| Internet-Draft | Comparison of CoAP Security Protocols | November 2020 |
| Mattsson, et al. | Expires 6 May 2021 | [Page] |
This document analyzes and compares the sizes of key exchange flights and the per-packet message size overheads when using different security protocols to secure CoAP. The analyzed security protocols are DTLS 1.2, DTLS 1.3, TLS 1.2, TLS 1.3, EDHOC, OSCORE, and Group OSCORE. The DTLS and TLS record layers are analyzed with and without 6LoWPAN-GHC compression. DTLS is analyzed with and without Connection ID.¶
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This document analyzes and compares the sizes of key exchange flights and the per-packet message size overheads when using different security protocols to secure CoAP over UPD [RFC7252] and TCP [RFC8323]. The analyzed security protocols are DTLS 1.2 [RFC6347], DTLS 1.3 [I-D.ietf-tls-dtls13], TLS 1.2 [RFC5246], TLS 1.3 [RFC8446], EDHOC [I-D.ietf-lake-edhoc], OSCORE [RFC8613], and Group OSCORE [I-D.ietf-core-oscore-groupcomm].¶
The DTLS and TLS record layers are analyzed with and without 6LoWPAN-GHC compression. DTLS is anlyzed with and without Connection ID [I-D.ietf-tls-dtls-connection-id]. Readers are expected to be familiar with some of the terms described in RFC 7925 [RFC7925], such as ICV. Section 2 compares the overhead of key exchange, while Section 3 covers the overhead for protection of application data.¶
This section analyzes and compares the sizes of key exchange flights for different protocols.¶
To enable a fair comparison between protocols, the following assumptions are made:¶
Section 2.1 gives a short summary of the message overhead based on different parameters and some assumptions. The following sections detail the assumptions and the calculations.¶
The DTLS overhead is dependent on the parameter Connection ID. The following overheads apply for all Connection IDs of the same length, when Connection ID is used.¶
The EDHOC overhead is dependent on the key identifiers included. The following overheads apply for Sender IDs of the same length.¶
All the overhead are dependent on the tag length. The following overheads apply for tags of the same length.¶
Figure 1 compares the message sizes of EDHOC [I-D.ietf-lake-edhoc] with the DTLS 1.3 [I-D.ietf-tls-dtls13] and TLS 1.3 [RFC8446] handshakes with connection ID.¶
===================================================================== Flight #1 #2 #3 Total --------------------------------------------------------------------- DTLS 1.3 RPK + ECDHE 150 373 213 736 DTLS 1.3 Cached X.509/RPK + ECDHE 182 347 213 742 DTLS 1.3 PSK + ECDHE 184 190 57 431 DTLS 1.3 PSK 134 150 57 341 --------------------------------------------------------------------- EDHOC RPK + ECDHE 37 46 20 103 EDHOC X.509 + ECDHE 37 117 91 245 =====================================================================
Figure 2 compares of message sizes of DTLS 1.3 [I-D.ietf-tls-dtls13] and TLS 1.3 [RFC8446] handshakes without connection ID.¶
===================================================================== Flight #1 #2 #3 Total --------------------------------------------------------------------- DTLS 1.3 RPK + ECDHE 144 364 212 722 DTLS 1.3 PSK + ECDHE 178 183 56 417 DTLS 1.3 PSK 128 143 56 327 --------------------------------------------------------------------- TLS 1.3 RPK + ECDHE 129 322 194 645 TLS 1.3 PSK + ECDHE 163 157 50 370 TLS 1.3 PSK 113 117 50 280 =====================================================================
The details of the message size calculations are given in the following sections.¶
This section gives an estimate of the message sizes of DTLS 1.3 with different authentication methods. Note that the examples in this section are not test vectors, the cryptographic parts are just replaced with byte strings of the same length, while other fixed length fields are replace with arbitrary strings or omitted, in which case their length is indicated. Values that are not arbitrary are given in hexadecimal.¶
In this section, a Connection ID of 1 byte is used.¶
Record Header - DTLSPlaintext (13 bytes):
16 fe fd EE EE SS SS SS SS SS SS LL LL
Handshake Header - Client Hello (10 bytes):
01 LL LL LL SS SS 00 00 00 LL LL LL
Legacy Version (2 bytes):
fe fd
Client Random (32 bytes):
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15
16 17 18 19 1a 1b 1c 1d 1e 1f
Legacy Session ID (1 bytes):
00
Legacy Cookie (1 bytes):
00
Cipher Suites (TLS_AES_128_CCM_8_SHA256) (4 bytes):
00 02 13 05
Compression Methods (null) (2 bytes):
01 00
Extensions Length (2 bytes):
LL LL
Extension - Supported Groups (x25519) (8 bytes):
00 0a 00 04 00 02 00 1d
Extension - Signature Algorithms (ecdsa_secp256r1_sha256)
(8 bytes):
00 0d 00 04 00 02 08 07
Extension - Key Share (42 bytes):
00 33 00 26 00 24 00 1d 00 20
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15
16 17 18 19 1a 1b 1c 1d 1e 1f
Extension - Supported Versions (1.3) (7 bytes):
00 2b 00 03 02 03 04
Extension - Client Certificate Type (Raw Public Key) (6 bytes):
00 13 00 01 01 02
Extension - Server Certificate Type (Raw Public Key) (6 bytes):
00 14 00 01 01 02
Extension - Connection Identifier (43) (6 bytes):
XX XX 00 02 01 42
13 + 10 + 2 + 32 + 1 + 1 + 4 + 2 + 2 + 8 + 8 + 42 + 7 + 6 + 6 + 6 = 150
bytes
¶
DTLS 1.3 RPK + ECDHE flight_1 gives 150 bytes of overhead.¶
Record Header - DTLSPlaintext (13 bytes):
16 fe fd EE EE SS SS SS SS SS SS LL LL
Handshake Header - Server Hello (10 bytes):
02 LL LL LL SS SS 00 00 00 LL LL LL
Legacy Version (2 bytes):
fe fd
Server Random (32 bytes):
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15
16 17 18 19 1a 1b 1c 1d 1e 1f
Legacy Session ID (1 bytes):
00
Cipher Suite (TLS_AES_128_CCM_8_SHA256) (2 bytes):
13 05
Compression Method (null) (1 bytes):
00
Extensions Length (2 bytes):
LL LL
Extension - Key Share (40 bytes):
00 33 00 24 00 1d 00 20
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15
16 17 18 19 1a 1b 1c 1d 1e 1f
Extension - Supported Versions (1.3) (6 bytes):
00 2b 00 02 03 04
Extension - Connection Identifier (43) (6 bytes):
XX XX 00 02 01 43
Record Header - DTLSCiphertext, Full (6 bytes):
HH ES SS 43 LL LL
Handshake Header - Encrypted Extensions (10 bytes):
08 LL LL LL SS SS 00 00 00 LL LL LL
Extensions Length (2 bytes):
LL LL
Extension - Client Certificate Type (Raw Public Key) (6 bytes):
00 13 00 01 01 02
Extension - Server Certificate Type (Raw Public Key) (6 bytes):
00 14 00 01 01 02
Handshake Header - Certificate Request (10 bytes):
0d LL LL LL SS SS 00 00 00 LL LL LL
Request Context (1 bytes):
00
Extensions Length (2 bytes):
LL LL
Extension - Signature Algorithms (ecdsa_secp256r1_sha256)
(8 bytes):
00 0d 00 04 00 02 08 07
Handshake Header - Certificate (10 bytes):
0b LL LL LL SS SS 00 00 00 LL LL LL
Request Context (1 bytes):
00
Certificate List Length (3 bytes):
LL LL LL
Certificate Length (3 bytes):
LL LL LL
Certificate (59 bytes) // Point compression
....
Certificate Extensions (2 bytes):
00 00
Handshake Header - Certificate Verify (10 bytes):
0f LL LL LL SS SS 00 00 00 LL LL LL
Signature (68 bytes):
ZZ ZZ 00 40 ....
Handshake Header - Finished (10 bytes):
14 LL LL LL SS SS 00 00 00 LL LL LL
Verify Data (32 bytes):
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15
16 17 18 19 1a 1b 1c 1d 1e 1f
Record Type (1 byte):
16
Auth Tag (8 bytes):
e0 8b 0e 45 5a 35 0a e5
13 + 102 + 6 + 24 + 21 + 78 + 78 + 42 + 1 + 8 = 373 bytes
¶
DTLS 1.3 RPK + ECDHE flight_2 gives 373 bytes of overhead.¶
Record Header (6 bytes) // DTLSCiphertext, Full:
ZZ ES SS 42 LL LL
Handshake Header - Certificate (10 bytes):
0b LL LL LL SS SS XX XX XX LL LL LL
Request Context (1 bytes):
00
Certificate List Length (3 bytes):
LL LL LL
Certificate Length (3 bytes):
LL LL LL
Certificate (59 bytes) // Point compression
....
Certificate Extensions (2 bytes):
00 00
Handshake Header - Certificate Verify (10 bytes):
0f LL LL LL SS SS 00 00 00 LL LL LL
Signature (68 bytes):
04 03 LL LL //ecdsa_secp256r1_sha256
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15
16 17 18 19 1a 1b 1c 1d 1e 1f 00 01 02 03 04 05 06 07 08 09 0a 0b
0c 0d 0e 0f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
Handshake Header - Finished (10 bytes):
14 LL LL LL SS SS 00 00 00 LL LL LL
Verify Data (32 bytes) // SHA-256:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15
16 17 18 19 1a 1b 1c 1d 1e 1f
Record Type (1 byte):
16
Auth Tag (8 bytes) // AES-CCM_8:
00 01 02 03 04 05 06 07
6 + 78 + 78 + 42 + 1 + 8 = 213 bytes
¶
DTLS 1.3 RPK + ECDHE flight_2 gives 213 bytes of overhead.¶
The differences in overhead compared to Section 2.2.1.1 are:¶
The following is added:¶
+ Extension - PSK Key Exchange Modes (6 bytes): 00 2d 00 02 01 01 + Extension - Pre Shared Key (48 bytes): 00 29 00 2F 00 0a 00 01 ID 00 00 00 00 00 21 20 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f¶
The following is removed:¶
- Extension - Signature Algorithms (ecdsa_secp256r1_sha256) (8 bytes) - Extension - Client Certificate Type (Raw Public Key) (6 bytes) - Extension - Server Certificate Type (Raw Public Key) (6 bytes)¶
In total:¶
150 + 6 + 48 - 8 - 6 - 6 = 184 bytes¶
DTLS 1.3 PSK + ECDHE flight_1 gives 184 bytes of overhead.¶
The differences in overhead compared to Section 2.2.1.2 are:¶
The following is added:¶
+ Extension - Pre Shared Key (6 bytes) 00 29 00 02 00 00¶
The following is removed:¶
- Handshake Message Certificate (78 bytes) - Handshake Message CertificateVerify (78 bytes) - Handshake Message CertificateRequest (21 bytes) - Extension - Client Certificate Type (Raw Public Key) (6 bytes) - Extension - Server Certificate Type (Raw Public Key) (6 bytes)¶
In total:¶
373 - 78 - 78 - 21 - 6 - 6 + 6 = 190 bytes¶
DTLS 1.3 PSK + ECDHE flight_2 gives 190 bytes of overhead.¶
The differences in overhead compared to Section 2.2.2.1 are:¶
The following is removed:¶
- Extension - Supported Groups (x25519) (8 bytes) - Extension - Key Share (42 bytes)¶
In total:¶
184 - 8 - 42 = 134 bytes¶
DTLS 1.3 PSK flight_1 gives 134 bytes of overhead.¶
The differences in overhead compared to Section 2.2.2.2 are:¶
The following is removed:¶
- Extension - Key Share (40 bytes)¶
In total:¶
190 - 40 = 150 bytes¶
DTLS 1.3 PSK flight_2 gives 150 bytes of overhead.¶
There are no differences in overhead compared to Section 2.2.2.3.¶
DTLS 1.3 PSK flight_3 gives 57 bytes of overhead.¶
In this section, we consider the effect of [RFC7924] on the message size overhead.¶
Cached information together with server X.509 can be used to move bytes from flight #2 to flight #1 (cached RPK increases the number of bytes compared to cached X.509).¶
The differences compared to Section 2.2.1 are the following.¶
For the flight #1, the following is added:¶
+ Extension - Client Cashed Information (39 bytes): 00 19 LL LL LL LL 01 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f¶
And the following is removed:¶
- Extension - Server Certificate Type (Raw Public Key) (6 bytes)¶
Giving a total of:¶
150 + 33 = 183 bytes¶
For the flight #2, the following is added:¶
+ Extension - Server Cashed Information (7 bytes): 00 19 LL LL LL LL 01¶
And the following is removed:¶
- Extension - Server Certificate Type (Raw Public Key) (6 bytes) - Server Certificate (59 bytes -> 32 bytes)¶
Giving a total of:¶
373 - 26 = 347 bytes¶
A summary of the calculation is given in Figure 3.¶
====================================================================== Flight #1 #2 #3 Total ---------------------------------------------------------------------- DTLS 1.3 Cached X.509/RPK + ECDHE 183 347 213 743 DTLS 1.3 RPK + ECDHE 150 373 213 736 =======================================================================
To enable resumption, a 4th flight (New Session Ticket) is added to the PSK handshake.¶
Record Header - DTLSCiphertext, Full (6 bytes):
HH ES SS 43 LL LL
Handshake Header - New Session Ticket (10 bytes):
04 LL LL LL SS SS 00 00 00 LL LL LL
Ticket Lifetime (4 bytes):
00 01 02 03
Ticket Age Add (4 bytes):
00 01 02 03
Ticket Nonce (2 bytes):
01 00
Ticket (6 bytes):
00 04 ID ID ID ID
Extensions (2 bytes):
00 00
Auth Tag (8 bytes) // AES-CCM_8:
00 01 02 03 04 05 06 07
6 + 10 + 4 + 4 + 2 + 6 + 2 + 8 = 42 bytes
¶
The initial handshake when resumption is enabled is just a PSK handshake with 134 + 150 + 57 + 42 = 383 bytes.¶
Without a Connection ID the DTLS 1.3 flight sizes changes as follows.¶
DTLS 1.3 Flight #1: -6 bytes DTLS 1.3 Flight #2: -7 bytes DTLS 1.3 Flight #3: -1 byte¶
======================================================================= Flight #1 #2 #3 Total ----------------------------------------------------------------------- DTLS 1.3 RPK + ECDHE (no cid) 144 364 212 722 DTLS 1.3 PSK + ECDHE (no cid) 178 183 56 417 DTLS 1.3 PSK (no cid) 128 143 56 327 =======================================================================
TODO¶
0x30 // Sequence
0x59 // Size 89
0x30 // Sequence
0x13 // Size 19
0x06 0x07 0x2A 0x86 0x48 0xCE 0x3D 0x02 0x01
// OID 1.2.840.10045.2.1 (ecPublicKey)
0x06 0x08 0x2A 0x86 0x48 0xCE 0x3D 0x03 0x01 0x07
// OID 1.2.840.10045.3.1.7 (secp256r1)
0x03 // Bit string
0x42 // Size 66
0x00 // Unused bits 0
0x04 // Uncompressed
...... 64 bytes X and Y
Total of 91 bytes
¶
0x30 // Sequence
0x59 // Size 89
0x30 // Sequence
0x13 // Size 19
0x06 0x07 0x2A 0x86 0x48 0xCE 0x3D 0x02 0x01
// OID 1.2.840.10045.2.1 (ecPublicKey)
0x06 0x08 0x2A 0x86 0x48 0xCE 0x3D 0x03 0x01 0x07
// OID 1.2.840.10045.3.1.7 (secp256r1)
0x03 // Bit string
0x42 // Size 66
0x00 // Unused bits 0
0x03 // Compressed
...... 32 bytes X
Total of 59 bytes
¶
In this section, the message sizes are calculated for TLS 1.3. The major changes compared to DTLS 1.3 are that the record header is smaller, the handshake headers is smaller, and that Connection ID is not supported. Recently, additional work has taken shape with the goal to further reduce overhead for TLS 1.3 (see [I-D.rescorla-tls-ctls]).¶
TLS Assumptions:¶
For the PSK calculations, [Ulfheim-TLS13] was a useful resource, while for RPK calculations we followed the work of [IoT-Cert].¶
Record Header - TLSPlaintext (5 bytes):
16 03 03 LL LL
Handshake Header - Client Hello (4 bytes):
01 LL LL LL
Legacy Version (2 bytes):
03 03
Client Random (32 bytes):
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15
16 17 18 19 1a 1b 1c 1d 1e 1f
Legacy Session ID (1 bytes):
00
Cipher Suites (TLS_AES_128_CCM_8_SHA256) (4 bytes):
00 02 13 05
Compression Methods (null) (2 bytes):
01 00
Extensions Length (2 bytes):
LL LL
Extension - Supported Groups (x25519) (8 bytes):
00 0a 00 04 00 02 00 1d
Extension - Signature Algorithms(ecdsa_secp256r1_sha256)(8 bytes):
00 0d 00 04 00 02 08 07
Extension - Key Share (42 bytes):
00 33 00 26 00 24 00 1d 00 20
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15
16 17 18 19 1a 1b 1c 1d 1e 1f
Extension - Supported Versions (1.3) (7 bytes):
00 2b 00 03 02 03 04
Extension - Client Certificate Type (Raw Public Key) (6 bytes):
00 13 00 01 01 02
Extension - Server Certificate Type (Raw Public Key) (6 bytes):
00 14 00 01 01 02
5 + 4 + 2 + 32 + 1 + 4 + 2 + 2 + 8 + 8 + 42 + 7 + 6 + 6 = 129 bytes
¶
TLS 1.3 RPK + ECDHE flight_1 gives 129 bytes of overhead.¶
Record Header - TLSPlaintext (5 bytes):
16 03 03 LL LL
Handshake Header - Server Hello (4 bytes):
02 LL LL LL
Legacy Version (2 bytes):
fe fd
Server Random (32 bytes):
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15
16 17 18 19 1a 1b 1c 1d 1e 1f
Legacy Session ID (1 bytes):
00
Cipher Suite (TLS_AES_128_CCM_8_SHA256) (2 bytes):
13 05
Compression Method (null) (1 bytes):
00
Extensions Length (2 bytes):
LL LL
Extension - Key Share (40 bytes):
00 33 00 24 00 1d 00 20
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15
16 17 18 19 1a 1b 1c 1d 1e 1f
Extension - Supported Versions (1.3) (6 bytes):
00 2b 00 02 03 04
Record Header - TLSCiphertext (5 bytes):
17 03 03 LL LL
Handshake Header - Encrypted Extensions (4 bytes):
08 LL LL LL
Extensions Length (2 bytes):
LL LL
Extension - Client Certificate Type (Raw Public Key) (6 bytes):
00 13 00 01 01 02
Extension - Server Certificate Type (Raw Public Key) (6 bytes):
00 14 00 01 01 02
Handshake Header - Certificate Request (4 bytes):
0d LL LL LL
Request Context (1 bytes):
00
Extensions Length (2 bytes):
LL LL
Extension - Signature Algorithms(ecdsa_secp256r1_sha256)(8 bytes):
00 0d 00 04 00 02 08 07
Handshake Header - Certificate (4 bytes):
0b LL LL LL
Request Context (1 bytes):
00
Certificate List Length (3 bytes):
LL LL LL
Certificate Length (3 bytes):
LL LL LL
Certificate (59 bytes) // Point compression
....
Certificate Extensions (2 bytes):
00 00
Handshake Header - Certificate Verify (4 bytes):
0f LL LL LL
Signature (68 bytes):
ZZ ZZ 00 40 ....
Handshake Header - Finished (4 bytes):
14 LL LL LL
Verify Data (32 bytes):
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15
16 17 18 19 1a 1b 1c 1d 1e 1f
Record Type (1 byte):
16
Auth Tag (8 bytes):
e0 8b 0e 45 5a 35 0a e5
5 + 90 + 5 + 18 + 15 + 72 + 72 + 36 + 1 + 8 = 322 bytes
¶
TLS 1.3 RPK + ECDHE flight_2 gives 322 bytes of overhead.¶
Record Header - TLSCiphertext (5 bytes):
17 03 03 LL LL
Handshake Header - Certificate (4 bytes):
0b LL LL LL
Request Context (1 bytes):
00
Certificate List Length (3 bytes):
LL LL LL
Certificate Length (3 bytes):
LL LL LL
Certificate (59 bytes) // Point compression
....
Certificate Extensions (2 bytes):
00 00
Handshake Header - Certificate Verify (4 bytes):
0f LL LL LL
Signature (68 bytes):
04 03 LL LL //ecdsa_secp256r1_sha256
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15
16 17 18 19 1a 1b 1c 1d 1e 1f 00 01 02 03 04 05 06 07 08 09 0a 0b
0c 0d 0e 0f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
Handshake Header - Finished (4 bytes):
14 LL LL LL
Verify Data (32 bytes) // SHA-256:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15
16 17 18 19 1a 1b 1c 1d 1e 1f
Record Type (1 byte)
16
Auth Tag (8 bytes) // AES-CCM_8:
00 01 02 03 04 05 06 07
5 + 72 + 72 + 36 + 1 + 8 = 194 bytes
¶
TLS 1.3 RPK + ECDHE flight_3 gives 194 bytes of overhead.¶
The differences in overhead compared to Section 2.3.1.3 are:¶
The following is added:¶
+ Extension - PSK Key Exchange Modes (6 bytes): 00 2d 00 02 01 01 + Extension - Pre Shared Key (48 bytes): 00 29 00 2F 00 0a 00 01 ID 00 00 00 00 00 21 20 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f¶
The following is removed:¶
- Extension - Signature Algorithms (ecdsa_secp256r1_sha256) (8 bytes) - Extension - Client Certificate Type (Raw Public Key) (6 bytes) - Extension - Server Certificate Type (Raw Public Key) (6 bytes)¶
In total:¶
129 + 6 + 48 - 8 - 6 - 6 = 163 bytes¶
TLS 1.3 PSK + ECDHE flight_1 gives 166 bytes of overhead.¶
The differences in overhead compared to Section 2.3.1.2 are:¶
The following is added:¶
+ Extension - Pre Shared Key (6 bytes) 00 29 00 02 00 00¶
The following is removed:¶
- Handshake Message Certificate (72 bytes) - Handshake Message CertificateVerify (72 bytes) - Handshake Message CertificateRequest (15 bytes) - Extension - Client Certificate Type (Raw Public Key) (6 bytes) - Extension - Server Certificate Type (Raw Public Key) (6 bytes)¶
In total:¶
322 - 72 - 72 - 15 - 6 - 6 + 6 = 157 bytes¶
TLS 1.3 PSK + ECDHE flight_2 gives 157 bytes of overhead.¶
The differences in overhead compared to Section 2.3.2.1 are:¶
The following is removed:¶
- Extension - Supported Groups (x25519) (8 bytes) - Extension - Key Share (42 bytes)¶
In total:¶
163 - 8 - 42 = 113 bytes¶
TLS 1.3 PSK flight_1 gives 116 bytes of overhead.¶
The differences in overhead compared to Section 2.3.2.2 are:¶
The following is removed:¶
- Extension - Key Share (40 bytes)¶
In total:¶
157 - 40 = 117 bytes¶
TLS 1.3 PSK flight_2 gives 117 bytes of overhead.¶
There are no differences in overhead compared to Section 2.3.2.3.¶
TLS 1.3 PSK flight_3 gives 57 bytes of overhead.¶
This section gives an estimate of the message sizes of EDHOC with authenticated with static Diffie-Hellman keys. All examples are given in CBOR diagnostic notation and hexadecimal, and are based on the test vectors in Appendix B.2 of [I-D.ietf-lake-edhoc].¶
message_1 = (
13,
0,
h'8D3EF56D1B750A4351D68AC250A0E883790EFC80A538A444EE9E2B57E244
1A7C',
-2
)
¶
message_1 (37 bytes): 0d 00 58 20 8d 3e f5 6d 1b 75 0a 43 51 d6 8a c2 50 a0 e8 83 79 0e fc 80 a5 38 a4 44 ee 9e 2b 57 e2 44 1a 7c 21¶
The typical message sizes for the previous example and for an example of EDHOC authenticated with signature keys and X.509 certificates based on Appendix B.1 of [I-D.ietf-lake-edhoc] are summarized in Figure 5.¶
===============================
RPK x5t
-------------------------------
message_1 37 37
message_2 46 117
message_3 20 91
-------------------------------
Total 103 245
===============================
To do a fair comparison, one has to choose a specific deployment and look at the topology, the whole protocol stack, frame sizes (e.g. 51 or 128 bytes), how and where in the protocol stack fragmentation is done, and the expected packet loss. Note that the number of bytes in each frame that is available for the key exchange protocol may depend on the underlying protocol layers as well as on the number of hops in multi-hop networks. The packet loss may depend on how many other devices are transmitting at the same time, and may increase during network formation. The total overhead will be larger due to mechanisms for fragmentation, retransmission, and packet ordering. The overhead of fragmentation is roughly proportional to the number of fragments, while the expected overhead due to retransmission in noisy environments is a superlinear function of the flight sizes.¶
To enable comparison, all the overhead calculations in this section use AES-CCM with a tag length of 8 bytes (e.g. AES_128_CCM_8 or AES-CCM-16-64), a plaintext of 6 bytes, and the sequence number '05'. This follows the example in [RFC7400], Figure 16.¶
Note that the compressed overhead calculations for DLTS 1.2, DTLS 1.3, TLS 1.2 and TLS 1.3 are dependent on the parameters epoch, sequence number, and length, and all the overhead calculations are dependent on the parameter Connection ID when used. Note that the OSCORE overhead calculations are dependent on the CoAP option numbers, as well as the length of the OSCORE parameters Sender ID and Sequence Number. The following calculations are only examples.¶
Section 3.1 gives a short summary of the message overhead based on different parameters and some assumptions. The following sections detail the assumptions and the calculations.¶
The DTLS overhead is dependent on the parameter Connection ID. The following overheads apply for all Connection IDs with the same length.¶
The compression overhead (GHC) is dependent on the parameters epoch, sequence number, Connection ID, and length (where applicable). The following overheads should be representative for sequence numbers and Connection IDs with the same length.¶
The OSCORE overhead is dependent on the included CoAP Option numbers as well as the length of the OSCORE parameters Sender ID and sequence number. The following overheads apply for all sequence numbers and Sender IDs with the same length.¶
Sequence Number '05' '1005' '100005' ------------------------------------------------------------- DTLS 1.2 29 29 29 DTLS 1.3 11 12 12 ------------------------------------------------------------- DTLS 1.2 (GHC) 16 16 16 DTLS 1.3 (GHC) 12 13 13 ------------------------------------------------------------- TLS 1.2 21 21 21 TLS 1.3 14 14 14 ------------------------------------------------------------- TLS 1.2 (GHC) 17 18 19 TLS 1.3 (GHC) 15 16 17 ------------------------------------------------------------- OSCORE request 13 14 15 OSCORE response 11 11 11
Connection/Sender ID '' '42' '4002' ------------------------------------------------------------- DTLS 1.2 29 30 31 DTLS 1.3 11 12 13 ------------------------------------------------------------- DTLS 1.2 (GHC) 16 17 18 DTLS 1.3 (GHC) 12 13 14 ------------------------------------------------------------- OSCORE request 13 14 15 OSCORE response 11 11 11
Protocol Overhead Overhead (GHC) ------------------------------------------------------------- DTLS 1.2 21 8 DTLS 1.3 3 4 ------------------------------------------------------------- TLS 1.2 13 9 TLS 1.3 6 7 ------------------------------------------------------------- OSCORE request 5 OSCORE response 3
This section analyzes the overhead of DTLS 1.2 [RFC6347]. The nonce follow the strict profiling given in [RFC7925]. This example is taken directly from [RFC7400], Figure 16.¶
DTLS 1.2 record layer (35 bytes, 29 bytes overhead): 17 fe fd 00 01 00 00 00 00 00 05 00 16 00 01 00 00 00 00 00 05 ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 Content type: 17 Version: fe fd Epoch: 00 01 Sequence number: 00 00 00 00 00 05 Length: 00 16 Nonce: 00 01 00 00 00 00 00 05 Ciphertext: ae a0 15 56 67 92 ICV: 4d ff 8a 24 e4 cb 35 b9¶
DTLS 1.2 gives 29 bytes overhead.¶
This section analyzes the overhead of DTLS 1.2 [RFC6347] when compressed with 6LoWPAN-GHC [RFC7400]. The compression was done with [OlegHahm-ghc].¶
Note that the sequence number '01' used in [RFC7400], Figure 15 gives an exceptionally small overhead that is not representative.¶
Note that this header compression is not available when DTLS is used over transports that do not use 6LoWPAN together with 6LoWPAN-GHC.¶
Compressed DTLS 1.2 record layer (22 bytes, 16 bytes overhead): b0 c3 03 05 00 16 f2 0e ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 Compressed DTLS 1.2 record layer header and nonce: b0 c3 03 05 00 16 f2 0e Ciphertext: ae a0 15 56 67 92 ICV: 4d ff 8a 24 e4 cb 35 b9¶
When compressed with 6LoWPAN-GHC, DTLS 1.2 with the above parameters (epoch, sequence number, length) gives 16 bytes overhead.¶
This section analyzes the overhead of DTLS 1.2 [RFC6347] with Connection ID [I-D.ietf-tls-dtls-connection-id]. The overhead calculations in this section uses Connection ID = '42'. DTLS recored layer with a Connection ID = '' (the empty string) is equal to DTLS without Connection ID.¶
DTLS 1.2 record layer (36 bytes, 30 bytes overhead): 17 fe fd 00 01 00 00 00 00 00 05 42 00 16 00 01 00 00 00 00 00 05 ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 Content type: 17 Version: fe fd Epoch: 00 01 Sequence number: 00 00 00 00 00 05 Connection ID: 42 Length: 00 16 Nonce: 00 01 00 00 00 00 00 05 Ciphertext: ae a0 15 56 67 92 ICV: 4d ff 8a 24 e4 cb 35 b9¶
DTLS 1.2 with Connection ID gives 30 bytes overhead.¶
This section analyzes the overhead of DTLS 1.2 [RFC6347] with Connection ID [I-D.ietf-tls-dtls-connection-id] when compressed with 6LoWPAN-GHC [RFC7400] [OlegHahm-ghc].¶
Note that the sequence number '01' used in [RFC7400], Figure 15 gives an exceptionally small overhead that is not representative.¶
Note that this header compression is not available when DTLS is used over transports that do not use 6LoWPAN together with 6LoWPAN-GHC.¶
Compressed DTLS 1.2 record layer (23 bytes, 17 bytes overhead): b0 c3 04 05 42 00 16 f2 0e ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 Compressed DTLS 1.2 record layer header and nonce: b0 c3 04 05 42 00 16 f2 0e Ciphertext: ae a0 15 56 67 92 ICV: 4d ff 8a 24 e4 cb 35 b9¶
When compressed with 6LoWPAN-GHC, DTLS 1.2 with the above parameters (epoch, sequence number, Connection ID, length) gives 17 bytes overhead.¶
This section analyzes the overhead of DTLS 1.3 [I-D.ietf-tls-dtls13]. The changes compared to DTLS 1.2 are: omission of version number, merging of epoch into the first byte containing signalling bits, optional omission of length, reduction of sequence number into a 1 or 2-bytes field.¶
Only the minimal header format for DTLS 1.3 is analyzed (see Figure 4 of [I-D.ietf-tls-dtls13]). The minimal header formal omit the length field and only a 1-byte field is used to carry the 8 low order bits of the sequence number¶
DTLS 1.3 record layer (17 bytes, 11 bytes overhead): 21 05 ae a0 15 56 67 92 ec 4d ff 8a 24 e4 cb 35 b9 First byte (including epoch): 21 Sequence number: 05 Ciphertext (including encrypted content type): ae a0 15 56 67 92 ec ICV: 4d ff 8a 24 e4 cb 35 b9¶
DTLS 1.3 gives 11 bytes overhead.¶
This section analyzes the overhead of DTLS 1.3 [I-D.ietf-tls-dtls13] when compressed with 6LoWPAN-GHC [RFC7400] [OlegHahm-ghc].¶
Note that this header compression is not available when DTLS is used over transports that do not use 6LoWPAN together with 6LoWPAN-GHC.¶
Compressed DTLS 1.3 record layer (18 bytes, 12 bytes overhead): 11 21 05 ae a0 15 56 67 92 ec 4d ff 8a 24 e4 cb 35 b9 Compressed DTLS 1.3 record layer header and nonce: 11 21 05 Ciphertext (including encrypted content type): ae a0 15 56 67 92 ec ICV: 4d ff 8a 24 e4 cb 35 b9¶
When compressed with 6LoWPAN-GHC, DTLS 1.3 with the above parameters (epoch, sequence number, no length) gives 12 bytes overhead.¶
This section analyzes the overhead of DTLS 1.3 [I-D.ietf-tls-dtls13] with Connection ID [I-D.ietf-tls-dtls-connection-id].¶
In this example, the length field is omitted, and the 1-byte field is used for the sequence number. The minimal DTLSCiphertext structure is used (see Figure 4 of [I-D.ietf-tls-dtls13]), with the addition of the Connection ID field.¶
DTLS 1.3 record layer (18 bytes, 12 bytes overhead): 31 42 05 ae a0 15 56 67 92 ec 4d ff 8a 24 e4 cb 35 b9 First byte (including epoch): 31 Connection ID: 42 Sequence number: 05 Ciphertext (including encrypted content type): ae a0 15 56 67 92 ec ICV: 4d ff 8a 24 e4 cb 35 b9¶
DTLS 1.3 with Connection ID gives 12 bytes overhead.¶
This section analyzes the overhead of DTLS 1.3 [I-D.ietf-tls-dtls13] with Connection ID [I-D.ietf-tls-dtls-connection-id] when compressed with 6LoWPAN-GHC [RFC7400] [OlegHahm-ghc].¶
Note that this header compression is not available when DTLS is used over transports that do not use 6LoWPAN together with 6LoWPAN-GHC.¶
Compressed DTLS 1.3 record layer (19 bytes, 13 bytes overhead): 12 31 05 42 ae a0 15 56 67 92 ec 4d ff 8a 24 e4 cb 35 b9 Compressed DTLS 1.3 record layer header and nonce: 12 31 05 42 Ciphertext (including encrypted content type): ae a0 15 56 67 92 ec ICV: 4d ff 8a 24 e4 cb 35 b9¶
When compressed with 6LoWPAN-GHC, DTLS 1.3 with the above parameters (epoch, sequence number, Connection ID, no length) gives 13 bytes overhead.¶
This section analyzes the overhead of TLS 1.2 [RFC5246]. The changes compared to DTLS 1.2 is that the TLS 1.2 record layer does not have epoch and sequence number, and that the version is different.¶
TLS 1.2 Record Layer (27 bytes, 21 bytes overhead): 17 03 03 00 16 00 00 00 00 00 00 00 05 ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 Content type: 17 Version: 03 03 Length: 00 16 Nonce: 00 00 00 00 00 00 00 05 Ciphertext: ae a0 15 56 67 92 ICV: 4d ff 8a 24 e4 cb 35 b9¶
TLS 1.2 gives 21 bytes overhead.¶
This section analyzes the overhead of TLS 1.2 [RFC5246] when compressed with 6LoWPAN-GHC [RFC7400] [OlegHahm-ghc].¶
Note that this header compression is not available when TLS is used over transports that do not use 6LoWPAN together with 6LoWPAN-GHC.¶
Compressed TLS 1.2 record layer (23 bytes, 17 bytes overhead): 05 17 03 03 00 16 85 0f 05 ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 Compressed TLS 1.2 record layer header and nonce: 05 17 03 03 00 16 85 0f 05 Ciphertext: ae a0 15 56 67 92 ICV: 4d ff 8a 24 e4 cb 35 b9¶
When compressed with 6LoWPAN-GHC, TLS 1.2 with the above parameters (epoch, sequence number, length) gives 17 bytes overhead.¶
This section analyzes the overhead of TLS 1.3 [RFC8446]. The change compared to TLS 1.2 is that the TLS 1.3 record layer uses a different version.¶
TLS 1.3 Record Layer (20 bytes, 14 bytes overhead): 17 03 03 00 16 ae a0 15 56 67 92 ec 4d ff 8a 24 e4 cb 35 b9 Content type: 17 Legacy version: 03 03 Length: 00 0f Ciphertext (including encrypted content type): ae a0 15 56 67 92 ec ICV: 4d ff 8a 24 e4 cb 35 b9¶
TLS 1.3 gives 14 bytes overhead.¶
This section analyzes the overhead of TLS 1.3 [RFC8446] when compressed with 6LoWPAN-GHC [RFC7400] [OlegHahm-ghc].¶
Note that this header compression is not available when TLS is used over transports that do not use 6LoWPAN together with 6LoWPAN-GHC.¶
Compressed TLS 1.3 record layer (21 bytes, 15 bytes overhead): 14 17 03 03 00 0f ae a0 15 56 67 92 ec 4d ff 8a 24 e4 cb 35 b9 Compressed TLS 1.3 record layer header and nonce: 14 17 03 03 00 0f Ciphertext (including encrypted content type): ae a0 15 56 67 92 ec ICV: 4d ff 8a 24 e4 cb 35 b9¶
When compressed with 6LoWPAN-GHC, TLS 1.3 with the above parameters (epoch, sequence number, length) gives 15 bytes overhead.¶
This section analyzes the overhead of OSCORE [RFC8613].¶
The below calculation Option Delta = '9', Sender ID = '' (empty string), and Sequence Number = '05', and is only an example. Note that Sender ID = '' (empty string) can only be used by one client per server.¶
OSCORE request (19 bytes, 13 bytes overhead): 92 09 05 ff ec ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 CoAP option delta and length: 92 Option value (flag byte and sequence number): 09 05 Payload marker: ff Ciphertext (including encrypted code): ec ae a0 15 56 67 92 ICV: 4d ff 8a 24 e4 cb 35 b9¶
The below calculation Option Delta = '9', Sender ID = '42', and Sequence Number = '05', and is only an example.¶
OSCORE request (20 bytes, 14 bytes overhead): 93 09 05 42 ff ec ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 CoAP option delta and length: 93 Option Value (flag byte, sequence number, and Sender ID): 09 05 42 Payload marker: ff Ciphertext (including encrypted code): ec ae a0 15 56 67 92 ICV: 4d ff 8a 24 e4 cb 35 b9¶
The below calculation uses Option Delta = '9'.¶
OSCORE response (17 bytes, 11 bytes overhead): 90 ff ec ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 CoAP delta and option length: 90 Option value: - Payload marker: ff Ciphertext (including encrypted code): ec ae a0 15 56 67 92 ICV: 4d ff 8a 24 e4 cb 35 b9¶
OSCORE with the above parameters gives 13-14 bytes overhead for requests and 11 bytes overhead for responses.¶
Unlike DTLS and TLS, OSCORE has much smaller overhead for responses than requests.¶
This section analyzes the overhead of Group OSCORE [I-D.ietf-core-oscore-groupcomm].¶
TODO¶
DTLS 1.2 has quite a large overhead as it uses an explicit sequence number and an explicit nonce. TLS 1.2 has significantly less (but not small) overhead. TLS 1.3 has quite a small overhead. OSCORE and DTLS 1.3 (using the minimal structure) format have very small overhead.¶
The Generic Header Compression (6LoWPAN-GHC) can in addition to DTLS 1.2 handle TLS 1.2, and DTLS 1.2 with Connection ID. The Generic Header Compression (6LoWPAN-GHC) works very well for Connection ID and the overhead seems to increase exactly with the length of the Connection ID (which is optimal). The compression of TLS 1.2 is not as good as the compression of DTLS 1.2 (as the static dictionary only contains the DTLS 1.2 version number). Similar compression levels as for DTLS could be achieved also for TLS 1.2, but this would require different static dictionaries. For TLS 1.3 and DTLS 1.3, GHC increases the overhead. The 6LoWPAN-GHC header compression is not available when (D)TLS is used over transports that do not use 6LoWPAN together with 6LoWPAN-GHC.¶
New security protocols like OSCORE, TLS 1.3, and DTLS 1.3 have much lower overhead than DTLS 1.2 and TLS 1.2. The overhead is even smaller than DTLS 1.2 and TLS 1.2 over 6LoWPAN with compression, and therefore the small overhead is achieved even on deployments without 6LoWPAN or 6LoWPAN without compression. OSCORE is lightweight because it makes use of CoAP, CBOR, and COSE, which were designed to have as low overhead as possible.¶
Note that the compared protocols have slightly different use cases. TLS and DTLS are designed for the transport layer and are terminated in CoAP proxies. OSCORE is designed for the application layer and protects information end-to-end between the CoAP client and the CoAP server. Group OSCORE is designed for group communication and protects information between a CoAP client and any number of CoAP servers.¶
This document is purely informational.¶
This document has no actions for IANA.¶
The authors want to thank Ari Keraenen, Carsten Bormann, Goeran Selander, and Hannes Tschofenig for comments and suggestions on previous versions of the draft.¶
All 6LoWPAN-GHC compression was done with [OlegHahm-ghc].¶