# Patent

## Title
Packet header compression for lossy channels

## Abstract

The disclosed systems and methods provide for the compression and decompression of packet headers. An uncompressed header can be compressed in size to form a smaller, compressed header, such that the compressed header can include values that can be used to re-produce the uncompressed header. A compressed header can include at least two such values. A first value of the at least two values can be computed based on a second, earlier uncompressed header and can be used to derive the uncompressed header. Similarly, a second value of the at least two values can be computed based on a third uncompressed header and can be used to derive the same uncompressed header. Accordingly, the uncompressed header can be derived based on the first value and the second uncompressed header, or based on the second value and the third uncompressed header. The uncompressed header, second uncompressed header, and third uncompressed header can be associated with different packets.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser. No. 10/803,002, filed on Mar. 17, 2004, which is hereby incorporated by reference in its entirety.


BACKGROUND
(1) Field
The disclosed methods and systems relate generally to compression of packet headers, and more particularly to packet header compression for lossy channels.
(2) Description of Relevant Art
There is a continuing interest in improving the performance of packet-based networks to handle ever-growing demands on the capability of such networks to carry larger capacities of data. Such packet-based networks comprise intricate inter-connections of network devices that operate based on a diverse collection of protocols that define and control the overall operation of the network. These networking protocols require that data carried over the networks be accompanied by control information, which are often quite extensive and can place non-negligible demands on network resources. Thus, one continuing effort to improve network performance is known as “header compression” and involves encoding and/or condensing network protocol control information to decrease the network resources required by such protocols and to allow the networks to handle greater proportions of data.
In general, a packet contains a control portion (the header) that includes various fields that indicate the manner in which the packet should be handled, and a data portion (the payload) that stores the data being transmitted, such as e-mail data, text messages, voice audio, pictures, or video data. It is possible to compress a packet header because there is often significant redundancy between header fields, both within the same packet and between consecutive packets that are part of the same packet stream. For example, with respect to Transmission Control Protocol (TCP) and Internet Protocol (IP), a header compression method described by V. Jacobson in RFC 1144 indicates that for TCP/IP headers, half of a header is likely to remain constant between consecutive packets in a packet stream. Accordingly, a reduction of one-half in the size of a TCP/IP header can be realized by storing a copy of the latest packet header for a packet stream at a receiver and transmitting to the receiver only the variable portion of a subsequent packet header. The method of Jacobson further compresses a TCP/IP header by realizing that the variable portions often change slowly or minutely. Thus, transmitting the amount of change (called “delta”) rather than the value of the variable portion can result in a further reduction in header size. A receiver can apply a received delta to the variable portion of an uncompressed packet header to produce the variable portion of the next header. The constant portion of this next header can be obtained from the constant portion of the uncompressed header.
As exemplified by the header compression method of Jacobson, a “compressed header” is generally a condensed and/or encoded version of a full packet header. Thus, an individual compressed header in a packet does not convey the same extent of control information as a full header and must rely on a context provided by previous header(s) to derive control information. There are, however, situations where such context may be unavailable, For example, it is commonly known that packets may be re-ordered prior to arrival at a receiver. Because a header is compressed based on a previous header, packet re-ordering prior to decompression may cause a context of previous packets to be unavailable and can result in delays and/or other complications at the receiver. In addition, poor transmission conditions may cause certain packets to be lost, which results in decompression errors if a receiver does not recognize the packet loss and continues to apply received deltas to other packets in place of the lost packets. However, even when a receiver recognizes that re-ordering or packet loss has occurred, it may still be unable to accommodate subsequent packets and may require the transmitter to re-send the lost and/or subsequent packets. Thus, there is a continuing interest in improving header compression technology to accommodate problems such as packet re-ordering and packet loss.
SUMMARY
The disclosed systems and methods provide for the compression and decompression of packet headers. An uncompressed packet can include an uncompressed header structured according to a networking protocol, such as Internet Protocol, Transmission Control Protocol, User Datagram Protocol, and Real-Time Protocol, among others. An uncompressed header can be compressed in size to form a smaller, compressed header, such that the compressed header can include values that can be used to re-produce the uncompressed header based on preceding headers. In one embodiment, a compressed header can include at least two such values. A first value of the at least two values can be used to derive the uncompressed header based on a second, earlier uncompressed header. Similarly, a second value of the at least two values can be used to derive the uncompressed header based on a third uncompressed header. Accordingly, the uncompressed header can be derived based on the first value and the second uncompressed header, or based on the second value and the third uncompressed header.
In one embodiment, the first value can be computed based on the uncompressed header and the second uncompressed header, such that the first value corresponds to a difference between a value representative of a portion of the uncompressed header and a value representative of a corresponding portion of the second uncompressed header. Similarly, the second value can be computed based on the uncompressed header and the third uncompressed header, such that the second value corresponds to the difference between a value representative of a portion of the uncompressed header and a value representative of a corresponding portion of the third uncompressed header. The first and second values can be can encoded by a variable-length code and/or a sign-based code.
In one embodiment, the uncompressed header, the second uncompressed header, and the third uncompressed header can be associated with different packets. The packets associated with the second and third uncompressed headers can be consecutive packets. In one embodiment, the at least two values in the compressed header can include other values, in addition to the first and second values, that are associated with other packets distinct from those associated with the second and third uncompressed headers. In one embodiment, a compressed header can include a destination address, a packet sequence number, and/or a packet stream identifier number.
In one embodiment, the uncompressed header can be maintained at a first (e.g., source) network node, and the second and/or third uncompressed headers can be maintained at a second (e.g., destination) network node. A packet containing a compressed version of the uncompressed header can be transmitted from the first network node and received by the second network node, where the compressed header can include a first value and a second value for deriving the uncompressed header at the second network node based on the second uncompressed header or the third uncompressed header, respectively. In one embodiment, the uncompressed header can be derived at the second network node by summing the first value with the second uncompressed header and/or summing the second value with the third uncompressed header. The packet can traverse a connection from the first node to the second node that includes no intervening nodes, or the packet can traverse a connection that includes one or more intervening nodes.
More generally, a compressed version of an uncompressed header can include a plurality of values for deriving the uncompressed header. The plurality of values can be generated by computing, for each of at least two uncompressed headers associated with previously transmitted packets, a corresponding value for deriving the uncompressed header. The uncompressed headers associated with the previous packets can be stored and updated to include new uncompressed headers associated with newly transmitted packets.
Other objects and advantages will become apparent hereinafter in view of the specification and drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of an exemplary network architecture employing header compression/decompression;
FIGS. 2A-2C are diagrams of exemplary packets having compressed headers and/or uncompressed headers;
FIG. 3 is a flow chart of an exemplary method of operating a compressor to receive and compress packet headers in a packet stream;
FIG. 4 is a flow chart of an exemplary method of operating a decompressor to receive and decompress compressed packet headers in a packet stream;
FIG. 5 is a diagram of the exemplary portions of an uncompressed header and a compressed header;
FIG. 6 is a diagram of an exemplary compressor storage space from which encoded header values can be computed;
FIG. 7 is a diagram of an exemplary encoded portion of a compressed header; and
FIGS. 8A-8B are diagrams of an exemplary decompressor storage space containing decompressed headers and compressed headers waiting to be decompressed.



DESCRIPTION
To provide an overall understanding, certain illustrative embodiments will now be described; however, it will be understood by one of ordinary skill in the art that the systems and methods described herein can be adapted and modified to provide systems and methods for other suitable applications and that other additions and modifications can be made without departing from the scope of the systems and methods described herein.
Unless otherwise specified, the illustrated embodiments can be understood as providing exemplary features of varying detail of certain embodiments, and therefore, unless otherwise specified, features, components, modules, and/or aspects of the illustrations can be otherwise combined, separated, interchanged, and/or rearranged without departing from the disclosed systems or methods. Additionally, the shapes and sizes of components are also exemplary and unless otherwise specified, can be altered without affecting the disclosed systems or methods.
The disclosed systems and methods provide for managing compression and decompression of packet headers in a manner that accommodates packet re-ordering and packet loss, among other things. Data such as e-mail, text messages, voice audio, digital music and pictures, video, etc., can be carried in a network according to networking protocols that apportion the data into segments and append one or more headers to those segments. The headers contain control information that facilitate the handling and delivery of the data segments and are structured according to defined arrangements. As used herein, a header that is structured according to an arrangement defined by a networking protocol can be understood to be an “uncompressed header.” In contrast, a header that is a condensed and/or encoded version of an uncompressed header can be understood to be a “compressed header.” The disclosed systems and methods provide for compressing an uncompressed header based on a number of other uncompressed headers such that the uncompressed header can be re-produced using one of the other uncompressed headers.
Referring now to FIG. 1, there is shown an exemplary network architecture 100 having a source network node 102 and a destination network node 104 in communication with a network 106. For the described methods and systems, the network 106 can be a physically connected network having devices connected by physical connection medium, such as copper wire, twisted-pair, coaxial cable, fiber-optics, USB cable, firewire, etc., and/or a wirelessly connected network having devices that employ wireless technologies such as Bluetooth, 802.11b/g, GSM/GPRS, etc. The network 106 can encompass a myriad of network configurations and devices, ranging from a simple cable locally connecting the source node to the destination node to a complex inter-connection of wireline and/or wireless network nodes. As used herein, a network node can be understood to be a device, or portion thereof, that interacts with and/or forms a part of a network, such a PC, workstation, laptop, PDA, modem, NIC, cellular telephone, router, hub, switch, etc. Although the source and destination network nodes 102, 104 are illustrated as being separate from the network 106, the nodes 102, 104 perform networking operations and are, in reality, encompassed within and/or part of the network 106.
As previously provided herein, a network 106 manages delivery of data 108 according to networking protocols 110, 112, such as Internet Protocol (IP), Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Real-Time Protocol (RTP), among others. A network node 102, 104 operating in accordance with one or more networking protocols 110, 112 can implement the protocols using hardware components, such as digital circuitry, and/or using software instructions executing on a processor. As shown in FIG. 1, data 108 located at the source network node 102 can be processed by a cascade of one or more networking protocols 110, 112 and a header compressor 114 to produce packets (not shown) to carry the data 108 across the network 106. The compressor 114 can include a storage space 116 for maintaining information such as compressor configuration and/or a history of one or more transmitted headers 117. Packets that traverse the network 106 may encounter conditions that result in some packets being re-ordered or completely lost. The packets that are received at the destination network node 104 can be accepted or discarded by the networking protocols 112 and/or the header decompressor 118 based on the extent and severity of the packet re-ordering and/or loss. Considerations for determining whether to process or discard received headers will be described herein. The decompressor 118 can include a storage space 120 for maintaining information such as decompressor configuration and/or a history of one or more decompressed packets 122. Decompressed packets 122 can be further processed by networking schemes to re-assemble the original data 108 at the destination node 104.
The illustrated architecture and components in FIG. 1 are exemplary and do not limit the scope of the described technology. Other configurations and arrangements are contemplated and include, for example, multiple header compressors/decompressors at the source and destination network nodes 102, 104. Further, those of ordinary skill will recognize that packets that are successfully decompressed at the destination node 104 may need to be compressed again and transmitted to another network node. Accordingly, although the network nodes 102, 104 are singularly designated as a source or destination node, it is understood that a network node can concurrently operate as both a source node and a destination node and can perform header compression and decompression. Additionally, the illustrated storage spaces 116, 120 can be physically located internal or external to the compressor 114/decompressor 118.
Now with reference to FIG. 2A, there is shown a packet A 200 containing an uncompressed header 202 and a data segment 204. The uncompressed header 202 can be condensed and/or encoded to form a compressed header 206 that is reduced in size when compared to the uncompressed header 202. The compressed header 206 can include all or a portion of the uncompressed header 202 in encoded form, and can also include a duplicate of a portion of the uncompressed header 202. Additionally, a compressor 114/decompressor 118 can also convey header compression control information (not shown) in the compressed header 206, After header compression, a packet B 208 containing the compressed header 206 and data segment 204 can be transmitted to the network 106 if no additional networking protocols wait to process the packet. Otherwise, one or more additional networking protocol headers 210 can be coupled to packet B 208 to form a packet C 212, which can then be transmitted to the network 106.
In one embodiment, an uncompressed packet may contain more than one uncompressed header, and a compressor can designate a subset of the headers for compression. For example, shown in FIG. 2B is a header compression scheme based on TCP and IP headers. The uncompressed packet comprises an uncompressed IP header 214, an uncompressed TCP header 216, and a data segment 218. TCP provides useful network services that a network may wish to utilize so that in one embodiment, the TCP header 216 can be maintained in uncompressed form. Accordingly, a compressor can be designed to recognize TCP and IP header structures 214, 216 and can compress only the IP header 220 while leaving the TCP header 216 uncompressed. Alternatively, in one embodiment, multiple uncompressed headers can be condensed and/or encoded into a single compressed header. As shown in FIG. 2C, a TCP header 216 and an IP header 214 can be compressed to form a single compressed header 222.
The illustrated structures and arrangements of compressed and uncompressed packets in FIGS. 2A-2C are exemplary embodiments, and other combinations of one or more compressed and/or uncompressed headers within a packet are also contemplated to be included within the scope of the disclosed systems and methods.
Now with reference to FIG. 3, there is shown a flow chart 300 of a method of operating a compressor to receive and compress packet headers in a packet stream. Also with continuing reference to FIGS. 1 and 2A, a packet 200 arriving at a compressor 114 has an associated packet stream that determines the networking protocol headers 202 used by that stream. Accordingly, packets that are associated with the same packet stream contain the same types and arrangement of headers. In one embodiment, a compressor 114 can receive information regarding a packet stream (302) prior to receiving packets from that stream. The information can specify, for example, the order and type of networking protocol headers in a packet, thereby allowing the compressor 114 to properly service the packet stream. In one embodiment, a compressor 114 can allocate storage space 116 (304) for storing a history of uncompressed packet headers 117 from a packet stream. For the sake of simplicity for this example, it is assumed that each packet contains a single uncompressed header 202 and a data segment 204, although the disclosed methods are not limited by such assumption.
Upon receiving an uncompressed packet 200 (306), a compressor 114 can identify the packet stream associated with the packet 200 and can determine whether the header 202 should be compressed (308) based on, for example, whether the decompressor 118 may need to receive an uncompressed header 202 for use as a reference. For example, a packet header 202 can remain uncompressed so that when it is received at the decompressor 118 at a destination node 104, it can serve as a reference/starting point for decompressing subsequently received headers. Accordingly, the first packet or a number of first packets in a packet stream can remain uncompressed. Before transmitting an uncompressed packet 200 to another networking protocol 112 or to the network 106, the compressor 114 can store the uncompressed header 202 in the allocated storage space 116 to maintain a history of uncompressed headers 117 for a packet stream. The compressor 114 can determine whether the allocated storage space 116 is full (312), and if not, can store the uncompressed header 202 in an available location in the storage space 116 (316). Otherwise, the compressor 114 can store the uncompressed header 202 in the storage space 116 in place of the oldest header in the storage space 116 (314). In this manner, the storage space 116 can maintain a history of the most recent packet headers 117. After passing the uncompressed packet 200 to the next stage (318), the compressor 114 can determine if the packet stream is complete or if there are more packets to receive and process (320). The compressor 114 can terminate servicing of a packet stream when there are no more packets to receive.
Other than those situations for which a packet header may remain uncompressed, a compressor 114 can perform header compression in a manner that provides some amount of tolerance for packet re-ordering and/or loss. In one embodiment, the compressor 114 can compute, for each stored header 117 in the storage space 116, a corresponding value (such as a delta) from which the uncompressed header 202 can be derived using the corresponding stored header (310). These values can be included together in a compressed header 206 and can be used by a decompressor 118 to re-produce the uncompressed header 202 using one of the corresponding headers 117 that was also decompressed and stored 122 at the decompressor 118. Accordingly, a compressed packet 208 can be decompressed at a decompressor 118 if at least one packet 117 on which its compression is based was also received and decompressed. In one embodiment, a subset of less than all of the headers 117 stored in the compressor storage space 116 can be used in the compression process, where the headers in the subset can be consecutive or non-consecutive. Before transmitting the compressed packet 208 to another networking protocol 112 or to the network 106, the compressor 114 can store the corresponding uncompressed header 202 in the allocated storage space 116 to maintain a history of uncompressed headers 117 for a packet stream. As previously provided, the compressor 114 can determine whether the allocated storage space 116 is full (312), and if not, can store the uncompressed header 202 (316) in an available location in the storage space 116. Otherwise, the compressor 114 can store the uncompressed header 202 in the storage space 116 in place of the oldest header in the storage space 116 (314). In this manner, the storage space 116 can maintain a history of the most recent packet headers 117. After passing the compressed packet 208 to the next stage (318), the compressor 114 can determine if the packet stream is complete or if there are more packets to receive and process (320). The compressor 114 can terminate servicing of a packet stream when there are no more packets to receive.
Referring now to FIG. 4, there is shown a flow chart 400 of a method of operating a decompressor 118 to receive and decompress packet headers in a packet stream. With continuing reference to FIGS. 1 and 2A, a decompressor 118 can receive information regarding a packet stream (402) prior to receiving packets from that stream. Upon receiving packet stream information, a decompressor 118 can allocate storage space 120 (404) for storing a history of decompressed packet headers 122. For the sake of simplicity for the present example, it is assumed that each packet contains a single header (compressed 206 or uncompressed 202) and a data segment 204, although the disclosed methods are not limited by such assumption.
Upon receiving a packet (406), a decompressor 118 can identify the packet stream associated with the packet and can determine whether the header is compressed (408). If the header is not compressed 202, the decompressor 118 can use the uncompressed header 202 to update the storage space 120 (414). For example, the uncompressed header 202 can be stored in the storage space 120 if it is more recent than at least one other packet 122 in the storage space If the header is compressed 206, the decompressor 118 can examine the compressed header 206 to identify the headers 117 on which the compression is based. The decompressor 118 can determine whether the compressed header 206 can be decompressed (410) by determining if another of the headers 117 on which the compression is based correspond to decompressed and stored headers 122 in the storage space 120. The decompressor 118 can derive the uncompressed header 202 by applying a computed value (e.g., delta difference) in the compressed header 206 to a corresponding header 122 in the storage space 120 (412). The uncompressed header 202 can be used to update the storage space 120 (414) before being provided to the next stage (e.g., another networking protocol) (416) in the destination node 104. If the compressed header 206 cannot be decompressed, then the compressed packet 208 can be temporarily stored in the storage space 120 (418) until subsequent packets are received and decompressed, which may allow the stored compressed headers to be decompressed. For example, a decompressor 118 can decompress newly received packets and, using those decompressed packets, can decompress another stored compressed packets (412). After processing a received packet, the decompressor 118 can determine if the packet stream is complete or if there are more packets to receive in the stream (420). The decompressor 118 can complete servicing of the packet stream when there are no more packets to receive.
With respect to the compressor/decompressor storage spaces 116, 120, the amount of space to allocate for storing packet headers 117, 122 can depend upon several considerations, such as the nature of noise interference affecting packets and the amount of variation between packet header content. For example, if noise interference in the network is pulsed and, on average, has a particular pulse duration, then storage space 116, 120 can be allocated to maintain a number of consecutive headers 117, 122 that span a transmission duration longer than the average pulse duration. A compression scheme based on compressing against each of the consecutive headers 117 would thus be able to endure an average level of interference and packet loss. One disadvantage is that the number of consecutive headers 117, 122 to store may be large, and a compression scheme using every one of the consecutive headers can result in a compressed header 206 that is greater in size than the uncompressed header 202. However, if the nature of the packet stream is such that packet header contents vary slowly, then a header 202 can be compressed using every other (or fewer than every other) header in the storage space 116. The compressed header 206 can thus be reduced in size by allowing delta values to remain small, while still accommodating an average level of interference and packet loss.
The particular sizes of the compressor/decompressor storage spaces 116, 120 and the number and selection of stored headers 117 to use in a compression scheme are thus variable and can be designed according to the needs and considerations of the embodiment. The following sections will describe a particular compressor/decompressor design. However, the described design is exemplary and non-limiting, and variations and modifications are contemplated to be included within the scope of the disclosed systems and methods.
Referring now to FIG. 5, there is shown an embodiment of an uncompressed header 500 having a static portion 502 and a variable portion 504. A static portion 502 can be understood to include those fields of the header 500 that are unchanged for all packets in a packet stream. In contrast, the variable portion 504 includes those fields that vary between packets in a packet stream. The static portion 502 can additionally include header fields that may be variable, but whose values can be inferred from the variable portion 504 or from the headers of other networking protocols. The static portion 502 of a header needs to be received by the decompressor only once and can be conveyed to the decompressor in an uncompressed header 500. As an example, as Bormann, et al. describe in RFC 3095, an Internet Protocol (IP) header has a number of fields that include a source address field, a destination address field, a packet length field, and a time to live field. The source and destination address fields must be constant for all packets in a stream and thus are included in the static portion 502 of an IP header. In contrast, the time to live field can vary between packets and is included in the variable portion 504. While the packet length field is not constant, it can be considered part of the static portion 502 because packet length fields also exist in other networking protocol headers (such as link layer headers) that are attached to an IP header. Thus, the packet length field in an IP header can be inferred from the packet length fields of other networking protocols and need not be included in a compressed IP header. Although the static and variable portions 502, 504 are each illustrated as being contiguous segments, they can include one or more non-neighboring header fields and can also be non-contiguous.
The variable portion 504 can be included in a compressed header 506 in original form (not shown) or in encoded form 508, which in most cases is lesser in size than the original form. For example, the values in the variable portion 504 of a header 500 can be encoded by computing delta differences with respect to the variable portions of preceding packet headers 117 stored in a compressor storage space 116. If the variable portion varies slowly, then the delta value can be smaller than the value of the variable portion. Since a delta can be positive or negative, it can be encoded using a sign-based code such as twos complement. In addition, the delta can also be encoded based on frequency of occurrence using a variable-length code such as a Huffman code. A compressed header 506 can also include control information 510 in addition to the encoded variable portion 508. The control information 510 can, for example, specify a sequence number to convey packet order and/or an indicator to convey whether a header is compressed or uncompressed.
Referring now to FIG. 6, there is shown an exemplary compressor storage space 600 containing uncompressed packet headers 602. The stored packet headers 602 can include both the static and variable portions of a header or can include only the variable portion. In the illustrated embodiment, the storage space 600 is allocated to maintain a history of N packet headers 602, where N is two or greater and can be selected based on an average noise interference duration. Accordingly, the storage space 600 can maintain a history of the N most recent uncompressed packet headers. In one embodiment, a new header can be inserted into one end of the storage space while the oldest header can be discarded from the other end. In this manner, the storage space 600 need not store the sequence numbers of the packet headers because the N packet headers 602 remain in order from newest to oldest. In general, it can be inferred that if a header to be compressed has sequence number s, then the newest stored header has sequence number (s−1) and the oldest stored header has sequence number (s−N). If there are less than N stored headers in the storage space 600, then the oldest header has sequence number “one”. Although the compressor storage space 600 as illustrated includes only uncompressed packet headers 602, the storage space 600 can also include sequence numbers and other information in addition to packet headers.
With continuing reference to FIG. 5, in one embodiment, a compressor can encode an uncompressed header 500 by computing delta differences between the variable portion 504 of the uncompressed header 500 and the variable portions of the stored headers 602. With reference also to FIG. 7, there is shown an encoded variable portion 700 of a compressed header s that includes a delta value 702 for each stored header 602 in the storage space 600, where delta (s−1) corresponds to the difference between the variable portion of header s and the variable portion of uncompressed header (s−1), and so on for delta (s−2) through delta (s−N). The delta values 702 can be represented using a signed, variable-length code or a fixed-length code. The length of such a code can depend on the range of values that a delta can assume. Additionally, the length of a variable-length code can depend on the probabilistic distribution of the range of delta values.
With reference to FIG. 8A, there is shown an exemplary decompressor storage space 800 containing received headers that have been decompressed 802. In one embodiment, storage space 800 can be allocated to maintain N decompressed headers 802. For example, the number N of stored headers can equal eight so that the decompressor storage space 800 maintains the eight most recently decompressed headers. The most recently decompressed headers 802 may not have consecutive sequence numbers since packets may be received out of order or may not be received at all. As shown in FIG. 8A, while header 810 is the oldest decompressed header in the storage space 800, header eight-hundred eleven, a later header, is absent from the list. Referring again to the delta values 702 of FIG. 7 with N equal to eight, a compressed packet header 700 can be decompressed by applying one of the delta values 702 to a corresponding one of eight preceding headers 602. Stated another way, an uncompressed header with sequence number s can be used to decompress headers (s+1) through (s+8). Accordingly, stored header 810 in FIG. 8A can be used to decompress header eight-hundred eleven, and stored header 818 can be used to decompress headers eight-hundred nineteen through eight-hundred twenty-six.
The decompressor storage space 800 can be updated when a newly received header is decompressed. Suppose that a decompressor having the storage space 800 shown in FIG. 8A receives compressed headers 819, 830, and 811, in that order. The decompressor is able to derive uncompressed header 819 based on stored header 818 and can store uncompressed header 819 in the storage space as the newest header. In the process, the oldest header 810 is discarded, resulting in the stored headers 806 of the updated storage space 804 of FIG. 8B. The decompressor also receives compressed headers 830 and 811 but is unable to decompress them because the storage space 804 lacks the necessary headers. In one embodiment, compressed packets 808 that have been received but that cannot be decompressed can also be stored in the storage space 804. Thus, compressed headers 830 and 811 can be stored in the storage space 804 while they wait to be decompressed. Compressed header 830 can be decompressed based on whether another of headers eight-hundred twenty-two through eight-hundred twenty-nine are received and decompressed. However, based on the uncompressed headers 806 in the storage space 804 of FIG. 8B, compressed header 811 cannot be decompressed regardless of which other compressed headers are received. Compressed headers that arrive with sequence numbers less than 811 also cannot be decompressed, and headers that arrive with sequence numbers greater than 811 cannot be used to decompress header 811.
In one embodiment, the situation of stored compressed packets 808 that cannot be decompressed can be mitigated by storing every other (or fewer than every other) decompressed header 806 in the decompressor storage space 804. For example, a decompressor can store every other N header in the storage space 804. Since a stored header s can be used to decompress headers (s+1) through (s+N), this storage scheme can accommodate decompression of headers from a range of N2 sequence numbers. Additionally, the amount of storage space 804 allocated for storing uncompressed headers 806 can be greater than N, such as an integer multiple of N. In one embodiment, compressed packets 808 stored in the decompressor storage space 804 can be marked after a certain amount of time or after a certain number of other packets have been received and decompressed. A decompressor can discard the marked compressed packets 808 and/or request that the compressor re-transmit uncompressed versions of a marked packet. If there are multiple marked packets 808 having consecutive or neighboring sequence numbers, then in one embodiment, a compressor may need to only transmit one uncompressed header to serve as a reference header for decompressing all of the marked packets 808. Additionally, a marked packet that has been decompressed can be used to decompress other marked packets. The disclosed systems and methods for storing and decompressing packets are not limited to the sizes, dimensions, numbers, and components illustrated and/or described herein, Variations and/or modifications to the disclosed embodiments for storing and decompressing compressed packets are also contemplated.
The methods and systems described herein are not limited to a particular hardware or software configuration, and may find applicability in many computing or processing environments. The methods and systems can be implemented in hardware or software, or a combination of hardware and software. The methods and systems can be implemented in one or more computer programs, where a computer program can be understood to include one or more processor executable instructions. The computer program(s) can execute on one or more programmable processors, and can be stored on one or more storage medium readable by the processor (including volatile and non-volatile memory and/or storage elements), one or more input devices, and/or one or more output devices.
The computer program(s) can be implemented using one or more high level procedural or object-oriented programming languages to communicate with a computer system; however, the program(s) can be implemented in assembly or machine language, if desired. The language can be compiled or interpreted.
Unless otherwise stated, use of the word “substantially” can be construed to include a precise relationship, condition, arrangement, orientation, and/or other characteristic, and deviations thereof as understood by one of ordinary skill in the art, to the extent that such deviations do not materially affect the disclosed methods and systems.
Throughout the entirety of the present disclosure, use of the articles “a” or “an” to modify a noun can be understood to be used for convenience and to include one, or more than one of the modified noun, unless otherwise specifically stated.
Elements, components, modules, and/or parts thereof that are described and/or otherwise portrayed through the figures to communicate with, be associated with, and/or be based on, something else, can be understood to so communicate, be associated with, and or be based on in a direct and/or indirect manner, unless otherwise stipulated herein.
Many additional changes in the details, materials, and arrangement of parts, herein described and illustrated, can be made by those skilled in the art. Accordingly, it will be understood that the following claims are not to be limited to the embodiments disclosed herein, can include practices otherwise than specifically described, and are to be interpreted as broadly as allowed under the law.

## Claims

1. A packet header compressor, comprising:
a computing device configured to send a packet including:
a compressed header including:
a first value associated with a second uncompressed header, said first value being configured to derive an uncompressed header for said packet based on said second uncompressed header; and
a second value associated with a third uncompressed header, said second value being configured to derive said uncompressed header for said packet based on said third uncompressed header,
wherein at least one of said second uncompressed header and said third uncompressed header are stored in a storage space configured to maintain a history of transmitted headers and at least one of said second uncompressed header and said third uncompressed header is stored in said history; and

wherein said packet further includes a first sequence number, said second uncompressed header includes a second sequence number, said third uncompressed header includes a third sequence number, and the availability of at least one of said second uncompressed header and said third uncompressed header in said history for said packet is determined based on said sequence numbers.

2. The packet header compressor of claim 1, wherein said first value is computed based on said uncompressed header of said packet and said second uncompressed header.

3. The packet header compressor of claim 1, wherein said first value corresponds to a difference between: a value representative of a portion of said uncompressed header of said packet, and a value representative of a corresponding portion of said second uncompressed header.

4. The packet header compressor of claim 1, wherein said second value is computed based on said uncompressed header of said packet and said third uncompressed header.

5. The packet header compressor of claim 1, wherein said second value corresponds to a difference between: a value representative of a portion of said uncompressed header of said packet, and a value representative of a corresponding portion of said third uncompressed header.

6. The packet header compressor of claim 1, wherein said first value and said second value are encoded by at least one of: a variable-length code and a sign-based code.

7. The packet header compressor of claim 1, wherein said uncompressed header of said packet, said second uncompressed header, and said third uncompressed header include at least one of: an Internet Protocol header, a Transmission Control Protocol header, a User Datagram Protocol header, and a Real-Time Protocol header.

8. The packet header compressor of claim 1, wherein said compressed header of said packet further comprises:
at least one of: a destination address and a packet stream identifier number.

9. The packet header compressor of claim 1, wherein said compressed header of said packet further comprises:
at least one other value distinct from said first and second values, said at least one other value configured to derive said uncompressed header of said packet based on at least one other uncompressed header distinct from said second uncompressed header and said third uncompressed header.

10. The packet header compressor of claim 1, wherein said second uncompressed header and said third uncompressed header are consecutive headers from a packet stream.

11. The packet header compressor of claim 1, wherein said computing device is further configured to send a fourth packet including:
a second compressed header including:
a third value associated with said uncompressed header of said packet, said third value being configured to derive an uncompressed header for said fourth packet based on said uncompressed header of said packet; and
a fourth value associated with an uncompressed header of a packet other than said packet and said fourth packet, said fourth value being configured to derive said uncompressed header for said fourth packet based on said uncompressed header of said other packet.

12. A method of communicating data, the method comprising:
storing a plurality of transmitted uncompressed packet headers;
providing an uncompressed header of a packet to be transmitted;
forming a first value associated with a stored second uncompressed header, said first value being configured to derive said uncompressed header for said packet based on said second uncompressed header;
forming a second value associated with a stored third uncompressed header, said second value being configured to derive said uncompressed header for said packet based on said third uncompressed header, wherein said packet includes a first sequence number, said second uncompressed header includes a second sequence number, and said third uncompressed header includes a third sequence number;
maintaining a history of uncompressed headers, wherein at least one of said second uncompressed header and said third uncompressed header is stored in said history;
determining the availability of at least one of said second uncompressed header and said third uncompressed header in said history for said packet based on said sequence numbers; and
transmitting said packet including said first value and said second value.

13. The method of claim 12, further comprising:
storing a predetermined number of transmitted packet headers.

14. The method of claim 1, further comprising:
replacing one packet header in said plurality of transmitted packet headers with said uncompressed header.

15. The method of claim 12, further comprising:
including said uncompressed header in said plurality of transmitted packet headers.

16. The method of claim 12, further comprising:
forming at least one other value distinct from said first and second values, said at least one other value configured to derive said uncompressed header of said packet based on at least one other uncompressed header stored in said plurality of transmitted headers and distinct from said second uncompressed header and said third uncompressed header.

17. A computer program product embodied on a non-transitory computer-readable medium for communicating data, the computer program product comprising instructions to cause a processor to:
store a plurality of transmitted uncompressed packet headers;
provide an uncompressed header of a packet to be transmitted;
form a first value associated with a stored second uncompressed header, said first value being configured to derive said uncompressed header for said packet based on said second uncompressed header;
form a second value associated with a stored third uncompressed header, said second value being configured to derive said uncompressed header for said packet based on said third uncompressed header, wherein said packet includes a first sequence number, said second uncompressed header includes a second sequence number, and said third uncompressed header includes a third sequence number;
maintain a history of uncompressed headers, wherein at least one of said second uncompressed header and said third uncompressed header is stored in said history;
determine the availability of at least one of said second uncompressed header and said third uncompressed header in said history for said packet based on said sequence numbers; and
transmit said packet including said first value and said second value.

18. The computer program product of claim 17, wherein a predetermined number of transmitted packet headers are stored.

19. The computer program product of claim 17, further comprising instructions to cause a processor to:
replace one packet header in said plurality of transmitted packet headers with said uncompressed header.

20. The computer program product of claim 17, further comprising instructions to cause a processor to:
include said uncompressed header in said plurality of transmitted packet headers.

21. The computer program product of claim 17, further comprising instructions to cause a processor to:
form at least one other value distinct from said first and second values, said at least one other value configured to derive said uncompressed header of said packet based on at least one other uncompressed header stored in said plurality of transmitted headers and distinct from said second uncompressed header and said third uncompressed header.

22. A packet header decompressor including a computing device and a storage space for maintaining a history, said decompressor configured to receive a packet, said packet comprising:
a compressed header comprising:
a first value associated with a second uncompressed header, configured for deriving an uncompressed header for said packet based on said second uncompressed header; and
a second value associated with a third uncompressed header, configured for deriving said uncompressed header based on said third uncompressed header,

wherein said packet includes a first sequence number, said second uncompressed header includes a second sequence number, said third uncompressed header includes a third sequence number, and the availability of at least one of said second uncompressed header and said third uncompressed header in said history for said packet is determined based on said sequence numbers.

23. The packet header decompressor of claim 22, wherein said first value and said second value are encoded by at least one of: a variable-length code and a sign-based code.

24. The packet header decompressor of claim 22, wherein said compressed header further comprises:
at least one other value distinct from said first and second values, said at least one other value for deriving said uncompressed header based on at least one other uncompressed header distinct from said second uncompressed header and said third uncompressed header.

25. The packet header decompressor of claim 22, wherein said packets associated with said second and third uncompressed headers are consecutive headers from a packet stream.