| QUIC | C. Krasic |
| Internet-Draft | |
| Intended status: Standards Track | August 3, 2017 |
| Expires: February 4, 2018 |
Header Compression for HTTP over QUIC
draft-krasic-quic-qcram-02
The design of the core QUIC transport and the mapping of HTTP semantics over it subsume many HTTP/2 features, prominent among them stream multiplexing and HTTP header compression. A key advantage of the QUIC transport is it provides stream multiplexing free of HoL blocking between streams, while in HTTP/2 multiplexed streams can suffer HoL blocking primarily due to HTTP/2’s layering above TCP. However if HPACK is used for header compression, HTTP over QUIC is still vulnerable to HoL blocking, because of how HPACK exploits header redundancies between multiplexed HTTP transactions. This draft defines QCRAM, a variation of HPACK and mechanisms in the QUIC HTTP mapping that allow QUIC implementations the flexibility to avoid header-compression induced HoL blocking.
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The QUIC transport protocol was designed from the outset to support HTTP semantics, and its design subsumes most of the features of HTTP/2. Two of those features, stream multiplexing and header compression come into some conflict in QUIC. A key goal of the design of QUIC is to improve stream multiplexing relative to HTTP/2, by eliminating HoL (head of line) blocking that can occur in HTTP/2. HoL blocking can happen because HTTP/2 streams are multiplexed onto a single TCP connection with its in-order semantics. QUIC can maintain independence between streams because it implements core transport functionality in a fully stream-aware manner. However, the HTTP over QUIC mapping is still subject to HoL blocking if HPACK is used directly as in HTTP/2. HPACK exploits multiplexing for greater compression, shrinking the representation of headers that have appeared earlier on the same connection. In the context of QUIC, this imposes a vulnerability to HoL blocking as will be described more below (Section 2.1).
QUIC is described in [QUIC-TRANSPORT]. The HTTP over QUIC mapping is described in [QUIC-HTTP]. For a full description of HTTP/2, see [RFC7540]. The description of HPACK is [RFC7541].
Readers may wish to refer to [RFC7541] Section 1.3 to review HPACK terminology, and [QUIC-HTTP], Sections 4 on “HTTP over QUIC stream mapping” and 4.2.1 on “Header Compression”. QCRAM extensions to HPACK allow correctness in the presence of out-of-order delivery, with flexibility to balance between resilience against HoL blocking and compression ratio.
QCRAM is intended to be a relatively non-intrusive extension to HPACK, an implementation should be easily shared within stacks supporting both HTTP/2 over (TLS+)TCP and HTTP over QUIC.
The following is an example of how HPACK can induce HoL blocking in QUIC. Assume two HTTP message exchange streams A and B, and corresponding header blocks HA and HB. Stream B experiences HoL blocking due to A as follows:
Continuing the example, QCRAM’s approach is as follows.
In HEADERS and PUSH_PROMISE frames, HPACK Header data should be prefixed by a pair of integers: Fill and the Evictions. Fill is the number of entries in the table, and Evictions is the cumulative number entries that have been evicted from the table. Their sum is the cumulative number of entries inserted. Each is encoded as a single HPACK integer (8-bit prefix):
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|Fill (8+)|
+---------------+
|Evictions (8+)|
+---------------+
Figure 1: Absolute indexing
Section 3.2 describes the role of Fill and Section 3.3 covers the role of Evictions.
HPACK indexed entries refer to an entry by its current position in the dynamic table. As Figure 1 of RFC7541 illustrates, newest entries have smallest indices, and oldest entries are evicted first if the table is full. Under this scheme, each insertion to the table causes the index of all existing entries to change (implicitly). Implicit index updates are acceptable for HTTP/2 because TCP is totally ordered, but it is is problematic in the out-of-order context of QUIC.
QCRAM uses a hybrid absolute-relative indexing approach. The prefix defined in Section 3.1 is used by the decoder to interpret all subsequent HPACK instructions at absolute positions for indexed lookups and insertions. It is also used for evictions (Section 3.3).
As was defined in Section 2.2 case 3, the encoder has the option to select indexed representations that are vulnerable to HoL blocking. Decoder processing of indexed header fields MUST block the encompassing header block if the referenced entry has not been added to the table yet.
To protect against buggy or malicious peers, a timer should be used to set an upper bound on such blocking and in treat expiration of the timer as a decoding error. However, if the implementation chooses not to abort the connection, the remainder of the header block MUST be decoded and output discarded.
Due to out of order arrival, QCRAM’s eviction algorithm requires changes (relative to HPACK) to avoid the possibility that an indexed representation is decoded after the referenced entry is already evicted. QCRAM employs a two-phase eviction algorithm, in which the encoder will not evict entries that have outstanding (unacknowledged) references. The QCRAM encoder maintains a counter as entries are evicted, which is the cumulative number of evictions so far, Evictions (Section 3.1). On arrival at the decoder, if Evictions is higher than previously seen, the decoder MUST evict all entries at or below. Unlike HPACK where the decoder follows the same logic as the encoder to perform evictions, in QCRAM the decoder evicts exclusively based on the encoder’s explicit guidance.
In some cases, the encoder must forgo eviction by selecting a literal representation (blocked eviction), namely in the event that the entry subject to eviction is referenced by one or more unacknowledged header frames. To assure that the blocked eviction case is rare, a form of thresholding MAY be applied that constrains selection of Indexed representations, such that the oldest entries in the dynamic table will largely be evictable. The constraint is applied when encoding header fields: comparing the cumulative position (in bytes) of the matching entry to a threshold, categorizing oldest entries (past threshold) as at-risk. Avoiding references to at-risk entries, the encoder SHOULD use an Indexed-Duplicate representation instead (see Section 3.5).
The QCRAM encoder has the option to select representations that might require blocking (Section 2.2 case 3), but the decoder must be prevented from becoming hung if the stream associated with the referenced entry is reset. On stream reset, the QCRAM encoder MUST check if the stream has unacknowledged headers, and if so resend them on the Control Stream ([QUIC-HTTP] Section 4.1). If header blocks are resent on the control stream, duplicate arrivals are possible due to reset-acknowledgment races. The decoder MUST ignore duplicate header block arrivals, which is straightforward because of unambiguous indexing (see Section 3.2).
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0|0|1|Index(5+)|
+-+-+-+---------+
Figure 2: Indexed Header Field with Duplication
Indexed-Duplicates are treated as an Indexed Header Field Representation (see [RFC7541] Section 6.1), additionally inserting a new duplicate entry. [RFC7541] allows duplicate HPACK table entries, that is entries that have the same name and value.
Figure 2 annexes the representation for HPACK Dynamic Table Size Update (see Section 6.3 of RFC7541), which is not supported by HTTP over QUIC.
To help mitigate memory consumption due to duplicate entries, HPACK for QCRAM is required to de-duplicate strings in the dynamic table. The table insertion logic should check if the new entry matches any existing entries (name and value), and if so, table accounting MUST charge only the overhead portion ([RFC7541] Section 4.1) to the new entry.
Specific de-duplication mechanisms are left to implementations, but using a map in conjunction with reference counted pointers to strings would be typical.
Implementations can speculatively send header frames on the HTTP Connection Control Stream. Such headers would not be associated with any HTTP transaction, but could be used strategically to improve performance. For instance, the encoder might decide to refresh by sending Indexed-Duplicate representations for popular header fields (Section 3.1), ensuring they have small indices and hence minimal size on the wire.
HPACK defines overhead as 32 bytes ([RFC7541] Section 4.1). QCRAM adds some per-entry state, to track acknowledgment status and eviction rank, and requires mechanisms to de-duplicate strings. A larger value than 32 might be more accurate for QCRAM.
In Section 2.2 case 3, an exception exists when the representation of HA[i] and HB[j] are delivered within the same transport packet. If so, there is no risk of HoL blocking and using an indexed representation is strictly better than using a literal. An implementation could exploit this exception by employing co-ordination between QCRAM compression and QUIC transport packetization.
TBD.
This document currently makes no request of IANA, and might not need to.
This draft draws heavily on the text of [RFC7541]. The indirect input of those authors is gratefully acknowledged, as well as ideas from:
| [QUIC-HTTP] | Bishop, M., "Hypertext Transfer Protocol (HTTP) over QUIC", August 2017. |
| [QUIC-TRANSPORT] | Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed and Secure Transport", August 2017. |
| [RFC7540] | Belshe, M., Peon, R. and M. Thomson, "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015. |
| [RFC7541] | Peon, R. and H. Ruellan, "HPACK: Header Compression for HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015. |