FCIP connection in the FCIP tunnel 206, the IP gateway devices 200 and 202 will activate the logical ISL over the FCIP tunnel 206. When the ISL has been established, the logical FC ports appear as virtual E_PORTs in the IP gateway devices 200 and 202. For FC fabric services, the virtual E_PORTs emulate regular E_PORTs, except that the underlying transport is TCP/IP over an IP network, rather than FC in a normal FC fabric. Accordingly, the virtual E_PORTs 216 and 220 preserve the “semantics” of an E_PORT.
Currently TCP will enter Fast Recovery when the receiver notifies the transmitter of out of sequence packets by sending duplicate acknowledgements for every out of sequence packet. The transmitter will have a threshold of the number of duplicate acknowledgements, typically three, before going into Fast Recovery to retransmit lost data. Extended Fast Recovery according to the present invention is an enhancement to the existing Fast Recovery mechanism in Standard TCP. Once in Fast Recovery, Extended Fast Recovery operation starts a timer on the retransmission of each packet. The time expires in one measured round trip time. If there has not been an acknowledgement for the retransmitted packet and the Extended Fast Recovery timer expires, it is assumed that the retransmitted packet was lost and must be retransmitted again. Extended Fast Recovery operation keeps retransmitting the packet, once every round trip time, until an acknowledgement is received or the slow recovery timer expires. The benefits to this enhancement are quick retransmission of lost packets instead of waiting for the slow recovery timer to expire or DUP ACKS or SACKs to be received. Recovering with Fast Recovery is preferred to Slow Recovery because while Fast Recovery is attempting to retransmit only the packets that are lost, new data is allowed to be transmitted because the transmit window remains open. In Slow Recovery, no new data is transmitted, only the retransmitted packets. Extended Fast Recovery operation can lead to quicker recovery on networks with high loss avoiding the need to go into Slow Recovery.
Operation of the preferred embodiment is illustrated in the flowcharts of
In
In
In
It is understood that the operations of
Segment Timing is an addition to Extended Fast Recovery where every sent packet is timed separately from the time of first transmission, not just retransmitted packets. If the time from first transmission has exceeded the measured round trip time a retransmission of that packet will be required. This is a more aggressive form of retransmission that does not require the fast recovery notification from the receiver to retransmit packets. It does however mean packets could be retransmitted that are not needed due to the original packet being delayed on a network longer than normal. This method is beneficial for applications that are sensitive to changes in latency due to the retransmission of lost data, because the faster the transmitter can recover from loss the better the application will perform. These types of applications can perform better even taking into account the overhead from occasional non-necessary retransmissions.
The preferred embodiment of Segment Timing for packet X is illustrated in
In
In
While separate ST and EFR timers are described, it is understood that a single timer can be used for both operations, with the possible addition of a flag to indicate ST or EFR use.
It is also understood that the flowcharts are a simplification of any actual embodiment and are provided to simplify operation according to the present invention. It is further understood that the flowchart operations can be performed by hardware logic, a processor and firmware or software or a combination.
By retransmitting a packet after just an RTT without receiving an ACK, either for the original transmission or for a fast recovery retransmission, packets can be provided to the receiver earlier than if waiting for fast recovery or slow recovery operations to be started and hopefully avoids entry into slow recovery and its great slowdown of effective data transfer.
The above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments may be used in combination with each other. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.”
This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 61/867,787, entitled “TCP Extended Fast Recovery and Segment Timing,” filed Aug. 20, 2013, which is hereby incorporated by reference.
Number | Date | Country | |
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61867787 | Aug 2013 | US |