RFC 2757
Long Thin Networks
Pages: 46
Informational
Informational
Part 2 of 2 – Pages 21 to 46
4.8 Active Queue Management
As has been pointed out above, TCP responds to congestion by closing down the window and invoking slow start. Long-delay networks take a particularly long time to recover from this condition. Accordingly, it is imperative to avoid congestion in LTNs. To remedy this, active queue management techniques have been proposed as enhancements to routers throughout the Internet [RED]. The primary motivation for deployment of these mechanisms is to prevent "congestion collapse" (a severe degradation in service) by controlling the average queue size at the routers. As the average queue length grows, Random Early Detection [RED] increases the possibility of dropping packets. The benefits are: - Reduce packet drops in routers. By dropping a few packets before severe congestion sets in, RED avoids dropping bursts of packets. In other words, the objective is to drop m packets early to prevent n drops later on, where m is less than n. - Provide lower delays. This follows from the smaller queue sizes, and is particularly important for interactive applications, for which the inherent delays of wireless links already push the user experience to the limits of the non- acceptable. - Avoid lock-outs. Lack of resources in a router (and the resultant packet drops) may, in effect, obliterate throughput on certain connections. Because of active queue management, it is more probable for an incoming packet to find available buffer space at the router. Active Queue Management has two components: (1) routers detect congestion before exhausting their resources, and (2) they provide some form of congestion indication. Dropping packets via RED is only one example of the latter. Another way to indicate congestion is to use ECN [ECN] as discussed above under "Detecting Corruption Loss: With Explicit Notifications." Recommendation: RED is currently being deployed in the Internet, and LTNs should follow suit. ECN deployment should complement RED's.4.9 Scheduling Algorithms
Active queue management helps control the length of the queues. Additionally, a general solution requires replacing FIFO with other scheduling algorithms that improve:
1. Fairness (by policing how different packet streams utilize the
available bandwidth), and
2. Throughput (by improving the transmitter's radio channel
utilization).
For example, fairness is necessary for interactive applications (like
telnet or web browsing) to coexist with bulk transfer sessions.
Proposals here include:
- Fair Queueing (FQ) [Demers90]
- Class-based Queueing (CBQ) [Floyd95]
Even if they are only implemented over the wireless link portion of
the communication path, these proposals are attractive in wireless
LTN environments, because new connections for interactive
applications can have difficulty starting when a bulk TCP transfer
has already stabilized using all available bandwidth.
In our base architecture described above, the mobile device typically
communicates directly with only one wireless peer at a given time:
the intermediate node. In some W-WANs, it is possible to directly
address other mobiles within the same cell. Direct communication
with each such wireless peer may traverse a spatially distinct path,
each of which may exhibit statistically independent radio link
characteristics. Channel State Dependent Packet Scheduling (CSDP)
[BBKT96] tracks the state of the various radio links (as defined by
the target devices), and gives preferential treatment to packets
destined for radio links in a "good" state. This avoids attempting to
transmit to (and expect acknowledgements from) a peer on a "bad"
radio link, thus improving throughput.
A further refinement of this idea suggests that both fairness and
throughput can be improved by combining a wireless-enhanced CBQ with
CSDP [FSS98].
Recommendation: No recommendation at this time, pending further
study.
4.10 Split TCP and Performance-Enhancing Proxies (PEPs)
Given the dramatic differences between the wired and the wireless
links, a very common approach is to provide some impedance matching
where the two different technologies meet: at the intermediate node.
The idea is to replace an end-to-end TCP connection with two clearly distinct connections: one across the wireless link, the other across its wireline counterpart. Each of the two resulting TCP sessions operates under very different networking characteristics, and may adopt the policies best suited to its particular medium. For example, in a specific LTN topology it may be desirable to modify TCP Fast Retransmit to resend after the first duplicate ack and Fast Recovery to not shrink the congestion window if the LTN link has an extremely long RTT, is known to not reorder packets, and is not subject to congestion. Moreover, on a long-delay link or on a link with a relatively high bandwidth-delay product it may be desirable to "slow-start" with a relatively large initial window, even larger than four segments. While these kinds of TCP modifications can be negotiated to be employed over the LTN link, they would not be deployed end-to-end over the global Internet. In LTN topologies where the underlying link characteristics are known, a various similar types of performance enhancements can be employed without endangering operations over the global Internet. In some proposals, in addition to a PEP mechanism at the intermediate node, custom protocols are used on the wireless link (for example, [WAP], [YB94] or [MOWGLI]). Even if the gains from using non-TCP protocols are moderate or better, the wealth of research on optimizing TCP for wireless, and compatibility with the Internet are compelling reasons to adopt TCP on the wireless link (enhanced as suggested in section 5 below).4.10.1 Split TCP Approaches
Split-TCP proposals include schemes like I-TCP [ITCP] and MTCP [YB94] which achieve performance improvements by abandoning end-to-end semantics. The Mowgli architecture [MOWGLI] proposes a split approach with support for various enhancements at all the protocol layers, not only at the transport layer. Mowgli provides an option to replace the TCP/IP core protocols on the LTN link with a custom protocol that is tuned for LTN links [KRLKA97]. In addition, the protocol provides various features that are useful with LTNs. For example, it provides priority-based multiplexing of concurrent connections together with shared flow control, thus offering link capacity to interactive applications in a timely manner even if there are bandwidth-intensive background transfers. Also with this option, Mowgli preserves the socket semantics on the mobile device so that legacy applications can be run unmodified.
Employing split TCP approaches have several benefits as well as
drawbacks. Benefits related to split TCP approaches include the
following:
- Splitting the end-to-end TCP connection into two parts is a
straightforward way to shield the problems of the wireless link
from the wireline Internet path, and vice versa. Thus, a split TCP
approach enables applying local solutions to the local problems on
the wireless link. For example, it automatically solves the
problem of distinguishing congestion related packet losses on the
wireline Internet and packet losses due to transmission error on
the wireless link as these occur on separate TCP connections.
Even if both segments experience congestion, it may be of a
different nature and may be treated as such. Moreover, temporary
disconnections of the wireless link can be effectively shielded
from the wireline Internet.
- When one of the TCP connections crosses only a single hop wireless
link or a very limited number of hops, some or all link
characteristics for the wireless TCP path are known. For example,
with a particular link we may know that the link provides reliable
delivery of packets, packets are not delivered out of order, or
the link is not subject to congestion. Having this information for
the TCP path one could expect that defining the TCP mitigations to
be employed becomes a significantly easier task. In addition,
several mitigations that cannot be employed safely over the global
Internet, can be successfully employed over the wireless link.
- Splitting one TCP connection into two separate ones allows much
earlier deployment of various recent proposals to improve TCP
performance over wireless links; only the TCP implementations of
the mobile device and intermediate node need to be modified, thus
allowing the vast number of Internet hosts to continue running the
legacy TCP implementations unmodified. Any mitigations that would
require modification of TCP in these wireline hosts may take far
too long to become widely deployed.
- Allows exploitation of various application level enhancements
which may give significant performance gains (see section 4.10.2).
Drawbacks related to split TCP approaches include the following:
- One of the main criticisms against the split TCP approaches is
that it breaks TCP end-to-end semantics. This has various
drawbacks some of which are more severe than others. The most
detrimental drawback is probably that splitting the TCP connection
disables end-to-end usage of IP layer security mechanisms,
precluding the application of IPSec to achieve end-to-end
security. Still, IPSec could be employed separately in each of the
two parts, thus requiring the intermediate node to become a party
to the security association between the mobile device and the
remote host. This, however, is an undesirable or unacceptable
alternative in most cases. Other security mechanisms above the
transport layer, like TLS [RFC2246] or SOCKS [RFC1928], should be
employed for end-to-end security.
- Another drawback of breaking end-to-end semantics is that crashes
of the intermediate node become unrecoverable resulting in
termination of the TCP connections. Whether this should be
considered a severe problem depends on the expected frequency of
such crashes.
- In many occasions claims have been stated that if TCP end-to-end
semantics is broken, applications relying on TCP to provide
reliable data delivery become more vulnerable. This, however, is
an overstatement as a well-designed application should never fully
rely on TCP in achieving end-to-end reliability at the application
level. First, current APIs to TCP, such as the Berkeley socket
interface, do not allow applications to know when an TCP
acknowledgement for previously sent user data arrives at TCP
sender. Second, even if the application is informed of the TCP
acknowledgements, the sending application cannot know whether the
receiving application has received the data: it only knows that
the data reached the TCP receive buffer at the receiving end.
Finally, in order to achieve end-to-end reliability at the
application level an application level acknowledgement is required
to confirm that the receiver has taken the appropriate actions on
the data it received.
- When a mobile device moves, it is subject to handovers by the
serving base station. If the base station acts as the intermediate
node for the split TCP connection, the state of both TCP endpoints
on the previous intermediate node must be transferred to the new
intermediate node to ensure continued operation over the split TCP
connection. This requires extra work and causes overhead. However,
in most of the W-WAN wireless networks, unlike in W-LANs, the W-
WAN base station does not provide the mobile device with the
connection point to the wireline Internet (such base stations may
not even have an IP stack). Instead, the W-WAN network takes care
of the mobility and retains the connection point to the wireline
Internet unchanged while the mobile device moves. Thus, TCP state
handover is not required in most W-WANs.
- The packets traversing through all the protocol layers up to
transport layer and again down to the link layer result in extra
overhead at the intermediate node. In case of LTNs with low
bandwidth, this extra overhead does not cause serious additional
performance problems unlike with W-LANs that typically have much
higher bandwidth.
- Split TCP proposals are not applicable to networks with asymmetric
routing. Deploying a split TCP approach requires that traffic to
and from the mobile device be routed through the intermediate
node. With some networks, this cannot be accomplished, or it
requires that the intermediate node is located several hops away
from the wireless network edge which in turn is unpractical in
many cases and may result in non-optimal routing.
- Split TCP, as the name implies, does not address problems related
to UDP.
It should noted that using split TCP does not necessarily exclude
simultaneous usage of IP for end-to-end connectivity. Correct usage
of split TCP should be managed per application or per connection and
should be under the end-user control so that the user can decide
whether a particular TCP connection or application makes use of split
TCP or whether it operates end-to-end directly over IP.
Recommendation: Split TCP proposals that alter TCP semantics are not
recommended. Deploying custom protocols on the wireless link, such as
MOWGLI proposes is not recommended, because this note gives
preference to (1) improving TCP instead of designing a custom
protocol and (2) allowing end-to-end sessions at all times.
4.10.2 Application Level Proxies
Nowadays, application level proxies are widely used in the Internet.
Such proxies include Web proxy caches, relay MTAs (Mail Transfer
Agents), and secure transport proxies (e.g., SOCKS). In effect,
employing an application level proxy results in a "split TCP
connection" with the proxy as the intermediary. Hence, some of the
problems present with wireless links, such as combining of a
congested wide-area Internet path with a wireless LTN link, are
automatically alleviated to some extent.
The application protocols often employ plenty of (unnecessary) round
trips, lots of headers and inefficient encoding. Even unnecessary
data may get delivered over the wireless link in regular application
protocol operation. In many cases a significant amount of this
overhead can be reduced by simply running an application level proxy
on the intermediate node. With LTN links, significant additional
improvement can be achieved by introducing application level proxies
with application-specific enhancements. Such a proxy may employ an
enhanced version of the application protocol over the wireless link.
In an LTN environment enhancements at the application layer may provide much more notable performance improvements than any transport level enhancements. The Mowgli system provides full support for adding application level agent-proxy pairs between the client and the server, the agent on the mobile device and the proxy on the intermediate node. Such a pair may be either explicit or fully transparent to the applications, but it is, at all times, under the end-user control. Good examples of enhancements achieved with application-specific proxies include Mowgli WWW [LAKLR95], [LHKR96] and WebExpress [HL96], [CTCSM97]. Recommendation: Usage of application level proxies is conditionally recommended: an application must be proxy enabled and the decision of employing a proxy for an application must be under the user control at all times.4.10.3 Snoop and its Derivatives
Berkeley's SNOOP protocol [SNOOP] is a hybrid scheme mixing link- layer reliability mechanisms with the split connection approach. It is an improvement over split TCP approaches in that end-to-end semantics are retained. SNOOP does two things: 1. Locally (on the wireless link) retransmit lost packets, instead of allowing TCP to do so end-to-end. 2. Suppress the duplicate acks on their way from the receiver back to the sender, thus avoiding fast retransmit and congestion avoidance at the latter. Thus, the Snoop protocol is designed to avoid unnecessary fast retransmits by the TCP sender, when the wireless link layer retransmits a packet locally. Consider a system that does not use the Snoop agent. Consider a TCP sender S that sends packets to receiver R via an intermediate node IN. Assume that the sender sends packet A, B, C, D, E (in that order) which are forwarded by IN to the wireless receiver R. Assume that the intermediate node then retransmits B subsequently, because the first transmission of packet B is lost due to errors on the wireless link. In this case, receiver R receives packets A, C, D, E and B (in that order). Receipt of packets C, D and E triggers duplicate acknowledgements. When the TCP sender receives three duplicate acknowledgements, it triggers fast retransmit (which results in a retransmission, as well as reduction of congestion window). The fast retransmit occurs despite the link level retransmit on the wireless link, degrading throughput.
SNOOP [SNOOP] deals with this problem by dropping TCP dupacks appropriately (at the intermediate node). The Delayed Dupacks (see section 4.5) attempts to approximate Snoop without requiring modifications at the intermediate node. Such schemes are needed only if the possibility of a fast retransmit due to wireless errors is non-negligible. In particular, if the wireless link uses a stop-and- go protocol (or otherwise delivers packets in-order), then these schemes are not very beneficial. Also, if the bandwidth-delay product of the wireless link is smaller than four segments, the probability that the intermediate node will have an opportunity to send three new packets before a lost packet is retransmitted is small. Since at least three dupacks are needed to trigger a fast retransmit, with a wireless bandwidth-delay product less than four packets, schemes such as Snoop and Delayed Dupacks would not be necessary (unless the link layer is not designed properly). Conversely, when the wireless bandwidth-delay product is large enough, Snoop can provide significant performance improvement (compared with standard TCP). For further discussion on these topics, please refer to [Vaidya99]. The Delayed Dupacks scheme tends to provide performance benefit in environments where Snoop performs well. In general, performance improvement achieved by the Delayed Dupacks scheme is a function of packet loss rates due to congestion and transmission errors. When congestion-related losses occur, the Delayed Dupacks scheme unnecessarily delays retransmission. Thus, in the presence of congestion losses, the Delayed Dupacks scheme cannot achieve the same performance improvement as Snoop. However, simulation results [Vaidya99] indicate that the Delayed Dupacks can achieve a significant improvement in performance despite moderate congestion losses. WTCP [WTCP] is similar to SNOOP in that it preserves end-to-end semantics. In WTCP, the intermediate node uses a complex scheme to hide the time it spends recovering from local errors across the wireless link (this typically includes retransmissions due to error recovery, but may also include time spent dealing with congestion). The idea is for the sender to derive a smooth estimate of round-trip time. In order to work effectively, it assumes that the TCP endpoints implement the Timestamps option in RFC 1323 [TCPHP]. Unfortunately, support for RFC 1323 in TCP implementations is not yet widespread. Beyond this, WTCP requires changes only at the intermediate node. SNOOP and WTCP require the intermediate node to examine and operate on the traffic between the portable wireless device and the TCP server on the wired Internet. SNOOP and WTCP do not work if the IP traffic is encrypted, unless, of course, the intermediate node shares
the security association between the mobile device and its end-to-end
peer. They also require that both the data and the corresponding
ACKs traverse the same intermediate node. Furthermore, if the
intermediate node retransmits packets at the transport layer across
the wireless link, this may duplicate efforts by the link-layer.
SNOOP has been described by its designers as a TCP-aware link-layer.
This is the right approach: the link and network layers can be much
more aware of each other than traditional OSI layering suggests.
Encryption of IP packets via IPSEC's ESP header (in either transport
or tunnel mode) renders the TCP header and payload unintelligible to
the intermediate node. This precludes SNOOP (and WTCP) from working,
because it needs to examine the TCP headers in both directions.
Possible solutions involve:
- making the SNOOP (or WTCP) intermediate node a party to the
security association between the client and the server
- IPSEC tunneling mode, terminated at the SNOOPing intermediate node
However, these techniques require that users trust intermediate
nodes. Users valuing both privacy and performance should use SSL or
SOCKS for end-to-end security. These, however, are implemented above
the transport layer, and are not as resistant to some security
attacks (for example, those based on guessing TCP sequence numbers)
as IPSEC.
Recommendation: Implement SNOOP on intermediate nodes now. Research
results are encouraging, and it is an "invisible" optimization in
that neither the client nor the server needs to change, only the
intermediate node (for basic SNOOP without SACK). However, as
discussed above there is little or no benefit from implementing SNOOP
if:
1. The wireless link provides reliable, in-order packet delivery,
or,
2. The bandwidth-delay product of the wireless link is smaller
than four segments.
4.10.4 PEPs to handle Periods of Disconnection
Periods of disconnection are very common in wireless networks, either
during handoff, due to lack of resources (dropped connections) or
natural obstacles. During these periods, a TCP sender does not
receive the expected acknowledgements. Upon expiration of the
retransmit timer, this causes TCP to close its congestion window
with all the related drawbacks. Re-transmitting packets is useless
since the connection is broken. [M-TCP] aims at enabling TCP to better handle handoffs and periods of disconnection, while preserving end-to-end semantics. M-TCP adds an element: supervisor host (SH- TCP) at the edge of the wireless network. This intermediate node monitors the traffic coming from the sender to the mobile device. It does not break end-to-end semantics because the ACKs sent from the intermediate node to the sender are effectively the ones sent by the mobile node. The principle is to generally leave the last byte unacknowledged. Hence, SH-TCP could shut down the sender's window by sending the ACK for the last byte with a window set to zero. Thus the sender will go to persist mode. The second optimization is done on both the intermediate node and the mobile host. On the latter, TCP is aware of the current state of the connection. In the event of a disconnection, it is capable of freezing all timers. Upon reconnection, the mobile sends a specially marked ACK with the number of the highest byte received. The intermediate node assumes that the mobile is disconnected because it monitors the flow on the wireless link, so in the absence of acknowledgments from the mobile, it will inform SH-TCP, which will send the ACK closing the sender window as described in the previous paragraph. The intermediate node learns that the mobile is again connected when it receives a duplicate acknowledgment marked as reconnected. At this point it sends a duplicate ACK to the sender and grows the window. The sender exits persist mode and resumes transmitting at the same rate as before. It begins by retransmitting any data previously unacknowledged by the mobile node. Non overlapping or non soft handoffs are lightweight because the previous intermediate system can shrink the window, and the new one modifies it as soon as it has received an indication from the mobile. Recommendation: M-TCP is not slated for adoption at this moment, because of the highly experimental nature of the proposal, and the uncertainty that TCP/IP implementations handle zero window updates correctly. Continue tracking developments in this space.4.11 Header Compression Alternatives
Because Long Thin Networks are bandwidth-constrained, compressing every byte out of over-the-air segments is worth while. Mechanisms for TCP and IP header compression defined in [RFC1144, IPHC, IPHC-RTP, IPHC-PPP] provide the following benefits: - Improve interactive response time - Allow using small packets for bulk data with good line efficiency
- Allow using small packets for delay sensitive low data-rate
traffic
- Decrease header overhead (for a common TCP segment size of 512
the header overhead of IPv4/TCP within a Mobile IP tunnel can
decrease from 11.7 to less than 1 per cent.
- Reduce packet loss rate over lossy links (because of the
smaller cross-section of compressed packets).
Van Jacobson (VJ) header compression [RFC1144] describes a Proposed
Standard for TCP Header compression that is widely deployed. It uses
TCP timeouts to detect a loss of synchronization between the
compressor and decompressor. [IPHC] includes an explicit request for
transmission of uncompressed headers to allow resynchronization
without waiting for a TCP timeout (and executing congestion avoidance
procedures).
Recommendation: Implement [IPHC], in particular as it relates to IP-
in-IP [RFC2003] and Minimal Encapsulation [RFC2004] for Mobile IP, as
well as TCP header compression for lossy links and links that
reorder packets. PPP capable devices should implement [IPHC-PPP]. VJ
header compression may optionally be implemented as it is a widely
deployed Proposed Standard. However, it should only be enabled when
operating over reliable LTNs, because even a single bit error most
probably would result in a full TCP window being dropped, followed by
a costly recovery via slow-start.
4.12 Payload Compression
Compression of IP payloads is also desirable. "IP Payload Compression
Protocol (IPComp)" [IPPCP] defines a framework where common
compression algorithms can be applied to arbitrary IP segment
payloads. IP payload compression is something of a niche
optimization. It is necessary because IP-level security converts IP
payloads to random bitstreams, defeating commonly-deployed link-layer
compression mechanisms which are faced with payloads that have no
redundant "information" that can be more compactly represented.
However, many IP payloads are already compressed (images, audio,
video, "zipped" files being FTPed), or are already encrypted above
the IP layer (SSL/TLS, etc.). These payloads will not "compress"
further, limiting the benefit of this optimization.
HTTP/1.1 already supports compression of the message body. For
example, to use zlib compression the relevant directives are:
"Content-Encoding: deflate" and "Accept-Encoding: deflate" [HTTP-
PERF].
HTTP-NG is considering supporting compression of resources at the HTTP level, which would provide equivalent benefits for common compressible MIME types like text/html. This will reduce the need for IPComp. If IPComp is deployed more rapidly than HTTP-NG, IPComp compression of HTML and MIME headers would be beneficial. In general, application-level compression can often outperform IPComp, because of the opportunity to use compression dictionaries based on knowledge of the specific data being compressed. Recommendation: IPComp may optionally be implemented. Track HTTP-NG standardization and deployment for now. Implementing HTTP/1.1 compression using zlib SHOULD is recommended.4.13 TCP Control Block Interdependence [Touch97]
TCP maintains per-connection information such as connection state, current round-trip time, congestion control or maximum segment size. Sharing information between two consecutive connections or when creating a new connection while the first is still active to the same host may improve performance of the latter connection. The principle could easily be extended to sharing information amongst systems in a LAN not just within a given system. [Touch97] describes cache update for both cases. Users of W-WAN devices frequently request connections to the same servers or set of servers. For example, in order to read their email or to initiate connections to other servers, the devices may be configured to always use the same email server or WWW proxy. The main advantage of this proposal is that it relieves the application of the burden of optimizing the transport layer. In order to improve the performance of TCP connections, this mechanism only requires changes at the wireless device. In general, this scheme should improve the dynamism of connection setup without increasing the cost of the implementation. Recommendation: This mechanism is recommended, although HTTP/1.1 with its persistent connections may partially achieve the same effect without it. Other applications (even HTTP/1.0) may find it useful. Continue monitoring research on this. In particular, work on a "Congestion Manager" [CM] may generalize this concept of sharing information among protocols and applications with a view to making them more adaptable to network conditions.
5 Summary of Recommended Optimizations
The table below summarizes our recommendations with regards to the main proposals mentioned above. The first column, "Stability of the Proposal," refers to the maturity of the mechanism in question. Some proposals are being pursued within the IETF in a somewhat open fashion. An IETF proposal is either an Internet Drafts (I-D) or a Request for Comments (RFC). The former is a preliminary version. There are several types of RFCs. A Draft Standards (DS) is standards track, and carries more weight than a Proposed Standard (PS), which may still undergo revisions. Informational or Experimental RFCs do not specify a standard. Other proposals are isolated efforts with little or no public review, and unknown chances of garnering industry backing. "Implemented at" indicates which participant in a TCP session must be modified to implement the proposal. Legacy servers typically cannot be modified, so this column indicates whether implementation happens at either or both of the two nodes under some control: mobile device and intermediate node. The symbols used are: WS (wireless sender, that is, the mobile device's TCP send operation must be modified), WR (wireless receiver, that is, the mobile device's TCP receive operation must be modified), WD (wireless device, that is, modifications at the mobile device are not specific to either TCP send or receive), IN (intermediate node) and NI (network infrastructure). These entities are to be understood within the context of Section 1.1 ("Network Architecture"). NA simply means "not applicable." The "Recommendation" column captures our suggestions. Some mechanisms are endorsed for immediate adoption, others need more evidence and research, and others are not recommended. Name Stability of Implemented Recommendation the Proposal at ==================== ============= =========== ================= Increased Initial RFC 2581 (PS) WS Yes Window (initial_window=2) Disable delayed ACKs NA WR When stable during slow start Byte counting NA WS No instead of ACK counting
TCP Header RFC 1144 (PS) WD Yes compression for PPP IN (see 4.11) IP Payload RFC 2393 (PS) WD Yes Compression (simultaneously (IPComp) needed on Server) Header RFC 2507 (PS), WD Yes Compression RFC 2509 (PS) IN (For IPv4, TCP and Mobile IP, PPP) SNOOP plus SACK In limited use IN Yes WD (for SACK) Fast retransmit/fast RFC 2581 (PS) WD Yes (should be recovery there already) Transaction/TCP RFC 1644 WD No (Experimental) (simultaneously needed on Server) Estimating Slow NA WS No Start Threshold (ssthresh) Delayed Duplicate Not stable WR When stable Acknowledgements IN (for notifications) Class-based Queuing NA WD When stable on End Systems Explicit Congestion RFC 2481 (EXP) WD Yes Notification NI TCP Control Block RFC 2140 WD Yes Interdependence (Informational) (Track research) Of all the optimizations in the table above, only SNOOP plus SACK and Delayed duplicate acknowledgements are currently being proposed only for wireless networks. The others are being considered even for non- wireless applications. Their more general applicability attracts more attention and analysis from the research community. Of the above mechanisms, only Header Compression (for IP and TCP) and "SNOOP plus SACK" cease to work in the presence of IPSec.
6 Conclusion
In view of the unpredictable and problematic nature of long thin networks, arriving at an optimized transport is a daunting task. We have reviewed the existing proposals along with future research items. Based on this overview, we also recommend mechanisms for implementation in long thin networks (LTNs).7 Acknowledgements
The authors are deeply indebted to the IETF tcpsat and tcpimpl working groups. The following individuals have also provided valuable feedback: Mark Allman (NASA), Vern Paxson (ACIRI), Raphi Rom (Technion/Sun), Charlie Perkins (Nokia), Peter Stark (Phone.com).8 Security Considerations
The mechanisms discussed and recommended in this document have been proposed in previous publications. The security considerations outlined in the original discussions apply here as well. Several security issues are also discussed throughout this document. Additionally, we present below a non-exhaustive list of the most salient issues concerning our recommended mechanisms: - Larger Initial TCP Window Size No known security issues [RFC2414, RFC2581]. - Header Compression May be open to some denial of service attacks. But any attacker in a position to launch these attacks would have much stronger attacks at his disposal [IPHC, IPHC-RTP]. - Congestion Control, Fast Retransmit/Fast Recovery An attacker may force TCP connections to grind to a halt, or, more dangerously, behave more aggressively. The latter possibility may lead to congestion collapse, at least in some regions of the network [RFC2581]. - Explicit Congestion Notification It does not appear to increase the vulnerabilities in the network. On the contrary, it may reduce them by aiding in the identification of flows unresponsive to or non-compliant with TCP congestion control [ECN].
- Sharing of Network Performance Information (TCP Control Block
Sharing and Congestion Manager module)
Some information should not be shared. For example, TCP sequence
numbers are used to protect against spoofing attacks. Even
limiting the sharing to performance values leaves open the
possibility of denial-of-service attacks [Touch97].
- Performance Enhancing Proxies
These systems are men-in-the-middle from the point of view of
their security vulnerabilities. Accordingly, they must be used
with extreme care so as to prevent their being hijacked and
misused.
This last point is not to be underestimated: there is a general
security concern whenever an intermediate node performs operations
different from those carried out in an end-to-end basis. This is not
specific to performance-enhancing proxies. In particular, there may
be a tendency to forego IPSEC-based privacy in order to allow, for
example, a SNOOP module, header compression (TCP, UDP, RTP, etc), or
HTTP proxies to work.
Adding end-to-end security at higher layers (for example via RTP
encryption, or via TLS encryption of the TCP payload) alleviates the
problem. However, this still leaves protocol headers in the clear,
and these may be exploited for traffic analysis and denial-of-service
attacks.
9 References
[ACKSPACING] Partridge, C., "ACK Spacing for High Delay-Bandwidth
Paths with Insufficient Buffering", Work in Progress.
[ADGGHOSSTT98] Allman, M., Dawkins, S., Glover, D., Griner, J.,
Henderson, T., Heidemann, J., Kruse, H., Osterman, S.,
Scott, K., Semke, J., Touch, J. and D. Tran, "Ongoing
TCP Research Related to Satellites", Work in Progress.
[AGS98] Allman, M., Glover, D. and L. Sanchez, "Enhancing TCP
Over Satellite Channels using Standard Mechanisms",
BCP 28, RFC 2488, January 1999.
[Allman98] Mark Allman. On the Generation and Use of TCP Acknowledgments. ACM Computer Communication Review, 28(5), October 1998. [AHO98] Allman, M., Hayes, C., Ostermann, S., "An Evaluation of TCP with Larger Initial Windows," Computer Communication Review, 28(3), July 1998. [BBKT96] Bhagwat, P., Bhattacharya, P., Krishna, A., Tripathi, S., "Enhancing Throughput over Wireless LANs Using Channel State Dependent Packet Scheduling," in Proc. IEEE INFOCOM'96, pp. 1133-40, March 1996. [BBKVP96] Bakshi, B., P., Krishna, N., Vaidya, N., Pradhan, D.K., "Improving Performance of TCP over Wireless Networks," Technical Report 96-014, Texas A&M University, 1996. [BPSK96] Balakrishnan, H., Padmanabhan, V., Seshan, S., Katz, R., "A Comparison of Mechanisms for Improving TCP Performance over Wireless Links," in ACM SIGCOMM, Stanford, California, August 1996. [BPK99] Balakrishnan, H., Padmanabhan, V., Katz, R., "The effects of asymmetry on TCP performance," ACM Mobile Networks and Applications (MONET), Vol. 4, No. 3, 1999, pp. 219-241. [BV97] S. Biaz and N. H. Vaidya, "Distinguishing Congestion Losses from Wireless Transmission Losses: A Negative Result," Seventh International Conference on Computer Communications and Networks (IC3N), New Orleans, October 1998. [BV98] Biaz, S., Vaidya, N., "Sender-Based heuristics for Distinguishing Congestion Losses from Wireless Transmission Losses," Texas A&M University, Technical Report 98-013, June 1998. [BV98a] Biaz, S., Vaidya, N., "Discriminating Congestion Losses from Wireless Losses using Inter-Arrival Times at the Receiver," Texas A&M University, Technical Report 98-014, June 1998. [BW97] Brasche, G., Walke, B., "Concepts, Services, and Protocols of the New GSM Phase 2+ general Packet Radio Service," IEEE Communications Magazine, Vol. 35, No. 8, August 1997.
[CB96] Cheshire, S., Baker, M., "Experiences with a Wireless Network in MosquitoNet," IEEE Micro, February 1996. Available online as: http://rescomp.stanford.edu/~cheshire/papers /wireless.ps. [CDMA] Electronic Industry Alliance(EIA)/Telecommunications Industry Association (TIA), IS-95: Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, 1993. [CDPD] Wireless Data Forum, CDPD System Specification, Release 1.1, 1995. [CM] Hari Balakrishnan and Srinivasan Seshan, "The Congestion Manager," Work in Progress. [CTCSM97] Chang, H., Tait, C., Cohen, N., Shapiro, M., Mastrianni, S., Floyd, R., Housel, B., Lindquist, D., "Web Browsing in a Wireless Environment: Disconnected and Asynchronous Operation in ARTour Web Express," in Proc. MobiCom'97, Budapest, Hungary, September 1997. [Demers90] Demers, A., Keshav, S., and Shenker, S., Analysis and Simulation of a Fair Queueing Algorithm, Internetworking: Research and Experience, Vol. 1, 1990, pp. 3-26. [ECN] Ramakrishnan, K. and S. Floyd, "A Proposal to add Explicit Congestion Notification (ECN) to IP", RFC 2481, January 1999. [Floyd95] Floyd, S., and Jacobson, V., Link-sharing and Resource Management Models for Packet Networks. IEEE/ACM Transactions on Networking, Vol. 3 No. 4, pp. 365-386, August 1995. [FSS98] Fragouli, C., Sivaraman, V., Srivastava, M., "Controlled Multimedia Wireless Link Sharing via Enhanced Class-Based Queueing with Channel-State- Dependent Packet Scheduling," Proc. IEEE INFOCOM'98, April 1998. [GPRS] ETSI, "General Packet Radio Service (GPRS): Service Description, Stage 2," GSM03.60, v.6.1.1 August 1998.
[GSM] Rahnema, M., "Overview of the GSM system and protocol architecture," IEEE Communications Magazine, vol. 31, pp 92-100, April 1993. [HL96] Hausel, B., Lindquist, D., "WebExpress: A System for Optimizing Web Browsing in a Wireless Environment," in Proc. MobiCom'96, Rye, New York, USA, November 1996. [HTTP-PERF] Henrik Frystyk Nielsen (W3C, MIT), Jim Gettys (W3C, Digital), Anselm Baird-Smith (W3C, INRIA), Eric Prud'hommeaux (W3C, MIT), Hon Lie (W3C, INRIA), Chris Lilley (W3C, INRIA), "Network Performance Effects of HTTP/1.1, CSS1, and PNG," ACM SIGCOMM '97, Cannes, France, September 1997. Available at: http://www.w3.org/Protocols/HTTP/Performance /Pipeline.html [IPPCP] Shacham, A., Monsour, R., Pereira, R. and M. Thomas, "IP Payload Compression Protocol (IPComp)", RFC 2393, December 1998. [IPHC] Degermark, M., Nordgren, B. and S. Pink, "IP Header Compression", RFC 2507, February 1999. [IPHC-RTP] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for Low-Speed Serial Links", RFC 2508, February 1999. [IPHC-PPP] Engan, M., Casner, S. and C. Bormann, "IP Header Compression over PPP", RFC 2509, February 1999. [ITCP] Bakre, A., Badrinath, B.R., "Handoff and Systems Support for Indirect TCP/IP. In Proceedings of the Second USENIX Symposium on Mobile and Location- Independent Computing, Ann Arbor, Michigan, April 10- 11, 1995. [Jain89] Jain, R., "A Delay-Based Approach for Congestion Avoidance in Interconnected Heterogeneous Computer Networks," Digital Equipment Corporation, Technical Report DEC-TR-566, April 1989. [Karn93] Karn, P., "The Qualcomm CDMA Digital Cellular System" Proc. USENIX Mobile and Location-Independent Computing Symposium, USENIX Association, August 1993.
[KRLKA97] Kojo, M., Raatikainen, K., Liljeberg, M., Kiiskinen, J., Alanko, T., "An Efficient Transport Service for Slow Wireless Telephone Links," in IEEE Journal on Selected Areas of Communication, volume 15, number 7, September 1997. [LAKLR95] Liljeberg, M., Alanko, T., Kojo, M., Laamanen, H., Raatikainen, K., "Optimizing World-Wide Web for Weakly-Connected Mobile Workstations: An Indirect Approach," in Proc. 2nd Int. Workshop on Services in Distributed and Networked Environments, Whistler, Canada, pp. 132-139, June 1995. [LHKR96] Liljeberg, M., Helin, H., Kojo, M., Raatikainen, K., "Mowgli WWW Software: Improved Usability of WWW in Mobile WAN Environments," in Proc. IEEE Global Internet 1996 Conference, London, UK, November 1996. [LS98] Lettieri, P., Srivastava, M., "Adaptive Frame Length Control for Improving Wireless Link Throughput, Range, and Energy Efficiency," Proc. IEEE INFOCOM'98, April 1998. [MNCP] Piscitello, D., Phifer, L., Wang, Y., Hovey, R., "Mobile Network Computing Protocol (MNCP)", Work in Progress. [MOWGLI] Kojo, M., Raatikainen, K., Alanko, T., "Connecting Mobile Workstations to the Internet over a Digital Cellular Telephone Network," in Proc. Workshop on Mobile and Wireless Information Systems (MOBIDATA), Rutgers University, NJ, November 1994. Available at: http://www.cs.Helsinki.FI/research/mowgli/. Revised version published in Mobile Computing, pp. 253-270, Kluwer, 1996. [MSMO97] Mathis, M., Semke, J., Mahdavi, J., Ott, T., "The Macroscopic Behavior of the TCP Congestion Avoidance Algorithm," in Computer Communications Review, a publication of ACM SIGCOMM, volume 27, number 3, July 1997. [MTCP] Brown, K. Singh, S., "A Network Architecture for Mobile Computing," Proc. IEEE INFOCOM'96, pp. 1388- 1396, March 1996. Available at ftp://ftp.ece.orst.edu/pub/singh/papers /transport.ps.gz
[M-TCP] Brown, K. Singh, S., "M-TCP: TCP for Mobile Cellular Networks," ACM Computer Communications Review Vol. 27(5), 1997. Available at ftp://ftp.ece.orst.edu/pub/singh/papers/mtcp.ps.gz [MV97] Mehta, M., Vaidya, N., "Delayed Duplicate- Acknowledgements: A Proposal to Improve Performance of TCP on Wireless Links," Texas A&M University, December 24, 1997. Available at http://www.cs.tamu.edu/faculty/vaidya/mobile.html [NETBLT] White, J., "NETBLT (Network Block Transfer Protocol)", Work in Progress. [Paxson97] V. Paxson, "End-to-End Internet Packet Dynamics," Proc. SIGCOMM '97. Available at ftp://ftp.ee.lbl.gov/papers/vp-pkt-dyn-sigcomm97.ps.Z [RED] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering, S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G., Partridge, C., Peterson, L., Ramakrishnan, K., Shenker, S., Wroclawski, J. and L. Zhang, "Recommendations on Queue Management and Congestion Avoidance in the Internet", RFC 2309, April 1998. [RLP] ETSI, "Radio Link Protocol for Data and Telematic Services on the Mobile Station - Base Station System (MS-BSS) interface and the Base Station System - Mobile Switching Center (BSS-MSC) interface," GSM Specification 04.22, Version 3.7.0, February 1992. [RFC908] Velten, D., Hinden, R. and J. Sax, "Reliable Data Protocol", RFC 908, July 1984. [RFC1030] Lambert, M., "On Testing the NETBLT Protocol over Divers Networks", RFC 1030, November 1987. [RFC1122] Braden, R., "Requirements for Internet Hosts -- Communication Layers", STD 3, RFC 1122, October 1989. [RFC1144] Jacobson, V., "Compressing TCP/IP Headers for Low- Speed Serial Links", RFC 1144, February 1990. [RFC1151] Partridge, C., Hinden, R., "Version 2 of the Reliable Data Protocol (RDP)", RFC 1151, April 1990.
[RFC1191] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191, November 1990. [RFC1397] Braden, R., "Extending TCP for Transactions -- Concepts", RFC 1397, November 1992. [RFC1644] Braden, R., "T/TCP -- TCP Extensions for Transactions Functional Specification", RFC 1644, July 1994. [RFC1661] Simpson, W., "The Point-To-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D. and L. Jones, "SOCKS Protocol Version 5", RFC 1928, March 1996. [RFC1986] Polites, W., Wollman, W., Woo, D. and R. Langan, "Experiments with a Simple File Transfer Protocol for Radio Links using Enhanced Trivial File Transfer Protocol (ETFTP)", RFC 1986, August 1996. [RFC2002] Perkins, C., "IP Mobility Support", RFC 2002, October 1996. [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, October 1996. [RFC2004] Perkins, C., "Minimal Encapsulation within IP", RFC 2004, October 1996. [RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP Selective Acknowledgment Options", RFC 2018, October 1996. [RFC2188] Banan, M., Taylor, M. and J. Cheng, "AT&T/Neda's Efficient Short Remote Operations (ESRO) Protocol Specification Version 1.2", RFC 2188, September 1997. [RFC2246] Dierk, T. and E. Allen, "TLS Protocol Version 1", RFC 2246, January 1999. [RFC2414] Allman, M., Floyd, S. and C. Partridge. "Increasing TCP's Initial Window", RFC 2414, September 1998. [RFC2415] Poduri, K.and K. Nichols, "Simulation Studies of Increased Initial TCP Window Size", RFC 2415, September 1998.
[RFC2416] Shepard, T. and C. Partridge, "When TCP Starts Up With Four Packets Into Only Three Buffers", RFC 2416, September 1998. [RFC2581] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion Control", RFC 2581, April 1999. [RFC2582] Floyd, S. and T. Henderson, "The NewReno Modification to TCP's Fast Recovery Algorithm", RFC 2582, April 1999. [SNOOP] Balakrishnan, H., Seshan, S., Amir, E., Katz, R., "Improving TCP/IP Performance over Wireless Networks," Proc. 1st ACM Conf. on Mobile Computing and Networking (Mobicom), Berkeley, CA, November 1995. [Stevens94] R. Stevens, "TCP/IP Illustrated, Volume 1," Addison- Wesley, 1994 (section 2.10 for MTU size considerations and section 11.3 for weak checksums). [TCPHP] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992. [TCPSATMIN] TCPSAT Minutes, August, 1997. Available at: http://tcpsat.lerc.nasa.gov/tcpsat/meetings/munich- minutes.txt. [Touch97] Touch, T., "TCP Control Block Interdependence", RFC 2140, April 1997. [Vaidya99] N. H. Vaidya, M. Mehta, C. Perkins, G. Montenegro, "Delayed Duplicate Acknowledgements: A TCP-Unaware Approach to Improve Performance of TCP over Wireless," Technical Report 99-003, Computer Science Dept., Texas A&M University, February 1999. [VEGAS] Brakmo, L., O'Malley, S., "TCP Vegas, New Techniques for Congestion Detection and Avoidance," SIGCOMM'94, London, pp 24-35, October 1994. [VMTP] Cheriton, D., "VMTP: Versatile Message Transaction Protocol", RFC 1045, February 1988. [WAP] Wireless Application Protocol Forum. http://www.wapforum.org/
[WC91] Wang, Z., Crowcroft, J., "A New Congestion Control Scheme: Slow Start and Search," ACM Computer Communication Review, vol 21, pp 32-43, January 1991. [WTCP] Ratnam, K., Matta, I., "WTCP: An Efficient Transmission Control Protocol for Networks with Wireless Links," Technical Report NU-CCS-97-11, Northeastern University, July 1997. Available at: http://www.ece.neu.edu/personal/karu/papers/WTCP- NU.ps.gz [YB94] Yavatkar, R., Bhagawat, N., "Improving End-to-End Performance of TCP over Mobile Internetworks," Proc. Workshop on Mobile Computing Systems and Applications, IEEE Computer Society Press, Los Alamitos, California, 1994.Authors' Addresses
Questions about this document may be directed at: Gabriel E. Montenegro Sun Labs Networking and Security Group Sun Microsystems, Inc. 901 San Antonio Road Mailstop UMPK 15-214 Mountain View, California 94303 Phone: +1-650-786-6288 Fax: +1-650-786-6445 EMail: gab@sun.com Spencer Dawkins Nortel Networks P.O. Box 833805 Richardson, Texas 75083-3805 Phone: +1-972-684-4827 Fax: +1-972-685-3292 EMail: sdawkins@nortel.com
Markku Kojo Department of Computer Science University of Helsinki P.O. Box 26 (Teollisuuskatu 23) FIN-00014 HELSINKI Finland Phone: +358-9-1914-4179 Fax: +358-9-1914-4441 EMail: kojo@cs.helsinki.fi Vincent Magret Corporate Research Center Alcatel Network Systems, Inc 1201 Campbell Mail stop 446-310 Richardson Texas 75081 USA M/S 446-310 Phone: +1-972-996-2625 Fax: +1-972-996-5902 EMail: vincent.magret@aud.alcatel.com Nitin Vaidya Dept. of Computer Science Texas A&M University College Station, TX 77843-3112 Phone: 979-845-0512 Fax: 979-847-8578 EMail: vaidya@cs.tamu.edu
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