Request for Comments: 2140
TCP Control Block Interdependence
Status of this Memo
This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
This memo makes the case for interdependent TCP control blocks, where part of the TCP state is shared among similar concurrent connections, or across similar connection instances. TCP state includes a combination of parameters, such as connection state, current round- trip time estimates, congestion control information, and process information. This state is currently maintained on a per-connection basis in the TCP control block, but should be shared across connections to the same host. The goal is to improve transient transport performance, while maintaining backward-compatibility with existing implementations.
This document is a product of the LSAM project at ISI.
TCP is a connection-oriented reliable transport protocol layered over IP . Each TCP connection maintains state, usually in a data structure called the TCP Control Block (TCB). The TCB contains information about the connection state, its associated local process, and feedback parameters about the connection's transmission properties. As originally specified and usually implemented, the TCB is maintained on a per-connection basis. This document discusses the implications of that decision, and argues for an alternate implementation that shares some of this state across similar connection instances and among similar simultaneous connections. The resulting implementation can have better transient performance, especially for numerous short-lived and simultaneous connections, as often used in the World-Wide Web . These changes affect only the TCB initialization, and so have no effect on the long-term behavior of TCP after a connection has been established.
The TCP Control Block (TCB)
A TCB is associated with each connection, i.e., with each association of a pair of applications across the network. The TCB can be summarized as containing :
Local process state
pointers to send and receive buffers
pointers to retransmission queue and current segment pointers to Internet Protocol (IP) PCB
Per-connection shared state macro-state
local and remote host numbers and ports
send and receive window state (size*, current number) round-trip time and variance
cong. window size*
cong. window size threshold*
max windows seen*
round-trip time and variance#
The per-connection information is shown as split into macro-state and micro-state, terminology borrowed from . Macro-state describes the finite state machine; we include the endpoint numbers and components (timers, flags) used to help maintain that state. This includes the protocol for establishing and maintaining shared state about the connection. Micro-state describes the protocol after a connection has been established, to maintain the reliability and congestion control of the data transferred in the connection.
We further distinguish two other classes of shared micro-state that are associated more with host-pairs than with application pairs. One class is clearly host-pair dependent (#, e.g., MSS, RTT), and the other is host-pair dependent in its aggregate (*, e.g., cong. window info., curr. window sizes).
The observation that some TCB state is host-pair specific rather than application-pair dependent is not new, and is a common engineering decision in layered protocol implementations. A discussion of sharing RTT information among protocols layered over IP, including UDP and TCP, occurred in . T/TCP uses caches to maintain TCB information across instances, e.g., smoothed RTT, RTT variance, congestion avoidance threshold, and MSS . These values are in addition to connection counts used by T/TCP to accelerate data delivery prior to the full three-way handshake during an OPEN. The goal is to aggregate TCB components where they reflect one association - that of the host-pair, rather than artificially separating those components by connection.
At least one current T/TCP implementation saves the MSS and aggregates the RTT parameters across multiple connections, but omits caching the congestion window information , as originally specified in . There may be other values that may be cached, such as current window size, to permit new connections full access to accumulated channel resources.
We observe that there are two cases of TCB interdependence. Temporal sharing occurs when the TCB of an earlier (now CLOSED) connection to a host is used to initialize some parameters of a new connection to that same host. Ensemble sharing occurs when a currently active connection to a host is used to initialize another (concurrent) connection to that host. T/TCP documents considered the temporal case; we consider both.
An Example of Temporal Sharing
Temporal sharing of cached TCB data has been implemented in the SunOS 4.1.3 T/TCP extensions  and the FreeBSD port of same . As mentioned before, only the MSS and RTT parameters are cached, as originally specified in . Later discussion of T/TCP suggested including congestion control parameters in this cache .
The cache is accessed in two ways: it is read to initialize new TCBs, and written when more current per-host state is available. New TCBs are initialized as follows; snd_cwnd reuse is not yet implemented, although discussed in the T/TCP concepts :
TEMPORAL SHARING - TCB Initialization
Cached TCB New TCB ---------------------------------------- old-MSS old-MSS old-RTT old-RTT old-RTTvar old-RTTvar old-snd_cwnd old-snd_cwnd (not yet impl.)
Most cached TCB values are updated when a connection closes. An exception is MSS, which is updated whenever the MSS option is received in a TCP header.
TEMPORAL SHARING - Cache Updates
Cached TCB Current TCB when? New Cached TCB --------------------------------------------------------------- old-MSS curr-MSS MSSopt curr-MSS old-RTT curr-RTT CLOSE old += (curr - old) >> 2 old-RTTvar curr-RTTvar CLOSE old += (curr - old) >> 2 old-snd_cwnd curr-snd_cwnd CLOSE curr-snd_cwnd (not yet impl.)
MSS caching is trivial; reported values are cached, and the most recent value is used. The cache is updated when the MSS option is received, so the cache always has the most recent MSS value from any connection. The cache is consulted only at connection establishment, and not otherwise updated, which means that MSS options do not affect current connections. The default MSS is never saved; only reported MSS values update the cache, so an explicit override is required to reduce the MSS.
RTT values are updated by a more complicated mechanism , . Dynamic RTT estimation requires a sequence of RTT measurements, even though a single T/TCP transaction may not accumulate enough samples. As a result, the cached RTT (and its variance) is an average of its previous value with the contents of the currently active TCB for that host, when a TCB is closed. RTT values are updated only when a connection is closed. Further, the method for averaging the RTT values is not the same as the method for computing the RTT values within a connection, so that the cached value may not be appropriate.
For temporal sharing, the cache requires updating only when a connection closes, because the cached values will not yet be used to initialize a new TCB. For the ensemble sharing, this is not the case, as discussed below.
Other TCB variables may also be cached between sequential instances, such as the congestion control window information. Old cache values can be overwritten with the current TCB estimates, or a MAX or MIN function can be used to merge the results, depending on the optimism or pessimism of the reused values. For example, the congestion window can be reused if there are no concurrent connections.
An Example of Ensemble Sharing
Sharing cached TCB data across concurrent connections requires attention to the aggregate nature of some of the shared state. Although MSS and RTT values can be shared by copying, it may not be appropriate to copy congestion window information. At this point, we present only the MSS and RTT rules:
ENSEMBLE SHARING - TCB Initialization
Cached TCB New TCB ---------------------------------- old-MSS old-MSS old-RTT old-RTT old-RTTvar old-RTTvar
ENSEMBLE SHARING - Cache Updates
Cached TCB Current TCB when? New Cached TCB ----------------------------------------------------------- old-MSS curr-MSS MSSopt curr-MSS old-RTT curr-RTT update rtt_update(old,curr) old-RTTvar curr-RTTvar update rtt_update(old,curr)
For ensemble sharing, TCB information should be cached as early as possible, sometimes before a connection is closed. Otherwise, opening multiple concurrent connections may not result in TCB data sharing if no connection closes before others open. An optimistic solution would be to update cached data as early as possible, rather than only when a connection is closing. Some T/TCP implementations do this for MSS when the TCP MSS header option is received , although it is not addressed specifically in the concepts or functional specification .
In current T/TCP, RTT values are updated only after a CLOSE, which does not benefit concurrent sessions. As mentioned in the temporal case, averaging values between concurrent connections requires incorporating new RTT measurements. The amount of work involved in updating the aggregate average should be minimized, but the resulting value should be equivalent to having all values measured within a single connection. The function "rtt_update" in the ensemble sharing table indicates this operation, which occurs whenever the RTT would have been updated in the individual TCP connection. As a result, the cache contains the shared RTT variables, which no longer need to reside in the TCB .
Congestion window size aggregation is more complicated in the concurrent case. When there is an ensemble of connections, we need to decide how that ensemble would have shared the congestion window, in order to derive initial values for new TCBs. Because concurrent connections between two hosts share network paths (usually), they also share whatever capacity exists along that path. With regard to congestion, the set of connections might behave as if it were multiplexed prior to TCP, as if all data were part of a single connection. As a result, the current window sizes would maintain a constant sum, presuming sufficient offered load. This would go beyond caching to truly sharing state, as in the RTT case.
We pause to note that any assumption of this sharing can be incorrect, including this one. In current implementations, new congestion windows are set at an initial value of one segment, so that the sum of the current windows is increased for any new connection. This can have detrimental consequences where several connections share a highly congested link, such as in trans-Atlantic Web access.
There are several ways to initialize the congestion window in a new TCB among an ensemble of current connections to a host, as shown below. Current TCP implementations initialize it to one segment , and T/TCP hinted that it should be initialized to the old window size . In the former, the assumption is that new connections should behave as conservatively as possible. In the latter, no accommodation is made to concurrent aggregate behavior.
In either case, the sum of window sizes can increase, rather than remain constant. Another solution is to give each pending connection its "fair share" of the available congestion window, and let the connections balance from there. The assumption we make here is that new connections are implicit requests for an equal share of available link bandwidth which should be granted at the expense of current connections. This may or may not be the appropriate function; we propose that it be examined further.
ENSEMBLE SHARING - TCB Initialization
Some Options for Sharing Window-size
Cached TCB New TCB ----------------------------------------------------------------- old-snd_cwnd (current) one segment (T/TCP hint) old-snd_cwnd (proposed) old-snd_cwnd/(N+1) subtract old-snd_cwnd/(N+1)/N from each concurrent
ENSEMBLE SHARING - Cache Updates
Cached TCB Current TCB when? New Cached TCB ---------------------------------------------------------------- old-snd_cwnd curr-snd_cwnd update (adjust sum as appropriate)
Current TCP implementations do not use TCB caching, with the exception of T/TCP variants . New connections use the default initial values of all non-instantiated TCB variables. As a result, each connection calculates its own RTT measurements, MSS value, and congestion information. Eventually these values are updated for each connection.
For the congestion and current window information, the initial values may not be consistent with the long-term aggregate behavior of a set of concurrent connections. If a single connection has a window of 4 segments, new connections assume initial windows of 1 segment (the minimum), although the current connection's window doesn't decrease to accommodate this additional load. As a result, connections can mutually interfere. One example of this has been seen on trans- Atlantic links, where concurrent connections supporting Web traffic can collide because their initial windows are too large, even when set at one segment.
Because this proposal attempts to anticipate the aggregate steady- state values of TCB state among a group or over time, it should avoid the transient effects of new connections. In addition, because it considers the ensemble and temporal properties of those aggregates, it should also prevent the transients of short-lived or multiple concurrent connections from adversely affecting the overall network performance. We are performing analysis and experiments to validate these assumptions.
Here we attempt to optimize transient behavior of TCP without modifying its long-term properties. The predominant expense is in maintaining the cached values, or in using per-host state rather than per-connection state. In cases where performance is affected, however, we note that the per-host information can be kept in per- connection copies (as done now), because with higher performance should come less interference between concurrent connections.
Sharing TCB state can occur only at connection establishment and close (to update the cache), to minimize overhead, optimize transient behavior, and minimize the effect on the steady-state. It is possible that sharing state during a connection, as in the RTT or window-size variables, may be of benefit, provided its implementation cost is not high.
There are several implications to incorporating TCB interdependence in TCP implementations. First, it may prevent the need for application-layer multiplexing for performance enhancement . Protocols like persistent-HTTP avoid connection reestablishment costs by serializing or multiplexing a set of per-host connections across a single TCP connection. This avoids TCP's per-connection OPEN handshake, and also avoids recomputing MSS, RTT, and congestion windows. By avoiding the so-called, "slow-start restart," performance can be optimized. Our proposal provides the MSS, RTT, and OPEN handshake avoidance of T/TCP, and the "slow-start restart avoidance" of multiplexing, without requiring a multiplexing mechanism at the application layer. This multiplexing will be complicated when quality-of-service mechanisms (e.g., "integrated services scheduling") are provided later.
Second, we are attempting to push some of the TCP implementation from the traditional transport layer (in the ISO model ), to the network layer. This acknowledges that some state currently maintained as per-connection is in fact per-path, which we simplify as per- host-pair. Transport protocols typically manage per-application-pair associations (per stream), and network protocols manage per-path associations (routing). Round-trip time, MSS, and congestion information is more appropriately handled in a network-layer fashion, aggregated among concurrent connections, and shared across connection instances.
An earlier version of RTT sharing suggested implementing RTT state at the IP layer, rather than at the TCP layer . Our observations are for sharing state among TCP connections, which avoids some of the difficulties in an IP-layer solution. One such problem is determining the associated prior outgoing packet for an incoming packet, to infer RTT from the exchange. Because RTTs are still determined inside the TCP layer, this is simpler than at the IP layer. This is a case where information should be computed at the transport layer, but shared at the network layer.
We also note that per-host-pair associations are not the limit of these techniques. It is possible that TCBs could be similarly shared between hosts on a LAN, because the predominant path can be LAN-LAN, rather than host-host.
There may be other information that can be shared between concurrent connections. For example, knowing that another connection has just tried to expand its window size and failed, a connection may not attempt to do the same for some period. The idea is that existing TCP implementations infer the behavior of all competing connections, including those within the same host or LAN. One possible optimization is to make that implicit feedback explicit, via extended information in the per-host TCP area.
These suggested implementation enhancements do not have additional ramifications for direct attacks. These enhancements may be susceptible to denial-of-service attacks if not otherwise secured. For example, an application can open a connection and set its window size to 0, denying service to any other subsequent connection between those hosts.
TCB sharing may be susceptible to denial-of-service attacks, wherever the TCB is shared, between connections in a single host, or between hosts if TCB sharing is implemented on the LAN (see Implications section). Some shared TCB parameters are used only to create new TCBs, others are shared among the TCBs of ongoing connections. New connections can join the ongoing set, e.g., to optimize send window size among a set of connections to the same host.
Attacks on parameters used only for initialization affect only the transient performance of a TCP connection. For short connections, the performance ramification can approach that of a denial-of-service attack. E.g., if an application changes its TCB to have a false and small window size, subsequent connections would experience performance degradation until their window grew appropriately.
The solution is to limit the effect of compromised TCB values. TCBs are compromised when they are modified directly by an application or transmitted between hosts via unauthenticated means (e.g., by using a dirty flag). TCBs that are not compromised by application modification do not have any unique security ramifications. Note that the proposed parameters for TCB sharing are not currently modifiable by an application.
All shared TCBs MUST be validated against default minimum parameters before used for new connections. This validation would not impact performance, because it occurs only at TCB initialization. This limits the effect of attacks on new connections, to reducing the benefit of TCB sharing, resulting in the current default TCP performance. For ongoing connections, the effect of incoming packets on shared information should be both limited and validated against constraints before use. This is a beneficial precaution for existing TCP implementations as well.
TCBs modified by an application SHOULD not be shared, unless the new connection sharing the compromised information has been given explicit permission to use such information by the connection API. No mechanism for that indication currently exists, but it could be supported by an augmented API. This sharing restriction SHOULD be implemented in both the host and the LAN. Sharing on a LAN SHOULD utilize authentication to prevent undetected tampering of shared TCB parameters. These restrictions limit the security impact of modified TCBs both for connection initialization and for ongoing connections.
Finally, shared values MUST be limited to performance factors only. Other information, such as TCP sequence numbers, when shared, are already known to compromise security.
The author would like to thank the members of the High-Performance Computing and Communications Division at ISI, notably Bill Manning, Bob Braden, Jon Postel, Ted Faber, and Cliff Neuman for their assistance in the development of this memo.
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Joe Touch University of Southern California/Information Sciences Institute 4676 Admiralty Way Marina del Rey, CA 90292-6695 USA Phone: +1 310-822-1511 x151 Fax: +1 310-823-6714 URL: http://www.isi.edu/~touch Email: firstname.lastname@example.org